Plywood can maintain a high performance under a wide variety of environmental conditions. The high strength to weight ratios are influenced by the species used, softwood and hardwood, and also on the glues used in production. Plywood is generally made from veneers that are peeled from a log and are bonded together with an adhesive, with the grain of adjacent veneers at right angles to each other. The adhesive is cured by pressing the panel using heated platens.
As defined in BS EN 309,
- Plywood Wood-based panel consisting of an assembly of layers bonded together with the direction of the grain in adjacent layers, usually at right angles.
- Veneer is a thin sheet of wood not more than 7mm in thickness.
- Ply is either a single veneer, or two or more veneers joined edge to edge or end to end.
- Layer is either one ply or two or more plies, glued together with their grain direction parallel, or with another material.
- Multi-ply Plywood formed of more than three layers.
Typical adhesives used are;
- urea-formaldehyde (UF) - only suitable for interior use.
- phenol-formaldehyde (PF) - has greater moisture resistance, suitable for use in humid or in exterior situations.
- melamine-urea-formaldehyde (MUF) - for exterior or even marine plywood.
Many different wood species can be used for plywood manufacture, where the log can be reliably peeled or sliced into veneer. Softwood such as spruce, pine, fir and hardwood such as birch, beech, poplar and eucalyptus. The quality of the plywood depends on the quality of the wood species and lay-up of the veneers and on the resin type and bonding technology.
Prior to peeling the veneer, the logs are soaked or steamed in order to increase the moisture content. The veneers are then dried to a moisture content of about 4%8%. Open defects may be repaired using plugs or filler to upgrade the panel. The dried veneers are sorted into grades, usually by visual inspection. Synthetic resin adhesive is applied to the veneers by appropriate technology and the veneers are assembled with the grain of each normally at 90 to the adjacent veneer, also known as lay-up.
- Structural plywood are used in, floor decking, flat roofing, wall sheathing, concrete formwork, external cladding.
- Marine plywood has a very high performance under severe exposure conditions, and used in ship or boat building.
- Utility plywood is intended for non-construction applications, for joinery, furniture and mostly for interior uses.
- Class 1 - for interior applications with no risk of wetting,
- Class 2 - for use in protected exterior applications,
- Class 3 - for use in unprotected external applications.
Plywood normally has higher mechanical properties in a direction parallel to the face grain. If plywood panels are laid edge to edge, it is essential that suitable expansion gaps are provided to allow for any dimensional movement during service. Plywood can be fixed by nails, screws and staples or by gluing, depending upon the application and requirements. It possible to use nails and other mechanical fastenings quite close to the panel edges without the risk of pull out. Ring shank nails give improved performance over plain wire nails, when fastening plywood to timber. Glued joints provide stiffer joints than ones made with mechanical fixings alone. Plywood can be sanded and painted.
Plywood being a wood product is safe when it is handled and used correctly. When cutting or machining plywood, measures must be taken to control the dust. The formaldehyde content of plywood is normally very low and emission of formaldehyde is not often an issue.