Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-01-16
2002-04-16
Lipman, Bernard (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C524S565000, C524S566000, C525S332700, C525S302000
Reexamination Certificate
active
06372856
ABSTRACT:
TECHNICAL FIELD
This invention relates to a dip-forming, vulcanizable rubber latex composition comprising a sulfur-containing vulcanizer an a vulcanization accelerator, which is used for making a vulcanized unsaturated nitrile rubber article without production of a nitrosamine which is restricted by regulation, and further relates to a rubber article dip-formed therefrom.
BACKGROUND ART
Certain N-nitrosamines (hereinafter abbreviated to “nitrosamines”) are carcinogenic, and a problem arises in that it is possible that nitrosamines are produced in vulcanized rubber articles made from a solid polymer rubber or a polymer rubber latex.
For example, as examples of the vulcanized rubber articles made from a polymer rubber latex, there can be mentioned those which are used in contact with the human body, such as a nipple, a balloon, gloves for operation, or medical examination or detection, a balloon sac, and other medical articles. It is possible that nitrosamines are detected in these vulcanized rubber articles. Especially a serious problem is caused in medical rubber articles which are used in direct contact with mucous membranes or organs.
At present, the restriction of nitrosamines contained in vulcanized rubber articles by regulation is an urgent problem in Japan and many other countries. Already in German, the maximum permissible amount of specific nitrosamines is 10 ppb or below. As such nitrosamines which are restricted by regulation, there can be mentioned seven species, which include, for example, N-nitrosodimethylamine, N-nitrosodiethylamine, N-nitrosodi-n-butylamine and N-nitrosomethylphenylamine.
It is known that the occurrence of nitrosamines in vulcanized rubber articles is due to the fact that a dithiocarbamic acid compound used as a polymerization terminator for emulsion polymerization or as a vulcanization promoter emulsifier remains in a solid polymer rubber or polymer rubber latex. Namely, the dithiocarbamic acid compound is hydrolyzed to produce a secondary amine which in turn reacts with NO
x
in the environment or nitrites or other NO
x
contained in food or the saliva to produce nitrosamines.
Thus it has been studied to use dithiocarbamic acid compounds as a vulcanization accelerator for rubber, which produce only a negligible amount of a nitrosamine or a secondary amine, i.e., a precursor of nitrosamine. As an example of such dithiocarbamic acid compounds, there can be mentioned zinc dibenzyldithiocarbamate. However, it is reported that when this vulcanization accelerator is used for a natural rubber latex, the resulting vulcanized rubbers generally have poor vulcanization properties as compared with vulcanized rubbers made with other conventional vulcanization accelerators (Polymer Digest, 1987, 12, p12-). The present inventors have also confirmed that when zinc dithiocarbamates are incorporated in a natural rubber latex, rubber articles dip-formed from the natural rubber latex have cracks and poor surface luster.
A vulcanization accelerator incapable of producing a nitrosamine, which is different from the dithiocarbamic acid compound vulcanization accelerator, such as zinc isopropylxanthogenate, has also been reported. This vulcanization accelerator is however little or no practicality because it has a low storage stability and an offensive smell, and results in rubber vulcanizates with poor vulcanization properties.
Other types of vulcanization accelerators such as thiophosphate compounds, thiazole compounds, benzothiazolesulphenamide compounds and guanidine compounds are known, but desired vulcanization properties cannot be obtained with these vulcanization accelerators (Polymer Digest, 1991, 1, p65-).
Thus there is an increasing demand for a vulcanization accelerator which is incapable of producing a nitrosamine or a secondary amine and giving a vulcanized rubber having good vulcanization properties.
DISCLOSURE OF INVENTION
To solve the problems encountered with the conventional vulcanization promoters, the present inventors have conducted researches into vulcanization accelerators and found that, when specified dithiocarbamic acid compounds are used as a vulcanization accelerator for an unsaturated nitrile rubber (NBR) latex, a vulcanized rubber article dip-formed therefrom has no crack occurrence, excellent surface luster and good vulcanization properties. Based on this finding, the present invention has been completed.
Thus in accordance with the present invention there is provided a vulcanizable dip-forming rubber latex composition characterized as comprising an unsaturated nitrile-conjugated diene copolymer rubber latex, a sulfur-containing vulcanizer, at least one vulcanization accelerator selected from (i) dithiocarbamic acid compounds represented by the formula (1):
wherein R
1
and R
2
are hydrocarbon groups having at least 6 carbon atoms which may be the same as or different from each other, and (ii) zinc dithiocarbamate compounds represented by the formula (2):
wherein R
1
and R
2
are hydrocarbon groups having at least 6 carbon atoms which may be the same as or different from each other, and an optional thiazole compound vulcanization accelerator.
Further, in accordance with the present invention, there is provided a rubber article dip-formed from the above-mentioned vulcanizable dip-forming rubber latex composition.
BEST MODE FOR CARRYING OUT THE INVENTION
The embodiments of the present invention will now be described in detail.
The main point of the present invention lies, for the production of a vulcanized rubber article by dip-forming a sulfur-vulcanizable polymer rubber latex, in the use of an unsaturated nitrile-conjugated diene copolymer rubber as the polymer rubber, and a sulfur-containing vulcanizer and, in combination therewith, a dithiocabamic acid compound of the formula (1) and/or a zinc dithiocarbamate compound of the formula (2) as the vulcanization accelerator.
Vulcanizable Dip-Forming Rubber Latex Composition
The unsaturated nitrile-conjugated diene copolymer rubber latex used in the dip-forming rubber latex composition is a latex of a copolymer rubber prepared by copolymerizing a conjugated diene monomer, an ethylenically unsaturated nitrile monomer, and optional copolymerizable ethylenically unsaturated acid monomer and/or other ethylenically unsaturated monomer.
The conjugated diene monomer used for the preparation of the unsaturated nitrile-conjugated diene copolymer rubber is not particularly limited and includes, for example, 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-pentadiene and chloroprene. Especially 1,3-butadiene and isoprene are preferable. These conjugated diene monomers may be used either alone or as a combination of at least two thereof. The amount of the conjugated diene monomer is usually in the range of 30 to 90% by weight and preferably 35 to 80% by weight, based on the total weight of the monomers. If the amount of the conjugated diene monomer is smaller than 30% by weight, the vulcanized rubber article dip-formed from the latex composition has rigid feeling. In contrast, if the amount thereof is larger than 90% by weight, the vulcanized rubber article dip-formed from the latex has a poor oil resistance and low tensile strength and tear strength.
The ethylenically unsaturated nitrile monomer is not particularly limited and includes, for example, acrylonitrile, methacrylonitrile, fumaronitrile, &agr;-chloroacrylonitrile and &agr;-cyanoethylacrylonitrile. These ethylenically unsaturated nitrile monomers may be used either alone or as a combination of at least two thereof. The amount of the ethylenically unsaturated nitrile monomers is usually in the range of 9 to 50% by weight and preferably 20 to 45% by weight, based on the total weight of the monomers. If the amount of the unsaturated nitrile monomer is smaller than 9% by weight, the vulcanized rubber article dip-formed from the latex composition has a poor oil resistance. In contrast, if the amount thereof is larger than 50% by weight, the vulcanized rubber article dip-formed from the latex has a rigid feeling.
Ohta Hisanori
Ozawa Yutaka
Birch & Stewart Kolasch & Birch, LLP
Lipman Bernard
Nippon Zeon Co. Ltd.
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