Note: Descriptions are shown in the official language in which they were submitted.
10ti5338
This invention relates to an improved method of preparing monomeric
esters of alcohols and ~ unsaturated carboxylic acids and, more particular-
ly, to the preparation of such esters using a polyalkylene glycol or a mono-
ether derivative of an alkylene glycol or polyalkylene glycol which has a
tendency to form peroxide impurities.
Conventional processes for the production of these esters invol~e
the direct esterification of the alcohol with the unsaturated acid. The re-
action is generally carried out in the presence of an acid catalyst and with
the aid of an additive to inhibit the formation of polymers of the unsaturated
reactant, the ester product, or both. Also, it is customary to conduct the
reaction in the presence of an organic solvent, such as benzene or heptane,
which forms an azeotrope with the water of reaction to facilitate its removal
as esterification proceeds.
Despite the use of the polymerization inhibitor, varying amounts of
polymeric by-products are ordinarily obtained in conventional esterification
processes. As a result of polymer formation, esterification tends to be in-
complete so that the crude reaction product, besides containing polymeric by-
products, contains substantial amounts of unreacted or partially reacted gly-
col. The polymeric by-products often form stable emulsions upon neutraliza-
tion of the crude reaction products, making product recovery by aqueous extrac-
tion impractical without resorting to additional separation techniques. The
presence of unreacted glycol in the crude reaction product also interferes
with product recovery using aqueous extraction since the glycol tends to in-
crease the water solubility of the diester.
Some of those skilled in the art believed that the polymeric by-
products were due in large measure to the high reaction temperatures
--1--
iO~;5338
that are customarily employed. Thus, for example, in U. S. Patent No.
3,639,459, which issued February 1, 1972, the patentee declares that the con-
ventional process should be substantially modified by eliminating the solvent
from the system and conducting the reaction at a temperature not exceeding
about half of the boiling point of the acid. This approach suffers various
disadvantages including the loss of acid reactant via the formation and re-
moval of the acid/water-of-esterification azeotropes.
Others have suggested using various types of polymerization in-
hibitors to ameliorate the problem. And still others have suggested abandon-
ing the conventional system altogether and substituting a very different ap-
proach involving transesterification. Transesterification also suffers seri-
ous disadvantages. The esterification reaction proceeds slowly in the presence
of acidic or neutral catalysts. The rate of reaction can be increased by the
use of a basic catalyst, but the use of such a catalyst results in a greater
yield of the product formed by the addition of an alcohol across the carbon-
carbon double bond of the , ~-unsaturated carboxylic acid (Micheal addition),
which is a serious competing reaction. It is also reported that the methyl
and ethyl esters of acrylic acid and methacrylic acid form hard-to-break
azeotropes with methanol and ethanol respectively.
It has now been discovered that the amount of polymer formation can
be greatly reduced simply by heating a mixture comprising the impure poly-
alkylene glycol, a hydrocarbon solvent, and a small amount of caustic or sodium
borohydride to a temperature and for a time sufficient to decompose the per-
oxide impurity associated with the polyalkylene glycol. The treated glycol
is then immediately reacted with the unsaturated acid or is temporarily stored
under con-
10~5338
ditions unfavourable to the formation of additianal peroxides. The formation
of polymeric by~products can be kept to a minin~m if this procedure is used
in combinatian with performing the esterification reaction in a non-oxygen
containing atmosphere and in the presence of certain free radical polymeriza-
tion inhibitors which can function without oxygen.
Accordingly the present invention provides a method of preparing
polymer;~hle mon0teric esters of a polyalkylene glycol or of the mono-ether
alcahol derivative of an alkylene glycol or polyalkylene glycol, which glycol
or derivative has a tendency to form thermally unstable peroxides, which
process ccmprises heating a mixture of said glycol or derivative, a hydro-
carbon solvent, and a small amount of a caustic or sodium borohydride for
a time and at a temperatulre sufficient to decompose said peroxide; and
reacting said treated glycol or derivative with an , ~ - unsaturaLted mono-
carboxylic acid selected frcm the group consisting of acrylic acid, aLkyl
su,bstituted acrylic acids and halogen substituted acrylic acids in a nan-oKygen
containing atmosphere in the presence of a suitable esterification catalyst
and polymerization inhibitor for a time and at a temperature sufficient to
abtain the desired yield of monomeric ester.
m e present invention also provides a method of preparing polymeriz-
able mon0Leric diesters of a polyalkylene glycol which has a te~dency to farm
a thermally unstable peraxide; which process comprises: treating a muxture of
said gl~col, a hydrocarbon solvent, and a caustic for a time and to a tempera-
ture sufficient to deoompo~e said peroxide; and reacting said purified glycol
in an a~ygen free atmosphere with an , ~-unsaturated mDrocarbcxylic acid
selected frcm the grouLp consisting of acrylic acid, alkyl-substituted acrylic
acids and halogen substituted acrylic acids in the presence of an esterifica-
tian catalyst and a polymerization inhibitor selected fram the group oonsisting
of phenothiazine, pyrngallol, p-FhLenylenediamine and methylene blue at a
temperature for a time sufficient to obtain the desired yield of the diester.
ThLe present invention further provides a method for preparing a poly-
merizable mLnomeric diester of a polyaLkyJene glycol which has a tendency
to form thermally unstable hydroperoxides ccmprising reacting a polyalky1ene
i - 3 -
5338
glycol, which has been pretreated with caustic or sodium bo mhydride to remcve
thermally unstable peroxides associated therewith, with an ~ ~-unsaturated
mcnocarboxylic acid selected from the group consisting of acrylic acid, alkyl-
substituted acrylic acids, and halogen substituted acrylic acids in an inert
atmosphere in the presence of a suitable esterification catalyst and poly-
merization inhibitor at a temperature and for a time sufficient to abtain the
desired yield of the diester.
The present invention additionally provides a method of preparing
tetraethylene glycol diacrylate using tetraethylene glycol which has a
tendency to form a hydroperoxide which process comprises: heating a mixture
of said tetreethylene glycol, a hydrccarbon solvent and sodium or potassium
hydroxide, in a small amount sufficient to promDte the deccnpositiQn of said
hydroperoxide, to a temperature and for a time sufficient to decompoae said
hydroperoxide; and reacting in a non-oxygen oontaining atmosphere said
tetraethylene glycol with acrylic acid in the presence of a suitable esterifica-tion catalyst and a free radical polymerization inhibitor at a temperature
and for a time sufficient to abtain the desired yield of tetraethylene glycol
diacrylate.
The prccess of this invention is predicated in part upon the apprecia-
tion of the fact that certain polyalkylene glycols, such as tetraethylene gly-
col, have a tendency to form hydrop~roxides under certain conditions and that
these peroxides may, under the conditions of an esterification reactian, de-
oompose to form free radicals. m ese free radicals can initiate polymeriza-
tion of vinyl functions, oDnsume polymerization inhibitors, generate color
and cause formation of a product which results in the formation of intractable
emLlsions during neutralization. Polyalkylene glycols readily form hydro-
peroxides in the presence of air at temperatures of from 25 to loo&. In
the temperature range of from 70 - 100C, thermal deoompositian of the
hydroperoxides to form free radical species prcceeds at an appreciable rate.
As in the o~nventional esterification reaction, the reaction may
occur in a mixture of benzene and hexane, e.g., 3:1 (weight ratio), ar other
solvent as more fully described hereafter, and the reaction is driven to comr
~,
~ - 3a -
5338
pletion by the removal of water of esterification as a solvent-water azeotrope.
m e organic phase is extracted with an excess of an aqueous base, such as
aqueous sodium carbonate ~10 - 15% by weight sodium carbonate), to remove
acidic impurities and some of the poly~erization inhibitor. After extraction,
the mcnomeric diester solution is preferably treated with 0.5 - 1.0 weight
percent
10f~5338
based on the amount of monomeric ester present of Fuller~s Earth and also
with 0.5 - 1.0 weight percent of a decolorizing charcoal for from 1 to 2
hours at a temperature of from 25 - 60 C to remove the remaining color bodies.
The Fuller~s Earth plays the principal role because it shows a very strong
and uncommon affinity for methylene blue, the referred inhibitor for this
process. The decolorizing agents are then removed by filtration.
After filtration, the solvent and water may be removed by vacuum
flash distillation followed by vacuum gas stripping in a rotary flash evapor-
ator. The monomeric diester temperature should be kept below 65 C and pre-
ferably should not be greater than 60C to avoid or minimize polymerization,
and pressures in the range of from 100 - 200 mm Hg~ are maintained. A gas
stream consisting of nitrogen or preferably nitrogen and air is sparged
through the stripping pot during this operation to strip out solvent. After
stripping, the final product is filtered, e.g., through Celite filter aid, to
remove suspended solids.
The glycol reactant used in the present invention may be any poly-
alkylene glycol or mono-ether derivative of an alkylene glycol or polyalkylene
glycol which has a tendency to form thermally unstable peroxides. The inven-
tion is particularly applicable to polyalkylene glycols having two free re-
active hydroxyl groups including mixtures of such glycols. The more commonly
encountered and readily available glycols are straight chained polyethylene
glycols which have the free reactive hydroxyl groups at each of their termi-
nal ends. Preferably, the polyal~ylene glycol comprises from 3 - 15 carbon
atoms and most preferably 8 carbon atoms. Illustrative of suitable glycols
are diethylene glycol, triethylene glycol, tetraethylene glycol, and penta-
ethylene glycol and the corresponding propylene glycols. Ordinarily, the
glycol reactant has a molecular weight not greater than about 600.
1~)tj5338
The process of this invention is also applicable ~o the use of the
ether-alcohol derivatives of alkylene and polyalkylene glycols, e.g. alkoxy-
alkanols or mono-ether derivatives of alkylene glyaols or polyaLkelene glycols.
Thus, esters such as butoxyethyl acrylate can be prepared.
m e unsaturated acid reactant used in the present method is an
~ unsaturated monocarboxylic acid. Examples of unsaturated acids particu-
larly suitable for use in preparing the diesters are acrylic acid (boiling
point 141.9 &) and substituted acrylic acids, suah as aIkyl and halogen-sub-
stituted acrylic acids, e.g., methacrylic acid (boiling point 163C),
ethylacrylic acid, cr~tonic acid (boiling point 189C), tiglic acid (boiling
point 198.5C) and ~-chloroacrylic acid (boiling point 176&).
This prccess generally results in the esterification of from about
97 - 99~ of the available hydroxyl functionality. m e reactio,n between the
polyaIkylene glycol and acrylic acid produces a mlixture of the diacrylate and
monoacrylate esters of the diol. An excess of the acid is regyired to drive
the reaction essentially to a~mpletion under practical aonditi~ns of time and
temperature. Generally an excess of 5 - 10 mole percent of acid should be
emplcyed based o,n the theoretical number of moles of hydroxyl function avail-
able for esterification. Greater excesses can be used but are undesirable
fnom an efficiency standpoint.
For a fuller understanding of the inventive features of the process
disclosed herein, reference should be had to the following detailed descrip-
tion.
In carrying out the inventive method, the polyalkylene glycol, a
hydrocarbon solvent, and a small amount of aaustic are charged to a suitable
reactor, that is, a reactor equipped with heating and stirring means, gas
sparger, temperature measuring means,
- 5 -
10~5338
distillation column, reflux condenser, and overhead phase separator rator.
The term "solvent" as used herein refers to the inert organic
liquids conventionally used in esterification reactions as azeotroping agents
for the water. Ordinarily, the organic liquids employed for azeotroping the
water of esterification are essentially non-polar, organic solvents or di-
luents such as benzene, hexane, heptane, toluene, xylene, cyclohexane and
trichloroethylene. Preferably, a mixed solvent system is employed in this
process comprising 40 - 60% (preferably 50%) by weight of the reaction mix-
ture; the solvent mixture should comprise 10 - 50% n-hexane and 50 - 90%
benzene by weight. The solvent serves the typical functions of providing a
medium in which the reaction occurs and serving as an azeotroping agent for
removing the water of reaction and thereby driving this equilibrium reaction
to completion. The solvent also provides a means of controlling the tempera-
ture of the reaction mixture without resorting to temperature control based
on operation under vacuum or close control of heat input. If the total sol-
vent charge, e.g., 75 weight percent benzene and 25 weight percent hexane,
comprises 50% by weight of the total reaction charge, the temperature of the
reaction mixture varies in the range of 70 - 85C during the run, gradually
increasing as the reaction progresses toward completion. If a 50% by weight
solvent charge consisting of benzene alone is employed, temperatures in the
range of 80 - 100C are experienced during the run. The lower reaction tem-
perature experienced when the mixed solvent system is employed substantially
reduces emulsion formation during neutralization and color in the final pro-
duct. The use of n-hexane alone as the reaction solvent is inadvisable be-
cause
10~5338
of the limited solubility of the reactants and products of this process in
that solvent. It has also been observed that the use of a mixed solvent
system comprising 50% by weight of solvent (consisting of 10 - 50 weight per-
cent n-hexane and 50 - 90 weight percent benzene) greatly reduces emulsion
formation during neutralization, relative to what is observed when benzene
alone is employed as the reaction solvent.
Any strong caustic that has appreciable solubility under the pro-
posed conditions can be used for the decomposition of the hydroperoxide.
Thus, for example, sodium hydroxide or potassium hydroxide may be employed.
The amount of caustic is not critical. As little as one part of caustic per
one thousand parts of glycol is sufficient for the purpose. A tenfold in-
crease or a twofold decrease probably has very little affect on the reaction.
The aqueous caustic solution typically employed is about 4% by weight of
sodium hydroxide (1.0 molar); concentrations in the range of from 1 - 50% by
weight should work equally as well. It has also been discovered that sodium
borohydride can be used to decompose the peroxides. Sodium borohydride should
be used in a concentration within the range of from about 100 - 1,000 parts
per million based on the polyalkylene glycol.
The esterification reaction proceeds at atmospheric pressure. The
mixture is heated at reflux throughout the esterification reaction; and the
temperature of the reaction mixture is controlled by varying the composition
of the reaction mixture, particularly the composition and concentration of
the solvent. Usually the temperature of the reaction mixture at reflux in-
creases throughout the run. Typical increases are on the order of 10 C over
the entire run. Boilup may be
10~5338
held constant over a whole run or it may be cut back towards the end of the
run as the rate of water formation decreases.
The preferred esterification reaction catalyst is methanesulfonic
acid. Other catalysts which may be satisfactorily employed include p-toluene-
sulfonic acid, benzenesulfonic acid, and special purpose strong acid ion
exchange resins of the sulfonic acid type such as the one sold by Rohm & Haas
as Amberlyst-15. Strong mineral acids such as sulfuric acid and phosphoric
acids may be employed, but their use may result in substantially increased
color levels in the product. The amount of catalyst employed will generally
vary from 0.5 - 5.0~ by weight of ~he total reaction charge. Preferred con-
centrations are in the range of from 1.0 - 2.0 weight percent.
Many of the polymerization inhibitors commonly used in conventional
esterification reactions may not be employed in the present method. Thus,
for example, inhibitors such as p-methoxyphenol, hydroquinone, pyrocathecol,
and picric acid are not adequately effective and the use of such inhibitors
is likely to result in massive polymerization within one half to two hours
after reflux is commenced. It is believed that these inhibitors are inade-
quate due to lack of oxygen in the system.
Satisfactory inhibitors for the system described herein include
phenothiazine, pyrogallol, p-phenylenediamine, and methylene blue in a con-
centration from about 100 - 2000 parts per million based upon the ~ unsat-
urated carboxylic acid. All these materials, although effective in inhibiting
polymerization, have the disadvantage of being intensely colored or may de-
compose under reaction conditions to form intensely colored products. These
colored products with the exception of methylene blue cannot be readily re-
moved by ordinary purification procedures. High color concentrations are un-
10t~5338
satisfactory in coating end uses because of aesthetic and cure rate consider-
ations. Methylene blue is, therefore, the most preferred reaction inhibitor.
It effectively inhibits the polymerization reactions and has the added advan-
tage of being able to be readily removed after the reaction step is complete.
The preferred concentration for methylene blue is in the range of from 500 -
1,000 parts per million based on the carboxylic acid.
It is desirable to also add to the reaction mixture p-methoxyphenol
(MEHQ) as an inhibitor in the range of from 100 - 200 parts per million based
upon the carboxylic acid. This inhibitor is not removed during processing and
is, therefore, present in the finished product to inhibit polymerization in
the stored product.
me reaction is conducted at atmospheric pressure at reflux which
ordinarily should be in the range of from 70 - 85C. A non-oxygen containing
atmosphere is provided by sparging an inert gas such as nitrogen through the
reaction mixture throughout the run or by sparging the system with nitrogen
initially to displace the oxygen and then running the reaction under a blanket
of nitrogen gas. The water of reaction is taken overhead as the solvent
water azeotrope, thereby driving the reaction to completionO At the end of
the reaction the principal components of the reaction mixture are diester,
monoester, free acid, catalyst inhibitors and solvents.
The present invention is further illustrated by the following
Example which describes a laboratory process for preparation and purification
of tetraethylene glycol diacrylate.
~065338
EXAMPLE
Summary
Tetraethylene glycol diacrylate (TEGDA) is prepared by the sulfonic
acid catalyzed direct esterification of tetraethylene glycol (TEG) with
glacial acrylic in a benzene-hexane solvent mixture. A mixture of methylene
blue (MB) and p-methoxyphenol (MEHQ) is used to inhibit polymerization, and
the reaction is conducted under an oxygen free nitrogen atmosphere. Immediate-
ly prior to esterification, the TEG is treated with dilute NaOH to decompose
the peroxides present in this material.
The crude product (TEGDA) is purified by (1) extraction with an
aqueous base to remove acidic impurities and most of the MB, (2) treatment
with Fullerls Earth and activated charcoal to remove MB and color; (3) vacuum
flashing and stripping to remove solvent and water; and (4) filtration to re-
move suspended solids.
Apparatus
TEG Pretreatment ~ Reaction
The reaction vessel is equipped with heating facilities, good
stirring apparatus, gas sparger, temperature measuring device, short distilla-
tion column, reflux condenser, and overhead phase separator.
~O Purification
a. Extraction - a vessel equipped with good stirrin~ apparatus and
bottom mounted liquid take off.
b. Fullerls Earth and Charcoal Treatment - a vessel equipped with
a good stirrer and an effective filtration device.
c. Solvent removal - a vessel equipped with an effective stirrer,
a nitrogen-air gas sparger, temperature measuring device, heating facilities,
effective overhead condenser, and vacuum capability.
--10--
10~;5338
Detailed Description
For pretreatment and reaction, a 12-liter round bottom flask equip-
ped with a heating mantle; mechanical stirrer; thcrmometer well; gas sparger;
5-tray, 2-inch Oldershaw column; efficient condenser; and overhead phase
separator was employed. Into the reaction vessel were placed the following
materials:
Tetraethylene Glycol 2328 g 12.0 moles
Benzene 3107 g
n-hexane 1035 g
aqueous 4% NaOH60 ml
The mixture was stirred and heated at reflux for 1-2 hours; during
the operation a small stream of N2 (0.1 SCFH) was sparged through the liquid
phase to exclude air from the system. The following materials were next add-
ed to the reaction vessel:
Glacial Acrylic Acid 1814 g 25.2 moles
(with 200 ppm-MEHQ)
Methylene Blue 1.8 g
Aqueous 70% 166 g
Methane Sulfonic Acid
The mixture was refluxed with rapid mechanical stirring under an
inert nitrogen atmosphere. The reaction was driven to completion by removing
the water of reaction overhead as the solvent-water azeotrope. After approx-
imately 11 - 12 hours the reaction was judged complete based on the amount of
water collected. Alternatively, the free acid concentration can be used as a
measure of the degree of esterification. Temperature range for entire re-
action was 73 - 78C.
The crude reaction mixture was cooled down to 25 - 35 C, combined
solvent concentration adjusted to 52 weight percent, and
10~;5338
the mixture extracted with 1 x 1300 ml of aqueous 13% Na2C03 ~-25% excess).
The mixture was stirred for-^30 minutes and then allowed to settle for an
additional 30 minutes. The dark blue colored aqueous phase was then drawn
off and discarded. This procedure removed the sulfonic acid, excess acrylic
acid, and much of the MB.
Next, 36 g. each (1% by weight of the theoretical TEGDA yield) of
Fuller~s Earth and activated charcoal (Pittsburgh Activated Carbon type RC
Pulverized) were added to the organic phase, and the mixture was stirred up
at ambient temperature for 1 - 2 hours. After properly contacting these
agents with the TEGDA solution, the Fullerls Earth and activated carbon were
removed by vacuum filtration without added filter-aid. This operation removed
the remaining methylene blue and most of the color bodies.
For the solvent removal step, a rotary evaporator equipped with a
gas sparger discharging below the liquid level, an efficient chilled conden-
ser, and a vacuum system able to lower the pressure on the system to about 125
mm HgA were employed. TEGDA samples (~1 kg each~ containing approximately
equal amounts of TEGDA and solvent (benzene and n-hexane) were vacuum flashed
and then vacuum stripped until the concentration of solvents (~ainly benzene)
in the TEGDA was ~== 0.5% weight. During this operation gas sparge rates of
0.1 SCFH air and 0.2 - 2.0 SCFH Nitrogen were employed. The bath temperature
was not allowed to exceed 65C and was generally kept at 60 - 65 C. Stripping
time under these conditions was in the range of 1 - 2 hours per sample.
After stripping, the product was slurried with 1/2 weight % filter aid and re-
filtered to remove the remaining suspended solids. Typical yields are on the
order of 95% based on the TEG and 90% based on the HACA.
-12-
iO~;5338
Products obtained by the method typically had the following pro-
perties:
Saponification No. (mg KOH/g) 350 - 370
Acid No. (mg KOH/g) '1.0
H20 (% weight) '0.1
Inhibitor
MEHQ (ppm) ~100
MB (ppm) ~1
Viscosity, 25C (cps) 17 - 19
APHA color 50 - 200
Peroxides 10 - 25 ppm
Density, 25 C (g/cc) 1.11
Residual Solvent (% weight) co.5
As will be apparent to those skilled in the art, certain changes
may be made in the above procedure while still achieving the benefits of the
present invention. For example, other polymerization inhibitors and other acid
esterification catalysts as conventionally used in the art, whether organic or
inorganic, may be employed.
Results similar to those obtained above may be achieved in the pre-
paration and recovery of other polymerizable monomeric diesters. Typical of
the diesters that may be produced according to the present method are die-
thylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene
glycol dimethacrylate, dipropylene glycol dimethacrylate, tripropylene glycol
dimethacrylate, and the corresponding diester of acrylic acid and of other
substituted acrylic acids.
10~;5338
The monomeric esters prepared in accordance with this invention may
be polymerized in the presence of peroxides or other polymerization catalysts
employing bulk, aqueous emulsion or dispersion, or solution polymerization
techniques. Accordingly, the polymerizable monomeric esters find utility in
the manufacture of castings of various shapes, as impregnating and laminating
compositions, as surface coating compositions and in the preparation of poly-
meric sheets, tubes and rods. These polymerizable polyfunctional monomers
can also be used in end uses such as radiation cured coatings and printing
inks.
Since certain changes may be made in the above method ~ithout de-
parting from the scope of the invention herein involved, it is intended that
all matter contained in the above description shall be interpreted as illustra-
tive and not in a limiting sense.
