Fluorine-Containing Elastomer Composition

1. Field of the Invention

This invention relates to a fluorine-containing elastomer composition and more particularly to a fluorine-containing elastomer compositon capable of producing vulcanized products with good vulcanization properties, especially high tensile strength and elongation.

2. Description of the Prior Art

Owing to useful characteristics at elevated temperatures, such as high heat resistance, high chemical resistance, high oil resistance, high weathering resistance, demands for vulcanized products of fluorine-containing elastomer have been drastically increasing in the form of sealing materials such as gaskets, O-rings, packings etc., hoses, sheets, etc. in the fields of the automobile industry, the oil hydraulic industry, the general machine industry, the aviation industry, etc. In other words, it can be said that the demands for the vulcanized products of fluorine-containing elastomers have become versatile and stringent in addition to a demand for more efficient molding and processing.

Vulcanization of the fluorine-containing elastomer was initially carried out with polyamine-based derivatives such as hexamethylenediamine carbonate and methylenebis-(cyclohexyl)amine carbonate, but these vulcanization systems gave a poor scorching resistance such a poor processing safety and a poor storage stability or the vulcanized products had a poor permanent compression strain. Thereafter, a process for crosslinking the fluorine-containing elastomer with a polyhydroxyaromatic compound in the presence of a vulcanization accelerator and an acid acceptor was proposed as another vulcanization system capable of overcoming the afore-mentioned disadvantages, and has been practically utilized up to now.

The vulcanization accelerators so far proposed for the vulcanization system are quaternary phosphonium salt compounds [Japanese Patent Application Kokai (Laid-open) No. 47-191], quaternary ammonium salt compounds [Japanese Patent Publication No. 52-38072 and Japanese Patent Application Kokai (Laid-open) No. 47-3831], quaternary ammonium salt compounds of 8-alkyl (or 8-aralkyl)-1,8-diazabicyclo[5,4,0]-undec-7-ene [Japanese Patent Publication No. 52-8863] and [Japanese Patent Application Kokai (Laid-open) No. 48-55231], or combinations thereof with such an amount of 1,8-diazabicyclo[5,4,0]-undec-7-ene as substantially not to vulcanize the fluorine-containing elastomer [Japanese Patent Publication No. 57-20333], etc.

However, even with these vulcanization systems, neither tensile strength nor elongation of the vulcanized product reaches the desired satisfactory level yet and also the storage stability of green elastomer compostion is not satisfactory yet.

A vulcanization system containing a vulcanization accelerator of the quaternary ammonium salt compound had a considerable susceptibility to moisture absorption and deliquescence at the storage in addition to the aforementioned disadvantages and thus needed a special consideration for the storing. This not only caused a handling trouble, but also often lowered the vulcanization characteristics and the physical properties of vulcanized products.

The present applicant have been so far made extensive studies on acceleration of the vulcanization rate without deteriorating the scorching resistance and the permanent compression strain of vulcanized products and also on solution of the problems appearing in the so far known vulcanization systems, and previously found that in a vulcanization system using a polyhydroxyaromatic compound as a crosslinking agent in the presence of an acid acceptor, not only good storage stability, processing safety and vulcanization characteristics (vulcanization flowability, vulcanization rate, etc.) as green elastomer composition, but also considerably improved mechanical strength and permanent compression strain of vulcanized product, which had been the problems to be improved in the so far known vulcanization system, could be obtained by using a specific quaternary ammonium salt compound and an N-alkyl-substituted amide compound as vulcanization accelerator components at the same time, or further using such an amount of 1,8-diazabicyclo[5,4,0]-undec-7-ene, 1,5-diazabicyclo[4,3,0]-non-5-ene or 4-dialkylaminopyridine whose an alkyl group has 1 to 4 carbon atoms as to substantially fail to vulcanize the fluorine-containing elastomer together with the said two vulcanization accelerator components, and proposed a vulcanization system based on this finding [Japanese Patent Publication No. 59-46986]. The vulcanization system was found to be effective for improving the dispersibility of the system at the kneading and the shapability at the vulcanization molding.

In the said vulcanization system, a quaternary ammonium salt compound represented by the following general formula [II]: ##STR2## wherein R.sub.1 is an aralkyl group having 7 to 20 carbon atoms, R.sub.2 is a hydrogen atom or a dialkylamino group whose alkyl group has 1 to 4 carbon atoms, and X.sup.- is an anion, was used as the said specific ammonium salt compound. A fluorine-containing elastomer compositon containing the quaternary ammonium salt compound and the N-alkyl-substituted amide compound had no fear at all of moisture absorption and diliquescence during the storage. Furthermore, the said fluorine-containing elastomer composition had good storage stability, processing safety, and vulcanization characteristics, and gave a considerably improved shapability at the vulcanization molding, mechanical strength and permanent compression strain to vulcanized products and also had a higher vulcanization rate without deteriorating the scorching resistance.

An object of the present invention is to provide a novel vulacanization system showing neither moisture absorption nor diliquescence even in a system using the quaternary ammmonium salt compound alone without the N-alkyl-substituted amide compound, but fully showing the preferable characteristics obtained in the afore-mentioned vulcanization systems.

The object can be attained by a fluorine-containing elastomer composition comprising the following components (a) to (d) or (a) to (e). That is, the present fluorine-containing elastomer composition comprises (a) a fluorine-containing elastomer, (b) at least one of an oxide and a hydroxide of divalent metal, (c) a polyhydroxyaromatic compound, and (d) a quaternary ammonium salt compound represented by the following general formula [I]: ##STR3## wherein R is an alkyl group having 1 to 24 carbon atoms, or an aralkyl group having 7 to 20 carbon atoms, R' is a piperidino radical or a pyrrolidinyl radical, and X.sup.- is an anion, or further (e) such an amount of 1,8-diazabicyclo[5,4,0]-undec-7-ene or 1,5-diazabicyclo[4,3,0]-non-5-ene as to substantially fail to vulcanize the fluorine-containing elastomer.

Fluorine-containing elastomer to be vulcanized is highly fluorinated copolymers in an elastomeric state, and for example, copolymers of vinylidene fluoride with other fluoroolefins, and more specifically include copolymers of vinylidene fluoride with at least one of hexafluoropropene, pentafluoropropene, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, vinyl fluoride, perfluoroacrylate ester, perfluoroalkyl acrylate, perfluoro(methyl vinyl)ether, perfluoro(propyl vinyl)ether, etc., and preferably copolymer of vinylidene fluoride-hexafluoropropene and terpolymer of vinylidene fluoride-tetrafluoroethylene-hexafluoropropene. It is preferable that the copolymer contains about 40 to about 90% by mole of vinylidene fluoride.

As the oxide and the hydroxide as an acid-acceptor in the present invention, generally about 1 to about 40 parts by weight, preferably about 3 to about 15 parts by weight of at least one of oxides and hydroxides of divalent metals such as magnesium, calcium, barium, lead, zinc, etc. is used per 100 parts by weight of the fluorine-containing elastomer in the present invention.

The polyhydroxyaromatic compound for use as a cross-linking agent in the present invention includes 2,2-bis(4-hydroxyphenyl)propane [bisphenol A], 2,2-bis(4-hydroxyphenyl)perfluoropropane [bisphenol AF], hydroquinone, catechol, resorcinol, 4,4'-dihydroxydiphenyl, 4,4'-dihydroxydiphenylmethane, 4,4'-dihydroxydiphenylsulfone, 2,2-bis(4-hydroxyphenyl)butane, etc., preferably bisphenol A, bisphenol AF, hydroquinone, etc. These may be in the form of an alkali metal salt or alkaline earth metal salt. The crosslinking agent is used in an amount of about 0.5 to about 10 parts by weight, preferably about 0.5 to about 6 parts by weight, per 100 parts by weight of the fluorine-containing elastomer. Below about 0.5 parts by weight, the crosslinking density becomes too small, whereas above about 10 parts by weight the crosslinking density becomes too high, with the result that the rubber-like resilience is lost.

The quaternary ammonium salt compound represented by the foregoing formula [I] includes, for example, 1-benzyl-4-piperidinopyridinium chloride, 1-benzyl-4-(1-pyrrolidinyl)pyridinium chloride, 1-benzyl-4-piperidinopyridinium bromide, 1-benzyl-4-piperidinopyridinium stearate, 1-ethyl-4-(1-pyrrolidinyl)pyridinium chloride, etc., and the anionic groups are exemplified above, but may include halide, hydroxide, alkoxide, carboxylate, phenoxide, sulfonate, sulfate, sulfite, carbonate, nitrate, phosphate, thiocyanate, borate, tetraphenylborate, tetrafluoroborate, hexafluorophosphate, hexafluorosilicate, benzoate, etc.

About 0.1 to about 10 parts by weight, preferably about 0.1 to about 2 parts by weight of at least one of the quaternary ammonium salt compounds is used per 100 parts by weight of the fluorine-containing elastomer. Below about 0.1 parts by weight, the crosslinkability will be deteriorated, whereas above 10 parts by weight various characteristics of vulcanized products are adversely influenced to a considerable degree.

At least one of 1,8-diazabicyclo[5,4,0]-undec-7-ene and 1,5-diazabicyclo[4,3,0]-non-5-ene, if contained as an additional component in the composition, has a remarkable effect or the improvement of permanent compression strain of vulcanized products. At least one of these additional components is contained generally in such an amount as to substantially fail to vulcanize the fluorine-containing elastomer, preferably in an amount of not more than about 0.2 parts by weight per 100 parts by weight of the fluorine-containing elastomers, and particularly preferably in a ratio of not more than about 10% by weight thereof to the quaternary ammonium salt compound. Above the upper limit, an adverse effect will appear on the scorching resistance of green elastomer composition, and the crosslinking density of vulcanized product will be too high, resulting in a decrease in elongation.

These components for the vulcanization system can be mixed together and kneaded as such, or diluted with and dispersed in carbon black, silica powder, clay, talc, diatomaceous earth, barium sulfate, etc. or may be used as a master batch dispersion with the fluorine-containing elastomer. Furthermore, so far known filler, reinforcing agent, plasticizer, lubricant, processing additive, pigment, etc. can be added, if desired, to the present composition besides the foregoing components.

Vulcanization is carried out by heating after the said components for the vulcanization system and the said various additives are added to the fluorine-containing elastomer and mixed according to a conventional mixing method, for example, by roll mixing, kneader mixing, Banbury mixing, solution mixing, etc. Generally, primary vulcanization is carried out by heating at a temperature of about 140.degree. to about 200.degree. C. for about 2 to about 120 minutes, and secondary vulcanization is carried out by heating at about 150.degree. to about 250.degree. C. for up to 30 hours.

The present fluorine-containing elastomer composition shows neither moisture absorption nor deliquescence when stored as a green composition, and therefore has not only good storage stability and processing stability, but also good physical properties of vulcanization product, particularly higher tensile strength and elongation than those of the vulcanization systems using 3,5-dimethyl substitute or 4-phenyl substitute of 1-benzylpyridinium chloride requiring no N-alkyl-substituted amide compound at the same time [Japanese Patent Application Kokai (Laid-open) No. 62-89,745 and U.S. Pat. No. 4,734,460].

The present invention will be described in detail below, referring to Examples and Comparative Examples. Examples 1 to 3 and Comparative Examples 1 to 3.

Copolymer obtained by copolymerization of vinylidene fluoride with hexafluoropropene in the presence of ammonium persulfate as a polymerization initiator in an aqueous medium, using acetone as a chain transfer agent [molar ratio of comonomers=78:22 (vinylidene fluoride:hexafluoropropene), solution viscosity .eta. sp/c=0.98 (35.degree. C. in acetone, c=1.0), Mooney viscosity of copolymer ML.sub.1+10 53 (121.degree. C.); fluorine-containing elastomer A] was mixed with the components shown in the following Table 1 according to the composition proportions shown therein through 8-inch mixing rolls to prepare fluorine-containing elastomer composition. The composition formulations are given in parts by weight in Table 1.

The thus obtained various fluorine-containing elastomer compositions were subjected to determination of moisture absorption and deliquescence of green elastomer composition (sheets of green elastomer compositions, 100.times.100.times.2 mm in size, obtained by adding 100 parts by weight of each of valcanization accelerator component to 100 parts by weight of the fluorine-containing elastomer were placed in a low temperature humidistatic and thermostatic chamber made by Hashimoto Seisakusho K. K. Japan, and the surface state of the sheets was inspected after being left at a temperature of 25.degree. C. and a humidity of 70% for 24 hours, and the non-wet state was evaluated as "none", the wet state as "yes", and the water droplet-deposited state as "considerable"); Mooney viscosity and scorching time (time required until the Mooney viscosity takes a minimum value of +5, serving as an index for the storage stability and the processing safety as measured at the temperature of 121.degree. C.) and vulcanization characteristics by means of an oscillating disk rheometer (ODR) made by Toyo Seiki K. K., Japan.

Furthermore, the elastomer compositions were vulcanized by pressing at 180.degree. C. for 5 minutes and then subjected to secondary vulcanization in an oven at 230.degree. C. for 22 hours, and various physical properties of vulcanized products were determined according to JIS K-6301. Permanent compression strain was measured by making O-rings of P-24 through vulcanization under the same conditions as described above and by 25% compression thereof. The results of the determinations are given in the following Table 2.