Understanding Polyester Resins for Coatings: Structure Design, Key Indicators, and Coating Film Performance

1. The Intrinsic Relationship Between Polyester Resin Structure and Coating Film Performance

In a coating system, the resin is the primary film-forming material that forms a continuous coating film. Pigments, fillers, additives, solvents, or aqueous media can affect application, leveling, appearance, storage stability, and certain end-use properties. However, the hardness, flexibility, adhesion, chemical resistance, weatherability, and hydrolysis resistance of the coating film are first and foremost closely related to the chemical structure of the resin itself.

Polyester resins are widely used in industrial coatings, coil coatings, powder coatings, packaging coatings, metal finishing, and some polyurethane coating systems because they offer strong structural design flexibility. By selecting different acid components, alcohol components, functional groups, molecular weights, glass transition temperatures, and branching structures, different balances can be achieved among hardness, flexibility, adhesion, chemical resistance, weatherability, hydrolysis resistance, and application performance.

To understand polyester resins for coatings, it is necessary to understand the following logic:

① Why changing the structure of the acid or alcohol component changes the hardness, flexibility, and chemical resistance of the coating film;

② Why resins that are all called polyester resins can differ significantly: some are suitable for post-forming, some for high-hardness coatings, and some for outdoor weather-resistant systems;

③ Why polyester resins usually have good adhesion and flexibility, yet still require attention to hydrolysis resistance and alkali resistance;

④ Why judging whether a polyester resin is suitable for a coating system cannot be based on the resin name alone, but must consider acid value, hydroxyl value, Tg, molecular weight, viscosity, degree of branching, and curing system.

The core value of polyester resins for coatings is not to maximize a single property, but to achieve a balanced combination of properties through molecular structure design.

2. What Is a Polyester Resin for Coatings?

Polyester resin is a class of polymers whose main chain contains ester bonds. It is typically prepared by polycondensation of polycarboxylic acids, polyacid anhydrides, or their derivatives with polyols. Its core structural unit can be simplified as:—C(=O)–O—

—CO–O— is the ester bond. Ester bonds give polyester resins a certain polarity, an adhesion basis, and structural tunability. At the same time, they also make polyesters susceptible to hydrolysis under water, acid, alkali, or high-temperature and high-humidity conditions.

Polyester resins for coatings are different from polyester materials used in plastics. Plastic-grade polyesters, such as polyethylene terephthalate, usually emphasize melt processing, mechanical strength, dimensional stability, and crystallinity. Polyester resins for coatings, by contrast, place greater emphasis on film formation, leveling, adhesion, flexibility, curing reactivity, and overall coating film performance.

Typical polyester resins for coatings are mostly low- to medium-molecular-weight film-forming resins that contain reactive functional groups such as hydroxyl or carboxyl groups and have appropriate viscosity. Depending on the functional groups and application methods, polyester resins for coatings may include hydroxyl-functional polyesters, carboxyl-functional polyesters, waterborne polyesters, polyesters for powder coatings, and polyesters for coil coatings. This article focuses on the structure–property relationship of saturated polyester resins commonly used in coatings.

3. Which Structural Factors Determine Polyester Resin Performance?

The performance of polyester resins for coatings is not determined by a single factor, but by the combined effect of multiple structural variables.

Structural factor	Main meaning	Main influence
Ester bonds and main-chain structure	Basic backbone of the polyester	Affects polarity, adhesion, hydrolysis resistance, and structural design space
Acid components	Aromatic acids, aliphatic acids, cycloaliphatic acids, and anhydrides	Affect rigidity, flexibility, chemical resistance, and durability
Alcohol components	Diols, triols, and other polyols	Affect flexibility, hydrolysis resistance, branching degree, and viscosity
Reactive functional groups	Hydroxyl and carboxyl groups; unsaturated double bonds are mainly used in free-radical curing systems such as unsaturated polyesters or polyester acrylates	Determine curing reactivity, crosslink density, and adhesion
Molecular weight and distribution	Resin chain length and distribution range	Affect viscosity, leveling, mechanical strength, and application adaptability
Tg	Glass transition temperature, reflecting segmental mobility	Affects hardness, blocking resistance, flexibility, and storage stability
Linear or branched structure	Whether the molecular chain contains branches	Affects flowability, reactivity, film fullness, and crosslinking ability
Curing system	Reaction with amino resins, isocyanates, TGIC, HAA, etc.	Determines the final crosslinked network and coating film performance

These factors are not simply additive. For example, increasing resin rigidity is usually beneficial for hardness, heat resistance, and some aspects of chemical resistance, but it may reduce bending, impact resistance, and post-forming performance. Increasing functional group content can enhance curing reactivity, but may also increase system viscosity, polarity, and water absorption tendency. The essence of polyester resin design is to balance multiple structural factors.

4. Ester Bonds: The Source of Polyester Advantages and Hydrolysis Risk

The ester bond is the most important structural unit in polyester resins. It gives the resin a certain polarity, enabling polyester to provide good wetting and interaction with metals, pretreated substrates, some polar plastic substrates, and pigments and fillers. This is one of the important reasons why polyester resins usually show good adhesion.

Ester bonds also provide a broad structural design space. By changing the acid and alcohol components connected on both sides of the ester bond, the rigidity or flexibility, polarity, hydrophobicity, weatherability, chemical resistance, and hydrolysis resistance of the molecular chain can be adjusted.

However, ester bonds also bring an issue that must be taken seriously: hydrolysis risk. Hydrolysis refers to the cleavage of ester bonds under the action of water, acid, alkali, or other media, resulting in a decrease in resin molecular weight or damage to the crosslinked structure of the coating film. Waterborne polyesters, especially amine-neutralized systems, require particular attention to ester-bond hydrolysis under slightly alkaline conditions. Hydrolysis of ester bonds may lead to gloss loss, whitening, blistering, adhesion loss, reduced coating film strength, and even reduced stability of aqueous dispersions.

Effect brought by ester bonds	Influence on coating performance
Certain polarity	Helps wetting, pigment/filler dispersion, and adhesion
Structure can be adjusted through acid/alcohol monomers	Helps design different levels of hardness, flexibility, and durability
Can form rigid or flexible segments	Helps balance mechanical properties
Can hydrolyze under specific conditions	Requires attention to water resistance, humidity and heat resistance, alkali resistance, and storage stability

5. How Acid Components Affect Hardness, Flexibility, and Chemical Resistance

Acid components are an important source of the polyester resin main chain and directly affect the rigidity, flexibility, chemical resistance, and durability of the resin backbone. Common acid components include aromatic dibasic acids, aliphatic dibasic acids, cycloaliphatic dibasic acids, and anhydrides.

5.1 Aromatic Acids: Improving Rigidity, Hardness, and Heat Resistance

Common aromatic acids or anhydrides include terephthalic acid, isophthalic acid, and phthalic anhydride.

Aromatic structures contain benzene rings, which have strong molecular rigidity and restrict polymer chain segment movement. Therefore, introducing aromatic acids usually helps improve the rigidity, Tg, hardness, heat resistance, stain resistance, and certain aspects of chemical resistance of polyester resins. For industrial baking enamels, metal coatings, some powder coatings, and high-hardness coatings, aromatic structures are often an important way to increase coating film rigidity.

However, when the proportion of aromatic structures is too high, the coating film may become brittle, and flexibility, impact resistance, and post-forming performance may decrease. For coil coatings, stamped-part coatings, packaging coatings, and other systems requiring bending, hemming, stamping, or forming, excessive rigidity increases the risk of cracking.

It should be noted that aromatic structures do not necessarily mean “inherently weather-resistant.” Aromatic acids have a clear role in improving rigidity and heat resistance, but outdoor weatherability also depends on the specific monomer structure, curing system, pigments and fillers, UV stabilization system, and coating film density. Weatherability cannot be judged based solely on the presence of aromatic structures.

5.2 Aliphatic Acids: Improving Flexibility and Impact Performance

Aliphatic dibasic acids such as adipic acid contain relatively flexible carbon-chain structures. Introducing such structures into the polyester main chain can increase segmental mobility, giving the coating film better flexibility, impact resistance, bending performance, and low-temperature toughness. The higher the proportion of aliphatic segments, the more likely the resin is to show softness, low Tg, and good processability. However, if there are too many flexible segments, coating film hardness, blocking resistance, stain resistance, and solvent resistance may decrease.

5.3 Cycloaliphatic Acids: Providing a Balance Among Rigidity, Low Yellowing, and Durability

Cycloaliphatic structures are intermediate between rigid aromatic structures and flexible aliphatic structures. Cycloaliphatic monomers represented by 1,4-cyclohexanedicarboxylic acid (1,4-CHDA) and 1,4-cyclohexanedimethanol (CHDM) do not contain benzene-ring conjugated structures. They can provide a design space different from that of aromatic monomers in terms of low color, yellowing control, hardness/flexibility balance, and durability such as chemical resistance, humidity and heat resistance, and hydrolysis resistance. However, the specific effects still depend on isomer composition, acid/alcohol ratio, resin molecular weight, and curing system.

Cycloaliphatic monomers can be used in polyester systems requiring a balance of hardness, flexibility, low color, chemical resistance, and hydrolysis resistance. They provide polyester design with a structural option different from aromatic acids and aliphatic acids, allowing the resin to achieve a new balance among rigidity, segmental mobility, and durability.

6. How Alcohol Components Affect Hydrolysis Resistance, Flexibility, and Application Performance

Polyols are another important structural source for polyester resins. They affect not only molecular-chain flexibility, but also hydrolysis resistance, weatherability, branching degree, viscosity, and application performance.

6.1 Neopentyl Glycol: Improving Hydrolysis Resistance and Overall Durability

Neopentyl glycol (NPG) is an important diol commonly used in polyester resins for coatings. NPG has a neopentyl structure and relatively large steric hindrance. Polyester structures formed with NPG generally help improve hydrolysis resistance, humidity and heat resistance, chemical stability, and overall durability. Therefore, NPG is often used in the structural design of polyester polyols for industrial coatings, powder coatings, coil coatings, and polyurethane coatings. The role of NPG can be summarized as follows:

① It helps improve the hydrolysis resistance of polyester structures;

② It improves coating film water resistance, humidity and heat resistance, and chemical stability;

③ It helps achieve a balance between hardness and flexibility;

④ It is suitable for the structural design of weather-resistant and highly durable polyester resins.

The final hydrolysis resistance, weatherability, and chemical resistance of polyester resins also depend on acid component selection, acid value control, resin molecular weight, hydroxyl value or functionality design, crosslinking completeness, pigment/filler system, coating film density, and actual service environment.

6.2 Flexible Diols: Improving Bending, Impact, and Low-Temperature Toughness

Diols such as diethylene glycol, 1,6-hexanediol, and 1,5-pentanediol usually help lower Tg and improve coating film bending performance, impact resistance, and low-temperature toughness. Short-chain and regular diols such as ethylene glycol and 1,4-butanediol may increase polarity, segment regularity, or crystallization tendency. Their influence on flexibility, hardness, and water resistance must be judged together with the acid component and the overall formulation.

The proportion of flexible diols must be controlled. When there are too many flexible segments, resin Tg decreases, and the coating film may show insufficient hardness, poor blocking resistance, reduced stain resistance, or inadequate chemical resistance. Flexible diols are important for coil coatings, packaging coatings, and post-formable metal coatings. However, for high-hardness, high-stain-resistance, or high-solvent-resistance systems, they must be used in combination with rigid monomers and crosslinking design.

6.3 Trifunctional Alcohols: Increasing Branching Degree and Crosslinking Potential

Trimethylolpropane (TMP) is a common triol used in polyester resins for coatings. TMP contains three hydroxyl groups and can introduce branching points into the polyester structure, increasing the average functionality of the resin and its subsequent crosslinking potential.

Moderate introduction of TMP helps improve coating film hardness, fullness, crosslinking density, chemical resistance, and solvent resistance. It also helps high-solids systems maintain certain reactivity at relatively low molecular weight. However, if the TMP dosage is too high or the branching degree is not properly controlled, resin viscosity may increase significantly, affecting leveling, application performance, and storage stability. In severe cases, it may also increase the risk of gelation during synthesis.

7. How Functional Groups Determine Curing Reactions and Crosslinked Networks

Polyester resins for coatings usually contain reactive functional groups. Common functional groups include hydroxyl groups, carboxyl groups, and unsaturated double bonds. Functional groups determine whether the resin can participate in subsequent crosslinking reactions, and they also affect adhesion, compatibility, curing speed, and final coating film performance.

7.1 Hydroxyl Groups: Determining the Crosslinking Reactivity of Hydroxyl-Functional Polyesters

Hydroxyl groups can react with amino resins, isocyanates, blocked isocyanates, or other suitable curing agents to form crosslinked structures. Hydroxyl content is usually expressed as hydroxyl value, generally in mg KOH/g.

The higher the hydroxyl value, the more hydroxyl groups are usually available per unit mass of resin to participate in crosslinking. The curing-agent ratio and crosslink density design must therefore be adjusted accordingly. The influence of hydroxyl groups on coating film performance is mainly reflected in the following:

① Improving crosslinking reactivity;

② Enhancing interaction with polar substrates;

③ Improving the basis for adhesion;

④ Affecting system polarity, viscosity, and compatibility;

⑤ Affecting final crosslink density and coating film compactness.

More hydroxyl groups are not always better. Excessive hydroxyl content may lead to overly strong system polarity, increased water absorption, higher viscosity, or reduced application adaptability. A reasonable hydroxyl value must be determined according to curing-agent type, curing conditions, application method, and target performance.

7.2 Carboxyl Groups: Determining the Reactivity and Water Dispersibility of Carboxyl-Functional Polyesters

Carboxyl groups can increase resin polarity and adhesion, and can also act as reactive functional groups in crosslinking. Carboxyl content is usually expressed as acid value, generally in mg KOH/g.

For powder coatings, carboxyl-functional polyesters commonly react with triglycidyl isocyanurate (TGIC), β-hydroxyalkylamide (HAA), or other curing agents to form crosslinked networks. TGIC and HAA are both common curing routes for polyester powder coatings. HAA reacts with carboxyl groups while releasing water; therefore, in thick films, systems with insufficient degassing, or unsuitable curing windows, attention should be paid to differences in pinhole risk, water resistance, and chemical resistance.

Acid value must be balanced among reactivity, dispersibility, and water resistance. If the acid value is too low, there may be insufficient carboxyl reaction sites, or inadequate neutralization and dispersibility in waterborne systems. If the acid value is too high, resin hydrophilicity and water absorption tendency increase, and coating film water resistance, humidity and heat resistance, and chemical stability may decline.

7.3 Unsaturated Double Bonds: Suitable for Free-Radical Curing Systems

Unsaturated double bonds can participate in free-radical reactions, allowing the resin to form a crosslinked network. Unsaturated polyesters and polyester acrylates often use this characteristic to form cured coating films.

However, the reaction mechanism, curing speed, oxygen inhibition, volumetric shrinkage, and formulation design of unsaturated polyesters and polyester acrylates are different from those of conventional hydroxyl-functional and carboxyl-functional polyesters. Therefore, when discussing conventional saturated polyester resins for coatings, the performance logic of unsaturated polyesters should not be directly applied to all polyester systems.

8. How Molecular Weight, Tg, and Branching Structure Affect Application and Coating Film Performance

Acid components, alcohol components, and functional groups determine the chemical basis of polyester resins, while molecular weight, Tg, and branching structure further determine the resin’s application adaptability and coating film performance.

8.1 Molecular Weight: Affecting Viscosity, Leveling, and Mechanical Strength

Molecular weight is an important structural parameter of polyester resins. Generally speaking, as molecular weight increases, chain entanglement becomes stronger, and coating film strength and toughness may improve. However, viscosity also increases, and leveling, high-solids application performance, and sprayability may decrease.

When molecular weight is too low, resin viscosity is low, and application and leveling are easier, but coating film mechanical strength, solvent resistance, and durability may be insufficient. When molecular weight is too high, coating film strength may improve, but application viscosity increases, and appearance and leveling may be affected.

Molecular weight level	Common characteristics
Lower molecular weight	Low viscosity, easier to increase solids content, good leveling, but coating film strength may be insufficient
Medium molecular weight	Good balance among film formation, application, and mechanical properties
Higher molecular weight	Better mechanical strength and toughness, but higher viscosity
Broad molecular weight distribution	More complex application and coating film performance; must be judged together with viscosity and curing system

8.2 Tg: Affecting Hardness, Blocking Resistance, Flexibility, and Storage Stability

Glass transition temperature, abbreviated as Tg, is an important indicator for evaluating resin segmental mobility. When Tg is high, the resin is closer to a glassy state at room temperature, and the coating film usually shows higher hardness, better blocking resistance, and better stain resistance. When Tg is low, resin segments move more easily, and the coating film is usually more flexible, with better bending and impact performance.

Higher Tg	Lower Tg
Higher hardness	Better flexibility
Better blocking resistance	Better low-temperature toughness
Better stain resistance	Lower film-formation stress
Bending performance may decrease	Surface hardness may be insufficient
Coating film may be more brittle	Blocking resistance may decrease

Tg is not simply “the higher the better” or “the lower the better.” For powder coatings, an excessively low Tg may cause storage caking. For coil coatings that require post-forming, an excessively high Tg may cause cracking during bending. A reasonable Tg depends on the application scenario, application method, curing system, and target performance.

8.3 Linear and Branched Structures: Affecting Flow, Fullness, and Crosslinking Ability

Polyester resins can have either linear or branched structures.

Linear polyester molecular chains are relatively regular and usually have good flexibility, flowability, and processing adaptability. Coil coatings, packaging coatings, and metal coatings requiring post-forming usually place greater emphasis on bending, impact, and forming performance, and therefore often require linear or moderately branched structures.

Branched polyesters contain branch structures introduced by multifunctional monomers. Moderate branching can increase average functionality, increase the number of crosslinking points, and improve hardness, fullness, chemical resistance, and solvent resistance. However, excessive branching increases resin viscosity and affects leveling, application performance, and storage stability.

Structure type	Advantages	Risks
Linear polyester	Good flexibility, good leveling, good processing performance	Hardness and chemical resistance may be insufficient
Moderately branched polyester	Relatively strong reactivity and good overall performance	Viscosity and storage stability must be controlled
Highly branched polyester	High functionality and high crosslinking potential	Higher viscosity, more difficult leveling and application

9. Explaining Core Coating Film Properties from Structure

The performance of polyester coating films is not determined by a single structural factor alone, but by the combined effects of resin structure, curing system, pigments and fillers, additives, application method, and substrate condition. Resin structure provides the performance foundation, while formulation and processing determine the final performance.

Coating film property	Main structural source	Risks requiring attention
Adhesion	Polar structures such as ester bonds, hydroxyl groups, and carboxyl groups; good wetting; complete curing	Substrate contamination, insufficient pretreatment, excessive internal stress, or overly high polarity causing reduced water resistance
Flexibility	Aliphatic flexible segments, suitable molecular weight, low or moderate Tg, suitable crosslink density	Hardness, blocking resistance, stain resistance, and solvent resistance may decrease
Hardness	Aromatic or cycloaliphatic structures, higher Tg, higher crosslink density	Coating film may become brittle; impact, bending, and post-forming performance may decrease
Chemical resistance	Stable backbone, dense crosslinking, low water absorption, suitable curing system	Ester-bond hydrolysis risk under strong alkali, high temperature and humidity, or long-term water-vapor exposure
Hydrolysis resistance	Sterically hindered monomers, low hydrophilicity, reasonable acid value, good crosslinking	Excessive acid value, alkaline environment, or improper storage conditions for waterborne systems may increase risk
Weatherability	Stable monomers, low-yellowing structures, weather-resistant pigments and fillers, light stabilization system, dense coating film	Outdoor weathering grade cannot be judged based only on the name “polyester”
Application performance	Molecular weight, viscosity, solids content, branching degree, solvent or waterborne system design	Excessive viscosity affects leveling; excessively low molecular weight affects coating film strength

10. Structural Design Trade-Offs in Different Application Scenarios

The value of polyester resins lies in performance balance. Different coating applications have different requirements for resin structure, and it is not appropriate to judge all resins by the same set of indicators.

Application scenario	Key performance requirements	Polyester structure design focus
Coil coatings	Bending, stamping, adhesion, weatherability, no cracking after post-forming	Moderate Tg, reasonable flexible segments, good adhesion and curing completeness, weather-resistant monomers when necessary
Powder coatings	Storage stability, melt leveling, curing reaction, mechanical properties, and weatherability	Control Tg, acid value or hydroxyl value, branching degree, and curing-agent matching, such as carboxyl polyester/TGIC, carboxyl polyester/HAA, hydroxyl polyester/blocked isocyanate systems
Packaging coatings	Adhesion, flexibility, media resistance, processing resistance, and regulatory suitability	Focus on flexibility, media resistance, low migration risk, and complete curing
Industrial baking coatings	Hardness, appearance, chemical resistance, application efficiency	Hydroxyl-functional polyesters are often cured with amino resins or isocyanates; balance Tg, viscosity, and crosslink density
Metal protective coatings	Adhesion, corrosion resistance, humidity and heat resistance, chemical resistance	Require the combined effect of resin polarity, coating film density, pigment/filler system, and substrate pretreatment
Outdoor weather-resistant coatings	Gloss and color retention, anti-chalking, humidity and heat resistance, stain resistance	Select stable monomers, control yellowing-prone structures, and combine with weather-resistant pigments and light stabilization systems

For example, coil coatings emphasize post-forming performance, so the resin must not be too brittle. Powder coatings emphasize storage stability and melt leveling, making Tg and viscosity control very important. Packaging coatings emphasize flexibility, media resistance, and regulatory suitability. Industrial baking coatings focus more on hardness, appearance, chemical resistance, and application efficiency. Excellent polyester resin design does not maximize one property to an extreme, but allocates structural factors reasonably according to application requirements.

11. Understanding Technical Indicators of Polyester Resins

Technical indicator	Representative meaning	Main influence	Key points for judgment
Acid value, AV	Carboxyl group content	Reactivity, water dispersibility, adhesion, water resistance	Must be judged together with resin type and curing system
Hydroxyl value, OHV	Hydroxyl group content	Curing-agent ratio, crosslink density, hardness, solvent resistance	Not the higher the better; polarity, viscosity, and water absorption must be controlled
Tg	Segmental mobility	Hardness, blocking resistance, flexibility, storage stability	Requirements differ among powder coatings, coil coatings, and industrial baking coatings
Molecular weight	Resin chain length and entanglement degree	Viscosity, leveling, mechanical strength, film formation	Must be judged together with solids content and application method
Viscosity	Resin flow and application adaptability	Spraying, roll coating, leveling, appearance	Affected by temperature, solvent, waterborne system, and solids content
Solids content	Nonvolatile content	VOC emissions, application efficiency, film-thickness control	Must be judged together with viscosity and application window
Average functionality	Average number of reactive functional groups per molecule	Crosslinking ability, hardness, chemical resistance, curing speed	Must be judged together with acid value, hydroxyl value, and molecular weight
Residual acid or low-molecular-weight substances	Unreacted or residual small-molecule components	Storage stability, odor, water resistance, humidity and heat resistance	Especially important for waterborne, packaging, and high-durability systems

To judge whether a polyester resin is suitable for a coating system, the following questions should be considered at the same time:

① Is the resin hydroxyl-functional, carboxyl-functional, or another reactive type?

② Is the curing-agent type compatible?

③ Are the acid value, hydroxyl value, and average functionality suitable for the target crosslink density?

④ Is Tg suitable for storage, application, and service temperature?

⑤ Are molecular weight and viscosity suitable for the target application method?

⑥ Does resin polarity balance adhesion and water resistance?

⑦ Does the monomer structure meet weatherability, chemical resistance, or post-forming requirements?

⑧ Can the curing conditions form a complete and stable crosslinked network?

12. Summary of the Structure–Property Relationship

The performance of polyester resins for coatings comes from the synergistic effects of ester bonds, acid components, alcohol components, functional groups, molecular weight, Tg, linear or branched structures, and curing systems.

Structure design direction	Common performance change
Increasing aromatic structures	Improves hardness, heat resistance, and some aspects of chemical resistance, but flexibility and post-forming performance may decrease
Increasing aliphatic flexible segments	Improves flexibility, impact performance, and low-temperature toughness, but hardness and stain resistance may decrease
Introducing cycloaliphatic structures	Helps balance rigidity, low yellowing, chemical resistance, and durability
Using sterically hindered diols such as NPG	Helps improve hydrolysis resistance, humidity and heat resistance, and overall durability
Increasing hydroxyl value or acid value	Increases reactive sites, but water absorption, viscosity, and storage stability must be controlled
Increasing molecular weight	Improves mechanical strength and toughness, but viscosity increases
Increasing Tg	Improves hardness and blocking resistance, but flexibility may decrease
Moderate branching	Improves reactivity, film fullness, and coating film density, but excessive branching affects leveling and application

13. Classification Tables of Representative Monomers and Additives Related to Structure–Property Research of Polyester Resins for Coatings

Table 1. Rigid Acid Components, Aromatic/Cycloaliphatic Anhydrides, and Special Durability-Oriented Structural Monomers

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Aromatic anhydrides	85-44-9	P116466	Phthalic anhydride	Premium grade reagent, ≥99%	Introduces phthalate structures to adjust resin polarity, adhesion, hardness, and coating film fullness. Suitable for synthetic experiments involving saturated polyesters, alkyd resins, and coating resins.
Aromatic dibasic acids	121-91-5	I104311	Isophthalic acid (IPA)	AR, ≥99%	Introduces isophthalic structures to improve polyester backbone rigidity, heat resistance, chemical resistance, and coating film hardness. Suitable for research on powder coatings, coil coatings, and durable polyester resins.
Cycloaliphatic dibasic acids	1076-97-7	C100837	1,4-Cyclohexanedicarboxylic acid (CHDA)	≥99%, mixture of cis- and trans-	Introduces cycloaliphatic dicarboxylic acid structures, helping balance rigidity, flexibility, low yellowing, and hydrolysis resistance. Suitable for the design of weather-resistant polyesters, waterborne polyesters, and powder coating resins.
Aromatic tetrafunctional anhydrides	89-32-7	P109616	Pyromellitic dianhydride (PMDA)	≥99%	Provides a high-functionality aromatic anhydride structure and can be used for exploratory research on high-heat-resistant, highly branched, or crosslinked structures. In conventional saturated coating polyesters, it should be used at low levels and with caution to avoid excessively high viscosity, uncontrolled branching, or gelation risk.
Aromatic dicarboxylate esters	120-61-6	D106859	Dimethyl terephthalate	≥99%	Serves as a terephthalate-structured diester monomer for polyester synthesis via transesterification, improving resin rigidity, heat resistance, and mechanical strength. Suitable for experiments on coating polyester synthesis processes and structural regulation.
Aromatic dibasic acids	100-21-0	P108506	Terephthalic acid (PTA)	≥99%	Introduces terephthalic acid structures to improve polyester main-chain rigidity, Tg, hardness, and heat resistance. Suitable for research on high-hardness powder coatings, industrial baking coatings, and chemically resistant polyester resins.
Heteroaromatic dibasic acids	3238-40-2	F119129	2,5-Furandicarboxylic acid (FDCA)	≥98%	Introduces furandicarboxylic acid structures, providing rigidity, a bio-based structural source, and relatively high polarity. Suitable for research on sustainable polyesters, coil coating polyesters, and structural replacement in coating resins. During application, attention should be paid to color, solvent compatibility, synthesis temperature, molecular weight, and crystallization risk at high usage levels.
Cycloaliphatic anhydrides	85-42-7	C124721	1,2-Cyclohexanedicarboxylic anhydride	≥97%, cis + trans	Introduces cycloaliphatic anhydride structures to adjust polyester hardness, flexibility, yellowing resistance, and coating film appearance. Suitable for synthetic experiments involving cycloaliphatic polyesters, weather-resistant coatings, and low-color resins.
Aromatic trifunctional anhydrides	552-30-7	B104832	Trimellitic anhydride	≥97%	Provides an aromatic trifunctional anhydride structure that can introduce branching points and carboxyl-reactive sites, improving crosslinking potential, hardness, and chemical resistance. Suitable for research on carboxyl-functional polyesters, powder coating resins, and branched polyesters.

Table 2. Flexible Aliphatic Acid Components, Unsaturated Acid Components, and Ring-Opening Polyester Monomers

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Aliphatic dibasic acids	124-04-9	A108267	Adipic acid	Pharmaceutical grade, PharmPure™, ≥99.6%	Introduces flexible aliphatic diacid segments to improve polyester flexibility, impact performance, and bending performance. Suitable for research on coil coatings, packaging coatings, and post-formable polyester resins.
Unsaturated dibasic acids	97-65-4	I106140	Itaconic acid	Chemically pure (CP), ≥99%	Provides unsaturated double bonds and carboxyl structures for introducing reactive sites and expanding resin functionalization possibilities. Suitable for experiments on unsaturated polyesters, bio-based polyesters, and modified coating resins.
Long-chain aliphatic dibasic acids	111-20-6	S108452	Sebacic acid	Chemically pure (CP), ≥98%	Introduces long-chain flexible diacid structures, lowering resin Tg and improving coating film flexibility, low-temperature toughness, and impact resistance. Suitable for research on flexible polyesters and elastic coating resins.
Aliphatic dibasic acids	110-15-6	S431423	Succinic acid	Moligand™, suitable for synthesis	Introduces short-chain aliphatic diacid structures to adjust polyester flexibility, polarity, and bio-based structural content. Suitable for comparative experiments on biodegradable polyesters, coating polyesters, and resin structures.
Long-chain aliphatic dibasic acids	123-99-9	A108440	Azelaic acid	Moligand™, industrial grade, ≥85% (GC)	Introduces long-chain aliphatic diacid structures to improve coating film flexibility, impact resistance, and low-temperature performance. Suitable for research on flexible polyesters, bend-resistant coatings, and modified resins.
Unsaturated anhydrides	108-31-6	M116389	Maleic anhydride	AR, ≥99% (GC)	Introduces cis-unsaturated anhydride structures, providing double bonds and carboxyl-reactive sites. Suitable for research on unsaturated polyesters, modified polyesters, and free-radical-curable coating resins.
Unsaturated dibasic acids	110-17-8	F110742	Fumaric acid	≥99.5%	Introduces trans-unsaturated diacid structures, improving the reactivity and segment rigidity of unsaturated polyesters. Suitable for research on free-radical-curable resins, modified polyesters, and crosslinked networks.
Lactone ring-opening monomers	502-44-3	C109521	ε-Caprolactone	≥99%	Introduces polycaprolactone flexible segments through ring-opening polymerization, improving polyester flexibility, low-temperature toughness, and film-forming elasticity. Suitable for research on flexible coating resins, polyurethane modification, and block polyesters.

Table 3. Diol Monomers for Segment Regulation

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Aliphatic diols	110-63-4	B1508458	1,4-Butanediol (BDO)	Anhydrous grade, ≥99%	Introduces linear aliphatic diol structures to adjust polyester flexibility, crystallization tendency, mechanical strength, and film-forming performance. Suitable for research on polyester polyols, coating resins, and elastic segments.
Short-chain diols	107-21-1	E119700	Ethylene glycol	Anhydrous grade, ≥99.8%	Introduces short-chain diol structures to improve polyester segment regularity, polarity, and hardness. Suitable for basic polyester synthesis, structural comparison, and coating resin performance research.
Aliphatic diols	504-63-2	P432773	1,3-Propanediol	Suitable for synthesis	Introduces three-carbon diol segments to adjust polyester flexibility, polarity, and bio-based structural content. Suitable for research on coating polyesters, flexible resins, and sustainable materials.
Side-group diols	57-55-6	P432968	1,2-Propanediol	Basic grade reagent, for preparation	Introduces diol structures containing a pendant methyl group to adjust polyester segment regularity, viscosity, flexibility, and resin solubility. Suitable for coating polyester synthesis and research on side-group effects.
Ether diols	111-46-6	D476199	Diethylene glycol	UltraBio™, ultrapure grade, ≥99% (GC)	Introduces ether bonds and flexible segments, lowering resin Tg and improving film-forming flexibility and low-temperature performance. Suitable for experiments on flexible polyesters, coating resins, and structural regulation of waterborne systems. Ether bonds and relatively high hydrophilicity may affect water resistance, humidity and heat resistance, and chemical resistance, so the dosage should be controlled.
Long-chain aliphatic diols	629-11-8	H103708	1,6-Hexanediol	≥98%	Introduces six-carbon flexible diol segments to improve polyester flexibility, water resistance, impact performance, and film toughness. Suitable for research on flexible polyesters, polyurethane coatings, and bend-resistant coatings.
Sterically hindered diols	126-30-7	N103689	Neopentyl glycol (NPG)	≥99%	Introduces neopentyl sterically hindered structures to improve polyester hydrolysis resistance, humidity and heat resistance, chemical resistance, and hardness/flexibility balance. Suitable for research on powder coatings, coil coatings, and weather-resistant polyester resins.
Bio-based rigid diols	652-67-5	I157515	Isosorbide	≥98% (GC)	Introduces bicyclic rigid diol structures to improve polyester Tg, hardness, heat resistance, and bio-based content. Suitable for research on high-Tg polyesters, heat-resistant coating resins, and sustainable polyesters.
Side-group diols	2163-42-0	M141411	2-Methyl-1,3-propanediol	≥98%	Introduces branched aliphatic diol structures to adjust resin viscosity, flexibility, water resistance, and application adaptability. Suitable for research on high-solids polyesters, coil coatings, and industrial baking coating resins.
Aliphatic diols	111-29-5	P105588	1,5-Pentanediol	≥97%	Introduces five-carbon flexible diol segments to adjust polyester flexibility, low-temperature toughness, and film-forming performance. Suitable for research on flexible coating polyesters, polyester polyols, and structural comparison experiments.
Cycloaliphatic diols	105-08-8	C105684	1,4-Cyclohexanedimethanol (CHDM)	≥99%, mixture of cis and trans	Introduces cycloaliphatic diol structures to adjust polyester segment rigidity, heat resistance, chemical resistance, low color, and hardness/flexibility balance. Suitable for research on high-solids polyesters, coil coatings, powder coatings, and weather-resistant coating resins.

Table 4. Multifunctional Branching Monomers, Waterborne Functional Monomers, and Special Hydroxyl Monomers

Category	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Triol branching monomers	56-81-5	G755728	Glycerol	Anhydrous grade, UltraBio™, molecular biology grade, ≥99.5% (GC)	Provides trihydroxyl structures that can introduce branching points and multifunctional reactive sites, adjusting polyester crosslinking potential, viscosity, and coating film fullness. Suitable for synthetic experiments on branched polyesters and coating resins.
Tetraol branching monomers	115-77-5	P103696	Pentaerythritol (regulated explosive precursor)	AR, ≥98%	Provides tetrahydroxyl structures to increase polyester branching degree, average functionality, and crosslinking potential. Suitable for research on high-solids resins, branched polyesters, alkyd resins, and high-hardness coating films.
Trifunctional heat-resistant hydroxyl monomers	839-90-7	T162476	Tris(2-hydroxyethyl) isocyanurate (THEIC)	≥98% (N)	Introduces isocyanurate rings and polyhydroxyl structures to improve resin heat resistance, crosslinking potential, and coating film hardness. Suitable for research on heat-resistant polyesters, powder coatings, and highly durable coatings.
Waterborne functional diols	4767-03-7	B104539	2,2-Bis(hydroxymethyl)propionic acid (DMPA)	≥98%	Provides both hydroxyl and carboxyl structures, enabling the introduction of hydrophilic sites and neutralizable carboxyl groups. Suitable for research on waterborne polyesters, waterborne polyurethanes, dispersion stability, and acid value control.
Triol branching monomers	77-99-6	T110597	Trimethylolpropane (TMP)	≥98%	Provides trihydroxyl branching points to increase polyester average functionality, crosslink density, hardness, and solvent resistance. Suitable for research on high-solids polyesters, powder coating resins, and industrial baking coating resins.
Sulfonate hydrophilic monomers	6362-79-4	S115342	Sodium 5-sulfoisophthalate (5-SSIPA)	≥98%	Introduces sulfonate hydrophilic groups and isophthalic structures to improve polyester water dispersibility, ionic stability, and compatibility with pigments and fillers. Suitable for research on waterborne polyesters and water-dispersible coating resins.
Sulfonate dicarboxylate esters	3965-55-7	D101418	Dimethyl 5-sulfoisophthalate sodium salt	≥98%	Provides sulfonate hydrophilic structures and diester reactive sites for introducing ionic hydrophilic units via transesterification. Suitable for experiments on waterborne polyesters, water-dispersible resins, and hydrophilic modification.

Waterborne functional monomers such as DMPA, 5-SSIPA, and sulfonate dicarboxylate esters help improve water dispersibility and particle stability. However, excessive hydrophilic groups increase water absorption tendency and may reduce coating film water resistance, humidity and heat resistance, salt-spray resistance, and chemical resistance. Therefore, dispersion stability must be balanced with final durability.

Note: The above products are representative Aladdin products. More product specifications can be searched on the Aladdin website using the product name, CAS number, or catalog number.

References

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

[2] Chemical Dynamics. Fundamentals of Polyester Resins. 2020.

[3] Eastman Chemical Company. Eastman NPG Glycol Technical Data Sheet.

[4] Eastman Chemical Company. Improved Hydrolytic Stability of Waterborne Polyester Resins with Eastman 1,4-CHDA and Eastman TMPD Glycol.

[5] SI Analytics. Determination of Hydroxyl Value (DIN EN ISO 4629-2) and Acid Value (DIN EN ISO 2114).

[6] SpecialChem. Glass Transition Temperature of Coatings: Essential Concepts.

[7] allnex. TGIC Powder Coating and HAA Polyester Resins.

[8] Sherwin-Williams Coil Coatings. Metal Coating Comparison Guide.

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

[10] allnex. Powder Coating Resins.

For more related articles, please see below.

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