Formation of Bound Rubber
of GR-S Type Polymers with Carbon Blacks JUNE DUKE AND W. IC. TAFT Government Laboratories, University of Akron, Akron, Ohio
I. M. ICOLTHOFF University of Minnesota, Minneapolis, Minn. T h e bound rubber-black complex formed by milling various GR-S polymers and carbon blacks at several temperature levels was studied. The amount of bound polymer increased with greater loadings of black, but per unit of carbon black, it decreased at the higher black loadings. The temperature of mixing likewise has a large effectat lower carbon black loadings, higher temperatures increase the amount of binding; .the effect is minimized as the loading is increased until at high loadings (100 to 125 parts of black per 100 parts of rubber) this effect is eliminated. By fractionation of the sol portion, it has been
shown that polymer of progressively lower molecular weight is bound as the black loading is increased. Polymer of high molecular weight does not replace bound polymer of lower molecular weight; the polymer of higher molecular weight is preferentially bound during mill mixing. Although more polymer appears to be bound as the conversion is increased from 50 to 72% at a loading of 50 parts of black, other factors besides conversion may be determinative. No differences in relationship were found for polymers made at 122" or 41" F.
T
in 100 ml. of benzene a t room temperature in an undisturbed state permits the determination of swelling index,
H E combination of various types of carbon black with natural and synthetic rubbers to form the bound rubberblack complex has been discussed by many authors. Recently, the effects of the temperature of mixing the rubber and black on the results obtained for blacks made by different processes and combined with GR-S made at 41 and a t 122O F. have been published ( 2 , . 6 , 1 2 ) . Sperberg (11)correlated the amount of bound rubber and the tightness of the combination with the resistance to abrasion encountered in actual road tests. Sweitzer (12) concurs in this view, and goes further t o state that the carbon gel complex Rets the pattern for the ultimate form and performance of the vulcanized stock. Previously published work has been concerned mostly with natural rubber or with commercial samples of GR-S, made at 122 O or at 41 O F. to the usual conversion and viscosity levels, combined with blacks at conventional loadings. This report is concerned with the study of more extensive loadings, as well as with the effect of the polymerization variables of temperature of reaction, percentage conversion, and viscosity of the GR-S polym e r ~on the polymer-black gel formed during milling. The experimental polymers in this investigation were standard GR-S made at 122' F. (X-539) and various polymers of the GR-S butadiene-styrene ratio made a t 41" and at 122" F. in 5- or 500-gallon reactors at the Government Laboratories. The variables in these substantially gel-free polymers are shown in Table I. PROCEDURE AND DISCUSSION
To substantiate the techniques used, the effect of milling time was determined with X-539 GR-S and Wyex easy prmessing channel black (EPC) on a 10 X 20 inch mill a t two temperature levels and a t two loadings of the black, 25 and 125 parts per 100 parts of rubber (p.h,r,), Results of gel determinations in a Soxhlet at the reflux temperature of benzene were 3 to 5y0 lower at both carbon black loadings than results obtained in the Baker cell. Since the Baker method (IO)of soaking 0.25 gram of stock
gel swelling index = weight of swollen gel weight of dry gel as well as the dilute solution viscosity of the sol fraction a t 25"C., this method was used for determination of all of the gel results reported, except where noted otherwise. This static method does not disperse the black as readily as do the mobile methods, The results indicate that equilibrium is obtained by milling for 30 minutes and that at a loading of 25 parts of black the effect of milling temperature on binding of the polymer and the black is quite noticeable, whereas at a loading of 125 parts, this effect is small. The preferential binding of polymer of higher molecular weight is also noted. This observation has been reported previously by many authors.
TABLE I. EXPERIMENTAL POLYMERS Polymerization Temperature, Polymer 6HS2M 33HP7C 5HH Bld 1 L2 8PCE3 x-539 3PC465 5HH45 5HH68 33HPA4C 5°C Bld 1 5HHC21
ConversLon,
F.
%
122 122 122 122 122 122 122 122 41 41 41
59 50 00 69 72 75 75 78 50 61
76
Viscosity, LML-4 72 49 50 43 50 24 39 56 50 54 55
DSV 2.43 1.94 1.97 2.01 2.04 1.59 1.90 2.22 1.71 1.98 2.26
EFFECT O F MIXING TEMPERATURE AT DIFFERENT LOADINGS
With X-539 polymer, a series of tests was made by mixing 25, 50, 100, and 125 parts of EPC black on a mill for a total time of 30 minutes, including 8 minutes to band and refine the raw polymer. The mill rolls were cooled or heated, as necessary, to ob-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 43, No. 12
TABLE11. EFFECT OF MIXIKG TEMPERATURE, TYPE, AND AMOUNTO F CARBON BLACKON THE FORMATION OF C.4RBOSPOLYMER GEL Mixing Temp., F.0
295 305 170 200 300 175 190 200 215 250 270 295 305 225 230 260 29.5 200 220 245 270 300 300
175 250 160 180 260 235 250
Black, Parts/
100 Partsi Rubber
10 17.5 25 25 25 50 50 50 50 50 50 50 50
Gel,
%
188 24b 35 37 45 57 56 56 56 57 58 60 58
76
100 100
75
75 78 85 83 81 81 84 83
100 100
125 125 125 125 125 125
26 270 49
25 25 50 50
47 47
50
69
100 125
72
34 34 34 43 55
Gel Swelling Index
DSV
EPC BLACK .,. 1.47 ... 1.42 34 1.43 23 1.33 19 1.21 12 1.03 12 1.14 13 1.05 1.20 l33 1.21 1 11 1.18 11 1.02 12 0.93 5 0.78 5 0.81 6 0.76 5 0.74 4 0.61 5 0.76 4 0.66 4 0.73 4 0.67 4 0.90
Bound Polymei
70
Bound Polymer, yo Black
106 lib 19
lOOb 63b 76
21 31
84 124
36 34 34 34 36 37
72 68 68 68 72 74 80 74 52
40
37 52 50 50 56 60 62 57 57 64 62
HhIF BLACK 1.70 24.5 30 1.63 12 5 1.46 11,5 l,45 14 1 36 6 1.08 4 0.78
21 38 36
HAP BLACK 28 1 46
17
7 9 24 21
50
50 56 53 50 46 46 51 50
28 36 48 42 42 38 29
I25 PHR O F E P C 401
201
160
180
200
220
240
260
MIXING TElvlPERATLRE
200
300
320
OF:
Figure 1. Bound Polymer Per Cent Black us. Mixing Temperature at Various Loadings of X-539
In Figure 1, the curves indicate the relationship for EPC black (from the data in Table 11),a t the loadings shown for the bound polymer expressed as percentage black, with variations in the final temperature of the mix. As the temperature was increased, 59 50 at loadings, of 25 and 50 parts of black, sorpt'ion bonds, or polar 52 100 76 78 0.68 56 100 attraction forces, or the chemical combination of the black with 84 2.5 0.70 64 51 125 the polymer possibly t,hrough oxygen linkage, as described by Sweitzer ( l a ) ,have become more potent, At loadings of 100 and ACETYLENEBLACK 125 parts of black, the effect, of temperature on these forces be30 1 9 1.43 1 3 52 25 180 21 47 11.5 1.25 comes much less marked. This observation would indicate that, 43 50 200 44 72 5 1.00 44 100 215 at loadings possibly up t o about 60 to 80 parts of black, the 44 75 4.3 0.90 35 125 230 temperature of mixing is significant in binding the polymer to the a Temperature of stock. black, whereas a t higher loadings, the important consideration is b Black solution. c Gray solution. not temperature. The spot checks with,HI\IF, HAF, and the acetylene black indicate that EPC and HAF are somewhat similar in their ability t o combine with the polymer to form gel a t the different temperatain the final temperatures s h o m in Table 11. The gel or benzene tures. The slope of the bound polymer per cent' black us. mixing insolubles (bound polymer plus black), gel swelling index, and temperature for HAF a t 25 and 50 parts per 100 parts rubber is dilute solution viscosity of the sol portion were determined bj the steeper than that for EPC (dotted lines in Figure 1). This Rtatic method (10) for gel determination after a resting period of observation should be related to the greater scorchinem of HAF 48 houre, which has been shown t o be about the time necessary for black compounds. At the higher loadings, HAF appears someequilibration ( 1 12). A fen. points were then checked with high what more reactive than the EPC, although the differences may modulus furnace black (HMF), Statex 93, high abrasion furnace well be within experimental error, In any case, these two blacks black (HAF), Philblack 0, and acetylene black in place of the are nearly alike, whereas the HMF and acet'ylene blacks are conEPC black. From these determinationp, the data in Table IT siderably less active. The relative reactiveness of these last were calculatrd as shown: two is not clear; at loadings of 25 and 100 parts per 100 part's rubber. acetylene is closer to the EPC curve, whereas % bound polymer plus black - parts of black/100 parts rubber X 100 at 50 and 125parts per 100 loo f parts Of parts rubber 100 parts rubber, the order is Bound polymer, % = parts of black/100 parts rubber X 100 100 possibly reversed. The 100 parts of black/100 parts rubber following surface areas, in square meters per gram, % bound polymer X 100 were determined for these blacks by Drogin and Bishop (3): Bound polymer, % black = parts of black/100 parts rubber EPC, 106.8; HAF, 69.2; HRIF, 38.6; and acetylene, 16.1. 195 200 220 260 210 265 220 350 260
25 25 25 25 50
28 29 22 13 12 3.5 5.5
1.49 1.50 1 01 1.13 0.90 0.73
18
18 29 33 39 52 56
68 72 72 116 66 78
~
+
1
December 1951
EFFECT OF MIXING TEMPERATURE AND LOADING ON THE GEL SWELLING INDEX
Not enough data have been obtained to correlate the blacks in accordance with the tendency of these polymer-black gels to swell in benzene. Much tighter polymer-black gel is formed at higher loadings, as shorn-n in Figure 2 and Table 11, and the
O\O
mer caused by the banding and refining process. Regardless of this possible discrepancy, it will be noted that as the amount of carbon black is increased, the higher molecular weight material is selectively removed. The results of mixing, a t various temperatures and times, the same weight of polymer alone as previously used with the blacks are shown in Table IV and indicate that, at mixing temperatures of 265' F. and lower, the formation of the black gel is not concurrent with the formation of true polymer gel. Above 280' F., it is possible that the gel formed is partially macropolymer gel, not combined with the black. However, in the results shown in Table 111,no appreciable amount of macropolymer gel was formed because temperatures were below 265" F. In the calculations throughout, it has been assumed, regardless of temperature of mixing, that the polymer gel formed with the
\
', \ 0'
50
76
100
125
150
\ '1
L O A D I N 5 OF BLkCKs P.H.R.
Figure 2.
\
Gel Swelling Index and DSV us. Black Loading
3oo*F.
effect of higher temperature is to cause somewhat tighter gel; the lower the carbon black loading, the greater the effect of temperature. RELATIONSHIP
\L
'\ \
10 60
25
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INDUSTRIAL AND ENGINEERING CHEMISTRY
2 0 0 ° F . ~
40
BETWEEN POLYMER BOUND AND CARBON BLACK LOADING
50
75
100
1'25
150
LOADING OF B L A C K , P H R The relationship between the polymer bound by the black and the amount of black used in the mix is shown in Figure 3. The Figure 3. Bound Polymer Per Cent Black z's. Black shape of this curve is opposite from that reported by Sweitzer ( l a ) , Loading orobablv because of the differences in the gel techniaies used. All of the sol Dortions discussed thus far were clear with all of the black retained by TABLE 111. MOLECULAR WEIGHTDISTRIBUTION OF FRACTIONS OF POLYMERS the gel, except for the loadings of 10 and 17.5 ' REMAINING UNBOUND BY EPC CARBOK BLACK parts of EPC black per 100 parts of rubber and the Black, It appears that p a f ~ ~ ~ o o Distribution between DSV, % polymer, Bound polymer, Total Temp,, Mixing 25-part loading Of HMF with decreasing amounts of black the blackRubber Sol 0-1 1-2 2-3 3-4 4-5 yo V? F. polymer gel becomes more dilute in the polymer Oa 2.04b 33 27 18 12 10 0 100 ... 25 1.44 36 27 19 , .. 18 100 170 portion of the gel, and hence the swelling of the 60 1.15 41 23 3 .. .. 33 180 100 0 78 37 11 .. ,. .. 52 100 loo 225 polymer portion becomes greater. This condition 125 0.67 38 5 .. .. ,. 57 100 245 is reflected in Figure 2 where, with benzene as the a Original polymer (X-538) not milled. solvent, the swelling index becomes greater with a * From distribution curve, this DSV IS calculated to be 1.89, assuming average vlscosity forpolymerineachrange~ decrease in the black content as well as with a reduction in the milling temperature. The validity TABLE IV. EFFECT OF MILLINQ TEMPERATURE o s FORMATION for the extrapolated portions of Figure 3, where a t OF MACROGEL loadings below 25 parts per 100 parts of rubber the curves are (X-539 polymer) shown to continue upwards, has been substantiated by using Milling Gel another solvent (data not presented). Temp., Gel, Swelling
"2'
F.
SELECTIVITY OF BINDING OF HIGH MOLECULAR WEIGHT FRACTION
The preferential binding of the higher molecular weight fraction of the original polymer shown in Figure 2 raised the question as to how selective was this binding. Consequently, X-539 was mixed on cooled mill rolls with 25, 50, 100, and 125parts of EPC per 100 parts of rubber. The polymer-black was removed by dissolving the soluble portion in benzene. The sol was fractionated by ethanol precipitation according t o Johnson's method (6). The amounts of these fractions plus the bound polymer were calculated and are shown in Table I11 as percentages of the total polymer in the mix. The distribution of the original unmilled polymer is not exactly comparable with the distributions after the 30-minute mixing period because, as shown in Table IV, there has been some decrease in average molecular weight of the poly-
%
Index
205 250 265 280 290 295 300 305 310
Stock Mixed for 30 Minutes 0 0 1 0 1 0 0 0 2 0 18 106 18 104 14 105 14 17 101 20 ,..
120n 300"
Stock Mixed for 3 Minutes to Band 2 2
n
zoo
... ... ...
DSV
2.04 1.80 1.82 1.66
1.76
1.22 1.23 1.43
...
1.22
...
2.05 2.02
Stock Mixed for 8 Minutes (5 Minutes of Refining:)
a
120a 3 300' 1 Temperature of the millrolls.
... ...
1.87 1.88
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2888
black is truly bound polymer, not a mixture of bound polymer and macrogel, In support of the validity of this point of view, the data for EPC shown in Figure 1 do not indicate a distinct change in slope a t temperatures above 265" F., which would be expected if free macrogel were present from the preliminary refining or
Vol. 43, No. 12
of EPC black the net change has been a gain of 3% of bound polymer in the first instance and a loss of 2% in the second. Since the distribution values for the raw polymer and sol portions indicate the disappearance of the higher molecular weight fractions of the raw polymer and generally show an increase in the amount of the sol at DSV values above 1.0, it is interpreted that breakdown alone has occurred during mixing procedure No. 2 and that an interchange in the molecular weight of the polymer bound to the black did not occur. This conclusion is considered valid, because if the high molecular veight polymer had replaced the low molecular weight bound polymer, there would have been a marked increase in bound polymer after the second mixing. These results appear to be contrary to those obtained by Kolthoff and Kahn (9) by sorbingpolymer from dilute solution on Graphon, where the high molecular weight polymer displaced the sorbed low molecular weight portions during time of contact, and indicate that sorption from dilute solution and the binding of the rubber by black during milling are nat similar.
/____
4020
Figure 4.
VISCOSITY, ML- 4
Effect of Viscosity of GR-S Made at 122" F.
50 parte of EPC black per 100 parte of rubber
from the further mixing at temperatures above 265" F. It must be assumed, therefore, that all of the gel has been bound on the black. Since macrogel may be formed during these periods, possibly because of oxygen cross linkage a t temperatures above 265" F., it must be assumed that the bonding is such as to allow the black to enter into the combination. Kolthoff and Kahn (9) have shown that GR-S type of rubber is sorbed by Graphon and that the amount of rubber sorbed after one hour changed only slightly, but that the inherent viscosity of the supernatant liquid increased initially and then decreased over a period of 40 hours. These same authors with Gutmacher (8)have also reported that there is no appreciable difference in tHe amount of sorption, under equilibrium conditions, of rubber fractions having average molecular weights varying between 32,000 and 230,000
-
-z
-
---2
-
4 ML-4
REMIXIKG O F BOUND POLYMER-BLACK WITH FRESH POLYMER
To determine whether bound polymer will be replaced by material of higher molecular weight when the polymer-black 24 M L - 4 complex is remixed with fresh polymer, the polymer-blackcomplex obtained by mill mixing X-539 with 126 parts of EPC black: per 100parts of rubber was recovered by dissolving the sol in benzene. > 180 200 220 240 2M1 280 300 This polymer-black complex was dried and then remixed with TEMPERATURE OF FI'IXING. 'F. sufficient raw X-539 so that 27.9 and 56.7 parts of black per 100 Figure 5. Bound Polymer Per Cent Black and DSV of parts of rubber were contained in the total mix. The results are Sol z's. Mixing Temperature for Polymers of Different Viscosities presented in Table V Molecular weight distributions before and after mill mixing 25 parts of EPC black per 100 parts of rubber show that the higher molecular fractions are split preferentially to lower molecular weight materials, but any breakdown in the raw polymer resulting TABLE V. EFFECTOF REMIXING: POLYMER-BLACK COMPLEXWITH from the milling has been ignored in calculating RAWX-539 the values presented in Table VI; the distributions Tntnl -Black were calcuiated from the results of Table V. Distribution Loading, These results, as well as those presented earlier Parts/100 &tilling DSV betwen Bound Total DSV, % Polymer, Polymer, Mixing Parts Temp., of in this paper, reveal that at both black loadings, Procedure Rubber F. Sol 0-1 1-2 2-3 % % the polymer of very low molecular weight has not No. 1" 25 170 1 . 4 4 36 27 19 18 100 No. 2 b 27.9 175 1 . 5 8 28 18 28 26 100 been bound bv the black during the mixing proNO. 14 50 190 1 . 1 5 41 23 33 100 cedure and thgt the highest molecular weigllt porNo. 2b 56.7 185 1 46 23 34 ..3 43 100 tions of the raw polymers have been broken Raw polymer. down. These calculations indicate that during b Bound polymer plus raw X-539. the mixing with approximately 25 and 50 parts Q
December 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
for the high molecular weight fractions, not an average DSV of polymer over the entire range. The curves in Figure 6 show that, as the molecular weight of the polymer is increased, the bound portion is of higher molecular weight. Conversely, as the molecular weight of the polymer mixed with the black is decreased, not only the amount of polymer bound but also the range of molecular weight of the bound polymer is decreased. From Figure 6 and the actual DSV values shown in Figure 5, it appears that there is a limiting value for the DSV of the polymer from which binding will take place. With a decrease in mixing temperature at low to practical loadings, the limiting value should increase somewhat. The results shown in Tables I1 and 111 indicate that the limiting DSV of a polymer that will be bound is in the neighborhood of 0.5. To determine this limiting value more exactly, the sol portions of X-539 that had been mixed with 125 parts of EPC black per 100 parts of rubber were separated from the polymer-blackportion and the polymer was recovered by evaporation. Also several portions of X-539 GR-S were dissolved in benzene, precipitated separately by ethanol, and the low molecular weight frac-
OF POLYMER DISTRIBUTION TABLE VI. CALCULATION
Carbon Black,
Bound Polymer, 0-1 1-2 2-3 3-4 4-5 7% For Loading of 27.9 Parts EPC/100 P a r t s Rubber Raw X-539 0 33 27 18 12 10 Iiil 60.8%. of raw X-539 in No. 2 mix 0 20 17 11 7 6 Nil Polymer-black complex b 55.5 44.5 39 2 7 a of polymer-black com&e$ in NO. 2 mix 22 . . . . . . . . . . 17 100% polymer (sol portion) 0 28 18 28 . . . . 26 22 22 14 22 . . . . 20 100% mix Mix picked up 20 - 17 = 3% of bound polymer From DSV distribution, loss in DSV range of 3 to 5 = 13% (amount of raw polymer bound) Polymer
Yo
. . . . . . . . . .
For Loading of 56.7 Parts EPC/100 Parts Rubber Raw X-539 0 33 27 18 12 10 Kil 34.7%a of raw X-539 in No. 2 mix 0 12 9 6 4 4 Nil Polymer-black complexb 55.5 44.5 65.3%: of polymer-black complex in No. 2 mix 36 29 0 23 34 43 100% polymer (sol portion) 36 15 22 27 100% mix Mix lost 29 27 = 2% of bound polymer From DSV distribution, loss in DSV r a n i e of 2 to 5 = 14% (amount of raw polymer bound) a Amounts, by weight, of X-539 and polymer-black complex required for desired carbon black loadings in the total mix. b Obtained from mill mix of 100 parts of X-539 and 125 parts of E P C black.
. . . . . . . . . .
. . . . . . . . . .
-
.. .. .. .. .. ..
It is apparent from Figures 4 and 5 that the rate of binding is increased as the viscosity or molecular weight of the raw polymer is increased or as the temperature of mixing is increased (also
03
,, ,' /
02
2889
,'
INDUSTRIAL AND ENGINEERING CHEMISTRY
2890
Vol. 43, No. 12
Kolthoff, Gutmcher, and Kahn (a), who shorn-ed that sorption TABLE VIII. EFFECTO F ~ 7 1 S C O S I T YO F POLYMER O S FORXfATION by black from dilute solutions of polymer is independent of the OF CARBON-POLYXER GELAT DIFFEREUT MIXINGTEMPERATURES molecular Tveight of the polymer Polymer Mill-Mixed with 2 5 Parts E P C Blach per 100 Parts Rubber
Mixing Temp,
F.
Gel
75
Gel Swelling Indev
DSV
24 AIL-4 Polymer, DS\
Bound Polymer
Bound Polymer % Black
%
159 36 56 76
190 230 310
31 33 36
43 ML-4 Polymer; DSV. 2.01 31 1.48 26 1.47 26 1.19
14 16 20
56 64 80
170 200 300
35 37 45
50 ML-4 Polymer; DSV, 2.04 34 1.43 23 1.33 19 1.21
19 21 31
76 84 124
185 235 305
32 36 39
72 ML-4 Polymer; DSV, 2.43 38 1.74 15 29 1.61 20 23 1.31 24
60 80 96
EFFECT OF CONVERSION
GR-Sspecimens (described in Table I) made at 122" F. to 50, 60, 72, and 78% conversion and substantially equivalent viscosities were mixed a t relatively low temperatures Rith 25, 50, 100, and 125 parts of EPC black per 100 parts of rubber. The results are shown in Table x. The only significant effect apparently caused by increase in conversion of polymers made a t 122" F. is evident at the loading of 50 parts of EPC black per 100 parts of rubber, where increasing the conversion to the 72% level has resulted in a greater amount of bound polymer per unit of black. However, at the 78% conversion level, the amount of bound polymer was not increased further. At loadings of 100 and 125 parts of black per 100 parts of rubber, the effect of conversion is not apparent. The failure of the two polymers of low conversion t o bind the 25 parts of black sufficiently t o prevent colloidal dispersion of the black eliminates the possibility of any conclusions for this series. It appears that variables other than degree of conversion are interfering and that over the range of 50 t o 78% conversion at 122" F., the conversion factor is probably not too significant in drtcrmining the formation of the polymer-black gel. In Table XI, data are presented for GR-S polymers made at 41" F. in a similar range of conversion and mixed at the same loadings of bIack shown in Table X. Interpretation of t h c s results is similar t o that for the polymers made a t 122 o F. Furihcrmore, except for the somex hat higher binding with 50 parte of EPC black per 100 parts of rubber obtained for the polyrnw made at 122" F compared to that for the polymer made a t 41 F., the effect of temperature of polymeriaation on the binding of the polymers of differing degrees of conversion is negligible. These
tion of each polymer was recovered. The dilute solution viscosity of each of these recovered polymers was determined and these polymers m r e also fractionated. Each of the polymers was mixed with 125 parts of EPC black per 100 parts of rubber. The gel contents were determined for each of the mill-mixed masterbatches and the sol portion of each was fractionated, The results for these determinations are shown in Table IX 1% here a corresponding order is used in prcsenting the raw polymers and the mixes. The fractions with DSV values of 0.50 to 0.75 were bound for the raw polymer with an initial DSV of 0.23, indicating the probability that polymer with a DSV value as low as 0.50 (corresponding to a molecular weight of approximately 30,000) is bound by the black, provided the black's capacity for TABLE Ix. BINDINGAND FR4CTIONATION O F LOW MOLECULAR ITEIGHT POLYMER binding has not been satisfied by polymer of higher molecular weight. These Milling Distribution between DSV, % Bound Bound ye$,%, Gel, 0.25- 0.50- 0 75Polymer, Polymer, fractions of the raw polymer were liquid, %DSV 0-0.25 0 . 5 0 0 . 7 5 1.0 1-2 2-3 76 % Black Since more polymer was shown to be Raw Polymers bound from higher viscosity solid stocks ... Nil 0.79 58 12 7 4 10 9 .. than from those of lower viscosity, the Nil 0.66b 18 22 21 22 14 .. .. . , Nil 0.23 50 33 17 .. ,, . , .. results for these liquid fract'ions were compared n-ith those for a known Mixed with 128 Parts EPC Black per 100 Parts of Rubber 260
68
0.32
39
19
15
,
,
,
.
.,
27
22
polymer. 300 72 0.61 13 19 21 10 ., 37 30 For raw polymer X-539, about 50% 200 62 0.19 46 40 .. ,. .. .. 14 11 Temperaturea could not be kept constant. of bound polymer, based on the b Recovered sol from mix of 100 parts of X-539 and 125 parts of E P C black. black (see Table 11),was bound during mill mixing, compared to 22, 30, and 11% (Table I X ) TABLEX. EFFECTOF CONVERSION OF POLYMER MADEAT 122' F. ON FORMATION OF CARBOX-POLYMER GEL bound by the same quantity of EPC black from these low fractions. Black, These observations confirm the Psrts/ Mixing Raw Polymer Gel Bound Bound 100 Parts Temp., Conversion, Viscosity, Gel, Smeliing Polymer, Polymer decrease in the amount bound a F. 70 RIL-4 DSY % Index DSV % % Black' Robber by the black as the viscosity of 50 49 25 175 1.94 26a 44 1.FOQ 85 32a 60 50 27a 25 175 1.97 39 1.62= 90 36= the p o l y m e r is d e c r e a s e d . 25 170 72 50 2.04 35 34 1.43 19 76 78 56 2.22 30 Other data (not presented) 25 170 47 1.56 13 52 confirm the relative amounts 50 49 1.94 48 22 1.28 50 250 22 44 60 50 1.97 61 18 50 255 1.15 27 54 of these low molecular weight 54 15 72 50 2.04 1.13 31 62 60 250 78 56 2.22 54 19 50 245 1.11 31 62 polymers bound comparpd to 100 265 50 49 1.94 75 7 0.96 50 50 that bound from X-539 at load1.97 77 5 1.10 54 54 100 265 60 ings of 25 parts of black per 50 2.04 75 100 260 72 6 0.76 50 50 100 255 78 56 2.22 77 6 1.06 54 54 100 parts of rubber. These 126 260 50 1.94 82 6 0.69 59 47 results, which indicate that the 49 1 . 9 7 83 5 0 67 62 50 I25 260 60 50 2.04 81 4 0.74 57 46 125 270 72 higher molecular weight poly125 290 78 50 56 2.22 82 6 0.74 59 47 mer is preferentially bound by a Graysolution. the black, are again contrary to the results presented by , ,
2891
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1951
which has been shown to be applicable by osmotic molecu 1a r w e i g h t measurements conducted on fractions of GR-S Bound Bound (7). The breakdown of the Pol mer, Polymer, polymers caused by the mill DSV % % Black 1.55a 9a 36= mixing was assumed t.0 be that 1.70Q loa 40a obtained after 8 minutes of 1.70a 14" 56a milling. These assumptions 1.49 13 26 1.58 25 50 have been used to calculate the 1.29 25 50 relative average number of 50 50 0.84 1 08 54 54 polymer chains bound per unit 0 86 48 48 of black for the stocks shown 0 90 64 51 in Table XII. 0.91 66 53 0 61 60 48 The same method of calculation was used for a series of GR-S type polymers of different viscosity levels. Breakdown of these polymers, caused by the mill mixing, was assumed to be about the same as that for the stocks in Table XII-approximately 10%. The results calculated on this basis are shown in Table XIII. The same approach was used to calculate the effect of various polymers made a t 41" F. to different conversion levels; the results are shown in x''.
OF POLYMER MADEAT 41' F. ON FORMATION OF CARBONTABLE XI. EFFECTOF COSVERSIOX POLYMER GEL
EPC
Black, Psrts/100 Parts Rubber
50 50
100 100 100 125 125 125
260 265 260 275 265 295
50
a
' F.
245 245 255 245 250 255
25
25
25
Raw Polymer Conversion, Viscosity, % ML-4 50 50 54 61 76 55 50 50 54 61 76 55 50 50 54 61 55 76 50 50 54 61 55 76
Mixing Temp.,
Gel, DSV 1.71 1.98 2.26 1.71 1.98 2.26 1.71 1.98 2.26 1.71 1.98 2.26
%I
27a 28" 31a 42 50 50 75
Gel Swelling Index 32 35 31
20 16 19
8
77
6 8 6 4 6
74 84 85 82
Gray solution.
results are contrary to expectation and indicate again either that variables other than conversion have been encountered or that conversion is not an important factor. CALCULATION OF NUMBER OF CHAINS PEF UNIT OF BLACK
It has been shown that higher molecular weight polymer is bound preferentially by carbon black and that this bound polymer is probably firmly held by the black. Also, it has been shown that the bound polymer percentage black increases with the viscosity of the polymer and decreases with the amount of black mixed with the polymer. The rate of increase with viscosity falls off somewhat at the higher viscosities. These findings suggest that the polymer is attached a t one, or a t most a very few, of the functional groups of the polymer chain, and that the number of chains per unit of black should be reasonably constant for the same type of polymer. If the bound polymer per unit of black is divided by the average molecular weight of the bound polymer, the relative number of chains of bound polymer per unit of black should be obtained. This condition should be true if the polymer chain is attached to the black a t one end. T o check this reasoning, samples of mill-mixed masterbatches made with X-539 and EPC black, over a range of carbon black loadings and mixing temperatures, have been selected at random. Since the intrinsic viscosity of a polymer is equal to the sum of the intrinsic viscosities of the fractions, the following method of calculation was used:
If x
tion
= intrinsic viscosity of the sol fraction [ q ] = intrinsic viscosity of the raw poly-
and b
TABLE XII. RELATIVE NUMBER O F POLYMER CHAINSBOUND PER UNIT OF BLACK FOR GR-S POLYMER (X-539)
E P C Black Parts/100 Pakts Rubber 25 25 50 50 100 100 125 125
Mixing Temp,
F. 200 300 175-200 270-305 225-230 260-295 200 300
Average Number of Chains per Unit of Black X 107 12 9 21 3 12 4 14 6 11.9 12.3 14 6 14 4
In the calculations of the number of polymer chains per unit of black, the factors which influence the accuracy include: effect of branching; limited accuracy of the intrinsic viscosity-molecular weight relationship at high values of both factors; breakdown of the raw polymer during milling; effect of carbon black on this breakdown; assumptions made in converting the dilute solution
= intrinsic viscosity of the bound frac-
y
a
'
mer fraction of the raw polymer that is bound = fraction of the raw polymer that is in the sol, =
then [ q ] = az
+ by
[SI - by and x = ___
A range of dilute solution viscosities, from 0.7 to 4.50, for various fractions of X-539 was used to determine the slope of the intrinsic viscosity versus concentration curve. An average value for this slope, considering possible errors, was found to be 1.05, and this factor was used to determine the intrinsic viscosities for the calculations. The relationship between intrinsic viscosity and weight average molecular weight was calculated from t h e equation (6) log M
=
4.93
+ 1.49 log [SI
TABLE XIII. RELATIVE NUMBER OF POLYMER CHAINSBOUKDPER UNITOF BLACKFOR POLYMERS OF VARIOUS VISCOSITIES MADEAT 122" F. Polymer
Viscosity, ML-4
Conversion,
3PC465 3PC465 5HH45 3PCE3 3PCE3 x-539 6HS2M 6HS2M 6HS2M
24 24 39 43 43 50 72 72 72
75 75 75 69 69 72 59 59 59
%
E P C Black, Parts/100 Parts Rubber 25 50 50 25 50 50 25 25 50
Mixing Temp.,
F. 300 185 165 310 205 205 185 305 205
Average Kumber of Chains per Unit of Black x 10' 17 3 9 6 11 2 10 7 10 4 11 5 6 9 10 1 10 2
TABLE XIV. RELATIVE NUMBER O F POLYRfER CHAINS BOUNDPER UNITOF BLACKFOR POLYMERS MADEAT 41' F. Polymer
Viscosity, ML-4
Conversion,
33HPA4C 33HPA4C 5°C Bld 1 5HHC Bld 1 5HHC21 5HHC21
5o 50
50 50
54
61
54 55 55
61 76 76
%
E P C Black, PitrtS/100 Parts Rubber 50 100
&'fixing T y p 245 260
-4vera e Number of c f a i n s per Unit of Black x 107 11 8 17 2
50
250
15 6
100 50 100
265 255 260
17 4 6 9 9 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
2892
viscosities to intrinsic viscosities; experimental errors encountered during the milling and in the analytical procedures; and variations in the types of polymer. When these factors are taken into consideration, the agreement in the calculated values shown in Tables XII, XIII, and XIV for the relative number of polymer chains attached to each unit of black is believed t o constitute reasonable substantiation of the assumption. Thus the polymer apparently attaches to the black at one, or a t most a very few, of the functional groups of the polymer chain and the number of chains is relatively independent of the loading of black above 25 parte per 100 parts of rubberprovided the polymer does not contain gel of a type that Fill not enter into the black complex and the molecular weight of the bound polymer is between 200,000 and 1,000,000 LITERATURE CITED
(1) Baker, IT. O., and Walker, R.
W.,private communication to
Office of Rubber Reserve.
(2) Dannenberg, E. M., and Collyer,
Vol. 43, No. 12
H.J., IND.ENG.CHEM.,41,
1607 (1949). (3) Drogin, I., and Bishop, H. R., “Today’s Furnace Blacks,’’ Charleston,W. Va., United Carbon Co., Inc., 1948. (4) Drogin, I., Bishop, H. R., and Wiseman, P., India RubbeT W o r l d , 120,693-7 (1949); 121, 67-66 (1949). ( 5 ) French, D. M., and Ewart, R. H., Anat. Chem., 19, 165-7 (1947). (6) Johnson, B. L., Iun. ENG.CHEX., 40,351 (1948). (7) Kennedy, T. J., and Higuchi, T., private communication b (8) (9) (10) (11)
Office of Rubber Reserve. Kolthoff, I. M., Gutmacher, R. G., and Kahn, A,, Ibid. Kolthoff, I. M., and Kahn, A., I b i d . Mullen, J. W., and Baker, W. O., Ibid. Sperberg, L. R., Svetlik, J. F., and Bliss, L. A , IND. ENQ.
CHEM., 41, 1641 (1949). (12) Sweitzer, C. W.,Goodrich, W.C., and Burgess, K. A,, Rubber Age, 65, 651-62 (1949).
RECEIVED March 7, 1961. This work was sponsored by the Office of Rubber Reserve, Reconstruction Finance Corp., in connection with the government synthetic rubber program.
anical Breakdown of SoapR. J. MOORE AND A. M. CRAVATH Shell Development Co., Emeryville, Cui$. o n e of the important practical characteristics of a soapbase lubricating grease is its progressive softening upon prolonged mechanical agitation. This investigation was undertaken in order to find out what causes this softening and what determines its rate. It was found, from electron micrographs of sodium-, calcium-, lithium-, and barium-base greases, that breakage of the soap fibers occurred during work softening. The ratio of length to diameter of the fibers was the factor of predominant influence on consistency, and its change when the fibers were broken appeared to explain the observed change in consistency. Usually, the consistency (reciprocal of the Shell microcone penetration) was found to be a linear function of the logarithm of the time of working. A theoretical explanation of this relation, which seems to apply to many other cases of the mechanical degradation of small particles, shows that the law is approximately followed when the breakage of particles is due not to gradual fatigue or wear, but to the fluctuating stresses to which the particles are subjected and which occasionally reach unusually high values exceeding the initial breaking strengths of the particles.
T
HE changes in consistency of lubricating greases which occur when they are subjected to shearing forces have been the subject of considerable speculation. Chemical and/or physical changes in the system soap-oil which comprises the usual greases have been alluded to in order to explain observed changes in consistency, but on the whole the system and its mechanical degradation have been poorly understood. Solvation of soap by oil, for example, has been a common expression in this field, as has hydration or dehydration as the occasion demanded; the use of the term dehydration stems no doubt from the chemistry of lime-base greases which show a marked change on going from the hydrate to the anhydrous form. More recent work has established that most soap-base greases are true gels, or two-phase systems comprising cryetalline soap
and more or less soap-free oil (7, 9). The electron niicroscope studies reported by Farrington and Birdsall ( 1 ) defined the form taken by the soap-namely, a mass of fibers of about 0.1- to 1.0-micron width and of varying length up to 0.1 mm. These microscopic soap fibers should not be confused with the visible structures in the so-called fiber greases, such as some of the sodabase greases. -4lubricating grease can be defined as an aggregation of soap crystals in which oil is held by capillary forces, and the specific grease properties-consistency and non-Newtonian floware a consequence of the distribution of the soap in fine filaments or fibers. The present paper describes the mechanical breakdown of grease in terms of the disintegration of the fibers constituting the thickening agent as observed n-ith the electron microscope. EXPERIMENTAL
In evaluating the various devices used to effect mechanical breakdown of greases, it became apparent that while the rate of breakdown might differ markedly, the mechanism was the same. Accordingly, most of the work described here was carried out with the Shell roll tester, although similar results were obtained with other types of apparatus. The shear conditions of the roll tester are not completely known, but the severity correlates well with field experience with truck wheel bearings. By using this apparatus, a convenient amount of grease-about 75 grams-can be thoroughly and reproducibly sheared to cause breakdown in a reasonable time. At convenient intervals the grease samples were removed from the roller and penetration values were obtained with the Shell microcone ( 2 ) . This is a 74” aluminum cone 21 mm. high, with a weight such that the total weight of cone and plunger of the ASTM penetrometer is 58.3 grams. This cone is preferred because of its accuracy in the consistency range of interest-Le., between manufactured consistency and failure-and also because as a simple cone, in contrast to the complex ASTM cone, it gives rise to penetration values, whose reciprocals are roughly a linear function of yield value (6). I n making a consistency determination, the grease is immediately removed from the rolling apparatus and quickly brought to 25” ct 0.2” C. by placing the grease on a steel plate in a refrigerator. The grease is then loaded into the test cup and promptly tested. In this way variable results due
