April, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY EXTENDED COLD AGING
Various reports have indicated that not all synthetics behave similarly when conditioned at low temperatures for long periods. The tensiometer has confirmed the belief that, a t least as far as Perbunan and neoprene bases are concerned, resistances to cold are quite dissimilar. Curves (Figure 5 ) were obtained for plasticized Perbunan and Neoprene GN according to the standard tensiometer technique, consuming approximately 3 hours. The same samples were then conditioned 40 hours a t -40" F., and the stiffness was measured a t this temperature. Perbunan lost little of its original flexibility, whereas neoprene stiffened to more
A n investigation was undertaken to determine some of the basic information required to aid in the application of Buna S as insulation on wires and cables. The results presented here include data on chemical analysis, dielectric characteristics, moisture absorption, vulcanization rates, effect of inorganic fillers and organic extenders, brittleness temperatures as related to styrene content, accelerated aging in air and oxygen, and power consumption and temperatures attained during Banbury mixing. Data are given on the coefficient of vulcanization as derived from combined sulfur determinations and on the relation between the development of tensile properties and the rate of sulfur combination.
LTHOUGH the acute rubber situation manifests itself to the,public primarily in terms of tire shortage, an extremely important use for rubber is insulation on wires and cables. Rubber is ideally suited for this purpose because of its resistance to moisture, excellent electrical characteristics, low brittleness temperature, flexibility, ease of application, and toughness. Little information has been made ,available with regard to the application of butadiene-styrene copolymers to wires and cables. I n 1940 Roelig (7) reported test data on the application of Buna S, Buna SS, and Perbunan to wires and cables. He found the resistivity of natural rubber and its compounds to be higher than that of Buna S and Buna SS, and the change in resistivity with temperature to be less for natural rubber. Tensile properties of natural rubber insulating compounds were superior originally, but the Buna compounds were considerably more resistant to deterioration on hot air aging. Roelig also reported comparative data on moisture absorption and permeability and on heat deformation tests. He indicated a preference for Buna SS oontaining about 37% styrene over Buna S for wire and cable insulation. The studies presented here include the results of analyses of butadiene-styrene copolymers and their relation to moisture absorption and electrical stability together with studies on vulcanization, physical properties, brittleness temperature, and aging characteristics. I n most cases comparisons are made with natural rubber and natural rubber compounds.
A
ANALYSIS OF BUTADIENE-STYRENE COPOLYMERS Samples of nine butadiene-styrene copolymers from a number of manufacturers were analyzed to determine the quantities of constituents present which might contribute to moisture absorption and affect adversely the electrical characteristics of compounds in which they were used. Water extract, fatty acid, soap, ash content, and the conductivity of the water extract of the ash were studied. The water extract was determined by the method of van Rossem, van de Leur, and Dekker (18). About 3 grams of each of the copolymer samples were cut into small particles and digested for 6 hours in 100 ml. of distilled water just below the boiling point. The copolymer particles were then filtered off and 1
Present Address, The Whitney Blake Company, New Haven, Conn.
361
than 400% of its original flexibility. These curves were made with the samples immersed in air. We have not at this time conducted a more comprehensive investigation of the effect of different low temperatures or different aging periods, nor have we conditioned samples immersed in gasolines. Evidence indicates, however, that those testing methods designed to measure freeze resistance and low-temperature flexibility of various synthetic stocks must be capable of estimating the effect of prolonged cold aging. PRISENTEDbefore the fall meeting of the Division of Rubber Chemistry, SOCIETY, New York, N. Y. 1943. AMERICAN CHBMICAL
A. R. KEMP, J. H. INGMANSONl, J. B. HOWARD V. T. WALLDER Bell Telephone Laboratories, Inc., Murray Hill,
N. J.
redigested for 2 hours with 50 ml. of distilled water. The filtrates were combined and evaporated to dryness, and the residue was heated for 1 hour a t 100" C., cooled in a desiccator, and weighed to give the amount of water extract present. Fatty acid and soap were determined in accordance with the method recommended by WPB (12). The ash content was determined on 2-gram samples in accordance with A.S.T.M. method D-297-41T (1). The total ash obtained from each sample was used for conductivity determinations. The ash was washed into a flask with 100 ml. of hot distilled water, the flask placed in a boiling water bath, and a stirrer inserted. Each mixture was stirred vigorously for exactly 5 minutes and then vacuum-filtered on a Buchner funnel through No. 42 Whatman filter paper, which had been washed previously with two 100-ml. portions of boiling distilled water. A suction bell jar was used in the filtration, and the filtrates were collected in 125-ml. tall-form Pyrex beakers. The beakers were closed with rubber stoppers coveted with aluminum foil and placed in a water thermostat at 25" C. When thermal equilibrium had been. established, a Leeds & Northrup No. 4920 dip-type conductivity cell was inserted, and the resistance measured on the most sensitive scale of a Leeds & Northrup No. 4960 conductivity bridge. The analytical results are given in Table I. These data show marked variation in all constituents for which tests were made. There was no apparent significance in these variations except in the cases of water extract and conductivity of water extract of ash, both of which appeared to be related to electrical stability as discussed later. ELECTRICAL PROPERTIES OF UNVULCANIZED BUNA S
Small quantities of three typical Buna S samples were milled for 5 minutes on cold rolls and then molded into smooth sheets about 0.075 inch thick between sheets of Du Pont No. 600 cellophane2 in a steel mold. The molding temperature was 134.5" C.
* Recent work with metal foils has indicated that the values reported here may be high due to impurities present in the cellophane, but since any error so introduced is common to all the samples, the comparisons made are valid. TABLE I. ANALYSISOF COPOLYMERS ButadieneStyrene Copolysler
Water Ext.,
Fatty
Ash,
Acidal % so%* % % 3.31 0.17 2.30 0.63 0.13 0.56 1.59 0 0.11 1.52 0.63 1.38 0.33 0.07 0.50 0.49 0.82 3.96 0.08 0 1.10 4.98 0.12 .O 4.18 0.31 0.69 0 0.46 0.04 0.90 0.41 0.06 5.07 0 0.41 a Caloulated as etearic acid. b Factvy-washed eamples. polymers.
Conductivity of Water ~ x t ' . of Ash, Mho/ Cm. X 10-6 16.0 7.6 9.8 6.5 7.8 25.0 27.0 9.7 8.I C High-styrene
362
Vol. 36, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE11. ELECTRICAL CHARACTERISTICS OF BUNA S COMPARED WITH f h l O R E D SHEETS, DRYAND E\' T
Buna 9 . 1 3
Days in Water 0 7 0
Buna S, C
0
Buna S. A
7
Smoked sheets (milled)
7 0 7
% Wt. Gain 0
1.91 0 6.46 0 2.62 0 2.10
Dielectric Constant 2.39 2.59 2.49 2.92 2.50 2.68 2.45 2.84
SI?.
% Power
Resistivity, Ohm-Cm. 1 . 1 x 1014 8.2 X 1012 1.5 X 1 0 1 6 2.1 X 101: 6.8 X 1014 1.7 X 1014 > 5 X 1016 > 5 X lois
Factor 0.11
0.35 0.23 0.33 0.14 0.27 0.35 4.19
TABLE 111. COXPARISON OF ELECTRICAL CHARACTERISTICS OF VULCANIZEDBUNAS GUN STOCKS AND A NATURAL RUBBER GUMCOMPOUND Elastomer Buna 5, A Buna 8, B Pale crepe
Days in Water 0 7 0 7 0 7
%Et.
0 0.92 0 1.94 0 1.42
Dielectric Constant 2.72 2.78 2.72 2.95 2.68 2.94
% Power Factor 0.16 0.41 0.17 0.89 0.22 0.26
Res%vity, Ohm-Cm. 3.84 X 1014 1.22 X lo" 8.05 X 1014 2.00 x 1014 > 1 x 1016 > 1 X 1016
rubber is diminishing rapidly, and reclairriad rubber does not meet the desired low-capacitance requirements because of the carbon black and m i n e d filler content. I n formulating low-capacitance insulating compounds, the quantity of mineral filler which may be added is limited since dielectric constant increases a t a more or less rapid rate depending on the nature of the filler and in direct proportion to the volume of filler added. I n the case of rubber-insulated conductors for use in multiconductor cables, it is usually desirable that the insulation be sufficiently stiff to resist appreciable deformation during cabling operations. If deformation occurs, the relation between the electrical characteristics of one insulated conductor with respect to others in the cable will tend t o vary, and the electrical requirements on which dimensions are based in designing the structure will not be realized.
and the pressure applied was about 800 pounds per square inch. Heat was applied for 5 minutes, and the sheets were then cooled in the mold under pressure. The molded sheets were conditioned for 24 hours a t 25' C. and 40% relative humidity, weighed, and tested for electrical characteristics (4). The sheets were immersed in distilled water a t 25' C. for 7 days, after which they were removed, carefully dried on the surface, weighed, and retested for electrical characteristics. The results of these tests are given in Table 11, along with those obtained with smoked sheet rubber specimens prepared in a similar manner. Alternating current tests were made a t 1kilocycleand direct current tests a t 400 volts. The power factors found for the Buna S samples are of the same order as that reported by Sebrell (9), and the values for the dielectric constants are in substantial agreement. The results show that unvulcanized Buna S is inferior to milled smoked sheet rubber in reaistivity, but compares favorably in dielectric constant and power factor. ELECTRICAL PROPERTIES OF VULCANIZED PURE GUM BUNA S
Two Buna S gum compounds of the composition given below were mixed on a mill, using elastomers of high and low moisture absorbing properties: Ingredient
Buns 8 Zino oxide (Kadox 72)
Stearic aoid Sulfur bie-N, "(2-benzothiazylthiomethyl) urea (ElSixty) Benzothiaayl 2-monocyclohexyl sulfenamide (Santocure)
Parts by Wt. 100.00 6.00 1.00 2.00 0.50 0.50 110.00
__
Duplicate test sheets, 0.075 inch thick and vulcanized for 60 minutes at 134.5' C., were tested for electrical characteristics before and after immersion in distilled water at 25' C. for 7 days. The values are given in Table I11 and compared with those for a vulcanized crepe rubber pure gum stock. Vulcanization reduced the moisture absorption of the Buna S and increased its electrical stability. This is shown by comparing the changes in dielectric constant and resistivity for unvulcanized and vulcanized Buna S compounds in Tables I1 and 111. The dielectric properties of vulcanized Buna S compare favorably with those of vulcanized pale crepe rubber, except that the resistivity is approximately two orders lower and that the power factor increases to a greater extent under the test conditions used.
MIXING TIME IN MINUTES w 120
n 6
gt- 100 z W
u
80
2 $
60
w 0
n
40 BUNA S COMPOUND N O 5
BUNA S IN LOW-CAPACITANCE INSULATIONS
A t present the utilization of Buna S is of particular importance for low-capacitance insulation on wires. The supply of natural
0
1
2
3
4
5
6
7
M I X I N G TIME I N MINUTES
8
9
IO
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
April, 1944
363 IO
FIG. 3
(RUWER)
PHYSICAL PROPERTIES OF 9 BUNA S COMPOUNDS COMPARED WITH A SIMILAR RUBBER COMPOUND
2403
TENSILE
I
STRENGTH) LB. /IN?
ELONGATION
- 'lo
STRESS AT 200
yo ELONGATION,
L 0 . /IN?
t
5
CURE IN MINS. AT 135OC.
Because of these considerations, the insulation test formula used in the present evaluation of the various Buna S type polymers described in Table I was of a low-capacitance type. To avoid the use of excessive amounts of mineral fillers and at the same time obtain the desired stiffness, a gilsonite of 132" C. melting point was used as the principal volume mer. The addition of gilsonite has little effect on dielectric constant, moisture absorption, or electrical stability under moist conditions. The test formula was as follows: Ingredient Butadiene-styrene copolymer Zinc oxide Stearic acid Sulfur Antisun-checking wax (Heliozone) Litharge Diphenyl p-phen lenediamine 0 35 (Akroflex c) Pheny! a-napth famine 0 : 65 sgm-Dt-@-naphtXyl p-phenylenediamine (AgeRite
f
Whitel
Gilsdz;; Water-ground whiting (Atomite) El Sixty Santocure
Parts by Wt. 100 * 00 6.00 2.00
2.00 2.00 5.00
1.00 1.00 50.00 59.00 0.50 0.50 229.00
-
Compounds w.ere prepared with each of the nine butadienestyrene copolymers listed in Table I. For comparison, the same composition with smoked sheet rubber substituted for the synthetic polymer was used as a control. A laboratory, size B, Banbury mixer was used. The gilsonite was first mixed with the synthetic copolymer as a master batch to obtain satisfactory dispersion, and this master batch was then used in a Banbury mix of the compound. The accelerators were added on a laboratory mill during a period of 6 minutes. Temperatures during mixing were measured by a thermocouple placed a t the apex of the Banbury plunger and connected to a Leeds & Northrup Micromax recorder. The same Banbury mixing schedule was used for all the gilsonite master batches; similarly, one schedule was used for mixing all the compounds. The standard Williams plasticities at 70" C. of the Banburymixed batches before acceleration are given in Table IV, as well as data for power consumed and peak power loads attained during
mixing. The maximum load occurred in the case of the gilsonite master batches just after the addition of gilsonite. I n the compound mixes, the maximum peak occurred a t the start of mixing, and there was a secondary and somewhat lower peak after addition of Wers and other ingredients. Typical curves showing variation in load as mixing progressed are given in Figure 1 and temperature variations in Figure 2. Both the average and peak power loads obtained in mixing gilsonite master batches with the Buna S type polymers were a little higher than for natural rubber. When these master batches were used in Banbury mixes of the compounds, however, the power demands of the synthetics were not appreciably different from that of natural rubber. Similar trends were observed for the temperature variations. While the plasticity values reported appear to indicate that the Buna compounds are comparably as plastic as the natural rubber compound, experience indicates that these values cannot be interpreted as indicating equally satisfactory extrusion characteristics. Physical test results on the vulcanized compounds are shown in Figure 3. Maximum tensile strengths for the Buna S type compounds varied from 860 to 1200 pounds per square inch; time of cure to attain maximum tensile strength varied from 20 to 60
TABLE IV. PLASTICITY AND POWER LOADS OF BANBURY-MIXED BATCHES
Com ound $0.
1 2 3
4 5
6 7 8" Qa
700 c. Plasticity. Mm. 3.99 3.31 3.46 3.97 4.11 3.79 4.23 3.46 3.66 3.77
Power Load Gilsonite master batch Compound Peak Av. Kw.Peak Av. Kw.kw. kw. hr. kw. kw. hr. 6.8 5.4 5.6 6.2 7.2 7.2 6.3 4.8 5.1 4.8 7.8 0.80 6.3 4.4 0.73 r
Av. Natural rubber control 8.38 7.7 4.2 a High-styrene content polymers.
0.70
6.8
4.4
0.74
INDUSTRIAL AND ENGINEERING CHEMISTRY
364 TABLE v. Cornpound No. 1
RESULTS O F
Days in Water at 250 c. 0
7 14 31
2
3
4
5
7
Moisture Absorption
Power Factor
SP. Resistivity, Ohm-Cm.
73
Dielectric Constant
0
0,536 0.649 0.667 0,740
3.13 3.24 3.27 3.33
0
3.17 3.27 3.28 3.33
6.9 4.3
"lo
0.58 0.80 1.14
1 3 1.3 1 1 5.9 1.0
x x x
x x x
1015 1015 1015 1014 10'6 1014 1014 1014
7 14 31
0.46 0.67 0.98
0.627 0.716 0,724 0.799
0 7 14 31
0 0.44 0.61 0.85
0.597 0.688 0.708 0.785
3.14 3.22 3.26 3.28
6 . 8 X 1014 7 . 0 X 10" 6 . 9 x 1014 6 . 0 x 1014
0
0 0.56 0.77 1.os
0.695 0.708 0.737 0.815
3.14 3.23 3.28 3.31
8.0 8.0
0
0 0.46 0.67 0.98
0.783 0,898 0.872 0.966
3.14 3.20 3.25 3.27
4.5 5.3 6.0 4.2
0
0
0.93 1.31 1.89
0.590 0.702 0.751 0.866
3.14 3.29 3.34 3.40
1.2 1.2 1.6 2.2
0
0 0.89 1.29 1.87
0.667 0.716 0.794 0.874
3.18 3.29 3.34 3.39
8.4 9.7 9.7 3.3
0
7 14 31 7 14 31
6
ELECTRICAL A S D MOISTURE ABSORPTION TESTS
7 14 31 7 14 31
8.8
7.7
2.1
x x
x x x x x X x
1014 1014 1014 10"
x x x
10'6 10'5 1014
x
x x x x x x x x x
101, 10" 10" 1014
1014
1014 1014
101' 1014
8
0 7 14 31
0
1.06 1.13 1.20 1.21
3.14 3.22 3.26 3.28
1.0 9.9 9.9 1 0
9
0
7 14 31
0
0.31 0.42 0.58
0.713 0.799 0.799 0.896
3.21 3.29 3.30 3.35
3.4 4.8 4 9 9.3
0
0 0.62 0.90 1.33
0.786 0.875 0.917 0.902
3.10 3.29 3.35 3.38
2 7 x 1014 8 . 3 X 1018
10, natural rubber
7 14 31
0.34 0.53 0.75
x
x x x
10'8
10'6 10"
10"
1015 1015 10" 1014
8 4 x lola 8 . 0 X 10'8
Vol. 36, No. 4
scribed (4). The thickness of the test sheets varied from 0.073 to 0.082 inch. Following these initial electrical tests, the sheets were immersed in distilled water maintained at 25" C. At intervals of 7, 14,and 31 days the sheets were removed from the water and weighed after careful drying of the surfaces. Electrical tests were also made a t these same intervals. The results are given in Table V. The moisture absorption of these vulcanizates appears to reflect the magnitude of the water extracts and the conductivities of the water extract of the ash of the corresponding raw polymers shown in Table I. Compounds 8 and 9, made with the highstyrene copolymers, showed the lowest moisture absorption. I n the uncompounded, unvulcanized state these copolymers gave the lowest water extracts and the lowest conductivities for water extract of ash. Similarly, the compounds made with copolymers 6 and 7 showed highest moisture absorptions, and the corresponding raw polymers displayed the highest conductivities for water extract of ash. Copolymer 7 also gave the highest water extract. The parallel between the moisture absorption of the vulcanized compound and the water extract of the raw copolymer does not follow rigorously for copolymer 6 or the remaining samples, but a general dependence is apparent. Dielectric constant, power factor, and resistivity of the compounds made with the synthetic polymers were of approximately the same order before immersion in water as those of the natural rubber compound. Increases in dielectric constant after water immersion were greatest for compounds made with polymers 6, 7, and 1, in that order. These increases correlate well with ash extract conductivities and water extract values. The same may be said for increases in power factor after water immersion. In general, resistivity decreased somewhat on immersion, as might be expected. Since insulated wire is subjected in service to an appreciable range of temperatures, the evaluation of insulating compoundrr should be made over a temperature range sufficiently wide to cover service conditions. In order to compare the effects of temperature variations on the electrical properties of Buna S type polymers with similar effects in natural rubber, the following compound was prepared with smoked sheet rubber, with GR-S, and
minutes. However, vulcanizing periods varying from 15 to 30 minutes might be considered satisfactory to give technical cures. By comparison, the natural rubber control compound gave a maximum tensile strength of 2620 pounds per square inch in 30 minutes, with a cure of perhaps 20 minutes being con1.6 I I I sidered as technical. Most of the Buna S compounds gave I .4 elongation valpes a t break which were comparable to 1.2 those of the natural rubber compound. Buna samples 2 I.o and 3, which were washed, were exceptions. I n some 0.8 cases modulus values were considerably lower for the 0.6 Buna S compounds than for the natural rubber. This 0.4 SURE was particularly true in the EFFECT OF TEMPERATURE O N POWER FACTOR case of the washed Buna S 0.2 -40 -20 0 20 40 -40 -20 0 20 samples. TEMPERATURE, 'c. TEMPERATURE, C. Duplicate sheets vulcanized 2 3.5 3.6 for the time periods corresponding to maximum tensile 2 $?3.4 3.5 strength were prepared for electrical and moisture ab8 2 3.3 3.4 sorption tests. These sheets a were conditioned for 24 t 3.3 :3.2 hours a t 25' C. and 40% J -40 EO 0 20 40 -40 -20 0 20 relative humidity before testt? T E MP E R A T U R E, ' C TEMPERATURE, 'c. n ing in accordance with the m e t h o d s p r e v i o u s l y deFIGURE 5 . E F F E C T OF T E M P E R A T U R E ON D I E L E C T R I C CONSTANT
I
-
.
I
I
40
40
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1944
with a mixture of equal parts bf GR-S and a higher-styrene polymer to give a styrene content of 37.5%: Ingredient Natural rubber or polymer Gilsonite Water-ground whiting (Atomite) AgeRite White Paraffin Heliozone Kadox 72 Stearic acid Sulfur Selenium diethyl dithiocarbamate (Selenac) Mercaptobenzothiazole (Captax)
Parts by Wt. 100.00 30.00 75.00 1.00 2.00 2.00 5.00 1 .oo 2.00 1.00 1.50 220.50
The compounds were extruded as insulation on stranded conductors equivalent in size t o No. 17 American wire gage to an outside diameter of about 0.105 inch. The wires were covered with powdered soapstone and vulcanized in open steam. They were then conditioned at 25" C. and 40% relative humidity for several days and tested for a.c. characteristics a t 1 and 12 kc., and for insulation resistance a t 400 volts d.c. over the temperature range -35" to f38" C. in concentrated calcium chloride solution. Following these original tests, the wires were immersed in distilled water a t 25" C. and tests a t this temperature were repeated a t intervals up to 14 days. The results are shownin Figures 4 through 7. PROPERTIES AT OPTIMUM CUREOF GR-S TABLE VI. PHYSICAL ELASTOMERS IN LOW-CAPACITANCE INSULATION GR-S A
B C
D E
Cure at 197' C., Sec. 20 15 15 16 80
Tensile Strength, Lb./Sq. In. 765 890 835 855 775
Elon ation,
55
545 560 500 515 470
Tensile Stress at 200%, Lb./Sq. In. 130 195 230 225 \ 250
The test data showed a higher power factor for the natural rubber compound at room temperatures at 1 and 12 kc. than for the synthetics, with the GR-S compound lowest of the three. At temperatures below zero the power factor of the rubber compound was slightly lower than that of G R S , but both were higher than that of the high-styrene compound. The peak in the power factor-temperature curve was ,most pronounced for GR-S and least pronounced for the natural rubber. With increasing frequency, power factor increased for all three compounds and the peaks tended to shift toward lower temperatures. The dielectric constant for the G R S compound was somewhat higher than for the higher-styrene mixture, with that of the natural rubber compound intermediate between them. The dielectric constant decreased with incrwing frequency and increased with increasing temperature for all three compounds. The synthetic insulations showed variations in specific r e sistivity with temperature paralleling those found for the natural rubber compound. The resistivity of the G R S compound was almost half an order higher than that of the natural rubber compound and the compound made with the mixture of polymers was still higher. It should be emphasized, however, that these values are not representative of those obtained on high-quality rubber insulations. Tests on many high-quality insulating compounds have shown consistently higher resistivity values for natural rubber than for Buna S. The minimum insulation resistance requirements of rubber-covered wire and cable insulation under A.S.T.M. specifications D-27-41 and D-353-41 are based on a specific resistivity of about 1 X 1016 ohm-cm. a t 25" C. I n distilled water at 25" C. there was little or no change in power factor during the 14-day immersion period. The GR-S compound showed the greatest increase in capacitance; the natural rubber compound showed an increase intermediate between that of GR-S and the higher-styrene compound,
365
RECENT GR-S STOCKS IN LOW-CAPACITANCE INSULATIONS
The foregoing tests on Buna S were made before current GR-S was available. Therefore a similar evaluation of recent GR-S copolymers from various manufacturers was included in this study. Samples of five GR-S elastomers representing the products of five different plants were obtained and compounded into the following low-capacibance-type insulation test stock: Ingredient GR-S Gilsonite Mineral rubber Atomite A eRite White A t roflex C Heliozone Para511
Parts by Wt. 100 25 25 75 1 1
2 2
Ingredient Kadox 72 Steario acid Sulfur Selenac Benzothiazyl disulfide (Altax)
Parts by Wt. 5 1 2 1
1.5 241.5
Compounds were prepared in a manner simiiar to those discussed in the previous section; a Banbury master batch of gilsonite and GR-S was used to prepare the base mix in order to ensure adequate dispersion, Each compound was accelerated on a 6 X 12 inch laboratory mill and extruded on a 0.045-inch stranded c w duotor to an outside diameter of approximately 0.105 inch. Duplicate samples were vulcanized in powdered soapstone a t low-pressure (40 pounds per square inch) and at high-pressure (200 pounds) steam. The insulations vulcanized in soapstone were used for the determination of dielectric characteristics; stress-strain properties were determined on those cured a t 200 pounds steam pressure. Table VI shows little variation in tensile strength, the maximum variation between the highest and lowest values being only 15% of the average of all five samples. Elongation varied somewhat more. GR-5 samples C, D, and E gave higher tensilestress values than did A and B. Samples B, C, and D gave the fastest cures a t 197" C.:E was slowest with sample A intermediate. The original dielectric properties were measured on samples dried for 24 hours a t 50" C. The wires were then placedin water a t 50" C. and electrical characteristics determined after immersion for 1,3,7, and 14 days. The general order of original dielectric p r o p erties (Table VII) was about the same for all compounds in the dry state, but the data indicate definite superiority for elastomers B and D from the standpoint of stability of electrical properties on w a t e r immersion. GR-S compound B showed an increase in dielectric constant of 14% after 14 days in water a t 50" C., and sample D showed a 13y0 gain over the s a m e period. T h e other elastomers showed gains of 55 to g r e a t e r t h a n 80% under equivalent test I I conditions. Mechani-20 0 20 40 cal imperfections apTEMPERATURE IN OC. pear to have developed
'
INDUSTRIAL AND ENGINEERING CHEMISTRY
366
, TABLE VU.
GR-S A
PROPERTIES OF GR-8 ELASTOMERS IN LOW-CAPACITANCE IMSULATION
DIELECTRIC
Days in Water at 50' C.
Power Factor,
0 1 3 7 14 0 1
0.56 0.47 0.66 0.68 0.74 0.55 0.73 0.71 0.71 0.68 0.66 0.83 0.77 0.96 4.5 0.76 1.13 1.16 1.17 1.29 0.56 0.71 0.75 0.76 0.95
R
3 7 14 C
0 1 3 7 14
D
0 1 3
7
14 0 1 3
E
7
14
%
Increase in Dielectric Constant, %
Dielectric Constant 3.21 3.60 3.75 4.18 4.96 3.26 3.41 3.49 3.59 3.70 3.32 3.61 4.09 5.32 11. 3.31 3.47 3.62 3.62 3.73
SP. Resistivity, Ohm-Cm.
2::;
..
2 . 8 4 X 1013 2 . 6 0 x 1013 2 . 8 0 x 1013 4 . 9 6 x 1013
; :1
76 .
..9
2::;
23 60 232
2 !$'
2 . 0 0 x 108 4 . 1 0 X 101~ 2 . 4 5 x 1013 2 . 6 0 x 1013
*.
5
6 '
2 ::::
3 . 4 4 X loi3 4 . 8 5 x 1013 4 . 2 7 x 1013 3 . 5 0 x 1oia
11 14
3.24 3.50 3.81 4.38 5.79
5 ::::
..
9 17 30 55
?j:i: 5
9 13 *.
*, 12
8 l7 35 79
1.00
lola 3 . 7 0 X 1019
2x :::: 10'0
Vol. 36, No. 4
mined by evaporating to dryness and weighing the residue from the filtrate obtained in the test on conductivity of water extract of ash. The values in Table VI11 are expressed as per cent, on the Original elastomer. The data show wide variations in moisture absorptions at 25" C;. among the several elastomers. G R S samples B and D were least affected by prolonged exposure to water, taking up 14 and 11.2y0, respectively, after 31 days; the other three samples were markedIy inferior, showing gains of 27.7 to 32.2y0 over the same period. These data are plotted in Figure 8, together with values obtained for smoked sheet rubber under similar conditions. Comparison of the dielectric properties of the compounded stocks with moisture absorption, water solubles in ash, and conductivity of water extract of ash of the raw polymers indicates a definite correlation. Those elastomers showing low moisture absorption of the raw polymer have low water-soluble ash and low conductivity of water extract of ash. When compounded into a typical wire insulation they show improved dielectric stability on exposure to water over extended periods. It therefore appears reasonable to select any one of these raw polymer properties &s a criterion of dielectric quality in the compounded stock. EFFECT O F FILLERS IN BUNA S
in samples C and E after a week in water, as indicated by the decrease in resistivity. Sample B shows less change in d.c. properties than the others, although the decreases, except for samples C and E, were less than one order. The somewhat higher initial increase in power factor observed for sample D on immersion in water is believed to be characteristic of the method of coagulation employed in the preparation of this GR-S. To determine whether there was a correlation between the dielectric properties of these GR-S insulations under moist conditions and the rate of water absorption exhibited by the raw polymers, sheets of each of the five GR-S samples (1.5 X 1.5 X 0.020 inch) were placed in distilled water a t 25' C. The gain in weight was observed after 1, 2, 7, 14, 21, and 31 days of immersion. I n addition, determinations of fatty acid, soap, ash, water solubles in ash, and conductivity of water extract of ash were made on these polymers. The results are shown in Table VIII. These determinations were carried out by the methods previously described. The water solubles in the ash were deter-
u
3.7
0
3.5 3.4 y
0
1.0
I
t i
Pu
2
0.8
I
n
I
I
I
I
1
I
4
6
8
10
12
14
l 1
i
4
6
GR-S +OS
n
I
I5 80.6
3
1
0
2
Po
-
1
8
T I M E OF IMMERSION
1
- 10
i GR-S
I
,
10
12
14
IN DAYS
Table IX gives tensile test data obtained on laboratory-millmixed Buna S compounds containing 27.7 volumes of various mineral fdlers. The formula used was as follows, with 27.7 volumes of filler added: Parts by Wt. 100.00
Ingredient Bum S Kadox 7 2 Mineral rubber Stearic acid Akroflex C
16.00 36.00 0.76 1 .oo
Parts by U't. 1 .oo 4.00 1.50
Ingredient AgeRite White Sulfur Santooure El Sixty
1 .oo
These data indicate that hard clay and magnesia are the outstanding reinforcing materials of those evaluated. Unfortunately the use of these fillers is otten limited to low-quality insulations because of their behavior under moist conditions. Additions of organic extenders such as gilsonite or blown asphalt improve the tensile properties of Buna S appreciably. A Buna S compound containing no organic or mineral filler other than zinc oxide for activation was found to give a maximum tensile strength of 520 pounds per square inch in 5 minutes at 134.5" C. The tensile stress a t 200% elongation was 50 pounds. With additions of 40 parts of gilsonite on the Buna content, the tensile strength increased to a maximum of 1115 pounds, and the tensile stress at 200% elongation increased to 210 pounds. With further additions of gilsonite the tensile strength decreased slowly. The elongation a t break decreased from 840% for the pure gum compound to 625% for the compound containing 40 parts of gilsonite. As the gilsonite content was increased, the time of cure required to attain maximum tensile strength increased from 5 minutes with the pure gum stock to 15 minutes for the compound containing 75 parts gilsonite. The effect of gilsonite additions on tensile properties is shown in Figure 9. The results confirm those recently published by Selker, Scott, and McPherson (10). Blown asphalt, cumar, and polystyrene additions also contribute to improvement in tensile properties, and it is believed that other similar organic extenders xould behave in like manner.
AND ANALYTICAL TESTS ON RAWGR-S ELASTOMERS TABLE VIII. RESULTSOF MOISTUREABSORPTION
% Moisture Absorption after Days in Distd. Water at 25' C.: Compound
GR-8, A
GR-9, B GR-S, C GR-S D GR-S' E Smokkd sheets
1 5.8 2.6 5.9 1.9 5.7 3.1
2 8.8 3.6 8.2 2.5 7.7 4.3
7 15.9 6.6 16.8 5.2 13.8 8.3
14 22.5 9.4 22.2 7.6 20.2 12.1
21 27.2 11.4 26.8 9.3 24.0 15.9
31 32.2 14.0 31.8 11.2 27.7
...
Fatty Acid, ?& ._ 3.92 3.86 3.86 4.69 4.52
...
Soap,
Ash,
0.78 0.36 0.86 nil 0.17
0.82 0.34 1.10 0.57 0.87
%
...
%
...
Water Sol. in Ash, o/, ._ 0,772 0.241 0.908 0.541 0.782
...
Conductivity of Water Ext. of Ash, Mho/Cm. 1 . 1 x 10-4 0 . 3 6 x 10-4 1 . 6 x lo-' 0 . 2 6 x 10-4 1 . 2 x 10-
..........
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1944
DAYS IN DISTILLED WATER AT 25%.
TABLEIX. EFFECTOF FILLERS ON TENSILE PROPERTIES OF BUNAS
.
Filler Dry ground whiting Water-ground whiting Atomite) Ti& (Titanox AWDL) Irt?-,oglde (Mapico
magnesia
0 timum Jure at 134.5.O C., Mtn. 20 18
Lb./Sq. 11; 820 1100
12 12
1170 1250
985 860
100
15
1265
835
106
15 15 25 18 25 60
1315 1375 1425 1440 1980 2440
805 815 670 745 765 670
105 105 310 140 310 500
S%g;&
Elongation a t Break,
%
570 680
Tensile at 3007 Lb./Sq. 195 130
f;.
combined-sulfur test data on the compound used above, with 2 parts sulfur and 75 parts whiting as filler, gave a value of 2.07 per 10"C., which is within the range of first-order chemical reactions. This value is in good agreement with that of Cheyneyand Duncan (8). Calculations based on tensile data for the 8-8 compound over the same temperature range gave a somewhat lower value. Other test data have been obtained on a variety of compounds containing from 0.75 to 3.30% sulfur, based on the elastomer content, and a number of different accelerator combinations. These data consist of tensile strength values obtained on sheets vulcanized at 134.5' C., and both tensile strength and compression resistance values obtained on wire insulations cured at 197' C. in open steam. Although the data obtained a t the several temperatures are not strictly comparable, it is of interest to note that the temperature coefficients of vulcanization calculated from them vary from 1.63 to 1.90 per 10' C. Previously it was shown (6)that for typical natural rubber insulating compounds the temperature coefficient of vulcanization varies between 2.0 and 3.0 per 10' C. over the range 142" to 198' C. The low temperature coefficient for Buna S obtained from tensile
45
1200 N.
z
1100
8 1000 900
EFFECT O N TENSILE PROPERTIES OF ADDING ClLSONlTE TO A PURE G U M
RATE OF VULCANIZATION
II
367
OF BUNA S BUNA S COMPOUND The baae compound used for the filler-loading studies, with 50 parts of water-ground whiting added, was prepared with sulfur levels of 2, 4, and 6 parts by weight, based on the polymer content. Test sheets were vulcanized for varying periods at 134.5' C. The tensile strength and elongation values obtained zoo ,wf, for the various cures are plotted in Figures 10 and 11. IO0 ,w# The rate of reaction of Buna S with sulfur was measured by 4determining the free sulfur content of specimens taken from the '0 5 IO 15 20 25 30 35 40 45 50 -55 60 6 5 70 75 PARTS GlLSONlTE PER 100 PARTS B U N A S sheets vulcanized for various periods. The method of Oldham, Baker, and Craytor (6) was used for determining free sulfur. Combined sulfur was taken as the difference between total added sulfur and free sulfur found. In COMBINATIONS FOR HIQH-SPEEDCURE TABLE ACCELERATOR AT 197" C. Figure 12 the percentages of total sulfur combined Time of Tensile ElonStrese Compression increase directly as time of vulcanization for the Cure, Strength gation, Accelerator at 2007 Resistance, compounds containing 4 and 6% sulfur on the Buna Combinations Sec. Lb./Sq. f;. Lb. % Lb./Sq. 11;. aontent. 20 620 Tetrone A 1.6 (sulfur 0.6) 795 160 875 20 770 620 210 1116 Altax 1.0 Zimate 1.0, The plotted data show that sulfur combines with litharge 6.0 20 820 665 Ethasaa 0.5, A-46 1.5 200 850 the copolymer a t a constant rate until a substantial 12 830 635 Altax 1.0 Zirnate 1.0 215 1265 portion of that present is combined. The amount 12 835 765 Acrin 2 0' 808 0.5 180 870 30 840 685 2 M T i.d Zimate 0.5, 230 845 of sulfur which combines in a given time varies with lithargl5.0 15 850 735 200 765 Ethasan 0.5, A 4 6 1.5, the concentration. Comparison of these data with litharge 5.0 those of Garvey and White (3)appears to indicate 12 910 510 Selenao 1.0 Captax 1.5 280 1300 15 935 550 Alias 1.0, I h t o 1.0, litharge 280 900 that the reaction taking place between Buna S and 5.0 . . . . . 2MT 2.0 Tuads 0.5 10 940 735 190 826 sulfur is similar, in general type at least, to that of 10 950 655 Selenao '1.0, Captax 1.5, 220 860 natural rubber-sulfur vulcanization, As sulfur conlitharge 5.0, Vandex 0.5 16 970 735 205 826 2MT 2.0 Zimate 0 5 tent was increased, the time required for vulcaniza15 980 665 205 878 Mqrfex $3 2.0, DPG 0.5, litharge 5.0 tion to maximum tensile strength decreased. With 10 980 680 220 800 Selenao 1.0 Captax 1.5, 2% of sulfur baaed on the polymer content, the time litharge 5:O Acrin 2.0, TMTMS 0.3 12 995 685 225 975 required was about 22 minutes; with 4% sulfur, 16 220 1015 2MT 1 0 Tetrone A 0.25, 15 1050 695 TMTitkS 0.25 minutes; with 6% sulfur, 12 minutes. 15 1135 775 205 1060 2 M T 2.0, 808 0.5, Calculation of the temperature coefficient of vulTMTMS 0.2 canization over the range 134.5' to 154.4' C.from
x.
. . . ..
.
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
368
Vol. 36, No. 4 brittleness temperature of many
+ I200
'
O N R A T E OF VULCANIZATION OF BUNA S (ELONGATION)
t E
a 1000
Y
9 cI1
2
1
800
z
a
9
5
W
a m
600
[Compound Made with Copolymer No. 1 2
0
-J
d
z I t; z
synthetics is appreciably higher than that of natural rubber and must be carefully considered for all low-temperature service applications. In the tests reported the method of Selker, Winspear, and Kemp ( f f ) was used. The brittleness temperatures of nine Buna S compounds prepared for electrical evaluation from the copolymers listed in Table I are:
3
400
4
I
5
c
6 7 8
v)
Natural rubber
T I M E OF V U L C A N I Z A T I O N
IN M I N U T E S A T 134.5'C.
W
n
W
d
zm
W
v)
t
z
I-
90
0
a
80
3
23
70
v)
-I
5
e
L
(TENSILE STRENGTH)
I
60 50
0
w 40
:
0
30
10
15
20
25
30
TIME OF VULCANIZATION IN MINUTES AT 134.5' C.
8
g P
data may be due to the failure of a given amount of combined sulfur to reinforce the polymer structure to the same extent as natural rubber. I t is important that the speed of vulcanizing Buna S insulations should approximate that of natural rubber insulations in the continuous vulcanizing process in order that production capacity be maintained when Buna S is substituted for natural rubber. Although a lower temperature coefficient of vulcanization has been found for Buna S than for natural rubber, it appears that with suitable combinations of accelerators vulcanizing speeds approximating those of natural rubber can be obtained. Data are given in Table X on accelerator combinations in a GR-S compound containing 30 parts mineral rubber, 120 parts water-ground whiting, and 3 parts sulfur based on 100 parts GR-S. Five parts zinc oxide and one part stearic acid were employed as curing aids. The same lot of GR-S was used in all these experiments. The compounds were extruded as insulation on No. 17 American wire gage tinned conductor to give a wall thickness of about 0.031 inch and vulcanized a t 200 pounds steam pressure. Several of the combinations resulted in sufficiently rapid vulcanization for continuous curing applications. I n some cases the compounds showed a greater tendency to scorch than do natural rubber compounds formulated for the same vulcanization speeds. BRITTLENESS TEMPERATURE
Many applications for iatural rubber involve service at low temperatures, but the brittleness temperature of most natural rubber compounds is so low that few failures resulting from l o w temperature brittleness have been encountered. However, the
-
9
b
Britt1e;esess Temp., C. -46 - 48 48 - 47 - 44 - 44 -43 10 - 27 - 50
For the compounds made with polymers 1 to 7 , inclusive, containing 20-257, styrene, brittleness temperatures varied from -43" to -48" C. ; for those made with polymers 8 and 9 (44and 37% styrene, respectively) the values were - 10" and -27" C. The brittleness temperatures were determined for another series of msulating compounds in which the styrene content of the polymer was varied from 25 t o 50% by using two 10 I2 14 16 18 20 polymers of 25 and 50% styrene VULCAN,ZATION IN content and mixtures of these MINUTES A T 1 3 4 . 5 O C . polymers. I n this series the total softener and dasticizer content was less than 10% of the polymer content. Brittleness temperatures of these compounds follow: Compound
Styrene,
Brittleness Temp., O C . 49 43
% '
-
25
F
-36 -
30 36
G H
Cornpound
Gtyrene,
I
40 45
J
K
% '
Brittle2ess Temp., C . - 23 16
-
50
- 8
The data show that the brittleness temperatures of the compounds rise with increasing styrene content of the polymer; they sugsest that for use in general-purpose insulations the styrene content should not exceed about 35%. AGING OF BUNA S COMPOUNDS
In general, compounds made with Buna S type polymers are more resistant to accelerated aging in some respects than similar natural rubber compounds. However, the brittleness temperature of Buna S compounds is raised appreciably on aging. A series identical in composition except for the polymer were tested for low-temperature brittleness, before and after accelerated aging at 100" C. in air, for 1, 3, and 5 days. These compounds were prepared with GR-S, OS-10 (a copolymer of approximately 50-50 butadiene-styrene), and combinations of the two t o give variable styrene content. The results of brittleness tests follow : Compound
F
G H
I
J K
Styrene,
%
Unaged
- 47
- 40 - 33 -- 23 14 - s
Brittleness Temp., C. 00' C. in air 1 d wAged at3 1days 5dsys
- 38 -32
- 26
18 -- 10 - 3
-35
- 27 -21
- 13
- 1
+ 7
- 35
- 24 - 18
-
12 + 2
i-7
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1944
369
ably lower rate than the natural rubber and deteriorated physically
TABLE XI. EFFECTOF ACCELERATED AGINGIN AIR ON PHYSI- to a lesser extent. While the tensile strength of the natural CAL
PROPERTIES OF NATURAL RUBBER AND BUNA~NSULATIONS
Insulation
Time of Aging, Days
Tensile Strength, Lb./Sq. In.
Elongation,
%
Stress a t ZOO%, Lb./Sq. In.
Compression Resistance, Lb. 25O C. SOo C.
Acing a t 80' C .
GR-S
320 555 310 285 270 375
os-10
405 435 380 375 385 240
Natural rubber
Buna S
Natural rubber
0 1 3 5 10 22 40 0 1
5 10 21 0 1 5 10 21
1290 1515 1375 1330 1150 800
465 475 435
220 305 340 310 435 340 395 390 350 Too brittle to test
Aging a t 100' C. 840 530 300 665 830 220 910 195 1010 110 1290 465 1340 410 445 230 310 125 510 60
185 410 725 920
...
680
735 680 730 665 560
*605 730 760 605 650
220
680
325 365
710 590 200 160
... ...
430 465 455 420 380 240
220 335 375 330 330 430 370 170 80 40
The brittleness temperature appears to rise to about the same extent as a result of accelerated aging, regardless of styrene content of the polymer. I n most cases reported, the greatest portion of the rise occurred during the early stages of aging. The compression-cutting resistance of Buna S insulations on wires appears to increase as a result of accelerated aging whereas that of natural rubber compounds decreases. In general, the compression-cutting resistance of Buna S type insulations seems to compare favorably with that of similar natural rubber compounds when tests are made a t 25" C. before aging. For some types of service, however, the crushing resistance at higher temperatures is of greater importance than at 25" C. When compression tests are made a t 80" C., natural rubber insulations show approximately 65% of the 25' C. value whereas Buna S compounds re tain only 30 to 35%. However, the compression-cutting resistance of natural rubber decreases on aging while that of Buna S compounds appear to change little or actually to improve. Data in Table XI show the effect on tensile properties and compression resistance of accelerated aging of insulations in air at 80" and 100' ,C. These data are for wires insulated with a GR-S compound, an OS10 compound, a compound made with a poor processing type of Buna S of about 30% styrene content, and a similar rubber compound. The compounds used were similar to those evaluated in the section on "Buna S in LowCapacitance Insulations", but in all cases the Buna content was higher than the natural rubber content. The test data show initial tensile values for the OS-10 compound which compare favorably with those of the natural rubber; those for both the other compounds were lower. On aging, the tensile strength of the natural rubber insulations decreased as did that of the compound made with high-styrene polymer. The GR-S and Buna S compounds, however, increased in tensile strength; after comparatively short periods of aging, these insulations showed higher compression resistance values than the natural rubber compound a t both 25' and 80" C. This was particularly true for the samples aged a t 100' C. I n oxidation tests conducted a t 70" C. and 300 pounds per square inch oxygen pressure on jacket-type compounds of natural rubber and Buna S, the Buna compound absorbed oxygen a t an appreci-
rubber compound decreased to 66% of the original value in the course of 10-day aging, the tensile strength of the Buna 8 compound increased to 125% of the original value. The elongations of the Buna S and natural rubber compounds decreased t o 75 and 81% of the original values, respectively, as a result of aging. The data indicating that Buna S compounds are more resistant to aging than similar natural rubber compounds confirm the results reported by Roelig (7) on insulated wire aged in air at 100' C. for periods up t o 12 weeks. He found that natural rubber insulations were completely deteriorated from the standpoint of tensile properties in this test after 6 weeks, but that Buna S and Buna SS insulations showed little or no deterioration in tensile strength in 12 weeks. SUMMARY
1. The moisture absorption and electrical stability of Buna S are shown t o be related t o the water-soluble content of the polymer. Data are presented which indicate a general relation between moisture absorption of the raw polymer and electrical stability of the vulcanized Buna S insulations. 2. The dielectric constant and power factor of the unvulcanized Buna S samples are about the same as for unvulcanized natural rubber, but values for specific resistivity are one to two orders lower. Dielectric constant, power factor, and resistivity of vulcanized insulating compounds are of the same general order as for similar natural rubber stocks. 3. Tensile properties of the Buna S insulating compounds studied vary from 860 to 1200 ounds per square inch, compared to 2620 pounds for the naturafrubber control. Modulus values for Buna S are, in general, considerably lower than for natural rubber but elongations are similar. 4. The compression-cutting resistance of Buna S vulcanizates shows reater temperature dependence than is the case for similar naturaf rubber compounds 5. Resistance of Buna'S insulations to accelerated air aging is superior to that of natural rubber compounds from the standpoint of maintenance of tensile strength, modulus, and compression resistance. The decrease in elongation on aging is less for natural rubber ihan for Buna 5. 6. A vulcanization coefficient of 2.07 per 10" C. for Buna S is obtained from combined sulfur data. A number of accelerator combinations were investi ated which resulted in rates of vulcanization of Buna S suitahe for continuous vulcanization. 7. Brittleness tem eratures determined on insulation test stocks indicate that tEe styrene content of Buna S should not exceed approximately 35% where good low-temperature performance is desired. ACKNOWLEDGMENT
The authors are indebted to D. B. H e r r m h n of these laboratories for the electrical tests reported, and to R. W. Walker for assistance in obtaining the low-temperature flexibility data. LITERATURE CITED
(1) Am. SOC.for Testing Materials, Supplement, Vol. 111, p. 466 (1941). (2) Cheyney, La V. E., and Duncan, R. W., Ibid., 36,33-6 (1944). (3) Garvey, B. S., and White, W. D., IND.ENG.CHEM.,25, 1042-6 (1933). (4) Kemp, A. R., and Herrmann, D. B., Proc. Rubber Tech. Conf.. London, 1938,893-909. (6) Kemp, A. R., and Ingmanson, J. H., IND. ENG. CREM.,29, 782-8 (1937). (6) Oldham, E. W., Baker, L. M., and Craytor, M. W., IND.ENQ. CEEM.,ANAL.ED.,8,41-2 (1936). (7) Roelig, H., Kautschulc, 16,26-33(1940). (8) Rossem, van, Leur, van de, and Dekker, Kolloidchem. Beihefte. 10,43(1918). (9) Sebrell, L. B., IND.ENG.CREM.,35, 736-50 (1943). (IO) Selker, A. H., Scott, A. H., and McPherson, A. T., J . Research Nutl. Bur. Standards, 31, 141-61 (1943). (11) Selker, M. L., Winspear, G. G., and Kemp, A. R., IND.ENG. 34,167-60(1942). CHE~M., (12) War Production Board, Rubber and Rubber Products Branch, release on compounding of butadiene-styrene copolymer rubber, Oct. 1, 1943.
,
PmmmwrED before t h e fall meeting of t h e Divieion of Rubber Chemiutry. AUERICANC H ~ M I C A SOCIETY, L New York, N.Y.,1943.
