Patent GB-776678-A (GB)
Title: Improvements relating to shaped polyacrylonitrile-containing articles such as filaments
IPC: D01F6/18
Filed: 1954-09-14
Abstract: Shaped articles having a major dimension, such as continuous filaments, especially mono filaments, containing at least 50 per cent by weight of polyacrylonitrile, produced by extruding solutions of homo- or co-polymers of acrylonitrile in concentrated aqueous solutions of inorganic salts into an aqueous coagulating bath preferably at below 10 DEG  C., are stretched when containing not less than 30 per cent of water by weight along their major dimension to at least double their length, to orient the molecules of the articles along the direction of the major dimension, and the articles are then contracted along this dimension at least 20 per cent by heating them to at least 105 DEG  C. while substantially free from tension.  The polyacrylonitrile content of the polymer is preferably at least 70 per cent and especially 85 per cent or more by weight. The acrylonitrile may be copolymerized with vinyl esters, especially the vinyl esters of saturated aliphatic monocarboxylic acids; vinyl and vinylidene halides; vinyl-substituted N-heterocyclic compounds; allyl-type alcohols; allyl, methallyl and other unsaturated monohydric alcohol esters of monobasic acids; acrylic and alkacrylic acids and esters and amides of such acids; methacrylonitrile, ethacrylonitrile and other hydrocarbon-substituted acrylonitriles; unsaturated aliphatic hydrocarbons; styrene and substituted styrenes; and alkyl esters of alpha-, beta-unsaturated polycarboxylic acids. The extruded article is preferably washed and then dried to 30-60 per cent moisture content before being stretched.  The article may also be partially dried to a moisture content of not less than 9 per cent, preferably 10 to 13 per cent, after the stretching step and before the contracting step.  The latter step is preferably carried out in an inert medium which is subsequently removed from the article by washing.  After the contracting step the article may be completely dried by further heating, and this may be followed by a second contraction step in which the article is maintained under no tension at at least about 150 DEG  C. and preferably above 180 DEG  C. up to 500 DEG  C. or more.  The coagulating bath may be water but is preferably a 0.3-25 per cent by weight aqueous solution of a lower monohydric alcohol or of one of the salts contained in the spinning solution.  The coagulated article may be passed through a countercurrent series of wash baths of aqueous solutions of an alcohol or a spinning solution salt, preferably at -5 DEG  C. to +5 DEG  C.  The presence of a small proportion of residual sodium thiocyanate or other salt in the article does not adversely affect the flexibility or knot strength of the final product.  The article may be dyed before being partially dried and stretched.  The article is preferably stretched to above 500 per cent and especially 950 to 1200 per cent of its previous length and may be stretched up to 2000 per cent or more.  The stretching temperature is preferably above 50 DEG  C., e.g. 65 to 100 DEG  C.  The percentage of subsequent contraction is preferably about 25 to 35 per cent.  The contraction temperature when a liquid bath is used is preferably 125-175 DEG  C., but when contraction is effected by passing the article through a hot-air zone, the temperature of the latter may be 150-300 DEG  C., the dwell period in the contraction zone being, for example, 0.01 to 10 seconds, e.g. 2-3 seconds in the case of monofilaments.  When contraction is carried out in two stages, a substantial and preferably a major proportion of the total contraction should take place in the first stage.  In the example a polyacrylonitrile homopolymer, dissolved at a concentration of 5 to 15 per cent by weight in a 55 per cent aqueous sodium thiocyanate solution, is spun into a 14 per cent by weight aqueous solution of isopropanol maintained at 0 DEG  to -5 DEG  C. The gel mono-filament thus formed is passed through a 0.5 per cent aqueous isopropanol solution, dried to a moisture content of about 40 per cent on internally heated converging cylindrical rolls under tension without stretching and then stretched to 600 per cent of its original length in a water bath at 95 DEG  C.  Next the monofilament is partially dried to a moisture content of 12 per cent over rolls heated to 60-70 DEG  C., passed through an electrically heated two-zone air oven maintained at about 125 DEG  C. to produce 25 per cent shrinkage, and completely dried by passage over converging rolls maintained at 90-120 DEG  C.  Finally the dried monofilament is subjected to further contraction in an air oven at 175 DEG  C. and collected on bobbins.  Specification 689,743 and U.S.A.  Specifications 2,404,713, 2,558,733 and 2,558,734 are referred to.