Polypropylene (PP) is a polymer with excellent properties such as easy processability, good stiffness and strength, high thermal stability or excellent chemical resistance, but with low ductility especially when it is foamed. In the case of 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, and avoiding the ductile to brittle transition when the material is foamed.
It is well known that impact resistance of solid materials can be modified by the incorporation of rubber type materials. It is reported in the literature that the inclusion of such secondary phases allows improvements in the strain and the energy at break with respect to those presented by unfilled solid polypropylene. Taking this fact into account, different types of elastomers such as poly(styrene–ethylene–butylene–styrene) (SEBS), ethylene–propylene rubber (EPR), ethylene propylene–diene monomer elastomer (EPDM) or poly(ethylene-co-octene) (POE) has been used in the existent literature to improve the impact behavior of unfoamed PP. It is known that the critical parameter that determines the toughness of these materials is the distance between the surface of the elastomer particles (matrix ligament thickness or interparticle distance), existing a critical value of this parameter below the ductile behavior appears. This occurs due to a plane-strain to plane-stress transition during the cavitation of the elastomer particles .
However, as far as the author knows, there are not previous publications in which the impact response of foams produced from a PP containing a rubber phase are studied. Therefore, in this work, a commercial PP with low impact strength has been blended with a polyolefin elastomer (POE) using different extrusion conditions. The produced solids have been foamed using the Improved Compression Moulding (ICM) technique and azodicarbonamide as chemical blowing agent, obtaining foams with an expansion ratio of 1.5. The cellular structure and dispersion of the rubber phase have been analyzed using SEM. The impact resistance has been tested using Notched Izod Impact and instrumented Falling Weight Impact (IFWI), achieving promising results.