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Abrasion Resistance in Paints and Coatings

Abrasion Resistance of Paints and Coatings
  1. What is Abrasion Resistance?
  2. What Factors Govern Abrasion Resistance of Paints and Coatings?
  3. Raw Materials That Improve Abrasion of Paints & Coatings
  4. Testing Abrasion Resistance – Popular Methods

What is Abrasion Resistance?

In today’s modern life, what can be more annoying than having ugly scratches on a smartphone screen, polished floors, or the new shiny car? This undesirable surface appearance is often caused due to the exposure to the external elements. Scratches not only greatly reduce the surface aesthetics of a coated product but can lead to premature failure of the substrate (e.g. accelerated corrosion due to substrate exposure).

Scratch is thin shallow cuts or marks made using a pointed or sharp object on the coated or uncoated surface, while mar involves the degradation of the coating layer due to fine, light scratches caused by something abrasive.

The resistance of a coating to scratch and mar is called abrasion resistance. The abrasion resistance of coating provides information about the ability of coating to withstand damages due to abrasive materials, such as:

  • Sand
  • Dirt
  • Chippings
  • Scouring brushes, etc.

Abrasion resistance is a basic factor in the durability of a coating. Abrasion-resistant coatings are applied to a substrate intended to prevent mechanical damage that can result in surface imperfections. They are widely used to reduce or eliminate wear and thus, extend the lifetime of coated parts.

Certainly, abrasion resistance holds practical importance for both formulators and consumers using coatings for protective purposes. It is imperative to have a quantitative understanding of surface change to better relate various external factors resulting in coatings degradation and how the damage can be avoided or reduced due to abrasion.

In general, abrasion resistance is needed for coatings that will be exposed to wear and mechanical damage in service.

Let's review the main factors, raw materials and test methods that govern abrasion resistance of paints and coatings.

What Factors Govern Abrasion Resistance of Paints and Coatings?

Abrasion resistance of a coating is related to a coating’s ability to resist wear or deformation due to erosion or impact. The better the abrasion resistance of coating, it provides better and long-term protection to the substrate.

Wear occurs during abrasion between two contact surfaces due to uneven or rough profiles. When bumpy surfaces contact each other, abrasive forces occur. It results in the appearance change, brought about by surface roughening, which in turn leads to a reduction in gloss and reflectivity. If a material does not meet wear requirements even with lubrication, coatings with improved anti-abrasive properties may be needed.

Abrasion resistance is not a unique property of a surface coating, but it is related to other physical properties as well. It is functionally linked to the elastic properties as well as cohesive forces.

The different factors that govern abrasion resistance of materials, amongst others, are:

  • Roughness - The rougher the surface of the coating, the more vulnerable it is to abrasion.

  • Hardness, toughness, and flexibility - Hardness must be balanced with flexibility as these properties work together. If a coating is not hard enough, an impact may fracture the system down to the substrate. And, having more hardness in a coating will make it more susceptible to brittle fracture. 

  • Elasticity - The elasticity of a coating is also an important factor in controlling wear. In general, the elastic modulus gives a measure of the material’s tendency to elastically deform. It is a measure of the elastic limit of strain, which is an indicator of the amount of strain a coating can experience without permanent deformation occurs. 

  • Cohesive and tensile strength - A successful wear-resistant coating must support high loads, provide low friction, and must not show cohesive fracture or loss of adhesion to the substrate.

  • Contact geometry - The size, length, and cross-sectional area of abrasive particle influence coating’s resistance to abrasion.

  • Pigment Volume Concentration - The gloss, cleaning ability, mechanical strength, wash fastness, and abrasion resistance decrease with increasing pigment volume concentration.

  • Apart from these, the thickness of a film can be a key factor as well.

Raw Materials That Improve Abrasion of Paints & Coatings

Each coating is designed to combat abrasive wear while maintaining enough ductility to resist impact. The mechanical and optical properties of a coating surface are primarily defined by the resin system.

In the case of pigmented systems, the dispersed pigment and extender particles also have an influence on these properties. The abrasion resistance can also be improved with addition of certain waxes and fillers.

Let us discuss some examples and their role in abrasion resistance of a coating.

Types of Resin Binder Systems to Improve Abrasion

The resin binder controls the mechanical hardness, scratch resistance, and scrub resistance of the coating surface as well as the surface slip.

Aqueous Inkjet Inks: Accelerate Formulation with Best Practices

Urethane elastomers allow the formulation of tough, abrasion-resistant coatings for many flexible substrates including textiles.
Cellulose acetate butyrate is used in coatings for plastic because of its toughness, good abrasion resistance, and good bonding characteristics. The surface of molded plastic parts is often coated to obtain properties that the plastic does not have.
Polyethylenes of lower molecular mass are added as slip and matting agents to paints and printing inks. They also produce dirt-repellent and abrasion-resistant effects.
Polyvinyl Chloride (PVC) resins favor mechanical properties, a high abrasion resistance, and high chemical resistance in the coatings. Vinyl chloride copolymers are physically drying binders that undergo film formation by solvent evaporation. Vinyl chloride copolymers films show good flexibility and abrasion resistance that improves with increasing molecular mass.
Urethane alkyd resins are used in paints due to their benefits, such as rapid drying, high hardness, very good film elasticity, and above-average abrasion resistance. They produced by reacting long oil alkyd resins containing excess hydroxyl groups with diisocyanates.
Polyester resin grades are available for special mechanical requirements, such as high impact resistance, hardness, abrasion resistance.
Alkyl silicates (esters of the hypothetical silicic acid) are used as a binder for solid particles, i.e. for pigments. Using these esters makes the paints extremely resistant to organic solvents due to the absence of organic polymer. Also, the high degree of hardness confers excellent abrasion resistance.
Combination of polyurethane and polyester resins produce tough, abrasion-resistant, and durable general-purpose finishes.
Polyester paints are highly resistant to abrasion, alcohols, and other chemicals (cleaning agents). In these formulations, the binder is based on unsaturated polyester resins (copolymerized with styrene), low emission of organic solvents, catalytic curing by organic peroxides or UV radiation. highly resistant to abrasion, alcohols, and other chemicals (cleaning agents).
Polycarbonate Diols (PCD) and Polycarbonate-based Polyurethane Prepolymers offer exceptionally good scratch & abrasion resistance and outstanding durability of polyurethane coatings.
ETERNACOLL® polycarbonate diols (PCD) and ETERNATHANE® polycarbonate-based polyurethane prepolymers by UBE

Effective Additives for Improved Abrasion Resistance

The abrasion resistance can be further improved by the addition of additives as discussed below.

    Characteristics of Waxes
  • Waxes can improve scratch resistance and anti-blocking characteristics of the dry film. Waxes are used to protect a coating and/or its substrate from cosmetic and physical damage. The hardness of the wax will determine the effectiveness in improving abrasion resistance.
  • In the paint, silicone-based agents strongly reduce the surface tension and increase the slip. They can help enhance coating’s weatherability and resistance to abrasion, chemicals, and solvents.
  • Colloidal silica improves the mar & scratch resistance by increasing the cross-linking density of reactive groups. They enhance the film build density and thus, improve the resistance.
  • Metal oxides such as silicone oxides and aluminum oxide are often incorporated within the resin film to suppress marring, scratching, or abrading of the coating.
  • Abrasion resistant fillers such as calcined alumina, quartz, silicon carbide reduce abrasion of coatings, for example in road markings and floor coverings
  • Nano oxide particles can enhance physical properties of coatings without affecting appearance. Nano silica and nano alumina particles used for surface protection can also be incorporated into pigmented coatings. They not only offer scratch resistance, but can help in improving wear resistance, adhesion, stain, and corrosion resistance, because the nanoparticles create a denser coating structure.

» View the Complete List of Abrasion Resistance Agents that Protect Your Coatings from Damage!

Protective Coatings

Ceramic coatings can be used on the surface of metal or ceramic materials. Since ceramic has relatively high hardness, corrosion resistance and heat resistance, it can be widely used as a protection coating offering resistance to wear and corrosion. Typical ceramic coatings include Al2O3/TiO2, SiO2/TiO2/Cr2O3, SiC, B4C, ZrO2, CaO, CrN/AlCrN, CrN/BCN, SiO2, WC, and TiC.

Diamond-like carbon (DLC) coatings show a combination of a low coefficient of friction and high micro-hardness, making them extremely effective in many tribological and wear applications. Diamond-like carbon (DLC) exists in amorphous carbon materials that show some typical properties of diamond.

Note: The most important thing to remember when formulating or choosing an abrasion-resistant coating system for a given application is to know all the possible the parameters (environment, substrate, application method, etc.) to avoid asset failure.

Benefits of abrasive resistance coatings

  • Widely used to reduce or eliminate wear
  • Extending the lifetime of products
  • Suitable for certain environments unsuitable for lubrications
  • Strengthen mechanical properties, such as hardness and toughness

Key Applications/Significance: automobile bodies, floor coatings, optical displays, optical lenses, packaging, aerospace components, building facades, marine coatings, etc.

Testing Abrasion Resistance – Popular Methods

Abrasion testing of coatings should be done routinely and used as the basis for comparison of a coating’s mechanical performance. These tests assess the ability of a surface coating to withstand scratching or marring and overall durability.

Many types of abrasive media and treatment are used for testing e.g., fast and slow movement, small or large load, high and low temperature, constant or intermittent contact.

Commonly used test methods include:

(Of course, there exists several other methods as well, but all of them are not discussed here)

Rotating Wheel Method

This test method may be useful for quality control analysis of a coating, and it can be used to evaluate the effects of processing variables, such as substrate preparation before coating, surface texture, coating technique variables, or post coating treatments, any of which may influence the susceptibility of the coating to particle shedding.

Referred to as the “Taber Test,” a coating is applied to a plane, rigid surface. The test specimen is mounted to the Taber Abraser turntable and rotated at a fixed speed under a weighted CS-10 or CS-17 abrading wheel. A vacuum system removes any loose debris generated during testing. Before each test and after every 500 cycles, the abrading wheels are resurfaced with an S-11 refacing disc to standardize the wheel surfaces. At the conclusion of the test, wear resistance can be expressed in terms of:

  • Wear Index, the weight loss per specified number of revolutions (usually 1000) under a specified load (500 or 1000g).
  • Wear cycles per mil, the number of cycles required to wear through a 1-mil thickness of coating; or,
  • Cycles to failure, the number of test cycles to wear away the coating or to expose the substrate

Taber Abrasion Test by Cerakote

Common standards associated with Taber test:

  • ASTM F1978: Standard Test Method for Measuring Abrasion Resistance of Metallic Thermal Spray Coatings by Using the Taber Abraser
  • ASTM D4060: Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser
  • ASTM D1044 - 19: Standard Test Method for Resistance of Transparent Plastics to Surface Abrasion by the Taber Abraser
  • ISO 7784: Paints and varnishes — Determination of resistance to abrasion
    • Part 1: Method with abrasive-paper covered wheels and rotating test specimen
    • Part 2: Method with abrasive rubber wheels and rotating test specimen

Falling Sand (Abrasive) Method

These test methods cover the determination of the resistance of organic coatings to abrasion produced by abrasive falling onto coatings applied to a plane rigid surface, such as a metal or glass panel.

The falling sand test is a simple method of testing abrasion. Standardized sand is released and guided through a tube onto the test specimen. The volume of sand required to obtain total erosion of a known coating thickness shows the abrasion resistance.

Falling Sand Tester by Taber Industries
Falling Sand Tester by Taber Industries

 Common standards associated with Falling sand method:

  • ASTM D968: Standard Test Methods for Abrasion Resistance of Organic Coatings by Falling Abrasive
  • DIN 1164

Abrasive Blast Method

This test method covers the determination of the resistance of organic coatings to abrasion produced by abrasive blasting on coatings applied to a plane, rigid surface.

Common standards associated with Abrasive blast method:

  • ASTM D7127: Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument
  • ASTM D7055: Standard Practice for Preparation (by Abrasive Blast Cleaning) of Hot-Rolled Carbon Steel Panels for Testing of Coatings

Gardner Wet Abrasion Method

Abrasion resistance can be tested by wet abrasion methods using scrub media or cleaning solutions. The most common applications are testing the scrub resistance of interior wall paints, floor tiles, shower stalls, and furniture surfaces. The Gardner-scrub abrasion tester offers a versatile design for abrasion and washability testing applications.

(Source: BYK)

Common standards associated with Gardner wet abrasion method:

  • ASTM D2486: Standard Test Methods for Scrub Resistance of Wall Paints
  • ASTM D3450: Standard Test Method for Washability Properties of Interior Architectural Coatings
  • ASTM D4213: Standard Test Method for Scrub Resistance of Paints by Abrasion Weight Loss
  • ASTM D4828: Standard Test Methods for Practical Washability of Organic Coatings
  • ISO 11998: Paints and varnishes — Determination of wet-scrub resistance and cleanability of coatings
  • DIN 53778: Emulsion paints for interior use; evaluation of cleanability and of wash and scrub resistance of coatings

Find Suitable Additive For your Paint to Achieve Desired Scratch / Scrub / Mar Resistance »

Scratch Resistance Test Method

Scratch resistance tests assess the ability of a surface coating to withstand scratching. A needle with a spherical steel point (of specified diameter), carrying a predetermined weight, is lowered onto the paint film, and drawn across the surface at a set speed.

The method has been used in the paint industry (and in government specifications) for many years. The test may be operated in  two ways:

  • Either as a ‘pass or fail’ test using a specific weight for the material under test
  • Or (in assessing new materials) with an increasing load until failure occurs

The apparatus consists of a horizontal sliding panel to which the test panel is clamped, coated face upwards. The sliding panel is moved beneath the needle point at a speed of 3–4 cm per second. The ‘needle’ itself consists of a shank to which has been soldered a 1mm diameter grade A1 steel ball. It is fixed into a holder at the end of a counterpoised arm, which is kept horizontal by adjusting the length of the needle in the holder.

Weights are placed over the needle in the holder and the panels set in motion. The forward travel lowers the needle gently onto the surface. At least 6 cm of scratch is needed for the test. Although manual and motor driven apparatus are available for the test, the latter is preferred since more reproducible results can be obtained.

Scratch Testing by Taber Industries

Common standards associated with Scratch resistance test method:

  • ASTM D7027: Standard Test Method for Evaluation of Scratch Resistance of Polymeric Coatings and Plastics Using an Instrumented Scratch Machine
  • ASTM D7187: Standard Test Method for Measuring Mechanistic Aspects of Scratch/Mar Behavior of Paint Coatings by Nanoscratching

Abrasion Resistance Agents for Paints, Coatings and Inks

View the full range of abrasion resistance agents available today, analyze technical data of each product, get technical assistance or request samples.

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1 Comments on "Abrasion Resistance in Paints and Coatings"
v v Nov 26, 2021
What a informational article! About the abrasion resistance, i still have some inquiry. Such as Taber Instruments for abrasion , it looks good to test the abrasion .

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