Technical Information

Technical Department

geral@ota-abrasivos.pt
Tel +351 226 178 423 / 443

Call to national fixed network

What are the main abrasive families?

Abrasives are used for various purposes and can be in solid or liquid form (actually grains in compounds). They cover functions from cutting and grinding to high-gloss polishing. In solid form, they are generally presented in the following formats.

Bonded abrasives – constitutes a solid mixture of abrasive grains with a binder, normally rigid, coming in many forms: discs (the most common), grinding wheels, wheels, blocks, plates, etc. The binder can also be flexible or elastic, using rubber or polyurethane.

Coated Abrasives – a coated abrasive represents an abrasive fixed to a support material, usually paper, cloth or even sponge, flexible by nature. Traditional sandpaper falls into this family. A resin is normally applied to the support into which the abrasive grain is integrated.

Non-woven abrasives – These products are often referred to as “Scotch®” or three-dimensional abrasives. The abrasive grains are bonded to flexible nylon fibres saturated with abrasive grains. This “open-weave” construction provides a continuous supply of new grains as the fibre and old grains wear out. Non-woven products are adaptable to contours. The abrasive grains are distributed throughout the thickness of the material, exposing the grain even more during normal wear. This ensures consistent performance throughout the product’s lifespan, can be used on both sides, and never loses maximum abrasive performance. Non-woven abrasives have a very high mineral content. The open non-woven structure minimises the tendency to clog. Non-woven abrasives can be used wet or dry or in conjunction with polishing fluids. Non-woven abrasives are ideal for applications involving low cutting speed and low contact pressure.

Felts and Cloths – are products intended for the final polishing of parts, and are normally used with soaps, compounds, or other polishing fluids.

What are the different types of most common abrasive grains?

Aluminium oxide (AO or Alox)
Aluminium oxide is the most common and typically least expensive grain. Provides a good starting point for most metal applications, such as iron, carbon steel, alloy steel, hard bronze and all woods. It is the ideal solution when toughness is desired, that is, the ability to resist fracturing. Aluminium oxide is generally brown or reddish, but may have other colours (blue, green, or yellow), which generally indicates the presence of additional coating layers for cooling, lubrication or to prevent sogginess. It is durable, with resistant cutting edges, tending to lose its edges during use. Aluminium oxide is available in a wide range of grains and is the type of abrasive with the most variants.

Silicon carbide (SiC)
Silicon carbide is the hardest and sharpest of the minerals used in coated abrasives. Its hardness and very sharp shape make this abrasive ideal for sanding non-ferrous metals (aluminium, brass, bronze, magnesium, titanium alloys, etc.) or even cast iron, not too hard, glass, paints, rubber, plastics, fibrous wood, and other relatively soft materials that do not require great pressure when cutting. SiC typically has a black or grey colour. Its grains are friable, breaking which continually exposes new cutting edges and allows rapid removal of the material. It is more aggressive than the AO but, as it wears more quickly, it presents a more pronounced wear curve over time.

Zirconium (Z or Zr)
Zirconium (Zirconia Alumina, Zirc or Zirconium – Zr) is suitable for coarse to medium particle size applications in metal, being an excellent choice for demanding applications, for example in stainless steel. The Zr is usually blue in colour. It works best under high pressure, which is necessary for the grain to break, exposing new sharp edges. It is a type of abrasive that has large fracture planes and self-sharpens during cutting. Zr is available in limited particle sizes and on strong supports.

Ceramic (Cer)
Ceramic aluminum oxide abrasives (Ceramic Alumina – Cer) have the longest life and fastest cut rate of all coated abrasives. They are the latest introduction in abrasive grains and present maximum performance, providing an aggressive and consistent cut under moderate to high pressure, as they break down into microparticles, keeping the abrasive surface permanently sharp. Cer is the most expensive of all abrasives and is generally red or orange in color. It is mainly used in metal applications, especially hard ones, such as stainless steel, or alloys with a high content of nickel and cobalt. Cer grains are available in a larger granulometry than Zr but much less extensive than Alox and SiC, not reaching grains as fine as these, which is natural, as in this range the polishing capacity becomes more important and less the material removal.

Compact Grains (Cmp)
Coated abrasives with compact grains are therefore very different from conventional flexible abrasives, constituting a cluster or agglomeration of several abrasive grains of the same size. In simple terms, the product has multiple layers of the same grain, compared to just one layer in a conventionally coated abrasive product. As one layer of abrasive grain wears away, another layer is exposed. The tight grain construction results in less load, longer life, and a very constant finish over the lifetime. Mainly used in metal applications, namely in automated grinding applications on stainless steel and light high alloy steels. They are typically more expensive than the conventional coated abrasives.

How is the particle size of abrasives classified?

How is the particle size of abrasives classified?

There are several standards for measuring the particle size of abrasives. The best known is FEPA – Fédération Européenne des Fabricants de Produits Abrasifs (Federation of European Abrasive Producers).
FEPA distinguishes between sanding grits, i.e. for use on substrates (FEPA P), and agglomerated grits on abrasive parts / bonded abrasives, namely grinding wheels or abrasive wheels (FEPA F).
Grain sizes are defined by the number of lines per inch (25.4 mm) on a sieve, or, in more simplistic terms, how many grains would fit, juxtaposed, and aligned, in one inch. For example, for a 120 grit abrasive, there would be 120 lines/inch on the sieve, or 120 juxtaposed and aligned grains per inch would fit. This means that the lower numbers (16, 24, …) correspond to the coarser and more aggressive grits, intended for grinding, and the higher numbers (1000, 1200, …) correspond to the finest grits, intended for final polishing.
The following table shows the equivalence between the various granulometric conventions. OTA uses, as a rule, the FEPA convention in which the grit number is preceded by a P. This is an important guarantee of quality, as the user knows, with complete precision, which grit he is using. Some manufacturers, in some exceptionally fine products, present the particle size directly in microns and others present their own particle size with conversion tables. The following table also includes the grits typically available on the market for the main types of coated abrasives: Alox, SiC, Zr, Cer and Cmp. Please note that this table is merely indicative.

What are the extra coatings most used in coated abrasives?

It is common for coated abrasives to have surface treatments to improve their performance. The main ones are as follows.

Top Size Layer
The Top Size Layer is recommended for dry grinding of stainless steels and non-ferrous metals. Thanks to the additional layer with its grinding-inducing additives, cutting performance is improved while the temperature in the contact area is reduced. Furthermore, the lifespan of the abrasive increases substantially.

Stearate Layer
The Stearate anti-adhesive layer is mainly used to work non-ferrous metals, for example aluminium, preventing particle adhesion and, therefore, clogging/sogginess of the abrasive. Increases the useful life of the abrasive.

Antistatic Layer
Abrasives with an antistatic layer are mainly used for working wood or other non-conductive materials. Normally its support is supplemented with electrically conductive substances that ensure that electrical charges can be safely discharged through the abrasive to the equipment.
They prevent the buildup of an electrostatic charge so that sanding dust does not adhere to the workpiece or machine. It aims to prevent the abrasive from blocking and extends its useful life. Most wood finishing products have this treatment, and some even include an anti-static finish on the abrasive side through the inclusion of conductive substances (for example, graphite) in the own binding resins, improving the effect.
Note: this treatment, although an extra, does not correspond to the graphic representation above, as it does not constitute a coating layer.

What types of support are used in flexible abrasives?

There is a wide variety of solutions that support coated abrasives.

Paper
Paper supports have different weights, are designated by letters, and are available for different applications. Standard paperweights are indicated by a letter that is sometimes included in the finished product code. The lighter the support, the greater the degree of flexibility, the heavier the paper, the greater the resistance to breaking. The nature of each paper (density, thickness, flexibility, etc.) varies from manufacturer to manufacturer, including the treatments applied to it. For example, a waterproofing layer intended to withstand wet abrasive applications changes the characteristics of the substrate.

Paper	Weight (g/m2)	Notes
A	70-80	Lightweight and flexible, A paper is mainly used for manual operation with dry and wet finishing. In the latter case, the paper is waterproofed and gains a little more weight. It is used in all particle sizes, but especially above (finer) P80.
B	90-105	Similar to A paper, but can support coarser grains than P80.
C	110-125	This medium and light paper is used when some flexibility and strength are required. Stronger and less flexible than papers A and B, this support is also chosen for manual work (dry or wet) and for use in small portable sanders. This support is used with fine and medium grits.
D	130-160	Stronger and less flexible than C paper. This paper is still chosen for hand sanding and for use in portable sanders. It is used on grits that are already relatively coarse P36 to P80.
E	220-250	This support is no longer used in manual applications, but is mainly used in rolls, belts, and discs, where high resistance to breakage and cutting is required.
F	300	It is the most resistant and least flexible standard paper support used on the market. Used on belts, rolls, and discs that require great resistance, for example on large belts.
G	400	It is a very heavy paper, rarely used.

Cloth
Cloth backings are more durable than paper, offer greater tear resistance and tolerate continuous bending, remaining flexible during use, but are more expensive. They can be made of cotton, polyester, or a mixture of both (polycotton) (PA). The body/thickness is also referenced by letters, which depend largely on each manufacturer. Typical references are as follows.

Cloth	Notes
JJF	Normally in cotton. It is a hyper flexible support. It is relatively rare and applies to very fine grains. It may correspond to a JF screen from another manufacturer.
JF	Normally in cotton, but also in a polyester mix (PA), when you want to give it more mechanical resistance. It is the most flexible support available on the market, with fine and medium grains. In practice, it constitutes a more flexible variant of the J reference.
J	It is one of the market references and its name comes from the word “jeans”. It is a light and flexible cloth, used where surface finish and uniformity are more important than material removal. It is ideal for finishing, where flexibility and conformity are essential, such as contour work, especially curved surfaces.
X	Stronger and more rigid when compared to J screen, this support is used in products designed for medium and thick grains and material removal applications and finishing with fine grains. It is one of the standard backings for all manufacturers.
Y	Stronger and more resistant to breakage and cutting, it is a backing used in products designed for severe applications, normally in belts. It is generally made of polyester to guarantee greater mechanical resistance. It is, along with the J and X screens, another reference on the market.
H	Stronger and more resistant than Y screens, it has less flexibility. It is relatively rare.
Z	The strongest and most resistant of cloths. It’s rare.

Regarding these references, there are several variants on the market:
JF cloths also have designations such as JJ, F or JFlex.
There are slightly more flexible X cloths, which manufacturers refer to as XJ, XF or XFlex.
There are intermediate screens, between the X and Y cloths, which are referred to as XY or YX.
Some manufacturers use reinforced Y cloths, which they distinguish by designating them YY. Others may eventually designate them as H.

Combination
It constitutes a backing combining, by lamination, fabric, and rigid paper (E or F) and is used where mechanical resistance to rupture and breaking/folding is a determining requirement. It is mainly used in large calibrating belts with coarse grits, for chipboard/MDF.

Film
Polyester film is used as a backing for precision polishing products. It is most often used for abrasive belts and discs for orbital sanders. Film is more durable, mechanically, and chemically resistant to attacks than paper, but more expensive and more difficult to recycle. It can be used wet or dry. It is typically intended for very fine grains, above P800, to ensure superior abrasive surface uniformity and better finish quality.

Fiber
Fiber is a hard and durable material with great mechanical strength, which can provide enough flexibility for some applications. Typically, the fiber is made up of several layers of paper bound by resin. Its thicknesses can vary between 0.6 and 0.8 mm and has the greatest resistance of coated abrasive backings. It is exclusively used to make resin fiber discs. It often has a top size layer.

What types of finishes are used on flexible abrasive supports?

The strict backing/abrasive combination must often be complemented with a finishing to give it flexibility, additional rigidity or, simply, to allow its use with certain tools or equipment. The following finishings can be used simply applied to the backing or with different combinations of them.

Velour (Vel) – is the most common situation and is intended to allow the use of discs or sheets on equipment or pads with a Velcro® type system to allow easy and quick operation. Velour can come in a wide range of weights and resistances.

Adhesive – typically PSA type, which allows a similar use to Vel, although it is intended for specific needs. Also typically used on discs and sheets.

Foam – to provide cushioning to the coated abrasive, normally with a paper backing. It can have different thicknesses and elasticities.

Reinforcements – of the most varied nature, both in paper and synthetic material, intended to provide more resistance or rigidity to the backing. It is not of general application, being designed for special situations.

How should I store abrasive products?

The best way to ensure that abrasive products maintain maximum performance and lifespan is to store them properly. The bonding and backing materials used in abrasives are sensitive to changes in temperature and humidity.

Abrasives must be stored in environments with a temperature between 15ºC and 25ºC and a relative humidity between 35% and 50% and protected from solar radiation and heat sources.

Products must be kept in their packaging until ready for use. Some of them even have well-defined expiration dates.

I like the abrasive I’m using, but I don’t see it on the OTA website.
How can I find it?

We suggest that you take the references from the product you are using, or want, and contact our commercial department at (+351) 226 178 423 or geral@ota-abrasivos.pt.

We will be happy to help you find the best choice.

Does OTA make customised belts or discs?

Yes. OTA manufactures special size bands every day and they are no more expensive than standard bands of similar sizes. The same applies to discs.

Just ask us for the size (diameter or perimeter x width) you need and we will provide you with a quote and respective delivery time. If you’re not sure how to measure a belt, we can easily guide you.

Contact us by phone (+351) 226 178 423 or by email geral@ota-abrasivos.pt.

OTA – Organização Técnica de Abrasivos

Rua António da Silva Marinho, 157
4100-064 Porto – Portugal

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