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Concern over pests in wood is limiting movement of products, including green railway crossties within the United States. Heat treatment is accepted for the phytosanitation of wood. Immersion in a hot borate solution achieved the heat treatment of green crossties in about 7 hours and significant borate retention was achieved. Hot borate immersion treatments of railway crossties could combine the benefits of heat treatment with the long-term advantages of borate treatment with little extra cost.
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There is increasing concern about the movement of exotic invasive pests on wood products. Pallets and wood packaging materials that are transported internationally are now required to have phytosanitary treatments. The trade in green wood products within the United States currently is also being examined; for example, there are restrictions on the movement of ash wood from areas affected by the emerald ash borer. Most recently there have been cases of green, untreated railway crossties from the southeast being denied entry into Oregon amid concern over the possibility of introducing pests into agricultural areas. (1)
Currently accepted phytosanitary treatments are described in the ISPM 15 Guidelines for Regulating Wood Packaging Material in International Trade developed by the International Plant Protection Commission (IPPC). Heat treatment (56[degrees]C at the core for 30 min.) is the most common method for the phytosanitation of green wood products. While it is effective in killing pests that might be in the wood, it adds a cost to the manufacturing process that does not provide any lasting value to the product. Heat-treated wood is susceptible to reinfestation (Haack et al. 2007), and recycled pallets must be retreated.
Heat treatment of wood products generally is accomplished using hot air or steam in kilns similar to those used for kiln-drying lumber. Slahor et al. (2005), however, showed that heat treatment of pallet lumber could be combined with borate preservative addition using a hot borate-solution bath. In addition to the temporary sterilizing effect of the heat, the borate in the pallet would provide long-term protection against a wide range of fungi and insects. This addition of a wood preservative would add significant value to the commodity in terms of extended life of use and as a barrier to reinfestation by pathogenic organisms after heat treatment. He et al. (1997) concluded that such a treatment would not be feasible for Douglas-fir logs but they limited treatment times to 3 hours.
Borates have been used successfully as wood preservatives and pest control products for many decades. Advantages of borates include broad spectrum efficacy against all wood-destroying organisms, low cost, low mammalian toxicity, and a low environmental impact. Adding borates to railroad crossties prior to creosote treatment has been shown to provide significant benefit to the tie in service (Amburgey et al. 2003). Significant benefits have also been shown during air-seasoning in poles (Dickinson et al. 1990) where incipient decay is prevented. Incipient decay reduces wood strength, can serve as inoculum going into service, and can cause poor final creosote treatments due to localized pockets of high moisture content (MC) (Taylor 1985). The objective of this study was to determine if borate treatment of green railway crossties could be combined with phytosantitation using a thermal borate solution immersion treatment and whether significant borate retention and penetration could be achieved in the process.
Materials and methods
Railway crossties (175 by 225 by 2550 mm) were trucked from the sawmill (Eastview, Tennessee) to the testing facility (Knoxville, Tennessee) immediately after sawing. After delivery, the ties were covered with a tarp to retard drying. Five ties each of three wood species were included in this study: gum (Liquidambar styraciflua), red oak (Quercus spp.), and white oak (Quercus spp.). The trials for each species were conducted separately.
A 250 mm-long section was cut from the end of each tie and discarded. A 900-mm-long sample was cut from the remaining tie section. A 30-mm-long section was cut from each end of the sample, weighed, ovendried to constant weight at 103[degrees]C, and reweighed to determine initial moisture content (MC).
A 4-mm hole was drilled to the center of each sample halfway along the length, and a type K, 22-gauge thermocouple was inserted to the bottom of the hole. The hole was tightly sealed using a tapered, round wood toothpick. A Kiethly Model 2700 multimeter/data acquisition system was used to acquire data. Temperature data were read directly into a matrix on a portable computer and recorded every 60 seconds.
The samples were then immersed, on end, into a 1000 L plastic tank filled with hot borate solutions (10%, 20%, or 30% disodium octaborate tetrahydrate [DOT]). The tank was attached to an in-line heater and pump which circulated the solution and maintained the temperature at 80[degrees]C. The samples were kept in the treatment tank until all of the ties had reached 56[degrees]C for 30 minutes at the core. After treatment, the ties were removed from the tank and a 30-mm-long section was cut from near the middle of the tie (avoiding the location of the thermocouple hole) for borate retention analysis.
Borate retention was measured on the outer 25 mm of each tie sample, the required assay zone of timbers (AWPA C31-00 [AWPA 2001]), using reflux extraction in water for 1 hour and subsequent potentiometric titration of the resulting solution using mannitol (modified AWPA A2-15 [AWPA 2005]).
Results and discussion
Initial tests were conducted with 10 percent and 20 percent borate solutions, at various temperatures. Because the preservative retentions achieved were variable and slightly lower than desired (Table 1), subsequent trials were limited to 30 percent DOT (at 80[degrees]C).
Heat treatment using the 80[degrees]C bath required 6 to 7 hours (Table 2). This is consistent with expected heating times (Simpson 2001). Using a 30 percent DOT solution at 80[degrees]C, borate retentions achieved over that time generally exceeded the 4.5 kg/[m.sup.3] (0.28 pcf) required by AWPA C 31 for Formosan termite protection, and were about 2 to 10 times greater than retentions required to provide protection against wood-destroying fungi and beetles (Lloyd 1997). Amburgey et al. (2003) dipped unseasoned crossties for 3 minutes in 30 percent borate at 55[degrees]C and achieved retentions above 2.3 kg/ [m.sup.3] in the outer 25 mm of red oak and white oak crossties. These retentions were sufficient to provide lasting benefit (over 15 years in service in most U.S. situations) to the crossties. Retentions were significantly longer when combined with subsequent creosote treatment. Combined with a creosote treatment, the borate could prevent the premature failure of ties due to internal decay resulting from poor creosote treatment or poor creosote penetration.
These data demonstrate that a combined phytosanitation/ borate treatment of green railway crossties is possible using a hot borate solution immersion treatment. But, the effects of variations in initial MC, initial temperature, and other factors need to be considered if this treatment is to be applied in an industrial environment. Borate is a relatively inexpensive wood preservative but the costs of handling, and for thermal energy for the sanitation process, add a significant cost to tie production. Yet, the assurance of phytosanitation from the heat treatment combined with the long-term benefits of the borate may provide sufficient value to offset the cost of the treatment.
Acknowledgments
The authors thank Matt Clarke of Gross and Janes for providing the crossties and Samantha Lloyd for performing the boron concentration analyses.
Literature cited
Amburgey, T.L., J.L. Watt, and M.G. Sanders. 2003. Extending the service life of wooden crossties by using pre- and supplemental preservative treatments. 15-year exposure report. Crossties (May/June): 1-5.
American Wood-Preservers' Assoc. (AWPA). 2005. Standard Methods for Analysis of Waterborne Preservatives and Fire-Retardant Formulations. AWPA Standard A2-05. AWPA, Selma, AL.
--. 2001. Lumber used out of contact with the ground and continuously protected from liquid water--treatment by pressure processes. AWPA Standard C31-00. AWPA, Selma, AL.
Dickinson, D.J., A.R. Zahora, and A.P. Dodson. 1990. The control and pretreatment decay in air seasoning Scots and Corsican pine poles in England. IRG/WP 1451. In: Proc. of the annual meeting of the Inter. Res. Group on Wood Preservation, Roturua, New Zealand.
Haack, R.A., T.R. Petrice, P. Nzokuo, and P. Kamdem. 2007. Suitability of heat treatment for controlling post-treatment insect colonization of log and lumber with varying amount of bark. Presented at the 61st Annual Meeting of the Forest Products Soc., June 10-13, 2007, Knoxville, TN.
He, W., W.J. Simonsen, H. Chen, and J.J. Morrell. 1997. Evaluation of the efficacy of selected thermal boron treatments in eliminating pests in freshly peeled Douglas-fir logs. Forest Prod. J. 47(3):66-70.
Lloyd, J.D. 1997. Inter. status of borate preservative systems. In: Second Inter. Conf. on Wood Protection by Diffusible Preservatives and Pesticides, Nov. 6-8, 1996, Mobile, AL. Forest Products Soc., Madison, WI. pp. 45-34.
Simpson, W.T. 2001. Heating times for round and rectangular cross sections of wood in steam. Gen. Tech. Rept. GTR-FPL130. USDA Forest Serv., Forest Products Lab., Madison, WI. 103 pp.
Slahor, J.J., L.E. Osborn, E.T. Cesa, B. Dowson-Andoh, E. Lang, and S. Grushecky. 2005. Using a hot water bath as an alternative phytosanitation method for wood packaging material. Forest Prod. J. 55(4):59-61.
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