Lignins and Epoxidized Lignin-Based Biopolymers

1. Introduction

Lignin, a common byproduct of the paper and pulp industry, serves as a renewable raw resource. It is a biomass-based phenolic copolymer, serving as a cost-effective and appealing source of aromatic chemicals. Lignin and cellulose are the most prevalent natural polymers on Earth. The primary physiological role of lignin is providing plant cell walls stiffness and durability. Lignin enhances mechanical strength to the degree that towering trees, exceeding 100 meters in height, can maintain their upright posture [1, 2, 3].

The growing apprehension over environmental challenges, including fossil fuel depletion and climate change, has positioned wood and wood-based materials in the spotlight because of their positive impacts on lowering carbon footprints and greenhouse gas emissions. The potential for wood to be utilized in innovative and challenging construction projects mostly hinges on adhesive technology. The critical concern must encompass the transition to eco-friendly adhesives, the reduction of carbon dioxide emissions, and the adoption of entirely sustainable solutions. The environmental sustainability of engineered wood products, recognized as the sole renewable and sustainable construction material, is influenced by the adhesives employed, which are often synthesized from oil-derived raw materials such as petroleum and natural gas [4, 5].

Certain compounds, like cellulose and sweet liquor from wood and its byproducts, are combined in order to produce lignin. The demand for lignin will continue to grow due to factors like rising construction costs and the growing demand for electronics and cars [6].

Because of their established pulp and paper industries, which produce significant amounts of lignin as a byproduct, Europe and North America are the top markets for lignin-based dispersants. Due to its high availability in those areas, a great deal of research and development has been done to identify alternative uses, which is anticipated to boost market expansion in the near future [7].

According to research, the size of the worldwide lignin market is expected to grow from almost 1.3 billion US dollars in 2024 to 2 billion US dollars in a decade [7].

2. Lignin

The term “lignin” originates from the Latin word “lignum,” which signifies wood. Lignin constitutes around 30% by weight of wood, making it the second most prevalent component after cellulose. In contrast to cellulose, which is a linear polymer with partially crystalline structure, lignin possesses a non-crystalline structure, and it is a non-carbohydrate polymer rather than a polysaccharide. Because of its qualities, lignin is a substance that shows promise for a number of industrial uses. Figure 1. provides a simplified depiction of lignin’s intricate chemical structure [1].

A method called delignification is used for extracting lignin from biomass. Lignin is mostly aromatic and almost insoluble in common organic solvents, such as pentane, benzene, toluene, and naphthalene. Lignin can be considered a mixture of several compounds that possess analogous chemical structures. Compounds with identical molecular weight but distinct molecular structures are referred to as isomeric compounds. Compounds with varying molecular weights but identical structures are referred to as homologs. Lignin is not classified as isomeric compounds nor as homologs [8, 9].

Most plants conduct a conversion process called lignification, which uses carbohydrate intermediates to synthesize the aromatic polymer lignin from carbon dioxide. The synthesis of lignin precursors, such as trans-p-coumaryl alcohol, trans-p-sinapyl alcohol, and trans-p-coniferyl alcohol, is the first step in the two-stage metabolic mechanism of lignification. Their chemical structures are presented in Figure 2. The second step includes the conversion of those precursors into lignin [10].

Because it is a polymer of such aromatic precursors, lignin processing can be used to provide a variety of aromatic compounds such as p-hydroxybenzoic acid vanillin, vanillyl alcohol, resveratrol, gallic acid, xiamenmycin, and gastrodin [11, 12, 13].

3. Lignin-based epoxy resins

Environmental legislation has focused on a green design methodology and the production of bio-based adhesives from raw materials to foster a more sustainable and conscientious society. Consequently, developing an eco-friendly adhesive from renewable materials is pivotal in addressing the current sustainability requirements of the wood industry. In structural applications, it is essential for the adhesive joint to possess strength equal to or exceeding that of wood. An efficient adhesive for wood necessitates an array of characteristics such as steady bond performance, good water resistance, suitable viscosity, and mass uniformity. Traditional wood adhesives are formulated using phenol formaldehyde, urea formaldehyde, melamine formaldehyde, and polymeric diphenylmethane diisocyanate resin. Epoxy resins are thermosetting polymers with a variety of characteristics related to their structure and uses, such as advanced composites, coatings, and adhesives in constructions. Epoxy resins serve as efficient chemical cross-linkers, demonstrating superior attributes such as robust bonding capability and corrosives and heat resistance. The predominant form of epoxy resin is diglycidyl ether bisphenol A due to its high reactivity with several curing agents, including amine hardeners, isocyanates, and acid anhydrides, all of which are often detrimental to the environment. Substituting those curing chemicals (hardeners) with more environmentally friendly alternatives presents currently a significant technological challenge for numerous researchers [4, 7, 14].

It is essential to identify bio/raw materials suitable for epoxy resin synthesis without significant modification or functionalization. Research and industry fields have shown a great deal of interest in bio-based thermoset polymers derived from natural resources such as sugars, polysaccharides, vegetable fats, lignins, lipids, proteins, and other monomers. In addition to promoting the development of effective and environmentally friendly technologies and consumer goods, the conversion of waste biomass and industrial byproducts, such as tannins and lignins, into specialty value-added chemicals and materials unwittingly draws attention to the issue associated with agricultural and forest waste. The primary focus is on natural phenolic compounds, tannins, and lignins, which are suggested as viable alternatives to commercial or manufactured epoxy resins and curing agents [4, 15].

Lignin-based epoxy resins can be obtained through several methods, primarily involving the chemical modification of lignin to introduce epoxy groups, followed by curing with a hardener. Those methods include direct epoxidation of lignin, epoxidation after lignin modification (e.g., methylolation), and blending lignin with petroleum-based epoxy resins.

Here is a more detailed look at the methods [16]:

1. Direct Epoxidation of Lignin:

That involves reacting lignin with an epoxidizing agent, like epichlorohydrin, to introduce epoxy groups into the lignin structure. That method can be applied to both unmodified and modified lignins.

1. Epoxidation After Lignin Modification

   • Methylolation: Lignin can be modified with formaldehyde or glyoxal to introduce methylol groups, which can then be epoxidized.

   • Phenolation: Lignin can be reacted with phenol to create phenolated lignin, which is then epoxidized.

   • Other Modifications: Other chemical modifications, such as demethylation, can also be used to enhance lignin’s reactivity for epoxidation.

2. Blending with Petroleum-Based Epoxy Resins:

To make lignin-epoxy composites, lignin can be mixed with commercially available epoxy resins, such as diglycidyl ether of bisphenol A. The addition of lignin in such formulations is used frequently to further improve the epoxy resin’s toughness or thermal stability [4, 17].

1. Other Methods:

   • Depolymerization: Lignin can be partially depolymerized to create smaller molecules with more reactive sites for epoxidation.

   • Organosolv Lignin: Lignin extracted using organic solvents (organosolv lignin) can be more easily epoxidized due to its lower polydispersity and higher purity.

   • Enzymatic Hydrolysis Lignin: That type of lignin, obtained from enzymatic hydrolysis residues, has shown potential for epoxy applications due to its chemical reactivity.

Key Considerations:

• Lignin Source:

The type of lignin (e.g., kraft lignin, organosolv lignin, and enzymatic hydrolysis lignin) can influence the properties of the resulting epoxy resin [18].

• Epoxidation Conditions:

The reaction conditions (e.g., temperature, catalyst, and solvent) during epoxidation are essential to the reaction’s effectiveness and selectivity.

• Curing Agents:

The choice of curing agent (hardener) is also important for achieving desired properties in the final epoxy composite.

In summary, the creation of lignin-based epoxy resins involves a variety of chemical and physical modifications to lignin, enabling its incorporation into epoxy formulations for various applications [14, 16, 19].

4. Epoxy modification of lignin

There are several methods for the epoxidation of acetone soluble fraction of kraft lignin (AKL). Effectively introducing epoxy groups into the lignin structure is achieved through lignin epoxidation through the reaction with epichlorohydrin. The best synthesis conditions were found to be 8 hours at 55°C with a 6.3 NaOH/L molar ratio that provided a 99% yield and a high epoxy content (approximately 8). The AKL reaction with epichlorohydrin was used to epoxidize lignin at a molar ratio of 1:1.5 (wt%) lignin to NaOH. Dimethylformamide (DMF) was employed as an additional solvent because all of the lignin samples had good solubility in it. 0.8 g of tetrabutylammonium bromide were added as catalyst to the lignin solution in DMF containing 80 g of epichlorohydrin after 8 g AKL sample had been dissolved in 40 g of DMF and stirred for 10 minutes at room temperature. At 60°C, the reaction mixture was refluxed and stirred for 3 hours. To the reaction mixture that had cooled, 50 mL of a 40 weight percent NaOH solution containing 1.2 weight percent tetrabutylammonium bromide was then added dropwise. When 1000 mL deionized water was added to the reaction mixture to precipitate epoxidized lignin, the reaction proceeded for 8 hours at room temperature. After vacuum filtering out the surplus solvent, the epoxidized lignin was extracted and repeatedly rinsed with deionized water to get rid of the salt that had been produced and the unreacted epichlorohydrin. The epoxidized lignin was then dried for 48 hours at 40°C and 76 kPa in vacuum [4].

In compliance with ASTM D 1652, the epoxy equivalent weight of the epoxy lignin was determined. In a 150 mL flask, 0.4 g of the substance was dissolved in 10 mL of methylene chloride. Next, a few drops of crystal violet indicator were added to the flask along with 10 mL of tetraethyl ammonium bromide 0.25 g/mL solution in glacial acetic acid. Using a standardized 0.1 M perchloric acid, the solution was titrated until the endpoint showed a color shift from vivid blue to green [4, 17, 20].

Lignin was effectively epoxidized under the specified reaction conditions, resulting in the formation of a black powder-epoxy lignin. The lignin epoxidation reaction shown in Figure 3 typically requires 1–8 hours, and it is generally conducted in organic solvents due to the limited solubility of epichlorohydrin in water. The inclusion of water may stabilize the highly polar intermediate product in the lignin epoxidation reaction, increasing the reaction rate [4].

Since the reaction is highly exothermic, it is negatively impacted by the elevated temperature. Unmodified lignin exhibits poor solubility in water at pH levels below 10, whereas the epoxidized lignin produced demonstrates water solubility at pH levels exceeding 8. Increased water solubility and decreased intermolecular hydrogen bonding are the outcomes of the reduction of phenolic hydroxyl groups and the addition of the epoxy group. It was determined that the epoxy lignin’s equivalent weight was 4.8 mmol per gram of resin, which is less than but equivalent to the commercial diglycidyl ether bisphenol epoxy content of 5.3 mmol/g. Consequently, that synthetic epoxy lignin may serve as an appropriate substitute for epoxy resins [4].

5. The importance of lignin in pharmaceutical industry

Lignin is garnering heightened interest in the medical field owing to its biocompatibility, biodegradability, antioxidant characteristics, and potential as a drug delivery system. It is being investigated for its potential usefulness in cancer treatment, wound healing, and as a prebiotic [21, 22].

There are numerous pharmacological applications of lignin:

1. Drug Delivery:

Lignin has been evaluated as carrier for active pharmaceutical ingredients. Lignin, especially in nanoparticle form, can encapsulate and transport pharmaceuticals, particularly hydrophobic compounds, thereby rendering them stability and enabling a controlled release.

Lignin can be utilized as an excipient in tablets to augment the drug release rate and enhance the bioavailability of specific drugs, such as aspirin.

Hydrogels with adjustable qualities for use in biomedical devices can be developed with lignin. Functional hydrophilic groups like hydroxyls, carbonyls, and methoxyls are abundant in it and allow for easy chemical modification.

Targeted Drug Delivery: The chemical structure of lignin, characterized by its aromatic and aliphatic hydroxyl groups, facilitates modification with ligands or other chemical entities, so permitting the targeted delivery of drugs to specific tissues or cells [3, 21, 22].

1. Additional Biomedical Applications:

Lignin possesses antioxidant capabilities that may safeguard against oxidative stress and associated disorders, while its antibacterial and antifungal properties can be utilized in wound healing and several other applications. It has been demonstrated that lignin and its derivatives are active against both gram-positive and gram-negative bacterial strains. The disruption of membrane potential, interaction with membrane proteins, inhibition of ATP synthesis, induction of cell death akin to apoptosis, activation of antimicrobial peptides, synergistic effects with other antimicrobials, and modification of microbial metabolic pathways are some of the many mechanisms that determine antimicrobial activity. Because of all of that, lignin may be used as a natural antibacterial or preservative in food, medical, and pharmaceutical products [3, 22].

Depending on its source, extraction technique, and level of modification, lignin’s antibacterial activity can change [21].

Combining lignin with conventional antibiotics can increase their effectiveness. Antibiotic resistance can be prevented, and the necessary dosages of antibiotics can be decreased because to the synergistic effects [3].

By chemically altering lignin through procedures such as oxidation, its antibacterial qualities can be improved. More effective and targeted antibacterial activity is demonstrated by modified lignin [3].

Lignin-based materials are applicable in wound dressings and other uses to enhance tissue regeneration and provide protection against infection. As an antibacterial ingredient in coatings, films, and other medical applications, lignin can be more effectively delivered and interact with microbial cells when it is incorporated into nanoparticles [22].

Lignin’s biocompatibility renders it a good candidate for applications in bone tissue engineering.

Lignin functions as a prebiotic, fostering the proliferation of advantageous intestinal bacteria, hence enhancing digestive health. That characteristic could be used in functional meals and dietary supplements and is crucial for preserving a healthy digestive tract [23].

It has been discovered that some lignin derivatives have cytotoxic effects on cancer cells, preventing them from proliferating and growing [24, 25].

Some research indicates that by regulating the activity of inflammatory mediators, lignin and its derivatives can lessen inflammation. Strong antioxidant properties are attributed to lignin’s phenolic structure, which allows it to scavenge free radicals. That activity can be used to generate medications and health supplements and is helpful in lowering oxidative stress in biological systems [3, 23, 24].

1. Green Chemistry and Sustainability—Renewable Resource:

Lignin is a renewable resource obtained from biomass, rendering it an eco-friendly substitute for certain synthetic compounds utilized in pharmaceuticals.

Lignin-based compounds possess biodegradability, hence diminishing the environmental impact of pharmaceutical production and disposal.

Lignin can be transformed into precursors for the synthesis of active medicinal compounds, hence promoting a more sustainable pharmaceutical sector [3].

Since it efficiently protects against oxidative stress brought on by environmental factors, such as pollution and UV rays, lignin’s antioxidant properties make it a crucial component of skincare products. A useful ingredient in anti-aging creams, serums, and sunscreens is lignin. Furthermore, lignin’s thickening and emulsifying properties enhance the texture and longevity of cosmetics [24, 26].

6. Environmental applications

Lignin is a useful tool in bioremediation efforts because it may be used by microorganisms that break down environmental contaminants like industrial chemicals and pesticides.

By strengthening soil structure, boosting nutrient availability, and offering defense against infections, lignin and its derivatives can promote plant growth. More sustainable farming methods may result from that [4, 27].

Lignin has been used as a binder for wood and organic municipal solid waste, improving the thermal and physical properties of the briquette. Typically, biomass, such as agricultural waste or coal, is used to make briquettes. and a binder is needed when a substance cannot produce a strong densified form. By encouraging inter-particulate bonding, it facilitates the formation of the densified form [4, 14].

Renewable basic materials such as bioplastics can be made from lignin. Increased thermal stability, firmness, and UV resistance are characteristics of lignin-based composites that are critical for electronics, packaging materials, and automotive components [3].

Because it may be structurally altered and combined with various polymers to improve strength, thermal stability, and biodegradability, lignin is a useful component for composites. Those lignin-based polymers, which provide sustainable substitutes for oil-derived plastics, are being utilized more and more in construction materials, automobile parts, packaging, and other applications [3, 26, 28, 29, 30].

Carbon fibers synthesized from lignin are being researched as a sustainable and affordable substitute for those made from polyacrylonitrile for the automobile, sporting goods, and aerospace industries.

Because of their high conductivity and surface area, lignin-based carbon fibers can also be utilized in energy storage applications, such as in battery and supercapacitor electrodes [31].

Lignin is a sustainable substitute for oil-based adhesives in wood products. Strong bonding qualities are offered by lignin-based adhesives, which are also eco-friendly and emit less formaldehyde [32].

By acting as a binder, lignin can increase the durability and lessen the environmental effect of building materials like concrete and asphalt.

Lignin-derived hydrogels are used for water purification and agriculture [33].

Chemical modification of lignin can yield bio-based solvents, including substitutes for dimethyl sulfoxide. Those solvents provide a more environmentally friendly option to conventional solvents and are used in a variety of industrial processes, such as chemical synthesis.

Lignin-based products can be employed to soil improvement. By boosting the soil’s organic matter content, lignin serves to improve soil stability, minimize erosion, and encourage healthy plant development while lowering the need for artificial fertilizers.

Lignin is helpful in water treatment and environmental remediation because of its capacity to absorb organic contaminants and heavy metals. Heavy metals in the soil can bind with lignin, which lowers their toxicity and bioavailability. Through that chelation process, contaminated soils can be cleaned up and made safer for use in agriculture [27, 32, 33, 34].

By encouraging the creation of soil aggregates, lignin can improve the structure of soil. By increasing soil aeration and porosity, those aggregates lessen compaction and erosion. Because of its ability to hold water, lignin helps to improve soil water retention. It can retain vital elements including calcium, magnesium, potassium, and ammonium because of its high cation exchange capacity [35, 36].

By stabilizing the soil’s surface, lignin lessens the effects of water and wind erosion.

Lignin supports a diverse and active microbial community in the soil, critical to the structure, nutrient cycling, and general health of the soil. Lignin has the ability to affect how quickly other organic components in the soil break down. Lignin can control the breakdown of plant leftovers by interacting with enzymes made by soil microbes, which over time helps to ensure a balanced release of nutrients.

Mulches based on lignin are used in agriculture to control soil temperature, inhibit weeds, and hold onto soil moisture. Compared to traditional plastic mulches, biodegradable lignin-based mulches are more beneficial since as they break down, they are adding organic matter to the soil, increasing its fertility over time. Although the biodegradability of lignin is a big plus, it might be difficult to make sure the mulch does not break down too quickly and lasts the entire growing season. By lowering evaporation, lignin mulches also aid in the retention of soil moisture [35, 37].

Because synthetic chemicals are avoided in organic farming systems, lignin-based mulches are particularly advantageous. Lignin-based mulches have use outside of agriculture, such as soil improvement, weed control, and esthetics in landscaping. They are a desirable alternative for sustainable landscaping techniques because of their organic look and advantages for the environment. Over time, lignin’s slow breakdown increases the amount of organic matter in the soil, enhancing its quality for subsequent plants [38].

Vanillin produced from the pods of the vanilla orchid represents roughly 5% of the world’s vanilla. It is the only phenolic compound that can be mass-produced from biomass. Thus, 95% of vanillin is produced artificially, with 15% of the synthetic vanillin coming from lignin. It can be synthesized through numerous methods using ferulic acid and Kraft lignin [13, 39, 40].

7. Conclusions

Lignin is frequently seen as a waste of the paper and pulp industries. Its transformation into profitable goods like minerals, chemicals, and biofuels, however, is consistent with the circular economy’s tenets of resource maximization and waste minimization. Industries may lessen their environmental effect and support more sustainable production methods by turning lignin into useful products. Its use in the production of biofuels, biochemicals, and bio-materials encourages sustainability and helps to lower carbon footprints.

Additional research is required to comprehensively examine lignin’s potential in medicines, enhance its extraction and modification techniques, and resolve existing restrictions.

The continuous study of lignin valorization is essential to the advancement of circular economy principles, which turn waste resources into useful products.

Conflict of interest

The authors declare no conflict of interest.