In the general context of the environmental impact assessment of a product’s lifecycle, biodegradability is an important feature that is often taken into account when judging the ecological acceptability of a material, particularly in the phase of its elimination and disposal. In recent times, even in the field of leather, this feature is becoming increasingly more important, but not actually measured, in order to better evaluate an easier way for the disposal or recyclability of leather tanned with alternative systems to chrome. Hence the need for a study that firstly develops a suitable test method for leather; and then compares the biodegradability of chrome-tanned leather with that of leather tanned with alternative systems (white-wet and metal-free). This study was carried out at the Experimental Station for the Leather Industry and Tanning Materials in collaboration with the Biology Department of the “Federico II” University of Naples, under the general coordination of Dr Biagio Naviglio. In his research activity, Dr Naviglio started from a factual point of view, as he explained at the AICC (Italian Association of Leather Chemists) national conference where he presented the study: «Everyone is talking about biodegradability. But scientific literature does not show the numbers indicating the degree of biodegradability. Therefore, we have tried to represent the biodegradability of leather with standardized methods».
A valuable “ecological operator”
The measurement of biodegradability, in terms of waste disposal, allows assessing the environmental impact. From the perspective of a product’s lifecycle, biodegradability can be considered an “ecological operator”, i.e. a useful characteristic to qualify the product from an environmental point of view, especially in the elimination and disposal phase. Because a biodegradable product allows less accumulation in the environment. This long term vision allows overcoming an apparent contradiction: what is the point of talking about biodegradability when tanning has the opposite goal, namely that of making leather rot-proof and less biodegradable? In reality, this is not an incongruity because it is important to predict a material’s ability to deteriorate and to know its impact on the environment.
The action of microorganisms
Biodegradability is the characteristic of substances and materials to be assimilated by microorganisms and therefore to be introduced into natural cycles. In other words, it is the tendency of a material to be converted into carbon dioxide through microorganisms. When the process takes place in the presence of air, biological degradation is called “aerobic” and the organic substance produces carbon dioxide; instead, in the absence of air (i.e. anaerobic degradation), carbon dioxide and methane is released.
The most commonly used measurement model, i.e. the UNI EN 14046 standard, is based precisely on the development of the carbon dioxide analysis. From purely an operational standpoint, the assessment of aerobic biodegradability of packaging materials under controlled composting conditions is performed with reactors inside of which are placed: in one, compost and leather in a given ratio (6:1); in another, only compost; and n the third, compost and the reference substance (cellulose). The temperature is set at around 58°C; the conditions are aerobic, i.e. with strong presence of air; and the duration of the test is fixed at six months, with measurement once every two days of the CO2 released. The investigation involved bovine leather (trunks), a wet-white finished product and a product tanned with an organic substance of the last generation.
Three characteristic moments
With regards to the microscopic and product analysis, the chrome sample has a clearly visible and uniform flower design and is very full and soft to the touch; in the wet-white sample, the flower design is less obvious and the leather appears to be somewhat emptied and reswollen, even if it has an acceptable coat; finally, the last sample, the one processed with an organic substance, has a flattened and undistinguishable flower design and is slightly dry to the touch. After two and a half months, the chrome leather measures a degree of biodegradability of 45%, which increases to 81% in wet-white leather and to 84% in leather tanned with organic substances. As shown in the graph, the investigation shows three characteristic moments, albeit with different time intervals, depending on the intrinsic properties of the samples: an initial “adaptation” phase, with no or very slow biodegradation, followed by a relatively fast biodegradation period which ultimately stops when the stationary phase is reached.
Analysis of the results
In conclusion, it can be said that chrome-tanned leather has a lower percentage of biodegradability than the other two samples examined. The method that, on the contrary, shows the highest value of biodegradability (84%) is the metal free process. But the most obvious result is that no sample reaches the minimum level of biodegradation of 90% within the times set by the reference standard. To be sure, it should be remembered that previous investigations have shown that vegetable-tanned leather has a biodegradability value of just under 70% and, therefore, lower than that found in wet-white and organic tanning.