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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to phosphate ester compounds containing
terminal vicinal epoxide functionality, and a process for the preparation
of these compounds. More particularly, the present invention relates to
hydrolytically stable compositions of matter comprising the reaction
product of an epoxide, such as a diglycidyl ether of a dihydroxy compound,
and a phosphoric acid ester. The compositions are readily cured by known
epoxy curing agents such as amines and anhydrides to form coatings and
cast articles. The cured compositions of the present invention are highly
useful due to their ability to form char upon application of heat, thereby
finding use in intumescent paints and in ablative coatings. Accordingly, a
further species of the present invention is an intumescent coating
comprising the above phosphate ester compounds containing terminal vicinal
epoxide functionality. Said intumescent coatings exhibit improved adhesion
to metal and glass substrates.
In U.S. Pat. No. 2,541,027, cured coatings are prepared by the reaction of
epoxy ethers such as the diglycidyl ether of bisphenol A with
orthophosphoric acid or a monoalkyl orthophosphate. The curing reaction
was initiated by mixing together the epoxy resin and the
phosphorus-containing curing agent and heating the resulting mixture at an
elevated temperature to totally eliminate all epoxide functionality.
In U.S. Pat. No. 4,256,844; 4,289,812; 4,164,487; and 4,316,922, additional
adducts of phosphoric acid and epoxides that form water-dispersible
neutralized adducts are described. In each reference, the epoxide
functionality is completely ring-opened thereby forming secondary
hydroxyl-containing compositions. Curing such ring-opened compositions was
effected by reaction with hydroxyl curing agents such as isocyanates.
In U.S. Pat. No. 4,425,451, monoesters of orthophosphoric acid and epoxy
ethers such as the diglycidyl ether of bisphenol A were prepared by
reaction of the ether with 85 percent orthophosphoric acid (in water) in
the presence of 2-butoxyethanol solvent. Residual phosphoric acid
functionality was neutralized by salification with an amine.
The presence of reactive phosphoric acid moieties in previously known
processes has resulted disadvantageously in the formation of various
cyclic phosphorus-containing epoxy functionalized esters of phosphoric
acid. See U.S. Pat. No. 3,639,545 and 3,652,743. In addition, it has been
found that gelling of the resin results by means of reaction between
phosphoric acid functionality and epoxide thereby resulting in a loss of
epoxide values over time.
In view of the deficiencies of the prior art processes, it would be
desirable to provide a process for effectively preparing epoxy derivatives
of monoesters of phosphoric acid in high yields. Additionally, it would be
desirable to provide a process for preparing epoxy derivatives of
phosphoric acid esters that are relatively unaffected by epoxide
degradation during storage.
SUMMARY OF THE INVENTION
According to the present invention there is provided a composition
comprising a novel, hydrolytically stable phosphate ester containing at
least one terminal vicinal epoxide moiety, and a process for the
preparation thereof. Preferred phosphate esters of the present invention
are represented generally by the formula:
##STR1##
wherein R' is a glycidyl ether of the general formula:
##STR2##
wherein R is a difunctional remnant of a dihydroxy compound formed by
removal of the hydroxyl groups thereof;
R" is a group as defined by R"' or R';
R"' is H or an organic group of up to about 20 carbons; and
n is a positive number less than about 20 and is equal to the number of
repeating units in the compound.
In another sense, the invention is a process comprising contacting under
reaction conditions, optionally in the presence of water, at least one
phosphoric acid ester with an amount of an epoxide having an epoxy
equivalent greater than one, said amount being sufficient to substantially
neutralize the phosphoric acid functionality of the phosphoric acid ester,
the contacting being such that there is formed a composition comprising a
phosphate ester containing terminal epoxide functionality corresponding to
a compositon described hereinbefore.
Surprisingly, compositions of the present invention can be cured via the
epoxide moieties to form a resin having intumescent properties and having
good adhesive properties.
DETAILED DESCRIPTION OF THE INVENTION
The compositions prepared according to the process of the present invention
are vicinal epoxy terminated thermoplastic advanced resins that are cured
to solid objects by conventional amine, carboxylic acid or anhydride
functional epoxy curing agents. These storage stable thermoplastic epoxy
resins desirably are further characterized by a phosphorus content of from
about 0.5 to about 2.6 weight percent and an epoxide content of from about
5 to about 20 percent by weight. Preferred is a process that prepares a
resin having a phosphorus content of from about 0.75 to about 1.5 weight
percent and an epoxide content, after storage at about 30.degree. C. for
30 days in the presence of air, of from about 4.5 to about 18 percent by
weight.
The process of the present invention basically requires contacting a
phosphoric acid monoester with an amount of an epoxy compound having an
epoxy functionality greater than one sufficient to substantially
neutralize the phosphoric acid functionality. An excess of the epoxy
compound, which preferably is an epoxy ether, desirably is employed. In a
preferred process, the phosphoric acid monoester is prepared by
hydrolyzing an organic-soluble pyrophosphate diester. Water is employed in
a preferred process of the invention. However, in all processes the amount
of water present is such that not all epoxy moieties of the final product
resin are hydrolyzed, i.e., the product of the invention contains terminal
epoxy moieties.
The term phosphoric acid ester as used herein includes monophosphate
polyesters, such as diesters, triesters, etc.; polyphosphate polyesters,
such as diphosphate diesters; and monophosphate monoesters. Of these, the
monoesters desirably are the esters contacted with an excess of epoxy
compound.
A preferred process comprises the steps of:
(1) forming an organic-soluble pyrophosphate diester corresponding to the
formula:
##STR3##
wherein R.sub.a is an organic group of up to about 20 carbon atoms, in an
organic solvent;
(2) contacting the organic-soluble pyrophosphate diester with from about 1
to about 16 moles of water per mole of pyrophosphate diester to form the
corresponding phosphoric acid monoesters; and
(3) contacting the phosphoric acid monoesters of step (2) with an amount of
an epoxy ether having an epoxy functionality greater than one sufficient
to substantially neutralize the phosphoric acid functionality.
According to a preferred process of the present invention, it is believed
that previously described steps (1) and (2) result in the formation of a
stable organic solvent-soluble orthophosphate monoester (phosphoric acid
monoester) corresponding to the formula:
##STR4##
wherein R.sub.a is as previously defined. The preparation of such organic
solvent-soluble, stable monoesters has heretofore been accomplished only
with great difficulty and in unacceptable yields. It has now been
discovered that these monoesters are readily prepared in highly stable
form by hydrolyzing the corresponding diester of diphosphoric acid
(alternatively referred to as the diester of pyrophosphoric acid).
The diester starting reactants of the preferred process can be conveniently
prepared by the reaction of phosphorus pentoxide with about 2 moles per
mole of phosphorus pentoxide of an alkanol, phenol, an alkyl or phenyl
monoether of a (poly)alkylene glycol, an inertly-substituted derivative of
such compounds, or mixtures thereof (referred to hereinafter as
hydroxyl-containing reactant). The preparation can be conveniently
performed under inert atmosphere by contacting the hydroxyl-containing
reactant with a slurry of phosphorus pentoxide in an inert solvent.
Preferred inert solvents are chlorinated hydrocarbons, e.g., chloroform,
ethylene chloride, methylene chloride, etc. A preferred solvent is
methylene chloride. Such diesters of diphosphoric acid and the method of
their preparation have been previously described in U.S. Pat. No.
4,396,555. The presence of the organic ether moiety (R.sub.a) in the
prepared compounds renders the invented compositions more soluble in the
above-described organic solvents than previously known epoxy derivatives
of phosphorus compounds. Preferably R.sub.a is alkyl or alkoxyalkyl.
Examples of typical R.sub.a moieties include phenyl, t-butyl, neopentyl,
isopropyl, decyl, butoxyethyl, ethoxyethyl, dimethylaminoethyl,
methoxyethyl, and the like. Most preferably, R.sub.a is a glycol ether.
The hydrolysis, step (2) of the previously described preferred process,
preferably is performed by contacting the above diester of diphosphoric
acid with water at or below ambient temperature. Preferred addition
temperatures, i.e., initial contacting temperatures, are from about
0.degree. C. to about 25.degree. C. The hydrolysis can be effected by
merely contacting the pyrophosphate diester and water, optionally with
heating. Typically, heating is not required due to the exothermic nature
of the hydrolysis reaction of step (2). Preferably the organic solvent
used in the preparation of the pyrophosphate diester is recovered by
distillation or other suitable means and the hydrolysis (step (2)) is
conducted in the substantial absence of an organic solvent. The water for
the hydrolysis reaction is typically added in a molar amount which is from
about 1 to about 16 times the molar amount of diester of diphosphoric acid
to be hydrolyzed. Preferably from about 2 to about 12 moles of water are
employed for each mole of diester. Most preferably from about 4 moles of
water to about 10 moles of water for each mole of diester are employed.
The exothermic hydrolysis reaction is allowed to proceed to substantial
completion, which is identified by a reduction in the amount of heat
evolved. Typically, the reaction is allowed to proceed until the reaction
mass returns to ambient temperature. Usually, completion of hydrolysis
requires from about 3 to about 24 hours depending on the reaction
conditions employed. In the preferred process, the hydrolyzed reaction
product comprising substantial amounts of the desired monoester of
orthophosphoric acid is thereafter contacted with the epoxy ether having
an epoxy equivalency greater than one.
It is preferred to first hydrolyze the pyrophosphoric acid diester by
reaction with excess water prior to reaction with the epoxy ether
according to the previously described preferred process in order to reduce
the formation of cyclic epoxy phosphate species thought to correspond to
the following formula:
##STR5##
wherein R"' is as previously defined and X is the remnant of the epoxy
ether,
##STR6##
after ring-opening of one epoxide functionality has occurred. When excess
water is employed to effect hydration as previously explained, the
occurrence of the above undesired species is largely eliminated. On the
other hand, the presence of alcohol or glycol ether solvents along with
the epoxy ether reactant has been found to reduce the theoretical epoxide
content of the resulting resin due to competitive reactions between the
epoxide and alcohol functionality. Accordingly, it is preferred that the
reaction of the epoxy ether with the phosphate ester be conducted in the
substantial absence of an organic solvent. However, in certain instances,
such as in the preparation of very viscous products, it can be desirable
to employ a diluent, such as toluene, hexane or a reactive diluent, such
as an aliphatic monoepoxide.
Suitable epoxy ethers include those previously known and taught, for
example, in US 2,541,027. Preferred epoxy ether compositions correspond to
the formula:
##STR7##
wherein R is as previously defined. Preferably R is an alkylene, arylene,
aralkylene or bisaralkylene radical of up to about 30 carbons optionally
inertly-substituted. The most preferred epoxy ethers are the diglycidyl
ethers of bisphenol compounds, especially the diglycidyl ether of
bisphenol A and partially advanced homopolymers thereof.
It has been found that an excess of epoxy ether compound is desirable to
prepare a composition that is stable to ring-opening and does not produce
gelled reaction products. Suitably, from about 4 equivalents to about 20
equivalents of epoxy ether per equivalent of ester reactant are employed.
Preferably, from about 6 equivalents to about 12 equivalents of epoxy
ether for each equivalent of ester are employed. The reaction is performed
by adding the orthophosphate ester compound to an excess of the epoxy
ether accompanied by vigorous agitation to prevent localized
stoichiometric imbalance.
The reaction between the epoxy ether and monophosphate ester is preferably
conducted at elevated temperatures. Most preferred temperatures are from
about 25.degree. C. to about 80.degree. C. By using elevated temperatures
and an excess of epoxy ether reactant, phosphorus acid moieties capable of
functioning as curing agents for cross-linking (cf. U.S. Pat. No.
2,541,027) are substantially eliminated. Resins according to the present
invention are extremely stable in the absence of additional epoxy curing
agents.
In the present invention, the amount of epoxy ether having an epoxy
functionality greater than one sufficient to substantially neutralize
remnant hydroxyl functionality of the hydrolyzed pyrophosphoric acid
diesters is defined as the amount necessary to result in a free-flowing,
organic-soluble product that is stable under normal conditions of storage.
Accordingly, as a test of such storage stability, the material should not
form appreciable amounts of epoxide ring-opened degradation products upon
exposure for 30 days to temperatures of about 30.degree. C. in the
presence of air. By the term "appreciable" is meant that the above testing
procedure does not result in sufficient ring-opening of epoxide
functionality so as to render the resin uncurable by standard epoxy curing
agents, e.g., aliphatic or aromatic amine, anhydride or carboxylic acid
curing agents. Preferably less than about 20 percent of the originally
available epoxide functionality is degraded according to the above testing
procedure. More preferably, less than about 10 percent of the originally
available epoxide functionality is degraded. Most preferably, greater than
95 percent of the originally available epoxide functionality remains after
exposure for 30 days to air at temperatures of about 30.degree. C.
In the practice of the present invention, the desired reaction product
advantageously is recovered from the reaction in commercially suitable
form. No purification is required before employing the epoxy phosphate
ester resin in a coating formulation or in other suitable applications.
The presence of residual water, such as that remaining from the hydrolysis
process, in the resulting composition has not been found to be
disadvantageous. The presence of water, within the limits described
hereinabove, e.g., water of hydrolysis, gives rise to monohydrolyzed
oligomers which contribute to the thixotropic and curing properties of the
resin. However, too much residual water can result in increased hydrolysis
of the epoxide functionality.
The absence of formation of viscous gels or nonsoluble precipitates even
upon dilution of solutions with methylene chloride to as much as 2 percent
resin content indicates the substantial absence of cross-linked reaction
products formed by reaction of phosphoric acid and epoxide functionality.
Liquid chromatography, mass spectroscopy, P.sup.31 nuclear magnetic
resonance, and fluorescence spectroscopy indicate that the orthophosphate
ester is chemically bonded to the resin backbone containing at least one
oxirane functionality.
Curing of resinous epoxy phosphates is readily accomplished by contacting
these phosphates with a curing agent, optionally at elevated temperatures.
Coatings and molded articles are readily prepared using conventional
equipment and techniques. Standard additives including fillers, pigments,
dyes, solvents, etc., may be incorporated into the coating or molding
composition according to techniques well-known in the art.
When employed as an intumescent coating, e.g., for metal parts employed in
construction and other applications, coatings comprising the epoxy
phosphate compositions prepared by the present process possess improved
adhesion to ferrous metals. Advantageously, mechanical means of increasing
adhesion such as attaching reinforcing mesh prior to coating may be
eliminated when employing coatings comprising the epoxy phosphate
compositions prepared by the present process. Therefore, labor costs
associated with the application of the resins of the present invention can
be significantly reduced as compared to labor costs for resins which
require mechanical adhesion aids.
SPECIFIC EMBODIMENTS
Having described the invention, the following examples are provided as
further illustration thereof and are not to be construed as limiting.
EXAMPLE 1
Phosphorus pentoxide in methylene chloride solvent is contacted with
butoxyethanol in the ratio of two moles of butoxyethanol for each mole of
phosphorus pentoxide. Removal of methylene chloride solvent by evaporation
under reduced pressure at a temperature of about 60.degree. C. results in
a dark brown oilish liquid identified by P.sup.31 nuclear magnetic
resonance spectroscopy (NMR) as a composition corresponding substantially
to the formula:
##STR8##
The diester is contacted (neat) with water in the molar ratio of water to
diester of 6:1 at a temperature of 25.degree. C. for three hours. The
resulting liquid product and excess unreacted water are controllably added
with vigorous stirring over a period of about one-half hour to a 12-fold
molar excess of a liquid epoxy ether (DER 330, available from The Dow
Chemical Company, can be suitably employed) at about 80.degree. C. under
nitrogen atmosphere. The reaction is continued for about two hours.
The resulting product is a thick, tan-colored resin having the consistency
of taffy at ambient temperatures. Epoxy content (percent epoxide by
weight) as determined by standard titration techniques is 14 percent
(theoretical maximum percent epoxide is 18.3). After storage for 30 days
at 25.degree. C. in contact with air, the percent epoxide is 13.8 percent.
Thus, the percent of original epoxide functionality remaining after 30
days is 13.8 .div.14 =98.6 percent. Phosphorus content of the resin is
approximately 1.3 percent by weight.
EXAMPLES 2-9
The reaction conditions of Example 1 are substantially repeated employing
various atmospheres, molar ratios of epoxide:pyrophosphoric acid diester,
and water:pyrophosphoric acid diester; these conditions are further
identified in Table I. Percent epoxide initially and after exposure for 5
and 30 days to ambient conditions, respectively, are contained in Table I.
Table I also indicates the amount of monophosphate ester present in the
reaction product as determined by P.sup.31 nuclear magnetic resonance.
Examples 1-2 demonstrate the preferred technique wherein the organic
solvent, methylene chloride, is removed prior to hydrolysis of the
diester. Examples 3-9 illustrate the technique wherein the organic solvent
is not removed prior to the hydrolysis. The products of Examples 1-2
exhibit superior resistance to degradation over a period of 30 days, i.e.,
the products of Examples 1-2 are very storage stable.
TABLE I
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Molar Ratios Area %
epoxide/ H.sub.2 O/di-
% Epoxide monophosphate
Example
diester
ester
ATM Theory
Initial
5-day
30-day
Stability*
ester
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1 12.0 6.0 N.sub.2
18.3
14.0
-- 13.8
98.6 96.2
2 12.0 10.0 N.sub.2
18.3
12.3
-- 12.0
97.6 95.5
3 8.0 3.5 N.sub.2
16.0
16.7
14.7
10.2
61.1 95.0
4 12.0 4.0 N.sub.2
18.3
16.7
12.4
12.8
76.6 95.5
5 16.0 4.0 N.sub.2
19.6
17.3
14.8
14.3
82.7 95.5
6 17.4 1.3 N.sub.2
20.1
18.1
16.4
16.2
89.5 62.0
7 17.4 2.5 N.sub.2
20.1
16.6
14.6
14.6
88.0 80.0
8 17.4 3.6 Air 20.1
16.3
14.5
14.0
85.9 91.1
9 17.4 5.0 N.sub.2
20.0
16.6
13.8
13.8
83.1 98.0
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*Percentage of original epoxide remaining after 30 days.
EXAMPLES 10-15
A sample of resin, which is prepared substantially according to the
procedure of Example 1 (1.3 percent phosphorus), is cured using
commercially available epoxy resin curing agents, dicyandiamide (DICY),
triethylenetetraamine (TETA), and tetrahydrophthalic anhydride (THPA). The
gel time of polymer formation is determined by adding 100 percent or 75
percent of a stoichiometric amount of the curing agent along with an
accelerator, benzyldimethylamine (BDMA). The curing agent in a solvent
solution is added to an acetone solution of the epoxy resin at an elevated
temperature. The reaction mixture is stirred until fibers are no longer
capable of being drawn from the mixture. The elapsed time to reach this
rubbery state is the gel time. Results are contained in Table II.
TABLE II
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Amount Temp Gel Time
Example Curing Agent
(%) (.degree.C.)
(sec)
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10 DICY/BDMA 100 175 509
11 DICY/BDMA 75 175 611
12 TETA 100 120 301
13 TETA 75 120 396
14 THPA/BDMA 100 175 347
15 THPA/BDMA 75 175 385
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EXAMPLES 16-23
Cured and uncured samples of epoxy resin are tested for residual char
formation by heating in a DuPont 1090 Thermal Analyzer at 600.degree. C.
in an air atmosphere to determine residual char. Accordingly, a weighed
amount of the cured resin prepared substantially according to the
provisions of Examples 1 or 11 and standard epoxy resins with or without
phosphorus additives (to give 1.3 percent phosphorus content) are tested.
Residual char formation is expressed as percent by weight of original
resin. Results are contained in Table III.
TABLE III
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Additive %
Example Epoxy Resin (1.3% P) Char
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16 Ex. 1 (uncured) -- 23.7
17 Ex. 11 (cured) -- 22.4
18 DER-330 (uncured).sup.1
-- 4.6
19 DER-330 (cured).sup.2
-- 9.5
20 DER-661 (uncured)
-- 2.1
21 DER-661 (cured).sup.1
-- 9.0
22 DER-661 (uncured)
Ph.sub.3 P .dbd. O
14.3
23 DER-330 (cured) Ph.sub.3 P .dbd. O
16.5
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.sup.1 An epoxy resin of approximately 360 mole weight available from The
Dow Chemical Company.
.sup.2 Cured with DICY, 100 percent of theoretical.
It is seen that epoxy resins according to the present invention have
significantly improved residual char compared to conventional epoxy resins
and, compared to conventional resins, contain equivalent amounts of
phosphorus-containing additive.
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