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Technical Information Article

​Edge sealants for wood-based boards

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Protecting board edges is a vital part of board performance in humid conditions. This wood information sheet gives guidance on suitable materials.Edge sealants are treatments or products applied to the edges of wood-based panel products such as plywood and particleboard to prevent or retard the absorption of water under damp or wet conditions. There are many materials which have a potential edge sealing role but none should be relied upon to compensate for poor design or detailing.

Good design should be the first means of reducing moisture penetration through the edges of wood-based boards with edge sealants being used as a supplement.

THE NEED FOR EDGE SEALING

Many of the problems which eventually beset wood-based panel products in service could be avoided if the edges were effectively sealed against moisture before being installed. Unfortunately, all too often the faces of the board are decorated and protected whilst the edges, often hidden from view, are neglected. This can lead to premature failure of decorative treatments, the board itself, or both.

Water is absorbed much faster into the edges of an unprotected board than into the faces. The reason is that, whether the board be plywood, blockboard or particleboard, much of the timber exposed at the edges can be end grain. Wood structure may be likened to close-packed tubes along which water can pass easily but between which it cannot easily permeate. Therefore water is absorbed into the end grain of timber much faster than it is across the grain. This difference can be from several times to several thousand times faster depending on the species of timber.

The effect of such water penetration through the edges can cause any or all of the following faults:

  • swelling of the board edges
  • discolouration and watermarking
  • failure of surface coatings
  • mould and stain development
  • decay
  • delamination or disintegration

Absorption of water by wood causes swelling. This is a reversible process, the original dimensions being recovered when the wood is allowed to dry out. Even in solid timber repeated swelling and shrinkage is likely to result in splitting and distortion but in the case of wood-based boards, the problem is rather worse. This is because, during the manufacture of plywood, particleboard, etc the boards are subjected to heat and pressure which compresses them somewhat. When these boards get wet the compression is relieved to some extent and the boards will never regain their original thickness, even though dried thoroughly. This sort of problem can be seen frequently, for example with formwork linings if the edges of the boards have not been sealed. The edges swell to become thicker than the rest of the board and this ridge is imprinted on the concrete as a furrow.

Water penetrating from the edge towards the centre of the board can redistribute some of the natural colouring materials (extractives) in the wood to form a (usually) dark zone line or watermark roughly parallel to the exposed edge of the board.

A similar discolouration may also arise due to the water redistributing acid or alkaline components of the adhesives in the board. These can then react with the wood extractives to cause colour changes. If the water which is absorbed by the board edges is dirty or contaminated by chemicals, eg iron salts, this can also lead to discolouration, usually in the zone of maximum penetration and is particularly apparent with light-coloured timbers and transparent finishes.

The presence of water, absorbed through board edges and trapped behind a film of paint or other coating, is a common cause of cracking, peeling or blistering of the decorative finish. Even if this does not happen, it is quite likely that moulds etc will grow and cause an unsightly appearance.

The spores of mould, stain and decay fungi are always present in the atmosphere and will get carried into the end-grain of board edges. Under favourable conditions of moisture, temperature and a susceptible timber these spores will develop to cause an infection which at best will be disfiguring and at worst will result in decay of the wood.

Many types of wood-based board rely on adhesives to keep them together and give them their mechanical and physical properties. Some of these adhesives are designed to withstand exposure to the weather and will not deteriorate under prolonged exposure. Others, although perfectly adequate for normal interior use, will decompose slowly when exposed to adverse conditions for long periods. This failure will result in delamination of plywood, blockboard, etc and disintegration of particleboard. Even where such products use the so-called ‘weather and boil-proof’ (WBP) adhesives in their manufacture, repeated wetting and drying of the board edges can eventually give rise to splitting, cracking and delamination.

Desirable features of an edge sealing treatment

No ideal edge sealing treatment exists. If it did it would probably be:

  • cheap and in plentiful supply
  • easy and safe to apply in a factory or on site by a variety of means
  • odour-free, non-toxic and safe to apply
  • flexible throughout its life and at all temperatures
  • capable of adhering permanently to dry or moist wood of all types
  • able to bridge core gaps and pores of board edges
  • available in a range of colours
  • compatible with all paints, varnishes, glues, etc
  • of indefinite shelf-life and capable of being applied in all weathers
  • rapid drying, adhering tenaciously to board edges but capable of being removed easily and without damage from where it was not wanted
  • when dry, a firm, non-tacky, non-adhesive, damage-resistant seal

Above all, it should be impervious to water.

Many of the products currently available with edge-sealing potential fall short of this ideal in several respects and selection is an exercise in compromise.

Contact: EXOVA BMTRADA
E-mail: timber@exova.com

Tel: 01494 569750
Web: www.exovabmtrada.com/en-gb

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