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How Powder Coated Surfaces Resist Scratches
Development of Hardness and Crosslinked Thermoset Structures
Scratch resistance of powder coated surfaces is due to the unique chemical composition of coatings. Coatings are cured at 180 to 200 degrees Celsius for approximately 10 to 20 minutes. A permanent chemical reaction occurs with the polymer chains which form resilient three dimensional networks. This gives coatings a hardness of 3H to 9H on pencil hardness scale, ASTM D3363. Hardness is greater than conventional liquid paints. Dense networks resist movement of polymer chains. Coatings do not allow sliding of chains when rubbed or scratched. Even sharp object impacts do not initiate chain movement. Coatings remain intact and do not become brittle. Powder coatings are demonstrated to withstand approximately double the scratching force than their alternative thermoplastics.
Effect of Film Thickness, Texture and Curing on Scratch Resistance
Scratch resistance is influenced by three dependent parameters:
Film thickness of 60-120 μm is ideal, providing a sacrificial layer that inhibits substrate exposure due to scratches.
Textured finishes allow impact energy to be distributed across the texture which reduces visible damage by 40-60%.
Controlled curing leads to complete crosslinking. Coatings that are under-cured are demonstrated to be 30% less mar resistant in the Taber abrasion test (ISO 1518).
These variables have been tested for consistency with manufacture guidelines for automotive/industrial trim and equipment and for things such as ASTM D3363 and ISO 1518.
Colors and Finishes of Coatings: UV Stability and Fade Resistance of Powder Coated Finishes
Polyester Passes UV Fluoropolymer Fails with Different Absorption and Color Retention Chemistry
Polyester powders have UV protective damage mechanisms but these do cause a chalking effect eventual. An upside for Fluoropolymer powders is these have UV protective damage mechanisms that are caused due to an output of UV smart little crystal domains. An upside for Fluoropolymer powders is due to high energy (breakage) compared to low energy (breakage) of polyester which results in better stability and in-turn better pigmentation and freshness. Testing down in Florida showed these coatings kept over 95% of their original color even after sitting out there for a full decade.
Practical Durability: QUV Weathering Data (ASTM G154) 5,000+ Hours
The goal of the Accelerated QUV (ASTM G154) testing is to simulate up to decades of sun exposure. After 3,000 hours of exposure to premium polyester powders, the gloss retention is at 90%. For fluoropolymers, after 5,000 hours, gloss retention is at 98%. Paint payments show the above averages regarding the Delta-E value of color change.
Applicable to 15+ years of outdoor service to architectural metals in a temperate climate. Coastal installations have a 20% faster degradation of materials due to salt-fog and UV compounding conditions.
Environmental factors that challenge the longevity of powder coatings.
Humidity, salt spray, and chemical exposure and their combined effects in a rapid fade/scratch degradation. When the conditions are ideal for multiple types of environmental attack, the coatings fail at an accelerated rate. When a coating is humid, the polymer bonds begin to fail through a process called hydrolysis. Then, weakened coating bonds become an entry point for salt spray to attack, ultimately, causing the coating to fail due to internal, electrochemical corrosion. Rapid decay occurs in environments that are able to provide alkaline or acidic attack as the combined environmental effects do not work in isolation. The rapid decay of these coatings leads to additional maintenance expenses and a shortened service life in industrial applications.
The presence of an oxidized substrate can cause an early color change due to pellets that have no pigment which alter light diffusion.
The absorption of moisture causes the coating to be 40% less flexible and more easily marred and scratched.
Deep subsurface damage accelerates five times under humid conditions due to chloride ion permeation, as per ASTM B117.
The premature failure of the case to protect against moisture, corrosion, and ultraviolet damage is documented. For instance, when combined, a few years of exposure to ultraviolet light, salt air, and high humidity can cause unprecedented flaky and chaotic surface deterioration. This corrosion is a staggering unexpected cost. NACE international documented that the corrosion of exposed surfaces costs about 740 thousand dollars annually per facility, across multiple sectors. This type of damage can only be mitigated by using specially formulated coatings. Epoxy hybrids are commonly formulated for use in chemical processing, while moisture dense areas are better served by polyurethanes. Edge areas are primary areas of moisture intrusion and should be sealed to prevent premature failure.
To maximize the scratch and fade resistance of powder coated applications, the following guidelines are recommended.
For optimum surface preparation, it is advisable to use thorough cleaning and abrasive blasting to maximize the bond that is formed between the substrate and the coating which is critical in minimizing failure due to adhearing.
To promote color retention, the use of UV absorbers combined with high energy HALS and technology, is recommended.
Control the thickness of the coating: The coating should be at a thickness of 60–120 μm, as films that are too thin cause the substrate to be exposed, and films that are too thick reduce flexibility and increase the propensity for microcracking.
Ensure a precise cure time: Ideally, the cure time should be 10-15 minutes at a temperature of 180–200°C to achieve full cure and increase surface hardness by 15–20% (in accordance to ASTM D3363).
Gentle cleaning: Use cleaning solutions that are neutral pH and a microfiber cloth. Cleansers that are too abrasive may cause micro scratches which will accelerate photochemical degradation.
Repairs should be done promptly: Repairs to scratches and chips should be made as soon as possible to prevent moisture ingress and corrosion.
FAQ
What are the reasons powder coating is more resistant to scratches compared to liquid paint? The scratch resistance properties found in powder coatings is due to the thermosetting polymer structure. The three dimensional structure derived network offers a superior level of scratch resistance compared to liquid paint.
What happens when powder coatings are exposed to ultraviolet rays? There are different methods that powder manufacturers use to protect the integrity of the color in the coating, as well as offer some level of ultraviolet protection. Polyester powder coatings have aromatic ring structures while fluoropolymer powder coatings contain crystalline domains, both of which offer some protection and color retention with fluoropolymer coatings having the best retention.
What conditions cause a powder coating to show signs of wear more quickly? All the mentioned these elements: humidity, salt, air, and powder coatings degrading chemicals cause the coating to wear.
What are recommended methods for maintaining powder coated surfaces?
Good techniques for surface preparation, effective formulations, control over film thickness, accurate curing, gentle maintenance, and timely touch-up are all important for bond and improve the longevity and aesthetics of powder coated surfaces.