**2. Biocomposites**

The biocomposites are materials formed by a polymer matrix and natural fibers, which act as reinforcements. Among their main advantages, we can highlight the following: low density, low cost, high resistance, and they are eco-friendly as well. However, they have a disadvantage, incompatibility between polymer matrix and natural fibers, because polymers are hydrophobic and natural fibers have a hydrophilic nature. This is reflected in the mechanical performance of biocomposites. Because of this, chemical and physical treatments have been developed to promote interfacial adhesion between polymer and natural fibers, in addition, to improve dimensional stability and water absorption capacity of biocomposites [2]. In comparison to chemical and physical treatments, biological treatments are considered efficient and environmentally friendly processes. In nature, a great variety of microorganisms capable of degrading lignin, cellulose, and hemicellulose are found [3]. Inside these microorganisms, we can find out fungi that have the enzymatic structure necessary to degrade this type of polymers [4]. The main applications of biocomposites are automotive parts (door panel/inserts, seatbacks, spare tyre covers, interior panels, etc.), circuit boards, aerospace industry, building materials, etc.

#### **2.1 Chemical treatment**

As mentioned above, the main objective of the chemical treatment is to improve the adhesion between the natural fibers and the polymer matrix, in addition, it is possible to reduce the absorption of moisture, therefore the mechanical properties are improved. Chemical treatments including alkali, silane, acetylation, benzoylation, acrylation, maleated coupling agents, isocyanates, and others are commonly used. *Alkali treatment.* This method changes the surface morphology of the fibers, due to the breaking of the hydrogen bonds causing a roughness surface. The aqueous sodium hydroxide (NaOH) applied to natural fibers promotes the ionization of the hydroxyl group to the alkoxide [5]. It also removes certain quantity of lignin, oils, and waxes from the fiber surface; this treatment depolymerizes the cellulose in such a way that the cellulose crystals are left exposed on the fiber surface, increasing the reaction sites. This type of treatment is widely used with natural fibers that act as a reinforcement in either thermoplastic or thermosets polymers. Vinayaka et al. [6] found that biocomposites containing alkali-treated castor plant fibers have better mechanical properties than those with untreated castor plant fibers. Finally, alkaline processing directly influences the cellulosic fibril, the degree of polymerization, and the extraction of lignin and hemicellulosic compounds [7]. Several studies have been focused on the accurate concentration of NaOH, the temperature, and the time of the treatment over the fibers surface, in order to

**39**

**2.2 Physical treatment**

*Getting Environmentally Friendly and High Added-Value Products from Lignocellulosic Waste*

obtain biocomposites able to present satisfactory mechanical properties [8]. *Silane treatment.* According to Xie et al. [9], to effectively couple the natural fibers and polymer matrices, the silane molecule should have bifunctional groups, which may respectively react with the two phases thereby forming a bridge in between them. The general chemical structure of silane coupling agents consists of R(4-n)▬Si▬(R'X)n (n = 1, 2), where *R* is alkoxy, *X* represents an organofuncionality, and *R'* is an alkyl bridge connecting the silicon atom and the organofuncionality. Silanes can be dissolved in organic solvents or in a water/solvent mixture; this solution can be sprayed on the surface of natural fibers. Silane coupling agents have been found to be efficient improving the compatibility between natural fibers and the polymeric matrix by increasing the tensile strength of the biocomposite. Nishitani et al. [10] studied the effects of silane coupling agents on surface of hemp fiber, and they found that the tribological properties of the biocomposites were improved with the surface treatment by the silane coupling agent. *Acetylation.* Acetylation is a reaction that introduces an acetyl functional group into an organic compound. In natural fibers, the acetyl group reacts with the hydroxyl groups of the fiber and an esterification is generated, which reduces its hydrophilic nature. The advantages of using this method is that it increases the thermal stability as well as the dispersion of the fibers in a polymeric matrix [11]. *Benzoylation.* Benzoylation is an important transformation in organic synthesis [12]. Benzoyl chloride is most often used in fiber treatment. Benzoylation of fiber improves fiber-matrix adhesion, thereby considerably increasing the strength of the composite, decreasing its water absorption, and improving its thermal stability [13]. *Maleated coupling agents.* These agents are mainly used to increase the compatibility between the polymeric matrix and the natural fiber. Generally, maleic anhydride is applied to modify the fiber's surface, and the polypropylene (MAAP) enhances the interfacial bonds, as a result of that the mechanical properties increase (Impact strength, young's modulus, flexural modulus, and hardness) [14]. *Permanganate.* Most of the permanganate treatments are conducted by using potassium permanganate (KMnO4) solution (with acetone) in different concentrations with a soaking duration from 1 to 3 min after alkaline pretreatment [15, 16]. Paul et al. [17] studied the electrical properties of short-sisal fiber-reinforced low-density polyethylene composites using different surface treatments. As a result of permanganate treatment, the hydrophilic nature of the sisal fibers is reduced, and therefore, the water absorption decreases. At higher concentrations of KMnO4, there are possibilities to lead to the degradation of cellulosic fiber by the formation of polar groups. The dielectric constant values increase as the concentration of KMnO4 increases. *Peroxide.* Organic peroxides tend to decompose easily to form free radicals and attack the most available hydrogen in the polymer matrix and natural fibers. Benzoyl peroxide (BP) and dicumyl peroxide (DCP) are used in natural fiber surface modifications [11]. As a result of peroxide treatment, the hydrophilicity of the fiber decreases [17] and the tensile properties increase. *Isocyanate.* The isocyanate functional group (▬N═C═O) is highly susceptible to react with the hydroxyl groups of cellulose and lignin in fibers. Joseph and Thomas [18] studied the chemical treatment of the cardanol derivative of toluene diisocyanate (CTDIC) in sisal fiber-LDPE composites. It was demonstrated that CTDIC composites show superior tensile properties than other chemically treated sisal fiber composites due to their better compatibility between sisal fibers and LDPE.

There are different types of physical treatments used to modify only the surface of natural fibers without changing their chemical composition. Physical treatments promote the separation of the fiber bundle into individual fibrils and thus increase the

*DOI: http://dx.doi.org/10.5772/intechopen.93645*

#### *Getting Environmentally Friendly and High Added-Value Products from Lignocellulosic Waste DOI: http://dx.doi.org/10.5772/intechopen.93645*

obtain biocomposites able to present satisfactory mechanical properties [8]. *Silane treatment.* According to Xie et al. [9], to effectively couple the natural fibers and polymer matrices, the silane molecule should have bifunctional groups, which may respectively react with the two phases thereby forming a bridge in between them. The general chemical structure of silane coupling agents consists of R(4-n)▬Si▬(R'X)n (n = 1, 2), where *R* is alkoxy, *X* represents an organofuncionality, and *R'* is an alkyl bridge connecting the silicon atom and the organofuncionality. Silanes can be dissolved in organic solvents or in a water/solvent mixture; this solution can be sprayed on the surface of natural fibers. Silane coupling agents have been found to be efficient improving the compatibility between natural fibers and the polymeric matrix by increasing the tensile strength of the biocomposite. Nishitani et al. [10] studied the effects of silane coupling agents on surface of hemp fiber, and they found that the tribological properties of the biocomposites were improved with the surface treatment by the silane coupling agent. *Acetylation.* Acetylation is a reaction that introduces an acetyl functional group into an organic compound. In natural fibers, the acetyl group reacts with the hydroxyl groups of the fiber and an esterification is generated, which reduces its hydrophilic nature. The advantages of using this method is that it increases the thermal stability as well as the dispersion of the fibers in a polymeric matrix [11]. *Benzoylation.* Benzoylation is an important transformation in organic synthesis [12]. Benzoyl chloride is most often used in fiber treatment. Benzoylation of fiber improves fiber-matrix adhesion, thereby considerably increasing the strength of the composite, decreasing its water absorption, and improving its thermal stability [13]. *Maleated coupling agents.* These agents are mainly used to increase the compatibility between the polymeric matrix and the natural fiber. Generally, maleic anhydride is applied to modify the fiber's surface, and the polypropylene (MAAP) enhances the interfacial bonds, as a result of that the mechanical properties increase (Impact strength, young's modulus, flexural modulus, and hardness) [14]. *Permanganate.* Most of the permanganate treatments are conducted by using potassium permanganate (KMnO4) solution (with acetone) in different concentrations with a soaking duration from 1 to 3 min after alkaline pretreatment [15, 16]. Paul et al. [17] studied the electrical properties of short-sisal fiber-reinforced low-density polyethylene composites using different surface treatments. As a result of permanganate treatment, the hydrophilic nature of the sisal fibers is reduced, and therefore, the water absorption decreases. At higher concentrations of KMnO4, there are possibilities to lead to the degradation of cellulosic fiber by the formation of polar groups. The dielectric constant values increase as the concentration of KMnO4 increases. *Peroxide.* Organic peroxides tend to decompose easily to form free radicals and attack the most available hydrogen in the polymer matrix and natural fibers. Benzoyl peroxide (BP) and dicumyl peroxide (DCP) are used in natural fiber surface modifications [11]. As a result of peroxide treatment, the hydrophilicity of the fiber decreases [17] and the tensile properties increase. *Isocyanate.* The isocyanate functional group (▬N═C═O) is highly susceptible to react with the hydroxyl groups of cellulose and lignin in fibers. Joseph and Thomas [18] studied the chemical treatment of the cardanol derivative of toluene diisocyanate (CTDIC) in sisal fiber-LDPE composites. It was demonstrated that CTDIC composites show superior tensile properties than other chemically treated sisal fiber composites due to their better compatibility between sisal fibers and LDPE.

#### **2.2 Physical treatment**

There are different types of physical treatments used to modify only the surface of natural fibers without changing their chemical composition. Physical treatments promote the separation of the fiber bundle into individual fibrils and thus increase the

*Biotechnological Applications of Biomass*

residues was carried out.

**2.1 Chemical treatment**

**2. Biocomposites**

(acid, alkali, organosolv), and biological (commonly used white-rot fungi) pretreatments. However, this review is focused on the different treatments used on the surface of natural fibers in order to improve their compatibility with a polymeric matrix and thus obtain materials with ecological, lightweight, and excellent mechanical properties, called biocomposites. It is important to mention that when carrying out some of these treatments, residues are generated, which can be processed to recover some high value-added compounds (antioxidants, sugars, bioactive phenols, organic acids, polysaccharides, and polyphenolics). Furthermore, the different types of biomaterials that can be obtained from cellulose (MCF, NFC, CNC, BNC) are described. Finally, an investigation of the market size of some of the products derived from lignocellulosic

The biocomposites are materials formed by a polymer matrix and natural fibers, which act as reinforcements. Among their main advantages, we can highlight the following: low density, low cost, high resistance, and they are eco-friendly as well. However, they have a disadvantage, incompatibility between polymer matrix and natural fibers, because polymers are hydrophobic and natural fibers have a hydrophilic nature. This is reflected in the mechanical performance of biocomposites. Because of this, chemical and physical treatments have been developed to promote interfacial adhesion between polymer and natural fibers, in addition, to improve dimensional stability and water absorption capacity of biocomposites [2]. In comparison to chemical and physical treatments, biological treatments are considered efficient and environmentally friendly processes. In nature, a great variety of microorganisms capable of degrading lignin, cellulose, and hemicellulose are found [3]. Inside these microorganisms, we can find out fungi that have the enzymatic structure necessary to degrade this type of polymers [4]. The main applications of biocomposites are automotive parts (door panel/inserts, seatbacks, spare tyre covers, interior panels, etc.), circuit boards, aerospace industry, building materials, etc.

As mentioned above, the main objective of the chemical treatment is to improve the adhesion between the natural fibers and the polymer matrix, in addition, it is possible to reduce the absorption of moisture, therefore the mechanical properties are improved. Chemical treatments including alkali, silane, acetylation, benzoylation, acrylation, maleated coupling agents, isocyanates, and others are commonly used. *Alkali treatment.* This method changes the surface morphology of the fibers, due to the breaking of the hydrogen bonds causing a roughness surface. The aqueous sodium hydroxide (NaOH) applied to natural fibers promotes the ionization of the hydroxyl group to the alkoxide [5]. It also removes certain quantity of lignin, oils, and waxes from the fiber surface; this treatment depolymerizes the cellulose in such a way that the cellulose crystals are left exposed on the fiber surface, increasing the reaction sites. This type of treatment is widely used with natural fibers that act as a reinforcement in either thermoplastic or thermosets polymers. Vinayaka et al. [6] found that biocomposites containing alkali-treated castor plant fibers have better mechanical properties than those with untreated castor plant fibers. Finally, alkaline processing directly influences the cellulosic fibril, the degree of polymerization, and the extraction of lignin and hemicellulosic compounds [7]. Several studies have been focused on the accurate concentration of NaOH, the temperature, and the time of the treatment over the fibers surface, in order to

**38**

surface area of the fibers and the compatibility with the polymer matrix. According to Ahmed et al. [11], these physical treatments can be classified as follows: mechanical treatment (stretching, calendaring, or rolling), solvent extraction treatment, and electric discharge (plasma treatment, corona treatment, ionized air treatment, thermal treatment, steam explosion, electron radiation, dielectric barrier, and ultraviolet). The *mechanical treatments* promote the interactions between the natural fibers and the polymeric matrix by increasing the surface area of the fibers and decreasing the density and stiffness; therefore, a better distribution of the fibers in the polymer matrix is achieved [19]. *Solvent extraction* can increase the surface area and remove soluble impurities for natural fibers and fillers. Hence, fibers with high cellulose content are obtained. However, this treatment is not widely used because it generates dangerous stems that pollute the environment [20]. *Electric discharge* improve the compatibility between the hydrophilic fiber and the polymer matrix through roughness of the natural fiber surface and structure [21]. Plasma treatment does not need the use of chemicals, which makes it environmentally friendly and cheaper as well. Fazeli et al. [22] modified cellulose fibers by using plasma treatment for the development of biocomposites using a thermoplastic starch matrix (TPS), obtaining a biocomposite with acceptable mechanical properties due to a good interfacial interaction between cellulose fibers and TPS, verified by scanning electron microscope (STEM). Corona treatment changes the surface of natural fibers (surface energy can decrease or increase and free radicals can be produced) by using different types of gases and cold plasma [23]. The steam explosion and alkaline extraction treatments are the most efficient for the removal of hemicellulose fibers. Ultraviolet rays treatment oxidizes the surface of the natural fibers and improves the mechanical properties due to a good interfacial adhesion between natural fibers and the polymer matrix [24].
