Wood finishing technologies must take into account the nature and structure of a substrate that, by definition, is a “living material.” The first step in wood finishing is always drying the wood to reduce the water content from 60-70% to 15-20%, a moisture content that is in balance with the atmosphere’s moisture.
Whether it is solid or veneer, wood is an unstable substrate and a “breathing” material; it changes in structure and color over time. Even when it originates from the same tree, its color is not consistent. Any protective coating used on wood must overcome these challenges and protect the wood from aging, which is not an easy task.
For this reason, a “wood finishing system” is not, and can’t be, a single coat of varnish or paint, but must instead be a combination of different chemical systems, each with its peculiar characteristics and properties.
Chemical SystemsNitrocellulose (NC) Lacquers
The NC lacquers were the first to be used in semi-industrial wood finishing. They were developed after the first World War using explosives technology. The nitrocellulose is obtained by nitration of cellulose; a high nitration degree (greater than 14) generates NC for explosives, while a lower nitration degree generates NC for coatings.
NC is easily handled, dissolves quickly in solvents, and returns to a solid state simply by solvent evaporation. It has poor chemical resistance but dries rapidly and applies easily with most kinds of equipment. NC is very brittle, so it must always be plasticized with other polymers, typically alkyd resins. Its ease of use, good performance and good compatibility with wood make NC lacquers still the most used wood finishing chemistry worldwide.
Acid Curing (AC) Lacquers
AC lacquers are two-component systems in which a plasticized urea/formaldehyde polymer is reacted with a strong acid (hydrochloric acid [HCl] or para toluene sulphonic acid [PTSA]) during application. The reacted blend has a limited pot life (a few hours). It produces a hard and durable film with good scratch resistance and is suitable for furniture, chairs and tables. However, due to its formaldehyde content and corrosive acid, this system has largely been abandoned for more modern systems.
Polyurethane (PU) Lacquers and Paints
PU lacquers and paints are two-component systems in which a hydroxyl-functional modified saturated polyester or an acrylic polymer is reacted during application with an aromatic or aliphatic polyisocyanate. The two components are blended just before use, and the reaction occurs quickly, even at room temperature. These systems have a pot life of anywhere from 1 to 24 hours, depending on the polymers used, and the final appearance is ideal for just about any wood item. Thanks to the wide choice of polymers available, PUs are flexible systems in formulation. They are widely used in Europe and Asia for a variety of applications by both industrial operations and craftsmen. The polyisocyanate can contain very low percentages of a potentially toxic monomer (toluene diisocyanate [TDI]), but its content is strictly monitored and limited by producers and by national authorities and legislation.
Unsaturated Polyesters and Acrylics (PE, UV-PE or UV-ACR)
Unsaturated polyesters and acrylics are based on the radical polymerization of unsaturated polyesters and/or acrylics by cobalt-peroxide radicals or by ultraviolet (UV) light. They are high technology systems that offer fast drying and curing and can be applied with industrial equipment to obtain up to 15-20,000 m2 of finished flat panels in eight working hours. As a result, they can be used in a high-production environment. The systems incorporate a reactive solvent and have a solids content near 100%, with a very high build and good scratch hardness. These are all characteristics required for modern industrial furniture finishes.
In some cases, the coatings are formulated with photoinitiators added to the polymer. When the coated film is exposed to high-pressure UV lamps, the photoinitiators generate radicals that start the curing reaction faster than the peroxides. This reaction occurs almost instantly. The polyesters can be blended or replaced by more reactive acrylic resins and monomers, so that a coated film up to 300 microns (12 mils) thick can be cured in a few seconds (a few minutes for opaque systems). The UV lamps typically used for curing are arc lamps that contain mercury (Hg) that passes to a gas phase under the application of a strong electrical potential difference. The metal gas phase emits an intense UV radiation with a main emission peak of 360 nm, which is capable of activating the photoinitiators and generating the radicals necessary for polymerization. Mercury vapor arc lamps are suitable for clear UV cure coatings. The Hg can be doped with gallium to produce an emission peak of 400-420 nm. These radiation levels are able to pass through and cure most opaque films.
Waterborne lacquers have been used for more than a century for wall painting, but only lately have they been applied to wood finishing to reduce or eliminate the solvent emissions from lacquers and paints. Due to its porous structure, wood tends to absorb water more than any other substrate, and this absorption modifies its dimension and structure.
The primary WB lacquers used for wood are acrylic polymers in water emulsion or dispersion. The curing is both physical and chemical; after the water evaporates from the coating, the polymer undergoes a self-crosslinking reaction that improves the coating’s hardness and chemical resistance. The latest development in this technology is to react a hydroxy-functional acrylic with a blocked isocyanate emulsified in water. Incorporating an isocyanate in water is not easy, but the technology is progressing and seems quite promising.
The main challenge with waterborne systems is extracting the water from the coated film. Water has a high boiling point compared to traditional solvents. As a result, evaporation at room temperature occurs slowly and is affected by the humidity of the environment. At a high relative humidity, a waterborne lacquer can never dry. Several methods have been studied to force water evaporation, including ovens with laminar and percussive hot air, alone or combined with microwave ovens, which can extract water from the lowest layers of the coated film. UV curing is often used with WB systems to produce films that are much harder than the best self-crosslinked, air-dried films.
One other challenge facing WB systems is that acrylic polymers are expensive. It can be difficult for wood finishing professionals to understand why a 35% solid waterborne lacquer is often double the price of a PU, NC or PE containing the same or higher amount of solids. However, the increasing cost and shortage of petroleum, along with the need to reduce solvent emissions into the atmosphere, are driving a greater acceptance and appreciation of WB lacquers. These systems are widely used for factory-finished joinery. Over the last 10 years, all of the producers of factory-finished shutters and windows in southern Europe have switched from solvent-based, air drying products to waterborne and have experienced significant benefits in terms of appearance, weather durability and ease of maintenance.
Finishing SystemsThe two main types of finishing systems used on wood are clear/transparent and opaque. The transparent finish is the most common. It allows the natural color and pore structure of the wood to remain visible. Alternately, the finish can be a transparent high-gloss with a thick layer of coating that fills the pores but reveals the natural veins of the wood - a particular advantage with exotic woods such as rosewood, walnut or oak. An opaque system consists of a pigmented finish in a full-tone color that completely conceals the wood substrate.
With transparent systems, a transparent stain is normally applied first to equalize the wood color. An insulating lacquer can also be used to prevent the acids or fat substances in the wood from inhibiting the cure of the overcoated film.
A filling sealer is then applied in one or more coats to fill the pores to the desired degree and to allow the first coats of the finish to be sanded. Sanding is always required in wood finishing because the wood fibers rise to a certain degree when a lacquer is applied to the bare wood, and these fibers stay outside the film after curing. Sanding (by hand or mechanically) cuts all of these raised fibers and levels the surface before the application of the topcoat. Sanding can be carried out only after the last sealer coat has been applied, or after each coat if the intercoat interval is long enough to prevent intercoat adhesion.
For old-style furniture, a patina or stain is applied after sanding to provide an antique look. The last coat, or topcoat, must be smooth and hard and provide long-term protection from use and aging. The topcoat can be matte or glossy and is normally applied in a single coat, always after sanding the sealer.
An opaque system is normally easier to apply than a transparent coating. First, a white or colored primer is applied in one or more coats to completely cover the substrate. For pigmented systems, the substrate can be wood, chipboard, hardboard, medium-density fiberboard (MDF) or high-density fiberboard (HDF), or chipboard laminated with colored paper. Different finishing technologies exist, but the final aim is to prepare a smooth surface (through sanding) for the application of the pigmented matte or glossy topcoat. The color of the topcoat should remain stable over time. The topcoat should also provide a good appearance and long-term protection from use and aging.
All of the transparent and opaque systems, with the exception of stains and patinas, are formulated based on the chemical systems listed in the beginning of this article. Part 2 of this article will discuss how these chemical systems and the formulated finishes are used in actual wood finishing applications.