F IBERS AND TEXTILES
PRODUCED FROM C HITIN
AND C HITOSAN
– A L ITERATURE STUDY FOR DIFFERENT PRODUCTION METHODS
BSc in Chemical Engineering – Applied Biotechnology
Doaa Hameed Tamar Manouel
Program: BSc in Chemical Engineering – Applied Biotechnology
Title: Fibers and Textiles Produced from Chitin and Chitosan - A Literature Study For Different Production Methods.
Year of publication: 2020
Authors: Doaa Hameed, Tamar Manouel Supervisor: Akram Zamani
Examiner: Jorge Ferreira
Keywords: Chitin, Chitosan, Fibers, Nanofibers, Textiles, Wound dressing, Sutures, Spinning.
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ACKNOWLEDGMENT
All thanks and appreciations go to our supervisor Akram Zamani, for her help and unconditional support which without her could this work never been accomplished.
Special thanks and gratitude to our examiner Jorge Ferreira who supported and guided us to do this work.
We would also like to deeply thank our caring, loving and supportive families, together with husband and fiancé, who were always there to support us during our education, which without them we would not succeed and pass all the difficulties.
Sammanfattning
Ökningen av världspopulationen har orsakat en ökad avfallsgenerering. Avfallet kan innehålla betydelsefulla ämnen, vilka kan användas som råvaror i många olika material och för olika ändamål. Därför har omfattande forskning genomförts för att ta till vara på avfall som orsakar miljöföroreningar och ur dessa utveckla mer hållbara och biologiskt nedbrytbara material.
Exempel på detta är fibrer och textilier framställda av polysackaridmaterial, särskilt från kitin och kitosan, som finns tillgängliga som biprodukt från såväl skaldjur som insekter och cellväggar från svampar.
Kitin är efter cellulosa den vanligaste aminopolysackarid-polymeren som har en liknande struktur, medan kitosan är den deacetylerade formen av kitin som är den mest välkända och det viktigaste derivatet av kitin. Kitosan kan framställas från kitin genom antingen kemisk deacetylering eller enzymatiska beredningar, men för kommersiell skala idag, är produktion av kitosan med kemisk metod som deacetylering av kitin med en alkali såsom NaOH, mer lämplig och att föredra. Både kitin och kitosan är biobaserade material som har speciella egenskaper såsom hög specifik styvhet och hållfasthet, samtidigt som biologisk nedbrytbarhet är möjlig.
Dessutom förekommer materialen rikligt i naturen, vilket gör dem till passande och konkurrenskraftiga ersättare till traditionella fibrer.
Textilier är en stor källa till koldioxidutsläpp på grund av massiv global produktion och att även icke-nedbrytbara fibrer i vissa fall används i produktionen. Fibrer är den elementära enheten i textilier förutom bomull, som traditionellt används för textilproduktion. Det finns olika typer av fibrer som vanligtvis delas in i syntet- och biobaserade fibrer härrörande från förnybara resurser. Dessa förnybara fibrer har skapat ett stort intresse från världens textiltillverkare för att ställa om sin produktion och exempelvis producera gasbindor med återvinningsbara och biologiskt nedbrytbara material. Användningen av kitin och kitosan i textilindustrin är mycket intressant och viktig, dels på grund av deras mångsidighet och stora överflöd i naturen, dels då materialen annars anses vara spill eller restprodukter utan signifikant betydelse.
Syftet med denna avhandling var att göra en litteraturöversikt om metoder för produktion av fibrer och textilier från kitin och kitosan, samt att undersöka hur de kan användas och dess miljövänliga aspekter.
I denna avhandling har olika metoder baserade på många undersökningar och experiment introducerats, för att förstå och utvärdera möjliga processer för bildning av kitin- och kitosanfibrer. Dessutom har egenskaperna hos de framtagna fibrerna såsom draghållfasthet och töjning undersökts.
För kitinproduktion har fem olika metoder studerats med användning av olika lösningsmedel av joniska vätskor såsom 1-etyl-3-metylimidazoliumacetat [C2mim] OAc, 1-butyl-3- metylimidazoliumklorid [C4mim] Cl och 1-etyl-3-metylimidazoliumklorid [C2mim] Cl, triklorättiksyra (TCA) och metylenklorid, en kombination av TCA, klorhydrat och metylenklorid, en blandning av myrsyra (FA), diklorättiksyra (DCA) och isopropyleter (iPE), liksom en direkt upplösning i NaOH/tiourea/urea.
Produktion av nanofibrer från krabbskal, räkskal, kommersiella kitinpulver och svamp har undersökts, samt sårförband som en icke-vävd textil, genom att undersöka två olika produktionsmetoder.
Många studier på kitosanproduktion har listats med fokus på typen av spinnteknik såsom våtspinning med användning av en cellulosa/kitosan-kompositlösning samt fibrer bildade av myrsyramodifierad kitosan. Dessutom listas olika typer av tekniker för torrspinning, torrstråle- våtspinning och elektrospinning. Slutligen har sårförbandsprocessen med användning av icke- vävda textilier av chitosan/hyaluronan också inkluderats.
Sammanfattningsvis är produktion av textilfibrer med kitin och kitosan möjlig och kan göras på olika sätt. På grund av deras egenskaper och antimikrobiella effekter blir de intressanta alternativ till medicinska tillämpningar såsom suturer, sårförband, vävnadsteknik och antimikrobiellt medel. I likhet med andra material har kitin och kitosan fördelar, men även vissa nackdelar såsom svag och låg draghållfasthet hos de framtagna fibrerna och att de är delvis lösliga i substanser med pH under 5,5.
Produktion av fibrer och textilier baserade på kitin och kitosan är fortfarande en utmaning på grund av de många modifieringssteg som krävs. Bland annat måste man ta hänsyn till lösningsmedlet som används för upplösning, välja rätt spinnteknik samt att använda ett lämpligt koagulationsbad följt av en flerstegs tvätt- och torkningsprocess. Dessa metoder hjälper till för att uppnå önskade fibrer med en mycket god kvalitet. För att uppnå en kostnadseffektiv, miljövänlig, konkurrenskraftig och storskalig textilproduktion - särskilt inom klädindustrin - krävs därför framtida arbete för att förfina och utveckla tekniken.
Abstract
The growing of the world population caused an increase in waste generation which may contain high-value substances that can be used as raw materials in many applications. Therefore, tremendous research has been done towards the conversion of those wastes, that cause environmental pollution, in more sustainable and biodegradable materials. Part of these materials are fibers and textiles produced from polysaccharide materials especially from chitin and chitosan. Both chitin and chitosan are available as a by-product of seafood as well as in insects and cell walls of fungi, and can be used in many different applications.
Chitin is the most abundant amino polysaccharide polymer after cellulose which has a very similar structure to cellulose, while chitosan is the deacylated form of chitin and it is the well- known and the most important derivative of chitin. Chitosan can be produced from chitin by either chemical deacetylation or enzymatic preparations. However, at commercial scale nowadays, the production of chitosan by chemical method like deacetylation of chitin with an alkali such as NaOH, is more suitable and preferable. Both chitin and chitosan are bio-based materials that have special properties such as high specific stiffness and strength, they are biodegradable and plentifully available in the nature, which make them an active competitive to the production of the synthetic fibers.
Textiles are a big source for carbon emissions because of their large volume production and origin, in some cases, from non-biodegradable fibers. Fibers are the elementary units of textiles besides cotton that is traditionally used for textile production. There are different types of fibers that are usually divided into synthetic- and bio-based fibers derived from renewable resources which are getting a lot of interest in order to produce more biodegradable materials. Therefore, using chitin and chitosan in the textile industry is very important due to their versatility and large abundancy in nature. Additionally, they are biodegradable, biocompatible, non-toxic, and they are essentially able to form fibers and textiles.
The purpose of this thesis was to make a literature review about the methods for the production of fibers and textiles from chitin and chitosan, including their applications and their environmentally friendly aspects.
Different methods have been introduced in this thesis based on many researches and experiments in order to understand and evaluate which are the possible processes for chitin and chitosan fiber formation as well as the properties of the resulted fibers such as tensile strength and elongation.
For fiber production from chitin has been studied by using different solvents including ionic liquids such as 1-ethyl-3-methylimidazolium acetate [C2mim]OAc, 1-butyl-3- methylimidazolium chloride [C4mim]Cl and 1-ethyl-3-methylimidazolium chloride [C2mim]Cl, trichloroacetic acid (TCA) and methylene chloride, a combination of TCA, chloral hydrate and methylene chloride, a mixture of formic acid (FA), dichloroacetic acid (DCA) and isopropyl ether (iPE), as well as a direct dissolution in NaOH/ thiourea/ urea.
Additionally, nanofibers production from crab shells, prawn shells, shrimp shells, commercial chitin powders and mushrooms has been studied. Finally, wound dressing which is one of the nonwoven fabrics applications is introduced by referring to two methods of production.
For fiber production from chitosan, many studies have been listed focusing on the type of the spinning technique such as wet spinning by using a cellulose/chitosan composite solution as well as fibers formed from formic acid modified chitosan. In addition, dry spinning, dry-jet wet spinning and electrospinning techniques have been studied. The wound dressing process by using chitosan/hyaluronan nonwoven fabrics has also been introduced.
In conclusion, the production of textile fibers made of chitin and chitosan is possible and can be made in different ways. And because of their properties as biocompatibility, nontoxicity as well as their antimicrobial effects, they become interesting candidates for medical applications such as in sutures, wound dressing, tissue engineering and as antimicrobial agent. Similar to other manufactural industries, the production of fibers and textiles from chitin and chitosan have many advantages such as good values for dry tensile strength and elongation at break, antimicrobial activity and many more. At the same time, this production has some disadvantages such as the weak and low tensile strength of the resulted fibers and that they are partially soluble at pH below 5.5.
Producing fibers and textiles based on chitin and chitosan is still a challenge because of the many modification steps that are needed. The modifications include the solvent used for dissolution, choosing the proper spinning technique as well as using an appropriate coagulation bath followed by the conditions of washing and drying steps. Thus, the desired fibers with a very good quality mentioned before would be achieved. Therefore, a lot of future work is needed in this manner because the intention is to achieve a cost-effective, environmentally friendly and a competitive technology for the large scale textile production especially in clothing industries.
TABLE OF CONTENTS
1 INTRODUCTION ... 1
1.1 Background ... 2
1.2 Objective ... 2
2 CHITIN AND CHITOSAN: STRUCTURES AND PROPERTIES ... 3
2.1 Chitin: Structure and properties ... 4
2.2 Chitosan: Structure and properties ... 4
3 CONVENTIONAL METHODS FOR FIBER PRODUCTION ... 5
4 PRODUCTION, EXTRACTION AND APPLICATIONS OF CHITIN AND CHITOSAN ... 7
4.1 Production and extraction of chitin and chitosan ... 7
4.2 Applications of Chitin and Chitosan ... 9
5 METHODS OF PRODUCING CHITIN-BASED FIBERS AND NANOFIBERS: ... 11
5.1 Different solvents used for dissolution of chitin before spinning ... 12
5.1.1 Dissolution of crustacean shells using ionic liquids to produce chitin fibers and films ... 12
5.1.2 Dissolution of chitin by trichloroacetic acid (TCA) to produce chitin-based fibers and surgical sutures.. 13
5.1.3 Dissolution of chitin by using a mixture of dissolution solvents for Chitin fibers production ... 14
5.1.4 Chitin/Cellulose blend fibers by direct dissolution ... 15
According to Zhang et al. (2009), a bio-fiber could be achieved by direct dissolution in NaOH/thiourea/urea aqueous solution of a blend of chitin and cellulose. The study showed that mixing cellulose and chitin is an attractive. The resulted fibers from this blend had higher thermal stability than that of fibers made from only cellulose. This is due to the strong intramolecular interactions between chitin and cellulose, which make them also interesting to be used in biomedical applications. But on the other hand, the tensile properties of the blended fibers were low due to the partial damage of the crystalline region of cellulose due to chitin addition. ... 15
5.2 Methods of producing chitin-based nanofibers ... 15
5.2.1 Production of chitin nanofibers by top-down approach ... 16
5.2.1.1 Chitin nanofibers (CNFs) from crab shells ... 16
5.2.1.2 Chitin nanofibers (CNFs) from Prawn shells ... 16
5.2.1.3 Chitin nanofibers (CNFs) from Mushroom ... 17
5.2.1.4 Chitin nanofibers (CNFs) from a commercially dry α-chitin powder of crab shell ... 17
5.2.2 Preparation of chitin nanofibers (CNFs) by bottom up approach from shrimp shells ... 18
5.3 Methods for production of nonwoven fabrics from chitin ... 19
5.3.1 Chitin-based fibers for wound dressing ... 19
5.3.2 Chitin/Chitosan-glucan complex (CHCsGC) for wound dressing ... 20
6 METHODS OF PRODUCING CHITOSAN-BASED FIBERS ... 21
6.1 Wet-spinning of chitosan fibers ... 22
6.1.1 Wet spinning technique ... 22
6.1.2. Cellulose/chitosan composite fibers by wet spinning ... 26
6.1.3 Fibers from formic acid modified chitosan ... 27
6.1.4 Properties of the chitosan fibers in the wet spinning ... 27
6.2 Dry spinning of chitosan fibers ... 28
6.3 Dry-jet wet spinning of chitosan fibers ... 29
6.4 Electrospinning of chitosan fibers ... 30
6.4.1 Electrospinning technique ... 30
6.4.2 Electrospinning of pure chitosan ... 31
6.4.3 Electrospinning of chitosan with poly (vinyl alcohol) (PVA) ... 31
6.4.4 Electrospinning method for production of chitosan- based composite fibers ... 32
6.5 Methods of producing wound dressing from chitosan ... 32
6.5.1 Chitin/chitosan nanofibers for composite wound dressing ... 32
6.5.2 Chitosan/hyaluronan/ nonwoven fabrics for wound dressing ... 33
7 CONCLUSIONS ... 34
REFERENCES ... 35
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1 INTRODUCTION
The growing of the world population caused an increase in waste generation which may contain high-value substances that can be used as raw materials in many applications. Therefore, tremendous research has been done to convert those wastes that cause environmental pollution, in more sustainable and biodegradable materials. Part of these materials are fibers and textiles produced from polysaccharide materials especially from chitin and chitosan (Yadav et al. 2019).
Chitin is the most plentiful amino polysaccharide polymer and the second most abundant polysaccharide after cellulose. Chitin can be found in nature and in the exoskeleton of seafood, insects and in the cell wall of fungi. Chitosan is the most known derivative of chitin that can be achieved by enzymatic or chemical deacetylation. Chitin and chitosan are non-toxic as well as biocompatible and biodegradable biopolymers. They are also antimicrobial and hydrating agents (Komi et al. 2017).
The physical and chemical properties of both chitin and chitosan vary depending on the different raw materials and the preparation methods. There are many important and unique properties that chitin and chitosan have which make them optimal candidates for the fabrication of polymeric tissue scaffolds. For instance, the biodegradability, predictable degradation rate and biocompatibility of chitin and chitosan. In addition, the high porosity, non-toxicity to cells, structural integrity and the ability to form films give them an enormous economic value.
Both chitin and chitosan have many advantages and can be used widely in many applications, as fibers and textiles in general, medical applications, wound dressing, controlled drug release and chelation of heavy metals for water purification (Abbas 2010).
Thus, it is important to mention that each application has its specific method of treatments of chitin and chitosan to make the final desired form of fibers, textiles or films; these methods will be discussed in the following sections.
Textiles are a considerable source for carbon emissions because of their large volume production and origin, in some cases, from non-biodegradable fibers. Furthermore, as mentioned in Rana et al. (2015), the energy used and carbon emitted to produce 1 ton of fibers is too high for synthetic fibers compared to biobased fibers. Fibers are the elementary units of textiles besides cotton that is traditionally used for textile production. There are different types of fibers that are usually divided into two groups, synthetic fibers derived from petroleum sources, and regenerated fibers derived from renewable resources. The production of biofibers and textiles from polysaccharide-based materials, which can be found in seafood and insects shell as well as cell walls of fungi have become an interesting trend in the research of biobased materials production from a secondary feedstock (Zhu et al. 2019).
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1.1 Background
Textile production is the most rapidly growing sector including medical textiles, because of their large utilization amount. However, textiles are one of the largest sources of greenhouse gas emissions. Therefore, the production of sustainable and biodegradable textiles is necessary for the protection of the environment. Textiles produced from polysaccharide polymers such as chitin and chitosan have been gathering increasing interest due to their renewability and versatility in the nature (Zhu et al. 2019).
Chitin and its well-known derivative chitosan, have been considered attractive alternatives to synthetic fibers. They are biobased materials that have unique properties that make them equivalent to the synthetic fibers such as low toxicity, biodegradability, biocompatibility and they have the ability to form films. Their large abundance in nature is one of the most important factors for using them in the textile industry. Thus, the commercial value of chitin has been increasing because of its unique properties mentioned above, which are beneficial and suitable for textile industries (Synowiecki et al. 2010).
Different methods have been used to isolate chitin and chitosan from crustaceans such as crabs and shrimps as well as from fungi. The isolation of chitin includes demineralization and deproteinization or vice versa, followed by decolourisation, washing and drying which results in chitin powder. Furthermore, chitin can be converted to chitosan generally by two ways either with homogenous deacetylation or heterogenous deacetylation with NaOH, followed by either dissolving the deacetylated chitin in acid, filtering, washing and drying that results in free amine form of chitosan; or lyophilization which results in chitosonium acid salts (Tharanathan et al.
2010).
Thereafter the chitin or chitosan powder is used to produce the fibers by different methods generally using different solvents for dissolution of chitin, then different spinning techniques that take place in a coagulant bath followed by washing the chitin solution and drying it. Various methods have been introduced in sections 5 and 6.
1.2 Objective
The purpose of this bachelor thesis is to research how fibers and textiles can be produced from chitin and chitosan as well as to provide an overview of their biodegradable and environmentally friendly applications.
The main questions for our literature study were as follows:
- What are the different methods for the production of fibers and textiles from chitin and chitosan, and what are the properties of the produced fibers?
- What are the different applications of these fibers and are they commercially competitive to synthetic fibers?
- What are the advantages of the production of these fibers regarding environmental and future aspects?
This work has been done by searching in different databases such as ScienceDirect, ResearchGate, PubMed as well as patent research in Espacenet. All references are listed in the reference section.
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2 CHITIN AND CHITOSAN: STRUCTURES AND PROPERTIES
Chitin is a natural polysaccharide that has a similar structure to cellulose, with a difference that it has 2-acetamido-2-deoxy-β-D-glucose as its monomer units instead of glucose which is the monomer of cellulose. Those are linked to each other via β(1-4) linkages, which gives it similar properties to those of cellulose including its high insolubility in water as well as in the most organic solvents, and low chemical reactivity.
Chitin usually is of a white and hard inelastic form, and it occurs in nature as crystalline microfibrils (Komi 2017). It is collected from the exoskeleton of shellfish, insects and from the cell wall of fungi. Furthermore, chitin can be found in granules, sheets and powder forms (Barikani et al. 2014).
Chitosan, known as the most important derivative of chitin, is the deacylated form of chitin. It differs from chitin in the acetyl content that is lower for chitosan polymers (Barikani et al.
2014). Chitosan can be produced from chitin by either chemical deacetylation or enzymatic preparations (Komi 2017). For commercial scale, the production of chitosan by chemical method of deacetylation with NaOH, either heterogeneously or homogeneously, is more suitable and preferable because it is cheaper and easier to be done in large scales (Yadav et al.
2019).
Chitosan is not soluble in water, but it is able to dissolve in aqueous acidic solutions, and this property is limiting its utilization in medical applications. Thus, chitosan can be chemically modified in order to improve its solubility and to produce different derivatives of chitosan that have better properties to be used in wider applications (Zhao et al. 2018).
Figure 1: Chitin and Chitosan Sources, adopted from (Jardine et al. 2017).
The structures and properties of both chitin and chitosan are listed in the following paragraphs.
4 2.1 Chitin: Structure and properties
Chitin, poly-(1-4)- β-linked N-acetyl-D-glucosamine, as mentioned before is a polysaccharide material that is available in living organisms. Chitin’s chemical and physical properties are dependent on the different raw materials and the method of isolation and extraction from different sources (Roy et al. 2017).
As mentioned in Casadidio (2019), chitin has three crystalline allomorphs, namely α-, β- and γ- forms, that differ from each other regarding the orientation of microfibrils. The most generally known form of chitin is α-chitin, which is mainly found in fungi, yeasts, krill, lobsters, crabs, shrimps and insects. Its structure is an orthorhombic unit with chains arranged in antiparallel sheets or stacks. β-chitin, that is less common, can be found in squid pens, and it has a monoclinic unit with parallel chains, while γ-chitin is found in Ptinus beetles and Loligo squids and its structure includes a unit cell with a random chain trend predominating (two up one down).
Chitin has various biological properties such as antimicrobial and antioxidant activities.
Furthermore, it can also function as wound-healing agents, anticancer, fungistatic, hemostatic, analgesic, antiacid, antiulcer, immunoadjuvant and many more. In addition, chitin is a non- toxic, biocompatible, biodegradable polysaccharide and chitin is known to form microfibrillar arrangements in living organisms which makes it an interesting candidate for fiber spinning and therefore to be used in fiber and textile industries (Barikani et al. 2014).
Figure 2: Chemical structure of chitin (Casadidio et al. 2019).
2.2 Chitosan: Structure and properties
Chitosan, poly(1,4)-linked 2-amino-β-D-glucose (GlcN), is a linear heteropolysaccharide derived from chitin with either chemical or enzymatic processes. It can be in solid semi- crystalline form or in solution (Casadidio et al. 2019).
Chitosan has many attractive biological and chemical properties. The chemical properties include the presence of amino groups along the chitosan backbone which differ from chitin. In addition, chitosan contains many reactive amino groups that allow inclusion in different chemical modifications. Chitosan is not soluble in water and organic solvents, but it is soluble in diluted aqueous acidic solutions. It forms salts with organic solvents and inorganic acids, and it has a thin film-forming ability (Casadidio et al. 2019). Biological properties include chitosan’s ability to inhibit the growth of bacteria (Shirvan et al. 2019), as well as the ability to form intermolecular hydrogen bonds that gives it high viscosity (Casadidio et al. 2019). In addition, chitosan is non-toxic, has antimicrobial and antioxidant activities, is biocompatible and it is biodegradable, which are important properties to produce biobased fibers and textiles (Yadu et al. 2017).
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Another important fact is that chitosan is the only positively charged polysaccharide that occurs in the nature. Therefore, chitosan as a polyelectrolyte can be used in the preparation of multilayered films, by using a technique called layer-by-layer deposition technique (Croisier et al. 2013).
Figure 3: Chemical structure of chitosan (Casadidio et al. 2019).
3 CONVENTIONAL METHODS FOR FIBER PRODUCTION
A textile fiber as Qin (2016) mentioned, is a unit of matter, and it can be natural or manufactured. Fibers are characterized by having a length at least 100 times its diameter or width. There are many methods to spin the fibers into yarn or even made them into fabrics.
Those methods may include weaving, knitting, braiding, felting and twisting. In addition, many requirements have to be filled to spun the fibers into yarn, for example; fiber length has to be at least 5 mm, it has to be flexible, cohesive, to have sufficient strength, elasticity, durability and luster.
In order to make yarn from fibers, different spinning processes and solvent systems are needed as listed below.
Spinning Process:
Generally the production of fibers can be done by a spinning process where a thick and viscous liquid can be pressed over a spinneret in order to get a continuous filament. The spinning technique used four essential methods, namely melt, wet, dry and gel spinning. In addition, there are new spinning techniques such as electrospinning (Qin 2016).
Wet Spinning:
In the wet spinning process, the spinning solution that is prepared by dissolving the polymer in a proper solution, is initially pumped to a gear pump with accuracy through a spinneret into a coagulation bath that precipitates the polymer in order to form filaments. The filaments are collected in bundles of the desired thickness from the exit part of the coagulation bath.
Thereafter, the filaments are washed in successive extraction baths in order to remove the residual solvent before they are stretched, dried and heat relaxed. This type of spinning is usually used to produce acrylic, modacrylic, rayon, aramid and spandex fibers (Qin 2016).
Dry Spinning:
As mentioned in Qin (2016) the polymer solution in the dry spinning process is heated before being extruded into a hot stream gas that is also continuously heated during its passage.
Thereafter, the filaments are formed by evaporating the solvent.
6 Melt Spinning:
Generally, the melt spinning process is used in the production of synthetic fibers such as nylon, polyester, polypropylene etc.. In this process, a rapid cooling system is used to convert melted base materials into long strands or filaments. A typical melt spinning process consists of a large spinning tower where a drum of the used polymers sits at the top of it. A metering pump and filtering system are placed below the drum. The melted polymer is pumped down with the pump through a metal die, or spinneret. Large numbers of microscopic holes are the main part of the used die or spinneret, which helps in the formation of thin strands from the molten polymer.
Thereafter, the filaments are passed through a cold air blower in order to cool down and solidify the strands. The strands move down into a series of advancing rollers, followed by many steps such as stretching, crimping, heat setting, etc. before they are wound up as rolls of continuous filaments or cut into short-staple fibers (Qin 2016).
Gel Spinning:
According to Qin (2016), this process is used to obtain high-strength fibers. The polymers that are used for the formation of fibers by this process are not in a true liquid state during extrusion.
The gel term is coming from the fact that the polymers are not completely separated as they would be in a true solution. However, the polymer chains are bond together at various points in liquid crystal form and these bonds produce strong inter-chain forces in the resulted filaments.
These forces can increase the tensile strength of the fibers. Gel spinning can be used to produce high-strength polyethylene and aramid fibers.
Electrospinning:
The electrospinning system contains a high-voltage direct current supply, a grounded electrode, a nozzle system with diameter controls, as well as a fixed or rotated target to which the spun fiber can adhere. In this method, fibers with excellent surface morphology and porosity are obtained, which is appropriate for medical applications. In addition, the fibers are usually spun directly onto the nonwoven structures. The nonwoven structures are flat porous sheets and have a high surface area that differs from the woven textile structures, which are formed by weaving on a loom and made by many threads woven on a weft such as cotton, linen, silk, etc. (Qin 2016).
Solvent system:
One of the most important basic steps to produce desired fiber structures and properties is to choose a proper type of solvent system. And for each solvent system many parameters, such as the concentration of the used polymers, pH, temperature, ionic strength, molecular weight, etc., must be considered since they influence the dissolution process and solution viscosity, (Pillai et al. 2009).
Chitin and its derivatives, including chitosan need also to be dissolved before spinning. Thus, there are a lot of solvent systems that have been studied by many researchers. Chitin is showing variations in the solubility depending on the different sources they have been extracted from.
Therefore, it is not soluble in water and in the most organic solvents, but it can be dissolved in dichloroacetic acid-isopropyl ether-formic acid, trichloroacetic acid, dimethylacetamide- lithium chloride and a mixture of trichloroacetic. While chitosan is also insoluble in water, it can be dissolved in solvents such as acidic solutions, dilute aqueous organic acid, ionic liquids, and dimethylsulfoxide/N-N-dimethylsulfoxide (DMF) and many more (Roy et al. 2017).
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The pH of the solvent medium, which is one of the parameters to choose an appropriate solvent, influences the aggregation behaviour of the used polymer, and this effect includes increased ionization when the pH moves to low values, namely less than 6.0. The high pH value (>6.0) leads to an increase in the number of deprotonated amines of chitosan in the solution and converts aggregation into agglomeration due to the presence of hydrogen bonds in chitosan (Roy et al. 2017). As Roy et al. (2017) also mentioned, increasing the ionic strength of chitosan increases the dissolution temperature because the dissolution temperature is proportional to the ionic strength. In general, increasing the temperature helps to dissolve the hydrogen bonds between acetyl and hydroxyl groups in chitosan and it becomes more soluble. Besides the pH and the ionic strength, the molecular weight affects the conformational changes and solubility of chitosan. The decrease of molecular weight increases the chitosan solubility (Roy et al.
2017). In this manner, the following paragraphs introduce how chitin and chitosan are produced, extracted as well as their applications in textile industry, followed by different methods to produce fibers.
4 PRODUCTION, EXTRACTION AND APPLICATIONS OF CHITIN AND CHITOSAN
4.1 Production and extraction of chitin and chitosan
Chitin can be extracted from marine waste and seafood with either chemical or biological methods. Chemical methods include many steps, namely grinding, deproteinization with an alkali such as NaOH, demineralization with acids such as HCl, decolourisation and drying. The biological method includes enzymatic deproteination and fermentation by using microorganisms. According to different parameters and conditions, chitin can get different characteristics regarding molecular weight, degree of deacetylation, purity and polydispersity index. The chemical extraction is preferable even though it is not eco-friendly, because of using concentrated acids and alkali under high-temperature conditions, unfeasible, and affects chitin’s chemical and physical properties. Thus, the biological extraction has been more interesting due to its cheaper process, because it offers simpler manipulation, low energy input, greater reproducibility as well as it is less time- and solvent-consuming. However, its disadvantage can be that the enzymes are not always cheap and this process is limited to laboratory scale (Yadav et al. 2019).
Chitosan can be extracted from chitin mainly by deacetylation reaction in two ways as shown in figure 4, namely homogenous deacetylation or heterogenous deacetylation by using an alkali such as NaOH at different temperatures. The first way produces chitosan directly by drying, while the second way needs a step where the sample is dissolved in acid (acetic acid 2%), then filtered. This also gives two products, chitosan (free amine form) and chitosonium acid salts (water-soluble), by using precipitation with NaOH, washing and drying to get the first product, and lyophilization for the second. The following steps include rinsing the solutions with distilled water in order to get a neutral pH value (Yadav et al. 2019).
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Figure 4: Production of chitosan from chitin, adopted from (Yadav et al. 2019).
Chemical extraction of chitin:
The chemical extraction as described in Yadav et al. (2019), has three main steps, namely deproteination, demineralization and decolourisation. In the deproteination step, the chemical bonds between proteins and chitin are broken by using chemicals such as NaOH that depolymerize the biopolymer and requires 16-48 hours. The demineralization step is mainly used to remove the minerals especially calcium carbonate, principally by acidic treatment using sulphuric acid, nitric acid, acetic acid, formic acid, and hydrochloric acid which is the most used reagent. This step takes 1-24 hours. Lastly, the decolourisation step is an optional step and it is done when a colorless product is desired. For this purpose, an organic solvent mixture or acetone are used in order to remove the pigments.
Biological extraction of chitin:
In this process, microorganisms and enzymes can be involved in order to remove proteins during the extraction of chitin and thus making the process eco-friendly by avoiding alkaline treatments. This step produces chitin and protein hydrolysates with nutritional value. The main biological methods that are used for chitin extraction include enzymatic deproteination and two different methods of fermentation by using microorganisms which are lactic acid fermentation and non-lactic acid fermentation. In the enzymatic deproteination, proteolytic enzymes such as proteases, which can be found in plants, animals and microbes, are used to remove proteins from chitin and minimize the depolymerization and deacetylation during the isolation of chitin.
The major proteolytic enzymes used in this process are papain (from plant source: papaya fruit), trypsin (present in pancreatic secretions of humans and in all animals), alcalase (from microbial source: Bacillus licheniformis) and pepsin (animal source). This method is a clean process, but it has lower efficiency compared to chemical extraction.
On the other hand, the fermentation method can be carried out by using different strains of microorganisms to reduce the costs of using enzymes in the enzymatic deproteination method.
Fermentation can be done by one or two stages, co-fermentation/subsequent fermentation or from endogenous microorganisms (auto fermentation). The fermentation method can be done in two ways, namely lactic acid fermentation by using Lactobacillus spp. such as L. plantarum, L. paracasei and L. helveticus, and non-lactic acid fermentation by using fungi and different types of bacteria such as Bacillus spp., Pseudomonas spp. and Aspergillus spp. with different conditions (Casadidio et al. 2019).
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After using a proper extraction method, the extracted and isolated chitin can be converted into chitosan by a deacetylation step which can be done either heterogeneously or homogenously.
Figure 5:Chitin and Chitosan extraction from marine waste, adopted from (Yadav et al. 2019).
The polysaccharide nature of chitin and chitosan and their reactive functional groups give them the capability of participating in many processes. In order to achieve this, chitin and chitosan are required to be suffer different chemical modifications such as acylation, quatemization, alkylation, hydroxylation, phosphorylation, thiolation and graft copolymerization. This results in processing chitin and chitosan polymers into gels, films, membranes, nanofibers and nanofibrils and many more (El Knidri et al. 2018). The focus of this thesis is to evaluate the production of textile fibers from chitin and chitosan, this will be discussed in detail in the following sections.
4.2 Applications of Chitin and Chitosan
Chitin and chitosan can be utilized in many applications including in medical textiles, wound healing, tissue engineering, water purification, cancer diagnosis, antimicrobial agents etc..
Medical textiles
Medical textiles are one of the technical textile industries that are developing very fast compared to other sectors (Shirvan et al. 2019). These textiles have many important requirements such as biocompatibility, non-toxicity, non-allergenicity, non-carcinogenicity, antimicrobial effects and many more. As chitin and chitosan have almost all these required chemical and biological properties, they are important candidates for this sector (Shirvan et al.
2019). Additionally, mixing chitin/chitosan powder with viscose pulp (known as crabyon omikenshi) followed by wet spinning, produces a composite material of chitin/chitosan and cellulose. Thus, the resulted composite fibers have some relevant properties such as higher moisture content in comparison to that of cellulosic fibers. They also have dyeability towards direct and reactive dyes. In addition, they can be used to produce textiles for babies and elderly people who have weak and sensitive skin, due to its ability to keep the skin from drying without giving any irritation to the skin (Roy et al. 2017).
10 Wound Dressing
Wound dressings as mentioned in Shirvan et al. (2019), are used for wound healing. In general, they have three main performances including providing protection against infection, absorbing blood and exudate, and in some cases, employing medication to the wounds. The adhesive nature of both chitin and chitosan besides their antimicrobial activity and oxygen permeability give them the advantage to be used in the treatment of injuries, wounds and burns.
Tissue engineering
Chitin and chitosan have been well applied in tissue engineering production, namely in the fabrication of polymer scaffolds. To do this, many requirements need to be filled such as high porosity (with a suitable pore size distribution), biodegradability, structural integrity, nontoxicity to the cells, biocompatibility and many more (Komi et al. 2017). Several methods have been evaluated in order to produce chitosan scaffolds such as phase separation and lyophilization technique, rapid prototyping technology, and the formation of microparticles and microspheres (Komi et al. 2017). On the other hand, chitin-based materials can be also applied in tissue engineering by fabricating them into tubular forms. They can be used in tissue engineering of nerves and blood vessels as a template for cells. Moreover, scaffolds based on chitin are also adaptable products that can be improved to be used in several developmental intentions (Komi et al. 2017).
Chitosan as an auxiliary in the textile dyeing process
The dyeing process is one of the most important processes that help to test the final fibers. There are different types of dyes used depending on the type of fibers, such as Jig dye, Pad dye, and Jet dye. The process of spreading the dye molecules requires very high energy and a large amount of water and salt. In general, the dyeing process is costly considering the aquatic environment-related health, because of effluents containing chemicals in the dyeing process.
Therefore, chitosan or modified chitosan is used to reduce the use of salts if the amount of untreated dye is increasing in the bath. Moreover, the presence of chitosan prevents fiber degradation due to the attack of the oxidizer and maintains the mechanical properties of the fabric such as tensile strength and elongation at break (Roy et al. 2017).
An antimicrobial agent for textile
Chitosan-treated fabrics and fibers have a major role in the production of antibacterial fabrics for working environments such as a hospital, biotechnology research lab, cosmetics, fundamental industries, and so on. Chitosan has charged amino groups that interact with the cell wall of microbes which causes protein degradation as well as intracellular constituents. In addition, it affects the permeability of essential nutrients changing the cell membrane and causing cell death (Roy et al. 2017). Generally, as Roy et al. (2017) mentioned, a single antimicrobial agent usually has some limitations in order to act against gram positive and gram negative bacteria including an extensive range of microbes. For that reason, chitosan was interesting to use by combining it with other antimicrobial components. For instance, the combination of chitosan with silver particles has gained a lot of attention because of their ability to be used for the synthesis of chitosan-silver nanoparticles. Thus, producing an excellent antimicrobial property on textile application. In the following Sections 5 and 6 several methods for producing textiles based on chitin and chitosan including the resulted fibers properties and the process conditions will be discussed based on our literature research.
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5 METHODS OF PRODUCING CHITIN-BASED FIBERS AND NANOFIBERS:
According to Pillai et al. (2009), similarly to cellulose, chitin can be converted to form fiber and films due to its linear chain structure. Additionally, another indication that chitin can be spun into fibers is that microfibrils of chitin can be found with diameters between 2.5 to 2.8 nm which are usually embedded in a protein matrix in chitin sources.
Chitin has to be able to dissolve in a proper solvent by breaking up its structure. In chitin fibers formation process, choosing the appropriate spinning technique is an important step. For example, melt spinning is not preferable because chitin decomposes before melting. Therefore, many studies have been done regarding the dissolution of chitin in appropriate solvents and choosing the best spinning conditions in order to form fibers with strong tensile strength. The washing step is also needed in order to rinse the resulted spinning solution from the undesired particles and to remove unreacted chitin. The following step is the drying; this step can be done in different ways, namely at room temperature or air-dried, or using higher temperatures to get the final dried fibers (Pillai et al. 2009).
According to Pathan et al. (2014), blending chitin with synthetic composites, can be accomplished by a combination of two or more materials without losing their original properties. Examples of such fibers are carbon fibers, aramids, polyolefins, glass fibers, ceramic fibers, etc. Thus, blending has been an interesting area because it resulted in fibers that have special properties, such as good values for dry tensile strength and elongation at break. Chitin fibers that have been treated with a silver nitrate aqueous solution showed good antibacterial activity against Staphylococcus aureus. Moreover, the crystallinity and surface change density of deacylated chitin increased when treated with hydrochloric acid. Additional improvements to the fiber properties can be done (Pillai et al. 2009). The most appropriate spinning techniques used for chitin-based fiber formation are wet spinning, dry-jet spinning and electrospinning which will be described in the following sections.
Wet spinning and dry jet spinning of chitin
In most of the studies that have been done to produce chitin and its derivative fibers, the wet spinning has been found to be a proper technique (Nierstrasz et al. 2010). The mechanical properties of chitin and chitosan fibers, produced with wet spinning process, depend on the chemical nature of the fiber e.g. degree of deacetylation and spinning conditions. A lower degree of deacetylation in the polymers results in fibers with higher dry and wet strength than fibers produced from more deacetylated polymers (Nierstrasz et al. 2010).
In order to achieve fibers with higher strength, as Nierstrasz et al (2010) mentioned, special spinning techniques such as dry-jet wet-spinning are necessary. In this technique, the biopolymer solutions are also extruded, thereafter the solutions are loaded into an air gap of different length and precipitated in a coagulation bath. Additionally, a special solvent system is used to achieve a liquid crystal phase of chitin and chitosan solutions. In general, the produced fibers from chitin by using the wet spinning process have low tensile strength and they are partially soluble at pH below 5.5. Therefore, more future improvements need to be done for this method.
Electrospinning of chitin
According to Nierstrasz et al. (2010), recent studies have shown that the use of electrospinning technique is generally an appropriate method to produce a fibrous material with favorable
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properties such as very fine diameters, enormous surface-to-weight area as well as excellent mechanical properties. The apparatus of electrospinning, as Barber et al. (2013) explained, includes a high voltage power supply connecting a needle and a collecting electrode plate and a fixed target where the spun fiber can adhere. By a syringe that is attached to the needle, the polymer solution is delivered through air pressure delivered from a syringe pump. In this work, different coagulation solvents can be used depending on the insolubility of chitin.
Electrospinning of chitin uses an electric field in order to stretch micro- and nano-sized fibers from a polymer solution. The process starts with pushing a solution of the polymer through a charged spinneret. Thereafter, a Taylor cone, which refers to the distorting the droplet into a conical shape (Wang et al. 2011), is formed by the drop of polymer solution with the help of high electric potential. By a software-controlled high voltage generator, a high electric potential is applied to the system. After that, the viscous jet of the polymer is formed and passed to the collecting electrode. The following step is the evaporation of the solvent and concentration of the polymer solution which allows the fibers to form on the electrode. The help of a combination of the polymer entanglement density, solution viscosity, and surface tension, the instability in the jet can be repressed, preventing beads, and producing smooth, continuous fibers (Barber et al. 2013). In the following sections, different methods of producing fibers and nanofibers from chitin have been listed and discussed based on our literature research and study.
5.1 Different solvents used for dissolution of chitin before spinning
According to many studies and as mentioned before, choosing an appropriate solvent is important to dissolve chitin in a satisfactory quantity as well as a proper rheology for spinning (Qin et al. 2010). Therefore, the focus was to find some improved processes that used different solutions in order to form chitin-based fibers.
5.1.1 Dissolution of crustacean shells using ionic liquids to produce chitin fibers and films
In the work by Qin et al. (2010), ionic liquids have been used as solvents to dissolve chitin that occurs in crustacean shells. 1-ethyl-3-methyl-imidazolium acetate [C2mim]OAc, showed its ability to dissolve raw crustacean shells completely. This led to the recovery of chitin powder with a high purity and high molecular weight. Due to this, it can be used in the production of fibers and films by spinning directly from the extracted solution. As Qin et al. (2010) reported, three different ionic liquids have been used in their experiment in order to compare the dissolution of chitin. These ionic liquids were 1-Ethyl-3-methylimidazolium acetate [C2mim]OAc, 1-butyl-3-methylimidazolium chloride [C4mim]Cl and 1-ethyl-3- methylimidazolium chloride [C2mim]Cl. Two samples of chitin have been used namely pure chitin (content of 94.7-96.4%) and pure grade chitin (PG-chitin; content of 78.9%), produced from crustacean shells by a chemical method involving acid demineralization of the shell followed by alkali treatment and decolourisation. Those samples have been added to the ionic liquids (IL) as follows, 1 g PG-chitin to 10 g (IL) and 0.5 g pure chitin to 2 g (IL); those mixtures were heated with magnetic stirring at 100°C for 19 h. Dissolution of chitin in the solvents [C2mim]OAc, [C4mim]Cl and [C2mim]Cl has been tested and the results showed that the highest solubility was obtained in [C2mim]OAc while [C4mim]Cl and [C2mim]Cl had only partially dissolved the chitin. The same thing happened to PG-chitin samples which could dissolve better in [C2mim]OAc than in[C4mim]Cl and [C2mim]Cl. The spinning of fibers from pure chitin solutions was not possible due to the low molecular weight of chitin polymers. Thus, according to Qin et al. (2010), this process was possible and showed successful results with PG-chitin solution and shrimp shell solution as well, by using a dry-jet wet-spinning method.
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Deionized water (DI) was used as the coagulant bath. Moreover, the produced fibers were soaked in warm DI water for 1-2 days in order to clear away any leftover from IL. Thereafter, the fibers have been dried, and the resulted fiber properties have been measured and introduced in table 1.
Table 1: Chitin- based fibers properties from different methods.
5.1.2 Dissolution of chitin by trichloroacetic acid (TCA) to produce chitin-based fibers and surgical sutures
In another study done by Kifune et al. (1981), trichloroacetic acid (TCA) and methylene chloride were used as solvents for chitin dissolution derived from pink crab in order to produce chitin fibers to be used as medical sutures.
Preparation and purification of chitin:
The experiment began with fully drying the crab in a hot air-drying chamber at 40 °C followed by grinding with a hammer. Thereafter, the achieved powder had been drowned in 99.5% acetic acid for 30 minutes in order to remove the proteins, followed by filtration and washing it with methanol. Thereafter, the powder was treated with 2M HCl for 3 hours at 25 °C, followed by neutralization with caustic potash and washed with water. This was followed by treatments with sodium hydroxide and hydrochloric acid, and washed again with water to obtain a white chitin powder (Kifune et al. 1981).
Dissolution of chitin
For comparison purposes, Kifune et al. (1981) took 3 parts, weighed and dissolved in a solution containing 50 parts by weight of trichloroacetic acid, and 50 parts by weight of methylene chloride at a temperature of 5 °C. This process resulted in a transparent and viscous solution of chitin dope. Thereafter, the chitin dope was filtered under pressure, in this case, it was 4 kg/cm2 by using a 1480 mesh stainless steel net, then it was defoamed by reducing the pressure.
Spinning of chitin
The next step according to Kifune et al. (1981), was transforming the defoamed chitin dope and extruding it in acetone as the first coagulation bath. This was maintained with specific
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parameters such as the temperature at 14 °C and the pressure of 2.5 kg/cm2. Thereafter, the chitin dope went through a nozzle consisting of a hole diameter of 0.08 mm and 40 holes at a discharge amount of 2.3 ml/min, by using a gear pump in order to form filaments. The produced filaments were taken out using a roller at a rate of 10 m/min. Thereafter, the filaments had been introduced into the second coagulation bath by treating them with methanol and maintaining the bath at a temperature of 15 °C on a conveyor that was moving at a rate of 0.5 m/min for 5 minutes. Thereafter, they have been wound on a winder with a rate of 9 m/min. The following steps were to take the wound filaments and neutralize them by bathing them for one hour in an aqueous caustic potash solution which had a concentration of 0.3 g/l. After that, the filaments were washed with water. The pH of the washed water was checked until it became neutral. A centrifugal dehydrator was then used in order to dehydrate the washed filaments, followed by drying under reduced pressure at room temperature, preferably for 24 hours. By these steps, chitin fibers were obtained with a dry tensile strength of 3.10 g/d and elongation of 20 %.
Suture formation
Twelve filaments were taken and have been twisted in order to make a braid, and according to the standard USP XX Ed. sutures of 3-0 were also formed which then produced collagen sutures that have a diameter of about 0.32 mm. Thereafter, the knot tensile strength was measured and was 1.84 g/d and the knot tenacity was 1.87 kg. The last step was to prepare a suture based on filaments of the used sample. The suture was then dyed and sterilized in order to use it to suture the abdominal muscle of a rabbit. The results showed that the suture was easy to handle and the resistance of sutures in the muscle of the rabbit was small so that the suture was performed smoothly (Kifune et al. 1981).
Chitin-based fibers formation
From another perspective has Ravikumar (1999) also introduced the suggestion of using trichloroacetic acid (TCA) as a solvent for chitin dissolution. Chitin was crushed and pulverized; it was then added to a solvent mixture with 2 parts of weight to the mixture that contains 87 parts by weight of 40% TCA, 40% chloral hydrate, and 20% methylene chloride.
The mixture stayed for 30 to 45 min at room temperature. The extruding of filaments was done by using a hypodermic needle and acetone as the coagulant. Thereafter, the filaments were neutralized in 2-propanol with potassium hydroxide (KOH); this was followed by washing with DI water and cold drawn. In order to test the produced filaments, Ravikumar (1999) mentioned that two tensile breaks were taken at 60% relative humidity as well as at room temperature. The results showed that the filament that had a cross-section of 0.08 x 0.10 mm gave rise to a tensile strength of 72 kg/mm2 and an elongation of 13%. The filament that had a cross-section of 0.014 x 0.740 mm appeared to have a collapsed core structure. Thus, it showed a tensile strength of 104 kg/mm2 and an elongation at break of 44%.
5.1.3 Dissolution of chitin by using a mixture of dissolution solvents for Chitin fibers production
Tokura et al. (1979) have studied the possibility of using a combination of formic acid (FA), dichloroacetic acid (DCA) and isopropyl ether (iPE) as a solvent to dissolve chitin powder (30- 45 mesh) that was pulverized from Alaska King Crab Shell. The experiment began with suspending 5 to 7 grams of chitin powder in 100 cm3 of 99% FA several times in order to get a clear chitin gel; this was done within a temperature range of –20 °C to room temperature.
Thereafter, a small amount of DCA was added to the chitin gel in order to distribute it. However, the solution of the dispersed chitin had poor spinnability at high concentrations of chitin. It was necessary to add isopropyl ether in order to reduce chitin solution viscosity to almost 100 poise directly before spinning it. After that, the solution was filtered into a soft woven fabric under
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pressure, and it was spun to two different coagulation baths by using a nozzle made of platinum at a pressure of 1.0 to 1.2 kg/cm2. Thereafter, the produced chitin fibers were boiled in water in order to remove any residuals of FA or DCA for one hour, washed and drown in water for 24 hours followed by drying. Two types of spinning methods were used, namely wet and dry spinning in order to measure the tenacity, elongation and knot strength of the fibers which are listed in table 1.
5.1.4 Chitin/Cellulose blend fibers by direct dissolution
According to Zhang et al. (2009), a bio-fiber could be achieved by direct dissolution in NaOH/thiourea/urea aqueous solution of a blend of chitin and cellulose. The study showed that mixing cellulose and chitin is an attractive. The resulted fibers from this blend had higher thermal stability than that of fibers made from only cellulose. This is due to the strong intramolecular interactions between chitin and cellulose, which make them also interesting to be used in biomedical applications. But on the other hand, the tensile properties of the blended fibers were low due to the partial damage of the crystalline region of cellulose due to chitin addition.
Preparation of the mixture solution
The solution mixture was prepared by dissolving cotton linter in an 8 wt.% NaOH, 6.5 wt.%
thiourea and 8 wt.% urea aqueous solution to get a 5 wt.% cellulose solution. Thereafter, the chitin solution was prepared in a 10 wt.% NaOH aqueous solution as described in Zhang et al.
(2009). Chitin preparation was done by adding 40 g of chitin powder and dissolving it in 154 ml 46 wt.% NaOH for 6 hours in an ice bath. Thereafter, the solution was stirred and frozen for 24 hours at –5 °C. After that, the resulted product was heated and stirred at room temperature which resulted in a solution of chitin with a concentration of 3.5 wt.%. Finally, the chitin solution was mixed with cellulose solution to get the desired solution that consists of 10 wt.%
of chitin weight. It was then stirred for 30 minutes at room temperature followed by degassing at 10 °C by centrifugation.
Spinning and blended fibers formation
The spinning machine that was used in this method was a wet spinning device on a laboratory scale. The mixture of the prepared solution was added to the cylinder at room temperature followed by using a pressure of 0.1 MPa to the spinning dope. Thereafter, the dope was extruded into a coagulation bath by using a spinneret with 12 orifices. The coagulation bath consisted of 12.5 wt.% H2SO4 and 10 wt.% Na2SO4 aqueous solution at a temperature of 20 °C. The following step was washing the resulted fibers in boiling water and then drawing out; the used post-drawing roller ratio was adjusted to 120%. Finally, the fibers were dried by a heating roller at a temperature range between 65-80°C and wound on a spool. The tensile strength and the elongation which are listed in table 1 are lower compared to those of cellulose fibers according to Zhang et al. (2009) experiment. This is because the addition of chitin inhibits the cellulose macromolecules orientation leading to a lower crystallization of 55.0±2.1%.
Chitin nanofibers can be produced from crab shells, prawn shells, shrimp shells, mushroom, commercial chitin powder, and many more. These methods will be discussed below.
5.2 Methods of producing chitin-based nanofibers
Nanofibers produced from chitin (CNFs) are used in different structural biological materials. In general, nanofibers have networks with a uniform width of almost 10 nm, and especially chitin nanofibers are known as fibers with less than 100 nm diameter and an aspect ratio of more than
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100. By having a quite high surface-to-volume ratio, unique optical as well as mechanical properties gives the CNFs different properties from those of microfibers. Thus it was interesting to establish new methods to prepare nanofibers from chitin. Mainly two approaches are used in the field of nanofabrication, the top-down and the bottom-up approaches. Thus, the top-down approach (breaking down a large molecule), refers to the manufacturing of the desired nanostructure from natural structures of larger sized materials by successive disintegration, which results in individual building blocks. While the bottom-up (self-assembly) approach refers to that the individual molecules are getting self-assembled in order to yield the desired product (see figure 6).
Figure 6: Top-down and Bottom-down approaches for chitin nanofibers preparation, adopted from (Yadav et al.
2019)
5.2.1 Production of chitin nanofibers by top-down approach 5.2.1.1 Chitin nanofibers (CNFs) from crab shells
Crab shells are interesting raw materials for producing nanofibers because of their hierarchical organization with various structural levels. Ifuku (2014) has reported a simple method to achieve homogenous chitin nanofibers extracted from waste of crab shells. This method mainly included a disintegration process, which started by purifying the shells by a series of chemical treatments followed by mechanical treatment. An aqueous NaOH and HCl treatment was done in order to remove the proteins and minerals. Ifuku (2014) used a grinder (Masuko Sangyo Co., Ltd., Kawaguchi, Japan) to disintegrate the aggregates of chitin and it formed a gel of chitin slurry which contained highly uniform nanofibers with a width of almost 10 nm and the yield of dry chitin in the chitin slurry was 12.2 wt.%.
5.2.1.2 Chitin nanofibers (CNFs) from Prawn shells
Since prawn shells are also made up of hierarchically organized structures as crab shells, they are an interesting source for chitin nanofibers preparation. The method that Ifuku (2014) used was applicable for using with different types of prawn shells. Therefore, the author did an experiment by using three types of prawn shells that are available in abundance from industrial waste, including Penaeus monodon (black tiger prawn), Marsupenaeus japonicas (Japanese tiger prawn), and Pandalus eous Makarov (Alaskan pink shrimp). The method was similar to the method of producing nanofibers from crab shells. This method started with a purification step to remove proteins and minerals. Thereafter, disintegration with a grinder, in order to get a uniform structure of chitin nanofibers within the range of 10-20 nm and the yield of dry chitin in the chitin slurry was 16.7 wt.%. Ifuku (2014) mentioned that the structures of nanofibers produced from prawn shells were greatly similar to nanofibers produced from crab shells, with a difference on the mechanical disintegration which was easier with prawn shells than with crab shells. This goes back to the exoskeleton of prawn shells which is made up of a finer structure than the crab shell exoskeleton.
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5.2.1.3 Chitin nanofibers (CNFs) from Mushroom
Chitin nanofibers are available in the cell walls of mushrooms as well as edible mushrooms which are a source of dietary fibers. They can be found as a food bulking or thickening agent, and a film-forming agent as well as a stabilizer. Again, Ifuku et al. (2013) used approximately the same method as for the crab shells, by using five types of mushrooms that are widely consumed by humans, including Pleuotus eryngii (king trumpet mushroom), Lentinula edodes (shiitake), Agaricus bisporus (common mushroom), Hypsizygus marmoreus (bunashimeji) and Grifola frondosa (maitake). Considering that the cell wall ingredients are truly different from crab shells and prawn shells, different chemical treatments were needed in order to extract the chitin from mushrooms and to remove the proteins, minerals as well as glucans. Thereafter, a grinding treatment in an acidic medium was performed. This method resulted in nanofibers with a width within the range of 20-28 nm depending on mushroom type as well as the yield of the dry chitin in the chitin slurry was between 1.3 to 3.5 wt.% and it was less than that in crab or prawn shells.
5.2.1.4 Chitin nanofibers (CNFs) from a commercially dry α-chitin powder of crab shell
Preparing nanofibers from chitin powder can be done by firstly keeping the powder wet in order to avoid strong inter-fibrillar coagulation which can be considered as a disadvantage for the marketing of chitin nanofibers. Thereafter, the dried chitin powder was suspended in acetic acid medium followed by passing it through a grinder. The aggregates of chitin nanofibers were easily changed to make homogenous nanofibers, even though the commercial chitin powder could be made of mixtures of nanofibers. Another reason for the easy conversion of the powders into homogenous nanofibers; is the repulsive forces from the cationization of amino groups in α-chitin because of electrostatic repulsion effect which eased the fibrillation into nanofibers. In this method, as Ifuku (2014) mentioned, a high waterjet system (called the Star Burst instrument equipped with a ball-collision chamber) was used where the solution of chitin in acetic acid could pass by ejecting the slurry through a small nozzle using high pressure. Thereafter, several mechanical treatments were done in order to get thinner nanofibers (Ifuku 2014).
In conclusion, chitin nanofibers (CNFs) that are prepared from crab shells, prawn shells, mushrooms, as well as commercial chitin powders, can be modified with several chemical modifications as acetylation, deacetylation, phthaloylation, maleylation and many more, in order to design functional materials. These CNFs have powerful biological activities as well as many applications in the biomedical field. Moreover, CNFs can be applied for skin improving applications, namely improving the epithelial granular layer, increasing granular density as well as resulting in lower production of the transforming growth factor beta family (TGF-β). This indicates that the nanofibers of chitin can be integrated into cosmetic and textile productions (Ifuku 2014). In order to be used in the textile industry, nanofibers need to be spun with a proper spinning method, but to the best of our knowledge, there are not a lot of papers that introduced the spinning of chitin nanofibers. However, Barber et al. (2013) suggested a method for electrospinning of chitin nanofibers extracted from shrimp shells which is introduced in the following paragraph. Therefore, the future biomedical applications of nanofibers include wound dressing, tissue engineering, drug delivery, biosensors, etc. In addition, blending chitin nanofibers with other polymers can give a good option because they can reduce the repulsive forces within the charged nanofibers solutions and allow fiber spinning (Pillai and Sharma 2009).
