Maderas. Ciencia y tecnología. 5(2): 145-152, 2003

NOTA TÉCNICA

THE EFFECT OF HEAT TREATMENT ON SOME PROPERTIES AND COLOUR IN EUCALYPTUS (Eucalyptus camaldulensis DEHN.) WOOD

O. Unsal¹; S. Korkut¹; C. Atik^1
1Istanbul University, Forestry Faculty, 34473 Bahcekoy, Istanbul – Turkey.

Corresponding Author: suleymankorkut@hotmail.com

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ABSTRACT

Heat treatment is often applied to some wood species to improve dimensional stability. This study evaluated the effect of heat treatment on some physical and mechanical properties and colour of Eucalyptus wood (Eucalyptus camaldulensis Dehn.), which has industrially high usage potential and large plantations in Turkey. Wood specimens from Tarsus, Turkey were subjected to heat treatment in varying temperatures and durations. After the heat treatment, hardness, swelling, ovendry density, and colour change of the wood specimens were tested in comparison with untreated specimens. The results showed that density, swelling, and hardness decreased with increasing treatment temperature and durations while heat treatment made the colour of the wood specimens darker.

Keywords: Thermal treatment; Eucalyptus wood; Colour; Physical and Mechanical Properties

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INTRODUCTION
In the previous years, Eucalyptus plantations have received a lot of attention because of their ability to dry bogs. In addition, the wood of this species is an important raw material for some industries. Eucalyptus wood has recently been included in the forest products industry in Turkey as a wood material. The wood is important in some usage areas such as pulp and paper, fibre and chip industry, and packing box production. Since it is a fast-growing species, Eucalyptus plantation is also encouraged by pulp and paper industry in the country. Despite many advantages, Eucalyptus wood has some disadvantages such as high swelling, low dimensional stability, and several drying problems limiting its use.

Considerable research has been carried out in order to improve dimensional properties of wood. The methods brought out based on the result of these studies are commonly named“Wood Modification Methods”. The aims of heat treatment as a modification method are: (1) to reduce swelling-shrinkage of wood, in turn, to increase its dimensional stability; (2) to increase biological resistance, permeability and performance of preservatives such as CCA and CCB, the quality of surface treatments, and (3) to reduce equilibrium moisture content at a given ambient condition of humidity and temperature.

Previous studies showed that shrinkage-swelling, strength, hardness and density of wood decreased with heat treatment while this treatment also improved its dimensional stability. In many studies, the changes on dimensional stability were investigated for many trees such as beech, poplar, oak, eucalyptus, alder, maple, spruce, birch, and Scots pine with the heat treatment applied at 100-230 °C and 2- 48 hr. In these investigations, an increased dimensional stability up to 55-90% was generally obtained with increasing of temperature and duration (Feist and Sell 1987; Giebeler 1983; Burmester 1973; Burmester and Deppe 1973; Stamm at al. 1960; Viitaniemi 1997).

Stamm and Hansen (1937), by applying heat treatment to dried-wood, and Tiemann (1920), by drying using high temperatures, found a decrease in hygroscopicity of wood material. In the other studies, weight losses were observed in wood material with increased temperature and duration. In the study on Spruce (Picea abies) wood applied heat treatment during 24 hr, weight loss was also found 0.8 % and 15.5% at 120 °C and 200 °C, respectively (Fengel 1966).

Colour of wood surface affected by heat treatment is also a major criterion for evaluating the quality of furniture, panels, door elements, and flooring. The colour compatibility of components is important when matching pair individual pieces into final products. In the past, the segregation of pieces on the basis of colour was carried out by trained personnel. However, the human judgement is not always consistent and certainly influenced by the surrounding as well as by the colour variations within a lot. For this reason, industry started to employ electronic colour measuring systems to keep colour variations of products within specified ranges (Resch et al. 2000).

Wood colour becomes darker based on temperature, treatment time and techniques (MacLean 1951; Mailum and Arenas 1974; Militz 2002). It was determined that evident colour changes start at 60 °C for hardwoods while at 90 °C for softwood (Kollmann 1955). In addition to colour changes, heat treatment also has some other negative effects. One of the major problems is strength losses explained by thermal degredation rate and material loss (Rusche 1973). Hardness and strength of wood decrease when wood is heated and increase when it is cooled. This effect is clearly achieved with prolonged treatments. The least to be affected property is modulus of elasticity while the most to be affected properties are impact and static bending strengths (MacLean 1953; Millett and Gerhards 1972; MacLean 1954; MacLean 1955).

In a study, maritime pine and poplar samples of dimensions 50x100x2000 mm were heat treated during 6 h at temperature varying from 200 to 250^o C depending on species. This infrared spectroscopy study has indeed revealed a modification of chemical bonds in treated wood: the number of oxygen containing groups (mainly hydroxyl groups) decreased while the number of C= double bonds increased. Cellulose cristallinity does not seem to be affected (Avat 1993).

In the current study, our objective was to determine the effect of heat treatment on some physical and mechanical properties, and colour in Eucalyptus wood.

MATERIALS AND METHODS

The logs (min. diameter 30 cm) of the survey were obtained from Tarsus region with supporting of Eastern Mediterranean Forestry Research Institute. Taken logs were sawn into lumbers in sawmill of University of Istanbul, Faculty of Forestry (Anonymous 1976). Sawn pieces were planed (knife angle 45°) and then the samples were cut for hardness, swelling and density measurements. Wood surface colour was measured on the hardness test samples.

Specimens were subjected to heat treatment at either 120, 150 or 180 °C for 2, 6 or 10 hours in a small heating unit controlled with ± 1 °C sensitively under atmospheric pressure. After heat treatment, treated and untreated samples were conditioned to 12% moisture contents (MC) in a conditioning room at 20 °C (± 2) and 65 % (± 5) relative humidity (RH). The number of specimens taken from each log was nearly equal (30, 30 and 40 samples for hardness, surface colour, density and swelling, respectively).

Tests of static hardness (Janka), density and swelling were carried out based on Turkish Standards (TS) TS 2479/1976 (Anonymous 1976), TS 2471, TS 2472 and TS 53 (Anonymous 1976; Anonymous 1976; Anonymous 1976) and TS 4084 (Anonymous 1982), respectively.

For all parameters except for colour, all multiple comparisons were first undertaken for an analysis of variance (ANOVA) and significant differences between mean values of control and treated samples were determined using Duncan¢s Multiple Range Test. For colour changing, only mean values were given.

To measure the colour of the thermal treated eucalyptus wood of 5x5x5 cm pieces were taken. The cross-sectional surface was regarded as the most representative plane for revealing the colour difference of samples. Surfaces of the treated samples were sanded down (sandpaper roughness: P100) 3 mm and brushed cleanly (Monnonen et al. 2002).

The colour measurements were performed using an Elrepho 3300 spectrophotometer at 23 °C temperature and 50% RH. The device was calibrated against a white working standard supplied with the instrument. Measurements were made using D65 illumination, 10° standard observer and 34 mm aperture.

The CIE L*, a* b* (Commission Internationale de l¢eacute;clairage) characters were determined and the colour changes as a function of heat treatment were calculated using the following equation:

where R = reference, S = sample tested

The CIE colour system utilize three coordinates L*, a*, and b*to locate colour in colour space. L*axis represents the lightness, it varies from 100 (white) to zero (black), a*and b*are the chromaticity coordinates, representing red-green and blue-yellow respectively.

RESULTS AND DISCUSSION

Table 1 shows the changes in the ovendry density, swelling, and hardness at varying treatment temperatures and durations. Results showed that all the parameters decreased as temperature and duration increased. Anova and Duncan¢s Multiple Range Tests showed that there were significant differences between the values (Table 1).

Table 1. Measured ovendry density, swelling and janka-hardnessa at different treatment temperatures and treatment durations.

Heat Treatment	Times	Units	Ovendry Density	Swelling		Janka Hardness
			Do g/cm³	Radial %	Tangential %	Cross-Section N/mm²	Radial /mm²	Tangential N/mm²
None		Avg.	0.680	6.59	7.67	73.6	36.2	42.3
		± s	0.05	1.33	1.72	138	54	71
		s²	0.003	1.78	2.96	19049	2876	4996
		V	8.37	20.28	22.43	18.7	15	17
		N	30	40	40	30	30	30
120°C	2 hr.	Avg.	0.658	6.38	6.79**	71.1	34.6	40.3
		± s	0.05	1.43	1.32	93.3	50.5	46.6
		s²	0.002	2.06	1.74	8711	2554.4	2168
		V	7.89	22.51	19.47	13.1	14.6	11.6
		N	30	40	40	30	30	30
	6 hr.	Avg.	0.653	6.31	6.65**	65.3**	29.4**	35.1**
		± s	0.003	1.25	1.51	94.3	45.2	61.2
		s²	0.05	1.56	2.29	8894	2045	3745.4
		V	9.008	19.82	22.76	14.4	15.4	17.5
		N	30	40	40	30	30	30
	10hr.	Avg.	0.648	6.21	6.44**	61.3**	24.5**	31.1**
		± s	0.05	1.3	1.14	111	36.1	28.8
		s²	0.002	1.7	1.3	12319	1304	827.3
		V	8.18	20.99	17.74	18.1	14.8	9.26
		N	30	40	40	30	30	30
150°C	2 hr.	Avg.	0.650**	6.33	6.71**	68.5	32.6**	38.7**
		± s	0.06	1.48	1.28	117	43	54.8
		s²	0.004	2.19	1.63	13789	1847	3005
		V	9.89	23.39	19.05	17.1	13.2	14.2
		N	30	40	40	30	30	30
	6 hr.	Avg.	0.649**	5.85*	6.61**	63.6**	28**	33.5**
		± s	0.05	1.34	1.36	95.2	46	45.9
		s²	0.002	1.8	1.86	9067.2	2132	2110
		V	8.03	22.94	20.67	15	17	13.7
		N	30	40	40	30	30	30
	10 hr.	Avg.	0.630**	5.76*	6.3**	58.9**	22.4**	29.6**
		± s	0.05	1.02	1.21	75.6	31.5	54
		s²	0.002	1.05	1.46	5717	991	2914
		V	8.12	17.83	19.23	12.8	14	18
		N	30	40	40	30	30	30
180°C	2 hr.	Avg.	0.632**	5.82**	6.26**	65.4**	30.9**	37**
		± s	0.05	1.35	1.05	90.6	49.3	63.8
		s²	0.003	1.83	1.11	8200.8	2428.3	4073
		V	9.44	23.28	16.86	13.8	16	17.2
		N	30	40	40	30	30	30
	6 hr.	Avg.	0.632**	5.82**	6.26**	65.4**	30.9**	37**
		± s	0.05	1.35	1.05	90.6	49.3	63.8
		s²	0.003	1.83	1.11	8200.8	2428.3	4073
		V	9.44	23.28	16.86	13.8	16	17.2
		N	30	40	40	30	30	30
	10 hr.	Avg.	0.600**	5.66**	6.02**	56**	20.2**	28.1**
		± s	0.11	1.13	1.05	75	38.4	22
		s²	0.01	1.29	1.10	5620	1472	473.2
		V	18.42	20.07	17.47	13	19	7.7
		N	30	40	40	30	30	30

Table 2 shows the measured colours using the CIE L*a*b* expression. The data represent average values of 20 readings.

a ± s =standard deviation; s²=Variance, V=coefficient of variation, N=number of specimens used in each test; asterisks denote significant difference compared with untreated control: *p=0.05, **p=0.01.

Table 2. Effect of heat treatment temperature and time on CIE L*a*b* colour properties measured on eucalyptus wood

Temperature (°C)	Treatment (hr.)	L*	a*	b*	ΔL*	Δa*	Δb*
None (control)	0	46.12	15.40	14.87	-	-	-
120	2	45.31	12.56	14.04	-0.81	-2.84	-0.83
	6	47.07	12.80	14.27	0.95	-2.60	-0.60
	10	39.70	12.37	14.23	-6.42	-3.03	-0.64
150	2	42.29	12.51	14.42	-3.83	-2.89	-0.45
	6	38.86	11.86	13.97	-7.26	-3.54	-0.90
	10	30.96	9.23	10.96	-15.16	-6.17	-3.91
180	2	42.22	12.16	13.51	-3.90	-3.24	-1.36
	6	30.61	8.84	9.86	-15.51	-6.56	-5.01
	10	23.21	5.94	7.24	-22.91	-9.46	-7.63

Table 2 indicates the changes in the lightness of the samples with respect to exposure time for each temperature. At any treatment temperature the lightness decrease as the treatment duration increase from 2 to 10 hours. At 120 °C, L* decrease steadily, and the loss of lightness was approximately 14% after 10 h of treatment. At 180 °C the significant lightness drop was observed during the first 6 hours and the lightness reached the lowest value (23.21) after 10 hours of treatment.

It was concluded that the decrease in L* values by heat treatment was due to not only lignin or lignin derivatives but also hemicelluloses (Mitsui et al. 2001).

Figure 1. Total colour difference D E* calculated between untreated (control) an thermal treated wood samples

Fig. 1 shows colour differences between untreated and treated wood specimens. At all temperature conditions, colour differences increase with increasing treatment duration, while the magnitude of the colour difference was higher at high temperature conditions.

It is known that the weight of the wood material and its swelling decreases when heat treatment is applied. According to a recent study on softwood heated at 210 ° C for 10 hours, weight loss and losses in hardness and shrinkage-swelling were 0.5%, 5% and 10%, respectively (Bozkurt et al. 1993). Another effect of heat treatment was colour change. In white coloured softwoods, as decrease in shrinkage-swelling reached up to 25%, colour turned to Walnut colour (Bozkurt et al. 1993).

According to the results obtained in this study, all properties changed with increased temperature and duration, and the changes were maximal at 180° C for 10 hr. Table 3 shows the decreases of these properties with increasing thermal treatment temperatures and durations.

Table 3. Decreases of wood properties as a function of temperature and duration in heat treatment

Treatment	Ovendry Density (%)	Swelling (%)		Hardness (%)
		Radial	Tangential	Cross section	Radial	Tangential
120° C-2 hr	3.23	3.19	11.47	3.97	4.42	4.73
150° C-10 hr	7.35	12.59	17.86	19.97	38.12	30.02
180° C-10 hr	11.76	14.11	21.51	23.91	44.20	33.57

A similar study was made by Yildiz (2002) with oriental beech and spruce. For oriental beech treated at 200 ° C for 10 hours, density decrease and swelling-reduction efficiency were up to 18.4% and 52.9%, respectively. At 180 ° C and 10 hr, maximum hardness loss in the cross section were measured to be 25.9%, and those in radial and tangential directions were 45.1% and 41.8% respectively. Also for oriental spruce, density decreasing was 10.5%. In another study swelling-reduction efficiency was found to be 24% in the study on Eucalyptus (Eucalyptus globulus) wood treated at 180° C (Santos 2000). These results can be explained by material losses in cell lumen and hemicelluloses degradation due to applied high temperature (Yildiz 2002).

Eucalyptus wood, in many countries, is not included in wood products industry because of its undesirable performance. When being heated, its swelling, density and hardness and consequently strength will decrease and moreover the colour will get darker. However, strength losses can be limited through advanced heat treatment techniques (Yildiz 2002). So, this tree species can be utilized using proper heat treatment techniques without any losses in strength values in areas that are important the working, stability. This study has investigated the effects of treatment temperature and duration on the wood properties concerned which will help to optimise the treatment conditions for the improvement of the wood performance.

ACKNOWLEDGEMENTS

This work was supported by the Research Fund of the University of Istanbul under the contract No: 108/15052003.

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