The role and mechanism of volatile solvents in UV coatings

1. The role of volatile solvents in coatings

The role of solvents in UV coatings is often not taken seriously by people, who think it is a volatile component. Finally, 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 for coatings are generally mixed solvents, consisting of three parts, namely true solvent, co-solvent and diluent. Solvents such as esters and ketones can dissolve various celluloses and UV resins and are true solvents. Alcohols are non-solvents (diluents) for UV resins, but for synthetic resins containing high polar groups such as hydroxyl 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, reactive diluent, and pigments and fillers in the UV paint are generally not suitable to be adjusted, but the solvent in the paint can be adjusted arbitrarily in proportion to achieve the best result. Construction viscosity.

2． Principles for selecting volatile solvents

● Principle of similar miscibility

Various polymer compounds and various solvents are It has different properties due to the influence of the configuration of its molecular structure, the type and number of polar groups, the length of the molecular chain and other factors. If the polymer compound is a polar molecule, it must be dissolved in a polar solvent; if the polymer compound is non-polar, it will be dissolved in the non-polar solvent. This is the law of like-like dissolving. Nitrocellulose molecules have strong polarity, so they are soluble in polar solvents such as esters and ketones, but insoluble in non-polar solvents such as hydrocarbons.

●Solubility parameter principle

Any kind of polymer material has intermolecular forces that can bring its molecular particles together. This effect can be called is the cohesive energy, the cohesive energy per unit volume is the cohesive energy density (CED), and the square root of the cohesive energy density is defined as the solubility parameter. The solubility parameter can be used as a reference index for selecting solvents. For non-polar polymer materials or polymer materials that are not very polar, when the solubility parameter is equal to or the difference between the solubility parameter of a certain solvent is not more than ±1.5, the polymerization The substance will be soluble in this solvent, otherwise it will be insoluble.

● Mixed solvent principle

When choosing a solvent, in addition to using a single solvent, you can also use a mixed solvent. Sometimes two solvents alone cannot dissolve a polymer, but the same polymer can be dissolved by mixing the two solvents in a certain proportion. Mixing solvents has synergistic effects and can be used as a method of solvent selection. Determine the ratio of the mixed solvent so that the solubility parameter of the mixed solvent is close to the solubility parameter of the polymer, which is finally determined by experimental verification.

●Solubility of the solvent

The solubility of a solvent to a polymer compound can be determined by the dissolution rate and viscosity of a solution of a certain concentration and the tolerance of the solution to non-solvents. degree (dilution ratio) and other aspects. The dilution ratio refers to the maximum number of parts of non-solvent that a solvent can tolerate. Beyond this value, the solubility will be completely lost. The resin precipitates and the solution is destroyed.

●Volatilization rate of solvent

Solvent is a volatile liquid, and the paint film spreading and flow in UV coatings containing volatile solvents are Flat surface ability is closely related to solvent evaporation, so the rate of solvent evaporation has a great impact on the appearance and quality of the paint film. The first thing that comes into contact during the construction process is the speed of solvent evaporation, which is proportional to the solvent evaporation time given at the construction site. If the time is short, the solvent must evaporate too fast, which will affect the leveling, gloss and other indicators of the paint film; volatilization Slower can ensure the leveling of the paint film and prevent orange peel, whitening, etc., but it may easily cause the hidden danger of incomplete evaporation of the solvent in the dry film.

The volatilization rate of a solvent is determined by three major factors: boiling point, molecular weight and molecular structure of the solvent itself. It is generally believed that low boiling point solvents have high vapor pressure at room temperature and evaporate quickly. Solvents are usually divided into low boiling point solvents, medium boiling point solvents and high boiling point solvents. Low boiling point solvents refer to solvents with a boiling point below 100°C; medium boiling points are between 110 and 145°C; high boiling points are between 145 and 170°C, and solvents with a boiling point above 170°C are called extra high boiling point solvents. But this is not entirely true. For example, the boiling points of ethyl acetate and ethanol are both around 78°C, which means that at 78°C, the saturated vapor pressure of both is 760mmHg, but at 30°C, the vapor pressure of ethanol is 79mmHg. column, ethyl acetate is a 120mmHg column, and the evaporation rate of ethanol is only 40% of that of ethyl acetate. The reason is that in addition to being affected by vapor pressure, the volatilization of the solvent is also related to the molecular weight of the volatile substance. The molecular weight of alcohol solvents is lower than that of esters, and their weight is much smaller when the same number of molecules is volatilized. In addition, if hydrogen bonds can occur between volatile substances, their volatilization speed will also be low.

There are two ways to express the evaporation rate of solvent. One is to take the volatilization time of unit mass of ether as 1, and the ratio of the volatilization time of unit mass of other solvents to the volatilization time of ether is the volatilization rate of the solvent. The second method is to use acetic acid for a certain period of timeThe line rises, and then suddenly turns to form an obtuse angle and changes to a low slope to rise. Such a solvent formula has greater disadvantages.

②The ratio of true solvent, co-solvent and diluent should be balanced. The ratio of true solvent, co-solvent and diluent has a great influence on the viscosity of paint. During the volatilization process, as the non-volatile content gradually increases, the viscosity of the paint increases. If a large amount of true solvent volatilizes at this time, the proportion of diluent will increase relatively, which will cause the viscosity of the paint to suddenly thicken and lose fluidity. Cause bubbles, orange peel and other paint film defects. In addition, the main function of the solvent is to keep all non-volatile components in a solution state before drying to form a film, and to prevent any component from being insoluble and precipitated. Otherwise, the continuous phase of the paint film will be destroyed and the surface will be rough. Loss of light and other phenomena. In order to prevent precipitation during the drying process, the solvent components must be well balanced according to the properties of the non-volatile components. It is necessary to save the true solvent and not use excessive diluent. When considering the formula, not only must there be enough true solvent, but it must also be balanced according to different evaporation rates, so that the solvent remaining in the paint film can remain no less than the original solvent ratio.

(Source: Paint Paint Network)