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Competitive polycondensation of model compound (MUF) resin system by 13C NMR

  • Corresponding author: Zhigang Wu, wzhigang9@163.com ; Taohong Li, lith.cool@163.com
  • Received Date: 2019-09-11
    Fund Project:

    Science-technology Support Foundation of Guizhou Prov-ince of China [2019]2308

    National Natural Science Foundation of China 31870546

    National Natural Science Foundation of China 31800481

    Science-technology Support Foundation of Guizhou Prov-ince of China [2019]2325

    Forestry Department Foundation of Guizhou Province of China [2017]14

    Education De-partment Foundation of Guizhou Province of China [2019]184

    Forestry Department Foundation of Guizhou Province of China [2018]13

  • Melamine-urea-formaldehyde (MUF) resin is an excellent adhesive in the field of wood adhesives, however the com-petition mechanism is questionable which affects the structure control and performance optimization of the resin. In this sduty, the competitive resin synthesis polycondensation reaction of MUF system under alkaline condition was studied based on the model compound 1, 3-dihydroxymethyl urea (UF2) and melamine (M) system, and the competitive reaction mechanism in the system was deduced by 13C NMR quantitative analysis. The results show that the energy barrier of hydroxymethylation of melamine is lower than that of urea, and the priority of hydroxymethylation is lower; the addition of melamine results in a large amount of hydrolysis of UF2, and the formed free formaldehyde, resulting in hydroxymethylation of melamine; there is obvious polycondensation reaction in UF2+M system, mainly from the relationship between Hydroxymethylurea and melamine or hydroxymethylmelamine. The type I bridge bond structure of polycondensation mainly comes from the reaction of UF2 and M, which is difficult to form the type II bridge bond. At low molar ratio, the formation of bridge bond is superior to that of ether bond. With the increase of molar ratio, the formation of ether bond shows advantages, but there is obvious competition between them. There may be competitive presence of the UF self-condensation products, melamine-formaldehyde (MF) self-condensation products and MUF co-condensed products after the polycondensation reaction.
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  • [1]

    Angelatos, A.S., Burgar, M.I., Dunlop, N., Separovic, F., 2004. NMR structural elucidation of amino resins. J. Appl. Polym. Sci. 91, 3504-3512. doi: 10.1002/app.13538
    [2]

    Cao, M., Li, T.H., Liang, J.K., Du, G.B., 2017. The influence of pH on the melamine-dimethylurea-formaldehyde Co-condensations:a quantitative 13C-NMR study. Polymers 9, 109. doi: 10.3390/polym9030109
    [3]

    Cui, J.Q., Xie, M.J., Yang, S., 2017. Properties of diethylene glycol ether toughening modified melamine-urea-formaldehyde resin. J. For. Eng. 31, 10-14.
    [4]

    Dunky, M., 1998. Urea-formaldehyde (UF) adhesive resins for wood. Int. J. Adhesion Adhesives 18, 95-107. doi: 10.1016/S0143-7496(97)00054-7
    [5]

    Dunky, M., 2004. Adhesives based on formaldehyde condensation resins. Macromol. Symp. 217, 417-430. doi: 10.1002/masy.200451338
    [6]

    Kim, M.G., 1999. Examination of selected synthesis parameters for typical wood adhesive-type urea-formaldehyde resins by 13C NMR spec-troscopy. I. J. Polym. Sci. A Polym. Chem. 37, 995-1007. doi: 10.1002/(SICI)1099-0518(19990401)37:7<995::AID-POLA14>3.0.CO;2-6
    [7]

    Kim, M.G., 2000. Examination of selected synthesis parameters for typical wood adhesive-type urea-formaldehyde resins by 13C-NMR spec-troscopy. II. J. Appl. Polym. Sci. 75, 1243-1254. doi: 10.1002/(SICI)1097-4628(20000307)75:10<1243::AID-APP5>3.0.CO;2-F
    [8]

    Kim, M.G., 2001. Examination of selected synthesis parameters for wood adhesive-type urea-formaldehyde resins by 13C NMR spectroscopy. III. J. Appl. Polym. Sci. 80, 2800-2814. doi: 10.1002/app.1397
    [9]

    Li, T.H., 2015. Study on the mechanisms of the reactions in synthesis of amino resins used as wood adhesives. Nanjing: Nanjing Forestry Uni-versity.
    [10]

    Li, T.H., Guo, X.S., Liang, J.K., Wang, H., Xie, X.G., Du, G.B., 2015. Competitive formation of the methylene and methylene ether bridges in the urea-formaldehyde reaction in alkaline solution:a combined experimental and theoretical study. Wood Sci. Technol. 49, 475-493. doi: 10.1007/s00226-015-0711-2
    [11]

    Liang, J.K., Li, T.H., Cao, M., 2017. Study on the competitive relationships of the base-catalytic melamine formaldehyde system. J For. Environ. 37, 251-256.
    [12]

    Liang, J.K., Li, T.H., Guo, X.S., 2014. Study on copolycondensation between dimethylol urea and melamine. China Adhesives 23, 1-4, 9.
    [13]

    Liao, J.J., Zhou, X.J., Du, G.B., 2016. Effects of different poly(amidoamine) s additions on the properties of hyperbranched polymer modified urea-formaldehyde resin. J For. Eng. 1, 21-26.
    [14]

    Park, B.D., Kim, J.W., 2008. Dynamic mechanical analysis of urea-formaldehyde resin adhesives with different formaldehyde-to-urea molar ratios. J. Appl. Polym. Sci. 108, 2045-2051. doi: 10.1002/app.27595
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    Siimer, K., Pehk, T., Christjanson, P., 1999. Study of the structural changes in urea-formaldehyde condensates during synthesis. Macromol. Symp. 148, 149-156. doi: 10.1002/masy.19991480113
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    Tomita, B., Hse, C.Y., 1995. Analyses of cocondensation of melamine and urea through formaldehyde with Carbon 13 Nuclear Magnetic Res-onance Spectroscopy. Journal of the Japan Wood Research Society 41, 349-354.
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    Wang, B., Zhang, Y. H., Tan, H. Y., 2018.Study on advance of melamine modified urea formaldehyde resin. New Chem. Mater. 46, 234-237, 241.
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    Zhang, J., Liang, J.K., Du, G.B., 2018. Melamine formaldehyde resin modified tannin furfuryl alcohol resin grinding wheel. J. For.. Environ. 38, 123-127.
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Competitive polycondensation of model compound (MUF) resin system by 13C NMR

    Corresponding author: Zhigang Wu, wzhigang9@163.com
    Corresponding author: Taohong Li, lith.cool@163.com
  • a. Kaili University, Qiandongnan 556011, China
  • b. College of Forestry, Guizhou University, Guiyang 550025, China
  • c. Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China
Fund Project:  Science-technology Support Foundation of Guizhou Prov-ince of China [2019]2308National Natural Science Foundation of China 31870546National Natural Science Foundation of China 31800481Science-technology Support Foundation of Guizhou Prov-ince of China [2019]2325Forestry Department Foundation of Guizhou Province of China [2017]14Education De-partment Foundation of Guizhou Province of China [2019]184Forestry Department Foundation of Guizhou Province of China [2018]13

Abstract: Melamine-urea-formaldehyde (MUF) resin is an excellent adhesive in the field of wood adhesives, however the com-petition mechanism is questionable which affects the structure control and performance optimization of the resin. In this sduty, the competitive resin synthesis polycondensation reaction of MUF system under alkaline condition was studied based on the model compound 1, 3-dihydroxymethyl urea (UF2) and melamine (M) system, and the competitive reaction mechanism in the system was deduced by 13C NMR quantitative analysis. The results show that the energy barrier of hydroxymethylation of melamine is lower than that of urea, and the priority of hydroxymethylation is lower; the addition of melamine results in a large amount of hydrolysis of UF2, and the formed free formaldehyde, resulting in hydroxymethylation of melamine; there is obvious polycondensation reaction in UF2+M system, mainly from the relationship between Hydroxymethylurea and melamine or hydroxymethylmelamine. The type I bridge bond structure of polycondensation mainly comes from the reaction of UF2 and M, which is difficult to form the type II bridge bond. At low molar ratio, the formation of bridge bond is superior to that of ether bond. With the increase of molar ratio, the formation of ether bond shows advantages, but there is obvious competition between them. There may be competitive presence of the UF self-condensation products, melamine-formaldehyde (MF) self-condensation products and MUF co-condensed products after the polycondensation reaction.

1.   Introduction
  • The alkali-acid-alkali process is the most commonly used process for urea-formaldehyde (UF) resin synthesis (Park and Kim, 2008; Liang et al., 2014; Li et al., 2015; Liao et al., 2016; Wang et al., 2018). In the initial alkaline stage, hydroxymethylurea was the main product from the addition reaction, meanwhile, methylene ether bond was formed after the condensation between hydroxymethylurea to some extent. Some researchers (Dunky, 1998; Dunky, 2004) found that the initial polymer formed in the alkaline stage has little effect on the final structure and performance of the resin; but results of some researchers (Kim, 1999; Siimer et al., 1999; Kim, 2000, 2001) showed that the initial polymer produced in this stage with a large number of ether bonds is involved in the later condensation in acidic conditions, resulting in a large number of ether bonds in the final structure of the resin, which would affect the performance of the resin. Previous research tended to support the latter point of view, and it is necessary to control the condensation reaction in the alkaline stage and reduce the formation of ether bond as much as possible to improve the resin performance (Li, 2015). According to the classical theory (Dunky, 1998; Kim, 1999; Siimer et al., 1999; Kim, 2000, 2001; Dunky, 2004; Li, 2015), the condensation reaction between hydroxymethylurea can only form the methylene ether bond (—CH2—O—CH2—), while the methylene bridge bond (—NR—CH2—NR—) cannot be formed. However, theoretical calculation and 13C NMR analysis show that the formation of bridge bond and ether bond is competitive. Theoretically, reducing the molar ratio of F/U in the alkaline phase may be beneficial to form bridge bonds and reduce the content of ether bonds in the final structure of the resin. According to the classical theory, formaldehyde belongs to carbonyl compounds, urea (U) has two nucleophilic centers (two amino groups), and melamine (M) show three nucleophilic centers. Nucleophilic addition reaction takes place between them, followed by polycondensation reaction. In fact, the synthesis of amino resin tends to be complex due to the spatial effect, the difference of activity of each reaction site, and the change of reaction system conditions, and there are a lot of competition or parallel reactions. However, the current theory cannot explain the reasons in detail.

    A certain amount addition of M in the synthesis process of UF resin can improve the resin performance and reduce the formaldehyde emission. Obviously, the introduction of a third reactant into the U-F system makes the reaction system more complex (Cui et al., 2017). At present, it is controversial whether or to what extent these three kinds of reactants can co-condensate. Tomita and Hse (1995) believed that there was a condensation reaction between melamine and dihydroxymethylurea (UF2) under laboratory conditions through the resin structure analysis, and proposed the possible structure of melamine-urea-formaldehyde (MUF) resin. While Angelatos et al. (2004) found it was more possible to form the self-condensation reaction of melamine and urea than the copolycondensation reaction between them. It is normal to have these controversies about the polycondensation reaction, although both urea and melamine have the active functional group of amino group, the two reactants have different chemical structures, and the reaction activity of their amino groups to formaldehyde may be different. The true copolycondensation refers to the form of resin molecule contains three reactant units which are combined by covalent bond at the same time. At present, the research question is, in different synthesis processes, what kind of reactions can happen between the starting reactants and the intermediate products? How do these reactions compete under different conditions? Which reactions are beneficial to the improvement of resin properties? How to make a favorable response dominant in the competition? In the field of wood adhesives, since these problems have not been solved, the design of synthesis route is blind because of lack of theoretical basis. The polycondensation reaction is still in the initial stage of exploration, so it is important to solve the basic theoretical problems for the synthesis and application of polycondensation resin in the future.

    The reaction types in MUF system are complex. In order to clarify the selectivity of the reaction, the model compound UF2 was used as the starting material to study the competitive polycondensation mechanism of resin synthesis in M-U-F system under alkaline conditions, the control factors of the formation of resin molecular structure was revealed. and 13C NMR was an effective method for structural evolution analysis in the field of wood adhesives (Dunky, 1998; Kim, 1999; Siimer et al., 1999; Kim, 2000, 2001; Dunky, 2004; Park and Kim, 2008; Liang et al., 2014; Li et al., 2015; Liao et al., 2016; Cao et al., 2017; Wang et al., 2018). In this work, combined with the relevant quantum chemical calculation results, the research results will further improve the theoretical system of resin synthesis and provide valuable theoretical reference for the control and optimization of resin structure.

2.   Materials and methods
  • Urea (99% purity), sodium hydroxide (AR), melamine (99% purity), formaldehyde (mass fraction 37%–40%). The above pharmaceutical reagents are all produced by Sinopharm Chemical Reagent Co., Ltd. Dihydroxymethylurea (98% purity, produced by bailingwei Reagent Co., Ltd.), distilled water.

  • (1) Adding 12 g of UF2 (0.1 mol), 12.6 g of mol/L (0.1 mol) and 50 mL distilled water were into the reactor, making the molar ratio UF2/M = 1, adjusting pH value to 8.5–9.0, controlling the temperature of water bath 90 ℃, stirring and reacting for 1 h, and then taking samples with the number MUF-1.

    (2) Adding 24 g of UF2 (0.2 mol), 12.6 g of mol/L (0.1 mol) and 50 mL distilled water into the reactor, making the molar ratio UF2/M = 2 (the same condition illustrated in step 1), stirring and reacting for 1 h, and then taking samples with the number of MUF-2.

    (3) Adding 36 g of UF2 (0.3 mol), 12.6 g of mol/L (0.1 mol) and 50 mL distilled water into the reactor, making the molar ratio UF2/M = 3 (same condition illustrated in step 1), stirring and reacting for 1 h, and then taking samples with the number of MUF-3.

    (4) Adding 25.83 g of M (0.205 mol), 12.3 g of U (urea) (0.205 mol) and 50 g formaldehyde (converted into F of 0.616 mol), M꞉U꞉F = 1꞉1꞉3 (same condition illustrated in step 1), stirring and reacting for 1 h, and then taking samples with the number of MUF-4.

  • The instrument is high-resolution superconducting superfrequency NMR (Bruker avance 600 M). The resin sample and the solvent DMSO-d6 were mixed in a certain proportion and tested. The relaxation time was 6 s, and the pulse sequence was zgig. The reverse gated decoupling technology was used in the zgig sequence for quantitative analysis. The pulse length is 8.85 db, the pulse amplitude is 11.80 μs, and the number of cumulative scans is 800–1200.

3.   Results and discussion
  • There are a lot of competitive or parallel reactions in UF2+M system, mainly including (1) polycondensation between hydroxymethylurea, (2) polycondensation between hydroxymethylmelamine, (3) polycondensation between various hydroxymethylurea compounds and urea or melamine, (4) polycondensation between hydroxymethylurea and hydroxymethylmelamine, and (5) polycondensation of various hydroxymethylmelamine with urea or melamine. Due to the similarity of the reaction mechanism between urea and melamine under alkaline condition, there are many possibilities for the reaction in the solution with U, M and F units, which makes the analysis more complicated, especially the polycondensation reaction can be self polycondensation or co polycondensation.

    We calculated the activation energy of the related elementary reactions, and the results are as follows:

    Under alkaline condition, urea and melamine can form negative ions with formaldehyde to participate in the reaction, and the formation process can be expressed by Eqs. (1) and (2). The energy barrier of hydroxymethylation is low. The energy barrier of M and F hydroxymethylation is 6.4 and 11.5 kJ/mol, respectively, indicating the superior selectivity of M hydroxymethylation. In the system of UF2+M, once the hydrolysis of UF2 releases free formaldehyde, the hydroxylation of M exists in theory due to the low energy barrier. In fact, according to the principle of reversible reaction, the existence of M will promote the hydrolysis of UF2 since the energy barrier of M is lower than that of U, thus the hydroxylation of M occurs. Eqs. (3) and (4) show the two reaction processes of the formation of methylene bridge (-NR-CH2-NR-) copolymerization structure. The energy barrier is relatively low, which is 3–4 kJ/mol lower than that of the formation of methylene bridge bond by UF self-condensation. From the energy barrier level, the competition between UF self-condensation and MUF co-condensation is obvious.

    The carbon spectrum of MUF-1 is shown in Fig. 1 (The molar ratio of UF2/M is 1). The clear absorption peaks at (166.77–167.33) × 10–6 of chemical displacement is attributed to hydroxymethylmelamine group. The appearance of these signal peaks indicates that the added melamine has been hydroxymethylated. At the same time, the 161.26 × 10–6 absorption peak corresponding to monosubstituted urea and 162.89 × 10–6 absorption peak corresponding to urea were also found, which indicated that UF2 was hydrolyzed in the system, and formaldehyde produced by hydrolysis made melamine hydroxymethylated. In addition, the absorption peaks of 155.56 × 10–6, 156.48 × 10–6 and 157.77 × 10–6 belong to the uron ring, indicating that the addition of melamine causes a lot of hydrolysis of UF2, and the formation of the uron ring is not the main product.

    Figure 1.  The 13C NMR spectrum for sample MUF-1

    Table 1 shows that there are three types of methylene ether bonds under this condition, indicating that the UF2 is hydrolyzed in the system and free formaldehyde participates in the reaction. In the study of the MF resin synthesis (Cao et al., 2017; Cui et al., 2017; Liang et al., 2017; Zhang et al., 2018), it is difficult to form II and III type ether bonds when the formaldehyde ratio is low, so the majority of II and III type ether bonds are generated by self-condensation or co-condensation of hydroxymethylurea. However, because the molar ratio of UF2/M is 1, it is difficult to form a gemodihydroxymethyl group on M, and the possibility of producing II and III type ether bonds by co-condensation is small, mainly due to the self-condensation of hydroxymethylurea. In Fig. 1, there are type I and type II bridge bond signals. Type I bridge bond is mainly formed by dehydration and condensation of free amino group and hydroxymethyl group. The content ratio of type I bridge bond and type I ether bond in Table 1 is 1.05, which is obviously a competitive relationship. However, the MF resin synthesis research shows that even under low F/M molar ratio, hydroxymethyl melamine mainly forms ether bond (Cao et al., 2017). At the same time, previous research has shown that under the initial alkaline condition, the ether bond structure is mainly formed in the system of UF2 (Li, 2015), so the type I bridge bond here is mainly formed by the reaction of UF2 and M, which is consistent with the molar ratio of them. There is no type III bridge signal in the figure, indicating that the type III bridge is not formed between hydroxymethylurea, but by the reaction of amidohydromethylmelamine and amidohydromethylurea. However, when the molar ratio of UF2/M is 1, it is difficult to form amidohydromethylmelamine, so there is no type III bridge signal in the figure. In Table 1, the content of total ether bond accounts for 11.2%, slightly lower than 13.4% of total bridge bond, indicating that the formation of bridge bond here show stronger advantage than that of ether bond.

    Structure Chemical shift (× 10–6) Content percentage (%)
    MUF-1 UF2/M = 1 MUF-2 UF2/M=2 MUF-3 UF2/M=3 MUF-4 M꞉U꞉F=1꞉1꞉3
    -NHCH2OCH2NH- (Ⅰ) 68–70 9.8 13.5 21.6 23.7
    -NHCH2OCH2N= (Ⅱ) 76–77 0.8 1.3 1.7 0.8
    =NCH2OCH2N= (Ⅲ) 79–80 0.2 1.3 2.0 0.7
    -NHCH2OH 64–65 72.0 66.1 63.9 65.0
    -NH(-CH2)CH2OH 71–73 2.9 4.3 2.6 3.7
    -NHCH2NH- (Ⅰ) 46–48 10.3 8.8 4.9 4.1
    -NHCH2N= (Ⅱ) 53–54 3.1 3.6 2.0 0.8
    =NCH2N= (Ⅲ) 60–61
    HOCH2OH 83–84 0.9 0.7 0.8 0.3
    HOCH2OCH2OH 87–88 0.3 0.6 0.5
    HO(CH2O)nH 90–92 0.5
    Formaldehyde polycondensation rate 24.2 28.5 32.2 30.1
    Degree of hydroxymethylation 74.9 70.4 66.5 68.7
    Formaldehyde and its polycondensate 0.9 1.0 1.4 1.3
    Ether bond/bridge bond ratio 0.9 1.3 3.7 5.1

    Table 1.  The integral structure attributable to its 13C NMR chemical shifts of corresponding samples and their percentage composition

    Fig. 2 shows the carbon spectrum of MUF-2, and the mole ratio of UF2/M increases from 1 to 2; Fig. 3 shows the carbon spectrum of MUF -3, and the mole ratio of UF2/M is 3. As shown in Table 1, with the increase of the mole ratio of UF2/M, the content of various types of ether bonds has increased. The proportion of type I/II ether bonds in the three samples is about 10. The proportion of type II/III ether bonds decreased from 3.3 to 1 and further to 0.9. It indicates that the increase of UF2 ratio is beneficial to the formation of type III ether bonds. It is found that the absolute proportion of bridge bonds decreases with the increase of UF2/M molar ratio. Compared with the relative proportion of different types of bridge bonds, the type I/II bridge bonds in MUF-1, MUF-2 and MUF-3 are 3.3, 2.4 and 2.5, respectively, indicating the increase of UF2/M molar ratio is unfavorable to the formation of branch chain bridge bonds, and the effect tends to be stable with the increase of UF2/M molar ratio. Compared with the percentage of ether bond and bridge bond content, as shown in Table 1, that the increase of initial UF2 content is conducive to the formation of ether bond. When the molar ratio of UF2/M is 1, the ratio of ether bond to bridge bond is 0.9, and when the molar ratio of UF2/M is 3, the corresponding ratio is 3.7. Obviously, there is competition between bridge bond and ether bond in the polycondensation reaction system. Results of previous study showed that ether bond is dominant in MF system (Cao et al., 2017). In UF system, only when the initial mole of F/U is very low, the formation of ether bond and bridge bond has an effective competitive relationship, and the ether bond has an absolute advantage in UF2 system. The reason of the formation of ether bond and bridge bond in some UF2+M system based on model compounds is competitive, and it may be: (1) the energy barrier of M is only 6.4 kJ/mol, the existence of M makes UF2 hydrolyze to produce UF1 and U, and the production of free amino group makes hydroxymethylurea self-condense to form methylene bridge bond and ether bond; (2) previous studies have shown that the energy barrier needed for the formation of copolycondensation structure is corresponding to UF (Li, 2015). The energy barrier between UF self-condensing structure and MF self-condensing structure. At the same time, mass spectrometry study shows that there are UF self-condensation product structure, MF self-condensation product structure and co-condensation structure (Li, 2015) in UF2+M system, so the formation of methylene bridge bond in the co-condensation reaction structure is competitive due to the multi free amino group structure in M. From Eqs. (4) and (5), we found that the higher the degree of hydroxymethylation of reactants is, the more favorable the reaction is and the lower the reaction energy barrier is. Because of the low energy barrier of M hydroxymethylation and the strong competition of M hydroxymethylation in UF2+M system, the monomers in the system are mainly hydroxymethyl M and hydroxymethyl urea. The mass spectrometry study showed that both self-condensation products and co-condensation products existed (Liang et al., 2014). For the formation mechanism of bridge bond products, we can use Eq. (5) to express that the reaction energy barrier of FU-+ MF = FUFM is only 124.9 kJ/mol, which is lower than that of 148.1 kJ/mol corresponding to bridge bond structure and 136.5 kJ/mol corresponding to ether bond structure in UF self-condensation products. Compared with the formaldehyde polycondensation rate, it is found that the increase of UF2/M is beneficial to the increase of the formaldehyde polycondensation rate in the alkaline stage (24.24%–32.2%), meanwhile, the degree of hydroxymethylation shows the opposite trend (66.5%–74.9%). The increase of polycondensation rate coincides with the decrease of hydroxymethylation, indicating that the increased UF2 has a high degree of participation in polycondensation.

    Figure 2.  The 13C NMR spectrum for sample MUF-2

    Figure 3.  The 13C NMR spectrum for sample MUF-3

    In order to confirm the selectivity and competitiveness of various reactions in MUF system, experiments were carried out with basic raw materials melamine, urea and formaldehyde based on the study of model compounds. Fig. 4 presents the carbon spectrum of sample MUF-4, M꞉U꞉F = 1꞉1꞉3, which is a one-time adding raw material. In Fig. 4, the absorption peaks at (166.12–167.89) × 10–6 corresponding to substituted melamine, (159.09–162.55) × 10–6 for substituted urea and (154.54–156.09) × 10–6 for uron ring are observed, indicating that hydroxymethylation of M and U occurs in the system, and the ratio of substituted melamine/urea is 3.15. The results show that M has an advantage over U. In Table 1, the corresponding type I ether bond has an absolute advantage, while the type II and type III ether bonds are few. Study results showed that the formation of type II and type III ether bonds is mainly from the condensation between hydroxymethylurea, and the condensation between hydroxymethylmelamine is mainly the formation of straight chain ether bonds, indicating that the concentration of polyhydroxymethylurea generated is small, which is not enough to form sufficient type II and type III ether bonds (Li, 2015). The ratio of the content of the ether bridge bond is 5.1, where the formation of the bridge bond is competitive to some extent. Previous studies have shown that the condensation between hydroxymethylmelamine is mainly based on the ether bond, and the ratio of substituted melamine to substituted urea is 3.15 (Li, 2015). Comprehensive analysis shows that the formation of the bridge bond under this condition is mainly due to the reaction of hydroxymethylmelamine and free urea.

    Figure 4.  The 13C NMR spectrum for sample MUF-4

4.   Conclusion
  • Based on the model compound UF2 and M as the starting materials, the competitive polycondensation mechanism of resin synthesis in M-U-F system under alkaline conditions was analyzed. The following conclusions are obtained: (1) Hydroxymethylation of melamine takes precedence over urea. In the UF2+M system, a large amount of hydrolysis of UF2 occurs, resulting in free formaldehyde, which makes hydroxymethylation of melamine. (2) In the UF2+M system, there is obvious copolycondensation reaction, which mainly comes from the relationship between hydroxymethylurea and melamine or hydroxymethylmelamine. (3) In the UF2+M system, the type I bridge bond mainly comes from the reaction between UF2 and M, so it is difficult to form the type II bridge bond. At low molar ratio, the formation of bridge bond is superior to that of ether bond. With the increase of molar ratio, the formation of ether bond shows superiority, but there is obvious competition between them. (4) The condensation rate of UF2+M system is generally higher than that of UF2 and lower than that of MF. And 5) There may be not only UF self-condensation products, but also MF co-condensation products and cocondensation products in the polycondensation products.

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