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.
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.
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.