Failure Mechanisms, Application Limitations, and Systematic Improvement Directions for Polyester Coatings

1. Application Positioning of Polyester Coatings

Polyester resins are widely used in coatings because they can provide a good balance among adhesion, flexibility, hardness, appearance, application adaptability, and cost. Polyester coatings are commonly used in powder coatings, coil coatings, industrial baking coatings, home appliance coatings, architectural aluminum profiles, metal products, and some packaging coatings. Among them, polyester resin is also a common and important resin type in powder coatings, and both TGIC and HAA curing systems are widely used in polyester powder coatings.

The reliability of polyester coatings mainly depends on the following factors:

① Resin structure, such as ester bond density, steric hindrance, hydrophilicity, acid value, hydroxyl value, and molecular weight;

② Curing system, such as curing agent type, reaction activity, crosslinking completeness, and curing window;

③ Coating film structure, such as crosslinking uniformity, film thickness, pinholes, craters, pigment and filler distribution, and interfacial bonding;

④ Substrate condition, such as degreasing, phosphating, chromating, zirconium treatment, silane treatment, or the quality of other pretreatments;

⑤ Application conditions, such as spraying parameters, baking profile, actual workpiece temperature, drying rate, and ambient humidity;

⑥ Service environment, such as water, heat, acids, alkalis, UV, salt spray, cleaning agents, pollutants, and mechanical damage.

Therefore, improvement of polyester coatings should not rely only on a single resin or a single additive. Instead, coatings should be systematically designed around “failure mechanism—typical manifestation—control focus—verification method.”

2. Overview of the Main Failure Mechanisms of Polyester Coatings

The main limitations of polyester coatings can be summarized into the following categories. It should be noted that these limitations do not mean that polyester resins have poor performance. Rather, they indicate that polyester coatings must be designed according to the application environment, application conditions, and protection objectives.

Failure Scenario	Core Mechanism	Typical Manifestations	Control Focus	Recommended Verification Methods
Damp heat, water immersion, or high-humidity environment	Water diffusion, plasticization, ester bond hydrolysis, weakened interfacial bonding	Whitening, gloss loss, blistering, adhesion loss, peeling	Low-water-absorption resin, appropriate acid value/hydroxyl value, sufficient curing, film thickness control, pretreatment	Damp heat test, water immersion test, adhesion after aging, water resistance test
Contact with alkaline media	Alkaline hydrolysis or saponification of ester bonds; interface damaged by high-pH media	Softening, tackiness, chalking, discoloration, blistering, edge peeling	Alkali-resistant structure, dense crosslinking, primer isolation, verification with actual media	Alkali solution immersion, cleaning agent test, adhesion after damp heat exposure
Outdoor aging	Combined effects of UV, moisture, oxygen, temperature variation, pollutants, and pigment stability	Gloss loss, discoloration, chalking, surface roughness, cracking	Weather-resistant polyester, weather-resistant pigments, UV absorbers, HALS, film thickness, and application quality	QUV, xenon arc aging, natural exposure, gloss retention, color difference, chalking grade
Insufficient curing	Insufficient temperature or time; incomplete crosslinking reaction	Low hardness, poor MEK resistance, poor water resistance, poor solvent resistance, unstable adhesion	Actual workpiece temperature, curing profile, activity matching between resin and curing agent	Oven temperature profile, MEK rub test, hardness, impact, adhesion, DSC
Over-curing	Thermal aging, embrittlement, or yellowing caused by excessive temperature or excessive time	Reduced impact resistance, cracking on bending, yellowing, gloss change, reduced intercoat adhesion	Control the over-baking window; avoid excessive crosslinking and thermal aging	Over-baking test, impact, bending, color difference, gloss
Powder application defects	Imbalance among melting, flow, degassing, and gelation rate	Orange peel, pinholes, craters, thin edge coverage, storage caking	Particle size, melt viscosity, degassing, spraying parameters, heating program	Appearance, film thickness distribution, pinhole detection, cross-section observation, storage stability
Failure of waterborne systems	Film formation affected by water evaporation, hydrophilic groups, pH, dispersion stability, and drying process	Craters, whitening, water marks, flash rusting, reduced water resistance, storage thickening	Dispersion stability, pH, conductivity, drying profile, crosslinking efficiency, flash rust control	pH/conductivity, damp heat, water resistance, flash rusting, storage stability
Application difficulty of high-solids systems	Difficult to balance low VOC, low viscosity, flow, sag resistance, and crosslinking performance	Poor flow, orange peel, sagging, narrow application window, reduced durability	Molecular weight, branched structure, low-viscosity resin, additives, and application viscosity window	Application viscosity, sagging, flow, mechanical and chemical resistance after curing
Insufficient metal corrosion protection	Water, oxygen, and ions enter through coating defects or interfaces, causing under-film corrosion	Edge rusting, scratch creep, blistering, under-film corrosion, peeling after salt spray	Pretreatment, primer, anticorrosive pigments, film thickness, edge protection, multi-coat system	Salt spray, cyclic corrosion, scratch creep, adhesion after aging

3. Chemical Medium-Induced Failure: Hydrolysis, Alkaline Hydrolysis, and Insufficient Chemical Resistance

3.1 Hydrolysis Is an Important Potential Failure Mechanism for Polyester Coatings in Damp Heat or Strong Acid/Alkali Environments

The main chain of polyester resin contains ester bonds. Ester bonds are relatively stable in ordinary dry environments, but hydrolysis may occur under the combined effects of moisture, heat, acidic conditions, or alkaline conditions. The hydrolysis rate of polyester materials is usually affected by pH, hydrophilicity, temperature, structural morphology, and polymer structure. Under sufficient moisture, temperature, acid/alkali conditions, or long-term exposure, hydrolysis may cause polymer chain scission and reduce the cohesive strength, mechanical properties, and adhesion retention of the coating film. Hydrolytic failure of polyester coating films usually proceeds through the following process:

① Moisture enters the coating film;

② The coating film swells or becomes plasticized;

③ Ester bonds gradually undergo hydrolysis;

④ Polyester chain segments or the crosslinked network are damaged;

⑤ The cohesive strength of the coating film and interfacial adhesion decrease;

⑥ Whitening, gloss loss, blistering, cracking, peeling, or under-film corrosion occurs.

It should be noted that heat itself is not a hydrolyzing agent, but elevated temperature accelerates moisture diffusion and chemical reaction rates. Acids and alkalis may act as catalysts or promoters. Water-induced coating failure includes not only visible phenomena such as blistering, corrosion, and peeling, but also hidden processes such as water diffusion, swelling, plasticization, hydrolysis, and interfacial bonding damage. In actual coatings, hydrolysis may occur in parallel with water diffusion, plasticization, interfacial weakening, and under-film corrosion.

The hydrolysis sensitivity of polyester is closely related to the resin structure. By selecting monomers with better hydrolysis resistance, reducing hydrophilicity, minimizing water-soluble residues, optimizing acid value and hydroxyl value, and improving crosslinking completeness, the stability of polyester coating films in damp heat or water immersion environments can be significantly improved.

3.2 The Risk Is Higher in Alkaline Environments

Under alkaline conditions, polyester is more prone to alkaline hydrolysis or saponification. Hydroxide ions in alkaline media attack ester bonds, causing polyester chain segments to break at the ester bond sites and generating hydroxyl and carboxylate end groups. Compared with neutral water environments, strong alkali or high-temperature alkaline environments are generally more destructive to polyester coating films. Special attention should be paid to the alkali resistance of polyester coatings in the following environments:

① Areas in contact with freshly mixed concrete, wet cement slurry, or mortar;

② Alkaline leachate, damp cement dust, or high-pH construction environments;

③ Equipment or metal panels that are in long-term contact with alkaline cleaning agents;

④ Industrial alkaline pollution environments;

⑤ High-temperature alkaline solutions, washing systems, or cleaning conditions;

⑥ Locations where architectural metal panels are in long-term contact with alkaline materials.

When alkali resistance is insufficient, the coating film may show gloss loss, tackiness, softening, color change, chalking, blistering, adhesion loss, or edge peeling. Such problems are usually not simply surface defects, but the result of chemical medium-induced damage to the resin structure, coating film crosslinked network, and coating/substrate interface.

3.3 Other Manifestations of Insufficient Chemical Resistance

In addition to water and alkalis, acidic media, organic solvents, cleaning agents, oils, food media, and high-temperature damp heat environments may also affect polyester coating films. Different media cause damage in different ways.

Medium Type	Possible Effects
Water and damp heat	Water diffusion, swelling, plasticization, hydrolysis, adhesion loss
Acidic media	Acid-catalyzed hydrolysis, gloss loss, discoloration, interfacial damage
Alkaline media	Alkaline hydrolysis, saponification, softening, chalking, peeling
Organic solvents	Swelling, softening, gloss loss, reduced rub resistance
Cleaning agents	Surface gloss loss, color change, softening, or adhesion loss
Oils and pollutants	Penetration, adhesion, difficult cleaning, localized discoloration

3.4 Improvement Directions

To improve the chemical stability of polyester coatings, the following aspects should be controlled:

Improvement Direction	Function
Optimize resin structure	Select monomers with better hydrolysis resistance, increase steric hindrance around ester bonds, and reduce hydrolysis sensitivity
Reduce water absorption tendency	Control hydrophilic groups, water-soluble residues, and migratable components to reduce moisture ingress
Optimize acid value and hydroxyl value	Avoid risks caused by residual acidic components, free hydrophilic groups, or acid-base imbalance in the formulation
Improve crosslinking completeness	Form a denser and more uniform coating film structure to improve water resistance, solvent resistance, and chemical resistance
Control film thickness and pinholes	Improve barrier properties and reduce the ingress of water, oxygen, and ions
Optimize pigments and fillers	Improve barrier properties and reduce medium migration pathways
Strengthen pretreatment	Improve interfacial stability and reduce the risk of blistering and under-film corrosion
Conduct actual-medium testing	Carry out immersion, damp heat, cleaning agent exposure, and post-aging evaluation according to the actual contact medium

Note: Increasing the degree of crosslinking is not always better. Excessive or non-uniform crosslinking may make the coating film brittle, increase internal stress, reduce impact resistance, or cause cracking during bending. A more reasonable goal is to improve the completeness and uniformity of crosslinking and its compatibility with the application scenario.

4. Outdoor Aging Failure: Combined Effects of UV, Moisture, Oxygen, and System Factors

4.1 Outdoor Aging Is Not Caused by a Single Factor

The weatherability of polyester coatings varies greatly and cannot be simply summarized as “polyester is weather-resistant” or “polyester is not weather-resistant.” There are significant differences in outdoor performance among standard polyester, weather-resistant polyester, super-durable polyester, polyurethane-modified systems, acrylic systems, and fluorocarbon systems. Evaluation of the weatherability of polyester coatings requires comprehensive consideration of resin structure, curing system, pigments, additives, film thickness, pretreatment, and application quality.

Outdoor aging is usually caused by the combined effects of the following factors:

① Ultraviolet light, i.e., UV;

② Moisture, condensation water, and damp heat cycling;

③ Oxygen and photo-oxidation reactions;

④ Temperature variation and thermal cycling;

⑤ Acid rain, salts, and industrial pollutants;

⑥ Stability of pigments and additives;

⑦ Degree of coating film curing;

⑧ Film thickness and edge coverage;

⑨ Quality of substrate pretreatment.

4.2 Typical Manifestations of Weathering Failure

The aging manifestations of outdoor polyester coatings mainly fall into three categories:

Type	Typical Manifestations	Main Causes
Appearance changes	Gloss loss, discoloration, chalking, surface roughness	Photo-oxidation of the surface resin layer, insufficient weather resistance of pigments, additive failure
Structural deterioration	Embrittlement, fine cracks, cracking, reduced impact or bending performance	Aging of the coating film crosslinked network, increased internal stress, degradation of polymer chain segments
Interfacial failure	Blistering, adhesion loss, localized peeling	Moisture vapor penetration, insufficient pretreatment, insufficient film thickness, weakened interfacial bonding

4.3 Improvement Directions

The weatherability of polyester coatings should be improved from the perspective of the complete system:

Improvement Direction	Function
Use weather-resistant or super-durable polyester	Improve the inherent UV resistance, oxidation resistance, and damp heat resistance of the resin
Select weather-resistant pigments	Avoid accelerated fading, discoloration, or chalking caused by low-weatherability pigments
Use a light stabilization system	UV absorbers and HALS can delay photo-aging
Optimize pigment-to-binder ratio	Prevent premature exposure of pigments and fillers due to insufficient resin encapsulation
Improve curing completeness	Reduce the ingress of moisture vapor and oxygen into the coating film and improve performance retention after aging
Control film thickness	Ensure barrier performance, appearance retention, and edge coverage
Optimize pretreatment and primer	Reduce adhesion loss and under-film corrosion after aging
Conduct post-aging evaluation	Focus on changes in gloss retention, color difference, chalking grade, adhesion, impact, and bending performance

UV absorbers and hindered amine light stabilizers can improve weathering performance, but they cannot replace weather-resistant resins, weather-resistant pigments, appropriate film thickness, and reliable application. If the resin structure, pigment stability, or curing conditions are unreasonable, it is difficult to achieve long-term stable performance by relying only on light stabilizers.

5. Curing-Related Failure: Under-Curing, Over-Curing, and the Low-Temperature Curing Window

5.1 Insufficient Curing Weakens Coating Film Performance

Many polyester coatings are thermosetting systems and need to form a crosslinked structure under specific temperature and time conditions. For powder coatings, the heating process usually includes melting, flow, degassing, and curing at the same time. Curing evaluation should not only look at the oven set temperature. It should also focus on the actual workpiece temperature, the holding time after the workpiece reaches the target temperature, and the actual reaction degree of the coating film.

If the curing temperature is insufficient, the curing time is insufficient, the actual workpiece temperature does not meet the requirement, or the reaction activity of the curing agent does not match the resin, under-curing may occur.

Common manifestations of insufficient curing include:

① Low hardness;

② Poor methyl ethyl ketone rub resistance. Methyl ethyl ketone is abbreviated as MEK and is also known as butanone;

③ Reduced water resistance and chemical resistance;

④ Surface tackiness;

⑤ Unstable adhesion;

⑥ Insufficient impact and bending performance;

⑦ Faster performance degradation after aging.

5.2 Over-Curing Can Also Cause Failure

Excessively high curing temperature or excessively long curing time does not necessarily bring better performance. Over-curing may lead to:

① Embrittlement of the coating film;

② Reduced impact resistance;

③ Cracking during bending;

④ Yellowing;

⑤ Gloss change;

⑥ Reduced intercoat adhesion;

⑦ Thermal aging of certain pigments or additives.

The key to curing control is to find a balance among temperature, time, melt flow, degassing, reaction rate, and final performance.

5.3 Value and Challenges of Low-Temperature Curing

Low-temperature curing polyester powder coatings can reduce baking energy consumption, shorten production cycle time, and help expand powder coating applications on heat-sensitive substrates such as heavy metal parts, medium density fiberboard (MDF), engineered wood, some plastics, and composite materials. The development of low-temperature curing technology is also related to energy conservation, sustainable production, and the coating needs of heat-sensitive substrates.

However, low-temperature curing is not simply a matter of lowering the baking temperature. When the temperature is reduced, resin melting, flow, degassing, and crosslinking reactions may all be affected.

Challenge	Possible Problems
Insufficient melting	Orange peel, poor flow, rough surface
Insufficient reaction	Reduced hardness, water resistance, and chemical resistance
Reaction too fast	Insufficient flow time, increased pinholes or craters
Insufficient degassing	Bubbles, pinholes, thick-film defects
Reduced storage stability	Powder caking or premature reaction
Increased cost	Higher cost of high-activity resins, curing agents, or catalysts

5.4 Improvement Directions

Curing-related problems should be controlled from the following aspects:

Control Direction	Function
Measure actual workpiece temperature	Confirm real curing conditions instead of relying only on oven settings
Establish a curing profile	Identify the relationship among time, temperature, and performance
Optimize the reaction activity of resin and curing agent	Avoid under-curing, overly rapid gelation, or over-curing
Control the melt-flow window	Ensure sufficient flow before complete crosslinking
Optimize the heating program	Reduce pinholes, bubbles, and localized over-baking
Match substrate heat capacity	Use different curing strategies for thick parts, thin parts, and heat-sensitive substrates
Conduct post-curing performance tests	Verify by hardness, MEK, impact, bending, adhesion, and water resistance

6. New Failure Risks in Low-VOC Systems: Powder, Waterborne, and High-Solids Systems

Low VOC is an important development direction for polyester coatings. Due to their highly designable structure, polyester resins play an important role in powder coatings, coil coatings, high-solids coatings, and some waterborne systems. However, low-VOC systems are not simply about reducing solvent. They also change issues related to film formation, application, drying, storage, and stability.

Waterborne coatings have been widely used in architectural, automotive, and other fields, but in some high-performance applications, waterborne systems may still face challenges in performance, cost, or commercial replacement.

6.1 Failure Risks of Polyester Powder Coatings

Polyester powder coatings offer advantages such as low VOC, high material utilization, and high application efficiency. They are usually electrostatically sprayed in solid powder form onto conductive substrates and are especially common on metal substrates. However, powder systems are highly sensitive to melt viscosity, particle size, flow, degassing, gelation rate, spraying parameters, and baking program.

Defect	Main Causes	Control Methods
Orange peel	High melt viscosity, insufficient flow time, unreasonable particle size distribution, curing too fast	Reduce melt viscosity; optimize leveling agent, particle size, and heating profile
Pinholes	Gas release, substrate porosity, insufficient degassing in thick films, gelation too fast	Control film thickness, use degassing agents, optimize preheating and baking program
Craters	Surface contamination, poor wetting, incompatible additives	Improve pretreatment, control contamination sources, optimize wetting and leveling system
Thin edge coverage	Electrostatic shielding, sharp-edge effect, insufficient coating of complex structures	Optimize spraying voltage, gun distance, particle size, and touch-up spraying process
Storage caking	Low resin Tg, high storage temperature, excessive fines, or moisture absorption	Increase Tg, control particle size, use anti-caking additives, improve storage and transportation conditions

β-Hydroxyalkylamide, abbreviated as HAA, is a commonly used TGIC-free curing agent for carboxyl-functional polyester powder coatings. TGIC refers to triglycidyl isocyanurate. HAA releases water when curing carboxyl-functional polyester. Therefore, under conditions of thick film, insufficient degassing, or overly rapid gelation, attention should be paid to the risk of surface defects such as pinholes and bubbles.

6.2 Failure Risks of Waterborne Polyester

Waterborne polyester uses water as the main dispersion medium, which helps reduce VOC. However, waterborne conversion does not automatically improve performance. The high surface tension of water, evaporation process, hydrophilic groups, pH changes, dispersion stability, and drying conditions all affect film formation quality.

Problem	Formation Causes	Control Methods
Craters and poor flow	High surface tension of water, insufficient wetting, contamination, or incompatible additives	Optimize wetting agents, leveling agents, and substrate cleanliness
Whitening or water marks	Moisture retention, uneven drying, incomplete film formation	Optimize drying profile and application environment
Flash rusting	Contact between water and metal substrate; insufficient initial protection	Use flash rust inhibitors; improve pretreatment and drying speed
Reduced water resistance	Excessive hydrophilic groups, residual moisture, or insufficient crosslinking	Control hydrophilicity and improve crosslinking efficiency
Storage thickening or settling	pH drift, insufficient dispersion stability, microbial effects, or electrolyte effects	Control pH, water quality, conductivity, preservative system, and dispersion system
Pigment flooding and floating	Poor pigment dispersion or unstable additive system	Optimize wetting/dispersing agents and pigment/filler system

Waterborne polyester systems require special attention to pH, hydrophilic groups, and storage stability. If the system contains excessive hydrophilic structures, or if crosslinking efficiency is insufficient, the water resistance, damp heat resistance, and long-term adhesion of the coating film may decrease.

6.3 Failure Risks of High-Solids Polyester

High-solids polyester reduces VOC by increasing solids content and reducing solvent. However, after solids content is increased, it becomes more difficult to balance system viscosity, flow, sagging, application, and crosslinking. The core contradictions of high-solids systems are:

① Reducing VOC requires reducing solvent;

② Reducing viscosity may require reducing molecular weight;

③ Maintaining mechanical strength requires sufficient molecular weight and crosslinked structure;

④ Maintaining flow requires appropriate application viscosity and open time;

⑤ Improving chemical resistance requires sufficient crosslinking and a dense coating film.

If molecular weight is simply reduced to lower viscosity, coating film strength and durability may decrease. If functionality or crosslink density is increased, viscosity may rise again and application properties may deteriorate.

Improvement directions for high-solids polyester include:

① Designing low-viscosity polyester resins;

② Controlling molecular weight and molecular weight distribution;

③ Using an appropriate branched structure;

④ Selecting low-viscosity crosslinking components or suitable co-solvents;

⑤ Introducing reactive low-viscosity components when necessary, while verifying migration, VOC, curing compatibility, and final coating film performance;

⑥ Optimizing flow, sag resistance, and defoaming systems;

⑦ Controlling curing reaction rate;

⑧ Establishing an application viscosity window according to the spraying method.

The common feature of powder, waterborne, and high-solids systems is that they can reduce VOC, but they also require more precise resin design, formulation control, and application management.

7. Limitations in Metal Corrosion Protection Applications: Polyester Is More Suitable as a Topcoat or as Part of a Coating System

Polyester coatings can be used to protect metal products. However, in severe corrosive environments, a single-layer polyester coating film usually cannot provide complete heavy-duty corrosion protection. Polyester is more suitable as a decorative layer, a weather-resistant layer, or a component of a composite system, rather than independently providing adhesion, rust prevention, barrier protection, corrosion inhibition, and chemical resistance in all corrosive scenarios.

Organic coatings are not completely impermeable. Water, oxygen, and ions may pass through the coating or enter the coating/metal interface through defects, thereby causing under-film corrosion. Common failures of polyester coatings on metal substrates include edge rusting, corrosion creep from scratches, under-film corrosion, blistering, adhesion loss, peeling after salt spray exposure, and premature failure at welds, holes, and sharp edges. These problems are often related to the following factors:

Cause	Impact
Insufficient pretreatment	Weak interfacial bonding; blistering and peeling are likely after water ingress
Insufficient edge film thickness	Sharp edges, welds, and holes become corrosion initiation sites
Insufficient anticorrosive performance of primer	Difficult to prevent corrosion spread after moisture vapor and ions enter
Pinholes or craters in the coating film	Localized barrier failure
Insufficient curing	Reduced coating film density and chemical resistance
Damage not repaired	Accelerated corrosion spread from scratches
Service environment too severe	Exceeds the design capability of a single-layer polyester coating

When used for metal protection, polyester coatings should be considered as part of a complete corrosion protection system. A more reliable design usually includes:

① Appropriate degreasing, phosphating, chromating, zirconium treatment, silane treatment, or other pretreatments;

② A primer with anticorrosive capability;

③ Sufficient dry film thickness;

④ Protection of edges, welds, holes, and cut edges;

⑤ A dense, pinhole-free topcoat;

⑥ A multi-coat system;

⑦ Scribed salt spray, cyclic corrosion, and adhesion testing after aging.

8. Application Verification: From Single Tests to Failure-Scenario Verification

Failure of polyester coatings does not necessarily come from the resin itself. It may also come from the formulation, application process, substrate, curing, or service environment. Reliable evaluation should not focus only on initial performance, but also on performance retention after aging and real service scenarios.

8.1 Post-Aging Performance Should Be Emphasized

When evaluating polyester coatings, the following items should be emphasized:

Test Item	Evaluation Significance
Adhesion after aging	Determines whether the interface remains stable
Impact performance after aging	Determines whether the coating film has become brittle
Bending performance after aging	Determines reliability after forming, thermal cycling, or long-term use
Water resistance after damp heat exposure	Determines coating film integrity after moisture vapor exposure
Appearance and adhesion after chemical exposure	Determines performance retention after contact with media
Gloss retention and color difference	Determines the degree of outdoor aging
Chalking grade	Determines the degree of surface resin degradation
Scratch corrosion creep	Determines corrosion protection capability after damage
MEK, hardness, and degree of curing	Determines whether curing is sufficient
Film thickness and edge coverage	Determines barrier performance and the risk of weak areas

8.2 Composite Environmental Verification Is Closer to Real Failure

Real service environments are usually not affected by a single factor. Outdoor metal parts may be exposed simultaneously to ultraviolet light, moisture vapor, salts, temperature variation, pollutants, and mechanical damage during use. Therefore, salt spray testing alone, damp heat testing alone, or aging testing alone usually reflects only one type of risk and cannot fully represent the actual failure process.

Verification of polyester coatings should combine test conditions according to the service scenario. For example, outdoor weatherability should focus on ultraviolet light and condensation cycling. Metal corrosion protection should focus on salt spray, wet-dry cycling, and corrosion creep from scratches. Formed parts should focus on adhesion and corrosion resistance after bending, impact, or forming. Areas that are cleaned frequently should include evaluation of appearance, gloss, and adhesion after contact with cleaning agents.

When evaluating polyester coatings, the focus should shift from single initial performance tests to “performance retention after aging” and “failure verification under composite environments.” Key results to observe include gloss retention, color difference, chalking, blistering, adhesion loss, scratch creep, and edge corrosion.

8.3 Application Process Data Are Key Evidence for Determining Failure Causes

Many failures of polyester coatings are not caused by incorrect resin selection, but by insufficient application process control. For example, insufficient curing reduces the density and chemical resistance of the coating film, insufficient film thickness weakens barrier performance, poor pretreatment leads to adhesion loss after damp heat or salt spray exposure, and insufficient edge coverage increases the risk of localized corrosion.

During failure analysis, key process data should be recorded and retained, including baking oven temperature profile, actual workpiece temperature, film thickness distribution, edge coverage, pretreatment quality, powder particle size distribution, pH value and conductivity of waterborne coatings, application viscosity, temperature and humidity of the spraying environment, as well as post-curing results for hardness, methyl ethyl ketone (MEK) rub test, impact, bending, and adhesion.

9. Classified Application Tables of Representative Chemicals Related to Polyester Coating Limitations, Failure Mechanisms, and Improvement Directions

Note: The following products are mainly intended as references for polyester coating structure design, failure mechanism studies, formulation screening, or testing and evaluation. For actual industrial applications, the target system, regulatory requirements, SDS/COA, compatibility, dispersibility, migration risk, and long-term performance verification should also be considered.

Table 1. Polyester Resin Structure Design and Bio-Based Monomers

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Polyester structural monomer	504-63-2	P432773	1,3-Propanediol	Suitable for synthesis	Diol monomer; used for flexible segment design of polyester resins, hydroxyl value regulation, and screening of hydrolysis-resistant structures
Bio-based polyol source	8001-79-4	C434218	Castor oil	European Pharmacopoeia (Ph.Eur)	Hydroxyl-containing natural oil; used in bio-based polyester modification, flexibility regulation, and research on introducing hydrophobic segments
Unsaturated dicarboxylic acid monomer	97-65-4	I106140	Itaconic acid	Chemically pure (CP), ≥99%	Double-bond-containing dicarboxylic acid; used in unsaturated polyester synthesis, functional modification, and studies on crosslinking reaction activity
Long-chain aliphatic dicarboxylic acid	111-20-6	S108452	Sebacic acid	Chemically pure (CP), ≥98%	Long-chain dicarboxylic acid; used to regulate polyester flexibility, hydrophobicity, low-temperature impact performance, and chemical resistance
Aliphatic dicarboxylic acid	110-15-6	S431422	Succinic acid	PharmPure™, ChP, JP, ACS, NF, crystalline	Dicarboxylic acid monomer; used in aliphatic polyester synthesis, acid value regulation, and sustainable polyester structure design
Hydroxy acid monomer	50-21-5	L108839	DL-Lactic acid	AR, 85–90%	Hydroxycarboxylic acid; used in polyester segment construction, degradable polyester research, and evaluation of ester bond hydrolysis sensitivity
Bio-based rigid diol	652-67-5	I157515	Isosorbide	≥98% (GC)	Rigid diol; used in bio-based polyester synthesis, hardness and glass transition temperature regulation, and heat-resistant structure design
Bio-based aromatic dicarboxylic acid	3238-40-2	F119129	2,5-Furandicarboxylic acid (FDCA)	≥98%	Furan dicarboxylic acid; used in bio-based polyester synthesis and research on barrier properties, hardness, and heat resistance
Unsaturated anhydride monomer	2170-03-8	I132081	Itaconic anhydride (ITA)	≥95%	Anhydride-type functional monomer; used in polyester functionalization, introduction of crosslinking sites, and regulation of reaction activity

Table 2. Monomers and Additives Related to Waterborne Polyester Systems: Dispersion, Film Formation, and Storage Stability

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Waterborne film formation and rheology control	25322-68-3	P615493	Polyethylene oxide	Viscosity 65–115 cps	Water-soluble polyether; used in waterborne polyester film formation, rheology control, and studies on moisture diffusion and hydrophilicity effects
Storage stability of waterborne systems	532-32-1	S104128	Sodium benzoate	Pharmaceutical grade, PharmPure™	Aqueous-phase preservative component; used for storage stability, antimicrobial preservation, and formulation stability evaluation of waterborne polyester systems
Neutralizing agent and acid-base adjustment	121-44-8	T140677	Triethylamine	Anhydrous, ≥99.5%, water ≤50 ppm	Organic amine neutralizing agent; used for neutralization of carboxyl-functional polyester, acid value adjustment, and preparation of waterborne dispersion systems
Neutralizing agent for waterborne resins	108-01-0	D109080	N,N-Dimethylethanolamine (DMEA)	Refined grade, ≥99.5%	Tertiary amine neutralizing agent; used in waterborne polyester dispersion, acid-base balance, and studies on post-drying water resistance
Internal emulsifying monomer for waterborne systems	4767-03-7	B104539	2,2-Bis(hydroxymethyl)propionic acid (DMPA)	≥98%	Carboxyl-containing diol; used in internal emulsification of waterborne polyester and waterborne polyurethane systems, and in studies on the balance between hydrophilicity and water resistance
Sulfonate-type waterborne monomer	6362-79-4	S115342	Sodium 5-sulfoisophthalate (5-SSIPA)	≥98%	Sulfonate dicarboxylic acid; used in water-dispersible polyester synthesis and research on ionic stability, water resistance, and dispersibility
Sulfonate-type polyester monomer	3965-55-7	D101418	Dimethyl 5-sulfoisophthalate sodium salt	≥98%	Sulfonate dimethyl ester monomer; used in waterborne polyester copolymerization, dispersion stability, and regulation of hydrophilic group content

Table 3. Chemicals Related to Curing Control and Failure Testing of Polyester Coatings

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Solvent for curing degree evaluation	78-93-3	B1506362	Methyl ethyl ketone (regulated precursor chemical)	For HPLC, ≥99.7%	Solvent for resistance testing; used to evaluate curing degree, crosslinking completeness, and rub resistance of polyester coating films
Test medium for alkali-induced failure	1310-73-2	S431793	Sodium hydroxide	Anhydrous, ≥98%, pellets	Strong alkali reagent; used in research on alkaline hydrolysis, saponification, alkali immersion resistance, and adhesion decay of polyester coating films
Phase transfer and reaction control	1643-19-2	T103374	Tetrabutylammonium bromide	Ion-pair chromatography grade, ≥99%	Quaternary ammonium salt reagent; used in polyester functionalization reactions, phase-transfer catalysis, and studies on the impact of ionic residues
Test medium for acid-induced failure	7647-01-0	H399545	Hydrochloric acid (regulated precursor chemical)	ACS, ≥37%	Strong acid reagent; used in control experiments on acid-catalyzed hydrolysis, acid immersion resistance, and medium-induced failure of polyester coating films
Powder degassing additive	119-53-9	B100919	Benzoin	≥99.5%	Degassing component for powder coatings; used in studies on pinhole control, thick-film degassing, and melt-flow defects
Curing catalyst	693-98-1	M104839	2-Methylimidazole	≥98%	Imidazole catalyst; used in polyester composite curing, gel time control, and low-temperature curing reaction window studies
Curing agent for carboxyl-functional polyester	6334-25-4	N158980	N,N,N',N'-Tetrakis(2-hydroxyethyl)adipamide	≥97%	Hydroxyalkylamide curing agent; used in polyester powder curing, TGIC-free systems, and thick-film pinhole evaluation

Table 4. Chemicals Related to Metal Corrosion Protection System Design and Corrosion Failure Evaluation for Polyester Coatings

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Flash rust inhibition and corrosion inhibition	7632-00-0	S433709	Sodium nitrite	Anhydrous, ACS, ≥97%	Nitrite corrosion inhibitor; used in flash rust control for waterborne polyester systems and initial corrosion inhibition research on steel substrates
Anticorrosive pigment	13939-25-8	A302801	Aluminum tripolyphosphate	P₂O₅ content 60–70%	Phosphate anticorrosive pigment; used in polyester primer corrosion protection, under-film corrosion inhibition, and salt spray performance evaluation
Corrosive medium	7647-14-5	C111533	Sodium chloride	AR, ≥99.5%	Chloride salt medium; used in salt spray testing, immersion corrosion, scratch creep, and edge corrosion evaluation
Anticorrosive pigment	7779-90-0	Z112909	Zinc phosphate hydrate	AR, ≥99%	Zinc phosphate pigment; used in metal primer corrosion protection, interfacial stability, and adhesion studies after salt spray exposure
Corrosion inhibition and flash rust control	10102-40-6	S104867	Sodium molybdate dihydrate	AR, ≥99%	Molybdate corrosion inhibitor; used in flash rust inhibition in waterborne coatings and corrosion control at metal interfaces
Metal corrosion inhibitor	95-14-7	B101002	Benzotriazole	≥99%	Triazole corrosion inhibitor; used in metal interface protection, anti-tarnish protection of copper and alloys, and under-coating corrosion research

Table 5. Chemicals Related to Weathering Stabilization and Thermal-Oxidative Aging Control of Polyester Coatings

Category	CAS No.	Aladdin Catalog No.	Name	Specification or Purity	Product Features and Applications
Phenolic antioxidant	128-37-0	D104363	2,6-Di-tert-butyl-p-cresol (BHT)	Chemically pure (CP)	Hindered phenolic antioxidant; used in research on thermal-oxidative aging inhibition, yellowing control, and storage stability of polyester resins
Hindered amine light stabilizer	129757-67-1	B166874	Bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate	Monomer ≥65%	Low-basicity hindered amine; used in polyester coating films for photo-oxidation inhibition, chalking delay, and outdoor weatherability evaluation
Benzotriazole UV absorber	2440-22-4	H157228	2-(2-Hydroxy-5-methylphenyl)benzotriazole	≥99%	UV absorber; used in studies on gloss loss, discoloration, photo-aging, and surface chalking of polyester coating films
Benzotriazole UV absorber	70321-86-7	H157227	2-(2H-Benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol	≥98% (HPLC)	High-molecular-weight UV absorber; used in polyester topcoat weatherability, low-migration light stabilization, and color difference control studies
Benzotriazole UV absorber	3896-11-5	C153529	2-(5-Chloro-2-benzotriazolyl)-6-tert-butyl-p-cresol	≥98% (HPLC)	Benzotriazole light-stabilizing component; used in the evaluation of UV aging, gloss retention, and yellowing of polyester coatings
Liquid UV absorber	104810-48-2	H302135	Poly(ethylene glycol) 300 ester of 3-[3-(2H-benzotriazol-2-yl)-4-hydroxy-5-tert-butylphenyl]propionic acid	≥98%	Liquid light-stabilizing component; used in weatherability, compatibility, and migration studies of waterborne and high-solids polyester systems
Composite light stabilizer	63843-89-0	B304248	Bis(1,2,2,6,6-pentamethyl-4-piperidyl) [[3,5-di-tert-butyl-4-hydroxyphenyl]methyl]butylmalonate	≥98%	Component containing hindered amine and phenolic structures; used in studies on synergistic photo-oxidative stabilization and weathering aging of polyester coatings
Phosphite antioxidant	31570-04-4	T161948	Tris(2,4-di-tert-butylphenyl) phosphite	≥98%	Secondary antioxidant; used in polyester processing thermal stability, peroxide decomposition, and yellowing control studies
Hindered amine light stabilizer	52829-07-9	B102211	Bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate	≥98%	Hindered amine light stabilizer; used in UV aging, chalking delay, and outdoor exposure evaluation of polyester coating films
Phenolic antioxidant	6683-19-8	P473547	Pentaerythritol tetrakis(3,5-di-tert-butyl-4-hydroxyhydrocinnamate)	≥98%	High-molecular-weight hindered phenolic antioxidant; used in research on thermal-oxidative stability and baking yellowing control of polyester resins
Phenolic antioxidant	2082-79-3	I106561	Antioxidant 1076	≥98%	Hindered phenolic antioxidant; used in thermal-oxidative aging, storage stability, and processing stability studies of polyester resins
Hindered amine light stabilizer	41556-26-7	B134649	Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate	≥95% (GC), sum of monoester and diester	Liquid hindered amine light stabilizer; used in polyester topcoat weatherability, gloss retention, and low-temperature compatibility studies
Triazine UV absorber	153519-44-9	U302981	UV Absorber UV400	≥85%	Triazine light-stabilizing component; used in long-term weatherability, color difference control, and gloss retention evaluation of polyester coating films
Liquid UV absorber	104810-47-1	H302134	UV Absorber 1130	≥84% (HPLC)	Liquid benzotriazole light-stabilizing component; used in weatherability and compatibility evaluation of waterborne and solventborne polyester systems

Note: The above are representative Aladdin products. For more product specifications, search by “product name/CAS/catalog number” on the Aladdin official website.

References

[1] Wicks, Z. W.; Jones, F. N.; Pappas, S. P.; Wicks, D. A. Organic Coatings: Science and Technology. 3rd ed. Wiley, 2007.

[2] Koleske, J. V., ed. Paint and Coating Testing Manual: 15th Edition of the Gardner-Sward Handbook. ASTM International, 2012.

[3] Jones, F. N. “Alkyd Resins and Polyester Resins.” Journal of Coatings Technology, 1995.

[4] Hydrolytic Stability of Unsaturated Polyesters. ScienceDirect chapter.

[5] Sabet-Bokati, K.; Plucknett, K. “Water-induced failure in polymer coatings: Mechanisms, impacts and mitigation strategies.” Polymer Degradation and Stability, 2024.

[6] Allnex. “TGIC Powder Coating and HAA Polyester Resins.”

[7] Powder Coated Tough. “Polyester Powder Coatings: TGIC vs. HAA.”

[8] Pieters, K.; Mekonnen, T. H. “Progress in waterborne polymer dispersions for coating applications: commercialized systems and new trends.” RSC Sustainability, 2024.

[9] Corrosion Under Organic Coatings. Springer Nature Link.

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A Panorama Guide to Synthetic Resins: Definitions & Polymerization Mechanisms, Classification Frameworks, Common Resins and Applications, Packaging Codes, and a Selection Roadmap (Tables 1–3)