Short fiber-reinforced rubber

Abstract: Disclosed is a short fiber-reinforced rubber wherein fibrilated polyacrylonitrile short fibers are uniformly dispersed in a rubber, and a process for producing the same.

Description: The present invention relates to a process for producing a short fiber-reinforced rubber. More particularly, it relates to a short-fiber reinforced rubber having high strength and modulus and a process wherein such a short fiber-reinforced rubber can be easily produced. A short fiber-reinforced rubber in which short fibers are included in order to provide strength or functionality (e.g. anisotropy, durability, etc.) to a rubber has been known. The properties of a reinforced rubber depend upon the kind, amount, orientation and dispersion state of the short fibers. Particularly, the improvement of properties such as strength, modulus, etc. are conducted, for example, by the improvement of short fiber-rubber interaction (e.g. adhesion, bonding property, etc.). The improvement of short fiber-rubber interaction can be conducted, for example, by subjecting the short fiber to a surface treatment [e.g. resolcin-formalin-latex (RFL), etc, etc.] or a surface modification (e.g. graft treatment, plasma treatment, etc). However, the surface treatment or surface modification of the short fiber is a complicated process in the production of the short fiber-reinforced rubber. In addition, it generally involves high costs and a long time to develop a suitable surface treating agent or a surface modification method. We have now developed a process for producing a short fiber-reinforced rubber having an excellent strength and modulus, in which the dispersion of short fibers in a rubber and the short fiber-rubber interaction are improved without subjecting the short fibers to a surface treatment or surface modification. The present inventors have found that, if a short fiber-formulated rubber composition is kneaded with the application of a strong shear force, the short fiber is torn, which results in a uniform dispersion and network, namely, uniform fibrilation. According to the present invention, there is provided a process for producing a short fiber-reinforced rubber which comprises kneading a rubber composition comprising a base rubber, polyacrylonitrile fibers of not less than 10 µm in length, a crosslinking co-agent and an organic peroxide with the application a shear force, wherein the kneading of the composition is carried out at a rotating speed of at least 20 rpm for at least 10 minutes. The process of the present invention is particularly useful for rubbers vulcanized by an organic peroxide although it can also be applied to those vulcanized by sulfur and the like. The base rubber used in the present invention is not specifically limited and it may be anyone which has been used for a rubber composition heretoforce. Examples thereof include isoprene rubber, natural rubber, butadiene rubber, styrene-butadiene rubber, nitrile rubber, ethylene rubber, ethylene propylene rubber, chloroprene rubber or mixtures thereof. A particularly preferred base rubber is cis-1,4-poly-butadiene, the percentage of the cis structure being not less than 90%. Further, it is preferred to use a base rubber which has been kneaded with the application of a shear force in a predetermined direction in advance, as described in Japanese Patent Application No. 2-106713. When such a rubber having a high hardness and modulus in which a strength is provided with anisotropy is used, the strength of the resulting short fiber-reinforced rubber is further enhanced. An amount of the base rubber formulated is preferably 70 to 99 volume % for the rubber composition. As the short fiber formulated in the rubber composition, polyacrylonitrile (PAN) fiber is suitable. Polyacrylonitrile is easily fibrilated by a shear force applied during kneading of the rubber composition, which provides high performances to the rubber. It is firmly fixed and adhered to a rubber matrix by fibrilation and, at the same time, its performance as the reinforcing fiber is maintained. Other fibers (e.g. aromatic polyamide fiber, polyester fiber, polyethylene fiber, etc.) may be added to the above-described polyacrylonitrile fiber in a small amount. As described above, it is not necessary that these fibers are subjected to a surface treatment or surface modification. However, in order to further enhance the strength of the short fiber-reinforced rubber, a heat stretching treatment or adhesion treatment may be conducted. The length (L) of the short fiber is not less than 10 µm, particularly 1 to 10 mm. Further, the ratio (L/D) of fiber length (L) to fiber diameter (D) is preferably not less than 10, particularly 50 to 1000. When the ratio is less than 10, the rubber can not be provided with sufficiently high strength. The amount of the short fiber formulated is preferably 1 to 30 volume % for the rubber composition. When the amount is less than 1 volume %, the rubber can not be provided with sufficiently high strength and, when the amount exceeds 30 volume %, a problem arises in formability. As the crosslinking co-agent which is used in the present invention, for example, there is a metal salt of α,β-unsaturated fatty acid. As the metal salt of α,β-unsaturated fatty acid, a metal of α,β-monoethyenically unsaturated carboxylic acid having 3 to 8 carbon atoms is preferred and examples thereof include metal salts of methacrylic acid, acrylic acid, itaconic acid and crotonic acid. As the metal, for example, divalent metals such as zinc and magnesium are preferred, although other metals (e.g. sodium, lithium and aluminium) may be used. The amount of the co-crosslinking agent is preferably 5 to 80 volume %, particularly 5 to 80 volume %, particularly 10 to 30 volume %. Further, this ingredient may be formulated in the form of metal oxide, hydroxide or carbonate with α,β-unsaturated fatty acid to form a salt in the rubber composition. Examples of the organic peroxide which are formulated in the rubber composition of the present invention, are perbenzoic acid, benzoyl peroxide, cumene peroxide and dicumyl peroxide. Dicumyl peroxide is preferred. The amount of the organic peroxide is preferably 0.1 to 5 volume % for the rubber composition. Further, additives such as fillers (e.g. silica, calcium carbonate, etc.), colorants, antioxidants and the like may be formulated in the rubber composition. The short fiber-reinforced rubber of the present invention is produced by kneading the above rubber composition with the application of a shear force, followed by vulcanization forming. Strong shear force is applied to the short fiber during kneading and, as a result, it is fibrilated and, therefore, dispersibility in the rubber is improved. As the method of kneading with the application of a shear force, a method comprising charging an appropriate amount (65 to 95%) of a rubber composition upon mixing and kneading with a kneader or Banbury mixer at a rotating speed of at least 20 rpm for at least 10 minutes, preferably at a rotating speed of 4 to 200 rpm for 10 to 50 minutes, is provided. When the rotating speed is less than 20 rpm and the mixing time is less than 10 minutes, the short fiber is not sufficiently fibrilated. Thereafter, the rubber composition is formed by passing it through an open roll in one direction, followed by a normal vulcanization (140 to 170°C, 10 to 40 minutes) to obtain a short fiber-reinforced rubber of the present invention. The short fiber-reinforced rubber thus obtained as described above has high strength and modulus, for example, it is suitably used for a timing belt, marine material, chain or rubber spring. According to the present invention, there is provided a process for producing a short fiber-reinforced rubber having excellent strength and modulus without subjecting the short fibers to surface treatment or surface modification. The following Examples and Comparative Examples further illustrate the present invention in details are not to be construed to limit the scope thereof. In the Examples and Comparative Examples, all "parts" are by weight unless otherwise stated. Examples 1 and 2 and Comparative Examples 1 to 3 Each rubber composition comprising the ingredients shown in Table 1 was charged in a mixing container at about 70 %, followed by kneading under the conditions of a rotating speed and kneading time shown in Table 1, respectively. The rubber composition thus kneaded was formed by passing it through an open roll, followed by vulucanization at 160°C for 25 minutes. The strength, modulus and hardness of each short fiber-reinforced rubber was determined and the results are shown in Table 1. Further, each measuring test was conducted according to JIS-K6301. Example 1 Example 2 Comp.Ex.1 Comp.Ex.2 Comp.Ex.3 Butadiene rubber 70 70 70 70 70 Natural rubber 15 15 15 15 15 Ethylene propylene rubber 15 15 15 15 15 Zinc oxide 32 32 32 32 32 Methacrylic acid 30 3 30 30 30 Dicumyl peroxide 1.0 1.0 1.0 1.0 1.0 PAN fiber (vol%) 10 10 10 - - Nylon 66 (vo%) - - - 10 - RFL-treated nylon 66 (vol%) - - - - 10 Rotating spead(rpm) 30 30 30 30 30 Kneading time (min) 10 25 5 10 10 Breaking strength (MPa) 33 36 23 26 29 Elongation at breakage (%) 19 17 18 57 21 10% Modulus (MPa) 19 25 16 7 13 Tear strength(kN/m) 92 99 83 89 73 Hardness (JIS-A) 95 97 94 90 91 As is clear from Table 1, a short fiber-reinforced rubber of Examples 1 and 2 has high strength and modulus in comparison with Comparative Example 1, because the short fiber is fibrilated by increasing the kneading time. Further, a short fiber-reinforced rubber of Examples 1 and 2 has high strength and modulus in comparison with Comparative Examples 2 and 3, because dispersibility of the short fiber in the rubber as well as rubber-short fiber interaction are improved.

Claims: A process for producing a short fiber-reinforced rubber which comprises kneading a rubber composition comprising a base rubber, polyacrylonitrile fibers of not less than 10µm in length, a crosslinking co-agent and an organic peroxide with the application of a shear force, wherein the kneading of the composition is carried out at a rotating speed of at least 20 rpm for at least 10 minutes. A process as claimed in claim 1 wherein the base rubber is cis-1, 4-polybutadiene having a cis structure of 90% or more. A process as claimed in claim 1 or claim 2 wherein the crosslinking co-agent is an α,β-unsaturated fatty acid. A process as claimed in any one of claims 1 to 3 wherein the crosslinking co-agent is present in an amount of 5 to 80 volume % and the organic peroxide is present in an amount of 0.1 to 5 volume %, based on the total volume of the rubber mixture. A process as claimed in any one of claims 1 to 4 wherein the short fiber is present in an amount of from 1 to 30 volume % of the rubber composition. A process as claimed in any one of claims 1 to 5 wherein the polyacrylonitrile fibers have a fiber length of from 1 to 10 mm. A process as claimed in anyone of claims 1 to 6 wherein the ratio (L/D) of fiber length (L) to fiber diameters (D) is not less than 10.

Inventors: NAKAHARA AKIHIRO, NOYAMA TOMOKO, NAKAHARA, AKIHIRO, NOYAMA, TOMOKO
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES, LTD.
Application year: 1992
Main classification: C08J5
IPC codes: C08K7, C08L21, C08J5, C08K5