Curable fluoropolyether base rubber compositions

Details Curable fluoropolyether base rubber compositions Curable fluoropolyether base rubber compositions

2001-07-03

2004-07-06

Robertson, Jeffrey B. (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...

C524S492000, C524S567000, C524S588000

Reexamination Certificate

active

06759467

ABSTRACT:

This invention relates to a curable fluoropolyether base rubber composition which is suited for liquid injection molding and cures into products having improved mechanical properties.
BACKGROUND OF THE INVENTION
In the prior art, linear fluoropolyether compounds containing at least two alkenyl groups per molecule and having a perfluoroalkyl ether structure in their backbone are used in a variety of applications owing to the excellent characteristics of the perfluoroalkyl ether structure. While silica fillers including dry silica (known as fumed silica) and wet silica (known as precipitated silica) are commonly used in silicone rubber for reinforcement purposes, it is known that blending such silica filler in the curable fluoropolyether base rubber can remarkably improve the mechanical properties of the cured product thereof. The blending of silica filler provides fluoropolyether base rubber compositions with a good balance of heat resistance, chemical resistance, solvent resistance, water repellency, oil repellency, and weather resistance. The resultant compositions perform well in most applications.
However, a problem arises in molding such rubber materials. When O-rings and diaphragms for use in semiconductor parts and hard disks are to be molded, liquid injection molding (LIM) featuring mass-scale productivity is often used. A limit is imposed on the viscosity of liquid rubber materials which can be molded. This, in turn, imposes a limit on the permissible loading of reinforcing silica. It is then difficult to find a compromise between good mechanical strength and ease of liquid injection molding.
SUMMARY OF THE INVENTION
An object of the invention is to provide a liquid curable fluoropolyether base rubber composition which is amenable to liquid injection molding and cures into products having improved mechanical properties.
The present invention provides a curable fluoropolyether base rubber composition comprising
(A) 100 parts by weight of a linear fluoropolyether compound containing at least two alkenyl groups in a molecule and having a perfluoroalkyl ether structure in its backbone,
(B) 10 to 50 parts by weight of a silica filler having a specific surface area of at least 100 m
2
/g and a bulk density of 100 to 200 g/l,
(C) an effective amount to cure component (A) of an organosilicon compound having at least two hydrogen atoms each bound to a silicon atom in a molecule, and
(D) a catalytic amount of a hydrosilylation catalyst. This fluoropolyether base rubber composition is prevented from a viscosity rise due to filler loading, is amenable to liquid injection molding, and cures into products having improved mechanical properties.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The respective components of the curable fluoropolyether base rubber composition are described below.
(A) Linear Fluoropolyether Compound
The linear fluoropolyether compound used herein as a base polymer in the composition is one containing at least two alkenyl groups in a molecule and having a divalent perfluoroalkyl ether structure in its backbone.
The alkenyl groups in the linear fluoropolyether compound are those having a CH
2
═CH— structure at an end such as vinyl, allyl, propenyl, isopropenyl, butenyl and hexenyl, with the vinyl and allyl being especially preferred. The alkenyl groups may be attached either directly to both ends of the backbone of the linear fluoropolyether compound or to the backbone through a divalent linking group such as —CH
2
—, —CH
2
O— or —Y—NR—CO—. Herein Y is —CH
2
— or a group of the following structural formula:
(the bond may be at o, m or p-position) and R is hydrogen, methyl, phenyl or allyl.
The perfluoroalkyl ether structure in the linear fluoropolyether compound includes those of the following general formula:
—(Rf—O)
q
wherein Rf is a straight or branched perfluoroalkylene group of 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, and letter q is an integer of 1 to 500, preferably 2 to 400, more preferably 10 to 200.
Examples of the recurring units —(Rf—O)— are shown below.
—CF
2
O—, —CF
2
CF
2
O—, —CF
2
CF
2
CF
2
O—,
—CH(CF
3
)CF
2
O—, —CF
2
CF
2
CF
2
CF
2
O—,
—CF
2
CF
2
CF
2
CF
2
CF
2
CF
2
O—, and —C(CF
3
)
2
—.
Of these, —CF
2
O—, —CF
2
CF
2
O—, —CF
2
CF
2
CF
2
O—, and —CH(CF
3
)CF
2
O— are preferred. It is understood that the perfluoroalkyl ether structure may consist of recurring units —(Rf—O)— of one type or recurring units of two or more types.
Typical of the linear fluoropolyether compound (A) are those of the following general formula (1).
In formula (1), X is independently selected from among —CH
2
—, —CH
2
O— and —Y—NR—CO—, letter p is independently equal to 0 or 1, r is an integer of 2 to 6, and m and n are integers of 0 to 200, preferably 5 to 100. Y is —CH
2
— or a group of the following structural formula:
(the bond may be at o, m or p-position), and R is hydrogen, methyl, phenyl or allyl. These linear fluoropolyether compounds have a molecular weight of about 400 to 100,000 and preferably about 1,000 to 50,000.
Illustrative examples of the linear fluoropolyether compound of formula (1) are given below. In the following formulae, m and n are as defined above.
These linear fluoropolyether compounds may be used alone or in admixture of two or more.
(B) Silica Filler
The filler (B) used for the reinforcement of the curable fluoropolyether base rubber composition is particulate silica. It should have a specific surface area of at least 100 m
2
/g as measured by the hydrogen adsorption BET method. From the molding standpoint, the silica filler should have a bulk density of 100 to 200 g/l, and preferably 130 to 160 g/l, in order to prevent the composition from increasing its viscosity as a result of blending of the silica filler. Too low a bulk density detracts from reinforcement effects whereas too high a bulk density results in thickening.
The silica filler may be either dry silica known as fumed silica or wet silica known as precipitated silica. The silica filler may be treated with organochlorosilanes, silazane compounds and cyclic silazane compounds which react with hydroxyl groups attached to silicon atoms on silica surfaces. Alternatively, the silica filler may be hydrophobized on the surface with dimethylpolysiloxanes having a low degree of polymerization.
An appropriate amount of the silica filler (B) blended is 10 to 50 parts, and preferably 15 to 30 parts by weight per 100 parts by weight of component (A). Less than 10 parts of component (B) fails to achieve sufficient reinforcement whereas more than 50 parts of component (B) invites an excessive rise of viscosity and is difficult to compound.
(C) Organosilicon Compound
The organosilicon compound (C) functions as a crosslinking agent and chain extender for component (A). Any organosilicon compound is useful as long as it has at least two hydrogen atoms each bound to a silicon atom, that is, hydrosilyl (SiH) groups in a molecule. With the compatibility with and dispersibility in component (A), and uniformity after curing taken into account, organosilicon compounds having at least one monovalent perfluorooxyalkyl group, monovalent perfluoroalkyl group, divalent perfluorooxyalkylene group or divalent perfluoroalkylene group as well as at least two, preferably at least three hydrosilyl groups (or SiH groups) are preferred.
The perfluorooxyalkyl, perfluoroalkyl, perfluorooxy-alkylene and perfluoroalkylene groups include the groups of the following general formulae.
monovalent perfluoroalkyl groups:
C
m
F
2m+1
—
m is an integer of 1 to 20, preferably 2 to 10.
divalent perfluoroalkylene groups:
—C
m
F
2m
—
m is an integer of 1 to 20, preferably 2 to 10.
monovalent perfluorooxyalkyl groups:
n is an integer of 1 to 5. divalent perfluorooxyalkylene groups:
m is an integer of 1 to 50, n is an integer of 1 to 50, and m+n is an integer of 2 to 100.
—(CF
2
O)
m
—(CF
2
CF
2
O)
n
—CF
2
—
m and n each are an integer of 1 to 50.
These perfluoro(oxy)alkyl and perfluoro(oxy)alkylene groups each may be attached either directly to a silicon atom or to a silicon atom through a divalent linki

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