Which Solvents to Use

Q. I’m new to formulating a sprayable solvent-based epoxy paint and my question mainly involves the choice of solvents. Although I know each solvent has its own evaporation rate and solvent power, why do most formulators use more than two types. For example, why would toluene, MEK, MIBK, xylene be used in one paint formulation? Couldn’t I get a specific evaporation rate, viscosity, sprayability, VOC’s, etc. by just using MIBK and MEK? Or are xylene, toluene or other solvents having more of a major play in a formulation than a carrier? B.G.

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A. There are functions that each solvent needs to perform and all need to work together to provide the optimal properties for a coating. Some of the properties a formulator is trying to achieve when formulating a paint would be evaporation rate (affects drying time), viscosity (affects spraying and application characteristics), solvent strength (affects the ability of the formulation to dissolve binders), environmental status (volatile organic content or VOC and hazardous air pollutants or HAPs) and cost. As you can see, it is a juggling act to see how you can optimize all these parameters. Many times there are trade offs as to how the formulation will take shape based on the priority of these requirements.

For example, the customer may require the paint supplier to provide a formulation with low VOC, no hazardous air pollutants and a relatively fast dry time. Given those requirements, it limits the amount and selection of some solvents available. For instance, the MEK would provide possibly the fastest dry time of the solvents (possibly too fast) but would likely also create problems with the environmental requirements.

Additionally, once the correct solvents are chosen to provide the required properties of the formulation, other solvent properties need to be taken into consideration. The relative solvent power of each possible solvent would also need consideration. The solvency power of industrial solvents is often reported as a K-B or Kauri-Butanol value. The higher this value, the higher the solvency power is for a given solvent. This value required for a given formulation is usually a function of the binder system in the paint. A solvent such as toluene will have a higher K-B value than something like mineral spirits.
So in the end, the formulator has to juggle the requirements of the customer, the physical properties and limitations of the solvents to optimize application efficiency and the overall cost of the system.