Handbook of Composites from Renewable Materials, Structure and Chemistry

Preface

The concept of green chemistry and sustainable development policy impose on industry and technology to switch raw material base from the petroleum to renewable resources. Remarkable attention has been paid to the environmental friendly, green and sustainable materials for a number of applications during the last few years. Indeed the rapidly diminishing global petroleum resources, along with awareness of global environmental problems, have promoted the way to switch towards renewable resources based materials. In this regards, bio-based renewable materials can form the basis for variety of eco-efficient, sustainable products that can capture and compete markets presently dominated by products based solely on petroleum based raw materials. The nature provides a wide range of the raw materials that can be converted into a polymeric matrix/adhesive/reinforcement applicable in composites formulation. Different kinds of polymers (renewable/nonrenewable) and polymer composite materials have been emerging rapidly as the prospective substitute to the ceramic or metal materials, due to their advantages over conventional materials. In brief, polymers are macromolecular groups collectively recognized as polymers due to the presence of repeating blocks of covalently linked atomic arrangement in the formation of these molecules. The repetitive atomic arrangements forming the macromolecules by forming covalent links are the building block or constituent monomers. As the covalent bond formation between monomer units is the essence of polymer formation, polymers are organic or carbon compounds of either biological or synthetic origin. The phenomenon or process of polymerization enables to create diverse forms of macromolecules with varied structural and functional properties and applications. On the other hand, composite materials, or composites, are one of the main improvements in material technology in recent years. In the materials science field, a composite is a multi-phase material consisting of two or more physically distinct components, a matrix (or a continuous phase) and at least one dispersed (filler or reinforcement) phase. The dispersed phase, responsible for enhancing one or more properties of matrix, can be categorized according to particle dimensions that comprise platelet, ellipsoids, spheres and fibers. These particles can be inorganic or organic origin and possess rigid or flexible properties.

The most important resources for renewable raw materials originate from nature such as wood, starch, proteins and oils from plants. Therefore, renewable raw materials lead to the benefit of processing in industries owing to the short period of replenishment cycle resulting in the continuous flow production. Moreover, the production cost can be reduced by using natural raw materials instead of chemical raw materials. The waste and residues from agriculture and industry have been also used as an alternative renewable resources for producing energy and raw materials such as chemicals, cellulose, carbon and silica. For polymer composites applications, an intensifying focus has been directed toward the use of renewable materials. Bio-based polymers are one of the most attractive candidates in renewable raw materials for use as organic reinforcing fillers such as flex, hemp, pine needles, coir, jute, kenaf, sisal, rice husk, ramie, palm and banana fibres which exhibited excellence enhancement in mechanical and thermal properties. For green polymer composites composed of inorganic reinforcing fillers, renewable resources based polymers have been used as matrix materials.

Significant research efforts all around the globe are continuing to explore and improve the properties of renewable polymers based materials. Researchers are collectively focusing their efforts to use the inherent advantages of renewable polymers for miscellaneous applications. To ensure a sustainable future, the use of bio-based materials containing a high content of derivatives from renewable biomass is the best solution.

This volume of the book series Handbook of Composites from Renewable Materials is solely focused on the Structure and Chemistry of renewable materials. Some of the important topics include but not limited to: carbon fibers from sustainable resources; polylactic acid composites and composite foams based on natural fibres; composites materials from other than cellulosic resources; microcrystalline cellulose and related polymer composites; Tannin based foam; Renewable feedstock vanillin derived polymer and composites; silk biocomposites; bio-derived adhesives and matrix polymers; biomass based formaldehyde-free bio-resin; isolation and characterisation of water soluble polysaccharide; bio-based fillers; keratin based materials in biotechnology; structure of proteins adsorbed onto bioactive glasses for sustainable composite; effect of filler properties on the antioxidant response of starch composites; composite of chitosan and its derivate; magnetic biochar from discarded agricultural biomass; biodegradable polymers for protein and peptide conjugation; polyurethanes and polyurethane composites from bio-based/recycled components.

Several critical issues and suggestions for future work are comprehensively discussed in this volume with the hope that the book will provide a deep insight into the state-of-art of Structure and Chemistry of the renewable materials. We would like to thank the Publisher and Martin Scrivener for the invaluable help in the organisation of the editing process. Finally, we would like to thank our parents for their continuous encouragement and support.

Vijay Kumar Thakur, Ph.D.

University of Cranfield, U.K.

Manju Kumari Thakur, M.Sc., M.Phil., Ph.D.

Himachal Pradesh University, Shimla, India

Michael R. Kessler, Ph.D., P.E.

Washington State University, U.S.A.

Chapter 1

Carbon Fibers from Sustainable Resources

Rafael de Avila Delucis1, Veronica Maria de Araujo Calado2, Jose Roberto Moraes d’Almeida3 and Sandro Campos Amico1*

1Mining, Metallurgical and Materials Engineering Post-Graduate Program (PPGE3M), Federal University of Rio Grande do Sul (UFRGS), Porto Alegre/RS, Brazil

2School of Chemistry, Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro/RJ, Brazil

3Materials Engineering Department, Pontificia Universidade Catolica Rio de Janeiro (PUC-Rio), Rio de Janeiro/RJ, Brazil

*Corresponding author: [email protected]

Abstract

Carbon fibers (CF) combine unique properties that have enabled their growing use as reinforcement in polymeric composites. CF based on polyacrylonitrile (PAN) are widely in use today, even though the first attempt to produce these fibers in 1878 employed cotton and other materials. Ongoing and steady research for green precursors for CF from available natural resources is motivated by the high cost and generation of toxic products related to PAN- or pitch-based fibers. This chapter reviews some of the work being carried out on lignin and other natural resources (rayon, wood, cotton, jute, ramie, wool, chitin, chitosan, tar pitches and sea squirts). Due to its importance and wide availability, lignin, from hardwood or softwood, is discussed in detail, including the various extraction methods available. The processing for obtaining CF varies but, for polymeric precursors such as PAN or lignin, three basic steps are common: thermal extrusion and spinning, thermal stabilization, and carbonization. This chapter also describes the use of blends of lignin with polymers, such as PEG, PEO, PET/PP, PVA, PAN and PLA, as precursor for CF.

Keywords: Lignin, cellulose, natural fibers, wood, rayon, precursors

1.1 Introduction

Carbon fibers (CF) combine unique properties such as dimensional stability, high strength, high stiffness, low thermal expansion coefficient, biological compatibility and elevated fatigue resistance (Chand, 2000; Wazir & Kakakhel, 2009). Due to these and other features, these fibers have been used in composites to replace plastics, steel, and other engineering materials in sectors/applications such as military, aerospace, marine, automotive, civil construction, petrochemical, offshore structural components, biomedical, sporting goods, pressurized gas storage, as well as supercapacitors, lithium-ion batteries and flywheels (Fitzer, 1989; Momma et al., 1996).

In 2005, the value of the worldwide carbon fiber market amounted to around $900 million, split into commercial grade (59%) and aerospace grade (41%). The numbers for 2015 reached $2 billion, with a production increase of 122%, and a relative expansion in the commercial grade fiber (71%) in relation to the aeronautical grade (29%) (Zoltek, 2015). Thus, we are seeing a trend towards mass production of less specialized CF.

Even though CF based on polyacrylonitrile (PAN) are in wide use, the first attempt to produce these fibers, in 1878, was based on cotton, when Thomas Edison produced filaments for incandescent lamps (Edison, 1880). The production process for CF can be broadly divided into precursor production/isolation, fiber spinning, fiber stabilization, fiber carbonization and fiber graphitization (Edie, 1998). However, each stage has specific features depending on the nature of the precursor used and the required final properties of the CF (Edie, 1998; Zhang, 2014). Considerations when selecting a precursor/process include:

Technical standpoint: Chemical composition of the precursor is vital; it should present a high carbon content, at least 92 wt% of anisotropic carbon (Frank et al., 2014). Also, it should not melt during carbonization (Park & Heo, 2015).

Economic standpoint: Compared with other artificial fibers (e.g., glass and polymeric fibers), CF are costly, which limits their use to a range of applications (Wu et al., 2013). Moreover, as reported by Mainka et al., (2015), more than 50% of the carbon fiber cost is related to the precursor, 15% to the oxidation process and 23% to the carbonization process. The price of the precursor is linked to its availability, as is its isolation process.

Environmental standpoint: Preferably, processing should not result in toxic wastes, and the precursor should come from a sustainable resource.

Currently, due to the high mechanical strength of the fibers produced, PAN – a synthetic non-renewable petroleum-based precursor – is the main commercial precursor, representing about 90% of the total carbon fiber production. Pitch and viscose rayon are also widely used precursors for CF (Chand, 2000). Although these precursors present relatively proven technical efficiency (Mora et al., 2002), they have drawbacks related to high cost and generation of toxic products during processing, e.g., hydrogen cyanide (HCN).

Thus, much of the current research regarding carbon fiber production focuses on defining alternative precursors, especially green ones from available natural resources. Indeed, the use of precursors from biomass that may lead to low price and eco-friendly CF could overcome the cited problems and increase the applications in which carbon fibers may be used. According to Langholtz et al., (2014), this could increase biorefinery gross revenue by 30% to 300% and reduce carbon dioxide (CO2) emissions.

In general, to be considered a potential candidate for carbon fiber production, a natural resource-based precursor must present high carbon content (Mavinkurve et al., 1995), resistance to high temperature (Dumanli & Windle, 2012), not more than one carbon atom between the aromatic rings (Chand, 2000), a high degree of order, orientation and flatness (Inagaki et al., 1991; Inagaki et al., 1992), simple release of non-carbon atoms and easy cyclization (Mavinkurve et al., 1995; Huang, 2009), high molecular weight (Morgon, 2005), and ash content lower than 1000 ppm (0.01%) (Frank et al., 2014). According to Chand (2000), in order to resist to high temperatures, the precursor should preferably be a heterocyclic aromatic polymer and the heteroatoms should not belong to the main molecule chain.

Forests, agricultural waste, crop residues, and wood chips, among others are all possible biomass sources (Agrawal et al., 2014). Primarily lignin, cotton, wool, jute, and ramie are regarded as potential sustainable precursors for CF. Among them, lignin is largely cited because of its high availability and ecological appeal since it is obtained as an industrial waste from the cellulose pulping process. The cited natural resources and the chemical modifications required to make them appropriate for processing are further exploited below.

1.2 Lignin and Other Sustainable Resources

Lignin is responsible for a mass corresponding to 300,000 Mton in the biosphere, and is one of the most abundant materials in nature, second only to cellulose (Gregorováa et al., 2006). Both are sustainable and naturally occurring renewable polymers (Fengel & Wegener, 1989 (Voicu et al., 2016). Its name comes from Latin lignum, meaning wood (Piló-Veloso, 1993). In its natural state, lignin is found in biological materials associated with carbohydrates such as cellulose and hemicellulose, and concentrated in intercellular spaces of all vascular plants, promoting the interconnection between anatomical characters (Evert, 2006). The estimated annual production of lignin reaches approximately 50 Mton (Thakur et al., 2014), especially as a co-product of pulping, and more recently, as a by-product of cellulosic ethanol production in biorefineries (Thakur & Thakur, 2015).

Many different structures have been proposed for lignin, which are believed to depend not only on source (hardwood, softwood and grass plants), but also on plant age, environmental conditions and the extraction process used (Kraft, organosolv, alkali, and so on). The molecular structure of lignin from hardwood allows good spinning and slow stabilization, whereas for softwood lignin, stabilization is easier, but the lignin is not readily spoolable.

The lack of an effective method to isolate lignin, makes it difficult to fully elucidate its chemical structure. Nevertheless, the lignin structure has been cited in recent decades as a complex, three-dimensional, heteropolymeric, amorphous, cross-linked and highly branched structure. More specifically, polyether-phenylpropane is considered the major unit of lignin (Silva et al., 2009), with carbon-carbon and ether linkages between monomeric units. Hydroxyl and methoxyl groups are often cited as substituents on the phenyl group.

Lignin has a carbon content greater than 60% (Mainka et al., 2015) and it may be classified as a function of the pretreatment used to fractionate the lignocellulosic matrix or based on its chemical configurations. The methods applied for biomass pretreatment are broadly divided into physical, chemical, physicochemical and biological and include steam explosion (Wang et al., 2010), microbial fermentation (Chang et al., 2012), alkali pretreatment (Xu et al., 2015), hydrolysis with diluted acid (Kim et al., 2015), hydrothermal treatment with hot water (Pelaez-Samaniego et al., 2015), microwave irradiation (Li et al., 2015), ionic liquids pretreatment (Zhang et al., 2015b), electron beam irradiation (Metreveli et al., 2014), wet oxidation (Klinke et al., 2002), supercritical fluid extraction (Assmann et al., 2013) and organosolv pretreatment (Santos et al., 2014).

It is out of the scope of this chapter to approach all chemical configurations wherein lignin is found artificially and for every pretreatment for the fractioning of lignocellulosic biomass. Instead, it will briefly discuss the main features of those lignins that are potential precursors for CF. The various configurations wherein lignin is found artificially are called technical lignins that, owing to processes carried out for the obtaining of the lignocellulosic raw material, may be classified into:

Extracted lignins: Milled wood lignin (MWL), Milled wood enzyme lignin (MWEL), Cellulase enzyme lignin (CEL) and Braun’s lignin.

Residual lignins: Klason lignin and Willstaller lignin.

Derived lignins: Thiolignins, Organosolv lignin, Kraft lignin and Sulfite lignin (lignosulfonates).

Depending on the method used, the obtained lignin will have distinct characteristics, leading to carbon fibers with particular properties for different applications. Some of them are discussed below.

Milled wood lignin (MWL), also called Björkman Milled Wood Lignin or Björkman Lignin, is obtained from sawdust-sized particles through extraction in aqueous p-dioxane (Björkman, 1956). The obtained lignin presents itself in its natural state (protolignin) (Ikeda, 2002), with a yield between 20 and 30% (Rencoret et al., 2009; Goundalkar et al., 2014). Increase in the milling extent increases the MWL yield and the lignin obtained becomes more representative of the original amount of lignin. However, severe chemical modification of the lignin may occur, including increase in carbonyl content and phenolic hydroxyl content (Bjorkman, 1957; Chang, 1975), decrease in molecular weight (Chang, 1975) as well as cleavage of aryl ether linkages (Gellerstedt & Northey, 1989), which is undesirable for CF production. A carbohydrate residue of approx. 10% remains in the obtained lignin, depending on plant species (Guerra et al., 2006; Goundalkar et al., 2014), and the final product, of high molecular weight, is not completely dissolved in commonly used solvents for lignin (El Hage, 2009; Bu et al., 2011).

In order to increase the final purity, MWL is often transformed into Milled Wood Enzyme Lignin (MWEL) by enzymatic hydrolysis of the polysaccharides (Gellerstedt, 1992) and fractioned by a soluble solvent, obtaining Cellulase Enzyme Lignin (CEL) (Ikeda, 2002). According to Guerra et al., (2006), the molecular weight decreases in the following order: MWEL, CEL and MWL, therefore possibly increasing the CF production capability.

Braun’s lignin (BNL), or native lignin, is obtained by extraction in methanol. Samples are flour-sized particles and, after ethanol extraction, the lignin can be purified by washing in cold water and diethyl ether to remove foreign components (Hiltunen et al., 2006). Compared to MWL, a lower yield (about 8%, depending of the degree of milling) of a lower molecular weight lignin is obtained (Agrawal et al., 2014), which is considered disadvantageous for CF production.

Classified as residual lignins, Willstätter lignin and Klason lignin are obtained by hydrolysis of soluble polysaccharides in water, which is performed by treating with sulfuric acid and hydrochloric acid, respectively. After the chemical reaction, the lignin is recovered as an insoluble residue. These methods for lignin isolation are mainly used for evaluating the lignin content (Goundalkar et al., 2014; Santos et al., 2014), because the obtained lignin presents a highly condensed structure (Rowell et al., 2012) which prevents its use for the manufacturing of new products.

In general, derived lignins are byproducts of pulp and paper production processes. Differently from extracted or residual lignins, these technical lignins are not isolated with the intention of representing the lignin in its natural state (i.e., protolignins). Therefore, after isolation and purification, it is very important to evaluate the degree of carbohydrate contamination. These lignins have economic and environmental appeal because they are waste from industrial processes being produced on large scale, and are considered environmental pollutants. After the pulping process, this lignin is often burned for energy production but the total amount of lignin produced is 60% greater than what is actually needed for internal power supply (Sannigrahi et al., 2010). This is often a bottleneck in the production process, thus favoring the insertion of lignin in other niches such as for CF production. In addition, according to Paananen & Sixta, (2015), pulping processes that use excess alkali generate a highly alkaline residual black liquorthat is very difficult to use for incineration, requiring an extra step in the recovery procedure to reduce alkalinity.

Kraft lignin is produced in the most widespread chemical process in the industry. In general terms, this lignin is obtained by dissolution and degradation of the lignocellulosic raw material in an alkaline sodium sulphite solution, called white liquor. It has low molecular weight and high content of phenolic compounds (Sadeghifar et al., 2012), which results in high reactivity and low potential to be used as CF precursor. On the other hand, the organosolv pulping process is considered the most environmental-friendly method for the fractioning of lignocellulosic materials, yielding value-added products (i.e., lignin and hemicelluloses) (Santos et al., 2014). For García et al., (2011), the production process for organosolv lignin is sulphur-free, the yield is about 78% depending mainly on the process temperature (Astner et al., 2015), and the derived lignin presents low molecular weight (El Mansouri & Salvadó, 2006).

The lignosulfonates, also called lignin sulfites, are the cellulosic waste from the sulfite pulping process. This process may be understood as a lignocellulosic matter treatment by immersion in an aqueous solution of sodium sulfite at high temperature. The lignosulfonates composition varies based on the extent of the lignin degradation and the number of sulfonic groups present (Gandini, 2008; Areskogh et al., 2010). Compared with Kraft lignin, it has lower α-cellulose content and higher molecular weight (Miao et al., 2014).

Steam explosion lignins are obtained by defibrillation of lignocellulosic matter based on a treatment with steam at high temperature and pressure followed by the rapid release of pressure, obtaining about 50% yield (Martin-Sampedro et al., 2011). The first CF obtained following the steam-explosion of lignin was reported by Sudo & Shimizu (1992). Treating lignin under such conditions, at very low pH, is considered undesirable because of its structural complexity and thermal instability. Elevated temperatures convert lignin into an unwanted condensed, inert and insoluble material (El Mansouri et al., 2011).

Besides lignin, other natural resources have been used for CF production such as viscose rayon, wool, cotton and bast fibers (Reinhardt et al., 2013). Shells of some crustaceans, chitin and their products can also be useful for this purpose. All these resources have environmental appeal and are largely available.

Rayon may be considered an array of manufactured fibers composed of regenerated cellulose. According to Sixta et al., (2013), more than 30% of the 4,2 Mton of dissolved wood pulps produced in the world annually were converted into rayon and its byproducts. It is mainly commercialized as four products, which are classified based on the fiber production method (Chen, 2014b), namely:

Viscose rayon: Produced by reacting caustic soda with wood pulp to make alkali cellulose solutionthat is mixed with carbon disulfide to produce sodium cellulose xanthate, which is later dissolved in a weak caustic soda solution. This material is largely pure cellulose (X-ray crystallinity between 55 and 65%) with a low degree of polymerization (300–450) (Kadolph, 2001; Ruan et al., 2004) that produces highly-oriented cellulose chains by wet-spinning (Siller et al., 2014). Viscose rayon is the most important regenerated cellulose fiber, including when used for composite materials (Reinhardt et al., 2013).

Lyocell rayon: Produced by directly dissolving cellulose into N-methylmorpholine-N-oxide (NMMO) in order to reduce several side reactions and the amount of byproducts produced, as well as to effectively recover the high-cost solvents employed (Ruan et al., 2004). Lyocell rayon fiber is among the new generation of regenerated cellulose fibers, due to its eco-friendly character. Lyocell fiber is commonly used in apparel application, paper industry, nonwoven fabrics manufacturing, blend production, etc. (Perepelkin, 2007; Singha, 2012) and its common composition is 50–60% NMMO, 20–30% water and 10–15% cellulose (Rosenau et al., 2001).

Cupro rayon: Produced by dissolving cellulose into a cuprammonium solution and then wet-spinning it to regenerate cellulose. This process, known as cuprammonium hydroxide process, requires the use of high-priced cotton cellulose and copper salts, so it is not competitive with viscose rayon, being produced in a much inferior amount (Gupta, 2007). Compared to Lyocell rayon, it is disadvantageous due to the production of highly toxic by-product waste and pollutants (Fink et al., 2001). Compared to viscose rayon, it has a higher polymerization degree (Eichhorn & Young, 2001) and is useful in certain specialty markets.

Acetate: This is a cellulose-derived fiber rather than a regenerated cellulose fiber. It is produced by acetylating cellulose using acetic anhydride and sulfuric acid. The resulting cellulose acetate is dissolved in acetone and dry-spun into fiber (Imura et al., 2014). Cellulose acetate has very good handling and comfort properties (Rana et al., 2014) and it is generally directed for films, plastics, coatings, cellulose ethers and cellulose powder (Sixta et al., 2013).

Currently, the world production of regenerated cellulose fiber is about 3 Mton per year, accounting for approximately 5% of the global man-made fiber production (Chen, 2014b; Pappu et al., 2015). Because of this, much research has been directed towards investigating CF manufactured from rayon-based precursors (Plaisantin et al., 2001). These precursors are still important nowadays because they are largely available, and have low cost and non-melting character (Li et al., 2007).

Wood is commonly classified as a natural composite material, comprising fibers, parenchymas, vessels, ray cells, drilling plates and pits arranged in three different planes (Evert, 2006). However, only the fibrous cells are significantly used to produce carbon products (Tondi & Pizzi, 2009; Huang et al., 2015).

At the ultrastructural level, wood fibers are built up of four layers, namely, middle lamella (mainly composed of lignin), primary wall, secondary wall and warty layer, which differ mainly in their microfibrils angle (Fengel & Wegener, 1989) (Singha and Thakur, 2009). The wood cell wall is chemically comprised of carbohydrates (65–75%) and lignin (18–35%). Elementary composition of the cell fibers from wood is carbon (≈50 wt%), oxygen (≈44 wt%), hydrogen (≈6 wt%) and other inorganic components (≈1 wt%) (Rowell et al., 2012). Compared to other natural fibers, fibers from wood are considered short and fine, and their morphological characteristics are influenced by factors like planting area, genetic heritability, hormone production, seed origin, weather conditions and silvicultural management (Bendtsen, 1978; Pande, 2013).

Based on anatomical and morphological characteristics, it is possible to classify wood species into:

Softwood: Wood species of the gymnosperm class. Its cell fibers are called tracheids that present mean length and width of about 2000–6000 µm and 20–40 µm, respectively (Dai & Fan, 2014). These anatomical elements represent over 90% of its volume (Wiedenhoeft, 2010).

Hardwood: Wood species of the angiosperms dicotyledones class. It has a much more complex structure than softwood. Its axial system comprises various types of fibrous elements, vessel elements of distinct sizes and arrangements, and axial parenchyma in various patterns and amounts (Wiedenhoeft, 2010). Its fibers represent approximately 50% of the wood volume, with mean length and width of about 1000–2000 µm and 10–50 µm, respectively (Dai & Fan, 2014).

The methods employed for the defibration of wood are called pulping processes. They can be classified in:

Mechanical pulping: The wood logs are lubricated by water and concomitantly pressed against one rotating stone. The friction between them causes two simultaneous mechanisms, the lignin concentrated in the intercellular spaces is softened, and the fibers are forced to separate from each other (Hellström et al., 2008; Hellström et al., 2009). This procedure commonly presents about 92–96% yield (Biermann, 1996).

Chemical pulping: The wood logs are chopped into chips and then treated under high pressure in the presence of chemicals. There are many routes, but the main methods are sulfite and Kraft sulfate (Lourençon et al., 2015). The yield levels obtained are commonly around 40–45% (Biermann, 1996). These processes are traditionally employed to obtain cellulose for paper production but, as previous mentioned, they can be used for lignin production.

Wood is also vastly used as wood flour due to cost and environmental aspects related to the reuse of industrial waste. Wood flour may be defined as a finely ground wood derived from various wood planer shavings, chips, sawdust, and other clean waste wood from saw mills and other wood processing industries (Matuana & Stark, 2015) (Singha & Thakur, 2010).

Cotton has been used since around 5000 BC in India and the Middle East; its annual production is around 25,000 Mton with a growth trend of 2% per annum. Cotton is one of the main materials for textile applications because it combines considerable strength with good absorbency (Dochia & Sirghie, 2012). Cotton is composed of cellulose and hemicelluloses (about 95–97% wt%), along with proteins, pectoses, pigments and small levels of oils, waxes and mineral substances, reaching a crystallinity of about 70% (Chen, 2014a).

Jute is a largely available natural fiber, being second to cotton in amount produced, reaching an estimated annual production of more than 3 Mton (Faostat, 2012). It grows in Southern Asia and it is mostly produced in Bangladesh, India, China and Thailand (Zhou et al., 2013). Jute fiber has been traditionally used for the manufacture of sacks, carpet, twines, and ropes, among others (Roy & Lutfar, 2012a; Chen, 2014b). However, it is also used as reinforcing material in the automotive, construction and packaging industries (Gon et al., 2012). In comparison to other multicellular fibers, jute fibers are known for their greater lignin content. Del Rio et al., (2009) evaluated their chemical composition as: holocelulose (81.6 wt%), Klason lignin (13.3 wt%), acid-soluble lignin (2.8 wt%), ash (1.0 wt%), hydrosolubles (1.0 wt%) and lipophilic extractives (~0.4 wt%).

Ramie, also known as China grass, is native to China, Japan and the Malay Peninsula, where it was reported in as early as 1300 AD, being used for a long period as a textile fiber. It represents the second most important fiber market in the world trade. In China, ramie is one of the main economic crops, reaching a production of 150 Mton of fibers per year, about 96–97% of the world production (Faostat, 2012). Although of tropical origin, ramie was successfully introduced in other environments, appearing suited to temperate conditions, giving satisfactory yields (Kipriotis et al., 2015).

Currently, ramie fibers are used in many niches, including industrial sewing threads, fabrics for household furnishings, high-quality papers, packing materials, fishing nets, parachute fabrics, woven fire hoses, and clothing, among others (Sen & Reddy, 2011; Roy & Lutfar, 2012b). Mohanty et al., (2000) reported ramie’s main constituents as cellulose (68.676.2 wt%), hemicelluloses (13.116.7 wt%), lignin (~0.7 wt%), pectin (1.9 wt%) and wax (0.3 wt%).

Wool, the most commonly used animal fiber, is a textile material mainly composed of insoluble and tough fibrous proteins, such as keratins (Fortier et al., 2012). In its natural state, keratin is present in the form of filaments, which are known as α-keratin fibers. These α-keratin fibers are aligned in the direction of the main fiber axis constituting macrofibrils of about 300 nm diameter, covered by a lipid membrane and linked by a intermacrofibrillar matrix called cell membrane complex (Mura et al., 2015). From an economic point of view, wool demand has been growing in many countries, like China, USA, Japan, South Korea, Italy and UK (Kuffner & Popescu, 2012), and is used in many applications including household textiles, garment materials and accessories (Zhang et al., 2015a). Elemental analysis of wool produces carbon (around 50 wt%), hydrogen (7 wt%), oxygen (22 wt%), nitrogen (16 wt%) and sulphur (5 wt%) (Popescu & Hoecker, 2007).

Chitin is produced from the exoskeleton of shrimp, crabs, squid, lobsters, crayfishes and cuttlefishes, as well from the bone plate of squid (Nwe et al., 2014). Chitin is commonly obtained at laboratory, as well as industrial scale (Rinaudo, 2006). Chemically, it can be described as a linear polymer of β(1–4) linked 2-acetamido-2-deoxy-D-glucopyranose.

Among the polymers obtained from chitin, chitosan (N-deacetylated) is the most important, combining bioresorption, absence of cytotoxicity and low environmental impact during processing (Szymańska & Winnicka, 2015; Thakur & Thakur, 2014a). From the chemical standpoint, it is a linear binary copolymer of (1–4)-linked A-units and 2-amino-2-deoxy-β-D-glucopyranose (GlcN; D-unit) (Badawy & Rabea, 2011). Chitosan is considered more versatile than chitin due to its reactive amino groups at the C-2 positions, which are responsible for the chitosan polycationic nature in aqueous media (Sibaja et al., 2015). Nevertheless, the physicochemical properties of this chitin product depend on its purification, which requires several steps, broadly referred as demineralization, deproteination, discoloration and deacetylation (Tajik et al., 2008). To be considered a chitosan, the chitin needs to present a degree of deacetylation of about 50%. It is traditionally used as antioxidative agent, edible film, and plant fertilizer, among others (Nwe et al., 2014; Trung & Bao, 2015).

1.3 Carbon Fibers from Lignin

After extraction, the lignin powder can be used to produce blends with other polymers, in resins and even for the production of carbon fiber, a very noble application first patented in 1969. The processing for obtaining CF varies but, for polymeric precursors such as PAN or lignin, three basic steps are common: thermal extrusion and spinning, thermal stabilization and carbonization. The graphitization step may also follow.

In the thermal extrusion step, the melting temperature or softening point must be inferior to the decomposition temperature of the lignin. This temperature is controversial in the literature. For Braun et al., (2005), it is around 190 °C, for Kadla et al., (2002) it is around 270 °C. Other researchers argue that degradation occurs within 200–500 °C (Brebu & Vasile, 2010). Above 400 °C, pyrolysis of lignin, decomposition reactions and condensation of aromatic rings occur. This variation may be related to different sources of lignin.

During thermostabilization, the glass transition temperature (Tg) increases and the thermoplastic lignin fiber becomes a thermoset. This step is necessary to promote infusibility of the fiber in the subsequent step (carbonization). The process is generally carried out in an oxidizing atmosphere, usually air due to its low cost (Hayashi et al., 1995). The treatment consists of inserting oxygen cross-links between molecules and incorporating oxidized groups, justifying the increase in Tg (Braun et al., 2005). This is one of the most expensive steps of the process and optimization studies have been carried out to minimize stabilization time and conversion costs, while ensuring the desired properties (Hayashi et al., 1995), i.e., microwave-assisted plasma processing (White et al., 2014).

Fiber quality is compromised if thermal stabilization is performed quickly (Edie, 1998), and low heating rate guarantees oxidation of the functional groups (Braun et al., 2005). For high temperatures and low heating rates, Tg increases faster than the system temperature, keeping the material in the glassy state. For high heating rates, the reactions cannot maintain a Tg higher than the temperature of the system, leading to fiber melting (Braun et al., 2005). It is also important to apply a tension to the fiber in order to limit relaxation of the polymer structure during heating (Edie, 1998). The thermal oxidation process of lignin is complex and still not fully understood (Braun et al., 2005).

During the carbonization step, flat sheets of graphene with high carbon content are produced (Kadla et al., 2002; Hayashi et al., 1995). This step occurs in an inert atmosphere and most of the non-carbonaceous elements in the fiber are volatilized as gases, increasing the amount of carbon-carbon bonds and consequently improving the mechanical, electrical and thermal properties of the fiber (Edie, 1998). The mass loss in this step varies with the precursor, being within 48–51% for lignin (Kadla et al., 2002). The maximum tensile strength is achieved at 1500 °C. Above this temperature, strength decreases even though modulus increases. A high carbonization rate causes structural defects to the fiber, while a low rate causes great loss of heteroatoms. Thus, process optimization is recommended (Huang, 2009).

To improve lignin spinning characteristics, some authors proposed modification of its structure, while others added plasticizers. In fact, lignin-based CF were first developed by Nippon Kayaku in 1969 (Otani et al., 1969). The pilot process consisted of dry spinning lignin dissolved in an alkaline solution with polyvinyl alcohol as plasticizer. Because of the poor mechanical properties owing to lignin with many impurities, the process was terminated (Kadla et al., 2002). In the 1970s, a similar process was patented by Mannsmann et al., (1973), using an aqueous solution of lignin or lignin salts with the addition of polyethylene oxide (PEO) as plasticizer.

In order to obtain CF with good properties, the Oak Ridge National Laboratory suggested that the precursor should have a lignin content higher than 99%, with less than 500 ppm of carbohydrates, less than 5 wt% of volatiles, and less than 1000 ppm of ashes. Therefore, lignin extracted from many sources need to be purified prior to the whole process (Luo et al., 2011), removing impurities that hinder the spooling process. In a recent paper, Nordström et al., (2013a) proposed the use of a ceramic membrane to fractionate black liquor from hard and softwood, reducing the processing temperature.

The lignin molecule is very complex, with different groups, so it is a very hard task to align it in order to promote mechanical properties. Different treatments have been proposed to obtain lignin, such as hydrogenation (Sudo & Shimizu, 1992), acetylation (Eckert & Abdullah, 2008; Zhang & Ogale, 2014), as well as esterification with octanoyl chloride (Lewis & Brauns, 1947) and lauroylchloride (Lewis et al., 1943). The results were considered satisfactory by the authors, reducing the Tg of lignin and increasing the carbon content of the treated lignin and the CF.

The literature presents many papers about lignin-based CF. Some authors use pure lignin from hardwood and softwood (Braun et al., 2005; Ruiz-Rosas et al., 2010; Xiaojun et al., 2010; Foston et al., 2013; Teng et al., 2013; Nordström et al., 2013a; Nordström et al., 2013b; Chatterjee et al., 2014) others use a blend of lignin and polymers, such as Poly(ethylene glycol) - PEG (Lin et al., 2012; Lin et al., 2014), poly(ethylene oxide) - PEO (Schreiber et al., 2014; Dallmeyer et al., 2014a; Dallmeyer et al., 2014b), poly(ethylene terephthalate)/poly(propylene) - PET/PP (Kubo & Kadla, 2005), Poly(vinyl alcohol) - PVA (Ago et al., 2012; Lai et al., 2014a; Lai et al., 2014b), PAN (Maradur et al., 2012; Xu et al., 2013; Xu et al., 2014; Hu et al., 2014; Jin et al., 2014a; Jin et al., 2014b; Oroumei et al., 2015), poly(lactic acid) - PLA (Thunga et al., 2014; Wang et al., 2015). The results are very promising.

Thus, there is a trend, in recent papers, to mix lignin with a polymer to improve the properties of lignin-based CF, especially related to their brittleness and mechanical properties. However, lignin-based CF with structural characteristics are still yet to be realized. An alternative is to manufacture lignin-based carbon mats by using melt-blown system or electrospinning. These techniques allow the production of carbon nanofibers to be used in high temperature processes (e.g., fireproofing fabrics), secondary structures of airplanes, car panels and hoods, boards for electronic devices, and the like. There are some papers in the literature that present very good results, especially related to the fiber spooling process (Ago et al., 2012; Xu et al., 2013; Lai et al., 2014a; Dallmeyer et al., 2014a; Dallmeyer et al., 2014b; Schreiber et al., 2014; Jin et al., 2014a).

The overall challenges in this area include techniques to isolate and purify lignin, reduction in brittleness of lignin fiber, spinning rate during the extrusion process, heating rates inside the oven, carbon yield in the final fiber, and homogeneity of properties considering that lignin comes from different sources.

Carbon fiber that has modulus between 40 GPa and 200 GPa is classified as low modulus and costs less than $22/kg. It can be used for nonstructural applications (Prince Engineering, 2015). For aerospace and military applications, ultra-high modulus (600–965 GPa) CF is required, and its price reaches $55/kg (Prince Engineering 2015). The papers published since the 1990s generally show an evolution in mechanical properties of the produced CF. In their study, Sudo & Shimizu, (1992) obtained lignin CF extracted by steam-explosion of birch wood chips reaching CF yields of about 15.7–17.4% in relation to the starting material. The values reported were: 7.6 ± 2.7 µm fiber diameter, 1.63 ± 0.19% elongation, 660 ± 230 MPa tensile strength and 40.7 ± 6.3 GPa modulus of elasticity.

Ten years later, Kadla et al., (2002), using lignin from soft and hardwood extracted by Kraft and organosolv methods, respectively, published the following values: 63 ± 7 µm fiber diameter, 1.25 ± 0.26% elongation, 339 ± 53 MPa tensile strength and 33 ± 2 GPa modulus of elasticity. Additionally, these authors performed thermal extrusions, with and without PEO, at temperatures between 130 and 240 °C, concluding that the glass transition temperatures (Tg) of the polymer blend with PEO decreased, all organosolv lignin-PEO fibers fused together on thermostabilization, and that hardwood Kraft lignin-PEO fibers with high PEO content were not thermally stable.

Ten years after that, this same research group, through the study published by Qin & Kadla, (2012), analyzed three types of lignin-based CF using organosolv lignin, Kraft lignin and pyrolytic lignin (from bio-oil from wood flour) as precursors. In this case, the authors reported 49 ± 1 µm fiber diameter, 412 ± 39 MPa tensile strength and 41 ± 3 GPa modulus.

Many other important papers in this area have been published recently. For instance, Lin et al., (2012) used lignin isolated from cedar wood by a H2SO4 solution, Zhang & Ogale, (2014) and Thunga et al., (2014) employed softwood Kraft lignin, and Wang et al. (2015), hardwood Kraft lignin. For their lignin-based CF, Lin et al., (2012) reported 11.5 ± 2.0 µm fiber diameter, 2.0 ± 0.5% elongation, 441 ± 100 MPa tensile strength and 23.0 ± 5.4 GPa modulus of elasticity.

For their lignin-based CF, Zhang & Ogale (2014) showed average tensile modulus and strength of 52 ± 2 GPa and 1.04 ± 0.10 GPa, respectively, and concluded that the fibers would be more reactive than those with a graphitic structure, which is beneficial for composites. On the other hand, Thunga et al., (2014) presented CF with high lignin content. They reported around 0.95 µm elongation, 32.0 GPa tensile strength and 32.5 GPa modulus of elasticity. In their study, Zhang et al., (2015) reported a mean strength of 258.6 GPa and modulus of 1.7 GPa. In this work, the authors produced PLA-based blends reinforced by their lignin-based CF and concluded that strength decreased with the increase in PLA content, while the opposite happened to tensile modulus. Their CF presented voids because of the volatilization of PLA during heating.

1.4 Carbon Fibers from Other Sustainable Resources

Cellulose-based CF were developed as early as the 1950s, and several types of natural-based materials can be used as precursors to manufacture commercial grade CF, or at least activated CF. Although traditional lignocellulosic fibers such as ramie, sisal and hemp were used in the past as cellulosic precursor fibers, these fibers are no longer important sources of CF (Chand, 2000; Huang, 2009) (Thakur & Thakur, 2014b). The main constraint of these and other cellulosic precursors that still hinders their use is their low carbon yield.

CF obtained from cellulose precursors amounts to only around 1–2% of the total carbon fiber production (Zhang et al., 2006). However, due to the increasing demand for a more green engineering approach towards a sustainable development, there is a renovated interest in renewable cellulosic CF precursors (Dumanli & Windle 2012).

Rayon, i.e., regenerated cellulose, was the basis for the development of CF in the late 1950s, but the interest in its use declined after the development of PAN-based fibers because PAN allows higher carbon yields and the hot stretching process of rayon fibers is costly (Dumanli & Windle, 2012). Also, PAN-based CF show superior physical properties compared to rayon-based fibers. Tensile properties of rayon-based CF are typically 1.25 GPa for strength and 170 GPa for Young’s modulus (Dumanli & Windle, 2012). Rayon-based CF typically show a very irregular surface, and this could partially account for their comparatively poor mechanical properties (Wu & Pan, 2002), i.e., 2.47 GPa for tensile strength and 221 GPa for Young’s modulus (Naito et al., 2008).

Recently, improvements in processing of rayon-based CF, especially regarding a controlled microstructural change throughout stabilization and carbonization processes, lead to a better set of properties although still very low compared to PAN-based fibers (Park & Heo, 2015). These fibers are, in fact, better tailored to be the source of activated CF for non-structural applications, such as gas absorption applications (Huang, 2009). Park & Heo, (2015) showed that, during pyrolysis, an amorphous structure is generated, destroying the previous crystalline order of the cellulosic precursor, which is later recovered with carbonization under tension. Karacan & Soy, (2013) showed that microstructural control should begin in the very first steps of the treatment process of rayon fibers. They reported that boric acid–phosphoric acid impregnation enhanced thermal stability and minimized volatile by-products during oxidation of the fibers. An increase in char yield was thus obtained. They also mentioned that hot stretching during graphitization influences both structure and properties of these fibers (Zhang et al., 2014).

Lyocell is also a purely cellulosic fiber spun from wood or from cotton pulp (Carrillo et al., 2004). Carbon fibers using Lyocell as a precursor were developed by Wu & Pan, (2002). These fibers are considered a very suitable precursor because they have homogeneous circular cross section, with an average diameter of 10.0 ± 0.9 µm, and a smooth surface. The process to produce CF is similar to that from rayon viscose. According to Kong et al., (2012), a typical methodology would follow these processes: stabilization in air at about 150–200 °C, carbonization at 1000 °C using inert argon atmosphere, and graphitization with coke dust medium at about 2500 °C. Lyocell-based CF with a uniform diameter of 7.5 ± 0.9 µm were successfully produced using a two-stage thermal treatment (i.e., low temperature in air + high temperature in nitrogen), obtaining better mechanical properties than rayon-based CF due to their greater degree of crystallinity, more round cross-sections and less defects (Peng et al., 2003). The tensile strength of these fibers was, however, low (around 900 MPa) compared to common CF, but high enough to be used in several composite parts where low to medium strength requirements are needed.

Bocell is a new thin high-modulus regenerated cellulose fiber with average diameter of about 11.8 µm. It shows Young’s modulus within 46.6–60 GPa (Mottershead et al., 2007; Gindl et al., 2008), higher than that presented by Lyocell (around 23.4 GPa) (Adusumali et al., 2006). Also, Bocell is a high strength fiber, in the 1.17–2.6 GPa range (Mottershead et al., 2007; Gindl et al., 2008). These properties are due to the high molecular alignment obtained by spinning the liquid crystalline solution of cellulose in phosphoric acid (Boerstoel et al., 2001). The high orientation of the final structure obtained at the graphitization step is very attractive and is related to the crystallite size of the cellulose precursor (Kong et al., 2012). Currently, pyrolized fibers present a skin-core structure consisting of a highly graphitized skin and a less graphitized inner core. Using Raman spectroscopy, Kong et al., (2012) indicated that the skin could show a modulus of 140 GPa, against 40 GPa for the core. Although this skin-core structure is detrimental to high-strength, high-modulus applications, Bocell-derived CF are very promising for medium to high strength structural applications.

Several other natural polymers have been investigated as possible precursors for CF. Khan et al., (2007) and Khan et al., (2009) studied silk fibers, which were first treated with iodine vapor at 100 °C for 12 h and heated from 25 to 800 °C in a multi-stage carbonization process. The process conditions were established based on the optimum thermal degradation rate of silk. The carbon yield of the process reached 36 wt%, and fibers with smooth surfaces but with irregular cross sections were obtained. Iodine treated fibers showed better tensile properties than untreated fibers, presenting a tensile strength up to 12.6 gf. Silk fibers could, therefore, be an interesting precursor since the process temperatures are low compared to those usual for PAN-based fibers (Chand, 2000).

Prauchner et al., (2005) used eucalyptus tar pitches to produce CF. A four-step production process was chosen, with a final carbonization step at 1000 °C under nitrogen atmosphere. The obtained fibers had smooth surfaces but large diameter (27 ± 2 µm) and low mechanical properties (130 MPa of tensile strength and 14 GPa of Young’s modulus). The authors suggested the use of these fibers as felts for electrical insulation. It could be highlighted that this precursor is a bio-pitch, available as a by-product of the charcoal industry, and abundant on many countries.

The use of waste wood as raw material to manufacture CF was described by Lin et al. (1995) and Okabe et al., (2005). They processed wood powder to obtain a phenolic resin, and this resin was blended with high density polyethylene (HDPE). The phenolic-HDPE polymer blend was mechanically homogenized and the fibers were obtained by melt-spinning. These fibers were stabilized using an acid solution and then carbonized under inert atmosphere at 1000 °C. During carbonization, HDPE was removed and fibers as thin as 1 µm were obtained. However, the obtained fibers presented a steady fraction of mesopores and were considered worthy only as activated CF.

Another possible natural precursor for CF is chitosan. Bengisu & Yilmaz, (2002) studied the oxidation and pyrolysis of chitosan fibers. The process used a maximum pyrolysis temperature of 700 °C and reached a carbon yield of ~20%, but the tensile strength of the fibers was very low (591 MPa). The authors proposed to stretch the fibers during the final pyrolysis step to enhance their mechanical properties. Schreiber et al., (2014) used chitosan blended with lignin to produce fibers. Polyethylene oxide was used to aid the electro-spinning of the fibers and it was later removed by a washing process. The obtained chitosan-lignin blended fiber showed homogeneous distribution of both components throughout the fiber, and was cited as a feasible carbon fiber precursor.

Wool fibers are also being investigated as possible precursor for CF (Hassan et al., 2015). As with other renewable precursors, a low yield was obtained, varying from 16.7% for an untreated fiber to 25.8% for a chemically treated fiber. Stabilization of the fibers was obtained by pyrolysis at 800 °C, but the tensile properties of treated or untreated fibers was low, varying between 143 to 219 MPa. The CF obtained were also very brittle, with an average tensile strain of 0.5%. The very irregular surface and cross-section of the obtained fiber is perhaps responsible for its low mechanical properties.

For last, Kim et al., (2001) showed that high crystalline native cellulose samples can be conveniently heat-treated to obtain graphite structures following a graphitization step at 2000 °C. A three-dimensional crystalline order was produced when cellulose obtained from the outer shell of sea squirt Halocynthia roretzi (sea pineapple). Long graphite microfibrillar units were also observed when cellulose from the green alga, Cladophora wrightiana, was used as precursor, indicating that certain regions of the original cellulose microfibrils retained their morphology after graphitization.

1.5 Concluding Remarks

The use of sustainable resources as precursor for carbon fibers is an alternative way to obtain less expensive fibers and to promote their use in comparison to glass and polymeric fibers. Regenerated cellulose and lignins from industrial wastes are the most common precursors due to their low cost, high carbon content and abundance. However, others sources such as chitosan, wood, and wool, along with more traditional rayon, presented interesting results.

The mechanical properties of bio-based CF showed improvement in the last ten to twenty years. However, they are still far from the properties obtained for PAN- and pitch-based fibers. In this sense, it is important to further optimize the processing of these fibers, for example manufacturing lignin-based carbon mats by melt-blown systems or electrospinning viscose rayon-based carbon fibers during graphitization.

Also, many natural resources require chemical modification in order to make them appropriate for processing and there may be large variations in the final properties of the carbon fibers derived from renewable resources. These factors undermine their utilization and lead to their use in less-demanding sectors. Nevertheless, a great effort has been devoted to these green carbon fibers and many researchers are working on related subjects. Perhaps these fibers will achieve greater application with more commercial competitiveness in the not too distant future.

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