Plywood Print E-mail
Plywood is produced by assembling veneers (thin sheets of wood of uniform thickness) and bonding them together to form a panel. The veneers are usually assembled with the grain direction in one veneer being at right angles to that in the adjacent veneer. To insure a stable balance construction free of warping tendencies, an odd number of veneers is used (e.g. 3-ply, 5-ply and 7-ply). The grain on the face and back veneer is parallel. The crossing of the grain at right angles tends to equalize the strength in all directions and the uniformity increases with the number of plies used.

Plywood is only as good as the adhesive used in bonding the veneers together. Plywood that is used in the manufacture of furniture or as interior panelling does not normally require a high water-resistant bond. Plywood that is to be used in a situation exposed to the weather such as exterior doors or cladding requires an adhesive that is not damaged by water and fungi and which is strong enough to resist the much severer stresses in the timber by the greater moisture content range in exposed positions. The most important consideration in choosing the right plywood for a particular use is the type of adhesive used to bond the veneers together. Bonds are classified into four types.

Type A: A waterproof bond that is able to withstand exposure to the weather for a long period without deterioration. Phenol formaldehyde, Resorcinol formaldehyde and tannin formaldehyde adhesives can provide such a bond.

Type B: A waterproof bond but one that is subject to deterioration after several years' continuous exposure to the weather. This bond is suitable for use in plywood in concrete formwork and for permanent applications where only occasional exposure to wet weather is involved, such as in exterior doors. Melamine urea formaldehyde adhesives are commonly used.

Type C: A bond that can withstand occasional dampness, and suitable for general internal plywood. Extended urea formaldehyde is used.

Type D: A bond that is only suitable for internal conditions completely free of dampness. Highly extended urea formaldehyde is the most commonly used adhesive.

Although the use of a durable adhesive provides a bond of long-term effectiveness, it does not imply that the veneers being bonded together will have any long-term durability. For example, if plywood is to be used for a marine craft and will experience service conditions that are liable to encourage decay, then it will also be necessary to ensure that the veneers are of a durable species or that the plywood be preservative treated to the appropriate hazard level. (See: Treated Timber)

The term waterproof plywood is often misunderstood. It doesn't necessarily mean that the plywood is waterproof but rather that the glue is waterproof.

Plywood can be made termite resistant by preservative treatment to H2 level.  (See: Treated Timber)

Australian Wood Panels Association
AWPA is an organisation formed to promote the use of reconstituted wood products such as MDF and Particleboard and further research and development to conserve the agricultural and forestry resources of Australia.