Elastomer. fibres (4).pptx

Elastomer fibres (expending fiber)

1. Elastomeric fibres

2. Elastomeric fibres are those fibres that possess extremely high elongations (400%-800%) at break and that recover fully and rapidly from high elongations up to their breaking point.  Elastomeric fibres are made up of molecular chain networks that contain highly amorphous areas joined together by crosslinks. On elongation, these amorphous areas become more oriented and more crystalline in nature.  Elongation continues until the crosslinks in the structure limit further molecular movement. If additional force is placed on the elastomeric fibres at this point, molecular scission occurs, causing a reduction in properties and ultimate breaking of the fibre . 

3. Elastomeric  fibres  include the  cross-linked natural and synthetic rubbers,  spandex  fibres / lycra  (segmented polyurethanes),  anidex   fibres  (cross-linked polyacrylates) and side-by-side  biconstituent   fibre  of nylon and spandex ( Monvelle ).  The  fibres  are all used in specialized applications where high elasticity is necessary within the textile structure. 

4. Clothing that stretches with the human body, supports it and conforms to it, almost certainly contains elastomeric fibres .  Elastomers are polymers possessing high extensibility, with rapid and substantially complete recovery.  At one time, fibres made from rubber were the predominant type of elastomeric fibre , but these have largely been superseded and elastomers are produced from several different polymeric types.  By far the most important in commercial terms are elastanes , which contain 85% or more by mass of segmented polyurethane

5. Polyurethanes are polymers containing urethane groups, –NH–CO–O–, along the length of each constituent chain.  These fibres are known for possessing up to 99% elastic recovery.  They also possess high extensibility (up to 500%), the actual value of which is determined by the composition of the elastane polymeric chains and the method used to produce them.  In Europe, these fibres are often known as Lycra fibres (though other brands such as Roica and Dorlastan also exist), and in the United States they are known as Spandex

6. Elastane Fibres The polymeric chains comprising elastane fibres each consist of alternate blocks of ‘hard’ and ‘soft’ segments.  The soft segments comprise either polyethers or polyesters.  Polyether elastane fibres are more resistant to detergents and mildew,  whereas polyester elastane fibres are more resistant to oxidation (e.g. by ultra-violet radiation) and the absorption of oils, such as suntan oils. 

7. In the relaxed state, the soft segment structures are loosely arranged, but when the fibre is extended, the segments unwind and become more aligned with the fibre axis   On release of the tension, the segments revert to their original state.  The hard segments, by contrast, contain rigid aromatic structures throughout.  Each hard segment can interact with hard segments in adjacent polymeric chains, chiefly through hydrogen bonding.  A strong network is, therefore, provided throughout the elastomeric fibre and slippage between adjacent chains is inhibited.  Hard and soft segments in each chain are joined to one another through urethane linkages. 

8. The balance between the proportion of soft and hard segments is important for determining the properties of the fibre . If the interaction between polymeric chains is too great, then the fibre will not achieve a sufficient degree of stretch. If the interaction is too small, the chains will slip relative to one another and the fibre will not recover properly after being stretched. Judicious control of the structure and length of each segment and of the ratio of hard to soft segments can however provide good control of the mechanical properties obtained. 

9. Elastane fibres can be spun in several ways: Solution dry spinning Solution wet spinning  Melt spinning  The commonest method is dry spinning.  The solvent used is either dimethyl formamide or dimethyl acetamide.  The solid filaments formed when the solvent evaporates are sticky and come together on touching.  This property has the advantage that yarn of required linear density can often be readily produced.  Once the required linear density is attained, yarns are treated with a finishing agent such as magnesium stearate or polydimethyl siloxane, to prevent further yarn adhesion. 

10. Chemistry of elastane with PU It  is made up of a long chain polymer called polyurethane, which is produced by reacting a polyester  with a diisocyanate.  The polymer is converted into a fiber using a dry spinning technique.

11. Polymer reactions 1. The first step in the production of spandex is the production of the prepolymer.  This is done by mixing a macroglycol with a diisocyanate monomer. Some macroglycol that can be used are poly tetramethyleneadiptate glycol, poly caprolactone glycol, poly hexamethylenecarbonate glycol, poly oxytetramethylene glycol, poly oxypropylene glycol, polyether, polyester,polyestercarboxylate , and polycarbonate.

12. Diisocyanate substances are smaller molecular weight, but it plays three roles in building the chains of spandex fiber.  First it acts as the coupling agent for the  macroglycol  component to produce the soft segments.  Then it acts a coupling agent for the chain extender component to produce hard segments.  Lastly, it creates the polyurethane chain by bonding with hard and soft segments  The compounds are mixed in a reaction vessel and under the right conditions they react to form a prepolymer.  Since the ratio of the component materials produces fibers with varying characteristics, it is strictly controlled.  A typical ratio of glycol to diisocyanate may be 1:2.

13. 2. In dry spinning fiber production, the prepolymer is further reacted with an equal amount of diamine. This is known as a chain extension reaction. The resulting solution is diluted with a solvent to produce the spinning solution. The solvent helps make the solution thinner and more easily handled. It can then be pumped into the fiber production cell.

14. Producing the fibers 3 The spinning solution is pumped into a cylindrical spinning cell where it is cured and converted into fibers. In this cell, the polymer solution is forced through a metal plate, called a spinneret, which has small holes throughout. This causes the solution to be aligned in strands of liquid polymer.  As the strands pass through the cell, they are heated in the presence of a nitrogen and solvent gas. These conditions cause the liquid polymer to chemically react and form solid strands.

15. 4 As the fibers exit the cell, a specific amount of the solid strands are bundled together to produce the desired thickness. This is done with a compressed air device that twists the fibers together. In reality, each fiber of spandex is made up of many smaller individual fibers that adhere to one another due to the natural stickiness of their surface.

16. 5 The fibers are then treated with a finishing agent.  This may be magnesium stearate or another polymer such as poly(dimethyl-siloxane). These finishing materials prevent the fibers from sticking together and aid in textile manufacture. After this treatment, the fibers are transferred through a series of rollers onto a spool. The windup speed of the entire process can be anywhere from 300-500 mi (482.7-804.5 km) per minute depending on the thickness of the fibers.

17. 6 When the spools are filled with fiber, they are put into final packaging and shipped to textile manufacturers and other customers.  Here, the fibers may be woven with other fibers such as cotton or nylon to produce the fabric that is used in clothing manufacture. This fabric can also be dyed to produce a desired color.

18. Fibre Structure Elastane fibres possess either round or square cross-sections. Linear density may range from 20 to 6000 dtex , depending on the application for which the fibres are required. When adjacent filaments come together during the spinning process, they are not joined along all of their length. Elastane filaments can be used in combination with yarns of other types of fibre . In some cases, elastane filaments form the inner core of a covered yarn.  In these covered yarns, the elastane cores are each individually covered with yarn of another fibre type.

19. FibreProperties They are quite weak in comparison to other textile fibres , but they possess good resistance to chemical attack. They can, however, be damaged by extended exposure to chlorine.  Elastane fibres based on polyether are more resistant to chlorine and are therefore preferred for swimwear. Elastane fibres are resistant to ultra-violet radiation and to micro-organisms. 

20. Applications Elastane fibre is used in lightweight support hosiery (20–250 dtex ), men’s hosiery (150–600 dtex ) and swimwear and foundation garments (80–2500 dtex ).  Usually, only a small proportion of elastane is present in a fabric (2–5%), to provide the elasticity required.  In foundation garments, however, as much as 45% elastane may be present. Covered elastane yarns are used extensively in cotton and woollen garments, often at a level of 2–5%. 

21. 1. What are elastomeric fibres ? 2. Describe elastomeric fibres   with reference to structure and strength. Give suitable examples. Describe the production method of one elastomeric fibre . 3. how is the polymeric chain of elastanes made? What are the advantages with each class? 4. mention the application area of spandex fabric