FLAME RETARDANT HEAT-SHRINKABLE TUBE

Abstract: A flame retardant heat-shrinkable tube comprising a crosslinked flame retardant resin composition which comprises: (a) 100 parts by weight of an ethylene/vinyl acetate copolymer having a vinyl acetate content of 20 wt% or more based on the total amount of the copolymer or 100 parts by weight of a resin mixture containing a polyolefin resin and an ethylene/vinyl acetate copolymer the resin mixture having a vinyl acetate content of 20 wt% or more based on the total amount of the resin mixture, and (b) from 150 to 250 parts by weight of magnesium hydroxide.

Description: FIELD OF THE INVENTION The present invention relates to a flame retardant heat-shrinkable tube having excellent flame retardancy without the generation of toxic gas and excellent mechanical properties. BACKGROUND OF THE INVENTION Recently, safety in electric cables has been highly desired, i.e., electric cables for atomic power plants, vehicles, various electronic devices, etc. have been desired to have excellent flame retardancy without the generation of toxic gas during combustion of the cables. In accordance with the above, heat-shrinkable tubes used for protection, insulation or the like of these electric cables have also been desired to have excellent flame retardancy without the generation of toxic gas during combustion of the tubes. In order to attain the above requirements, methods for attaining flame retardancy of heat-shrinkable tubes have been proposed wherein a large amount of a metallic hydrate, such as aluminum hydroxide and magnesium hydroxide, is added to polyethylene (as described in Japanese Patent Publication No. 41367/87); or the combination of metallic hydrate and red phosphorous or wherein the combination of magnesium hydroxide and carbon powder is utilized (as described in Japanese Patent Publication Nos. 13832/85 and 10898/82). However, if an organic flame retardant such as metallic hydrate is added to a resin composition such as polyethylene in an amount capable of attaining excellent flame retardancy passing the Standard All-tubing Flame Test of Underwriters Laboratories (UL Standard), the mechanical properties of the resin composition, particularly the elongation, are deteriorated, such that the UL Standard (elongation of 200% or more) is not satisfied. Moreover, in the methods using red phosphorus or carbon powder are not suitable for improving the flame retardancy of heat-shrinkable tubes which are required to be colored several different colors depending on their objective use, because the resin composition is colored red or black. GB-A-2041960 discloses ethylene-based resin compositions filled with hydrated metal oxide for use as coating materials for fire-retardant wire cables or as materials for fire-retardant foams. Amongst a variety of ethylene-based resins, ethylene-vinyl acetate copolymers are disclosed where the vinyl acetate content is 5 to 75% by weight. As the hydrated metal oxide, ten metal oxides, including aluminium hydroxide and magnesium hydroxide, are concretely disclosed. The amount of such metal oxide is 80 to 250 parts by weight per 100 parts by weight of the resin. Of 61 examples disclosed in GB-A-2041960, example 11 discloses a resin composition comprising 20 parts by weight of ethylene, 80 parts by weight of an ethylene-vinyl acetate copolymer having a vinyl acetate content of 25% by weight, and 100 parts by weight of magnesium hydroxide. There is no suggestion to use these compositions for heat shrinkable tubes. US-A-3922442 discloses flame retardant compositions for coating electrical wires and cables and for moulding flame retardant products. Such compositions contain one or more curable ethylene-vinyl acetate copolymers , one or more silanes and one or more hydrated organic fillers. As the copolymer, ones having at least 9% and up to 40% or more of vinyl acetate are mentioned, with a preference for a vinyl acetate content not exceeding 28%. Of the hydrated organic fillers, mention is made of aluminium hydroxide, hydrated magnesia and hydrated calcium silicate, with a preference being expressed for aluminium hydroxide. The amount of the filler is 80 to 400 (most preferably 125 to 150) parts by weight per 100 parts by weight of the copolymer. In the specific examples of filled compositions, the ethylene-vinyl acetate copolymer contains 17% vinyl acetate, 125 parts by weight of the filler are used per 100 parts by weight of the polymer, and none of such examples uses hydrated magnesia. There is no suggestion to use these compositions for heat shrinkable tubes. GB-A-2110696 discloses flame retardant claddings which are in dimensionally recoverable form, eg heatshrinkable and which comprise a substantially cross-linked blend of a vinyl acetate/alkene copolymer with a thermoplastic polyalkene or alkene-alkene copolymer and incorporating an effective amount of a halogen-free inorganic flame retardant. The blend contains at least 66 weight% vinyl acetate/alkene copolymer having a vinyl acetate content of greater than 40 weight%. The flame retardant can be used in an amount of from 10 to 400 parts by weight per 100 parts by weight of the polymeric component(s), with a preference being expressed for flame retardant in an amount of 80 to 150 parts by weight per 100 parts by weight of the polymeric component. Of the flame retardants specifically disclosed mention is made of a number of materials including hydrated aluminium oxides and hydrated magnesia, with particular preference being expressed for α-alumina trihydrates, such material being used in all of the specific examples in an amount of 100 parts by weight per 100 parts by weight of the polymeric component. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a flame retardant heat-shrinkable tube having excellent flame retardant without the generation of toxic gas and excellent mechanical properties. Other objects of the present invention will be apparent from the following description. The above objects of the present invention have been attained by a flame retardant heat-shrinkable tube comprising a crosslinked flame retardant resin composition which comprises: (a) 100 parts by weight of an ethylene/vinyl acetate copolymer having a vinyl acetate content of 20 wt% or more based on the total amount of the copolymer, or 100 parts by weight of a resin mixture containing a polyolefin resin and an ethylene/vinyl acetate copolymer, the resin mixture having a vinyl acetate content of 20 wt% or more based on the total amount of the resin mixture, and (b) from 150 to 250 parts by weight of magnesium hydroxide, said tube being obtainable by inflating it at or above its melting point and then cooling it whilst maintaining the inflated shape so as to render the tube heatshrinkable. DETAILED DESCRIPTION OF THE INVENTION The general procedure for attaining the heat-recovery property of a heat-shrinkable tube comprises heating a polyethylene tube which is previously crosslinked to its melting point or more, inflating to a desired extent, and cooling while maintaining the inflated shape so as to maintain the inflated shape after cooling by crystals thus formed. The heat-shrinkable tube obtained can return to its original shape by heating again to its melting point or more. Accordingly, the crystalline component and the crosslinking of the molecules are necessary in the materials having a heat-recovery property. The ethylene/vinyl acetate copolymer used in the present invention has a vinyl acetate content of 20 wt% or more, preferably from 20 to 40 wt%, based on the total amount of the copolymer. If the vinyl acetate content is less than 20 wt%, the amount of magnesium hydroxide required to pass the Standard All-tubing Flame Test is more than 250 parts by weight. As a result, an elongation of 200% or more cannot be obtained. If the vinyl acetate content is more than 40 wt%, it is not advantageous that the amount of the crystalline component becomes small and the shape maintaining property after cooling as a heat-shrinkable tube becomes not quite good. Examples of the polyolefin resin used in combination with the ethylene/vinyl acetate copolymer having a vinyl acetate content of 20 wt% or more in the present invention include a polyethylene resin, an ethylene/vinyl acetate copolymer resin having a vinyl acetate content of less than 20 wt%, an ethylene/ethyl acrylate copolymer resin, an ethylene/butene-1 copolymer resin and combinations thereof. The resin composition used in the present invention is formed by mixing one or more of polyolefin resin and an ethylene/vinyl acetate copolymer having a vinyl acetate content of 20 wt% or more so that the total vinyl acetate content of the resin composition is 20 wt% or more. If the vinyl acetate content of the resin composition is less than 20 wt%, the amount of magnesium hydroxide required to pass the Standard All-tubing Flame Test becomes more than 250 parts by weight so that the elongation of 200% or more cannot attained. The amount of magnesium hydroxide added to the resin composition of the present invention is from 150 to 250 parts by weight per 100 parts by weight of the resin composition. If the amount is less than 150 parts by weight, a flame retardancy passing the Standard All-tubing Flame Test cannot attained. If the amount is more than 250 parts by weight, the elongation of 200% or more cannot be attained. The average particle diameter of the magnesium hydoxide is preferably from 0.2 to 20 µm, more preferably from 0.5 to 10 µm. The surface of the magnesium hydoxide is preferably treated with, e.g., a surface active agent, a silane coupling agent or a titanate coupling agent, in view of dispersing such into the resin composition (as described in Japanese Patent Publication No. 30262/75, Japanese Patent Application (OPI) No. 243155/85, U.S. Patent 4,322,575, etc.) (the term "OPI" used herein means a published unexamined Japanese patent application). The method for crosslinking which can be used in the present invention is for example crosslinking by an organic peroxide compound (as described in Japanese Patent Application (OPI) No. 34036/86), crosslinking by an electron beam, silane graft water crosslinking (as described in Japanese Patent Publication No. 19459/76). In the flame retardant heat-shrinkable tube of the present invention, an antioxidizing agent, a pigment, a lubricating agent, inorganic or organic fillers may be used. The All-tubing Flame Test is carried out by: inserting a conductive material, having an outer diameter the same as the inner diameter of the heat-shrinkable tube, into the heat-shrinkable tube; and heating the tube with a burner. The tube is disqualified if the tube burns for 1 minute or more after putting out the burner; if the predetermined mark (i.e., the indicator flag defined in Standard All-tubing Test) of the tube burns out; or if cotton placed beneath the tube burns as a result of a matter dropped from the tube. The present invention is explained in more detail referring to the following Examples and Comparative Examples. EXAMPLES 1 TO 8 The ingredients indicated in Table 1 below were mixed in the proportion indicated in Table 1 and formed into pellets. The pellets were formed into a tube having an outer diameter of 4.0 mm and an inner diameter of 2.8 mm by using an extruder so as to prepare samples of Examples 1 to 8. The tubes obtained each was irradiated by an electron beam of 24.10⁴ J/kg (24 Mrad) so as to crosslink the tube. The tensile test and the Standard All-tubing Flame Test, according to the UL Standard, were carried out on the tubes thus-obtained. The results obtained are shown in Table 1 below. As shown in Table 1, all of the samples of Examples 1 to 8 according to the present ivention have an elongation of 200% or more and pass the Standard All-Tubing Flame Test. COMPARATIVE EXAMPLES 1 TO 7 By using the ingredients indicated in Table 2 below, the same procedures as in Examples 1 to 8 were repeated so as to prepare samples of Comparative Examples 1 to 7. The tensile test and the Standard All-tubing Flame Test according to the UL Standard were carried out on the tubes thus-obtained. The results obtained are shown in Table 2 below. As shown in Table 2, the samples of Comparative Examples 1 to 7, which are the outside of the scope of the present invention, cannot simultaneously attain an elongation of 200% or more and pass the Standard All-tubing Flame Test. As stated in the foregoing, the flame retardant heat-shrinkable tube according to the present invention has both excellent flame retardancy without the generation of toxic gas and an excellent mechanical properties.

Claims: A flame retardant heat-shrinkable tube comprising a crosslinked flame retardant resin composition which comprises: (a) 100 parts by weight of an ethylene/vinyl acetate copolymer having a vinyl acetate content of 20 wt% or more based on the total amount of the copolymer, or 100 parts by weight of a resin mixture containing a polyolefin resin and an ethylene/vinyl acetate copolymer, said resin mixture having a vinyl acetate content of 20 wt% or more based on the total amount of said resin mixture, and (b) from 150 to 250 parts by weight of magnesium hydroxide, said tube being obtainable by inflating it at or above its melting point and then cooling it whilst maintaining the inflated shape so as to render the tube heat-shrinkable. A heat-shrinkable tube as claimed in claim 1, wherein said ethylene/vinyl acetate copolymer has a vinyl acetate content of from 20 to 40 wt%. A heat-shrinkable tube as claimed in claim 1, wherein said polyolefin resin is selected from a polyethylene resin, an ethylene/vinyl acetate copolymer resin, an ethylene/ethyl acrylate copolymer resin, an ethylene/butene-1 copolymer resin, and a combination thereof. A heat-shrinkable tube as claimed in claim 1, wherein said magnesium hydroxide has an average particle diameter of from 0.2 to 20 µm. A heat-shrinkable tube as claimed in claim 4, wherein said magnesium hydroxide has an average particle diameter of from 0.5 to 10 µm. A heat-shrinkable tube as claimed in claim 1, wherein said magnesium hydroxide has been treated with a surface active agent, a silane coupling agent, or a titanate coupling agent. A heat-shrinkable tube as claimed in claim 1, wherein said crosslinking is carried out using an organic peroxide compound, an electron beam, or silane graft water crosslinking. A heat-shrinkable tube as claimed in claim 1, wherein the resin composition has been crosslinked by an electron beam.

Inventors: SAKAMOTO, YOSHITO C/O OSAKA WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: C08L23
IPC codes: C08K3, H01B7