Process for Producing Acrylic Rubber by Copolymerizing Acrylic Ester and Malonic Acid Derivative Having Active Methylene Group

The present invention relates to a process for producing vulcanizable acrylic rubber.

Conventional acrylic rubbers are stable to heat and oilproof because of high polarity of their ester structure, but are unvulcanizable with sulfur because they do not have any unsaturated groups or double bonds in their polymer backbone. Therefore, the conventional process for producing acrylic rubbers is by copolymerizing acrylic esters with a suitable amount of a cross-linking monomer which reacts with a vulcanizing agent such as soaps, ethyltetramines and tetraethylpentamines, and curing the copolymer with such a vulcanizing agent. Such monomers include those which contain halogen such as .beta.-chloroethyl vinyl ether and vinyl chloroacetate, and those which contain an epoxy group such as allyl glycidyl ether, glycidyl acrylate and glycidyl methacrylate.

Such conventional acrylic rubbers are, however, liable to scorch during storage and have low resistance to cold and poor processibility. Particularly, the acrylic rubbers produced by use of liquid polyamine as a vulcanizing agent have undue adhesiveness to a mixing roll, poor bin stability, high corrosiveness, and offensive odor and toxicity coming from the amine.

It is an object of this invention to provide a process of producing vulcanizable acrylic rubber which does not have such disadvantages.

According to the present invention, an acrylic ester or esters are copolymerized with a malonic acid derivative having an active methylene group to produce acrylic rubber which can be cured with vulcanizing agents in the thiuram series for cross-linking.

FIG. 1 is a graph showing the vulcanization curves for an acrylic rubber produced according to this invention and for the conventional acrylic rubber.

FIG. 2 is a similar graph for the acrylic rubbers prepared in Example 9.

The malonic acid derivative having an active methylene group utilized in the present invention has the following general formula: ##STR2## wherein R.sub.1 represents vinyl, allyl or methallyl group and X represents COOR.sub.2 or cyano group.

First, if X represents COOR.sub.2 in the formula (1), the malonic acid derivative has the following general formula: ##STR3## wherein R.sub.2 represents methyl, ethyl or propyl group. Thus, the derivatives are malonic acids with an active methylene group having one of two acid radicals esterified with an unsaturated alcohol such as allyl alcohol, and having the other acid radical esterified with a saturated alcohol. Such derivatives include allyl ethyl malonate and allyl methyl malonate, for example.

A process for producing the former will be described by way of example. A mixture of 1 mole of ethyl cyanoacetate, 1 mole of sulfuric acid and 1 mole of water is kept at 80.degree. C. or lower for about four hours under stirring. 1.5 mole of allyl alcohol is added, the mixture being allowed to react with slow stirring at room temperature for about 72 hours. The mixture is then rinsed, dehydrated and distilled under reduced pressure. During distillation, ethyl cyanoacetate distills off first and allyl ethyl malonate distills off last. In this process, if allyl cyanoacetate and ethyl alcohol are used as the starting materials the reaction product contains only allyl cyanoacetate and allyl ethyl malonate, containing no ethyl cyanoacetate which has a bad effect on cross-linking.

Next, if X represents cyano group in the formula (1), the malonic acid derivative has the following general formula: ##STR4## wherein R.sub.1 represents vinyl, allyl or methallyl group. The malonic acid derivatives include esters of cyanoacetic acid (that is, malonic acid mononitrile) having an active methylene group with an unsaturated alcohol, such as allyl cyanoacetate or methallyl cyanoacetate, and esters thereof with hydroxyethyl acrylate or hydroxyethyl methacrylate.

Such malonic acid derivative having the general formula (3) may be produced by the conventional processes, one of which will be described by way of example. One part by weight of p-toluene sulfonic acid as a catalyzer is added to a mixture of 100 parts of cyanoacetic acid, 100 parts of allyl alcohol, 50 parts of benzene and 50 parts of cyclohexane. The mixture undergoes esterification at 70.degree.-80.degree. C. for about 24 hours while refluxing by means of a phase separator to remove water. After reaction, it is cooled, rinsed, dehydrated and distilled to remove the solvents. Thereafter it is further distilled under reduced pressure of 10 mmHg. The derivative aimed at is obtained by collecting the fraction at 110.degree.-112.degree. C.

The acrylic esters used in this invention are esters of acrylic acid with alkyl alcohols, .beta.-ethoxy ethyl alcohol or .beta.-methoxy ethyl alcohol.

In the copolymerization according to this invention, the amount of the malonic acid derivative having the general formula (2) or (3) is preferably 2-10% by weight, and more preferably 2-6%, relative to the acrylic ester. If it were less than 2%, the addition of malonic acid derivative would not have a sufficient effect, whereas for more than 10% the curing rate would be much higher and the tensile strength would increase owing to over-cure, but the hardness would increase, thus resulting in lower elongation and elasticity.

The reaction temperature for copolymerization is 5.degree.-95.degree. C., preferably 50.degree.-70.degree. C., and the reaction time is preferably 30 to 120 minutes.

As vulcanizing agents used for the acrylic rubber produced according to the present invention, tetramethylthiuram disulfide and tetraethylthiuram disulfide are preferable. Also, tetramethylthiuram monosulfide or thiazole is preferably used as a vulcanizing accelerator.

The acrylic rubbers produced according to the present invention show much higher curing rate and marked plateau effect in comparison with the conventional acrylic rubber cured with amines, as will be seen in FIG. 1 wherein (A) is the vulcanization curve for the acrylic rubber produced according to this invention and (B) is the curve for conventional acrylic rubber. The present acrylic rubbers also retain resistance to heat, oil, ozone, weathering and bend-cracking which the conventional acrylic rubber has. Furthermore, they have additional advantages of better processibility with a mixing roll, freedom from scorch during processing or storage, no corrosiveness to a curing mold and easy adhesion to metal inserts. Besides the present acrylic rubbers allow use of white carbon in the silica or talc series as well as the conventional carbon black as a reinforcing agent. This provides greater flexibility for production of colored rubber.

The acrylic rubber produced according to the present invention may be formed into rolls, seals, gaskets, "O"-rings, hoses and so on.

The following examples are included merely to aid in the understanding of the present invention. Unless otherwise stated, quantities are expressed as parts by weight.

A. In a flask were put 200 parts of water, 0.5 part of sodium laurylsulfate and 2 parts of polyoxyethylene lauryl ether as emulsifiers, 5 parts of allyl ethyl malonate, 0.05 part of potassium persulfate as polymerization initiator and 0.05 part of sodium hydrogen bisulfite as redox catalyst. The mixture was heated to 50.degree.-70.degree. C. while blowing nitrogen gas thereinto and 95 parts of ethyl acrylate was added drop by drop, taking 30 to 40 minutes, for emulsion polymerization to give vulcanizable acrylic rubber.

B. To 100 parts of the acrylic rubber thus prepared in (A) were added 50 parts of MEF (medium extrusion furnace) carbon, 1 part of stearic acid, 2.4 parts of tetramethylthiuram disulfide and 3.3 parts of dibenzothiazolyl disulfide. After kneading well in an open roll, the mixture was put into a curing mold and heated at 170.degree. C. for 10 minutes. The rubber slab thus made was subjected to post cure at 150.degree. C. for 16 hours. Table 1 summarizes the physical properties of the cured acrylic rubber in an original test, an air heat aging test at 150.degree. C. for 70 hours and oil resistance tests, respectively.

Except that 4 parts of allyl methyl malonate and 96 parts of ethyl acrylate were used, the same mixing ratio and reaction conditions as in Example 1 were used to prepare vulcanizable acrylic rubber.

To 100 parts of the acrylic rubber were added 50 parts of white carbon, 1 part of stearic acid, 2.4 parts of tetraethylthiuram disulfide and 3.3 parts of dibenzothiazolyl disulfide. After kneading, the mixture was pre-cured at 170.degree. C. for 10 minutes and post-cured at 150.degree. C. for 4 hours. Table 2 shows the physical properties of the cured acrylic rubber measured as in Example 1.

Except that 5 parts of allyl ethyl malonate, 80 parts of ethyl acrylate and 15 parts of butyl acrylate were used as monomers, the same mixing ratio and reaction contitions as in Example 1 were used. The acrylic rubber thus made was cured in the same manner as in Example 1 except that the post cure time was 7 hours. Table 3 shows the physical properties of the cured acrylic rubber.

Except that 8 parts of allyl ethyl malonate and 92 parts of ethyl acrylate were used, the same mixing ratio and reaction conditions as in Example 1 were used to produce vulcanizable acrylic rubber. It was then cured as in Example 1 except that tetraethylthiuram disulfide was used instead of tetramethylthiuram disulfide. Table 4 shows the physical properties of the cured acrylic rubber.

A. In a flask were put 200 parts of water, 0.5 part of sodium laurylsulfate and 2 parts of polyoxyethylene lauryl ether, 5 parts of allyl cyanoacetate, 0.05 part of potassium persulfate and 0.05 part of sodium hydrogen bisulfite. The mixture was heated to 50.degree.-70.degree. C. while blowing nitrogen gas thereinto and 95 parts of ethyl acrylate was added drop by drop, taking 30 to 40 minutes, for emulsion polymerization to give vulcanizable acrylic rubber.

B. To 100 parts of the acrylic rubber thus prepared were added 50 parts of MEF carbon, 1 part of stearic acid, 2 parts of tetramethylthiuram disulfide and 2 parts of dibenzothiazolyl disulfide. After kneading well in an open roll, the mixture was put into a curing mold and heated at 170.degree. C. for 10 minutes. The rubber slab thus made was subjected to post cure at 150.degree. C. for 16 hours. Table 5 shows the physical properties of the cured acrylic rubber.

Except that 7 parts of allyl cyanoacetate, 15 parts of methyl acrylate and 78 parts of ethyl acrylate were used as monomers, the same mixing ratio and reaction conditions as in Example 5 were used.

To 100 parts of the acrylic rubber thus prepared were added 50 parts of white carbon as a reinforcing agent, 1 part of stearic acid, 3 parts of tetraethylthiuram disulfide and 3 parts of dibenzothiazolyl disulfide. The acrylic rubber was cured as in Example 5 except that the post cure time was 4 hours.

Except that 5 parts of allyl cyanoacetate, 10 parts of acrylonitrile and 85 parts of butyl acrylate were used, the same mixing ratio and reaction conditions as in Example 5 were used.

The acrylic rubber thus prepared was cured as in Example 5 except that 1.5 parts of dibenzothiazolyl disulfide were used and the post cure time was 7 hours. Table 6 shows the physical properties of the cured acrylic rubber.

Except that 7 parts of allyl cyanoacetate and 93 parts of ethyl acrylate were used as monomers, the same mixing ratio and reaction conditions as in Example 5 were used.

To 100 parts of the acrylic rubber thus prepared were added 50 parts of MEF carbon, 1 part of stearic acid, 2 parts of tetraethylthiuram disulfide and 2 parts of dibenzothiazolyl disulfide. The acrylic rubber was cured as in Example 5 except that the post cure time was 4 hours.

The vulcanizable acrylic rubber prepared in step (A) of Example 5 was cured at 170.degree. C. by use of such vulcanizing agent, accelerators and retarder as shown in Table 7.

FIG. 2 shows the vulcanization curves for these three tests. These test results show that the acrylic rubber produced according to the present invention has a large advantage over the conventional acrylic rubber in that the rise or start of vulcanization is adjustable by using a vulcanization accelerator or retarder.