Abstract
This effort is focused on work completed publicly and privately within academic research and industrial sectors on the utilization of lignin to produce thermosets, thermoplastics, foams, hydrogels, and rubbers. The size of the plastics market and the current and projected influence of lignin on it were evaluated. Further, an analysis of patent activity was employed to show the direction of and interest for lignin in these markets. The market trends documented in the literature, when coupled with detailed patent research, offer a new approach to evaluate potential markets and future directions. The analysis of the commercial market sizes of bioplastics and segmentation showed low penetration of actual lignin-based bioplastics. This exposed the contradiction between the abundance of technologies for lignin-based materials and their little practical use. In addition, this finding highlighted a severe gap between lignin research and development and the actual market.
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A Perspective of Lignin Processing and Utilization Technologies for Composites and Plastics with Emphasis on Technical and Market Trends
Marcel Kropat, Mochen Liao, Hyeonji Park, Khandoker S. Salem, Shelly Johnson, and Dimitris S. Argyropoulos *
This effort is focused on work completed publicly and privately within academic research and industrial sectors on the utilization of lignin to produce thermosets, thermoplastics, foams, hydrogels, and rubbers. The size of the plastics market and the current and projected influence of lignin on it were evaluated. Further, an analysis of patent activity was employed to show the direction of and interest for lignin in these markets. The market trends documented in the literature, when coupled with detailed patent research, offer a new approach to evaluate potential markets and future directions. The analysis of the commercial market sizes of bioplastics and segmentation showed low penetration of actual lignin-based bioplastics. This exposed the contradiction between the abundance of technologies for lignin-based materials and their little practical use. In addition, this finding highlighted a severe gap between lignin research and development and the actual market.
Keywords: Biopolymers; Bioplastics; Lignin; Market trends; Patents; Renewable resources
Contact information: Department of Forest Biomaterials, North Carolina State University, 2820 Faucette Drive, Raleigh, North Carolina, 27606, USA; *Corresponding author: dsargyro@ncsu.edu
GRAPHICAL ABSTRACT
INTRODUCTION
Global environmental awareness and sustainability efforts have gained momentum, which has increased the need for environmentally friendly primary resources for chemicals, materials, and energy. Thus, the concept of biorefinery has stemmed from the research and development of biomass products to tackle such key societal and technological challenges (Passoni et al. 2016). International Energy Agency Task 42 coined a formal definition for biorefinery as “the sustainable processing of biomass into a spectrum of bio-based products (food, feed, chemicals, and/or materials) and bioenergy (biofuels, power, and/or heat)” (Jungmeier et al. 2015). Biorefinery can be a concept, a facility, a process, a plant, or even a cluster of facilities that provide a sustainable approach to valuable products. It can improve biomass processing economics and environmental impact (e.g., greenhouse gas savings) if a fully integrated concept is developed.
Among the various products of biorefinery, lignin is an indispensable and valuable component. Both renewable energy and renewable chemical practices must be adopted for a sustainable economy based on biorefineries. However, due to the current low price of oil, biorefineries require improved profitability to compete with fossil fuels. This can be accomplished by manufacturing diversified products and effectively utilizing byproducts for materials applications (Ho et al. 2018). Petroleum-based chemicals have been the primary feedstock for the production of commodity plastics since the 1940s. Due to limited petroleum resources, the focus has shifted towards the identification of renewable feedstocks as substitutes for the currently used non-renewable petroleum-based materials.
In an effort to increase sustainability, the U.S. Department of Energy and the U.S. Department of Agriculture are strongly committed to expanding the role of biomass for the energy and chemical industry. An increase in the proportion of commodity chemicals and materials made from renewable feedstocks from 5% in 2005 to 25% in 2030 is predicted (Perlack et al. 2005). Lignocellulosic biomass can be utilized for a variety of different materials, such as chemicals and biofuels, and it is the most accessible renewable carbon-based feedstock (Upton and Kasko 2016).
The three main components of lignocellulosic biomass are cellulose, hemicellulose, and lignin, whereby the mass percent composition is dependent on the plant species. Cellulose, a polymer of glucose, incorporates 30% to 50% of the biomass. Hemicellulose, a heteropolymer of various monosaccharides, represents a smaller portion of 20% to 35%. The remaining 15% to 30% of the lignocellulosic biomass consists of lignin. Lignin is a cross-linked heteropolymer that promotes structural integrity in the bio-composite. The aromatic-based amorphous polymer consists of various phenyl propane units linked by carbon-carbon and ether bonds. As the second most abundant natural polymer, lignin accounts for approximately 30% of organic carbon in the biosphere. According to Tuck et al. (2012), as lignin is the only scalable renewable feedstock composed of aromatic units, it is highly underutilized. In 2010, only 2% of the 50 million tons of lignin produced in pulp- and papermaking processes was utilized for specialty products. The largest proportion (48%) was used as low-value fuel (Upton and Kasko 2016). This underutilization indicates a need to develop processes to efficiently isolate lignin from biomass and engineer lignin-based products with increased commercial value.
However, the purity and consistency of the products obtained from petroleum-based starting materials are redeeming qualities. Though lignin is a bio-based product that contributes to more renewable and sustainable products, its heterogeneity is a major technical drawback. Furthermore, the established infrastructure and favorable economics of petrochemical feedstocks inhibit the ability of lignin to be considered as a viable substitute. In this analysis, the patent search and subsequent market analyses were solely based on lignin and current commodity polymers. Overall, this work did not examine the uses of lignin as direct replacement molecules (of monomeric or oligomeric nature as would be derived by catalytic lignin degradation approaches). Further, this approach, if technically and economically resolved, could be a substantial contributing factor to the emergence of “lignin-to-polymer” products.
To comprehensively explore these issues, this review examined the various stages of development in using lignin in different polymer markets, which included the full range from laboratory-scale to industrial, commercial, and market considerations.
LIGNIN PRODUCTION AND MODIFICATION
Current Production of Lignin
The heterogeneity of lignin offers extreme difficulties for technical processes that require a uniform lignin product (Argyropoulos 2014). For example, polymerization processes often require precise stoichiometry that cannot be met by “unrefined” technical lignins. This can be attributed to the fact that the pulping processes are aimed at creating paper or other products, not lignin. The use of a variety of feedstocks, process conditions, manufacturing processes, and pulping sequences, etc., tends to deliver heterogeneous lignin products (Cui et al. 2014). This is further compounded by the fact that the delignification statistics cause the inherent production of lignin fragments, whose molecular weights and distributions (Mw/Mn) are governed by the Flory-Stockmayer equations that contain critical exponents. Such exponential processes are very hard to precisely control (Argyropoulos and Bolker 1987a,b,c). This situation results in tremendous structural, polymeric, and functional group variations that need to be addressed prior to any successful implementation of value-added processes with lignin. In this respect, one may cite a variety of efforts of lignin utilization within the confines of the enumerated variations (Pan et al. 2013; Du et al. 2014; Duval and Lawoko 2014; Laurichesse and Avérous 2014; Thakur et al. 2014; Mendis et al. 2015; Kun and Pukánszky 2017; Tribot et al. 2019). However, recently, a welcome review has appeared in the literature that precisely addresses the various fractionation processes available (Gigli and Crestini 2020).
Because lignin is a major bio-based aromatic material, its importance and relevance to the petrochemical-based industrial economy are immense. Most valorized lignins include materials produced from the following processes: lignosulfonate, kraft, organosolv, and soda. Other methods include steam explosion and processes using various catalysts, which are called catalytic processes. The first step in isolating lignin from the native biomass is delignification, which involves the separation of the lignin from the cellulose, hemicellulose, and extractives. After delignification, the modification or lignin fractionation stage follows. Each pathway in a process influences the quality and purity of the lignin product. The following sections of the review summarize the delignification and the ensuing modification stages.
Delignification
The delignification stage is used to separate lignin from the other biomass components, which are mainly cellulose and hemicellulose. The pulp and paper and bioethanol industries are large-scale operations that use commercially proven techniques. The delignification methods differ based on the active species and process conditions used. The soda and kraft processes each treat the biomass at high temperatures (ranging from 155 °C to 175 °C) for up to 5 h under basic conditions (OH–) for the soda process and a combination of OH– and SH– nucleophiles for the kraft process. The sulfite process operates under reaction times and temperatures similar to the soda and kraft processes but under acidic conditions using acid bisulfite or bisulfite as the reactants. Steam explosion is a process that treats the biomass with steam before explosively decompressing the biomass/steam mixture. Finally, the organosolv process uses polar solvents, such as ethanol, methanol, formic acid, and/or acetic acid, at elevated temperatures. Common to all methods is the final lignin precipitation stage. Each of these processes results in an extracted lignin product with a large variety of average molecular weights, polydispersity indices (PDI = Mw/Mn), and functional groups depending on the nature and the intensity of the delignification process applied, which exemplifies various heterogeneity concerns (Argyropoulos 2014; Cui et al. 2014).
Chemical Modification of Lignin
Various lignin modification stages have been proposed and mainly developed to address its compatibility with synthetic polymers via improving the mechanical and thermal stability characteristics (Figueiredo et al. 2018). Such lignins have seen utilization as fuels (Wang et al. 2015; Cheng and Brewer 2017), fuel additives (Chuck et al. 2013; Ogunkoya et al. 2015), chemicals (Schutyser et al. 2018), and polymers (Pan et al. 2013; Su et al. 2013; Mendis et al. 2015).
The functional groups of lignin (hydroxyl, methoxyl, carbonyl, carboxyl groups, etc.) are usually targeted to create various lignin derivatives. The precise modification is generally decided depending on the intended application, the reactivity of the functional groups, and the type of lignin used in the reaction. The chemical modification of lignin usually offers enhanced solubility for the modified lignin in organic solvents, makes the lignin production process more straightforward, and decreases the brittleness of the polymer with lignin. Therefore, lignin has emerged as a possible precursor to a variety of polymer applications, including higher value end uses. Chemistries that can be applied to lignin are, among others, methylolation (hydroxymethylation), demethylation, amination, and phenolation (Gonçalves and Benar 2001; Zhang et al. 2013). Sulfomethylation is the means by which methyl sulfonate groups are attached to lignin using a variety of pH and temperature levels and/or reactants (Konduri and Fatehi 2015; Chen et al. 2019). Depending on the conditions, the solubility of the sulfomethylated lignins in water is controlled. For example, such lignins have been applied to cement admixtures as a dispersant by Huang et al. (2018). A mixture of formaldehyde and sodium sulfite was used to generate sodium hydroxymethylsulfonate, the reactive reagent for the sulfomethylation.
Amination uses the Mannich reaction, which is a relatively simple and straight-forward reaction among the possible lignin modification reactions. The amination of lignin can be achieved under acidic, neutral, or alkaline conditions (Matsushita and Yasuda 2003; Du et al. 2014; Wang et al. 2016; Wang et al. 2018; Jiao et al. 2019). Depending on the actual conditions, the reaction can either occur between the deprotonated phenolic lignin and the used N,N-dialkylated amine alcohol or between the phenolic lignin and the iminium ion. Aminated lignins are developed for use as emulsifiers or surfactants (Figueiredo et al. 2018).
There is a relative abundance of phenolic and aliphatic hydroxyl groups within the various technical lignins. Substitution reactions using these groups is another major method used to modify technical lignins to obtain lignin polyol derivatives. Esterification is a facile and simple reaction that allows the formation of lignin macromolecular forms of lignin-based polyesters (Duval and Lawoko 2014). Esterification can be carried out by ring-opening reactions, condensation, or dehydration polymerization chemistries. In addition, epichlorohydrin or alkylene oxides have been co-polymerized with lignin. Lignin alkylation (Liu and Li 2006) is another type of modification that uses reactants, such as diazoalkanes and alkylsulfates, in the presence of sodium hydroxide. The phenolation of lignin is among the most widely examined reactions used to replace phenol with technical lignin (Tan 1996; Effendi et al. 2008). Phenolation is used to increase the reactivity of lignin for a variety of applications. Lignin is reacted with phenol under acidic conditions, resulting in the condensation of phenol with lignin side chains. The increase in phenolic hydroxyl groups as well as the readily available reactive sites lead to an improvement of the reactivity of the molecules (Jiang et al. 2018). Lignosulfonates and kraft lignin are commonly investigated lignin starting materials to which phenolation is applied. Efforts to remove methyl groups from the methoxyl substituents in the aromatic rings of lignin to create demethylated lignins have also appeared in the literature (Duval and Lawoko 2014; Laurichesse and Avérous 2014).
Lignin Applications
Lignin-derived thermoplastics
Thermoplastics are polymers that can be liquefied via melting and then reversibly cooled for multiple cycles without notable effects on their inherent properties. Thermo-plastic material is composed of polymer chains that are associated via noncovalent intermolecular forces. Thus, these materials offer low melt viscosities, excellent processing ability, good thermal stability (at processing temperatures and sometimes at operating temperatures), and robust mechanical properties (Mendis et al. 2015; Upton and Kasko 2016).
Lignin-based thermoplastic materials have shown improved processing abilities and recyclability compared to lignin-based thermosetting polymer networks (Wang et al. 2016). As such, they have gained increased interest as sustainable, cost-effective, and biodegradable alternatives for petroleum-based thermoplastics. However, lignin has a high glass-transition temperature (Tg) and undergoes radical-induced self-condensation at high temperatures (Pouteau et al. 2004; Sadeghifar et al. 2012; Sadeghifar and Argyropoulos 2015), which limits its thermal processing ability and eventually hinders the development of high-performance thermoplastic materials. To address these issues, plasticizers or compatible polymers with low Tg values have been added to lignin via blending or chemical modification.
Presently, lignin or its derivatives are blended or mixed with different thermoplastic polymers to prepare lignin-based thermoplastic materials. The most commonly used thermoplastic polymers are polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), polyester, starch, and protein (de Chirico et al. 2003; Pouteau et al. 2003, 2004; Liu and Li 2006; Mu et al. 2009; Hu et al. 2011; Mansouri et al. 2011; Barzegari et al. 2012; Huo et al. 2012; da Silva et al. 2013; Thakur et al. 2014; Domínguez-Robles et al. 2018). Most thermoplastics are prepared using melt-mixing and solution-mixing techniques at temperatures of approximately 170 °C to 210 °C. To produce thermoplastics with melt-mixing, the standard methods used are extrusion, compression, injection, and blow-molding. In contrast, melt-mixing uses casting molding techniques. However, lignin/polyolefin thermoplastic composites are brittle materials with relatively high Tg values with the polar groups in lignin, which limit their compatibility within the nonpolar polyolefin matrix. Plasticizers used in melt-mixing are usually applied during blending with the goal of increasing the interactions on the interfaces of the blended polymers, decreasing micro-phase separation within the composite, and improving the performance of such composite materials (Su et al. 2013). Bouajila et al. (2006) reported that hydrogen bond forming molecules are suitable plasticizers for dry lignin and that aromatic molecules with a structure similar to monolignols (e.g., vanillin) are more appropriate for hydrated lignin (Gandini et al. 2002). In addition, the mechanical properties of lignin-based thermoplastics can be improved using polymer blending techniques (Wang et al. 2016). Few polymers are miscible with lignin through hydrogen bonding and acid-base interactions. Miscibility effects depend on the lignin content, as phase separation occurs at high lignin contents (Nahmany and Melman 2004; Effendi et al. 2008). Lignin/polyester composite systems, in contrast to other materials, display improved compatibility and overall performance due to the formation of hydrogen bonds between the carbonyl groups of the polyester matrix and the hydroxyl groups of the lignin (Canetti and Bertini 2007, 2009). Lignin can also be incorporated into thermoplastics and composites as a filler. The presence of its aromatic moieties improves the antioxidant, flame retardant, and (some) mechanical properties of the ensuing materials; further, water resistance and thermal stabililty are improved (de Chirico et al. 2003; Pouteau et al. 2003; Košíková et al. 2007; Ago et al. 2012; Barzegari et al. 2012). However, some of the observed improvements (i.e., enhanced thermal stability) depend on the improved miscibility of lignin with various polymers. The compatibility of lignin-thermoplastic blends could be easily improved by selecting a suitable structure of the polymer matrix in conjunction with the mixing technique. Further, the compatibility of lignin-based thermoplastics depends on the hydrogen-bonding interactions between the hydroxyls of lignin and thermoplastic polymers. It varies with the abundance of the different aromatic and aliphatic hydroxyl groups in lignin. Better compatibility can be obtained through modification of the macromolecules to create new active chemical sites via methods such as alkylation (Liu and Li 2006), hydroxyalkylation (Mu et al. 2009; Hu et al. 2011; Mansouri et al. 2011; da Silva et al. 2013), amination (Huo et al. 2012), and nitration (Huang and Zhang 2002). Furthermore, the compatibility can be improved by selective masking of the reactive hydroxyl groups in lignin by different chemical modification methods, such as esterification (Gandini et al. 2002; Nahmany and Melman 2004), phenolation (Tan 1996; Effendi et al. 2008), etherification (da Silva et al. 2009; Ahvazi et al. 2011; Sadeghifar et al. 2012), urethane formation (da Silva et al. 2009; Hatakeyama et al. 2013), polymer grafting (Kim and Kadla 2010; Wang et al. 2011), and copolymerization (Gao et al. 2012; Cinelli et al. 2013; Laurichesse and Avérous 2014). Recently, specific molecular weight fractions of kraft lignin have been shown to offer beneficial melt, thermal stability (Sadeghifar and Argyropoulos 2016), and antioxidant characteristics (Sadeghifar and Argyropoulos 2015) for blends with polyethylene, which has opened novel utilization possibilities.
Lignin-derived thermosets
Thermosets are three-dimensionally cross-linked polymeric materials formed via a polyfunctional polymerization (curing) process of reactive resins. Usually, before curing, the components are of lower viscosity than thermoplastics, which makes such systems ideal for the creation of fiber-reinforced composites. Due to its highly branched structure and its large number of functional groups, lignin appears to be a suitable bio-based component for resin formulations of thermosets and thermoset composite materials. In particular, for phenol-formaldehyde (PF), polyurethane (PU), and epoxy (EP) resin systems, lignin may act as a reinforcement and/or a possible replacement of petroleum-based resources. The preparation of lignin-based thermosets follows two general approaches. In one method, lignin is chemically modified to enhance its reactivity. Such lignin materials can be utilized to partially substitute conventional resin components or act as a reactive components, such as toughening or curing agents, in resin systems. Among other methods, chemical modification is typically accomplished by esterification (de Oliveira and Glasser 1994; Hirose et al. 2002; Matsushita et al. 2011; Chung et al. 2013), phenolation (Nimz 1969; Gierer and Pettersson 1977; Kratzl and Oburger 1980; Tan 1996; Lin et al. 2001; Effendi et al. 2008), and oxypropylation (Nadji et al. 2005; Cateto et al. 2009; Sadeghifar et al. 2012). In another approach, petroleum-derived components are partially substituted with crude lignin. However, due to the polarity of the lignin units and the resulting self- interactions, the degree of substitution is limited. High substitution rates with crude lignin result in the compounded bioplastic exhibiting poor mechanical properties (Laurichesse and Avérous 2014; Kun and Pukánszky 2017). Both approaches are partial solutions, as the conventional petroleum-based components are not entirely substituted. Detailed reviews have been conducted on the topic of lignin-derived thermosets and are highly recommended for further reading (Feldman 2002; Lora and Glasser 2002; Matsushita and Yasuda 2003; Raquez et al. 2010; Sen et al. 2015; Ma et al. 2016; Wang et al. 2016; Kun and Pukánszky 2017).
Phenolic resins
Phenolic resins (PF-resins) are well known for their dynamic stability and water resistant, chemical resistant, and electrical insulating characteristics. Typical applications for PF-resins are coatings, adhesives, and molded products. These resins are produced either in acidic or basic media (Olivares et al. 1988; Gardziella et al. 2000; Sen et al. 2015). The synthesis of PF-resins occurs via the reaction of formaldehyde with the ortho- and para- positions of phenol. The result is a three-dimensional, cross-linked network (Duval and Lawoko 2014). For a reaction with formaldehyde to occur, at least one of the ortho- groups of the lignin-monomers must be unsubstituted (Sen et al. 2015).
Due to the similar chemical structures of polyphenolic lignin and phenol, the use of lignin to prepare phenolic resins has been extensively examined, and the topic has been subjected to numerous literature reviews (Wang et al. 1992; Belgacem and Gandini 2008; Lora 2008; Doherty et al. 2011; Duval and Lawoko 2014). The need for low-cost adhesives of reliable supply and durability is a driving factor for the substitution of phenolic components with different lignin types. Primary application targets for these lignin-based phenolic resins are composites, adhesives, insulation, lamination, wood bonding, molded parts, and plywood-adhesives (Barrett 2018). The degree of substitution in phenolic resin systems by crude lignin is limited, as the deterioration of the properties is proportional to the amount of lignin substituted. As a result, recent research has mainly focused on purified, functionalized, and fractionated lignins with higher reactivity for application in phenolic resin systems (Kun and Pukánszky 2017). Even with modified lignin, the substitution rate of classic phenols in the industrial-scale production of lignin-based PF-resins is limited. However, UPM recently announced a 100% lignin-based phenolic resin (Wisa BioBond) that is produced on a commercial scale (Barrett 2018).
Epoxy resins
Epoxy resins account for 70% of the global production of thermosets (Ma et al. 2016). Approximately 90% of epoxy precursors use bisphenol A and epichlorohydrin. However, bisphenol A is an environmentally hazardous molecule, as it can cause cancer, infertility, and impotence, and it has been reported to be hazardous to young children and infants (Asada et al. 2015). The use of bisphenol A in materials that come into contact with food has been banned in several countries, and the use of non-toxic, renewable alternative resources is under serious consideration (US FDA 2010; Flint et al. 2012). The production of epoxy precursors based on renewable resources, such as lignin, has been reported and accordingly reviewed (Duval and Lawoko 2014; Thakur et al. 2014; Sen et al. 2015; Ma et al. 2016; Sun et al. 2018). Modified lignins can be used as both the epoxy component and the curing agent for the synthesis of epoxy resins. However, due to its high molecular weight, crude lignin is not suitable as an epoxy component alone. The resulting epoxy resins from crude lignin are usually characterized by low curing reactivity and unstable properties due to the complex structure and the low mobility of the macromolecules.
Further, the compatibility of crude lignin with curing agents is rather low (Hofmann and Glasser 1993, 1994). However, lignin derivatives of a relatively low molecular weight and enhanced reactivity are currently a prominent feedstock for epoxy resins (Ma et al. 2016). The epoxidation of lignin is performed by reacting its hydroxyl groups with epichlorohydrin, which is followed by epoxide ring closure in alkaline media. The created lignin epoxides are then mixed with curing agents, which causes cross-linking of the resin components. Curing agents contain reactive hydrogen atoms in the form of an amine, anhydride, carboxyl, or hydroxyl groups. Notably, unmodified lignins can also be adequate curing agents without chemical modification, as they contain carboxyl and hydroxyl groups (Kun and Pukánszky 2017).
Polyurethanes and foams
Polyurethanes (PU) are synthesized via the reaction of di- or poly-isocyanates and polyols that possess terminal hydroxyl groups (Engels et al. 2013). As lignin contains many phenolic and aliphatic hydroxyl functionalities, it can be used to replace the polyol component of polyurethanes (Zhang et al. 2015). Lignin-based PUs are more biodegradable than petroleum-based PUs (Cateto et al. 2014; Hu et al. 2014). Both the aliphatic and phenolic hydroxyl groups of lignin react with an aliphatic isocyanate to form polyurethanes with low Tg (Gandini et al. 2002; Bonini et al. 2005). However, high molecular weight polyurethane can be produced by controlling the feedstock (kraft, alcoholysis, lignosulfonate, or solvolysis lignin), polyols (PEG or PPG), polymeric methyl-diisocyante, reaction time, and temperature (Thring et al. 1997; Hu 2002; Duong et al. 2014). However, lignin content was found to be the dominant determinant of material properties over other factors, such as the molecular weight of the polyols, whereas the ratio of isocyanate to hydroxyl groups influenced the tensile properties to some extent and improved the thermal stability (Thring et al. 1997; Vanderlaan and Thring 1998; Pohjanlehto et al. 2014). Lignin-based rigid and flexible bio-foams have been prepared with better thermal conductivity than traditional fossil-feedstock-based polyurethanes due to their larger cell size and lower density.
Lignin is often functionalized to generate new active sites and to improve its chemical structure, which increases its solubility in organic solvents and improves its processing ability and reactivity. A particularly straightforward functionalization approach was reported by Chung and Washburn (2012). The authors demethylated the lignin with Lewis acid catalysts and generated 28% more hydroxyl groups, which produced polyurethanes with significantly improved cross-link densities (Chung and Washburn 2012). Esterification is another simple method that has been used to produce esterified lignin. These esterified lignins can generate lignin-oleic acid-based polyols that form polyurethanes with relatively low Tgs when reacted with polyisocyanates (Laurichesse et al. 2014). Further, the hydroxyl groups of lignin have been functionalized to develop rigid polyurethane foams with comparable insulating and physical properties to those derived from petroleum-based products (Nadji et al. 2005). The water solubility and durability of polyurethane products can be improved using the Mannich reaction, which introduces an amine group in the lignin structure using formaldehyde and a primary or secondary amine or ammonia (Liu et al. 2013). The development of lignin-based phosphate melamine polyurethanes has also been reported, as they showed improved compression strength, thermal stability, char residue formation, and self-extinguishment (i.e., flame retardancy) (Zhu et al. 2014).
Overall, lignin-based polyurethanes have tremendous potential as commodity materials because they display promising (and many times better) physical properties than their petroleum-based counterparts. However, the properties of lignin-based materials can vary due to the batch to batch variability of technical lignins. This is an important drawback that necessitates the development of processes that offer consistent lignin products. Lignin fractionation is such a process that may provide lignin fractions with tunable and coherent specifications and properties to the end users before entering the market (Argyropoulos 2014; Cui et al. 2014).
Hydrogels
Hydrogels are typically polymeric materials that can absorb water, which results in distinct swelling due to the onset of osmotic pressure within them (Passauer 2012). Acrylic acid has been used as a substantial component of commercial hydrogels. Many research efforts have been carried out on the use of biomaterials to create hydrogels. Lignin has been considered a potential source for hydrogel formation, which is mainly due to its documented biodegradability (Thakur et al. 2014; Collins et al. 2019; Meng et al. 2019). The topic of lignin-based hydrogels has been reviewed by several authors who investigated specific applications, such as water purification (Thakur et al. 2017), tissue engineering (Figueiredo et al. 2018; Witzler et al. 2018), biomedicine (Witzler et al. 2018), and biosensors or electrodes (Meng et al. 2019). Water purification aims to remove pollutants and ions from water, and lignin-based hydrogels are a promising resource in this area (Thakur et al. 2017). Lignosulfonate-based graphene hydrogels are composed of a highly porous structure and high specific surface area that has been reported to be able to capture Pb2+ with good endurance and reusability (82% adsorption efficiency after 10 cycles) (Li et al. 2016). Cellulose-lignin composite hydrogels and bentonite/sodium lignosulfonate/ acrylamide/maleic anhydride hydrogels were also used to adsorb Pb2+. Lignosulfonate-grafted-acyclic acid-based hydrogels have also been examined for the adsorption of dyes in water (Thakur et al. 2017). Though excellent chelating performance was observed, the presence of bubbles within the hydrogel, due to entraining air during stirring, was problematic for its transparency. Lignin sulfonate-based mesoporous materials were found to address this problem with improved chelating efficiencies. Overall, Thakur et al. (2017) conclude that because lignins have shown promises in this area, technologies to produce quality lignin feedstocks are needed to promote interest in lignin-based hydrogels for the removal of pollutants from water (Thakur et al. 2017).
Another proposed use of lignin-based hydrogels was demonstrated by Figueiredo et al. (2018), who examined the use of lignin-based hydrogels for tissue engineering and found that operating temperature was a determinant of their functionality. Temperature-induced gelation transitions (thermo-gelation) is a material behavior that occurs at temperatures lower than that of the human body and higher than room temperature. Atom transfer radical polymerization (ATRP) techniques, in which lignin is used as a core, have been developed to make a temperature-sensitive lignin-based hydrogel (Figueiredo et al. 2018). Furthermore, Xu et al. (2018) created lignin-based hydrogels using three-dimensional printing techniques for various biomedical applications. The authors claimed that lignin has a high potential for use in applications such as cell adhesion using direct ink writing techniques (Xu et al. 2018). Notably, several lignin derivatives, including agarose-lignin hydrogels with epichlorohydrin as the cross-linker, have been developed in steam-cell-based tissue engineering (Witzler et al. 2018). Overall, Witzler et al. (2018) state that value-added lignin applications are currently of increased interest due to the fact that lignin-based hydrogels show considerable promise for the controlled or sustained release of pharmaceuticals (Witzler et al. 2018).
Meng et al. (2019) classified the various areas of lignin-based hydrogels into four applications: absorption, controlled release, stimuli-responsive, and biosensors and electrodes. The preparation of lignin-based hydrogels towards biosensor application was conducted by copolymerization of organosolv lignin, lignosulphonates, and kraft lignin with poly(propylene oxide), blended with multiwall carbon nanotubes (Meng et al. 2019).
To enable the hydrogel to display conductivity, the lignin was modified with cations using ethyltrimethyl ammonium chloride. Graphene and lignosulfonate composites have also been introduced as metal-free supercapacitors (Li et al. 2017). Lignosulfonates have been reported to increase conductivity considerations in such hydrogels when compared to pure graphene hydrogels. Single-wall carbon nanotubes were also combined with lignosulfonates to display pressure-sensitive characteristics (Peng et al. 2018). The cited authors emphasize that lignin offers good possibilities for the creation of novel hydrogels, but the synthesis routes need to be improved in terms of simplicity, eco-friendliness, and efficiency.
Rubbers
Rubber is a vulcanized heterogeneous material, the property characteristics of which rely heavily on its reinforcement and binding components. Carbon black is the typical reinforcing agent used in rubbers, but lignin could potentially replace it. Such a replacement assumes that the added lignin will be of acceptable uniformity to offer economical and consistent reinforcement characteristics within stringent specifications (Kakroodi and Sain 2016).
The literature contains abundant accounts and reviews that demonstrate the beneficial effects of lignin on the mechanical and other properties of rubbers (Košíková et al. 2007; Bova et al. 2016; Datta et al. 2017) with possible commercial ramifications (Bova et al. 2016). For example, Košíková et al. (2007) conclude that the role of lignin in rubbers is both that of a strengthening and reinforcing agent and a thermal stabilizer (Košíková and Gregorová 2005; Košíková et al. 2007). Other efforts have also accounted for the resistance of lignin-containing vulcanized natural rubber against thermo-oxidative degradation effects (Barana et al. 2016). The work of Wang et al. (2018) with industrial alkali lignin reported that, when blended with carbon black, a lignin-rubber elastomer was produced with processing energy savings and notable benefits in the mechanical properties of the elastomer.
The miscibility of the lignin with various polymeric components within the rubber is of great importance in this area because such miscibility and compatibility considerations have a profound effect on the interaction between the various polymeric components, which is eventually reflected in the mechanical properties of the blend (Wang et al. 2018). Tran et al. (2016) attempted to enhance the miscibility of lignin in rubber composites and showed that improvements could be realized by using a fractionated melt-stable lignin (Tran et al. 2016). In addition, Yu et al. (2016) attempted to enhance the miscibility of lignin in rubber compounds with a fine dispersion of lignin by utilizing latex-compounding methods. Efforts to fundamentally understand and validate these considerations led Setua et al. (2000) to compare the properties imparted on rubber composites by crude and modified lignin preparations. They concluded that the modified lignin preparations offered better thermal stability and oil/fuel resistance than phenolic resins or carbon black additives (Setua et al. 2000). Other studies have also shown that methylated lignins offer good miscibility with natural rubber (Barana et al. 2018).
MARKET ANALYSES
According to the Statistical Review of World Energy (BP Global 2018), the actual global oil reserves in 2017 consisted of 1,700 billion barrels, which would be sufficient to for 50.2 y of global production at the 2017 level of consumption. However, the demand for oil in the global market is growing. For example, between 2016 to 2017, the growth rate was determined to be 1.8% per day, whereas a decade prior, the rate was 1.2% per day. China and the US were the largest contributors to this growth (BP Global 2018).
As resources based on fossil fuels are declining, global society has focused on alternative sources of energy and chemicals. Biomass is the most abundant renewable alternative to petroleum-based feedstock (L’udmila et al. 2015). Approximately 150 billion tons to 180 billion tons of biomass are produced annually. Approximately 75% of biomass is polysaccharides (cellulose and hemicellulose) and 20% is lignin. The global annual increase of lignin is estimated at 20 billion tons per y, and the total lignin availability in the biosphere is estimated at 300 billion tons (Blažej and Košík 1993; Gregorová et al. 2006; Rossilo-Calle et al. 2007; L’udmila et al. 2015; Bruijnincx et al. 2016).
Lignin Markets
Though approximately 95% of global lignin production is combusted to produce energy, only the remaining 5% is marketed for substantial applications (Tribot et al. 2019).
A market report published by Grand View Research (2020) valued the global lignin market at approximately US$ 954.5 million in 2019. The report included a recent market analysis for the period of 2016 to 2018 and a forecast of the market for 2020 to 2027. The lignin market is expected to reach around US$ 1.12 billion by 2027. The Compounded Annual Rate of Growth (CARG) (in terms of revenue) is projected to be 2% between 2020 and 2027. Macromolecules and aromatics are major applications in the lignin market. Macromolecules application accounted for approximately 58% market share in 2019. Water-soluble sulfonates accounted for the largest market segment in macromolecules for 2019 and are estimated to maintain market dominance until 2027 (Grand View Research 2020). Approximately 85% of the existing lignin markets are dominated by water-soluble sulfonates, although sulfite pulping has virtually been replaced by kraft delignification in most countries. This preference of sulfite over kraft lignin is due to the nature of each process, whereby one process involves structural modification (via installation of sulfonate groups on lignin) and the other is based on structural degradation (via hydrolysis of alkyl aryl ether bonds). A CARG of 1.4% is anticipated for the demand for water-soluble sulfonates from 2020 to 2027. The market share of kraft lignin was over 7% in 2019 (Grand View Research 2020).
The broad application range for products with lignin includes paints and coatings, building and construction, agrochemicals, and many others. Bioplastics made from lignin were not explicitly mentioned in the report (Grand View Research 2020). The growth of the global lignin market is attributable to the strong demand from these applications. The global lignin market is classified based on application segments. The application segments include concrete additives, dyestuffs, animal feed, and others. Over 45% of the global lignin market was used for concrete additives in 2019. According to the report, the North America region was dominant in the global lignin market, followed by Europe. However, Europe is expected to grow rapidly. Europe was followed by the Asia Pacific and Latin American regions, respectively. The fastest growing market during the forecast period is estimated to be the Asia Pacific market (Grand View Research 2020).
Bioplastics Market
The bioplastics industry requires feedstocks based on renewable resources, such as polysaccharides and lignin from biomass. According to the European Bioplastics Report (European Bioplastics 2019), bioplastics represent less than 1% of the current 335 million tons of plastic produced annually. The report predicted an increase in global production capacities from around 2.11 million tons to 2.62 million tons for 2023. Polylactic acid (PLA) and polyhydroxyalkanoates (PHA) were the main predicted drivers of this growth. Whereas the production capacities of PHAs were estimated to quadruple within the next 5 y, the production capacities of PLA were predicted to grow 60% by 2023. Bio-based polypropylene (PP) production on a commercial scale is expected to enter the market in 2023 (European Bioplastics 2019). Figure 1 shows the shares of the total global production capacities of 2.11 million tons in 2018 by material type (European Bioplastics 2019).
Fig. 1. Global production capacities of bioplastics in 2018 by material type; this figure was created with permission based on data from European Bioplastics (2019)
Fig. 2. The global production capacity of bioplastics in 2018 by region; this figure was created with permission based on data from European Bioplastics (2019)
The report describes Asia (with 55%) as the largest contributor to the production capacities of bioplastics, followed by Europe and the U.S., respectively. Figure 2 shows the distribution of the global production capacities of bioplastics by region.
The IBISWorld Industry Report (2018) for bioplastics manufacturing in the U.S reported data about the products and services segmentation of bioplastics in the U.S. The central segment for bioplastics (34.1%) is starch-based bioplastics, whereas bioplastics from lignin were merged in the category of synthetic/others. Figure 3 shows the market shares of the different products and service segments in U.S. bioplastics production (IBISWorld 2018). The report illustrates the scarcity of lignin in current industry-scale applications because most bioplastics are made from polysaccharides. However, the market offers some lignin-based plastic composite materials (e.g., ARBOFORM® and ARBOBLEND®). These composites consist of natural fibers and a thermoplastic made from lignin (Naegele et al. 2016). Though Asia is the most substantial contributor to the bioplastics market in general, Europe is the largest contributor to the lignin-based production of bioplastics and composites, followed by the U.S. (Barrett 2018).
Fig. 3. The product and service segmentation of the U.S. bioplastics market in 2018; this figure was created with permission based on data from European Bioplastics (2019)
TECHNICAL TREND ANALYSES
Based on the analysis of the bioplastics commercial market sizes and segmentation examined above, the amount of actual lignin-based bioplastics that have penetrated the market is low. This exposes the contradiction between the abundance of technologies for lignin-based materials and their little practical use. Further, this reveals the existence of a serious gap between lignin research and development and the actual market. In the following sections, more informed data analysis is offered to reduce the gap between lignin research and the actual market by providing further insights to connect researchers, project managers, and stakeholders.
To address this gap, patent searches and a patent trend analysis methodology developed for this review were employed. Patents, which are globally the most popular means of intellectual property protection, may offer distinct advantages if used as a “technology indicator” (Mogee 1991). Because patents are the bridge that connects industrial applications to academic research, the analysis of patent data can be helpful for both academia and industry. Previous studies performed patent data analyses to determine technological trajectories in different domains, which included the low emission vehicle (LEV), radio frequency identification, and semiconductor industries (Almeida 1996; Oltra and Saint-Jean 2009; Trappey et al. 2011). Similar patent data analyses are applied in the ensuing text aimed to offer further insights.
Methodology
Patent search engine
Previous studies performed their patent data analyses based on the data provided by the European Patent Office (EPO), which offers a comprehensive collection of documents and the correlated search engine results in esp@cenet (ESPACENET) (Oltra and Saint-Jean 2009). The ESPACENET database contains patents applied for and granted all over the world (more than 70 countries and regions). It is a popular database for relevant technical analyses used in previous studies (Archibugi and Pianta 1992; Maurseth and Verspagen 2002). Though ESPACENET contains more than 110 million patent publications, another patent source with a broader range of patent data (European Patent Office 2019) was used, namely the Google Patent database. Google Patents, developed by Google Inc., includes the patents and patent applications from 17 different patent offices, including the United States Patent and Trademark Office (USPTO), China’s National Intellectual Property Administration (CNIPA), Japan’s Patent Office (JPO), Korean Intellectual Property Office (KIPO), World Intellectual Property Office (WIPO), Deutsches Patent- und Markenamt (PDMA), Canadian Intellectual Property Office (CIPO), EPO, and others (Google Patents 2019). To avoid replication, the patent data analysis was completed using both the ESPACENET and Google Patent databases/search engines separately. The detailed analysis method is shown in Fig. 4. The first step was a search to determine the number of patents related to specific search term(s), which was correlated to certain key-phrases.
Data collection, refinement, and analysis
As the present review article focused on the utilization of lignin-based materials, the key-phrases included “lignin” and specific material types. In Table 1, the key-phrases were divided into three groups, which corresponded to the feedstock of the production, the different applications of the produced materials, and the different categories of the produced polymer materials.
Table 1. Key-phrases Used for the Patent Data Analysis
Based on these key-phrases, the patent data was then further refined via the following three stages, which had an increasing order of dependency:
- Stage 1: Patents whose content covered the key-phrase in Group 1 and the required key-phrase in Group 2 / Group 3
- Stage 2: Patents in Stage 1 whose title covered the key-phrase in Group 1 or the required key-phrase in Group 2 / Group 3
- Stage 3: Patents in Stage 2 whose title covered the key-phrase in Group 1 and the required key-phrase in Group 2 / Group 3
Such refining of the patent data was instrumental in ensuring that the discovered patents had a strong correlation to the topics of lignin-based materials. Institutions may not always provide relevant titles to their patents for a variety of reasons. Such citations may partly invalidate some of the data derived from Stage 2 and Stage 3 of the patent search. However, such disparities, if they occurred, were anticipated to be of minor consequence. However, this limitation was included to ensure clarity. Further, some deficiencies may have existed in the presented method. For example, because the data in Stage 3 had a stronger dependence on the desired topic, it may have omitted some patents that use abbreviations of the key-phrases in Groups 2 and 3. Therefore, an additional analysis method was performed on the data extracted from Stage 2. The analysis method used the grant ratio (Eq. 1) and the spatial and temporal distributions of the refined patent data, the details of which are clarified in the ensuing discussion:
(1)
Based on the process flow chart shown in Fig. 4, the patent data were initially collected from ESPACENET and the Google Patent search engines. The input parameters were the key-phrases in Groups 1, 2, and 3 (Table 1). Then, Stage 1 refinement was applied, and the data were downloaded and saved. For example, for a search of “lignin-based polyethylene,” the input was “(lignin) AND (polyethylene),” which yielded all of the patents whose contents cover “lignin” and “polyethylene.” Both Google Patent and ESPACENET provided downloadable CSV-files that allowed the detailed data for Stage 1 for a given topic to be collected. After obtaining the Stage 1 data, further analyses were conducted by proceeding to Stages 2 and 3 to arrive at spatial and temporal distributions and the grant ratios. All analyses were carried out by using the Python 3.6 programming language (Python Software Foundation, Version 3.6, Beaverton, OR, USA).
Fig. 4. Patent data analysis methodology
The spatial distributions categorized the patents by their countries of origin or the non-government organizations that published them (e.g., EPO and WIPO). The countries with the most patents are provided, discussed, and analyzed in the following text. The temporal distribution trends were examined for two different periods (a short one and a longer one). The short period showed the patent activity for the recent past (20 y), highlighting recent technical trends. The longer period was used to display the patent activity from the beginning of the last century to offer a view of historical developments for a given research topic. Finally, the grant ratio provided insights into the possible commercialization of the relevant applications.
RESULTS AND DISCUSSION
Data Refinement
Table 2 provides the data refinement for Group 2 using the key-phrases shown in Table 1. The values in column Stage 1 directly reflect the relative size of the databases.
The size of the ESPACENET database is considerably smaller than the Google Patents database. Further, for some specific types of lignin-based materials, some notable differences were apparent. For example, in the Google Patents database, the number of patents for lignin-based foaming materials was 16571 (Stage 1). This was a much larger number than that for other types of lignin-based materials. However, in the ESPACENET database, the total number of lignin-based foaming materials patents was only 289 (Stage 1), which was lower than the number for lignin-based rubber (615 for Stage 1). Due to the relatively small size of the ESPACENET database, the grant ratio calculated from this database may not have been representative (e.g., lignin-based thermosets). Because of these discrepancies, further analyses used the results obtained from the Google Patents database.
Table 2. Group 2 Patent Data Refinement Results From Google Patent and ESPACENET
To understand the degree of technical maturity of different types of lignin-based materials, the grant ratio was calculated. This is the number of patents granted in Stage 2 divided by the number of overall patents obtained in Stage 2 (× 100%) for a specific type of lignin-based material. Higher grant ratios indicate industry-preferred areas, in which technologies are likely more mature.
Data analysis
The grant ratios for different types of lignin-based materials based on the Stage 2 refined patent data are provided in Table 2. The patents for lignin-based thermoplastics and lignin-based thermosets had higher grant ratios (33.2% and 29.1%, respectively) than other types of lignin-based materials. These two market segments are likely more mature than other types of lignin-based materials.
Tables 3 and 4 show the spatial distribution for lignin-based material patents. The top 5 countries with the highest patent activity in a given area are shown first, and the rest are combined and displayed as “Others.” China, the United States, and Japan are at the top of the list and responsible for 64% to 83% of the total number of patents for all the different types of lignin-based materials examined.
Through comparing the number of patents (Stage 2 refinement) between China and the United States, it was evident that China had considerably higher activity in lignin-based foaming materials and lignin-based aerogels/hydrogels. In contrast, the United States had higher patent activity in the area of lignin-based thermoplastics, thermosets, and rubbers, which are more traditional materials than foaming material and aerogel/hydrogel applications.
Table 3. Group 2 Patent Data Spatial Distribution Results as Obtained From Google Patent
Table 4. Group 2 Patent Data Spatial Distribution Results From ESPACENET
The developing trends for different lignin-based materials are also plotted as a function of two time domains to show recent and long-term trends. Figure 5 displays patent activities in the past 20 y (Fig. 5a.) and the past 90 y (Fig. 5b).
There has been notable recent activity in lignin-based foaming materials, aerogels, and hydrogels (Fig. 5a). These trends have accelerated in recent years, and lignin-based aerogels and hydrogels have been in a leading position since 2012. The patent activities for lignin-based thermoplastics, thermosets, and rubbers have stabilized during the past 20 y.
In summary, lignin-based foaming materials and lignin-based aerogels and hydrogels have been subject to increased recent patent activity, which likely indicates commercial interest and possible opportunities.
Fig. 5a. Group 2 Google Patent data distribution during the past 20 years
Fig. 5b. Group 2 Google Patent data distribution during the past 90 years
The long-term patent activity trends (Fig. 5b) for the different types of lignin-based materials showed a steadily increasing interest, which can be interpreted as an indicator of the prospective development of lignin-based materials. Interestingly, the number of patents for lignin-based rubber materials exceeded that of other types of materials from the 1950s to the 1970s, but the activity decreased in the 21st century. Overall, the trends discovered and displayed in Fig. 5 can provide a valuable guideline for decision makers (industry, funding agencies, and academics) to determine the direction and importance of projects related to different types of lignin-based materials.
An analysis of the patent activities for lignin-based blends and composites with various types of synthetic polymers follows. Similar approaches to those applied to Stage 2 were also used on the data accumulated in the patent data analysis of Group 3, which aimed at specific types of materials. In Table 5, the data refinement results based on key-phrases in Groups 1 and 3 are shown. The number of patents for lignin-based polyethylene (PE) (65781 in Stage 1) and polypropylene (PP) (53432 in Stage 1) far exceeded the number of patents for other types of polymers, which followed the same trend due to the high market penetration ratios of PP and PE (Witter 2015). The grant ratios of different lignin-based polymers are also listed in Table 2.
However, some grant ratio data may have been unrepresentative due to small data sizes; thus, it was not included. For example, the number of patents in Stage 2 for polylactic acid, polybutylene adipate terephthalate, and polybutylene succinate was less than 50, whereas the largest number was higher than 1000 (1081 for polyethylene). Therefore, further analysis may be necessary to fully comprehend and validate the technological maturity of these lignin-based polymers. Table 6 shows the spatial distribution for the patent activity of lignin-based polymers. Like the results of Table 4, the top 5 countries with the highest patent activity are provided, and the number under “Others” is the sum of the patents from the remaining countries.
Table 5. Group 3 Patent Data Refinement Results From Google Patent
Table 6. Group 3 Patent Data Spatial Distribution Results From Google Patent
The sizable activities of China and the United States were apparent, as these countries had the most patent activity for all of the lignin-based polymers. They were followed by WIPO, Japan, and EPO. Interestingly, China showed higher activity in lignin-based polyethylene, polypropylene, polylactic acid, polybutylene succinate, and terephthalate, which have higher annual production (Witter 2015) and higher absolute numbers of patents than other types of lignin-based polymers. The United States was dominant in patent activity for the remaining types of lignin-based polymers, which may indicate differences in lignin-based polymer development between China and the United States.
Figure 6 displays the development trends for lignin-based polymers in short-term (Fig. 6a) and long-term (Fig. 6b).