The effect on the impact properties of adding long chain branched polypropylene in pp/poe foams produced by core-back injection mouldingThursday (08.10.2020) 11:30 - 11:50
Linear Polypropylene (L-PP) is a polymer with excellent properties such as easy processability, good stiffness and strength, high thermal stability and excellent chemical resistance, but with low ductility especially when it is foamed. In the case of L-PP compounds, ductile solids become brittle foams even for very low expansion ratios. Thus the objective of this work is to implement different strategies allowing improving the ductility of foams with low expansion ratios avoiding the ductile to brittle transition when the material is foamed.
There are several strategies to improve the impact response of foams. One strategy is to enhance the impact behavior of the PP producing blends with impact modifiers as elastomers. This is known as elastomer toughening. One of the most promising materials use nowadays for this purpose are ethylene/ α-olefin copolymers, and especially, Octene-Ethylene copolymers (POE).
The other possible used strategy to improve the impact behavior is to produce foams with an improved a better cellular structure. Core-back injection molding foaming allows obtaining finer cellular structures than low pressure foaming. However, the cellular structure can be improved even more if, in the formulations, a long chain branched polypropylene (LCB-PP) is included. This material exhibits higher melt strength than L-PP, avoiding cell degeneration mechanisms and providing materials with smaller cell sizes.
The combination of these strategies could be useful to avoid the beforementioned ductile-brittle transition in foams. To evaluate it, solids and foams with different density reductions composed of L-PP/POE and L- PP/LCB-PP/POE have been used. The used method to produce the foams was core- back injection molding foaming. Impact properties were tested by Instrumented Falling Weight Impact (IFWI). Extensional rheology (figure 1) and crystallization kinetics provided useful information to understand the cellular structure, that was crucial in the mechanical and impact response of the produced materials.
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