Direct Steam Mixing Jet Pasting of Slurries, or Peeling the OnionAll hydratable adhesives and colloids share a common denominator: They are composed of multiple compacted layers of dehydrated, high molecular weight polymer chains. Starch, polyvinyl alcohol and other natural or synthetic gums are purchased as granular solids varying in particle size from 10 Km to 0.03125" dia. The objective of jet pasting (cooking) is to hydrate and disperse each successive polymer layer in each granule by stripping away fully hydrated individual polymer chains to form a stable colloidal sol.
Figure 1 illustrates a functional cross-section of a modern version of Coppock's starch pasting device. It closely resembles the jet pumps employed to move liquids and some slurries. High pressure saturated steam, ranging from 60 to 200 psig, is injected through a steam nozzle into the center of a venturi mixing tube. Instantaneously, the entire slurry mass is pulled into the annulus gap formed by the steam nozzle and venturi opening. It is heated above the gelatinization temperature of the colloid as it accelerates to sonic velocity within the mixing tube.
Immediately, partial condensation of the steam occurs as it combines with the much cooler slurry. However, much of the steam remains at a pressure high enough to provide a temperature well in excess of the polymer's gelation temperature. As the highly swollen adhesive particles, water and steam pass through the cooker's mixing tube, several dynamic reactions occur.
Initially, a high velocity (1,500 FPC) conical-shaped steam jet forms, which may be as much as 15 to 25 mixing tube diameters long. Its outer wall - adjacent to the inner wall surface of the mixing tube - moves at a slower rate than its core adjacent to the continuously decreasing conical steam gas jet.
During passage through the mixing tube, each granule spins and hydrates. The swollen, hydrated gels on the granule's outer surface are stripped off by the vigorous mixing action to form a viscous paste that, at the elevated temperatures used in jet pasting, is relatively fluid. Final paste characteristics are a function of the interaction of all the jet-pasting process variables outlined in table 1.
The internal dimensions of the pasting jet venturi (figure 2) determine the shear velocity applied to the heated slurry as a function of its cross-sectional area and throughput. The smaller the mixing tube inside diameter, the higher the shear rate within, for any throughput flow rate. The higher the shear rate, the greater the resulting polymer dispersion. Winfrey established this using a specifically sized Coppock jet cooker.
Much closely held information exists relating cornstarch pasting temperature and retention time to final paste viscosity. Just how much attention has been given to shear rate obtained through mixing intensity variation is unknown. It is known, however, that increasing the throughput velocity from 1,500 ft/sec to 2,500 ft/sec by reducing the venturi boor diameter using excess steam flow, yields a completely dispersed paste from partly cracked #2 Dent corn starch. The resulting paste is quite operable on a size press at normal (7 to 8%) solids concentration.
Data available about the cooking variables for #2 Dent corn starch derivatives are shown in table 2. These data were developed from the industrial results contained in the personal archives of several well-informed starch authorities.
The pressure cooking or digestive action exerted upon the starch granules effectively reduces the viscosity of the resulting paste to such an extent that it is operable in the equipment for which it is intended. Producing jet pasted modified corn starch for coating binder at high paste solids (37 to 38%) is another story. Two approaches are currently practiced. In the first method, the starch is pasted in the presence of a portion or all of the coating's mineral content. In the second method, the starch is pasted to achieve final solids as high (38 to 39%) as possible.
Table 3 summarizes the three major attributes of jet pasting hydrocolloids. It is an efficient, economic method to produce high quality, stable suspensions or sols from granular hydratable colloid slurry on a continuous basis.
The spectrum of jet pastable polymers differs greatly, as does the concentration of hydroxy (OH) end groups available for hydrogen bonding. Polyvinyl alcohol, starch and other hydratable colloids do not form true ionized solutions as does table salt in water. Instead, hydrated adhesive pastes are suspensions of polymer chains in water, wherein the polymer is completely hydrated and dispersed. Essentially no molecular ionization occurs. The hydrogen bonding between water molecules and polymer end groups achieves stability.
Of all the jet pastable polymers, polyvinyl alcohol is the most demanding of pasting temperature and retention time. Eight minutes retention at 330°F (166°C) is needed for super-hydrolyzed high molecular weight polyvinyl alcohol.
1. Coppock: May 28, 1940; U.S. No. 2,202,573
2a. Etheridge: February 8, 1966; U.S. No. 3,234,046
2b. Etheridge: April 18, 1961; U.S. No. 2,980,576
2c. Etheridge: December 29, 1959; U.S. No. 2,919,214
2d. Etheridge: January 27, 1959; U.S. No. 2,871,146
2e. Etheridge: September 10, 1957; U.S. No. 2,805,966
2f. Etheridge: U.S. No. 2,582,188
3. Winfrey, et al: May 19, 1964; U.S. No. 3,133,836
4. Lauterbach: October 12, 1965; U.S. No. 3,211,564
5a. Maurer: October 28,1969; U.S. No. 3,475,215
5b. Maurer: U.S. No. 3,219,483
6. Cummins and Perry: April 28, 1998; U.S. No. 5,743,638
7. Cummins and Perry: October 13, 1998; U.S. No. 5,820,259
B.W. Rowland*, "Notes on the Use of Starch in Papermaking," August 20, 1940, Paper Trade Journal - TAPPI, Section Pages 101-104.
*Research Associate, The Institute of Paper Chemistry, Appleton, Wl
Craig, Oltmanns and Loppnow, "Mill Experience with Ammonium Persulfate Oxidation of Unmodified Starches for Paper Coatings," November 1968, TAPPI, Pages 82A-88A.