Philip David Evans
Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - Mar 2019)
Adhesive is a costly and critical component of wood composites. The relationship between adhesive distribution and properties of wood composites has been explored, but few studies have attempted to alter the distribution of adhesive in wood composites as a way of improving their properties. In this thesis, I hypothesize that creating a 3-dimensionally inter-connected adhesive network by introducing adhesive Z-connections will improve two key properties of wood composites (thickness swelling and fracture toughness). Both experiments and computer simulation (finite element analysis) were carried out to test this hypothesis. I developed a methodology to precisely perforate veneer to facilitate the creation of adhesive Z-connections when the composite was pressed. Adhesive Z-connections are defined as the cured adhesive distributed in the Z- (thickness) direction (in addition to the X-Y directions) of the laminated wood composite due to the perforation in veneer. I examined factors affecting the ability of Z-connections to improve dimensional stability and fracture toughness of a model wood composite. I visualized the adhesive distribution in the composite in 2D and 3D using macro-photography, X-ray micro-computed tomography and scanning electron microscopy. Significant improvements in dimensional stability and fracture toughness of some of the composites were observed. Key parameters affecting the ability of adhesive Z-connections to reduce thickness swelling were diameter and spatial arrangement of Z-connections, adhesive level and wood species used to make the composite. Key parameters affecting the ability of adhesive to increase the fracture toughness of a model wood composite were area-density of Z-connections and reinforcement of the adhesive in the composite. I conclude that introducing adhesive Z-connections can reduce thickness swelling and enhance fracture toughness of wood composites, but the effectiveness of such an approach is affected by wood species, area-density and spatial arrangement of the Z-connections. I discuss the implications of my findings for the development of wood composites with enhanced dimensional stability and fracture toughness and further research needed to capitalize on the concept of creating an inter-connected 3D adhesive network in wood composites by introducing adhesive Z-connections.
In this thesis I hypothesize that plasma will etch wood surfaces, produce new cell wall microstructures, and change the surface chemistry of wood because of differential etching of wood’s polymeric constituents. I also examine factors affecting the etching of wood by plasma, and applications of plasma etching for wood processing. Scanning electron and light microscopy and white light confocal profilometry were used to examine etching of wood surfaces. Wet chemical analysis, FTIR and XPS spectroscopy were used to analyze chemical changes at the surface of plasma-treated wood. Experiments were also performed to examine the effect of plasma treatments on the color of blue-stained wood, the morphology of fungal hyphae and the adhesion and performance of coatings on hot-oil modified wood. Exposure of wood to plasma caused etching of wood cell walls and created new surface microstructures. Regions of cell walls that were rich in lignin such as the middle lamella were etched more slowly by plasma. Confocal profilometry of wood exposed to plasma revealed a strong relationship between plasma treatment time and etching of cell walls, and same technique found that lignin pellets were etched more slowly than cellulose pellets. Plasma reduced the levels of carbohydrate at the surface of modified wood, which resulted in a relative increase in lignin content. Plasma treatment improved the effectiveness of hypochlorite bleach at removing blue-stain from wood and it prevented the discoloration of a white acrylic paint on hot-oil modified wood exposed to natural weathering. However, plasma treatment of hot-oil modified wood did not have positive effects on the adhesion and exterior performance of a range of other coatings (mainly semi-transparent stains). I conclude that prolonged exposure to plasma can etch wood cell walls, but cell wall layers that are rich in lignin are degraded more slowly. Plasma etching of wood mainly depends on treatment time and also on the structure and chemical composition of wood. Plasma treatment is an efficient pre-treatment for bleaching of blue-stained wood and reducing the discoloration of white acrylic paint on hot-oil modified wood.
In this thesis I hypothesized that the graying of wood exposed outdoors is due to the presence of melanized fungi that are relatively resistant to UV-light. To test this hypothesis I examined the color and chemical changes at wood surfaces exposed to the weather and filtered solar radiation, isolated and identified fungi colonizing wood samples by DNA analysis and microscopy and examined the survival, growth and melanin production of staining fungi under UV, visible or no light. The ability of isolated fungi to decay wood was also tested by evaluating changes in the microstructure, mechanical, viscoelastic and chemical properties of spruce and lime wood incubated with fungi. Finally, I tested a novel non-biocidal approach to reduce the staining of wood by fungi, which employed melanin biosynthesis inhibitors (MBIs). My results support the general hypothesis (above) and reveal that weathered wood surfaces are grayed by the interactive effects of solar radiation and fungal colonization. UV-radiation increased the production of melanin by the fungus most frequently isolated from weathered wood (Aureobasidium pullulans), which leads to darker weathered wood surfaces. Decay tests showed that species of Cladosporium, Coniochaeta, Epicoccum, Lewia, Mollisia and Phialocephala, were able to degrade wood tissues. In artificial media, MBIs in combination with UV-radiation blocked the growth of staining fungi, but at wood surfaces MBIs reduced fungal staining irrespective of the type of light that samples were exposed to. I conclude that UV-radiation and melanized fungi interact to influence the color of weathered wood surfaces. Degradation of wood by surface fungi is possible, but the extent of damage probably depends on the presence of conditions that favor microbial decay. Finally, the use of MBIs is a promising approach to control graying of weathered wood surfaces, but further research is required to optimize the treatments and test them outdoors.
Master's Student Supervision (2010-2017)
When wood becomes wet and then dries, restraint of shrinkage at the surface of wood by wetter surface and also sub-surface layers causes tensile stresses to develop resulting in micro-checking. Wood elements in composites such as oriented strand board (OSB) also swell and shrink when they become wet and dry. Furthermore, some wood elements are large enough to develop unbalanced surface and sub-surface tensile stresses. Hence, moisture changes and swelling and shrinkage of OSB might result in micro-checking. I test this hypothesis in this thesis. I also examine whether micro-checking contributes to thickness swelling of OSB. I used macro-photography, X-ray micro-computed tomography, and field emission scanning electron microscopy to probe the microstructure of OSB exposed to wetting and drying. These techniques were used to visualize and quantify the thickness swelling of OSB and the dimensions of micro-checks and other voids in OSB during and after wetting and drying. The spatial micro-distribution of a zinc borate biocide in OSB was also examined before and after samples were exposed to wetting and drying. Numerous surface and internal micro-checks developed in OSB exposed to wetting and drying as I hypothesized. Micro-checks developed during wetting, unlike the pattern of checking found in solid wood. Enlargement of voids was also observed during wetting and drying. Micro-checks occurred at the interface between latewood and earlywood and in the rays of softwood flakes and, less commonly, in the rays of aspen flakes. My results indicate that the pattern of micro-checking of OSB is different in some respects to that of solid wood, and suggest that micro-checking contributes to the irreversible thickness swelling of OSB. I briefly discuss the implications of my findings for the development of treatments designed to reduce the irreversible thickness swelling of OSB.
When oriented strandboard (OSB) absorbs moisture its compressed wood strands swell up and recover strains that were induced during hot pressing. Adhesive bonds that hold the strands together rupture and permanent thickness swelling occur. Few post-treatments are able to effectively reduce the thickness swelling of OSB. OSB is porous, with inter-strand voids up to 1.5 mm in diameter. Surface coatings are less effective at restricting moisture ingress into OSB than into solid wood, because of the irregular surface of OSB. Molten wax has a low viscosity and may be able to flow into and block the inter-strand voids of OSB. In this thesis I hypothesize that the water repellency and dimensional stability of OSB may be improved by spraying molten wax onto the hot surface of the board. Furthermore, I hypothesize that polar and low melting point waxes will form more effective water repellent barriers than nonpolar or high melting point waxes. The properties of 13 different waxes and five custom made wax blends were characterized with emphasis on properties likely to influence the water repellency of OSB. Molten waxes were sprayed onto the surface of hot OSB and the water absorption and thickness swelling of the samples were measured. Wax treatments were able to reduce the rate of water absorption and the rate of thickness swelling, but not the extent thickness swelling. I conclude that wax treatments are able to increase the water repellency of OSB and reduce the thickness swelling during short-term periods of exposure to water. Waxes with high melting point temperatures tended to form more effective water-repellent barriers, especially for short-term exposure periods, contrary to my hypothesis. Blends of pure beeswax, which contains polar functional groups, and strongly hydrophobic waxes, such as paraffin wax, formed excellent water-repellent barriers. The results suggest that a combination of polar and nonpolar wax is more effective at reducing the thickness swelling of wax-treated OSB than waxes that contain only hydrophobic components. Errata: http://hdl.handle.net/2429/46708.
Wood is one of the main materials of choice to reduce the environmental footprint of the building and construction sector. Wood is aesthetically pleasing and hence there is great demand for finishing systems that do not mask wood’s color and texture. Unfortunately, clear coatings used on wood outdoors fail within one to two years mainly because of photodegradation of the underlying wood substrate. Theperformance of clear coatings on wood can be enhanced by photostabilizing thewood before coating or by modifying the clear coating to improve its photostability and flexibility. However, these approaches have seldom been combined. In this thesis I hypothesized that pre-treatments that can photostabilize wood without adversely affecting coating performance could, when combined with state-of-the-art clear coatings, create coating systems that are a significant improvement on those that are currently available. I compared the ability of nine different chemical treatments to photostabilize wood. I examined the effect of the same wood pretreatments on coating properties (wettability and adhesion) and the permeability of three different coating types. The most effective photostabilizing pre-treatments were benzoyl chloride, vinyl benzoate and chromic acid, and a PF-resin containing a hindered amine light stabilizer. The solvent based polyurethane was more compatible than the water-based acrylic and alkyd coatings with treated wood surfaces. Wood treatments had no effect on coating adhesion, but coating adhesion was highest for the polyurethane. Subsequently, I examined the outdoor performance of seven different clear coatings on photostabilized wood substrates. Benzoyl chloride, vinyl benzoate, chromic acid and the PF-resin/HALS treatments effectively enhanced the clear coat performance during a year-long weathering trial in Australia. Clear coat performance was also improved by using coatings with double the amount of UVA/HALS additives. My findings confirm that clear coat performance on wood exposed outdoors depends on the combination of wood retreatment and coating type. I conclude that superior clear coat systems (pretreatment plus coating) for wood can be developed by combining very effective treatments that can photostabilize wood together with state-of-the-art photostableand flexible clear coatings.
Wood is susceptible to photodegradation, particularly by ultraviolet (UV) light which triggers photochemical reactions in wood up to 100 µm from the surface. Painting may protect the wood from sunlight, however, consumers prefer clear-coatings because they reveal wood’s grain. Unfortunately, clear finishes on wood used outdoors fail within 1-2 years because sunlight transmitted through the clear-coating degrades the underlying wood. I hypothesized that the performance of clear-coatings on wood used outdoors would be improved if the clear-coating could penetrate deeply into the wood and bond with sub-surface layers of wood that are less affected by UV light. Glow discharge plasma derived from water was used to etch and open up flow paths at the surface of the refractory wood species, black spruce. Image analysis was used to assess the penetration of two polyurethane clear-coatings into wood, and the adhesion (pull-off) of clear-coatings to unweathered wood was measured. A second experiment examined the changes in color, gloss and integrity of clear-coats on plasma treated and coated specimens subjected to artificial accelerated weathering. Plasma treatment of black spruce for 20 min significantly increased the penetration of clear coatings into sub-surface layers of wood (50-75 µm). However, the adhesion of the polyurethane clear coatings on wood depended much more on coating type (adhesion of the water-borne was better than that of the oil-borne coating). Plasma pre-treatments decreased film failure of coatings on flat-sawn specimens exposed to artificial weathering, but increased the surface yellowing of coated wood specimens. The oil-borne polyurethane performed better in terms of three performance criteria (redness, yellowness and gloss), while the water-borne was superior in terms of changes in lightness. I conclude that plasma treatment can increase the penetration of coatings into wood if the plasma can etch the wood and open up important flow-paths. However, plasma treatments have no positive effect on the adhesion of polyurethane coatings to black spruce wood and adhesion is influenced more by the type of coating. Finally, plasma treatments can have positive and negative effects on the exterior performance of coatings on wood depending on the criteria used to assess performance.
Optical confocal profilometry is a new technology for characterizing the surfaces of materials. In this thesis, I hypothesize that confocal profilometry will be able to more accurately measure the erosion of wood during weathering than optical microscopy. Confocal profilometry may be able to screen photoprotective chemicals, and in this thesis I use confocal profilometry to test the hypothesis that PF resin can photostabilize wood. Confocal profilometry was used to measure the erosion of untreated western red cedar wood exposed to natural and artificial weathering. The erosion of western red cedar specimens increased with time. Specimens exposed in a xenon-arc weatherometer eroded significantly faster than specimens exposed in a QUV weatherometer. The profilometer was able to measure the erosion of specimens exposed for only 100 h in a xenon-arc weatherometer. There was a positive correlation between the size of the area of wood exposed to weathering and the erosion of wood during artificial and natural weathering. The erosion rate was about 2-20 times faster during artificial weathering compared to natural weathering. Profilometry was able to discern differences in the erosion of untreated specimens and specimens treated with PF resin. However, profilometry was not able to detect differences in erosion of specimens treated with different PF resin formulations. In contrast, the thin strip technique, which measured weight and tensile strength losses of treated wood veneers exposed to weathering, was able to discern differences between formulations. I conclude from measurements on western red cedar that confocal profilometry is a more accurate and less labor-intensive way of measuring the erosion of wood during weathering than optical microscopy. The exposure time required to produce erosion that can be measured using the profilometer is much less than that needed when using an optical microscope. But specimens need to be flat, free of checks and have eroded areas that are distinct from unweathered areas. Low molecular weight PF resin shows great promise as a treatment for photostabilizing wood.
Recent Tri-Agency Grants
The following is a selection of grants for which the faculty member was principal investigator or co-investigator. Currently, the list only covers Canadian Tri-Agency grants from years 2013/14-2016/17 and excludes grants from any other agencies.
- Surface engineering to enhance the performance of exterior wood decking - Natural Sciences and Engineering Research Council of Canada (NSERC) - Collaborative Research and Development Grants - Project (2015/2016)
- Explaining, exploiting and controlling the staining of wood by UV tolerant fungi - Natural Sciences and Engineering Research Council of Canada (NSERC) - Discovery Grants Program - Individual (2013/2014)