Color change of selected wood species affected by thermal treatment and sanding

Abstract

The aim of the research was to evaluate the impact of various temperatures of thermal modification and sanding treatment on the color change of sessile oak, Norway spruce, and Red meranti. Thermal modification was carried out at various temperatures. Subsequently, one side was sanded. The measurements were recorded using a BFS 33M-GSS-F01-PU-02 color reader. A Konica Minolta CR-10 Plus colorimeter and Nikon D3200 camera were used in conjunction with the Matlab program. The assessments were conducted in the color space of CIE L* a* b*. The measured values confirmed that the decrease in lightness from natural to thermally modified wood (220 °C) was the largest for non-machined spruce samples (ΔL = 42.47) and the smallest was for sanded spruce samples (ΔL = 31.64). The relative change in sample lightness was the largest for sanded oak samples (51%). The trends of the color values a* and b* were different for individual wood species. Overall, the average color change ΔE was the lowest for the non-machined meranti species (ΔE = 33.06), and the largest for the non-machined spruce (ΔE = 43.06). Comparing the individual methodologies, it was found that all methodologies provided relevant results and can be used in practice.

Full Article

Color Change of Selected Wood Species Affected by Thermal Treatment and Sanding

Mária Hrčková,^a,* Peter Koleda,^a Pavol Koleda,^a Štefan Barcík,^a and Jaroslava Štefková ^b

The aim of the research was to evaluate the impact of various temperatures of thermal modification and sanding treatment on the color change of sessile oak, Norway spruce, and Red meranti. Thermal modification was carried out at various temperatures. Subsequently, one side was sanded. The measurements were recorded using a BFS 33M-GSS-F01-PU-02 color reader. A Konica Minolta CR-10 Plus colorimeter and Nikon D3200 camera were used in conjunction with the Matlab program. The assessments were conducted in the color space of CIE L* a* b*. The measured values confirmed that the decrease in lightness from natural to thermally modified wood (220 °C) was the largest for non-machined spruce samples (ΔL = 42.47) and the smallest was for sanded spruce samples (ΔL = 31.64). The relative change in sample lightness was the largest for sanded oak samples (51%). The trends of the color values a* and b* were different for individual wood species. Overall, the average color change ΔE was the lowest for the non-machined meranti species (ΔE = 33.06), and the largest for the non-machined spruce (ΔE = 43.06). Comparing the individual methodologies, it was found that all methodologies provided relevant results and can be used in practice.

Keywords: Color; Thermal modification; Temperature; Lightness; L*a*b* color values; Oak; Spruce; Meranti

Contact information: a: Department of Manufacturing and Automation Technology, Faculty of Environmental and Manufacturing Technology, Technical University in Zvolen, Studentska 26, 96053 Zvolen, Slovakia; b: Institute of Foreign Languages, Technical University in Zvolen, T. G. Masaryka 24, 96053 Zvolen, Slovakia; * Corresponding author: hrckova@tuzvo.sk

INTRODUCTION

Wood is an important natural renewable material (Dos Santos et al. 2016), and the possibilities for its use are tremendous. It is as important a building material for the interior as it is for the exterior. Wood is used in the furniture industry, and it is an important raw material in the paper industry as well as the power industry. As a natural material, wood is a chemical compound of mostly organic substances. It is mainly composed of organic macromolecular substances: cellulose (35% to 50%), hemicelluloses (20% to 35%), and lignin (15% to 35%) (Geffert 2013). All together, these components make up 90% to 97% of the absolute weight of dry wood. Furthermore, wood also contains extractive substances: carbohydrates, starch, and oils; proteins, inorganic salts, waxes, tanstuffs, and resins; and turpentine, ethereal oils, and colorants, as well as others that influence the wood color (Geffert 2013). The content of extractives ranges up to 10% (Čunderlík 2009). Wood is a readily machinable and mechanically resistant material. However, it has some unfavorable properties, such as hygroscopicity, anisotropy, and poor resistance to biological degraders. Currently, the need is increasing to improve the properties of wood and especially its durability and resistance towards biological agents. Thermal modification seems to be a suitable solution for this requirement.

The color of wood is an important component of its appearance (Babiak et al. 2004), and therefore it figures crucially in the final decision of a customer (Sahin et al. 2011; Jankowska and Kozakiewicz 2014; Kubovský and Igaz 2014; Barcík et al. 2015). Color is one of wood’s basic physical properties (Čunderlík 2009; Dzurenda 2018). The color is determined by the chemical components of wood. Besides genetic factors, the color of the wood is influenced by the environmental conditions (the humidity, solar radiation, pollution, and wind) where the tree and the wood grew (Čunderlík 2009; Valverde and Moya 2013). The color is also one of the parameters of the quality of the surface assessment.

At present, color and shade are measured by colorimetry, which is based on the standards and technical requirements stated and issued by the International Commission on Illumination (Commision Internationale de l`Éclairage). The color space CIE L*a*b* is the closest to the human perception. Its construction is based on the theory of opposite colors. The color space CIE L*a*b* is characterized by three parameters: L*, a*, and b*. The vertical axis L* represents the lightness (100 = white, 0 = black). The chromatic axes are represented by the a* and b* components. The a* axis represents the shift from green (-) to red color (+), and the b* axis represents the shift from blue (-) to yellow (+) (Babiak et al. 2004; Tuong and Li 2010; Dzurenda 2013). The CIE L*a*b* is the most frequently used color space for measuring wood surface color (Brischke et al. 2007). The color measurement is executed by apparatuses called spectrophotometers or colorimeters (Babiak et al. 2004) or optic color readers.

Thermal modification of wood is the process by which wood is modified by high temperatures (Barcík et al. 2015). This technology is considered an ecological wood treatment because it does not use any chemical substances (Boonstra 2008; Tuong and Li 2010; Aydemir et al. 2012). It is crucial to carefully determine the conditions of modification, including maximum temperature, exposure time, wood dimensions, and suitable equipment. The changes to wood and its chemical composition occur based on set conditions (Kučerová et al. 2016). The properties of thermally modified wood have been investigated for a rather long time. Thermally modified wood is more resistant to microorganisms, and it has greater dimensional stability, durability, and insulating and hygroscopic properties (Johansson 2005; Reinprech and Vidholdová 2008; Mitani and Barboutis 2014). Based on the newly-established properties, the interest in its use in various areas, e.g., floorings, sauna furniture, garden furniture, exterior and interior siding, windows, doors, roofing systems, etc., is constantly growing (Vančo et al. 2016).

Thermal modification uses the thermal and hydrothermal effects of high temperatures (150 to 260 °C) and in various environments (vacuum, inert atmosphere, air, water, or oil) (Reinprecht and Vidholdová 2008; Kačíková and Kačík 2011). In several countries in Europe and in Canada, different methodologies of thermal modification have been patented, for example Thermo Wood in Finland, which uses air, PlatoWood in Germany, which uses oil, and Rectification in France, which uses inert gases (Tuong and Li 2010).

Changing the chemical composition of wood due to thermal modification also determines the color change of the wood. The change in color is a function of temperature and time (ITWA 2003; Hill 2006). The increase in temperature and treatment time leads to darker colorification (Brischke et al. 2007; Klement and Marko 2008; Esteves and Pereira 2009). Under the effect of high temperature, the wood takes the color of yellow-brown to brown-black and often resembles that of tropical wood species. The wood acquires darker shades due to the changes in the basic constituents of wood and extractive substances (Tuong and Li 2010). The degree of color change may indicate the quality of the thermal modification (Hill 2006; Kamperidou and Barmpoutis 2015). After mechanical machining (cutting, planning, milling, or sanding), the wood retains the original shade acquired by the thermal modification. The measurement of the wood surface color is a quick, precise, and reproducible process (Brischke et al. 2007; Hrčka 2008). The acquired information about a color of a thermally modified wood together with the knowledge of the temperature used can provide a wide range of possibilities for further use. They can serve as the foundations for monitoring the quality of thermally modified wood, as well as the input parameter to determine the temperature of modification, reflecting the demands for the wood appearance for further processing.

The aims of the present experiments are as follows: first, to determine the color change in the CIE L*a*b* color space of selected wood species, namely, of Sessile oak, spruce, and Red meranti, under the influence of thermal modification at 160, 180, 200, and 220 °C, second, to assess the influence of sanding on the color of the wood species, and third, to verify whether the use of other methods of wood color evaluating demonstrates significantly different results compared to the Conica Minolta CR-10 Plus. This sensor is normally used to detect the color of wood species as well.

EXPERIMENTAL

Materials

The choice of the three wood species corresponded to the requirements of the VEGA project 1/0315/2017 “The Research of the Relevant Properties of Thermally Modified Wood in the Contact Processes of Machining Addressing the Optimal Surface Achievement”. The samples were prepared from both traditional European and exotic wood species (Sfarra et al. 2017). To make generalizations, the emphasis was placed upon the heterogeneity of the selection of species and colors – wood species native to Slovakia – deciduous, coniferous, and an exotic wood species. The color of the wood of these trees was also considered; they were divided into three groups according to color of wood – pale wood (spruce), brown wood (oak), and red wood (meranti) (Čunderlík 2009). The three wood species used for the experiment were sessile oak (Quercus petraea), which has a distinct grain and interesting wood texture, Norway spruce (Picea abies), which has a pale wood color and minimal colour differences, and Red meranti (Shorea acuminata), which has an exotic grain and wood of various shades of color. The exotic wood species meranti resembles a wood species native to Slovakia of oak in its appearance as well as in its properties. The wood species of sessile oak and Norway spruce were harvested in Slovakia, in the locality of Vlčí jarok (440 meters above sea level) near Budča. The samples of the wood were made in the Research and Development Workshops of the Technical University in Zvolen. Radial boards 25-mm-thick were cut out of heartwood of the logs by a MEBOR HTZ 1000 (Mebor d.o.o., Železniki, Slovenia) saw and then dried to 8% moisture content. The 8% moisture selection was based on the environment properties (interior) in which the wood will be used. The test samples, with dimensions 20 mm × 100 mm × 700 mm, were made from these boards. Red meranti wood species in Slovakia was imported from Malaysia and Indonesia. The red meranti used in the research was purchased from the importer (Wood Store, Prague, Czech Republic) in the requested dimensions. The country of origin was not specified by the importer. The density of the studied wood species is in Table 1.

Table 1. Densities of the Studied Wood Species

The prepared samples were divided into five groups. The first group was left in their natural state. The other groups of samples were thermally treated using the ThermoWood® technology, each at a different temperature (160 °C, 180 °C, 200 °C, and 220 °C). The process of the thermal treatment was executed in the Arboretum of the Faculty of Forestry and Wood Sciences (Czech Agricultural University, Prague, Czech Republic) in Kostelec nad Černými lesy in a LAC S 400/03 type chamber (Katres s.r.o., Říčany, Czech Republic), (Fig. 1). The basic parameters are given in Table 2.

Fig. 1. Chamber S400/3

Table 2. Parameters of LAC S 400/03 Chamber

The samples were stored at the temperature of 10 °C. The process of thermal modification was managed by the program. The samples remained in the chamber until they had cooled to 60 °C, and then they were removed. The process of the temperature change itself (heating, temperature exposure, and cooling) in time is illustrated in Fig. 2.

Fig. 2. The stages of thermal modification of wood species: a) sessile oak, b) Norway spruce, and c) red meranti