Improved Polymerization Techniques Device f o r Sampling Latex during Polymerization ROBERT J.
HOUSTON, The B. F. Goodrich Company, Akron, Ohio
A procedure is described for the precise and accurate sampling of latex during the ~ o u r s eof a n emulsion polymerization without interrupting the reaction. The sampling device, which consists of a hypodermic syringe equipped with a stopcock, and a “stirrup” for stopping the plunger, is used in conjunction with punoture-sealing caps or closures on the polymerization vessels. The technique is simple and rapid and of general applicability. It is particularly useful for followi n g the progress of a polymerization by determination of total solids and for removing samples for special investigations such as mercaptan analysis or gel and intrinsio viscosity studies.
ARIOUS procedures have been employed for following the of error. The “tots1 solids” method is capable of high precision, provided that a truly representative sample of latex is course of emulsion polymerisation reactions oonducted in the evaporatcd. The analytical sampling of latex containing laboratory. In the tube technique described by Fryling (I), the progress of the polymerization was followed by periodic observavolatile monomers, such as butadiene, may he complicated by tions of the height of the latex column. !Although simple and relatively high pressures in the reactor, rapid loss in weight mpid, this method can be considered only semiqusntitahive of sample due to evaporation of monomer, and tendency t o foam because of the presence of emulsifving The variparticularly a t low conversions where tl lati:x is usually’ foamy. . . agent. . ous venting procedures, employed t o More refined dilatometric procedures may result in increased preciston but remove free butadiene from the latex prior to sampling, are tedious and become rather laborious. All such methods suffer from a t least a time-consuming. Medalia (4) has detheoretical disadvantage in the ease scribed a stopoock device for samof two-monomer systems, in that the pling which obviates the difficulties reduction in volume of the latex may inherent in the venting problem. The depend upon the combining ratio of necessity of weighing the bottle bethe monomers as well as upon the fore and after sampling, however, imtotal hydrocarbon conversion. More poses a limit upon the precision which serious limitations from the practical is atteinahle and application of the point of view, however, are the exmethod is restricted to small reactors, tremely mild agitation and the low such as 4ounce bottles. upper limit upon the amount of polyA procedure has been developed mer which can he prepitred from each for the precise and accurate Samcharge in such apparatus. pling of latex as it is formed in an Screw-capped or c r o wn-eappe d emulsion polymerization process withbottles, with capacities varying from out interrupting the polymerization. a few ounces to ahout one quart, It is possible, therefore, not only t o have come into rather general use as follow the progress of a Single reacreactors for emulsion polymerizations. tion throughout its entire course, but The progress of a given reabtion has to remove samples at any time for usually been folloned by analysis a t special studies such 8.8 mercaptan appropriate intervals of members of analysis or gel determinations. The * a series of duplicate charges, and oontechnique is simple and rapid and is .. .. versions have been determined either f any size. from the yield of coagulum or from the total solids obtained upon evapor a t i o n of a weighed portion of “vented” latex. The apparent couversion cdculttted from the yield of The sampling device consists of a hypodermic syringe equipped with B dried polymer is subject to variations one-way stopoock and a “stirrup” for depending upon the method of eostopping the plunger when a sample agulation, the nature of the coagulum, of the desired size has been admitted. and the cere exercised during washIt is used in conjunction with puncturesealing closures such as those deing. Occlusion of coagulant or emulscribed by Harrison and Meincke (8). sifying agent, loss of low molecular Syringes and fittings are available weight polymer through solubility in from Becton, Diekinson and Comthe ooagulation medium, and mechanpany, Rutherford, N. J., or their reFigtrre 1. Hypodermic Syringe tail outlets. Although the assembly Sampling Devise ical loss of product are possible sources
49
ANALYTICAL CHEMISTRY
50
90r------
admits of many variations, an example of a satisfactory combinat ion is tabulated : B&D Catalog No. Item 5-ml. syringe with Luer-Lok One-way stopcock Hypodermic needle
ao-
5YL L/S1 LIiRH, 20 gage, 1 inch
The stirrup can be made in a few minutes from a strip of thin metal, attached t o the syringe by a metal hand tightened with a small machine screw, or it can be machined from light metal, as illustrated in Figure 1. k
PROCEDURE
The syringe plunger and the stopcock are lubricated lightly with fairly heavy stopcock grease. One lubrication will usually suffice for a considerable number of samples. In removing samples of latex from bottles, the usual sequence of operations is as follows: 1. Shake the bottle and invert. 2. Insert the hypodermic needle through the bottle cap and open the stopcock. 3. AUlowthe syringe to fill or, if the pressure in the bottle is low, draw the plunger out until stopped by the stirrup. 4. Close the stopcock and withdraw the needle from the bottle. 5. Wipe latex from the end of the needle and weigh the syringe assembly to the nearest centigram. 6. Eject the latex into a tared dish or into shortstop or coagulant, close the stopcock, and reweigh the syringe assembly.
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70
1
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z w u
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E 50n
z z 40-
2
ul
a w 30>
z 0 u
20-
10-
0 0
I
I
2
4
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6
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8
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10
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TIME IN HOURS
Figure 3. Effect of Oxygen ;n Polymerization Rates i n a Butadiene-Styrene Emulsion System A . Control; no oxygen B . 25 m l . of oxygen (atmospheric pressure) per 100 grams of mono-
c.
Figure 2. Sampling Deviations in Syringe Sampling Technique A. B.
Samples withdrawn as rapidly as possible Samples withdrawn 30 seconds after those i n series A
For latex a t l o r conversions, because of the instability of most emulsions, sampling must be very rapid if a representative sample is to be assured. In such cases the needle is f i s t inserted with the stopcock open and the plunger of the syringe held down securely with the thumb; the bottle is shaken vigorously and inverted, and the plunger is released. The sample can thus be taken within a few seconds and before appreciable separation of phases takes place. The importance of rapid sampling is emphasized by the data plotted in Figure 2, where the apparent conversions obtained
mers
50 m l . of oxygen (atmospheric pressure) per 100 grams of monomers
by sampling as rapidly as possible are compared with those obtained when the bottle was allowed to remain inverted for an additional 30 seconds before the sample was withdrawn. In determining total solids, the latex is ejected into a weighed dish containing a suitable shortstop, such as hydroquinone, and evaporated to constant weight on a hot plate or in an oven. It is unnecessary to rinse t'he syringe between samples, provided the conversions are similar. If such is not t,he case, the syringe must be rinsed and dried or it may be flushed n-ith latex of suitable solids concentration before it is used again. The mercaptan content of a synthetic latex can be determined conveniently by amperometric titration TTith silver nitrate, according to the method of Kolthoff and Harris (3). In such an analysis, rapid sampling at lov conversions is extremely important because the concentration of mercaptan in the monomer phase is usually high. Improved precision can be obtained in such a case by modifying the sampling procedure. The syringe is weighed, ' filled with sample, and reweighed; it is then rinsed m-ith xvater and alcohol in order to transfer the latex and free nionomers quantitatively. DISCUSSION
The utility of the syringe sampling technique has been proved by use over nearly three years by various investigators in the author's laboratories and elsewhere. Hydrocarbon conversions can be determined by the total solids method over most of the range with an accuracy of +0.5y0 conversion. Although precision and accuracy decrease slightly at low conversions, the technique is very useful for studying and distinguishing between inhibition and retardation phenomena. For illustration, the data plotted in Figure 3 for a particular butadiene-styrene emulsion system demonstrate unequivocally the inhibiting effect of
V O L U M E 2 0 , N O . 1, J A N U A R Y 1 9 4 8 oxygen, as evidenced by definite induction periods, with no retardation once normal polymerization has begun. ACKNOWLEDGMENT
The author is indebted to S. A. Sundet for the experimental data chosen to illustrate the hypodermic syringe sampling technique.
51 LITERATURE CITED
(1) Fryling, C. F., ISD. ENG.CHEM.,ASAL.ED.,16, 1-4 (1944). ( 2 ) Harrison. S. A , . and Meincke. E. R.. Ibid.. 20, 47 (1948). (3) Kolthoff, I. M , , and Harris, FV. E., Ibid., 18, 161-2 (1946). (4) Medalia, -1.I., J . Polymer Sci., 1, 245-6 (1946).
R~~~~~~~~~l~ 8 , 1947. Work done under the research program sponsored by the Office of Rubber Reserve, Reconstruction Finance Corporation.
Chromatographic Separation of Beta-Carotene Stereoisomers as a Function of Developing Solvent E. M. BICKOFF Western Regional Research Luborutory, Albany, Culif. 'The quantitative treatment of chromatography as developed by LeRosen has been utilized for studying the separation of @-carotenefrom its cis- trans stereoisomers. The relative efficiency of a number of solvents as developers for this system has been determined. Certain aromatic ethers have been found superior to most of the more common developers for separating the stereoisomers adsorbed on a lime column.
K"""
KT WORK has shown that the provitamin X activit,y of carotenoids is influenced to a considerable degree by their stercochemical configuration (3, 4). Since purified carotene extracts may contain up to 44'3, of the cis-trans stereoisomers ( 6 ) , it is essential to include their estimation in any analytical procedure for carotene. Chromatographic adsorption on calcium hydroxide columns will separate the isomers from one another ( I O ) . However, the column must then be extruded, and each zone separately carved out and transferred t o petroleum ether. In a collaborative study of this method of separating the isomers, four laboratories reported results which were in poor agreement and which indicated that the method must be further studied and simplified before it can be recommended for general routine use ( 5 ) . Chromatographic separation of the isomers would be simplified if it were possible to collect them separately by continuing the development until each zone passes completely through the column. The experiments herein reported were undertaken in order to obtain more information on the role of the developing solvent in increasing the separation of the stereoisomers during chromatographic adsorption analysis. The relative efficiency of the solvents as developers has been determined under standardized reproducible condit,ions. Quantitative data on the relative eluting strength of the solvents have also been obtained. 31ATERIALS AND APPARATUS
Adsorbent. The adsorbent used in this xork was Shell Brand lime, chemical hydrate, 325-mesh as recommended by Polgar and Zechmeister ( I O ) . -1100-pound sack of the fresh material JTas repacked in tightly closed bottles until used. h-o difficulty due to change in activity viith age was noted. Precautions were always taken to minimize the exposure of the lime to the atmosphere prior to use. Through the courtesy of PolgAr, a sample of the lime used in most of his published work was obtained and compared x i t h this material ( I O ) . The adsorptive properties of the tlvo lime preparations ton-ard the carotene isomers were found t,o be very similar. Isomerized-Carotene Solution. This solution was prepared from S.M.A. crystalline @-carotene. Iodine (equivalent to 2% of the \wight of the cnrot,ene)was added to the carotene dissolved in petroleum ether (87.8" to 98.6" C.). The concentration of the carotene was 53.6 mg. per 100 ml. of solution. Only this concentration of carotene was studied. Solutions viere stored i n amber-colored flasks in the refrigerator until used. Polgar
and Zechmeister have also prepared such iodine-isomerized solutions and chromatographed them on lime columns. Their report gives the relative positions of the isomers on the adsorpt'ion columns (10). In the present report,, the isomers are identified by comparison vrith the previous work. Similar chromatograms were obtained with solutions stored for periods up to 3 weeks. Solvents. The petroleum ether (87.8" t o 9 8 . 6 O C.) was dried over sodium and used without further purification. Acetone \vas treated n-ith silver oxide, dried, and redistilled. Benzene was dried over sodium and redistilled. Ethyl ether was sodiumdried, passed through an alumina column (g), and redistilled. Methyl and ethyl alcohols were refluxed over magnesium and redistilled. Carbon tetrachloride was water-washed, dried, and redistilled. Chloroform was shaken with concentrated sulfuric acid, washed, dried, and redistilled. Phenyl ether and cetyl alcohol were recrystallized prior to use. All other solvents were redistilled, prior t,o use, in a highly efficient,all-glass fract'ionating apparatus (11). Apparatus. The chromatographic tubes (KO. 1, 9 mm. in inside diameter x 130 mm.) were described by Zechmeister ( I S ) . Three S o . 1 tubes were used in this work. Each tube was mounted by means of adapter and rubber stopper on a 1-liter suction flask which was conntxcted to a water aspirator. A Zimmerli gage was used for determinat,ions of t'he pressure a t intervals during the chromatographic analysis. PROCEDURE
Column Characteristics. LeRosen's technique 11-ith only slight modifications was used for obtaining the quantitative data presented in this paper ('7, 8, 9). He introduced the following terms to assist in characterizing the systems studied:
T50 = time in seconds required for a solvent to penetrate 50 mm. into an initially dry column under vacuum given by a water aspirator
s
length of adsorbent column containing one unit volume = length of tube requlredof tosolvent contam same volume of solvent (S indicates average packing of column)
5 volunie of
adsorbent = 100 -.('
s
I ) , gives a measure of per-
centage of tube volume occupied by adsorbent
V,
= rate of flon. of developing solvent through column when a
state of constant flow has been reached, mm. per minute
