The Materials and Technologies (GMT) research group has found a new, sustainable material for sandblasting denim. They found that the grit obtained from grinding mussel shells has better abrasive qualities than other materials used in the textile industry. It is more environmentally friendly because a waste product is turned into a raw material.

A major multinational in the textile manufacturing sector was looking for a sustainable technique for abrading denim, and the Materials and Technologies (GMT) research group of the University of the Basque Country (EHU) came up with a solution in collaboration with the Massachusetts Institute of Technology (MIT). “We concluded that the grit comprising ground mussel shells was an effective abrasive and sustainable material. It offers the chance to replace other industrial processes currently used to abrade denim, and which have a greater environmental impact or are more toxic,” said the researchers Cristina Peña and Juan Luis Osa.

Indeed, the treatments used to achieve that worn look of denim, which is so aesthetically attractive, pose many obstacles. Traditionally, denim was abraded by blasting the fabric with silica sand at high pressure. But the workers who used this technique developed silicosis as a result of the inappropriate use of safety measures and protection. As silicosis is a disease with a high mortality rate, the abrasive technique based on sandblasting fell into disrepute and was abandoned by the textile industry. So alternative methods began to be used. Oxidising chemical treatments or thermal laser treatments, for example. However, these were found to be potentially more toxic and did not achieve such good abrasive results.

“Bearing in mind that there is no reason why the sandblasting technique should harm the workers as long as they are properly protected, that brand that produced the clothing wanted to revisit that technique, but by using a more sustainable material; in other words, more sustainable than the garnet (quartz-free silicate mineral) normally used as an abrasive,” explained Peña. Initially, they conducted the first tests using the buttons of garments destined for the landfill, but that option was not viable. So they decided to look for a renewable material, and at the suggestion of the very textile company that commissioned the work, they started testing with mussel shells. Specifically, the shells discarded by the food industry. “Taking advantage of the fact that I live very near a bar called La Mejillonera [The Mussel Bed] in Donostia, we did some tests using the mussel shells they usually throw away. This is the process: the shells are washed, sterilized using a thermal treatment, ground in a mill, sifted, and finally, the grit produced is projected under pressure at the denim using a compressed air gun. We saw that by doing so, the grit responded very well in wearing away the denim. Better than the garnet usually used in this sandblasting process,” said Osa.

They concluded that the mussel shell grit was less brittle than the silicate mineral, and therefore performed better. “It is less likely to break during the blasting than the garnet. And that is a positive feature. Indeed, less mussel shell grit than the garnet is needed to abrade the same number of jeans. The fact is, during the sandblasting process, after the garment has been blasted with the grit, the grit used is collected so that it can be reused. Collected and reused over and over again. But with each use the grit gradually becomes smaller and from time to time new grit has to be added. So what we saw was that the grit made up of mussel shells lasts longer than the garnet and therefore less material is used to achieve the same abrasion,” explained the researchers.

They also confirmed that the finish achieved by blasting the fabric with mussel shells is of good quality. In other words, it is capable of creating the colours that fashion designers and consumers love so much and, unlike other chemical treatments, leaves the fabric smooth to the touch.

From a waste product to a raw material

The sustainability of the abrasive material discovered is another strong point in the EHU research: “It has a lower impact on the environment than traditional and chemical abrasives. For example, garnet is not renewable whereas shells are. The first is a mineral that has to be extracted from nature, whereas the other ones are a sub-product. As a result of mussel consumption, 1.5 million tonnes of shells are produced per year worldwide, and, as far as we know, to date they are not used for anything,” said Peña.

To take advantage of the mussel shells that end up on landfills, the scientific community is working to find new uses, but the EHU researchers have been among the first to find an effective solution. “It was a major piece of work and it took us time to achieve these results, but we are delighted with the achievement. We have verified a specific application to abrade denim, but in addition to that, the results have revealed that mussel shell grit has many other applications; it could be a suitable abrasive material and could be useful in other industries, such as the cleaning of mechanical parts or ships. What is more, it does not require any special equipment. We did our tests on machines routinely used in sandblasting processes and they went well," explained Osa.

The EHU researchers stressed that the research has given the circular economy a boost: “It is a good starting point to begin to carry out the processes currently done in industry differently. It is not just to have a more sustainable process, but also to harness the waste we create. It shows that we need to have a broad perspective on the use of raw materials, and beyond the raw materials that have always been used, it’s worth looking at what alternatives there are, to see what waste we generate and to explore what we could do with it.”