Drying times

Volatility is one of the most important properties of a hydrocarbon solvent. Volatility has a direct relation to the time it takes to evaporate the solvent and, therefore, to the drying time for the dissolved product. The desired value of volatility varies greatly with the nature of the dissolved product and its application temperature. Therefore, whether it be an ink that needs to dry at ambient temperature, sometimes very fast, or whether it be an extraction solvent, the volatility needs are not the same. 

Finishing sprays and spritzes are sprayed on clean, dry hair and tested for drying time, stiffness, combing ease, flaking after combing and stickiness upon contact with moisture. Additionally, hair hoi ding properties are evaluated by measurements of the cud retention at various humidities. Standardized water-set cuds are sprayed with standard amounts of product under controlled circumstances. They are then dried, placed into vadous humidity levels, and the cud fall monitored at vadous time pedods. 

The drying-time comparisons are based on the use of ethyl acetate as standard at 10. Acetone is about twice as fast as ethyl acetate and is Hsted as 5. 

To reduce drying time, maximum airflows are used during the first portion of the kilning cycle until the exit air is no longer saturated with moisture. Airflow then is reduced or recirculated to conserve energy. Average fuel consumption for a United States kiln is ca 5.9 x 10 kJ/t (1.4 x 10 kcal/t) ) malt, with a range of 2.9-10 X 10 kJ/t of malt. 

Solvents. Solvents in house paints serve several essential purposes. They keep the binder dispersed or dissolved and the pigments dispersed in an easy-to-use state. Solvents allow the paint to be appHed in the correct thickness and uniformity, and evaporate from the paint film after the paint is apphed. Solvent choice is limited mainly to a solvent that is compatible with the binder system and that has the desked evaporation rate and toxicity profile. The volatility or evaporation rate of a solvent determines to a large extent the open-time and dry-time properties of a paint (6). 

Bonding properties of water-based contact adhesives are similar to those of solvent-based systems, but are free of flammabihty ha2ards. However, drying times are longer and the bond is sensitive to moisture. 

The environmental concerns associated with the use of toluene, a toxic and flammable aromatic hydrocarbon, as a gravure ink solvent must be addressed. Whereas ink manufacturers are working on the development of water-based inks, the slow drying times and poor printing quaUties of the prototype products have impeded commercia1i2ation. Furthermore, the high cost of these materials is seen as a barrier to their introduction. 

Inks. The main components of the inks ate typically water, colorants, and humectants. Additives ate used to control drying time, waterfastness, lightfastness, and consistency of drop formation. Water is an excellent vehicle for ink jet because of its high surface tension and safety in all environments. 

Spray-dried powders find application in adhesives to build solids, increase viscosity, improve tack, and decrease drying time (145). A primary use is in joint compounds with other applications in mastics and grouts, and patching compounds (146). 

Table 5. Typical Softwood Kiln-Drying Time Schedules for 2.5-cm Ponderosa Common Pine...

Active Dry Yeast (ADY). The production of active dry yeast is very similar to the production of compressed yeast. However, a different strain of yeast is used and the nitrogen content is reduced to 7% of soHds compared with 8—9% for compressed yeast. The press cake made with the active dry yeast strain is extmded through a perforated plate in the form of thin strands with a diameter of 2—3 mm and a length of 3—10 mm. The strands are dried on endless belts of steel mesh in drying chambers (a continuous process) or in roto-louvre dryers (a batch process), with the temperature kept below 40°C. Drying time in drying chambers is 3—4 h and in roto-louvre dryers is 6 h or more. The final moisture level attained is 7.5—8%. 

Instant Active Dry Yeast. Instant ADY (lADY or HADY) production is similar to ADY production but requires a different strain of yeast. After pressing, the yeast is extmded into noodles 0.2—0.5 mm in diameter and 1—2 cm long and deposited on a metal screen or perforated plate in a fluid-bed air dryer. Drying time is shorter than with ADY, about 1—2 hours in practice, with a final moisture level of 4—6%. Instant active dry yeast does not require separate rehydration. It is always packaged in a protective atmosphere or under vacuum. On an equivalent soHds basis, the activity of lADY is greater than that of regular ADY, but stiU less than that of compressed yeast. 

Besides the solvent composition, the vehicle system is responsible for various drying deficiencies associated with water-borne coatings, such as slow initial dry time, loss of dry, poor through drying, and hardness (see Coatings). 

Loss of Dry. When the initial dry time of a solvent-based coating becomes substantially longer after aging, it is said to lose dry. The primary cause of this problem has been identified as adsorption of the drier on the pigment surface. Pigments with large surface areas are the worst offenders. 

For materials that foUow equation 30 the drying rate is inversely proportional to material thickness falling rate drying time is estimated,... 

Fig. 8. Drying time and rate profiles for leather pasted on glass plates and dried in two temperature stages. Gas velocity = 5 m/s in parallel flow, 71°C in the first stage, 57°C in the second. The falling rate, drying rate is proportional to residual moisture content.

---