Ink evaporation

The partial ionization of carbonic acid produces hydronium ion, H+, driving the indicator equilibrium to the weak acid form. A colorless solution results. As the water in the ink evaporates, the white residue of sodium carbonate remains. 

Jetting reUabiUty also Umits the molecular weight of the binder used. In the vicinity of the nozzle, the more volatile components of ink evaporate. When the resin drops out of solution this wiU cause nozzle blockage. In addition, high molecular weight polymers may lead to viscosities outside the print head range. Polymers used normaUy have a molecular weight below 100 000 and often below 50 000. Common polymers used are vinyl chloride/vinyl acetate copolymers, acryUc resins and polyketone resins. 

Figure 1 illustrates how this new type of paper allows for color writing with a colorless ink. The paper is typically a crystalline lattice of polystyrene (PS) beads whose voids are con iletely filled widi poly(dimediylsiloxane) (PDMS). The ink is a silicone fluid or any odier liquid capable of swelling PDMS (11). As the ink is applied to die surface of the p er, the position of stop band will be shifted too a new wavelength, and thus the color Splayed by crystalline lattice will change. If the colors of these two states are sufficiendy different to be distinguishable by the naked eye, one can use dieir contrast to achieve color writing with materials that are colorless by themselves. As the ink evaporates. 

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. 

The variant of the cylindrical model which has played a prominent part in the development of the subject is the ink-bottle , composed of a cylindrical pore closed one end and with a narrow neck at the other (Fig. 3.12(a)). The course of events is different according as the core radius r of the body is greater or less than twice the core radius r of the neck. Nucleation to give a hemispherical meniscus, can occur at the base B at the relative pressure p/p°)i = exp( —2K/r ) but a meniscus originating in the neck is necessarily cylindrical so that its formation would need the pressure (P/P°)n = exp(-K/r ). If now r /r, < 2, (p/p ), is lower than p/p°)n, so that condensation will commence at the base B and will All the whole pore, neck as well as body, at the relative pressure exp( —2K/r ). Evaporation from the full pore will commence from the hemispherical meniscus in the neck at the relative pressure p/p°) = cxp(-2K/r ) and will continue till the core of the body is also empty, since the pressure is already lower than the equilibrium value (p/p°)i) for evaporation from the body. Thus the adsorption branch of the loop leads to values of the core radius of the body, and the desorption branch to values of the core radius of the neck. 

An ink is considered dry when a print does not stick or transfer to another surface pressed into contact with it. Drying is accompHshed by one or more of the following physical or chemical mechanisms absorption, evaporation, precipitation, oxidation, polymerization, cold setting, gelation, and radiation curing. 

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. 

Infrared and Microwave Inks. These ate inks which have been formulated to absorb these radiant energies. The energy causes the inks to heat and dry through the partial evaporation of solvent. Absorption of the ink into a porous substrate can also be part of the overall drying mechanism with these inks. They have not found wide commercial success due to the variabiHty of the it absorption with ink color and the energy inefficiency of microwave systems in drying nonwater-based inks. 

The printing of newspapers is conducted at very high speeds, often reaching 3000 feet per miaute. AH three printing processes utilize similar quaHty newsptint which, essentiaHy, is made of groundwood or thermomechanical pulp. Presses are fed a continuous web of newsptint that unwiads from a feed roUer. Inks dry by absorption of Hquid iato the porosity of the substrate. Some evaporation of water ia a flexo pubHcation ink can accelerate the dryiag process. 

Web Heat-Set Publication and Commercial Inks. Almost all heat-set inks are now printed on web offset presses, and are based on vehicles containing synthetic resins and/or some natural resins. These are dissolved in hydrocarbon solvent fractions which are specially fractionated for use in the ink industry. They vary in boiling range between 180 and 300 °C. Small percentages of alkyd resins (qv) may be contained in these inks. They dry in less than one second by means of solvent evaporation in a heatset oven. These ovens utilize high velocity hot air to raise the web temperature to 120-150 °C. 

Water-Based Writing Inks. These consist of very fine pigment dispersions in aqueous media containing small amounts of glycol or glycerol and a dispersing aid. They dry mainly by evaporation and quick wetting of ceUulosic fibers in paper substrates. 

Printing Inks. Printing ink preparation is similar to many coating systems. The resin is dissolved in the solvent, followed by pigment dispersion to produce the ink. In most printing operations, the solvent must evaporate fast for best production speed. Alcohol—hydrocarbon solvent combinations are used with polyamide resins for some printing processes (see Inks). 

Many of the materials used for boiler water antifoams are also employed as synthetic lubricant replacements for mineral oils, brake fluids, emulsion polymerizers, emolients, solvents for inks, cleaners, and antifoams for saltwater evaporators. These materials have been... 

In this context, the question has to be raised about the contribution offset heatset inks make to the mineral oil problem in RCP. The inks themselves can contain a significant mineral oil content even when mineral oil-free inks are available on the market. During the drying process, the majority of the oils evaporate. Nevertheless, in RCP from offset heatset printed magazines, mineral oil constituents will exist even though on a much lower level than in recovered newspapers and despite the... 

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