HMPSA TAPES AND LABELSTOCKS WITH IMPROVED DIE-CUTTABILITY

Jacques Lechat, ExxonMobil Chemical, Machelen, Belgium Hot Melt Pressure Sensitive Adhesives (HMPSA) are appreciated in the Hot melt Pressure Sensitive Adhesive (HMPSA) market for their aggressive tack and resistance under humid conditions, as well as their adhesion on "spoiled" surfaces. They also offer high adhesive performance over a broad range of temperature and provide high adhesion to "difficult" substrates such as rubbery and polyolefinic materials. Labelstock manufacturing includes the coating of a release liner, such as a silicone coated paper and laminating, with a facestock to produce a useful labelstock. LABEL DIE-CUTTING The converting operations include the printing and die-cutting, as well as stripping of the matrix, also called the skeleton to leave the useable label on a release liner. Poor die cutting might have different origins (Figure 1): - Poor cutting at the cutting stage under the knife (worn tool) - Adhesive recovery between cutting and stripping - Peeling force at the stripping phase higher than the paper tear strength

FIGURE 1

~ 100 cm

1) CUTTING STAGEadhesive is not

well cut

2) BETWEEN CUTTING AND STRIPPINGadhesive rejoins over the cut

3) STRIPPING STAGEpeeling forces > paper tear strength

label

adhesive

release paper

RHEOLOGYRHEOLOGYK. Mc Elrath,M. Fun Tse

COMPUTER NUMERICAL SIMULATION OF DIE CUTTING PROCESS To identify what occurs at the cutting stage and study the phenomena involved, we performed a numerical computer simulation of the die-cutting process and looked, using finite element calculations, at the stresses involved in the face paper, the adhesive layer and siliconized backing. Some assumptions were made to limit the calculation times (Figure 2): - the cutting tool/cutting cylinder was considered completely rigid - the y and z axis was considered as symmetrical axis - R>>> Δ (= 80 microns) ===> α very small thus Vx = V Cutting tool has a vertical motion Vs moving paper

Counter/anvil cylinder considered as a flat surface Face paper: orthotropic behavior (Ex= 6.25 GPa>>>Ey=Ez=2.8 GPa)

FIGURE 2

To simulate the experimental data of the adhesive viscoelastic behavior, we used for the numerical simulations, the Maxwell model, a known rheology model available from literature. Figure 3 indicates that the model is matching the experimental data.

FIGURE 3

Face paper

Release paper

Adhesive

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E-05 1.E-03 1.E-01 1.E+01 1.E+03 1.E+05 1.E+07 1.E+09

frequency (rad/s)

D Ñ Experimental data Model

20 μ m

v x

v y v

α

α 80 μ m 60 μ m

R

Δ

R

60°

R = 6 µm new tool R = 50 µm used tool

Face paper

Release paper

Adhesive

Results of computer numerical simulation: The bar at top of the face paper represents the knife For each of the nets of the finite element network, the stresses below the knife (while it was indenting the face paper) were calculated. The higher the stress, the color turns from dark black to grey, to light grey and to white.

FIGURE 4

It can be noted that (Figure 5): - The paper is much more rigid than the adhesive - The highest stresses build up apart from the knife, and in the more rigid paper - The adhesive is pushed aside by the face paper, e.g. to have a good convertability, the adhesive has to creep away from the cut line by viscous flow between the face paper and the release; in other words, the adhesive should have a low elastic modulus.

Release paper (60µm)

Adhesive (20 µm)

Face paper (80 µm)

Knife

FIGURE 5

RELATIONSHIP BETWEEN POLYMER RHEOLOGY AND MOLECULAR STRUCTURE From these preliminary observations, we established the relationship between the polymer rheology and its molecular structure. Dynamic Mechanical Analysis (DMA) was used to determine the storage modulus G' and the loss modulus G" in function of frequency at 20° C. HMPSA for labels are usually based on about 30% of SBC block copolymers, which act as the backbone of the adhesive, together with tackifying resins (of hydrocarbon or natural origin) which provide the tack properties, and potentially some plasticizing oils, as well as the required antioxidants. Typical SBC block copolymers are of the Styrene-Isoprene-Styrene (S-I-S) type and could be pure triblock S-I-S or blends of triblock with Styrene-Isoprene diblock. It can be noticed that (Figure 6): - For an acrylic, G' and G" have similar moduli and are continuously decreasing with frequency - HMPSA behaves like acrylics in the glassy (high frequency) and glass transition regions, while they present a plateau at very low frequency (< 10-3 rad/sec).

Release paper (60µm)

Adhesive (20 µm)

Face paper (80 µm)

FIGURE 6 - For HMPSA based on SIS - SI block copolymers (Figure 7):

• the diblock content does not affect, the storage and loss moduli in the glassy and glass transition regions

• When increasing the percentage of diblock in the SIS-SI copolymer, the storage modulus plateau at low frequency decreases, e.g.; the higher the diblock content the lower the permanent elasticity, and the rheological behavior of SIS-SI becomes closer to those of acrylics.

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09 1.00E+11 1.00E+13

frequency (rad.s-1)

G' G

" (P

a)

G' HMPSAG" HMPSAG' acrylicG"acrylic

HMPSA formulation

%blockcopolymer = 31%Escorez 1310 = 42%Liquid hydrocarbon resin =27

Acrylic

HMPSA

FIGURE 7 To develop new polymers, one needs to understand what is the physical origin of permanent elasticity in SI and SIS-SI systems. SIS triblock is composed of a polyisoprene block trapped between two polystyrene sequences. The styrene block is in its glassy state at room temperature, while the isoprene has a rubbery behavior, in other word the SIS is composed of rigid PS domains dispersed in a rubbery and dense network of isoprene entangled molecules (Figure 8).

FIGURE 8 One can draw an analogy between the rheological behavior of such styrene "fillers" dispersed in a rubbery phase, and the rheology of polymers in solution.

1.E+02

1.E+04

1.E+06

1.E+08

1.E+10

1.E-05 1.E-02 1.E+01 1.E+04 1.E+07 1.E+10 1.E+13

aT ω (Rad/s)

G'( ω

) and

G''(

ω)

(Pa)

0 % diblock19 % diblock42 % diblock54 % diblock

diblocks reduce permanent elasticity

diblocks do not not affect glass transition and glassy regions

S

S

I

According to this theory, the storage modulus of SIS block copolymers would be described as:

f: Volume fraction of styrene in the dispersed phase r = Density of [SIS] molecule Me (molecular weight of entanglement)~ 4500 g/mole R = universal gas constant For SIS-SI triblock-diblock blends, the SI diblocks are tethered e.g. the polyisoprene part of the molecule is attached on one side to the polystyrene block while the other end is free to explore new configurations (Figure 9).

FIGURE 9

This has two consequences: • The SI diblock does not contribute to the permanent elasticity of the SIS-SI polymer.

Thus, one can envisage to correct in the equation valid for SIS polymers, the density factor (r) by the concentration (c) of the SIS in the SIS-SI blend.

• The diblocks dilute the polyisoprene entanglements and therefore increase the average molecular weight of entanglements. It can be considered to correct the Me term of the equation valid for SIS by Me/c.

Considering the above assumptions, the storage modulus of SIS-SI polymers could be described by the following equation:

eSIS M

RTfG ρφ ⋅= )('

2/ '

/)(' cG

cMcRTfG SIS

eSISIS ⋅=⋅= ρφ

SI swelling effect of the SIS network

Experimental results: Influence of the SI diblock content: We evaluated the rheology profile of SIS-SI copolymers which have the same triblock and diblock species, but have a different percentage of diblock content (figure 10).

FIGURE 10 By plotting the secondary plateau modulus Gn

0 as a function of the logarithm of the SIS concentration, we found a linear relationship between the storage modulus and the diblock content with a slope of 1.8. This validates the assumptions we made above (Figure11), and confirms that the free isoprene block of the SI molecule plays the role of a polymeric "solvent" by swelling the SIS triblock network.

FIGURE 11

1.00E+02

1.00E+04

1.00E+06

1.00E+08

1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09

Frequency [rad.s-1]

G'

[Pa]

0% SI

1.00E+02

1.00E+04

1.00E+06

1.00E+08

1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09

Frequency [rad.s-1]

G'

[Pa] 50% SI

1.00E+02

1.00E+04

1.00E+06

1.00E+08

1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09

Frequency [rad.s-1]

G'

[Pa]

80% SI

Secondary plateau modulus decrease when SI content increases

Gn0 = 1,81.%SIS/SIS+SI + 5,7

R2 = 0,9834

4

4,2

4,4

4,6

4,8

5

5,2

5,4

5,6

5,8

6

-0,8 -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0 0,1% SIS in the blend SIS+SI

Gn0

: se

cond

ary

plat

eau

mod

ulus

[Pa]

Influence of the triblock molecular weight (Figure 12): We compared the rheological behavior of two SIS-SI copolymers having different molecular weights for the triblock, but the same diblock molecular weight and the same diblock and styrene contents. There is no apparent effect of the SIS triblock Mw on the rheology behavior of the SIS-SI polymer.

FIGURE 12

Influence of the styrene content: We looked at the variation of G' for various SIS-SI polymers, in a range of frequencies below the glass transition temperature (Tg), zone where the styrene has a strong effect. When increasing the styrene content of SIS-SI polymers, we observed an increase of the storage modulus, confirming the "filler" effect played by the styrene nodules (Figure 13).

FIGURE 13

1,00E+04

1,00E+05

1,00E+06

1,00E-04 1,00E-02 1,00E+00 1,00E+02 1,00E+04 1,00E+06

Frequency [rad.s-1]

G' [

Pa]

Mw (SI) = 68 000 g.mol-1same % S in SIS

Same Tα

Mw (SIS) = 234 000 g.mol-1

Mw (SIS) = 143 000 g.mol-1

1.00E+05

1.00E+06

1.00E+07

1.00E+00 1.00E+02 1.00E+04 1.00E+06 1.00E+08

Frequency [rad.s-1]

G'

[Pa]

An equation with a quadratic term represents the variation of the secondary plateau modulus normalized by the modulus of a pure isoprene (figure 14), in function of the styrene concentration. This correlation derived from the Einstein Law valid for diluted solutions takes into account the styrene-styrene interactions, e.g. filler effect of the styrene. It is close to the results found by Palierne in studying the influence of fillers in polymeric blends.

0

0.5

1

1.5

2

2.5

3

3.5

4

0.000 0.100 0.200 0.300 0.400Φ (styrene %)

Gn0 '/G

n0

EINSTEIN

PALIERNE

1 + 2.5 x Φ+10 x Φ2

FIGURE 14 Influence of the diblock molecular weight: Finally, we studied the influence of the SI diblock molecular weight, while looking at the variation of G' for SIS-SI polymers having similar triblock molecular weight and similar diblock content (Figure 15). When increasing the length of the isoprene chain of the diblock, the terminal relaxation plateau is translated to lower frequencies and the Mw of the SI diblock has no influence on the Tg of the SIS-SI polymer. Thus indicating that in this zone, one measures essentially the relaxation of the free isoprene sequence of the SI diblock.

FIGURE 15

Influence of the rheological behavior of SIS-SI block copolymers on the cuttability of HMPSA coated labels: Having identified the influence of the block copolymers structure on their rheological behavior, we studied the influence of this structure on the die cuttability of HMPSA- based labels. Creep compliance and recovery of HMPSA after cutting (figure 16): We simulated/calculated the creep compliance J(t) under a constant stress, for HMPSA based on SIS-SI of varying content of SI diblock. HMPSA, based on copolymers with high diblock content, creeps more than those based on pure triblock resulting in a closer behavior to those of acrylics. Simulating the recovery of the adhesive deformation after removing a stress applied during a certain time (t0= 100 s), indicates that the more diblock in the SIS-SI polymer, the less the tendency to recover the deformation.

1,00E+03

1,00E+04

1,00E+05

1,00E+06

1,00E-05 1,00E-03 1,00E-01 1,00E+01 1,00E+03 1,00E+05 1,00E+07

Frequency [rad.s-1]

G' [

Pa] Same Tα

Mw (SI) = 105 000 g.mol-1

Mw (SI) = 68 000 g.mol-1

Mw (SIS) = 143 000 g.mol-1G ’ corrected of effect of %S

Relaxation of Isoprene of SI

FIGURE 16 Peeling forces: We also evaluated the peeling force of HMPSA-based labels at various peeling speeds (Figure 17).

FIGURE 17 The higher the diblock content of SIS-SI polymers, the higher the peeling forces; but this could be adjusted by modifying the silicone coating release force. MODELING OF THE RHEOLOGICAL BEHAVIOUR OF SIS-SI COPOLYMERS Based on above experimental data, we developed a theoretical model which describes the rheological behavior of SIS-SI block copolymers. The model is based on molecular dynamics developed for linear homopolymers. It basically describes the rheological properties of the molecule within the whole range of frequencies (Figure 18). From high to low frequency, four distinct relaxation domains and processes were considered (polyisoprene molecule):

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.3 0.5 0.7 0.9 1.1 1.3 1.5LOG (V) (m/mn)

LOG

(F) (

N)

54 % diblock42 % diblock19 % diblockpure triblock

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0 10 20 30 40 50recovery tim e (s)

J(t)

(Pa-

1)

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-03 1.E-01 1.E+01 1.E+03 1.E+05time (s)

J(t)

(Pa-

1)

54 % diblock42 % diblock19 % diblockpure triblock

54 % diblock42 % diblock19 % diblockpure triblock

acrylic acrylic

• Domain 4 describes the behavior of polyisoprene in its glassy state, which corresponds to the sequential motion of C-C bonds

• Going down in frequency (domain 3) we are looking at the reequilibrium of the monomer density (per unit length) between two entanglements

• Domain 2 is equivalent to domain 3 except that we consider the whole scale of the (polyisoprene) molecule

• At the lowest frequency, the relaxation process is done by reptation (e.g. a snake like motion of the molecule along the path defined by the constraints of other surrounding molecules). This process is related to the molecular weight.

For pure isoprene

FIGURE 18 Following a similar approach we modelized the rheological behavior of SI diblock and SIS triblock: • We took into account the "filler" effect of the styrene blocks [f(f) = volume fraction

of styrene] • For the SI diblock molecules, we drew an analogy with the behavior of a star

polymer, where each arm of the star is also tethered at one of its end. Consequently, we replaced the reptation process by the one of a star molecule (Figure 19).

• For the SIS triblock molecule, we replaced the relaxation term of the equation by a relaxation process describing the rheology behavior of crosslinked rubbers (Figure 20).

) ( * ) ( * ) ( * ) ( * ) ( * ... ω ω ω ω ω tgBArept G G G G G + + + =

C C C C

1.E+01

1.E+04

1.E+07

1.E+10

1.E-02 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10aT ω (Rad/s)

G'( ω

) and

G''(

ω) (

Pa)

Mw Me

FIGURE 19

FIGURE 20 Finally, the molecular dynamic models developed for SIS triblock and SI diblock were combined to describe the behavior of SIS-SI polymers using a double reptation mixing rule (Figure 21).

1.E+02

1.E+04

1.E+06

1.E+08

1.E+10

1.E-04 1.E-01 1.E+02 1.E+05 1.E+08 1.E+11aT ω (rad/s)

G'( ω

) and

G''(

ω) (

Pa)

[ ])(*)(*)(*)(*)()(* ... ωωωωφω tgBAreptSI GGGGfG +++=

assuming polyisoprene + styrene effect

)(* ωstarG

)exp(~/ bMνω1

[ ])(*)(*)(*)(*)()(* .. ωωωωφω tgBAreptationSIS GGGGfG +++=

1.E+02

1.E+04

1.E+06

1.E+08

1.E+10

1.E-02 1.E+00 1.E+02 1.E+04 1.E+06 1.E+08 1.E+10aT ω (rad/s)

G'( ω

) and

G''(

ω) (

Pa)

This mixing rule was made consistent with the limiting behaviors, e.g.:

G SIS = GSI when w Ø 0

Gn0 ∂ c2 (GSI = 0 when w Ø 0)

FIGURE 21

NEW COPOLYMERS WITH IMPROVED DIE CUTTABILITY Using above model and adapting it to describe the rheological behavior of polymers having different molecular structures, 2 new copolymers have been designed and developed. • A styrene - isoprene linear block copolymer with a high diblock content • A 4 arms radial block styrene isoprene styrene block copolymer with a high diblock

content Table 1 describes above polymer properties

1.E+02

1.E+04

1.E+06

1.E+08

1.E+10

1.E-07 1.E-04 1.E-01 1.E+02 1.E+05 1.E+08 1.E+11aT ω (rad/s)

G'( ω

)and

G''(

ω) (

Pa)

212121 1 /// )()()()( tGctcGtG SISISblend −+=

Description and characteristics of new polymers:

TABLE 1 HMPSA blends based on the above polymers were prepared in a sigma blade mixer at about 145o C, by blending the polymers with a solid and a liquid C5 aliphatic hydrocarbon resins having respectively softening points of 91oC and 10 oC). Blending time was approximately 40 minutes. Table 2 reports the blends used.

Weight % Copolymer

31

Escorez® 1310 42 Liquid C5 aliphatic resin 27 Antioxidant 0.3

TABLE 2

Escorez® is an ExxonMobil trademark The HMPSA were coated on a release paper, using a laboratory coater, and transferred to a vellum frontal paper of 80 g/m2. The coating weight was about 20 g/m2. Similar coatings were done on a pilot coater. In this later case, a semi-gloss paper of 80 g/m2 was used. Adhesive performances The adhesive performances of the blends were measured according to FINAT testing methods. The 180o peel adhesion was performed using glass and polyethylene as reference surfaces. The migration through the frontal paper was measured according to a test method developed by ExxonMobil Chemical, which measures the whiteness of the frontal coated paper versus a non-coated paper as reference.

Grade DPX 565 DPX 586

Copolymer type SIS - SI(Plant

production)

(SI)4 - SIPilot

productProperty Testing method Unit Typical Value

Styrene Dexco Method % Mass 15 17

% Diblock Dexco Method % Mass 55 76

Melt flow ASTM D-1238 g/10 min 16 46

Pellets form Densepellets

Densepellets

Laboratory coatings Pilot Coating trials

DPX 565 DPX 586 DPX 586

Viscosity

Brookfield Viscosity (175°C – mPa.s)

12900

5380

5380

180° peel strength - N/25mm

Room temperature - glass 36 cf 35 cf 30 pt

Room temperature - PE 24.5 af 26 af 27 pt

Loop Tack - N

- glass at room temperature 27 af 33 af 31.5 af+pt

Loop Tack - N

- Polyethylene at room temperature 20.5 af 20 af 30 pt

Shear - room temperature - hours

Stainless steel - 25*25mm -1kg 150 cf 8-90 cf >600

Migration - % reflection

1 week 60°C 89 89.5 79

2 weeks 60°C 90 87.5 71

1 week 70°C 84 81 67

2 weeks 70°C 85 70 44.8

af adhesion failure TABLE 3 cf cohesion failure pt paper tear HMPSA based on the above polymers demonstrate: • Low melt viscosity, allowing low coating temperature as well as short adhesive

mixing time and increased efficiency • High peel strength including polyolefinic substrates • Excellent tack performances • Low migration characteristics • HMPSA based on DPX 586 show high shear characteristics associated with broad

formulation latitude

The dynamic rheological properties of above HMPSA were compared to those of linear triblock with varying amounts of diblock (Figure 22). As it can be seen, the plateau modulus at low frequencies is decreasing with the level of diblock (or equivalent), and has a much lower modulus than those of an adhesive based on Vector 4114, a linear SIS with a diblock content of about 42 %.

FIGURE 22 The die-cutting performances of the adhesives based DPX 586 were tested on a printing die-cutting machine, and compared with a commercial HMPSA based on linear block copolymers having an overall diblock content of 75% and with an acrylic- based adhesive. The tests were performed using different types of die shapes.

The results in figure 23, demonstrate that the reduced elastic behavior as shown in figure 22, provides to HMPSA based on DPX 586 better die-cutting performances than commercially available copolymer-based adhesive formulations and is close to the die-cutting behavior of water-based acrylic adhesives.

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E-05 1.00E-03 1.00E-01 1.00E+01 1.00E+03 1.00E+05 1.00E+07 1.00E+09 1.00E+11 1.00E+13

frequency (rad.s-1)

G' (

Pa)

0.00E+00

5.00E-01

1.00E+00

1.50E+00

2.00E+00

2.50E+00

3.00E+00

3.50E+00

Tan

delta

G' - HMPSA based on Vector 4114

G' - HMPSA based on DPX 565

G'- HMPSA based on DPX 586

Tan delta - HMPSA based on Vector 4114

Tan delta - HMPSA based on DPX 565

Tan delta - HMPSA based on DPX 586

FIGURE 23 CONCLUSION: The rheological properties of HMPSA, based on block copolymers are highly dependent of the block copolymer structure. The understanding of these parameters and properties allowed to build a theoretical model based on molecular dynamics. Using such a model, we have developed 2 new block copolymers of different molecular structure: • A styrene - isoprene linear block copolymer with a high diblock content • A 4 arms radial block copolymer HMPSAs based on such copolymers exhibit: • low melt viscosity, allowing low coating temperature as well as reduced mixing time

and improved efficiency • high adhesion properties including polyolefinic substrates • low migration characteristics and improved die-cuttability performances for

labelstock applications • HMPSA based on DPX 586 shows high shear characteristics allowing high

formulation flexibility and broad range of tape applications.

0

1

2

3

4

5

6

7

8

9

10

Die 1 Die 2 Die 3

WB acrylic Commercial HMPSAHMPSA based on DPX 586

ACKNOWLEDGEMENT I would like to extend my thanks to O. Georjon and N. Kappes for their contributions and valuable support in preparing this report. REFERENCES: F.X. Gibert, PhD thesis, (2001) N. Kappes, PhD thesis, (1998) Einstein A, Ann. Phys, 34, 591-592, (1911) Palierne, J.F., Rheol. Acta, 29, 204, (1990) De Gennes, P.G., J. Chem. Soc. 55, 572, (1971) K.L. Ngai, DJ Plazek, J. of Polymer Sciences, 24, 619, (1986) P.E. Rouse, J. Chem. Phys., 21, 1272, (1953) RC Ball, TCB McLeish, Macromolecules, 26, 5264, (1993)