The present invention relates generally to blowing agent concentrates for forming porous, molded thermoplastic items, and, more particularly to blowing agent concentrates that are compatible with polycarbonate homopolymers, but which can be extruded without causing thermal decomposition of the blowing agent.
Porous, thermoplastic molded articles are useful for various applicatioins including computer housings and outdoor telecommunication devices. Methods for making such articles typically employ a blowing agent, which is conventionally extruded together with a polymer to make polymer pellets comprising the blowing agent. These pellets are then subjected to heat and pressure in a molding process used to form useful articles. Alternatively, the pellets comprising the blowing agent (i.e., the blowing agent concentrate) may be combined with pellets of a second polymer before introducing the mixture into a molding machine. The heat and pressure employed in the molding process must be sufficient to cause the blowing agent to decompose into gaseous chemicals which do not harm the resin. These gaseous chemicals are trapped within voids present in the molded article upon cooling.
It has heretofore been difficult to employ polycarbonate homopolymer as the principle ingredient in said porous, thermoplastic molded articles because polycarbonate must be extruded at a high temperature due to its relatively high softening point. Specifically, the extrusion temperature necessary for polycarbonate is often sufficiently high to cause thermal degradation of the blowing agent prior to molding.
For the foregoing reasons, there is a need for a combination of a thermoplastic polymer and a blowing agent which can be extruded together to form a blowing agent concentrate that is compatible with polycarbonate homnopolyimer, without causing degradation of the blowincg agent.