Method for the preparation of one-package room-temperature-curable silicone elastomer compositions

One-package room-temperature-curable silicone elastomer compositions that do not slump prior to their cure, that provide a suitable processing or working time, that do not crack or fissure during their cure even when deformed by an external force, and that do not yellow during storage or after curing are prepared by mixing (A) the reaction mixture of PA0 (a) hydroxyl-terminated diorganopolysiloxane and PA0 (b) alkyl-containing oximosilane PA0 or optionally (b) alone with (B) the reaction mixture of PA0 (a) hydroxy-terminated diorganopolysiloxane and PA0 (c) vinyl-functional oximosilane and by thereafter mixing in (C) inorganic filler.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for preparing one- package 
room-temperature-curable (hereinafter referred to as OP/RTC) silicone 
elastomer compositions. More specifically, this invention relates to a 
method for preparing OP/RTC silicone elastomer compositions that exhibit 
an excellent processability in that they do not slump prior to cure and 
provide a suitable processing or working time, that do not crack or 
fissure during their cure even when deformed by an external force, and 
that do not yellow during storage or after curing even when subjected to 
thermal episodes. 
2. Background Information 
Description of the Prior Art and Problems to Be Solved 
OP/RTC silicone elastomer compositions are widely used as sealants, 
coatings, and adhesives in various sectors, such as the construction and 
civil engineering sectors, general manufacturing, and electronic and 
electrical sectors. In particular, the so-called oxime-eliminating 
room-temperature- curable silicone elastomer compositions, which produce a 
ketoxime by-product at the time of curing, have entered into broad use by 
virtue of their low corrosion of the contacted substrate and their 
excellent storage stability. The use of these OP/RTC silicone elastomer 
compositions involves their storage in a sealed container, such as a tube 
or cartridge; then, at the actual point of application, extrusion of the 
silicone elastomer composition as a paste; and thereafter finishing the 
surface to smoothness using, for example, a spatula. Thus, the surface of 
the composition must not cure for the particular period of time elapsing 
from extrusion into the atmosphere until finishing. Nor must the 
composition flow downward when filled into a vertical or downward-slanting 
position, i.e., it must be slump-free. Moreover, even when the surface has 
begun to cure, additional time is required for the curing region to 
develop adequate mechanical strength, and deformation of the composition 
by outside forces is problematic during the time interval extending from 
cure initiation at the surface until the development of mechanical 
strength. In specific terms, when subjected to a stretching or 
elongational deformation, the curing region will rupture due to its 
inadequate mechanical strength. This occurrence of rupture in one location 
can lead to fracture of the entire body after its cure due to stress 
concentration at the said rupture site. 
The occurrence of rupture during the course of curing can be prevented by 
increasing the cure rate of subject silicone elastomer compositions, but 
simply increasing the cure rate functions to shorten the working time 
available for spatula finishing. The use of this approach is also 
associated with a ready tendency for the silicone elastomer composition to 
yellow during storage. The development is therefore desired of a OP/RTC 
silicone elastomer composition that will provide an acceptable working 
time but which will rapidly develop mechanical strength once curing has is 
started. 
Japanese Patent Application Laid Open (Kokai or Unexamined) Number Hei 
4-53902 (1992) proposes a composition in which the crosslinker consists in 
part of a compound having four oxime groups bonded to one silicon atom. 
This composition exhibits appropriate rates of surface cure and mechanical 
strength development. However, the tetraoximosilanes are associated with 
an explosion risk, etc., when the organic solvent used in their synthesis 
is removed, but avoiding this forces the production of a composition that 
contains organic solvent. Therefore, the use of trioximosilanes and the 
omission of tetraoximosilanes, as in the examples of the prior art 
provided below, is preferred for economic and safety reasons. 
Sattlegger et al in U.S. Pat. No. 4,419,484, issued Dec. 6, 1983, 
equivalent to Japanese Patent Application Laid Open (Kokai or Unexamined) 
Number Sho 57-149355 (1982) provides by way of example a method for the 
preparation of compositions by admixing the filler after trioximosilane 
and hydroxyl- terminated diorganopolysiloxane have already been mixed. 
Japanese Patent Application Laid Open Kokai or Unexamined! Number Hei 
241361 41,361/1990! discloses that this method can produce 
slumping-inhibited room-temperature-curable polyorganosiloxane 
compositions. This preparative method, however, suffers from a number of 
other problems. Thus, it yields compositions that are prone to exhibit an 
extremely rapid surface cure rate and that require a long period of time 
for the skinned film to develop mechanical strength. 
Improvements to Japanese Number Hei 2-41361 are proposed by Arai et al in 
U.S. Pat. No. 5,266,631, issued Nov. 30, 1993 claiming priority for both 
Japanese Patent Application Laid Open (Kokai or Unexamined) Numbers Hei 
4-366171(1992) and Hei 5-105813 (1993). Methods are proposed therein that 
provide slump inhibition, a suitably controlled surface cure rate, and a 
suitably adjusted time to mechanical strength development by the skinned 
film. This is achieved by using a moisture-depleted filler or by bringing 
the quantity of crosslinker added prior to filler addition into an 
appropriate range. However, these methods still do not yield an acceptable 
problem resolution. Thus, when the yellowing-resistant and economically 
advantageous methyltrioximosilanes are used as crosslinker, as shown later 
in the working examples the time to mechanical strength development by the 
skinned film becomes too long and surface cracking cannot be prevented 
under difficult curing conditions. As described in the working examples of 
these proposals, these drawbacks can be solved by changing the crosslinker 
over to the highly active vinyltrioximosilanes or by their combined use 
with the methyltrioximosilanes. However, large amounts of 
vinyltrioximosilane must be used in order to prevent surface cracking, 
which in turn causes new problems, for example, the composition now has a 
pronounced tendency to yellow during storage and is uneconomical. 
Like Sattlegger et al '484, Dziark et al in European Patent Application 
Publication No. 0599616, published Jun. 1, 1994 and equivalent to Japanese 
Patent Application Laid Open (Kokai or Unexamined) Number Hei 6-234148 
(1994) proposes a method in which the filler is admixed after the 
crosslinker has been mixed with hydroxyl-terminated diorganopolysiloxane, 
but the latter method then continues with the addition of a nonreactive 
diorganopolysiloxane. This method solves two problems in that it inhibits 
slump and improves the extrudability. However, it cannot solve the problem 
of inhibiting surface cracking while maintaining a suitable working time. 
Moreover, in order to avoid post-cure bleed and a reduction in adherence, 
the amount of nonreactive diorganopoly-siloxane added after the filler 
cannot be too large, but this restriction diminishes the flexibility to 
adjust the various properties. 
SUMMARY OF THE INVENTION 
Problems to Be Solved by the Invention 
The inventors achieved the present invention as the result of extensive 
investigations directed to solving the problems described above. 
The object of the present invention is to provide a method for the 
preparation of OP/RTC silicone elastomer compositions that do not slump 
prior to their cure, that provide a suitable processing or working time, 
that do not crack or fissure during their cure even when deformed by an 
external force, and that do not yellow during storage or after curing even 
when subjected to thermal episodes. 
Means Solving the Problems and Function Thereof 
The present invention relates to a method for the preparation of 
room-temperature-curable silicone elastomer compositions comprising mixing 
(A) 0.5 to 75 parts by weight of the reaction mixture of (a) and (b) or 
composition (b) prepared from 
(a) 0 to 55 parts by weight hydroxyl-terminated diorganopolysiloxane with a 
viscosity at 25.degree. C. of 0.5 to 300 Pa.s and 
(b) an alkyl-containing oximosilane with the formula R.sup.1 Si(OX).sub.3 
in which R.sup.1 represents an alkyl group and X is an organic group of 
the formula --N.dbd.CR.sup.2 R.sup.3 in which each R.sup.2 and R.sup.3 
represents a monovalent hydrocarbon groups having no more than 6 carbon 
atoms; an organic group of the formula 
##STR1## 
in which R.sup.4 represents a divalent hydrocarbon group having no more 
than 10 carbon atoms; or a C.sub.1 to C.sub.4 monovalent hydrocarbon 
group, wherein C.sub.1 to C.sub.4 monovalent hydrocarbon groups make up no 
more than 30 mole % of X, wherein the amount of the said component (b) is 
within the range of 0.5 to 20 parts by weight for each 100 parts by weight 
of the total weight of component (a) in components (A) and (B) and the 
total number of moles of oximo groups in component (b) exceeds the total 
number of moles of hydroxyl groups in component (a) with 
(B) 45.5 to 120 arts by weight of the reaction mixture of 
(a) 45 to 100 parts by weight hydroxyl-terminated diorganopolysiloxane with 
a viscosity at 25.degree. C. of 0.5 to 300 Pa.s, with the proviso that the 
total amount of component (a) used in components (A) and (B) is 100 parts 
by weight and 
(c) vinyl-functional oximosilane with the formula CH.sub.2 
.dbd.CHSi(OX).sub.3 in which X is defined above, wherein the amount of 
said component (c) is within the range of 0.5 to 20 parts by weight for 
each 100 parts by weight of the total weight of component (a) in 
components (A) and (B) and the total number of moles of oximo groups in 
this component exceeds the total number of moles of hydroxyl groups in 
component (a); and 
by thereafter blending in 
(C) 1 to 200 parts by weight inorganic filler.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The diorganopolysiloxane (a) in component (A) is hydroxyl-terminated 
diorganopolysiloxane. Its pendant organic groups consist of substituted 
and unsubstituted monovalent hydrocarbon groups, which are specifically 
exemplified by alkyl groups such as methyl, ethyl, and so forth; aryl 
groups such as phenyl and so forth; haloalkyl groups such as 
trifluoropropyl and so forth; and alkenyl groups such as vinyl, allyl, and 
so forth. The viscosity of this diorganopolysiloxane must be in the range 
of 0.5 to 300 Pa.s at 25.degree. C. Viscosities below 0.5 Pa.s cause a 
reduced post-cure mechanical strength, while viscosities in excess of 300 
Pa.s cause the silicone elastomer composition to exhibit an extremely 
reduced pre-cure workability. The subject diorganopolysiloxane is in fact 
well known as a starting material for room-temperature-curable silicone 
elastomer compositions. As long as at least 50 mole % of the molecular 
chain terminals of this diorganopolysiloxane carry the hydroxyl group, the 
remainder may be endblocked by inert groups such as trimethylsiloxy and 
the like. 
Component (b), which functions as a crosslinker, is an alkyl-containing 
oximosilane with the formula R.sup.1 Si(OX).sub.3 in which R.sup.1 is a 
C.sub.1 to C.sub.4 alkyl group such as methyl, ethyl, or propyl, and X is 
an organic group of the formula --N.dbd.CR.sup.2 R.sup.3 in which each 
R.sup.2 and R.sup.3 represents a monovalent hydrocarbon group having no 
more than 6 carbon atoms, e.g., methyl, ethyl, propyl, etc.; an organic 
group of the formula 
##STR2## 
in which R.sup.4 represents a divalent hydrocarbon group having no more 
than 10 carbon atoms, e.g., methylene, ethylene, propylene, etc.; or a 
C.sub.1 to C.sub.4 monovalent hydrocarbon group wherein C.sub.1 to C.sub.4 
monovalent hydrocarbon groups make up no more than 30 mole % of X. Typical 
examples of the oximosilanes of component (b) are methyltri(methyl ethyl 
ketoximo)silane and ethyltri(methyl ethyl ketoximo)silane. The present 
invention may use only a single oximosilane selection or a mixture of two 
or more oximosilane selections defined by R.sup.1 Si(OX).sub.3. The amount 
of component (b) is within the range of 0.5 to 20 parts by weight for each 
100 parts by weight of the total weight of the component (a) used in 
components (A) and (B), and, moreover, the total number of moles of oximo 
group present in component (b) should exceed the total number of moles of 
hydroxyl group present in component (a). When the number of moles of oximo 
group present in component (b) is less than the number of moles of 
hydroxyl group present in component (a), problems will occur such as one 
or more of the following problems gelation during the production of the 
component (a) and component (b) reaction mixture or the viscosity 
increases prior to the ensuing steps. Component (A) can be obtained simply 
by stirring components (a) and (b) together either at ambient temperature 
or with heating. The reaction between components (a) and (b) can be 
analyzed by techniques such as nuclear magnetic resonance spectroscopic 
analysis and so forth. 
The component (A) used in the present invention includes the case in which 
use of component (a) is omitted, in which case component (A) consists only 
of component (b) and is not a reaction mixture of components (a) and (b). 
The diorganopolysiloxane of (a) of the reaction mixture in (B) is the same 
as that described above for component (A). 
Component (c), which also functions as a crosslinker, is a vinyl-functional 
oximosilane with the formula CH.sub.2 .dbd.CHSi(OX).sub.3 in which X is 
defined above. A typical example of this oximosilane is vinyltri(methyl 
ethyl ketoximo)silane. The amount of component (c) is within the range of 
0.5 to 20 parts by weight for each 100 parts by weight of the total weight 
of the component (a) used in components (A) and (B), and, moreover, the 
total number of moles of oximo group present in component (c) should 
exceed the total number of moles of hydroxyl group present in component 
(a). When the number of moles of oximo group present in component (c) is 
less than the number of moles of hydroxyl group present in component (a) 
within the range of 0.5 to 20 parts by weight for each 100 parts by weight 
of the total weight of the component (a) in components (A) and (B), 
problems will occur such as one or more of the following problems, 
gelation during the production of the component (a) and component (c) 
reaction mixture or viscosity increases prior to the ensuing steps. 
Component (B) can be obtained simply by stirring components (a) and (c) 
together either at ambient temperature or with heating. This mixing is 
preferably carried out under an inert gas such as nitrogen. The reaction 
between components (a) and (c) can be analyzed by techniques such as 
nuclear magnetic resonance spectroscopic analysis and so forth. 
The components (b) and (c) of the composition according to the present 
invention react not only with the hydroxyl groups (silanol groups) present 
in component (a), but also with the adsorbed water and surface silanol 
present in component (c) and with moisture infiltrating during composition 
storage. When these reactions occur and there is also no excess of 
components (b) and (c), problems will occur with the composition prepared 
by the method according to the present invention such as curing or gel 
production within the storage container and difficult extrusion due to an 
increased viscosity. 
Components (A) and (B) are prepared as described above and are mixed in an 
ensuing step in the present invention. The mixing ratio for components (A) 
and (B) is 45.5 to 120 parts by weight component (B) per 0.5 to 75 parts 
by weight component (A). This mixing is a simple mixing that is 
unaccompanied by chemical reactions. However, since components (b) and (c) 
involved here are readily hydrolyzed by moisture, this mixing should be 
done in at atmosphere of an inert gas, such as nitrogen, or more 
preferably is carried out using a sealed mixing device. 
The present invention proceeds first with the mixing of the components (A) 
and (B) described above and thereafter are admixed with the inorganic 
filler, (C). The inorganic filler (C) used here functions to improve the 
mechanical strength properties of the composition afforded by the 
preparative method when it is a reinforcing filler. Component (C), when it 
is a reinforcing filler, it will ordinarily be a reinforcing silica 
micropowder, for example, a dry-process silica or wet-process silica. 
Other fillers, such as extending fillers, may be used here, for example, 
calcium carbonate. When reinforcing silica micropowder is used, the 
desirable silica has a specific surface of 50 to 400 m.sup.2 /g by the BET 
method. Moisture adsorbs quite readily to the surface of such silica 
micropowders, and when mixed into the composition this moisture can cause 
a loss of performance by the composition resulting from this method by 
reacting with components (b) and (c). The adsorbed moisture is therefore 
desirably minimized as much as possible prior to admixture. Subject silica 
micropowder may be directly used without additional processing, but may 
also be used after its surface has been subjected to a hydrophobicizing 
treatment. Hydrophobicized silica is exemplified by 
hexamethyldisilazane-treated silica, dimethyldichlorosilane-treated 
silica, dimethyldimethoxysilane-treated silica, 
methyltrimethoxysilane-treated silica, and so forth. Reinforcing silica 
micropowders are used in amounts of from 1 to 50 parts by weight per 100 
parts by weight of (a), preferably from 5 to 25 parts by weight per 100 
parts by weight of (a). Extending fillers can be used in amounts of from 1 
to 200 parts by weight per 100 parts by weight of (a). Component (C) must 
be added at the rate of 1 to 200 parts by weight per 100 parts by weight 
total component (a) in components (A) and (B). Adequate reinforcement of 
the cured silicone elastomer is not obtained at an addition below 1 part 
by weight, while additions in excess of 200 parts by weight cause a loss 
of elasticity in the cured product and make it difficult to extrude the 
composition from its container. 
The characteristic features of the present invention reside in the 
procedure and proportions for intermixing the components (A) to (C) 
described above. The goal here is to induce the appearance of the effects 
of the vinyl-functional trioximosilane (c) at the lowest possible 
concentration. Component (c) is essential for shortening the time to the 
appearance of mechanical strength by skinned sections (the shortening 
effect), but at the same time is very prone to cause yellowing. More 
particularly, the goal here is to induce a selective bonding of the 
vinyl-functional oximosilane with the silanol groups at the terminals of 
the base polymer. This is based on the knowledge, gained during the course 
of the investigations that led to the present invention, that the 
shortening effect of vinyl-functional oximosilane appears when the 
vinyl-functional oximosilane is directly bonded to the hydroxyl at the 
polymer terminals. 
The inorganic filler is admixed in a third step into the mixture thus 
prepared. There are no particular restrictions on the mixing technique 
used here, but it will be desirable to run this mixing under an atmosphere 
that excludes atmospheric moisture. Heating is not required; rather, it is 
recommended that means be implemented to prevent the increase in 
temperature during mixing caused by the generation of heat due to shear. 
The preparative method according to the present invention is completed by 
removing the air entrained with the inorganic filler by a degassing 
procedure during or after this mixing, thus yielding a OP/RTC silicone 
elastomer composition having the desired characteristics. 
In addition to components (A) to (C), the addition of a curing catalyst, a 
component (D) is recommended in the method of the present invention for 
the purpose of accelerating the cure. Any catalytic compound heretofore 
known in the art may be used as component (D) insofar as the functions of 
the invention composition are not impaired. Component (D) is exemplified 
by tin catalysts such as the dialkyltin dicarboxylates, titanate esters 
such as tetrabutyl titanate, and amine catalysts such as 
tetramethylguanidine. While component (D) will ordinarily take the form of 
a single selection, combinations of two or more selections may also be 
used. This component, when added, must be added at the rate of 0.01 to 5 
parts by weight per the 100 parts by weight total component (a) in 
components (A) and (B). Additions in excess of 5 parts by weight 
frequently bring out negative effects such as yellowing and a loss of 
water resistance and heat resistance. No effect is obtained for the 
addition of less than 0.01 parts by weight. The timing of component (D) 
addition is not crucial. 
The following can be added on an optional basis to the compositions of the 
present invention: silanol-free diorganopolysiloxanes, silicone resins, 
fluidity adjusters, adhesion promoters, pigments, heat stabilizers, flame 
retardants, antimolds, organic solvents, and the like. 
Compositions according to the present invention as described above are 
characterized by an excellent processability or workability, freedom from 
yellowing, and freedom from surface cracking during their cure even when 
deformed by an external force. In consequence thereof they are 
particularly useful as adhesives, coatings, sealants, and the like. 
The present invention will be explained in greater detail in the following 
through working and comparative examples, in which the reported viscosity 
values were measured at 25.degree. C. and Pa.s is an abbreviation for 
pascal-seconds. Polymer A was 70 weight % dimethylpolysiloxane (viscosity 
17 Pa.s) endblocked at both terminals by hydroxyl and 30 weight % 
dimethylpolysiloxane (viscosity=17 Pa.s) endblocked at one terminal by 
hydroxyl and at the other by trimethylsiloxy. Crosslinker V refers to 
vinyltri(methyl ethyl ketoximo)silane and Crosslinker M refers to 
methyltri(methyl ethyl ketoximo)silane. The properties of the one-package 
silicone compositions were evaluated using the following methods. 
Tack-free-time 
The tack-free time was measured as an index of the working time. The test 
method was based on JIS A 5758. 
Surface cracking time 
The surface cracking time was evaluated as an index of the tendency for the 
composition to crack during the course of its cure. The test method 
consisted of first applying the composition on an aluminum panel, curing 
for a prescribed period of time at 25.degree. C., and then executing a 
180.degree. fold in the aluminum panel. The value measured was the time 
until there was no occurrence of cracking in the surface of the 
composition at this point while keeping the specimen folded. Surface 
cracking times of 120 minutes or less may be taken as indicative from a 
practical standpoint of a low probability of cracking, while times of 60 
minutes or less are even more desirable. 
Post-cure durometer 
A sheet with a thickness of approximately 2 mm was fabricated and cured at 
25.degree. C. for 5 days. The durometer of this sheet was then measured in 
accordance with JIS K 6301. 
Yellowing 
After its preparation, the composition was filled into a 1/3-L plastic 
cartridge and held for 8 weeks in a 95% humidity/40.degree. C. atmosphere. 
The cartridge was then cut open, and the color change in the composition 
was inspected. 
EXAMPLE 1 
Using a mixer, 99.8 g Crosslinker M was added under a nitrogen blanket to 
600 g Polymer A and this was mixed for 30 minutes at room temperature to 
yield a Polymer A/Crosslinker M mixture. 15.5 g Crosslinker V was added 
under a nitrogen blanket to 600 g Polymer A with mixing at room 
temperature for 30 minutes followed by the addition of 467 g of the 
previously prepared Polymer A/Crosslinker M mixture while exercising care 
to avoid contact with moisture. After then mixing at room temperature 
under a nitrogen blanket for 30 minutes, 115 g dry- process silica (BET 
specific surface =200 m.sup.2 /g) that had been dried at 120.degree. C. 
for 3 hours was added with thorough mixing under a nitrogen atmosphere. 
8.6 g gamma-(2-aminoethyl)aminopropyl-trimethoxysilane as adhesion 
promoter and 2.5 g dibutyltin dilaurate as curing catalyst were then added 
with thorough mixing under nitrogen. The resulting OP/RTC silicone 
elastomer composition was slump free and had an excellent appearance. The 
results from the evaluation of this composition were as reported in Table 
1. 
.sup.29 Si-NMR measurements were also run on small samples (Polymer 
A/Crosslinker M mixture, Polymer A/Crosslinker V mixture, and Polymer A) 
taken during the course of production. With respect to the former two 
samples, the --SiMe.sub.2 OH present in Polymer A was not detected at all, 
while in its place --O-- SiMe(OX).sub.2 and --OSiVi(OX).sub.2 were 
detested, respectively. 
EXAMPLE 2 
Using a mixer, 93.1 g Crosslinker M was added under a nitrogen blanket to 
700 g Polymer A and this was mixed for 30 minutes at room temperature to 
yield a Polymer A/Crosslinker M mixture. 15.5 g Crosslinker V was added 
under a nitrogen blanket to 500 g Polymer A with mixing at room 
temperature for 30 minutes followed by the addition of 567 g of the 
previously prepared Polymer A/Crosslinker M mixture while exercising care 
to avoid contact with moisture. After then mixing at room temperature 
under a nitrogen blanket for 30 minutes, 115 g dry- process silica (BET 
specific surface =200 m.sup.2 /g) that had been dried at 120.degree. C. 
for 3 hours was added with thorough mixing under a nitrogen atmosphere. 
8.6 g gamma-(2-aminoethyl)aminopropyl-trimethoxysilane as adhesion 
promoter and 2.5 g dibutyltin dilaurate as curing catalyst were then added 
with thorough mixing under nitrogen. The resulting OP/RTC silicone 
elastomer composition was slump free and had an excellent appearance. The 
results from the evaluation of this composition were as reported in Table 
1. 
EXAMPLE 3 
Using a mixer, 15.5 g Crosslinker V was added under a nitrogen blanket to 
1,000 g Polymer A and this was mixed for 30 minutes at room temperature. 
66.5 g Crosslinker M was thereafter added while exercising care to avoid 
contact with moisture. After then mixing at room temperature under a 
nitrogen blanket for 30 minutes, 115 g dry-process silica (BET specific 
surface=200 m.sup.2 /g) that had been dried at 120.degree. C. for 3 hours 
was added with thorough mixing under a nitrogen atmosphere. 8.6 g 
gamma-(2-aminoethyl)aminopropyltrimethoxy- silane as adhesion promoter and 
2.5 g dibutyltin dilaurate as curing catalyst were then added with 
thorough mixing under nitrogen. The resulting OP/RTC silicone elastomer 
composition was slump free and had an excellent appearance. The results 
from the evaluation of this composition were as reported in Table 1. 
Comparative Example 1 
Using a mixer, 99.8 g Crosslinker M was added under a nitrogen blanket to 
1,050 g Polymer A and this was mixed for 30 minutes at room temperature to 
yield a Polymer A/Crosslinker M mixture. 15.5 g Crosslinker V was added 
under a nitrogen blanket to 300 g Polymer A with mixing at room 
temperature for 30 minutes followed by the addition of 767 g of the 
previously prepared Polymer A/Crosslinker M mixture while exercising care 
to avoid contact with moisture. After then mixing at room temperature 
under a nitrogen blanket for 30 minutes, 115 g dry- process silica (BET 
specific surface =200 m.sup.2 /g) that had been dried at 120.degree. C. 
for 3 hours was added with thorough mixing under a nitrogen atmosphere. 
8.6 g gamma-(2-aminoethyl)aminopropyl-trimethoxysilane as adhesion 
promoter and 2.5 g dibutyltin dilaurate as curing catalyst were then added 
with thorough mixing under nitrogen. The resulting OP/RTC silicone 
elastomer composition was slump free and had an excellent appearance. The 
results from the evaluation of this composition were as reported in Table 
1. 
Comparative Example 2 
15.5 g Crosslinker V and 66.5 g Crosslinker M were simultaneously added 
under a nitrogen blanket to 1,000 g Polymer A and this was mixed for 30 
minutes at room temperature. 115 g dry-process silica (BET specific 
surface=200 m.sup.2 /g) that had been dried at 120.degree. C. for 3 hours 
was added with thorough mixing under a nitrogen atmosphere. 8.6 g 
gamma-(2-aminoethyl)amino propyltrimethoxysilane as adhesion promoter and 
2.5 g dibutyltin dilaurate as curing catalyst were then added with 
thorough mixing under nitrogen. The resulting OP/RTC silicone elastomer 
composition was slump free and had an excellent appearance. The results 
from the evaluation of this composition were as reported in Table 1. 
Comparative Example 3 
82.0 g Crosslinker V was added under a nitrogen blanket to 1,000 g Polymer 
A and this was mixed for 30 minutes at room temperature. 115 g dry-process 
silica (BET specific surface=200 m.sup.2 /g) that had been dried at 
120.degree. C. for 3 hours was added with thorough mixing under a nitrogen 
atmosphere. 8.6 g gamma-2-aminoethyl)aminopropyltrimethoxysilane as 
adhesion promoter and 2.5 g dibutyltin dilaurate as curing catalyst were 
then added with thorough mixing under nitrogen. The resulting OP/RTC 
silicone elastomer composition was slump free and had an excellent 
appearance. The results from the evaluation of this composition were as 
reported in Table 1. In Table 1 and following tables, CX=crosslinker. 
TABLE 1 
______________________________________ 
Examples Comparative Examples 
1 2 3 1 2 3 
______________________________________ 
Polymer weight ratio 
(1) CX V + polymer 
60 50 100 30 -- 100 
(2) CX M + polymer 
40 50 0 70 -- 0 
Addition weight ratio 
Crosslinker V 
20 20 20 20 20 100 
Crosslinker M 
80 80 80 80 80 0 
Slump, mm none none none none none none 
Tack-free-time 
5 5 5 6 5 5 
minutes 
Surface cracking 
50 80 25 at 200 25 
time, minutes least 
240 
Durometer 27 27 25 24 24 27 
Yellowing no no no no no major 
______________________________________ 
EXAMPLE 4 
Using a mixer, 75.3 g Crosslinker M was added under a nitrogen blanket to 
450 g Polymer A and this was mixed for 30 minutes at room temperature to 
yield a Polymer A/Crosslinker M mixture. 15.5 g Crosslinker V was added 
under a nitrogen blanket to 700 g Polymer A with mixing at room 
temperature for 30 minutes followed by the addition of 350 g of the 
previously prepared Polymer A/Crosslinker M mixture while exercising care 
to avoid contact with moisture. After then mixing at room temperature 
under a nitrogen blanket for 30 minutes, 115 g hydrophobic silica (BET 
specific surface 130 m.sup.2 /g, prepared by treating the surface of 
dry-process silica with dimethyldichlorosilane) was added with thorough 
mixing under a nitrogen atmosphere. 8.6 g 
gamma-(2-aminoethyl)aminopropyltrimethoxy- silane as adhesion promoter and 
1.0 g dibutyltin dilaurate as curing catalyst were then added with 
thorough mixing under nitrogen. The resulting OP/RTC silicone elastomer 
composition was slump free and had an excellent appearance. The results 
from the evaluation of this composition were as reported in Table 2. 
Comparative Example A 
Using a mixer, 69.5 g Crosslinker M was added under a nitrogen blanket to 
900 g Polymer A and this was mixed for 30 minutes at room temperature to 
yield a Polymer A/Crosslinker M mixture. 15.5 g Crosslinker V was added 
under a nitrogen blanket to 350 g Polymer A with mixing at room 
temperature for 30 minutes followed by the addition of 350 g of the 
previously prepared Polymer A/Crosslinker M mixture while exercising care 
to avoid contact with moisture. After then mixing at room temperature 
under a nitrogen blanket for 30 minutes, 115 g hydrophobic silica (BET 
specific surface =130 m.sup.2 /g, prepared by treating the surface of 
dry-process silica with dimethyldichlorosilane) was added with thorough 
mixing under a nitrogen atmosphere. 8.6 g 
gamma-(2-aminoethyl)aminopropyltrimethoxy- silane as adhesion promoter and 
1.0 g dibutyltin dilaurate as curing catalyst were then added with 
thorough mixing under nitrogen. The resulting OP/RTC silicone elastomer 
composition was slump free and had an excellent appearance. The results 
from the evaluation of this composition were as reported in Table 2. 
TABLE 2 
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Example 4 
Comparative Example 4 
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Polymer weight ratio 
(1) CX V + polymer 
70 35 
(2) CX M + polymer 
30 65 
Addition weight ratio 
Crosslinker V 23 23 
Crosslinker M 77 77 
Slump, mm none none 
Tack-free-time, min 
15 20 
Surface cracking time, min 
20 150 
Durometer 26 26 
Yellowing no no 
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EFFECTS OF THE INVENTION 
OP/RTC silicone elastomer compositions according to the present invention 
have the following characteristic features because they are prepared by 
mixing specific quantities of each of components (A) and (B) and 
thereafter blending in component (C): absence of precure slump, provision 
of an appropriate working time, absence of cracking during the course of 
curing even under deformation by external forces, and no yellowing during 
storage or after curing even when challenged by thermal episodes.