Process for curing thermoset resins using phenyl esters of carboxylic acids as latent catalysts

Latent catalysts comprising the phenyl esters of certain carboxylic acids are disclosed for use in the cure thermosetting resins curable by an acid catalyst.

The present invention involves the novel use of certain phenyl esters of 
carboxylic acids as catalysts in the cure of thermosetting resins. These 
catalysts are latent, being catalytically inactive at lower temperatures, 
but become catalytically active at temperatures encountered during the 
curing of the resin. 
Thermosetting compositions have been known for many years and have been 
employed in many applications because of such advantageous properties as 
light weight, high heat resistance and excellent dimensional stability. 
The traditional method for processing thermoset molding compounds involves 
blending with fillers, pigments and other additives, followed by 
compounding and granulating. The granulated compositions are then 
fabricated by any of the well known methods such as compression, transfer, 
or injection molding. This multi-step process is cumbersome and energy 
intensive. Furthermore, the compounding step often causes considerable 
attrition of the reinforcing fiber and thus the reinforcing action of the 
fiber is not effeciently utilized. As a consequence, ultimate high impact 
resistance is often not obtained. 
Recently, there have been devised thermoset fabrication methods in which 
liquid thermosetable compositions are injected directly into a mold where 
cure takes place resulting in the formation of a fabricated part. 
Depending on the injection process and the nature of the compositions, 
these methods are known as liquid injection molding (LIM), reaction 
injection molding (RIM), or resin transfer molding (RTM). 
Another process for manufacturing high strength thermoset composites is the 
sheet-molding compound (SMC) method. In this process, a liquid 
thermosetting resin, such as an unsaturated polyester, reinforcing fiber 
and other additives are mixed under low shear conditions. The resulting 
viscous mixture is partially cured to non-tacky sheets. Final cure to 
finished parts is then carried out in a mold. The commercial applications 
of this method have been generally limited to unsaturated polyesters 
which, unless they are specially treated, exhibit poor flammability 
resistance. In addition, careful formulation is required to obtain good 
processability and smooth surfaces. 
These processes are rapid and adaptable to high speed production 
requirements, and since the curing reaction is generally exothermic, these 
processes are less energy intensive than the traditional methods for 
processing thermosets. Furthermore, because these relatively new thermoset 
fabrication methods are low pressure processes, there is required 
considerably lower clamping forces than those required for the injection 
molding of engineering plastics, and thus lower capital and operating 
expenditures are required. 
It has recently been found that certain resins are particularly adaptable 
for use in injection molding processes because of their low viscosity and 
their having a low amount of unbound water. These resins are disclosed in 
U.S. Patent Applications Ser. No. 340,853 and 340,855 filed concurrently 
herewith by Brode and Chow, and Brode, Chow and Hale, respectively. These 
resins are high-ortho resoles containing hemiformal groups. They are of 
low viscosity, are essentially free of unbound water and can be cured to 
phenol-formaldehyde thermoset shapes and when mixed with reinforcing 
material to composite shapes. 
Another class of thermosetting compositions found suitable in injection 
molding processes are hemiformals of phenol and methylolated phenol and 
solutions of these hemiformals as disclosed in U.S. Patent Applications 
Ser. No. 340,719 filed by Covitz, Brode and Chow, Ser. Nos. 340,790 and 
340,720 filed by Brode and Chow and Ser. No. 340,695 filed by Brode, Chow 
and Hale, all on Jan. 19, 1982. These compositions are hemiformals formed 
by the reaction of formaldehyde with the phenol hydroxy and methylol 
groups of phenol. These compositions are stable and are of low viscosity 
and can be used to form phenol-formaldehyde resin. They can also be mixed 
with a co-reactive polymer to form solutions useful in liquid injection 
molding and are curable to solid shapes. 
Generally, the catalyst used for curing the above phenol-formaldehyde 
resins and other thermosetting compositions has been acids or bases. 
Suitable acid catalysts that have been used include sulfuric acid, phenyl 
sulfonic acid, phosphonic acid, oxalic acid, ferric chloride, and toluene 
sulfonic acid. These acids have been used successfully with thermosetting 
resins, such as those disclosed above. However, these acids have 
significant catalytic activity, even at ambient temperatures 
(20.degree.-30.degree. C.). Therefore, a resin that contains an acid 
catalyst very quickly begins to gel and form a solid resinous product 
after the catalyst has been added. Therefore, when used in liquid 
injection molding processes the acid catalyst is typically mixed with the 
resin shortly before injection into the mold. A less expensive and more 
convenient method would be to mix the catalyst with the resin and then 
store it until it is needed. However, because of the activity of 
conventional acid catalysts at storage temperatures, an attempt to store a 
resin containing an acid catalyst would risk gelling and solidification of 
the resin in the storage areas and process lines. It would, therefore, be 
highly desirable to have available a catalyst that is latent, namely one 
that exhibits little or no activity at ambient conditions in which the 
resin is stored, but becomes catalytically active at the cure conditions 
found in a mold. Then the catalyst could be added to the resin and the 
resin stored until used without danger of the resin gelling. 
It has now been found that certain phenyl esters of carboxylic acids can be 
used as latent catalysts in the curing of thermosetting resins that are 
curable by an acid catalyst. 
These catalysts when mixed with thermosetting resins show little or 
insignificant activity at storage temperatures, allowing storage of the 
catalyst-containing resin. However, when exposed to curing temperatures, 
they become very catalytically active and facilitate rapid curing of the 
resin. 
The catalysts useful in the method of the invention are phenyl esters of 
carboxylic acids that hydrolyze to form acids having a pK.sub.2 of about 2 
or less when exposed to water and temperatures greater than about 
100.degree. C. These compounds when mixed with a resin at storage 
temperatures, about 60.degree. C. or less, are neutral or only weakly 
acidic. Therefore, their catalytic activity is very small at storage 
conditions. However, when exposed to curing temperatures, about 
100.degree. to 200.degree. C., they produce in situ strong acids that 
catalyze the curing of the resin. 
At storage conditions the latent catalyst exists as the unhydrolyzed phenyl 
ester which is neutral or only weakly acidic. This is due to the 
temperature being too low to activate a hydrolysis reaction and/or the 
absence of water in the resin. When the resin is heated to cure 
temperatures, it undergoes a heat activated condensation reaction in which 
water is released. The phenyl ester latent catalyst hydrolyzes at the cure 
temperature and in the presence of the water from the condensation 
reaction, and other water that may be present, to form a carboxylic acid 
that acts as an acid catalyst. This process is accelerated as the 
carboxylic acid catalyzes the condensation reaction, producing more water. 
Thus, a highly active acid catalyst is produced which effectuates a rapid 
cure of the resin. The condensation reaction, whereby the resin is cured, 
is further accelerated by the removal of water from the system. The rate 
of cure is inversely proportional to the concentration of water, so 
removal of water by hydrolysis of the phenyl ester increases the rate. 
The preferred phenyl esters of carboxylic acid include phenyl 
trifluoroacetate and phenyl hydrogen maleate. Phenyl trifluoroacetate is 
neutral and phenyl hydrogen maleate is a weak acid (pK.sub.a greater than 
3). Both yield strongly acidic products upon hydrolysis, namely, 
trifluoroacetic acid (pK.sub.a =0.27), and maleic acid (pK.sub.a =1.94), 
respectively. 
The phenyl esters may be introduced as such or made in situ by reacting an 
anhydride of a suitable carboxylic acid, such as maleic anhydride, with 
phenol to form the phenyl ester. 
Other suitable phenyl esters include the phenyl esters of the following 
acids: 4-chloro-o-phthalic acid (pK.sub.a =1.6), dibromosuccinic acid 
(pK.sub.a =1.5), 2,6-dihydroxybenzoic acid (pK.sub.a =1.3), and 
but-2-yne-1,4-dicarboxylic acid (pK.sub.a =1.75). 
Esters of polymers that have acid groups incorporated into their structure 
can also be used, providing they have the above recited hydrolysis 
properties at elevated temperatures. Examples include copolymers of vinyl 
alkyl ether-maleic anhydride and styrene-maleic anhydride. These polymeric 
acid-esters when used in sufficient quantities can also serve to modify 
polymer properties. 
The phenyl ester latent catalyst is introduced in an amount to give a 
concentration of about 0.2 to about 10 weight percent, preferably about 
0.5 to about 5 weight percent, based on the weight of the catalyst free 
resin. 
Resins suitable for use in the method of the invention are thermosetting 
resins that are curable using an acid catalyst. These include 
phenol-formaldehyde, urea-formaldehyde, and melamine-formaldehyde resins. 
Preferably the resins used in the method of the invention are usable in 
sheet molding compounds or in injection molding processes. If used in 
sheet molding compound processes, the resin used in the method of 
invention should have a viscosity less than about 500,000 centipoise 
(Brookfield) at 25.degree. C. 
When used in liquid injection molding processes, the resin should have a 
viscosity less than about 10,000 centipoise (Brookfield) at 25.degree. C. 
Resins suitable for use in the method of the invention include the 
hemiformals of phenol and methylolated phenol and solutions of these with 
a co-reactive polymer as disclosed in the above cited United States, 
Applications, Ser. Nos. 340,719, 340,790, and 340,720. These resins are 
described as having any one of the formulas below, 
##STR1## 
wherein R is any substituent typically employed in conjunction with a 
phenolic structure, n is a positive number of at least 1, preferably about 
1 to about 5, most preferably about 1.2 to about 2.5. b is 1 to about 5, c 
is 1 to about 3, and d is 0 to about 2, x is 0 to 3, the sum of c and d 
is at least 1 and no greater than 3 and the sum of c, d, and x is at least 
1 but no greater than 5, where x=0 or at least 50 mole percent of the 
hemiformal, and with respect to the R substituent, at least 2 of the 
ortho- and para- positions are free in relation to the --OH and 
--O(CH.sub.2 O).sub.n H groups. With respect to R, it is preferably a 
monovalent radical which includes alkyl of from 1 18 carbon atoms, 
cycloalkyl from 5 to 8 carbon atoms, aryl containing from 1 to 3 aromatic 
rings, aralkyl, alkaryl, alkoxy containing from 1 to 18 carbon atoms, 
aroxy containing 1 to 3 aromatic nuclei, halide such as chloride, bromide, 
fluoride, and iodide; alkyl sulphido having from 1 to 18 carbon atoms, 
aryl sulphido having from 1 to 3 aromatic nuclei, and the like, as well 
as, a radical derived from an oil such as linseed oil or tung oil. 
Examples of co-reactive polymers are phenol-formaldehyde resoles, 
phenol-formaldehyde novolacs, aromatic polyesters, aromatic 
polycarbonates, unsaturated polyesters, poly(aryl-ethers), 
urea-formaldehyde resin, and melamine-formaldehyde resins. 
Also included in the resins suitable for use in the invention are the 
phenolic resins disclosed in U.S. Pat. No. 3,485,797, issued to Robins on 
Dec. 23, 1969. These are phenol formaldehyde resins having the general 
formula 
##STR2## 
wherein R is hydrogen, hydrocarbon radical, oxyhydrocarbon radical or 
halogen, meta to the hydroxyl group of the phenol; m and n are numbers the 
sum of which is at least two and the ratio of m to n is greater than one; 
and A is a hydrogen or a methylol group, the molar ratio of said methylol 
group to hydrogen being at least one. 
A preferred resin for the method of the invention is disclosed in the above 
cited application, U.S. Ser. No. 340,853. These resins are high-ortho 
phenol-formaldehyde resole polymers containing hemiformal groups and 
having the general formula 
##STR3## 
wherein a is from 0 to 3, b is 0 to 1, the sum of a and b does not exceed 
3, the sum of c and d is from 2 to about 20, the mole fraction d/(c+d) is 
0.4 to 0.9, preferably 0.6 to 0.8, R is --CH.sub.2 O(CH.sub.2 O).sub.e H, 
e is 0 to about 5, and X is a monovalent radical, wherein for at least one 
of the R or O(CH.sub.2 O).sub.e H groups e is at least 1 and wherein at 
least 50 mole percent of the 
##STR4## 
X is any substituent typically employed in conjunction with a phenolic 
structure. With respect to X, it is preferably a monovalent radical which 
includes alkyl of from about 1 to about 18 carbon atoms, cycloalkyl from 5 
to 8 carbon atoms, aryl containing from 1 to about 3 aromatic rings, 
aralkyl, alkaryl, alkoxy containing from 1 to about 18 carbon atoms, aroxy 
containing from 1 to about 18 carbon atoms, aroxy containing 1 to 3 
aromatic nuclei, halide such as choride, bromide, fluoride, and iodide, 
alkyl sulphides having from 1 to about 18 carbon atoms, aryl sulphides 
having from 1 to about 3 aromatic nuclei, and the like. These resins are 
essentially free of unbound water and volatile organic compounds and have 
a low viscosity, as low as 1,000 centipoise (Brookfield) at 25.degree. C. 
By essentially free of unbound water is meant that the resin contains less 
than about 5 weight percent, preferably less than about 2 weight percent, 
of unbound water and volatile organic compounds, based on the total weight 
of the resin. Concentrations less than 1 weight percent are achievable. By 
unbound water is meant that water present as an impurity and is 
distinguished from the water produced from the condensation reaction. 
Volatile organic substances are those that volatilize to form a gas when 
the resin is exposed to elevated temperatures, about 100.degree. C. These 
include formaldehyde not incorporated in methylol groups, hemiformal 
groups on methylol linkages of the resin. Also includes are the solvents 
typically used to reduce the viscosity of a resin, such as alcohol or 
aromatic hydrocarbons. Also included are such substances such as methanol 
that may be introduced as contaminants in the formaldehyde or phenol used 
in manufacture of the resin. 
The amount of water that can be tolerated in the resin used in the method 
of the invention depends on the susceptibility of the phenyl ester 
catalyst towards hydrolyzation. For example, phenyl hydrogen maleate at 
temperatures below about 60.degree. C. will hydrolyze insignificantly, 
even in the presence of a significant amount of water in the resin. 
However, at elevated temperatures and when in the presence of water, 
phenyl hydrogen maleate will dissociate to form maleic acid. Therefore, 
when phenyl hydrogen maleate or a phenyl ester having similar hydration 
properties is used, resins having significant amounts of water may be 
used. Resins containing up to about 5 weight percent water have been found 
satisfactory. 
On the other hand, phenyl trifluoroacetate hydrolyzes and forms acids quite 
readily at low temperatures. Therefore, when using a phenyl ester that 
hydrolyzes readily, such as phenyl trifluoroacetate, as a latent catalyst 
the resin should contain less than about 1 weight percent, based on the 
weight of the resin, of unbound water. Such a suitable resin is that 
disclosed in the above cited United States Ser. No. 340,853, wherein the 
resin is made under azeotropic distillation conditions to remove residual 
water. 
The resin containing the latent catalyst can be cured using known methods 
in the art. The preferred curing method is any one of the liquid injection 
molding processes wherein the resin is injected into a mold, and cured by 
heating the mold. The catalyst may be premixed with the resin and stored 
before the curing. The catalyst containing resin is stable at storage 
temperatures with stability decreasing as the temperature rises. Storage 
temperatures lower than 60.degree. C. are suitable although storage 
temperatures lower than 30.degree. C. are preferred. 
The catalyst containing resins can be cured at temperatures between about 
100.degree. C. and about 200.degree. C., preferably between 120.degree. C. 
and about 180.degree. C. The optimum curing temperature for the most rapid 
cure rate depends in large part on the kinetic properties of the 
particular latent catalyst used. For phenyl hydrogen maleate, the optimum 
curing temperature is about 150.degree. C. to 160.degree. C. For 
trifluoroacetate, the optimum curing temperature is about 140.degree. C. 
The rate of the cure can be regulated by the cure temperature and the 
concentration of the catalyst. 
Optionally, resins containing the latent catalyst may be blended with a 
reinforcing material such as glass fiber, graphite fiber, carbon fiber, 
wollastonite, cellulousic fibers such as wood fiber and the like, organic 
fibers such as aromatic polyamide fibers, and mica. The preferred 
reinforcing materials are glass fiber, graphite fiber, carbon fiber, and 
aromatic polyamide fiber. These fibers may be in any form common in the 
art, such as chopped fiber, mat, and woven cloth. 
When used in sheet molding compound processes, the catalyst-containing 
resins can be blended with a chopped fiber and cured to a B-stage resin 
and then later to finished composite shapes. 
In liquid injection molding processes, the fiber may be placed in any 
suitable form into the mold before injection of the catalyst-containing 
resin. 
The following examples demonstrate the invention and are not intended to 
limit the invention in any way.

EXAMPLE 1 
Evaluations of phenyl hydrogen maleate and phenyl trifluoroacetate as 
latent catalysts were made by obtaining differential scanning calorimetric 
data (DSC) and the viscosity vs. time relationships. 
DSC is a method for determining the catalytic activity of a catalyst. The 
cure of phenol-formaldehyde resins is exothermic; therefore, the 
temperature of the exothermic peak of a DSC spectrum is a measure of the 
temperature where the most rapid cure rate occurs and the activity of the 
catalyst during cure. 
The relationship of viscosity vs. time is a measure of stability at a given 
temperature. The stability can be measured by the time it takes the 
viscosity to double. An increase in viscosity indicates reaction of the 
resin to form higher molecular weight products. 
Two latent catalysts, phenyl hydrogen maleate and phenyl trifluoroacetate, 
were evaluated as well as two conventional acid catalysts, diphenyl 
hydrogen phosphate and sulfuric acid. Each catalyst was added to a resin 
at a concentration of 2 weight percent, based on the weight of the 
catalyst free resin. The phenyl hydrogen maleate was prepared from maleic 
anhydride and excess phenol and was added as a phenol solution. The 
concentration of the phenyl hydrogen maleate was about 4 weight percent. 
The polymer used was a phenol-formaldehyde resole resin having hemiformal 
groups as disclosed in the above cited U.S. Application Ser. No. 340,853. 
It was made by charging into a 5 gallon vessel equipped with a water 
separator 7058 grams of phenol, 35.3 grams of zinc acetate dihydrate and 
494 grams of toluene. The solution was stirred and heated to about 
100.degree. C., after which 9206 grams of an aqueous solution of 
formaldehyde containing 48.9 weight percent formaldehyde were metered into 
the reaction mixture over five hours. There was an initial mild exotherm, 
which was easily moderated by regulating the source of heat. Water added 
with the formaldehyde was then removed azeotropically with the toluene 
using the water separator and a condenser. After all the aqueous 
formaldehyde solution had been added, the mixture was azeotropically 
distilled for about 1.5 hours at atmospheric pressure to a temperature of 
about 110.degree. C. The toluene and water were removed as an azeotropic 
vapor mixture which was condensed using the water separator. The 
heat-source was then removed and the distillation continued under a 
reduced pressure of 50 mm Hg for about 1/2 hour. Total water recovered 
corresponded to about 106% of the water added in the aqueous formaldehyde 
solution. The resulting composition has a viscosity (Brookfield Model RVT) 
of about 4000 cp at 29.degree. C. 
Resins containing the catalysts and a catalyst-free resin, for comparison, 
were evaluated for activity at cure temperatures by using standard 
differential calorimeter apparatus and the peak exotherm determined. The 
calorimeter was a Dupont Differential Thermoanalyzer Model 990 equipped 
with a pressure cell. The data are summarized in Table I. As seen by the 
peak exotherms, shown in Table I, the curing activity of the latent 
catalysts of the invention approaches the activity of a conventional 
catalyst used in the art, diphenyl hydrogen phosphate. 
Catalyst containing resins were evaluated for stability at storage 
temperatures by allowing them to stand at a temperature of 25.degree. C. 
to 30.degree. C. and the time determined in which the viscosity doubled. 
These times (t.sub.2) for each resin are shown in Table I. As shown by 
these times, the stability of the resins containing the latent catalysts 
used in the invention are significantly greater than resins containing 
conventional catalysts. The decreased stability of the phenyl 
trifluoroacetate containing resin is probably due to its extremely facile 
hydrolysis with the trace amounts of water in the resin. The decreased 
stability of the phenyl hydrogen maleate is probably due to the fact that 
phenyl hydrogen maleate is itself a weak acid. 
TABLE I 
______________________________________ 
Peak Evotherm 
Catalyst Temperature (.degree.C.) 
t.sub.2 (days) 
______________________________________ 
None 219 250 
Phenyl hydrogen maleate* 
158 3 
Phenyl trifluoroacetate* 
133 1 
Diphenyl hydrogen 
phosphate** 127 0.17 
Sulfuric acid** -- &lt;0.1 
______________________________________ 
*Latent catalyst 
**Conventional acid catalyst 
EXAMPLE 2 
A thermosetting resin was prepared as in Example 1. Phenyl hydrogen 
maleate was prepared by reacting excess phenol with maleic anhydride and 
was added to the above resin. 
The resin was cured using a liquid injection molding apparatus described in 
U.S. Patent Application Ser. No. 139,906, filed Apr. 14, 1980 by Angell. 
Fiberglass mat was placed into a mold, the mold was preheated to 
150.degree. C. and clamped with a hydraulic press. An exothermic reaction 
ensued. When the mold cooled to its initial preheated temperature of about 
150.degree., the cured composite plaques were removed from the mold. The 
cycle times were measured and are shown in Table II. 
The plaques were evaluated for their physical properties. In these 
evaluations the following standard tests were used: 
Flexural Modulus--ASTM D790 
Flexural Strength--ASTM D790 
Notched Izod (Impact)--ASTM D256 
Tensilte Modulus--ASTM D638 
Tensile Strength--ASTM D638 
Elongation--ASTM D638 
The glass fiber used was glass fiber mat, designated as type AKM available 
from PPG Industries, Pittsburgh, Pennsylvania. 
In Table II are summarized the results of the tests, the glass content 
expressed as weight percent glass, based on the weight of the cured 
composite, mold cycle time in seconds, and the concentration of the 
catalyst expressed as weight percent phenyl hydrogen maleate based on the 
weight of the catalyst-free resin. 
The data in Table II show the use of the latent catalysts of the invention 
in curing resins to form composites. Excellent physical properties of the 
composites as well as short cycle times are shown. 
TABLE II 
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Catalyst Mold Flexural 
Flexural 
Tensile 
Tensile Notched 
Conc. 
Wt. % 
Cycle 
Modulus 
Strength 
Modulus 
Strength 
% Izod 
(wt. %) 
Glass 
(sec) 
(psi .times. 10.sup.-6) 
(psi .times. 10.sup.-3) 
(psi .times. 10.sup.-3) 
(psi .times. 10.sup.-3) 
Elong. 
(ft-lbs/in) 
__________________________________________________________________________ 
9.8 51 150 1.69 33.3 1.45 23.0 2.2 26 
9.8 53 120 1.84 36.3 1.55 24 2.1 25 
9.8 52 90 1.45 29.5 1.47 22.8 2.0 28 
3.9 52 300-400 
1.48 24.3 1.50 22.7 1.9 21 
3.9 60 300-400 
2.2 35.7 1.67 25.6 2.1 35 
__________________________________________________________________________