Temperature, Time, and Interactions between Them in Relation to Colour Parameters of Black Poplar (Populus nigra L.) Thermally Modified in Nitrogen Atmosphere.

Olga Bytner,Janusz Zawadzki,Michał Drożdżek,Agnieszka Laskowska

doi:10.3390/ma15030824

Abstract

Thermal modification of wood in nitrogen atmosphere permits its usability value to be improved. The aim of the research was to determine the impact of technological modification parameters at different levels on the colour of black poplar (Populus nigra L.). Black poplar was thermally modified in nitrogen atmosphere at a range of temperatures from 160 °C to 220 °C, at times between 2 h and 8 h. The parameters of wood colour were measured according to the CIE L*a*b* colour space model. The changes in a* and b* had a non-linear profile. The maximum value of a* for black poplar wood was achieved after a modification at the temperature of 200 °C, while the maximum value of the b* parameter was achieved after modification at 190 °C. Colour changes in the ΔE of black poplar after modification at 160 °C and 170 °C were similar, and the dynamics of changes increased after modification at the temperature of 180 °C. The highest value of ΔE, around 40, was observed after modification at the temperature of 220 °C and time of 8 h. There were no statistically significant differences between the ΔE for radial and tangential sections. Statistical analysis showed that modification temperature was responsible for the variability of the L* value in 90%, and in ca. 70% for the changes in parameters a* and b*. The influence of the modification time on the colour parameters was minor—below 4%. The influence of the interaction between modification temperature and time on the colour parameters was below 10%. As a consequence, in the case of ΔE of black poplar wood, the influence of temperature was at a level of ca. 80%. On the other hand, the influence of time and the interaction between temperature and time of modification was low—below 3%.

Highlights

The development of the wood industry depends, to a large extent, on the implementation of innovative solutions in the scope of production technology
Bytner et al [12] showed that the share of hemicelluloses in non-modified black poplar wood amounted to ca. 30%
After a modification in 160 ◦C for 2 h, the share of hemicelluloses amounted to ca. 24%, and after the modification in 220 ◦C for 2 h, the share amounted to only 3%, so it was 10 times lower than in non-modified black poplar wood

Summary

Introduction

The development of the wood industry depends, to a large extent, on the implementation of innovative solutions in the scope of production technology. One of the methods to achieve those goals is through processes of wood modification, mostly thermal processes. Thermal modification is used to improve the aesthetical and usability properties of local wood species or to increase their resistance to destructive environmental factors [4]. In recent years, a tendency to substitute wood of high mechanical properties with lower quality wood can be seen. The development of such modifications has been accelerated by a growing ecological awareness, high prices, and difficult access to tropical wood. Modified wood can replace the tropical species due to the good functional properties and dark colour achieved as a result of modification [6]

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