Film Formation Mechanism of Coatings: Types and Factors

28th December 2023
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What is film formation?

The film formation or curing of coatings primarily refers to the process of film formation after applying coatings onto a painted substrate. As solvents evaporate and crosslinking occurs, the liquid coatings solidify into a film.

2 Types of film formation

The film formation mechanisms vary among different coatings and are closely related to their composition. There are mainly two types of film formation, based on evaporation and chemical reaction. There is no chemical change in the structure before and after the film formation based on evaporation, such as thermoplastic resin or heat-induced film formation, and non-crosslinking emulsion film formation. On the other hand, chemical film formation involves film-forming materials undergoing chemical reactions, resulting in the crosslinking into three-dimensional macromolecular films.

1. Based on Evaporation

Traditional thermoplastic solvent-based coatings, such as chlorinated polyolefin, nitrocellulose, acrylic resin, CAB, and polyvinyl acetal, are dissolved in solvents to prepare coatings with less than 50% solid content. After application, the coatings cure into films through solvent evaporation.

Polymers generally exhibit better physical properties at higher molecular weights, but with higher molecular weights come elevated Tg(Glass Transition Temperatures). To facilitate coating, it's necessary to lower the Tg by using a sufficient amount of solvent, ensuring that the T-Tg value is large enough to enhance the flowability of solution.

Solvents matter a lot to achieve a smooth and glossy coating. If the solvent evaporates too quickly, the surface coating may lose its flowability due to increased viscosity, resulting in an uneven paint film. Additionally, rapid evaporation may cause excessive heat loss and lower the surface temperature to the dew point, leading to water condensation within the film.

Film formation of coating based on evaporation

2.Based on Chemical Curing

Resins undergo chemical reactions during the film formation process, leading to the formation of a continuous coating with a three-dimensional macromolecular structure, known as chemical film formation.

Typically, coatings formed through the chemical film formation process exhibit superior overall performance compared to coatings formed based on evaporation. These film-forming materials are often referred to as thermosetting resins. Except for powder coatings, they are low-molecular-weight oligomers with low construction viscosity. As the crosslinking density increases, viscosity and Tg rise, and free volume decreases, eventually resulting in the formation of a continuous and uniform solid coating.

  • 1K thermosetting  coatings

These coatings are popular in the market due to their convenient application, labor, time, and material savings. Examples include alkyds, modified alkyds, epoxy esters, and urethane oils, such as polyurethane-modified alkyds, which undergo oxidative crosslinking by absorbing oxygen from the air. One-component moisture-curing polyurethane coatings absorb water from the air, react with excess NCO in the film-forming material, and form polyurea-polyurethane coatings. Another example is the closed isocyanate film-forming system, which remains stable at room temperature, and upon heating and catalysis, releases one NCO for rapid reaction and crosslinking to form a film.

  • Free Radical Polymerization for Film Formation

Film-forming materials composed of unsaturated polyesters, acrylics, or epoxies modified with allyl groups, polyurethanes, low molecular weight oligomers, and epoxy compounds, along with active diluents, undergo polymerization and crosslinking under the action of a free radical initiator or, alternatively, under the influence of high-energy light beams such as ultraviolet or electron beams, which trigger the decomposition of photosensitizers to generate free radicals or reactive ions. The entire process is completed within seconds to minutes. During film formation, there is no organic solvent evaporation, making the process environmentally friendly and energy-efficient.

  • 2K Coatings

Epoxy resin, when combined with amine curing agents or polyfunctional alcohols or amines along with polyisocyanate curing agents, undergoes addition polymerization and crosslinking to form a film. They are typically packaged as two-component systems, requiring proportional mixing before application, leading to the coating and film formation. Although two-component coatings generally lack storage stability, it's crucial to handle polyisocyanate curing agents with care during production, packaging, and storage due to their sensitivity to moisture.

Read more to learn the film-formation of polyurethane coatings.

Factors of film formation

  • Tg (Glass transition temperature)

Crosslinking film-forming materials are typically low-molecular-weight oligomers with a low Tg, which increases to over 100°C as the crosslinking density rises after the reaction. Thermoplastic film-forming materials have a specific glass transition temperature (Tg), and the Tg of the film-forming material at room temperature must be above 25°C to form a coating with sufficient strength. However, film formation is not possible above the Tg temperature. It only occurs well below the Tg temperature, where the coating exhibits the necessary flowability and film-forming properties.

  • Temperature

Coatings primarily relies on solvent evaporation drying or undergoing crosslinking reactions to form a film. When the ambient temperature exceeds 35°C, the dry time of the coating shortens. At this point, the solvent accelerates evaporation, carrying away a significant amount of heat.  Condensation may occur on the coating surface after curing, leading to defects on the coating appearance.

On the other hand, when the ambient temperature drops below 15°C, solvent evaporation slows down, prolonging the dry time of the coating. If the coating surface remains uncured for an extended period, it not only causes dust contamination but also affects the adhesion of the coating. Therefore, the curing environment temperature for such coatings should be controlled between 20-25°C.

  • Moisture&Humidity

Water-based coatings generally use water as a solvent or dispersing medium, resulting in the presence of strongly polar functional groups in the coating with high hydrophilicity. When the relative humidity exceeds 65%, the hydrophilic functional groups absorb moisture from the environment, which can lead toincrease in the wet film thickness. After curing, issues such as foaming and even delamination may occur. Therefore, it is crucial to strictly control the relative humidity between 35% and 65% during spraying.

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