Transverse Compression Behavior of Wood in Saturated Steam at 150-170°C

The transverse compression behavior of wood in high temperature (150, 160, and 170°C) and saturated steam conditions was studied. The effect of the temperature on the stress-strain response, nonlinear strain function, and relative density change was examined by a modified Hooke's law based on the load-compression behavior of flexible foams. The influence of environmental conditions during compression on the set recovery of the compression deformation was determined. It was found that temperature and moisture content affected the compression behavior of wood in saturated steam conditions. A small difference in moisture content of specimens compressed at 160 and 170°C caused approximately the same stress-strain and relative density curves with minimum temperature affect on the compression behavior. The compressive modulus of the wood and cell wall modulus were found to decrease with increasing temperature from 150 to 160°C with no change when increased to 170°C. The densification region was entered at notably lower stress levels at 160 and 170°C when compared with 150°C. The results established that temperature and moisture content did not affect the nonlinear strain function at strain levels lower than 0.63. Furthermore, it was found that the set recovery of compressive deformation decreased with increasing temperature of compression from 150 to 160°C. In addition, the results showed that compression at 160 and 170°C significantly lowered the equilibrium moisture content

Moisture Dependent Softening Behavior of Wood

An improved understanding of material behavior during the manufacture of wood-based composites can increase the efficiency of wood utilization and provide insight into the development of new processes and products that manipulate the viscoelastic nature of wood. One specific area where additional knowledge can be of great benefit is the influence of heat and moisture on the softening behavior of wood.The thermal softening behavior of wood at four moisture levels from 0 to 20% was evaluated using dielectric thermal analysis (DETA). Coincident in situ relaxations attributed to the softening of amorphous wood components in the range of 20 to 200°C were observed and found to exhibit the characteristics of a glass transition. The moisture dependence of this transition was characterized, and differences in the observed Tg were detected between juvenile and mature wood. Time-temperature superposition was also shown to be applicable to the wood and water system

Investigations of Flakeboard Mat Consolidation Part II. Modeling Mat Consolidation Using Theories of Cellular Materials

This work tested the applicability of theories designed to predict the compressive stress-strain behavior of cellular materials for modeling the consolidation of a wood flake mat. Model mats designed to simulate narrow sections of randomly aligned and preferentially oriented flake mats were compressed at ambient temperature and moisture conditions in a specially designed apparatus fitted to a servo-hydraulic testing machine. Load and deflection data were collected in real time, and theoretical equations designed to predict the compression of cellular materials were fit to the experimental data. Wood flake mats are cellular-cellular materials, exhibiting two overlapping phases of densification and a highly nonlinear stress-strain response. No differences in the observed stress-strain responses of mats resulted from variations in flake orientation. Theoretical models developed for the stress-strain relationships of cellular foams were fairly effective in predicting the stress-strain relationships of wood flake mats at strains less than 70%. At higher strain levels, the relative density surpassed the initial flake density, causing a violation of model assumptions and forcing the predicted stress levels to increase asymptotically. Combining one cellular material model for the densification of the mat with another for the densification of the wood flakes may be an effective way to model the complex mechanical behavior occurring during consolidation of a wood flake mat

Characteristics of Phenol-Formaldehyde Adhesive Bonds in Steam Injection Pressed Flakeboard

A better understanding of the mechanisms involved in phenol-formaldehyde resin-wood bonding is needed to design adhesive systems that can adequately develop bond strength in a humid environment. This study was performed to determine how the molecular weight distribution of a liquid resole phenol-formaldehyde adhesive affects mechanical properties and adhesive flow in flakeboard bonded during steam injection pressing. The performance of three adhesives, differing only in molecular weight distribution, was studied. For all adhesives, mechanical properties of specimens located on the edge of the panel were found to be superior to those located in the center of the board. Excessive moisture present in the center of the mat was believed to be responsible for poor bonding. Edge internal bond strength improved with higher weight average molecular weight adhesive. Fluorescence microscopy and image analysis techniques were used to measure flow of adhesive into the wood substrate before and after exposure to a steam injection pressing environment. Flakes wetted with adhesive and not exposed to a pressing environment had more adhesive penetration with the lowest weight average molecular weight adhesive. Deeper and less concentrated adhesive penetration was measured in flakes exposed to a steam injection environment, with a smaller apparent difference between the three adhesives

Densified radiata pine for structural composites

A novel wood-based composite has been developed for use in structural applications. The process was designed to utilize rapidly-grown, low density, wood species. Plantation grown radiata pine is particularly well suited to this process. This is a laminated composite, where the lamina may be comprised of various materials, some of which have been treated with the viscoelastic thermal compression (VTC) process. The VTC process increases the density of wood, without causing fractures in the cell wall, thus increasing strength and stiffness of the wood material. The process may be applied to veneer, sawn wood, or strand composites. The VTC lamina is then bonded to other lamina to produce the final product. The strength and stiffness of this VTC composite exceeds any wood-based composite that is currently on the market. For example, modulus of elasticity in bending of over 20 GPa is easily obtainable

High-frequency Heating of Wood with Moisture Content Gradient

The influence of moisture content (MC) gradient on the development of a temperature gradient in wood heated in a high-frequency (HF) electromagnetic field was investigated. Fifteen layers of 1.6-mm-thick beech veneer (Fagus sylvatica L.), with dimension 400 X 400 mm, were used to simulate a moisture content (MC) gradient. A uniform MC of 5, 10, 15, and 20%, and two MC gradient schemes ranging from 20% to 5%, were used in the experiment. The dielectric constant and loss tangent were measured before HF heating at 6.3 MHz. During HF heating of wood, the magnitude of the MC, the shape of the MC gradient, and the potential for thermal losses influence the development of the temperature gradient. An MC gradient in a laminated composite could be used to control the shape and severity of the temperature gradient during HF heating

Engineering analysis of a rotary dryer : drying of wood particles

Rotary dryers are the most commonly used wood drying system in the particleboard industry. These dryers also play an increasingly important role in drying wood residues for fuel. Many potential benefits may be realized through an improved understanding of the rotary drying process. A rotary dryer simulation model was developed, in the form of a computer program, for the purpose of analyzing the drying behavior of wood particles. The model is applicable to single pass rotary drums, with or without a centerfill flighting section. Modifications to the base program could be made to allow for alternative rotary drum designs, such as multiple pass drums. The approach used in the model development analyzed the rotary drying process in a sequential manner. Beginning with a study of particle residence time in a rotary drum, the process of heat transfer, and then mass transfer, were incorporated to yield a complete rotary dryer simulation model. The resultant computer program does not require empirical constants or equations developed for a particular rotary dryer system. Experiments on a commercially manufactured rotary dryer were performed to check the performance of the simulation model as a predictor of overall residence time and drying behavior. The variables tested were drum rotation rate, gas flow rate, and inlet gas temperature. Measurements of gas temperature, particle temperature, and particle moisture content were obtained along the drum length. Comparison between the predictions and the measured results were good, indicating a percent root mean square error of 22.2 in the prediction of the outlet particle moisture content. A series of computer simulation trials were performed to check the affect of inlet particle moisture content, blend-box gas temperature, drum diameter, air leakage, drum length, gas volumetric flow rate, particle size, particle sphericity, drum speed, and angle of repose on dryer behavior. It was discovered that an optimal gas flow rate exists at which the greatest extent of drying may be achieved. In addition, the presence of centerfill flights enhances the extent of drying in a rotary dryer. The rotary dryer simulation model developed in this study should prove useful for optimizing process parameters in the drying of wood particles

Fundamental Aspects of Wood Deformation Pertaining To Manufacture of Wood-Based Composites

During processing, wood-based composites are pressed using extreme heat and pressure for varying lengths of time. Evidence exists that the environmental conditions under which the wood densifies can alter the properties of both the solid wood and the composite product. Given the larger number and extreme nature of variables that exist during composite manufacture, it is imperative that the deformation process be understood from a fundamental standpoint. The objective of this research was to determine the applicability of basic materials engineering theory to the viscoelastic deformation of wood in transverse compression under a variety of temperatures and moisture contents.Theories of cellular solids were used to model the nonlinear compression behavior of small wood elements. For low-density woods, it was determined that cellular collapse can result from elastic buckling of the cell wall. The dependence of inelastic behavior of the gross wood on the elastic properties of the cell wall allows the time, temperature, and moisture dependence to be modeled with classical linear viscoelastic theory of amorphous polymers. Time-temperature-moisture superposition was shown to be applicable to stress relaxation data collected for temperatures between 39 and 99 C and moisture contents between 3 and 16%. The shift factors derived were described using free volume and entropy-based equations. This research demonstrates that wood behaves similarly under those conditions to the general class of cellular amorphous polymers. This conclusion opens many possibilities for experimentally and mathematically modeling the pressing of wood-based composites