What is Oxo-Biodegradable Technology
All polymers derive their mechanical properties, especially toughness, from the entanglement of their long chains. Polymer chains long enough to confer useful mechanical properties are usually too large to be able to cross the cell walls of bacteria or fungi. All polymer biodegradation thus requires that there be some extra-cellular chemical process to cleave the chains and to release them from the entangled mass as fragments small enough to be transported into the cell and metabolised.
There are two well-recognised biodegradation pathways for plastics, hydro- and oxo-biodegradation. The hydro- and oxo- prefixes are inserted to emphasise that biodegradation of a plastic always involves two stages, and both mechanisms are influenced by the environments to which the materials are exposed.
In natural hydro-biodegradable (HBD) polymers, chain scission occurs by hydrolysis, as in the case of polyesters like the poly(hydroxyalkanoate)s. In natural oxo-biodegradable (OBD) polymers, scission is by oxidation, as in the case of natural rubber and lignin.
In either case, this initial degradation may be purely chemical or it may be mediated by enzymes released from the cell. Both hydrolytic enzymes (typically esterases) and oxidising enzymes (e.g. cytochrome systems) are well known. There is certainly nothing in nature to exclude oxidative scission as a precursor to biodegradation; it is the way nature disposes of both natural rubber latexes and the lignin fractions of wood and other plant matter.
Aside from the basic distinction between hydrolytic and oxidative cleavage, the main differences between OBD and HBD technologies are;
- The lifetime of an OBD polymer, before biodegradation starts, can be regulated by varying the antioxidant / pro-oxidant ratio.
- Because of the induction period required for oxidation to produce biodegradable materials, biodegradation of an OBD material is inevitably slower than that of an HBD. Although this excludes OBD plastics from applications requiring, or merely specifying, very rapid bio-assimilation, there are equally many applications where the rapid and uncontrolled biodegradation of HBD plastics is a problem.
In summary, the basic technology of an OBD material involves:
- An induction period during which oxidation catalysis by the pro-oxidant(s) is prevented by the antioxidant(s). During this period there is no change in the polymer but the antioxidants are consumed.
- A rapid oxidation of the polymer during which chain scission produces low molecular weight fragments, which are oxidised, hydrophilic, dense and polar.
- A period of bio-assimilation of the oxidised fragments leading to mineralisation
It is important to emphasise that these are overlapping processes. In particular, once significant oxidation starts, it is faster in biotic than in abiotic environments, so that lifetime predictions from simple oven ageing or light exposure testing will tend to predict over-long breakdown times in natural exposure.
The important points to consider from the above mechanism, is that the material molecular weight is continually reduced through a catalytic oxidative process and will continue to decrease while aerobic conditions prevail. Therefore, the material does not just fragment, due to the loss in mechanical integrity from the reduced Mw of the material, but the very chemical nature of the material is changed due to oxidation processes. This produces low Mw oxidised molecular species which become evermore hydrophilic and available as carbonaceous food sources for microbes. Microbes then digest these species to not only produce CO2 but to use a material for cell growth and conversion to humus in a similar manner to wood and leaf matter.
The oxo-biodegradable process is in contrast to compostable bio-based polymers which essentially use an industrial composting system at high temperature to firstly breakdown the material through hydrolytic processes to produce fragmented polymeric material and then to incinerate at least 90% of the polymer over a period of up to 6 months to form CO2, a problematic greenhouse gas.