Spandex Fiber and its Production Process

Spandex is a synthetic fiber that has an exceptional characteristic of elasticity due to which it is also known as elastane. It is lightweight, soft, strong and very stretchable. In fact, spandex fiber was developed as an alternative to rubber but has a better quality than it. The name Lycra has also come to be a synonymous of spandex. However, Lycra is the trademark brand but it has become so popular that all the varieties of spandex are popularly referred to as Lycra. Due to its durability and stretchability, spandex is mostly used to make sports wear and work wear, specially for factory workers. It wears like a second skin and for risk involved jobs like that of machine operators, Lycra clothing is like a boon.

Characteristics of Spandex

The most significant characteristic of spandex is its stretchability. It can be stretched to a great length and then also recovers its near to original shape. It can, in fact, be stretched to almost 500% of its length. It is lightweight, soft, smooth, supple and more durable and has higher retractive ability than rubber. As such, when spandex is used for making any clothing, it gives the best fit and comfort and also prevents bagging and sagging of the garment. It is also heat-settable which means that it facilitates transforming puckered fabrics into flat fabrics, or flat fabrics into permanent rounded shapes. Spandex fibers or fabrics can be easily dyed and they also resist damage by body oils, perspiration, lotions or detergents. These fabrics are also abrasion resistant. When spandex is sewn, the needle causes little or no damage from "needle cutting" compared to the older types of elastic materials. The spandex fiber diameters range from 10 denier to 2500 denier and can be found in both, clear and opaque lusters..

Basic Principle for Making Spandex Fiber
Federal Trade Commission defines Spandex Fiber as a manufactured fiber in which the fiber forming substance is a long-chain synthetic polymer comprised of at least 85% of a segmented polyurethane. The polymer chain is a segmented block copolymer which contains long, randomly coiled, liquid, soft segments that move to a more linear, lower entropy, structure. The rigid segments act as "virtual cross-links" that tie all the polymer chains together into an infinite network. The network thus formed, prevents the polymer chains from slipping past each other and taking on a permanent set or draw. When the stretching force is no longer applied, the linear, low entropy, soft segments move back to the original randomly coiled, higher entropy state, causing the fiber to recover its original shape and length. This segmented block copolymer is made in a multi-step proprietary process. It is extruded into a fiber as a monofilament threadline or for most products into a multiplicity of fine filaments that are coalesced shortly after they are formed into a single threadline.

Materials Used for Spandex Fibers
Many raw materials are used to make the unique stretchable spandex fibers. They include prepolymers which produce the backbone of the fiber, stabilizers which protect the integrity of the polymer, and textile colorants to give attractive shades to the fibers and the spandex fabrics. Two kinds of prepolymers are reacted to produce the spandex fiber polymer back-bone- the flexible macroglycol and the rigid diisocyanate. Macro-glycol can be anything from among polyester, polyether, polycarbonate, polycaprolactone or some combination of these. These are long and flexible chain polymers having hydroxyl groups (-OH) on both ends, responsible for the stretching characteristic of spandex. The polymeric diisocyanate is a shorter and rigid chain polymer, having an isocyanate (-NCO) group on both ends. This molecule provides strength to the spandex fibers. Catalyst such as diazobicyclo octane is used to initiate reaction between the two types of prepolymers. Other low molecular weight amines are used to control the molecular weight of the fibers.

Apart from the basic prepolymers, catalysts etc. stabilizers, such as antioxidants, are added to protect the fibers from damaging sources like heat, light, atmospheric contaminants, and chlorine. Ultraviolet (UV) screeners such as hydroxybenzotriazoles are added to protect it against light degradation. Other stabilizers such as for preventing discoloration caused by atmospheric pollutants may also be used. As spandex is often used for making swim wear, antimildew additives are also added to it. Stabilizers added to the spandex fibers, are resistant to solvent exposure as this could have a damaging effect on the fiber. Dispersed and acid dyes are typically used for giving color to spandex fibers. However, special dyeing methods are adopted if the spandex fibers are interwoven with other fibers such as nylon fiber or polyester fiber.

Manufacturing Process
Spandex fibers are manufactured in four different ways- melt extrusion, reaction spinning, solution dry spinning, and solution wet spinning. The initial step in all these methods is that of reacting monomers to produce a prepolymer. Prepolymer is then reacted further, in a variety of ways, and drawn out to produce a long fiber. The most commonly used method is the solution dry spinning that produces over 90% of the world's spandex fibers.

• First of all prepolymer is produced by mixing a macroglycol with a diisocyanate monomer. They are mixed in a reaction vessel and need the perfect conditions so that they may react to form a prepolymer. Ratio of the component materials is responsible for giving different characteristics to the fibers. As such the ratio is strictly controlled. The ideal ratio of glycol to diisocyanate may be 1:2.
• In the chain extension reaction, the prepolymer is reacted with an equal amount of diamine. It results in a solution which is diluted with a solvent to produce the spinning solution. The solvent makes the solution thinner which can be easily handled. It can then be pumped into the fiber production cell.
• In the fiber production cell, the polymer solution is pumped through a metal plate, called a spinneret, which has small holes throughout its structure. The solution gets aligned in strands of liquid polymer. The strands passing through the cell, are heated in the presence of a nitrogen and solvent gas. The liquid polymer gets chemically reacted and forms into solid strands.
• A specific amount of the solid strands are bundled together to produce the desired thickness with the help of a compressed air device that twists the fibers together. As such, it can be said that each fiber of spandex is made up of many smaller individual fibers that join one another due to the natural stickiness of their surface.
• The fibers are finally treated with textile finishing chemicals that can be magnesium stearate or other polymer such as poly(dimethyl-siloxane). These finishes prevent the fibers from sticking together and help in the process of textile manufacturing. Fibers are then transferred through a series of rollers onto a spool. The windup speed of the entire process depends on the thickness of the fibers that can be anywhere from 300-500 mi (482.7-804.5 km) per minute.
• The spools with fiber, are put into final packaging and shipped to textile manufacturers or any other customers. The fibers here, may be blended with other fibers such as nylon or cotton fiber to produce the fabric that is used for clothing purposes. These fabrics can also be dyed in order to give a desired color to them.

Applications of Spandex Fiber
Spandex Fiber is mainly used to make such garments that require great comfort and fit. As such, they find applications in manufacturing of hosiery, swimsuits, aerobic or exercise wear, ski pants, golf jackets, disposable diaper, waist bands, bra straps and bra side panels etc. They are even great for making shaped garments like bra cups. Spandex fabrics are also used to make compression garments, such as surgical hose, support hose, bicycle pants, foundation garments etc.