Molding compositions based on polystyrene are used to an increasing extent in many fields of applications, predominantly in the construction, packaging, electrical and automotive industries. In particular, foamed polystyrene found a major role in construction and packaging applications because of their favorable mechanical and physical properties, such as insulation against heat, sound and electricity. The major limitation of molded or foamed polystyrene based articles is their combustibility.
Polymer foams have become available in a wide variety of forms, especially foam sheets, films, profiles and slabs for uses such as packaging, pipe and tubing, garment trimmings, construction and insulation. Foamed polystyrene is currently used in the insulation of freezers, coolers, trucks, railroad cars, buildings, roof decks and housing. Polystyrene foams are also used as the core material for structural multilayered panels. There is an increasing demand, partially driven by legislation, to improve the fire retardant properties of polymers in such applications.
The use of halogenated organic compounds as additives to polystyrene products is a well-known practice. In order to render the products fire resistant, brominated organic compounds have been used in both foamed and non-foamed polystyrene compositions. Of the various brominated organic flame retardant agents commercially available mainly brominated aliphatic compounds are utilized with vinyl-aromatic polymer foams. Hexabromocyclododecane, HBCD, and 1,2-dibromomethyl-4-(1,2-dibromomethyl)cyclohexane, BCL-462, Albemarle, are the most common flame retardants used in foamed polystyrene articles.
Dow patent, WO 91/19758, describes the limited fire retardancy of HBCD and discloses the use of a mixture of aliphatic bromine compounds, especially HBCD, and aromatic bromine compounds such as decabromodiphenylether as flame retardant for PS foams as a method to improve the flame retardancy known in the prior art. Another Dow patent, U.S. Pat. No. 6,579,911, discloses an application of HBCD and phosphate or phosphorous compounds and flow promoter, to improve the flame retardance efficiency known in the prior art. The patent also emphasizes that, typically, only brominated aliphatic compounds are utilized with vinyl-aromatic based foams, with HBCD being the most common.
U.S. Pat. No. 5,639,799 and U.S. Pat. No. 5,717,001 disclose methods to improve the thermal stability of HBCD for application in styrenic polymer foam compositions.
The amounts of flame retardant additives and synergists incorporated in polystyrene compositions used in foams must be strictly controlled, since they can negatively affect the structural qualities and skin quality of the foam and reduce the strength of the foam or its insulating properties at high levels. In non-foamed polystyrenic compositions the typical loading of flame retardant additives is significantly higher than in foamed compositions. Therefore flame retardants for foamed polystyrene compositions must have a high degree of efficiency, or in other words, the suitable organic compounds must release, when subjected to fire, the appropriate amount of bromine at the suitable temperature in order to prevent the foamed polystyrene resin from combustion.
DE 2,064,677A discloses a low inflammability molding composition of a styrene polymer that can be obtained, if the styrene polymer contains brominated polyalkylbenzenes. The brominated polyalkylbenzenes disclosed therein contain substituents on the aromatic ring of bromomethyl, and dibromomethyl in numbers varying between 2 and 4, and alkyl of 1 to 4 carbon atoms, bromine, and chlorine in numbers between 1 and 4. However, DE 2,064,677 does not mention a use of a brominated alkylbenzene having a single bromomethyl group on its ring.
Other properties that should be fulfilled by a brominated organic compound, specially designed for flame retarding foamed polystyrene articles, are:    1. Suitable thermal stability of the brominated organic flame retardant is another crucial property, since additives of low thermal stability will limit the possibilities for processing, regrinding and recycling of flame retarded material. Flame retardant additives of insufficient thermal stability will cause degradation of the polystyrene resin, by reducing the molecular weight of the styrene polymer, during processing, and this in turn will immediately cause a drop in all mechanical and insulating properties of the foam, and even corrosion of the equipment in the most severe cases.    2. For most of the existing brominated organic flame retardants it is common practice to apply a specially stabilized material, mainly by using acid scavenger type additives.    3. Good chemical compatibility of the brominated organic compound with the polystyrene matrix is usually achieved by applying criteria of chemical structure resemblance or by solubility measurements.
Obviously one would expect a brominated aromatic compound to be the most suitable flame retardant additive for foamed polystyrene formulations, while the most common existing additives are aliphatic brominated structures.
Several types of additives are known to those experienced in the art as increasing the efficiency of brominated aliphatic flame retardants, so that the amount of bromine added to the polystyrene foam for fulfillment of flammability standards is reduced. Among these additives several types are inherently relevant to any brominated flame retardant designed for foamed polystyrene, and not limited to HBCD or aliphatic bromine compounds:    1. Flow promoters, melt flow modifiers, may be included in the polystyrene flame retardant formulation; in addition it is also known that they can increase the efficiency of the flame retardant compound. It is commonly accepted that such flow promoters are also capable of providing a source of reactive free radicals that are formed at temperatures lower than the formation of bromine radicals from the flame retardant additive. Such flow promoters are therefore also applied as part of the flame retarding system. The addition of such “free radical starters” enables, therefore, the use of lower levels of brominated flame retardant additive. WO 91/19758 and U.S. Pat. No. 6,579,911, incorporated here by reference, disclose the use of flow promoters together with HBCD. Typical flow modifiers include 2,3-dimethyl-2,3-diphenylbutane; bis(alpha-phenylethyl) sulfone; 1,1′-diphenylbicyclohexyl; 2,2′-dimethyl-2,2′-azobutane; 2,2′-dibromo-2,2′-azobutane; 2,2′-dichloro-2,2′-azobutane; 2,2′-dimethyl-2,2′-azobutane-3,3′4,4′-tetracarboxylic acid; 1,1′-diphenylbicyclopentyl; 2,5-bis(tribromomethyl)-1,3,4-thiadiazole; dioctyl tin maleate and dibutyl tin maleate.    2. Processing aids commonly applied together with flame retardants in foamed polystyrene are epoxy oligomers and most preferably brominated epoxy oligomers (BEO). BEO encompasses, inter alia, brominated oligomers containing epoxy groups, based on bisphenol A, such as TBBA. An illustrative example is the commercial product F-2200, supplied by the Dead Sea Bromine Group, Israel. These processing aids reduce the viscosity during foaming and enable the building of fine cells. Moreover BEO's act as heat stabilizers.    3. Phosphate and phosphorous compounds are known as flame retardants in some applications, including blends of styrenic resins. U.S. Pat. No. 5,204,394 relates to a polymer mixture which comprises an aromatic polycarbonate, a styrene-containing copolymer and/or a styrene-containing graft polymer and oligomeric phosphate flame-retardants, wherein the mixture has improved properties. As mentioned above, phosphate and phosphorous compounds are also incorporated together with halogenated flame-retardant compounds.
In other applications, synergism of phosphorous and bromine has been described. The use of phosphorous compounds together with HBCD was described in U.S. Pat. No. 6,579,911. The use of phosphorous compounds together with the brominated flame retardant results in another clear advantage, when processing the foamed polystyrene composition. The good solubility of the phosphate compound in the styrenic resin results in a lowering of the glass transition temperature of the polystyrene resin, and consequently the processing temperature can be lowered while the dispersion of the flame retardant in the resin is kept optimal and the density of the foam is kept low even at lowered processing temperature.
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