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.

Preparation of liquefied wood-based resins and their application in molding material

To investigate value in use of liquefied wood-based resin applications in molding material, Chinese fir （Cunninghamia lanceolata） and poplar （Populus tomentosa） wood meal were liquefied in phenol. The reactant was co-condensed with formaldehyde to obtain liquefied wood-based resin. For this paper, we investigated the characterization of the resin and its application in molding material. The result shows that the basic properties of liquefied wood-based resin were satisfactory; the bonding strength of plywood prepared with liquefied Chinese fir and liquefied poplar resin can reach 1.54 and 1.00 MPa, respectively. The compression strengths of the molding material prepared with two kinds of liquefied wood resin were 73.01 and 73.58 MPa, almost the same as that of PF resin molding material. The limiting volume swelling of molding material made with liquefied Chinese resin and liquefied poplar resin were 8.5% and 8.3%, thickness swelling rates of water absorption were 3.3% and 4.2%, and the maximum weight ratios of water absorption were 25.9% and 26.2%, respectively. The soil burial test result shows that the weight loss rate of the molding materials made with liquefied Chinese resin and liquefied poplar resin were 8.3% and 9.1% and that of the PF resin molding material was 7.9%. After the soil internment test, the reduction ratio of compression strength of the two kinds of molding material achieved 16.9% and 17.7%, while that of the PF resin molding material was 15.4%. The test results of wood fungi inoculation on the three surfaces of the molding material indicate the breeding rate of molding material prepared with liquefied Chinese resin and liquefied poplar resin were at level 4 and that of PF resin molding material was at level 1 of the ISO standard.

A design and application of compound multi-functional sensor in wood-based panel processing

A compound multi-functional sensor was designed by the study on the on-line testing technology of wood-based panels, and its properties of shape, functions, size, resistance to special environment were studied in details. The operational principles of different sensors, technical flow of manufacturing, development of software systems of special functions, and the assessments of technical specification were also be introduced. This sensor adopted many new technologies, such as the applications of piezoresistant effect and heat sensitive effect can effectively measure the pressure and temperature, digital signal processing technology was used to extract and treat signals, and resist interference, encapsulation technology was used to keep the normal run of sensor under a harsh environment. Thus, the on-line compound multi-functional temperature/pressure sensor can be applied better to supervise the production of wood-based panels. All technical specifications of the compound multi-functional sensor were tested and the results met the requirements of the equipments.

Recent development and challenges in enhancing fire performance on wood and wood-based composites:A 10-year review from 2012 to 2021

Due to their durability,versatility,and aesthetic value,wood and wood-based composites are widely used as building materials.The fact that these materials are flammable,however,raises a major worry since they might cause fire hazards and significant loss of life and property.The article investigates the variables that affect fire performance as well as the various fire-retardant treatments and their mechanisms.The current developments and challenges in improving the fire performance of wood and wood-based composites treated with fire-retardant materials are summarized in this paper.Nanoparticles,organic chemicals,and densification are some recent developments in fire-retardant treatments that are also emphasized.Key points from the review are summarized,along with potential areas for further research and development.

Improvement of controlled pollination techniques of poplar

Over a number of years, in order to find substitutes for two traditional poplar pollination techniques： outdoor bridging trees and indoor cutting with water culture, research into two new pollination methods of uprooted outdoor seed trees and outdoor cutting branches was carried out. The advantages of two new and improved techniques were of efficiency, economy, safety and ease of operation. The methods can be applied in hybridization and breeding of poplar and other easy-to-root trees.

Studies on the sound absorption and transmission loss performances of wood-based, natural and wastematerials

The sound absorption and sound transmission loss performances of the natural woods,hard and soft processed woods with attachment of the various natural orwastematerials were investigated in the present study using impedance tube with American Society for Testing Material(ASTM)standards.The sound absorption performances of all natural and all hard processed woods were very poor.It was found that filter mat made by the coconut fibre was the best material for sound absorption improvement of the hard processed woods.The sound absorption performance of the soft processed wood(cork)was better than all natural and all hard processed woods.Among all tested woods,it was found that the cork with attached tea bag made by corn fibre is the best selection for sound absorption application.The transmission loss performances of all natural woods were good.The effects of various materials on the transmission loss performances of all hard processed woods were not very significant.For cork with and without attachment of various materials,their transmission loss performances were not as good as the hard processed woods.Among all tested woods,it was found that Pterocarpus soyauxii and Quercus spp.(natural woods)are the best woods to be used in those applications when prevention of sound transmission is needed.It is recommended that cork is the best wood to be used in those applications where sound absorption and prevention of sound transmission are needed at the same time due to its good sound absorption performance while its ability on prevention of the sound transmission is also acceptable.

Mechanical Properties of a Eucalyptus-Based Oriented Oblique Strand Lumber for Structural Applications

Wood and wood-based composite materials have gained increasing attention in the sustainable building industry because of their renewability and environmental friendliness.Oriented oblique strand lumber(Eucalyptus Strand Wood,ESWood),which is manufactured from fast-growing small diameter eucalyptus wood(Eucalyptus urophylla×E.grandis),is introduced in this paper.Small clear specimen tests were conducted to determine the mechanical properties of ESWood material while full-scale component tests were performed to observe the structural performance of ESWood beams.A comparison of mechanical properties of ESWood with other wood/bamboo-based materials is then reported.From the results presented herein,it appears that the strength and stiffness properties of ESWood are affected by grain directionality and glued layers.However,it still has preferable mechanical properties as a building material,which is comparable or superior to those of other engineered wood/bamboo-based products(e.g.,Sitka spruce,LVL,OSL,Glulam,and Glubam).Furthermore,results from full-scale component tests show the stable mechanical performance of beams made by ESWood.This study makes a significant contribution to a potential utilization of fast-growing eucalyptus for general use in construction,and the presented mechanical tests results can serve as a fundamental data for more applications of ESWood in practical engineering.

Study on the Preparation Process Optimization of Plywood Based on a Full Biomass Tannin-Sucrose Wood Adhesive

Biomass adhesive is conducive to decreasing the dependence of the wood adhesive industry on synthetic resin based on fossil resources and improving the market competitiveness of adhesives.It is also a critical breakthrough to realize the goal of carbon peaking and carbon neutrality in the wood industry.In this study,a full biomass wood adhesive composed of tannin and sucrose was developed and applied successfully to the preparation of ply-wood.The preparation technique of plywood was optimized,and the chemical structure,curing performance,crystallization property and thermal performance of the adhesive were investigated.Results showed that:(1)hot-pressing temperature played a decisive role in the performances of tannin-sucrose composite adhesives and it also had a very significant influence on the water resistance of plywood.(2)The preparation of tannin-sucrose composite adhesive was a process in which sucrose was transformed into furan aldehydes and then made cross-linking reaction with tannin.These composite adhesives could only get good bonding performances when the curing temperature was above 210℃.(3)The optimal plywood preparation technique was:hot-pressing tem-perature of 220℃,hot-pressing time of 1.2 min/mm,m(tannin):m(sucrose)of 60:40,and adhesive loading of 160 g/m^(2).The wet bonding strength in boiling water of the prepared plywood was 0.83 MPa,meeting the strength requirements of Type-I plywood in the standard of GB/T 17657-2013.(4)The curing temperature of tannin-sucrose composite adhesive was further decreased by lowering the temperature during the transformation of sucrose into 5-HMF,which was a key in subsequent research.

Current Research and Development on Deformation and Fixation of Bended Wood and Wood-based Composites

There are many advantages of bended wood, such as good-looking shape, simple process and low cost. The product, however, is easy to get recovery, which is urgent to be dealt with. This paper concludes the features of deformation and recovery of bended wood and wood-based composites and summarizes four treating methods to keep dimensional stability. Compared to bended solid wood, some elementary perspectives on the research of bended wood-based composites are presented. The purpose of this paper is to suggest： 1） to investigate the optimum heating time and temperature that bended wood and wood-based composites need from the formation of deformation to the recovery and to the permanent fixation, according to its changes of dimensional stability such as curvature radius; 2） to measure the composites comprised of wood and adhesives on the changes of stress relaxation, dynamic viscoelasticity and crystallization field; 3） to quantitatively analyze changes of the major components in wood cell wall polymers as well as the composites under heat/steam treatment and untreated conditions. It will be helpful for subsequent research to clarify on the mechanisms of permanent fixation of bended wood and also contribute to that of wood-based composites.

Overview of Wood-based Panel Industry in China

The production of wood-based panel was presented firstly in this paper. Some features of China’s wood-based panel industry were reviewed, including production bases, raw materials and main markets of wood-based panel. In addition, the trade flow of wood-based panel was described in the end.

Review on Preservative Treatment and Decay Resistance of Wood-based Composites

To extend the service life and explore the use of wood-based composites, it is quite necessary toimprove the decay resistance of wood-based composites through preservative treatments. Researchers havestudied the methods of preservative treatment and the decay resistance of treated wood-based composites.In this paper, the categories of wood-based composites are briefly summarized and the previous and presentinvestigations on the preservative treatment and decay resistance of different categories of wood-basedcomposites are introduced. Some problems are also put forward and suggestions are given for future studies.

Research Status and Prospect of Binderless Board

The latest research progress of binderless board was summarized in the view of the existing binderless technology at home and abroad,and according to the bonding mechanism of binderless board,the problems an d questions in its manufacture and application at present were summed up,at last,the effect of bindless board forming process on the development of artificial board industry in China was prospected.

Review on Volatile Organic Compounds Emission from Wood Composites

The problem of indoor air quality (IAQ) is mainly caused by the volatile organic compounds(VOC) emission from the wood-based composites. As a material for decoration, furniture manufacturing orbuilding, wood-based composite is one of the sources of VOC emissions. Most of them are formaldehyde,terpene, ketone and benzene. The paper reviews on VOC emission of wood-based composites at home andabroad, including the source of the VOC, its impacts on IAQ, its emission during processing and using, theusual sampling and analyse methods of VOC in different conditions. Meanwhile, main problems existed inthe past researches are summarized and some suggestions are put forward.