EFFECT OF EXTRACTIVES ON WOOD COLOR OF HEAT TREATED Pinus radiata AND Eucalyptus pellita

Heat treatment can change wood color without any use of environmentally harmful chemicals, and the efficiency of this process depends on the raw material to be treated. The objective of this study was to evaluate the influence of extractives on the color change of Eucalyptus pellita and Pinus radiata wood during heat treatment. The extractives were extracted in cold water and in dichloromethane as well as totally removed and the wood was treated at 170°C and 200°C for three hours under atmospheric pressure and in presence of air for evaluating the lightness (L*), green-red coordinate (a*), blue-yellow coordinate (b*), color saturation (C) and tonality angle (H) values. Pinus radiata wood was more resistant to discoloration by heat treatment. The removal of total and cold water-soluble extractives before heat treatment changed the L* value of Pinus radiata, a* value of Eucalyptus pellita, and b*, C, and H values of both species. Removal of extractives soluble in dichloromethane did not affect the color of heat treated wood. Thus, understanding the influence of extractives on heat treated wood can allow adapting the raw material to the process for enhancing the applicability of heat treatment for changing wood color.


INTRODUCTION
The traditional methods of changing wood color involved use of chemicals, like varnishes and paints, resulting in generation of toluene and xylene ( Korkut 2012).In comparison, heat treatment is an environmentally friendly and convenient method for changing the color of wood.This process involves application of heat of 160-230°C on the wood to be treated (Kocaefe et al. 2008, Esteves and Pereira 2009, Pétrissans et al. 2014).This temperature range degrades the hemicellulose and extractives (Brito et al. 2008, Zanuncio et al. 2014c).As positive effect, heat treatment reduces hygroscopicity and increases the dimensional stability (Esteves et al. 2013, Palermo et al. 2014, Ratnasingam and Ioras 2012, Bal and Bektaş 2012), as negative effect, it damages the mechanical properties of wood (Garcia et al. 2012, Dundar et al. 2012, Cademartori et al. 2012, Dubey et al. 2012).
The wood extractives influence the color of heat treated wood (Chen et al. 2012), they vary between wood species, part of the tree, and environmental conditions (Alañón et al. 2011, Zanuncio et al. 2013).Other factors such as difference between radial and longitudinal section, species and conditions of treatment also influences the color of the wood heat treated (Picelli et al. 2012, Cademartori et al. 2013, Sun et al. 2013).The CIELAB system (1976) is the most commonly used method to evaluate color of materials.In this system, "L*" indicates lightness with a scale of 0 to 100 and values increasing from black to white; "a*" indicates the green-red coordinate with a positive number, indicating the red and negative green colors; "b*" indicates blue-yellow coordinate with a positive number, indicating yellow and the negative blue colors; "color saturation" (C) indicates the purity of the color; and "tonality angle" (H) indicates the dominance of any component in the color.
The effect of extractives on color change during heat treatment of wood is not well known.Thus, the objective of this study was to evaluate the influence of extractives on the color change of P. radiata and E. pellita wood during heat treatment.

MATERIAL AND METHODS
Disks of 10-cm thickness were cut at 1,3 m above the ground level from three 8-year-old E. pellita trees and from three 14-year-old P. radiata trees.The average diameter of the discs were 16,1 cm and 22,2 cm for E. pellita and P. radiata, respectively.The samples were triturated using Willey mill, sieved through 40-and 60-mesh sieves, and the fraction retained in 60-mesh sieves was conditioned in a climatic chamber at 23 ± 1 °C and relative humidity of 50 ± 2%.
The extractives soluble in dichloromethane were determined according to the American Society for Testing and Materials ASTM D-1108ASTM D- -94 (1994)); extractives soluble in cold water according to ASTM D-1110ASTM D- -94 (1994) ) and the total extractives according to ASTM D-1105ASTM D- -94 (1994)).Then, the samples were heat treated at 170°C and 200°C for three hours in an oven at atmospheric pressure and in the presence of air.The extraction and heat treatment were carried out with five replicates per treatment.In each sample, the colorimetric analysis was performed three times.
The means were subjected to analysis of variance (ANOVA) and, when established significant differences (p <0,05); the treatments were compared by Scott-Knott test 0,05 probability level, according to Melo et al. 2015.
Table 1.Extractives content of E. pellita and P. radiata soluble in cold water, dichloromethane and total extractives.
The proportion of extractives soluble in cold water in E. pellita was higher than in dichloromethane, which represents the pattern for most hardwoods (Moya and Tenorio 2013, Kilic and Niemz 2012), mainly of Eucalyptus species (Morais andPereira 2012, Zanuncio et al. 2014b).On the other hand, the proportion of extractives soluble in dichloromethane in P. radiata was higher than that in E. pellita.In general, conifers have a higher proportion of extractives soluble in dichloromethane than hardwoods, as reported for Pinus oocarpa and Pinus taeda in relation to Eucalyptus urophylla, Swietenia macrophylla, Tectona grandis and Vochysia guatemalensis (Morais et al. 2005, Brand et al. 2011, Moya and Tenorio 2013, Zanuncio et al. 2014b).
Heat treatment changed the color in P. radiata and E. pellita wood in all parameters evaluated (Table 2).The behavior of heat-treated wood color of these plants differed as the temperature increased, showing the effect of species in this process.Heat treatment decreased the value of L* of E. pellita and P. radiata, especially at 170°C and 200°C, respectively, but with higher impact on the first specie.Softwoods are more resistant to L* change by heat treatment (Ayadi et al. 2013, Esteves et al. 2008, 2009, 2014, Zanuncio et al. 2014a), which explains greater reduction of P. radiata at higher temperatures.
The removal of total extractives and extractives soluble in cold water increased the L* of P. radiata treated at 200°C.The L* of wood without removal of extractives or after extraction with dichloromethane was lower than of those with total extraction or after extraction in cold water.The extractives soluble in dichloromethane decompose between 250°C and 550°C (Mészáros et al. 2007), which is higher than the temperature applied in this work, explaining the similar L* value of heat treated wood with or without dichloromethane extraction.
The green-red coordinate (a*) of E. pellita wood was higher than that of P. radiata.Heat treatment reduced the a* value of E. pellita and increased that of P. radiata, with similar values between wood species treated at 170°C.At 200°C, the a* values of P. radiata were higher than those of E. pellita, similar to that for Pinus caribeae, Pinus sylvestris, Eucalyptus grandis and Eucalyptus saligna (Kamperidou et al. 2013, Picelli et al. 2012, Cademartori et al. 2013).
The reduction of a* values in heat-treated hardwoods is associated with phenolic extractives degradation (Pincelli et al. 2012, Moura andBrito 2011).These extractives are removed by total extraction or by extraction in cold water.Thus, the a* values in E. pellita wood with total extraction or after extraction in cold water were higher than those of heat-treated control woods.The removal of extractives soluble in cold water did not affect the a* value of heat-treated P. radiata due to its low extractive content of 1,25%.
The heat treatment reduced the blue-yellow coordinate (b*) in E. pellita and P. radiata wood, mainly at 170°C and 200°C, respectively, the values was higher for P. radiata wood at all temperatures.Softwoods are more resistant to change in b* value compared to hardwoods (Esteves and Pereira 2009, Moura andBrito 2011, Picelli et al. 2012).Wood with total and cold water extraction showed similar b* value after heat treatment, but with higher values than those of timber without extraction or with dichloromethane extraction.P. radiata wood showed higher values for color saturation (C) than E. pellita wood at all temperatures (Table 2).C value result from a* and b* coordinates, and the higher value of blue-yellow coordinate (b*) in P. radiata wood made the color saturation (C) follow (b*) values.The removal of total extractives and extractives soluble in cold water increased the color saturation (C) of P. radiata wood treated at 200°C, which is similar to the tendency for b*, and increased that of E. pellita wood treated at 170°C and 200°C, for which a* and b* showed similar trends.P. radiata wood showed higher tonality angle (H) in all treatments, followed by b* values (Table 2).Removal of extractives affected the H value of E. pellita and P. radiata wood treated at 200°C due to changes in the a* and b* coordinates at this temperature.The removal of total or soluble extractives with cold water reduced the H value of E. pellita, while the opposite was true for P. radiata wood.
The extractives should be considered during heat treatment for the purpose of changing wood color.Detailed knowledge about the raw materials can optimize, increase the applicability, and the economic viability of the heat treatment process in such cases.