Electrostatic spraying is divided into powder spraying and fluorocarbon spraying. Powder spraying uses materials such as polyurethane, polyurethane resin, epoxy resin, hydroxy polyester resin, and epoxy/polyester resin, and can be formulated into a wide variety of colors. Powder spraying features include manual and automatic hanging spraying equipment, simple application, coating thickness of 30 microns or greater, and excellent impact, abrasion, corrosion, and weather resistance. The paint is cheaper than fluorocarbon. Powder spraying's biggest weakness is its sensitivity to ultraviolet radiation. Long-term exposure can cause natural fading, leading to noticeable color differences between the sun-facing and non-sun-facing sides of aluminum panels after a few years, typically within 2-5 years. Currently, so-called colored aluminum profiles are available on the market for aluminum doors and windows. These are powder-coated aluminum profiles. This increases the color variety of aluminum doors and windows while also enhancing corrosion resistance.
Another electrostatic spraying method is liquid spraying, also known as fluorocarbon spraying or, in Hong Kong, as curium oil. This high-end, expensive spray coating has long been used abroad. However, it has only been widely used on aluminum curtain walls in China in the past two years. Due to its superior properties, it is gaining increasing attention and favor among the construction industry and users. Fluorocarbon spray coating offers excellent resistance to fading, blooming, corrosion from atmospheric pollution (such as acid rain), UV resistance, cracking, and the ability to withstand harsh weather conditions. This is unmatched by ordinary coatings.
I. Fluorocarbon Spray Coating Materials and Structure
Fluorocarbon spray coatings are made using polyvinylidene fluoride resin (nCH2CF2)n (PVDF) as a base material or with aluminum powder as a pigment. The chemical structure of the fluorocarbon base material is based on fluorine/carbon bonds. This short-bond structure is believed to provide the most stable and strong bond with hydrogen ions. This stability and strength of the chemical structure distinguishes fluorocarbon coatings from ordinary coatings in terms of physical properties. In addition to excellent mechanical properties such as wear resistance and impact resistance, fluorocarbon coatings exhibit long-term resistance to fading and UV light, particularly in harsh climates and environments. In the early 1960s, it began to be used in engineering plastics, such as wires and cables. In 1965. Pennwalt Chemical Company in the United States first combined polyvinylidene fluoride (PVDF) with aluminum powder to create a topcoat for architectural applications, also known as metallic paint, under the trademark Kynar 500. Subsequently, manufacturers worldwide began using Kynar 500 to create their own fluorocarbon paint topcoats for the interior and exterior coatings of high-end aluminum structures. Its wide range of colors, beautiful and elegant appearance, and durability have enhanced the brilliance of many magnificent curtain wall structures worldwide. Paint manufacturers' lifespan guarantees for coatings have evolved from 10 or 15 years to 20 years.
A US research institute conducted comparative tests on fluorocarbon coatings, super coatings, and conventional coatings. Samples of each coating were exposed to the scorching sun and the harsh environment of humid, salty air in Florida for 12 years. The results demonstrated that fluorocarbon coatings exhibited stability and durability that were 30 and 80 percentage points higher than the other two coatings, respectively, ensuring their suitability for use in a variety of harsh environments.
Many fluorocarbon coating manufacturers around the world commonly use Kynar 500 or Hylar 5000 as the base for their neon-carbon coatings. There are no officially certified manufacturers in China. Major fluorocarbon coating manufacturers include:
1. 1CI (UK);2. PPG (US); 3. Valspar (US); 4. DNT Singapore (Dai Nippon Paint Singapore); 5. Kawsai (Kansai) Paints (Japan); 6. Korea Chemical (South Korea); 7. Fengyuan Paint (Taiwan).
II. Fluorocarbon Spraying Technical Requirements and Standards
As a high-end surface coating process, fluorocarbon spraying requires extremely stringent quality requirements throughout the entire process. Fluorocarbon coating manufacturers must obtain a quality license to produce Kynar 500-based fluorocarbon coatings. The entire surface treatment process, including pretreatment, spraying, and curing, must be rigorously controlled by the coating manufacturer to ensure the quality of the aluminum coating. The fluorocarbon spray coating industry currently relies primarily on the American Architectural Manufacturers Association standard AAMA-605.02.90. which is widely recognized as the primary international standard for evaluating coating quality. The following are the key indicators for fluorocarbon spray coatings from AAMA-605.02.90.
The AAMA standard frequently mentions South Florida as the testing standard because its climate, known for its humidity, high temperatures, and direct sunlight, is considered a harsh environment for coating quality testing.
There are also other similar standards worldwide that are used to assess fluorocarbon spray coating quality. These are:
1. British standard BS6496-1984. "Standard for the Coating of Architectural Aluminum Profiles and Sheets."
2. German GSBRAL-RC 631 1994 Standard "Quality Inspection Standard for Architectural Aluminum."
3. Australian Standard AS3715-1989 "Standard for Coating Metal Materials."
4. South African Standard SABS-1993 "Standard for Coating Exterior Aluminum Materials."
5. Swiss Standard "Standard for Coating Aluminum Materials Used in the Construction Industry," 1994.
American Architectural Manufacturers Association Standard (Abstract) AAMA-605.02.90
III. Fluorocarbon Spraying Equipment and Process
Due to the inherent properties of fluorocarbon coatings, spraying equipment must ensure excellent atomization to ensure a consistent and consistent spray coating. Uniformity and the distribution of metallic particles in fluorocarbon coatings directly impact the coating's appearance. High-quality, uniform fluorocarbon coatings exhibit a metallic luster, vibrant colors, and a distinct three-dimensional effect. However, fluorocarbon coatings applied with inappropriate spray equipment can produce uneven colors, surface shadows, or a loose coating, significantly impacting the decorative effect. To achieve optimal spraying results, high-voltage electrostatic electric spray guns are used. Various types and brands of spray guns are produced in the United States, Japan, Germany, and other countries, each with its own unique characteristics.
Fluorocarbon spraying processes often utilize multi-layer spraying. To fully utilize the durability and weather resistance of Kynar 500 metallic paint, strict quality control is applied throughout the entire coating process, from the aluminum's pre-surface treatment to the final product, ensuring it meets the American Architectural Manufacturers Association (AAMA) standard 605.02.90.
The fluorocarbon spray coating process is as follows:
Pretreatment: Degreasing and cleaning the aluminum → Water washing → Alkaline washing (degreasing) → Water washing → Acid washing → Water washing → Chromizing → Water washing → Pure water washing
Spraying: Primer coating → Topcoat → Clear coat → Baking (180-250°C) → Quality inspection
The multi-layer spray coating process consists of three spray coats (referred to as "three sprays"): primer, topcoat, and clear coat, followed by a secondary spray coat (primer and topcoat).
1. Purpose of Pretreatment: To achieve a high-quality finish on aluminum alloy profiles. Before spraying, the surface of the workpiece must be degreased, cleaned, and chemically treated to create a chromized film. This enhances the adhesion between the coating and the metal surface and its anti-oxidation properties, thereby extending the lifespan of the paint film.
2. Primer Coating: As a sealant for the substrate, the primer coat enhances the coating's resistance to penetration, strengthens substrate protection, stabilizes the metal surface layer, strengthens the adhesion of the topcoat to the metal surface, and ensures color uniformity. The paint layer thickness is generally 5-10 microns.
3. Topcoat: The topcoat is a critical layer of the spray coating, providing the desired decorative color for the aluminum, ensuring the aluminum's appearance meets the design requirements, and protecting the metal surface from the erosion of the external atmosphere, acid rain, pollution, and UV penetration. It significantly enhances aging resistance. The topcoat is the thickest layer of the spray coating, generally 23-30 microns thick.
4. Clearcoat: The clearcoat, also known as the varnish, The main purpose is to more effectively enhance the paint layer's resistance to external corrosion, protect the topcoat, and enhance the metallic luster of the topcoat, resulting in a more vibrant and eye-catching appearance. The coating thickness is generally 5-10 microns. The total thickness of the three-spray coating is generally 40-60 microns, but can be thicker if required.
5. Curing: Three-spray coatings generally require a secondary curing process. The aluminum material enters a curing oven. The curing temperature is generally between 180°C and 250°C, and the curing time is 15-25 minutes. Different fluorocarbon coating manufacturers will provide optimal temperatures and times based on their coatings. Some chlorocarbon spray coating plants (curing oil plants) may change the double curing of the three-spray coating to a single curing process based on their own experience.
6. Quality Inspection: Quality inspection should comply with AAMA-605.02.90. Strict quality inspection guarantees high-quality spray coating products.
