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Article

Crosslinking Mechanism of Tannin-Based Adhesives Based on Model Compounds: Copolycondensation of Resorcinol with Dimethylol Urea

by
Jiankun Liang
1,2,†,
De Li
1,†,
Xiao Zhong
1,
Zhigang Wu
1,3,*,
Ming Cao
3,4,*,
Guifen Yang
1,
Shuang Yin
1 and
Feiyan Gong
1
1
College of Forestry, Guizhou University, Guiyang 550025, China
2
College of Civil Engineering, Kaili University, Qiandongnan 556011, China
3
International Joint Research Center for Biomass Materials, Southwest Forestry University, Kunming 650224, China
4
The Yunnan Province Key Lab of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2024, 15(1), 98; https://doi.org/10.3390/f15010098
Submission received: 30 November 2023 / Revised: 25 December 2023 / Accepted: 2 January 2024 / Published: 4 January 2024
(This article belongs to the Special Issue Advances in the Cultivation, Protection and High-Value Utilization of Forest Resources)
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Abstract

:
This study focuses on the competition reaction rules of a system containing resorcinol (as a tannin model compound) and dimethylol urea (as a urea–formaldehyde resin model compound) under various alkaline and acidic environments. The aim is to investigate the crosslinked modification mechanism of urea–formaldehyde resin with tannin adhesive. The study delves into the competitive relationship between self-condensation polymerization reactions and co-condensation polymerization reactions. It specifically highlights the conditions for the copolycondensation reaction of dimethylolurea and resorcinol and validates its rationality through an examination of the resorcinol–urea–formaldehyde system’s reaction rules. The results show that (1) under strongly acidic conditions, the activity of carbocation intermediates produced by hydroxymethyl resorcinol for the resorcinol phenol ring is higher than the electrophilic reactivity of nitrogen atoms on hydroxymethyl urea, which is more beneficial for the resorcinol–formaldehyde self-polycondensation reaction, and the co-polycondensation structures do not play a dominant role. (2) Under weakly acidic conditions, the co-polycondensation structures are evidently advantageous over self-polycondensation structures, and the degree of the co-polycondensation reaction is positively correlated with pH below the neutral point of resorcinol. (3) Under alkaline conditions, the self-polycondensation between resorcinol and formaldehyde is dominant in the system. (4) The concentration of hydroxymethyl urea carbocation is the key factor to determine the degree of the co-polycondensation reaction.

1. Introduction

With the gradual exhaustion of petrochemical raw materials, the concept of green environmental protection and sustainable development has become a common concept in scientific and technological research. In the field of wood-based composite resins, it has become the new research trend to replace part or all of the existing petrochemical raw materials with renewable biomass resources [1,2,3,4,5,6,7,8,9]. On this basis, tannin, which is of a biomass nature, is quite a representative natural raw material. As a secondary metabolite with a preservative effect, tannin widely exists in stems, skins, roots, leaves, and fruits of plants. Tannin adhesives can be prepared with high-tannin-content plants as the main raw materials [10,11,12,13,14], showing broad application prospects. According to the chemical composition and structure, condensed tannins account for 90% of the world tannin yield, with economic development value in the production of adhesives and resins [15,16,17,18]. In addition, condensed tannins are composed of flavonoid monomers differing in degree of condensation in a fixed structural mode, and their basic structural units are displayed in Figure 1.
The reactivity of tannins is mainly determined by the reactivity of tannin ring A, while ring B usually does not participate in the reaction. According to the presence or absence of a hydroxyl group in the C5 position of ring A, the structural units of condensed tannins can be divided into resorcinol A ring type and phloroglucinol A ring type. Under alkaline conditions, tannin and formaldehyde can react at room temperature and be further condensed into polymers with larger molecular weights, thus producing tannin–formaldehyde (TF) resin [19,20,21,22]. The C6 and C8 positions of ring A mainly participate in the reaction, while ring B only does so under strongly acidic or alkaline conditions. The nucleophilic reactivity of the aromatic ring is obviously enhanced by multiple activating groups attached to it. Due to the too-fast reaction between tannin and formaldehyde and the relatively large molecular weight of tannin itself, however, the molecular weight of polymers increases rapidly, the viscosity of the reaction system is enhanced sharply, and, consequently, the molecules at unreacted sites lose fluidity, making it hard to continue the polycondensation reaction and finally leading to the low degree of polymerization of the system [23,24,25,26]. In addition, the polycondensation reaction is impeded by the steric hinderance effect at active sites in the tannin structure. As a result, the final bonding strength of TF resin is not ideal. Meanwhile, it shows poorer water resistance and higher brittleness than traditional adhesives [18,27,28,29,30].
Co-polycondensation is an effective method to solve these problems of tannin adhesives. In the field of wood adhesives, urea–formaldehyde (UF) resin is the most widely used basic adhesive because of its low price and excellent performance. Introducing UF resin into tannin adhesives is a possible direction [31,32,33], but the basic reaction mechanism therein remains uncertain. Considering the complex chemical structure of biomass tannin, polycondensation products with different structures and properties can be obtained by different polymerization degrees and reaction sites of its structural monomers, which is too complex to investigate. With this background, reaction mechanisms based on model compounds are an effective method. In this study, the crosslinking modification mechanism of UF for tannin adhesives was explored by taking resorcinol as a tannin A ring model compound [34,35,36,37] and dimethylol urea as a UF model compound from the perspective of the reaction of such model compounds. At present, most scholars believe that the co-polycondensation reaction exists in the alkaline stage [38,39,40,41,42], and the research direction mostly focuses on the influence on the final resin properties, while the co-polycondensation reaction conditions have been investigated less. Co-polycondensation structures remain ambiguous, and the co-polycondensation conditions and mechanism have not been clearly described [43,44,45]. Does the co-polycondensation of resorcinol–urea–formaldehyde (RUF) resin occur under strongly acidic, weakly acidic, weakly alkaline, or strongly alkaline conditions? In the case of co-polycondensation, how is the reaction catalyzed by H+ and OH- in acidic and alkaline stages? This has not been systematically investigated or explained. Most studies involving the modification of polyphenols have been established in co-polycondensation under alkaline conditions. According to our early study and the relevant literature, these co-polycondensation conditions and structures are controversial. In this study, therefore, the co-polycondensation conditions and structures of the resorcinol–dimethylol urea system were quantitatively analyzed and deeply studied using a 13C–NMR instrument. Then, the possible reaction mechanism was reasonably speculated and experimentally verified, and high-reproducibility experimental data were acquired so as to theoretically support the synthesis of RUF with excellent properties and provide reference for further synthesizing tannin–formaldehyde–urea adhesives. Based on the selectivity of intermediates for conditions, how to accurately regulate the synthesis of co-condensed tannin resin adhesive by regulating the experimental parameters provides a new path.

2. Materials and Methods

2.1. Materials

N,N′-dimethylol urea (purity: 98%) was from J & K Technology Co., Ltd. (Beijing, China); formaldehyde, analytically pure (purity: 37%–40%), was produced by Chongqing Chuanjiang Chemical Reagent Factory (Chongqing, China); sodium hydroxide, analytically pure, was produced by Shantou Dahao District Fine Chemicals Co., Ltd. (Shantou, China); formic acid came from Zhangzhou Xiangcheng Sanan Chemical Co., Ltd. (Zhangzhou, China); oxalic acid, Weifang Oxalic Acid Company (Weifang, China); resorcinol, analytically pure, was from Henan Zhengyao Chemical Products Co., Ltd. (Zhengzhou, China).

2.2. Sample Preparation

The test reaction volume was converted into the same proportion based on the amount of actual adhesive synthesized in ordinary times, and the molar ratio of each reactant was mainly controlled; the reaction conditions were to simulate the reaction conditions in the actual synthesis stage. The final effect was to use model compounds to simulate the complex competitive reaction process during the synthesis process. Synthesis schemes of resorcinol (R) and formaldehyde (F), resorcinol (R) and dimethylol urea (UF2), resorcinol (R), formaldehyde (F), and urea (U) are shown in Table 1, Table 2 and Table 3, respectively.
(1) Initially, 11 g (0.1 mol) of resorcinol, 67.5 g (0.832 mol) of 37% formaldehyde, and 50 g of distilled water were weighed and put into a three-necked flask and stirred with a magnetic stirrer until resorcinol was completely dissolved. After the pH value was regulated to 2~2.5 with 88% formic acid solution, the mixture was put into a 50 °C water bath for 1 h reaction, followed by sampling for 13C–NMR analysis, and the sample was numbered RF1. Similarly, the pH value was regulated to 3~4, 5~6, 7~8, and 9~10, and the samples acquired were respectively numbered RF2, RF3, RF4, and RF5.
(2) Then, 11 g (0.1 mol) of resorcinol, 2.4 g (0.02 mol) of N,N′-dimethylol urea (UF2), and 20 g of distilled water were weighed and placed into a three-necked flask and stirred with a magnetic stirrer until resorcinol and N,N′-dimethylol urea were completely dissolved. After the pH was adjusted to 1~2 with oxalic acid solid, the mixture was put into a water bath at 60 °C for 1 h reaction, followed by sampling and analysis, and the sample was numbered UFR1. Similarly, the pH was regulated to 2~3, 3~4, 5~6, 8~9, and 10~11, and the samples acquired were respectively numbered UFR2, UFR3, UFR4, UFR5, and UFR6.
(3) Further, 11 g (0.1 mol) of resorcinol, 8.11 g (0.1 mol) of 37% formaldehyde, and 100 g of distilled water were weighed and put into a three-necked flask and stirred with a magnetic stirrer until resorcinol was completely dissolved. The pH value was adjusted to 2~2.5 with 88% formic acid solution, followed by reaction in the water bath at 60 °C for 30 min. Then, the solution prepared using 12 g of urea (0.2 mol) and 20 g of water was added for continuous reaction for another 30 min, and the sample obtained was numbered RUF1. Similarly, the pH value was adjusted to 3~4, 5~6, 8~9, and 10~11, and the samples acquired were numbered RUF2, RUF3, RUF4, and RUF5.

2.3. 13C–NMR Test

The sample was evenly mixed with dimethyl sulfoxide deuterated-d6 (DMSO-d6) according to the volume ratio of 3:1 and then tested on a Bruker AVANCE 600 NMR spectrometer (Mürzzuschlag, Switzerland). The test was performed through the inverse gated decoupling technique (quantitative analysis technique) under the following parameters: relaxation time (6 s), pulse sequence (zgig), pulse length (8.85 dB), pulse amplitude (11.80 μs), and accumulative number of scanning times (800~1200).
The relative content of various types of methylene carbon was calculated through the normalized quantification method:
W % = ( Ap / ∑ p Ap ) ∗ 100 %
where Ap is the peak area integral value of a methylene carbon, and ∑Ap is the sum of the peak area integral values of all methylene carbons. The relative content of various substitutive urea carbonyl carbons was solved through a similar method.

3. Results and Discussion

3.1. The Reaction Law of Model Compounds Resorcinol and Formaldehyde

Resorcinol is a type of polyphenolic substance, and the two phenolic hydroxyl groups in its structure are more active to benzene rings than phenol [46,47] such that its electrophilic substitution reaction is more active than phenol. The reaction conditions and structure formation mechanism of UFR in the wood-based panel industry have been widely studied. In order to figure out the co-polycondensation reaction mechanism regarding resorcinol–urea–formaldehyde, it is necessary to study the reaction mechanism and competitive relationship of structure formation between resorcinol and formaldehyde first. As for the structural analysis method, 13C–NMR, which has been widely used at present, calibrates different carbons by different chemical shifts in the carbon nucleus in the strong magnetic field according to the electron cloud density around the carbon nucleus. This method, which is of a relatively high identification degree, is more accurate for structural analysis [48,49,50] and more feasible for the quantitative analysis of the content of chemical structures and can provide effective data support for the reaction mechanism study. Figure 2 shows the 13C–NMR graphs of the reaction products of formaldehyde and resorcinol under pH = 2~2.5, 3~4, 5~6, 7~8, and 9~10, and the attribution of the corresponding chemical structures is listed in Table 4.
It can be known by combining Figure 2 and Table 4 that resorcinol and formaldehyde show different reactivity at different pH values. The phenolic hydroxyl groups on resorcinol are o-p-positioning groups, and the two phenolic hydroxyl groups are located at meta-positions, so resorcinol has three active reaction sites: 1 o-position and 2 o-p-positions, respectively. Similar to phenol, the resorcinol–formaldehyde system can undergo hydroxymethylation and polycondensation; ether bond polycondensation and bridge bond polycondensation exist in the polycondensation products, and its reaction mechanism is different in the active reaction intermediates under acidic and alkaline conditions, as shown in Figure 3. This has been reflected in the team’s previous research.
Formulas (1) and (2) are the hydroxymethylation mechanisms of resorcinol and formaldehyde under alkaline and acidic conditions, respectively. Formulas (3) and (4) are the mechanisms of forming self-polycondensation bridges and ether bonds under acidic conditions, and Formula (5) is the mechanism of polycondensation into bridges under alkaline conditions. There are intermediates of resorcinol benzene ring anion and hydroxymethyl resorcinol carbocation in the reaction, which is the reason why resorcinol and formaldehyde can be condensed under both acidic and alkaline conditions.
The 13C–NMR attribution of resorcinol is based on the test rules of the literature [51,52,53,54,55,56] and NMR chemical shift, in which the chemical shift of resorcinol o-hydroxymethyl is 54–55 ppm, that of o-p-hydroxymethyl is 60–61 ppm, and that of resorcinol o-o-o-o bridged methylene is 14–15 ppm; the absorption peak of o-p-o-o bridged methylene is 23–24 ppm, and the chemical shift of hydroxymethyl hemiacetal is 66–67 ppm. In Table 4, no absorption peaks appeared at the chemical shifts of 54–55 and 14–15 ppm in all the samples because the resorcinol o-o-steric hindrance was so great that reaction could hardly occur. However, at pH of 2–2.5 and 9–10, a small number of methylene absorption peaks of resorcinol o-o-o-p bridged bonds appeared, the content of which reached 3.06% and 20.32%, respectively, indicating that the o-o-resorcinol had strong reactivity under strong acidic and alkaline conditions, and the reactivity of this position under alkaline conditions was stronger than that under acidic conditions.
The absorption peak of the resorcinol o-p-o-p-methylene bridge bond at the chemical shift of 29–30 ppm was obvious, and the content reached 32.75% at pH of 2–2.5, decreased to 3.82% at pH of 3–4, increased to 4.17% at pH of 5–6, and further increased to 19.78% and 74.87% at pH of 7–8 and 9–10. With the increase in pH, the absorption peak content of resorcinol o-p-o-p-methylene bridge bond first decreased and then increased, reflecting that the reactivity of resorcinol was very low within a specific range in the weakly acidic stage, and this pH range was most likely 4–5 in terms of content. Below this range, the acidic catalytic reaction played a dominant role, and the reactivity of resorcinol declined with the increase in the pH value. Above this range, the alkaline catalytic reaction dominated, and the reactivity of resorcinol was continuously enhanced as the pH value was elevated.
With the increase in pH, the residual content of o-p hydroxymethyl at a chemical shift of 60–61 ppm was 0, 0.64%, 12.50%, 73.83%, and 4.81% respectively. When the pH value was 2–2.5, the residual content of hydroxymethyl was 0, indicating that the reactivity of hydroxymethyl carbocation was very high under strongly acidic conditions, and the residual content of methylene glycol and its self-polycondensation products reached 64.19%. At pH of 7–8 and 9–10 under alkaline conditions, the residual amount of methyl glycol was very little and even zero, reflecting that the reactivity of formaldehyde with resorcinol was much higher under alkaline conditions than under acidic conditions. The speed and degree of strongly acidic catalytic polycondensation of resorcinol depended on the hydroxymethylation degree of formaldehyde and resorcinol. Once resorcinol was hydroxymethylated, polycondensation could occur all the time, further proving that pH = 4–5 was a turning point for the reactivity of resorcinol. However, 93.6% of the formaldehyde reacted with resorcinol at pH of 7–8 under alkaline conditions, but the residual content of hydroxymethyl reached 73.83%, and the content of polycondensation products was only 19.78%. When the pH value was elevated to 9–10, all the formaldehyde reacted with resorcinol, and the content of polycondensation products also reached 95.09%. This revealed that hydroxymethylation could be fully carried out under weakly alkaline conditions, and the polycondensation reaction required a high concentration of resorcinol benzene ring anions, which coincided with the results of relevant literature research [44].

3.2. The Polycondensation Law of Model Compounds Resorcinol and Dimethylol Urea

In the resorcinol–formaldehyde–urea system, the self-polycondensation reaction between hydroxymethyl resorcinol and hydroxymethyl urea and the co-polycondensation reaction of resorcinol–formaldehyde–urea existed, both of which were complicated [57]. To study the co-polycondensation mechanism of UFR and resorcinol more effectively, it is necessary to introduce model compounds with relatively simple structures to reduce the complexity of the reactions. N,N′-dimethylol urea is a common model compound with a symmetrical hydroxymethyl structure, and thus the co-polycondensation reaction can be more representatively investigated [58,59]. Since the UF2 system can be easily gelatinized under strongly acidic conditions, making the reaction difficult, the molar ratio of UF2/R was chosen as 1/5 considering the stability of the final test sample. Figure 4 displays the 13C–NMR graphs of the reaction products between the model compounds UF2 and resorcinol at pH values of 1~2, 2~3, 3~4, 5~6, 7~8, and 9~10. The attribution of the corresponding chemical structure is exhibited in Table 5.
In the system of UF2 and resorcinol, two kinds of self-polycondensation structures of hydroxymethyl resorcinol appeared under both acidic and alkaline conditions, i.e., (2.4.) self-polycondensation bridge peak at 23–24 ppm and (4.4.) self-polycondensation bridge peak at 29–30 ppm, respectively. The reaction mechanisms of resorcinol–formaldehyde–urea under acidic and alkaline conditions were shown in Figure 5 and Figure 6 respectively. The appearance of these two absorption peaks showed that, first, model compound UF2 was hydrolyzed in the solution system and produced free formaldehyde, as shown in Formula (6); second, resorcinol experienced hydroxymethylation with formaldehyde, and protonated formaldehyde was formed under acidic conditions, as shown in Formulas (7) and (8). Moreover, resorcinol benzene ring anions were formed under alkaline conditions, as shown in Formulas (17) and (18); third, the self-polycondensation structure of resorcinol could be formed under both acidic and alkaline conditions. The acidic active intermediates came from hydroxymethyl resorcinol carbocation, as shown in Formulas (9)–(12), and the active intermediate under alkaline conditions was resorcinol benzene ring anion, which reacted with the resorcinol benzoquinone structure, as displayed in Formulas (19) and (20).
With the change in pH, the content of the resorcinol self-polycondensation structure at (4.4.) position corresponding to the chemical shift of 29–30 ppm ranged from 58.97% at pH = 1–2 to 28.49% at pH = 2–3, and then to 7.78% at pH = 3–4. The self-polycondensation structure of resorcinol at (2.4) position only existed in a small amount (only 1.29%) when pH = 1–2, and it was hardly produced with the decrease in pH. This reflected that hydroxymethylresorcinol carbocation was highly sensitive to pH, and the concentration of carbocation was highly consistent with the concentration of H+.
At the chemical shift of 35–36 ppm was the co-polycondensation structure of resorcinol 2 and urea, and its reactive intermediate could be hydroxymethylresorcinol carbocation, with the reaction mechanism as shown in Formula (13); it could also be hydroxymethyl urea carbocation, as shown in Formula (15). With the increase in pH, the content of this co-polycondensation structure increased slightly (7.13%–7.26%–7.59%), reached the highest when pH = 4, and decreased to 2.98% when pH = 5–6, further indicating that a pH of 4–5 was the neutral point of resorcinol; the acidic catalytic mechanism was below the neutral point, while the alkaline catalytic mechanism was above the neutral point. The law was not that obvious, possibly because the reaction failed to proceed rapidly under the steric effect of resorcinol 2. The concentration of hydroxymethyl resorcinol carbocation would decrease with the increase in pH below the neutral point, and the structural content of the corresponding reaction products also decreased, so the co-polycondensation structure at a chemical shift of 35–36 ppm was unlikely generated due to the reaction in which hydroxymethyl resorcinol carbocation participated; otherwise, why the content of the co-polycondensation structure increased contrarily with the increase in pH could not be explained. Therefore, the active intermediate of the co-polycondensation reaction here was more likely to come from hydroxymethyl urea carbocation. Due to the steric hindrance effect of resorcinol 2, however, the law was not obvious and could not be accurately explained, making it necessary to consider the co-polycondensation structure of resorcinol 4 with weaker steric hindrance.
At the chemical shift of 40–41 ppm was the co-polycondensation structure of resorcinol 4 and urea, or that of resorcinol 2 and urea, and the active intermediate might be hydroxymethyl resorcinol carbocation or hydroxymethyl urea carbocation, as shown in Formulas (14) and (16), respectively. According to Table 5, the structural content presented evident change laws: below the neutral point of resorcinol, the content of the co-polycondensation structure rapidly increased with the growth in pH, reaching 83.26% when pH = 3–4, and rapidly declined to 45.29% when pH = 5–6. The raw materials of the reaction system were model compounds UF2 and resorcinol. The main form of resorcinol below the neutral point was the resorcinol molecular structure. With the increase in pH, the concentration of H+ decreased, the degree of hydroxymethylation of resorcinol decreased greatly, and the concentration of hydroxymethyl resorcinol carbocation formed subsequently decreased greatly. The tendency of UF2 hydrolysis to produce formaldehyde decreased, and most UF2 could be protonated to generate hydroxymethyl urea carbocation to participate in the reaction. Below the neutral point, with the increase in pH, the concentration of hydroxymethyl urea carbocation increased while that of hydroxymethyl resorcinol carbocation decreased, which could reasonably explain the above questions. Meanwhile, a new problem was raised. The concentration of hydroxymethyl urea carbocation would continuously increase with the increase in pH below the neutral point of resorcinol; the content of the self-polycondensation structure of the bridge bond (Formula (12)) and ether bond (Formula (13)) between hydroxymethyl urea was not found to increase, but 1.37% of I type bridge bond (47–48 ppm) was generated below and near the neutral point. This showed that the relative reactivity of hydroxymethyl urea carbocation to resorcinol molecules was much greater than that of p-hydroxymethyl urea itself, and only in the system with extremely high concentration of hydroxymethyl urea carbocation could a very small amount of hydroxymethyl urea self-polycondensation structure be produced.
In the synthesis process of UFR, the pH value in the resinifying stage was generally 5–6. At this pH value, the resorcinol -UF2 system presented different competitive relationships. In this case, resorcinol was above the neutral point and went through ionization to produce phenoxy anions, and H+ preferentially bound phenoxy anions but rarely protonated with hydroxymethyl resorcinol hydroxyl to produce carbocation. Under these conditions, hydroxymethyl resorcinol carbocation could hardly be formed, but hydroxymethyl urea carbocation could be formed. When pH = 5–6, the absorption peak content of co-polycondensation reached 45.29% at 40–41 ppm, so it could be determined that the active intermediate for the formation of the co-polycondensation structure derived from hydroxymethyl urea carbocation instead of hydroxymethyl resorcinol carbocation. This pH range was above the neutral point of resorcinol, and the catalytic reaction of resorcinol belonged to alkali catalytic reaction. However, the alkali catalysis of resorcinol was weak here, the acid catalysis of hydroxymethyl urea was not strong, and the absolute concentration of hydroxymethyl urea carbocation was also decreasing, so the overall reactivity was not strong, resulting in a small amount of free formaldehyde (3.54%) in the system. At the same time, free hydroxymethyl resorcinol also appeared, with the content of 2# position reaching 0.81% and that of 4# position reaching 5.36%. There was also a small amount of resorcinol self-polycondensation products (8.54%). In this case, a competitive relationship was observed between the polycondensation products of hydroxymethyl urea, and the content of type I bridge bond and type I ether bond was 7.4% and 3.83%, respectively.
When the pH of the system was 7–8, both UF2 and resorcinol experienced alkaline catalytic reactions. The reaction of the former under alkaline conditions depended on urea anions and hydroxymethyl oxygen anions, with the reaction mechanisms displayed in Formulas (21) and (22). However, its concentration was very low, the reaction was very slow, with only 13% of hydroxymethyl urea, and no polycondensation products were obviously found between hydroxymethyl urea. In this case, however, the pH was alkaline enough for resorcinol, which could form resorcinol benzene ring anions with sufficient concentration, which further went through hydroxymethylation with formaldehyde generated by hydrolysis and also self-polycondensation with benzoquinone resorcinol, with the reaction mechanisms expressed by Formulas (17)–(20). Due to the high concentration of benzene ring anions of resorcinol, a large number of polycondensation products were formed, and the content of the self-polycondensation structure reached 53.43% at the chemical shift of 29–30 ppm, and 8.30% at the chemical shift of 23–24 ppm. If the co-polycondensation structure could be formed under alkaline conditions, the most likely active intermediates were resorcinol benzene ring anions, and the possible reaction mechanism is shown in Formulas (23) and (24). No obvious co-polycondensation structure was found in Table 5 and Figure 4, indicating that the reactivity between resorcinol benzene ring anions and hydroxymethyl urea was not great, but the reaction between resorcinol benzene ring anions and hydroxymethyl resorcinol was more advantageous.
Following this idea, the alkalinity was elevated to pH = 10–11. Under this circumstance, the concentration of benzene ring anions formed by resorcinol was higher, but no evident co-polycondensation structure was observed. In Table 5 and Figure 4, the self-polycondensation structure between resorcinol almost played a dominant role, indicating that the reactivity between resorcinol benzene ring anions and hydroxymethyl urea was less competitive than that between resorcinol benzene ring anions and hydroxymethyl resorcinol. This experiment cleverly demonstrates the different sensitivity of different intermediates to pH values, providing the possibility for finely regulating the reaction process to obtain the desired adhesive. It provides a path for the precise synthesis of adhesives.

3.3. Verification of Resorcinol–Urea–Formaldehyde Reaction Rule

The results of Section 3.1 and Section 3.2 showed that the co-polycondensation reaction between resorcinol and UF2, which were model compounds, mainly occurred in the acidic stage, and the reactive intermediate mainly derived from hydroxymethyl urea carbocation instead of hydroxymethyl resorcinol carbocation. In order to further verify this conclusion, resorcinol was first subjected to the reaction with formaldehyde for a period of time under certain conditions, and then urea was added for further reaction to obtain samples for 13C–NMR analysis. The test results are shown in Table 6 and Figure 7.
When pH = 2–2.5, the product structure was dominated by resorcinol self-polycondensation products (29–30 ppm) at (4.4.) position. If the formation of the co-polycondensation structure benefited from hydroxymethyl resorcinol carbocation, the added urea would undergo the co-polycondensation reaction with it, which, however, was not the truth. A possible reason was that the self-polycondensation between resorcinol was more advantageous, and the reaction was already complete before the addition of urea. From the equilibrium of reaction, moreover, a small amount of free hydroxymethyl resorcinol would appear in the system, and the addition of urea was supposed to produce a small number of co-polycondensation structures, which, however, was not the case. Therefore, the co-polycondensation reaction intermediates at this position were unlikely to derive from hydroxymethyl resorcinol carbocation.
When the pH value was elevated to 3–4, hydroxymethyl urea and resorcinol were both catalyzed by acid. At this time, the self-polycondensation structure of the system only accounted for 3.98%, which was much lower than the previous 100%. Meanwhile, the content of the co-polycondensation structure at 2# position reached 5.58% (35–36 ppm), while that at 4# position reached 39.84% (40–41 ppm), and the content of urea–formaldehyde resin I-bridge bond (47–48 ppm) reached 50.60%, meaning that comprehensive competitive relationships were manifested in the reaction between systems. When pH = 3–4, the concentration of H+ declined and the concentration of hydroxymethyl resorcinol carbocation sharply dropped, which could be obviously observed from the decline in the content of the resorcinol self-polycondensation structure from 100% to 3.98%. However, many co-polycondensation structures appeared at this position. Thus, the production of co-polycondensation structures in quantity mainly relied upon hydroxymethyl urea carbocation.
When the pH grew to 5–6, resorcinol mainly experienced an alkaline catalytic reaction, but this acidity was weak for the formation of hydroxymethyl urea carbocation, which led to the sharp decrease in the co-polycondensation structure to 5.63%. This further proved that the co-polycondensation structure mainly derived from hydroxymethyl urea carbocation. Moreover, pH of 7–8 contributed to the alkaline catalytic reaction for both of them. Resorcinol could form enough phenol ring anions, while urea had a low concentration of anions, without competitiveness. The results in Table 6 and Figure 7 also showed that formaldehyde was almost completely hydroxymethylated with resorcinol, without an obvious polycondensation reaction. When pH = 9–10, the free hydroxymethyl resorcinol almost disappeared, 24.26% of the formaldehyde was converted into the self-polycondensation structure at (2.4.) position, and 69.07% of the formaldehyde was converted into the self-polycondensation product at (4.4.) position, reflecting that the phenol ring anion could be generated with sufficient concentration when the alkalinity was strong enough so that the polycondensation reaction between hydroxymethyl resorcinol and the phenol ring anion could fully take place. However, only 6% of the co-polycondensation absorption peaks (40~41 ppm) were observed in Table 6 and Figure 7. This only revealed that phenol ring anions could participate in the co-polycondensation reaction in a small amount in theory to a limited degree, and they were not obviously competitive compared with acid carbocation.
To sum up, the results in Table 6 and Figure 7 reflected that the reaction law of the resorcinol–urea–formaldehyde system verified the rationality of the polycondensation reaction law of resorcinol and dimethylol urea as model compounds in Section 3.1 and Section 3.2. The reactivity of hydroxymethyl resorcinol carbonium ion and hydroxymethyl urea carbonium ion has a significant impact on the reaction. Previous studies have not provided clear explanations, and this paper provides some evidence to support the preference of hydroxymethyl urea carbonium ion. The research conclusion echoes the literature research [39,57,59,60,61].

4. Conclusions

Resorcinol was used as a tannin model compound and dimethylol urea as a UF model compound. The reaction system of dimethylol urea and resorcinol in alkaline, weakly acidic, and strongly acidic environments was systematically studied in this paper. The competitive relationship between the self-polycondensation reaction and the co-polycondensation reaction of dimethylol urea and resorcinol under different conditions was clarified, and the conditions for the co-polycondensation reaction were clearly pointed out. The results showed that
(1) Under strongly acidic conditions, the activity of carbocation intermediates produced by hydroxymethyl resorcinol for the resorcinol phenol ring is higher than the electrophilic reactivity of nitrogen atoms on hydroxymethyl urea, which is more beneficial for the resorcinol–formaldehyde self-polycondensation reaction, and the co-polycondensation structures do not play a dominant role.
(2) Under weakly acidic conditions, the co-polycondensation structures are evidently advantageous over self-polycondensation structures, and the degree of the co-polycondensation reaction is positively correlated with pH below the neutral point of resorcinol.
(3) Under alkaline conditions, the self-polycondensation between resorcinol and formaldehyde dominates in the system.
(4) The formation of co-polycondensation structures mainly depends on the difficulty in the formation of different active intermediates and their relative activity in self-polycondensation and co-polycondensation reactions.
(5) The concentration of hydroxymethyl urea carbocation is the key factor to determine the degree of the co-polycondensation reactions.
(6) The reaction law of the resorcinol–urea–formaldehyde system verified the rationality of the polycondensation reaction law of model compounds resorcinol and dimethylol urea. The results provide theoretical support for the modification of tannin adhesives and pave the way for future innovations in tannin adhesive modification. Looking ahead, this research opens avenues for developing more sustainable, efficient bonding solutions, aligning with the evolving demands of environmental stewardship and industrial efficiency.

Author Contributions

Conceptualization, methodology, validation, formal analysis, resources, and visualization, J.L., D.L., X.Z., G.Y., S.Y. and F.G.; writing—original draft preparation and writing—review and editing, J.L., M.C. and Z.W.; supervision, M.C. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (31800481 and 32160413), Science–Technology Support Foundation of Guizhou Province, China ([2020]1Y128 and [2019]2308), the Forestry Science and Technology Research Project of the Guizhou Forestry Bureau, China ([2020]C14), the Qiandongnan Basic Research Program Project ([2021]15), the Outstanding Youth Science and Technology Talent Project of Guizhou Province, China(YQK[2023]003), Yunnan Provincial High-level Talents Training Support Plan Youth Top Talent Project (YNWR-QNBJ-2020-144), the Talents from Guizhou Science and Technology Cooperation Platform ([2019]01-3), Kaili University ‘Practical Engineering’ Special Project(2020gkzs01), Qiandongnan Prefecture Science and Technology Plan Project ([2019]107), and the Agriculture Joint Research Program of Yunnan Province (2017FG001 (-079)), International Joint Research Center for Biomass Materials Open Fund (2023-GH03).

Data Availability Statement

All the data are provided in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Chemical structure of condensed tannin unit.
Figure 1. Chemical structure of condensed tannin unit.
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Figure 2. The 13C–NMR curve of samples. Note: (a), RF1; (b), RF2; (c), RF3; (d), RF4; (e), RF5.
Figure 2. The 13C–NMR curve of samples. Note: (a), RF1; (b), RF2; (c), RF3; (d), RF4; (e), RF5.
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Figure 3. The reaction of resorcinol with formaldehyde under acidic and alkaline conditions.
Figure 3. The reaction of resorcinol with formaldehyde under acidic and alkaline conditions.
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Figure 4. The 13C–NMR curve of samples. Note: (a), UFR1; (b), UFR2; (c), UFR3; (d), UFR4; (e), UFR5; (f), UFR6.
Figure 4. The 13C–NMR curve of samples. Note: (a), UFR1; (b), UFR2; (c), UFR3; (d), UFR4; (e), UFR5; (f), UFR6.
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Figure 5. The correlation mechanism of resorcinol–formaldehyde–urea under acidic conditions.
Figure 5. The correlation mechanism of resorcinol–formaldehyde–urea under acidic conditions.
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Figure 6. The correlation mechanism of resorcinol–formaldehyde–urea under alkaline conditions.
Figure 6. The correlation mechanism of resorcinol–formaldehyde–urea under alkaline conditions.
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Figure 7. The 13C–NMR curve of samples. Note: (a), RUF1; (b), RUF2; (c), RUF3; (d), RUF4; (e), RUF5.
Figure 7. The 13C–NMR curve of samples. Note: (a), RUF1; (b), RUF2; (c), RUF3; (d), RUF4; (e), RUF5.
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Table 1. Synthesis scheme of reaction of resorcinol (R) and formaldehyde (F).
Table 1. Synthesis scheme of reaction of resorcinol (R) and formaldehyde (F).
Synthesis SchemeSample Name
R + F, pH = 2~2.5RF1
R + F, pH = 3~4RF2
R + F, pH = 5~6RF3
R + F, pH = 7~8RF4
R + F, pH = 9~10RF5
Table 2. Synthesis scheme of reaction of resorcinol (R) and dimethylol urea (UF2).
Table 2. Synthesis scheme of reaction of resorcinol (R) and dimethylol urea (UF2).
Synthesis SchemeSample Name
R + UF2, pH = 1~2UFR1
R +UF2, pH = 2~3UFR2
R + UF2, pH = 3~4UFR3
R + UF2, pH = 5~6UFR4
R + UF2, pH = 8~9UFR5
R + UF2, pH = 10~11UFR6
Table 3. Synthesis scheme of reaction of resorcinol (R), formaldehyde (F), and urea (U).
Table 3. Synthesis scheme of reaction of resorcinol (R), formaldehyde (F), and urea (U).
Synthesis SchemeSample Name
R + F + U, pH = 2~2.5RUF1
R + F + U, pH = 3~4RUF2
R + F + U, pH = 5~6RUF3
R + F + U, pH = 8~9RUF4
R + F + U, pH = 10~11RUF5
Table 4. The peak value on 13C–NMR curves of resorcinol and formaldehyde under different pHs.
Table 4. The peak value on 13C–NMR curves of resorcinol and formaldehyde under different pHs.
StructuresChemical Shifts/ppmRF1RF2RF3RF4RF5
Forests 15 00098 i00114–1500000
Forests 15 00098 i00223–243.0600020.32
Forests 15 00098 i00329–3032.753.824.1719.7874.87
Forests 15 00098 i00454–5500000
Forests 15 00098 i00560–6100.6412.5073.834.81
Forests 15 00098 i00666–67004.1700
Forests 15 00098 i00762–6300000
Forests 15 00098 i00862–6371–7200000
Forests 15 00098 i00970–7100000
Methyl diol and its self-condensation polymers83–9564.1995.5479.176.400
Total100100100100100
Table 5. The peak value of 13C–NMR in R and UF2 under different pHs.
Table 5. The peak value of 13C–NMR in R and UF2 under different pHs.
StructuresChemical Shifts/ppmpH Value
1~22~33~45~67~89~10
Φ-CH2-Φ oo-o′o′14–15
Φ-CH2-Φ oo- p′o′23–241.29 8.3011.70
Φ-CH2-Φ po- p′o′29–3059.8728.497.785.5653.4383.47
Φ-CH2-U oo- U35–367.137.267.592.98
Φ-CH2-U po- U40–4131.7264.2583.2645.29
-NH-CH2-NH- (I)47–48 1.377.42
-NH-CH2-N= (II)53–55
=N-CH2-N= (III)59–60
-NH-CH2OCH2NH- (I)69–70 3.83
-NH-CH2OCH2N= (II)75–77
Uron78–80
oo-R-CH2OH54–55 0.818.304.82
po-R-CH2OH61–62 5.3616.97
-NH-CH2OH (I)65–66 25.2113.00
-NH(-CH2)-CH2OH (II)71–72
-NH-CH2-O-CH372–73
HO-CH2-OH83–84 3.54
HOCH2-O-CH2-OCH2OH86–87
HOCH2-O-CH2-OCH2OH90–91
H(CH2O)nOCH2OCH394–95
Total100.00100.00100.00100.00100.00100.00
Table 6. The peak value of 13C–NMR in R, U, and F under different pHs.
Table 6. The peak value of 13C–NMR in R, U, and F under different pHs.
StructuresChemical Shifts/ppmpH Value
2~2.53~45~67~89~102~2.5
Φ-CH2-Φ oo-o′o′14–15
Φ-CH2-Φ oo- p′o′23–241.29 8.3011.70
Φ-CH2-Φ po- p′o′29–3059.8728.497.785.5653.4383.47
Φ-CH2-Uoo- U35–367.137.267.592.98
Φ-CH2-Upo- U40–4131.7264.2583.2645.29
-NH-CH2-NH- (I)47–48 1.377.42
-NH-CH2-N= (II)53–55
=N-CH2-N= (III)59–60
69–70 3.83
-NH-CH2OCH2NH- (I)75–77
-NH-CH2OCH2N= (II)78–80
Uron54–55 0.818.304.82
oo-R-CH2OH61–62 5.3616.97
po-R-CH2OH65–66 25.2113.00
-NH-CH2OH (I)71–72
-NH(-CH2)-CH2OH (II)72–73
-NH-CH2-O-CH383–84 3.54
HO-CH2-OH86–87
HOCH2-O-CH2-OCH2OH90–91
HOCH2-O-CH2-OCH2OH
H(CH2O)nOCH2OCH394–95
Total100.00100.00100.00100.00100.00100.00
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Liang, J.; Li, D.; Zhong, X.; Wu, Z.; Cao, M.; Yang, G.; Yin, S.; Gong, F. Crosslinking Mechanism of Tannin-Based Adhesives Based on Model Compounds: Copolycondensation of Resorcinol with Dimethylol Urea. Forests 2024, 15, 98. https://doi.org/10.3390/f15010098

AMA Style

Liang J, Li D, Zhong X, Wu Z, Cao M, Yang G, Yin S, Gong F. Crosslinking Mechanism of Tannin-Based Adhesives Based on Model Compounds: Copolycondensation of Resorcinol with Dimethylol Urea. Forests. 2024; 15(1):98. https://doi.org/10.3390/f15010098

Chicago/Turabian Style

Liang, Jiankun, De Li, Xiao Zhong, Zhigang Wu, Ming Cao, Guifen Yang, Shuang Yin, and Feiyan Gong. 2024. "Crosslinking Mechanism of Tannin-Based Adhesives Based on Model Compounds: Copolycondensation of Resorcinol with Dimethylol Urea" Forests 15, no. 1: 98. https://doi.org/10.3390/f15010098

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