What is abrasion resistance?
In today's modern life, what's more annoying than an unsightly scratch on your smartphone screen, a polished floor, or a shiny new car? This undesirable surface appearance is often the result of exposure to external elements. Scratches not only greatly detract from the surface aesthetics of coated products, but can also lead to premature failure of the substrate (e.g. accelerated corrosion due to substrate exposure).
Scratches are shallow cuts or marks made on a coated or uncoated surface using a pointed or sharp object, whereas scratches are degraded coatings due to microscopic scratches caused by certain abrasives.
The scratch and mar resistance of the coating is called abrasion resistance. The wear resistance of a coating provides information on the ability of the coating to withstand damage from abrasive materials such as:
sand
dirt
debris
Brush etc.
Abrasion resistance is an essential factor in coating durability. Abrasion-resistant coatings are applied to substrates and are designed to prevent mechanical damage that could lead to surface defects. They are widely used to reduce or eliminate wear, thereby extending the life of coated parts.
Of course, abrasion resistance is of practical importance to both formulators and consumers who use coatings for protective purposes. A quantitative understanding of surface changes is needed to better correlate the various external factors that lead to coating degradation and how damage from abrasion can be avoided or reduced.
In general, abrasion resistance is required for coatings that are subject to abrasion and mechanical damage in use.
Let's review the main factors, raw materials and testing methods that determine the abrasion resistance of paints and coatings.
What factors affect the abrasion resistance of paints and coatings?
The wear resistance of a coating is related to the ability of the coating to resist wear or deformation due to erosion or impact. The more abrasion resistant a coating is, the better and long-term protection it provides to the substrate.
Wear occurs during wear between two contact surfaces due to uneven or rough profiles. Friction occurs when uneven surfaces come into contact with each other. It causes a change in appearance due to roughening of the surface, resulting in reduced gloss and reflectivity. If the material cannot meet the wear requirements even under lubrication, a coating with improved anti-wear properties may be required.
Abrasion resistance is not an exclusive property of surface coatings, but it is also related to other physical properties. It is functionally related to elasticity and cohesion.
The different factors that determine a material's wear resistance include:
Roughness - The rougher the surface of the coating, the easier it is to wear.
Hardness, Toughness, and Flexibility - Hardness needs to be balanced with flexibility because these properties work together. If the coating is not hard enough, the impact may crack the system into the substrate. Moreover, the higher the hardness of the coating, the more prone to brittle fracture.
Elasticity - Elasticity of the coating is also an important factor in controlling wear. In general, the modulus of elasticity gives a measure of a material's tendency to deform elastically. It is a measure of the strain elastic limit, which is an indicator of the amount of strain a coating can withstand without permanent deformation.
Cohesion and Tensile Strength - Successful abrasion resistant coatings need to be able to withstand high loads, provide low friction, and not cohesively fracture or lose adhesion to the substrate.
Contact Geometry - The size, length and cross-sectional area of the abrasive particles can affect the wear resistance of the coating.
Pigment Volume Concentration - Gloss, cleaning power, mechanical strength, wash fastness and abrasion resistance decrease as pigment volume concentration increases.
In addition, the thickness of the film is also a key factor.
Raw materials for improving paint and coating wear
Each coating is designed to resist abrasive wear while remaining ductile enough to withstand impact. The mechanical and optical properties of the coated surface are mainly determined by the resin system.
In the case of colored systems, dispersed pigment and extender particles also affect these properties. The addition of certain waxes and fillers can also improve abrasion resistance.
Let's discuss some examples and their role in coating abrasion resistance.
Types of Resin Binder Systems for Wear Improvement
The resin binder controls the mechanical hardness, scratch and scrub resistance, and surface slip of the coated surface.
Polyurethane elastomers allow the formulation of tough, abrasion-resistant coatings for many flexible substrates, including textiles.
Cellulose acetate butyrate is used in plastic coatings for its toughness, good abrasion resistance and good adhesion properties. The surfaces of molded plastic parts are often coated to obtain properties not found in plastics.
Lower molecular weight polyethylenes are added to paints and printing inks as slip and matting agents. They are also stain and abrasion resistant.
Polyvinyl chloride (PVC) resins contribute to the coating's mechanical properties, high abrasion resistance and high chemical resistance. Vinyl chloride copolymers are physically drying binders that form films by solvent evaporation. Vinyl chloride copolymer films exhibit good flexibility and abrasion resistance, which increase with increasing molecular weight.
Polyurethane alkyd resins are used in coatings for their fast drying, high hardness, very good film elasticity and above average abrasion resistance. They are 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 (hypothetical esters of silicic acid) are used as binders for solid particles (ie pigments). The use of these esters results in coatings that are extremely resistant to organic solvents due to the absence of organic polymers. In addition, high hardness imparts excellent wear resistance.
A combination of polyurethane and polyester resins produces a tough, hard wearing and durable general purpose finish.
Polyester coatings are highly resistant to abrasion, alcohol and other chemicals (cleaners). In these formulations, the adhesive 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, alcohol and other chemicals (cleaning agents).
Polycarbonate diol (PCD) and polycarbonate-based polyurethane prepolymers have very good scratch and abrasion resistance and very good durability of polyurethane coatings.
Effective additives for increased wear resistance
As described below, the wear resistance can be further improved by adding additives.
Waxes can improve the scratch and anti-block properties of the dry film. Waxes are used to protect coatings and/or their substrates from cosmetic and physical damage. The hardness of the wax will determine how effective it is at improving wear resistance.
In coatings, silicone-based agents strongly reduce surface tension and increase slip. They help improve the weather and abrasion resistance, chemical and solvent resistance of coatings.
Colloidal silica improves scratch resistance by increasing the crosslink density of reactive groups. They increase the build density of the film, which increases electrical resistance.
Metal oxides such as silicon oxide and aluminum oxide are often incorporated into resin films to inhibit damage, scratching or abrasion of the coating.
Wear-resistant fillers such as calcined alumina, quartz, silicon carbide reduce wear of coatings, e.g. in road markings and floor coverings
Nano oxide particles can enhance the physical properties of the coating without affecting the appearance. Nano-silica and nano-alumina particles for surface protection can also be added to colored paints. Not only are they scratch resistant, but they also contribute to improved abrasion, adhesion, stain and corrosion resistance as the nanoparticles create a denser coating structure.
protective coating
Ceramic coatings can be applied to surfaces of metal or ceramic materials. Due to their high hardness, corrosion resistance and heat resistance, ceramics are widely used as protective coatings for wear and corrosion resistance. 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 display a combination of low coefficient of friction and high microhardness, making them very effective in many tribological and wear applications. Diamond-like carbon (DLC) exists in amorphous carbon materials that exhibit some of the typical properties of diamond.
NOTE: An important thing to remember when formulating or selecting an abrasion resistant coating system for a given application is to understand all possible parameters (environment, substrate, method of application, etc.) to avoid asset failure.
Advantages of wear-resistant coatings
Widely used to reduce or eliminate wear
Extend the life of the product
Suitable for some environments not suitable for lubrication
Enhanced mechanical properties such as hardness and toughness
Main applications/significance: automotive body, floor coating, optical display, optical lens, packaging, aerospace components, building exterior, marine coating, etc.
