Cellulosic Varnishes

Wood Material Surface Treatment Substances: Cellulosic Varnishes  

1. General Information

Natural resources rank among the materials that mankind has utilized since ancient times. Cellulose, the basic cell wall material of plants with photosynthetic capability ranging from primitive annual plants to highly organized trees, is also the most abundant natural polymer on Earth [1]. The widespread availability of cellulose has attracted the attention of researchers and, depending on technological developments, it has been utilized in many fields. Cellulose has been utilized without changing its structure (paper, furniture, composites), and today it is possible to produce advanced engineering design products (nanocellulose, artificial organs, cellulose derivatives) [2-3]. The extensive and widespread application areas of cellulose can be attributed not only to its being a natural and inexpensive raw material, but also to its derivation from biomass with self-renewable properties. Although each biomass contains cellulose of the same chemical composition, it enables the production of cellulose raw material with different physical properties [1]. Nitrocellulose, one of the esterification products of cellulose, is one of the important compounds used in surface treatment of wood and other materials. The properties of the product obtained depend on the degree of esterification of cellulose [1,4].

2. Varnishes Used in Wood Material Surface Treatment

Wood and other material surface treatment applications are among the most effective methods used to keep the aesthetic appearance as close as possible to natural appearance, and to provide inexpensive and easy protection. For this purpose, transparent surface treatments, namely varnishes and polishes, are utilized. Varnishes are defined as solutions containing at least two elements (solvent and solid matter) that form a transparent/translucent layer after hardening on the surfaces to which they are applied [5,6]. For this reason, varnish application can be summarized as creating a transparent protective layer on material surfaces. Paints and varnishes used in wood material surface treatment can be classified in the most general way as follows [5-7]: • According to drying and hardening types (physical, chemical, or both), • According to their effects on the surfaces to which they are applied (glossy, matte, translucent), • According to their area of use (furniture, decoration, automotive), • According to the characteristics of the raw material from which they are obtained (alkyd, urethane, cellulosic) • According to the application sequence (primer, filler, topcoat). The most widely used varnish type today is two-component solvent-based varnishes (oil-based varnishes) due to their ease of application (physical and chemical drying) [5]. However, the use of this type of varnish has increasingly come under scrutiny due to the toxic chemical compounds it contains (VOCs). As a proposed solution, in recent years alternative varnishes have been developed by changing the solvent and resin used in varnish formulations. Water-based varnish systems are products that have come to be used in increasingly high volumes as a result of these efforts [5-8]. Table 1 shows a comparative presentation of some varnish types used in wood surface treatment. Table 2 shows cellulosic varnish types produced for different purposes and their application areas.  

2.1. Cellulose-Based Varnishes and General Properties

Cellulosic varnishes are generally known as nitrocellulose and have been used since the 1920s. Generally, nitrocellulose is produced through esterification reactions of cellulose in sulfuric and nitric acid environments. In this reaction type, the free hydroxyl groups (-OH) in the cellulose structure react with nitro groups (–NO3), forming cellulose nitrate ester in an aqueous resin solution [1,4]. This resin is treated with certain solvents that have rapid drying properties to obtain nitrocellulose, or cellulosic varnish. Figure 1 shows the esterification reaction from cellulose to nitrocellulose [1]. As seen in Figure 1, each anhydroglucose unit in the cellulose structure carries three OH groups capable of forming an esterification reaction with a nitro group. Depending on the degree to which these -OH groups react with the nitro group, nitrocellulose can be named as mononitrocellulose, dinitrocellulose, and trinitrocellulose [1,4]. In this transformation (esterification), the presence of other cell wall chemicals in the cellulose structure (lignin, hemicellulose, minerals) has a reducing effect on the quality of the resulting varnish.     Nitrocellulose varnish was initially marketed in wood surface treatment as a fast-drying alternative and was also formulated with pigments as spray paint. Today, due to its reasonable price, rapid drying, and compatibility with the natural texture and coloring of various wood materials, it is still widely used as a wood surface varnish and paint substance [3,14]. Nitrocellulose is colorless, transparent, leaves a flexible film, and is easily soluble in organic solvents, but it has weak structure and flexibility. To improve these properties, chemicals called resins and plasticizers can be added to provide the desired level of elasticity and color stability. Additionally, resistance to water and chemicals can be provided, and fast-drying nitrocellulose of different types can be produced [6-10]. However, in general, the more elastic, colorless, and durable nitrocellulose-based varnish is, the higher its cost becomes [3,14]. The properties of nitrocellulose are generally defined by its drying method rather than which resins or plasticizers are added. Complete hardening occurs when the solvent and varnish thinner evaporate. Since the solvent evaporates rapidly, drying occurs quickly. However, due to the absence of cross-linking on the cured surface, nitrocellulose varnishes are not very resistant to heat, solvents, and chemical damage [14]. Since the evaporation rates of thinners or solvents used in the preparation of nitrocellulose varnishes can differ, the hardening time of the varnish can be controlled depending on the selected/prepared formulation. For this reason, the control of nitrocellulose varnish hardening/curing is easier compared to other varnish and paint systems. The general advantages of nitrocellulosic varnishes can be summarized as follows [6-10, 14]; • Can be easily applied in spray form, • Dries quickly, • Suitable for multiple coat application, • Can be used with different thinner/solvent mixtures, • Has very high film transparency, • Can be formulated with many colored pigments and used as paint, • Its cost is competitive with other varnish types. One of the fundamental problems related to nitrocellulose varnish is the quality of the solvent and thinners used in the varnish formulation. The solvents used cause toxic emissions (VOC), creating adverse conditions for human and environmental health, and also have easy flammability/flash properties. Some important disadvantages of nitrocellulose varnish are [6-10, 14]; • The use of some toxic, flammable, air-polluting solvents is necessary in applications, • It has limited heat, chemical, water, acid, and alkali resistance, • It has limited scratch resistance, • It has weak film structure due to low solid content, • Only 10-20% of the applied liquid varnish remains on the surface as a solid film.  

3. Conclusion and Recommendations

Nitrocellulose varnish is a varnish type with many application areas and is widely used on wood material surfaces. Its most important property is that it dries very quickly. However, in recent years, research has increasingly continued on varnish types that are environmentally compatible and have minimized toxic substance emissions. With the development of solvent systems that allow nitrocellulosic varnishes to be formulated with different properties, and by adjusting their formulations to reduce toxic chemical emissions entering the atmosphere, increased interest in this varnish type can be expected in the future.       References 1. Fengel, D. &Wegener, G. (1984).Wood, Chemistry, Ultrastructure, Reactions, Walter de Gruyter Public. Berlin, Germany. 2. Bowyer J.L, Shmulsky, R &Haygreen J.G. (2003).Forest Product sand Wood Science-An Introduction. Fourthedition, Iowa State University, Ames, IA, 553p. 3. Williams, R. S. (1999).Finishing of wood. Woodhandbook: wood as an engineering material. Madison, WI: USDA FPL; GTR-113: Pp.15.1-15.37. 4. Sjostrom, E. (1993). Wood chemistry: fundamental sand applications. Gulf Professional publishing. 5. ŞAHİN H. T., Yılmaz S., Özçelik G., Erbil İ. (2019). Ahşap Malzeme Yüzey İşlem Maddeleri: Su Bazlı Vernikler. Turkcoat, Ekim-Kasım 2019, 2-7. 6. Kurtoğlu, A. (2000). Ağaç Malzeme Yüzey İşlemleri, I. Cilt, Genel Bilgiler, İÜ Orman Fakültesi Yayınları, Yayın no: 463, İstanbul. 7. Sönmez, A.& Budakçı M. (2004). Ağaç işlerinde Üst Yüzey İşlemleri II. Koruyucu Katman ve Vernik Sistemleri, Gazi Üniversitesi Teknik Eğitim Fakültesi Yayını, Ankara. 8. Ulay, G. & Budakçı, M. (2015). Ahşap Yüzeylerde Kullanılan Su Bazlı Vernikler İle Türkiye'de Yapılan Çalışmalar, Düzce Üniversitesi Bilim ve Teknoloji Dergisi, 470-480. 9. Aykaç, S. & Sofuoğlu, S. D. (2020). Selülozik, sentetik, poliüretan ve su bazlı vernik uygulanmış bambu malzemesinde yüzey pürüzlülüğü parametrelerinin karşılaştırılması üzerine bir çalışma. Mobilya ve Ahşap Malzeme Araştırmaları Dergisi, 3(2), 84-92. 10. Kaygin, B. &Akgun, E. (2008).Comparison of conventional varnishes with nanolacke UV varnish with respect to hardness and adhesion durability. International Journal of Molecular Sciences, 9(4), 476-485. 11. URL 1. http://dekra-mobilya.blogspot.com/2008/01/st-yzey-ilemleri.html (Accessed: 26.01.2022). 12. URL 2.https://teknikrehber.com/, (Accessed: 26.01.2022) 13. Kubler, H. (1980).Wood as Buildingand Hobby Material, John Wiley&SonsInc, NY, 270p. 14. Flexner, B. (2021). Understanding wood finishing: How to select and apply the right finish. Fox Chapel Publishing.     Prof. Dr. H. Turgut Şahin - Isparta Uygulamalı Bilimler Üniversitesi Faculty of Forestry Department of Forest Industry Engineering   Merve Cambazoğlu - Research Assistant Isparta Uygulamalı Bilimler Üniversitesi Faculty of Forestry Department of Forest Industry Engineering   Uğur Özkan - Research Assistant Isparta Uygulamalı Bilimler Üniversitesi Faculty of Forestry Department of Forest Industry Engineering