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Article

Melamine–Urea–Formaldehyde Resin Adhesive Modified with Recycling Lignin: Preparation, Structures and Properties

by
De Li
1,†,
Liping Yu
1,†,
Lifen Li
1,
Jiankun Liang
2,*,
Zhigang Wu
1,*,
Xiaoxue Xu
1,
Xiao Zhong
1 and
Feiyan Gong
1
1
College of Forestry, Guizhou University, Guiyang 550025, China
2
College of Civil Engineering, Kaili University, Qiandongnan 556011, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2023, 14(8), 1625; https://doi.org/10.3390/f14081625
Submission received: 18 July 2023 / Revised: 3 August 2023 / Accepted: 8 August 2023 / Published: 11 August 2023
(This article belongs to the Section Wood Science and Forest Products)
Browse Figures
Versions Notes

Abstract

:
The wettability, bonding strength and flexibility of modified melamine–urea–formaldehyde resin adhesive with hydroxymethyl lignin (LMUF) were investigated. Moreover, the curing performance, thermal properties and chemical structure of LMUF were analyzed by differential scanning calorimetry (DSC), thermogravimetry (TG), scanning electronmicroscopy (SEM), X–ray diffraction (XRD), Fourier–transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance (13C–NMR). The synthesis mechanism of LMUF was also discussed. The results demonstrated that: (1) LMUF resin is characterized by low free formaldehyde and a short pot life. (2) With the increase in hydroxymethyl lignin, the bonding strength and flexibility of the LMUF increased first and then decreased. They reached their maximum when the hydroxymethyl lignin content was 6%–8%, which represented increases of 36% and 102%, respectively. (3) The DSC and TG analyses showed that the LMUF resin had a high hot-pressing temperature, but its thermostability was increased. (4) The XRD and SEM analyses proved that the LMUF resin was characterized by flexibility failure, especially at 6% of hydroxymethyl lignin. (5) Hydroxymethyl lignin is a type of flexible long-chain molecule, which was introduced into spaces between the rigid triazine rings through adhesion and penetration, thus improving the regular single structural form of MUF resin and increasing the compactness of the system. As a result, the resin had stronger deformability and cohesion in the molecules. The bonding strength and flexibility were improved to different degrees.

1. Introduction

With the increasing market demand for high-quality, multi-function and diversified wood products, wood–based panel products are developing from single products toward diversified and high–performance products continuously. Moreover, wood–based panel products are being updated and reformed continuously, as timber resources shift from natural forest resources to man–made forest and agricultural waste products [1,2,3,4,5,6]. The production of wood-based panels is a process of separation first and then synthesis. Adhesives are an essentially important component, accounting for 30%~50% of wood–based panel production costs [7,8,9,10,11,12]. Furthermore, adhesives consumption is used to measure the wood industrial technological level of a country or region.
In 2021, formaldehyde-based adhesive consumption for wood–based panels in China reached 16.57 million tons. Formaldehyde–based adhesives still represent the major market share (>98%) [13,14,15]. Traditional formaldehyde-based adhesives mainly refer to urea–formaldehyde (UF) resin adhesives, phenol–formaldehyde (PF) resin adhesives and melamine–formaldehyde (MF) resin adhesives. They are used as traditional single–component adhesives and have their own advantages and disadvantages. Although PF has excellent water resistance and weather resistance, and it can meet the requirements under relatively harsh conditions, it has a deep color after curing, high brittleness, a long curing time, high curing temperature, high requirement of board moisture content, and even high free phenol content and strong toxicity [16,17,18,19,20]. Although MF has excellent performance, it has high brittleness, a high cost and poor storage stability [21,22,23,24,25]. Formaldehyde and urea are major ingredients of UF, which is advantageous for simple synthesis technology, has a low production cost, fast curing and a light color after curing [26,27,28,29,30,31,32]. Hence, UF is still the most commonly used wood adhesive in China.
Traditional single–component formaldehyde–based adhesives can no longer meet the production and usage requirements for wood–based panels. Recently, co-condensed resin has become one of the main development trends in wood adhesives, and it can integrate the advantages of multiple resins by building the co-condensed resin system to achieve effective connections with existing technological conditions and give products excellent comprehensive performance. Studies on co-condensed resins mainly focus on melamine–urea–formaldehyde (MUF) [33,34,35,36,37,38,39,40], phenol–urea–formaldehyde (PUF) [41,42,43], phenol–melamine–formaldehyde (PMF) [44,45], phenol–urea–melamine–formaldehyde resin (PMUF) [46,47], and so on. On one hand, MUF resin can improve the performance of UF resins in humid environments effectively and decrease the cost of MF resins effectively. On the other hand, MUF has the unique advantages of fast curing and light colors, and it can replace some PF to be applied in outdoor applications.
The inclusion of natural macromolecular compounds into the building of a co-condensed resin system can improve the environmentally friendly performance of adhesives and decrease dependence on fossil resources [48,49]. Hence, biomass–based co-condensation technology attracts more and more attention. Mixing biomass materials like soybean proteins, polysaccharides, tannin and lignin with MUF, or establishing biomass-based co-condensed resin by using biomass materials and MUF, can achieve the ideal modification effect. Both methods can not only improve the bonding strength and water resistance of MUF resin but also have lower costs and free formaldehyde [50,51,52,53,54,55,56].
Lignin, which is a kind of natural polymer that exists in vascular plant species, is the main body of the plant skeleton which is formed through materialization with cellulose and semi–cellulose. Lignin has considerable reserves in nature, only next to cellulose. In the molecular structure of lignin, there are many active groups like phenolic hydroxyl groups, alcoholic hydroxyl groups, carbonyls, methoxy groups and conjugated double bonds for many chemical reactions, including oxidization, hydrolysis, acylation, sulfonation, polycondensation or graft copolymerization [57,58,59]. Lignin is mainly used as industrial lignin in wood adhesives. Industrial lignins like alkali lignins, enzymatichydrolysis lignins and organic solvent lignins can only be dissolved completely in asolvent system with a high pH value. Therefore, they are mainly applied to the synthesis of PF adhesives. However, most industrial lignin is difficult to use in co-condensation fully due to the complexity of the MUF co-condensation reaction system, thus resulting in incomplete co-condensation among different components and thereby influencing the performance of the resins [20,57,58]. Hydroxymethyl lignin has good solubility in aqueous solutions, which can assure good reactivity, and participates in co-condensation with MUF fully. In this study, the wettability, bonding strength, curing performance and thermal properties of modified MUF resin adhesive with hydroxymethyl lignin (LMUF) are discussed.

2. Materials and Methods

2.1. Materials

Alkali lignin with a pH of 10.5 in a 50% aqueous solution and brown powder was bought from Nanjing Durai Biological Co., Ltd., Nanjing, China. The formaldehyde (wt 37%), NaOH, melamine and urea were analytically pure and were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Hydroxymethyl lignin with a viscosity of 110 mPa·s was made in the laboratory with a formaldehyde–to–lignin mass ratio of 1:1. Populus spp. Veneer, with a size of 400 mm × 400 mm × 1.5 mm and a moisture content of 8%–10%, was purchased from Qunyou Wood Co., Ltd., Suzhou, China and, after drying, knotless and sapwood materials were selected.

2.2. Preparation of Melamine-Urea–Formaldehyde Resins

Melamine–urea–formaldehyde resin adhesive with hydroxymethyl lignin (LMUF) was prepared as shown in Table 1: Briefly, in a 50 °C water bath environment, 334.8 g of formaldehyde (wt 37%) was added into a three-nozzle flask with a thermometer. The pH was adjusted to 9.0, and the first urea (U1 = 75.2 g), the first melamine (M1 = 11.90 g) and the hydroxymethyl lignin were added; then, the temperature was raised to 90 °C. The pH was adjusted to 5.0–5.2 and reacted for 60 min. Then, the pH was adjusted to 8.7–8.9, and the second melamine (M2 = 69.50 g) was added. When the water tolerance at room temperature of the adhesive reached 150~200%, the pH was adjusted to 9.0 immediately, and the temperature dropped to 45 °C. The second urea (U2 = 17.1 g) was added and reacted for 10 min, and the pH was adjusted to 8.0–8.5. Finally, the resin was obtained when it cooled to room temperature. Then, 2%, 4%, 6%and 8% of hydroxymethyl lignin were added, respectively; the different LMUF resins were prepared and are denoted as LMUF1, LMUF2, LMUF3 and LMUF4, respectively. An MUF resin adhesive without hydroxymethyl lignin was prepared in a similar way for comparison purposes and was named LMUF0. The solid content, viscosity, pH and free formaldehyde of the adhesives were tested according to the Chinese national standard (GB/T 14074–2006) [3,20]. The flexibility of the adhesives was measured according to the references [20,53]. The viscosity was investigated with an NDJ–1 rotary viscometer from Flora Automation Technology Co., Ltd. (Tianjin, China) with a 4# rotor at a speed of 60 r/min. The pot life was the time from the successful synthesis of the MUF to the time of hardening of the MUF.

2.3. Preparation of Plywood and Test of Bonding Strength

A three-layer plywood (400 mm × 400 mm × 5 mm) was prepared in the laboratory under a hot–pressing temperature of 130 °C, hot–pressing pressure of 1.5 MPa, hot–pressing time of 5 min, and double–sided adhesive loading of 160 g/m2.The bonding strength of the plywood was tested according to the standard (Class I plywood, GB/T 17657–2013) [3,20]. As seen in Figure 1, a mechanical testing machine (WDS-50KN) with a loading speed of 1.5 mm/min was used to determine the shear strength of the plywood specimens. The final reported bonding strength was the mean of 12 specimens, and the standard deviation was less than 5%.

2.4. Test of Wettability

The wettability of an adhesive includes the surface tension and contact angle. The surface tension of the LMUF resin was measured using the hanging drop method: The picture of the MUF hanging drop was taken with the JC2000A dropping contact angle measuring instrument. Three points on the left (A), right (B) and bottom (C) of the hanging drop were selected, and a horizontal line (DE) would appear at the top of the hanging drop intersecting both sides. The information on intersections D and E could then be read (Figure 2), and the surface tension was calculated with reference to the literature.
Contact angle test using static drop method: A poplar veneer with a smooth surface was cut into a size of 2 cm × 3 cm. The contact angle was measured by the JC2000A static droplet measuring instrument (Shanghai Zhongchen Company, Shanghai, China) at the third second of the drop on the surface of the veneer with are solution of 800 × 600.

2.5. Test of Fourier–Transform Infrared Spectroscopy (FTIR)

The LMUF was baked at 120 °C for 2 h, then crushed and passed through a 40~60 mesh sieve. The LMUF powder and KBr were ground evenly with a mass ratio of 1:100, then pressed into tablets and put into a dryer to remove the water. A Varian 1000 (United States) Fourier transform infrared spectrometer was used for testing, as per the following parameters: wave number range: 400~4000 cm–1; resolution: 4 cm–1; scanning times: 32 times. The indoor temperature was 22~25 °C, and the relative humidity was 60%.

2.6. Test of Differential Scanning Calorimetry (DSC)

The differential scanning calorimeter (DSC 204 F1) produced by Nez was used for the test. The test parameters were as follows: temperature 25~210 °C; heating rate 10 °C/min; N2 protection; sample mass 5–8 mg.

2.7. Test of Thermogravimetry (TG)

The LMUF resin was baked at 120 °C for 2 h, crushed, and then tested with a 40–60 mesh sieve. A TG 209 F3 thermogravimetric analyzer produced by Neutsch Company in Germany was adopted, and the test conditions were: N2 protection; temperature 30~700 °C; heating rate 10 °C/min.

2.8. Test of X-ray Diffraction (XRD)

The LMUF resin was baked at 120 °C for 2 h, crushed, and then tested using a TTR X-ray diffractometer (RIGAKU, Tokyo, Japan) with a 40–60 mesh sieve. The test parameters were the same as the previous test [20].

2.9. Test of Scanning Electron Microscopy (SEM)

The section of cured MUF resin was observed with a Hitachi S–3400N scanning electron microscope. The section was sprayed with gold, and the accelerated voltage was 12.5 kV. The locations to be observed were photographed at different resolutions (200 µm, 100 µm, and 500 µm).

2.10. Test of 13C Nuclear Magnetic Resonance (13C–NMR)

Quantities of 100 µL of LMUF resin and 300 µL of DMSO–d6 were injected into a nuclear magnetic tube, dissolved and shaken, and tested by a Bruker Avance high–resolution superconducting over frequency nuclear magnetic resonance instrument. The test parameters were as follows: pulse sequence, zgig; internal standard, DMSO–d6; cumulative times, 500–800 times; spectral width, 39,062.5 Hz.

3. Results and Discussion

3.1. Basic Performance Analysis

The appearance and basic performance of the MUF resins are shown in Figure 3 and Table 2. It can be seen from Table 2 that the viscosity of the unmodified MUF resin was 68.4 mPa·s. The viscosity of the LMUF modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin was 88.6, 107.9, 232.6 and 412.4 mPa·s, respectively. The viscosity improved for the following reasons: (1) Lignin might contain some impurities that don’t participate in synthetic reactions. For example, the viscosity of the resin system might be increased when vegetable glue encounters water. (2) Due to its high molecular weight, lignin has an electrostatic force and hydrogen bond association with them [20]. (3) The benzene rings on lignin molecules are hydrophobic, and the affinity of lignin molecules for water is low, thus resulting in the high viscosity of the modified MUF resin. (4) Hydroxymethyl lignin participates in a polycondensation reaction with the MUF resin to produce polymers with a higher molecular weight. (5) Hydroxymethyl lignin and hydroxymethyl melamine molecules form “groups” through hydrogen-bond interactions to increase viscosity. In a word, the viscosity of the MUF resin increased continuously over the extension of the reaction time due to the above five factors, which surely influenced the pot life of the adhesives. The pot life of the unmodified MUF resin reached as long as 90 days, but the pot life of the MUF resins modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin decreased to 41, 36, 23 and 19 days. Nevertheless, the pot life of the modified MUF resin could still meet the production requirements, with a minimum application period of nearly 20 days.
Additionally, the free formaldehyde content of the unmodified MUF resin was 0.12%, which decreased to 0.08%, 0.06%, 0.06% and 0.07% in the LMUF modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin. This proved that hydroxymethyl lignin could play the role of formaldehyde–catching agent adequately, which achieved the original intention of preparing environmentally friendly wood adhesives by using agricultural and forest wastes.

3.2. Bonding Strength Analysis

As shown in Figure 4, the bonding strength of the control MUF resin was 1.27 MPa, and that of the LMUF modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin increased to 1.45, 1.63, 1.73 and 1.63 MPa, respectively. With the increase in hydroxymethyl lignin content, the bonding strength increased first and then decreased, but the bonding strength of the MUF resin modified by different amounts of hydroxymethyl lignin increased to different extents. The bonding strength reached the maximum (1.73 MPa), and it increased by 36% at 8% of hydroxymethyl lignin content. An appropriate amount of hydroxymethyl lignin could realize the graft copolymerization effect. It participated in the condensation reaction of the MUF resin and increased the degree of cross–linking of the resin, thus increasing the bonding strength. In addition, the wood failure percentage of all the samples was above 60%, and the variation trend of the wood failure percentage was consistent with that of the bonding strength, which showed that the bonding reliability of the modified MUF resin was improved. However, excessive hydroxymethyl lignin was disadvantageous for improvement of the bonding strength, because (1) the active sites on the benzene rings of the lignin, which had not been replaced, still reacted with the formaldehyde, and the excessive content of hydroxymethyl lignin consumed more formaldehyde from the system, thus influencing the urea–formaldehyde and melamine–formaldehyde reactions, affecting the degree of cross–linking of the resin system, and thereby influencing the bonding strength of the resin; (2) lignin has a complicated structure and abundant active groups are wrapped in molecules. The substituent groups on the aromatic ring had great steric hindrance and relatively low chemical reaction activity. Hence, excessive lignin hindered the normal polymerization of melamine, urea and formaldehyde. (3) Adhesives must have some wettability and diffusivity on the wood surface for bonding to form a thin and uniform glue layer and form enough bonding points to the wood, thus creating the essential conditions to realize good bonding performance [60,61,62]. However, the modified MUF resin had high viscosity, thus increasing the cohesion, but weakening the wettability. Additionally, high viscosity might also influence the operational performance of the MUF resin.

3.3. Flexibility Analysis

The stress–strain curves of the resins are shown in Figure 5. The MUF resin molecules after curing had short lattice chains, high cross–linking density, great steric hindrance, difficult bending of the molecular chain against external stress–strain, great brittleness of the curing layer and a decreased ability to absorb impact energy [20,25]. The triazine ring in the melamine molecule is a kind of heterocyclic ring with a similar structure to an aromatic ring. Due to the great steric hindrance and high rigidity of the triazine ring, the MUF resin has poor flexibility. In addition, there were hydroxymethyls that did not participate in the reaction in the curing of the MUF resin, which added hygroscopicity and generated stresses and fatigue due to frequent moisture absorption and desorption, finally developing brittleness [49]. Hence, the unmodified cured MUF resin had high strength and brittleness. In the tensile test, cracks developed inside before the stress reached the yield limit, and the samples were broken, although there was no obvious yield on the stress–strain curve, and the elongation at the break was only 5.1%.
After the flexible hydroxymethyl lignin was introduced into the spaces between the rigid triazine rings, the LMUF resin had a greater ability of intramolecular deformability. The elongations at the break of the LMUF modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin were valued at 6.0%, 10.3%, 9.5% and 7.7%, respectively. With the increase in the hydroxymethyl lignin content, the elongation at the break presented an inverted V–shaped variation pattern, and it was generally higher than that of unmodified MUF resin. When the hydroxymethyl lignin content was 4%, 6% and 8%, there was an obvious yield phenomenon on the stress–strain curve, indicating that the flexibility of the MUF resins was improved.

3.4. Wettability Analysis

There are more sparse molecules on the liquid–gas interface than in the internal structure. With the increase in intermolecular distance, molecules attract mutually, and the surface molecules bear inward tensile stresses [61,62]. Hence, there is an automatic shrinking trend on the liquid surface, which is manifested as surface tension. The unmodified MUF resin system contained a lot of hydroxymethyl melamine and methylol urea. The aggregation of these hydroxymethyls easily causes the aggregation of hydrogen bonds, thus resulting in a high surface tension of 109.8 mN/m. The surface tension on the MUF modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin decreased to 94.0, 101.1, 99.4 and 96.0 mN/m (Figure 6). This was because the hydroxymethyl lignin could result in polycondensation with the hydroxymethyls, thus decreasing the hydroxymethyl content and weakening the acting force of the hydrogen bonds. As a result, the surface tension decreased, which also reflected indirectly that the hydroxymethyl lignin participated in the synthesis reaction of the MUF resin.
Due to their liquidity and surface tension, adhesives’ wettability on wood surfaces makes the molecules on the bonding interface contact tightly. Subsequently, the adhesive molecules diffuse and penetrate, crossing the boundaries. The contact angle can reflect the wettability of the adhesives, and the smaller the contact angle is, the better the wettability is. The contact angle of the unmodified MUF resin was 45.3°, and the contact angles of the LMUF modified with 2%, 4%, 6% and 8% of hydroxymethyl lignin were 64.5°, 47.0°, 44.8° and 50.3°, respectively. The contact angles of the LMUF resins differed slightly but they were all less than 90°, indicating that the modification with the hydroxymethyl lignin had a slight influence on the wettability of the LMUF resin.

3.5. Curing PerformanceAnalysis

The curing degree is an important index to measure the curing reaction of adhesives. The curing degree curve of different MUF resins can be obtained (Figure 7b) by integrating the DSC peaks in Figure 7a. The curing degree curves of LMUF0, LMUF2 and LMUF4 had an intersection at 65%. With respect to the curing reaction rate, LMUF0 < LMUF4 < LMUF2 below 65%, and LMUF2 < LMUF4 < LMUF0 over 65%. Hence, the introduction of hydroxymethyl lignin could facilitate the curing of MUF resins in the early stage, because it increased the active hydroxymethyl contents in the system. However, such a promotion effect was not obvious in the late stage, because the reaction activity of hydroxymethyl lignin was weaker than those of methylol urea and hydroxymethyl melamine. The curing peak temperature of the unmodified MUF resin was 109.8 °C, and the curing peak temperatures of LMUF2 and LMUF4 were 114.2 °C and 111.9 °C, respectively. Hence, the introduction of hydroxymethyl lignin generally increased the curing temperature of the resins.

3.6. Thermal Properties Analysis

The TG/DTG curves of the cured products of the MUF resins are shown in Figure 8. All the TG/DTG curves generally showed the same trend. Stage 1: 35–150 °C—this stage mainly had steam evaporation and the decomposition of micromolecular and unstable chemical bonds. There was a relatively low loss of mass in this stage. Stage 2: 150–400 °C—this was a stage of thermal decomposition of the major skeleton of the MUF resins, involving the breakage and decomposition of ether bonds, the release of formaldehyde, and the breakage and decomposition of polymethylene bonds [49,50,51]. The oxygen–containing groups were broken down and formed amorphous carbon at this stage. LMUF2 had a low mass loss ratio in this stage. LMUF0 and LMUF2 achieved the maximum weight loss ratios at 241.3 and 251.9 °C. This reflected that MUF resins modified with hydroxymethyl lignin produced stable cross-linking products. LMUF4 achieved the maximum weight loss ratio at 246.1 °C, indicating that excessive hydroxymethyl lignin affected the thermostability of the resin. Hence, the cross-linking network structure formed after the polycondensation reaction of the MUF resins became more compact and stable by adding an appropriate amount of hydroxymethyl lignin.

3.7. XRD and SEM Analysis

It can be seen from Figure 9 that the unmodified MUF resin had a relatively sharp absorption peak and a high diffraction intensity at 002, without the diffraction peaks of other impurity phases, and the resin presented high rigidity. The MUF modified with 2% and 6% hydroxymethyl lignin developed new absorption peaks at 2θ = 10.5° and 8.4°, which were peaks formed by polycondensation between the hydroxymethyl lignin and the MUF. The different positions of the peaks also reflected different degrees of polycondensation of the final product indirectly. The appearance of new peaks indicated that the resin structure became more disordered, and the rigidity was weakened. In other words, the flexibility was enhanced.
It can be seen from Figure 10 that the unmodified MUF resin had smooth and flat sections, indicating a brittle fracture. The fracture surfaces of the LMUF had obvious wrinkle and waveform structures, which were attributed to the absorption of abundant energy and fracture yield during the breakage of resins, which is a typical characteristic of flexibility failure. Although the MUF resin modified with 8% hydroxymethyl lignin (LMUF4) still showed flexibility failure features, the wrinkle and waveform textures decreased obviously. Theoretically, adding hydroxymethyl lignin, which is a kind of flexible long–chain molecule, into triazine rings could give the resin a greater ability of intramolecular deformability and gradual strengthening of flexibility. In fact, there was no proportional relationship between the hydroxymethyl lignin content and flexibility, because the excessive hydroxymethyl lignin influenced the polycondensation reaction.

3.8. Modification Mechanism Analysis

The FTIR curves before and after the hydroxymethylation of the lignin are shown in Figure 11a. As a kind of natural macromolecular compound with a very complicated chemical structure, lignin is composed of three structural units, including syringyl, guaiacyl propane and phydroxyphenyl propane. Figure 11 shows the stretching vibration absorption peak of O–H at 3406.9 cm−1, the stretching vibration absorption peak of C–H in the methylene of lignin at 2935.3 cm−1, the skeleton vibration peak of the benzene ring, the feature band of the lignin at 1601.3 cm−1, the C–H stretching vibration of the methoxy group on the benzene ring of the lignin at 1459.9 cm−1, and the deformation vibration absorption peak of C–H on the phenylpropane aromatic ring of the lignin at 1124.9 cm−1 [20]. The peaks at 1046.1 and 980.5 cm−1 are the aromatic C–H series.
The FTIR curves of the MUF and LMUF are shown in Figure 11b, with the vibration absorption peak of C=O at 1662.2 cm−1, the deformation vibration absorption peak of the triazine ring at 1558.4 cm−1, the bending vibration absorption peak of –CH2 at 1376.3 cm−1, the symmetrical stretching vibration peak of C–O–C at 1162.7 cm−1, the absorption peak of –CH2OH at 1009.3 cm−1, the peak of C–H of the triazine ring side chain at 899.5 cm−1, and the vibration characteristic absorption peak of the out–of–plane ring of melamine at 816.7 cm−1. Specifically, the characteristic peaks at 1558.4 and 816.7 cm−1 are important symbols of the triazine ring of melamine. The FTIR of the MUF and LMUF were basically consistent, indicating that adding lignin had no great influence on the formation of basic functional groups of resins. Except for the basic characteristic peaks, the characteristic peaks of the LMUF at 1558.4 and 1009.3 cm−1 weaken obviously. This reflected that adding hydroxymethyl lignin created different substitution structures on the triazine ring of melamine. Additionally, the aromatic characteristic peak at 1302.6 cm−1 indicated that the hydroxymethyl lignin participated in the polycondensation reaction with MUF.
Since FTIR had overlaps in many zones, the resins were further analyzed based on 13C–NMR. The chemical displacement of 83 ppm (methylene glycol) was used as the reference peak, and the integration of all the absorption peaks was carried out. Next, the sum of the integral areas of all the mesomethylene carbon was calculated. The ratio between the integral value of different types of chemical bonds and the integral value of total methylene carbon was calculated as the percentage content of different mesomethylene carbons. The test results are shown in Figure 12 and Table 3. Hydroxymethyl was the basis and premise for increasing the molecular chains and cross–linking reaction of the MUF resins. The higher hydroxymethyl content represents that the addition reaction is more thorough and complete. Since the methylenebridge bonds and methylene ether bonds are transformed from the hydroxymethyl consumption, the proportion of methylenebridge bonds and methylene ether links is relatively high [49,63,64,65]. The higher degree of polycondensation indicates the higher strength of the resin.
The chemical displacement of the hydroxymethyl group was 63~65 ppm. The chemical displacement of the methylenebridge bond was 54~56 and 46~48 ppm, while the chemical displacements of the methylene ether links were mainly distributed within 67~70 ppm. The chemical displacement of the methylene ether links produced by the co-condensation between the melamine and the urea were 74~75 and 77~78 ppm. The hydroxymethyl content, methylenebridge bonds, methylene ether links and the methylene ether links produced by co-condensation between melamine and urea in the unmodified MUF resin were 48.8%, 17.3%, 19.0% and 9.8% respectively. In the LMUF, the hydroxymethyl content, methylenebridge bond, methylene ether links and the methylene ether links produced by the co-condensation between the melamine and the urea were 44.3%, 20.1%, 19.3% and 10.3%, respectively. Therefore, the methylenebridge bonds and methylene ether links in the MUF and LMUF resins were 46.1% (17.3% + 19.0% + 9.8%) and 49.7% (20.1% + 19.3% + 10.3%), respectively. This indicated that the LMUF resin system had a high condensation degree, the formaldehyde consumption by the LMUF resins was high, and the free formaldehyde residues in the resins were relatively low. These agreed with Section 3.1. The LMUF resins had a high degree of condensation, which also explained the significant growth in bonding strength.
Take the guaiac–based propane structural unit that composes lignin, for example, to interpret the reaction route for hydroxymethyl lignin to participate in the synthesis of MUF (Figure 13). Firstly, the lignin units that had not participated in the reaction could be used as the filler and were mixed in the pores of the MUF structure through electrostatic absorption. This could increase the compactness of the system, absorb the microcrack energy, and decelerate the expansion ability of cracks, thus improving the flexibility of MUF resins. Secondly, the lignin was activated through the hydroxymethylation of formaldehyde in the system. The formaldehyde mainly came from the free formaldehyde in the system or terminal hydroxymethyl separating from the MUF system. The lignin further formed a stable covalent bond system with the hydroxymethyl structure in the MUF through bridged bonding. Since there were many active sites in the lignin, the further reaction structures were relatively complicated and could form a 3D system structure, which greatly improved the brittleness of the MUF resins caused by the regular single structural forms. The overall resistance of the system to impact loads was strengthened. Finally, the bonding strength and flexibility of the MUF resins were increased to different extents through the adhesion to and penetration of lignin structures.

4. Conclusions

In this study, the effects of hydroxymethyl lignin on the wettability, bonding strength and flexibility of LMUF were analyzed. Moreover, the curing performances, thermal properties and chemical structure of LMUF resins were investigated. The synthesis mechanism of LMUF was also discussed. The results demonstrate that (1) the LMUF resin was characteristic of low free formaldehyde and short pot life. (2) With the increase in hydroxymethyl lignin, the bonding strength and flexibility of the LMUF increased first and then decreased. (3) The LMUF resin had a high hot pressing temperature, but its thermostability increased. (4) Hydroxymethyl ligninis a type of flexible long–chain molecule. Flexible chain segments were introduced into spaces among the rigid triazine rings through adhesion and filling, which improved the regular single structural form of the MUF resins and increased the compactness of the system. Due to the adhesion and filling of the hydroxymethyl lignin, the LMUF had greater intramolecular deformability and cohesion, thus increasing its bonding strength and flexibility to different extents. (5) MUF resin adhesive modified with hydroxymethyl lignin has great practical application prospects in the field of glued wood, and this modification method can also be used to strengthen and toughen other co-polycondensation resins.

Author Contributions

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

Funding

This work was supported by the Outstanding Youth Science and Technology Talent Project of Guizhou Province, China (YQK[2023]003), Science–Technology Support Foundation of Guizhou Province, China ([2019]2308), the National Natural Science Foundation of China (31800481), the Forestry Science and Technology Research Project of the Guizhou Forestry Bureau ([2017]14 and [2020]C14), the Talents from Guizhou Science and Technology Cooperation Platform ([2019]01-3) and the Qiandongnan Basic Research Program Project ([2021]15).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The schematic diagram of the bonding strength test of the plywood.
Figure 1. The schematic diagram of the bonding strength test of the plywood.
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Figure 2. The schematic diagram of the surface tension test, based on the pendant–drop method.
Figure 2. The schematic diagram of the surface tension test, based on the pendant–drop method.
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Figure 3. Appearance of the MUF resins.
Figure 3. Appearance of the MUF resins.
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Figure 4. The bonding strength of MUF resins.
Figure 4. The bonding strength of MUF resins.
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Figure 5. The stress–strain curves of MUF resins.
Figure 5. The stress–strain curves of MUF resins.
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Figure 6. The surface tension and contact angle of MUF resins.
Figure 6. The surface tension and contact angle of MUF resins.
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Figure 7. The curing performance of MUF resins. (a). DSC curves; (b). curing degree.
Figure 7. The curing performance of MUF resins. (a). DSC curves; (b). curing degree.
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Figure 8. TG and DTG curves of MUF resins.
Figure 8. TG and DTG curves of MUF resins.
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Figure 9. XRD curves of cured MUF resins.
Figure 9. XRD curves of cured MUF resins.
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Figure 10. SEM images of fractured cross–sections of cured MUF resins.
Figure 10. SEM images of fractured cross–sections of cured MUF resins.
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Figure 11. FTIR curves of lignin, hydroxymethyl lignin (a) and MUF resins (b).
Figure 11. FTIR curves of lignin, hydroxymethyl lignin (a) and MUF resins (b).
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Figure 12. 13C-NMR curves of MUF resins.
Figure 12. 13C-NMR curves of MUF resins.
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Figure 13. Diagram of synthesis mechanism of LMUF resin.
Figure 13. Diagram of synthesis mechanism of LMUF resin.
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Table 1. Synthesis scheme of MUF resins.
Table 1. Synthesis scheme of MUF resins.
Synthesis SchemeMUFLMUF
Step 1F + U1 + M1, pH = 9.0, 90 °CF + U1 + M1 + Lignin, pH = 9.0, 90 °C
Step 2pH = 5.0–5.2, 60 minpH = 5.0–5.2, 60 min
Step 3+M2, pH = 9.0+M2, pH = 9.0
Step 4+U2, pH = 9.0, 45 °C+U2, pH = 9.0, 45 °C
Table 2. Properties of the MUF resins.
Table 2. Properties of the MUF resins.
AdhesivesSolid Content
/%		Viscosity
/mPa·s		Free Formaldehyde
/%		Pot Life
/d
LMUF051.868.40.1290
LMUF152.788.60.0841
LMUF253.1107.90.0636
LMUF353.3232.60.0623
LMUF454.8412.40.0719
Table 3. Percentage values for various methylenic carbons of MUF resin.
Table 3. Percentage values for various methylenic carbons of MUF resin.
StructuresChemical Shifts/ppmMUFLMUF2
-NH-CH2-NH-46–4811.210.5
-N(CH2-)-CH2-NH-54–566.12.2
CH3OH/-CH2OCH349–50
M-NH-CH2OH/U-NH-CH2OH63–6548.844.3
-NHCH2OCH2(NH-/-OH)67–7019.019.3
-NHCH2OCH372–734.35.8
M-N(CH2-)-CH2-O-CH2-N(CH2-)-U74–759.810.3
M-NH-CH2-O-CH2-N(CH2-)-U77–78
HOCH2OH82.4–82.50.80.2
HOCH2OCH2OH86.2–86.3
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Li, D.; Yu, L.; Li, L.; Liang, J.; Wu, Z.; Xu, X.; Zhong, X.; Gong, F. Melamine–Urea–Formaldehyde Resin Adhesive Modified with Recycling Lignin: Preparation, Structures and Properties. Forests 2023, 14, 1625. https://doi.org/10.3390/f14081625

AMA Style

Li D, Yu L, Li L, Liang J, Wu Z, Xu X, Zhong X, Gong F. Melamine–Urea–Formaldehyde Resin Adhesive Modified with Recycling Lignin: Preparation, Structures and Properties. Forests. 2023; 14(8):1625. https://doi.org/10.3390/f14081625

Chicago/Turabian Style

Li, De, Liping Yu, Lifen Li, Jiankun Liang, Zhigang Wu, Xiaoxue Xu, Xiao Zhong, and Feiyan Gong. 2023. "Melamine–Urea–Formaldehyde Resin Adhesive Modified with Recycling Lignin: Preparation, Structures and Properties" Forests 14, no. 8: 1625. https://doi.org/10.3390/f14081625

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