Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-01-19
2002-12-31
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S154000, C528S024000, C528S032000, C528S043000
Reexamination Certificate
active
06500874
ABSTRACT:
This invention relates to a silicone rubber composition of the organic peroxide curing type, especially such a silicone rubber composition suited for wire coating, a silicone rubber sponge composition, and a silicone rubber-covered wire.
BACKGROUND OF THE INVENTION
In the prior art, silicone rubber compositions of the organic peroxide curing type use organic peroxides such as bis(2,4-dichlorobenzoyl)peroxide, 2,5-di(t-butylperoxy)hexane and dicumyl peroxide as the curing agent. An appropriate type and amount of curing agent is selected and used depending on the molding technique of a silicone rubber composition as well as molding temperature and other conditions, and the type of silicone rubber.
Of these organic peroxide curing agents, bis(2,4-dichlorobenzoyl)peroxide is a superior curing agent and widely used in the industry. This is because bis(2,4-dichlorobenzoyl)peroxide is stable at room temperature, achieves the fastest vulcanization as compared with other curing agents and offers high productivity. As the significant feature, this curing agent is most effective under atmospheric hot air vulcanization (HAV) conditions.
However, bis(2,4-dichlorobenzoyl)peroxide produces chlorine by-products when thermally decomposed. The molding time is undesirably prolonged because long-term heat treatment is needed to remove the chlorine by-products. It is believed from the environmental standpoint that the use of peroxide curing agents which will form such chlorine by-products becomes difficult or inhibited.
From such a standpoint, there is a need for a peroxide curing agent substitute for bis(2,4-dichlorobenzoyl)peroxide. JP-A 62-185750 discloses bis(p-methylbenzoyl)peroxide as the halogen-free peroxide curing agent. JP-A 63-130663 discloses 1,3-bis(t-butylperoxy-carbonyloxy)-2,2-dimethylpropane as a halogen-free peroxide curing agent. These peroxide curing agents have high decomposition temperatures and suffer from the drawbacks of a slow vulcanization rate and poor productivity.
For the vulcanization of wire coating silicone rubber compositions, various techniques are used depending on the type of silicone rubber composition and the physical properties required for the cured rubber. In general, heat treatment in the presence of organic peroxides is widely used. Such organic peroxides include benzoyl peroxide, bis(p-chlorobenzoyl)peroxide, bis(2,4-dichlorobenzoyl)peroxide, dicumyl peroxide, di-t-butyl peroxide, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, t-butyl perbenzoate, and t-butyl cumyl peroxide. As the vulcanizing agents which ensure that cost effective silicone rubber-covered wires having good properties are manufactured through extrusion hot air vulcanization, bis(2,4-dichlorobenzoyl)peroxide and other halogen-containing peroxides are customary. However, when halogen-containing organic peroxides are used, decomposed products thereof are left in molded parts after curing, which will bleed out on the surface with the lapse of time. Because of their own poisoning effect, long-term post-curing is necessary.
To solve these problems, for example, JP-A 59-18758 proposes the use of bis(o-methylbenzoyl)peroxide as the curing agent in silicone rubber compositions. The use of bis(p-methylbenzoyl)peroxide is also proposed as the curing agent in silicone rubber compositions. These curing agents in the form of methyl-substituted benzoyl peroxide have a high decomposition temperature and a low vulcanization rate so that silicone rubber compositions using such curing agents tend to generate voids upon curing. As a consequence, silicone rubber molded parts often have varying physical properties, and coated wires undergo a lowering of breakdown voltage such as spark-over.
Meanwhile, silicone rubber sponge has physical properties inherent to silicone rubber as well as excellent properties including heat resistance, freeze resistance, electrical insulation, flame retardance and compression set. Basically, silicone rubber sponge is manufactured by combining a heat-curable silicone rubber composition with a curing agent and a blowing agent, and heating the composition for blowing and curing, thereby forming a sponge. In this process, a blowing ability, a uniform fine cell structure, and a skin layer having a smooth tack-free surface are important, and the physical properties inherent to silicone rubber must be retained.
With respect to the molding method, the compositions are cured and expanded in atmospheric hot air so that continuous molding is possible. Organic peroxides enabling atmospheric hot air vulcanization are generally halogen-containing peroxides such as bis(2,4-dichlorobenzoyl)-peroxide. When halogen-containing organic peroxides are used, there arise the problems that decomposed products left in molded parts after curing will bleed out on the surface with the lapse of time, and long-term post-curing is necessary because of the decomposed products' own poisoning effect as previously mentioned.
Halogen-free benzoyl peroxide allows for atmospheric hot air vulcanization, but fails to provide satisfactory sponge properties with respect to expansion and cell structure.
Then JP-A 10-182972 proposes the use of alkyl-substituted benzoyl peroxides as a halogen-free curing agent. These peroxides have a high decomposition temperature and a low vulcanization rate, leading to low productivity, and fail to provide satisfactory sponge properties with respect to expansion and cell structure.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a silicone rubber composition, especially for electrical wire coating, which has a high vulcanization rate and hygienic safety, and can be extrusion vulcanized in a continuous atmospheric hot air vulcanization manner into a silicone rubber part without voids or surface tack. Another object is to provide a silicone rubber-covered wire using the silicone rubber composition and having improved withstand voltage properties.
A further object of the invention is to provide a silicone rubber sponge composition which can be cured with halogen-free organic peroxides without detracting from working efficiency and has a reduced post-cure time, as compared with halogen-containing organic peroxides, and which is effectively expandable into a silicone rubber sponge having a uniform fine cell structure and a skin layer with a smooth, tack-free surface.
It has been found that when organic peroxides of the following structural formula (2), which are halogen free, are used as the curing agent in silicone rubber compositions, these organic peroxides create no environmental problem, have a high vulcanization rate satisfactory for productivity, and function well under atmospheric hot air vulcanization conditions. These organic peroxides are thus effective for producing silicone rubber or silicone rubber sponge with satisfactory physical properties. When wires are covered with the silicone rubber compositions, extrusion vulcanization can be effected without giving rise to such problems as voids and surface tackiness, and the resulting silicone rubber coating has improved withstand voltage properties. The invention is predicated on these findings.
According to the invention, there is provided a silicone rubber composition comprising (A) an organopolysiloxane of the average compositional formula (1), (B) an inorganic filler, and (C) an organic peroxide of the structural formula (2).
R
1
a
SiO
(4−a)/2
(1)
Herein R
1
is a substituted or unsubstituted monovalent hydrocarbon group and “a” is a positive number of 1.8 to 2.3.
Herein R
2
is independently hydrogen or alkyl, R
3
is alkylene, and n is an integer of 1 to 3.
Also provided herein is a silicone rubber-covered wire comprising a wire around which the silicone rubber composition has been extrusion molded.
The invention also provides a silicone rubber sponge composition further comprising (E) an organic blowing agent in addition to the above components.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Component (A) of the silicone rubber composition according to the invention is an organopolysi
Satoh Terukazu
Yaginuma Atsushi
Birch, Stewart, Kolasch & Birch. LLP
Foelak Morton
Shin-Etsu Chemical Co. , Ltd.
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