Environmental concerns over depletion of the ozone in the atmosphere have prompted calls for the elimination of chlorofluorocarbons as blowing agents for closed-cell polyolefin foams. Hydrocarbons, which are not known to have ozone depletion potential, are currently being studied as alternatives to chloroflourocarbons.
Hydrocarbons and other flammable blowing agents present their own unique set of problems in use as a blowing agent for closed-cell polyolefin foams. Chief among these is their relatively slow permeation from the foam. Slow permeation out of the foam results in slow decrease in flammable blowing agent content in the foam. The more flammable blowing agent entrapped in the foam, the more flammable the foam.
One means of accelerating permeation or release of flammable blowing agent from polyolefin foam is to extrude a foam structure having channels extending longitudinally therethrough such as seen in U.S. Pat. Nos. 3,573,152 and 4,824,720, neither of which appreciated the advantage of accelerated blowing agent release. The channels provide enhanced gaseous exchange between the interior of the foam structure and the environment outside of it.
Closed-cell polyolefin foam structures having channels therethrough present processing problems because too much of the blowing agent is lost too quickly upon severance of newly-extruded continuous foam structure into discrete foam structure portions at the time of manufacturing. Severance of the continuous foam structure into discrete portions exposes the open ends of the channels in the discrete portions causing the discrete portions to shrink to an unacceptable degree due to rapid loss of blowing agent.
Current manufacturing processes address the problem of foam structure shrinkage by intermittently crimping the continuous foam structure, and severing the continuous foam structure in the middle of the crimps to form crimped discrete foam structure portions. The crimps effectively seal off the ends of the crimped discrete portions, and, thus, the open ends of the channels within. The end user may excise or remove the crimps from both ends of the crimped portions before using them. The problem of shrinkage is avoided because the discrete portions have long since cooled to an extent sufficient, i.e., below the melting point of the polyolefin comprising the foam, that the discrete portions can maintain their form despite the loss of blowing agent.
Disadvantages of crimping the foam structure include inconvenience to the end user in having to excise the crimps from the foam structure portions, and delayed release of flammable blowing agent from the foam structure until time of actual use of the structure.
It would be desirable to have a method for more quickly releasing hydrocarbons and other flammable blowing agents from closed-cell polyolefin foams. It would be further desirable to have a method for quickly releasing such blowing agents from closed-cell polyolefin foams wherein removal of crimps by the end user is eliminated.