PHA: plastic the way nature intended
Recent media coverage has very effectively highlighted the issues associated with waste plastic in the environment. The pervasive images of our plastic-filled oceans and the suffering caused to sea life have pricked our collective consciences and highlighted disposable plastics as the villain of our time.
As well as the obvious problem of how we dispose of our waste, the enduring nature of plastics has called into question our wisdom in using them in the first place. Numbers in the region of thousands of years are frequently quoted for the time it will take plastic bottles and bags to decompose in our fields and waterways. Scientists and engineers are berated for being the architects of this curse.
So what role can scientists play to make amends? We can’t ignore the fact that the benefits plastics give our modern lives are themselves great. We have fallen in love with the convenience that these light, inert materials give us – and we have also fallen in love with the low cost of this commodity and the savings it offers us as a way of packaging our food and household goods. But the choice we face may not be as harsh as a world with or without plastics. Scientists have also developed plastic materials which only persist in our environment for a matter of weeks or months. A range of biodegradable plastics are now on offer and the variety is growing day by day.
Engineering the difference
So, does this mean that the scientists have done their jobs and the engineers can now just specify biodegradable plastics in all their products?
Well, you’ve probably guessed that it’s not that simple.
We currently use hundreds of different plastic materials (strictly, we should probably call them polymers – because most rubbers and foams give us the same problems). These materials are selected for their specific properties: stiffness, strength to weight ratio, the ability to stop the fizz escaping from our diet cola. If we want to replace our conventional plastics with biodegradable alternatives, then we must also find alternatives with similar properties.
The first biodegradable plastics had very limited properties. Starch-based and cellulose-based materials provided some promise but tended to suffer from poor moisture stability and limited mechanical strength. The increasing prevalence of polylactic acid (PLA) has provided a far wider range of physical properties with striking similarities to the PET used in drinks bottles - in all but maximum operating temperature and gas barrier properties. More recently a renaissance in the production and use of polyhydroxyalkanoates (PHAs) has provided us with a vast range of potential properties from biodegradable plastics. This series of polyester materials derived from bacterial fermentation can be copolymerised and blended to cover most of the conventional mechanical and thermal design spaces.
But nothing is ever an exact replacement. If you tailor a new polymer to have the same stiffness and density as a conventional material, then the chances are that it will melt at a different temperature and have a different hardness or strength. If you blend your bioplastics to give you the same clarity and strength as the plastic currently used in cola bottles, then you’ll likely find that your favourite fizzy drink goes flat after a week on the shelf – or that the acidity in the drink causes the bioplastic to go hazy.
So, the engineers still have a lot of work to do. The scientist can provide them with a wide range of biodegradable plastics to play with, but the engineers are still going to need to make very clever compromises to get their products to work. Engineers can work out just how much thicker you need to make a beam to give it the right stiffness - when you’ve made your plastic a little bit softer to stop it being quite so brittle.
To design more complicated parts, engineers will often make use of computer simulations to make sure that suitable design changes are made to compensate for subtle differences in materials properties. Even something as simple as a screw cap for a bottle might undergo intricate analysis in finite element modelling software to make sure that it can withstand the same pressures, be compliant enough to make an airtight seal and allow you to break the tamper evident seal without needing muscles like the Incredible Hulk.
But the role of the engineer doesn’t only stop with the mechanical properties of the product. The optical, frictional and thermal properties of the product must also be considered. What use is a plastic coffee cup if it melts into a gooey lump as soon as you pour boiling water into it?
Perhaps the least obvious and often most difficult work that engineers must do to replace conventional materials is in the manufacturing. Different polymers behave very differently when you try to make things out of them. They melt at different temperatures. They flow differently into moulds. Some may require ten times as much force to push them into a particular form – others may start to degrade because of the frictional heating when you inject them. Engineers also have to worry about all of this. How you make parts from biodegradable plastics and solve all of the problems that come with the manufacturing processes – that’s the work that you’ll never see when you unscrew the bottle cap and hear the hiss of fizz escaping from the bottle.
At Cambridge Consultants we have synthetic biologists who enthuse about getting bacteria to turn waste into polymers. We have chemists and materials scientists who bug the biologists about which polymers they want the bacteria to make to give the engineers the properties they need to replace the plastics that are currently in our cups, bottles, toothpaste tubes, tooth brushes, straws, spoons and wetwipes. And we also have the engineers who analyse the stresses and strains, modify the designs, re-analyse, re-design and worry about how to make things – so that our clients can say that their widget is now made of biodegradable plastic!
If you’d like to learn more about bioplastics and PHA in particular then download our whitepaper: PHA: plastic the way nature intended.