New Curing System of Urea-Formaldehyde Resins with Polyhydrazides III.
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1 742 [ Mokuzai Gakkaishi Vol. 35, No. 8, p (1989) (Original Article)] New Curing System of Urea-Formaldehyde Resins with Polyhydrazides III. Curing reaction mechanism* 1 Katsumasa MIYAKE* 2, Bunichiro TOMITA* 3, Chung Y. HSE* 4 and George E. MYERS* 5 A new curing system for urea-formaldehyde (UF) resins has been developed using hydrazide polymer and dihydrazide compounds as curing agents reported in the previous papers of this series. The reaction mechanism of this system was investigated with the carbon nuclear magnetic resonance technique and torsional braid analysis. The results are summarized as follows : 1) It was concluded that the reaction of hydrazide compounds with UF resin involves two stages. The first stage is the reaction of hydrazide group with free formaldehyde which makes hydrazide compounds or hydrazide polymer self-condense through methylene linkages. Therefore the initial gelation takes place even at low temperatures. The second stage, occurring at a rather high temperature, is due to the cocondensation of hydrazide groups with methylol groups of UF resins which gives the crosslinking networks between UF resin and hydrazide compounds. 2) It was confirmed that hydrazide groups have three functions in regard to formaldehyde and Report II: This Journal, 35, (1989) * 1 Received December 8, A part of this paper was presented at the 37th Annual Meeting of The Japan Wood Research Society, Kyoto, 1987, p * 2 Present address Recruit Co. Ltd., Chiyoda-ku, Tokyo 110 * 3 Faculty of Agriculture, University of Tokyo, Tokyo 1 * 4 USDA Forest Service, Southern Forest Experiment Station, Pineville, LA 760, U. S. A. * 5 USDA Forest Service, Forest Products Laboratory, Madison, WI U. S. A.
2 Vol. 35, No. 8, 1989] New Curing System of Urea-Formaldehyde Resins 743 methylol groups. Keywords : urea-formaldehyde resin, polyhydrazide, neutral curing, C-NMR, TBA. 1. INTRODUCTION In our previous paper of this series, 1,2 ) we reported that a new curing method for urea-formaldehyde (UF) resins had been developed using dihydrazide compounds and hydrazide polymers as curing agents. The method is based on the simple mixing of UF resins with the curing agents under neutral conditions without using the conventional curing agents such as ammonium chloride or acids. The practicality of this system was confirmed by adhesive strength tests on plywood, and a remarkable reduction of formaldehyde emission from the plywood also was recognized. It has been suggested also that the reaction of hydrazides with UF resins involves two stages. This paper explains the reaction mechanism of this new curing system. The chemical reactions were analyzed by carbon nuclear magnetic resonance ( C-NMR) techniques using model compounds, and the curing process was followed by a torsional braid analysis (TBA). 2. EXPERIMENT 2.1 Model Compounds, UF Resin, and Poly (Methacryloyl Hydrazide) Two kinds of hydrazide compounds were used as model compounds. Benzoyl hydrazine and formalin were available commercially, and the preparation of adipoyl dihydrazide was described in the initial paper of this series. 1) Monomethylolurea was prepared according to the method reported by Meyer. 3) UF resin was the commercially available U1 and poly (methacryloyl hydrazide) (PMAH) was the same as described in the previous paper. 2) 2.2 C-NMR Measurements A FX-100 spectrometer (Japan Electron Optics Lab. Co. Ltd.) was used at an observation frequency range of 5 khz, an operating frequency of 25.0 MHz, an irradiation frequency of 51.5 khz, and a flip time of 7 µs. The spectra of complete decoupling of protons were taken using a 10 mm sample tube. Each reactant from the model compound and formaldehyde was dissolved in DMSO-d6 or D 2O. The chemical shift was calculated by defining a chemical shift of external dioxane as 67.2 ppm in a D 2O solution and that of internal DMSO-d6 as 39.5 ppm. Each reaction was made under the neutral conditions of ph Other reaction conditions are noted in the figures. 2.3 Torsional Braid Analysis Torsional braid analysis (TBA) was conducted with a torsional pendulum apparatus (RESCA Co. Ltd.). About 60 mg of a mixture of UF resin and PMAH was brushed onto a glass braid. The measurement was initiated immediately after mixing resin. The rate of the increase of temperature was 2 C/min. Results were plotted in terms of relative rigidity (l/p) 2, where P was the frequency interval. The relative rigidity can be considered proportional to the storage modulus (E' ). 3. RESULTS AND DISCUSSION 3.1 Reaction of Hydrazide Group and Formaldehyde To confirm the new curing system chemically, several attempts were made by C-NMR techniques. The reaction of hydrazide with formaldehyde was not realized completely besides the formation of hydrazone. Figure 1 shows the C-NMR spectrum of the water-insoluble part obtained from the equimolar reaction of benzylhydrazine and formaldehyde at 60 C. The appearance of the signal at ppm indicated the presence of a double bond, and its mass spectrum (MS) showed the molecular ion of 148. Fig. 1. C-NMR spectrum of the water-soluble part of the reactant from phenylhydrazine and formaldehyde. Note : Reaction conditions : Molar ratio : hydrazine (1) : formaldehyde (1). Reaction temperature : 60 C.
3 744 Katsumasa MIYAKE, Bunichiro TOMITA. Chung Y. HSE and George E. MYERS [ Mokuzai Gakkaishi Therefore this product was identified as phenylhydrazone. The reaction of excess benzylhydrazine (2 mol) with formaldehyde (1 mol) also gave a waterinsoluble product. Its NMR spectrum is shown in Fig. 2, where the new signal is observed at 73.6 ppm. This chemical shift well agreed with the calculated value of methylene carbon of the dimer, the formation of which also was supported by MS data. From these results it was concluded that an excess amount of hydrazide produces the methylene linkage between hydrazide groups. Figure 3 is the C-NMR spectrum of the reactant from adipoyl dihydrazide and formaldehyde. The co-existence of the hydrazone with a methylene linkage also are recognized at and 71.2 ppm. Fig. 2. Note : C -NMR spectrum of the water-soluble part of the reactant from phenylhydrazine and formaldehyde. Reaction conditions : Molar ratio : hydrazine (2) : formaldehyde (1). Reaction temperature : 60 C. resins react with the hydrazide group or not. AS monomethylolurea has one functional group, its reaction with benzylhydrazine was not considered to produce polymeric materials or crosslinking networks. Therefore, it also was considered that products from these two model compounds are easy to analyze in a solution state. The bottom spectrum in Fig. 4 was taken immediately after dissolving two model compounds in water at 20 C under neutral conditions. After heating of the solution at 60 C for 5 min, the new signal appeared at 57.1 ppm in the upper spectrum. Moreover, the signal at ppm, derived from the carbonyl carbon of monomethylolurea, and the signal at ppm, derived from the carbonyl carbon of benzylhydrazine, became split. If monomethylolurea reacts with benzylhydrazine to form a methylene bridge, its chemical shift can be estimated by a simple calculation using the additivity relationship. 4-6) The chemical shift of the methylene carbon in the dimer of benzylhydrazine is 73.6 ppm as determined in Fig. 2, and that of methylenediurea is 47.9 ppm. 6) Thus the chemical shift of the methylene carbon between these two model compounds is considered to be about 60.8 ppm [( )/2]. As this calculated value to the chemical shift was in fair agreement with the observed value of 57.1 ppm within a small error, the cocondensation is ascertained to be between methylol groups attached to urea residue and the hydrazide group. It also should be noted that Fig. 3. C-NMR spectrum of the reactant from adipoyl dihydrazide and formaldehyde at the molar ratio of 1 : Reaction of the Hydrazide Group with Monomethylolurea It is a matter for developing the new curing system to determine whether the methylol groups of UF Fig. 4. C - NMR spectra of the mixture of monomethylolurea and phenylhydrazine at the molar ratio of 1 : 1. Notes : Bottom spectrum : before heating. Upper spectrum : after heating at 60 C for 5 min.
4 Vol. 35, No. 8, 1989] New Curing System of Urea-Formaldehyde Resins 745 other chemical reactions, such as self-condensation and dissociation of monomethylolurea, could not be recognized in Fig. 4. Further NMR attempts were made on the reaction of adipoyl dihydrazide with monomethylolurea. In this case, the crosslinking or polymeric material is not produced by the monofunctionality of monomethylolurea although adipoyl dihydrazide is multifunctional. Three kinds of reactions different in the molar ratios of the hydrazide group to monomethylolurea, were performed under the same conditions as in Fig. 4. At the molar ratio of 1.2 in Fig. 5, the signal at 57.2 ppm was observed with the same manner as Fig. 4. The appearance of this key signal also proves the cocondensation between two model compounds through a methylene linkage. The chemical shift of the methylene linkage was changed slightly when the molar ratio decreased. As an excess amount of monomethylolurea can be considered to cause further substitutions on the monosubstituted hydrazide group, the signal at 59.4 ppm was assigned to the methylene carbon attached to the tertiary nitrogen atom. Similarly, the signal at 47.4 ppm, observed in the bottom spectrum of Fig. 5, safely was assigned to the third substitution to the same hydrazide group. These assignments were well consistent with calculations using substituent effects. The results of assignments can be postulated as shown in Fig. 6. It is very important that the hydrazide group has the three functions against monomethylolurea. Therefore, the crosslinking networks seemed to form even with a small molar ratio of the hydrazide group to formaldehyde, including both free formaldehyde and methylol groups in the UF resin as described in the previous paper. 2) 3.3 TBA of the Process of Curing The curing process of this system was investigated with a TBA, which is well-known in pointing out the dynamic mechanical variations of gelation and curing of thermosetting polymers as a function of time or temperature. 7) The changes of relative rigidities of PMAW, UF resin and UF resin mixed with ammonium chloride are compared in Fig. 7. The temperature dependency of PMAH was not recognized until the relative rigidity began to fall at about 170 C. The curing process of UF resin with ammonium chloride is demonstrated well by a remarkable in- Fig.5. Note : C-NMR spectra of the reactants from adipoyl hydrazide and monomethylolurea at various molar ratios of the hydrazide group to formaldehyde. Numbers in parentheses are the molar ratios. Fig. 6. C-NMR assignment of various methylene carbons between urea residue and the hydrazide groups observed in Fig. 5. Legend : U : urea residue. crease of relative rigidity. Both gelation and the curing process involving the removal of water were complete.until the relative rigidity began to fall slightly. This falling may have been due to shrinkage of the cured resins. On the other hand the curing of UF resin under neutral conditions without a hardner has two stages. The first, between 20 C and 100 C, seemed to be attributable to the removal of water and to gelation by the formation of weakly crosslinked networks. The second stage above 120 C produced a gradual formation of strongly crosslinked
5 746 Katsumasa MIYAKE, Bunichiro TOMITA, Chung Y. HSE and George E. MYERS [ Mokuzai Gakkaishi Fig. 7. TBA of PMAH and UF resin. Legend : PMAH : PMAH itself. UF : UF resin itself. UF+NH 4Cl: curing of UF resin with NH 4Cl. networks as indicated by the increase of rigidity. It was concluded that curing under neutral conditions required considerably higher temperatures than those of the conventional curing method under acidic conditions although a smaller degree of curing resulted. The characteristic figures of several kinds of curing methods different in the molar ratios (-CONHNH 2/ HCHO) are compared in Fig. 8. At the small molar ratio of 0.1 : 1, the relative rigidity changed almost like UF resin itself and showed two transitional stages. However, note that the first stage took place at a rather lower temperature than that in the curing of UF resin without a hardner. The shift of the first stage to a lower temperature implies the gel formation by self-condensation of the hydrazide polymer with the free formaldehyde of the UF resin. This idea well agrees with the conclusions we previously reported. 2) All of the hydrazide groups were completely consumed in reacting with free formaldehyde and in producing self-condensation between the main chains of hydrazide polymers. Therefore, the second stages are observed to be similar as in the case of UF resin itself. As increasing amounts of hydrazide polymer were added, the temperature of the second stage became lower, and the first stage became obscure. The first stage, in the case of larger molar Fig. 8. TBA of the PMAH-UF resin curing system at various molar ratios. Legend : UF : UF resin itself. Note: Each number is the molar ratio of the hydrazide group to formaldehyde of UF resin as determined by iodometry. ratios, was finished at room temperature before a TBA measurement was initiated as shown in Fig. 8. This only one transition due to the second stage can be observed in the TBA data of the larger molar ratios. The second stage shifted in the direction of lower temperatures as the molar ratio increased. Finally, at the molar ratio of 1.1 : 1 it displayed a pattern as in the curing of UF resin with ammonium chloride. From these TBA data, it also was concluded that hydrazide polymer is available as a crosslinking agent, a hardner, of UF resins. 4. CONCLUSIONS The reaction mechanism of the new curing system was analyzed with a TBA and the C-NMR technique using model compounds. It was concluded that the reaction of hydrazide compounds with UF resin involves two stages as illustrated in Fig. 9. The first stage is the reaction of the hydrazide group with free formaldehyde, which makes the hydrazide compound or polymer self-condense through a methylene linkage to result in the formation of the initial gelation within the short reaction time of a few minutes. The second
6 Vol.35, No. 8, 1989] New Curing System of Urea-Formaldehyde Resins 747 Acknowledgement This research was performed under the Research Agreement FP between the University of Tokyo and the USDA Forest Service. A part of the work was supported by a Grantin-Aid for Scientific Research (No , ) from the Ministry of Education, Japan. REFERENCES Fig. 9. Typical two-stage reaction of hydrazide polymer and UF resins. www : main chain of PMAH. -U-: urea resin. stage at rather high temperature is due to the cocondensation of hydrazide groups with methylol groups of UF resins to give crosslinking networks between UF resin and the hydrazide compounds. It also was explained by the C-NMR study that the hydrazide groups have three functions in regard to formaldehyde and methylol groups.
New Curing System of Urea-Formaldehyde Resins with Polyhydrazides I.
455 Note [ MokuzaiGakkaishi Vol. 35, No. 5, p. 455-459 (1989) (Original Article)] New Curing System of Urea-Formaldehyde Resins with Polyhydrazides I. Curing with dihydrazide compounds* 1 Bunichiro TOMITA*
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