Note: Descriptions are shown in the official language in which they were submitted.
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SEALANTS AND POTTING FORMULATIONS INCLUDING POLYMERS
PRODUCED BY THE REACTION OF A POLYTHIOL AND POLYVINYL
ETHER MONOMER
Cross-Reference to Related Ap~~lications
This application is a continuation-in-part of U.S. Patent Application No.
08/928,972 filed September 12, 1997, which is a continuation-in-part of now
U.S. Patent No. 5,912,319. Also, this application is a continuation-in-part of
U.S. Patent Application No. 09/318,500 filed May 25, 1999, which is a division
of now U.S. Patent No. 5,912,319. This application also claims the benefit of
U.S. provisional application no. 60/182,396 filed February 14, 2000 and U.S.
provisional application no. 60/215,548 filed June 30, 2000.
Field of the Invention
The present invention relates to a sealant or potting formulation
prepared from a polymer produced by the reaction of polythiol(s) and polyvinyl
ether monomers) and being terminated with at least one reactive functional
group other than a mercapto group, the formulation having good low
temperature flexibility and fuel resistance.
Background of the Invention
Commercially available polymeric materials which have sufficient sulfur
content to exhibit desirable sealing and fuel resistance properties for
aerospace sealants and electrical potting compounds are the polysulfide
polyformal polymers described, e.g., in U.S. Pat. No. 2,466,963, and the alkyl
side chain containing polythioether polyether polymers described, e.g., in
U.S.
Pat. No. 4,366,307 to Singh et al. Materials useful in this context also have
the desirable properties of low temperature flexibility characterized by a low
glass transition temperature (T9) and liquidity at room temperature.
An additional desirable combination of properties for aerospace
sealants which is much more difficult to obtain is the combination of long
application time (i.e., the time during which the sealant remains usable) and
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short curing time (the time required to reach a predetermined strength). Singh
et al., U.S. Pat. No. 4,366,307, disclose such materials. Singh et al. teach
the
acid-catalyzed condensation of hydroxyl-functional thioethers. The hydroxyl
groups are in the ~i-position with respect to a sulfur atom for increased
condensation reactivity. The Singh et al. patent also teaches the use of such
hydroxyl-functional thioethers with pendant methyl groups to afford polymers
having good flexibility and liquidity. However, the disclosed condensation
reaction has a maximum yield of about 75% of the desired condensation
product. Furthermore, the acid-catalyzed reaction of ~i-hydroxysulfide
monomers yields significant quantities of an aqueous solution of thermally
stable and highly malodorous cyclic byproducts, such as 1-thia-4-oxa-
cyclohexane which limits the suitable application of the disclosed polymers.
Another desirable feature in polymers suitable for use in aerospace
sealants is high temperature resistance. While incorporating sulfur to carbon
bonds into a polymer generally enhances high temperature performance, the
polysulfide polyformal polymers disclosed in U.S. Pat. No. 2,466,963 have
multiple -S-S- linkages in the polymer backbones which result in compromised
thermal resistance. In the polymers of Singh et al., U.S. Pat. No. 4,366,307,
enhanced thermal stability is achieved through replacement of polysulfide
linkages with polythioether (-S-) linkages. However, the thermal resistance of
these polythioethers is limited as a result of residual acid condensation
catalyst.
Morris et al., U.S. Pat. No. 4,609,762, describes reacting dithiols with
secondary or tertiary alcohols to afford liquid polythioethers having no
oxygen
in the polymeric backbone. Cured polymeric materials formed from these
polymers have the disadvantage, however, of reduced fuel resistance due to
the large number of pendant methyl groups that are present. In addition, the
disclosed process generates undesirable aqueous acidic waste.
Cameron, U.S. Pat. No. 5,225,472, discloses production of
polythioether polymers by the acid-catalyzed condensation of dithiols with
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active carbonyl compounds such as HCOOH. Again, this process generates
undesirable aqueous acidic waste.
The addition polymerization of aromatic or aliphatic dithiols with diene
monomers has been described in the literature. See, e.g., Klemm, E. et al.,
J. Macromol. Sci.-Chem., A28(9), pp. 875-883 (1991); Nuyken, O. et al.,
Makromol. Chem., Rapid Common. 11, 365-373 (1990). However, neither
Klemm et al. nor Nuyken suggest selection of particular starting materials to
form a polymer that is liquid at room temperature and, upon curing, has
excellent low-temperature flexibility (low Tg) and high resistance to fuels,
i.e.,
hydrocarbon fluids. Nor do Klemm et al. suggest production of a polymer that
also is curable at room or lower temperatures. Moreover, the reactions
disclosed by Klemm et al. also generate undesirable cyclic byproducts.
There exists a need in the art for sealant, coating and electrical potting
formulations or compositions which can provide good pot life as well as good
performance properties, such as fuel resistance, flexural strength, thermal
resistance and longevity in use.
Summar~~ of the Invention
The present invention relates to sealant and electrical potting
formulations prepared from components comprising: (a) at least one ungelled
polymer prepared by reacting reactants comprising at least one polyvinyl
ether monomer and at least one polythiol material, the ungelled polymer being
terminated with functional groups other than a thiol group; (b) at least one
curing agent reactive with the functional group of (a); and (c) at least one
additive selected from the group consisting of fillers, adhesion promoters,
plasticizers and catalysts..
Another aspect of the present invention is a sealant formulation
prepared from components comprising: (a) at least one ungelled polymer
prepared by reacting reactants comprising diethylene glycol divinyl ether and
dimercapto dioxaoctane, the ungelled polymer being terminated with
functional groups other than a fihiol group; (b) at least one curing agent
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reactive with the reactive functional group of (a); and (c) at least one
additive
selected from the group consisting of fillers, adhesion promoters,
plasticizers
and catalysts.
Other than in the operating examples, or where otherwise indicated, all
numbers expressing quantities of ingredients, reaction conditions, and so
forth
used in the specification and claims are to be understood as being qualified
in
all instances by the term "about'. Also, as used herein, the term "polymer" is
meant to refer to oligomers, homopolymers and copolymers.
Detailed. Description of the Preferred Embodiments
The sealant and potting formulations of the present invention comprise
one or more ungelled polymers prepared from reactants comprising at least
one polyvinyl ether monomer and at least one polythiol material, the ungelled
polymer being terminated with functional groups other than a thiol group. It
has surprisingly been discovered that polythioethers prepared from the
combination of polythiol(s) with polyvinyl ether monomers) according to the
present invention results in ungelled polymers that are liquid at room
temperature and pressure and that have desirable physical and rheological
properties, and that furthermore are substantially free of malodorous cyclic
by-
products. The inventive materials also are substantially free of deleterious
catalyst residues, and can have superior thermal resistance properties.
The ungelled polymers useful in the sealant and potting formulations of
the present invention are preferably liquid at room temperature and pressure
and cured sealants including such polymers have excellent low temperature
flexibility and fuel resistance. As used herein, the term "room temperature
and pressure" denotes conditions of approximately 77°F (25°C)
and 1
atmosphere (760 mm Hg) pressure.
By"ungelled" is meant that the ungelled polymer is substantially free of
crosslinking and has an intrinsic viscosity when dissolved in a suitable
solvent, as determined, for example, in accordance with ASTM-D1795 or
ASTM-D4243. The intrinsic viscosity of the ungelled polymer is an indication
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of its finite molecular weight. A gelled reaction product, on the other hand,
since it is of essentially infinitely high molecular weight, will have an
intrinsic
viscosity too high to measure.
Preferably, the ungelled polymer has a glass transition temperature
(Tg) that is not higher than -50°C, more preferably not higher than -
55°C and
most preferably not higher than -60°C. Generally, it is preferred that
the glass
transition temperature of the ungelled polymer ranges from -85°C to-
50°C,
and more preferably -70°C to -50°C, as determined by
differential scanning
calorimetry (DSC).
Low temperature flexibility can be determined by known methods, for
example, by the methods described in AMS (Aerospace Material
Specification) 3267 ~4.5.4.7, MIL-S (Military Specification) -8802E ~3.3.12
and MIL-S-29574, and by methods similar, to those described in ASTM
(American Society for Testing and Materials) D522-88, which are incorporated
herein by reference. Cured formulations having good low temperature
flexibility are desirable in aerospace applications because the formulations
are
subjected to wide variations in environmental conditions, such as temperature
and pressure, and physical conditions such as joint contraction and expansion
and vibration.
An advantage of the formulations of the present invention is that they
exhibit very desirable fuel resistance characteristics when cured, due at
least
in part to the use of the ungelled polymers discussed herein. The fuel
resistance of a cured sealant can be determined by percent volume swell after
prolonged exposure of the cured sealant to a hydrocarbon fuel, which can be
quantitatively determined using methods similar to those described in ASTM
D792 or AMS 3269, which are incorporated herein by reference. For fuel
resistance testing, the cured sealant can be prepared from 100 parts by
weight of ungelled polymer containing functional groups other than thiol such
as an amine group or a hydroxyl group, 50 parts by weight of precipitated
calcium carbonate and a curing agent, for example an epoxy curing agent for
the amine functional polymer or a polyisocyanate for the hydroxyl functional
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polymer, in a 1:1 equivalent ratio of functional groups to coreactive
functional
groups. For the epoxy curing agent, one prepared from a 60:40 weight ratio
of EPON 828 bisphenol A diglycidyl ether (available from Shell Chemical) to
DEN 431 bisphenol A novolac resin (available from Dow Chemical) can be
used.
In a preferred embodiment, the cured sealants of the present invention
have a percent volume swell not greater than 40%, and preferably not greater
than 25% after immersion for one week at 140°F (60°C) and
ambient pressure
in jet reference fluid (JRF) type 1. More preferably, the percent volume swell
of the cured polymers is not greater than 20%, and more preferably ranges
from zero to 20%. Jet reference fluid JRF type 1, as employed herein for
determination of fuel resistance, has the following composition (see AMS
2629, issued Jul. 1, 1989), ~3.1.1 et seq., available from SAE (Society of
Automotive Engineers, Warrendale, Pennsylvania) (which is incorporated
herein by reference):
Toluene 28 t 1 % by volume
Cyclohexane (technical) 34 t 1 % by volume
Isooctane 38 t 1 % by volume
Tertiary dibutyl disulfide
(doctor sweet) 1 t 0.005% by volume
Tertiary butyl mercaptan 0.015% t 0.0015 by weight of the other
four components
Preferably, the ungelled polymer has a number average molecular
weight ranging from about 500 to about 20,000 grams per mole, more
preferably from about 1,000 to about 10,000, and most preferably from about
2,000 to about 5,000, the molecular weight being determined by gel-
permeation chromatography using a polystyrene standard.
Liquid ungelled polymers within the scope of the present invention can
be difunctional, that is, linear polymers having two end groups, or
polyfunctional, that is, branched polymers having three or more end groups.
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The ungelled polymers are prepared by reacting reactants comprising
one or more polyvinyl ether monomers and one or more polythiol materials.
The ungelled polymer has one or more reactive functional groups other than a
polythiol group, such as hydroxyl, amino, and/or vinyl groups.
Useful polyvinyl ether monomers include divinyl ethers having the
formula (V):
CHZ = CH-O-(-R2-O-)m-CH = CH2 (V)
where R2 is C2_6 n-alkylene, CZ_6 branched alkyiene, C6_$ cycloalkylene or
Cg_10
alkylcycloalkylene group or -[(CH2-)p-O-]q-(-CH2-)r- and m is a rational
number
ranging from 0 to 10, p is an independently selected integer ranging from 2 to
6, q is an independently selected integer ranging from 1 to 5 and r is an
independently selected integer ranging from 2 to 10.
The materials of formula V are divinyl ethers. Such divinyl ether
monomers as described herein can provide polymers having superior fuel
resistance and low temperature performance as compared to prior art
polymers prepared from alkenyl ether and conjugated dienes such as 1,3
butadiene copolymerized with a dithiol such as DMDS. Divinyl ether (m=0) is
operative herein. Preferred divinyl ethers include those compounds having at
least one oxyalkylene group, more preferably from 1 to 4 oxyalkylene groups
such as those compounds in which m is an integer from 1 to 4. More
preferably, m is an integer from 2 to 4. It is also possible to employ
commercially available divinyl ether mixtures in producing ungelled polymers
according to the invention. Such mixtures are characterized by a non-integral
average value for the number of alkoxy units per molecule. Thus, m in
formula V can also take on rational number values between 0 and 10.0;
preferably between 1.0 and 10.0; very preferably between 1.0 and 4.0,
particularly between 2.0 and 4Ø
Suitable polyvinyl ether monomers include divinyl ether monomers,
such as divinyl ether, ethylene glycol divinyl ether (EG-DVE) (R2=ethylene,
m=1 ), butanediol divinyl ether (BD-DVE) (R2=butylene, m=1 ), hexanediol
divinyl ether (HD-DVE) (R2=hexylene, m=1), diethylene glycol divinyl ether
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(DEG-DVE) (R2=ethylene, m=2) (preferred), triethylene glycol divinyl ether
(R2=ethylene, m=3), tetraethylene glycol divinyl ether (R2=ethylene, m=4),
cyclohexanedimethanol divinyl ether, polytetrahydrofuryl divinyl ether;
trivinyl
ether monomers such as trimethylolpropane trivinyi ether, tetrafunctional
monomers such as pentaerythritol tetravinyl ether and mixtures thereof. The
polyvinyl ether material can have one or more pendant groups selected from
alkyl groups and alkoxy groups.
Useful divinyl ethers in which R2 is C2_6 branched alkylene can be
prepared by reacting a polyhydroxy compound with acetylene. Exemplary
compounds of this type include compounds in which R2 is an alkyl-substituted
methylene group such as -CH(CH3)- (for example"PLURIOLC~' blends such
as PLURIOL~ E-200 divinyl ether (BASF Corp. of Parsippany, New Jersey),
for which RZ=ethylene and m=3.8) or an alkyl-substituted ethylene (for
example -CH2CH(CH3)- such as"DPE" polymeric blends including DPE-2 and
DPE-3 (International Specialty Products of Wayne, New Jersey)).
Other useful divinyl ethers include fluorinated compounds or
compounds in which R2 is polytetrahydrofuryl (poly-THF) or polyoxyalkylene,
preferably having an average of about 3 monomer units.
Two or more polyvinyl ether monomers of the formula V can be used in
the foregoing method. Thus in preferred embodiments of the invention, two
polythiols of formula IV (discussed below) and one polyvinyl ether monomer of
formula V, one polythiol of formula IV and two polyvinyl ether monomers of
formula V, two polythiols of formula IV and two polyvinyl ether monomers of
formula V, and more than two compounds of one or both formulas, can be
used to produce a variety of polymers according to the invention, and all such
combinations of compounds are contemplated as being within the scope of
the invention.
Generally, the polyvinyl ether monomer comprises greater than 50
mole percent of the reactants used to prepare the ungelied polymer.
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Suitable polythiol materials for preparing the ungelled polymer include
compounds, monomers or polymers having at least two thiol groups. Useful
polythiols include dithiols having the formula (IV):
H S-R'-S Fi ( I V)
where R' can be a C2_6 n-alkylene group; C3_6 branched alkylene group,
having one or more pendant groups which can be, for example, hydroxyl
groups, alkyl groups such as methyl or ethyl groups; alkoxy groups, C6_$
cycloalkylene; C6_~o alkylcycloalkylene group; -[(-CH2)p-X]q-(-CH2)~-; or
-[(-CH2)p-X]q-(-CH2)~- in which at least one -CH2- unit is substituted with a
methyl group and in which p is an independently selected integer ranging from
2 to 6, q is an independently selected integer ranging from 1 to 5 and r is an
independently selected integer ranging from 2 to 10.
Further preferred dithiols include one or more heteroatom substituents
in the carbon backbone, that is, dithiols in which X includes a heteroatom
such as O, S or another bivalent heteroatom radical; a secondary or tertiary
amine group, i.e., -NR6-, where R6 is hydrogen or methyl; or another
substituted trivalent heteroatom. In a preferred embodiment, X is O or S, and
thus R~ is -[(-CH2-)P-O-]q -(-CH2-)~- or -[(-CH2-)P-S-]q-(-CH2-)~-.
Preferably, p
and r are equal, and most preferably both have the value of 2.
Useful polythiols include but are not limited to dithiols such as 1,2-
ethanedithiol, 1,2-propanedithiol, 1,3-propanedithiol, 1,3-butanedithiol, 1,4-
butanedithiol, 2,3-butanedithiol, 1,3-pentanedithiol, 1,5-pentanedithiol, 1,6-
hexanedithiol, 1,3-dimercapto-3-methylbutane, dipentenedimercaptan,
ethylcyclohexyldithiol (ECHDT), dimercaptodiethylsulfide, methyl-substituted
dimercaptodiethylsulfide, dimethyl-substituted dimercaptodiethylsulfide,
dimercaptodioxaoctane, 1,5-dimercapto-3-oxapentane and mixtures thereof.
The polythiol material can have one or more pendant groups selected from
lower alkyl groups, lower alkoxy groups and hydroxyl groups. Suitable alkyl
pendant groups include C~-C6 linear alkyl, C3-C6 branched alkyl, cyclopentyl,
and cyclohexyl.
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Preferred dithiols include dimercaptodiethylsulfide (DMDS) (p= 2, r=2,
q=1, X=S); dimercaptodioxaoctane (DMDO) (p=2, q=2, r=2, X=0); and 1,5-
dimercapto-3-oxapentane (p=2, r=2, q=1, X=O). It is also possible to use
dithiols that include both heteroatom substituents in the carbon backbone and
pendant alkyl groups, such as methyl groups. Such compounds include
methyl-substituted DMDS, such as HS-CH2CH(CH3)-S-CH2CH2-SH, HS-
CH(CH3)CH2-S-CHZCH2-SH and dimethyl substituted DMDS such as HS-
CH2CH(CH3)-S-CH(CH3)CH2-SH and HS-CH(CH3)CH2-S-CH2CH(CH3)-SH.
Two or more different polythiols can be used if desired to prepare
useful polythioethers.
Preferably, the polythiol material has a number average molecular
weight ranging from 90 to 1000 grams per mole, and more preferably 90 to
500 grams per mole.
Relative amounts of dithiol and divinyl ether materials used to prepare
the polymers can be chosen to yield reactive functional groups such as
terminal vinyl groups that further can be reacted to provide other reactive
functional groups such as hydroxyl groups or amino groups in a manner
discussed below. Preferably, the equivalent ratio of divinyl to polythiol
compounds is greater than 1:1 (i.e. greater than 50 mole percent, with the
thiol material preferably being less than 50 mole percent), resulting in a
vinyl-
terminated polymer that can be reacted to incorporate other functional groups,
such as hydroxyl, amino, and/or epoxy.
Hydroxyl- or amino-functional terminal polymers can be produced, for
example, by reacting a vinyl terminated material with mercaptoalcohols such
as 3-mercaptopropanol or mercaptoamines such as 4-mercaptobutylamine,
respectively.
Preferably, the ungelled polymer comprises 30 to 90 weight percent of
the sealant formulation on a basis of total weight of the sealant formulation,
and more preferably 30 to 60 weight percent.
The reactants from which the ungelled polymers are prepared can
further comprise one or more free radical catalysts. Preferred free radical
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catalysts include azo compounds, for example azobis-nitrite compounds such
as azo(bis)isobutyronitrile (AIBN)9 organic peroxides such as benzoyl
peroxide and t-butyl peroxide; inorganic peroxides and similar free-radical
generators. The reaction can also be effected by irradiation with ultraviolet
light either with or without a cationic photoinitiating moiety. Ionic
catalysis
methods, using either inorganic or organic bases, e.g., triethylamine, also
yield materials useful in the context of this invention.
Ungelled polymers within the scope of the present invention can be
prepared by a number of methods. According to a first preferred method, a
material having the formula IV:
HS-R'-SH (IV)
or a mixture of at least two different compounds having the formula IV, are
reacted with a material having the formula V:
CH2 = CH-O-(-R2-O-)",-CH = CH2 (V)
or a mixture of at least two different compounds having the formula V, in the
presence of a catalyst in a CH2=CH-/-SH equivalent ratio greater than 1. This
method provides an uncapped, vinyl-terminated difunctional polymer.
According to a preferred embodiment, a difunctional vinyl-terminated
polymer is prepared. Thus, the polythioether has the following structure:
CH2 = CH -[- R~ - S -(CHz)2 - O -[- R2 - O -]m- (CHz)2 - S - R~ -]"- CH=CHZ
In a preferred embodiment, R~=-[(-CH2)p-X]q-(-CH2)~-, where p=2, X=O,
q=2 and r=2, R2 is ethylene group, m=2 and n is about 9.
The foregoing polymers are produced, for example, by reacting a
stoichiometric excess of divinyl ether or mixture thereof with dithiol or
mixture
thereof, as discussed in detail below.
Preferably, the ungelled polymers are essentially free of sulfone, ester
or disulfide linkages, and more preferably free of such linkages. The absence
of these linkages can provide good fuel and temperature resistance and good
hydrolytic stability. As used herein, "essentially free of sulfone, ester or
disulfide linkages" means that less than 2 mole percent of the linkages in the
ungelled polymer are sulfone, ester or disulfide linkages. Disulfide linkages
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are particularly susceptible to thermal degradation, sulfone linkages are
particularly susceptible to hydrolytic degradation.
Besides vinyl groups, the ungelled polymer can contain other functional
groups which can be prepared by reacting the vinyl terminated polymers with
one or more functionalizing agents.
The term "functionalizing agenf' as employed herein denotes a
compound having one moiety that is reactive with -CH=CH2 groups and one
moiety that contains at least one functional group such as hydroxyl and
amino, that is not reactive or is comparatively less reactive with -CH=CH2
groups.
Inventive polymers as described above have a wide range of average
functionality. For example, functionalizing agents can be chosen to give
average functionalities from about 2.05 to 3.0, preferably about 2.1 to 2.6.
Wider ranges of average functionality can be achieved by using
tetrafunctional or higher functionality. The functionalizing agent can be
reacted with the vinyl terminated polymer. Typically, the reaction product is
prepared in one step. The polyvinyl monomer, polythiol and functionalizing
agent are reacted together, with this stoichiometry and reaction conditions
being controlled, so as to obtain an ungelled polymer with the desired
functionality and molecular weight.
Preferably, the ungelled polymer has a viscosity of less than about 500
poise at a temperature of about 25°C and a pressure of about 760 mm Hg
determined according to ASTM D-2849 para. 79-90 using a Brookfield
viscometer.
The ungelled polymer or combination of ungelled polymers as detailed
herein preferably is present in the polymerizable sealant composition in an
amount from about 30 wt% to about 90 wt%, more preferably about 40 to
about 80 wt%, very preferably about 45 to about 75 wt%, with the wt% being
calculated based on the weight of total solids of the composition.
The sealant or potting formulations of the present invention further
comprise one or more curing agents that contain reactive functional groups
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that are reactive with the functional groups associated with the ungelled
polymer. Useful curing agents include polythiols, such as those mentioned
above, for the vinyl terminated ungelled polymers; polyisocyanates such as
isophorone, diisocyanate, and hexamethylene diisocyanate including mixtures
thereof and including isocyanurate derivatives thereof, and polyepoxides for
amine terminated ungelled polymers. Examples of polyepoxides include
hydantoin diepoxide, bisphenol-A epoxides, bisphenol-F epoxides, novolac
type epoxides, aliphatic polyepoxides, and any of the epoxidized unsaturated
and phenolic resins.
Depending on the nature of the ungelled polymers) used in the
composition, the equivalent ratio of curing agent to ungel(ed polymer can be
from 0.05-1.5/1, preferably 0.1-1/1.
Fillers useful in the polymerizable compositions of the invention for
aerospace application include those commonly used in the art, such as
carbon black and calcium carbonate (CaC03). Potting compound fillers
illustratively include high band gap materials such as zinc sulfide and
inorganic barium compounds. Preferably, the compositions include about 10
to about 70 wt% of the selected filler or combination of fillers, more
preferably
about 10 to 50 wt% based upon the total weight of the composition.
The sealant and potting compositions of the present invention can
comprise one or more adhesion promoters. Suitable adhesion promoters
include phenolics such as METHYLON phenolic resin available from
Occidental Chemicals, organosilanes such as epoxy, mercapto or amino
functional silanes such as A-187 and A-1100 available from OSi Specialities.
Preferably, an adhesion promoter is employed in an amount from 0.1 to 15
wt% based upon total weight of the formulation.
Common substrates to which the sealant compositions of the present
invention are applied can include titanium, stainless steel, aluminum,
anodized, primed, organic coated and chromate coated forms thereof, epoxy,
urethane, graphite, fiberglass composite, KEVLAR~, acrylics and
polycarbonates.
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Preferably, a plasticizer is present in the sealant formulation in an
amount ranging from 1 to 8 weight percent based upon total weight of the
formulation. Plasticizers that are useful in polymerizable compositions of
fihe
invention include phthalate esters, chlorinated paraffins, hydrogenated
terphenyls, etc.
The formulation can further comprise one or more organic solvents,
such as isopropyl alcohol, in an amount ranging from 0 to 15 percent by
weight on a basis of total weight of the formulation, preferably less than 15
weight percent and more preferably less than 10 weight percent.
The foregoing sealant or potting formulations preferably are cured at
ambient temperature and pressure, however the formulations generally can
be cured at a temperature ranging from about 0°C to about 100°C.
In addition to the foregoing ingredients, polymerizable sealant
compositions of the invention can optionally include one or more of the
following: pigments; thixotropes; retardants; catalysts; and masking agents.
Useful pigments include those conventional in the art, such as carbon
black and metal oxides. Pigments preferably are present in an amount from
about 0.1 to about 10 wt% based upon total weight of the formulation.
Thixotropes, for example fumed silica or carbon black, are preferably
used in an amount from about 0.1 to about 5 wt% based upon total weight of
the formulation.
An additional advantage of sealant formulations according to the
invention is their improved curing behavior. The extent of cure of a sealant
formulation as a function of time is often difficult to measure directly, but
can
be estimated by determining the extrusion rate of the composition as a
function of time. The extrusion rate is the rate at which a mixed sealant
formulation, i.e., a sealant formulation together with an accelerator system,
is
extruded from an applicator device. As the sealant formulation is mixed with
the accelerator system, curing begins, and the extrusion rate changes with
time. The extrusion rate thus is inversely related to the extent of cure. When
the extent of cure is low, the viscosity of the mixed ungelled sealant
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formulation is low and thus the extrusion rate is high. When the reaction
approaches completion, the viscosity becomes very high, and the extrusion
rate thus becomes low. The extrusion rate can be measured according to
AMS Method 3276 (section 4.5.10), which is incorporated herein by reference.
Sealant formulations according to the present invention can have,
depending on the particular formulation, initial extrusion rates as high as
500
g/min or higher, together with low extrusion rates on the order of about 5 to
10
g/min or less after curing times on the order of one hour.
Another preferred curable sealant formulation combines one or more
plasticizers with the ungelled polymer(s), curing agents) and fillers)
described above. Use of a plasticizer allows the polymerizable formulation to
include ungelled polymers which have higher Tg than would ordinarily be
useful in an aerospace sealant or potting compound, i.e., use of a plasticizer
effectively reduces the T9 of the formulation, and thus increases the low-
temperature flexibility of the cured polymerizable formulation beyond that
which would be expected on the basis of the T9 of the ungelled polymers
alone.
The compositions of the present invention are useful in aerospace
applications such as aerospace sealants and linings for fuel tanks; and as
electrical potting or encapsulant compounds. An aerospace sealant material
according to the present invention can exhibit properties including high
temperature performance, fuel resistance and flexural strength. The
formulations detailed herein are well suited for use as potting compounds to
encapsulate electrical and electronic components which can experience
temperature extremes, chemically harsh environments and mechanical
vibrations.
The foregoing description is illustrative of particular embodiments of the
invention, but is not meant to be a limitation upon the practice thereof. The
following claims, including all eguivalents thereof, are intended to define
the
scope of the invention.
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