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General view

As aramid or aromatic polyamides, polyamides are not defined in the main chain with aromatic groups, but they are defined as per a definition of the US Federal Trade Commission. In the main chain only long-chained synthetic polyamides at which at least 85% of the amide groups are directly bound to two aromatic rings are named.Aramid is mainly manufactured as fibre and rarely as a film. Aramid fibres are golden-yellow organic synthetic fibres. These fibres were developed in 1965 from Stephanie Kwolek at the company DuPont and they were launched with the brand name Kevlar.These fibres characterise by extremely high stability, high impact resistance, high elongation, good vibration absorption as well as resistance against acids and solvents. Furthermore they are very heat- and flame resistance. Aramid fibres do not melt at high temperatures but they start to coal at about 400°C. Well-known brand names for aramid fibres are Nomex and Kevlar from DuPont or Teijinconex, Twaron and Technora from Teijin.It is differed between meta-aramids (Teijinconex and Nomex), para-aramids (Twaron, Kevlar) and para-aramid-copolymers (Technora). The full name of para-aramid is polyphenylene terephthalate amide and sometimes they are named with the abbreviation PPTA.

 

Para-Aramid manufacture

The manufacture of fibres can only be realised of solvents because the melting point often is far above the decomposition point. A high polymer concentration in the spinning bath is favourable for the fibre manufacture and can lead to high orientations. A good solvent for aramids in high concentration, so a solvent with anisotropic character, is concentrated sulphuric acid. The process of direct spinning of the polymer solvent is not recommended, polymers which are para-orientated, aromatic dicarboxylic acids and diamines are more economic. The fibre manufacture through polymerisation and the usage of sulphuric acid as a solvent is shown in the picture. During polymerisation of these aramids mainly aromatic dicarboxylic acid halides are used, for example paraphenylenediamines (PPD) and terephthaloyl dichloride (TDC). The polymer is formed through reaction of both monomers in a solvent. A possible solvent is hexamethylene phosphoric acid triamide but it does not seem suitable because of assumed carcinogen effects. However the usage of this solvent can be avoided if the polymerisation is realised in a suspension of calcium chloride CaCl2 in the solvent N- Methylpyrrolidon. The spinning process is the common wet spinning process. The usage of an air gap between spinning nozzles and spinning bath like it is known from the acrylic spinning, has some advantages. After drying the yarn has high stability and high elasticity modulus. During the second process step the yarn can be drawn at temperatures of 300 to 400°C. This leads to an even high modulus by remaining to the stability and having lower elongation. This aramid fibre model is used for different applications.

 

Applications

The best-known applications for para-aramid fibres are found in the safety area (shatter- and bulletproff vests, head protections, armoured protection for vehicles, cut-resistant gloves. But they also began to use aramid fibres at modern high-power loudspeakers. Damages at the membranes made of aramid fibres can be avoided because of the high load capacity. Aramid fibres are also used as alternative for asbestos in brake coverings and gaskets as well as reinforcing material e.g. for glass fibre cables or rubber materials. The electrical insulation is also a wide application field for aramid papers. The products are used as slot closures, slot insulations and phase insulations in electrical motors as well as layer insulations in transformers.Aramid fibres are also used as e.g. stadium roofs in the construction area. In this case they are the basic material which is covered with PVC or PTFE, so it becomes an UV- and weather resistant, partially transparent membrane. They are also well-suitable for sport equipment because of their tear- and tensile strength and their low weight. Aramid fibres are often used for ropes, painters for paragliders, for sails of sailing boats and surfboards, for hockey sticks or as strings for tennis rackets. Certain bicycle tires are equipped with Kevlar inserts to protect them against damages of broken glass or similar. Folding tires contain a Kevlar bundle instead of wire. Climbing ropes cannot be made of aramid because of the required elongation. Furthermore they are used as fibre-reinforced plastics in the aircraft construction; mainly they are used for the construction of gliders. Aramid fibres are also used among others as engine mount coatings in the Airbus A310 and A380 as well as for helicopters like MBB Bo 105 and BK 117.Meta-aramid fibres are used especially for the fire protection. They became well-known through fire-resistant clothing (like protective suits at the fire brigade or similar).Another application for meta-aramid is the processing in the fibre composite to honeycombs made of Nomex papers. The mentioned aramid fibre is also used in the magician area where it is named “Invisible Kevlar Thread” and it is used as a non-visible thread.

 

Properties

The fibres have, similar to carbon fibres, a negative coefficient of thermal expansion in machine direction. That means that they become shorter and thicker during warming. Their specific stability and their elasticity modulus are obviously lower than these of carbon fibres. High dimensionally stable construction parts can be manufactured in combination with positive coefficient of thermal expansion of the matrix resin. In comparison to carbon fibre reinforced plastics, the pressure resistance of aramid fibre composites is clearly lower. Moreover aramids are heat resistant, they endure temperatures of more than 370°C without any problems and they do not melt. Aramid fibres are also well-heat-resistant even if the compound is not close-fitted. It is differed between two modifications which vary especially through their different elasticity modulus:

"Low modulus": Tensile strength 2800 N/mm² and E-Module 59 kN/mm²

"High Modulus": Tensile strength 2900 N/mm² und E-Module 127 kN/mm²

Both modifications have a density of 1,45 g/cm³. The high-modulus fibres are mainly used for impulse and impact stressed construction parts. Low-modulus fibres are mainly used as bullet-retardant vests. Aramid films are often calendered of fabrics but they are also manufactured directly as thin films. They are used as insulating material (e.g. in transformers of the insulating class C, temperatures up to 220 °C), as basic material for flexible conducting plates and as window material at accelerators and detectors.

Processing

When processing and dealing with these materials, the slight humidity absorption and the poor UV-resistance has to keep in mind. The original golden yellow fibres get a bronze-brown colour during contact with UV radiation (sun light) and they lose up to 75% of their stability. The fibres can absorb up to 7% water, according to the storage of them. Fibres with high moisture can be dried. A moisture content of fewer than 3% is common in the aerospace.

To cut aramid fibres special micro-serrated cutting dies are required. The mechanical treatment also effects with high-class machining tools or through water jet cutting.

Fibre composites are usually manufactured with epoxy resins. Chemical adhesives are not known.

Source: Wikipedia

 


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