1. Basic Definition of Acrylic Resins

Acrylic resins are polymers or copolymers formed through the polymerization of acrylic acid, methacrylic acid, and their ester monomers as the main structural units. In coatings, acrylic resins are commonly used as film-forming materials or primary binder resins. They influence coating-film hardness, flexibility, adhesion, weatherability, gloss and color retention, transparency, water resistance, and resistance to chemical media.

In coatings, the term “acrylic resin” usually does not refer to a single polymer, but rather to a class of resin systems that can be adjusted through monomer composition, molecular weight, functional groups, glass transition temperature, particle size, and crosslinking mode. Their performance differences mainly arise from three aspects:

1. Different monomer compositions

Different monomers determine the hardness, flexibility, polarity, hydrophobicity, and reactivity of polymer segments.

2. Different structural parameters

Parameters such as Tg, molecular weight, acid value, hydroxyl value, particle size, and crosslinking density determine the film-forming ability of the resin and the performance of the coating film.

3. Different film formation and curing methods

Acrylic resins can form thermoplastic coating films through physical film formation, or they can form thermoset crosslinked coating films through functional-group reactions.

2. Main Types and Classification Logic of Acrylic Resins

In practical coating formulations, acrylic resins are often referred to by various names, such as all-acrylic emulsion, styrene-acrylic emulsion, hydroxyl acrylic resin, thermoplastic acrylic resin, thermosetting acrylic resin, and waterborne acrylic dispersion. These names do not belong to the same level of classification.

A common source of confusion is the distinction between “thermoplastic/thermosetting” and “waterborne/solventborne.” These are not the same classification dimension. A waterborne acrylic resin may be a thermoplastic emulsion, or it may be a thermosetting or self-crosslinking system containing crosslinkable functional groups. Similarly, a solventborne acrylic resin may be a thermoplastic resin, or it may be a crosslinkable resin such as a hydroxyl acrylic resin.

3. Monomer Structure: The Starting Point of Acrylic Resin Performance Design

The performance of an acrylic resin first depends on its monomers. Acrylic resins for coatings are usually not polymerized from a single monomer, but are composed of hard monomers, soft monomers, and functional monomers.

3.1 Hard Monomers

Hard monomers generally increase the glass transition temperature of the resin, giving the coating film higher hardness, blocking resistance, stain resistance, and surface strength.

The core role of hard monomers is to establish coating-film hardness and resistance to deformation. However, the higher the proportion of hard monomers, the more difficult polymer-segment motion becomes. The coating film is usually harder and more brittle, while low-temperature film formation and crack resistance may decrease.

3.2 Soft Monomers

Soft monomers generally reduce the Tg of the resin and increase polymer-segment mobility, giving the coating film better flexibility, elongation, and low-temperature film-forming ability.

The main value of soft monomers lies in improving film formation and flexibility. In waterborne emulsion systems, soft monomers can also reduce the minimum film-forming temperature, which is beneficial for low-temperature application. However, if the proportion of soft monomers is too high, the coating film may become soft and tacky, with reduced stain resistance and blocking resistance.

3.3 Functional Monomers

Functional monomers are used to introduce functional groups such as carboxyl, hydroxyl, epoxy, carbonyl, and silane groups, enabling acrylic resins to provide adhesion, dispersion, crosslinking, or special interfacial effects.

Functional monomers are key to upgrading the performance of acrylic resins. Ordinary thermoplastic acrylic resins mainly rely on the polymer itself to form a coating film. After functional monomers are introduced, the resin can react with curing agents or with its own functional groups to form a stronger crosslinked structure.

4. Key Structural Parameters: From Monomer Composition to Coating-Film Performance

Monomers determine the basic structure of the resin, but final performance also depends on a set of key structural parameters. For coating formulations, important parameters include Tg, MFFT, molecular weight, acid value, hydroxyl value, particle size, and crosslinking density.

4.1 Glass Transition Temperature, Tg

Tg is one of the important structural parameters of acrylic resins. It reflects the temperature range in which polymer segments transition from a glassy state to a rubbery state. Tg directly affects coating-film hardness, flexibility, blocking resistance, stain resistance, and low-temperature film-forming ability.

The essence of Tg design is balancing hardness, flexibility, and film-forming ability. For acrylic resins, hard monomers increase Tg, while soft monomers reduce Tg. In formulation design, it is not appropriate to simply pursue either a high Tg or a low Tg. Instead, a suitable range should be determined according to the target performance of the coating film.

4.2 Minimum Film-Forming Temperature, MFFT

MFFT is mainly used to evaluate the film-forming ability of waterborne emulsions or waterborne polymer dispersions. It is the lowest temperature at which a waterborne emulsion or dispersion can coalesce and form a continuous coating film during drying. Below the MFFT, polymer particles have difficulty deforming and fusing sufficiently, and the coating film may show whitening, powdering, cracking, reduced adhesion, or insufficient water resistance.

Tg is related to MFFT. In addition to polymer Tg, MFFT is also affected by particle size, coalescing agents, emulsifiers, residual monomers, polymer phase structure, and drying conditions.

4.3 Molecular Weight and Molecular Weight Distribution

Molecular weight affects resin viscosity, film strength, leveling, drying speed, and application properties. In solution acrylic resins, at the same solids content, the higher the molecular weight, the higher the system viscosity usually is. A narrower molecular weight distribution usually makes viscosity and performance easier to control.

For emulsion acrylic resins, system viscosity is not mainly determined by polymer molecular weight. It is more strongly influenced by particle size, particle size distribution, thickening system, pH, and electrolytes.

4.4 Acid Value

The acid value reflects the carboxyl group content in the resin and is usually expressed as the number of milligrams of KOH required to neutralize 1 g of resin. Carboxyl groups can improve resin adhesion to polar substrates and also help with water dispersion, water dilution, and emulsion stability.

Acid value design should not consider only water-dispersion stability. The more carboxyl groups present, the easier it is for the resin to achieve water dispersion or alkali swelling, but the tendency of the coating film to absorb water may also increase. Acid value is a balancing indicator among waterborne compatibility, adhesion, and water resistance.

4.5 Hydroxyl Value

The hydroxyl value reflects the content of hydroxyl functional groups in the resin and is an important indicator for hydroxyl acrylic resins. Hydroxyl groups can react with curing agents such as polyisocyanates and amino resins to form crosslinked structures. The higher the hydroxyl value, the more hydroxyl groups are theoretically available for crosslinking reactions, and the greater the design space for crosslinking.

Hydroxyl value cannot be evaluated independently of the curing agent. The final performance of hydroxyl acrylic resins also depends on curing-agent type, reaction equivalent ratio, catalyst, curing temperature, humidity, and film-forming conditions.

4.6 Particle Size and Particle Size Distribution

Particle size mainly applies to waterborne acrylic emulsions and dispersions. The particle size of latex particles affects emulsion stability, viscosity, gloss, transparency, film compactness, and early water resistance.

The particle size of a waterborne emulsion is jointly determined by emulsifiers, initiators, monomer feeding method, seed emulsion, pH, and polymerization process. Particle size design is not simply a matter of making particles as small as possible; it should serve coating-film appearance, storage stability, application viscosity, and film compactness.

4.7 Crosslinking Density

Crosslinking density is a key indicator for thermosetting acrylic resins. After crosslinking, chemical connections are formed between polymer chains, and the hardness, solvent resistance, resistance to chemical media, heat resistance, and stain resistance of the coating film usually improve. The goal of crosslinking design is not simply to increase crosslinking density, but to achieve a reasonable balance among solvent resistance, resistance to chemical media, hardness, flexibility, and application conditions.

5. Polymerization Methods and Resin Forms

Acrylic resins are usually prepared through free-radical polymerization. Different polymerization methods affect resin form, molecular weight, particle size, solids content, viscosity, and subsequent film-forming behavior.

5.1 Solution Polymerization

Solution polymerization is the polymerization of monomers in an organic solvent. It is commonly used to prepare solventborne thermoplastic acrylic resins, hydroxyl acrylic resins, and high-solids acrylic resins.

Solution polymerization is suitable for preparing acrylic resins with well-defined structures, controllable functional-group content, and good leveling properties.

5.2 Emulsion Polymerization

Emulsion polymerization is carried out in the aqueous phase using emulsifiers, initiators, and a monomer dispersion system. It is an important preparation method for waterborne acrylic emulsions. Waterborne acrylic resins can be prepared through emulsion polymerization, solution polymerization, bulk polymerization, suspension polymerization, and other methods, among which emulsion polymerization is widely used. For waterborne acrylic emulsions used in coatings, emulsion polymerization is the most typical preparation route. Water-dispersible or water-reducible acrylic resins, on the other hand, often obtain waterborne compatibility by introducing hydrophilic groups and then undergoing neutralization and dispersion.

Emulsion polymerization produces a dispersion of polymer particles in water. Its coating-film performance depends not only on the polymer itself, but also on whether the particles can sufficiently deform, fuse, and form a continuous film during drying.

5.3 Water-Dispersible or Water-Reducible Acrylic Resins

Water-dispersible or water-reducible acrylic resins are usually formed by introducing hydrophilic groups such as carboxyl groups into the polymer chain, followed by amine neutralization to form a stable aqueous dispersion or water-reducible system.

Water-dispersible acrylic resins are different from ordinary emulsions. Emulsions are mainly dispersions of polymer particles in water, while water-dispersible or water-reducible resins rely more on hydrophilic groups and neutralized structures on the polymer chain to maintain stability.

6. Film Formation and Curing Mechanisms: The Essential Difference Between Thermoplastic and Thermosetting Systems

Acrylic resins ultimately need to transform from a liquid, dispersed, or solid state into a continuous coating film. According to film formation and curing mechanism, they can be divided into thermoplastic acrylic resins and thermosetting acrylic resins.

6.1 Thermoplastic Acrylic Resins: Mainly Physical Film Formation

Thermoplastic acrylic resins usually do not contain obvious reactive crosslinking groups. Even if they contain a small amount of polar groups, chemical crosslinking is not the main curing mechanism. Their film formation mainly depends on the evaporation of solvent or water, followed by polymer-chain entanglement, particle fusion, and intermolecular interactions to form a continuous coating film.

The performance of thermoplastic acrylic resins is mainly controlled by Tg and molecular weight. Increasing Tg helps improve hardness and blocking resistance, but is unfavorable for flexibility and low-temperature film formation. Increasing molecular weight helps improve cohesive strength, but also increases system viscosity.

6.2 Thermosetting Acrylic Resins: Improving Coating-Film Strength Through Chemical Crosslinking

Thermosetting acrylic resins contain functional groups that can participate in reactions, such as hydroxyl, carboxyl, epoxy, carbonyl, or silane groups. After the coating film is formed, these functional groups can react with external curing agents or undergo self-crosslinking reactions to form a three-dimensional crosslinked structure.

Common crosslinking methods include:

1. Reaction between hydroxyl acrylic resin and polyisocyanates

Forms a polyurethane crosslinked structure, improving hardness, solvent resistance, and resistance to chemical media.

2. Reaction between hydroxyl acrylic resin and amino resins

Commonly used in baking-curing systems to improve coating-film hardness, chemical resistance, and heat resistance.

3. Reaction between carboxyl groups and epoxy groups

Can increase crosslinking density, adhesion, and resistance to media.

4. Reaction between carbonyl groups and hydrazide-type crosslinkers

Commonly used in waterborne self-crosslinking acrylic emulsions to improve water resistance and solvent resistance.

5. Silane hydrolysis-condensation crosslinking

Improves water resistance and bonding to inorganic surfaces by forming Si—O—Si structures.

The performance advantages of thermosetting acrylic resins come from the crosslinked network, but excessive crosslinking density can reduce flexibility and impact resistance. Therefore, the core of thermosetting design is not that “more crosslinking is always better,” but that the degree of crosslinking should match the application requirements of the coating film.

7. Main Performance Advantages of Acrylic Resins and Their Structural Origins

The advantage of acrylic resins does not lie in one single outstanding property, but in their ability to achieve a good overall balance through structural design.

7.1 Good Weatherability and Gloss/Color Retention

Acrylic resins generally have good resistance to UV aging, gloss retention, and color stability. In particular, all-acrylic systems based mainly on acrylate and methacrylate monomers usually provide better outdoor durability and yellowing resistance than styrene-acrylic systems with higher styrene content.

The main reasons include:

1. The C—C backbone of acrylic/methacrylic ester polymers is relatively stable against photo-oxidation, and most ester side groups are relatively stable under normal coating-use conditions.

2. The resin is light in color and has good transparency.

3. When the coating film contains no or only a small amount of structures prone to yellowing, its color retention is better.

4. Proper crosslinking can further improve stain resistance and long-term durability.

7.2 Wide Adjustable Range Between Hardness and Flexibility

Acrylic resins can adjust Tg through the ratio of hard monomers to soft monomers, thereby controlling coating-film hardness, flexibility, low-temperature film-forming ability, and blocking resistance.

1. Increasing hard monomers improves coating-film hardness and blocking resistance.

2. Increasing soft monomers improves coating-film flexibility and film-forming ability.

3. Increasing functional monomers can further introduce adhesion, crosslinking, or special interfacial effects.

This adjustability enables acrylic resins to cover a wide performance range, from soft film-forming systems to highly hard crosslinked coating films.

7.3 Good Transparency and Decorative Performance

Acrylic resins are usually light in color and have good transparency, making them suitable for forming highly transparent or light-colored coating films. Their good gloss and color retention also help maintain the long-term appearance stability of coating films.

Transparency is related not only to the resin itself, but also to particle size, compatibility, film integrity, and crosslinking uniformity. For waterborne emulsions, overly large particle size, insufficient film formation, or obvious phase separation may all affect transparency and gloss.

7.4 Adhesion Can Be Improved Through Functional-Group Design

Acrylic resins can improve adhesion through functional monomers containing carboxyl, hydroxyl, epoxy, phosphate ester, silane, and other groups.

Different substrates require different adhesion mechanisms:

1. Polar substrates usually require polar interactions from groups such as carboxyl, hydroxyl, and phosphate ester groups.

2. Inorganic substrates can use silane structures to strengthen interfacial bonding.

3. Low-surface-energy plastic substrates rely more on resin polarity, wetting ability, swelling effect, and interfacial compatibility.

Therefore, adhesion cannot be solved by a single monomer alone. It is the combined result of resin polarity, substrate surface condition, application wetting, and film-formation shrinkage.

7.5 Can Be Designed as Either a Physical Film-Forming or Chemically Crosslinking System

Acrylic resins can be designed either as thermoplastic physical film-forming resins or as thermosetting resins containing reactive functional groups.

1. Physical film-forming systems emphasize drying speed, application properties, transparency, and appearance.

2. Chemically crosslinked systems emphasize solvent resistance, resistance to chemical media, hardness, heat resistance, and long-term performance.

3. Waterborne systems also need to balance dispersion stability, MFFT, early water resistance, and film compactness.

This is also an important reason why acrylic resins have strong adaptability in coatings.

8. Main Limitations of Acrylic Resins and Their Structural Origins

The limitations of acrylic resins usually do not come from a single defect, but from performance trade-offs inherent in structural design.

8.1 Water Resistance of Waterborne Systems Is Easily Affected by Hydrophilic Components

Waterborne acrylic emulsions or dispersions usually require emulsifiers, carboxyl groups, neutralizing agents, or other hydrophilic structures to maintain stability. These hydrophilic components are beneficial for waterborne compatibility and storage stability, but they may increase the tendency of the coating film to absorb water, resulting in reduced early water resistance, whitening resistance, or barrier performance.

Improving water resistance usually requires an integrated design approach, including reducing hydrophilic components, optimizing the emulsification system, improving film compactness, and introducing appropriate crosslinking.

8.2 Limited Solvent Resistance and Chemical Resistance of Thermoplastic Systems

Thermoplastic acrylic resins mainly rely on physical entanglement to form coating films, and there is no stable three-dimensional chemical crosslinked network between polymer chains. Therefore, under strong solvents, chemical media, or high-temperature conditions, the coating film is prone to swelling, softening, or performance degradation.

To improve solvent resistance and resistance to chemical media, it is usually necessary to introduce reactive groups such as hydroxyl, carboxyl, epoxy, and carbonyl groups, and to form crosslinked structures through isocyanate, amino resin, epoxy, or self-crosslinking reactions.

8.3 Trade-Offs Among Hardness, Flexibility, and Low-Temperature Film Formation

The most common trade-offs in acrylic resin design are as follows:

1. Increasing Tg is beneficial for hardness, stain resistance, and blocking resistance.

2. Decreasing Tg is beneficial for flexibility, crack resistance, and low-temperature film formation.

3. Increasing crosslinking density is beneficial for solvent resistance and resistance to chemical media.

4. Excessively high crosslinking density may make the coating film brittle and reduce impact resistance.

Therefore, acrylic resin design should not pursue a single indicator alone. Instead, it should achieve an overall balance around the target performance of the coating film.

8.4 Limited Contribution to Barrier Performance in Heavy-Duty Anticorrosive Coatings

The advantages of ordinary acrylic resins are mainly reflected in weatherability, gloss and color retention, transparency, and decorative protection. Compared with high-adhesion and high-barrier resins such as epoxy resins, ordinary acrylic resins are usually not the strongest resin type for primer protection in heavy-duty anticorrosive coating systems.

If acrylic resins need to meet higher protective requirements, they usually need to be enhanced through the following approaches:

1. Increasing crosslinking density.

2. Optimizing the pigment, filler, and anticorrosive pigment systems.

3. Introducing adhesion-promoting monomers.

4. Forming composite systems with other resins.

5. Using a matched primer-topcoat coating structure.

9. Core Structure–Performance Relationships of Acrylic Resins

10. Representative Chemicals Related to Acrylic Resin Structural Design (Tables 1–4)

Table 1. Polymerization Control and Auxiliary Materials for Emulsion Polymerization

Table 2. Basic Carboxylic Acid Monomers and Hardness-Regulating Monomers

Table 3. Soft Monomers and Hydrophobic Regulating Monomers

Table 4. Functional Monomers and Crosslinking-Reactive Monomers

Note: The above are representative Aladdin products. More product specifications can be searched on the Aladdin official website by “product name / CAS / catalog number.”

References

[1] SpecialChem. Acrylic resins: How to select the right grade for coatings? Last update: Apr. 27, 2026.

[2] Jiao C., Sun L., Shao Q., Song J., Hu Q., Naik N., Guo Z. Advances in Waterborne Acrylic Resins: Synthesis Principle, Modification Strategies, and Their Applications. ACS Omega, 2021, 6, 2443–2449. DOI: 10.1021/acsomega.0c05593.

[3] SpecialChem. Minimum film forming temperature (MFFT): A characteristic property of coatings. Last update: Jul. 15, 2025.

[4] Polynt. Thermoset Solution Acrylics / Thermosetting Acrylic Resins.

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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)