The Impact of Wood Choice and Preparation on Coating Performance
As consumer preferences and trends change, the types of wood used to manufacture industrial wood products such as kitchen cabinets, furniture, and flooring change to reflect the current desires and expectations. The dynamic nature of the industrial wood market—combined with the wide range of wood types and different finishes required for different applications—makes formulating industrial wood coatings challenging. The need for proper preparation of wood surfaces can make achieving excellent coating performance even more difficult. The key for producers of wood products is to work closely with their coating supplier to identify the best solutions for their individual product needs.
The species of wood used in industrial wood products typically follow consumer trends and the type of market, according to James Monroe, market segment manager for Furniture and Flooring with BASF. He notes that a few years ago, dark colors were favored by designers for commercial spaces, so walnut (Juglans nigra) was popular. Lower cost wood species such as yellow poplar (Liriodendron tulipifera) were also used in combination with a dark stain. “Today, however, light colors and a natural appearance are favored for applications where a ‘wood look’ is desired,” Monroe says. “We are therefore seeing increasing use of maple (Acer saccarum), and oak (Quercus rubra & Quercus alba) is experiencing a come-back,” he adds.
The application definitely dictates the choice of wood. For high-end furniture, such as office or hotel furniture, high-value timbers are used. However, for high-volume, commoditized segments, flat stock such as medium density fiberboard (MDF) and lower-value processed timbers such as acacia or recycled timber pallets are used as basestock, according to Michael Law, global business director for Waterborne Technologies with allnex. Reclaimed or repurposed wood is trending along with a natural or weathered appearance, adds Monroe. “Painted kitchens and furniture account for approximately 50% share of the coatings used in this application. In these cases, it is common to coat directly over the MDF or other wood composite materials to lower costs,” he explains.
In the home-furnishing market, Sherwin-Williams is seeing a resurgence of walnut in high-end pieces, while mindi, pine, oak, and acacia are popular choices for more affordable furnishings, according to Nick Bartoszek, global marketing director for New Product Development for Sherwin-Williams’ Industrial Wood Coatings. Mindi wood in particular has grown in popularity recently. In the kitchen cabinet market, Bartoszek notes that maple, alder, oak, and cherry remain popular, largely due to predominately domestic U.S. manufacturing and the proximity to the wood source. “Maple has been the best-selling wood for cabinetry for several years,” he adds.
The choice of wood species is an important factor when selecting coatings for industrial wood products because the quality of wood varies from one species to another. While many of today’s industrial wood coatings are formulated to accommodate most wood species, there are certain species that may require special sealers or additional steps in finishing, according to Monroe. Ipe (Handroanthus spp.), for example, is used in wood flooring and may require a special sealer or primer to obtain proper adhesion for following coats. “Lower-value processed timbers often have a lot of knots and higher levels of tannin. For painted products, knots must be sealed and tannin and resin exudation barriers are needed to block their penetration through the coating,” says Law. The wood type can also affect the coating cure profile. Traditionally, two-component solvent-based polyurethanes have been used as block sealers. The industry is transitioning to waterborne coatings, however, which presents additional challenges given that tannin is water soluble, according to Law. “The key is to understand the characteristics of the wood and to establish appropriate finishing schedules,” asserts Bartoszek.
While the grain raising effect was initially found to be a problem, some people today that employ waterborne systems have learned to use this effect to their advantage, leveraging this behavior to promote adhesion.
Surface preparation is key for a quality finish regardless of the species of wood or the type of finish desired. For a quality finishing job, the wood surface must be clean and properly sanded prior to finishing, according to Monroe. “While advances in coating technologies may permit less or different preparation requirements, for a coating to achieve its potential, the surface must be properly prepared. As in most coating applications, hand creams, pneumatic lubricants, and other contaminants can cause finishing defects. Having a clean environment along with proper sanding of the surface allows achievement of a quality finish,” he says. Not all preparations are the same, however. Different methods are needed when finishing with solventborne and waterborne systems, for example, mainly due to the fact that water will raise wood grain. “While the grain raising effect was initially found to be a problem, some people today that employ waterborne systems have learned to use this effect to their advantage, leveraging this behavior to promote adhesion,” Monroe comments. He adds that in general a 120/150/180 or 150/180/220 sanding grit sequence is a good starting point for all finish types. Of course, achieving a high depth of image (DOI) and gloss requires different wood preparation techniques and resin selection criteria than are used when producing a commoditized, pigmented, low-sheen furniture finish, according to Law. High DOI coatings also often require post-cure polishing.
The best approach, according to Bartoszek, is to consider each application individually. “Our team of technical representatives strives to understand the desired result and deliver a finishing system that meets quality, cost, and production requirements,” he explains. Overall, his recommendation to industrial wood products manufacturers is to partner with a quality coatings supplier that is investing in developing new technologies to answer challenges of regulations and improvements in raw materials and that can provide solutions to help improve their businesses.