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Mechanical properties and deformation behaviors of acrylonitrile-butadiene-styrene under izod impact test and uniaxial tension at various strain rates.

INTRODUCTION

Acryionitriie-butadiene-styrene (ABS) resin is a rubber-toughened thermoplastic, the dispersed rubbery phase is poly-butadiene (PBD), and the continuous rigid phase is acrylonitrile-styrene copolymer (SAN). PBD particles are grafted with SAN to achieve the necessary interaction with the SAN matrix. The rubber toughening of polymer alloys has long been studied. Investigations on deformation yield and fracture mechanisms in rubber-toughened polymers have mainly concentrated on the analysis of the interactions between dispersed-phase domains and the deformation processes occurring in the continuous phase. Macroscopically, the toughness of the rubber-toughened polymers is affected by the matrix toughness (1), composition and concentration of rubber (2), rubber particle size and particle/matrix adhesion, and the matrix ligament thickness (matrix thickness between neighboring rubber particles) (3). However, because rubber-toughened materials are viscoelastic, they generally exhibit a geometry and strain-rate dependent mechanical properties such as fracture toughness and yield stress, which has to be kept in mind when using data for design. Leevers (4) presented a theory of impact and dynamic fracture in semicrystalline polymers that correlates impact resistance to bulk material properties. He concluded that there appears to be no mechanism that is truly independent of geometry or strain. Jar and Wu (5) studied the toughne (6) studied the mechanical deformation of two high-impact polystyrene samples with different average rubber particle size under uniaxial tension at various strain rates. They concluded that crazing is the dominant deformation mechanism for high impact polystyrene (HIPS) blends, regardless of the strain rate, and the difference in capability to initiate crazes is a dominant factor for the significantly different fracture energy of the two HIPS at the highest strain rate.

The objective of this paper is to investigate the mechanical properties and deformation behaviors of ABS under Izod impact condition and tension at various strain rates, Microstructure of stress-whitening zone in ABS blends is observed by using transmission electron microscope (TEM) and scanning electron microscope (SEM) to find the correlation among the mechanical properties, deformation mechanisms, and the test conditions.

EXPERIMENTAL

Materials

Two kinds of PBD latex with the average latex particle sizes of 92 and 325 nm were supplied by Jilin Chemical Company, China. PBD-g-SAN impact modifiers were synthesized by grafting styrene and acrylonitrile monomers onto those PBD latex rubber particles by seeded emulsion polymerization.

The SAN resin, which contains about 25 wt% AN was also supplied by Jilin Chemical Company, China. [M.sub.w] and [M.sub.n] for SAN resin are 148,000 g/mol and 49,400 g/mol, respectively.

ABS blends with a constant rubber concentration of 15 wt% were produced by compounding these PBD-g-SAN impact modifiers with SAN resin in a twin-screw extruder at 220[degrees]C, The samples with small and large rubber particles are referred as ABS-S and ABS-L, respectively. The characteristics of the two ABS blends are summarized in Table 1.
TABLE 1. Compositions of ABS blends.

                                                    ABS-S  ABS-L

Average PB latex particle size (nm)                   92    325
PB concentration in PB-g-SAN impact modifier (wt%)    30     60
St/AN in PB-g-SAN impact modifier (wt%)             75/25  75/25
PB concentration in ABS blend (wt%)                   15     15


Examination of Mechanical Properties of ABS Blends

The dimensions of all the specimens obtained by injection molding for notched Izod impact strength test were 63.5 mm x 12.7 mm x 6 mm. The test was conducted on XJU-22 Impact tester (Chengde Puhui testing equipment manufacturer Co., Ltd.) at 23[degrees]C in accordance with ASTM D256.

All materials for tensile test were injection-molded into dumb-bell type specimens whose dimensions of the parallel part were 60 mm in length with a cross-section of about 12.90 x 2.90 [mm.sup.2]. Tensile tests were conducted on AGS-H 5kN Electrical Testing Machine at various strain rates from 2.564 x [10.sup.-4] [S.sup.-1] to 1.282 x [10.sup.-1] [S.sup.-1] at 23 C in accordance with ASTM D638.

Morphology

A JEM-2000EX TEM operated at 200 kV was used to study the deformation mechanisms of ABS blends under Izod impact and tensile tests. The samples were ultromicrotomed from the stress-whitening zone and then stained with [OsO.sub.4] solution for 8 h before observation.

Plastic deformation behaviors in ABS samples were observed by SEM on machine JSM-5600. The specimen was prepared by cryogenically splitting the impact and tensile tested ABS blends, and the cryogenic fracture surface was perpendicular to fracture surface and passed through the stress-whitening zone.

RESULTS AND DISCUSSIONS

Mechanical Properties of ABS Blends

The typical stress-strain curves of ABS-S and ABS-L blends under various tensile strain rates are shown in Fig. 1. To distinguish the curves, the starting strains are shifted horizontally to 0%, 4%, and 8% in the order of increasing strain rate, respectively. For the two samples, yield stress increases with the strain rate, as expected. The Eyring equation has been found to describe the yield stress variation with strain rate for a number of glassy polymers (7). The Eq. 1 is given by

[epsilon] = A exp {[[-([DELTA]H - [sigma][upsilon])]/kT]} (1)

[FIGURE 1 OMITTED]

where [epsilon] is the strain-rate, [sigma] is the applied stress, [upsilon] is the activation volume, T is the absolute temperature, k is Boltzmann's constant, A is a constant, and [DELTA]H is the activation energy for the process. The above equation can be rearranged to

[[sigma].sub.y] = [[DELTA]H/[upsilon]] + [kT/[upsilon]]ln([[epsilon]/A]) (2)

From the equation we can see that the yield stress increases linearly with the increase of the logarithmic strain rate, provided that [DELTA]H, [upsilon], and T are constant. The relationship between the yield stress and strain rate of ABS-S and ABS-L blends is shown in Fig. 2. It approximately follows the trend that the plot of yield stress versus strain rate at a constant temperature gives parallel straight lines. Besides, the activation volumes are also obtained from the slopes of the curves and the results are listed in Table 2. It is obvious that there is a tendency of increase in activation volume with the increase of rubber particle size.
TABLE 2. Activation volume for ABS blends.

Code of the ABS blends  [[Angstrom].sup.3] Activation volume

        ABS-S                           1235
        ABS-L                           2173


[FIGURE 2 OMITTED]

Compared with ABS blend containing small rubber particles, the sample containing large rubber particles yields at a slightly lower stress, which is probably because of the large volume fraction of rubbery phase. The original volume fraction of the rubber particles with different size is similar. After the grafting reaction of styrene and acrylonitrile onto PBD latex rubber particles to prepare PBD-g-SAN impact modifiers, the occlusion in the large-sized rubber particles caused by the internal grafting, which will be shown by TEM, enhances its effective volume fraction. On the contrary, there's no occlusion in the small-sized rubber particles. According to the Ishai-Cohen model (8), the tensile yield stress, [[sigma].sub.yt]([PHI]) of a composite containing a volume fraction, [PHI], of low modulus inclusions can be expressed as follows:

[[sigma].sub.yt]([PHI]) = [[sigma].sub.yt](0)(1 - 1.21 [[PHI].sup.[2/3]]) (3)

where [[sigma].sub.yt](0) is the yield stress of the matrix. Applying this model to ABS polymer, in this case [PHI] is the effective volume fraction of rubber particles, we can see that the increased volume fraction of rubber phase resulted from the increased internal grafting and decreases the yield stress of the ABS polymers. On the other hand, the sample containing large rubber particles yields at a slightly lower stress than the sample containing small rubber particles, which can be explained by the deformation mechanisms. It is suggested (9) that craze may be preferentially initiated at the occlusion-rubber interface in materials containing highly occluded rubber particles compared with small rubber particles without occlusions. That is, the rubber particles can be deformed by a relatively lower stress, so that craze may be initiated at the occlusion-rubber interface at a lower stress (10). This issue on deformation mechanisms will be discussed further in the following sections by analyzing the morphology and deformation behaviors of ABS.

Results of Izod impact strength are shown in Fig. 3. The toughness for ABS-S and ABS-L blends is ranked in the order of ABS-S < ABS-L (39.7 and 202.8 J/m, respectively). It is very apparent that the variation of impact strength is inconsistent with that of the activation volume. That is, the larger the activation volume is, the tougher the ABS blend is. Toughness of rubber-modified polymers is dependent on rubber particle size (1), (11), dispersion of rubber particles in the matrix, the volume fraction of the rubbery phase (8), (12), and so on. From the results it can be concluded that compared with the rubber particles with the diameter of 325 nm, small rubber particles with the diameter of 92 nm cannot toughen SAN resin, and consequently results in the occurrence of brittle fracture and absence of stress-whitening zone. Many researchers have concentrated upon the effect of rubber particle size on the tougheness of glassy polymers. And a significant body of work has convincingly established that the particle size of the modifier must lie within an optimum range to toughen a given matrix (13-16).

[FIGURE 3 OMITTED]

Morphology and Deformation Behaviors of ABS Blends

TEM micrographs of ABS-L under impact condition are shown in Fig. 4. The internal cavitation of rubber particles can be seen from the pictures. Crazes are formed not only nearby but also away from the equator of the rubber particles, and the propagation direction of crazes is nearly orthogonal to the deformation direction of the rubber particles. The importance of voids formed within the rubber particles in rubber-modified polymers has long been the focus of discussion (17-25). Bucknall et al. (26) suggested that void formation is an essential prerequisite for crazes. Okamoto et al. (27) also emphasized the role of internal rubber particle cavitation. They proposed that the deformation sequence of HIPS is crazing, followed by particle cavitation. The same result has been obtained by Takaki et al. (28) who studied the deformation behavior of rubber-toughened poly(vinyl chloride). But for rubber-toughened polycarbonate of which matrix deformation is dominated by shear yielding, cavitation was suggested to occur first (29). Based on the results shown in Fig. 4, it is conceivable that crazes are initiated first in the matrix in the stress field of the rubber particles according to the original model by Argon (30), (31). But the centers of craze initiation cannot open to macroscopic crazes unless the adjacent rubber particles form voids to compensate for the dilatation of the craze. That is, crazes and internal cavitation of rubber particles are synergistic processes. On the one hand, rubber particle, which acts as stress concentrator initiates crazes, and on the other hand rubber particle promotes the growth of craze by the process of cavitation.

[FIGURE 4 OMITTED]

Freeze fracture technique is a useful method to study the rubber particle cavitation of ABS (32). Figure 5 shows the SEM micrographs of the ABS-L under Izod impact test. The information derived from the micrographs taken at various locations suggests that far away from the fracture surface shown in the right picture no rubber particle cavitation is visible. In the region near the fracture surface, shown in the left picture, the rubber particles have cavitated. The cavitation occurs inside the rubber particles as well as in the boundary between the matrix and rubber particles. Several voids and cavities in the region near the fracture surface are much larger than the original rubber particles. It implies that coalescence of neighboring voids has occurred, which may be initiated by individual rubber particle.

[FIGURE 5 OMITTED]

For ABS, it is well known that the deformation mechanisms include crazing (33), (34), shear yielding (35), cavitation of rubber particles, and the induced yielding in matrix (36). Combining the results of TEM and SEM obtained here, it is found that craze, cavitation of rubber particles, and the induced shear yielding in SAN matrix coexist in ABS-L blend under impact condition. It has been found (21) that rubber particle cavitation is influenced by the bulk modulus of the rubber whose Poisson's ratio is close to 0.5. Because of the high bulk modulus of rubber particles, small deformation can produce a high biaxial stress state in the rubber particles, thus encouraging the cavitation of rubber particles, consequently enhancing the shear deformation in the matrix and the toughness (37). In many rubber-modified polymers such as polyamide 6 (38), PC (39), (40), PVC (41), and epoxy (42) cavitation of rubber particles also happens when they are submitted to a triaxial stress state. Besides, Li et al. (43) have found that for rubber-modified epoxy, fracture toughness can not be improved if the cavitation of rubber particles is suppressed. Besides, compared with TEM, more cavitated rubber particles can be observed by SEM. Therefore, it can be inferred that when the sample is ultromicrotomed, contraction of rubber particles occurs, which leads to the disappearance of voids within the rubber particles.

Figures 6 and 7 summarize the SEM micrographs of ABS-S and ABS-L blends under tensile test at various tensile rates. For ABS-L tested at the strain rate of 2.564 x [10.sup.-4] [S.sup.-1] and 1.282 x [10.sup.-1] [S.sup.-1] shown in Fig. 7, the rubber particle cavitation is visible and the cavitated rubber particles are distorted in the loading direction, indicating that the shear deformation occurs in the matrix between the cavitated rubber particles. As the fracture surface is approached, the cavitated rubber particles deform strongly. Different from ABS-L, ABS-S shown in Fig. 6 only exhibits shear yielding in the matrix and little rubber particle cavitation, which probably results from individual large rubber particles. It indicates that small rubber particles with the diameter of 92 nm cannot initiate cavitation. Many studies have shown that rubber particle size is one of the main factors in controlling rubber particle cavitation (21), (34), (44). These tests are based on the blends containing rubber particles with a range of sizes and show that voids form preferentially in the larger particles.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The TEM micrographs of the ABS-S and ABS-L under tensile test are shown in Figs. 8 and 9. In ABS-S, as seen in Fig. 8, regardless the strain rate, no craze has occurred. However, elongation of rubber particle along the direction of stress and shear yielding in the matrix can be observed obviously. The higher the strain rate is, the stronger the shear yielding is, which implies that high tensile strain rate promotes the shear yielding rather than craze. Different from ABS-S, a lot of crazes and rubber particle cavitation can be found in ABS-L shown in Fig. 9. It indicates that compared with small rubber particles, rubber particles with large size can initiate crazes and voids formation within rubber particles more easily. Another fact illustrated by the pictures is that more crazes are formed when ABS-L is under low strain rate. Breuer et al. (45) noticed that the contribution of crazes was reduced with the increase of the strain rate of testing. The development of craze, which requires chain scission and/or slippage via chain disentanglement for fibrillation to occur, will take place over a longer time scale. Thus, craze will be suppressed with the increase of strain rate.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Under impact conditions, chain scission is likely to become important (37), thus it can be speculated that crazing will be favored and voiding and fibrillation will be aided by the scission processes rather than by viscous disentanglement via repetition, as occurred in slow strain-rate tests such as tensile test. According to this, it can be understood easily why many crazes are formed in ABS-L when it is under impact condition, shown in Fig. 4.

As to ABS-S under impact test, although shear yielding happens, crazes as well as rubber particle cavitation are suppressed, which results in the occurrence of brittle fracture and absence of stress-whitening zone.

By comparing the micrographs of TEM, it is found that rubber particles are deformed more strongly in SEM. So one more issue, which can be clarified here, is that craze closure has occurred either because of the relaxation of unloaded sample or the sectioning. Thus, we conclude that neither TEM nor SEM can enable us to study the deformation mechanism of ABS blends as effectively as the two methods in combination.

CONCLUSIONS

Mechanical properties and deformation behaviors of ABS blends under Izod impact test and tensile test at various strain rates were studied. The tensile test indicates that compared to the sample with small rubber particles, the ABS blend with large rubber particles yields at a slightly lower stress. The impact test shows that small rubber particles with the diameter of 92 nm cannot toughen SAN resin. SEM and TEM results indicate that as for ABS blend containing large rubber particles, the deformation mechanisms, which include crazes, rubber particles cavitation, and shear yielding in the matrix, do not change with the strain rate. As far as ABS blend containing small rubber particles is concerned, only shear yielding of the matrix has occurred.

SEM and TEM are two useful methods to study the deformation mechanisms of ABS blends. Crazes can be found by TEM, and rubber particles cavitation can be observed by SEM. But only combination of TEM and SEM does make the deformation behaviors more conceivable.

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Correspondence to: X.Y. Xu; e-mail: xu_xinyu@hotmail.com

Contract grant sponsor: National Natural Science Foundation of China.

Published online in Wiley Online Library (wileyonlinelibrary.com).

[C]2011 Society of Plastics Engineers

X.Y. Xu, (1) X.F. Xu (2)

(1) Department of Polymer Materials and Engineering, School of Chemistry and Materials Science, Liaoning Shihua University, Fushun, Liaoning, People's Republic of China

(2) CREPEC, Departement of Chemical Engineering, Ecole Polytechnique, Montreal, Quebec, Canada

DOI 10.1002/pen.21908
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Author:Xu, X.Y.; Xu, X.F.
Publication:Polymer Engineering and Science
Date:May 1, 2011
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