Catalysis of Urea-Formaldehyde Condensation I. J. GRUNTFEST AND E. M. YOUNG, JR. Rohm & Haas Co., 5000 Richmond St., Philadelphia 37, Pa.
O n e of the traditional dilemmas of resin chemists arises from the fact that the B stage of the resin-the soluble or formable precursor of the final product-must be sufficiently stable to ship and handle and yet must be sufficiently reactive to go over to the C or final stage without the use of severe conditions of cure. A technique for meeting this dilemma is suggested which exploits the acid neutralizing capacity of the solvent-in the present case, water-to produce a latency of the catalyst. In particular, certain cationic acids (in the Bronsted sense) such as pyridinium ion show no catalytic activity in the presence of water but are very effective in promoting the conversion of B stage to C stage resin when the water is removed. Typical data show that a considerable number of amine ions having acid dissociation constants in water between 1 O-' and 10-4 are effective, while carboxylic acids o f the same strength are completely ineffective. Activity is destroyed or reduced under certain conditions where intramolecular neutralization or cyclization with formaldehyde is likely. The possibility of applying these catalysts to reactions other than the urea-formaldehyde condensation i s considered.
T H E use of urea-formaldehyde resina for the productio: of wet strength paper leads to one of the traditional dilemmas of resin chemists. That is, the B stage ( I ) , the soluble or formable precursor of the final product, must be sufficiently stable to ship and handle and yet it must be sufficiently reactive to go to the C or final stage in a short time without the use of severe conditions of cure. In the applications of these resins in paper (7') this problem is particularly acute. The performance of the resin depends on its ability to be adsorbed by the paper fibers at the wet end of the paper machine, and this adsorption is most efficient for the most highly condensed resins. Practically, compromises are made. Catalyst is withheld from the resin until just before application. Moderate pot life is accepted and moderately severe curing cycles are used. In some cases, for example in wood glues, a catalyst with some degree of latent activity is used. Of these, ammonium salts of strong acids are most common (3). Here the formaldehyde in the resin reacts slowly with the ammonium ion and liberates free acid which is the effective catalyst. Recently some ways have been discovered to meet this dilemma. These methods are particularly useful for aqueous resins but may be more generally applicable to other, nonaqueous, ureaformaldehyde preparations and indeed to a variety of other acid catalyzed processes such as those involved in the celluloseformaldehyde reaction and the cure of phenolic resins. This approach can be considered to be an exploitation of the acid neutralizing capacity of the solvent-in this case water, although in principle other solvents would work as well-to produce a latency of the catalyst. According to a current view, in the urea-formaldehyde mixtures that are used for the production of resins there is a relatively rapid, reversible reaction between urea and formaldehyde to give methylol ureas, which is both acid and base catalyzed. The subsequent condensation of the methylol ureas is a slower, acid catalyzed process ( 4 , 6). From an analogy with esterification, which is suggested by the kinetics of the second condensation, it is suspected that the acid January 1956
catalyst reacts with the methylol compound to give a complex which leads to the production of polymer. Since the methylol compounds can be expected to have a base strength, like other urea derivatives, comparable with that of water (6) and the concentration of water is relatively high, the reactions of an acid, A, in an aqueous B stage urea-formaldehyde resin can be formulated as follows: A
+ Methylol urea + activated complex A f HzO eA.Hz0
(1) (2)
where the larger arrow for the Equation 2 indicates that this is the major reaction. Now if A is a strong acid A.H20 may be sufficiently strong to catalyze the condensation and produce premature cure. Furthermore, in a practical situation, if A.HaO is a strong acid, special problem of corrosion and damage to the material being treated may arise. On the other hand if A . HzO is sufficiently weak, the mixture of acid, water, and resin may be stable for a satisfactorily long time. Of course, in order to be useful, the catalyst must be a strong enough acid to promote the condensation of the resin when the water concentration is reduced. A substantial number of acids of appropriate strength have been found. However, the acid strength as indicated by the p H of an aqueous solution is not an adequate criterion for activity. It is also necessary that the acid be cationic (B), although even among cationic acids there is a somewhat less than perfect correlation between activity and acid strengths as shown by the p H of aqueous solutions.
Effect of Pyridinium Ion A catalyst fulfilling the conditions for both latency and activity is the pyridinium ion. I n order to show the high degree of activity which can be achieved the following experiment with Uformite 470 (Rohm & Haas Co.) which is a commercial cationic wet strength resin for paper, and a water leaf bleached sulfite paper, 36-pound basis weight, waa conducted.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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The resin was applied to the finished sheet of paper by a tub sizing operation. A solution containing 3% of resin solids was applied and a weight of solution equal to the weight of the dry paper was picked up. The sheet n-as then dried in the laboratory air at room temperature. Separate samples were allon-ed to stand for different lengths of time at room temperatxe, and others were given heat treatments of different severity. Three different resin solutions were used for this experiment. One was adjusted to p H 4 with hydrochloric acid, the second \vas similar but contained 0,3% ammonium chloride n-hich is a commonly used catalyst for urea-formaldehyde resins, and the third was adjusted to pH 4 with hydrochloric acid after 0.45% of pyridine-equivalent to 0.3 % ammonium chloride-was added. Percentage wet strength, measared by Tappi methods ( 7 ) , of the untreated and treated sheets after various curing periods are shown in Table I. These values are used here as indexes of the state of cure of the resin. The use of percentage wet strength as a criterion of cure is not altogether satisfactory and no measnrements of molecular weight or solubility were made. HoTvever, it had been shown earlier ( 4 ) that low molecular weight resins or reactants which can diffuse the paper fibers are much less effective for producing wet strength than high molecular weight resins which must remain in a separate interfiber phase, This provides some basis for the exclusion of reaction with cellulose or perhaps other nonpolymer forming processes as decisive factors in the development of wet strength. The measurement recorded in the zero day column was obtained on the same day the resin was applied and generally as soon as the sample felt fairly dry to the touch. At that time it might still contain about 15% water.
Table 1.
stituted for the pyridine. Before treatment the pH of the solution was adjusted to integral values between 4 and 7. The wet strength values obtained after zero and 7 days aging at room temperature are listed in Table 11.
Table I!.
Dependence of Wet Strength Development on p H PH
S o Catalyst
5 6
1.3 1.1
0.8 0.5
0.:
7
0.5 a-Picoline
4 5
5.5
3.9 5.6 4.4
6
7
5 5
5,5 5,s
1.8
Dimethylaniline 4 5 6 7
5 9
5 6 5,3 4.2 1.3
5.4 1.5 0.7
Notice that oi-picoline and dimethylaniline are equally effective at pH 4 and perhaps 5. The former, however, is superior at the higher p R values of 6 and 7. Since the pH of half neutralization of the picoline is 5.7 and that of the dimethylaniline is 4.1, at the higher pH values, the concentration of acidic cation is substantially higher in the case of the picoline. Both cations are adequate for producing very rapid cure at the low pH values. -4more precise criterion for state of cure might show different relative activities for the different ions.
Wet Strength of Resin Treated Paper
Untreated paper 3% Resin, pH 4 3% Resin NHaCl 3% Resin pyridine adjusted t o p H 4
10% 13 9
54
..
36 52 55
i5
..
The rapid rate of rure shown by the pyridine-containing sample of urea resin can also be observed with Uformite 470 solutions containing relatively large amounts of alum-12% by weight on resin. With this preparation the zero day wet strength was found to be 41%. Here the alum probably provides a cationic acid TThich could act in the same way as the pyridinium ion. Buffers that do not contain a cationic acid show no catalytic effects at pH 4. For example, addition of sodium acetate and acetic acid, sodium acetate and hydrochloric acid, or McIlvaines mixtures (citric acid and potassium acid phosphate) t o Uformite 470 solutions yields exactly the same result as an uncatalyzed preparation at the same pH. Furthermore, the activity of the pyridinium ion itself is markedly reduced by the presence of carboxylic acid anions such as formate, acetate, or tartrate. I n connection with latency, the pyridine salt catalyzed solutions were generally as stable as those without catalyst. In addition the drift of pH toxard the acid side, which is characteristic of ammonium salt catalyzed resin solutions, is completely absent. Effect of pH
Some further detail of the action of cationic acids of the appropriate strength is revealed by the following. Paper is treated with 3% resin solution ag in the other experiment except that equivalent quantities of a-picoline and dimethylaniline are sub-
108
0.5 0.5
4
Effect
++
Wet Strength Lb./Inoh After Zero days 7 Days
OF
Different Amines
Since the test method is extremely simple, a large number of amines, which were available in this laboratory, were screened for catalytic activity in the urea-formaldehyde coildensation. Base strengths were determined by titration with strong acids in dilute aqueous solution. All those amines described gave equivalent weights in good agreement with the theoretical values. Yo other tests of purity or identity were applied. Uniform titration practices mere observed so that the relative base strengths should be significant except in cases where solubility was a problem.
Table Ill. Base Strength and Catalytic Activity of Amine Containing Nuclear Nitrogen
2 4 6-Collidine 2:4:Lutidine 2,B-Lutidine a-Picoline Pyridine 2 (6-Hydroxyethyl) pyridine 2 (P-carbamyloxyethyl) pyridine Quinoldine 2- (1,3-dihydroxy-2-propyl)pyridine dicarbamate 2- (1,3-dihydroxy-2-propyl)pyridine monooarbarnate
pK 6.9 6.2 6.2 5.7 5 4 5.4 5 2
5.2
Wet Strength, % 0 Day 14 Day. 52 59 56
54 50 54 17 24 41
59 58
60 55 50 58
4.5
8
z2
4.4
18
49
Many amines react with formaldehyde under the conditions existing in the resin solutiox Since this complicates the interpretation of the results, such amines will not be considered in the present discussion. The compounds listed in Table I11 do not by any means exhaust the possible cationic acids which are available, nor indeed
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 1
THERMOSETTING RESINS Table IV.
Base Strength and Catalytic Activity of Various Amines and Miscellaneous Bases Compound
PK
For information on two new specialpurpose thermosetting resins, see
Wet Strength, % 0 Day 14 Days
Aromatic Amines Diet hylaniline p-Ureido-dimethylaniline N-Methyl-N-(hydroxyethyl)aniline N N-di-(fl-hydroxyethyl)-p-toluidine N:N-Di(@-hydroxyethyl)aniline Dimethylaniline
5.0 4.8 4.8 4.6 4.2 4.1
55 23 20 32 29 61
59 54 58
43 39 15 45 17 32 15 10
59 54 62 51 42 49 42
7.8 6.1 6.1 5.2
34 13 57 19
54 62 51 62
Miscellaneous Bases Triethanolamine 7.8 2-Methyl-5-isobutyloxazoline 5.5 Hexamethylenetetramine 4.8 N-(tert-ootyl) Furfuraldimine 4.2
14 37 13 43
eo
N-(cyanoalkyl) Amines Cyanomethyl-diisopropylamine 5.6 C yanomethyl-pyrrolidine 5.6 Di- (cyanoethyl)methylamine 4.8 Cyanomethyl-dimethylamine 4.5 N-oyanomethyl-N-methvlethanolamine 3.6 C yanomethyl-diallylam
