The major innovations in plywood manufacturing at present among the plywood industries are to increase productivity and decrease costs. One is, innovations in the drying operation of the veneer during the manufacturing process. The need to increase drying throughput and the opportunity to improve product quality by reducing de-grade due to over-drying, has stimulated the development of adhesives tolerant to high veneer moisture.
The urea formaldehyde adhesives used at present are satisfactory for gluing dry veneers having moisture contents of 0% to 6%, they cannot be reliably employed with wet veneers having moisture contents as high as 10% or higher. With the latter, blisters develop on hot pressing, or even complete failure to form a bond under the usual conditions of temperature and pressing time which are employed in plywood manufacturing. In the case of plywood manufacturing, if the moisture content of the veneer is high, e.g. 10% or higher, excessive penetration of the adhesive into the veneer adversely affects bond quality.
The process of gluing of veneer-based products, as plywood 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 of plywood is generally produced with adhesives based on urea-formaldehyde resins and veneers that should be dried to 4 to 6 % 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.
In wood based Panel Industries drying of veneer to reduce the moisture content of core veneers. It is an important process which requires constant supervision as it effects the quality of plywood directly. Drying of veneers for a longer duration resulting in a situation called surface inactivation or case hardening. It is a situation in which all the moisture from molecular level of wood evaporates leaving the cell walls inactivated. Glue comprises of around 50% solid content and 50% moisture. When the inactivated veneer comes in contact with this glue, it absorbs a lot of moisture from the glue resulting in lower flow time.
Due to this, higher volume of glue is absorbed by the veneers. This results in overall increase in price of plywood. If the moisture content is optimal then the cell walls of veneers are not inactivated and comprise of some moisture. This results in better flow time for the glue and hence lower consumption. Furthermore, the process of plywood manufacture is characterized by significant energy costs for veneer drying (almost 50%) and hot pressing (10%), which significantly affects the cost of production. Therefore, using veneers with higher moisture would result in substantial savings of energy. There is a necessary to modify the resin or by using additive technology to manufacture the ply board by gluing the veneer having higher moisture content up to 15% to provide desired strength properties of plywood.