Wood Surface Treatment Agents: Waterborne Varnishes

 

1. Introduction

Wood surface is one of the natural materials that humans have benefited from since ancient times. One of the most important reasons for this is its ability to renew and serve as a sustainable material. Because each wood species has its own aesthetic appearance, color, pattern and anatomical structure, these characteristics are primary reasons for its preference in many useful applications. Therefore, thousands of wood species have been used for many purposes throughout the world (Sahin and Mantanis 2016). However, there are a number of reasons to choose wood as a material for many uses such as: being a light and durable material with easy processing and low energy requirements, dry wood exhibiting insulating properties with suitable resistance and mechanical properties for many construction works, and so on. Some examples of its uses include construction and housing works, furniture and decorations, energy and industrial products (Bowyer, et al. 2003). Despite the many advantages mentioned above, wood also has some important disadvantages that have a limiting effect on many applications. Since wood is an anisotropic material that shows different physical properties in three directions (transverse, tangential, radial), and because it is hygroscopic in nature, it can lose or gain moisture from the atmosphere. In addition, due to their natural nutritious ingredients, they can be destroyed by organisms such as fungi, insects and bacteria (Bowyer, et al. 2003; Kubler, 1980). In addition to the properties described briefly above, one of the most important disadvantages of wood use in furniture and decoration works is that it can be easily degraded by atmospheric conditions (weathering) and its natural aesthetic appearance may be impaired. Weathering effects include solar radiation (UV rays), humidity (rain, snow, moisture, dew) and mechanical effects (wind, sand, clay). As a result of these degradations, wear, roughness and cracking may occur on surfaces which may lose their natural appearance (Bowyer, et al. 2003; Feist, 1983).

2. Wood Varnishes

One of the most effective ways to protect the natural texture of wood material is by applying transparent surface treatments. This can be done through the application of varnishes to wood surfaces. After drying and hardening, a transparent layer can be formed. Varnishes are transparent surface treatment agents that typically consist of at least two elements (solvent and solid matter) as in solutions (Kurtoglu, 2000). The primary aim of applying varnish to a wood surface is to preserve the natural aesthetic appearance of color by creating a protective layer on the wood surface. Varnishes can be classified in many ways (Kurtoğlu, 2000; Sönmez and Budakçı 2004).

Most generally:

• According to drying and curing conditions (physically, chemically, combination of both hardenings), • According to the effect on surfaces (glossy, matte, transparent), • Depending on the place of use (furniture, decorative, water vessels), • According to the characteristics of the solid polymeric materials (alkyd, urethane, cellulose), • The application order (pre-layer, topcoat, filler). It is important to make the right choice in forming a protective layer in accordance with the end use of the wood material. Generally, the most commonly used varnish types are two-component solvent-based (oily varnishes) that harden through a combination of both physical and chemical processes. However, as a result of volatile organic compounds (VOC) released into the atmosphere during preparation or use, there is a need to utilize more environmentally friendly varnishes. Hence, many varnish types with different formulations have emerged in recent years. As a result of these research efforts, water-based (waterborne) varnish systems have become increasingly common in recent years (Kurtoğlu, 2000; Sönmez and Budakçı 2004; Ulay and Budakçı, 2015).

2.1. Waterborne Varnishes and General Properties

As described briefly above, varnishes are solutions consisting of at least two different components (solvents and solid matter). The solvents are usually used as the dissolving medium in formulations. However, solvents can release some volatile organic compounds (VOCs) into the atmosphere during or after application from the wood surface. These adversely affect human and environmental health. In recent years, many studies have been carried out especially on the reduction of solvent use in varnish systems with the aim of lowering VOC emissions to the atmosphere (Kurtoğlu, 2000; Ulay and Budakçı, 2015). As a result of these studies, water-based varnish systems (waterborne) containing very low levels of volatile organic compounds have been prepared from various polymers (Kurtoğlu, 2000; Sönmez and Budakçı 2004; Ulay and Budakçı, 2015). In this regard, waterborne varnishes are generally produced from resins such as alkyd, polyester, acrylic and polyurethanes. Transparent waterborne varnishes do not contain color pigments but in dull types, some elements are added to ensure matting (Kurtoğlu, 2000; Sönmez and Budakçı 2004; Ulay and Budakçı, 2015). After the removal of water (hardening) from the suspension in which small polymeric substances are dispersed, the polymeric particles combine with the wood surface to form a thin transparent film. The resins used in the preparation of these varnishes contain functional groups such as hydroxyl (-OH), carboxyl (-COOH) and amide (-CONH2) with long carbon chains (-R). These groups are connected between molecules by reacting at free functional groups (-OH) on the wood surface (Fengel and Wegener, 1984; Sönmez and Budakçı, 2004).

Figure 1 schematically shows the chemical structure and bonding of alkyd and urethane resins with the wood surface.

After a number of research studies conducted on waterborne varnish systems, the use of such varnishes on wood materials has increased in recent years. Waterborne varnishes are among the varnish systems that provide protection and transparency closest to the natural color of wood. Since they harden both physically and chemically, they can form a durable layer on surfaces with easy application, repair and mark-free appearance. Waterborne varnishes usually create fewer resistant layers on wood surfaces than other solvent-based varnishes (Kurtoğlu, 2000; Ulay and Budakçı, 2015). Due to moderate adhesion to wood surfaces, appropriate hardness and the ability to form a thick and protective layer is not easy. This is due to its low solid content with special chemical properties that adversely affect the reaction. The pH level of waterborne varnish is usually in the alkaline range (pH: 8-10), which can cause discoloration on some wood surfaces (i.e. tannin in oak) (Ulay and Budakçı, 2015). Table 1 shows the general properties of water-based varnishes.

Advantages	Disadvantages
The proportion of volatile organic compounds in chemical formulations is low.	Multiple layers must be formed to provide effective protection on wooden surfaces.
During their use, there is almost no emission of volatile organic compounds (VOCs) into the atmosphere.	In outdoor atmospheric conditions, protection factors are generally lower.
They can be used for softwood and hardwood species.	As it is alkaline, it can react with some wood compounds (tannin).
They are easy to apply and clean.	The degree of bonding to wood surfaces is moderate.
They can provide protection without changing the natural color of the wood.	Hardening/drying times are longer than chemical drying systems.
They are cheaper than other varnish types.

 

3. Conclusion and Recommendations

One of the best ways to maintain the natural appearance of wood is to use synthetic varnish. However, many varnish types other than waterborne can produce more or less color change on the wood surface. In this respect, there has been a rapid increase in the application of waterborne varnish systems to wood products in recent years. This is mainly due to the development of binder resin properties, which allow formulation with different characteristics, and the avoidance of toxic chemical emissions into the air due to its formulation. Because in the structure of other varnishes, volatile organic compounds (VOC) are generally present and VOCs mix with air during the application and/or use of the varnish. Although varnish manufacturers use some chemicals in their formulations to keep VOC levels limited, water-based varnishes contain much less VOCs than oil-based ones and are therefore much safer.

Prof. Dr. H. Turgut Şahin - Isparta Applied Sciences University - Forest Faculty - Department of Forest Industry Engineering

Sevim Yılmaz - Forest Industry Engineer - Isparta Applied Sciences University - Forest Faculty - Department of Forest Industry Engineering

Gamze Özçelik - Forest Industry Engineer - Isparta Applied Sciences University - Forest Faculty - Department of Forest Industry Engineering

İsmail Erbil - Forest Industry Engineer - Isparta Applied Sciences University - Forest Faculty - Department of Forest Industry Engineering

References 1- Bowyer J.L, Shmulsky, R&Haygreen J.G. (2003). Forest Products and Wood Science-An Introduction. Fourth edition, Iowa State University, Ames, IA, 553p. 2- Budakçı, M. & Sönmez, A. (2010). Bazı Ahşap Verniklerin Farklı Ağaç Malzeme Yüzeylerindeki Yapışma Direncinin Belirlenmesi, Gazi Üniversitesi Mühendislik Mimarlık Fakültesi Dergisi, 25 (1). 3- Feist, W.C. (1983). Weathering and Protection of Wood, AmericanWood- Preservers' Association, In: Proceedings, Seventy-ninths Annual Meeting of AWPA, Kansas City, MO, Stevensville, pp.195- 205. 4- Fengel, D. & Wegener, G. (1984). Wood, Chemistry, Ultrastructure, Reactions, Walter de Gruyter Public. Berlin, Germany. 5- Kubler, H. (1980). Wood as Building and Hobby Material, John Wiley&Sons Inc, NY, 270p. 6- Kurtoğlu, A. (2000). Ağaç Malzeme Yüzey İşlemleri, I. Cilt, Genel Bilgiler, İÜ Orman Fakültesi Yayınları, Yayın no: 463, İstanbul. 7- Özçiftçi, A., Atar M. & Uysal, B. (1997). Ağaç Malzemede Renk Açmada Kullanılan Kimyasalların Yüzey Parlaklığına ve Verniklerin Yapışma Mukavemetine Olan Etkileri, Türk Tarım ve Ormancılık Dergisi, 23 (3): 763-770. 8- Sahin, H.T. (2016). Environmentally Friendly Alternative Pulp and Paper Technologies, Chapter 11, In: Developments in Science and Engineering, R. Efe, L.Matchavariani, A. Yaldır, L. Lévai (Editors), ST. Kliment Ohridski University Press, Sofia, ss.125-145. 9- Sahin, H.T. (2018). Chemical Properties and Utilization of Tree Barks, Chapter 8, In: Academician Publisher Scientific Researches Book, Educational Science, Oğuz Kutlu (Editor), Akademisyen Kitabevi, Ankara, ss.91-107. 10- Sahin, H. T. & Mantanis, G. I. (2016). Colour Changes Of Pin And Fir Wood Treated With Several Titanium and Zinc-Oxide Based Nanocompounds. Advences in Forestry Letters, 5, 17-23. 11-Söğütlü, C. & Sönmez, A. (2006). Değişik Koruyucular ile İşlem Görmüş Bazı Yerli Ağaçlarda UV Işınlarının Renk Değiştirici Etkisi. Gazi Üniversitesi Mühendislik Mimarlık Fakültesi Dergisi, 21(1): 1, 151-159. 12- Söğütlü, C., Nzokou, P., Koc, İ., Tutgun,R. & Dongel,N. (2016). The Effects Of Surface Roughness On Varnish Adhesion Strength Of Wood Materials, J. Coat. Technol. Res., 13(5): 863–870. 13- Sönmez, A. (1989). Ağaçtan Yapılmış Mobilya Üst Yüzeylerinde Kullanılan Verniklerin Önemli Mekanik Fiziksel ve Kimyasal Etkilere Karşı Dayanıklılıkları, Doktora Tezi, Gazi Üniversitesi Fen Bilimleri Enstitüsü, Ankara. 14- Sönmez, A., Budakçı, M., Atar, M. (2002). Karaağaç Odununda Renk Açma İşleminin Vernik Katmanına Etkisi, Teknoloji Dergisi, 1-2: 35-42. 15- Sönmez, A. & Budakçı M. (2004). Ağaç işlerinde Üst Yüzey İşlemleri II. Koruyucu Katman ve Vernik Sistemleri, Gazi Üniversitesi Teknik Eğitim Fakültesi Yayını, Ankara. 16- Sönmez, A. (2005). Ağaç İşlerinde Üst yüzey İşlemleri I, Hazırlık ve Renklendirme, Gazi Üniversitesi Teknik Eğitim Fakültesi Yayını, Ankara. 17- Şener, N. (2006). Mobilyada Kullanılan Ahşap Malzeme, Yüzey, Üst Yüzey İşlemler ve Koruma, Yüksek Lisans Tezi, Mimar Sinan Güzel Sanatlar Üniversitesi Fen Bilimleri Enstitüsü, İstanbul. 18- Ulay, G. & Budakçı, M. (2015). Ahşap Yüzeylerde Kullanılan Su Bazlı Vernikler ile Türkiye'de Yapılan Çalışmalar, Düzce Üniversitesi Bilim ve Teknoloji Dergisi, 470-480. 19- Yakın, M. (2001). Su Bazlı Verniklerde Sertlik, Parlaklık ve Yüzeye Yapışma Mukavemetinin Tespiti, Yüksek Lisans Tezi, Gazi Üniversitesi Fen Bilimleri Enstitüsü, Ankara. 770.