6 Factors For Choosing The Right Solvents for Coatings

23rd February 2024
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What is the role of solvents in coating&paints?

Solvents are an indispensable component in coating formulation. It helps to dissolve compounds including resin, additives and pigment and form a liquid solution that can be evenly applied to the substrate. Solvents also serve as important ingredient to optimize the overall performance the coating system.

To be specific, the role of solvents can be summarized as follows:

  • Dissolving resins.
  • Improving the wetting properties of pigments and fillers, reducing pigment floating.
  • Extending the shelf life of coatings.
  • Adjusting viscosity during production to optimize coatings and reduce issues.
  • Enhancing the flow and gloss of coatings, adjusting the surface state for surfaces with specific requirements.
  • Facilitating wetting between the coated surface and the paint during application, particularly beneficial for unprepped surfaces, increasing adhesion.
  • Correcting sagging and physical drying properties when coating vertical surfaces.
  • Minimizing brush marks, bubbles, seams, and coating opacity.
  • Selecting appropriate active solvents can effectively reduce VOC emissions during the drying process.

Film formation of coating

There are normally 6 factors to consider when choosing the right solvents for coating formulations. Coating manufacturers should take the balance of the following factors to select the most suitable solvents based on the actual applications.

Solvating power

Solvating power refers to the solvent's ability to dissolve resins and form a homogeneous solution of high molecular weight polymers. The solvent's capacity to disperse polymers into small particles, forming a uniform solution, the rate at which a resin solution of a certain concentration is formed, the viscosity of a solution at a certain concentration, and the mutual solubility between solvents are the primary considerations when selecting solvents for designing paint formulations.

Viscosity

In the coatings industry, we are not only concerned about whether the resin can dissolve in the solvent to form a uniform solution but also about the viscosity of the resulting resin solution. We aim for lower viscosity in resin solutions of the same concentration (or solids content). This allows for higher solids content in the paint solution at the same application viscosity, thus improving application efficiency and reducing solvent emissions into the atmosphere, resulting in less environmental pollution.

Solvents typically influence the viscosity of resin solutions in two ways: Solvent's solvency towards polymers and viscosity of the solvent itself.

Volatility

The drying film is formed during the solvent evaporation process. During this process, solvents control the flow characteristics of the film formation. If the solvents evaporate too quickly, the film will neither flow out nor wet the substrate sufficiently, resulting in poor adhesion. Solvents evaporating too rapidly can also cause the formation of a whitish film due to rapid cooling and condensation of water vapor on the wet film surface.

On the other hand, if solvents evaporate too slowly, not only will the drying time be prolonged, but the film may also sag and become thin. Undesirable changes in solvent composition during evaporation can lead to resin precipitation and film defects. Therefore, the solvent evaporation rate is a critical factor affecting the quality of coatings and films.

Surface Tension

Surface tension, when applied to liquids, refers to the work required to form a unit area of surface between the liquid phase and the gas phase. Alternatively defined as the force acting perpendicular to a unit length of line on the liquid surface. Therefore, surface tension is measured in J/m² or N/m.

In coatings, surface tension is an important parameter. Low surface tension of resin solutions and liquid coatings is undoubtedly beneficial and manifests in several ways:

  1. Resin solutions (materials) with low surface tension facilitate wetting of pigments, aiding in pigment dispersion in paint, enhancing color paint manufacturing and grinding dispersion efficiency, and promoting paint stability.
  2. Liquid coatings with low surface tension facilitate wetting of the substrate, thereby promoting leveling of the coating and improving adhesion of the coating to the substrate.
  3. The impact of surface tension of high solids content coatings on their atomization performance during spraying is more significant than the effect of viscosity. Due to the easy rupture and atomization of liquid coatings with low surface tension during spraying, high solids content coatings with low surface tension are more likely to achieve satisfactory spraying results.

Resistivity

Due to the advantages of electrostatic spraying, such as uniform coating, excellent decoration, high productivity, suitability for mass production, high coating utilization rate, and reduction of solvent diffusion pollution, many users choose electrostatic spraying for coating products. When formulating electrostatic spraying coatings, resistivity becomes an important indicator, and optimal coating resistivity is one of the necessary parameters for electrostatic spraying construction.

Various components of coatings, including resins, pigments, additives, and solvents, all affect the resistivity of the coating. However, the selection of resins and pigments is often determined by considerations of the decorative, mechanical, and aging resistance properties required for the coating film. Adjusting the resistivity of the coating by changing these components is often impractical in most cases. The use of additives is generally limited, and specific additives are chosen for specific purposes, making it necessary to adjust the resistivity of the coating through solvent selection.

Different types of solvents have different resistivities depending on their polarity. Alcohol solvents, ketone solvents, and alcohol ether solvents have strong polarity and low resistivity; hydrocarbon and ester solvents have weak polarity and higher resistivity.

Toxicity and Safety

When selecting solvents and designing paint formulations, it is crucial to consider the odor, toxicity to humans, air pollution restrictions, and safety of the solvents. Solvents with unpleasant odors, high toxicity to humans, flammability, and those that do not meet air pollution regulations should be avoided as much as possible.

  1. Odor: The odor of a solvent is not necessarily correlated with its toxicity to humans. For example, ethyl acetate is a highly toxic gas that, despite its pleasant fruity odor, can be lethal. On the other hand, despite its unpleasant odor, acetone is much less toxic than benzene, which has a pleasant aroma. Solvents with unpleasant odors in paint products are generally undesirable for users, especially if the odor persists and affects the application of the paint product.
  2. Toxicity: Toxicity refers to a substance's ability to cause harm to the body. The toxicity of a substance depends on the amount of exposure, its physical and chemical properties, and the route of exposure to the body.

Solvents can cause harm to the body through absorption via the skin, digestive system, and respiratory system. Most organic solvents exhibit narcotic effects on the body when there is high concentration vapor exposure. Prolonged and high-concentration exposure to the vapor of any volatile substance is harmful. With increasing severity and prolonged exposure, acute and chronic poisoning can occur. Solvents with high volatility rates at room temperature have much higher concentrations in the air compared to solvents with lower volatility rates. Therefore, solvents with lower toxicity to humans and lower volatility rates are relatively safer, but ingestion or absorption through the skin can still lead to poisoning if precautions are not taken.

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