Wettability – contact angle Results and Discussion

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 37 Figure 20 Average contact angle on veneer loose side from pith to bark of unboiled sengon by using water a, boiled sengon by using water b, unboiled sengon by using PVAc c and boiled sengon by using PVAc d Wetting is directly related to the oxygen: carbon OC ratio and inversely related to the C1C2 ratio Sernek 2002. The C1 component is related to C-C or C-H bond and C2 component represent single C-O-bond. A high OC ratio and low C1C2 ratio reflect a high concentration of polar wood component on the wood surface, which made the wood surface hydrophilic. When wood is heated, hemicellulose begins to degrade, results the production of methanol, acetic acid and various volatile heterocyclic compounds Hill 2006. This condition could lead to prevention veneer surfaces inactivation. According to Aydin 2004 surface inactivation could lead to the interference wetting, flow and penetration of the adhesive. Ideally, the shape of a liquid droplet is determined by the surface tension of the liquid. In a pure liquid, each molecule in a bulk is pulled equally in every direction by neighboring liquid molecule, resulting in a net force of zero Yuan and Lee 2013. However, the liquid exposed at the surfaces of boiled sengon and jabon veneer have neighboring molecules in all direction caused by high OC ratio. As a results, the liquid voluntarily spread its surface area which lead to low contact angle. Lower values of contact angle are caused by the difference between wood and adhesive surface tension. In order to have high bonding quality, surface tension of wood must be lower than surface tension of adhesive Yuan and Lee 2013. 20 40 60 80 100 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Unboiled sengon 1st segment Unboiled sengon 2nd segment Unboiled sengon 3rd segment Unboiled sengon 4th segment Unboiled sengon 5th segment Unboiled sengon 6th segment Unboiled sengon 7th segment a 20 40 60 80 100 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Boiled sengon 1st segment Boiled sengon 2nd segment Boiled sengon 3rd segment Boiled sengon 4th segment Boiled sengon 5th segment Boiled sengon 6th segment Boiled sengon 7th segment 60 80 100 120 140 160 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Unboiled sengon 1st segment Unboiled sengon 2nd segment Unboiled sengon 3rd segment Unboiled sengon 4th segment Unboiled sengon 5th segment Unboiled sengon 6th segment Unboiled sengon 7th segment 40 60 80 100 120 140 160 180 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Boiled sengon 1st segment Boiled sengon 2nd segment Boiled sengon 3rd segment Boiled sengon 4th segment Boiled sengon 5th segment Boiled sengon 6th segment Boiled sengon 7th segment b c d 38 Figure 21 Average contact angle on veneer loose side from pith to bark of ‘lambro’ by using water a, ‘lambro’ by using PVAc b, ‘soligo’ by using water c and ‘soligo’ by using PVAc d Figure 22 Average contact angle on veneer loose side from pith to bark of ‘taro’ by using water a and ‘taro’ by using PVAc b The same with sengon and jabon, contact angle values decreased as a function of time on poplar cultivars Figure 21 and 22. The average initial contact angle from pith to bark when we dropped water onto the veneers were 93.7° ‘taro’, 91.3° ‘lambro’ and 91.0° ‘soligo’. The average initial contact angle from pith to bark by using PVAc were for 124.8° ‘taro’, 131.2°’lambro’ 20 40 60 80 100 120 140 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Taro 1 st segment Taro 5th segment Taro 10th segment Taro 15th segment Taro 20th segment Taro 25th segment Taro 30th segment b 20 40 60 80 100 120 140 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Lambro 1st segment Lambro 5th segment Lambro 10th segment Lambro 15th segment Lambro 20th segment Lambro 25th segment Lambro 30th segment 80 100 120 140 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Lambro 1st segment Lambro 5th segment Lambro 10th segment Lambro 15th segment Lambro 20th segment Lambro 25th segment Lambro 30th segment 20 40 60 80 100 120 140 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Soligo 1st segment Soligo 5th segment Soligo 10th segment Soligo 15th segment Soligo 20th segment Soligo 25th segment Soligo 30th segment 60 80 100 120 140 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Soligo 1st segment Soligo 5th segment Soligo 10th segment Soligo 15th segment Soligo 20th segment Soligo 25th segment Soligo 30th segment 20 40 60 80 100 120 140 20 40 60 80 100 120 140 160 180 C on tact an gle ° Time seconds Taro 1st segment Taro 5th segment Taro 10th segment Taro 15th segment Taro 20th segment Taro 25th segment Taro 30th segment a b c d a 39 and 129.7° ‘soligo’. We concluded that poplar cultivars produced lower wettability also when PVAc was dropped into veneer loose side compare with water. Mature veneer surface had higher contact angle than juvenile veneer, both for water and PVAc. Juvenile veneers surface had rough surface, so that water and PVAc easily penetrated and spread on to the surface. Moreover, higher frequency of lathe check near the pith could also contribute to rough surface of juvenile veneers which lead to low contact angle values. Therefore, poplar juvenile veneers had higher wettability than veneers from poplar mature wood. 3.4.5 Correlation between equilibrium contact angle and frequency of lathe check and surface roughness The correlation between dependent variable equilibrium contact angle and independent variables frequency of lathe check and Ra values could observe from multiple linear regressions Table 5. Generally, frequency of lathe check and surface roughness were correlated negatively with contact angle. The more frequency of lathe check, the contact angle would be lower. The same with surface roughness, the rougher the surface, the contact angle would be lower. Table 5 Correlation between dependent variable equilibrium contact angle and independent variables frequency of lathe check and Ra values at a 95 confidence level p0.05 Wood Species Equation R 2 P Unboiled jabon Y= 114.195 – 0.0877544X 1 – 0.191574 X 2 98.19 0.013 Boiled jabon Y= 25.3831 – 1.65247 X 1 + 10.2127 X 2 89.5 0.032 Lambro Y = 136.389 – 3.24764 X 1 – 0.0115521 X 2 92.68 0.002 Soligo Y = 148.043 – 4.13325 X 1 – 0.606963 X 2 84.59 0.010 Taro Y= 125.11 – 0.563179 X 1 – 0.313378 X 2 92.89 0.002 where, Y = equilibrium contact angle X 1 = frequency of lathe check X 2 = Ra values surface roughness R 2 = coefficient determination The results in Table 5 indicate that the surface roughness did not show the negative effect on equilibrium contact angle in boiled jabon. We suspected that boiled jabon veneer surfaces were more porous compared to unboiled veneers and 75°C boiling treatment prevented surface inactivation. The boiled veneer contained high OC ratio and low C1C2 ratio that could lead to low values of contact angle. We could conclude that veneers from boiled logs had higher wettability than veneers from unboiled logs. This characteristic would contribute to better LVL glue bond and bending strength.

3.5 Conclusion

The frequency, depth and length of lathe check, surface roughness and contact angle were influenced by juvenility. The frequency of lathe check, surface roughness and contact angle decreased from pith to bark, while the value of length 40 and depth of lathe check tended slightly fluctuate from pith to bark. These results were occurred in all wood species that were used in this study. In general, 3 mm sengon and jabon veneers from 5 years old boiled sengon and jabon logs had better veneer quality lower lathe check frequency, better surface roughness however better wettability than unboiled logs. In most of samples, frequency of lathe check and surface roughness were correlated negatively with equilibrium contact angle except for boiled jabon.