There are three main methods that can be found in industry and published media for the degradation of products down to natural by-products.  Composting of plastics and oxodegradable additives for plastics have been around for several decades and probably explains why they are still prevalent.  Landfill biodegradable additives are relatively new and are still making headway into the design process of plastics.  They all have different characteristics and the target disposal method needs to be considered carefully before any one method is selected.

On this page:

• What is the difference between Biodegradable vs Degradable?
• Landfill Biodegradable Plastic
• Compostable Plastics
• Oxodegradable Plastics

 

What is the Difference between Biodegradable vs Degradable?

Degradable is a general term used to denote a product will break down into smaller parts or become transformed to another form  For example steel degrades (rusts) to an iron oxide.  In the case of a plastic, if the plastic degrades down to smaller and smaller particles to the point they are not readily seen, it doesn’t mean the plastic has gone away.  In that condition it is more than likely a greater environmental problem.   Biodegradable is a more specific term that explains the process of how a product degrades.  When a product biodegrades it is digested by bacteria, fungi or algae and it is broken down into a biogas, (CO2 for aerobic degradation and CH4 for anaerobic degradation) and a soggy biomass or humus which makes plant fertiliser.  The biodegradable by-products are then naturally recycled back into the cycle of earth.

In A Nutshell

Landfill-biodegradable plastics

1. Made by adding an organic additive to a conventional plastic at time of manufacture.
2. Allows the plastic to be digested by naturally occurring microorganisms in the landfill.
3. Will not break down on the shelf.
4. Have no shelf life issues.
5. Can be put back into the main stream recycling of plastics.
6. Many have FDA approval for food containers.
7. Consumers can dispose of it in the way they are used to disposing of rubbish, i.e. putting it in a bin.

Compostable plastics (BioPlastics)   Generally

1. Made form plant materials not petroleum products.
2. Will only biodegrade in a commercial compost facility.
3. When they do biodegrade in a commercial compost facility they degrade aerobically to CO2 and biomass.  What this means however is the molecular energy in the plastic has forever been lost and cannot be used to generate electricity anymore.  Compare this to either combusting the plastic in a Waste to Energy plant that recovers most of the energy from the plastic and coverts it to CO2 and steam or electricity.  Or the anaerobic biodegradation of a plastic in a landfill that produces methane which can be harvested and combusted to produce CO2 and energy.   Hence while the biomass of a commercial compost facility has large beneficial values for soils, the loss of potential energy reclamation is a considerable detriment.
4. Often touted as green plastics due to their plant origins but they are no more green than a conventional plastic if the bioplastic is not sent to a commercial compost facility.
5. Will not biodegrade in a landfill as they need oxygen, high temps (>60 deg c) and moisture to biodegrade.
6. Are marked with the #7 on their recycling symbol.  #7 means ‘Other Plastic’ and is not recycled and send to landfill or incineration.
7. There is no special collection facility for bioplastics hence they cannot be sorted form other plastics and end up in landfills.
8.  There are few commercial compost facilities that will take bioplastics.
9.  Consumers cannot be expected to drive for miles to drop off a compostable bag and a few items of cutlery.  Hence they will dispose of the items to trash where they will not biodegrade.

Oxodegradable Plastics

1. Made by the addition of a chemical additive to a conventional plastic at the time of manufacture.
2.  A slow chemical reaction starts that causes the plastic to fragment down in to little pieces over 12-24 months.  The degradation time is controllable by the amount of additive added.
3. Will only fragment in the presence of oxygen and often sunlight.  Will not fragment once buried in a landfill but last 100’s of years like a conventional plastic.
4. Are not recyclable in mainstream recycling as the additive could cause the new products to start to fragment even they may be items that we do not want to break down like an electrical insulator.
5.  Claims of the little fragments of plastic then become biodegradable have not be proven by a recognised ASTM type test.
6.  The little fragments of plastic can easily be blown around or washed down water ways potentially making more of a environmental hazard.
7.  Will fragment faster if they get exposed to hotter ambient temperatures.  Which may mean they fragment before their design life is over.
8.  Have a limited shelf life after which they will start to fragment apart.  Makes planning of their design life complicated as their final application will not be known by the manufacturer in many cases.

In-Depth

Landfill Biodegradable

Imagine, a plastic that will biodegrade away in a few years and be returned to the earth as natural plant food, (biomass or humus).  Within recent years, this has now become possible and may be a large part of the answer to how we dispose of all future plastic.

Landfill Biodegradable Plastics      Take a normal plastic like PET used to make bottles.  If disposed to a landfill, this would last 100’s of years.  However if a small amount of landfill-biodegradable additive was included to it at the time of manufacture, this bottle would biodegrade away by naturally occurring bacteria in the landfill.  The bacteria seeking out the additive to eat, simultaneously cleave the large plastic molecules into smaller molecular chains, down to the point where the bacteria will eat them too as food.  Finally, the bottle is gone!

 

How long will it take to biodegrade away?

The question that people always ask is; how long will it take to biodegrade away? Our view is simple and based on a generational concept.   We do not want our trash to be left around for our grandkids or their grandkids to deal with one day.  We need to be responsible for our problems in our life time.  So do we care if a bottle takes 2 years or 10 years to biodegrade away?  Even if it took 20 years to biodegrade away that is still a far better situation than taking 500 to 1000 years.  (We have only been making plastics seriously for 40 odd years, therefore we really have no idea how long normal plastics will last.  Some estimates are over 500 years).

Other factors that affect biodegradation are the amount of bacteria present.  This is governed by the level of organic material present, the moisture level, (the more there is the better the bacteria like it) and the temperature of the landfill.  If it is too cold or too hot, they do not survive.  Biodegradation with aerobic (oxygen breathing) bacteria only occurs in the top meter or so of a landfill.  Below that the bacteria are anaerobic and anaerobic biodegradation occurs.

Most plastics that are discarded, especially one off disposable consumer used plastics like bags, bottles, cups, food containers, etc. are thrown out to trash if they are not recycled.  A certain percentage of the recycled plastic is dumped to trash as well, depending on its content and the operational status of the recycling facility.  Therefore having a plastic with the ability to landfill biodegrade without requiring any special handling procedures or asking consumers to change their disposal habits will yield the maximum amount of plastic waste biodegrading away over time.

Recyclability
Landfill recyclable plastics can readily be recycled in a conventional recycle stream.  In fact it is encouraged as it spreads the additive to all the plastic that doesn’t have an additive in it.  This makes all the new products become biodegradable.  As the additive concentration becomes diluted with other plastic that has no additive in it, the biodegradation rate will slow down.   However slow biodegradation is better than no biodegradation.

Suitability for Food Containers
A large part of our disposable plastic comes from food containers.  Many of these are recyclable, however a good number are still not.  The landfill biodegradable additives are USA FDA compliant to be used as food storage containers.  Using a landfill biodegradable additive in them will allow those containers that do get recycled to still be recycled, and those that get disposed directly to trash to biodegrade down in a landfill.

Other Degradation Methods
There are two other types of degradation methods that are promoted.  However there are numerous issues with both of these and the practicality of their use is limited or even more damaging to the environment.

Compostable Plastics (Bioplastics)

Composting is biodegradation in the presence of oxygen, moisture and warm temperatures of approximately 60 deg C (140 deg F).  Commercial compost facilities must rotate their materials frequently or inject oxygen to keep the materials exposed to oxygen and thereby keeping the biodegradation reactions processing along.  The biodegradation rate in a compost pile can be quite fast, just tens of days for a plastic bag to fully degrade.

Compostable plastics such as the corn starch derived plastic PLA have a low working temperature of about 60-70 deg C (140-160 deg F)  This has led to problems of them being used as cutlery for hot foods.

There are several major problems with composting that make it a non viable option for effective plastics disposal.

1. There is a lack of commercial composting facilities available.   They are few and far between and very few people are going to go out of their way to take their few compostable items to a distant special facility.
2. There is no special recycling guide for consumers to follow for compostable plastics.
   

   Compostable plastics have a type 7 in their triangular chasing windows symbol which is miscellaneous plastics.  These can’t be recycled because the materials in the mix may have different melting points and such.

3. There is no special curb side pickup for compostable plastics.
4. When they do biodegrade in a commercial compost facility they degrade aerobically to CO2 and biomass.  What this means however is the molecular energy in the plastic has forever been lost and cannot be used to generate electricity anymore.  Compare this to either combusting the plastic in a Waste to Energy plant that recovers most of the energy from the plastic and coverts it to CO2 and steam or electricity.  Or the anaerobic biodegradation of a plastic in a landfill that produces methane which can be harvested and combusted to produce CO2 and energy.   Hence while the biomass of a commercial compost facility has large beneficial values for soils, the loss of potential energy reclamation is a considerable detriment.
5. When discarded to a landfill, compostable plastics behave no different to any other non biodegradable plastic and will last for the same 100s of years.

All this leaves consumers little choice but to throw their compostable plastic out in the trash where it will last for many generations.

Oxodegradable Plastics

As its name implies, this is a process requiring oxygen to work.  In addition it is a slow rate chemical reaction which does not require bacteria.  The reaction embrittles the plastic over time so that eventually it gets so brittle that it breaks down into smaller and smaller pieces.  At some point the pieces become so small they are not readily seen by the eye.  Some oxo manufacturers then claim that once the plastic molecules are so short they will be eaten by bacteria and biodegrade away.  This is called Oxo-bio degradation.  The oxodegradable plastic is manufactured by adding a small amount of the oxo additive to a plastic at the time of extrusion.  Thereafter the chemical reaction starts to take place.  The amount of additive used controls the reaction rate, the more additive, the faster the fragmentation process occurs.  A lesser amount of additive causes it to fragment slower.

These oxodegradable plastic items also require sunlight or oxygen to degrade.  This implies they need to ‘lie’ around in the environment to breakdown.  Case in point, one of the major additives manufacturers, mentions a test they did to illustrate the degradation of plastic bags with their additive.  For the test they ‘hung the bags on a fence’ in the sunlight!  In trying to control plastic debris we do not want to have plastics that require to be left on the ground or in the ocean where they can get sunlight and oxygen.  Yes, there they would break down to smaller and smaller pieces, but the increased litter aspect of this process is very questionable.  Also for the case with oxodegradable plastics, this is degradation based on time.  So it precludes a plastic item that has been treated with an oxodegradable additive from being recycled, as the new product would start degrading whether the user wanted it to or not.  The most common method of disposal for one-off use plastic items is for them to be disposed to a landfill and oxo-additives suppliers themselves admit the additive does not work in an anaerobic landfill setting.  They claim they work in the aerobic part of a landfill, but that is a small part of the landfill volume and as material becomes buried that aerobic environment changes to an anaerobic situation with no oxygen.  Then the degradation of the plastic ceases.  At this point the oxo plastic will last in the landfill as long as an original untreated piece of plastic does, i.e. many 100s of years.

The oxo proponents also say their method is the best method to control plastic litter as it will cause the litter to break down to small pieces then biodegrade away.  For plastics that do end up as litter and become caught in a tree or stuck in a waterway, the idea of having that plastic item break down into many smaller pieces seems a worse effect than just leaving the original plastic item in one piece.  One oxo manufacturer wrote; “all short-life plastic products should be oxo-bio so that they will degrade much faster than ordinary plastic if they do get into the open environment. Of course they will not degrade immediately, because they are designed to be re-used many times.”  This statement has two problems with it.  First the oxo additive manufacturer has no control of how the final product manufacturer designs (amount of additive added) their product, so whether it is designed for many reuses is not under their control.  Second, if the product was designed for a life time equivalent for many uses, then the litter with the oxo additive in it will last that long too.  This typically would be a few years.  We do not want to leave litter lying around two years or so before it starts to break down.  When we see litter, most people pick it up then and there.  Many city councils also operate weekly litter clean ups.  Litter is ugly to most of us and we are not prepared to let it remain for a few years just because it has some additive in it that will break it down at some point in the future.  The only time litter is not picked up soon after it is dropped is in some locations where people do not often walk.  The side of a highway is an example.  But even there on busy highways, there are cleanup crews periodically out there picking up litter.  Hence the oxo manufacturers’ claims of it being a way to dispose of litter seem not to be viable.

Another claim made was “If all the plastic in the ‘Great Pacific Garbage Patch’ had been made with oxo-bio, it would not be there at all.”  It is not in question whether a plastic with an oxo additive in it would have broken down or not.  However what really has to be asked is this: what happens to these now small plastic fragments in the ocean.  Saying they are ‘inherently biodegradable’ is fine, but saying they will biodegrade is quite a different thing.  If there are insufficient bacteria in the ocean they will not biodegrade.  Research shows that as the plastic pieces float through the water they absorb toxins with which they come in contact. Research articles from scientists such as Captain Charles Moore and others present results of expeditions which have trawled through the garbage patch with nets.  Following their journeys they then dissected the fish. Their results show the fish are ingesting these small pieces of plastic.  The record holder was one fish with 83 pieces of plastic in its stomach.  The really small shards of plastic, that are the same size as plankton, are also being ingested by the fish and mammals that swim through the water as they search for plankton.  In the largest gyre, located in the central North Pacific, Neuston (surface) trawls lined with 0.333mm mesh yielded the astounding figure of six kilos of plastic fragments for every kilo of zooplankton  >0.333mm in size.  Detritus feeders, like the Laysan albatross, have been demonstrated to feed primarily in and around the North Pacific subtropical gyre, and the stomach contents of their chicks, receiving nutriment only by regurgitation from adult birds, contain alarming quantities of plastic.  This presents a real problem for those who eat fish. Not only are they eating fish that have small pieces of plastic in them, they are also digesting the toxins that were on those plastic shards. We would be better off to not cause those plastic items to break down but leave them in their original large size and mechanically collect the trash from the ocean.  Another comment the oxo group use which seems to mislead people is that once the item has broken down it is gone.  This is not fully true. It may be gone from sight, but the particles still remain and can be causing damage that we cannot see with our eyes. Only our health in the future will show this, and by then, it will be too late to make any changes.

One point that needs clarification is the biodegradation.  If there are no bacteria around then there is no biodegradation. There are not many bacteria on the ground on the side of a highway.  So this claim by oxo people of biodegradation has to be qualified to biodegrade in the presence of oxygen and bacteria.  If the environment is such that there are bacteria on the side of the highway, then surely, using a biodegradable additive instead is a better alternative.  It may be a little slower due to less surface area to act upon, but the object remains in mainly a single piece during the degradation. The oxo product would break it down into thousands of pieces which are more likely to be picked up and blown around, or get washed down a waterway.   This scenario would seem a lot worse. There is no additive solution that will solve a litter problem with plastics and the oxo people should cease to say they have a solution for litter. Their claim is based on “out of sight, out of mind” thinking.

The biodegradable claim on these small pieces is in question from some sources.  Also the plastic item cannot be put back in the recycle stream.   It is difficult to comprehend how a plastic item that disintegrates into smaller and small pieces of plastic is good for the environment.  These pieces become so small and will blow around in the wind, or flow in moving water.  Wildlife are then particularly vulnerable to ingesting these particles.  There are numerous reports of birds, fish, turtles and other species that have multiple pieces of plastic in their stomachs.  The plastic is not broken down by their digestive systems, and can accumulate in their bodies.  In cases such as turtles, the plastic makes them more buoyant until they can no longer dive underwater to find food.  They get trapped on the surface and starve.  Researchers have estimated that for every 1 kilogram (2.2 pounds) of plankton in the Great Pacific Garbage Patch in the Pacific Ocean, there is 6 kilograms (13.2 pounds) of plastic.
It is easy to understand how most consumers who have no knowledge of the ins and outs of these treated plastics become confused.  Prior to the time non degradable plastic bags started to become banned in various locations, there were few options available to produce a degradable bag.  This is the time the entry of oxo-degradable additives seems to have found their place.  South Australia is a case in point.  They allow oxo-degradable bags but not non degradable bags.  Considering the above comments, it is not clear what benefit South Australia has picked up at all from allowing oxo-degradable additives.    One UK study reported ‘Surely a ban on oxo-degradable plastic would be more appropriate and the issue of non-biodegradable plastic tackled through recycling incentives and/or a switch to truly biodegradable or compostable plastic?’
From the OxoAlliance themselves, when replying to the question, “Oxo products biodegrade, but do they in the landfill?”
Answer: This is a tricky issue.  Are we talking about a dry landfill?  What about a wet landfill?  Cold?  Hot?  What about a regularly turned landfill? Deep inside a landfill nothing biodegrades. It is like a tomb. No air and no UV light, which are the triggers to start the oxobiodegradation process.
The goal of Oxo is for the biodegradation to take place out in the open environment where we find unsightly trash, and on the top levels of landfills or in regularly turned landfills.
Oxobiodegradable plastic does not biodegrade deep in a landfill because there is no oxygen.  The OxoAlliance does not support companies who claim oxo biodegrades in a landfill.