Haixiang Liu, Jun Zhang, Chunlei Dong, Gang Zhu, Guanben Du, Shuduan Deng*

Yunnan Key Laboratory of Wood Adhesives and Glue Products, Southwest Forestry University, Kunming 650224, China

Keywords:

ABSTRACT

1. Introduction

Formaldehyde-based wood adhesive resins mainly include urea–formaldehyde(UF),phenol–formaldehyde(PF)and melamineurea–formaldehyde (MUF) resins. Among these resins, UF resin is still one of the most widely used adhesives in wood-based panels nowadays owing to its standout advantages including excellent bonding performance, easy usage, light color, fast curing, and low cost.Nevertheless, the fatal defect of UF resin is high formaldehyde emission that not only does harm to the environment but also people’s health,which restricts its applications in chemical engineering and industry.Along with the increasing awareness of environmental protection and green development,the non-toxic,eco-friendly wood adhesive has been attracted more and more attentions all over the world. In order to avoid the formaldehyde emission of UF resin adhesives,some measures have been done,such as chemical modification [1,2], changing hardener [3], biomass modification [4–7],nano-materials modification [8], and liquid condensates (obtained during drying of wood materials) modification [9]. In addition, the wood adhesives without formaldehyde have been produced by various methods and formulations [10–13], in which the natural extracts from plants and vegetables, such as lignin or tannin, were used to substitute for some proportions of formaldehyde-based resins gradually. Even so, the main measure to reduce or eliminate formaldehyde emission of UF resins is to use low molar ratio of formaldehyde to urea(F/U)in the synthetic UF resin,while its bonding strength and water resistance of wood panels would be dropped to more extent.Moreover,the formaldehyde emission of UF resins is the inevitable problem forever as far as formaldehyde is used as the main reactant in UF resins’manufacturing.Even if a small amount of free formaldehyde is emitted from UF resin, which will cause a potential threat to environment and human health.

Besides decreasing formaldehyde emission of UF resins, more and more researches focus on the development of the alternative formaldehyde to using the non-toxic and non-volatile aldehyde in the wood adhesive’s production, like glyoxal [14], glutaraldehyde [15] and dimethoxyethanal [16]. Among them, glyoxal(OHC-CHO),as one of the most simple dialdehydes with combined merits of low volatilization, non-toxicity, low cost and mature preparation technology, has been widely used in textile industries[17,18], paper industry [19], and wood industry [20]. Comparing with formaldehyde, glyoxal with non-toxicity and low volatilization can be deemed as a pleasant alternative to formaldehyde.Our research group has attempted to do a lot of work on glyoxal-based resins. The reaction mechanism of glyoxal-urea(GU) and glyoxal-monomethylolurea (G-MMU) resin was firstly studied by quantum chemical calculations in our previous work[21]. On this basis, the friendly environmental G-MMU resins and GUF resins were prepared for the first time, and the bonding performance was further improved [21–24]. In addition, melamineglyoxal(MG)resin with high viscosity was prepared and its structures were studied before and after curing, which demonstrated the formation of —NH—CH=CH—NH—, —NH—CHR—O—CHR—N H— and —NH—CHR—NH– bridges [25].

In this paper, the high reactivity of melamine, which has been used to modify or prepare formaldehyde-based resins,such as melamine–formaldehyde (MF) resin [26] and melamine-ureaformaldehyde (MUF) resin [27,28], was chosen as a raw material for improving G-MMU resin. Comparing with urea, its functional group(—NH2)with higher reactivity is easier to react with glyoxal,which can improve the condensation degree of the G-MMU resin.Additionally, the introduction of the triazine rings with a hyperconjugation structure would improve the low resistance of the G-MMU resins to humidity and water.Up to now,there was rather scarce literature about the synthesis and structural analysis of glyoxal-monomethylolurea-melamine (G-MMU-M) resin. Herein,G-MMU-M resin is firstly prepared through the reaction system of G, MMU and M. Effects of MMU/G molar ratio and the addition of melamine on the basic properties and the bonding strength of resins are fully tested according to the corresponding standard requirements.At the same time,the structures of G-MMU-M resin are characterized via Fourier transform infrared spectroscopy(FTIR), nuclear magnetic resonance spectroscopy (13C NMR),X-ray photoelectron spectroscopy (XPS) and electrospray ionization mass spectrometry(ESI-MS)analyses.It is expected to lay foundation for develop a novel and eco-friendly glyoxal-based resin with excellent properties as wood adhesive in wood based panel.

2. Experimental

2.1. Synthesis of G-MMU-M resins

Glyoxal (G, 40% water solution) was added to the reactor equipped with a condenser, thermometer and agitator. The pH range was adjusted to 3.0–4.0 by adding 30%NaOH water solution at room temperature. A certain amount of monomethylol urea(MMU) powders was added to the reactor according to a certain molar ratio of G: MMU. Melamine (M) was placed in the reaction system when the reaction temperature slowly increased to 60 °C and then keep the temperature at (80 ± 3) °C for 3 h. Lastly, the temperature of the mixture was decreased to 40 °C and the pH was adjusted to 7.0–8.0 for use.The G-MMU-M resins were placed for 24 h to determine the basic performance.

2.2. Determination of basic properties of G-MMU-M resins

The viscosity of the resins was measured using SNB-2 digital rotary viscosity meter (Shanghai Hengping Instrument, China) at room temperature. The state of the resins was determined by observing the appearance of resins such as color, transparency and purity. According to Eq. (1), the solid content of the G-MMUM resin was calculated by the mass of resins before and after drying, obtaining the average of three parallel tests of different formulations.

where m1is the mass of the container, m2is the total mass of the container and resin before drying, and m3is the total mass of the container and resin after drying.

2.3. Three-layers plywood preparation

Poplar veneer layers with dimensions of 330 mm ×220 mm × 2 mm were used to prepare the three-layers plywood.G-MMU-M resins were mixed with NH4Cl hardener(3%total mass of resins’ solid) and cassava starch (10% total mass of adhesive).The first and third veneers were coated with the above mixture of 200 g?m-2. Then, the three layers were assembled and pressed at 140 °C and 1.50 MPa pressing pressure for 5 min. After hot pressing, the plywood was balanced at room temperature for 24 h. Triplicate panels were prepared for each resin formulation.

2.4. Measurement of bond strength of the G-MMU-M resins

The specimens were prepared and the bond strength of adhesive was measured according to the standard requirement of GB/T 17657–2013 about the test method of physical and chemical properties of wood-based panels and surface decorated wood panels.Twelve specimens(25 mm×100 mm)of each adhesive formation were prepared for testing dry shear strength and wet shear strength. The dry and wet shear strength of the specimens was measured using a UTM5105 tensile machine (Shenzhen Suns,China). For the measurement of shear strength, specimens were tested after immersing in cool water fully at (20 ± 3) °C for 24 h.The shear strength was calculated by Eq. (2):

2.5. Characterization of FTIR, 13C NMR, and XPS

The chemical structure of G-MMU-M resin was characterized by FTIR,13C NMR, and XPS. FTIR of G-MMU-M resins from different formulations was carried out on a Nicolet iS50 FTIR spectrometer(Thermo Fisher Scientific,USA).The samples for the test were prepared by spreading the resin solution to KBr pellets from 500 to 4000 cm-1at room temperature of 22–25 °C.

For13C NMR measurement,a little of G-MMU-M resin was dissolved in dimethyl sulfoxide (DMSO), and measured on a Bruker Avance III NMR spectrometer 400 MHz (Bruker, Germany) with a relaxation delay of 2 s at room temperature of 22–25 °C.

The elements and functional groups of G-MMU-M resin were determined by an XPS K-Alpha spectrometer(Thermo Fisher Scientific). The patterns were taken in the CAE mode of this equipment with a spot size of 400 μm,at the energy of 150 eV with the step of 1 eV.

2.6. Differential scanning calorimetry (DSC) analysis

DSC analysis of the resins’sample was performed by DSC 204 F1 thermal analyzer (NETZSCH, Germany). The G-MMU-M adhesive,10% cassava starch by mass on the total mass of adhesive, and 3%NH4Cl by the mass of the resins’solid were mixed well in the beaker,then the mixture of 5–10 mg was placed in the DSC cups. The DSC scans were recorded at the heating rate of 15°C?min-1and in the temperature range from 20 °C to 180 °C under an N2atmosphere with a rate of 20 ml?min-1.

2.7. ESI-MS analysis

The ESI-MS spectra of the range of mass-to-charge ratio (m/z)150–2250 were determined by Thermo Scientific Q Exactive Orbitrap mass spectrometer (Thermo Fisher Scientific) with 1.4 × 105resolution. The corresponding parameters of the ESI source in positive ion scanning mode are 3.5 kV spray voltage, 3.5 MPa sheath gas flow rate,1.0 MPa aux gas flow rate,0.2 MPa sweep gas flow rate, 350 °C capillary temperature, and 300 °C spray temperature.

3. Results and Discussion

3.1. Basic properties of G-MMU-M resins

In order to obtain an optimum synthesis condition of the G-MMU-M resin,a series of G-MMU-M resins were prepared under different melamine addition amounts and the molar ratio of MMU/G.

Firstly,at a fixed molar ratio of MMU/G of 0.9:1.0 and at the pH range of 3.0–4.0, G-MMU-M resins were prepared with different addition amounts of melamine (0, 3%, 5%, 8%, and 10% (mass) relative to the mass of the MMU), and the resin’s basic properties were listed in Table 1. The stability of all samples was more than 30 d at room temperature and the appearance of all resins was homogeneous and red wine color. From Table 1, the incorporation of M into G-MMU resins does not impact solid content and viscosity obviously.

Considering the cost and performance of the G-MMU-M resin,in the following experiments,the G-MMU-M resins with 8%(mass)melamine (relative to the mass of MMU) and the different molar ratio of MMU/G were prepared at the pH of 3.0–4.0 condition and their basic properties were listed in Table 2.Similarly,the stability of all samples was more than 30 d at ambient temperature and the appearance of all resins was homogeneous and red wine color. From Table 2, the solid content and viscosity of the G-MMU-M resins increase with the molar ratio of MMU/G in general,which is probably due to the higher co-condensation degree of the resins under the higher molar ratio of MMU/G. The viscosity increases slightly as the value of MMU/G increases from 0.7:1.0 to 0.9:1.0. This is probably due to the complex reactions in G-MMU-M resin owing to the steric hindrance effect of two aldehyde groups in glyoxal. However, the viscosity of the molar ratio MMU/G of 1.1:1.0 is 50% higher than that of the molar ratio MMU/G 0.9:1.0.

3.2. DSC analysis of G-MMU-M resins

DSC test is a technology to study thermophysical properties of materials by altering temperature, which has been widely used to investigate the curing process of wood adhesive resins, such asurea–formaldehyde(UF) resin[29],melamine-urea–formaldehyde(MUF)resin[30],urea-glyoxal(UG)resin[31]as well as melamineglyoxal (MG) resin [32].

Table 1 Basic properties of G-MMU-M resins with different addition amounts of melamine

Table 2 Basic properties of G-MMU-M resin with the different molar ratios of MMU/G

The influence of melamine addition amount and MMU/G molar ratio on the curing behavior of the G-MMU-M resins was investigated by the DSC method,and the corresponding DSC curves were shown in Fig. 1. For the G-MMU-M resins, the exothermic peaks could be attributed to the fact that the resins’curing is an exothermic process and the temperature for exothermic peaks (TPeak)appears at the range of 105–115 °C, which was conformed to the related researches about our previous dynamic mechanical analysis (DMA) of G-MMU [22]. Fig. 1(a) shows that the G-MMU-M5%resin begin to cure at approximately 100 °C, and the peak curing temperature is only about 105°C,which may be caused by the fact that the melamine with three highly active groups(—NH2)and the triazine ring promots the formation of crossing structure among G,MMU and M. Meanwhile, G-MMU-M accelerates the curing process of the resins comparing with the pure G-MMU resin.

As shown in Fig. 1(b), the peak curing temperature of the G-MMU-M0.7:1.0resin is dropped to 105 °C. Nevertheless, those of all other samples are higher than 109 °C, which indicates that the molar ratio of MMU/G could also impact the curing process of the G-MMU-M resins. Thus, the initial molar ratios of MMU/G should be controlled to achieve appropriate curing temperature and better bonding performance.

3.3. Bonding strength of plywood bonded with the G-MMU-M resins

To further investigate the bonding performance of the G-MMUM resins prepared under different conditions, the dry and wet bonding strength of the plywood bonded with different G-MMUM resins were determined and shown in Fig. 2.

It can be seen from Fig.2(a)that,with the additional amount of melamine being lower than 8%, the dry and wet shear strength of the plywood bonded with the G-MMU-M resins is much higher than the requirements (0.70 MPa) of GB/T 9846–2015. The dry and wet shear strength of the plywood increase with the addition amount of melamine at first, and then decrease when melamine’s amount exceeded 8%,which illustrates that the appropriate amino groups and triazine rings introduced could improve the water resistance of wood panels. Comparing with G-MMU resin, the GMMU-M8%exhibits better bonding performance of dry and wet shear strength of 1.98 MPa and 1.27 MPa, increased by 34% and 63%, respectively. Fig. 2(b) reveals that the dry and wet bonding strength of the G-MMU-M resins with different MMU/G molar ratios can meet the requirements of Chinese National Standard GB/T 9846–2015.It’s clear that the increase of MMU/G molar ratio impacted the water resistance and dry bonding strength of resin.Among these resins, the wet bonding strength of G-MMU-M0.9:1is the best up to 1.27 MPa. Obviously, the proportion of melamine and MMU/G molar ratio would impact the bonding strength of the resins. Meanwhile, compared with the dry shear strength of the plywood bonding with UG (0.98 MPa) [21], GUF (0.96 MPa) [23],and G-MMU(0.90 MPa)[22]resin which were prepared in our previous works, the G-MMU-M resins with 8% melamine and at the molar ratio of 0.9:1.0 was 102%, 106%, and 120% higher than that of UG, GUF, and G-MMU resins, respectively, and presented the great wet bonding strength, which also confirmed that the introduction of melamine played a significant role in bonding strength.

Fig.2. The shear strength of the plywoods with different resins:(a)the resins with different addition amount of melamine;(b)the resins with different MMU/G molar ratio.

3.4. FTIR analysis of G-MMU-M resin

FTIR is widely used to characterize the composition and structure of polymers according to the characteristic absorption peaks of materials, which is still widely employed to investigate numerous adhesives in recent years[33–37].Fig.3 shows FTIR spectra of melamine and a series of G-MMU-M resins with different addition amounts of melamine.And the main peak assignments were summarized in Table 3.

As shown in Fig. 3, FTIR spectra of the prepared G-MMU-M resins have similar absorption band,indicating that there is almost no change for the main functional groups, that is, the addition of melamine has little effect on the main functional groups of GMMU-M resins.By observing FTIR spectra of G-MMU-M,the broad and strong adsorption peak at 3413 cm-1is attributed to the stretching vibration of the O—H and N—H groups. Some of the peaks of G-MMU-M resins shift to lower wave numbers such as C=O stretching vibration at 1716 cm-1, N—H deformation vibration at 1619 cm-1, and C—O—C stretching vibration at 1245 cm-1, probably due to the p-π conjugative effect. It should be noted that the absorption of aldehyde groups is not presented on the spectrum, which implies that the free glyoxal is almost absent due to the complete reaction of glyoxal with melamine and monomethylolurea in the reaction system.By comparing with the FTIR spectra of melamine,the main absorption peaks of the GMMU-M resins have shifted to some extent, such as N—H stretching vibration and the skeleton vibration of triazine ring,which confirms the reaction among melamine, monomethylolurea and glyoxal.

Fig. 3. FTIR spectra of G-MMU-M resins with different M addition amount:(a) melamine; (b) M:1%; (c) M:3%; (d) M:5%; (e) M:8%; (f) M:10%.

3.5. XPS analysis of G-MMU-M resin

XPS equipped with extremely high sensitivity and resolution accurately performs the chemical stations, such as the measurement of chemical composition and content of substances,and analysis of the information of chemical bonds and functional groups[38–41]. To further investigate the chemical structures of GMMU-M resins, XPS analysis of G-MMU and G-MMU-M8%resins are shown in Fig. 4 and Fig. 5, respectively.

Obviously shown in Fig. 4(a) and Fig. 5(a), the spectra suggest that O 1s, N 1s, and C 1s are mainly located at about 533.08 eV,401.08 eV, and 288.08 eV, respectively. The increase of the contents of N 1s in G-MMU-M resin and the decrease of C 1s and O 1s indicate a new chemical bond formed in the system through the reaction among melamine, monomethylolurea, and glyoxal.In addition, according to the published literatures about GU [23],G-MMU [42] and MG resin [23,42,43], there are two states of C 1s in the resins, that is, the saturated carbons (C—C/C—H/C—O/C—N) and unsaturated carbons (C=O/C=N). Thus, the C 1s spectra of G-MMU resin (Fig. 4(b)) and G-MMU-M resin (Fig. 5(b)) are fitted to three kinds of bonding energy peaks in 285.23/285.33 eV for C—C/C—H species,in 287.73/287.83 eV for C—O/C—N species from methylol groups(—CH2OH),methylene ether bridges(—C—O—C—),the triazine ring (N=C—N) and MMU (—C—NH2), in 288.68/288.88 eV for C=O/C=N species from the triazine ring,MMU and G.It’s apparent that the introduction of melamine leads to the increase of the intensity of C=O/C=N peaks and the decrease of the intensity of C—O/C—N peaks, which could be attributed to the fact that the dehydration reaction between methylol groups and amino groups predominantly form C=N structure,which leads to a significant decrease in the contents of C—N— and —C—O—structures. At the same time, the introduction of the triazine ring also increased the C=N bond content in the G-MMU-M resins.

For the N 1s spectra, the peak of 399.98/400.18 eV could be assigned to the C=N bond in the triazine ring and the—NH—from the MMU and melamine molecules, as well as the peak of 400.73/401.28 eV assigned to the —NH2groups. The intensity of—NH— increased significantly, which not only indicated that the melamine could promote the co-condensation reaction betweenglyoxal and amino groups but also was consistent with the above analysis about the decrease of the intensity of C—O/C—N peaks.In the case of O 1s spectra Fig. 4(d) and Fig. 5(d), the chemical bonds of C—O and C=O species could be distinguished in 532.28/532.43 eV and 533.28/533.33 eV, respectively.

Table 3 FTIR assignments of G-MMU-M resin and melamine

Fig. 4. (a) XPS spectra, (b) C 1s core-level spectra, (c) N 1s core-level spectra, and(d) O 1s core-level spectra of G-MMU resin.

Fig. 5. (a) XPS spectra, (b) C 1s core-level spectra, (c) N 1s core-level spectra and(d) O 1s core-level spectra of G-MMU-M8% resin.

3.6. 13C NMR analysis

As the reaction conditions change, the structure and performance of resins would also change as well. Structure studies of resins under different conditions can provide theoretical guidance and a basis for controlling and optimizing the resins’structure.13C NMR is one of the most effective methods for analyzing the resins’structure [44–46].

Fig. 6 shows the13C NMR spectrum of G-MMU-M resins with MMU/G 0.9:1.0. The main13C NMR shifts assignments of G-MMU-M resins are summarized in Table 4, and their formation process were shown in Figs. 7-9, respectively.

As seen in Fig. 6, the highest absorption in low-magnetic-filed areas was no more than 174, and the lowest absorption in highmagnetic-filed areas was no less than 61. According to previous research about the reaction of glyoxal with urea in weak acid solution, glyoxal would have protonated reaction and a series of addition reactions with H2O, that is, there is no free glyoxal in the solution [21,24]. The predominant addition reaction between G and MMU include the reactions of MMU with protonated glyoxal(p-G) and 2, 2-dihydroxyacetaldehyde (p-G1) to form the important carbocation intermediates of C-p-GMMU and C-p-G1MMU[22,23]. Accordingly, comparing with the13C NMR spectra of G-MMU resin, the absorptions of the aldehyde-group carbon, protonated glyoxal, and its additive products to H2O disappear at the chemical shift range of 92–108 in Fig. 6, which indicates that a great proportion of glyoxal participated in the reaction with melamine and monomethylolurea. The absorption of 61–64 corresponded to —C*H(—NH—)—NH— groups. Moreover, the absorption peaks in the range of 61–64 could also be due to the signal of —CH2OH from the —C(=O)—NH—C*H2OH. It is of particular interest to note the absorption peaks at around 75–78 region.This specie could correspond to the (a) M—C*H(—CH—)—NH—C(=O)—NH— and (b) M—C*H(—CH2OH)—NH—C(=O)—NH— through the condensation reaction and their formation mechanism was shown in Fig.7.It’s possible that the signals at 82 represent the formation of ether bridges of (a) M—C*H(—CH—)—O—C*H2—NH— and (b)—(C=O)—NH—C*H2—O—C*H2—NH—(C=O)— through the reaction among melamine, glyoxal, and MMU or by dehydration reaction between two MMU molecules, as shown in Fig. 8. The signals at 83–85 were likely to belong to two structures, namely, (a) the —NH—C*H(—OH)—C*H(—OH)—NH groups formed by the connection of glyoxal with two MMU molecules,(b)the M—C*H(—OH)—C*H(—OH)—NH—C(=O)— groups formed by the additive reaction of glyoxal with melamine and MMU, and Fig. 9 shows the formation of these structures.

The signal of —NH—(C*=O)—NH— is presented in the 157–162 region definitely, which is due to the fact that the change in the chemical environments of —C=O leads to its absorption in this region. The chemical shift of more than 162 could be attributed to the structures of the melamine [47], and the peaks at 169 and 174 represent the carbons of monosubstituted and disubstituted triazine ring of melamine and unsubstituted melamine, respectively[48,49].Noticeably,their weak absorption intensity indicates that the great majority of melamine participated in the reaction with G and MMU and that only a tiny minority of unreacted melamine presents in the reaction system.

3.7. ESI-MS analysis of G-MMU-M resins

13C NMR and FTIR are usually combined with MALDI-TOF-MS or ESI-MS to investigate the structure of the resin, which was widely used in wood adhesives in terms of previous research [50,51]. In order to determine the oligomers distribution range of the resins,to further confirm the structure, and deduce the reaction mechanism of the G-MMU-M resin, the ESI-MS spectrum of G-MMU-M0.9:1resin is shown in Fig. 10, and the assignments of main species of the G-MMU-M resin are listed in Table 5.

Fig. 6. 13C NMR spectrum of G-MMU-M with MMU/G molar ratio 0.9:1.0.

Table 4 13C NMR main assignments of G-MMU-M resin

Table 4 (continued)

It’s worth noting that the m/z value of each peak in the ESI-MS spectrum is the value of the molecular weight of corresponding oligomer species adding the Na+ion (the molecular weight 23) from the electrolyte NaCl.The addition reaction between glyoxal(G)and MMU in acid solution predominantly contains the reaction of amino groups in MMU with protonated glyoxal (p-G) and protonated 2,2- dihydroxyacetaldehyde (p-G1) to form the main carbocation intermediates C-p-G-MMU, C-p-G1-MMU and other compounds of G-MMU, G1-MMU. According to the organic reaction mechanism, melamine reacting with G can form a melamine-glyoxal (MG) compound with a hydroxyl group at m/z 198. Simultaneously, the condensation reactions among MMU molecules can form an ether bridge at m/z 159 or methylene bridges at m/z 183.

Fig. 7. The mechanism of structure in 75–78 region.

Fig. 8. The formation of structures in 82.

Fig. 9. The formation of structures in 83–85 region.

Fig. 10. ESI-MS spectrum of G-MMU-M with MMU/G molar ratio of 0.9:1.

In addition,a series of addition reactions occur among G,M,and MMU molecules to produce the carbocation intermediate of M-CH(-+CH-MMU)-O-MMU and M-CH(–OH)-+CH-MMU-O-MMU structures, which are presented at the experimental value of m/z 353 in the ESI-MS spectra. In the same way, the peak of m/z 353 also has another possibility, that is, M-CH(—CH2OH)-MMU-O-MMU(m/z 353), in which G links with M and MMU by –CH-NH- bridge formed through the dehydration reaction between hydroxyl groups of the protonated G and amino group of MMU. According to the previous studies [48], the specie of M—CH(—+CH-MMU)-MMU-p-G at m/z 381 equipped with —CH—NH— bridge was also deduced.On the basis of the above analysis,in this reaction system,the four main basic structures were M—CH(—+CH-MMU)-O-MMU,M—CH(–CH2OH)-MMU-O-MMU, M—CH(—+CH-MMU)-MMU-p-G,and M—CH(—OH)—+CH-MMU-O-MMU.

The peak of m/z 411–413 probably corresponded to the MCH(-+CH-MMU)-O-MMU-p-G (m/z 411), M-CH(–CH2OH)-MMU-OMMU-p-G (m/z 412) and M—CH(—OH)—+CH-MMU-O-MMU-p-G(m/z 411). And the peak of m/z 437 is probably related to the species of M—CH(—CH—(MMU)2)-O-MMU and M—CH(—OH)—CH(-M MU)-MMU-O-MMU. The peak of m/z 485 is the structure of M—CH(—CH2O-MMU-p-G)-MMU-O-MMU by the condensation reaction of MMU-p-G and M—CH(—CH2OH)-MMU-O-MMU. For the peak of m/z 499, the probable structures are M—CH(—CH-(MMU)2)-O-MMU-p-G (m/z 500), M—CH(CH2OH)-MMU-O-MMUG-MMU (m/z 501) and M—CH(—OH)—CH(-MMU)-MMU-O-MMUp-G (m/z 500). Moreover, the peaks of m/z 559 and 591 might correspond to the M—CH(—CH-(MMU)2)-MMU-G-MMU (m/z 559)andM—CH(—CH-(MMU)2)-O-MMU-G-MMU(m/z589),M—CH(—OH)—CH(-MMU)-MMU-O-MMU-G-MMU(m/z589),respectively. With regard to the peaks of more than m/z 600, they were probably assigned to the subsequent reaction products of the above oligomers.

Table 5 ESI-MS main assignments of G-MMU-M resin

Table 5 (continued)

From the ESI-MS spectra of the G-MMU-M resins,it can be also deduced that the oligomers with molecular weights lower than m/z 500 predominates in the resins and those nucleophilic addition reactions of melamine with glyoxal, and of p-G with MMU and co-condensation reactions between hydroxyl and amino groups are the main way to form G-MMU-M resins. All the above structures show that melamine participates in the co-condensation reaction with glyoxal and monomethylolurea, which is consistent with the results obtained from FTIR, XPS and13C NMR studies.

4. Conclusions

The addition amount of melamine and MMU/G molar ratio has some effects on the viscosity of the G-MMU-M co-condensed resins, while the solids content almost remains stable. The appropriate introduction of melamine not only diminished the curing temperature of the G-MMU resins and promoted the crossing link rate, but also improved the dry and wet shear strength of the three-layer plywood bonded with the G-MMU-M resins. Through the FTIR, XPS,13C NMR, and ESI-MS analysis, melamine molecule participated in the condensation reactions between glyoxal and monomethylolurea, meanwhile, would form the predominantly oligomers in G-MMU-M resins that is, M—CH(—+CH-MMU)-OMMU,M—CH(—OH)—+CH-MMU-O-MMU,M—CH(—CH2OH)-MMU-O-MMU and M—CH(—+CH-MMU)-MMU-p-G. The further reaction to form higher molecular products is carried out through the condensation reaction and additive reaction among the oligomers.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Funding support from National Natural Science Foundation of China(31860188),Special Project of‘‘Leading Talents of Industrial Technology”of Yunnan Ten Thousand Talents Plan(80201408)and Yunnan Agricultural joint project (202101BD070001-105) are acknowledged.