Process for preparing gas-, vapor- and odor-proof coupled and coextruded multilayer articles of thermoplastic material and closed-surface bodies comprising the same

A process for preparing multilayer articles consisting of at least two layers made of different thermoplastic materials and characterized in being impermeable to gases, vapors and odors, by coextruding at least two different thermoplastic polymers at least one of which is polyvinyl alcohol having a polymerization degree of from 600 to 1,500 and a hydrolysis degree of at least 85% by mols, and successively molding the coextruded article, said process comprising the following steps:

• (a) feeding to an extruder connected with a coextrusion head a polyvinyl alcohol having the stated hydrolysis degree pre-mixed with an aqueous mixture of plasticizing compounds in an amount not higher than 50% by weight with respect to the polyvinyl alcohol and heating the polyvinyl alcohol/aqueous plasticizing compounds mixture in the extruder under pressure to bring it to the plastisol state;
• (b) raising the temperature of the polyvinyl alcohol/aqueous plasticizing compounds mixture, before the mixture enters the coextrusion head, to the temperature at least equal to that reached by the mixture in the coextrusion head, and subjecting the mixture, while it is at such relatively high temperature, to rapid decompression with simultaneous venting of the evolved vapors;
• (c) separately feeding at least one other thermoplastic polymer in the molten state and selected from the group consisting of polystyrene, polyvinyl chloride, foamed polystyrene, nylon 6, nylon 66, polyethylene terephthalate, chlorinated polyethylene, polyvinylidene chloride, polyvinyl acetate, polyolefins obtained by polymerizing ethylenically unsaturated monomers of the formula CH.sub.2 .dbd.CHR, in which R is hydrogen or a hydrocarbon radical containing 1 to 20 C atoms either alone or with one another, and polymers obtained by polymerizing said monomers CH.sub.2 .dbd.CHR with other comonomers copolymerizable therewith and containing from 0.05 to 20% by weight of comonomer, to the same coextrusion head;
• (d) adjusting the temperature of the polymers when they are in contact so that the temperature of the polyvinyl alcohol is at least equal to the temperature of the other coextruded polymer or polymers;
• (e) adjusting the flow rate of the polymers so that the outflow linear velocities are equal or differ no more than .+-.10%;
• and
• (f) coextruding the polyvinyl alcohol/aqueous mixture of plasticizing compounds and the other thermoplastic polymer and molding the article comprising the coextruded and bonded layers leaving the coextrusion head to obtain a shaped, manufactured article.