A comprehensive guide to Abrasion Resistance

1 Introduction

We are confronted with abrasion resistance every day. Whether it is your shiny new bathroom floor that is beginning to fade or a scratch on the case of your new phone or car (Figure 1). Abrasion can ruin a day in a moment. So how do we test and improve our coatings on these appliances to mitigate the chance of this happening? In this article we will discuss what abrasion resistance is, how it relates to other physical properties, and what tools and methods are needed to  measure it.

1.1 What is Abrasion resistance
Resistance to abrasion is a basic factor in the durability of a coating. It occurs, when a hard surface slides across a softer surface causing friction. It is defined as a loss of material due to hard particles that are moved along a solid surface.^[1] There are two general types of abrasion:

1.     Mar abrasion
2.     Wear abrasion

Mar abrasion consists of permanent deformations that have not ruptured the surface of the coating. Mar abrasion resistance takes the form of a coatings ability to withstand scuffing actions that change the appearance of its surface. Typical examples include sliding of an object across a furniture surface or rubbing of a rough fabric on a car finish. 

Wear abrasion is caused by a mechanical action that removes material from the surface of a coating through repetitive, gradual motion. Typical examples include damage from scouring brushes or sand.^[2]

Mathematically, abrasion can be described by the Achard wear equation which states, that the volume of removed debris is proportional to the work done by friction forces along a distance (Equation 1).^[3]

Q=KWL/H

Equation 1: Achard equation. Q = total volume of debris, K = constant, W = total normal load, L = sliding distance, H = hardness of the softest contact surface.

The constant K may be obtained experimentally and is dependent on the substrate, its hardness, and its chemical properties. The Achard equation is widely used to predict wear in engineering practices, however it is restricted to sufficiently long wear durations. Recently, an addendum has been published, to remedy this limitation.^[4]

1.2 Abrasion resistance in the coating industry
In the context of wall paints, abrasion refers to the resistance of the paint to being worn away by frictional forces, such as scrubbing, cleaning, or physical contact without losing its color, gloss, or protective qualities. Abrasion wear resistance is a critical property for wall paints, especially in high-traffic areas or environments where surfaces are frequently cleaned or touched and is important for maintaining the aesthetic and functional integrity of painted surfaces over time. 
Coated surfaces which are exposed to the elements, like automotive body panels, may be abraded by dust, flying sand or small debris. Coated floors or tiles may suffer from abrasion through continuous use while furniture coats might be damaged through abrading cleaning fluids. It is obvious, that each of these cases requires different testing conditions, and as such it is not possible to define one standard method to determine the abrasion resistance. In a later section we will discuss several applications in detail with their respective standard testing method.
 

Abbildung 1

2 Factors that affect abrasion resistance

Abrasion resistance is not an isolated property of a substrate, it is connected to many different properties. These can broadly be categorized into factors that affect the coating itself and external factors. Internal factors include binders, pigment filles, additives and the curing and drying process, while external factors include physical properties like hardness, flexibility, elasticity, toughness, tensile strength, and the roughness of the surface itself (Figure 2).

2.1     How hardness, flexibility, elasticity, and coating thickness affect abrasion resistance
Abrasion resistance is related to hardness. It is obvious, that a harder substrate will be harder to scratch or abrade. But the issue is not that simple. Hardness and flexibility have to be in balance in order to achieve good abrasion resistance. If a coating is not hard enough, it will fracture when impacted. If it is too hard however, it might become brittle and break off. 
Another factor that must be considered is the elasticity. Steel is much harder than rubber but tires out of steel would make for a rather uncomfortable journey. Rubber tires have higher abrasion resistance than steel even though they are an order of magnitude softer. This can be traced back to its ability to redirect incoming energy throughout the system much easier than steel can. Where steel would fracture and yield because of too much applied force at one point, rubber will yield and redistribute the force.
The smoothness of the substrate is another important factor. Since abrasion happens when two surfaces come into contact, the roughness of these surfaces plays an important role. Likewise, the thicker the applied coating is, the higher its protective properties because more material has to be abraded before any damage to the substrate can happen.

2.2    How raw materials can affect abrasion resistance
The chemical makeup of each coating has to be designed around its application. The mechanical properties of the coating are primarily defined by its resin system. Common types of resin systems include polyurethanes or polyesters as they offer good impact resistance and hardness. Additionally, solid metal oxides like TiO₂ might be added to suppress marring and scratching. 
 

Abbildung 2 A scanning electron microscope (SEM) image of a rough surface.

3 How to measure abrasion resistance

3.1    Overview of methods
From the above theory, it should be obvious, that there is no simple way to accurately and reproducibly measure abrasion resistance according to one all-defining method. Each use case has to be specified and tested in a very defined and controlled environment. This led to the development of many different types and methods of measurement. These include, falling sand abrasive tests, air blast abrasive tests, gravel projecting machines, rotating wheel abrasives and wet scrub abrasive methods (Figures 3, 4 and 5). For the purpose of this article, we will focus on rotating wheel method as well as the wet scrub method, which are mostly used in the coatings and (architectural) paints industry.

3.2    Rotating wheel method (Taber abraser)

This test method involves the testing of a sample in the form of a 10.2 cm or 4 inch diameter disc or 10.2 cm or 4 inch square. These are mounted on a turntable, which is rotated at a fixed speed under a weighted abrading wheel (Figure 4). As the turntable rotates, the wheels are moved over the sample in opposite directions about a horizontal axis displaced tangentially from the axis of the sample. One abrading wheel rubs the specimen outward toward the periphery and the other, inward toward the center while a vacuum system removes loose debris during the test. The weight on the abrading wheels can be varied by the use of additional weights and counterweights. The wheels have to be refaced after a specified time of use with the help of a refacing tool in order to refresh and standardize the contact surfaces. The samples are evaluated according to their weight loss after a specified amount of cycles, the taber wear index (which corresponds to the loss of weight times 1000 divided by the number of test cycles) or depth of wear.^[5]
Typical substrates include plastics, textiles, coated panels, laminated wood and rubber.

In addition to the weight loss after a set number of cycles, the abrasion resistance can be objectively measured using the BYK-Gardner haze-gard. Especially for plastics, it is important to control and measure the haziness, in order to guarantee the quality and functionality of a product. The taber abrasion test is specified in several international standards such as ASTM D4060, ASTM D1044, DIN 52347, DIN 53109 and many others.

3.3    Wet scrub method (Gardner scrub)

Another method for the determination of the abrasion resistance of coating surfaces is a special wet abrasion scrub tester. Originally used for dispersion paints this method has found considerable acceptance for the testing of automotive solid color top coats. It tests the cleanability and washability of these finishes coats. The instrument is made of a mechanical arm on which several special brushes, sponges or sandpapers can be mounted (Figure 5). This arm is then moved along the substrate surface at a defined speed over a defined number of cycles. Additional scrubbing liquids may be added to further test the abrasion resistance of the sample. 
Several international standards have been established for this method, several of which we will described in detail in the next section. These include the most commonly known ISO 11998, DIN EN 13300, ASTM D2486 and the withdrawn, but still sometimes used, DIN 53778.^[6]

Abbildung 3 A falling sand abrasion tester (left) and a Single impact mar tester (right).

Abbildung 4 A taber abrader.

4 International standards for the determination of abrasion resistance using the Gardner scrub

4.1    DIN EN 13300
The DIN EN 13300 standard is a European norm that is used as a system for the classification of water-based paints and varnishes for walls and ceilings in interior areas. These paints and varnishes are classified according to the following four different aesthetic and protective functions.^[7]

4.1.1    Wet scrub resistance according to DIN EN ISO 11998
This will be discussed in the next section in more detail.

4.1.2    Gloss
There are four different levels of gloss defined in the DIN EN 13300 standard as seen in Table 1. The measurements are made with a single or multi angle gloss meter on a smooth sample. BYK-Gardner offers a variety of gloss meters, the micro-gloss family, for objective, precise and reliable gloss data. 

4.1.3 Hiding power
The hiding power is evaluated by measurement of the opacity of a coated substrate. The higher the value the better the opacity and with it the hiding power (Table 2). These measurements are performed with a spectrophotometer such as the BYK-Gardner spectro2guide. The hiding power is given in % of opacity per m²/L of paint.

Walls, that are coated with low gloss paint will hide protrusions in the wall easier than glossy coatings will.

4.1.4    Maximum fineness of grind
Grind gages are used to indicate the fineness of the grind or the presence of coarse particles or agglomerates in a dispersion. It does not determine particle size distribution. Paints are classified according to their maximum particle size according to Table 3.

4.2    DIN EN ISO 11998
This standard describes the ability of coatings to withstand wear cause by repeated cleaning operations and their ability to withstand penetration by soiling agents with the use of an automatic scrubbing tester (Figure 6). These properties depend not only on the quality of the coating but also on the substrate. Therefore, the method of application, the drying methods and other factors must be considered in order to obtain repeatable results. The following standards are used to guarantee this repeatability. ISO 1513 and 15528 for the examination and preparation of samples and ISO 3270 for the temperature and humidity control. For further information on these methods refer to the standards themselves.^[8]

First, the film has to be applied to the substrate. This is best done via an automatic film applicator such as the byko-drive. The film length must be at least 10 mm longer than the maximum scrub length of the scrub tester while the film width should be at least 60 mm wide. The film thickness has to be so, that after 200 scrub cycles, the substrate is not exposed. The film has then to be dried according to ISO 3270 for the specified period of time. Afterwards, the panel is weighed. The sample is then transferred to the automatic scrub tester and fixated with a clamp.
The washing liquid is a solution of 2.5 g/L of sodium n-dodecylbenzenesulfonate in water which has been allowed to stand to dissipate any air-bubbles or foam that may have formed.
The washing liquid is applied with a soft paint brush and left in contact with the coating for 60 s. Afterwards, the abrasive pad is likewise saturated with washing liquid to a final mass of about 4 g. The abrasive is now fixated on the machine so that the unprinted side is in contact with the coating. The mass of the scrub pad holder which exerts the force on the panel has to be 135 g. The machine is run for 200 cycles at a speed of 37±2 cycles per minute and afterwards the panel is detached, thoroughly rinsed with water, dried, and weighed. 
 

To calculate the loss of coating during the scrubbing process calculate the area traversed by the scrub during operation (Equation 2). Afterwards, calculate the mass loss per area by subtracting the starting weight from the weight after testing. (Equation 3)
A = SW / 10⁶
Equation 2: Calculation for the area traversed by the scouring pad. S = scrub length in mm, W = scrub width in mm.

L = (m₁ - m₂) / A
Equation 3: Calculation for the loss in coating mass per area. m₁ = initial weight, m₂ = weight after testing, A = area traversed by scouring pad.

To calculate the mean loss in film thickness, divide the loss in coating mass per area by the dry film thickness (Equation 4).

L(dft) =  L / p_(df)
Equation 4: Calculation for the mean loss in film thickness. L = loss in coating mass per area, p_(df) = film thickness in g/cm³.

This mean loss in film thickness is used in DIN EN 13300 to determine the wet scrub resistance class (Table 4). 

For the testing of the cleanability of a paint, the procedure is identical. The only difference is, that a soiling agent (pencil, crayons, etc.) is applied before the washing liquid is applied (Figure 7). Visual evaluation of the soiled area gives the result. If the soiling agent is fully removed, compare the scrubbed area with a control panel. If it is not fully removed, the coating can be rated in cleanability classes in accordance with another suitable standard. 

4.3    ASTM D2486
This standard is used to determine the resistance of wall paints to erosion caused by scrubbing. It describes two different testing methods (Figure 8).^[9]

Method A:
Three drawdowns of the sample paint are made on a black plastic panel with a film applicator of 0.18 mm or 7.0 mil gap clearance. This application should take between 3 to 4 seconds. Afterwards the panels are air dried at 23 °C and 50% relative humidity in accordance with D3924 for seven days. Before the scrubbing test can begin, the brush has to be soaked in water overnight and then preconditioned for 400 scrubbing cycles. 
The dried panel is placed onto the scrubbing tester. A brass shim is placed perpendicular to the path of the brush and the panel is fixated onto the machine. The combined weight of the nylon brush and the pad holder must be 454 g. 10 g of scrubbing media are spread over the brushes of the bristle and the brush is placed on one end of the path. Then the panel is wet with 5 mL of water on the path of the brush.
The test is started and run until a continuous thin line of paint film across the width of the shim is visible (Figure 9, right). If no line is visible after 400 cycles, repeat the addition of scrubbing media and run the machine for another 400 cycles and repeat until failure is observable. This whole process is repeated with one more coated panel and if the results differ by more than 30%, a third panel is tested.

Method B:
Is almost identical to method A, however here two panels are tested side by side at the same time. One panel is coated with the test paint and another panel is coated with a reference paint. In the test report the ratio of failure cycles of the test paint in relation to the reference paint are reported in order to determine which paint has the better scrub resistance.

4.4    ECE R43 & ISO 5685
This standard describes the testing of a glazing surface against abrasion by a wiper rubber under the influence of a dust suspension. It is used to determine the abrasion resistance of windshields in normal driving conditions against wind shield wipers. Additional testing like taber abrasion and falling sand tests are described in ECE R43. The evaluation is performed by haze measurement before and after the test using BYK-gardner haze-gard.^[10]

4.5    ASTM D3450
This test method determines the ease of removing soilant discoloration from interior coatings. The coating is drawn down on a black plastic panel and allowed to dry for seven days. A specified soilant medium is applied. The coating is scrubbed with an abrasive or non-abrasive media using a cellulosic type sponge for 100 cycles. The soilant removal is assessed by measuring the CIE Y standard tristimulus value before and after the test.^[11]

4.6    ASTM D4213
The purpose of this method is to measure scrub resistance. The primary differences from ASTM D 2486 method are: The scrub resistance is determined by weight loss of the paint film relative to a standard calibration panel. The test panel and calibration panel are scrubbed simultaneously. The scrubbing device is a Scotch-Brite™7448 abrasive pad.^[12]

4.7    ASTM D4828
This test method determines the relative ease of removing soil and stains from interior coatings. The coating is applied to a black plastic panel and dried for seven days. The soilant can be user defined or the soilant described in ASTM D 3450 can also be used. A user defined liquid or powder cleaner is applied. The panel is scrubbed 100 cycles with a sponge. The soilant removal is assessed using gloss or color measurement.^[13]

4.8    Outdated standard DIN 53778
This standard was recalled and replaced with ISO11998, but it is still used in some testing conditions by manufacturers. It is similar to ISO11998, but it uses a hog bristle brush instead of a scotch scouring pad, it runs over 1000 cycles for cleanability and 5000 cycles for scrub resistance instead of the 200 cycles of the ISO11998. The evaluation of the test results is done visually only.^[14]
 

Abbildung 5 Gardner-scrub with ISO11998 attachment kit.

Abbildung 6 Gardner-scrub tester: Accessories needed for an abrasion test according to ISO 11998. Scotch brite pads (top right), automatic scrubbing tester (middle), black scrub panels (bottom left) and pad holder (bottom right).

Abbildung 7 A testing panel before scrubbing that has been soiled by crayons via a line perpendicular to the scrubbing direction.

Abbildung 8 Gardner-scrub ASTM D2486 - accessories needed for a scrubbing test according to ASTM D2486. Black scrubbing panels (front left), brass shims (front middle, on top of panels), nylon brush and pad holder (bottom right), automatic scrubbing tester (back middle).

Abbildung 9 Abrasion resistance test according to ASTM D2486. On the left is the reference panel, with clearly visible abrasion. On the right is the test panel, which shows one continuous line between the shims, indicating the point of failure for this coating.

5 Conclusion

Abrasion resistance in coatings and paints refers to the ability of a surface to withstand mechanical action such as rubbing, scraping, or erosion. It’s a key property of a coating, as it determines the durability and longevity as well as the protective properties. The principle behind it is the relationship between the hardness and flexibility of the coating and the roughness of the surface. It’s important to measure because high abrasion resistance means the painted surface can maintain its appearance and integrity under tough conditions for longer. The tests are typically done using specific abrasion testing methods, according to a variety of different international standards. Each standard defines a slightly different application and uses a different experimental setup. 

BYK-Gardner offeres a variety of abrasion testers with different accessory kits to meet ISO and ASTM standards.  In addition, the test results can be objectively evaluated using BYK-Gardner high quality spectro2guide family for color measurement, micro-gloss family for high to matt gloss evaluation and haze-gard for haze measurement of transparent products. All you need from one source.