Selection of solvents for coatings

0 IntroductionThe author in the process of coating research and development and after-sales technical services, often encountered customers do not use manufacturers supporting solvents, but the use of some small workshops produced cheap solvent phenomenon, very random, often problems, this paper to analyze this problem.characteristics of a 1 solvent is a liquid that is used to dissolve or disperse a membrane substance, facilitates construction, and is volatile during film formation, customaryly referred to as volatile. Although the solvent is not a permanent part of the coating product, the solvent's solubility to the film determines the uniformity of the resin solution formed, the viscosity of the lacquer solution and the storage stability. In the process of coating film drying, the volatility of solvent greatly affects the drying speed, structure and appearance of the coating film. At the same time, the viscosity of solvents, surface stress, chemical properties and their effects on the properties of resin solutions, as well as the safety of solvent use, toxicity to the human body, etc. are must be considered.1.1 Solubilitydifferent types of solvents are required for different coating systems. Solvent-to-resin dissolution follows the principles of similar polarity, similar solubility parameters, and solventization. The solvent to be prepared should be able to fully dissolve the selected resin, and take into account costs and other factors to achieve a comprehensive balance.1.2 viscosityin the coating preparation process, it is hoped that the same concentration or solid content of the resin solution less viscosity, the better. In this way, when the same construction viscosity is achieved, the solid content of the lacquer liquid is higher, thus improving the construction efficiency, while the amount of solvent volatile into the atmosphere is less, reducing environmental pollution. Because of its solubility to high-concentrations and their own viscosity, solvents have a certain effect on the viscosity of coatings.1.3 Volatility Ratethe role of solvents in film drying is to control the flow characteristics of the coating film. If the solvent evaporates too fast, the coating film is too late to level, nor can the substrate have enough moisture, so it can not produce a good adhesion, but also due to rapid cooling led to water vapor condensation on the surface of the wet film and make the coating film white, if the solvent evaporation is too slow, not only will delay the drying time, but also cause the coating film to hang. Solvent in the evaporation process should also maintain a suitable volatility gradient, if the composition of the solvent is not ideal, it will cause resin precipitation and coating defects. Therefore, the evaporation rate of solvents is a very important factor affecting the quality of coatings and coatings.1.4 Surfacesurface stress of solvents can affect the surface stress of the coating. In coatings, low surface pressure solvents have a positive effect on the wetting dispersion of pigments, wetting of substrates, flat coating, adhesion of substrates, atomization properties of spraying, and preventing surface defects of coating films (e.g. sinks, shrink holes, frames, etc.). The surface strength of the solvent must be fully taken into account when designing coatings.1.5 resistivitydifferent solvents have different resistivity, in the case of electrostectrical spraying, resistivity becomes an important indicator. When using electrostectrical spraying for construction, you will often encounter some paints that are not easily charged, and by adjusting the solvent formulation, such as adding a small amount of deatonol to the paint formulation, its resistance can be reduced to a value suitable for construction.
1.6 Toxicity and safetySolvents are volatile organic substances (VOCs) that are more or less toxic to the human body. Depending on the effect on the human body, its classification is shown in Table 1.In coatings, the content of solvents directly represents the content of VOC, and different solvents in the air the highest permissible concentration, all countries have standards to limit. In addition, solvents are flammable and explosive substances, so the safety of coating products using different solvents must be considered in the production, storage, transportation and coating process.2 When designing a paint formulation, the choice of solventto produce the coating, the above factors must be fully taken into account in order to achieve the best coating formulation. In addition, solvents can have the following effects on coatings.2.1 Solvent reactivesome solvents will react with certain components in the coating, some of which are what we hope, such as unsaturated polyester resin and photo-curing resin, self-leveling epoxy flooring paint in the active thinner or monomer, wherein the active thinner or monomer can play a dilution, degluing effect, but also with the main resin reaction, cross-linking, so as to achieve high solid content, high film thickness, low CVO requirements. However, part of the reaction is not desirable, such as in polyurethane paint, if the use of alcohol-containing solvents, which contain hydroxyl and curing agent isocyanate base reaction, affecting cross-linking, while generating low relative molecular mass substances, so that the hardness of the coating film, durability, solvent resistance and other performance is reduced. Therefore, such reactions should be avoided in coating formulations.2.2 affects the storage stability of the coating if the paint is made, adding the right amount of alcohol solvent will improve its storage stability, while adding ester solvent will affect its storage stability. When used in aolic acid resins, it promotes drying and improves storage stability.
2.3 Some solvents have moisture absorption or high moisture content, the prudent use of
for polyurethane coatings, the solvent used needs to be dehydrated and other treatment, to meet the urethane grade standard before it can be used, otherwise it will seriously affect its storage stability, and affect the performance of coating.2.4 OdorFor some solvents, such as ketone solvents, may contain unpleasant odors, the choice of solvents should be avoided as far as possible.3 Anatomy using non-matching solvents General formal paint manufacturers in the design of coatings and solvent formulations, will try to consider the above factors. At the same time, according to the variety of coatings, construction conditions and other factors to adjust the formulation of solvents. Even in the same type of paint, different paint manufacturers use different resins, paint solvents and related solvents between manufacturers will also be different.themarket is riddled with many low-cost solvents, and the following problems may exist in applications: (1) Low-cost solvents contain so many impurities that it is difficult to avoid problems such as not containing alcohols or alcohol ether solvents, or high water content. If equipped with polyurethane coatings, may react with isocyanate curing agent, which affects the storage stability of the coating and coating performance.
(2) may be made with recycled solvents. The recovery solvent itself is not strictly controlled, and the stability between batches is poor, resulting in the uncontrollability of its products.(3) the solvent used by the general manufacturer will adjust the rate of solvent volatility according to the climate. Low-cost solvents rarely consider this problem, which will cause the solvent is too low or too high when the solvent does not match, the formation of paint film ills.(4) the general recovery solvents contain benzene, methanol and other harmful solvents, will have an impact on human health and the environment.
(5) when electrostectrical spraying, the suitable solvent formulation has a very good effect on adjusting the resistivity of the coating. On the contrary, low-cost solvents do not take this into account and are uncontrollable.(6) the volatility gradient of the mixture solvent in the coating filming process will affect the final state of the paint film. If, in the process of film-forming, the solvent with good solubility is volatile, and the solvent with poor solubility remains, it may cause the resin to be dissoquented due to insufficient solubility, resulting in particles on the surface of the coating film; Therefore, in solvent formulation design, it is necessary to take into account that the relative volatilization rate of the components of the mixed solvent should be maintained with the composition of the solvent, that is, to achieve solvent balance. Cheap solvents may be less considered in this regard.(7) the volatile rate of cheap solvents is generally relatively fast, if the control is not right or the ambient humidity is large, it is very easy to appear paint film whitening and other ills. (8) Some inexpensive solvents contain large amounts of toxic and harmful substances, have a pulient odor, and can have adverse effects on the human body and the atmospheric environment. 4 Conclusion In the process of coating use, although the solvent is volatile, only a small amount will remain in the dry film, however, solvent on the final performance of the coating has a great impact. The use of non-matching solvents may cause inconvenience to paint construction or cause paint film maladies. Cheap solvents on the market contain a lot of impurities, there is a risk in application, while the toxicity may be greater, adverse effects on the human body and the environment, users are advised to be cautious.