GB877885A - Improvements relating to the treatment of aromatic polyamide structures - Google Patents

Abstract

Uniformly oriented structures of aromatic polyamides are treated to improve their crystallinity by being subjected to a temperature of at least 220 DEG C. but below the melting point of the polyamide for at least 0,2 secs. while being held under tension sufficient to prevent retraction of the structure by more than 10%. Aromatic polyamides are defined as consisting of the recurring structural unit: <FORM:0877885/IV (a)/1> in which Ar1 and Ar2 are the same or different and are substituted or unsubstituted divalent aromatic radicals in which the chain extending bonds are not ortho-oriented with respect to each other or to bonds linking one aromatic ring directly or through a linking group to another aromatic ring, the substituent groups, if any, in the aromatic rings being groups which do not react with amine or halide groups during polymerization and the total number of carbon atoms in substituent groups attached to any one aromatic ring not exceeding nine, subject to the proviso that at most 10 mol per cent of the Ar1 radicals and at most 10 mol per cent of the Ar2 radicals may be replaced by non-aromatic radicals, and that the inherent viscosity of the polyamide, measured in concentrated sulphuric acid at 30 DEG C., is at least 0,6, preferably at least 0,8. Preferably, the polyamide is wholly aromatic, i.e. none of the Ar1 or Ar2 radicals are replaced by non-aromatic radicals. Specified polyamides include poly(m-phenylene isophthalamide), the wholly aromatic polyamide made from 4-41-diamino diphenyl ether and terephthaloyl chloride, and polyamides of the structures: <FORM:0877885/IV (a)/2> where n is a large whole number of about 30 or more. The structure is preferably a filamentary material, e.g. a multifilament yarn, a monofilament, film or ribbon, and may be oriented by drawing a substantially unoriented structure to at least 3 X its original length while in contact with steam or hot water. An unoriented polyamide yarn may be drawn and heat-treated in a single continuous operation. Preferred heat-treatment conditions comprise 0,3-5,0 secs. at 300 DEG -350 DEG C. In an example, poly (m-phenylene isophthalamide) is dry spun from a 17% solution in dimethylacetamide containing 2,9% calcium chloride, based on the total solution weight. The amorphous unoriented yarn is drawn 4,0 X while passing through a bath of hot water which also extracts residual solvent and calcium chloride. Extraction is completed by a second pass through hot water, without further drawing, and the yarn is then drawn 1,2 X further, to a total draw of 4,8 X, while passing in contact with a heated plate at 200 DEG C. This step orients the yarn, at a temperature insufficient to cause crystallization. The yarn is then crystallized by being passed over a second hot plate at 330 DEG C. for a contact time of 0,6 secs., while under a tension of 50 grams, which causes a stretch of 5,8%. Treatment improves tenacity, tensile modulus and the constancy of elongatability over a wide range of temperatures, and reduces shrinkage on repeated wet and hot-dry treatment. Specification 871,581 is referred to.