Karl Borgin

The effect of aging on the ultrastructure of wood

SummaryWood samples varying in age from 900 to 4400 years were investigated with the electron microscopes (TEM, SEM) and polarized light microscopy as well as by analysis of the mechanism of fracture. The weakest parts of the structure, and therefore the most susceptible to failure, are the middle lamella/S1 region and the interfibrillar matrix. Parts of the wall exhibit fissures, cracks and loss of adhesion, not found in recent wood. Studies of the fractured surfaces confirm a change in the micromorphology of the old wood samples. In spite of the breakdown of certain elements at ultrastructural level the samples had retained almost their normal macroscopic appearance and properties. As long as the main reinforcing structural elements, the microfibrils, remain intact, the major properties of wood do not apparently undergo drastic changes.

The use of the scanning electron microscope for the study of weathered wood

The use of the scanning electron microscope for the study of the changes in the structure and ultrastructure of weathered wood was evaluated in a study of wood exposed to natural weathering for several hundred years.

The cohesive failure of wood studied with the scanning electron microscope

The purpose of the present investigation was to establish the value and suitability of the scanning electron microscope for research on the mechanism of fracture and cohesive failure of the structure of wood. Wood samples, mainly consisting of various pines, were stressed to the point of failure in tension and shear parallel or perpendicular to the main fibre direction. The rate of loading and the temperature were varied between wide limits. The fractured surfaces were studied and gave the following results:

The hydrophobic and water-repellent properties of wattle bark extractives

SummaryBased on the fact that bark is an almost perfect water barrier and assuming that bark is impregnated with materials which aid in rendering it impermeable to water, the hydrophobic and water-repellent properties of several bark extractives were investigated. Products obtained after extraction with polar and non-polar solvents showed widely different properties. Some were extremely hydrophobic and water-repellent with a water-solid-air contact angle higher than 100°. These were the products obtained from extraction with benzene and mineral turpentine. Alcohol and acetone extractives had contact angles as low as 23° but coatings from some of these materials nevertheless had a high efficiency as water barriers. The efficiency of the various extractives as water repellents or water barriers was determined by coating wood with 10 per cent solutions of the extractives in different solvents and determining the extent to which water could be prevented from being absorbed and causing swelling of the wood and thereby reduce the dimensional changes of the wood. The extractives obtained with nonpolar solvents were extremely efficient as true water repellents while the polar aleohol extracts were far better than could be expected from their almost hydrophilic properties. Nature has therefore provided for hydrophilic film-formers combined with hydrophobic materials which act as a double line of defence against water absorption. The hydrophilic extractives, in addition, act as a bridging agent between the strongly hydrophobic extractives and the hydrophilic cell wall material.Since some of the extractives from wattle bark have excellent hydrophobic properties, and since this bark is available in large quantities as a waste product after tannins have been extracted, the utilization of certain wattle bark extractives as water repellents could be economically attractive.

The hydrophobic properties of bark extractives

SummaryThe hydrophobic properties of benzene-soluble extractives from the bark of Pinus radiata were studied by determining the water-solid-air contact angle and the resistance to water penetration.The material contained in the bark extract possessed wax-like and film-forming properties. It was highly hydrophobic with a contact angle with water of about 98°. Wood samples were impregnated with bark extract in order to study the ability of the hydrophobic bark extractives to prevent preferential wetting with water and the displacement of the impregnant from the hydrophilic wood surface. The wax-like material deposited from the bark extracts showed a remarkable affinity for wood as revealed by the high degree of resistance to water penetration and wetting of the treated wood even after several wetting-drying cycles which otherwise tend to break down the bonds and adhesion between a hydrophilic solid and a hydrophobic coating.Hydrophobic or film-forming additives did not improve the ability of bark extractives to protect a hydrophilic solid from water. The properties of the bark extractives as a water repellent and a water barrier could therefore be assumed to be as near perfect as possible.Nature seems to have solved the problem of producing one single, although complex, material with two properties which are extremely difficult, maybe impossible, to combine in a single synthetic material: one of high affinity for a hydrophilic solid and another of extreme water repellency and resistance to water.The in situ conditions prevailing in the bark tissue, combining bark extractives and cell walls of various anatomical structures, must therefore represent a rather unique and for this specific purpose highly efficient two-phase hydrophobic-hydrophilic solid system.

Improvement of capillary penetration of liquids into wood by use of supersonic waves.

SummaryThe effect of supersonic waves on the capillary penetration of liquids into wood was found to be positive or negative depending upon a number of factors. The penetration of non-polar compounds like paraffins and aromatic hydrocarbons was not improved by using supersonic waves. The effect of supersonic waves, however, became positive by introducing one or several hydrophilic groups into straight hydrocarbon chains.Diluting hydrocarbons containing hydrophilic groups with non-polar solvents reduced the positive effect of supersonic waves until it became negative below a certain concentration. Polyfunctional alcohols exhibited an optimum effect at certain concentrations when diluted with water. These concentrations correspond to alcohol-water blends of maximum affinity to the surface of wood.Supersonic waves improved the capillary penetration of water when the surface tension was reduced by using surface active agents.In fields were the rapid and thorough impregnation of wood by various liquids is important, supersonic waves can under certain conditions aid this process and be of practical use.

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.

The Strength of the Glue Bond of Industrial Laminates made from South African Pine with Resorcinol Adhesives

SUMMARY The frequency distribution and the average strength of the glue bond in shear were determined for a large number of glued laminated beams made by several South African lamination plants using different pine species and resorcinol adhesives. The results are compared with the data obtained in a previous investigation using laboratory-made laminates which were made under strictly controlled and accurately reproducible conditions. The comparison shows that the industrially-made laminates are very uniform indeed with the average value of the failing load of the glue bond in shear as high as 1250 p.s.i. while 90 per cent of the laminates failed at loads higher than 1000 p.s.i. in shear. The average strength and the uniformity of the quality of the glue line of laminates made by the South African lamination industry are therefore highly satisfactory; and glued laminated products made in this country can be used by architects, engineers and designers with great confidence. The results also reflect favoura...