In Detail
Synthetic polymers like polyethylene and polypropylene are not normally regarded as biodegradable. The properties that make them attractive as a packaging medium- strength, flexibility, and water and air resistance for example - are the result of their molecular structures.
Both materials are a hydrocarbon, which is to say that their molecular backbones are constructed of hydrogen atoms bonded together by carbon atoms in long entangled chains. It is these long chains that provide flexibility, strength -and significantly prevent oxygen from attaching to the carbon and hydrogen atoms and causing oxidation; which in turn leads to degradation.
The molecular mass of a material can be a good indication of the complexity of its molecular chains and thus resistance to oxidation.
Molecular mass is, simply, the weight of the atoms that make up an individual molecule of the material. So, for example, water is two hydrogen atoms and one oxygen atom - H2O. The atomic mass of hydrogen is 1.00784 and that of oxygen is 15.9994; therefore, the molecular mass of water with formula H2O is (2 x 1.00784) + 15.9994 = 18.01508. Therefore, one molecule of water weighs 18u. The molecular mass of a typical polyethylene is 300,000u!
Eventually even polymers like polyethylene and polypropylene will degrade - through oxidation and then bio-degradation but this will take decades of exposure to heat and light that will slowly break down the polymer molecular chains.
The technology behind d2w™ totally degradable plastics introduces a pro-degradant into the polymer that acts as a catalyst and causes a rapid breakdown of the long molecular chains. This pro-degradant is in the form of a metal salt that causes a breakdown of the carbon-carbon bonds in the molecular chains - i.e. chain cleavage, or scission is activated. The plastic product will become brittle and quickly disintegrate into tiny flakes. As the chains continue to reduce in size, oxygen is permitted to bond with the carbon and produce CO2. The molecular mass quickly descends to below 40,000u and at that stage, the material effectively becomes water wettable and micro-organisms can access the carbon and hydrogen. Carbon is used for cell wall structure etc. and it is exhausted as CO2 and the hydrogen as H2O. This stage can accurately be described as bio-degradation.
The pro-degradant is introduced at the extrusion stage of manufacture, when polymer granules are heated and melted to form packaging films. The d2w™ additive itself is provided as a masterbatch and only small amounts are required to cause the degradation reaction. In percentage terms, only between 1 to 3% is typically required. Degradation is triggered by the extrusion process but is sufficiently slow during the initial stages of the scission for it to have no effect on the properties of the film.
Stabilisers are also included in the additive masterbatch that protect the pro-degradant throughout the melting process, and then determine the time scale to the onset of the degradation process. Therefore, different periods of 'fit for purpose' can be engineered depending on the final application of the material.