Improvements in or relating to a method of insolubilising artificial filaments, threads, fibres and the like obtained by the spinning of solutions of proteins

Insolubilized protein filaments are produced from a bundle of filaments which has been obtained by spinning solutions of protein in aqueous alkaline or other suitable aqueous solvent media into suitable acidified saline coagulating baths, hardening the filaments in the bundle with at least one aqueous saline solution of formaldehyde having a pH up to 7, impregnating the hardened filaments with an aqueous solution of formaldehyde, stretching the filaments, holding the filaments in the stretched condition while drying at a temperature and exceeding 140 DEG C. and at least for such a period of time that they no longer tend to shrink, further heating said filaments to a temperature not exceeding 220 DEG C., and preferably in the relaxed condition and/or at least such a period of time that the filaments are insolubilized, and with or without thereafter subjecting the thus insolubilized filaments to a second impregnation with an aqueous solution of formaldehyde and repeating the drying and further heating steps while the filaments are in the relaxed condition, at least for such a period of time that the filaments are not reduced in dry or wet strengths to any substantial degree on drying after heating in a bath containing 0.1 per cent by weight sulphuric acid and 0.25 per cent sodium sulphate when immersed therein for 90 minutes at 97 DEG C., characterized in that the hardening is effected so that 100 parts by weight of the hardening filaments have a formaldehyde content of 0.2 to 0.5 parts by weight. The hardening solution may contain either sodium chloride or sodium sulphate, preferably in saturated state and the hardening may be effected between 30 DEG and 70 DEG C. The pH of the saline formaldehyde solution may be adjusted with sulphuric acid. Preferably the aqueous solution of formaldehyde used for impregnating is free from salts. It may, however, contain hydrochloric or sulphuric acids, preferably such that the pH is not greater than 1.5. In addition, the bundle of coagulated filaments may be treated with an aqueous solution having a pH up to 4 of a salt preferably selected from the group comprising alkali metal salts of hydrochloric acid, sulphuric acid or phosphoric acid, magnesium sulphate, ammonium sulphate, aluminium sulphate and sodium zirconyl sulphate. Such a treatment would precede hardening of the filaments. Specifications 741,497, 742,261, 742,890 and 787,588 are referred to.