Aramid fibers are a class of heat-resistant and strong synthetic fibers. They are commonly used in aerospace and military applications that require high strength and heat resistance. These fibers are produced by solid-state synthesis and their molecules are highly oriented along the fiber axis. Some key characteristics of it include their high tensile strength, low weight, resistance to flame, heat and corrosion.
Chemical Composition and Structure
They belong to the family of polyamides, but are neither meltable nor fusible. They are synthesized from a diphenyl groups containing diamine and a dichloro-containing aromatic diacid. The most common diamine used is p-phenylenediamine and the diacid used is terephthaloyl chloride, which forms poly(phenyleneterphthalamide). Their chemical structure contains rigid aromatic nuclei connected by hydrogen bonds along the fiber axis which gives them their exceptional strength.
Properties and Applications
High Strength and Stiffness
Aramid Fibers have one of the highest strength-to-weight ratios of any materials. Their tensile strength is around 3.5-4.0 GPa, with some fibers reaching over 5.0 GPa. They are also very stiff, with a Young's modulus of around 75-120 GPa. This high strength and stiffness makes aramid fibers an ideal reinforcement material for composites.
Heat and Flame Resistance
They do not melt even at very high temperatures, maintaining their strength up to around 300°C. They also have high glass transition temperatures, around 250-290°C depending on the type. This heat resistance comes from the aromatic rings in their chemical structure. They also have very low flammability and self-extinguish upon removal from ignition source. Due to this, they find applications requiring heat or flame protection.
Electrical Insulation
They do not conduct electricity and hence are used as insulators for electrical cables and high voltage equipment. The hydrogen bonds between the rigid aromatic units are responsible for this insulating property.
Bulletproof Armor
The exceptional combination of lightweight, strength and stiffness makes aramid fibers ideal for bulletproof vests and other military armor applications. Multiple layers of aramid fabric can absorb the kinetic energy of high-speed projectiles without breaking. Kevlar is the commercial brand most commonly used for this purpose.
Composites Reinforcement
As mentioned before, the high strength and modulus of aramid fibers make them excellent reinforcements for composite materials. They are widely used to strengthen plastics, rubbers and cement in applications ranging from aircraft wiring sleeves to fishing rods and boat hulls. Some common composites reinforced with them include fiberglass, aluminum, graphite and carbon fiber reinforced polymers.
Aircraft Components
They find extensive use in the aerospace industry due to their ability to withstand impacts, temperature variations and fire. Composites containing aramid fibers are used to make aircraft components like fuselages, wings, rotorcraft blades and interiors owing to their light weight and mechanical properties. Their strength to weight ratio helps reduce the weight of aircraft significantly.
Other Applications
A few other applications of them include filtration media, optical fibers, tires, guitar strings and ropes requiring high strength, abrasion resistance and low weight. Composite cements reinforced with them have been used in dentistry for crown and bridge work as well. Their insulating properties have also allowed use in cryogenic storage tanks.
Production Process
The most common production method for manufacturing aramid fibers involves melt-spinning of the polymer through a spinneret, followed by stretching and drawing to align the molecules. A diagram summarizing the four main steps is given below:
1. Polymerization: The diamine and diacid monomers are reacted in a polar solvent like m-cresol to form the polyamide polymer.
2. Spinning: The spinning dope containing 20-30% solution of high molecular weight polymer is extruded through a spinneret into a coagulating bath and fibers are formed.
3. Drawing: The as-spun fiber is passed through successive godets with increasing speeds to extract and orient the polymer chains along the fiber axis. This increases the strength.
4. Winding: The drawn fiber is wound onto spools under controlled tension for packaging and shipment to final product manufacturers.
Some additional post-drawing treatments may also be given to improve properties. The properties of them can be tailored by controlling factors like draw ratio and relaxation annealing.
Production
Currently, the two most common types of commercial aramid fibers are Kevlar and Twaron. Kevlar is produced by DuPont and accounts for over 80% of the global . Twaron is the brand name for fibers produced by Teijin Group. The worldwide production capacity for it is estimated to be around 130,000 tons per year. The major consumers are North America, Western Europe and Asia Pacific regions. The aerospace industry remains the single largest end-user segment consuming about 30% of the total production. Composites and protective fabrics are other major application areas.
Environmental and Safety Aspects
Being completely synthetic, aramid do not occur naturally. However, their production involves hazardous chemicals that need proper handling. Spent residues also require safe disposal as per environmental regulations. As reinforcing additives, they improve the durability and lifespan of composite materials. But these composites are not readily
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Money Singh is a seasoned content writer with over four years of experience in the market research sector. Her expertise spans various industries, including food and beverages, biotechnology, chemical and materials, defense and aerospace, consumer goods, etc. (https://www.linkedin.com/in/money-singh-590844163)
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