A comparison of NIR spectroscopy and colour measurements for the classification of radiata pine timber thermally modified over a narrow temperature range

Abstract

The New Zealand Building Code is unique in the world in having prescribed durability requirements for all building elements, including interior timber such as flooring. Heavy metal preservatives are still used extensively for this purpose, despite being banned in many other countries. Thermal modification of wood increases timber durability without relying on chemical additives. This involves heating the wood in a carefully controlled environment with the modification settings being specific to the process and the species. Thermal modification induces chemical changes in the wood structure that can be identified using spectroscopy. Correlation of chemical changes to specific processing conditions for a particular wood species may then enable independent verification of the modification temperature. This is a crucial aspect of product compliance with the New Zealand Building Code. Without being able to verify the modification temperature, evidence of compliance with these stringent durability requirements is extremely difficult. In this study, near-infrared (NIR) spectroscopy was investigated as a tool for developing a quality assurance/quality control (QA/QC) methodology, to ascertain the claimed heat treatment levels have been applied to thermally modified timber products. NIR spectroscopy is also compared to techniques based on colour changes of heat-treated timber. It was found that a predictive classification model, based on NIR spectra from solid wood surfaces, was able to predict 100% correctly the 3 treatment levels of thermally modified timber used in this study (210 °C, 220 °C, 230 °C). It was superior to a L*, a*, and b* colour model (87% correct) and a visible colourimetry spectrum model (95% correct).