Note: Descriptions are shown in the official language in which they were submitted.
1335617
-1- 60SI-721
SILICONE CONFOR~AL COATINGS~
BACKGROUND OF THE INVENTION
The present invention relates to protective silicone
coating compositions and articles coated therewith. More
specifically, the present invention relates to silicone
compositions comprising the reaction product of ~1) a
hydroxy-containing resinous copolymer of R3SiOo 5 units
and SiO2 units and ~2~ a linear organopolysiloxane
containing terminal s;licon-bonded hydroxy groups.
Additionally, the invention relates to articles coated
with such compositions and methods for protecting articles
such as electrical and electronic devices ~y coating
such devices with the composition of the present
lnventlon.
Recent advances in the arts of electrical and
electronics engineering have resulted in very complex
semiconductor circuits contained in much smaller areas
than heretofore possible. Such circuits place increased
demands upon electrical insulation to minimize mechanical
2Q damage due to joltlng or jostling and to minimize the
ability of the system to withstand extreme environment
conditions such as heat, cold and moisture. Furthermore,
it is important that the insulating material have
very low leyels of ionic impurities so as to minimize
electrical cont~mjn~tion.
Among the more useful insulative materials
presently utilized are silicone compositions such as
greases, resins and room temperature yulcanizable
polysiloxanes. Although such silicone compositions give
,_ ~
1 3 3 5 6 1 7 6QSI-721
--2--
the desired protection to sensitive electrical and
electronic components, they h~ve the shortcoming that
once they are in place it is yirtually impossible to
remo~e all or a portion of the coating so as to provide
access for rep~ir or seryicin~ of the components.
~ hile silicone compositions formed from a Iinear
hydxoxy-chainstopped diorgano~olysiloxane ~nd an MQ
resin are descri~ed in the arts of pressure sensitiye
adhesives and room temperature vulcanizable ~RTV~
compositions, so f~r as is known, none have been provided
which are tough dur~ble coating, yet capable of being
removed by a solvent.
It is, therefore, an object of the present invention
to provide a composition for protecting electrical and
electronic equipment from adverse environmental conditions,
but which composition is removable by a solvent so as to
proYide access for repair and servicing of such electronic
components and circuits.
Other objects and advantages of the present invention
2Q will be obvious from the following detailed description.
SUMMARY OF THE INVENTION
The present invention provides a novel silicone
conformal coating composition, a method for protecting
electronic and electrical components utilizing such
composition, and articles having such composition cured
thereon. Briefly stated,and in its broadest sense, the
silicone conformal coating composition of the present
invention comprises the co-condensation Xeaction product
of (1~ from approximately 55 to 7Q parts by weight of
an or~anopolysiloxane cohydxolysis product of a trialkyl
hydrolyzable silane and an alkyl silicate or sodium
silic~te, in which the hydrolyzable silane and the silicate
are reacted in the ratio Qf from about Q.33 to about 0.55
mole of hydxolyzable silane per mole of silicate~ said
alkyl ~adicals ha~ing at most four carbon atoms, and C2~
3~ to 45 parts by weight of a linear diorganopolysiloxane
t
3 13 3 5 617
fluid having terminal silicone-honded hydroxy groups~
said organo ~roups being selected fxom the ~roup consisti.ng
of alkyl radicals, aryl radicals, alk~xyl radicals,
aralk~l xadicals, haloaryl radic~ls and alkenyl radicals
and mixtures thereof, ~nd said diorganopQlysiloxane
having a ~iscosity of from about 200,0Q0 centipoise to
about 2,QQ0,QQQ centipoise at 25C.
DES~K~ O~ OF ~HE IN~ENTION
In accordance with the present invention there are
provided noYel silicone compositions which are solvent
removable and exhibit primerless adhesion to a wide
yariety of substrates. Generally, such compositions are
the co-condensation reaction product of a mixture
containing ~1~ approximately 55 to 70 parts and preferably
6Q to 65 parts by weight of a hydroxy containing resinous
copolymer of R3SiOQ 5 units (M units~ and SiO2 (:Q units~,
wherein the ratio of R3SiOo 5 units per SiO2 units varies
from approximately 0.33 to 0.55:1 and preferably 0.35 to
0.45:1, and wherein R is an alkyl radical preferably
having not more than four carbon atoms, and ~2)
approximately 30 to 45 parts, and preferably 35 to 40
parts, by weight of a hydroxy end-stopped diorgano-
polysiloxane having an average weight of about two
organic radicals per silicone atom, said organic radicals
being selected from the group consisting of alkyl radicals,
aryl radicals, alkaryl radicals, aralkyl radicals,
haloaryl radicals and alkenyl xadicals and mixtures
thereof, said diorganopolysiloxane haying a viscosity
of from about 200,QQQ centipoise to about 2,Q0Q,QQ0
centipoise at 25~C, and the sum of ~1~ and C2~ equalling
100 parts.
The resinous copQlymer of R3SiOQ 5 units and SiQ2
units employed in the practice of the p~esent inYention
is ~ell known in the prior art and is descxibed in
U.S. Patent NQ. 2,736,72I, issued Fehruary 28, lq56
to Dexter, U.$. Patent NO. 2,857,356, issued
13 3 5 61 7 605I-721
Octo~er 21, 1958 to Goodwin, Jr., U.5. Patent No.
2,676,182, issued April 2Q, 1954 to Daudt et al., and
U.S. Patent No. 3,017,384, issued January 16, 1962
to Modic. Although a Yariety of methods are ayailable
for producing such MQ resins, the present invention does
not depend upon the p~axticular method by which the
resinous copolymer is made ~or patentability. Howeyer,
it has been found that MQ resins made according to the
process of Goodwin, Jr., U.S. Patent No. 2,857,356, are
lQ particularly prefera~le as they directly provide a
hydroxy cont~inlng resin. The production of such resins
invol~es the cohydrolysis of a trialkyl hydrolyzable
silane and an alkyl silicate or sodium silicate; that
is, the trialkyl hydrolyzable silane and alkyl silicate
or sodium silicate are added to a suita~le solvent and
thereafter a sufficient amount of water is added to
effect the desired cohydrolysis and co-condensation and
ethylorthosilicate to obtain such hydroxy-containing
MQ resin. Of course, the propo~tions of hydrolyza~le
silane and silicate must be such that there results a
resinous copolymer containing from approximately 0.33
to Q.55 R3SiOo 5 unit per SiO2 unit.
Briefly, the MQ resin is obtained by dissolving the
two ingredients in a suitable solvent for example,
toluene, xylene or an aliphatic hydrocarbon, and then
adding the mixture with stirring, to water maintained at
a temperature on the order to 60 to 85C. Thereafter,
the resulting two-phase system is processed to remoYe
the resultin~ water-alcohol layex and the resinous
material is neutralized with an amount of sodium bicarbonate
or other alkaline material. The resinous solution is
then filtexed and the resinous solids content adjusted
to the desired leYel~ usin~ where necessaXy~ ~dditional
amounts of solyent. Those interested in ~reater detail
or additional information are ur~ed to consult the
aforementioned patents:.
1335617 6QSI-72l
--5--
Regardless of the method ~y which the resinous
copolymer is formed, it is preferred that the alkyl
groups be lower alkyl groups such as methyl, ethyl,
propyl and ~utyl since highex alkyl radicals undesira~ly
slow down the hydrolysis of the silane ~nd cause a
type of co-condensation with the silicate w~hich leads
to less desirable product$. HoweYer, such alkyl groups
may ~e the same or different alkyl groups. It is also
important that in the fin~l product there should be
lQ approximately Q.33 to a . 55 M units per Q unit so that
optimal properties, such as toughness, tensile strength
and freedom from tack;ness, are obtained.
The linear, high viscosity organopolysilane used
for co-reacting with the hydroxy-containing MQ resin
mu$t of necessity have texminal silicon-bonded hydroxy
groups to permit ready copolymerization with the resin.
Such organopolysilxanes may also be prepared by any of
the known methods, for example, acid or alkali catalyzed
polymerization of the corresponding cyclic siloxanes.
2Q Ag~in, the processes for preparing hydroxy-endstopped
polysilxanes are well known in the art, for example as
described in Goodwin, Jr., U.S. Patent No. 2,857,356.
The Goodwin, Jr. process involyes heating cyclic
organopolysilxanes at a temperature of 125~C to 150C
in the presence of a small amount of rearrangement
catalyst such as potassium hydroxide. In general, the
polymerization is carried out for a time sufficient to
obtain a high molecular weight product, prefexably having
a viscosity within the xange of about 75,QOQ to 125,QQ0
3~ centipoise. After such polymerized product is obtained,
it is treated to proYïde termin~l silicon-bonded hydroxy
groups on the molecules of the organopolysiloxane for
co-reaction with the hydroxyl groups of the xesin. This
can be acco~plished ~y blowing steam through or across
the surface of the polymer. Howeyer, this decreases the
viscosity of the polymex while ~t the same tLme increasing
13356~7 60SI-721
--6--
the silanol content of the organopolysiloxane. Although
the organopolysiloxane could be used in this form, it is
preferable to reheat the organosiloxane, which still
contains the rearrangement catalyzed, to a tempexature
of about 125~C to 150C to obtain a material haying a
viscosity of from 200,000 tQ 2,000,000 centipoise. The
preferred range of ~iscosities is from 400,000 to 800,000
centipoise. Once the desired viscosity has been reached,
the organopolysiloxane should be treated in order to
inactiV~te the siloxane rearrangement catalyst.
The organo groups attached to the silicon atoms of
the diorganopolysiloxane are selected from the class
consisting of alkyl radicals such as methyl, ethyl,
propyl, butyl and isobutyl, aryl radicals such as
phenyl and naphthyl; alkaryl radicals such as tolyl,
xylyl and ethylphenyl; aralkyl radicals such as benzyl
and phenylethyl; haloaryl radicals such as chlorophenyl,
tetrachlorophenyl, and difluorophenyl; and alkenyl
radicals such as vinyl and allyl. It is also intended
that mixtures of the foregoing radicals are also within
the scope of the present invention. Preferably the organo
groups are an alkyl group selected from methyl, ethyl,
propyl and butyl and most preferably is methyl.
Preparation of the silicone conformal coating
composition from the MQ resin and diorganopolysiloxane
is relatively simple in that it merely requires mixing
the two together and heating the mixture to effect
co-condensation. In practicing the present invention,
the critical consideration is that the ratio of resin to
organopolysiloxane be within the range of 1.22 to 2.33:1.
Utilization of higher leyels of organopolysiloxane will
result in a final product which is too tacky, ~hereas
utilization of higher leyels of MQ resin will result
in a final product which is too brittle.
To accompli$h the co-condensation ofresin and
organopoly$iloxane, the xesîn is heated, for instance, at a
temperature of about 100 to 150C for from one half to six
_7 1335617 60SI-721
hours or until a product of desired characteristics is
obtained. After a suitable material is obtained the
product is dissolved in a solvent such as toluene,
xylene, an aliphatic hydrocarbon, or a halogenated
aliphatic hydrocarbon, to a convenient solids content,
for example from 10 to 50 percent. The amount of soIvent
in the mixture can varY ~idely as its only function is
to facilitate handling and application of the conformal
coating composition to substrates such as electronic
circuits and device$.
The present invention further provides a method
for pxotecting electrical and electronic components
from adyerse environmental conditions comprising
applying a 0.00254 to 0.254 millimeter thick coating
of the composition of the present invention to such
electronic equipment. Throughout the present disclosure
the terms electrical and electronic devices, components,
equipment and the like are used interchangeably and are
meant to include, but not limited to, devices such as
circuits, transistors, diodes, resistors, capacitors
and the like. Following application of the formulation
to the equipment or devices by conventional means such as
spraying, brushing and dipping, the formulation is air
dried for 10 to 60 minutes or until "tack free" and
subsequently oven dried at a temperature in the range
of 75 to 150C for 10 to 60 minutes.
Optionally, a catalyst or curing agent is included
in the composition. Pxeferably such curing agent is a
peroxide catalyst such as benzoyl perioxide or
dichlorobenzoyl perioxide at a level of 1 to 2% based on
the silicone content of the coating compositions.
Howe~er, any organic pexioxide present in amounts
ranging from about 0.01 to 3.0% by weight is effective.
Of course, any suitable catalyst can be employed. In
application, the catalyst is mixed into the 10 to 50
percent solid$ solution of the conformal coating
-8- 1335 605I-721
composition. The solution is then coated on the desired
electronic components, and thereafter the coating
component i5 air dried and then heated at a temperature
of from about 75 to 15Q9C for 10 to 60 minutes to
evaporate the solvent and effect curing of the confprmal
coating composition. The solvent must be removed before
the te~perature is r~ised to actiVate the peroxide.
The article thus ,f,o~med is effective for protecting
electronic and electrical components from adverse
environmental conditions such as heat, cold and moisture
as well as protecting against mechanical damage caused by
jostling or jolting of the equipment. An important
~dvantage of utilizing the composition of the present
invention on electronic components, circuitboards and
the like is that the composition is easily removable by
exposing it to a solvent such as toluene, xylene, or an
aliphatic hydrocarbon. Thus, in the event that the
coated article malfunctions, it is now possible to
remove all or a part of the silicone conformal coating
composition so as to provide access for repair of the
electronic device or circuit. Moreover, it is now
possible to gain access to such coated electronic devices
for routine servicing, if necessary, rather than only for
making repairs. Although it is envisioned that the
composition of the present invention will primarily be
employed to protect and insulate electronic and electrical
equipment; other components, devices, substrates and the
like which can be protected by the instant silicone
conformal coating composition are also within the scope
of the invention.
-
