Glass Polymerization Vessel for Small Scale Studies J. D. SUTHERLAND
AND
J.
P. McKENZlE
Copolymer Rubber and Chemical Corp., Baton Rouge, la.
T
HE earliest work in laboratory scale polymerizations was con-
ducted in sealed glass tubes, which were rotated end over end in a constant temperature bath, The use of eealed tubes soon gave way to the use of crown cap beverage bottles ( 1 ) or screw cap bottles (2), tumbled end over end a t from 17 to 35 r.p.m. in what has become one of the most important research tools in the synthetic rubber industry, the bottle polymerizer. The self-sealing gasket and hypodermic syringe were introduced as important accessories to this polymerizer (2). These accessories have also been retained in the half-gallon reactor design. Numerous innovations to the original bottle polymerizer have been suggested. Among these are the use of stainless steel screw cap bottles (5)with self-sealing gaskets and an all-glass bottle ( 4 ) with an inverted spherical ground-glass joint in the neck which operates as a check valve. This eliminated the self-sealing gasket which could have an inhibiting effect on the reaction due to extraction of plasticizer in the gasket by the hydrocarbons present. With the advent of cold high solids latex, the need for better agitation in the bottle polymerizer became apparent. The need for an improved bottle polymerizer has been recognized, and under sponsorship of the Office of Synthetic Rubber a great deal of time and effort was devoted to the development of an internally stirred bottle polymerizer using pint size Mason jars with both magnetically coupled agitators and direct-drive agitators using packing-gland seals (3). Apparently this polymerieer did not prove entirely satisfactory for high solids latex work, and there was also a serious pressure limitation of the Mason jars used. Important reactor design considerations included flexibility of agitation and visibility
The reason for the development of a small glass reactor was to provide a laboratory method of high solids latex polymerization. It has been found that the tumbling type bottle polymerizer is entirely inadequate for high solids latex work from the standpoint of agitation. Agitation. During the early development of high solids latex recipes it was necessary to work in 5-gallon or 20-gallon reactors because of lack of agitation in the bottles. Even in these reactors, the agitators had to be changed from the shrouded turbine to marine type impellers in order to produce the desired turbulence. Agitator modifications were also made in 500-gallon reactors as well as in 3750-gallon plant high solids latex reactors. As a result of an exhaustive study of the effects of agitation on high solids latex viscosity, it was found possible to lower the viscosity at 60% solids of X-667 type latex (now Cop0 2102) from over 5000 cp. to less than 400 cp. by changes in the speed of agitation and impeller design. I n the design of the laboratory size reactor, the flexibility of the agitation was one of the most important considerations. January 1956
Ease of Cleaning. I n the course of development of a high solids latex recipe, the reaction vessel is quite frequently filled with precoagulum before the optimum point of increment soap addition has been accurately established; therefore ease of cleaning was another importsnt consideration. I t may take as long as 12 hours using craftsmen of as many as five different trades to clean a 20-gallon reactor. The half-gallon reactor can be cleaned in a half hour by laboratory personnel. A stream of high pressure water directed on the internal cooling coil will quickly strip off any coagulum. Visibility. The third design consideration was the need for good visibility inside the reactor. Since the time a t which increments of soap should be added is determined by observations as to the point a t which floc begins to form, the formation of particles of coagulum should be easy to detect. However, once floc has formed, addition of increment soap will not correct the situation. The recipe corrections are made in subsequent polymerizations. Size. In making the transition from the conventional bottle polymerizer to the laboratory scale reactor, the minimum volume capacity of l/Z-gallon waa selected for several reasons. First of all, frequent samples are taken from the polymerization batch for control tests during the course of the reaction, causing a depletion of the reactor contents. Secondly, it is desirable to have sufficient concentrated latex for routine laboratory tests such as determinations of viscosity, gel time, foam density, latex stability, as well as for the preparation of a foam sponge sample on individual batches. Since the laboratory srale reactor can be filled to almost its full capacity whereas a tumbled bottle requires considerable free space, the 1/z gallon vessel adequately fills the capacity requirement. Explosion-resistant light globe provides strong reaction vessel
The l/rgallon reactor consists of a borosilicate glass bowl to which is clamped a stainless steel top carrying a hydraulically driven agitator, an internal cooling coil, and various pipe connections. Hydraulic power for the agitation in this reactor was selected after several years of successful pilot plant use in other applications. The operation of similar hydraulic motors was first seen on a visit to the Government Pilot Plant a t Akron some 4 or 5 years ago. These motors, although quite small, are capable of delivering high horsepower. The same type of motor is adequate in driving 12-inch marine type impellers in 50-gallon latex stripping vessels. It is evident that there will be no shortage of power for agitation in the l/rgallon vessel. These motors also have the advantage of controllable speeds from 0 to 3700 r.p.m., and they are safe for use in hazardous atmospheres.
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ENGINEERING, DESIGN, AND EQUIPMENT Figure 1 shows a cutaway section of a hydraulic motor with the agitator attached. There is no need for a separate shaft seal or packing gland on the reactor as the motor's self-contained seal, backed up by the hydraulic fluid, is quite effective. The motor contains seven cylinders, with a unique system of converting reciprocating motion into rotary motion. The entire cylinder block of this motor is free to rotate just as the cartridge cylinder of a revolver type pistol. Ball joints are used on each end of the connecting rods. As high pressure oil forces a piston down, a rotary motion occurs and is applied to a plate corresponding to a crankshaft. The block is forced to rotate since it is connected to this plate by a shaft and universal joints. As the cylinder block rotates the piston reaches the bottom of its stroke, and as the piston continues around it discharges the oil into the return line. There is some oil leakage past the pistons in spite of the precision of these motors. This oil is returned through the case, past the bearings, and backs up the motor seal. The case is vented back to the low pressure oil return line. It is possible to regulate the oil return pressure so that it will be equal to the internal pressure in the reactor, thus giving a zero pressure differential across the seal. This technique is similar to that used in the Duraseal units of standard plant reactors and may be used in high pressure applications; however, the reactor has been used successfully at pressures as high as 50 pounds per square inch without any regulation of back pressure. The minimum oil pressure required for operation of the hydraulic motor in this application is about 400 pounds per square inch. The agitator is held in place by a tight fitting splined shaft furnished with the motor, and rotation of the shaft is such that the thrust from the propellers is directed toward the motor. The hydraulic motors were obtained from a war surplus equipment supplier at a cost of $25 each instead of at the market value of $292. There is a good supply of these surplus motors, as they were used quite extensively on airplane controls during the war. This motor was first used on a 1-gallon wide-mouthed, screw cap pickle jar and several successful polymerizations were conducted in this type vessel. However, the breaking point of the jar was only 32 pounds per square inch, and the glass threads tended to strip 0.Y when the lid was tightened enough to prevent leakage. The price of specially made heavy glass jars was prohibitive because of high initial tooling costs.
Figure 1 , 18
Agitator and cutaway of motor
It mas found that explosion-resistant light globes such as Crouse Hinds EV-530, made of 1/2-inch-thick borosilicate glass, would withstand pressures as high as 300 pounds per square inch. The globes are flanged at the top and have a ground-glass edge, making them easy to seal off. The cost of these standard globes is only about $10 each, and they can readily be obtained from any electrical dealer handling explosion-resistant electrical equipment. These globes are the strongest standard glass vessels that can be obtained-stronger than glass pipe fittings which have a working pressure of only about 60 pounds per square inch gage. The top to fit this globe (Figure 3) was made from '/rinch stainless steel plate. On it were mounted the hydraulic motor, the internal cooling coil, a thermowell and thermometer, a screw cap for a self-sealing gasket, a sample line, and a vent. About 16 man-hours of machinist work were required for making the top and assembling the various components. No external heating or cooling bath is required with the reactor as the necessary heat transfer is accomplished by an internal coil. This coil consists of 15 feet of 1/4-inchstainless steel tubing made to fit loosely in the reactor. The inlet to and outlet from the coil as well as those to the hydraulic motor are snap-on fittings, making it possible to make and break connections without the use of tools. Temperature control is maintained by alcoholwater coolant passing through the coil, the coolant flow being controlled by a temperature recorder controller, the thermobulb of which is inserted in the open thermobvell. This control unit was used because of its availability. However, other types of temperature controllers, such as self-activated control valves, would be much lower in cost. Two 3-inch marine type impellers are mounted on the agitator shaft, but any desired type of impeller may be substituted for these, The combination of widely variable speed and the use of different types of impellers makes this unit a useful tool for studying the effect of agitator design and/or speed on the properties of the latex or polymer produced. After reactor i s charged, incremental additions may be made by hypodermic through a self-sealing gasket
The reactor is easily assembled by laboratory personnel and may be pressure tested for leaks by introducing a few grams of
Figure 2.
Assembled reactor ready for charging
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Figure 3.
Figure 4.
Reactor parts disassembled
Charged reactor showing method
of incremental addition
butadiene, closing off all outlets, and immersing the entire reactor in water. Butadiene vapor bubbling out will reveal any leaks. Soft neoprene gaskets are easiest to seal properly, but oil-resistant asbestos composition gaskets are more durable. Figure 2 shows the reactor assembled and ready for charging. The closure mechanism of this reactor consists of four '/rinch bolts welded to the ring which holds the glass bowl. These bolts also serve as the legs for the reactor. Wing nuts are used for clamping the top securely to the glass bowl. All the reagents except butadiene and the hydroperoxide are weighed and charged to the reactor through a funnel in the screw cap. The entire reactor is then placed on a weigh scale and the butadiene charged through the funnel. Flashing of butadiene sweeps air out of the reactor. The oil and coolant lines are then connected to the reactor and, when temperature is brought down, the batch is initiated by the addition of the hydroperoxide with a hypodermic syringe through the self-sealing gasket. Samples for solids determinations are obtained through the sample line which is also the drop-out line since it extends all the way to the bottom of the reactor. Incremental soap and activator shots are added by hypodermic syringe as shown in Figure 4. The largest syringe available has a 50-cc. capacity which is sufficient for soap shots. Incremental butadiene may be added by attaching a bomb containing a weighed amount of butadiene to the vent line. This vent line may also be used for applying nitrogen pressure or for attaching a pressure gage. The combination vacuum stripper and concentrator used in connection with this reactor consists of an agitated three-necked, 12-liter, round-bottomed flask with necessary heating, vacuum, and condensing accessories. Reactor i s suitable for polymerizations from -70'
to
300' F. and pressures from vacuum to 200 pounds During the short while this reactor has been in use, butadiene/ styrene emulsion polymerizations have been conducted in which temperature control was easily maintained a t 41 " F. on batches completed in as little as 2 hours. The temperature of the initial charge may be brought down from ambient temperature to 41° F. in 3 to 4 minutes. A series of cord dip polymers has been prepared in this reactor for testing in a single strand cord dip apparatus.
January 1956
Several high solids latex batches have been prepared and agitation, even during the very viscous stages of the reaction, seems to be as good as that obtained in plant scale reactors. This reactor is of a convenient size for work being conducted using very high cost monomers such as vinylpyridine. We believe this reactor to be useful also in bulk type polymerizations which are now experiencing renewed interest because of recent developments in this field of polymerization. In fact, this reactor will be useful in any polymerization system within a temperature range of -70" to 300" F. and a pressure range from vacuum to 200 pounds pressure. Summary
The essential features of the laboratory scale glass reactor are as follows: The '/*-gallon size is ideally suited for work with high cost monomers but still yields sufficient latex or polymer for initial small scale evaluation. The reactor can easily be cleaned by lahorsitory persannel. There is sufficient agitation for the study of any type reaction such as high solids latex studies in which there is quite often a phase inversion which results in temporarily high viscosities. The use of a glass polymerization vessel affords unrestricted visibility of the contents of the reactor. However, the vessel does not have the pressure limitation normally associated with glass as it is made of '/tinch-thick impact-resistant borosilicate glass and is capable of withstanding pressures up to 300 pounds per square inch. - This type reactor is useful for studying any type of polymerization requiring turbulent agitation combined with good heat transfer over a wide range of temperatures and pressures. Literature cited (1)
(2) (3)
(4) (5)
Bishop, D. L., Gould, R. T., and h i s s , R. T., private communication to Reconstruction Finance Corp., Feb. 23, 1943. Frank, R. L., and Shepherd, D. A,, Ibid., March 2, 1944. Hobson, R. W., Clowney, J. Y., Foley, H. K., and Pierson, R. M., Ibid., Oct. 25, 1954. Medalia, A. I., and Johnson, W. F., Ibid., May 26, 1948. Simmons, E. S., Parker, J. P., and Wheelock, G. L., Ibid.. June 25, 1947.
R E C E I ~for ~D review November 15, 1955. ACCBIPTED November 18, 1955. Division of Rubber Chemistry, ' ACS, 68th Meeting, Philadelphia, Pa., November 1955.
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