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: amico@ufrgs.br

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:

1. 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).
2. 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.
3. 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),...