WETTABILITY AND SURFACE ROUGHNESS CHARACTERISTICS OF MEDIUM DENSITY FIBERBOARD PANELS FROM RHODODENDRON (RHODODENDRON PONTICUM) BIOMASS

Wettability and surface roughness properties of medium density berboard (MDF) panels made from Rhododendron biomass were examined in this study. Rhododendron dominates the understory layer of the forests throughout the Black Sea Region in Turkey with substantial biomass potential. is study was conducted to evaluate suitability of Rhododendron ber for MDF production. For the experiments, rhododendron and commercially-manufactured-chip (Pinus sylvestris L. and Quercus robur L.) with 11% moisture content were used. e mixing ratios of rhododendron with commercially-manufactured-chip were 100:0, 75:25, 50:50, 25:75 and 0:100 %, respectively. Commercial urea formaldehyde (UF) adhesive was used as a binder. A stylus method was employed to evaluate the surface characteristics of the samples. Roughness measurements by the stylus method were taken perpendicularly to the ber. ere was a signi cant di erence (p=0.05) between surface roughness parameters (Ra, Rz, Rq, and Ry). e results obtained in this study revealed that all the panels have met the general purpose-use requirements of European Norm (EN). It was found that panels made with a 25/75 ratio of rhododendron to commercially-manufactured-chip had a signi cantly lower contact angle (88.14o) than panels made with a 100/0 ratio (117.91o). Surface roughness measurements are based on four roughness parameters, average roughness (Ra), mean peak-to-valley height (Rz), root mean square roughness (Rq), and maximum peak-to-valley height (Ry) were considered to evaluate the surface characteristics of the panels and supported the above ndings as the panels made with a 25/75 ratio had a slightly rougher surface with average values of 2.929 μm (Ra). From the tests performed, we conclude that increasing the rhododendron mixing portion increased surface roughness and decreased wettability. Based on these results, rhododendron biomass could be an alternative raw material for MDF manufacturing.


INTRODUCTION
e demand for wood in the forest industry has been growing, but the production of industrial wood from natural forests continues to decline. e decline in forest resources in developing countries is due to depletion of the resources while in developed countries it is due to the conversion of forested areas from industrial production to other uses such as recreation. A signi cant pressure on standing forest resources on the other hand, results in higher demand for wood in the forest industry due to increasing population and new application areas (Akgul and Tozluoglu 2008).
Because of a continuing lack of raw material in the forest products industry, medium density berboard (MDF) production and other sectors are being negatively a ected. e current trend is for berboard manufacturers to use wastes as raw material. Additionally, there is a need to nd alternative raw materials or use wood resources more completely including harvesting residues, barks, annual plants, lumber and furniture plant residues, residues of pulp plants, and recycled paper etc. (Akgul and Camlibel 2008).
Turkish forestry is based mainly on wood production. e most signi cant part of the forestlands in Turkey is located in the Black Sea Region. Rhododendron (Rhododendron ponticum L.), with about 40 metric ton ha -1 biomass (Yildiz et al. 2005), has invaded the understory layer of the forest ecosystem throughout and has become the dominant species in the understory of forests in the Black Sea Region of Turkey (Esen et al. 2006. Medium density berboard (MDF) is one of the most rapidly growing composite panel products in the forest products market (Ayrılmış 2008). MDF is a dry formed panel product manufactured from lignocellulosic ber. While MDF is a relatively new product in Turkey, its production has been increasing in recent years. Turkey has produced more than 4.5 million cubic meters of annually, the 2nd largest MDF production capacity in Europe (FAO, 2009). MDF is used extensively in factoryassembled and ready-to-assemble furniture, as well as in cabinets, underlayment, drawer fronts, molding, and countertops due to its ability to be produced in molded form, as well as in straightedged at panels.
Data on the chemical composition of wood material is generally preferred and required for many processes and applications in the wood industry (Fengel and Wegener 1989). Species of origin has been considered a major variable in the manufacture of MDF with the density of the species being the most in uencing factor (Xu et al. 1998).
Birch (Betula), ash (Fraxinus), lime (Tilia), Douglas-r (Pseudotsuga), spruce (Picea), and larch (Larix) have been recognized as high quality material for MDF production (Chow et al. 1992). In Turkey, low quality oak, beech, and pine are used either as a single species or as a mixture of raw materials for MDF manufacture (Akbulut et al. 2000). In addition, a study with oak (Ouercus robur L.) showed that production of MDF from oak biomass is technically feasible, but supply of raw materials cannot meet the industry demand as is the case for the other wood industries.
Surface roughness of the substrate is very important when the panels are to be coated with thin overlays such as melamine impregnated papers, foils, and thin lms. Roughness refers to the minute and periodic irregularities on the surface other than those defects resulting from the manufacturing techniques (Hiziroglu 2005). Any surface irregularities on the substrate may show through the overlay and in uence the quality of nal products. e surface of wood products may be characterized by either topography or pro le. Pro les are more widely used in evaluating surface irregularities due to the less-expensive data acquisition equipment required for pro le measurement compared to topography measurements. Stylus technique is one of several methods, such as pneumatic, laser, and acoustic emission, that is accurate, practical, and repeatable (Hiziroglu 1996, Stumbo 1963. Quantitative roughness parameters can be accurately calculated from the graph obtained from the surface and all standard parameters can be generated to gather objective information about the surface measured (Goker et al. 1997). Di erent techniques including stylus method were used in various studies to measure and quantify surface characteristics of wood and wood-based panels (Stumbo 1963, Peters and Cumming 1970, Peters and Mergen 1971. Wettability and surface roughness properties of wood and wood composite panels produced with 100% wood materials were previously investigated by Aydin and Colakoglu (2002) Wettability is de ned as a surface condition that determines how fast a liquid will wet and spread on the surface or be repelled. Wettability is crucial for good adhesion in wood bonding. e adhesive has to wet, ow, and penetrate the cellular structure of wood in order to establish intimate contact between molecules of wood and adhesive. e wettability of wood can be characterized by contact angle analysis. is analysis is important to determine the adhesive and coating properties of wood and wood-based composite surfaces. Wetting quality of wood is in uenced by many factors including wood macroscopic characteristics (e.g., porosity, surface roughness, wood surface polarity, pH value, moisture content, grain orientation, and extractives), surface quality of wood (e.g., virgin, aging, and contamination), processing temperature, and properties of adhesives (e.g., acidity, rheology, and viscosity) (Lu 2003). e objectives of this study include (1) Determine surface roughness and wettability properties of the panels made from rhododendron wood and compare them with conventional mixtures (oak and pine) for possible usage for MDF production, and (2) to recommend the result of the experiment to the industry. erefore, the aim of the current study is to nd alternative raw materials for the MDF industry in Turkey.

Materials
e material for this study included rhododendron wood biomass obtained from forests in the western Black Sea Region of Turkey, and commercially-manufactured-chip (Pinus sylvestris L and Quercus robur L).
Rhododendron wood and commercially-manufactured-chip were divided into pieces to make chips of 20 mm x 25 mm x 5 mm. In order to convert the materials into ber an Asplund de brator using 7.8 bar of steam pressure at 175 o C was used for 3.5 minutes. en, 1 % wax, 0.8 % NH 4 Cl (catalyst), and 11 % urea-formaldehyde (UF) resin (binder) were added into the ber mixture. Mats with 11 % average moisture content were pressed under 38 kg cm -2 pressure at 175 o C for 4 minutes. A er cooling, the panels were sanded using 50, 80 and 120 grit size sandpaper. en, the panels were conditioned at 20± 2 o C and 65± 5 % relative humidity (RH) until they reached 12 % moisture content.
Eight experimental panels with dimensions of 2100 mm by 4900 mm by 18 mm with target densities of 0.73 g/cm 3 for each type of raw material combination were manufactured at Divapan Integrated Wood Products Inc. in Duzce, Turkey. Five di erent ratios of two ber types, namely 75 %, 50 %, 25 % rhododendron bers were mixed with oak and pine to manufacture the panels. In addition, panels were produced entirely from each species.

Methods
e TS EN 326-1 (1999) standards were used for preparing the samples from the panels. Each panel was rst divided into pieces bigger than 800 mm by 1600 mm. en sub-samples were taken from these as per the standard cited above. Following TS EN 325 (1999) standards, sample thickness and length were measured using a digital micrometer and compass grading 0.01 mm.

Determination of surface roughness
Forty samples were measured from each group. Surface roughness of the samples was measured using a pro lometer (Mitutoyo Sur est SJ-301) with the pro le method using a stylus device standard.
Four roughness parameters, mean arithmetic deviation of pro le (R a ), mean peak to-valley height (R z ), root mean square roughness (R q ), and maximum roughness (R y ) were commonly used in previous studies to evaluate surface characteristics of wood and wood composites such as veneer (Stumbo 1963). R a is the average distance from the pro le to the mean line over the length of assessment. R q is the square root of the arithmetic mean of the squares of pro le deviations from the mean line. R z can be calculated from the peak-to-valley values of ve equal lengths within the pro le while maximum roughness (R y ) is the distance between peak and valley points of the pro le which can be used as an indicator of the maximum defect height within the assessed pro le (Mummery 1993). erefore, such parameters which are characterized by ISO 4287 (1997) andDIN 4768 (1990) were recorded.
Speci cation of roughness parameters is described by Hiziroglu (1996) and Hiziroglu and Graham (1998). Roughness values were measured with a sensitivity of 0.5μm. e length of scanning line (Lt) was 15mm and the cuto was λ=2.5mm. e measuring force of the scanning arm on the surfaces was 4mN (0.4 g), which did not signi cantly damage the surface according to the Mitutoyo Sur est SJ-301 user manual (Mitutoyo surface roughness tester 2002). e points of roughness measurement were randomly marked on the surface of the samples. Measurements were made perpendicular to the ber of the samples. Measurements were repeated whenever the stylus tip fell into the cell lumen during the tests. Measurements were performed at room temperature and the pin was calibrated before the tests.

Determination of wettability
Contact angle (CA) tests were performed to evaluate the wettability characteristics of the MDF panels made from rhododendron biomass used in this study. e contact angle value was de ned as the angle through the liquid phase formed between the surface of a solid and the line tangent to the droplet radius from the point of contact with the solid. CA values were obtained by using a goniometer system connected with a digital camera and computer system (KSV Instrument, Finland). e liquid employed for the measurements was 5 μL distilled water at 20°C with a surface tension of 72.80mN/m. e CA was determined for each image by digital image analysis so ware. e image was captured immediately a er the droplet of distilled water was placed on the surface, and then every 1 second for the duration of 30 seconds. e mean CA value for each sample was calculated from thirty photo images. A total of y testing samples, ten samples for each panel group, were used for the CA analysis.

Statistical Analyses
For the surface roughness and wettability all multiple comparisons were rst subjected to an analysis of variance (ANOVA) at p<0.05 and signi cant di erences between mean values of the mixed of rhododendron and industrial furnish groups were determined using Duncan's multiple range test. Table 1 shows the results of surface roughness for all groups. According to the averages, all the parameters decreased with the proportion of rhododendron mixed. e ANOVA and Duncan's multiple range tests showed these changes to be signi cant. Homogeneity groups: same letters in each column indicate that there is no statistical di erence between the samples according to the Duncan's multiply range test at P<0:05. Comparisons were done between the each control and its test.

RESULTS AND DISCUSSION
It is evident from table 1 that the surface roughness values decrease with a decreasing mixture of rhododendron bers under the conditions in this study. e maximum decrease for all parameters was recorded at a mix the 25% rhododendron bers were mixed with oak and pine to manufacture the panels in addition to board produced 100% of each species. e panels made from 25% rhododendron wood had the lowest average surface roughness value (2.929) while panels containing 100% rhododendron wood had the highest. Average surface roughness was higher for control samples containing more industrial furnish. R z , R q and R y parameters had similar trends to the R a values. Increasing the mixing ratio in the mixture negatively a ected the surface roughness parameters of the panels. Ciencia y tecnología, 14(2): 185-193, 2012 Maderas. Table 2 shows the levels of decreases in surface roughness values in the MDF Panels with decreased mixed of rhododendron biomass. e initial results of this study suggest four roughness parameters can be used to quantify and improve roughness properties of samples for further manufacturing steps. For the direct painting and other surface nishing treatments of MDF to be successful, especially in furniture industry, the surfaces must to be smooth, stable, and not highly absorbent.
In general, the results of this study describing the e ect of mixed ratios of two ber types on the panel are comparable to the ndings of other studies. Nemli et al. (2005) found that raw material type a ected surface roughness of particleboard. Also, an increase in shelling ratio, panel density, and press pressure improved the surface roughness of particleboard. Buyuksari et al. (2010) found that the average R a values of the panels increased as a greater ratio of pine cone was added to the mixture. Panels made from 100% wood particles had the smoothest surface (9.77 µm), and the roughest surface (15.50 µm) was obtained from the panels containing 50% pine cone. e R y and R z parameters of panels had similar trends to the Ra values. ese values also increased with increasing the pine cone ratio in the mixture.
Surface roughness is a function of raw material characteristics, species, particle size and distribution and manufacturing variables, press parameters, resin content, face layer densi cation, and sanding process of the panels. Di erences in the average surface roughness of the produced particleboards were most likely due to the morphological properties of wood.
It is recommended that the surface roughness values of the panels made from rhododendron wood be improved by sanding and by increasing panel density, shelling ratio, and pres pressure.
e CA values of the samples were maximum at the beginning of the tests. e angles decreased with increasing time. e values were minimum at the end of the tests (at 30rd second). As seen from table 3, the CA values of the panels made from rhododendron wood signi cantly increased with increasing mix ratios of rhododendron bers. e results acquired in this study also indicate the wettability property of the panels decreased with the mixed ratios of rhododendron bers. e CA values were greater in the case of the panels containing rhododendron bers compared to those of the control panels. is change of wettability could have signi cant consequences on the use of the material, in particular for the adhesion of paints and coatings. Wettability of the MDF panels can provide good information on their ability to bond (Hakkou et al. 2005). ere is evidence about the positive relationship between wood wettability and adhesion. When contact angle is zero, perfect wetting of a surface occurs. Contact angle is a useful index of adhesive e ectiveness (Ayrılmış and Winandy 2009).

CONCLUSIONS
e results of this work showed that a stylus pro lometer can be used to evaluate surface roughness variations of the panels. e average R a value of the produced panels increased with increasing the rhododendron ber ratio in the mixture. e increase in the surface roughness of the panels should be considered if the MDF panels containing rhododendron bers are used in manufacturing overlaid panels for the furniture industry.
It was concluded that the MDF panels made from rhododendron biomass had poor wettability performance.
e results also indicate the wettability property of the panels decreased as the rhododendron wood ratio increased.
Finally, it could be suggested that rhododendron biomass could be used an alternative raw material for MDF manufacturing so that sustainable forest management could be achieved.