Lignin Chemistry
Lignin History
Lignin is a plant-derived biopolymer with several applications. It is considered an effective alternative to chemically synthesized plastics that pose serious environmental problems during manufacture and disposal. Lignin is often obtained as a byproduct in paper and pulp manufacture. Lignin has to be dissolved to obtain cellulose from wood. In 1838, Anselme Payen discovered that wood contained cellulose, and oxidizable coatings (later called lignin) that bind the cellulose together (McCarthy and Islam, 2000). This coating material could be removed from wood after acid treatment and washing with an aqueous alkali solution (McCarthy and Islam, 2000). Later, several researches tried to decipher the structure and biosynthesis of lignin by adopting processes similar to Payen’s, and studying the soluble residues. Finally, it was found that lignin is a polymer composed of several phenyl propane-like residues in combination with various carbohydrates (McCarthy and Islam, 2000).
Composition and Structure of Lignin
Lignin is a polymer, and its monomeric units are phenyl propane units with hydroxyphenyl and methoxy substitutions at various positions on the aromatic ring (Sen, Patil and Argyropoulos, 2015). The monomers of lignin (monolignols) are classified into p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) residues, which are depicted in Fig 1. The percentage of each of the H, G and S residues in lignin varies based on the wood type. Softwood trees have mostly G units in lignin, while hardwood trees have G and S type residues, whereas grass and other smaller plants have all three types of lignin monomers (Sen, Patil and Argyropoulos, 2015). The H, G and S monolignols undergo bio-polymerization by generating ether linkages, biphenyl linkages or other type of bonds (Kai et al, 2016). Based on the type of pulping process sulfur based or non-sulfur based, there is difference in bonding between lignin residues (Kai et al, 2016). The Kraft or sulfur based process alters most of the lignin residues from their natural state (Kai et al, 2016). Lignin obtained from Kraft process has more phenolic residues with –OH group due to breakage of the aryl bonds, and lignosufonates are also formed due to use of sulfur in the process (Sen, Patil and Argyropoulos, 2015). However, lignin obtained from non-sulfur process i.e. soda-lignin is more close to the native lignin, and it can be easily separated by extraction in organic solvent (Sen, Patil and Argyropoulos, 2015).
Physical and Chemical Properties of Lignin
The basic physical property of a polymeric material is the glass transition temperature (Tg) i.e. the temperature range at which the material transforms from a hard glass like form to a soft rubberlike form. This Tg value is high (138-160°C) for softwood lignins with large number of hydrogen bonds between molecules compared to hardwood lignin (110-130°C) (Sen, Patil and Argyropoulos, 2015). Addition of moisture improves plasticity of lignin, and also oxidation prior to heat treatment improves thermal stability of lignin (Sen, Patil and Argyropoulos, 2015). Oxidized lignin can withstand up to 300°C without any weight loss (Sen, Patil and Argyropoulos, 2015). Additionally, Lignin can act as thermoplastic as well as thermosetting plastic. Due to the intermolecular hydrogen bonds lignin behaves like a thermoplastic, but its flowing property is poor. So, to use lignin as a thermoplastic it has to be mixed with other synthetic polymeric materials (Sen, Patil and Argyropoulos, 2015). However, if lignin is heated above its Tg, it can undergo self-polymerization, and there will be enormous increase in its molecular weight. This property can be used to make thermoset materials from lignin, as self-polymerized lignin is insoluble in organic solvent too (Sen, Patil and Argyropoulos, 2015). Basically lignins are aromatic compounds and biodegradable. So, they can be used as eco-friendly alternatives to conventional plastics (Sen, Patil and Argyropoulos, 2015). Further the presence of –OH groups allows lignin chains to be extended or modified based on the application. Thus, lignin can be exploited in various ways.
Ecological Function of Lignin
Lignin has very important structural and supportive function in plants. Lignin acts as the binding material that holds cellulose and hemicellulose together, and it provides rigidity to the plant’s vascular system (Sen, Patil and Argyropoulos, 2015). It helps plants have a tough stem, and withstand environmental stress, wind, pressure etc. Lignin in plant cell walls helps transport water and nutrients, as well as provides defense against microbial infections and insect attacks (McCarthy and Islam, 2000). About 20-40% of plant cell mass is contributed by lignin (McCarthy and Islam, 2000). Thus, lignin is essential for basic survival of trees and plants, and it can act as a valuable material for various applications that benefit mankind.
Lignin as a Value Added Material
Lignin can be used as a substitute for various polymeric materials such as polyesters, polyurethanes, polyethylene terephthalates (PET), PF resins, epoxy resins, and various composites. They have wide applications in bio-medical, electrical and electronic industries as well as in sustainable building materials. However, the potential of lignin needs to be exploited further through intensive research.
Polymer synthesis
Lignin has a very high Tg value, so co-polymerizing lignin with other synthetic polymers such as polyesters and polyurethanes or modifying lignin through alkylation or alkoxylation can greatly reduce its Tg, and make it a useful polymeric material (Sen, Patil and Argyropoulos, 2015). Polyester is formed by esterification i.e. reaction of acid (RCOOH) with alcohol (R’OH) to form an ester RCOOR’. Commercially used polyesters are obtained from petroleum or other fossil fuels (Sen, Patil and Argyropoulos, 2015). Hardwood Kraft Lignin (HKL) already has several –OH groups, and various thermoplastics have been derived in laboratory by reacting different polyester-amine compounds with HKL (Sen, Patil and Argyropoulos, 2015). Thermal and flow properties of lignin co-polyesters are superior compared to native lignin. In a lignin thermoplastic copolymer, lignin imparts strength, while the synthetic polymer improves flow properties. Lignin polyurethane copolymer can be used to substitute rubber as sealants, adhesives, insulators and coatings. Several researchers have developed lignin polyurethane films, and they have very high thermal stability.
Resin synthesis
Phenol formaldehyde (PF) resins are commercially used in several thermoset plastic applications, and resols formed with excess formaldehyde yield the required three-dimensional cross-linked structure to the resin (Sen, Patil and Argyropoulos, 2015). G-lignin with free ortho position abundantly available in softwood is thus ideal candidate to substitute phenol, and manufacture bio PF resins. Further, epoxy resins derived from bisphenol - A are widely used in adhesives, paints, composite building materials, and have wide application in electrical and electronic as well as automobile industries (Sen, Patil and Argyropoulos, 2015). However, bisphenol epoxy resin is considered a toxic pollutant that can adversely affect human health. Lignin or lignin derivatives can be used as safe alternatives to prepare epoxy compounds.
Biological and medical field applications
Lignin has antioxidant properties due to its aromatic nature and functional groups (Kai et al, 2016). Hence lignin can be impregnated in food packaging to prevent spoilage due to oxidation (Kai et al, 2016). Another important medical application of lignin is in bio-implants placed inside bodies of patients. These implants often trigger immune response due to foreign nature of the material, and there will be peroxide mediated oxidative stress response or reactions (Kai et al, 2016). Optimum level of lignin impregnated into polymeric implants will reduce the cytotoxic immune response (Kai et al, 2016). Lignin has UV absorption properties, and can be used in sunscreen lotion or cream preparations to protect against harmful UV light exposure (Kai et al, 2016). Further lignin’s antimicrobial properties have long been known, and the change in antimicrobial nature with lignin’s degradation rate is being researched extensively (Kai et al, 2016). Lignin based hydrogels can be used in tissue culture as well as for targeted drug or gene delivery (Kai et al, 2016). Lignin has a suitable scaffold to support growth of desired cells or tissues in laboratory that can later be implanted into the patient. Thus, lignin is a very useful biomaterial, and extensive research is being done to evaluate its safety in medical applications.
Smart materials
Smart materials are those that change their properties according to changes in external environmental conditions such as pH, temperature, pressure, moisture or electricity (Kai et al, 2016). Lignin can change its plasticity according to moisture, and temperature. Hence, it has potential application in shape memory polymers (SMPs), that shift between temporary shapes or configurations, and return to their original state upon heating (Kai et al, 2016). SMPs are used for various bio-medical applications.
Economic Aspects of Lignin Renewable Material
Lignin can be subjected to carbonization, electrical or thermal treatment, and used in activated carbon or carbon fiber or tube manufacturing (Kai et al, 2016). These carbon materials can be used as absorbents, catalysts or electrodes in various energy generation and waste management applications. Catalysts and electrodes are usually exhausted soon, and they decide the operating cost of a system. Cost associated with safe disposal of these materials is also a burden. Lignin mat electrodes can help in developing novel energy storage devices that are eco-friendly, and do not pose serious disposal issues. Further, lignin based activated carbon absorbents can be used improve efficiency of water or wastewater treatment techniques. As lignin is a natural polymer available in abundance, and it is biodegradable, it can be sourced cheaply (Kai et al, 2016). Thus, lignin based renewable materials can lift the pressure off existing synthetic materials that are associated with high manufacturing costs, energy usage as well as disposal issues.
Future Perspective of Lignin Based Material
Lignin applications are still only in laboratory scale or pilot scale levels. While its biosafety and biocompatibility is well documented, studies on its practical applications are still minimal. There is a need to find novel approaches to extract lignin without much modification in its residues, and a need to identify optimal level of copolymerizing lignin with other polymers to improve its characteristics (Kai et al, 2016). Lignin is a unique natural material whose potential can provide unique medical implants, targeted drug delivery systems and shape shifting plastics to mankind. While fossil fuel derived polymeric raw materials can no longer be depended, lignin offers to be a sustainable resource for our future needs.
Conclusion
Lignin, which is often considered a waste from paper pulp manufacture, can be put to several novel uses. The biopolymer has unique physic-chemical characteristics, which can be slightly modified to suit various thermoplastic as well as thermoset plastic applications. Lignin can be grafted with existing polymeric materials and used in various engineering applications. Owing to its abundant natural occurrence and biodegradability, lignin proves to be a cheap and sustainable alternative to synthetic polymers. For the benefit of mankind extensive research is essential to exploit this natural polymer to its fullest.
References
Kai, D., Tan, M. J., Chee, P. L., Chua, Y. K., Yap, Y. L., & Loh, X. J. (2016). Towards
lignin-based functional materials in a sustainable world. Green Chem., 18(5), 1175-1200. doi:10.1039/c5gc02616d
McCarthy, J. L., & Islam, A. (2000). Lignin Chemistry, Technology, and Utilization: A Brief
History. In Lignin: Historical, Biological, and Materials Perspectives (Vol. 742, pp. 2-99). ACS. Retrieved May 11, 2016, from http://pubs.acs.org/doi/pdf/10.1021/bk-2000-0742.ch001
Sen, S., Patil, S., & Argyropoulos, D. S. (2015). Thermal properties of lignin in copolymers,
blends, and composites: A review. Green Chem., 17(11), 4862-4887. doi:10.1039/c5gc01066g
