The Application of Solvents in the Coatings Industry

Solvents play a crucial role in the coatings industry, accounting for approximately 47% of materials used in coating applications. Their primary function is to dissolve and disperse film-forming materials, imparting fluidity to coatings for proper surface application. Although solvents evaporate after film formation and are not part of the final coating film, they significantly influence critical film properties such as gloss, leveling, and adhesion. The selection of solvents varies depending on the type of coating. This article will delve into the diverse classifications of solvents used in the coatings industry and their specific applications across various coating types, including natural oil-based paints, synthetic resin coatings, and cellulose-based coatings.

Classification of Solvents Used in Coatings

Solvents continue to play a crucial role in the coatings industry, accounting for approximately 47% of the materials used in coatings. The solvents used in coatings vary depending on the type of coating. During the coating process, solvents dissolve and disperse the coating materials, imparting fluidity to enable surface application. After the coating film forms, the solvents evaporate. Although solvents are not components of the coating film, they significantly influence the performance of the coating film (such as gloss, flowability, and adhesion).

Solvents used in coatings are generally classified using the following methods:

Classification by chemical structure

Hydrocarbon solvents (alkanes, alkenes, cycloalkanes, aromatic hydrocarbons); alcohols, esters, ketones, and ethers; halogenated hydrocarbons; nitrogen-containing compounds; and aldehydes, furans, acids, and sulfur-containing compounds.

Classification By boiling point

low-boiling-point solvents (boiling point below 100°C at atmospheric pressure); Medium-boiling-point solvents (boiling point between 100°C and 150°C); High-boiling-point solvents (boiling point above 150°C).

Classification by solvent polarity 

Polar solvents (referring to ketones, esters, etc., which have polarity, a large dielectric constant, and a large dipole moment); non-polar solvents (such as hydrocarbons with non-polar functional groups, low dielectric constants, and small dipole moments).

Classification based on solvent solubility 

1. Solvents: solvents that can dissolve solutes independently, generally without the inclusion of co-solvents or diluents.

2. Co-solvents (latent solvents) cannot dissolve solutes on their own but exhibit solubility when mixed with other components (e.g., alcohols dissolving nitrocellulose).

3. Diluents have no solubility for solutes, can dilute solutions without causing solutes to precipitate or settle, and are sometimes referred to as non-solvents. For example, hydrocarbons such as toluene, xylene, and heptane can all serve as diluents for nitrocellulose.

Plasticizers

Solvents with very slow evaporation rates can be used as plasticizers. Due to their slow evaporation rates, plasticizers remain in the coating film for extended periods, exerting softening and plasticizing effects. Their softening and plasticizing effects depend not only on the molecular weight and evaporation rate of the plasticizer but also on the mutual solubility between the plasticizer and the components of the coating film. Under the influence of the plasticizer, the intermolecular forces in the coating film are reduced, and the glass transition temperature of the coating film is lowered. However, this enhances the adhesion of the coating film and improves its elongation and bending properties.

Natural oil-based coatings

This category includes paints, varnishes, and enamel paints. The solvents used in these coatings must have the following properties:

• High solubility for drying oils and resins;

• Appropriate evaporation rate;

• Colorless or light-colored;

• Vapor evaporation without unpleasant odors or toxicity;

• Sulfur-free.

Generally, drying oils are easily soluble in hydrocarbon solvents. These solvents include petroleum-based hydrocarbons (such as 200" solvent gasoline, kerosene, crude gasoline with high aromatic hydrocarbon content, etc.), coal tar-based hydrocarbons (such as benzene, toluene, xylene, naphtha, etc.), and plant-based hydrocarbons (such as turpentine, etc.). Among these, coal tar-based hydrocarbon solvents have the strongest solubility, followed by plant-based solvents, with petroleum-based hydrocarbon solvents having the weakest solubility. However, due to the abundance of raw materials and low cost of petroleum-based hydrocarbon solvents, most general-purpose drying oils and natural resins still primarily use petroleum-based hydrocarbon solvents. When natural resins are used in combination with synthetic resins such as alkyd resins and urea resins, due to the poor solubility of petroleum-based hydrocarbon solvents, it is necessary to simultaneously add aromatic petroleum crude gasoline, coal tar-based solvents, as well as fiber-dissolving agents and butanol solvents.

Solubility of major natural resins in solvents

Synthetic Resin Coatings

(1) Thermoplastic Resins

Vinyl Resins

The high-molecular-weight resins used in coatings include polyvinyl acetate, vinyl chloride-vinyl acetate copolymers, polyvinyl alcohol formaldehyde, and acrylic or methacrylic polymers. These resins do not undergo chemical reactions before or after the formation of the coating film. Among these, polyvinyl acetate resin has poor water resistance and is not used alone as a coating. It is miscible with nitrocellulose. Low-molecular-weight polyvinyl acetate can be used as a solvent for mixed paints (with excellent adhesive properties, primarily used as an adhesive). Polyvinyl acetate resins are soluble in esters, halogenated hydrocarbons, nitropropane, and low-boiling-point aromatic hydrocarbons, but insoluble in anhydrous ethanol and petroleum-based hydrocarbons. They are insoluble or swell in ethers, easily soluble in propionic acid, and their solubility in alcohols is influenced by the water content in the alcohol.

Chlorinated vinyl and vinyl acetate copolymer resins are soluble in ketones, nitropropane, and other solvents, but tend to gel in ester solvents. Dilutants can be aromatic hydrocarbons, or a small amount of aliphatic hydrocarbons can be added. In copolymers, solubility generally decreases as the chlorinated vinyl content increases. Polyvinyl alcohol formaldehyde resins have excellent water resistance, adhesion, and strength, making them widely used in coatings. However, the properties of formaldehyde resins vary depending on the type of aldehyde, degree of polymerization, and degree of formaldehyde modification.The figure below shows the solubility of three types of aldol resins in various solvents.

Solubility of three types of aldol resins in solvents

Acrylic Resins

This class of resins is primarily composed of acrylic acid, methyl methacrylate, ethyl methacrylate, and butyl methacrylate, and exhibits similar properties. The solubility of these monomers and polymers in solvents is as follows: Solvents that can dissolve both monomers and polymers include aromatic hydrocarbons (benzene, toluene, xylene), chlorinated hydrocarbons (chloroform, dichloroethylene), esters (methyl acetate, ethyl acetate, butyl acetate), ketones (acetone, dialkyl ketones), and fiber-dissolving solvents.

Solvents that dissolve the monomers but not the polymers include: methanol, ethanol, isopropanol, butanol, gasoline, ethylene glycol, etc. As the degree of polymerization increases, the solubility of this type of resin in solvents decreases. However, as the number of alkyl carbon atoms in the resin increases, its solubility in alcohols and aliphatic hydrocarbon solvents increases. For example, poly(methyl methacrylate) can dissolve in gasoline and turpentine, while poly(methyl methacrylate) is insoluble.

Acrylic resins can be used alone or blended with other resins as coatings. Table 5-3 shows the formulation of acrylic varnish.

Dispersion coatings

This type of coating disperses high-molecular-weight resins using dispersants to form low-viscosity formulations, which are then heated to form continuous coatings after application. The dispersion types include:

• Organic dispersions:

  Organic sol—dispersing polymers in plasticizers and dispersants, then diluting with volatile solvents.

  Plastic sol—dispersing polymers using plasticizers.

• Aqueous dispersions.

  Latex—forming an aqueous dispersion of resin polymerized from monomers using an aqueous emulsion.

  Aqueous sol—forming an aqueous dispersion using dried polymers.

In these dispersible coatings, organic solutes primarily utilize solvents. In dispersible coatings based on ethylene resins, the primary resin is polyvinyl chloride (PVC). The resin particles are dispersed in a dispersant to form a sol. Dispersants with excessive swelling force result in high sol viscosity; dispersants with insufficient swelling force cause resin particles to settle, leading to an unstable dispersion system. Therefore, the type of resin and the dispersing ability of the dispersant must be appropriately selected and matched. Dispersants are generally polar compounds with moderate resin solubility, such as ketones, esters, ethers, alcohols, and nitroalkanes for polyvinyl chloride (PVC) resins. Dispersants with high solubility yield high-viscosity dispersions; those with low solubility yield low-viscosity dispersions. The role of diluents is to dilute the resin dispersion to an appropriate viscosity after dispersion. Aromatic hydrocarbons and aliphatic hydrocarbons are commonly used as diluents.

(2) Thermosetting Resins

Thermosetting resin coatings based on alkyd resins, urea resins, and phenolic resins require a particularly large amount of solvent. These resins have a low molecular weight (approximately 5,000) before use and only transform into large molecular structures through air or heat after application. Therefore, from the perspective of solubility performance, the solvent requirements can be referenced from those of oil-based coatings and varnishes.

Alkyd Resin

Alkyd resins are polymerized from phthalic anhydride and glycerol. They can also be produced using maleic anhydride, adipic acid (rosin acid) can partially or fully replace phthalic anhydride, pentaerythritol or ethylene glycol can replace glycerol, or oils or fatty acids can partially replace phthalic anhydride to modify alkyd resins. These resins have good miscibility with other resins and excellent coating properties, making them widely used in the coatings industry. The most widely used are oil-modified alkyd resins, such as oxidized resins modified with drying oils or their fatty acids; and paint resins modified with non-drying oils or their fatty acids. The solubility of these oil-modified alkyd resins varies depending on the ratio of oil to resin. Short-oil alkyd resins are easily soluble in aromatic hydrocarbons, with high solution viscosity; long-oil alkyd resins are easily soluble in aliphatic hydrocarbons, with low solution viscosity.

Urea resins and melamine resins

These resins are similar to alkyd resins in that they initially have low molecular weights and are easily soluble in alcohol-based solvents. However, unmodified urea resins and melamine resins tend to crack and age when using butanol or octanol as solvents, making them unsuitable for use as coatings. Generally, they are modified with alcohol to form alkylated urea resins and alkylated melamine resins. After modification, they have good miscibility with other resins, especially when mixed with alkyd resins for use in baked coatings, resulting in excellent film properties.

Butanol-modified urea resins and melamine resins are soluble in hydrocarbon solvents. Coatings using alkyd resins can add butanol or higher alcohols (20–25%) to enhance miscibility and ensure long-term stability. This coating can also incorporate small amounts of ethylene glycol ethers (e.g., ethylene glycol butyl ether, ethylene glycol acetate, diethylene glycol ether, etc.) to enhance the film's density, water resistance, solvent resistance, and chemical resistance, making it suitable as a substitute for nitrocellulose spray paint in magnetic coatings for automobiles, refrigerators, sewing machines, and other applications.

 Phenolic Resin

1. Alcohol-soluble resin: A resin formed by the condensation of phenol and formaldehyde under acid or alkali catalysis, divided into thermoplastic phenolic resin (acid-catalyzed) and soluble phenolic resin (alkali-catalyzed). The former is soluble in alcohols, while the latter is soluble in acetone.

2. Oil-soluble resins: Oil-soluble phenolic resins are produced by modifying acid-catalyzed phenolic resins with natural resins or drying oils, or by replacing phenol with phenol derivatives in the condensation reaction with formaldehyde. When used as a varnish in combination with drying oils, these resins can be dissolved in 200" solvent gasoline.

3. Benzene-soluble resins: The hydroxymethyl groups in alkali-catalyzed soluble phenolic resins are etherified to produce a product that is insoluble in alcohols

4. but soluble in benzene. It is generally used in combination with drying oils, alkyd resins, etc., as a baking coating.

Epoxy resins

The best solvent for epoxy resins is ethylene glycol acetate. They are generally soluble in ketones, esters, ketone-alcohols, and organic epoxides, but insoluble in hydrocarbons and alcohol-based solvents. Epoxy resins used in coatings are often blended with alkyd resins and melamine resins for baking applications, using ethylene glycol acetate and toluene as mixed solvents.

Cellulose-based coatings

The solvents used in cellulose-based coatings should be capable of dissolving the three main components—cellulose derivatives, plasticizers, and resins—that form the coating film, resulting in a low-viscosity solution suitable for application. Solvents evaporate on their own after application and do not participate in film formation. However, the selection of solvents significantly affects the performance of the coating film, and the choice of solvents also varies depending on the application method (e.g., brushing, spraying, dipping, etc.). Therefore, the solvents used in cellulose-based coatings are more complex than those used in oil-based coatings, varnishes, and thermosetting synthetic resins.

Conditions that solvents for cellulose coatings should meet:

• They should have a certain dissolving ability for cellulose derivatives and resins (plasticizers are generally soluble in most solvents);

• They should form low-viscosity solutions using as little solvent as possible;

• They should have moderate evaporation rates and produce good coating film properties (using a single solvent is difficult, so they are generally used in combination with appropriate solvents);

• The solvent should retain its solubility for the dissolved components until complete evaporation;

• The solvent should be neutral and not react with other components;

• It should have no hygroscopicity or moisture absorption;

• The vapor should be non-toxic and odorless;

• Good storage stability.

Solvents, co-solvents, and diluents

Solvents used in cellulose coatings are classified into three types based on their solubility of cellulose derivatives: solvents, co-solvents, and diluents. Solvents can dissolve cellulose derivatives; co-solvents cannot dissolve cellulose derivatives on their own but are added to solvents to enhance their solubility and reduce solution viscosity; Diluents do not dissolve cellulose derivatives and do not precipitate when added to a certain extent. They can be used as solvents for resins and plasticizers to reduce costs. Table 5-4 lists the solvents, co-solvents, and diluents used in nitrocellulose spray paint, and Table 55 lists the solvents used in cellulose acetate coatings.

Solvents, co-solvents, and diluents for nitrocellulose lacquer

Solvent for cellulose acetate coatings

Paint strippers

Organic flammable paint strippers

This type of paint stripper and the corresponding paint film are shown in the table below. The disadvantages are that it has weak paint stripping ability, is not effective on baked paint films, is flammable, and the paraffin used as an evaporation inhibitor is difficult to treat after paint stripping.

Organic combustible paint stripper

Organic flame-retardant paint strippers

This type of solvent primarily consists of chlorinated hydrocarbons (such as dichloromethane, chloroform, carbon tetrachloride, dichloroethane, trichloroethylene, and chlorobenzene), which exhibit strong solvent properties and excellent penetration and stripping performance on paint films. In particular, dichloromethane, when used alone or mixed with 20% alcohols or esters, demonstrates excellent stripping performance on baked melamine paint films. Its characteristics include strong paint removal capability, ease of water washing, and non-flammability. However, this type of solvent has poor paint removal performance on urea resin paint films.

Organic flame retardant paint stripper