Noise detection

Exploring efficient ways to extract cellulose crystalline regions from fibers has attracted plenty of attention of authors during the last years, fact that can be observed in the growing number of patents related to this field published since year 2000 (Charreau et al., 2013).

Bengt G. Rånby is thought to be the first author to obtain colloidal suspensions of cellulose crystals by controlled sulfuric acid hydrolysis of cellulose fibers (Rånby, 1951). Previously, Nickerson and Habrle (1947) observed that the degradation induced by boiling cellulose fibers in acidic solution reached a limit after a certain time of treatment. Transmission electron microscopy (TEM) images of dried suspensions revealed for the first time the presence of aggregates of needle-shaped particles, while further analyses of these rods with electron diffraction demonstrated that they had the same crystalline structure as the original fibers (Mukherjee and Woods, 1953). Another milestone in the development of this material was performed by Marchessault et al. in 1959, who observed that colloidal suspensions of nanocrystalline cellulose showed birefringence beyond a critical concentration. Decades later Revol and co-workers demonstrated that this birefringence was produced due to a chiral nematic liquid-crystalline phase (Revol et al., 1992) formed by nanoliquid-crystalline cellulose generated by sulfuric acid hydrolysis. This discovery, together with observation that NCC could strongly improve the mechanical properties of nanocomposites and the general growing interest in using renewable resources for our everyday products (Charreau et al., 2013) produced a renaissance of interest among the scientific community towards this material up to our days (Klemm et al., 2011).

1.4.1. Special features and applications

Nanocrystalline cellulose (NCC), also referred as cellulose nanocrystals, microcrystals, whiskers, nanoparticles, microcrystallites, or nanofibers are rod-like cellulose crystals with widths among 5–70 nm and lengths between 100 nm and several micrometers (Klemm et al., 2011) (Figure 1-8). The interest in producing NCC lies on its physicochemical characteristics making it a very promising material for several applications. Firstly, the already explained chiral nematic liquid crystal behavior

confers it special optical properties. Other properties strongly depend on the used raw material used and the preparation procedure. However, generally it could be said that NCC has a high degree of crystal structure (more than 70%) (Fan and Li, 2012), a very high aspect ratio, i.e. length-to-diameter ratio (up to 300) (Tanaka et al., 2014), a large surface area (above 150 m²/g), a very high elastic moduli, (estimated to be over 130-150 GPa) (Sakurada et al., 1962) and a low thermal expansion coefficient (6 ppm/K) (Hori and Wada, 2005).

Nanocrystalline cellulose finds applications in many sectors. One of the most studied applications of this material is its use as filler in nanocomposites, in order to improve their mechanical, thermal and barrier properties. There are several references addressing this topic in bibliography (Hamad, 2006; George et al., 2011; Moon et al., 2011; Brinchi et al., 2013; Mariano et al., 2014). Synthetic polymeric matrixes (such as polypropylene, PP or polyvinyl chloride, PVC) and natural ones (such as starch, or polylactic acid, PLA) have been used for templating with NCC in a vast number of examples existing in bibliography.

Regarding mechanical properties, NCC presence has been found to increase tensile strength, young modulus and elongation break of nanocomposites (Cao et al., 2008). Thermal properties of NCC containing nanocomposites have also been studied.

Thermal degradation, or the reduction of mechanical properties at high temperatures are among the major problems limiting NCC application as fillers in nanocomposites (Moon et al., 2011). However, the improvement of glass–rubber transition temperatures (Tg), melting point (Tm), and thermal stability has been investigated by several authors through differential scanning calorimetry (DSC) (Brinchi et al., 2013).

In some cases, thermal performances of nanocomposites showed improvements after addition of nanocellulose. Furthermore, barrier properties of materials are very important, for example, for meeting safety regulations in packaging products. Several works have been carried out investigating this property. Saxena and Ragauskas 2009, found that water permeability of xylan/NCC nanocomposites decreased considerably when adding and increasing NCC content on the composite. Morevocer, optical properties of nanocomposites (e.g. optical transmittance) are often properties aimed to be preserved, regardless of the gain in mechanical properties provided by the filler.

Studies using NCC have demonstrated that the gain in mechanical properties by a nanocomposite did not necessarily imply a loss in light transmittance, due to the

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unique optical properties shown by this material (Moon et al., 2011). Finally, the fact that NCC are usually electrically charged on surface allows their use in biomedical applications, due to the possibility of electrostatically absorb enzymes or proteins, or for drug delivery uses (Lin and Dufresne, 2014).

1.4.2. Cellulose sources and dimensions

Cellulose sources for NCC preparation may vary, and their degree of crystallinity strongly conditions the further dimensions of the released crystals. Sources such as wood, cotton or Avicel yield a narrow distribution of highly crystalline (90%

crystallinity) nanorods with: 5–10 nm in width and 100–300 nm in length (Dong et al., 1996; Beck-Candanedo et al., 2005; Elazzouzi-Hafraoui et al., 2008). Other sources, such as tunicin (extracted from tunicates, sea animals producing cellulose), bacterial cellulose, and algae, generate NCC with larger polydispersities and dimensions (width 5–60 nm, length: 100 nm up to several micrometers) (Yoshiharu et al., 1997; Elazzouzi-Hafraoui et al., 2008; Hirai et al., 2009). NCC obtained from these sources appears to be similar to NFC in size terms (Turbak et al., 1983). However, in contraposition to NFC, they have very limited flexibility, as they do not contain amorphous regions but instead exhibit elongated crystalline rodlike shapes.

Figure 1-8: SEM micrograph of a NCC film showing chiral nematic organization, from Majoinen et al., 2012, (A). NCC film showing birefringence from Kelly et al., 2013 (B).

A B

1.4.2.1. High-cellulose content fibers

As explained in previous sections, non-wood cellulose sources are a promising source for high-quality fibers with a broad range of environmental advantages.

However, their usage as cellulose source for NCC requires the previous elimination of non-cellulosic components. Hemicelluloses are an undesirable impurity in pulps intended for production not only of nanocellulose but also of viscose rayon or other derivatives, as they interfere with chemicals during manufacturing processes, reducing their yield and also quality of final product.

Concerning NCC in particular, high-cellulose content fibers are the preferential source for its production due to a series of reasons. In the first place, other components of lignocellulosic biomass such as hemicelluloses or lignin are known to hinder the acid-cellulose interaction, modifying the kinetics of the acid hydrolysis reaction and thereafter reducing its efficiency (Yoon et al., 2014). Secondly, hemicelluloses or lignin presence in biomass would reduce the NCC yield of the reaction as they are non-cellulosic components. Lastly, hemicelluloses presence in NCC has been related to a decrease in their quality, for example, by increasing their thermal degradability (Jonoobi et al., 2015).

Traditionally, pulps with low hemicelluloses content have been obtained through acid sulphite or pre-hydrolysis Kraft processes (Li et al., 2015). On them, hemicelluloses that are present on fibers suffer a stronger attack than during alkaline processes such as Kraft or NaOH-AQ, reducing their presence on final product.

Generally, pulps obtained through acid processes have some drawbacks related to quality of final product or the pollution they generate. Also, these they imply higher costs than alkaline ones in terms of chemical consumption, production rate, inventories and storage space (Barlow and Hillman, 2006). For these reasons, several methods have been studied in order to carry out the selective elimination of hemicelluloses from alkaline pulps (Jackson et al., 1998; Bajpai and Bajpai, 2001; Kopcke et al., 2008). These methods include nitren, cuen and alkaline extraction or enzymatic hydrolysis (Ibarra et al., 2010; Quintana et al., 2013). Also among these alternatives, the use of biotechnology by means of enzymatic hydrolysis of different components of lignocellulose has attracted special attention because of its potential as a “green”

process by virtue of its high specificity and environmental friendliness.

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1.4.3. NCC Preparation

The most extended method for preparation of nanocrystalline is controlled hydrolysis using mineral acids (Beck-Candanedo et al., 2005; Habibi, 2014). During this hydrolysis polysaccharides bound at the fibril surface are removed first and then more readily accessible amorphous regions are cleaved and destructed in order to liberate rod-like crystalline cellulose sections (Klemm et al., 2011). This differentiated susceptibility to acid attack is thought to be provoked by differences in the kinetics of hydrolysis between amorphous and crystalline domains, where the first ones are more rapidly accessible by acid and thereafter, hydrolyzed first (Habibi et al., 2010). When a desired depolymerization level is reached, hydrolysis reaction is usually quenched by diluting the acid concentration with distilled water and by reducing sample temperature (Klemm et al., 2011). After this, residual acid is eliminated washing samples by centrifugation. Hydrolysis is usually followed by a mechanical process, typically sonication, which disperses the nanocrystals until a uniform stable suspension is formed. Finally, samples are extensively dialyzed against distilled water for complete acid elimination.

Hydrolysis reaction strongly influences structure, properties and phase-separation behavior of nanocrystalline cellulose suspensions. The type of mineral acid used, its concentration; and hydrolysis time and temperature are key parameters determining final properties of NCC (Bondeson et al., 2006; Fan and Li, 2012;

Kargarzadeh et al., 2012; Chen et al., 2015). NCC produced with acids such as phosphoric, hydrochloric (HCl) or hydrobromic (HBr) present a low colloidal stability (Araki et al., 2000; Habibi et al., 2010; Beck et al., 2015), whereas the use of sulfuric acid leads to well-stable suspensions produced by strong electrostatic repulsions between negatively charged NCC. This electrical charge, only present in NCC obtained with sulfuric acid is explained by a side reaction occurring during hydrolysis with this acid. In this reaction, sulfate (SO^42-) moieties are incorporated onto NCC surface throughout an esterification with free OH- groups of cellulose (Abitbol et al., 2013;

Beck et al., 2015). Besides the improvement in suspension stability, sulfur content has been related to a higher NCC thermodegradability (Roman and Winter, 2004). Sulfate groups also influence, among other characteristics, the properties NCC could confer to composites if used as fillers. Recently, new NCC preparation methods have been

reported, such as a hydrolysis with phospotungstic acid followed by and extraction and purification (Liu et al., 2014), hydrolysis using a cationic-exchange resins (Tang et al., 2011) and biotechnology, obtaining NCC trough enzymatic hydrolysis (Anderson et al., 2014).

Generally NCC preparation procedures have been characterized by low yields.

Studies such as those performed by Bondeson et al. (2006), Chen et al. (2015) or Fan and Li (2012) addressed this issue by studying different reaction conditions in order to increase yield. With this same objective and also with the aim of reducing the environmental impact of NCC manufacture it was considered that the use of biotechnology could be an effective tool to improve the efficiency of NCC isolation.