Wood Surface Treatment Materials: Water-Based Varnishes

General Information

Wood material ranks among the natural materials that humanity has benefited from since its existence and whose use continues to increase today. Wood's wide and extensive range of applications stems primarily from its ability to regenerate and its natural aesthetic appearance. Indeed, each wood species possesses its own distinctive aesthetic appearance, color and pattern. This characteristic makes it preferred in many applications. Today, thousands of wood materials with different natural colors, anatomical structures, and physical and mechanical properties are used for various purposes (Sahin and Mantanis 2016). Other advantages in the selection of wood material include ease of processing and low energy requirements, lightness and durability, insulating properties when dry, and possession of suitable mechanical resistance properties for many construction applications. In this respect, it is preferred in building and construction works, furniture and decoration, engineering composite material production, and industrial and manufacturing product development (Bowyer, et al., 2003). Despite the many important properties listed above, wood has certain significant disadvantages that limit its use in many applications. First, wood has an anisotropic structure and exhibits different physical and mechanical properties in different directions (tangential, radial, cross-grain). Additionally, as a hygroscopic material, it can exchange moisture with atmospheric conditions. Because it contains natural nutrients, it can be damaged by organisms such as fungi, insects and bacteria (Bowyer, et al., 2003; Kubler, 1980). Beyond the characteristics briefly explained above, the greatest limitation to its use in furniture and decoration is its deterioration through open-air exposure (abiotic damage) and the loss of its natural aesthetic appearance. Atmospheric or open-air effects, also referred to as "weathering," include solar radiation, UV rays, rain, snow, moisture, dew, wind, sand, clay, temperature, etc. Through these factors, wood material can lose its natural distinctive aesthetic appearance, and surfaces can experience wear, roughness and cracking (Bowyer, et al., 2003; Feist, 1983).

Wood Varnishes

One of the most effective methods of protecting wood material's natural aesthetic texture and color without major color change is through the application of transparent surface treatments. The most preferred method for this purpose is the application of varnishes to wood surfaces. Varnishes are solutions containing at least two elements (solvent and solids) that, after drying and hardening on the surface, typically form a transparent/translucent layer (Kurtoğlu, 2000). The fundamental purpose of varnishing wood material is to protect its aesthetic appearance and color without alteration by creating a transparent protective layer on the surfaces. While many classification methods exist for varnishes, they are most commonly classified as follows (Kurtoğlu, 2000; Sönmez and Budakçı 2004).

• According to drying and hardening types (physical, chemical, or both methods),
• According to effects on applied surfaces (gloss, matte, translucent),

• According to application area (furniture, decoration, watercraft/water structures),
• According to properties of raw materials used (alkyd, urethane, cellulosic),
• According to application sequence (primer, filler, first-final coat).

Proper selection of the protective layer based on the final intended use of the wood material is important. The most commonly used type of varnish is solvent-based, two-component products that harden/dry both physically and chemically (oil-based varnishes). However, during the preparation or application of these types of varnishes, hazardous chemicals are released into the environment. In recent years, due to the growing need for more environmentally friendly varnish use, various varnish and paint products with different formulations have emerged. As a result of these developments, the use of water-based varnish systems in combination with wood material has shown increasing growth (Kurtoğlu, 2000; Sönmez and Budakçı 2004; Ulay and Budakçı, 2015).

2.1. Water-Based Varnishes and General Properties

As briefly defined above, varnishes are generally solutions consisting of two-component systems. Solvents are used as the solvent component. However, the vast majority of these solvents release volatile organic compounds (VOCs) that negatively affect human and environmental health both during application and after application from the wood surface into the atmosphere. In recent years, particularly regarding the reduction of solvent use in varnish systems, considerable research has been conducted and, as a solution, the use of certain polymers in varnish production has increased (Kurtoğlu, 2000; Ulay and Budakçı, 2015). As a result of this research, water-based varnish systems containing very low levels of volatile organic compounds (VOCs) from different synthetic resins such as alkyds, polyesters, acrylics and polyurethanes have become feasible (Kurtoğlu, 2000; Sönmez and Budakçı 2004; Ulay and Budakçı, 2015). Generally, water-based varnishes are produced from synthetic resins such as alkyd, acrylic and polyurethane. Since one of the objectives in using these varnishes is to prevent color change in the wood material, they typically contain no color pigment, though some elements may be present to provide a matte finish (Kurtoğlu, 2000; Sönmez and Budakçı 2004; Ulay and Budakçı, 2015). Figure 1 shows schematically the chemical structure of alkyd and urethane resins and their bonding to wood. As water is removed from a suspension of very small polymer droplets dispersed in water, polymer particles unite with the wood surface to form a film layer. The resins used in preparing these varnishes typically contain functional groups such as hydroxyl (-OH), carboxyl (-COOH) and amide (-CONH) in their long carbon chains (-R). These groups react with free groups (-OH) on wood material surfaces, forming bonds between molecules (Fengel and Wegener, 1984; Sönmez and Budakçı, 2004). Figure 1 shows schematically the chemical structure of alkyd and urethane resins and their bonding to wood.   Water-based varnish systems have shown rapid growth in use in recent years due to intensive research and the ability to formulate resins with varying characteristics. Water-based varnishes rank among the varnish systems that provide protection and gloss closest to wood material's natural color. Because their hardening occurs both physically and chemically (combined), they can form an appropriate layer on surfaces, are easy to repair and maintain, and leave no marks on surfaces. However, the adhesion, hardness and protective layer-forming properties of these varnishes on wood material surfaces, and consequently their resistance to mechanical stress, is lower than that of solvent-based varnishes (Kurtoğlu, 2000; Ulay and Budakçı, 2015). The main reason for this is the low solids content and lower wood-reacting formulation capability. Additionally, the alkaline nature of water-based varnish (pH: 8-10) can lead to unwanted reactions with certain woods (for example, tannins in oak wood) (Ulay and Budakçı, 2015). Table 1 provides a summary of the general properties of water-based varnishes. Table 1. General properties of water-based varnishes

Advantages/Advantages	Disadvantages/Disadvantages
The proportion of volatile organic compounds in chemical formulations is low.	Multiple coats are required to provide effective protection on wood surfaces.
Volatile organic compound emissions to the atmosphere during application are negligible.	Protection factors in external atmospheric conditions are generally lower.
They can be used for softwood and hardwood species.	Because of their alkaline properties, they can react with certain wood components (tannins).
Application and cleaning are easy.	Their degree of adhesion to wood material surfaces is not very high.
They can provide protection without changing the natural color of the wood material.	Drying times are longer compared to chemical drying systems.
They are less expensive than other types of varnish.

 

3. Conclusion and Recommendations

One of the best ways to preserve the natural appearance of wood material is to use synthetic varnish. However, many types of varnishes other than water-based varnishes can cause some color change on wood material surfaces. In this respect, rapid growth has been seen in recent years in the application of water-based varnish systems to wood products. The main reason for this is the improvement in the properties of binder resins that allow formulation with varying characteristics and the absence of airborne toxic chemicals due to formulation. Indeed, other varnishes contain higher levels of volatile organic compounds (VOCs) in their structure and these mix into the air during varnish application and/or use. Although varnish manufacturers add certain chemicals to chemical formulations to keep VOC levels in oil-based synthetic varnishes at limited levels, water-based varnishes contain significantly less VOCs and are therefore much safer.

	Prof. Dr. H. Turgut Şahin Isparta University of Applied Sciences Faculty of Forestry Department of Forest Industry Engineering
	Sevim Yılmaz Forest Industry Engineer Isparta University of Applied Sciences Faculty of Forestry Department of Forest Industry Engineering
	Gamze Özçelik Forest Industry Engineer Isparta University of Applied Sciences Faculty of Forestry Department of Forest Industry Engineering
	İsmail Erbil Forest Industry Engineer Isparta University of Applied Sciences Faculty of Forestry Department of Forest Industry Engineering

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