i) KEVLAR

is one of the most important man-made organic fibers ever developed. Because of its unique combination of properties, KEVLAR is used today in a wide variety of industrial applications. KEVLAR para-aramid fiber possesses a remarkable combination of properties that has led to its adoption in a variety of end uses since its commercial introduction in the early 1970s.

Fibers of KEVLAR consist of long molecular chains produced from poly-paraphenylene terephthalamide. The chains are highly oriented with strong inter chain bonding, which result in a unique combination of properties.

Properties of Kelvar

• High tensile strength at low weight
• Low elongation to break
• High modulus (structural rigidity)
• Low electrical conductivity
• High chemical resistance
• Low thermal shrinkage
• High toughness (work-to-break)
• Excellent dimensional stability
• High cut resistance
• Flame resistant, self-extinguishing

The physical properties of Kevlar make it a suitable material for many applications, such as:

• Body armour: bullet-proof vests and helmets
• Ropes and cables.
• Belts and hoses for industrial applications
• Composites for aircraft body parts, boats, and sporting goods (e.g. skis)
• Fibre-optic cables for communication, data transmission and ignition.
• Friction products such as brake pads, clutch linings, gaskets. It is often employed as a replacement for the carcinogenic asbestos
• Sailing/motorcycle outerwear
• Adhesives and sealants

The large benzene (R) groups force the C-N bonds in the trans-conformation, resulting in a linear polymer structure. The cis-conformation, shown at the end of the second chain is sterically hindered; the benzene groups (not to scale) are close to each other, resulting in an energetically unfavourable conformation.

The linear structure of Kevlar results in a highly ordered or crystalline structure. This ordering makes the Kevlar fibers exceptionally strong; randomly cluttered strands would be easy to pull apart, but lined up they can withstand enormous tensile forces.

Another reason for the tremendous strength of Kevlar is the formation of hydrogen bridge bonds between the O and H groups on the chain. These hydrogen bridges (symbolized by ":") help the structural rigidity of the polymer:

The synthesis of Kevlar itself is a relatively simple polymerization. However, because of its tendency to crystallize into rigid structures, it was difficult to process into a useful product. Kevlar does not dissolve easily, and it does not melt (it decomposes at 500 C). Spinning the fibers requires the product to be in a liquid state.

ii) PMMA

Poly (methyl methacrylate) (PMMA) is known as Plexiglass. It is a transparent thermoplastic.

PMMA is prepared with the polymerization of methyl methacrylate.

It is being done by bulk or suspension polymerization method using peroxide as initiators.

The reaction involved is: