Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2000-07-13
2002-05-07
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S031000, C528S028000, C428S447000
Reexamination Certificate
active
06384171
ABSTRACT:
This invention relates to urethane resin coating compositions which effectively bond to surfaces of various substrates, especially silicone rubber substrates, to form a surface protective layer having improved wear resistance thereon, and silicone rubber members surface protected with the coating compositions.
BACKGROUND OF THE INVENTION
Cellular phones and remote controllers have silicone rubber key pads. On the top of the keys, characters or images are often printed with silicone rubber base ink having satisfactory bonding and coloring abilities (a character ink layer). The character ink layer subject to key touch is worn by contact with fingers whereby the characters become blurred and unreadable. It is also a problem that sweat components such as water, salts and oil are conveyed from the fingers to the silicone rubber key pad whereupon they penetrate through the pad to cause contact failure.
As the rolls in electrophotographic copiers and printers, silicone rubber rolls are commonly used because they are highly heat resistant and their properties such as hardness and elasticity can be adjusted over a wide range. Silicone rubber, however, lacks print durability due to poor wear resistance. During service, a minor amount of low-molecular-weight siloxane contained in the silicone rubber oozes out on the surface, degrading the image quality of the print.
One common approach for imparting wear resistance or providing a countermeasure against contact failure or low-molecular-weight siloxane is to apply a resin coating material to such a silicone rubber member to form a surface protective layer. More particularly, it was proposed to use as the resin coating material a urethane resin characterized by very good wear resistance and minimized penetration of water, salts, oil and low-molecular-weight siloxane.
However, the urethane resin gives rise to a problem when the substrate is silicone rubber having parting properties. Unless the surface of silicone rubber substrate is subjected to cumbersome pre-treatment such as primer treatment, ozone treatment or etching, the urethane resin does not adhere to the substrate surface and thus fails to exert wear resistance. It is desired to overcome this problem.
SUMMARY OF THE INVENTION
An object of the invention is to provide a coating composition which firmly bonds to a variety of substrates, especially silicone rubber substrates, and readily forms thereon a surface protective layer having improved wear resistance. Another object of the invention is to provide a silicone rubber member which is surface treated with the coating composition.
It has been found that by blending (A) a urethane polymer having at least one aliphatic unsaturated hydrocarbon group on a side chain in a molecule, (B) an organohydrogenpolysiloxane having at least two hydrosilyl groups in a molecule, and (C) a hydrosilylation catalyst, there is obtained a coating composition which firmly bonds to a variety of substrates, especially silicone rubber substrates, and readily forms thereon a surface protective layer having improved wear resistance without a need for cumbersome pre-treatment. When (D) a polyfunctional isocyanate compound is further blended, the coating composition can form a surface protective layer of higher quality having better adhesion. A silicone rubber member having a surface protective layer formed of the coating composition is effectively utilized as key pads and rolls in copiers and printers.
Among a variety of silicone rubbers, millable silicone rubbers and injection molding liquid silicone rubbers are typical silicone rubbers for use as key pads and rolls. The vulcanizing and crosslinking mechanisms of these rubbers are peroxide vulcanization and hydrosilylation crosslinking, respectively. The peroxide vulcanization generally follows the mechanism that radicals originating from a peroxide act on vinyl groups in silicone gum for effecting crosslinking, leaving some unreacted vinyl groups in the rubber after curing. In the hydrosilylation crosslinking, a vinyl-containing silicone fluid and a hydrosilyl-containing crosslinking agent are reacted in the presence of a hydrosilylation catalyst, leaving some unreacted vinyl and/or hydrosilyl groups in the rubber after curing.
When the urethane resin coating composition of the invention is applied to a surface of such silicone rubber, the components of the coating composition partially penetrate into the silicone rubber. Then hydrosilylation reaction takes place in a combined fashion among aliphatic unsaturated hydrocarbon groups in the urethane polymer (A), hydrosilyl groups in the organohydrogenpolysiloxane (B), and residual vinyl and/or hydrosilyl groups in the silicone rubber in the presence of the hydrosilylation catalyst (C), accomplishing an anchoring effect near the surface of silicone rubber. Through this mechanism, the composition forms a firm bond to a variety of substrates, especially silicone rubber substrates.
When the polyfunctional isocyanate compound (D) is further blended, terminal hydroxyl or amino groups in the urethane polymer (A) react with isocyanate groups in the polyfunctional isocyanate compound (D) to form a three-dimensional structure. Consequently, the urethane resin is further improved in properties and adhesion.
Accordingly, the invention provides a coating composition comprising (A) 100 parts by weight of a urethane polymer having at least one aliphatic unsaturated hydrocarbon group on a side chain in a molecule, (B) 0.1 to 100 parts by weight of an organohydrogenpolysiloxane having at least two hydrosilyl groups in a molecule, and (C) a catalytic amount of a hydrosilylation catalyst. In a referred embodiment, the coating composition further includes (D) 1 to 150 parts by weight of a polyfunctional isocyanate compound.
Also contemplated herein is a silicone rubber member comprising a silicone rubber substrate and a surface protective layer of the above-defined coating composition formed on a surface of the substrate.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Component (A) of the coating composition according to the invention is a urethane polymer having at least one aliphatic unsaturated hydrocarbon group on a side chain in a molecule.
The urethane polymer (A) is preferably obtained by reacting (a) a straight chain polymer having a hydroxyl or amino group at each end and at least one aliphatic unsaturated hydrocarbon group on a side chain, preferably 1 to 50, and more preferably 2 to 20 aliphatic unsaturated hydrocarbon groups on side chains with (b) a difunctional isocyanate compound. In this reaction, the molar ratio of terminal functional groups in the respective reactants, that is, (terminal hydroxyl or amino groups in reactant (a))/(NCO groups in reactant (b)), simply represented by (a)/(b), is set greater than 1.0, preferably 2.0≧(a)/(b)>1.0.
Examples of the straight chain polymer (a) having a hydroxyl or amino group at each end and at least one aliphatic unsaturated hydrocarbon group on a side chain include diols such as polyether diols, polyester diols and polycarbonate diols, and modified polysiloxanes such as both end carbinol-modified polysiloxanes and both end amino-modified polysiloxanes. They may be used alone or in admixture of two or more.
Exemplary polyether diols are represented by the following general formula (1).
HO—(R
2
O)
n
—(R
1
O)
x
—(R
3
O)
m
—H (1)
Herein, R
1
is a residue of a dihydric alcohol; R
2
and R
3
are independently a ring-opened residue of a cyclic alkylene oxide; and any one of R
1
, R
2
and R
3
is a group having an aliphatic unsaturated hydrocarbon group on a side chain. Letter x is equal to 0 or 1, each of n and m is an integer of at least 1, preferably 1 to 300, more preferably 5 to 100, and further preferably 10 to 50.
Exemplary polyester diols are represented by the following general formulae (2) and (3).
HO—(R
4
O—COR
5
COO)
k
R
6
OH (2)
HO—(R
8
COO)
i
—R
7
O—(COR
9
O)
j
—H (3)
Herein, R
4
, R
6
and R
7
are independently a residue of a dihydric alcohol; R
5
is independently a residu
Nakamura Tsutomu
Yamazaki Toshio
Yoshida Takeo
Millen White Zelano & Branigan P.C.
Moore Margaret G.
Shin-Etsu Chemical Co. , Ltd.
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