Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
2000-06-09
2001-09-04
Michl, Paul R. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S432000, C524S433000
Reexamination Certificate
active
06284829
ABSTRACT:
The present invention relates to silicone elastomers of high thermal conductivity and to the organopolysiloxane compositions which enable them to be obtained. These elastomers are in particular applied in filling materials for heat transfer, in particular with respect to automobile components and components for domestic electrical appliances, in adhesives for electronic components and in two-component products used in moulding.
The thermal conductivity of composite materials is commonly obtained by incorporation of a large amount of filler. As a general rule, to have available a thermally-conducting silicone elastomer material, it is universally agreed that the level of filler by volume must be greater than 35-40% and not more than 65-70%. The nature of the filler is chosen according to its compatibility with the polymer matrix, its ease of incorporation and its own thermal conductivity.
The fillers are also chosen according to the final destination of the elastomer, which may be either electrically conducting or electrically insulating.
The main fillers used in silicone elastomers of high thermal conductivity are: Be, Si, Al, Zn, Mg, Fe and Ti oxides, boron, aluminium or silicon nitrides, silicon carbide, quartz, calcium carbonate, graphite, Ca and Mg fluorides and Al and Cu powders.
Japanese Patent JP-A-56/000834 describes the effect of the distribution of the size of the filler particles in the elastomer on the thermal conductivity of the latter. They use a trimodal distribution of alumina particles:
Example 1
Example 2
Composition
parts by weight
parts by weight
Alumina, from 0.8
600
300
to 2.5 &mgr;m
Alumina, from 4 to
300
300
18 &mgr;m
Alumina, from 25
300
600
to 40 &mgr;m
Siloxane
100
100
Peroxide
0.4
0.4
% of filler in
92.3
92.3
materials (mass)
% of filler in
75
75
materials (volume)
Thermal
3.7
2.3
conductivity
(W/m.K)
The authors find that the thermal conductivity is improved by the use of a greater amount of particles of small diameter.
However, the amount of fillers used in these elastomers is very large and the elastomeric properties are lost or greatly weakened. Indeed, this composition should not be regarded as being classifiable as a silicone elastomer.
Patent U.S. Pat. No. 4,518,655 describes a composition comprising an &agr;,&ohgr;-hydroxypolydimethylsiloxane silicone oil, tabular alumina and calcined alumina. The tabular alumina is finely divided and must not exceed 100 mesh (i.e. approximately 168 &mgr;m), the finest being indicated at 325 mesh, i.e. a maximum size of approximately 48 &mgr;m. The example cites the use of tabular alumina with a size of between 100 and 325 mesh. As regards the calcined alumina, its size is less than one micrometre.
The main problem for thermally-conducting elastomers is thus the relationship between the amount of fillers incorporated, which is the factor which increases the thermal conductivity of the material, and the elastomeric properties, which are inversely proportional to the amount of fillers. The problem therefore becomes increasingly difficult to solve as the desired thermal conductivity increases.
Generally, true heat-conducting industrial silicone elastomers have a thermal conductivity, measured at 25° C. by the “Flash” method (N. J. Parker et al., J. of Applied Physics, 32, pp. 1679-1684, 1961), of between 0.8 and 1.2 W/m.K.
The object of the present invention is therefore to develop an elastomeric material of high thermal conductivity, which can reach and even exceed 1.2 W/m.K, which thus contains a high level of filler but which retains elastomeric properties and in particular an elongation at break (measured according to ISO standard R37 at 25° C. with H2-type test specimens) which is entirely satisfactory and in particular greater then 30%.
Another object of the invention is to develop such materials on the basis either of electrically conducting fillers or of fillers having electrically insulating properties.
In contrast to the teaching of the Japanese patent mentioned above, the Applicant Company has found, surprisingly, that the more the size of the particles of the filler was increased, the better was the thermal conductivity of the elastomeric material comprising it and that this conductivity, as well as the compactness of the material, could be further improved by the incorporation of smaller filler particles.
The Applicant Company has, in addition, found an optimum compromise between high thermal conductivity and elastomeric nature by limiting the size of the filler particles.
The Applicant Company has further observed a synergy effect resulting from the combination of large-sized particles, in a predominant amount, and of small-sized particles.
Finally, the achievement of the desired result involved the determination of the volume of filler to be incorporated, that is to say of the volume which will be occupied by the filler in the final elastomer.
The subject of the present invention is therefore a polyorganosiloxane composition resulting in a silicone elastomer of high thermal conductivity, which can reach and even exceed 1.2 W/m.K, the elastomer retaining an elongation at break of greater than 30%, comprising at least one functional polyorganosiloxane (I) which crosslinks by a polyaddition or polycondensation reaction or by the radical route, optionally a polyorganohydrosiloxane (II), a catalyst (III) and at least one pulverulent filler (IV) intended to increase the thermal conductivity of the final elastomer, as well as, optionally, a reinforcing filler (V), characterized in that the filler (IV) intended to increase the thermal conductivity is present in the composition in the proportion of 35 to 70% by volume, preferably of 45 to 65% and more preferentially still of 50 to 60%, with respect to the total composition and in that this filler comprises at least two groups of particles of very different mean diameters, a first group having a mean particle diameter of between 10 and 40 &mgr;m, preferably of 15 to 35 &mgr;m, present in a predominant amount in the filler, and a second group having a mean particle diameter of less than 5 &mgr;m, preferably of between 0.1 and 5 &mgr;m.
Preferably, when speaking of a group of particles having a mean diameter within a given range, it should be understood that more than 50% by weight of the particles have a diameter within the range (from 50 to 100% by weight of the particles).
Predominant amount is understood to mean in particular an amount of the order of 60 of 90% by volume, preferably of the order of 75 to 90% by volume, with respect to the total amount of fillers intended to increase the thermal conductivity.
According to an advantageous form of the invention, the small-sized particles can be distributed according to a bimodal distribution, in particular with a first mean diameter domain of between 1 &mgr;m and 5 &mgr;m, in particular of the order of 2 &mgr;m, and a second domain with a mean particle diameter of between 0.1 &mgr;m and 0.5 &mgr;m, in particular of the order of 0.2 &mgr;m.
The distribution between particles of the first domain and particles of the second domain is preferably from 85 to 95% by volume for the first particles and from 5 to 15% by volume for the second particles.
The preferred fillers (IV) are ground quartz, Al
2
O
3
, MgO, ZnO and mixtures of these.
The polyorganosiloxanes (I) and the optional polyorganohydrosiloxanes (II), the main constituents of the compositions according to the invention, are composed of siloxy units of general formula:
Z
x
R
y
SiO
4
-
(
x
+
y
)
2
(
1
)
optionally all the other units being siloxy units of mean formula:
R
n
SiO
4
-
n
2
(
2
)
in which formulae the various symbols have the following meaning:
the R symbols, which are identical or different, each represent a group of non-hydrolysable hydrocarbon nature, it being possible for this radical to be:
an alkyl or haloalkyl radical having from 1 to 5 carbon atoms and containing from 1 to 6 chlorine and/or fluorine atoms,
cycloalkyl and halocycloalkyl radicals having from 3 to 8 carbon atoms and containing from 1 to 4 chlorine an
Dalbe Bernard
Giraud Yves
Michl Paul R.
Rhodia Chimie
Seugnet Jean-Louis
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