The application of thermo-mechanical densification in production of curved laminated veneer lumber and the effect on mechanical properties

In this study, it was investigated to produce densified and curved-densified laminated veneer lumber (dLVL and cdLVL) from poplar veneers by using of the thermo mechanical (TM) densification, lamination and bending processes together which require high temperature and pressure. Thus, it is aimed to find new usage areas for poplar wood by improving the mechanical properties. For this purpose, primarily an answer was sought to the question of how to produce dLVLs and cdLVLs. Due to the many advantages, the first step of the study focused on the production of dLVL from pre-densified veneers (discrete method) in a vacuum press. For this, the effect of low press pressure on the bonding strength of dLVL produced from densified poplar veneers was investigated. In this study, I-214 clone hybrid poplar (populus euramericana cv.) veneers were compressed at three different levels, 50%, 75 and 100%. Then, dLVL were produced from these veneers under 0.25, 0.50 and 0.75 N/mm2 pressure by using urea formaldehyde (UF) and polyvinyl acetate (PVAc) adhesives. As a result, it has been observed that the densification process has a positive effect on the bonding strength, but as the press pressure decreases, the bonding strength decreases as well. On the other hand, considering the high production costs, long processing time and difficulties in bending due to the increase of modulus of elasticity (MOE) as a result of the pre-densification, the discrete method was pushed to the second plan and it was decided to try the combined method. In the second stage, dLVL was produced by combining TM densification and lamination processes and the production conditions for best mechanical properties were determined. For this purpose, optimum levels were determined for compression ratio, press temperature, press time and amount of adhesive for dLVLs production using Taguchi based gray relational analysis. dLVLs were produced from Samsun (I-77/51) clone American poplar (populus deltoides) veneers by using UF and phenol formaldehyde (FF) adhesives. As a result of the study, it has been determined that the most effective production factors on mechanical properties are compression ratio and press time. The best results were obtained from dLVL at compression rate of 50% and at press time of 3 minutes for UF, 7 minutes for FF. Beside of the improvements up to 71% in mechanical properties, because of the potential to decrease production cost and time, it was decided to use of combined method in cdLVL production. In the final stage of the study, cdLVL was produced from Samsun (I-77/51) clone American poplar (populus deltoides) by combining the TM densification, lamination and bending processes. Within the scope of the study, the moment carrying capacity and rigidity of produced cdLVL under static load were determined and the production parameters such as the type of adhesive, coating thickness and press temperature are optimized. Finally, the cdLVLs with best multiple performances were compared with the control groups according to the fatigue strength under cycling load. As a result, it was determined that the most effective production factor on mechanical properties was veneer thickness and multiple performances decreased as veneer thickness increased. The best results were obtained when the UF adhesive, 1.8 mm veneer thickness and 150 ⁰C press temperature were used. Compared to poplar control group, a 63% improvement in moment carrying capacity under diagonal compression load was achieved, while a limited increase (25%) was observed under diagonal pull load.