Vulcanizable rubber blends

Abstract: A sealing part formed a peroxide vulcanizate of a rubber compound having as a polymer component an EPDM mixture of 50 to 90 parts by weight of an EPDM having a Mooney viscosity (ML₁+₄ 100°C) of 35 to 55, and 50 to 10 parts by weight of a low-Mooney EPDM (a sub-polymer) having a Mooney viscosity (ML₁+₄ 100°C) of 5 to 25. The sealing part at the present invention has a high resistance to low-temperature compression permanent strain and is excellent in the kneadability of a rubber compound for producing the sealing part.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to vulcanizable EPDM rubber blends, and, in particular to peroxide vulcanizates of a rubber blind comprising a mixture of high Mooney viscosity and low Mooney viscosity EPDM rubbers and articles of manufacture, such as mechanical seals for hydraulic systems formed therefrom. 2. Background Information The tendency of polymeric materials used in the sealing parts for hydraulic cylinders, to wear is well known. This creates a variety of problems including a loss of mechanical properties such as low resistance to abrasion, erosion, and sealing capability. Consequently, it has been proposed to improve the mechanical properties of the sealing parts by using other polymeric materials, such as styrene-butadiene rubber (SBR). Although the use of styrene-butadiene rubber has been proposed for use in sealing parts, this use is subject to a variety of constraints. For example, such components cannot be used at higher temperatures or with high boiling substances (such as boric ester-added ethylene glycol and silicon oil) for brake fluids. As a consequence, manufacturers have now switched from traditional styrene-butadiene rubber components to rubber components comprising ethylene propylene diene monomer (EPDM). Recently, due to the installment of electronic lockproofing mechanisms (e.g. the "ESC system") to guard against impact when braking, a need has arisen for sealing parts having great strength (particularly high resistance to erosion and to abrasion) and satisfactory sealing properties. Such sealing parts, however, do not retain these mechanical properties over long periods of time. It has been propsed to improve sealing parts for a hydraulic cylinder comprising a composition formed of a peroxide vulcanizate of a rubber compound comprising as a polymer component an ethylene propylene diene monomer. The polymer component satisfies the following requirements: (a) an ethylene content of 60 to 70 wt%, an α-olefin content of 40 to 30 wt%, a Mooney viscosity of (ML₁+₄ 100°C; hereinafter referred to as "ML") 35 to 55, and an iodine value of 20 to 35; (b) a molecular weight distribution where M w /M n ≦ 5.0; and (c) an α-olefin content in the polymer component which is 20 wt%, each on the opposite sides of the α-olefin composition distribution where C₃E - C₃I ≦ 5.0 wt%, where C₃I stands for the high molecular weight side α-olefin component and C₃E for the low molecular weight side α-olefin component (see Japanese Patent Application Laid-open SHO 61(1986)-221249). The rubber compound comprising ethylene propylene diene monomer as the polymer component, however, exhibits poor kneading processability. That is, dispersal of additional (supplementary) materials in the rubber compound during the primary kneading step with a Banbury mixer or the like prior to roll milling is not satisfactory, and the composition obtained by the primary kneading does not exhibit roll retention during secondary kneading, i.e., the roll milling. Furthermore, the peroxide vulcanizate does not possess sufficient resistance to low-temperature compression permanent strain in some cases when excellent physical properties are required. The EP-A-0 246 745 discloses a curable composition comprising a high molecular weight EPDM having a particular ethylene/propylene ratio and a low molecular weight EPDM, each in a specific amount, and a peroxide curing agent. Applicants have now devised a sealing part for a hydraulic cylinder which eliminates or substantially overcomes the aforementioned problems. This sealing part exhibits an improved resistance to low-temperature permanent compression strain and permits improvement in the kneading processability of a rubber compound used for producing the sealing part. Accordingly, the present invention provides a vulcanizable rubber blend comprising: 50 to 90 wt parts of an EPDM rubber having a Mooney viscosity (ML₁+₄ 100°C) of 35 to 55; 10 to 50 wt parts of an EPDM rubber having a Mooney viscosity (ML₁+₄ 100°C) of 5 to 25; and a peroxide as a vulcanizing agent; the EPDM rubber having the Mooney viscosity of 35 to 55 having an ethylene content of 45 to 59 wt%, an iodine value of 20 to 35, a molecular weight distribution of M w /M n ≦ 5.0, an α-olefin content of 41 to 55 wt%, and an α-olefin composition distribution in the case of which 20 wt% of the α-olefin content of the EPDM at opposite ends of the molecular weight distribution comply with the equation C₃E - C₃I ≦ 5.0 wt%, wherein C₃I and C₃E corresponds to the α-olefin content at the ends of the high molecular weight region and the low molecular weight region, respectively. The present invention further provides a sealing part formed of a peroxide vulcanizate of a rubber mixture comprising: 50 to 90 parts by weight of an EPDM having an ethylene content of 45 to 59 wt%, a Mooney viscosity (ML₁+₄ 100°C) of 35 to 55, a molecular weight distribution of M w /M n ≦ 5.0, an iodine value of 20 to 35, an α-olefin content of 41 to 55 wt%, and an α-olefin composition distribution in the case of which 20 wt% of the α-olefin content of the EPDM at opposite ends of the molecular weight distribution comply with the equation C₃E - C₃I ≦ 5.0 wt%, wherein C₃I and C₃E corresponds to the α-olefin content at the ends of the high molecular weight region and the low molecular weight region, respectively; and 50 to 10 parts by weight of a low-Mooney EPDM (a sub-polymer) having a Mooney viscosity (ML₁+₄ 100°C) of 5 to 25. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a plate type piston cup for a hydraulic cylinder. Fig. 2 shows an annular type piston cup for a hydraulic cylinder. DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS As noted above, the invention relates to vulcanizable EPDM rubber blends, and, in particular to peroxide vulcanizates of a rubber blend comprising a mixture of high Mooney viscosity and low Mooney viscosity EPDM rubbers and articles of manufacture, such as mechanical seals or sealing parts for any system requiring properties of improved resistance to low-temperature permanent compression strain and permits improvement in the kneading processability. The blending proportions are all by weight unless otherwise specified. (1) The main polymer ethylene propylene diene monomer (hereinafter referred to as "the main EPDM") is obtained by adding ethylene, an α-olefin and at least one diene to an inert solvent in the presence of a suitable catalyst and polymerizing under atmospheric pressure or slightly increased pressure. This resulting polymer has an ethylene content of 45 to 59 wt%, an α-olefin content of 41 to 55 wt%, a diene (third component) content of not more than 5 wt%, a Mooney viscosity (ML) of 35 to 55 and an iodine value of 20 to 35. Generally, propylene is used as the α-olefin. Optionally, 1-butene or other similar α-olefin may be used instead. As the diene, a non-conjugated diene or a mixture of non-conjugated dienes selected from the group consisting of ethylidenenorbornene, propenyl norbornene, vinyl norbornene, dicyclopentadiene, 1,4-hexadiene, 2-methyl-1,5-hexadiene, 1,6-octadiene, 1,5-octadiene, 1,7-octadiene, 1,4-octadiene, and methyl hydroindene is used. The diene may also be used in combination with conjugated dienes such as butadiene, isoprene, etc. When the ethylene content in the composition is less than 45 %, the mechanical properties of the vulcanizate will deteriorate. When it exceeds 59 wt%, the low temperature resistance is lowered. (2) In the main EPDM, the molecular weight distribution is so sharp as to be defined by the equation (M w /M n ≦ 5.0 and the α-olefin composition distribution is so sharp as to be defined by the equation C₃E - C₃I ≦ 5.0 wt% , where C₃I stands for the high molecular weight side α-olefin content and C₃E for the low molecular weight side α-olefin content respectively at opposite sides of the molecular weight distribution wherein each of the C₃I and C₃E fraction represents 20 wt% of the α-olefin content of the EPDM. In the EDPM polymer used in conventional sealing parts, the molecular weight distribution satisfies the following equation M w /M n = 8 - 20 and the α-olefin composition distribution satisfies the equation C₃E - C₃I = 8 - 15 (wt%) . If the requirements mentioned above are not met, the improvement particularly in the resistance to erosion and to enervation cannot be obtained. (3) The low-Mooney EPDM (referred to as "the sub-EPDM") is blended as a sub-polymer with the main EPDM. The low-Mooney EPDM, i.e., the sub-EPDM, is an EPDM having a Mooney viscosity (ML) of 5 to 25 (preferably 5 to 15) which is much lower than that of conventional EPDM's (30 or more). The ethylene content of the sub-EPDM is preferably close to that of the main EPDM from the viewpoint of compatibility between these EPDM's. The proportions of the main EPDM and the sub-EPDM are 50 to 90 parts and 50 to 10 parts, respectively. When the proportion of the sub-EPDM is less than 10 parts, the effects of the addition of the sub-EPDM (mainly an improving effect on the kneading processability) cannot be obtained. When it exceeds 50 parts, the sub-EPDM has an undesirable influence on the strength properties of the vulcanizate. (4) The polymer component obtained by blending the sub-EPDM having a low Mooney viscosity with the main EPDM are properly incorporated with compounding ingredients such as carbon black, zinc oxide, processing aid, antioxidant, and coloring material which are generally used in the peroxide vulcanizate of EPDM. An organic peroxide is also added to the polymer component together with a co-crosslinking agent. By compression, transfer or injection molding and vulcanizing the rubber compound thus obtained, the sealing part for the hydraulic cylinder aimed at by this invention is produced. As processing aids, conventional higher fatty acids and the like can be used. As the antioxidant, conventional amine type antioxidants and the like can be used. Specific examples of the organic peroxide are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, n-butyl-4,4-bis(t-butylperoxy)-valerate, dicumyl peroxide, di-t-butylperoxy-diisopropylbenzene, 2,5-dimethyl-2,5-(t-butylperoxy)-hexane, and 2,5-dimethyl-di(t-butylperoxy)-hexine-3. Specific examples of the co-crosslinking agent are sulfur, trimethacrylate, trimethylol propane, triallyl isocyanurate, and phenothiazine. As described above, the sealing part for a brake cylinder according to the present invention is a sealing part for a hydraulic cylinder, formed of a peroxide vulcanizate comprising an EPDM mixture as a polymer component, wherein the polymer component is a blend of the main EPDM and the sub-EPDM having a low Mooney viscosity, and the ethylene content of the main EPDM is made lower than that of the EPDM described in the above reference (conventional example: Japanese Patent Application Laid-open SHO 61(1986)-221249), namely, the α-olefin content of the main EPDM is increased as much. Consequently, when compared with the conventional sealing part for a hydraulic cylinder, the sealing part of the present invention exhibits improved resistance to low-temperature permanent compression strain, and improved kneadability (processability) during preparation of the rubber compound, i.e., a material for forming the sealing part of the present invention. The present invention is further illustrated by the following examples. Examples 1-4 and Comparative Examples 1-7 As the main EPDM; copolymers of varying composition, attributes and distributions as indicated in Table 1 were used. The data on M w /M n was derived from molecular weight distributions determined by the gel permeation chromatography (using o-dichlorobenzene as solvent). The data on C₃E - C₃I were calculated from molecular weight distributions determined by the fractional precipitation method (number of fractions: not less than 10, solvent: cyclohexanol/icopropyl alcohol). As the sub-EPDM, a copolymer having a Mooney viscosity of ML 8, an ethylene content of 61 wt%, a propylene content of 39 wt%, and an iodine value of 22. The basic rubber formulation was as follows. As the furnace black, a normal type one (iodine adsorption capacity: 53 mg/g, dibutylphthalate adsorption capacity: 58 ml/100 g) was used. A varying rubber compound of the formulation shown above was kneaded and than compression molded (vulcanizing conditions: 170°C x 20 minutes), whereby test pieces were prepared. The test pieces were subjected to the following tests. (1) Normal physical properties [H S (hardness), T B (tensile strength), and E B (elongation)] - By the methods of JIS K 6301 (using dumbbel test pieces, No. 3), with necessary modifications. (1) Tear resistance - By the method of tear tests (B type) specified by JIS K6301, with necessary modifications. (3) Permanent strain by compression - (1) JIS Method: The compression permanent strain test (150°C x 22 hours) specified in JIS K6301 was carried out with necessary modifications. (2) Standing method: In the compression permanent strain test of JIS K6301, a given test piece was heated at 150°C for 22 hours, compressed, and then left standing in a compressed state until it cooled to room temperature, after which it was left standing at room temperature for another 5 hours. (4) Low-temperature permanent strain by compression - The low-temperature permanent strain test (-30°C x 22 hours) specified in JIS K6301 was carried out with necessary modifications. (5) Abrasion resistance - With an Acron type tester, a given test piece was abraded 1,000 times under the conditions (number of rotations of friction wheel; 33 rpm, temperature of the ambient air; 23°C). After the test, the test piece was measured to determined loss of volume. (6) Processability (1) Kneadability was determined by observing the state of materials after primary kneading (Banbury mixer) of each rubber compound. The rating (mark) shown in the Table is as follows; O: united in a body, ▵: not united in a body. (2) With respect to roll retention, the state of retention of rubber during secondary kneading (roll milling) was determined. The rating (mark) shown in the Table 1 is as follows; O: substantially not peeled, ▵: sometimes peeled. From Table 1 showing the results of all the tests, the following can be seen. In all of Examples 1 to 4 of the present invention, the low-temperature permanent strain by compression is small, and the processability is satisfactory. In Comparative Examples 1 to 7 in which the sub-polymer is not added, the processability is not satisfactory. In both Comparative Examples 2 and 3 in which the ethylene content of the main EPDM is outside the range specified in the present invention (namely, it exceeds 59 wt%), the low-temperature permanent strain by compression is large. In Comparative Example 4 in which the ethylene content of the main EPDM is in the specified range of the present invention but the iodine value and C₃E - C₃I are outside the ranges of the present invention, the ordinary-temperature permanent strain by compression is large, and the low-temperature permanent strain by compression is large as well. In Comparative Examples 1, 5, 6 and 7 in which the sub-EPDM is not added though other constituents are in the ranges specified in the present invention, the low-temperature permanent strain by compression is small. Examples 5 and 6 Each of the rubber compounds of the formulations of Examples 1 to 4, respectively, was kneaded and then transfer molded (vulcanizing conditions: 170°C x 20 minutes) to produce the piston cups shown in Fig. 1 and Fig. 2. While the present invention has been illustrated by detailed descriptions of preferred embodiments thereof, it will be obvious to those skilled in the art that various changes in form and detail can be made therein without departing from the true scope of the invention. For that reason, the invention must be measured by the claims appended thereto.

Claims: A vulcanizable rubber blend comprising: 50 to 90 wt parts of an EPDM rubber having a Mooney viscosity (ML₁+₄ 100°C) of 35 to 55; 10 to 50 wt parts of an EPDM rubber having a Mooney viscosity (ML₁+₄ 100°C) of 5 to 25; and a peroxide as a vulcanizing agent; the EPDM rubber having the Mooney viscosity of 35 to 55 having an ethylene content of 45 to 59 wt%, an iodine value of 20 to 35, a molecular weight distribution of M w /M n ≦ 5.0, an α-olefin content of 41 to 55 wt%, and an α-olefin composition distribution in the case of which 20 wt% of the α-olefin content of the EPDM at opposite ends of the molecular weight distribution comply with the equation C₃E - C₃I ≦ 5.0 wt% , wherein C₃I and C₃E corresponds to the α-olefin content at the ends of the high molecular weight region and the low molecular weight region, respectively. A vulcanizate comprising the rubber blend of claim 1. An article of manufacture formed from the vulcanizate of claim 2. An article according to claim 3 which is a mechanical seal or sealing part. A sealing part formed of a peroxide vulcanizate of a rubber mixture according to claims 1-4 comprising: 50 to 90 parts by weight of an EPDM having an ethylene content of 45 to 59 wt%, a Mooney viscosity (ML₁+₄ 100°C) of 35 to 55, a molecular weight distribution of M w /M n ≦ 5.0, an iodine value of 20 to 35, an α-olefin content of 41 to 55 wt%, and an α-olefin composition distribution in the case of which 20 wt% of the α-olefin content of the EPDM at opposite ends of the molecular weight distribution comply with the equation C₃E - C₃I ≦ 5.0 wt% , wherein C₃I and C₃E corresponds to the α-olefin content at the ends of the high molecular weight region and the low molecular weight region, respectively; and 50 to 10 parts by weight of a low-Mooney EPDM (a subpolymer) having a Mooney viscosity (ML₁+₄ 100°C) of 5 to 25.

Inventors: HAYASHI HIDENORI, HONDA HIDENOBU TAKARA KUSUNOKI, NAKATA RIKIZOU, TERASHIMA KIYOMITSU, HAYASHI, HIDENORI, HONDA, HIDENOBU, TAKARA KUSUNOKI HAITSU 608, NAKATA, RIKIZOU, TERASHIMA, KIYOMITSU
Applicants: TOYODA GOSEI KK, TOYODA GOSEI CO., LTD.
Application year: 1991
Main classification: C08L23
IPC codes: F16J15, C09K3, C08L23