The daily grind: how minerals play a key role in the production of coated abrasives

Grinding and sanding are necessary actions in the production of almost all everyday products that require surface finishing. Whether you’re an industrial grinder or supplier, a professional manufacturer or someone with an interest in DIY, coated abrasives like sandpaper and sanding belts are typically used to carry out these jobs – and at the heart of these products are synthetic abrasive grains such as fused aluminum oxide. 

One of the oldest needs of humankind has been to shape and grind tools in order to make them sharper or change their form. Our ancestors did this by using abrasive – materials, typically minerals, that shape or finish objects through a rubbing motion – and the method has continued to this day.

As civilization evolved, the development of grinding machines and techniques made the process more efficient, and the abrasives market split into two major categories: bonded and coated. 

Bonded abrasives are abrasive grains held tightly together by a bonding agent and formed into a specific shape – typically that of a wheel. Coated abrasives, on the other hand, contain abrasive grains that are bonded to base materials like cloth or paper with adhesive, taking the form of sandpaper and sanding belts. 

Raphael Gasser, an abrasives specialist at Imerys, says: “Bonded abrasives are typically used for cutting and grinding various work materials with defined shapes, while the uses for coated abrasives are more commonly for grinding large surfaces, polishing applications and finer surface finishes. Both coated and bonded abrasives are used for a wide range of industrial applications as well as in the DIY market.” 

Up until around 100 years ago, all abrasive articles were made from natural minerals, but when the needs of manufacturers and consumers became more sophisticated, research was done to create synthetic abrasive minerals that could provide the requirements desired. 

Using natural minerals to create enhanced manufactured minerals  

The most common synthetic mineral in circulation today is Fused Aluminum Oxide (FAO). 

FAO is made from processing minerals such as raw bauxite, the first step of which involves passing the mineral through an electric arc furnace. The resulting product is then cooled, crushed and sized, and then, finally, blended. Various product enhancements. such as mechanical, chemical, ceramic or thermal treatments are also possible.

The main quality criteria that manufacturers look for in the resulting grain include:

• Chemical composition
• Homogeneity (guaranteeing a constant quality for the customers)
• Hardness
• Toughness
• Size distribution (according to international standards)
• Quality of the coating on the grain.

The type of synthetic mineral being produced will play a role in some of the resulting criteria. 

“Our various fused aluminum oxide types are different in terms of hardness and toughness”, explains Raphael. “FAO is in general a very hard mineral. It's number nine on the Mohs scale of mineral hardness, while number 10 is diamond. When considering regular FAO qualities, it is usually the case that a harder material shows less toughness whereas a tough material has less hardness. Which material the customer chooses depends mainly on the work-material that will be sanded with the abrasive article. Tougher grains have a longer lifetime, while harder grains are more aggressive. If we look at white fused alumina as an example, it is very hard but not that tough, making it good for sanding wooden surfaces. On metal it would be too brittle, however, so a tougher grain would be the better choice.”

Producing fused aluminum oxide that caters to a wide range of needs 

Imerys is the world’s largest producer of FAO based products, predominantly for the bonded and coated abrasives market, with a product portfolio that is unmatched in terms of size and variety. Its Alodur® range is designed to serve the varying needs of abrasives manufacturers.

Chemical elements change the properties and stability of the FAOs, and each combination can be identified by its colour – from brown to white to pink – allowing manufacturers to mix and match in order to get the performance they need.

In addition to regular fused aluminum oxide, Imerys also offers high-performance abrasive minerals like Sol-Gel – a microstructured ceramic grain that provides a unique aggressive grinding performance. This grinding performance combines the high toughness and high hardness of the grain with the ability to self-sharpen. 

Abrasive grains’ edges are naturally very sharp. Specific grain shapes can be produced by using different milling technology, resulting in pointed, angular or cubic forms. 

“In general, pointed and angular grains are most commonly used for coated abrasives”, says Raphael, “as they have a more aggressive grinding behaviour. But each customer will have different requirements.”

Abrasive grains can also be coarse or fine. “There has been a general trend over the past few decades towards finer grains as the demand for finer surface finishes grows, but, of course, finer grains do not replace the coarser ones”, explains Raphael. 

“For example, if someone is interested in DIY and wants to grind wood with sandpaper, they’ll likely choose a coarser option first as that will result in quicker progress. They will then be left with a rough surface, so they’ll probably opt for a finer grade that will give them a smoother finish.”

Making production more sustainable and effective

Imerys is a high-quality supplier when it comes to abrasive grain production, taking care to study the quality of the product and its impact on the environment. 

“During the grinding process, when you use a higher quality grain you get better stock removal, which means you end up needing less of the coated abrasive to do the same amount of work”, says Raphael. “This means the product lasts longer, which results in less waste. From time to time we also record grinding parameters, and it’s been proven that a good aggressive grinding behaviour – gained by using a sharp grain – reduces energy consumption. Furthermore, we offer a projectability treatment for coated abrasives that helps to increase the amount of grain captured onto the backing material during production. This means higher productivity to make the coated abrasives, and it also leaves the customer with fewer by-products.” 

Imerys also provides a number of other treatments that further boost the efficiency, productivity and lifetime of the grains. These include thermal treatments to improve toughness and lifetime, ceramic treatments to increase the surface area for coating the grain, helping the grain to bond better with the backing material, and chemical treatments that improve the flowability and prevent grains agglomerating. 

“Our customers can customize their products, choosing from a wide portfolio of grains and treatments according to market requirements. Abrasives are part of our daily life, even if we do not realize it all the time. Whether you drive your car or prepare a nice meal, you can be sure that many of the tools you are using have already made contact with abrasive grain.”