Self-extinguishing modacrylic polymer filaments and fibres consist of a copolymer of 35-85 (wt.)% CH2:CHCN, 5-45% CH2:CHCl, CH2:CHBr and/or CH2:CCl2, 5-30, pref. 8-20% carboxyphosphinic acid deriv. (I) of formula: R1R2PO-O-CH2CH2O-CO-CR3=CH2 (in which R1 and R2 are 1-8C alkyl; R3 is H or Me) and 0-15% other unsatd. cpd. with activated double bond. The use of (I) gives as good flame retardancy as the use of phosphine oxide cpds., i.e. much better than the corresp. phosphonic acid derivs., which have almost no synergistic effect on the flame retardancy. The thermal stability is much higher than when phosphine oxide cpds. are used, so that there is much less discoloration at elevated temp. and fibres with a much higher degree of whiteness can be produced. The filaments and fibres are produced by copolymerising the monomers, dissolving the prod. in an aprotic solvent and spinning, opt. after adding other auxiliaries.
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