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The present invention relates to a method for improving the adhesion of
organopolysiloxane elastomers, particularly to a method for improving the
adhesion of organopolysiloxanes which crosslink at room temperature and
more particularly to primer compositions for improving the adhesion of
crosslinkable organopolysiloxanes.
BACKGROUND OF THE INVENTION
Silicone elastomers have been bonded to various substrates by coating the
substrates with various silanes such as alkylacyloxysilanes,
alkylalkoxysilanes, alkoxyacyloxysilanes or vinyltriacyloxysilanes prior
to the application of the crosslinkable organopolysiloxanes.
U.S. Pat. No. 2,979,420 to Harper discloses applying a silane of the
formula RSi(OZ).sub.3, where R is a phenyl or an aliphatic hydrocarbon
having less than 4 carbon atoms and Z is an acyl radical having less than
4 carbon atoms to a masonry substrate and thereafter applying a room
temperature curable silicone rubber to the coated substrate.
U.S. Pat. No. 3,306,800 to Plueddemann discloses a method for bonding a
room temperature vulcanizable organopolysiloxane to a thermoplastic resin
by coating the thermoplastic resin with a silylated polymethylmethacrylate
bonding agent and then applying a vulcanizable organopolysiloxane to the
coated thermoplastic resin.
U.S. Pat. No. 3,505,099 to Neuroth discloses bonding a vulcanizable
organopolysiloxane composition obtained from the polymerization of organic
monomers in the presence of an organopolysiloxane and a free radical
generator to a metal substrate by first applying a zinc dust primer to the
metal substrate, then applying an acyloxysilane to the zinc dust primer
and thereafter applying the vulcanizable organopolysiloxane composition to
the coated substrate.
U.S. Pat. No. 4,133,938 to Bingham discloses a method for bonding silicone
elastomers to a porous substrate, which comprises coating the substrate
with a composition obtained from the reaction of an acrylate with an
acryloxyalkylalkoxysilane in the presence of a free radical generator and
thereafter applying an organopolysiloxane which is crosslinkable at
ambient temperature.
One object of this invention is to provide a method for improving the
adhesion of organopolysiloxane elastomers to substrates. Another object of
this invention is to provide a method for improving the adhesion of
organopolysiloxanes which are crosslinked at room temperature to solid
substrates. Still another object of the present invention is to provide a
method for improving the adhesion of organopolysiloxane elastomers to
substrates which are exposed to water. A further object of the present
invention is to provide a method for improving the adhesion of
organopolysiloxane elastomers to substrates which are exposed to water
containing living organisms.
SUMMARY OF THE INVENTION
The foregoing objects and others which will become apparent from the
following description are accomplished in accordance with this invention,
generally speaking, by providing a method for improving the adhesion of
organopolysiloxane elastomers to substrates which comprises (1) coating a
substrate with a primer composition containing (a) a mixture consisting of
a silane having an SiC-bonded aliphatic multiple bond and a silane having
an SiC-bonded epoxy group; or (b) a stratum consisting of (i) at least a
partially crosslinked organopolysiloxane having SiO.sub.4/2 units,
trimethylsiloxane units and vinyldimethylsiloxane units and (ii) at least
a partially crosslinked organopolysiloxane elastomeric coating over said
stratum, in which the crosslinked organopolysiloxane is obtained from a
crosslinkable composition which is stable under anhydrous conditions, but
crosslinks when exposed to moisture at room temperature and (2) applying a
crosslinkable composition over said primer, in which the crosslinkable
composition contains a diorganopolysiloxane, a crosslinking agent, a
catalyst and at least one additional component, in which the component is
flowable after the composition has crosslinked and/or the component is a
rod-shaped copolymer obtained from the copolymerization of styrene and
n-butylacrylate in the presence of a diorganopolysiloxane by means of free
radicals.
DETAILED DESCRIPTION OF THE INVENTION
Organopolysiloxane compositions which crosslink at room temperature
containing a diorganopolysiloxane, a crosslinking agent and a catalyst are
generally known as "two-component compositions" or "two-component
systems". In the "two-component systems", the crosslinking agent and
catalyst are incorporated in the composition just prior to use.
Two-component compositions which can be crosslinked to form
organopolysiloxane elastomers and which in addition to a crosslinkable
diorganopolysiloxane contain a crosslinking agent, a catalyst, and a
rod-shaped copolymer of styrene and n-butylacrylate, which is obtained
from the copolymerization of styrene and n-butylacrylate in the presence
of free radicals and a diorganopolysiloxane, have been described in the
literature and are commercially available. The preparation of these
compositions are described, for example, in U.S. Pat. No. 4,133,938 to
Bingham; U.S. Pat. No. 3,555,109 to Getson; and U.S. Pat. No. 4,032,499 to
Kreuzer et al.
It is preferred that the rod-shaped copolymers which are produced in the
presence of diorganopolysiloxanes and free radical initiators, comprise
between 45 and 75 percent by weight of styrene-derived units with the
remainder consisting of units that are derived from n-butylacrylate.
It is preferred that the diorganopolysiloxane be present in an amount of
from 20 to 60 percent by weight based on the total weight of the
diorganopolysiloxane and the copolymer consisting of styrene and
n-butylacrylate.
Because of their availability, it is preferred that at least 80 percent of
the organic radicals in the diorganosiloxane units be methyl radicals.
It is preferred that the diorganopolysiloxane in whose presence a
rod-shaped copolymer is produced from styrene and n-butylacrylate, have an
average viscosity of from about 150 to 6000 mPa.s at 25.degree. C.
The substance, which is still flowable after the organopolysiloxane
composition has crosslinked, may consist of a triorganosiloxy endblocked
diorganopolysiloxane which is a liquid at room temperature (British Pat.
No. 1,470,465, published Apr. 14, 1977) or the flowable substance may
consist of organic compounds which are devoid of metal and silicon (German
Patent Application No. 2,756,495, published June 22, 1978), or mixtures of
the flowable organic compounds and flowable organopolysiloxanes. Also,
other two-component systems containing crosslinkable organopolysiloxane
compositions consisting of triorganosiloxy endblocked
diorganopolysiloxanes which are liquid at room temperature, or which
contain organic compounds that remain flowable after the crosslinkable
organopolysiloxanes are crosslinked, other than the rod-shaped copolymers
obtained from the copolymerization of styrene and n-butylacrylate by means
of free radicals in the presence of diorganopolysiloxanes are known. (See
for example European Patent Application No. 0032597, published July 29,
1981).
Examples of triorganosiloxy endblocked diorganopolysiloxanes which are
liquid at room temperature, are trimethylsiloxy endblocked
dimethylpolysiloxanes, trimethylsiloxy endblocked methylphenylsiloxanes,
trimethylsiloxy endblocked copolymers consisting of dimethylsiloxane and
methylphenylsiloxane units, trimethylsiloxy endblocked copolymers of
dimethylsiloxane and diphenylsiloxane units, and trimethylsiloxy
endblocked copolymers of dimethylsiloxane, methylphenylsiloxane end
diphenylsiloxane units.
Examples of flowable organic compounds which are free of metal and silicon
are polyisobutylene having a molecular weight of about 350, technical,
white mineral oil and tricresylphosphate.
The substance which is still flowable after the organopolysiloxane
composition is crosslinked, is preferably present in the composition in an
amount of from about 10 to 80 percent by weight, based on the total weight
of the crosslinkable organopolysiloxane and the rod-shaped copolymer of
styrene and n-butylacrylate, when present.
Examples of crosslinking agents which are used in the two-component
compositions of this invention are polyethylsilicate having an SiO.sub.2
content of from about 34 to 40 percent by weight and hexaethoxydisiloxane.
Catalysts which may be used in the two-component compositions of this
invention are tin compounds such as dibutyltin dilaurate and dibutyltin
acylates, in which the acylate groups are derived from a mixture of
carboxylic acids containing from 9 to 11 carbon atoms per molecule and the
carboxyl group is bonded to a tertiary carbon atom in at least 90 percent
by weight of the acids (dibutyltin diversatate).
Silanes having an SiC-bonded aliphatic multiple bond are, for example,
vinyltriacetoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane,
vinyltris-(beta-methoxyethyleneoxy)silane,
beta-methacryloxypropyltrimethoxysilane and
beta-methacryloxypropyltris-(beta-methoxyethyleneoxy)-silane.
Examples of silanes having an SiC-bonded epoxy group are
gamma-glycidoxypropyltrimethoxysilane and
beta-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane.
It is preferred that the silane having an SiC-bonded epoxy group be
employed in an amount of from 1.2 to 1.4 moles for each mole of silane
having an SiC-bonded aliphatic multiple bond.
Methods for preparing coatings consisting of at least partially crosslinked
organopolysiloxanes consisting of SiO.sub.4/2 units, trimethylsiloxane
units and vinyldimethylsiloxane units and the utilization of such coatings
as primers are generally known and are commercially available. (See for
example, the publication "Wacker Silicone" SM 12-121, 803, April 1980,
pages 1 and 2).
Methods for preparing coatings from organopolysiloxane compositions which
are stable under anhydrous conditions, but when exposed to atmospheric
moisture at room temperature, crosslink to form elastomers, have been
described in the literature and are commercially available. Among the
compositions which are stable under anhydrous conditions, but which
crosslink when exposed to atmospheric moisture at room temperature, are
those which release acetic acid or an amine on crosslinking, such as, for
example, sec-butylamine or cyclohexylamine.
In the process of this invention, it is preferred that primer (a) be
employed when the organopolysiloxane compositions contain a substance
which is still flowable, after crosslinking.
When it is desired to improve the adhesion of organopolysiloxane elastomers
on substrates when said compositions comprise a rod-shaped copolymer
obtained from the copolymerization of styrene and n-butylacrylate by means
of free radicals in the presence of the organopolysiloxane, then primer
(b) is preferably employed.
Primers (a) and (b) can be dissolved or dispersed in an organic solvent
such as toluene or in a mixture of xylene isomers. Furthermore, the
two-component compositions which can be crosslinked to form an
organopolysiloxane elastomer used in the process of this invention can be
dissolved or dispersed in an organic solvent such as benzene.
The compositions employed in the process of this invention can be applied
to the substrates by coating, pouring, spraying or calendering.
The method of this invention is especially useful in coating substrates
having solid surfaces that are exposed to water containing living
organisms which may grow on the coated surfaces. Examples of such surfaces
are those consisting of glass, wood, plastics, such as an epoxy resin,
including adhesive varnishes based on epoxide-tar, or
fiberglass-reinforced polyesters, inorganic buidling materials such as
concrete, metal, such as iron or steel, ship bottoms, buoys, sea mines,
supports for drilling platforms or bridges, and containers such as
bathtubs and pipes. Substrates treated in accordance with the method of
this invention may be exposed to stationary or flowing water as well as
salt water, such as ocean water.
In the following examples all parts and percentages are by weight unless
otherwise specified.
EXAMPLE 1
About 72 hours after an epoxide-tar based adhesive has been applied to a
steel plate, a mixture containing 1 part of vinyltriacetoxysilane and 1.3
parts of gamma-glycidoxypropyltrimethoxysilane is applied to the coating.
About one hour after the application of the silane mixture, a
crosslinkable composition containing a diorganopolysiloxane, a
crosslinking agent, a catalyst and at least one additional component which
remains flowable after the composition is crosslinked is applied to the
silane coating with a bar. (The crosslinking agent and the catalyst air
incorporated in the composition just prior to the application of the
composition to the silane coating.) The crosslinkable composition
contains:
(A) 100 parts of a mixture consisting of:
62.5 parts of a dimethylpolysiloxane having terminal Si-bonded hydroxyl
groups and a viscosity of about 1000 mPa.s at 25.degree. C.;
62.5 parts silicon dioxide;
2 parts ferric oxide;
4 parts calcium carbonate;
20 parts of a dimethylpolysiloxane having trimethylsiloxy terminal groups
and an average molecular weight of about 16,000; and
(B) 3 parts of a mixture which is incorporated just prior to the
application of the composition to the silane coating consisting of;
3 parts of polyethylsilicate having an SiO.sub.2 content of about 34
percent; and
1 part dibutyltindiacylate, in which the acylate groups are derived from a
mixture of carboxylic acids containing from 9 to 11 carbon atoms per
molecule, in which the carboxyl group of at least 90 percent of the acids
is bonded to a tertiary carbon atom.
Before this coating begins to crosslink, a thin strip of steel plate which
has been coated with the epoxide-tar adhesive and which subsequently has
been coated with the silane coating, is applied with pressure to the
crosslinkable composition.
The structure obtained is then stored for 7 days at 23.degree. C. and in a
relative humidity of 50 percent. The metal strip is subsequently removed
at a rate of 200 mm per minute. The resistance to shear is 125.6
N/cm.sup.2.
EXAMPLE 2
A steel plate is coated with an epoxide-tar based adhesive and then after
about 72 hours, a mixture consisting of the following ingredients:
10 parts of polyethylsilicate having an SiO.sub.2 content of about 34
percent;
10 parts of butyltitanate;
10 parts of an organopolysiloxane consisting of SiO.sub.4/2 units,
trimethylsiloxane units and vinyldimethylsiloxane units in a molar ratio
of 1:0.8:0.1,
is applied to the plate containing the adhesive coating as a 60 percent
toluene solution.
After one hour an organopolysiloxane composition which is stable under
anhydrous conditions, but which crosslinks when exposed to moisture at
room temperature, is applied to the coated steel plate. The
organopolysiloxane consists of the following ingredients:
100 parts of a dimethylpolysiloxane having terminal Si-bonded hydroxyl
groups and a viscosity of about 80,000 mPa.s at 25.degree. C.; and
20 parts of silicon dioxide having a surface area of 150 m.sup.2 /g, in
which the silanol groups are partially substituted with trimethylsiloxy
groups;
5 parts of methyltriacetoxysilane; and
0.5 parts of di-tert-butoxydiacetoxysilane.
Two hours after the composition has been applied, it is then coated with
the crosslinkable composition described in Example 1 with the aid of a
bar. Before the coating begins to crosslink, a thin strip of steel plate
which has been coated with the adhesive, then with the
organopolysiloxane-containing mixture and then with the crosslinkable
organopolysiloxane composition, is pressed slightly into the coating.
After seven days of storage at 23.degree. C. and at 50 percent relative
himidity, the resistance to shear is 116 N/cm.sup.2.
COMPARISON EXAMPLE
The process described in Example 1 is repeated, except that instead of the
mixture containing the two silanes, a mixture containing the following
ingredients:
17 parts of butyltitanate;
11 parts of tetra-n-propoxysilane;
8 parts of tetra-(methoxyethyleneoxy)-silane;
4 parts of methyltris-(methoxyethyleneoxy)-silane
as a 10 percent benzene solution is used.
After seven days of storage at 23.degree. C. and at 50 percent humidity,
the resistance to shear is 92.4 N/cm.sup.2.
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