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Landry, V., Blanchet, P., and Cormier, L. M. (2013). "Water-based and solvent-based stains: Impact on the grain raising in yellow birch," BioRes. 8(2), 1997-2009.

Abstract

Water-based finishes are slowly replacing solvent-based finishes in the wood industry. Wood grain raising is an important issue associated with the use of water-based stains. In this paper, water-based and solvent-based stains were applied on yellow birch veneers and hardwood samples that had been previously sanded. Grain raising phenomena were studied by profilometry and microscopy. This study demonstrated that the appearance of wood surfaces treated with water-based and solvent-based stains is affected by a number of factors, including grain raising, surface preparation quality, and substrate type. Main observations are: 1) the sanding method has an important role in the grain raising generation and finish quality; 2) profilometry experiments revealed that developed interfacial area parameter can provide valuable information, as it captures both grain roughness and small-scale roughness due to raised fiber fragments; 3) differences between sawn lumber and peeled veneer appeared minor, although the lumber exhibited less significant differences between water-based and solvent-based finishing systems; and 4) wood fragments on the wood surface would be difficult to eliminate.


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Water-Based and Solvent-Based Stains: Impact on the Grain Raising in Yellow Birch

Véronic Landry,a,* Pierre Blanchet,a and Lyne M. Cormier b

Water-based finishes are slowly replacing solvent-based finishes in the wood industry. Wood grain raising is an important issue associated with the use of water-based stains. In this paper, water-based and solvent-based stains were applied on yellow birch veneers and hardwood samples that had been previously sanded. Grain raising phenomena were studied by profilometry and microscopy. This study demonstrated that the appearance of wood surfaces treated with water-based and solvent-based stains is affected by a number of factors, including grain raising, surface preparation quality, and substrate type. Main observations are: 1) the sanding method has an important role in the grain raising generation and finish quality; 2) profilometry experiments revealed that developed interfacial area parameter can provide valuable information, as it captures both grain roughness and small-scale roughness due to raised fiber fragments; 3) differences between sawn lumber and peeled veneer appeared minor, although the lumber exhibited less significant differences between water-based and solvent-based finishing systems; and 4) wood fragments on the wood surface would be difficult to eliminate.

Keywords: Grain raising; Water-based stains; Profilometry; Wood surface preparation; Fibre fragments

Contact information: a: FPInnovations, Secondary Manufacturing, 319 rue Franquet, Québec City, Québec, G1P 4R4, Canada; b: FPInnovations, Pulp, Paper & Bioproducts, 570 boul. Saint-Jean, Pointe-Claire, Québec, H9R 3J9, Canada; * Corresponding author: veronic.landry@fpinnovations.ca

INTRODUCTION

Solvent-based finishes have long been the standard practice in a number of industries, including the secondary processing wood sector. Solvents are used to dissolve and disperse resins, pigments, and additives. They serve as a transfer vehicle for final application to the substrate. Organic solvents used in finishing products create issues with respect to the environment and human health, as well as handling and storage (Anon. 2010). As a result, European, American, and Canadian architectural regulations have recently been modified to restrict usage and encourage less hazardous alternatives.

The proposed alternatives entail significant disadvantages, and these restrict their adoption by the wood industry. One major such disadvantage relates to the appearance of water-based coatings, which typically differs from that of solvent-based coatings (Landry and Blanchet 2010). The literature cites several factors accounting for such appearance differences. They can be listed under two different headings. The first category includes factors relating to the wood/coating interface (e.g., wood grain raising), while the second category consists of factors relating to the inherent properties of the finishing products.

The application of water or water-based finishes onto a sanded wood surface is known to raise the grain of the some woods (e.g., cotton wood) more than others (e.g., oaks) (Koehler 1932). Wetting induces the development of lint, thus increasing roughness. Grain raising varies with several factors, including: wood species; solids contents and surface tension of the finishing products; coating thickness; resin solubility and glass transition temperature; minimum film-forming temperature; drying speed and method; surface preparation; resin viscosity; resin application method; and ambient conditions (humidity and temperature) (van Ginkel 2002).

Despite being a common phenomenon, wood grain raising has attracted limited interest from the scientific community. The first published work on the topic (Koehler 1932), concluded that grain raising actually involves individual fibers, groups of fibers, or fiber fragments lifting as a result of wetting and sanding. Individual fibers tend to twist and lift as the wood dries down, with some species being more prone to grain raising than others. Grain raising also seems to relate to fiber damage caused by the sanding process.

Marra’s group (Marra 1943) identified three main causes for grain raising: cell wall collapse, swelling of sanded fibers, and inter-fiber separation. They have also shown that sanding perpendicular to fibers leads to greater grain raising, that grain raising is more pronounced on the pith side of a board than on the bark side, and that the degree of grain raising varies with the roughness of the sandpaper.

In another study, Nakamura and Takachio (1961) compared the effects of various sanding parameters and made similar observations to previous studies. Coarser sandpaper (e.g., 60 to 80 grit) induces greater grain raising. For the finer sandpaper (e.g., 150 to 240 grit), the grain raising varies significantly with different wood species. In addition, they observed that increased sanding pressure generates greater grain raising.

More recently, Evans (2009) also investigated wood grain raising and arrived at the conclusion that grain raising is greater in low-density wood species than in those with high-density wood. He hypothesized that sanding abrasives tear wood cells along their length, leaving behind strips of loosened cell wall material that tends to lift when exposed to moisture, this being particularly true for low-density wood species. Another conclusion from this work was that fiber sections resulting from the sanding operation swell when wetted, but far less than the swelling due to damage to cell walls. He found that sanding may cause micro-structural damage to the wood surface and induce grain raising, that the degree of grain raising is proportional to grit size, and that grain raising can be reduced by choosing sanding parameters appropriate for each wood species. He also concluded that specimens conditioned under higher moisture conditions tend to yield darker wood hues.

Several patents have been issued in relation to preventing grain raising. One of them describes a process using β-eleostearic acid (Rippey and Dike 1939), while another relies on an aluminum salt (Beane and Safta 1996). These two products tend to decrease the water absorption of the wood fibers. Stains based on milk protein have also been investigated (Burwell 1967).

The objectives of this project are to study the impact of water-based finishes on wood grain raising and more precisely to characterize the grain raising phenomenon by microscopy and profilometry. Another objective is to study the difference in grain raising for solid wood panels and veneers.

MATERIALS AND METHODS

Materials

Yellow birch wood was selected for this research since it is a widely used species for furniture and kitchen cabinets in Canada. Two substrate types were used: sliced veneer and solid sawn wood. Table 1 lists the water-based and solvent-based finishing products used in this study and supplied by CanLak (Canada).

Table 1. Finishing Products Selected for this Study (supplied by CanLak)

* Stains pigment volume concentrations were adjusted to obtain the same stain opacity.

Specimen Preparation

The specimens were first conditioned to constant mass at 20 °C and 50% relative humidity (RH) for an equilibrium moisture content (EMC) of 8%. Solid wood was purchased in the wood market and was kiln dry. Sliced wood also was bought in the market, glued with a PVA (type 3) on a HDF substrate, and then conditioned. Visual inspection confirmed that specimens were free of microcracks resulting from drying.

Following preliminary sanding in a wide-belt sander (P100-P120-P150 grit), the veneer specimens were sanded to P180 grit with a Sioux band orbital sander rotating at 12,000 rotations per minute (rpm) with an orbit of 3/32″. The aluminum oxide SiaDrive 1949 sand paper was provided by Sia Abrasifs JJS. The lumber panels were sanded to P150 grit with the orbital sander. This selection of grit for both materials (solid wood and sliced veneer) was made to reflect industrial practices.

The wiping stains were sprayed onto the substrates. After one minute, they were wiped with a cloth. The sealers and lacquers were applied with a Falconi reciprocator by the finishing product supplier. One coat of sealer was applied for the solvent-based system, as opposed to two for the water-based system as prescribed by the coating manufacturer. Samples were also prepared with the stains only in order to study the impact of their solvent (water or organic) on the roughness.

Specimen Analysis and Characterization

Overall surface appearance

Specimen surfaces (stained-only specimens) were imaged by optical microscopy in dark-field illumination mode. The microscope used was a Zeiss Axio Imager Z1 model.

The effect of the water-based stain on surface roughness was measured with an Altisurf 520 profilometer from Altimet. This is a non-contact profilometer using confocal chromatic aberration sensors. 3D surface profiles were measured on one specimen in every set of 10. 2D line profiles were measured on all samples but will not be reported here as the trends were similar to the 3D measurements. The measured area was 20 mm x 20 mm with a step size of 5 µm/point. Roughness parameters defined in ISO Standard 25178 (Table 2) were calculated from the height distributions on sub-areas 5 mm x 5 mm in size and averaged. The profiles were corrected for slope and shape (large-scale waving) by fitting a 2nd order polynomial before the roughness parameters were calculated. See ISO 25178 for parameters calculation details.

Table 2. Surface roughness parameters described in ISO Standard 25178

The Developed Interfacial Area Ratio, Sdr, is less well known than other parameters. It is defined as the percentage of surface area contributed by the roughness of the wood as compared to a perfectly flat surface the same size of the measured area. This is a useful parameter to differentiate wood surfaces that have similar roughness amplitude but have different sharpness of their peak to trough transitions. Figure 1 illustrates the schematic conditions of two surfaces with near-identical average roughness, one of them showing double the developed interfacial area ratio of the other due to sharper transitions across peaks and troughs.

Fig. 1. Comparison of roughness parameters for two hypothetical surfaces

Wood surface morphology was assessed by scanning electron microscopy (SEM). Samples were metalized with a thin gold layer (10-15 nm). Images were recorded at 30 kV. SEM observations were performed on a JEOL 6360.

RESULTS AND DISCUSSION

General Appearance of Stained Surfaces

The appearance of stained-only specimens is shown in Fig. 2. The solid birch specimens treated with water-based and solvent-based stains are shown in Fig. 2a and 2b, respectively, while Figs. 2c and 2d display the veneer specimens treated with water-based and solvent-based stains, respectively. At first sight, few differences could be observed across the different specimens. Stain hues were similar, and greater pigment concentrations were noticeable in vessel sections. All surfaces also displayed arc-shaped marks due to orbital sanding.