Heat recoverable articles

Abstract: The invention relates to a heat recoverable article, such as heat-shrinkable tubing which is used as an insulating sleeve for an electrical cable connection or terminal arrangement. The tubing is formed from a polymer material, and has an inner adhesive layer formed on its inner surface. A resin composite containing ethylene alkyl (meth) acrylate carbon monoxide copolymer is used for the inner layer. A heat shrinkable tubing is obtained which possesses outstanding adhesive properties in relation to an associated adherent material, without suffering from foaming during the manufacturing process since this inner layer exhibits relatively little hygroscopic properties.

Description: This invention relates to heat recoverable articles, such as heat shrinkable tubing suitable for joints or terminal arrangements of electrical cables and the like. Heat recoverable articles for example heat shrinkable tubings are used in applications such as insulating sleeves for joints or connections of electrical cables, terminal protection arrangements for wiring in many types of equipment, or for anti-corrosion treatment of steel pipes. When a connection or joint part between electrical cables is sleeved with the heat shrinkable tubing, and the tubing is then heated, the tubing shrinks around the shape of the joint part and contracts so tightly against the part that it can be protected from external injuries. When a greater adhesive property is required, for example when the part is to be waterproofed, a heat shrinkable tubing having an inner layer of an adhesive resin is used. Ethylene-vinylacetate copolymer (EVA), ethylene-enthylacrylate copolymer (EEA), hot-melt type adhesive resins based on these copolymers and hot-melt adhesive resins comprising polyamide or saturated ester copolymer are conventionally used as the inner layer (for example, see Japanese Laid-open Patent Publications SHO 54-84294, SHO 56-17218 and SHO 60-22413). The adhesive resin to be used as the inner layer is appropriately selected from the above-described resins depending on the material of the heat shrinkable tubing on which the inner layer is coated or painted and on the materials of the substrates to be treated, respectively. A conventional method for manufacturing the heat shrinkable tubing comprises the following steps, i.e. (1) molding a resin composition into a tubular shape; (2) cross-linking the polymer material by irradiating with an ionizing radiation such as an electron beam or τ-ray, or by means of chemical cross-linking or silane type cross-linking; (3) expanding the tube in an environment of high temperature; and (4) cooling the tube while maintaining it in its expanded shape. Among these manufacturing methods, irradiation with ionizing radiation, in particular, has been widely used in industries as a cross-linking method for the heat shrinkable tubing because it exhibits a short cross-linking reaction time, as well as excellent productivity. The heat shrinkable tubing having the inner adhesive layer is also cross-linked by ionizing radiation. Polyamide based hot-melt adhesive resins have been most widely used as the inner layer of the heat shrinkable tubing. Their adhesive properties in relation to adherents, i.e. the materials or substrates to which they are to adhere, are outstanding. However, the polyamide based hot-melt adhesive resins have, in addition to their high cost, a deficiency in that they are very hygroscopic and therefore are liable to foam during their manufacturing process. Moreover, since saturated ester copolymer based resins showed similar properties to the polyamide based hot-melt adhesive resins, they are not satisfactory materials to solve the foaming problem. Because copolymers mainly polymerized from ethylene and vinyl acetate (VA) exhibit low hygroscopic properties and good adhesive properties in relation to many kinds of substances, EVA based copolymers were sometimes used as the inner layer. Since most of the EVA based copolymers are, however, deficient in that they exhibit poor adhesive properties in relation to polyvinyl chloride (PVC), they were seldom used for the adhesive resin when the adherent were made of PVC. Therefore, development of an adhesive resin composition with excellent adhesive properties in relation to many adherent substances, which is inexpensive, and has low hygroscopic properties, is desired. Accordingly, an object of this invention is to provide a heat recoverable article employing a relatively inexpensive inner layer having relatively low hygroscopic properties and good adhesive properties in relation to many adherent materials. The present inventors have found that the above object could be attained by using a resin, comprising an ethylene-alkyl (meth) acrylate-carbon monoxide copolymer as a main component for the inner layer. According to the present invention, a heat recoverable article comprising a polymeric material or materials is formed with a layer containing an ethylene-alkyl (meth) acrylate-carbon monoxide copolymer. When the article is tubular, the layer is an inner layer formed on the inner surface of the tube. The heat recoverable articles, such as heat shrinkable tubes embodying this invention, are not particularly limited, and may be manufactured from a wide variety of different polymer materials by conventional methods. Examples of the polymer materials are polyethylene, polyvinyl chloride (PVC), ethylene-vinyl acetate copolymer (EVA), ethylene-ethyl acrylate copolymer (EEA), chlorinated polyethylene, various kinds of thermoplastic elastomers (polyester elastomer, for example) and the like. Various kinds of additives such as cross-linking agent or fillers may be contained in these polymer materials, if required. The substrate to be treated by the heat recoverable articles embodying this invention is not limited, and examples of the substrate materials are metals such as copper, iron, aluminium, tin or nickel, glass or various kinds of polymer materials. Ethylene-alkyl (meth) acrylate carbon monoxide copolymers are used as the inner layer in this invention. The term "alkyl (meth) acrylate" in this invention means alkyl acrylate or alkyl methacrylate. Examples of ethylene-alkyl (meth) acrylate-carbon monoxide copolymers are ethylene-methyl acrylate-carbon monoxide copolymer, ethylene-ethyl acrylate-carbon monoxide copolymer or ethylene-methyl methacrylate-carbon monoxide copolymer. The composition of these ethylene alkyl (meth) acrylate carbon monoxide copolymers to be used for the inner layer is not particularly limited, but it is desirable that the melt-flow index (MI) as measured at 190°C under a load of 2.16kg is usually in the range of 1 to 500 g/10 min, preferably from 5 to 400 g/10min. If the MI value is less than 1, the adhesion to the treated substrate is poor since its melt flow property is insufficient. If the MI value is over 500, on the other hand, the inner layer will flow out during the tube expanding process. When the heat shrinkable tubing is cross-linked by irradiating with ionizing radiation, decrease in the melt-flow property by irradiating ionizing radiation can be more effectively prevented by adding anti-oxidants such as (hindered) phenols, organic sulfur compounds, amine compounds or phosphite compounds of hydroquinone derivatives to the ethylene-alkyl (meth) acrylate-carbon monoxidecopolymers. Various kinds of additives, such as flame retarder or color pigments, can be further blended with the copolymers, if required. The ethylene-alkyl (meth) acrylate-carbon monoxide copolymers are observed to have a tendency to be colored when they have been exposed to an environment of high temperature. As a result, the heat recoverable articles having the copolymer as an inner layer have a problem in that the transparency of the material deteriorates. It has been found that the problem could be alleviated by adding a light stabilizer to the ethylene-alkyl (meth) acrylate-carbon monoxide copolymer. Examples of the light stabilizer are hindered amine compounds, salicylic acid derivatives, benzophenone derivatives and benzotriazole derivatives, the hindered amine compounds are preferably used among them. The amount of the light stabilizer to be used is, based on 100 parts by weight of the ethylene alkyl (meth)) acrylate carbon monoxide copolymer, 0.1 to 10 parts by weight and preferably from 0.5 to 5 parts by weight. When the amount used is less than 0.1 parts by weight, the efficiency of coloring prevention becomes small, while an amount over 10 parts by weight is not preferable since the light stabilizer blooms. For the purpose of forming a layer comprising the ethylene-alkyl (meth) acrylate-carbon monoxide copolymer on the inner surface of the heat recoverable articles, it is preferable to apply a melt-extrusion molding method by means of co-extrusion of the polymer material forming the tubing with the material including the copolymer which forms the inner layer. The heat shrinkable tubing having an inner layer can be cross-linked by irradiating the ionizing radiation. Of course, it is no problem to use the ethylene-alkyl (meth) acrylate-carbon monoxide copolymer for the inner layer even when the heat shrinkable tubing is cross-linked by a chemical cross-linking method or a silane type cross-linking method. Needless to say, a blended material of the ethylene-alkyl (meth) acrylate-carbon monoxide copolymer with a polymer material compatible with the copolymer can be used for the inner layer embodying this invention. This invention is described hereinafter in more detail referring to particular examples. This invention is, however, not limited to these examples. The methods for evaluating the properties of the articles are as follows. Adhesive Property After the sample was applied to cover various kinds of electrical cables, and heat-shrunk to allow it to contract, the covering tubing and its inner layer were cut longitudinally of the cable into half-cylindrical form. The tip of the heat-shrunk tubing, with its inner layer, was peeled off from the cable and bent to an angle of 180° and, followed by holding the tip, the peeling strength when the tubing and its inner layer were peeled off from the cable at a tensile velocity of 50 mm/min was measured. The results were evaluated in three grades as follows. o ○ : The peeling strength is 5 kg/25mm or more. ○ : The peeling strength is 1 kg/25mm or more and less than 5 kg/25mm. X : The peeling strength is less than 1 kg/25mm. Buried degree and foaming degree of inner layer resin The sample was cut and its cross section was inspected for evidence of any crevices or foaming. As for the evaluation of buried degree, the samples in which no crevices were observed were ranked good, and those having some crevices were ranked not good. Resistivity against thermal aging As for the evaluation of coloring after heat-aging, several sheets 1 and 2 mm in thickness were prepared by using each composition of the inner layer. After allowing the sheets to stand in a gear oven at 140°C for one week, an evaluation was carried out visually as to whether the transparency of the sheet was maintained or not. The evaluation standard was as follows. o ○ : The sheet is transparent even when the thickness is 2mm. ○ : The sheet is transparent when its thickness is 1mm. X : The sheet is not transparent in both cases described above. [Example 1] An outer layer was co-extruded with an inner layer by using a flame retarder EVA resin composition (MI = 3 g/10min., vinylacetate content = 25 % by weight) as an outer layer and a resin composition, prepared by adding 3 part by weight of triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] as an antioxidant and 1 parts by weight of 2,4-dihydroxybenzophenone as a light stabilizer to 100 parts by weight of ethylene-ethyl acrylate-carbon monoxide copolymer (MI = 10 g/10min.), as an inner layer, thereby manufacturing a tubing having an inner layer with an outer diameter of the outer layer of 6mm⊘ (inner diameter of the outer layer of 5mm⊘ ) and an inner diameter of the inner layer of 4mm⊘ . The tubing was irradiated with a dose of 15 Mrad at 1 MV accelerate voltage by electron accelerator. Then, the tubing obtained was expanded in a constant temperature chamber so that the inner diameter of the inner layer became 8mm⊘ , thereby manufacturing a heat shrinkable tubing. An electric cable (5 mm⊘ coated with polyvinyl chloride (PVC), polyethylene and polyester resins was covered with the heat shrinkable tubing obtained, and the tube was allowed to contract by heating. Buried degree, foaming property, adhesive property and coloring after heat aging were examined by using the resin-coated electric cable covered with the heat shrinkable tubing tube thus obtained. The results are shown in Table 1. [Example 2] A test sample was manufactured by the same method as described in Example 1, except that an ethylene-ethyl acrylate-carbon monoxide copolymer with MI = 100 g/10 min. was used as a copolymer for the inner layer. [Example 3] A test sample was manufactured by the same method as described in Example 1, except that an ethylene-methyl acrylate-carbon monoxide copolymer with MI = 10 g/10 min. was used as a copolymer for the inner layer. [Example 4] A test sample was manufactured by the same method as described in Example 1, except that a resin composite in which 20 parts by weight of decabromodiphenyl ether and 5 parts by weight of antimony trioxide were further added to the afore-described resin composite as flame retarder was used for the inner layer. [Example 5] A test sample was manufactured by the same method as described in Example 1, except that the light stabilizer to be added to the inner layer was replaced by a hindered amine light stabilizer of bis(2,2,6,6-tetramethyl-4-piperidyl)cebacate. [Example 6] A test sample was manufactured by the same method as described in Example 1, except that a resin composition prepared without adding the light stabilizer was used as an inner layer. [Comparative example 1] A test sample was manufactured by the same method as described in Example 1, except that a polyamide of hot-melt inner type adhesive resin with MI = 10 g/10min. was used as an inner layer instead of ethylene-ethyl acrylate-carbon monoxide copolymer. [Comparative example 2] A test sample was manufactured by the same method as described in Example 1, except that a saturated ester copolymer with MI = 10 g/10min. was used as an inner layer instead of ethylene-ethyl acrylate-carbon monoxide copolymer. [Comparative example 3] A test sample was manufactured by the same method as described in Example 1, except that an ethylene vinyl-acetate copolymer (EVA) with MI = 10 g/10min. VA content-28% by weight was used as an inner layer instead of ethylene-ethyl acrylate-carbon monoxide copolymer. Example Comparative Example 1 2 3 4 5 6 1 2 3 (Blending of inner layer) •Ethylene-ethyl acrylate-CO copolymer (MI=10) 100 100 100 100 •Ethylene-ethyl acrylate-CO copolymer (MI=100) 100 •Ethylene-methyl acrylate-CO copolymer (MI=10) 100 •Polyamide (MI=!0) 100 •Saturated copoly-polyester (MI=10) 100 100 •EVA (MI=25, VA=28 wt%) •Decabromodiphenyl ether 20 •Antimony trioxide 5 •Antioxidant 3 3 3 3 3 3 3 3 3 •Benzophenone series light stabilizer 1 1 1 1 1 1 1 •Hindered amine series light stabilizer 1 Buried degree of the inner layer good good good good good good good good good Foaming of the inner layer Non Non Non Non Non Non Yes Yes Non Transparency of the inner layer after heat aging ○ ○ ○ ○ o ○ X ○ ○ o ○ Adhesive property to the adherent to PVC cable o ○ o ○ o ○ ○ o ○ o ○ o ○ o ○ X to polyethylene cable ○ ○ ○ ○ ○ ○ X X ○ to polyester cable ○ ○ ○ ○ ○ ○ ○ ○ X The heat shrinkable tubings according to Examples 1 to 6 are outstanding in relation to the characteristics of the inner layer, since the heat shrinkable tubings have a good buried degree, foaming problems do not arise and their adhesive properties in relation to many adherents are excellent, especially in relation to PVC. As for the coloring after the heat aging, however, the heat shrinkable tubing according to Example 6 in which no light stabilizer was added showed a large degree of coloring as compared with the heat shrinkable tubings according to Examples 1 to 5, in which light stabilizers, especially hindered amine light stabilizers, were added, and little coloring was observed. The heat shrinkable tubings according to Comparative examples 1 to 2 have a poor adhesive strength in relation to polyethylene, besides exhibiting foaming. Moreover, the heat shrinkable tubing according to Comparative example 3 possesses poor adhesive properties in relation to polyester or PVC although it has no problem in respect of the buried degree, foaming and coloring after the aging treatment. The heat shrinkable tubing embodying the invention possesses outstanding adhesive properties in relation to many adherents, without suffering from foaming during the manufacturing process because the material has a low hygroscopic property This is achieved by using a resin composition whose main component comprises an ethylene-alkyl (meth) acrylate-carbon monoxide copolymer as the inner layer of the heat recoverable article, for example heat shrinkable tubing or the like.

Claims: A heat recoverable article comprising a polymer material or materials, characterized in that an inner layer containing an ethylene-alkyl (meth) acrylate-carbon monoxide copolymer is formed on its inner surface. A heat recoverable article according to claim 1, wherein the inner layer contains a light stabilizer in the range of 0.1 to 10 parts by weight based on 100 parts by weight of the ethylene-alkyl (meth) acrylate-carbon monoxide copolymer. A heat recoverable article according to claim 1 or 2, which comprises a heat shrinkable tubing. A heat recoverable article according to claim 1, 2 or 3, wherein the ethylene-alkyl (meth) acrylate-carbon monoxide copolymer is ethylene-ethyl acrylate-carbon monoxide copolymer. A heat recoverable article according to any preceding claim, wherein a flame retarder agent is added to the inner layer.

Inventors: ADZUMA SHUJI, HAYAMI HIROSHI, HOSOKAWA TAKEHIRO, ADZUMA, SHUJI, HAYAMI, HIROSHI, HOSOKAWA, TAKEHIRO, Adzuma, Shuji, c/o Osaka Works, HAYAMI, HIROSHI, C/O OSAKA WORKS, Hosokawa, Takehiro, c/o Osaka Works
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1992
Main classification: B32B27
IPC codes: B29C65, B29C61, H01B3, B32B27, H01B17, B29K105, B29C63, B29L9, F16L47, C08L73