Plywood consists of fine sheets of wood, or veneers bonded with glue. Plywood is a versatile product that can combine attractive surface appearance with superior performance under hazardous conditions while retaining comparatively high strength-to-weight ratios. It is available in a range of wood species, including hardwood and softwood species and combinations of the two, and a range of resin types for interior, high humidity and exterior conditions. Plywood was the first type of board developed to provide panels with dimensional stability and good strength both along and across the panel.
How is it made?
Plywood is generally made from veneers that are peeled from a log. These are bonded together with an adhesive that is appropriate to the end use, with the grain of adjacent veneers generally at right angles to each other, known as “cross grain”. The adhesive is cured by pressing the panel using heated platens. Plywood is processed on a worldwide basis using a wide range of timber species including softwoods and temperate or tropical hardwood.
What is this used for?
Plywood is commonly used in construction, for transportation, for packaging, for furniture and for decorative applications. Completely versatile, it is also suitable for the most extreme uses, amongst others for ship and boat building.
Light and easy to install, plywood offers excellent resistance to creep (the disforming of the wood due to weight bearing). Its cross-layer construction offers good stability and high resistance to impacts and weathering in all board directions. Depending on the type, it is suitable for both interior and exterior applications.
Light and easy to install, plywood offers excellent resistance to creep. Its cross layers gives it a good stability and high resistance to impacts and weathering.
Completely versatile, it is suitable for the most extreme uses: nautical construction, decoration, building, transport industry…
A recyclable material
Manufactured from logs coming from forests under long–term durable management, plywood is above all a material which can be recycled. It actively contributes both to the protection of the environment and the ecological and economic balance of the wood channel.
The mechanical characteristics of plywood panels vary according to their compositions (species used, lay-out and thickness of the plies). Nevertheless, they generally remain very close to those of solid timber while having the advantage of a greater regularity and a greater stability. This results from the elimination of important defects during the preparation of the veneers and the dissemination of the residual minor defects in the panel.
There are thus no standard values for the mechanical properties of plywood. Each manufacturer provides his own values according to the composition of his products.
2 series of different values can be provided:
• those resulting from controls in the factories on small samples, used for classification as defined in standard EN 636
• those resulting from tests on semi-size samples (tests according to standards EN 789 and EN 1058) or derived from the values measured in the Factory Production Control (using standard EN 12369-2).
Only such values are usable for structural design.
EN 314-2 defines the tests to be carried out to check the bonding class of a panel.
These classes correspond to the following conditions of use:
Class 1: dry interior
Class 2: humid interior
Class 3: exterior
Usually, urea-formaldehyde resins are used for class 1, melamine-urea-formaldehyde for class 2, melamine-urea-formaldehyde or phenol-formaldehyde for class 3.
Classes of formaldehyde release
The adhesives used for the manufacture of plywood contain synthetic resins (aminoplastic or phenoplastic).
These resins are manufactured from formol ( a soluble formaldehyde in water) and contain small quantities of free formaldehyde, necessary for a good hardening of the adhesive.
Although the hardened adhesive only contains traces of free formaldehyde, very weak quantities of formaldehyde can be released, particularly during the first weeks following the manufacture of plywood.
According to the reference standard, EN 636, a panel is classified according to formaldehyde release test results:
Class E1: less than 0,1 ppm according to EN 717-1
Class E2: more than 0,1 ppm according to EN 717-1
All the manufacturers, members of EPF only, put products on the market matching this class.
Plywood is a product in great demand because it offers a large variety of finishings; from the most traditional to the most sophisticated, you have the choice of all the different types of plywood:
• Raw and natural
• Covered with a decorative veneer
• Moulded into shape
The choice of a plywood for a given use is in general done according to 2 criteria:
– according to its basic characteristics (bonding class, specific properties)
– according to the wood species constituting its faces
Choice of plywood according to the bonding type
Plywood for interior use
All wood species may be used for this type of use.
Panels with a bonding class 1 may only be used in dry conditions.
For humid conditions, a bonding class 2 is necessary.
The reference standard for this type of panel is EN 636 (-1 or –2).
Plywood for exterior use
Certain species, due to their insufficient natural resistance against the wood-destroying fungi, may not be used for the manufacture of plywood for exterior conditions, unless they have undergone an appropriate treatment.
It is imperative to use the bonding class 3.
In all cases, the durability of a plywood panel exposed to bad weather can only be assured by respecting the installation rules and having a suitable finishing.
The reference standard for this type of panel is EN 636 (-3).
Choice of plywood according to the wood species
The use of Okoumé
Okoumé is a tropical wood species which is harvested in Gabon, in Equatorial Guinea and in Congo.
It is a light wood (density 0,4 to 0,5), pink-red colored and easy to peel.
It is appreciated for the manufacture of plywood because its quality is homogeneous, and it provides a consistency of surface quality giving an excellent finish (varnish, paint). Okoumé-based plywood is easy to machine and, with an adapted bonding, can be used in external applications.
The use of Maritime pine
Maritime pine has been used for the manufacture of plywood since the 1970’s.
The high mechanical characteristics of this species make this type of plywood particularly suitable for structural applications in building (racking in timber-framed houses, load-bearing floors, I-beams…) but also for industrial packing and boxes.
Grooved, it is used in the interior covering of walls and its good resistance to fungi enables its use for cladding.
The use of Poplar
A fast-growing species which presents an excellent aptitude for peeling, poplar is very much used for the manufacture of light packaging (cheese boxes, small crates for fruit and vegetables…).
Particularly light (density from 0,4 to 0,45), white, resistant, poplar plywood is particularly valued in packaging and furniture.
The use of Spruce
Spruce plywood is mainly used as construction panel such as concrete formwork and scaffolding and in the vehicle industry. Spruce is a good panel for general applications where strength, stability and lightweight properties are needed, such as for floors, lining and roofs.
Spruce plywood is used uncoated (mainly grades II and III) or coated by phenolic films, usually 120 g/m², 170 g/m² and 220 g/m². Spruce plywood is suitable for internal and external sue, depending on the level of protection required.