1 Core Issues in Film Formation of Waterborne Resins

1.1 Film formation is not simply “water drying off”

After a waterborne coating is applied, water evaporation is only the beginning of the film-formation process. For emulsion-type resins and waterborne dispersions, the resin is usually dispersed in water in the form of polymer particles, colloidal particles, or resin microparticles. After application, these dispersed particles need to move closer together, pack, deform, coalesce, undergo chain-segment diffusion, and then harden in the later stage, ultimately forming a continuous, dense coating film with the desired performance.

The core question in waterborne resin film formation is: how do resin particles dispersed in water transform into a continuous coating film and develop properties such as hardness, flexibility, adhesion, water resistance, and chemical resistance? If only water evaporates while the resin particles do not sufficiently deform and coalesce, the coating film may exhibit problems such as chalking, whitening, cracking, poor adhesion, and poor water resistance.

1.2 Differences between film formation of waterborne resins and solventborne resins

The film-formation processes of waterborne resins and solventborne resins are clearly different.

The difficulty in waterborne resin film formation lies in the fact that the particles must remain stably dispersed before application, but after application, the boundaries between particles must gradually disappear to form a continuous film.

2 Film-Formation Process of Waterborne Emulsions and Dispersions

For ease of understanding, the film-formation process of waterborne emulsions and polymer water dispersions can usually be divided into several stages: water evaporation, particle concentration and packing, particle deformation, particle coalescence and chain-segment diffusion, and post-hardening or crosslinking. In practice, these stages may overlap, and their relative importance and rates may vary among different resin systems.

2.1 Stage One: Water Evaporation

After the coating is applied to the substrate surface, water begins to evaporate first. As water content decreases, the polymer particles, pigments and fillers, and additives in the system gradually become concentrated. At this stage, the coating film is still in a wet-film state, and the resin particles remain relatively far apart. The rate of water evaporation is affected by the following factors:

If water release is too slow, it may lead to long drying time, sagging, slow early-stage development of water resistance, or whitening under low-temperature and high-humidity conditions. If water release is too fast, the surface layer may close too early, making it difficult for internal water or bubbles to escape, thereby increasing the risk of pinholes, micropores, internal stress, or cracking. Cratering is usually also related to substrate contamination, insufficient wetting, surface-tension differences, or additive compatibility.

2.2 Stage Two: Particle Concentration and Packing

As water continues to evaporate, the distance between polymer particles gradually decreases. When the particle concentration increases to a certain level, the particles begin to contact each other and form a packed structure. Ideally, the particles can pack relatively uniformly, creating favorable conditions for subsequent deformation and coalescence. If particle packing is non-uniform, voids, microcracks, or localized defects may form in the coating film.

Particle packing is affected by particle size, particle-size distribution, emulsion stability, pigment/filler volume concentration, and rheological state. An appropriate particle-size distribution helps improve packing density, but an excessively broad particle-size distribution may also affect system stability and application rheology.

2.3 Stage Three: Particle Deformation

After the particles come into contact, the polymer particles must undergo a certain degree of deformation in order to form a continuous film. Particle deformation reduces the voids between particles and gradually densifies the film layer. Whether particles deform easily mainly depends on the softness or hardness of the resin, application temperature, minimum film-forming temperature MFFT, glass transition temperature Tg, coalescing agents, and particle surface state.

If the particles cannot deform sufficiently, obvious particle boundaries and voids remain, and the coating film may appear opaque, whitened, chalky, low in strength, and poor in water resistance.

2.4 Stage Four: Particle Coalescence and Chain-Segment Diffusion

After particle deformation, polymer chain segments still need to diffuse and interpenetrate between particles. This process gradually weakens the interfaces between particles, transforming dispersed particles into a more continuous polymer phase. Chain-segment diffusion is closely related to the mobility of polymer molecules. The closer the temperature is to, or the higher it is above, the resin Tg, the easier chain-segment motion becomes. At excessively low temperatures, chain-segment motion is restricted; even if the particles are in contact, it is difficult to form a strong and tough continuous film.

Chain-segment diffusion has an important influence on coating-film performance. Emulsion film formation is not simply particles “sticking together”; polymer chain segments must cross particle interfaces to form a continuous phase. The more complete the chain-segment diffusion, the better the integrity of the coating film generally becomes.

2.5 Stage Five: Post-Hardening and Performance Development

After the coating film forms a continuous phase, its properties continue to develop. Different systems develop performance in different ways, mainly including physical hardening, oxidative drying, and chemical crosslinking.

Post-hardening and crosslinking affect hardness, blocking resistance, water resistance, solvent resistance, chemical resistance, and stain resistance. For high-performance waterborne coatings, merely forming a continuous film is not enough; sufficient film density and a crosslinked structure are also needed to support final durability.

3 Tg, MFFT, and the Film-Formation Window

3.1 Tg determines resin softness/hardness and chain-segment mobility

Tg is an important indicator for judging the softness or hardness of a polymer. When the resin is below its Tg, chain-segment motion is restricted, and the material behaves as harder and more brittle. When the resin is above its Tg, chain-segment mobility increases, and the material is more likely to deform and coalesce. In waterborne resins, Tg directly affects film formation and dry-film performance.

Tg is not simply “the higher the better” or “the lower the better.” The challenge in designing waterborne resins is to enable the resin to form a continuous film at the application temperature while also allowing the dry film to achieve sufficient hardness and durability.

3.2 MFFT determines minimum film-forming capability

MFFT refers to the minimum temperature at which an emulsion or polymer water dispersion can form a continuous, uniform film without cracking or powdering under specified conditions. It is an important indicator for evaluating the low-temperature film-forming capability of waterborne resins. When the application temperature is lower than the MFFT, resin particles have difficulty deforming and coalescing sufficiently, and the coating film may exhibit whitening, chalking, cracking, reduced adhesion, reduced water resistance, and lower gloss and transparency.

When the application temperature is higher than the MFFT, the resin is more likely to form a continuous film, but final performance still needs to be evaluated together with Tg, crosslinking, film thickness, humidity, and formulation composition. In practical applications, a resin with low MFFT may have good low-temperature film formation, but if its Tg is low and crosslinking is insufficient, the dry film may show poor blocking resistance or insufficient stain resistance. A resin with high Tg may have better hardness, but if the application temperature is insufficient, film integrity may be compromised. Tg and MFFT are related, but they are not equivalent. MFFT is also affected by coalescing agents, water plasticization, particle size, core-shell structure, emulsifiers or hydrophilic components, molecular weight, and test conditions.

3.3 The film-formation window is determined by multiple factors

The film-formation window refers to the range of conditions under which a waterborne resin can form an intact coating film under actual application conditions. The film-formation window is jointly determined by resin structure, environmental conditions, and formulation factors.

4 Particle Size, Particle-Size Distribution, and Core-Shell Structure

4.1 Particle size affects film density and appearance performance

The particle size in emulsions and dispersions affects coating-film packing, gloss, transparency, permeability, and stability.

Particle size is not simply “the smaller the better.” Smaller particles may improve surface fineness and film density, but they may also lead to increased viscosity, higher sensitivity of stability, and changes in additive requirements.

4.2 Particle-size distribution affects particle packing

During water evaporation, particles must gradually transition from a dispersed state to a tightly packed state. Particle-size distribution affects the way space is filled between particles. An appropriate particle-size distribution can improve packing density, reduce voids, and help form a dense coating film. However, if system stability is insufficient, particles may flocculate before drying, resulting in a rough film layer, lower gloss, or localized defects. Therefore, particle-size distribution is not only a film-formation issue but also a storage-stability and application-stability issue.

4.3 Core-shell structure is used to balance film formation and performance

Core-shell structure is a common structural design approach in waterborne emulsions. By designing different compositions in the core and shell of the same particle, the resin can meet multiple performance targets.

The value of a core-shell structure lies in partially separating the two requirements of “film formation requiring softness” and “dry film requiring hardness.” A core-shell structure is not simply intended to improve one property; rather, it improves the overall performance balance through internal particle-structure design.

5 Hydrophilicity, Water Resistance, and Coating-Film Density

5.1 Hydrophilic structures help dispersion stability

Waterborne resins need to remain stable in water; therefore, hydrophilic groups, ionic groups, emulsifiers, or protective colloids are often introduced. These structures help resin particles remain dispersed in water and prevent flocculation, sedimentation, or emulsion breaking. Common sources of hydrophilicity include:

5.2 Hydrophilic residues may reduce dry-film water resistance

The challenge of waterborne resins is that hydrophilic stabilization is needed before application, but after film formation, the coating film is expected to be as water-resistant as possible. If excessive hydrophilic groups, free emulsifiers, hydrophilic additives, or water-soluble residues remain in the dry film, water can more easily enter the film. This may lead to increased water absorption, reduced resistance to water whitening, lower wet adhesion, reduced corrosion resistance, lower alkali resistance, and film softening or reduced strength.

Therefore, the water resistance of a waterborne resin depends not only on whether the resin itself has a hydrophobic structure, but also on whether film formation is sufficient, whether the coating film is dense, whether hydrophilic components are immobilized or reduced, and whether effective crosslinking is present.

5.3 Coating-film density is the foundation of water resistance

The water resistance of a coating film is closely related to film density. Even if the resin itself has a relatively good hydrophobic structure, water can still easily penetrate into the film if film formation is insufficient, particle boundaries remain obvious, or the film contains many voids. The main factors affecting coating-film density include:

6 Effects of Functional Groups and Crosslinking on Performance Development

6.1 Difference between physical film formation and chemical crosslinking

After a waterborne resin forms a continuous film, performance can be developed through physical hardening or further improved through chemical crosslinking. Physical film formation solves the problem of “forming a continuous film,” while crosslinking further addresses whether the film is resistant to water, solvents, chemicals, and long-term use.

Common functional groups in waterborne resins include hydroxyl groups, carboxyl groups, epoxy groups, isocyanate-reactive structures, ketone carbonyl groups, and hydrazide groups. Different functional groups have different effects on adhesion, crosslinking, water resistance, and chemical resistance.

6.3 Crosslink density must be appropriate

Crosslinking can significantly improve coating-film performance, but a higher crosslink density is not always better. A reasonable crosslinking design should serve the target application. Flexible substrates require a balance between elongation and water resistance. Anti-corrosion and flooring systems place greater emphasis on crosslink density, film density, and chemical resistance. Wood coatings need to balance hardness, feel, transparency, and blocking resistance.

7 Film-Formation-Related Defects and Causes

When waterborne resin film formation is poor, common problems include whitening, chalking, cracking, poor blocking resistance, poor water resistance, and insufficient adhesion. These problems are usually not caused by a single factor, but by the combined effects of resin structure, application environment, film-formation conditions, and formulation factors.

7.1 Common film-formation defects

7.2 Diagnostic sequence for film-formation defects

When diagnosing film-formation problems, it is advisable to first determine whether the issue is related to film-formation integrity, and then analyze the effects of resin structure and formulation. A common diagnostic sequence is shown below. This sequence helps distinguish whether the problem originates from insufficient film-formation temperature, mismatched resin structure, hydrophilic residues, insufficient crosslinking, or improper application conditions.

8 Summary

The performance of waterborne resins is not naturally obtained after water evaporation; rather, it is gradually developed during the film-formation process. For emulsion-type resins and waterborne dispersions, film formation usually involves water evaporation, particle concentration, particle packing, particle deformation, particle coalescence, chain-segment diffusion, and post-hardening. The core content of this article can be summarized in the following five points:

① Waterborne resin film formation is not simply drying.

After water evaporates, resin particles still need to sufficiently deform, coalesce, and undergo chain-segment diffusion in order to form a continuous coating film.

② Tg and MFFT determine the film-formation window.

Tg affects resin softness/hardness and chain-segment mobility, while MFFT determines low-temperature film-forming capability. Both must be evaluated together with application temperature, coalescing agents, and final performance.

③ Particle size, particle-size distribution, and core-shell structure affect film density.

Appropriate particle size and structural design help improve packing, coalescence, gloss, transparency, and performance balance.

④ Hydrophilicity helps stability in water, but may affect dry-film water resistance.

Waterborne resins require hydrophilic structures to maintain dispersion stability, but excessive hydrophilic residues in the dry film increase the risk of water absorption, whitening, and reduced water resistance.

⑤ Crosslinking is an important route for developing high performance.

Physical film formation creates a continuous film, while crosslinking further improves water resistance, solvent resistance, chemical resistance, hardness, and blocking resistance. However, crosslink density must be balanced with flexibility, pot life/application window, and application requirements.

9 Representative Chemical Categories and Application Table Related to Film-Formation Mechanisms and Performance Development of Waterborne Resins

Note: The products listed below are mainly representative raw materials for waterborne resin synthesis, film-formation mechanism studies, crosslinking reaction research, or formulation screening. When used in actual production of waterborne coatings, selection should also be based on the product SDS, COA, water dispersibility, reaction activity, VOC considerations, regulatory requirements, application process, and formulation validation.

Table 1 Film Softness/Hardness Adjustment and Comonomers

Table 2 Functional Monomers, Self-Crosslinking Monomers, and Raw Materials for Waterborne Performance Development

Table 3 Coalescing Agents, Co-Solvents, and Latent-Crosslinking Auxiliary Products

Table 4 Polyurethane Crosslinking, Epoxy Curing, and Products for High-Performance Coating-Film Development

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

References

[1] Keddie J. L. Film formation of latex. Materials Science and Engineering: R: Reports, 1997, 21(3): 101–170.

[2] Winnik M. A. Latex film formation. Current Opinion in Colloid & Interface Science, 1997, 2(2): 192–199.

[3] Steward P. A., Hearn J., Wilkinson M. C. An overview of polymer latex film formation and properties. Advances in Colloid and Interface Science, 2000, 86(3): 195–267.

[4] Routh A. F., Russel W. B. A process model for latex film formation: limiting regimes for individual driving forces. Langmuir, 1999, 15(22): 7762–7773.

[5] Croll S. G. Drying of latex paint. Journal of Coatings Technology, 1986, 58(734): 41–49.

[6] Wicks Z. W., Jones F. N., Pappas S. P., Wicks D. A. Organic Coatings: Science and Technology. 3rd ed. Hoboken: John Wiley & Sons, 2007.

More related articles are listed 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)

Isocyanate-Functional Silane Coupling Agents: Structural Features, Classification, Applications, and Selection

A Complete Guide to Selecting Epoxy Curing Systems: Amines vs. Anhydrides vs. Latent Curing — with Aladdin’s Recommended Selection Table

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

Epoxy Resin: From Reactive Resin to High-Performance Material System

Analysis of the Epoxy Resin Curing Mechanism: From Ring-Opening Reaction, Gelation and Crosslinking to Post-Curing and Property Development

How Matting Agents in Coatings Build Low-Gloss Coating Surfaces: Mechanisms, Formulation Trade-Offs, and Product Selection

Alkyd Resins: From Oil/Fatty-Acid-Modified Polyesters to Autoxidative Drying — Understanding Their Structure and Film-Formation Mechanism

Adding a Temperature-Triggered Switch to NCO: How Blocked Isocyanates Affect the Storage, Curing, and Film Performance of 1K Baking Coatings