**3. Wood preservatives**

As aforementioned, one of the major drawbacks of wood is its susceptibility to biological deterioration. Wood can be attacked by decay fungi inadvertently in natural conditions and as such its durability is accordingly reduced, accompanied by some decay hazards and others such as losses amounting to billions of dollars each year. It is the primary reason that wood needs to be treated aiming to prolong its service life. Based on the background, the chemical wood preservative and protection technology have been greatly developed worldwide [36].

#### **3.1 Conventional wood preservatives**

Generally, conventional wood preservatives can commonly be classified into three types, i.e. oil type wood preservatives, oil-borne type wood preservatives and water-borne type wood preservatives. Coal tar and creosote, which are usually known as traditional oil preservatives, are toxic and effective in resistance against wood decay fungi, insects and other microorganisms. Oil-borne preservatives, which dissolve some toxic water-insoluble organic fungicide compounds in organic solvents [37, 38], are also efficient but limited utilized in certain wood products due to their volatile organic compound problems. During the actual production and application process, some traditional preservatives (oil preservatives or oil-borne preservatives) are poisonous to the environment and human health, due to their containing some toxic chemicals [39]. Therefore, water-borne preservatives have been developed and applied instead of traditional preservatives in many areas.

Water soluble preservatives can protect wood efficiently in most environments, which dissolve some active chemical ingredients in an aqueous solution, which is valid in the inhibition of the harmful microorganisms to wood due to its toxicity to fungi, bacteria, insects, and other biological erosion. Besides, water-borne preservatives can also penetrate into wood cell walls due to their good permeability, and effectively protect the entire wood from the erosion of microorganisms, then provide long-lasting protective effects. Wood water-borne preservatives are usually prepared in liquid form, which can be applied to wood and/or wood surfaces through simple methods such as soaking, spraying, or brushing. Compared with other types of preservatives (oil preservatives or oil-borne preservatives), the application of water-borne preservatives is very convenient. Thus, water-borne preservatives attract increasing attention due to their well-treatment feature, lower toxicity, less environmental impact and pollution after use. Collectively, up to date, water-borne preservatives are the most widely applied wood preservative.

However, there also are some disadvantages to water-borne preservatives, including limited fixation and durability due to their high solubility in water, and low

protection compared to some oil preservatives or oil-borne preservatives, especially in extreme environmental conditions, such as high humidity.

#### **3.2 Environmentally benign wood preservatives**

Wood is easy to be deteriorated for ubiquitous organisms, such as fungi, bacteria and insects. For this reason, wood products require chemical treatment rather than soil direct contact to prolong their operating lives. Though water-borne preservatives are widely utilized attributing to their inexpensive cost and good permeability, their high solubility also brings negative effects such as low stability and fixation, even risk to the environment due to the toxic leachable chemicals. Chromated copper arsenate (CCA) has been extensively used to effectively protect wood for nearly 100 years. Arsenic and copper compounds are used as toxic elements to the microorganisms and insects in CCA components, while the chromium salt is applied to fixable agents and prevent them from leaching from the CCA-treated lumber into the environment. However, CCA-treated wood products need careful use and cautious disposal for it is a toxic waste and harmful to humans, animals and the environment due to chromium and arsenate in CCA elements are inaccessible to standard toxicity characteristic leaching profile (TCLP) tests [40]. Since 2004 the U.S. Environmental Protection Agency prohibited CCA for residential purposes due to its hazard during manufacture and treatment. As a consequence, it is urgent to develop and research feasible, effective, environment-friendly and cost-competitive wood preservatives to substitute for traditional preservatives, such as CCA.

Under the above background, environmentally benign wood preservatives are researched and developed by many wood science researchers and wood preservative companies during the past few years. Copper salts, which are poisonous to microorganisms and insects, have been used most frequently in wood preservatives and could react with and/or bind to lignin, tannin, or protein consequently fixed in the wood. Boron salts are the oldest preservatives and are still used as effective fungicides and insecticides nowadays on account of their low toxicity [41, 42]. Due to the preservative active ingredients and low toxicity, copper and boron salts attracted more and more attention [43–45]. However, the leachates (copper and/or boron elements) from the treated products during the long-time application are inevitable for their water solubility [14]. Thus, copper and/or boron-based wood preservatives need to be developed to stabilize and fix the active ingredients (copper and/or boron elements) onto wood structures.

Recently, proteins such as soy isolates, okara protein, and feather protein, have been used to interact with the preservative active ingredients by coagulation, autocondensation, and/or other chemical reactions to increase the durability of the preservative in treated wood [36, 46]. When the copper-boron-protein preservatives impregnate the wood, copper and boron can interact with wood components and be fixed in the wood matrix by gelling of protein via heating or other methods. Mazela et al. (2003) and Thevenon et al. (1998) used proteins and tannin compounds to fix the boric acid during two impregnation stages [47–49]. Sye et al. (2008) prepared a wood preservative by formulating copper and/or borax with organic waste okara to substitute the high-price copper azoles (CuAz) and alkaline copper quaternary (ACQ ) [50]. Yang (2006) studied the feasibility of using soy protein instead of toxic chromium and arsenic to formulate wood preservatives with copper and boron [51]. The aforementioned studies proved that the protein-based wood preservatives could penetrate the wood block and protect the wood products against fungal attack as effectively as traditional preservatives, such as CuAz. In addition, they are environmentally friendly and have been considered as an interest alternative to CCA.

Based on this theory, the authors developed a kind of environmental friendly wood preservative based on chicken feather protein, which was used as the source of protein for its environmental benign character and low cost. The preservative formulations were composed of hydrolyzed chicken feather protein, copper sulfate (CuSO4·5H2O) and sodium borate (Na2B4O7·10H2O). Chicken feather powder was hydrolyzed at 140°C for 4 h, then the protein hydrolysate was obtained. The condensed hydrolyzate was added into the suspension of copper sulfate and sodium borate. Then the wood preservative solution based on feather protein was achieved with the dissolving agent ammonium hydroxide (NH4OH) [36].

The results showed that chicken feather proteins can be successfully used to prepare the protein-based wood preservative, which can penetrate wood structures and are stable against water leaching. The interactions between chicken feather protein-based wood preservatives and wood components were also confirmed. Therefore, chicken feather protein could be used as a source of protein and an efficient chelating agent to prepare low-cost, effective and environmentally benign wood preservatives, and the chicken feather protein-based preservative can effectively protect the wood against decay fungi and prolong the service life of the treated wood blocks, which provides a new source of protein using natural components as potential wood preservatives. For exploring the ground-contact protection of the chicken feather protein-based preservative, field trials with much longer processing time need to be conducted in the future in order to evaluate the long-term effectiveness of this kind of preservative.

Natural materials attract more and more interest as a source of preservatives due to their simple way to obtain, low cost and environmentally friendly characteristics. There are some new preservatives prepared from natural materials, due to their competitive cost, low toxicity and low environmental impact, such as plant-derived wood preservatives. For instance, Tiina Belt investigated the extractives of heartwood of Scots pine, containing extractives, such as pinosylvins, and suggested that pine heartwood extractives have the potential to inhibit the white rot fungi [52]. The ethanol extractives of teak heartwood residues also showed promising antifungal abilities as wood preservatives [6]. Tchinda reported that plant essential oil showed positive antifungal activities, using natural plant extracts to protect wood [38, 53]. Senmiao Fang mixed chitosan and cinnamaldehyde as a kind of natural wood preservative and proved that the new kind of preservative can effectively protect the test sample, which can be easily used and overcomes the volatilization problem. Salicylic acid, also a natural organic substance extracted from plants, possessing antibacterial functions, can also be utilized and formulated as a kind of wood preservative. Li Yan formulated a salicylic acid/silica microcapsule and studied its decay resistance as well as the stability of modified poplar wood. The decay resistance of treated poplar was greatly improved compared to untreated poplar [54].

### **4. Conclusions**

Wood is susceptible to being infected, decayed and deteriorating in the natural environment due to the ubiquitous microorganisms. Many microorganisms, such as wood decay fungi and bacteria, could attack and damage wood products leading to enormous commercial waste and property losses. Wood decay, an inevitable natural

#### *Wood Degradation by Fungi and Environmentally Benign Wood Preservatives DOI: http://dx.doi.org/10.5772/intechopen.112033*

phenomenon caused by the activity of some microorganisms, brought about various hazards, such as wood structural deformation, and the losses of original strength and stability, which could lead to deconstruction and collapse of wooden products, causing economic impact and property damage even harm and danger to citizens. The decay mechanisms of wood are different depending on the different decay organisms (fungi, bacteria or insects) and various wood species including diverse decay stages. It is very important to understand the decay mechanisms of wood. Through the elucidation of the decay mechanisms of wood, scientific protection technology or measures could be performed in order to prolong the service life of wood products, which is crucial both for sustainable forest resource management and the research and development of wood preservatives. To enhance the resistance of wood against the decay fungi or bacteria, chemical treatments and preservatives have frequently been applied in the wood industry. Environmental concerns have prompted the development of wood preservatives based on natural materials, which are with high efficacy, low cost, low health risks and low environmental impacts. Some protein-based wood preservatives or wood extractives have received great attention due to their low cost, toxicity and environmental impact. As scientific research and technologies advance, the decay mechanism of wood and its relationship with wood preservatives have been further developed, providing an improved understanding of the wood degradation process by various microorganisms (fungi, bacteria) and promoting scientific wood protection and maintenance.
