Wood Surface Treatment Agents: Cellulosic Varnish
1. General Information
Natural resources are one of the materials that human beings have benefited from since the first ages. Cellulose, the basic cell wall elements of plants capable of photosynthesis, from primitive plants to highly organized trees, is also the most abundant natural polymer on earth [1].
The fact that cellulose is so common source which has attracted the attention by researchers and it has been tried to be used in many areas, resulting technological developments.
While it is possible to use without changing its structure (paper, furniture, composite), today, advanced engineering design products (nanocellulose, artificial organ) can also be produced [2-3].
The fact that cellulose has such a wide and widespread use can be given that it is natural and cheap raw material, as well as being obtained from biomass with self-renewal feature.
Although each biomass has cellulose, in the same chemical structure, it enables the production of cellulose raw materials with physically different properties [1].
Nitrocellulose, one of the esterification products of cellulose, is very important compounds used in the surface treatment of wood and other materials. Depending on the degree of
esterification of cellulose, the properties of the product obtained vary [1,4].
2. Wood Material Varnishes
One of the most effective methods used to the aesthetic appearance of wood and other materials is closest to nature and to provide inexpensive and easy protection is top surface
treatments application. For this purpose, transparent surface treatments, namely varnishes, are used [5].
Varnishes, on the other hand, are solutions that form a transparent layer after curing on the applied surfaces and contain at least two elements (solvent and solid matter) [5-6]. The aim of varnish application can be predict as creating a transparent protective layer on material surfaces.
Although there are many approaches in the classification of paints and varnishes, they can be generally classified as follows [5-7].
• According to drying and hardening types (physical, chemical or both methods),
• According to the effects on the applied surfaces (glossy, matte, permeable),
• According to the place of use (furniture, decoration, automotive),
• According to the nature of the raw material (alkyd, urethane, cellulosic)
• According to the application order (primer, filler, first, top coat).
Today, the most used varnish type is the two-element solvent- based varnishes (oil-based varnishes) due to their easy drying properties (physical and chemical drying) [5]. However,
the use of this type of varnish has begun to be questioned day by day due to the toxic chemical compounds (VOCs) they contain.
As a solution proposal, some alternative varnishes have been tried to be developed in recent years by changing the type and amount of solvent used in formulations. As a result
of these studies, water-based varnish systems are among the varnishes that are increasingly used today [5-8]. Table 1 show some varnish types comparatively.
2.1. Cellulose Based Varnishes and General Properties
Cellulosic varnish or lacquer commonly known nitrocellulose became a viable finish since 1920s. It is a esterification products of cellulose. However, it typically made by treating cellulose with a mixture of sulfuric and nitric acids. This impact modification of the hydroxyl groups (–OH) in the cellulose backbone (nitrate ester) to nitro groups (–NO3) which can
be strained to produce a watery resin [1,4].
This resin is then combined with a number of fast drying solvents to produce the finished lacquer. Figure 1 summarizes the conversion of cellulose to nitrocellulose. The properties of the cellulose is important while other cell wall components of hemicellulose, lignin and minerals give inferior to nitrocelluloses.
As seen in Figure 1, each anhydroglucose units of cellulose backbone carry three OH groups, each of which can form a nitrate ester [1]. Thus, nitrocellulose can denote mononitrocellulose, dinitrocellulose, and trinitrocellulose[1,4]. The chemical equation for the formation of the trinitrate is:
Nitrocellulose lacquer was initially marketed as a fast-drying alternative to wood surface treatments such as shellac and varnishes. However, it was also combined with pigment to
produce spray type paints. In these days, it is still widely used as a wood surface varnish and coloring matter due to its affordable price, fast drying and adaptability to the natural
texture and coloring of various wood materials.
Nitrocellulose dissolves readily in organic solvents, which upon evaporation left a colorless, transparent, flexible film. However, nitrocellulose has sensitive structure and flexibility. To improve these properties, the desired level of elasticity and color stability can be achieved by adding chemicals called resins and plasticizers. [6-10].
Therefore, manufacturers vary the amounts and types of these resins and plasticizers to produce nitrocellulose that cure with varying degrees of elasticity, color, and resistance to water and solvents. Generally the more elastic, colorless, and resistant the lacquer is,
the more it costs [3,14].
The properties of nitrocellulose are usually defined more by the way it cures than by which resins or plasticizers are added. The curing occurs when the solvent or thinner, evaporates.
Since the solvent evaporates curing occurs rapidly. Due to the fact that there is no cross-linking in the cured finish so that nitrocellulose lacquers are not very resistant to heat,
solvent, or chemical damage. Some general advantages of lacquer could be briefly given as [6-10, 14];
• It could be applied with spray equipment easily,
• It could be dried quickly,
• It is suitable to apply multicoatings,
• Large variety of thinner/solvent blends could be useful,
• It has very high film clarity,
• It has wide range of possible formulations including in many colors to be used like paints,
• Its cost is comparable to other synthetic similar agents.
One of the main problems with lacquer is the high percentage of thinner required to put the lacquer into solution. Not only does the lacquer thinner cause air pollution, which is leading to lacquer being restricted, but it is also highly flammable and bad for human health. Some disadvantages of these lacquer could be briefly given as [6-10, 14];
• Some toxic, flammable, air-polluting solvents must be used during applications,
• It has limited heat, solvent, acid, and alkali resistance,
• It has limited scratch resistance,
• It has poor film-build due to low solids content. Only about 10 to 20% of the liquid lacquer you apply will remain as a solid film.
There are a wide range of solvents that dissolve lacquer. These solvents evaporate at different rates, so you can control how fast the lacquer will cure by which solvent blend
you choose. No other common finish gives you this control. However, the solvents are combined by manufacturers into three broad categories: evaporates at more slowly, normal
and faster drying than standard lacquer.
3. Conclusions
Nitrocellulose varnish is a type of varnish that has been widely used on wood material surfaces. The most important feature of this varnish is that it dries very quickly. However, in recent years, studies on varnish types that are friendly to the environment and the emission of toxic substanceminimizing have increasingly continued.
In nitrocellulosic varnishes, it can be expected that the interest in this type of varnish will
increase in the future as a result of the development of solvent systems that allow formulation with different properties and the reduction of toxic chemical emissions into the atmosphere by regulating their formulations.
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Prof. Dr. H. Turgut Şahin – Isparta University of Applied Sciences
Faculty of Forestry
Department of Forest Products Engineering
Merve Cambazoğlu – Research Associate
Isparta University of Applied Sciences
Faculty of Forestry
Department of Forest Products Engineering
Uğur Özkan – Research Associate
Isparta University of Applied Sciences
Faculty of Forestry
Department of Forest Products Engineering
