C. Cuddington
ExxonMobil,
United States
Summary:
Expanded polypropylene bead foam (ePP) is valued for its high durability, reusability, and streamlined recyclability potential compared to expandable polystyrene (ePS); however, ePP remains a smaller, more niche market compared to ePS due in part to the high steam chest pressures and long cycle times required to attain adequate bead fusion during the molding step. ePS molding machines are significantly less expensive to acquire and operate and as such out number ePP molding machines roughly 10-to-1. ePP beads are unable to be molded in that higher throughput equipment due to the relatively high molding temperature of random copolymers of polypropylene (RCPs) (>135oC) compared to PS (<120oC). Lower melting temperature polyolefins such as ethylene-propylene copolymers and linear low-density polyethylene (LLDPE) could feasibly be molded in ePS steam chests, but the low flexural modulus of these polymers translates to a lack of compressive strength in the final foamed part and in some cases, the resin is unable to be properly expanded into a bead. At ExxonMobil, we have designed a new generation of polyolefin, one that has comparable stiffness to the RCPs used in ePP while also having a significantly lower melting temperature. This gives the potential for a foamed part that has the durability and rigidity customers have come to expect from ePP, while drastically reducing the time and energy that goes into the molding step. This new grade not only has the ability to improve the processing parameters of ePP production, but the melt temperature is low enough to possibly be molded in ePS machinery. Preliminary testing on foamed beads made from this new polyolefin grade has shown adequate welding at steam chest pressures below 1.5 bar-g. The potential therefore exists that this new polyolefin could be a drop-in replacement into the ePP expansion process and can then be molded into final foam parts leveraging the scale, speed, and efficiency of ePS steam chests.