Inadequate and non-uniform penetration of preservatives into refractory wood species is a main concern in the wood preservation industry for pressure processes. Although the treatability of wood is greatly related to its porous structure, it may be influenced by other parameters like wood moisture content, drying technique, preservative formulation and treatment process. General principles of the wood treatability measurement by macro- and microscale approaches as well as by analytical techniques are discussed. Several innovative strategies have been developed to enhance the treatability of wood. Some strategies are currently being used in the industry, some have a potential for industrial upscaling, while others still remain at the laboratory scale. This review also involves a comprehensive review of recent progress to date in improving this physical property of wood.

Treatability of wood for pressure treatment processes: a literature review

The purpose of this study is to analyze the pore structure for the heartwood, intermediate wood, and sapwood sections in three species of softwood (hinoki, Douglas fir, and hemlock) and to investigate the correlation of gas permeability depending on pore structure. For this study, gas permeability and pore size were measured by capillary flow porometry, and classification of a novel method was performed to determine the type of pores (through pores, blind pores, and closed pores) based on International Union of Pure and Applied Chemistry (IUPAC). Gas permeability, through pore porosity, and pore size increased from heartwood to sapwood. The results of multiple regression analysis showed that through pore porosity, mean pore size, and bulk density were significant factors affecting gas permeability.

/pdf/an-experimental-study-on-change-of-gas-permeability-3p1dawjeg8.pdf

An experimental study on change of gas permeability depending on pore structures in three species (hinoki, Douglas fir, and hemlock) of softwood

A 2-D mathematical model was developed to simulate moisture movement and heat transfer in width and thickness directions within a softwood board during drying. The model is based on wood physiological features and behaviour observed during drying. In sapwood, liquid water movement is assumed to be a consequence of capillary action between liquid and gas phases inside the cell lumens. However, liquid flow does not occur in wood close to the exposed surfaces because at timber surfaces the wood cells are damaged during the sawing process and consequently the liquid column is broken. In heartwood, liquid flow is negligible since the pits are normally aspirated during the formation of heartwood in the growing tree. Water vapour moves under a partial vapour pressure gradient while bound water diffuses within the wood material due to differences in chemical potential. The model was solved numerically to predict moisture-content profiles. Experiments were undertaken to measure the moisturecontent gradient. Samples were removed from a tunnel dryer at intervals throughout drying, frozen overnight and then cut into slices for moisture-content determination. The experimental results were used to verify the model.

Moisture content gradient in a softwood board during drying : simulation from a 2-D model and measurement

Scots pine (Pinus sylvestris L.) planks were dried in industrial progressive, conventional batch and high temperature kilns. The timber was further impregnated in an industrial autoclave with three preservatives used for above ground use. Samples dried by the three test methods and control samples were thereafter processed for scanning electron microscopy observations. Small clear specimens were sawn for determination of impact bending strength, modulus of elasticity and rupture (MOE and MOR) and hardness. The high temperature dried wood (at 115°C) was characterised by partially damaged apertures in some bordered pits and nano- (10-20 nm) and micro checks (1-2 μm) in the warty and S 3 layers of the cell walls. It is probable that certain modifications in the structure of the wood polymer complex also occurred due to hemicellulose thermal degradation. The above-mentioned structural changes facilitated the penetration of the preservative during impregnation and its evacuation during the final stage of vacuum. The industrial progressive and conventional batch kiln drying had no visible effect on the microstructure of Scots pine wood. There was no critical reduction of the impact bending strength, hardness and MOE of the dried untreated wood regardless of the drying method. MOR was significantly decreased after the conventional kiln drying, but not following high temperature drying. Impregnation with Tanalith E and conventional hatch kiln drying aggravated the MOE and MOR of the high temperature dried wood. but both MOE and MOR did not differ significantly from those of progressive and conventional batch kiln dried samples.

Industrial Kiln Drying and its Effect on Microstructure, Impregnation and Properties of Scots Pine Timber Impregnated for Above Ground Use. Part 2. Effect of Drying on Microstructure and Some Mechanical Properties of Scots Pine Wood

Radiata pine sapwood and heartwood were dried using high-temperature, conventional-temperature, and air drying schedules with and without pre-steaming. They were then impregnated by vacuum treatment with double-distilled water, toluidine blue, and fluorescein dye. For sapwood, there were only minor differences in uptake between drying methods and when pre-steaming was used. Using microscopy, the primary flow pathways in sapwood were found to be the resin canal network and ray parenchyma cells, which provided conduction without large resistance. In heartwood, uptake was strongly influenced by pre-steaming the green lumber. After pre-steaming heart-wood, there was an increase in uptake from all surfaces but especially from the radial surfaces. Lower extractive contents, disruption of epithelial and ray parenchyma cells, and alteration of the condition of bordered pits were also associated with pre-steaming. It was therefore possible to classify flow paths in radiata pine heartwood five ways, according to uptake values and wood anatomical features.

/pdf/impregnation-of-radiata-pine-wood-by-vacuum-treatment-ii-1gmprabdw6.pdf

Impregnation of radiata pine wood by vacuum treatment II: effect of pre-steaming on wood structure and resin content

The effect of air-,kiln-, and high-temperature drying on the radial permeability ofPinus radiata D. Don was studied. It was found that for both heartwood and sapwood the permeability increased as the severity of drying increased. The sapwood permeability rose from 5.3×10−14m2 for air-dried timber to 10.9×10−14 m2 for high-temperature-dried timber, an increase of approximately 100%. The rise in heartwood permeability was approximately 280%, with the air-dried permeability being 1.3×10−14 m2 and the high-temperature-dried permeability being 5.3×10−14 m2. It is considered that this higher radial permeability of high temperature-dried wood is responsible for its increased ease of impregnation with preservatives. As the predominant route for gas movement inP. radiata is the resin canal network, it is suggested that this increase in permeability is due to movement and modification of the resin. The flow through the samples was entirely laminar, with no evidence of either molecular or turbulent flow for pressure differentials from 10 to 100 kPa.

The effect of drying schedule on the radial permeability ofPinus radiata D. Don

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The effect of drying schedule on the radial permeability ofPinus radiata D. Don