Effect of Accelerators and Antioxidants on Electrical Characteristics and Water Absorption of Vulcanized Rubber Insulation J. H. INGMANSON, C. W. SCHARF,AKD R. L. TAYLOR, Bell Telephone Laboratories, New York, N. Y. Typical 30 per cent vulcanized-rubber insulating The variations in moisture-absorbing characteristics compounds hare been prepared in which were incor- of the compounds, which are attributed to the parporated separately ten well-known accelerators and ticular accelerator or antioxidant used, are not satistwelre commercial antioxidants. The moisture factory criteria for judging degree of electrical absorption, specific resistivily, specific a. c. con- stability. It is shown that power factor increases ductance, dielectric constant, and power factor for with time of cure in compounds containing thiurams each have been determined both i n the dry condition as accelerators and 1.35 per cent of sulfur. and after immersion in distilled water at 70' C. f o r Of the accelerators tested, highest resistivity values have been obtained on compounds ac5 days. The data obtained indicate that the choice of celerated with thiurams. The variations in the accelerator or antioxidant among those tested is not electrical characteristics of the compounds concritical as regards moisture absorption, but m a y be taining antioxidants cannot be attributed disomewhat critical as regards electrical characteristics rectly to differences in chemical structure of the imparted to a high-grade soft rubber insulation. antioxidants.
T
HE major improvements effected in manufactured rubber articles during recent years can be largely attributed to the introduction of accelerators and antioxidants into rubber compounds. Despite the well-known advantages resulting from the use of these materials in rubber used for various commercial purposes, the introduction of organic accelerators and antioxidants into insulating stocks is only now becoming general. The hesitancy of the wire trade to accept these materials has been partly due to the fact that technical societies' specifications prohibited their use. Another important reason is undoubtedly the fact that information has not been available regarding the electrical characteristics imparted by these materials to insulation stocks under the varied conditions of required service. The work herein reported, which is a continuation of studies carried on in this laboratory some years ago under the direction of A. R. Kemp, was undertaken for the purpose of determining the effect that various representative commercial accelerators and antioxidants have upon the electrical charxcteristics of a typical rubber-insulating compound. SURVEYOF PREVIOUS WORK In 1925 Curtis and McPherson @) showed that increased resistivity, lowered Dower factor. and lowered mecific inductive capacity could 6e obtained in the case of puie gum stocks by vulcanizing with a combination of tetramethylthiuram disulfide and sulfur rather than sulfur alone. In a later paper the same authors with Scott (3) showed that dielectric constant and power factor vary over a wide range, depending on percentage of combined sulfur. Kemp (5) has shown that the most desirable electrical characteristics are obtained with low sulfur ratios in the case of soft rubbers, and with very high sulfur ratios for hard rubber. Aizawa and Tacheuchi (1) have shown that the maxima of these characteristics are
shifted with increasing temperatures in the direction of greater combined sulfur ratios. Since the use of accelerators in soft rubber compositions makes it possible to obtain maximum physical properties with lower sulfur ratios, it follows that improved electrical characteristics may be expected to result from their use. Kemp (4) showed that the physical properties and useful life of rubber-insulated wire used by the telephone industry have been greatly extended, largely as a result of the use of accelerators and antioxidants in the rubber. This was the first work reported involving an evaluation of the electrical characteristics of antioxidants. CHolCE
PREPARaT1oN
OF
There are available commercially today over fifty organic accelerators and a t least a score of antioxidants for use in rubber. It has been necessary to limit the number of materials investigated a t this time. Accordingly, ten well-known accelerators and twelve antioxidants have been selected as representing several types of these materials. The electrical characteristics of a 30 per cent rubber-insulating compound in which each antioxidant and accelerator was incorporated separately have been determined in a drv and wet condition. The base compounds given in Table i are modeled after A. 8. T. M. and A. R. A, 30 per cent compound requirements,
TABLE I. BASECOMPOUNDS .kCCELERATOX TESTS Smoked sheets 31.00 32.35 z;T$?de 32 .OO 2.00 1.35 Accelerator 0.30 COMPOUND FOR
~~~~~
COMPOUND FOR ANTIOXIDANT TESTS Smoked sheets Whiting Zinc oxide Paraffin
Sulfur
Tetramethylthiuram monosulfide Antioxidant
31.00 32.35 32.00 2.00 1.35 0.30 1.00
For testing the accelerators, ten compounds were prepared, each containing 0.3 part of an accelerator. Similarly, twelve 83
INDUSTRIAL AND ENGINEERING
8-1
other compounds were prepared for testing the antioxidants, each compound containing 1.0 part of a n antioxidant. The base compound containing the antioxidants was the same as was used for the accelerators, except that 0.3 part of tetramethylthiuram monosulfide was added in all cases to give shorter curing time.
C3Pl
2
1
1
6 TIME
8
1
0
,
2
1
4
6
a
OF CURE Phi MINUTES
FIGURE 1. CHAKGEIN PO\VER F4cTOR WITH TINE OF CURE IN VULCANIZED RUBBERACCELERATED RITH THIURAM SULFIDES
Electrical test sheets, 1.5 cm. square and 0.128 cm. thick, were vulcanized in a mold a t 135" C. Duplicate sheets were prepared at the optimum cure as determined by tensile. The weighing of the ingredients, milling, and the preparation of the molded sheets were carried out with painstaking care and minute attention to details to insure uniformity in the test sheets.
TESTING PROCEDURE AKD METHODOF ELECTRICAL MEASUREMENT The electrical test sheets were dried over calcium chloride several days after being vulcanized. They were then cleaned b y wiping the surface with petrolic ether. After the cleaning operation, measurements were carried out in the following order: thickness, area, Capacitance and conductance, insulation resistance, and original J+eight of sheets; and weight, capacitance and conductance, and insulation resistance after immersion in distilled water for 6 days a t 70" C. The temperature ranged from 23" to 26' C. and the relative humidity from 40 to 60 per cent during the tests. RIEASUREMEST OF AREAAKD THICKNESS. The same electrodes were used for each sheet tested. The area of the guarded electrode was 98.4 sq. cm. The thickness of the specimens h-ias measured with a Schopper micrometer which could be read to *0.0002 inch (0.0005 cm.). Ten measurements were made on a systematic pattern over the sheet within the space covered by the upper electrode. The average thickness of the 48 sheets was 0.128 cm., and the percentage deviation of the mean, 0.6. The percentage deviations of the mean of the ten thickness meas-
CHEMISTRY
Vol. 2 5 , No. 1
urements made on tlie qingle sheets averaged about 0.9 per cent. DIELECTRIC COSST \ST, A. C. COKDUCTAKCE AND P O W E R FACTOR. Measurements of dielectric constant and conductance mere made 011 a shielded capacitance bridge described by Shackelton and Ferguson (6). This bridge equipment consists of a bridge balance unit and a capacitanceresistance standard, together with an oscillator and a detector. The balance unit consists of shielded input and output transformers, two equal shielded resistance ratio arms, a variable input attenuator, two variable balancing air condensers, a conductance compensator, and ratio-arm and impedancearm reversing switches. The resistance-capacitance standard consists of an AD resistance of five ten-step decades, a n adjustable CD resistance, a mica decade box of two ninestep decades, two ten-step air decade condensers, one continuously variable air condenser, a direct capacitance switch, and a switch to ground either the C or D point of the bridge. The oscillator and detector were standard Western Electric equipment. The capacitance was measured to 0.1 micromicrofarad and the conductance to mhos. All measurements were made at a frequency of 2000 cycles per second. d toolmaker's surface plate was used as the lower electrode. The upper electrode consisted of a 41/rin~h(11.4-em.) brass ring, inch (0.48 cm.) high, filled with mercury surrounded by a mercury guard ring of the same height and of inside diameter 45/8inches (11.76 em.). The brass ring was beveled on the outside to a inch (0.04cm.) edge to obtain better contact with the sheet. Care was taken to fill the ring to the same height with mercury for each measurement in order that the pressure should be the same on each sheet. The dielectric constant was calculated using the equation: K=- CXt
0.08858
where G
= t =
A
=
capacitance, micromicrofarads thickness of sheet, cm. area of guarded electrode, sq. cm.
The specific conductance is given by the equation: SP.
G
=
Gt A
where G = conductance, mhos The pox-er factor is given by the equation: G P. F. = __ 2Kj c
where C f
= =
capacitance, farads frequency, cycles per second
The area is precise to 0.5 per cent; the individual thickness measurements, to 0.4 per cent; and the capacitance measurements, although individually precise to 0.02 per cent, may be considered reproducible x-ithin 0.1 per cent. The precision of the dielectric constant will >hen be ahout 0.7 per
O F J'ULC.4SIZED RUBBER COMPOBITIOSS COXTAIXISG VARIOUS ASTIOXIDASTs 1\IOISTURE SP..$, COSDCCTANCE DIELECTRIC COSSTlST SP. RESISTIVITY POWPER FACTOR ABSORPTION D r y Wet Dry TYet Dry Ket ( X 102) % Mhos/cm.3 X 10'2 Ohms/cm.3 X 10-15 Dry Wet
CHARACTERISTICS TABLE 11. E,LECTRICAL
c.
.~TTIOXIDIST
Xeosone D or Age-Rite powder (phenyl-8naphthylamine) Age-Rite resin (aldol-0-naphthylamine) Age-Rite white (sym-di-8-naphthyl-p-phenylenediamine) Zalba (formula unknown) Antox (butyraldehyde aniline derivative) BLE (amine reaction roduct) Neozone standard (pKeny1-a-naphthylamine, n-toluenediamine and stearic acid) Oxynone ( Unsym-di'aminodiphenylamine) Resistox (aldol aniline) Stabilite (diphenylethylene diamine) V. G. B. (acetaldehyde ethylene diamine) Stabilite Alba (di-o-tolylethylene diamine)
2.83 3.03
16.1 16.6
74.8 83.3
4.20 4.26
5 42
5.49
3.20 3.85
1.50 1.15
0.344 0.350
1.241 1.365
2.98 2.09 2.43 2.78
14.7 19.6 16.9 16.6
61.8 89.6 96.5 93.8
4.15 4.30 4.27 4.17
5.26 5.72 5.60 5.i4
5.70 1.90 0.72 3.30
1.77 0.69 0.16 0.59
0.318 0.409 0.356 0.357
1.055 1.410 1,545 1.470
2.47 2.37 2.88 2.10 2.91 2.26
15.9 17.6 17.5 16.8 18.9 16.1
53.6 52.7 92.3 98.8 92.3 102.3
4.18 4.15 4.24 4.27 4.22 4.31
5.li
5.70
5.04 5.56 5.71 5.50 5.68
4.75
1.78 1.36 1.39 0.79 1.58 0.82
0,342 0.380 0.370 0.352 0.404 0,335
0,932 0.940 1.488 1.555 1.507 1.619
3.85 0.85 5.00 1.85
Siiiiilarly, the jrerceiitage cifie B.C. coiiductancc n-ill be ia the order power factor, 0.4 per cent. Tliis gives ioh one ahcet may he compared nith ite capacitance and conduct,anccvxhies, -inn: not all stray eurrcnts a i d capacitimccu are accounted for with the electrodes used. The variation bet~veeiicluplicatt: slicets of the uairrr oo111puuird wnu less before immersion tliaii after soa iii water :it 70" C. The average ~ierceiitaged m!an of the mrioiis clcctriciil iiit~nsiin~niciits heforc and niter immersion is ais follo\vs: twit or approximately *0.1)4 unit.
Iieeousc of the nriation in ~ihysiciilcliaraoteristics \rliieli dupIicat,e I,at,rhrs or vulc2iriix,d rui,ber of identiral compcisicmtniniug tetrametlryltion somctiines show, the cu~~ijimnd t1,iusam disulfide as an accelerator wns prepred six times, diiplicnte sbi+ct,sfor i:lectricnl test bring prepared Sroin each ciiinpoui~l. Duplication of electrical va.lues was excellent. arid well nithin the limits of precisjcm expressed ahove. It is brlieveii tliat deviations of approxiniately the m i n e order would result f r m i similar te ounds eiiritainiiig the otlier accelerators and the ii
up t.u 2.2 x 10'3 oimis eodd lie measured. This app m t i i s is ncc~uri~t.e ryithin 5 per ceiit for irormal deflections \ut.the error may be greater than this wl~eiithe ileiiections arc
a1w5
mdl. The sairic electrodes were used
a8 iii
the case
uf
Lire a. c.
.tivity in olims per cwitiirieter culic is gii-cm /
