PCL Cable Ties

biodegradable polycaprolactone


Polycaprolactone synthesis

These biodegradable cable ties are made from polycaprolactone, an unbranched waxy polymer that resembles polyethylene. PCL was first reported by Frank van Natta, Julian Hill, and Wallace Carothers around 1935, and is made from a ring-shaped hydrocarbon monomer (caprolactone) that contains a carbon dioxide or “ester” group. When these cyclic esters are split and linked together (typically a few hundred), the resulting polymer contains CO2 groups at regular intervals.

Such polymers are known as “aliphatic polyesters”, a broad class of material that includes many microbially digestible plastics such as PGA, PLA and PHB. These exhibit a wide range of physical properties, as determined by the CH2-to-CO2 ratio and the presence or absence of hydrocarbon side-chains.

Generally, short monomers and short side-chains produce stiffer materials with higher melting points, whereas longer, unbranched monomers yield more flexible polymers.

Although many aliphatic polyesters can be consumed to biomass by microbes, they can be difficult and expensive to produce. Only a few are commercially available, with the most common being poly lactic acid. Unfortunately, PLA is fairly durable in the natural environment and can only be rapidly digested via deliberate hot composting. (Where applications allow, PLA can be blended with other materials to improve its degradability, but the process can still take many years to complete.)

Despite being slightly weaker than PLA, the polycaprolactone used in these cable ties has a lower melting point of just 60C, and remains flexible down to -60C. Importantly, these temperatures are low enough for the material to be digested by microbes under ambient conditions. PCL is therefore widely regarded as one of the best alternative plastics for single-use products that may end up in the environment.

(Side note: P4HB may one day supersede PCL in this respect, however it is currently produced commercially by just one company (Tepha Inc), and is used almost exclusively for biomedical applications. Unlike PCL, there is no simple industrial route to synthesising this polymer, and "TephaFlex" is instead produced by genetically engineered bacteria. Production of P4HB can be stimulated by feeding the bacteria with gamma-hydroxy-butanoic acid (GHB) and 1,4-butane diol (BDO). In many countries, both these monomers are classed as illegal party drugs! For more information about this fascinating material see the 2020 paper by Utsunomia et al. listed in the references.)

Biodegradable aliphatic polyesters