Polyphenylene ether resins are known in the art as a class of thermoplastics which are characterized by excellent physical properties, including hydrolytic stability, dimensional stability and excellent dielectic properties. In general, they are prepared by the oxidative coupling of a phenolic compound with complex metal catalysts, e.g., a complex copper catalyst. The preparation of polyphenylene ether resins is described in Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875, which is incorporated herein by reference.
It is known that when polyphenylene ether resins are combined with styrene resins, e.g., rubber-modified high-impact polystyrene, the resulting compositions provide properties which are superior to those of each of these resins alone. Such combinations are disclosed in Cizek, U.S. Pat. No. 3,383,435, the disclosure of which is incorporated herein by reference. However, the addition of styrene resins to the normally self-extinguishing and non-dripping polyphenylene ether resin results in molding compositions and articles which are normally flammable, i.e., unable to meet minimum requirements established by various testing groups such as Underwriters' Laboratories. Moreover, these compositions drip flaming resin while burning.
Self-extinguishing, i.e., flame retardant, thermoplastic compositions of a polyphenylene ether resin and a styrene resin are known. It has been proposed, for example, to render polyphenylene ether-polystyrene combinations flame retardant with the addition of combinations of aromatic halogen compounds, e.g., chlorinated biphenyls, and aromatic phosphates. Such compositions are disclosed in Haaf, U.S. Pat. No. 3,639,506, which is incorporated herein by reference.
More recently, it has been proposed that normally flammable polyphenylene ether-polystyrene combinations can be rendered self-extinguishing and non-dripping with a flame retardant combination of an aromatic halogen compound, e.g., hexambromobenzene, and an antimony containing compound, e.g., antimony oxide. Such compositions are disclosed in Reinhard, U.S. Pat. No. 3,809,729, incorporated herein by reference.
These flame retardant prior art compositions, particularly those which employ antimony oxide, are not entirely satisfactory, however. In such compositions the flame retarding agent decomposes at elevated temperatures. Moreover, articles molded from such decompositions are somewhat brittle, apparently because of the presence of the flame retarding agent. Although it has been found that by lowering the processing temperatures of the molding compositions decomposition of the flame retarding agent is minimized, embrittlement still occurs.
It has now been surprisingly discovered that certain materials can be added to flame retardant polyphenylene etherpolystyrene compositions which promote the retention of mechanical properties, without adversely affecting flame resistance properties. More particularly, it has been found that when the additives of this invention are added to polyphenylene ether-polystyrene compositions containing a flame retardant combination of a halogenated aromatic compound and an antimony compound, compositions are obtained which have good flame resistance properties and unexpectedly improved mechanical properties, including improved impact strength.