Revista Mexicana de Ciencias Forestales Vol. 15 (84)

Julio - Agosto (2024)

Fecha de recepción/Reception date: 14 de septiembre de 2023.

Fecha de aceptación/Acceptance date: 23 de mayo de 2024.

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¹Facultad de Ciencias Forestales, Universidad Autónoma de Nuevo León. México.

*Autor para correspondencia; correo-e: wibke.himmelsbach@uanl.edu.mx

*Corresponding author; e-mail: wibke.himmelsbach@uanl.edu.mx

Abstract

Wood is a material widely used in construction, furniture, and other applications. Technologies are used to protect its quality and durability against biological damage and the effects of water, temperature and radiation that affect its physical and mechanical properties. The present work reviews the available treatments, evaluates their advantages and disadvantages, and defines the criteria for their use. The theme was divided into two sections: (I) Wood degrading agents, and (II) A classification of protection technologies that included both the traditional methods and novel approaches such as nanotechnology. The conclusions obtained with this approach point to the fact that several traditional chemical treatments substantially reduce biological damage and moisture absorption in wood. However, potential health and environmental effects should be considered. On the other hand, the dimensional stability of the wood is improved through the use of heat treatments. The use of nanometric composites for wood protection is a very promising technique that is under increasing development. However, it is a technology that requires special care because the nanomaterials must be toxic to the agents causing biodeterioration, but harmless or less hazardous to humans and the environment.

Key words: Wood degradation, durability, nanotechnology, wood protection, chemical treatments, heat treatments.

Resumen

La madera es un material ampliamente usado en construcción, muebles, entre otras aplicaciones. Para mantener su calidad y durabilidad se emplean tecnologías de protección contra daños biológicos y los efectos del agua, la temperatura y la radiación que afectan sus propiedades físicas y mecánicas. En el presente trabajo se realizó una revisión de los tratamientos disponibles y se evaluaron sus ventajas e inconvenientes, además se definieron los criterios para su utilización. La temática se dividió en dos secciones: (I) los agentes degradadores de la madera, y (II) una clasificación de tecnologías de protección que incluyó tanto métodos tradicionales, como enfoques novedosos, tal es el caso de la nanotecnología. Bajo este enfoque, las conclusiones obtenidas apuntan a que diversos tratamientos químicos tradicionales reducen, sustancialmente, el daño biológico y la absorción de humedad en la madera. No obstante, deben considerarse los posibles efectos a la salud y al ambiente. Por otra parte, si se emplean tratamientos térmicos, la estabilidad dimensional de la madera mejora. El uso de compuestos nanométricos para la protección de la madera es una técnica muy prometedora y en creciente desarrollo. Sin embargo, es una tecnología de especial cuidado porque los nanomateriales tienen que ser tóxicos para los agentes causantes del biodeterioro, pero inocuos o menos peligrosos para los humanos y el ambiente.

Palabras clave: Degradación de la madera, durabilidad, nanotecnología, protección de la madera, tratamientos químicos, tratamientos térmicos.

Introduction

According to FAO (2022), the annual global industrial production of wood in 2020 was approximately 3.9 billion cubic meters destined for fuel or roundwood, 473 million cubic meters were produced as sawn timber, and 367 million cubic meters, as materials derived from the wood. The demand for these products, mainly roundwood, is expected to reach 6 billion cubic meters by 2050 (Barua et al., 2014).

Wood protection is crucial in the global timber market, which faces the challenge of preserving it against biodegradation and exposure to water. This challenge can be solved with the help of protection technologies (Chen et al., 2020). As a response, methods have been developed to treat the wood using different protection strategies to improve its resistance to biodegradation.

In this regard, the quality, protection and resistance to degradation of wood products depends on such factors as humidity, temperature, wood density, and protection treatments (Gérardin, 2016).

UNE-EN 350 (Asociación Española de Normalización, 2017) classifies wood into four categories according to its ease of treatment: Class I, easy to treat; Class II, moderately easy to treat; Class III, difficult to treat; and Class IV, extremely difficult to treat. This standard establishes methods for assessing and classifying the durability of wood, understood as its ability to resist decay and decomposition against fungi, termites, and marine organisms (Reinprecht, 2016), and it is applicable even to treated or modified wood. The use of such a standard is crucial to provide a recognized standardized framework allowing comparison, as well as adding credibility by supporting the information with accepted standards and offering practical guidance through specific criteria and tests. In addition, its reference is essential to determine the commercial quality of the wood.

In this sense, in recent years, several works have been carried out to show affordable and environmentally friendly alternatives for the protection of wood. Some of these protection treatments are well-known and widely used in the industry, such as the traditional ones described by Peraza (2002), while others are more novel and involve nanotechnological techniques such as those described by Teng et al. (2018) and Jasmani et al. (2020).

The objective of this paper is to provide a general review of the main techniques currently available for wood protection and to evaluate their advantages and disadvantages. First, wood degrading agents will be discussed, followed by a classification of protection technologies ranging from traditional methods to modern and novel approaches such as nanotechnology.

Wood degrading agents

Table 1 presents a classification of the most important agents affecting the durability of wood, their effects and the most recent reference research proposing different protective treatments. In general, a distinction is made between biotic and abiotic agents. The former refer to living organisms, and the latter, to physical and chemical components of the environment.

Table 1. Factors influencing wood degradation, damages, and proposed protective treatments.

The main factors of wood degradation, understood as damage to the wood structure that can be initiated at both higher and molecular levels, include abiotic or atmospheric agents such as moisture, UV rays, and temperature, together with the presence of xylophagous organisms (Reinprecht, 2016). Exposure to sunlight and thermal decay affect the adhesion of coatings and the appearance of wood structures, leading to early replacement (McKinley et al., 2019).

On the other hand, moisture variations in the environment favor the growth of microorganisms that damage wood and affect its quality and properties, especially outdoors and in contact with the ground (Marais et al., 2022).

The main wood degrading agents are fungi, which cause different types of rot, such as white rot, brown rot, soft rot, mold rot, and blue stain, the latter having only an aesthetic effect on the wood. Fungi, with the exception of those causing blue stain, damage the structure of the wood, reducing its strength and visual appeal (Broda, 2020).

Insects also damage wood, the most economically relevant being termites. Although only a small percentage of these cause damage, their global economic impact in 2010 was estimated at US $40 billion (Rust and Su, 2012). In addition, marine borers, such as crustaceans and bivalves, bore into the wood of ships, destroying it over time (Marais et al., 2022).

Development and Discussion

In order to establish a certain type of wood protection treatment or preservative to be used, several aspects must be considered. These include the type of wood to be preserved (coniferous or broadleaf), the level of risk of deterioration to the particular service environment, the function the wood will serve (structural, ornamental, container, etc.), and the required service life.

On the one hand, there are hardwoods that are resistant and do not require any treatment for protection. On the other hand, there are softwoods with less natural durability. Figure 1 shows a classification of wood protection technologies.

Based on Gérardin (2016), Sandberg et al. (2017), Teng et al. (2018), Papadopoulos et al. (2019), Teacă et al. (2019), Jasmani et al. (2020), and Khademibami and Bobadilha (2022).

Figure 1. Treatments used for wood protection.

Traditional treatments by chemical modification

In chemical modification treatments, as a traditional treatment, the cell wall of the wood is reacted with low molecular weight active monomers or oligomers under certain conditions such as high temperature heating. Chemicals can also be introduced into cell cavities such as lumens and vessels in such a way as to block the physical channels and reduce the access of water into the cell walls of the wood (Xie et al., 2013) .

In this regard, it is worth mentioning that acetylation is a chemical process in which the free hydroxyl groups of the wood cell wall are converted into acetyl groups and all the weight gained by the acetyl is converted into units of occluded hydroxyl groups. This technique can significantly reduce water absorption and improve resistance to fungal and insect attacks (Yang et al., 2022) . However, acetylation presents some disadvantages, such as partial lignin degradation, and deformation and cracking of refractory woods. This, in turn, leads to poor quality and performance (Martins et al., 2019) .

Another method is furfurylation, which consists of impregnating the wood with furfuryl alcohol, obtained by processing furfural, a compound derived from biomass by-products. This technique reduces water absorption and, therefore, fungal attack (Martha et al., 2021) . However, its limitations include the fact that the weight of the catalyst must be small for it to penetrate the pores effectively (Bi et al., 2021).

Furfurylation is suitable for wood species with higher porosity and loose and tidy structures (Dong et al., 2016). Wood treated with furfuryl alcohol has higher hardness and stiffness, good appearance and texture similar to tropical timbers. It can be used for decking boards. Acetylated wood, on the other hand, has greater biological durability and dimensional stability. Therefore, it is good joinery products and for various structural applications Mantanis, 2017); however, the manufacturing costs are higher (Bi et al., 2021).

Among the impregnating materials, resins are a widely used and versatile group in the protection of wood. Their main purpose is to stabilize or reinforce the dimensions of the wood because they polymerize or cross-link easily (Wang et al., 2021) . However, their use may present disadvantages, such as possible degradation due to weathering, difficulty in achieving uniform application, and the potential environmental impact associated with chemical compounds present in certain resins (Stefanowski et al., 2018).

Schardosin et al. (2020) point out that impregnation with kerosene emulsions could be a substitute for acetylation, if the objective is to reduce water absorption. However, they also indicate that particle size influences the penetration of the wax into the wood.

Traditional treatments by thermal modification

Thermal modification of wood began in 1915 in Wisconsin, USA, but did not become known throughout the world until the 1970s and 1980s. The process usually occurs at a temperature of 150 to 240 °C, and its main objective is to improve dimensional stability and microbial resistance (Hill et al., 2021) . However, these treatments have certain drawbacks. First, they significantly affect the fracture toughness of the wood (Khademibami and Bobadilha, 2022) ; in addition, they modify the color of the wood, especially that of tropical woods. The current challenge is to find the balance between improved protection against agents, the loss of resistance, the preservation of the original color, and the improvement of the equipment used in the application of these treatments (Gu et al., 2019) .

Currently, there is a wide variety of processes for the thermal modification of wood. Table 2 lists the main commercial heat treatment processes used in Europe.

Table 2. Treatments used for thermal modification of timbers.

*The initial moisture content of all processes varies between 0 and 30 %. The stages of each process are separated by “/” and depend on the treatments and authors. Modified and expanded from Sandberg et al. (2017).

The choice of the best heat treatment is sometimes complicated, as all processes have some technical or economic limitations or disadvantages. The species and moisture content of the wood, as well as the intensity of the treatment, must be considered when making the selection. Pockrandt et al. (2018) compared different heat treatments of hardwoods and conclude that the TVT process is less destructive than ThermoWood; however, the durability of the wood is not significantly improved with the TVT process. Jebrane et al. (2018) , for their part, cite that for softwoods both processes lead to similar results.

Thermogravimetric analysis has shown that hardwoods such as beech, poplar, ash, and eucalyptus are more susceptible to thermal degradation than softwoods such as pine and spruce (Candelier et al., 2016) . This is due to the hemicellulose content in hardwoods containing highly acetylated functional groups, compared to softwoods (Martínez-Abad et al., 2018) .

Traditional treatments of natural origin

Traditional treatments of natural origin are usually based on water or oily substances. Water-soluble preservatives are mainly used when the preservation of the color of the wood is an important factor, as is the odor of the preservative substance once applied to the wood. These preservatives have the disadvantage that they do not confer dimensional stability, while they may increase the corrosion rate of nails or metal fasteners (Reinprecht, 2016).

Oil-soluble wood preservative methods are a promising alternative as impregnators and binders in paints or in combination with other formulations (Cesprini et al., 2022). They are preservatives that fill the cavities of the wood by capillary action, i. e., they do not chemically bind to the cell walls; therefore, a high retention capacity must be ensured to achieve the desired protection (Woźniak, 2022) .

There are other very effective preservatives such as creosote and PCP (pentachlorophenol); despite not being of natural origin, these were widely used in Europe and North America, but their use has been banned since 2018 due to health and environmental concerns (Khademibami and Bobadilha, 2022) . Likewise, although the current acceptance of copper/chromate/arsenic salts (CCA), acid/copper/chromate (ACC), arsenate/cupric/ammonia (ACA), and arsenate/cupric/zinc/ammonia (ACZA) is limited by environmental concerns, they have played a crucial role in the conservation of timber (Tarmian et al., 2020).

Natural compounds, on the other hand, are renewable and easily obtainable substances with beneficial antimicrobial properties and less ecological impact than traditional chemical products (Broda, 2020) . (Cesprini et al., 2022; Ella et al., 2022) .

The heterogeneity from which the compounds are derived, their lower retention within the wood, their easy leaching, and their high susceptibility to degradation are some of the disadvantages of natural preservatives. Therefore, they are generally costly and not very cost-effective, and their use is limited.

Nanotechnological treatments

The main advantage of nanotechnologies in wood preservation is the high capacity of nanoparticles to penetrate wood structures completely and uniformly, resulting in a product with high physical and mechanical performance (Papadopoulos et al., 2019) . Therefore, they can improve wood bonding and durability, moisture resistance, UV absorption, structural performance, fire protection, and reduce excessive leaching (Jasmani et al., 2020) .

One of the applications of nanotechnology in wood protection is the use of polymeric nanocarriers, which act as a storage and transport medium for fungicides and bactericides to penetrate the wood, with the polymeric matrix controlling the release rate of the fungicides and bactericides (Teng et al., 2018) . However, there are certain limitations such as the need to maintain control of the size and stability of the nanoparticle suspension throughout the process, as well as to improve the surfactant system of the nanoparticles (Bi et al., 2021) .

Potential nanocarriers include carbon nanotubes (CNTs) and halloysite, a natural nanotubular material made of aluminosilicate clay, which is inexpensive and has no toxicity or negative environmental impact (Lisuzzo et al., 2021) .

Furthermore, certain nanometals can be synthesized by chemical methods in their gas and liquid states and used in mixtures with other nanometals or even in traditional heat treatments (Teng et al., 2018). They improve the durability of wood in three ways: first, they interact with bacteria or deactivate the enzymes necessary for degradation reactions; second, they do not recognize fungus in the presence of nanometric metal and therefore prevent its development; and third, they generate reactive oxygen species in fungal cells (Bi et al., 2021).

On the other hand, there are nanoadditives used in coatings to improve the durability of wood. Applied alone or with traditional coatings, they enhance the wood’s mechanical properties, its fire resistance and protect against water and UV damage (Jasmani et al., 2020). They can be applied by brushing, dipping, or in situ polymerization to achieve better adhesion (Bi et al., 2021).

When these materials are used in a coating treatment, the slow and controlled release of the active ingredient is important due to its long-lasting effect and minimal environmental impact (Papadopoulos et al., 2019). In this sense, the incorporation of bio-based nanoparticles could significantly improve the performance of existing compounds in traditional markets and promote the development of new types of biocomposites and markets (Pacheco-Torgal et al., 2019).

Conclusions

The choice of the best wood protection treatment depends on the weighing of ecological, economic, and protection aspects.

To improve the dimensional stability of wood against moisture absorption, acetylation is suggested over furfurylation because it is more environmentally friendly, as it utilizes less aggressive compounds, and potentially more cost-effective in the long term. Both methods improve the effectiveness of the protection; acetylation stands out for its resistance to humidity and decomposition, and furfurylation for its improved stability and resistance, although the latter depends on the quality of the products and the precision of the process.

In the choice of heat treatment for wood, it is suggested to consider several options depending on the specific properties of the project. To minimize environmental impact, FWD, PLATO and ThermoWood all offer environmentally friendly alternatives, as they largely avoid the use of aggressive chemicals. If economic efficiency is crucial, ThermoWood could be a cost-effective selection in the long term, while FWD and Le Bois Perdure involve more substantial upfront investments. In terms of protection, all heat treatments provide significant improvements in wood strength and durability. However, it is important to consider that heat treatment leads to the deterioration of certain mechanical properties.

Natural, mostly environmentally friendly treatments, offer effective protection against insects, fungi, and decay. The economic feasibility depends on the specific substance and its availability; however, the possibility of reducing long-term maintenance costs should be considered. Therefore, it is recommended to use these treatments together with other methods.

If the application of advanced treatments for wood protection is desired, the cautious use of nanotechnology with a balanced approach is suggested. It is crucial to assess the sustainability of nanomaterials, minimizing environmental impacts. Despite the initial investment, nanotechnology promises long-term protection and potential cost savings. Its adoption requires addressing environmental considerations and must align with specific objectives to allow profiting from its innovative potential in a gradual and conscious manner.

The importance of adopting environmentally friendly practices is recognized; therefore, it is recommended to focus research on the generation of solutions for the protection of wood that minimize the ecological impact and prioritize treatments of natural origin. The treatment choice must meet the durability and resistance standards indicated in international quality standards. Finally, ways must be sought to optimize costs without compromising the environmental integrity or human health, as well as the effectiveness in protecting the environment.

Acknowledgments

The authors are grateful to Conahcyt for the doctoral scholarship granted and to the UANL Foundation for the scholarship provided for an international stay. In addition, they thank the anonymous reviewers for their constructive comments, which have played a key role in improving this literature review.

Conflict of interests

The authors declare that they have no conflicts of interest.

Contribution by author

Víctor Daniel Núñez-Retana: development of the idea and the manuscript; Marco Aurelio González-Tagle, Humberto González-Rodríguez and María Inés Yáñez-Díaz: revision of the manuscript; Wibke Himmelsbach: structuring and revision of the manuscript.

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