Plasticizer for chloroprene rubber and plasticized chloroprene rubber

A plasticizer for chloroprene rubber represented by the following general formula (I): EQU R.sub.1 (O--R).sub.m S(R--O).sub.n R.sub.2 (I) PA1 wherein R.sub.1 and R.sub.2 independently represent each a saturated or unsaturated aliphatic monoacyl group having 4 to 24 carbon atoms; PA1 R represents an alkylene group having two to four carbon atoms; and PA1 m and n represent each an integer of 1 to 4: is disclosed.

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: 
EQU R.sub.1 (O--R).sub.m S(R--O).sub.n R.sub.2 (I) 
wherein R.sub.1 and R.sub.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.sub.1 and R.sub.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.degree. 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.