Performance, Reaction Mechanism, and Characterization of Glyoxal

Article pubs.acs.org/IECR

Performance, Reaction Mechanism, and Characterization of Glyoxal−Monomethylol Urea (G−MMU) Resin Shuduan Deng,†,‡ Guanben Du,*,† Xianghong Li,†,§ and Xiaoguang Xie§ †

Yunnan Key Laboratory of Wood Adhesives and Glue Products, Southwest Forestry University, Kunming 650224, P.R. China College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, P.R. China § School of Chemical Science and Technology, Yunnan University, Kunming 650091, P.R. China ‡

ABSTRACT: The nonvolatile and nontoxic aldehyde glyoxal (G) was chosen to react with monomethylol urea (MMU) to prepare a glyoxal−monomethylol urea (G−MMU) resin. The basic properties of the resin and bonding strength of bonded plywood panels were tested. The reaction mechanism of the addition of G to MMU was investigated theoretically at the BLYP/ DND/COSMO level of quantum chemistry using density functional theory (DFT) method. The results showed that the addition reaction of G with MMU under weakly acidic conditions mainly involves the reactions of MMU with protonated glyoxal (p-G), protonated 2,2-dihydroxyacetaldehyde (p-G1), and protonated bis-hemidiol (p-G2) to form the two important reactive carbocation intermediates C-p-G−MMU and C-p-G1−MMU and the two important hydroxyl compounds N-p-G−MMU and N-p-G1−MMU. The G−MMU resin was characterized by FTIR and 13C NMR spectroscopies, and the results were found to be quite compatible with those obtained by quantum chemical calculations. Based on the theoretical and experimental results, polycondensation reactions for G−MMU resin are proposed.

1. INTRODUCTION As the most important and most common type of so-called amino plastic resins, urea−formaldehyde (UF) resins are currently the major wood-adhesive binders used in the manufacturing of interior-use wood-composite boards such as particleboards, medium-density fiberboards, and plywood because of their low cost, fast curing, and good performance.1,2 However, UF resins present the major disadvantage that toxic formaldehyde is emitted from UF-bonded products. Formaldehyde (F) is a human carcinogen and causes irreversible health effects [LD50(rat) ≥ 100 mg/kg; LD50(mouse) ≥ 42 mg/kg], so the extensive use of UF resins has resulted in free formaldehyde polluting the environment and harming human health. Because of this fatal defect, research on decreasing formaldehyde emissions from UF resins has been ongoing. Thus, to decrease emissions of formaldehyde, many approaches have been tried during the past several decades, and much progress has been achieved.3−5 Among the strategies for reducing formaldehyde emissions, use of a low molar ratio of formaldehyde to urea (F/U) in the manufacturing of UF resins is the main method, and currently, this ratio is at a low value of about 1.15. Lowering the F/U ratio would certainly decrease formaldehyde emissions, but it inevitably reduces the bond strength and water resistance of boards and increases the curing time because of the limited crosslinking in the resulting resin.6 The main drawback of UF resins is their principal component of formaldehyde. The unreacted F remains in the final resin products. Furthermore, both the methylolation and condensation reactions occurring in UF resins are reversible, and the methylene ether linkages (CH2O CH2) in the condensed resins are unstable at high temperature.7 Therefore, formaldehyde emissions from UF resins are unavoidable as long as formaldehyde is used as a main reactant in the manufacturing of UF resins. The small amount of free © 2014 American Chemical Society

formaldehyde will still be a potential threat to the environment and human health. With the improvement of people’s living standard and the strengthening of environmental awareness, increasingly strict standard regulations on formaldehyde emissions have induced considerable research focusing not only on decreasing formaldehyde emissions of UF resins but also on developing alternatives to UF resins.8−16 One possibility that has not been explored much is the use of alternative, nontoxic, nonvolatile aldehydes to produce new amino resins for wood adhesives. To date, the alternative aldehydes dimethoxyethanal,8,9 propionaldehyde,17 succinaldehyde,18 glutaraldhyde,19,20 and isobutyraldehyde21 have been reported. However, these alternative aldehydes still have problems: Either they are toxic to some extent, or they are volatile, or they present other problems such as low reactivity or low solubility. These problems restrict their applications, so identifying an appropriate alternative aldehyde is the most important problem regarding the industrialized application of such alternative resins. As the simplest aliphatic dicarbonyl compound, glyoxal (G) is nontoxic [LD50(rat) ≥ 2900 mg/kg; LD50(mouse) ≥ 1280 mg/kg]22 and nonvolatile.23 Because of its advantages of having a mature production technology, low price, and easy biodegradation, G has been widely used in the paper and textile industries as an ideal green environmental agent.24−29 In the wood-adhesive field, G is mainly used to substitute partially or completely for F as a cross-linking agent or curing agent in the preparation of natural wood adhesives such as tannin-based, 30−34 lignin-based,35−39 and protein-based40−42 adhesives. However, Received: Revised: Accepted: Published: 5421

December March 11, March 12, March 12,

18, 2013 2014 2014 2014

dx.doi.org/10.1021/ie404278d | Ind. Eng. Chem. Res. 2014, 53, 5421−5431

Industrial & Engineering Chemistry Research

Article

the ambient environment (20 °C and 12% relative humidity) for 48 h before being tested. Three panels of plywood were made for each adhesive formulation. 2.4. Measurement of Dry Shear Strength. After being conditioned for 48 h at room temperature, the plywood panels were cut into 12 specimens according to China National Standard GB/T 17657-1999, Test Methods of Evaluating the Properties of Wood-Based Panels and Surface-Decorated WoodBased Panels, and all specimens were chosen randomly for the testing of dry shear strength by compression loading on a WDS-50KN mechanical testing machine (Shimadzu, Kyoto, Japan). The values of the dry shear strength were calculated according to the equation

the literature available to date on the application of this new glyoxal resin in wood adhesives and its reaction mechanism is scant. On the other hand, monomethylol urea (MMU) is one of the most important intermediate products in the preparation of UF resins, and it can further participate in addition and condensation reactions owing to the active amino (NH2) and hydroxymethyl (CH2OH) groups in its structure. Thus, MMU can be regarded as a potential reactant to produce amino plastic resins. In the present work, to decrease or eliminate formaldehyde emissions from UF resins at the source, glyoxal−monomethylol urea (G−MMU) resin was synthesized under weakly acidic conditions. The basic properties of the resin and its bonding strength in bonded plywood were tested. Meanwhile, the curing process of G−MMU resins was studied by dynamic mechanical analysis (DMA). Furthermore, the addition mechanism of G and MMU was theoretically investigated at the BLYP/DND/ COSMO level of quantum chemistry. The relative energy (ΔE), protonated affiliation energy (PA), Gibbs free energy change of reaction (ΔrG), and enthalpy change of reaction (ΔrH) were calculated and are discussed in detail. The structure of G−MMU resin was characterized by FTIR and 13C NMR spectroscopies. According to the theoretical and experimental results, polycondensation reactions for G−MMU resin are proposed. It is thus hoped that this work will help to accumulate useful experimental data and provide theoretical guidance for the synthesis of zero-formaldehyde-emissions wood adhesives containing G−MMU resin.

shear strength (MPa) =

maximum force (N) effective gluing area (m 2)

(1)

and the average strength was calculated for 18 test specimens from three panels. 2.5. Dynamic Mechanical Analysis (DMA). Because the G−MMU resin adhesive is an aqueous solution, the use of DMA requires solidification of the resin adhesive after its impregnation into a substrate, and in this study, the same poplar veneer asprepared plywood was used as the substrate to simulate the hotpressing process in plywood manufacturing. For sample preparation, 2% NH4Cl (25% solution) and 15% cassava starch were added to G−MMU resin and then mixed thoroughly. This mixture was symmetrically spread on one surface of the poplar veneer with dimensions of 50 mm × 10 mm × 1.5 mm. The spreading density of the adhesive was 250 mg/m2. The coated veneer was covered by an uncoated veneer of the same size with the same grain direction. The stacked veneers were cold-pressed at 1.0 MPa and 20 °C for 15 min. The prepared specimens were placed on the grips of a DMA 242C dynamic mechanical thermal analyzer (NETZSCH Corporation, Selb, Germany) for DMA measurements. The DMA measurements were made in bending mode with the specimen clamped in a horizontal plane between the ends of two parallel arms. Fixed-displacement mode with a 60-μm amplitude and a 10-Hz oscillation frequency was used. For a dynamic scan of DMA, the temperature was increased from room temperature to 300 °C at a heating rate of 10 °C/min. Nitrogen gas was used to prevent any oxidation of the sample and to purge the DMA chamber at a rate of 200 mL/min. Duplicate scans were performed for the G−MMU resin adhesive, and this resulted in similar curves without any significant difference. Thermomechanical parameters such as the storage modulus (E′), loss modulus (E″), and tan δ (ratio of E″/E′) were obtained from the DMA curves. 2.6. Quantum Chemical Calculations. Quantum chemical calculations were performed with DMol3 numerical-based density functional theory (DFT) in the Materials Studio 4.0 package from Accelrys Inc.43 Geometrical optimizations and frequency calculations were carried out with the generalized gradient approximation (GGA) functional of Becke exchange plus Lee−Yang−Parr correlation (BLYP)44 in conjunction with the double-numerical plus d functions (DND) basis set.45 Fine convergence criteria and global orbital cutoffs were employed on basis set definitions. For all calculations, the spin-unrestricted formalism was used. The convergence tolerances for energy, maximum force, and maximum displacement and the SCF convergence criterion were 1.0 × 10−5 Ha, 2.0× 10−3 Ha/Å, 5.0× 10−3 Å, and 1.0 × 10−6, respectively. The k-point set was 1 × 1 × 1. Considering solvent

2. EXPERIMENTAL SECTION 2.1. Materials. The glyoxal (40% water solution) and monomethylol urea (MMU) used were of analytical grade (AR) from China National Pharmaceutical Group Corporation. Cassava starch was purchased from Guangzhou Jinguang Chemical Plant, Guangzhou, China. Poplar veneer with a moisture content of 8−10% was obtained from Wen’an Veneer Factory of Hebei Province, China. 2.2. Preparation of G−MMU Resin and Determination of Basic Properties. G−MMU resin was prepared at a G/MMU molar ratio of 1:1. G (40% water solution) was placed in a reactor, and the pH was adjusted to 6−7 using 30% NaOH. Subsequently, MMU was added, and the mixture was heated to 75 °C for 3 h. The reaction mixture was made weakly alkaline (pH 7−8) and cooled to room temperature. About 1 g of G−MMU resin was poured into a disposable aluminum dish and then dried in an oven at 120 ± 1 °C for 2 h. The nonvolatile solid content was determined by the measurement of the weight of the G−MMU resin before and after drying. An average of three parallel specimens is presented. The viscosity of the G−MMU resin was measured with an NDJ-1 rotary viscosity meter at 25 °C. 2.3. Preparation of Three-Layer Plywood. The performance of the G−MMU resin was tested by preparing laboratory plywood and evaluating its dry shear strength. Duplicate threelayer laboratory plywood panels of 300 mm × 220 mm × 4.5 mm were prepared using the G−MMU resin and poplar veneers. To all of these glue mixtures were added 2% NH4Cl hardener, by weight based on the total weight of G−MMU resin, where NH4Cl was predissolved to a 25% solution in water, and 15% cassava starch by weight based on the total weight of G−MMU resin. The amount of glue used was 250 g/m2 (single side), the hot pressing time was 5 min at 160 °C, and the pressing pressure was 1.5 MPa. After being hot-pressed, the plywood was stored in 5422



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effects, all geometries were reoptimized at the GGA/DND level using COSMO (conductor-like screening model)46 and defining water as the solvent. All optimized species including reactants, intermediates, and products were confirmed to have no imaginary frequencies. Zero-point vibrational energies (ZPVEs) were used to correct all relative energies (ΔE). 2.7. FTIR and 13C NMR Spectroscopies. FTIR spectra were recorded with an AVATAR-FTIR-360 spectrophotometer (Thermo Nicolet Company, Madison, WI). Because G−MMU resin is a liquid, its spectrum was recorded using the OMNI sampler accessory. The 13C NMR spectra of the samples were obtained on a Bruker Avance III 400 M NMR spectrometer with a relaxation delay of 5 s. G was mixed with an equal amount of deuterium oxide (D2O) with tetramethylsilane (TMS) added as an internal standard. MMU and G−MMU resin were dissolved in dimethyl sulfoxide (DMSO). Figure 1. Relationship between storage modulus (E′) and temperature for the samples.

3. RESULTS AND DISCUSSION 3.1. Properties of G−MMU Resin. The G−MMU resin was synthesized under weakly acidic conditions of pH 6−7 to largely avoid the self-polycondensation reaction of MMU, and its properties are summarized in Table 1. As reported in the table, Table 1. Properties of G−MMU Resin property

value

appearance viscosity (mPa·s) nonvolatile solid content (%) dry shear strength (MPa)

light yellow liquid 41.0 54.7 0.90

the viscosity of G−MMU resin was 41.0 mPa·s. For the preparation of plywood panels, 15% cassava starch (by weight based on the total weight of G−MMU resin) was added to adjust the viscosity of the adhesives as well as to lower the cost. The dry shear strength of the plywood bonded with G−MMU resin was found to be higher than 0.70 MPa. This dry shear strength could meet the type III plywood requirement of China National Standard GB/T 9846.3-2004, and the bonded plywood could be directly used as interior decoration and furniture materials in dry conditions. 3.2. Dynamic Mechanical Analysis (DMA). Dynamic mechanical analysis (DMA) tests were conducted through an oscillating force applied to a sample, and the material’s response to the force was analyzed.47 The DMA 242C dynamic mechanical thermal analyzer can continuously measure the storage modulus (E′), loss modulus (E″), and tan δ (ratio of E″ to E′) of a resin sample as the temperature of the sample chamber is increased and/or maintained. E′ is a measure of the stored energy of a material, that is, the rigidity, depending on the polymer type, temperature, and frequency of oscillation, whereas E″ measures the dissipated energy of a sample due to the molecular friction occurring in viscous flow. Although the DMA method does not accurately simulate the hot-pressing conditions of boards, some useful resin curing parameters can be obtained. For a thermoset material, DMA can be used to monitor the curing process by tracking modulus changes with temperature.48 This technique has been used to investigate the curing process of phenol− formaldehyde,49−52 urea−formaldehyde,53,54 and urea−melamine− formaldehyde (UMF)48 resins. In the present work, DMA tests were used to characterize the modulus changes of samples by simulating the hot-pressing

Figure 2. Relationship between tan δ and temperature for the samples.

process in plywood manufacturing. The changes in storage modulus (E′) and tan δ of a sample with temperature are shown in Figures 1 and 2, respectively. The peak values are also marked in the corresponding figures. As can be seen from Figure 1, at temperatures lower than 99.4 °C, the storage modulus (E′) remained approximately constant at about 2000 MPa because the adhesive did not cure completely in this temperature range. Then, as the temperature increased, E′ increased rapidly to a maximum value of 7212 MPa at 137.6 °C and basically remained constant from 137.6 to 150 °C, which is attributed to the complete curing of the adhesive and the maintainance of the mechanical properties of sample after curing within this temperature range. When the temperature was higher than 150 °C, E′ decreased sharply to a minimum value of about 2029 MPa and then basically remained about the same, which is probably due to the degradation of the cured adhesives at high temperature. The peak temperature of tan δ often represents the phase transformation of a sample. Two peaks were found in the tan δ curve of the sample with temperature as shown in Figure 2. The first peak at 109.4 °C is the initial temperature of vitrification of the adhesive. At temperatures lower than 109.4 °C, the evaporation of water and the curing of the adhesive greatly 5423



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increased the viscosity and decreased the flowability of the adhesive, and the adhesive changed from an aqueous solution state to a softening state with poor rigidity. When the temperature was higher than 109.4 °C, the adhesive entered a glassy state from the softening state as the curing proceeded further, which resulted in the fastest growth of rigidity of the sample, namely, the storage modulus, and reached a maximum value. A similar process was reported in DMA of phenol− formaldehyde (PF) resins.55,56 The other peak observed at 222.7 °C was probably the degradation temperature of the adhesives; it was just this degradation that led to the fastest decrease of the storage modulus occurring at high temperature. These results are in agreement with those in Figure 1. 3.3. Reaction Mechanism of G with MMU. 3.3.1. Chemical Compounds of G in Acidic Solution. The availability of water plays a key role in glyoxal chemistry, as glyoxal (G) molecules in water solution exist mainly in the form of one and two hydrates.57−59 First, G reacts with H2O to form 2,2dihydroxyacetaldehyde (G1) and then adds H2O to form the bishemidiol (G2). In acidic solution, the protonation of G, G1, and G2 should be considered, forming the corresponding species pG, p-G1, and p-G2. Generally, the calculated portonation ability of a compound is determined by the protonated affiliation energy (PA). Through quantum chemical calculations, the PA values of G, G1, and G2 were determined to be 947.1, 990.8, and 1021.6 kJ mol−1, respectively. These large PA values indicate that each of the three studied compounds, G, G1, and G2, can be easily protonated in acidic solution. The reactions of G in acidic solution and the corresponding optimized chemical structures of G, G1, G2, p-G, p-G1, and p-G2 are shown in Figure 3. All

however, it is impossible for G2 to react with monomethylol urea (MMU) through a nucleophilic addition mechanism because G2 contains no reactive carboxyl group (CO). This result is similar to that of our recent work on the addition reaction between methanediol and urea.7 Accordingly, the reactions of the five compounds G, G1, G2, p-G, p-G1, and p-G2 with MMU are schematically shown in Figure 4, and the corresponding quantum chemical parameters are also listed in Table 2. The optimized chemical structures of MMU reactant and intermediate products G−MMU, G1−MMU, N-p-G−MMU, C-p-G−MMU, N-pG1−MMU, and C-p-G1−MMU are presented in Figure 5. The relative energies (ΔE) are listed in Table 3. Vibrational frequency calculations on all of the reactants, intermediates, and products were carried out, yielding thermochemical data within the temperature range 25−1000 K. Then, the enthalpy change of reaction (ΔrH) and the Gibbs free energy change of reaction (ΔrG) were calculated as Δr H =

∑ υBEt(B) + ∑ υBH(B) B

Δr G =

B

∑ υBEt(B) + ∑ υBG(B) B

B

(2)

(3)

where υB is the stoichimetic coefficient of component B and Et is the total electronic energy. H and G are the enthalpy and Gibbs free energy (including zero-point vibrational energies), respectively. Considering that the experimental temperature of the synthesized resin was about 350 K, the thermodynamic reaction parameters of Gibbs free energy change of reaction (ΔrG) and enthalpy change of reaction (ΔrH) at 350 K were calculated at the BLYP/DND/COSMO level and are also listed in Table 3. The nucleophilic addition mechanism of aldehyde and monomethylol urea is a two-step pathway that involves the attack of a nitrogen atom on the carbon atom of the aldehyde to form a CN bond, followed by the transfer of a hydrogen atom from nitrogen to oxygen to form an OH bond. According to previous reports,7,63 the addition reaction between unprotonated methanediol and urea is not favored under neutral conditions. Inspection of Table 3 reveals that the values of ΔE and ΔrH are negative for the reactions G + MMU → G−MMU and G1 + MMU → G1−MMU, which indicates that these reactions are exothermic. However, the positive values of ΔrG suggest that the reactions are thermodynamically unfavorable at 350 K. This finding can be explained as follows: The strong p−π conjugation between the nitro group (NH2) and the carbonyl (CO) in MMU significantly delocalizes the electrons on the nitrogen atom and subsequently decreases the nucleophilicity of MMU. To confirm this viewpoint, experiments on the reaction between G and MMU in neutral media were performed, and the results indicated that there was almost no reaction in this system. On the contrary, under acidic conditions, protonated gloxyl (p-G) carrying a positive charge can act as a strong electrophile, favoring the attack of MMU. Specifically, the addition reaction between G and MMU is general acid catalysis. In acidic solution, for the reaction p-G + MMU → N-p-G−MMU, the ΔrG value is negative to a greater extent, which indicates that this reaction can easily occur because p-G is a cation that lacks electrons and MMU has a lone pair of electrons on the nitrogen atom. Accordingly, a stable complex can easily be formed through a direct collision between them. In the subsequent reaction N-pG−MMU → C-p-G−MMU + H2O, the ΔrG value is also large and negative (−35.12 kJ/mol); thus, the N-p-G−MMU complex

Figure 3. Possible different reactions of glyoxal (G) with H2O and protonated reactions in acidic solution.

optimized species had no imaginary frequencies, and all bonds were found to be stable. The quantum chemical parameters of these compounds are listed in Table 2. Clearly, the G molecule is in one plane and has two trans-carbonyl groups, which is in good agreement with the experimental data60,61 and theoretical results,62 which indicates that the selected method is reliable for studying the reaction system. All CO bond distances were found to be in the normal range, except for p-G2 with relative longer CO bond (1.661 Å). Thus, p-G2 should easily lose one H2O molecule. 3.3.2. Addition Reactions of G, G1, G2, p-G, p-G1, and p-G2 with MMU. In summary, G in acidic solution mainly exists in the different forms G, G1, G2, p-G, p-G1, and p-G2. Because chemical equilibrium exists between the neutral and protonated molecules, the reactions of G, G1, G2, p-G, p-G1, and p-G2 with MMU should be fully considered. Among these reactants, 5424



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Table 2. Quantum Chemical Parameters Obtained at the GGA/DND/BLYP/COSMO Level in the Liquid Phase

a

species

Et (Haa)

ZPVE (kcalb/mol)

H at 350 K (kcalb/mol)

G at 350 K (kcalb/mol)

G MMU G−MMU G1 G1−MMU p-G N-p-G−MMU H2O C-p-G−MMU p-G1 N-p-G1−MMU C-p-G1−MMU G2 p-G2

−227.8859843 −339.8580532 −567.7491795 −304.3398253 −644.1980846 −228.2599667 −568.1612526 −76.4467976 −491.6972734 −304.7295033 −644.6143084 −568.1637981 −380.7924701 −381.1938201

22.289 58.653 84.564 39.579 100.703 30.719 92.787 12.670 74.597 47.404 109.172 91.822 56.061 63.868

26.432 65.200 94.212 44.923 112.162 34.847 102.079 15.458 83.837 52.946 120.895 102.023 36.351 70.875

1.814 34.445 55.584 17.418 69.396 10.613 64.403 −1.307 46.537 25.302 77.016 62.241 31.865 39.438

1 Ha = 627.51 kcal/mol. b1 cal = 4.186 J.

Figure 5. Optimized chemical structures of MMU, G−MMU, G1− MMU, N-p-G−MMU, C-p-G−MMU, N-p-G1−MMU, and C-p-G1− MMU.

Interestingly, for the reaction p-G1 + MMU → N-p-G1− MMU, the ΔrG value is slightly positive (2.02 kJ/mol) and approximately equal to zero, which implies that this reaction between p-G1 and MMU is in a state of dynamic nearequilibrium. Further inspection of Table 3 reveals that ΔrG for the second reaction N-p-G1−MMU → C-p-G1−MMU + H2O is lower than −48.0 kJ/mol (−57.57 kJ/mol); thus, N-p-G1− MMU could easily be changed into C-p-G1−MMU by eliminating H2O. Therefore, the dynamic equilibrium p-G1 + MMU ↔ N-p-G1−MMU would be easily destroyed owing to a decrease of the product N-p-G1−MMU, and then the reaction would go toward the products (toward the right), which suggests

Figure 4. Reaction pathways of G, G1, p-G, p-G1, and p-G2 reacting with MMU.

easily results in the elimination of a water molecule, producing the reactive carbocation intermediate C-p-G−MMU. 5425



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Table 3. Relative Energies (ΔE) and Protonated Affiliation Energies (PA) for the Reactions in the Liquid Phase reaction

ΔE (kJ/mol)

ΔrH (kJ/mol)

ΔrG (kJ/mol)

G + HMU → G−HMU G1 + HMU → G1−HMU p-G + HMU → N-p-G−HMU N-p-G−HMU → C-p-G−HMU + H2O p-G1 + HMU → N-p-G1−HMU N-p-G1−HMU → C-p-G1−HMU + H2O p-G2 + HMU → N-p-G1−HMU + H2O

1.63 9.80 −99.27 22.03 −57.23 −9.84 −27.10

−2.72 7.99 −105.06 31.89 −58.76 −4.54 −23.09

67.37 72.85 −32.59 −35.12 2.02 −57.57 −16.61

that the p-G + MMU → N-p-G−MMU reaction would also occur. For the reaction between p-G2 and MMU, the ΔrG value is negative, indicating that these reactants could react to form N-p-G1−MMU, as p-G2 can easily lose one H2O molecule. In summary, according to thermochemical analysis, reactions of the protonated reactants p-G, p-G1, and p-G2 with MMU can easily take place in acidic solution. Furthermore, these reactions are exothermic, as ΔrH < 0, which indicates that these reactions are favorable at elevated temperature. For the subsequent reactions, the N-p-G−MMU → C-p-G− MMU + H2O reaction is endothermic, as ΔrH > 0, whereas the N-p-G1−MMU → C-p-G1−MMU + H2O reaction is exothermic, as ΔrH < 0. Both C-p-G−MMU and C-p-G1−MMU are important intermediate compounds that play important roles in the subsequent formation of G−MMU resin. All bond parameters of intermediate compounds were calculated, and it was found that all bond length values were in normal ranges. Thus, C-p-G−MMU and C-p-G1−MMU are stable. In addition, Mulliken population analysis of C-p-G−MMU and C-p-G1−MMU, as shown in Figure 6, indicates that the positive charges in these two carbonium ions, which play important roles in the subsequent reactions, are highly deloclized throughout the whole molecule. In the experiment of Nair and Francis,64 the formation of NH2CONHCH2+ was also suggested. In the quantum calculations of UF by our research team, the highly stable methylocarbonium (NH2CONHCH2+) plays an important role in the subsequent formation of methylene and methylene ether linkages.7 Noticeably, either N-p-G−MMU or N-p-G1−MMU could produce hydroxyl compounds G−MMU or G1−MMU through loss of a proton (H+), which would also play an important role in the condensation reaction. 3.4. FTIR Spectroscopy of G−MMU Resin. As one of the earliest methods used to characterize UF resins and one of the most important and most mature ways to investigate the composition and structure of polymers,65,66 Fourier transform infrared (FTIR) spectroscopy has been widely used to characterize the structures of wood-adhesive resins and other macromolecular compounds.67−70 In this work, FTIR spectroscopy was used to investigate the functional groups in the G−MMU resin structure, thus obtaining information about the reaction of G with MMU in this system. Figure 7 shows FTIR spectra of MMU and G−MMU resin, and the main assignments are listed in Table 4. Based on a comparison of panels b and a of Figure 7, the strong and wide absorption band at 3200−3600 cm−1 is attributed to the superposition of NH in amino groups and the OH stretching vibration in hydroxyl groups. Owing to the p−π conjugation effect between NH2 and CO in MMU, the CO peak shifts to lower wavenumber of 1661 cm−1. However,

Figure 6. Mulliken population analyses of C-p-G−MMU and C-p-G1− MMU.

Figure 7. FTIR spectra of (a) MMU and (b) G−MMU resin.

in G−MMU resin, the CO peak is observed at 1714 cm−1 because of the difference in structure between G−MMU resin and MMU. Similarly, other absorption bands of NH deformation vibration and CO stretching vibration also exhibit corresponding shifts. It is worth noting that the disappearance of the absorption at 1297 cm−1 and the appearance of the absorption at 1236 cm−1 for the resin indicate the presence of COC bonds in G−MMU resin.67,70 3.5. 13C NMR Spectroscopy of G−MMU Resin. The structure and property of the resins were found to vary as the synthesis conditions changed. Investigation of the structure formation process under different conditions can provide the theoretical basis for controlling and optimizing the resin structure. Structure studies of both amino copolycondensation resins and new amino resins are based on the structure studies of UF resins. As one of the most effective methods for studying resin structure, nuclear magnetic resonance (NMR) spectroscopy, especially 13C 5426



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Table 4. FTIR Assignments of MMU and G−MMU Resin absorption band (cm−1) 3441 3339 2962 2897 1661 1575 1458 1297 1044 1004 32003400 2962 1714 1558 1466 1236 1053

chemical structure assignments MMU OH stretching vibration NH stretching vibration CH asymmetric stretching vibration in CH2 CH symmetric stretching vibration of secondary amide CO stretching vibration NH deformation vibration of secondary amide CH deformation vibration of CH2 composite absorption of CN and NH in secondary amide CN stretching vibration CO stretching vibration of methylol groups G−MMU stretching vibration of OH and NH CH asymmetric stretching vibration in CH2 CO stretching vibration NH deformation vibration of secondary amide CH deformation vibration of CH2 COC stretching vibration in ester CN stretching vibration

Figure 9. 13C NMR spectrum of MMU.

NMR spectroscopy, has been successfully used to characterize the structures of wood-adhesive resins such as UF,71−74 PF,75 phenol−urea−formaldehyde (PUF),76−78 and melamine−urea− formaldehyde (MUF)79 resins. To characterize the G−MMU resin and confirm the reaction pathways proposed by quantum chemical calculations, 13C NMR spectra of G, MMU, and G−MMU resin are shown in Figures 8−10, respectively. On the basis of the 13C NMR analysis

Figure 10. 13C NMR spectrum of G−MMU resin.

in UF resins,71−74,80 the absorption of sp3-hybridized carbon in this system appeared in lower-magnetic-field areas of 90−110 ppm. The effects of greater electronegativity from having more oxygen (O) atoms in more hydroxyl groups in their structures (Figure 8) made the signals move to lower-magnetic-field areas. These results agree with those of quantum chemical calculations of the addition mechanism of MMU with G and the corresponding protonated products. To study the structure changes of the G−MMU resin before and after reaction, the 13C NMR spectrum of MMU (Figure 9) was examined. From Figure 9, it can be seen that there are two main groups of absorption peaks, as the MMU molecular structure contains only two types of carbons with different chemical environments, and their 13C NMR assignments are listed in Table 5. Comparing the 13C NMR spectrum of the G−MMU resin (Figure 10) with those of G and MMU, one can see that there are many new absorption peaks not appearing in Figures 8 and 9, which indicates that the reaction of MMU with G is probably very complicated. On the basis of the 13C NMR analysis of amino resins, especially the 13C NMR spectrum of UF resin, as well as a full consideration of the influencing factors of the chemical shifts of 13C and the quantum chemical calculation results, the 13C NMR spectrum of G−MMU resin was also mainly assigned in Table 5. The peaks at 160.04−160.89 ppm are definitely the

Figure 8. 13C NMR spectrum of glyoxal (G).

of the amino resins, especially the 13C NMR spectrum of UF resin,71−74 as well as a full consideration of the factors influencing the 13C chemical shifts and the quantum chemical calculation results, assignments of the 13C NMR spectra of G, MMU, and G−MMU resin are summarized in Table 5. From the 13C NMR spectrum of G (Figure 8), there are no absorptions in low-magnetic-field areas higher than 110 ppm and in high-magnetic-field areas lower than 80 ppm, especially in the vicinity of 195 ppm without absorption of the aldehyde-group carbon (CHO), which indicates that G does not exist in the free form in acidic water solution but, rather, is present in the form of protonated glyoxal and the additve products to H2O. It is worth noting that, compared with other sp3-hybridized carbons 5427



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Table 5. 13C NMR Assignments of G, MMU, and G−MMU Resin

signal of CO, which moved to slightly lower magnetic field owing to the change of its chemical environment through reaction. Moreover, the peaks in the range of 46.95−48.83 can be ascribed to the signals of CH2 from the C(O) NHC*H2HNC(O) groups formed through the

dehydration reaction between two MMU molecules, which is similar to the result reported for the 13C NMR analysis of UF resin.71−74,80 In addition, based on the absorption positions of the peaks and an analysis of influencing factors of the chemical shifts of 13C, the signals in the sp3-hybridized range 5428



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The NMR signals of 13C in different CH(OR)x groups vary from the influence of oxygen atom’s (O) electronegativity.81,82 These results are in accordance with those of quantum chemical calculations and further confirm the correctness of the results of the quantum chemical calculations. 3.6. Polycondensation Reactions. According to the polycondensation reactions for UF resins using experimental64 and theoretical7 methods, methylene linkages (NHCH2 NH) and methylene ether linkages (CH2OCH2) are the bridges that link the reactants and intermediates to form UF polymers. Similarly, it would be reasonable to deduce that these linkages are also important for G−MMU resin and can form through reactions of the intermediates with MMU or through self-condensation of the intermediates. In this work, some polycondensation reaction pathways are proposed based on the theoretical and experimental results. Based on the results obtained by quantum chemical calculations, the two important carbon cations C-p-G−MMU and C-p-G1−MMU could play important roles in the polycondensation reactions. As shown in Figure 11a,b, these two important carbon cations could react directly with MMU to form larger molecular structures of the forms C-p-G−MMU−MMU and C-p-G1−MMU−MMU, respectively. Then, they could transform into G−MMU−MMU and G1−MMU−MMU, respectively, by losing H+. These chemical structures contain the important linkages NHCHNH. The methylene ether linkage (CH2OCH2) is another type of bridge formed through the self-condensation of intermediates. According to the above quantum calculation results, both N-p-G− MMU and N-p-G1−MMU are important hydroxyl compounds in G−MMU resin. They could form hydroxyl compounds G− MMU and G1−MMU, respectively, through loss of a proton (H+), which would also form linkages of the type CH2O CH2 in G−MMU−G−MMU and G1−MMU−G1−MMU, as shown in Figure 11c,d. Noticeably, the self-condensation of G1− MMU has other pathways as presented in Figure 11e,f owing to the fact that it contains more hydroxyl groups (OH) in its molecular structure. These proposed pathways were confirmed by 13C NMR spectroscopy.

4. CONCLUSIONS (1) Plywood panels bonded with G−MMU resin with a dry shear strength of 0.90 MPa could meet the type III plywood requirement of China National Standard GB/T 9846.3-2004 and could be directly used as interior decoration and furniture material in dry conditions. (2) The maximum storage modulus (E′) was found to be 7212 MPa at 137.6 °C and remained constant in the temperature range from about 137 to 150 °C, after which it rapidly dropped to a minimum value of about 2029 MPa. (3) The addition reaction of G with MMU in acidic solution mainly involves the reactions of MMU with p-G, p-G1, and p-G2 to form the important intermediates C-p-G− MMU, C-p-G1−MMU, N-p-G−MMU, and N-p-G1− MMU. (4) The results of 13C NMR and FTIR analyses of G−MMU resin are quite compatible with the results of quantum chemical calculations. (5) Linkages of the types NHCHNH and  CH2OCH2 could be formed by polycondensation reactions in G−MMU resin.

Figure 11. Polycondensation reactions in G−MMU resin.

of 70.96−100.81 ppm correspond to CH(OR)x groups in different reaction products, where R represents different substituted group and x is the number of OR group (detailed in Table 5). 5429



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-871-63863472. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out within the framework of research projects supported by the National Natural Science Foundation of China (No. 31260160), State Forestry Administration 948 Project (2014-4-40), and Key Program of Science Foundation of Educational Department of Yunnan Province (No. 2013Z087).

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Performance, Reaction Mechanism, and Characterization of Glyoxal

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