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Case studies / Friction
Bismuth sulfides, the new contenders against strong LME dependent metals?

Spoiler, they are! Despite the current circumstances, the increase in volume production of Bi metal divided it’s base price times 3 and stabilized there, making its usage feasible for the friction industry.

After a big drop in the consumption of commodity metals, (such as copper, tin, aluminium, among others), as a consequence of a global pandemic,  it is expected base metals markets to return to normality in 2021, supporting increases in prices, as the world economies start to pick up.

This situation has brought a significant unbalance between demand and production capacity. In 2021, the demand-side will be also supperted by an increase of spendings of governments around the world, primarily through investment in infrastructure projects, driving increases in base metal prices. This is one of the main causes for the price rise in many raw materials but also in unexpected hassles to stock up from several raw materials.

Therefore, moving some consumptions to products with more stable trends and less LME dependency became a new driver in many industrial fields.

Base metals — such as iron ore, copper, aluminum and nickel — are listed in stock markets. Beyond supply and demand, the quotation in the stock market is the third factor influencing the short-run fluctuations in commodity prices.

Prices of strategic metals for friction industry, such as tin and antimony present pronounced variability both at short and long term, even though its demand has been continuously increasing during the 21st century. Meanwhile, unlike other metals, the base level price of bismuth has been largely reduced during the last decade and it’s pretty stable. Bismuth is not listed in stock markets.

Bismuth is a remarkable eco-friendly metal despite its location on the periodic table

Scientific literature concurs that bismuth and most of its compounds are less toxic compared to other heavy metals (lead, antimony, etc.) and so, the inflection point came when several industries realized about that and started to massively replace lead on a wide range of industries. Although bismuth had few commercial applications at the beginning of the century, which made this product very expensive in the past, it, new applications followed afterwards.

Therefore, since then, new applications followed afterwards. And now we stretch a bit more the focus on the friction industry and realize that some commonly used products, have now a serious contendant, which not only is able to provide a similar effect, or even an improvement, but also it is reaching its price level, if it has not already done so.

If we target some of the “top environmental challenges” of nowadays, as it is antimony replacement (Sb2S3) or reducing tin dependency, we can analyse if its analogue in bismuth (Bi2S3) has potential to replace them. Results turned out positive in this regard and that’s why this product is already being used for the most advanced friction material manufacturers from a technical perspective, and of course at rimsa we are ready for it!

Chemical properties
To begin with, Bismuth (III) sulfide is isostructural with antimony (III) sulfide and both crystallize in an orthorhombic structure. Bi2S3 has also a close oxidation temperature range, to Sb2S3 and tin sulfides. Also, its reaction mechanism with oxygen is the closest one to antimony trisulfide.
AKM Critical sections
Therefore, because they all share a very similar chemistry, they behave similarly when included in a friction formula. As these sulfides have a very similar interaction mechanism with the phenolic resin, if we take a look on the high temperature sections of the AKM test, we’ll see they provide the same capacity of reducing the CoF amplitude and reducing the in-stop variability, which will have immediate consequences on the NVH properties of the overall formula.
Disc roughness
It has been observed that the distinctive effect of Sb2S3 is its ability to achieve a very smooth finishing on rotors compared to other established additives. Bismuth-based sulfides, are able to reduce the disc rugosity, with comparable values to tin sulfide as well.
Pad wear
Besides cutting the cost of tin sulfide in half, bismuth sulfides are able to achieve similar wear rate on the pad.

At rimsa, pure (BI81) and composite (BI65) compositions are available, to match the requirements of your application. BI65 was designed to reduce the density of the product, and therefore its price. It’s unique composition provides the same friction behaviour with an additional contribution of thermal conductivity.

Thanks to our production technology, we ensure consistent quality and very stable chemical composition, without impurities and free of heavy metals

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