The term ‘Oxobiodegradability’ is a hybridisation of two words, oxidation and biodegradability. It defines clearly a two step process initiated in this case by the Reverte® 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.
Reverte® 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 Reverte® is now within the plastic at a predetermined amount.
The film is 'blown' as normal with no operational change to the process equipment or settings, this means that Reverte® 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.
After the check out bag is discarded the Reverte® 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.
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.
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.