The present invention relates to a novel flame-retardant rubber composition or, more particularly, to a rubber composition mainly composed of a polyolefin-based synthetic rubber imparted with remarkably improved flame retardancy.
As is well known, synthetic rubbers of the polyolefin type are generally excellent in the electric and mechanical properties so that they are widely used as a material for electric insulation. On the other hand, polyolefin-based synthetic rubber compositions are defective in their combustion behavior since they are highly inflammable and, once they have taken fire, the fire rapidly propagates with dripping of the molten rubber composition. Therefore, the fields of their application are greatly limited in this regard to those where flame retardancy is not essential and it has been eagerly desired to develop a novel polyolefin-based rubber composition with high flame retardancy.
The flame retardancy of a polyolefin-based rubber composition can be improved by incorporating a flame retardant agent into the rubber composition and the most conventional flame retardant agents are various kinds of halogen-containing organic compounds and antimony oxide or combinations thereof. These flame retardant agents are practically not quite satisfactory because sufficiently high flame retardancy can be obtained only by incorporating a considerably large amount of the agent accompanied by an economical disadvantage due to the expensiveness of these flame retardant agents. In addition, these flame retardant agents are not acceptable from the standpoint of human health and environmental pollution because large volumes of toxic or corrosive gases and smoke are produced when a rubber composition impregnated with these flame retardant agents is set on fire.
Various attempts have been made for improving the flame retardancy of the polyolefin-based rubber compositions by incorporating other inorganic compounds into the rubber composition. Unfortunately, none of them is so effective as to impart sufficiently high flame retardancy with a relatively small amount while a polyolefin-based synthetic rubber composition impregnated with a large amount of such an inorganic compound can no longer exhibit the excellent electrical and mechanical properties inherent to the rubber compositions of this class.
Alternatively, various kinds of phosphorus-containing compounds are proposed as a flame retardant agent but incorporation of a large amount of such a flame retardant agent into a rubber composition causes a problem of blooming of the compound on the surface of the rubber article in addition to the decrease in the electric properties.
Thus, no satisfactory solution has been found in this problem of improving the flame retardancy of a polyolefin-based rubber composition.