Increasing concern about environmental pollution by persistent chemicals and materials is one of the drivers towards developing recyclable materials. However, various polymeric materials cannot be efficiently recycled due to their specific uses, which are too dispersed (e.g. thickeners in home care products) or complex (thin protective coatings on e.g. furniture or flooring). Furthermore, so-called thermoset materials, which can be found in e.g. cushions, coatings and rubbers, are difficult to recycle due to their specific molecular structure. Since many of these materials have a high probability of ending up in the environment due to abrasion or degradation (e.g. chipping of paint, wear of rubber tyres), it is becoming increasingly important to ensure that they do not accumulate and persist in nature.
One of the core objectives of the CHAMPION project is to replace high-performance fossil feedstock-derived polymers with bio-based and biodegradable alternatives obtained through so-called aza-Michael (AM) addition chemistry. While researchers in the project have recently shown that AM chemistry is suitable for making polymeric materials with the desired mechanical properties (see details here), preliminary results show that AM materials also have increased biodegradability compared to more conventional polymeric materials.
Accelerated soil biodegradation tests (at 37 °C) performed by project partner OWS show that a bio-based pre-polymer cured by CHAMPION’s AM chemistry (green curve in the graph) biodegraded considerably faster than the reference material obtained from the same pre-polymer but cured by conventional radical-mediated polymerisation (red curve). The reference polymer reached less than 6% biodegradation after 135 days, while a biodegradation of more than 55% was achieved for the AM-polymer. Furthermore, the tests indicate that the AM polymer is continuing to degrade even after 135 days. In comparison, the bio-based pre-polymer itself showed a relatively quick degradation (black curve), only slightly slower than the biodegradable benchmark material cellulose (blue curve), which easily reached the validity criterion of 60% degradation within 28 days.
These preliminary results show that it is possible to use AM chemistry to develop materials that are stable enough during their desired use phase, and yet sufficiently biodegradable to prevent accumulation in the environment or lead to the formation of persistent microparticles.
Definitions and further explanations
Industrial composting: the harmonized European standard EN 13432 defines the requirements for a product to be called industrially compostable. It consists of four criteria. Two criteria are related to degradation: biodegradation and disintegration, and the other two are related to environmental safety, which, in the case of composting, falls back to compost quality, including chemical analyses and ecotoxicity.
Biodegradation: microbial conversion of an organic, carbon-based compound to carbon dioxide, new microbial biomass and mineral salts under oxic conditions (i.e. oxygen is present) or to carbon dioxide, methane, new microbial biomass and mineral salts, under anoxic conditions (no oxygen is present).
Disintegration: disintegration or fragmentation is the degradation of a product on a physical, product level and is only the visual disappearance of a material.
Organic recycling of polymer products: the aerobic treatment (industrial composting) or anaerobic treatment (biogasification) of these products.
Both the rate and the maximum level of biodegradation of a specific material are determined by the environmental niche. These environmental niches can differ with regard to:
• Moisture content: ranging from water to high-solids;
• Oxygen availability: aerobic or anaerobic;
• Temperature: e.g., high in industrial composting process, ambient to low in soil and water;
• Types of micro-organisms: bacteria, fungi, or actinomycetes;
• Concentration of micro-organisms: e.g., high in a wastewater treatment plant, low in the open sea;
• Salt concentration.
A material that is biodegradable in one environment is not necessarily biodegradable in another environment. A typical example is the thermoplastic polyester polylactic acid (PLA), one of the most used biodegradable polymers. This material needs a thermal trigger before biodegradation starts and therefore it biodegrades well under industrial composting conditions, but not in soil or water without any modification. The graph below gives a general overview of the biodegradation of most applied biopolymers in relation to the environment.