Processes and production apparatus for rigid cellular materials comprising polyvinyl chloride

In a process for producing rigid cellular plastic by treatment of an embryonic block of PVC containing a polyisocyanate, with a hot fluid comprising water selected from the group consisting of liquid water and water vapor, the improvement comprising:

• introducing said embryonic block into a closable expansion box into which the hot fluid can pass and having side walls, a bottom wall and a top wall, and further containing a centering and holding means for the embryonic block, said expansion box being of predetermined, fixed internal maximum dimensions, closing said expansion box and centering and holding said embryonic block in place with said centering and holding means;
• introducing said closed box containing said embryonic block into a first bath of said hot fluid at a temperature of about 80.degree.-85.degree. C. for a sufficiently long period to pre-heat said embryonic block to a uniform temperature;
• removing said closed expansion box from said first bath and immersing it into a second bath of said hot fluid at a temperature of about 90.degree.-95.degree. C. for a period sufficient to cause pre-expansion of said block;
• removing said expansion box from said second bath and immersing it into a third bath of said hot fluid held at a temperature of about 95.degree.-100.degree. C. for a period sufficiently long to bring about expansion of said block to the internal dimensions of said expansion box and also to bring about hardening of said expanded block by reaction between said water and said polyisocyanate, the expansion of said block being performed in said closed expansion box and being controlled by means of said centering and holding means and without transferring of said block from one expansion box to another;
• and
• cooling said expansion box containing said expanded block for a period sufficiently long to dimensionally stabilize said expanded block.