INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
August, 1944
301
LITERATURE CITED
theae changes caused by aging are the same as those of the less stable products of the optimum cure. These results indicate that perhaps a revision of some of our prevailing ideas on the correct state of cure for maximum service of this type of GR-S stocks is in order.
(1) Bierer, J. M.,and Davis, C. C., IND. ENQ.CHEJM., 21,1008(1929). (2) Ingmanson, J. H., and Kemp, A.R., Ibid.,28,890-1 (1936).
i(5) :; gy; ~ ~ Nagai, H.,J .
~ . t B.~s., Inb ~ ~.36, , $212-15 ~ (1944). ~ ~ , SOC.Rubber Id.Japan,9,147-52 (1936).
(6) Olsen, 8.R., private communication, 1943. (7)Prettyman, I. B.,IND.ENQ.CHEJM., 36,29-33 (1944). (8) Shelton, J. R., and Winn, H.,Zbid., 36,728 (1944). (9) Williams, Ira, and Ned, A. M., Ibid.,22,877(1930).
ACKNOWLEDGMENT
The writers wish to thank J. N. Street, Director of Research, md the Management of The Firestone Tire & Rubber Company 'Or permission to this They wish to thank R. M.Hill for supplying Pa& of the data on rate of Cut growth.
before the spring meeting of the Division of Rubber Chemistry, C H ~ ~ I CSOCIETY. AL in New York, N.y.,1944.
PalalNTBD AMlaICAN
RELATION TO A. R. KEMP
AND
W. G. STRAITIFF Hill, N. J.
Bell Telephone Laboratories, Murray
The nonsymmetry in the chain structure of Buna S hydrocarbon i s dimcussed in relation to the prevention of crystallization and the impeding of cross linking during vulcanization. This lack of chain wmmetry i s put forward to account for the poor quality of Buna S vulcanizates in comparison with corresponding vulcanizates prepared from natural rubber. Fractionation data on a regular benzenesoluble crude Buna S indicates the presence of an objectionable broad range of polymer sizes. It is shown that the lowest-molecularweight polymer fractions in Buna S are not chemically bound in the vulcanizate but remain soluble in chloroform. By removing most of this low polymer from Buna S, the chloroform extract of its vulcanizate decreases accordingly. Vulcanizates were prepared from highand low-molecular-weight fractions of Buna The high fractions weae tough, dry, and difficult to handle on the mill! the lowermolecular-weight fractions was soft and sticky. The tensile strength e f vulcanizates from the high fraction was somewhat greater than that of the whole polymer, but the modulus was considerably increased. For the low-Tolecular-weight polymer both tensile and modulus values were much lower. Vulcanizates prepared b y mixing natural rubber and gutta-percha hydrocarbons show lower strength #hat neither of the hydrocarbons separately tested in the same formula.
s.
HE low tensile strength of Buna S gum stocks is generally believed to be due to failure to obtain effective cross linkage as the result of vulcanization with sulfur and accelerators. Combined with this is the complete absence of Crystallization of Buna S upon stretching which, in the case of natural rubber, reinforces and strengthens the vulcanized gel structure. The absence of crystallization in Buna S can be explained on the basis of nonsymmetry along the polymer chains. Strictly speaking, Buna S is not a true polymer since ozonolysis shows that the styrene units are not evenly spaced in the chain but are grouped together in some locations ( I ) . Ozonolysis also has proved the presence of vinyl groups attached to the chain, resulting from the polymerization of butadiene in the 1,2 instead of the 1,4 position ( I ) . These vinyl groups must be unevenly spaced along the chain, and mixed trans and cis isomers must be present.
PHYSICAL PROPERTIES Figure 1 illustrates the chemical units present in Buna S, rubber, and gutta-percha hydrocarbons. I n a Buna S copolymer containing 24.6% styrene, there are six butadiene to one styrene unit. It appears ( 1 ) that about one butadiene in five polymerizes in the 1,2 position in the chain. It should be emphasized that, in Buna S, ozonolysis (1) has shown that no regular order exists in the location of A , B, and C units in the polymer. An entire lack of symmetry in the positioning of these units in the chain would be expected in view of the nature of the polymerization reaction. Consequently, we can readily see how impossible it is for Buna S to crystallize. Since double bonds in the butadiene chain units and in the vinyl groups are unevenly spaced along the chain, they cannot be uniformly or frequently matched up with those in neighboring chains. This fact may result in an ineffectual cross linking of Buna S chains with active sulfur atoms during vulcani~ation. I n contrast, the more or less perfect chain symmetry of sol rubber hydrocarbon should result in a higher frequency of alignment of the double bonds and, therefore, a more effective vulcanization reaction to produce more frequent and more evenly spaced cross links in the vulcanizates. Where vinyl and benzene side groups are located in the Buna S chain, the separation of adjacent chains will be considerably greater than in natural rubber chains with shorter methyl side groups. This wider chain separation of Buna S chains may also reduce effective cross linking with sulfur during vulcanization and thereby cause weak are= in the vulcanizate structure. Good physical characteristics of Buna S vulcanizates can be obtained only by pigment reinforcement. The factors determining the extent and nature of this reinforcement are amount of loading, size and size distribution of pigment, shape of pigment particles, and dispersion quality or uniformity of' separation of pigment particles in the vulcanizate. Because Buna S does not wet pigments so readily as rubber, it is more difficult to obtain a uniform Buna S stock in conventional rubber mixing equipment. The physical properties of Buna S compounds deteriorate faster with a rise of temperature than do those of natural rubber compounds. This behavior can be explained on the basis that the physical properties of Buna S stocks depend to a greater extent on association forces and less on chemical cross linking than
708
INDUSTRIAL AND ENGINEERING CHEMISTRY
do those of similar natural rubber vulcanizates. These association forces between the polymer chains and between the pigment particles and Buna S chains are greatly reduced a t elevated temperatures; a weaker gel structure results in Buna S than in the more highly and effectively cross-linked rubbcr gel. SYNTHETIC BUNA-S
NATURAL RUBBER HYDROCARBON C M ~
c=c
7
cy3
H,
c=C
C\H ~ H ,
C-C
CIS
NATURAL GUTTA- PERCHA HYDROCARBON
TRANS
Figure 1.
Chemical Unib in Bund S, Rubber, and Gutt.-Percha Hydrocarbons
Gelation resulting from branch chaining and cmss linking may be present in Buna S in widely varying degrees, depending up011 the polymerizing conditions and subsequent treatment of thc polymer. I n cases where the polymer is completely soluble in benzene, it will become increasingly insoluble in benzene or chloroform if antioxidant is absent and the polymor is exposed to air. For ex ample, a pure white Buna S polymer changed from 100% benzene solubility to 39% solubility upon exposure to air a t room temperature for 4 days, whereas no gain in weight could be dcWted on an ordinary analytical balance. The rate of oxidation of purified Buna S polymer is indicated by the weight gain curves for two different preparations in finely cut form in Figure 2. After &day exposure to air in the laboratory, the material became yellowish and was hard and brittle on the outside surfaces. Gelation will also takc place in solution in the absence of antioxidants. Pure rubber hydrocarbon will absorb oxygen somewhat more rapidly than Buna S hydrocarbon when exposed to air, but unlike the Buna S hydrocarbon, it remains soluble in chloroform(2). Possibly the vinyl side groups in OR-S may be the seat of oxygen crow linking which results in gelation. It is well known that a complete gelation of linear polymers can result from a limited number of cross links. Improvements in Buna S from a structural point of view are made extremely difficult if not impossible by the fundamental nature of the polymerization reaction which leads to the features of chain asymmetry just described. Aside from a small amount of gel, not only does the rubber hydrocarbon in crude rubber possess chain symmetry, but it is all in the cis configuration. Another weakness of Buna S is poor processing characteristics. It is very nervy, and its compounded stocks show a large shrinkage when slabbed off a mill following mixing and cooling. A shrinkage of 40% in length is not uncommon. A lack of tack or cohesive bonding to itself is also a well known defect of Buna S.
Vol. 36, No. 8
The nerve or poor processing is due to the presence of a higbmolecular-weight fraction associated with a considerable portion of soft, liquid, low polymers. In other words, Buna S has too broad a distribution of polymer sizes. These large moleculee are highly clasbic and, if deformed, spring back when the distortion force is removed. The presence of the liquid low polymers together with the excessive amount of stearic acid and other nonhydrocarbon constituents is also objectionable; they contribute to poor wetting of pigments and poor vulcanization which, in turn, lead, to deficient physical properties. When crude rubber is broken down by mastication the large molecules are placed under strain and, in the presence of ozone and oxygen, arc split into shorter segments. A well masticated crude rubber is made up of polymer units with a narrow range in chain length (6)sinco tho longer chains are apparently preferentially split, A considerable improvement in the processing Characteristics of Buna S would result if a narrower range of polymer sizes could be effeoted. This now range should contain a lower-molecularweight high fraction and should eliminate the lowest-molecuiarw i g h t fractions which vary from oils to a taffy-like substance. In any case, gelation of the polymer should be prevented. As ‘long as the present higher-polymer fractions remain, the removal of the lower fractions results in a dry, nervy polymer which is practically unmanageable on the mill. This paper presents data on the fractionation of a benzenesoluble type of Buna S polymer. Data are given on the effect of fractionation of Buna S on the nature of vulcanizates made from different fractions. The effect of polymer size on the chloroform extract obtained after acetone extraction of the vulcanizates is also reported. This chloroform extract is a soft pasty solid having a composition like the vulcanized hydrocarbon. It has a very low molecular weight and tends to crystallize in long fern-like needles. Its presence appears to result from the smallcr polymer 4 .O
!i W
z z
2.0
0
I-
B 0
10
Figure 9 .
20
30 40 EXPOSURE TIME IN DAYS
50
w
70
Rate of Oxidation of Bun. S Hydrocarbon
100.
VOLUME OF CHBOH
IN PERCENT
Figure 3. Relation of Alcohol Content to Precipitation of Bun. from Benzene Solution
S
Table CHaOH Added % by Vbl. 19.6
molecules which r e a t with sulfur intramolecularly but not intermolecularly, and therefore remain soluble in chloroform. A small amount of chloroform-extractable crystalline substance is pesent in vulcanized crepe rubber.
1. Fractionation of Buna S %-
-Yield, Hydrooarbon Fraction
On total hydrocarbon 40.0
On Buns 8 37.0
Description Moderately tough and elaatic, not tacky. 10.0 10.9 Somewhat elaatic, not tacky 2 20.3 9.3 10.1 Soft, plastic, stringy, not 3 21.2 tacky 8.7 9.6 Soft, taffy-like, somewhat 4 22.3 tacky 8.0 8.6 Soft, taey-like, very plaatic 5 24.5 and tacky Highly viscous,sticky liquid 6 28.5 8.0 8.6 6.3 6.8 Viscous, sticky liqufd 7 38.0 8" b.0 b.b Viscous oil 24.9 Tough and elbstic like 23.0 1A) 19.3 smoked eheeta =Supernatant liquid from fraction 7 waa evaporated to drynwa, m d raclidue repeatedly washed with hot methanol followed by one cold acetone wash to remove nonhydrocarbon aubstsncw ; reeidy wastraction 8. b Fraction 1 was redissolved in bensene. containpg antioxidant, to form a 0.b 0 solution A 60-bo methanol-benzene solution waa added gradually vigorous 'mechanical stirring to precipitate partially the higber-moleoular-weight portion. 1
....
wtri
Tabk
11. Chemical Analysis of Buna S Fractions
Fraction
Carbon,
Hydrogen,
1 2 3 4 6
88.90 88.97 88.97 89.26 89.27 88.67 88.06 87.74
10.37 10.32 10.27 10.26 10,29 10.33 10.30 10.2b
Ratlo 8.b7 8.61 8.66 8.70 8.64 8.58 8.56 8.65
88.27
10.26
8.63
No.
7 8 Whole hydro-
.
oarhonb
70
Styrenecontent
%
(465
I
C/H
Ash,
%
0.88 0.19 0.27 0.18 0.29 0.89 0.88 0.36 0.80
- iodine No.) X 100 466
Iodine styrene* %from NO. C/H I NO." 3b6.1 21.3 28.4 368.3 22.b 22.7 22.8 369.1 24.1 22.6 369.8 26.4 23.b 22.7 369.6 35b. 6 21.6 23.6 362.1 2 1 . 0 24.3 348.1 20.7 26.2 366.7 23.2 23.3
Acetone-extracted GR-S.
TIble
709
INDUSTRIAL AND ENGINEERING CHEMISTRY
August, 1944
z c
111. Polymer Length of Buna S Fractions in Comparison with Natural Rubbev
Log N+
6.09 2.54
166.2 92.1 68.4 51.2 34.8 22.1 12.0 8.0 245.1 103.6
3677 2108 1566 1172 787 bo8 275 183 5613 2372
2.23 1.95 1.93 1.84 1.70
90.0 78.7 77.7 74.1 69.2
.... ..... ... ..... ., .
1b42 1858 1340 1286 1194
cm3?ole sol 0.049 0.000527 1.41 6.99 283.1 6286 Milled 15 min., 0.221 0.00288 1.70 2.40 96.9 21b2 y o C.b Mdled 16 min., 0.476 0.00612 1.81 1.24 50.3 1117 3b0 C.b Milled 200 min., 0.313 0.00337 1.32 0.83 88.8 796 3Q0C.b Nomenclature: C grsmq per 100 cc. of solution N average number of carbon atoms in chains 9 cc. of solute Der cc. of solution log vrW. - log qrWa +Koa ' Na =- +Reo W 0.734 based on 23.4% styrene in Bun8 S used W 0.71 in case Of natural rubber Wi 0.6b2, asaurmng 30% 1,2 addition of butadiene in Bun8 8
.... .. .. ....
b 6 7
8 1-A
Whole
0
vr
(1)
0.218 0.300 0.432 0.400 0.609 0.446 0.642 2.610 0.101 0.186
0.00234 0.00322 0.00466 0.00430 0.006b5 0.00469 0.00690 0.02801 0.00109 0.00197
2.32 1.98 2.08 1.66 1.69 1.27 1.21 1.66 1.86 1.60
3.84 2.28 1.69 1.26
0.304 0.348 0.349 0.842 0.850
0.00327 0.00374 0.00376 0.00368 0.00376
1.97 1.97 1.W 1.88 1.82
0.88
0.64 0.30 0.19
4
Buna 8 ~~
Hydrocarbon Unmilled bmhoncoolroll 10min.oncoolroll 3Omin.oncoolroU 60min.oncoolroll
-----
a$ 2z W
Ni 2693 1Q89 1180 883 600 381 207 188 4228 1787
C
50
a
qr
Polymer Fraction 1 2 3 4
FRACTIONATION BY PRECIPITATION
A benzene-soluble type of Buna S waa dissolved in benzene to form a 2y0 solution. A 60-50 mixture of methanol-benzene by volume was added slowly with vigorous stirring until cloudiness appeared. All solutione were kept a t 20' C. The solution was warmed to 40' C. t o redissolve the precipitated polymer, cooled to 20' C., and allowed to stand overnight; during this time the fraction precipitated and settled. The fraction was separated by decantation and coagulation with pure alcohol. Data on the fractionation, analysis, and relative chain lengths of these fractions are given in Tables I, 11, and 111. The h t fraction is large in comparison with the others because only a small spread in the quantity of alcohol used for precipitation was required to throw a large part of the high-molecular fraction out of solution. Figure 3 shows the effect of alcohol on the precipitation of Buna S hydrocarbon from benzene solution. Each point on the curve represents a fraction of precipitated hydrocarbon which was removed by decantation of the supernatant liquid followed by the addition of more pure alcohol to harden the coagulum eo it could be removed. The fractions were dried under vacuum at 50' C. in a stream of lamp nitrogen and weighed.
....
Kea- 0.0m
*b S.Mill k f t e m p and Peten (6). roll temperature. Table IV. Viscosity of Benzene Solutions of Extracts 1, 9 , and 3 Extract Concd. CHrOH Extraction % -L No. in CaHa, ' 7 Time, Weeka Extracted C 1 20.2 1 8.0 19.2 2 2 16.0 O:bi2 20.0 1 3 16.b 0.372
3
g z
3d
Y
d
5 0
POLYMER FRACTION IN PERCENT
Figure 4.
Molecular Welght Distribution of Buna
S
I n Figure 4 average chain length is plotted against percentage of each fraction. It must be realized that each fraction still contains polymer units of varying sizes, as one fractionation is only partially effective in separating the different sized specimens present. I n no case can one expect to approach perfection in any number of repeated fractionations. If Buna S with narrow polymer distribution could be prepared with average chain lengths in the range of fractions 2 or 3 (Table 111), a much needed improvement in processing would result. Better control of the polymerination reaction is needed to accomplish this. The physical properties of pigment-loaded stocks probably would not be greatly improved by such a change for reasons already stated although hysteresis and heat build-up might be improved.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
710
voi. 36. N ~ a.
The expression of polymer size by chain atom lengths (Table 111) is desirable; it places polymers of questionable chain linearity on a more comparable basis with those whose chains are 6 more certain to be linear. Since averakw polymer Iength deter... mines viscosity, the argument for using t>histerm in comparing ... lOb'.bO 1'0.00 polymers is apparent. 5.00 EFFECT OF REMOVING LOW POLYMERS 0.30 0.75 Two hundred grams of Buna S were shected out to a thickness 0.10 of about 0.002 inch on a laboratory mill, cut into 1-inch strip8, Lb./Sq. In. and dropped separately into a 5-liter round-bottom flask conComRate of ModElongapound Cure Extension, ulus at Tensile tion a t Hardness. taining 4 liters of a 20.2% by volume mixture of methanol in No. Min. (" 'F.) In./Min. 300% strength Break, % Shore A benzene. The flask and its contents were placed in the dark and 1 45 280) 22 50 875 1125 25 kept at room temperature (18-23" C ) . On standing, the Bum 2 60 i280, 22 1350 3150 575 67 S slowly changed into a liquid, swollcn gel which collected on the 3 45 (280) 22 250 670 520 37 610 22 180 750 bottom of the flask. After one week the extract was decanted, and 650 22 260 920 610 12 290 760 4 liters of fresh solvent were added. This time the concentration 650 12 210 820 of methanol in benzene was reduced to 19.2Oj,in order to hasten 4 00 (280) 22 2400 3380 350 66 the extraction process. From time to time during the next two 3070 320 . 22 2370 22 2360 3380 370 weeks the mixture was thoroughly agitated, and a t the end of this 370 12 2470 3350 12 2340 8040 350 time the extract was replaced with fresh solvent. The concentra5 40 (280) 22 50 ... ... 26 tion of methanol in benzene was then 20% by volume. During 900 22 50 1400 the following week the mixture was agitated as before. Finally, 900 22 50 1300 12 50 1720 950 the extract was decanted, 1.5% Neocone D was added to the 12 50 1460 970 gelatinous mass, and it was coagulated with pure methanol. The coagulum was pressed and placed in a vacuum oven a t 40' C. Table VI. Data o n Acetone and Chloroform Extractions of until free of solvent. Its weight was 116 grams or .58% of the Compounds 1 to 5 original material. ~~~t~~~ Chloroform Ext., % Aliquots from the various extracts were dried in a weighed Compound No. Ext., % 0-24 hr. 24-48 hr. Total, % crystallizing dish, weighed, and then dissolved in 1.00ml. of pure 1 (regular Buns S urn) 10.33 Continuously 16.4 benzene. The viscosity of the benzene solutions was determined 2 (regular Buna 6-%tatex) 7.36 3.13 0.81 3.94 3 (hinh-mol.-wt. fraotion Buna 7.01 4.44 1.67 6.11 . ._ in a n Ostwald viscometer at 25" C. The results are given in S gum) 0.83 1.65 4 (high-mol.-wt. fraotion Buns 4.21 0.82 Table IV. S-Statex) The extracted Buna S was very tough, had little plasticity, 1.14 6.68 5 (natural rubber gum) 6.04 5.54 and sheeted out on the laboratory mill rolls with difficulty. It ComChloroform Extract pound resembled closely the higher-molecular-weight fractions of Buna No. Aoetone Extract 0-24 hr. 24-48 hr. S rubber which had been separated by fractional precipitation. 1 Brown and oily Colorless, viscous, tacky It was soluble to the extent of 99% in benzene, and the log 2 Brown, vipoua. partly Colorless, tacky, VIS- Colorless, glassy crystalline COUS qJC value for this solution was 1.13. The average log s,/C value 3 Brown viscoua, partly Colorless, tacky, vis- Colorless, glassy orysialline cow for the highest-molecular-weight fractions of Buna S, which Colorless, glassy 4 Brown, viscous. partly Colorless, viscous represent 60% by weight of the polymer, was found to be 1.34. crystalline 3 Yellowish. partly crys- Colorless, partly cry8- Colorless, glassy The lower log v,/C value of the former probably resulted talline talline from breakdown during the sheetirrg-out operation. It is a h probable. that more of the low-molecular-weight eopolyrnor Table VII. Effect of Accelerators on Physical Properties molecules remained in the extracted material than in the case Compound No. 6 7 8 where precipitation procedure was employed. Table V gives the composition of compounds prepared with extracted Buna S, two similar compounds made with the original Buna S, and one made with natural rubber. The two regular Buna S cbmpounds (1 and 2) were mixed according to the War -Production Board mixing schedule. The two high-molecular113.5 113.5 113.5 weight Buna S compounds (3 and 4) and natural rubber comAir-Aged 7 Days at 100LCL Original Teats pound 5 were mixed on the regular 12-inch laboratory mill with Cure. Lb./sq. in. ElongaLb./sq. in. Elongaspecial guides to reduce the working length of the mill rolls to 5 Min. at Modulus Tonsile tion at hlodulus Tensile tion a t 292O F. at 300% strength break, % at 300% strength break, 7% inches. Compound 6 (Captax) The mixed compounds were permitted to stand overnight, 220 920 ... ... 26 20 refined twice through tight mill rolls, and vulcanized in a 0.075180 650 ... ... 50 30 150 510 ... ... 70 40 inch-thick mold. There was just enough compound to make one ... 180 610 ... 50 70 ... 180 420 ... 60 70 6 X 6 inch sheet cure. After vulcanization the cured sheets were ... 167 377 ... 120 I20 placed in the aeration rack in a constant temperature-humidity room for 24 hours. They were then removed, and five tensile 620 40 130 20 None 60 610 None 210 30 test specimens were cut from each sheet with a razor-sharp 250 100 650 None 40 290 530 100 50 17 '/,-inch dumbbell die. The results of tensile strength tests on 180 360 23 140 60 optimum cures are shown in Table V. The rate of extension of 220 295 220 60 120 the specimens was changed from 22 to 12 inches per minute. The Compound 8 (808) 20 575 715 230 340 370 96 resuIts show that this had no effect on tensile strength values. 30 210 690 650 340 360 90 Table V. Tensile Strength Data on Optimum Cures Compound No. 1 2 3 4 Regular Buna S 100.00 100.00 High-mol.-wt. Buna S ... .. IOO.'OO ldd.00 . . . 60.00 Smoked sheet Statex black 60.00 Zinc oxide X X 7 2 i6.00 10.00 iO.'Oo 10.00 Stearic acid 2.00 2.00 5.00 5.00 2.00 0.50 2.00 0.50 Sulfur 0.75 0.75 0.76 Accelerator 808 0.75 TMTMS 0.10 0.10 0.10 0.10
.
I
40 60 60 120
116 120 110 150
520 450 610 420
620 575 610 600
226 200 220 220
285 380 400 290
360 420 420 350
ACETONE AND CHLOROFORM EXTRACTION
One-gram samples of the optimum cured compounds (1 to 6 ) were diced and extracted with acetone in the regular extraction
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1944 Table VIII.
cornound $0.
6 (Captax) 7
(DPG)
Data on Acetone and Chloroform Extraction of Compounds 6, 7, and 8 Cure ~ iAt ~ 292* F. 20 60 120
Acetone . Ext., 5% Unaged Aged 14.89 ... 12.77 ... 11.98
20
SO
120
.
8 (808)
20
SO
120
. I .
COMPOUND 12
iioc
Chloroform Exl.. % Unaged Aged 13.90 6.04 .. 4.34 ..
..
14.03 13.80 11.41
9.03 7.60 6.43
26.81 19.47 7.29
10.66 4.56 1.86
11.03 10.78 10.37
7.36 7.00 6.66
7.91 6.94 4.84
2.39 2.64 2.41
71 1
IOOC
9W
-z
6
E u)
800 COMPOUND 13 700
0
Table IX.
Chemical Analysis of Chloroform Extract of DPG Stock 7, Cured 50 Minuter at 299' F. Styrene Ext., from Hr.of C H S Ash, Iodine C/H Ratio I/No. % % No. % Extn. $ $ 18 87.57 10.31 1.26 0.10 360.3 8.49 22.2 11.8 ~ - 4 6 87.45 9.87 1.31 0.27 344.7 8.84 23.3 4.14
3
2 z
E L
6oa
500
si 400
Table
w z
X. Viscosity of Benzene Solutions of Extracts 6, 7, and 8
Extraat
Cure, Min. 20 60 20
Concn., Q./lOO MI. 1.264 2.012 0.769
Log vr
300
E
1.61 2.19 1.15
0.143
0.169 0.079
aoo COMPOUND
IO0
apparatus for 20 hours. They were then chloroform-extractedfor 8 total time of 48 hours, the chloroform being replaced at the end of 24 hours. Results and physical properties are given in Table VI. During the chloroform extractions compounds 2 and 4 (Statex black) swelled very little, and compounds 1 and 3 (pure gum) swelled considerably rand to about the same extent. Compound 5 (natural rubber) swelled to about the same extent as did the Buna S g u m compounds. The acetone and chloroform extracts of both pure gum and Stetex black high-molecular-weight Buna S compounds were red u d considerably. It is of intereat to note the lower chloroform extract of those stocks containing Statex black. It appears poseible that carbon black tends t o bond the smaller Buna S molecules. The physical properties of the extracts are also described. Eemp and Peters (4) showed that crepe rubber contains a small percentage of low-molecular polymer which, combined with the low sulfur content in compound 5, probably accounts for ita substantial chloroform extract. EFFECT O F DIFFERENT ACCELERATORS
me three compounds in Table VI1 were prepared to teat the &e& of acid and alkaline type accelerators on the vulcanization of Buns 9. The mixing schedule was slightly modified in that Bune S was allowed to work only 1.5 minutes before the compounding ingredients were added, as compared with 10 minutes recommended by the WPB schedule. Table VI1 gives the physiacrl properties of the cured sheets after conditioning a t 75" F. and 46% relative humidity and of the same cure8 after aging for 7 days in the hot air oven a t 100' C. (212' F.). The acetone and chloroform extraction data on these compounds are given in Teblea VI11 and IX. Compound 7 swelled most when ohloroformsxtracted, compound 6 was intermediate, and compound 8 swelled least. The 7-day-aged samples swelled only about half a8 much as did the unaged samples. The acetone extract of each unaged compound waa brown and viscous; the chloroform extracts were colorless Viscous liquids which, on standing, crystallized into radial monoclinic crystals arranged into a fern-shaped pattern.
0
cc
200
400 600 800 1000 STRAIN I N PERCENT ELONGATION
Figure 5. Stresr.Sbain Curves of Regular and Fractionated Buna S Pure-Gum Compounds, Cured PO Minuter at 460' F.
The relative viscosities and log qJC value8 of benzene solutions of the chloroform extracts of each of the three compounds are given in Table X. The 80Saccelerated Buna S compound 8 possessed the best physical properties of the three studied. Its acetone extract was lower than that of the other two stocks. With an increase in time of cure, the chloroform extracts decreased. After air-oven aging at 100" C. for 7 days, all the stocks had lower acetone and chloroform extracts. The tensile strength a t optimum cure of the DPG stock remained unchanged. Under the same conditions compound 8 decreased in tensile strength and elongation a t break but showed a substantial increase in modulus.
Table XI.
Formulas of Tread Com oundr 9, 10, 11, and of Pure Gum CompoundS lP,13,14
Compound No. Buna S Bardol Zino oxide XX-4 Kosmobile 77 Sulfur Captat Stearia acid Cure Compound No. Buna S Zinc oxide X X 7 2 Steario acid Sulfur
808 TMTMS Cure
Q
9
10
12 100 10 2
ia
14
100.
100s
0.76 0.76 0.10
10 6
0.7b 0.76
0.10 20 minutea at 260" E.
R i d mol. weight. b Low mol. weight.
I1
10 0 0.76 0.7b 0.10
Vd. 38, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
712
Table XII. Compound NO. 9
100%
... 240
200%
478 715 10 11 110 270 Dry extract partly crystalline.
Modulus, Lb./Sq. In. 300% 400% 500% 990 1500 2050 1500 2475 3585 815 1125 550
Table XIII. Compound
N ~ . 100%
..
200%
...
300%
12 40 60 220 465 13 14 a Dry extract partly crystalline.
..
...
...
Physical Properties of Tread Compounds 9, 10, and 11
400% 65 750
...
800% 2615
700%
i2o
1730
...
I . .
Tensile Strength, Lb./Sq.In. 2870 3770 1900
Elongation,
%
..
815 815 760
Chloraform%* Ext.,
Acetone% Ext.,
Shore A Hardnose
i.*&
6 . YO
57 47
6.56
8.73
Physical Properties of Pure Gum Compounds 12, 13, and 14
Modulua, Lb./Sq. In. No% 600% 700% 100
... ...
800%
900%
Tensile Strength
1000% LbJSq.1;.
170
250
400
620
lo00
1170 825
25
60
105
165
265
a68
...
...
...
When the chloroform extracts of the three stocks were dissolved in benzene and their relative viscosities determined, the chloroform extract of the DPG-accelerated stock had the highest log qr/C value and the 808-accelerated stock had the lowest. The degree of swelling of the compounds in hot chloroform was inversely proportional to thc time of cure and aging of Buna S.
...
...
As a further check on the effect of molecular weight on the vulcanization of Buna S, 124 grains of the high-molecular-weight portion of Buna S were prepared as follows, using benzene soluble Buna S from the same lot employed in the previous fractionation: Five W g r a m portions of Buna S were dissolved in 2500 cc. of C.P. thiophene-free benzene in separate 4-liter Erlenmeyer flasks. The solution was heated to 40' C., and 1525 cc. of a 50-50 bensenemethanol solution were slowly added with vigorous stirring. This resulted in a methanol concentration of 18.95% which was sufficient to precipitate the high-molecular-weight fraction of Buna S.
-.
1025 425 1075
Acetone Ext.9 %
Ex$.,%"
10.23 5.23 8,14
10.18 2.46 16.89
40 17
Chlorw form
Table XIV. Data on Pure Gum Crepe Rubber Stocks 16 17 Compound No. 16 100
*.... .. ..
Sulfur Neozone A Curing temp.,O F. 10-minute cure Tensile, lb./s in. Elongation Modulus ai 500%, lb./sq. in. 20-minute cure Tensile lb./s .in. Elongaiion Modulua ad d o % , lb./sq. in. 30-minute cure Tensile, lbhf in. Elongation Modulus ad 00%. lb./sq. in. 40-minute cure Tensile, lb./s in. Elongation Modulus ai SOO%, lb./sq. in. Hardness, Bhore A (20 min.) Acetoneext % Chloroform &t,, %a a Chloroform extracta were partly
2. 4
2.
0
0.2
0.4
0.6
... 100 ,.10.
18
... ...
. I .
ibb
... ...
ibi,
2 3 0.76 0.10 1 287
10 2 3 0.76 0.10 1 287
2 3 0.76 0.10 1 287
-
5186 775 915
4775 790 760
393s 830 516
4070 735 810
4830 685 1465
4770 716 1280
3930 670 1260
4280
4175
4340 660 1680
... ...
...
... ... ...
... ...
... ... ...
10
%%MS
HIGH- AND LOW-MOLECULAR WEIGHT FRACTIONS
Elonaa- Shore tion, A Hard% neas
660
1S50
(OS0 4160 650 640 1680 1640 43 48 6.28 5.86 0 . 7 6 0.77 orysystslline,
0.0
1.0
t.2
10
2 3 0.711 0.10 1 287
706
1260
... 38
4.a
6.28 1.15
1.4
1.6
6.87 0.90
1,e
LOG bf. _ I _
C
Figure 6. Stress-Strain Curves of Re ular and Fractionrted Bunr Tread Compounds, Cured 5 8 Minutes r t 299' F.
S
Figure 7.
Tensile Strength of Tread Compounds vs. Molecular Weight Index of Starting Bun. S
Augud,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1944
Table XV.
Data on Crepe Rubber Tread Stocks
Compound No.
19
20
=mobile
77 -0 oxide XX72 Stmulo aoid Sdw
Curing temp., * F. 2O-minute oure Tensile,.lb./~in. Elongation Modulus ai %, Ib./sq. IO-minute aura Tensile lb./a in. Elonpalion Modulus ai g o % , lb./nq. (IO-minute cure Twile,,lb./s in. Elongation Modulw ai 300%, lb./aq. SO-minub a r e Tensile Ib./s .in. mongaiion Modulus ai 380%, lb./nq.
8
4
in.
4280 666 1210 4910
in.
660
2000
2936 so0 1360 3646 606
1840
Table XVI, Data on Rubber-Gutta-Percho Stocks Compound No. Q u t k BR-19 Crepe rubber Neonone A Sulfur P n o pxida (Kadox B.L.) Steano acid 808 TMTMS Cornpound No. 21
Cureat 287' F., Unoured
22
Unoured
Min.
in.
2000
8400 466 2030
in.
4600 620 2180
3840 420 2120
66 4.7 0.84
66 4,s 1.22
Hudnc9a. Shore A (40-min.) Awtoneext % Chloroform 'Axe.. %
666
16 20
23
Across
10 16
20 24
IJnarrr6d $0
16
pireotion of Qrnin With Across With Across With Aorosa With Across With
10
4730
713
&%. 4476 4000 4040 3676 3985 4600 4366 4886 a970
4110
With Aorcm With
1 m 620
With With With With With With F.
~6 8076
2400 2510
3600 8640 4060 3896 3156 2980
100% 1800
426 400
1800 1600
506 640 640 640
480
616 730 800 720 726 676 680
1800
760 876 700 700 600 626
76 76 76 76
110 100
&)
140 676
1
686
606
600 770 770 785 710 680
670
360 276 266 230 200 170
3 10 2 0.76 0.10
Modulua, Lb./Sq. In. 200% 300% 400% 2200 3100 2200 3100
... ...
2136 1660 1000 1160
2900 2600 1976
lo00 826
1800
960 950
176 125 200
200 a26 B26
aim
3800
isbb
1776
3050 3060 3000 3100 3900
300 200 800 300 400 871
476 326 660 600 700 626
1960 1960
500%
... ... ... ...
iddo
3876 3860
... ...
1025 626
1200 1060
1560 1400
... 260
... ... ... ... iik 360 276 300 260
600 476 476 476
400 860 320 270 260 200
460 480 416 336 320 266
780
. . I
200 126 126 76
24 60 50 1
3 10 2 0.75 0.10
3 10 2 0.76 0.10
Elonpation, % 360 410
23 100
...
... 1
e..
Tennile.
4876 4600 47a~ 4910 4800 4640
With Across
22
100
...1 ... ... ... ...
with Aorou With Acrosa With Across
Aoras
The flasks were set aside to cool slowly to room temperature, which resulted in precipitation. The 24 4' 6O lkeks were placed in water at 20" C. for several 8' hours, and the supernatant liquid was then deloa 120 canted from the precipitate. A 0.4-gram portion 14' of Neoaone D was added to the gelatinous precip 0 Cured at 274O itate in each flask, and the coagulum w w solidified by adding more 50-50 benzene-alcohol and finally pure alcohol. The high-polymer fm- - tion waa dried under an &m. vacuum in a stream of lamp nitrogen. The yield waa 4b.0% of the original Buna 8. To the decantate was added pure methanol to a 40.13y0alcohol concentration. After the temperature was brought to 20' C. as before, the supernatant liquid waa decanted from the coagulum. The coagulum was dried aa before, and the yield was 40.1 % of a soft, tacky, plastic product. The log qr/C value of the high fraction was 1.86, of the low fraction 0.50, and of the whole Buna S 0.97. Compounding waa carried out under carefully standardized oonditions on a smell Thropp mill. The compositions of the t r d stocks (9, 10, and 11) are given in Table XI; results of physical tests are listed in Table XII. Additional stearic acid wan added to each fraction to replace that removed by fractionation. Exhaustive acetone and chloroform extraction waa made to ehow the lack of cmsa linking of the low polymer8 during vul&ration, as indicated by the amount of polymer extractable with chloroform. In Table XI and XI11 the pure gum vulcani-
21 100
200 210 160
e..
810
630
660 680 130
975 1100 1060
1300 1230 1210 1160
1120 830
STRAIN IN RERCENT ELONGATION
MILLING VJMC 8N MINUTES
Fiswe 8. Effect of Milling Time at 95. F. on Vircoaity Index ond Molcular Weight of Pale Crepe
Figure 9. Stress-Strain Curves of Pale Cepe Pun-Gum Compoundr Made with R u b k d Different Molecular Weishti
INDUSTRIAL AND ENGINEERING CHEMISTRY
714
Vol. 36, No. 8
EFFECT OF MILLING CREPE RUBBER
High-, medium-, and low-molecular-weight rubber were prepared by milling pale crepe 3 minutes at 212' F., 10 minutes at 95" F., and 200 minutes at 95" F., respectively. The average molecular weights determined by the viscosity procedure ( 9 ) were 62,000, 32,000,and 20,000,respectively. Compounds were also prepared by using a 50-50mixture of high- and low-molecular-weight rubber. Effect of milling time on viscosity index and molecular weight of pale crepe is shown in Figure 8. The data on pure gum stocks are given in Table XIV and on tread stocks in Table XV; they are plotted in Figures 9, 10,and 11. The effect of physical properties on reducing the molecular weight of crepe by excessive milling is not so pronounced M is shown by the high and low fractions of Buna S. The same ie true for tread stocks 19 and 20. The low-chloroform extracta found in the natural rubber compounds along with their superior physical properties is evidence that natural rubber is more effectively cross-linked than Buna S during vulcanization.
Table XVII. Compound No. Balata hydrocarbon Sulfur Zinc butyl xanthate Curing temp.. F.
STRAIN IN PERCENT ELONGATION
Figure 10. StrescStrain Curves of Pale Crepe Tread Compounds, Cured 40 Minutes at 287' F.
sates (12,13, and 14) are compared, using the original Buna S and its high- and low-molecular-weight fractions. These results show that the presence of the low-molecular-weight portion of Buna S decreases modulus and tensile strength and increases chloroform extract. The amount of chloroform extract based on the polymer is some'what less when carbon black is present. This may be due in part to the higher content of sulfur present in the tread stock. These data are plotted in Figures 5 , 6,and 7.
MOLECULAR WEIGHT
Figure 11.
Tensile Strength of Pale Crepe Pure-Gum Compounds vs. Molecular Weight
D a b on Balata Compound,
287
26 96.5 2.8 1.0 287
0.5-hour cure Tensile, lb./sq. in Elongation, % Permanent set
3920 480 245
39413 446 250
1-hour cure Tensile, lb./8q. in. Elongation, yo Permanent aet
3465 615 225
3615
2-hour cure Tensile, lb./eq. in Elongation, % Permanent set
3125 670 156
3330 470 230
3-hour cure Tensile lb./s Elongalion Permanent'set
""
2440 635 26
338.5 520 206
17-hour cure Tensile lb /E Elongation', in' Permanent set
... ... ..,
3
2'
26 94
6 1
440
24b
1685
650
10
EFFECT OF MIXING RUBBER AND GUITA-PERCHA
By mixing crepe rubber and leaf gutta-percha hydrocarbon BR-19 from the Tjipetir, Java, plantation i t was thought that crystallization of each hydrorarbon would be inhibited. Rubber hydrocarbon having the cis c-onfiguration would not so readily form crystals in the presence of gutta-percha which has the trans configuration. The object was not to prevent crystallization of the gutta and rubber entirely but to reduce i t considerably. The compositions and test data are given in Table XVI. The sulfuraccelerator combination was selected after the preparation and evaluation of numerous preliminary compounds. The gutta was presoftened in water a t 195' F., and the mill rolls were maintained at this temperature. Compound 24 was prepared by blending equal weights of compounds 22 and 23. Compound 21 served a s blank. The best cures (plotted in Figure 12) show that the mixing of cis and trans isomers results in a somewhat lowered tensile strength which is very sensitive to overcure. It is not certain, therefore, whether this lowering is a structural effect or due to failure to find best cure. The importance of crystallization as a factor in contributing to strength is indicated in the greatly reduced strength shown by compound 24 in the uncured state. In this case the gutta hydrocarbon loses over 70% of its strength when mixed with an equal amount of lightly milled rubber. When vulcanized, this mixture
August, 1944
INDUSTRIAL AND ENGINEERING
,
25 50 75 RUBBER CONTENT IN PERCENT
Figure 12.
100
Effect of Mixing cis and trans Polyprene lsomen on Tensile Properties
CHEMISTRY
715
XVII shows conversion of balata to an elastomer as a result of curing over a long period. The corrdsponding reduction in tensile strength is a further indication’that the effect is the result of reducing the degree of crystallization. The nature and extent of cross linking is also an important factor in contributing to tensile strength and elastic quality. Although only a few cross links are effective in eliminating plastic flow, tensile strength is probably not enhanced greatly by cross links unless they are sufficient in number and properly placed in the three-dimensional gel structure. In other words, the strength of a structure is determined by that of its weakest member. LITERATURE CITED
increases in strength to about 90% of that of the original vulcanized whole gutta or whole rubber since the cross linking of the rubber has served to prevent its flow. The vulcanized gutta loses most of its strength when heated a t 140-2oOD F. A t the higher temperatures it becomes very short and tender, and is similar to vulcanized pure gum Buna S in strength and general appearance. Vulcanized pure gum rubber, on the other hand, retains a large part of its strength a t these elevated temperatures. Since vulcanized rubber crystallizes on stretching, the importance of crystal reinforcement on the strength of polymers is indicated. Gutta or balata hydrocarbons change permanently to a rubber-like vulcanizate when cured under certain conditions.
(1) Alekseeva, E. N., and Belitzkaya, R. M., J. Urn. C h m . (U.8 . 8. R.),11, No. 4, 358 (1941): Rubber C h m . Tsch., 15, 693 t1942). (2) Kemp, A. R., Bishop, W. S., and Lasselle, P. A., IND. ENQ. CHBM., 23, 1444 (1931); Rubber C h m . Tech., 5,51 (1932). (3) Kemp, A. R.,and Peters, H., IND.ENQ.CEDM.,33, 1263 (1941); Rubber Chem. Ted*., 15, 60 (1942). (4) Kemp and Peters, IND.ENC).CHBM.,33, 1391 (1942); Rubber C h m . Tech., 15, 1 (1942). (5) Kemp and Peters, IND.ENQ.CHBM.,35, 1108 (1943). (0) Kemp and Peters, J. Phy8. Chem., 43, 1063 (1939); Rubbur Chem. Tech., 13, 11 (1980). Presented before the spring meeting of the Division of Rubber Chemistry , A m B X O A N C A ~ M I C Socr&TY, AL in New York, N. Y.,1944.
S. D. GEHMAN The Goodyear Tire & Rubber Company, Akron, O h i o Results of measurements on gum stocks in compression, particularly those dealing with the effect of the shape of test piece on stiffness, furnish an explanation of the stiffening effect of pigment loadings. In a loaded stock, films of rubber exist between the pigment particles. The films are attached to the particles, and their thickness i s of the same order ol magnitude as the particle diameter. Under these Circumstances it i s to be expected that the stiffness of the rubber will be enhanced and the rubber will b e working under conditions analogous to those existing when a test piece in compression has a high shape factor-Le., high ratio of load area to free area. The idea i s applied t o calculate the relative effect of the volume loading of carbon blacks on stiffness in compression. Agreement with experimental curves i s good. The theory explains the systematic change i n shape of load-compression curves as pigment loading is increased. I t accounts qualitatively for the divergence between the dynamic and static modulus for stocks with carbon black loadings. Essentially, the theory seeks t o extend the compression results on large-scale test pieces to the microstructure i n the loaded stock.
HE use of reinforcing pigments in rubber, especially in synthetic rubber, is important in securingdesirable physical properties. The earlier literature on the subject has been reviewed by Shepard, Street, and Park (16). Several theories were advanced to explain the effectiveness of reinforcing pigments in rubber. Some of these emphasized the importance of particle size and shape, and others, the chemical composition and surfam activity. The eleotron microscope has now furnished more reliable information on the particle size and shape of pigments, especially of carbon blacks, than was available in the earlier work (I,18,18). The carbon blacks used in rubber compounding are essentially spherical; the mean diameter varies from 28 to 274 mp for different types. Systematic studies have been carried out to show the influence of particle size and surface area on various physical testing indices used to evaluate rubber compounds (8, 19, 19, M). The x-ray structure of rubber stocks containing carbon black has been investigated (a. Beveral articles dealing with the theory of pigment reinforcement have Itppeared (14.18)
7
This additional work has not clarified the original concepts advanced to explain filler reinforcement, but the research of Wiegand shows beyond doubt that particle size is the predominating factor for explaining differences in the properties of rubber compounded with various blacks. The present paper will ettempt to explain some of the effects of pigment reinforcement of rubber, particularly increased stiffnew, by simple mechanical considerations which may be regarded as extensions or extrapolations of the results of measurements on rubber in compression. EFFECT OF SHAPE ON SnFFNESS OF COMPRESSED RUBBER
The analysis of what happens when a piece of rubber is compressed between parallel plates has disclosed a complicated set of circumstances. Because of the practical interest in such deformations of rubber, the work haa been thorough (9, IO, li, 26) end the principles are well understood. It has been found that, if the bearing surfaces are lubricated, the oompression modulus
