Effect of the elastomer viscosity on the morphology and impact behavior of injection molded foams based on blends of polypropylene and polyolefin elastomers

Abstract

The impact resistance of injection‐molded polypropylene (PP) parts is severely reduced when they are foamed. It is necessary to implement strategies, such as elastomer toughening, to increase the impact behavior of foamed parts. However, the knowledge on the effect of elastomer addition on the morphology, cellular structure, and impact of injection‐molded cellular parts is very limited. In this work, foamed parts based on blends of PP and polyolefin elastomers have been produced and characterized. A high and a low viscosity octene‐ethylene copolymer (EOC) and a high viscosity butene‐ethylene copolymer (EBC) were employed. The blends have been thermally and rheological characterized. Solids materials and foams (relative density 0.76) were injection‐molded. The solid phase and cellular structure morphologies were studied using scanning electron microscopy. The results showed that elastomer toughening has been successful to obtain an improvement of the impact behavior in solid and cellular polymers. In this case, EOC materials provide an appropriate interfacial adhesion and optimized cellular structure which results in high impact resistance. The optimum elastomer to improve the properties is the EOC with a higher viscosity which provides impact resistance with n values below 3 due to the toughening of polymer matrix, thick skin thickness, and low cell size.

Fundamental influences of propylene‐based elastomer on the foaming properties of high melt strength polypropylene based on extrusion foaming

“…Generally, it can be modified by introducing long‐chain branching, [ 10–13 ] crosslinking, [ 14–17 ] blending, [ 18–24 ] and inorganic filling, [ 25,26 ] and so forth, to improve the melt strength of PP and obtain foam with a better cell structure. Stange et al compared the effect of rheological properties of PP with different molecular structures on the foaming performance and found that the expansion ratio of HMS‐PP with a long‐chain branching was larger than that of linear PP.…”

Effects of different polyolefin copolymers on properties of melt mixed polypropylene blends

“…The appropriate selection of polymers/copolymers to blend with PP plays an important role in determining the final properties and applications 14 . It has been indicated that the impact strength of PP can be significantly improved by the combination with rubber phase such as ethylene‐propylene rubber, ethylene‐propylene‐diene rubber, ethylene–propylene‐styrene rubber as well as various olefin‐originated copolymers including ethylene‐propylene copolymer, ethylene‐vinyl acetate copolymer, styrene‐butadiene‐styrene, styrene‐ethylene‐butadiene‐styrene copolymers, and poly(ethylene‐octene) 4,15–18 . Moreover, the impact PP copolymers can be produced in a two‐reactor system to yield a homopolymer with an ethylene‐propylene rubber 19,20 .…”

“…Intensive efforts on studies on the role of several factors such as processing parameters, 24,25 matrix nature (glass transition temperature, crystallinity degree), 25–27 and elastomer percentage 27,28 in achieving the final PP blended with elastomers have been made. Notwithstanding, the influences of the particle size of immiscible elastomers and the correspondence between PP microstructure and elastomer viscosity, on the impact response of the PP blends, are very limited 18,29 . Polyolefin‐based elastomers can be thermodynamically miscible or mechanically compatible.…”

“…14 It has been indicated that the impact strength of PP can be significantly improved by the combination with rubber phase such as ethylene-propylene rubber, ethylene-propylene-diene rubber, ethylene-propylene-styrene rubber as well as various olefin-originated copolymers including ethylenepropylene copolymer, ethylene-vinyl acetate copolymer, styrene-butadiene-styrene, styrene-ethylene-butadienestyrene copolymers, and poly(ethylene-octene). 4,[15][16][17][18] Moreover, the impact PP copolymers can be produced in a two-reactor system to yield a homopolymer with an ethylene-propylene rubber. 19,20 Among these copolymers, octene-based ethylene copolymers have received huge attention owing to their unique molecular characteristics and their physical properties lie between plastic and elastomer.…”

“…Notwithstanding, the influences of the particle size of immiscible elastomers and the correspondence between PP microstructure and elastomer viscosity, on the impact response of the PP blends, are very limited. 18,29 Polyolefin-based elastomers can be thermodynamically miscible or mechanically compatible. While completely miscible blends are relatively rare given the unfavorable thermodynamics of polymer mixing; immiscible blends are therefore highly prevalent.…”