Flame retardants halogen-free systems

Flame retardants halogen free systems (including phosphorus... 

Encapsulation resins from Dow Chemical are solvent-free systems sold under the Voratron name. Flame-retarded halogen-free versions are available that meet UL94 V-0 requirements. These polyurethane crosslinked resins are both insulating materials and act as a construction material and housing for fixing electrical components in power distribution, transformers and cable joints. 

Less than 10% of the polyamide produced is made in a flame retardant version. The best system is composed of a combination of red phosphorus and zinc borate (see table above). The only drawback of this system is its color which is restricted to brick red or black. If other colors are required, ammonium polyphosphate is used either in combination with organic flame retardants or with antimony trioxide. It is possible to manufacture a very wide range of colors in the halogen free system. Some systems make use of the addition of novolac or melamine resins. For intumescent applications, ammonium polyphosphate, in combination with other components, is the most frequently used additive. Figure 13.6 shows that fillers such as calcium carbonate and talc (at certain range of concentrations) improve the effectiveness of ammonium polyphosphate. This is both unusual and important. It is unusual because, in most polymers, the addition of fillers has an opposite influence on the efficiency of ammonium polyphosphate and it is important because ammonium polyphosphate must be used in large concentrations (minimum 20%, typical 30%) in order to perform as a flame retardant. 

Flame Retardants Processors learn to ivork ivith halogen-free systems. Modem Plastics International, 23,9, pp. 39-41 (1993). 

The most important representative of the antimony flame-retardants is antimony trioxide (Sb203 or Sb406). It has very little if any effect on non-halogenic polymers or in halogen-free systems. In the presence of halogens, however, a very strong synergetic effect multiplies the fiame-retardancy. 

Joseph Storey has introduced a zinc hydroxystannate flame retardant, which it found to be the closest non-toxic alternative to antimony trioxide in halogenated systems. The new material also has excellent smoke suppression properties. In halogen-free systems, tin char formation occurs, leading to reductions in filler loadings and improved physical properties. 

APP and APP-based systems are very effident halogen-free flame retardants mainly used in polyolefins (PE, PP), epoxies, polyurethanes, unsaturated polyesters, phenolic resins, and others. APP is a nontoxic, environment friendly material and it does not generate additional quantities of smoke due to intumescence. Compared to other halogen-free systems, APP requires lower loadings. In thermoplastic formulations, APP exhibits good processability, retention of good mechanical properties. 

K. Shen and E. Olsen, eds.. Recent Advances on the Use of Borates as Fire Retardants in Halogen-Free Systems Proceedings of the 16th Annual BCC Conference on Flame Retardancy (Stamford, CT BCC Research, 2005). 

There are three essential conditions to be met if a polymer, once ignited, is to continue burning. There must be a supply of heat to the bulk polymer, a generation of fuel (typically volatile decomposition products) and there must be a flame. Halogen-based systems act by a well-documented flame poisoning mechanism in the vapour phase. The alternative halogen-free systems, which encompass a wide variety of additives, tend to act by mechanisms which disrupt heat flow and the supply of fuel to the flame. Here the mechanisms are not always understood in great detail but two broad types of flame retardant action can be defined. 

The practical development of halogen-free systems will continue to exploit the potential of additive blends, particularly where synergistic effects can be seen. Such blends offer the best opportunity of tailoring a combination of flame retardant mechanisms to achieve an efficient, cost-effective performance in each plastics application. 

Halogen-free flame retardants additive delivery systems... 

In particular in halogen-free systems, there are major advantages with respect to smoke suppression, afterglow suppression, corrosion inhibition, by combining zinc borates with other flame retardants in certain polymers, such as ethylene vinyl acetate, polyfvinyl chloride), poly(amide) (PA). 

The performance of aluminium hydroxide/magnesium hydroxide-filled systems can be enhanced by incorporation of zinc hydroxystannate in halogen-free rubbers giving reduced smoke and toxic gas emission, coupled with higher flame retardancy. This action will be complimentary to the water release and endothermic effects of aluminium hydroxide/magnesium hydroxide filler systems. 

In general, tin compounds do not exhibit flame-retardant properties in halogen-free polymer systems, unless the composition contains a high inorganic filler loading. However, tin additives often act as smoke suppressants in non-halogenated polymers. 

Shen, K.K. 1988. Zinc borate as a flame retardant in halogen-free wire and cable systems. Plastics Compounding, Edgell Communication, Cleveland, OH, November/December. 

Xie, R.C. and Qu, B.J. 2001. Expandable graphite systems for halogen-free flame-retarding of polyolefins. I. Flammability characterization and synergistic effect. Journal of Applied Polymer Science 80(8) 1181—1189. 

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