Deep eutectic solvents in the pre-treatment of Kraft eucalyptus pulp for the pre-production of nanocellulose

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Faculty of Science and Forestry

DEEP EUTECTIC SOLVENTS IN THE PRE-TREATMENT OF KRAFT EUCALYPTUS PULP FOR THE PRE-PRODUCTION OF NANOCELLULOSE

Pedro Viriato Parigot de Souza Neto

MASTER’S THESIS

WOOD MATERIALS SCIENCE

JOENSUU 2019

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Pedro Viriato Parigot de Souza Neto, 2019, Deep eutectic solvents in the pre-treatment of Kraft eucalyptus pulp for the pre-production of nanocellulose, Faculty of Science and Forestry, Wood Materials Science, 44 p.

ABSTRACT

The aim of this study was to investigate how commercially available bleached, unbleached and bale dried Kraft eucalyptus pulps pre-treated with three different deep eutectic solvent systems (DESes), in practice combinations of choline chloride and lactic acid, choline chloride and ethylene glycol, lithium chloride and urea, and what kind of morphological and chemical changes occur when treated pulps were further sonicated with a predetermined and equal energy.

Eucalyptus pulp provided by Fibria, Brazil, was washed with ethanol and oven dried at 60 °C.

The DES systems were prepared by bath heating at 90°C until a clear, colorless liquid was formed. Pulp was then added at a concentration of 1% (w/w) and stirred for 2 hours.

Succinic anhydride was also added for half the samples. Then the beaker was removed from the oil bath and the samples washed with ethanol. After filtration, samples were diluted to a consistency of 1%, pH was adjusted to 8.0, and samples diluted to 0.5% and sonicated using a probe type high-intensity ultrasound with 200 kJ. Analyzes done were Valmet FS5 fiber morphology analyzer, FTIR and XRD.

Results show that choline chloride and lactic acid (CC:LA) was the most efficient DES system used, bringing fiber width and length down and increasing the % of fines. FTIR analyses indicate that both CC:LA and lithium chloride and urea (LC:urea) chemically changed samples, indicated by the esterification reaction, where choline chloride and ethylene glycol did not. Calculated crystallinity indexes did not change significantly confirming that crystallites remained intact. Choline chloride and lactic acid DES system shows great promise on the pretreatment of fibers for nano and micro fibrillated cellulose.

Key words: deep eutectic solvents, sonification, eucalyptus, nanocellulose

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ACKNOWLEDGEMENTS

I could not have completed this thesis and earned the title of Master of Science without strong support and motivation. First of all, my parents, who supported me with love and understanding. And secondly, my friends and colleagues, each of whom has provided patient advice and guidance throughout the duration of my studies. Thank you all for your unwavering support.

Special thanks to my supervisor, Associate Professor Antti Haapala, for his time and support.

For the people in Fibria, especially to Paulo Pavan, who gave me the opportunity to visit the company and suggested this research topic. And last, but not least important, to all Finnish citizens who worked hard to finance my studies and gave me the opportunity to live and study in this magnificent country.

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ABSTRACT CONTENTS

1 Introduction ... 6

2 Literature review ... 8

2.1 Cellulose ... 8

2.1.1 Characteristics and refining of cellulose... 9

2.1.2 Eucalyptus as a source of cellulose pulp ... 11

2.1.3 Nanocellulose as a material ... 12

2.1.4 Nanocellulose production ... 14

2.2 Deep eutectic solvents ... 19

2 Materials and Methods ... 22

2.1 Feedstocks and pre-treatment cellulose pulps ... 22

2.1.1 Eucalyptus pulps ... 22

2.1.2 Deep eutectic solvents ... 23

2.1.3 High-intensity ultrasonic fibrillation ... 24

2.2 Analyses ... 25

2.2.1 Fiber morphology ... 25

2.2.2 Crystallinity of cellulose by X-ray diffraction ... 26

2.2.3 Fourier Transform Infrared Spectroscopy ... 26

3 Results ... 27

3.1 Fiber morphology ... 27

3.1.1 Untreated pulps ... 27

3.1.2 Choline chloride and lactic acid DES system ... 27

3.1.3 Choline chloride and ethylene glycol DES system ... 28

3.1.4 Lithium chloride and urea DES system ... 29

3.2 Crystallinity of cellulose by X-ray diffraction ... 29

3.3 Fourier Transform Infrared Spectroscopy ... 32

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4 Discussion ... 35

4.1 Fiber morphology ... 35

4.1.1 Untreated vs. treated fibers ... 35

4.1.2 DES component mixing ratio ... 36

4.1.3 Presence of succinic anhydride in the DES ... 36

4.2 Crystallinity of cellulose by X-ray diffraction ... 36

4.2.1 Untreated vs. treated fibers ... 36

4.2.2 DES component mixing ratio ... 37

4.2.3 Presence of succinic anhydride in the DES ... 37

4.3 Fourier Transform Infrared Spectroscopy ... 37

5 Conclusions ... 39

6 References... 40

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1 INTRODUCTION

Micro- and nanocelluloses are novel renewable materials made of woody biomass although it has been shown that such materials can be manufactures from virtually all possible virgin and recycled material that contains cellulose. They can be combined with many different materials to change desired properties and hence there is significant interest in scientific community to investigate processing methods that might make it faster and cheaper to produce these materials. Uses include paper industry, composite manufacturing, food industry, as it is safe for human and animal consumption and many more. Some of these uses are possible since nanocellulose has unique characteristics like huge surface area, high aspect ratio and high Young’s modulus.

Micro- and nanoscale celluloses are commonly manufactured from chemically pretreated pulp which is mostly made of cellulose, although some amount of hemicelluloses, lignin and extractives residue are most often included. Mostly the manufacturing process consists of chemical and mechanical forces and in many cases the combination of them. Mechanical disintegration requires an intensive amount of energy, hence chemicals can be used to help this process. Common equipment for this are microfluidizer, homogenizer and high intensity ultrasonication.

This thesis aims to investigate how the performance of selected deep eutectic solvents (DES) systems impact the pretreatment of three different eucalyptus pulps of different purity and processing history: never-dried bleached, never-dried unbleached and bale dry sheets.

DESes have been used to help release fiber bundles and microfibrils in other studies, maybe because they increase fiber swelling, so it is expected that fibrillation will be stronger in samples treated with these solvents, thus decreasing fiber dimensions and crystallinity after sonication. Likewise, it is expected that chemical changes should happen, indicated by different peaks shown on the FTIR spectra results.

The responses to pre-treatments to three different DESes, choline chloride-ethylene glycol (CC:EG), choline chloride-lactic acid (CC:LA) and lithium chloride-urea (LiCl:urea), either with or without succinic anhydride present was characterized by Valmet FS5 analyzer via the dimensional changes – swelling and shortening – of the fibers. Also, changes in the pulp’s

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chemical and physical structure were studied using Fourier transform infrared spectroscopy (FTIR) and x-ray diffractometer (XRD).

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2 LITERATURE REVIEW

2.1 Cellulose

Cellulose is by far the most abundant renewable polymer on earth (Dufresne 2010). It is the main component of plant cell walls and it is responsible for the structural strength of plants and can also be synthesized by animals, bacteria and fungi (Heux 1990). The other main components of plant cell wall are hemicelluloses, which in contrast to cellulose are branched heteropolymers, and lignin, which is an aromatic branched polymer base on hydroxyphenylpropane. Pectin, pigments and extractives are found in small quantities (Satyanarayana et al. 1990).

Cellulose is a linear polymer consisting of β-D-glucopyranose units linked together by β-1-4- linkages (Brännval 2007). The degree of polymerization varies widely depending on the specie and even inside the same plant and can present values higher than 20.000 glucopyranose units for certain tree species (Daniel 1985). Microfibrils are the basic structural component of cellulose and have diameters ranging from 2 to 20 nm and several tens of micrometers in length (Azizi et al. 2005) as seen in Fig. 1.

Figure 1. Scheme of the cellulose cell wall and microfibril organization (U.S. Department of Energy 2005).

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2.1.1 Characteristics and refining of cellulose

Cellulose is embedded in a matrix of other polysaccharides, such as hemicelluloses, and lignin. The gaps between microfibrils are occupied with hemicelluloses. These hemicelluloses are heterogeneous polymers which represent between 15-35% of plant biomass and are composed of pentoses, hexoses and uronic acids (Girio et al 2010).

Cellulose is routinely referred to as a natural fiber, therefore it is important to clarify that these fibers are in fact tracheids in softwoods and libriform fibers in hardwoods (Satyanarayana 2007). Plants cell wall is made of different layers as shown in figure 2. The separation process of these fibers from the starting material, for example wood, is called pulping. In gymnosperms, or conifers, softwoods, the cells of the axial system are most frequently tracheids and in angiosperms, hardwoods, the presence of fibers and vessel elements are more common. Other cell types include vessel-associated cells, parenchyma and different ray cells. Ninety percent of the wood is made of tracheids, and the remainder is composed of ray parenchyma and longitudinal parenchyma cells (Plomion 2001). For the pulp and paper industry, it all comes down to fibers, even though in angiosperms, they are in fact tracheids, therefore in this study the term fiber is adopted. In other fields of study, such as biology and wood anatomy, this simplification is not valid.

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Figure 2. Cellulose structures in trees from logs to molecules (Moon 2008).

There are basically four methods for separating fibers from lignocellulosic materials:

mechanical, chemi-mechanical, semi-chemical and chemical pulping. Mechanical pulping processes, as the name suggests, use only mechanical forces applied usually by grinding to separate fibers. In this process no chemical besides water or steam is added to the process.

It has a very high yield of 90–98% and all lignin is retained in the pulp. Chemi-mechanical pulping method consists of two stages: a mild chemical treatment followed by mechanical forces. In this method the yield is also very high, around 85–95%, lignin is retained but small amounts of hemicellulose and extractives are lost. Semi-chemical pulping process consists of impregnating wood chips with chemicals and cooking it in steam atmosphere. After that the chips are mechanically refined to complete pulping action. This process has medium yield of 60–80%. Chemical pulping process consists of reacting chips with chemicals at high pressure. Most lignin and hemicellulose are dissolved and removed, causing the yield to be lower than other methods, but fibers remain largely intact. (Biermann 1996)

The most used chemical pulping method is the Kraft pulping, which preserves most of the cellulose, around half of hemicelluloses and some lignin in the fiber material. Several variations exist today, using different types of additives or process parameters such as

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temperature and pressure, resulting in pulps with different qualities for specific end uses (Lindström 1992). In this study, only eucalyptus Kraft pulps were used, thus further discussion will focus on this product and process.

Kraft pulping method, or sulfate process, consists in adding sodium hydroxide and sodium sulfide to wood chips in equipment often called digester, in which hemicelluloses and lignin are extracted to an extent. The presence of sodium sulfide improves efficiency but it is not essential. The effect of the reaction is to depolymerize the lignin and hemicelluloses molecule. An essential factor that contributes to the success of the Kraft method is that the chemicals can be easily recovered. The black liquor, which is the mixture of inorganic salts and the solubilized part of wood, is concentrated and combusted. Remaining inorganic ash if then dissolved in water, calcium hydroxide is added to precipitate out calcium carbonate and to convert the sodium carbonate to sodium hydroxide for reuse. Calcium carbonate is separated by sedimentation and is combusted to give calcium oxide, which provides the calcium hydroxide which is used in the precipitation process (Roberts 1996). Surface structure of Kraft pulps fibers corresponds mainly to the primary and S1 layers of the fiber wall, which are conserved during this type of pulping. Opposing to the outer layers of the fiber wall, the S2 layer has a microfibrillar structure arranged in a helical manner (Heyn 1969).

Many different species of plants, and even organisms such as bacteria, have been used to obtain cellulose. In the pulp and paper industry, the main tree species of hardwoods are acacia, aspen, birch, beach, ash and eucalyptus. For softwoods, they are loblolly pine, slash pine and spruce. Other possible sources are tunicate, straw, cotton, bacterial cellulose, bamboo, sisal and hemp (Tonoli 2012).

2.1.2 Eucalyptus as a source of cellulose pulp

In the genus Eucalyptus, there are around 500 species, and only two of these are not found in Australia, which are Eucalyptus urophylla, found in Timor and E. deglupta, found in Papua New Guinea. Some species are bush-like and others very tall, with crowns soaring up to 90 m tall and trunks to 6 m in diameter (Kelly 1993). Some species of Eucalyptus are fast growing, with good fiber qualities and relatively cheap market price (Campinhos 1999).

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Genetic improvements have been done for the past decades to achieve high productivity Eucalyptus hybrids that yield desired fiber characteristics and higher wood density. Basic density is usually the most important aspect that researches try to improve (Lima 2000).

This characteristic is under strong genetic control in the genus Eucalyptus (Raymond 1995).

The vast majority of Eucalyptus planted today for cellulose production is comprised of hybrids of E. grandis x E. urophylla, which is commonly referred to as E. urograndis.

The potential of short fiber wood, like Eucalyptus species, for nanocellulose production is receiving more attention in the past years. Among kraft pulps, Eucalyptus kraft pulp, is the most abundant pulp in Brazil and is becoming increasingly available in other countries. The adequate selection of starting material can reduce drastically the energy consumption in nanocellulose production. Euca pulps are composed of shorter fibers and higher hemicellulose content than the other widely available pulp in Brazil, which is pine pulp (Campinhos 1999). As Iwamoto (2008) describes high contents of hemicelluloses and can facilitate the release of nanofibrils (Iwamoto 2008).

2.1.3 Nanocellulose as a material

Authors often classify nanocellulose in two categories, microfibrillated cellulose (MFC) and cellulose nanocrystals (CNC) although other categories are also used, such as bacterial cellulose. The term nanofibrillated cellulose (NFC) is sometimes used interchangeably with MFC, although differences exist in respect to size. About 36 individual cellulose molecules are combined into larger units to form elementary fibril, also called microfibril, which in turn are packed into larger units called microfibrillated cellulose (Dufresne 2008).

Cellulose microfibrils are the basic structural component of cellulose. Individual microfibrils can be considered as a string of cellulose crystals linked along the microfibril axis by amorphous regions (Azizi et al. 2005). The main portion of cellulose is formed by crystallites.

It can be classified in cellulose I which is the crystalline cellulose and cellulose II, which is regenerated cellulose, or cellulose that has been precipitated out of solutions (Daniel 1985).

Cellulosic fibers’ properties are influenced by factors such as chemical composition, cell morphology, location in the plant, microfibril angle and others (Dufresne 2008).

The diameter of a microfibril is considered to be about 5 nm and of the MFC ranges from 20 to 50 nm and has several micrometers in length (Lavoine et al. 2012). Microfibrils (figure 3)

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are considered as a flexible strand with cellulose crystals linked along the axis by disordered domains (Azizi 2005). The crystalline regions are ordered in cellulose packages that are stabilized by strong and complex network of hydrogen bonds that look like rods (Habibi 2010). Cellulose nanocrystals (CNC) can be also referred to as, nanocrystalline cellulose, cellulose whiskers, microcrystals, etc. In this study the term cellulose nanocrystals will be used.

Figure 3. Structure of wood pulp fibers: (a) Network of microfibrills, (b) Cross section of a fiber, (c) Microfibril in the S2 layer (Reproduced from Chinga-Carrasco 2011).

The characterization of nanocellulose can be done using many different methods based on the parameters that are interesting for each study. Properties such as size, surface charge,

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optics, rheology and crystallinity can be determined using different procedures. For dimensions and size distributions, electron microscopy (SEM, TEM) and atomic-force microscopy (ATM) can be used. Crystallinity can be accessed by X-ray diffraction (XRD) and nuclear magnetic resonance (NMR). Degree of polymerization is usually measured using viscosity.

2.1.4 Nanocellulose production

The first researchers to identify crystalline zones in cellulose materials were Nageli and Schwendener in 1870. Many decades later Rånby and Ribi (1950) were able to produce a stable suspension of colloidal cellulose crystals using wood and cotton by sulfuric acid hydrolysis. The nanocrystals were 50–60 nm in length and had a diameter of 5–10 nm. Since then a growing number of researchers have contributed to the production and analyses of such material (Lavoine 2012).

The main production process of cellulose nanocrystals (CNC) is based on strong (sulfuric) acid hydrolysis and further dialysis under controlled conditions of temperature, agitation and time. The disordered domains, or amorphous regions, which can be considered as structural defects, are attacked leaving the crystalline regions intact (Lavoine 2012). These fibers are rod-like, very stiff, have a small size distribution and are much smaller than MFC (Habibi 2010).

As shown in Figure 4 the routes to produce CNC and MFC are different. Cellulose microfibrils (CMF) obtained by mechanical disintegration from wood was first achieved by Herrick et al.

and Turbak et al., both in 1983. MFC is constituted of high-volume cellulose, moderately degraded and greatly expanded in surface area (Nakagaito 2005). Differently than CNC, MFC is long, flexible and is composed of individualized microfibrils that are around 10–100 nm in diameter and several micrometers in length (Andresen 2006). MFC comprise both amorphous and crystalline cellulose regions and presents a web like structure (Lu 2008).

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Figure 4. The main steps involved in the preparation of cellulose nanoparticles (Siqueira et al. 2010).

Microfibrillated cellulose can be manufactured from wide variety of different cellulosic sources and using different production methods. Currently, wood is the main raw material used to produce MFC with Kraft bleached pulp being the most used starting material (Habibi 2010). Other starting materials include sugar beet pulp, wheat straw, soy hulls, sisal, sugarcane bagasse, bamboo, palm trees, carrots, etc. (Lavoine 2012).

Even though microfibrils are the main constituent of microfibrillated cellulose, several studies have shown that fibrillation produces a material which is heterogeneous, containing fibers, fiber fragments, fines and fibrils. The treatment applied to the fibers before fibrillation, the fibrillation method or equipment and other parameters chosen such as pressure and temperature will affect directly the fraction of each component. Higher degree of fibrillation can be indicated by an increase in the transparency of the final material (Herrick 1983).

The first mechanical treatment was the Gaulin homogenizer (Figure 5), which is still used today (Lavoine 2012). It consists of pumping cellulose slurry with high pressure through a

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spring-loaded valve assembly, which opens and closes in fast succession making the fibers subject to a large pressure drop under high shearing forces (Turbak 1983).

Figure 5. Operating principle of a spring-loaded valve homogenizer (Visanko 2015).

The microfluidizer (Figure 6) is another mechanical treatment to produce MFC and has been developed more recently. In this process, the cellulose slurry passes through thin z- or y- shaped chambers under high pressure. The shear rate applied is very high resulting in the production of very small fibers. This equipment makes it possible to produce more uniformly sized fibers (Siqueira 2010).

Figure 6. Schematic presentation of microfluidizer equipment (Microfluidics Inc. 2018).

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The production of nanocellulose is typically performed by the addition of two or more methods, such as refining and high-pressure homogenization (Nakagaito 2005). These mechanical methods cause irreversible changes in the fibers, modifying their morphology and size. Also, they tend to damage in some part microfibril structure by reducing molar mass and degree of crystallinity (Henriksson et al. 2007).

Another method to produce MFC is based on a grinding process. Two grinding stones, one rotating and the other static, generate shear forces that break the fibers into smaller pieces.

This method is not very used since it degrades fibers and decreases their length (Lavoine 2012). One common example is shown on figure 7. The Masuko Supermasscolloider creates a high shear zone that helps to liberate nanofibers present in natural lignocellulosic fibers.

Figure 7. Ceramic grinding stone used in Masuko Supermasscolloider (Masuko Inc. 2018).

High-intensity ultra-sonication is another suitable mechanical method to produce nanocellulose, although it consumes a lot more power and hence is not very interesting for industrial scales. Ultrasound is a part of sound spectrum that ranges between 20 kHz and 10 MHz. This technique uses sound energy to disintegrate fibers by producing bubbles in the

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medium. High-intensity waves can produce a strong oscillating power, which forms, expands and implodes microscopic gas bubbles. These bubbles implode and produce high amounts of energy, a phenomenon denominated cavitation. Ultrasonic radiation is hence used in many processes, including emulsification, catalysis, homogenization, disaggregation, scission, and dispersion (Osong 2015). One method used to fibrillate cellulosic fibers is shown on figure 8.

Figure 8. One viable High-intensity ultra-sonication method for making nano cellulose (Reproduced from Chen 2011).

Sonication has been used for many different chemical reactions involving carbohydrates, such as hydrolysis and depolymerisation, including cellulose (Kardos 2001). Modification of cellulosic fibers using ultrasound has also been studied. Sonication in papermaking has been reviewed by Thompson and Manning (2005). Its effect on pulp fibrillation has been reported by Iwasaki et al. (1962). Findings in these studies include fiber morphology modification, in which, it could take place in four stages: deformation of cell wall, removal of S1 layer, swelling of the fibers and fibrillation.

Nanocellulose production requires a high amount of mechanical energy and therefore finding a way to reduce it could make this material cheaper and consequently broaden the horizons for its industrial scale uses. The high energy requirement limits nanocellulose application and the fibrillation of cellulosic fibers is a challenging process to improve. Using purely mechanical disintegration processes produces thick microfibril bundles despite the high energy input. For these reasons, different pretreatments have been used such as

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mechanical cutting, acid hydrolysis, enzymatic pretreatment and introduction of charged groups through carboxymethylation or TEMPO-mediated oxidation (Dufresne 2013).

Conventional chemicals used in the pretreatment of cellulosic fibers for nanocellulose production are often toxic, non-biodegradable and expensive. Novel pretreatment methods have been recently developed to overcome the limitations of conventional chemicals. Deep eutectic solvents have been recently used and they have been recognized as a potential, cheap and green solvent for this mean (Sirviö et al. 2015).

2.2 Deep eutectic solvents

Deep eutectic solvents (DESes) are considered as a new class of ionic liquid (IL) analogues since they have many characteristics and properties in common with ILs. DESes are systems formed from a eutectic mixture of Lewis and Brønsted acids and bases, which can contain a variety of anionic and cationic species that can form a synergic effect with each other and act as solvents, reactants and catalysts. Compared to ILs, research into DESes is still in the beginning with the first paper being published only in 2001 by Andew P. Abbott (Smith 2014). Many components used in DESes are biodegradable, have low vapor pressures and have relatively low toxicity, all of which are aspects that fit in the 12 Principles of Green Chemistry, elaborated by Anastas and Warner (1998).

A DES system is a fluid generally composed of two or more components that are capable of association, often through hydrogen bonding, to form a eutectic mixture that has a melting point lower than that of individual components (Zhang et al. 2012). DESes are easy to produce by simply mixing common commodity chemicals such as choline chloride and urea and the mixtures are effectively eutectics made using two components, but different than most molecular eutectic mixtures, the interaction between the ammonium salt and the hydrogen-bond donor results in a very large depression of the freezing point (Abbott 2006).

Several combinations of components forming DESes have been described, the most popular and studied examples are mixtures that involve choline chloride with urea, ethylene glycol, generally in a 1:2 molar ratio (Wagle 2014).

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Abbott et al. (2003) coined the term Deep Eutectic Solvent initially to describe eutectic systems composed of urea and choline chloride. Other studies later extended the range of DESes by the combination of metal halide salts with and without hydrates, quaternary ammonium salts and hydrogen bond donors (Abbott 2007). Hydrogen bond acceptors (HBA) contain a cation and an anion. The charge delocalization results from forming hydrogen bonding, which leads to a lower melting point of the mixture, when compared to HBD and HBA alone (Abbott 2001). The interaction between anion and hydrogen bond donors, as well as the individual lattice energies of HBD and HBA, are mainly responsible for the low melting point of DES systems (Harris 2009).

DESes are biodegradable, cheap and easy to prepare, which helps explain its many different uses. Applications range from material preparation, where DESes are used as dispersion media, urea synthesis, in which DES is used to carry out specific reactions and in separation processes of gas or liquid (Abbott 2007). Advances in the use of deep eutectic solvents to generate nanoscale materials are being made. As Zhang (2012) reviews, DESes aspects and properties include benign media for biotransformations, organic synthesis, biodiesel preparation and polymer synthesis. DESes are recognized to dissolve many highly polar species, e.g., metal salts, amino acids, glucose, citric acid, benzoic acid and ethylene glycol, and as such, show promise for biomass processing in the dissolution of many biopolymers (lignin, chitin, cellulose, starch) and pretreatment of cellulosic biomass (Zhang 2012).

As early as 1934, Graenacher discovered that molten N-ethylpyridinium chloride, in the presence of nitrogen-containing bases, could be used to dissolve cellulose. This might be the first example of cellulose dissolution using ionic liquids. Though by that time, it was thought of little practical value. Until a couple of decades ago, the dissolution of cellulose in ionic liquids was of little study. (Zhu 2006). In 2003, Swatloski concluded that cellulose can be dissolved and regenerated from ionic liquids. The regenerated cellulose has almost the same degree of polymerization and polydispersity as the initial one. In his study, Swatloski found that morphology is significantly changed and microfibrils of the regenerated cellulose are fused into a relatively homogeneous structure. Regenerated cellulose varies depending on the processes used to solubilize and regenerate it. Properties such as degree of crystallinity can be manipulated during these processes (Swatloski 2003). Zhang has also conducted extensive research in this field (Zhang 2012).

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Wood, which is composed of several hardly soluble polymers, is also being investigated as a material to be solubilized using ionic liquids. In 2007 Fort reported eucalyptus, oak, poplar and pine wood (figure 8) was partly dissolved using [C4mim]Cl and DMSO at 100°C. The dissolving degree was about 70% (w/w). The extract material was a mixture of polysaccharides and lignin (Fort 2017). Also in 2007, Kilpeläinen reported on wood dissolution by [C4mim]Cl and [Amim]Cl. He treated Norway spruce sawdust and southern pine thermomechanical pulp at temperatures between 80 and 130°C for 8 and 13 h, respectively, and observed that biomass was partially dissolved (Kilpeläinen 2007). Wang, using a room temperature IL, [Amim]l to extract cellulose-rich material from pine, poplar, Chinese parasol and catalpa, showed that pine was the most suitable wood species for cellulose extraction, reaching 62% (w/w) (Wang 2011).

Although ionic liquids could dissolve wood biomass only partly in early stage studies, complete dissolution was achieved in 2009 by Sun and co-workers using carboxylate salts under heating. They found that [C2mim][OAc] completely dissolved southern yellow pine and red oak after 46 and 25 h heating at 110°C, respectively. They also evaluated the effects of microwave and ultrasound pretreatment. With microwave pulses, the time for complete dissolution was reduced to shorter than half and ultrasound also significantly reduced the time needed, in about half (Sun 2009). Kilpeläinen (2007) also reported complete dissolution.

DESs have been used to pretreat biomass efficiently compared to some traditional solvents.

Sirviö et al. reported that DESs can be used as hydrolytic media for lignocellulose and pulp pretreatment to produce cellulose nanofibers (CNFs) and cellulose nanocrystals (CNCs) (Sirviö 2015). Li and Haapala concluded that nanofibrils with widths ranging from 13.0 to 19.3 nm were successfully fabricated from DES-pretreated fibers (Li 2017). The significance of such studies was to demonstrate the positive role of DESs as a pretreatment in fiber fibrillation.

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2 MATERIALS AND METHODS

2.1 Feedstocks and pre-treatment cellulose pulps 2.1.1 Eucalyptus pulps

Commercially available eucalyptus Kraft pulp from Fibria Cellulose SA (Brazil) was used in this study: never-dried bleached, never-dried unbleached and bale dry sheets (Fig. 9). The fibers were disintegrated in Milli-Q water (this deionized water was used throughout this study) and filtered. After that the pulp was collected by filtration, washed with technical ethanol, stirred in ethanol for 30 min, collected by filtration again, and dried in an oven at 60 °C. The chemical composition of the pulps, as per manufacturer specifications, is given in Table 1.

Figure 9. Eucalyptus pulps used in the thesis study.

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Table 1. Chemical composition of pulps used in this thesis

Bleached Unbleached Bale

Carboxylic Acids (meq/100g) 6.8 13.1 6.6

Ash (%) 0.4 10.2 0.38

Acetone extractives (%) 0.09 0.23 0.09

Ethanol Toluene extractives

(%) 0.15 0.49 0.09

Total lignin (%) - 2.05 -

Pentosans (%) 17.14 16.70 17.14

Pulp viscosity (dm³/kg) 831 1171 800

Kappa µKappa 0.50 13.6 0.36

Arabinose (%) 0.01 0.03 0.1

Galactose (%) 0.1 0.2 0.1

Glucose (%) 83.7 80.2 83.6

Manose (%) 0.07 0.05 0.07

Xilose (%) 15.28 15.38 15.85

2.1.2 Deep eutectic solvents

Chemicals used were choline chloride ((CH3)3N(Cl)CH2CH2OH) (>98.0%) from Acros, ethylene glycol (HOCH₂CH₂OH) (>99.0%) from VWR, lactic acid in aqueous solution (H₃CCH(OH)COOH) (~90%) from VWR, lithium chloride (LiCl) (>99.0%) from VWR, succinic anhydride (>99.0%) ((CH₂CO)₂O) from Acros and urea (>98.0%) (NH2CONH2) from Sigma Aldrich (Fig. 10).

Figure 10. Chemicals used in the thesis study.

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The DESes systems were prepared by heating and mixing the components at different molar ratios (1:2 and 1:9) in a beaker at 90°C in an oil bath until a clear, colorless liquid was formed. After that, pulp was added at a concentration of 1% (m/m) and stirred for 2 hours.

Half of the samples were also treated with succinic anhydride during the DES treatment at the concentration of 6.18% (w/w) (6.18 g of succinic anhydride for each 100 g of DES). After 2 hours the beaker was removed from the oil bath and ethanol (400 ml) was added to the stirring mixture. After that the pulp was filtered with 6 liters of deionized water.

For the DES systems that used lactic acid, the lactic acid was first heated in an oil bath at 105°C until stable mass was achieved. This was done to remove most of the water from the aqueous solution.

Table 2. DESes systems used in this study

Hydrogen-bond acceptor Hydrogen-bond donor DES systems Molar ratios in DES

Choline chloride Lactic acid ChCl:LA 1:2 and 1:9

Lithium chloride Urea LiCl:Urea 1:2 and 1:9

Choline chloride Ethylene glycol ChCl:EG 1:2 and 1:9

2.1.3 High-intensity ultrasonic fibrillation

After the samples were treated with DES, they were diluted to a consistency of 1% in deionized water, after which the pH was adjusted to 8 by the addition of dilute NaOH solution, and the mixture was further diluted to 0.5% consistency. Samples were sonicated with 200 kJ using an ultrasonic processor shown on figure 11 (model VCX 750, Vibra Cell Sonics, Newtown, Connecticut, USA) with a frequency of 20 kHz and amplitude of 100% (70 micrometer). 25 mm (1 inch) high grade titanium alloy probe was used to sonicate 400 ml of pulp suspension at 0.5% consistency. The beaker was immersed in ice bath to keep the temperature lower than 40 °C and magnetic stirring was used throughout the sonication.

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Figure 11. Sonication with magnetic stirring and ice bath.

2.2 Analyses

2.2.1 Fiber morphology

The equipment used to analyze fiber morphology is a prototype of the Valmet FS5 Fiber Image Analyzer updated for UHD resolution and image area of 16.2 x 13.5 mm. Images from a high resolution camera are evaluated for e.g. fiber length, coarseness, kink and curl.

Samples were tested 6 times and the average measurement was used on this study. The results can follow both TAPPI (Technical Association of the Pulp and Paper Industry) and ISO standards.

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2.2.2 Crystallinity of cellulose by X-ray diffraction

Measurements were conducted with a Rigaku Smart Lab 9 kW rotating-anode diffractometer with CoKa radiation (40 kV, 123 mA, l=1.79 nm). Samples were prepared first by freeze-drying, then pressed to form small discs and then placed in the equipment. The degree of crystallinity indicates the percentage of crystalline part in the sample by X-ray diffraction. It is calculated from the peak intensity of the main diffraction of the crystalline plane θ(200 at 2θ=26° and from the peak intensity at 2θ=21.6° associated with the amorphous fraction of cellulose (Iam) with equation by Segal L (1959):

Crl (%)=[(I200-Iam)/I200]x 100% (1)

where I00 is the intensity of the diffraction from the (200) plane at 2θ=21.6°, and Iam is the intensity for amorphous material taken at 2θ=26°.

2.2.3 Fourier Transform Infrared Spectroscopy

The surface of the prepared composites was investigated with a FTIR spectrometer Vertex 70 (Bruker, Leipzig, Germany) equipped with an ATR platinum diamond, a sensitive 2 × 2 mm diamond crystal surface, and a sample detector RT-DLaTGS. The same ATR-FTIR parameters were used in the OPUS 6.5 software (Bruker, Leipzig, Germany) throughout the measurements. Infrared absorbance spectra of the samples were recorded within the range 4000–800 cm^-1.

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3 RESULTS

3.1 Fiber morphology 3.1.1 Untreated pulps

Table 2 shows the results for the Valmet FS5 analysis for the untreated samples. Length in the range of 0.7–0.8 mm and width in the range of 10–12 µm, as expected, for these types of commercially available Kraft eucalyptus pulps from Brazilian plantations were provided by the company Fibria. Fines as a percentage of total fiber count are also shown. Both fibre and fines analysis is based on several tens of thousands of analyzed particles for each sample.

Table 3. Fiber widths, lengths and % of fines for the control samples of commercially available eucalyptus pulp

No. Pulp Lc(l)ISO [mm] Fiberwidth [µm] Fines [%]

7 Bleached 0.76 11.00 92.63

8 Unbleached 0.80 11.21 93.19

9 Bale 0.75 10.64 92.71

3.1.2 Choline chloride and lactic acid DES system

Table 4 shows the results for the fiber morphology for the choline chloride and lactic acid DES system based on Valmet FS5 analysis. It is possible to see a great reduction in fiber dimensions and an increase in percentage of fines. These changes are seen in all samples treated with CC:LA, independent whether it had a different ratio, the presence of succinic anhydride or different pulp.

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Table 4. Fiber widths, lengths and % of fines for samples treated with choline chloride and lactic acid

No. Pulp Treatment Lc(l)ISO [mm] Fiberwidth [µm] Fines [%]

22 Bleached CC 1 : 2 LA 0.74 3.89 100.00

23 Bleached CC 1 : 9 LA 0.62 3.85 100.00

24 Unbleached CC 1 : 2 LA 0.60 4.48 100.00

25 Unbleached CC 1 : 9 LA 0.67 5.86 99.99

26 Bale CC 1 : 2 LA 0.44 4.31 10000

27 Bale CC 1 : 9 LA 0.61 4.18 100.00

28 Bleached CC 1 : 2 LA + SA 0.27 3.72 100.00

29 Bleached CC 1 : 9 LA + SA 0.50 3.97 100.00

30 Unbleached CC 1 : 2 LA + SA 1.14 4.82 100.00

31 Unbleached CC 1 : 9 LA + SA 0.58 4.99 100.00

32 Bale CC 1 : 2 LA + SA 0.49 3.56 100.00

33 Bale CC 1 : 9 LA + SA 0.46 3.84 100.00

3.1.3 Choline chloride and ethylene glycol DES system

Table 5 shows the results for the fiber morphology for the choline chloride and ethylene glycol DES system. Pulps treated with CC:EG had no apparent significant change when compared to untreated pulps.

Table 5. Fiber widths, lengths and % of fines for samples treated with choline chloride and ethylene glycol

34 Bleached CC 1 : 2 EG 0.76 11.21 92.64

35 Bleached CC 1 : 9 EG 0.76 11.29 92.29

36 Unbleached CC 1 : 2 EG 0.80 11.86 91.23

37 Unbleached CC 1 : 9 EG 0.80 11.76 91.95

38 Bale CC 1 : 2 EG 0.76 11.29 90.50

39 Bale CC 1 : 9 EG 0.76 11.03 91.68

40 Bleached CC 1 : 2 EG + SA 0.76 10.79 92.82

41 Bleached CC 1 : 9 EG + SA 0.75 10.76 92.93

42 Unbleached CC 1 : 2 EG + SA 0.80 11.27 91.34

43 Unbleached CC 1 : 9 EG + SA 0.81 11.09 92.14

44 Bale CC 1 : 2 EG + SA 0.75 10.26 92.70

45 Bale CC 1 : 9 EG + SA 0.76 10.59 90.97

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3.1.4 Lithium chloride and urea DES system

Table 6 shows the results for the fiber morphology for the lithium chloride and urea DES system. There is no visible change in fiber width, length and percentage of fines. Comparing these results for different pulps, ratios and presence of succinic anhydride is further discussed in the next chapter.

Table 6. Fiber widths, lengths and % of fines for samples treated with lithium chloride and urea

10 Bleached LiCl 1 : 2 Urea 0.76 11.56 90.32

11 Bleached LiCl 1 : 9 Urea 0.77 11.10 92.65

12 Unbleached LiCl 1 : 2 Urea 0.81 11.61 92.39

13 Unbleached LiCl 1 : 9 Urea 0.79 11.29 93.07

14 Bale LiCl 1 : 2 Urea 0.76 11.16 90.45

15 Bale LiCl 1 : 9 Urea 0.75 11.34 89.92

16 Bleached LiCl 1 : 2 Urea + SA 0.76 11.08 88.72

17 Bleached LiCl 1 : 9 Urea + SA 0.83 12.12 89.13

18 Unbleached LiCl 1 : 2 Urea + SA 0.81 11.15 91.19

19 Unbleached LiCl 1 : 9 Urea + SA 0.79 11.13 91.92

20 Bale LiCl 1 : 2 Urea + SA 0.76 10.82 87.88

21 Bale LiCl 1 : 9 Urea + SA 0.78 11.16 88.69

3.2 Crystallinity of cellulose by X-ray diffraction 3.2.1 Untreated pulps

Table 7 shows the results for the degree of crystallinity, or crystallinity index, for the untreated samples. The degree of crystallinity indicates the percentage of crystalline part in the whole sample by X-ray diffraction and is calculated using equation 1.

Table 7. Crystallinity of the control samples of commercially available eucalyptus pulp.

Sample number Pulp Cristallinity

7 Bleached 71%

8 Unbleached 68%

9 Bale 69%

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Table 8 shows the results for the degree of crystallinity, or crystallinity index, for the samples treated wth the choline chloride and lactic acid DES system. The calculated degree of crystallinity indicates the percentage of crystalline part in the whole sample by X-ray diffraction and is calculated using equation 1, Crl (%)=[(I200-Iam)/I200]x 100%. Some samples appear to have their calculated crystallinity index increased while others apper to have decreased.

Table 8. Crystallinity for samples treated with choline chloride and lactic acid

No. Pulp Treatment Cristallinity

22 Bleached CC 1 : 2 LA 72%

23 Bleached CC 1 : 9 LA 71%

24 Unbleached CC 1 : 2 LA 69%

25 Unbleached CC 1 : 9 LA 68%

26 Bale CC 1 : 2 LA 73%

27 Bale CC 1 : 9 LA 72%

28 Bleached CC 1 : 2 LA + SA 74%

29 Bleached CC 1 : 9 LA + SA 72%

30 Unbleached CC 1 : 2 LA + SA 65%

31 Unbleached CC 1 : 9 LA + SA 60%

32 Bale CC 1 : 2 LA + SA 71%

33 Bale CC 1 : 9 LA + SA 69%

Table 9 shows the results for the degree of crystallinity, or crystallinity index, for the choline chloride and ethylene glycol DES system. The calculated degree of crystallinity indicates the percentage of crystalline part in the whole sample by X-ray diffraction and is calculated using equation 1, Crl (%)=[(I200-Iam)/I200]x 100%. The calculated crystallinity indes for pulps treated with CC:EG seem to be in the same range as control samples.

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Table 9. Crystallinity for samples treated with choline chloride and ethylene glycol.

34 Bleached CC 1 : 2 EG 74%

35 Bleached CC 1 : 9 EG 73%

36 Unbleached CC 1 : 2 EG 69%

37 Unbleached CC 1 : 9 EG 71%

38 Bale CC 1 : 2 EG 73%

39 Bale CC 1 : 9 EG 76%

40 Bleached CC 1 : 2 EG + SA 72%

41 Bleached CC 1 : 9 EG + SA 76%

42 Unbleached CC 1 : 2 EG + SA 67%

43 Unbleached CC 1 : 9 EG + SA 66%

44 Bale CC 1 : 2 EG + SA 72%

45 Bale CC 1 : 9 EG + SA 73%

Table 10 shows the results for the degree of crystallinity, or crystallinity index, for the lithium chloride and urea DES system. The degree of crystallinity indicates the percentage of crystalline part in the whole sample by X-ray diffraction and is calculated using equation 1, Crl (%)=[(I200-Iam)/I200]x 100%. Calculated crystallinity index for pulps treated with LiCl:urea seem to be in the same range as control samples.

Table 10. Crystallinity for samples treated with lithium chloride and urea

10 Bleached LiCl 1 : 2 Urea 74%

11 Bleached LiCl 1 : 9 Urea 73%

12 Unbleached LiCl 1 : 2 Urea 71%

13 Unbleached LiCl 1 : 9 Urea 67%

14 Bale LiCl 1 : 2 Urea 72%

15 Bale LiCl 1 : 9 Urea 73%

16 Bleached LiCl 1 : 2 Urea + SA 67%

17 Bleached LiCl 1 : 9 Urea + SA 71%

18 Unbleached LiCl 1 : 2 Urea + SA 69%

19 Unbleached LiCl 1 : 9 Urea + SA 66%

20 Bale LiCl 1 : 2 Urea + SA 70%

21 Bale LiCl 1 : 9 Urea + SA 69%

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3.3 Fourier Transform Infrared Spectroscopy 3.3.1 Untreated pulps

The DRIFTs spectra of control samples, without chemical treatment, are shown in Figure 12.

The bands at wavenumber ṽ ≈ 3400, 2900, 1430, 1371, and 899 cm^-1 are associated with native cellulose.Each absorption has its own chemical groups attribution; the OH stretching (ṽ = 3400 cm^-1), CH stretching (ṽ = 2900 cm^-1), the HCH and OCH in-plane bending vibrations (ṽ =1430 cm^-1) and the CH deformation vibration (ṽ = 1371 cm^-1). Finally, the COC, CCO, and CCH deformation modes and stretching vibrations (ṽ = 899 cm^-1).

Figure 12.DRIFT SPECTRA comparison for untreated control samples.

Figure 13 shows the DRIFT spectra of samples treated with choline chloride and lactic acid DES system. There is a visible difference at band 1740 between treated and untreated samples but essentially there are no differences between treatments.

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Figure 13. DRIFT SPECTRA comparison for the control sample and DES system CC:LA.

Figure 14 shows the DRIFT spectra of samples treated with choline chloride and ethylene glycol DES system. There is no visible difference between treated and untreated samples.

Figure 14. DRIFT SPECTRA comparison for the control sample and DES system CC:EG.

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Figure 15 shows the DRIFT spectra of samples treated with lithium chloride and urea DES system. There is a visible difference at band 1740 between treated and untreated samples.

Figure 15. DRIFT SPECTRA comparison for the control sample and DES system LiCl:urea.

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4 DISCUSSION

Three different deep eutectic solvents pretreatment and further sonication of three distinct Kraft eucalyptus pulps were investigated to understand chemical – infrared spectroscopy – and physical – length, width and crystallinity index – changes. These analyses include dynamic and limiting viscosity, conductometric titration and electronic microscopy. They would have been of immense value to help validate or reject current findings of this study.

Furthermore, analyses should have been done between chemical pretreatment and sonication, to help understand the role of each step on the final results. Nonetheless, with the remaining analyses and results it is still possible to draw important conclusions.

4.1 Fiber morphology

4.1.1 Untreated vs. treated fibers

Compared with control sample pulps that were not chemically treated, the DES system CC:LA, showed the greatest changes in fiber morphology. Fiber length and width of control samples were in the range 0.7–0.8 mm and 10.0–11.5 µm, went down to the range of 0.4–

0.7 mm and 3–5 µm, respectively. The other two DES systems did not cause any noticeable change in fiber length.

The structural change in cellulose fibers after sonication is consistent with what Wand et al.

(2009) concludes in his study, which shows that sonication alone, causes significant fibrillation in four different cellulose sources (Lyocell, pure cellulose fiber, MCC and pulp).

He also concludes that the temperature of fiber suspension affects fibrillation, with higher temperatures causing higher fibrillation. More important, longer raw fibers, the lower the fibrillation (Wang 2009).

Tonoli et al. (2012) showed that Eucalyptus micro and nanofibers can be produced using sonication and acid hydrolyses processes. The action of hydrodynamic forces caused by sonication was effective to open the structure of fibers, releasing micro and nanofibrils from the fiber cell wall (Tonoli 2012).

Fiber width for the samples treated with LiCl:urea and CC:EG DES systems showed a small increase when comparing to untreated control samples, probably because of fiber swelling.

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As mentioned earlier, fiber morphology should have been also analyzed before sonication, and not only after IR, to understand the difference in fiber swelling after chemical treatment.

In respect to the fines percentage, control samples went from a range of 83–93% to 100%

when the DES system CC:LA was used. The other two DES systems did not cause any noticeable change. Previous study using lactic acid and choline chloride DES system showed that cellulose is not soluble in this media, but lignin is. That indicates that the chosen DES system is suitable for nanocellulose production since it does not breakdown the crystalline part of cellulose (Francisco 2012).

4.1.2 DES component mixing ratio

Fiber length, width and percentage of fines were analyzed comparing different ratios of chemicals used in the DES systems. There was no noticeable change in fiber morphology.

4.1.3 Presence of succinic anhydride in the DES

Fiber length, width and percentage of fines were analyzed comparing the presence or absence of succinic anhydride with DES systems. It was found that for the LiCl:urea DES system some samples with succinic anhydride (SA) had higher fiber length than samples treated without SA. Samples 17 and 21 had higher fiber length, while 16, 18, 19 and 20 the length is exactly the same. Comparing fines percentage results, there is no visible difference between samples with and without SA.

4.2 Crystallinity of cellulose by X-ray diffraction 4.2.1 Untreated vs. treated fibers

Calculated crystallinity index of control and treated samples were all very close to 70% with no apparent differentiation when comparing control and treated samples, indicating that the cellulose I crystalline structure remained intact and no rearrangement of the cellulose structure into another crystalline form occurred during DES treatment and mechanical disintegration. This indicates that no significant dissolution of cellulose occurred during the chemical or during the sonication treatments. Though small changes, which are presented below, might have happened in the calculated crystallinity index, it is possible to say that

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the DES systems used are a mild pretreatment media for cellulose nanofibrillation. This is consistent with conclusions made by Sirviö et al. (Sirviö 2015).

4.2.2 DES component mixing ratio

Calculated crystallinity index for CC:LA DES system showed that samples treated with a 1:9 ratio had lower crystallinity index than samples treated with a 1:2 ratio. This DES system was the only to show any difference.

4.2.3 Presence of succinic anhydride in the DES

For the LiCl:urea DES system, the presence of SA brought the calculated crystallinity index down, from the range of 67–74% to 66–71%. This is visible on all samples treated with LiCl:urea, from sample number 10 to 21.

For the CC:EG DES system, the presence of SA brought the crystallinity index down. This is valid for 5 comparisons (samples: 34 vs 40, 36 vs 42, 37 vs 43, 38 vs 44 and 39 vs 45) but not valid for only one comparison, which is sample 35 vs. sample 41. Finding of this study are comparable to the ones from Selkälä et al. (2016) in which the crystalline structure of cellulose remained intact during the modification in lithium chloride – urea DES system.

Selkälä concluded that calculated crystallinity index was stable after chemical pretreatment and further nanofibrillation using microfluidizer (Selkälä 2016).

4.3 Fourier Transform Infrared Spectroscopy

Not all DRIFT spectra graphs are presented in this study for simplification manners, since there were no visible differences between the three control samples (figure 11), different ratios (1:2 to 1:9) and presence or absence of succinic anhydride.

On the other hand there was a very significant change at the band at 1740 cm^-1 which is visible on figures 12 and 14. All samples treated with CC:LA and LiCl:urea DES show this peak at band 1740 cm^-1, which can be related to the ester (C=O) stretching vibration. This result is consistent with what Tjeerdsma (2005) and what Gellerstedt (1999) confirm, which is, the occurrence of the esterification reaction between hydroxyl groups of cellulose and the chemicals used. Esterification is a reaction that introduces an ester functional group (COO-)

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on the surface of cellulose nanoparticles by condensation of a carboxylic group (COOH) and alcohol group (OH) (Dufresne 2017).

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5 CONCLUSIONS

Three different commercially available eucalyptus Kraft pulps were pre-treated with three different DESes systems, with the presence and absence of succinic anhydride, and further sonicated. Fiber morphology, chemical composition and crystallite structure were evaluated. Based on the results, the following conclusions can be made:

Fiber morphology:

 Fibers treated with the CC:LA system showed great reduction in width and length.

The percentage of fines increased significantly.

 Fibers treated with LiCl:urea and CC:EG showed a small increase in width, indicating fiber swelling.

 The presence of succinic anhydride in LiCl:urea system made fibers have higher length.

Crystallinity:

 Calculated crystallinity index for CC:LA system indicated that samples treated with 1:9 ratio had lower crystallinity than samples treated with 1:2 ratio.

 For the LiCl:urea DES system, the presence of SA brought the calculated crystallinity index down.

 For the CC:EG DES system, the presence of SA brought the crystallinity index down.

Chemical composition:

 All samples treated with CC:LA and LiCl:urea DESes show the occurrence of the esterification reaction between hydroxyl groups of cellulose and the chemicals used.

Choline chloride and lactic acid deep eutectic system did affect fiber width and length much more than the other systems studied, so it should be further investigated. Also, morphological changes should be analyzed after chemical treatment but before fibrillation, to understand the effect of each step.

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6 REFERENCES

Abbott, A. P., Boothby, D., Capper, G., Davies, D. L., & Rasheed, R. K. (2004). Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids. Journal of the American Chemical Society, 126(29), 9142-9147.

Abbott, A. P., Capper, G., & Gray, S. (2006). Design of improved deep eutectic solvents using hole theory. ChemPhysChem, 7(4), 803-806.

Andresen, M.; Johansson, L.S.; Tanem, B.S.; Stenius, P. Properties and characterization of hydrophobized microfibrillated cellulose. Cellulose 2006, 13, 665-677.

Azizi Samir, M.A.S.; Alloin, F.; Dufresne, A. (2005) Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field. Biomacromolecules, 6, 612-626.

Biermann, C. J. (1996). Handbook of pulping and papermaking. Academic press.

S. Katz, R. P. Beatson, A. M. Scallan, Sven. Papperstidn., 87,R48 –53

Boerjan, W., Ralph, J., & Baucher, M. (2003). Lignin biosynthesis. Annual review of plant biology, 54(1), 519-546.

Bose, S. K., Omori, S., Kanungo, D., Francis, R. C., & Shin, N. H. (2009). Mechanistic differences between kraft and soda/AQ pulping. Part 1: Results from wood chips and pulps.

Journal of wood chemistry and technology, 29(3), 214-226.

Brännvall, E. Aspect on Strength Delivery and Higher Utilisation of Strength Potential of Soft Wood Kraft Pupl Fibres. Ph.D. Thesis, KTH, Royal Institute of Technology: Stockholm, Sweden, 2007.

Campinhos, E., Jr. (1999). Sustainable plantations of high-yield Eucalyptus trees for production of fiber: The Aracruz case. New Forests, 17, 129–143

Chen, W., Yu, H., Liu, Y., Chen, P., Zhang, M., & Hai, Y. (2011). Individualization of cellulose nanofibers from wood using high-intensity ultrasonication combined with chemical pretreatments. Carbohydrate Polymers, 83(4), 1804–1811.

Chinga-Carrasco, G. (2011). Cellulose fibres, nanofibrils and microfibrils: The morphological sequence of MFC components from a plant physiology and fibre technology point of view.

Nanoscale Research Letters, 6(1), 417. doi:10.1186/1556-276x-6-417

Daniel, J.R. Cellulose structure and properties. In Encyclopedia of Polymer Science and Engineering; Kroschwitz, J.I., Ed.; Wiley-Interscience Publication John Wiley & Sons: New York, NY, USA, 1985; Volume 3, pp. 86-123.

Dufresne, A. (2013). Nanocellulose: from nature to high performance tailored materials.

Walter de Gruyter.

(41)

Dufresne, A. Polymer nanocomposites from Biological Sources. In Encyclopedia of Nanoscience and Nanotechnology, 2nd ed.; Nalwa, H.S., Ed.; American Scientific Publisher:

Valencia, CA, USA, in press.

Dufresne, Alain. (2017). Nanocellulose: From nature to high performance tailored materials, 2. Edition. 10.1515/9783110480412.

Fort DA, Remsing RC, Swatloski RP, Moyna P, Moyna G, Rogers RD. (2007). Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3- methylimidazolium chloride. Green Chem., 9:63–9.

Francisco, M., van den Bruinhorst, A., & Kroon, M. C. (2012). New natural and renewable low transition temperature mixtures (LTTMs): screening as solvents for lignocellulosic biomass processing. Green Chemistry, 14(8), 2153.doi:10.1039/c2gc35660k

Gellerstedt, F. & Gatenholm, P. Cellulose (1999) 6: 103.

https://doi.org/10.1023/A:1009239225050

Gírio, F. M., Fonseca, C., Carvalheiro, F., Duarte, L. C., Marques, S., & Bogel-Łukasik, R.

(2010). Hemicelluloses for fuel ethanol: a review. Bioresource technology, 101(13), 4775- 4800.

Habibi, Y., Lucia, L.A., & Rojas, O. J. (2010). Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chemical Reviews 110: 3479–3500.

Henriksson M, Henriksson G, Berglund LA, Lindström T (2007) An environmentally friendly method for enzymeassisted preparation of microfibrillated cellulose (MFC) nanofibers. Eur Polym J 43:3434–3441.

Herrick FW, Casebier RL, Hamilton JK, Sandberg KR. (1983). Microfibrillated Cellulose:

Morphology and accessibility. J Appl Polym Sci Appl Polym Symp, 37:797-813.

Heux, L.; Dinand, E.; Vignon, M.R. (1999). Structural aspects in ultrathin cellulose microfibrils followed by 13C CP-MAS NMR. Carbohydr. Polym., 40, 115-124.

Heyn AN: The elementary fibril and supermolecular structure of cellulose in soft wood fiber.

(1969). J Ultrastructure research, 26:52-68

Iwamoto, S., Abe, K., & Yano, H. (2008). The effect of hemicelluloses on wood pulp nanofibrillation and nanofiber network characteristics. Biomacromolecules, 9, 1022–1026.

Kardos, N., & Luche, J.-L. (2001). Sonochemistry of carbohydrate compounds. Carbohydrate Research, 332(2), 115–131. doi:10.1016/s0008-6215(01)00081-7

Kelly, S. 1993. Eucalypts (Volume 1). Van Nostrand Reinhold Company, New York.

Kilpeläinen I, Xie H, King A, Granstrom M, Heikkinen S, Argyropoulos DS. (2007). Dissolution of wood in ionic liquids. J Agric Food Chem., 55:9142–8.

(42)

Lavoine, N., Desloges, I., Dufresne, A., & Bras, J. (2012). Microfibrillated cellulose–Its barrier properties and applications in cellulosic materials: A review. Carbohydrate polymers, 90(2), 735-764.

Lima, J.T., M.C. Breese & C.M. Cahalan. (2000). Genotype-environment interaction in wood basic density of Eucalyptus clones. Wood Sci. Technol. 34: 197–206.

Lindström T. (1992). Chemical factors affecting the behaviour of fibres during papermaking.

Nord Pulp Pap Res J 4:181-192.

Lu, J.; Askeland, P.; Drzal, L.T. (2008). Surface modification of microfibrillated cellulose for epoxy composite applications. Polymer, 49, 1285-1298.

Moon, R.J. McGraw-Hill Yearbook in Science & Technology, McGraw-Hill, 2008, pp. 225–228.

Nakagaito AN, Yano H. (2005). Novel high-strength biocomposites based on microfibrillated cellulose having nanoorder-unit web-like network structure. Appl Phys A-Mater Sci Process 80:155–159.

Osong, S.H., Norgren, S., Engstrand, P. (2015). Processing of wood-based microfibrillated cellulose and nanofibrillated cellulose, and applications relating to papermaking: a review.

Cellulose, 1–31, http://dx.doi.org/10.1007/ s10570-015-0798-5.

P. T. Anastas, J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, Oxford, 1998.

Li P, Sirviö JA, Haapala A and Liimatainen H. (2017). Cellulose Nanofibrils from Nonderivatizing Urea-Based Deep Eutectic Solvent Pretreatments. ACS Applied Materials &

Interfaces, 9 (3), 2846-2855. DOI: 10.1021/acsami.6b13625

Plomion, C., Leprovost, G., and Stokes, A. (2001). Wood formation in trees. Plant Physiol.

127: 1513–1523.

Raymond, C.A. 1995. Genetic control of wood and fibre traits in Eucalyptus. In: B.M. Potts, N.M.G. Borralho, J.B. Reid, R.N. Cromer, W.N. Tibbits & C.A. Raymond (eds.), Eucalypt plantations: improving fibre yield and quality: 49–52. CRC for Temperate Hardwood Forestry, Hobart.

Roberts,J. C., The Chemistry of Paper, The Royal Society of Chemistry, 1996.

Rojas, J., Bedoya, M., & Ciro, Y. (2015). Current trends in the production of cellulose nanoparticles and nanocomposites for biomedical applications. In Cellulose-Fundamental Aspects and Current Trends. Intech.

Satyanarayana, K.G.; Guilmaraes, J.L.; Wypych, F. (2007). Studies on lignocellulosic fibers of Brazil. Part I: Source, production, morphology, properties and applications. Compos. A-Appl.

Sci. Manufact., 38, 1694-1709.

Satyanarayana, K.G.; Sukumaran, K.; Mukherjee, P.S.; Pavithran, C.; Pillai, S.G.K. (1990).

Natural fibre-polymer composites. Cem. Concr. Compos., 12, 117-136.

(43)

Selkälä, T., Sirviö, J. A., Lorite, G. S., & Liimatainen, H. (2016). Anionically Stabilized Cellulose Nanofibrils through Succinylation Pretreatment in Urea–Lithium Chloride Deep Eutectic Solvent. ChemSusChem, 9(21), 3074-3083.

Siqueira, G., Bras, J., & Dufresne, A. (2010). Cellulosic bionanocomposites: a review of preparation, properties and applications. Polymers, 2(4), 728-765.

Sirviö, J. A., Visanko, M., Liimatainen, H. (2015). Deep eutectic solvent system based on choline chloride-urea as a pre-treatment for nanoﬁbrillation of wood cellulose. Green Chemistry, 17, 3401–3406.

Smith, E. L., Abbott, A. P., & Ryder, K. S. (2014). Deep eutectic solvents (DESes) and their applications. Chemical reviews, 114(21), 11060-11082.

Sun N, Rahman M, Qin Y, Maxim ML, Rodriguez H, Rogers RD. (2009). Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3-methylimidazolium acetate.

Green Chem. 2009;11:646–55.

Swatloski, R.P., Rogers, R. D., Holbrey, J.D. Dissolution and processing of cellulose using ionic liquids, cellulose solution, and regenerating cellulose, World Pat., WO 2003 029329 (2003) Syverud, K., Chinga-Carrasco, G., Toledo, J., & Toledo, P. G. (2011). A comparative study of Eucalyptus and Pinus radiata pulp fibres as raw materials for production of cellulose nanofibrils. Carbohydrate Polymers, 84(3), 1033-1038.

Thompson, R & Manning, A. (2005). A review of ultrasound and its applications in papermaking. Progress in Paper Recycling. 14. 26-42.

Tjeerdsma BF, Militz H. (2005). Chemical changes in hydrothermal treated wood: FTIR analysis of combined hydrothermal and dry heat-treated wood. Holz als Rohund Werkst 63:102–111. https://doi.org/10.1007/s00107-004-0532-8

Tonoli, G. H. D., Teixeira, E. M., Corrêa, A. C., Marconcini, J. M., Caixeta, L. A., Pereira-da- Silva, M. A., & Mattoso, L. H. C. (2012). Cellulose micro/nanofibres from Eucalyptus kraft pulp: Preparation and properties. Carbohydrate Polymers, 89(1), 80–

88.doi:10.1016/j.carbpol.2012.02.052

Turbak, A.F.; Snyder, F.W.; Sandberg, K.R. Microfibrillated cellulose, a new cellulose product:

Properties, uses, and commercial potential. J. Appl. Polym. Sci. Appl. Polym. Symp. 1983, 37, 815-827.

Velmurugan R., K. Muthukumar, Ultrasound-assisted alkaline pretreatment of sugarcane bagasse for fermentable sugar production: optimization through response surface methodology, Bioresour. Technol. 112 (2012) 293–299.

Visanko, M. (2015). Functionalized nanocelluloses and their use in barrier and membrane thin films. 10.13140/RG.2.1.4859.9769.

(44)

Wagle, D. V., Zhao, H., & Baker, G. A. (2014). Deep Eutectic Solvents: Sustainable Media for Nanoscale and Functional Materials. Accounts of Chemical Research, 47(8), 2299–

2308. doi:10.1021/ar5000488

Wang S, Cheng Q. (2009). A novel process to isolate fibrils from cellulose fibers by high- intensity ultrasonication, part 1: process optimization. J Appl Polym Sci 113(2):1270–1275 Wang X, Li H, Cao Y, Tang Q. (2011). Cellulose extraction from wood chip in an ionic liquid 1- allyl-3-methylimidazolium chloride (AmimCl). Bioresour Technol., 102:7959–65.

Zhang, Q., Vigier, K. D. O., Royer, S., & Jérôme, F. (2012). Deep eutectic solvents: syntheses, properties and applications. Chemical Society Reviews, 41(21), 7108-7146.

Zhu, S., Wu, Y., Chen, Q., Yu, Z., Wang, C., Jin, S., … Wu, G. (2006). Dissolution of cellulose with ionic liquids and its application: a mini-review. Green Chemistry, 8(4), 325.doi:10.1039/b601395c