1.What is the core guarantee of weather resistance?
Galvanized coating: Serves as a protective base layer, protecting the steel sheet from corrosion through the sacrificial anodic action of zinc. Its thickness directly impacts the corrosion resistance of the base layer (e.g., zinc coating weight ranges from 80-275g/m², depending on the corrosive environment).
Chemical conversion coatings (e.g., passivation layers): Enhance the adhesion between the galvanized coating and the primer, while further blocking moisture and oxygen penetration.
Primer: Typically epoxy or polyester, it bonds tightly to the galvanized coating, providing corrosion resistance and permeation resistance, and is crucial for connecting the coating to the topcoat.
Topcoat: Directly exposed to the environment, it must exhibit resistance to UV aging, acid and alkali, chalking, and discoloration. Common types include polyester, silicone-modified polyester (SMP), and polyvinylidene fluoride (PVDF, also known as fluorocarbon paint). PVDF topcoats offer the best weather resistance.
2.What are the differences in weather resistance performance in different environments?
Normal atmospheric environments (urban and rural)
Suitable for building exteriors, roofs, appliance casings, and other applications. ◦ Polyester topcoat: Generally guarantees 5-10 years of coating integrity with no noticeable chalking or discoloration. ◦ Silicone-modified polyester (SMP): Offers superior weather resistance compared to standard polyester, maintaining stable appearance and performance for 10-15 years. ◦ PVDF topcoat: Exceptionally weather-resistant, with a service life of over 20 years in a dry, clean atmosphere, with virtually no chalking or discoloration.
Harsh environments (coastal areas, industrial areas, and high-humidity areas) ◦ Coastal areas (high salt spray): Chloride ions in salt spray accelerate coating aging and plating corrosion. PVDF topcoat, with its strong chemical stability, resists salt spray erosion. Combined with a thick zinc coating (e.g., 275g/m²), it can last 15-20 years in coastal areas. Standard polyester topcoats may experience blistering and rust in such environments after only 3-5 years. ◦ Industrial areas (containing sulfur dioxide and dust): Acidic pollutants can corrode the coating. SMP or PVDF topcoats can reduce the rate of corrosion through their acid and alkali resistance, while ordinary polyesters are prone to paint peeling. ◦ High-humidity areas (such as the rainy southern regions): Hot and humid environments can easily lead to moisture penetration between the coating and the substrate. High-quality primers (such as epoxy primers) can effectively block moisture and, when combined with a dense topcoat, reduce the risk of blistering and peeling.
3.What are the key weather resistance indicators and testing standards?
Salt spray resistance: PVDF coatings typically pass salt spray tests for over 5,000 hours, simulating coastal or highly corrosive environments, according to GB/T 13448 or ASTM B117. They show no rust or blistering; ordinary polyester typically lasts for 1,000-2,000 hours.
Accelerated weathering (QUV) testing: simulating the aging effects of ultraviolet light and thermal cycling on coatings, according to ASTM G53, shows that after 3,000 hours, PVDF coatings typically exhibit a color difference (ΔE) of less than 3 and a chalking rating of >4 (virtually no chalking). Ordinary polyester may exhibit noticeable chalking and discoloration after 2,000 hours.
Moisture and heat resistance: When tested in a high-temperature, high-humidity environment (e.g., 40°C/90% RH), high-quality color-coated coils can survive for over 1,000 hours without blistering or peeling. Acid and alkali resistance: Resistant to acid rain or industrial acid and alkali corrosion. After the PVDF topcoat is immersed in a solution with a pH of 3-11, the paint film has no obvious damage.
4.What are the key factors affecting weather resistance?
Coating Materials: Topcoat type is crucial: PVDF (fluorine content ≥ 70%) > SMP > conventional polyester. Compatibility between primer and topcoat is also crucial (for example, an epoxy primer + PVDF topcoat combination offers superior weather resistance).
Coating Thickness: Topcoat thickness is generally required to be ≥ 20 μm; thinner thickness can lead to premature aging. Primer thickness should be ≥ 5 μm to ensure adhesion to the coating.
Production Process: Improper control of coating curing temperature and time can lead to inadequate crosslinking of the film, reducing weather resistance. Incomplete surface pretreatment (such as passivation of the galvanized layer) can affect coating adhesion and accelerate aging.
Maintenance: Long-term accumulation of dust and oil will accelerate coating aging. Regular cleaning can extend the service life. Scratches on the coating during installation that are not promptly repaired can become a starting point for corrosion and reduce overall weather resistance.
5.What is the difference between the weather resistance of color-coated galvanized coil and ordinary galvanized coil?
Ordinary polyester color-coated coil is suitable for dry, low-corrosion environments and has a service life of 5-10 years.
SMP color-coated coil is suitable for moderately corrosive environments and has a service life of 10-15 years.
PVDF color-coated coil is a high-end option, offering stable service for 15-25 years in harsh environments such as coastal areas and industrial areas. It is the preferred choice for long-term outdoor use in building exterior walls and rooftops.