High-Performance Materials from Bio-based Feedstocks. Группа авторов

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СКАЧАТЬ some of the biggest challenges in using biomass as feedstock for high‐performance applications are its heterogeneity, seasonal variation, and complexity regarding separation. In many cases, biomass needs to undergo some form of processing prior to being used as high‐performance materials. Typically, the processing of biomass can be performed using chemical, biochemical, and thermochemical processes. These challenges are being tackled as part of the growth in holistic biorefineries, and such approaches that generate no waste are vital for maximizing the value of biomass.

However, unlike petrochemical feedstocks that require significant functionalization, bio‐based feedstocks are blessed with an abundance of functionalities. As such, the development of high‐performance materials from biomass requires different chemistries compared to those from fossil resources. The benefits of biomass utilization for industrial‐scale production of high‐performance materials are that they can potentially reduce waste and production costs, in addition to being carbon neutral, low cost, versatile, and renewable.

1.2 High‐performance Bio‐based Materials and Their Applications

      Bio‐based materials can be synthesized directly from biomass constituents (such as polysaccharides and other polymers, proteins and amino acids, and active biological compounds) that are directly extracted from biomass, but also from biomass‐derived materials (e.g. bio‐derived polymers, porous carbons, or ashes) that require additional processing steps such as polymerization, carbonization, or combustion.

      1.2.1 Biomass Constituents

       1.2.1.1 Polysaccharides

      Polysaccharides are biopolymers that are made up of monosaccharide units connected by glycosidic linkages. Polysaccharides can be obtained from plants (e.g. cellulose, starch, and pectin), algae (alginate), animals (chitin/chitosan), bacteria (bacterial cellulose), and fungi (pullulan). In contrast to synthetic polymers, polysaccharides are abundant in nature, renewable and biodegradable, and are therefore considered as promising replacements of nonrenewable fossil fuel‐based materials in a wide range of applications [9]. Yet, polysaccharides alone frequently present insufficient physicochemical properties, necessitating physical and/or chemical modifications to meet the required product specifications.

      The hydrophilic nature of many polysaccharides presents a poor mechanical strength, which is a major hurdle for their widespread use. For this reason, polysaccharides are often blended with other polymers and/or chemically modified to improve their properties. In addition, inorganic additives can also play a key role to enhance materials properties from boosting strength to improving oxygen barrier and antibacterial properties. For example, esterification, etherification, acetylation, hydroxylation, and oxidation of starch, as well as the blending of starch with other biodegradable polymers can improve its physical and mechanical properties for food‐packaging applications (Chapter 15). Similarly, the substitution of hydrogen by alkyl groups to form cellulose ethers including ethyl cellulose (EC) and carboxymethyl cellulose (CMC) increases the hydrophobicity of cellulose. The addition of CMC into food packaging can improve mechanical, thermal, and barrier properties (Chapter 15). Although biopolymers account for less than 1% of the total plastic production [9], there is a strong growth potential toward their wider application driven by the circular economy trend. Novel biopolymers with improved properties and new functionalities for various applications such as packaging films and coatings as well as textile applications have been developed for commercialization and are reviewed in Chapter 15 with the emphasis on the packaging.

      Similar modifications to the hydrophobicity of biopolymers have been used in coating materials in controlled release fertilizers. In coated fertilizers, the hydrophilicity of native biopolymers results in the buildup of osmotic pressure inside the fertilizer beads. This causes the coatings to break prematurely, resulting in burst release of the nutrients. Naturally obtained biopolymers are thus not ideal fertilizer‐coating materials owing to the lack of hydrophobicity and modifications are necessary to help control the nutrient release. By replacing hydroxy groups with ester groups, cellulose acetate (CA), which is a common cellulose derivative, can be prepared. The presence of ester introduces its pH sensitivity property to the materials and can be beneficial for applications not only in slow‐release fertilizers (Chapter 16) but also in drug delivery (Chapter 8).

In other applications, the hydrophilicity and swelling behavior of biopolymers are used as an advantage. A different class of fertilizers is that of hydrogel‐based fertilizers, which absorb and retain high amounts of water. Hydrogel composites with plant nutrients embedded in their network structure using alginate or chitosan have been developed to reduce the frequency of irrigation as well as controlling the rate of nutrient release (Chapter 16). Hydrogels also find applications in targeted drug delivery. The biocompatibility, biodegradability, and non‐toxicity as well as high affinity for water make biopolymers like alginate, hyaluronic acid, pectin, and carrageenan attractive in controlled release and targeted drug delivery systems for HIV prophylaxis (Chapter 8). Alginate microparticles and films have also been used for anti‐HIV drug delivery due to their excellent biocompatibility and biodegradability (Chapter 8), and alginate has been blended with other polymers to adjust the hydrophobicity of polyelectrolyte films for food‐packaging applications (Chapter 15).

      An interesting group of oligosaccharides with both hydrophilic and hydrophobic properties is that of cyclodextrins. The truncated cone structure of these circular oligosaccharides exhibits a hydrophobic inner cavity, while the upper and lower rims are hydrophilic. These unique properties enable cyclodextrins to contain hydrophobic molecules, but the high costs limit the applicability in foods (Chapter 14). Cyclodextrins can also be used to form metal–organic frameworks (MOFs) that have been investigated for a variety of potential applications including molecular separations, drug delivery, and biomedicine. Organic ligands are the main factors that determine if an MOF is bio‐based, but the sustainability and safety of the metal ions should also be considered (Chapter 12).

      Biodegradability is preferred in some applications such as packaging and fertilizers. The biodegradability needs to be tuned, however, to meet the desired product specifications. For example, in fertilizers, polysaccharide‐based coatings based on, e.g. starch or cellulose, are too biodegradable, leading to premature nutrient release into the soil. To reduce the rate of degradation in soil and extend its service time, the biopolymer can be grafted with rubber or a different polymer with a lower biodegradability and higher hydrophobicity (Chapter 16). In food packaging, many packaging materials are based on blends of biodegradable polysaccharides such as starch, cellulose, and chitosan. In addition, cellulose nanocrystals, nanofibers, and bacterial cellulose have been used as biodegradable reinforcing fillers in various packaging films (Chapter 15).

      Due to their biological nature, the biocompatibility of polysaccharides can be taken advantage of in applications where the polymer requires intimate contact with cells. For example, mucoadhesive films based on derivatives of cellulose, alginate, or chitosan can provide sustained release of several antiretroviral agents (Chapter 8). Moreover, biomaterials are critical СКАЧАТЬ