Philip D. Evans

Studies of the degradation and protection of wood surfaces

SummaryLarge weight losses occurred in thin veneers of radiata pine (P. radiata) during natural weathering due mainly to loss of lignin and hemicelluloses. This was demonstrated by direct chemical analysis and by spectrocopic techniques. Treatment of veneers with dilute aqueous solutions of the recognized surface stabilizing compound chromium trioxide was found to dramatically restrict weight loss during weathering, but similar applications of ferric compound were less effective (Evans, Schmalzl 1989). In order to rationalize these protective effects FTIR internal reflectance spectroscopy was used to study chemical changes taking place at the wood surface upon treatment and during weathering. The spectra obtained provide direct evidence for the modification and stabilization of the lignin aromatic system with aqueous chromium trioxide and to a lesser extent with ferric salts. It is postulated that photostable lignin complexes are formed. The implications of these findings for the development of improved surface stabilizing compounds for wood are discussed briefly.

Degradation of wood surfaces during natural weathering. Effects on lignin and cellulose and on the adhesion of acrylic latex primers

SummaryRadiata pine veneers and blocks were exposed to natural weathering under Australian summer conditions over a period of 30 days. Infrared spectroscopy revealed that there was perceptible surface delignification after 4 hours exposure, substantial surface delignification after 3 days exposure and almost complete surface delignification after 6 days. Rapid lignin degradation was also suggested by measurements of the acid insoluble lignin content of weathered veneers. Viscometry determinations on holocellulose samples from weathered veneers and unweathered controls indicated significant depolymerisation of cellulose after 4 days exposure. A tape test was used to assess the adhesion of acrylic latex primers to weathered blocks. The adhesion of exterior acrylic primers decreased on weathered wood surfaces and was significantly lower on specimens that were weathered for 5 to 10 days. An oil-modified acrylic primer showed greater adhesion to weathered wood surfaces. Primer adhesion was lower on weathered radial surfaces than on similarly exposed tangential surfaces. The practical implications of these findings for the coating of exterior wood with acrylic latex primers are discussed briefly.

Weathering and photostability of benzoylated wood

Abstract Chemical modification shows promise as a means of protecting wood from deterioration by fungi, but the systems tested to date have shown limited ability to photostabilise wood. In this study wood was esterified with benzoyl choride and the photostability of the modified wood was assessed. Benzoyl chloride was chosen because it was thought that the introduction of benzoyl groups into wood might act as a UV screen protecting wood from photodegradation. Benzoylation of wood to high weight gains (∼70%) was effective at protecting wood from photodegradation and there was an inverse relationship between weight gains due to benzoylation and mass losses of modified Scots pine wood veneers during weathering. FTIR spectroscopy and SEM of benzoylated wood exposed to natural weathering provided strong evidence for the stabilisation of lignin as a result of benzoylation. Benzoylation to high weight gain, however, caused extensive swelling of the wood cell wall and large losses in the tensile strength of veneers. The UV absorption characteristics of wood were modified by benzoylation and ESR spectroscopy of UV irradiated veneers clearly showed that benzoylation reduced the concentration of free radicals that are involved in the photodegradation of wood.

Weathering of chemically modified wood surfaces - Natural weathering of Scots pine acetylated to different weight gains

Summary Scots pine wood veneers were acetylated to weight gains of 5, 10, 15 or 20% and exposed to natural weathering. Veneers acetylated to low weight gains of 5 and 10% and exposed to the weather showed greater losses in mass and tensile strength due to increased delignification and depolymerisation of cellulose than similarly exposed, untreated controls. Acetylation to 20% weight gain restricted the loss of veneer mass and holocellulose during exposure. Initially it also slightly reduced the depolymerisation of cellulose, and this was reflected in lower losses in veneer zero-span tensile strength after 35 days exposure. However, the photoprotective effects of acetylation, with the exception of veneer mass, were lost with prolonged exposure of veneers to the weather. Acetylation of wood blocks to 20% weight gain caused a bulking of the wood cell wall, particularly the S3 layer of the secondary wall, and reductions in lumen size. Acetylated latewood cells maintained their shape and radial orientation during exterior exposure, but acetylation did not prevent erosion of the middle lamella. Mechanisms to explain how acetylation may affect the weathering resistance of wood are suggested.

Violet light causes photodegradation of wood beyond the zone affected by ultraviolet radiation

Abstract The limited penetration of wood by light explains why the weathering of wood exposed outdoors is a surface phenomenon. Wood is rapidly degraded by short-wavelength UV radiation, but the penetration of light into wood is positively correlated with its wavelength. Hence, sub-surface degradation is likely to be caused by longer-wavelength light that still has sufficient energy to degrade wood. In this paper we test this hypothesis and determine the wavelengths of visible light that extend photodegradation into wood beyond the zone affected by UV radiation. Sugi (Cryptomeria japonica) earlywood was exposed to UV and visible light with narrow band gaps (20 nm) and the penetration of light into the wood was measured using a photodetector. Photodegradation was depth-profiled using FT-IR microscopy. There was a positive correlation between the penetration of light into sugi earlywood and the wavelength of the incident radiation within the range 246–496 nm. The depth of photodegradation also increased with wavelength up to and including the violet region (403 nm) of the visible spectrum. Blue light (434–496 nm) penetrated wood to a greater extent than violet light and was capable of bleaching the wood, but it did not significantly modify lignin, and hence it was not responsible for sub-surface photodegradation of wood. We conclude that violet light is the component of the visible spectrum that extends photodegradation into wood beyond the zone affected by UV radiation. Accordingly, surface treatments designed to protect wood used outdoors should shield wood from the effects of violet light.

Photostabilisation of wood surfaces using a grafted benzophenone UV absorber

Abstract The photostability of polymers can be greatly improved using grafted photostabilisers. This approach was used here to improve the photostability of wood. 2-Hydroxy-4(2,3-epoxypropoxy)-benzophenone (HEPBP) was synthesised and the reaction conditions required to graft HEPBP to wood were examined. The photostability of grafted veneers during natural weathering was compared with veneers treated with 2,4-dihydroxybenzophenone (DHBP) or chromium trioxide. In the presence of dimethylbenzylamine, at temperatures in excess of 80 °C, HEPBP reacted with wood resulting in permanent weight gains. FTIR and XPS spectra suggested grafting of the benzophenone to wood. Grafting of HEPBP restricted losses in veneer mass and tensile strength during weathering, whereas DHBP was ineffective. Grafting was generally as effective as chromium trioxide in restricting losses in veneer mass during weathering, and superior to chromium trioxide in terms of its effects on tensile strength. Grafting also reduced photochemical changes at exposed wood surfaces and improved the performance of clear finishes.

Surface deterioration of wood-flour polypropylene composites by weathering trials

The market for wood-fiber plastic composites (WPCs) is expanding rapidly in many countries including Japan, where WPCs are mainly used for exterior products. In such applications, WPCs undergo undesirable color change, chalking, and shrinkage and swelling, and accordingly there is a need to better understand the mechanisms responsible for the weathering of WPC and develop methods of improving their weathering resistance. In this study, weatherability of WPC was assessed by natural and accelerated weathering trials. Discoloration (whitening) of WPC during exposure was caused by degradation of both wood and plastic. Darker color pigments as additives improved the color stability of WPC; however, chalking on the surfaces still occurred. The color stability of WPC was improved by application of exterior coatings. Preweathering of WPC (before coatings were applied) increased the absorption of coatings by the WPC and had a positive effect on the color stability and prevented chalking of the composites.

Wood surface protection with some titanium, zirconium and manganese compounds

Thin radiata pine veneers were treated with a range of titanium, zirconium and manganese compounds and exposed to natural weathering. Veneer weight and tensile strength losses were used to assess the protective efficacy of the compounds. The oxidative manganese compounds potassium permanganate and manganic acetate restricted both weight and tensile strength loss of treated wood veneers during weathering. FTIR internal reflectance spectroscopy indicated that lignin was retained at the surface of weathered veneers treated with both manganese oxidants. A number of titanates including tetrabutyl and tetraisopropyl titanate enhanced the tensile strength of wood veneers both before and after weathering, perhaps in part due to the complexing of cellulose in wood. Most of the titanates and zirconates were not very effective in restricting weight loss, indicating that they were unable to protect lignin and FTIR internal reflectance spectroscopy of weathered titanate and zirconate treated veneers supported this conclusion. SEM on many of the titanate and zirconate treated wood surfaces showed extensive deposits formed by hydrolysis of the reagents and such deposits may shelter the underlying wood matrix.

Degradation of wood surfaces by water Changes in mechanical properties of thin wood strips

Thin strips of pine and lime were exposed to deionised water in the temperature range 50–90°C. Losses in wet tested tensile strength and toughness occurred rapidly at 65°C. Dry tested specimens and those tested wet but at zero-span retained a much higher proportion of their original strength. Scanning electron micrographs revealed that these specimens failed mainly by intra-fibre fracture, whereas specimens tested wet but at finite span showed marked interfibre shear failure. Strength losses in water saturated with nitrogen and in water saturated with air were similar and strength losses in the absence of water were small. pH measurements on water solutions revealed a marked increase in acidity during exposure.ZusammenfassungProben aus Kiefer und Linde wurden deionisiertem Wasser bei Temperaturen von 50 bis 90°C ausgesetzt. Bei 65°C traten rasch Verluste der im feuchten Zustand getesteten Proben hinsichtlich Zugfestigkeit und Zähigkeit auf. In trockenem Zustand geprüfte Proben und naß geprüfte, jedoch bei einer „freien Einspannlänge” von null, behielten einen wesentlich höheren Anteil ihrer ursprünglichen Festigkeit. Raster-Elektronen-Mikroskopiebilder zeigten, daß diese Proben meist in der Faser selbst brachen, während die naß getesteten mit einer bestimmten freien Einspannlänge einen deutlichen Scherbruch zwischen den Fasern erkennen ließen. Festigkeitsverluste in Stickstoff-gesättigtem und in Luft-gesättigtem Wasser waren ähnlich hoch, Festigkeitsverluste in Abwesenheit von Wasser niedrig. pH-Messungen des verwendeten Wasser ließen einen deutlichen Anstieg des Säuregrades während der Exposition erkennen.

Etching of wood surfaces by glow discharge plasma

This research tests the hypothesis that plasma will cause differential etching of wood cell walls because of variation in the susceptibility of aromatic and aliphatic polymers to degradation by plasma. Wood was exposed to glow discharge plasma, and scanning electron microscopy and chromatic confocal profilometry were used to examine etching of cell walls. Plasma etched cell walls and made them thinner, but the middle lamella was more resistant to etching than the secondary wall. Plasma created small voids within the secondary wall, which were separated by thin lamellae connected to the middle lamella and tertiary wall layers. Larger voids were created in cell walls by the etching of bordered and half-bordered pits. Etching of the uppermost layer of cells at wood surfaces occurs first and when large voids are created in the walls of these cells then significant plasma etching of the underlying cells occurs. Etching of wood cell walls can be quantified using confocal profilometry, and using this technique a strong relationship between applied plasma energy and volume of cell wall etched by plasma was observed. It is concluded that all of wood’s polymers can be degraded by plasma even though cell wall layers that are rich in lignin are etched more slowly than other parts of the cell wall.