Preparation of high fatigue endurance liquid silicone rubber composition

A liquid silicone rubber base including (A) an alkenyl-containing organopolysiloxane, (B) a reinforcing silica filler, (C) a mixing assistant in the form of a silanol-containing compound, (E) an organohydrogen polysiloxane, (f) a platinum catalyst and optionally, (D) a wetting auxiliary catalyst is prepared by previously mixing 10-120 parts of component (A) with 1-35 parts of component (C) and 0-35 parts of component (D) to form a premix liquid, mixing the premix liquid with 100 parts of component (B) to form a premix power, and kneading the premix powder with the remainder of component (A). By adding a curing agent of organohydrogenpolysiloxane/platinum catalyst to the liquid silicone rubber base, there is obtained a liquid silicone rubber composition having improved fatigue endurance.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for preparing a silicone rubber base 
useful as a source of silicone rubber for use in electric, automotive and 
business machine applications and a silicone rubber composition comprising 
the silicone rubber base. More particularly, it relates to a method for 
preparing a high fatigue endurance liquid silicone rubber composition 
within a relatively short time which maintains a stable viscosity during 
storage and cures into an elastomeric product having improved fatigue 
endurance. 
2. Prior Art 
In the current silicone rubber market, attention is paid to liquid silicone 
rubber compositions which are smoothly flowing, readily applicable through 
injection molding machines and automatically processable. Among others, 
many methods have been developed for the preparation of addition curing 
type liquid silicone rubber compositions of the versatile product 
formulation based on alkenyl group-containing organopolysiloxane. These 
methods are designed to comply with a line for the manufacture of various 
types of products. A liquid silicone rubber base is utilized as a common 
intermediate, and a particular silicone rubber composition is prepared by 
blending the base with appropriate curing agents and other ingredients. 
The silicone rubber base is prepared by various methods which are either 
batchwise or continuous although the continuous methods suffer from more 
problems. For example, JP-B 47664/1991 and JP-A 130344/1986 propose to mix 
predetermined amounts of an alkenyl group-containing organopolysiloxane as 
a base component and a powdery silica filler. These methods take a long 
time for the manufacture of a desired composition because an initial 
mixing time of more than 1 hour and a heat treating time of 1 to 30 hours 
above 140.degree. C. are generally required for a batchwise formulation. 
For the manufacture of silicone rubber base using a large scale 
manufacturing installation, about 30 hours is sometimes necessary when 
both the treatment times are combined. On the other hand, the continuous 
methods do not insure a sufficient treatment time because the residence 
time within the kneader is very short. Then the liquid silicone rubber 
base thickens and loses shelf stability. For the cured product (silicone 
rubber) obtained from such a liquid silicone rubber base, there also occur 
losses of basic rubbery properties such as elongation and tensile strength 
and a loss of fatigue endurance due to insufficient dispersion of the 
filler. 
Solutions to this problem have been proposed. JP-A 32909/1994 corresponding 
to EP 568891 A discloses a method for continuously preparing a liquid 
silicone rubber while providing an average residence time of more than 15 
minutes. JP-A 102007/1990 corresponding to U.S. Pat. No. 5,198,171 
discloses a method for continuously preparing a base compound comprising a 
polyorganosiloxane having a higher viscosity (raw rubber) as a main 
component in admixture with a reinforcing silica filler and various mixing 
assistants (i.e., so-called "wetter" or "wetting agent" which can improve 
affinity or wettability and dispersibility between an organopolysiloxane 
and silica fillers). Since simply mixing the components in a continuous 
kneader takes a long time until a uniform mix is obtained, this method 
involves previously uniformly dispersing the polyorganosiloxane and the 
silica filler in a high speed mechanical shearing mixer to form a free 
flowing powder and continuously feeding the powder into a twin screw 
extruder, thereby preparing a silicone compound within a short time. 
The above method uses organosilanes (e.g., diphenylsilane diol, 
dimethylsilane diol, dihydroxypolydimethylsiloxane and 
dimethoxypolydimethylsiloxane) or low viscosity polysiloxanes as the 
mixing assistant. With only these mixing assistants used, it was difficult 
to impart satisfactory flow and good shelf stability to the liquid 
silicone rubber base. 
When hexamethyldisilazane is used instead of the silicone oil, the 
resulting composition is improved in flow, but becomes deteriorated in 
cured properties, loses viscosity stability during storage, and fails to 
provide fatigue endurance. 
SUMMARY OF THE INVENTION 
Therefore, an object of the present invention is to provide a method for 
continuously preparing a liquid silicone rubber composition which method 
enables the preparation of a liquid silicone rubber base within a 
significantly reduced residence time for kneading and the preparation of a 
liquid silicone rubber composition from the base which is capable of 
minimizing a viscosity increase during storage (that is, having improved 
storage stability) and curing into a product having improved fatigue 
endurance. 
The invention pertains to a method for preparing a liquid silicone rubber 
base by continuously mixing (A) an alkenyl group-containing 
organopolysiloxane, (B) a reinforcing silica filler, (C) a mixing 
assistant (i.e., wetting agent), and optionally (D) an auxiliary catalyst 
capable of promoting a wetting effect. We have found that the liquid 
silicone rubber base can be continuously prepared within a significantly 
reduced residence time by previously mixing a part of the alkenyl 
group-containing organopolysiloxane with the mixing assistant and 
optionally, the auxiliary catalyst in a specific proportion to form a 
premix liquid, mixing the premix liquid with the powdery silica filler to 
form a premix powder, and uniformly mixing the premix powder with the 
remainder of the alkenyl group-containing organopolysiloxane in a specific 
proportion in a continuous kneader. Silicone rubber obtained from the base 
has improved physical properties. A liquid silicone rubber composition 
obtained using the base experiences a minimized viscosity increase during 
shelf storage, that is, has improved storage stability and cures into a 
product having improved fatigue endurance. 
In connection with the continuous preparation of a liquid silicone rubber 
base by mixing (A) an organopolysiloxane having at least two alkenyl 
groups each attached to a silicon atom in a molecule and having a 
viscosity of 100 to 300,000 centistokes at 25.degree. C., (B) a 
reinforcing silica filler having a specific surface area of at least 50 
m.sup.2 /g as measured by the BET method, (C) a mixing assistant of the 
following general formula (1) or (2): 
##STR1## 
wherein R.sup.1 is a methyl, trimethylsiloxy, vinyl or trifluoropropyl 
group, and optionally, (D) an auxiliary catalyst capable of promoting a 
wetting effect, 
the present invention provides a method for preparing a high fatigue 
endurance liquid silicone rubber composition comprising the steps of 
previously mixing 10 to 120 parts by weight of component (A) with 1 to 35 
parts by weight of component (C) and 0 to 35 parts by weight of component 
(D) per 100 parts by weight of component (B) to form a premix liquid; 
mixing the premix liquid with 100 parts by weight of component (B) to form 
a premix powder; kneading 100 parts by weight of the premix powder with at 
least 20 parts by weight of component (A) to form a liquid silicone rubber 
base; and thereafter, adding a curing agent to the liquid silicone rubber 
base. The curing agent consists of (E) an organohydrogenpolysiloxane 
having at least two hydrogen atoms each attached to a silicon atom in a 
molecule and (F) a platinum catalyst. 
In one preferred embodiment, the step of kneading the premix powder with 
component (A) is carried out by means of a first single, twin or multiple 
screw continuous kneader having an entire length between an inlet and an 
outlet. An aft region of the kneader extending from the outlet to a 
position of 10 to 80% of the entire length toward the inlet is maintained 
at a temperature of 200 to 350.degree. C. A forward region of the kneader 
extending from the inlet to a position of less than 50% of the entire 
length toward the outlet and not overlapping the aft region is maintained 
at a temperature of not higher than 60.degree. C. Preferably, the premix 
powder and a part of the component (A) to be kneaded therewith are fed 
into the forward region of the kneader, and the remainder of the component 
(A) is fed into the aft region. A vacuum deaerator is preferably disposed 
near the outlet of the kneader for removing low molecular weight materials 
and unreacted materials from the liquid silicone rubber base. More 
preferably, the first kneader at the outlet is connected to a second 
single, twin or multiple screw continuous kneader which is maintained at a 
temperature of 150 to 300.degree. C. for further kneading the silicone 
rubber base.

DETAILED DESCRIPTION OF THE INVENTION 
Component (A), organopolysiloxane is a main component of the liquid 
silicone rubber composition of the invention. The organopolysiloxane 
should have at least two alkenyl groups each attached to a silicon atom in 
a molecule. 
The preferred organopolysiloxane is represented by the following average 
compositional formula (3): 
EQU R.sub.a SiO.sub.(4-a)/2 (3) 
wherein R is a substituted or unsubstituted monovalent hydrocarbon group 
attached to a silicon atom, preferably having 1 to 12 carbon atoms, more 
preferably 1 to 8 carbon atoms. Examples include alkyl groups such as 
methyl, ethyl, propyl, isopropyl, butyl, pentyl, neopentyl, hexyl, heptyl, 
octyl, nonyl, decyl and dodecyl; cycloalkyl groups such as cyclopentyl, 
cyclohexyl, and cycloheptyl; alkenyl groups such as vinyl, allyl, 
propenyl, isopropenyl, butenyl, isobutenyl, hexenyl, and cyclohexenyl; 
aryl groups such as phenyl, tolyl, xylyl, naphthyl, and biphenyl; aralkyl 
groups such as benzyl, phenylethyl, phenylpropyl, and methylbenzyl; and 
substituted ones of these groups wherein some or all of the hydrogen atoms 
are replaced by halogen atoms such as fluorine, chlorine and bromine or 
cyano groups, for example, chloromethyl, 2-bromoethyl, 
3,3,3-trifluoropropyl, 3-chloropropyl, and cyanoethyl. 
In the organopolysiloxane of formula (3), at least two of the organic 
groups represented by R are alkenyl groups. Preferably, the alkenyl group 
accounts for 0.001 to 20 mol %, especially 0.01 to 10 mol % of the entire 
R groups. Outside the range, a smaller proportion of the alkenyl group 
would result in a composition which becomes less curable whereas a larger 
proportion of the alkenyl group would result in a cured product which 
becomes low in physical properties including tensile strength, tear 
strength, elongation, and fatigue endurance. It is noted that the alkenyl 
group may be attached to either a silicon atom at the end of a molecular 
chain or a silicon atom intermediate of a molecular chain or both. 
In formula (3), letter a is a positive number of 1.65 to 2.35, preferably 
1.8 to 2.2, more preferably 1.95 to 2.05. The structure of the 
organopolysiloxane is often preferred to be linear and have an alkenyl 
group at the end of a molecular chain although a branched structure may be 
partially contained. More preferably, the organopolysiloxane is blocked at 
the end of its molecular chain with a triorganosilyl group such as 
trivinylsilyl, methyldivinylsilyl, dimethylvinylsilyl, and trimethylsilyl. 
An appropriate molecular weight may be selected for the organopolysiloxane. 
In most cases, the organopolysiloxane should have a viscosity of 100 to 
300,000 centistokes at 25.degree. C., preferably 1,000 to 100,000 
centistokes at 25.degree. C. because it must cure into a rubbery elastomer 
and form a liquid silicone rubber composition. 
Component (B) is a reinforcing silica filler having a specific surface area 
of at least 50 m.sup.2 /g as measured by the BET method. Useful silica 
fillers include fumed silica, fired silica, and precipitated silica alone 
or in admixture of two or more. The silica filler may also be surface 
treated with suitable agents such as linear organopolysiloxanes and 
hexamethyldisilazane. 
The silica filler (B) should have a specific surface area of at least 50 
m.sup.2 /g as measured by the BET method. Particularly for providing 
transparency and reinforcement to the silicone rubber composition, fumed 
silica having a specific surface area of 50 to 600 m.sup.2 /g, especially 
100 to 400 m.sup.2 /g is desirable. Also, in view of the cost and physical 
properties such as elasticity of the silicone rubber composition, 
reinforcing precipitated silica having a specific surface area of 50 to 
600 m.sup.2 /g, especially 100 to 400 m.sup.2 /g is desirable. 
In preparing a premix powder to be described later (in the second step), 
component (A) is preferably blended in an amount of 10 to 120 parts, more 
preferably 40 to 80 parts by weight of the organopolysiloxane (A) per 100 
parts by weight of the silica filler (B). If the amount of component (A) 
is more or less than this range, the resulting silicone rubber composition 
becomes difficult to mold and cures into a product having rather poor 
mechanical strength such as tensile strength and tear strength. 
Component (C) as a mixing assistant (i.e., a wetting agent for components 
(A) and (B)) is a silanol group-containing compound of the following 
general formula (1) or (2). In the manufacture of a premix liquid which is 
carried out prior to the manufacture of a premix powder, component (C) is 
blended with component (A) and optional component (D) when they are 
previously mixed to form a premix liquid (i.e., before component (A) is 
mixed with component (B)). Component (C) blended at this stage is 
effective for improving the fatigue endurance, flow and viscosity 
stability during storage of the silicone rubber composition. 
##STR2## 
R.sup.1 is methyl, trimethylsiloxy, vinyl or trifluoropropyl. 
As described in JP-A 228782/1995 corresponding to U.S. Pat. No. 5,597,853, 
the compound of formula (1) can be prepared by well-known methods, for 
example, by hydrolyzing 1,1,1,3,5,7,7,7-octamethyltetrasiloxane or 
3,5-bis(trimethylsiloxy)-1,1,1,7,7,7-hexamethyltetrasiloxane in the 
presence of a Pd/C catalyst. 
Also, the compound of formula (2) can be prepared by well-known methods, 
for example, by hydrolyzing 1,1,1,3,5,5,5-heptamethyltrisiloxane or 
3-(trimethylsiloxy)-1,1,1,5,5,5-hexamethyltrisiloxane in the presence of a 
Pd/C catalyst. 
The compounds of formulae (1) and (2) may be used alone or in admixture of 
two or more. The amount of the compound (C) blended is 1 to 35 parts, 
preferably 1 to 20 parts, more preferably 2 to 10 parts by weight per 100 
parts by weight of the silica filler (B). With less than 1 part of the 
mixing assistant (C), the uniformity of the premix powder and the 
compatibility between the organopolysiloxane and the powder silica filler 
become insufficient, and the composition finally obtained is not improved 
in fatigue endurance and viscosity stability during storage because the 
high activity of silica is not suppressed. With more than 35 parts of the 
mixing assistant (C), no further improvement by the mixing assistant is 
expected and the step of positively removing an excess of the mixing 
assistant becomes necessary, which increases the cost and time of the 
manufacturing process, resulting in an economical disadvantage. 
Additionally, the mechanical properties of a cured product are adversely 
affected and its fatigue endurance is reduced. 
Component (D), which is optional, is an auxiliary catalyst. The auxiliary 
catalyst is effective not only for facilitating the feed to a continuous 
kneader of a premix powder or intermediate obtained by mixing components 
(A), (B), and (C), but also for promoting reaction of silanol groups on 
the silica filler surface of component (B) and silanol groups of component 
(C). Examples of the auxiliary catalyst include ammonia, aqueous ammonia, 
ammonium salts such as tetrabutylammonium hydroxide, phosphorus-siliconate 
salts as shown below, potassium-siliconate salts as shown below, tin 
compounds such as tin octylate and dibutyltin dilaurate, titanium compound 
such as tetrabutyl titanate, and zinc compounds such as zinc octylate and 
zinc naphthylate. Ammonia and aqueous ammonia are preferred among others. 
Phosphorus-siliconate salts 
##STR3## 
d: an integer of 3 to 100 X: H or (C.sub.4 H.sub.9).sub.4 P-- 
Potassium-siliconate salts 
##STR4## 
e: an integer of 3 to 100 Y: K or H 
The amount of component (D) blended is 0 to 35 parts, preferably 1 to 10 
parts by weight per 100 parts by weight of component (B) and 0 to 1.0 
part, preferably 0.1 to 0.8 parts by weight per part by weight of 
component (C). More than 1.0 part of component (D) per part by weight of 
component (C) would adversely affect curing and mechanical properties. 
The mixing assistant (i.e., wetting agent) as component (C) and optional 
auxiliary catalyst as component (D) are added not only for making the 
organopolysiloxane (A) and the powdery silica filler (B) miscible to 
facilitate the formation of a premix powder, but also for facilitating the 
subsequent step of kneading the premix powder with the organopolysiloxane. 
They react with silanol groups on the surface of the powdery silica filler 
(B) to promote the dispersion of the silica filler, thereby achieving a 
silicone rubber composition having improved fatigue endurance, a minimized 
viscosity increase during storage, enough storage stability, and good 
flow. 
Next, the method for preparing a liquid silicone rubber composition is 
described. 
First, a liquid silicone rubber base is prepared by continuously mixing (A) 
an alkenyl group-containing organopolysiloxane having a viscosity of 100 
to 300,000 centistokes at 25.degree. C., (B) a powdery silica filler, (C) 
a mixing assistant, and optionally, (D) an auxiliary catalyst. According 
to the invention, the liquid silicone rubber base is prepared by 
the first step of previously mixing 10 to 120 parts, preferably 40 to 80 
parts by weight of alkenyl group-containing organopolysiloxane (A) with 1 
to 35 parts, preferably 2 to 20 parts by weight of mixing assistant (C) 
and 0 to 35 parts, preferably 1 to 10 parts by weight of auxiliary 
catalyst (D) per 100 parts by weight of powdery silica filler (B) to form 
a premix liquid, 
the second step of mixing the premix liquid (i.e., the mixture of 
components (A), (C) and optionally (D) described in the first step) with 
100 parts by weight of powdery silica filler (B) to form a premix powder, 
and 
the third step of kneading 100 parts by weight of the premix powder 
described in the second step with at least 20 parts by weight of alkenyl 
group-containing organopolysiloxane (A). 
The mixing assistant (C) is not directly mixed with the powdery silica 
filler (B), but previously mixed in the first step with a part of the 
alkenyl group-containing organopolysiloxane (A) and optionally, the 
auxiliary catalyst (D) to form a uniform premix liquid before the premix 
liquid is added to the powdery silica filler (B) to form a premix powder 
in the second step. The first step of forming the premix liquid does not 
require a complex operation since both component (A) and component (C) are 
liquid. The components can be mixed into a fully uniform premix by simple 
mixing techniques using a simple stirring rod or hand drill in a mixing 
tank. When continuous mixing is desired, a suitable mixer, for example, a 
static mixer may be incorporated in piping. 
In the second step of forming the premix powder, 100 parts by weight of 
powdery silica filler (B) is mixed with the premix liquid, preferably 11 
to 140 parts, especially 40 to 100 parts by weight of the premix liquid 
consisting of the organopolysiloxane (A), mixing assistant (C) and 
optionally, auxiliary catalyst (D). If the amount of the premix liquid 
exceeds 140 parts, the mixture resulting from the mixing of the premix 
liquid and the powdery silica filler would become liquid or bulk rather 
than powder. Less than 11 parts of the premix liquid would be too small to 
achieve effective kneading of the mixture with the remainder of the 
alkenyl group-containing organopolysiloxane (A) in the third step. A 
continuous mixer is suitable for forming the premix powder. Such a 
continuous mixer may be selected from vertical and horizontal, high speed 
(.gtoreq.1,000 rpm), medium speed, and low speed (50 to 100 rpm) rotating 
mixers. As long as the selection of the components and the amounts thereof 
is appropriate as mentioned above, the premix powder can be readily 
obtained. The residence time during mixing is usually 10 seconds to 5 
minutes. It is possible that the second step be directly followed by the 
third step. 
The third step is to knead 100 parts by weight of the premix powder with at 
least 20 parts, preferably 30 to 200 parts, more preferably 40 to 150 
parts by weight of the alkenyl group-containing organopolysiloxane (A) in 
a continuous kneader until uniform, thereby forming the liquid silicone 
rubber base. The amount of the alkenyl group-containing organopolysiloxane 
(A) added in the third step varies depending on the desired viscosity of 
the liquid silicone rubber base, the desired rubber strength of a final 
product, etc. The alkenyl group-containing organopolysiloxane (A) which is 
the main component may be fed to the continuous kneader in various ways, 
for example, by adding the entirety or by adding divided portions at 
forward and aft stages. The addition of divided portions is preferred for 
increasing the rubber strength of a final product. It is more preferred to 
add a less amount at the forward stage. 
The continuous kneader may be a single, twin or multiple screw continuous 
kneader which can be heated, with the twin screw continuous kneader being 
especially preferred. Ceramic stone mills and disc type shearing mixers 
having stones mills axially mounted in multiple stages (e.g., KCK 
continuous kneader by KCK K.K.) are also useful as the continuous kneader. 
The preferred continuous kneader is of the following construction. The 
continuous kneader defines an interior having an entire length between an 
inlet and an outlet. An aft region of the kneader interior that extends 
from the outlet (i.e., the position of 0%) to a position of 10 to 80%, 
more preferably 50 to 80% of the entire length toward the inlet is 
maintained at a relatively high temperature of 200 to 350.degree. C., more 
preferably 250 to 300.degree. C. In this sense, the aft region may also be 
referred to as a heated region. With temperatures below 200.degree. C., 
the liquid silicone rubber base would experience a viscosity rise during 
shelf storage, that is, lose storage stability. With temperatures above 
350.degree. C., the main chain of the alkenyl group-containing 
organopolysiloxane would be scissored so that a final product based on the 
liquid silicone rubber base might lose rubbery physical properties. 
Removal means, for example, a vacuum deaerator may be disposed near the 
outlet of the kneader for removing unreacted low molecular weight 
materials contained in the alkenyl group-containing organopolysiloxane 
and/or oligomeric low molecular weight materials, unreacted mixing 
assistant, and auxiliary catalyst from the liquid silicone rubber base. 
Furthermore, a forward region of the kneader interior that extends from the 
inlet (i.e., the position of 0%) to a position of less than 50% of the 
entire length toward the outlet and not overlapping the aft region is 
maintained at a temperature of not higher than 60.degree. C. If the 
continuous kneader is entirely heated above 60.degree. C., the premix 
powder upon entry into the kneader would not be smoothly taken into the 
kneader inlet, but kept away therefrom, resulting in the liquid silicone 
rubber base having a non-uniform composition. In the initial stage of 
kneading where the premix powder and the alkenyl group-containing 
organopolysiloxane are contacted, intermixed and merged with each other, 
the temperature is preferably set below 60.degree. C., preferably 5 to 
60.degree. C., most often near room temperature (e.g., 10 to 35.degree. 
C). 
In the kneader whose temperature is set to the abovementioned distribution, 
the premix powder is fed and the alkenyl group-containing 
organopolysiloxane is added and kneaded therewith. In one way, the entire 
amount of the alkenyl group-containing organopolysiloxane to be added may 
be introduced into the forward region of the kneader which is maintained 
below 60.degree. C., preferably near room temperature. In another way, 
which is preferred, the entire amount of the organopolysiloxane to be 
added is divided into two portions which are added at the forward and aft 
stages. A first portion corresponding to 1/4 to 3/4 of the entire amount 
of organopolysiloxane is introduced into the forward region of the kneader 
and the remainder is introduced into the heated region. 
An appropriate kneading time is generally 30 seconds to 10 minutes, 
especially 1 to 5 minutes although the kneading time may be properly 
determined without undue experimentation. 
In a further preferred embodiment, the kneader (designated first kneader) 
at the outlet is connected to a second single, twin or multiple screw 
continuous kneader for further kneading the silicone rubber base which has 
been kneaded in the first kneader. The second kneader is maintained at a 
temperature of 150 to 300.degree. C. An appropriate kneading time in this 
stage is generally 3 to 30 minutes. 
Referring to FIGS. 1 and 2, there are illustrated exemplary systems for 
preparing a silicone rubber base. The system includes a liquid mixing tank 
1 for forming a premix liquid. To the mixing tank 1 are connected a line 2 
for supplying the alkenyl group-containing organopolysiloxane as the main 
component, a line 3 for continuously supplying the mixing assistant, and a 
line 4 for supplying the auxiliary catalyst. A blade agitator 5 is 
disposed in the tank 1. A line 6 is connected between the mixing tank 1 
and a horizontal mixer 7 for discharging the premix liquid from the tank 1 
to the mixer 7. The horizontal mixer 7 is equipped with a blade agitator 8 
like the liquid mixing tank 1. A line 9 is connected to the mixer 7 for 
supplying the powdery silica filler. In the mixer 7, the premix liquid is 
mixed with the powdery silica filler to form a premix powder. The mixer 7 
is connected through a line 11 to a powder constant delivery meter 10 for 
metering the premix powder. The premix powder is metered and fed to a twin 
screw continuous kneader 12 through a hopper 14. Also connected to the 
hopper 14 is a line 13 for supplying the alkenyl group-containing 
organopolysiloxane from a main component tank 21. The kneader 12 also has 
an outlet 15 for delivering the liquid silicone rubber base kneaded 
therein. A line 16 for supplying the alkenyl group-containing 
organopolysiloxane from the main component tank 21 is connected to the 
kneader 12 near its outlet 15. A vent 17 for removing low molecular weight 
materials is disposed near the outlet 15 of the kneader 12 and connected 
to a vacuum pump 18 via a condenser 19 for effecting vacuum deaeration. 
In the embodiment of FIG. 2, a second continuous kneader 20 is serially 
connected to the first continuous kneader 12. 
According to the invention, a curing agent is added to the liquid silicone 
rubber base thus obtained, thereby forming a high fatigue endurance liquid 
silicone rubber composition. 
The curing agent used herein consists of (E) an organohydrogenpolysiloxane 
having at least two hydrogen atoms each attached to a silicon atom in a 
molecule and (F) a platinum catalyst. The organohydrogenpolysiloxane (E) 
and the platinum catalyst (F) may be known ones commonly used in the 
preparation of conventional addition reaction curing type liquid silicone 
rubber compositions. 
The organohydrogenpolysiloxane as component (E) serves as a crosslinking 
agent for the silicone rubber composition. More particularly, the hydrogen 
atom attached to a silicon atom in component (E) undergoes addition 
reaction to the alkenyl group attached to a silicon atom in component (A) 
in the presence of the platinum catalyst (F), to effect crosslinking, 
thereby curing the silicone rubber composition. The 
organohydrogenpolysiloxane (E) should have at least two hydrogen atoms 
each attached to a silicon atom (that is, SiH groups) in a molecule. 
Preferably the organohydrogenpolysiloxane has 2 to 200 SiH groups, 
especially 3 to 50 SiH groups. 
The preferred organohydrogenpolysiloxane is represented by the following 
general compositional formula (4). 
EQU R'.sub.b H.sub.c SiO.sub.(4-b-c)/2 (4) 
Herein, R' is a substituted or unsubstituted monovalent hydrocarbon group 
having 1 to 12 carbon atoms, preferably 1 to 8 carbon atoms, for example, 
those groups exemplified for R, preferably such groups free of an 
aliphatic unsaturated bond. Especially preferred are alkyl, aryl, aralkyl 
and substituted alkyl groups. Letters b and c are positive numbers 
satisfying 0.8.gtoreq.b.gtoreq.2.2, 0.002.gtoreq.c.gtoreq.1.0, and 
1.0&lt;b+c.gtoreq.3.0. The molecular structure of the 
organohydrogenpolysiloxane (E) may be linear, cyclic, branched or network 
while the SiH group may be positioned at the end or intermediate the 
molecular chain. Although no particular limit is imposed on the molecular 
weight of component (E), it preferably has such a molecular weight as to 
give a viscosity in the range of 1 to 1,000 centipoise at 25.degree. C., 
more preferably 3 to 500 centipoise at 25.degree. C. 
The organohydrogenpolysiloxane (E) is blended in such an amount that the 
ratio of the number of hydrogen atoms each attached to a silicon atom 
(that is, SiH groups) in component (E) to the number of alkenyl groups 
each attached to a silicon atom in component (A) may range from 0.5:1 to 
20:1, especially from 1:1 to 3:1. If this ratio of SiH group to alkenyl 
group is too low, full curing is not always expectable. Too high a ratio 
has the risk of foaming. 
The platinum group catalyst (F) is to catalyze the curing of the silicone 
rubber composition. Such platinum group catalysts include finely divided 
metallic platinum catalysts as disclosed in U.S. Pat. No. 2,970,150, 
chloroplatinic acid catalysts as disclosed in U.S. Pat. No. 2,823,218, 
platinum-hydrocarbon complexes as disclosed in U.S. Pat. Nos. 3,159,601 
and 3,156,946, chloroplatinic acid-olefin compounds as disclosed in U.S. 
Pat. No. 3,516,946, and platinum-vinylsiloxane complexes as disclosed in 
U.S. Pat. Nos. 3,775,452 and 3,814,780. The catalyst (F) is preferably 
added in a sufficient amount to give 0.1 to 1,000 ppm, especially 1 to 100 
ppm of metallic platinum based on the weight of components (A) and (E) 
combined. Less than 0.1 ppm of the catalyst would be ineffective for the 
composition to cure whereas more than 1,000 ppm of the catalyst would add 
to the cost. 
The silicone rubber composition of the invention is a liquid composition 
having fluidity. Although no particular limit is imposed on the viscosity 
of the composition, an appropriate viscosity is 50 to 50,000 poise at 
25.degree. C., especially 200 to 20,000 poise at 25.degree. C. The 
silicone rubber composition may optionally contain various additives 
commonly used in compositions of this type, for example, reaction 
controlling agents, pigments, flame retardants, and mold release agents 
insofar as the benefits of the invention are not lost. Of these, liquid 
additives may be added during the preparation of the premix liquid or when 
the premix powder is kneaded with component (A), and powder additives may 
be added upon mixing of component (B). Alternatively, the additives may be 
added to and mixed with the silicone rubber base. 
The method for preparing a liquid silicone rubber composition according to 
the present invention permits a reduction in the time required for the 
preparation of a liquid silicone rubber base by significantly reducing the 
residence time and permits preparation from the liquid silicon rubber base 
of a liquid silicone rubber composition having improved viscosity 
stability during storage and curing into a product having improved fatigue 
endurance. 
EXAMPLE 
Examples of the invention are given below by way of illustration and not by 
way of limitation. In the Examples, parts are by weight and the viscosity 
is a measurement at 25.degree. C. In preparing a silicone rubber base 
according to the invention, a system as shown in FIGS. 1 or 2 was used. 
Example 1 
The system shown in FIG. 1 was used. Using a mixing tank equipped with an 
agitator blade, 45 parts of a linear dimethylpolysiloxane blocked with a 
dimethylvinylsilyl group at each end of its molecular chain and having a 
viscosity of 10,000 centistokes as a main component was mixed with 3 parts 
of 1,1,1,3,5,7,7,7-octamethyl-3,5-dihydroxytetrasiloxane to form a premix 
liquid. The premix liquid was mixed with 55 parts of silica powder (Nipsil 
LP by Nippon Silica K.K.) in a horizontal continuous mixer (Blow Shear 
Mixer) for a residence time of 1 minute to form a premix powder. The 
premix powder was then fed to a twin screw continuous kneader through a 
constant delivery meter. At this point, the linear dimethylpolysiloxane as 
the main component was fed to the kneader in divided portions. That is, 37 
parts was added to the premix powder at the inlet and 79 parts was added 
near the outlet of the kneader. The total amount of the main component 
added at the stage of the continuous kneader was 116 parts. The interior 
of the continuous kneader had the following temperature distribution. The 
forward region extending from the inlet (0%) to 30% of the entire length 
was maintained at a temperature of 10 to 50.degree. C. and the aft region 
extending from the outlet (0%) to 55% of the entire length was maintained 
at a temperature of 280.degree. C. A vent was disposed near the kneader 
outlet for removing low molecular weight materials and connected to a 
vacuum pump which was operated to effect vacuum deaeration. The residence 
time in the continuous kneader was within 90 seconds. 
Example 2 
The system shown in FIG. 2 was used. Using a mixing tank equipped with an 
agitator blade, 45 parts of a linear dimethylpolysiloxane blocked with a 
dimethylvinylsilyl group at each end of its molecular chain and having a 
viscosity of 10,000 centistokes as a main component was mixed with 3 parts 
of 1,1,1,3,5,7,7,7-octamethyl-3,5-dihydroxytetrasiloxane to form a premix 
liquid. The premix liquid was mixed with 55 parts of silica powder (Nipsil 
LP by Nippon Silica K.K.) in a horizontal continuous mixer (Blow Shear 
Mixer) for a residence time of 1 minute to form a premix powder. The 
premix powder was then fed to a first twin screw continuous kneader 
through a constant delivery meter. At this point, the linear 
dimethylpolysiloxane as the main component was fed to the kneader in 
divided portions. That is, 37 parts was added to the premix powder at the 
inlet and 79 parts was added near the outlet of the kneader. The total 
amount of the main component added at the stage of the continuous kneader 
was 116 parts. The interior of the continuous kneader had the following 
temperature distribution. The forward region extending from the inlet (0%) 
to 30% of the entire length was maintained at a temperature of 10 to 
50.degree. C. and the aft region extending from the outlet (0%) to 55% of 
the entire length was maintained at a temperature of 280.degree. C. A vent 
was disposed near the kneader outlet for removing low molecular weight 
materials and connected to a vacuum pump which was operated to effect 
vacuum deaeration. The residence time in the continuous kneader was within 
90 seconds. The resulting compound was further heat kneaded in a second 
continuous kneader serially connected to the first kneader. The second 
kneader was maintained at 250.degree. C. over its entire region and the 
residence time was 13 minutes. 
Example 3 
The system shown in FIG. 2 was used. Using a mixing tank equipped with an 
agitator blade, 45 parts of a linear dimethylpolysiloxane blocked with a 
dimethylvinylsilyl group at each end of its molecular chain and having a 
viscosity of 10,000 centistokes as a main component was mixed with 3 parts 
of 1,1,1,3,5,7,7,7-octamethyl-3,5-dihydroxytetrasiloxane to form a premix 
liquid. The premix liquid was mixed with 55 parts of silica powder (Nipsil 
LP by Nippon Silica K.K.) in a horizontal continuous mixer (Blow Shear 
Mixer) for a residence time of 1 minute to form a premix powder. The 
premix powder was then fed to a first twin screw continuous kneader 
through a constant delivery meter. At this point, the linear 
dimethylpolysiloxane as the main component was fed to the kneader in 
divided portions. That is, 37 parts was added to the premix powder at the 
inlet and 79 parts was added near the outlet of the kneader. The total 
amount of the main component added at the stage of the continuous kneader 
was 116 parts. The interior of the continuous kneader had the following 
temperature distribution. The forward region extending from the inlet (0%) 
to 30% of the entire length was maintained at a temperature of 10 to 
50.degree. C. and the aft region extending from the outlet (0%) to 55% of 
the entire length was maintained at a temperature of 200.degree. C. A vent 
was disposed near the kneader outlet for removing low molecular weight 
materials and connected to a vacuum pump which was operated to effect 
vacuum deaeration. The residence time in the continuous kneader was within 
90 seconds. The resulting compound was further heat kneaded in a second 
continuous kneader serially connected to the first kneader. The second 
kneader was maintained at 150.degree. C. over its entire region and the 
residence time was 13 minutes. 
Comparative Example 1 
The system shown in FIG. 1 was used. Using a mixing tank equipped with an 
agitator blade, 45 parts of a linear dimethylpolysiloxane blocked with a 
dimethylvinylsilyl group at each end of its molecular chain and having a 
viscosity of 10,000 centistokes as a main component was mixed with 3 parts 
of hexamethyldisilazane to form a premix liquid. The premix was mixed with 
55 parts of silica powder (Nipsil LP by Nippon Silica K.K.) in a 
horizontal continuous mixer (Blow Shear Mixer) for a residence time of 1 
minute to form a premix powder. The premix powder was then fed to a twin 
screw continuous kneader through a constant delivery meter. At this point, 
the linear dimethylpolysiloxane as the main component was fed to the 
kneader in divided portions. That is, 37 parts was added to the premix 
powder at the inlet and 79 parts was added near the outlet of the kneader. 
The total amount of the main component added at the stage of the 
continuous kneader was 116 parts. The interior of the continuous kneader 
had the following temperature distribution. The forward region extending 
from the inlet (0%) to 30% of the entire length was maintained at a 
temperature of 10 to 50.degree. C. and the aft region extending from the 
outlet (0%) to 55% of the entire length was maintained at a temperature of 
280.degree. C. A vent was disposed near the kneader outlet for removing 
low molecular weight materials and connected to a vacuum pump which was 
operated to effect vacuum deaeration. The residence time in the continuous 
kneader was within 90 seconds. 
Comparative Example 2 
The system shown in FIG. 2 was used. Using a mixing tank equipped with an 
agitator blade, 45 parts of a linear dimethylpolysiloxane blocked with a 
dimethylvinylsilyl group at each end of its molecular chain and having a 
viscosity of 10,000 centistokes as a main component was mixed with 3 parts 
of hexamethyldisilazane to form a premix liquid. The premix was mixed with 
55 parts of silica powder (Nipsil LP by Nippon Silica K.K.) in a 
horizontal continuous mixer (Blow Shear Mixer) for a residence time of 1 
minute to form a premix powder. The premix powder was then fed to a first 
twin screw continuous kneader through a constant delivery meter. At this 
point, the linear dimethylpolysiloxane as the main component was fed to 
the kneader in divided portions. That is, 37 parts was added to the premix 
powder at the inlet and 79 parts was added near the outlet of the kneader. 
The total amount of the main component added at the stage of the 
continuous kneader was 116 parts. The interior of the continuous kneader 
had the following temperature distribution. The forward region extending 
from the inlet (0%) to 30% of the entire length was maintained at a 
temperature of 10 to 50.degree. C. and the aft region extending from the 
outlet (0%) to 55% of the entire length was maintained at a temperature of 
280.degree. C. A vent was disposed near the kneader outlet for removing 
low molecular weight materials and connected to a vacuum pump which was 
operated to effect vacuum deaeration. The residence time in the continuous 
kneader was within 90 seconds. The resulting compound was further heat 
kneaded in a second continuous kneader serially connected to the first 
kneader. The second kneader was maintained at 250.degree. C. over its 
entire region and the residence time was 13 minutes. 
Tests on Liquid Silicone Rubber Base 
The liquid silicone rubber bases obtained in Examples 1-3 and Comparative 
Examples 1-2 were examined by the following tests. 
Using B type rotating viscometer by Tokyo Keiki K.K., a liquid silicone 
rubber base was measured for viscosity before and after heating at 
105.degree. C. for 6 hours. 
For the measurement of the physical properties and fatigue endurance of a 
liquid silicone rubber base, a test sheet was prepared by mixing 100 parts 
of the liquid silicone rubber base with 25 parts of the linear 
dimethylpolysiloxane, 3 parts of methylhydrogenpolysiloxane of the formula 
shown below as a crosslinking agent, 0.3 part of a 1% isopropyl alcohol 
solution of chloroplatinic acid as a platinum catalyst, and 0.3 part of 
ethynyl cyclohexanol as a reaction inhibitor, forming the mixture into a 
sheet, and curing at 120.degree. C. for 10 minutes. 
##STR5## 
Physical properties were measured according to JIS K-6301. Constant 
elongation fatigue was determined by punching a No. 3 dumbbell specimen 
out of the sheet, setting the specimen on a de Mattia machine (Toyo Seiki 
K.K.) according to JIS K-6301, subjecting the specimen to reciprocal 
motion of 100% elongation (repetitively elongated between 0% and 100%) at 
a rate of 300 cycles/min. until the specimen was ruptured. 
The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Base compound Hard- Tensile Elonga- 
Fatigue 
viscosity (poise) 
ness strength tion endurance 
Initial 105.degree. C./6 hr. 
(JIS-A) (kgf/cm.sup.2) 
(%) (.times.10.sup.4 
______________________________________ 
cycles) 
E1 800 1400 40 80 370 1200 
E2 800 900 39 80 400 1350 
E3 800 2000 40 70 280 1100 
CE1 790 1600 40 65 260 700 
CE2 850 2600 41 61 260 650 
______________________________________ 
It is evident that the method of the invention is successful in briefly 
preparing a liquid silicone rubber base which is improved in viscosity 
stability during storage and producing a liquid silicone rubber 
composition which is significantly improved in fatigue endurance. 
Although some preferred embodiments have been described, many modifications 
and variations may be made thereto in the light of the above teachings. It 
is therefore to be understood that within the scope of the appended 
claims, the invention may be practiced otherwise than as specifically 
described.