Additionally, the surface of the wood also can be modified to improve its hydrophobicity. While modifying treatments can usually improve the resistance against fungal attack and mechanical properties of wood, most of them do not provide sufficient long-term protection against wood damaging insects.
Therefore, chemical protection from preservatives is still currently regarded as indispensable for wood preservation.
A preservative system refers to any wood preservation treatment that applies wood preservatives chemical substances to protect wood against fungi and insects. These preservatives can be distinguished by their respective solvents such as oil-borne, water-borne, and organic solvent-borne preservatives.
Each of the preservatives has different characteristic and chemical properties. These preservatives have excellent thermal and chemical stability and are highly resistant to leaching as they are insoluble in water. Therefore, oil-borne preservatives are usually only applied on poles, train railways, and other outdoor applications that present no risk of human contact and minimal effects on the environment Roman Water-borne preservatives, which mainly include metallic preservatives that can be solubilized in water, are effective against a wide range of wood-destroying organisms.
Metallic preservative additives are also easily leached through rainfall, and they represent an environmental hazard because they contain heavy metals Mercer and Frostick , As a result, they are used for indoor applications. Usually, these additives are delivered by using a light organic carrier solvent such as white spirit or petroleum-based hydrocarbon.
Additionally, they can be emulsified in solvent-emulsifier mixtures before being dispersed in water Cui and Preston However, the relatively high cost of organic solvents and emulsifiers limits their use, so most of the relevant existing industrial facilities still support the water-based formulations. Therefore, they are not intended for use where the treated commodity comes into contact with the soil Cookson The authors stated that phenolic compounds in filtrates could provide resistance against fungi.
However, their availability and economic feasibility have not promoted their extensive use and hence, more efforts are required to commercialize this type of preservative in future. In the following section, some of the common treatments used to incorporate wood preservatives into wood are outlined. There are many methods to treat wood with wood preservatives.
A pressure treatment uses a combination of vacuum and pressure to force chemical preservatives into the cellular structure of the wood Salamah and Dahlan ; Tripathi and Poonia By using this treatment, uniform preservative retention and deep penetration are achieved, along with long-term performance and a substantial increase in service lifetime of the wood product. Table 1 compares the different pressure processes commonly in use.
Generally, the moisture in the wood is first reduced and the wood is then transferred to a horizontal cylindrical pressure treatment tank.
A vacuum or initial pressure is applied before the cylinder is flooded with respective preservative solutions. Further pressure is then applied to force the solution to diffuse deep into the porous structures of the wood until the required level of preservative retention is achieved. Non-pressure processes are conducted under atmospheric pressure, without the use of artificial pressure. The differences between several non-pressure processes is shown in Table 2.
Various non-pressure processes differ widely in their penetration and retention levels and in the degree of protection provided by each. Most of the non-pressure treatments, particularly those involving surface applications, generally do not produce a good level of protection in comparison with pressure treatments. Nevertheless, these various non-pressure processes do serve a useful purpose when more thorough treatments are impractical, or when little protection is required.
Although many wood preservatives have been developed, only a few of them have been implemented in current commercial treatments. This is because most of the established wood treatment plants use water-based preservative treatments for practical reasons. Hydrophobic biocides, for example, are less preferable to industries due to their relatively high cost, which is caused by the use of an emulsifier or organic solvent. In contrast, hydrophilic biocide also faces the excessive leaching problem.
For these reasons, researchers are currently proposing the utilization of nanotechnology to overcome the shortcomings in the present methods of wood preservation. Nanotechnology has become a popular topic recently.
Figure 4 shows the number of publications based on the Scopus search engine, where the keywords used were nano and wood. Based on the graph, it can be observed that there has been an increasing trend in the number of publications on wood-related nanotechnology. The number of publications increased more than fold from year to from 6 to This shows that this field is getting more attention from researchers and industry. Fig 4. Number of wood-related nanotechnology publications based on Scopus from the year to Nanotechnology shows great potential to be introduced into wood preservation to overcome problems associated with the existing methods of wood preservation.
Such applications can be achieved by using a variety of nanomaterials such as nanosized metal, polymeric nanocarriers, nanotubules, and other nanomaterials. Copper compounds, such as copper II carbonate, can be micronized through mechanical grinding using a commercial grinding mill Zhang and Leach Suitable nanosized material can also be obtained through chemical means with a co-biocide to enhance its efficacy. As is well known, the synthesis methods of metal nanoparticles play a very significant role in determining the physicochemical characteristics of nanosized metals e.
In some cases, Cu-tolerant wood-destroying fungi may not be able to recognize copper nanoparticles. Once nanoparticles enter fungal cell walls through endocytosis or diffusion through the membrane, they form a reactive oxygen species, or have disruptive effects on mitochondria, proteins, and deoxyribonucleic acid DNA within the fungus cell.
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The nanoparticles may also undergo dissolution, and thereby interfere with homeostatic processes within the fungal cell. Moreover, in a study done by Mclntyre and Freeman , nano-micronized copper quaternary formulation significantly outperformed achieving a better rating amine copper quaternary formulations in a five-year field stake test in Finland.
This outcome was further verified by a study by Akhtari and Nicholas The authors found that nano-micronized copper formulation can greatly reduce the weight loss of wood due to termite attack from Wood treated with micronized copper is less corrosive to metal fasteners and is lighter in color Kofoed and Ruddick Additionally, nano-micronized copper systems have proven qualities, such as reduced leaching and reduced selective adsorption of active ingredient, and work effectively in field tests Freeman and Mclntyre For example, Akhtari and Ganjipour investigated and compared the effects of nanosilver, nanocopper, and nanozinc oxide on the resistance of Paulownia Paulownia fortunei wood against white rot fungus Coriolus versicolor.
Wood specimens were impregnated with a ppm aqueous suspension of nanoparticles with particle sizes ranging from 10 to 80 nm. A chemical retention of 0. These studies demonstrated that the nanometals can provide sufficient biological resistance to the treated woods. It may then be indicated that nanometal may have the potential to improve drying conditions and decrease drying stresses in convective kilns. Polymeric nanocarriers also present the industry with interesting properties in improving the impregnation of wood preservatives.
For instance, a polymeric nanocarrier loaded with a hydrophobic active ingredient has excellent colloid dispersity in water Li and Huh Types of polymeric nanocarriers for active ingredient delivery. The hydrophobic core functions as a reservoir for hydrophobic drugs, whereas the hydrophilic shell region stabilizes the hydrophobic core and renders the polymer water-soluble. Although polymeric nanocarriers have been widely applied in the pharmaceutical sector in controlled drug delivery systems, they have received far less attention from researchers in the wood preservation sector as compared to the attention given to nanometal preservatives.
The treated wood was exposed to brown rot Gloeophyllum trabeum and white rot wood decay fungus Trametes versicolor for 55 days.
Unasylva - No. 90 - Importance of wood preservation in tropical countries
The weight losses of wood after 55 days are given in Table 4. Table 4. The most interesting observation was that the active ingredient-containing nanoparticles were found to provide good resistance against fungal attack on treated wood, even at very low levels of active ingredient incorporation 0. This observation indicated that an active ingredient introduced into wood using the nanoparticles had unexpectedly high activity.
Such unique properties have the potential to reduce cost and minimize the side effects of active ingredients on non-target organisms. However, the cause of this observation is still unknown. Different biocides, such as 4,5-dichloron-octylisothiazolone RH and chlorpyrifos, were included in these robustness studies. The method was generally robust where nanoparticles could be prepared from several different polymers, copolymers, and polymer blends.
However, it was found that decreasing the hydrophilicity of polymers decreased the active ingredient release rate, and concurrently increased the size of the nanoparticles. In contrast, the delivery efficiency of nanoparticles on wood decreased with increasing suspension loading and matrix hydrophobicity. Large-sized nanoparticles blocked the pit pores of wood and thus, prevented the further introduction of nanoparticles into the wood.
Undelivered nanoparticles were found to have undergone aggregation. Greater aggregation occurred in the more hydrophobic formulations than in the hydrophilic formulations. In this method, the initiator fragments serve to ionically stabilize the nanoparticles.
The team found that surfactant-free formulations afforded nanoparticles with significantly smaller median particle diameters and more stable aqueous suspensions 6 months than their surfactant-stabilized counterparts few weeks. Although surfactant-free suspensions had significantly broader particle size distributions, the surfactant-free nanoparticles were still delivered more efficiently into the wood than the surfactant-stabilized formulations. The surfactant-free nanoparticles appeared to afford more resistance to decay than the surfactant-stabilized formulations.
This observation was ascribed to faster active ingredient release from the surfactant-free nanoparticles than those stabilized with the surfactant, which would allow the active ingredient to reach threshold levels more rapidly. Nanotubules can be a promising material for use as a carrier for biocides, due to their hollow structure and high contact surface area. However, it appears that no research has been done on the combination of other nanocarriers with biocide in wood preservation.
Carbon nanotubes CNTs are one of the best known nanotubules. Surface functionalization of CNTs may be covalent or non-covalent. Meanwhile, non-covalent functionalization exploits favorable interactions between the hydrophobic domain of an amphiphilic molecule and the CNTs surface. This nanocarrier exhibited excellent intracellular drug delivery properties in cancer cells.
Wood preservation with gold hydroxyapatite system
Another potential carrier is halloysite. Halloysite is a naturally occurring aluminosilicate clay nanotubular material. Aside from its low cost, it is routinely utilized in the sustained release of chemical agents in pharmaceutical areas due to its non-toxicity. The mortar containing the biocide-loaded nanotubes showed reduced water capillary absorption and prolonged resistance to microbiological growth after being exposed to natural contamination in outdoor conditions for two years.
All these nanocarriers exhibited superior performance in drug controlled-release formulation, albeit in the pharmaceutical sector. It is obvious that the application of nanotechnology in wood preservative requires more attention from researchers and industry, if the development of a safer, more efficient, and low-cost biocide delivery system is to become a reality.
Employing nanotechnology in wood modification, especially coating treatment, also can result in next-generation products having hyper-performance and superior service ability. The coating treatment can be done through physical or chemical approach. In physical approach, pre-synthesized nanomaterials can be used directly or added into existing wood coating.
One of the popular coating materials in wood preservations is the development of hydrophobic surfaces on wood. Their study showed there was a reduction in water absorption and volumetric swelling which suggested that nano-ZnO provided substantial water resistance and dimensional stability.
As water droplets cannot adhere to such superhydrophobic surface but easily roll off, it can take away pollutants and thus, exhibits self-cleaning effects. In their study, silicon dioxide nanoparticles were first modified with poly- dimethylsiloxane and heptadecafluoro-1,1,2,2-tetradecyl trimethoxysilane. Furthermore, wood polymers in surface of natural wood will undergo dissociation and cause rapid color changes when exposed to solar radiation.
In a study conducted by Salla and co-authors , maleic anhydride graft polypropylene MAPP based coating was blended with 7. Rubberwood Hevea brasiliensis specimens were treated with the coating and then exposed to UV light to assess its effectiveness for protection of wood against UV degradation. MAPP contains acid anhydrides groups that can associated with wood constituents by reacting with hydroxyl groups of wood. In addition, polypropylene content in MAPP may make wood surfaces hydrophobic.
The photostability test showed dispersion of ZnO nanoparticles in MAPP restricted the color changes and photodegradation of wood polymers. This UV-absorbing material is essential for outdoor applications to increase their effective operation life or durability. Although these nano-based coatings provide exterior protection to wood, maintenance frequency and their appearance need to be take into consideration. The coating also needs to be flexible and has strong adhesion with wood as exterior wood shrinks and swells with moisture changes.
Both short- and long-term effects of nano-based coatings is important for the safety of consumers. These literatures are essential for developing standardized risk assessment methods for wood preservation industries. Furthermore, effect of these nanomaterials on human health will still remain as a critical question with the commercialization of nanotechnology. As such, these nanoparticles may be inhaled or ingested by consumers when they are in contact with the nano-treated wood surfaces. Even though a number of reports have been published lately, the actual exposure level of nanoparticles is difficult to be determined, leading to inconclusive findings.
Moreover, nano-based formulation is not adherently more dangerous than conventional formulation. In a study conducted by Platten III and co-workers , the dermal release of copper and copper particles was examined from the surfaces of lumber pressure-treated with micronized copper. The authors found that the micronized copper azole and copper azole formulations released similar quantities of total copper, resulting in similar exposure levels.
Therefore, more studies are required to provide regulators a scientific foundation for environmental and human health policy regarding the application of nanoparticles in wood products in the future. Wood preservation can be achieved by treating wood with various wood preservatives or by forcing it to undergo thermal or chemical modification. Generally, wood preservatives can provide biological resistance, while wood modification can enhance physical properties of wood by altering the chemical structure of the lignocellulose component.
Suitable wood preservation methods by necessity are adopted based on the end-product applications. Nanotechnology has been observed to have a great potential for wood preservation applications. The use of nanosized metal preservatives allow for deeper penetration and a more homogenous uptake of particles in the wood. In addition, the incorporation of biocides in nanocarriers allows biocides to be safely stored within the interior of nanoparticles, where loss due to factors, such as leaching and the random degradation of biocides, can be avoided.
Therefore, the development of more effective, safe to use, and environmentally friendly preservative formulations are encouraged to safeguard the environment in the long run. Moreover, nano-based wood coating can provide exterior protection such as UV resistance and hydrophobicity for wood. In short, it can be concluded that nanotechnology is likely to have a major impact on the wood protection industry, through the future design of nanomaterials with the necessary unique properties to enhance the performance of wood preservatives, prolonging the wood product service life.
Economic aspects of wood preservation
However, consideration towards the environmental impact and potential health risk of the nanotechnology is essential and crucial to ensure this emerging market is sustainable. Accsys Group Accessed 1 August Akhtari, M. DOI: Especially local coniferous timbers, such as spruce, fir, pine or larch can be used for roof trusses, wood-based construction materials like particle boards or plywood, facades, fences, patio decks, posts and masts, bridges, avalanche barriers, landing stages and platforms, cooling towers and many other applications. Used externally, these timber species of mainly low durability are at risk from wood-destroying fungi and insects.
Dry or wet rot, house long horn and common furniture beetles pose a threat to wood used in indoor construction. Here, modern wood preservation provides a valuable contribution to maintaining the value of timber structures and preserving resources. You can find out here which processes and products are suitable for the long-term protection of wood and in which particular applications. Modern, fixing Wolman products, which have undergone extensive ecological testing, are guaranteed to provide wood with reliable and long-lasting protection. In many buildings, wooden structures carry out supporting functions.
Preventive preservatives offer lasting protection from wood-destroying insects and fungi. The impregnation of constructional timber, boards, laths and other wood-based materials in dipping processes with coloured or colourless Wolman wood preservatives is the classical, tried and tested approach. Other processes, such as coating and spraying are also used in this field. However, in many cases, constructive protection is not enough to guarantee the long-term stability of supporting and reinforcing elements in particular.
In such cases, chemical protection is also required. Chemical wood protection measures make a substantial contribution to the conservation of resources and the sustainable use of local wood types, making it possible to do without the use of tropical timbers. The risk of timbers being attacked by wood-destroying or staining fungi depends on the moisture content of the wood, as these organisms require a minimum moisture level to be able to infest the wood. By applying constructive measures which contribute to reducing the moisture content, the risk of fungal infestation can be reduced.