This article systematically introduces the concept, types, characteristics, functions, application fields, precautions for selecting powder coatings, and solutions to common problems of polyester powder coatings, with a focus on the specific applications of polyester powder coatings, to help everyone better understand what polyester powder coating is and what its characteristics and functions are.
What Is Polyester Powder Coating
Polyester powder coating is a thermosetting powder coating that uses carboxyl-terminated polyester resin as the main film-forming material, combined with curing agents (such as TGIC or HAA), pigments, fillers, and additives. It is manufactured through melt extrusion, pulverization, and sieving, and is electrostatically sprayed onto metal substrates, where it is heated, melted, and cured to form a film.Types of Polyester Powder Coating
According to the curing system, polyester powder coatings can be divided into:1. TGIC-Cured Polyester System
This is the most classic outdoor weather-resistant system, using triglycidyl isocyanurate (TGIC) as the curing agent. Its overall performance is mature and stable, with excellent weather resistance, heat resistance, and mechanical properties, making it the dominant choice for outdoor powder coatings in China. However, TGIC has been subject to controversy regarding biological toxicity. Its use has gradually been restricted in Europe, while it is permitted in the U.S. market.
2. HAA-Cured Polyester System (TGIC-Free)
This alternative system was developed in response to environmental protection requirements and uses β-hydroxyalkylamide as the curing agent. Its reaction activity is stronger than that of TGIC, enabling lower curing temperatures, while being non-toxic and environmentally friendly. Its weather resistance is comparable to that of the TGIC system, and it has better storage stability. However, its heat resistance is slightly lower, and when the coating is too thick, the curing by-product (water) can easily cause pinholes. Its corrosion resistance on steel substrates is also relatively weaker.
3. Other Curing Systems
There are also systems cured with IPDI (blocked isocyanate), which have excellent leveling and outdoor weather resistance, as well as good pigment wetting properties. In addition, there are indoor polyester systems cured with epoxy (hybrid systems), but these belong to the category of indoor applications and do not have outdoor weather resistance.
Characteristics of Polyester Powder Coating
The main characteristics of polyester powder coatings are as follows.Excellent outdoor weather resistance: Polyester systems based primarily on isophthalic acid can effectively resist UV aging and provide good gloss and color retention, making them a mainstream choice for outdoor powder coatings.
Good mechanical properties: The coating has excellent impact resistance and flexibility, strong adhesion, and can withstand certain mechanical stresses and deformation.
Excellent decorative performance: It has good leveling properties and can be produced in various appearance effects, such as high gloss, flat, sand texture, and metallic finishes.
Low tendency to yellow during baking: Compared with epoxy systems, polyester powder coatings are less likely to yellow during curing and baking, making them suitable for light-colored and white coatings.
Moderate chemical resistance: They can resist general acid, alkali, and solvent attack, but their corrosion resistance is insufficient in highly corrosive environments.
Functions of Polyester Powder Coating
Which fields use polyester powder coatings? Their specific applications are as follows:
1. Protective Functions
(1) Outdoor weather protection: Resists UV radiation, rainwater, temperature differences, and other climatic factors, preventing coating chalking, fading, and cracking. It is the core protective method for outdoor metal facilities.
(2) Corrosion and rust protection: Forms a dense coating on the surface of metal substrates, isolating moisture and oxygen and slowing down the corrosion process.
(3) Mechanical protection: With good impact resistance and flexibility, it resists daily impacts, scratches, and deformation, protecting the substrate from mechanical damage.
(4) Chemical protection: Resists the attack of general acids, alkalis, solvents, and cleaning agents, making it suitable for moderately corrosive environments.
2. Decorative Functions
(1) Appearance enhancement: Provides various surface effects such as high gloss, flat, sand texture, and metallic finishes to meet different product decoration requirements.
(2) Color and gloss retention: Maintains color and gloss after long-term outdoor use, preventing facilities from appearing aged due to coating deterioration.
(3) Low yellowing during baking: Suitable for light-colored and white coatings, maintaining a clean appearance.
Application Fields of Polyester Powder Coating
The main application fields of polyester powder coatings are as follows:1. Building Materials and Aluminum Profiles
This is the largest application market for polyester powder coatings. Specific applications include aluminum alloy doors and windows, curtain walls, architectural aluminum profiles, steel building components, cladding, and doors. Super-durable polyester systems, such as products meeting Qualicoat Class 2 or AAMA 2604 standards, are specifically used for architectural exterior components with strict requirements for color and gloss retention.
2. Outdoor Facilities and Garden Products
The weather resistance advantages of polyester make it a preferred choice for outdoor metal facilities. Typical applications include garden furniture, garden fences, railings, gazebos, road sign posts, highway guardrails, bicycle frames, and outdoor lighting facilities.
3. Home Appliance Housings
Used for decorative and protective coating of home appliance products such as refrigerators, washing machines, air conditioner housings, vacuum cleaners, and instrument housings. The low yellowing tendency of polyester systems during baking makes them suitable for light-colored home appliance finishes.
4. Automotive and Transportation Components
Used for surface coating of automotive wheels, bumpers, motorcycle components, bicycle frames, and transportation equipment.
5. General Industrial and Agricultural Equipment
Covers fields such as agricultural machinery, utility poles, chemical equipment, metal furniture, office shelving, and energy storage equipment.
How to Choose Polyester Powder Coating
When selecting polyester powder coatings, we may face the problem of not knowing how to choose. Based on our industry experience, we recommend focusing on the following aspects when selecting polyester powder coatings.1. Confirm Whether It Is for Outdoor Use — This Is the Prerequisite
For outdoor applications, pure polyester must be selected, and epoxy or epoxy-polyester hybrid systems should absolutely not be used. Epoxy systems rapidly chalk and yellow under UV radiation and are only suitable for indoor applications. The core advantage of polyester powder coating is its excellent resistance to UV radiation, making it the preferred choice for outdoor metal facilities.
2. Determine the Weather Resistance Grade According to the Severity of the Environment
Even for outdoor applications, the requirements for ordinary courtyards and coastal curtain walls can be completely different. The AAMA (American Architectural Manufacturers Association) standards can be used as a reference for determining the weather resistance grade:
| Grade | AAMA Standard | Typical Weather Resistance Performance | Application |
| Standard Weather Resistance | AAMA 2603 | Approximately 1 year of Florida exposure | General outdoor facilities, indoor and outdoor applications |
| Super Weather Resistance | AAMA 2604 | Approximately 5 years of Florida exposure | Architectural curtain walls, high-requirement coastal applications |
| Extreme Weather Resistance | AAMA 2605 | Approximately 10 years of Florida exposure | Landmark buildings, extreme climate regions |
If you have a specific facility location and expected service life, you can select the corresponding grade accordingly.
3. Curing Agent Selection — TGIC vs HAA
This is the most important formulation choice for polyester powder coatings:
(1) TGIC (Triglycidyl Isocyanurate)
Advantages: Excellent overall performance, outstanding weather resistance, chemical resistance, and heat resistance, with no by-products during the reaction, making thick coatings less prone to pinholes.
Disadvantages: It has biological toxicity, can cause skin irritation, and is relatively expensive.
Current status: It remains a mainstream choice for outdoor weather-resistant powder coatings, but its use has been restricted in regions such as Europe where environmental requirements are strict.
(2) HAA (Hydroxyalkylamide)
Advantages: Non-toxic and environmentally friendly, with stronger reaction activity than TGIC, enabling lower curing temperatures (reaction can occur at 150°C), and lower formulation costs.
Disadvantages: Water is generated as a by-product during curing, making thick coatings prone to pinholes; it is also more likely to yellow at high temperatures.
Trend: Preferred for environmentally sensitive applications, such as children's facilities and food-contact equipment, and is also a mainstream alternative under the current "TGIC-free" trend.
One-sentence decision: For conventional outdoor applications, choose TGIC (mature and stable performance); for environmentally sensitive applications or when low-temperature curing is required, choose HAA (while paying attention to the risk of pinholes in thick coatings).
4. Verify Additional Requirements for Specific Application Scenarios
Different application scenarios have different requirements for polyester powder coatings:
(1) Architectural aluminum profiles: Priority should be given to products that meet Qualicoat Class 2 or GSB certification, as these certifications directly correspond to long-term weather resistance requirements for architectural exteriors.
(2) Agricultural machinery/construction equipment: Pay attention to resistance to over-baking and excellent mechanical properties.
(3) Heat-sensitive substrates (MDF/wood): Low-temperature-curing polyester is required, such as TGIC-free low-temperature types or UV-curing types.
Common Problems and Solutions for Polyester Powder Coating
The most common problems encountered during the use of polyester powder coatings mainly occur in the following areas. Based on our industry experience, we propose corresponding solutions to help effectively solve the powder coating problems you encounter.1. Insufficient Curing (Soft Coating, Poor Adhesion, and Insufficient Corrosion Resistance)
Problem description: The coating has low hardness and poor scratch resistance, and rusting and peeling may occur prematurely after salt spray or humidity resistance testing. The most difficult aspect is that an insufficiently cured coating looks almost identical to a fully cured coating, making it difficult to distinguish with the naked eye.
Possible causes:
(1) Only oven temperature is measured, not workpiece temperature: The curing conditions specified in the powder data sheet refer to the metal temperature of the workpiece, not the air temperature inside the oven. For heavy workpieces or densely loaded racks, the actual workpiece temperature may be far below the required level.
(2) HAA curing systems are more sensitive to curing conditions: Non-TGIC polyester systems based on HAA (hydroxyalkylamide) have a lower tolerance for insufficient curing than TGIC polyester systems and require stricter compliance with curing conditions.
(3) Variation in oven loading: Changes in workpiece loading and rack density can significantly alter the heating curve, resulting in insufficient curing of some workpieces within the same oven.
Solutions:
(1) Measure workpiece temperature using an oven temperature profiler: Use the actual measured metal temperature at the thickest and most densely loaded areas as the basis, and confirm that it reaches the requirements of the data sheet (typically 180–200°C for TGIC polyester, with sufficient holding time).
(2) Verify curing by solvent rubbing: Use acetone or MEK to rub the coating in an inconspicuous area. Well-cured TGIC polyester can generally withstand 25 double rubs with acetone without obvious color loss or softening; HAA systems are more sensitive to curing and require stricter verification.
(3) Re-verify whenever the product/process changes: The curing conditions need to be reconfirmed after changes in oven loading, line speed, or workpiece type.
2. Pinholes (Small Holes on the Coating Surface That Extend to the Substrate or Underlayer)
Problem description: Small circular holes appear on the coating surface and are visible to the naked eye, becoming a starting point for insulation or corrosion failure.
Possible causes:
(1) Water as a by-product of HAA curing: Water is generated as a by-product during HAA curing. When the coating is thick, water vapor has difficulty escaping, and when the film thickness exceeds approximately 100 μm (4 mils), the risk of pinholes increases significantly.
(2) Substrate outgassing: Moisture or air trapped inside porous substrates such as galvanized steel and castings escapes when heated during curing, breaking through the molten coating and forming pinholes or bubbles.
(3) Moisture absorption by the powder: Moisture absorbed by the powder vaporizes and escapes during curing.
Solutions:
(1) Control film thickness: When using an HAA system, control the dry film thickness below 100 μm, preferably below 80 μm.
(2) Pre-baking for degassing: For galvanized parts or castings, preheat them to 10–15°C above the recommended curing temperature before spraying to remove volatile substances from the substrate, and spray after cooling.
(3) Use degassing-type powder: Add wax-based degassing agents to the formulation to allow gas to escape smoothly during the molten stage of the coating, after which the coating can self-heal.
3. Craters (Circular Depressions with the Center Exposing the Substrate)
Problem description: Small circular depressions appear on the coating surface, resembling volcanic craters, with the substrate or underlying coating visible at the center.
Possible causes:
(1) Surface tension problems of low-cost polyester: Low-cost polyester resin has relatively high surface tension (approximately 43 mN/m), while the surface tension of iron-based substrates is approximately 40 mN/m. The coating has difficulty spreading over the substrate, resulting in craters.
(2) Crystallization tendency of low-cost resin: Low-cost formulations use large amounts of ethylene glycol, which has low steric hindrance and high symmetry, giving it crystallization capability. The presence of trace microcrystals causes deterioration of surface properties.
(3) Oil and water in compressed air: This is the most common external source of craters, as oil mist or water droplets are sprayed onto the workpiece together with the powder.
Solutions:
(1) Use anti-cratering additives containing active functional groups: Ordinary leveling agents and gloss-enhancing agents (acrylate-based, without active functional groups) are ineffective against cratering in low-cost polyester. Copolymer additives containing active functional groups should be selected to interact with polyester resin, inhibit crystallization, and reduce surface tension.
(2) Purify compressed air: Install and maintain a two-stage filtration system—first pass through a 5 μm particulate filter to remove particles and some moisture, then through a coalescing filter to capture oil mist particles, with an additional final filter installed before the spray gun.
4. Insufficient Edge Coverage (Exposed Substrate at External Corners and Thin Coating at Internal Corners)
Problem description: The coating is too thin or missing at sharp edges, welds, and holes, and these areas become starting points for rust. Polyester coatings themselves have insufficient edge coverage, and this weakness is amplified in corrosive environments.
Possible causes:
(1) Excessive powder melt flow: To achieve large-area leveling, the powder has relatively low viscosity during curing, and the coating at external corners flows toward both sides under the influence of surface tension.
(2) Faraday shielding: Internal corners and grooves are located in electrostatic field shielding zones, making it difficult for charged powder to reach them.
Solutions:
(1) Use a multi-coat system: For workpieces in corrosive environments, a single-layer polyester coating is often insufficient. In a two-coat system consisting of primer powder and topcoat powder, the primer can use an edge-coverage formulation to significantly improve corrosion protection at edges and corners.
(2) Optimize the spraying method: Apply additional manual touch-up spraying to edge and corner areas, or adjust the spray gun angle to reduce the effects of electrostatic shielding.
5. Unstable Gloss in Matte Systems
Problem description: The gloss of two-component polyester matte powder coatings (dry-blending method) varies significantly between batches, making it difficult to consistently control the gloss at the target value.
Possible cause:
The ratio of Components A/B reaching the substrate deviates from the blending ratio: The high-acid-value polyester (A) and low-acid-value polyester (B) flakes have different brittleness. After the flakes are pulverized, their particle size distributions are inconsistent, and the proportions drawn away by the induced draft fan during grinding also differ. Under the influence of the electric field, gravity, initial powder velocity, and other factors, the actual A:B ratio deposited on the substrate is no longer the same as the ratio during flake blending.
Solution:
Grind the A/B flakes separately and then blend the powders: First grind the two types of flakes separately and adjust them to basically consistent particle size distributions (such as D50 of 41 μm for both), and then dry-blend them according to the required ratio. This eliminates component deviation caused by differences in brittleness and provides relatively stable gloss.
If you encounter some difficult-to-solve problems during the use of polyester powder coatings, please feel free to contact us at any time for professional technical support. We can discuss solutions together and promote the development of the powder coating industry.
We hope this article can provide you with a professional and reliable reference regarding the powder coating industry. We sincerely welcome you to consult us regarding powder coating product performance, industry standards, application methods, precautions, or any other related questions. We look forward to hearing from you at any time through messages or direct contact, so that we can provide you with more detailed product information, demonstration videos, or customized solutions to help you fully understand the functions and advantages of our products.
