Abrasives for Polishing Applications

Introduction to polishing basics

Polishing is a mechanical process aimed at reducing imperfections from a surface such as scratches, oxidation and dullness. This is achieved by the removal of a thin layer of the surface aided by an abrasive, this process is also known as cutting or stock removal. This is followed by the use of a softer, milder abrasive to “polish” and smooth out the surface to increase the gloss.

Selection of an abrasive largely depends on the hardness of the surface being polished. Stock removal of a car coating will usually be achieved by soft, low calcined alumina grades and a polished high gloss finish will be achieved by a milder abrasives such as a calcined kaolin.

For metal polishing applications, medium and hard alumina agglomerates are used to achieve the desired level of cut and polish as the surface is typically a lot harder than a polymer coating therefore a harder, more abrasive material is required. 

This article will focus mainly on abrasives within car care applications but the same general principles will apply across the board.

Types of alumina in polishing

Alpha alumina, or α-alumina (Al₂O₃), is a crystalline form of aluminium oxide known for its hardness and stability. Its content in a polishing material can significantly influence the effectiveness of the polishing process. Alpha alumina is extremely hard, with a Mohs hardness of around 9 which makes it effective as an abrasive in polishing applications.

The calcined alumina refers to aluminium oxide that has been subjected to high temperature processing (calcination), typically above 1000 degrees. This process removes water, impurities and other volatiles resulting in a more stable and dense form of alumina.

The calcination degree determines the primary crystal size and affects how hard the alumina agglomerate will be, with lower calcined grades producing a softer, more gentle abrasive action to achieve high gloss where a high calcined grade will find application where an aggressive level of cut is required. The additional benefit of calcination is a more uniform particle size distribution, key to achieving a more consistent finish.

Alteo are a worldwide leader in speciality aluminas and have different grades of low-calcined aluminas for different levels of polishing with different particle sizes. See Table 1 below for a more in-depth overview of these.

Figure 1: Alteo's grade of calcined alumina, A4G70 a range of d50 shown above by the SEM image.

The ability of an abrasive to effectively cut is determined by its inherent hardness, crystal size and shape. A larger, less uniformly shaped particle will achieve a greater cut whereas a fine uniform particle will provide a more consistent gentle polish to smooth out the surface imperfections.  

What makes a surface matte or glossy?

If we examine the fundamentals of what makes a surface matte or glossy, it is merely a measure of how much light can reflect from a given surface, which is directly correlated to surface roughness. The more rough and uneven the surface is, the less light will be directly reflected and a more matte effect will be the result. 

Figure 1: A standard car coating system with some wear and tear before polishing. The surface appearance is slightly dull with some swirls and scratches.

This effect can be achieved by design during coating formulation by the use of matting agents, which work by intentionally introducing surface roughness and affecting how incident light is reflected.

In contrast, the more even and smooth a surface is, the higher the gloss, as more light will reflect from the surface giving a shiny appearance. In car care applications, detailers will achieve a high gloss by cutting and polishing the coating to achieve a smooth surface.

Figure 2: A standard car coating system after polishing. The surface has a higher gloss, the appearance of scratches and swirls is reduced.

A high gloss finish in an automotive setting is important as it gives the effect of a mirror like cleanliness, which adds to the perception of high quality.

Types of abrasive products

In recent years, high-performance clear coats have been applied to cars to provide additional gloss, UV resistance and scratch protection. Over time, these coatings can become weathered from exposure to natural elements such as UV rays from the sun, rain etc. which can fade the paint work by creating a thin oxidized layer on the surface. In addition, swirls and scratches may appear from cleaning the car as well as general wear and tear.

The appearance of the coating may be restored with the use of “cutting compounds” or “polishing” products which contain alumina, or kaolins as abrasives.

Car detailers typically use rotary or dual-action machine polishers to apply these abrasive products to help remove a thin layer of the coating and restore the showroom finish.

Figure 3: An example of an oxidised, worn coating surface with marks and swirls (left) compared to after polishing, restoring the coating to its original appearance.

As the coating durability performance has been increased over the years, the need for higher-performance abrasives has also increased to achieve the same level of cut and polish as before.

A “cutting compound” will generally use high-performance, calcined alumina as an active abrasive – the loading can range anywhere from 10-20% dependent on the level of cutting ability needed based on the condition of painted surface. The cutting compound is used in the preparation of a surface and will do most of the heavy stock removal.

A “polish” will typically use a milder abrasive such as a calcined kaolin or super ground grades of low calcined alumina. The smaller particle size lends itself to a more gentle abrasive action which is required for finishing and achieving the high gloss.

The abrasives can be applied in a range of forms:

• Solid compounds
• Liquid compounds
• Dry powders
• In suspension as a slurry

Care must be taken when applying these abrasives so as not to cut too much and too often as there is only a finite coating layer available, after which the underlying colour coating will be exposed and vulnerable to degradation.

A general rule of thumb is to apply the least abrasive material first and progress upwards.

Abrasive selection

The abrasive selection is largely dependent on the condition and hardness of the surface you are trying to improve. It is important to choose a particle size that balances cutting ability and finish quality.

Generally, the following materials are recommended for car care applications and the  different desired effects:

Table 1: The recommended material selection table for different levels of polishing in car care.

Formulation considerations

Whether the product is a cutting compound or a polish in car care applications, the abrasive component will need to be suspended within the formulation and exhibit good stability. The greater the loading percentage of the abrasives, the higher the viscosity is required to prevent settling.

The choice of solvent is important within a polish formulation as it acts as a carrier for the active ingredients, and helps to wet and clean the surface. The compatibility with the surface as well as drying rates should be considered when selecting a solvent system to avoid the polish drying out too quickly during use.

Waxes are also commonly found in polish formulations as they may help reduce smearing, provide durability and improve film continuity. These can be natural (carnauba) or synthetic (silicone, polyethylene) and can promote overall stability by acting as a thickener.

Silicones such as polydimethylsiloxanes, amino-functional silicones and other silicone resins are generally used as active ingredients in polish formulations. They can help enhance the glossy finish, provide a hydrophobic effect, helping water beading and provide overall protection. Key things to consider when selecting a silicone is the molecular weight (Mw).

High viscosity and high Mw will lead to high durability but can cause smearing whereas lower Mw silicones can help with the ease of application and increase surface wettability. It is typically advised to use a combination of both to achieve the best of both worlds in terms of durability and ease of application.

A blend of materials can be used within a single formulation to achieve both a cut and polish action. Dependent on the type of product, this can be a blend of different particle sized materials, or a combination of both calcined alumina (hard) and calcined kaolin (softer) to achieve the desired effect.

Generally, no two coatings will be the same, and therefore the cut and polish rating abilities of materials only act as a rough guide – typically, newer cars have harder clear coats and will require harder abrasives that demonstrate a greater cutting ability, whereas older cars might only require softer abrasives. The best way to find the abrasive you are looking for is to try a range of hard and soft materials, as well as different particle sized grades to determine what works best for your application. 

If you would like to find out more or request a sample, contact our technical team who would be happy to assist in recommending the right grade for your needs.