Hygroscopic characteristics of wood products play an important role in their effective utilisation. Although there are many methods to enhance stability and appearance of wood, finishing is still considered as the most popular one among the others.
Coating the surface of wood with different types of finishes will not only improve its appearance but also extend its service life. Final quality of coating applied to the surface of wood is influenced by various parameters; namely, species, roughness, porosity, density, and interaction between coating and the substrate. Wood, being a nonhomogeneous material, sapwood, and heartwood ratio will also affect overall quality of coating.
Adhesive strength of coating on the surface can be evaluated with several techniques such as tape peel, observing the crosscut, and pull-off test. The last one was effectively employed to evaluate adhesion strength of wood and wood products coated with different finishes.
In general it is expected that rougher surface of the substrate results in better bonding ability of peak and valley points of the surface. As it is well known, surface roughness of the substrate will also influence the amount of finishing used controlling overall production cost. Therefore, subjective numerical information on the surface quality of the wood would provide valuable information so that not only amount of finishing chemical can be controlled but also final product can be manufactured with a better quality.
Finding The Right Grind
A total of 240 defect-free, 60 for each species were prepared from beech, alder, spruce and fir. Samples with dimensions of 400 mm by 100 mm by 200 mm were conditioned in a climate room having a relative humidity of 65 percent and a temperature of 20 degC until they reach equilibrium moisture content of 12 percent.
Conditioned specimens were sanded with 80-grit and 180-grit sandpaper using a commercial drum type sander. Although there are various roughness measurement techniques including pneumatic, laser, and light scattering methods to evaluate surface quality of wood and wood products, stylus type of profilometer is the most commonly used one due to its practicality and providing numerical result with an accuracy. Therefore, aprofilometer was employed to measure roughness of the samples.
A total of 25 random measurements with a span of 15 mm were taken from the surface of each radial and tangential sample across the grain orientation. Mean peak-to-valley height which is well accepted roughness parameter was used as an indicator of the surface quality of the samples.
The specimens were coated with cellulosic varnish with 35 percent solid content having viscosity of 300s. Coating was applied to the samples in two sequential steps, namely initial coating and the final coating. After the initial coating, samples were dried in room temperature and sanded with sandpaper having 220-grit sandpaper. In the next step, the final coating was applied and dried specimens were also sanded using aluminium oxide sandpaper with 400-grit sandpaper.
An adhesion pull-off type tester was employed for adhesion strength evaluation of the specimens. Twenty random measurements were taken from the surface of the samples by gluing steel head with 20 mm diameter using epoxy resin on the samples. The equipment was run at a constant speed of 100 mm/min and applied the force to the surface layer by pulling the coating from the surface.
From each sample, 20 mm by 200 mm by 30 mm specimens were cut to measure their density. These small samples were weighed and their dimensions were measured.
Finding The Strength
The highest value was found for alder samples sanded with 80-grit sandpaper in tangential direction followed by fir specimens sanded with the same grit size. Once these samples were sanded with 180-grit sandpaper, their average values reduced ranging from 10.5 percent to 25.5 percent.
Spruce and fir species had smoother surface quality than those of beech and alder when they were sanded with higher grit size sandpaper. This could be related to low density of both softwood species. Based on the analysis, no significant difference was found between values determined from the tangential and radial surfaces for all four types of species.
The highest adhesion strength value was determined for beech samples sanded with 80-grit size sandpaper in tangential direction. It is a well-known fact that anatomical structure is one of the major parameters influencing overall interaction between coating and substrate. Both alder and beech being semi-porous structure would result in better absorption of the varnish causing higher magnitude of interaction between the two elements.
No significant difference was found between adhesion strength values of two softwood species based on tests. Even though they were sanded with finer sand paper, their strength values did not go down in contrast to alder and beech. It seems that varnish had deeper penetration due to more porous structure than that of spruce and fir samples
Based on the findings, it seems that adhesion strength of the samples improved when they were sanded with higher-grit sandpaper with increasing their surface roughness characteristics evaluated using a stylus type of equipment.
Tangential and radial grain orientations of the specimens did not make any significant differences in their roughness and adhesion strength values. Anatomical structure of the species would be considered one of the factors influencing roughness as well as development of bonding between coating and the samples. Softwood species having lower density levels resulted in smoother surface when they were sanded with rougher sandpaper.