Relative humidity , solvent system evaporation

Precipita.tlon. An ink may also be caused to dry by precipitation of its binder rather than by evaporation of solvent. This can be accompHshed by a dding a diluent, such as water in the form of steam or humidity, to a hygroscopic solvent ink system, which causes the solubiHty of the resin in the ink film to decrease sharply and causes it to precipitate when its tolerance for the diluent is reached. Eurther drying is accompHshed by absorption of the solvents into the stock and then by evaporation. Another form of precipitation setting is the quick-set mechanism. This utilizes resins held in solution in a relatively poor solvent, by means of a small amount of an exceUent solvent (called a sweetener) blended with it. When the ink film is printed on the paper, an amount of the solvents is absorbed reducing the content of the sweetener solvent to a point which causes the resins to precipitate and the ink to set. 

Figure 7.1.25. Initial evaporation rate from waterborne coating vs. relative humidity for two solvent systems. [Data from S Kojima, T Moriga, Polym. Eng. Set, 35, No.l3,1098-105 (1995).]...

An increase in relative humidity does not affect the evaporation rate of xylene (xylol) but with water the evaporation rate decreases with increasing relative humidity. The vapour pressure of water increases with increasing temperature, hence the atmosphere above the coating system holds more moisture (solvent) as the temperature is increased and thus greater air flow rates are required for efficient flash off and stoving times. 

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