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
1998-09-04
2001-01-23
Moore, Margaret G. (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, C264S211200, C264S211210, C264S211230, C264S211000, C264S210600, C366S091000
Reexamination Certificate
active
06177506
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an efficient method for preparing heat-curable silicone rubber compounds.
2. Prior Art
Heat-cured silicone rubber is improved in weather resistance, durability, heat resistance, physiological activity and colorability. It is used in a variety of applications including building materials, electronic materials, electric materials, business machine parts, automotive parts, and medical parts.
A variety of methods are known for the preparation of heat-curable silicone rubber compounds which heat cure into silicone rubbers. Most customarily, they are prepared using large size, open kneader/mixers having a pair of mixing blades in a chamber.
The kneader/mixers have the advantage that a large amount of mass can be produced in a single pass on account of its maximum capacity reaching several thousands of liters, but the drawback that a long time is needed until the mixture becomes homogeneous, and specifically, the mixing step takes several hours to several tens of hours. The throughput rate at which heat-curable silicone rubber compounds are prepared by means of kneader/mixers can be increased by increasing the size of the apparatus or using a multi-stage apparatus. These modifications, however, are difficult to implement partially because of limits on the installation space.
With respect to the preparation of heat-curable silicone rubber compounds, a number of methods have been investigated for the purpose of improving productivity. For example, JP-A 56736/1989 discloses a continuous production process comprising the step of feeding a diorganopolysiloxane gum to a twin-screw kneader/extruder along with an inorganic filler which has been surface treated with an organic silicon compound under pressure. The drawback of this method is that the pretreatment of the filler takes a long time. JP-A 102007/1990 discloses a continuous production process comprising the steps of continuously pulverizing raw materials in a high-speed shearing machine and feeding them to a continuous twin-screw kneader/extruder. In order to obtain uniform particles, the pulverizing conditions must be strictly controlled. This makes the process complex.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a novel and improved method for the efficient preparation of a heat-curable silicone rubber compound through simple steps within a short time.
We have found that when basic components including an organopolysiloxane gum, a reinforcing silica filler, and a processing aid are first fed to a batchwise closed mixer for mixing them into a uniform mixture, and the mixture is continuously fed into a kneader/extruder for continuously heat treating it, the components can be mixed, homogenized and heat treated within a short time and by simple steps and a heat-curable silicone rubber compound having improved curing properties is prepared.
In well-known methods for preparing silicone rubber compounds, a series of steps from mixing and homogenization of raw ingredients to heat treatment are carried out in a kneader/mixer. Since the kneader/mixer is rather incompetent in mixing and homogenization operation, the present invention carries out mixing and homogenization in a batchwise closed mixer capable of achieving homogenization within a short time, as the first step. In the kneader/extruder of the second step, only heat treatment is continuously carried out. The two steps are assigned to the respective apparatus. This enables brief production of silicone rubber compounds, achieving an increased throughput rate.
The invention provides a method for preparing a heat-curable silicone rubber compound, comprising:
a first step of feeding basic components to a batchwise closed mixer for mixing them into a uniform mixture, said basic components including (A) 100 parts by weight of an organopolysiloxane gum, (B) 5 to 100 parts by weight of a reinforcing silica filler, and (C) 0.1 to 30 parts by weight of a processing aid, and
a second step of continuously feeding the mixture of the first step into a continuous kneader/extruder for continuously heat treating the mixture.
DETAILED DESCRIPTION OF THE INVENTION
The heat-curable silicone rubber compound to be prepared by the method of the invention is a millable compound containing basic components: (A) an organopolysiloxane gum, (B) a reinforcing silica filler, and (C) a processing aid.
The organopolysiloxane gum as component (A) is preferably a linear high viscosity organopolysiloxane of the general formula (1):
R
2
(R
1
2
SiO)
n
SiR
1
2
R
2
(1)
wherein R
1
is a monovalent hydrocarbon group having 1 to 10 carbon atoms, R
2
is selected from methyl, vinyl and hydroxyl groups, and letter n is a number of at least 1,000.
In formula (1), R
1
is selected from substituted or unsubstituted monovalent hydrocarbon groups having 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms. Included are unsubstituted hydrocarbon groups, for example, alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl and dodecyl, alkenyl groups such as vinyl, allyl and butenyl, aryl groups such as phenyl and tolyl, and aralkyl groups such as benzyl and &bgr;-phenylethyl, and substituted hydrocarbon groups in which some or all of the hydrogen atoms in the foregoing groups are replaced by halogen atoms or cyano groups, such as 3,3,3-trifluoropropyl and cyanoethyl. Of these, methyl, vinyl, phenyl and 3,3,3-trifluoropropyl are preferred. R
2
is selected from methyl, vinyl and hydroxyl groups. Letter n, representative of an average degree of polymerization, is an integer of at least 1,000, preferably 3,000 to 20,000.
With the properties of silicone rubber taken into account, it is preferred that at least 50 mol %, especially at least 80 mol % of the entire R
1
and R
2
groups in formula (1) be methyl and 0.025 to 0.5 mol %, especially 0.05 to 0.3 mol % of the entire R
1
and R
2
groups be vinyl.
Component (B) is a reinforcing silica filler which is selected, for example, from fumed silica, fired silica, precipitated silica (or wet silica) and mixtures thereof. Fillers having a specific surface area of at least 50 m
2
/g are desirable. Such fillers may be used alone or in admixture of two or more.
The silica fillers may have been surface treated with suitable agents, for example, linear organopolysiloxanes, cyclic organopolysiloxanes, hexamethyldisilazane and reactive silanes. Since the reinforcing silica filler becomes expensive on account of the surface treatment, such surface treatment is unnecessary unless it is desired to impart special properties. From the standpoints of transparency and reinforcement of the reinforced silicone rubber, fumed silica having a specific surface area of 100 to 400 m
2
/g is desirable. From the standpoints of cost, elasticity and physical properties of the reinforced silicone rubber, reinforcing precipitated silica having a specific surface area of 50 to 800 m
2
/g is especially desirable.
An appropriate amount of the reinforcing silica filler blended is 5 to 100 parts by weight, preferably 10 to 50 parts by weight, per 100 parts by weight of organopolysiloxane (A). Outside this range, less or larger amounts of the filler result in silicone rubber compositions which have poor properties and cure into rubber parts having insufficient mechanical strength (such as tensile strength and tear strength).
Component (C) is a processing aid for improving the dispersibility of the reinforcing silica filler (B) in the silicone rubber and imparting various desirable properties to the silicone rubber. Exemplary are low-molecular weight organic silicon compounds blocked with a hydroxyl group at each end, low-molecular weight organic silicon compounds blocked with an alkoxy group at each end, and silazanes. The processing aid used herein is typically an organic silicon compound represented by the general formula (2):
R
4
O—(R
3
2
SiO)
m
—R
4
(2)
wherein R
3
is a monovalent hydrocarbon group having 1 to 10 carbon atoms, R
4
is methyl, ethyl or h
Hagiwara Yutaka
Igarashi Minoru
Shibata Keiji
Takahashi Masaharu
Millen White Zelano & Branigan P.C.
Moore Margaret G.
Shin-Etsu Chemical Co. , Ltd.
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