Biodegradability? - what triggers the change from performance polymer to degradable product?
Most plastics / polymers are designed to resist performance degradation when exposed to moisture, temperature changes, sunlight etc. to provide a useful product life without premature failure. When the useful life is over, biodegradable polymers are expected to behave like uncoated paper, food, or yard trimmings, and quickly break down into simple molecules that can be safely consumed by bacteria, plants and animals. How can a plastic molecule be designed that has a distinct on/off switch that changes the performance profile from resisting moisture, humidity, heat, sunlight to allowing degradation? Can that be done at acceptable cost and will it work within the existing post consumer waste management & recycling systems?
By definition, biodegradable plastics are those that can be completely degraded in landfills, composters or sewage treatment plants by the action of naturally occurring micro-organisms. A truly biodegradable plastics should not leave no toxic, visible or distinguishable residues following degradation. Biodegradable polymers are broadly synthesized from bio-polymers, synthetics polymers (esters) and mixture of both. The material design of the polymers depends on the purpose of use, such as for different medical applications, the composition varies because of specific requirements of material properties. Therefore, not a single polymer can serve the purpose.
I’ll present a few points here
- Biodegrability need not be fast. A tree innocuously decreases over 50 years or more.
- Environmental resistance is highly variable in need but also context. Wooden structures globally last over 1000 years. Out that same wood in a bog and it’s gone in a few.
- Recyclability can trump biodegradability.
- Being innocuous is more important than biodegradability.
All can can had by design through a product’s life cycle.
The ISO (International Organization for Standardization) has defined six types of degradable plastics.
Degradable – breaks down in some way.
Photodegradable, broken down by light
Oxidatively degradable broken down by oxygen.
Hydrolytically degradable. Broken down by water
*Biodegradable – can be broken down by microbes to mass, water and co2 but with no indication of how long that might take. May also need chemical addatives to make this process possible.
*Compostable – degrade at a rate that’s similar to other types of compostable materials, and they result, again, in water, carbon dioxide, humus, and inorganic compounds. Compostable plastics biodegrade naturally.They do not need additonal addatives to break down the polymers as they made from natural materials that microorganisms recognise.
This is a confusing list because the last two (*) seem to refer to the natural process of biodegrading while the others refer to plastic with added degradation Initiators.
The difficulty is of course ensuring that the plastic doesn’t start biodegrading in normal conditions so that the strength of the plastic product is not jeopodised. Biodegradation is designed to start in certain extreme conditions.
As 75% of plastic ends up in landfill, most additives are designed to work in landfill conditions.
While products may start to degrade outside of the specified conditions but the process will take much longer.
The obvious flaw in this solution is the wrong product in the wrong place. For example plastic that has been manipulated to degrade quickly in a landfill conditions ending up as litter on the roadside where it will not degrade quickly.
But yes there are quite a few discoveries made around the globe that help us keep hope that some day, the plastics we are going to use are fully biodegradable. Check out the video, where in Indonesia, plastics are made from Cassava!
To answer the question directly, I do not feel that there is a technlogy yet acting as "on/off" switch, meaning the plastics do not biodegrade at all during service and yet degrade rapidly after service. Most of the plastics are exposed to environments where bacteria may exist during application, and during this time biodegradibility is not desired. To me the focus is to find ways in landfill or treatment plants to biodegrade plastics that are otherwise not readily biodegrade (there are some reports indicating certain bugs can eat plastics, though it's still at very early stage and cost is not low).
As far as I know, there is nothing triggering the change. The degradation process is something gradual which take place faster or slower depending on exposure conditions and material composition. Manufacturers can play with this composition (upon certain limit) to modulate degradation behaviour.
For products with short-life cycle (e.g. packaging) it can be formulated a polymer with relatively fast degradation time and these materials will degrade in the environment fast. For other products with longer life cycle, formulation should avoid a very fast degradation and it will affect therefore to end-of-life. That is why main applications of biodegradable plastics are focused on short-life cycle products such as packaging or bags.
For those products such as engineering structural polymers, load bearing materials, protective coatings, that require long service life, biodegradability is not the way to go, but biostability and chemical stability are essential (and the long and reilable service save energy, waste, and cost, and the environmental benefits they provide far exceed the non-degradability issue). For those only serve temporary or short-term usage, biodegradability can be purposely designed. One of the focus (and is a challenging one) is to find methods that can degrade stable plastics in treatment plants or sites.