The interpretation of solvents by a senior coating technician

2. Classification of solvents?

Solvents can be divided into true solvents, co-solvents, film-forming solvents and pseudo-solvents. It is true solvents that can dissolve polymers. In industrial water-soluble paint, it is the co-solvent that enhances the solubility of the true solvent. In the latex paint, it is the film-forming solvent that helps the latex particles flow, elastically deform and form a film. Almost 100% of the solvents used in UV coatings are film-forming solvents. What cannot dissolve the resin is a pseudo solvent. False solvents are used for dilution, viscosity reduction and cost reduction. But mixing a suitable pseudo-solvent can dissolve a specific resin.

Solvents are classified in other ways into hydrocarbons and non-hydrocarbons, polar and non-polar, oxygen-containing and non-oxygen. Mixed classification descriptions, for example, hydrocarbons include aliphatic hydrocarbons and aromatic hydrocarbons, non-hydrocarbons include oxygen-containing ketones, alcohols, esters, and nitrogen-containing solvents and halogen-containing solvents. Hydrocarbons are non-polar or very low polarity, and oxygenated solvents are medium or high polarity. Solvents containing hydrogen bonds can cause polarization.

3. What is the role of the solvent in the paint?

●Solvents are used as carriers for polymers, pigments and other ingredients in coatings

The solvent provides the necessary fluidity for film formation

●Due to the low surface tension, the solvent can improve the wetting of the pigment and the substrate

●Lower alcohols are used for the storage stability of amino resins and silane-containing coatings, the most commonly used is n-butanol

●A small amount of hydrogen-bonded solvent can improve the thixotropy of fumed silica, and bentonite as a thixotropic agent also requires a small amount of polar solvent to activate

The correct solvent can improve the stability of the color paste, and the wrong solvent can cause flocculation, flow, floating or other defects

●Solvent contributes to the coating with high gloss and smooth appearance

●The bite of the solvent makes the top coat adhere to the primer, and the interface water is also a super solvent to eliminate the traces of repair overlap

●Different solvent combinations make the coating easy to apply under different temperature and humidity

● In non-aqueous dispersion coatings, the carrier solvent can keep the particles stable without dissolving the polymer particles

●The thinner used for spraying can reduce the viscosity after being added to facilitate construction, and most of the volatilization during spraying and flashing

●Aluminum powder needs to be immersed in a solvent to stably exist in the paint

4. Why is the choice of solvent important?

Solvents have important effects on the toxicity, flammability, cost, workability, flow and leveling, substrate wettability, appearance, hardness, flexibility, and VOC of coatings. Decades ago, it was very easy for formulators to choose various traditional solvents for solvent-based coatings. However, in recent years, as people recognize the toxicity of traditional solvents to the human body and the impact on the environment, as well as the application of new solvents, new resins and new processes, how to combine various solvents for a given coating and process has become relatively complicated and difficult. Selecting a solvent for a coating or resin requires consideration of a series of conditions, including toxicity, availability, flammability, boiling point, solubility, volatility, cost, odor, hydrogen bonding, viscosity, and surface tension.

Nowadays, the formulations of traditional solvent-based paints are easily obtained by Baidu. In the past, broad laws and regulations, traditional solvent-based paints tend to use high-molecular-weight resins (such as CAB resin, high-molecular-weight acrylic resin and polyester resin), with a large amount of solvent, low Apply under the condition of low solids viscosity to obtain good performance. The high molecular weight resin prevents sagging during the drying process and provides good durability, and the extensive use of solvents makes the paint have low viscosity, low surface tension and other good application properties. This design allows a wider range of solvent options and a wider application range. The basic principle of choosing a solvent for a specific resin system is to match a suitable fast-drying diluent to the true solvent that evaporates slowly.

The volatilization speed of the diluent needs to be equal to or faster than that of the real solvent in order to maintain the balance of the dissolving power during the drying and curing process. As the diluent volatilizes, the solvent dissolving power gradually increases. The peak of the dissolving power should preferably occur after 70% of the solvent volatilizes. Before it happened.

5. Replacement of solvent

Each company has its own set of emergency management methods for emergencies. Paint manufacturers will prepare a variety of measures to ensure the stability of their products. Among them, the supply of solvents in the formula is not timely, and the purchase price of raw materials has risen sharply. Prepare the second set of plans, namely, carry out solvent replacement and produce paint according to the standby formula. However, it is not easy to replace the solvent, because no two solvents have the same performance, and the solvent affects many properties of the coating. It is not enough to consider only the solubility when replacing, because the volatilization speed, viscosity, and surface tension will be different. The first element of replacement consideration should be the volatilization speed. The key to replacement is to design a mixed solvent that can obtain the necessary performance. Sometimes a single solvent can be used to replace it, but usually multiple mixtures are required, and even the entire solvent system needs to be redesigned to remove A problematic solvent.

Ideally, the replacement solvent needs to have the same or very close volatilization rate to the replaced solvent in the formula, and the dissolving power is similar, but in fact the acceptable range is relatively wide. Solubility is an important parameter, and the range of solvency parameters is wide. If the solvency parameter is within 1-1.5 units, it is generally acceptable. A good solvent replacement workflow is to find some solvents with similar volatilization rates in the solvent manual first, and then check the other properties of the solvent. Depending on the product category and resin type, some solvents are excluded due to the presence of reactive functional groups, excessive toxicity, odor, price, viscosity, and surface tension. By comparing various parameters among solvents, the selection range can be narrowed to two to three solvents, which can be added to the formula for verification.