BIOPLASTICS | BIODEGRADABLE POLYMERS
Figure 1: The main commercially-available biodegradable polymers showing their principal sourcing routes Source: Carmine Di Fiore
Polyglycolic acid (PGA) is the simplest linear
aliphatic polyester. It is produced by polymerisa- tion through opening of a cyclic lactone, the glycolide. The molecular weight of the repeat unit is 116 and the carbon content 41.4% (carbon content is important when calculating the renew- able content). PGA has been known since 1954. It has a glass
transition temperature between 35-40°C and melting temperature between 225-230°C. The polymer is characterised by a high degree of crystallinity (around 45-55%), which gives it some resistance to hydrolysis. The PGA biodegradation process converts the polymer back to glycolic acid. In the first phase, water infiltrates the amorphous regions and splits the ester bonds; in the second phase the crystalline portions are exposed to the water and the chain dissolves. PGA can also be degraded by certain enzymes.
Glycolic acid is non-toxic and converts via the
Krebs cycle in living organisms, including humans, to water and carbon dioxide. For this reason, it is used in products such as absorbable sutures. Studies show PGA sutures halve their strength in around two weeks and are completely reabsorbed by the body in 4-6 months. Polycaprolactone (PCL) is a biodegradable polyester with a low melting point of around 57°C and glass transition temperature of -60°C. The molecular weight of the repeat unit is 114 and the percentage of carbon is about 63%.
42 COMPOUNDING WORLD | September 2021
PCL is obtained by polymerisation through opening of the ε-caprolactone ring by heat in the presence of a catalyst and high molecular weight polymers can be obtained. It is a semi-rigid material with a flexural modulus of about 300MPa at room temperature. Tensile strength is typically low and elongation at break high (> 800%). Polycaprolactones have been produced for
many years and have various uses, the most significant of which are in the medical area — it degrades by hydrolysis of its ester bonds under physiological conditions in the human body. Due to its fast biodegradability, PCL polymers have been used for production of films but their low melting point compromises performance. To overcome this, PCL is more typically used in blends with PLA, PGA, or starch. Perhaps the best known biodegradable poly-
mer, polylactic acid (PLA) is a linear aliphatic polyester with a repeat unit molecular weight of 72 and a carbon content of around 50%. It was first polymerised in 1931 by DuPont scientist Wallace Carothers but the difficulty in obtaining high molecular weights and the tendency of the polymer to degrade meant development halted. It is now produced commercially by Natureworks and Total-Corbion and is one of the most promising biopolymers with applications extending from packaging through to consumer durables. Typically, the lactic acid monomer (2-hydroxy
propionic acid) is obtained from corn starch or sugar cane by chemical synthesis or by fermenta-
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