Flame-resistant flexible resin compositions for electrical cable coatings

Abstract: A flame-resistant resin composition is prepared for use as a coating material and applied to electrical cables. The resin composition is not crosslinked, and contains no halogen. It is highly formable, flexible, heat-resistant and oil proof. The flame-resistant flexible resin composition at least comprises 100 parts by weight of polymeric materials containing 50 to 90 parts by weight of thermoplastic elastomer comprising a polyolefin segment and a rubber segment in the same molecule, 5 to 25 parts by weight of copolymers comprising ethylene and a vinyl-based compound containing at least one carbonyl group, and 25 parts by weight at the most, excluding zero, of polyolefins having a crystallinity of at least 55 %, and 30 to 200 parts by weight of metal hydroxides the surface of which is not treated with stearic acid.

Description: The present invention relates to flame-resistant flexible resin compositions used for coatings, which do not generate halogen-type noxious gases even when accidentally burned. The resin compositions of the invention are in particular suitable for use as coatings for electrical cables, especially as substantially thick coatings for electrical cables. The invention also relates to electrical cables that use such resin compositions as fire-proof coatings. Poly(vinyl chloride), also known as PVC is commonly used as an insulating coating material for electrical cables. The reason for this preferential use is that PVC satisfies the prerequisites for coating materials, such as high electrical insulation, flame resistance, oil-proofing, waterproofing and ozone resistance. Further, by blending with appropriate additives such as a plasticizer or a filler, PVC can easily provide a wide spectrum of products, depending on the purpose, ranging from heat-proof to cold-proof, or from hard products to soft products. However, PVC suffers from the fact that it is a halogen-containing polymer. When PVC is burned accidentally, it generates noxious gases such as hydrogen chloride. Accordingly, when an electrical cable containing PVC is set on fire, hydrogen chloride gas is generated from the PVC and may cause secondary damages such as corrosion of the metals used for wiring. Moreover, in our environmentally conscious age, it has become necessary to reduce or recycle industrial wastes such as plastic materials. As is well known, PVC is already used for coating automobile wiring-harness cables. However, the hydrogen chloride gas generated from PVC when incinerated has recently caused serious damage. As a result, it is now required to use halogen-free-type, flame-resistant coating materials that do not generate toxic gases such as hydrogen halide gas. Typical halogen-free-type flame-resistant materials include a mixture consisting of a polyolefin resin, and a metal hydroxide as a halogen-free flame-resistant agent. However, electrical cables obtained by coating with known halogen-free type flame-resistant materials are less flexible and less resilient, in comparison with PVC-based products. Moreover, in order to guarantee flame resistance, a considerable amount of metal hydroxide has to be added to the flame-resistant materials. Now, an increase in the amount of added flame-resistant agents (flame retardants), is to the detriment of mechanical strength of the product, such as wear resistance or tensile strength. In order to confer wear resistance to such halogen-free materials, the latter may be hardened, or coatings with these materials may be made thicker. For instance, an electrical cable can be given a thick coating with a layer thickness of 0.5 to 0.8 mm, so that, even if wear resistance is decreased by addition of a metal hydroxide, the risk for the wearing away coating remains small. The conductive portion of the electrical cable may thus be protected from possible exposure. However, when the coating containing such a hard material is thickened, the coated electrical cable becomes too hard, and makes the wiring process more difficult. It thus appears that to solve these problems, an electrical cable should first be coated thickly with a flexible material. Examples of appropriate materials include ethylene-vinyl acetate copolymer (EVA), ethylene-ethylacrylate copolymer, and the like. They can be used alone or in a mixture. However, such flexible materials usually have a very low melting point, poor heat resistance and poor oil proofing capacity. They are thus unsuitable as a coating material for electrical cables used in oily environments such as in automobiles. To remedy these shortcomings, such materials may be cross-linked by electron beams, or through chemical treatments such as vulcanization. However, to cross-link the coatings of an elongate electrical cable, a large scale equipment is required, both for electron beam irradiation and for a vulcanization treatment. This means that the productivity of electrical cable manufacture is lowered and the production has to be entrusted to a specialized manufacturer. Accordingly, there exists a strong need for non-cross-linked coating materials which are oil- and heat-proof, and highly flexible. Document EP-A-0 383 147, US-A-5 561 185 and EP-A-0 490 394 disclose a flame-resistant flexible resin composition containing (a) a thermoplastic elastomer comprising a polyolefin segment and a rubber segment, (b) a copolymer comprising ethylene and a vinyl-based compound containing one or a plurality of carbonyl groups, (c) a polyolefin and (d) a metal hydroxide, the surface of which is not treated with stearic acid. In order to ensure a high heat resistance without proceeding to cross-linking, polypropylene having a relatively high melting point may be blended with a high-density polyethylene. However, the blending of these two polymers impairs flexibility. Further, electrical cables have to be subjected to end portion conditioning in later stages. As illustrated in FIG. 1, when an end of a cable is peeled off or stripped of the insulating coatings made of such polymers in such a conditioning process, there forms a cut-out face after the coating 1 that is partially chipped away. Such a cut-out face leaves behind whisker-like trails 2 of insulating coating 1 (hereafter referred to as whiskers) along a conductive portion 3. This gives rise to the so-called "necking" phenomenon. When the conductive portion 3 with such whiskers 2 is forcibly connected to a terminal, the whiskers get caught between the conductive portion 3 and the terminal. As a result, contact resistance increases and conductivity deteriorates. Accordingly, when the above-mentioned polymer mixture is used, its whisker-trailing effect due to low formability creates problems. The present invention has therefore a primary object to remedy these shortcomings of the prior art, and to provide a non cross-linked resin composition containing no halogen, which has a good formability, flexibility, heat resistance and oil-proofing, as well as to provide an electrical cable coated with such a resin composition. To this end, there is provided a flame-resistant flexible resin composition at least comprising 100 parts by weight of polymeric material containing about 50 to 90 parts by weight of thermoplastic elastomer, about 5 to 25 parts by weight of copolymer comprising ethylene and a vinyl-based compound containing one or a plurality of carbonyl groups, and about 25 parts by weight at the most, excluding zero, of polyolefin having a crystallinity of at least about 55 %, and about 30 to 200 parts by weight of at least one metal hydroxide, the surface of which is not treated with stearic acid. According to the invention, the thermoplastic elastomer is a block copolymer comprising a polyolefin segment and a rubber segment in the same molecule. Preferably, the thermoplastic elastomer comprising a polyolefin segment and a rubber segment in the same molecule has a melting point of at least about 130 °C and a JIS-A hardness of about 90 at the most. Preferably also, the copolymer comprising ethylene and a vinyl-based compound containing one or plurality of carbonyl groups is at least one compound selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer and ethylene-methacrylate copolymer. The polyolefin having a crystallinity of at least about 55 % preferably comprises at least one of a high density polyethylene or a polypropylene. The electrical cable according to the present invention utilizes the above-mentioned flame-resistant resin composition as a coating material. Preferably, the coating is about 0.5 to 0.8 mm thick. There is also provided a use of the flame-resistant resin composition of the present invention as a coating material for electrical cables. The above and the other objects, features and advantages of the present invention will be made apparent from the following description of the preferred embodiments, given as non-limiting examples, with reference to the accompanying drawings, in which: FIG. 1 is a view illustrating the formation of coating whiskers when the coating is stripped off the electrical cable; and FIG. 2 is a view illustrating a flame-resistance testing method. The polymeric material used in the flame-resistant resin composition of the invention comprises a thermoplastic elastomer comprising a polyolefin segment and a rubber segment in the same molecule; a copolymer comprising ethylene and a vinyl-based compound containing at least one carbonyl group, and a polyolefin having a crystallinity of at least 55 %. The flame-resistant resin composition further comprises a metal hydroxide as a flame-resistant agent. The thermoplastic elastomer comprising a polyolefin segment and a rubber segment in the same molecule according to the present invention is a block copolymer which comprises a polyolefin as a hard segment (H) and a rubber as a soft segment (S). This elastomer confers flexibility to the coating. Typical polyolefins used as the hard segment (H) include polyethylene and polypropylene, and typical rubbers used as the soft segment (S) include an ethylene propylene binary copolymer (EPR), an ethylene propylene diene terpolymer (EPDM), butadiene and isoprene. The type of block copolymer may be either a di-block type H-S, a tri-block type H-S-H, a multi-block type (H-S)n or a star type. Typical elastomers used in the present invention include PE-EPR, PE-EPDM, PP-EPR, PP-EPDM, PE-polybutadiene, PE-polyisoprene and PP-polybutadiene elastomers. They may be used alone or in a mixture of at least two elastomers. Among the above-mentioned thermoplastic elastomers, the compound having a JIS-A hardness of 90 at the most is preferably used in the present resin composition. When the JIS-A hardness exceeds 90, the elastomer does not confer sufficient flexibility to the coatings. JIS-A hardness is measured by using a Spring-type Hardness Test Machine A, defined in JIS K6301 Standards. Even when two thermoplastic elastomers are composed of the same hard segment and the same soft segment, JIS-A hardness may be differentiated by changing the proportion of hard segment to soft segment. JIS-A hardness may also be differentiated by selecting the molecular weight (polymerization degree) of each segment. The melting point of the thermoplastic polyolefin-rubber elastomer according to the invention is preferably at least 130 °C. When it is less than 130 °C, heat resistance of the coating is too low. Such a thermoplastic polyolefin-rubber elastomer accounts for 50 to 90 % by weight of the polymeric material. When its amount is less than 50 % by weight, the coating is not sufficiently flexible. When its amount exceeds 90 % by weight, the coating's formability is not good. As a result, when end or middle portions of the electrical cable are stripped off their coatings, necking occurs, giving rise to coating whiskers. The addition of thermoplastic elastomer thus confers flexibility to the coatings, but lowers their formability. The addition of the copolymer comprising ethylene and a vinyl-based compound containing at least one carbonyl group can compensate for this lowered formability. Examples of such a copolymer include an ethylene-acrylate copolymer such as ethylene-methyl acrylate copolymer or ethylene-ethyl acrylate copolymer, an ethylene-methacrylate copolymer such as ethylene-methyl methacrylate copolymer or ethylene-ethyl methacrylate copolymer, and ethylene-vinyl acetate copolymer (EVA). The copolymer comprising ethylene and a vinyl-based compound containing at least one carbonyl group accounts for 5 to 25 % by weight of the polymeric material. When its amount is less than 5 % by weight, the formability of the coatings is not sufficiently improved. Conversely, when its amount is in excess of 25 % by weight, oil-proofing of the coatings is not sufficient. Addition of the copolymer comprising ethylene and a vinyl-based compound containing at least one carbonyl group thus tends to lower the oil-proofing property of the coatings. Conversely, polyolefins having a crystallinity of at least 55 % have a good oil-proofing property. Addition of the latter thus compensates the lowering of the oil-proofing property of the coatings. Examples of such polyolefins include a high density polyethylene (crystallinity of about 65 to 90 %) and an isotactic polypropylene (crystallinity of about 60 %). A polyolefin having high crystallinity usually hardens the product to which it is added. Such polyolefins account for 25 % by weight at the most, but excluding zero, of the polymeric material. When its amount is in excess of 25 % by weight, the flexibility of the coatings is impaired. In addition to the polymeric material, the resin composition of the present invention comprises metal hydroxides as flame-resistant agents. Typical metal hydroxides include magnesium hydroxide and aluminum hydroxide. However, when added massively, these metal hydroxides deteriorate the physical properties of the base polymer, in particular its tensile strength, elongation and flexibility. Accordingly, a metal hydroxide is added in an amount of 30 to 200 parts by weight, relative to 100 parts by weight of the polymeric material. In order to raise mechanical strength, the surface of heat-resistant metal hydroxides may be treated, prior to use, with a surface-treatment agent. However, this treatment deteriorates the product's formability. For instance, coatings containing a metal hydroxide whose surface is treated with higher fatty acids, especially stearic acid, are likely to cause necking. Therefore, the metal hydroxides used in the invention are not usually treated with these surface-treatment agents. The resin composition of the invention further comprises, where necessary, anti-oxidants, copper-pollution preventing agents, colorants, lubricants or similar compounds. However, additions of these compounds should be limited to a certain level so as not to deteriorate the physical properties necessary for use as cable coatings, i.e. flexibility, formability, flame resistance, oil proofing, tensile strength and elongation. The resin composition prepared as described above is suitable for use as electrical cable insulating coatings, in particular the insulating coatings as thick as 0.5 to 0.8 mm. An electrical cable coated with such a resin composition is oil proof, flexible in spite of its thickness, and easy to use for wiring. It also has a good formability: when a coated cable is subjected to an end portion conditioning or a middle-zone preparation for external connection, corresponding portions of the coatings must be stripped off. When a prior art coated cable is used, such a stripping process creates coating whiskers. By comparison, an electrical cable coated with the inventive resin composition forms no such whisker. As described above, the electrical cable of the invention is coated with the resin composition according to the present invention. Coatings are prepared from the inventive resin composition, without passing through the cross-linking steps. Notwithstanding, the cable coatings according to the invention are secure as regards the necessary oil proofing and heat resistance. As its coating is flexible, the inventive electrical cable is well adapted for a heavy coating as thick as 0.5 to 0.8 mm. Accordingly, the present cable is particularly suitable for electrical cabling in automobiles. Examples [Crystallinity measurement] Crystallinity is usually calculated from density. To obtain a precise density, "density-gradient tube" methods are preferably used. One example of such methods is described in Standard "JIS K 7112", according to which the crystallinities mentioned in the present invention are measured. When a density is measured, crystallinity is calculated as follows: Crystallinity (%) = (d - da)/(dc - da) x (dc/d) x 100 where d, da and dc signify a measured density, the density of perfect amorphous bodies and the density of perfect crystals, respectively. [Evaluation Method] Evaluation methods of the examples of the present invention are described hereinafter. 1) Flame resistance The flame resistance is assessed by Horizontal Burning Tests according to JASO D611 Standards. A Bunsen burner having a nozzle diameter of about 10 mm is adjusted so as to obtain a reducing flame length of about 35 mm. A test sample is kept horizontal as shown in FIG. 2. The middle zone of the test sample is heated by the flame from underside of the sample, such that the test sample lights up within 30 seconds. The flame is then removed, and the time elapsed before the lit-up flame fades away is counted. When this time is equal to, or less than, 15 seconds, flame resistance is considered to be good and denoted with a "O" in TABLE II. Otherwise, the result is denoted with an "X". 2) Tensile strength (MPa) The tensile strength is measured by tensile tests of insulating materials according to JASO D611 Standards. A dumbbell type or tubular test sample is prepared. The test sample is drawn from both ends of the test sample by a Pendulum Tensile Test Machine at a speed of 300 mm/min (in the case of dumbbell type test samples) or 500 mm/min (in the case of tubular test samples), and the load at breaking point is measured. When the load is in excess of 15.7 MPa, the test sample has a good tensile strength, and is denoted with an "O" in TABLE II. Otherwise, the result is denoted with an "X". For use as an electrical cable, the material of the test sample should have a tensile strength of at least 15.7 MPa. 3) Elongation (%) In the tensile test mentioned above, the elongation of the broken test sample is measured, and elongation rates (%) with regard to the initial sample length are calculated. Satisfactory results are denoted with an "O". Otherwise, the results are denoted with an "X". 4) Oil proofing A coating sample is dipped in engine oil at 70 °C. After 24 hours, the sample is withdrawn from the oil and allowed to stand for natural cooling. When the sample's tensile strength or elongation after dipping, compared to what it was before dipping, is altered by an amount less than or equal to 40 %, a mark "O" is given. When the alteration exceeds 40 %, a mark "X" is given. 5) Flexibility Flexibility is estimated by "hand" feeling when a cable sample is flexed. When the flexibility is felt sufficiently upon flexing, the result is denoted with an "O", otherwise it is denoted with an "X". 6) Formability A coated cable is stripped off its coatings at a middle portion along its length, and the formation of coating whiskers is observed with the naked eye. When whiskers are not formed, an "O" mark is given, and when they are formed, an "X" mark is given. [Preparation of electrical cables] A conductive wire is prepared by twisting 65 soft copper threads, each having a diameter of 0.32 mm, to obtain a cross-section of 5 mm 2 . A flame-resistant resin composition is extruded on the wire surface, so as to form a coating of 0.7 mm thickness. The resin compositions each contain the constituent parts as shown in TABLE I. In these resin compositions, components of the polymeric material part and magnesium hydroxide part (a flame-resistant agent) are varied in amounts as shown in TABLE II. Extrusion is carried out at a die temperature of 210 °C, a cylinder temperature of 200 °C and a line speed of 50 m/min. Electrical cable Samples 1 to 12 thus prepared have coatings consisting of different components. Among them, Samples 1 and 2 correspond to the examples according to the present invention, whereas Samples 3 to 12 correspond to comparative examples. Constituent parts Amounts by weight Polymeric material 100 Magnesium hydroxide Variable Anti-aging agents 1 Zinc stearate 0.3 Calcium stearate 0.3 Thermoplastic polyolefin-rubber elastomer used is "P.E.R. T310J" produced by Kabushiki Kaisha Tokuyama (Tokuyama Corp.), which corresponds to polypropylene-EPR having a JIS-A hardness of 90, and "NEWCON NB 2100" produced by Chisso Corporation, which corresponds to polypropylene-EPR having a JIS-A hardness of 95. These products differ by JIS-A hardness due to different propylene contents. The anti-aging agent used is "Tominox TT" which is a phenol-type product produced by YOSHITOMI FINECHEMICALS, LTD.. The electrical cables thus prepared are each subjected to the tests concerning flame resistance, oil proofing, flexibility, tensile strength, elongation and formability according to the evaluation methods described above. The results obtained are shown in TABLE II, together with the amounts of components included in the resin compositions. Samples 1 2 3 4 5 6 7 8 9 10 11 12 Elastomer PP-EPR(90) 60 50 60 60 100 60 40 50 50 60 80 - PP-EPR (95) - - - - - - - - - - - 60 Polyolefin PP(60) 20 - 30 - - - - 25 25 20 20 - High density PE (70) - 25 - - - - 30 - - - - 20 Low density PE (50) - - - - - 20 - - - - - - E-VA 20 25 10 40 - 20 30 25 25 20 - 20 Mg(OH) 2 Non-treated 100 150 100 100 100 150 100 - 20 250 100 100 Treated with stearic acid - - - - - - - 100 - - - - Evaluation Flame resistance O O O O O O O O X O O O Oil proofing O O O X O X X O O O O O Flexibility O O X O O O X O O X O X Tensile strength O O O O O O O O O O O O Elongation O O O O O O O O O X O O Formability O O O O X O O X O O X O By comparing the results obtained with Samples 1 and 3, it can be concluded that when a higher amount of polypropylene having a high crystallinity is used, the resulting product becomes less flexible. Conversely, Sample 4 indicates that when the product does not contain highly crystalline polyolefin, but contains proportionally high EVA portion, flexibility is satisfactory, but the oil proof property is not sufficient. Further, comparison of the results from Samples 1 and 6 indicates that even when the product contains an appropriate amount of polyolefin, its oil proof property is not satisfactory if the polyolefin has a crystallinity of 50 %. By comparing the results from Samples 1 and 12, it can be concluded that when an elastomer having a hardness of 95 is used, the resulting product lacks flexibility. Consequently, in order to secure a necessary amount of flexibility, the elastomer should have a hardness of less than 95, preferably less than 90. According to the results from Sample 5, the elastomers, when used alone, fulfill the prerequisite conditions for oil proofing, flexibility, tensile strength and elongation, but not for formability. According to Sample 11, when an elastomer and a polyolefin are combined, oil proofing, flexibility, tensile strength and elongation are satisfied even if EVA is not contained, but not formability. According to Sample 7, when the elastomer accounts for less than 50 % by weight of the polymeric material, the resulting product is not flexible enough. Further, when EVA content is in excess of 25 % by weight of the polymeric material, the product is not sufficiently oil proof. On the basis of the results from Sample 9, the content of flame-resistant agents should not be too low. However, as seen from Sample 10, when the content exceeds 200 parts by weight, mechanical characteristics such as elongation and flexibility deteriorate. According to the results from Sample 8, when the surface of magnesium hydroxide used as a flame-resistant agent is treated with stearic acid, the formability of resulting product is lowered. The flame-resistant resin composition according to the invention contains no halogen for flame prevention, so that it is environment-friendly. The resin composition is oil proof and flexible without being cross-linked. The inventive resin composition is therefore suitable for coatings requiring such features. The electrical cable using such a resin composition is also environment-friendly, flame-resistant, oil proof, and has a high mechanical strength. Flexibility is maintained intact even when the electrical cable is sized up and its coating made thicker. The wiring process can thus be facilitated. Moreover, end portion conditioning and a middle-length preparation of the electrical cable are easily performed.

Claims: A flame-resistant flexible resin composition at least comprising 100 parts by weight of polymeric material containing about 50 to 90 parts by weight of thermoplastic elastomer, about 5 to 25 parts by weight of copolymer comprising ethylene and a vinyl-based compound containing one or a plurality of carbonyl groups, and about 25 parts by weight at the most, excluding zero, of polyolefin having a crystallinity of at least about 55 %, and about 30 to 200 parts by weight of at least one metal hydroxide the surface of which is not treated with stearic acid; characterised in that said thermoplastic elastomer is a block copolymer comprising a polyolefin segment and a rubber segment in the same molecule. The flame-resistant resin composition according to claim 1, wherein said thermoplastic elastomer comprising a polyolefin segment and a rubber segment in the same molecule have a melting point of at least about 130 °C and a JIS-A hardness of about 90 at the most. The flame-resistant resin composition according to claim 1 or 2, wherein said copolymer comprising ethylene and a vinyl-based compound containing one or a plurality of carbonyl groups is at least one compound selected from the group consisting of ethylene-vinyl acetate copolymer, ethylene-acrylate copolymer and ethylene-methacrylate copolymer. The flame-resistant resin composition according to any one of claims 1 to 3, wherein said polyolefin having a crystallinity of at least about 55 % comprises at least one of a high density polyethylene or a polypropylene. An electrical cable including a coating comprised of the flame-resistant resin composition defined in any one of claims 1 to 4. The electrical cable according to claim 5, wherein said coating is about 0.5 to 0.8 mm thick.. Use of the flame-resistant resin composition defined in any one of claims 1 to 4 as a coating material for electrical cables.

Inventors: HASE TATSUYA, MATSUMOTO SHINICHI, SENOO RYOU, YAMANO YOSHIAKI, HASE, TATSUYA, MATSUMOTO, SHINICHI, SENOO, RYOU, YAMANO, YOSHIAKI
Applicants: SUMITOMO WIRING SYSTEMS, SUMITOMO WIRING SYSTEMS, LTD.
Application year: 1999
Main classification: H01B3
IPC codes: C08K3, H01B3, C09K21, C08L53