CHAPTER 1INTRODUCTION1.1 BackgroundA great challenge mankind faces as it enters the twenty-first century is the waste management (Xiong et al., 2012), for example, textile wastes and agro-wastes. A good illustration of agro-waste is from banana plants. Banana plants are a crucial fruit crop in the world yet only 12% w/w is edible, with more than 80% of the non-edible parts such as pseudostems, are discarded as agricultural wastes (Bolio-López et al., 2011; Mueller et al.
, 2014; Zuluaga et al., 2007). Textile wastes largely from clothing manufacturers are often buried or burned, which could then lead to environmental pollution (Xiong et al.
, 2012). Cellulose is the most copious biopolymer on Earth (Grishkewich et al., 2017) that can be found in plants, lignocellulosic biomass and agro-wastes (Börjesson & Westman, 2015). Cellulose is also a fibrous, tough, water-insoluble substance which is essential to ensure the preservation of structure of natural fibres (Trache et al.
, 2017). Cotton contains the highest cellulose content among plants with about 90% cellulose as compared to wood, with about 40 to 50% cellulose (Börjesson & Westman, 2015). One of the most significant characteristics that differentiate cellulose from other biopolymers is that each of its anhydroglucose unit bears three hydroxyl groups (Figure 1.1). Due to the linear structure and the presence of hydroxyl group, it can form an ordered crystalline structure held by hydrogen bonds in Figure 1.2 (Camarero Espinosa et al.
, 2013) Figure 1.1: Cellulose structure with three hydroxyl groups. Figure 1.
2: Ordered crystalline structure held by hydrogen bonds. Production cellulose nanocrystals from agro-wastes, such as banana pseudostems is gaining popularity recently for their properties. However, the extraction of cellulose nanocrystals (CNCs) from these natural resources generally requires the removal of non-cellulosic components such as lignin, pectin, hemicellulose and wax (Leung et al., 2011).
Common methods that are used to obtain and aggregate CNCs require more tedious steps and treatments as it needs a pure cellulosic starting material. Herein, a more straightforward method is used to produce nanocrystals by using ammonium persulfate (APS). Through APS oxidation, these non-cellulosic compounds can be removed without the need of any pre-treatment. Moreover, this oxidant shows long term low toxicity, high water solubility, cheap and can produce high quality cellulose nanocrystals of high thermal stability in a single step (Leung et al., 2011) CNCs have surfaced as a new class of nanomaterials for polymer reinforcement and nanocomposite composition due to their outstanding mechanical strength (modulus of 100-140 GPa), low density (1.6 g cm-3), chemical tunability and low cost (Leung et al.
, 2011). In addition, CNCs can be readily modified with chemicals, which may include converting them to carboxylic acid, amine, aldehyde or thiol functionalities, allowing further modifications to take place (Grishkewich et al., 2017; Siqueira et al., 2017; Tekinalp et al., 2014).1.
2 Problem statementOverflowing landfills have been a serious problem faced by the agricultural and textile sectors for years. These wastes are often tossed aside in landfills or burned which contributes to environmental pollution and turning into an eye sore. The volume of these wastes keeps increasing with years and growing population, which require immediate attention, especially for textile wastes which takes a longer time to decompose. Acid hydrolysis is the most common procedure used to obtain cellulose nanocrystals. It requires very strong acids which is quite dangerous to the operators. Moreover, more processing steps are needed to prepare the raw material for conversion into nanocellulose since the sample must be pure in order to produce high yield of good quality nanocellulose.
1.3 Significance of studiesThe finding of this study will contribute to the benefit of the society considering that the management of waste is very crucial in a developing country such as Malaysia. Textile agro-wastes can be put into good use by producing cellulose nanocrystals which can further benefit the society due to its unique properties in a variety of applications such as drug delivery, 3D printing and water treatment. In a way, the environmental problems can be overcome through a reduction of waste generation and recycling of raw materials.1.
4 ObjectivesThis study aims to:1. extract cellulose nanocrystals from cotton and banana pseudostem using Ammonium Persulfate (APS) oxidation.2. vary the APS concentration and temperature during the APS oxidation of cotton and banana pseudostem.3. characterize the cellulose nanocrystals derived from cotton and banana pesudostem using Fourier-transform Infrared (FTIR) spectroscopy, Nuclear Magnetic Resonance (NMR) spectroscopy and Transmission Electron Microscope (TEM).
CHAPTER 2LITERATURE REVIEW2.1 NanotechnologyNanotechnology opens doors to more opportunities and sustainable advances. The core of nanotechnology is the capability to work at the atomic, molecular, and super-molecular levels targeting in manufacturing, exploring and using materials, devices and systems with the foundation of a new property and function due to the expression of properties and structure at nano-scale (Balakrishnan et al., 2013).
However, nanotechnology research is still slowly developing, particularly in developing countries like Malaysia (Hashim et al., 2009). In order to ensure a prosperous and innovative future in this country, opportunities to explore more in the field of nanotechnology should not be turned down. Through nanotechnology, the health quality, lifestyle and productivity of society all over the world can be enhanced (Zhao et al., 2003) Nanocomposite materials can be made from readily available natural fibres, such as grasses, reeds, stalks and woody vegetation. The use of natural fibres derived from renewable resources provide a positive environmental impact with respect to ultimate disposability and use of raw materials (Eichhorn et al.
, 2001; Kamel, 2007). There are a variety of nanocomposites such as zero-dimensional (nanoparticle), one-dimensional (nanofibre), two-dimensional (nanolayer) and three-dimensional (interpreting network) (Kamel, 2007; Schmidt et al., 2002). In general, nanotechnology is attractive because it requires minimal space, materials and energy. It is also faster and more efficient in terms of length scale for manufacturing and it has excellent properties and phenomena (Kamel, 2007). 2.
2 Classification of Nanocellulose StructuresNanocellulose can be seen in different forms. The most common ones are cellulose nanocrystals, cellulose nanofibrils and bacterial nanocellulose.2.2.1 Cellulose Nanocrystals (CNCs)Cellulose nanocrystals (CNCs) are known by many names such as microcrystals, nano whiskers, nanocrystals or nanoparticles (Habibi et al., 2010).
CNCs from different types of lignocellulosic resources have a width ranging between 2-30 nm and hundreds of nanometres in length (Börjesson & Westman, 2015) in the form of rods or “needle-like” particles with high crystallinity and specific surface area (Trache et al., 2017). The microscopic properties of CNCs have an important bearing on their microscopic properties which are summarised in Figure 2.1 (Tang et al.
, 2017) Figure 2.1: A summary of the physical and chemical properties of CNCs (Tang et al., 2017) Depending on the desired application, CNCs can be easily modified due to the presence of hydroxyl groups. Some of these reactions include sulfonation, oxidation (Habibi et al., 2006), grafting (Habibi et al., 2008) via acid chloride (De Menezes et al.
, 2009; Habibi et al., 2008), acid anhydride (Pandey et al., 2009), isocyanate (Siqueira et al., 2008) and silylation (Goussé et al., 2002) (Figure 2.2).
High surface area of CNCs allow strong interactions with surrounding species, such as, water, organic and polymeric compounds, nanoparticles and living cells (Klemm et al., 2011; Xiong et al., 2012).
As such, CNCs are emerging as one of the most promising sustainable building blocks for future advanced materials (Walther et al., 2011; Xiong et al., 2012; Yano et al., 2005; Yi et al.
, 2009; T. Zhang et al., 2010). Figure 2.
2 Possible routes for chemical modification of CNC (clockwise from right): (a) sulfonation; (b) oxidation by TEMPO; (c) ester linkages via acid chloride; (d) cationization via epoxides; (e) ester linkages via acid anhydrides; (f) urethane linkages via isocyanates; and (g) silylation (Lam et al., 2012)2.2.2 Cellulose Nanofibrils (CNFs)Cellulose nanofibrils (CNFs) can be extracted from cellulosic fibres through three types of processes which are mechanical treatments, chemical treatments or a combination of both (Abitbol et al., 2016). When CNF is produced by mechanical treatment, the non-crystalline parts as well as the length of the fibrils are preserved (Borjesson & Westman, 2015).
Common mechanical treatments include homogenisation and pulp refining, whereby both procedures individualise the nanofibrils and give a stable dispersion when diluted in water. Different treatments may vary the properties of an obtained CNFs, such as their crystallinity or length (Foster et al., 2018). Figure 2.
3 shows the production of CNFs by mechanical treatment (Zhang et al., 2018) CNFs consists of micro-meter long fibrils that are entangled, with both amorphous and crystalline domains, in contrast to CNCs which has a near-perfect crystallinity (Abitbol et al., 2016). Figure 2.
3: Production of CNFs by mechanical treatment (Zhang et al., 2018)2.2.3 Bacterial Nanocellulose (BNC)Bacterial nanocellulose (BNC) are composed of fibrils with lateral dimensions, ranging from 25 to 86 nm (Bespalova et al.
, 2017). Qualities of BNC include extremely fine and pure fibre network structure and a substantial degree of polymerisation (up to 8000). BNC also has good mechanical properties such as high mechanical strength, biocompatibility and water holding capability (Börjesson & Westman, 2015; Kamel, 2007; Klemm et al., 2011). BNC is most useful for medical health and surgical applications, for example, bandages for wound healing or skin burns or as a proxy for medical materials, such as blood vessels (Börjesson & Westman, 2015).
Other than that, it can also be used in food Industry or paper and packaging industry BNC has many advantages, it cannot be produced in larger quantities thus not meeting the requirements for commercialisation (Börjesson & Westman, 2015; Czaja et al., 2006). 2.3 Sources of NanocelluloseVariations in cellulose nanomaterials arise when nanocellulose is extracted from different kinds of cellulosic biomass.
Most common source include plants, algae, tunicates and bacteria. 2.3.
1 PlantsLignocellulosic biomass is mainly woody and non-woody plants. Their natural fibres can be categorised based on the origin of the plant, for example, bast or stem, leaf, seed or fruit, grass and straw fibres (Chowdhury & Hamid, 2016; Trache et al., 2017).
Chemical structures and internal fibrous composition notably influence the properties of natural fibres, not to mention the changes between different parts of plants and among different plants too (Trache et al., 2017). Typically, wood is an extremely crucial industrial raw material used in buildings, furniture and other construction materials which gradually decreases the supply of hard wood.
Recently, non-woody plants become the alternate source of cellulose. In comparison to woody plants, non-woody plants contain less lignin and therefore the bleaching process is unnecessary (Siró & Plackett, 2010). Herein, cellulose nanocrystals will be derived from banana pseudostems since the other parts of the banana, such as rachis and banana peels, have been conducted previously and it will be compared with a commercially available cellulose, such as cotton.2.3.2 Algae Cellulose can also be derived from algae.
In fact, cellulose extracted from marine green algae, for example, Valonia or Cladophora, exhibit a substantial degree of crystallinity up to 95%. Cladophora algae, grow well on submerged rocks and stones, and readily multiply their population. As these algae age and die, they drift into the water forming a dense mat, which may harm other forms of aquatic life (Mihranyan, 2011). Cellulose extracted from this algae is believed to possess high degree of crystallinity attributed to the presence of thick microfibrils, which measures to about 10 to 30 nm in width, in contrast to that of wood at only 5nm (Mihranyan, 2011). On crystallinity, Cladophora absorbs very little moisture from the air at ambient relative humidity. 2.
3.3 TunicatesTunicates are invertebrate filter feeding animals. They compete for food and space with other filter feeding animals, for example, mussels and scallops. Generally, tunicates are known as two of the most damaging invasive species.
They grow and procreate in 10 weeks, releasing more than 10,000 eggs. Their vast reproduction creates a threat to the biodiversity (Dunlop et al., 2018). The subphylum Tunicata comprises three classes, whereby two of the classes Ascidiacea and Thaliacea, have tunics. However, the third class which does not possess any tunic, Appendicularia, secretes cellulosic materials. Tunicates are known to be the only animal that can produce cellulose (Habibi et al.
, 2010; Trache et al., 2017; Y. Zhao & Li, 2014). The skeletal structure in the tunic tissues consists of cellulose, which makes it different in terms of the integumentary tissue that covers the whole part of the epidermis (George & Sabapathi, 2015). As such, that part of the tunicate can be extracted to create cellulose nanocrystals (Dunlop et al.
, 2018). The shape of the cellulose microfibril depends on the species. 2.3.4 BacteriaBacterial Nanocellulose (BNC) is a different kind of nanocellulose.
The most promising producers of bacterial cellulose are Gram-negative acetic acid bacteria, such as Acetobacter xylinum, which is also referred to as genus Gluconacetobacter (Kumbhar et al., 2015; Trache et al., 2017). These bacteria are aerobic, non-photosynthetic and are able to transform glucose and other organic substrates into cellulose in a short period of time (Keshk, 2014; Trache et al., 2017). Bacterial cellulose (BC) is basically composed of chain molecules connected by cellobiose, which is similar to other celluloses.
In contrast to plant derived cellulose, BC is free from contaminant molecules such as lignin, hemicellulose and pectin. It has a relatively high tensile strength, extremely hydrophilic surface, homologous structure with native extracellular matrix, special nanostructure, high chemical purity and outstanding biodegradability. Moreover, the biocompatibility of BC is the key attribute that makes BC a favourable material in biomedical applications (Huang et al., 2013).