Rubber composition containing reclaimed silicone rubber

A novel method is proposed for the effective utilization of scraps of cured silicone rubbers, of which no method of disposal is known heretofore other than discarding as a waste material. Namely, scraps of cured silicone rubbers can be compounded in a considerably large amount with a vulcanizable composition based on an organic rubbery elastomer such as fluorocarbon rubbers, e.g., binary copolymers of hexafluoropropylene and vinylidene fluoride, and dispersed therein in such fineness that particles of the cured silicone rubber can no longer be recognized by the naked eyes only when the organic rubbery elastomer composition has a specific Mooney viscosity (ML.sub.1+4 100.degree. C.) of at least 70 to give a uniform curable rubber composition capable of being cured into vulcanized rubber articles having various properties as good as or comparable with those of the vulcanized rubber articles from the same organic rubbery elastomer without admixture of cured silicone rubber scraps.

BACKGROUND OF THE INVENTION 
The present invention relates to a rubber composition or, more 
particularly, to a rubber composition which is a composite consisting of a 
matrix phase of an unvulcanized rubbery elastomer such as fluorocarbon 
rubbers, EPDM rubbers and the like and a finely dispersed phase therein 
which is a cured silicone rubber comminuted so finely that particles of 
the silicone rubber are no longer visible. The rubber composition of the 
invention is capable of giving vulcanized rubber articles having excellent 
rubbery properties despite the outstanding inexpensiveness since scraps of 
cured silicone rubbers occurring in the molding and curing process of 
silicone rubber articles or reclaimed as a waste of silicone rubber 
articles after use can be used to provide the dispersed phase in the 
matrix of the unvulcanized rubbery matrix. 
It is usual that scraps of vulcanized rubbers occurring in the molding and 
vulcanizing process as burrs or unacceptable products or obtained as a 
waste material after prolonged use of vulcanized rubber articles are not 
always discarded as such but are recycled and re-used as a reclaimed 
rubber after refining by washing, pulverization, desulfurization and the 
like into a rubber powder having a particle size of 30 to 80 mesh, which 
can be compounded in a considerably large amount with an unvulcanized 
rubber stock under a shearing force to be dispersed in the matrix of the 
unvulcanized rubber to such an extent that the particles of the reclaimed 
rubber are no longer visible to the naked eyes. Such a composite rubber 
stock containing a powder of the reclaimed rubber can be again molded and 
vulcanized into vulcanized rubber articles having rubbery properties 
comparable to those prepared from a fresh rubber alone so that economical 
advantages are obtained owing to the inexpensiveness of the reclaimed 
rubber as compared with a fresh rubber. 
As to silicone rubbers as a class of synthetic rubbers, the production and 
consumption of silicone rubber articles are rapidly growing more and more 
in a variety of application fields by virtue of the excellent properties 
thereof as compared with other organic synthetic rubbers such as heat and 
cold resistance, weatherability, electric insulation, etc., such that the 
amount of scraps of cured silicone rubbers, occurring as burrs and 
unacceptable products in the molding process of silicone rubber articles 
is also rapidly increasing and as a waste of used-up silicone rubber 
articles. Nevertheless, no efficient way has yet been established for 
recycling and re-using scraps of cured silicone rubber articles. Scraps of 
cured silicone rubber articles can of course be converted into a fine 
powder in a way similar to that for organic rubbers mentioned above but, 
when such a powder of reclaimed silicone rubber scraps is blended with a 
fresh uncured silicone rubber and the composite silicone rubber stock is 
molded and cured into cured silicone rubber articles, the reclaimed 
silicone rubber can never be fully comminuted to such a fineness that the 
particles thereof are no longer visible to the naked eyes. Instead the 
particles remain as a heterogeneous phase in the matrix of the fresh 
silicone rubber and eventually the particles of the reclaimed silicone 
rubber fall out of the cured silicone rubber body. Accordingly, the result 
is that such a cured silicone rubber article prepared by blending a powder 
of reclaimed silicone rubber scraps is very inferior in mechanical 
strengths as well as in the permanent compression set and heat resistance. 
Thus, no practically applicable way is known for the efficient utilization 
of scraps of cured silicone rubbers so that the only way to dispose scraps 
of cured silicone rubbers is just to discard them as a waste material of 
nuisance. Therefore, it is eagerly desired to develop an efficient way to 
effectively utilize such cured silicone rubber scraps. 
SUMMARY OF THE INVENTION 
The present invention accordingly has a primary object to efficiently 
utilize scraps of cured silicone rubbers so as to reduce the problems 
accompanying disposal of such scraps. The object can be achieved by 
preparing a curable rubber composition which comprises an organic rubbery 
elastomer-based composition having a Mooney viscosity (ML.sub.1+4 
100.degree. C.) of at least 70 and a cured silicone rubber finely divided 
and dispersed in the matrix of the organic rubbery elastomer in an amount 
in the range from 0.1 to 50 parts by weight per 100 parts by weight of the 
organic rubbery elastomer-based composition. 
A cured silicone rubber can be finely dispersed by milling under a shearing 
force in the matrix of an organic rubbery elastomer only when the organic 
rubbery elastomer-based composition has a Mooney viscosity (ML.sub.1+4 
100.degree. C.) of at least 70. This provides a composite rubber 
composition capable of giving a cured rubber article having rubbery 
properties as good as or comparable with those of the cured rubber article 
prepared from the same organic rubbery elastomer without admixture of the 
cured silicone rubber scraps despite the high loading amount of the cured 
silicone rubber scraps. This unexpected effect is very remarkable when the 
organic rubbery elastomer is a fluorocarbon rubber such as a binary 
copolymer of hexafluoropropylene and vinylidene fluoride, a ternary 
copolymer of hexafluoropropylene, vinylidene fluoride and 
tetrafluoroethylene or an EPDM rubber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As is described above, the primary object of the present invention is to 
provide a means for the effective utilization of scraps of cured silicone 
rubbers, where discarding as a waste was the only way heretofore to 
dispose the same. This object can be achieved by compounding scraps of 
cured silicone rubbers with an organic rubbery elastomer having a 
specified value of the Mooney viscosity under a shearing force. The cured 
silicone rubber can be finely comminuted and dispersed in the matrix of 
the organic rubbery elastomer in such a fineness that the particles of the 
cured silicone rubber can no longer be recognized by the naked eyes. Thus, 
there is provided a seemingly homogeneous rubber composition capable of 
giving a cured or vulcanized rubber article exhibiting mechanical 
strengths, permanent compression set and heat resistance which are not 
significantly decreased compared with a rubber article prepared from the 
same organic rubbery elastomer not compounded with scraps of cured 
silicone rubbers. 
Although it is a well established technology to prepare a rubber blend of 
an organic rubbery elastomer such as natural rubber, SBR, fluorocarbon 
rubbers, EPDM rubbers and the like with a silicone elastomer, the silicone 
elastomer used in the prior art as a constituent of such a rubber blend is 
limited to uncured silicone elastomers because no uniform dispersion of 
the silicone elastomer in the matrix of the organic rubbery elastomer can 
be obtained when the silicone elastomer is in a cured state. It was a 
quite unexpected discovery accordingly that a cured silicone rubber could 
be very finely and uniformly dispersed in the matrix of an organic rubbery 
elastomer only when the organic rubbery elastomer-based composition has a 
specific value of the Mooney viscosity (ML.sub.1+4 100.degree. C.) which 
should be at least 70 or, preferably, at least 100. 
The Mooney viscosity value of the organic rubbery elastomer referred to 
above is obtained by the measurement of a composition of the organic 
rubbery elastomer already compounded with various additives including 
reinforcing fillers and other ingredients excepting the cured silicone 
rubber. Accordingly, it is not always necessary that the organic rubbery 
elastomer per se has such a high Mooney viscosity. An organic rubbery 
elastomer having a low Mooney viscosity can be used because a rubber 
composition based thereon may have the required high Mooney viscosity when 
the organic rubbery elastomer is compounded with a relatively large amount 
of a reinforcing filler and the like. 
Although the type of the organic rubbery elastomer is not particularly 
limitative, it is preferable that the organic rubbery elastomer is 
selected from those which can be improved in one or more properties or can 
be imparted with a new advantageous property by compounding with an 
uncured silicone elastomer. Needless to say, any organic rubbery 
elastomers of which no improvement in the properties can be expected by 
blending with a silicone rubber can be used in the present invention, if 
desired. Examples of preferable organic rubbery elastomers include 
fluorocarbon rubbers, EPDM rubbers, acrylic rubbers and the like. 
When a fluorocarbon rubber is used as the organic rubbery elastomer in the 
present invention, the resultant composite rubber composition can be 
improved with respect to the low-temperature characteristics, since one of 
the major defects in most fluorocarbon rubbers is their poor 
low-temperature characteristics. In this regard, examples of fluorocarbon 
rubbers suitable for use in the inventive rubber composition include 
binary copolymers of hexafluoropropylene and vinylidene fluoride and 
ternary copolymers of hexafluoropropylene, vinylidene fluoride and 
tetrafluoroethylene since these fluorocarbon rubbers are particularly poor 
in the low-temperature characteristics and the effect of improvement to be 
obtained by compounding with a cured silicone rubber is highly remarkable. 
The cured silicone rubber to be compounded with an organic rubbery 
elastomer according to the present invention is not particularly 
limitative in respect of the type of the curing reaction, which may 
include peroxide curing, addition reaction curing and condensation 
reaction curing provided that the principal ingredient thereof is an 
organopolysiloxane. It is optional that the organopolysiloxane as the 
principal ingredient of the silicone rubber is compounded with a minor 
amount of an organic rubbery elastomer and various kinds of additives 
including finely divided silica fillers conventionally used for the 
reinforcement of silicone rubbers and other modifier agents used to 
improve the properties of the silicone rubber each in a limited amount. 
The organopolysiloxane mentioned above has a chemical composition 
represented by the average unit formula R.sub.a SiO.sub.(4-a)/2, in which 
R is an unsubstituted or substituted monovalent hydrocarbon group 
exemplified by alkyl groups such as methyl, ethyl, propyl and butyl 
groups, alkenyl groups such as vinyl, allyl and butenyl groups and aryl 
groups such as phenyl and tolyl groups as well as those substituted groups 
obtained by replacing a part or all of the hydrogen atoms in the above 
named hydrocarbon groups with halogen atoms, cyano groups and the like 
such as chloromethyl, chloropropyl, 3,3,3-trifluoropropyl and 2-cyano 
ethyl groups and the subscript a is a positive number in the range from 
1.95 to 2.05. It is optional that a part of the groups denoted by R are 
hydrogen atoms, hydroxy groups, alkoxy groups, e.g., methoxy and ethoxy 
groups, acyloxy groups, e.g., acetoxy group, and the like which may serve 
to provide the crosslinking sites to the organopolysiloxane molecules. 
The process by which the silicone rubber is cured is not limitative and any 
scraps of cured silicone rubbers can be used in the present invention 
regardless of the process of curing including extrusion molding, injection 
molding, compression molding and the like to give a shaped body followed 
by curing. It is more or less unavoidable in these molding and curing 
processes that a considerable amount of burrs and unacceptable cured 
silicone rubber articles are produced for which no means of disposal other 
than to discard them as a waste material was previously known, since they 
have no reclaimability as such. Moreover, the amount of worn-out silicone 
rubber articles is rapidly increasing year by year and they also must be 
discarded heretofore as a waste material. These scraps of cured silicone 
rubbers can be efficiently utilized according to the present invention. 
As is mentioned before, the blending work of the organic rubbery elastomer 
and scraps of cured silicone rubbers is performed under a shearing force 
as high as possible. Suitable blending machines include two-roller mills, 
pressurizable kneaders, Banbury mixers and the like. When scraps of cured 
silicone rubbers are blended in these blending machines with an organic 
rubbery elastomer-based composition having a specified Mooney viscosity 
under a shearing force, the silicone rubber scraps are readily comminuted 
and dispersed in the matrix of the organic rubbery elastomer in such a 
fineness that the particulate form of the cured silicone rubber can no 
longer be recognized at least by the naked eyes. 
Since the primary object of the present invention is to provide a means for 
the effective utilization of scraps of cured silicone rubbers, the amount 
of the scraps of cured silicone rubbers is desirably as large as possible 
although the properties of the cured rubber articles prepared from the 
composite rubber composition would be greatly decreased when the amount of 
the cured silicone rubber scraps blended with the organic rubbery 
elastomer is too large. In this regard, the amount of the cured silicone 
rubber scraps should not exceed 50 parts by weight or, preferably, 30 
parts by weight per 100 parts by weight of the organic rubbery elastomer. 
As to the lower limit in the amount of the cured silicone rubber, the 
amount is at least 0.1 part by weight or, preferably, at least 2 parts by 
weight per 100 parts by weight of the organic rubbery elastomer. This is 
especially the ease when certain improvements are desired in the 
properties of the cured rubber articles as in the case of fluorocarbon 
rubbers which can be improved in the low-temperature characteristics and 
EPDM rubbers which can be improved in the heat resistance by blending with 
a cured silicone rubber. 
The composite rubber composition according to the present invention can be 
shaped and vulcanized by a known rubber vulcanization method such as 
compression molding, extrusion molding, calendering, transfer molding, 
injection molding and the like into any desired forms such as pipes, 
sheets, rods and the like as well as various irregular forms followed by 
curing or vulcanization in a conventional manner. The thus obtained rubber 
articles have various properties almost equivalent to those of the rubber 
articles prepared from a blend of the same organic rubbery elastomer with 
uncured silicone rubber and an improvement can even be expected in some 
properties such as low-temperature characteristics, in particular, when 
the organic rubbery elastomer is a fluorocarbon rubber. 
Since the properties of the vulcanized rubber articles obtained from the 
composition according to the present invention depend to some extent on 
the chemical composition of the cured silicone rubber scraps blended with 
the organic rubbery elastomer, it is desirable from the standpoint of 
quality control that the lots of scraps of cured silicone rubbers are 
adjusted to be uniform as far as possible in respect of the kinds of the 
organic groups bonded to the silicon atoms, content of the 
organopolysiloxane constituents in the cured rubber and other chemical 
constituents. 
EXAMPLES 
In the following, the present invention is illustrated in more detail by 
way of examples and comparative examples, in which the term of "parts" 
always refers to "parts by weight" and the values of viscosity are all 
those obtained by the measurement at 25.degree. C. 
In the following, two cured silicone rubbers, referred to as cured silicone 
rubbers A and B, are prepared to simulate cured silicone rubber scraps. 
Cured silicone rubber A 
A silicone rubber cornposition was prepared by uniformly compounding 100 
parts of a diorganopolysiloxane having a viscosity of 10,000,000 
centistokes consisting of 99.875% by moles of the dimethyl siloxane units 
of the unit formula (CH.sub.3).sub.2 SiO and 0.125% by moles of the methyl 
vinyl siloxane units of the unit formula (CH.sub.2 .dbd.CH)(CH.sub.3)SiO 
with a dimethyl vinyl silyl group at each molecular chain end with 40 
parts of a fumed silica filler (Aerosil 200, a product by Nippon Aerosil 
Co.) and 4 parts of diphenyl silane diol as a dispersion aid to give a 
base compound which was subjected to a heat treatment at 150.degree. C. 
for 4 hours and then mastication and plasticization on a two-roller mill. 
The thus obtained base compound was admixed with 0.4% by weight of 
2,5-bis(tert-butylperoxy)-2,5-dimethyl hexane as a curing agent by 
kneading on a two-roller mill to give a curable silicone rubber 
composition which was cured by compression molding at 165.degree. C. for 
10 minutes into a cured silicone rubber sheet having a thickness of 2 mm 
followed by post-curing at 200.degree. C. for 4 hours. This cured silicone 
rubber sheet was cut into 2 cm by 2 cm wide square pieces as a simulation 
of scraps of cured silicone rubbers. 
Cured silicone rubber B 
The formulation of a base compound of silicone rubber was the same as in 
the preparation of the cured silicone rubber A described above excepting 
replacement of the diorganopolysiloxane with the same amount of another 
diorganopolysiloxane having a viscosity of 10,000,000 centistokes 
consisting of 99.85% by moles of the methyl 3,3,3-trifluoropropyl siloxane 
units of the unit formula (CF.sub.3 CH.sub.2 CH.sub.2)(CH.sub.3)SiO and 
0.15% by moles of the methyl vinyl siloxane units with a hydrogen atom 
directly bonded to the silicon atom at each molecular chain end. This base 
compound was admixed with 0.6% by weight of the same curing agent as in 
the cured silicone rubber A and cured into a cured silicone rubber sheet 
under the same conditions as in the preparation of the cured silicone 
rubber A followed by cutting into 2 cm by 2 cm square pieces of the cured 
silicone rubber sheet. 
EXAMPLE 1 
An EPDM rubber composition having a Mooney viscosity (ML.sub.1+4 
100.degree. C.) of 79 was prepared by uniformly compounding, on a 
two-roller mill, 100 parts of an EPDM rubber having a Mooney viscosity 
(ML.sub.1+4 100.degree. C.) of 35 and an iodine value of 14, of which the 
content of the propylene moiety was 22% by weight (Esprene 514, a product 
by Mitsui Petrochemical Co.) with 60 parts of a precipitated silica filler 
having a specific surface area of 190 m.sup.2 /g (Nipsil VN3 LP, a product 
by Nippon Silica Co.) and 15 parts of a process oil (Sunpar 2280, a 
product by Nippon Sun Petroleum Co.). This EPDM rubber composition and 10% 
by weight of the cured silicone rubber A were milled together on a 
two-roller mill with the result that the cured silicone rubber was finely 
comminuted to such a fineness that the particulate form of the cured 
silicone rubber could no longer be recognized by the naked eyes. 
COMATIVE EXAMPLE 1 
The base compound prepared in the course of the preparation of the cured 
silicone rubber B, which had a Mooney viscosity (ML.sub.1+4 100.degree. 
C.) of 40, was blended and milled with 10% by weight of the cured silicone 
rubber A on a two-roller mill to find that the cured silicone rubber could 
hardly be comminuted and remained in a clearly recognizable particulate 
form in the matrix. 
Comparative Example 2 
An EPDM rubber composition having a Mooney viscosity (ML.sub.1+4 
100.degree. C.) of 50 was prepared in the same manner as in Example 1 
excepting a decrease of the amounts of the precipitated silica filler and 
the process oil to 20 parts and 5 parts, respectively. This EPDM rubber 
composition was blended and milled together with 10% by weight of the 
cured silicone rubber A on a two-roller mill to find that the cured 
silicone rubber could hardly be comminuted and remained in a clearly 
recognizable particulate form in the matrix. 
EXAMPLES 2 AND 3 AND COMATIVE EXAMPLES 3 TO 5 
In each of Examples 2 and 3, a fluorocarbon rubber composition was prepared 
by uniformly blending 80 parts of a fluorocarbon rubber, which was a 
binary copolymer of hexafluoropropylene and vinylidene fluoride and 
contained 66% by weight of fluorine, having a Mooney viscosity (ML.sub.1+4 
100.degree. C.) of 75 (FC-2260, a product by Sumitomo 3M Co.) with 25 
parts or 20 parts, respectively, of a fumed silica filler (Aerosil 300, a 
product by Nippon Aerosil Co.), 3 parts of calcium hydroxide as an acid 
acceptor, 1.3 parts of 2,5-bis(tert-butylperoxy)-2,5-dimethyl hexane as a 
vulcanizing agent, 1.1 parts of triallyl isocyanurate as a vulcanization 
aid and 30 parts of the cured silicone rubber A or B, respectively. These 
fluorocarbon rubber compositions were examined for the state of dispersion 
of the cured silicone rubber and a particulate form of the cured silicone 
rubber could not be found by the naked eyes. 
These rubber compositions were compression-molded into a sheet at 
165.degree. C. for 10 minutes followed by a post-curing treatment at 
200.degree. C. for 4 hours and the test sheets were subjected to 
measurements of the mechanical properties as well as to the immersion 
tests according to JIS K 6301 in a JIS Fuel C oil at 25.degree. C. for 70 
hours and in methyl alcohol at 25.degree. C. for 70 hours and a 
low-temperature torsion test to give the results shown in Table 1 below. 
In Comparative Examples 3 and 4, the formulations of the fluorocarbon 
rubber compositions were the same as in Examples 2 and 3, respectively, 
excepting replacement of the cured silicone rubber A or B with the same 
amount of the same silicone rubber composition before admixture of the 
curing agent. The formulation in Comparative Example 5 was the same as in 
Example 2 excepting omission of the cured silicone rubber A and an 
increase of the amount of the fluorocarbon rubber from 80 parts to 100 
parts. 
The test sheets prepared from these comparative rubber compositions were 
subjected to the same tests as in Examples 2 and 3 to give the results 
also shown in Table 1. 
TABLE 1 
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Example Comparative Example 
2 3 3 4 5 
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Hardness, JIS 88 92 89 92 89 
Ultimate elongation, % 
260 240 290 270 360 
Tensile strength, kgf/cm.sup.2 
140 200 151 182 260 
Tear strength, A, kgf/cm 
34 50 35 45 44 
Permanent compression 
26 38 26 31 44 
set, %, after 22 hours at 
180.degree. C. 
Immersion test in JIS Fuel C 
Increment, %, in volume 
+40.2 
+11.8 +39.5 
+11.5 +3.6 
Increment, %, in weight 
+18.3 
+5.4 +18.6 
+5.5 +1.6 
Immersion test in methyl alcohol 
Increment, %, in volume 
+95.0 
+144 +96.1 
+120 +164 
Increment, %, in weight 
+46.5 
+64.8 +45.0 
+53.5 +69.7 
Low-temperature torsion 
-30.5 
-22.1 -30.8 
-21.7 -20.2 
test, T-10, .degree.C. 
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