Bernard S. W. Dawson

THE MECHANISM OF FAILURE OF CLEAR COATED WOODEN BOARDS AS REVEALED BY MICROSCOPY

Regions of wood-coating interface from clear coated Pinus radiata boards were examined by a combination of light and transmission electron microscopy techniques after six months exposure outdoors in order to understand the mechanism of clear coating failure. The primary cause of coating failure was the separation of cells in the surface layers of wood underlying the coating that was caused mainly by the photodegradation of the middle lamella. These results are discussed in the light of known effects of solar radiation on lignin and other components of wood cell walls.

The Relationship Between Pit Membrane Ultrastructure and Chemical Impregnability of Wood

Summary The woods of Alder and Eucalypt were examined by light microscopy before and after a chemical treatment by the Indurite process to increase the hardness of the wood. The pattern of wood cell impregnation for Alder differed significantly from Eucalypt in some respects. In Alder wood all cell types eg. vessels, fibres and rays, were impregnated in similar proportions. In comparison, in Eucalypt wood the impregnation material was largely confined to ray cells and the lumina of vessels; other cell types were either not impregnated or impregnated in very small numbers. Transmission electron microscopy of Alder and Eucalypt woods suggests that ultrastructural differences in the texture and porosity of pit membranes may be the main reason for the observed differences between these wood species with regard to their impregnability by the impregnation material used.

Interlaboratory determination of copper, chromium and arsenic in timber treated with preservative

Precision analyses of timber treated with preservative are an important part of quality control in the New Zealand timber preservation industry. Determinations of Cu, Cr and As remain important criteria in this industry. Repeatability and reproducibility values for two interlaboratory trials are calculated. The repeatability for the Rotorua interlaboratory trial ranged from 0.0109 to 0.0497% for Cu, 0.0190 to 0.600% for Cr and 0.0121 to 0.0346% for As, while reproducibility values ranged from 0.0121 to 0.0663% for Cu, 0.0389 to 0.1951% for Cr and 0.0342 to 0.1298% for As. The Queensland interlaboratory trial also produced repeatability and reproducibility values similar to those for the Rotorua trial. Significant differences between laboratories were found for the determination of Cu, Cr and As. Overall, the data from both interlaboratory trials were similar. Increased awareness of quality assurance programmes is seen as essential to improve the performance of laboratories involved in the timber preservation industry.

Effect of specimen dimension and pre-heating temperature on supercritical CO2 dewatering of radiata pine sapwood

Abstract Removing water from wood is a critical requirement for applications in building and construction and for chemical modifications. Normally, green radiata pine (Pinus radiata D. Don) timber, with a moisture content (MC) range at harvest between 150% and 200%, is kiln dried to below fiber saturation point (FSP) to 10–14% MC. In the present work, a physical-chemical-mechanical dewatering process is presented, which involves pressure cycling with supercritical CO2 to remove water to near the FSP. When the CO2 was cycled from ∼4 MPa into the supercritical state, at pressures up to 20 MPa, specimens of cross-sectional dimensions of up to 52 mm were successfully dewatered from a MC of 174%, typical of the green state, to approximately 39% in seven cycles. The specimens with the smallest cross-sectional dimensions dewatered more slowly than the larger specimens. Preheating the green wood before loading it into the dewatering vessel increased the rate of dewatering. The final MCs were similar in all experiments and were independent of specimen dimension (15–52 mm) or preheating temperature between 40°C and 60°C. Pressure-temperature phase diagrams show that it is necessary to compress the CO2 to the supercritical state for efficient dewatering. Diffusion rates and solubility of CO2 in sap were important, but channel opening within specimens was proposed to be a critical factor in the dewatering process. The reason why pressure-based experiments remove water from wood to an MC greater than the established FSP of 30% is not yet clear.

Interlaboratory determination of boron compounds in preservative treated timber

Abstract Accurate analysis for preservatives in treated timber is an integral part of quality assurance in the New Zealand timber preservation industry. The continued high volume use of boron-treated timber has highlighted the requirement for reliable methods to determine boron-containing preservatives in timber. Several aspects of a leaching-titration method are discussed in an effort to validate and standardize laboratory analysis of samples for the timber preservation industry. Steam volatility was found not to be a problem. The limit of detection was 0.007% (w/w) H 3 BO 3 and the relative standard deviation of 50 replicate analyses was 2.2%. Statistical outliers and stragglers were identified by Cochrans test. The precision data (repeatability and reproducibility) from an interlaboratory determination of boric acid in timber were 0.0133–0.0273 and 0.0194–0.0535% H 3 BO 3 , respectively, over six concentration levels. Recommendations for maintaining active quality assurance programmes are made.

The Anatomy of raised grain on Pinus Radiata weatherboards

Surface irregularities on machined Pinus radiata weatherboards, visible as localised protrusions, were examined by field-emission scanning electron microscopy (FE-SEM) to understand the anatomical basis for the surface deformities present. The SEM observations confirmed that the location of protruded regions corresponded with raised grain. Raised grain is related to moisture content changes of wood after machining and this can cause problems in the acceptability of affected boards on buildings when the raised grain is visible as surface protrusions. A comparison of the SEM images obtained from sections cut sequentially from the same protruded regions prior to and after a brief (five seconds) dipping of the blocks in water suggested that the raised grain resulted from spring back of compressed earlywood tissues underlying the thin, tapered part of latewood bands.

Effect of supercritical CO2 dewatering followed by oven-drying of softwood and hardwood timbers

Supercritical CO2 dewatering of green wood creates timber with unique properties due to the removal of sap directly from cell lumens as a result of cycling between supercritical and gas phases. The susceptibility of 22 softwoods and hardwoods to shrinkage, collapse and checking during dewatering and oven-drying was investigated. The results were compared to green control specimens that were directly oven-dried. Dewatering efficiency was highly variable amongst species and was highest (93–94%) for the permeable sapwood of four softwoods and lowest for the impermeable heartwood of two hardwoods and two softwoods (4–27%). In general, there was less collapse after dewatering followed by oven-drying than after oven-drying alone, more so in hardwoods than softwoods. Six species (a softwood and five hardwoods) displayed strong collapse. Checking was more prevalent after CO2 dewatering and oven-drying than after oven-drying alone. The supercritical dewatering treatment alone did not induce collapse or internal checking; however, for collapse-prone timber either collapse or internal checking, or a combination of both, was induced on subsequent oven-drying.

Primer blocking and pre-service adhesion of acrylic coating systems applied to light organic solvent preservative (LOSP) treated radiata pine sapwood boards

2) for the relationship between residual solvent level and time was 0.98. Rate of solvent loss was dependent on the magnitude of the solvent uptake level. For an uptake level of 34 L/m3, the rate of loss was 2.3 L/m3/day after three days; 1.8 L/m3/day after 28 days; and 0.1 L/m3/day after 51 days. Within 3 to 14 days of preservative treatment, primed 20 mm thick boards can be expected to retain 35–83% of the absorbed solution. For 34 mm boards, retention levels will be higher (42–87%).P. radiata) wurden mit Holzschutzmitteln in leichten organischen Lösemmitteln behandelt und grundiert. Ein Teil der grundierten Bretter wurde danach mit einer von vier Acrylfarbtypen bestrichen. Die Lösemittelaufnahme wurde über einen Zeitraum von 51 Tagen gemessen. Ebenso wurde die Haftfestigkeit der Anstriche und der Einfluß der Grundierung auf das Verkleben der Bretter abgeschätzt. Letzteres wurde bei Grundierungen auf Acryl- jedoch nicht auf Öl-Basis festgestellt. Die Farb- haftfestigkeit unterschied sich in den vier Systemen und war beeinflußt von der Restmenge an Lösemitteln in den Brettern. Keine Haftungsfehler ergaben sich bei einem System mit Hybrid-Grundierung (mit einer Charakteristik sowohl von wasserlöslichen als auch von Lösemittellöslichen Anstrichen). Zwei andere Acryl-Systeme zeigten sich Fehler in einer oder zwei Farbschichten, jedoch war dies in einem Fall auf die ersten 14 Tage beschränkt. Bei anderen Acrylfarben konnten Haftungsfehler auf eine fehlerhafte Auftragmethode zurückgeführt werden. Ein System auf Alkydbasis zeigte die größten Haftungsfehler über die Dauer von 51 Tagen. Eine exponentielle Gleichung wurde entwickelt, die die Abgabe der Lösemittel aus den behandelten Brettern beschreibt. Der Korrelationskoeffizient für die Beziehung zwischen Restmenge und Zeitdauer betrug: R2=0,98. Die Abgaberate war abhängig von der Höhe der Lösemittelaufnahme. Bei einer Aufnahmemenge von 34 L/m3 betrug die Rate der Lösemittelabgabe 2,3 L/m3 nach drei Tagen, 1,8 L/m3 nach 28 Tagen und 0,1 L/m3 nach 51 Tagen. Innerhalb von 3 bis 14 Tagen nach Schutzmittelbehandlung kann für 20 mm dicke Bretter eine absorbierte Lösemittel-Menge von 35–83% der absorbierten Lösung erwartet werden. Bei 34 mm starken Brettern liegt die Menge höher (42–87%).

Primer cure and adhesion after light organic solvent preservative treatment of radiata pine

This study examined the effect of preservative treatment and wood drying history on paint primer cure and adhesion for New Zealand radiata pine (Pinus radiata D. Don). Double Vacuum LOSP treatments resulted in a significantly lower LOSP uptake than did Lowry treatments, for both sapwood and heartwood. Wood drying history had no effect on uptake by sapwood, but did affect uptake by heartwood; high-temperature-kiln-dried heartwood retained more solvent than air-dried or conventionally dried heartwood. For sapwood, LOSP treatment method and wood drying history did not significantly affect either primer cure or primer adhesion. For heartwood, primer adhesion in the early stage of cure was better with Double Vacuum treatments. Double Vacuum treatments also resulted in a higher cure of primer, two days after priming, than did Lowry treatment when either conventionally or high-temperature-kiln-dried heartwood was used, but no significant difference was found with air-dried heartwood. It was concluded that, for radiata pine heartwood, Double Vacuum treatment, combined with either air-drying or conventional drying, will reduce LOSP uptake and give better primer performance, so long as effective ventilated solvent removal is achieved before priming.ZusammenfassungIn dieser Arbeit wird der Einfluß der Vorbehandlung mit Holzschutzmitteln und der Trocknungsgeschichte auf Aushärtung und Haftung eines Grundanstrichs auf Kiefernholz untersucht. Die zweimalige Vakuum-Imprägnierung mit Tributyl-Zinn in organischem Lösemittel (LOSP) brachte eine deutlich höhere Aufnahme in Splint- und Kernholz als die Lowry-Behandlungen. Die Art der Trocknung hatte keinen Einfluß auf die Retention im Splintholz. Das Kernholz nahm jedoch nach Hochtemperaturtrocknung mehr Lösemittel auf als nach Lufttrocknung oder konventioneller Trocknung. Bei Splintholz zeigte sich auch kein Einfluß der Behandlung und der Trocknung auf die Haftung und Aushärtung des Primers. Auf Kernholz war die Haftung nach zweifacher Vakuumbehandlung besser; auch die Aushärtung war nach zwei Tagen weiter fortgeschritten als nach Lowry-Behandlung, und zwar nach konventioneller und nach Hochtemperatur-Trocknung, nicht jedoch nach Lufttrocknung. Für Kernholz von Pinus radiata ist demnach eine doppelte Vakuum-Imprägnierung in Kombination mit Lufttrocknung oder konventioneller Trocknung zu empfehlen, da hierbei weniger Lösemittel aufgenommen und eine bessere Qualität des Grundanstrichs erreicht wird, wenn zuvor eine effektive Entfernung des Lösemittels durch entsprechende Belüftung gewährleistet ist.