Quantitative Studies of Toughening Mechanisms in ABS and ASA

15 Quantitative Studies of Toughening

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 22, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0154.ch015

Mechanisms in ABS and ASA Polymers C. B. BUCKNALL, C. J. PAGE, and V. O. YOUNG Cranfield Institute of Technology, Cranfield, Bedford MK43 OAL, England The kinetics and mechanisms of tensile deformation in ASA and ABS polymers were studied using high accuracy creep tests. Crazing was detected by volume strain measurements. In both polymers, creep of compression-molded specimens is caused mainly by crazing, with shear processes account ing for less than 20% of the total time-dependent deforma tion. Crazing is associated with an increasing creep rate and a substantial drop in modulus. The effects of stress upon creep rates are described by the Eyring equation, which also offers an explanation for the effects of rubber content upon creep kinetics. Hot-drawing reduces creep rates parallel to the draw direction and increases the relative importance of shear mechanisms. A crylonitrile-butadiene-styrene (ABS) and acrylonitrile-styrene-acrylate (ASA) are rubber-toughened plastics based upon the styreneacrylonitrile (SAN) copolymer matrix. The combination of the stiffness and toughness exhibited by these materials has made them increasingly attractive in engineering applications, and the activity of the patent literature testifies to a continuing interest in improving properties through modifications of structure. The aim of this paper is to discuss a quantitative approach to structure-property relationships in ABS and ASA polymers. Electron microscope studies have shown that the toughness of ABS polymers is caused largely by multiple craze formation (1,2). The rubber particles appear both to initiate and to control craze formation, so that impact energy is dissipated in the production of numerous small crazes (3). However, this theory does not exclude the possibility of contributions from other mechanisms. The observation that many ABS polymers tend to neck during a tensile test suggests that shear mechanisms are also significant. 179 In Toughness and Brittleness of Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.


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TOUGHNESS A N D BRITTLENESS O F PLASTICS

The first quantitative study of deformation mechanisms in ABS polymers was made by Bucknall and Drinkwater, who used accurate extensometers to make simultaneous measurements of longitudinal and lateral strains during tensile creep tests (4). Volume strains calculated from these data were used to determine the extent of craze formation, and lateral strains were used to follow shear processes. Thus the tensile deformation was analyzed in terms of the two mechanisms, and the kinetics of each mechanism were studied separately. Bucknall and Drinkwater showed that both crazing and shear processes contribute significantly to the creep of Cycolac T—an ABS emulsion polymer—at room temperature and at relatively low stresses and strain rates. The same techniques were used in the present work to study the effects of orientation and rubber content upon the creep behavior of rubber-toughened SAN polymers at room temperature. As in previous work, the tests were conducted at low strain rates and were terminated at longitudinal strains between 5 and 6%. Experimental Two ASA polymers were studied: Luran S 757R and Luran S 776S; both were made by BASF. The polymers have similar SAN matrices but respectively contain ca. 30 and 40% of the acrylic rubber-toughening agent. The ABS polymer (ABS 500) was made by the Dow Chemical Co. It contained SAN-filled rubber particles ca. 1.0 /mi in diameter, suggesting that it was manufactured by bulk or suspension polymerization. Dumbell creep specimens with a parallel gage portion 40 mm long and 5 mm wide were milled from compression-molded sheet. Some of the ABS sheets were subjected to uniaxial hot-drawing at 116 °C, and creep specimens were cut parallel to the draw direction. Creep tests were carried out at 20° -b 0.5 °C using high accuracy lever-loading creep rigs developed by Darlington and Saunders (5) (see Figure 1). Longitudinal strain e was measured in the central 20 mm of the specimen, and lateral strain e± was measured simultaneously at the center of the gage portion; d is usually negative in a tensile test. The lateral strain e was not measured. In the calculations all specimens, including those cut from a drawn sheet, were assumed to be transversely isotropic— i.e., ex = e . On the basis of this assumption the volume strain A V / V was calculated from the expression: 3

2

2

AV/V

= (1 + e ) (1 + 3

61)

2

-

1

(1)

Each long-term creep test was preceded by a loading-unloading program at successively increasing loads in order to obtain a 100 sec isochronous curve of tensile creep modulus against a 100 sec longitudinal strain within the low strain region. During this preliminary program stress levels were kept well below those used for long term testing. Long-term tests were terminated when e was ca. 5%; the specimen was then unloaded and allowed to recover. The unloading program was then re3

In Toughness and Brittleness of Plastics; Deanin, R., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

15.

B U C K N A L L

E T

A L .

Toughening Mechanisms

181

Upper Grip Longitudinal


Ex ten so meter

Lower

Figure 1.

Grip

Darlington-Saunders creep apparatus

peated in order to obtain another 100 sec isochronous curve, to illustrate the effects of creep history on the polymer. Results ASA Polymers. Creep tests were performed on Luran S 757R at stresses from 19-26 M N / m . Over this range, the time to reach 5% ex2

Figure 2.

Creep and recovery of ASA polymer Luran S 757R: stress = 20 MN/m 2


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TOUGHNESS

A N DBRITTLENESS

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PLASTICS

tension varied from 12,700 to 40 sec. A typical set of results is shown in Figure 2. The creep rate is low initially but increases at a strain of ca. 2% and then becomes approximately constant. After the initial deformation, the lateral strain — e± changes comparatively slowly, whereas the volume strain A V / V changes in a manner similar to that of the longitudinal strain e .


3

The creep behavior of Luran S 776S is similar to that of 757R, except that creep rates are higher. Tests were made at stresses between 19 and 24 M N / m ; the times to reach 5% extension varied from 4700 to 20 sec. Increased creep rates are to be expected in view of the higher rubber content of the polymer. 2

Elastic and viscoelastic deformation, crazing, and shear band formation all contribute to the tensile creep of rubber-toughened plastics. In order to separate the contributions of these various mechanisms, certain assumptions are needed. In the following analysis, it is assumed that the volume strain measured in the specimen at the time of the first strain determination is caused entirely by the elastic and viscoelastic response of the polymer to the hydrostatic component of stress and that all subsequent volume changes are caused by crazing. The duration of the creep tests is long compared with the time at which the first strain measurements are made, and the observed kinetics of crazing strongly suggest that very little crazing occurs during this initial period. The subsequent time-dependent volume changes are very large and can be explained only in terms of craze formation. Generalized volume relaxation effects, and any volume changes that might be associated with shear band formation, can be neglected. In support of this statement, it should be noted that creep occurs at constant volume in Cycolac T grade ABS under a tensile stress of 26.5 M N / m (4). At this stress, the polymer reaches a strain of 5% by elastic deformation and time-dependent shear processes only, showing no visible evidence of crazing and no further volume change after the initial elastic response to loading. 2

In discussing shear deformation, it is convenient to distinguish between the initial elastic and viscoelastic response of the polymer to the applied load and the subsequent time-dependent response. However, the distinction is somewhat arbitrary and is not as fundamental as that between elastic volume response and crazing. Viscoelastic shear deformation continues throughout the period under load. The observed timedependence of lateral strain reflects both generalized viscoelastic relaxation and shear band formation. Since crazing consists simply of displacement in the tensile stress direction, it makes no contribution to lateral strain; therefore — e specifically measures deformation by shear processes. x


15.

BUCKNALL ET A L .

The most convenient way to analyze the contributions of crazing and shear processes to creep is to plot the volume strain recorded at any given stage of the creep test against the corresponding longitudinal strain as shown for Luran S 757R in Figure 3. The slopes of the lines are 0.85 at 19 M N / m and 0.89 at 22 M N / m , indicating that crazing is responsible for 85-90% of the time-dependent part of the creep. If there were no contribution from crazing, the slope would be zero, whereas if crazing were the only deformation mechanism operating during the period of the measurements, the slope would be unity. Results for Luran S 776S are similar to those obtained for 757R. 2


183


2

2 3 4 LONGITUDINAL STRAIN (%)

Figure 3.

Creep mechanisms in Luran S 757R

Earlier work (4,6) demonstrated that a high gradient in the volume strain-longitudinal strain curve is associated with a large drop in the modulus of the material. This is to be expected, as the crazes have much lower moduli than the material from which they were formed (7). As a result of the creep tests described above, the 100 sec tensile modulus of Luran S 757R at a strain of 0.5% fell from 1.99 to 1.30 G N / m , and the modulus of Luran S 776S at a similar strain fell from 1.65 to 0.98 GN/m . The effect of stress upon the kinetics of crazing can be represented by two rate quantities obtainable from the creep data. The linear portion at the end of the volume strain-time curve defines a maximum rate of 2

2

crazing,-^-^-

max

, and the beginning of this period of rapid creep defines



184

TOUGHNESS A N D BRITTLENESS O F PLASTICS

STRESS

Figure 4.

2

(MNrrf )

Eyring relationships for crazing

an induction period, T . Graphs of the logarithms of the maximum rate and of reciprocal induction period against stress are linear over the range of stresses used. This agrees with the Eyring equation: e = A exp (yva/kT)

(2)

where e is the rate, A is a constant, y is a stress concentration factor, v is the activation volume, a is the stress, k is Boltzmanns constant, and


15.

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E T

A L .

185


T is the absolute temperature. These plots are shown in Figure 4 for both ASA polymers. Both rate quantities give approximately the same value, ca. 3500A , for the apparent activation volume yv for both polymers. A similar graph can be plotted for the maximum lateral strain rate d/dt (— d) (see Figure 5); this gives an apparent activation volume of ca. 3000 A for the shear processes in both ASA polymers. 3


3

1 0

-7|

18

1

20

1

1

22 24 STRESS (MNrrT )

1

1

26

28

2

Figure 5.

Eyring relationships for shear deformation


186

TOUGHNESS

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6

2 1

-

*1

1

0 0

1000

2000

3000

TIME (s)

Figure 6.

Isotropic Dow 500 ABS: creep at 30.0 MN/m recovery

2

and

Isotropic ABS Polymer. Isotropic Dow 500 ABS polymer has the same general creep characteristics as the ASA polymers described above. Some typical results are shown in Figure 6. The longitudinal strain e increases relatively slowly at first, but then accelerates at a strain of ca. 2% and becomes approximately linear with time. Apart from the initial elastic response, most of the deformation is caused by crazing: the volume strain follows the same pattern as the longitudinal strain while the lateral strain remains almost constant throughout the test. The relationship between the volume strain and the longitudinal strain (see Figure 7) reinforces the conclusion that the creep of this grade of ABS polymer 3

STRESS (MNrrT ) 2

320 260

0

0

2 3 4 5 LONGITUDINAL STRAIN

Quantitative Studies of Toughening Mechanisms in ABS and ASA

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