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...
Reexamination Certificate
2000-06-09
2001-12-18
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...
C524S588000, C528S015000, C528S032000, C528S031000, C523S209000, C525S478000
Reexamination Certificate
active
06331588
ABSTRACT:
This invention relates to addition curing type liquid silicone rubber compositions which prevent thickening after mixing, have an extended pot life, and are especially suited for injection molding.
BACKGROUND OF THE INVENTION
In the prior art, addition curing type silicone rubber compositions are typically molded by press molding, transfer molding, and injection molding techniques. Under the current demand for cost reduction and mass production, it is desired to increase the cure rate of rubber to shorten the molding cycle. In the case of addition curing, however, the cure rate is increased at the sacrifice of a pot life. A silicone rubber composition is prepared by admixing its components. No problems arise if the silicone rubber composition is molded immediately after admixing. However, if the molding machine is stopped and kept quiescent for a long period of time, the composition will thicken during that period. When the molding machine is operated again, clogging or gelation can occur due to thickening or a short pot life before the composition reaches the mold, causing detrimental effects on molded parts.
To impart physical properties such as strength and durability to silicone rubber, the blending of reinforcing silica is requisite. In the application where transparent parts are necessary, large amounts of reinforcing silica must sometimes be blended in order to provide a high hardness. After blending, the silica starts to agglomerate with the lapse of time, causing thickening. In the case of an addition curing type silicone rubber composition comprising a base oil, a crosslinking component (organohydrogenpolysiloxane) and an addition reaction catalyst, the silica blended gives rise to the problem of a short pot life. There is a desire to overcome these problems.
SUMMARY OF THE INVENTION
An object of the invention is to provide an addition curing type liquid silicone rubber composition comprising an addition curing type organopolysiloxane component and reinforcing silica fines, which is prevented from thickening after mixing and has an extended pot life.
It has been found that by blending an addition curing type organopolysiloxane component with reinforcing silica fines which have been surface treated with an organosilicon compound in the form of a hydrolyzable group-containing monomer or a partial hydrolyzate thereof so as to bear at least 3% by weight of carbon on their surfaces, there is obtained a liquid addition curing type silicone rubber composition which is prevented from thickening after mixing and has an extended pot life even when the cure rate is increased. Over a long period of time, the composition has little or no detrimental effects on the line through which unvulcanized rubber is fed to a molding machine or on molded parts.
Accordingly, the invention provides an addition curing type liquid silicone rubber composition comprising (A) an addition curing type organopolysiloxane component, and (B) reinforcing silica fines having a specific surface area of at least 50 m
2
/g as measured by the BET method and surface treated with an organosilicon compound such that the content of carbon borne on the surface may be at least 3% by weight. In one preferred embodiment, the organosilicon compound with which the reinforcing silica fines are surface treated is a monomer having a hydrolyzable group capable of covering surfaces of the silica fines with (CH
3
)
n
—Si— groups wherein n is an integer of 1 to 3, or a partial hydrolyzate thereof and more preferably an organosilazane. While the composition is prepared by admixing component (B) with component (A) or an alkenyl group-containing organopolysiloxane in component (A), preferably component (B) is further treated with a second surface treating agent at the same time as this admixing.
According to the invention, the use of reinforcing silica fines having a surface carbon content of at least 3% by weight and hence, a high degree of hydrophobicity provides the liquid addition curing type silicone rubber composition in an uncured or unvulcanized state with a substantially extended pot life at room temperature. The composition is especially suited for injection molding.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The addition curing type organopolysiloxane component (A) used in the composition of the invention is preferably one primarily comprising
(a) an organopolysiloxane containing on the average at least two alkenyl groups in a molecule, represented by the following average compositional formula (1):
R
1
a
SiO
(4−a)/2
(1)
wherein R
1
is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 12 carbon atoms, 0.01 to 20 mol % of R
1
being alkenyl, and a is a positive number of 1.5 to 2.8,
(b) an organohydrogenpolysiloxane containing at least two hydrogen atoms each attached to a silicon atom in a molecule, represented by the following average compositional formula (2):
R
2
b
H
c
SiO
(4−b−c)/2
(2)
wherein R
2
is a substituted or unsubstituted monovalent hydrocarbon group of 1 to 10 carbon atoms, b is a positive number of 0.7 to 2.1, c is a positive number of 0.002 to 1, and b+c is 0.8 to 3, and
(c) a catalytic amount of an addition reaction catalyst.
The alkenyl group-containing organopolysiloxane of formula (1) is a base polymer of the addition curing type organopolysiloxane component (A). R
1
stands for substituted or unsubstituted monovalent hydrocarbon groups of 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, for example, alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, neopentyl, hexyl, cyclohexyl, octyl, nonyl and decyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl, isobutenyl, hexenyl, cyclohexenyl and octenyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; and halo- or cyano-substituted hydrocarbon groups such as chloromethyl, bromoethyl, 3,3,3-trifluoropropyl, 3-chloropropyl and cyanoethyl. The R
1
groups may be the same or different. Alkenyl groups should preferably account for 0.01 to 20 mol % (i.e., 0.0001 to 0.2), and more preferably 0.1 to 10 mol % (i.e., 0.001 to 0.1) of the entire organic groups (i.e., R
1
groups). The organopolysiloxane should contain on the average at least two alkenyl groups. Although R
1
may be any of the foregoing groups, it is preferred to introduce vinyl as the alkenyl group, and methyl, phenyl and 3,3,3-trifluoropropyl as the other substituent group. Letter a is a positive number of 1.3 to 2.8, preferably 1.8 to 2.5, and more preferably 1.95 to 2.05.
The molecular structure of the organopolysiloxane of formula (1) may be a straight chain or a branched structure containing R
1
SiO
3/2
and/or SiO
4/2
units. Typical is a linear diorganopolysiloxane having a backbone consisting essentially of recurring diorganosiloxane units: R
1
2
SiO
2/2
and blocked with a triorganosiloxy unit: R
1
3
SiO
1/2
at each end of its molecular chain. The alkenyl groups in the molecule may be attached to either or both of the silicon atoms at the end and intermediate of the molecular chain. It is preferred from the standpoints of curability and cured physical properties that the organopolysiloxane have at least alkenyl groups attached to the silicon atoms at both ends of the molecular chain.
The alkenyl group-containing organopolysiloxane of formula (1) desirably has a viscosity of 100 to 1,000,000 centipoise at 25° C., and especially 500 to 500,000 centipoise at 25° C.
The alkenyl group-containing organopolysiloxane can be prepared by any well-known method, for example, by effecting equilibration reaction between a cyclic and/or linear organopolysiloxane and a hexaorganodisiloxane in the presence of an alkali or acid catalyst.
In formula (2) representing the organohydrogenpolysiloxane, R
2
is as defined for R
1
and preferably free of aliphatic unsaturation, with methyl, phenyl and 3,3,3-trifluoropropyl being especially preferred. Letter b is a positive number of 0.7 to 2.1, preferably 1 to 2, c is a
Azechi Syuuichi
Taira Yujiro
Yoshida Takeo
Birch & Stewart Kolasch & Birch, LLP
Dawson Robert
Peng Kim-Liang
Shin-Etsu Chemical Co. , Ltd.
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