Investigating the biodegradability of footwear

Explaining in detail how key biodegradability tests are conducted and what the results demonstrate.

by Nicola Pichel-Juan

Manufacturing materials and products with a lower environmental impact is a huge challenge being faced by the footwear industry, with one of the biggest problems to solve being what happens to items at the end of their life. Of the approximately 24 billion pairs of footwear produced every year, current estimates suggest that more than 85 per cent ultimately ends up in landfill, which is clearly not sustainable. Finding alternatives to landfill and incineration for discarded products is vital. One solution is to develop biodegradable materials that can be composted, with that compost ultimately being used to support the growth of nutritious crops.

In the February 2022 issue of this magazine, SATRA announced the launch of compostability and biodegradability testing. This article explains the key services in more detail.

It is important to note that not all material types are likely to be biodegradable. For material types that do have the potential to be biodegradable, the end result will depend on exactly how a given material has been manufactured and processed, and what, if any, treatments may have been applied to it. Materials that can be good candidates for biodegradability testing include leather (generally not including chrome-tanned), cotton, wool, some ethylene vinyl acetate (EVA) foams with bio-based fillers, rubber, paper and cardboard.

The quick screen disintegration test

While most disintegration tests take at least 90 days, the 20-day ‘quick screen disintegration test’ (a modified version of ISO 20200:2015) provides an indication of whether or not a material is a good candidate for a compostability certification by simulating the first 20 days of the certified test. Patterns of the breakdown are then matched against previous successful or unsuccessful candidates.

The test material is prepared and placed into a controlled composting environment (using a control synthetic compost) at a temperature of 58°C to simulate industrial composting. The compost containing the material is then mixed and watered at set intervals, with changes to the material observed according to the following criteria: i) degree of disintegration, ii) colour, iii) appearance, iv) smell, v) finish quality, vi) size/shape and vii) softness. These observations – when they are combined with the expertise that is available from the testing team – will indicate if the material is ever likely to be compostable.

Disintegration test

The ‘disintegration test’ (ISO 20200:2015) is a laboratory-scale industrial composting simulation that is used to determine the extent to which a material breaks down when composting.

The test material is prepared and placed into a controlled composting environment using a synthetic compost at a temperature of 58°C (the ‘thermophilic incubation period’). Control tests on items with known disintegration properties are conducted at the same time, and these are used to confirm the validity of the test.

The following initial measurements are taken from the compost before starting the test to confirm that its formulation is viable, and again at the end to confirm that the compost has grown correctly and that the test is valid: i) carbon:nitrogen ratio, ii) compost pH value, iii) dry matter value and iv) volatile solids value. The compost is mixed and watered at set intervals over a three-month period, with changes observed as per the quick screen above. There is also an option to continue the test for a further three months at 25°C (the ‘mesophilic incubation period’), depending on the extent of decomposition seen after the first three months.

The degree of disintegration of the test material is measured by how much of it remains in a 2 mm sieve after the compost is sifted, with the results showing if the material breaks down into something recognisable as compost. However, further testing would be required to understand if the compost contains harmful substances or if it is viable to grow plants in.

Ecotoxicity test

This assessment looks at how safe the biodegraded material is when added to an ecosystem, and determines if there are any chemicals of concern present that are toxic, carcinogenic, mutagenic or toxic to reproduction from immediate or prolonged exposure or from bioaccumulation (the gradual accumulation of substances, such as pesticides or other chemicals, in an organism). The material is placed into a controlled composting environment as above at 58°C for 90 days, and is mixed and watered at set intervals.

The final compost is then analysed for more than 200 substances in line with soil requirements from around the world, including the UK’s PAS 100 and the US USCC scheme. Some of the substances that the compost is tested for include metals, volatile organic hydrocarbons, phenols, polycyclic aromatic hydrocarbons, volatile halogenated hydrocarbons, chlorobenzenes, chlorophenols, polychlorinated biphenyl, miscellaneous chlorinated hydrocarbons, nitrogen and other pesticides, phthalate esters and petroleum hydrocarbons.

Plant response test

The ‘plant response test’ (REAL CCS 3.1) evaluates how well plants – typically tomato (but also different types of legumes) – grow and respond to being grown in compost (mixed with soil) with the biodegraded material. The material is placed into a controlled composting environment as above for 90 days, after which time the compost is mixed with other growing media and plants are grown in it for 28 days.

Their growth parameters are then compared with plants grown at the same time in a control compost. The final assessment includes looking at the amount of overall growth, the number of leaves and the presence of any abnormalities (such as deformation, wilting and stunted growth) to understand if healthy and viable plants can be grown in compost which contains the biodegraded material.

Leather determination of degradability

This leather-specific test (ISO 20136:2020) involves the leather being ground up and put into a bacteria ‘soup’ comprised of nutrients, water and inoculum from tannery effluent. The mixture is placed in an incubator at 23°C for 28 days and is shaken at 150 rotations per minute (figure 1). The amount of CO₂ given off is measured by infrared equipment and compared to a collagen control sample with known biodegradation properties (see figure 2). This relative biodegradability indicates how biodegradable the material is.

In conclusion

While there is no quick and simple answer to the challenge of making truly sustainable products, having materials that can under the right conditions enhance rather than harm a given ecosystem must be a key consideration as part of a circular or closed loop solution. As well as the tests detailed above, SATRA is also able to offer biodegradability and compostability certification for a number of product types and has further testing in development.

Publishing Data

This article was originally published on page 10 of the June 2022 issue of SATRA Bulletin.

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