Key Factors in Wood Material Surface Treatment Processes

Important Factors in Wood Material Surface Treatments

1. General Information

Wood, with its natural colour, aesthetic appearance that varies according to cutting direction, high mechanical strength combined with ease of processing and compatibility with other materials, makes it a highly valued material sought for many structural and decorative applications (Bowyer et al., 2003). However, wood also has certain disadvantages that limit its use. Because it has an anisotropic structure, its physical and mechanical resistance properties can vary in different directions (radial, tangential, transverse), and it can exchange moisture with atmospheric conditions (hygroscopic). Due to its sensitive natural chemical composition (cellulose, hemicellulose and lignin), it can be damaged by living microorganisms (biotic factors) such as fungi, insects and bacteria that use it as a natural food source (Kurtoğlu, 2000; Sönmez and Budakçı 2004 and 2005). When used without any protection treatment, especially in external conditions, the chemical components in its structure are affected by moisture, heat and light, resulting in photochemical reactions (abiotic factors) that cause deterioration and deformation in its natural structure (Feist, 1983, Fengel and Wegener 1984). The fundamental aim of all methods developed to increase the service life of wood material and its service properties in the final use location can be summarized as creating an environment unsuitable for microorganisms and external atmospheric conditions by making it more stable against abiotic and biotic damage. To increase the success rate of these treatments, good knowledge of wood material and the protection methods to be applied, and their interaction being controllable and compatible is one of the basic requirements (Bozkurt et al., 1993; Şahin et al., 2019).

2. Wood Surface Treatments

Many methods, techniques and approaches have been developed to prevent quality degradation or service loss occurring first on the surface and then throughout the wood material. Depending on developments in the technological field, wood preservation technology has undergone intensive development. The simple application methods previously used for surface coating/dipping have been replaced by more modern protection treatments. Today, wood materials can be impregnated under pressure with specialized programmes, their chemical structure can be modified, or morphological changes can be made through thermal treatments, extending their service life to many years. Wood surface treatments, which are particularly inexpensive and in some cases highly effective, are processes that have been applied since ancient times. However, similar surface treatment substances may not have the same effect or level of protection on different wood materials. This is because the interaction of the surface treatment substance with wood and its bonding potential differs from wood to wood (density, film-forming properties, structural heterogeneity). Additionally, natural defects in wood material such as knottiness, fibre waviness, and presence of reaction wood can cause surface treatment failures. When wood material damaged by abiotic or biotic factors does not uniformly absorb surface treatment substances, there are disadvantages in the protective substance demonstrating uniform effectiveness (Bozkurt et al., 1993; Kurtoğlu, 2000; Sönmez and Budakçı 2004 and 2005). Table 1 below provides a summary of some surface treatments applied for wood protection and their characteristics (Bozkurt et al., 1993; Kurtoğlu, 2000; Sönmez and Budakçı, 2004 and 2005; Şahin et al., 2019). In wood material surface treatments, polymers dispersed in a solvent system (water, alcohol or solvent) bond with the wood surface as the solvent is removed, forming a film layer. Resins commonly used in the preparation of transparent varnish systems typically contain long carbon chains (-R) and functional groups such as hydroxyl (-OH), carboxyl (-COOH) and amide (-CONH2) attached to these chains. These functional groups react with free functional groups (-OH) in the wood structure and form bonds between molecules (Fengel and Wegener, 1984; Kurtoğlu, 2000, Sönmez and Budakçı 2005). Figure 1 shows schematically the chemical structures of alkyd and urethane resins and their bonding to wood (Şahin et al. 2019).

3. Important Considerations in Wood Surface Treatments

Wood material can be treated with surface finishes such as paint, varnish and shellac. However, for a successful treatment to take place, good knowledge of the wood material and the method to be applied is required (Feist, 1983; Kubler, 1980). In addition to protecting the wood material against moisture, temperature, radiation and other atmospheric conditions, the surface treatment substance to be used should ideally reflect its aesthetic beauty. In opaque surface treatments, the paint or colour substance applied to the surface is visible rather than the natural patterned structure of the wood material (Kurtoğlu, 2000; Sönmez and Budakçı 2005). Surface treatment chemicals vary in terms of their characteristics and quality and usage properties. However, most failures in these treatments stem from insufficient knowledge of wood properties and inadequate preparation. The most common factor causing insufficient wood-surface treatment substance bonding is improper moisture control before and after application (Kurtoğlu, 2000). Generally, wood moisture has significant effects during surface treatment substance application. The suitability of moist or excessively dried wood materials for surface treatments is limited. When wood moisture is high, the surface treatment substance does not absorb well and strong adhesion/cohesion interaction cannot form. When it is too dry, the surface treatment substance penetrates excessively, resulting in an insufficient protective layer on the surfaces. Generally, for most surface treatment applications, wood moisture of 12-18% is adequate. Woods containing different chemical compounds in their structure, for example resin-containing woods, can react with surface treatment substances, causing staining, colour irregularities or incompatible surface reactions. Similarly, woods containing tannins can cause irregular discolouration on surfaces when they come into contact with metal. Additionally, wood samples obtained from different cutting directions of the same tree may have different effects with the same surface treatment substance. As a general rule, wood cut in the tangential direction and annual rings visible as broad bands (summerwood) are more prone to roughness and crack formation. The type and degree of surface treatment application should be determined taking into account the conditions the wood material will encounter during use. Especially for external use, the degree of rainfall and presence of sunlight should be considered. Since the source of moisture in external use is rain and dew, in the use of wood material with surface treatment applied; the direction of the building, its height and other protective measures (roof, etc.) are effective. In indoor use, as air moisture increases, condensation on doors and windows can increase the moisture content of the wood material.

3. Conclusion and Recommendations

Wood surface treatments are preferred for many wood preservation purposes due to the ease of application and the fact that the degree of effect is suitable for many uses. However, for successful application of these treatments, the selected method must be compatible with the wood. First and foremost, it is important to determine for what purpose the wood receiving surface treatment will be used (decorative, aesthetic, structural) and where (indoor/outdoor, humid/dry environment). Similarly, the wood must be prepared under suitable conditions for surface treatments (ensuring surface smoothness, removing dirt and stains) and the surfaces should be as uniform as possible. Wood material having a moisture level significantly higher or lower than the required degree causes a reduction in the effectiveness of protective substances after their application. In recent years, efforts to develop environmentally friendly varnishes have intensified. For this purpose, many varnish and paint varieties less harmful to human health have emerged. In particular, the use of water-based varnish systems in wood surface treatments has been increasing. Although the protection levels of these systems are somewhat lower than other solvent-based systems, the ease of preparation and the fact that their formulations are less harmful to human health suggest that demand for these products is expected to increase in the future.

References

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