Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invelltion relatcs to organopolysiloxanes and more
particularly to organopolysiloxane compositions wllich may be cross-linked
to form neutron-absorbing elastomers.
Neutron-absorbing elastomeric compositions which are obtained
from compositi.ons capabl.e of being cross-linked containing d;.organopoly-
siloxanes having SiC--bonded phenyl groups and neutron-absorbillg fillers
are known in the art. For example, German Patent Application No.
2,822,494, published December 7, 1978, (Brand Industrial Services, Tnc.)
discloses neutron-absorbing compositions containing diorganopolysiloxanes
having SiC-bonded phenyl groups; however, it does not disclose the pro-
portion of SiC-bonded phenyl groups present on the diorganopolysiloxane.
Organopolysiloxane compositions which may be cured to form
elastomers containing a polydiorganosiloxane gum having up to lO mole per-
cent of phenyl radicals, reinforcing silica filler, an organic peroxide,
a platinum-containing material and carbon black are described in United
States Patent No. 3,652,488 to Harder.
It is an object of one aspect of this invention to provide a
cross-linked organopolysiloxane composition which is capable of absorbing
radiation.
An object of another aspect of this invention is to provide an
elastomer ~hich is more resistant to gamma radiation.
An object of still another aspect of this invention is to pro-
vide a cross-linkable organopolysiloxane composition which is capable of
forming neutron-absorbing elastomers.
An object of still another aspect of this invention is to pro-
vide neutron-absorbing elastomers having increased elongation and
increased tear resistance.
An object of a further aspect of this invention is to ?rovide
neutron-absorbing elastomers having improved flame retardancy.
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In accordance with an aspect of this invention, compositions
are provided which are capable of being cross-linked to form neutron-
absorbing elastomers, the composition consisting essentially o: (a) a
diorganopolysiloxane having an average of from 15 to 20 mole percent of
SiC-bonded phenyl groups and an average viscosity of from 105 to 107 mPa.s
at 25C.; (b) a neutron-absorbing filler selected from the group con-
sisting of boron carbide, boroxide, boric acid, cadmium oxide, lithium
oxide and mixtures thereof which is present in an amount of from 30 to
70 percent by volume based on the total volume of the diorganopolysiloxane
containing SiC-bonded phenyl groups and the neutron-absorbing filler;
and (c) a cross-linking agent.
By one variant, the SiC-bonded phenyl groups are present as
diphenylsiloxane units.
By another variant, the diorganopolysiloxane is represented by
the formula
(HO)XSiR3 X(siR2)nsiR3_X(H)x
where R is selected from the group consisting of monovalent hydrocarbon
radicals and substituted monovalent hydrocarbon radicals, with the proviso
that from 15 to 20 percent of the number of R radicals are phenyl radicals,
n is an integer such that the diorganopolysiloxane has an average viscos-
ity of from 105 to 10 mPa.s at 25C. and x is 0 or 1.
By another variant, the cross-linking agent is a peroxide com-
pound.
By another variant, the diorganopolysiloxane contains at least
2 SiC-bonded alkenyl groups and the cross-linking agent is a methyl-
hydrogenpolysiloxane and a platinum compound.
It has been found that the cross-linked compositions of aspects
of this invention, when cornpared with known dimethylpolysiloxane composi-
tions in the preparation of neutron-absorbing materials have certain
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advantages. For example, neutron-absorbing elastomers obta;ned from the
compositions of aspects of this invention may conta;n greater amounts of
neutron-absorbing fillers and are more resistant to gamma radiation.
Moreover, even with the same amount of neutron-absorbing fillers, the
elastomers obtained from the compos;tions of aspects of th;s invention
exhibit increased elongation at break and increased tear r2sistance,
thereby providing elastomers l~hich are less sensitive to stresses caused
by vibration and benidng. Furthermore, the elastomers obtained from the
compositions of aspects of this invention exhibit greater flame retardancy
than similar compositions containing dimethylpolysiloxanes.
As discussed above, the diorganopolysiloxanes present in the
compositions of aspects of this invention are preferably represented by
the following general formula:
(Ho)XSiR3 X(siR20)nsiR3_x)O )x
where R represents the same or different monovalent hydrocarbon radicals
or substituted monovalent hydrocarbon radicals, with the proviso that
from 15 to 20 percent of the number of such R radicals are phenyl groups,
n is an integer such that the diorganopolysiloxanes have an average vis-
cosity of from 105 to 107 mPa.s at 25C. and x is 0 or 1.
Although this is not generally shown, siloxane units other than
the diorganosiloxane units (SiR20~ may be present within or along the
siloxane chain of the above-cited formula. Examples of other siloxane
units which are generally present only as impurities, are those having
the formulas RSiO3/2, R3SiOl/2 and SiO4/2, where R is the same as above.
It is preferred that such other siloxane units be present in an amount
less than 1 mole percent.
Although it is preferred that the phenyl groups be present as
diphenylsiloxane units, they may also be present, for example, in units
of the follol~ing general formula
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~here R' represents a monovalent hydrocarbon radical other than the phenyl
groups or a substituted hydrocarbon radical.
It is preferred that the monovalent hydrocarbon radicals other
than the phenyl group and the substituted hydrocarhon radicals each con-
tain from 1 to8 carbon atoms per radical.
Exa~ples of suitable hydrocarbon radicals represented by R and
R', other than the phenyl groups, are alkyl radicals, e.g , the methyl and
the ethyl radicals, as well as the propyl, butyl and hexyl radicals,
alkenyl radicals, e.g., the vinyl, the allyl, the ethylallyl and the buta-
dienyl radical; alkaryl radicals, e.g., the tolyl radicals and the
aralkyl radicals, e.g., the beta-phenylethyl radical.
Examples of substituted hydrocarbon radicals represented by R
and R' are especially halogenated hydrocarhon radicals, e.g., the 3,3,3-
trifluoropropyl radical, chlorophenyl and bromotolyl radicals; and
cyanoalkyl radicals, e.g., the beta-cyanoethyl radical.
Because of their availability, it is preferred that at least 80
percent of the number of the SiC-bonded organic radicals other than phenyl
groups be methy radicals.
In the same or different molecules having the above formula,
the values for x may be the same or different. M;xtures of molecules
having various values for n may be present.
The compositions of aspects of this invention may contain any
of the neutron-absorbing fillers which have been or could have been
incorporated into neutron-absorbing elastomers. A preferred filler is
-~ boron carbide (B4C) having a particle size of from 5 to 500 micrometers.
Additional examples of neutron-absorbing fillers are boroxide, boric acid,
cadmium oxide and lithium oxide. Mixtures of various neutron-absorbing
materials may also be employed.
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In addition to the diorganopo]ysiloxane and the neutron-
absorbing fillers, the compositions of aspects of this invention may con-
tain otller materials, e.g., cross-linking agents, fillers other than
neutron-absorbing fillers, heat-stabilizers, anti-oxidants, flame retar-
dants, processing aids and pigments.
The preferred cross-linking agents are the peroxide compounds.
Suitable examples of peroxide compounds which may be used as cross-linking
agents are acylperoxides, e.g., dibenzoyl peroxide, bis-(4-chlorobenzoyl)-
peroxide and bis-(2,4-dichlorobeonzoyl~-peroxide; alkyl peroxides and
arylperoxides, e.g., di-tert-butyl peroxide and dicumyl peroxide;
perketals, e.g., 2,5-bis-(tert-butylperoxy)-2,5-dimethylhexane, as well as
peresters, e.g., diacetyl peroxydicarbonate, tert-butyl perbenzoate,
tert-butylperoxy isopropyl carbonate and tert-butylperisonanoate. Also,
tert-butyl-beta-hydroxyethylperoxide may be used as a cross-linking agent.
Additional examples of cross-linking agents which may be used in the com-
positions of aspects of this invention are azo compounds which form
radicals, e.g., a~oisobutyric acid nitrile. However, when the composition
contains diorganopolysikoxanes having at least 2 SiC-bonded alkenyl groups,
particularly vinyl groups, per molecule, it is possible to use methyl-
hydrogenpolysiloxane and platinum catalysts.
Tf the compositions of aspects of this invention contain per-
oxide compounds as the cross-linking agent, it is preferred that they be
present in an amount of from 0.5 to 5 percent by weight based on the total
weight of the composition.
Examples of fillers other than neutron-absorbing fillers which
may be employed in the compositions of aspects of this invention are rein-
forcing fillers, e.g. pyrogenically obtained silicon dioxide having a
surface area of from 100 to 300 m2/g and non-reinforcing fillers, e.g.,
quartz meal. It is preferred that the compositions contain ?yrogenically
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obtained silicon dioxide having a surface area of from 100 to 300 m2/g in
an amount which does not exceed more than 30 percent by weight, based on
the total weight of the other constituents of the composition.
Examples of flame retardant agents which may be employed are
graphite, alum;num oxide trihydrate which may contain organosiloxy groups
on its surface, and platinum or platinum compounds, or plat;num complexes
and mixtures of at least two of such substances~
If the compositions of aspects of this invention contain graph-
ite, then the amount of graphite should be in the range of from 1 to 10
percent by weight, based on the total weight of the other constituents of
the composition.
If the compositions of aspects of this invention contain plati-
num, platinum compounds or platinum complexes, particularly in combination
with graphite, then the platinum may be present in an amount of from 1 to
10 parts by weight of platinum (calculated as elemental platinum) per
mil]ion parts by weight of the composition.
When the compositions of aspects of this invention contain
aluminum oxide trihydrate, then the amount of aluminum oxide trihydrate
present in thè composition may range from 10 to 40 percent by weight,
based on the total weight of the other constituents of the composition.
~xamples of processing aids which may be incorporated in the compositions
of aspects of this invention are organopolysiloxanes having an average
viscosity of from 100 to 1,000 mPa.s at 25C. and from 10 to 20 mole per-
cent of SiC-bonded phenyl groups. ~en the compositions of aspects of
this invention contain such processing aids, they should be present in an
amount of from 1 to 10 percent by weight based on the total weight of tlle
other constituents of the composition.
In prepa~ing the compositions of aspects of this invention, all
of the constituents may ~e mixed in any desired se~uence, in a conventional
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mixing device, e.g., for example, a kneader. Mixing may tatie place at
room temperature. I-lowever, mixing may also be carried out at higher
- temperatures, for example, at temperatures in the range of from 35 to
200C, However, heat-sensitive constituents such as the organic peroxide
- compounds may, of course, be mixed only at temperatures at which they
remain unchanged.
The compositions of aspects of this invention may be shaped by
any technique known in the art for shaping cross--linl;able compositions
containing diorganopolysiloxanes having a viscosity of at least lO mPa.s
at 25C. Examples of suitable shaping techniques are injection molding,
transfer molding or other methods involving pressure or extrusion.
The compositions of aspects of this invention may be cross-
linked by any suitable means known for cross-linking compositions contain-
ing the particular cross-link ng agent. For example, when a peroxide
cross-linking agent is used, cross-linking may be accomplished by heating
the compositions to between 120 and 180C. It is preferred that heating
be continued (so-called tempering), for example, for 4 hours at 200C.
However, if the objects prepared from the compositions of aspects of this
invention are thicker than 8 mm, then it is preferred that they be heated
for from 4 to 6 hours at 150C. and then for 4 to 6 hours at 200C.
Neutron-absorbing elastomers prepared from the compositions of
aspects of this invention are resistant to gamma radiation up to 1011 rad
and can absorb at least 10 7 neutrons for each square centimer of surface.
Example
A 500 liter kneader was used for mixing 180 kg of a diorgano-
polysiloxane containing vinyldimethylsiloxy terminal units and consisting
of 82.9 mole percent of dimethylsiloxane units, 17 mole percent of
diphenylsiloxane units and 0.1 mole percent vinylmethylsiloxane units
(17 mole percent SiC-bonded phenyl groups and 0.1 mole percent SiC-b-llded
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vinyl groups) and having a viscosity of 15-106 mPa.s at 25C., with 450 kg
(52 percent by volume) of boron carbide, and 20 kg of pyrogenically
obtained s;licon dioxide having a surface area of 150 m2/g, and 3 kg of a
diorganopolysiloxane containing vinyldimethylsiloxy terminal units which
consists of 92 mo]e percent dimethylsi]oxane units and 18 mole percent
diphenylsiloxane units (18 mole percent of SiC-bonded phenyl groups) and
a viscosity of 100 mPa.s at 25C. at a temperature of 150C. After the
mixture has cooled to room temperature, 20 kg of graphite having a surface
area of 10 m /g, 1 kg of a 1 weight percent solution of H2PtC16 6H2O in
ethylene glycol monomethyl ether, and 7 kg of dicumylperoxide are added
to the mixture. The resultant composition is formed into 2 mm thick .
plates, then heated for 15 minutes to 165C. under a pressure of 100 bar
(abs.) and then heated for 4 hours at 200C. in the absence of pressure.
; Comparison Example
The process described in the preceding example is repeated,
except that 180 kg of a trimethylsiloxy end-blocked diorganopolysiloxane
which consists of 99.9 mole percent dimethylsiloxane units and 0.1 mole
percent of vinylmethylsiloxane units having a viscosity of 10-106 mPa.s at
25C. are substituted for the diorganopolysiloxane containing 17 mole
percent SiC-bonded phenyl groups used in the preceding example.
Properties of the resultant elastomer are shown in the table. The
table shows the extent of flame retardancy as measured by the LOI (_imited
Oxygen Index) factor, as determined in accordance with ASTM-D 28 63-70.
The higher the factor, the higher the flame retardancy.
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'ABLE
xam~le_o~arison F. a ple
Shore-A Hardness 80 82
Elongation at break, percent 160 50
Tensi.le strength, N/mm 1.9 2
Resistance to tearin~ N/~m 15 6
LOI factor, percent 60 52
