Natural fibres shapes

Natural fibres, such as wool or cotton, have a curled or irr ular shape. Yams and fabrics made from these fibres are bulky and have a high thermal insulation and a pleasant grip and appearance. Many methods have been developed to give synthetic yams and fibres simUar properties. This is called texturisation. In texturisation processes unannealed yams or fibres are usually deformed by twisting, stuffing or knitting. They are then heat-set in the deformed state, which makes the deformation more or less permanent Most of those processes can be applied to split fibres but we shall only deal with techniques in which orientation effects play an important role. 

Many shapes are to be found in natural fibres. Cotton fibres are C-shaped and have a hole in the centre. Flax and hemp are relatively smooth. Wool fibres are more or less round but have a scaled surface and may be used for felting, a process in which the scales are made to adhere to each other by mechanical forces, assisted by the application of a surface solvent chemical. 

This chapter first gives an overview of cellulose raw materials and their molecular and supermolecular structures. The principles of shaping cellulose into fibres, films, and nonwovens by means of solution techniques are then outlined followed by a section on properties and market applications of these materials. Derivatives of cellulose are presented with special emphasis on thermoplastic cellulose esters, typical plasticizers, and promising reinforcing materials. Finally, recent developments and future prospects of cellulose materials are reviewed as far as the above applications are concerned. This book does not cover the important applications of cellulose and ligno cellulose fibres for reinforcing thermoplastics, like wood plastic composites (WPC) and natural fibre reinforced plastics (NFRP), since in these cases cellulose does not substitute a thermoplastic. 

Natural fibres can be obtained from various parts of a plant, as shown in Table 13.2. Natural fibres can be continuous fibres, short fibres, whiskers or particles, as shown in Figure 13.4. Types of polymer composite based on shape and structure are divided into particle, fibre and structural. 

Figure 13.4 Various shapes of natural fibres (author s experiment).

When native natural fibres are used in conventional textile yams, the manufacturer has control over the macroscopic degree of twist imparted to the filaments, and (within limits) the length of filaments used. However, (s)he at best has only partial control over stmeture and properties at length scales smaller than that of the filaments. At the.se smaller length scales, nature controls the stmctural variables that will dictate fibre strength the primary stmeture (monomer sequence) of the polymer chains, the conformation (shape) of the chains, and the supramolecular organisation of the chains. Often the chains adopt hierarchical helical stmetures, exemplified by those in keratin (Fig. 1). Combined with the macroscopic twist in yams, the molecular and... 

Moisture is introduced by the use of steam. Steam at different pressures has different moisture contents the higher the steam pressure, the lower the moisture in the steam. The presence of moisture is required to aid in fibre swelling and thus shape stabilisation. Different fibres require different amounts of moisture. For example, natural fibres such as cotton and wool and regenerated cellulose fibres such as bamboo viscose and viscose rayon require the presence of moisture in the steam, and therefore steaming tables are usually preferred. On the other hand, synthetic fibres require heat to promote swelling and therefore relaxation of the structore. Excessive moisture may cause fabric shrinkage and colour bleeding. 

Many other properties have to be considered, especially for apparel fibres, e.g. moisture absorption, dyeability, drape, texture, weaving characteristics, etc. Many of the properties are strongly influenced by cross-sectional shape cotton is a round hollow fibre, whereas silk has a triangular shape giving it a fine lustre and rustle. Wool has a scaly surface and appears to be made up of two components which have different water absorption characteristics. This gives wool its crimpability and bulk. From studies like these, synthetic fibres can minimic natural fibres and can, in some cases, be more versatile. For example ... 

The heat deflection temperature (or heat distortion temperature) is an important material property mostly used to determine a material s useful temperature operating range. It refers to the temperature below which a moulded object can hold its own shape. It can be determined using a dynamic mechanical analyser set to apply a constant force. The HDT was determined for blends of plasticized soy flour (52% protein) and polyamide (nylon) as 45 °C when plasticized with 20 wt% sorbital, 35 °C when plasticized with 20 wt% glycerol, and 39 °C when plasticized with 10 wt% of each." When such a blend is used to make composites with natural flbres, increasing content of natural fibres also increased the HDT. 

Natural fibres have a number of inherent disadvantages. They exhibit large variations in staple length, fineness, shape, crimp and other physical properties,... 

From the earliest times man depended upon nature for polymeric materials like wood as fuel, furs and fibres as clothing, grain and flesh as food. Many polymeric materials behave as plastics, i.e., in some stage of their fabrication they are soft and putty like and can be moulded into any desired shape and then set to retain that shape. 

The muscle fibres of the ciliary bodies are innervated by the mrd or oculomotor nerve. As explained above, when these fibres contract they allow the lens to take up its natural shape. Direct stimulation of the Illrd nerve therefore produces accommodation for near objects. Parasympathomimetic drugs have a similar action, whereas atropine paralyses this effect and so accommodates the lens for seeing distant objects. 

The fixed nature of the collection fibres in a conventional SORS experiment limits the range of spatial offsets available. An alternative approach (inverse SORS) swaps the laser and collection fibre geometries Raman light is collected through a group of fibres (arranged in a disc shape) contained within the centre of a probed area defined by a ring-shaped laser beam (see Fig. 3.5). 

Industrial separation membranes and ion-exchange resins can be made from chitin, especially for water purification. Chitin is also used industrially as an additive to thicken and stabilize foods and pharmaceuticals. Since it can be shaped into fibres, the textile industry has used chitin, especially for socks, as it is claimed that chitin fabrics are naturally antibacterial and antiodour (www.solstitch.net). Chitin also acts as a binder in dyes, fabrics and adhesives. Some processes to size and strengthen paper employ chitin. 

Crystal habit modification. Several crystal habits have been reported in the open and patent literature for zeolite omega. Elongated hexagonal rods (15,16) or fibres (17) have been reported when the zeolite resulted from the recrystallization of another zeolite, Y (15) or S (17), or of clays (16). Moreover the natural counterpart of zeolite omega, mazzite, appears as bundles of needle-shaped particles (18). All these solids have been grown at low supersaturation levels, hence under conditions in which our results show that the growth in the direction <001> prevails on the growth normal to the c-axis (Fig.8). 

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