Hardening phenol-formaldehyde resols in the presence of aniline

In a process for hardening phenol-formaldehyde resol resins, comprising conducting the hardening of the resins with an acidic solution of water and/or an organic solvent having dissolved therein an aromatic sulphonic acid and/or concentrated sulphuric acid, the improvement comprising the hardening in the presence of aniline added to the phenolic resin beforehand in a quantity of between about 0.5 and 2% relative to the weight of the phenolic resin whereby the pot life is extended and the cure time is satisfactory, the resultant products being especially useful as prepegs.

BACKGROUND OF THE INVENTION 
The present invention relates to a new system for curing phenolic resins, 
and in particular to a system for using phenol-formaldehyde resols. 
The curing of phenolic resols by the addition of strong acids is known. For 
example, hydrochloric acid, sulphuric acid, phosphoric acid, 
trichlorolacetic acid, and sulphonic acids are employed as acids, either 
alone or as mixtures. Most frequently, these acids are used in the form of 
aqueous solutions at concentrations ranging between 20 and 70%. (Such 
curing agents are also referred to herein as hardening agents or 
catalysts.) 
For some applications, the phenolic resins are conventionally cured in the 
presence of reinforcing agents, e.g. cellulose or glass fibers, non-woven 
fibers made of high molecular weight polester or of polyvinyl chloride for 
example, or glass mat, or cloth made, for example, of an aromatic 
polyamide, glass or asbestos. After preliminary curing of the phenolic 
resin, the composite obtained is then finally hardened at a temperature 
generally in the region of 90.degree. C. 
In the manufacture of some reinforced compositions it is sometimes 
essential to be able to increase the pot or shelf life of the phenolic 
resin at ambient temperature, on the one hand, but also to permit a high 
rate of hardening of the resin when the composition is cured at higher 
temperatures. It can be seen therefore that in practice there are two 
opposing problems to be solved. On the one hand, the resin must not harden 
too quickly in the cold but, on the other hand, it is also necessary that 
it retains a hardening potential which is sufficient to enable it to be 
quickly cross-linked when heated. 
As examples of reinforced compositions where it is necessary to solve the 
problems outlined above, mention can be made of the manufacture of 
phenolic prepegs, the manufacture of phenolic materials prepared by 
filament winding, and the manufacture of articles by the "pultrusion" 
technique, a technique consisting of pulling the finished product by 
making it pass through a die. 
The equipment employed for the manufacture of materials prepared by 
filament winding comprises, in brief, an impregnation trough containing 
the phenolic resin and curing agent, in which the glass fibers are 
impregnated. After impregnation the fibers are drained and then reeled 
around a mandrel before being hardened in an oven at temperature of 
between 60.degree. and 90.degree. C. The techniques employing pultrusion 
involve the impregnation of reinforcing agents with the aid of resins. 
This impregnation is carried out with impregnation troughs which contain 
the resin to which a curing agent has been added. The impregnation is 
followed by a heating stage which make it possible to carry out the 
polymerization of the resin and then a stage of drawing with the aid of a 
heated die in which the polymerization of the resin is continued. If 
appropriate, the heating stage may be carried out after the passage 
through the die. The reinforcing agents employed in the pultrusion 
technique generally consist of glass fiber rovings. The phenolic resins 
are particularly suitable for the use of the pultrusion technique. Whether 
the materials are manufactured by the filament winding techniques or by 
the pultrusion technique, it is essential, in order to avoid losing large 
quantities of resins that the phenolic resin does not harden too quickly 
in the impregnation troughs which are kept at ambient temperature. 
Furthermore, it is also necessary and advantageous that after impregnation 
of the fibers, the phenolic resin can harden when heated and that it does 
so quickly in order that the material obtained occupies the oven for a 
minimum time. 
To solve these problems, it is possible to reduce the concentration of the 
acid hardener solution and to employ aqueous solutions which have an acid 
concentration below 20%. In this case, large quantities of water are 
carried along, which effectively increase the pot life at ambient 
temperature but which have the disadvantages of reducing the rate of 
hardening when heated and imparting imperfections to the finished 
products, in the form of blisters and micropits. It has also been proposed 
to replace the water completely with organic solvents to obtain dilute 
acid solutions. In this case, an increase in the pot life at ambient 
temperature is, in fact, observed, but so is a reduction in the rate of 
hardening when heated; moreover, it is observed that excessive quantities 
of solvents are found in the finished products and cause, in particular, a 
decrease in the mechanical properties of these products, which in most 
cases is reflected in cracks. In addition, the use of these solvents 
presents storage and toxicity problems which can only be solved by the use 
of costly extraction equipment. From an economic point of view, there is, 
therefore, no advantage in using curing agents dissolved in organic 
solvents. 
SUMMARY OF THE INVENTION 
An object of this invention, therefore, is to provide an improved system, 
incorporating both process and product aspects, which will result in a 
more satisfactory combination of a long pot life and a high thermal curing 
rate. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art. 
The hardening of the resins is carried out with the aid of solutions in 
water and/or organic solvents of an acid, especially an aromatic sulphonic 
acid and/or concentrated sulphuric acid, wherein the improvement comprises 
conducting the acid hardening in the presence of aniline previously added 
to the phenolic resin. 
It is also important that the quantiy of aniline employed is between 0.5 
and preferably not more than about 2% relative to the weight of the 
phenolic resin and preferably between 0.8 and 1.5%. Below 0.5% no 
substantial effect is observed on the pot life at ambient temperature or 
on the rate of hardening when the resin containing the hardener is heated. 
Above 2% by weight, it has been found that the content of free aniline can 
no longer be tolerated insofar as the toxicity of the resin is concerned. 
Moreover, with higher quantities of aniline, e.g. 3%, the resol produces a 
gel which does not harden when heated for industrially acceptable 
residence times. 
By this invention, it is possible to carefully adjust the pot lives and the 
rates of thermal hardening. (By thermal hardening is meant at a 
temperature of generally about 70.degree. to 100.degree. C., especially 
90.degree. C.) 
Surprisingly, it has been found that the aniline employed within the limits 
claimed according to the invention not only has a positive effect on the 
abovementioned characteristics of the phenolic resins but, in addition, 
but also that it was present, after being added to the resin, in free form 
within limits which can be tolerated from the point of view of toxicology. 
It is as if the aniline was combined with the products present in the 
resol while having no negative effect on the reactivity of these resols. 
Given that the resols contain free formaldehyde, it might have been 
thought that the addition products of formaldehyde and aniline prepared 
beforehand and then added to the resol would have had the same effect as 
the aniline added to the resol according to the process of the invention. 
But this is not the case; in fact it has been found that the products of 
addition of formaldehyde and aniline which are prepared beforehand and 
then added to resol are insoluble in the latter. 
According to an important characteristic of the process of the invention, 
aniline is therefore added to the phenolic resin not at the time of its 
use, that is to say just before being catalyzed with the aid of hardening 
catalysts, but it is added before sale. In fact is has been found 
surprisingly that the aniline added in this manner has a beneficial effect 
on the pot life at ambient temperature of the mixture constituted with 
phenolic resin, catalyst and aniline and also on the hardening when heated 
with the aid of conventional catalysts. 
Furthermore, it might have been thought that since the invention produced 
very good results with aniline, a chemical compound containing an amine 
function, that straight-chain amines or other aromatic amines might also 
be useful to the same degree. It has been found, however, that amines such 
as diethylamine, methylethylaniline, or polyamines, had no effect on the 
resols, because they are incompatible with the latter from the point of 
view of miscibility or, as in the case of diethylaniline, have only a very 
limited effect. 
The hardening of the phenolic resins is conducted conventionally with the 
aid of catalysts consisting essentially of an acid and water and/or and 
organic solvent. Without restriction being intended, conventional acids 
are para-toluenesulphonic, ortho-toluenesulphonic, benzenesulphonic and 
xylenesulphonic acids. By organic solvents is meant compounds containing 
an alcohol function, methanol, ethanol, propanol, isopropanol, as well as 
polyols such as glycerol, dipropylene glycol and triethylene glycol. The 
concentration of the acid is generally on a weight basis about 20 to 80%, 
preferably 30 to 60%. 
The quantities of hardening catalysts employed are those conventionally 
employed for hardening resols. These quantities (meaning the acid with 
water and/or solvent) are between 4 and 25% by weight relative to the 
weight of the resol and preferably between 6 and 12%. The resols employed 
are conventional resols prepared by the condensation of formaldehyde with 
phenol in the presence of an alkaline catalyst. They have a molar ratio 
(f/p) of between 1.5 and 2.5 and may, if appropriate, contain conventional 
additives such as plasticizers, surfactants, fillers, and the like. 
Accordingly, a composition aspect of this invention consists essentially of 
a resol having a formaldehyde to phenol ratio (f/p) between about 1.5 to 
2.5; and about 0.5 to 2% preferably about 0.8 to 1.5% of aniline relative 
to weight of the phenolic resin. 
The present invention is perfectly suitable in some processes for 
manufacturing phenolic prepegs and in processes for manufacturing 
reinforced composition employing equipment in which the phenolic resin to 
which the hardening catalyst has been added is stored in a vessel through 
which glass fibers which are thus impregnated with phenolic resin pass 
continuously. In general, the invention is suitable in all cases where the 
phenolic resin should have an extended pot life but should also be capable 
of being rapidly hardened when heated. 
Another aspect of this invention relates to the hardened products produced 
on the basis of this invention. 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore, to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever. In the following 
examples, all temperatures are set forth uncorrected in degrees Celsius; 
unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1 
A phenol-formaldehyde resin with a molar ratio f/p of 1.5 was mixed in a 
rotary drum with variable quantities of aniline, 8% of a curing agent 
(also called catalyst in the following examples) consisting of 
paratoluenesulphonic acid dissolved at a concentration of 60% in water was 
added, the following results were obtained: 
______________________________________ 
Gel time at 20.degree. C. 
Aniline % by weight 
(minutes) 
______________________________________ 
0 8 
1 18 
1.2 20 
1.5 23 
______________________________________ 
EXAMPLE 2 
Example 1 is repeated with the use of a resol of an identical molar ratio 
f/p but which has a reactivity of 80.degree. C. instead of 110.degree. C. 
The reactivity is defined by the exothermic peak of a mixture consisting 
of 100 parts of resin and a 50% strength solution of sulphuric acid. The 
following results are obtained by using 10% by weight of catalyst 
consisting of a 50% strength solution of sulphonic acid, 5% of phosphoric 
acid, the remainder being water. 
______________________________________ 
REACTIVITY WHEN 
HEATED 
ANILINE % GEL TIME AT Temperature 
Peak 
BY WEIGHT 20.degree. C. (minutes) 
.degree.C. (minutes) 
______________________________________ 
0 10 116.degree. C. 
14 
1 21 120.degree. C. 
29 
2 40 104.degree. C. 
40 
3 Gel very soft 40.degree. C. 
after 4 h 
______________________________________ 
It is thus found that at high aniline contents namely 3% there is no 
formation of soft gel at ambient temperature. Thus, if a cure of this 
resin is to be carried out, it will therefore be necessary to heat for a 
very long time to obtain the hardening of the resin, which is industrially 
unacceptable. 
EXAMPLE 3 
Example 1 is repeated but with the use of a phenolic resin with a molar 
ratio f/p of 2. The resin is then hardened with the aid of 15% by weight 
of the same catalyst employed in Example 2. 
The following table shows the results obtained: 
______________________________________ 
REACTIVITY 
ANILINE CATALYST GEL TIME WHEN HEATED 
% BY % BY AT 20.degree.C. 
Temper- Peak 
WEIGHT WEIGHT (minutes) ature (.degree.C.) 
(minutes) 
______________________________________ 
0 15 12 92 17 
1 15 25 70 32 
______________________________________ 
EXAMPLE 4 (COMATIVE) 
A phenolic resin with a molar ratio f/p of 1.5 is employed. Various 
quantities of aniline and diethylaniline are added to the resol. The resin 
is catalyzed with the aid of 10% by weight of the catalyst employed in 
Example 2. The following table summarizes the results obtained: 
______________________________________ 
REACTIVITY 
ANILINE GEL TIME WHEN HEATED 
% BY DIETHYL- AT 20.degree. C. 
Temper- Peak 
WEIGHT ANILINE (minutes) ature (.degree.C.) 
(minutes) 
______________________________________ 
0 10 116 14 
1 21 120 29 
2 40 104 50 
1 14 126 20 
2 25 115 35 
______________________________________ 
It is found that diethylaniline is less effective than aniline. Moreoever, 
when the resin is heated, an unpleasant odor of diethylaniline is 
released. 
EXAMPLE 5 
Instead of adding aniline directly to the resol as done according to the 
present invention, an addition product of formaldehyde and aniline was 
prepared beforehand which was then added to the resol. This intermediate 
product is made by adding 46.5 g of aniline (0.5 mole) to 50 g of a 30% 
strength solution of formaldehyde (0.5 mole). An exothermic reaction is 
then observed which results in a viscous product to which dipropylene 
glycol is added. The product obtained is insoluble in water and also 
insoluble in the resol and in dipropylene glycol. 
Relating to "reactivity when heated" this is a reactivity test. According 
to this test phenolic resin and catalyst are mixed and placed in an 
isotherm cell. A thermocouple is placed inside the mixture. When catalyst 
is added, there is an exothermic reaction. Temperature is measured then a 
courbe is traced with temperature in Y-axis and time in X-axis; at the end 
of the reaction, an exothermic peak appears: this peak definite the 
reactivity. 
For a given resin when time is short, this meaning that resin is reactive. 
In examples, it is noted that when anilin is added time is long: this 
meaning that the resin is less reactive. In others words the pot life is 
improved. 
The reactivity of starting resol is determined according to the identical 
test described hereabove but sulfuric acid is used instead of catalyst 
used in examples and moreover ethyl alcohol is added in resol. This test 
is achieved in order to determine the reactivity and the condensation 
degree of phenolic resin. When a resin has a reactivity of 110.degree. C. 
(example 1) this meaning that resin is more reactive and less condensed 
than a resin which has a reactivity of 80.degree. C. (example 2). 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding example. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.