The Journey of Polyamide Fibers
(From Nylon to Aramid)
Nylon Fiber Identification
Burn Test
Nylon is a polyamide made from petroleum. Nylon melts and then burns rapidly if the flame remains on the melted fiber. If the flame is kept on the melting nylon, it smells like burning plastic. Its residue is initially a hard, cream-colored bead that becomes darker in course of burning.
Solubility Test
Aliphatic polyamides, such as nylon fibers are usually soluble in phenols, formic acid, fluorinated alcohols and mineral acids. They are also soluble in methanol under pressure. Aromatic polyamides (aramids) are somewhat different from aliphatic polyamides in solubility. For example, they are not soluble in formic acid.
Nylon Fiber Properties
The end uses of a fiber depend on its physical as well as chemical properties. Nylon fibers are superior to all natural and regenerated fibers with respect to high tenacity, elongation, abrasion resistance and insensitivity to rotting and moth attack.
a. Action of Water: Water absorption of nylon fibers varies with relative humidity and crystallinity and, also to a little extent with temperature at a given with relative humidity. Though there is a relatively strong attraction of water molecules by the polar amide groups, the very crystalline polymer system of nylon allows few water molecules to be absorbed. Typical nylon 6,6 or 6 absorb ca. 9-10% water at 100% rh and ca. 4-4.5% at 65% rh.
b. Action of Acids: The amide groups in the nylon fibers are prone to hydrolysis under strong acidic conditions. Dilute acids do not affect nylon under the conditions encountered in practical operation. Exposure to hot mineral acid will, however, decompose nylon. The solubility of nylon 6,6 and nylon 6 in concentrated solution of hydrochloric acid differs from each other; nylon 6 is readily soluble at 20°C, whereas nylon 6,6 only goes into solution when heated.
c. Action of Alkalis: Nylon is more resistant to alkalis than it is to acids. It can be boiled in concentrated caustic soda solutions without any remarkable damage. Experimental results showed that nylon loses its strength only by 5% if it is treated with 10% solution of caustic soda at 85°C for ten hours.
Difference between Nylon 6.6 and Nylon 6
Nylon 6,6 and nylon 6 differ from each other in various ways. The differences in the structural composition of these two fibers are reflected to their physical as well as chemical behavior that lead them to be used in different areas of application according to the requirements of a particular end-use.
| Property | Nylon 6,6 | Nylon 6 |
|---|---|---|
| Crystallinity | More crystalline than nylon 6 | Less crystalline |
| Melting point | Higher melting point (about 250 °C) | Lower melting point (about 215 °C) |
| Glass transition temperature | Typical Tg values are 47–57 °C | Typical Tg is around 40 °C |
| Color fastness | Dye diffusion is slower, but dye bonds strongly, so better color fastness | Dye diffuses faster but strips out easily; lower color fastness |
| Resilience | Shows 100% recovery after 24 hours at 100,000 psi; very resilient | Also shows up to 100% recovery, but generally less resilient |
Application/Uses of Nylon
Nylons are extensively used in-
Hosiery
Lingerie
Underwear
Sweaters
Other knitted goods
Nylon also have been extensively used in-
Light and sheer apparel (e.g., windbreakers)
Home furnishings
Carpets
Upholstery
Tie cords
Parachutes
Sails
Ropes
Thread
Outdoor wear
Aramid (Kevlar) Fibers
The aramid polyamide fibers are formed from a long chain of synthetic polyamides in which at least 85% of the amide linkages are attached to aromatic rings. These essentially fully aromatic polyamides are characteristically high melting and have excellent property retention at high temperatures and excellent durability. They are unaffected by moisture and most chemicals and are inherently flame retardant. The fibers have high strength and can be used in a number of unique high-strength applications.
Structural Properties of Aramid
Aramids are formed through step growth polymerization of aromatic diacid chlorides with aromatic diamines in a polar aprotic solvent such as N,N-dimethylformamide (DMF) to a DP of 100-250. The meta– and para-substituted benzene dicarboxylic acid chlorides and diamines are characteristically used for aramid fibers presently in production, but other fully aromatic ring systems are possible future sources of aramid polymers for fibers:
The resultant aramid polymers are spun in suspension through a spinneret into hot air (dry spinning) or a coagulating of both (wet spinning) to form the fibers, followed by fiber stretching and orientation. The aromatic units along aramid polyamide chains confer stiffness to the polymeric chains and limit their flexibility and mobility. Hydrogen bonding between amide groups on adjacent chains and extremely strong van der Waals inter-actions between aromatic rings planar to adjacent aromatic rings provide a tightly- packed, strongly held molecular structure and account for the strength and thermal stability of the aramids. The aramids are usually spun in round or dumbbell cross section.
Application/End Uses of Aramid Fiber
Aramid fibers are a class of heat-resistant and strong synthetic fibers.
They are fibers in which the chain molecules are highly oriented along the fiber axis, so the strength of the chemical bond can be exploited, having the tenacity level 20 g/d.
They are used in aerospace and military applications, for ballistic-rated body armor fabric and ballistic composites, in bicycle tires, and as an asbestos substitute.
Aramid fibers are highly crystalline aromatic polyamide are manufacture by air-gap/wet spinning method.
Aramid fibers have very low density and high specific tensile strength when compared to commonly available reinforcing fibers.
