oxo-biodegradability

ReverteTM is well suited to a wide range of processes and applications, has extensive food contact approvals and has been used for many years to provide a reliable and reproducible oxo-biodegradable property to plastics around the world.

Molecular Weight > Time

The term ‘Oxo-biodegradability’ is a hybridisation of two words, oxidation and biodegradability. It defines clearly a two step process initiated in this case by the ReverteTM additive to degrade the polymer chain (break up) and make it available for biodegradability within the environment when a treated item has finished its useful life.

The phase of oxidation reduces the molecular weight and introduces oxygen into the structure. This process transforms the polymer from long strands to much smaller lengths. By reducing the chain length of the polymer the material loses its physical strength and elongation properties making it brittle and none ‘plastic’. The biodegradability aspect refers to the conversion of these lower molecular weight species by bacteria into biomass, CO2 and H2O in an aerobic environment, or in the case of an anaerobic environment, CH4.

ReverteTM is introduced to a plastic article such as a check out bag at the manufacturing stage, the complex formulation is dosed in at a very low level, and the carefully developed complex additive package within ReverteTM is now within the plastic at a predetermined amount.

study of a reverteTM checkout bag

ReverteTM can modify polymer recipes...

The film is 'blown' as normal with no operational change to the process equipment or settings, this means that ReverteTM can modify polymer recipes making them oxo-biodegradable with only a small impact on cost and with minimum effort.

Once manufactured the check out bag now begins its useful life going from the production process through to warehousing, stock holding despatch and finally into the supermarket. It is used by the customer and discarded into the waste stream in some form or another.

reverteTM additive is activated by natural sunlight...

After the check out bag is discarded the ReverteTM additive is activated by natural sunlight, this accelerates catalytic reaction to break the carbon-carbon (aliphatic) bonds at a molecular level and introduce oxygen into the polymer chain. This simple but effective reaction does two things, first it reduces the molecular weight of the polymer by 'chemically cutting' the chain, secondly it makes the molecular fragment more 'hydrophilic'.

The first physical observation is that as the chain scission occurs and the molecular weight of the polymer is reduced the plastic begins to lose its properties, its strength is lost and its elongation properties reduce, in effect the material becomes brittle. As the chain scission reaction continues the bag simply falls apart having no integral strength.troduced to a plastic article such as a check out bag at the manufacturing stage, the complex formulation is dosed in at a very low level, and the carefully developed complex additive package within ReverteTM is now within the plastic at a predetermined amount.

the molecular weight has dropped...

It is a common misconception that this resultant powder is just smaller pieces of plastic, this is not the case. The molecular weight has dropped and oxygen has been introduced into these species, producing a complex mixture of carboxylic acids, ketones and alcohols. These lower molecular weight entities are not polymeric (the do not have sufficient repeating units to be classed as a polymer) and the material does not have any of the plastic properties which we associate with the polymer. It has therefore irreversibly changed beyond recognition.

The remnants are also hydrophilic which means they can be wetted out, unlike a normal polyolefin which is extremely hydrophobic. This is a critical parameter as it means that the carbon that was originally bound up in the polymer chain is now accessible and can be utilised by microbes, unlike when it was a polymeric chain and highly hydrophobic.

available for biodigestion and are biodegradable...

In essence the polymer has been altered into a group of materials which are both available for biodigestion and are biodegradable. These materials if in a suitable environment which is rich in bacteria can now be safely biodegraded into the environment.

The biodegradation step rate is dependent on the environmental conditions in which it is exposed, this in fact is true for all biodegradable materials whether it be grass cuttings, straw or an oxo-biodegradable check out bag!

This final step of converting the available carbon from the polymer into CO2 is called mineralisation, the rate at which CO2 is generated can be measured and provided with a ‘rate’ for this stage of biodigestion.

technology

degradation profile ..

The key attribute of the ReverteTM technology is the degradation profile and how it compares with a more conventional ‘oxo-biodegradable material’.

Wells Plastics laboratories measure the first stage of degradation (oxidation) using carbonyl indexing and embrittlement point determination, processes which have been developed and exploited at Wells for many years and are now adopted by the industry.

The growth of the carbonyl index relates to the level of oxygen which has been incorporated into the polymer and wherever a carbonyl group is introduced the polymer undergoes a chain scission, i.e. a molecular weight reduction and a change in its physical and chemical properties.

Graph showing direct relationship between carbonyl index and molecular weight aged in accordance with ASTM 6954-04

carbonyl index ..

The more carbonyls introduced the further removed it becomes from its polymeric nature. Once a critical point has been achieved in carbonyl index the polymer loses its flexibility and breaks apart under the minutest of stresses. At this time it is considered to be at its ‘embrittlement point’.

the profile of degradation...

What makes ReverteTM stand out is the profile of degradation, the curve plotted of carbonyl index versus time starts off very flat with a period of no change in the carbonyl index over time. This period is known as the ‘dwell time’ another phrase coined by Wells Plastics.

After the dwell time has been passed the degradation then proceeds rapidly until the embrittlement point is reached and beyond that the plastic falls apart. The molecular weight is lowered and the material available for biodigestion.

biodegradability...

Biodegradability of a ReverteTM treated film occurs once the molecular weight is below 5000.

At this level the polymer no longer exists and is a wide range of discrete oxidised species. Bacteria and fungi find these species suitable for a source of carbon and can be assimilated into the lifecycle resulting in the formation of biomass and CO2 under aerobic conditions.

Biodegradation can only occur (whether this is for ReverteTM plastics or for grass cuttings) in environments which have warmth, bacterial activity and moisture. This fact is often overlooked when marketing oxo-biodegradable or hydrobiodegradable materials.

*The marketing claims which can be made about Oxo-biodegradable plastics may vary from country to country and in some regions may be legislated through trade and consumer bodies. In some cases these 'green claims codes' may restrict how the benefits of oxo-biodegradable technology can be described or depicted in marketing claims. It is therefore the responsibility of the end user to satisfy themselves that they are marketing the product in line with the market destination for which the product is intended. Wells Plastics Limited cannot be held responsible for the incorrect marketing of end products by users of the technology and we recommend that all marketing statements are checked by local lawyers familiar with the local legislation.