This special edition of the Forest Products Journal contains invited papers selected from the International Conference on Wood Adhesives, held October 25–27, 2017, in Atlanta, Georgia. At the meeting, the presentations were enlightening, the 241 attendees were highly engaged, and many good discussions took place during breaks. The keynote speaker, Johan Bruck from IKEA, energized the attendees by discussing IKEA's commitment to use all bio-based adhesives in their products. This was especially relevant to the many companies and academics working on bio-based adhesives. Other plenaries highlighted the myriad opportunities available from highly reactive bio-based cross-linking agents, the long
The development of new and improved wood adhesives is hindered by an incomplete mechanistic understanding of what makes a wood–adhesive bond able to withstand changes in moisture levels. Although common methods are established to test and report the bulk-level properties of bond lines, such as bond-line shear strength and wood failure analysis, the development of experimental tools to study wood–adhesive bond lines and the effects of moisture at the nanometer to millimeter length scales remains an active area of research. Here we introduce and briefly review four recently developed tools that, when combined, we have found capable to facilitate the study of bond lines and the effects of moisture across these length scales. The tools are X-ray computed tomography, X-ray fluorescence microscopy, nanoindentation, and small-angle neutron scattering. Their combined utility has been demonstrated by studying model bond-line systems made using loblolly pine and phenol-formaldehyde adhesives. The results led to a new insight that adhesive infiltration into cellulose microfibrils at nanometer length scales likely plays an important role in the moisture durability of wood–adhesive bond lines. A new model to facilitate the discussion of potential interactions between adhesive and wood nanostructure was also developed.Abstract
Small-Angle Neutron Scattering as a New Tool to Evaluate Moisture-Induced Swelling in the Nanostructure of Chemically Modified Wood Cell Walls*
Wood-based products can be a sustainable and more environmentally friendly alternative to traditional construction materials because of their reduced contribution to air and water pollution. An integral component of these products is often an adhesive. Because wood is hygroscopic, moisture-induced swelling in the cell walls near the wood–adhesive bond lines can lead to durability and performance issues. Unfortunately, researchers working toward improving the moisture durability of forest products are hindered by an incomplete understanding of the nanoscale mechanisms that contribute to moisture-induced swelling in wood and how chemical modifications affect the swelling. Therefore, we developed small-angle neutron scattering (SANS) into a tool that can study the 1- to 100-nm structure of unmodified and chemically modified wood cell walls and can measure the effects of moisture in this structure. In this study, SANS was used to reveal the nanostructure of a deuterium-labeled phenol-formaldehyde (dPF) adhesive infiltrated into wood cell walls. The results revealed that the dPF infiltrated the water-accessible regions between the elementary fibrils inside the wood cell walls. These results provide the new insight that adhesive infiltration into the cellulose microfibril (a bundle of elementary fibrils) may be a key to designing moisture-durable wood adhesives.Abstract
Correlation of Adhesive Performance between Automated Bond Evaluation System Tests and Plywood Tests: A Case Study of Lignin-Phenol-Formaldehyde Adhesives*
The automated bond evaluation system (ABES), which recently became ASTM D7998-15 standard test method, is an effective tool for screen testing of different water-based wood adhesive formulations. This method enables rapid evaluations of mechanical responsiveness of different adhesive formulations to various press temperatures and/or press times, providing an efficient and realistic comparison of bondability and reactivity among the adhesive formulations. Based on extensive testing work, this article provides experimental findings and evidence for the use of this method to evaluate bonding performance of lignin as a major ingredient in the phenolic adhesive system. The relationship between bond strength development and press temperature can be established for a particular adhesive formulation using this method, which can then help the formulation and optimization of a wood adhesive containing lignin. Softwood plywood experiments demonstrated that there is a strong correlation between ABES test results and adhesive performance in the panel products.Abstract
With an increasing prevalence of renewables used as binders in panelboard products, it is of interest to know how these renewables may impact manufacturing environments and product performance. In this study, soy and zein proteins together with a kraft lignin and condensed tannin extract have been evaluated for contributions to volatile organic compounds arising in press emissions from processing hardwood veneer plywood. Analysis revealed aldehydes tended to dominate press emissions, with both proteinaceous materials producing significantly higher acetaldehyde and hexaldehyde emissions relative to the wood veneer used. Other aldehydes, including formaldehyde, valeraldehyde, and propionaldehyde, were also produced but in relatively lower quantities. The use of polyphenolics led to lower formaldehyde content in press emissions with tannin, also reducing acetaldehyde and other aldehydes in press emissions. Chamber testing plywood panels found the composition of VOC emissions initially released from panels to be prominent compounds in press emissions produced during panel manufacture.Abstract
Impact of Molecular Weight of Kraft Lignin on Adhesive Performance of Lignin-Based Phenol-Formaldehyde Resins*
This study describes the influence of improved lignin homogeneity on the adhesive properties of lignin-based phenolic resins. Softwood kraft lignin was separated by applying an ethanol-based solvent fractionation to obtain three lignin fractions with a narrow molar mass distribution (smallest [Mw 1,590 g/mol] to largest [9,570 g/mol]). Lignin-phenol-formaldehyde (LPF) adhesives were prepared by 50 percent (by weight) substitution of phenol with an adjusted formaldehyde-to-phenol molar ratio. For investigating the storage stability of the resin, viscosity was monitored, curing behavior was determined by differential scanning calorimetry (DSC), and development of the bonding strength was analyzed via tensile shear strength as a function of press time. An acceleration of the condensation process for LPFs with higher molecular mass was observed. DSC signal indicated that LPFs need a slightly higher temperature for a complete curing than the phenol-formaldehyde reference resin. Furthermore, it was found that the tensile shear strength decreases with the use of fractionated kraft lignin in phenolic adhesives. No significant difference in reactivity was found for the resins making up the different lignin fractions.Abstract
Distinctive Impact of Processing Techniques on Bonding Surfaces of Acetylated and Heat-Treated Beech Wood and Its Relation to Bonding Strength*
In this study, the tensile shear strength of untreated, acetylated, and heat-treated beech (Fagus sylvatica L.) wood joints was investigated as a function of different surficial processing techniques. It was hypothesized that differentiating patterns of surface texture are induced by specific processing techniques directly affecting the bonding performance of adhered assemblies. Surface processing was implemented either by peripheral planing with sharp and dull knives, or by sanding (P100). Process-dependent surface textures were visualized by scanning electron microscopy and a digital light microscope was applied to display the structural integrity of surficial wood tissues. In dependence on wood modification techniques, process-related patterns of surface texture were observed. Laser scanning data of surface morphology was used to derive area-related functional roughness parameters defining complex surface textures quantitatively. For tensile shear testing, lamellae were bonded either with a two-component melamine-urea-formaldehyde adhesive or with a one-component moisture-curing polyurethane adhesive. Single lap-joint specimens were prepared following EN 302-1:2013 by the Deutsches Institut für Normung considering a material-adapted specimen geometry. Bonding strength was evaluated with respect to differentiating regimes of moisture. Specific dependences of modified beech wood properties on surface morphologies subsequent to surface processing and, therewith, on the associated bonding performance could be verified. As a result, universal relationships between bonding performance and surface processing technique could not be identified. Thus, individual studies of bonding performances in dependence on adherend- and processing-related surface textures are inevitable.Abstract
At present, the evaluation of wood–adhesive bonds lacks a method that is able to predict the long-term load carrying capacity in shear of a bond in a comparatively short testing time. For this reason, a new test approach was investigated to determine the time to failure of wood–adhesive bonds. In our research, lap joint specimens were prepared with a melamine-urea-formaldehyde (MUF) adhesive at two mixing ratios (100/100 and 100/20 [resin/hardener]). The specimens were subjected to tensile shear stresses at load levels between 30 and 90 percent of their mean wet short-term strength while being immersed in water at temperatures of 60°C and 90°C. The time to failure and the wood failure percentage were determined. The analysis showed good correlations between time to failure and load level as well as between time to failure and temperature. The adhesive mixing ratio, however, showed no influence on the failure characteristics. The wood failure percentage highly depended on the test duration. With prolonged test duration, the mode of failure increasingly changed from wood failure to adhesion failure. Overall, the test method proved to be promising for a detailed performance evaluation of wood–adhesive bonds.Abstract
Some Interfacial Interactions between Isocyanates and Metals that Affect Release in Composite Wood-Panel Production*
Isocyanate- and polyurethane-based adhesives for composite wood-panel production are well known. They are useful for the manufacture of high-performance assemblies, particularly for moisture resistance, durability, and strength. However, isocyanate-based adhesives have several opportunities for improvements related to composite panel manufacture. Of particular note is the ability of isocyanates to bond, not only to wood, but to metal press platens used to produce the composite panels. Although several methodologies exist to overcome this, there is a lack of understanding of the nature of the interactions between isocyanates and metal. The purpose of the current study was to provide insights into these interactions and to help to develop strategies for releasable isocyanate-based resins. Variable-thickness isocyanate–metal and metal–wood assemblies were prepared and the interface exposed by various methods, e.g., surface analysis combined with cluster beam etching and simple peel tests. The interfaces were then analyzed by X-ray photoelectron spectroscopy or time-of-flight–secondary ion mass spectrometry (or both). These surface analytical methods have revealed several chemical interactions between the isocyanates and the metal surfaces, including carbamate-type links and metal–nitrogen linkages. Further, the surface analysis has revealed the compositional character of the locus of failure in wood–metal joints. The locus of failure depends on the presence of wood extractives. With nonextracted wood, the locus of failure is within the wood, which is characterized by a certain concentration of the penetrated resin. With solvent-extracted wood, the locus of failure is within the glue line.Abstract
Recently, the usage of wood–plastic composite (WPC) products has increased, mainly for exterior decking. The shape of fillers is one of the important factors deciding the mechanical and physical properties of WPCs. Surface-fibrillated wood flour (WF) improves the mechanical properties of WPCs, although it requires a lot of energy and time to produce the fibrous structure during the pulverization process. Therefore, the adsorptive interaction between cellulose nanofiber (CNF) and WF by hydrogen bonding was investigated. We considered that CNF could form the fibrous structures on the WF surface by mixing CNF and WF. Also, it is thought that the addition of CNF could increase mechanical and physical properties of WPC because CNF has better physical and mechanical properties than most other fibers. The objective of this study was to produce WF-CNF fillers and to evaluate the mechanical and physical properties of WPCs containing WF-CNF fillers. WF-CNF fillers could be produced by freeze-drying after mixing WF, CNF, and water. The fibrous structures on the WF surface were observed through scanning electron microscope images of WF-CNF filler containing 3 weight percent CNF. A WPC with WF-CNF filler containing 22 percent (by weight) WF and 3 percent (by weight) CNF showed improved mechanical properties compared with WPCs without CNF. The water absorption of WPCs containing CNF was found to decrease with increasing CNF content.Abstract
This study investigated the effects of board thicknesses and time in service on formaldehyde emission for different wood-based boards sampled from standard particleboard (PB) and medium-density fiberboard (MDF). The test samples were subjected to formaldehyde emission by multi-RAE multiple gas analyzer at a temperature of 20°C and at 65 percent relative humidity for a period of 3 days (Treatment 1 [T1]) and 6 months (Treatment 2 [T2]) after production in accordance with European Committee for Standardization (CEN) EN 13986 and Turkish Standards Institute (TS) EN 717-1. PB with a thickness of 18 mm yielded the highest value of formaldehyde emission (1.1078 ppm for T1; 0.5089 ppm for T2), while 18-mm MDF gave the lowest emission (0.2311 and 0.1378 ppm). After 6 months of production, the reduction was 54 percent for PB and 40 percent for MDF. A significant increase was detected with respect to time in service for all board types, the highest values of formaldehyde emission (1.2900 and 0.5800 ppm) were found in 18-mm PBs that were treated for 3 hours, while the lowest values (0.2433 and 0.1600 ppm) were obtained in the samples of 18-mm MDF that were treated for 1 hour. Accordingly, after 6 months, the reduction in formaldehyde emission was 55 percent for PB and 34 percent for MDF. All values were found above the limits of E1 (≤0.10 ppm, EN 717-1). In conclusion, thickness, time in service, and type of wood-based boards significantly affect formaldehyde emission.Abstract
Waste medium-density fiberboard (MDF) is mostly disposed of in landfills and left for incineration. The consequences of filling MDF waste to the landfill include undesirable associated costs and environmental problems caused by incineration. In this study, a prediction method is used for calculating the thermal energy required to recycle MDF fibers. The recycling method consists of a high-temperature treatment in a preheater at an operating temperature of 100°C, which melts the resin and separates the fibers. The cost reduction and energy savings of virgin wood material are calculated for MDF that has been manufactured by replacing 10 and 20 percent of the wood fiber with recycled fiber. Results show that the benefits of MDF production using 10 percent recycled fiber result in a 10 percent reduction in virgin wood material costs and an energy savings of 3.9 percent. Using 20 percent recycled fiber results in an estimated 20 percent reduction in the cost of virgin wood material and an energy savings of 7.8 percent for MDF production. The predicted amounts of thermal energy required to produce MDF are consistent with those of previous studies.Abstract
Formaldehyde-based resin and petroleum-based plastic are often used as an adhesive in plywood manufacturing, which has an adverse effect on people's health and the global environment. We have investigated the plywoodlike composite with wood veneer (WV) and poly-β-hydroxybutyrate film (PHBF) as a novel formaldehyde-free and environmentally friendly composite (WV-PHBF). The composite was prepared with three veneers, and the PHBF placed between adjacent veneers varied from 22.5 to 112.5 g/m2. The composite was hot pressed at 170°C for 70 s/mm under a pressure of 1.0 MPa. Furthermore, the WV-PHBF with an adhesive dosage at 67.5 g/m2 was modified with three modification methods: silane coupling agent (KH-550), alkaline solution (NaOH), and polydiphenylmethane diisocyanate (p-MDI). Results showed the following: (1) under the untreated condition, a composite with more than 67.5 g/m2 adhesive will meet the Chinese National Standard (GB/T) 9846-2015 of bonding and bending strength requirements, and 90.0 g/m2 was the best; (2) the KH-550 and p-MDI treatment had a positive effect on WV-PHBF, but NaOH mainly had an adverse effect; (3) the physical–mechanical properties of the composite with 67.5 g/m2 adhesive content being processed by 1 percent by weight (wt%) KH-550 or 2 wt% p-MDI was better than WV-PHBF with 90 g/m2 adhesive content, especially in bonding strength; (4) statistical analysis shows that 1.0 wt% KH-550 significantly improved the bonding strength properties of WV-PHBF, and water-resistance properties were significantly improved by 2.0 wt% of p-MDI also; and (5) morphological observation shows that KH-550 and p-MDI treatments increase the interfacial compatibility between WV and PHBF of the composites.Abstract
Interest in the wood industry of African mahogany (Khaya spp.) has increased in Brazil because of the quality of the wood and the similarity to the highly demanded Brazilian mahogany (Swietenia macrophylla King). The objective was to study the shear strength and wood failure of joints of 19-year-old plantation African mahogany species (K. ivorensis and K. senegalensis) bonded with four different commercially used adhesives in order to better evaluate the potential use of this material as a suitable replacement for Brazilian mahogany wood. The resins used in this study were urea-formaldehyde (UF), melamine-urea-formaldehyde (MUF), and emulsion polymeric isocyanate (EPI), and cross-linking polyvinyl acetate (PVAc) was used as a thermoplastic adhesive. PVAc yielded statistically higher shear strength for both species. For K. ivorensis, MUF, EPI, and PVAc reached the minimum value for shear strength as specified by ASTM International. However, with K. senegalensis, PVAc was the only adhesive to meet the standard. For K. ivorensis, PVAc and MUF resulted in statistically higher wood failure, and MUF, EPI, and PVAc met the required percentage of wood failure. In K. senegalensis, MUF and PVAc met the minimum requirement for wood failure. Based on the classification given by ASTM standards, the adhesives MUF, EPI, and PVAc can be used for nonstructural lumber products for K. ivorensis, and PVAC can be used in both species tested.Abstract
The aim of this study was to clarify the wood properties of two interspecific Eucalyptus hybrids, Eucalyptus grandis × Eucalyptus pellita (G×P) and E. grandis × Eucalyptus urophylla (G×U), developed in Indonesia. The growth characteristics and stress-wave velocity (SWV) were evaluated for 102 trees from three clones of the G×P hybrid and 105 trees from three clones of the G×U hybrid. Wood properties, such as basic density, shrinkage, compressive strength, modulus of elasticity, and modulus of rupture, were evaluated from nine selected trees of each clone. The G×U hybrid had better tree growth and SWV compared with those of the G×P hybrid. No negative correlation coefficients were observed between growth characteristics and SWV, indicating that the selection of clones for increased wood volume does not reduce their mechanical properties. Hybridizations between E. grandis and E. pellita or E. urophylla showed that E. grandis dominated the growth characteristics with some refinements in the wood properties, especially compressive strength, in the G×P hybrid. Selection based on the superior clones for tree growth would result in higher wood productivity for timber use in these two hybrids.Abstract
In this work, a comparative study on microwave–vacuum-drying (MVD) and hot-air-drying (HAD) of bamboo culms was conducted. Related physical and mechanical properties of bamboo culms were investigated. MVD results showed no drying defects of bamboo culms. Compared with that of the HAD method, the drying time of MVD was reduced. The shrinkage of both bamboo culm diameter and wall thickness increased with drying time. The crystallinity was changed with drying time between MVD and HAD. The CIELAB color system (L*, a*, b*) was used for analyzing bamboo color changes. All the values of L*, a*, and b* were increased after MVD, which caused the bamboo culms to become golden yellow. However, after HAD, L*, a*, and b* decreased and samples became brown. The mechanical properties of samples were improved after MVD, and did not show any significant changes after HAD. The pore structure after MVD showed higher adsorption capacity, higher specific surface area, and greater volume compared with HAD. Therefore, taking into consideration the dried product quality and short drying time, MVD provides a potential method for drying bamboo.Abstract
Leaders in the wood pallet industry have faced numerous trends over the past few decades that have challenged their ability to maintain success without adopting new business models and innovations. Past literature would predict that industry managers who instill entrepreneurial processes such as innovation, risk-taking, and proactiveness into their operations should be better equipped to identify, positively react to, and take advantage of changing environmental conditions. Firms that emphasize such processes are described as being strategically orientated toward entrepreneurship. The objective of this study was to determine whether pallet manufacturing firms that exhibit entrepreneurial processes are more successful than those that do not. As an example of how an entrepreneurial orientation might influence business operations, this study also analyzed revenue-generating methods of managing wood waste. Data for the study were collected from executives of North American pallet manufacturers. Findings suggest pallet firms that continually work to improve their products and services, frequently make decisions that involve risk, and proactively seek out new opportunities have higher performance than firms that do not. Furthermore, manufacturing firms that monetize waste products exhibit greater proclivity for entrepreneurial processes than firms that pay to dispose of or give away wood waste. Results are discussed and potential implications for managers in the industry are provided.Abstract
Wood grain direction is a three-dimensional quantity that is defined by its angles within and into the plane of the measured surface. These are respectively called the surface and dive angles. An interesting method to measure these angles involves measuring the spatial reflection from the wood surface when illuminated by concentrated light. The cellular shape of the wood microstructure causes the light reflection to be greatest perpendicular to the wood grain. This effect allows the surface and dive angles to be determined by analyzing the spatial variation of the reflected light. The conventional method for doing this involves sampling the reflection intensities around a circle above the wood surface. However, this method is effective only for small dive angles. A new method is described here where light reflection intensity variation is measured along two parallel lines on either side of the illuminated area. It is able to measure the ranges of surface and dive angles of interest in strength grading applications. A laboratory device for making the required spatial reflection measurements is described and experimental results are presented.Abstract
We used microtextures to develop a novel antifriction technology for wood cutting tools. Northeast China ash (Fraxinus spp.) was selected as the research species, and the influence of the microtexture on the friction coefficient of cemented carbide samples with and without the texture was studied using various loads and wood moisture content. The results showed that a reasonable microtexture form (d = 40 μm) can effectively reduce the friction coefficient between wood and cemented carbide regardless of the moisture content. The moisture in the wood is conducive to reducing the friction coefficient between the cemented carbide and the wood surface. Its mechanism of action is affected by the wood moisture content, the load used, and other factors. The friction coefficient significantly decreased when the wood moisture content was above the fiber saturation point.Abstract