The absence of commercial facilities to recycle or recover value from wood treated with metal-based wood preservatives at the end of its service life is one of the most significant negative points in the generally positive life cycle analysis of treated wood. Wood treated with carbon-based preservatives (metal-free) may be far easier to recycle or recover value from since the preservatives are relatively vulnerable to thermal, chemical and biological breakdown. As a result they might be destroyed by kraft pulping, combustion or composting of treated wood. The present research evaluates the use of carbon-based preservative-treated wood in these processes.
Kraft pulps produced from wood freshly treated with recommended loadings of carbon-based preservatives contained significant quantities of didecyldimethylammonium carbonate (DDAcarbonate), propiconazole and tebuconazole. However, lower preservative concentration in the wood and intensive pulping may be able to produce pulps without detectable preservatives. The azoles were also detected in significant quantities in the black liquor (DDAcarbonate was not analysed in black liquor).
No azoles were found in the ash produced from combustion, but significant quantities were detected in the filtered smoke. DDAcarbonate was not detected in the filtered smoke. Analysis of DDAcarbonate in ash was inconclusive.
A composting experiment has been set up and is in progress. Data on preservative breakdown during composting is expected next year.
Recent work at FPInnovations within the Transformative Technologies program has focused on developing information to assist the Canadian Wood Preservation industry to transition to carbon-based preservation systems. One of the aims of the research is to identify ways of processing wood treated with carbon-based preservatives at the end of its service life. There is also concern regarding biodegradation of carbon-based preservatives during the service life leading to premature failure. This review summarizes what is currently known about the biodegradation of carbon-based actives and identifies knowledge gaps to guide future work in this area.
Lacking the UV protection provided by copper, carbon-based preservative-treated wood used in many above-ground applications will require coating to meet consumer demand for weather resistance. While earlier metal-based preservatives were true solutions, many of the formulations of carbon-based preservatives rely on surfactants for solubility or dispersion in water. These surfactants can potentially react badly with the dispersion agents in the existing coatings on the market. The present work investigates the performance of six selected coatings on white spruce heartwood and ponderosa pine sapwood untreated and treated with one of three carbon-based preservatives. After 500 hours of artificial weathering there were few differences in coating performance between untreated and treated samples. Further weathering should be completed to potentially detect more subtle effects that these preservatives may have on coating performance.
The absence of commercial facilities to recycle or recover value from wood treated with metal-based wood preservatives at the end of its service life is one of the most significant negative points in the generally positive life cycle analysis of treated wood. An experiment was initiated to determine whether wood treated with carbon-based preservatives could be degraded by composting without residual preservative contamination. After one year the concentration of triazoles remained high, while the concentration of DDAcarbonate was greatly reduced.
This report describes the depletion of carbon-based actives from compost mixtures after a further six months of storage. Triazoles remained at high concentrations in the wood, while the concentration of DDAcarbonate was reduced. Overall, composting wood treated with carbon-based preservatives, under the conditions present in this study, was neither wholly effective nor efficient in breaking down these preservatives.
Transformative Technologies - Element I identifierSeries Progress Report 2009/10i
Project No. 201000356
Vancouver, British Columbia
Some black stain fungi are known to exhibit resistance to carbon-based preservatives. However, the cause of this resistance is unclear. The fungi may have mechanisms to tolerate these chemicals, or they may be able to degrade and detoxify them. Fungi capable of degrading carbon-based preservatives are of interest for possible bioremediation of carbon-based preservative-treated wood at the end of its service. Moreover, such fungi could potentially facilitate the colonization of carbon-based preservative treated wood by decay fungi. Two strains of Aureobasidium pullulans and two strains of Epicoccum purpurascens were evaluated for their ability to degrade three commonly used carbon-based preservatives: propiconazole, tebuconazole, and DDAC. At low and medium concentrations propiconazole and tebuconazole were found to be vulnerable to degradation by one of the Epicoccum purpurascens isolates. Where biocides are needed to control the growth of black stain fungi on wood, a combination should be used to minimize the risk posed by resistant strains.
The long-term performance of carbon-based preservatives is anticipated to be limited by detoxification through the action of microorganisms. This study identified primary colonizers of wood treated with carbon-based preservatives that may contribute to detoxification of the active ingredients. Wood coupons treated with three different carbon-based preservatives were subjected to a repeated wetting and drying cycle in an accelerated field simulator. Samples were harvested at different times: two, four, and eight weeks (16 and 32 week samples remain to be sampled). Samples were analysed by HPLC to determine the amount of chemical remaining at the surface of the sample. Chemical concentrations in the samples remained above reported toxic thresholds, and no depletion was seen between two and eight weeks. Wood slivers were chiselled from beneath the surface of the wood and plated. Isolates were identified by sequencing ribosomal DNA. Samples were dominated by bacteria that appeared morphologically similar between samples. DNA sequencing identified these isolates as a Pseudomonas species. Only three fungal isolates were found, one a Trichoderma sp., and the others are yet to be identified. Isolations and identifications will continue.
Field tests of wood coatings in Maple Ridge, BC and Saucier, Miss. were evaluated. Coatings on spruce heartwood generally performed better than those on pine sapwood. Treatment with carbon-based preservatives was associated with better coatings performance. The protective pre-coats were associated with better coatings performance. Plasma treatment had a small positive association with coating performance, but not as much as the use of a protective pre-coat. An 18-month evaluation is recommended.
Field tests of wood coatings in Maple Ridge, BC and Saucier, MS were evaluated. Coatings on spruce heartwood generally performed better than those on pine sapwood. All preservative treatments were associated with improved performance of coatings F1 and F3. Coating F2 was so degraded that there were no apparent differences. All of the protective pre-coats were associated with better coatings performance. In addition, pre-treatment with various plasmas was associated with better coatings performance, similar to that obtained with the protective pre-coats. Colonisation by black stain fungi was the predominant mode of failure for all coating at both test sites. Samples coated with F2 and F3 have largely failed; however, many samples coated with F1 are still in good shape. Samples coated with F1 should be re-evaluated after 36 months of exposure to improve estimates of service life and enable a full cost benefit analysis of these technologies.
There is increasing demand for natural looking wood in exterior applications. If wood finished with transparent and semi-transparent coatings is to compete with other building materials that claim to be low maintenance, the time between re-finishing has to be considerably extended. The use of low-nutrient wood materials, carbon-based preservative treatments, and protective pre-coats that reduce photo-degradation may be able to reduce the maintenance requirements for exterior wood products. The effectiveness of various combinations of protection systems were investigated in field tests set up in Maple Ridge, BC and Saucier, Mississippi. This report describes the initiation of these tests and results after six months of exposure.
In general spruce heartwood samples performed better than the pine sapwood samples. The semi-transparent film-forming finish (F1) performed better than the transparent film-former (F2), which performed much better than the semi-transparent penetrating stain (F3). In general, preservative-treated wood performed better as a substrate than untreated wood. The protective pre-coats were relatively effective under the transparent film-former. Further evaluations are needed to determine the efficacy of the protective pre-coats under the semi-transparent film-former.
A ground-contact field test was installed to evaluate the effect of wrapping the below-ground portion of a post on the decay resistance of a carbon-based preservative system. After two years of exposure at two locations in Canada, the unwrapped untreated Pacific silver fir 4x4 posts showed significant decay, with half the posts failing at one location. In contrast, all the wrapped untreated posts remained sound at both test sites. Treated posts were in excellent condition, with no visible decay.