No.119: Types and Features of Solvents

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About Solvents and Their Types 

1. What is a Solvent?

A solvent is a medium that dissolves substances and evaporates into the air if left exposed. While water, which dissolves water-soluble resins and metal surface treatment agents, is also a solvent, this section focuses on organic solvents that dissolve oils and resins. Water used as a solvent is detailed on another page

 Solvents not only dissolve resins and additives but also disperse pigments and additives, adjust the viscosity of the paint, ease the application process, and regulate the drying speed of the coating, significantly affecting the final appearance. Solvents are included in the resin varnish that is part of the paint ingredients and are present in the finished paint products, also used as thinners. Thinner, crucial for painting operation, finishing appearance, adhesion of the paint film, and durability, must be specifically designated for each paint type. 

2-1. Types of Solvents and Their Classification Methods 

 2-1-1. Classification by Solubility

    • True Solvents: Have the capacity to dissolve solutes (e.g., resins) on their own. Examples include aliphatic hydrocarbon solvents for oil-based paints, aromatic hydrocarbon solvents for amino alkyd resin paints, and ester solvents for nitrocellulose lacquers.
    • Co-solvents: Enhance the solubility when used in conjunction with true solvents, despite inadequate solvency on their own. For instance, alcohols for nitrocellulose.
    • Diluents: Used to reduce the concentration of solutes within a solution. For example, toluene in an ester solvent and alcohol solution dissolving nitrocellulose lacquer adjusts the concentration without causing separation or precipitation of nitrocellulose. Styrene monomer added to unsaturated polyester resin paints acts as a reactive diluent, helping in the curing process while reducing viscosity.

 2-1-2. Classification by Polarity

    • Polar Solvents: Contain polar groups such as hydroxyl, carboxyl, carbonyl, ketone, and ester groups. These solvents have high solvency for high-polarity binders and are also used in electrostatic painting.
    • Non-polar Solvents: Do not contain polar groups and include aromatic hydrocarbons (e.g., toluene, xylene) and aliphatic hydrocarbons (e.g., mineral spirits, petroleum naphtha). These solvents cannot dissolve polar compounds like nitrocellulose but can dissolve non-polar resins such as oil-based and long-oil alkyd resins.

 2-1-3. Classification by Boiling Point

    • Low Boiling Point Solvents: Boiling point under 100°C,

(e.g., ethyl ether, acetone, ethyl acetate, methanol, petroleum benzene.)

    • Medium Boiling Point Solvents: Boiling point from 100°C to under 150°C,

(e.g., toluene, xylene, MIBK, butyl acetate, butanol, Cellosolve.)

    • High Boiling Point Solvents: Boiling point over 150°C,

(e.g., diisobutyl ketone, carbitol acetate, cyclohexanol, Solvesso 100, 150, turpentine oil.) 

 2-1-4. Classification by Chemical Structure

The chemical structures of solvents are classified into the following groups:  

    • Aliphatic hydrocarbons
    • Cycloaliphatic hydrocarbons
    • Mixed hydrocarbons
    • Aromatic hydrocarbons
    • Alcohols
    • Ketones
    • Esters
    • Ether alcohols
    • Ester ethers

 In the following section, we will explore the general properties of solvents (2-2). 

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