Heretofore, various methods for producing foamed thermoplastic articles by continuous extrusion have been known. As for the foaming agent, both physical agents which volatilize by heating and chemical agents which decompose and generate gas by heating have been used.
However, it has been difficult to obtain uniform, fine-celled, highly expanded foam from thermoplastic resins because such resins sharply decrease in viscosity with heating and melting.
In order to avoid this defect, a crosslinking-foaming process wherein a thermoplastic resin is crosslinked to adjust its melt viscosity to a desired degree for foaming has been offered. For example, a thermoplastic resin comrising a chemical foaming agent and a crosslinking agent which has a lower decomposition temperature than the chemical foaming agent is formed into a desired shape. The shaped article is heated at atmospheric pressure or under controlled pressure, firstly, to cause crosslinking by decomposition of the crosslinking agent and, secondly, to cause expansion of the resin by decomposition of the chemical foaming agent.
An atmospheric pressure, rapid foaming causes foaming of only the surface portion of the article. The foamed surface portion acts as an insulation layer and prevents thermal conduction to an inner portion of the resin and the chemical foaming agent cannot decompose smoothly. Even if a resin compound is heated slowly, decomposition of a chemical foaming agent in the innermost portion of an article thicker than 10 mm is difficult.
Accordingly, the upper limit of the thickness of such a foamed article is about 20 mm and a foamed article thicker than 20 mm is usually made by lamination.
On the other hand, for the production of crosslinked foamed articles under pressure there are known a batchwise method and a continuous method. In the batchwise method, as described in the Japanese Patent Publication No. 29381/70 (Yoshida et al), a thermoplastic resin compound comprising a crosslinking agent and a foaming agent is heated in a mold through one step or two steps to cause crosslinking and foaming of the resin. In this process, even though thick, uniform, fine-celled, foamed articles can be obtained, productivity remains low due to the use of a batchwise system.
In the continuous method, Japanese patent Publication No. 1531/83 (Matsui et al) shows a method for foaming a polyolefin in which a polyolefin containing an organic peroxide and foaming agent is extruded into a long-land die, formed into a tubular shape, crosslinked and exposed to a lower pressure zone than in the long-land die to cause expansion. This process uses an oil lubricating method as described in U.S. Pat. No. 3,928,525 (Matsui et al) in which a thermoplastic resin is crosslinked in a long-land die and extruded continuously therefrom. In this process, however, bubbles which are generated from the foaming agent cause thermal conduction to an inner portion of the resin to become low and prevent uniform crosslinking.
Many kinds of foaming agents normally used in a crosslinking-foaming process may generate bubbles in a resin by volatilizing or decomposing partially during crosslinking to decrease the flow rate of the resin and, as the result, to decrease thermal conduction and retard the crosslinking reaction. If bubbles are generated in a resin before the flow rate of the resin becomes sufficiently low due to crosslinking, the dimensions of cells in the finally produced foamed article are extremely nonuniform.
The present invention has been made to overcome the aforesaid problems in the prior art. An object of this invention is to provide a method for producing a crosslinked foamed article in which, even in a thick article, uniform expansion can be achieved in the innermost portion.
Another object of this invention is to provide a method for producing a foamed article having a uniform, fine cell structure at high and low expansion rates.
Another object of this invention is to provide a method for making such a crosslinked foamed article which has high productivity.