Biopolymers as Coating Additives for Engineered Wood Products

Abstract

Engineered wood products are used as a construction material due to enhance performance, faster and higher construction of buildings, durability, and less impact on the environment. However, its flammability and resistance to mold, insects and water limits its use in construction, and especially in exterior use. Thus, the necessity of developing wood coating formulations that reduce the impact of the environment and increases the durability of engineered wood products. Biopolymers have attracted considerable interest as alternatives in coating applications for engineered wood products due to their availability, environmentally friendly and compatibility with the main wood components. The focus of this book chapter is to give an overview of the treatment methods and bio-based coating of the engineered wood, with special emphasis on lignin-based coating. Lignin/lignin nanoparticles, due to the presence of functional groups, is a promising polymer for coating formulations and applications. Lignin can produce a significant quantity of char when heated at high temperatures. This is important combustion characteristic when lignin is to be used as coating additive for wood. As such, lignin-based fire retardant and antibacterial action of lignin are important properties when lignin-based coating formulations are developed, and they are discussed in this chapter.

1. Introduction

Engineered wood products are building materials that are made by laminating layers of wood together or by binding wood fibers together into a composite material, typically with an adhesive usually involving heat and/or pressure [1]. Engineered wood products offer consistency of structural performance and dimensional stability, making it possible to integrate them successfully with other construction materials on large and complex projects [2]. Engineered wood products are making it conceivable to build taller and bigger wood structures, which is highly asked by the building market [3]. There are many different types of engineered wood products, which can be categorized according to the type of feedstock used in their manufacture. The engineered wood association offer guidance on the properties and applications of engineered wood products being classified in three primary categories, as shown in Figure 1. Advantages of using engineered wood products compared to alternative building materials are: (i) natural, renewable, sustainable and reduced carbon footprint; (ii) lighter-weight and greater flexibility in design and construction; (iii) faster and quitter construction; (iv) cheaper construction, warmth and esthetically pleasing with health benefits. Engineered wood products are considered to be renewable construction materials due to their composition [2].

Figure 1.

The classification of engineered wood products, according to APA.

A major factor supporting growth in the use of engineered wood products in construction is the increased environmental and sustainability concern [3, 4], which is influencing construction techniques and the choice of building materials. As such, wood is a renewable and sustainable building material used for modern engineering solutions and functional and decorative applications [5]. Wood has better insulating properties and a positive carbon balance compared to other building materials. Wood has much smaller carbon footprint than other construction materials, and increased use may reduce CO2 emissions by 14% [6]. Moreover, the greenhouse gas emissions have been shown to be as the same level as of concrete and lower than steel [7]. If properly maintained, wood can storage carbon for long lifespan. The engineered wood products used in exterior applications are maintained in time due to the use of preservatives such as creosote, halogenated carbamates, benzothiazoles, pentachlorophenol, (alkyl) imidazoles, bis(tributyltin) oxide, or salt-based impregnates [8, 9]. In addition, fire retardancy is an important behavior if the engineered wood products that are used in construction applications. Currently, the halogenated products are used as fire retardant additives for wood applications [10, 11, 12, 13].

Both the currently used preservatives and halogenated fire-retardant additives used today are considered toxic, both for humans and environment. Preservative leaching problem when wood is encountering water and recyclability of wood materials when preservatives are used have been discussed by several authors [14, 15, 16]. As such, these concerns contribute to the necessity of developing non-toxic bio-based alternatives. Issues of sustainability and carbon sequestration opens the possibility for new green technologies which can improve durability, stability, and performance of wood, especially in exterior applications [17]. The wood preservative industry is interested in finding low-cost, environmentally friendly methods for the treatment of the wood [18]. Engineered wood products (EWP) producers recognize the potential to greatly expand their market-share if more optimal and affordable wood protection options can be found [1].

Extending the service life of wood and wood-derived products by using environmentally friendly biopolymers represents an attractive approach for wood protection from the perspectives of human health and environmental protection [19, 20]. The modern coating market is dominated by acrylic, polyurethane, and polyester polymer resins produced from unsustainable fossil resources. These coating additives are still used on the market due to their properties and low price. In the recent years, sustainable solutions such as vegetable-oil based coatings such as tall linseed, coconut, soybean, and castor-oil have been introduced on the market [21, 22] These oils are often used in different coating combinations to improve their properties. Usually, most of the additives used in coating applications requires chemical modification [21, 22, 23, 24].

An environmentally friendly solution for wood preservation could be the use of biopolymers [25]. Biopolymers, due to the compatibility with the main wood components are considered as interesting alternatives to be used in coating applications [20]. Biopolymers not only that can enhance the performance of adhesives derived from petroleum in different ways [26], but also they can be used to develop environmental friendly and sustainable bio-based alternatives. Biopolymers are categorized as a function of their monomer unit in polysaccharides (cellulose, hemicellulose, glucans, starch), proteins (gelatin, casein), derived polypeptides (collagen, peptides) and polyphenols (tannins, lignin) [25, 27]. Biopolymer-based coatings can be directly deposited onto the substrate surface or by chemical reactions between the biopolymer and the substrate. The chosen coating technique is decided by a specific application, thus depending on several factors, described by Song et al. [28]. Biopolymers, due to the superior compatibility with the main components of wood, have a positive effect on the penetrability of biopolymer into the wooden mass and can enhance the biopolymer biocidal activity. Many other benefits, such as wood recyclability have been pointed out in the work of Patachia and Croitoru [29]. As such, this book chapter is important as it gives an overview on the use of biopolymers in coatings formulations and how these formulations can protect the wood against fire, insects, mold and water. Using natural biopolymers to replace the conventional preservatives and fire-retardant additives in wood protection is highly recommended, as the engineered wood product will be completely renewable and recyclable.

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.

Figure 2.

Schematic figure of engineered wood treatment and surface coating.

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 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, 39, 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].

Figure 3.

Illustration of lignin nanoparticles, chemical/enzymatic modification of lignin.

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, 54, 55].

3.1.1 Lignin-based fire retardant

In recent research literature, it has been found that intumescent flame retardants, such as ammonium polyphosphate (APP), as a non-reactive, inorganic material can be added to polymers as a substitute for halogen flame retardant, being compatible with many polymers and biopolymers [56]. APP is a reaction results of ammonia and phosphoric acid. Therefore, as an additive used for intumescent coating in flame-retardant applications, APP has both function of acid and gas source. When a product containing APP meets fire, APP acts as a flame retardant by a chemical effect in the condensed phase called intumescence. As a result, a carbon foam is formed at the surface of the material which acts as an insulating layer, preventing further decomposition of the material. It has been shown that APP has high content of phosphorus and nitrogen, environmentally friendly, good thermal stability, low smoke, and nontoxicity [11]. These characteristics makes the intumescent flame superior to conventional flame retardants. In recent years, lignin and chitosan has been used as a carbon source in different flame-retardant formulations [12, 13, 57].

Lignin based flame retardants can be prepared by directly physical blending or by chemical modification [58, 59]. A disadvantage of the physical method is uneven multicomponent mixing, which has a negative impact on flame retardancy. During the chemical modification, the hydroxyl groups present in lignin structure will react with desired functional groups, such as ammonia, phosphoric acid given a lignin with modified structure which is suitable to be used in intumescent flame-retardant formulations [60, 61, 62, 63]. Zhang et al. [64] showed that lignin modified with urea and combined it with ammonium polyphosphate (APP) was successfully used as a novel intumescent flame retardant (IFR) system to improve the flame retardancy of polylactic acid (PLA). Moreover, Liu et al. [63] showed that novel lignin-based flame retardant was done by chemically grafting nitrogen, phosphorus and copper elements into lignin structure to improve the flame retardance of wood- plastic composites. Lignin nanoparticles can also be used in different intumescent flame-retardant formulations. Collet et al. used for the first-time lignin nanoparticles modified with phosphor in intumescent flame-retardant formulations [65].

Char yield during combustion of a polymer is an important characteristic when the polymer is to be used as a flame retardant or as additive for intumescent coating. We have observed that during thermal decomposition lignin produces high char yield up to 45%, as seen in Figure 4. We believe that the high char layer has a positive effect on smoke suppression and therefore er lignin a promising additive in intumescent flame-retardant formulations.

Figure 4.

Thermal decomposition of lignin.

Our hypothesis on formation of a larger and denser charring layer helps in improving smoke suppression is in accordance with literature results of Dai et al. [59]. The mechanism involved here is similar when APP is used, where the hydroxyl groups present in lignin structure reacts with phosphoric acid and ammonia, as illustrated in Figure 5.

Figure 5.

Illustration of incorporation of phosphor and nitrogen in lignin structure.

As such, both phosphor and nitrogen are introduced in lignin structure, having a function of an acid and gas source in intumescent flame-retardant formulations. We strongly believe that the synergic effect of both nitrogen and phosphor incorporated in lignin structure can improve the fire-resistance properties.

3.1.2 Antibacterial activity of lignin

The antimicrobial property of biopolymers has been shown literature to depend on several factors, such as molecular mass, concentration, ability to be fixed into the wood structure and electrical charge [66]. Biopolymers with higher molecular masses have low biocidal activity, compared to high molecular mass biopolymers [67]. Literature studies shown that the antimicrobial activities of lignin can be inhibited by the presence phenolic monomers in lignin [68]. The lignin’s antimicrobial activity depends on biomass source, the presence of hydroxyl and methoxy groups, and the extraction methods as follows: softwood organosolv > softwood kraft > grass organosolv due to the effect of acid-soluble lignin content [69]. Lignin as an antimicrobial agent is being used in commodity products like in plastic production [70], textile [71, 72, 73], medical materials, pest control, and healthcare products [74]. Lignin’s and lignin nanoparticles chemical modification and combination with metals, for example Cu-lignin combination, have been shown to increase antimicrobial activity [75]. Thus, the use lignin as an antibacterial agent is believed to be a high value approach for lignin valorization.

4. Conclusions

Biopolymers are promising bio-based alternative to be used as biocides or barrier-forming compounds into the structure of wood. Designing a coating system with better performance on wood depends on understanding the interaction among individual wood constituents with the coating components. By using biopolymers as coating additives to protect the wood has environmental benefits and avoids the issue of wood recycling. Lignin based coating shows great potential in the future as a promising alternative to fossil-based polymers.

Acknowledgments

The author wishes to acknowledge the financial support from the Research Council of Norway (FME Centre for Environmentally friendly Energy Research Bio4Fuel, project number 25622).

Conflict of interest

The authors declare no conflict of interest.