A polyamide is a macromolecule with repeating units linked by amide bonds. Polyamides 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 yielding materials such as nylons, aramids, and sodium poly(aspartate). Synthetic polyamides are commonly used in textiles, automotive applications, carpets and sportswear due to their high durability and strength. The transportation industry is the major consumer, accounting for 35% of polyamide (PA) consumption.
Classification
Polymers of amino acids are known are polypeptides or proteins.
According to the composition of their main chain, synthetic polyamides are classified as follows:
All polyamides are made by the formation of an amide function to link two molecules of monomer together. The monomers can be amides themselves (usually in the form of a cyclic lactam such as Caprolactam), α,ω-amino acids or a stoichiometric mixture of a diamine and a diacid. Both these kinds of precursors give a homopolymer. Polyamides are easily copolymerized, and thus many mixtures of monomers are possible which can in turn lead to many copolymers. Additionally many nylon polymers are miscible with one another allowing the creation of blends.
Nomenclature
The nomenclature for the resulting polyamide is write after "PA" for polyamide number(s) indicating the number of carbon atoms in aliphatic monomers, and letters for cyclic and aromatic monomers. One number after "PA" for a homopolymer based on one monomer, and two numbers or sets of letters where there are two monomers. For copolymers the comonomers or pairs of comonomers are separated by slashes, as shown in the examples below.
- homopolymers :
- PA 6 : [NHâˆ'(CH2)5âˆ'CO]n made from ε-Caprolactam ;
- PA 66 : [NHâˆ'(CH2)6âˆ'NHâˆ'COâˆ'(CH2)4âˆ'CO]n made from hexamethylenediamine and adipic acid;
- copolymers :
- PA 6/66 : [NH-(CH2)6âˆ'NHâˆ'COâˆ'(CH2)4âˆ'CO]nâˆ'[NHâˆ'(CH2)5âˆ'CO]m made from caprolactam, hexamethylenediamine and adipic acid ;
- PA 66/610 : [NHâˆ'(CH2)6âˆ'NHâˆ'COâˆ'(CH2)4âˆ'CO]nâˆ'[NHâˆ'(CH2)6âˆ'NHâˆ'COâˆ'(CH2)8âˆ'CO]m made from hexamethylenediamine, adipic acid and sebacic acid.
Homopolymer polyamides derived from one monomer
Homopolymer polyamides derived from pairs of diamines and diacids (or diacid derivatives). Shown in the table below are polymers which are or have been offered commercially either as homopolymers or as a part of a copolymer.
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.
Polymerization chemistry
In living organisms, amino acids are condensed with one another by an enzyme to form amide groups (known as Peptides). The resulting polymers are known as polypeptides or proteins.
For the industrial production of synthetic polyamides with at least one aliphatic monomer ("nylon polymers"), the amide link is produced from the condensation reaction of an amino group and a carboxylic acid group. water is eliminated. For fully aromatic polyamides or 'aramids' e.g.Kevlar , the acid chloride is used as a monomer. The polymerization reaction with the amine group eliminates hydrogen chloride. The acid chloride route can be used as a laboratory synthesis to avoid heating and obtain an almost instantaneous reaction.
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 (if the difference between R groups is ignored) reacting with identical molecules to form a polyamide:
Aramid (pictured below) is made from two different monomers which continuously alternate to form the polymer and is an aromatic polyamide:
Current market and forecast
As one of the largest engineering polymer families, the global demand of polyamide resins and compounds was valued at roughly US$20.5 billion in 2013. The market is expected to reach US$30 billion by 2020 by following an average annual growth of 5.5%.
See also
- Polyamide-imide
References
Bibliography
Kohan, Melvin I. (1995). Nylon Plastics Handbook. Hanser/Gardner Publications. ISBN 9781569901892