Ind. Eng, Chem. Prod. Res. Dev. 1985, 24, 1-5
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SYMPOSIA SECTION
I.
Symposium on "Phenolics Revisited, 75 Years Later" Louis A. Pilato, Chairman 186th National Meeting of the American Chemical Society Washington, DC, August 1983 (Continued from September 1984 Issue)
Cocondensation between Resol and Amino Resins Bunlchlro Tomita' and Tadashl Matsurakl Depafiment of Forest Prcducts, Facuity of Agricukure, University of Tokyo, Yayoi 1- 1- 1, Bunkyo-ku, Tokyo 1 13, Japan
The cocondensations of phenol-formaldehyde-melamine and phenol-formaldehyde-urea were investigated with 13C NMR spectroscopy and gel permeation chromatography. The occurrence of cocondensation was determined by the appearance of the key signal at 44.2 ppm due to the carbon of methylene linkage between aromatic rings and amino groups and also by the GPC pattern. In the case of cocondensation of phenol-formaldehyde-urea, the synthetic method of reacting methylolphenols with an excessive amount of urea under acidic conditions was effective. In the case of the phenol-formaldehyde-melamine system, the cocondensation was achieved by reacting methylolphenols with methylolmelamines at a pH below neutral.
Introduction Recently, so-called phenol-melamine cocondensed resins have been used as adhesives for the manufacturing of plywood in Japan. Aqueous phenol-melamine resins have good gluability to soft wood and resinous tropical hard wood such as apiton, and they have an advantage over resol itself from the aspect of saving energy in the making of plywood (Tamura et al., 1981). This is due to the fastcuring nature of melamine resins. Phenol-melamine resins can be cured at lower temperatures than resol. However, since they are commonly synthesized by simple blending of melamine resins with resol and used at pH below neutral, resol itself is apt not to cure completely. This condition is suitable for the curing of melamine resin but not for resol. Therefore, it seemed to be necessary to produce a sufficient amount of cocondensationbetween phenol and melamine at the time of preparing resins. Phenol-melamine resins have a fairly long history of being utilized in fields such as the production of laminated sheets. Recently, Chen et al. (1982) reported on the synthesis and temperature properties of phenol-melamine resins. However, it has never been established that chemical cocondensation really happens. The main purposes of this paper are to analyze the chemical reactions of phenol-formaldehydemelamine and phenol-formaldehydeurea systems and to determine the conditions necessary to produce their cocondensation. Experimental Section Samples. Commercial grade urea, melamine, phenol, and 37% formaline were used in all experiments. Authentic 0- and p-methylolphenols were synthesized according to Freeman (1952) and identified by melting 0196-4321 /05/1224-0001$01 .SO10
points, IR, and 13C NMR. The resol of the initial condensation stage was synthesized with the molar ratio formaldehyde/phenol/NaOH = 1.5/1.0/0.4 by heating at 80 "C for 10 min. The mixture of methylolmelamine was synthesized at the molar ratio formaldehyde/melamine = 2 / 1 by heating at 80 "C for 10 min at pH 8.5. All other reaction conditions are cited in the respective figures. 13C NMR Measurement. Each reactant was diluted directly with solvents (pyridine-& DzO, MePSO-d,), and its 13C NMR spectrum was obtained with an FX-100 spectrometer (JEOL) at a frequency of 25.0 MHz with the complete decoupling of proton. Chemical shifts were calculated by defining internal methanol as 50.0 ppm. Gel Permeation Chromatography. Each readant was also diluted with DMF and its chromatogram was obtained with a Liquid Chromatograph ALC/GPC-201 with an R-401 Differential Refractometer (Waters Associates). Four Shodex GPC-AD-802s columns (Showa Denko Co., Ltd.) were connected in series and kept at 60 "C in a constant-temperature bath. The flow rate of DMF was 1.0 mL/min. Results and Discussion 1. Calculation of 13C NMR Chemical Shift of Methylene Carbons in Cocondensation. The structural analysis of formaldehyde resins including urea, melamine, and phenol resins has been extensively advanced with the 13CNMR spectroscopic techniques by many workers (e.g., de Breet et al., 1977; Slonim et al., 1977; Tomita and Hatono, 1978; Sojka et al., 1979; Ebdon and Heaton, 1977; Pasch et al., 1981; Siling et al., 1977; Mukoyama and Tanno, 1973; Dradi et al., 1978), and the assignment of all the signals due to the reacted formaldehyde has been completely established. 0 1985 American Chemical Society
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Figure 2. 13CNMR spectra of reactant from urea-methylolphenol at pH 4.8: (1)o-methylolphenol (0.3 g)-urea (2 9); (2) p-methylolphenol (0.3 g)-urea (2 9). Figure 1. 13C NMR spectra of resol (1)and urea resin (2) in D,O. Table I. Observed and Calculated Chemical Shifts of Carbons in Methylene Linkage in Resol, Urea Resin, and Their Cocondensed Resin chemical shift, PPm resin and structure obsd" calcd resol o - C ~ H ~ C H ~ C ~ H ~ - O 29.6 o - C ~ H ~ C H ~ C ~ H ~ - ~35.9 40.5 p-C6H4CH2C6H4-p cocondensed resin -NHCH~C~H~-O 40.2 38.7 44.2 44.1 -NHCH~C~H~-P 44.8 -N(CH~)CH~C~H~-O -N(CH~)CH~C~H~-P 50.3 urea resin 47.1 -NHCHZNH53.5 -NHCHZN(CHZ)-N(CHZ)CHZN(CH.& 60.0 "The observed chemical shifts of resol and urea resin were measured in D20 and those of cocondensed resin were measured in pyridine-d5.
In Figure 1the 13CNMR spectra of resol and urea resin are compared. The 13CNMR spectral data of methylene carbons in resol and urea resin are compared in Table I. The chemical shifts of methylene carbon in =NCH2N= of urea resin are varied according to the method of substitution of other carbons to the adjacent nitrogen atoms. It should be noted that the chemical shift of each corresponding carbon of combined formaldehyde is almost identical between urea resin and melamine resin and that their spectra are superimposable (Tomita and Ono, 1979). On the other hand, the chemical shift of methylene carbon between aromatic rings (C6H4CH2C6H4) is changed by the position of substitution to the aromatic ring (de Breet et al., 1977). The chemical shift of the cocondensed methylene carbon represented by =NCH2C6H4 can be calculated. If we start from the basic chemical shift (A) of unsubstituted methylene carbon (or methane carbon) and use two different substituents, X and Y, three kinds of chemical shift, C,,, Cy , and C,, are expressed by eq 1-3 from the additivity refationships, respectively. X-CH2-X .......... C,,, = A + 2X (1)
+ 2Y .......... C,,y = A + X + Y
Y-CH2-Y .......... C,,? = A X-CH2-Y
(2) (3)
Here x and y are the substituent effects of X and Y, respectively. From these equations the simple eq 4 is obCx,y
= (CXJ
+ CYJP
(4)
tained, from which the chemical shift in cocondensation structure, C,, can be calculated using known values of C,, and Cy,y. For example, the value of -NHCH2C6H4-ois calculated by (47.7 + 29.6)/2 using chemical shifts of -NHCH2NHand o-C6H4CH2C6H4-o.The calculated values for all possible cocondensed structures are summarized in Table I. As previously described, since the chemical shift of each corresponding carbon of combined formaldehyde is superimposable between urea resin and melamine resin, each cocondensed methylene carbon of melamine-phenol resin is also considered to have almost the same chemical shift as the corresponding methylene carbon of urea-phenol resin. 2. Determination of Chemical Shift for Methylene Carbon in Cocoadensation. Authentic o- and p methylolphenols were, respectively, reacted with an excessive amount of urea under various pH levels. Figure 2 shows the 13CNMR spectra of the reaction mixture at pH 4.8. The chemical shifts of methylol carbon in o- and p-methylolphenol are 61.1 and 64.7 ppm, respectively. In the case of the o-methylolphenol-urea reaction system, two signals appeared a t 40.5 and 35.9 ppm. The latter is attributed to o-C6H4CH2C6H4-p which was caused by selfcondensation of the model compound between methylol groups at the ortho position and the free para position. The chemical shift of the former may agree with p C6H4CH2C6H4-p.This possibility, however, is denied by the absence of the p-methylol group in the model compound and also by the absence of free formaldehyde in the reaction system. The chemical shift agrees with the calculated value for -NHCH2C6H4-owhich is originated from cocondensation. So it is concluded that the cocondensed methylene carbon of -NHCH2C6H4-oappears at almost the same magnetic field as self-condensed methylene carbon of p-C6H4CH2C6H4-p. In the case of the p-methylolphenol-urea reaction system, the cocondensation is easily ascertained by the appearance of the signal a t 44.2 ppm that is completely identical with the calculated value for -NHCH,C6H4-p. It is important that this signal is not overlapped with other signals due to self-condensations. Therefore the real chemical cocondensation is judged from the occurrence of this key signal in spectra. 3. Conditions Required for Cocondensation of Phenol-Formaldehyde-Urea. In order to determine the optimum condition producing cocondensation, it was of importance to know whether the cocondensation is induced by phenol's methylol group or by urea's methylol group. Further, the acidity was considered to relate the reaction. Then the following reaction systems were investigated at various pH levels. (a) Reaction of Methylolphenols a n d Methylolureas. First, the mixture of urea and phenol was reacted
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, 1985 3
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Figure 3. 13CNMR spectra of urea-phenol-formaldehyde reaction mixture (UPF) and urea resin (UF). Reaction conditions of UPF: pH 10.3,80 " C , 1 h, molar ratio F:P:U = 4:l:l.
Figure 5. 13CNMR spectrum of resol at initial condensation stage in pyridine-d,. Synthetic conditions: 80 OC, 15 min, molar ratio PF:NaOH = 1:1.60.4.
PPM
50
LO
Figure 6. 13CNMR spectra of resol excess urea reaction mixture at pH 4.8, 7.0, and 10.0. Reaction conditions: 80 "C, 1 h, resol (8 g)-urea (25 9). O' PPM
50
30
Figure 4. 13CNMR spectra of reactant from dimethylolurea and excess phenol. Reaction conditions: 80 O C , 1 h, dimethylolurea (0.5 g)-phenol (59).
with formaldehyde under basic medium. Its spectrum is shown in Figure 3. The occurrence of cocondensation is denied, because the key signal is not recognized in the region of 40-50 ppm. Though the same reaction was performed under acidic medium, the key signal due to cocondensation could not be observed. (b) Reaction of Methylolureas and Phenol. Dimethylolurea was reacted with an excessive amount of phenol under various pH levels, and the spectra of the reactants are shown in Figure 4. In the basic condition of pH 10.5, the methylol group of dimethylolurea was dissociated and the resulting formaldehyde was added to phenol. This is certified by the presence of the signal at 64 ppm. Then the self-condensation between phenols occurred, which is also judged from two signals at 36 and 40.5 ppm. In the acidic state of pH 5.0, the self-condensation of urem dominates the reaction. This is confirmed by the appearance of the two signals at 47.7 and 53.5 ppm. ( c ) Reaction of Methylolphenol and Urea. Next, in order to investigate the reactivity of methylolphenol, resol of initial condensation stage was synthesized, and its spectrum is shown in Figure 5. The resol was reacted with an excessive amount of urea at three pH levels. The cocondensation could be confirmed clearly by the presence of the signal at 44.2 ppm in Figure 6. At pH 4.8 the self-condensation of phenols is negligible from the absence of the signal at 35.9 ppm due to O-CgH4CH&6H4-p, while
Figure 7. 13C NMR spectrum of cocondensed resin between resol and excessive amount of urea in pyridine-&
it is observed at pH 10.0 and 7.0. Therefore in the case of pH 4.8 the signal of 40.5 ppm is not considered to be caused by the p-C6H4CH2C6H4-p, but attributed to the cocondensed methylene carbon between the ortho position and amino group. These facts and the relative intensities of the key signals due to cocondensation indicate that an acidic environment is desirable and that the cocondensation is induced by the electrophilic attack of methylol group. Though the signal of 47.5 ppm at pH 10.0 and 7.0 may correspond with the self-condensed methylene carbon between urea residues, the basic condition without free formaldehyde and the presence of an excessive amount of urea easily exclude this possibility. So the signal implys the new chemical structure; e.g., the formation of hydroxybenzylamines from methylolphenols and ammonia is suggested (Sojka et al.,
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Figure 8. 13C NMR spectra of melamine-phenol-formaldehyde reaction mixture in pyridine-d5 and Me2SO-da. Reaction conditions: pH 10.3, 80 "C, 1 h, molar ratio F:P:M = 6.6:2:1.
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Figure 9. 13C NMR spectrum of melamine-phenol-formaldehyde reaction mixture in pyridine-& Reaction conditions: pH 6.7.80 "C, 1 h, molar ratio F:P:M = 6.6:2:1.
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NMR spectrum of commercial melamine-phenol Figure 11. cocondensed resin in pyridine-d5.
pH 103
90
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Figure 10. 13C NMR spectrum of reactant of methylolmelamines with reaol of initial condensation stage. Reaction conditions: 2 min, 70 "C, pH 5.0 by AcOH, methylolme1amine:resol = 1:9 (w/w).
1981). Ammonia may be derived from hydrolysis of urea. Figure 7 is the whole spectrum of the cocondensed resin between resol and urea. Cocondensation is confirmed by the appearance of the large signal a t 44.1 ppm. 4. Cocondensation of Phenol-Formaldehyde-Melamine. The suitable condition required for the cocondensation between phenol and urea was considered applicable to the melamine-phenol system, but the same reaction could not be done because of poor solubility of melamine. Then the mixture of melamine and phenol was reacted directly with formaldehyde under various pH levels. The absence of the key signal in Figure 8 denies cocondensation at the alkaline state. Next, the same reaction was performed at pH 6.7. The key signal due to cocondensation is observed in Figure 9. Then the mixture of methylolmelamines was synthesized at a lower degree of addition. It was reacted with the resol of the initial
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20
30 m l
Figure 12. Variation of gel permeation chromatogram of melamine-phenol-formaldehyde mixture in a condensation step. Reaction conditions: pH 10.3, 80 "C, molar ratio F:P:M = 6.6:2:1.
condensation stage under the various pH levels. In the case of the reactant at the pH 5.0, the key signal is clearly observed as shown in Figure 10. On the other hand, the reactions of methylolmelamines with phenol did not produce the cocondensation at every pH level. From these results it is also concluded that the cocondensation of melamine and phenol is induced by the reaction of phenol's methylol group with the unsubstituted amino group. Figure 11 shows the 13C NMR spectrum of the commercially so-called melamine-phenol cocondensed resin. This resin seems to be made by the simple blending of resol with melamine resin, since the real cocondensation is not recognized. 5. Gel Permeation Chromatographic Analysis on Cocondensation of Phenol-FormaldehydeMelamine. Gel permeation chromatographic analysis of melamine-, urea-, and phenol resins using DMF as eluent was previously reported by Matsuzaki et al. (1980). The mixture of melamine and phenol was reacted with formaldehyde under three pH levels. Reaction conditions were the same as in the NMR study and the change of their chromatograms in a condensation step was followed by the same method as reported. In Figure 12 it is clear that selfcondensation of resol dominated the reaction at pH 10.3. The high-molecular-weightpeak at 17 mL is due to resol, and the low-molecular-weight peak at 27 mL is due to methylolmelamines. The cocondensation seems to occur under the weak base of pH 8.4, since the high-molecular-weight peak at 17 mL due to self-condensation of resol is smaller than under the strong base, though the unreacted phenol is observed as the peak at 33 mL in Figure 13. The same reason leads to show the sure cocondensation under the neutral condition of pH 6.7 in Figure 14,
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24,
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spectroscopic technique. The calculated 13CNMR chemical shifts of methylene carbons in cocondensation by the simple additivity relationship were generalized by the model experiments. In the case of cocondensation of phenol-formaldehydeurea, the synthetic method to react methylolphenols with an excessive amount of urea under acidic condition was desirable. In the case of phenolformaldehydemelamine, the cocondensation was achieved by reacting methylolphenols with methylolmelamines at pH below neutral. Acknowledgment
F i g u r e 13.
Variation of gel permeation chromatogram o f mel-
aminephenol-formaldehyde mixture in a condensation step. Reaction conditions: pH 8.4, 80 “C,molar ratio FP:M = 6.62:l.
A part of this paper was presented at the 30th Annual Meeting of Japan Thermosetting Plastics Industry Association and the 31th Annual Meeting of the Japan Wood Research Society. The whole of this paper was presented at ACS Symposium on “Phenolics Revisited, 75 Years Later”, 186th National Meeting of the American Chemical Society, Washington, DC, Aug 1983. This work was supported by the Grant-in-Aid for Scientific Research (1981-1982) from the Ministry of Education, Japan. Registry No. (Phenol).(melamine).(fonnaldehyde)(copolymer), 25917-04-8; (phenol)-(urea)(formaldehyde)(copolymer), 25104-55-6.
Literature Cited
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io
I
1hr
io
io
mi
F i g u r e 14. Variation of gel permeation chromatogram of melamine-phenol-formaldehyde mixture in a condensation step. Reaction conditions: pH 6.7, 80 O C , molar ratio F:P:M = 6.6:21.
though unreacted phenol is observed slightly. Conclusion The real cocondensation was proved to produce some new signals due to the carbon of methylene linkage between aromatic rings and amino groups by the lSC NMR
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Received for review February 15, Accepted September 4,
1984 1984
