JPH0458498B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates to a method for producing rubber foam with improved surface texture. Rubber foams are used in a wide range of applications, including automobiles and industrial products, and are generally required to have a certain density, soft feel, water absorption resistance, abrasion resistance, low compression set, and high mechanical strength. In particular, the surface texture of rubber foam is a very important issue, and rubber foams with good surface texture are
It not only increases its commercial value, but also has good properties such as abrasion resistance and water absorption resistance. By the way, such rubber foams are generally produced by preforming an unvulcanized rubber composition containing a blowing agent using an extruder, roll, press, etc. It is manufactured by heating in a heating tank such as a salt bath, fluidized bed bath, high frequency bath, or steam pipe bath to perform vulcanization and foaming in a timely manner. However, in such a method, if vulcanization precedes foaming too much, the viscosity of the compound increases and then foams, making it difficult to obtain a high expansion ratio (low density). The vulcanization rate changes due to variations in the kneading conditions, and the density of the rubber foam changes significantly due to slight variations in the heating conditions, making it difficult to maintain stable shape and performance, increasing the defective rate of the product (reducing productivity). Cause. When the vulcanization rate is slow, it becomes easier to balance vulcanization and foaming, and the defect rate of the product can be reduced, but there is a problem that the surface texture deteriorates. On the other hand, from an industrial standpoint, what is important is to produce rubber foams having a certain level of performance and good surface texture at a high yield and in a stable manner with high productivity. For this reason, in the conventional manufacturing method of rubber foam, it is necessary to precisely control the vulcanization rate and foaming rate, which is different from the case of non-foamed materials, where only the vulcanization rate is the key point. By the way, this is also the reason why advanced technology is required. In view of this, the present inventors have studied to solve the problems of these conventional methods and to easily manufacture rubber foams with good surface texture without deteriorating their properties or productivity. As a result, we have completed the present invention. That is, in the present invention, the surface of a molded article of an unvulcanized rubber composition containing a blowing agent is irradiated with a dose of 0.1 to 50 Mrad.
The present invention provides a method for producing a rubber foam having an improved surface texture, which comprises irradiating the rubber foam with an electron beam to form a crosslinked surface layer within a range of 100 to 100 ml, followed by heating, vulcanization, and foaming. The method of the present invention involves irradiating the surface of a molded article of an unvulcanized rubber composition containing a blowing agent and a vulcanizing agent with an electron beam to form a crosslinked skin layer on the surface layer, and then heating to vulcanize and foam. Therefore, it is possible to make the surface of the rubber foam smooth regardless of the speed of vulcanization and foaming or the balance thereof. The unvulcanized rubber composition for rubber foam used in the present invention is not particularly limited, and can be used in a kneading machine such as a Banbury mixer, a kneader mixer, or a mixing roll, in the same way as conventionally produced for rubber foams. Used in rubber, reinforcing agents, fillers, softeners,
Manufactured by kneading blowing agents, vulcanization aids, stabilizers, etc. Examples of the rubber include natural rubber, isoprene rubber, styrene-butadiene rubber, butadiene rubber, styrene-propylene rubber, chloroprene rubber, isoprene-isobutylene rubber, acrylonitrile-butadiene rubber, silicone rubber, fluorine rubber, and the like. Typical blowing agents include dinitrosopentamethylenetetramine, azodicarbonamide, p,p'-orthobisbenzenesulfonylhydrazine, etc., but other blowing agents can also be used without limitation. There are no restrictions on the vulcanization system of the rubber foam composition to which the present invention can be applied, and any vulcanization system such as sulfur vulcanization, peroxide vulcanization, quinoid vulcanization, etc. can be used. Further, various commonly used reinforcing agents, fillers, softeners, vulcanization aids, colorants, stabilizers, anti-aging agents, etc. can be used in the rubber composition used in the present invention. Further, such an unvulcanized rubber composition is molded into a desired molded article by preforming, and the molding method is not particularly limited, and can be performed by any conventionally known method such as an extruder, press, or roll. In the conventional method, such a molded article is directly subjected to the next step of vulcanization and foaming, but in the method of the present invention, the surface of the molded article is irradiated with an electron beam prior to vulcanization and foaming to create a crosslinked surface layer (crosslinked skin layer). Its characteristic lies in the fact that it allows the formation of In this electron beam irradiation, the optimal accelerating voltage and irradiation dose vary depending on the type of rubber in the unvulcanized rubber composition that is the raw material, the type of compounding agents, the compounding ratio, etc., and are determined appropriately according to each condition. However, the thickness of the crosslinked skin layer formed on the surface is influenced by the accelerating voltage of the electron beam, and the higher the accelerating voltage, the thicker the skin layer becomes. The accelerating voltage applied to the present invention is usually 1 to
3000KV, preferably in the range of 10-500KV,
The thickness of the skin layer formed at this time varies depending on the rubber composition, but generally ranges from several microns to several hundred microns. If the accelerating voltage is too low, the thickness of the crosslinked skin layer will not be sufficient, and if the accelerating voltage is too high, the crosslinked skin layer will be too thick, making it difficult to obtain a good rubber foam. The degree of crosslinking of the crosslinked skin layer varies depending on the amount of electron beam irradiated. In the present invention, the irradiation dose is more important, and if the irradiation dose is too small, a sufficient crosslinked skin layer will not be formed and the effect of improving the surface skin of the foam will be small. Furthermore, if the irradiation dose is too large, crosslinking may proceed, increasing the strength of the skin layer and inhibiting foaming. The electron beam irradiation dose applied to the present invention is usually in the range of 0.1 to 50 Mrad, preferably 1 to 20 Mrad. A crosslinked skin layer is formed on the surface of the molded article by such electron beam irradiation, and then heated, vulcanized, and foamed. As heating equipment for this vulcanization and foaming, there are various heating equipment that have been used conventionally, such as heating tanks such as a heat press, a hot air tank, a molten salt tank, a fluidized bed tank, a high frequency tank, and a steam pipe tank. used, but not limited to. Moreover, two or more heating methods can be used in combination as necessary. The heating temperature is higher than the vulcanization temperature and the foaming temperature, and is selected depending on the respective conditions. Thus, according to the method of the present invention, a rubber foam with a good surface texture can be obtained even with an unvulcanized rubber composition whose vulcanization rate is slower than that of conventional rubber compositions. Therefore,
Variations in density during the rubber foam manufacturing process are also small, reducing the occurrence of product defects. In addition, since a crosslinked layer is formed on the surface of the blowing agent before it decomposes, it is possible to greatly reduce the amount of decomposed gas from the blowing agent that escapes from the surface. can be obtained. The present invention will be explained below with reference to Examples. Example 1

[Table] The composition shown in Table 1 was kneaded using a Banbury mixer and a mixing roll, and
An EPDM composition was obtained. This rubber composition is extruded into a tube shape using an extruder, the surface of the extruded product is irradiated with an electron beam, and then heated in a hot air bath at 230°C for 5 minutes to perform vulcanization and foaming to form a rubber foam. I got it. Table 2 shows the evaluation of the surface texture of the obtained rubber foam and the specific gravity of the rubber foam.

[Table] *2 Rubber foam surface texture ◎ Excellent, ○ Good, △ Fair As shown in Table 2, electron beam irradiation dramatically improves the surface texture of rubber foam without changing the expansion ratio. can be done, and its effects are significant.

Claims (1)

[Claims] 1. The surface of a molded article of an unvulcanized rubber composition containing a foaming agent is irradiated with an electron beam at an irradiation dose of 0.1 to 50 Mrad to form a crosslinked surface layer, and then heated to vulcanize and foam. A method for producing a rubber foam having an improved surface texture.