The two authors contributed equally to this work
Based on the ESI-MS and 13C-NMR analysis of the forms of glyoxal in acidic and alkaline solutions, the soy-based adhesive cross-linked by glyoxal was prepared in this work. The results showed that glyoxal existed in water in different forms at different pH levels. Under alkaline conditions, glyoxal transformed to glycolate through the intramolecular disproportionation reaction. Under acidic conditions, although some of glyoxal transformed to glycolate as what happened under alkaline conditions, most of glyoxal molecules existed in the form of five- or six-membered cyclic ether structure. No ethylene tetraol or free aldehyde group was actually detected under these conditions. Although glyoxal reacted with soy protein under both acidic and alkaline conditions, alkaline conditions were more favorable for the improvement of mechanical performance and water resistance of soy-based adhesives than acid conditions.
The development of soy protein-based adhesives has been a hot topic in wood industry today. Some cross-linkers are used to prepare soy-based adhesive with acceptable mechanical performance and water resistance [
Compared with formaldehyde, glyoxal is relatively nontoxic and nonvolatile [
As the simplest dialdehyde, glyoxal has no α-H structure. It exists in water in different forms and even shows no aldehyde group [
Soy flour with protein content of 53.4% was from Yuxin Soybean Protein Co., Ltd., China. Glyoxal solution with solid content 40%, NaOH, NaHSO3 and other chemical reagents in this work were all analytic grade and from Sinopharm Chemical Reagent Co., Ltd., P.R. China. Poplar veneers with a thickness of 1.5 mm and moisture content 8–10% were purchased from Qunyou Wood Co., Ltd., China for the preparation of plywood.
The 30% diluted glyoxal was adjusted to pH = 2–3 and pH = 9–10, respectively, with either 10% hydrochloric acid or 10% sodium hydroxide solution. Then these glyoxal solutions were charged to different flasks with magnetic stirrer bars separately and kept at 75–80°C for 1 h under the designed pH levels.
The glyoxal solution samples at different pH levels were dissolved in chloroform to get a solution with a concentration of 10 μl/mL (10 μl sample and 1 mL chloroform). The solution was then injected into the ion trap mass spectrometer with ESI via a syringe at a flow rate of 5 μg/s. Spectra were recorded in positive mode with ion energy of 0.3 eV on a Waters Xevo TQ-S instrument and the scan range was 0–1000 Da.
The glyoxal solution samples at different pH levels for 13C-NMR analysis were prepared by mixing 300 μl glyoxal liquid solution sample with 100 μl dimethyl sulfoxide-d6 (DMSO-d6). Liquid 13C-NMR (zgig) spectra were generated using a Bruker Avance spectrometer with a frequency of 600 MHz. To achieve a sufficient signal-to-noise ratio, inverse-gated proton decoupling method was applied. The spectra were obtained at 39062.5 Hz for a number of transients of 800–1200. All the analyses were run with a relaxation delay of 6 s and the chemical shifts were accurate to 1 ppm.
Eighty parts of defatted soy flour, 300 parts of water and 1.6 parts of NaHSO3 were sequently charged into a flask equipped with a condenser, thermometer and a magnetic stirrer bar. The mixture was kept at 50°C for 1h to get a soy-based adhesive sample. Then 9.6 parts of glyoxal solution were added to the flask. After this, the mixture was divided into four parts and the pH of each part were adjusted to 1, 3, 11 and 13, respectively. Corresponded samples with the name of SPG1, SPG3, SPG11 and SPG13 were obtained.
Four soy protein-based adhesives modified with glyoxal solution, that is, SPG1, SPG3, SPG11 and SPG13, were used for the preparation of three-layer plywood samples with the dimensions of 400 mm × 400 mm × 4 mm. Soy protein-based adhesives were applied to both sides of core veneers with a resin loading rate of 300 g/m2. Two surface veneers without adhesives were added to both sides of the core veneers. The three-layer plywood assemblies were left at room temperature for 15–20 min before loaded into a XLB type single-opening hot press. The plywood assemblies were pressed under 1.5 MPa at 160°C for 3.5 min.
Before being cut into shear strength specimens of dimensions of 100 mm × 25 mm, plywood samples were conditioned at the chamber of 20 ± 2°C with relative humidity of 65 ± 5% in the laboratory for 1 day. The bonded area of each specimen was 25 mm × 25 mm (
The ESI-MS spectra of glyoxal solution prepared at pH = 9–10 and pH = 2–3 are given in
Experimental (Da) |
Structures |
---|---|
121 | CH2OH-COONa |
219 | 2 CH2OH-COONa |
317 | 3 CH2OH-COONa |
415 | 4 CH2OH-COONa |
513 | 5 CH2OH-COONa |
611 | 6 CH2OH-COONa |
709 | 7 CH2OH-COONa |
807 | 8 CH2OH-COONa |
98*n+23(n = 1,2,3…15) | n CH2OH-COONa |
An obvious regularity was observed from the ESI-MS spectra for the glyoxal in alkaline solution, as in
The possible reaction mechanism for the transformation of glyoxal under alkaline conditions is shown in
Both of the two reactions respect the unimolecular elimination of conjugate base (Sn1) reaction rules. CH2OH-COOH is the main structure following the path of Reaction 1. It was also the main types of the main ion peaks observed from the ESI-MS spectra of glyoxal solution under alkaline conditions (
Being different from Reaction 1, for the transformation of glyoxal in acidic solution, it was H2O rather than OH- that attacked the carbon atom of carbonyl groups to result the disproportionation reaction, as shown in Reaction 3. Although H2O was not a common nucleophile, the reaction between H2O and carbonyl groups of one glyoxal molecule was possible when the eletrophilicity of the carbon atom of carbonyl groups increased, caused by the combination between the oxygen atom of the glyoxal molecule and H+ in the acidic solution. However, as shown from
However, no obvious ion peak at around 117 Da from ethylene tetraol was detected in
An obvious regularity was also observed from
13C-NMR was used to analyze the possible chemical structure of glyoxal solution in this study. Considering many of the possible isomers, 13C-NMR will give more information on the chemical structure than ESI-MS [
The possible chemical shifts of carbon atoms from different structures calculated by the software of ACD/Labs are given in
In
Besides the structure of five-membered rings, the six-membered rings existed in the solution, too. If there were only structures of five-membered rings in the solution system, the molecular weight of resulted combination products would not exceed 210 Da. It was not what had been observed from its ESI-MS results. Therefore, the absorption at 91.46 ppm, 92.15 ppm, 92.19 ppm, 93.59 ppm in
The performances of soy-based adhesives modified by glyoxal solution at different pH levels are given in
Soy-based adhesive | Dry strength/MPa | Wet strength (60°C)/MPa |
---|---|---|
SP | 0.46 ± 0.06 | -- |
SPG1 | 0.70 ± 0.08 | 0.57 ± 0.01 |
SPG3 | 0.94 ± 0.09 | 0.57 ± 0.01 |
SPG11 | 1.10 ± 0.16 | 0.63 ± 0.03 |
SPG13 | 1.17 ± 0.16 | 0.85 ± 0.01 |
The molecular structure of soy protein is very complex. It was too difficult to see anything whatsoever of the reaction product of soy protein and glyoxal by NMR or ESI-MS. The former results of ESI-MS and 13C-NMR indicated that glyoxal molecules transformed to some different derivatives in water at different pH levels. The reaction mechanism of the soy-based adhesive with glyoxal solution under alkaline or acidic conditions would then be different, too, which might be responsible to the different performances of soy-based adhesive modified by glyoxal solution. It was presumed that soy protein owned some reactive groups, such as -NH2, -CONH, -COOH and -OH groups. CH2OHCOOH was the main form of glyoxal in alkaline solution. In theory, its -COOH and -OH groups could react with soy protein. The possible reaction mechanism is shown in
Ethylene tetraol and their products from the etherification reaction were the main forms of glyoxal in acidic solution. The etherification reaction was reversible. The possible reaction mechanism between soy protein and glyoxal solution under acidic conditions is shown in
However, under acidic conditions, the glyoxal would react mainly with the amino groups of protein and the reaction between glyoxal and carboxyl groups of protein would be very difficult. At the same time, under acidic conditions, the amino groups of soy protein had the intendancy to combine with proton. It led the reactivity of soy protein to decrease further [
Based on the ESI-MS and 13C-NMR analysis on the forms of glyoxal in acidic and alkaline solutions, the soy-based adhesives cross-linked by glyoxal were prepared in this work. Some conclusions could be drawn. Glyoxal existed in water in different forms at different pH levels. Under alkaline conditions, glyoxal transformed to glycolate through the intramolecular disproportionation reaction, which reduced the activity of aldehyde group of glyoxal. Under acidic conditions, although some of glyoxal transformed to glycolate as what happened under alkaline conditions, most of glyoxal molecules existed in the solution in the form of five- or six-membered cyclic ether structure. The six-membered cyclic ether consisted of C,C'-p-dihydroxyl intercyclization and triethylenetetraol intercyclization. No ethylene tetraol or free aldehyde group was actually detected. Although glyoxal reacted with soy protein under both acidic and alkaline conditions, alkaline conditions were more favorable for the improvement of mechanical performance and water resistance of soy-based adhesives than acid conditions, which was due to the adhesives had gotten high crosslinking density and cohesive strength.