Forming and crosslinking high-density polyethylene - by very high pressure compaction, moderate heating and then further heating

摘要

Powdered thermoplastic prods. are prepd. using a cross-linking agent and high pressure exerted upon the mixt. as it passes along and through a nozzle, followed by cross-linking after it leaves the nozzle. High pressure si exerted along the nozzle upon the powdered mixt. contained in it, thus causing the mixt. to sinter; the mixt. is melted while still within the nozzle, but at a temp. not high enough to initiate cross-linking; after leaving the nozzle the material then passes through a bath fo molten salt at a temp. high enough to activate the cross-linking agent and thus cross-link the thermoplastics material. Pref. a piston is used to apply an axial pressure of 1000 bar along the nozzle. The process is partic. for high-density polyethylene, where normally the material could not be made sufficiently fluid in the early stages of passage through the nozzle without producing such a high temp. that cross-linking would occur prematurely. The method using sintering by application of high pressure enables the material to be compressed through the nozzle and carries out the preforming at this stage, without reaching the activation temp. of the cross-linking agent.