PRODUCTION OF MOLDED VULCANISED RUBBER ARTICLES

Abstract: A rubber composition is prepared containing a rubber (such as styrene-butadiene rubber, ethylene-propy­ lene rubber, acrylonitrile-butadiene rubber, chlorinated ethylene-propylene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, silicone rubber, fluorinated rubber, natural rubber or chloroprene rubber), a peroxide (e.g. dicumyl peroxide) and other additives, and is molded to shape (e.g. to a sheet); the mixture is heated in the presence of oxygen (air) to 140 - 260°C (e.g. 150°C for 15 minutes) to cause vulcanisation by crosslinking due to the peroxide; and usually before but optionally after the heating the molding is irradiated with electron beams, usually of 100 - 500 kV and at a dose of 1 to 40 Mrad so as to fully vulcanise the surface layer only, to a depth of 0.05 - 2 mm. The product obtained is not sticky on its surface and has excellent tensile properties.

Description: The present invention relates to a process for producing rubber products by vulcanizing a molded article composed of a rubber composition containing a peroxide as a crosslinking agent in the presence of oxygen, wherein the molded article is irradiated with electron beams. Compared with vulcanization of rubbers with sulfur, the technique of vulcanization with peroxides used as crosslinking agents offers superior heat resistance, compression set, and electrical insulation for molded articles and superior properties in compounding with colored substances, e.g., coloration and stability of color. Vulcanization with peroxides has therefore enjoyed extensive commercial use in applications where the aforementioned properties are particularly important. However, as is well known, vulcanization with peroxides is hindered by oxygen and, if the vulcanization is carried out in the presence of air, crosslinking reaction does not proceed to an adequate extent on the surface of the resulting molded article which is in contact with air, leading to various problems such as causing extreme tackiness, reduction in strength, and insufficient hardness. These problems cause the defects that the surface of the molded article is readily scratched and is poor in wear resistance and slipperiness. Accordingly, vulcanization with peroxides is generally applied to the production of rubber products using a vulcanization step shielded from air, such as press molding, injection molding or transfer molding. In the case of autoclave cure which is a commercial technique for fabricating hoses and electric cables, products satisfying practical requirements can be obtained by replacing air (oxygen) with steam. However, in order to yield products of consistent quality, utmost care must be exercized in process control. Continuous vulcanization of rubbers has recently gained commercial acceptance, and industrially important rubber products such as hoses, weather strips, and roofing sheets are currently produced by the continuous vulcaniza­ tion method. While continuous vulcanization can be performed by various methods, the most commonly employed techniques are hot-air vulcanization (HAV) which may be in a fluidized bed (FBV), and ultra-high frequency vulcanization (UHF). However, in each of these methods the vulcanization is in the presence of air and, hence, they are not suitable for vulcanization with peroxides. We have now devised a continuous method of vulcanization with peroxides which is capable of providing rubber products with excellent heat resistance and compression set. The present invention provides a process for producing a rubber product by continuously vulcanizing a molded article composed of a rubber composition containing a peroxide as a crosslinking agent in the presence of oxygen, wherein the molded article is irradiated with electron beams. The irradiation with electron beams is typically carried out in a step which is preliminary to the vulcanization with a peroxide but, if desired, the molded article may be irradiated with electron beams after the vulcanization with a peroxide. In accordance with the process of the present invention, a molded rubber article is subjected, either before or after vulcanization with a peroxide, to irradiation with electron beams to an appropriate degree so as to vulcanize a surface layer of the molded rubber. The term "vulcanization" as used herein means vulcanization which is performed in the presence of air (or oxygen). The vulcanization may be carried out by using a vulcanization system wherein a heating medium such as steam or heated nitrogen gas, air (oxygen) is not fully replaced by the heating medium such as steam or heated nitrogen gas. Vulcanization may be carried out in the present invention by employing the aforementioned HAV, UHF or FBV process. Vulcanization may also be performed in a batch system by, for example, autoclave cure or oven cure. The method and conditions of irradiation with electron beams are not critical, and appropriate acceler­ ation voltages, irradiation doses and irradiation equip­ ment may be selected depending on specific use of the final product. The acceleration voltage typically is from 100 to 500 KV and more suitably from 150 to 400 kV, and the irradiation dose from 1 to 40 Mrad and more suitably from 2 to 30 Mrad. However, these ranges may be suitably adjusted as needed. The electron beam causes vulcanisation of only the surface layer, to a depth of about 0.05 to 2 mm. The rubber composition treated by the present invention is characterized by the presence of a peroxide as a crosslinking agent. While the peroxide usable as a crosslinking agent is not limited to any particular compounds, illustrative examples include dicumyl peroxide, 1,3-bis(t-butylperoxyisopropyl)benzene, 2,5-dimethyl-­ 2,5-di(t-butylperoxy)hexane, 1,1-di-t-butylperoxy-3,3,5-trimeth­ ylcyclohexane, cumene hydroperoxide, di-t-butyl peroxide, t-butyl­ cumyl peroxide, p-chlorobenzoyl peroxide, t-butyl peroxybenzoate and t-butylperoxyisopropyl carbonate. A suitable vulcanization temperature is from 140 to 260°C, and an appropriate peroxide is chosen depending upon the vulcanization temperature employed. The rubber composition used in the present invention contains, in addition to the peroxide as a crosslinking agent, a variety of ingredients such as rubber bases, resins, carbon black, talc, clay, calcium carbonate, silica, process oils, processing aids, activators, crosslinking aids, antioxidants, pigments, foaming agents, foaming aids, and any other necessary components. If desired, a variety of short fibers may be incorporated in the rubber composition. Illustrative rubber bases include styrene-­ butadiene rubber, ethylene-propylene rubber, acrylo­ nitrile-butadiene rubber, chlorinated ethylene-propylene rubber, chlorinated polyethylene, chlorosulfonated poly­ ethylene, silicone rubber, and fluorinated rubber. Natural rubber and chloroprene rubber may be used as required. Illustrative resins include polyethylene, polypropylene, an ethylene-vinyl acetate copolymer, and polyvinyl chloride. Illustrative processing aids include agents that serve to improve the dispersibility of fillers during mastication, tackifiers, and extrusion aids. Illustrative activators include a variety of metal oxides such as zinc white and zinc stearate, glycols, and amines. Illustrative crosslinking aids that assist the vulcanization with peroxides include a variety of sulfur compounds, oximes, maleimides, and methacrylic acid esters. The types and amounts of these ingredients and the peroxide used as a crosslinking agent are properly selected and determined in accordance with the specific properties required for the final rubber product. The composition used in the present invention may be prepared by blending appropriate ingredients with rubber by means of a suitable machine such as a Banbury mixer, a kneader, or a roll, and is sub­ sequently molded by means of a calendar roll or an extruder. By employing the process of the present invention, rubber products such as hoses, rubber sheets, roofing sheets, canvas sheets, weather strips, sealing sponge, protector tubes, and protector sponge having excellent heat resistance, compression set, and electrical insulation can be obtained. In addition, the rubber products obtained are not sticky on the surface and exhibit excellent tensile properties. The following examples are provided for the purpose of further illustrating the present invention and are by no means intended to limit the invention. Example 1 A peroxide-loaded rubber composition having the recipe indicated below was masticated on rolls (10 in ), and the admixture was fed into an appropriate extruder for rubber (ID: 45 mm) from which a sheet having a thickness of 3 mm and a width of 40 mm was extrusion molded. Both sides of the sheet were irradiated with electron beams at an acceleration voltage of 150 kV to give a total dose of 20 Mrad. The irradiated sheet was then vulcanized in a gear oven (in air) at 150°C for 15 minutes. Data on the surface stickiness, tensile strength, and elongation of the vulcanized rubber sheet are given in Table 1, which clearly demonstrates the improvement attained by irradiation with electron beams. Example 2 A sample of an unvulcanized (green) rubber sheet which was of the same type as that extrusion molded in Example 1 was vulcanized in a gear oven (in air) at 150°C for 15 minutes. Thereafter, both sides of the sticky vulcanized sheet were irradiated with electron beams at an acceleration voltage of 150 kV to give a total doese of 20 Mrad. The results are shown in Table 1. The surfaces of the oven-vulcanized sheet ceased to be sticky after irradiation with electron beams. A measurable increase in the tensile strength of the sheet also occurred as a result of irradiation with electron beams. Example 3 A sample of an irradiated rubber sheet which was of the same type as obtained in Example 1 was vulcanized by autoclave cure at 150°C for 15 minutes. The results are shown in Table 1. Comparative Example 1 A sample of a green rubber sheet which was of the same type as that extrusion molded in Example 1 was vulcanized in a gear oven (in air) at 150°C for 15 minutes. The vulcanized rubber sheet was very sticky and had reduced tensile properties, as is evident from Table 1. Comparative Example 2 A sample of a green rubber sheet which was of the same type as that extrusion molded in Example 1 was subjected to autoclave cure at 150°C for 15 minutes. The results were unsatisfactory as shown in Table 1. Comparative Example 3 A rubber composition having the recipe shown in Example 1 was vulcanized on a heat press at 150°C for 10 minutes. Data on the surface stickiness and tensile properties of the vulcanized rubber are shown in Table 1. The vulcanization system employed in this comparative example permitted no contact with oxygen and, hence, produced satisfactory results.

Claims: 1. A process of producing a molded rubber article by vul­ canizing a molded rubber composition in the presence of oxygen, charact­ erised in that the composition contains a peroxide as a cross­ linking agent for the rubber, and the molded composition is treated to be cross-linked by the action of the peroxide and is also, before or after the crosslinking, irradiated with electron beams so as to vulcanize a surface layer of the article. 2. A process as claimed in Claim 1, wherein the irradiation is at an acceleration voltage of 100 to 500 KV and a dose of 1 to 40 Mrad. 3. A process as claimed in Claim 1 or 2, wherein the crosslinking is effected by treating the article with hot air or nitrogen, steam or ultrahigh frequency radiation. 4. A process as claimed in Claim 1, 2 or 3, where­ in said peroxide is selected from dicumyl peroxide, 1,3-­ bis(t-butylperoxyisopropyl) benzene, 2,5-dimethyl-2, 5-­ di(t-butylperoxy) hexane, 1,1-di-t-butylperoxy-3,3,5-­ trimethylcyclohexane, cumene hydroperoxide, di-t-butyl peroxide, t-butylcumyl peroxide, p-chlorobenzoyl peroxide, t-butyl peroxybenzoate and t-butylperoxyisopropyl carbon­ ate. 5. A process as claimed in any preceding claim, wherein the rubber in the composition is styrene-butadiene rubber, ethylene-propylene rubber, acrylonitrile-butadiene rubber, chlorinated ethylene-propylene rubber, chlorinated polyethylene, chlorosulfonated polyethylene, silicone rubber, fluorinated rubber, natural rubber or chloroprene rubber.

Inventors: AOSHIMA, MASASHI, JINNO, TADASHI
Applicants: SUMITOMO CHEMICAL COMPANY, LIMITED
Application year: 1986
Main classification: B29C35
IPC codes: C08J3, B29C59, B29C35