Plywood
Thin wood veneers glued with their grain crossed — which removes the one weakness wood has, and it is the oldest engineered composite there is.
Plywood is an odd number of thin wood veneers glued face to face with the grain of each turned ninety degrees to its neighbours. That single arrangement fixes wood's defining problem.
Wood is roughly twenty times weaker across the grain than along it, and it moves with moisture across the grain and hardly at all along it. Cross-laminating means every ply is restrained by the one next to it, so the panel is strong in both directions, barely moves with humidity, and does not split.
The odd number is not arbitrary: it puts the face grain in the same direction on both sides, so the panel is balanced and does not cup as it takes up moisture.
Processing
A log is steamed and turned against a knife on a lathe, peeling a continuous veneer off it like unrolling a carpet — which is how a panel wider than any board can be made from a tree. The veneers are clipped, dried, coated with adhesive, stacked in alternating grain and hot-pressed.
The adhesive decides the grade. Urea-formaldehyde is cheap and not water-resistant; phenol-formaldehyde is what makes exterior and marine plywood exterior and marine. It is the resin rather than the wood that is being specified when somebody asks for marine ply.
Uses
Structural sheathing, flooring, roofing and formwork in construction, which is most of it by volume. Furniture and cabinetry. Boat hulls, aircraft in the 1930s and 1940s — the de Havilland Mosquito was a plywood aeroplane and was fast because of it — packaging, and concrete shuttering.
It is the archetype of a class: taking a natural material with a directional weakness and engineering the direction out of it, which is the same idea as a composite laminate arrived at a century earlier.
History
Cross-laminated veneer is ancient — Egyptian furniture uses it — and industrial plywood dates from the nineteenth century with the rotary lathe, which is the enabling invention. Synthetic resin adhesives from the 1930s are the second: animal glue plywood delaminates in damp, and phenolic resin does not.
Environmental impact
Wood is the only common structural material that starts as a carbon sink, and a panel that stays in a building keeps that carbon out of the air for the building's life. Against that, plywood requires large, straight, high-quality logs, which is a demand on forests that engineered products made from smaller wood do not make.
Formaldehyde emission from the adhesive is the standing indoor air quality issue, controlled by emission classes — the European E1 and the Californian CARB standards — and modern panels emit far less than those of the 1970s and 1980s. The panels already in older buildings do not improve.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is composed of
- Wood — material · peeled into veneer on a lathe, which is how a panel wider than any board is made from a log
- Phenolic resin — material · the adhesive, and it is the resin rather than the wood that is specified when somebody asks for marine ply
is used as
- Structural engineering — application · sheathing, flooring and formwork, and the cross-lamination is what makes a panel strong in both directions
is used in
- Construction — industry · sheathing, flooring and concrete formwork, which is most of it by volume
is an alternative to
- Wood — material · strong in both directions and dimensionally stable where a board is neither, and it needs large straight logs and a synthetic adhesive to be either
is associated with
- The wartime materials programmes — event · the de Havilland Mosquito was a plywood aeroplane, and it was fast because of it
is produced by
- Heat treatment — process · veneers peeled from a log, laid with the grain of each at right angles to the last, and pressed with adhesive under heat
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing