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2-2. General Properties of Solvents
Organic solvents exhibit a variety of characteristics that are important to understand not only for the formulation and application of paint but also for maintaining the performance and finish of the paint film, and for safety and hygiene considerations. Below are the main properties:
2-2-1. Solubility
Solubility indicates the ability of a solvent to dissolve a certain amount of solute, which is dependent on temperature and pressure. It is typically expressed as the maximum amount of solute that can be dissolved per 100 units of solvent. In the case of high molecular amorphous substances, solubility may increase the viscosity and sometimes may not be clearly defined.
2-2-2. Solubility Parameter
The solubility parameter is defined as the square root of the cohesive energy density of molecules, known as the SP value. Substances with similar SP values mix more readily. Even if the SP values of a solvent and a resin differ, solubility can be achieved by adjusting the SP value in combination with other solvents, which is a useful technique in paint formulation. More details on the solubility parameter can be found on this page.
2-2-3. Evaporation Rate and Boiling Point
The evaporation rate indicates the speed at which a solvent evaporates, affecting the workability of painting and the finish of the paint film. Generally, solvents with a low boiling point have a higher evaporation rate, but solvents with a high boiling point can also evaporate quickly if they have high vapor pressure. Extensive use may lead to whitening of the paint film, which can be prevented by adding solvents with a lower evaporation rate (retarders, non-brushing).
2-2-4. Surface Tension
Molecules inside a liquid are subjected to equal forces, but surface molecules only experience inward attraction, causing the liquid to contract and create surface tension. Solvents with low surface tension have higher penetrability. In paints, the surface tension affects the ‘wetting’ between pigments and binders, and in painting for the ‘wetting’ between the surface of the object being painted and the paint itself. This significantly impacts the dispersion of pigments and the adhesion of the paint film. More information about surface tension is also available on this page.
2-2-5. Flammability of Solvents
Organic solvents generally catch fire easily, and their handling is strictly regulated under fire safety laws. Solvents with lower flash points are more prone to ignition, and explosions can occur within specific concentration ranges. Proper storage and handling are crucial.
2-2-6. Toxicity of Solvents
Organic solvents can cause irritation and anesthetic effects, which are transient and recoverable with exposure to fresh air. The Occupational Safety and Health Law and related regulations mandate the use of equipment, protective gear, and training for handling solvents. The allowable air concentration levels of organic solvents indicate their toxicity, which should be regularly measured and reflected in facility management.
Understanding and properly managing these general properties of solvents are essential for safe and effective use of paints on factory production lines.