Samples of plywood with and without a defect were used for the research. It has been determined that the most expedient and economically beneficial flaw detection method for plywood is the impact method. The results indicated that, inspired by the stiffness difference in the “sandwich structure” of wood, preparation of the SD plywood based upon hydro-thermal treatment is feasible. Comparing to some domestic Chinese hardwood species, SD plywood possessed greater surface hardness with less average density. The greatest increment of MOR, MOE and hardness was 54, 104 and 144%, respectively. The enhancements of MOR, MOE and hardness could be attributed to its high-density surfaces. Mechanical properties – including modulus of rupture (MOR), modulus of elasticity (MOE) and hardness – of the SD plywood were significantly higher than the CK plywood. No significant variation was observed for the bonding strength among SD and control (CK) plywood. Final MCs of the SD plywood met the requirement of the Chinese national standard. A greater degree of surface densification could be achieved at higher hot-pressing temperatures or higher surface veneer MCs. The densification was attributed to the thermal softening and mechano-sorptive behavior of constituents in the wood cell wall. Great surface densification was observed according to density distribution through plywood thickness.
INDOOR VEEER SERIES
A series of veneer moisture contents (MCs) and hot-pressing temperatures were selected to prepare the SD plywood. Inspired by stiffness difference in the “sandwich structure” of earlywood and latewood, surface-densified (SD) plywood was manufactured based on a hydro-thermal approach. The use of densified veneers for the production of plywood can lead to a shorter pressing time (17%-50% reduction), lower glue consumption (33.3% reduction) and a lower pressing pressure (22.2% reduction) without negatively impacting the bonding strength of the plywood. At a high pressing pressure, the heating rate of the densified veneer stacks was faster than that of non-densified veneers at the same pressure. The heating rate of the veneer stacks increased as the pressing temperature increased and decreased markedly with an increasing number of veneer layers. The temperature evolution inside the veneer stacks was measured for birch veneers for different pressing temperatures and pressures for different numbers of veneer layers. Bonding quality was evaluated by determining the shear strength of the plywood samples. The effect of pressure and time of pressing on bonding quality of the plywood was determined. Rotary-cut, non-densified and densified birch veneers and phenol formaldehyde (PF) adhesive were used to manufacture plywood samples. The influence of selected parameters including veneer treatment (non-densified and densified), plywood structure, temperature, time and pressure of pressing, on the bonding quality and temperature evolution within the veneer stacks during hot pressing was investigated. This research optimizes the process of plywood production to determine its effectiveness in reducing energy and adhesive consumption for more efficient production with the required quality. Findings of this work are useful for industrial applications to optimize the plywood production. The shear strength values of the plywood made of densified veneers were found two times higher than the normalized value of 1.0 MPa, meeting the EN 314-1 standard requirements. This will reduce the pressing time of plywood made of densified veneer by 2-29% depending on the number of veneer layers. It was found that the multi-layers plywood made of densified veneers was heated faster when compared to the plywood made of non-densified veneers. Plywood products of 3, 5, 7, 9 and 11-layers were manufactured under laboratory conditions. The multi-layers plywood structures made of densified and non-densified veneers with and without adhesive were investigated. Rotary-cut birch veneer and phenol-formaldehyde resin were used to make the plywood samples.
In addition, the shear strength of the plywood was determined. In this study the effect of veneer densification on the temperature change inside the plywood during the hot pressing step was analyzed.