Discolouration of Timber
Discolouration of wood can occur with both raw and coated timbers
Freshly cut timber can change colour when exposed to light in the presence of oxygen. The quinones and phenolic compounds present in the timber discolour on exposure to light, causing bleaching of dark timbers or yellowing of light timbers. The degree of change will depend on the light intensity and the species of timber.
Article supplied by NZ Mirotone branch
Discolouration – Darkening when Coated
Surface coatings, when applied to the timber, change the colour of the wood considerably, usually darkening the timber. This is due to the coating filling the voids in the timber. Coatings have a higher refractive index than air, thus the darker appearance upon initial coating. The faster the coating dries, the less pronounced the darkening. A slower drying system penetrates the pores and displaces more air pockets; thus a polyurethane coating often shows a darker initial colour than a fast dry lacquer.
Discolouration – After Coating
Discolouration faults after coating with clear systems are largely confined to the solid wood or timber veneer substrates. They fall into two kinds in terms of cause and effect, Reddening and Yellowing – Fading.
Most wood species contain naturally occurring tannins called leucoanthocyanins. These normally absorb light in the ultraviolet region of the spectrum and hence appear colourless. Under acidic conditions the tannins undergo a chemical change (becoming anthocyanins) and appear visible in blue region of the spectrum, developing as irregular reddish or purple patches on the underlying substrate.
Reddening of the wood occurs predominantly with:
- Acid catalysed coatings
- Urea formaldehyde glues
- Some fast curing PVA adhesives
- Other coatings with a high acid value
Not all woods are susceptible and the fault has mainly been encountered with tawa, beech, blackwood, cherry and maple. Other timber species may be susceptible and testing is recommended prior to full-scale production.
Reddening may not appear until several weeks after the manufacture of the wooden componentry. This reddening effect is particularly common on timber veneers where, due to high adhesive coating weight, excess glue can be forced through the grain of the veneer to near the surface of the timber veneer.
Reddening associated with the use of acid-catalysed coatings is generally less intense and more uniform in distribution than adhesive induced reddening.
However, it should be mentioned that of timbers with a natural red tone, the application of a clear coating system would enhance the timbers natural discolouration.
Yellowing – Fading
Clear coatings offer limited protection to wood with regards to yellowing and fading. This is because the coating is transparent and allows ultraviolet light to penetrate through to the wood surface discolouring the timber substrate, causing:
- Fading or bleaching in dark timbers
- Yellowing or darkening in light timbers
It is possible to put barriers in place to either reflect or to absorb sunlight however these only offer temporary protection. Ultraviolet light absorbers work by capturing ultraviolet light sacrificially, gradually becoming less effective. This prolongs the onset of the discolouring process, but eventually as the absorbers have been utilised, the protection to the timbers diminishes.
In general terms resin systems used in clear coatings discolour gradually over a period of time. However certain polymers (resins used in coatings manufacture) yellow when light is not present. This leads to the phenomena of yellowing under an object placed over a coated substrate. This yellowing fades on re-exposure to light.
The discolouration effect induced by light on many items including fabrics, carpets, curtains, furnishing, timbers, etc. is well recognised. The fundamental fact that all articles, subjected to light, change colour and appearance in some way as they age. Generally, the surface closest to the light source is affected more rapidly than that area furthest away.
The degree of colour change on clear coated timber articles varies with the amount of light to which the coating and timber substrate are exposed. Yellowing and fading of timber does occur, always has occurred and is often considered to add to the mellowness of this natural substrate.
Minimising Discolouration - Reddening
- Before undertaking the construction of a timber article, consider carefully that the species of timber chosen is suitable for the intended environment and end use application.
- Select only those veneer boards where the adhesive coating weight used is the minimum required to glue the veneer to the base board.
- Stain the timber substrate with a MIROSTAIN 2616 before finishing. Although this modifies the natural appearance of freshly exposed wood substrates, the resulting colour is more stable.
- Use clear coatings with low acid value. (Contact your nearest Mirotone sales or technical representative for details).
Minimising Discolouration - Yellowing
- Before undertaking the construction of a timber article. Consider carefully that the species of timber is suitable for the intended environment and end use application.
- Apply a very thin wash of MIROSTAIN 2616 Wiping Stain onto the substrate. This puts a barrier of pigment (although very thin) at the interface of the timber and the subsequent coating system. This reduces the penetration of ultra violet light.
- Use a clear coating system formulated with UV absorbers.
- Shade clear coated furniture or panelling from direct sunlight.