The Journey of Polyamide Fibers
(From Nylon to Aramid)
Polyamide Fiber
Polyamide fibre is a synthetic fibre of polymer containing amide linkages. The polyamide fibres include the nylons and the aramid fibres. Both fibre types are formed from polymers of long-chain polyamides. Polyamides containing less than of amide linkages directly attached two aliphatic groups are termed as nylon fibres. 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.
History of Polyamide
- In September 1931, American chemist Wallace Carothers a researcher of the Du Pont Company carried out research on polymeric fiber referred to simply as “66”, a number derived from its molecular structure.
- Carothers was also the first to propose the melt spinning method or nylon 6,6 in 1937 within Du Pont. Du Pont began commercial production of nylon in 1939.
- By 1938, Paul Schlack of the IG Farben Company in Germany polymerized caprolactam and created a different form of the polymer, identified as “nylon 6”
- Nylon 6 fiber was commercialized by Toray in 1951 with introduction of spinning technology for nylon 6,6.
Nomenclature of Polyamide
Nylon represents the generic name for all synthetic fiber-forming polyamides. It can be formed in two methods. In one approach, molecules with an acid (COOH) group on each end are reacted with the molecules containing amine (NH2) on each end.
The resulting nylon is made on the basis of the number of carbon atoms separating the two acid groups and the two amines. Thus the nylon 6,6 which is widely used for fibers is made from adipic acid and hexamethylene diamine. Since hexamethylene diamine has six carbon atoms, and adipic acid also contains six carbon atoms, the nylon they form is called nylon 6,6 or polyamide 6-6. Before the name “nylon” was used, nylon 6,6 was called by a codename, “fiber 66”.
An earlier nylon that the Carothers group studied was made from pentamethylene diamine and sebacic acid.
In another approach, a compound containing both an amine and an acid group is polymerized to form a chain with repeating units of {NH-(CH₂)x-CO-}n. If x=5, the nylon is referred to as nylon 6. For example, caprolactam is a lactam with six carbon atoms. So nylon made from caprolactam is called nylon 6 or polyamide 6.
Nylon Fibers/What is Nylon Fiber?
- Nylons are the first synthetic fiber of importance. They are aliphatic polyamide fibers produced by the melt spinning method. Their tenacity level is at most 10 g/d.
- Nylon is a generic designation for a family of synthetic polymers known as polyamides. First produced on February 28, 1935.
- There are several forms of nylon fibre depending on the chemical synthesis such as nylon 6; 6.6; 5.10 and 11.
- The nylons are generally tough, strong, durable fiber useful in wide range of textile applications.
- Nylon is found in clothes all the time, but also in other places in the form of thermoplastic materials.
Physical Structure of Nylon
Nylon is a regular, translucent, fine filament or staple fibers whose diameter depending on the end use requirements ranges from approximately 15 to 25 denier. The surface of nylon fibers is smooth with no striations. Their longitudinal structure is very regular and appearance like a glass rod. The fibers are commonly of almost round cross section.
The extent of each state for a given fiber structure depends on the spinning conditions, drawing and operation parameters that affect the ultimate characteristics of the fibers.
The maximum crystallinity that can be obtained varies with the nature of the polymer repeat unit. High crystallinity (40-60%) can be obtained with nylon 6.6; 6.10 and 6, whose regular structure permit good chain alignment and a high degree of hydrogen bonding.
Chemical Structure of Nylon
It is to be noted that the molecules of nylon are long and straight and there are no side chains or cross linkages. When nylon is spun into fibers, the long chain-like macromolecules line up parallel to each other: The amide groups on adjacent chains then form hydrogen bonds with each other. The chains are oriented in such a way as to maximize hydrogen bonding. These hydrogen bonds hold the adjacent chains together and allow the chains to line up in orderly fashion to form strong fibers. The formation of hydrogen bond also makes the polymer less responsive and more impervious to dyeing.
Comparison of Physical Properties- Nylon 6,6 vs Nylon 6
| Properties | Nylon 6.6 | Nylon 6 |
|---|---|---|
| Tensile strength, g/d | ||
| – Dry | 5.0-6.5 | 4.8-6.4 |
| – Wet | 4.2-5.0 | 4.2-5.9 |
| Tensile elongation, % | ||
| – Dry | 25-38 | 24-45 |
| – Wet | 30-52 | 26-55 |
| Young’s modulus, g/d | 30-52 | 20-45 |
| Elastic recovery (at 3% elongation), % | 88-100 | 90-100 |
| Loop strength, g/d | 8.5-11.5 | 8.5-11.5 |
| Knot strength, g/d | 4.5-6.0 | 4.3-6.0 |
| Softening point, °C | 230-235 | 180-185 |
| Specific heat, J/(g·K) | 1.67 | 1.67 |
| Dielectric constant, 10² H₂ | 7.0 | 7.0 |
| Sp. gr. | 1.14 | 1.12-1.15 |
| Water absorption, % | ||
| – 65% RH | 3.5-4.5 | 3.5-4.5 |
| – 100% RH | 9-10 | 9-10 |
Manufacturing Process of Nylon
Polymerization
Two basic reactions are used to synthesize nylon polymers: (1) polycondensation of a dicarboxylic acid and a diamine to produce nylon 6,6 or (2) the ring-opening polymerization of caprolactam to produce nylon 6. Many other routes have been used in the laboratory, but only these two methods have gained commercial importance.
Spinning
