5 декабря, 2015 
Pokraskin Some important physical and chemical characteristics of solvents from the standpoint of their use in paints and coatings are discussed below.
Dissolving a resin to yield a solution with viscosity suitable for application is one of the primary roles of solvents in paints and coatings. Therefore, solvency is the foremost performance requirement of a solvent. The solvency or solvent power is the efficiency with which a solvent can reduce the intermolecular forces between polymer molecules and separate them to solvate them in the solvent. One of the methods of presenting the solvency of a solvent (or solvent mixture) is by a viscosity profile of a solution of resins. The data are normally represented by a plot of viscosity of resin solutions in a given solvent (or solvent mixture) against the percent resin solids at a constant temperature. In coating formulations, the normal aim is to have as little solvent as possible while achieving a viscosity sufficiently low for efficient application. Even though the Hansen three dimensional approach based on solubility parameters is a more scientific way to understand the solvency of solvents, some empirical test methods have traditionally been used as a quick tool to rate solvency.
The Kauri-butanol value, used to rate solvency of hydrocarbon solvents, is defined in ASTM Method D 1133 as the volume in milliliters of the hydrocarbon solvent at 25 °C that is required to produce a defined degree of turbidity when titrated into a specified quantity of a standard clear solution of kauri resin in n-butyl alcohol. Kauri — butanol values range from approximately 20 for the weakest hydrocarbon solvents to more than 100 for stronger aromatic solvents. The test is limited to hydrocarbon solvents and is not applicable to oxygenated solvents. As Kauri resin (a natural resin) is no longer easily available, the test has not maintained its acceptance in the industry. Aniline point (or mixed aniline point), another measure of solvency of hydrocarbon solvents, is more accepted. The aniline point is defined as the minimum temperature at which equal volumes of aniline and a solvent sample become completely soluble. The strongest solvent exhibits the lowest aniline point, while the weakest one exhibits the highest. To avoid measurement below 0 °C, a mixed aniline point is measured in which the solvent being tested is blended with an equal volume of n-heptane.
The toluene dilution ratio is used for rating solvency of oxygenated solvents. This test method involves the determination of the volume ratio of toluene to active solvent required to cause precipitation in a standard solution of nitrocellulose. Toluene dilution ratios vary from approximately 6 for very strong solvents to approximately
1.5 for weak solvents.
All these empirical test methods for arbitrary rating of solvency have the common limitation that they compare the ability of a solvent to keep in solution one solute, namely kauri resin, aniline and nitrocellulose for Kauri-butanol value, aniline point and toluene dilution ratio, respectively.
Опубликовано в рубрике 