Moisture Content: The moisture content of wood and wood products is defined as the mass (weight) of water expressed as a fraction (%) of the oven-dry mass of the wood product. 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. Wood placed in an environment of constant atmospheric conditions and isolated from liquid water, will eventually reach a moisture content in equilibrium with that environment, termed “equilibrium moisture content” (EMC). EMC is a function of the ambient relative humidity, and, to a lesser extent, the temperature. The EMC at any set of atmospheric conditions below 100% RH, will be below the fiber saturation point.
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.
The factors which affect the quality of the plywood due to changes in moisture content of the core and face veneers used for the manufacturing of plywood are:
1. Glue consumption
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 don’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.
2. Mechanical Properties
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 would be the strength.
3. Thickness Variation
Thickness variation is the maximum thickness difference observed at different points on the plywood. The plywood having less thickness variation is considered to be of good quality. Drying of core veneer plays an important part in controlling the variations in plywood thickness. When a veneer is dried, it starts to lose moisture and shrinks as a result. Prolonged drying leads to wavy veneers. These wavy veneers and cross bands when pressed in the hot press start to straighten which leads to gaps and overlaps in plywood. These gaps and overlaps lead to thickness variations.
4. Visual Defects
Failing to control the moisture content can also lead to visual defects in plywood. Some of the visual defects can be as follows –
Air pockets: Air pockets are bumps and cracks on face veneer caused by the release of vapors from core veneers. These are caused when the moisture content of veneers is high. The moisture in veneers starts to evaporate when hot pressed in the form of vapors. These vapors get trapped between core and face veneers which lead to air pockets and cracks.
Face cracking: Face cracking usually occurs due to high moisture content of face veneer. When the moist face veneer is hot pressed it starts losing moisture and starts contracting rapidly. This causes it to crack at places.
Warpage of plywood: Warping the plywood sheets get affected by many factors during the whole process of production. Particular stages starting from the storage of construction veneers to the storage of finished plywood sheets. An actual material for the production of plywood sheets (veneers), which is characterized by moisture and temperature, is a basic monitored factor. The moisture and temperature of veneers and plywood rank among the most important factors affecting the shape & stability.
Wood destroying organism effect: Moisture content (MC) in excess of 20% is a major predictor of performance problems with wood and wood products, influencing their susceptibility to fungal and insect infestation and the susceptibility of fasteners embedded in them to corrosion. High MC and repetitive, large fluctuations in MC are also implicated in creep deflection.
To prevent these deleterious effects and to provide for the long service life of solid wood in use, an MC of 20 % or less has therefore long been recommended. This level is approximately the equilibrium MC value of wood at 90 % relative humidity and room temperature. Maintaining wood and wood products below these target MCs will prevent the establishment and growth of decay fungi and will preclude the possibility of objectionable creep deflection in adequately sized members.