Abstract:
A plasticizer for chloroprene rubber represented by the following general formula (I): 
     
       R.sub.1 (O--R).sub.m S(R--O).sub.n R.sub.2                 (I) 
     
     wherein R 1  and R 2  independently represent each a saturated or unsaturated aliphatic monoacyl group having 4 to 24 carbon atoms; 
     R represents an alkylene group having two to four carbon atoms; and 
     m and n represent each an integer of 1 to 4: 
     is disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a plasticizer for chloroprene rubber. More particularly, it relates to a plasticizer for chloroprene rubber which comprises an ester of a glycol having a thioether bond with aliphatic monocarboxylic acid(s). 
     2. Description of the Prior Art 
     Chloroprene rubber, which is excellent not only in general physical properties of a rubber but also in various properties such as weather resistance, chemical resistance and flame resistance, has been widely applied to, for example, industrial products, building gaskets, high-way joint seals, automotive parts, electrical cables and adhesives. 
     However the ozone resistance and cold resistance of the chloroprene rubber are unsatisfactory. Thus it has been required to improve these properties thereof. 
     Similar to other commonly employed rubbers, chloroprene rubber contains plasticizers such as process oil or various ester oils. It is known that ester plasticizers such as unsaturated vegetable oils, butyl oleate or dioctyl sebacate improve the cold resistance of chloroprene rubber. 
     However none of these plasticizers can exert any satisfactory effect. Therefore there has been proposed to improve the cold resistance of chloroprene rubber by, for example, using an unsaturated carboxylic acid ester of a glycol (cf. Japanese Patent Laid-Open No. 122532/1984) or by employing a dibasic acid ester of an ether alcohol as a plasticizer therefor (cf. Japanese Patent Publication No. 44695/1979). 
     Although the cold resistance of chloroprene rubber can be improved to a certain extent by using these plasticizers, each of these plasticizers has an insufficient heat resistance, causes a significant physical change upon prolonged heating and shows a considerably poor ozone resistance. Thus these plasticizers are unsatisfactory from the practical viewpoint. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a plasticizer for chloroprene rubber which is excellent not only in cold resistance and heat resistance but also in ozone resistance and causes little physical changes when chloroprene rubber containing the same is treated at a high temperature for a prolonged period of time. 
     Under these circumstances, the present inventors have conducted studies in order to develop a plasticizer which is excellent not only in cold resistance and heat resistance but also in ozone resistance. As a result, they have found that a diester of a glycol having a thioether bond with aliphatic monocarboxylic acid(s) as represented by the following general formula (I) is an excellent plasticizer capable of solving all of the above problems, thus completing the present invention: 
     
         R.sub.1 (O--R).sub.m S(R--O).sub.n R.sub.2                 (I) 
    
     wherein R 1  and R 2  independently represent each a saturated or unsaturated aliphatic monoacyl group having 4 to 24 carbon atoms; 
     R represents an alkylene group having two to four carbon atoms; 
     and m and n represent each an integer of 1 to 4. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of the alkylene group represented by the R in the compound of the above general formula (I) include ethylene, 1,2-propylene, 1,3-propylene, 1,2-butylene and 1,4-butylene groups. Thus examples of the glycol having a thioether bond include thiodiethanol, thiodipropanol, thiodibutanol, thiodidiethylene glycol, thioditetraethylene glycol and hydroxyethoxyethylthioethanol. 
     As the aliphatic monocarboxylic acid supplying the saturated or unsaturated aliphatic monoacyl groups represented by the R 1  and R 2  in the above general formula (I), natural or synthetic aliphatic monocarboxylic acids may be employed. Examples thereof include butyric, caproic, caprylic, 2-ethylhexanoic, capric, isononanoic, lauric, myristic, stearic, isostearic, lauroleic, oleic, elaidic, erucic, linoleic, linoelaidic, eleostearic, myristoleic and linolenic acids as well as aliphatic monocarboxylic acids obtained from natural fats, such as tall oil fatty acids which is a mixture of the abovementioned acids. 
     One or more of these aliphatic monocarboxylic acids may be employed as the aliphatic monocarboxylic acid(s) in the plasticizer of the present invention. When it is necessary to particularly improve the ozone resistance of chloroprene rubber, it is preferable to use unsaturated aliphatic monocarboxylic acid(s) or a mixture comprising at least 30% by mol of the same. Although unsaturated aliphatic monocarboxylic acid(s) may be exclusively used, the compatibility with the rubber would be somewhat lowered in this case. Thus it is particularly preferable to use a mixture of 30 to 90% by mol of unsaturated aliphatic monocarboxylic acid(s) with 70 to 10% by mol of saturated aliphatic monocarboxylic acid(s) as the aliphatic monocarboxylic acid(s). 
     Accordingly examples of the plasticizer to be used in the present invention include thiodiethanol dioctoate, thiodiethanol dioleate, thiodiethanol tall oil fatty acid diesters, thiodiethanol di(oleate/octoate) (1:1 by mol), thiodi(diethylene glycol) dioleate and thiodi(diethylene glycol) di(oleate/octoate) (1:1 by mol). 
     These plasticizers may be prepared by known esterifying processes. For example, they may be readily obtained by condensation of a glycol having a thioether bond with an aliphatic monocarboxylic acid through elimination of water molecule or transesterification between a glycol and an aliphatic monocarboxylic acid lower alkyl ester. 
     The following Synthetic Example will show a particular synthesis of a plasticizer to be used in the present invention. 
     SYNTHETIC EXAMPLE 
     SYNTHESIS OF THIODIETHANOL 
     di(oleate/octoate) (1:1 by mol) 
     268.4 g (2.2 mol) of thiodiethanol, 380.2 g (2.64 mol) of 2-ethylhexanoic acid, 620.4 g (2.2 mol) of oleic acid, 12.7 g of active carbon and 1.27 g of tetraisopropyl titanate were stirred together under a nitrogen stream at 220° C. for eight hours while removing generated water. After removing excessive 2-ethylhexanoic acid under reduced pressure, the product was filtered through Celite. Thus a product in the form of a brown liquid having an acid value of 0.58, a hydroxyl value of 0.9 and a viscosity of 38 cp was obtained. 
     The amount of the plasticizer of the present invention is not particularly restricted but may be determined depending on the purpose. In general, approximately 2 to 50 parts by weight, preferably 5 to 40 parts by weight of the plasticizer is employed per 100 parts by weight of chloroprene rubber. 
     The plasticizer of the present invention may be used together with other known plasticizer(s). In this case, the total amount of the employed plasticizers may be generally adjusted within the range as defined above. 
     The plasticizer of the present invention may be readily blended with chloroprene rubber by a conventional rubber kneading method by using, for example, an open roll, a Banbury mixer or a kneader blender. In this blending step, other additives conventionally employed in the art, for example, vulcanizer, vulcanizing accelerator, filler, aging inhibitor, processing aid or pigment may be added thereto. 
    
    
     To further illustrate the present invention, and not by way of limitation, the following Examples will be given. 
     EXAMPLE 1 
     
         ______________________________________Composition              part by weight______________________________________chloroprene rubber       100(Neoprene W. mfd. by Du Pont-Showa Denko)SRF carbon               50magnesium oxide          4zinc oxide               5stearic acid             1mixed dialkyl-p-phenylenediamine                    22-mercaptoimidazoline    1tetramethylthiuram disulfide                    0.5plasticizer              20______________________________________ 
    
     The above components were mixed together in an open roll and press-vulcanized at 150° C. for 30 minutes to thereby give a test piece. 
     The cold resistance (brittle temperature) of each test piece was determined according to JIS K 6301. 
     Each test piece was elongated by 0 to 30% in a testing machine at 40° C. and an ozone concentration of 0.5 ppm at a rate of 60 times per minute to thereby examine the dynamic ozone resistance thereof. The ozone resistance was determined by observing the cracking on the surface of the test piece according to the standards of the Item 16 of JIS K 6301. In the following Table, N represents no cracking while the number of cracks increases in the order of A, B and C and the size thereof increases in the order of 1 to 5. 
     The heat resistance of each test pieces was evaluated from the weight loss, change in hardness (JIS A hardness) and residual elongation thereof after heating the same at 120° C. for 168 hours and 300 hours. 
     Table 1 shows the results. 
     
                                           TABLE 1__________________________________________________________________________                Brittle        After heating for 120                                            After heating for 300                                            hr                temp.                    Ozone resistance after                               wt. hardness                                        resid.   hardness                                                      resid.No.   Plasticizer    (°C.)                    48 hr                        96 hr                           168 hr                               loss                                   change                                        elong.                                            wt. loss                                                 change                                                      elong.__________________________________________________________________________Comp. butyl oleate   -48 A-2 B-3                           C-3 6.5%                                   +27   24%                                            8.3% +28   23%Ex. 1-11-2   1,6-hexanediol dioleate                -51 A-2 B-2                           C-3 2.2 +22  56  4.0  +25  521-3   butyldiglycol adipate                -50 A-3 B-4                           C-4 4.7 +28  33  6.5  +29  28Ex. 1-1 thiodiethanol dioctoate                -52 NC  A-1                           B-2 1.6 +14  73  2.8  +18  681-2   thiodiethanol dioleate                -52 NC  NC A-1 0.1 +7   88  1.1  +14  771-3   thiodiethanol (oleate/octoate)                -54 NC  NC B-1 1.2 +9   81  2.1  +15  75 (1:1)1-4   thiodiethanol tall oil fatty                -53 NC  NC A-1 0.1 +6   89  1.0  +13  77 acid diester1-5   thiodiethylene glycol                -53 NC  NC A-1 0.9 +7   83  1.8  +15  76 (oleate/octoate) (1:1) thiodiethanol (oleate/octoate) (1:1) 10 pts.1-6   propylene glycol (oleate/                -54 NC  NC B-1 1.5 +10  77  2.6  +17  70 octoate (1:1) 10 pts.1-7   thiodiethanol (tall oil fatty                -54 NC  NC A-1 0.8 +7   88  1.3  +12  79 acid/2-ethylhexanoic acid) esters (1:1)__________________________________________________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________                       part byComposition                 weight______________________________________chloroprene rubber          100(Neoprene WRT mfd. by Du Pont-Showa Denko)FEF carbon                  60magnesium oxide             4zinc oxide                  5stearic acid                0.52-mercaptoimidazoline       0.5plasticizer                 30______________________________________ 
    
     Test pieces were prepared from the above composition in the same manner as the one described in Example 1. 
     The weight loss, residual elongation, change in hardness and brittle temperature of each test piece were determined in the same manners as those described in Example 1. 
     Table 2 shows the result. 
     
                                           TABLE 2__________________________________________________________________________                 Weight loss (%)                 based on            Brittle temp. (°C.)                 hardened                      based on                            Resid.                                Hardness                                     before                                          afterNo.   Plasticizer     matter                      plasticizer                            elong.                                change                                     heating                                          heating__________________________________________________________________________Comp. butyl oleate    9.4  63     27%                                +25  -52  -32Ex. 2-12-2   1,6-hexanediol dioleate                 2.9  19    58  +20  -54  -442-3   butyldiglycol adipate                 6.2  41    36  +27  -51  -37Ex. 2-1 thiodiethanol dioctoate                 2.0  13    73  +12  -55  -502-2   thiodiethanol dioleate                 0.1   1    87   +6  -56  -522-3   thiodiethanol (oleate/octoate                 1.1   7    82   +8  -58  -55 (1:1)2-4   thiodiethanol tall oil fatty acid                 0.2   1    89   +6  -57  -52 diester2-5   thiodiethylene glycol                 0.8   5    84   +7  -56  -51 (oleate/octoate) (1:1) thiodiethanol (oleate/octoate) (1:1) 15 pts.2-6   propylene glycol (oleate/octoate)                 1.7  12    76  +11  -58  -52 (1:1) 15 pts.2-7   thiodiethanol (tall oil fatty acid/                 0.6   4    87   +6  -58  -55 2-ethylhexanoic acid) esters (1:1)__________________________________________________________________________ 
    
     The plasticizer of the present invention not only significantly improves the cold resistance and ozone resistance of chloroprene rubber but also has an excellent heat resistance and thus causes little physical changes when the chloroprene rubber containing the same is treated at a high temperature for a prolonged period of time.