INFLUENCE OF VAPORIZATION AND IMPREGNATION OF SILVER NANOPARTICLES ON THE DRYING RATE OF Eucalyptus pellita F. MUELL.

radiais madeiras realizadas avaliar o efeito taxa de secagem foi determinada faixas de umidade antes e após o ponto de saturação das fibras (PSF). geral, os tratamentos não modificaram as características anatômicas, densidade e permeabilidade da madeira nas três regiões radiais; contudo, a vaporização por 24h reduziu o teor de extrativos totais na madeira. Esses resultados contribuíram para que ganhos na taxa de secagem antes e após o ponto de saturação das fibras fossem obtidos. O efeito da impregnação de nanopartículas com vácuo e os dois períodos de vaporização resultaram nas maiores taxas de secagem, sendo que o tempo de 24h de vaporização obteve as melhores médias entre todos os tratamentos. A vaporização por 24h e a impregnação de nanoparticulas apresentaram efeitos positivos na taxa de secagem. Abstract The aim of this study was to evaluate the effect of vaporization and impregnation of silver nanoparticles on the Eucalyptus pellita wood properties. For that, samples of three radial regions of the wood from three trees were vaporized for 12 and 24 hours and later on, they were immersed in solution of silver nanoparticles with and without application of vacuum (750 mm.Hg). Anatomical, physical and chemical analyzes of the wood were carried out in order to evaluate the effect of the treatments. The drying rate of the wood was determined in moisture bands before and after the fiber saturation point. Generally, the treatments did not modify the anatomical characteristics, permeability, and wood density in the three radial regions; however, the vaporization for 24h reduced the total extractive content in the wood. These results contributed in obtaining gains in the drying rate before and after the fiber saturation point. The effect of impregnation of nanoparticles with vacuum preceded by two periods of vaporization resulted in higher drying rates and the time of 24h stood out, resulting in the best averages among all treatments. The impregnation of nanoparticles had positive effects on the drying rate.


INTRODUCTION
During the drying process it is necessary to minimize the undesirable effects to the factors related to the wood, such as the thermal diffusivity, the related physical properties with the dimensional movement, the drying tensions, density and wood permeability (TARMIAN et al., 2012;LUIS et al., 2017). 580 FLORESTA,Curitiba,PR,v. 49,n. 3,jul Drying rate is an important variable in wood behavior control during the drying process. In general, eucalyptus wood presents low permeability which is responsible for the significant humidity gradient and consequently tension formation that complicates its drying process (ELEOTÉRIO et al., 2014;REZENDE et al., 2015). According to Bal and Bektas (2012), vaporization is a treatment possibility that modifies wood hygroscopic properties. The application of elevated temperatures initially causes carbohydrates degradation, mainly hemicelluloses and amorphus regions of cellulose chains (LUÍS et al., 2017), and by extended periods and higher temperatures. It modifies completely the wood permeability due to the appearance of microcracks in cellular wall .
Vapor treatment allows drying process optimization due to permeability increase, resulting from pits and vessels clearance (ALEXIOU et al., 1990). In such a way, the permeability is directly related to anatomical structures (BARAÚNA et al. , 2014), as well as with the balance of the water entrance and exit from the wood and the easiness of preservative fluid penetration (BREADS et al., 2013;GAO et al., 2015).
The wood impregnation with silver nanoparticles has revealed a worthwhile process, promoting alterations in wood characteristics internally and superficially, providing, in some cases, improvements in durability as well as physical and mechanical properties TAGHIYARI et al., 2012;MONTAZER, ALIMOHAMMADI, 2012;TAGHIYARI et al., 2014;TAGHIYARI et al., 2015;GAO et al., 2015).
Regarding what was exposed, it was evaluated the hypothesis that vaporization and impregnation of wood with silver nanoparticles promote modifications in the drying process, influenced by the material chemistry and anatomy. Thus, the objective of this experiment was to evaluate the wood technological behavior of Eucalyptus pellita F. Muell. treated previously with vapor and impregnated with silver (Ag) nanoparticles.

Delineation, collection and preparation of the material
For the acquisition of wood samples, three 22-year-old trees of Pellita Eucalyptus had been cutted, planted 3 x 2m of spacing in a population located in the campus of the Rural Federal University of Rio De Janeiro -UFRRJ in Seropédica, state of Rio de Janeiro, Brazil (Lat 22.7604°, Lon 43.7078°). Samples of these trees were deposited in the xylotheque of the Forest Institute of the Rural Federal University of Rio de Janeiro, under the registration number: 7711, 7712 and 7713. After the cutting, the first log of each tree, with 4.5 m of length, was conducted to the primary cut with the assistance of vertical band saw in order to obtain planks with 3 inch radial thickness. After that, they were parted in pieces with the following dimentions: 33 x 2.5 x 5 cm (length x thickness x width), enclosing three radial positions from the pith towards the vascular cambium, denominated: Heartwood/Sapwood Transition Zone (HST), Intermediate Heartwood (ITH) and Internal Heartwood (INH) The samples of each region were distributerd in three groups: 1) No treatment samples; 2) samples treated only with nanoparticles; and 3) samples treated only with vapor (Table 1), totalizing 5 treatments per radial position. Vaporizarion was applied in the wood pieces in two constant periods with 12h and 24h of duration. Tabela 1. Delineamento experimental aplicado para as três regiões radias da madeira: Transição Cerne/Alburno (TCA), Cerne Intermediário (CIT) e Cerne Interno (CIN).

Vaporization and impregnation of nanopartículas
Initially, the wood was vaporized using an horizontal autoclave with capacity of 0.18 m³ approximately, providing temperature and pressure control. The maximum temperature applicated was 98°C ± 2 and the relative humidity was 90% ± 5 proceeding from vapor generation through a boiler with the capacity of 12 Kg vapor/hour. The sample impregnation with silver nanoparticles (NPs) was performed through simple immersion, using an acrylic chamber with 45 x 40 x 50 cm of dimention. The wood immersion in destilled water with silver NPs in suspension (22 ppm) occurred with and without initial vacuum application of 750 mmHg for five minutes. This solution consisted by silver NPs with size between 5 and 20 nm, which characterizes as colloid with exceptional stability and purity, i. e., without surfactants presence, avoiding precipitates formation. The wood remained submerged in the solution for 30 min.

Anatomical analyses
For the anatomical analyses, a sample of 2 cm of thickness was removed from the extremity of each treatment parts. Later, this sample was cutted in 1 x 1 cm blocks for observation in Scanning Electron Microscope (SEM), aiming at the verification of nanoparticles impregnation and vascular contents and the obtention of transversal histological cuts in slide microtome.
Semi-permanent blades were prepared to the measurement of the tangential diameter of the vessels through digital images captured by a monochromatic camera connected to the trilocular microscope.1 The used procedures followed the International Association of Wood Anatomy Committee (IAWA) (1989) recomendations.
At first, the frequency of the vessels (tissue.mm -2 ) also followed the methodology proposed by IAWA (1989). For that, digitalized images from the transversal face of cilindrical samples were used, being prepared to the air permeability test.

Chemical analyses
For the determination of total extratives content, the procedures described by Abreu et al. (2006) were used, following an eluotropic sequence with the solvents: cyclohexane, acetate and methanol. Each extraction was performed during six hours. After each extraction the solution was placed in a rotary evaporator (Rotavapor) so it was possible, using vacuum and heat, to concentrate and weigh the extracted portion in each type of solvent. The contents of lignin (soluble and insoluble in acid), uronic acid, acetyl group and carbohydrates (xylan, mannan, galactan, arabinan and glucan) were determined according to the procedures proposed by Tappi T222 (2000), Scott (1995), Solar et al. (1987) and Wallis et al. (1996). Additionally, chemical analyses were performed in the vaporized and without treatment samples.

Wood drying rate
After vaporization and nanoparticle impregnation treatment, all samples went through a drying process in climatized room with temperature at 20°C ± 2 and relative humidity at 65% ± 5. During the drying process, at each 2 hours the mass and respective dimentions (width, thickness and length) of all samples were measured. This process was used in order to register water mass loss and it was 54 days long until the obtention of 15% humidity.
According to the water mass loss registered after each 24 hours, it was determined the wood drying rate with the following equation: in which: Dr = drying rate for a determined humidity interval (kg/cm 2 .h), Mwater = water mass removed from wood (kg), t = drying time (h), A = evaporation area (cm²).
The wood drying rate in each treatment was calculated using the area of a prism to the humidity intervals varying from saturated until 30%, from saturated until 15%, and from 30% until 15% humidity, according to the formula: = 2( . + . + . ) in which: TA = total area (cm²); a, b and c = prismatic sample faces' measures (cm).

Wood density and permeability
After the drying process, the prismatic samples were lathed in order to obtain cylindrical pieces with 2.0 cm of diameter. Then, they were sectioned in 5.0 cm to obtain the sample to density and permeability test.
The volume of cylindrical samples was obtained through Mercury (Hg) imertion in order to determine the aparent density (15%) of all samples already in balance and, after the permeability test, the basic density through the gravimetric method was performed. Mercury temperature was measured after each eight weighing with a digital thermometer (±0,2°C).
To the permeability test the same dimentions described by other authors were adopted (BARAÚNA et al., 2014;TAGHIYARI et al., 2012;TAGHIYARI et al. 2015 linked in series in the following scales and sequences were used to determine the wood permeability regarding atmospheric air: 0.04 to 0.5 LPM (Liters per Minute); 0.2 to 2.5 LPM; 0.4 to 5.0 LPM and 2.0 to 25.0 LPM. Then, in one of the flow meters series extremities it was connected a vacuum pump, and on the other extremity the samples were connected. The following equation was used for the determination of air wood permeability: = Q. L. Pi A. ∆P. Pa in which, Kg = gas permeability (cm³/cm.atm.s), Q = gas flow volume that travels the species (cm³/s), L = length of sample (cm); Pi = entrance pressure, the one from the environment (atm); A = transversal section area (cm²); ΔP = pressure difference (atm); Pa = average pressure in the sample (atm).

Statistical analyses
For the anatomical, chemical, density and drying rate variables, being accepted residues statistical requirements of normality (Shapiro-Wilk, at 5% of significace) and homogeneity of variance (Bartlett, at 5% of significace), methods of parametrical analyses (ANOVA) with completly randomized design were adopted, considering: three radial positions; two vaporization periods (12 and 24 hours); and two nanoparticles impregnation levels (with and without vacuum impregnation). Tukey test was used for comparison of the averages at 95% level of reliability, all the times that nullity hypothesis was rejected. The statistical analysis used for permeability was the non parametric test of Kruskal-Wallis (95% probability) for the comparison of the average stages, due to the fact that the data did not follow a normal distribution (teste de Lillefors). After this test, the analysis by Dunn test was carried out in order to compare averages (95% probability).

Anatomical and chemical analyses
Diameter, area and vessels analyses did not present significant differences (at 95% probability) between three radial regions, highlighting that statistically anatomical elements morphology did not modify in the radial direction of the wood and with vaporization use. At first, carbohydrates' content reduced with vaporization period increasing, as well as total extractives' content, which resulted in a total percentage increase of lignin in the samples (Table 2).
The total extractives content average for wood was 9.07%. A significant increase of contents was observed, when analyzing extractives content in pith towards the vascular cambium direction, because of the heartwood presence in the most interior wood parts. The vaporization caused a significant reduction in total extractives' content. In vaporized wood for 12h only the intermediate heartwood (ITH) region presented reduction, while vaporization for 24h resulted in an average decreasing of 29,31% in total extractives' content. FLORESTA,Curitiba,PR,v. 49,n. 3,jul

Air permeability and density
Wood basic density in the three radial regions did not differ statistically, even when submitted to vaporization and nanoparticles impregnation. Density varied between 0.843 to 0.857 g/cm 3 . The results obtained to air permeability (k) of E. pellitta wood showed a variation between the radial position. The heartwood regions (INH and ITH) present values significantly inferior to the portions that contain sapwood (Table 3). Wood permeability in sapwood region was about 15 times superior to internal heartwood (INH) region. This denotes that capillary contents affect significantly fluids' and gases' flow.

Treatment
Basic density (g/cm 3 ) Air permeability ( However, even with reduction of extractive content and carbohydrates caused by vapor application, there was not significant increase in air flow in vaporized wood in relation to the non-vaporized one. The treatments with nanoparticles impregnations did not improve air longitudinal permeability. The radial profile of E. pellita wood permeability presents a natural tendency, once the heartwood region present vascular cells with the lumen obstructed by thyllos, as well as by extractives (Figure 1).
Wood vaporization for 24h was more efficient than 12h period in total drying rate (saturated at 15%), also in capilarity and diffusion. However, silver nanoparticles incorporation in wood with previous vacuum application resulted in higher increase in drying rate because of the benefits before and after saturation point of fibers. It was possible to observe nanoparticles' deposition location, after previous vacuum application, in the surface of vessels walls as well as inside fibers' walls ultrastructure (Figure 2).

DISCUTION
In all vapor treatments the sugar content reduced. This effect can be explained by molecular mass loss that occurs in hemicellulose polymer when vapor is directed towards the wood (PERSSON; JÖNSSON, 2017).