Rebecca E. Ibach

Understanding decay resistance, dimensional stability and strength changes in heat treated and acetylated wood

Abstract Reductions in hygroscopicity, increased dimensional stability and decay resistance of heat-treated wood depend on decomposition of a large portion of the hemicelluloses in the wood cell wall. In theory, these hemicelluloses are converted to small organic molecules, water and volatile furan-type intermediates that can polymerize in the cell wall. Reductions in hygroscopicity and improved dimensional stability of acetylated wood depend on esterification of the accessible hemicelluloses in the cell wall reducing hydrogen bonding with water and bulking the cell wall back to its green volume. Stability is not 100% since the water molecule is smaller than the acetyl group so water can access hydroxyl sites even when the wood is fully acetylated. The cell-wall moisture content is too low in acetylated wood to support fungal attack so the initial enzymic attack starting the colonization does not take place. Strength properties are reduced in heat-treated wood owing to the degradation of the cell-wall matrix resulting from the hemicellulose loss. Strength properties are not significantly changed in acetylated wood and acetylation results in greatly improved wet strength and wet stiffness properties.

Improvements in Decay Resistance Based on Moisture Exclusion

Abstract Moisture content has an effect on the biological decay of wood. The literature states that serious decay occurs when the moisture content of wood is above the fiber saturation point (FSP), which is the measurement of the moisture content of wood when the cell walls are saturated and the cell cavities free from water (average 30%). We can chemically modify wood hydroxyls by various treatments (i.e., acetylation, isocyanates, and epoxides) which result in the lowering of the FSP. If we modify the availability of water in the cell wall, we can reduce or eliminate biological degradation. So is biological protection as simple as removing a water molecule at the glycosidic hydrolysis site required by the degrading enzyme? Investigations are underway to chemically modify wood and fiber samples and evaluate them biologically by the soil block test, as well as by the FSP and the equilibrium moisture content (EMC). EMC is the moisture content of wood at any given relative humidity and temperature. Potential correlation between moisture exclusion and biological protection will be discussed.

Wood preservation based on in situ polymerization of bioactive monomers. Part 1. Synthesis of bioactive monomers, wood treatments and microscopic analysis

Summary In situ polymerization of bioactive monomers was investigated as an alternative to conventional preservative treatments. The results are presented in a series of two papers. In Part 1 of the study, six acrylate monomers with covalently bonded, potentially bioactive moieties were synthesized: (1) pentachlorophenolyl acrylate (PCPA), (2) tributyltin acrylate (TBTA), (3) 8-hydroxyquinolyl acrylate (HQA), (4) 5,7-dibromo-8-hydroxyquinolyl acrylate (DBHQA), (5) diethyl-N1N-bis(acryloxyethyl) aminomethyl phosphonate (Fyrol 6 acrylate, F6A), and (6) tetrabromobisphenol A acrylate (TBBPAA). All of these acrylates, except F6A, were purified. Southern pine sapwood samples were treated with acrylate solutions (except TBBPAA) at different retention levels and various amounts of crosslinker (trimethylolpropane trimethacrylate, TMPTM), polymerized in situ, and then acetone leached. The relative amount and location of the polymer in earlywood and latewood of selected samples were determined by scanning electron microscopy and x-ray analysis. Distribution of the compounds varied with treatment. Biological and thermal properties of the treated wood are discussed in Part 2 of this series.

Laboratory and exterior decay of wood–plastic composite boards: voids analysis and computed tomography

ABSTRACT After exposure in the field and laboratory soil block culture testing, the void content of wood–plastic composite (WPC) decking boards was compared to unexposed samples. A void volume analysis was conducted based on calculations of sample density and from micro-computed tomography (microCT) data. It was found that reference WPC contains voids of different sizes from the micrometer range up to several cubic millimeters. Large voids were unevenly distributed within the composite sample. Void size and volume increased after conditioning the WPC in water at 70°C. Depending on the effect of exposure conditions, fungal decay during laboratory soil block testing increased the size and volume of voids. For laboratory samples, the calculated void volume was much higher compared to microCT-detected voids because of the limited resolution of the instrument on relatively large samples with many nano- and microvoids present in the material. In both laboratory and field samples, the creation of the voids resulted in a significant decrease in composite density. Decay damage observed as an increase in the size and volume of voids was particularly severe for boards exposed in the field. The calculated void volume in such samples was in reasonable agreement with voids detected by microCT.

Magnetic resonance imaging used for the evaluation of water presence in wood plastic composite boards exposed to exterior conditions

Abstract Two wood plastic composite (WPC) boards, one experimental and one commercial, were exposed to exterior conditions and evaluated non-destructively using a clinical magnetic resonance imaging (MRI) unit for moisture content (MC) and distribution. The experimental board was exposed in Vancouver, British Columbia, for more than 8 years, and the commercial board was exposed near Hilo, Hawaii, for 2 years. Both boards were characterized in terms of wood content, density, water uptake properties and voids content. The experimental board was additionally destructively analysed for water absorption of the WPC and MC calculated based on the wood content for verification of MRI results. MRI detected the presence of free water and its distribution in both of the WPC boards. Fibre saturation in the experimental board was found to be about 22–24%, in comparison to 25–30% present in most wood species. There was good correlation between the detection of free water by MRI and by destructive testing. Magnetic resonance images showed various major points of water entry in the WPC boards such as the support area, the cut ends, the dripping edge and the sides of the boards. For the experimental board, significant water entry also occurred at the upper exposed surface.

Field and Laboratory Decay Evaluations of Wood–Plastic Composites

Abstract Experimental wood–plastic composites (WPCs) were made so that they matched the manufacturing process, dimensions, and water absorption of some commercial decking boards. WPC samples from selected formulations were divided into two identical groups. The first group was exposed in exterior conditions in Vancouver, British Columbia, and Hilo, Hawaii, at sun and shadow sites. Water absorption and biological activity were monitored by field inspection, density change measurement, and optical and scanning electron microscopy. The second group was used for soil block culture testing performed according to AWPA E10 (or ASTM D1413). Specimens were conditioned by immersion in water at room and elevated temperatures. Results of fungal decay activity are reported as specimen weight loss or corresponding density decrease. Observed density changes during field exposure and soil block culture testing are compared. Samples exposed to aggressive exterior conditions underwent decay, which was detected by microscop...

Wood Preservation Based on In situ Polymerization of Bioactive Monomers. Part 2. Fungal Resistance and Thermal Properties of Treated Wood

Summary This paper is the second in a two-part series on in situ polymerization of bioactive monomers as an alternative to conventional preservative treatments. In this part of the study, bioactive monomers were evaluated for their ability to provide resistance to decay and protection against fire. Five bioactive monomers were synthesized: (1) pentachlorophenolyl acrylate (PCPA), (2) tributyltin acrylate (TBTA), (3) 8-hydroxyquinolyl acrylate (HQA), (4) 5,7-dibromo-8-hydroxyquinolyl acrylate (DBHQA), and (5) diethyl-N1N-bis (acryloxyethyl) aminomethyl phosphonate (Fyrol 6 acrylate, F6A). Southern pine sapwood samples were treated with acrylate solutions at different retention levels and with various amounts of crosslinker (trimethylolpropane trimethacrylate, TMPTM), then polymerized in situ. Methyl methacrylate (MMA) was used as the control. Biological resistance to the brown-rot fungus Gloeophyllum trabeum was determined on acetone-leached and unleached samples. PCPA showed some biological efficacy in the absence of crosslinker, but otherwise provided no more protection than did MMA alone. TBTA was biologically effective at all retention levels except with crosslinker concentration ≥10 %. HQA was biologically effective at ≥ 2% retention. F6A was not biologically effective, although unleached wood treated with 10% F6A and 5% or no crosslinker showed some resistance to decay. The 5% DBHQA plus 5% crosslinker treatment was biologically effective in both leached and unleached wood. The effects of the highest treatment level of each monomer, after polymerization, were also evaluated by thermogravimetric analysis. All treatments provided some resistance to fire. The best treatment was 10 % F6A, which resulted in the lowest mass loss (67.0 %) and the lowest maximum temperature of pyrolysis (308.5 °C).

Evaluating physical property changes for small-diameter, plantation-grown southern pine after in situ polymerization of an acrylic monomer

Because of the large percentage of juvenile wood in small-diameter southern pine, this material has lower strength properties compared with the historic published values in the ASTM Standard D2555. Finding new, simple, and inexpensive ways of increasing these strength properties would increase the use of this material for residential construction. For this study, we chose in situ polymerization using the monomer 1,6-hexanediol dimethacrylate to enhance bending strength and stiffness. After determining the lower range of density, modulus of rupture (MOR), and modulus of elasticity (MOE) of juvenile wood from small southern pine logs, southern pine specimens were polymerized using both a vacuum-impregnation and a surfaceapplication approach. The results showed some significant physical property increases for the fully impregnated material that used a large amount of monomer. Although the surface-application approach used less monomer, the physical properties of the juvenile wood did not increase as expected. Only the 1-minute dip treatment showed a significant increase in both bending stiffness and strength, with a weight gain of 11.9 percent. For the surface-application approach, monomer moving to the wood surface during polymerization reduced their effectiveness in increasing MOR and MOE to the expected levels. Therefore, the challenge is finding a method that maintains polymer loading inside the wood structure during the curing process.

Exterior Decay of Wood–Plastic Composite Boards: Characterization and Magnetic Resonance Imaging

Abstract Magnetic resonance imaging (MRI) was used to evaluate free water content and distribution in wood–plastic composite (WPC) materials decayed during exterior exposure because moisture is an ...

Laboratory and environmental decay of wood–plastic composite boards: flexural properties

ABSTRACT The flexural properties of wood–plastic composite (WPC) deck boards exposed to 9.5 years of environmental decay in Hilo, Hawaii, were compared to samples exposed to moisture and decay fungi for 12 weeks in the laboratory, to establish a correlation between sample flexural properties and calculated void volume. Specimens were tested for flexural strength and modulus, both wet and dry, at 23°C and 52°C. Some specimens degenerated to only 15% of original flexural strength. UV radiation had no impact on flexural properties of field-exposed boards; loss occurred mainly on the side opposite to the sun-exposed surface. The mechanism of the aging process on colonization of WPC by fungi was examined and is consistent with development of slow crack growth in the polyethylene matrix combined with wood decay by fungi. Wood particle decay, moisture, and elevated temperature were the major factors causing composite degradation, indicated by accumulation of voids and a severe decrease in flexural properties. To simulate long-term field impact (including decay) on WPC flexural properties in the laboratory, conditioning of specimens in hot water for an extended period of time is required. Exposure to water (70°C/5 days) was adequate for simulating long-term composite exposure in Hawaii of 4 × 15 × 86 mm3 specimens.