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Influence of drying processes on the treatability and CCA distribution in the heartwood of five Canadian softwoods An, Yuxian

Abstract

The first objective of the study was to determine whether drying could improve the treatability of sawnwood with respect to CCA and borate preservatives. Heartwood boards of lodgepole pine (Pinus contorta Dougl.), white spruce (Picea glauca [Moench.] Voss.), amabilis fir (Abies amabilis [Dougl] Forbes), western hemlock (Tsuga heterophylla Raf. Sarg), and Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) were used in this study. The treatability of each board was measured and the boards ranked for permeability. The seven drying processes were: air drying (used as reference), dehumidification drying, conventional drying, presteaming plus conventional drying, high temperature drying, radio frequency/vacuum drying, and superheated steam/vacuum drying. After drying, the boards were pressure treated with either 2.5% CCA or 4.2% disodium octaborate tetrahydrate, and analysed for preservative penetration and retention. It was concluded that: • No single drying regime consistently improved the treatability o f all wood species, for either chemical; • in most cases, CCA and borate preservative penetrations were similar: • for Douglas-fir and western hemlock RF/V drying gave the best penetration for both CCA and borate; The second objective was to examine the microdistribution of the CCA components in different cells of the various softwoods using light microscopy, and scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectrometer (EDX). From the visible and SEM - EDX microscopic examination it was found that: the Cu:As:Cr ratio differed markedly for specific analysis locations in treated wood, confirming the different fixation reactions in specific cell types; ray cells, resin canals (if present) and pits played key roles in enhancing CCA penetration in sawnwood; copper and chromium were located in the highly lignified cell corners and compound middle lamellae; arsenic was precipitated in the secondary wall; enhanced copper content was found in the resin canals, suggesting a preferential reaction with extractives; Copper and arsenic concentrations were enhanced in the pit areas; a high concentration o f chromium was found at the crassulae, which is highly lignified; deposits at the tracheid lumens of amabilis fir were shown to contain mainly arsenic and copper.

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