Inflammability of modacrylic fibre

Abstract

Modacrylic fibres were defined as those containing 35-85 wt% acrylonitrile. There can be a wide variation in quantities of comonomers, and the modacrylics commercially available contain 25-26% monomers, such as vinyl chloride or vinylidene chloride, and consequently possess a high degree of flame resistance. Since the acrylonitrile and vinylidene chloride copolymerization can be carried out at atmospheric pressure, the production of these copolymers is extremely advantageous from the industrial viewpoint due to the low apparatus cost and its high production efficiency. Furthermore, vinylidene chloride copolymer contains a larger limiting oxygen index (LOI-value) than vinyl chloride copolymer [1]. The chlorine content of vinylidene chlorideacrylonitrile copolymers or vinyl chloride-acrylonitrile copolymers can be examined by thermal analysis [2]. It is well known that the modacrylic fibre and its powder have a different amount of weight loss during thermogravimetric analysis (TGA). In this letter the relationship between weight loss in TGA curves and LOI-value, and between the surface area of modacrylic fibre and LOI-value, are discussed. The acrylonitrile-vinylidene chloride copolymers used in this study were copolymerized by the suspension method with redox catalyst, such as sodium hydrogen sulphite, ammonium persulphate and ferrous sulphate, at 25 °C under nitrogen gas. The slurry of polymer was filtered by centrifuge to form polymeric powder, simultaneously the polymeric powder was washed several times by water, then dried in a vaccum oven at 100 *C. The spinning dope contained 24% of the polymer in N,Ndimethylacetamide, and was spun through a coagulation bath to form modacrylic fibre which was stretched in boiling water and dried. The diameter of the spinneret hole was 0.06 mm (Lid = 1). The chlorine content in the copolymer was obtained from titrating the absorbent liquid, in which copolymer was burned in oxygen flask then was absorbed, by using mercury ion. The average molecular weight and polydispersity (Mw/Mn) were determined by a Waters model 440 gel-permeation chromatograph (eluting solvent N,N-dimethylformamide containing 0.5 wt% LiBr; column #Bondagel E-Linear). Monodispersed polystyrenes were used as standards. Thermograms of copolymers were obtained by using a Perkin-Elmer system 7/4 thermogravimeter (TG). Samples of 3 mg weight were heated under an air purge at a rate of 20 °Cmin -~. A Cambridge S-360 scanning electron microscope (SEM) was used to examine the crosssection of the fibre. The LOI was determined by the method of [3]. The surface area of modacrylic fibre and powder was measured by a micromeritics Flowsorb II 2300. As a general rule, modacrylic fibre with a higher chlorine content has a higher LOI, but in this study modacrylic fibre with a higher chlorine content had no increase in LOI-value due to the effect of the cross-section of the fibre, as shown in Table I and Fig. 1. Modacrylic fibre (sample PNDC 81-4) with a cross-section containing many pores (see Fig. 1) had a higher LOI-value, because it had a higher surface area. Halogenated flame retardants (MX) are postulated to function primarily by the following mechanisms [4], which produce HX