Suspension Polymerization
of Chlorotrifluoroethy J 0 H N R. ELLIOTT, ROBERT L. MYERS, AND GEORGE F. ROEDEL Research Laboratory, General Electric Co., Schenectady, !Y. Y.
S
EVERAL disclosures ( I , 3 ) in the patent literature indicate that aqueous polymerization of chlorotrifluoroethylene is feasible, but details are generally lacking. More recently, Hamilton ( d ) reported an aqueous bisulfite silver ion system. The results of the work on the hydrolysis of the monomeric chlorotrifluoroethylene, reported in the accompanying paper ( 5 ) , directed attention to an oxygen-free, acid-side polymerization employing an organic peroxide.
polymerization, even u i t h these, the rate of conversion was excessively slow. It was apparent that a polymerization activator was necessary. Of the various heavy metal ions. including F e + + , Co-+, X+-, MnT+, AI+++, 4g+, and the various reducing agents available, many pairs were effective in accelerating the aqueous polymerization of chloiotrifluoroethjlene; but one pair, soluble ferric phosphate and sodium bisulfite, ~1ere outstanding.
EXPERIMENTAL
All experiments were conducted in heavy-walled borosilicate glass tubes described in the accompanying paper ( 5 ) . The tubes were charged with the aqueous phase, attached to a vacuum manifold, and degassed to a residual pressure of less than 3 microns a t - 196' C. by sequentially freezing in liquid nitrogen, evacuating, and t'hawing. The frozen tubes were pressurized to atmospheric pressure with pure nitrogen and withdrawn from the manifold momentarily to add the peroxide, previously weighed into short melting point tubes. The tubes were again evacuated, and chlorotrifluoroethylene was condensed from n calibrated surge t'ank into the tube, by the use of liquid nitrogen. The tubes were again evacuated and sealed. Purification of the monomer has been described ( 5 ) . It was subsequently observed that a two-stage sulfuric acid scrubbing was a satisfactory substitute for the distillation step in removing the terpene inhibitor. Water treatment and glassware cleaning methods were the same as in the preliminary experiments ( 5 ) . -4fter being charged, the tubes were placed in a water thermostat and rot'ated end over end a t 28 r.p.m. The tubes were opened a t the conclusion of the run by a device consisting of a brass tube fitted with a gate valve. The polymerization tube was placed in the opener with the neck of the tube in the open gate valve. The whole assembly was pressurized to 90 pounds per square inch gage and the gate valve closed. Conversions were determined by weighing the powdery polymer or, when a latex was produced, an aliquot v a s evaporated t o determine t'otal solids. Specific viscosities, qsp, were determined a t 140" C. in dichlorobenzotrifluoride by a modified Ostwald viscometer. The specific viscosity, qsp, is defined by the equation 'Isp =
tdt, - 1
n
TOP VIEW CAPILLARY DISSOLVING CHAMBER
I
EXPLODED 'VIEW
Figure 1. Modified Ostwald Viscometer
Original Formulation. The original formulation utilizing soluble ferric phosphate and sodium bisulfite was as follows :
___
where t s is the flow time of the solution, to the flow time of the solvent, and c the concentration in grams per 100 ml. Unless otherwise specified, values of vaP Rere determined at 1% concentration. ,4 dissolving chamber equipped with a sinteredglass filter R as attached to a conventional Ostwald viscometer (Figure 1). In practice, the polymer was added to the dissolving chamber, and the dichlorobenzotrifluoride then added. The viscometer was shaken a t low amplitude until solution was effected. Nitrogen pressure \\as applied to transfer the solution through the sintered-glass filter into the Ostwald side of the viscometer. A 6-inch length of 0.013-inch Trubore capillary tubing was used, resulting in flow times in excess of 200 seconds for the purr solvent. R 4 T E OF POLYMERIZATIOV
Very early in the investigation it became obvious that a SJ stem employing only an organic peroxide would be impractiral. Only peroxides t h a t had significant decomposition rates a t 25" C.-e.g., acetyl peroxide-were able to produrr an appreciable rate of
SINTERED GLASS FILTER (C)
Chlorotrifluoroethylene Water Soluble ferric phosphate Sodium bisulfite tert-Butyl perbenzoate
Parts by Wt. 12.5 25.0 0.030 0,030 0,0125
The original system produced a conversion of 70 t o 85% in 18 to 24 hours. Water-Monomer Ratio. Two effects were observed when the mater-monomer ratio was varied between the limits of 1 to 1 and 7.5 t o 1. EFFECT o s RATEOF COKVERSIOS.Below a ratio of 2 t o 1, the rate of conversion falls off markedly, but above this value little improvement is observed. This is illustrated in Table I. EFFECTO N FLOCCULATION THRESHOLD. It was observed in the systems mith a 2 to 1 water-monomer ratio that a t lo^ conversion the polymer was present as a typical emulsion pollmerization latex, bluish-opalescent in appearance. However, as the polymerization proceeded all of the polymer flocculated as a dry, Lvhite powder. To obtain the polymer as a latex would be very 1786
August 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE I. EFFECTOF WATER-MONOMER RATIOON RATEOF CONVERSION (20 Hours a t 25’ ( 2 . 9 Water/Monomer 1 /1
Yield, % 57 Zii 90 92 3/1 92 92 93 6/ 1 Contained 0,020 gram NaHSOs, 0.005 gram soluble FeP04, 3.0 ml. 0.01 N HC1, and 10 grams chlorotrifluoroethylene.
desirable from an application standpoint, in view of the fine particle size of the powder, and also from an engineering viewpoint, since latices may be handled as fluids during manufacturing and storage. Since the inclusion of dirt and foreign particles in this polymer is rather serious, this latter point may be very important. This latex can be obtained in two ways: by operating with a low water-monomer ratio and interrupting the reaction a t a conversion a t which the solids content does not exceed 12% or by operating the reaction with a high water-monomer ratio so that a t high conversions the solids content does not exceed 12%. This is illustrated in Table 11. F i g u r e 2.
TABLE11. EFFECTOF WATER-MONOMER RATIO O N FLOCCULATION THRESHOLD Water/ Monomer
Solids a t 1 0 0 ~ oConversion, Conversion,
%
20
4/1 4.6/1
17.7 16 15 14 13
5.2/1 5.6/1 6.2/1 6.7/1
Amt. Preflocculation Large Large Large Large 0.39% 0.36%
%
90
92 91 94 93 94
The particle size of a typical latex produced in this fashion is approximately 0.2 micron. A photomicrograph (Figure 2 ) illustrates their appearance. Effect of Soluble Ferric Phosphate on Rate of Conversion. When the iron concentration is lowered below 3 parts in 7500 parts of monomer the rate of conversion fans off markedly. Above this concentration, the rate also “tails off,” but less seriously. This is illustrated in Table 111. Although it is entirely possible that the tail-off is genuine, it might also be ascribed t o a catalyst exhaustion, since the amount of peroxide was not increased in the same ratio as the activators and thus could have been consumed at a faster rate.
Electron P h o t o m i c r o g r a p h of Typical Latex
Peroxide Concentration. As far as rate of conversion alone is concerned, a peroxide concentration of 0.05% based on monomer is sufficient t o reach 90% conversions. The peroxide concentration exerts a considerable effect on the molecular weighb. 90
70
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TABLE 111. EFFECT OF SOLUBLE FERRIC PHOSPHATE CONCENTRATION O N CONVERSION
F i g u r e 3. Effect of I n i t a l pH on R a t e of Conversion 15 Hours at 25” C.
(15 Hours a t 25’ C.a) Sol. FePOa ~arts/750d Parts Monomer
Conversion,
%
21 5 86.0 81.5 79.0 65 0 42.0 Contained 12.5 grams ohlorotrifluoroethylene, 25 grams HzO, 0.0125 gram tert-butyl perbenaoate, 0.030 gram NaHSOs. a
Effect of Soluble Iron Phosphate-Sodium Bisulfite Ratio on Rate of Conversion. Since it is the function of the reducing agent t o convert the ferric ion back t o the ferrous state where i t can once more react with the peroxide, it is not surprising that a minimum concentration is observed for the sodium bisulfile, below which the reaction does not proceed so rapidly. This is illustrated in Table IV.
Effect of pH on Rate of Reaction. The influence of the hydrogen ion concentration on this system was observed early in the investigation. When attempts were made t o translate the glass tube experiments to a */,gallon stainless steel autoclave, the system resulted in low yields or none a t all. The only difference appeared t o be the initial and final p H of the aqueous phase. I n a series of runs in which the initial p H was adjusted over a rather narrow range, a sharp maximum in rate was observed a t an initial p H of 3.0. This is illustrated in Table V and Figure 3. A second determination more clearly established this maximum a t pH 3. I n spite of this pronounced dependence on the initial pH, there does not appear t o be any great significance in the final pH of the system. This is partly explained b y the fact t h a t the greater the extent of the reaction, the greater the acidity that is developed by the bisulfabe ion produced in the redox cycle: thus,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 45, No. 8
All of the iron complexes listed in Table VI1 served to replace TABLEIV. EFFECTOF FERRIC PHOSPHATE-SODIUM BISULFITE the soluble iron phosphate. The ferric benzoate system was R.4TIO
rather extensively investigated, since it produced the fastest polymerization observed. However, it had one serious drawback-an extreme sensitivity to pH. This is illustrated in Table VIII. A difference of 5 X 1 0 - 6 moles of potassium hydroxide effected a difference of 73% in conversion. Since monomer from various sources varies by as much as 5 X 10-6 moles of H+, it is almost surprising that this system was ever observed. This effect must be related to the formation of the iron complex, which permits the iron to be carried across the monomer-uater interface. I n a mixture of chloroform and water, ferric benzoate is soluble in the oil phase, but when sodium bisulfite is added, the TABLE V. EFFECT OF INITIAL HYDROGEN IONCONCENTRATION iron is in the aqueous phase (Fe++). This is in effect a modificao s RATEOF CONVERSION tion of the redox cycle proposed by Wall and Swaboda (6). (15 Hours a t 25' C.) Some iron complex is absolutely essential to the successful 0 288 AT HCI, Initial Conversion, Final operation of this system. The problem that originally prompted MI. PH % PH the investigation of these other complexes-that is, the gas 0 3.50 66.5 2.45 0.25 2.95 81.6 2.15 evolution caused by the citrate resolved itself when the soluble 0.75 2.30 67.6 2.38 2.0 2.00 20.7 2.00 ferric phosphate concentration was reduced to the optimum value 7.0 1.35 8.3 1.35 listed in Table I X , at which point no gas evolution was observed. 0 . 1 N HCI, Optimum Formulation. Incorporating all of the changes RI1. indicated by the previous work, the following optimum formula-1 (NaOH) 5.7 43 2.18 0 3.46 85.7 ... tion was obtained: (15 Hours a t 25' C.Q) Sol. FePOh, NaHS03, Conversion, G. G. FePOdNaHSOa 9% 8 0.015 0.003 5/1 23 0.015 0.007 2/1 75 1/1 0.015 0.015 77 0.015 0,030 1/2 0.015 0.060 79 0.015 0.075 72 a Contained 12.5 grams chlorotrifluoroethylene. 25 ml. H20, 0.0125 gram tert-butyl perbenzoate.
0.2 0.4 0.6 1.0
3.30 3.12 2.98 2.76
85.0 88.4 88.5 82.2
2.38 2.14 2.30 2.35
those experiments starting nearer the maximum proceed further and generate more hydrogen ions. One difficulty with p H control was monomer acidity. Long after the sensitivity of this system to changes in pH had been demonstrated, duplicate runs using different monomer sources would behave very erratically, seldom agreeing in rate of conversion. After careful analysis of the various monomer sources by analytical distillation and mass spectrometer data and after no detectable differences were found, the amount of acidity developed by different monomers was determined by shaking with degassed water. It was found that these various monomers were contributing trace amounts of hydrogen halide acids, varying by as much as 5 X 10-5 moles of acid per 7.5 grams of monomer charged. In this polymerization system the change in p H of 3.5 to 3.0 was accomplished by adding 7.2 X 10-5 moles of hydrochloric acid, resulting in a net change in reaction rate of 1yo per hour, It is not surprising, then, that this monomer acidity had such a vigorous effect on the rates of reaction. After complete removal of the volatile hydrogen halides by passing the monomer through a soda lime adsorption tube, the optimum hydrogen ion concentration was found to be 5 X 10-5 mole, or 0.001792 gram on the 7.5-gram monomer charge. Organic Complexing Agent. It is significant that all references to the iron phosphate used in this system are prefixed by the word "soluble." The compound used was a complex salt of sodium ferricitrophosphate, containing 12 t o 15% iron, 1570 phosphorus pentoxide, and 45y0 citric acid ( 4 ) . At one point of this investigation, when relatively large amounts of this material were employed, it was noticed t h a t gas evolution from the polymers invariably began at 265" C. It was found that the citric acid present produced this gas evolution. In an effort to replace citric acid the experiments shown in Table VI were run, replacing the soluble iron phosphate with equimolar amounts of other iron salts. The failure of the inorganic salts to promote this system is somewhat remarkable. When this unusual dependence on the presence of the organic complexing agent was observed, other organic complexing agents were evaluated. Some of these are illustrated in Table VII.
Parts by K t 7.5 54.5 0.003 0 012 0.004 0 001792
Chlorotrifluoroethylene Water Soluble ferric phosphate Sodium bisulfite tert-Butyl perbenzoate Hydrochloric acid
3.0 9 90
$;ne, hours Conversion. %
The effect on the rate of incorporating these optimum conditions is illustrated in Figure 4. MOLECUL4R WEIGHT
All of the preceding work was directed toward accelerating the rate of polymerization without particular regard for molecular weight, principally because it was felt that this would not be a very serious obstacle. Although a variety of physical methods have been reported for relating the molecular weights of various chlorotrifluoroethylene polymers. including melting point and melt viscosity, the methods have serious experimental drawbacks. A routine technique for determining solution viscosities permitted the evaluation with some precision of efforts t o produce high molecular weight polymer. -4plot of the reduced viscosity versus
TABLET7I. EFFECTO F
IRON
S.4LTS
ON
CONVERSION Conversion,
Iron Salt Fez(S0ds. (NH4)2SOa.24HzO FeS04. ( N H ~ ) ~ S O I . G H ~ O KaFe (CN)6 KnFe(CN)G FeSOa.7HzO citric acid FeS04.7H20
+
c/a
18 10 10 0
15 87.5
OF ORGANIC COMPLEXING AGENTS TABLE VII. EFFECT
(16 Hours a t 25' C.) Yield, Compound Ferric stearate Ferrous acetvl acetonate Ferric acetyiacetonate Ferric benzoate a
7 hours.
%
83 57 7 8 . 5a 85.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TIME,HRS il
Figure 4.
I
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Effect of Optimum Conditions on Rate of Conversion
concentration indicates the precision of the method for four molecular weight ranges (Figure 5). I n the absence of chain regulation by chain transfer or monomer diffusion rate, the viscosity molecular weights should be controlled by the rate of radical generation. Several methods were evaluated for controlling the number of radicals produced per unit of time. Lowered Peroxide Concentration. The first approach to higher molecular weights was accomplished by lowering the peroxide concentration, as shown in Table IX. Some improvement was obtained, but a t a rather serious reduction in rate and extent of conversion. Attempts were made t o add the peroxide incrementally, so that a given monomer charge might be carried t o a high conversion by using low peroxide concentrations a t any given time. This was accomplished by sealing small capillaries, into which the peroxide had been weighed, t o a glass-encased Alnico magnet. This interior magnet was held in place by a second larger magnet taped to the exterior of the tube. At the appropriate time the outer magnet was loosened and one of the capillary tubes crushed on the bottom of the polymerization tube. The results of one of these experiments are listed in Table X. The only effect of these various permutations was on the extent of conversion, with little effect on the molecular weight. Further investigations along thia same line failed t o produce higher molecular weight or higher conversion.
TABLE VIII. EFFECTOF 0.01 N POTASSIUM HYDROXIDE ON FERRIC BENZOATE SYSTEM (10 Hours) 0.01 N KOH M1. 6 0
Conversion,
%
n
0 11.4 84.1 66.0
0 9 t
t
aO"s
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Figure 6.
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Effect of Temperature on Molecular Weight Variable peroxide concentration
Iron Concentration. The results of the study of regulating the rate of radical generation by the reduction of the iron concentration are listed in Table XI. Although these experiments were not conducted at the optimum pH, it is still significant that the molecular weight did not change appreciably over the range investigated. The only real effect was a reduction'in the extent of conversion. Temperature. From the previous attempts t o elevate the molecular weight, it was concluded that the peroxide was decomposing at a rate faster than monomer was being supplied t o the reacting zone. This was especially suggested by the trend o f increased molecular weight with the reduction of the peroxide. A correlation of molecular weight with polymerization temperature was observed. This is illustrated by Figure 6. The higher molecular weight materials are produced at the lower temperatures. To date, this has not been accomplished without a sacrifice in rate and final extent of conversion. The extent of
TABLE IX. EFFECT OF PEROXIDE CONCENTRATION ON MOLECULAR WEIGHT
(Optimum formulation a t 25' C.) tert-Butyl Perbenzoate
%
Excessively long;
tubes.
Timea, Hrs.
TABLE X. EFFECT OF INCREMENTAL PEROXIDE ADDITIONON MOLECULAR WEIGHT
Conversion,
%
a t t e m p t to produce equivalent conversion i n all
Initial 0.02 0.01 0.01 0.02
e. 04
0.02
(Optimum formulation, 17 hours a t 25' C.) Cqntert-Butyl Perbenzoate, % ' version, 4 Hr. 7 Hr. Total % 0.0 0.0 0.02 37 0.01 0.01 0.03 45 0.01 0.01 0.03 59.6 0.0 0.01 0.03 63.5 0.02 0.0 0.06 88.5 0.04 0.0 0.06 81.5
WP(l%) 1.21 1.17 1.03 1.18 1.05 1.11
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The system consists of chlorotrifluoroethylene, water, sodium OF IROK CONCENTRATION O N MOLECULAR bisulfite, soluble ferric phosphate, and tert-butyl perbenzoate as TABLE XI. EFFECT
WEIGHT
Soluble FePOa, Parts b y Wt. 0.0030 0.0024 0.0018 0.0012
0.0006 0.0003
initiator, operating a t an initial p H of 3.0. duces a 90% conversion in 9 hours a t 25" C.
(15 Hours a t 25' C.) Conversion,
%
tlSP(l%)
64 64 54.7 54 58
45
ACKNOWLEDGMENT
0.953
...
...
... 0:935
conversion is affected by the reduction of the amount of peroxide employed, so that the system exhausts the peroxide prior t o completion of the conversion to polymer. It is entirely likely, although not demonstrated t o date, that this feature can be overcome by incremental addition of catalyst so that a t any given time the amount of peroxide present is small, but enough may be added to convert all of the monomer to polymer.
This system pro-
The authors wish to express their appreciation to D. JV. Caird and K. Goldblum, Chemical Division, General Electric Co., Pittsfield, K.Y., for their many helpful suggestions in the course of this work. They are also indebted t o Patricia Morrison for her valuable assistance in the specific viscosity determinations. LITERATURE CITED
( 1 ) A m e r i c a n Viscose Corp., B r i t i s h Patent 578,168 (June 18, 1946). (2) H a m i l t o n , J. M. (to E. I. du Pont de N e m o u r s & Co.), U. S.
Patent 2,569,524 (Oct. 2, 1951). (3) I. G. F a r b e n i n d . , B r i t . Patent 465,520 ( M a y 3, 1937). (4) Merck Index, 6th ed., p. 429, Merck 8: Co., R a h w a y , N. J., 1952. (5) Myers, R. L., ISD. ENG.CHEM.,45, 1783 (1953). (6) W a l l , F. T., and S w a b o d a , T. J., J . Am. Chem. Soc., 71, 919 (1949).
SUMMARY
A system has been developed for the suspension polymerization of chlorotrifluoroethylene which produces high yields of high molecular weight polymer in reasonably short times.
RECEIVED for review January 8, 1953. ACCEPTED April 23, 1963. Presented as part of t h e Symposium on Chlorotrifluoroethylene before the Division of Polymer Chemistry at t h e 122nd Meeting of the AMERICAN CHEMICAL SOCIETY, At,lantic City, S . J.
Electro horetic Mobility Study of Fresh Hevea Latex WILLIAM W. BOWLER1 T h e Firestone Plantations Co., Harbel, Liberia, West Africa
T
0 BOTH the producer of natural rubber latex and the manufacturer of latex goods, the stability of Hevea latex is one of its most important properties. Two aspects of this important latex property are the mechanical stability and the chemical stability. The former is measured by the sensitivity of the latex to mechanical agitation, the latter by the sensitivity t o addition of ionic material. Although there is no clear relation known to exist between these two stabilities, the electrical charge on the rubber particles is considered t o be important in either case. As electrophoretic mobility determinations furnish one of the most direct methods for studying the charge on colloidal particles such as those in rubber latex, the effect of seasonal and clonal variations, latex aging, ionic strength, pH, and the presence of added materials upon the electrical mobility of fresh Hevea latex was studied, At the same time the mechanical stability of many latices was measured in order t o determine what correlation exists between mobility and mechanical stability. EXPERIMENTAL WORK
The micromethod described by Abramson, Moyer, and Gorin ( 1 ) was used t o study the mobility of latex.
A flat, horizontal, glass microelectrophoresis cell of the Briggs (5') type was used, with a mercuric nitrate solution in the lower
part of the electrode compartment and half-saturated potassium nitrate in t h e upper. The cell width was 23.6 mm. and depth 0.785 mm., giving a width-to-depth ratio of 30. The glass cell was mounted on a metal frame which rested on the microscope stage. A 20X Spencer objective and a 2 0 X Bausch and Lomb ocular provided a magnification of 400 X. The fine adjustment 1
Present address, The Firestone Tire & Rubber Co., Akron 17, Ohio.
scale of the microscope was calibrated by means of a slide of known thickness. An eyepiece micrometer, calibrated with? a stage micrometer, was used t o measure the velocity of the moving particles. For measuring the conductivity of the dispersions, an Industrial Instruments, Inc., Model RC-BC conductivity bridge was used along with a dip-type cell having a cell constant of 0.916. A series of velocity measurements was made on one dispersion a t various depths in the glass cell. The best parabola was drawn through the points of the velocity-depth plot, and the resulting curve was integrated graphically. The true electrophoretic velocity was thus found t o occur at the 0.20 and 0.80 fractional depth levels. The theory of Komagata ( 5 ) predicts values of 0.205 and 0.795 for a cell whose width-to-depth ratio is 30. All velocity measurements were therefore made by averaging t h e values measured a t the 0.20 and 0.80 levels. p H was measured with a Beckman Model G glass electrode assembly. Room temperature varied between 28.5" and 32.5" C. during the course of all experiments, and the temperature of each dispersion was maintained a t 30.0" i. 1.0' C. while measurements were being made. All mobility measurements except those after various aging periods were made by allowing approximately 3 ml. of latex to flow directly from the tapping cut of the tree being studied into 500 ml. of boiled distilled water 5 minutes after the flow of latex had started. The dilute suspension thus obtained was then taken immediately to the laboratory, where dispersions in buffer solutions of the desired concentrations and p H values were made. The mobility measurements were then made immediately upon these dispersions and the conductivity and p H measurements were all completed on the same day the latex was obtained. An acetate buffer was used i n the p H range 3 to 6, although it was difficult t o obtain satisfactory results a t p H values above 5.5 with this buffer. At pH values above 6.0, a disodium hydrogen phos-
