Practical Solutions for Painting Defects (Part 16): Measures to Ensure Coating Performance in Powder Coating (Part 2)
In this section, the Painting Quality Improvement Subcommittee of Top Industry Co., Ltd., a specialized painting manufacturer based in Kawagoe City, Saitama Prefecture, that focuses on the development and manufacturing of industrial painting and related equipment, provides knowledge and advice useful for industrial painting sites.
In the first part, we explained the initial coating performance of powder coating, specifically "appearance" and "adhesion." In this second part, we will introduce "corrosion resistance" and "weather resistance," which are the reliability performance factors of painted products.
Corrosion Resistance
The corrosion resistance of painted products is evaluated using accelerated corrosion testing equipment. Cross-cut scratches are made on the test panel, and tests such as salt spray testing are performed to observe the state of rust formation and coating peeling.
<Example of Test Method>
・Place the cross-cut test panel into the salt spray testing equipment
・Determine the corrosion state after the specified time (within 3mm of corrosion width on one side of the cut)
[Countermeasure Examples]
(1) Zinc phosphate treatment and corrosion resistance
Using test panels (TP) of cold-rolled steel sheet (SPCC) and galvannealed steel sheet (GA material), we prepared samples with only alkaline degreasing and samples with [alkaline degreasing + zinc phosphate treatment] as pre-painting treatments. A polyester-based powder coating was applied to these, baked and cured at 200°C for 20 minutes, and cross-cuts reaching the substrate were made.
These test panels were placed in a salt spray testing machine, and an adhesion test using adhesive tape was conducted after 500 hours. As a result, the samples with only degreasing showed clear red rust and coating blistering/peeling, while the latter showed no blistering or peeling. It was confirmed that zinc phosphate treatment is an important painting substrate treatment for ensuring coating adhesion and corrosion resistance.
(2) Corrosion resistance of edge parts
For workpieces with many edge burrs, powder coating has superior edge coverage compared to solvent-based coating because it forms a thicker film.
In corrosion tests as well, it has been confirmed that while solvent-based coatings tend to develop rust early from the edges, powder coatings have the effect of delaying rust formation.
Weather Resistance
To determine the weather-resistant lifespan of painted products used outdoors, accelerated weather resistance tests and outdoor exposure tests are conducted to evaluate the weather resistance performance of the coating.
<Example of Test Conditions>
After placing the test panel in a xenon weather meter and allowing the specified time to pass, the color difference from the initial coating, gloss change, and presence of chalking are determined.
[Countermeasure Examples]
(1) Baking/curing temperature and weather resistance
Generally, the higher the baking temperature, the lower the gloss and the better the corrosion resistance.
Weather resistance decreases if the baking is insufficient, and it also decreases if the resin deteriorates due to exposure to excessively high temperatures. To obtain good weather resistance, it is important to set and maintain optimal baking conditions.
(2) Film Thickness Reduction by Paint Resin Type
Outdoors, one may sometimes see epoxy resin-coated products where the substrate is exposed. It is presumed that this occurs because the coating surface repeatedly undergoes degradation and chalking due to the influence of ultraviolet rays, gradually thinning the film until the object being coated is exposed.
As a verification experiment, test panels were prepared using epoxy resin powder coating and acrylic resin powder coating. They were placed in a sunshine carbon arc weatherometer based on JIS D 0205, and every 200 hours, the coating surface was wiped 10 times back and forth with paper wipers to remove surface dirt (chalking).
The graph below shows the transition of film thickness reduction measured at that time.
The film thickness of the epoxy powder decreased as the test time progressed, and after 1,000 hours, it became a thin film of 10 μm or less, reproducing the exposure of the substrate of the coated object.
In contrast, the acrylic powder showed a film thickness reduction of only about a few μm, and no chalking was observed, confirming its superior weather resistance.
The characteristics of powder coating are summarized as follows.
While powder coating is excellent in corrosion resistance and edge coverage due to its thick film, the coating film performance is greatly influenced by the paint resin and baking conditions.
By understanding the characteristics of powder coating and performing appropriate coating specification design and process management, stable coating film quality can be ensured. Furthermore, powder coating is a coating method for which demand is expected to increase in the future from the perspective of VOC reduction.
References
Japan Powder Coating Industry Association (Ed.), "Powder Coating Technology Handbook," Toryo Houchi Shimbun Co., Ltd., 1994
Minoru Tsubota, "Easy-to-Understand Basics and Practice of the Latest Paints and Coating," Shuwa System, 2016
Nippon Paint Co., Ltd. Paint Division, "Easy Technical Explanation Series, New Edition 1: Overview of Automotive Coating," Nippon Toryo Shimbun Co., Ltd., 1995
Top Industry Co., Ltd... Founded in Toshima-ku, Tokyo in October 1955. Later relocated to Kawagoe, and currently operates the Kawagoe Head Office Factory and the Misato Factory in Misato-machi, Kodama-gun. Main businesses include metal coating (antibacterial processing possible), resin coating (antibacterial processing possible), electrodeposition coating, and the development of coating equipment and related devices and systems. With the new entry into the contract paint stripping business, the expansion of services other than coating is also accelerating.