The Influence On The Bonding Interface By The Wood Drying Quality

Due to the increasing dependency extent of the modern wood industry on the wood adhesives, the bonding quality has become one of the most important factors showing great influence on the final quality of wood products. Although numerous researchers have paid more attention to the bonding properties of wood products, the study associated with the interrelationship between timber drying quality and bonding quality still remains a research vacancy now.As results of the different drying schedules and also different conditioning parameters, the final dried timbers will show particular drying quality. The wood drying quality characters include the following ones,1. The final MC; 2. The MC gradients along the wood thickness direction; 3. The drying stress; 4. The visual drying defects. Different drying quality will have characteristic influences over the actual properties of material surface, namely, wet ability, surface tension, surface energy, the numbers and properties of activated atoms. These physical and chemical changes will, in turn, have the direct influences on the properties of bonding interface.Based on the specified wood drying quality, this paper mainly aimed to investigate the influences of the timber drying quality and the surface changes of wood materials on bonding interface in the microscopic level, trying to establish the inter-relationship between the bonding interface properties and the bonding mechanical strength in the macroscopic level. According to the theoretical analysis and research results of the experimental practices, the influence of drying quality on the bonding mechanical properties will be well documented. The theoretical backgrounds for the improvement of bonding interface qualities were put forward by making use of the mathematical analysis method. In consequence of these works, the drying technology and drying process control strategy can be optimized accordingly, so that the resulting drying quality can be applicable to the bonding design process.The main research results are as follows:1. Moisture content (MC) of woodThe MC tolerance on the wood thickness:it shows the undulation change with the storage time. For the higher MC wood (the MC is higher than that of the local Equilibrium Moisture Content), the MC tolerance is higher too, with the storage time pass on, it shows to become lower trend. In this progress, the core MC degressive speed is 3.1 percent per day, it is faster than that of the surface one 1.47 percent per day. This will bring about the core MC and the surface MC come together faster and let the MC tolerance become minished quickly. For the lower MC wood, the MC tolerance is lower too, with the storage time pass on; it shows to become higher trend. In this progress, the core MC increased speed is 0.85 percent per day, it is faster than that of the surface one 0.60 percent per day. This will bring about the core MC and the surface MC bigger distance and let the MC tolerance become big quickly. The MC tolerance decreased to the lowest volume in 3 to 4 days for the higher MC lumber; on the contrary, the MC tolerance increased to the highest volume in 3 to 4 days for the lower MC lumber. Both of them, the MC tolerance become to the stable in 5 to 6 days.For the MC on the wood surface, the higher MC become to lower with the storage time pass on; on the contrary, for the lower MC on wood surface, it will become higher. After 5 days, both of them will be stable. The research showed the best MC of wood bonding is 9-10%.2 Wood drying stressThe trend of the wood stress will become higher after completed drying, this is fluctuated change:it has the close relationship to the changing of the MC.MC changes and the stress changes accordingly. The MC is stable and the stress is stable too. When the wood drying schedule's highest temperature is 90℃,the MC tolerance is around 1%, the drying stress is in the smallest condition. It will be increased when the wood dried at higher temperature as well as the big MC tolerance.3 Bonding strength (compressive and shear)The wood bonding strength has the close relationship to the MC as well as the wood drying stress. If the stress higher, the bonding strength will be lower. The MC tolerance will become main factor to the bonding strength when the stress is similare.The bonding strength will be the highest when the N1s content around 3% and O1s 23-24% on the bonding interface. The wood drying schedule temperature is 90℃accordingly.4 Wet ability on the wood surfaceThe cosine contact angle will become lower and lower with the wood storage time pass on, in another words, the wood surface wet ability is decreased. The wet ability will be affected by the wood surface MC changing, the more MC changes on the wood surface, the more decreased to the wet ability, on the contrary, the small MC changes on the wood surface, the even changes to the wet ability. The higher MC wood wet ability is better than that of the lower MC one.Undered the same MC condition, The wood, which dried in the higher temperature,the wet ability will be lower than that of the one which dried in the lower temperature. The wet ability will be in the best when the MC is 8-10% and the dried temperature is around 90℃. When the MC is less than 7%, no mater how lower temperature used to dry the wood, the wet ability could not has a higher volume.5 Wood surface free energyThe wood surface free energy will become lower and lower with the storage time pass on. It has the close relationship with the MC changing:the higher MC wood surface free energy is better than that of the lower one. The wood surface free energy will be in the highest volume under the 8-9% MC as well as the dried temperature is 90℃. There is lower free energy volume if the wood MC is less than 6%.6 Wood surface active functional groupWith the storage time pass on, the wood surface active functional group will be changed as below:C1s changed in fluctuate way, normally it is decreased at first then increased; O1s content volume is changed from 35% to 27% then increased to 30%.The Nls content is small on the wood surface. The wood drying temperature will give a great effect to the C1s volume on the wood surface:C1s volume will be less when the dried final temperature is lower than that of it dried under the higher final temperature. O1s on the wood surface will be in highest volume when the MC is around 10%, the dried final temperature is 90℃. When the MC is less than 6%, there is no big change of the O1s on the wood surface with the different dried temperature.7 Bonding interface active functional groupCompared with the wood surface,C1s, O1s and N1s atom volume are changed obviously on the bonding interface, C1s is decreased; Nls is increased more; O1s is decreased. This indicated that there are some chemical changes between the wood and the adhesive surface. C1s volume is in the best condition under the 9-10% MC and the dried final temperature is 90℃. The dried temperature give less affect to O1s volume on the bonding interface, the wood surface MC gives a big effect to O1s.