Fluorosilicone rubber compositions

Details Fluorosilicone rubber compositions Fluorosilicone rubber compositions

2001-08-31

2003-08-26

Dawson, Robert (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C528S042000, C528S024000, C525S479000, C525S474000, C560S302000, C556S458000, C544S221000, C549S215000, C524S492000

Reexamination Certificate

active

06610778

ABSTRACT:

This invention relates to fluorosilicone rubber compositions, and more particularly, to fluorosilicone rubber compositions which can be briefly molded and vulcanized into products having improved swell resistance upon immersion in fuel oil and lubricating oil as well as satisfactory compression recovery and mechanical strength.
BACKGROUND OF THE INVENTION
Because of heat resistance, freeze resistance, oil resistance, fuel oil resistance, and compression recovery, fluorosilicone rubber is widely used as parts for automobiles, aircraft and other transporting vehicles and parts for petroleum-related equipment.
Fluorosilicone rubber compositions can be molded and vulcanized as are dimethylsilicone rubber compositions. Usually, a fluorosilicone rubber composition is molded into any desired shape by a conventional rubber molding technique such as compression molding, transfer molding, injection molding, extrusion molding or calender molding and then vulcanized and cured by a conventional technique. In this way, rubber parts such as O-rings, diaphragms and gaskets are obtained. The predominant molding method involves molding to the desired shape by applying heat and pressure in a mold as in compression molding, transfer molding and injection molding, followed by vulcanization curing. The vulcanization curing may use prior art well-known organic peroxides as in dimethylsilicone rubber compositions, and most often, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and dicumyl peroxide are employed.
For reducing the manufacturing cost of rubber parts, a number of studies have been made how to reduce the vulcanization curing time. In the process of applying heat and pressure within a mold, the manufacturing cost can be reduced by reducing the vulcanization curing time to increase the turnover of the mold.
As compared with dimethylsilicone rubber compositions, the fluorosilicone rubber compositions are slow in vulcanization curing and require a long time for vulcanization curing. If the vulcanization curing time is short, the desired crosslinking density is not reached. This results in shortage of hardness, shortage of strength, and poor heat resistance and still worse, the compression set and oil swell which are important as sealing material are also exacerbated.
Vulcanization curing can be accelerated by increasing the amount of organic peroxide used as the vulcanizing or curing agent. However, even if the organic peroxide is increased beyond the appropriate level, the crosslinking density is no longer increased. Also the molded parts are rather exacerbated in compression set, heat resistance and oil resistance under the influence of the excess of organic peroxide and decomposed residues thereof.
Use of an organic peroxide having a lower decomposition temperature is also under consideration with the aim to accelerate vulcanization curing at the same molding temperature. For dimethylsilicone rubber compositions, the vulcanization curing time can be reduced using peroxy ketal organic peroxides such as 1,1-bis(tert-butylperoxy)-3,3,5-trimethylcyclohexane or percarbonate organic peroxides such as 1,6-bis(tert-butylperoxycarbonyl-oxy)hexane. For fluorosilicone rubber compositions, similar vulcanization curing gives rise to problems that the rubber physical properties are poor as compared with vulcanization curing with customary peroxides such as 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane and dicumyl peroxide, and that vulcanization curing is so fast that the rubber composition has been cured before it fully flows and fills in the mold, resulting in molded parts of undesired shape.
Raising the vulcanization curing temperature is an effective measure for reducing the vulcanization curing time because decomposition of the organic peroxide is promoted. At the elevated temperature, however, vulcanization curing is so fast that the rubber composition has been cured before it fully flows and fills in the mold. This often gives rise to problems that the molded parts have undesired shape, are distorted, discolored or unevenly colored, and seized to the mold.
For dimethylsilicone rubber compositions, the vulcanization curing time can also be reduced by vulcanizing the compositions through addition reaction using platinum base catalysts. This is not effective or practical to most fluorosilicone rubber compositions because little vulcanization proceeds with these catalysts.
SUMMARY OF THE INVENTION
An object of the invention is to provide fluorosilicone rubber compositions which are significantly reduced in vulcanization molding time.
The invention is directed to a fluorosilicone rubber composition comprising an organopolysiloxane having a trifluoropropyl group and a silica filler. It has been found that the time required for the composition to complete vulcanization molding can be significantly reduced without detracting from its physical properties when an organic peroxide of percarbonate type is used as a crosslinking agent and concurrently, a compound having a molecular weight of up to 10,000 and bearing in a molecule at least two substituents selected from among allyl, acryloyl, methacryloyl, epoxy and alkoxy groups, at least one of the substituents being selected from among allyl, acryloyl, methacryloyl and epoxy groups, is used as a co-crosslinking agent.
The invention provides a fluorosilicone rubber composition comprising
(A) 100 parts by weight of an organopolysiloxane represented by the following average compositional formula (1):
R
1
a
R
2
b
R
3
c
SiO
(4-a-b-c)/2
  (1)
 wherein R
1
is trifluoropropyl, R
2
is a substituted or unsubstituted, monovalent, aliphatic unsaturated hydrocarbon group having 2 to 8 carbon atoms, R
3
is an unsubstituted, monovalent, aliphatic saturated hydrocarbon or aromatic hydrocarbon group having 1 to 8 carbon atoms, “a” is a positive number of 0.96 to 1.01, “b” is a positive number of 0.0001 to 0.01, “c” is a positive number of 0.96 to 1.06, and the sum of a+b+c is 1.98 to 2.02, and having a viscosity of at least 10,000 cs at 25° C.,
(B) 5 to 100 parts by weight of a silica filler,
(C) 0.01 to 5 parts by weight of an organic peroxide of the following general formula (2) or (3):
 wherein each of R
4
and R
5
, which are the same or different, is a monovalent hydrocarbon group having 3 to 10 carbon atoms,
 wherein each of R
6
and R
8
, which are the same or different, is a monovalent hydrocarbon group having 3 to 10 carbon atoms, and R
7
is a divalent hydrocarbon group of 2 to 8 carbon atoms which may contain an oxygen atom, and
(D) 0.01 to 2 parts by weight of a co-crosslinking agent having a molecular weight of up to 10,000 and bearing in a molecule at least two substituents selected from the class consisting of allyl, acryloyl, methacryloyl, epoxy and alkoxy groups, at least one of the substituents being selected from the class consisting of allyl, acryloyl, methacryloyl and epoxy groups.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Component (A) of the fluorosilicone rubber composition according to the invention is an organopolysiloxane represented by the following average compositional formula (1).
R
1
a
R
2
b
R
3
c
SiO
(
4-a-b-c)/2
  (1)
Herein R
1
is trifluoropropyl. R
2
stands for substituted or unsubstituted monovalent aliphatic unsaturated hydrocarbon groups of 2 to 8 carbon atoms, for example, alkenyl groups such as vinyl, allyl, propenyl, butenyl and hexenyl. R
3
stands for unsubstituted, monovalent, aliphatic saturated hydrocarbon or aromatic hydrocarbon groups of 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, butyl and hexyl, aryl groups such as phenyl and tolyl, and aralkyl groups such as benzyl. Letter “a” is a positive number of 0.96 to 1.01, “b” is a positive number of 0.0001 to 0.01, “c” is a positive number of 0.96 to 1.06, and a+b+c is 1.98 to 2.02.
The organopolysiloxane of formula (1) should have at least two aliphatic unsaturated hydrocarbon groups represented by R
2
in a molecule while R
2
may be located at the ends of the backbone and/or on side chains.

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