THULE / Shanks Natural Fibre Composites

Materials, Processes and Properties
1. Auflage 2014
ISBN: 978-0-85709-922-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 408 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

The use of natural fibres as reinforcements in composites has grown in importance in recent years. Natural Fibre Composites summarises the wealth of significant recent research in this area. Chapters in part one introduce and explore the structure, properties, processing, and applications of natural fibre reinforcements, including those made from wood and cellulosic fibres. Part two describes and illustrates the processing of natural fibre composites. Chapters discuss ethical practices in the processing of green composites, manufacturing methods and compression and injection molding techniques for natural fibre composites, and thermoset matrix natural fibre-reinforced composites. Part three highlights and interprets the testing and properties of natural fibre composites including, non-destructive and high strain rate testing. The performance of natural fibre composites is examined under dynamic loading, the response of natural fibre composites to impact damage is appraised, and the response of natural fibre composites in a marine environment is assessed. Natural Fibre Composites is a technical guide for professionals requiring an understanding of natural fibre composite materials. It offers reviews, applications and evaluations of the subject for researchers and engineers. - Introduces and explores the structure, properties, processing, and applications of natural fibre reinforcements, including those made from wood and cellulosic fibres - Highlights and interprets the testing and properties of natural fibre composites, including non-destructive and high strain rate testing - Examines performance of natural fibre composites under dynamic loading, the response of natural fibre composites to impact damage, and the response of natural fibre composites in a marine environment

Autoren/Hrsg.

Thule, Alma

Shanks, Robert

Leseproben

2 Chemistry and structure of cellulosic fibres as reinforcements in natural fibre composites
R.A. Shanks,     RMIT University, Australia Abstract:
Cellulose fibres, derivatives and regenerated compositions have traditionally been applied in many materials for structural, decorative and disposable products. Cellulose structure–property–performance begins with glucose covalent bonding, and properties extend to supramolecular assemblies. Chirality dependent stereochemistry distinguishes cellulose from synthetic polymers. Cellulose fibre structure, separation and purification from plant materials are described. Processing dependent properties, enhanced performance, shaping into regenerated fibres, sheets or nano-fibres and crystals are reviewed. Cellulose chemistry extends into crystallinity, polymorphism, solubility and environmental dependent characteristics. Cellulose, as with all biopolymers, exhibits typical polymer characteristics expected from theory; however, its unique structural complexity gives cellulose an additional property set. Strongly hydrogen-bonding solvents including ionic liquids are adopted for processing. New cellulose technologies contribute value-added materials derived from trees and crops, with emphasis on nanostructured celluloses. Key words natural fibre cellulose structure cellulose solution regenerated fibre cellulose derivative 2.1 Introduction
Cellulose is the most abundant organic chemical. Cellulose is a polymer with complex monomeric units that have a precise stereochemistry arising from a group of chiral carbons arranged in a six-membered ring. This complex monomer and its reproducible attachments along cellulose chains provide a set of structural levels that each contribute to the formation of cellulose crystals, the chemical properties and consequential physical and mechanical properties. The complexity and order within cellulose needs to be developed from the primary covalent bonded monomer to perceive the many contributions of stereochemistry and intra-/intermolecular interactions that constitute the cellulose fibre structure and properties. Within each of the levels of structure from primary bonding in the monomer, stereochemistry, polymer chain linkages, macromolecular conformation and supramolecular assembly, cellulose is a chemical marvel. The stereochemical complexity brought about from four chiral carbon atoms plus a fifth chiral carbon arising from cyclic structure and interchain link ensures the selective conformation and consistent molecular arrangement within cellulose structures. Cellulose is the structural material of plants, algae and bacterial species. It is a resource derived from trees, plant stalks and agricultural waste. Formation of cellulose by photosynthesis utilizes atmospheric carbon dioxide, and this carbon capture secures the carbon within plants and any products derived from them. Cellulose fibre composites are used in many durable materials for building, transport and furnishings where carbon contained in the cellulose is removed from the atmosphere. Because cellulose is a natural structural material, cellulose sources are already available with suitable properties for application after separation, purification, shaping, where applicable. In many cases minor modification is required to meet an application. More processed forms have been developed for specialized use, such as treated timber, reconstituted fibres and paper making. Cellulose is combined with other substances in plants, therefore to utilize cellulose fibres directly they must first be separated and purified. A further step is to disassemble the structure of native cellulose through solution processes and then reconstitute it into new forms such as textile cellulose fibres, films and moulded shapes. The aim of this chapter is to review the structure, chemistry and properties of cellulose based on chemical structure, stereochemistry and supramolecular structures, through to bulk properties such as lower critical solution properties, ionic solvents, solution characteristics, nano-cellulose and high performance materials. This review will provide a context for inclusion of cellulose fibres from natural fibres, regenerated fibres, modified fibres and nanostructured fibres, in composite materials. Other composites utilize cellulose and derivatives as a matrix phase in fibre and inorganic particle containing composites, where typically one of the reinforcements will be a type of cellulose fibres. Microcrystalline cellulose (MCC) and nano-forms of cellulose including cellulose nano-fibres (CNF) and cellulose nano-crystals (CNC) are included. 2.2 Glucose monomer
Glucose is the monomer that forms cellulose via a biological-enzyme catalysed step-growth or condensation polymerization. The repeat unit is ß-1,4-anhydroglucopyranose. This chapter will briefly consider the mechanism of glucose biopolymerization; however, emphasis is on the structure and properties. Glucose is a hexose, 6-carbon sugar, a diastereoisomer of 2,3,4,5,6-pentahydroxyhexanal, where each of carbons 2, 3, 4, 5 are chiral, giving 16 possible configurations comprising diastereoisomers and enantiomers (a special case of being mirror image structures). The 5-hydroxy combines with the aldehyde to form a six-membered cyclic hemiacetal called the pyroanose form of glucose, shown in Fig. 2.1.
2.1 ß-Glucopyranose The cyclic structure is not planar and, in the case of glucose only, all of the hydroxyl groups pendent to the cyclic structure are in the most stable conformation, being approximately planar (equatorial) to the puckered ring. The hemiacetal hydroxy on carbon 1 on the right in Fig. 2.1 is a new chiral centre, giving a total of 32 diastereoisomers and enantiomers. The conformation is called ß-glucopyranoside and is the conformation that occurs in cellulose. The opposite conformation is called a-glucopyranoside, which creates the more bent link that occurs in starch. The difference between cellulose and starch (amylose and amylopectin) is the chirality about carbon 1. In glucose the a-anomer is more stable than the ß-anomer by about 2:1 even though the a-hydroxy is in a normally less stable axial configuration. This reversal of stability is called the anomeric effect, which is brought about by interaction of the dipole from the non-bonding electrons on the adjacent hemi-acetal-ring oxygen aligning with the hemiacetal hydroxyl on C1. The planar structure of cellulose suits it to regular close-packing and crystal formation. The bent conformation of starch makes it suitable to form loose coils that crystallize with space for water and other complexed molecules within the coils. Each carbon in the glucopyranose structure has a pendant hydroxy or –CH2OH (carbon 5) and hydrogen, where the small hydrogen atoms are directed out of the cyclic plane, because since hydrogens are small they are less crowded in this conformation than if the hydroxyls were the non-planar (axial) groups. This is illustrated in the more complete structure of ß-glucopyranose in Fig. 2.2. The in-plane groups are called equatorial, while the out-of-plane groups are called axial, the descriptions approximating to the actual directional orientation of the pendant groups.
2.2 ß-Glucopyranose showing out of plane hydrogen atoms. The hydroxyl groups of cellulose appear to be typical alcohol functional groups; however, each hydroxyl has other hydroxyls in proximity including the hemiacetal ether type oxygen. Attraction of electrons by oxygens renders each hydroxyl group more acidic than a typical alcohol. The hydroxyl groups form salts with strong bases such as sodium hydroxide, and lithium or potassium homologues. The salts form in low concentration even in strong base because the hydroxyl groups are only marginally acidic. The acidic nature of cellulose hydroxyls is utilized for swelling, partial dissolving, polymorphic transitions, dyeing and derivatization reactions. The cyclic structure of glucose is oxidized at the C6 hydroxyl to form an aldehyde or carboxylic acid, generally a carboxylic acid with many mild oxidizing agents such as sodium perborate. The carboxylic acid, glucuronic acid, shown in Fig. 2.3 can form salts the same as any carboxylic acid; however, in carbohydrate structures gluconic acid and other carboxylic acids from isomeric hexapyranoses convert these materials into gels, viscose solutions, or with divalent metal ions such as calcium, magnesium or zinc, ionic crosslinking takes place. Figure 2.3 shows gluconic acid formed by oxidation of the aldehyde at carbon 1 in open chain form, and oxidation of both carbon 1 and 6 to form open chain glutaric acid. Glucuronic acid is the carboxylic acid form found in polysaccharides since carbon 6 can be oxidized to carboxylic acid without preventing or disrupting polymer formation.
2.3 (a) Glucuronic acid, (b) gluconic acid, (c) glutaric acid, (d) cyclic form ß-glucuronic acid. Other hexoses are found in polysaccharides often as copolymers, such as hemicelluloses, pectins and gums (Kaplan, 1998). Common...