Adhesives have played and will continue to play an important role in the efficient utilization of wood resources. Early Egyptians used adhesives to attach rare veneers to wood furniture. Today, adhesives play a vital role in allowing almost all types and sizes of wood to be converted to functional products. For centuries, wood was bonded using biobased adhesives. In the 20th century, synthetic adhesives gradually took over because they were typically more effective and cost less. Refinement of these adhesives led to a great expansion in the bonded wood products industry by growing existing markets and allowing development of new
Although it is well known that wood can absorb liquids, the full impact of this phenomenon on resulting adhesive cure and performance is not well understood. In a related soy adhesive study, aspects of this phenomenon were not fully anticipated and resulted in a significant impact on adhesive bond quality. In essence, the wood being bonded acted like chromatography media, filtering relatively smaller molecules from the surrounding adhesive matrix and altering the local composition of the adhesive material. Through the use of soy adhesives containing glycerin, the infiltration of glycerin analogs into wood cell walls has been determined and related with the impact on bondline and wood properties. The impacts of infiltration and chromatographic separation on the cured adhesive are also discussed.Abstract
Urea-formaldehyde (UF) resin is the predominant adhesive used in the manufacture of medium-density fiberboard (MDF). While it is well known that this resin is susceptible to moisture and hydrolysis, the present study determined that MDF panels lose a significant quantity of UF resin components in panel water soaking tests. Panel mass and %N losses indicate that MDF panels lose 48 to 66 percent of resin components during water soaking tests. Chemical analysis revealed that the labile extractable resin components included urea and monomeric and low-molecular-weight UF oligomers. Panel formaldehyde emissions were linked to the proportions of extractable UF resin oligomers. The results have implications for panel performance and the extent of resin cure achievable in manufacturing MDF.Abstract
The disposal problem of bark residues and the shortage of wood raw material supply in some regions make the use of bark an attractive option for the industry. In this study, medium-density particleboards were manufactured from mixed black spruce bark and spruce-pine-fir wood particles at a weight ratio of 50/50. Different adhesive systems were used as binders for the bark-based panels, including commercial phenol-formaldehyde (PF), urea-formaldehyde (UF), polymeric methylene diphenyl diisocyanate (pMDI) resins, and a laboratory-synthesized lignin-PF resin containing 30 wt% lignin substitution for phenol. The objective was to investigate the suitability of utilizing bark residues in the manufacture of particleboards for the floor underlayment (PBU) application. The resulting boards were evaluated according to the ANSI A208.1‐2009 standard for internal bond (IB), modulus of rupture, modulus of elasticity, hardness, thickness swell, and linear expansion. The test results indicate that all panels made with these resin systems can meet the PBU requirements in terms of IB and hardness and that those bonded with 8 percent PF or 5 percent pMDI can meet all the requirements by the ANSI A208.1‐2009 standard for the floor underlayment application in terms of the properties evaluated. Results of this study imply that it is feasible to use bark as a raw material to manufacture medium-density particleboard for the floor underlayment application.Abstract
Although proteins are naturally good adhesives, often the most economical source has a significant carbohydrate component. Our prior studies have shown that commercial soy protein isolates (CSPI) give very good dry and wet bond strength for wood bonding, but the strengths are much lower for soy flour, especially under wet conditions. One large difference between these soy products is the percentages of carbohydrates, which generally provide poor bond strength under wet conditions. A variety of commercial isolates, concentrates, and flours were examined for their adhesive properties using a small-scale bond test that emphasizes cohesive bond strength. In studying how much the carbohydrates weaken the bond strength, we learned that the carbohydrate interference is only part of the difference between commercial soy flour and purified soy proteins (isolate and concentrate). An even larger factor is the denaturation of the isolate in the CSPI. Thus, it is important to realize that the CSPI performance may not be a good predictor of properties that can be expected from soy flours.Abstract
The development of biobased adhesives without harmful chemical agents is very important for the future. In this study, citric acid was used as a wood adhesive. Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid) is a very safe substance, and it is commercially produced by the fermentation of glucose or glucose- and sucrose-containing materials. This article shows the bonding properties of citric acid in wood-based moldings and particleboards. In wood-based molding, wood and bark powders were used as elements. Citric acid powder was mixed with wood or bark powders, and the mixture was hot pressed at 200°C for 10 minutes. The wood-based moldings had good mechanical properties and excellent water resistance. When wood powder was used as an element, the specific modulus of rupture and modulus of elasticity values were 38.1 kN·m/kg and 4.9 MN·m/kg, respectively. In the case of particleboard, sucrose was used in addition to citric acid; both were dissolved in a water solution and used as an adhesive. Particleboard was manufactured at 200°C for 10 minutes. The board had good mechanical properties and water resistance. When bonded with a 25/75 ratio of citric acid and sucrose, the mechanical properties of the board were comparable to the 18 type JIS A 5908 standard. We conclude that citric acid could be used as a bioadhesive for wood.Abstract
Organosolv lignins were modified with different fungal species. The modified lignins were used as raw materials for preparing lignin-phenol-formaldehyde (LPF) resins. Oriented strandboard (OSB) panels were produced with these laboratory-synthesized LPF resins for evaluating the bond performance of the LPF resins in the manufacturing of wood composites. Ultraviolet spectroscopy results show that the phenolic hydroxyl contents in the lignins were changed after the lignins were treated with fungi. The lignin modified with the brown-rot fungi extended the gel time of the LPF resin compared with the corresponding unmodified lignin, while the lignin modified with the white-rot fungi shortened the gel time. The OSB test results show that the internal bond strength and the bending properties of the panels bonded with the LPF resins containing the modified lignin were comparable to or better than those of the panels bonded with the commercial phenolic resin or the LPF resin containing the unmodified lignin. It is worth noting that the fungi-modified lignin reduced the thickness swell and water absorption of the OSB panels, implying the water resistance of the LPF resins was improved with the fungi-modified lignin. It is also suggested that up to 50 percent of phenol can be potentially replaced with fungi-modified lignin in phenolic resins used as wood adhesives.Abstract
Methylene Malonates and Cyanoacrylates: Energy-Efficient, High-Performance Sustainable Adhesive Systems*
More robust and eco-friendly adhesive options are growing in demand. Monomer-based adhesive systems provide an alternative to solventborne adhesives residentially and industrially used. The systems discussed here include cyanoacrylates and methylene malonates. Although not typically used for woodworking, novel one-part cyanoacrylate formulations have been developed for millwork applications offering improved sustained strength and durability. Mechanical and environmental testing highlight the feasibility of using one-part cyanoacrylate systems in lieu of traditional solventborne wood glues or two-part cyanoacrylate adhesives. Similar to cyanoacrylates, methylene malonates provide supplementary environmental resistance that increases the range of overall industrial application.Abstract
Novel Analytical Method to Determine Factors Causing Unwanted Sticking of Glued Wood Particles onto Machinery Parts*
“Sticking” of glued particles onto machine parts (blender, conveyor, forming station) during wood panel production increasingly occurs when applying low-emission aminoplastic adhesives. Therefore, a new and reproducible method to investigate sticking effects was developed. With this setup, both drying out of the adhesive and the tendency of particles to adhere onto machinery components can be described. The method consists of a rotating cylinder made out of different materials and of diverse surface roughness in order to simulate machine parts, which runs over a wood surface where resin is applied. The wood surface represents the wood particles. The temperature of the wood surface and the ambient air as well as the relative humidity can be controlled. Before starting the experiment, a defined amount of adhesive is applied onto the wood surface and with this a “glue line” is built up. Drying out of the adhesive causes an increase of the rolling resistance up to a peak value. Afterward, an abrupt or gradual decrease of the rolling resistance is observed. This rolling resistance is accurately measured and recorded. Using the approach described above, influences of different materials, climate conditions, surface temperature and roughness, as well as adhesive properties can be observed. The initial results provide strong evidence that climate conditions of the ambient air as well as material and surface properties of machine parts show a significant contribution to the phenomenon.Abstract
Creep Testing of One-Component Polyurethane and Emulsion Polymer Isocyanate Adhesives for Structural Timber Bonding*
Moisture hardening one-component polyurethane (PU) adhesives and emulsion polymer isocyanate (EPI) adhesives have recently gained a considerable market share in structural timber gluing, especially in Europe. The creep behavior of bond lines between solid timber adherends with said adhesives, specifically nine PU and five EPI products, along with two phenolic resorcinol-formaldehyde (PRF) adhesives used for reference purposes, were studied. Most of the adhesives investigated are approved for gluing of structural timber components in Europe. All of the reported 34 bending creep test series (with 340 specimens), were performed with two-part laminated spruce (Picea abies) specimens according to the European structural adhesive creep test standard EN 15416‐3. The standard addresses the ratio RC of creep between PU and EPI adhesives' bonds versus the minimally creeping bond lines of matched specimens glued with a reference PRF adhesive. Mainly glue line thicknesses of 0.3 mm were investigated, with some additional tests having been performed with 0.2- and 0.5-mm-thick glue lines as well. The investigations provide a substantial insight into the creep behavior of the two regarded polyaddition adhesive families. Additionally, the evaluation proves, for PUs and EPIs for test glue line thicknesses of 0.3 mm and less, that it is well justified to reduce the presently prescribed creep periods of 26 and 52 weeks down to 13 and 26 weeks, respectively. The results will serve as a basis for a change of the European test and requirement standards on structural PU and EPI adhesives.Abstract