Biodegradable shoes. Different components

The shoe, however, is made of many different components, let’s try to list some of them: the material used for the upper and that used as a liner, the strings if present and also the eyelets in which the strings are inserted. Buckles, zippers, ribbons and elastic bands, ornaments such as stones and studs. Assembly insoles, heels and soles. But also glues, nails, reinforcement materials, sewing threads. What are all these components made of? It is clear that if some of these components are not biodegradable, the famous shoe put in the soil will not completely disappear but some parts of it may remain unchanged even after a long time.   

Is it necessary that the shoe is entirely made of natural materials in order to be biodegradable?

Not all “natural” materials are biodegradable, cork for example is not. However, we can say that in general many of the organic materials, in other words, created by nature are biodegradable. It is the “circle of life” of the Lion king: at the end of life, matter transforms and becomes nourishment for new life.
Therefore, a shoe only made with organic materials is likely to be biodegradable. But how many of the shoes sold each year are made using only natural materials? It is therefore correct to try to understand how biodegradable man-made materials can be.
Man-made materials (for example plastic materials derived from petroleum) are not always able to decompose and therefore biodegrade. The main reason is that in nature there are no microorganisms capable of decomposing these materials. There is, however, another reason: if some materials were decomposed they would produce substances hazardous to the life of the microorganisms themselves.
Now let’s try to take a step forward and delve into a little bit about what is written above, trying to give some more scientifically accurate reasons.

1. Putting in soil

Decomposition occurs thanks to the work of microorganisms. Imagine living organisms, for example bacteria, capable of initiating chemical reactions in the molecules of the material, breaking the bonds between the atoms and creating different molecules such as carbon dioxide, water etc.
Technically, we are talking about putting the object into a compost, basically the soil you buy and put into a pot to make a little plant grow at home. It is not exactly earth but a mixture of different things containing the famous microorganisms capable of initiating chemical reactions and decomposing the materials of which the object is made.

2. What we put in the soil

ISO standards that describe how biodegradability tests are made do not say so. There remains therefore the question whether to put a shoe in the soil or to first tear the shoe into pieces and then bury the pieces.
It is clear that in that case size matters. The thickness of a material can determine the time it takes to biodegrade. And since the test is timed, in order to say that an object is biodegradable it is necessary that the reaction takes place within a certain time.
The question we all ask ourselves though is: “but when we throw away a pair of shoes does someone take care of tearing them up before trying to decompose them?”. The answer is “no” and therefore the theme related to the collection of used shoes and what happens to them following the landfill remains open.

3. Waiting a little while

The rule says that everything must happen within 6 months. There are accelerated tests but they are not suitable for shoes: in this case shoes simply do not decompose. It, therefore, takes a lot of time to prove that the shoe is biodegradable. In a world where fashion companies offer different products every month, who has the time to carry out these tests? You can test the materials of which the shoe is made but to do things well it would be better to put the whole shoe in the soil. Also because the materials used are far too many.

4. It turns into something else

What does the shoe become once you put it in the soil? The most used test checks that most of the mass is transformed into carbon dioxide, that is, the gas that plants use to start the photosynthesis that generates sugars thanks to the energy of the sun. Both materials of natural origin and many of the plastics contain carbon in their chemical composition as one of the main elements. If the reaction occurring “in the soil” manages to break the bonds between the atoms of the material, the carbon can react with the oxygen that is in the air creating CO2, that is, carbon dioxide.

The biodegradability test is carried out under particular temperature conditions (at about 60 degrees) and with a humidity close to 100%.

Watch out, it is not enough for the material to disintegrate, that is, for it to shatter into tiny pieces, even invisible ones. In this case, in fact, no reaction took place and the mass of the material is all still there. It is really necessary that the mass disappears, i.e., that it turns into gas. Plants will then use that gas to generate life.

5. But is it necessary for the shoe to disappear completely?

 We leave it up to you. The test regulated by the ISO standard (an agreement that scientists have reached worldwide) says that to be valid, the test must include at least 70% of the mass transformation.
Can we say that in that case the shoe is biodegradable? Technically we can not: it would be better to affirm that the shoe biodegrades by 70% (or by a greater percentage if this is the case)
The presence of these two elements indicates the seriousness of statements created by the marketing of the material companies: the percentage of biodegradability and the time it takes to biodegradate. Saying that a material biodegrades by 80% within 6 months is more accurate than simply saying that a material is biodegradable. 

6. Why are some materials non-biodegradable?

Simply because in nature there is no micro organism capable of triggering the transformation reaction of the carbon present in the material into carbon dioxide. The reason is therefore not the absence of carbon in the chemical composition of the material but rather the fact that carbon atoms are so strongly bonded to other atoms that it is not possible to break these bonds and activate the transformation reaction. There are materials that we call bio-plastics that are completely similar to plastics in terms of physical characteristics, appearance and touch and that can be biodegraded precisely because there are microorganisms that can decompose them.

7. But is it appropriate to biodegrade anything?

 Of course not. Some materials are stable in their original nature but contain atoms that could react and form molecules that give rise to toxic substances. Attention should therefore be paid to what is biodegraded. Also because the compost we use to biodegrade is enriched by substances generated by the biodegradation process. And if we want to use this compost to grow vegetables that we will eat it is better not to leave toxic substances inside.

8. Is it possible to biodegrade any type of shoe when it comes to shoes?

We have now understood that the answer is not simple: firstly it would be correct to declare in what percentage the object biodegrades in its original form (i.e. without being torn to pieces before) and in what time and under what conditions this reaction takes place. What we can say is that some materials will never biodegrade unless some microorganism is discovered that manages to crumble the material transforming it into carbon dioxide in a reasonable time without dispersing other harmful substances into the environment.

What are the parts of the shoes that will not biodegrade in the current state of knowledge? The metals we use for eyelets and buckles and a whole series of components based on plastic polymers that we use to make soles and heels: vulcanised rubber, EVA (the light midsole of running shoes), the microporous sole (which in any case is EVA), polyurethanes and PLA contained in heels. When used for production, all these materials undergo chemical reactions that alter their molecular structure to form bonds that microorganisms at our disposal are not able to dismantle.

9. And is skin biodegradable?

It depends.

Of the animals we eat, we know that beyond bones the skin is what remains. If the skin was immediately put in the soil after skinning the animal, it would tend to rot and unhealthy reactions for the environment and human health would be created. The tanning process that transforms the animal’s skin into the material we use to produce shoes and bags, blocks the rotting reaction which is nothing more than a biodegradation process.

There are tanning processes that produce leathers that can be biodegraded. Not all tanning processes allow it. And in any case pay attention to how hides are finished: tanning is not enough if retanning and finishing processes occur with chemicals that are not compatible with a following biodegradation