Major innovations in plywood manufacturing at the recent time are to increase the productivity and decrease the costs. One result of these innovations has been a shift of the process bottleneck to the drying operation. The need to increase drying throughout and the opportunity to improve product quality by reducing degrade due to over drying, has stimulated the development of adhesives tolerant to high veneer moisture.
The process of gluing of veneer-based products, as plywood or laminated veneer lumber is significantly affected by the moisture content in wood combined with water in an adhesive. This moisture directly influences the curing process and properties of the used adhesive, economic costs (consumption of glue, pressing time and costs for veneer drying) as well as physical and mechanical properties of veneer-based products. Nowadays in India, construction plywood is generally produced with adhesives based on phenol-formaldehyde resins, urea formaldehyde resin and veneers that should be dried up to 8 % moisture content. For such moisture content the conventional thermo-reactive adhesives provide high quality bonding of plywood with physical and mechanical performances that meet Indian standard requirements. Gluing of veneers with lower moisture content ( < 8±2 %) involves the disadvantages of high consumption of energy for drying, brittleness of veneers, rapid adhesive viscosity increase by diffusion of water into the cellular structure, worsened wetting and insufficient transfer of adhesive to another surface. Gluing of veneer with higher initial moisture content (> 8±2%) has the disadvantages to increasing adhesive penetration by the flow inside the vessel network through the veneer thickness, high vapor pressure causing steam blisters or blows, decreasing the viscosity of the applied adhesive layer, and loss of wood mass as well as thickness due to compression. There is a risk of forming of discontinuous glue layer on veneer surface, retardation of glue curing or creation of insufficient joints. Furthermore, the process of plywood manufacture is characterized by significant energy costs for veneer drying (almost 60%) and hot pressing (15%), which significantly affects the cost of production. By using resin after modification like fast curing using lignin, tannin etc shows that, it has failed to provide pre press tackiness to the veneer and creates blister on the surface when plywood manufactured at higher moisture content. Drying of core veneer to bring down the moisture content within 6% is highly expensive by using utility. Therefore, using veneers with higher moisture would result in substantial savings of energy, cost saving, minimizing formaldehyde emission value and to improve the quality also. Therefore it needs to develop the adhesive compositions that allow gluing veneers with higher moisture content (up to 15 %) to provide the desired strength properties of plywood and veneer-based materials.
Some of the recent developments and trends in the field of eco-efficient bonding technologies contribute to manufacturing of plywood having high moisture content developed so far as per literature review reports are as follows.
Addition of polysaccharides or soy proteins to traditional synthetic wood adhesives after partial hydrolysis and modifications has been reported (Pizzi, 2006).
Some articles (Besinova et al., 1997; Elbez, 1997) were oriented on benefit of gluing high moisture veneers. More studies were performed on possibilities of bonding high moisture veneers using phenol-formaldehyde resin filled by hydrolyzed soy protein, starch and tannin (Vijayendran et al., 2000)
Other researchers prepared high solid PF adhesive mixtures with increased reactivity by grafting resorcinol (Clark et al., 1988) or they modified PF resin (Steiner et al., 1993) that consisted of an alkaline insoluble but swellable dispersed PF phase, an alkaline soluble continuous phase and propylene carbonate additive. Some systems of filling with pecan shell flour and two furfural-process residues were tested (Sellers et al., 1990).
One of them involves mixing a high molecular weight resin with alkylene carbonates or phenol-resorcinolformaldehyde resins (Clarke et al., 1990).
The wood porosity is one of the main physical indexes in wood science. Bonding of wood elements with higher moisture content is a difficult process; the details of the penetration of the hardening adhesives into the porous wood skeleton are rather complicated. It is strongly infl uenced by factors as wood species, grain orientation and surface roughness, adhesive factors as type of adhesive, solid content and viscosity, and process factors as applied pressure and temperature, which have the most significant influence on the bonding performance (Kamke and Lee, 2007; Varivodina et al., 2010).
Some studies were performed on possibilities of bonding high moisture veneers using phenol formaldehyde resin filled by hydrolyzed soy protein, wheat starch, rye flour and tannin.
Based on the obtained results, PF resin modified by soy protein isolates can be recommended for gluing veneer with 15% moisture content by hot-pressing (pressing temperature 130 or 150 °C).
Modified PF resin with selected modifying agents allows using veneer with moisture content of 15 % for the manufacture of plywood in the laboratory scale, if their properties meet the standard requirements. Modifications of phenol-formaldehyde resin, in the way presented in this study, are possible with the use of wheat starch, phenol-resorcinol-formaldehyde resin and 30 %, 40 %, 50 % concentrations of resorcinol. The results of tests of shear strength properties of plywood show that mainly PRF glue compositions are most appropriate for the gluing of veneer with higher moisture content.
The positive results from the laboratory experiments gave the base to plan experiments in industrial conditions, and however the application of some of them is limited due to their higher viscosity, mainly from the view of glue spread control. These glues have the solid content higher in comparison with reference PF resin.
Further research and experimental work is under progress to manufacturing of plywood in industrial scale having veneer having high moisture content by using both amino and phenolic resin by modifying adhesive with using of additive technology without altering desired strength properties of plywood.
AUTHOR: S. C. Sahoo, Scientist, Indian Plywood Industries Research &Training Institute, (IPIRTI)