Global plastic production was 260 million tonnes in 2007 while plastics use has historically grown at a rate of 9% p.a. indicating that plastic consumption will continue its rapid increase. There have been a number of alternatives to oil based plastics developed; polylactic acid (PLA) is produced by the ring opening polymerisation of lactide which is produced via a three step process, polyhydroxy butyrate (PHB) which is produced naturally by various microorganisms such as alcaligenes eutrophus in response to conditions such as stress. These polymers can be used to produce plastics which possess some useful material properties while originating from sustainable resources as well as being biodegradable, unlike most conventional petroleum based plastics.
One alternative to this is thermoplastic starch (TPS) which is formed by the incorporation of small polar organic compounds such as water, glycerol, urea, ethanolamine and formamide. These compounds help to plasticise the starch by breaking the internal hydrogen bonding between the glucose rings in the starch breaking up its crystallinity and making the structure more amorphous. The modifier is usually incorporated into the structure by extrusion, compression molding, casting or melt processing.
In our work we have incorporated simple organic salts to act as plasticisers. These so-called salt modified starch materials have increased flexibility. One such salt is choline chloride (Ch Cl) which is a bulk chemical used in a variety of consumer products. It was found that heating a 70:30 starch/ plasticiser by weight mixtures at 140 oC under 10 tonnes pressure for 10 minutes could produce a transparent material (A) with a UTS of 5MPa and an elongation at break of 20%. We have also been able to blow this material into a foam with improved resilience compared to commercial blown starch which is commonly used as a packaging material.
One advantage of salt modified starch is that they can be dyed using food colouring (B). It can also be used for high strength composites with hemp or flax (C). In this form the composites have strengths of 60 MPa and are comparable in strength with nylon. This is the first all-natural high-strength material and may find application in cladding or flooring. In addition to the use of virgin starch the group is also investigating the use of waste starch from sources such as potato, banana and orange peel (D). The group is also looking towards natural fillers such as eggshell to produce all-natural plastics. The aim is to be bio-inspired and look to the range of natural materials for ways of constructing starch and cellulose composites.
The materials produced in this way are fully compostable.