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Investigation of heat treatment on wood veneers by NIR spectroscopy

Hikaru Kobori 1 ; Emilia-Adela Salca 2 ; Ayaka Kajita 1 ; Tetsuya Inagaki 3 ; Yoichi Kojima 1 ; Shigehiko Suzuki 1
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INTRODUCTION
Heat treatment is applied to wood to improve its surface texture and mechanical properties. Most of heat treated wood present a darker appearance compare to untreated. It allows them to replace some tropical species from the aspect of color appearance. Wood veneers can be used as overlaying material for laminations of particleboards and medium density fiberboard (MDF) for furniture products. Due to their thickness, shorter duration of heat treatment will be needed for veneers compared to solid wood, however, there are little information for heat treatment of veneer. In this study the effect of heat treatment on color changes of veneers of various wood species was investigated by using CIELab and NIR spectroscopy.

EXPERIMENTAL
50 mm x 50mm x 0.2mm thickness veneer samples of Romanian and Japanese wood species were prepared. They were subjected to heat treatment in a regular oven at 190°C with different span times after one week of curing period at 20°C and 60%RH. After heat treatment, color measurements were performed according to ISO 7724-2 standard by using a Chroma Meter Konika Minolta CR-410 device. Diffuse reflectance NIR spectra ranging from 10,000-4,000cm-1 with 3.85cm-1 interval was also acquired from the surface of the veneers by FT-NIR spectrometer MatrixF-Bruker Optics. The relationship among L*a*b* values, heat treated duration and absorbance at NIR region were investigated. Data was processed by Matlab R2014a.

RESULTS AND DISCUSSION
As the duration of heat treatment increased, absorption around 5220-5150cm-1 which is assigned H2O decreased while at the same time absorption due to crystalline regions of cellulose (5464cm-1), phenolic groups of lignin (6874cm-1) relatively increased. As the duration of heat treatment increases, L* value decreased. In the case of European Beech veneer, although a* value gradually increased as the duration of heat treatment increased, the relationship between duration and b* value was unclear. In the case of Japanese cypress veneer, variance of L*, a* and b* for 40minites treated samples were lower than that of untreated samples. Heat treatment may improve the quality stability of appearance. PLS predictive models for L*a*b values were built from NIR spectra. Prediction models of L* value showed proper prediction accuracy. However, prediction models of a* for Hinoki cypress and that of b* for European Beech did not work. Predicted values of these models were just separated into two classes; heat treated and untreated. These results indicated that the relationship between chemical change and color appearance during heat treatment differ from species.