Plywood - an overview

2.4.1 Plywood

Plywood is constructed from plies or veneers of wood glued together, a technique that may have been used in some form for thousands of years. Actual structural plywood has a more limited history with first mentions made in the 18th century. Subsequently, plywood has been used in construction, interior furnishings and even aircraft (Stark et al., 2010).

Manufacture. Plywood is a board material constructed from an odd number of (usually three, five, seven, etc.) sheets of wood, termed plies, that are bonded together using a glue (traditionally a urea or PF −   5%–10% by weight dependent on type). In recently developed plywood (UPM Grada), the thermoplastic film glue can be hot-melted again (90°C or 130°C), which makes this material thermoformable. When arranging the sheets, the layers are aligned so that the grain of the wood of each ply is perpendicular to that of the plies it is bonded to. Therefore, all odd-numbered layers have a grain orientation parallel to the long dimension of the panel, whilst all even numbered plies have a grain orientation perpendicular to the long dimension. The wood used can be either soft or hard wood origin, based on the timber availability and desired end use. Spruce, pine, fir, birch, beech, poplar and eucalypt are all commonly used. (For a more detailed description, see Thoemen et al., 2010; Hughes, 2015.)

A simple scheme of manufacturing plywood includes a number of processes; the logs are debarked, and veneer is peeled or sliced, checked for defects, cut to obtain the required sorted quality and dimensions and dried; glue is applied; and then, the boards are cold and hot-pressed to cure the glue. Finally, the boards are finished by sanding or by application of coatings or laminates.

Properties. The characteristics of plywood are heavily influenced by the wood species, the number of plies and the type of resin used for glueing (Stark et al., 2010; Wood Panels Industry Federation, 2014a). In general terms, plywood has good mechanical properties, which are based on the strength of the wood species but enhanced by the cross-laminated design (Cai and Ross, 2010). This gives good bending and shear strength both along and across the panel. The good mechanical properties give good resistance to splitting, which enable strong fastenings to be made, even with fasteners near to the edge of the panels. The higher strength-to-weight ratio of plywood compared with the comparable solid wood makes it ideal for flooring use. Plywood also shows good and uniform dimensional stability across the panel with low edge swelling seen with increased moisture. The actual moisture resistance of plywood is more heavily dependent on the adhesive used. For waterproof grades, PF glues, impregnated paper laminates (for the surface finish) and painted edges are used. If exposed to moisture extremes, the cross-laminated nature reduces shrinkage and warping of the panel in contrast to comparable solid woods. It can also be treated relatively easily to increase its resistance to chemicals, fire or biological attack.

Plywood can be categorised based on its use, with common classifications including structural, decorative (exterior and interior) and marine grades. In Europe, plywood is categorised (EN 636 (CEN, 2015)) based on the usage of the product (Eurocode 5 (CEN, 1995)), hazard class of the environment (EN 335 (CEN, 2013)) and the adhesive bonding of the plywood (EN 314 (CEN, 1993)) as shown in Table 2.2.

Table 2.2. European classification of plywood, according to EN 636 (2015)

Plywood classificationEN 314 bonding classEurocode 5 service classEN 335 use classEN 636-1Dry conditions111EN 636-2Humid conditions221 and 2EN 636-3Exterior conditions331, 2 and 3

Applications. As noted, plywood has inherently good strength properties that can be engineered for certain uses. In structural terms, plywood can be used for partitions, floors, ceilings and sheathing. It is also used for decorative purposes both in exterior positions, such as cladding and doors, and internally as cabinets, shelves and furniture. The highest specification of plywood is used for marine environments, such as boats and docks—this uses veneers made from durable wood species, for example, okoume and PF resins. Requirements for untreated tropical hardwood veneers and glues for marine grade plywood are specified in the European standards EN1088-1 and EN 1088-2 (CEN, 2003a,b).