History

Discuss about the Resourced From The Internet for Future In Aerospace Industry?

KWOLEK (1965) researching at the chemical company DuPont, found that that some polymers a special structure in a specific condition and thus showed extra strength. She was actually working on the shortage of petrol resources and thus was trying for creating a new fibre for tires. This had to be lightweight and strong as well. She was working on the polymers poly-p-phenylene-terephthalate and poly benzamide. Now the solution formed in such cases used to be cloudy and less viscous. But KWOLEK (1965) found something unusual about this. When this solution was stirred at high speed the fibres inside the beaker remained intact unlike those in the case of other polymers like nylons. She therefore realised that accidentally a material with exceptional properties has been discovered. But it was only after 7 years in 1971, that Kevlar was introduced commercially as a fibre material.

It is a category of aramid synthetic polymers. Another similar polymer with close structural and mechanical properties is Nomex etc. Kevlar was used in the beginning only for the purpose it was discovered i.e. as the replacement for the steel tires used in automobiles. However, slowly its use has extended to military purposes or aircraft as well because now it available in sheets or spun ropes too.

Kevlar is a polymer whose constituent monomers are 1,4-phenylene-diamine (para-phenylenediamine) and terephthaloyl chloride. It is produced by the reaction of these monomers in the liquid phase. This is a condensation reaction in which the two molecules condense to give out hydrochloric acid also. The polymer hence formed is crystalline even in the liquid form. These monomers react in the presence of  Hexamethylphosphoramide (HMPA). Although it has now been ceased to use due to the corrosiveness of its solution in the polymer. These days a solution of N-methyl-pyrrolidone and calcium chloride is utilised for the solvent purpose.

Now since concentrated acid is produced in the solution along with the polymers it complicates the process and put the polymers at the risks of being corroded. Also since the polymers formed in strands it gets separated out of the solution creates issues during synthesis. It also degrades in the presence of ultraviolet light and hence can be corroded in the presence of sunlight. This limits the usage of Kevlar in outdoor areas directly under sunlight.

QUINTANILLA (1990) has found that the fibre which is formed after spinning Kevlar has a tensile strength of about 3,620 Mpa. Its density with respect to water is  1.44. The formed has various inter-molecular bonds including the high strength hydrogen bonds as depicted in the figure above. Such inter-molecular bonds lead to the formation of a very high strength polymer.

Structure and synthesis

These intermolecular hydrogen bonds are formed between the carbonyl groups and the nitrogen-hydrogen centres. Since there is also an aromatic stacking of the molecules of adjacent strands it gives an additional strength to the bonds .Such interactions are way stronger than the regular Van-der walls forces and therefore have a greater influence on Kevlar .These also affect the chain length which typically influence the properties of other synthetic polymers and fibres such as Dyneema.

If impurities like ions of other salts are present then it can hinder the direct interaction of the molecules. This in turn will decrease the strength of the polymers. So it is made sure that polymers are pure and free of other ions.

As is apparent from the figure the Kevlar’s structure has planar shaped and the molecules have a very closely stacked structure which makes it extra strong.

Even very low temperatures up to -196 °C cannot affect the molecular strength of the Kevlar structures as may be the case of other polymers due to brittleness. In fact at times it becomes stronger as the temperature is brought down. Although  increasing the temperatures could lead to some serious decrease in the strength of Kevlar maybe up to 10-20% but it still very less when compared to polymers like Nylon which might even melt at high temperatures.

The above graph as cited on PRINCETON (2015) shows the Mechanical strength of Kevlar as compared to other such materials used in the industry.

MRAZOVA (2013) has pointed out some very important advantages of Kevlar which are listed as follows:-

The strands of Kevlar are very light in weight and can reduce the overall weight of the equipment up to 20-50%.

Kevlar can stay unharmed even in direct impact accidents like fired bullet so it is also used as a shield in the military.

Kevlar has high mechanical strength so it can sustain damages without being broken which is very useful in accident scenarios.

4.If we use metals for hard structures they have various corrosion problems and hence there is a loss of materials with time. Kevlar however is long lasting and has no such issues.

Following could be the disadvantages of using Kevlar

Since Kevlar involves very complex processes from production to separation it is really expensive in the market and hence using Kevlar in bulk becomes very expensive for some cases.

Usage of kevlar could also involve some very high recurring costs.

Mechanical Properties

Since Kevlar does not break immediately on impact so the damage on the material may not be visible immediately. This could lead to some hidden damages in the material.

The damages in the kevlar material are difficult to analyse or find out. Therefore the repairs are difficult and since it is a fibrous material it has different repair techniques than the metal itself.

USAGE IN AIRCRAFT INDUSTRY

1) Since the floorings and interiors form the base to the strength of the Aircraft they are made up Kevlar to give extra strength and better looks that metal would give.

2) Filament-wound pressure bottles :- Since aircraft work at very high pressures so the bottles made with a normal material could cause catastrophic situations in case those bottles burst hence Kevlar is used for such rupture proof bottles as well.

3)Engine nacelles

The jets fly at very high speeds, sometimes even supersonic speed. At such high speeds even a small vibration or extra stress could prove deadly.The nacelles use in the  jet engines must withstand such forces or thrust.Being strong and light compared to metallic structures it is widely used in such cases.

4)Aircraft tires

An aircraft takes off at a very high speed and in such cases the attrition is not only due to friction but also due to extra forces exerted due to the takeoff. Similar things happen when the aircraft lands. Such continuous cycles can lead to severe damage in the aircraft. The tires are hence covered with a layer of Kevlar to prevent any such damage.

5) Rotor blades

The rotor blades are made up of the honeycomb structured Kevlar so give a stiff blade and keep the weight within control.

 6) Spacecraft: Since Kevlar can sustain very low temperatures as well as high temperatures too it becomes the perfect choice to be used in space where the temperature could vary in a wide range. It has been used in the various missions to mars to construct the rovers and to avoid impact attrition.

7)Navy vessels- The navy vessels work in a very corrosive environment in the sea hence a normal material would sustain serious damages therefore Kevlar is used in such cases.

The various composites like Kevlar have proved to be very efficient for aircraft purposes and advantageous too but could also be a potential danger to the environment and the organisms. Such composites like Kevlar are made up of fibres which individually are very small in structure when handled during production. Being so small in size they can penetrate through the nostrils making way to the lungs and hence causing lung diseases. They could damage the internal tissues or could cause clotting in the arteries. Another effect of this could be their adsorption on the skin and hence causing cancer or irritation. Aircraft structures made of such stuff when crash and burned release very small fibres of dust into the air causing air pollution. This is also the case when an aircraft is disposed of. They again could release toxic vapours in the atmosphere and hence cause skin, lung or eye problems. Therefore very serious precautions are required while producing, handling or disposing of the Kevlar made stuff.

References

DUPONT(2013), Resourced From The Internet on 13th Feb,16 from  https://www.dupont.com/products-and-services/fabrics-fibers-nonwovens/fibers/brands/kevlar.html

MARIA MRAZOVA(2013), Advanced composite materials of the Future In Aerospace Industry, The Faculty of Operation and Economics of Transport,  Slovak Republic

PRINCETON(2015) Ch. 3—Polymer Matrix Composites Page- 7 7 resourced from the internet on 13th Feb,16 from https://www.princeton.edu/~ota/disk2/1988/8801/880106.PDF

QUINTANILLA, J. (1990). “Microstructure and properties of random heterogeneous materials: a review of theoretical results”. Polymer engineering and science 39: 559–585

STEPHANIE KWOLEK “Inventing Modern America: Insight —:”. Lemelson-MIT program. Archived from the original documents on May 24, 2009