Figure 1.10 Structure of epoxidized cyclohexene‐derivatized linseed oil.
Compositions based on modified vegetable oils, hardened by photopolymerization, are mainly intended for coating materials. However, it has been shown that it is also possible to use epoxidized soybean and linseed oils together with cycloaliphatic epoxy resin as binders for glass fiber‐reinforced composites and cross‐linked with visible or UV light [47].
1.3 Substitutes for Bisphenol A Replacement
1.3.1 Lignin‐Based Phenols
Lignin (Figure 1.11) is the relatively large‐volume renewable aromatic feedstock. Next to heteropolysaccharides, it is one of the most abundant biopolymer on Earth, which is found in most global plants [48, 49]. It is deposited in the cell walls and the middle lamella.
Lignin, whose concentration systematically decreases from the outer layer to the inner layer of the cell wall, is generally responsible for reinforcing the plant structure. It is described as a water sealant in the stems, playing an important role in controlling water transport throughout the cell wall. Additionally, lignin of outer layers acts as a binding agent, holding the adjoining cells together, whereas the lignin within the cell walls gives rigidity by the chemical bonding with hemicellulose and cellulose microfibrils [50]. Moreover, it protects plants against decay and biological attacks [51].
Figure 1.11 Simplified structure of softwood lignin (including three monolignols, the building blocks of lignin) [48, 49].
Lignin is a complex and amorphous, three‐dimensional network of hydroxylated phenylpropane units. Its contents vary with different types of plants, and overall, it is about 15–40% of the dry weight of lignocellulosic biomass [52]. Lignin is cross‐linked with cellulose and hemicellulose through covalent and hydrogen bonds [53]. Generally, because of the complex structure and variety of possible degrees of polymerization, lignin is called by the term “lignins,” which refers to the complex and diverse chemical composition and structure [54]. Mentioned properties, along with amorphous and hydrophobic nature, have an influence on difficult process of the isolation of lignin in unaltered form [55]. That is why, ball‐milled wood lignin (MWL), isolated from finely powdered wood via the application of mild, neutral solvents, is considered to be the closest to in vivo lignin. In general, lignins contain a variety of alkyl‐ or aryl‐ether interunit linkages (∼60–70%), carbon–carbon (∼25–35%), and small amounts of ester bonds, which include β‐O‐4, β‐5, β‐β, β‐1, β‐5, β‐6, α‐β, α‐O‐4, α‐O‐γ, γ‐O‐γ, 1‐O‐4, 4‐O‐5, 1‐5, 5‐5, and 6‐5 (Figure 1.11) [56, 57]. Respectively, β‐O‐4‐aryl ether (β‐O‐4), β‐O‐4‐aryl ether (β‐O‐4), 4‐O‐5‐diaryl ether (4‐O‐5), β‐5‐phenylcoumaran (β‐5), 5‐5‐biphenyl (5‐5), β‐1‐(1,2‐diarylpropane) (β‐1), and β‐β (resinol) are major linkages, which are present within lignin macromolecules.
The lignin content is usually higher in softwoods (27–33%) than in hardwoods (18–25%), and herbaceous plants such as grasses (17–24%) have the lowest lignin contents [51]. Moreover, lignin originated from softwood and hardwood has different contents of methoxyl groups. Softwood lignin is composed of guaiacyl units, resulted from a polymerization of coniferyl alcohol (one methoxyl group per phenylpropane unit), whereas hardwood lignin is a copolymer of coniferyl and sinapyl alcohols (two methoxyl groups per phenylpropane unit). Additionally, hardwood lignin, on the one hand, contains fewer free phenolic hydroxyl groups but, on the other hand, contains more free benzyl alcohol groups than softwood lignin (Table 1.2).
Based on the literature [58], there is also a third type of lignin, which is formed by the polymerization of p‐coumaryl alcohol. However, the resulting p‐hydroxyphenyl lignin, is usually found in the form of a copolymer with guaiacyl lignin only in certain trees and tissues.
Even though lignin is one of the most abundant natural polymers, its industrial applications are rather limited. That is why, recently, in the era of greater ecological awareness, as well as unstable and diminishing petrochemical resources, the intensive research is being performed on application of lignin and its valuable compounds. However, the strong chemical bonding of lignin with hemicellulose and cellulose microfibrils makes it hard to isolate for effective utilization. Hence, great effort is being put on the development of pretreatment methods for more effective separation of lignin. Generally, the isolation of lignin is performed by its extraction using different methods, such as Kraft, soda, lignosulfate, organosolv [59–61], hydrolysis, enzymatic, ionic liquids [62], and ultrafiltration by membrane technology. Because all the mentioned isolation methods require specific conditions such as pH, temperature, pressure, reagents, time, and variety of different solvents, the isolated lignin is characterized by diverse structural and chemical properties [63]. Utilization of lignin might be performed: (i) without its chemical modification (via the incorporation of lignin into matrix to give new or improved properties) and (ii) with the chemical modification to prepare a large number of smaller chemicals, which might be used to obtain other chemical compounds including polymers. The chemical modification of lignin (Figure 1.12) is performed by (i) fragmentation or lignin depolymerization to use lignin as a carbon source or to split the structure of lignin into aromatic macromers; (ii) creation of new chemical active sites, and (iii) chemical modification of hydroxyl groups.
Table 1.2 Proportions of interunit linkages in softwood and hardwood [50].
| Lignocellulosic material | ||
|---|---|---|
| Structure (%) | Softwood | Hardwood |
| Phenylpropane unit | ||
| Coumaryl | — | — |
| Coniferyl | 90–95 | 50 |
| Sinapyl | 5–10 | 50 |
| C9‐O‐C9 | ||
| β‐O‐4 | 46 | 60 |
| α‐O‐4 | 6–8 | 6–8 |
| 4‐O‐5 | 3.5–4 | 6.5 |
| C9‐C9 | ||
| β‐5 | 9–12 | 6 |
| β‐1 | 7 | 7 |
| β‐β |
2
| |