Film Formation and Curing Mechanisms of Polyester Resins for Coatings: From Functional Groups and Curing Systems to Crosslinked Networks

1. The Key to Determining the Film-Formation Route of Polyester Resins Is the Reaction Pathway

Polyester resins used in coatings are not a single, uniform category. Different polyester resins vary in resin structure, reactive functional groups, curing-agent type, application method, and final coating-film structure.

In coating formulation, determining how a polyester resin forms a film should begin with three key questions:

① What reactive functional groups are present on the resin?

② What type of curing agent does it need to react with?

③ Does the final coating film mainly rely on physical film formation, or on chemical crosslinking and curing?

In high-performance industrial coatings, coil coatings, powder coatings, and automotive coatings, polyester resins usually do not simply form films by leaving a resin layer after solvent or water evaporation. Instead, reactive structures such as hydroxyl groups, carboxyl groups, or carbon-carbon double bonds react with curing agents to form a three-dimensional crosslinked network. Compared with uncrosslinked resin films, crosslinked coating films generally offer better solvent resistance, chemical resistance, hardness, heat resistance, and mechanical strength.

2. Classification Logic of Polyester Resins for Coatings

Polyester resins for coatings can be classified from multiple perspectives. These classifications are not necessarily on the same hierarchical level, nor are they mutually exclusive.

Classification Perspective	Representative Types	Key Point for Evaluation
By reactive functional group	Hydroxyl polyester, carboxyl polyester	What type of curing agent it reacts with
By reactive structure in the main chain	Saturated polyester, unsaturated polyester	Whether it contains carbon-carbon double bonds capable of free-radical polymerization
By modified structure	Alkyd resin, polyester acrylate	Whether it is modified with fatty acids, acrylates, or other structures
By application form	Solventborne polyester, waterborne polyester, powder polyester	How the resin is dispersed, applied, and formed into a film
By curing method	Amino curing, isocyanate curing, epoxy curing, TGIC curing, HAA curing, oxidative drying, UV curing	The specific film-forming reaction pathway

For example, a powder polyester can be both a saturated polyester and a carboxyl polyester, and it can react with triglycidyl isocyanurate — TGIC, also known as tris-glycidyl isocyanurate — or β-hydroxyalkylamide (HAA) to form a thermoset powder coating film. A waterborne polyester can also be a hydroxyl polyester and can crosslink with an amino resin under baking conditions.

3. Thermoplastic Film Formation and Thermosetting Film Formation

To understand how polyester resins form films, it is necessary to distinguish between thermoplastic film formation and thermosetting film formation.

3.1 Thermoplastic Film Formation

Thermoplastic film formation mainly relies on solvent evaporation, water evaporation, or melt cooling to form a continuous coating film. During film formation, resin molecules usually do not undergo significant chemical crosslinking. Thermoplastic film formation has the following characteristics:

① The film-forming process is mainly a physical change;

② The coating film may soften again when heated;

③ The coating film may redissolve or swell when exposed to a suitable solvent;

④ Solvent resistance, heat resistance, and chemical resistance are usually lower than those of fully crosslinked thermoset coating films;

⑤ Application and film formation are relatively simple, but use in high-performance industrial coatings is limited.

3.2 Thermosetting Film Formation

Thermosetting film formation relies on chemical reactions between the resin and the curing agent to form a three-dimensional crosslinked network. After curing is complete, the coating film usually no longer melts and is not easily redissolved by the original solvent. Thermosetting film formation has the following characteristics:

① The film-forming process involves chemical reactions;

② The degree of curing directly affects final performance;

③ Solvent resistance, chemical resistance, heat resistance, and mechanical strength are usually better;

④ The amount of curing agent, curing temperature, curing time, catalyst, film thickness, and application conditions must be controlled.

Most high-performance polyester coatings are thermosetting systems, such as hydroxyl polyester/amino resin systems, hydroxyl polyester/isocyanate systems, carboxyl polyester/TGIC systems, carboxyl polyester/HAA systems, and polyurethane powder systems.

4. Saturated Polyesters: Hydroxyl Polyesters and Carboxyl Polyesters Are Core Types

In coating applications, saturated polyesters generally refer to polyester resins whose main chains do not contain aliphatic carbon-carbon double bonds, such as those from maleic acid or fumaric acid, that can participate in free-radical copolymerization. Saturated polyesters are usually prepared by polycondensation of polyacids and polyols. They rely on functional groups such as hydroxyl and carboxyl groups in the molecule to react with curing agents and form thermoset coating films. Saturated polyesters are widely used in powder coatings, coil coatings, industrial baking finishes, packaging coatings, metal decorative coatings, automotive coatings, and transportation coatings.

The performance of saturated polyesters can be adjusted through the acid component, alcohol component, molecular weight, glass transition temperature, hydroxyl value, acid value, and degree of branching. Among these parameters, hydroxyl value and acid value are particularly important indicators in formulation design.

A hydroxyl polyester is a polyester resin containing hydroxyl groups, expressed as —OH. Hydroxyl groups allow the polyester to react with amino resins, isocyanates, or blocked isocyanates to form a crosslinked coating film.

A carboxyl polyester is a polyester resin containing carboxyl groups, expressed as —COOH. Carboxyl groups can react with epoxy-functional curing agents, such as epoxy resins or TGIC, and can also be cured with HAA through esterification crosslinking to form a thermoset coating film.

Hydroxyl polyesters and carboxyl polyesters are two important categories of saturated polyesters used in coatings. Hydroxyl polyesters are commonly used in polyester-amino baking finishes, polyester-polyurethane coatings, and polyurethane powder coatings. Carboxyl polyesters are especially important in powder coatings and are commonly used in polyester/epoxy hybrid powders, TGIC outdoor powders, and HAA outdoor powders.

Type	Main Functional Group	Typical Curing Agents	Common Applications
Hydroxyl polyester	Hydroxyl —OH	Amino resin, isocyanate, blocked isocyanate	Industrial baking finishes, coil coatings, polyurethane coatings, polyurethane powders
Carboxyl polyester	Carboxyl —COOH	Epoxy resin, TGIC, HAA	Powder coatings, industrial coatings, some coil-coating systems

5. Main Curing Systems for Hydroxyl Polyesters

The core feature of hydroxyl polyesters is the reactivity of hydroxyl groups. Hydroxyl groups can react with amino resins, isocyanates, or blocked isocyanates to form different types of crosslinked structures. Different curing systems have different reaction conditions, coating-film properties, and application requirements.

5.1 Curing of Hydroxyl Polyesters with Amino Resins

Amino resins are common thermosetting curing agents in baking coatings. They mainly include melamine-formaldehyde resins, urea-formaldehyde resins, and benzoguanamine resins. Among them, melamine-formaldehyde resins are commonly used in industrial baking finishes, coil coatings, and automotive coatings.

Under heating and acid-catalyzed conditions, hydroxyl polyesters and alkyl-etherified amino resins usually crosslink mainly through transetherification between hydroxyl groups and alkoxymethyl groups in the amino resin. Competing reactions, such as self-condensation of the amino resin, may also occur, ultimately forming a crosslinked network.

This system can be simplified as follows:

Hydroxyl polyester + amino resin + heat + acid catalyst → crosslinked coating film

The hydroxyl polyester/amino resin system has the following characteristics:

① Suitable for baking coatings;

② Relatively fast curing speed;

③ Good coating-film hardness, gloss, and leveling;

④ Good solvent resistance and chemical resistance;

⑤ Suitable for continuous, high-speed coating processes;

⑥ Sensitive to baking temperature, baking time, and catalyst.

This system is commonly used in coil coatings. Coil-coating production requires rapid coating, rapid baking, and stable film formation. Therefore, resin reactivity, amino resin type, acid catalyst, and baking conditions are critical. Insufficient crosslinking can lead to low hardness, poor solvent resistance, and inadequate chemical resistance. Excessive crosslinking or improper baking conditions may cause the coating film to become brittle, crack during bending, or lose post-forming performance.

5.2 Curing of Hydroxyl Polyesters with Isocyanates

The isocyanate group is usually represented as —NCO. Hydroxyl polyesters can react with isocyanates to form urethane bonds, thereby forming polyurethane structures. The basic reaction can be expressed as:

Hydroxyl —OH + isocyanate —NCO → urethane bond

Hydroxyl polyester/isocyanate systems are commonly used in two-component polyurethane coatings. Before application, the hydroxyl polyester base component is mixed with the isocyanate curing agent, after which crosslinking occurs. This system has the following characteristics:

① Can cure at room temperature or at low-to-medium temperatures;

② Provides good film fullness and build;

③ Offers good adhesion, abrasion resistance, and mechanical properties;

④ Provides good chemical resistance;

⑤ Suitable for industrial coatings, wood coatings, automotive refinish coatings, and high-performance topcoats.

For this system, pot life must be carefully controlled. After the base component and curing agent are mixed, the system continues to react and the viscosity gradually increases. Once the pot life is exceeded, leveling, appearance, and coating-film performance may be affected. In addition, isocyanates are sensitive to moisture. Moisture may react with isocyanates as a side reaction and release carbon dioxide, leading to bubbles, pinholes, or reduced performance. Therefore, during application and storage, environmental humidity, solvent water content, and moisture on the substrate surface must be controlled.

5.3 Curing of Hydroxyl Polyesters with Blocked Isocyanates

Blocked isocyanates temporarily block the isocyanate groups, making them less likely to react with hydroxyl groups at room temperature. When heated to a certain temperature, the blocking agent dissociates and releases active isocyanate groups, which then react with hydroxyl polyesters to form polyurethane crosslinked structures. This process can be simplified as follows:

Blocked isocyanate + heat → active isocyanate → reaction with hydroxyl polyester to form a film

Blocked isocyanates are suitable for one-component baking systems and are also commonly used in polyurethane powder coatings. This system has the following characteristics:

① Good storage stability;

② Suitable for one-component baking coatings;

③ Forms polyurethane structures after curing;

④ Provides good leveling, abrasion resistance, chemical resistance, and mechanical properties;

⑤ Requires reaching the deblocking temperature and maintaining sufficient curing time.

Polyurethane powder coatings usually use hydroxyl polyesters in combination with blocked isocyanates or uretdione curing agents. Conventional blocked isocyanates usually involve deblocking and release of the blocking agent. Uretdione curing agents are often regarded as internally blocked isocyanate systems; upon heating, they can release isocyanate groups and react with hydroxyl resins. Because they usually do not rely on the release of an external blocking agent, the risks of odor and pinholes caused by volatile blocking agents are generally lower than those of conventional blocked isocyanate systems.

The film-forming process includes powder melting, leveling, curing-agent deblocking or reaction activation, reaction between hydroxyl groups and isocyanate groups, and formation of a crosslinked network. This system requires attention to deblocking temperature, volatile release, application odor, pinhole risk, and baking conditions. If used for outdoor coatings, the effect of the isocyanate structure on weatherability and yellowing performance should also be considered.

Curing System	Main Reaction	Film-Formation Characteristics	Main Points of Attention
Hydroxyl polyester/amino resin	Crosslinking between hydroxyl groups and amino resin	Baking cure, good hardness and appearance	Catalyst, baking conditions, flexibility
Hydroxyl polyester/isocyanate	Reaction between hydroxyl groups and isocyanate groups	Polyurethane structure, good mechanical properties	Pot life, moisture sensitivity, mixing ratio
Hydroxyl polyester/blocked isocyanate	Reaction between hydroxyl groups and deblocked isocyanate groups	One-component baking or powder systems	Deblocking temperature, volatiles, completeness of cure

6. Curing Systems of Carboxyl Polyesters in Powder Coatings

Carboxyl polyesters are very important in powder coatings. Powder coatings are usually composed of solid resins, curing agents, pigments and fillers, and additives. They are manufactured into powder through melt extrusion, cooling, grinding, and sieving. During application, powder particles adhere to the substrate surface through electrostatic spraying or similar methods, and then undergo melting, leveling, and crosslinking cure upon heating.

The film-forming process of polyester powder coatings usually includes four stages:

① Thermal softening: after powder particles enter the oven, the resin gradually softens;

② Melt coalescence: powder particles fuse with each other and form a continuous molten film;

③ Leveling and wetting: the molten resin flows, wets the substrate, and forms a relatively smooth surface;

④ Crosslinking cure: resin functional groups react with the curing agent to form an insoluble and infusible thermoset coating film.

The film quality of powder polyesters depends not only on the chemical reaction, but also on melt viscosity, gel time, leveling time, curing temperature, film thickness, pigment and filler dispersion, degassing performance, and substrate temperature.

6.1 Curing of Carboxyl Polyesters with Epoxy Resins

Carboxyl polyesters can react with epoxy groups in epoxy resins to form crosslinked structures. This reaction is an important basis for polyester/epoxy hybrid powder coatings. This system can be simplified as follows:

Carboxyl polyester + epoxy resin → crosslinked coating film

Polyester/epoxy hybrid powder coatings are commonly used for indoor applications, such as household appliances, office furniture, metal shelving, instrument housings, and general metal products. Their advantages include a good balance of leveling, mechanical properties, decorative appearance, and cost. Because epoxy components have limited outdoor weatherability, this system is usually not the preferred powder coating for long-term outdoor exposure.

6.2 Curing of Carboxyl Polyesters with TGIC

TGIC, or triglycidyl isocyanurate, contains multiple epoxy groups and can react with carboxyl polyesters to form a crosslinked network. The carboxyl polyester/TGIC system is one of the classic systems for outdoor weather-resistant powder coatings. This system can be simplified as follows:

Carboxyl polyester + TGIC → crosslinked coating film

Carboxyl polyester/TGIC systems usually have the following characteristics:

① Good outdoor weatherability;

② Good mechanical properties;

③ Good leveling and surface appearance;

④ Mature curing process;

⑤ Suitable for architectural aluminum profiles, metal curtain walls, outdoor facilities, traffic guardrails, agricultural machinery, and outdoor metal products.

For TGIC systems, close attention should be paid to polyester acid value, TGIC dosage, curing temperature, curing time, leveling window, weatherability of pigments and fillers, and safety and regulatory requirements. TGIC-free refers to powder-curing systems that do not use TGIC. A common representative is the HAA/Primid-type system, but TGIC-free systems are not limited to HAA. Specific performance still depends on resin structure, curing-agent type, and formulation design.

6.3 Curing of Carboxyl Polyesters with HAA

HAA is β-hydroxyalkylamide. It is a common TGIC-free curing agent for carboxyl polyester powder coatings. Commercial HAA curing agents are also often referred to as Primid-type curing agents. The core reaction between HAA and carboxyl polyester is esterification crosslinking, and water molecules are released during curing. This process can be simplified as follows:

Carboxyl polyester + HAA + heat → crosslinked coating film + water

Because water is released during curing, HAA systems are more prone to pinholes, bubbles, or surface appearance problems under thick-film application, rapid heating, low-temperature curing, or insufficient degassing conditions. Therefore, HAA systems require careful control of film thickness, heating rate, leveling window, degassing additives, and curing conditions. HAA polyester powder coatings usually have the following characteristics:

① TGIC-free;

② Good outdoor weatherability;

③ Good mechanical properties;

④ Suitable for architectural and general outdoor metal coating;

⑤ Compared with TGIC, HAA/Primid-type systems usually offer advantages in toxicological risk profile and regulatory compliance; however, specific safety, use restrictions, and compliance requirements should still be determined based on the product SDS and the regulations of the target market;

⑥ Pinholes and degassing require close attention during thick-film application.

6.4 Comparison of Common Powder Polyester Systems

Powder System	Resin Type	Curing Agent	Film-Formation Characteristics	Main Applications
Polyester/epoxy hybrid	Carboxyl polyester	Epoxy resin	Good leveling and decorative appearance, mainly for indoor use	Appliances, furniture, metal products
Pure polyester TGIC type	Carboxyl polyester	TGIC	Good outdoor weatherability, mature system	Architectural aluminum profiles, outdoor metal parts
Pure polyester HAA type	Carboxyl polyester	HAA	TGIC-free, releases water during curing	Architectural and general outdoor powders
Polyurethane powder	Hydroxyl polyester	Blocked isocyanate or uretdione curing agent	Good appearance, leveling, and mechanical properties	High-appearance, high-performance powder coatings

Both TGIC and HAA can be used in outdoor weather-resistant powder coatings, but they differ in reaction by-products, leveling window, thick-film application, surface appearance, regulatory requirements, and formulation tolerance.

Item	TGIC System	HAA System
Curing substrate	Carboxyl polyester	Carboxyl polyester
Main reaction	Reaction between carboxyl groups and epoxy groups	Esterification crosslinking between carboxyl groups and HAA
TGIC-free?	No	Yes
Release of small molecules	Usually not characterized by water release	Releases water during curing
Appearance control	Mature leveling and surface control	Thick films and degassing require close attention
Outdoor application	Mature application	Widely used
Regulatory considerations	TGIC safety and hazard-labeling requirements must be considered	Relatively more favorable

7. Alkyd Resins: Oxidative Film Formation of Typical Air-Drying Polyesters

Alkyd resins are fatty-acid- or oil-modified polyesters. Unlike conventional saturated polyesters, typical air-drying alkyd resins form films through autoxidative chain reactions involving oxygen from the air.

Unsaturated fatty acid segments in alkyd resins undergo autoxidation in air to form hydroperoxides, which then further generate crosslinked structures through free-radical reactions. This process usually requires promotion by metal driers, such as metal carboxylate driers based on cobalt, manganese, iron, zirconium, and other metals. The process can be simplified as follows:

Alkyd resin + oxygen from air + drier → oxidatively crosslinked coating film

Typical air-drying alkyd resins have the following characteristics:

① Can dry at room temperature;

② Good application tolerance;

③ Good wetting of substrates;

④ Good coating-film fullness;

⑤ Relatively moderate cost;

⑥ Drying speed is affected by oxygen, temperature, humidity, film thickness, and driers.

Alkyd resins are commonly used in architectural coatings, wood coatings, metal protective coatings, and general industrial coatings.

It should be noted that oxidative air drying mainly applies to air-drying alkyd resins. Some short-oil or non-drying alkyd resins can also be crosslinked with amino resins, isocyanates, and other curing agents for baking or two-component systems. Therefore, not all alkyd resins should be simply understood as air-oxidation-drying resins.

Item	Typical Air-Drying Alkyd Resin	Conventional Thermosetting Saturated Polyester
Structural characteristics	Fatty-acid- or oil-modified polyester	Mainly polycondensation of polyacids and polyols
Film-formation method	Oxidative crosslinking in air	Crosslinking reaction with curing agent
Oxygen dependence	Usually dependent on oxygen	Usually not dependent on oxygen
Common applications	Architectural coatings, wood coatings, metal protection	Powder coatings, coil coatings, industrial baking finishes
Main control factors	Drier, oxygen, film thickness, temperature and humidity	Curing agent, temperature, time, mixing ratio

8. Unsaturated Polyesters and Polyester Acrylates: Free-Radical Curing of Double Bonds

Both unsaturated polyesters and polyester acrylates rely on carbon-carbon double bonds to participate in free-radical polymerization and form crosslinked structures. However, they differ in resin structure, initiation method, and application scenarios.

8.1 Unsaturated Polyesters

Unsaturated polyesters are polyester resins containing polymerizable carbon-carbon double bonds. They are usually prepared by polycondensation of unsaturated dibasic acids or anhydrides with polyols. Common sources of unsaturated structures include maleic anhydride and fumaric acid.

Unsaturated polyesters are usually used together with reactive diluents. Under the action of initiators and promoters, they undergo free-radical polymerization to form a three-dimensional crosslinked network. In traditional unsaturated polyester systems, styrene is commonly used as a reactive diluent. However, with increasing environmental and occupational health requirements, low-styrene, styrene-free, and low-VOC systems are also being developed. This process can be simplified as follows:

Unsaturated polyester + reactive diluent + initiator → free-radically crosslinked coating film

Unsaturated polyester systems have the following characteristics:

① Fast curing speed;

② Can form relatively high crosslink density;

③ Good coating-film hardness and chemical resistance;

④ Suitable for thick-film or special protective systems;

⑤ Application odor, volume shrinkage, and curing exotherm must be controlled;

⑥ Sensitive to oxygen inhibition, initiator dosage, and formulation stability.

Unsaturated polyesters are valuable in composite surface coatings, gel coats, special thick-film systems, and some industrial protective systems.

8.2 Polyester Acrylates

Polyester acrylates are a class of resins in which acrylate double bonds are introduced into the polyester structure. They are commonly used in radiation-curable coatings.

UV curing systems are usually composed of oligomers, reactive diluents, photoinitiators, and additives. After absorbing UV energy, the photoinitiator generates active free radicals, which initiate polymerization of acrylate double bonds and rapidly form a crosslinked coating film. The curing process includes:

① The photoinitiator absorbs UV energy;

② The photoinitiator generates active free radicals;

③ Free radicals initiate polymerization of acrylate double bonds;

④ Oligomers and reactive diluents crosslink together;

⑤ The coating film rapidly changes from a liquid state to a solid state.

This process can be simplified as follows:

Polyester acrylate + photoinitiator + UV energy → rapidly crosslinked coating film

Polyester acrylate UV-curable systems have the following characteristics: fast curing speed; relatively low energy consumption; suitability for continuous production; ability to produce high-gloss, high-hardness coating films; potential for lower VOC emissions; and sensitivity to light penetration, pigment hiding power, film thickness, and oxygen inhibition.

This article discusses acrylate-based free-radical UV curing. In the field of UV curing, cationic curing systems and hybrid free-radical/cationic systems also exist.

The key to UV curing is not “drying” in the traditional sense, but photoinitiated polymerization. Full curing can be achieved only when light intensity, photoinitiator type and dosage, double-bond concentration, film thickness, and formulation transparency are properly matched. For colored systems, thick-film systems, or systems with high levels of light-blocking pigments, special attention must be paid to the balance between through-cure and surface cure.

8.3 Differences Between the Two Types of Free-Radical Curing Systems

Item	Unsaturated Polyester	Polyester Acrylate
Main reactive structure	Polymerizable double bonds in the polyester main chain	Acrylate double bonds
Typical initiation method	Initiator, promoter, heat, or redox initiation	UV or electron-beam curing
Film-formation characteristics	Can cure thick films and form relatively high crosslink density	Fast curing, suitable for continuous production
Main concerns	Shrinkage, exotherm, odor, oxygen inhibition	Light penetration, film thickness, pigment hiding power, oxygen inhibition
Common applications	Composite surface coatings, gel coats, special protective coatings	UV coatings, inks, clearcoats

9. Waterborne Polyesters: Combining Water-Dispersed Forms with Crosslinking Systems

Waterborne polyesters are resin systems in which hydrophilic groups, ionic groups, or external emulsification methods are introduced so that polyester resins can be stably dispersed in water. Waterborne polyesters are commonly used in waterborne baking finishes, waterborne industrial coatings, waterborne coil coatings, and waterborne packaging coatings.

The key challenge for waterborne polyesters is to balance water-dispersion stability, application leveling, the drying process, and water resistance after curing. Common film-formation routes for waterborne polyesters include: forming a continuous film after water evaporation; baking crosslinking with amino resins; crosslinking with blocked isocyanates; reacting with other suitable crosslinkers; and further curing through self-crosslinking structures.

Waterborne polyester is not a single curing mechanism, but a class of polyester systems that use water as the main dispersion medium. It can be either a physically drying system or a baking-crosslinking system. It can be a hydroxyl polyester, or it can contain carboxyl groups or other hydrophilic structures.

Waterborne polyester systems require close attention to the following factors:

Key Factor	Effect
Water-dispersion stability	Affects storage, application, and coating-film uniformity
Neutralizing-agent type	Affects pH, odor, volatility, and water resistance
Hydrophilic-group content	Affects dispersibility, water resistance, and coating-film compactness
Curing-agent compatibility	Affects crosslinking uniformity and final performance
Water evaporation process	Affects leveling, cratering, pinholes, and drying speed
Baking conditions	Affect degree of crosslinking, chemical resistance, and water resistance

The difficulty of waterborne polyester systems lies in balancing dispersion stability with final water resistance. Hydrophilic structures help the resin remain stably dispersed in water, but excessive hydrophilic structures may reduce the water resistance of the coating film. Therefore, the film quality of waterborne polyesters depends on the combined effects of resin structure, water evaporation process, curing-agent compatibility, and baking conditions.

10. Summary of Degree of Cure, System Selection, and Film-Formation Logic

Selecting the right resin and curing agent does not guarantee good coating-film performance. Only when curing is sufficient and the reaction is uniform can the structural advantages of the resin be truly translated into coating-film performance.

When curing is insufficient, common problems include insufficient hardness, poor solvent resistance, poor chemical resistance, tacky coating films, reduced water resistance, reduced adhesion, and accelerated outdoor aging.

Overbaking or improper curing conditions may also cause problems, such as coating-film brittleness, cracking during bending, yellowing, loss of gloss, reduced adhesion, increased surface defects, and reduced post-forming performance.

Film formation in polyester coatings is the combined result of resin melting, flow, wetting, degassing, crosslinking, and stress release. For powder coatings, the melt-leveling time must be balanced with the gelation and curing time. For liquid baking coatings, solvent or water evaporation, leveling, and crosslinking reactions must be balanced. When selecting a polyester resin, evaluation should follow the sequence below.

10.1 First, Determine the Application Method

Determine whether the coating is solventborne, waterborne, high-solids, powder, or UV-curable; whether it dries at room temperature, cures at low temperature, or requires high-temperature baking; and whether it is applied by spraying, roller coating, dip coating, electrodeposition, electrostatic powder spraying, or continuous coil coating. The application method directly affects the selection of resin molecular weight, viscosity, glass transition temperature, melt flow, drying speed, and curing window.

10.2 Next, Determine the Target Performance

Coil coatings emphasize flexibility, adhesion, solvent resistance, and post-forming performance. Outdoor powder coatings emphasize weatherability, gloss and color retention, and resistance to chalking. Industrial protective coatings emphasize chemical resistance, abrasion resistance, and corrosion protection. Decorative coatings focus more on appearance, leveling, gloss, and touch. Different target properties require different resin structures and curing systems.

10.3 Then, Determine the Curing System

Hydroxyl polyesters are suitable for reaction with amino resins, isocyanates, or blocked isocyanates. Carboxyl polyesters are suitable for reaction with epoxy resins, TGIC, or HAA. Air-drying alkyd resins are suitable for oxidative drying. Unsaturated polyesters are suitable for free-radical curing. Polyester acrylates are suitable for UV curing. Waterborne polyesters require further determination of whether they are physically drying or crosslinking-curing systems. Powder polyesters require further determination of whether they are carboxyl-curing systems or hydroxyl polyurethane powder systems.

10.4 Finally, Determine the Process Conditions

Consider curing temperature, curing time, pot life, storage stability, film thickness, leveling, pinholes, degassing, substrate temperature, production cycle time, and application environment.

The core differences among different polyester curing systems can be summarized as follows:

Film-Formation Mechanism	Representative Systems	Core Film-Formation Process	Main Points of Attention
Hydroxyl crosslinking	Hydroxyl polyester/amino resin; hydroxyl polyester/isocyanate	Hydroxyl groups react with crosslinkers to form a network	Catalyst, mixing ratio, curing temperature, pot life
Carboxyl crosslinking	Carboxyl polyester/epoxy; carboxyl polyester/TGIC; carboxyl polyester/HAA	Carboxyl groups react with epoxy groups or HAA	Acid value, curing-agent dosage, leveling window, degassing
Oxidative crosslinking	Air-drying alkyd resin	Autoxidative crosslinking of unsaturated fatty acid segments	Drier, oxygen, film thickness, temperature and humidity
Free-radical crosslinking	Unsaturated polyester; polyester acrylate	Free-radical polymerization of carbon-carbon double bonds	Initiation method, oxygen inhibition, shrinkage, light penetration
Combination of application form and chemical curing	Waterborne polyester; powder polyester	Further crosslinking after water evaporation or melt leveling	Dispersion stability, leveling, curing window, defect control

11. Representative Chemicals Related to Polyester Resin Film Formation (Tables 1–5)

Note: The following products are representative chemicals used in research on polyester resin film formation and curing. They are mainly intended for mechanism studies, synthesis research, model curing experiments, or formulation screening. Before industrial scale-up, it is necessary to further verify product grade, SDS information, regulatory requirements, formulation compatibility, and applicable processing conditions.

Table 1. Chemicals Related to Baking Cure of Hydroxyl Polyester/Amino Resin Systems

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Raw material for amino resin synthesis	108-78-1	T431382	2,4,6-Triamino-1,3,5-triazine	Suitable for synthesis	Used in research on the synthesis of melamine-formaldehyde amino resins; suitable for crosslinker structure design in hydroxyl polyester baking-crosslinking systems.
Raw material for amino resin synthesis	50-00-0	F111934	Formaldehyde solution	AR, contains 10–15% methanol as stabilizer	Used in the synthesis of amino resins, phenolic resins, and related crosslinking resins; suitable for studying the baking-curing reaction between hydroxyl polyesters and amino resins.
Acid catalyst	6192-52-5	T104293	p-Toluenesulfonic acid monohydrate	Chemically pure (CP), ≥98%	Used for acid-catalyzed curing in hydroxyl polyester/amino resin baking systems; can promote transetherification and condensation crosslinking reactions.
Amino resin crosslinker	3089-11-0	T162539	2,4,6-Tris[bis(methoxymethyl)amino]-1,3,5-triazine	≥98% (HPLC)	Used for baking crosslinking of hydroxyl-containing resins such as hydroxyl polyesters and acrylic resins; suitable for curing studies of industrial baking finishes and coil coatings.
Structural monomer for amino resins	91-76-9	D139438	2,4-Diamino-6-phenyl-1,3,5-triazine	≥98% (HPLC)	Used in research on the synthesis of benzoguanamine resins and modified amino resins; suitable for adjusting the hardness, flexibility, and chemical resistance of baked coating films.
Acid catalyst	104-15-4	T684184	4-Toluenesulfonic acid	≥98%	Used as an acid catalyst in amino resin baking-curing systems; can be used to study the effect of catalyst dosage on the crosslinking rate and coating-film performance of hydroxyl polyesters.
Acid catalyst	27176-87-0	D106550	Dodecylbenzenesulfonic acid (DBSA)	≥90%, mixture	Used for acid-catalyzed curing in hydroxyl polyester/amino resin systems; suitable for regulating crosslinking speed, storage stability, and coating-film performance in baking coatings.

Table 2. Chemicals Related to Hydroxyl Polyester/Isocyanate and Blocked Isocyanate Curing

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Polyisocyanate curing agent	28182-81-2	P485967	Poly(hexamethylene diisocyanate) (PolyHDI)	Viscosity 900–1500 cP (25 °C)	Used in curing studies of two-component hydroxyl polyester polyurethane coatings; can form crosslinked coating films with good abrasion resistance, chemical resistance, and mechanical properties.
Blocking agent	105-60-2	C111698	Caprolactam	Chemically pure (CP)	Used in research on isocyanate blocking reactions; suitable for blocked isocyanate curing agents, one-component baking polyurethane coatings, and polyurethane powder systems.
Aliphatic diisocyanate	822-06-0	H106723	Hexamethylene diisocyanate (HDI)	Moligand™, ≥99%	Used to synthesize aliphatic polyurethane curing agents, biurets, trimers, and blocked isocyanates; suitable for research on weather-resistant hydroxyl polyester polyurethane systems.
Blocking agent	67-51-6	D139167	3,5-Dimethylpyrazole	≥99%	Used in research on isocyanate blocking reactions; can be used for the design of one-component baking polyurethane coatings and polyurethane powder curing agents.
Blocking agent	96-29-7	B105233	Butanone oxime	≥99%	Used in research on the synthesis of blocked isocyanates; suitable for studying deblocking temperature and curing behavior in hydroxyl polyester baking polyurethane systems.
Cycloaliphatic diisocyanate	4098-71-9	I109582	Isophorone diisocyanate, mixture of isomers (IPDI)	≥99%	Used to synthesize weather-resistant polyurethane resins, blocked isocyanates, and polyurethane curing agents; suitable for research on high-performance hydroxyl polyester coating films.
Aromatic diisocyanate	26471-62-5	T135411	Toluene diisocyanate, 2,4- and 2,6-isomers (TDI)	≥98% (GC)	Used in research on the synthesis of polyurethane resins and curing agents; suitable for experiments on the reaction between hydroxyl polyesters and isocyanates and the formation of urethane bonds.
Aromatic diisocyanate	101-68-8	M106783	4,4′-Methylenebis(phenyl isocyanate) (MDI)	≥98%	Used in research on the synthesis of polyurethane materials and curing agents; suitable for studying hydroxyl polyester reactivity, crosslink density, and mechanical properties of coating films.
Isocyanurate-type curing agent	3779-63-3	H694586	1,3,5-Tris(6-isocyanatohexyl)-1,3,5-triazine-2,4,6-trione	≥95%	Used in hydroxyl polyester polyurethane curing systems; suitable for studying the effect of trifunctional isocyanate structures on crosslink density, chemical resistance, and weatherability.
Polyurethane catalyst	77-58-7	D100274	Dibutyltin dilaurate (DBTDL)	≥95%	Used to catalyze the reaction between hydroxyl groups and isocyanates; suitable for studying curing speed, pot life, and film-formation performance of polyester polyurethane coatings.
Cycloaliphatic diisocyanate	5124-30-1	D155475	Dicyclohexylmethane 4,4′-diisocyanate, mixture of isomers (HMDI)	≥90% (GC)	Used in research on the synthesis of weather-resistant polyurethane resins and curing agents; suitable for evaluating the coating-film performance of hydroxyl polyester cycloaliphatic polyurethane systems.

Table 3. Chemicals Related to Carboxyl Polyester/Epoxy, TGIC, and HAA Powder Curing

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Model epoxy compound / epoxy resin raw material	1675-54-3	B131786	Bisphenol A diglycidyl ether (BADGE)	Moligand™, ≥85%	Can be used as a model epoxy compound for carboxyl/epoxy reactions or as a research raw material related to epoxy resins; suitable for studying crosslinking reactions between carboxyl groups and epoxy groups.
Phase-transfer catalyst	1643-19-2	T103372	Tetrabutylammonium bromide	AR, ≥99%	Used in research on epoxy ring-opening, esterification, and quaternary ammonium salt-catalyzed reactions; suitable for experiments promoting carboxyl polyester/epoxy curing reactions.
Epoxy-curing catalyst	603-35-0	T104475	Triphenylphosphine	≥99% (GC)	Used to catalyze reactions between carboxyl groups and epoxy groups; suitable for studying reaction activity in polyester/epoxy powder coatings and TGIC curing systems.
Epoxy-curing catalyst	1100-88-5	B109283	Benzyltriphenylphosphonium chloride	≥99%	Used to catalyze reactions between epoxy resins and carboxyl resins; suitable for studying curing speed, gel time, and degree of crosslinking in powder coatings.
Epoxy-curing accelerator	693-98-1	M104839	2-Methylimidazole	≥98%	Used to accelerate epoxy ring-opening curing; suitable for curing studies of polyester/epoxy hybrid powder coatings and epoxy-modified systems.
TGIC curing agent	2451-62-9	T102749	Triglycidyl isocyanurate	≥98%	Used for curing carboxyl polyester outdoor powder coatings; suitable for studying carboxyl/epoxy reactions, crosslinked structures of weather-resistant powder coating films, and curing windows.
HAA curing agent	6334-25-4	N158980	N,N,N′,N′-Tetrakis(2-hydroxyethyl)adipamide	≥97%	Used for curing carboxyl polyester TGIC-free powder coatings; suitable for studying esterification crosslinking, water release, pinhole control, and thick-film application performance.

Table 4. Chemicals Related to Oxidative Drying of Alkyd Resins and Metal Driers

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Auxiliary drier	22464-99-9	Z282613	Zirconium 2-ethylhexanoate	In mineral spirits (~6% Zr)	Used in oxidative drying systems for alkyd resins; can promote late-stage hardening after surface drying and support studies on overall drying performance.
Primary drier	136-52-7	C282475	Cobalt(II) 2-ethylhexanoate solution	65 wt.% in mineral spirits	Used for oxidative crosslinking of air-drying alkyd resins; can promote surface drying and early-stage oxidation reactions; suitable for studies on drier formulation ratios.
Primary drier	13434-24-7	M283339	Manganese(II) 2-ethylhexanoate	40% solution in mineral spirits (6% Mn)	Used in oxidative drying systems for alkyd resins; can participate in peroxide decomposition and oxidative crosslinking; suitable for research on cobalt-free or low-cobalt drier systems.
Auxiliary drier	136-51-6	C283314	Calcium 2-ethylhexanoate	40% in 2-ethylhexanoic acid (3–8% Ca)	Used in alkyd resin drier systems; can improve drier dispersion, pigment wetting, and drying uniformity of coating films.

Table 5. Chemicals Related to Free-Radical Curing of Unsaturated Polyesters and Polyester Acrylates

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Thermal initiator	78-67-1	A104256	2,2′-Azobisisobutyronitrile (AIBN)	Recrystallized, ≥99%	Used in research on free-radical polymerization of unsaturated polyesters, acrylates, and vinyl monomers; suitable for experiments on double-bond curing reactions and crosslinked network formation.
Photoinitiator	119-61-9	B431545	Benzophenone	Suitable for synthesis	Used in free-radical photocuring systems; a hydrogen-abstraction photoinitiator that usually needs to be used with an amine synergist or another hydrogen donor; suitable for studying photocuring reactions of polyester acrylates.
Peroxide initiator	1338-23-4	B707058	Methyl ethyl ketone peroxide	Active oxygen content 9%	Used for room-temperature or low-to-medium-temperature curing of unsaturated polyesters; suitable for studying initiator dosage, curing speed, and exothermic behavior.
Reactive diluent	100-42-5	S110375	Styrene	CP, contains 10–15 ppm 4-tert-butylcatechol as stabilizer	Used for free-radical crosslinking of unsaturated polyester resins; suitable for resin dilution, copolymerization curing, and adjustment of crosslink density.
Peroxide initiator	94-36-0	B104630	Benzoyl peroxide (BPO)	AR	Used for free-radical initiation in unsaturated polyester, acrylate, and styrene systems; suitable for thermal curing reactions and polymerization kinetics studies.
Photoinitiator	119313-12-1	B138084	2-Benzyl-2-dimethylamino-1-(4-morpholinophenyl)butanone	≥98% (HPLC)	Used in ultraviolet curing systems; suitable for photocuring studies of polyester acrylates, colored coatings, and thick films.
Photoinitiator	947-19-3	H110064	1-Hydroxycyclohexyl phenyl ketone	≥98%	Used in polyester acrylate photocuring systems; suitable for transparent coatings, clearcoats, and rapid surface curing studies.
Photoinitiator	7473-98-5	H110280	2-Hydroxy-2-methylpropiophenone	≥97%	Used in free-radical photocuring systems; suitable for studying surface drying, curing speed, and photoinitiation efficiency of polyester acrylate coating films.
Photoinitiator	75980-60-8	T107643	Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide	≥97%	Used in ultraviolet curing systems; suitable for deep-curing studies of polyester acrylate thick films and colored systems.
Photoinitiator	162881-26-7	P138333	Phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO)	≥97%	Used in free-radical photocuring systems; suitable for polyester acrylate thick films, pigmented systems, and high-crosslink-density coatings.
Hydroxy acrylate monomer	818-61-1	H104535	2-Hydroxyethyl acrylate	≥96%, contains 200–600 ppm MEHQ as inhibitor	Used to synthesize hydroxy acrylic resins and polyester acrylate-modified materials; suitable for hydroxyl reaction studies and research on introducing photocurable double bonds.
Difunctional acrylate monomer	13048-33-4	H102721	1,6-Hexanediol diacrylate (HDDA), stabilized with MEHQ	≥90%	Used as a reactive diluent and crosslinking component in polyester acrylate photocuring systems; can adjust coating-film hardness, shrinkage, and curing speed.
Trifunctional acrylate monomer	15625-89-5	T102522	Trimethylolpropane triacrylate	≥85%, contains 600 ppm MEHQ as stabilizer	Used for multifunctional crosslinking in polyester acrylate photocuring systems; suitable for high-crosslink-density, high-hardness, and chemical-resistant coating-film studies.

Note: The above are representative Aladdin products. For more product specifications, search by “product name/CAS/catalog number” on the Aladdin 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] allnex. Amino Crosslinkers: Technologies.

[3] allnex. TGIC & HAA Polyester Resins for Powder Coating.

[4] UBE Corporation. Trixene® Blocked Isocyanates.

[5] Powder Coating Institute. Technology Interchange: Polyester Powder Coatings: TGIC vs. HAA.

[6] EMS-Griltech. Primid® Crosslinkers for Powder Coatings.

[7] Soucek, M. D.; Khattab, T.; Wu, J. Review of Autoxidation and Driers. Progress in Organic Coatings, 2012, 73, 435–454.

[8] Rolph, M. S.; Markowska, A. L. J.; Warriner, C. N.; O’Reilly, R. K. Blocked Isocyanates: From Analytical and Experimental Considerations to Non-Polyurethane Applications. Polymer Chemistry, 2016, 7, 7351–7364.

[9] Duran, J.; et al. Free-Radical Photopolymerization for Curing Products for Refinish Coatings Market. Polymers, 2022, 14, 2856.

For more related articles, see below.

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Understanding Amine Curing Agents: Structure, Types, and Application Selection