Effects of heat pretreatment of starch on graft copolymerization reaction and performance of resulting starch-based wood adhesive

Optimization of Starch-Tannin Adhesives for Solid Wood Gluing

Bio-based solutions for solid timber gluing have always been a very sensitive topic in wood technology. In this work, we optimize the gluing conditions of a starch-tannin formulation, which allows high performance in dry conditions and resistance to water dipping for 3 h, allowing for the D2 classification to be reached according to EN 204. It was observed that the starch-tannin formulations enhanced their performance by increasing the heating temperature, achieving satisfactory results at 140 °C for 13 min. The proportion of polyphenols in the mixture enhances the water resistance but is only tolerated until 20-30%. In particular, the addition of 10% tannin-hexamine enhances the water-resistant properties of starch for both quebracho and chestnut extract. The application of the jet of cold atmospheric plasma allows for good results with more viscous formulations, increasing their penetration in wood. Solid-state 13C-NMR analysis was also performed, and the spectroscopic information suggests establishing a coordination complex between starch and tannin.

Development and Characterization of Bio-Based Formaldehyde Free Sucrose-Based Adhesive for Fabrication of Plywood

In order to solve the problem of excessive consumption of petrochemical resources and the harm of free formaldehyde release to human health, biomass raw materials, such as sucrose (S) and ammonium dihydrogen phosphate (ADP) can be chemically condensed in a simple route under acidic conditions to produce a formaldehyde free wood adhesive (S-ADP), characterized by good storage stability and water resistance, and higher wet shear strength with respect to petroleum based phenolic resin adhesive. The dry and boiling shear strength of the plywood based on S-ADP adhesive are as high as 1.05 MPa and 1.19 MPa, respectively. Moreover, is Modulus of Elasticity (MOE) is as high as 4910 MPa. Interestingly, the plywood based on the developed S-ADP adhesive exhibited good flame retardancy. After burning for 90 s, its shape remains unchanged. Meanwhile, it can be concluded from thermomechanical analysis (TMA) and thermogravimetric analysis (TGA) that the S-ADP acquired excellent modulus of elasticity (MOE) and good thermal stability. It is thus thought promisingly that the use of S-ADP adhesive as a substitute for PF resin adhesive seems feasible in the near future.

Facile preparation of high-performance plywood adhesive from gelatinized corn starch crosslinked with ammonium dihydrogen phosphate

A flame retardant high-performance gelatinized starch (GS)-ammonium dihydrogen phosphate (ADP) wood adhesive, named GS-ADP adhesive was prepared by condensation of GS and ADP under acidic condition. The preparation process of GS-ADP adhesive is very simple by mixing and stirring GS and ADP evenly at room temperature. The results revealed that the GS-ADP adhesive has good storage stability and water resistance, and its wet shear strength is much higher than that of phenolic resin (PF) adhesive. Markedly, the cone calorimeter test results show that G-ADP adhesive has good flame retardancy, and the plywood based on GS-ADP adhesive has good flame retardancy. Meanwhile, it can be seen from dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA) that GS-ADP has excellent modulus of elasticity (MOE), high glass transition temperature (Tg) and good thermal stability. The findings suggest that GS-ADP could be a viable substitute for PF resin in structural wood fabrication.

Preparation and characterization of the bonding performance of a starch-based water resistance adhesive by Schiff base reaction

Starch is one of the important raw materials for the preparation of biomass adhesives for its good viscosity and low-cost properties. However, the drawbacks of poor water resistance and bonding performance seriously restrict its application in the wood industry. To resolve those problems, an environment-friendly renewable, and high water resistance starch-based adhesive (OSTH) was prepared with oxidized starch and hexanediamine by Schiff base reaction. In order to optimize the adhesive preparation process, the effect of different oxidation times and oxidant addition on the mechanical performance of plywood were investigated. In addition, the curing behavior characteristics, thermomechanical properties, and thermal stability of the OSTH adhesives were analyzed by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TG). Fourier-transform infrared (FTIR) spectrometry and Liquid Chromatography-Mass Spectrometry (LC-MS) were used to explain the reaction mechanisms involved. The results show this adhesive has an excellent bonding performance at the oxidation time of 12 h with 11 % (w/w, dry starch basis) NaIO4 as an oxidant. The dry shear strength, 24-hour cold water, and 3-hour hot water (63 °C) soaking shear strength of the plywood bonded with this resin were respectively 1.87 MPa, 0.96 MPa, and 0.91 MPa, which satisfied the standard requirement of GB/T 9846-2015 (≥0.7 MPa). Thus, this study provided a potential strategy to prepare starch-based wood adhesives with good bonding performance and water resistance.

Facile Synthesis of Formaldehyde-Free Bio-Based Thermoset Resins for Fabrication of Highly Efficient Foams

Bio-based biodegradable foams were formulated from a crosslinkable network structure combining starch, furfuryl alcohol, glyoxal, and condensed tannin in the presence of p-toluenesulfonic acid (pTSA) and azodicarbonamide (AC) as a foaming agent. More importantly, the reinforcement of gelatinized starch-furanic foam using tannin, originating from forestry, resulted in an excellent compressive strength and lower pulverization ratio. Moreover, the addition of tannin guaranteed a low thermal conductivity and moderate flame retardancy. Fourier transform infrared (FTIR) spectroscopy approved the successful polycondensation of these condensing agents under the employed acidic conditions. Moreover, the catalytic effect of pTSA on the foaming agent induced liberation of gases, which are necessary for foam formation during crosslinking. Scanning electron microscopy (SEM) showed foam formation comprising closed cells with uniform cell distribution and appropriate apparent density. Meanwhile, the novel foam exhibited biodegradation under the action of Penicillium sp., as identified by the damage of cell walls of this foam over a period of 30 days.

Relevance between Cassava Starch Liquefied by Phenol and Modification of Phenol-Formaldehyde Resin Wood Adhesive

A novel type of phenol-formaldehyde (PF) resin was prepared by utilizing the liquefaction products liquefied by phenol under acidic conditions and then reacted with formaldehyde under alkaline conditions. The relationship between the liquefaction behavior of cassava starch and the properties of modified PF resin wood adhesive was studied. The effects of the liquid–solid ratio of phenol to cassava starch, sulfuric acid usage, and liquefaction time on the liquefaction residue rate and relative crystallinity of cassava starch were determined. The results showed that the bonding strength of modified PF resin decreased gradually with the decrease of the liquid–solid ratio. It was a great surprise that bonding strength still met the requirement of the national standard of 0.7 MPa when the liquid–solid ratio was 1.0. The detailed contents were analyzed through FT-IR, SEM, and XRD. In terms of the utilization of bio-materials for liquefaction to synthesize wood adhesive, cassava starch may be superior to the others.

Effects of various durations of enzyme hydrolysis on properties of starch-based wood adhesive

Improving the performance of wood adhesive is important for the development of the veneer industry. This work investigated the effects of various durations of enzymatic hydrolysis to improve and enhance the properties of starch-based wood adhesive (SWA). The results showed that moderate enzymatic hydrolysis for 2 h of starch molecule could improve the properties of SWA. The bonding strength of SWA was increased from 2.72 MPa (0 h) to 6.87 MPa (2 h) in the dry state and from 0.87 MPa (0 h) to 2.69 MPa (2 h) in the wet state. A significant decrease in the viscosity of SWA was also observed after 2 h hydrolysis of starch molecules, which allowed smooth spreading and penetration of adhesive through the wood surface. Meanwhile, the dynamic mechanical analysis and scanning electron microscopy showed that SWA with 2 h enzymatic hydrolysis exhibited better elastic deformation and smooth surfaces compared with SWA with un-hydrolysis starch. This study provides important information regarding the possible applications of SWA in the wood industry and presents a potential alternative to less environmentally friendly formaldehyde-based wood adhesives.

Polyurethane Wood Adhesives Prepared from Modified Polysaccharides

This study investigated the performance of polyurethane adhesives prepared with various combinations of wheat starch that had been modified by isophorone diisocyanate (MS), two polyol types (1,3-propanediol (PD) and glycerol (Gly)), native wheat starch (NS), and 4,4′-diphenylmethane diisocyanate (pMDI) at a NCO:OH weight ratio of 1:1. Two more adhesives were also synthesized with NS, PD, or Gly and pMDI blends and served as controls. The thermal behavior of the adhesives before and after the curing process, as well as their rheological performance and lap shear strength, were analyzed. Differential scanning calorimetry (DSC) showed a reduction in curing temperature and heat by adding MS. The thermal stability of the cured adhesives was slightly increased by MS addition. The viscosity of the adhesives that contained MS substantially increased at a linear ascendant ramp of shear, while the controls exhibited relatively low viscosity during the whole shear rate spectrum from 0.1 to 100 s⁻¹. The tensile shear strength of wood veneers was also significantly increased by the incorporation of MS under both dry and wet measuring conditions. The maximum dry shear strength was obtained for the adhesive with Gly polyol and a higher content of MS and was comparable to the control adhesive with pMDI.

Biomimetic water-in-oil water/pMDI emulsion as an excellent ecofriendly adhesive for bonding wood-based composites

In this study, a novel and environmental strategy inspired by mussels is reported for the construction of a high-performance water-in-oil water/pMDI adhesive with strongly adherent catechol groups. It is found that the design of the biomimetic water-in-oil emulsion significantly increases the apparent viscosity and storage stability of the adhesive, which in turn influences the handling and bonding performance in practical application. Additionally, although the biomimetic emulsion design consumes part of active isocyanate groups in the pMDI, the contained catechol can serve as a reactive platform to induce secondary crosslinking interactions with the wood substrate to further improve the mechanical and adhesion performances of the modified pMDI adhesives. Consequently, compared to the pristine pMDI sample, the wet shear strength of the biomimetic water-in-oil water/pMDI adhesive is increased by 129.7 %, exhibiting that the obvious optimization of adhesion and water resistance properties. Overall, our findings provide new insights into exploiting novel and superior wood adhesives, and the constructed high-performance adhesive presents potential applications for sustainable wood products.

Preparation, characterization and properties of starch-based adhesive for wood-based panels

A new biodegradable, renewable, and environmentally friendly starch-based adhesive for wood-based panels was synthesized. The synthesis was conducted by grafting polymerization of vinyl acetate (VAC) monomer onto corn starch and crosslinking polymerization with N-methylol acrylamide (NMA). Compared with the traditional starch-based wood adhesive, the water resistance of starch-based adhesive with NMA (SWA-N) was greatly improved to more than 1 MPa; this exceeds the Chinese standard by 40%. The results from various analyses, including particle size, contact angle, thermogravimetric analysis (TGA), Fourier-transform infrared (FTIR), confocal Raman microscopy (CRM) and ¹³C-CPMAS, indicated that such improved performance is due to increased crosslinking density and formation of complex network structure. Such complex network structure was found to inhibit excessive expansion of the adhesive during high temperature pressing and water absorption. As a result, the internal structure of the adhesive remained intact when subjected to hot pressing and placed in wet conditions.

Synthesis and characterization of starch-g-poly(vinyl acetate-co-butyl acrylate) bio-based adhesive for wood application

Enhancing the performance of wood adhesive is important for its industrial applications. Accordingly, we designed and demonstrated the use of two co-monomers vinyl acetate (VAc) and butyl acrylate (BA) for promoting the graft copolymerization while improving the bonding performance of wood adhesive. The results showed that the addition of co-monomers in the ratio of VAc/BA 6:4 (v/v, volume basis of VAc) could improve the shear strength to 6.68MPa and 3.32MPa in dry and wet states, respectively. ¹H-nuclear magnetic resonance (¹H NMR) and fourier transform infrared spectroscopy (FT-IR) analysis revealed successful graft copolymerization reaction while the morphologies were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, the grafting reaction and thermal stabilities of wood adhesive were analyzed by X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA). The results showed that the properties of wood adhesive could improve dramatically by using two co-monomers VAc and BA during the graft copolymerization reaction.

Development of sustainable bio-adhesives for engineered wood panels – A Review

Stricter legislation on formaldehyde emissions as well as growing consumer interest in sustainable raw materials and products are the main driving factors behind research on bio-based adhesives, as alternatives to amino-based ones, for wood panels.