Degradaation can be classified in two ways:
• According to the way that they degrade, for example whether they require the actions of microorganisms (i.e. are biodegradable), or whether they require heat, ultraviolet light, mechanical stress or water in order to break down; and
• According to the materials they are manufactured from, for example whether they are made from natural polymers, synthetic polymers or from a blend of a conventional polymer with an additive to facilitate degradation.
There are five different types of degradable polymers:
- Biodegradable polymers are those that are capable of undergoing decomposition into carbon dioxide, methane, water, inorganic compounds or biomass in which the predominant mechanism is the enzymatic action of micro-organisms that can be measured by standardized tests, in a specified time, reflecting available disposal conditions.
- Compostable polymers are those that are degradable under composting conditions. To meet this definition they must break down under the action of micro-organisms (bacteria, fungi, algae), achieve total mineralization (conversion into carbon dioxide, methane, water, inorganic compounds or biomass under aerobic conditions) and the mineralization rate must be high and compatible with the composting process.
- Oxo-biodegradable polymers are those that undergo controlled degradation through the incorporation of ‘prodegradant’ additives (additives that can trigger and accelerate the degradation process). These polymers undergo accelerated oxidative define degradation initiated by natural daylight, heat and/or mechanical stress, and embrittle in the environment and erode under the influence of weathering.
- Photodegradable polymers are those that break down through the action of ultraviolet (UV) light, which degrades the chemical bond or link in the polymer or chemical structure of the plastic. This process can be assisted by the presence of UV-sensitive additives in the polymer.
- Water-soluble polymers are those that dissolve in water within a designated temperature range and then biodegrade in contact with microorganisms.
(The composition of degradable bags also varies, with the main categories)
- Thermoplastic starch-based polymers, made with at least 90% starch from renewable resources such as corn, potato, tapioca or wheat.
- Polyesters, manufactured from hydrocarbons (oil or gas). All polyesters degrade eventually, with degradation rates ranging from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET).
- Starch – polyester BLENDS that mix thermoplastic starch with polyesters made from hydrocarbons.