This invention relates to thermally conductive heat curable organopolysiloxane compositions containing from 30 to 95 percent by weight of filler based on the weight of the composition of which at least 10 percent by weight of the filler is silicon nitride particles.

The present invention relates to silicone elastomers having thermal 
conductive properties and more particularly to heat curable 
organopolysiloxane compositions containing thermally conductive particles. 
BACKGROUND OF THE INVENTION 
Various materials have been incorporated in organopolysiloxane compositions 
to improve the thermal conductivity of the resultant elastomers. For 
example, U.S. Pat. No. 3,499,859 to Matherly discloses adding boron 
nitride particles to room temperature vulcanizable organopolysiloxane 
compositions to form thermally conductive silicone elastomers. Also, U.S. 
Pat. No. 4,292,225 to Theodore et al, describes highly filled 
organopolysiloxane compositions containing boron refractory powders having 
good thermal conductive properties. 
In contrast to the teachings of U.S. Pat. Nos. 3,499,859 and 4,292,225, it 
has been found that organopolysiloxane compositions having increased 
filler loadings provide elastomers having desirable physical properties as 
well as excellent thermal conductivity. Moreover, the compositions of this 
invention are useful for insulating electrical components because they 
permit any heat which is generated to be conducted away from the 
electrical component. Furthermore, the compositions of this invention are 
unique in that filler loadings in excess of 60 percent by weight based on 
the weight of the composition can be achieved using silicon nitride 
particles and the resultant elastomers have superior thermal conductivity 
and desirable physical properties. 
It is, therefore, an object of this invention to provide a highly filled 
thermally conductive vulcanizable organopolysiloxane composition 
containing silicon nitride particles. Another object of this invention is 
to provide a thermally conductive composition having unique physical 
properties by virtue of the use of silicon nitride particles in filler 
loadings in excess of 60 percent by weight based on the weight of the 
composition. Still another object of this composition is to provide a 
highly filled composition which is sufficiently pliable that it can be 
extruded and cured. A further object of this invention is to provide 
highly filled heat curable compositions having improved thermal 
conductivity. 
SUMMARY OF THE INVENTION 
The foregoing objects and others which will become apparent from the 
following descriptions are accomplished in accordance with this invention, 
generally speaking, by providing heat curable compositions containing an 
organopolysiloxane capable of being cured at an elevated temperature, a 
vulcanizing agent and from 30 to 95 percent by weight of filler based on 
the weight of the composition, of which at least 10 percent by weight of 
filler is silicon nitride particles. The resultant composition is heated 
to an elevated temperature to form a thermally conductive elastomer. 
DESCRIPTION OF THE INVENTION 
The organopolysiloxanes employed in the compositions of this invention 
generally have recurring structural units of the general formula 
##STR1## 
where R is selected from the group consisting of monovalent hydrocarbon 
radicals and substituted monovalent hydrocarbon radicals having from 1 to 
18 carbon atoms and n is an integer of from about 1.7 to 2.2, with an 
average value of from about 1.8 to 2.1. 
It is preferred that the hydrocarbon radicals and substituted hydrocarbon 
radicals represented by R each contain from 1 to 18 carbon atoms. Examples 
of suitable hydrocarbon radicals are alkyl radicals, such as the methyl, 
ethyl, n-propyl and isopropyl radicals, as well as octadecyl radicals; 
cycloalkyl radicals such as the cyclohexyl and the cycloheptyl radicals; 
aryl radicals such as the phenyl radical; alkaryl radicals such as the 
tolyl radicals and aralkyl radicals such as the benzyl and 
beta-phenylethyl radicals. Examples of substituted hydrocarbon radicals 
reprsented by R are halogenated hydrocarbon radicals, such as the 
3,3,3-trifluoropropyl radical and o-, p- and m-chlorophenyl radicals. 
Because of their availability, it is preferred that at least 80 percent of 
the R radicals be methyl radicals. 
The viscosity of the organopolysiloxanes employed in the compositions of 
this invention may range from about 300 mPa.s at 25.degree. C. up to a gum 
having a Williams plasticity number up to about 250. More preferably, the 
organopolysiloxanes have a viscosity of from about 1000 mPa.s at 
25.degree. C. up to a plasticity number of about 200. These 
organopolysiloxanes are essentially linear polymers containing 
diorganosiloxane units of the formula R.sub.2 SiO; however, they may also 
contain minor amounts, generally not more than about 2 mol percent of 
other units, such as RSiO.sub.3/2 units, R.sub.3 SiO.sub.0.5 and/or 
SiO.sub.4/2 units, in which R is the same as above. Included specifically 
in the above formula are the dimethylpolysiloxanes, 
methylphenylpolysiloxanes, methylvinylpolysiloxanes, and copolymers of 
such units, such as copolymers containing dimethyl- and 
phenylmethylsiloxane units and copolymers containing phenylmethyl-, 
dimethyl- and vinylmethylsiloxane units. These organopolysiloxanes are 
well known in the art and methods for producing such materials are old and 
widely described in the literature. 
Vulcanizing agents which may be added to the organopolysiloxane 
compositions to effect rapid conversion of the compositions to an 
elastomer are organic peroxides, such as benzoyl peroxide, t-butyl 
perbenzoate, bis-(2,4-dichlorobenzoyl)peroxide, dicumyl peroxide, dialkyl 
peroxides, such as di-t-butyl peroxide, p-chlorobenzoyl peroxide, etc. 
These vulcanizing agents may be present in amounts ranging from about 0.1 
to as high as 4 to 8 percent by weight or even more based on the weight of 
the organopolysiloxane polymers. 
The silicon nitride particles employed in the compositions of this 
invention are polycrystalline or amorphous materials having an average 
particle size of from about 0.5 to about 350 microns and more preferably 
from about 40 to 250 microns. The particle size is not critical as long as 
the particles are not so large as to be difficult to mix with the 
organopolysiloxane to form a homogeneous mixture. 
The silicon nitride particles may be used with other fillers such as 
reinforcing fillers, i.e., fillers having a surface area of at least 50 
m.sup.2 /gm. Examples of such fillers are precipitated silicon dioxide 
having a surface area of at least 50 m.sup.2 /gm and/or pyrogenically 
produced silicon dioxide. Examples of other reinforcing fillers are the 
aerogels, alumina, carbon blacks and graphite. 
A portion of the fillers can be semi- or non-reinforcing fillers, i.e., 
fillers which have a surface area of less than 50 m.sup.2 /gm. Examples of 
semi- or non-reinforcing fillers are metal oxides, metal nitrides, glass 
beads, bubbles or fibers, metallic flakes, powders, and fibers such as 
copper, nickel and aluminum, cork, organic resins, 
polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinyl chloride, 
bentonite, diatomaceous earth, crushed quartz, mica, and mixtures thereof. 
Preferred examples of metal oxide fillers are zinc oxide, ferric oxide, 
alumina and titanium oxide. The fillers may also be treated with, for 
example, triorganoalkoxysilanes, such as trimethylethoxysilane to coat the 
surfaces with organosiloxy groups. 
The amount of fillers including silicon nitride particles which may be 
incorporated in the compositions of this invention is not critical and may 
vary over a wide range. Thus, the amount of filler including silicon 
nitride particles may range from about 30 to 95 percent by weight and more 
preferably from about 40 to 90 percent by weight, based on the weight of 
the composition, in which at least 10 percent by weight of the filler is 
silicon nitride particles. More preferably, the amount of silicon nitride 
particles may range from about 30 to 100 percent by weight based on the 
weight of the filler. It is, however, preferred that the other fillers 
employed in the composition not interfere with the thermal conductivity of 
the resultant composition. 
Other additives which can be incorporated into the compositions of this 
invention include pigments, compression set additives, oxidation 
inhibitors, adhesion promoters, and other materials commonly employed as 
additives in the silicone rubber art. Such additives are preferably 
present in an amount below about 15 percent by weight based on the weight 
of the composition. 
Various antistructuring agents may also be incorporated in the compositions 
of this invention to prevent hardening or "crepe aging" of the materials 
prior to vulcanization. Examples of suitable antistructuring agents are 
water; hydroxyl-terminated silanes and siloxanes having a viscosity of 
from about 30 to 100 centistokes, such as diphenylsilane diols, 
methylphenylsilane diols, hydroxylated methylpolysiloxanes, hydroxylated 
methylphenylpolysiloxanes, hydroxylated diphenylpolysiloxanes; methyl 
endblocked dimethylpolysiloxane fluids, low molecular weight alkoxylated 
siloxanes; phosphate fluids, such as tripropylphosphate and 
tributylphosphate; glycols, such as methylene glycol and propylene glycol; 
esters; and anhydrides, such as phthalic anhydride. 
The amount of antistructuring agents employed in these compositions 
generally ranges from about 2 to 30 percent, preferably from about 5 to 20 
percent by weight based on the weight of the organopolysiloxane polymers. 
The manner in which the present invention may be practiced may be widely 
varied. The silicon nitride particles may be incorporated in the curable 
organopolysiloxane compositions before the addition of the reinforcing 
and/or non-reinforcing fillers or it may be incorporated simultaneously 
with the other fillers. Vulcanizing agents and other additives, such as 
dyes, pigments, and flame retardants, may be added to the 
organopolysiloxane compositions during the milling operation. 
Alternatively, the silicon nitride particles may be added to the 
organopolysiloxane compositions and then milled with the reinforcing and 
non-reinforcing fillers at some future time. 
The organopolysiloxane compositions of this invention may be molded or 
extruded and cured in accordance with the conventional techniques known in 
the art. They may, for example, be cured at temperatures ranging from 
about 100.degree. to 200.degree. C. or more for varying periods of time 
ranging from less than 1 minute up to several hours. 
The compositions of this invention can be used for any electrical 
insulation where it is desirable to conduct the heat away from the source. 
The compositions of this invention are especially useful for encapsulating 
semiconductors since they both insulate and conduct the heat away from the 
semiconductor. These compositions may also be injection molded to form 
thermally conductive pads for use in various electrical applications.

The embodiments of this invention are further illustrated in the following 
examples in which all parts and percentages are by weight unless otherwise 
specified. 
EXAMPLE 1 
A heat curable organopolysiloxane composition is prepared by mixing 5 parts 
of a vinyl-terminated dimethylpolysiloxane having a viscosity of about 
18000 mPa.s at 25.degree. C. and having a vinyl content of about 0.075 
weight percent with 5 parts of a vinyl-terminated dimethylpolysiloxane 
having a viscosity of about 132,000 mPa.s at 25.degree. C. and a vinyl 
content of about 0.58 weight percent, 0.4 parts of a 
methylvinyldimethylpolysiloxane diol having a viscosity of about 40 mPa.s 
at 25.degree. C. and containing about 3.5 weight percent OH and having a 
vinyl content of about 2 weight percent, 45.5 parts silicon nitride 
particles (325 mesh) and 0.1 part of synthetic amorphous calcium silicate. 
The resultant composition is then mixed with 8.4 parts of a 
trimethylsiloxy-terminated dimethylpolysiloxane having a viscosity of 
60,000 mPa.s at 25.degree. C. and 1.4 parts dicumyl peroxide to form a 
homogenous composition. The resultant composition is molded and cured for 
30 minutes at 175.degree. C. The physical properties and thermal 
conductivity are shown in the following table. 
EXAMPLE 2 
The procedure of Example 1 is repeated, except that the 
trimethylsiloxy-terminated dimethylpolysiloxane is omitted and 0.056 parts 
of dicumyl peroxide is substituted for 1.4 parts of dicumyl peroxide. The 
physical properties and the thermal conductivity are shown in the 
following table. 
TABLE 
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Physical Example Example 
Properties 1 2 
______________________________________ 
Tensile strength, psi. 
331 587 
Elongation, percent 
129 84 
Durometer, Shore A 
50 91 
Tear strength, lb./in. 
38 68 
Thermal conductivity, 
7.8 .times. 10.sup.-4 
1 .times. 10.sup.-3 
cal. cm..sup.-1 sec..sup.-1 .degree.C..sup.-1 
______________________________________ 
EXAMPLE 3 
The procedure of Example 1 is repeated except that 10 parts of silicon 
nitride particles and 50 parts of fumed silica are substituted for the 
45.5 parts of silicon nitride particles. The resultant elastomer exhibits 
good physical properties and thermal conductivity. 
EXAMPLE 4 
The procedure of Example 1 is repeated except that 40 parts of silicon 
nitride particles and 5.5 parts of nickel flakes are substituted for the 
45.5 parts of silicon nitride particles. The resultant elastomer exhibits 
good physical and thermal conductivity.