EFFECT OF MICROWAVE TREATMENT ON THE IMPREGNABILITY AND MECHANICAL PROPERTIES OF Eucalyptus globulus WOOD

Application of microwave energy to wood can increase permeability in order to improve the impregnation of treatments, particularly important in refractory species such as eucalyptus. However, microwave treatment can also affect the physical and mechanical properties of wood. This paper discusses the application of microwave treatments of different intensities to improve impregnability (uptake and retention) and examines the effects on the mechanical properties of eucalyptus wood ( Eucalyptus globulus ). Microwave treatments significantly improve uptake and retention of the protective agent, especially in the most severe treatment. In contrast, there was a clear decrease in mechanical properties (modulus of elasticity and bending, compression, tension and shear strength) as a function on the energy applied. The effect of microwaves on the properties of wood varies depending on moisture content at the time of treatment.

Microwaves consist of electromagnetic energy at frequencies between 300MHz and 300GHz.MW energy absorption produces the random vibration of polarized molecules, leading to an increase in temperature.Although water molecules are significantly affected, unsurprisingly given their high dielectric permittivity, other polar molecules in wood (cellulose, hemicellulose and lignin) can also be affected (Hansson and Antii 2003).The heat generated is absorbed in different ways depending on the type cell affected (tracheids, fibers).Pressure causes fractures and the creation of micro-holes in thinwalled cells, increasing the permeability of the wood.
MW energy applied for wood modification needs to be in the range of 250 to 1200 MJm -3 , depending on the purpose.For example, MW processing to increase the uptake of water-based preservatives for some softwood species (Stika spruce, radiata pine) needs energy in the range of 250 to 400 MJm -3 and this energy needs to be higher for highly refractory hardwood species (Torgovnikov and Vinden 2010).It is important to select MW energy that does not excessively damage the wood structure or significantly affect the mechanical properties, but still improves permeability.The energy required depends on the size of the piece and initial moisture content.
The objective of this paper was to study the application of different energies based on MW treatments to Eucalyptus globulus heartwood (a refractory species) in order to improve permeability, while at the same time evaluating the influence on the mechanical properties.

MATERIAL AND METHODS
Heartwood samples of Eucalyptus globulus L. from the Northwest of Spain were obtained from commercial boards.The objective was the evaluation of the standard wood existing on the market, without previous selection by tree or log.A total of 350 samples (20x30x350 mm 3 ) were prepared avoiding the presence of knots, cracks or other defects.The samples were divided into 7 groups of 50 specimens, one group corresponding to each MW treatment configuration.The initial moisture content of all the samples was over 30%.
Four of the groups were conditioned to an approximate moisture content of 12% (dry sample groups D1, D2 and D3 for treatments 1, 2 and 3 respectively and a reference group/control, R), while the moisture content of the other three groups remained above 30% (green sample groups G1, G2 and G3 for the treatments 1, 2 and 3 respectively).Samples were treated in a laboratory MW measuring 400x280x240 mm 3 (inner chamber), with a frequency of 2,45 GHz and a magnetron with a power rating of 500W and homogeneous distribution of energy within the chamber.A frequency of 2,45 GHz is suitable for treating pieces with thicknesses up to 90 mm (Torgovnikov and Vinden 2009).Treatment times were defined based on the power supplied and the volume of the samples in order to reach the desired energies (Table 1).
Immediately after the treatment, the samples were weighed and their moisture content (using hygrometer TESTO 616) and temperature (T) measured.Subsequently, all samples were conditioned to achieve a moisture content of 12% in preparation for impregnation and mechanical tests.

Evaluation of impregnability
10 samples from each treatment (and control) were sealed at their ends with paraffin and treated in autoclave with 2% solution of water-soluble salts TANALITH E (vacuum conditions of 0,3 bar and pressure of 9 bar for 120 minutes).
After the treatment, samples were weighed and the differences between pre-and post-treatment were used to calculate uptake (%) and retention (kgm -3 ) rates.Retention was calculated considering the concentration of active ingredients in the product (2%).

Evaluation of mechanical properties
Remaining 40 samples from each MW treatment were tested to evaluate the mechanical properties: modulus of elasticity parallel to the grain (E m ), bending strength (f m ), compression strength (f c,0 ), tension strength (f t,0 ) and shear strength (f v ).Modulus of elasticity and bending strength tests were performed as defined in ISO 13061-4,2 and ISO 13061-3,2; respectively, using a universal testing machine, model ELIB-30, with a load capacity of 30 kN.Shear strength tests were made according to UNE 56543.Compression and tension strength tests were performed according to EN 408 (sample dimensions were modified to 10x10x150 mm 3 for compression and tension tests according to the standard).

Moisture content, temperature and visual damages
Table 2 shows the means of percentage of moisture content loss and of surface temperature increase following treatment.In dry samples (12% MC) the maximum moisture content loss was observed at the maximum MW energy, with an average value of 2,12%.In green samples, (initial moisture content above 30%) the greatest drying effect was observed, with losses ranging from 4,74% in treatment 1 to 11,87 % at the highest intensity.Temperature increases on the wood surface were higher in dry samples.This causes the thermal degradation of the wood during exposure to MW energy (Smith et al. 1996).The degradation occurs in wood heated above 100 o C, and can affect the chemical constituents of the cell wall or cause major structural damage (Saporiti 2006).Increasing energy seems to be associated with increasing splitting and cracks, particularly in the dry samples (Figure 1).

Uptake and retention
The results for the 7 groups tested are shown in table 3 and figures 2-3.Both absorption and retention showed significant differences depending on the MW treatment applied.These differences tended to be greater when the treatment occurred in the green samples (G1, G2 and G3).
Table 3.Effect of microwave treatment on permeability of E. globulus as measured by uptake of a salt solution applied by pressure treatment.
Group n Uptake (Lm -3 ) Uptake (%) Retention (kgm -3 ) R 10 55,9 (±15,1) (a)  6,4 (±2,0) (a) 1,7 (±0,3) (a) D1 10 84,9 (±32,3 4,2 (±0,6) (c) Values followed by the same letter do not differ significantly at α=0,05 Effect of microwave treatment ..: Hermoso y Vega  Torgornikov and Vinden 2010, showed that energy ranges between 570 and 850 MJm -3 were suitable for improving the impregnability of highly refractory species such as messmate (Eucalyptus obliqua), increasing uptake by a factor between 8 and 14.Uptake increases in the samples of this study were 2,7 times the reference values, though the energy applied was lower (430 MJm -3 ).However, more intense treatments were not used because of concerns about degradation of the wood.Treu et al. (2008), demonstrated that MW irradiation of spruce wood led to increased water uptake after vacuum and pressure impregnation.The authors proposed decreasing the exposure time to avoid cracking and cell wall degradation but using higher power at shorter intervals and multiple cycles.In contrast, Hansson and Antti (2003), proposed longer treatment times at a lower power.Further studies on the treatment configuration for Eucalyptus globulus wood need to be performed in the future to better define the optimum parameters to increase permeability with minimum effect on wood properties.

Mechanical properties
The results from the mechanical tests are shown in table 4. Modulus of elasticity (E m ), bending strength (f m ), compression strength (f c,0 ), tension strength (f t,0 ) and shear strength (f v ) decrease as a function of the MW energy applied (Figure 4).
Compression, tensile and shear strength, with one exception, all showed significant changes for treatment 3, in both green and dried samples, in terms of a significant decrease in all values, the exception being green sample shear strength.
These results were similar to those reported by Torgovnikov and Vinden (2009), showing a decrease in both E m and f m of messmate (Eucalyptus obliqua) wood after applying microwave energy between 250 and 600 MJm -3 .The decrease in the values of these variables was significant in treatments above 300 MJm -3 , with decreases of 35% and 12% respectively when applying energy of 400 MJm -3 .Furthermore, Saporiti (2006) has indicated a notable decrease in compressive strength in oak (Querqus pyrenaica) when applying three MW treatments similar to those of the present work (550 W and 2,45 GHz).

CONCLUSIONS
Microwave treatment (430 MJm -3 ) on Eucalyptus globulus wood before the conventional impregnation process results in a 2,28-fold increase in uptake and a 2,50-fold increase in retention compared to the values of the reference samples.
Wood stiffness and strength decreased significantly, especially for the most severe treatment (430 MJm -3 ).The effect of MW treatment on the properties of wood is variable, depending on the initial moisture content, being lower in moisture contents above 30%.
In view of the results, the range of energy that provides a balance between improving impregnability without affecting other properties of the material is between 360 and 430 MJm -3 .These conclusions refer to studies of samples of small dimensions free of defects of Eucalyptus globulus heartwood.Future new works evaluating structural size wood are therefore necessary.

Figure 2 .
Figure 2. Effect of microwave treatment on permeability as determined by changes in solution

Figure 3 .
Figure 3.Effect of microwave treatment on permeability as determined by changes in solution retention during pressure treatment.

Figure 4 .
Figure 4. Relationships between MW energies, moisture content and mechanical properties (modulus of elasticity and bending strength) of Eucalyptus globulus.

Table 1 .
MW energies (E) and initial moisture content (MC) conditions tested for their effects on permeability and mechanical properties of Eucalyptus globulus.

Table 2 .
Moisture content loss and temperature increase.

Table 4 .
Effect of microwave treatment on mechanical properties of Eucalyptus globulus treated at 12% or >30% moisture content.Values represent means and standard deviation.