The following designation was adopted in 1974 by the United States Federal Trade Commission to describe aromatic polyamide-based fibres under the generic term aramid:‘a manufactured fibre in which the fibre-forming substance is a long chain synthetic polyamide in which at least 85% of the amide (—CO—NH—) linkages are attached directly to two aromatic rings. Aramids are prepared by the generic reaction between an amine group and a carboxylic acid halide group.
Simple homo-polymers may be synthesised according to the scheme below:
nNH2-Ar-COCl → [NH-Ar-CO]- n + nHCl
Aromatic polyamides became breakthrough materials in commercial applications as early as the 1960s, with the market launch of the meta-aramid fibre Nomex by DuPont which opened up new horizons in the field of thermal and electrical insulation. In the 1970s, based on an aromatic polyamide–hydrazine composition, Monsanto developed an aromatic co-polyamide fibre under the code X500 which almost reached the market. A much higher tenacity and modulus fibre was developed and commercialised, also by DuPont, under the trade name Kevlar in 1971. Another para-aramid, Twaron (Twaron is a registered product of Teijin), similar to Kevlar, and an aromatic co-polyamide, appeared on the market towards the end of the 1980s Teijin, after a remarkable scientific interpretation of the prior art by Ozawa and Matsuda, who pioneered the development of the aromatic co-polyamide fibre, commercialised the Technora (Technora is a registered product of Teijin) fibre.
This passage dives into the world of commercially available aramid compounds. Three main types are mentioned: MPDI (poly-(m-phenylene isophthalamide)), PPTA (poly(p-phenylene terephthalamide)), and ODA-PPTA (co-poly(p-phenylene-3,4-diphenyl ether terephthalamide)). For MPDI fibers, the most popular brand is Nomex (DuPont), while Kevlar (DuPont) reigns supreme for PPTA fibers. Teijin also offers Twaron (PPTA-based) and Technora (ODA-PPTA copolymer). Interestingly, the chemical makeup of Kevlar and Twaron is identical (PPTA). The standard method for creating aliphatic polyamides isn’t suitable for aramids due to their high melting points. Therefore, a special process using NMP (N-methyl-pyrrolidone) and CaCl2 (calcium chloride) solvents is required to synthesize PPTA molecules from 1,4-phenylenediamine (PPD) and terephthaloyl dichloride (TDC) monomers. Aramid production involves three stages: polymerization, filament yarn spinning, and conversion to usable fiber forms like staple, short-cut, or pulp. MPDI aramids are simpler to produce, using a low-temperature polycondensation process with m-phenylenediamine and isophthaloyl chloride in an amide solvent. These fibers can be spun using either dry or wet methods. In wet spinning, the polymer solution goes through tiny holes into a coagulating bath, followed by washing, stretching, and drying steps. Finally, Technora, a co-polymer aramid by Teijin, incorporates three monomers: terephthalic acid, p-phenylenediamine (PDA), and 3,4-diaminodiphenyl ether. This ether monomer adds flexibility to the backbone chain, resulting in a fiber with slightly better compression properties compared to PPTA aramid fibers produced via the liquid crystal route. An amide solvent with a small amount of calcium chloride or lithium chloride is used in this process.
Aramid Fiber Formation: A Twist on Wet Spinning
Aramids take a unique route when it comes to fiber formation, using a dry-jet wet-spinning system. This method differs significantly from the traditional wet-spinning process. In regular wet-spinning, the nozzle creating the fibers dips directly into a coagulating bath. Dry-jet wet-spinning, however, takes a different approach. The aramid solution is extruded through a spinneret positioned just above the coagulating bath (usually water or diluted sulfuric acid). This air gap allows for further alignment of the polymer chains within the solution before it starts to solidify. The specific design of the spinneret capillary and the air gap work together to induce a rotation and alignment of the polymer domains. This results in highly crystalline and oriented fibers right from the start (as-spun fibers). The secret behind aramid’s remarkable strength and modulus (stiffness) lies in the anisotropy (directional dependence) of its solutions and the presence of liquid crystals within them. These factors contribute to an exceptionally high level of orientation and association between the polymer molecules. Imagine long, strong chains all neatly lined up and tightly connected – that’s the key to aramid’s impressive properties.
The properties of Aramid Fiber: The Strength of Structure in Poly(p-phenylene terephthalamide), Poly(p-phenylene terephthalamide), often abbreviated as PPTA, owes its remarkable strength to its very rigid building blocks. These stiff “phenylene rings” are linked together in a specific “para” position, maximizing their stability. Another key advantage of PPTA is the presence of amide groups stationed like beads along its long backbone. These amide groups act like tiny magnets, attracting neighboring PPTA chains through a powerful force called “hydrogen bonding.” This extensive sideways bonding between chains creates a super-stable network, contributing significantly to the material’s strength. Here’s a point to remember: similar materials called “meta-aramids” don’t quite reach the same level of strength as PPTA. This is because the way their chains are linked (in a “meta” position) makes them more flexible, resembling textile fibers. While they may not be the strongest, meta-aramids offer excellent thermal stability, making them valuable in different applications.
Chemical Quirks of Aramid Fibers:- Aramids are all drawn together by a common thread – the “amide link.” This special connection loves water (hydrophilic), but the amount of moisture absorbed varies depending on the specific aramid. For example, PPD-T (poly-phenylene terephthalamide) fibers are champions when it comes to resisting many nasty chemicals like organic solvents and salts. However, strong acids can be their kryptonite, causing a significant loss of strength. Dyeing aramids can also be a tricky business. Their high “Tg” (glass transition temperature) makes them stubborn when it comes to taking on color. Here’s another interesting fact: the aromatic structure of para-aramids makes them susceptible to reactions with oxygen when exposed to ultraviolet light. This can lead to a change in color and, unfortunately, a decrease in their strength.
Aramid’s Thermal Resilience :- Aramids are not like other materials – they don’t melt in the traditional sense. Instead, they decompose at high temperatures. Setting them on fire is no easy feat either, thanks to their low “oxygen index values.”Impressively, some aramid types can retain around 50% of their strength even at scorching temperatures of 300 degrees Celsius. They also boast high crystallinity, which means they barely shrink even when the heat is on. This makes them ideal for applications where maintaining their shape under high temperatures is crucial.
Mechanical Property: Aramid yarn packs a serious punch! With a breaking tenacity of 3045 MPa, it boasts a strength more than 5 times that of steel underwater (and 4 times stronger even when dry). That’s double the strength of glass fiber or nylon! This incredible toughness comes from a winning combination of factors: the aromatic and amide groups within its structure, and a highly ordered crystalline arrangement.Here’s the amazing part: aramid retains its strength and elasticity (modulus) even at scorching temperatures as high as 300 degrees Celsius. Imagine staying strong and flexible even in the heat! Aramid behaves predictably under tension, stretching like a spring within a certain range. However, when it comes to sharp bends, it exhibits a non-linear plastic deformation – meaning it won’t spring back perfectly but might take on a permanent bend.But that’s not all! Aramid is a champion of endurance. Under repeated stress (tension fatigue), it shows no signs of failure even at remarkably high loads and extended cycles. Talk about impressive stamina! And to top it off, aramid experiences minimal creep strain (only 0.3%), meaning it resists deforming under constant load over time. This makes it ideal for applications that demand unwavering stability.
Aramid fibers, with their remarkable strength and heat resistance, have woven their way into a wide range of applications. In the aviation industry, they’re used for everything from airplane panels to parachutes. Their impressive strength makes them ideal for ropes and cables, from mooring lines for massive ships to delicate wires in electronic devices. Aramid fibers even play a role in construction, reinforcing concrete and suspending bridges. The automotive industry utilizes them for car parts and tires, while sporting goods like hockey sticks and skis benefit from their durability. Even medical applications and everyday items like conveyor belts and fireproof clothing rely on the unique properties of aramid fibers.
