Cold Rubber Emulsion Polymerizations EFFECT OF ELECTROLYTES ON COPOLYMERIZATION RATE C. F. FRYLING AND W. M. ST. JOHN, JR. Phillips Petroleum Company, Bartlesuille, Okla. Electrolytes are required in certain low temperature emulsion polymerization systems to serve two purposesacceleration of the rate of reaction and repression of gelation of latex. A so-called activator solution is prepared by complexing ferrous sulfate and sodium pyrophosphate under definite conditions of time, temperature, and concentration. Amounts of these electrolytes molecularly equivalent to each other and to the cumene hydroperoxide employed as an initiator of polymerization have been found to be optimum. Although any neutral, nonoxidizing or nonreducing electrolyte can be employed for re-
pression of gelation, potassium chloride exhibits properties which commend its use. When incorporated in the activator solution prior to ripening of the solution, potassium chloride increases the over-all rate of polymerization by preventing cessation of reaction at low conversions. This makes it possible to employ low initiator concentrations, thereby lowering the iron content of the resulting elastomer. The advantage of using potassium chloride in rosin soap recipes conducted at -10" C. has been demonstrated despite the fact that such recipes are generally conceded to be intolerant to the presence of electrolytes.
T
olds and associates (7). The circumstances of this discovery are worth relating since it is one of a few instances in emulsion polymerization research where deductions based on theory, though contrary to any previously observed behavior, were verified by experiment. The effect was also the first well authenticated example of acceleration of the emulsion polymerization process by addition of electrolytes to the recipe. The increase of reaction rate with increased soap concentration and the correlation of emulsger efficacy with solubilization had led to the formulation of the micelle theory of initiation of polymerisation. This possibility had been suggested by Fryling and Harrington ( 8 ) from their work on the butadiene-acrylonitrile system and had subsequently received thorough confirmation by Harkins and associates (4). Reynolds, reasoning that the micellar state is intermediate between solubility and insolubility, came to the conclusion that low molecular weight fatty acid soaps and sulfates, such as sodium caprylate and sodium octyl sulfate, would promote emulsion polymerizations provided an electrolyte were present in sufficient concentration to micellize the soap. Such proved to be the case. The lower molecular weight soaps required progressively more electrolyte to exhibit a maximum when conversions at a constant time were determined as a function of electrolyte content. At sufficiently high concentrations, retardation occurred. The foregoing summarizes the effect of electrolytes on the rate of emulsion polymerization as known prior to the development of low temperature redox formulations. These effects are exhibited by simple electrolytes and can be described as suppression of gelation, which may be a type of peptization, and enhancement of micelle formation. The remainder of this report deals with the adjustment of the electrolyte components of the cumene hydroperoxide-ferrous pyrophosphate initiator system and with a new and as yet unexplained effect of simple electrolytes on such systems. Activation of the initiators is exhibited by complexing agents such as pyrophosphates and ethylenediamine tetiaacetates (6) The recently observed effect is of the nature of a catalyst promoter action. Thus, electrolytes exhibit four recognized effects in the emulsion polymerization reaction; these can be termed peptization, micellization, activation, and promoter action.
HE influence of electrolytes on emulsion copolymerization systems is complicated and far from being completely understood. That a considerable familiarity with the behavior of electrolytes in the manufacture of synthetic rubber is of practical importance becomes evident from the following: insufficient electrolyte may cause gelation of the latex within the reactor while an excess may result in undue prolongation of the reaction. Recent investigations have shown that new and beneficial results may follow from the use of carefully controlled concentrations of electrolytes. Thus the simple expedient of adjusting electrolyte contents contributes greatly to accelerating low temperature emulsion polymerization systems. I t is now common knowledge that a small concentration of electrolyte reduces the viscosity of synthetic rubber latex whereas larger concentrations bring about flocculation. This phenomenon was investigated by the senior author in 1937 during an investigation of factors responsible for the formation of gelled latex. With certain recipes, the resulting latex exhibits a consistency similar to that of vaseline-for example, cationic emulsified systems exactly neutralized with hydrochloric acid and fatty acid soap systems also exactly neutralized with sodium hydroxide when nonelectrolyte initiators are used. Diaeoaminobeneene initiated systems were conspicuous in this respect. However, the addition of excess acid in the one case or excess alkali in the other brought about the formation of free-flowing latices. Then it was found that any highly ionized electrolyte would exhibit the same behavior, and it became customary to include from 0.05 to 0.2 part of sodium sulfate in recipes to prevent gelation. If the reactor charge gelled during the process, it could be liquefied by the addition of salt. However, in this early work the addition of electrolyte was always attended by retardation, frequently of serious magnitude. With the general adoption in 1942 of the standard or mutual GR-S recipe, which contained 0.30 part of potassium persulfate, gelation of latex ceased to be important. For this reason the phenomenon has not received the attention which it undoubtedly merits. Gelation of latex has again become important as a result of the recent development of low temperature recipes, and much of the information presented here was obtained from efforts to circumvent this difficulty. Another important electrolyte effect was discovered by Reyn-
2164
Odober 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
MOLAR RELATIONS EXHIBITFD BY INITIATOR INGREDIENTS IN FERROUS PYROPHOSPHATE.GUMENE H Y D R O P m O X I D E RECIPES
The development of cumene hydroperoxide-fmus pyrophosphate redox recipes for operation at low temperatures has been described recently (9). With recipes containing from 1 to 3 parts of a reducing sugar, no reaction occurred when the molar ratio of cumene hydroperoxide to ferrous iron was less than unity. With variation of this ratio the conversion in a definite time increased abruptly to a maximum value at unity and then decreased slowly with further increase of hydroperoxide content. This phenomenon focused attention on the relations which exist between initiator ingredients. It became convenient to die cum molar ratios of initiating ingredients and to record concentrations in terms of millimoles per 100 g r a m of monomers. Eventually the expression "millimole level" of initiators was adopted. In sugar recipes, no molar relation was observed for the sodium pyrophosphate. As the content of this ingredient was increased to about 3 parts, the rate of conversion increased to a flat maximum. There was an insufficient increase in rate to warrant using above 1 part. Ferrous sulfate contents higher than 0.10 part (approximately 0.3 millimole) were not required with sugar recipes. An explanation of this was afforded by the assumption that the ferrous ions were regenerated by reduction with sugar, thereby acting as a redox catalyst. As a matter of fact almost equally fast results were obtained with ferric salts. With ferric salts, however, the curve did not exhibit an abrupt break at the 1:l ratio. Reaction rates obtained with recipes containing reducing sugars such as levulose were disappointingly low at temperatures below 0' C. Faster reactions were obtained by removing the sugar and increasing the ferrous sulfate-cumene hydroperoxide contents to approximately the 1 millimole level as illustrated by the formulation in Table I. This recipe exhibited two unfavorable characteristics-a tendency 'toward gelation of the latex and the necessity of having a high ferrous sulfate content. Because of gelation, investigations to determine the optimum initiator contents were conducted at low conversions. The 43% conversion reported in Table I is a consequence of this characteristic. At conversions of 30% and higher, particularly at low millimole levels, gelation of the latex interferes with efficient heat transfer. The temperature of the system tends to rise above that of the bath, the reaction is accelerated, and i t becomes impossible to obtain consistent kinetic results when the concentration of any ingredient is varied. Therefore the recipe was balanced by determining the rate of copolymerization at conversions lower than those at which gelatioh appears. Succow in establishing molar relations is attributed to this procedure more than to any other. Experimental details have been described in previous publications (1, 6). However, an adequate published description of many details is still unavailable. In a recipe such as that given in Table I, highest accuracy in measuring reaction rates is .attainable by withholding one of the components of the redox system until the remainder of the ingredients have been brought to reaction temperature. Although some prefer to add the cumene hydroperoxide last, the procedure in this laboratory has been that of starting the reaction by the addition of the activator solution. The activator solution is the product obtained by adding the sodium pyrophosphate solution to an aqueous solution of ferrous sulfate. Concentrations of the activator solution are adjusted so that about 25 ml. are required per 100 grams of monomers. This amount can readily be added to the bottles from a syringe. The activator solution so prepared consists of an aqueous phase containing 9 light colored gray-green precipitate. The precipitate can be filtered out under nitrogen and used dry, if desired, or the activator solution can be sufficiently dilute that all of the ferrous pyrophosphate formed is apparently in solution.
2165
T A B LI.~ F ~ o u PYROPHOSPEATE-CIJMEN~ s HYDROPEROXIDE RECIP~ Parts 70
Butadiene
Styrene Water total Met hahol Potassium laurate, 95% neutralized Mixed tertiary meroaptans'
30
192 48 5.00 0.25
-
0.60 0.084 0.223 0.139 10 7.6 34
Millimole Level 1.00 0.167 0.446 0.278 10 7.6 43
-
-
2.50 0.418 1.118 ' 0.885 10 7.6 62
However, fastest results have been obtained by using the solution with its solid phase well dispersed by shaking just ,prior to addition with a syringe to the reaction mixture. For consistent results, shaking is essential. With a system as complex as that under investigation, different results might be expected if the activator solution were prepared in different ways. One procedure which contributes to obtaining fast reactions is that of warming the activator solution (dispersion) and thereafter cooling it. When working on a small laboratory scale, this is done by placing the activator solution in a 100-ml. flask in the absence of air for 40 minutes in a circulating air oven adjusted to 60" C. Forty minutes is just, about sufficient time to permit the contents of the flask to reach the temperature of the oven. Longer or shorter periods of heating result in slower reactions. After cooling to room temperature, the activator solution is ready for use. Experimental data showing the influence of variable concentrations of sodium pyrophosphate, ferrous sulfate, and cumene hydroperoxide have been previously presented (3). In additSon, information was obtained on the influence of variable millimole levels, variable time of heating the activator solution, variable pH, and the variation of the modifier content on conversion. Maximum rates of polymerization were demonstrated a t 1:l:l molar ratios of cumene hydroperoxide, ferrous sulfate, and sodium pyrophosphate. The curves exhibited by the ferrous sulfate and sodium pyrophosphate rise to rather symmetrical maxima; the cumene hydroperoxide curve rises rather abruptly to the maximum and the@levels off. Consequently it has been considered a practical expedient to use a slight excess of cumene hydroperoxide, thereby avoiding the irregularities which can be expected by operating too close to the abrupt downward trend of the. curve. For this reason 0.55, 1.10, and 2.75 millimoles of cumene hydroperoxide are included in the recipes presented in Table I. The equality of ferrous sulfate and sodium pyrophosphate concentration indicates that sodium ferrous pyrophosphate (NanFePtO,) is the reductant involved in initiation. Conclusive proof of this may be difficult to obtain. However, it is evident that for every molecule of ferrous sulfate employed, a molecule of sodium sulfate in the recipe can be expected. This is important, as will become evident later, in considering the electrolyte content of the system. Table I1 shows that equality of molar concentrations results in maximum rates of conversion over a range of millimole levels. Since the maximum obtained by varying the ferrous sulfate content was more pronounced than that shown by the other two initiator ingredients, ferrous sulfate was varied at the 0.5 and 2.5 millimole levels. The ferrous sulfate content was varied from 0.5 to 2.5 times that of the other initiator ingredients as indicated by the factor column. At low convergions, up to 5.4
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
2166
hours, the maximum occur as expected at equimolar concentrations. At the 0.50 millimole level this regularity disappears at high conversions because of gel formation. At the 2.5 millimole level, however, the maxima occur at equimolar concentrations at all conversions because the high electrolyte concentrations suppress gelation. A tendency for the reaction to die out at low and
1
L1 'O1 ,"'.,Z.SO
f
,
10 .I
I
MILLIMOLE LEVCL
\'\
'
Vol. 42, No. 10
an equivalent of sodium sulfate is formed in the activator solutioii through reaction with the pyrophosphate. Since attention was focused on the rate of reaction at low conversions, it was not immediately apparent that the results obtained at the 1 millimole level and lower were extremely erratic with respect to cessation of reaction at low conversions. It appeared, too, that the ferrous sulfate content required with the sugar-free recipe was considerably higher than rubber technologists would tolerate because of deleterious effects of iron on the aging properties of rubber. These difficulties were circumvented by information derived from the further study of electrolytes.
0.30 MILLIMOLE LEVEL
ACTIVATION O F SODIUM-FERROUS-PYROPHOSPHATE REDUCTANT BY NEUTRAL SALTS
a
',
. o
0.26
030
1.8s
P
u)
01s 1 ?J
IO0
so0
I26
ew
FeSO4 Millimoles per 100 G. of Monomer
Figure 1. Variation of Conversion at -10' C. as Function of FeSOd Content Initiator levels, 0.50 and 2.50 millimoles
high ferrous sulfate contents is also evident. This is presumably due to exhaustion of ferrous iron in the first instance and of cumene hydroperoxide at the high iron levels. The 5.4-hour conversions are plotted in Figure 1. OF FERROUS SULFATE CONTENT AT Two TABLE 11. VARIATION MILLIMOLE LEVELSAT -10" C.
Millimole Level 0.50 0.60 0.50 0.50 0.50 0.50 2.50 2.60 2.50 2.50 2.50 2.50
(ReciDes from Table I) Percentage F e s 0 m z O ___ 2.6 5.4 Parts Millimoles Factor hr. hr. 6 0.5 8 0.25 0.070 10 16 0.75 0.37 0.104 24 12 1.0 0.50 0.139 23 1.5 9 0.75 0.209 2.0 18 7 1.00 0.278 4 2.5 5 1.25 0.348 0.5 7 18 1.25 0.348 24 10 0.75 1.88 0.522 34 1.0 14 2.50 0.695 11 1.5 23 3.75 1.043 1 0 2.0 LOO 1.390 2.5 0 3 6.25 1.738
Conversion a t 7.6 10.3 23.8 hr. hr. hr. 10 9 9 17 18 18 38 51' 34 64 37 48 25 20 20 10 7 7 71 36 27 51 36 a5 52 70 98 48 a5 83 3 4 13 12 8 7
-~
Other data have shown that when ell three initiator ingredients are held in equimolar proportion and varied simultaneously, a maximum in the rate of conversion occurs at the 2.50 millimole level (3). At the higher initiator levels the electrolyte content of the system is sufficiently high t o I I bring about retardation (Figure 2). These investigations indicate that the ferrous sulfate and sodium pyrophosphate components of the initiator system should be present in molar concentrations which are equal to or only slightly less , than that of the 0 I 2 3 4 8 cumene hydroperU I L L W O L E 8 OF IN11IA1ORS PER 100 0. OF YOYOYERS oxide. For each Figure 2. Variation of Conversion equivalent of ferRate with Millimole Level of rous sulfate used, Initiator Ingredients a t -10' C.
Before the recipe under consideration was exactly balanced with respect to initiat,or ingredients, an effort was made to s u p press gelation of the latex by addition of sodium sulfate to the soap solution. When sufficient electrolyte was used to be effective on the gel, severe retardation of the reaction occurred. However, the results obtained by adding 0.14 part of a number of electrolytes to the activator solution were surprising. The majority of experiments showed increased rates of reaction, and perfectly fluid latices were obtained at high conversions in a number of cases (Table 111). In considering the condition of the latex, it should be realized that a highly viscous latex may differ only slightly from a gelled latex. Although many of the additives were chosen with the idea that they formed complexes with iron salts, this cannot be the explanation of the accelerations observed as sodium carbonate would not have been almost as effective as sodiufi fluoride. The data of Table TI1 show that many electrolytes accelerate the polymerization of the ferrous pyrophosphate system, that sodium fluoride is one of the most effective i n this respect, and that it contributes to the formation of a fluid latex. The term "promoter action" is suggested for this accelerating effect. The results of many esperimcnts conducted with sodium fluoride showed that the accelerating effect of this electrolyte was
TABLE111. ADDITIOKOF ELECTROLYTES TO FERROUS PYROPHOSPHATE RECIPEAT -10" C. Recioe B;tadiene/styrene Water/methanol Potassium laurate (95% neutralized)
WTM"
Cumene hydroperoxide (100%) Activator solution heated 40 min. i n 60" C. oven ?r'a&O?. lOHlO FeS04.7HzO Added electrolyte Time, hours
Parts 70/30 180/40 5.0 0.12 0 . 1 7 7 (1.16 millimoles) 0 . 6 6 1 25 millimoles) 0.31 [I: 10 millimoles) 0.14 17.5
Conversion,
%
Latex6
..
76 74 72 70 70 69 64
b
Potassium chiorate,-KClOJ ControlC no added electrol te Sodium &carbonate NaHdbr 60 Sodium tetraborate 'NazB'07 58 Sodium bisulfate, NaHSOr 55 Potassium ferncyanide, KdFe(CK)s 49 Potassium ayanide, KCN 38 8 Potassium persulfate, KzSlOa Potassium iodate, KIOJ 1 Sodium erborate NaBOa.lH20 0 Sodium grornate, NaBrOa 0 Mixed tertiary mercaptans as shown in Table I. v viscous. f = fluid. pc = precoagulum; g = gelled. Control coniained 0.7 ;art of Nad'z07.10HzO.
-
V
V V V
f V V V IJC, V
f,
p" V
..
October 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
real and in addition to the accelerating effect resulting from the use of sodium pyrophosphate. The average increase of conversion obtained by the addition of 0.14 part of sodium fluoride a t different sodium pyrophosphate contents amount to 10% in a p proximately 17 hours. Fluid latices were obtained in all cases. A more thorough understanding of the effect of electrolytes was obtained from experiments conducted on the recipes presented in Table I with initiator ingredients at the 0.5 millimole level. These were undertaken to compare the acceleration obtained by increasing the emulsifier content with that resulting from the addition of an equivalent amount of potassium chloride. Before such a comparison could be made, it was necessary to determine whether addition of electrolyte to the soap solution gave the same no results aa addition to the activator solution (Table IV). The experiments in which potaasium chloride was added to the activator solution showed no tendency to die out at low conversions wherew po0 * IO I6 90 Pa tassium chloride TIYE - HOURS added to the soap Figure 3. Reaction Curves .Obtained with 0.3 Part KCl at has no per-10' C., 0.5 Millimole Level ceptible effect on the 0 = 0.4 KCI added to activator solution rate Of polymeribefore heating sation Or On the tendX = 0.4 KCl added to system in other ways ency of the reaction to die out. In Table V are data which show that the only requirement for obtaining this beneficial result is that the potassium chloride be added to the activator solution containing ferrous sulfate and sodium pyrophosphate prior to heating for 40 minutes in an oven at 60' C. It appears evident that the electrolyte must influence either the composition or the structure of the ferrous pyrophosphate complex formed during heating. In addition it exerts a typical electrolyte effect in preventing gelation of the latex. By plotting the average conversions from Table V, the efficacy of 0.40 part of potassium chloride in preventing cessation of reaction is readily evident (Figure 3). The imporfiance of this new electrolyte effect is also evident, on comparing its magnitude with that of an equimolecular quantity of soap. One part of soap is stoichiometrically equivalent to approximately 0.3 part of potassium chloride. The results of adding these quantities of potassium chloride and soap to the recipe are presenhd in Table VI Here it is shown that increasing the soap content does not prevent the reaction from dying out and that at higher conversions the acceleration induced by the potassium chloride is much greater than that of the soap.
TABLE IV. ADDITIONOF KCI TO ACTIVATORAND SOAP SOLUTIONS
-
(Millimole level, 0.5; temperature, 10" C.) Percentage Conversion a t 2.5 4.2 5.4 7.6 10.5 hr. hr. hr. hr. hr. Variable KCI in activator solution, part 0.4 10 24 51 38 0.4 24 10 . . 40 54 0.4 27 11 42 55 KCl in soap solution, patt 0.4 ,. 24 13 29 30 13 0.4 25 .. .. 30 32 0.4 23 11 32 39 Control 12 . . 25 29 33 No KCI 23 10 No KCl .. 30 36 36 No KCI 11 30 25
..
..
23.8 hr. 83 83 83 37 41 49 47 45 45
2167
TABLEV. VARIOUSMETHODSOF ADDINGKCI
TO
SYSTEM
(Millimole level, 0.50; KCI, 0.40part; temperature, -loo C.) Percentage Conversion a t 2.5 5.4 7.4 10.3 24.5 hr. hr. hr. hr. hr. Variable KCl in ferroua solution add NmPzO7 solution, heit 9 24 37 52 84 KCI In NadP107 solution, add ferrous solution, heat 10 24 39 53 84 NarPzOr and ferrous in solu11 26 40 54 81 tion, add KC1, heat NaaP201 and ferrous in solution, add KCI, heat 10 26 37 54 82 1 83 * 1 Av. 1LO t 0 . 5 25 t 1 38 * l 53 f
Na4PzO7 and ferrous in solution, heat, cool,. add KCI KCI in 8oap solution KCl injected in bottles after activator KCI injected in bottles before activator No KCl (control)
TABLE VI. KCI
AND
11
23 21
10
29 29
34 35
41 43
11
23
29
35
45
12
24 22
30 31
37 37
44 47
8
SOAPEFFECTSAT 0.5 MILLIMOLE LEVEL
Variable 5 parts soap no KCl 6 parts soap: no KCI 5 parts soap, 0.3 KC1 in &etivator 6 parts soap, 0.3 KC1 in activator solution
2.6 hr. 12 16
Percentage Conversion a t 5.4 hr. 7.6 hr. 10.3 hr. 25 29 33 28 32 33
23.7 hr. 37 41
10
24
40
56
83
14
38
49
64
87
Another possibility revealed by the addition of electrolyte to the activator solution is that of going to lower initiator levels without undue sacrifice of rate of reaction. Conversion data obtained at several millimole levels are presented in Table VII. Even at the lowest initiator level, potamium chloride added to the activator solution prior to heating was efficacious in preventing cessation of reaction.
TABLEVII. EFFECTO F 0.40 PARTKCI ON SYSTEMSWITH INITIATOR LEVELSOF 0.5 MILLIMOLE AND LESS Initiator Level, Millimole/100 0. 2.5 hr. 0.20 8 0.20 9 0.30 10 0.30 0.40 lo9 0.40 9 0.50 10 0.50 11
Percentage Conversion at 4.2 hr. 7.6 hr. 10.5 hr. 35 20 l9 28 35 24 36 48 23 36 49 24 37 b3 26 39 54 54 55
;;
*g
23.8 hr. 60 59 72 75 80 80 83 83
The beneficial effects of adding electrolyte to the activator solution at low initiator levels have been demonstrated. Data obtained with sodium hypophosphite (NaH2POn),one of the more effective electrolytes from Table 111, show that the effect is reversed at high millimole levels. From Table VI11 it is evident that a positive effect is shown at the 0.5 millimole level, there is little to be gained by addition at 1.0 millimole, and at 2.5 millimoles the addition of electrolyte is deleterious. Determinations of optimum concentrations of electrolytes at various initiator levels have required considerable effort. Table IX presents a summary of 12 such investigations. Except for sodium carbonate, the various electrolytes exhibit similar effects. Some of the series exhibit maxima; with the others the electrolyte concentrations have not been carried high enough to show this behavior. More complete data for five electrolytes are presented in Table X. When the conversions are plotted RS functions of molecular concentrations, some suggestive resemblances become apparent (Figure 4). The c'urves obtained
INDUSTRIAL AND ENGINEERING CHEMISTRY
2168
I
I
I
1
4 6 0 )o ELECTROLYTP MINOEMTOATION,YILLIYOLE(I
I
I
Vol. 42, No. 10
I
IO I4 Id PER 100 0. OF YQMOYCR
Figure 4. Variation of 10-Hour Conversion with Electrolyte Concentration at 0.5 Millimole Level KCI CONCENTRATION, MILLIMOLE8
Recipe Table I * Temperature, -IOo C. 0 2 KCI = NaF NaCl = 2 X NazSOr A = 1 X NasSOc
---6
EFFECTOF
MONOMER
Figure 5. Variation of Conversion with KCl Concentration at 0.20 Millimole Level Recipe Table I; temperature, - l o o C.
for sodium chloride and sodium fluoride reserpble each other closely as they pass through the maximum. In both cases, at 10-hour reaction, the maxima occur at 5.6 millimoles. With sodium sulfate plotted on the same basis, the maximum occurs at 2.8 millimoles. When the sodium equivalence of sodium sulfate is plotted, the curve falls between the curves for NaF and NaCI. The maximum conversions for these three sodium %Its lie between 53 and 560Jo-that is, they do not differ by much more than experimental error. These resemblances suggest that the acceleration can be attributed to the sodium ion and that the anions are not involved. The most marked difference between the behavior of potassium chloride and the sodium salts is that the former exhibits a flat maximum. The observation that both sodium and potassium ions accelerate to about the same amount further suggests that the accelerating effect does not depend on a change of composition, such as substitution of part of the sodium in the sodium-ferrous-pyrophosphate complex by potassium, but rather some change in structure is brought about by heating the complex in an environment modified by the presence of a high concentration of cations.
TABLE VIII.
PER ID0 0 . O F
VARIABLESODIUMHYPOPHOSPHITE AT
THREE INITIATOR LEVELS
(Recipe from Table I ; temperature, -10' C.f NaHzPOs, Percentage Conversion a t Part 4.5 hr. 7.2 hr. 0.00 0.05 0.10 0.15 0.20 0.25
At 0.50 Millimole 19 21 21 22 22 23
24 33 35 35 38 39
0.00 0.04 0.08 0.12 0.20 0.24
At 1.00 Millimole 20 22 21 22 21 22
38 42 42 44 43 43
At 2.50 Millimolee 28 29 25 23 19 16
49 49 45 42 35 30
Considerable difficulty was experienced in obtaining consistent results when potassium chloride was varied at the 0.2 millimole level. The data were measured after 16.6 and 23.8 hours and the averages of four series of experiments are presented in Figure 5. The maxima appear to be reached at the same molecular concentration as previously observed (5.6 millimoles potassium chloride per 100 grams of monomers) but increasing the potassium chloride content up to 67 millimoles did not retard the reaction as seriously as was the case with sodium chloride, sodium sulfate, and sodium fluoride at the 0.5 millimole initiator level. Since activation of the sodium-ferrous-pyrophosphate complex by cations has been shown to occur during the ripening of the activator solution, experiments were conducted to determine whether 40 minutes in a 60' C. oven for the small sample involved were sufficient to obtain the maximum possible degree of activation (Table XI). A slight advantage would have resulted if the activator solution had been ripened for 55 instead of 40 minutes but the differencewas not sufficient to cause concern. ACTIVATION OF ROSIN SOAP RECIPE BY KCI
The activation of rosin soap recipes as compared with fatty acid soapemulsified systems presents certain difficulties. Dresinate S-134 is a potassium rosin soap especially prepared for low temperature emulsion polymerizations. Its physical properties indicate that it is composed largely of the potassium salts of di- and tetrahydroabietic acids. The activity of the rosin soap
TABLEIX. VARIABLE ELECTROLYTE CONCENTRATIONS AT 0.5 MILLIMOLE INITIATOR LEVEL (Percentage cohversions, appr0,ximately 7.2 hours; temperature, recipe from Table I) ElectrolyteD Part 1 2 3 4 5 6 7 8 9 10 0.00 24 26 26 28 28 25 24 24 25 25 0 . 0 5 33 31 34 29 31 32 29 34 23 26 0.10 35 30 36 27 32 30 30 33 26 0 . 1 6 35 34 36 28 34 8 33 34 31 33 0 . 2 0 38 31 36 33 33 0 33 33 33 37 0 . 2 5 39 32 37 32 34 0 34 32 29 39 7 = Bodium benzoate 4 1 = sodium h pophosphite, 8 = potassium tartrate NsHnJOYOn 9 sodium salicylate 2 = otassium citrate 10 sodium formate 3 = kSCN 11 = Nan504 4 NaPOs 12 = NaF 5 = Na~HP04.12HsO 6 = NazCO:.HnO
..
--
- 10" C.; 11 21 26 30 34 34 38
12 21 27 33 36 40 42
INDUSTRIAL AND EN GINEERING CHEMISTRY
October 1933
2169
TABLE XII. ROSINSOAP RECIPE Parts 70 30 192 48 3.50 1.50 0.25
Butadiene Styrene Water Methanal Dreainate 8-134 (05 Potassium laurate MTM
neutralized) (8% neutralised)
Cumene hydroperoxide, 1007,part Sodium yrophosphate.lOHt8, part FeSor.7k0, part Temperature, O C. KCl
0
I
I t
I
4
I
I
I
I
I
b 8 10 IO 101 C O N C C N ~ ~ A T I O N . Y I L L I M OFIR L ~ ~ 100
1
I@
14
a OF
1
Ib
YOIIOYCI
Figure 6. Rosin Soap Recipe Variation of Conversion with KC1 Concentration at Four Initiator Levels Time, 10.5 hours; temperature, Millimole level 0 50 -AX -- = 0.70 --0 -
- o - =
--
- 10' -
C.
1.00 = 2.00 =
recipe can be increased markedly by including a small amount of fatty acid soap in the formulation. Since fatty acids are usually added to rosin rubber during compounding, no harm is apparent in this procedure and the advantages to be gained are important. However, prior to this work there was serious doubt regarding the advisability of adding any electrolyte to the recipe. The rosin soap recipes are in general more susceptible to retardation by electrolytes than are fatty acid soap recipes and concern has even been expressed regarding the presence of the small amount of sodium sulfate in the recipe resulting from the reaction between ferrous sulfate and sodium pyrophosphate.
TABLE x. VARIABLE CONCENTRATIONS O F FIVEELECTROLYTDS AT 0.5 MILLIMOLE INITIATOR LEVEL (Recipe from Table I) Electrolyte Percentage Conversiona a t Nsme Parts Millimoles 2.5 hr. 5.0 hr. 10.0 hr. 20.0 hr. 0.00 O.OO(aontro1) 10 15 19 30 NarSOr 10 23 36 45 NanSOr 0.10 0.70 0.25 1.76 11 28 51 71 10 24 55 81 NarSOr Nans04 0.40 2.82 9 16 24 33 0.75 5.28 NatS01 1 2 11 30 NaaSOr 1.00 7.04 0.10 2.38 12 24 45 66 NaF 0.25 5.85 12 26 56 79 NaF 0.40 9.52 10 18 28 40 NaF 6 12 22 34 0.75 17.8 NaF NaF 1.00 23.8 0 1 2 10 0.10 1.71 14 26 46 60 NaCl 0.25 4.28 15 27 52 74 NaCl 0.40 6.84 11 25 52 83 NaCl 0.75 12.8 1 2 6 8 NaCl 0 0 1 3 NaCl 1.00 17.1 0.10 1.34 10 24 52 73 KCl 0.25 3.35 11 26 56 78 KCl 0.40 5.36 11 27 57 79 KCl 11 28 80 0.75 10.1 KCl KC1 1.00 13.4 12 29 58 81 64 0 94 11 24 NaHnPOs.Hn0 0.10 40 71 2 36 11 25 48 NsHnPOz.Ht0 0.25 3.78 12 25 55 80 NaHaPOa.Ha0 0.40 7.04 9 16 26 37 NaH,POz.HaO 0.75 9.44 2 8 19 32 NaHaPO%.HsO 1.00
TABLE XI. VARIABLE TIMEOF HEATINQ ACTIVATOR SOLUTION (Recipe. Table I: 0.20 millimole level: KC1, 0.4 part) Percentage Conversion a t In Oven a t 23.4 hr. 16.3 hr, 60° C., min. 15 29 31 42 37 30 43 51 45 45 51 60 34 45 90
Millimole Level 0.50 0.70 1.00 2.00 0.084 0.12 0.167 0.334 0.223 0.32 0.446 0.892 0.140 0.20 0.278 0.556 10 Variable
-
The recipes employed are presented in Table XII, and the data obtained at several initiator levels are given in Table XIII. Again the results show that advantages can be derived both with respect to rate and continuation of reaction at low initiator levels. Freedom from gelation of latex, although not specifically nbted in Table XIII, constitutes another advantage. At the high initiator level, slight retardation appears to result from the addition of potassium chloride to the recipe. The data a t 10.5hours are plotted in Figure 6. At the 0.5 millimole initiator level the maximum may be as high as 8 millimoles of potassium chloride. It shifts to lower values at higher initiator levels and disappears entirely at 2.0 millimoles. CONCLUSIONS
The importance of electrolytes in low temperature redox recipes has been stressed. Careful adjustment of ferrous sulfate and sodium pyrophosphate concentrations to a 1:l molar ratio is required t o obtain maximum rates of reaction a t low conversions in the cumene hydroperoxide-sugar-free system developed for
TABLEXIII. VARIABLE KCI CONCENTRATIONS AT SEVERAL MILLIMOLELEVELSIN ROSINSOAPRECIPE KC1 Parts
Millimoles
0.00 0.10 0.20 0.40 0.60 0.80 1.00 1.25
0.00 1.34 2.68 5.36 8.04 10.7 13.4 16.8
0.00 0.10 0.20 0.40 0.60 0.80 1.00 1.25
0.00 1.34 2.68 5.36 8.04 10.7 13.4 16.8
0.00 0.05 0.075 0.10 0.25 0.50 0.75 1.25
0.00 0.67 1.00 1.34 3.35 6.70 10.0 16.8
Part 0.00 0.025 0.050 0.075 0.100 0.25 0.50 1.00
2 hr.
Percentage Conversion a t 4 hr. 7 hr. 10.5 hr.
0.50 Millimole Initiator Level 9 12 14 15 12 12 9 8 0.70 Millimole Initiator Level 8 16 26 10 18 32 8 18 32 9 15 30 8 16 27 15 26 6
5
10 7
14 11
1.00 Millimole Initiator Level 14 14 16 16 17 15 11 *. 7 9
KCl Millimoles
.. .... ....
..
24 hr. 12 24 34 55 59 63 53 20
33 46 48 46 41 37 15 13
44 77 87 80 71 66 18 18
33 34 43 45 47 39 32
11
Percentage Conversion a t 2.6 hr. 4.5 hr. 8.0 hr. 24 hr.
2.00 Millimole Initiator Level 21 0.00 16 0.39 17 0.67 20 1.00 19 1.34 6 3.35 6 6.70 1 1 13.4
36 30 29 34 32 9 10 5
2170
INDUSTRIAL A N D ENGINEERING CHEMISTRY
operation at -10' C. The hydroperoxide content can be carried to higher molar ratios than 1:l with respect to these electrolytes, but it cannot be used effectively in lower amounts. Neutral salts activate the sodium-ferrous-pyrophosphate reductant at low initiator levels if present in controlled amounts during the heat ripening of the activator solutions; at high initiator levels a reverse effect is sometimes noted. The effect on the reductant-activator is exhibited in an outstanding manner by potassium chloride. With this new method of activation it is possible to conduct the cumene hydroperoxide-sugar-free recipe with a ferrous sulfate decahydrate content as low as 0.10 part per 100 parts of monomers in a reasonable length of time (24 hours) at - 10" C. The most important advantage resulting from this use of electrolytes is the suppression of the tendency of the reaction to die out at low conversions. So marked is this effect at low electrolyte concentrations that it is now realized that much of the lack of reproducibility previously observed at low initiator levels can be attributed to the accidental presence or absence of small amounts of electrolytes. At the same time the gelation tendency of low initiator level recipes is suppressed. Activation
Vol. 42, No. 10
with potassium chloride has also been demonstrated for a rosin soap recipe. ACKNOWLEDGMENT
A part of the experimental work referred to in this paper was carried out under the auspices of the Office of Rubber Reserve, Reconstruction Finance Corporation. Acknowledgment is gratefully made for this assistance. LITERATURE CITED
(1) Fryling, C. F., IND. ENG.CNEM.,40, 928 (1948). (2) Fryling, C. F., and Harrington, E. W., Ibid.,36, 114 (1944). (3)Fryling, C. F., Landes, S.H.. St. John, UT.M. Jr., and Uraneck, C. 4 . , I b i d . , 41, 986 (1949). (4) Harkina. W. D., J . A m . Chern. SOC.,69, 1428 (1947). (5) Houston, R. J., A n d Chern., 20, 49 (1948). (6) Mitchell, J. M., et ai.,IND.ENG.CHEM.,41, 1592 (1949). (7) Reynolds, W. B., A.A.A.S. Conference, Gibson Island. Md.
(July 1946). RECEIVED April 29, 1950.
Dialkyl Tetrachlorophthalates as Plasticizers Evaluation in a Vinyl Chloride-Acetate Copolymer J. K. STEVENSON, L. E. CHEYNEY',
AND M.
M. BALDWIN
Rattelle Memorial Institute, Columbus, Ohio
A
series of dialkyl tetrachlorophthalates has been investigated as plasticizers for vinyl chloride-acetate copolymers. These esters are flame resistant and exhibit practically no tendency to migrate. They are not so efficient as di-2-ethylhexyl phthalate and produce compositions having somewhat poorer low-temperature flexibility. Their apparent efficiency varies somewhat with the method of rating. Efficiencyis highest with the lower members of the n-alkyl series and decreases as the n-
alkyl chain increases. Compatibility of the tetrachlorophthalate esters in Vinylite VYDR is excellent until the aliphatic chain reaches eight carbon atoms, when compatibility diminishes. Best flame resistance is obtained with the lower members of the series, but even the octyl tetrachlorophthalates offer considerable flame resistance. The tetrachlorophthalates are not so flame resistant as tricresyl phosphate in mixtures with dioctyl phthalate. The esters are also compatible with several others resins.
I
from tetrachlorophthalic anhydride as a starting material. The study was confined to aliphatic derivatives, inasmuch as it has been shown by the work of others that aromatic rings in a plssticizer molecule are undesirable. A series of esters prepared from normal alcohols containing from 3 to 8 carbon atoms formed the basic part of the study, although the effects of branching and presence of ether oxygen were also investigated to wme extent.
T IS apparent that the alkyl phthalates represent one of the most useful groups of ester plasticizers, particularly for vinyl chloi ide polymers and copolymers. Special requirements, such as lowtemperature flexibility, electrical properties, flame resistance. etc., often result in the selection of other types of plasticizers for special applications. Flame resistance is improved by certain types of plasticizer strurtures. Tricresyl phosphate has been widely employed for this purpose, with other phosphates and certain chlorinated compounds employed to a limited extent. The various chlorinated phthalate esters have been the subject of recent interest, especially as applied to the plasticization of vinyl chloride polymers (6, IO). One of the more interesting studies ww that of Lawrence and McIntyre (6) who showed, for a series of dibutyl esters, that efficiency, low-temperature flexibility, volatility, and migration into surface finishes all decreased rapidly as the number of chlorinp atoms in the molecule was increased from zero to three. The study described in this paper was an attempt to determine the effect of varying the alkyl radical in a series of esters derived 1 Present address, Minnesota Minim & Manufacturing Company, Detroit, Mich.
EXPERIMENTAL PROCEDURE
The esters studied were piepared in this laboratory by standard esteiification procedures. The properties of the compounds used in this investigation are shown in Table I. The n-propyl and the n-butyl esters have been studied briefly in a vinyl chloride-acetate copolymer by Scheer ( I O ) . Evaluation of the experimental plasticizers in a vinyl chlorideacetate copolymer was carried out in general by the procedures of Kent and Weaver ( 5 ) , employing Vinylite VYDR (a 95 to 5 vinyl chloride-vinyl acetate copolymer, marketed by the Bakelite Corporation) aa the base polymer. Three plasticizer concentrations (35, 50, and 65 parts by weight per 100 parts resin) were employed in all cams. I t was found necessary to reduce the molding
