Media Blast Cleaning - University of Toronto T-Space · Starch Media blast c1e-g is an...
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Starch Media Blast Cleaning: Properties and Performance of Masking Tapes and Aged Paint Films Payarn Tangestanh A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Department of Mechanid and Industrial Engineering University of Toronto O Copyright by Payarn Tangestanian 1999
Transcript of Media Blast Cleaning - University of Toronto T-Space · Starch Media blast c1e-g is an...
Starch Media Blast Cleaning:
Properties and Performance of Masking Tapes and Aged
Paint Films
Payarn Tangestanh
A thesis submitted in conformity with the requirements for the degree of
Master of Applied Science
Department of Mechanid and Industrial Engineering
University of Toronto
O Copyright by Payarn Tangestanian 1999
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Abstract
Starch Media blast c1e-g is an enViTonmentally benign method for
coatiog removal nom ahraft. The main objective of this work was to provide
a method for predicting the paht stripping rate of an aged paint system,
througb a know1dge of some fundamental physicai properties of the paint
substrate system, i.e., hardness and modulus of eldcity. Ii was shown that
aging of aluminum panels painted with polyurethane significantly increased
the hardness of the coatïngs, while the process did not have a significaat
effect on the coating modulus of elasticity. It was a h shown that the c o a ~ g
became more erosion resistant as it aged. Finally, it was shown that it is
possible to predict the paint removal rate by in situ measufement of coating
dynamic hardness ( d c i e n t of restitution) using Leeb's principle.
The effectiveness of masking tapes in resisting erosion and edge Lifting
during media blasting was investigated. It was concluded that the erosion
resistance of masking -tapes depends on the coefficient of restitution; the
higher the coefficient of restitution the l e s the plastic deformation of the tape
and the greater the incident kinetic energy restored in the springback
Finally, a mode1 was developed to explain the erosion resistance of the
masliag tapes and aged coatuigs based on two material properties, the
modulus of elasticity and the dynamic hardness. It was concluded that the
higher the coefficient of restitution, the more erosion resistant is the coating.
TABLE OF CONTENTS
.. Absîract .............O.............. ......................................................................... II
.. 0
Table of Contents ..................................................................................... III
List of Figures .......................................................................................... vi
List of Tables .....ww......-......1...-...............................o......-..................... S ... .................................................................................. Acknowtedpcnts UIR
Iiib9duction ...... ....................H..e..............................e....................... 1
.................................................... . 1 -1 Starch Media Blast Leaning 1
.................................................................. 1.2. Thesisobjectives 2
................................................................. 1 .3 . Literawe Review 3
1.4. Thesis Outiine ....................................................................... 6
Experimental Setups . . ~ ~ . . . . . . . . . . m ~ ~ . . . . ~ ~ ~ ~ ~ ~ m ~ t ~ ~ ~ ~ t ~ ~ m ~ ~ ~ ~ ~ 7
2.1. Blast CleaniBg Equipment ................................................. 7
............................ 2.1.1. Definitions of Blasting Parameters 9
............................ . 2.1 .2 Blasting Experimental Procedure 1 1
................................................................... 2.2. Peel Test Setup 12
................................................. 2.3. Vkkers Hardness Testing 13
............................. 2.4. Uhrasonic Time of F1ight Measurement 16
............................................................ 2.5. Gas Gun Apparatus 18
.............................................. 2.6. Velocity Measurernent Setup 20
PART 1
3 . Effectiveness of Masking Materials in Starch Media Blasting ... 23
...................................................... 3.1. General Observations .. 24
....................................... 3 .2 . Perforation and Edge-lifting T m 25
..................................................... 3 .2.1. Perforation Tests 25
3.2.2. Mass-fossTests ................................................... 31
.................................... . 3 .2.3 Scratch Hardness Tests 38
.................................... 3 .2.4. Coefficient of Restitution ... 39
3.2.5. Efféct of Contamination on Edge-lifting .................. 42
3 .2.6. Edge-iifting and the Hot Melt Adhesive ................. -44
.......................................................................... 3 -3 . Peel Tests 46
... 3.3.1. Effêct of Adhesion Time on the Peel Strength .. .... 46
......... 3 -3 .2 . Effect of Contamination on the Peel Strength -48
... 3.3 .3 . Effect of Relative Humidity on the Peel S trength -49
3 -4- Summary and Discussion of Masking Tape Pefiormance ... -54
4 . Changes in Orgnic Coiting Roperties Due to Aging .............. 57
4.1 . Accelerated Aging of Painted Panels ................................. 57
................... 4.1.1. The Paint System Used in Experiments 58
4.2. Changes in Psint Hardness .................... .... ................... 59
.................................. 4.2. 1. Pend Scratch Hardness Test 60
............................................ 4.2.2. Vickers Hardness Test 61
............................ 4.2.3. Dynamic Hardness Measurement 62
........................................ 4.3. Changes in Moduius of Elasticity 67
BIast Cieaniirg EsperUaenîs with Fresb and A g d Coatings ...... 70
.......................................................... . 5.1 Paint Stripping Rates 71
.................................................................. 5.2. Paint Thickness 72
..................................*.. .......................... 5.3. Work Exposure .. 73
5.4. Results ............................................................................... 73
.................................................................. 5.5. Observations .... 77
.......................................................... 5 .6 . Uncertainty Anaiysis 84
........................ 5 .6.1. Uncertainty in Paint Stripping Rates 84
56.2. Uncertainty in the Work Exposure .......................... 84
5.6.3. UnceWnty in Coahg tbichess Maisurement ...... - 8 5
Discussion rad Conclusions .....O ...............................~.................. 87
................. 6.1. Analysïs of Impact on Tapes and Painted Panels 87
6.2. Mechanid Propertïes Meeting Coating System Behaviour 91
.................................................. . 6.2.1 Dynamic Hardness 91
6.2.2. Young's Modulus ................... .. ....................... 94
........................................ 6.2.3. Coefficient of Restitution 94
.................................. 6.2.4. Leeb's Principle of Operation 95
............................................................. . ..... 6.3 Conclusions ..... 96
......................................................... 6.3.1. Making Tapes 96
..................... 6.3 .2 . Effécts of Agiag on Painted Panels .... 97
LIST OF FIGURES
Page
Figure 2.1 : Blast deaning ficility ................................................................... 8
Figure 2.2. Angie of attack and standoff distance ........................................... 9
............................................................... Figure 2.3 : Needle pressure gauge 10
Figure 2.4: Slidiog table setup, 1- Nozzle, 2- Adjustable clamp, 3- SIiding
table, 4- Screw shaft, 5- DC motor .............................................................. I l
Figure 2.5. Layout of the traces on the sample panels ...........................ces...... 12
Figure 2.6. Peel test apparatus ..................................................................... 13
Figure 2.7: Experimental setup for dtrasonic timesf-flight measurements .. 1 7
Figure 2.8: Gas gun setup used for rneasurement of particle's incident and
....................................................................................... rebound velocities 19
Figure 2.9. Experimental sehip for velocÏty meastuement ............................ 21
Figure 2.10: Multiple nposure picture obtained with ELlVifostrip 12/30, 207
@a, 4.08 kB/m in. ........................................................................................ 22
Figure 3.1 : Promac tape &a 5 seconds exposure to the blast ..................... 27
Figure 3.2. 3M tape &a 5 seconds expom to the blast ............................ 27
Figure 3 -3 : Bron tape after 5 seconds exposure to the blast ......................... 28
................... Figure 3 -4: Prornac tape after 10 seconds exposure to the blast -28
Figure 3.5. 3M tape &er 10 seconds exposure to the biast .......................... 29
Figure 3 -6: Bron tape after 10 seconds -sure to the blast ........................ 29
Figure 3.7. Angle of attack a is negative when the blasting particles and the
sliding table are moving in the same direction ..... .. .................................. 3 1
Figure 3.8: Angle of aîtack a is positive when the blssting particles and the
sliding table are moving in the opposite direction -..................ectiecti..ecti.......ectiecti.. .ecti.ecti. 32
Figure 3 -9. ï h e n o d e is moving towards edge A and away fiom edge B. ... 32
Figure 3.10: Blasting Promac tape at 70° (lefi) and 45O (right) ..... ..... ..----..-.. 33
Figure 3.1 1 : 3M tape blasted at four different orientation .. ...... . .. . . . . . 36
Figure 3.12: Pend scratch test marks on the Bron tape .....--....--.- .. ...-.-.-..-..- 39
Figure 3-13 : Coefficient of rrstitution as a hction of incident normal velocity
for masking tapes. . . . . . . . . . . . . . . . . . . . .- -. *. -. . -. . . -. . -.. . - - -. . - - - - -. - - -. . -. -. . . . . . . - - -. - - - -. . . - -- - - - - - -. .41
Figure 3.14: Edge-Ming of Promac tape on a contmhated panel, top view 43
Figure 3.15 : Edge-lifting of Promac tape, side view . . . . . . . . . . . . . . . -. . . - - - -. -. . . - -. . - - -. 44
Figure 3-16: Edge-Lifting of Bron tape on a contambated panel. ............ .... . -44
Figure 3 - 1 7: Edge of the Bron tape is protected by hot melt. . . . .. . . . . . . . . . . . . . . -. . - 45
Figure 3.18: The effect of bonding time on 4S0 p e d stxngth of Bron tape at
a rate of 12-7 mmlmin, . , . .. . ,. , . , . -. -. , -. - min, min, min, -. . ..min,min,. -. min, . . -. -. . . min,. . . . -. . . . . . . . .min, min, min,. min, min, -. . min, min, min, min, min, min,-. -. -. -- 47
Figure 3.19: Peel test results at digerent IeweIs of contamination, 0 Bron
dean, O Bron contaminateci, X 3M clean, A 3M contaminateci, OPromac
cleanin, 0 Promac contaminated.-. . .- . . . . -. -- -- - . - - -. -. - - .- -. . . -. . -. . -. . -. -. -. . . - . . . . . . -. . . . . . -49
Figure 3.20: Humidity control chamber, note that in view A-A the silicone
sealant and the sealing tape around the foam disk and cover edges are not
s h o w . .. .. . . . . -. . . . , . . . . . . . . -. -. -. - -. . - - - ..-.. .--.-- --.. . -.. . . -. .--....--- -. . . . . . -. -. - -. . - - -- - -. - -. . . . - - -. . . - 5 1
Figure 3.21: Promac tape peel test at 45O, 6.35 mmlmïn (0.25 idmin.),
0 sample 1 RH. 90'36, 30°C, O sample 2 RK W h 30°C, A sample 3 lab. - -
condtt~oo- ...................-.--..--.-*. ..-.... ...-........-..........-.-..--.....-...--........-.--.--.-.+.. 52
Figure 3.22: Bron tape peel test at 4S0, 6.35 d m i n . (0.25
idmin.), O samp1e 1 RH. Wh, 30°C, a sample 2 RH. 90% 30°C, A lab.
condition .....,.......,................-m..--..--- .. -.-.-----.-....... ..................................... 52
Figure 3.23: 3M tape peel test at 4S0, 6.35 d m i n . (0.25 i n . / . ) , O sampie
1 RH. 90%, 30°C, simple 2 RH- 90% 3O0C, A lab. condition .........---.-. 53
Figure 4- 1: Vickers hardness values (25 gf) of paint films due to aging (100
OC), old exterior airmaft panels, O fie& and artificially aged paneis. ..... . . -6 1
Figure 4.2: CoeScient of restitution, e, for the aged and fiesh panels. Error
bars represent + lstandard deviation in the 3 trials for each exper imd data
points. .......................... .. ...................................................................... 63
Figure 4.3. Threedhensional profile of an impact site. ............. .. ... ,.. . eeeeeeee..ee.ee 64
Figure 4.4: WYKO two-dimensional profile . -. . . . - . . . . . . . . . . . . . -. . -. . . . .- . . . . -. . . - -. -. . nsi -. -64
Figure 4.5: Two-dimensional cross-section of impact site in coating ......... ... 65
Figure 4.6: Velocity of sound in paint fiims, using ultrasonic timeof-flight
measurement. Error bars represent *I standard deviation (I30 d s ) in the 5
trials for each experimental data point. ...-........... .-...rimrimrimrim-rimrimrim.rim..rim.rimrim..rim.....rim-..-..~..~. 68
Figure 4.7: Modulus of elasticity of painted panels. Emor bars represent *1
standard deviation (* 100 MPa) in the 5 trials for each experimental data
point. ............................................................... .. ...-...--..---.-.*-. 69
Figure 5.1: Thickness of topcoat renioved fkom fieshly painted afuminum
substrate as a hct ion of work exposure, using Envirostrip 30/1ûû and two
Merent noale angles. The available data fiom DjurovÏc et ai. [2] are plotted
for cornparison. ...... .. .. . . . .. ... .... ,.. . .- .. -.---.-... -- --. --. .. -. --. -.-.-..-. .- .- .. . .. . . .*-. . . . .- -. .- 74
Figure 5.2: Coating thichess removal vs. work exposure for fksh and aged
panels, noale angle of attack at 4S0. ................... ... ................................. 76
Figure 5.3 : Fresbly painted panel, table speed 1 -2 mlmin (4 Wmin), media
flow rate 5-44 kg/& (12 Iblmin), pressure 207 kPa (30 psi), angle of attack
20°. . ,,. . . . . ....,.-.- - .. . . . - - - - -. . -* -. . -. - -. - -- . . . . . . . . . . . . - . . . . . . - . . . . . . . . . - . . . . 7 8
Figure 5.4: Panel aged for 1 day, table speed 1.2 m/mh (4 ft/min), media flow
rate 5.44 kg/min (12 lblmin), pressure 207 kPa (30 psi), angle of attack 20" 78
Figure 5.5: Panel aged for 2 days, table speed 1.2 mimin (4 Wmin), media
flow rate 5.44 kg/min (12 Ibhin), pressure 207 kPa (30 psi), angle of attack
............................................................................................................. 20° 7 9
Figure 5.6: Panel aged for 4 days, tabIe speed 1.2 m/min (4 Wmin), media
flow rate 5.44 ks/min (12 Iblmin), pressure 207 kPa (30 psi), angie of attack
............................................................................................................. 20° -79
Figure 5.7: Panel aged for 8 days, table speed 1.2 m/min (4 Wmin), media
flow rate 5.44 kgmin (12 Iblmin), pressrue 207 kPa (30 psi), angie of attack
............................................................................................................. 20" 8 0
Figure 5 -8: Old aircrafk exterior panel, table speed 1.2 dmin (4 fümin), media
flow rate 5.44 kghin (12 lblmin), pressure 207 kPa (30 psi), angle of attack
20" .............................................................................................................. 80
Figure 5.9: Freshly painted panel table speed 1.5 d m i n (5 ftlmin), media
flow rate 5 -44 kg/min (1 2 Ibhin), pressure 207 kPa (30 psi), angle of attack
20° ...-........-.......-......................................................................................... 81
Figure 5.10: Panel aged for 1 day, table speed 1.5 dmin (5 Wmin), media
flow rate 5.44 kgmin (12 Ibhin), pressure 207 kPa (30 psi), angle of attack
.............................................................................................................. 20" 8 1
Figure 5.11: Panel aged for 2 days, table speed 1.5 mimin (5 Nmin), media
flow rate 5.44 kgmin (12 Ib/min), pressure 207 kPa (30 psi), angle of attack
.............................................................................................................. 20° 82
Figure 5.12: Panel aged for 4 days, table speed 1.5 m/mh (5 Wmin), media
flow rate 5.44 kg/& (12 lb/mh), pressure 207 kPa (30 psi), angle of aaadt
20" .............................................................................................................. 82
Figure 5.13: Panel aged for 8 days, table speed 1.5 mhin (5 Wmin), media
flow rate 5.44 kg/rnin (12 lb/min), pressure 207 kPa (30 psi), angle of attack
20" .............................................................................................................. 83
Figure 6. l (a)Assumed (elastic-piastic), actud (elastic-plastic), and f d y
plastic m e s of: (a) forcedeflection, and (b) mean pres~~~edeflection. ...... 89
Figure 6.2 : Geometry of assumed revefsl'ble rebound process ..................... 90 Figure 6.3: Plot of mean contact pressure vs. depth of penetrattion, for
comparing two coatings with the same Young's modulus and different
dynarnic hardness. ............. ... ....................................................................... 92
LIST OF TABLES
Page
.............................................................. Table 3-1: Perforation test results 25
Table 3 -2: The range of sliding table speeds, U (dmin), for complete removal
................................................................................................... of the tape 30
Table 3 .3 . Mass loss with different blasting parameters . Blasting pressure was
.................................................................... 1 3 8 kPa for al1 the expaiments 34
.... Table 3 -4: Mass loss of tapes at Merent orientations on the sliding table 37
................................................................ Table 3.5 : P e n d hardness vaiues 38
Table 3-6: Coefficient of restitution, e, of three masking tapes for an impact
test with steel sphere of 1.5 mm diameter at six differmt impact velocities .
Number of repetitions N, and standard deviation of the results SD are given in
............................................................................................... parentheses -40
Table 3.7: The effkct of bonding time on the 4S0 peel strength at a rate of 6.3
d m in. The peel force is shown in N f m width of the masking tapes ...... 47
Table 3.8 : Peel stfength (Nfmm of tape width) as a h c t i o n of contamination
....................... leveis (obtained by applying molybdenum desulnde greax) 48
Table 3 -9: P d strength @Umm) as a fùnction of relative humidity .............. 50
Table 3.10. Results of 4S0 peel test at a rate of 6.35 rnm/min ................... 5 4
............................................. Table 4.1 : Pend scratch hardness test results 60
Table 4.2: Velocity of sound and Young's modulus in paint films, N=5
.............................................................................................. S M 0 m/s 6 8
.......................... Table 5.1 : Paint stripping rate of panels (topcoat remod) 72
Table 5.2: The average coating thickness removed fiom the paiated paneis . 75
Table 5.3: Average paint ttrickness removed (i standard deviation) with the
flat nozzle at 4.08 ks/min and 138 Wa, table speed u=1.2 mhin (4 ftlmin),
work exposure 1 -790 ~JM. average particle veloQty 1 57 mls ................... -86
......................................... Table 6.1 : Cornparison of fksh and aged paneis- 93
xii
ACKNOWLEDGMENTS
1 would Wce to express my appreciation for the guidance and support of my
supervisor Professor Jan K Spelt dwing the course of this project. 1 wouid
ais0 like to thanic my coiieagues Marcello Papin4 Boris Djurovic, Yijun Tu
and Shuwen Wang for their technid assistance and fiiendship.
This work had been made possiile by the financial and material support of the
Naturd Sciences and Engineering Research Councii of Canada, Department
of Nationai Defence, ADM/ûgiivie Ltd., and CAE Electronics Ltd.
C h a p t e r 1
I n t r o d u c t i o n
Paint stripping and repainting of a i r d surfaces are required periodidy during
the operating Metirne of an a i r c d Histoncaiiy, paint removai has been achieved with
chemical strippers, however, the use of methylene chloride and phenol-based chemical
strippers for aircraft paint removal generates large quantities of hazardous waste and
creates bealth and s a f i problems for operating personnel.
Strict environmental regdations banning widely-used but toxk chemical paint
strippers are forcing the aerospace industq to find new ways to remove paint fkom
aircraft. The new methods developed for removing coatings f b m aircraft and aimaft
components include emrûonmentally d e chemical strippers and paint softeners; optical
methods such as lasers and fia& lamps; and mechanid methods using abrasive media
such as plastic, wheat starch, walnut sheüs, ice and dry ice.
1.1 Starch Media Bast Clesmiag
One alternative method is wheat starch blast cleaLUcLg, which is being used
increasingly to remove organic coatings fiom substrates. In tbïs paint stripping method, a
Stream of wheat starch particles is directed towards the coated substrate, and the coating
is removed by means of erosion
Wheat starch blast cleaning is an environrnentaiiy benign method since the media is
non-toxic and biodegradable, The dust waste produced can often be classified as non-toxic
by app1-g bio-remediation methods to reduce the amount of waste and to separate
hazardous components such as paint, sealants, and masking tapes.
Due to its chernical inertness and the softness of the medium, the use of wheat
starch for dry stripping of aircraft paint is applicable to aluminum alloys and polymer
matrix composites. The process is also capable of selective stripping, removing only the
top layer of a paint system and leaving the primer intact, thus reducing the amount of
waste generated and the cost of repainting-
1.2 Thesis Objectives
The diff idty of removing paint fiom aircraft varies greatly due to m o r s such as
the number of coats, age and condition of paint, and the presence of other coverings such
as decals. Efficient automation of paint stripping r e m s the prediction of paint stripping
rates at various places over the aircraft.
One of the objectives of this work was to provide a method for predicting the paint
stripping rate of an aged paint system, tbrough a knowledge of some hdamental physical
properties of the paint substrate system, i-e., hardness and moduhis of elasticity.
Particle velocity at the exit f?om the n o d e is one of the main parameters
goveming the paint stripping process. An objective of this research was to develop and
test the feasibility of a method for optical velocity measurement of blasting particles of
varying sizes. The objective was to observe the velocity distribution of different particle
sizes in a stream of blasting starch media
Another important issue in stripping paint kom a i r d is protection Erom media
ingress to sensitive areas such as engines as control systems. Masking materials such as
plastic sheets, plugs and covers, PSA tapes and hot melt adhesives must resist media
penetration and erosion
The present research investigated the effectiveness of difEerent types of PSA
masking materials in ressting erosion and edge-lifting during starch media blasting.
1.3 Literature Review
Starch media paint stripping technology appeared in the early 1990's. As with the
erosion of organic coatings in general, most pubiished studies have been semi-ernpiricai
and have not addressed questions of a fbdamentai nature.
Shipway and Hutchings Cl] evaluated coating durability by measuring the dose of
erodent particles required to penetrate the coating. They noted that for meanin@ results
in these type of tests, the total depth of disruption of the coating by particle impact must
be only a small fiaction of the coating thickness. They also provided an excellent review of
the generai problem of solid particle erosion of coatings.
Djurovic et ai. 121 studied the erosion of a typical aerospace urethane paint and an
epoxy primer fiom duminm and composite substrates using streams of wheat starch
media. They examined the impact sites, measurd the particle size, shape and velocity, and
determined the paint stripping rates as a function of strearn power and impact angle. For
both substrates the following were concluded:
The rate of coating removai increased as the average particle size decreased,
iargely as a result of an increase in the average particle velocity.
For a given mix of the wheat starch particles, the rate of coating removal increased
with the increasing blasting pressure and decreasing mass flow rate, since both of these
tended to ïncrease the average particle velocity.
The rate of mathg removal did not correlate with the average stream power alone;
for a given total power, the aggressiveness of the stream was proportional to the average
velocity .
The urethane topcoat was eroded gradualiy, with several impacts being required at
a single site before it was penetrated to the primer-
Preliminary evidence suggested that both the average particle size and the size
distribution play a role in determining the erosion aggressiveness of a mOr of wheat starch
particles. [2]
Papini and Spelt [3] d y z e d the erosion of the same aerospace paint system used
by Djurovic et al. on ahimiaum substnite due to the impact of individual steel spheres.
They adjusted the incident velocity so that the impacthg particle did not completely
penetrate the coating to the substrate; therefore, it was possible to mode1 the impacts to
predict crater size, shape and rebound parameters as fùnctions of incident velocity and
angle. They observed that this type of coating cannot be made to delaminate, regardless of
impact conditions due to their high hardness and interfacial strength. These coatings must
be removed by mechanicd erosion
Ln another paper, Papini and Spelt [4] examineci the collision of single glass beads
with steel samples coated with an alkyd paint. The examination of impact sites fiom these
experiments revealed behaviour consistent with coating delamination due to the buckliog
of the coating. They concluded that the normal velocity of the particles is the dominant
factor in modeling the coating removal.
Al1 these investigations were on fiesh paints, while in practice, it is usually the
aged paint which is being stripped.
Surprisingly, in the field of organic coatings, physical aging has not been
exteasively investigated in the published Literature. Accordkg to J.M.Hutchùison [SI,
"physical aging" is a change in a property of the polymer as a fùnction of time, at an
elevated temperature, at zero stress and under no infiuence fkom any other extemal
conditions. Perera and Schutyser [6] studied the e f f i of physical aghg on thennal stress
development in coatings. The studies, mainly with plastic materials showed that the
density, elastic moduhis and hardness increase with aging, while the stress relaxation rate
and ultimate elongation decrease- It is well established that, contrary to chernical aging
(iduced by W radiation, temperature and moimire) which usuaiiy provokes irreversible
changes, the effects of physical aging can be eliminated by heating the material at a
temperature above Tg for sufficient time.
Saimoto and van Prooijen [7'j studied the physical aging of paints using micro-
indentation testing. They measured the hardness of silica modified paints aged fiom 2 to
IO years at 3 locations. They reporteci that the perfocmance of the paint is more sensitive
to aging time than to the exposure site (the ambient temperature and degree of ultra-violet
radiation).
Nicholas and Darr [8] investigated the effkct of accelerated weatheriag on the
stress distribution and mechanicd performance of automobile paint systems. They used
h i t e element anaiysis to compute the stress distriiution in automotive paint systems. They
mentioned that a bwledge of the stress distribution in paint systems is crucial to
designing accelerated w e a t h e ~ g tests and understanding the differences between failures
in out door exposures and accelerated weathering.
The eEect of substrate on the adhesion of pressure-sensitive adhesives (PSA) has
been studied by many authors 19, 23, 241. It is well documented that the bond strength of
a PSA is highly dependent on the buik properties of the PSA, such as Young's modulus,
E, and fracture energy, G c , as weii as the substrate roughness and surface energy.
Other factors that are known to affect the bond strength of a PSA on a given
substrate are the adhesive contact time and the adhesive Iayer thickness (the "coat
weight").
RH. Mann and J.T. Tse [9] examined the combined effect of adhesive contact
time, adhesive coat weighf substrate surface energy and substrate roughness on PSA
adhesion. They showed that adhesion on a given substrate increased with higher adhesive
coat weight and longer contact the. Also, adhesion was proportional to the substrate
surface energy and was greater on smooth surfaces.
In order to explain the stability of PSA against aging and water attack, Brockrnann
and Huther [IO] presented a mode1 of dynamic adhesion for PSAs. Pressure sensitive
adhesives are able to replace physicai bonds, destroyed by environmental influences, due
to their rheology and high molecular mobility, therefore the behaviour of PSAs is strongly
infiuenced by rheological factors. They 1101 noted a great diilicuity in using commercially
available adhesives because the compositions were not known, therefore they
recommended the use of a mode1 PSA with hown adhesive and backing material
pro perties.
The erosion of masking tapes has not been investigated in the past, Buciinski [ 1 11
qualitatively investigated the resistance to particle abrasion of selected plastics. His generai
result was that the more easily the material deforms in contact with a particuiar abrasive,
the better is the abrasion resistance.
The thesis is divided into six chapters. Cbapter 2 describes the experirnental
facilities and procedures that were used in the present research.
In Chapter 3, the e f f i e n e s s of masichg materiais in starch media blasting is
investigated, and the perfiormance ofthree types o f masking tapes is evaluated.
Chapter 4 discusses the changes in organic coating properties, (hardness, and
modulus of elasticity) that can occur in service and during accelerated aghg.
In Chapter 5, paint stripping test reSuLts for d i f rent aged panels are represented,
and the paint stripping rate is correlated to the agùïg of coating,
Chapter 6 is dedicated to discussion of the results of the experiments performed on
dflerent aged panels, the overail conclusions are also presented in this Chapter.
C h a p t e r 2
E x p e r i m e n t a l Setups
In order to characte- the response of paint films and masking materials to blast
deaning, several experimental hacilies were used: blast cleaniog equipment, peel testing
apparatus, Vickers micro-indentation hardness testing machine, ultrasonic time-of-flight
measurement setup and a gas gun. A detailed description of these setups and the
experimental procedures are given below.
2.1 Blast Cleaning Equipment
Starch media blast cleaning equipment is gmilar to that used for other media such
as plastic or g l a s beads. Figure 2.1 ilhistrates the main components of the blast cleaning
faciiity used in the present research. It consisteci of thne major components:
1 . a cabinet enclosure in which the test panels were blasted;
2. a reclaimer and blast machine which is used for storage of media, recyclhg used
media, and creating the blast Stream; and
3. a dust collection system consisting of a group of tube filters for wliecthg dust and
releasing clean air into the ewironment.
The starch media was initiaMy stocked in the blast machine. The media valve
controIIed the mass flowrate of the media, which was fed into the compresseci air stream
at the bottom of the blastkg machine- The air-abrasive Stream was brought to the blast
cabinet via a flexible rubber hose at the end of which a nozzle accelerated the starch
p articles.
AU the blasting was done inside the cabinet enclosure to prwent the dispersion of
potentially hamfùl du* and to allow the media to be recycleci. The used media was
collected at the bottom of the cabinet, and was carried to the reclaimer, where the fine
particles were separated from larger ones, which feli into the blast machine to be reused.
The h e r particles ended up in the dust wllector, where they were discarded.
The cabinet used for the present research was manufâctwed by Clemco (model
PCN 4050). The c o m p r d air for blasting was supplied by an air cornpressor
(Broomwade model V750, 200 hp) up to a maximum pressure of 35 psi, and was dried to
a dew point of 4.4 OC 140 by aa air drier (Atlas Copco model FD80). The node was
provided by CAE Electronics Ltd. It was a flat node (Vista, rectangular cross section)
with an exit sue of approximstely 38 mm x 6.3 mm [1.5* x W ] , and was designed to
provide uniform energy distnbuton aaoss the Stream of blasting particles.
2.1.1 Definitions of Blasting Parameters
The following parameters are generaliy acccpted as sigdkant the dry hpping
process :
nozzle standoff distance
angle of attack
blast pressure
media flowrate.
The nozzle standoff distance was m+rsiacd dong the axk of the nozzle, frwi the
nozzle tip to the test surface (Figure 2.2)- 2). parameter was held constant at 15 cm [6
inches ] throughout ai i experiments.
The angle of attack (a) is the smailest angle between the axis of the nozzle and the
surface to be depointeci. The angle of attack a was dehed to be positive when the sliding
table was moving in the direction opposite to the strearn of blasting particles, as
illustrated in Figure 2.2, and it was negative whca the blasting particles and the sliding
table were moving in the same direction,
Standoff
is positive
, L Direction of motion of the siiding table
Figure 2.2: Am& of attdc and stamdoff distance
Blast pressure was the pressure of the ai . measued at a point just Wore the
nozzie using a needle pressure gauge, as shown in Figure 2.3. The pressure at the blast
machine was kept constant by a pressure regulator, but it bad to be adjusted with
reference to the n o d e pressure to take in account the pressure tosses in the blast hose.
Figure 2.3: Ntedle pressure gauge
O
Media flowrate m was the mass of blasting particles flowing nom the blast nozzle
per unit the. The media flowrate was adjusted by the media control vahre of the blast
machine. Any change of the blast pressure or media size led to a signincant change in the
media flow rate, so the flowrate was re-adjusted for each experiment. O
To masure the flowrate m , the blast cabinet was fkst wmpletely emptied of
media, then the cabinet was filled with a known m a s of media, M. By pressing the foot
valve, the blasting process was started and a timer was used to measure-the duration of
the blasting, At, (the blast machine made an easily identifiable sound when media were
exhausted). The flownite was then caldated using the formula
The pressure and flowrate were unstable during the e s t and last few seconds of
blasting, so panels were not blasted during these periods.
Early experiments showed that the control of the mass flowrate obtained with the
media valve was not very good, aad that any change in the type of media or in the
pressure led to large changes in the mass fiow rate. Therefore, the actual mas fiowrate
was monitored during the arpaiments by weighing the media input to the blasting
machine and measuring the time of blasting.
2.1.2 Blasting Experimental Procedure
The p a . stripping rate of the panels were determined by the foflowing
experimental procedure:
To Vary the exposure of the samples to the abrasive Stream, a sliding table was
used, as shown in Figure 2-4.
Figure 2.4: Sfidbg tabk m p , 1- Nde , 2- Adjustabk damp, 3- Sliduig table, 4- Screw shaft, 5- DC motor
Nonle was heid rigidly in position by an adjustable clamp, and the sample moved
under it (see Fig 2.4). The siidhg table was driven by a DC motor which aiiowed precise
control of the speed fkom about 5 mmls [l ftfrnin] up to 115 mm/s C22.5 A/mio].
The sliding table speed was monitored by measuring the time it travels a distance
of 30 cm, using a digital chronometer, with an accuracy of 3 percent.
The adjustable clamp was used to prrcisdy Iocate the nozzle above the
predetermined position of the traces on the panels. The nozzle angle was adjusteci wiih an
accuracy of f 0.5 degree- Up to a maximum of eight traces were made on each face of the
panel (see Figue 2.5).
Each trace was 15 cm long and wverd half the length of the panel. When doing
experiments on one haif of the panel, the other halfwas masked with an alumiaum plate to
prevent interference between the tests. The lateral distance betwezn the center of the
traces was 7.5 cm, which was sufEcient to lave a band of undamageci paint between
adjacent traces, as shown in Figure 2.5.
Figure 2.5: Layout of tbe traces on tbe swpk pin&
2.2 Peel Test Setup
In order to mesure the peehg force of different pressure sensitive adhesives, a
test apparatus for peei testing at an angle of 45" and an Instron-1000 testing machine were
used. The peel specimen was clamped on a 45" angle block and the Bmbe pressure
sensitive adhesive (PSA) tape was held using a fiction grip, as shown in Figure 2.6. The
fiiction grip was comected to a long flexible wire, approximatey 60 cm, which was
attached to the cross-head of the Instron-1000 ttsting machine. The long steel wire
ensured that the peel angle was kept to 45 * 3" when the flexi'ble PSA tape was peeled
fiom aiuminum su bstrate.
Steel wire (0.6 m long) T- - cross-head
l Angle Block l Fi- 2.6: Peel test apparatus
nie peel testing was conducteci at a rate of 6.4 d m i n (0.25 idmin), and the
flexible PSA tape was peeled 6rom the alimùnum substratt for a length of at least 50 mm.
The peel force data w m remrded at 10 second intervais as the flexible strip was peeled
The final resuit was in the fomi of the pcel force W. the cross-head movemeat. The p l
force was reported as the average of data coiiected aftcr the peel trace micheci a steady
value, and it was e x p d pet unit width of the specimen.
2.3 Vickers Hardness Testing
Equipment for Vickers micro~bardaess testiag was simüar in -ce to a
bench microscope, but d B e d in that it has a spcciai indenter objective.
It consisteci of a tcsting machine which supportcd the specimen; a lording
mechanism which hught the indenter md the specimen itao contact graduaiiy and
smoothly; and a measuring microscope which was mounted on the machine in such a way
that the impression in the speciimen may be readily locaîed in the optical field.
The standard Vickers indenter is a square-based pyramidal diamond with an apex
angle of 136" (with the angle between opposite fices being 136").
A bading mechanism forces the indenter, into the surface of the material under the
test, at a predetemhed load. The Ioading mechanism utilizes a da&-pot system which
aiiows the indenter and the specimen to be brought into contact gradually and smoothiy.
After a predetermined period, the load is released, the indenter is removed, and the
resufting impression after removal of the load is observed under the microscope. Using a
f3ar micrometer, the diagonais of the indents are then measured.
mckers hardness number, HV, is obtained by dividing the applied load in
kilograms-force by the d a c e ana of the indentation in square millimeters computed
fkom the mean of the rneasured diagonds of the indentation. Thus, the Vickers hardness
number, W, is given by:
where P is the applied load (kgf) and d is the length of the diagonal of the indent
(-1-
Experimental Method
In order to investigate the e f f ' of aging on the hardness of the painted panels,
Vickers micro-indentation hardness tests were performed.
Aluminum panels painted with MItC-83386 Tempomatte gray polyurrthaae paht
were artificially aged in an oven at 100% for periods of 24, 48, 96 and 192 hours, (U.S.
Air force T.O. 118, coatings and coaîhg removal document suggests baking at 1 0 0 ~ ~ for
96 hours).
Sample specimens for hardness testhg were prepared fiom these artificiaüy aged
panels. Each specimen was approximately 4 c d The samples were mounted on the
hardness tester in such a manner that no movement of the specimen occurred during
application of the load. Hardness reaAings were taken wd away fiom fiee edges over
scratch-fiee areas on the sample. The space between each indentation was more than 0.5
m m
A Ioad of 20 grams-force was applied for a period of 25 seconds, forcing the
Vïckers indneter into the surface of the paint. Then, the indentation was centered in the
field of the microscope. The diagonals were measwed with a precision of 0.4 microns. The
average of the two diagonals were taken and the HV number was calculated through
Equation 2.3.
Paint is a visco-elastic material, Therefore when the load was removed the
deformed paint film tended to retriwe to its on- shape. It has been shown in the
literature [12] that the removal of the indenter is accompanied by a signiscant amount of
time-dependent recovery. It is interestkg to note that the vast majority (90%) of the
recovery occurs in the first few seconds &er the load is removed, prior to optical
measurement of the indent. This recoveq is unlikely to be observeû and wodd certainly
be difficult to masure-
Crawford [12] measured the indentation depth using a Wallace Hardness tester,
and compared the results with bardness numbers obtaiaed by the Vickers instrument. He
concluded that a relatively large recovexy occurs in the depth of the indentation but not
nearly to the same extent on the diagonals. In general the d e s of hardness indiate a 5%
recovery in the length of the diagonal.
In the present research, in order to investigate the effect of visco-elasticity of the
paint a control surfàce with the fie& paint nIm was coated with a thin layer (Les than 10
micron) of gold. The hardness of the gold-coated p d was compared with that of a fiesh
paint fi. Since the measured hardness on the gold coated panel was less dependent on
the viso-elasticity of the paht film, the comparison between th& the Iength of the
diagonal on the two surfiices would indicate the amount of spriogbaclc However, the
Merence between the length of the indentation diagonds measured on the gold coated
d a c e and the nesh paint film was l e s than 3%. Therefore it wu verifid that the
springback on diagonals was insignïfïcant.
2.4 Ultrasonic Toimeof-Fiight Technique
In the present research, an ultrasonic tirneof-tlight technique was used to study
the change in the Young's moddus of thia coatings due to physicaî agïng. Ultrasonic
time-of-fiight techniques are widely used in the chanictaization of the stifii~ess propertïes
of materiais. The basis of these techniques is that the speed of sound for compression and
transverse waves of various polarization uui be daermined dong different axes of a
specimen by measurernent of the time-qf-flight of each wave through a known ttiickness of
materiai. From these sound velonties, both principal and shear propertïes of the materiai
may be determineci.
The velocity of sound, V, is determineci by the moduli and the density, p, of the
material supporting the sound wave through an equation of the form
V=WP)? (2.4)
where M is a hct ion of the elastic moduli of the material, that depends on the
mode of vibration being propagated.
Papini and Spelt [4] measured the Young's moduhis of a t h coating using a tirne-
of-fiïght method. It was reasoned that the combination of a high fiequency and low strain
amplitude wouid produce an elastic response in the coating approxhating that created by
the very hi& strain rates of particle impact.
In the present expriment, the wave was longinidid with velocity VI propagated
through the thickness of the coating. ïhe sample wating thickness was several
wavelengths at the high uitrasonic fiequency used.
For isotropie materials Young's moduius E, the shear modulus G, and Poisson's
ratio p are calculateci from meanirements of density and ultrasonic shear and longitudinal
wave velocities, by using Equations 2.5 and 2.6 113 1.
Figure 2.7 shows the experimentai setup used for normal incidence measurements.
It consisteci of a tilt table that could rotate about two horizontai axes to align the
transducer precisely normal to the samp1e. A micrometer was used to move the
transducerMt table assembly in the vertical (2 iuti~) direction
The transducer was excited by a Panametncs 5601 NST higbfiequency pulser,
which also acted as receivedampMer. The amplineci echo was digitized by an HP 545031).
osciüoscope and then fd to a personai cornputer for M e r analysis.
Wiiter
Figure 2.7: Experimentai sctup for ultnsonic tirnoof-flight musunments 114)
2.5 Gas-Gan Apparatos
This section describes the experimentai apparatus that was designeci and used by
Papini and Spelt [3] in studying the mliision of single particles with painted steel and
aluminum targets. In order to investigate the change in the dynamic hardness of the
coatings due to physical agihg, this setup was used to measufe the incident and rebound
velocities of steel spheres upon impact on Mirent aged panels-
The incident velocity of the particle was used to calculate the kinetic energy of the
particle. As the steel spheres were w t deformed in the collision, the dynamic hardness of
the coating was estimated by setting the kinetic energy equal to the work done in
plasticaiiy deforming the coatbg:
where Vi is the incident velocity, m is the mass of the incident particle, and P(6) is
the load as a fùnction of the indentation depth 6.
Figure 2.8 itlustrates the gas-gun and high-speed photographie setup. A single
particle was loaded into a cyIiadrical urethane sabot (6.3 mm in diameter and 10 mm in
Iength) which was, in turn, loaded into a 6.3 mm inner diameter, 50 cm long steel barre1
via a breech- The target specimens (20 mm X 20 mm) were clamped onto a specimen
holder which was attacheci to a long steel rod, pemitting adjustments in the height of the
target.
The barrel and breech were attacheci to a solenoid valve which was comected to a
compressed air cylinder. The fast-acting solenoid valve (Mode1 73216 BNSTOO,
Honeyweil) created a burst of compressed air to accelerate the urethane sabot (and the
particle) to the end of the barrel where a urethane ring stopped the sabot but aiiowed the
steel sphere to exit. A series of holes dded in the top of the barrel relieved the pressure
behind the sabot as it stopped, and minimEced the amount of air exiting the end of the
barrel with the particle.
As the mass of the steel sphaes was vey Sman wmpared to that of the sabot, the
speed of the particles exithg the baml was independent of the size and deasity of the
particles.
Top View
Sidc View c m
Figure 2 8 : Gu gun wtup used for memurement of putide's incident .ad rebouad
do*.
Four high-speed flashes and a black and white video camera were used ta obtain
images of the particle. An infiarad trigger mounted at the end of band (VIS II, OptiLon
Inc.) se& the particles as they left the barrei, and sent a signai to an y0 board (Opto 22
PB8) attached to the computer and to a delay geoentor. The VO board as C O M ~ C ~ ~ to
the fiame grabber and the h e grab seyence was teminateci upon triggering. The
trigger also caused the delay unit to generate pulses to trigger four high-speed flashes
(flash duraàon r 0.5 ps) mounted near the target at adjustable delays (imcrements of 1 ps).
The delays were timed so that two images of the particles were just before collision, and
two just after collision A smaH program was written to controi the trigger of flashes,
capture the video pictwe on the fame grabber and transfe~ed the multiple exposure
picture to the computer.
By ushg image d y s i s software (Image Pro Plus, Media cyberoetics Inc.), the
precise distance between successive particle h g e s @e., tbe distance the particle traveled
between flashes) were measured. This distance divided by the delay between flashes gave
the incident and rebound velocities of the particle [ 151-
2.6 Velocity Measrirement Setup
Several attempts have been made in the past to mcasure the velocity of the blasting
particles. At Pauli & GnBin 1161 in 1993 a high speed camera and image recording device
that ailowed fiirn speeds up to 1/1ûûû of a second was used, but the camera speed was not
fast enough. In 1996 Jean [17] trïed a strobo~~~pic method and several time intervals to
take images of the particles. The quality of these pictures vaned considerably depending
upon the blasting conditions (Le., pressure, msss flow rate and media) due to hi@ Ievel of
dust inside the blast cabinet, and it was therefore impossible to measure the velocity of
s m d particles.
Experimental setup
In order to take high quaiity pictures of d particlu whiie they were traveling at
high speed, the foiiowing qerimmtal setup was used, as shown in Figure 2.9.
Three hi&-speed flashes (Strobotac, type 1539A) and a vide0 camcorder (JVC)
were put in fiont of the blast cabinet window. A flash delay wntroller, adjustable in
increments of 1 ju, triggered the tbree fiashes in q e n c e .
Top Vkw Isolathg Box
Sidt View
Compressai air Stream of bluting ~uticlea and starch media
'\
I I I I
F i 2.9: ~ r i m e e t r i l setup for velocity measunmtit
The rectangular nozzle was clamped near the blast cabinet window in h n t of the
camera in such that it 38 mm side was in parailel to the lem field of view. The lem
mapification and focus were adjusted so that the focal plam of the lem was at the center
of the stream width. In order to elimhate dust h m the image, an isolating box was built
around the nozzle with an opening at the end to let the blastîng particles out.
When the flash sequenccs were triggered manually, the image of each particle was
captured at three points, and the rnultipIolexposurt image was recordeci on a videotape.
The tape was then screened on a cornputer using au image analysis software (Image Pro
Plus Inc.) and a fiame grabber.
The images capturd on the vide0 tape showed that it was possible to obbmre
paaicles with thrce size categories md measme their traveled distance on the fiame. A
typical image is shown in Fig. 2.10.
Figure 2.10: Muitiple crposure pi- obtained with EnvîrostripQD 12/30,201 kPa, 4.08 kgIrnia, 20 ps between flashes. The ncnzlt is at tbe right side o f the picture. Each three multiple images rcprescnts one particle.
C h a p t e r 3
Effectiveness of Masking Materials in Starch Media Blasting
The overd objective of this pari of the research was to evaluate the effectiveness
of masking tapes used in the protection of aircraft components during wheat starch bfast
cleaning. The properties and puformance of the three most commody used masking tapes
(i-e., 3M tape model YR510, Bron tape model BT-858 and Promac tape model Y389)
were rneaswed and used to develop a deeper understanding of the mechanism of tape
erosion and fdure. This facilitates the specification of the tape properties necessary for
good masking performance-
The experîments performed on the masking tapes were dMded into two main
types: Section 3 -2 disaisses the experiments performd in the Clemco blast cabinet, while
Section 3.3 presents the peel tests- To mhbke the possiile influence of factors such as
relative humidity and temperature, the experiments were grouped so that cornparisons
could be made using specimens that were tested within a short time spae
3.1. General Observations
The Promac Y389 tape was a pressure sensitive adhesive (PSA) masking tape used
in the aerospace industry to protect substrates during wheat starch blast cleaning. It was
red in color and had a thickness of 0.5 mm. The Promac tape had the greatest aâhesion
strength among the three tapes which were examineci. Its backing material was also the
hardest among the three tapes, and its absoiute erosion mass-loss was the least. As the
Promac tape thickness, 0.5 mm, was slightly more than bîlf that of the 0th- two tapes, its
percentage mass loss was relatively large and it had the minimum time to perforate to the
substrate. Another disadvantage of the Promac tape was that it was very sensitive to
substrate contamination and relative humidity. An undean substrate resulted in poor
adhesion strength with the tape falling off under its own weight.
The Bron tape BT-858 was another PSA masLing tape used in these experiments.
It was white in color with a thiclmess of 0.90 mm The adhesion strength of the Bron tape
was the lowest among the three tapes, but the adhesion strength was not Skcted
significantiy by relative humidity and substrate contamination. It had the softest backing
materiai among the three tapes and its time to erosion perforation was the greatest. It was
observed that when the applied pressure for bonding the Bron tape was hi& such as at
locations where the tape was gripped by clamps to the substrate, a significant amount of
adhesive residue remaineci afkr peeling which was difficult to remove nom the substrate.
The 3M tape YR5 10 was the t k d PSA tape that was tested in these experiments.
It was green in color with a thickness of 0.95 mm. The 3M tape ranked in the middle of
other two tapes in ail the expaiments, but the nature of its behavior was comparabIe with
the Bron tape. Peeling the 3M tape after a prolonged period of bonding to the substrate
ofien resulted in the tearing of the tape; this happaied many tiws during the pecl tests. It
also lefi adhesive residues on the substrate, though less than with the Bron tape,
particuiariy when the bondhg pressure was hi@
Eflectiveness of M d - n g Matenals in Stwch Media Blasting
3.2. Perforation and Edgc-Lifting Tests
3.2.1. Perforation Tests
In order to evaiuate and compare the durability of the pressure sensitive adhesive
(PSA) tapes against erosion, a series of perforation tests were conducteci. Sample tapes, 5
cm x 10 cm, were cut fiom tape roiis, and were applied to cfeaned, painted duminum
panels (AA 2024-T3 ahunhum panels painted with MIL-C-83286 topcoat and MIL-P-
23377 primer provided by the Department of National Defence for the present research)
which were then installai in the blast cabinet on the sliding table.
The blasting parameters were adjusteci as fofiows:
Nozzle (rectangufar exit) standoff distance: 1 5 cm.
Angle of attack (a): 45".
Media flow rate: 4.08 kg/& (9 Ib/min).
BIast pressure: 172 kPa (25 psi).
New wheat starch media Envirostrip 30/100 was used in ail blasting experiments.
A shutter was used to controi the exposure time of PSA tapes to the blast media The
Stream of wheat starch media was Mocked by the shutter for the fkst 15 seconds of each
experiment until the pressure and flow of particles were stabilized and veri6ed. The
masking tapes were exposed to the jet of blasting particles by removing the shutter.
Table 3.1 shows the time fkom the start of the exposure to the first obsewation of
the painted substrate through the tapes.
Tape T i e (s)
Bron BT-858 20
3M YU10 12
Promac Y389 6
Tabk 3.1: Pdoration test resuits
Eflectiveness of MPkklng MolerÏaIs in Stmcir Media Bim'ng
In another approach to observe the resistancc of the tapes to perforation, a second
set of blasting experiments were perfonned on the three mashg tapes. Each tape was
blasted under the same conditions for periods of 5 seconds and 10 seconds in two separate
experiments. The blasting parameters were as follows:
Nozzle standoff distance: 15 cm
Angle of attack (a): 4S0.
Media flow rate: 5.44 kg/& (1 2 ib/mh).
Blast pressure: 172 kPa (25 psi.
Media: Wheat Starch EnWostrip 30/100.
Once again, a shuttcr blocked the stream of wheat starch particles mtil the flow
was stabilized (after approximately 15 seconds). Figures 3.1, 3.2 and 3 -3 show the tapes
after 5 seconds of exposure to the blast of wheat starch media It can be seen in Figure
3.1 that the Promac tape becornes perforated to the substrate &er a short penod of
exposure, less than 5 seconds; while Figures 3.2 and 3.3 show that the 3M tape erodes
more than the Bron, but both protect the substrate. Figwes 3.4, 3.5, and 3.6 show the
tapes after 10 seconds exposure. The Promac and the 3M are puforated; while the Bron
continues to protect the substnite. Therefore, it is conciuded that, under these conditions
the ranking of the perforation resistaace is again: Bron > 3M > Promac.
Figure 3.1: Promrrc tape aiter 5 s+coads erpwure to tôe Mirst
Figure 33: 3M tape after S secoads erposiuc to tbe bhst
F i 33: Bron tape riltcr 5 seconds apoaure to î k bhst
Eïgure 3.4: Pro- tape &r 10 seconds erpoervc to tbe M.st
Effèctiveltess ofMm&ing Materials in Starch Media Bfasn-ng
t r i 3.5: 3M tape a b r 10 seconds eqnmivlc to tk bbst
Efleciiveness of Masking M'e~ iaIs in Starch Media Blasting
Table speed for complete tape removd
To investigate the rate of tape erosion as a fhction of the speed of the siiding
table, the following experiment was performed. Four samples, 5 cm x 7 cm, were cut
fiom each tape roll of the three masking tapes (Bron, 3M and Promac), and were bonded
to a cleaned, painted aliiminum panel (AA 2024-T3 aiuminum panels painted with MIL-C-
83286 topcoat and MZGP-23377 primer). The panel was clamped to the sliding table and
blasted with new wheat starch ~ n v i r o s t r i ~ ~ 30/100 media. The blasting parameters were
as follows:
blasting pressure: 138 kPa (20 psi),
nozzle standoff distance: 15 cm,
media mass flow rate: 5.44 kg/&.
For each set of blasting condition, the speed of the sliding table was progressively
increased fiom 0.1 5 to 0.30 druin, at steps of 0.05 m/min, und complete tape removal
was observed. The above experiment was performed at three angles of attack (a): 20°,
45' and 70". The table speed ranges at which the tapes were completely eroded are shown
in the Table 3 -2.
Tape a=20° a=4S0 a=70°
Bron 0.20 + 0.05 less than 0.1 5 less than O. 1 5
3M 0.20 + 0.05 0.20 +, 0.05 0.20 + 0.05
Promac more than 0.30 0.25 i- 0.05 0.20 k 0-05
Table 3.2: The range of siidhg table speds, U (mlmin), for complete removd of the tape.
The results show that blasting is more erosive at 20" than 70". For the Promac
tape, the speed of the siïduig table for complete eroîion of the tape iacreased significantly
with a decrease in the angle of attack nom 70' to 20°. For the Bron tape, the speed of the
sliding table for complete erosion was l e s than 0.15 m/min at 4S0 and 70°,
although the damage to the Bron tape at 4S0 was more than the damage at 70°. The speed
of the sIiding table for wmplete erosion was in the range of 0.15 to 0.25 m/min for the
3M tape, but it was observed that the damage to tht 3M tape also increased with a
decrease in the angle of atîack k m 20" to 70°. Table 32 shows that the erosion of the
Promac tape was a f f i e d by the angle of attack more than the other two masking tapes.
The ranking of the erosion resistance accordhg to this test at ail angles was again: Bron >
3M > Promac.
3.2-2. Mass-toss Tests
A m e r series of experiments were undertaken to measure the rate of tape
erosion by measiaing the mass loss fiom the tapes as a hction of the speed of the siiding
table, angle of attack and the direction of the exposure to the blast Stream. Figures 3.7,3.8
and 3.9 display the angle of attack and the direction of the expoçure to the blast stream.
4- Direaion of motion of the siidhg table
F i 3.7. Angle of ath& 8 U negativc when the bluting p u t i c l u and tbe
sliding table are movbg in the u i l e direction.
,L Direction of motion of the sliding table
Figiue 3.8: Angk of attack a b positive when the bluting particles and tbe
sliding table are moving in the opposite direction.
, D~cctim of motion of the dicihg table
Figure 3.9. The node is moving to- d g e A and away from eûge B.
The resuits for ciiffirent blasting conditions are slmimarized in Table 3.3.
No edge-lifting was observed in any of these expcriments. This suggests thai,
when tape adhesion is good (as it was to these clesn surfaces), the tapes erodc without
edge-lifting.
The relative resistance to perforation can be assesseci by comparing the percentage
mass removed under like conditions- For example, at 5.44 kg/* 70°, 0.30 dmin, the
Bron tape had 200/0 los, while the 3M and Proniac tapes bad 35% and 43% los,
respectively. At 5-44 kg/- 453 0.15 dmin, the Bron tape had 60% mass loss, while
the 3M and Promac tapes both bad approximately 90% loss. Figure 3.10 shows the
Prornac tape blasted at 45' and 70°, revealing that 4S0 is more damaging than 70°; a result
that is similar to the wheat starch stripping behavior of the methane topcoat on aluminum
substrates [2],
Feure 3.10: Blrrsting Pro- tape at 70° (kit) ami 4 5 O (right).
As expected, Table 3.3 shows that an increase m the sliding table speed wiil reduce
the mass los. At 4.08 kg/min, -70°, 0.90 m/min, the Bron and the 3M had 2% and 8%
rnass loss respectively; while at 0.30 dmin the Bron had a mass loss of 16% and for the
3M it was 40%.
Another interesting r d t is that an increase in the mass flow rate did not
correspond to an increase in the niass los. Comparison of the mass loss of the Bron and
3M tapes at 4.08 kg/min, -70° and 0.30 m/min with their rnass l o s at 5.44 kg/- with the
same angle of attack and table speed showed a 4% krease in mass l o s for the Bron and
a 5% decrease in mass loss for the 3M tape. Note tbat, m both cases, the biastjng pressure
Efecîivenem of Masking Materials in Starch Media Blartr-ng
was same. A previous investigation by Djurovic et al- [Z] showed that an increase in the
mass flow rate will reduce the velocity of blasting particles, while it wiU increase the
number of the particle impacts per unit area of the tapes.
Tape Mass Angle of Attadc Table Speed Mass Removed M u s
Flow a s (degr-) (mfmin) (fi!) Loss
(kg/&) (error = 8%) O/.
Bron 4.08 -70 0.90 0.06
Bron 4.08 -70 0.30 0.49 16
Bron 5 -44 70 0.30 0.62 20
Bron 5 -44 -45 0.30 0.83 27
Bron 5 -44 45 O. 15 1.80 60
Promac 4.08 -70 0-90 O. 13 IO
Promac 4.08 -70 0.30 0.56 40
Promac 5 -44 70 0.30 0.59 43
Promac 5 -44 -45 0.30 0-64 50
Promac 5.44 45 O. 15 1.22 95
Table 3.3. Mass loss witb different blasting parameters. Blasîhg pressure wlls 138 kPa for al1
the experiments.
The foUowing points should be noted:
the angle of attack is relative to the plane of the surface and is defineci positive
when the sliding table is moving in the direction opposite to the Stream of blastiag
particles (Figures 3.7 and 3.8).
Eflectivrness o f M ' i r g Màtrnrna& in Starch Media BIasîïng
Percentage of mass l o s is the ratio of mass removed to the original mas. The
weight of the PSA tape was measured after the test and compareâ to the intact
tape weight. Aiso, the results were checked by measuring the thickness of the
tapes at various points and caicuiating the vohune removeci-
The results of Table 3.3 indicated that once again, the erosion resistance could be
ranked according to percentage mass loss as: Bron > 3M > Promac.
Effect of Tape Orientation
To m e r d y z e the effect of tape orientation on the resistance to erosion, I
samples of the three tapes were applied to a po1ywethane-coated ahimiaum panel at four
dserent orientations with respect to the blasting direction as shown in Figure 3.1 1.
Samples A and B were bonded so that the taperoll direction was dong the
stripping path For sample 4 the angle of attack, a, was 45' (the table was moving
opposite to the blasting direction) and the nozzie was moving towards edge 1; while for
sample B, the angle of attack was 4 5 " (table was moving in the same direction as the
blasting particles) and the n o d e was moving away fkom edge 2. The tape-roll direction of
sample C was across the stripping path, and sample D was oriented at 45" to the stripping
path.
L' ire 3.11: 3M îape blrsfad at ber didFcnat orkohtion.
The tapes were blasted wÏth wheat starch Envirostnp 30/1ûû media under the
following blasting conditions:
Media flow rate: 5.44 kghin (1 2 Wmin).
Blast pressure: 138 kPa (20 psi).
Angle of attack (a): 45".
Table speed: 0.30 mhin (1 .O ftlmin).
The m a s Ioss due to the erosion of the tapes was then calculated by weighing the
remains of the tapes after bkmg, and subtracting fiom the originai weight. Table 3.4
shows the results as the mass removeci and the percentage of the original rnass.
Eflecîbeness of Màsking Mrrtmen& in S t d Media BIasting
Tape Orientation Orifid Miss Aftcr Bluting M u s R e m o 4 M a s
O 0 e, Loss '!!
Bron A 3.00 2.21 0.79 26
Bron B 3.00 2.16 0.84 28
Bron C 3.18 2.34 O. 83 26
Bron D 3.06 1.97 1-09 36
3M A 3 -04 2.04 1.00 33
3M B 3.17 2.28 0-90 28
3M D 3.24 2.1 1 1.13 35
Promac A 1.38 0.79 0.59 43
Promac B 1 -43 0.95 0.48 34
Promac C 1 -42 1.00 0.42 30
Promac D 1.10 0.67 0.43 39
Table 3.4: M t s s loss of tapes at d i f f i t orientations on the sliding table
Referring to Table 3.4, the tapes can be d e d accordhg to the amount of mass
removd. The Promac tape was the most resïstanî to blasting with a range of 0.42 - 0.59 g
removed. Both the Bron tape, with mass loss in the range of 0.79 - 1 .O9 g, and the 3M
tape, with mass loss in the range of 0.90 - 1.13 g, were o f comparable resistance. It should
be noted that the thickness of the Promac tape was 0.50 mm, which was siïghtly more than
half the thickness of the Bron and the 3M tapes (0.90 to 0.95 mm, respectively).
Therefore, while the Promac mass loss was less than the other two tapes. it was perforated
much sooner. in other words, as before, on a percentage mass loss basis, Table 3.4
indicates the erosion resistance ranks as : Bron > 3M > Promac.
It is interesthg to see tht the différence between the amount of tape removed due
to blasting at Werent orientations was weii within the experimental mors, * 8%. Also
note that, once again, no edge-lifting was obsened in these tests, connrming that this was
not a problem as long as the tapes were bonded to a clean substt-ate.
3.2.3. Scratch Hardness Tests
Hardness is the most important physical property affecting impact erosion
resistance.. The three masking tapes were tested according to ASTM D3363 (Standard
Test Method for Film Hardness by Pend Test, American Society for Testing and
Materials, 1992). A set of Staedtler MARS Lumograph pencils were used with the
foilowing hardness vahes:
(softest) 6B-5B4B-3B-2B-B-HB-F-H-2H-3H4HH5HH6H (hardest).
Two pend hardness vaiues were observed: The "gouge hardness7', defined as the
hardest pend that leaves the tape uncut, and the "scratch hardness", dehed as the
hardest pencil that does not lave a permanent scratch on the tape. Table 3.5 shows the
results of the pencil hardness tests on the three types of masking tapes.
Tape Gouge Hardntss Scratch Hardness
Bron BT-858 Hl3 4B
3M YRS10 2H 3B
Promac Y389 4H B
Table 3.5: Pend hardncss values
The ranking of the tape hardness was the same for both gouge and scratch
hardness measures; Le., Promac > 3M > Bron Figure 3.12 shows the pend scratch test
marks on the Bron tape. It is interestiog to note that this ranking is opposïte to that seen in
the erosion tests (i. e., Bron>3M>Promac).
Egectiveness of M'king MrrterrerroLr in Starch Meda Blasring
Etgare 3.12: P e d scratch test riiuks on tbt Bron tape
3.2.4. Coefficient of restitution, e
in order to characterise the impact bebaviour of the masking tapes, a gas gun setup
capable of launchhg single particles at speeds up to 120 m l s and a high-speed
photographie setup capable of measuring inbound and rebouad velocities were use& as
descriid in Section 2.5.
impact tests were conducted at impact velocities (Vi) of 18.6, 27.9, 32.4, 45.6,
52.4 and 65.0 mk The maximum variation in the velocities was approximately f 7%. The
masking tape specimens were prepared by cutting 50 mm x 50 mm pieces fiom tape rob,
and a d h e ~ g them to painted aluminum p a d s of the same dimensions. The specimens
were cl+ to a test jig and oriented normal to the gun b m l at a distance of 20 an.
Impacting steel spheres with a diameter of 1 -5 + 0.5 mm were seiected through a
two step process. First, spheres with a diameter between 1.4 and 1.7 mm were obtained
Impacting steel spheres with a diameter of 1.5 f 0.5 mm were selected through a
two step process. Fiist, spheres with a diameter between 1 -4 and 1 -7 mm were obtained
using two sieves, mesh 12 and me& 14, according to ASTM designation code E323. The
diameter of each single s t e d sphere was checked with a digitai micrometer before loading
on the sabot. The average mass of the incident particle, measured by a digitai baiance
(Sarturius, type 17 12) was 13 -8 mg.
Incident and rebound vel&es were measured using a video camera and strobe
lights as desmieci in Section 2.5. Using Equation 3.1, the coefficient o f restitution, e, of
the tapes was calculateci.
Table 3 -6 shows the codncient of restitution, e, of three PSA masking tapes, at six
different impact velocities.
Impact Velocity Bron Tape 3M Tape Promac Tape
(mm BT-858 YR 510 Y 389
Table 3.6: Coefficient of restitution, t, of three muking tapes for rn impact test witb sted sphere of 1.5 mm diameter rt s u difftmt impact vdocities. Number of repetitions N, and standard deviition of the nsults SD uc givcn in parentheses.
As shown in Table 3.6, the coefficient of restitution decreased with increasing
incident normal velocity. At each impact velocity, the Promac tape had a much lower
coefficient of restitution, wMe the vaiues of e for the Bron and 3M tapes were
comparable.
It is interesthg to note that the Promac tape, with a smaller of coefficient of
restitution tha. the other two tapes, failed at impact velocities hïgher than 50 d s . The
other two tapes protected the subsîrate at the maximum impact velocity reached in this
test, i-e. 65 m/s. The erosion tests in the blast cabinet aIso showed that the Promac tape
was less erosion resistant than the Bron and 3M tapes. Figure 3.13 shows a graphitai
presentation of the results.
1 -C Romac 1
Incident normal velocity ( m l s )
Figure 3.13: Coefficient of nstitution w r fianetion of incident normal vdocity for masking tapes.
In these experiments, the stal sphaes were launched n o r d to the maslting tapa,
and they did not d e r m y plastic deformation Thdore, bis coefficient of restitution
Eflectiveness ofMaxking Muteriuk in Starch Media BipsnPsnng
In these experiments, the steel sphers were launched normal to the xnasking tapes,
and they did not suffer any plastic deformation Therefore, this coefficient of restitution
represented the amount of plastic deformation that the tape had undergone. The higher the
coefficient, the less the plastic deformation of the tape, and more of the incident kinetic
energy was restored in the spring back. These r d t s are discussed in more detail in
Section 6.2.3.
3.2.5. Effect of contamination on edge-üf'g
AIthough edge-lifting did not occur on a clean Surface, the &kt of a contaminated
substrate on the effectiveness of the three types of masking tape was examined in the
following experïments-
A 30 cm x 30 cm aluminwn panel, painted with the standard polywethane topcoat,
was coated with a solution of 300 ml of methylethyl ketone (MEK) mixed with 1 g of
molybdenum disuffide grease (3.3 m g h l molybdenum disuifide in MEK). Raised edges of
caulkùig on the edges of the panel held the solution in place until the MEK had evaporated
(under the fume hood for 18 hours), gemrating a uniforni contamination l m 1 on the panel
of 1 .1 mg/cm2 (similar to the hi@ lewel of contamination in section 3.B). Four samples, 5
cm x 7 cm, were cut fiom each roll of the three &g tapes (Bron, Promac and 3M),
and were apptied to the panel. It was noticcd immediately that the Promac tape was very
sensitive to the c o n ~ t i o n and codd not be properly bonded to the panel. The tapes
were then blasted with starch media Envirostrip 30/100 under the foilowing conditions:
Mass flow rate: 5.44 kg/min (12 1bImin)-
Blasting pressure: 138 kPa (20 psi).
Angle of attack (a): 45".
Noaie stand-off distance: 15 c m
Table speed: 0.1 5 m/mia (0.5 Rlmin).
Blasting of the contaminated panel caused the Promac tape to be liftexi with the air
blast alone (no starch media) as shown in Figures 3.14 and 3.15. As mentioned above, the
Efectfiveness of Mmking h&rtaLÎ in Stumh Media Bhting
edge-lifting tban other two tapes. But when the Promac tape was properly bonded, edge-
Lifting did not occur.
Figure 3.16 shows that edge-lifting mis minimal with the Brou tape. No edge-
lifting was observeci with the 3M tape, imücating that was better able to bond to
contamùiated surfàces. It is concluded that edge-liftnig is a fhdon of cl eadkssofthe
substrate, and proper adhesion of the tapes prevents edgelifthg. A simple method for
checking the effkctive cleanliaeSS of an aircraf€ surfàce wouid be to measure the peel force
using a portable force gauge.
Figure 3.14: Edge-üiüng of Prompf tape on a c o m ~ t c d panet, top view
'ing
Edge
Fmre 3.15: Eàpliiang of h n m c tape, side view
Fwre 3.16: Edgdübg of Bron tape on a contauhateai pluid
-lifting and the Use of a Hot Melt Adhaive
It bas becorne a common practice to apply a hot melt adhesive to the edges of
masking tapes in order to prevent edge-lifting. The following experiment was conducteci
to investigate the role of a hot melt adhesive, Bostik, m this application. Samples of the
three masking tapes were applied to a cleaned painted (poiyurehane topcoat) aiuIllinum
panel, For each tape, three different edge conditions were prepad: One sample bad hot
melt covering both the edge and the painted panel; another had hot meit only on the edge
of the tape (not on the panel); and a third sample did not have any edge protection as a
control for cornparison purposes. The tapes were then biasted with wheat starch media
Envirostrip 3011 00. The blasting parameters were as foiiows:
Mass flow rate: 5.44 kgmin (12 Wmin).
Blastmg pressure: 138 kPa (20 psi).
Angle of attack (a): 45".
Nozzle stand-off distance: 15 cm
Table speed: 15 cdmin (0.5 Wh).
Figure 3.17 shows hot melt on the edge of the Promac tape after the blasting. The
angle of attack was 45" with the bhst Stream towards edge A. Note that the substrate
topcoat was stripped but that the hot melt protected the leading edge of the mas- tape
fiom erosion (erosion was not expected on the trailing edge shown below edge A).
Figure 3.17: Edge of tbe Bron tape is hfccted by tbe Hot mclt,
Eflectnreness of Marking Mafmerr& in Sta~ch Media BIasting
These tests showed that the hot melt protected the edge as an erosion barrier.
Adhesion strength of the hot meIt to a çontaminated surfàce qualitatively compared with
that of the Bron, 3M and Promac tapes. It was concluded that the hot meIt was also
sensitive to substrate contamination-
It was therefore concluded that the hot melt adhesive protects the edges of the
masking tapes in three ways:
it deflects the Stream of blasting particles therefore preventing the collision of the
blasting particles to the edge of the tape;
it acts as an erosion barrier by embedding the blasting particles; and
the hot melt increases the tapes resistance to edge lifting by increasing the stiffiiess
of the edge of the tape,
3.3. Peel Tests
3.3.1. Effect of Adhesion Time on the Peel Strength
Ln order to investigate the effect of bonding time on the adhesion strength of the
three masking tapes, samples were applied to the panels and a 45" peel test was performed
after bonding times of 1 hour, 20 hours and 1 week (in normal lab conditions), as
described in chapter 2, Before applying the tapes, the painted surfaces were clea~led by
wiping thoroughiy with a tissue soaked in ethyl alcohol. The cleanfiness of the d a c e s
was then checked by measuring the advancing contact angle of distilled, deionized water
[7 1" + 3"]. Table 3 -7 shows the average values of the measured peel strengths.
Eflectiveness of Manking Materiais in Stmch Media BIPsting
--- --
Tape Tirne1 h TimdO h Tirne= 1 week
3M YR5 10 0.53 0-70 0-90
Bron BT858 0.25 0.25 0.24
Promac Y389 1.2 1.26 1.87
Table 3.7: Tk efftct of bonding timc on the 4S0 p d stnagth rt a rite of 6.3 d m i n . Tbe pcd forcc is shown in NImm width of the masking tapes,
For the 3M and Prornac tapes, there was a significant increase in peel strength with
tirne; while this was not true for the Bron tape.
Fig- 3.18 shows the r d t s of the 45O p a l test on the Bron tape with a rate of 12.7
d m i n (0.5 in./min), where it is seen that the peel force was relatively constant after the
initial increase associated with the establishment of steady state.
Time (su)
Figure 3.18: The effcct of bonding thne on 4S0 p d strcngth of Bron tape rt a rate of 12.7 d m i n .
3.3.2. Effect of Contamination on the Peel Strength
In order to investigate the &ect of contamination, a series of test were performed
by applying masking tapes to substrates contambted at two different levels- The
resulting peeI strengths were then compared to those fiom very clean substrates.
The panels (2.5 cm x 15 cm) were contaminated with moiyûdenum disuifide
grease. The grease was fist dissohd in methylethyi ketone (MEK) and a controfled
amount of the solution was applied over the entire suTf=dce of the panel. The panels were
then placed under a h e h o o d for a few hours so that the soivent could evaporate leaving
a unifonn amount of grease over the entire surface of the test panels. Contact angle
measurements at various locations verified that the contamination of the panels was
unifonn.
Two levels of contamination were used: a low level of 0.2 mg/cm2, and a high level
of contamination of 1.0 mglcd. MIL-PM-87937C Part 4-6-21 -4 gives a recommended
contamination level of about 0.56 mg/cm2 (50 mg on 2"xT' am), for the testing of
cleaning effectiveness of solvents.
The sample masking tapes were appiied to the contaminated panels and kept in
ambient laboratory conditions for one week prior ta t&gg An Instron load fiame was
then used to perform the peel tests at a 45" peel angle and a rate of 6.35 mdrnin (0.25
incwmin). The r d & are SullLnaeized in Table 3.8. Figure 3.19 shows some
representative peel force auves on the clean and contaminated substrates.
Tape Clan Low Levd Contamination High Levd Contamination
Bron BTS58 0.24 0.22 0.20
Prornac Y389 1.85 1.15 0.63
Table 3.8: P d strength (Nimm of tape rpidth) as r hinction of coatUnin.tion k d s (obtabd by applying molybdcnum dcsuifide grtrst).
Eflectïveneness of M d n g M4tenen& in Stamh Media BIasting
The Bron tape was not sensitive to these l d s of contamination, dthough its peel
strength was much less than that of the other tapes under aii conditions. The 3M and
Promac tapes were guite sensitive to even low I d of contamination, but thek peel
strengths remained greater than that of the Bron tape even at high contamination Ievels.
Figure 3.19: P d test rcsults a different Iw& of contrmination, 0 Bron dun, 0 Bron contaminated, X 3M dean, A 3M contaminated, OPromac ckanin, Promac contriminated,
3.3.3. Effect of Relative Humidity on the Peel Strength
Ln order to investigate the effm of ambient humidity on tape adhesion, samples of
the three mashg tapes were applied to clean panels. The cleaniiness of the panels was
verified by contact angie measurements. Three peel specimens were prepared for each of
the masking tapes: One panel was kept in normal laboratory conditions (approximaîely
50% RH-), one was sealed in a chamber with a relative humidity about 200/., whiie the
third sarnple was put in a chamber containing a desiccant to produce 0% relative
humidity. After one week unda these conditions, a 45' ped test at a rate of 6.35 d m i n
Eflecîiveness of Masking Materi& in Stcvch Media Bfarting
(0.25 incVrnin) was performed on the tapes. Table 3.9 shows the resulting peeling force in
NI- width of the tape.
Tape 0% RH 20% RE Lab. Air
3M YR510 0.86 0.90 0.90
Bron BTS58 0-24 0-24 0.24
Promac Y389 1.71 1.87 1.85
Table 33: P d sbcagth (N/mm) as a funetion of ditive bumidity.
It is seen that wne of the tapes was affectecl significantly by the ambient relative
humidity over a one week period. This is contrary to some of the conventional wisdom
associated with the use of these tapes in hangars during the winter, Le., some operators
have reported that tape penormance was adversely by low relative humidity.
Measurement of Peel Strength at High Humidity
In order to observe the e f f l of high humidity on adhesion strength, samples of
the tapes bonded to dean panels were left in an environment chamber at 30°C, 90%
relative humidity for a period of 5 days.
The humidity in the chamber was controled by a solution of sodium hydroxide
(NaOH) in distilled water. The relative humidity (RH.) is dehed as:
where P,, is the equiiibrium partial pressure of HzO over the aquews solution, and
P,, is the saturation vapor pressure of water at the correspondhg ds, bulb temperature
(30°C). A solution of 20 g of NaOH in 1 0 g of Hs wiü maintain a relative humidity of
90% at 30°C dfy bulb temperature 11 81.
Samples of the three masking tapes, two samples for each tape, were bonded to
clean polyurethane-coated panels and were put in a seaieci container on a tray above the
NaOH so1ution (50 g of NaOH in 250 g distiIled water), as shown in Figure 3.20. The
container was kept in an oven at 30°C for 5 days. The humidity inside the chamber was
measured through an openhg in the lid ushg an electronic humidity probe. Another se$ of
samples was prepared and kept under normal laboratory conditions as a measurement
control.
- NaOH solution
VICW A-A
Figure 3.20: Humidity control chamber, note tbat in view A-A the silicone seaimt and the seaiing tape around the t o m disk and cover dges ue not showa.
After five days, peel tests were performed at a rate of 6.3 5 &min (0.25 idmin)
at a peel angle of 4S0. Figures 3.21, 3.22 and 3.23 show the peel test r d t s on three
samples of each tape.
Figure 3.21 : Promrc tape ped test rt 4S0, 6.35 ~ d m i n . (0.25 inJmin_), 0 suaplt 1 RH. 90%, 30°C, Osample 2 RH. 90% 30°C, A sampk 3 Id. condition.
Figure 3.22: Bron tape ped test at 4S0, 6.35 d m i n . (0.25 iaJmia), Osample 1 RH, 90°/., 30°C, O sampk 2 RH. 90% %OC, w p k 3 1.b. condition-
Figure 3.23: 3M tape peel test rt 4S0,6.35 d m i n . (0.25 iaJmin.), 0 slmplc 1 RH. 90%, 30°C, a a m p k 2 RH. 90% WC, A srmpk 3 hb. condition.
Figures 3 -2 1,3.22 and 3 -23 show that the Bron tape was the least sensitive to high
humidity arnong the three tapes with an average of 8% decrease in adhesion strength,
while 3M had ao average decrease of IO0!% in adhesion strength, and Promac had an
average decrease of 45% in adhesion strength. It is intaesting to note that while the
Promac tape had a higher peel strength than the other two tapes, its high semitkity to
humidity caused it to debond fiom the substrate at some points with a much lower peel
force. Note that in Figure 3.2 1, the peel strength of the Promac specimen 2 decreased to a
quarter of its onginal value (specimen 3). a d the p a l strength of -le 1 û relativdy
unstable, due to tocai bond weakness. Table 3.10 summarizes the resuits of the peel tests
in N/mm of tape width.
Masking Tape Sampk 1,s &YS Simple 2, S days rt Coatrol Smple
at 300 C and 90% RH, W C and 90% RH. rt Irboratory condition
Bron 0.30 0.27 0.3 1
Promac 1.22 0.36 1 -44
Table 3.10: Results of 4S0 peel test at a rate of 6.35 mm/&
3.4. Summary and Discussion of Masking Tape Performance
A good tape for masking during wheat starch blast cleaning of aircraft shouid have
the following characteristics:
a. Perforation and Edgclifting: The tape should protect the underiying
substrate fiom the media under normal blasting conditions for a reasonable penod of tirne,
equivalent to severai passes of the blast nozzle.
b. Tape Adhesion Strength: The tape should adhere well to painted surfaces and
be reiatively insensitive to contamination, ambient humidity, and the method of
application.
c. Residues After Removd: The tape should be removed easily without leaving
adhesive residues.
The peflormance of the three masking tapes is summan'zed below under these
three categories.
a. Perforation and Edge-lifting
The foiiowiog experiments were performed to evduate the tape resistance to the
wheat starch blasting:
E_Crectiveness of Màskïng Materiafs in S b c h Media BIasîïcrrlrng
Scratch test (hardness)
Perforation and Mas-loss
Edge-Lifting
The results of the pencil scratch hardness tests (ASTM D3363) showed that the
Promac tape had the highest effectjve hardness. Its backing material was harder and stiner
than that of the other tapes. The 3M and Bron tapes ranked second and third,
respectively, in gouge and scratch hardness.
The wheat starch perforation tests gave the tirne required to penetrate the tape to
the undeilying substrate. It is intereshg to note that in this case, the Bron tape had the
best pefiormance, with approximately twice the resisîance to wheat starch of the other
two tapes. The 3M and Promac tapes ranked second and third, respectiveiyY The result
was also supported by the percentage mas-loss experirnents, dthough in these the
Promac ranked somewhat bigher because its relative thinness was not a consideration; ie.,
a 50Y0 mass loss in the Promac tape left a much thinner layer than did a 50% m a s loss
with the other tapes.
This result is similar to Burkioski's conclusion [Il] , "the more easily the matenal
deforms in contact with a particular abrasive, the better is the abrasion resistance". It
should be noted that the Promac tape with the hardest backing has the least erosion
resistance. This unexpected result wiil be disaissed in Chapter 6.
The backing materials of the Bron and 3M tapes are sofier and more elastic than
the Promac. The elastomenc backing material absorb the impact energy of blasting
particles in the form of elanic energy and returns t to the particles with less amount of
plastic deformation tban that of the Promac tape.
The thickness of the tape was a sipikant &or in protecting the substrate. The
thicker Bron and 3M tapes were more rcsistaat to the blasting particles, m n though the
relatively thinner Promac had a harder backhg materiai (fiom the pencil tests).
Blasting experiments showed that edge-lifting did not occur if the tapes wae
applied properiy to cl- substrates.
b. Tape Adhesion Strengtb
Peel tests showed that the Promac tape bad a much p a t e r adhesion strength thui
the 3M tape, and that the Bron tape was the weakest.
The Promac tape, howmr, d a e d the greatest percentage decrease in peel
strength as a result of contamination and also due to its sensitivity to high levels of
humidity. Nevertheles, because its initial strength was so hi& evea with a heady
contarninated substrate, it still had the greatest peel strength on average.
c. Residues Afttr Removaî
It was obsemed that aAer blasting if the masking tape backing materid was
damaged it was very difncult to remove the PSA residues.
The Promac tape removal did not leave residues if the backing matenal was not
perforated. When the applied pressure for bonding the Bron tape was bigb, suc6 as
locations where the tape was gripped by clamps, there was a sigdicant amount of residue
left which was difficult to remove f?om the substrate.
Peeling the 3M tape after a prolonged period of bonding to the substrate resulted
in tearing of the tape. This phenornenon was more obvious when the tape had been in a
humid environment for over a one week period. The 3M tape also Iefi an amount of
residues on the substrate, although less than the amount of residue left by the Bron tape,
particularly when the applied bonding pressure was hi&
C h a p t e r 4
Changes in Org- Coating Properties Due to Aging
The dBcuity of removing paint fiom aircraft varies greatly due to f ~ o r s such as
the number of coats, age and condition of paint, and îhe presence of other coverings sucb
as decals. Efficient automation of paint stripping requires the prediction of paint stripping
rates at various places over the airctaft.
One of the objectives of this work was to provide a method for predicting the paint
stripping rate of an aged paint system, through a knowledge of some fhdamental physical
properties of the paint substrate system, i-e., hardness and m d u s of elasticity.
This Chapter disaisses the changes in organic coaîing properties, (hardness, and
modulus of elasticity) that can occur in Service and during acceferated aging.
4.1. Accelerated Aging of Painted Panels
During its seNice Life, the a i r d is exposeci to severe environmental effects
including ultra-violet radiation at high altitudes, rapid temperature changes, and also
driving rain and ice which cause erosion and cracking in the paint:
Changes in Organic CoLtrng Prapnhpnhes Due to Agïng
Many of the materials that corne in contact with the body of a i r c m are also
aggressive to paint films; in parti&, hydraulic fluids, engine hibricants, brake fluids, and
alwhol in de-icing fluids can soften and dissolve surface coatings, while burning fiieis
discharged through exhaust can have the reverse effect and harden the wating [19].
Exposure to UV radiation, heat, moisture, and pollution wiii cause changes in the
properties of the paim film which is callexi weathering effécts or aging. The amount of
exposure to these factors varies over the aircraA; therefore, the characteristics of paint film
and hence its response to the paht stripping proces is not d o m
In addition to the above factors, a typical d h y paht system should be capabie
of withholding the aerodynamic shear forces encountered at speeds above 1000 k d h .
Thick coatings of elastomeric polyurethanes show the best resistance to these forces.
4.1.1. The Paht System Used in Experiments
In order to simulate some of the changes that occur during actual outdoor
exposure of the panels, an acceierated aging (dciai weathering) method was used. In
addition to reproducïbiiity, a gnat advantage of accelerated weathering was that the
results were available more quickly than with naturai weathering, which is a phenornenon
that may take years.
The paint model used in this research was a typical aerospace coating system:
The primer, MIL-P-23377, was a two-component epoxy-polyamide. It was yeliow
in color and had a nominal thickness of 25 pm (1 mil),
The topcoat, MLC-83286, was a two-compoaent urethane aliphatic isocyanate.
It was matt gray color and its thickness was 50-75 p (2-3 mils). The nIm density of the
polyurethane topcoat was 1 -27 g/mi 0.02.
The substrate was AA2024-T3 clad aluminum preaeated with a chernical
conversion coating. The panels were painted on both sides by the Canadian Department of
National Defence and measured 30 cm x 30 cm x 3 -2 mm thick
In order to accelerate the aging pracess, painted ahmhum panels were heated in
an oven (Fisher precision scieatific model 214) at 100 3 OC , for periods of 1 ,2 ,4 and 8
Changes in Organic Coating Propcrhpcrhes Due to Aging
days. The temperature of the oven was kept constant, and was monitored by a digital
thermometer ( OMEGA type H226).
The behaviour of paint films cxposed to heat varies with the nature of the paint and
the character of the -sure, partinilady the temperature. Mon coathgs wiii yellow, and
if exposure to heat is severe and prolonged, hardening Win occur. Darkening and
embrittlement are the wmmon outcome of the organic binders subjected to heat The
number of organic binders which will withstand prolonged heating to ternperatures above
120- 130 OC without appreciable change is relatively srnall. A study of the heat resistance
of a number of M h e s was carried out by Berger [19] who aposed coated md.1 panels
at 90 OC and at 120 OC for 1600 hours. Only a Silicon resin remained unchangeci and
except for polyurethanes based on aliphatic isocyanate, other coatings developed yellow
or brown discoloration
Polyurethane coatings am availabie either as two-component systexns thet are
mked shortly before use and cured by direct cross-linling or as single-package materials
that cure when exposed as a film to moisture, oxygen, kat, etc. The former type is
commonly used by the military and can be M e r classified as aliphatic or aromatic
polyurethanes [20]. The fiutdamentai urethane reaction is that of an isocyanate with an
alcohol. The combination of different isocyanates with the large number of avaüable
polyols makes possible the formation of a wide variety of polymers with properties to
meet specific applications; c o a ~ g s can be made extremely hard or sofi by varying the
molecular weight of the polyol. Prolonged heating process at devateci temperatures causes
the polymerization proces and cross-iinking to continue, thereby c h g i a g the
chacteristic of the paint film.
4.2 Changes in Paht Hardness
Hardness is one of the most important physical properties affeding impact erosion
resistance. The present hypothesk was that the hardness of the paiat film characterizes its
response to the paiat stripping procws. ïhis was imrestigated by measuring the hardness
Clianges in Organic Coating h p w t i e s Due fo Aging
and stripping resistance of fiesbly painted paneis and comparing it with panels coming
fkom the field, and also with artificiaiiy aged panels.
4.2.1, Pencil Scratch Hardness Test
A simple method to evduate the hardness of painted surfaces is the p e n d scratch
test. This test was performed as a prelimimq measure of the changes in bardoess of
panels. The pend scratch hardness test r d t s are somewhat subjective.
The ftesh and aged painted paneis were tested according to ASTM D3363
(Standard Test Method for Film Hardness by Pencil Test, American Society for Testhg
and Materials, 1992).
A set of Staedtler MARS Lumograph pends were used with the foiiowing
hardness values:
(so Itest) 6B-SB-4B-3 B-2B-B-HB-F-H-2H-3 WH-SH-6H (hardest).
Two pend hardness values were observed: The "gouge hardness", defined as the
hardest pencil that Ieaves the coating uncut, and the "scratch hardness", deked as the
hardest pencil that does not leave a permanent scratch on the coabng. Table 4.1. shows
the resuits of the pencil hardness tests on the paint films.
Panel Scratcb Hardncss ~ou~e-Ëardneu
Fresh B 3H
Old Aircraft Panel F 3H
1 Day aged F 3H
2 days Aged F 4H
4 Days Aged H 4H
8 Days Aged H 5H
Table 4.1: Pcncii scntcb b u d a e s test rcs-
Changes in Organic Coonirg Properties Due IO Aging
The results in Table 4.1. show that both hardness values (Le., Scratch and Gouge
hardness) increase as the paint film ages. It also shows that the scratch hardness value of
the painted panel fiom the field is comparable with one and two days aged panels, and its
gouge hardness value is comparable wîth one &y aged panel. In general, the paint nIm
hardness increases as the paint ages-
4.2.2. Vickers Hardaess Test
To ùivestigate the effect of aging on the hardness of the painted panels, Vickers
micro-indentation tests were perfonned on fksh and aged panels, as described in Section
2.3. Figure 4- 1 shows the Vickers Hardness of painteci panels.
Agiag Time ( Days )
Figure 4.1: Vidcers Hardnas Vahm of Paint Films Due 10 Aging (100 OC)
Changes in Organic Coating Properties Due to Aging
The results show that the average Vickers hardness of the k s h panel was 125
MPa and it gradually increased to an average value of 158 MPa for a panel aged eight
days, which shows a significant increase in hardness value due to aging. Another
interesting result is the Vickers Hardness of the field panels, which was in the range of
140 to 150 MPa. Therefore, according to the Vickers Hardness test, the paint f?lmn from
the field was comparable with two and four days aged panels.
Using the Vickers Hardness test, it was possible to measure the hardness value of
the paint film accurately, however this method had two main drawbacks:
First, Vickers Hardness measurement required the samples to be cut h m the
aircraft body, flattened and prepared for the apparatus; which is a time consuming ,
destructive process.
Second, in this method the applied load was static, and did not indicate the
dynamic hardness value effective during impact, which is a high strain rate loading.
4.2.3. Dynamic Hardness Measurement
One of the most important material properties of the coating is the dynamic
hardness. It is defined as the constant pressure which opposes the plastic flow of material
around a penetrating particle,
In order to characterise the impact behaviour of the coated panels, a gas gun
capable of launching single particles at speeds up to 120 m/s and a high-speed
photographic setup capable of measuring inbound and rebound particle velocities were
used, as described in Section 2.5.
Using the gas gun, steel spheres (diameter 1.5 f 0.05 mm, mass = 13.8 + 0.8 mg)
were launched normal to these coated panels at relatively low velocities ( 30-35 d s ) , to
limit the penetration depth to within the coating. Incident and rebound velocities were
measured using a video camera and strobe Lights as described in Chapter 2.
Cliunges in Organic Couting Roperîies Due to Aging
Using Equation (4. l), the coefficient of restitution of the panels were ealdated.
e= V * d (4- 1) v-*
In these experiments the steel spheres were launched normal to the urethane
coating, and they did not d e r any plastic deformation Therefore, dis coefficient tao
represent the amount of plastic deformation that the c o a ~ g has undergone. The higher
the coefficient, the l e s the coating damage, and more of the incident kinetic energy is
restored in the spruig back The coefficients of restitution of the panels are shown in
Figure 4.2.
0.30 ! I
Aging Timc pays)
Figure 4.2: Coefficient of Restitution for tbe Aged rnd Frcsh Puwls, Error bus rcprescnt + lstandard deviation in tbe 3 tri& for crch uptrimentd data points.
In order to examine the impact sites in more detail, the three-dimensional profiles
of the impact craters were a h obtained with an optical d a c e pronlometer (WYKO),
with a resolution of 3 nm. An example of a three-dimensional pronlometer scan can be
found in Figure 4.3.
Mag: 5.3 X Mode: VSI 3-D Plot
Date: 02/17/99 13:4332
Ra: 4.18 um Rq: 6.87 um Rz: 5 1 .O6 um Rt: 56.50 um
Set-up Parameters: Size: 368 X 236 Sarnpling: 3.20 um
Fkcesed Options: Terms Removed: Tilt Filtering: None
Title: Fresh Coating Note: Impact site, 1.5 mm sphere steel
urn
Figure 4.3: Tbrtc dimeasionai profüe of an impact site
v v r g Mode: VSI Mag : 5.3 x
o z 1 7/99 2D Profiles X-Profile 12 Pt / Radial 13:43:32
Size: 368 X 236
Title: Fresh Coating Note: Impact site, 1.5 mm sphere steel
Figure 4.4: Two-dimeasionai profile of an impact site form WYKO
Changes in Orgmic Coaîing Pn,gmïies Due fo Aging
As cm be seen in Figure 4.3. the impact caused the coating to shift to the sides, and raised
edges were created aromd the crater. Dunng the paint stripping process, the raised
material at the edges of the crater would be knocked-off by subsequent impacts-
Figure 4.4. shows the WYKO opt id profilorneter two-dimensionai cross-section
used to measure the diameter of the crater shown in Figure 4.3. The diameter was taken at
a height equal to the undisnirbed coating d a c e , as shown in Figure 4.5.
Direction of Impact 1
Urethane Topcoat
Epoxy Primer
Figure 4.5: Two-dimensionai cross-section of impact site in c o h g
Changes in manie Coating Propertl-es Due fo Agîng
In order to &d the dmc hardness in the present study, a method similar to that
of Tkupatiah and Sunda-an 1211 was used: if the Sze of the impact crater at a
particular velocity and normal incidence are known, then the dynamic hardness can be
estimated by settiag the incident kinetic energy quai to the work done in plastically
deforrning the coating,
where V; is the incident velocity, m is the mass of the incident particle; and P@) is
the load as a fùnction of the indentation depth 6.
If the pileup of the material adjacent to the crater edges is neglected, the
indentation depth, 6, and hence the force-depth relationship, P(6), cm be expressecl in
ternis of the contact radius, a Making the appropriate substitutions and rearranging
Equation (4.2) r e d t s in the following expression for the dynamic harchess [15],
where R is the particle radius, and ;L, is the maximum contact radius reached at
deepea penetration, 6,.
#en a particle strikes, the coating below the particle will experience compressive
plastic deformation in the direction of the impact, and plastic radial expansion. On
rebound, some of the compression in the direction of impact will be recovered due to
elastic effects, but much less radiai expansion wili be recovered. The matenal surroundhg
the craters will thus be leR in a state of residual bi-anal compressive stress, whicb will
inhibit radial recoveq within the crater. Furthemore, the elastic deformations up ta the
point of yield WU be much greater in the direction of impact than in radid direction[l5],
and thus the elastic recovery will alsa be gnater. For these reasons, it cm be assumed that
the final crater radius measured after impact Cie., af€er elastic recovery) wi l be
Ciranges in Organic Coatittg Propertïies Due tu Aging
approximately equal to the maximum contact radius reached dwing impact, a- in
Eqyation (4.3). In this manner, measurements of the Crater diameter can be used to
estimate the dynamic hardness.
For the present coating system, dynamic hardness of the fiesh and aged panels
was dculated by measuring a- of the impact sites using the WYKO profiiometer, and
measuring incident velocity with the gas gun setup and image analysis software.
For the impact of a 1.5 mm steel sphere, with a velocity of 34 mk, the dynamic
hardness was found to change fiom 1.4 GPa for a fie& panel to 1.9 GPa for 8 days aged
panels.
4.3. Changes in Moddiis of Elasticity
In order to investigate the change in the modulus of elasticity of the paint nims, an
ultrasonic the-of-fligbt method was used calculate the moduhis of elasticity.
For isotropie materials, Young's mociuius E, the shear modulus G, and Poisson's
ratio, p, can be caicuiated fkom measurements of density and ultrasonic shear and
longitudinal wave veiocîties ushg Equations 2.5 and 2.6.
However in the present researcb, the poison ratio and the film density of the
polyurethane topcoat were known to be 0.376 and 1.27 g/d 0.02, respectively [IS].
Therefore by meamring the longitudinal velocity of sound wave, VI, the Young's moduhis
of the coatings were calculated by Equation 2.5.
Sample coatings, 10 x 10 cm, were ait from the fiesh and aged panels. The
coating thickness was measured at five points on the samples and the same locations were
used for messuring the speed of sound, as desaibed in Section 2.4. The measured veiocity
of sound was in the range of 2400 m/s. 'Iae average vahie of f i e separate measurements
Changes in Uganic Coatïng Properîies Due to Aging
where used to calculate the Young's modulus of the ccating, using Equation (2.5). Table
4.2, and Figures 4.6 and 4.7 show the measured veiocity and the calculated Young's
modulus.
Panels Velocity of Sound (mis) Young's modulus (GPa)
Fresh 2410 4-03
1 Day Aged 2520 4.42
2 Days Aged 2450 4-16
4 Days Aged 2500 4.35
8 Days Aged 2430 4-09
Table 4.2: Velocity of sound and Young's modulus in paint films, N=S S M 0 &S.
2360 ! 1 . 4
Numbcr of days aged
Figure 4.6: Vclocity of sound in paht fiims, using uitrasonic timesf-fligbt measurcment. Error bus nprcseat *1 standard dcvirtioa (Sû mis) in tbe 5 triais for each upen*mentd data point.
It was originaüy thought that the paint film would become barder and M e r as the
paht aged, and therefore that the momihis of elasticity of the paim film would ïncrease.
Changes in Uganic Coating hperhperhes Due to Aging
However, by cornparhg the velocity of sound in paint nIms baween dinerent aged pands,
it became obvious that the paint film modulus of elasticity remaineci almost constant.
1 1 Day Aged
Number of days aged
Figure 4.7: Modulus of dasticity of printed pan&. Error bars represent *1 standard deviation (* 100 MPa) in the S birls for eacb upcrimcntrl data point.
C h a p t e r 5
Blast Cleaning Experiments with Fresh and Aged Coatings
In order to investigate the effect of aging on the paiot stripping rates, alumiaurn
panels, painted with the standard aerospace paint system, that exp1a.ine.d in Chapter 4,
were artincially aged for one, two, four and eight days in an oven at a temperature of
100°C, as describeci in Section 4.1. The panels were then blasted in the cabinet as
described in Section 2.1.
A blasting system was used to strip the painted panels with ~nvirostrï~@ 30/100
media under three controiieâ blasting conditions, at various relative nozzle speeds. The
system aiiowed control of the media flow rate, air pressure, angle of attack, and table
speed.
This chapter disaisses the changes in the paint stripping rate and rate of coatuig
thickness removai, and compares the p e t stripping results obtained on old aircraft
extenor panels with those of the fieshiy painted and artificially aged paaels.
Bfast C h i n g EjrpenEjrpenrnenis with Fresh and Aged Coafïngs
5.1. Paint stripping rates
The paint stripping rate was detennined as the maximum area of paint removed
down to a spenfic layer per unit of tirne, Le., to the primer or to the substrate, while the
nozzie was moving at an optimum speed relative to the panel.
The tbreshold for topcoat removal was reached when no topcoat was visible on the
top of the primer, and the threshold for compkte stripping was reached when no primer
was visible on the substrate. For the paint system chosen for the present research, the
topcoat was grey and the primer was yeliow, which made it very easy to determine which
degree of stripping was achieved. The experiments were performed on two separate
batches of painted panels. Each batch was taken fiom the same sealeci package of panels
that had been painted at the same the.
When using the blasting system, describeci in Chapter 2, the paint stripping rates,
psr, were caldated as foiiows:
where u was the speed of the nozzie relative to the surface, and w was the average
width of the trace left by the abrasive Stream on the sample.
Table 5-1 shows the paint stripping rates of the panels blasted with EnvirostrïpB
3 O/ 1 00, at a media flow rate of 5.44 kg/min (1 2 Ib/min), an air pressure of 207 kPa (30
Psi), nozzle angle of attack 20°, and a table speed of 1.2 mhin (4 fümin). In this blasting
condition, the average partic1e velocity was taken fiom previous measurernents by
Djurovic et al. [2] to be 197 m/s. The estimateci maximum error in w andpsr was f 3 and
+ 5 percent, respectively.
Bfmt Cfeaning FJrpriments wih Fresh andAged Coatings
Panels Fresb 1 Day Ageà 2 Days Aged 4 Days A g d 8 Davs Aged Old Trace width, w 46.5 39.7 35.2 36.3 32.3 37.6 [Ulm]
Paint stripping 3 -44 2-90 2-65 2.66 2-52 2-76 rate, psr [m2/h]
Table 5.1 : Paint stripping rate of pmds (topcort nmovd).
Table 5-1 shows that the paint stripping rate for topcoat removd decreased fiom
3.44 mz/h, for fieshly painted paaei, to 2.52 m2/h, for a pawl aged for 8 days. The average
paînt stripping rate of old panels taken &om scrapped a i r d extenor wmponents was
2.76 m2/h.
5.2. Paint thickness
The thickness of the paint on the aluminum substrates, before and after exposure
to the blasting media, was measured every 1.3 cm dong the intendeci centerlùie of the
blasting trace (total of 11 measurements), using an eddy-current paint thickness gage
(DeFelsko Corp., Positector mode1 6000), which had a resolution of *1 p x ~ These data
were used to measure the amount of coating thickness removed.
In order to assess the reproducibiüty of the thickness removal resultts, two panels, a
fieshly painted panel and a panel aged four days, were blasted with the same blasting
parameters on two different dates. The reproducibility of the resuit was good, with a
maximum difference of 10 percent.
To determine the variability in a single set of blasting experiments, a set of fieshly
painted and aged panels were blasted three times, with a single set of blasting parameters.
The standard deviation in the amount of coating thickness removal varied fiom 4 to 15
percent. However, the absofute vahre of the error was less than 6 microns.
B f m Cfeaning Erpcn'menlir wïth Frcsii autiAged Caonhgs
5.3. Work Exposure
In order to quanti& îhe blasting conditions, and to fàcilitate cornparisons between
hem, the "work exposure7', Wq7 was used to express the amount of energy O wbich a
unit area of coating was exposed. It encompasseci the &kcts of media type, mass flow
rate, air pressure, average particle velocity, and relative nozzle speed, and was calculated
as follows:
wbere lir was the mass flow rate @@s) and V was the average particle velocity
( d s ) . The overd uncertaïnty in caldathg W, due to meamernent uncertainties in rh,
V , and psr, was estimated to be at most f 16 percent.
The paint stripping experïments were performed on fieshly painted and aged paneis
using the flat node, a standoff distance of 15 cm, and table speeds of 0.75 to 1.50 &min,
under the foilowing three Merent blasting conditions:
A a mas flow rate of 5.44 kg/min (12 Ib/min), a pressure of 138 kPa (20
psi), and an ande of attack of 45";
B. a mass flow rate of 4.08 kg/min (9 lb/min), a pressure of 138 kPa (20
psi), and an angle of attack of 45";
C. a mass fiow rate of 5.44 kghin (12 Iblmio), a pressure of 207 kPa (30
psi), and an angle of attack of 20°.
The three blasting conditions were selead in order to facilitate cornparison with
the data available from the preMws re!search 12, 17,221.
The average particle velocities for calculaihg W, were taken from previous
rotathg disk measurements by Qurovic n al. [Z]; for conditions A, B and C they were
13 1, 1 57 and 197 mls, respectively. The estimated maximum emor in average velocities
measured by this method was reporteci to be k 7?4 1221.
B h t Cfeaning Erpcrimenik with Fnsh mdAged Coatings
Figure 5.1 shows the coating removal r d t s for different work exposures at two
different angles. The previous r d t s availabie in the literature are also superimposed on
this Figure for comprison
- -=- - N d e Angle 45
I 1 1 1 I o f 1 1 6 i 1 I I i
O 1 2 3 4 5 6 7
Work Exposure (k.J/m2)
Figure 5.1: Tbickness of topcoat nmoved from fmhly painted duminum substrate as a fmction of work exposure, using Envirostrip 301100 and two diffiient n d e angles. Tbe available data from Djurovic et ai. [ZJ are plottd for cornpuison.
As show in Figure 5.1, the previous resuits h m Djurovic et ai421 were
consistent with the present research. At low work exposures, the thickness removed fiom
the topcoat was smali, and it hcreased with hcrpilsing work exposure. AU curves for the
results corn the present and past research reached a plateau at thicknesses of 50 to 75 pm,
representing the cornplete removal of the topcoat with no apparent removal of primer.
Another conclusion was that the thickness removed was strongly dependent on the
nozzle angle of attack, and the impact angle at 45' was more effective tbaa 20'. in other
words, both the normal and tangentid components of velocity contri'buted to paint
removal on the aluminum substrate 121.
Table 5.2 presents the average coating t h i c h removed frorn panels under
different bliuùng conditions expressed as the work exposure.
BI& Cfeaning ~ ~ m e n f s with F m and Aged Cootings
Blasting Work Fresb 1 Day
Condition Exposure Aged
w f m Z )
Table 5.2: The average corting thickness nmoved from the printed pan&.
Table 5.2 shows that the amount of paint thickness removed decreases as the paint
ages. The amount of paint removed is comparable for the panel fiom the field and 1 days
and 2 days aged panels. The increase in the amount of thickness removed dong each
column ais0 indicates the direct relatiomhip between the W- and the amount of thickness
removed. Fig 5.2 gives a graphitai interpretation of the r d t s .
BI& CZeaning Experrnentr with Fmsh and Aged Coatings
A 1 Day + 2 Days A 4 Days + 8 Days - Old
Work Exposun (w/m2)
Figure 5.2: Coating thidmess removd vs. work uposure for frcsh and r g d p h , n d e angk of attadc at 45O.
Figure 5.2 illustrates that the vahres of thickness removd fali between two
extremes. The minimum amount of coating removai belonged to a panel aged for 8 days,
and the maximum amount was for a Ereshly painted panel. It also shows that the behaviour
of an old panel ftom the field was comparable with a panel aged for 1 or 2 days.
The a w e for fiesh painted panel in Figure 5.2 reached a plateau at W, vaiues
over 1.5 k.J/m2, wmplete topcoat removai, whiie an 8-days-aged panel. did not reach a
plateau for W, values as high as 6 kJ/m2.
It is noted that the paint film thickness measurement with the eddy-current probe
was a maximm value beuuise the probe tip rested on asperity peaks. Nevertheless, the
change in average peak htight shouid be equal to the change in the average paint
thickness,
Bfasi Cfemiriig Expen'ments wiîh Fmsh and Aged Coatings
5.5. Observations
Figures 5.3 to 5. I 4 show the blast trace left by the abrasive Stream on the
duminum substrates. The yeliow-colored primer under the grey-colored topcoat made it
very easy to detenniw which degree of stripping was achieved.
Figures 5.3 to 5.7 show fieshly painted, 2 days, 4 days, and 8-days-aged panels.
Ail these panei were stripped with the same blastïng parameters, Le., with the nonle angle
of attack 20°, at 5.44 kg/min and 207 kPa, table speed 1 -2 dmin (4 fthnin), W, 3 -760
k.l/m2, average particle velocity 197 mis. Figure 5.3 ïilustnites complete topcoat removal
âom a freshly painted panel. Figure 5.4 ,a panel aged for 1 day, shows partial topcoat
removd with primer visible in some areas. The area of visible primer is decreasing
graduaily Erom Figure 5 -4, a May-aged panel to Figure 5.7, an 8-days-aged panel. Partial d
topcoat removal is visible in Figures 5.4 to 5.7, for 1, 2, 4, and 8-days-aged panels, whiie
the area of topcoat removed decreases as the paint ages. Figure 5.8 iiiustrates the trace left
on an old aircraft panel. The amount of partial topcoat removal is comparable with a panel
artificially aged for 1 day.
Figures 5.9 to 5.13 show the traces on fieshly painted, 1, 2, 4, and 8-days-aged
panels, wder a milder blasting condition. It cm be seen that while 4 and 8 days aged
panels were in the topwat removai period, the primer became visible in the 2-days-aged
panel, with the a rough surface, and the peaks of the topcoat is removed in 1 day. Figure
5.10 shows the topcoat par&idy removed nom the freshly painteci panel.
Another interestkg visual assessrnent is seen by cornparhg Figures 5.3 and 5.9.
The effect of decreased work exposure on reducing the paint stripping rate is clear. This
could be observed for other panels a b , by c o m p a ~ g Figures 5.4 and 5.10 for 1 -day-
aged panels, Figures 5.5 and 5.1 1 for 2days-aged panels, Figures 5 -6 and 5.1 2 for 4-days-
aged panels, and Figures 5.7 and 5.13 for 8-days-aged panels.
Figure 53: Freshly paiated panel, table sped 1.2 mlmh (4 ftlmin), media flow rate 5.44 kg/mia (12 IWmin), pressure 207 kPa (30 psi), mgle of rttack 2W
Figure 5.4: Pliad rged for 1 &y, taMe s p d 1.2 m/mh (4 fvmin), media flow rate 5.44 kg/min (12 IWmin), pmssure 207 kPa (30 psi), angk of tttack 2û0
Figure 515: Pamei agai for 2 days, tabk spacd 1.2 dmia (4 fimin), media flow nate 5.44 kgIrnia (12 IWmin), prusurr 207 IcPa (30 pari, angle of attack 20°
Figure 5.6: Paad rgcd for 4 àavs, h b k speai 1.2 &min (4 filmin), media fiow rate 5.44 wmin (12 IWmia), prwsare 207 kPa (30 psi), aagk of attack 20°
Figure 5.7: Paael aga# for 8 days, tabk speed 12 d m i a (4 Wmia), media flow rate 5.44 Wmia (12 IWmin), pmssare 207 kPa (30 psi), .egk of attack 20°
Figure 5.8: Oid a i r c d prwi, tabie s p d 1.2 dmin (4 ftfmin), medi. flow rate 5.44 kglmin (12 IWmia), prrssrim 207 kPi (3û pi, uigk of rttack 2O"
igure 5.9:- Fresôîy paintd pane!, taMe speed 1 5 m/mia (5 Wmin), media
~w -te 5.44 kg/min (12 IWmin), pressure 207 kPI (30 psi), angle of attack
ire 5-10: Paad rgcd for 1 day, tabk s p u d 15 d m i n (5 ft/mim), m d i a rate 5.44 kgMn (12 IWmin), piessure 207 kPa (3û psi), angle of athck 2(1
Figure 5.11: Pand rgcd for 2 dam table speed 1.5 &min (S Wmh), media flow rate 5-44 wmin (12 IWmin), pmssure 207 kPa (30 paix angle of attrck 20°
Figure 5.12: Panel agd for 4 days, tabie spsed 1.5 &min (S Wmin), media
flow mte 5.44 wmin (12 Iblmia), p-i, 207 kPa (30 psi), uigk of aîtack 20"
Figure 5-13: Panel aged for 8 &ys, table speed 1.5 d m i a (5 fümin), media
flow rate 5-44 Wmin (12 IWmin), pressure 207 kP. (30 pi), sngk of attack 2a0
Blast CIeaning Eqerimrnts wiîh F w h and Aged Coaîings
5.6. Uncertainty Analysis
The foiiowing analysis pertains to the uncertainty in the masurement of the
stripping rate, the paint thickness measurement, and the work exposure.
5.6.1. Uncertahty in paint stripping rates
The paint stripping rates were caiculated by EQuation 5.1,
p S T = W X U , 6 II
where u was the speed of the nozzie relative the d a c e , and w was the average
width of the trace lefi by the abrasive stream on the sample. From this equation, the
possible percentage error in determining stripping rate is given by:
6w 6u where -is the relative enor in determining w, and -is the relative error in
W U
determining u. The relative error in w was estimateû to be less than 3%. Relative errors in
u were estimated to be between 2 and 4 percent for table speeds of 0.9 to 1.5 mimin (3 to
5 ft/min), respectivefy. Therefore, the maximum uncertainty in the paint stripping rate was
'between 4 and 5 percent.
5.6.2 Uncertainty in the Work Exposure
Work exposure, Wq was calcuiated as the amount of energy applied to a uait
area, as foiiows:
where h was the mass flow rate, Y was the average particle velocity.
Therefore the percentage error in deteminhg Work Exposure is given by:
BIast Cleaning l5perirnenîs witA Fresh and Aged Coatings
Grir SV where - was the relative error in measuring flow rate, y was the relative
m v
Gpsr was the relative mor in measuring the error in the average particle velocity, and - Pm
paint stripping rate.
To set the flow rate, m, a known amount of media was biasted for a certain
amount of time. The mor in measuring the mass of media was ne&-gible. The error in
time was up to 3 s, causing a percentage error in the flow rate of up to 5%.
The error in measuring the particle veiocity for the tested blasting conditions was
up to 7%, pjurovic thesis]. Therefore, the maximum overall uncertainty in work exposure
was f 6 percent.
5.6.3. Uncertainty in Coating Thickness Measurement
Before the experiments were done, the paint thickness was measured every 1.3 cm
dong the intended centerline of wery trace, for a total of 11 measures using an eddy-
m e n t paint thickness gage (DeFelsko Corp., Positector mode1 6000), whicb had a
resolution of *1 pm. Mer the experhents were wmpleted, the paint thickness was
measured again at the same points and the tbickaess of paht removed was caldated. The
average and standard deviation of the amount of tbickness removed for each trace were
caiculated.
The error in the average paint thïckness removed was estimated using a confidence
level of 95%, and v = n - 1 = 10 degrees of fiieedom.
The estimated error was
h=Kf0.6718.?, (5.5)
where h is the average thickness removed and i is the standard deviation for each
trace.
Bfast Cfeaning Eperï9nenîs with FnJlh and Aged Coatings
It shouid be noted that the thickness measurement using the thickness gage
rneasured the average of the maximum peak heights, since the top of the gage was always
supporteci by the peaks on the rough h c e of the exposed area. Therefore the maximum
paint thickness removed at very low flow rates wuid be possibly higher than the amornt
measured by the thickness gauge-
The variabiiity in a singie set of blastulg expexhent was estimateci as follows: three
different traces were bhsted with the same blasting parameters on each panel. The
standard deviation and the average paint thickness removed are shown in Table 5.3. The
variability had a maximum of 15 percent error for the 8-days-aged panei.
Panels Fresh 1 Day A g d 2 Days Aged 4 Days Agtd 8 Days Aged Old Paint 6 9 k 3 - 50f6 41 + 5 19+3 - thickness removed r ~ r
Table 5.3: Average paint thidmess removed (* standard dcviation) with tbe îirt no& rt 4.08 kgmin and 1 3 kPa, tabk speed u=1.2 mlmin (4 fimin), work crposure 1.79û kJ/m2, average partide vclocity 157
In order to assess the reproducibility of the thickness removal results, two panels, a
fieshly painted panel and a panel aged four days, were blasted with the same parameter on
two different dates. The reproducibility of the r d t s had a maximum error of 10 percent.
C h a p t e r 6
D i s c u s s i o n a n d C o n c l u s i o n s
The experiments, descriied in Chapter 4, showed that aging the duminum panels
painted with pofyurethane significantiy i n c r d t h e dynarnic hardness of the coatings,
while the process of aging did not have a significant e&a on the coatîng's modulus of
elasticity. It was also observed, in Chapter 5, that aging would decrease the paint
stripping rate and the amount of coating removal. In other words, the coatulg became
more erosion resistant as it aged. A possible reason for tbis coating behaviour is
explained in this Chapter. A method for prediaing the paint stripping rate of Urcraft
panels by performing in situ measurement on the watings is also recommended.
6.1. Analysis of impact on tapes and painted panels
Previous research on starch media strippiag of the eaospacc paint system h t
was used in the present research (MIL-C-83286 topcoat with MIGP-23377 primer on
AA 20244'3 aluminum panels) by Djurovic et al. [2], revealed that the topcoat was not
removed by deIamination fiom the primer nor by brittie erosion, but in a cumulative
fashion, as severai impacts were needed to remove it wqletely.
Papini and Spelt [3] reported that the same aemspace coating system w s
removed by ploughiog erosion when struck with spherical panicfes, in wbich plasticaily
defomed material was pushed to the Lips at the edges of the impact miter, where it
became available for knock-off by subsequent impact. For particle impact against these
coated substrates at moderate speeds, such as those found in blast cleaning (50-150 mis),
the dynamic effects wexe negligible, and the collision could be tmted as quasi-static. A
summary of this quasi-static impact anaiysis wouid hdp explain the coating behaviour in
blastuig. A complete analysis can be found in [3]
A typical curve for the collision of a sphere with a substrate at normal incidence
is shown in Fig. d.l(a). If the instantaneous contact force, P, is divided by the
instantaneous contact area, then the mean contact pressure, pm, can also be plotted as a
fiinction of the penetration depth, 6. Figure 6.1 (b)
(elastic-piastic) elastic plastic
i J w ,
i
s: Figure 6.1 (a):Assud (elastic-plastic), rctud (dutic-plastic), and fully plastic curves ot: (a) force-defldon, and (b) mcrn pressure-de~on 13)
The amount of elastic energy stored atid returned to the system cm be calculated
by integrating the Pb auve using the appropriate iimïts. When the coaàng bas M y
yielded, the contact pressure reaches a constant vaiue, pd, calleci the dyaamic hardness or
plastic flow pressure.
For normal incidence collisions, acwrding to Figure 6.1, the particle first
encounters an elastic retardhg force, then an elastic-plastic transition retarding force, and
Gnally a plastic retarding force. The elastic-plastic transition portion of the curve is
difficult to obtain, so the collision is assumed to follow the elastic path ut i l fûly plastic
conditions have been reached, as shown in Figure 6.l@). The M y plastic condition
continues until the point of maximum penetration, where t is assumeci that the particie
normal velocity is zero. At this point, an efastic rebound contact force causes the sphere
to accelerate away tiom the coating.
The elastic rebound P-S relationship can be modeled as the reverse of an elastic
indentation of the final (relaxeci) crater to the point where the rebound begins (L)-
6, =L -6, (6- 1)
Where 6-= is the elastic deformation for a coating of thickness h (equal to the
final thickness at the bottom of the crater) pressed to the depth of &. (see Figure 6.2).
A
Coating H
Figure 6.2 : Geometry of rssumeà revenibk rebound pro-
6.2. Mechanical properties aflecting coating system behaviour
A single particle, with mass m and velocity Vi hPs an impact energy of +rnKZ.
Upon incidence, assuming the tnction is negligible and there is no particle deformation,
the total impact energy will be converted to the work required to displace the materid.
For the ðane system, the amount of coathg pushed &O lips at the edge of the crater
depends strongly in the coating dyaamic hardness. If two coatings bave the same
Young's modulus, E, then the one with higher dynamic hardness, pd, would have a
smaiier indentation upon impact. 11 51
Figure 6.3 shows a plot of the mean contact pressure, p, vs. the penetration
depth, 6, for two coatings having the same Young's moduhs, E. Coating 2 has a higher
dynamic hardness than coating 1, but the impact conditions (i-e., particle properties and
kinetic energy) are identical
Referrïng to Figure 6.3, the work done is quai to the area under the P ô cuve to
the point of maximum deflection for coating 1. It is also equal to the area under the
P â curve for coating 2, tiil the maximum deflection 6-. It is clear fiom the Figure 6.3
that coating 2, with higher dynarnic hardness, suffers less indentation for the same
arnount of impact energy, provided that the coating is thick enough to prevent the
effective hardness fiom hcreasing due to proximity of the substnite. At the point of
maximum penetration, it is assumed that the particle n o d velocity is zero, and an
elastic rebound contact force causes the sphere to accelerate away nom the coating-
Elastic Energy Elastic Energy Absorbed Returned Figure 6.3: Plot of mean contact pressun vs. dcptb of ptocbation, for cornpuing two coatings with the samc Young's rnodulus and diff'ennt dyuamïc hudness.
Now the elastic portion of the energy wiii be returned to the particle, it is clear
fiom Figure 6.3 that the particle with a higher dynamic hardness has a larger amount of
elastic energy returned, which, when trandormed to kinetic energy, would mean a hîgher
rebound velocity. Therefore, the mathg with higher dynamic hardness has a bigher
coefficient of restitution.
Chapter 4 reported on the impact o f a 1.5 mm steel sphere, with a velocity of 32
f 2 d s , on painted panels. The dynamic hardness was found to change fiom 1.4 GPa for
a fieshly paiated panel to 1.9 GPa for an 8-days-aged panei, while the moduhis of
elasticity of the panels alrnost remained the same, between 4.03 and 4.42 GPa (The
differences between the values of Young's moduhis were within experimental enor,
which was approximately k 1W as shown in Figure 4.6).
The measu~ed properties of a ikeshiy painted paad aad aged paneis are @en in
Tabte 6.1 for cornparison
Frtsbly painted 2 drys rged 8 days aged
panel prael pmd
Young's Moduius, E (Gpa) 4.03 4-16 4.09
(N=3, SD=6%) (N=5, SD=7%) (N=5, SD=12%)
Dynamic Hardness Pd (Gpa) 1.43 1 -64 1.91
(N=3, SD=5%) (N=3, SD=5%) (N=3, S M % )
Incident Velocity, V; (ds ) 34 34 34
(N=3, S M ) (N=3, S M % ) (N=3, SD=2%)
Rebound Velocity, V, (ds ) 12 16 15
(N=3, SM'%) (N-3, S M % ) (N=3, SD=4%)
Coefficient of Restitution, e 0.3 5 0.46 0-44
(N=3, S M % ) (N=3, SD=4%) (N=3, S M % )
Penetration Depth (jun) 32 26 37
(N=3, SD=12%) (N=3, SD=lS%) (N=3, SD=lO%)
Paint Stripping Rate (m2/hr) 3 -44 2.65 2.52
at W, of 1790 kJ/m2 (N=3, SD=3%) (N=3, SD=4%) (N=3, SD=5%)
Thickness Removed (pm) 69 50 19
at W,, of 1790 kYm2 (N=3, SD=4%) (N=3, SD=12%) (N=3, SD=16%)
Table 6.1: Cornparison of h b .nd rgd p d , Numbtr of data points and the stuidud Mation are givto m parenthesis.
It can be seen that although al1 panels have almost the same value of Young's
moddus (3% ciifference), the coating with the higha dyaamic hardness was signSantly
more resistant to erosion.
As expected, the coefficient of restitution increased with agiag fkom 0.3 5 to 0.46.
The paint stripping results for these panels are consistent with the increased dynamic
hardness of the older paint fj.ims, The average coating thickness removed fkom 2-days-
aged is approxirnately 80% of that of the Eiesh painted panel with the same blasting
condition, and the 8-days-aged panel with the highest dynamic hardness has the most
erosion resistaut coating among the panels.
It has become clear that the dynamic hardness, pd, of a coating is an important
property that controls its response to blasting. If two coatings have the same Young's
modulus, E, then the one with higher dynamic hardness, pd, would be more erosion
resistant,
6.2.2. Young's Moddus
For the same dynamic hardness, a low Young's modulus (E) will promote
springback (and hence less material pushed into lips at the edge of the impact crater),
because the amount of recoverable elastic deformation will be increased. The conclusion
is that a lower E coating is more erosion resistant.
If the coating is an elastomer (Le., a material that has a very high elongation to
yield and hence Iow E), then most of the incident kinetic energy WU be returned to the
particle on rebound. In this case, oniy a s m d amount of energy is available to damage the
coating. This is probably, one main reason for good erosion resistance of masking tapes,
which had an elastomeric backing material.
6.2.3. Coefficient of restitution
The coefficient of restitution, e, is defined as the ratio of the normal rebound
velocity to the no& incident velocity. A high coefficient of restitution means h t the
impact was predominantly elastic, and therefore the minimum damage was done to the
coating. On the ot&er hand, a low coefEcient of restitution mems that there was a
signifiant amount of plastic defomation in the wating.
In the experiments with the tapes, Section 3.23, the s tel spheres were launched
normal to the masking tapes, and the spheres did not suffer any plastic deformation.
Therefore, this coefficient of restitution represented the amount of plastic defonnation
that the tape had undergone. The higher the coefficient, the Iess the plastic deformation
of the tape, and more of the incident kinetic energy was restored in the springback.
At lower incident velocities, the impact was more elastic, and therefore tess
damage was done to the masking tapes. As can be seen in Figure 3.13, the Promac tape,
had the lowest coefficient of restitution, me-g that it had more plastic deformaton and
iess spnngback due to a harder bachg matenal, srnalier thickness, and less ductility. On
the other hand, the coefficients of restitution of the Bron and the 3M tapes were
comparable, and they both showed a higher level of erosion resistance in the erosion
tests.
However, according to the scratch test results, the Promac tape had the hardes
d a c e (backing material) among the three tapes; both the 3M and Bron tape had a much
softer d a c e than the Promac. This would sean to imply that the Promac would be
more erosion resistant, although the opposite was seen.
One possible explanation is that, for the maskiag materials, the hardness of the
backing material is not the dominant fkctor in determinhg erosion resistance. The erosion
resistance of the tapes also depends on the amount of remverable elastic deformation,
which depends, in turn, on the tape yield strength and its Young's modulus. if the
backing material is ehstomeric (i-e., has a very high elongation to yield), then more of the
incident kinetic energy will be retumed to the particle on rebound, with vey fittle damage
to the tape. In this case, a very srnall amount of the energy is avdable to damage the
tape.
The conclusion is that the high e may be due to a high dynamic hardness (e-g.,
aged coatings), or due to a low modulus of elasticity (e-g. masking tapes), however in
both cases it means a higher erosion resistance. Therefore, it is suggested that the
measurement of this coating pro- would diredy reveal the coating's resistance to
erosion.
6.2.4. Leeb's Principle of Operation
The coefficient of restitution, e, of a d a c e can be measured directty usiag a
method known as Leeb's principle. To apply Leeb's principle, compact, commercially
avaiiable devices use a s p ~ g to propel a metal sphere through a guide tube toward the
test surface. An induced current is generated within a wil encircling the guide tube. M e r
impact, the sphere rebounds fiom the surface inducing a second signal in the coi]. The
instrument calculates coefficient of restitution, e, using the ratio of the voltages
(proportiod to velocities of sphae). In order to measure the coating dynamic hardwss,
the mass and veIocity of the impact body shouid be carehily adfisteci to control the depth
of penetration. It is suggested that this simple method of measurement wouid help the
operator to predict the paint stripping rate of panels in various locations over the aircraft.
For example, upper wing Surface exposed to direct sunlight and areas near engine
exhausts are expected to undergo more rapid aging, and hence have a greater dynamic
hardness.
6.3. Conclusions
6.3.1. Masichg Tapes
The effectiveness of three different types of PSA masking materials in resisting
erosion and edge lifting during starch media blasting was investigated.
It was wncluded that the erosion resistance of the tapes depended on the
coefficient of restitution, which represented the amount of plastic deformation that the
tape had undergone upon impact. The higher thc coefficient of restitution, the less the
plastic deformation of the tape, and more of the incident kinetic energy was restored in
the springback.
The edge lifting of the tapes did not occur on a c lan surface, however this
phenornena was obxrved on contaminateci substrates. The problem of edge LiRiog can be
eliminated by proper cleaning of the substrate before applying of the tapes, however due
to difncuity in maintaining the cleanliness in the work environment, the use of a hot mek
adhesive for protection of the edges is recommended.
The hot melt adhesk protects the edges of the mashg tapes in three different
ways:
1) it deflects the Stream of blaotuig particles, therefore preventing the coilision of
the blasting particles to the edeg of the tape,
2) it acts as an erosion barrier by embedding the blasting particles, and
3) the hot melt increases the tape resistance to edge üfting by increasing the
stiffiiess of the edge of the tape-
6.3.2 Effects of Paint Agkg
It was shown that aging of the polyurethane paint signincantly increase the
hardoess of the coatings, while the process did not have a significant e f f i on the coating
modulus of elasticity.
The aging process decreased the paint stripping rate aud the amount of coating
removal; Le., the coating became more erosion resistant as it aged.
It became clcar that the dynamic hardness, pd, of a caating is an important
property that controls its response to blasting. If two coatings have the same Young's
modulus, E, then the one with higher dynamic hardness, ph will be more erosion
resistant .
It is possible to correlate the paht stripping rate with the coefficient of restitution
or dynamic hardness of the paiat film. The coefficient of restitution (and dynamic
hardness) c m be measured on the suffàce us@ a method known as Leeb's principle;
therefore it is possible to predict the paint stripping rate by the in situ measurement of the
coefficient of restitution-
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