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OCR for page 91
91
RABBI AT ~ ~
3. T. Blake and R. I.yle
Simplex dire & (able Comply
Cambridge, Mass.
The question of the supp] y of natural and synthetic rubber and their uses
in electrical insulation continues to be an important One. In the country as a
whole, the proportion of synthetic rubber cased fell to about 42 per cent in 19470
About 80 per cerLt of the rubber used in electrical insulation was synthetic, since
this material :Ls preeminently aciapted to this use. :Et wouic] have been shill greater
had not a shortage of GR~S developed. This shortage was caused by the premature
closing by the Government of some synt;het,ic rubber p' ants.
At the present time, GR-S :Ls in adequate supply due to the curtailment of
tire production schedules, and to the proposed reactivation of several producing
plants .
There was a relatively high rate of usage of GR-S in spite of. the fact
that9 for a portion of the years the price of natural rubber was as much as 5 cants
less than that of GR-S. At the present time, stockPi].irlg policies of the Govern-
ment are holding the price of natural rubber slightly higher than that of OR-So It
is protiable that on a supply and demand basis' the price of natural rubber drill
drop In the next fey? years. Even at the present price, the rubber plantations are
not happy, since their costs have increased aT,precia'~ly since before the -zero In
the past, plantation costs leave had little to do pith selling prices. There are
indications, however, that~the price of GR-S could be lowered several cereals and
still be profit&tile.
A1 though manufacturorp of rubber insulation ares in general' v~>e3.~-satis-
fied with GP`-S' consumers are not entirely educated to its use. They still remember
the higher tensile strength insulations obtainable With natural rubber, although
this is, of course, an unimportant property for insulation. Prob2.bl,nr-more attention
should be paid by the consumer to the better aging properties of GR-S9 and tile
better resistance to sunlight of such ~nsulation..,In ad.,dition9 the greater c~ean-
liness and uniformity of GR-S are.arg~ments for 'its use'. '
. . . . . . . . . .
The recent announcement of types of GR-S ~ol'~mer~zed at loper temperatures
should make it much more desirable for insulation. These 'never polymers, a.libough
. . . .
not yet available in quantity and not yet made in electrical grades, should give
vu~canizates With greater touchiness, and the tensile strength will, incidentl,y) be
higher. They 'v'~73)1 have higher compression resistance at elevated temperatures, and
the good aging qualities and resistance to sunlight of regular GR-S should be main-
tained. In addition to the materials already -in sights it its certain that better
polymers mill be produced in the near future and remove finally any possible advan-
ta~Fe that natural rubber might posses.
OCR for page 92
c>?
This year ~ as in the past se`Jere1 Steals ~ the most significant improve -
ments in rubber insulations were in the field of synthetic rubbers. The properties
of sem~-conductir~g rubbers have continued to provide considerable interest.
LThe marketing of b;lty1 rubL-er insulated. cables for high Frontage uses Was
announced . The properties of such an insulation Were discussed bay Eaton sold
others in a paper ~.'h~c~h was abstracted at some length last years Such cable. are
recommended for operation s.t a copter temperature of 80°CO Bo-;~ev=-r9 the most
stringent test reported requires orally 7 days aging in an ox,rger~ bomb at 70°C. No
data are given for aging at 80°C or higher.
The use of poi.yeth:ylene as a filler in busy]. rubber vitas annolmced3O Polar_
ethylene filled butyl rubber insulation is claimed to have the sane excellent corona
resistance and aging characteristics as betel rubber. The physical properties are
cc~llsi-derab~y improved, particularly hardness and modu:Lus9 a~r~roximating9 it is
claimed, heat-resistant natural rubber compounds. These compounds are recor ted as
retaining 60 per cent of their tensile strength after 20 days at 100°C in an air
ovens and 30 per cent after 2Q days at 121 °CO The air pressure aging at 127°O
shored ~ retention of 70 per chant of original tensile strength after 20 I1OUrS9 SD]
30 per cent after 80 hours O After 28 days in an o~-~gyg~n bomb at BORIC, the compound
retained 65 per cent of its original tens)]= strength. The dielectric strength of
a polyeth:y'ene-fi11~d butyl rubber its not high, and is reported to be 289 volts
per mi3! --- about tile same as for buty]~ insu:~&tion.
Advances were made in the preparation of GR-S For electrical r~ur~oses~
TVilson4 and co-workers report that the addition of about 1 per cent of Q unprotective
agent's such as Dixie C3-,ay during the coagul`~-tion of GR-S latex results in a much
lower electrolyte canteens in the coagulant The amount of electrolyte gal, estimated
from resistivity measurements of a slurry of hater and;the ash obtained from for
temperature incineration of the rubterO The values thus ob+,~ined are compared Lithe
a set of s+..anclard v.?.lues obtained from sar.^ples Faith known a~,ou.~ts of e~ectroly'c.
kali extraction of the rubber is recommended to;~emove.fa~ty acid. The clays it.
is claimed' aids in the formation of' a smaller, more uniform' and more paroles crumb
than usual permitting better extraction of electrolyte. The deposition of epic- -
tro~yto ir1 the crumb through syneresis is retarded. The rubber is said bo be su-
~erior to CTR-S 65. NTm~canizates of salr.plcs processed in this manner showed lower
mater absorptions (A.S,T.M. method), and greater electrical~stabilit~- in water,
Ten Broeck and June' have reported the development or ~ net-? boll moistures
absorptive GR-S (X-361) using alum coagulation from dispute solution (4 per cent
latex solids). This technique reduces the soap and electrolyte content of- the
coagulum. Water absorptions are reported from 0.51 to I.4 ing.,~sq. cm. in 2-0 homers
at. 700C. The change in S.:OC. from soaking in Hater at 70°C and 90°5 is reduced
considerably. GR-S compounds prepared from this rubber are said to be superior to
those prepared from GROSS 650
A scuds of the dielectric properties of mixtures of polystyrene and poly-
butadiene was reported by Boyor and co-workers. The Over factor vs. frequency
curves Were found to pass through a maximum at 100 megacycles' and the Sam.. maximum
was found to occur in natural rubber. The theory is-;presented that a long chain of
conjugated double bonds is a perfect conductor, and a regular distrib-~ltion of double
· . .
OCR for page 93
93
bonds as in a long po2yisoprene chain causes a Metric di.s~ors~on of electrons
wher' their transit time along the po'~ner is equal to the freqliex~ey of the applied
field.
r1
Bonner' has developed a method of testing snail pressed sheet samples of
rubber co~.pounds for they r electr~.ca]~ property. es. lie recommends using, l,infoi]
electrodes which are assured Or good contact ~.r~th the sample by wetting with pet,:o-
le~'m jel].y. Reproducible results Were obtained for lTollune res~st,'~ity7 poker factor
arid S O I . I, . No correlate ons between resllits obtained on pressed sheets and respites
obtained on ex+~ruded wire revere given.
In the i'ield of cond~lct:.ng rubbers Rack and othereg made measurements of
the DC cond~lcti.~rity of rubbers containing ShaT`~;n~gan;acetylene back arid Ccntinen-
tal. R-40. The resistive ties Were st,ud~ed as functions of time, t~em~erature9 cons
certration of black and elongation of the s+,ock. The res~st~vit~r at first decreased
very rawer With ages and then Revealed off at a veer sIoyT rate of change. Ttle
importance of annealing samples to remove strains wars stressed. Te~nperature co-
efficients were positive for Shaw~.nigan and negative for R-40. Temperature CG-
e~ficients (defined as ~X: 50°C = ~ C) tended to increase With increasing
concentration of black arid with in -~;asf~g elongation The resistivit,.- increased
with elongation for R-40, but decreased with Shawinigan at low concent~at,ons, but
after 20 per cent elongation increased with increasing extension.
Hanson9 has developed a method for measuring the resistivi.~.y c. conduct-
ing blacks which eliminates the Usual high electrode contact resistances It in-
volves a double elec+,rode system With two circuits, (13 a current ci.rcuiL, and (2)
a potential measuring circuit. The resisti.~rity Was determined f' om the potential
drop "long a portion of the sample (circuit 2) Chill ~ current was being passed
through the sample (circuit l)0 ITanson found the fol.lowi?~g factors affected the
resistibility: ]. comporting ingred~en~s~ 2. terse of black' 3. amok Of
flexing of the sample g 4. red ative humidity 5 0 temperature ~ 6. age of sample.
He foment it necessary to anneal the same to eliminate the effect of fic`~.~, and to
store samples in a dessicator before testing. Testing at a starboard tempera4;;u?:e is
recommended for reproducible res its.
s
OCR for page 94
94
BIBLIOGRAPHY
1. S=uper 5oronol. Publicity release of General Electric Company' Apparatus
Department g Schenectady' AT, Y.
2c The Electrical Properties of BU{Y: Rubber. I. C. Eaton, A. lT. Roberts and
3. E. Sheldon, India Rubber World' 1149 No. 29 227 (1946~9
(Abstract).May, 19469 AoCeSe seeing High Poisoner Foruan.
See also ~Jolur.qe X, Digest of I!iterature on Dielectrics, c.
3. be ABP High Voltage Insulation. L. F. H~ckernell, Publication toe C-639
Anaconda fire 8 Cable Company
4. Develo~b~on Process for the Reduction of- Electrolyte
in GR-S. E. A. bisons [. ~. Riggs' G. [a ~heelock and B. M. Go bicker'
Rubber Age - Vo1. 623 No. 29 pp. 183-:LS69 November (1947 J
S. Cow Moisture Absorptive GR-S. To T ~ Ten 3xoeck and R. D. Juve9 India
_ ~ _ O ~
- Rubber World - VolO 116, No. 6, pp. 78~-7229 Septe~her (1'P47)
. . .
6, Some Dielectric Properties of Bu diene-Conta~n~n.~ P~r_and Cocolsr,ers.
R. F. Boyer' :~. BO Baker and PO C4 Woodland. Paper presented
before the Division of IIigh-Polymer Physics of The American
Physical Society' January? 1947. Rubber Age - -Iol. 60, Into. 49
p. 455 (Abstract one
7. Electrical Tuber. Ro BonnerO Transactions of the :Tnstitutior.
. _..,.
of the Rubber Industry - Vow.. 23' p. 155-16] 9'' (October 1947)
. . . . . .
80 Electrical Conductiv~.t~r of GR-S and M_a1 Rubber Stocks Loaded w it
Shaw~L~|acksO P. 13. Sac R. Tto Ar~t''~ony and
E. Guth, Journal of Applied Pl~y.sics, box. 1.89 pie' 45i6-~69
(May ].S47)
9. Measurement of Resist~onductive Rubber. A. C. lIansonO Paper
_.. ~ it,
presented before the Division of Rubber Chemistry, A~r.erican
Cheek Cal Society9 September, ~Q,./~70 Rubber Age g VolO 63.
NoO 5, p. 576 (abstract only) (August 1947)
Representative terms from entire chapter:
tensile strength
