Bio-based polyamides

Polyamides are the engineering plastics of this cluster, and their behaviour comes from one structural feature applied repeatedly along the chain.

Hydrogen bonds between chains, not within them#

A polyamide backbone alternates methylene sequences with amide groups, each carrying an N–H donor and a C=O acceptor. Adjacent chains align and hydrogen-bond to each other in sheets.

This interchain bonding is what makes nylon a structural material. It drives high crystallinity, and the crystallites act as physical crosslinks that resist chain slippage — hence stiffness, tensile strength, abrasion resistance and a melting point far above what the chain’s van der Waals forces alone would give. Polyethylene, chemically a nylon without the amide groups, melts around 130 °C; PA66 melts near 260 °C, and the difference is hydrogen bonding.

Amide density explains the whole family#

The property ladder across polyamides follows from how frequently amide groups occur along the chain — that is, how many CH₂ units separate them.

High amide density (PA6, PA66: short methylene runs) gives more hydrogen bonds per unit length, so higher melting point, higher stiffness and higher strength.

The same density has a cost. N–H and C=O groups hydrogen-bond to water as readily as to each other. Absorbed water inserts itself between chains, breaking interchain bonds and acting as a plasticiser: the polymer swells, dimensions change, stiffness falls and impact resistance rises. PA66 can take up several per cent of its weight in water at equilibrium, and a moulded part’s dimensions are therefore a function of ambient humidity.

Low amide density (PA11, PA12: ten and eleven methylenes between amides) inverts every term. Fewer hydrogen bonds mean a lower melting point and lower stiffness, but also much lower moisture uptake and far better dimensional stability, plus better low-temperature toughness and chemical resistance. This is why PA11 and PA12 dominate applications where a part must hold its size in a wet or variable environment — fuel lines, offshore flexible pipe, tubing.

That is a useful thing to know for a different reason: the bio-based polyamides happen to sit at the low-amide-density end, so they are not substitutes for PA66 but occupants of a different property band.

Where the renewable carbon comes in#

Castor oil is the anchor, and its chemistry is unusual. Ricinoleic acid, about 90% of castor oil’s fatty acids, carries a hydroxyl group mid-chain — a functional handle almost no other plant oil provides. Pyrolysis cleaves it to undecylenic acid, which is converted to 11-aminoundecanoic acid: a single monomer with an amine at one end and an acid at the other, polymerising to PA11 with fully renewable carbon. Castor also yields sebacic acid by alkali cleavage, giving PA610 and PA1010, partly or fully bio depending on the diamine.

Fermentation supplies diamines. Bio-pentanediamine is produced by decarboxylating L-lysine with lysine decarboxylase, and with adipic acid it gives PA56.

There is a specific environmental argument for bio-adipic acid that is worth stating because it is not about carbon. Conventional adipic acid production, by nitric acid oxidation of a cyclohexanol–cyclohexanone mixture, generates nitrous oxide as a byproduct — a greenhouse gas with a global warming potential roughly three hundred times that of CO₂ and an ozone-depleting substance. Abatement equipment on modern plants destroys most of it, but the routes that avoid the oxidation chemistry avoid the problem rather than treating it.

None of these polyamides is biodegradable. The amide bond is hydrolysable in principle, but crystallinity and hydrogen bonding make it effectively inaccessible.