Improving Drying Quality of Eucalyptus Nitens Timber
By R Sargent, S Riley, J Harrington, D Gaunt, July 2016.
Download SWP-T008 (pdf)
Eucalyptus nitens has a very high incidence of drying degrade, primarily caused by water tension stress occurring early in drying. This degrade reduces the recovery of sawn timber from nitens logs and is a barrier to producing significant volumes of sawn timber.
If drying degrade could be reduced, this would improve the economics of producing sawn timber from nitens, and could potentially lead to decreased drying times, increasing the throughput of nitens processors. Research to date into reducing degrade has primarily focussed on altering the drying conditions (air temperature etc.) during conventional drying. This has not produced promising results, so a different approach is required. Some promising research has been done freezing wood prior to drying to reduce water tension, but water tension forces are very difficult to quantify which hampers research in this area. Scion has an improved method of measuring water tension forces, which can be combined with existing methods of quantifying degrade to assess methods of reducing degrade more accurately than has been possible in the past.
This project aims to reduce drying degrade in nitens using non-traditional means. As an initial step, brief periods of freezing prior to drying are trialled.
A review of processing issues for eucalypt species has been covered in Cown, et al. (2015) “Wood Processing Challenges and Opportunities for Douglas-fir and Several Eucalyptus Species”
The current work is based on the findings from Riley, et al. (2016) “Using freezing techniques to minimise collapse during drying”
In this project five small boards were cut from each of twenty different nitens trees from South Wood Exports in Southland. Four sets of twenty boards were briefly frozen (1 week or less), with the freezing temperature, freezing rate and time frozen varied between sets. One set of boards was not frozen at all. Boards were air dried under controlled conditions and the surface strain measured during drying to quantify the level of collapse-causing water tension stress. Once boards reached 30% MC they were kiln dried and conditioned to ~12%MC. The level of collapse and checking was visually assessed for each board, and the level of water tension stress calculated. Comparing the different freezing treatments there was no obvious differences in drying degrade or drying rate between the different treatments. Overall levels of degrade were lower than we would have expected, with around 20% of boards having unacceptable degrade. Reports of percentages of nitens boards affected by degrade vary considerably: 15-40%: Innes, et al. (2008), 15% Satchell (2015), and at Scion we have previously had 50% of commercially-sourced boards affected by degrade. These variable levels of degrade in nitens need to be understood better before further research can be done in this area.
Implications for SWP
None of the pre-freezing treatments had a noticeable effect on drying degrade or drying rate. There was no indication that pre-freezing had any effect on the wood stiffness during drying.
Overall levels of degrade were lower than expected, making it difficult to compare treatments, and suggesting that the drying method used was not representative of conditions used in practice. Future work of this kind would need to investigate lab scale drying techniques to ensure results are applicable to industrial scale drying.
It is known that checking behaviour varies widely between trees, and using a screening tool to identify trees prone to checking would ensure a greater proportion of check prone trees in each experiment.
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