One of the most successful outcomes of polymer blends is the improvement in impact resistance of polypropylene (PP) at room and low temperatures, generally achieved by incorporating an elastomer into the PP matrix. One way the incorporation of the rubbery phase in PP can be achieved is by mechanical blending with elastomers.

Thermoplastic elastomers such as styrene-butadiene-styrene (SBS) or styrene-ethylene-butylene-styrene (SEBS) have higher service temperatures for plastic manufacturing processes. Blends of recycled polypropylene with SEBS or SBS can be designed specifically for the purpose of re-establishing the impact resistance of recycled PP, discussed in the study below.

Experimental

Recycled polypropylene (RPP) and elastomers (Table 1) were pre-blended in an intensive mixer, fed into a 25-mm co-rotating twin-screw extruder, and mixed at 250 rpm. The temperature profile of the extruder zones was controlled at 160°C, 180°C, 200°C, and 220°C.

The granules obtained from extrusion were used for melt flow index tests and were fed into an injection molding machine to mold plates for impact tests. Gardner impact-resistance testing was performed using the falling dart impact procedure (ASTM D-5420-04). The dimensions of the impact specimens were 90 mm x 55 mm x 2 mm. The tests were performed at room temperature (25^oC).

Melt flow index measurements were performed in a melt flow index tester at typical polypropylene test conditions (230^oC and 2.16 kg). Further morphological analysis by scanning electron microscopy was performed in tested compound samples.

Results

The Gardner impact resistance of RPP in blends with different SBS and SEBS elastomers is shown in Figure 1. Low-viscosity SEBS and medium viscosity SEBS achieved the best impact resistance in the polymer blends.

Melt flow properties of RPP blends with SBS and SEBS showed 10% decreased melt flow when the recycled polymer was blended with elastomers, except with the use of linear SBS, low viscosity. However, the melt flow rate obtained at these rubber contents are suitable for typical PP injection processes, due to minimum melt flow values for injection molding of about 30 g/10 min. under these conditions.

Scanning electron microscopy (SEM) was carried out with a resolution of 3 nm (high vacuum mode) and 4 nm (variable pressure mode), with an acceleration voltage of 0.5-30 kV and variable pressures of 1 to 270 Pa, using a detector of secondary and backscattered electrons, an X-ray detector, and the image processing system Scandium.

SEM images of the RPP/low-viscosity SEBS mix show greater homogeneity, and a compacted and defined structure. SEM images of the RPP/low-viscosity SBS mix show a rough surface, with agglomerates of different sizes and shapes (Figure 2).

Theory & Conclusions

The incorporation of rubber particles within the matrix of brittle plastics improves their impact resistance. The degree of impact resistance depends on the quantity of rubber incorporated in the method to form the polymer blend. The higher the rubber content of the system, the higher the impact resistance.

When a material undergoes an impact force, micro-fissures are developed until it fractures. When elastomeric low-dimension particles are uniformly distributed in the polypropylene matrix, the probability of small particles encountering the micro-fissures is higher, preventing their propagation and therefore improving the impact strength. The compatibility of materials is a key factor for obtaining a good blend with excellent homogeneity. 

The presence of ethylene-butylene blocks in SEBS makes it compatible with polyolefins such as PP. This differentiation in terms of impact strength is significantly improved in SEBS/RPP mixtures versus SBS/RPP mixtures. The high degree of phase compatibility for the SEBS polymer type results in relatively high impact resistance. 

Low molecular weight SEBS is more effective in increasing the impact strength of SEBS/PP blends than the high molecular weight SEBS. The increase in the impact strength of the composites with increasing low molecular weight SEBS elastomer content is higher than the increases from low, medium, and high SBS elastomers, due to the formation of a better-dispersed matrix.

References

1. Ajji, A. and Utracki, L.A. “Interphase and compatibilization of polymer blends.” Polymer Engineering & Science, 36(12), 1574–1585, 1996.
2. Duell, J.M. “Impact testing of advanced composites.” Advanced Topics in Characterization of Composites. Trafford Publishing, pp. 97-112, 2006.
3. Hong, B. K. and Jo, W.H. “Effects of molecular weight of SEBS triblock copolymer on the morphology, impact strength, and rheological property of syndiotactic polystyrene/ethylene propylene rubber blends.” Polymer 41(6), 2069–2079, 2000.
4. Jang, B.P, Huang, C.T., Hsieh, C.Y., Kowbel, W., and Jang, B.Z. “Repeated impact failure of continuous fiber reinforced thermoplastic and thermoset composites.” Journal of Composite Materials 25(9), 1171-1203, 1991.
5. Jiménez, A., Torre, L., and Kenny, J.M. “Processing and properties of recycled polypropylene modified with elastomers.” Plastics, Rubber and Composites 32(8), 357-367, 2003.
6. Roylance, M., Roylance, D.K., and Sultan, J.N. “The role of rubber modification in improving high rate impact resistance.” Toughness and Brittleness of Plastics (eds. R.D. Deanin and A.M. Crugnola), Vol. 154, American Chemical Society, 1976.