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Thermal Modification of Specialty Species Results of Scion’s SSIF-funded experiments

By Rosie Sargent, Jamie Agnew, Liz Cunningham, September 2019.

Download SWP-T086 (pdf)

Executive summary

The problem

The objective of this work is to improve the properties of two specialty wood species (Eucalyptus nitens and Cupressus lusitanica) through thermal modification. Thermal modification darkens the colour of wood, increases dimensional stability, and at high levels of modification increases durability. Increased durability has been a primary focus for this work, but due to poor durability results for E. nitens, there has been a shift in focus for this species, to see if there is a place for thermally modified timber in interior markets, where durability is not a concern, so severe levels of modification are not required.

Key results

Eucalyptus nitens

Interim (33 month) fungus cellar results for E. nitens modified at Scion using an atmospheric steam process are not looking promising. The durability of the modified wood is greater than that of the unmodified controls, but not as great as H3.2 CCA-treated radiata pine. A more severe level of modification is not possible for this modification process, as the existing modification process is already producing excessive levels of degrade in the wood. An alternative pressure steam modification has achieved a similar degree of modification to the atmospheric steam process, while producing no degrade in the wood. This means there is an opportunity to trial a more severe pressure steam modification to increase the degree of durability.

Market research was performed to better understand New Zealand market requirements for interior timbers and how thermally modified wood fits within these. In general the responses were not positive for thermally modified timber, there is considerable market pull for very pale coloured timbers, and high consumer demand for traditional species such as white oak. Dimensional stability is not seen to be an issue with solid wood flooring, and there was a general feeling that existing flooring timbers were able to meet the range. There is anecdotal evidence that some overseas markets have very different drivers for interior wood products, and there could be opportunities for thermally modified wood in interiors in other markets.

Cupressus lusitanica

Interim (12 month) fungus cellar results for C. lusitanica are very promising with modified heartwood and sapwood both showing increased durability over unmodified heartwood, and showing similar durability to H3.2 CCA treated radiata pine. Mechanical testing results are in line with what we would expect for a severe level of modification - a slight (5%) decrease in MOE and a substantial (40%) decrease in MOR. A pilot (2.4m) modification has been performed to produce enough material to begin outdoor durability testing. Initial results (colour change and anti-shrink efficiency) show no significant difference in properties between the lab scale and pilot scale modifications. The lab scale modifications showed a high degree of degrade (checking) following modification, but this was eliminated in the pilot scale modification by drying the wood to a very uniform moisture content prior to modification.

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