effect of the structure of latex particles on adhesion. part i: synthesis and characterization of...

Effect of the Structure of Latex Particles on Adhesion. Part I: Synthesis and Characterization of Structured Latex Particles of Acrylic Copolymers and Their Peel Adhesion Behavior

ANDRE MAYER, THA PITH, GUO-HUA HU,* and M O R A N D LAMBLA

Ecole d'Application des Hauts PolymGres, lnstitut Charles Sadron (CRM-EAHP), 4, rue Boussingault, 67000 Strasbourg, France

SYNOPSIS

This is a series of articles that deals with fundamental aspects of the effects of the structure of latex particles of acrylic copolymers on their adhesion behavior. Specifically, relationship or analogy between rheological properties and adhesion performance of the acrylic copolymers was demonstrated. The first part of this series concerns the synthesis and characterization of latex particles with desired structures and compositions, and the experimental results of peel adhesion. The second part develops an analogy between the peel adhesion performance of the adhesives and rheological properties of the corresponding copolymers. The third part addresses the generalities and particularities of three major tests for adhesion: peeling, blistering, and spontaneous peeling. Three types of structured latex particles were synthesized by three different emulsion polymerization processes: the first type had a uniform composition over the entire particles with a glass transition temperature (T,) varying between -40°C and O'C, depending upon the compositions of monomers involved in the copolymer; the second type was of core-shell structure. As for the third type, the composition of monomers varied gradually across the particle radii. The glass transition behavior and the dynamic mechanical properties in the solid state of the copolymers confirmed the structures of the corresponding latex particles. On the other hand, the peel adhesion performance of the films of these latex particles varied with the dynamic mechanical properties of the corresponding copolymers. This implies that a correlation could be found between the structure of the latex particles, dynamic mechanical properties in the solid state of the corresponding copolymers, and the peel adhesion performance of the adhesive films. 0 1995 John Wiley & Sons, Inc. Keywords: adhesion acrylic copolymers latex particles rheology peeling

INTRODUCTION

The first conference organized by the Pressure Sen- sitive Tape Council in 1978 on adhesives synthesized in dispersed media marks the formal entrance of acrylic adhesives obtained by emulsion polymerization into the market of pressure-sensitive adhesives. Since then, acrylic adhesives have gained numerous industrial and academic interests. Nevertheless, the relationship between the structure of latex particles and the adhesion behavior of the resulting copoly-

* To whom all correspondence should be addressed. Journal of Polymer Science: Part B Polymer Physics, Vol. 33,1781-1791 (1995) 0 1995 John Wiley & Sons, Inc. CCC 0887-6266/95/121781-11

mers after coalescence remains unclear. One way toward establishing such a relationship is to verify whether the adhesion behavior of an adhesive film can be characterized by some of its mechanical properties. If so, the job of chemists is to select an appropriate emulsion polymerization process that allows the attainment of structured latex particles, whose film obtained after coalescence satisfies cri- teria set forth for those mechanical properties. The most important goal of this study was to establish a relationship between the dynamic mechanical properties of an adhesive film and its peel strength as a function of peel speed. The methodology developed by Aubrey and Sherriff for tackified nitril rubber',' was followed.

1781

1782 MAYER ET AL.

Table I. AA) Used for the Syntheses of the Latex Particles of Series H , (Uniform Compositions)

Monomer Compositions (BA, MMA, and

Hl Hz H3 H 4

BA (8) 372 300 272 240 MMA (g ) 16 88 116 148 AA lg) 12 12 12 12

Three types of model latex particles with desired structures and relatively simple compositions were synthesized by different emulsion polymerization processes. The structures and selected properties of those latex particles after coalescence were characterized by physical and rheological means. On the other hand, their adhesion behavior and peel behavior in particular were investigated. An attempt was then made to relate dynamic mechanical properties to the variation of peel strength as a function of peel speed. This article concerns the synthesis, characterization, and peel adhesion of the following three types of structured latex particles: the first series, denoted as H,, consisted of particles whose compositions were uniform; the second series (S,) was of core-shell structure; and the third series (PF,) had compositions that varied gradually across the particle radii.

EXPERIMENTAL

Synthesis of latex Particles

Series H, (Table I; x = 1 to 4, denoting four different compositions in monomers) was synthesized in a semicontinuous mode and the monomer compositions were kept constant during the entire feeding period. Three acrylic monomers were used butyl acrylate (BA), methyl methacrylate (MMA), and

acrylic acid (AA). The first two monomers allowed the attainment of copolymers that covered a large range of Tg by varying the compositions of the monomers involved in the copolymers. AA was used for potential reactivity, which was very helpful for improving anchorage of coated latex films to the face stock (film support) (PE or PET). Two sets of core- shell latices were synthesized with a composition of the shell identical to that of H, (series S,; Table 11). This latter allowed a comparison of latices H2 and S, in terms of dynamic mechanical properties and adhesion behavior with the same surface and interfacial properties. The core of one type of latices (S ly) was poly(buty1 methacrylate) (PBMA; Tg = 30°C) and that of the other (S,) polystyrene (PS; Tg = 100°C). Three weight fractions were chosen for each core. The latex particles of series PF, (Table 111) were synthesized using the same compositions as those of H,, H3, and H4. To obtain a gradient in composition across the entire particle diameter, which in our case was characterized by a decrease of Tg from the center to the shell, various amounts of BA were taken out of the original compositions of H, and fed separately during the entire monomer feeding period (power feed mode). All the monomers used in this study were purchased from Aldrich (Saint Quentin Fallaria, France) and purified before use. As our objective was to vary the viscoelastic properties of the copolymers without changing the surface characteristics of the “adhesives,” the amounts of acrylic acid, surfactant, buffer, and solids content were the same for all the syntheses. The surfactant was 33AD, a commercial grade of am- monium nonyl phenyl ether sulfate. This minimized possible effects of the surfactant on the adhesion perf~rmance.~ The initiator (potassium persulfate, KPS) and the buffer (NaHC03) were commercial grades, and they were used as received.

Table 11. or PS) Used for the Syntheses of the Latex Particles of Series S, (Core-Shell Structures)

Compositions (Shell: the Same Composition as that of H2; the Core: PBMA

Reactor:

Reservoir: seed ( H 2 ) 85 g 173 g 260 g 270 g 240 g 210 g

BA ( g ) 270 240 210 270 240 210 MMA (9) 79.2 70.4 61.6 79.2 70.4 61.6 AA (d 10.8 9.6 8.4 10.8 9.6 8.4

LATEX PARTICLES AND ADHESION-PART I 1783

Table 111. AA) Used for the Syntheses of the Latex Particles of Series PF, (Radially Varying Compositions)

Monomer Compositions (BA, MMA, and

Reservoir I: H2 H3 H4 BA (g) 140 112 79.2

AA (g) 12 12 12 MMA (g) 88 115 148.8

Reservoir 11: BA (9) 160 160 160

Determination of the latex Particle Sizes and the Glass Transition Temperatures of the Corresponding Copolymers

The size of the latex particles was measured by light scattering of type Malvern autosizer IIc. The glass transition temperature Tg of these latex particles after coalescence was determined using a Perkin El- mer DSC4 at a heating rate of 10"C/min.

Rheological Characterization of the Films of the Adhesive Copolymers

The dynamic mechanical properties of the copolymers synthesized in emulsion were characterized in terms of storage and loss moduli, G' and G". Mea- surements were carried out on a rheometer of type Rheometrics RMS 605. The frequency of deformation varied between and lo2 rad/s. To obtain moduli ranging from 104 to 10' Pa, two geometries were used torsion pendulum and plate-plate. Sam- ples were prepared by coalescing the latex particles in a Teflon mold. This latter helped reduce adher- ence. For the peel measurements, the adhesives had to be coated onto a polyethylene (PE) face stock or film support. This necessitated a specific formulation for the latices: 200 g of latex (50% solids), 20 g of a solution containing 5% of a demolding agent, 0.2 g of a solution containing 15% NH3, and 2 g of a solution containing 50% of a crosslinking agent dissolved in ethyl acetate. The same formulation was adopted for the preparation of samples for the dynamic mechanical measurements. The thickness of the samples varied between 2 and 3 mm, and the samples were dried for a week. An additional 3 weeks at room temperature was needed for the samples to reach a quasistable state. This latter aimed at min- imizing possible effects of migration or diffusion of the surfactant, or any other small molecules on the adhesion or dynamic mechanical properties.

Peel Testing Procedures

The basis for evaluating the peel performance of most pressure-sensitive tapes is ASTM D1000-66 or later version^.^ The procedures of interest for peel testing include sample preparation, adhesion to steel plate (180" peel test), drum adhesion (90" peel test), and other tests. The 180" peel test is used most often to characterize the pressure-sensitive tapes. It is easier to carry out than the 90" test, for example, and the data show slightly less scattering. It is also more sensitive to variations in tape construction. In this study, the 180" peel test with a rigid backing (a stainless steel plate) was used. Specifically, adhesives were coated onto a film support (also called face stock), which was corona-treated. This latter was either a low density polyethylene (LDPE) or poly(ethy1ene terephthalate) (PET). The adhesives were deposited onto a film support by a laboratory coating machine of type DIXON. The coating weight varied between 1 g/m2 and 30 g/m2. A problem was encountered when the adhesives were coated onto LDPE: a specific formulation of the latices had to be developed to improve anchorage to this film support. The compositions of this formulation were presented above. After being coated, the edges of those adhesive tapes were cut off with a width between 15 and 20 cm. Those tapes were then stored at 25°C for 3 weeks, allowing them to reach the equilibrium of maturation before testing.

The peel strength of those tapes was characterized by a peel force as a function of peel force in the 180' peel test (Fig. 1). The peel test was carried out using a 4-cm wide band of adhesive. To keep the film support curvature constant regardless of the value of the peel force, a small rigid cylinder (1 mm in diameter) was placed just behind the peeled tape. This allowed the assumption that whatever the level of adhesion that varied during peeling, the geometry of the rupture zone was unchanged. As opposed to a normal contact time of 20 min between adhesives and the rigid backing, it was extended to half a day.

Figure 1. Schematic representation of the 180' peel test. Rigid backing: stainless steel or polycarbonate; film support (face stock) of the adhesives: LDPE or PET.

1784 MAYER ET AL.

The standard peel speed range of the machine was relatively narrow (between 0.3 and 35 cm/min). To widen it, tests were carried out at various temperatures, and master curves were then constructed based upon the principle of time-temperature eq~ivalence.~,~ This technique necessitated relatively long times (30 min) to reach a required temperature equilibrium. This forced us to extend the contact time prior to each measurement to reduce the effect of time in the thermostated chamber on the accuracy of measurement. Two measurements were carried out for each given peel speed, and the average was used. The peeling results were generated from a ten- sile machine of type ZWICK equipped with a thermostated chamber. The temperature could be reg- ulated between -20°C and 60°C with an accuracy of k1"C.

RESULTS AND DISCUSSION

Size of the latex Particles

The diameters of all the latex particles obtained in this study were relatively close, as shown in Table IV. This was important as our objective was to in- vestigate the effects of the structures of latex particles on the peel adhesion performance of the corresponding adhesives without interferences of the effects of other parameters, such as those of the amount of the emulsifier and the diameter of particles.

Glass Transition Temperatures

Glass transition temperature characterizes, to some extent, the structure and uniformity in composition of latex particles. For example, a single Tg is expected for latex particles involving a single (co)polymer whose composition is homogeneous over the entire particle radii. If the latex particles are of core-shell structure, and the Tg of the (co)polymer corresponding to the core and that of the (co)polymer corresponding to the shell differ, then two Tgs should appear. If the composition of the latex particles containing a single (co)polymer varies with their radii, then a single and wide glass transition zone should exist. It should be emphasized that the reverse of the above statements is not necessarily correct. For example, if the Tgs corresponding to the expected core and shell are seen, this does not necessarily mean that the latex particles are of perfect core-

shell structure. Their structure or morphology could be of other irregular shapes as well (inverted core- shell, poow, inverted poow, sandwich, raspberry, or even two phase separated blend7v8). In these cases, other independent techniques, such as transmission electron microscopy, have to be used.

The DSC curves of all the copolymers H,, S,, and PF, are shown in Figure 2. The glass transition zones of the copolymers of series H, are relatively narrow due to their uniform compositions. Also, the Tg increases with increasing MMA content in the latex particles: it is -4O"C, -20°C, -1O"C, and 0°C for H I , H,, H3, and H4, respectively.

The latex particles of series S, are characterized by the Tgs of the shells, which are very close to that of H2. This is expected as the composition of the shells of series S, is identical to that of H2. On the other hand, when the core is made of PBMA (series Sly), its Tg is not seen when its weight fraction is 10%; it appears at about 35°C when the weight fraction of the core is 20% or 30%, corresponding to the Tg of pure PBMA. When the core is composed of PS (series SZy), its Tg is close to that of pure PS (100°C). In this case, the Tg of the core is already seen when the core weighs only 10%. This is due to that fact that the core of PS is less compatible with the shell (which is made of the composition of H2: BA, MMA, and AA) than the core of PBMA. In any event, all these results are in line with the expected core-shell structures of series S,. Recently, the expected structures of this type of latex particles obtained by the same emulsion polymerization processes were confirmed using transmission electron microscopy, as described elsewhere.'

As opposed to the latex particles H,, those belonging to series PF, are characterized by wide glass transition zones due to gradients in composition within these latex particles. This is better seen for latex particles PF, whose gradient in composition is the highest.

Rheological Characterization

The storage and loss moduli, G' and G , of the acrylic copolymers of the latex particles H,, Sly, S2,, and PF, are shown in Figures 3-10, respectively, on the logarithmic scale.

Series H,

The 10s' as a function of temperature for the acrylic copolymers of this series (Fig. 3) confirms the results


Table IV. Determined by Light Scattering

Diameters of the Various Latex Particles Synthesized in this Study as

4(nm) 210 230 220 230 252 208 179 292 233 215 245 240 236

obtained from the DSC that the glass transition zones are narrow (within a 10°C interval). The Tg values obtained from the dynamic mechanical measurements agree with those obtained by DSC. They also fit Fox’s equation” well (Table V). The rubber- like plateau zone is virtually identical and relatively wide for all four copolymers, due to the presence of the crosslinking agent in the latex formulations. This crosslinking agent determines not only the width of the rubber-like plateau zone but its modulus as well. This indicates that the crosslinking agent has equal access to the functional monomer (AA) introduced to the copolymers. The log G” as a function of the logarithm of reduced frequency, log(w - uT)

for series H, is shown in Figure 4 (reference temperature: 20°C; the values of the shift factor UT used in this and other figures of this paper were deter-

mined experimentally upon construction of master curves logG”(T) as a function of temperature, and they are reported in Part 11”). The slope of these curves characterizes the homogeneity of the copolymers, as does the width of the transition zone of the DSC curves or that of the logG’ curves. According to Rouse theory,” the slopes would be 0.5. In our case, they vary between 0.46 and 0.57, which are judged to be close to 0.5.

Series S,, Similar to what is observed in the DSC curves, there appears also a second glass transition zone at about 20°C in the logG‘ as a function of temperature (Fig. 5) curves for the copolymers belonging to series Sly (core-shell structure with PBMA as the core). This

I

_ _ _ H 3 _ _ _ _ _ H4 ’ ,’ /

I I -60 -20 $0 60 -20 20 $0 100

TEMPERATURE (“C) TEMPERATURE (“C)

-60 -20 2b 60 100 140 TEMPERATURE (“C)

I

-60 -10 20 60 1 TEMPERATURE (“C)

I0

Figure 2. (latex particles H,, S,, and PF, after coalescence).

DSC curves of the acrylic copolymers obtained by emulsion polymerization

1786

9.5

8.5

2 7.5 W

b 06.5 -

5.5

4.5 4.5

MAYER ET AL.

7 I I I I I I I , I I I I , I I I I I I I , I , I I

-1.00 -;o 0 50 100 150 -lo',' "-id ' ' ' 0 ' ' ' ' ' 50 I ' ' ' ' 160 ' ' ' ' 1iO ' ' TEMPERATURE ("C)

-4.0

Figure 3. function of temperature for the copolymers of series H,.

Logarithm of storage modulus, logG', as a

' -d.0 ' 0.b ' 2.b 4.b 6.b 8.'0

transition zone corresponds to the Tg of the PBMA in the material. As expected, the transition is better seen in the case of S13, which has the highest content in PBMA. The difference in Tg between pure PBMA (30°C) and that dispersed in the copolymer H2 matrix (20°C) is due to a partial interpenetration between the core (PBMA) and the shell (Hz) . Such a situation is often encountered in interpenetrating polymer networks in which two networks often display two T8 closer than they would be between these two homopolymers.'3 When comparing the logG' curve of H2 with that of S13, it is seen that the ten- dency in the first zone (10' and lo7 Pa) between these two curves is very similar, confirming that the matrix of SI3 copolymer is indeed made of H2. Al- though the models of Kerner14 and Dickie15-17 are

8.5

7.5

h o 6.5 n

b m5.5

W

-

4.5

3.5

Figure 4. Logarithm of loss modulus, logG", as a function of the logarithm of reduced deformation frequency, log(w - aT), for the copolymers of series H,.

9.5

8.5

2 7.5 - W

b ~ 6 . 5 -

5.5

'\8t0 ' - 0

\ O \ o

\@ O \o* 0

supposed to be able to confirm matrix/dispersion structures, they cannot be applied to the present system because the Tgs of the core and the shell are too close.

The logG" as a function of log(w.uT) (Fig. 6 ) shows a second glass transition zone at low frequencies. The slope of the logG" curves decreases with increasing inclusion of PBMA because the logG" increases with it at low frequencies: 0.54 for H2, 0.47 for Sll, 0.44 for S12, and 0.29 for SI3. Also noteworthy is the similarity of the master curves of logG" (o - uT) between series Sly and H2. This similarity indicates that the properties of the materials formed after coalescence of the latex particles Sly are indeed dictated by the shell made of the composition of H2, which constitutes the matrix.

8.5

7.5

- 2 6.5 v

b ,5.5 0 -

4.5

3.5

*u s12 0-0 5 1 3 - _ _ H2

T,ef = 20°C

I ~ I ~ I ~ I ~ I ' I -2.0 0.0 2.0 4.0 6.0 8.0

log(w.aT) (s-')

Figure 6. Logarithm of loss modulus, IogG", as a function of the logarithm of reduced deformation frequency, log(w - aT), for the copolymers of series S,.


b m6.5 - -

5.5 -

9.5 ,

0 '0 \ m o o \ 0

\D -. 0

-F - 0 - : 0 - \o

o o - - - - - .... - -..

3.5

4.5 ~ 1 1 1 1 1 , , , , 1 1 , , , 1 , 1 , , 1 , , , 1 1 , ,

- 100 -50 0 50 100 150 TEMPERATURE ("C)

Figure 7. Logarithm of storage modulus, logG', as a function of temperature for the copolymers of series S,.

I I I ,

Series Szy

The logG'(T) as a function of temperature (Fig. 7) yields two visible glass transition zones, with the first one corresponding to H2 and the second one located around 100°C corresponding to pure PS. The second transition zone for the copolymers of this series is more distinct than those of series Sly, whose core is PBMA. This is not only due to the fact that PS has a higher Tg than PBMA, but also due to a higher incompatibility between the core (PS) and the shell (H2), being in agreement with what is seen in the DSC. When the logG' curve for the core-shell structure S23 is compared to that of H2, it is seen that both copolymers SZ3 and H2 behave virtually the same in the first part of the glass transition zone,

8.5

7.5

A 2 6.5 v

0 ,5.5 0 -

4.5

3.5

!2LL! s 2 1 ..... s22 ODDOO S23 _ _ _ H 2

20°C

-4.0 ' -21.0 ' o h ' 2.0 4.0 ' 6.0 a.0 iog(w.ar) (s-')

Figure 8. Logarithm of loss modulus, logG", as a function of the logarithm of reduced deformation frequency, log(w - uT), for the copolymers of series S,.

9.50

8.50

A D 7.50 a v

g6.50 -

5.50

Figure 9. Logarithm of storage modulus, logG', as a function of temperature for the copolymers of series PF,.

This is true for the whole series, implying that the matrix of series SZy is made of H2.

As opposed to series Sly, the difference in Tg and incompatibility between the core and the shell of series SZy is large enough to use Kerner's model for predicting the storage modulus in the rubber-like plateau zone. According to this model, the storage modulus of the composite material Gc is related to those of the matrix (G,) and the inclusion (Gi) by

where u, denotes the volume fraction of the matrix, ueff the effective volume fraction of the inclusion. This latter is given by

8.5

7.5

h CJ 6.5 a

23

v

m5.5 -

4.5

, -0 PF2 ).... PF3 -0 PF4 _ _ _ H 2

1788 MAYER ET AL.

Table V. Uniform Latex Particles of Series H, as Measured by DSC

Glass Transition Temperatures (T,) of the

TgFor (“c) -40 -20 -10 0 T~DSC (“C) -39 -21 -12 -1 TgRhe (“c) -40 -18 -9 3

U,ff = ui + U p ( 1 - uim)/ui”m

(0.6 < uim < 0.65 in our case),

where ui and uim represent the volume fraction and the corrected volume fraction of the inclusion, respectively. The coefficient a is expressed as

Q! = 2(4 - 5 ~,)/(7 - 5 urn),

where urn is the Poisson coefficient of the matrix. In this study, the values of urn and Gi were taken as 0.5 and 1 X lo9 Pa, respectively.

As shown in Table VI, the calculated and experimental storage moduli, Gccal and Gcexp, are close when the core is made of either 10% or 20% PS. However, when the core is made of 30% PS (S23) ,

the elastic modulus is underestimated with respect to the experimental value. Despite this discrepancy between the calculated and the experimental values of G‘ for SZ3, the relatively good agreement between the experimental and calculated results for the latex particles with 10 and 20% PS support the expected core-shell structures of the latex particles.

The curves logG”(o.aT) of series S, in Figure 8 display vaguely a second dissipated area due to the presence of PS, and they converge at high frequencies. This is because all the shells of the latex particles of this series are made of copolymer H,, which constitutes the matrices. On the other hand, at low

frequencies the loss modulus increases with increasing PS content in the core. Similar to series Sly, the slope of the logG” curve decreases with increasing volume fraction of the inclusion (0.54 for H,, 0.52 for SZ1, 0.42 for SZ2, and 0.35 for S23) .

Series PF,

The shapes of the curves logGI( T ) of this series (Fig. 9) confirm the results of the DSC that the glass transition zones are much wider than those of series H,. Series PF, and H, have similar storage modulus values a t low frequencies. This indicates that 3% of acrylic acid incorporated in the copolymers, regardless of the emulsion polymerization process chosen, is equally accessible to the crosslinking agent that dictates the storage modulus at low frequencies. The master curves logG”(w - uT) were also constructed as before (Fig. 10). As the polymers of series PF, cor- respond more to polymer blends than copolymers, one would expect that the principle of time-temperature equivalence may not apply. Nevertheless, this is not the case here due probably to a relatively narrow frequency range (10 decades). Similar to the results obtained from the other copolymers, the slope of the curve logG”(w - uT) decreases with increasing content in MMA in the H, formulations.

In summary, the structures of the latex particles obtained from the various emulsion polymerization processes were confirmed by the results of DSC and by their storage and loss moduli. The dynamic mechanical properties of these structured latex particles are more sensitive to their particular structures than their glass transition temperatures.

Peeling Behavior

To have a preliminary appreciation of the peel adhesion behavior of the various structured latex particles synthesized in this study, presented below are only the peel adhesion versus peel speed results

Table VI. of Series SZv

Comparison of Moduli Between the Experiments and Kealble’s Model for the Core-Shell Latex Particles

0.10 0.125 1.35

1.7 x 105 2.2 x lo5 2.2 x lo5

0.20 0.30 2.07

3.8 x 105 3.5 x 105

0.30 0.525 3.78

1.5 X lo6 6.4 x 105

Note: The elastic modulus of the matrix G,,, for S, materials was taken as 1.7 X lo5 Pa, the value of the elastic modulus of H2 that constitutes the matrix of these S, materials.


2.5

2.0

n

E 1.5 \ z W

LLal .o rn 0 -

0.5

0.0 I I I I I I i -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -

hlv, ( d s )

Figure 11. Logarithm of peel force, log,, as a function of the logarithm of reduced peel speed, log( up - aT), for the copolymers of series H, using PE or PET as a film support and stainless steel as a rigid backing.

of the acrylic copolymers of latex particles H,. Those of the other acrylic copolymers will be shown in Part I1 of this series when the intrinsic mechanical properties of these materials will be addressed. Shown in Figure 11 are the peel force as a function of peel speed results plotted on a log-log scale for the adhesive film of H2 coated onto PE or PET as a film support. It is seen that as temperature increases, the curve logF, versus logu, shifts to the left, and the peel strength decreases. It appears that the principle of time-temperature equivalence can be applied to the present system or master curves logFp versus log(u,.aT) can be constructed. This was indeed confirmed by our peeling results that covered a widest peel speed range possible. The lowest peel speeds were limited by the sensitivity of the machine, and the upper peel speeds were related to the onset of the stick-slip zone (the yielding point of the peel force was about 2.2 N/m). The shift factor aT was obtained by constructing master curves as a function of temperature. Examples of such master curves are shown in Figure 12. It is seen that lo#, varies lin- early with log(u, - aT) at low peel speeds. When the peel speeds are high, lo#, reaches a maximum be- yond which the stick-slip zone occurs, which is typ- ical of peel adhesion testing. It is noteworthy that the curves have similar slopes in the linear region, regardless of the nature of the film support (PE or PET). This is the case for all the adhesives belonging to series H,. It follows that the nature of a film support does not affect the peel force as a function of

2.5

2.0

- E 1.5

'=. v

LLn1.o Ul -

0.5

REFERENCE TEMPERATURE: 22°C

d

0- HZ/FILM PE; METAL HZ/FILM PET; METAL

0 0 ~ 0 0 H2/FILM PE; PC 0.0 1 I 1 I I

log(V,.ar) ( 4 s )

Figure 12. Logarithm of peel force, log,, as a function of the logarithm of reduced peel speed, log(u,- aT), for the copolymers of H2 and H3 using PE or PET as a film support and stainless steel or polycarbonate as a rigid backing.

peel speed of the adhesives of interest. The differ- ences of the peel force at a given peel speed among various film supports are simply due to the differ- ences in thickness of the adhesives and modulus of the film supports. This is true if the "rigid backing" in Figure 1 is truly rigid, as appears to be the case with stainless steel. If polycarbonate is used instead for the rigid backing, the slope of the curve l o g p versus log( up * uT) at high peel speeds differs greatly from the cases where a stainless steel plate was used as the rigid backing. It appears that polycarbonate as a rigid backing also deforms, thus contributing to the overall energy dissipation during a peeling test. The peeling results of the adhesives of series

2.5

2.0

E 21.5 v

0. LL

g1.0 -

0.5

REFERENCE TEMPERA TURE: 22°C ./

* .

Figure 13. Logarithm of peel force, log(F,), as a function of the logarithm of reduced peel speed, log(u,-aT), for the copolymers of series H, using PE as a film support and stainless steel as a rigid backing.

1790 MAYER ET AL.

H, using LDPE as a film support and a stainless steel plate as the rigid backing are shown in Figure 13. Keep in mind that the latex particles of series H, are characterized by uniform compositions across the entire diameter of the particles and by Tg, which increases from HI to H4 (-4O"C, -2O"C, -1O"C, and 0°C). The increase in Tg shifts the curve logFp versus log(up. uT) toward the left (the onset of stick-slip) and causes a decrease in the maximum of adhesion. This latter reveals the effect of the physical and rheological properties of the adhesives on their adhesion performance, which is the subject of the second part of this series of papers.

CONCLUDING REMARKS

To establish a relationship among emulsion polymerization processes, rheological /mechanical properties, and adhesion performance of adhesives, model latex particles of acrylic copolymers with desired structures and compositions were synthesized using appropriate polymerization processes: structures with uniform composition ( series H,) ; core- shell structures (series s,) ; and structures with radially changing composition (PF,) . Their structures and compositions were confirmed by their DSC curves. They were further supported by the dynamic mechanical properties (storage and loss moduli) of the corresponding acrylic copolymers obtained after coalescence of the latex particles. For series H,, a change in Tg brings about a shift for the logG' ( T ) and logG" ( w uT ) curves, as expected. Series S, are actually composite materials. Their structures are related to the nature and compatibility of the phases (core and shell) : for Sly, the core ( PBMA) and the shell ( H 2 ) are somewhat compatible, therefore forming a third zone that results from the interdif- fusion between the core and the shell; as for SZy, the compatibility between the core ( P S ) and the shell ( H 2 ) is much less important; thus, the third zone is expected to be very narrow. Inclusion having a Tg higher than that of the matrix increases the storage modulus of the rubber-like plateau. The magnitude of increase is related to the inclusion content, as shown by Kerner or Dickie's model for the cases where the Tgs of the two phases (core and shell) differ appreciably. The last series, PF,, is more like a polymer blend, displaying a large glass transition zone. As far as the loss modulus G" is concerned, the value of logG"( o - aT) shifts to lower frequencies when the Tg of the copolymer H, increases. This is also true for copolymers S, and PF,. The slope of the logG" versus log (o * u T ) curves of the copolymers

of the structured latex particles is smaller than that for the uniform latex particles of H,. The magnitude of the decrease in slope depends on the amount and the nature of the core. As for the results obtained from the peeling tests, the nature of the face stock does not affect the peel force as a function of peel speed. These preliminary results have already re- vealed the applicability of the principle of time- temperature equivalence to peel adhesion phenom- ena, which will be discussed in great detail in Part I1 of this series.

This work was supported by Novacel. The authors are grateful for this support.

NOMENCLATURE

AA BA MMA PBMA PE PET PS Hx

s,

S1Y

S 2 Y

PF,

acrylic acid butyl acrylate methyl methacrylate poly (butyl methacrylate) polyethylene poly ( ethylene terephtalate) polystyrene homogeneous latex particles made of BA,

MMA, and AA, corresponding copolymers thereof, or adhesive tapes thereof ( x = 1 to 4 denoting four different compositions in monomers; see Table 1 )

latex particles of core-shell structure in which the shell has the same composition as that of H2 (see Table 2 )

whose core is poly (butyl methacrylate ) or PBMA (y = 1 to 4 denoting four different weight fractions in PBMA)

whose core is polystyrene or PS (y = 1 to 4 denoting four different weight fractions in PS)

latex particles whose compositions in BA, MMA, and AA vary with their radii (see Table 3)

REFERENCES AND NOTES

1. D. W. Aubrey and M. Sherriff, J. Polym. Sci., Polym.

2. D. W. Aubrey and M. Sherriff, J. Polym. Sci., Polym. Chem., 16,2631 (1978).

Chem., 18, 2597 (1980).


3. C. L. Zhao, Ph.D. dissertation, Universitk Louis Pas- teur de Strasbourg, France ( 1987).

4. Standard Method of Testing Pressure Sensitive Ad- hesive Coated Tapes Used for Electrical Insulation,

5. D. H. Kaelble, in Physical Chemistry of Adhesion,

6. J. Schultz and A. N. Gent, J. Adhesion, 3,281 (1972). 7. J. C. Daniel, Makromol. Chem., Suppl. l O / l l , 359

(1985). 8. D. C. Sundberg, A. P. Casassa, J. Pantazopoulos, and

M. R. Muscato, J. Appl. Polym. Sci., 41,1425 (1990). 9. F. Vazquez, H. Cartier, K. Landfester, G. H. Hu, T.

Pith, and M. Lambla, Reactive blends of thermoplas- tics and latex particles, The Third International Symposium on Radical Copolymers in Dispersed Me- dia, April 17-22, Lyon, France.

ANSI/ASTM D1000-78.

Wiley Interscience, London, 1971, pp. 450-498.

10. T. G. Fox, Bull. Am. Phys. Sco., 1,123 (1956).

11. A. Mayer, T. Pith, G.-H. Hu, and M. Lambla, Effect of the structure of latex particles on adhesion. Part I1 Analogy between peel adhesion and rheological properties of acrylic copolymers, accepted by J. Polym. Sci., Polym. Phys. Ed. (1995).

12. J. D. Ferry, in Viscoelastic Properties of Solid Polymers, John Wiley, New York, 1980, pp. 224-263.

13. H. L. Sperling and Chiu Tai-Woo, J. Appli. Polym. Sci., 17, 2443 (1973).

14. E. H. Kerner, Proc. Phys. SOC., 69B, 808 (1956). 15. R. A. Dickie, J. Appli. Polym. Sci., 17, 45 (1973). 16. R. A. Dickie, M.-F. Cheung, and S. Newman, J. Appli.

17. R. A. Dickie, J. Appti. Polym. Sci., 1 7 , 79 Polym. Sci., 17, 65 (1973).

(1973).

Received October 12, 1994 Revised March 7, 1995 Accepted March 22, 1995

effect of the structure of latex particles on adhesion. part i: synthesis and characterization of...

Documents