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Steam accelerated borate diffusion: Optimizing dry tie treatment
2015 - IRG/WP 15-40713
A small laboratory study was carried out which demonstrated that dry wood dip treated in a high concentration borate solution could not be subsequently treated properly with oil borne copper naphthenate unless it was subsequently steam treated to accelerate borate penetration into the wood and re-dry the wood beforehand. A larger study with commercial sized cross ties (railway sleepers) was carried out to see if the same steaming approach could be successful in large dimensions, at a commercial treatment plant and with creosote. It was found that dry ties dip treated in a 25 % borate solution at ambient temperature easily met AWPA standard borate retention and railway specifications as determined by gauge and could then be subsequently and successfully creosote treated after a two hour steaming process. The steaming would not increase total treatment time in the cylinder as it could be considered part of the sterilization time and the treatment by dip had the advantage of treating all species approximately the same with regard to borate preservative retention, overcoming the weakness of pressure treating with borate which can reduce creosote penetration and retention by making the ties too wet and cannot properly treat the white oak and hickory ties to appropriate retentions when treated in the same charge as red oak ties. Some slight loss of borate seamed to occur as a result of steaming, as shown by assay, but the process was modified to avoid condensation in the process for full commercial adoption.
J-W Kim, J D Lloyd

Protecting Sustainable Wood Infrastructure ̶ Improving the Performance of Poles, Piles, Ties & Timbers with Dual Treatments
2019 - IRG/WP 19-30735
Wood is by far the best structural material in terms of environmental impact, as shown by a number of LCA studies. This environmental impact superiority can be improved if the longevity of the commodity is extended. The main durability concern in large cross-section treated materials is the susceptibility of the heartwood to decay, as it is not typically treatable using traditional methods. This can lead to premature failure, for example of Douglas-fir, pine and spruce poles, oak and hickory crossties (sleepers) and pine bridge ties. Traditionally these issues have been managed by high cost remedial treatments, or simply by accepting the short service life and the required product replacement costs. Research over many decades has led to the growing commercial practice of dual treatment. Dual treatment is defined here as combining two (dual) preservative systems, a water-diffusible system and an oil-borne pressure-impregnated system being applied to the same piece of wood. This is often done as separate treatments at different times (‘two-step’); in this case it is essentially the same as using an oil-borne preservative with a remedial diffusible treatment (typically a sodium borate salt), but has the potential benefit of preventing decay before it happens. Adding borates to railroad crossties prior to creosote treatment has been shown to provide significant benefit to crossties (Amburgey et al. 2003, Anon. 2010) and utility poles (Dickinson and Murphy 1991). Currently, more than 50 % of the 22 million wood ties produced annually in the United States are dual treated with an initial diffusible borate dip treatment. The multi-billion-dollar potential savings possible with this approach, especially in the high-hazard southeast of the USA, have been documented (ZetaTech 2011). The inclusion of borate has also been used to justify reducing typical copper naphthenate or creosote retention specifications from 0.96 kg·m-3 as copper and 128 kg·m-3 to 0.64 kg·m-3 as copper and 96 kg·m-3 respectively This means that the cost of the borate treatment is more than paid for with the copper naphthenate or creosote reduction (savings). Treatment of bridge timbers has now also been commercialized using a further modification of this approach, with even higher unit cost savings (Lloyd et al. 2017) and commercial testing is now underway in utility poles.
J Lloyd, A Taylor, C Brischke, N Irby, M Manning