Wood is a hygroscopic material and has ability to exchange its moisture content with air. Many mechanical properties are affected by changes in moisture content. It is an important parameter of quality which is often overlooked by plywood manufacturers.
The exchange of moisture depends upon the relative humidity and temperature of air and on the amount of water present in the wood. This relation has great influence on the properties and quality of manufactured plywood.
The Bureau of Indian Standards (BIS) recommends a moisture content of 5% – 15% by weight for plywood. Moisture content above or below this range can have adverse effects on the manufactured products and the environment.
Below mentioned are some of the factors which are affected due to changes in moisture content –
Drying is a process used in plywood industry 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. Companies in general doesn’t lay much emphasis on this process and usually dry the 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. This inactivation has a lot of effect on the glue consumption of veneer.
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.
Mechanical properties like bending strength, shear strength and modulus of elasticity are important parameters of plywood quality. These parameters ensure prolonged usage of plywood without failure.
There has been research that the value of these properties is inversely proportional to the moisture content of veneers. More the moisture content, lesser the strength.