Curable elastomer compositions with improved mold release

Abstract: The invention relates to curable and cured compositions comprising : (a) an elastomer, (b) a crosslinking-effective amount of a metal salt of an α, β-ethylenically unsaturated carboxylic acid, (c) a metal salt of a fatty acid, and (d) a cure-effective amount of an organic peroxide. Such compositions show an improved mold release.

Description: The present invention relates to elastomers with improved mold release. More specifically the present invention relates to curable and cured elastomers in which metal salts of α, β -ethylenically unsaturated carboxylic acids are used as curing coagents and in which fatty acid metal salts are incorporated to improve mold release. It is known namely by EP-A-487397 that the use of metal salts of α, β-ethylenically unsaturated carboxylic acids as curing coagents in peroxide-curable elastomers improves the properties of the cured elastomers. However, it has been observed that such use also increases the adhesion of the elastomers to metals, as noted for example in EP-A-570277. This increased adhesion may result in severe mold release problems when curing of the elastomer composition is effected in a metallic mold. Conventional mold release agents that may be used to coat the mold prior to molding an elastomer are well known. However, there are a number of disadvantages to coating the mold with a mold release agent. These disadvantages include the accumulation of deposits of the agent on the mold and adherence of the agent to the surface of the molded elastomeric article. Additionally, the coated mold release agents make the molding process less efficient and more expensive. Certain elastomeric compositions which incorporate mold release agents are also known. US-A-3,549,744 discloses a silicone elastomer composition which is non-adherent to metal molds. The silicone elastomer composition comprises silicone rubber, reinforcing silica fibers, an organic peroxide vulcanizing agent and 0.05 to 2 weight percent of a metal salt of a C 16 to C 26 carboxylic acid. US-A-3,201,361 discloses a mold release composition for sulfur-cured elastomers comprising 11 to 16% of a zinc, magnesium, lead or calcium oleate salt, 50 to 60% of a petroleum jelly or paraffin wax, 4 to 6% of a C 8 to C 18 monohydric branched chain alkyl or a C 12 to C 20 monohydric fatty alcohol, 4.5 to 6.7% potassium stearate, and 14 to 26% excess stearic acid. JP-A-06 107 740 describes a rubber composition for expanded material comprising : from 10 to 60 parts by weight of ethylenic unsaturated carboxylic acid metal salt ; from 0.5 to 20 parts by weight of higher aliphatic acid metal salt ; from 1 to 15 parts by weight of foaming agent ; and from 0.2 to 10 parts by weight of organic peroxide per 100 parts of hydrogenated acrylonitrile-butadiene copolymer. The present invention provides for the improved mold release of elastomers in which an α, β-ethylenically unsaturated carboxylic acid metal salt is present as a curing coagent. The improved mold release is achieved through the incorporation of a fatty acid metal salt into the elastomer. The present invention provides curable and cured elastomer compositions in which α, β-ethylenically unsaturated carboxylic acid metal salts are present as curing coagents, which compositions exhibit improved mold release as well as good cured properties. It has been discovered that the improved mold release, without loss of cured properties, may be achieved in these elastomer compositions by incorporating a fatty acid metal salt into the composition. The term "elastomer" as used herein, is intended to include without limitation, elastomeric organic high polymers, including natural rubber and the various synthetic rubbers which cure, vulcanize or crosslink, by the addition of a free radical generating compound. In general, these curable rubbers are polymers of conjugated dienes or polymers with abstractible hydrogen, wherein the monomers are linked through carbon-carbon bonds. Exemplary elastomers include, without limitation, ethylene vinyl acetate copolymers, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, styrenebutadiene elastomer, polybutadiene elastomer, nitrile elastomer, hydrogenated nitrile elastomer, polyisoprene elastomer, silicone elastomer, polyurethane elastomer, polysulfide elastomer, chlorinated polyethylene, chlorosulfonyl polyethylene, acrylonitrile butadiene styrene terpolymers, ethylene butylacrylate copolymers, fluorinated elastomers, bromobutyl elastomers, and the like. These elastomers are all commercially available. By way of illustration, suitable organic peroxides, advantageously used in the present invention, include, without limitation, diacetyl peroxide, dibenzoyl peroxide, dicapryl peroxide, di-(p-chloro-benzoyl)peroxide, didecanoyl peroxide, di-(2,4-dichlorobenzoyl)peroxide, diisobutyl peroxide, diisonanoyl peroxide, dilauroyl peroxide, dipelargonyl peroxide, dipropynyl peroxide, methyl ethyl ketone peroxide, cyclohexanone peroxide, dihydroxydimethyl-dioxacyclopentane, t-butyl peroxide, t-butyl peroxy (2-ethylhexanoate), t-butylperoxyisobutylate, 0,0-t-butyl-0-isopropylmonoperoxy-carbonate, t-butylperoxy pivalate, dimethyldibenzoylperoxy-hexane, t-butyl-peroxy (2-ethylbutylate), di-t-butyl peroxide, dicumyl peroxide, 2,5-bis(t-butylperoxy)-2,5-dimethylhexane, t-butyl hydroperoxide, cumyl hydroperoxide, α,α-bis-(t-butyl peroxy)diisopropyl benzene, 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane and the like. The amount of organic peroxide used in the present invention may be varied depending on the elastomer and crosslinking coagent selected. The required amount of organic peroxide to practice the present invention is a cure-effective amount readily ascertainable by one of ordinary skill in the art. Generally, an amount of from about 0.1 to about 15 parts, preferably from about 0.5 to about 10 parts, by weight per 100 parts by weight of the elastomer is used. Also used in the present invention are metal salts of certain α, β-ethylenically unsaturated carboxylic acids, specifically metal salts of acrylic and methacrylic acid. These metal salts are known to improve the physical properties of the elastomer as well as to increase the adhesion of the elastomers to polar surfaces, such as metal. They may be incorporated into the composition of the invention as such or may be formed in situ as is already known in the art. Examples of the metal component that may be used for the metal salts of the α, β-ethylenically unsaturated carboxylic acids include, without limitation, zinc, magnesium, sodium, potassium, calcium, barium, cobalt, copper, aluminum, and iron. Typically, zinc is preferred due to its effectiveness. Generally, the amount of α, β-ethylenically unsaturated carboxylic acid metal salt used is a crosslinking-effective amount. More specifically, an amount from about 0.1 to about 100 parts, preferably from about 0.1 to about 20 parts, by weight per 100 parts by weight of the elastomer is used. The precise mechanism by which the fatty acid metal salts improve mold release is unknown. One possible theory is that the metals of the fatty acid metal salt and the α, β-ethylenically unsaturated carboxylic acid metal salt compete for active sites on the surface of the mold. If the metal of the fatty acid metal salt preferentially consumes the active sites, the adhesion imparted by the α, β-ethylenically unsaturated carboxylic acid metal salt would be reduced. By the term "fatty acid", as used herein, is meant those carboxylic acids derived from, or contained in, the fats or oils found in animals or vegetables. Such fatty acids generally have 4 to 22 carbons alkyl groups that terminate in a carboxyl radical. Both saturated and unsaturated fatty acids may be used in the present invention. Exemplary saturated fatty acids include, without limitation, butyric acid, caproic acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, and stearic acid. Illustrative unsaturated fatty acids include, without limitation, palmitoleic acid, oleic acid, linoleic acid, linolenic acid, and erucic acid. The metal component of the fatty acid metal salts may be, without limitation, zinc, calcium, barium, lithium, magnesium, sodium, aluminum, and tin. The fatty acid metal salts may be incorporated into the elastomer or the fatty acid and the metal component may be added to the elastomer separately and reacted to form the metal salt of the fatty acid in situ . The preferred fatty acid metal salt is zinc stearate. The amount of the fatty acid metal salt to be used is critical because too little of an amount will not produce improvement in mold release while too great of an amount will result in the loss of cured properties. The precise amount of fatty acid metal salt used will depend on the amount of α, β-ethylenically unsaturated carboxylic acid metal salt in the composition. It has been discovered that a mold releasing-effective amount of the fatty acid metal salt is, generally, from about 1.5 parts to about 3 parts fatty acid metal salt for 1 part α, β-ethylenically unsaturated carboxylic acid metal salt. Preferably, an amount of about 2 parts fatty acid metal salt for each 1 part α, β-ethylenically unsaturated carboxylic acid metal salt is used. Without departing from the scope of the present invention, and as appreciated by those skilled in the art, inert fillers may be included in the compositions of the invention. If an inert filler is desired, any known or conventional filler which does not interfere with the vulcanization process described herein may be used, and such fillers are desirable in finely divided form. Suitable fillers include, without limitation, silica fillers, thermal blacks, clays, kaolin, diatomaceous earth, zinc oxide, cork, titania, flocs, fibrous fillers such as glass and synthetic fibers, metal, oxides, carbonates, and the like. The amount of inert filler is dictated by its type and the intended end-use of the elastomeric composition and, generally, is less than about 30 parts, by weight per 100 parts by weight of the elastomer. Other additives that may be used in the compositions of the present invention include, without limitation, scorch retardants, antioxidants, UV stabilizers, antiozonants, plasticizers, tackifers, anti-tack agents, processing acids, coloring agents, and the like. In the present invention, the aforementioned ingredients may be mixed by any of the conventional means. Mixing may be accomplished by charging the ingredients to a Banbury mixer or a rubber mixing mill and intimately mixing the ingredients until the composition is uniform. The temperature of the mixing operation is not critical, but should be below temperatures at which the curing reaction commences. Generally, normal elastomer milling practice may be employed. Curing times, in general, range from about 1 to 30 minutes, preferably from about 5 to 15 minutes. Cure temperatures and pressures should be sufficient to melt or soften the reactants and to thermally decompose the organic peroxide. Thus, the selection of the cure temperature will be predicated upon the elastomer, coagent and organic peroxide selected. The temperatures useful in the present invention may vary between wide limits such as from 50°C to 250°C. For curing large rubber rolls, cure times of 24 hours are common to avoid stressing the roll. The invention will be further clarified by a consideration of the following examples. EXAMPLES In the following tables, all of the ingredients are expressed in parts by weight, unless otherwise noted, and each column corresponds to a numbered example. The formulations were tested, as indicated on Tables 1 and 2, in the following manner. The unvulcanized elastomer compound strip was gauged to about 0.635 mm with the two roll mill and cut into strips for lap shear adhesion testing. Cold rolled steel adhesion coupons 25.4 x 76.2 mm and about 0.762 mm thick were methanol washed and dried. The coupons were overlapped by 25.4 mm with the compound strip between and placed in a 1.9 mm plaque mold, preheated to 160°C. and then cured for twenty five minutes. Lap shear adhesion was measured on a Thwing-Albert tensile tester in accordance with ASTM D-412 using a jaw separation speed of 1.0 IPM. The results obtained may be found reported on Tables 1 and 2. Mold release was tested, subjectively, by compression molding the compound in a clean 1.9 µm plaque mold for 20 minutes at 160°C. The mold was then opened. Mold release was considered poor if the halves of the mold opened by prying them apart with a screw driver. Mold release was considered poor if the mold halves had to be separated by hammering a wedge between the halves. Vulcanization characteristics were determined with a Tech PRO ODR using a 1.0 °arc. Vulcanization characteristics were determined at 160°C. Vulcanization characteristics measured include, TS 1 . TC (90), and maximum torque (MHF) and the results are reported on Tables 1 and 2. Specimens of 1.9 µm thickness were compression molded and cured for 20 min. at 160°C, the specimens were air cooled and aged one day before being tested on a Thwing-Albert tensile tester in accordance with ASTM D-412. Modulus values at 100 percent elongation were recorded in accordance with ASTM D-412. Shore A hardness values were obtained in accordance with ASTM D-1415. The results obtained may be found on Tables 1 and 2. EXAMPLE 1 A batch of ethylene vinyl acetate elastomer available as Elvax™ 240, was masticated on a two roll mill until an acceptable bead was established in the conventional manner. Thereafter, for each 100 parts of ethylene vinyl acetate elastomer, and in accordance with the formulation list on Table 1, 25 parts of Hisil™ 230, a hydrated amorphous silica reinforcing pigment, 3 parts of zinc oxide, 3 parts of stearic acid, 3 parts of trioctyl trimillitate, 2 parts of bis(tert-butyl peroxy isopropyl)benzene peroxide available as Perkadox™ 1440, were added slowly to the flux roll and the band was cut and mixed until a uniform, smooth band developed. Thereafter the unvulcanized compound strip was cut and divided for testing as above-described. EXAMPLES 2 - 11 Elastomer compositions were produced by repeating the procedure of Example 1 except that the formulations for Examples 2 - 11, which may be found on Table 1 were followed. Example 2 involved the addition of 2.8 parts zinc diacrylate and 0.2 parts zinc stearate to the flux roll. Example 3 involved the addition of 2.8 parts zinc diacrylate and 2.2 parts zinc stearate to the flux roll. Example 4 involved the addition of 2.8 parts zinc diacrylate and 5.2 parts zinc stearate to the flux roll. Example 5 and 6 involved the addition of 2.8 parts zinc diacrylate and 10.2 parts zinc stearate to the flux roll. Example 7 involved the addition of 3.8 parts of zinc dimethacrylate to the flux roll. Example 8 involved the addition of 3.8 parts of zinc dimethacrylate and 10.0 parts zinc stearate to the flux roll. Example 9 involved the addition of 2.4 parts of zinc diacrylate and 0.6 parts of zinc stearate to the flux roll. Example 10 involved the addition of 2.4 parts of calcium diacrylate to the flux roll. Example 11 involved the addition of 3 parts of calcium diacrylate and 10.0 parts of calcium stearate. EXAMPLE 12 Example 12 involved substituting ethylene propylene diene (EPDM) terpolymer, available as Nordel™ 1040, in place of the EVA of Example 1. The EPDM was masticated as for Example 1 and, thereafter, for each 100 parts of EPDM, and in accordance with the formulation listed on Table 2, 100 parts of semi-reinforcing carbon black N762, 50 parts of SUNPAR™ 2280, a proprietary plasticizer, 5 parts of zinc oxide, 1 part of stearic acid, 1 part of polymerized, 1,2 - dihydro-2,2,4-trimethylquinoline, an antioxidant resin available as AGERITE RESIN D, and 2 parts of PERKADOX™ 1440 peroxide were added slowly to the flux roll. The band was cut and tested as for Example 1. EXAMPLES 13 - 16 Compositions were produced according to Example 12 except that the formulations for Examples 13 - 16, which may be found on Table 2 were followed. Example 13 involved the addition of 2.8 parts zinc diacrylate to the flux roll. Example 14 involved the addition of 0.9 parts zinc diacrylate and 1.9 parts zinc stearate to the flux roll. Example 15 involved the addition of 2.8 parts zinc diacrylate and 6.0 parts zinc stearate to the flux roll. Example 16 involved the addition of 4.8 parts zinc diacrylate and 10.2 parts zinc stearate to the flux roll. As is apparent from the data reported on Tables 1 and 2, the use of high levels of a fatty acid metal salt produced a marked improvement in mold release. For those compositions containing an α, β-ethylenically unsaturated carboxylic acid metal salt the absence of, or low levels of, the fatty acid metal salt resulted in an elastomer composition having high lap shear adhesion. Further, in such compositions, the cured elastomer plaque stuck badly to the mold had to be hammered apart In those compositions in which a high level of fatty acid metal salt was utilized, a marked decrease in lap shear adhesion and improved mold release was noted. At the same time, no loss of crosslink density was noted as was indicated by the fact that ODR torque measurements remained high.

Claims: A curable composition comprising : (a) an elastomer ; (b) 0.1 to 20 parts by weight of a metal salt of an α,β-ethylenically unsaturated carboxylic acid, based on 100 parts by weight of the elastomer (a); (c) a metal salt of a fatty acid at a weight ratio to the α,β-ethylenically unsaturated carboxylic acid metal salt (b) which is from 1.5 to 3 ; and (c) 0.01 to 15 parts by weight of an organic peroxide based on 100 parts by weight of the elastomer (a). The curable elastomer composition according to claim 1, wherein the organic peroxide (d) is present in an amount of from 0.5 to 10 parts by weight based on 100 parts by weight of the elastomer (a). The curable elastomer composition according to claim 1 or 2, wherein the fatty acid of the fatty acid metal salt (c) is a saturated fatty acid. The curable elastomer composition according to any of claims 1 to 2, wherein the fatty acid of the fatty acid metal salt (c) is an unsaturated fatty acid. The curable elastomer composition of claim 3, wherein the fatty acid is stearic acid. The curable elastomer composition of claim 5, wherein the fatty acid metal salt (c) is selected from zinc stearate and calcium stearate. The curable composition according to any of claims 1 to 6, wherein the α,β-ethylenically unsaturated carboxylic acid of the α,β-ethylenically unsaturated carboxylic acid metal salt (b) is selected from acrylic acid and methacrylic acid. The curable elastomer composition of claim 7, wherein the metal salt of methacrylic acid or acrylic acid is selected from zinc diacrylate, calcium diacrylate and zinc dimethacrylate. The curable elastomer composition according to any of claims 1 to 8, wherein the elastomer is selected from ethylene vinyl acetate copolymers, ethylene-propylene copolymers, ethylene-propylene-diene terpolymers, styrene butadiene elastomer, polybutadiene elastomer, nitrile elastomer, hydrogenated nitrile elastomer, polyisoprene elastomer, silicone elastomer, polyurethane elastomer, polysulfide elastomer, chlorinated polyethylene, chlorosulfonyl polyethylene, acrylonitrile butadiene styrene terpolymers, ethylene butylacrylate copolymers, fluorinated elastomers and bromobutyl elastomers. A cured composition provided by curing a composition according to any of claims 1 to 9.

Inventors: COSTIN RICHARD, NAGEL WALTER, COSTIN, RICHARD, NAGEL, WALTER
Applicants: SARTOMER CO INC, SARTOMER COMPANY, INC.
Application year: 1995
Main classification: C08K5
IPC codes: C08K5, C08L23