Лако-красочные материалы — производство

While the solvency of solvents is important in dissolving resins and controlling paint viscosity for ease of application, their evaporation rate is equally important, as a solvent must leave the coating film by evaporation at a controlled rate in order to give a uniform and defect-free dry film. Selection of solvents is important in controlling gloss, appearance and a number of coating defects that are related to evaporation rate of solvent during application and film formation.

As a solvent begins to evaporate, the viscosity of the wet film incre­ases and its ability to flow spontaneously decreases. For good film formation, therefore, the solvent must evaporate at a controlled rate such that good flow and leveling occurs before the wet film beco­mes excessively high in viscosity. Fast evaporating solvents help in reducing sagging of coatings on vertical surfaces and require shor­ter flash-off time and tack-free time, but may present difficulty in flow-out to form films without defects, such as orange peel or brush marks. If solvent evaporates very rapidly from a wet film, its visco­sity will rapidly rise and there will not be sufficient time to flow and level, resulting in non-uniform film formation that results in poor gloss and appearance. Furthermore, use of very fast evaporating sol­vents, such as acetone, may result in sudden cooling of the surface upon solvent evaporation, resulting in condensation of atmospheric moisture and its subsequent diffusion into the wet film. This causes blushing, a defect characterized by undesirable white opalescence, especially in environments of high humidity.

Slow evaporating solvents, on the other hand, provide better flow and leveling properties, but the coating may sag on vertical sur­faces and solvents may be trapped in the film during curing, which may result in longer tack-free time. In extreme conditions, solvents trapped in the film may cause blistering, a serious defect in which vapors of trapped solvent put sufficient pressure on the film to cause formation of a blister and subsequent breakage of the film. Because of conflicting advantages and disadvantages of both fast and slow evaporating solvents, in most coatings, formulators often use a blend of solvents with different evaporation rates to balance the require­ments of the film formation process and performance of coatings.

Evaporation of solvents from a wet film takes place in essentially two stages: initially, the solvent loss is primarily dependent on the volatility (related to vapor pressure) of the solvent, and at the later stage, as film formation proceeds, solvent loss by evaporation become increasingly difficult and essentially becomes diffusion controlled, a slow process. This latter stage may be dominant when as much as 20 % of the solvent is retained in the film.

In general, among the important factors affecting evaporation rate of a solvent from a wet film are vapor pressure of the solvent, tem­perature, the air flow rate at the surface, and the wet film thickness.

Measurement of evaporation rates is done very simply by a gravime­tric method. Shell developed an automatic evaporometer described in ASTM D 3539. Two methods are commonly used: evaporation from a thin film of the liquid, or evaporation from filter paper, using equal volumes of liquids in each case. The filter paper method is pre­ferred. The evaporation time of a given amount of solvent is deter­mined experimentally under identical conditions and compared with that of butyl acetate or diethyl ether. The relative evaporation rate is defined as:

E (butyl acetate) = t90 (butyl acetate) / t90 (test solvent)

E (diethyl ether) = t (test solvent) / t (diethyl ether)

Where t90 = the time when 90 % of the test solvent or butyl acetate evaporates in a given type of equipment under the test conditions and t = evaporation time of the test solvent or diethyl ether in seconds.

The results relative to diethyl ether = 1 are termed evaporation number (EN), which is more commonly used in Europe. On the basis of these numbers, solvents are classified into four categories: high volatility (EN < 10), medium volatility (EN 10 to 35), low vola­tility (EN 35 to 50) and very low volatility (EN > 50). In the United States, the volatility of a solvent is normally evaluated relative to butyl acetate = 1, and is classified as Class 1 (rapid evaporation, E > 3.0), Class 2 (moderate evaporation, 0.8 < E < 3.0) and Class 3 (slow evaporation, E < 0.8).