July 13, 2003/ This paper compares the modification mechanism provided by ethylene-octene (EO) copolymer to that of ethylene-propylene-diene terpolymer (EPDM) rubber. Within the limits of this study, the highest impact strength was achieved at 30-40% rubber content, regardless of the rubber type. An increase in rubber melt viscosity resulted in overall greater impact strength. At the optimum concentration, the high-viscosity (MFI = 1 to 5) EO rubber provided modification mainly via a crazing mechanism, while the EPDM rubber functioned by energy dissipation through the three-dimensional network structure formed with the polypropylene matrix. This paper also discusses the effects of the processing conditions on the physical properties of PP/EPR copolymer. An increase in processing temperature and screw speed resulted in a reduced number of discrete rubber particles and nearly no or very slight increase in impact strength, but a very significant reduction in tensile strength and tensile modulus.

INTRODUCTION

Polypropylene is widely used in various applications requiring good impact properties. High impact strength and ductility are typically achieved by chemical modification (copolymerization) or by mechanical blending with a number of elastomers. Most commonly used are EPR and EPDM and most recently other polyolefin elastomers were introduced, including ethylene-butene, ethylene hexene and ethylene- octene metallocene catalyzed rubbers. Many studies were done to understand the mechanism of PP impact modification and fracture mechanics, but there is much disagreement on this subject. While some authors claim that shear banding is the main mechanism (1-3), others report that cavitation and crazing are prevalent in impact modified PP (4). The effects of rubber particle size, melt flow index (2, 4-7), crystallinity (7, 8), and dynamic crosslinking (1) on performance of the rubbers in polypropylene are also discussed in the literature.

It is well known that the processing conditions including temperature, shear rate and other variables, significantly effect morphology and properties of polymeric materials, such as PVC, ABS, and PS. However, very little information is available for impact modified polypropylene. It was reported in the literature, that the extrusion process results in polypropylene orientation, that an increased shear rate can result in re-agglomeration of the rubber particles (9), increased impact properties but no effect on tensile strength and modulus (10). However, temperature effects were not considered in the above studies.

The purpose of this paper was to gain a better understanding of the polypropylene modification mechanisms with various rubbers and effect of process temperature on its properties.

For evaluation of relative modifier efficiency and analyses of the fracture morphology, PP/rubber blends were prepared on the Brabender laboratory twin screw extruder fitted with the 4-inch x 0.020-inch strip die. The extrusion was conducted at the temperatures 190[degrees]C to 199[degrees]C and 15 RPM screw speed. For the study on the effect of heat history on physical properties of PP copolymers, they were extruded on the same equipment but at two different conditions: low heat history-at 185[degrees]C, 10 RPM screw speed, and high heat history-at 220[degrees]C, 15 RPM.

Then the extruded strips were plied to achieve desirable thickness of 3.2 mm for physical testing, and compression molded at the same melt temperature as the previous heat history.

The compression-molded plaques were tested for hardness, ASTM D2240; room temperature Izod impact, ASTM D 256; and tensile properties, ASTM D 638.

Scanning electron microscopy (SEM) analyses of the fracture surfaces were used to examine the samples and their fracture morphology.

RESULTS AND DISCUSSION

Figure 1 shows the effect of ethylene-octene (EO) and ethylene propylene diene terpolymer (EPDM) rubber concentration on Izod impact of PP/rubber blends. Within the limits of this study, the Izod impact strength and ductility of all polypropylene/rubber blends increased with an increase in rubber concentration. However, this increase reached a limit beyond which further increase in rubber content resulted in a reduction of Izod impact. The maximum Izod impact strength was achieved at approximately 30-40% rubber content, which is consistent with earlier findings (1). In addition, the EO rubbers of lower melt flow index (higher melt viscosity) demonstrated greater impact strength and overall greater modifying efiiciency at any concentration. Finally, the impact performance of EO rubber (MFI = 5) and EPDM rubber of similar melt viscosity (MFI = 10) appeared to be also similar.

Izod impact strength as a function of rubber content for PP/EO and PP/EPDM blends.

SEM of the freeze fracture surface of PP/EO blends. a) 15% EO, Izod = 6 J/cm. Shore D hardness = 70. b) 30% EO, Izod = 8.4 J/cm, Shore D hardness = 66. c) 60% EO, Izod = 7.8 J/cm, Shore D hardness = 46.

SEM analyses were performed to better understand the rubber behavior in polypropylene. Figure 2 shows a PP/EO blend at various rubber concentrations. It clearly demonstrates that an increase in rubber content caused a change in the blend morphology. At low concentrations (15%), the rubber was present in the form of discrete particles (Fig. 2a). As the concentration increased to 30%, the number of rubber particles increased (Fig. 2b), thus resulting in higher impact strength. At high, above optimum, rubber concentration (60%), however, the blend morphology changed from the discrete rubber particles dispersed in the PP matrix to the co-continuous rubber-PP phase (Fig. 2c). This resulted in reduced surface hardness, greater flexibility at room temperature and thus easier bending of the sample (lower energy required for deformation) in the Izod test with no failure. These findings generally agree with the literature (5, 11, 12).

In order to understand modification mechanism, further SEM analyses of the Izod fracture surface were performed on the samples under study.

SEM of the Izod impact fracture of 70% PP/30% EO. (Rubber MFI = 1 g/10 min, Izod impact = 8.7 J/cm.)

Morphology of PP homopolymer, containing 30% high viscosity ethylene/octene rubber and having excellent Izod impact and ductility. One can clearly see the 0.5 to 3 micron rubber particles, dispersed in the polymer matrix during processing (Fig. 3a). Adhesion of the rubber to the matrix is somewhat limited, as indicated by the cavities formed in the matrix upon freeze fracture in sample preparation for microscopy. Next, Figs. 3b and 3c, where the micrographs of the Izod fracture surface and the surface perpendicular to the fracture surface were taken midway along the crack, clearly show cavitation and void formation in the polymer matrix. There is clear evidence that these changes in polymer morphology are caused by separation of the rubber particles from the matrix and their deformation. Figures 3d and 3e show extensive crazing, where the fibrils pulled out from the matrix across the crack are severely torn (fibrils sticking out from the fracture surface). The crazes are formed not only across the crack, but also in the direction perpendicular to the crack (long crazes with fibers across the opening not only on the fracture surface but also going deep into the matrix). The presence of many voids and/or crazes is also evident from the appearance of the stress whitening in the Izod bars. The beginning of craze formation (elongation of the material in the direction across the opening and the crack formation perpendicular to the opening) is also very evident in Fig. 3[function of] of the fracture surface closest to the tip of the crack, where the speed of crack propagation is much reduced.

Based on the above SEM analyses of the fracture surfaces, we developed a schematic representation of fracture mechanism in high viscosity PP modified with high viscosity E/O rubber (Fig. 4). When the PP/rubber blend is under the impact stress, rubber particles deform and tend to separate from the PP matrix. Deformation of the rubber particles results in fiber and void formation. At a high rate of crack growth, such as at the beginning of the crack, the newly formed fibers break allowing further crack growth. As the crack growth rate decreases as a result of energy absorption, fewer fibers are torn, and instead, the crazes, where the fibers are pulled out from the matrix across the crack opening, are formed. This crazing fracture mechanism is quite common for many plastic materials, such as PVC (13), HIPS (14), and polyolefins (15, 16).

Crazes absorb much energy by creation of new surfaces, and therefore are responsible for high impact strength and ductility of PP modified with EO rubber under this study. Of course, the modification mechanism and rubber modifying efficiency will depend on its affinity to and degree of dispersion in the polymer matrix, presence of the discrete rubber particles and particle size as shown in Fig. 5. Here, low viscosity, high melt flow index rubber is better dispersed in the PP and has much smaller particle size than the high viscosity rubber, but still is present in the form of discrete particles.

The claims in the literature are that smaller rubber particles are beneficial for improving impact properties, because instead of crazing, theycause more efficient modification mechanism of shear banding (2, 4). However, in contrast to these reports, our study showed lower impact properties with low viscosity smaller particle size EO rubber, no evidence of shear banding or crazing, but rather small cavities and voids. This absence of energy absorbing crazing and/or shear banding explains its lower impact modifying efficiency.

SEM of Izod impact fracture surfaces of 70% PP/30% EO blends. (Rubber MFI = 30 g /10 min., Izod impact = 2 J/cm.)

Schematic representation of the impact fracture mechanism for EO rubber modified PP.

The Izod fracture morphology of PP homopolymer modified with 30% EPDM rubber and having high impact strength. Unlike PP/EO blend, which had stress whitening under stress, caused by crazes, PP/EPDM blend did not have any stress whitening. In this case, the crosslinked EPDM rubber is well dispersed in the PP and under stress forms the three-dimensional network structure with the PP matrix. Some voids are formed, but they are very small, and the crazing, if any, is minimal. These results are in agreement with the previous findings (3). The SEM pictures indicate that the energy dissipation and possibly shear banding mechanism is more likely in this case. When impacted, the energy is dissipated down the branches of the network structure. The crack, if formed, grows along the branches until it cannot travel any more in that direction. Then it changes direction and travels down another branch. Thus, the crack can propagate through the material in the zigzagging manner only until the material breaks. This mechanism of crack propagation and shear banding appears to be responsible for good impact performance of PP/EPDM blends.

SEM of the Izod impact fracture surface of 70% PP/30% EPDM blend. (Rubber MFI = 10 g/10 min., Izod impact = 7 J/cm.)

Effect of processing temperature on physical properties of PP impact copolymers

The effect of processing temperature on fracture morphology of PP/ EPR copolymer. As in the case of PP/E/O blend, the rubber phase in the copolymer is in the shape of the discrete rubber particles. At higher processing temperature, the number of the rubber particles decreases, indicating greater rubber solubility in the polymer matrix. This may be due to material orientation during the extrusion process. Lower extrusion temperature is closer to the crystallization point in PP. Thus, the extrudate retains frozen-in molecular orientation to a greater degree than the higher melt temperature. A highly oriented polymer matrix may result in lower rubber solubility, preserving discrete rubber particles in the PP matrix.

Polymers with a wide range of melting temperatures, such as PVC, typically have greater ductility, impact strength and tensile elongation at higher temperatures. This is due to improved fusion between particulate melt flow units (17). In contrast, these properties of PP/EPR copolymers are not significantly affected by processing conditions (Table 1). This is because in polypropylene, which has a fairly sharp melting point, the melt is continuous and the fusion is of no consideration. This study also showed that an increase in processing temperature of the PP/EPR copolymer resulted in a very significant reduction in tensile modulus and tensile strength, believed to be due to an increased rubber solubility in PP matrix.

SEM of the extrudate fracture surface for PP/EPR copolymer. Effect of process conditions on fracture morphology.

Effect of Process Conditions on Physical Properties of the PP/25% EPR Copolymer.

CONCLUSIONS

Within the limits of this study the following observations and conclusions are made:

* As the rubber concentration increases the impact strength of polypropylene homopolymers or copolymers increases. The optimum rubber content for achieving the highest impact strength is 30-40%. Higher concentration results in reduced stiffness and lower impact strength due to easier deformation.

* SEM analyses of the PP blend morphology helped provide an understanding of impact modification and fracture mechanisms. Generally, the efficient EO rubber modifier is present in the form of discrete 0.5 to 3 [mu]m diameter rubber particles, dispersed in PP matrix. It improves impact properties by partial de-bonding, cavitation, voiding, deformation, and eventually extensive crazing. This mechanism absorbs much energy and therefore is responsible for good impact properties.

* An increase in melt viscosity (reduction in melt flow index) of EO rubber results in an increases impact modifying efficiency.

* EPDM rubber is more compatible with the polypropylene than EO rubber. The modification mechanism of EPDM rubber is mainly energy dissipation through the three-dimensional network structure formed with the PP matrix.

* An increase in processing temperature and shear rate results in greater rubber solubility, reduced tensile strength and tensile modulus, but has insignificant effect on impact properties of PP/ EPR copolymer.

ACKNOWLEDGMENTS

We thank PolyOne Corporation for support and for permission to publish this work.

REFERENCES

1. A. K. Jain et al., J. Appl. Poly. Sci., 78, 2089-2103 (2000).

2. A. Van der Wal and R. J. Gaymans, Polymer, 40, 6067-6075 (1999).

3. S. H. Jafari and N. K. Gupta, J. Appl. Poly. Sci., 78, 962- 971 (2000).

4. B. Z. Jang, Polym. Eng. Sci., 25:10, 643 (1985).

5. J. Karger Kocsis and A. Kallo, Polymer. 25:2, 1480-1484 (1984).

6. K. C. Dao, Polymer, 25:10, 279-286 (1984).

7. T. C. Yu, ANTEC '95. 2374-2385 (1995).

8. T. Inoue and T. Suzuki, J. Appl. Poly. Sci., 56, 1112-1125 (1995).

9. S. Danesi and R. S. Porter, ANTEC '78. 240-242 (1978).

10. S. Paul and D. D. Kale, J. Appl. Poly. Sci., 76, 1480-1484 (2000).

11. N. K. Gupta et al., J. Appl. Poly. Sci., 78, 2104-2121 (2000).

12. A. Lucia et al., J. Appl. Poly. Sci., 75, 692-704 (2000).

13. J. W. Summers et al., Polym. Eng. Sci., 20:2, 155 (1980).

14. C. B. Bucknall, Brit. Plast., 40, 84 (1967).

15. J. Lu et al., J. Appl. Poly. Sci., 76, 311-319 (2000).

16. Y. Liu et al., Polymer, 38:11, 2797-2805 (1997).

17. E. B. Rabinovitch and J. W. Summers, J. Vinyl Tech., 2:3, 165 (1980).

ELVIRA B. RABINOVITCH*1, JAMES W. SUMMERS**1, and GREG SMITH2

1PolyOne Corporation Avon Lake, Ohio 44012

2Polymer Diagnostics Inc. Avon Lake, Ohio 44012

*Current address: 31925 Tracy Lane, Salon, OH 44139.

*Current address: 29751 Wolf Road, Bay Village,