HIGH POLYMER ENGINEERING
POLVMERS Manufacturing Operations J. L A W R E N C E AMOS A N D A. F. R O C H E POW CHEMICAL CO.,
I
N THE highly competitive plastics market manufacturers
continually are bombarded with demands from fabricators and customers for new plastics products as well as for improved properties and quality in established lines. As a result, increasing attention is being given to studies of several types of unit operations involved in plastics manufacture and in developmental work t o obtain fundamental data for t h e best exploitation of the commercial processes. Some aspects of this processing and engineering for plastics are presented here. H e a t Transfer
Heat transfer is an important operation in the manufacture of addition polymers because of the large amount of heat generated in this process. Since polymers of good quality are demanded by fabricators and customers, good control of heat during processing is a necessity. Some heats of polymerization as well as the possible temperature rises during adiabatic polymerizations are given in Table I (37).
T a b l e I. Heats of Polymerization a n d Adiabatic Polymerization T e m p e r a t u r e Rises Heat of Approx. Temp. Rise Polymerization, (Adiabatic), Monomer
w
Acrylonitrile Vinyl acetate
Isobut lene
Methyrrnethacrylate Styrene 2,5-Diohlorostyrene a-Methylstyrene
B.t.u./Lb. 588 446
322 308 286
175 145
F. 1176 892 640
suspension polymer will have an over-all heat transfer coefficient of 30 t o 70 B.t.u./hr./sq. ft./" F., with circulating water as a heat transfer medium in the jacket. In the early stages of o solution or mass polymerization the heat transfer is again much like t h a t of water in that turbulent flow can be achieved in tubes, and convection takes place in kettles. However, as the per cent solids increases t h e viscosity rises rapidly, This is illustrated in Figure 1 which shows the viscosity of solutions of molding grade polystyrene in styrene monomer. At 20% concentration and 25' C. a throughput of 3370 pounds per hour in a 2-inch pipe would be required to create turbulent flow (Reynolds No. of 2100). Therefore, heat transfer coefficients t o polymers and polymer solutions are described by functions similar to those of Seider and Tate for viscous flow ( 4 2 ) . The average polymer film coefficient, which is usually much smaller than the wall or heat transfer medium coefficients, rises with velocity and varies inversely with the tube length. These heat transfer coefficients can be increased by mechanical means. By combining a good mixing action with wall scraping the film coefficient on the plastic side of a heat exchange surface becomes solely a function of the thickness of the clearance between the scraper and the wall. This function is given by h,
=
K p-
Ar
e
616
572
350
290
A major portion of the polymerization field can be covered by four types of processes: emulsion, suspension, solution, and mass. For purposes of heat transfer design the emulsion and suspension processes are much alike. Similarly, solution processes have many problems in common with mass reactions. Emulsions consist of very finely divided (about 1 micron) monomer or polymer dispersed in water by means of an emulsifier. Suspensions are much coarser particles, approximating inch in diameter, held in suspension b y such agents as inorganic powders. Both of these systems have essentially the same viscosity as water which makes the calculation and design of heat transfer surfaces the same as t h a t of a n all-water system. For instance, a Pfaudler kettle used in the polymerization of a December 1955
MIDLAND. M I C H .
where h, is the plastic film coefficient, K , is the thermal conductivity in B.t.u./hr./sq. ft./" F./ft., and r is the thickness of the polymer film or radial clearance of the rotor in feet. I n most cases the wall film coefficient and the heat transfer medium film coefficient are so much greater than the film coefficient of the polymer that the polymer film is the limiting resistanoe. This makes h, approximately equal to the over-all coefficient, U. In one particular application a unit which had U of 2 with no scraper, had U of 15 with the scraper. This same unit by Equation 1 should have had an over-all U of 13. Movement of Polymer Solutions
Movement of most liquid materials is accomplished by pum s I n the plastics industry the viscosities of the liquids handlexis again a major Problem. For instance, in Pumping a thin liqllid either a gear pump Or a pump may be used. sions and suspensions are thin liquids; consequently, they can be handled in much the same manner as water, but the equipment should be designed t o minimize coagulation of the latex or grind-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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ENGINEERING, DESIGN, AND EQUIPMENT I06
105
ing of the beads. The centrifugal pump cannot be used for many of the high viscosity materials encountered in high polymer processing. As the viscosity becomes greater even the gear pumps must be of special design. For low and medium viscosity liquids a pump can be chosen if its pumping capacity for water is known a t various back pressures. This capacity is concerned only with the volumetric capacity of the gear teeth and the resistance to back flow offered by the pump clearances. However, as the viscosity of the liquid approaches 10,000 poise another factor makes its appearance. This is the ability of the liquid t o flow into the gear openings during the time they are open t o the inlet side of the pump. This ability depends on the speed of the pump, the inlet head pressure, the viscosity of the medium, the size of the gear teeth, and the size of the opening into the pump, The dimensions of the gear teeth are usually such that they, s o t the throat of the pum create the controlling resistance. This resistance can be calcukted using the contours of the gear teeth. It is found in this way t h a t the velocity of the polymer solution entering the tooth diminished progressively with the depth t o which it has entered. Consequently gear pumps possess a nonlinear capacity with r.p.m. such as t h a t indicated in Figure 2 . Pictures of a gear tooth being filled with a polystyrene solution are shown in Figure 3.
I
c
S O L U T I O N S OF COMMERCIAL POLY STY R E N E IN ETHYL BENZENE
Process Controls
0
20
40
80
60
IO0
% POLYMER
Figure 1
400
0:
/
300
.
3 0
L v)
n
z 200 3
0
a
IO0
0
I
0
3
2
4
5
R EN.
Figure 2.
Capacity of polymer gear pumps
Table 1 1 .
6
The large changes in viscosity with concentration offer a method of measuring and controlling the composition of solutions and the extent of conversion of polymerizing materials. Useful recording and controlling viscometers are commercially available. However, most concentrated polymer solutions are non-Newtonion in character. Therefore, if the instrument depends on a high rate of shear it will record a lower viscosity than that indicated by capillary or falling ball techniques. T o use a viscometer in the measurement of concentration of a polymer solution requires information as to the variation of viscosity with the following variables: per cent solids, solvent t o be used, type polymer, and molecular weight of polymer. A second method of controlling or recording the per cent solids of a solution is the refractive index. Instruments are commercially available for this measurement. They require the same calibration as the viscometer except that they are not influenced by the molecular weight of the polymer involved. Infrared analyzers and mass spectrometers are commercially available and could be used as control instruments in plastic processing. These would be of particular value in controlling the feed composition and analyzing the recycle streams of continuous polymerization processes. I n many phases of the handling of polymer solutions it is necessary to have some idea of the vapor pressure of the solutions. If these data are not readily available from some experimental source, a fair approximation may be obtained by using an equation based on the statistical theory of polymer solutions. A good review of this subject appears in “Solubility of Nonelectrolytes” ($3).
Methods of Removing Volatile frOm Plastics
Method Spray drying
Continuous
Steam atomization Vacuum
Steam adds heat for evaporation: easy recovery of volatile .41lows lower temperatures during processing
Compound rolls under vacuum
Allows lower temperatures during processing. allows addition of heat b y mechanical energy: good mixing action
Extruder, devolatilizers under vacuum Alcohol precipitation
Sllows lower temperatures during processing: allows addition of heat b y mechanical energy: good mixing: pumps product from vacuum zone Allows fractionation of polymers
Inert pas devolatiliaation Extraction with nonsolvent
Excellent for latex products; can be operated a t atmospheric pressure: when steam is used, recovery of monomer is easy Does not destroy originalshape of polymer particle: removes volatile t h a t vacuum techniques would not reoover
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Advastages
References
Disadvantages Some materials cobweb; recovery of volatile from gases Cost of steam: difficult t o obtain clean steam Air leaks and contamination: difficulty of removing polymer from vacuum system Expensive unit: air leaks a n d contamination: difficulty of removing polymer from vacu u m unit Expensive unit; air leaks and contamination
(a, 6 , 8 ,
Alcohols form azeotropes with most volatiles; cost of handling large amounts of alcohol Some systems require large amounts of inert gas Difficulty of purification of nonsolvent berecovery of noncause of azeotropes; solvent from polymer
(13 17 18 66 50 sk,sQ,4 i , 4 4 6oj (6,81)
INDUSTRIAL AND ENGINEERING CHEMISTRY
(16)
(9,87)
13, 20, 86, 29 40 42 47) ( 1 , 4: 10: 14,’88,36,
46) (19,38) (6 11 15 94, 8 1 , h4, i8,44)
Vol. 47, No. 12
HIGH POLYMER ENGINEERING Table I I I.
Methods of Polymerization
Method Batch reaction
General Characteristics Usually pure chemicals used
Advantages Simplicity; flexibility
Continuous reaction
Usually pure chemicals used
More uniform product; process control
Reaction in solution
Usually pure chemicals used in a solvent Solvent generally used with pure chemicals and catalyst
Better control of heat dissipation
Ionic polymerization Suspension polymerization
Water used as carrier with stabilizing agents and catalyst
Emulsion polymerization
Water used as carrier with emulsifying agent and catalyst
better
Rapid rate of polymerization. sometimes only known method of oolvmerization: control of h e i t froblem ' No difficulty i,n dissipating heat; polymer is in granular form Rapid polymerization; many copolymerizations not possible b y other techniques; adaptable t o continuous polymerization; polymer in latex form
The simplified equation used in these calculations is
where p is the vapor pressure of solution; p , is the vapor pressure of solvent; cpo is the volume fraction of solvent; p P is the volume fraction of polymer; and p is a parameter measuring the solvent power of solvent for polymer. For many commonly encountered polymer solutions where the polymer is nonpolar in character and there is no phase separation as the concentration changesfrom 100% polymer to 100% solvent the value will be in the neighborhood of 0.4. This is true over a wide range of temperatures-i.e., polystyrene in styrene from 25' to 180" C. Figure 4 shows this function as a graph. Figure 5 shows the application of this function as it applies to polystyrene in styrene.
Disadvantages Heat transfer in large scale operation. broad distribution of mol.'wt. Mechanical problems of moving viscous materials; channeling in process vessel Difficulty in removing last 10% of solvent; solvent reduces mol. wt. and rate of polymerization Removal of catalyst and solvent
Contamination from water and stabilizing agent. sometimes requires volatile ;emoval; drying and pelletizing required Contamination: drying pelletizing sometimes required
Examples of Products Made Polystyrene; styrenated drying oils. styfenated alkyd resins; phenolid resins Polystyrene; some styrene copolymers Polyethylene Poly-a-methylstyrene: butylene copolymer; nitrile
styrene-isopolyacrylo-
Polystyrene; poly(viny1 chloride) ; polystyrene beads for foaming resins; ion exchange resins Polymers-copolymers of styrene; latex paints and coatings; poly(viny1 chloride) ; vinylidene chloride polymers a n d copolymers; synthetic rubbers; polyethylene
Process, market, and economic demands complicate operations in the manufacture of plastic products because so many types of polymerization techniques must be used. Table I11 gives some idea of the different commercial methods of polymerization and copolymerization that are used to fulfill the demand for products of the quality and price required in the plastic industry. One can see that many processing techniques are required if a position is to be maintained in the plastics field. Sometimes the process used is dictated by the end use of the product. For example, the use of plastics in latex paints and coatings requires the use of a film forming latex-hence the need of emulsion polymerization. The Dow Chemical Co. uses all six methods of plastic processing to meet the demands of customers.
P a r t ia I Polymerirati on
Partial polymerization processing of monomers and mixtures of monomers has received considerable attention during the last 10 years (1-8, 7 , 14, ?6, 42, 46,46, 48). Here the partial conversion of the monomers or monomer mixtures to polymers and copolymers is involved. This type of processing has several advantages over that generally used where the monomers are completely converted to polymeric materials: 1. Allows average polymerization rates u p to eight times that of a process polymerizing to 100% conversion 2. Solutions are fluid and readily pumped 3. Can form polymers under isothermal condition Good control of molecular weight of polymers produced a. Necessity for some comonomer systems 6. Flexible system 7. Allows good control of polymerization
4.
This process, like others used in the plastic processing industry, has disadvantages. The major drawback of the partial polymerization process is the necessity of removal, recovery, and recirculation of the unpolynierized material in the partially polymerized monomer mixture. In some partial polymerization systems the characteristics of the monomers and polymers allow recycle of the recovered monomer without any purification after i t leaves the recovery train of the devolatilization system. Styrene is a good example of this system. I n many applications of partial polymerization, purification (usually by distillation) is combined with the polymer devolatilization processing in order t o remove objectionable impurities from the monomers prior to recycle to the process. Table I1 lists methods for removing volatile materials from plastics; some of these are in commercial use and others are of interest because they are described in the literature and patent art.
December 1355
Figure 3.
Polymer p u m p gear tooth filling operation
The trend in plastic processing is to build into plastic compositions certain properties which, for some specific reasons, the fabricator and customer want. Some of the tailor-made properties in plastics are: Improved heat distortion Moldability (short fabrication cycle) Light stability Physical appearance (smooth glossy surface) Solvent resistance Craze resistance
Plasticizer retention Ease of dry blending Improved granulation Extrudabilit y Ease of vacuum forming Static resistance Improved heat stability
Market demands force plastic manufacturers to modify their existing processing facilities t o produce improved products. This is accompliehed in some cases by change in processing conditions, by the use of addition agents before and/or after processing, and by adding new equipment to old facilities. However, for some products facilities of an entirely new design will be needed. An example of this is the current interest
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING, DESIGN, AND EQUIPMENT I .O
.9
.8 1,000
.7
P
.6 P/P, .5 .4
i
VAPOR PRESSURE
I
POLYMER SOLUTIONS
J 01
3
x
.3
10
20
30
40
50
60
80
100
VOLUME % S O L V E N T
Figure 4
in special catalyst complexes for carrying out polymerization reactions-Le., Ziegler’s ( 4 9 ) aluminum triethyl and Morton’s (33) alfin catalysts. The one interesting highlight t h a t has come aa the result of the creation of new techniques of plastic manufacturing processing is t h a t not only new and improved products are formed but also that new markets are established which help many of the other plastics and particularly suppliers of addition agents and raw materials for the rapidly growing industry. Acknowledgment
The authors wish t o thank R. M. Riley, W. W. Hunt, and others in Dow’s research organization for supplying information that is published in this paper. The authors are grateful to the Dow Chemical Co. for permission t o publish this report. References
Allen, I., Marshall, W. R., and Wightman, G. E. (to Union Carbide & Carbon Corp.), U. s. Patents 2,496,653 (Feb. 7, 1950)and 2,614,910 (Oct. 21, 1952). Amos, J. L., and Miller, C. T. (to Dow Chemical Co.), U. S. Patents 2,638,465 and 2,638,466 (May 12, 1953). Amos, J. L., McCurdy, J. L., and McIntire, 0. R., Ibid., 2,694,692 (Nov. 16, 1954). Blaw-Knox Co., Bluflovak Equipment Division, Buffalo, N. Y . , vacuum drum dryer. Britton, E. C., and LeFevre, W. J. (to Dow Chemical Co.), U. S. Patent 2,255,729 (Sept. 9, 1941). Carpenter, AI. J., and Feuchter, C. F. (to Standard Oil Co. Indiana), U. S.Patent 2,374,272 (April 24, 1945). Chaney, D. dv. (to American Viscose Corp.), U. S. Patent 2,537,031 (Jan. 9, 1951). Collings, W. R., Gibb, D. L., and Schmelter, G. P. (to Dow Chemical Co.), U. S.Patent 2,283,539 (AVay 19, 1942). Collings, W. R., Schmelter, G. P., and Dulmage, F. E., Ibid,, 2,270,182 (Jan. 13, 1942). Conaway, R. R. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,088,577 (Aug. 3, 1937). Cox, H. L., and Matlack, J. D. (to Carbide and Carbon Chemical Carp.), U. S.Patent 2,202,481 (May 28, 1940). Craig, D. (to R. F. Goodrich), U. S. Patent 2,362,052 (Nov. 7, 1944).
Crane, P. W., and Fields, R. T. (to E. I. du Pont de Nemours & CO.), U. S. Patent 2,146,532 (Feb. 7, 1939). DeBell, J. M., Goggin, W. C., and Gloor, W. E., “German Plastics Practice,” chap. 2, DeBell and Richardson, Springfield, Mass., 1946 Elwell, W. E., and Snow, A. L. (to California Research Corp.), u. S.Patent 2,482,056 (Sept. 13, 1949). Ferris, C. R , and Carte, E. T. (to Carbide and Carbon Chemical Corp.), U. S. Patent 2,187,877 (Jan. 23, 1940). Fisk, C. F. (to U. S. Rubber Co.), U. S.Patent 2,505,353 (April 25, 1950).
Froyan, J. E. (to Phillips Petroleum Co.), U. S. Patent 2,615,010 (Oct. 21, 1952).
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IO0
W U P
10
0
20
40
60 80 100 T E M P E R I T U R E , ‘C.
140
180
Figure 5
(19) Fuller, L. J. (to Welding Engineers), U. S. Patents 2,458,068 (Jan. 4, 1949) and 2,615,199 (Oct. 28, 1952). (20) Graves, G. W. (to E. I. du Pont de Nemours & Co.), U. S. Patent 2,273,188 (Feb. 17, 1942). (21) Grim, J. M. (to Koppers Co.), U. S. Patent 2,691,008 (Oct. 5, 1954) * (22) Hanson, A. W. (to Dow Chemical Co.), U. S.Patent 2,488,189 (Nov. 15, 1949). (23) Hildebrand, J. H., and Scott, R. L., “Solubility of Nonelectrolytes,” chap. 20, Reinhold, New York, 1950. (24) Hooper, G. S., and hlcFarren. G. A. (to Industrial Rayon Corp.), U. 8. Patent 2,570,237 (Oct. 9, 1951). (25) Hutchinson, H. M. (to Distillers Co., Ltd.), U. S.Patent 2,650,912 (Sept. 1 , 1953). (26) Johnson, C. R. (to Firestone Tire & Rubber Co.), U. S.Patent 2,514,207 (July 4, 1950). (27) Johnston, W. S., and Keen, A. W. (to Naugatuck Chemical Co.), U. 5. Patent 1,673,685 (June 12, 1928). (28) Johnson, C. R., and Otto, W. M., Chem. Eng. Progr., 45, 407-14 (1949). (29) King, W. A. (to Allied Chemical and Dye Corp.), U. S.Patent 2,350,400 (June 6, 1944)’. (30) Kitani, K., and Takashima, N. (to General Electric Co.), U. S. Patent 2,338,517 (Jan. 4, 1944). (31) McFarren, G. A. (to Industrial Rayon Corp.), U. S. Patent 2,570,257 (Oct. 9, 1951). (32) Marshall, W. R. (to Bakelite Corp.), U. S. Patent 2,434,707 (Jan. 20, 1948). ENG.CHEM., 42, 1488 (1950). (33) Morton, A. A., IND. (34) Ostromislensky, I . (to Naugatuck Chemical Co.), Can. Patent 265,328 (Oct. 26, 1926). (35) Ostromislensky, I., and Gibbons, W. A. (to Naugatuck Chemical c o . ) , U. S. Patent 1,855,413 (-4pril26, 1932). (36) Overton Machine Co., Dowagaic, Mich., vacuum drum dryer. (37) Roberts. D. E.. J . Research Natl. Bur. Standnrds. 44. 221 (1950). (38j Robertson, H. F. (to Carbide and Carbon Chemical Gorp.;, U. S. Patent 2,010,963 (Aug. 13, 1935). (39) Rumbold, J. S. (to U. S. Rubber Co.), C . S. Patent 2,484,425 (Oct. 11, 1949). (40) Sans, M., and Michon, Robert (to SOC.Anonyme des Mfg. des Glaces et Produits Chimiques), U. S. Patent 2,467,055 (April 12, 1949). (41) Seider, E. K., and Tate, G. E., IND.ENG.CHEM.,28,1429 (1936). (42) Shriver, L. C., and Fremon, George H. (to Carbide and Carbon Chemical Corp.), U. S. Patent 2,420,330 (May 13, 1947). (43) Speir, F. W., and Darrin, M. (to H. Koppers Co.), U. S. Patent 1,263,813 (April 23, 1918). (44) Stobbe, Hans, and Posnjak, George, Ann., 371, 259-87 (1909). (45) Stober, K. E., and Amos, J. L. (to Dow Chemical Co.), U. S Patent 2,530,409 (1950). (46) Soday, F. J. (to The United Gas Improvement Co.), U. S. Patent 2,345,013 (March 28, 1944). (47) Tyson, C. W. (to Standard Oil Development Co.), U. S.Patent 2,235,127 (Mar. 18, 1941). (48) Whitby, G. S., Davis, C. C., and Dunbrook, R. F., “Synthetic Rubber,” chap. 7, Wiley, New York, 1954. (49) Ziegler, Karl, Z . angew. Chem., 64, 323 (1952). (50) Zimmer, J. C. (to Standard Oil Development Co.), U. S. Patent 2,379,268 (June 26, 1945).
RECEIVED for review March 9, 1955.
ACCEPTEDOctober 10, 1956.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
