Anionic Redox Activation of Bu tadiene-Styrene Copolymerization EFFECT OF SUBZERO POLYMERIZATION O N POLYMER PROPERTIES 6.H. JOHNSON, R. R. BROWN, AND R. L. BEBB The Firestone Tire & Rubber Company, Akron, Ohio Redox recipes have been developed for the copolymerization of butadiene and styrene a t temperatures ranging from 40' to -40" C., using glycerol as the antifreeze in the subzero charges. The total amount of glycerol present as well as the glycerol-water ratio was found to have a critical relationship to the speed of reaction. However, the ratio is limited, in the case of -40" C. polymerizations, to the minimum freezing mixture of glycerol and water. With proper adjustment of the polymerization recipe, 60qo conversion was observed a t -25" and -40" C. in 16 and 96 hours, respectivelj In general, a more processable polymer with more uniform breakdown characteristics was obtained a t -25" C. and -4O'C. than a t 10'C. Vulcanizates of polymers showed trends in most properties with changes in polymerization temperature. Modulus increased with increasing temperature, while elongation decreased; tensile strength was substantially unaffected. Normal cut growth resistance was essentially independent of temperature but aged cut growth resistance was improved as the temperature was lowered. X-ray diffraction studies of the various polymers showed increased orientation as the polymerization temperature was decreased. An increase in lateral spacing between chains with increased conversion is thought to be due to cross linking between the chains.
.
I
GR-S included higher molecular weights, rapid breakdown on milling, and improved tensile strength, rebound and crack growth resistance. Although the 10 C. polymers continued to be more difficult to process than GR-S, there was a tendency for processing to improve with lolver polymerization temperatures. Strict reproducibility of complete mill breakdown, however, was not obtained with polymers prepared a t 10 C. regardless of the polymerization variables considered. I n view of the improvements which had been obtained by lowering the polymerization temperature t o 10" C., i t was considered desirable to polymerize a t still lower temperatures. ConsequentIv, the work described in this paper is concerned primarily with polymerizations in an unmodified redox system at subzeio ternperatures. APPARATUS AND M E T H O D S
The polymerizer contained two stainless steel wheels each fitted to hold 16 one-quart bottles rotating in a bath insulated with 10 inches of cork (Figure 1). $he bath liquid was isopropyl alcohoI cooled by a Freon refrigeration unit capable of maintaining temperatures in the cabinet down to -40" C. Agitation of the charges was effectedby the rotation of the wheels. The complete solution of water-soluble ingredients was subjected to a n aging period at room temperature for 18 to 20 mrnUtes prior to the addition of monomers. Aging was carried out in the presence of the ambient air with gentle agitation to prevent entrapment of excessive amounts of air by foaming. When the charges reached the desired conversion the bottlee were removed from the polymerizer, the latex stabilized by the addition of a dispersion of 20/, phenyl-@-naphthylamine and coagulated by a mixture of salt and aluminum chloride. The coagulated samples were mill washed, dried at 70" C., mixed in the following tread stock recipe, and cured at 280" F.
T JTAS established in a previouq paper ( 2 ) that improved properties resulted from the preparation of butadienestyrene copolymers in a 10" C. redox activated system. This type of charge was selected for further study at low temperatures The investigation of subzero polymerization required adjuatrng because of the satisfactory rates that could be obtained at temthe system by replacing part of the water by a n antifreeze agene peratures much below the range where the GR-8 charge became n-hich would not interfere with the reaction. Methanol, ethanol, ineffective. Thus it was possible t o study a single system over a and acetone inhibited pol-wide range of polymerizamerization in the redox tion temperatures, with a system; glycerol was found minimum adjustment of the ingredients, investigating to be satisfactory and therethe bearing of polymerizafore was used throughout tion temperatures on the the investigation. When the proper amount physical characteristics of of glycerol was introduced, the resulting rubbers. the rate at -10" C. comThe f i s t phase of the redox work in this laborapared favorably with that tory was conducted at 30" of a 50" C. G R S control; to 50" C . until a system at lower temperatures. it that produced very rapid became necessary to vary reaction rates was develthe concentrations of the oped. The temperature ingredients in order to obwas then reduced to 10" C. tain the optimum rate. where 60% conversion in 6 I Considerable work was done hours was observed, and at -25" C. establishing a where a distinct improveI desirable balance between ment in properties resulted. the ingredients of the charge The improvements over as a background for polyFigure 1. Low Temperature Polymerization Unit
1617
1618
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE I. TREAD STOCK RECIPE Polymer Sulfur E P C black Stearic acid Bardol Pine t a r Phenyl-P-naphthylamine Zinc oxide Santocure
100.0 1.7 45.0 2.5 4.0 2.6 0.6 2.4 1.2
__
160.0
merization at -40" C. The forniulas given in Table I1 represent the ones used at each temperature. Further improvement in the rate a t each temperature could he effected by continued study.
TABLE 11. REDOX RECIPES 0 a n d above
Butadiene 70 * 30 Styrene 0.5 Benzovl peroxide MP-189-S 3.6 Sodium oleate 1.5 0 0 Pota-sium oleate NarPnOi (anhydrous) 0 2 Sorbose 0.6 Fe(h"dn(S0a)z ~ H z O 0.35 \Tater 200 Glycerol 0 Wt. 5$ glyccrol in glycerolwater solution Freezing point, glycerolwater solution, ' C. n Time to reach 60% conversion 18 hr., O n C. 6 hr., 10' C. 30 niin., SOo C .
Temperature, C. - 10 -25 70 30
0.5 3 5 1.5 0 0 0.2
0 6 0.35 120 101 45 6
70 30
0.5 3 5 0 5 0 0.35
0.6 0.6 120 156 36 5
-19.4
-32
22 hi.
16 hi.
3
-10
70 30 0.5 3 5 0 5 0 0 35 0.6 0.6 113 263
70 0 -38
9
Vol. 41, No. 8
in respect to reaction rate shows that the optimum conditions are met by charging 180 parts of glycerol per 100 parts of monomer, using a n-atcr-glycerol ratio of 5 to 7. Although this ratio was opt,imum for - 2 5 O C. it was impossible to take this recipe to -40" C. because the charges froze. Hence, i t became necessary to compromise with the optimum rate effect of the glycerol-water ratio in order to produce a recipe that mould effect, polymerization a t that low temperature. -4 successful formula was developed by using a 3 to 7 xater-glycerol ratio. It should be pointed out that this particular mixture is the lowest freezing combination possible with these materials, having a freezing point of -38.9 C., very near the -40" C. polymerization temperature. SUB ZERO PO LYhIERI Z 4TIOY CHARACTERISTICS
The piesence oi glycerol nith its high boiling point in thc subzero charges made it impossible to run total solids in the usual manner of following the course of polymerization. Multiplc bottles nere loaded for removal a t seveial stages and percentage conversion obtained by dry coagulum weight. Since this required several hours of drying in the 70" C. oven before the dly weight could be obtainrd, another method mas evolved which gave approximate conversion results within 1 to 2 hours. The latex from one bottle was divided and coagulated; these small samples were then washed on a mill and dried to constant neight under the heat lamps and the percentage conversion calculated from the dry solids content. The final conversion was always determined by the weight of the total polymer yield after washing and drying.
96 Iir.
EFFECT O F GLYCEROL ON POLYMERIZATIOh- RATE
The rate at -25' C. was dependent upon the amount of glycerol present as well as upon the ratio of water to glycerol. Table I11 gives the data supporting this statement and Figure 2 shows the relationship graphically. Four groups of bottle runs were made at -25' C. I n each group a constant ratio of water to glyccrol was maintained, x-hile the individual charges within the group varied in the total amount of glycerol. The emulsifier concentration, based on the monomer charge, was held constant. Essentially this constituted greater dilution of the charge with the water-glycerol mixture. I n all cases there was an optimum dilution although the effect TTas more marked in systems containing 130 or more parts of glycerol for 100 parts of n-ater. An examination of the curves
O K REACTIOE; RATEAT TABLE 111. EFFECTOF GLYCEROL -25" C. Run so. 39-1 39-2 39-3 39-4
39-5 37-1 37-2 37-3 37-4 38-1 35-1 35-2 3,5-3 35-4 38-2
Time, Hr. 18 18 18
168
Watcv : Glycerol Proportion 50 : 60 2 0 : GO -30: 60 50: GO BO : 60
120
50:65
18
60:65 50:65
18
TT-ater,
Glycerol,
100 1i n 120 130 140 100 110 120 130 150 100 110 120 130 150
120 132 141 156
G.
G.
143 156 169 195 140 151 168 182 210
50365
50:65 5 0 : 70 - 0 : 70
,i0:70 50:70 5 0 : 70
18
18 18 18 18 18 18 18 18 18
%
Conversion 57.4 38.1 38.9 42.7 40.6 38.7 52.7 61.5
62,I 48.5
.X,9
59.3 67.0 70.0 58.5
L
50t
O'
I20 140 160 180 200 220 GMS. GLYCEROL P E R 100 GMS. MONOMERS
Figure 2.
Proportion and Amount. of Glycerol zs. Rate
Polymerization temperature, - 2 5 O hours
C. 5
time, 18
GENERALAPPEARASCE OF LATEX. The appearance of the finished latex in the subzero charges \vas cha.nged appreciably because of the presence of glycerol. Instead of resembling milky emulsions, they looked like viscous polymer solutions lyhich were stable and free of prefloc up to GOYoconversion. COSGULATIOS, ~ ~ A s I i I X GBND , DRYIXG.The technique involved in coagulation, washing, and drying of the subzero polymers was essentially the same as at) higher temperatures: st>ahilization by phenyl-@naphthylamine, coagulat'ion by a mixt'ure of salt and aluminum chloride, mill washing, and drying in a 70' C,. oven. The presence of glycerol, however, necessitated longer mill washing to ensure a glycerol-free dried polymer. This was alleviated whenever possible hy the alternate rinsing of the freshly coagulated rubber in lukewarm tap water. EFFECT OF CONVERSION ON PHYSICAL PROPERTIES AT -25" C.
The -25 C. redox system vas st,udied at several conversions to establish an optimum for use a t lower temperatures. The ingredients of the activation system were aged at room tempera-
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1949
TABLE IV. PHYSICAL PROPERTIES OF -25 ' C. REDOXPOLYMERS -25' Polymer No. Conversion, % ' Gel, %
OR-S Control 75
C. Redox Polymer
30 40
43 60
34 71
46 91.5
Raw
5.2
0.5 0.3
6.50 2.47
25.0 10.3
50.8 6.2
Raw
...
.,. ...
5.02 3.17
4.81 a.14
3.18 2.88
7.01
3.10 0.94
6.34 3.96 1.00 0.46 3.85 0.40
4.71 0.82
6.36 4.14 2.89 0.69 3.88 0.33
7.34 5.61 3.56 2.33 4.80 1.06
7.7
6.5
8.0
10.0
18.0
...
675 850 975
775 975 1100
750 950 1025
1100 1225 1275
3725 3650 3575
4050 3875 3825
3700 3750 3675
3775 3575 3700
...
640 580 060
640 560 560
620 580 560
540 520 500
i:
56 66
54 67.6
57 69
5l,5 67
0.22 3.7
0.21 4.53
0.22 2.49
0.20 3.21
220 35.5 43
246
-43 43
240 40.5 42
- 34 39
Ten passa Intrinsic viscosity Ten pass Williams plasticity
Ya
Ten passa Williams recovery, mm. Ten P R S S ~ Compounded Williams, Ya Compounded recovery Compounded extrusion plasticity, Tio i n sec. Modulus a t 300% Cure minutes a t 280" F. 40 60
80
.
.
I
3.6
...
0.56 , . e
600
...
Tensile, lb./sq. inch Cure minutes a t 280' F. 40 ... 2625 60 ... 80 Elongation % Cure mihutes a t 2800 F 40 720 60 80 Rebound, % At 72" F. At 212O F. Crack growth 0.77 Normal 3.6 Aged 4 days a t 212' F. Runnin%temp. a t 60-minute 290 cure F. BlowoAt time, mind 13 43 Low temp. index, C. Ten passes through a cold, tight mill.
...
-
... ...
3.89
-
1619 being higher than a GR-S control in all cases. Tensile strength and rebound were fairly constant throughout the series and were better than the values for a GR-S control. Unaged cut growth was better than the control regardless of the conversion; aged values were generally better, especially a t the higher conversions. Running temperature was below the control value in all cases, although an increase was evident with the higher conversions. Blowout times were much better than the G R S value with a slight superiority shown by the 60% sample. Low t e m p e r a t u r e characteristics were identical with those of GR-S except for the 91% sample.
EFFECT OF POLYMERIZATION TEMPERATURE ON POLYMER PROPERTIES
252
BREAKDOWN CHARACTERISPolymers made a t 40 C. were high in gel content and Williams plasticity values. No significant breakdown in these properties accompanied light milling on a cold mill (Table V). At 10" C., however, breakdown did orcur but somewhat irregularly. A statistical study was made of the breakdown characteristics
-
TICS.
ture for 18 minutes and the bottles were loaded in the normal manner. Samples collected a t 40, 60, 71, and 91.570 conversion showed the properties listed in Table IV. The gel content of these samples increased with increasing conversion but the gel was capable of being rapidly dispersed even a t 91% converTABLEV. PHYSICAL PROPERTIES OF REDOX POLYMERS PREPARED AT VARIOUSTEMPERATURES sion. This was accompanied Polymer No. 99/86 11 43 59 GR-S by the characteristic drop in control intrinsic viscosity after the gel ??olymerization temp., C. +40 - 10 - 25 - 40 50 10 Gel, % point had been passed. The Raw 84.5 68.5 0.94 61.7 6.50 15.0 Ten passa 54.2 2.74 29.7 0.0 2.47 1.92 plasticity values were all higher Intrinsic viscosity 2.74 2.53 2.13 5.02 4.90 than those for the GR-S conTen passo 1.75 2.60 1.62 2.80 3.17 2.95 8 10 Williams plasticity, Ya 8 63 8.38 3.75 7.01 6.00 trol but 10 passes through a Ten passa 6.54 5.55 6.73 3.40 ... 4.22 Williams recovery, mm. 3.18 2.35 4 32 3.89 1.00 1.98 tight mill improved even the Ten passa 3.42 3.16 2.51 ... 0.23 0.30 h i g h e s t conversion sample. . .. Compounded Williams, Y S .. 4.71 ... 3.33 3.10 Compounded recovery ... ... ... 0.94 0.82 0.24 The latter was rather nervy, Compounded extrusion plas. . . ticity, Tia in sec. ,.. 8.4 7.7 even after 10 passes, as shown 8.0 Modulus a t 300% by the high recovery value. Cure minutes a t 280' F. 40 2100 1250 1100 775 625 500 Compounded s t * x k s from 60 2400 1400 950 1700 975 925 80 2400 1925 1400 1200 1100 1050 these samples increased in stiff2225 120 .. 1525 1300 1125 1175 new with rising conversion, a Tensile, lb./sq. in. BOY0 sample being roughly comCure minutes a t 280' F. 40 3640 4030 3600 4059 3975 2350 parable with the GR-S control. 60 3050 3970 3750 3975 3200 3870 80 2600 3750 2475 3875 2700 This characteristic, in particu3825 120 3720 2600 3775 3925 2800 lar, justified the selection of 60% conversion for unmodified 550 520 640 640 797 redox work. It also agrees 474 GOO 673 560 500 430 with the tendency for higher 560 550 560 410 400 560 487 540 400 conversion samples to show in... 57.5 54 53 44 creasing gel formation and may 73 67.5 67 55 ..~ be related to an increase in 0.08 0.098 0.21 1I O 0 1.5 cross links between the polymer 4.53 7.4 2.88 6.2 7.9 ... 246 259 300 239 chains, 61 44 43 84 13 An increase in modulus ticmill. companied the conversion rise,
+
O
...
.
I
.
1620
INDUSTRIAL AND ENGINEERING CHEMISTRY
of 10" and -25' C. polymeis, using gel and Williams plasticity ( 4 ) data. The averages and standard deviations of conversion, raw and teB-pass gel, Williams plasticity, and Williams recovery are given in Table I7I.
TABLE
VI.
irARIABILITY O F
Measured Quality
BREAKDOWN -4T +lo" Mean,
Polymerization temp., 0 C. Conversion Raw gel Ten-pass gel R.aw Ya Ten-pass Y S Raw recovery Ten-pass recovery Nomber of individual dereiminations
4
AKD
c.
-25'
Standard Deviation, u
10 54.9 64.3 29.7 8.03 6.02 3.37 3.06
-25 57.9 32.75 4.52 6.68 4.74 3.24 1.74
8.2
-25 3.5 16.7 3.1
0.22
0.024 0.42 0.24
34
8
34
6
10
4.8 6.2
0.83 0.56 0.38
0.48
If a normal distribution is assumed, the mean * 3 a includes over 99% of the cases. For polymers made a t 10" C. about 570broke down to a gel valuc of less than 12%, whereas all of the polymers made a t -25' C. reached a value less than 14VG gel, Greater uniformity in gel breakdown has been accomplished by polymerizing at lower temperatures. This is also reflected by the average ten-pass Williams plasticity value of 4.74 for - 25 ' as compared to 6.02 at 10" C. Ten-pass recovery was also significantly different in the polymer made a t -25 C. From the overall point of view provided by the data, it can be said that a more processable and more uniform polymer was obtained at -25" C. than at 10" C. O
* 5 1Q
51,
SI
-IU 0 TEMPERATURE
,
IO OC.
Figure 3. Effect of P o l y m e r i z a t i o n T e m p e r a t u r e on Osmotic Molecular Weight
POLYMERIZATIOK AT -40" C. (CONTROLLED TO * 3 O (2.). Since substantial improvement in the uniformity of breakdown was observed a t -25" C. it was considered possible that this desirable quality would be enhanced a t lower temperatures while other characteristics were simultaneously being improved. Consequently, polymerizations were conducted a t -40" C. using the recipe developed for this temperature. I t has appeared that the -40" C. polymers were equivalent to those made at -25' C. in respect to breakdown and processability as measured by raw and ten-pass gel and Wlliams plasticity. At this stage in the development of the low temperature formula, i t was considered advisable to investigate the bearing of thc emulsificr systcm on the polymerization rate. Several loadings were made to cover complete removal, as well as increasing concentrations of MP-189-S. The elimination of MP189-5 from the system retarded the polymerization appreciably even when the oleate concentration vias raised. A decrease in the potassium oleate charge likewisp retarded the polymerization rate. Run P;o. 41 P- 189-5 Potassium oleate ConiTersion (93 hr. a t -40° C.)
61-1 3.5
2.0 il .il
61-2 61-3 0 0 5 , ~8 . 5 36, i) 4 2 . 5
61-4 3.5 1.5 42 .O
MODIFICATION. The irregularity shown by the breakdown of higher temperature redox polymers could be removed by the introduction of modifiers such as dodecyl mercaptan (DDM) (thiol), diisopropylxanthogen disulfide (Dixie), and tertiary hexadecyl mercaptan without retarding the polymerization rate. Although the products of such systems were much more processable than the unmodified redox rubbers, their physical properties more nearly resembled those of GR-S than those of typical redox polymers. Consequently, the work was continued in the direction of improving the breakdown characteristics of unmodified redox polymers. MOLECULAR WEIGHTDISTRIBUTION.The various polymerr were milled just enough to render them completely soluble and were fractionated by precipitation from solution. Intrinsic viscosities were run on each of the fractions and the results grouped as shown below. Visaasits range GR-8 Redox, + l o 0 C., modified Redox, + I O o C. Redox -10' C. Redox: -25' C. Redox. -40° C.
61-5 5.0 1.5
57.5
61-6 8.8 1 5
