Process for heat stabilizing oxymethylene copolymers

#This invention relates to a process for heat stabilizing oxymethylene copolymers. It is known to produce an oxymethylene copolymer by copolymerizing trioxane with a cyclic ether and/or a cyclic acetal. The resulting crude oxymethylene copolymer as such, however, cannot become a serviceable resin for the reason that it has the group (OCH.sub.2).sub.n OH at its molecular chain terminals, which terminals easily decompose on heating. As known methods of stabilizing this crude oxymethylene copolymer, there is a method of acetylating, etherifying or urethanizing the terminals of the copolymer, or a method of decomposing the copolymer until the oxyalkylene unit other than the oxymethylene unit, the constituent portion derived from the comonomer and contained in the molecular chain, forms a terminal, thus eliminating the instable portions.

#This method of stabilization involving the decomposition of the terminals is advantageously employed for stabilizing the crude oxymethylene copolymer.

#In view of the fact that, in the case of especially the method of decomposing and eliminating the instable portions by rendering the crude oxymethylene copolymer into a molten state (hereinafter to be referred to as the direct heat treatment method), a stabilized copolymer can be obtained directly without using a solvent, this method is most advantageous for commercial operations.

#An example of a degassing apparatus, i.e., a heat stabilizing apparatus, to be used in the present invention will now be described by reference to the accompanying drawings.

#FIGS. 1 and 2 are a side elevation and a plan view, respectively, of an example of a degassing apparatus; and

#FIG. 3 is a cross-sectional view taken along line X--X in FIG. 1.

#In FIG. 2 the degassing apparatus is made up by coupling a screw extruder A and a surface renewing-type mixer B. It must be understood however that the relative sizes of A and B are not necessarily shown as actually practiced. Further, it should be understood that various methods of connecting A and B are available.

#The extruder A is an ordinary used extruder provided with a single or plurality of shafts. Included are not only those of the paddle-equipped type as in the case of the ZSK extruder marketed by Werner & Pfleiderer Engineers but also the double intermeshing screw-type extruder such as the ZDS extruder of the same company. A powder of a crude oxymethylene copolymer is heat-melted by the aforesaid extruder, and the molten resin is fed to the mixer via a coupling part. The decomposed gas resulting from the decomposition of the instable portions of the resin in the extruder is removed via a vent 3.

#The surface renewing-type mixer B has a plurality of scraping blades 2 fitted to each of the two stirring shafts 1 and 1'. In FIGS. 2 and 3 the plurality of blades 2 are so disposed that they do not collide with one another when the stirring shafts 1 and 1' rotate, and whereby the blades rotate while maintaining a slight clearance between their tips and the inside surface of the casing and the surface of the other stirring shaft.