Flame retardance synthetic polymers

Clays, natural or synthetic, are the most widely investigated and understood nanoadditives used to enhance the flame retardancy of polymers through nanocomposite technology, because of their unique properties, particularly the ease of surface treatment and application in polymer matrices. Clay can be cationic and anionic materials, in accordance with the charge on the clay layers. In this chapter, the focus is on two kinds of clays montmorillonite (MMT), a naturally occurring cationic clay that belongs to the smectite group of silicates, and LDH, an anionic clay that does occur naturally but for which the synthetic form is more common. Other clays will also be mentioned as appropriate. 

The use of flame retardants came about because of concern over the flammabiUty of synthetic polymers (plastics). A simple method of assessing the potential contribution of polymers to a fire is to examine the heats of combustion, which for common polymers vary by only about a factor of two (1). Heats of combustion correlate with the chemical nature of a polymer whether the polymer is synthetic or natural. Concern over flammabiUty should arise via a proper risk assessment which takes into account not only the flammabiUty of the material, but also the environment in which it is used. 

Considerable effort is being made (ca 1993) to develop satisfactory flame retardants for blended fabrics. It has been feasible for a number of years to produce flame-resistant blended fabrics provided that they contain about 65% or more ceUulosic fibers. It appears probable that blends of even greater synthetic fiber content can be effectively made flame resistant. An alternative approach may be to first produce flame-resistant thermoplastic fibers by altering the chemical stmcture of the polymers. These flame-resistant fibers could then be blended with cotton or rayon and the blend treated with an appropriate flame retardant for the ceUulose, thereby producing a flame-resistant fabric. Several noteworthy finishes have been reported since the early 1970s. 

Phosphonium Salt—Urea Precondensate. A combination approach for producing flame-retardant cotton-synthetic blends has been developed based on the use of a phosphonium salt—urea precondensate (145). The precondensate is appUed to the blend fabric from aqueous solution. The fabric is dried, cured with ammonia gas, and then oxidized. This forms a flame-resistant polymer on and in the cotton fibers of the component. The synthetic component is then treated with either a cycUc phosphonate ester such as Antiblaze 19/ 19T, or hexabromocyclododecane. The result is a blended textile with good flame resistance. Another patent has appeared in which various modifications of the original process have been claimed (146). Although a few finishers have begun to use this process on blended textiles, it is too early to judge its impact on the industry. 

Phosphoms compounds are effective flame retardants for oxygenated synthetic polymers such as polyurethanes and polyesters. Aryl phosphates and chloroalkyl phosphates are commonly used, although other compounds such as phosphonates are also effective. The phosphoms compounds can promote char formation, thereby inhibiting further ignition and providing an efficient thermal insulation to the underlying polymer. 

Another fire-related problem that has seen some research effort is that of smolder resistance of upholstery and bedding fabrics. Finishing techniques have been developed to make cotton smolder-resistant (152—156), but the use of synthetic barrier fabrics appears to provide a degree of protection. Work also has provided a means of producing cotton fabrics that have both smooth-dry and flame-retardant performance (150,151). In this case, the appHcation of FR treatment should be performed first, and DP treatment should be modified to accommodate the presence of the FR polymer on the fabric. 

Nonwoven products ranging from medical disposables to automotive fabrics are required to meet specific flammability standards. These fabrics are generally composed of cellulosic and/or synthetic fibers which are flammable. Additionally, polymer coatings are applied to the fabric to impart properties such as strength, abrasion resistance and overall binding. It is the purpose of this paper to describe the various polymer coatings commonly used in the nonwovens industry and their effect on flammability of the substrates. Additionally, the effect of flame retardant additives, commonly used in latex formulations, will be discussed. 

At the present time, inorganic tin compounds find a relatively small use in natural polymers, particularly as flame-resist treatments for woollen rugs and sheepskins (8,9). Although certain other metal derivatives have received more attention, there has been much interest recently in the potential use of tin chemicals as flame retardants and smoke suppressants for synthetic polymers (10). 

The production of flame retardant quahty aluminium hydroxide has recently been reviewed [98]. Various crystal forms of aluminium hydroxide exist, but that used for polymer appHcations is Gibbsite. This occurs widely in nature, usually in the rock bauxite, but the natural form is usually not suitable for direct use and synthetic products are nearly always employed. Most aluminium hydroxide is manufactured through the Bayer process used to make alumina for refractory applications. 

PBBs were also widely used as flame retardant additives in polymer formulations, e.g., synthetic fibers, molded plastics and plastic housings also in the manufacture of polycarbonates, polyesters, polyolefins and polystyrenes. Nixed ABS polymers (acrylonitrile -butadiene - styrene), plastics, coatings and lacquers also contained added PBBs to enhance fire-retardancy. 

L. Haurie, A.I. Fernandez, J.I. Velasco, J.M. Chimenos, J.M. Lopez-Cuesta, and F. Espiell, Thermal stability and flame retardancy of LDPE/EVA blends filled with synthetic hydromagnesite/aluminium hydroxide/aluminium hydroxide/ montmorillonite mixtures, Polym. Degrad. Stabil., 2007, 92 1082-1087. 

As the title suggests, this approach is applicable to synthetic fibers only where either one of the monomer/ homopolymer can be flame retarded or the FR molecules can be attached to the polymer chain during... 

New trends involve the use of nanoparticles in synthetic fibers. Polymer-layered silicates, nanotubes, and POSS have been successfully introduced in a number of textile fibers, mainly poly-amide-6, polypropylene, and polyester. Although they reduce the flammability of these fibers, but on their own are not effective enough to confer flame retardancy to a specified level. However, in presence of small amounts of selected conventional FRs (5-10 wt %), synergistic effect can be achieved. With this approach fibers having multifunctional properties can also be obtained, e.g., water repellency or antistatic properties along with fire retardancy. Most of the work in this area at present is on the lab scale and there is a potential to take this forward to a commercial scale. 

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