Effect of supercritical CO2 treatment and kiln drying on collapse in Eucalyptus nitens wood

Bernard S W Dawson,Alan R Dickson,Bruce Davy,Mark O Kimberley,Hamish Pearson

doi:10.1007/s00107-020-01500-5

Bernard S W Dawson, Alan R Dickson + Show 3 more

Open Access

https://doi.org/10.1007/s00107-020-01500-5

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Abstract

Collapse-prone timbers such as species of Eucalyptus are poorly utilised due to low conversion rates that necessitate long pre-drying times. A supercritical CO2 lumen water expulsion pre-treatment prior to kiln drying is proposed to bypass lengthy pre-drying. After drying (air, kiln or oven drying), shrinkage, collapse, washboard depression and checking of Eucalyptus nitens were determined using image analysis of 0.8 mm thick wafers and 5 mm thick biscuits. Lumen water expulsion-kiln drying reduced collapse by 75% and washboard depression by 71%, compared to drying from green. As water is removed from the water conductive tissue (vessels, rays, and fibre-tracheids) by lumen water expulsion, the water column is broken throughout the specimen, thereby disrupting the development of meniscus-induced water tension as subsequent drying occurs. Remaining water is proposed to reside in the non-water-conductive fibre tissue. If the process can be applied on large scale to Eucalyptus nitens, there is the opportunity for higher conversion rates to increase the commercial viability of solid wood products.

Highlights

The moisture content in living trees can exceed 200% (m/m) with large in-tree variation (Simpson and TenWolde 1999)
Collapse is thought to be the result of negative water tension that develops in boards during the early stages of drying while still at very high moisture contents (Kauman 1964; Chafe et al 1992)
Previous work has shown that lumen water expulsion removes the majority of the cell lumen water in Pinus radiata (Dawson and Pearson 2017) resulting in moisture contents (MCLWE) close to the fibre saturation point

Summary

Introduction

The moisture content in living trees can exceed 200% (m/m) with large in-tree variation (Simpson and TenWolde 1999). Yang and Liu (2018) reviewed the history of collapse in the genus Eucalyptus They considered the morphological characteristics of collapse and gave an overview of the liquid tension theory of collapse, which involves a differential pressure across the menisci of saturated cell lumens (a lumen is the inside space of a cell). They listed treatments designed to reduce collapse as well as variants of drying processes. Collapse is thought to be the result of negative water tension that develops in boards during the early stages of drying while still at very high moisture contents (Kauman 1964; Chafe et al 1992).

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