Alain Cloutier

Experimental determination of the convective heat and mass transfer coefficients for wood drying

Abstract The knowledge of the convective heat and mass transfer coefficients is required for the characterization of the boundary conditions of the heat and mass transfer equations of a wood drying model based on water potential. A new experimental method for the determination of the convective mass transfer coefficient is presented. This method is based on the measurement of the moisture content, and indirectly the water potential, at the surface of a wood specimen at different drying times. Drying experiments were performed on red pine (Pinus resinosa Ait.) sapwood from nearly saturated to dry conditions at 56 °C, 52% relative humidity and air velocities of 1.0, 2.5 and 5.0 m s−1. The results show that the convective mass transfer coefficient is constant until the wood surface moisture content reaches about 80% and then decreases more or less gradually as the moisture content decreases further. The convective mass transfer coefficient increases with air velocity. A regression analysis shows that there is no significant improvement in considering the water potential gradient near the wood surface when the difference in water potential between the surface and the surrounding air (ψs − ψ∞) is used to determine the convective mass flux at the surface. Also, ψs − ψ∞ is more appropriate than the water vapour pressure difference (pvs − pv∞) as the responsible driving force of the moisture flux leaving the wood surface. The convective heat transfer coefficient was determined during the same experiments. A plateau is observed at high values of moisture content corresponding to the constant drying rate period.

Genetic control of wood properties in Picea glauca - an analysis of trends with cambial age

We investigated the genetic control of wood properties as a function of cambial age to enable improvement of juvenile wood attributes in white spruce (Picea glauca (Moench) Voss). Increment cores were taken from 375 trees randomly selected from 25 open-pollinated families in a provenance–progeny trial repeated on three sites. High-resolution pith-to-bark profiles were obtained for microfibril angle (MFA), modulus of elasticity (MOE), wood density, tracheid diameter and cell wall thickness, fibre coarseness, and specific fibre surface with the SilviScan technology. Heritability estimates indicated that genetic control of cell anatomy traits and wood density increased with cambial age, whereas the genetic control of MFA and MOE remained relatively low across growth rings. Wood density, radial cell diameter, cell wall thickness, and specific fibre surface were highly heritable, indicating that significant genetic gains could be expected in tree improvement programs, although cambial age at selection may stro...

Particleboard made from hammer milled black spruce bark residues

Summary The disposal of bark residues is an important problem for the forest industry. An important proportion of the bark produced by the paper and lumber industries is used for energy production, but a significant amount of bark is still unused. The objective of this study was to determine the technical feasibility of making particleboards from black spruce bark residues bonded with urea formaldehyde resin and meeting the indoor performance requirements for wood particleboards. In the positive case, this would define a new use for black spruce bark residues. Fresh black spruce bark residues were obtained from a sawmill located in the northeast part of the province of Quebec, Canada. The bark was kiln-dried at 60 °C, the particles were generated from a hammermill and sieved. Particles from 0.02 to 2.0 mm were used in the surface layers and particles from 2.0 to 6.0 mm were used in the core layer. Particleboards of 540 × 560 × 16 mm were made with a laboratory hot press following a factorial design with two manufacturing variables at three levels: (1) wood particles content of the surface layers (0, 25, 50 percent); and (2) UF resin content of the surface layers (12, 14 and 16 percent) with a UF resin content in the core of 8 percent. This resulted in a factorial design of 9 different combinations repeated 3 times for a total of 27 boards. It was observed that the heating kinetics varied according to the wood particles content in the surface layers. The compression ratio of the mat and the board internal bond, modulus of elasticity, modulus of rupture, linear expansion and thickness swell were determined. The results show that it is technically possible to make particleboard from bark residues meeting the American National Standard Institute indoor requirement for wood particleboard under certain conditions. The modulus of rupture of the boards was the most critical property in this study. The best mechanical properties were obtained with a 50 percent wood content and 14 percent resin content in the surface layers. The particleboards produced in this study did not meet the minimal requirements for linear expansion. The temperature measurements performed in the core of the mat during hot pressing show that heat transfer improves with an increase in wood particles content in the surface layers.

Moisture content—water potential relationship of wood from saturated to dry conditions

SummaryThe water potential concept as applied to wood-water relations is presented. The gradient in water potential can be used as the driving force of moisture in wood in a model of drying in isothermal conditions provided the moisture content — water potential relationship is known. This relationship is established for aspen sapwood in desorption from saturated to dry conditions at 20, 35 and 50 °C for two specimen orientations. The tension plate, pressure plate and pressure membrane methods were used at high moisture contents and equilibration over saturated salt solutions was used at low moisture contents. The results obtained demonstrate that these methods can be used in combination in order to establish the relationship within the whole range of moisture contents. The equilibrium moisture contents obtained by the tension plate, the pressure plate and the pressure membrane methods for tangential desorption were slightly higher than those measured for radial desorption. The water potential increased with temperature at a given moisture content. This effect cannot be solely explained by the variation of surface tension of water with temperature.

A model of moisture movement in wood based on water potential and the determination of the effective water conductivity

SummaryA model of isothermal moisture movement in wood during drying using the gradient in water potential as the driving force is proposed. The moisture transport coefficient used in this model is the effective water conductivity. It is a function of moisture content, temperature, and direction of flow. The boundary desorption curve of the effective water conductivity function is established in the radial and tangential directions of aspen sapwood from nearly saturated to dry conditions at 20, 35, and 50 °C using the instantaneous profile method. The results show that the effective water conductivity increases exponentially with moisture content and temperature. The effect of temperature cannot be solely explained by the variation of the viscosity of water. The variation of the moisture content-water potential relationship with temperature would explain a large part of this effect. The effective water conductivity was generally higher in the radial direction than in the tangential direction in a ratio varying from 1/1 to 25/1 depending on moisture content and temperature. The flux-gradient relationship obtained at given moisture contents were found to be linear, confirming the validity of the model for the experimental conditions considered in the present work.

THE WATER POTENTIAL CONCEPT A WOOD DRYING FINITE ELEMENT MODEL BASED ON

ABSTRACT A two-dimensional finite element model of isothermal wood drying based on the water potential concept is presented. An original algorithm iS used to update moisture content and water potential during the computations. The moisture content water potential relationship and the effective water Conductivty functions obtained during independent experiments are used as model parameters. In order to assess the possibilities and limitations of this approach. the calculated drying curves and moisture content profiles are compared to results obtained during convective drying of aspen sapwood at 20, 35, and 50°C, from full saturation to a final moisture content of about 10 %. The experimental and calculated results are in reasonable agreement although differences are observed at intermediate drying times. These differences could be attributed to the description of the convective boundary condition or to possible thermal effects.

Analysis of Among-Species Variability in Wood Fiber Surface Using DRIFTS and XPS: Effects on Esterification Efficiency

Abstract Variability in the chemical composition of surface properties of various wood fibers (eastern white cedar, jack pine, black spruce, and bark) was investigated using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS). Both DRIFTS and XPS showed high variability in fiber surface composition between species and between fiber types (sapwood, heartwood, and bark). Fiber surface was modified by esterification reaction using a maleic anhydride polyethylene (MAPE) treatment. DRIFTS failed to assess surface modification, whereas XPS results showed that MAPE treatment increased the surface hydrocarbon concentration of jack pine wood fiber, indicated by a decrease in oxygen–carbon ratio and an increase in relative intensity of the C1 component in the C1s signal. Lignin concentration variability on the fiber surface was determined as the major factor that prevents esterification from taking place.

Dimensional stability of MDF panels produced from heat-treated fibres

Abstract The objectives of this study were to improve the dimensional stability of medium-density fibreboard (MDF) by heat treatment and to determine the effects of the treatment on the mechanical properties and vertical density profile of the panels. MDF panels were produced from untreated fibres and fibres treated at two different temperatures (150 and 180°C) for 15, 30 and 60 min. Panels produced from heat-treated fibres showed an important reduction in thickness swelling and water absorption after water soaking. Linear expansion and contraction were not improved by the treatment. Thickness swelling after repeated cycles of adsorption and desorption increased, while thickness shrinkage under the same conditions was not changed by the treatment. The springback of panels after repeated cycles of adsorption and desorption was not improved, either. Statistical analyses did not show significant differences in the modulus of rupture, modulus of elasticity and internal bond strength of panels following heat treatment. No significant effect was found for the vertical density profile of panels following heat treatment, although the profiles for panels produced from heat-treated fibres were flatter than those produced from untreated fibres.

Measurement of urea-formaldehyde resin distribution as a function of MDF fiber size by laser scanning microscopy

A technique has been developed to show the extent of the distribution of the adhesive on the fiber surface. This involves treatment of the resinated fibers with the dye Toluidine blue O, to quench the autofluorescence of the wood materials, to show only the fluorescence of the UF resin in the presence of the dye. The results indicate that this method is a simple and practical way to understand UF resin coverage on wood fibers. With image analysis, the adhesive has been shown to cover a small percentage (3.5%) of the fiber surface, although the percent by weight (based on wood fiber) of resin is much more (13%). The fiber size strongly affects the resin coverage ratio and the chance to collect resin droplets, but it is not significant in the resin-droplet’s size distribution. Decreasing the proportion of shorter fibers is beneficial in improving resin coverage uniformity. A model for calculating the resin-coverage ratio was developed. The resin coverage is about 4.8% for the resin content of 13% by the model.

Antioxidant properties and polyphenol contents of trembling aspen bark extracts

Trembling aspen (Populus tremuloides Michx) bark was extracted with water and the crude extract fractionated with tert-butyl-methyl ether (TBME), ethyl acetate (EtOAc) and n-butanol (BuOH) to obtain four different fractions. The antioxidant properties of the bark hot water extracts and its fractions were determined by three in vitro experiments: DPPH assay, phosphomolybdenum assay and canola oil thermoxidation assay by DSC analysis. Most of the results of the reported tests showed that the crude hot water extract and its fractions exhibited a strong antioxidant activity, higher than the synthetic antioxidant BHT. The results of this study confirm that antioxidant activity is a property that strongly depends on the oxidation conditions used in the particular oxidation test. Among the fractions separated from the aqueous extracts of bark, BuOH, TBME and EtOAc soluble fractions exhibited the best antioxidant efficiency, in phosphomolybdenum assay, DPPH assay and canola oil thermoxidation, respectively. Total phenol, flavonoid and flavanol contents were also evaluated and the results confirmed that the polyphenols contained in the hot water extract of this bark are mainly composed of non-flavonoid compounds.