H. F. Vermaas

Drying Eucalypts for Quality: Material Characteristics, Pre-drying Treatments, Drying Methods, Schedules and Optimisation of Drying Quality

SYNOPSIS Drying today is recognised as a vital element in the value-added processing of solid wood products and there is currently a strong emphasis on improving drying quality and reducing drying costs. Eucalypts with their characteristically high shrinkage and low mass diffusivity values are notoriously difficult to season without degrade, especially backsawn boards. Considering the premium price which can be fetched for appearance-grade timber, this publication reviews the drying characteristics of eucalypts, various possible pre-drying treatments to enhance ultimate quality and drying rate, the most prevalent drying methods/kilns used, drying schedules to achieve optimum drying conditions, and various measures which can be applied to improve drying quality. The major problem in hardwood drying remains the development of economical ways of drying appearance-grade backsawn timber, free of degrade and in various thicknesses.

Fundamentals of Superheated Steam and Superheated Vapour Drying

SYNOPSIS A discussion is given of the principles involved when drying with dry bulb temperatures above 100°C in either an air-steam mixture (i.e. superheated vapour drying) or in an atmosphere without any air present (i.e. superheated steam drying). The drying mechanisms involved and drying rates and -quality attainable are considered, as well as equilibrium moisture content/temperature/relative humidity relationships of importance in practical drying operations. This theoretical background will serve as a basis for a research project in the Department of Wood Science on high temperature and high temperature/high pressure drying of S. A. pine.

D.C. Resistance Moisture Meters for Wood

SYNOPSIS Regression equations are given for the relationship between logarithm of resistance measured with pin electrodes and reciprocal of absolute temperature for Pinus pinaster and P. radiata. The most important finding of the present work was that the relationship between log R and 1/T at lower moisture contents was curvilinear and not linear as found by most other workers. The curvilinear relationship at higher moisture contents was also confirmed by the present results. Regression equations expressing the functional relationship between log R and moisture content at various temperatures are also given for the two species as well as a set of pooled regression equations which permit the design of a temperature compensated moisture meter for P. pinaster and P. radiata.

Case for the Low Temperature Kiln Drying of Eucalyptus grandis

SYNOPSIS The main constraints in the wider application of E. grandis for the manufacture of better grades of furniture, joinery and other higher value markets as well as for possible import replacement are growth stresses and drying properties. Current practice is to air-dry this species to approximately 300 g/kg before final kiln-drying with resulting long drying times and unsatisfactory drying quality. This article describes the alternative of using low temperature kiln drying, either as a complete drying method or as a pre-dryer, to produce quality dried lumber on order. Capital costs are low, interest costs on large air-drying inventories are reduced substantially and the pay-back period may be two to three years.

The Loss Tangent of Wood and its Importance in Dielectric Heating

SUMMARY The loss tangent of Pinus pinaster was determined as a function of wood density, moisture content, frequency of applied field, grain direction and extractive content. Results are presented in the form of graphs, and the variation of loss tangent with wood variables is discussed. The factors governing heat development in dielectric heating are discussed, particularly the loss tangent as the most important of these. It is stressed that the dielectric constant gives no indication of the heat generated in a dielectric, but that loss tangent is instead the important factor.

Creep in Laminated Wooden Members

SUMMARY Laminated wooden structural members subjected to constant loads for long periods of time experience a gradual increase in deformation with time. This flow or creep is associated with a reduction in strength. In this study the standard block shear sample was used to establish whether creep occurs in the glueline itself, in the interface or in the wood, and to evaluate the effect of various materials added to reduce creep. It was concluded that the actual movement takes place in the glueline itself. The amount of creep was significantly reduced by glass-fibre, asbestos or a glass-fibre mat in the gluelines of thermosetting adhesives but shear strength was not affected by these additives.

The Determination and Use of the Volumetric Change per Unit Mass Moisture Change, for Some South African Grown Pine Species

SUMMARY The volumetric change per unit mass moisture change at 10% moisture content was determined for various pine species. If the average value determined, is used for calculating ro from Ru for 0,08≥u≥0,12, the method is limited to an accuracy of ± 0,7%.

Volume Determination of Wood Samples by Soaking in Mineral Turpentine: Effect on Dry Mass

SUMMARY An accurate method for the determination of the volume of wood samples was developed which involves soaking in mineral turpentine. The effect of this soaking procedure on the dry mass of the samples was determined and found to be negligible.

A Comparison of Two Methods for Determining the Proportional Limit of Wood in Compression Parallel to the Grain

SUMMARY Two methods for determining the proportional limit of compression parallel to the grain were compared. No significant difference was found between results from force-strain and force-d-c-resistance curves.

Research Note on the Correlation between the Density and Extractive Content of Wood

SUMMARY The statement commonly found in textbooks on wood science that wood density increases with increasing extractive content could not he verified since no statistical correlation could he found between these two variables.