 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a solvent system and to the use of the
solvent system for dissolving one or more curing agents for epoxy resins
and/or one or more curing catalysts and/or one or more cure inhibitors.
It is a well known technique to utilize an organic solvent for reducing the
viscosity of liquid epoxy resins or liquid curing agents therefore or to
solubilize solid resins and/or curing agents in organic solvents in order
to facilitate the handling of the epoxy resins and/or of the curing
agents.
U.S. Pat. No. 3,679,465 teaches the production of reinforced, hardenable
epoxy compositions by continuously passing a reinforcement material
through an epoxy solution system, thereby impregnating the reinforcement
material with the epoxy solution system. The impregnated reinforcement
material is subjected to a heat treatment. The epoxy solution system
contains a curable epoxy resin, a curing agent therefore and a low boiling
solvent. Various types of conventional curing agents or hardeners as well
as accelerators are listed, such as primary and secondary amines, amides,
polyamines, polyamides, dicyandiamide, benzoguanamine, imidazole,
tetramethyl diamine, etc. As useful low boiling organic solvents are
mentioned acetone or methyl ethyl ketone, or a mixture of dimethyl
formamide, acetone and water.
It is known that mixtures containing an epoxy resin and certain curing
agents therefore, such as dicyandiamide, have excellent storage stability
at 20.degree. C. Therefore, these mixtures are useful for producing
so-called "one-component systems" which cure upon heating.
East German patent 134 446 teaches that a major disadvantage of known
one-component systems is the low solubility of dicyandiamide in the
solvents which are used in these systems. The amount of dicyandiamide
required for curing the system is often close to or even above its
saturation concentration in the solvent. Precipitation of dicyandiamide
from the one-component system at low temperature, such as 0.degree. C., is
quite frequent. In order to overcome this disadvantage, the East German
patent suggests the production of a one-component system by dissolving an
epoxy resin and dicyandiamide in a mixture consisting of a glycol ether
and water. Water is used in an amount of from 1 to 30 percent, preferably
5 to 15 percent, by the weight of the glycol ether. Useful glycol ethers
are said to be ethyl glycol and/or methyl glycol.
WORLD PATENT INDEX, Abstract No. 79-26180B by Derwent Publications Ltd.,
which abstracts East German patent 133 955, discloses that a hardener for
epoxy resins consists of dicyandiamide dissolved in a solvent mixture of
glycol ethers, especially ethyl glycol and/or methyl glycol, containing 1
to 50, preferably 5 to 20 wt. -percent, water. It is disclosed that the
presence of water doubles the dicyandiamide solubility. Presently, the
monomethyl ether and monoethyl ether of ethylene glycol as well as
dimethyl formamide are widely used in the industry for dissolving epoxy
hardeners like dicyandiamide.
However, the wide use of ethylene glycol monomethyl ether, ethylene glycol
monoethyl ether and dimethyl formamide has raised some concern among
environmentalists.
It is known from Swiss patent 257,115 to heat an epoxy resin in the
presence of a solvent with dicyandiamide and an aldehyde condensation
product containing etherified methylol groups, such as alkyl ethers of
methylolmelamine or of methylolureas. It is suggested to dissolve the
epoxy resin in an organic solvent, such as 2-methyl-pentanediol-(2,4),
toluene, cyclohexanol, etc. and to add the other components.
Alternatively, the epoxy resin can be combined with a mixture of
dicyandiamide, aldehyde condensation product and solvent. From 2 to 20
percent, preferably from 6 to 10 percent, of dicyandiamide is used for
curing, based on the weight of the epoxy resin. The Swiss patent teaches
that the amount of the aldehyde condensation product must be high enough
to enable a complete absorption of dicyandiamide in the resin solution.
Lacquer resin solutions are produced which can be used for applying coats
of lacquer on metals. These coats of lacquer have good properties, such as
good adherence to metals and high resistance to chemicals.
WORLD PATENT INDEX LATEST, Abstract No. 91-003223 by Derwent Publications
Limited, abstracting published Japanese patent application JP-A-2,279,776
discloses an ink composite containing an epoxy resin, a hardening agent
and a solvent. Dicyandiamide, imidazole compounds, triazine compounds,
urea compounds, aromatic amine compounds and one or more kinds of
photo-cationic polymerization catalysts are used as a hardening agent. The
compound of formula R.sup.1 --(OR.sup.2).sub.n OR.sup.3 and one or more
kinds of solvent naphtha are used as a solvent. R.sup.1 is hydrogen or
C.sub.1-8 -alkyl, R.sup.2 is methylene or ethylene, R.sup.3 is hydrogen or
--C(O)--R.sup.4 wherein R.sup.4 is C.sub.1-8 -alkyl and n is 1 to 4.
Published German patent applications DE-A-2,545,149 and DE-A-2,650,408
suggest the use of a mixture of diacetone alcohol and water for dissolving
dicyandiamide. It is recommended to dissolve 3 weight parts of
dicyandiamide in 65 weight parts of diacetone alcohol and 13.3 weight
parts of water. However, trials made by the Applicants have shown that
dicyandiamide dissolves very slowly in the suggested mixture of diacetone
alcohol and water. Apparently no more than 3 weight parts of dicyandiamide
can be dissolved in the suggested solvent mixture. Furthermore, upon
addition of an epoxy resin solution described in detail in the Examples
further below, the dicyandiamide solution in diacetone alcohol and water
turns turbid. The turbidity is an indication that dicyandiamide
crystallizes.
The published Belgian patent application BE-A-677,466 discloses an epoxy
resin and 1-cyanoguanidine dissolved in a solvent mixture of a low boiling
point. The solvents in the solvent mixture are mainly polar. One solvent
mainly serves to dissolve the epoxy resin. Acetone and/or ethyl acetate
are suggested. The other solvent mainly serves to dissolve
1-cyanoguanidine. Water or a mixture of water and acetone or a low boiling
alcohol, such as methanol and/or ethanol are suggested. However, trials
made by the applicants have shown that the solubility of the expoxy resin
and 1-cyanoguanidine in several of the suggested solvents, such as water,
methanol, ethanol or acetone, is undesirably low.
Accordingly, one object of the present inventions is to find a new solvent
system. Another object of the present invention is to find a solvent
system which is useful for dissolving a curing agent for an epoxy resin,
for dissolving a curing catalyst or a cure inhibitor for dissolving a
blend thereof.
Applicants have found that an essentially water-free mixture comprising
methoxy acetone and an organic protic solvent is a very useful solvent for
a curing agent, such as a dicyandiamide, or for a curing catalyst, such as
an imidazole, or for a cure inhibitor, such as boric acid, or for a
mixture of two or more of such compounds.
Accordingly, one aspect of the present invention is an essentially
water-free solvent system comprising methoxy acetone and an organic protic
solvent.
Another aspect of the present invention is a method of preparing a solution
of one or more compounds selected from the group consisting of
a) curing agents for an epoxy resin,
b) curing catalysts, and
c) cure inhibitors,
in the indicated solvent system by contacting one or more such compounds
with a solvent system comprising methoxy acetone and an organic protic
solvent.
Yet another aspect of the present invention is a solution containing the
indicated solvent system and one or more such compounds.
Yet another aspect of the present invention is an epoxy resin composition
containing the indicated solution of the present invention and an epoxy
resin.
The essentially water-free solvent system comprises methoxy acetone and one
or more organic protic solvents. By the term "essentially water-free" is
meant that the solvent system does not contain an essential amount of
water. This means that the water content of the solvent system is not more
than about 1 percent, preferably not more than about 0.1 percent, more
preferably not more than about 0.01 percent, by the total weight of the
solvent system. Most preferably, the solvent system does not contain any
extractable amount of water.
The weight percentage of methoxy acetone in the solvent system of the
present invention preferably is from about 25 to about 95 percent, more
preferably from about 30 to about 65 percent, most preferably from about
40 to about 60 percent, by the total weight of the solvent system. Methoxy
acetone, also named 1-methoxy-2-propanone, has the chemical formula
H.sub.3 C--C(O)--CH.sub.2 --OCH.sub.3. Methoxy acetone has a boiling point
of about 114.degree. C. at atmospheric pressure. The boiling point of the
solvent system depends on the amount and kind of the organic protic
solvent and of any optional compounds present. Generally, the solvent
system has a boiling point of from 100.degree. to 150.degree. C.,
preferably from 105 to 140.degree. C. This is very favorable because on
one side the boiling point is low enough to allow removal of the boiling
system after usage without substantial difficulties and on the other hand
the boiling point is high enough that premature evaporation of the solvent
system and a resulting quality loss generally is not experienced.
Methoxy acetone is a known compound. The author of JP-A-2,279,776 believes
that methoxy acetone forms its peroxide under mild conditions. It is
suggested to use peroxide-containing methoxy acetone as a radical
polymerization initiator. The peroxide-containing methoxy acetone is said
to have high dissolving power. Equilibrium constants for addition of water
and 3-mercaptopropionic acid to various methyl ketones including methoxy
acetone in D.sub.2 O have been made by Th. J. Burkey et al., "Equilibrium
studies of water and thiol addition to ketones: substituent and solvent
effects for methyl ketones"; Journal of the American Chemical Society,
vol. 105, no. 4, 23 Feb. 1983, pages 868-871. The potential biological
importance of thiol addition to ketones is discussed in the article.
Preferably, the solvent system comprises from about 5 to about 75 percent,
preferably from about 10 to about 70 percent, most preferably from about
10 to about 60 percent of one or more organic protic solvents, based on
the total weight of the solvent system. If the solvent system comprises a
blend of two or more organic protic solvents, the indicated percentages
refer to their total weight. As will be described further below, the
solvent system of the present invention may comprise other components in
addition to methoxy acetone an an organic protic solvent. Generally, the
solubility of an epoxy curing agent, such as dicyandiamide, or of a curing
catalyst, such as an imidazole, in the solvent system increases when the
content of the organic protic solvent in the solvent system increases.
However, if the content of the organic protic solvent in the solvent
system is too high, the compound dissolved in the solvent system tends to
precipitate if the solution of the compound is mixed with an epoxy resin.
Useful saturated, aliphatic unsaturated or aromatic organic protic solvents
generally contain one or more hydroxyl, mercapto, or acid groups, such as
carboxyl. Preferably, the organic protic solvent contains one or two, more
preferably only one, hydroxyl, mercapto or acid group. The protic organic
solvent should be inert, i.e. at the chosen conditions for dissolving a
curing agent, curing catalyst and/or cure inhibitor in the solvent system,
it should neither undergo a chemical reaction with such compounds to an
essential degree nor should it react with an epoxy resin which is chosen
for preparing an epoxy resin composition. Based on the present teaching
the skilled artisan knows how to select an organic protic solvent that
does not react with the chosen curing agent, curing catalyst, cure
inhibitor and/or epoxy resin.
Preferred organic protic solvents are monofunctional alcohols, such as
saturated open-chain or cyclic alcohols, preferably methanol, ethanol, the
propanols, such as n-propanol or isopropanol, the butanols, such as
n-butanol or isobutanol, the hexanols, such as n-hexanol or cyclohexanol,
the heptanols, octanols, decanols, dodecanols, such as lauryl alcohol, or
the octadecanols, such as stearyl alcohol; or unsaturated alcohols,
preferably the allyl or furfuryl alcohol. The alcohols preferably have 1
to 12, more preferably 1 to 8, most preferably 1 to 4 carbon atoms. Other
useful alcohols are glycols or glycol monoethers. Preferred glycols are
ethylene, propylene or butylene glycols, such as ethylene glycol,
diethylene glycol, propylene glycol, dipropylene glycol, butylene glycol
or dibutylene glycol.
Preferred glycol monoethers are the propylene or butylene glycol
monoethers, most preferably ethers represented by formula I
R.sub.1 O--(CH.sub.2 --CHR.sup.3 O).sub.n --R.sub.2 (I)
wherein
one of the substituents R.sub.1 and R.sub.2 is an alkyl group having from 1
to 12, preferably from 1 to 6, more preferably from 1 to 4 carbon atoms
and the other of the substituents R.sub.1 and R.sub.2 is hydrogen,
R.sup.3 in each occurrence independently is methyl or ethyl and
n is from 1 to 4, preferably 1, 2 or 3.
The alkyl groups may be branched or unbranched. Exemplary of the alkyl
groups are methyl, ethyl, n-propyl, isopropyl, the butyl groups, such as
n-butyl or isobutyl, and the pentyl, hexyl, octyl, decyl or dodecyl
groups. Of the propyl and butyl groups n-propyl and n-butyl are preferred.
Preferably, one of the substituents R.sub.1 and R.sub.2 independently is
methyl or n-butyl.
Preferred monoethers of formula I are propylene glycol methyl ether,
propylene glycol n-butyl ether, dipropylene glycol methyl ether and
dipropylene glycol n-butyl ether.
Corresponding thiols containing one or more mercapto groups instead of one
or more hydroxyl groups in the above-mentioned organic protic solvents are
useful as well, such as 1,3-propane dithiol, ethane thiol or propane
thiol.
Preferred acid groups are sulfenic, sulfinic or sulfonic acid groups,
phosphinic or phosphonic acid groups or the carboxyl group. Carboxylic
acids, such as formic acid, acetic acid, propionic acid, butanoic acid or
pentanoic acid are preferred.
The preferred organic protic solvent are methanol, ethylene glycol and,
most preferably, ethanol.
The solvent system of the present invention may comprise one or more
organic protic solvents.
The solvent system may additionally comprise one or more aprotic
oxygen-containing solvents other than methoxy acetone. Preferably, methoxy
acetone, the organic protic solvent(s) and, if present, the aprotic
oxygen-containing solvent other than methoxy acetone together amount to
about 90 percent or more, more preferably to about 95 percent or more,
most preferably to about 99 percent or more, of the total weight of the
solvent system. Most preferably, the solvent system consists essentially
of methoxy acetone, the organic protic solvent(s) and, optionally, the
aprotic oxygen-containing solvent(s) other than methoxy acetone. In this
case the above indicated ranges of the content of the organic protic
solvent are based on the total weight of methoxy acetone, the organic
protic solvent(s) and aprotic oxygen-containing solvent(s) other than
methoxy acetone, if present.
Preferably, the weight ratio between the aprotic oxygen-containing solvent
(other than methoxy acetone), if present, and methoxy acetone is from
about 0.1:1 to about 2:1, more preferably from about 0.3:1 to about
1.5:1, most preferably from about 0.5:1 to about 1:1. Preferably, the
weight ratio between the aprotic oxygen-containing solvent, if present,
and the protic organic solvent is from about 0.01:1 to about 30:1, more
preferably from about 1:1 to about 20: 1, most preferably from about 2:1
to about 6:1. If the solvent system of the present invention does not
comprise an aprotic oxygen-containing solvent (other than methoxy acetone)
it preferably comprises from about 25 to about 95 percent, more preferably
from about 30 to about 65 percent and most preferably from about 40 to
about 60 percent of methoxy acetone and preferably from about 5 to about
75 percent, more preferably from about 35 to about 70 percent and most
preferably from about 40 to about 60 percent of an organic protic solvent.
If the solvent system comprises an aprotic oxygen-containing solvent in
addition to methoxy acetone, it preferably comprises from about 25 to
about 80 percent, more preferably from about 30 to about 65 percent and
most preferably from about 40 to about 60 percent of methoxy acetone,
preferably from about 5 to about 60 percent, more preferably from about 10
to about 45 percent, most preferably from about 10 to about 20 percent of
an organic protic solvent and preferably from about 15 to about 70
percent, more preferably from about 25 to about 60 percent and most
preferably from about 30 to about 50 percent of an aprotic
oxygen-containing solvent (other than methoxy acetone).
Useful aprotic oxygen-containing solvents generally contain one or more
ether or carbonyl functionalities. Aldehydes and ketones are preferred,
such as formaldehyde, acetaldehyde, acetone, methyl ethyl ketone, methyl
iso-butyl ketone or cyclohexanone.
Other useful aprotic oxygen-containing solvents are ethers, preferably
propylene or butylene glycol diethers, most preferably ethers represented
by formula II
R.sub.4 O--(CH.sub.2 --CHR.sup.3 O).sub.n --R.sub.5 (II)
wherein
the substituents R.sub.4 and R.sub.5 each independently are an alkyl group
having from 1 to 12, preferably from 1 to 6, more preferably from 1 to 4
carbon atoms,
R.sup.3 in each occurrence independently is methyl or ethyl and
n is from 1 to 4, preferably 1, 2 or 3.
Preferred alkyl groups are listed above with reference to formula I.
Preferred diethers of formula II are propylene glycol dimethyl ether,
propylene glycol methyl n-butyl ether, dipropylene glycol dimethyl ether
and dipropylene glycol methyl n-butyl ether.
The solvent system may comprise other additives, such as viscosity
modifiers, for example N-methyl pyrrolidone, thickeners, for example high
molecular polyalkylene glycols, or plasticizers, for example dioctyl
phthalate or chlorinated paraffin. If present, their amount preferably is
from about 0.1 to about 9 percent, more preferably from about 1 to about 4
percent, based on the total weight of the solvent system.
The solvent system of the present invention is very useful as a solvent for
a) a curing agent for an epoxy resin; and/or
b) a curing catalyst; and/or
C) a cure inhibitor.
Curing agents for epoxy resins, commonly also called epoxy hardeners, are
well known in the art. Useful classes of curing agents are for example
amides, acid anhydrides, boron trifluoride complexes, dicyandiamide,
substituted dicyandiamides, polyester resins, novolacs or phenolic
hardeners, i.e. compounds containing more than one aromatic hydroxyl
group. Another class of curing agents well known in the art comprises
prereacted adducts of epoxy resins with amines or anhydrides or
dicyandiamide or phenolic resins. Preferred phenolic hardeners are
described on pages 6-8 of European patent specification 0,240,565, the
teaching of which is incorporated herein by reference. Other known curing
agents are primary or secondary amines, hydrazides or hydrazine,
preferably the multifunctional, more preferably the di- to hexafunctional
primary amines, amides and hydrazides. Such curing agents are listed on
column 5, lines 47-68 and column 6, lines 14-19 of U.S. Pat. No.
4,789,690, the teaching of which is incorporated herein by reference.
Further useful curing agents are listed on page 11, lines 41-58 and page
12, lines 1-40 of the published European patent application
EP-A-0,458,502, the teaching of which is incorporated herein by reference.
Other preferred curing agents are cyanamide, dicyanamide, derivatives of
cyanamide or dicyanamide, dihydroxy phenols, biphenols, halogenated
bisphenols, alkylated bisphenols, trisphenols, phenol-aldehyde resins,
halogenated phenol-aldehyde novolac resins, alkylated phenol-aldehyde
novolac resins, hydrocarbonphenol resins, hydrocarbon-halogenated phenol
resins, hydrocarbon-alkylated phenol resins or a combination of two or
more thereof.
The solvent system of the present invention is particularly useful for
dissolving a dicyandiamide, such as a substituted dicyandiamide or
non-substituted dicyandiamide (cyanoguanidine). The low solubility of
dicyandiamide in other, known solvents or solvent compositions is well
known. Therefore, there was a particular need for providing a solvent
system in which dicyandiamide can be dissolved to a sufficient extent. It
has been found that the solvent system of the present invention is a very
good solvent for dicyandiamide. The solvent system of the present
invention is also useful for dissolving substituted dicyandiamides, such
as dicyandiamides wherein some, but not all, hydrogens bonded to a
nitrogen are replaced by alkyl, preferably C.sub.1-6 -alkyl, more
preferably methyl, ethyl or the propyl groups; or by aryl, preferably
benzyl, more preferably 2-methylbenzyl. Preferably, the dicyandiamide
carries only one of the above-listed substituents. Most preferably, the
dicyandiamide is not substituted.
The solvent system of the present invention is useful for dissolving two or
more of the above-mentioned curing agents for epoxy resins. The term "a
curing agent" as used herein also comprises mixtures of two or more
compounds which act as a curing agent for an epoxy resin.
If the solvent system is used for dissolving a curing agent, preferably
from about 1 to about 20 percent, more preferably from about 2 to about 15
percent, most preferably from about 3 to about 12 percent of a curing
agent is dissolved in the solvent system, by the weight of the solvent
system. It is to be understood that the solubility of the curing agent in
the solvent system of the present invention depends on various factors,
such as the type of the curing agent, the specific composition of the
solvent system and the amounts and types of compounds which may
additionally be dissolved in the solvent system, such as a curing catalyst
or a cure inhibitor. The solubility of a specific curing agent in a
specific solvent system of the present invention can be evaluated by
series trials.
Curing catalysts or curing accelerators which increase the speed of
reaction between the curing agent and the epoxy resin are also well known
in the art.
Curing catalysts or curing accelerators which increase the speed of
reaction between the curing agent and the epoxy resin are also well known
in the art. Preferred are tertiary amine-containing or heterocyclic amine
containing compounds. Some curing catalysts may have an effect as a curing
agent per se, such as benzoguanamidine, imidazoles, benzodimethylamine,
metaphenolene diamine, or N,N,N',N'-tetramethyl-1,3-butadiamine. Preferred
imidazoles are 2-methyl imidazole, 2-ethyl-4-methyl-imidazole or
2-phenylimidazole. 2-Methyl imidazole is the most preferred curing
catalyst. Other curing catalysts which may be dissolved in the solvent
system of the present invention are heterocyclic nitrogen compounds,
phosphines, sulfides or ammonium, phosphonium or sulfonium containing
compounds. Such curing catalysts are listed on page 12, lines 41-50, on
pages 13-16 and on page 17, lines 1-22 of the published European patent
application EP-A-0,458,502, the teaching of which is incorporated herein
by reference. Exemplary of such curing catalysts are ethyltriphenyl
phosphonium acetate, ethyltriphenyl phosphonium acetate.acetic acid
complex, tetrabutyl phosphonium acetate, tetrabutyl phosphonium
acetate.acetic acid complex, ethyltriphenyl phosphonium chloride, ethyl
triphenyl phosphonium iodide, tetrabutyl phosphonium chloride, tetrabutyl
phosphonium iodide, tetrabutylphosphonium hydroxide, tetramethylammonium
hydroxide, ethyltri(2-ethoxyethyl)ammonium hydroxide,
triethyl(2-thioethylethyl)ammonium hydroxide,
N-methyl-N-methylenemethanaminium acetate,
N-methyl-N-methylenemethanaminium acetate.acetic acid
complex,-N-methyl-N-methylenemethanaminium chloride,
N-methyl-N-methylenemethanaminium iodide, N-methylpyridinium acetate,
N-methylpyridinium acetate.acetic acid complex,N-methylpyridinium
chloride, N-methylpyridinium iodide, 1-ethyl-2,3-dimethylimidazolium
acetate, 1-ethyl-2,3-dimethylimidazolium acetate.acetic acid complex,
1-ethyl-2,3-dimethylimidazolium chloride, 1-ethyl-2,3-dimethylimidazolium
iodide, N-methylquinolinium acetate, N-methylquinolinium acetate.acetic
acid complex, N-methylquinolinium chloride, N-methylquinolinium iodide,
N-methyl-1,3,5-triazinium acetate, N-methyl-1,3,5-triazinium
acetate.acetic acid complex, N-methyl-1,3,5-triazinium chloride,
N-methyl-1,3,5-triazinium iodide, ethylamine, diethylamine, triethylamine,
n-propylamine, di-n-propylamine, tri-n-propylamine, isopropylamine,
diisopropylamine, triisopropylamine, butylamine, dibutylamine,
tributylamine, methyldibutylamine or an imidazole, an imidazolidine, an
imidazoline, an oxazole, a pyrrole, a thiazole, a pyridine, a pyrazine, a
morpholine, a pyridazine, a pyrimidine, a pyrrolidine, a pyrazole, a
quinoxaline, a quinoazoline, a phthalozine, a quinoline, a purine, an
indazole, an indole, an indolazine, a phenazine, a phenarazine, a
phenothiazine, a pyrroline, an indoline, a piperidine, a piperazine; or a
combination of two or more thereof.
The solvent system of the present invention is useful for dissolving two or
more of the above-mentioned curing catalysts. The term "a curing catalyst"
as used herein also comprises mixtures of two or more compounds which
influence the speed of reaction between an epoxy resin and an epoxy
hardener.
If the solvent system is used for dissolving a curing catalyst, generally
from about 0.1 to about 40 percent, preferably from about 0.5 to about 35
percent, more preferably from about 1 to about 20 percent and most
preferably from about 2 to about 12 percent of a curing catalyst is
dissolved in the solvent system, by the weight of the solvent system. It
is to be understood that the solubility of the curing catalyst in the
solvent system of the present invention depends on various factors, such
as the type of curing catalyst, the amount and type of curing agent which
is optionally present in the solvent system, the specific composition of
the solvent system and the amounts and types of compounds which may
additionally be dissolved in the solvent system. The solubility of a
specific curing catalyst in a specific solvent system of the present
invention can be evaluated by series trials. In the case of imidazoles
such as 2-methylimidazole, preferably from about 1 to about 20, more
preferably from about 2 to about 12 percent of an imidazole is dissolved
in the solvent system, by the weight of the solvent system.
Useful cure inhibitors are boric acid, metaboric acid, boric acid anhydride
or maleic acid or a mixture of (meta)boric acid(anhydride) with at least
one acid having a weak nucleophilic anion, such as fluoroboric acid
(HBF.sub.4). Cure inhibitors are described on page 17, lines 18-50 and
page 18, lines 1-38 of the published European patent application
EP-A-0,458,502, the teaching of which is incorporated by reference. If a
cure inhibitor is dissolved in the solvent system, it is preferably
dissolved in the solvent system in an amount of from about 0.1 to about 12
percent, more preferably from about 0.5 to about 6 percent, most
preferably of from about 1 to about 4 percent, by the weight of the
solvent system.
If the solvent system of the present invention contains substantial amounts
of a curing agent, for example between about 3 and about 12 percent of
dicyandiamide, the solubility of the curing catalyst and/or the cure
inhibitor in the solvent system is generally smaller, usually between
about 0.1 and about 3 percent, typically between about 0.5 and about 2
percent, based on the weight of the solvent system.
As indicated above, one aspect of the present invention is a method of
preparing a solution of one or more compounds selected from the group
consisting of a) curing agents for an epoxy resin, b) curing catalysts,
and c) cure inhibitors by contacting one or more such compounds with an
effective amount of a solvent system comprising methoxy acetone, an
organic protic solvent and, optionally, an oxygen-containing solvent other
than methoxy acetone. The solution may be prepared in a known way. Useful
curing agents, curing catalyst and cure inhibitors as well as their useful
concentrations in the solvent system are indicated above. Generally
methoxy acetone, an organic protic solvent and, optionally, an
oxygen-containing solvent other than methoxy acetone are mixed in the
ratios indicated above at a temperature of from about 1 to about
80.degree. C., preferably from about 15.degree. to about 40.degree. C.,
until a homogeneous mixture is obtained. Then the desired amount of curing
agent(s) and/or curing catalyst(s) and/or cure inhibitor(s) is added to
the solvent system and the resulting mixture is stirred until a clear
solution is obtained. The compounds to be dissolved can be added together
or alternatingly to the solvent system of the present invention.
Alternatively, each compound can be dissolved individually in the solvent
system and the resulting solutions can be combined if desired. Preferred
compositions of the resulting solutions of the present invention are
described above.
The above described solutions of a curing agent and/or a curing catalyst
and/or a cure inhibitor in the solvent composition of the present
invention can be mixed with an epoxy resin for preparing an epoxy resin
composition. Typically the epoxy resin is dissolved in a solvent. Although
the solvent for the epoxy resin can be added simultaneously or after the
epoxy resin has been mixed with the solution of the curing agent, curing
catalyst and/or cure inhibitor, the epoxy resin is preferably pre-mixed
with the solvent. The epoxy resin solution is then mixed with the solution
of the curing agent, curing catalyst and/or cure inhibitor for producing
an epoxy resin composition. Such a resin composition is typically
designated in the art as "one-component epoxy resin composition".
The epoxy resin composition of the present invention may comprise a wide
variety of epoxy resins, provided that they are curable, preferably with
dicyandiamide. Curable epoxy resins are well known in the art.
Suitable examples include epoxy resins from the reaction of polyphenols and
epihalohydrins, polyalcohols and epihalohydrins, amines and
epihalohydrins, sulfur-containing compounds and epihalohydrins,
polycarboxylic acids and epihalohydrins, polyisocyanates and
2,3-epoxy-1-propanol (glycide) and from epoxidation of olefinically
unsaturated compounds. Preferred epoxy resins are the reaction products of
polyphenols and epihalohydrins, or polyalcohols and epihalohydrins or of
polycarboxylic acids and epihalohydrins. Mixtures of polyphenols,
polyalcohols, amines, sulfur-containing compounds, polycarboxylic acids
and/or polyisocyanates can also be reacted with epihalohydrins.
Illustrative examples of epoxy resins useful herein are described in The
Handbook of Epoxy Resins by H. Lee and K. Neville, published in 1967 by
McGraw-Hill, New York, in appendix 4-1, ppgs through 4-56 and U.S. Pat.
Nos. 2,633,458; 3,477,990; 3,821,243; 3,970,719; 3,975,397; 4,071,477; and
4,582,892, and GB Patent Specification No. 1,597,610, all of which are
incorporated herein by reference.
Epoxy resins of particular interest include diglycidyl ethers of bisphenol
compounds, particularly those compounds represented by the following
structure III: wherein each A is independently a divalent hydrocarbon
group having from 1 to 8 carbon atoms, preferably methylene or, more
preferably, isopropylidene (--C(CH.sub.3).sub.2 --), --C(O)--, --O--,
--S--, --S--S--, --S(O)--, --S(O).sub.2 --or a covalent bond; each X is
independently hydrogen, an alkyl group of 1 to 6 carbon atoms such as
methyl, ethyl, propyl, butyl, pentyl or hexyl, or halogen, preferably
chlorine or bromine; and n has an average
##STR1##
value of from 0 to 35, preferably from 0 to 10, most preferably from 0 to
2.
The average epoxy equivalent weight is advantageously from 149 to 3000,
preferably from 170 to 950, most preferably from 170 to 450. The molecular
weight is a weight average molecular weight.
Other polyhydric phenols which may be co-reacted with an epihalohydrin to
provide these epoxy polyethers are such compounds as resorcinol,
hydroquinone and substituted hydroquinones, e.g. methylhydroquinone.
Further useful liquid epoxy resins are those obtained from the reaction of
polyhydric alcohols with epihalohydrins. These alcohols can be polyether
polyols or polyester polyols.
Another useful class of polymeric resins includes liquid epoxy novolac
resins. The epoxy novolac resins can be obtained by reacting, preferably
in the presence of a basic catalyst, e.g. sodium or potassium hydroxide,
an epihalohydrin, such as epichlorohydrin, with the resinous condensate of
an aidehyde, e.g. formaldehyde, and either a monohydric phenol, e.g.
phenol itself, or a polyhydric phenol. Further details concerning the
nature and preparation of these epoxy novolac resins can be obtained in
Lee, H. and Neville, K., Handbook of Epoxy Resins, McGraw Hill Book Co.
New York, 1967. The epoxy resin compositions of the present invention may
contain two or more different epoxy resin.
Useful solvents for the epoxy resin are well known in the art. Preferred
examples are 2-metyl-pentanediol-(2,4), toluene, o-dichlorobenzene,
cyclohexanone, cyclohexanol or, more preferably, methyl ethyl ketone or
methyl iso-butyl ketone. Mixtures of different solvents are also useful
for dissolving the epoxy resin.
Generally, from about 50 to about 95 percent, preferably from about 60 to
about 90 percent, more preferably from about 70 to about 85 percent epoxy
resin is dissolved in a suitable solvent, based on the total weight of
epoxy resin and solvent.
The epoxy resin composition of the present invention preferably comprises
from about 0.5 to about 20 percent, more preferably from about 1 to about
10 percent, most preferably from about 2 to about 6 percent of a curing
agent, such as dicyandiamide, based on the weight of the epoxy resin. The
epoxy resin composition preferably also comprises from about 0.05 to about
2 percent, more preferably from about 0.08 to about 1 percent, most
preferably from about 0.1 to about 0.5 percent of a curing catalyst, such
as a 2-methylimidazole, based on the weight of the epoxy resin. The epoxy
resin composition may comprise a cure inhibitor, such as boric acid, for
modifying the curing catalyst. The amount of such a cure inhibitor, if
present, preferably is from about 0.05 to about 2 percent, more preferably
from about 0.08 to about 1 percent, most preferably from about 0.1 to
about 0.5 percent, based on the weight of the epoxy resin. The epoxy
resin composition optionally contains known auxiliary compounds, such as
colorants, fillers and the like.
The weight ratio between the epoxy resin and methoxy acetone generally is
from 0.5-20:1, preferably from 2-15:1, more preferably from 4-10:1. The
weight ratio between the epoxy resin and the organic protic solvent
generally is from 1-60:1, preferably from 5-40:1, more preferably from
10-30:1.
The epoxy resin compositions of the present invention are useful for
various known applications, for example for preparing electrical
laminates, coatings etc. At least the preferred embodiments of the epoxy
resin composition of the present invention are homogeneous and generally
have a viscosity that is low enough to allow a good impregnation of a
reinforcing material, such as glass rovings, reinforcing mats etc. to
produce reinforced epoxy compositions which cure upon heating. Techniques
of impregnating reinforcing materials with epoxy resin compositions and
curing the epoxy resin compositions are well known in the art.
The present invention is further illustrated by the following examples
which should not be construed to limit the scope of the present invention.
All parts and percentages are by weight unless otherwise mentioned.
Examples 1 to 36 and Comparative Examples A to G
The solubility of dicyandiamide in various solvents and solvent systems is
tested at room temperature. The amounts of the component(s) of the solvent
system and of dicyandiamide are listed in the following table as weight
percentage, based on the total weight of the solvent system. The produced
dicyandiamide solutions are evaluated after they have been freshly
prepared and after storage at 0.degree. C. during 24 hours. The results
are listed in the following Table. "Y" and "N" mean that some (Y) or no
(N) crystallization or precipitation in the dicyandiamide solution is
observed.
Varying amounts of a dicyandiamide solution (from 30 to 40 parts) are
blended with 100 parts of an epoxy resin solution. The epoxy resin
solution contains 80 percent of a solid reaction product of a liquid epoxy
resin and tetrabromobisphenol A and 20 percent of methyl ethyl ketone. The
epoxy resin solution is commercially available as D.E.R. 535 EK 80 epoxy
resin from The Dow Chemical Company. The produced epoxy resin compositions
are evaluated after they have been freshly prepared and after storage at
0.degree. C. during 24 hours. The results are listed in the following
Table. "Y" and "N" mean that some (Y) or no (N) crystallization or
precipitation in the epoxy resin composition is observed.
__________________________________________________________________________
(Comparative) Example
A B C D E F G 1 2 3 4
__________________________________________________________________________
methoxy acetone
100 44 44 44 44
propylene glycol
100 44 44 44 44
methyl ether
methanol 100 12 12
ethanol 100 12 12
ethylene glycol 100
methyl ethyl 100
ketone
acetone 100
acetic acid (99%)
propionic acid
dicyandiamide
1 1 5 1 15 2 1 7.0
8.0
7.0
8.0
Cryst./prec.
fresh N Y Y Y Y Y Y N N N N
after 24 hrs
N Y Y Y Y Y Y N N N N
epoxy resin
--.sup.1)
-- -- -- -- -- -- 100
100 100
100
dicyandiamide
-- -- -- -- -- -- -- 34.4
30.0
34.4
30.0
solution
Cryst./prec.
fresh -- -- -- -- -- -- -- N N N Y
after 24 hrs
-- -- -- -- -- -- -- N Y N Y
__________________________________________________________________________
(Comparative) Example
5 6 7 8 9 10 11 12 13 14 15 16
__________________________________________________________________________
methoxy acetone
44 44 40 35 40 35 40 35 44 44 44 44
propylene glycol
44 44 48 53 48 53 48 53
methyl ether
methanol 12 12
ethanol 12 12 12 12
ethylene glycol
12 12 12 12 12 12
methyl ethyl 44 44
ketone
acetone 44 44
acetic acid (99%)
propionic acid
dicyandiamide
7.0
8.0
7.0
7.0
7.0
7.0
8 | | |
