The heart of leather biodegradability – Part 4 of 4
Updated: Nov 26, 2021
In Part 4 of the Heart of Leather Biodegradability, we looked how measures to manage bacteria and fungi in tanning processes can be used to understand and enhance biodegradability. In this final part, we look at the role that biodegradability has in leather lifecycles, the tests that can be used to assess it, and future innovation opportunities.
Lowering the barriers
It is pivotal when designing future leather products that we look to both ends of the leather products spectrum. The tanner will be designing for long-lived products with maximum durability for leather articles that are destined for longevity models (say upholstery, some leather goods and dress shoes). The tanner may also be designing leathers (say shoe upper and clothing) that is on a short life-cycle, that must be as biodegradable as possible.
Biodegradable leathers should have built in protections during the working life and should rapidly decompose in composting or digesting environments. The resistance to breakdown in these biological environments can be readily tracked using the metrics that currently exist in the standard methods or will need tweaking to ensure that the lowest possible environmental impact results.
Signs of breakdown
It is important to understand what the perfect outcome for biodegradability is for bacteria and fungi to release their primary metabolising enzymes, break their substrate down, and absorb the nutrient and convert it into their own biomass. Fertile compost is predominantly bacterial/fungal biomass.
The next level of degradation is the bacteria/fungi release primary and secondary metabolising enzymes, the substrate is wholly or partly digested, and the remainder is mostly biomass. The difference between the two is time and energy required.
In the next scenario, the biodegradation proceeds with primary and secondary metabolism, but the enzymes are not capable of breaking down the substrate to varying degrees. The substrate then appears resistant and the processor would describe the scenario as long-term biodegradable.
The last scenario, the substrate appears unchanged over extremely long periods of time and the final breakdown is more dependent on disintegration by chemical and physical means as opposed to biological. The material is described as inert and highly resistant to micro-organisms.
The ISO standard methods
The ISO 20200 and 20136 tests are two of the more useful tests for assessing leather breakdown. ISO 20200 is a measure of disintegration and is designed for plastics - but is commonly used for leather. It has a few weaknesses, such as it is time consuming, long, and appears to have moisture uniformity issues.
ISO 20200 is an open system and if the test house is not careful can have overdried samples. The test depends on the replication element and if the deviation between samples is high then great doubts must result. The test house will only be good as the sieve calibration - as the test measures the result by sieving the contents.
ISO 20136 depends on very specific equipment. It measures CO2 and is a test of ultimate biodegradability. The highly specialist equipment is its major weakness as the test will be expensive. It is faster than ISO 20200 and in Authenticae’s opinion should have one important change. The inoculum for the test should make use of a tannery sludge seeded synthetic digestate, not the aerobic digestant sludge that is currently required as tannery digestants will vary on the activity and type of the tannery. A synthetic digestion medium, like the medium in ISO 20200 will be easier to standardise.
The leather industry is ready to fill the increasing gap left by consumers abandoning plastic due to the Blue Planet Effect. Leathers has an inherent biodegradability that can be enhanced by definite technical strategies in the design with a specific life-cycle in mind.
The addition of tannage and how it affects the collagen is primary. Research from the gelatine and archival industries tell tanners what the collagen weaknesses are. Future generation tanners will start with the level of biodegradability in mind - at the beginning of the product development cycle. These will be closely linked with the leather article designer who will design with deconstruction in mind.