In the USA, borates are increasingly being applied prior to air seasoning and creosote treatment of railroad ties (railway sleepers). Borates are typically applied to green ties by vacuum pressure application at 1 to 10% disodium octaborate tetrahydrate (DOT) concentration or by traditional dip-diffusion treatments at elevated temperatures. Higher temperatures are used to maintain the 30 to 50% solution concentrations needed to deliver appropriate cross-sectional retentions with only a topical application. In our study, a thickened borate system similar to that developed in New Zealand in the 1970s for the treatment of framing lumber was investigated as a possible method for the treatment of the larger timbers used for rail ties. This paper reports on the penetration and retentions achieved with three wood species dip-treated green in DOT at ambient temperature in a high-concentration borate emulsion. In an initial test, freshly-sawn crosstie sections of three common wood species (sweet gum, red oak and white oak) were dipped for three minutes in one of five different concentrations (25-50%) of borate emulsion at approximately 10oC. It was found that dipping in a 30% concentration liquid provided weight retentions that approximated to American Wood Protection Association (AWPA) standard retentions (2.7 kg•m-3). In the subsequent study, tie sections dipped in 30% DOT liquid were stacked and stored outside, with moisture-barrier wraps or roof covers. Moisture content and boron retention and penetration were monitored over time. During 3 months of monitoring, 10-20% reduction occurred from the 65-75% initial moisture content. White oak samples had the lowest borate retention of the three species and gum had the highest. Penetration also varied according to wood species. The data suggest that an ambient temperature dip-diffusion treatment of green railroad ties with DOT could provide cost-effective creosote pretreatment with the additional benefit of protecting the wood during the air seasoning period.

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