In halogen-free polymers

In halogen-free polymers, the zinc borate in combination with ATH at high loadings can also function as a flame retardant and smoke suppressant by releasing water and forming a porous ceramic residue, which acts as a thermal insulator. 

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. 

There has been a great recent market demand for halogen-free fire-retardant polymers. Zinc borates are also multifunctional fire retardants in halogen-free polymers. They can promote char formation, reduce the Rate of Heat Release, smoke evolution, carbon monoxide generation, and afterglow combustion. When used in conjunction with metal hydroxides, they can also display synergy in fire test performance. 

Recent studies of coated fillers have focused on their activity in halogen-free polymer formulations and encouraging results have been obtained in ethylene-vinyl acetate (EVA), - ethylene-ethyl acrylate (EEA) and epoxy resins/ particularly for applications in the electronics industry. 

It has been demonstrated that the combination of zinc borate and ATH can be used as an effective flame retardant and smoke suppressant in halogen-free polymers such as EVA, polyethylene, EPDM, ethylene-ethyl acrylate copolymer (EEA), epoxy, and acrylics. In some pol5miers such as silicone rubbers, zinc borate used alone is very effective as a fire retardant. 

K. Shen and E. Olsen, eds.. Borates as FR in Halogen-Free Polymers Proceedings of the 15th Annual BCC Conference on Flame Retardancy (Stamford, CT BCC Research, 2004). 

B. Perret and B. Schartel, The effect of different impact modifiers in halogen-free flame retarded polycarbonate blends - ii. fire behaviour, Polym. Degrad. Stab., In Press, Corrected Proof, 2009. 

Both ammonium fluoroborate (NH4BF4) and potassium fluoroborate (KBF4) were reported to be flame retardants in both Cl/Br-containing and halogen-free polymers. Ammonium fluoroborate sublimes at 238°C and potassium fluoroborate has a melting point of 359°C. 

Zinc borate in conjunction with ATH or MDH is used in many halogen-free polymers. Based on DTA and DSC analyses, it was also demonstrated that the partial replacement of ATH with Firebrake ZB in cross-linked EVA can delay and reduce the thermal oxidative peak (Figure 9.17). The mode of action of this zinc borate and metal hydroxide is summarized as follows ... 

L. Du, B. Qu, and Z. Xu, Flammability characteristics and synergistic effect of hydrotalcite with microencapsulated red phosphorus in halogen-free flame retardant EVA composite, Polym. Degrad. Stabil., 2006,91 995-1001. 

Qiang, W., Jianping, 1., Baojun, Q., 2003. Preparation and characterization of microencapsulated red phosphoms and its flame — retardant mechanism in halogen free flame retardant polyolefins. Polymer International 52 (8), 1326—1331. 

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. 

Recent studies have demonstrated that there are major advantages in using a combination of ATH and zinc borate in a variety of halogen-free polymer systems. 

With halogen-containing systems, zinc borate can partially or completely replace the antimony synergist in PVC, notably wire and cable, wallcoverings, roof membranes and tarpaulins. It is also effective in polyolefins, elastomers, polyamides and epoxy polymers while, in halogen-free systems, it can be used in conjunction with alumina trihydrate, magnesium hydroxide or red phosphorus. 

C. M. Jiao, Z. Z. Wang, X. L. Chen, and Y. Hu, Synthesis of a magnesium/ aluminum/iron layered double hydroxide and its flammability characteristics in halogen-free, flame-retardant ethylene/vinyl acetate copolymer composites. Journal of Applied Polymer Science, 107 (2007), 2626-31. 

L. Ye and B. J. Qu, Flammability characteristics and flame retardant mechanism of phosphate-intercalated hydrotalcite in halogen-free flame retardant EVA blends. Polymer Degradation and Stability, 93 (2008), 918-24. 

Although there have been relatively few studies of tin additives in non-halogenated plastics, it has been found that tin compounds, including ZHS, ZS and Sn02, when utilized at low addition levels, can significantly improve the flame-retardant and, particularly, the smoke-suppressant properties of halogen-free polymer formulations. 

The tin additives exert their fire-retardant action in both the condensed and vapour phases, by promoting the formation of a thermally stable carbonaceous char and (in halogen-containing polymer formulations) by generating volatile metal halide species which assist in free radical scavenging reactions in the flame. 

Zinc borate can also change the oxidative decomposition pathway of halogen-free polymers. It is not completely clear if this is happening because of an inhibition effect of boron oxides toward the oxidation of hydrocarbons or the oxidation of graphite structures in the char, or is due purely to the formation of a protective sintered layer. In combination with ATH, zinc borate creates a porous ceramiclike residue, which has much better insulative properties than those of pure anhydrous alumina. It was shown that zinc borate accelerates dehydration of magnesium hydroxide and creates a ceramiclike structure with dehydrated MgO. 

Wu, Q. Lu, J.P. Qu, B.J. Preparation and characterization of microcapsulated red phosphorus and its flame-retardant mechanism in halogen-free flame retardant polyolefins. Polym. Int. 2003, 52(8), 1326-1331. 

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