Polyamide


A polyamide is a macromolecule with repeating
units linked by amide bonds. They can occur both naturally and artificially. Examples of naturally occurring polyamides
are proteins, such as wool and silk. Artificially made polyamides can be made through
step-growth polymerization or solid-phase synthesis, examples being nylons, aramids,
and sodium poly(aspartate). Synthetic polyamides are commonly used in
textiles, automotives, carpet and sportswear due to their extreme durability and strength. Transportation is the major consumer, accounting
for 35% of polyamide consumption. Classification
According to the composition of their main chain, polyamides are classified as follows:
According to the number of repeating units’ types, polyamides can be:
homopolymers : PA 6 : [NH−(CH2)5−CO]n made from ε-Caprolactam ;
PA 66 : [NH−(CH2)6−NH−CO−(CH2)4−CO]n made from hexamethylenediamine and adipic
acid; copolymers :
PA 6/66 : [NH-(CH2)6−NH−CO−(CH2)4−CO]n−[NH−(CH2)5−CO]m made from caprolactam, hexamethylenediamine
and adipic acid ; PA 66/610 : [NH−(CH2)6−NH−CO−(CH2)4−CO]n−[NH−(CH2)6−NH−CO−(CH2)8−CO]m
made from hexamethylenediamine, adipic acid and sebacic acid. According to their crystallinity, polyamides
can be: semi-crystalline:
high crystallinity : PA46 et PA 66 ; low crystallinity : PA mXD6 made from m-xylylenediamine
and adipic acid; amorphous : PA 6I made from hexamethylenediamine
and isophthalic acid. According to this classification, PA66, for
example, is an aliphatic semi-crystalline homopolyamide. Production from monomers
The amide link is produced from the condensation reaction of an amino group and a carboxylic
acid or acid chloride group. A small molecule, usually water, or hydrogen
chloride, is eliminated. The amino group and the carboxylic acid group
can be on the same monomer, or the polymer can be constituted of two different bifunctional
monomers, one with two amino groups, the other with two carboxylic acid or acid chloride
groups. Amino acids can be taken as examples of single
monomer reacting with identical molecules to form a polyamide: Aramid is made from two different monomers
which continuously alternate to form the polymer and is an aromatic polyamide: See also
Polyamide-imide References