Solvent-water blends evaporation

Evaporation Rate of Solvents Evaporation of Solvent-Water Blends Flash Point of Solvents Air Quality Regulations and Solvents Formulation of Solvent Blends Solvents for High-Solids Coatings... 

Evaporation of Solvent-Water Blends. The evaporation of blends of solvents and water presents several problems, the most prominent of which is the fact that the relative rates of evaporation of solvents and water in a given blend vary greatly under conditions of varying humidity. Under dry conditions the water evaporates relatively rapidly, but under high humidity the water evaporates relatively slowly in comparison to solvent. Usually it is desired that the water and solvent evaporate at rates such that the residual liquid remains constant in composition (perfect solvent balance). Often, enrichment of solvent in the residual liquid is desired, but depletion of solvent in the residual liquid might lead to resin kick-out or loss of ability of the resin to coalesce into a smooth, continuous film. One means of minimizing premature loss of... 

Computer Methods for Finding Solvent Blend Replacements and for Predicting Water/Cosolvent Evaporation at Any Humidity... 

Rocklin and Bonner (65) developed a computer method that predicts solvent balance and evaporation times of water-solvent blends at any humidity with any number of water-soluble organic solvents. The method also can be used for regular water-free solvent blends but Ignores humidity. Key considerations of the method are the following it uses the UNIFAC method for calculating activity coefficients it computes the actual evaporation temperature on the filter paper substrate it calculates evaporation rates at the calculated temperature by using the activity coefficients at that temperature humidity is accommodated by applying a correction factor to the water evaporation rate. Experimental data on several systems verified the computer calculations. 

Figure 13. Effect of relative humidity on solvent balance upon evaporation of sec-butyl aIcoho1-2-butoxyethano1-water blend. Reproduced with permission from Ref. 62. Copyright 1978 Federation of Societies for Coatings Technology.

Evaporation characteristics Actual evaporation time and solvent balance during evaporation of organic solvent and water based blends can be predicted under different humidity conditions. The most important parameters (see above) can be calculated and water pick-up by blends of oxygenated, polar solvents during evaporation predicted. 

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. 

Weigh 2.5 or 5 g of crop matrix into a blending vessel. Fortify samples at this point with the appropriate analytical standards. Allow the solvent to evaporate. Add 100 mL of acetone-water (4 1, v/v) and blend the mixture using an Omni mixer equipped with a macro generator for 5 min at 6000-7000 rpm. Filter the sample through a Whatman 934 AFI glass-fiber filter paper on a Buchner funnel/vacuum flask setup. Rinse the blending cup and filter cake with 100 mL of acetone. Transfer the filtrate into a 200-mL TurboVap vessel. 

Formulators can use the tendency of creams, gels, and other systems to evaporatively concentrate to advantage. Solvents are chosen and blended so that the drug remains soluble in the formed film long after application is made. This can be accomplished by replacing a fraction of the water or other highly volatile solvent found in these systems with solvents of far lower volatility. As previously pointed out, 5-15% propylene glycol is found in many topical corticosteroid creams and lotions just for this reason. 

The Water Base Paint in an 80/20 Blend. In this case, a 45 solids coating will be utilized with an 80 water and 20 solvent base. As before, 68.4 gallons of water and solvent will be removed per hour of operation for a total of 456 pounds of water. This, in turn, will require a heat input of 0.5) x 10° BTU/hr for evaporation. 

Figure 2a. Computed evaporation curve at 15% humidity for a solvent blend composed of 70% w water and 10% w each of 2-butoxyethanol, dimethylformamide, and 4-methoxy-4-methyl-2-pen-tanone. 90% evaporation time = 1178 at25°C.

Fig. 51 Preparation of the pristine polymer and polymer-polymer blend nanoparticles is presented. Step 1 a by ultrasonication a polymer solution and water/surfactant solution are blended to a miniemulsion and transformed by solvent evaporation to an aqueous polymer-in-water nanoparticle dispersion. Step 2 b Films are spin cast from the dispersion. (Reprinted with permission from [239], 2003, Macmillan Publishers Ltd)...

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