Preservative treatment to achieve termite and decay resistance has potential to develop new products and markets for post-MPB lumber. The objective of this work was to determine the feasibility of through treating post-MPB lumber to reduce Canada’s competitive disadvantage versus US southern pine and New Zealand radiata pine in markets where termite and decay resistance is important. Post-MPB sapwood has increased permeability but the heartwood is unchanged from its normal low permeability. There is an opportunity to take advantage of the increased permeability of the sapwood in treated lumber with minimal heartwood content. Another potential advantage of treating is the ability to mask bluestain discolouration. Post-MPB lumber was sorted into a heavy stain group (stain across three sides) and a non-sorted group of mixed proportions of heartwood and sapwood. Both non-dried (ND) and kiln-dried (KD) lumber, the latter with planed or combed surface, was pressure treated with borate using a conventional process then stored to allow moisture equilibration. Additional KD lumber was pressure treated with borate at a higher solution temperature and with alkaline copper quaternary (ACQ). Further material, stained and mixed, planed and combed, was treated with a novel accelerated process involving a dip followed by 72 hrs kiln conditioning.
The stained sort, either pressure treated with DOT or with the dip plus 72hr kiln conditioning gave through treatment (over 85% cross section) after one week storage, comparable to requirements for Southern Pine. Lumber sorted for heavily stained sapwood content resulted in about twice the uptake from pressure treatments. ACQ treatment gave substantial penetration only in sapwood, with a very thin shell in the heartwood. A higher temperature DOT solution did not increase uptake from pressure treatment. Pressure treatment uptakes were lower in non-dried wood than in kiln-dried wood. Pressure treatment of kiln-dried lumber with DOT at 20°C met penetration and retention targets only with stained sort combed samples. Pressure treatment of kiln-dried lumber with DOT at 35°C met penetration and retention targets only with stained sort planed (combed not tested) samples. None of the pressure treatments had heartwood penetrations with 80% at or over 10mm even after 2 weeks storage whereas dip-plus-kiln-conditioning treatment met this requirement for mixed sort planed and combed with Treatment 1 and mixed sort combed with Treatment 2 after one week.
Dip-plus-kiln-conditioning uptake was greater on combed surfaces but, strangely, this was not reflected in increased borate loading. Dip-plus-kiln-conditioning Treatment 2 uptakes and retentions were greater than in dip-plus-kiln-conditioning Treatment 1. The dip-plus-kiln-conditioning process increased the wood moisture content to 20% to 30% MC, with the greater increases resulting from dip-plus-kiln-conditioning Treatment 2. Process modifications will be needed to reduce the moisture uptake or bring the moisture content down to below 20% before shipping to prevent problems with stain and mold.
The dip-plus-kiln-conditioning process met penetration and retention targets with stained sort planed or stained sort combed samples treated with dip-plus-kiln-conditioning Treatment 2. Targets were met immediately after treatment. None of the data adequately reflects the difference in uniformity of treatment between the pressure treatment and the dip-plus-kiln-conditioning treatment. Dip-plus-kiln-conditioning treatment gave a much more uniformly penetrated shell around the whole cross section, though this was not reflected in the analyzed heartwood retentions.
Further refinements are needed to the dip-plus-kiln-conditioning process, but these will likely be customized for each sawmill depending on whether spray or dip is used to apply the formulation and the type of kiln used for conditioning. The lack of relationship between analysed retentions, uptakes and penetrations in the dip-plus-kiln-conditioning process requires further investigation.