Abstract
The purpose of this study was to determine how resin, a side compound of wood, and resin cleaning methods affect the adhesion strength of water-based varnishes. For this purpose, scots pine (Pinus sylvestris L.), black pine (Pinus nigra subsp.), larch (Larix decidua Mill.), and spruce (Picea abies (L.) H. Karst.), woods with different amounts of resin in their anatomical structure were examined. Physical and chemical resin cleaning procedures were applied to the samples using acetone, cellulosic (lacquer) thinner, sodium hydroxide (NaOH), sodium hydroxide + hydrogen peroxide (NaOH + H2O2), and soft soap chemicals. Later, single-component and double-component water-based varnishes were applied to these sample surfaces. The samples were then subjected to a hot and cold-check test in accordance with the principles set forth in ASTM D 1211 (1997). In the examples, the changes in adhesion strength were examined according to TS EN ISO 4624. According to the results, resin cleaning chemicals and methods reduce the adhesion strength of water-based varnishes.
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Effect of Resin Cleaning Process on Adhesion Strength of Water-Based Varnishes
Mehmet Budakçı,a,* Emre Saygin,a and Süleyman Şenol b
The purpose of this study was to determine how resin, a side compound of wood, and resin cleaning methods affect the adhesion strength of water-based varnishes. For this purpose, scots pine (Pinus sylvestris L.), black pine (Pinus nigra subsp.), larch (Larix decidua Mill.), and spruce (Picea abies (L.) H. Karst.), woods with different amounts of resin in their anatomical structure were examined. Physical and chemical resin cleaning procedures were applied to the samples using acetone, cellulosic (lacquer) thinner, sodium hydroxide (NaOH), sodium hydroxide + hydrogen peroxide (NaOH + H2O2), and soft soap chemicals. Later, single-component and double-component water-based varnishes were applied to these sample surfaces. The samples were then subjected to a hot and cold-check test in accordance with the principles set forth in ASTM D 1211 (1997). In the examples, the changes in adhesion strength were examined according to TS EN ISO 4624. According to the results, resin cleaning chemicals and methods reduce the adhesion strength of water-based varnishes.
Keywords: Resin; Resin cleaning methods; Water-Based Varnishes; Cold-Check test; Adhesion strength
Contact information: a: Department of Wood Products Industrial Engineering, Faculty of Technology, Düzce University, 81060, Düzce, Turkey; b: Department of Design, Kütahya Fine Arts Vocational School, Dumlupinar University, Kütahya, Turkey; * Corresponding author: mehmetbudakci@duzce.edu.tr
INTRODUCTION
Wood is a natural continuously renewable material. Although there are many alternatives for wood, over the centuries it has never lost its importance due to its superior properties (Kaygin and Akgun 2008; Priadi and Hiziroglu 2013; Kesik and Akyıldız 2015). There are many side compounds in wood material with organic structure depending on the chemical structure. Some of these are starches, oils, tanning materials, phenolic and dyed materials, etheric oils, and resins. Wood resin is a solid or semi-fluid, thermoplastic organic material. It usually exists in coniferous trees and occurs in the middle lamella between the parenchyma cells or as a result of any injury in the wood material (Rowell et al. 2005).
The effect of resin on the surface treatment is greater than that of the other side compounds. As the resin in the cell wall reduces the internal surface area and plugs the pit membrane, it has negative effects in the top surface processes. In coloring processes, the paint solution cannot penetrate the depths of the wood material as the passages are clogged. Therefore, the color remains light in the resin-intensive areas. If the internal surface area of the wood material is filled with resin, then surface adhesion to the outer layer of wood becomes weak due to the decrease of the mechanical adhesion in the applied varnish layer. The disadvantage is most noticeable after the top surface processing is completed. The resin in the wood structure tends to ooze to the surface due to its thermoplastic structure because of the temperature effect in the environment. Often, it pierces the protective layer, thereby causing weakening and damage to the varnish/paint layer. In this way, the resin accumulated in the form of bud-like pitch particles is separated from the surface together with the varnish/paint layer in various forms and thus, a convenient way for water and moisture transition in the layer is opened. In order to minimize such defects, it is necessary to perform resin cleaning before the top surface treatment (Sönmez 2005).
Resin cleaning does not protect the wood. After resin cleaning, the surfaces must be covered with a protective layer such as varnish/paint in order to protect the wood material against external influences (Budakçı and Sönmez 2010; Budakçı et al. 2012; Budakçı and Taşçıoğlu 2013; Demirci et al. 2013; Kesik and Akyıldız 2015). Protective layers (paint/ varnish) have limited strength to resist external effects, and their continued integrity depends on the type and severity of the exposure. Factors such as humidity and sudden temperature changes, which cause rapid deterioration in the protective layers, lead to such problems as brightness, color change, cracking, and exfoliation. This especially reduces the life span of furniture and decoration elements made of wood materials and it affects the aesthetic value negatively (Budakçı et al. 2010).
One of the most complex parameters determining the long-term durability performance of the wood’s protective layers, such as varnish and paint, is adhesion (Williams et al. 1987; Williams et al. 1990; Awaja et al. 2009). Adhesion and cohesion must be balanced for the protective layer to have a long life. This balance can deteriorate during the production phase due to errors made by producers in the formulation of the protective layer. In the layers unnecessarily thickened by the users, as a result of excessive cohesion, the surface tension coefficient increases. This, in turn, leads to cracking in the layers and reduces adhesion (Corcoran 1972; Nelson 1995; Sönmez and Budakçı 2004; Budakçı 2006; Kúdela and Liptáková 2006; Lee et al. 2006).
Solvents are generally preferred in paint and varnish production. Their use has been reduced by many European countries due to their damage to the environment and human health. The current legislation has accelerated the use of water-based polymers in paint and varnish production (Wicks et al. 2007; Chen et al. 2011; Pan et al. 2011; Ma et al. 2014; Dai et al. 2015; Saygin and Budakçı 2017). However, until now, experimental studies have not tested the relationship between resin and the performance of water-based varnish on wood.
The effects of cellulosic, polyurethane, and water-based varnishes applied to the wood material moisture on the surface adhesion strength have been investigated. The highest adhesion strength value has been found in the polyurethane varnish applied to oak wood with 8% moisture content (Sönmez et al. 2009). Some cellulosic, polyurethane, acrylic, and water-based varnishes have been examined to different wood species at various layer thicknesses to determine the effect on adhesion strength. The effect of wood species and varnish type is significant, but the effect of the layer thickness is insignificant on the surface adhesion strength of different varnish layers applied to wood material surfaces (Budakçı and Sönmez 2010). Water-based varnishes prepared for wood surfaces have a lower adhesion strength than solvent-based polyurethane and acrylic varnishes. Water-based varnish undergoes a visible color change, especially on oak surfaces. Alkali-based water-based varnish may interact with the tannin substance in the oak wood, causing a single-step chemical coloring, and this may be the source of the problem (Budakçı 2006). In the study by Çakıcıer (2007), single-component and double-component water-based varnishes coated with different layer thicknesses were subjected to an accelerated aging process using the xenon-arc lamp for yellow pine (Pinus sylvestris L.), Iroko (Chlorophora excelsa), and Anatolian chestnut (Castanea sativa Mill.), and the performance characteristics of the varnish layer were determined. In the experiments performed, adhesion to the surface and hardness values increased. In the Gezer (2009) study, water-based varnish was applied to yellow pine (Pinus sylvestris L.), eastern beech (Fagus orientalis Lipsky), and chestnut (Castanea sativa Mill.) subjected to heat treatment at different temperatures to see the effect of the heat treatment on varnish hardness, brightness, and adhesion to the surface. In all wood types, the double-component varnish was superior to the single-component varnish (Gezer 2009).
The aim of this study was to determine the effect of the resin in scots pine (Pinus sylvestris L.), black pine (Pinus nigra subsp.), larch (Larix decidua Mill.), and spruce (Picea abies (L.) H. Karst.) on the adhesion strength of the water-based varnish layers. The effect of the resin cleaning process was investigated.
EXPERIMENTAL
Wood Materials
Scots pine (Pinus sylvestris L.), black pine (Pinus nigra subsp.), larch (Larix decidua Mill.), and spruce (Picea abies (L.) H. Karst), all widely used in the furniture and decoration industry in Turkey, were preferred while preparing the samples. The samples in 12% moisture content were cut out from the wood parts of randomly selected first class knotless, crack-free wood material that exhibited smooth fiber without color and density difference. The wood samples had smooth and fresh annual rings in the sizes of 320 × 110 × 14 mm (TS 2470 1976). The samples were stored in a climate cabinet at 20 ± 2 °C temperature and 65 ± 3% relative humidity until constant weight was reached, and then adjusted to the net size of 310 × 100 × 10 mm (TS 2471 1976). After the machine operations, the wetting and sanding operations were carried out according to the finishing principles. The sanding process was performed in the calibrating sanding machine, first using 80 grit followed by 100 grit sandpaper (Fig. 1).
Fig. 1. Sample preparation
Resin Cleaning Process
Sodium hydroxide (NaOH), hydrogen peroxide (H2O2), and soft soap were preferred as chemically effective (saponifying) cleaners, while acetone and cellulosic (lacquer) thinner were preferred as physically effective cleaners in the resin cleaning process.
Table 1. Mixing Ratios of Resin Cleaning Chemicals
The chemical solutions used in the resin cleaning process were prepared at the manufacturer’s concentration for acetone and cellulosic thinner and at the concentration of 18% by weight (Mg) or by volume (V mL) for the others (Table 1). For those in solid state,
(1)
where Mg is the amount of chemical substance (g), Mç is the amount of solution intended to be prepared, %M/M is the percentage by weight of the intended solution, and %S is the impurity ratio % of the chemical substance. For liquids,
(2)
where Vml is theamount of chemical substance (mL), Vç is theamount of solution intended to be prepared, %V/V is the percentage by volume of the intended solution, and d is the solution density (Demir 1991).
The prepared solutions were applied to the specimens, the dusts of which were cleaned with a sponge, as 100 ± 10 mL/m2 first parallel to the fibers, then perpendicular to the fibers, and again parallel to the fibers. While applying the NaOH + H2O2 solution, the solution-forming elements were applied separately, and the second solution was applied after 2 min to increase the effect of the first applied substance. After the resin cleaning, the samples were left at room temperature for 2 days to increase the effect depth of the chemicals, and then the samples were neutralized with distilled water (Fig. 2). After this procedure, the samples were stored again in the climate chamber at 20 ± 2 °C and 65 ± 3% relative humidity until reaching the constant weight.
Fig. 2. Resin cleaning procedure
Varnish
After resin cleaning, the sample surfaces were varnished using Aquacoll brand FX 6150 coating (acrylic), FX 7680 single-component (acrylic aliphatic polyurethane), and FX 980 double-component (aliphatic polyurethane) water-based bright varnishes (Dual Paint Varnish Ind. Trade Co. Ltd., Istanbul, Turkey). The solid material ratio and manufacturer’s recommendations were decisive in determining the amount of varnish to be applied to the surface. Some properties of varnishes are given in Table 2.
Table 2. Properties of Varnishes Used in Experiments
The samples were varnished in accordance with ASTM D 3023 (2011) and the manufacturer’s recommendations. The varnish was made with a spray gun as FX 7680 single-component and FX 980 double-component varnish application on FX 6150 primer varnish. After the application of the primer coating, the surfaces were sanded slightly with 220 grit sandpaper using a sanding pad on a smooth surface, and the final coating of the varnish application was performed after the dusts were cleaned. The time interval between the coatings was 24 h. The amount of varnish applied was determined by weighing with 0.01 g precision analytical balance (Fig. 3).