A process for the production of foamed polypropylene,and articles produced therefrom

摘要

Foamed polypropylene is prepared by feeding polypropylene and a nucleating agent containing a CO2-liberating component and an acid to the transport zone of an extruder, injecting and mixing a liquid organic foaming agent into the plastic mass in the extruder and extruding the mixture with formation of foam the pressure in the extruder at the point where the foaming agent is injected being at least 100% higher than the pressure at the end of the transport zone, i.e. at the point where the flow passage in the extruder is first restricted. The nucleating agent is preferably made up of 0.5% by weight of the feed of sodium bicarbonate and 0.4% citric acid. The foaming agent is preferably isopentane or butane: and the extrusion temperature is preferably close to the melting point of the polymer. A cigarette filter may be made of the foamed polypropylene.ALSO:Cigarette filters are made from foamed polypropylene by cutting extruded strands of the foam into short lengths. The foam has a continuous or substantially closed skin and a low resistance to passage of gases in the longitudinal direction, and the volume of open cells in the foam should be at least 60% of the total filter volume the density being between 10 and 50 gms per litre.