**2. Treatment methods of engineered wood products**

The drivers of using biopolymers-based additives in wood coating applications consist of sustainable concerns, such as increasing the durability of engineered wood products without the use of toxic compounds. In general, as described in the introduction part, wood can be protected against fungi, molds or insects by impregnation using different natural substances [30, 31]. Impregnation has been mostly applied on the solid wood, where the wood structure was chemically modified so that water penetration in the wood structure was limited. Chemical structure and composition of the engineered wood products allows efficient chemical modifications both at surface and inside the wood structure [32, 33]. As such, there is possible to tailor and

synthesis recyclable and renewable engineered wood products with specific properties [18]. The uses of engineered wood products for exterior applications, such as in house-holds buildings requires both a surface and chemical treatment. Exterior wood coatings represent the second largest segment accounting 25% of the global architectural wood coatings market [7]. Different coatings formulations are designed to protect the wood from weathering degradation and preservation in outdoor conditions [34]. The coating agents act at the wood surface as barriers against environmental factors action, such as attack of insects, moisture and fire) and to maintain the aesthetical appearance of the wood. Plant oil-type wood preservatives, such as wood and plants extractives, vegetable oils, natural waxes, different biopolymers and biological control agents are the most applied one [31].

Both, high and low molecular mass biopolymers protect the wood against moisture, oxygen and biological attack. However, the mechanism between low and high molecular mass biopolymers is different. Biopolymers with high molecular mass can be used for surface impregnation, forming viscous biopolymer solutions which can minimize the leaching of biocidal compounds from the treated wood [35]. As such, biopolymer coating formulations can protect the environment and prolonging the lifecycle of wood [35]. Natural biopolymers, with low molecular mass generate solutions (aqueous or organic solvent based) with low viscosity, are proposed to be used as impregnation agents by diffusion into the wood. As such, by creating a film inside wood lumen and closing the pores, allows protection of the treated engineered wood products against water and biological attack. These low-molecular biopolymer solutions could be introduced into wood either by immersing (superficial impregnation) or by high-pressure impregnation [36, 37]. As such, enzymatic polymerization of essential oils with lignin in wood and treatments with nanoparticles [18], represents a promising solution to the engineered wood treatment as illustrated in **Figure 2**.

Currently, biopolymers are used in wood impregnation as aqueous dispersion or emulsions. However, new techniques for using biopolymers in wood modifications are developed in the last years such as biopolymer hydrogels, nanoparticles or biopolymer insertion by using an organic solvent as carrier. The hydrogels or the nanoparticles

*Biopolymers as Coating Additives for Engineered Wood Products DOI: http://dx.doi.org/10.5772/intechopen.113049*

can be loaded with biocides and within controlled conditions of temperature and moisture favors the swelling and the diffusion of hydrogels into the holes in the wood structure. As such, the biocide is fixed in the wood structure avoiding the leaching problem. When an organics solvent is used as carrier for the biopolymer, the same swollen mechanism was observed [38–40].

### **3. Bio-based coatings**

In the last years, have bio-based adhesives gained considerable interest in the bonding of engineered wood products, as environmentally friendly approach compared to the conventional based adhesives. Natural biopolymers such as, cellulose, protein, lignin, and tannin and their modifications with different dispersing agents and cross-linkers have been successfully applied as adhesives for bonding of the engineered wood products [41]. Because of its hydrophobicity, lignin can be used as raw materials for coating [42]. Lignin, due to the presence of phenol groups in its structure, can successfully replace phenol in lignin-based adhesives formulations. Siahkamari et al. [43] developed a bio-based phenolic adhesive by entirely substituting both fossil-based phenol and formaldehyde with lignin and glyoxal.

#### **3.1 Lignin-based coatings**

Lignin, as a natural biopolymer from wood is produced as a by-product in many biorefinery processes. Currently, only about 1 million ton is used for value-added purposes, which mainly comprise in dispersants, adhesives, and fillers [44, 45]. Lignin has a complex chemical structure which includes hydroxyl, carboxyl and phenolic groups. The presence of these groups depends on the lignin isolation process. Chemical/ enzymatic modification of lignin is often a necessity to introduce new functional groups that will increase compatibility between the components in the final material, illustrated in **Figure 3**. Interest in substituting fossil-based polymers with biopolymers in coating industry represents a great market opportunity in channeling recent developments into the production of green coating additives for engineered wood products. Lignin conversion to high quality products is critical to a biorefinery's profitability and sustainability. Organosolv lignin was esterified using dodecanoyl chloride to synthesize a hydrophobic coating for wood [46]. Literature studies show that lignin-based coatings have improved water repellent properties compared to conventional coatings formulations [46]. Henn et al. [47] demonstrated the preparation of fully particulate coatings without the use of binding matrix using lignin instead of metal oxides. Furthermore, colloidal lignin particles were exploited to prepare water-based, solvent free, and multiresistant surface coatings. Due to their hydroxyl groups, the colloidal lignin particles acted as hardener and required no binder to adhere to the substrate. As such, organosolv lignin has been successfully employed to prepare lignin-based epoxy resins [48].

Micro- and nanostructured coatings, such as colloidal lignin particles or lignin nanoparticles have gained attention because they disperse easy and due to their often excellent anticorrosion, antibacterial, anti-icing, and UV-shielding properties [49, 50]. It has been shown that high surface roughness of nanostructured coatings is one important factor contributing to their exceptional hydrophobicity [51, 52]. Hydrophobicity and abrasion of nanostructured coatings can be improved by binding or encapsulating the particles to a polymer/biopolymer matrix obtaining in this way a covalently particle-polymer matrix. As such, particle-polymer matrix can be applied

for special applications being shown in the literature to give a very good water and abrasion protection, but at higher price [50, 53–55].
