Surface Roughness Results and Discussion

35 Juvenile wood on ‘soligo’ had fiber diameter of 34.5 µm compared to 29.7 µm mature wood. This condition could cause the rougher surface on juvenile veneers. Also higher frequency of lathe check near the pith could also contribute to rough surface of juvenile veneers. Further Tanritanir et al. 2006 investigated the effect of steaming time on surface roughness of beech veneer and they also found that the roughness of veneer sheets taken from heartwood near pith had higher values than those of sapwood near bark. Average Ra values of the samples manufactured from the jabon logs with a temperature of 75°C from pith to bark was 13.8 µm, while for unboiled was 14.8 µm. Findings in this study suggest that surface roughness of the veneer improved with boiling the log at that temperature. This result corresponded with Aydin et al. 2005, who discovered the same phenomena on spruce veneer. It seems that boiling temperature resulted in better surface properties of the samples based on the results of the tests. This finding would also contribute to reduced resin consumption during the gluing. Faust and Rice 1986 found that the use of rough veneers in LVL decreased bonding quality approximately 33 percent compared to LVL made of smooth surface veneers. Rough veneers reduce contact between the layers resulting in a weak glueline and low strength properties of the plywood Kantay et al. 2003. Veneer with a rough surface can also cause excessive resin use and may result in resin bleeding through the face veneer. Roughness of face veneer can be improved to a certain extent by sanding; however, this increases overall production costs Lebow and Winandy 1998, Taylor et al. 1999.

3.4.4 Wettability – contact angle

In general, contact angle values decreased as a function of time in sengon, jabon and poplar cultivars Figure 19, 20, 21 and 22. This result was in line with Shi and Gardner 2001 who stated that the contact angle changes as a function of time due to spreading and penetration of the liquid. The average initial contact angle of unboiled jabon when we dropped water onto the veneers was 89°, while for boiled jabon was 53° Figure 19a-b. The average initial contact angle of sengon was 84° unboiled and 63° boiled Figure 20a-b. Further, when we dropped PVAc, the initial contact angle were more than 90° for both sengon and jabon Figure 19c-d and 20c-d. We could conclude that PVAc on surface of sengon and jabon wood had lower wettability compared to water. It was due to PVAc had higher viscosity than water so that adhesive was slower and more difficult penetrating jabon veneers. Viscosity value of water was 0.008poise, while PVAc was 90-110poise. Surface wettability would decrease as viscosity value increase Gavrilovic-Grmusa et al. 2012. According to Yuan and Lee 2013, contact angle less than 90° indicates that wetting of the surface is favorable, and the fluid will spread over a large area on the surface, while contact angles greater than 90° generally means that wetting of the surface is unfavorable so the fluid will minimize its contact with the surface and form a compact liquid droplet. For example, complete wetting occurs when the contact angle is 0°, as the droplet turns into a flat puddle. For superhydrophobic surfaces, water contact angles are usually greater than 150°, showing almost no contact between the liquid drop and the surface. 36 Jabon produced higher contact angle than sengon both for water and PVAc. It was due to sengon had lower density than jabon. According to Shi and Gardner 2001, liquid penetration in the phase of wetting is mainly related to the wood structure. Sengon was more porous than jabon. Figure 19 Average contact angle on veneer loose side from pith to bark of unboiled jabon by using water a, boiled jabon by using water b, unboiled jabon by using PVAc c and boiled jabon by using PVAc d The contact angle values of sengon and jabon veneers near bark were larger than that of veneers near pith Figure 19 and 20. Veneers near pith had rougher veneer surface than near bark. Surface roughness affected contact angle. These results were correspond to Airylmis et al. 2010, who concluded that rough surface roughness had higher wettability compare to smoother surfaces Though, the surface roughness of boiled sengon and jabon were lower than unboiled sengon and jabon, however the contact angle value of boiled sengon and jabon was smaller than unboiled Figure 19 and 20. It was due to the boiled veneers become more porous compared to unboiled veneers. This condition made liquids water and PVAc were easy to spread and penetrate into sengon and jabon boiled veneer surfaces. Trung 2014 found that large holes are created at radiata pine array areas at softening condition of wood moisture content of 30 and temperature 100°C. 100 120 140 160 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Boiled jabon 1st segment Boiled jabon 2nd segment Boiled jabon 3rd segment Boiled jabon 4th segment Boiled jabon 5th segment Boiled jabon 6th segment Boiled jabon 7th segment d 20 40 60 80 100 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Unboiled jabon 1st segment Unboiled jabon 2nd segment Unboiled jabon 3rd segment Unboiled jabon 4th segment Unboiled jabon 5th segment Unboiled jabon 6th segment Unboiled jabon 7th segment 20 40 60 80 100 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Boiled jabon 1st segment Boiled jabon 2nd segment Boiled jabon 3rd segment Boiled jabon 4th segment Boiled jabon 5th segment Boiled jabon 6th segment Boiled jabon 7th segment b a 100 120 140 160 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Unboiled jabon 1st segment Unboiled jabon 2nd segment Unboiled jabon 3rd segment Unboiled jabon 4th segment Unboiled jabon 5th segment Unboiled jabon 6th segment Unboiled jabon 7th segment c