1570
INDUSTRIAL AND ENGINEERING CHEMISTRY
Calibrated gasometers may be used to measure product gases, but their best use appears t o be t o store product gases after metering and prior t o sampling. S.4MPLIh-G PRODCCT GASES. Samples of product gases may be taken as stream samples or as portions of the entire gas. Spot samples taken in evacuated 250-cr. bulbs can be collected in the minimum time. Continuous samples are representative only if the rate of collection is a constant fraction of the ratil of production. Often the difference is not important. d n evacuated bottle fitted with a gage, drying tube, and needle valve niay be utilized to collect a dry sample, the rate of pressure rise giving a n estimate of the rate of collcction. Stirred gasometers are useful for collection of good repreaeritative samples, especially if it is desired t o blend in the gas produced during stabilization of the liquid sample. The stirrer is carried near the top of the gasometer bell on a shaft passing through the top of the bell. Experience in preparing samples of mixed gases shoved that some form of agitation is needed if a representative sample is required in a reasonable time.
Vol. 39, No. 12
MI SCELLASEOUS FACTORS
A few details contribute t o smoother operation and greattLr production. h s far as posnihlc general scrvircc: should Iw provided at one point for all reactors in a n area and piped t o the units. This applies to compressed gases, refrigerants, and p o w r and vent lines. Standardization of parts permits operators on shift t o make replacements as needed and do minor maintenance. Furthermore break-in time in changing from one unit t o anothrr is greatly reduced. Like metals, particularly stainless steels, arc s u b j ( ~tto sieziriy or galling when run in contact. This can be :ivoidod by using Ai.I.S.I.type 116 steel for onc of the parts and .i.I.S.I. type 301 or similar steel for the other. XI1 the apparatus and proccdures d ibed have beeri in use for several years, and continued efforts have been made t o simplify equipment and operation's as far as possible, consistent with good results. RECEIVED April 14, 1947.
... Preparation of high-molecularweight addition polymers
BENCH SCALE EQUIPMENT AND TECHR-IQUES
A. I. Goldberg P O L l T E C H h I C I\STITUTE,
HIGH-molecular-w eight pols mers may be prepared in various w as s, such as hulL, solution, suspension, or eniulsion. The equipment depends bomew hat on the mode of preparation but is essentially a function of the phjsical properties of the monomers. Temperature control of pol? merization reactions is especiall: important inasniuc h as the alerage molecular weights, and therefore the phgsical properties. Iarj considerablj with temperature. Because of the exothermic nature of the reaction, good heat transfer is required, which necessitates adequate agitation. The large number of \olatile mononiers existing in the gaseous state at the desired polymerization temperaturei indicate the need for specialized equipment. Sealed tubes, screw -cap pressure bottles, Coca Cola hottles. aiitoclaies, etc., require the use of \arious techniques for agitation and heating. ifew types of apparatus used in low pressure (5 t o 10 atmospheres) and moderate pressure, up to about 30 atmospheres, are described in some detail. Technique* for handling rolatile monomers, isolation of pol?mer*, arid low temperature polymerization are brieflj discusced.
T
HE formation of high-molecular-n.eight substances froiii
unsaturated organic compounds is well knon-n (6'). In general, these compounds are rqiresented by the furniula RCR,=CH,, and the reaction niay conveniently be tc,riiicd vinyl polymerization. The foilon-ing discussion r e v i e w the techniques t h a t have been devised for carrying out sucih reactions in the laboratorj-. Polymerization may be carried out iri any of a riuiiiber. oi \\-ays such as bulk, solution, suspension, or cmul$ion. The latioratory equipment necessary is somewhat dependent upon the mode of preparation, but for the most part it is a function of the physical
BROOKLYN, N. Y .
properties uf the niuiioniers. It is necessary to control the tenipcwaturc of pol>-merization reactions fairly closely inasmuch as tlic average molecular n-eights, and conscquentl- the physical properties, vary considerably n-ith the temperature of the reaction. Furtlierniore, the polymerization involvcs a chain reaction lvhich ia esotherinic in nature, with the evolution of about 20,000 calories per mole. Thus, it is necessary to escrcise precautions to maintain the desired temperature duriiig the reaction and aderj- to secure efficient heat trarihfer.. BULK POLYhIERIZhTION
Bulk or block polymerization is a term used to describe the polymerization of vinyl compounds, with or without catalyst, which is performed in the absence of solvents or other dispersing media. The dissipation of the heat evolved in the reaction is a major problem in bulk or block polymerization. -4s the reaction proceeds, the fluid monomer becomes more and more viscous. The hcat traiisfcr Iiwomes poorer and poorcr as the viscosity , arid xgitation becomes less and less effective. -4 tempci,ature i i v Of 30" C . or more may be noted in hulk pnly11 \vlic.rc, prr,caution? have been taken for heat dissipation. Yucli reactions ai'c carried out i n the laboratory by utilizing :i high t.atio C J ~ ' surface t o volunie. It is advisable t o work n.ith a nioiionicr layer which is no mow than one inch thick. Small quaiititic,i iiiay he ctrnveniently polynic~iizedin test tubes about o ~ i ciuch in cliameter. For the hulk pol>-nierization of larger quantities (600 to 900grarns) the use of a prcssurc cvolxr ha3 been found satisfactory foi systems having a maximum p r i v u r e of 2 atiiiosphere,~. .I pressure cookilr utilizing a mclral-to-mctal seal has been uaccl. The usual rubber gaskets will not withstand the action of hot nioriomeric suhstances, and a substitute gasketing material (silicone) should he secured for such typcs T o facili-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1947
tatc polymer removal, the monomer charge is poured into a shallow baking pan that fits within the pressure cooker. The sealed pressure c o o l w is placed in a n oven Fhich is held a t the polynicrization temperature for the required time. K h e n polpierization is desired at or near the boiling point of the monomer, the pressure gage incorporated in many cookers provides an iritlicalion of the course of the reaction. SUSPENSION POLIMERIZATION
T h e problem of heat dissipation in bulk polymerization niay br solved by the application of the suspension method (4). The monomer or iiioiiomers t o be polymerized are suspended in a nonsolvent medium-water in most casrs, although salt solutions, ethylene glj-col, and glycerol have been used. Upon agitation the inononicr is dispersed into droplets. These globulcs tend to coalcsce during the polymerization, and the use of dispersing agctrits has generally been found necessary to prevent this. These agc'nts have bceri of t\vo types: difficultly soluble inorganic ciinipounds such as magnesium carbonatc, tricalcium phosphate, talc, and bentonite, or organic substances such as gelatin or polyvin)-l alcohol. The basic unit of laboratory equipment here, i i i well as in solution arid c,niulsion polymerization for high boiling monomers, is the t h r e e - n c ~ krouiitl-bottom flask equipped Kith thermometer, condensc.r, aiid stirrer. d typical formulation for suspension polymerization is : 300 grams water, 30 grams monomer, 0.3 gram s u ~ p e n d i n gagt'rit, and 0.3 gram catalyst (organic). -4 satisfactory proccdure is to add the suspending agent to the water, and commence stirring and heating. K h e n the polymerization temperature (u.sually betxveen 60" and 90" C.) is attained, the solution of catalyst (e.g., benzoyl peroxide) in monomer is added. The polynii,r is isolated i n the form of globules or pearls gc siz(', depending upon the conditions: of the
1571
are among the fastest reactions known to the organic chemist. T h e reaction is characterized by a very low energy of activation, and once the reaction begins, it tends to proceed with explosive violence. Because of this high rate of reaction, the problem of dissipating the heat of polymerization becomes much morc acute, so that these reactions are almost always carried out in solution. Furthermore, it has been found necessary in sonic cases to add the catalyst 11y spraying dilutc: catalyst solutions into the reaction chamber. These reactions are often carried out a t Ion- temperatures, from -50" to - 130" C. Hersberger, Reid, and Heiligmann (.?I recently presented a schematic diagram of an ionic catalyzed apparatus with a thorough description of the technique. E3IULSION PO LYBI ERI ZATION
The commercial production of huge quantities of emulsionpolymerized synthetic rubber during the past few years was preceded and accompanied by a large volume of laboratory work. Much of it has already been published (2, 4, 9, 10, 111, and the equipment and technique have been adequately described. The use of reduction-activation systems which furthtxr acri~li~ratc the rate of emulsion polymerization may be nii~ntionrtlas B recent development by Evans and co-workers ( 1 ) . By the incorporation of reducing substances in addition to the persulfate catalyst of the typical formulation, thc rate of reaction increases many fold. This is applicable to other methods of polynierization as vie11 : but many reduction-activation systems, such as sulfite-persulfate and ferrous sulfate-peroxide, are water soluble, and t h r emulsion procedure facilitates their use. Hohenstein and Mark (4)recently reviewed polymerization literature with special empha.sis on the suspension and emulsion nic,thods, and discussed the mechanism of such polymerizatior!s. PO LYM ERIZATION E QUIPM E 5 T
The maxiniuni ratio of niononier to suspending medium depends upon the partivular niononier, varying from 1:l to 1 : 10 in accordance lvitli the degree of heat dissipation found necessary. For example, s t y r c w permits the use of high monomer ratios (1 :2j, wherea? cth?-l acrylate agglo~nrratesunless Ion-er ratios (13103 ~ r ustd. c T h e amount of suspending agent required to avoid agglomeration depends upon such factors as the nionomerR-ater ratio, t h e monomer, the suspending agent, etc.; in general 15 on the iiionoimxr i i sufficient for efficient agents such as magnesium carbonatc, tricalcium phosphate, etc. The problem ent coagulation of the pearls during polymerization, and one of the variab1c.s involved is the rate of stirring. T o nditions, a variable-s from n-hich as many a ear transmission. By , the actual stirring rate niay be detcmiined. SOLUTION POLT3IERIZATIO5-
The solution method is the simplest means of preparing polymers in the laboratory. Good heat transfer is obtained by diluting the n i o n o n i c ~rvith solvent. L-sually, a homogeneous system is secured w1io.w viscosit?- increase is relatively small n-lien the ~iiononierconcentration is about 2 0 5 . In some cases the solvent takcs part in polymerization and acts as a chain transfer agent; in that c a solvent molecule such a s carbon tetrachloride niay tw chemically incorporated into a polynier molecule to form (8) H R (73((
i I
(-L-(
'
!--j
n('l
I
H I< Ionic c a t a l y c d polymerizations involving the use of acidic catalysts, such a s boron fluoride, titanium tetrachloride, etc.,
Since niariy of the coniuion niononic~r.~, such as butadicnc, viiiyl chloride, isoprene, etc.. are in the gaseous state at the desircd polymerization temperature, it bCcOlneS necessary to nwrk under pressures above atmospheric. Some Inonomera such as ethylene,, trtrafluoroethylene. et(:., require high pressure apparatus which is out3ide the scope of this di-cu.csion. The use of scdcd tubes, screw-cap bottles, Coca Cola bottle autoclaves, etc., nccessitates the use of specialized ecquipnient or agitation and hcating. The technique used in the handling of volatile niouoiiwrs has alrcady heen described by Fryling ( 2 ) ,and thew is littl(1 further that need be added. The method i11 use a t t h r x 1:hrLitorics is to pass the gaseous mononi('r froni the cylinder tlirougli :iwtoncextracted rubber tubing through two xvash bottles containing 5% sodium hydroxide and tlieii rhrough t x o wash bottles filled n-ith a desiccant such a$ Driei,irc. This process reinow substances and water vapor. The dry gas is then liquc,ficd in a condenser coil which is sealed directly into the receiving flask. The condenser flask is kept ill ti vide-mouth Dewar bottle filled with crushed dry ice. Aftcr the necessary quantity has been condensed, the distillation is stopped and the c a d e n s e r is disconnected. By applying prwsure from a nitrogen tank, the monomer can be forced out through a glass tube which rrach(!s t o the bottom of the flask, directly into the reaction x The reaction vessel is placed on a balance, and a sniall csxiws of volatile monomer is added t o the vessel which has been previously filled with all the other less volatile ingredients. The excess is allowed to boil off, and the vessel is inimediately cooled in a dry ice-solvent mixture. The reaction vessel is then capped or scaled n-ith a hand torch. T h e use of test, tubes with necks which may bc readily sealed has the advantage that, when properly sealed, there is no leakage. However, Coca Cola bottles n-hich have been capped with a hand-capping machine are very convenient. The use of heavyFall screiv-cap bottles has also been found satisfactory (?).
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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Figure 1.
Polymerization Apparatus
Hoxever, their convenience is offset, by the possibility of leakage Most of these applications have been to butadiene copolymer systems which were polymerized a t low temperatures and correspondingly low pressures. The capped bottles often leak, b u t where the heat dissipation is poor, the pressure may 1 rapidly and serious explosions have been reported ( 7 ) . The polymerizations are conveniently carried out in a constant temperature cabinet or liquid bath. The latter (Figure 1) provides better heat transfer and makes it possible t o detect leaks readily. The vessels are either rotated end over elid or agitated in a forward and backward oscillatory niann
