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A. K. PRINCE and R. D. SPITZ The Dow Chemical Co., Midland, Mich.
Synthetic Rubber Production Chelating Agents in Sulfoxylate Polymerization EDTA is apparently unique in its capacity to sustain sulfoxylate polymerization reactions activated by low iron levels. Utilization of the most economical form of this agent in the proper amount offers synthetic rubber producers a possible avenue of economy
PUBLICATIOS
of the fact that the iron chelate of (ethylenedinitri1o)tetraacetic acid (ethylenediaminetetraacetic acid, EDTA) is a poiverful activator for peroxide-catalyzed emulsion polymerization reactions (5) and the subsequent development and wide-spread use of the sulfoxylate-based activator system ( 2 ) have made chelating agents impcrtant raw maierials for virtually every rubber producer. The data presented here may allow these producers to reduce their chelating agent cost substantially. T h e sulfoxylate system is activated by low iron levels and might be expected to be sensitive to small changes in the iron level. The effect on the reaction rate of a change in the chelating agent (and the resultant change in iron chelation) appears worthy of investigation. In the early article (2) the effect of varying the chelant concentration was investigated, but more extensive work on the role of the chelant is unreported in the general literature. The present investigation was undertaken to answer the following questions: What conversion rates are obtained with chelating agents based on aminopolycarboxylic acids other than EDTA. how does the molar ratio of chelant to iron affect the conversion rate, and how important is chelant purity? Of the chelating agents evaluated, only those based on EDTA supported and sustained the polymerization reaetion under the conditions studied-i.e., attained linear conversion curves reaching 60Yo conversion in 6 to 8 hours. T h e effectiveness of commercial mixtures of Na4EDTA and NaDHEG is apparently due to the presence of the former. The conversion rate undergoes relatively minor changes when the EDTA-iron molar ratio is changed frcjm 2 to 0.5, and seems unaffected by differences of purity of the agent used. Because of the high degree of hydrolysis of trivalent iron, the chelation efficiencies of these chelants for ferric iron are low a t the p H of polymerization, Thus, the major effect of the chelant must be the contrcl of the ferrous ion
Literature Tabula tion Subject Ref. Iron chelate of EDTA i s a powerful activator for peroxide-catalyzed emulsion polymerization reactions (6) Sodium formaldehyde sulfoxylate is a useful ferrous ion regenerant in peroxide-catalyzed emulsion polymerization system activated by EDTA-iron chelates (2) Contributions of components in cold GR-S rubber recipes (8) Specification by the Rubber Reserve for fatty acid emulsifiers sold for GR-S rubber manufacture (4) Conversion rate in sulfoxylate system unaffected by pHinregion 10.0-11.0 (2) Tabulation of stability constants from (1) available chemical literature
level. T h a t portion of the total iron bvhich is present a t any time as ferric ion does not utilize chelating agent, so that the agent-iron ratio may be lowered below stoichiometric with no effect on polymerization rate. If the ferric ion concentration is unaffected by chelating agent-i.e., controlled by the pH or maintained at a steady state by the balanced oxidant-reductant system-it should be possible to correlate the efficiency of the chelating agent in the activating system with the ferrous ion level maintained by the chelant at the p H of polymerization. Apparently, for the: system studied, EDTA maintains
Table I. This Recipe Was Selected as Typical of the Sulfoxylate System (Temp., 5' C . ; pH, 10.5-10.6)
Ingredient Butadiene Styrene Deionized water Diisopropylbenzene hydroperoxide Sodium formaldehyde sulfoxylate dihydrate Potassium palmitate tert-Dodecyl mercaptan Potassium chloride Ferrous sulfate heptahydrate Chelating agent in various molar ratios to iron
Parts 70 30 180 0.10 0.07 4.50
0.30 0.30 0.02
the ferrous ion a t the concentration resulting in optimum radical generation rate. Of prime importance is the magnitude of the ferrous ion chelate stability constant, which fixes the concentration level a t which the ferrous ion is maintained. The further secondorder changes due to mass action when the chelant-iron ratio is changed result in changes within experimental deviation.
The Recipe A typical sulfoxylate recipe was chosen after considering the literature on redox rubber polymerization systems (2) and emulsion polymerization systems in general ( 3 ) , and after discussions with rubber producers. The levels of ingredients were those most often used for this type of polymerization system (Table I ) . Styrene and butadiene were Dow production materials free from inhibitors. T h e distilled water was boiled to expel oxygen. Diisopropylbenzene hydroperoxide (DIBH) was chosen because it was available. Conversion rates obtained with this and other organic hydroperoxides are comparable. Sodium formaldehyde sulfoxylate (SFS) was the reductant. Potassium palmitate \vas chosen as the emulsifier in preference to disproportionated resin acid soaps because fatty acid soaps give more reproducible results in laboratory scale evaluations. Potassium palmitate was freshly prepared each day as an aqueous solution from palmitic acid [Seo Fat 16 served as a source of palmitic acid, which conformed ta specifications originally set by the Rubber Reserve ( 4 . 1 and potassium hydroxide. Potassium chloride was used as the electrolyte to maintain a n all-potassium system, which gives somewhat higher conversion rates than sodium systems. Analytical grade ferrous sulfate heptahydrate was the iron source and an aqueous phase p H of 10.5 was used. p H values of 10.0 to 11.O apparently give comparable results (2). A temperature of 5" C. is standard for sulfoxylate polymerizations and was carefully controlled. VOL. 52, NO. 3
MARCH 1960
235
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Figure 1. Pure EDTA at concentrations equimolar (or greater) to iron produced desired conversion curves with nominaI-deviation
The Procedure A method modeled after the "pop bottle" polymerization method was used. Prior to each run the bottles were washed thoroughly with soap and water and chromic acid, rinsed with distilled water, and allowed to dry. Emulsifier solution was prepared by adding 29.2 grams of the palmitic acid and 3.0 grams of potassium chloride to approximately 1 liter of water (previously boiled to expel the dissolved oxygen and cooled to 60" to 70' C.), adding sufficient potassium hydroxide solution to dissolve the palmitic acid and adjust the p H to 10.5, and diluting to a final weight of 1600 grams. Then 160 grams of this emulsifier solution was added to each bottle, and the bottle was flushed with nitrogen, corked, and set aside. The bottles containing the emulsifier were placed in a deep freeze and cooled to 0" C. (approximately 4 hours). A stock iron solution containing 6.4 mg. of iron per ml. a t p H 1.0 was prepared from ferrous sulfate heptahydrate, distilled water, and sulfuric acid. Chelating agent stock solutions, equimolar to this iron solution, were prepared by dissolving the required amount of chelant in distilled water and adjusting the p H to 10.0 with sulfuric acid or potassium hydroxide. p H adjustment of these stock solutions resulted in clear solutions when they were mixed to form the activator. Activator solutions were prepared by mixing 10.0 ml. of the stock iron solution with sufficient chelant stock solution to give the desired chelant-iron molar ratio, adding 1.12 grams of SFS dihydrate, and diluting to 400 grams with previously boiled distilled water. This activator solution was cooled in a deep freeze for approximately '/z hour and
236
Figure 2. Only EDTA-based che'lants sustained polymerization at the desired rate
added in 25-ml. amounts to each bottle containing the chilled emulsifier. Next, 29.1 grams of a solution of 1% tert-dodecyl mercaptan in styrene was added to each bottle. The bottles were flushed with nitrogen and to each was added 70 grams of butadiene. Immediately after this addition the bottles were capped and placed in a refrigerated (5" C.) constant temperature bath. After rotation in the bath for 45 minutes, each bottle was injected with 1 ml. of a styrene-DIBH solution containing 0.10 gram of DIBH per ml. of styrene. Because DIBH initiates polymerization, the time of injection was taken as zero time. Duplicate samples were removed from the bath at predetermined time intervals and shortstopped by injecting 1 ml. of a 50Yo aqueous solution of a commercial shortstop agent based on sodium polysulfide and dimethyldithiocarbamate. After steam distillation to remove unreacted monomers, the latex was coagulated by addition of a 10% alum solution. The coagulated latex was removed by vacuum filtration and dried in a vacuum oven at 65" to 70" C . The weight of the dried latex was recorded and the data were analyzed by consideration of graphs of per cent conversion us. time. The only criteria used in comparing the efficiencies of the activator systems were plots of per cent conversion us. time. A nearly linear curve reaching 60% conversion in 6 to 8 hours was desired. No attempts were made to study the properties of the latex. Preliminary Investigation After standardizing upon the above recipe (Table I) and procedure, five consecutive runs using pure EDTA at a 1 to 1 molar ratio of chelating agent to iron gave the results plotted in Figure 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
The average per cent conversion at the various conversion times, bracketed by the average deviation, appears as the shaded area. Any concentration level of any given chelating agent yielding curves of per cent conversion us. time which fall within this shaded area would be considered as giving results indistinguishable, by this method, from those obtained with pure EDTA at a 1 to 1 molar ratio to iron. Figure 1 also presents results of preliminary investigations on the effect of changing the chelant-iron ratio. These results indicated that 2 to 1 molar ratios of EDTA to iron gave conversion rates comparable to 1 to 1 ratios; 0.5 to 1 molar ratios gave slightly slower rates. Effect of Other Chelants The chelating agents evaluated in this work, together with the logarithm of the formation constants for their iron chelates, are listed in Table 11. The constants listed are for the so-called normal chelates, the complexes formed between the simple ionic forms of the metal ions and the completely ionized forms of the chelating agents. Because of hydrolysis, these normal chelate stability constants do not, in themselves, determine the relative effectiveness of these agents at the alkaline p H used in this investigation. Rather, they indicate that these agents have differing iron-binding capacities and, consequently, different activating influences on polymerization. T h e latter three chelants listed were evaluated in the system at 2 to 1 molar ratios of agent to iron because of their greater stability. Other agents were evaluated at the normal 1 to 1 ratio. Figure 2 presents conversion results obtained at the agent-iron molar ratios described above. For this series of
S Y N T H E T I C RUBB6R PRODUCTIOW
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Figure 4. Essentially equivalent conversion rates are obtained when equal weights of commercial solutions of NahEDTA and Na4EDTA-DHEG mixtures are compared
Figure 3. Average deviation range of seven runs using commercial NadEDTA eauimolar to iron (Table 111) illustrates ~. reproducibility of method bottles containing samples using Na4control of the refrigerated bath. The measurements commercially available EDTA as chelating agent and bottles resultant reduction in temperature forms of the sodium salts of EDTA, containing samples using the NadEDTAfluctuation greatly improved the uniDTPA, HEDTA, and DHEG were used NaDHEG mixture as chelating agent formity of subsequent conversion rates as the chelants, while the HEIDA and were run simultaneously as a part of a (Figure 3). Comparing Figure 3 with NTA salts were prepared from laborasingle batch. Thus, with the exception the shaded area in Figure 1 shows imtory samples of these acids. Also preof the chelating agent used in the activaprovement in reproducibility. sented, as the upper shaded area, is the tor solution, the sets of samples were Figure 4 presents the comparisons of range of deviation obtained earlier using identical. This approach allowed a commercial NadEDTA solution and 1 to 1 molar ratios of pure EDTA to direct comparison of the curves of commercial Na4EDTA-NaDHEG rnixiron (Figure 1). Of the chelants tested, per cent conversion us. time obtained tures as agents in the polymerization only EDTA gave the desired conversion with two chelating agents, since the system. Sufficient Na4EDTA solution rates-i.e., essentially linear curves with samples using the different agents were was utilized to produce an equimolar 60y0 conversion in the desired 6 to 8 run simultaneously and, therefore, under agent-iron ratio. For the comparisons, hours. The curve for DTPA most nearly identical conditions. Essentially superan equal weight amount of a commercial approaches the one obtained with EDTA imposable conversion curves are obtained and represents the highest rate attained Na4EDTA-NaDHEG solution (Versene with equal weight amounts of these two in the determinations using DTPA. Fe-3 liquid) was used. As these agents agents (Figure 4), although the All other agents gave curves in the lower give conversion rates so nearly identical, shaded area of Figure 2. One of the interesting results of this comparison was the ineffectiveness of Table II. Formulas and Stability Constants for Agents Investigated DHEG as an activator in the polymerizaLog Stability Constants tion. Yet this compound is a compoChelating Agent Used Fe +a Fe+' nent of one of the most widely used HOOCCHz (EDTA) /CHzCOOH chelating agents in sulfoxylate SBR 25.1 14.3 )N-CH~ CH-N production. For this reason it was H O 0 CCHn \CH~COOH desired to compare conversion curves (HEDTA)
