Overview of the Synthesis, Characterization, and Application of Tannin-Glyoxal Adhesive for Wood-Based Composites

More than a century after its initial synthesis,urea-formaldehyde(UF)resins still have dominant applications as adhesives,paints,and coatings.However,formaldehyde in this industry produces formaldehyde emissions that are dangerous to health.Scientists have spent the last decade replacing formaldehyde and phenol with environmentally friendly substances such as glyoxal and tannin to create bio-based adhesives.This review covers recent advances in synthesizing glyoxal tannin-based resins,especially those made from sustainable raw material substitutes and changes made to synthetic processes to improve mechanical properties.The efficacy of using tannin-glyoxal adhesives in producing wood-based composites has been proven.The glyoxylate reaction forms cross-linked bridges between the aromatic sites of the tannin and glyoxal molecular structures.Glyoxal tannin adhesive with a greater percentage of glyoxal than tannin will produce an adhesive with better characteristics.The gel time reduces as the hardener concentration rises from 7.5%to 15%when glyoxal is used in adhesives.However,excessive amounts of glyoxal will result in a decrease in viscosity values.Glyoxal exhibits faster delivery degradation when it reaches a maximum temperature of approximately 130°C,although it initiates the curing process slightly slower at 110°C.Adding glyoxal to tannin-based adhesives can improve the mechanical properties of composite boards.The wet shear strength of the resulting plywood is increased by 105.4%with the addition of 5-weight percent tannin-based resin with glyoxal as a cross-linker in Soy Protein Adhesive.With glyoxal as a hardener,the panels produced showed good internal bond strengths(>0.35 MPa)and met the international standard specifications for interior-grade panels.

Crosslinking Mechanism of Soy Protein-based Adhesives based on Glyoxal and a Compound of Protein Model

The crosslinking mechanism of glyoxal and asparagine was analyzed,and the relationship between the mechanism and practical performances of soy protein-based adhesives was also discussed.It is shown that when pH=1 and 3,glyoxal reacted with asparagine in the form of major cyclic ether compounds.When pH=5,glyoxal reacted with asparagine in two structural forms of sodium glycollate and cyclic ether compounds.However,amidogens of asparagine were easy to develop protonation under acid conditions.Supplemented by the instability of cyclic ether compounds,the reaction activity and reaction degree between glyoxal and asparagine were relatively small.Under alkaline conditions,glyoxal mainly reacted with asparagine in the form of sodium glycollate.With the increase of pH,the polycondensation was more sufficient and the produced polycondensation products were more stable.The reaction mechanism between glyoxal and asparagine had strong correspondence to the practical performances of the adhesives.Glyoxal solution could develop crosslinking reactions with soy protein under both acid and alkaline conditions.Bonding strength and water resistance of the prepared soy protein-based adhesives were increased significantly.When pH>7,glyoxal had relatively high reaction activity and reaction intensity with soy protein,and the prepared adhesives had high crosslinking density and cohesion strength,showing relatively high bonding strength,water resistance and thermal stability.

Theoretical Studies on Mechanism and Rate Constant of Gas Phase Hydrolysis of Glyoxal Catalyzed by Sulfuric Acid

The gas phase hydration of glyoxal （HCOCHO） in the presence of sulfuric acid （H2SO4） were studied by the high-level quantum chemical calculations with M06-2X and CCSD（T） theoretical methods and the conventional transition state theory （CTST）. The mechanism and rate constant of the five different reaction paths are consid- ered corresponding to HCOCHO＋H2O, HCOCHO＋H2O… H2O, HCOCHO… H2O＋H2O, HCOCHO＋H2O… H2SO4 and HCOCHO… H2O＋H2SOa. Results show that H2SO4 has a strong catalytic ability, which can significantly reduce the energy barrier for the hydration reaction of glyoxal. The energy barrier of hydrolysis of glyoxal in gas phase is lowered to 7.08 kcal/mol from 37.15 kcal/mol relative to pre-reactive complexes at the CCSD（T）/6- 311＋＋G（3df, 3pd）//M06-2X/6-311＋＋G（3df, 3pd） level of theory. The rate constant of the H2SO4 catalyzed hydrolysis of glyoxal is 1.34×10-11 cm3/（molecule.s）, about 1013 higher than that involving catalysis by an equal number of water molecules, and is greater than the reaction rate of glyoxal reaction with OH radicals of 1.10×10-11 cm3/（molecule·s） at the room temperature, indicating that the gas phase hydrolysis of glyoxal of H2SO4 catalyst is feasible and could compete with the reaction glyoxal＋OH under certain atmospheric condi- tions. This study may provide useful information on understanding the mechanistic features of inorganic acid-catalyzed hydration of glyoxal for the formation of oligomer.

Chlorine-radical Induced Oxidation of Glyoxal and Glyoxal-S(IV) Adducts in the Aqueous Phase

A laser flash photolysis study of the reactivity of Cl˙with glyoxal, glyoxal mono- and dibisulfite adducts, 1-hydroxy-2, 2-diol-ethanesulfonate and 1, 2-dihydroxy-1, 2-ethanedi sulfonate in the aqueous phase was carried out. The obtained rate constants can be used for atmospheric modeling.

Oxidation of glyoxal to glyoxylic acid by oxygen over V_2O_5/C catalyst

A novel vanadium oxide catalyst supported on active carbon was prepared by an incipient wetness impregnation method, and the precursor was obtained from oxalic acid aqueous solutions of NH4VO3. The catalyst was applied liquid phase oxidation of glyoxal to glyoxylic acid. It was found that V2O5/C catalyst exhibited obvious activity for glyoxal oxidation. Glyoxylic acid could be obtained without pH regulation during the reaction. By using this catalyst, the conversion of glyoxal and the yield of glyoxalic acid were 29.2% and 13.6%, respectively at 313 K and oxygen flow 0.1 L/rain after reaction for 10 h.

Study on the Soy Protein-Based Adhesive Cross-Linked by Glyoxal

Based on the ESI-MS and ^(13)C-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 fiveor 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 soybased adhesives than acid conditions.

Reaction Mechanism,Synthesis and Characterization of Urea-glyoxal(UG) Resin

The reaction mechanism of glyoxal （G） with urea （U） under weak acid condition was theoretically investigated at PW91/DNP/COSMO of quantum chemistry using density functional theory （DFT） method. The results show that the addition reaction of G with U under the conditions mainly involves the reactions of U with protonated glyoxal （p-G）, protonated 2,2-dihy- droxyacetaldehyde （p-G 1） and protonated bis-hemdiol （p-G2） to form two important carbocation reactive intermediates of C-p-UG and C-p-UG1, and two important hydroxyl compounds of UG and UG1. These compounds play important roles in the formation of UG resin. According to the result of quantum chemical calculation, UG resin was synthesized successfully under weak acid conditions. The UG resin was characterized by matrix assisted laser desorption ionization time of flight mass spectrometry （MALDI-TOF-MS）, ultraviolet and visible spectroscopy （UV-vis）, Fourier transform infrared spectroscopy （FT1R） and nuclear magnetic resonance spectroscopy （13CNMR and 1HNMR）. These instrumental analytical results agree with each other and further confirm the addition reaction pathway of glyoxal with urea proposed by quantum chemical calculation.

Study on Various Compositions of Polyvinyl Alcohol and Starch Blends by Cross-Linking with Glyoxal

The aim of this study is to analyze the various compositions of polyvinyl alcohol (PVA) and starch blends. The blends have been cross-linked with glyoxal to enhance its properties. The hydroxyl groups of PVA and starch react with glyoxal via formation of acetal bonds;hence crosslinking could take place. The cross-linking of glyoxal is observed in various analytical methods such as DSC and FTIR. The cross-linked blends showed better thermal and mechanical properties. Viscosity, tensile shear strength, pencil hardness and ultimate stress were evaluated to estimate the changes due to cross-linking. It was observed that the cross-linking is directly proportional to starch, since the starch hydroxyl groups are easily accessible for reacting. The cross-linked blend showed better cohesion between its chains, thereby increasing glass transition temperature. It was reflected in the subsequent increase in tensile strength properties.

Three competitive dissociation pathways of glyoxalate

Three possible lower barrier dissociation pathways of the deportonated glyoxylic acid in aqueous solution are predicated by means of B3LYP and CCSD（T） （single-point） methods. The calculated results suggest that the formation of formate ion is the energetically most favorable process, and that the decomposition reaction is occurring through nucleophilic attack of negatively charged carboxylic oxygen on the α-carbon. This is in good agreement with the proposed mechanisms of the mass spectrometry experiment.

Glyoxal induced advanced glycation end products formation in chicken meat emulsion instead of oxidation

Advanced glycation end products(AGE) are potential harmful substances formed in the advanced Maillard reaction and increasingly investigated in muscle foods. However, the contribution of oxidation to the AGE formation is controversial. Moreover, reports on glyoxal(GO) induced AGE formation in chicken meat emulsion(CME) are limited. Thus, the effects of GO on emulsifying properties, rheological behavior and AGE formation in CME were investigated. Our findings exhibited that levels of Nε-carboxymethyllysine(CML) and Nε-carboxyethyllysine(CEL) were associated with lipid oxidation but not significantly(P > 0.05). Levels of AGE peaked when GO concentration ranged from 5 mmol/L(CML) to 10 mmol/L(CEL). The droplets’ aggregation associated with the disulfide bond when the concentration of GO was at 0.5–30 mmol/L while non-disulfide bond association occurred at 30–50 mmol/L GO concentration. In conclusion, compared to the effect of oxidation, GO exhibited the main role in the AGE formation of CME. This study will provide theoretical significance for further understanding and controlling the formation of AGE in CME.

Study of the Crosslinking of PVA with Glyoxal in LbL Nanocomposites

Crosslinking is a common practice to improve the barrier properties of polymers. In this study, Montmorillonite (MMT) was used with Polyvinyl alcohol (PVA) to deposit nanocomposite coatings which were crosslinked with glyoxal (Gly) by Layer by Layer (LbL) on a PET substrate. Two crosslinking conditions were studied, under mild condition and with an acidic environment. Mild condition was useful to identify the reversibility steps and the optimum crosslinking times while the acidic environment was essential to investigate the crosslinking mechanism, by determining the permeability for different crosslinking times. PVA and PVA-MMT coatings showed a strong correlation between the permeability coefficients for different crosslinking times and the FTIR results.

环氧树脂增强苯酚−尿素−乙二醛共缩聚树脂的粘接性能研究

为了改善乙二醛基树脂胶黏剂的胶合性能及耐水性,以环氧树脂(EPR)、苯酚(P)、尿素(U)、乙二醛(G)为原料,合成了一种新型EPR−PUG多元共缩聚树脂胶黏剂,探究树脂的基本性能、固化性能、胶合性能,以确定P/U/G摩尔比对树脂胶黏剂体系性能的影响和树脂胶黏剂的较佳合成配方。通过接触角、傅里叶变换红外光谱(FT−IR)、核磁共振碳谱(13C NMR)和X−射线光电子能谱(XPS)方法分析了共缩聚树脂胶黏剂的结构特征和合成机理。结果表明:原料的官能团之间发生了有效交联反应,分子间形成了致密的网络结构,致使树脂胶黏剂体系整体性能得以提升;在P∶U∶G摩尔比为0.1∶1.0∶1.5、反应pH为4~5、反应温度为70~80℃、反应时间3 h、EPR添加量为13%时,所制备的EPR−PUG共缩聚树脂胶黏剂制备的胶合板性能最佳,干/湿强度分别为2.48 MPa、1.33 MPa(冷水)及0.72 MPa(63℃热水),符合国标对II类胶合板的要求,且无甲醛释放。

温度和湿度对乙二醛聚合物加固饱水梓木文物性能和微观结构的影响

乙二醛聚合物加固饱水梓木文物在自然环境或者博物馆存放时,受环境因素如温度和湿度的影响会导致文物发生劣化。本研究采用傅里叶红外光谱(FTIR)、扫描电镜(SEM)等方法研究了乙二醛聚合物脱水加固饱水梓木试样在不同温度和湿度环境中性能与微观结构变化。结果显示,饱水梓木文物材料经乙二醛聚合物脱水加固后,乙二醛聚合物与梓木细胞壁结合紧密,部分填充于梓木细胞内加固梓木细胞壁。试样在25℃和50%RH环境中试验30 d其结构和性能无明显变化。在60℃和50%RH环境中试验30 d,试样内部的乙二醛聚合物逐渐均匀附着于细胞壁且表面平滑,试样横截面断口平整;但试样表面的乙二醛聚合物逐渐消失,暴露出木材本体。在25℃和95%RH环境中乙二醛聚合物吸湿性明显增强,其与水的结合能力高于梓木细胞壁,乙二醛聚合物与细胞壁之间的结合被破坏,失去加固效果,但仍填充于细胞中。最后分析了温湿度对乙二醛聚合物加固饱水梓木性能与结构的影响机制,并提出了试样在吸湿和脱水过程中乙二醛聚合物与梓木细胞壁的结构变化模型。

食源性乙二醛毒性和抑制研究进展

乙二醛(glyoxal,GO)是糖基化过程中产生的活性二羰基,外源性GO来源为食物、饮料和香烟烟雾等,由于其高活性和高毒性而备受关注。在食品的热加工和贮存过程中生成的α-二羰基化合物,进一步增加了人类通过饮食摄入这些化合物以及由此产生的晚期糖基化终末产物的风险。鉴于GO多重和复杂的毒性作用,深入阐明GO的毒性机制和相关的解毒策略是十分必要的。该文概述了食品中GO的分布、危害和毒性机制,对目前GO抑制剂的抑制机理和应用进行综述,为新型抑制剂开发和应用提供参考。

异氰酸酯增强尿素-乙二醛树脂的结构与性能研究

为提升尿素-乙二醛树脂(UG)的耐水性能,本研究在UG树脂中添加高活性的异氰酸酯(MDI)共混制备了异氰酸酯/尿素-乙二醛(MDI-UG)木材胶黏剂。采用傅里叶变换红外光谱(FT-IR)、核磁共振波谱仪(NMR)和X射线光电子能谱(XPS)对其固化结构进行表征,通过差式扫描量热分析(DSC)、动态热机械分析(DMA)和扫描电镜(SEM)对其固化性能进行分析,以MDI-UG混合体系制备胶合板并测定其力学性能。结果表明UG树脂中含有的活泼氢基团与异氰酸酯反应形成新的氨基甲酸酯;与UG树脂相比,MDI-UG树脂的固化温度降低且热稳定性提高;采用扫描电镜下可观测到MDI-UG树脂的固化颗粒更加密集且交联支化程度明显提高;当MDI的添加量为3%且乙二醛与尿素的摩尔比为1.3:1时,所制得胶合板的热水湿状剪切强度达到0.72 MPa,表明MDI的引入显著改善了UG树脂的耐水性,而且充分发挥了UG树脂优异的环保性能和异氰酸酯的高强度特点。

尿素-乙二醛树脂改性大豆蛋白胶黏剂的制备及性能研究

以尿素-乙二醛(UG)树脂和大豆分离蛋白(SPI)为主要原料制备大豆蛋白胶(SUG),并对其固体含量、黏度、表面张力系数、接触角及胶合性能进行测试;利用XPS、FT-IR和^(13)C NMR对合成的SUG的结构进行表征;采用DSC及DMA对SUG的固化性能进行测定。研究结果表明:改性后大豆蛋白胶的固体含量、黏度、表面张力系数及接触角随着SPI添加量的增加而增大,其中黏度的变化最显著;合成大豆蛋白胶中主要含有C—C、C—O—C、C=O、C=N、C=O等官能团;结合^(13)C NMR及FT-IR分析结果,可以得出UG树脂与SPI二者之间产生了交联;当SPI用量为30 g,UG树脂用量为50 g时,制备的大豆蛋白基胶黏剂性能较佳且固化后储能模量较大,为4082 MPa,在160和180℃热压温度下制备的胶合板冷水24 h湿强度分别为1.23和1.48 MPa,热水3 h湿强度分别为1.05和1.22 MPa,远大于国家标准GB/T 9846—2015对II类胶合板的要求,且具备一定的耐沸水性能。

乙二醛法脱水加固饱水榉木的机制研究和温度对性能的影响

乙二醛法是我国目前应用最广的脱水加固木质文物保护方法。采用傅里叶红外光谱(FTIR)、扫描电镜(SEM)和X射线衍射(XRD)等方法研究了乙二醛法脱水加固饱水榉木的试验过程,分析了脱水填充机制以及温度对脱水加固饱水榉木性能的影响规律。结果表明:乙二醛进入饱水榉木内部后交联聚合形成乙二醛聚合物直接作用于细胞内部,与细胞壁结合,支撑细胞加固细胞壁;试样的增重率升高,收缩率降低,色差明显增加。乙二醛法处理的温度升高(40℃相对于20℃)能提高乙二醛在榉木试样内部的扩散和浸入量,增加乙二醛聚合物在榉木内部的含量,提高试样增重率和弯曲强度,降低收缩率,有利于木材细胞形状的恢复和保持,缩短脱水加固工艺处理的时间,对表面活性剂的效果影响较轻。

高效液相色谱法测定生活用纸中甲醛、乙二醛、戊二醛的迁移量

建立了高效液相色谱测定生活用纸中的甲醛、乙二醛、戊二醛含量的方法,优化了色谱分析的参数,考察了不同的衍生条件,包括检测波长的选择、流动相的选择、衍生反应酸的选择、衍生反应试剂的用量、衍生反应溶剂的用量、衍生反应时间对衍生产物的影响,并对衍生物的稳定性进行了考察。本方法中甲醛、乙二醛、戊二醛在5mg/L~25mg/L范围内线性相关系数均大于0.995,方法检出限分别为0.01135mg/dm2、0.02462mg/dm^(2)、0.04841mg/dm^(2),高、中、低3个水平加标回收率在90.6%~99.0%之间,相对标准偏差范围为0.36%~2.88%,可满足生活用纸中甲醛、乙二醛、戊二醛检测要求。

Investigating the impact of Glyoxal retrieval from MAX-DOAS observations during haze and non-haze conditions in Beijing

This study presents the Multi Axis Differential Optical Absorption Spectroscopy(MAXDOAS) measurements for Glyoxal(CHOCHO) in Beijing, China(39.95°N, 116.32°E). CHOCHO is the smallest compound of di-carbonyl group. As a primary sink of CHOCHO, its photolysis with NOx(oxides of nitrogen) results in the production of tropospheric ozone. Therefore,the focus of CHOCHO DOAS measurements is increasing in trend. We did the measurements from 09 May 2017 to 09 September 2017. The study was conducted to compare different retrieval settings in order to reveal best DOAS fit settings for CHOCHO;furthermore, effect of haze and non-haze days on CHOCHO concentration was examined.The root mean square of residual and Differential Slant Column density(dSCD) error was reduced when measurements were done with lower wavelength limit around 432–438 nm and upper intervals around 455–460 nm. Thus, lower wavelength intervals around432–438 nm and upper intervals around 457–460 nm were best for the retrieval of dSCDs for CHOCHO. Meteorological conditions like haze or non-haze days did not have significant effect on DOAS fit parameters. The CHOCHO vertical column densities range from 1.33 E +14 to 9.77 E + 14 molecules/cm2 during the study period with average of 6.16 E +14 molecules/cm2. The results indicated that during haze days CHOCHO concentration was higher because of lower rate of photolysis and atmospheric oxidation potential. Our results did not show any significant weekend effect on CHOCHO atmospheric concentration.

Vertical distribution and temporal evolution of formaldehyde and glyoxal derived from MAX-DOAS observations:The indicative role of VOC sources

Formaldehyde(HCHO)and glyoxal(CHOCHO)are important oxidization intermediates of most volatile organic compounds(VOCs),but their vertical evolution in urban areas is not well understood.Vertical profiles of HCHO,CHOCHO,and nitrogen dioxide(NO_(2))were retrieved from ground-based Multi-Axis Differential Optical Absorption Spectroscopy(MAXDOAS)observations in Hefei,China.HCHO and CHOCHO vertical profiles prefer to occur at higher altitudes compared to NO_(2),which might be caused by the photochemistry-oxidation of longer-lived VOCs at higher altitudes.Monthly means of HCHO concentrations were higher in summer,while enhanced amounts of NO_(2)were mainly observed in winter.CHOCHO exhibited a hump-like seasonal variation,with higher monthly-averaged values not only occurred in warm months(July-August)but also in cold months(November-December).Peak values mainly occurred during noon for HCHO but emerged in the morning for CHOCHO and NO_(2),suggesting that HCHO is stronger link to photochemistry than CHOCHO.We further use the glyoxal to formaldehyde ratio(GFR)to investigate the VOC sources at different altitudes.The lowest GFR value is almost found in the altitude from 0.2 to 0.4 km,and then rises rapidly as the altitude increases.The GFR results indicate that the largest contributor of the precursor VOC is biogenic VOCs at lower altitudes,while at higher altitudes is anthropogenic VOCs.Our findings provide a lot more insight into VOC sources at vertical direction,but more verification is recommended to be done in the future.