| 摘要 |
A foamed polyolefin resin is produced by treating, under conditions of elevated temperature, the product resulting from the extrusion of a foamable mixture comprising a polyolefin resin and a volatile pneumatogen, with a gas substantially inert to the polyolefin. The preferred polyolefine is high or low pressure polyethylene though other suitable resins are derived from olefines containing not more than four carbon atoms. The volatile pneumatogen is a liquid with an atmospheric boiling point lower than the extrusion temperature, e.g. aliphatic hydrocarbons containing up to four carbon atoms such as ethane, ethylene, propane, n-butane, iso-butane and butadiene. n-Pentane, neopentane and carbon dioxide are also specified. Nucleating agents such as finely divided silica or alumina optionally mixed with zinc stearate or a mixture of sodium bicarbonate and tartaric or citric acid may also be incorporated. The extruded product is obtained by a conventional extrusion process in the temperature range 90-120 DEG C. and is treated immediately or shortly thereafter with a gas at a temperature in the range 50-100 DEG C. preferably either at atmospheric pressure or under a pressure, which is increased and released gradually, in the range 40-100 p.s.i.g. for a period lasting from 5 minutes up to 20 hours. The preferred gases are air, nitrogen or a fluorocarbon such as chlorotrifluoromethane. |
