Filter cake before and after syneresis. Steam savings which may be realized are also illustrated
I
P.
M. LINDSTEDT
Cake discharge from continuous production filter
and H. 1. GUNNERSON
The Goodyear Tire & Rubber Co., Akron, Ohio
Dewatering of Thermoplastic Resins by Syneresis By a practical engineering process up to 80% of the moisture in filter cake is removed without evaporative or-mechanical methods D E v a o m m T of a series of copolymers known as “rubber-reinforcing resins” came as a logical outgrowth of the war-fostered synthetic rubber industry. These resins were copolymers of 5 to 15 parts of butadiene and 95 to 85 parts of styrene. The increased proportion of styrene compared to that used in GR-S gave a hard, thermoplastic resin. One of the first descriptions of compositions made from rubber-reinforcing resins was published in 1946 (7). Pliolite S-3 and S-6 are trade names of The Goodyear Tire & Rubber Co. for two rubber reinforcing resins. Pliolite S-5 differs from these resins in its solubility characteristics, as it is made specifically for use in solvent-base paints. The rubber-reinforcing resins are insoluble in solvents, because of high amounts of copolymer in gel form. A demand for commercial quantities of resins of this type developed a t about the time the postwar demand for synthetic rubber dropped sharply. T h e surplus plant capacity created an incentive to develop a manufacturing process for the new resins, requiring only minor conversion of existing plant facilities. This was accomplished in a practical way by utilizing a property of colloidal dispersions known as “syneresis” for dewatering the resin filter cake.
The Engineering Problem The manufacture of thermoplastic resins in synthetic rubber processing equipment presented filtration and drying problems. These resins produce a filter cake having a fine particle size and, consequently, high moisture. The conventional air-drying equipment used in synthetic rubber manufacture consists of a perforated conveyor traveling through several zones in which heated air is circulated downward, through the bed of material. These dryers have
been built with a single conveyor deck as well as with double and triple conveyor decks, one above the other. The crumb of wet rubber must be uniformly distributed on the conveyor to a depth of 2 to 3 inches, in a layer sufficiently porous and coarse to allow air circulation through the bed without excessive loss of fines falling through the conveyor perforations. When the resin filter cake was dried under these conditions in the pilot plant, large shrinkage caused cracking in the bed of material, resulting in nonuniform
Figure 1. Syneresis chamber in plant dryer
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thickness and maintaining an air velocity of 200 feet per minute through the bed (Figure 1). A close-up view of the finished product is shown in Figure 2. Equipment
Conversion of the production conveyor dryer to this new dewatering process required partitioning of the high humidity zone, dividing the overhead steam tubes into a separate zone for high humidity control, installing stainless steel drainage pans beneath the conveyor in the dewatering zone, and installing a recording controller for both wet- and dry-bulb temperatures in the dewatering zone. Pilot Plant and Plant Performance
Figure 2.
Dried Pliolite granules
drying. The product was too dusty for this type ofdrying equipment or for satisfactory handling of the resin during its end use. I t was observed during the pilot plant stage that the wet filter cake could not be stored or shipped for drying tests, because the water soon separated from the resin. A search was then begun for a practical method of accelerating this removal of water. When the temperature of the wet resin was raised to its softening point, rapid shrinkage occurred and water drained away easily. This could not be done in a stream of warm, dry air because the evaporative cooling of the resin kept the wet-bulb temperature below the softening point of the resin. Drying Aided b y Moist Air
Humidification of the circulating air to nearly 100% of saturation elevated the temperature of the resin without allowing it to dry. A sudden agglomeration of the resin again resulted, expelling the water as a free liquid from the interstices of the resin. The ex-
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udation of a supernatant layer of water from colloidal gels upon standing was first described as syneresis by T. Graham in 1861. The similarity of the agglomeration of Pliolite resin filter cake to the behavior of certain colloidal gels led to the naming of the process “dewatering by syneresis.” I t is not reversible in the sense that water-soluble gels undergo syneresis reversibly with temperature. Water is exuded so spontaneously during syneresis that flooding in the bottom layer of resin creates a drainage problem. To overcome this difficulty, a porous bed of material 6 to 8 inches deep is formed, which gives adequate drainage. The resin latex is coagulated and filtered a t a temperature which gives an extrudable filter cake containing approximately 400% moisture on a dry basis. The wet material is then extruded through a perforated plate, such as a rolling performer device. T o achieve maximum dewatering, the temperature of the resin must be raised to the agglomeration point as quickly and as uniformly as possible. This is accomplished by carefully controlling the bed
N O R M A L DRYING
\
Figure 3. Drying time of Pliolite S-3 resin by
p 7 5 Yo 10
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30 DRYING
1 824
40 50 60 T I M E I N MINUTES
0.75 %
70
INDUSTRIAL AND ENGINEERING CHEMISTRY
80
syneresis process and by evaporation
The drying curves in Figure 3 compare the results obtained in the pilot plant batch dryer by conventional air drying with those of syneresis dewatering prior to air drying. Syneresis dewatering reduces drying time by more than 5070. Figure 4 compares the moisture results from the laboratory dryer with those from the converted plant dryer operating a t equilibrium conditions. Because the syneresis dewatering step was done a t a wet-bulb temperature of 165’ F. in the plant dryer instead of a t 150” F. used for the pilot plant test, dewatering was completed more rapidly in plant equipment. Consequently, the time allowed in the plant design for dewatering and drainage appears more than ample, as can be seen by the flattening of the moisture curve in Figure 4 for the syneresis zone. In actual plant practice, the relative humidity is controlled a t 75% of saturation to prevent condensation on the wall of the dewatering chamber. As would be expected, a slight amount of evaporation continues to lower the moisture content of the bed of resin as it passes through the syneresis zone. Estimating Steam Savings
Unfortunately, no data on steam consumption are available, as a basis for calculating the amount saved after converting to the syneresis dewatering process. However, steam savings are easily estimated. The steam required to remove 4 pounds of water per pound of dry resin in the filter cake, assuming that 1.5 pounds are required to evaporate each pound of water, would be 6 pounds. The steam required to remove 87.5’% of the water by the syneresis process is that necessary to heat the resin filter cake to the softening temperature. A liberal estimate of this would be 0.65 pound of steam per pound of dry resin. An additional 0.75 pound of steam per pound of dry resin would be required to remove the remaining water by evapora-
E N G I N E E R I N G ASPECTS OF P O L Y M E R PROCESSES tion. This gives a total of only 1.4 pounds of steam per pound of dry resin required for the syneresis process, and represents a saving of 4.6 pounds of steam per pound of dry resin.
Effect of Temperature on Dewatering Pilot plant data were obtained a t several different wet-bulb temperatures while 1OOyorelative humidity was maintained to avoid evaporation (Figure 5 ) . As the wet-bulb temperature is raised above the softening point of the resin, the amount of moisture removed increases. Pliolite s-5 resin, which has a lower softening point than Pliolite S-6, is more readily dewatered a t the same wet-bulb temperature.
20
IO DRYING
Mechanism of Syneresis
-
Maron and Moore (7) reported a similar phenomenon in the “stockpudct” method of concentrating low solids latex. This process consists of controlled gelation of latex-electrolyte mixes by cooling until the gel point is reached. Continued cooling below this temperature results in a viscosity increase until the “thin point temperature” is reached. The viscosity then suddenly drops and the mass becomes fluid. At this point the fine latex particles are held together as spherical agglomerates by a transparent phase. Syneresis may be observed if the mixture is allowed to stand undisturbed in constant temperature. This process is reversible, but otherwise resembles colloidal resin by becoming viscous during agglomeration of the latex and undergoing syneresis upon standing. Shaler (8, 9) studied the coalescence of copper spheres by considering the simplified case of one pair of isolated particles. The mechanism of sintering is attributed to viscous flow of metal under the influence of surface tension, modified by gas pressure. Dillon, Matheson, and Bradford (2) present a similar theory in regard to the sintering of synthetic latex particles: that the surface tension of the polymer particle exerts the force necessary to cause viscous flow of the polymer for the formation of latex films. The authors consider the sintering of powdered metals to be somewhat analogous to the coalescence of resin particles in a film of latex, in which interstitial water is removed by evaporation. In order for sintering to occur according to Kuczynski’s theory of viscous flow (4, the particles must first be in contact. The Pliolite resin particles, which are discrete and unagglomerated in the latex (Figure 6), are in contact in the filter cake (Figures 7 and 9). Consider the pair of sintered latex
Figure 4.
30
TIME
(MIN.)
Laboratory and plant performance of syneresis process
particles in Figure 8. Dillon, Matheson, and Bradford (2) concluded that the mechanism which applies to the sintering of latex particles in the formation of a film is viscous flow of the thermoplastic polymer. If viscous flow is to occur, a shearing stress is necessary. Dillon has shown that this shearing stress is produced by the surface tension force acting over the enormous surface present in the colloidal polymer particles. The surface tension is the force on a surface tending to minimize that surface. Thus two drops of mercury will coalesce completely when brought together. The surface tension of mercury is high and the viscosity of the fluid mercury in the drops is low; therefore, the shearing stress exerted by the surface tension forces at the point of contact causes the mercury drops to flow quickly together.
8“i
A similar mechanism must also be involved when Pliolite resin filter cake is dewatered by the syneresis process. The excess pressure due to surface tension within a sphere of liquid of radius R over the pressure outside of the droplet’s surface can be calculated. In Figure 8, the surface tension, y1, of the resin operating over the circumference, 27rR, tends to pull the two halves together with a force of 2nRyI dynes. The resin particles are surrounded by water which, because of its surface tension, yz, exerts an additional contracting force on the particle. This force is approximately twice that due to the resin and amounts to 27rRy~dynes. Neglecting a small additional force due to hydrostatic pressure, the total contracting force equals 27rR(y, yz)dynes. Before two particles come into contact, a pres-
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$80
30‘
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Figure 5.
170
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BULB TEMPERATURE
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Effect of syneresis temperature on residual moisture content VOL. 49, NO. 1 1
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Figure 6.
Electron micrograph of Pliolite latex, 7000X Dvs = 900 A.
TA2P
=
Figure 7. Electron micrograph of filter cake showing individual resin particles in agglomerate, 7000 X 27rR(7l
+
72)
and
This expression relates the surface tension with the internal pressure of the colloidal Pliolite particles as they exist in the original latex. After coagulation and filtration have brought the particles together, they appear as showm in Figure 9. Each particle is in contact with several others, forming a three-dimensional cluster. A relationship for the coalescence of spheres by viscous flow has been developed by Frenkel (3):
Figure 8. Two resin particles in contact, representing sintering of resin particles by viscous flow
sure P is distributed over the area of the circle irR2sq. cm. resulting in a balancing force of nR2P dynes pushing the two halves of a particle apart. Equating these two forces,
Figure
where 0 is the angle shown in Figure 8, y1 is the surface tension, 7 is the viscosity coefficient of the liquid or plastic, R is the sphere radius, and t is the time factor. Thus it is hypothesized that once the two particles of resin are in contact, elevating the temperature of the thermoplastic resin lowers the viscosity, ?, with very little effect on the surface tension value, and allows the plastic to flow together through the rapidly growing neck of radius r. As Equation 1 can be written to represent the differential pressure, p , at the neck as follows:
9. Resin particles in contact in filter cake, 30,000 X
it becomes apparent that the transient pressure differentia1 is much larger a t the area of the neck when neck radius r is small. Also, as I? > r, p > P, the pressure at the neck is greater than the pressure at the center of the individual particles which have joined. The average diameter of these resin latex particles as determined by electron microscope count was 900 A. (see Figure 6). If it is assumed that two such particles have just come into contact and have formed a neck with r = 10 A., an idea of the magnitude of the pressure differential, Pd, which tends to pull these two particles together into one larger sphere, may be obtained.
The surface tension of the syneresis effluent water, y ~ ,was found to be 64.4 dynes per cm. at 160’ I?. A value of 25 dynes per cm. for surface tension was used by Dillon (2) as applied to plastic materials. Therefore, using Dillon’s figures, Pd = 1.77 X 109 dynes per sq. cm. or 25,600 pounds per square inch. Of course, as 1’ approaches R, Pd decreases. However, a capillary force holding
Figure IO. Electron micrograph of dry Pliolite resin, 3000X Individual resin particles may still b e identified in sintered agglomerates
1826
INDUSTRIAL AND ENGINEERING
CHEMISTRY
ENGINEERING ASPECTS the water in the interstitial voids between particles in the filter cake represents an opposing force to the coalescing or sintering process. The average capillary radius might be estimated from the expression presented by Maatman and Prater (6): r,=2(
)
total volume of voids surface area of voids
(5)
and from ro pressure P, tending to hold the water in the capillaries may be calculated from an expression similar to that of Equation 1. The y1 term is dropped. The surface tension determining the capillary force is either the tension of the water or the interfacial tension between the water and resin, and is the smaller of the two. Because the resin is hydrocarbon, it is logical that the interfacial tension, y,, 2) is smaller than that of the water. Therefore,
The filter cake contains approximately 400Yo moisture by weight of a dry basis.
This is approximately 80 cc. of water or void volume per 100 cc. of filter cake. A conductometric titration of the coagulated latex slurry with potassium phosphate solution indicated a total surface area of 4 X 107 sq. cm. per 100 cc. of resin solids. Assuming that this would not change when compacted into a filter cake, it would be equivalent to 8 x 106 sq. cm. per 100 cc. of filter cake. In the absence of interfacial tension data, a maximum for the capillary pressure may be calculated by assuming that the interfacial tension between water and resin approaches that of the water and is 64.4 dynes per cm. Therefore.
64.4 X 105 dynes per sq. cm.
or 93.3 pounds per square inch as a maximum pressure opposing agglomeration of the particles. As coalescence proceeds a t a given temperature, the volume and radius of the capillaries decrease, causing the average capillary pressure, Pc,to increase. Simultaneously, internal differential pressure, p , at the neck of the two coalescing particles decreases as the neck widens, until water pressure within the capillaries equals the internal pressure a t the neck. When these pressures become equal, the coalescence process ceases and dewatering ends. The more complete dewatering obtained at elevated temperatures (Figure 5) can perhaps be explained by reasoning that the resin viscosity, 7 Equation 2, is lowered by a greater amount than the
OF POLYMER PROCESSES
surface tension of the resin. In each instance, the coalescence ceases when the internal and external pressures of the resin particles are equal. Therefore, the dewatered agglomerate contains residual moisture-filled voids formed by the spaces left by the necks between particles and by imperfections in the random spacial arrangement. The sintered agglomerates shown in Figure 10 reveal a considerable number of the original latex particles which have not been destroyed-a further verification in a qualitative way that the foregoing hypothesis is valid. Effect of Porosity
This incomplete fusion of the resin particles is an important advantage in end-use applications. The porous structure permits the solvents used for dissolving Pliolite S-5 to penetrate the entire mass, thus accelerating the rate cf solution. A fragile, porous structure is also important in the performance of the Pliolite S-3 and S-6 as rubber-reinforcing resins. because this friable form of the resin disintegrates during mixing operations with rubber, allowing the attainment of a completely fluxed dispersion in the least time. Although this effect can be accomplished with a finely ground resin powder, such powders are disagreeable to handle in production equipment because of the ease with which the material becomes an air-borne dust. Conclusions
A dewatering process utilizes the surface energy of the colloidal thermoplastic resin to coalesce the particles, forcing out a corresponding amount of water, which amounts to as much as 80% of that originally present (5). Plant drying equipment was successfully converted, based upon a design from pilot plant data. The original production unit has been in operation for 8 years with no unusual difficulties. Product quality was improved because of the ease of dispersion in rubber and increased dissolving rate in solvents due to the porous structure of the granules, and even the dust nuisance normally associated with finely ground resin powders was substantially reduced, thus improving plant cleanliness. A mechanism has been proposed for the syneresis process. based upon the viscous flow relationship developed by Frenkel ( 3 ) , Equation 2. Although no attempt was made to verify this proposal here, work done by Dillon, Matheson, and Bradford (2) showed that the vis-. cous flow theory governs the coalescence process in thermoplastic latex films of similar composition.
Acknowledgment
Sincere appreciation is extended to the following, who made the presentation of this paper possible. Proctor and Schwartz, Inc., supplied pilot plant test data; W. G. Best and L. H. Willisford of Goodyear Research supplied the electron photomicrographs with supplementary data; The Goodyear Tire & Rubber Co. gave permission to publish this paper; and several members of the Goodyear Chemical Engineering Division cooperated in making this information available. The counsel and criticism offered by M. B.' Palmer. Kent State University, in developing the theory for the proposed mechanism are also sincerely appreciated. Nomenclature
P
pressure within a resin particle due to forces acting on particle Pc = pressure holding water in capillary interstitial voids pd = (p - p) p = pressure at neck where two resin particles are joined together R = radius of resin particle r = radius of neck formed when two particles join together rc = average radius of interstitial capillaries in filter cake = time factor in viscous flow y1 = surface tension of resin ya = surface tension of syneresis effluent water y1,2 = interfacial tension between resin and effluent water e = angle between common axis of two spherical particles joined together and a line from the center of either particle to the circumference of the neck = viscosity coefficient of resin 7 =
literature Cited (1) Borders, A. M., Juve, R. D., Hess, L. D., IND. ENG. CHEM.38, 955 (1946). (2) Dillon, R. E., Matheson, L. A., Bradford. E. B.. J . Colloid Sci. 6, 108 ( 1951 ). (3) Frenkel, J., J . Phys. (U.S.S.R.) 9, 385 (1943). (4) Kuczynski, G. C., J . Metals 7 5 , 169 (1949). (5) Lindstedt, P. M., U. S. Patent 2.615,206 fOct. 28. 1952). (6) Maatman, R. W., Prater, C. D., IND. ENG.CHEM. 49, 254 (1957). (7) Maron, S. H., Moore, C., J . Collozd Sci. 7, 94 (1952). (8) Shaler, A . J., J . Metals 75, 796 (1949). (9) Shaler. A. J.. Wulff, J., IND. ENG. CHEM.40, 838 (1948) \
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RECEIVED for review April 25, 1957 ACCEPTEDAugust 20, 1957 Division of Industrial and Engineering Chemistry, Symposium on Engineering Aspects of Polymer Processes and Applications. Joint with Divisions of Paint. Plastics, and Printing Ink and Polymer Chemistry, 131st Meeting, ACS, Miami, Fla., April 1957. VOL. 49, NO. 1 1
NOVEMBER 1957
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