Wednesday, 26 September 2012

Biodegradable plastics

Back in the seventies, I was given a couple of golf tees made from polyhydroxybutyrate (PHB), a plastic produced by microorganisms under controlled conditions of physiological stress. It is similar in appearance to polypropylene and as far as the eye could tell, these golf tees were ordinary plastic tees. But because they were made of PHB, they were biodegradable. Tees lost in the rough would slowly rot away.

I tried leaving them in sewage sludge and if I remember rightly, within a week or so they had turned brown and the smooth surface had begun to degrade. Wikipedia lists is properties as:-

  • Water insoluble and relatively resistant to hydrolytic degradation. This differentiates PHB from most other currently available biodegradable plastics, which are either water soluble or moisture sensitive.
  • Good oxygen permeability.
  • Good ultra-violet resistance but poor resistance to acids and bases.
  • Soluble in chloroform and other chlorinated hydrocarbons.
  • Biocompatible and hence is suitable for medical applications.
  • Melting point 175°C., and glass transition temperature 2°C.
  • Tensile strength 40 MPa, close to that of polypropylene.
  • Sinks in water (while polypropylene floats), facilitating its anaerobic biodegradation in sediments.
  • Nontoxic.
  • Less 'sticky' when melted, making it a potentially good material for clothing in the future

PHB can be made in industrial quantities by fermentation, although it has taken decades for the process to become cost effective for certain specialized applications. However, the biodegradable tag has market value and there are products available which could surely appeal to a middle class market prepared to pay premium prices.

So these materials can be made and markets for them may be developed. Oil is not the only possible source of plastics - merely the cheapest.

Until someone decides to subsidize PHB of course, which may already be going on via carbon credits. In the seventies we knew those golf tees were unlikely to be anything more than curiosities - possibilities worth investigating as university research projects, but with only a marginal chance of commercial success.

But in the world as it is now - who knows?


James Higham said...

I've been looking at these sorts of things for pontoons but they appear to be UV susceptible, let alone heat.

A K Haart said...

James - I can imagine this kind of material being very UV susceptible without additives. Even PVC is UV susceptible of course - it's only the additives which keep it from going brittle and prone to cracks.

Sam Vega said...

I don't know what it is made out of, but I bought a soft drink in a plastic bottle which prided itself on being biodegradable. It said that if buried in landfill, it just disappeared. I put it in the composter, thinking it would be gone in a couple of weeks.

Two years later it is still there. It looks in better shape than the plastic composter. If the composter and bottle manufacturers swapped plastics, we could all benefit.

A K Haart said...

Sam - I'm not sure if there is a standard for this type of claim. Most drinks bottles seem to be made from PET which may well degrade eventually, but it could be decades.

Your drinks bottle may really be photodegradable like Tesco bags, so burying it in the composter could slow down the process.

Woodsy42 said...

Don't I remember some years ago one of the car companies, Fiat maybe, were experimenting with biodegradable car bodies. By which I mean plastic ones, not the traditional ones that convert themselves to rust.

A K Haart said...

Woodsy - I know car companies have increased the amount of plastics in cars, but I don't know if any of it is biodegradable.