By Penny Carmichael, on 24 October 2013
– Article by Jack Humphrey
After last week’s journey into the world of beer we shrug off our hangovers to ponder the environment and the future of plastics. Professor Andy Abbott from the University of Leicester is interested in the novel chemistry of ionic liquids and their applications in renewable materials.
The Plastic Problem
Oil is running out but plastic consumption is rising year on year. This is compounded by the fact that a plastic bottle will sit in landfill for centuries before breaking down. Biodegradable polymers do exist though, as nature creates them in the form of starch and cellulose. If only we could make use of them. Creating a functional biodegradable plastic from abundant natural products would solve the plastic problem, especially if the plastics could made cheaply and efficiently and from sources that would otherwise be thrown away.
The Chemistry of Ionic Liquids
Ionic liquids are classified as a mixture of salts that are liquid below 100°C. Common salts like sodium chloride melt at around 800°C due to the strong ionic bonds but by taking a bulky organic salt and adding an agent to complex one of the ions the strength of ionic bonding can be reduced. Although earlier ionic liquids used toxic or expensive ingredients Professor Abbott’s lab hit upon using natural and biodegradable ingredients. The naturally occurring ammonium salt choline chloride, also used for chicken feed and body building, can be mixed with the hydrogen bond donor urea to make a very cheap and non-toxic ionic liquid. There are also no waste products formed in the reaction.
Creating Plastic from Scratch Starch
Ionic liquids are very versatile solvents. Their ability to dissolve carbohydrates means that you can alter the properties. Cellulose can be acetylated to create a much more hydrophilic material. Starch is a different matter. The professor was inspired by the kitchen chemistry of salt dough, where the salt encourages strong bonds between the starch molecules to create a tough material used for sculpting. By mixing the components of an ionic liquid with starch molecules a thermoplastic is created as the structure of the starch molecules is disrupted. This can be compressed and heated into a clear plastic material By altering the ratios of choline, starch, and urea different properties can be encouraged. The so-called Salt Modified Starch (SMS) can be formed into pellets and moulded just like any other plastic. It is also completely biodegradable. The one drawback to this seemingly miraculous material is its susceptibility to water. But as Professor Abbott explained it’s then possible to create a composite with another material to give you the desired properties. SMS can be made hydrophobic by incorporating calcium carbonate from waste eggshells. The strength of the plastic can also be improved by using flax or hemp. The source of starch can be from common food waste such as orange and banana peels. Samples were given out but sadly they didn’t smell of anything.
Goodbye to MDF?
As well as creating new plastics, Professor Abbott has set his sights on another outdated and environmentally unfriendly material: Medium Density Fibreboard or MDF, a material made from hardwood fibres mixed with a formaldehyde resin. Not only is formaldehyde carcinogenic it is also very difficult to work with in a processing plant. Salt Modified Starch plastics can be designed to have similar properties as MDF and even improve on it as SMS doesn’t need sanding after cutting and it can be vacuum formed. A display cabinet made out of thermoplastic wood was produced a year ago and is still standing.