Detailed explanation of the role of solvents in coatings and current problems

The role of solvents in coatings

The role of solvents in coatings is often not taken seriously by people because they are considered to be volatile components. It always evaporates without remaining in the paint film, so it will not have a great impact on the quality of the paint. In fact, factors such as the solubility and volatilization rate of various solvents have a great impact on the production, storage, construction, gloss, adhesion, surface condition and other aspects of the finished paint.

Solvents used in coatings are generally mixed solvents, consisting of three parts, namely true solvent and co-solvent and thinner. Solvents such as esters and ketones can dissolve both nitrocellulose and synthetic resins, such as acrylic resin, which are true solvents. Aromatic hydrocarbons and chlorinated hydrocarbons are true solvents for synthetic resins and non-solvents (diluents) for nitrocellulose. Alcohols are co-solvents for nitrocellulose and non-solvents (diluents) for synthetic resins. However, for synthetic resins containing high polar groups such as hydroxyl groups and carboxyl groups, alcohols are also true solvents. Aliphatic hydrocarbons (petroleum solvents) cannot dissolve general acrylic resins (except for polymers with longer side chain alkyl carbon chains).

When the paint is being constructed, the resin, pigment, and plasticizer in the paint are generally not suitable to be adjusted, but the solvent in the paint can be adjusted in proportion to achieve the best construction viscosity.

Current problems with solvents used in the coating industry

　1. Inadequate understanding of the performance differences of the solvent itself

The same solvent also has different quality specifications, resulting in large differences in performance when used in coatings: For example, mixed xylene has solvent type and mixed isomeric grade. The former contains more than 50% ethylbenzene. The solvent type xylene has less solubility than the mixed isomeric grade and cannot be used in plastic coatings and automotive coatings that require high quality. ; Another example is mixed aromatics S-100: it can come from cracking C-9 and reforming C-9 respectively. Although the olefins in cracked C-9 are saturated by hydrogenation, the total aromatic content is not enough, which is manifested by a downward shift in the initial boiling point and insufficient solubility. There is no problem in using them in ship coatings and anti-corrosion coatings, but the volatilization rate is affected by the use of them. The quality of wood coatings that have certain requirements and resins that also have solubility requirements will definitely be affected when they are manufactured. Resin factories usually use aniline point or mixed aniline point measurement to check and accept mixed aromatics S-100. The lower the aniline point or mixed aniline point, the stronger the solvent’s solubility and the higher the aromatic content.

　2. Inadequate understanding of the quality requirements of the solvent itself

　 In order to pursue price advantages, some factories do not hesitate to use low-quality solvents in formulas. For example, in the formula for mixed isomeric-grade xylene, solvent xylene with high ethylbenzene content is used, and even a large amount of non-standard low aromatic content dimethylbenzene is used. Toluene; many factories still use CAC, which is cheap but internationally banned.

3. Not very clear about the solvent quality requirements of the coating itself

　Different coatings have different requirements for the same solvent due to the different resins used. The main function of mixed aromatic hydrocarbons S-100 in nitrocellulose wood coatings is to adjust the volatilization speed. Too wide distillation range and low initial boiling point cannot work well; mixed aromatic hydrocarbons S-100 is not very suitable in ship coatings and anti-corrosion coatings. It is required to adjust the volatilization speed. Its main function is to reduce costs and adjust the construction viscosity. An appropriately wide distillation range has little impact on use.

4. Pay attention to the shelf life of certain solvents

　 Some active solvents have a shelf life such as diacetone alcohol. In principle, they should not be used after the shelf life because their internal active groups have changed; butyl acetate as a curing agent diluent requires The moisture content is low. If butyl acetate is stored for too long and improperly or during transportation, the moisture content may increase; MEK is also sensitive to moisture. Canned coatings have high moisture requirements, and MEK with excessive moisture content will cause Causes some canned paints to “discolor”.

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