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Surface modification of high-performance aramid andpolyethylene fibres for improved adhesive bonding to epoxyresinsCitation for published version (APA):Mercx, F. P. M. (1996). Surface modification of high-performance aramid and polyethylene fibres for improvedadhesive bonding to epoxy resins. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR455550
DOI:10.6100/IR455550
Document status and date:Published: 01/01/1996
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https://doi.org/10.6100/IR455550https://doi.org/10.6100/IR455550https://research.tue.nl/en/publications/0091d6c7-6763-4312-848f-a34b393af1fc
SURFACE MODIFICATION OF
HIGH-PERFORMANCE ARAMlD AND
POL YETHYLENE FIBRES FOR
IMPROVED ADHESIVE BONDING TO
EPOXY RESINS
Cover:
Typical surface structure of air-plasma-treated PE tapes, showing many
small pits (see chapter 5)
Omslag:
Karakteristieke oppervlaktestructuur van een met lucht-plasma behandelde
PE film (zie hoofdstuk 5)
SURFACE MODIFICATION OF
HIGH-PERFORMANCE ARAMlD AND
POL YETHYLENE FIBRES FOR
IMPROVED ADHESIVE BONDING TO
EPOXY RESINS
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof. dr. J.H. van Lint,
voor een commissie aangewezen door het College van Dekanen in het openbaar te verdedigen op donderdag 7 maart 1996 om 16. 00 uur
door
Franciscus Petros Maria Mercx
Geboren te Halsteren
Dit proefschrift is goedgekeurd door
de promotoren prof. dr. P.J. Lemstra
prof. dr. ir. J. van Turnhout
en de copromotor dr. ing. A.A.J.M. Peijs
Contents
Contents
Chapter 1 Introduetion
1.1 Fibre-Reinforced Polymers
1.2 Adhesion 1.3 Developments in Aramid Fibre-Matrix and PE Fibre-Matrix Adhesion
1.3.1 Aramid Fibre-Matrix Adhesion 1.3.2 Polyethylene Fibre-Matrix Adhesion
1.4 Objective of the Present Investigation 1.5 Outline of the Thesis 1.6 References
Part A: Aramid Fibres
Chapter 2 The Selective Introduetion of Specific Organic Groups at the Surface of Aramid Fibres: A Model Compound Study
2.1 Introduetion 2.2 Experimental
2.2.1 Materials 2.2.2 Reactions 2.2.3 Characterization Methods
2.3 Results and Discussion 2.3.1 Chemica! Structure 2.3.2 Higher Homologues 2.3.3 Thermal Stability 2.3.4 Condusion
2.4 References
1
1 2 4 4 7
8
9 10
15
15
15
15
16 17
17
17
23 24 24 26
i i Contents
Chapter 3 Surface Modification of Aramid Fibres 27
3.1 Introduetion 27 3.2 Experimental 27
3.2.1 Reactions 27 3.2.2 X -ray Photoelectron Spectroscopy 28 3.2.3 Scanning Electron Microscopy 28 3.2.4 Determination of Acthesion 28 3.2.5 Determination of Mechanical Properties 29
3.3 Results and Discussion 29 3.3.1 Chemical Structure 29 3.3.2 Acthesion and Mechanical Properties 33
3.4 References 35
Part B: Polyethylene Fibres
Chapter 4 Oxidative Acid Etching 39
4.1 Introduetion 39 4.2 Experimental 40
4.2.1 Prepararlon of Tapes 40 4.2.2 Acid Treatment 41 4.2.3 Determination of Acthesion 41 4.2.4 Determination of Mechanica} Properties 42 4.2.5 X-ray Photoelectron Spectroscopy 42 4.2.6 Infrared Spectroscopy 42 4.2.7 Scanning Electron Microscopy 42
4.3 Results 43 4.3.1 Acthesion versus Mechanical Properties 43 4.3.2 Scanning Electron Microscopy 45 4.3.3 Weight Loss 47 4.3.4 Infrared Spectroscopy 47 4.3.5 X-ray Photoelectron Spectroscopy 48
4.4 Discussion 50 4.5 References 52
Contents
Chapter 5 Air- and Ammonia-Plasma Treatment
5.1 Introduetion 5.2 Experimental
5.2.1 Polyethylene Tapes 5.2.2 Plasma Treatment 5.2.3 Adhesion, Mechanica! Properties and
Chemica! Characterization
5.2.4 Scanning Electron Microscopy 5.3 Influence of Process Parameters 5.4 Results and Discussion
5.4.1 Tape Charaterization 5.4.2 Acthesion and Failure Mode 5.4.3 Mechanism of Acthesion
5.5 Conclusions 5.6 References
Chapter 6 Corona Grafting of Acrylic Acid
6.1 Introduetion 6.2 Ex perimental
6.2.1 Polyethylene Tapes 6.2.2 Corona Grafting 6.2.3 Characterization
6.3 Results and Discussion 6.3.1 Tape Characterization 6.3.2 Acthesion and Mechanica! Properties 6.3.3 Surface Treatment and Shear Strength
6.4 References
Epilogue The Role of Fibre Anisotropy and Adhesion on Composite Performance
iii
55
55 56 56 56 57
57 58 59 59 65 67 71
72
75
75 75 75 76 76 76 77
80 80 81
83
iiii Contents
Summary 88
Samenvatting 92
Curriculum Vitae 96
Dankwoord 97
Introduetion
Chapter 1 Introduetion
1.1 Fibre-Reinforced Polymers
1
The use of fibre-reinforced polymers has rapidly grown over the past few decades and there
is every indication that this will continue. This growth has been achieved mainly by the
reptacement of traditional construction materialsas metals, wood and concrete and was driven
by the superior properties per unit weight (specific properties) of fibre-reinforced polymerie
materials. The higher specific modulus and strength of fibre-reinforced polymers means that
weight savings can be realized when constructing with these composite materials, which
results in a greater efficiency and energy savings. Initially applied in military and aerospace
applications, fibre-reinforced composites have now penetrated other segments of the market
as well, including the automotive industry. Some examples of the various realized applications
are given in table 1.1.
Table 1.1 Applications of fibre-reinforced polymeri·5
Industry Examples
Aerospace Antennas, wings, radomes, helicopter blades, landing gears
Marine Hulls, decks, masts
Automobile Bumpers, drive shafts, seats, trailers
Sport Tennis and squash rackets, fishing rods, skis, canoes, golf clubs
Fumiture and equipment Chairs, tables, lamps, ladders
Chemica! Pressure vessels, pipes
2 Chapter 1
Partienlady the inexpensive glass-fibre-reinforced polymers contributed much to the growth
of polymerie composites in the last decade. The more actvaneed composites, based on carbon
and/or aramid fibres, are stilllimited intheir commercial use because of high material costs.
However, they are widely applied in the aerospace industry to satisfy requirements for
enhanced performance and reduced maintenance. Moreover, since the sports industry
discovered these advanced polymerie composites, the number of applications and consequently
their commercial importance is growing2•3.s.
The reptacement of traditional materials as metals by polymerie composites was not
achieved easily. It was in fact preceded by elaborate research to optimize the (mechanical)
properties of fibre reinforeed polymers. The development of new high-performance fibres
with improved strengthand stiffness to weight ratios was but one important step. Decisive
for the evolution of fibre reinforeed polymers to its present accepted status as competitive
construction material were, however, the developments in the area of fibre-matrix adhesion.
1.2 Adhesion
The first applications of fibre reinforeed polymers can be traeed back to 1940s when glass
fibres were first used as reinforcement in polyester resins. It soon became apparent that these
polymerie composites may loose much of their strength in every day practice, resulting in
premature failures6•7• The in-depth investigations that foliowed traeed this back to the low
initia! adhesion, that could not withstand the intrusion of water. Eventually this leads to the
debonding of resin from the hydrophillic glass, causing the observed deterioration in
properties. Following the recognition that the level of fibre-matrix adhesion was the key factor to composite performance, a search began for glass fibre sizings that could improve
the adhesion between such dissimHar matenals as glass and polyester. To this end numerous
compounds were evaluated. Not surprisingly, organofunctional silanes, which are hybrids of
silica and organic matenals related to resins, were among the compounds tested. They proved
to be highly effective in increasing both the dry- and wet-strengthof glass-fibre-reinforced
polyesters6•7• Moreover, by tailoring the organic part of these silanes, it proved to be
relatively easy to optimize the adhesion of the glass fibres to other polymerie materials,
including epoxy resins, polyamides and even polyolefms6•7• It was these developments in the
area of adhesion that increased the (long-term) performance and ensured the reliable use of
glass-fibre-reinforced polymers in every day practice.
Experiments conducted at the end of the fifties showed that carbonization of fibrous materials yielded a continuons carbon fibre with exceptional specific properties8 ( see fig 1.1).
Analogous to glass fibres, these carbon fibres could be used to provide a reinforcemet;lt in various resin systems for the fabrication of structural composites. However, the initial carbon-
Introduetion 3
fibre-reinforced polymers did not achieve the expected mechanica! properties derived from the properties of fibre and matrix separately. Similar to glass-fibre-reinforced polymers, this could be traeed backtoa lack of acthesion between the carbon fibres and the polymer matrix. Again, numerous surface treatments were developed to overcome the initia! weak bond
strength of the as-made carbon fîbres. Of these, only the oxidative pretreatrnents gained commercial importance8•9 . Electrochemical oxidation is now the most widely used industrial technique and has replaced other wet methods as immersing the fibres in oxidizing agents such as nitric and chromic acid or dry methods as the oxidation in air or oxygen9• Owing to the increased adhesion, the full potential of the carbon fibres could finally be exploited, leading to the penetration of these polymerie composites in high-performance markets such
as the aerospace, military and sporting goods2•3•5• The success of carbon fibres in these appealing markets inspired the development of new
families of high-performance fibres. Research mainly focused on the orientation of linear polymers and was driven by the theoretically high mechanical properties of a fully aligned polymer chain and the low density of polymerie materials in general10·11 • Although these efforts resulted in a number of high-performance fibres, only two have gained commercial
importance. These are, in chronological order of development, aramid fibres (1973) and polyethylene (PE) fibres (1980). The Dutch companies Akzo and DSM played a leading role
in the development and commercialization of these fibres. Although several aramid fibres
exist, the term aramid fibre will be used in this thesis to indicate poly(p-phenylene terephthalamide) (PPTA) fibres, the most important representative of this class of fibres.
Figure 1.1 shows the specific properties of the high-performance polymerie fibres compared to glass and carbon fibres, various metals and some bulk polymers.
Following the research on glass and carbon fibres, it was generally accepted that the level of fibre-matrix acthesion is the key factor for the translation of fibre properties to composite
performance. However, the chemica! nature of the as-made aramid and PE fibres in combination with the smooth surface provides only a moderate acthesion at best. Therefore, the improverneut in the acthesion of these fibres was thought to be of major importance for the successful introduetion as reinforcement in polymerie composites. As a direct result, this PhD study, directedat improving the acthesion of aramid and PE fibres to epoxy resins, was started in 1986. The importance of fibre-matrix acthesion research both from a scientific and economie point of view, may also be illustrated by the fact that the advisory board of the Junovation Oriented Research Programmes (IOPs), an initiative of the Dutch Ministry of
Economie affairs, ranked fibre-matrix bonding as one of the primary areas for research in the first phase of this prograrmne ( 1987-1991). The incentive of these programmes is to develop new technical-scientific background knowledge and expertise in areas which are valuable for the consolidation and growth of the Dutch industry.
4 Chapter 1
Figure 1.1 Specific strength vs. specific modulus of various high-performance fibres
(N/Tex=GPa.p-1, p=density in g.cm-3)
1.3 Developments in Aramid Fibre-Matrix and PE Fibre-Matrix Adhesion
The most important developments in the adhesion of aramid and PE fibres to polymerie
matrices, prior to the investigations described in this thesis, are summarized below.
1.3.1 Aramid Fibre-Matrix Adbesion
lnvestigations on the effect of surface treatments on the adhesion of aramid fibres started in
the mid-seventies. Since then a lot of different methods have been developed. Rougbly, these
methods can be divided into three groups, i.e. the use of coupling agents, surface roughening
and the introduetion of functional groups. The majority of the investigations concentrared on
epoxy resin as a matrix material and the results presented here are for epoxy resin composites
unless stated otherwise.
Introduetion 5
Initia! attempts to improve the adhesion of aramid fibres focused on the use of coupling
agents12-16_ Preferentially, low molecular weight organic compounds were applied. Generally,
these compounds proved to be of limited use and increase the acthesion only marginally. This
was attributed to the fact that although immobilized on the surface, most of these compounds
do not penetrate or react with the aramid fibre 13 • Positive effects were only noted for highly
reactive coupling agents. Examples include the use of polyfunctional aziridines1\ which more
than double the interlaminar shear strength (ILSS) of polyester composites, and the actdition
of diisocyanates15 in case of aramid reinforeed rubber. Martin et al. 16 synthesized
blockcopolymers consisting of a rigid polybenzamide block and a flexible copolyamide 6/6.6
block which markedly improved the acthesion to polar thermoplastic resins. The proposed
metbod bas, however, the drawback that sulphuric acid used to apply the blockcopolymer
attacks and partially dissolves the surface of the aramid filaments. Although this ensures a
strong acthesion between the blockcopolymer and the aramid, it has a detrimental effect on
the tensite strength of the aramid fibres.
Surface roughening of fibres will increase the mechanica! keying effect but it will also
adversely affect the tensile strength. Consequently, only a few studies have been devoted to
this subject. Roebreeks et aL 17 used sandpapers attached to a rotating drum for controlled
fibrillation of strands and fabrics. Inherently related to the metbod employed, only the
outermost filaments of a strand or a fabric are fibrillated. Although this gave a large effect
on the lap-shear strength values measured, the effectiveness of this metbod for improving the
in-plane shear strength of real composites must be doubted. An interesting metbod was
developed by Breznick et al. 18 who first absorbs bromine in the outer surface layers foliowed
by the neutralization with an ammonium salt solution. The gaseous nitrogen formed, and
initially occluded under the fibre surface will pierce the surface, leading to the many small
pores detected by scanning electron microscopy (SEM). This treatment produces a 20%
improverneut in ILSS value but also invokes a drop in fibre tensile strength of 15%.
Although both described approaches (coupling agents and surface roughening) have been
successful to some extent, the absolute values of the acthesion strength as well as the scanning
electron micrographs of the fracture surfaces indicate that for composites based on these
fibres interfacial failure still dominates. In other words adhesion is still the limiting factor in
the performance of these composites. A higher level of adhesion can generally be obtained
through the introduetion of functional groups at the fibre surface by either physical or
chemical methods. Especially amino, carboxylic acid and epoxy groups were found to be
effective. Amino groups can be introduced by either ammonia19-21 , monomethyl amine19 or
nitrogenlhydrogen21 plasma treatment, a nitration-reduction cycle22·23 or bromination followed
by aminolysis22 • The amino groups introduced in these ways are almost exclusively attached
to the phenyl rings19-22 • Improved peel strength19·22 , pull-out strength20·23 and ILSS values2L24
up to 67-70 MPa we re attributed to improved physico-chemical interactions20•21 and
6 Chapter 1
c-Q-c_HU\._Z-J 11-11·~ 0 0 n
+ 2n Cf\-S-CHÏ No"'
8
R R I I CHONo CHONo
tc-o-J~_r.i. 11 li~J 0 0 n R'X
t c-Q-cl'o!'l 11 11 - J 0 0 n R'• ollyl or vinylbenzyl
Figure 1.2 Grafting of aramid polymer
Introduetion 7
chemical22•23 bonding. Hydroxyl, carbonyl and particularly carboxylic acid groups can be identified on the surface of oxygen and air plasma treated PPTA fibres, resulting from the
oxidation of the phenyl groups21 • Scanning electron micrographs of the fractured surfaces of ILSS samples indicate that the raise in ILSS value from 45-50 MPa for the untreated aramid
fibres to 67-70 MPa for the air, oxygen, ammonia and nitrogen/hydrogen plasma treated fibres is accompanied by a change in failure mode from interfacial controlled to failure inside the aramid fibre21 . A chemical methad for the selective introduetion of a variety of functional
groups was developed by Takayanagi25 • The method is schematically shown in figure 2 and
camprises two successive stages. In the first step, PPTA is reacted with methylcarbanion in dimethylsulfonyloxide (DMSO) to yield a metalated PPTA. Subsequently, this intermediate
is converted with alkyl halides or epoxies. Depending on the chemical nature of the epoxy or the alkyl halide used, different functional groups can be introduced, such as carboxylic acid25-27 , epoxy27 , allylic28 , acrylonitril26 and octadecyl26 groups. This approach allows the
tailoring of PPTA fibres for the impravement in acthesion to a number of resins, such as epoxies, polyesters, phenolics and thermoplastic resins. Although the methad developed by Takayanagi is appealing in its versatility, the corrosive nature and the high costs of the
chemieals used present a serious drawback for application. The reactivity of the amide group towards diacid chlorides such as oxalylchloride and its
application for the surface modification and enhancement of adhesion, as described in this
thesis, has not yet been investigated.
1.3.2 Polyethylene Fibre-Matrix Adhesion
Even though polyethylene is an apolar material that poorly bonds to most polymer matrices,
surface modification via oxidation is relatively easy and has been employed with great success for improved metal-plating and printability of low and high density polyethylene in the past 30 years30•33 • Consequently, oxidative pretreatments were among the first methods to be considered for improving the weak bond strength of high-performance PE fibres to polymerie matrices.
Ladizesky and Ward34 investigated the effect of oxygen-plasma and chromic acid treatment on the acthesion of melt-spun polyethylene fibres to an epoxy matrix. Both treatrnents markedly improved the adhesion, although plasma treatment was far more effective as
evidenced by a change in failure mode from interface failure to shear failure within the melt-spun PE fibres. The higher effectiveness of plasma treatment was attributed to the resulting pitted surface which allowed penetration of the resin to produce a mechanical keying between fibre and matrix. Similar results following air- or oxygen-plasma treatrnents were reported by Nguygen et al. 35 , Nardin et al. 36 , Kaplan et alY and Jacobs et al. 38 . A marked increase
8 Chapter 1
in wettability and adhesion was observed after ammonia-plasma treatment, although scanning electron microscopy showed no changes in surface structure39• Plasma treatment affects the fibre tensile strength negatively, decreases up to 20% have been noted34•36• Corona discharge resulted in approximately a two-fold increase in interlaminar shear strength40•41 • Postema et al. 42 reported a five-fold increase in the adhesion of gel-spun PE fibres to gypsum plaster
after chlorosulfonation. According to the authors this improverneut could be related to surface
roughening of the fibres. Based on evidence gathered on LDPE and HDPE, it is expected that the above described surface treatments will lead to oxidation or amination of the surface. Although some remarks concerning the introduetion of functional groups were made, no attempts were undertaken to monitor the changes in surface chernical composition, nor to reveal the nature of the chemical groups incorporated. In view of the large effects that functional groups can have on the adhesion, it seems premature to attribute the increased adhesion to surface roughening without the exact knowledge of the changes in surface chemica! composition and surface topography brought about by the different surface
treatments.
1.4 Objective of the Present Investigation
The main objective of the research described in this thesis is to improve the adhesion of high-performance aramid and PE fibres to epoxy resins via surface modification of the reinforcing fibres. An obvious requirement of any surface treatment procedure is that it should not affect the mechanica! properties of the reinforcing fibres, or at least not to a large extent {i.e. ::;; 10%). Consequently, the effect of the surface treatments on all relevant mechanica! properties of the high-performance fibres has been studied. Attention is focused on the relationship between surface chemistry, surface topography, adhesion and faiture mode. In this way the
mechanisms responsible for the increased adhesion as well as the failure mode cao be assessed. The latter will indicate whether adhesion, the transverse or shear strengthof these high-performance fibres, or the (shear) strength of the matrix is the limiting factor in the
performance of these composites.
Introduetion 9
1.5 Outüne of the Thesis
The thesis is divided in two parts dealing with high performance aramid and (gel-spun) PE fibres, respectively.
Part A: Aramid Fibres
Chapter 2 describes the results of a model compound study, undertaken to evaluate the feasibility of a novel two step chemica] modification procedure for the selective introduetion of specific organic groups at the surface of aramid fibres.
Following the methodology developed in chapter 2, the selective introduetion of acid, ester, amine and epoxy groups at the surface of aramid fibres is reported on in chapter 3.
Furthermore, the effect of these surface modifications on the adhesion to epoxy resin and the mechanica! properties of the aramid fibres is evaluated.
Part B: Polyethylene Fibres
Chapter 4 deals with the oxidative acid etching of high-performance PE fibres. Attention is given to the effect of the treatment on surface morphology, surface chemica] composition and mechanica! properties of the fibres and interfacial bond strength to epoxy resin.
In chapter 5, the influence of air and armnonia plasma treatment on surface chemical composition, surface morphology, mechanica! properties and interfacial bond strengthof high-
performance PE fibres is discussed.
Chapter 6 describes a novel method for the selective introduetion of carboxylic acid groups and the effect on the interfacial bond strength to epoxy re sin. Furthennore, some remarks are made with respect to the effect of oxidative processes on the shear strength of the outer PE surface layers.
In the epilogue, the role of fibre anisotropy and fibre-matrix acthesion on composite performance is commented upon.
10 Chapter 1
1.6 References
1. D. Huil, 'An Introduetion to Composite Materials', Cambridge University Press, Cambridge (1981)
2. I.C. Visconti, Polym. Plast Technol. Eng. 31, 1-59 (1992)
3. Composites, Engineered Materials Handbook-Vol. 1 (Eds. C.A. Dostal and M.S.
Woods), ASM International, USA (1987)
4. J.D. Packer-Tursman, Adv. Comp. 2(2), 26-28 (1994)
5. C. Petersen, Adv. Comp. 2(2), 20-21 (1994)
6. E.P. Plueddemann, 'Silane Coupling Agents', Plenum Press, New York (1982)
7. G. Tesoro and Y. Wu, J. Adhesion Sci. Techno!. 2_, 771-784 (1991)
8. J.B. Donnet and R.P. Chandal, 'Carbon Fibres, International Fiber Science and
Technology Series-Vol. 3' (Ed. M. Lewin), Marcel Dekker Inc., New York (1984)
9. J.D.H. Hughes, Comp. Sci. Technol41, 13-45 (1991)
10. P.J. Lemstra, R. Kirschbaum, T. Ohta and H. Yasuda in 'Developments in Oriented
Polymers-2' (Ed. I.M. Ward), Elsevier, London (1987), p. 39-77
11. H. Jiang, W. W. Adams and R.K. Eby in 'Materials Science and Technology-Vol. 12
Structure and Properties of Polymers' (Ed. E.L. Thomas), VCH, Weinheim (1993),
p.597-652
12. D.J. Vaughan, Polym. Eng. Sci. 18, 167-169 (1979)
13. L.S. Penn, F.A. Bystry and H.J. Marchionni, Polym. Comp. :!:. 26-31 (1983) 14. F.M. Lognllo and Y-T. Wu, United StatesPatent 4,418,164 (1983)
15. C. Hepburn and Y.B. Aziz, Int. J. Adhesion and Adhesives 2_, 153-159 (1985)
16. R. Martin, W. Götz and B. Vollmert, Angew. Makromol. Chem. 133, 121-140{1985)
17. G. Roebroeks and W.H.M. van Dreumel in 'Materials Science Monograhs: 35' (Eds.
K. Brunsch, H-D. Gölden and C-M. Herkert), Elsevier, Amsterdam (1986), p.95-102
18. M. Breznick, J. Banbaji, H. Guttmann and G. Marom, Polym. Comm. 28, 55-56 (1987)
19. R.E. Allred, DSc Thesis, Massachusetts Institute of Technology (1983)
20. L.S. Penn and T.K. Liao, Comp. Technol. Rev. Q, 133-136 (1984} 21. E. Logtenberg and D. Deventer, Unpublished results TNO Delft
22. Y. Wu and G.C. Tesoro, J. Appl. Polym. Sci. 31, 1041-1059 (1986)
23. L.S. Penn, G.C. Tesoro and H.X. Zhou, Polym. Comp. 2. 184-191 (1988) 24. T.J.J.M. Koek and J.J.G. Smits, European Patent 0,006,275 (1982)
25. M. Takayanagi and T. Katayose, J. Polym. Sci., Polym. Chem. Ed. 19, 1133-1145 (1981)
26. M. Takayanagi, T. Kajiyarna and T. Katayose, J. Appl. Polym. Sci. 27, 3903-3917 (1982)
Introduetion 11
27. M. Takayanagi, S. Ueta, W-Y. Lei and K.Koga, Polym. J. 19,467-474 (1987) 28. H. Ishizawa and Y. Hasuda, ACS 59, 362-366 (1988) 29. M. Takayanagi, S. Ueta and Y. Nishihara, Reports on Progress in Polym. Phys. in
Japan 28, 343-346 (1985) 30. D.M. Brewis and D. Briggs, Polymer 22, 7-16 (1981) 31. S. Wu, 'Polymer Interface and Adhesion', Marcel Dekker, New York (1982), p.279 32. J.A Lanauze and D.L. Myers, J. Appl. Polym. Sci. 40, 595-611 (1990)
33. P. Gatenholm, C. Bonneropand E. Wallström, J. Acthesion Sci. Technol. :!, 817-827 (1990)
34. N.H. Ladizesky and I.M. Ward, J. Mater. Sci. 18, 533-544 (1983) 35. H.X. Nguygen, G. Riahi, G. Wood and A. Peursartip in 'Proceedings of 33th
International SAMPE Symposium', Anaheim (1988), p.1721-1729
36. M. Nardin and I.M. Ward, Mater. Sci. Techno!. 38, 814-827 (1987)
37. S.L. Kaplan, P.W. Rose, H.X. Nguygen and H.W. Chang, SAMPE Q. 19(4), 55-59 (1988)
38. M.J.N. Jacobs and H.J.J. Rutten, Eur. Pat. Appl. EP 311197 A2, Dyneerna V.o.f. (1989)
39. S. Holmes and P. Schwartz, Comp. Sci. Technol. 38, 1-21 (1990)
40. R.J.H. Burlet, J.H.H. Raven and P.J. Lernstra, Eur. Pat. Appl. EP 144997 A2, DSM Stamicarbon (1985)
41. M.J.N. Jacobs and H.J.J. Rutten, Eur. Pat. Appl. EP 311198 A2, Dyneerna V.o.f. (1989)
42. A.R. Postema, A.T. Doornkamp, J.G. Meijer and H.D. Vlekkert, Polym. Bull. 1-6 (1986)
12
13
PART A: ARAMlD FIBRES
14
The selective introduetion of .. 15
Chapter 2 The Selective Introduetion of Specific Organic Groups at the Surface of Aramid Fibres: A Model Compound Study
2.1 Introduetion
In 1980, Vekemans and Hoornaert1 reported on a new synthetic route to isoquinolinetriones
starting from benzamides. Basically, benzamides were reacted with oxalyl chloride to yield
N-aroyloxamoyl chlorides2, which were subsequently converted to isoquinolinetriones
(cyclization) by raising the temperature. Of particular interest are the N-aroyloxamoyl
chloride intermediates, which still contain a reactive acid chloride group. If aramids react in
a similar way, the acid chloride group can be used for various derivatizations enabling the
introduetion of specific organic groups. With this consideration in mind, an extensive model
compound study was undertaken to evaluate the feasibility of such an approach, the results
of which are reported herein. The majority of the investigations was conducted on benzanilide
as model compound for aramid, butsome control experiments on higher homologues were
also performed.
2.2 Experimental
2.2.1 Materials
With the exception of diethyl ether and dichloromethane, all materials used were of reagent
grade and were used without further purification. Diethyl ether was dried and stored over
sodium, whereas dichloromethane was distilled and stored over 3 and 4 A molsieves.
16 Chapter 2
2.2.2 Reactions
Key intennediate: N-benzoyl-N-phenyloxamoyl chloride 1 A solution of 7.3 g (57 mmol) oxalyl chloride in 30 mi of carbon tetrachloride was added to 1 g (5 mmol) benzanilide and heated at 40 oe for 1 h. The benzanilide slowly dissolved after which the excess oxalyl chloride was removed by vacuum distillation. This solution was used
for the subsequent reactions described below.
Reaction of 1 with water: N-benzoyl-N-phenyloxamic acid 2 Upon actdition of water, a white solid precipitated. The product was fittered off, dissolved
in dichloromethane, dried over magnesium sulphate and filtered. Evaporating of the solvent
afforded white needle-like crystals which were driedunder vacuum at 40 oe and stored in a desiccator. Yield: 91%. Anal. calc. for e,5H110 4N: e 79.17; H 5.62; N 7.10. Found: C 79.11; H 5.67; N 7.02.
Reaction of 1 with methanol: methyl N-benzoyl-N-phenyloxamate 3 The white precipitate fonned after actdition of methanol was collected, rinsed with methanol,
dried at 40 oe under vacuum and stored in a desiccator. Yield: 87%. Anal. calc. for e,JI130 4N: e 67.84; H 4.63; N 4.94. Found e: 67.78; H 4.64; N 4.86.
Reaction of 1 with glycidol: 2.3-epoxypropyl N-benzoyl-N-phenyloxamate 4 Prior to the actdition of an equimolar amount of glycidol, dissolved in a small amount of
carbon tetrachloride, triethylamine was added to neutralize the hydrochloric acid formed
during the course of the reaction. This may otherwise cause ring opening and polymerization
of the epoxy groups of glycidol. All volatile substances were then removed under reduced
pressure with a rotary evaporator. The remaining residue was taken up in acetone. Piltration
of this suspension foliowed by evaporation of acetone yielded a sticky solid which, after drying, was stored in a desiccator. Yield: 64%. Anal. calc. for e,8H150 5N: e 66.46; H 4.65; N 4.31. Found: e 66.16; H 5.01; N 4.53.
The selective introduetion of .. 17
2.2.3 Characterization Methods
The infrared (IR) spectra were recorded on a Perkin Elmer 297 spectrophotometer applying
either K.Br disks or NaCl mounted liquid cells. 1H-NMR spectra were recorded with a 200 MHz Broker AC-200 spectrometer using
deuterated (D7) dimethylformamide as solvent. The signal of the deuterated methyl groups
was used as internat standard. When solutions in carbon tetrachloride or sulfolane were
measured, deuterated chloroform was added as internat standard and locking agent. The
spectra had a speetral width of 2400 Hz and were generally obtained after accumulating 64
scans. The digital resolution amounted to 0.15 Hz, corresponding toa datalengthof 16K. The 50 MHz 13C-NMR spectra were also measured on the Broker AC-200 spectrometer with a pulse delay of 10 sec.
Thermal gravimetrie analysis (TGA) was carried out on a Du Pont 951 Thermal Gravimetrie Analyzer with a heating rate of 10 °C/min in a nitrogen atmosphere. The
temperature of 1 % weight loss was taken as the onset of decomposition.
2.3 Results and Discussion
2.3.1 Chemical Structure
The first step in the modification procedure is the reaction between benzanilide, used as
model compound for aramid, and oxalylchloride giving 1 (scheme 2.1}. Figure 2.1 shows the IR spectra of the starting compound and the reaction product 1. The stretching vibrations of the N-H group at 3340 cm·1 and of the amict 11 group at 1530 cm·1 present in the spectrum of benzanilide are absent in the spectrum of 1. This points towards N-substitution. Furthermore, two strong absorption bands located at 1830 cm·1 and 1750 cm·1 appear in the
infrared spectrum of 1. These bands were, referring to the Sadtler standard spectra of oxalylchloride and derivates, ascribed to C=O stretching vibrations ofthe O=C-C=O group. The amid I band (C=O) located at 1660 cm·1 in benzanilide is shifted 40 cm·1 to higher field
as a result of this electron-withdrawing N-substitution.
Variabie temperature measurements did notchange the 1H-NMR spectrum of 1 as shown in figure 2.2. This rul es out the possibility that the position of the hydrogen atom of the N-H group (o=10.22 ppm in benzanilide), wbich depends on temperature, concentration and type of solvent, is located underneath the aryl-H bands in the 1H-NMR spectrum of 1. The absence of the N-H peak substantiates tbe IR results.
18 Chapter 2
Substitution with a strongly electron withdrawing group generally results in a deshielding
of neighbouring protons. Contrary hereto a shielding effect is observed for the aromatic
protons in benzanilide (compare fig. 2.2 a and b). This effect is thought to arise from the loss
in coplanarity upon N-substitution3• In a coplanar structure, the carbonyl group exerts a de-
shielding effect". However, in non-coplanar structures a shielding effect ofthe carbonyl group
is observed4• The opposed effect of the carbonyl group in compound 1 and benzanilide dominates over the deshielding effect exerted by the electron withdrawing group and explains
the overallshielding effect observed.
0
o-~-ë-o C=O I C=O I OH
2
Scheme 2.1
0 0 11 11
Cl-C-C-Cl .....
0
o-~-g-o c=o I C=O I Cl
0
o-~-ë-o~ c=o I C=O I OCH3
3
+ HCI
4
The reaction most likely proceeds through the 71"-electron system of the amide group to produce an 0-acylated product, which by intramolecular rearrangement gives the N-acylated
product'. This is interesting in view of the apparent difficulty of a direct chemica} attack at
the amide group of aramid due to the sterical bindrance of the neighbouring phenyl groups lying in the same plane as the amide group5•
The selective introduetion of ..
4000 3000 2000 1800 1600 1400 1200 1000 800 600 cm·1
b
19
Figure 2.1 lnfrared spectra of (a) benzanilide and (b) its reaction product with oxalyl
chloride 1
20 Chapter 2
a
12 10 8 6 4 2 0 PPM
a ,...--, a a 0 a a aQ-~-~-oa
C=O I C=C I Cl
b
12 10 8 6 4 2 0 PPM
Figure 2.2 1H-NMR spectra of (a) benzanilide and (b) its reaction product with oxalyl chloride I (* == solvent peaks)
In the secoud step, the highly reactive acid-chloride group is converted witheither water,
methanol or glycidol to introduce acid, ester and epoxy groups. As expected, no absorption
bands attributable to the N-H group and amid 11 are present in the IR spectra of these
products (fig. 2.3). In addition, all spectra show more than ohe absorption attributed to C=O
stretching vibrations (1650-1850 cm·1). A broad absorption band ranging from 3350 to 2500
cm·1 is seen in the spectrum of 2. Absorptions showing this characteristic are distinctive for
carboxylic acids6• The broadening is thought to be related to internal hydrogen bonding. H-
CH stretching vibrations at 2920 and 2860 cm·1, present in the spectra of 2 and 3, indicate
the presence of alkyl groups. The assignment of the bands in the fingerprint region is
hampered by the large number of bands present and was therefore not tried.
The selective introduetion of ..
4000 2000 1800 1600 1400 1200 1000 800 600 cm-1
Figure 2.3 Infrared spectra of (a) 2, (b) 3 and (c) 4
21
a
b
c
Conclusive evidence for the structure of the reaction products could be derived from 1H-
NMR and 13C-NMR spectroscopy (fig. 2.4 and 2.5). The assignment is basedon the 1H- and 13C-NMR spectra of the starting compounds and on tabulated increments 7 . Of special interest
is the 13C-NMR spectrum of 3 (fig. 2.5) which shows 3 carbonyl resonances, as expected.
22 Chopter 2
a a 0 a a a
ao-~-~-oa ,.......,
C=O I c=o I OHb
b a b
J lJ J 12 10 8 6 2 0
0 PPM
a a a a ao-~-~-oa a ,.......,
C=O I c=o I OCHsb
b
.I. 12 10 8 6 4 2 0
PPM a a 0 a a ao-~-~-oa a
C=O ,.....,..
I c=o I 0 I
cH-CHb I
dH-C, I 0 b c
6H-C/ c
I H,
12 10 8 6 4 2 0 PPM
Figure 2.4 1H-NMR spectra of (a) 2 (b) 3 and (c) 4 (* =solvent peaks)
The selective introduetion of .. 23
de f!!llÎ' k
b c
~~~~~.._;..l\IIIL~lloof"!-ni.._~~~~~~""""""",_,.,.~--~.~ÜU DMF
1 DMF
200 175 150 125 100 75 50 PPM
Figure 2.5 13C-NMR spectrum of 3
All features of the NMR and IR spectra are consistent with the conversion of the remairring
acid chloride group of 1, following well known chemica! reactions, thereby introducing
carboxylic acid, ester and epoxy groups onto benzanilide (scheme 2.1). Additional support
for the structure of these compounds comes from elemental analysis, which shows that the
calculated and found weight percentages C, H and N are within experimental error identical
(see experimental).
2.3.2 Higher Homolognes
To perform the above reactions on aramid fibres, the reaetauts have to be brought into close
proximity of the surface of the fibres. This requires the swelling of the surface by a suitable
solvent, which is a difficult task given the chemica! inertness and high crystallinity of aramid
24 Chapter 2
fibres. Sulfolane is one of the very few solvents capable of swelling aramid fibres. Moreover,
it is chemically inert to oxalyl chloride, which explains the choice for this solvent when
performing the experiments described below.
Benzanilide is the simplest model compound for aramid. To check the validity of the above
reaction sequence for the modification of aramid fibres, we performed some of the reactions
on higher homologues, ha ving more than 1 amide group in para position and hence even more
reminiscent of aramid. For these higher homologues, an oxalylchloride in sulfolane solution
and temperatures of 80 "C were used to carry out the first step of the reaction sequence. The
first step is the most crucial one in the proposed reaction sequence. The conversion of the
remaining acid chloride group in the secoud step follows classica! organic chemistry.
Basically the results were identical to those obtained for benzanilide as model compound.
Figure 2. 6, showing the 1H-NMR spectra of di(1 ,4-methylbenzene)terephthalamide before and after the reaction with oxalylchloride, may serve as an example of this. The absence of the
resonance of the amid proton (ó = 10.06 ppm in di(1 ,4-methylbenzene )terephthalamide) in the spectrum of the reaction product suggests N-substitution similar to the reaction product of
benzanilide and oxalyl chloride.
2.3.3 Thermal Stability
A prime requirement of any modification procedure that aims at improving the adhesion is
that the modification should be able to withstand the processing temperatures of the reinforeed
composites. We therefore investigated the thermal stability of the model compounds 2, 3 and 4 by TGA. The onset of decomposition (temperature of 1% wt loss taken) under a nitrogen atmosphere starts at 140 oe for 3, 143 oe for 4 and 146"C for 2. Initially, the degradation proceeds slowly but increases progressively when heated above 150 "C.
2.3.4 Condusion
In conclusion, this model compound study shows, that the chemical procedure outlined is a
versatile metbod for the selective introduetion of a variety of organic groups onto benzanilide,
used as model compound for aramid, among them carboxylic acid, ester and epoxy groups. Tlie procedure is not limited to these examples. Due to the limited thermal stability, the
applicability is, however confmed to those areas where the processing and/or the use
temperatures will not exceed the 140 "C. Still, this temperature is high enough for the enhancement of the acthesion in the majority of the aramid-fibre-reinforced epoxy and unsaturated polyester composites.
The selective introduetion of . . 25
eb OaaO bc dHsC -o- ~ -~-o-~ -~-o- CHsd He He
a
d
b a c
11 10 9 8 7 6 5 4 3 2 0 PPM
aa OaaO aa H3c-Q- ~ -~-o-~-~-o-CHs C=O C=O I I C=O a C=O I ,------, I Cl Cl
b
11 10 9 8 7 6 5 4 3 2 0 PPM
Figure 2.6 1H-NMR spectra of (a) N,N'-bis(4-methylphenyl)terephthalamide and (b) its
reaction product with oxalyl chloride (* =solvent peaks)
26 Chapter 2
2.4 References
1. J. Vekemans and G. Hoornaert, Tetrabedrou 36, 943-950 (1980) 2. A.I. Speziale and L.R. Smith, J. Org. Chem. 28, 1805-1811 (1963} 3. V.N. Tsvetkov, M.M. Koton, I.N. Shtennikova, P.N. Lavrenko, T.V. peker, O.V.
Okatava, V.B. Novakowski and G.l. Nosova, Polymer Sci. U.S.S.R. 1883-1893 (1980)
4. Private communications J. Vekemans 5. E.G. Chatzi, M.W. Urban, H.lshida and J.L. Koenig, Polymer 27, 1850-1854 (1986) 6. D.H. Williams and I. Fleming, 'Spektroskopische Methoden zur Strukturaufklärung',
George-Thieme Verlag, Stuttgart, 1979, p.40-79 7. Idem, p.80-161
Surface modification of aramid fibres 27
Chapter 3* Surface Modification of Aramid Fibres
3.1 Introduetion
A novel two-step chemica! procedure for the selective introduetion of various functional groups onto the surface of aramid fibres was proposed in the previous chapter1. lts feasibility
was demonstrated using benzanilide and some higher homologues as model compounds for PPTA. In this chapter, the actual surface modification of aramid fibres according to this novel two-step chemica! procedure will be discussed. Attention will focuss on the characterization
of the modified aramid fibres in terms of the effect of the different functional groups on the acthesion to epoxy resin and the effect of the surface modification procedure on the
mechanical properties of the aramid fibres.
3.2 Experimental
3.2.1 Reactions
The sizing of the aramid fibres used throughout this study (Twaron D1000) was removed by Soxhlet extraction in dichloromethane, prior to all experiments. The surface treatment
procedure comprised two successive stages. At first the aramid fibres, loosely wound around a glass cagelike support, were immersed in a hot (50-60 °C) salution of sulpholane/ oxalylchloride (9: 1 vol/vol) for 1 hour. Next, the fibres we re reacted with water, methanol, ethylenediamine and glycidol, respectively. For the reaction with water and
methanol, the oxalylchloride-treated fibres were simply immersed inthereagent soulutions, distilled water or methanol, followed by rinsing with distilled water or methanol. A slightly different procedure was used in the other cases. Before the oxalylchloride-treated fibres were
Reproduced in part from: F.P.M. Mercx and P.J. Lemstra, Polymer Commun. 31, 252-255 (1990)
28 Chapter 3
immersed in ethylenediamine, the fibres were rinsed with dry dichloromethane to remove excess oxalylchloride adhering to the fibre surface, which otherwise would give rise to (homo )polymer formation. Por the same reason, rinsing with dry diethyl ether was performed prior to the immersion of the oxalylchloride-treated fibres in a glycidolldiethyl ether solution. Excess glycidol was removed by subsequent rinsing with diethyl ether. All the surface
modified fibres were dried in vacuo and stored in a desiccator.
3.2.2 X-ray Photoelectron Spectroscopy
X-ray photoelectron spectroscopy (XPS) was performed on a Physical Electronics 550 XPS/ ABS spectrometer equipped with a magnesium X-ray souree and a double pass cylindrical analyser. Spectra were recorded in steps of 0.05 eV. The pressure did not exceed 6.7xl0-6 Pa, and the eperating temperature was approximately 293 K. Operating conditions of the X-ray souree were 10 kV and 40 mA. A sweeptime of 10 min was used for complete speetral scans, while for detailed recordings a sweeptime of 20 min per element was used.
The sample was placed at an angle of 50° to the analyzer, giving a probing depthof about
4 nm for the electrous of the C1, XPS line.
3.2.3 Scanning Electron Microscopy
Scanning electron microscopy (SEM) was performed using a Cambridge Stereoscan 200 microscope, eperating at a voltage of 25 kV. The aramid fibres were coated with a gold/palladium layer approximately 20 nm thick. The gold/palladium coated samples were pressed in silverpaint to ensure good conductivity.
3.2.4 Determination of Adhesion
The effect of the surface treatment, described above, on the fibre-matrix bonding was
measured using a multifilament pull-out testl. Specimen preparation consisted of taking two strands of aramid fibres, which were subsequently twisted by 1 turn/cm and embedded in a disk of epoxy resin (1.5-2 mm thick). A medium-viscosity resin, Ciba Geigy LY 556, together with an amine hardener, Ciba Geigy HT 972, were used througbout this study. The following heating cycle was used to cure the resin: (1) heating from room temperature to 80 oe with 2 °C/min, (2) 2 hours at 80 oe, (3) raising the temperature with 4/3 °C/min to 120 oe, (4) 2 hours at 120 oe and (5) cooling toroom temperature by 6,67 °C/min. After curing
Sulface modification of aramid fibres 29
and prior to testing, the samples were stored in a conditioned room (23 oe, 50% relative humidity). Tests were run on an lnstron tensile testing machine. The epoxy disc was tïxed on a specially designed grip by applying a slight (pre )strain. The crosshead speed was 10 mm/min. To compare the different results, the bundie pull-out shear strength (BPS) was
calculated. The BPS is defined as:
BPS p
n dl
where P is the maximum force measured during pull-out (N), d the fibre bundie diameter (mm) and I the embedded length of the fibre bundie (mm). At least six measurements were
made for each average value of the BPS.
3.2.5 Determination of Mechanical Properties
The aramid fibres used for the determination of the mechanica! properties were twisted by 1 turn/cm. Tensile tests were performed on a Zwick Rel tensile testing machine. Closed loop
operation made accurate constant strain rate experiments possible. The aramid fibres were tested at a strain rate of 10%/min in accordance with ASTM D-76. Initia! cross-sectional areas, used for the calculation of Young's modulus and tensile strengthwere obtained from the mass and the length of the fibres, assuming a crystal density of 1440 kg/m3• The values given are the average of at least six experiments.
3.3 Results and Discussion
3.3.1 Chemical Structnre
Evidence gathered on benzanilide as model compound for PPT A indicate that the following reactions will proceed on the surface of the aramid fibres1, following the experiments described above, see scheme 3.1. The formation of intermediate I (scheme 3.la) is the key-
step in the reaction sequence in that it provides a highly reactive intermediate. In a subsequent reaction step the 1-surface modified fibres are substituted with different functional groups by reaction with water (scheme 3.lb,c), methanol (scheme 3.lc), ethylene diamine (scheme 3.ld) or glycidol (scheme 3.1e). The reaction of intermediate I with water may be foliowed by a decarboxylation, yielding an aldehyde-modified aramid surface (scheme 3 .lc).
30
f~-o-~-~-o-r.l + H HO oJ. n 0 0 11 H
Cl-C-C-Cl _.., t~-0-~J--0--~J c~c=o~ I I c c=o I I Cl Cl (o) n
I
~-0-~J-0-~j and/or C~C:O~ I I c c=o I l
OH OH
(b) n r~-o-~-g 0--~j c c=o~ I I H H
(c) n
t~-0-~J-0-~j c-~c=o~ I I c c:o I I 30 OCH3 (d) n
0 0
~-o-~-H-Q-a o=c c=o
I l O:C C=O
' ' HN NH ' ' ~ ?i>
H.9 ?i> H,N NH,
(e) n
(f)
Scheme 3.1
0 I\
HO-CH,-C-C
Chopter 3
+HCI
Surface modijication of aramid fibres 31
XPS is a highly sensitive technique for surface analysis3•4 . With a sampling depth of 4 nm,
the results presented in tab ie 3. 1 represent surface plus some subsurface materiaL XPS does
not analyze for hydrogen. eonsequently, the atom percentages are computed only on the basis
of the analyzed elements. Tab ie 3.1 shows the surface composition of the aramid fibres and
surface-modified aramid fibres as measured with XPS and expressed as the carbon to nitrogen
to oxygen ratio, together with the calculated values according to reaction scheme 3 .1. The
experimental error depends strongly on the absolute atom percentages of the elements present.
When a particular element constitutes less than 10 atm% of the material the experimental
error in the value given is about 15%. For elements which constitute about 20 and 80 atm%
of the compound, the experimental error in the value given is about 10% and 5%,
respectively. Note that the stoichiometry of the untreated sample points towards an oxidized
surface. Similar findings were reported by Penn and Larsen3 and Allred4 and seems typical
for all commercial aramid fibres. The surface composition of the treated aramid fibres is
within experimental error identical to the calulated theoretica! values according to reaction
scheme 3.1.
Table 3.1 Effect of the various treatments on surface composition of Twaron D 1000 aramid fibres
%C
Treatment meas.
None 77
Oxalylchloride-water 70
Oxalylchloride-methanol 67
Oxalylchloride-ethylenediamine 65
Oxalylchloride-glycidol 70
•calculated according to reaction scheme 3 .1 b bCalculated according to reaction scheme 3 .I c
cal cd.
77.8 64.3./72.7b
66.7
64.7
66.7
%N %0
meas. cal cd. meas. calcd.
8 11.1 15 11.1
5 7.1./9.1b 25 28.6./18.2b
5 6.6 28 26.7
17 17.6 18 17.6
6 5.6 24 27.7
Detailed information about the nature of the incorporated groups can be obtained from high
resolution els• 0 1s and N1s spectra. The els spectra, shown in figure 3.1, are the most
informative. The binding energy of carbon (1s) in hydrocarbons is 285 eV. Introduetion of
oxygen induces a chemica! shift to higher binding energies for those carbon atoms chemically
bonded to oxygen. These shifts relative to els (hydrocarbon) are 1.5 eV for ether/epoxy, 3
eV for carbonyllaldehyde and 4.5 eV for carboxylic acid/ester groups3•4 • The chemica! shift
32 Chapter 3
of carbon in amide groups O=Ç-NH amounts to 3.5 eV3• Note that the observed differences
between the C1, spectra of surface-modified aramid fibres and untreated aramid fibres, when viewed in terms of the introduetion of the afore-mentioned carbon-oxygen groups are
consistent with the reaction schemes outlined above. From line-shape analysis of the C1, spectrum of oxalyl chloride-water-treated aramid fibres and the XPS determined surface composition, it can be concluded that reaction scheme 3.1 b prevails, yielding mainly
carboxylic acid-modified aramid fibres. The evidence presented by XPS thus verifies that the methodology developed is effective for the selective introduetion of carboxylic acid, ester, amine and epoxy groups.
c e
b d
a a
289 285 281 289 285 281 Binding energy f eV J Binding energy leV)
Figure 3.1 High resolution C1s spectra ofTwaron D 1000 aramidftbres: (a) untreated, (b)
oxalylchloride-water treated, (c) oxalylchloride-methanol treated, (d)
oxalylchloride-ethylenediamine treated, (e) oxalylchloride-glycidol treated
Surface modification of aramid fibres 33
3.3.2 Adhesion and Mechanical Properties
The introduced amine, epoxy or carboxylic acid groups may or may not participate in
subsequent co valent bonding with a curing epoxy resin network. Even if this is not the case,
these groups as well as the ester group are capable of forming hydragen bonds with the
hydroxyl groups of the resin network. Table 3.2 shows the effect of the various surface
treatments on the acthesion to epoxy resin. The maximum impravement in acthesion relative
to untreated aramid fibres is 70%. As evidenced by the extensive fibrillation of the epoxy-
modified aramid fibres subjected to the pull-out test, shear failure inside the aramid fibre
occurs, indicating that the acthesion is no longer the limiting factor in these composites.
Similar results with regard to the bundie pull-out test and failure mode were reported by
Elkink et al. 2 for a non-disclosed modification procedure.
Table 3.2 Tensile strength and adhesion to epoxy resin for treated and untreated Twaron
DJOOO aramid fibres
Treatment
None
Oxalylchloride-water
Oxalylchloride-methanol
Oxalylchloride-ethylenediamine
Oxalylchloride-glycidol
"Standard deviation given in parentheses
Tensile strength (GPa)
2.2 (O.l)a
2.2 (0. 1)
2.2 (0. 1)
2.1 (0.1)
2.1 (0.1)
Bundie pull-out shear
strength (MPa)
28.3 (2.1)"
43.0 (1.6)
38.6 (1.8)
38.3 (1.2)
52.2 (2.1)
SEM micrographs show that the fibre surface remains just as smooth after the treatment as
it was before (fig. 3.2). This is consistent with the improved acthesion being caused by the
introduetion of the functional groups mentioned earlier. Of the different groups introduced,
the epoxy groups are by far the most effective. Similar results were obtained by Takayanagi
et aL 7 forT-peel tests performed on untreated, epoxy treated and carboxymethylated aramid
fibres. They also noted that for the epoxy-modified aramid fibres, the skin layer was peeled
during testing, representing the limit of acthesion at which the fibre itself can notendure the
applied force. The amine and carboxylic acid groups, which can also form chemical bonds
with the epoxy resin, give smaller improvements in adhesion. In fact, the results are roughly
camparabie to the results obtained for the aramid fibers modified withester groups. These
34 Chapter 3
last groups are only capable of hydrogen bonding. This might suggest that the amine and
carboxylic acid groups do not form covalent boncts with the epoxy resin. The rather low
increase in acthesion following the introduetion of amine groups is rather surprising given the
excellent results that were previously reported for amine-modified aramid fibres6·8 This could point towards (partial) internal cyclization of the amine group with the carbonyl groups,
sim i lar to the cyclized structures found in y-aminopropyl silanes when coated onto glass fibres
from solutions of pH= 1 and 79, as aresult of which the amine groups are not available for
chemica! reaction with the curing epoxy resin.
The tensile strength of the aramid fibres is not affected (table 3.2), suggesting that the
procedure is limited to the outer surface layers .
Figure 3.2 Typical examples of scanning electron micrographs of (a) untreated and (b)
treated aramid fibres
In conclusion, pull-out tests showed that this newly developed surface treatment procedure
markediJ improves the acthesion to epoxy resins. Moreover, the improved acthesion is not
achieved at the expense of a decrease in tensile strength of the aramid fibers.
Surface modification of aramld fibres 35
3.4 References
1. Chapter 2
2. F. Elkink and J.H.M. Quaijtaal in 'Integration of Fundamental Polymer Science and Technology-3' (Eds. L.A. Kieintjens and P.J. Lemstra), Elsevier Applied Science Publishers, London (1989), p.228-234
3. C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder in 'Handbook of X-ray
Photoelectron Spectroscopy' (Ed. G.E. Muilenberg, G.E.), Perkin-Elmer, U.S.A. (1979)
4. D. Briggs in 'Practical Surface Analysis' (Eds. D. Briggs and M.P. Seah), Wiley, Chichester (1983), p.359
5. L. Penn and F. Larsen, J. Appl. Pol. Sci. 23, 59-73 (1979) 6. R.E. Allred, 'Surface Chemica! Modifications of Polyaramid Filaments with Amine
Plasmas', DSc Thesis, Massachusetts Institute of Technology (1983) 7. M. Takayanagi, S. Ueta, W-Y. Lei and K. Koga, Polym. J. 19, 467-474 (1987)
8. Y. Wu and G.C. Tesoro, J. Appl. Polym. Sci. 31, 1041-1059 (1986) 9. D.Wang and P.R. Jones, J. Mater. Sci. 28, 2481-2488 (1993)
36
37
PART B: POLYETHYLENE FIBRES
38
Oxidative acid 39
Chapter 4* Oxidative Acid Etching
4.1 Introduetion
Pretreatments are generally necessary to enable a polyethylene to be bonded, coated or printed upon. Oxidative acid etching is one of the most widely used commercial treatments and causes chemica! and physical changes in a thin surface layer. Hydroxyl, carbonyl, carboxylic acid and sulphonic acid groups are found at the surfaces of chromic acid1•4 ,
permanganate acid5 or potassiumchlorate acid5 treated polyethylene. The formation of
carbonyl and carboxylic acid groups increases wîth increasîng oxidative power of the acid
solutîon used and with the time of exposure2·3•5 and is accompanîed by chain scission1·5 • Eventually, chain scission will lead to the formation of small fragments that will go into
solution. As the rate of oxidation is much faster for the amorphous than for the crystalline regions, oxidative acid etching preferentially removes the amorphous regions and increases
the surface roughness1•2. There has been a lively discussion in the literature on the importance of the introduetion
of polar groups, surface roughening and the increased wettability that results from these
factors in improving the adhesion of oxidative acid treated PE4• Although weak boundary
layers, that may result from impurities or low molecular weight material, have often been mentioned as the major cause for the difficulty in bonding PE, evidence gathered in recent years clearly shows that this is not the casé6•
There is a great difference in the surface morphology of the polyethylenes used in the studies mentioned above, which were mostly isotropie ftlms or films of low draw ratio, with the high-strength, high-modulus PE structures produced by gel-spînning. These differences are for instanee retlected in the extremely high crystallinity of the gel-spun PE structures
exceeding 90%, compared to 20-60% for conventional LDPE-HDPE. Hence the question arises whether the above mentioned treatments are also effective in improving the adhesion
Reproduced in part from: F.P.M. Mercx, A. Benzina, A.D. van Langeveld and P.J. Lemstra, J. Mater. Sci. 753-759 (1993)
40 Chapter 4
of gel-spun PE structures.
Ladizesky and W ard7 were the first to investigate the effect of chromic acid treatment on
the adhesion of ultra-drawn PE structures to epoxy resin. Although the acthesion was
markedly improved, the effect of acid treatment was less than that of oxygen plasma
treatment. Surface roughening of the PE fibres following chlorosulphonic acid treatment was
reported by Postema et al. 8 resulting in a five-fold increase in the acthesion to gypsum plaster.
Recently, Hsieh et al. 9 attempt to improve the adhesion of gel-spun PE fibres to epoxy resin
by pretteatment of the fibres with chromic acid and chromic trioxide solution. The wettability
and the interfacial adhesion to the epoxy resin were both improved.
The purpose of the present study is to explore whether oxidative acid treatment can
improve the acthesion of gel-spun PE structures to epoxy resin and to relate this to the
changes in surface chemical composition and surface topography.
4.2 Experimental
4.2.1 Preparation of PE Tapes
Oriented PE tapes were employed in this study as they offer better signal to noise ratios in
the spectroscopie techniques used compared to fibres. The tapes were obtained by ultra-
drawing cast films as described previously10, except that decatin was replaced by xylene in
the prepatation procedure. The cast films were drawnon hotshoes (T=125 °C) to À=60.
The PE used was Hostalen Gur 412 with a weight average molar mass (Mw) of about 1.5xl03
kg/rooie. Stabilizer and remaining xylene were removed by subsequentextraction with hexane
(15 hr) and methanol (5 hr). The tapes prepared possessed a Young's modulus of 140 GPa,
and a tensile strength of 2.4 GPa at room temperature (measured at a strain rate of 10
%/min). It should be noted bere, that the tapes obtained by this batchwise process are
identical to those obtained by gel-spinning, precluding that the concentration of the PE
solution and the draw ratio are the same.
Oxidative acid etching 41
4.2.2 Acid Treatment
PE tapes were irnmersed in chlorosulphonic acid, chromic acid i.e., K2er20iH20/H2S04 (7:12:150 by weight) or KMn0iH20/H2S04 (1:12:150 by weight) at room temperature for different exposure times. The chlorosulphonic acid-treated tapes were rinsed with
concentrated sulphuric acid, whereas the KMnOiH20/H2S04-treated tapes were rinsed with concentrated Hel. Next, all tapes were rinsed with distilled water. Finally, the PE tapes were rinsed with acetone, dried and stored in a desiccator.
4.2.3 Determination of Adhesion
Pull-out tests were performed on specimens as illustrated in figure 4.1. A medium-viscosity resin, Europox 730, tagether with an aliphatic amine hardener XE-278 (both obtained from Schering) in the ratio 100115 wt/wt were used throughout this study. The resin was cured for 1 h at room temperature foliowed by heating to 80 oe at a rate of 2 oe/min and kept at this
temperature for 1.5 h. After curing, and prior to testing, the samples were stored in a conditioned chamber (23 oe, 50% RH). Tests were run on an Instron tensile testing machine
using specially designed grips. The crosshead speed was 10 mm/min. The adhesion was
defined as the failure load divided by the interface area. At least 6 measurements were made for each average value of the adhesion strength.
PE- tape
010 mm
Resin cylinders
Figure 4.1 Pull-out specimen
42 Chapter 4
4.2.4 Determination of Mechanical Properties
Tensile tests were perfonned on a Zwick Rel tensile machine. Closed loop operation made accurate constant strain-rate experiments possible. The PE tapes were tested at a strain rate of 10%/min in accordance with ASTM D-76. Initial cross-sectionat areas, used for calculating Young's modulus and tensile strength, were obtained from the mass and the length of the tapes assuming a crystal density of 103 kg/cm3• The values given are the average of at least
6 experiments.
4.2.5 X-ray Photoelectron Spectroscopy
See § 3.2.2.
4.2.6 lnfrared Spectroscopy
Fourier transfonn reflection-infrared spectra were obtained using either a Perkin-Elmer 1750 equipped with a 1 GE-TRG attenuated total reflection (ATR) unit or a Nicolet 20 SXB equipped with a Specac ATR unit. A germanium crystal (45° face angle) was used at a
nominal angle of incidence of 45°. Under these conditions the penetratien depth was about
400 nm at a wave length of 10 p.m.
4.2. 7 Scanning Electron Microscopy
Scanning electron micrographs (SEM) were taken with a Camscan 4-DV. A voltage of 20 kV was used, while the tapes were pressed in silver paint to ensure a good conductivity. The samples were first coated with carbon using an Emscope TB-500 Carbonstring coater. Secondly a gold/palladium (80/20 wtlwt) coating was applied in a Polaron E-5000 diode
sputtercoater. The coating thus applied had a total thickness of about 50 nm.
Oxidative acid etching 43
4.3 Results
4.3.1 Adhesion versus Mechanica! Properties
The etiect of exposure time to acids on the acthesion and tensite strength is shown in figures
4.2 and 4.3 and table 4.1. The time of exposure had no influence on the Young's modulus
of 140 GPa. Chlorosulphonic acid and chromic acid only slightly affects the tensile strength
of PE even after prolonged exposure. Postema et al. 8 reported a greater deercase in tensile
strengthafter exposure of gel-spun PE fibres to chlorosulphonic acid. This difference can be
explained by the more severe conditions, i.e. higher temperatures, used in these studies. The system K.Mn04/H20/H2S04 had a marked influence on the tensile strengthof the PE tapes.
Table 4.1 Adhesion, tensile strength and sulface composition of acid-etched oriented PE tapes
Treatment Time Pull-out Tensile Surface composition 0/C
(min) strength strength atomie% atomie
(MPa) (GPa) ratio c 0 s (xl02)
None 0.31 2.42 97.5 2.5 2.6
Chlorosulphonic 0.5 0.45 95.0 4.6 0.4 4.8 acid 0.54 92.9 6.8 0.3 7.3
5 0.65 93.2 6.4 0.4 6.9 30 1.00 2.33 90.3 8.7 1.0 9.6
240 1.07 1.98 91.3 7.4 1.3 8.1
Chromic acid 0.5 1.02 87.7 11.5 0.8 13.1 I 1.16 87.3 12.2 0.5 14.0
5 1.46 83.6 15.3 1.1 18.3 30 1.73 2.21 88.6 10.6 0.8 12.0
240 1.70 1.91 90.9 8.3 0.8 9.1
KMnO.fHp!H2S04 0.5 1.33 86.1 13.9 16.1 1.84 87.7 12.0 0.3 13.7
5 1.86 89.0 10.9 0.1 12.2 10 1.72
30 1.90 1.32
240 Tape brok en
44 Chapter 4
25 0 ,... .. CiS 20 0. è - 15 .c C)
~v-.-------x-------------.a Broken c
b
c: e -(/) 10 -:I a 0 •
"3 5 0.
0 0 10 20 30 40 240 250
Time of treatment (min)
Figure 4. 2 Pull-out strength as a function of treatment time for (a) chlorosulphonic acid, (b) chromic acid and (c) KMn0/H201H2S04
2.50
2.25 CiS 0. SZ 2.00 D a .c 4. b Ë> e 1.75 '!ij
.m 1.50 ïn
c: t! x c
1.25
1.00 0 50 100 150 200 250
Time of treatment (min)
Figure 4.3 Tensile strength of the PE tapes as a function of treatment time for: (a) chlorosulphonic acid, (b) chromic acid and (c) KMnO/HPIH~04
Oxidative acid 45
Consequently, tensile failure rather than pull-out occurred for the PE sample exposed to KMn04/H20/H2S04 for 240 min (fig. 4.2, table 4.1). Figure 4.2 also shows that the leveHing off acthesion value increases in the order chlorosulphonic acid, chromic acid,
KMn04/H20/H2S04, i.e., with the oxidation power ofthe acids applied5
•11
• The opposite order
is found for the time needed to reach this levelling off value.
4.3.2 Scanning Electron Microscopy
The surface of an untreated PE tape, as shown in figure 4.4a, is rather smooth except for the typical microfibrillar structure caused by the hot-drawing process. No change in surface
roughness was observed up to 10 min exposure to KMn0iH201H2S04 (fig. 4.4d). Upon
further exposure a distinct texture developed, the result of degradation and dissolution of material (fig. 4.4e). Prolonged exposure (240 min) is accompanied by an extensive loss of material producing a highly irregular surface (fig. 4.41). In contrast, no evidence of an
increase in surface roughness was found after prolonged exposure to chlorosulphonic or chromic acid (fig. 4.4b, 4.4c).
The regionsof the PE tapes embedded in the epoxy resin and subjected to the pull-out test
were also exarnined. Apparently, there was no difference in the appearance of the chlorosulphonic and chromic acid treated PE surfaces before and after pull-out, suggesting that failure occurred at the interface. A typical example of the groove in the epoxy matrix,
left after pull-out of untreated, chromic or chlorosulphonic acid-treated PE tapes is seen in figure 4.5a. Note that even the typical microfibrillar structure present at the surfaces of these tapes are faithfully replicated in the matrix. It thus appears that even untreated PE tapes are completely wetted by the liquid resin. In the case of KMn04/H20/H2S04 (5 min)-treated PE the situation is quite different. Localized spots of drawn material are visible at the surface of the pulled tapes as well as on the surface of the groove left after pull-out (fig. 4.5b), suggesting the forcible removal of the top surface layer during pull-out. Further evidence for this is obtained by treating the groove left after pull-out of a KMn04/H20/H2S04 (5 min)-treated PE tape with hot (120 °C) xylene, a known solvent for PE (fig. 4.5c). Clearly the adhering PE has dissolved, leaving a surface that shows a quite good resemblance with the original KMn04/H20/H2S04 (5 min)-treated PE surface.
46 Chapter 4
Figure 4.4 Scanning electron micrographs of acid-etched PE tapes: (a) untreated; (b)
chlorosulphonic acid, 240 min; (c) chromic acid, 240 min; (d) KMnO/HPI
H2S04 , JO min; (e) KMn0/HPIH2S04 , 30 min; (f) KMnO/HPIH2S04 , 240 min
Oxidative acid erehing 47
Figure 4.5 Typical examples of scanning electron micrographs of the grooves left ajter
pull-out of (a) untreated, chromic or chlorosulphonic acid-treated PE tapes, (b)
KMnO/ HPIH2S04 (5 min)-treated PE tapes and (c) as (b) followed by treatment
with hot (120 °C} xylene
4.3.3 Weight Loss
No weight toss was detectable for chromic or chlorosulphonic acid-treated PE samples, not
even after prolonged exposure (18 h) . Prolonged exposure to KMn04/H20 /H2S04, however,
resulted in the partial degradation and dissalution of the PE tape (up to 67 % weight toss) .
4.3.4 lnfrared Spectroscopy
The ref1ection infrared spectra of treated and untreated PE tapes were identical in all cases,
i.e . no oxidation products could be detected. Contrary to reflection infrared spectroscopy,
48 Chapter 4
XPS showed oxidation to have taken place (see § 4.3.5). The sensitivity of surface analysis
is far better for XPS with shallow penetradon ( 4 nm) than for reflection infrared
spectroscopy, because of its deep penetradon (400 nm). The fact that reflection infrared
spectroscopy failed to detect any chemical changes, indicates that the etching/oxidation is
conf'med to the outermost surface layers.
4.3.5. X-ray Photoelectron Spectroscopy
The amount of carbon, oxygen and sulphur as detected by XPS is shown in table 4.1. In the
cases of chlorosulphonic acid treatment, traces of chlorine ( < < 1 %) were found. The amount of oxygen incorporated at the surface initially increases with time of treatment. It
seems that the gradient of this initial increase is proportional to the oxidadon power of the
acids applied. For longer treatment times the oxygen content reaches a maximum, after which
it slowly levels off in all cases.
The surface chemica! composition of three differently treated PE samples prior to, and after
pull-out were determined. The samples investigated were chosen in such a way that a stepwise
increase in adhesion level was obtained. No traces of nitrogen were detected, indicating that
epoxy, i.e. amine hardener, was not present at the surface of the pulled samples.
Consequently, matrix failure during pull-out can be ruled out. The 0 1.:C1• peak intensity
ratios forthese samples are displayed in table 4.2. Comparison ofthe two columns shows that
the degree of oxidation before and after pull-out is within the experimental error (12%), for
both the chlorosulphonic and chromic acid-treated samples. This suggests that failure occurs
at the interface. The KMn04/H20/H2S04-treated sample, on the other hand, showed a significant decrease in oxygen content. This is attributed to faiture inside the PE tape,
removing the highly oxidized surface layer, in agreement with the results obtained by SEM.
Table 4.2 The amount of oxygen relative to carbon present at the surface of the etched PE tapes before and ajter pull-out
Treatment Time Pull-out strength 0/C atomie ratio (xl02)
(min) (MPa) before pull-out after pull-out
Chlorosulphonic acid 30 1.00 9.6 8.5 Chromic acid 5 1.46 18.3 16.2 KMn04/H20/H2S04 5 1.86 12.2 5.8
Oxidative acid etching 49
The binding energy of carbon (ls) in hydrocarbons is 285 eV. Introduetion of oxygen
induces a chemical shift, only for those carbon atoms chemically bonded to oxygen, to higher
binding energies. These shifts are 1.5 eV for hydroxyl, 3.0 eV for carbonyl and 4-4.5 eV for carboxylic acid groups12.1 3 • The preserree of sulphonic acid (S03H) groups was concluded
from the position of the S2P peak and tabulated data3
•12
•13
. The e1s spectra of several treated PE tapes as well as untreated PE tape are shown in figure 4.6. Note the tailing of the e1s peak on the high-energy side of the treated samples. In almost all cases shown this tail
extends up to 5 eV, indicating the presence of hydroxyl, carbonyl and carboxylic acid groups.
The time of exposure had no influence on the general shape of the els spectra of chromic acid
and KMn04/H20/H2S04-treated samples. Hence, it is concluded, that the same functional
groups are present in all samples. This does not imply that the number of individual
functional groups present are not subject to change. Differences were encountered when the
els spectra of PE tapes exposed to chlorosulphonic acid for less than 30 minutes were
examined; in this case the tailing is limited to 3.5 eV, indicating that carboxylic acid groups
are not present.
c e
b d
a a
289 285 281 289 285 281 Binding energy (eV) Binding energy (eV)
Figure 4.6 High resolution C1s spectra of acid-etched PE-tapes: (a) untreated; (b)
chlorosulphonic acid, 5 min; (c) chlorosulphonic acid, 30 min; (d) chromic acid,
5 min and (e) KMn0/HPIH2S04 , 5 min
50 Chapter 4
4.4 Discussion
As can be inferred from table 4.1 and tigure 4.2, adhesion of PE to epoxy resin is greatly
enhanced by pretteatment of PE with oxidizing acids. The maximum increase in adhesion,
as determined by pull-out, is 600% for KMnOiH20/H2S04 treatment. Chlorosulphonic and chromic acid treatment improves the adhesion by 300% and 550%, respectively. Of course, these values are only valid for the reaction conditions used. It is interesting to note that this
improverneut in adhesion can be achieved without a severe loss in tensile strength and
modulus. This is illustrated by table 4.3, which gives the etching time necessary to reach
maximum adhesion as well as the corresponding tensile strength of the PE tapes. The
remaining tensile strength was, regardless of type of treatment, 2.2-2.3 GPa, a drop of 10% or less compared to the initial value of 2.4 GPa. Prolonged exposure resulted in a further loss
in tensile strength without further improverneut in adhesion. It is therefore peculiar that
Silverstein in bis studies on the wetting14 , adhesion15 and failnre16 of etched PE fibres uses a
4 hour chromic acid etch, well beyond the optimum conditions with respect to acthesion and
mechanica! properties.
Table 4.3 The etching time required to reach maximum adhesion (fig. 4.2) and the corresponding tensile strength (fig. 4.3) of the PE tapes
Treatment
Chlorosulphonic acid
Chromic acid KMnO.fH20/H2S04
Time (min)
30
30
1
Tensile strength (GPa)
2.3 2.2 2.2
The etching of polyolefins and model compounds by chromic acid is well
documented1•4•17•18• According to the literature, hydroxyl groups are the first species formed,
and further oxidation causes chain scission to give carbonyUaldehyde or carboxylic acid
groups. It is likely that the other two acids follow the same scheme, although no
comprehensive information is available. The presence of carbonyl and carboxylic acid groups,
at the surface of the treated tapes, as detected by XPS, indicates that chain scission bas taken
place. Consequently, these broken chain ends act as flaws, initiating failure of the PE tape.
This explains the observed decrease in tensile strength of the PE tapes upon exposure to
chlorosulphonic acid, chromic acid, KMn04/H20/H2S04•
Oxidative acid 51
The improved acthesion of polyolefins to epoxy resin after acid etching can, in general, be
related to: 1) surface roughening; 2) an increase in the surface free energy, and consequently the wettability of the
surface is improved and the interfacial energy increases;
3) the introduetion of specific functional groups, giving rise to an increase in the
physico-chemical interactions at the interface;
or a combination of these2·6·19 . However, it should be noted here that these factors are
mutually dependent. This makes it difficult to distinguish the influence of one specific factor
from the others.
SEM observations showed that chlorosulphonic and chromic acid treatment do notproduce
significant changes in surface topography. Consequently, surface roughening can be ruled out
as a reason for the improved adhesion. A somewhat different situation is encountered for the
KMn04/H20/H2S04-treated tapes. The surface of a 1 or 5 min etched tape is quite smooth and
comparable to an untreated tape, whereas a 30 or 240 min treated tape is visibly etched.
These differences are not reflected in the acthesion values which are equal within experimental
error (fig. 4.2, table 4.1). Examinatien by SEM and XPS of the 5 min KMn04/H20/H2S04-treated tapes after pull-out, revealed that shear failure occurred inside the PE tape and not at
the interface. Consequently, the increase in acthesion is not brought about by surface
roughening. All the PE surfaces, treated as well as untreated, were completely wetted by the
epoxy resin as shown by SEM examinatien of the groove left after pull-out. Hence,
differences in wetting as an explanation can be ruled out too. The increase in acthesion is
solely caused by the introduetion of functional groups. XPS showed these groups to be
hydroxyl, carbonyl, carboxylic acid and sulphonic acid. They are the result from oxidation
of the PE by the various acid treatments. As a first approximation we tried to relate the
improved acthesion to the amount of oxygen, relative to carbon, i.e. the 0/C ratio. Figure 4. 7
shows, that a linear correlation exists during the initia! stages of oxidation. No such
correlation is observed when the overall 0/C acthesion data are plotted in the same figure.
This is not surprising. With increasing oxidation the type and number of functional groups
are subjected to changes. Furthermore, the various groups differ in their efficiency to
improve the adhesion to epoxy resins19 • Consequently, an exact knowledge of the type and
number of the different groups present at the surface is required to re late the differences in
adhesion to time or type of treatment, rather than the amount of oxygen introduced.
52
20
0 ..... . CiS 15 a.. è E Cl c: 10 ! s 0
5 "S a.. 0
0 0
a
5
[J
a
C A
10
0/C * 100
A
Chapter4
A
x
15 20
Figure 4.7 Pull-out strength as a function of the 0/C ratio; data taken from table I. o control; D chlorosulphonic acid, 0-30 min; "' chromic acid, 0-5 min; x KMn04 IHPIH~04 , 0-0.5 min
In conclusion, SEM and XPS studies showed that the improved acthesion to epoxy resin after pretteatment with chlorosulphonic acid, chromic acid or KMnOiH20/H2S04 is brought about by the introduetion of functional groups. Furthermore, at the highest level of acthesion obtained (1.8-1.9 MPa), the limiting factor is no longer the adhesion, but the rather low shear strengthof the (treated) PB-tapes.
4.6 Relerences
1. S. Wu, 'Polymer Interface and Acthesion', Marcel Dekker, New York (1982), p.279-336
2. P. Blais, D.J. Carlsson, G.W. Csullog and D.M. Wiles, J. Colloid and Interface Sci. 47' 636-649 (1974)
3. D. Briggs, D.M. Brewis and M.B. Konieczo, J. Mater Sci. ll , 1270-1277 (1976) 4. D.M. Brewis and D. Briggs, Polymer 22, 7-16 (1981) 5. C-G. Gölander, PhD-thesis, The Royal Institute of Technology, Stockholm (1986) 6. D.M. Brewis, Int. J. Acthesion and Acthesives 13, 251-256 (1993} 7. N.H. Ladizesky and I.M. Ward, J. Mater. Sci. 18, 533-544 (1983)
Oxidative acid etching 53
8. A.R. Postema, A.T. Doornkamp, J.G. Meijer and H. v.d. Vlekkert, Polymer Bulletin
16, 1-6 (1986) 9. Y-L. Hsieh, S. Xu and M. Hartzell, J. Acthesion Sci. Technol. ~, 1023-1039 (1991) 10. P.J. Lemstra, N.A.J.M. van Aerle and C.W. Bastiaansen, Polym J. 19, 85-98 (1987)
11. Handhook of Chemistry and Physics, 66th ed. (Ed. R.C. Weast), CRC Press, Boca
Raton (1985), p.Dl51-D158 12. D. Briggs in 'Practical Surface Analysis' (Eds. D. Briggs and M.P. Seah), John Wiley,
Chichester (1983), p.359-396
13. C.D. Wagner, W.M. Riggs, L.E. Davis and J.F. Moulder in 'Handbook of X-Ray Photoelectron Spectroscopy' (Ed. G.E. Muilenberg), Perkin Elmer, USA (1979)
14. M.S. Silverstein and 0. Breuer, Polymer 34, 3421-3427 (1993) 15. M.S. Silverstein and 0. Breuer, J. Mater. Sci. 28, 4718-4724 (1993) 16. M.S. Silverstein and 0. Breuer, J. Mater. Sci. 28, 4153-4158 (1993)
17. F. Holloway, M. Cohen and F.H. Westheimer, J. Am. Chem. Soc. 73 (1951) 65 18. K.B. Wiberg and R. Eisenthal, Tetrahedron 20 (1964) 1151
19. A. Chew, D.M. Brewis, D. Briggs and R.H. Dahm in 'Adhesion 8.' (Ed. K.W. Allen), Elsevier, London (1984), p.97-114
54
Air- and ammonia-plasma treatment 55
Chapter 5 Air- and Ammonia-Plasma Treatment
5.1 Introduetion
Plasma treatment is the most widely used technique both commercially and scientifically to improve the adhesion of high-modulus PE structures. In a plasma process, gas molecules are dissociated into ions, electrons, free radicals and neutral species. The interaction of these
species with the surface of the PE causes chemica! and/or physical changes in a thin surface layer (1-100 nm). The type of plasma employed depends toa large extent on the chemistry of the resin used. For PE-reinforced epoxy and polyester composites mainly air-1•3 , oxygen-4• 10, and ammonia-plasma9-13 treatments are used to increase the level of adhesion.
Air or oxygen plasma contains a mixture of active oxygen species, mainly atomie oxygen14 , and leads to oxidation of the PE. As a result, a variety of functional groups are introduced onto the surface, including hydroxyl, carbonyl, ester and carboxylic acid groups1·3•7•9 . Surface roughening of the PE fibres, after air- or oxygen-plasma treatment, was noted in some of the
investigations1.4-8. Recently, Tissington et al. 7 reported the formation of a crosslinked skin, which was associated with the intense UV radiation of the oxygen plasma at higher input
powers. Treatment of the high-modulus PE fibres with air or oxygen plasma markedly improved the acthesion to epoxy and polyester resins and resulted in a change in faiture mode from interface controlled to internat shear within