Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-10-08
2001-08-14
Moore, Margaret G. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S588000
Reexamination Certificate
active
06274648
ABSTRACT:
This invention relates to a hollow filler-containing silicone rubber composition suitable for use as rubber material in applications where weight reduction is of importance, such as transporting vehicles, business machines, and electric appliances.
BACKGROUND OF THE INVENTION
In general, rubber materials are more shock absorbing than metals and plastics. Of the rubber materials, silicone rubber is expected to find use in a wider variety of fields because its heat resistance, weather resistance and electrical insulating properties are excellent. However, the rubber material mainly absorbs impact energy by way of distortion due to its viscoelasticity and thus requires a thickness in the impact applied direction, which represents a certain weight. Under the current trend of promoting weight reduction in such applications as transporting vehicles, business machines, and electric appliances, there is a desire to have a material which is lightweight and sufficiently elastic and permits a number of molded parts to be manufactured.
One known material is silicone rubber foam. The silicone rubber foam is typically prepared by adding pyrolytic blowing agents. Alternatively, a silicone rubber composition is molded and cured in such a manner that hydrogen gas may evolve upon curing as a by-product whereby a foam is obtained. However, the addition of pyrolytic blowing agents has the problem that decomposition gases are toxic and odorous. Where a platinum catalyst is used as the curing catalyst, undesirably the blowing agent acts to retard curing. The method of utilizing hydrogen gas evolving upon curing suffers from the problems that hydrogen gas is explosive and the uncured composition requires careful handling during storage.
Silicone rubber foam can also be formed by injection molding a silicone rubber composition in a mold whereby the rubber is expanded in the mold. This method is difficult to produce a silicone rubber foam having uniform micro-cells.
SUMMARY OF THE INVENTION
An object of the invention is to provide a hollow filler-containing silicone rubber composition which cures into a silicone rubber having improved shock-absorbing properties and compression properties as well as weather resistance, freeze resistance and heat resistance inherent to silicone rubber.
Another object of the invention is to provide a hollow filler-containing silicone rubber composition which is lightweight, experiences minimal rupture of a hollow filler during molding, and cures into a silicone rubber having improved impact resistance.
In a first aspect, the invention provides a hollow filler-containing silicone rubber composition comprising, in admixture, 100 parts by weight of a curable organopolysiloxane composition and 0.1 to 20 parts by weight of a hollow filler, typically in the form of microballoons, having a mean particle size of up to 200 &mgr;m and an elasticity of at least 10%. This silicone rubber composition cures into a silicone rubber part having improved shock-absorbing properties and compression properties as well as the properties inherent to silicone rubber.
According to the invention, filler microballoons containing gas in the interior, having a specific particle size and an elasticity of at least 10% as measured by a method to be described later, and especially formed of a thermoplastic resin are blended in a curable organopolysiloxane composition. The resulting silicone rubber composition has many advantages that it quickly cures, that when impacted by a small force, it absorbs the impact by way of deformation of microballoons, that when impacted by a large force enough to break microballoons, it absorbs the impact by way of the internal gas, and that it fulfils the desired weight reduction while it does not compromise the weather resistance, freeze resistance and heat resistance inherent to silicone rubber.
In a second aspect, the invention provides a hollow filler-containing silicone rubber composition comprising
(1) 100 parts by weight of an organopolysiloxane having on the average at least two alkenyl radicals in a molecule and an average degree of polymerization of up to 1,200,
(2) 0.1 to 50 parts by weight of an organohydrogenpolysiloxane of the following average compositional formula (2):
R
b
H
c
SiO
(4-b-c)/2
(2)
wherein R is a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 10 carbon atoms, b is a positive number of 0.7 to 2.1, c is a positive number of 0.001 to 1.0, and the sum of b+c is from 0.8 to 3.0, said organohydrogenpolysiloxane having at least two hydrogen atoms each attached to a silicon atom in a molecule,
(3) a catalytic amount of an addition reaction catalyst, and
(4) 0.5 to 10 parts by weight of a hollow filler in the form of microballoons having a mean particle size of 20 to 60 &mgr;m.
This hollow filler-containing silicone rubber composition is fully lightweight and elastic, and can be effectively molded, without substantial rupture of the hollow filler, and cured into a silicone rubber part having improved impact resistance. That is, the composition is a material capable of meeting the demand of weight reduction in such applications as transporting vehicles, business machines, and electric appliances.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the first and second embodiments of the invention, the hollow filler to be blended in the silicone rubber composition acts such that gas-filled cells are created in a cured part to enable impact absorption like sponge rubber. Exemplary hollow fillers include glass balloons, silica balloons, carbon balloons, alumina balloons, zirconia balloons, shirasu (or silicious) balloons, and plastic balloons such as phenolic resin balloons and vinylidene chloride resin balloons.
Preferably, the hollow filler is formed of such a material that the hollow filler itself may have elasticity. Therefore, the preferred hollow filler is in the form of microballoons of a thermoplastic resin, especially microballoons formed of a homopolymer of vinylidene chloride, acrylonitrile, methacrylonitrile, an acrylate, or a methacrylate, or a copolymer of two or more of these monomers. With the thermoplastic resin microballoon filler used, an effective impact absorbing capability is obtainable. This is because the microballoons themselves have elasticity so that even when a weak force is applied, they may deform and thus absorb the impact.
The hollow filler in the form of microballoons has a mean particle size of up to 200 &mgr;m, typically 10 to 200 &mgr;m, preferably 10 to 100 &mgr;m, and more preferably 20 to 60 &mgr;m. The first embodiment of the invention favors the use of relatively large microballoons having a mean particle size in excess of 60 &mgr;m as well. Microballoons having a mean particle size in excess of 200 &mgr;m can be crushed during molding, failing to retain a sufficient impact absorbing capability. Microballoons having a mean particle size of less than 10 &mgr;m would be too small to achieve a sufficient impact absorbing capability. In the second embodiment wherein a composition can be molded, as by injection molding without substantial rupture of the hollow filler, into a cured part having improved impact resistance, it is preferred to use a microballoon filler having a mean particle size of 20 to 60 &mgr;m, more preferably 25 to 50 &mgr;m, and most preferably 30 to 40 &mgr;m. Microballoons having a mean particle size of less than 20 &mgr;m would be too small to achieve a sufficient impact absorbing capability. Microballoons having a mean particle size in excess of 60 &mgr;m can be crushed during injection molding and other molding processes involving application of a high pressure. The mean particle size can be determined as a weight average value (or median diameter) by means of a particle size distribution meter using laser light diffractometry and similar analysis means.
In the first embodiment, the hollow filler should have an elasticity of at least 10% and preferably at least 15%, as measured by the following method. A hollow filler with an elasticity of less than 10% is too rigid t
Ide Shin-ichi
Meguriya Noriyuki
Birch & Stewart Kolasch & Birch, LLP
Moore Margaret G.
Shin-Etsu Chemical Co. , Ltd.
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