Product derived from wood

There are a number of manufactured products derived from wood which make use of some property of the wood, such as its inherent strength, while at the same time overcoming problems, such as warping or splitting along the grain, which arise from wood's anisotropic character. In the case of plywood, for instance, this is overcome by bonding wood plys together with the grain running alternately at right angles.

The addition of a hollow or solid core to the manufactured product can also add to the material's rigidity and to its acoustic and thermal insulation properties. Where a veneer is applied to the face of the manufactured board the decorative effect of a rare or expensive wood can be achieved economically.

Particle boards are characterised by the regularity of their surface and strength properties which make them suitable for flooring, roof-decking, partitions, joinery and so on.

Products derived from wood contain wood in a number of different physical forms combined with glues, resins and other additives. The coating characteristics of these derived products may be significantly different from the parent wood and many of them are not suitable for exterior use - a point which should always be checked.

Plywood

Plywood retains many of the characteristics of wood, including a susceptibility to the effects of moisture, though movement is considerably reduced. Preservative treatment is required where non-durable wood species have been used.

Paint performance can be unsatisfactory because of checking but grades of plywood are available with an impregnated paper overlay which largely overcomes this problem. Woodstains are less affected by the checking characteristics but may offer less protection. Although plywoods usually have one or both faces sanded as standard it is possible to obtain rough-surfaced plywood which will be more suited to stains.

Some plywood adhesives contain water-soluble salts which are brought to the surface by moisture, this is more likely to happen with woodstains than with paint. In areas sheltered from the rain white deposits will build up. They can be removed by washing but will re-appear until the salts source is exhausted.

Performance of coated plywood is very dependent on good edge scaling.

Plywood is defined internationally as an arrangement of panels which consists of an assembly of plys bonded together. The direction of the grain in alternate plys is usually at right angles. The outer and inner plys are generally placed symmetrically on both sides of a central ply or core.

Plywood is further divided into veneer plywood, in which all the plys are made up of veneers of up to 7mm thick, arranged with their planes parallel to the surface of the panel, and coreplywood which includes further sub-divisions.

Wood Core Plywood

Battenboard-core is made of strips of solid wood more than 30mm wide, which may or may not be glued together.

Blockboard-core is made of strips of solid wood, more than 7mm wide, but not more than 30mm, which may or may not be glued together.

Laminboard-core is made of strips of solid wood or veneer not wider than 7mm, placed on edge and glued together.

Cellular Plywood

The core is of cellular construction, with at least two cross-banded plys on both sides of the core.

Composite Plywood

The core (or certain layers) is made of material other than wood or veneers, with at least two cross-banded plys on each side.

The 'sandwich' type of construction of the various forms of plywood gives the material a number of properties important for use in building. They include:

• High uniform strength. Wood is 25 - 40 times as strong along the grain as across it, and crossing the grain tends to equalise the strength in both directions.
• Less deformation and moisture movement. The balanced construction leads to less than 1 per cent change in moisture movement.
• Non-splitting. Again, because of its cross grain construction, nails, screws and rivets can be driven through it. But there is considerable resistance to any attempt to pull screw or nail heads through this form of plywood.

Only certain species of wood are suitable for the manufacture of plywood. The logs are pre-treated by steaming or boiling and are then rotated against an adjustable knife which produces a fulllog width of continuous sheet veneer, up to 7mm in thickness. The veneers are evenly dried and graded for appearance - knots may be removed and splits or patches replaced. Prepared veneers are then glued together, to build up the required sandwich. As many as 17 plys are used to produce 25mm plywood.

Plywood is available in a variety of sheet sizes up to about 3.1 x 1.6m; larger sheets may be produced by scarfed joints. The surface can be finished in unsanded, sanded or scraped form, or it can be processed further to produce plastic, papered, painted and other surfaces.

Where panels are likely to pick up moisture, the use of scalers and/or coating is essential. Special attention must also be paid to protecting the edges of the board.

The durability of plywood is related to the type of adhesive used in construction, and these have been classified as follows:

• INT. INTERIOR. Adhesives are resistant to cold water but are not required to resist attack by micro-organisms. They give a durable bond under dry conditions but have limited resistance to moisture.
• MR. MOISTURE AND MODERATELY WEATHER RESISTANT. Adhesives will survive full exposure to the weather for limited periods but will fail the boiling water test. They are generally resistant to attack by micro-organisms.
• BR. BOIL RESISTANT. Adhesives have good resistance to moderate weathering and boiling water, will withstand cold conditions for many years and are highly resistant to attack by micro-organisms.
• WBP. WEATHER AND BOIL PROOF. These adhesives are resistant to weather and micro-organisms, cold and boiling water, steam and dry heat. They are based on phenolic resin and provide the basis for fully exterior grade plywoods.
• GRADE E. Natural surface remaining visible.
• GRADE I. Surface which may remain visible.
• GRADE II. Surface which may be directly overlaid or painted.
• GRADE III. Surface generally intended to be unseen, painted or coated. Open defects permitted according to requirements of each species.
• GRADE IV. No requirements for surface appearance (e.g. for packing cases etc).

Hardboard

Hardboard or fibre building board is usually manufactured from woody fibres. Some is also made from other structures, such as sugar cane. All are felted together, normally without resin. The resulting products are free from defects; they present an ideal surface for finishing and/or painting, offer greater resistance to fungal. decay and pest infestation (most of the sugars and starches are removed during manufacture), and attain a lower equilibrium moisture content. Moisture movement is normally below 0.3 per cent but it is still important that the product is allowed to reach equilibrium before fixing. A slightly high moisture content enables the board to tighten on its fixings rather than to expand and distort.

Fibre building boards are classified as either soft or insulating boards, where a high degree of sound or thermal insulation is required, or as hardboards, which are further graded into medium, standard and tempered types. Tempered hardboard is composed of standard hardboard impregnated with hot oil or resin and subsequently heat cured to produce greater strength and resistance to water and abrasion.

Tempered fibreboard is the only fibreboard which is acceptable as an external cladding material; it should have a density of 960 kg/m3 or above.

Tempered hardboard may be overcoated with both solvent-borne and waterborne opaque finishes.

Particle Board

The variegrated pattern of chipboard can be highlighted with coloured varnish.Particle boards are manufactured from a range of materials, such as wood, flax and hemp and are bonded with synthetic resins, usually urea formaldehyde. They are distinguished by their regularity of surface, thickness, size and moisture content, making them ideal for use in flooring, roof decking, partitioning and wall linings, joinery and built-in fitments. Particle board can be supplied in several grades (for example flooring grade, ready-to-paint) or ready faced with wood veneer, plastic etc.

All-wood particle board, usually called chipboard, is manufactured from wood chips which are sprayed with resin and then compressed in a forming press. This method, known as platen-pressing, is the most common (up to 98 per cent of the market) and results in the chips lying perpendicular to the board surface. An alternative method of manufacture by extrusion produces a weaker product with much greater moisture movement. The main use for extruded boards is as core stock for veneers.

Careful storage and practice are required for best results on site. When particle boards are unprotected from moisture penetration any treatment should be applied to the edges as well as to the face and back. Material should not be left on site for long periods before being used and should be stored flat to prevent permanent deformation.

Chipboard is not suitable for exterior use unless protected by a suitable laminate or covering; surface coatings alone are not enough.

Chipboard is typically used for interior use as a flooring material, but is also a major component of furniture, usually covered by a decorative veneer.

The surface of chipboard is pitted and careful preparation with fillers or undercoat is necessary to achieve a smooth paint finish.

It is possible to make a feature of chipboard's inherent pattern by using coloured varnish, for example in flooring, shelving, toy boxes, etc. When using satin or matt finish the surface should be sealed with a gloss varnish first; this prevents excess sinkage and consequent white deposits.

Veneers

Veneers are thin strips of wood cut in such a way as to reveal the wood's best natural features - grain, colour, figure. They are produced either by rotary cutting, where the knife acts against the full length of the log turning on a lathe (veneers are so cut tangentially to the growth rings of the log) or, as in the case of most decorative veneers, sliced across the face of the log.

The advantages of veneers include the wide variety of choice from among the world's hardwoods and softwoods, and their economy when used as a decorative facing on other cheaper woods. Correctly-made veneered panels give few problems, even in the most exacting conditions such as centrally-heated houses.

Veneered panels are supplied ready for use. The core may be blackboard, laminboard, hardboard, plywood, particle board or solid timber. The veneer is laid at right angles to the grain on the face of the core. Balanced construction removes the possibility of shrinkage and warping and, when laid and polished, decorative veneers require little or no maintenance.

Veneers from different logs of the same species of tree can vary widely. If the final appearance is important it is necessary to give an exact specification when ordering.