Effect of Rubber Particle Size and Graft Ratio on the Morphology and Tensile Properties of ABS Resins

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1 Materials Science Research International, Vol.6, No.2 pp (2000) General paper Effect of Rubber Particle Size and Graft Ratio on the Morphology and Tensile Properties of ABS Resins Hideki YAMANE*, Zen-ichiro MAEKAWA* and Hajime SAKANO** * Division of Advanced Fibro-Science, Graduate School, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto Japan ** Kotec Corporation Ltd Cho, Chayamadai, Sakai, Osaka Japan Abstract: Tensile properties and the phase of ABS resins with various rubber particle sizes and graft ratios of SAN are examined. Large rubber particles disperse well in ABS resins irrespective of the graft ratio. On the other hand, the graft ratio affects the dispersion state of the small rubber particle, particles with high graft ratio disperse well and those with low graft ratio tend to coalesce into large aggregates. These differences of the morphology strongly alter the deformation mechanism, such as craze formation and cavitation in the rubber particles. The effects of mixing of ABS resins with various graft ratio and particle size on the mechanical properties and phase morphology are also discussed. Key words: ABS resin, Graft ratio, Rubber particle, Morphology, Tensile property 1. INTRODUCTION It had been considered that the reinforcing mechanism of the rubber modified amorphous polymers is that the rubber particles absorb the fracture energy and keep the cracks from growing [1], until electron microscope observations revealed the role of craze formation [2, 3] and void cavitation [4]. Bucknall et al. [5] examined the creep behavior of ABS, high impact polystyrene, and rubber modified PMMA and found that both the macroscopic deformation due to the craze formation and the shear deformation of rubber particles occur during the creep tests. Yee et al. [5] studied the deformation mechanisms of the rubber modified epoxy resins. They found that the rubber particles simply enhanced shear deformation at low strain rates, and that the rubber particles cavitated and subsequently promoted further shear deformation at high strain rates. Among the various mechanical properties of ABS resin, most researches are focused on the effects of rubber content, rubber particle size, and SAN graft ratio on the impact properties. Although the tensile behavior of ABS resins is also important in terms of the practical aspects, there have been a few reports on the tensile behavior of ABS resins or their blends with respect to the rubber particle size and graft ratio. This report deals with the effect of rubber particle size, graft ratio, and the mixing of the ABS resins with various rubber particle size and the graft ratios on the tensile properties and their. 2. EXPERIMENTALS 2.1. Preparation of ABS Resins Preparation of poly (butadiene) particles Poly (butadiene) (PB) latex was prepared in the emulsion polymerization at 65 Ž for 16 `55 hs by using potassium persulfate as an initiator. Various rubber particle sizes were obtained by addition of t- dodecylmercaptan (TDM) as a chain transfer reagent. Conversion was determined to be 95% and the gel fraction was 90%. Particle size of PB was determined by the transmission electron microscope (TEM 1200EX, JOEL) observations of the rubber particles stained in osmium tetraoxide (OsO4). Number average particle sizes were determined from image analyses of the photographs Graft polymerization of styrene/acrylonitrile (SAN) onto the rubber particles Graft polymerizations of acrylonitrile/styrene (SAN) onto the rubber particles were carried out at 65 Ž for 10hs in the presence of t-butylhydroxyperoxide (TBHP) as an initiator and TDM as a chain transfer reagent. The comonomer ratio of acrylonitrile/styrene in load was 27/73 (wt/wt) and the ratio of PB rubber/comonomers was 60/40 (wt/wt). The graft ratio was controlled by the reaction temperature and the amount of TDM added Estimation of the graft ratio The graft ratio of SAN onto PB rubber was defined by the following equation Graft ratio (%)= Amounts of SAN grafted rubber were determined by the following procedure [6]. Products obtained in the graft polymerization was dissolved in a large excess of xylene. Xylene insoluble part was filtered and dried, and then it was again dissolved in a large excess of Received August 13, 1999 Accepted April 25,

Morphology and Tensile Properties of ABS Resins Table 1. Characteristics of four SAN grafted. 3. RESULTS AND DISCUSSION Fig. 1. Schematic diagram of injection molded part. acetone.

2 Morphology and Tensile Properties of ABS Resins Table 1. Characteristics of four SAN grafted. 3. RESULTS AND DISCUSSION Fig. 1. Schematic diagram of injection molded part. acetone. Acetone insoluble part was regarded as SAN grafted rubber. Characteristics of four SAN grafted rubbers A, B, C, and D are listed in Table 1. To examine the effect of mixing of SAN grafted rubber with various particle size and SAN graft ratio, A and C and A and D are blended. These are denoted as AC and AD, respectively Preparation of ABS resins The four products obtained in the graft polymerization and two blends were further melt blended with AS copolymer synthesized separately in the emulsion polymerization (Mn=85000) to be ABS resins with 15 wt% rubber content. These ABS resins were then injection molded into ASTM1 dumbbell samples at 240 Ž for tensile tests, whose dimensions are described in Fig. 1. The samples finally obtained are denoted as ABS-A, ABS-B, ABS-C, ABS-AC and ABS-AD Measurements and Observation Tensile tests Tensile tests of ABS dumbbell samples were carried out using a tensile test apparatus (Autograph AG-1500, Shimadzu) at tensile speeds between 5 `500mm/min at room temperature Observation of phase morphology Phase morphology of the ABS resins was observed with a transmission electron microscope (TEM 1200EX, JOEL). Test specimens were microtomed from the core part of injection molded parts in the plane parallel to the flow direction as illustrated in Fig. 1. These specimens were stained in osmium tetraoxide (OsO4) Density measurements Densities of the stretched and unstretched samples were determined pycnometrically with water at 25 Ž. Changes in volume during the stretching were calculated following the equation: 3.1 Stress-Strain Behavior of ABS Resins Figure 2 shows the stress-strain curves of these ABS samples during stretching. ABS-A and C, which contain the highly grafted rubber particles, show the higher elastic moluli than ABS-B and D. Igawa et al. [7] found that the crosslinking reaction was induced in the poly (butadiene) particle by radicals due to an initiator for the graft reaction. Besides, the rubber particles contain some SAN occlusions at high graft ratio. These factors make the elastic modulus of the rubber particles higher. Higher elastic moduli of ABS-A and C are considered to be due to these hard rubber particles. These ABS samples show the yield points, which increase with the elongation rate, after an initial elastic region and then are stretched in neck deformation. Sample ABS-C has a highest yield strength among four ABS sample, and rest of three samples have similar yield strengths. ABS-C and -D, which have small rubber particles, show the broad yield peaks indicating that the stress concentrated points disperse in these samples. Figure 3 shows the elongation at break of these samples stretched at various elongation rates. There is a general trend that the elongation at break decreases monotonously with increasing elongation rate at low elongation rates. At higher elongation rates, the test condition reaches to impaction rather than to static tensile deformation. Morphological studies revealed that ABS-A and -B have large rubber particles and the small rubber particles in ABS-D agglomerated and acted as large rubber particles. The results of impact tests, which are not described in this paper, indicate these three ABS show good impact properties, and ABS-C which involves well dispersed small rubber particles shows fairly poor Fig. 2. Stress-strain curves of four ABS samples stretched.

Hideki YAMANE, Zen-ichiro MAEKAWA and Hajime SAKANO low strain. With increasing strain, particles are more elongated in the elongation direction.

3 Hideki YAMANE, Zen-ichiro MAEKAWA and Hajime SAKANO low strain. With increasing strain, particles are more elongated in the elongation direction. It is surprising to note that no craze is observed even at the breaking Fig. 3. Elongation at break of ABS samples stretched at various elongation rates. strain. Small rubber particles in ABS-C well adhere to the matrix due to the high graft ratio at a large interfacial area. For this sort of ABS sample, shear deformation of the rubber particles is considered to be a major mechanism of the deformation to the applied stress rather than the craze formation. In sample ABS-D, small particles coalesced and the resulted large aggregates deform in elongation direction. Small number of crazes initiated at the surface of the coalesced rubber particles. At higher strain, small voids are observed in the aggregates. At the breaking strain, most of the rubber aggregates are highly deformed and filled with voids. It is considered that the deformation of the rubber aggregates at a low strain and the formation of crazes and voids at high strain suppress the total fracture of this samples and give rise to the higher elongation at break. impact properties [8]. These facts explain the reason why the elongation at break of ABS-A, -B, and -D increase again at higher elongation rates. ABS-D shows higher elongation at break than other three samples Change in the Phase Morphology during Tensile Deformation Stabenow et al. [9] reported the studied the effect of particle size and graft ratio with AS on the dispersion state of the rubber particles, and found that the small rubber particle with low graft ratio tended to coalesce into aggregates. Figures 4 to 7 show the phase of ABS-A, -B, -C, and -D after stretching at 5mm/min to various strains. Samples ABS-A and -B contain a fairly uniform dispersion of large rubber particles irrespective of the graft ratio. The effect of increasing graft ratio is found to be an increase in the amount of occluded SAN domains in these samples. On the other hand, the graft ratio strongly affects the dispersion state of the small rubber particles. The rubber particles in ABS-C are well dispersed, while those in ABS-D coalesce to form continuous areas of rubber with some indication of individual particles. In samples ABS-A and -B, are similar and many crazes, which initiate at the equators of large rubber particles, are observed even at a low strain ( `5%). At a higher strain, these crazes grow and the voids occur in the rubber particles. In these samples, once crazes occur at the rubber particle surfaces, the tensile stress concentrates on them and the crazes grow with increasing strain. Finally the rubber particles are subject to the shear stress resulting in the occurrence of void cavitation. At the breaking strain, the rubber particles are highly stretched in elongation direction being filled with voids. When rubber particles are small, the effect of graft ratio is significant. Rubber particles in ABS-C are well dispersed and aligned in the elongation direction at a 3.3. Volume Change during Tensile Deformation Figure 8 shows the changes in the volume ƒ V of ABS-A, -B, and C as functions of the tensile strain. ƒ V of ABS-A and -B increase linearly up to the strain about 20% and then level off. As already shown in the previous section, ABS-A and -B already show a lot of crazes and voids even at 5% of strain. TEM observations revealed that crazes become wider with increasing strain and the amount of voids further increases. These reflect the change in the volume of ABS-A and -B. Similar behavior has been reported by Bucknall et al. [5], who found the linear increase in volume of the high impact polystyrene which contain large rubber particles. In such rubber modified plastics, large rubber particles are not subject to the shear deformation but the matrix deforms due to the craze formation. On the other hand ABS-C shows only slight increase in volume up to the strain about 45%. At a lower strain the rubber aggregates deform instead of causing the craze formation in Void formation occurs significantly just before the breaking strain. Since there is no factors to increase its volume before breaking strain, ABS-C shows only slight increase in volume up to high strain Blend of the Rubber Particles with Various Sizes and Graft Ratios Figures 9 and 10 show the change in the of the blend samples ABS-AC and -AD, respectively during tensile deformation, and the mechanical properties of these samples are listed in Table 2. In ABS-AC, because of the high graft ratio with SAN both large and small rubber particles disperse well and small particles are aligned in elongation direction at a low strain. At a higher strain, large particles deform significantly to the elongation direction and include large voids. However there is no indication of crazes

4 Morphology (a) Strain 5% Fig. 4. Phase and Tensile (b) Strain (a) Strain 5% Properties 10% of ABS-A after stretching Fig. 6. Phase (b) Strain 10% 5% (b) Strain of ABS-C after Resins (c) 45% (Breaking to the strains (c) 24% (Breaking 107 to the strains indicated. 10% stretching indicated. (c) 37% (Breaking Fig. 5. Phase of ABS-B after stretching (a) Strain of ABS to the strains indicated.

Hideki YAMANE, Zen-ichiroMAEKAWA and Hajime SAKANO (a) Strain 5% Fig. 7. (b) Strain 10% Phase of ABS-D after (c) 48% (Breaking stretching to the strains indicated.

5 Hideki YAMANE, Zen-ichiroMAEKAWA and Hajime SAKANO (a) Strain 5% Fig. 7. (b) Strain 10% Phase of ABS-D after (c) 48% (Breaking stretching to the strains indicated. even at the breaking strain, although ABS-A shows a lot of crazes at high strain as was already shown in previous section. This is due to the inclusion of small and highly grafted rubber particle and similar mechanism to that of ABS-C can be considered. Because of the lack of the crazes even at high strain, the elongation at break of ABS-AC is very small. On the other hand, small particles in ABS-AD coalesce similarly to those in ABS-D. Since large rubber particles A contain a lot of inclusions of AS forming a salami like structure, they have higher elastic modulus than aggregates of small particles and the aggre- Fig. 8. Changes in the functions volume of the (a) Strain 5% Fig. 9. Phase of ABS strain. samples gates deform easier than large particles. Because of this reason, crazes occur only from the aggregates of small particle and the aggregates deform to the elongation direction at low strain. No craze is observed at the surface of large particles. At a higher strain there are a lot of voids in the aggregates of small particle and in large particles. as (b) Strain 10% of ABS-AC (c) 10.6% (Breaking after stretching 108 to the strains indicated.

Morphology and Tensile Properties of ABS Resins (a) Strain 5% (b) Strain 10% (c) 16.3% (Breaking Fig. 10. Phase of ABS-AD after stretching to the strains indicated. Table 2.

6 Morphology and Tensile Properties of ABS Resins (a) Strain 5% (b) Strain 10% (c) 16.3% (Breaking Fig. 10. Phase of ABS-AD after stretching to the strains indicated. Table 2. Mechanical properties of ABS-AC and-ad. 4. CONCLUSIONS The effects of rubber particle size, graft ratio, and the mixing of the ABS resins on the tensile properties and their are examined. Phase morphology of ABS resins with large rubber particles was not affected by the graft ratio significantly. In such ABS resins, crazes occur at low strain and then shear defomation of the rubber particles occur at higher strain followed by the cavitation in the rubber particles. When the rubber particles are small, particles with high graft ratio disperse well in ABS resin, while those with low graft ratio coalesce into large aggregates. Strain dependence of the volume is strongly related to the occurrence of the crazes and cavitation in the rubber particles. ABS resins with large rubber particles tend to make a lot of crazes during tensile deformation and show a large linear increase in volume, while shear deformation of the rubber particles is dominant in ABS resin with slightly grafted small particles and the volume change is rather small. REFERENCES 1. E.M. Merz et al., J. Polym. Sci., 22 (1956) M. Dillon and M. Bevis, J. Mat. Sci., 17 (1982) M. Dillon and M. Bevis, J. Mat. Sci., 17 (1982) Y. Hung and A.J. Kinloch, J. Mat. Sci., 27 (1992) C.B. Bucknall, H. Ogura, and I. Narisawa, J. Macromol. Sci., Phys., B-19 (1981) B.D. Gesner, J. Poly. Sci., A3 (1965) K. Ikawa, H. Sakano, S. Tamai, and H. Kojima, Kobunnshi Ronbunshu, Vol.54, No.3 (1997) H. Sakano, Ph. D. Dissertation, Kyoto Institute of Technology (2000). 9. J. Stabenow and F. Haf, Ang. Makromol. Chem. 29/30 (1973)

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