In this issue of Technical Matters, we will investigate the use of coatings and finishes and what types there are available for use on wood. The global wood coatings market in 2021 was estimated to be worth approximately USD 4.2 billion and the highest growth rate in this market is in the Asia-Pacific region. Steady economic growth factors and improvement in the standards of living have driven the demand for these products in an upwards trajectory.
Types of Coatings
There is a vast array of coatings and wood finishes that are on the market to suit a range of different applications. The main objective of a coating is to protect the timber and also to enhance its aesthetic appeal. The one thing that is important for all of these coatings and finishes to work properly and give longevity in service is the surface preparation of the substrate on which the coating is to be applied. If the timber surface is not properly prepared, then performance will be significantly reduced.
For outdoor timbers there are two main types of exterior coatings that are available; paints and stains. The modern day formulations of these types of coating are known as ‘microporous’ or non-film forming. These products are ‘breathable’, allowing any retained surface moisture to evaporate through them, and as they do not form a film but penetrate the timber substrate prolonging the durability of the coating. They have more flexibility, eroding gradually with time rather than peeling or cracking. This makes maintenance an easier task when the time comes.
Stains tend to be translucent in that they allow the surface appearance of the timber to show through the coating, whereas paints tend to be more opaque in that they conceal the timber surface. For best results it is better to apply coatings in a factory environment by experienced applicators so that the correct film thickness of the coating is applied. For outdoor timbers you can also add penetrating oils to the list. These usually contain some pigment or ultraviolet (UV) inhibitors to minimize weathering. As the name suggests, these penetrate deeper into the wood, while offering good water repellency. It is not advisable to use clear coatings on exterior timber.
For both exterior and interior coating products the growing trend in coating technology is towards the development of water-based coatings and away from those that are solvent-based. Increasing regulations on volatile organic compounds (VOC) is the main driver for moving this development in technology forward. This transition also helps with insurance and leads to a better and safer working environment safeguarding the operators and the end users health. Drying times for water-based coatings are also vastly improved and industrial users, in particular, are benefitting from this as they are able to provide shorter production times, increased capacity and energy savings.
The market for waterborne coatings has increased significantly in recent years and the quality of these products has improved considerably, particularly for interior finishes as higher end furniture manufacturers demand better products for their more environmentally-aware customers and where appearance is very important. Today’s waterborne finishes result in finishes that cannot really be differentiated from those obtained with solvent-based products, ensuring a high level of mechanical wear and scratch resistance built in, meaning that they can be used with confidence in all interior furniture and joinery applications.
Another more recently developed option for coating and finishing wood is the polyurethane range of finishes. These can either be water-based or solvent-based and usually come in a two-pack system. Instead of just drying, polyurethanes cure by a chemical reaction usually in the presence of UV light. These products can be clear or pigmented, gloss or matt. The surface performance can withstand the most demanding situations, such as kitchen environments, with their excellent stain resistance, or bathrooms, with their exceptional moisture resistance.
All of the different types of coatings and finishes described above can be specified or bought in a wide range of different shades and colors. Some coatings companies even have dedicated global color and design teams to create aesthetic wood coatings and finishes for furniture, cabinetry, flooring and building products. There seems to be a preference for less grey and a trend to more color according to one global coatings manufacturer.
The coatings products described above are all really associated with the industrial application of timber products. We have not discussed the wide range of other finishes for smaller individual bespoke timber products. These will include a range of finishing oils, shellac, dyes and waxes together with the traditional skill of French polishing. So whatever timber product needs to be finished there is a wide range of products to choose from.
AHEC has joined forces with three Indian-based coatings companies and will be showing some of their stain finishes on different American hardwoods at the forthcoming INDIAWOOD show in Bangalore. The results and images will be documented and published in a report later this year.