By Ziniu Yu, BASF
Knots—the circular imperfections in the wood grain—often cause issues in coating appearance due to their high concentration of potent extractives that can degrade a coating system over time. Unfortunately, knots are unavoidable in softwood species, such as pine, which is often used for furniture and cabinetry applications.
Knots occur naturally when branches die and become encapsulated as the tree grows. During the encapsulation process, extractives—the tree’s natural resins—seal off the former branch’s location and provide a biological boundary between this area and the rest of the tree. Wood typically contains up to 4 wt % extractives of total weight of the wood. However, in knots, the extractives are concentrated and can reach as high as 40 wt %. When the wood is coated, the knot area is prone to exudate the extractives and bleed through the coating. Combined with the effect of UV radiation oxidizing some of these components, the typical result is brownish discoloration bleeding through the coated surface at the knot area.1 Figure 1 shows the knots of pinewood, wood extractives, and the resulting knot bleeding effect through the coating.2,3,4
Knot bleeding effects are a problem in the wood coating industry because they can significantly impact the appearance of the coatings, as shown in Figure 1. In addition, bleeding can compromise coating performance, causing a coating to become brittle, crack, or peel.5 The discoloration over the knots is typically not seen immediately but can take months. The time it takes for the discoloration to appear depends on several factors, including the wood species, environment humidity, temperature, and exposure to UV radiation.6,7,8
Knot bleeding should be differentiated from tannin bleeding, which is another very common defect from extractives causing discoloration. Tannin bleeding is typically a problem for hardwoods such as oak, cedar, or merbau, where knots are not typically an issue.9 Figure 2 is an example of coated panels that show tannin bleeding.
Figure 3 shows an overview of different structures of wood extractives. Extractives can be grouped as either hydrophobic extractives, such as terpenes and resin acid, and hydrophilic components, such as tannins. Depending on the wood types and growth conditions, the exact composition of the extractives in the wood can vary.10
There are a few approaches historically used to prevent knot bleeding through a coating. One solution is to use a solvent-based two-part (2K) coating. These coatings typically have high crosslinking densities, which provide good barrier properties to hinder the extractives from migrating to the surface. However, their solvent emissions and high VOC contents are disadvantages. A second option is to use water-based cationic dispersions; however, this technology is not commonly used for industrial wood coatings, because they can require special equipment or additives.11 A third solution is to use UV primers and topcoats. Like 2K coatings, UV coatings have high crosslinking densities, providing good barrier properties, but UV coatings require special curing equipment, which is often cost-prohibitive.
In this article, a new approach—an anionic acrylic dispersion—is introduced.
Continue reading in the March-April digital issue of CoatingsTech.