Polyamides, additives Fibres

Polymer Blends.—In addition to the work on polyester—polyamide blends reported in Section 2, several other papers describe the characteristics of various polymer formulations with polyamides. Biconstituent fibres have been formed from nylon-6 and poly(ethylene terephthalate). The same polyamide and nylon-12 have been blended with acrylonitrile-butadiene-styrene copolymer and the temperature and the concentration dependence of the dynamic modulus evaluated. The rheological properties of acrylonitrile-styrene copolymer/nylon-6 mixture have also been reported. Fourier transform infrared studies of nylon-6 and PVC have indicated the presence of specific interactions between the two polymers in both the molten and solid states. Finally X-r y studies carried out on injection-moulded blends of nylon-6, -12, and -66, have revealed that the addition of small amounts of the second component initiates formation of the y-crystalline phase within the nylon-6 polymer matrix. ... 

Since large tonnage production is desirable in order to minimise the cost of a polyamide and since the consumption of nylons as plastics materials remains rather small, it is important that any new materials introduced should also have a large outlet as a fibre. There are a number of polyamides in addition to those already mentioned that could well be very useful plastics materials but which would be uneconomical for all but a few applications if they were dependent on a limited outlet in the sphere of plastics. Both nylon 7 and nylon 9 are such examples but their availability as plastics is likely to occur only if they become established fibre-forming polymers. This in turn will depend on the economics of the telomerisation process and the ability to find outlets for the telomers produced other than those required for making the polyamides. 

Some three decades ago, scientists from the Du Pont company developed polycondensation methods which allowed the preparation of high molecular weight wholly aromatic polyamides. The first commercially produced wholly aromatic polyamide fibre was poly(m-phenyleneisophthalamide) (Nomex, Du Pont, 1967) [la, c]. Some years later, development of the preparation and processing of poly(p-phenyleneterephthalamide) (PPTA) led to the commercialization of the para product Kevlar (Du Pont) in the early seventies [lb, c]. While Nomex shows excellent thermal stability and flame-retardance, and indeed is referred to as a heat and flame resistant aramid fibre, Kevlar fibre also has similar properties, but in addition it has exceptional tensile strength and modulus, and is referred to as an ultra-high strength, high modulus aramid fibre. 

In addition to the common applications of PVC in flooring and floor tiles, there are also applications of PU (coatings for flooring), polyamide (PA) (woven carpets and rugs), linoleum (manufactured by oxidising linseed oil mixed with pine resin and wood flour in the form of sheets on jute backing), wood/thermoset composites (thermosets like PE, UF, polyisocyanates used as veneers, in plywood, particleboard, fibreboard, laminates), wood fibre/thermoplastic composites (for industrial flooring with PE). 

As stated above, conventional synthetic fibres may be rendered inherently flame retardant during production by either incorporation of a flame retardant additive in the polymer melt or solution prior to extrusion or by copolymeric modification before, during, or immediately after processing into filaments or staple fibres. Major problems of compatibility, especially at the high tanperatures used to extrude melt-extruded fibres like polyamide, polyester, and polypropylene and in reactive polymer solutions such as viscose dope and acrylic solutions, have ensured that only a few such fibres are commercially available. A major problem in developing successful inherently flame retardant fibres based on conventional fibre chemistries is that any modification, if present at a concentration much above 10wt% (whether as additive or comonomer), may seriously reduce tensile properties as well as the other desirable textile properties of dyeability, lustre and appearance, and handle, to mention but a few. 

The general paucity of FR polyamides reflects their high melt reactivities and hence poor potential flame retardant additive compatibilities. The only additive currently marketed as a potential flame retardant for polyamide fibres is Clarianf s Exolit OP930/935, which is based on a fine particulate (Dj(, 2-3pm), aluminium diethyl phosphinate. This phosphinate may be used alone or combined with melamine polyphosphate, although in bulk polymers total levels of 15wt% or so are required for acceptable levels of flame retardancy. To date it is not known whether commercially successful PA6 and PA6.6 fibres based on this agent are available. 

Polyamides are less combustible plastics due to their chemical composition. Unfilled and unmodified PA 6 and PA 66 are rated V-2 according to UL 94, with an oxygen index of about 25 per cent without any added agents. One peculiarity is that glass fibres, mineral fillers, and some additives (such as the impact modifiers) actually enhance the flammability of polyamides they are rated only HB when not flame-retarded. A drawback of polyamides is dripping during the combustion. 

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