Comparison of Natural and Synthetic Hard Rubbers G. G. WINSPEAR’, D. B. I-IERRlI-iXS, F. S. l L 4 L l I . ~ N D4. R . K E U P Bell Telephone Laboratories, Murrtry H i l l , V. J . nitrile, and natural hard rubbers are compared as regards compounding, processing, vulcanization, and physical and dielectric properties. S a t u r a l rubber and GR-S compounds intermediate in sulfur content between hard and soft rubber also are rompared. G K - S and nitrile rubber compobitions suitable for comrncrciiil ebonite fabrication are described. Extensive breakdow t i of the basic copolymers has little effect on the phy.ical properties of synthetic ebonites. The time rcquirecl for the beginning of esotherniic reaction iii vulcanizatiqn ia longer for GR-S U o r l , ~c l l hnrdrieas is than for natural rubher ebonite.. greater for G H - 5 , Sonie G H - S elmiiitcs are penetrated to the same depth as natural elionitec, M ith :I Freatcr teiid-
GR-S,
ency toward instantaneous recovery. The two are similar in impact strength, h u t the ahility to withstand a sharp bend is characteristic of natural ebonites alone. The l a t ter are superior to GR-S ebonites i n heat deformation helow 60‘ C., b u t above this temperature the reverse is true and nitrile ebonites are superior to hoth. GR-S ebonite. are more stable and nitrile ebonites less stahle cheniic~all? than natural ebonites. GR-S ebonite dust as a filler i n creases brittleness. .\ tliatomaceot~searth inipro\es the processing properties of GR-S hard ruhhers. The n t l \ e r - e effect of ultraviolet light on surface resisti\ity is rediicecl when a GR-S hard ruhher is filled with whiting. Natural and GR-S hard rubbers are alike in dielectric heha\ior.
T
standard procedure-: nhich were being folloived in the conimt~rcial fabrication of hard rubber. St6cklin (,?a), Gartner (?), and Roclig (18) discussed tht. use of t h e Bunn synthetic rubhers in German hard rubber ni:inufacture, and English and Russian intere.-t was shown in the work ~ t ’ Scott (20, Zf),Iiashin ( I I ) , and Xlexandrov and Lazurkin ( I ) . These reports disclosed t h a t certain properties of hard product. from synthetic rubbers of foreign origin, including rcaistancc t ( J moisture, organic liquids, acids, and to deformation a t elevated tcxmperatures, arc superior t o those of natural hard rubbvr-. Consistcntly mentioned shortcomings were difficulties in fabrication and a pronounced brittleness in fully cured synthetic hard rubbers. Limited information on GR-S hard rubbers was published by Cheney ( 5 ) , who studied various vulcanizates including conipounds containing 25 and 35Yc sulfur. The vulcanization tempcrature was 157” C., and mention was made of the vigorous evolution of hydrogen sulfide during the cures; this behavior indicated that the reaction involvcd considerable substitution. Morris ( 17 )and co-workersdescribbd a series of experimental GR-S hard rubbers in which types and amounts of sulfur were varied, and the effects of different fillers and softeners on physical properties were investigated. The compounding and properties of nitrile hard rubbers were described by Garvey and Sarbach (81.
HE wartime replacement of natural rubber Ivitli $yilt tictirs required a n unusual expenditure of eflort by tlic haNrubber industrl- in a short time. .it first, curtailment of nwnial protluction, coupled irith War Production Board reytrictions of forniulatioiis, mitigated the urgency for synthetic hard rubber remwcli. I t soon became evident, however, t h a t a complete line of synthetic hard rubbers would be desirable. These materials could be fabricated Lrith standard rubber proceesing equipment, and ivould offer physical and electrical equivalents for the various grades of natural hard rubber developed during nearly a century. h program vias started in these laboratories with the realization t h a t rapid progress might be difficult; rexarch on the compounding of natural hard rubber over the years had failed to produce improvements in over-all properties compared with the original “2bonites”. The latter, according t o the accepted nomenclature (f4),are simple mixtures of rubber with large amounts of sulfur vulcanized by heating until chemical saturation of the rubber is almost complete. T h e first approach to the problem was through a study of vulcanizing characteristics, arid through examination of the hard products resulting from tllc ieaction of sulfur n i t h butadiene-styrene copolymers*. Aisthe program progressed, the work was extended t o cover the processing of GR-S for ebonite fabrication and the compounding of GR-S hard rubbers for specific applications. Studies also were conducted relating to the compounding and proces.5ing of nitrile hard rubbers, and new tests were developed t o supplemcnt standard procedures used in the physical evaluation of hard rubhcrb. Earlier work in these laboratories had indicated that approximate dielectric equivalents of standard hard rubbers could be obtaided; the method suggested was direct replacenic~Iitof firht quality smoked sheet rubber with polybutatliene anti GI!-S synthetic rubbers from both foreign and domestic -ourecs. I,,arlic,r work had indicated also that the dielectric propcrticy of t1ie.e materials could be improved by washing the ruhl)er prior to compounding in order to remove water-soluble impurit iei. The processing characteristics of these materia!> had their bc.havior in production would neccqsitate a r
COMPOUNDING AND VULC.AYIZ.AT1ON OF GR-S EBONITES
T h e GR-S used had 10.7670 acetone-extractable matter and ’2.50yc ash. The iodine number after acetone extraction was
mixtures. Individual botches consi5ting of 300 grams of GR-S and the required amounts of sulfur were mixed o n a water-cooled standard laboratory mill ( 2 ) , nllon.ing 10 minutes for mastication arid 5 minutes for the addition of sulfur. After overnight storage the batcht.s were given a n additional 10-minute remix before vulcani-
‘ P r e s e n t address, R. T. Vanderbilt Company. 230 P a r k h r e n u e , Sea York. N. Y . *Unless otherwise s t a t e d , t h e GR-S was obtaine3 f r o m t h e Goodyear (.%kron, Ohio) plant.
687
688
INDUSTRIAL AND ENGINEERING CHEMISTRY A.lOO G R - S , 4 0 SULFUR
pounded in ebonite proportions (G8:32) and subjected to a vulcanizing temperature of 148' C., the internal heat formation became evident after 80 minutes and reached a maximum of 170" C. after 138 minutes. Increasing the temperature of vulcanization to 151' C. advanced the st.art of exothermic reaction t o 50 minutes; a peak temperature of 191O C. was observed a t 116 minutes. Sections were cut from the centers of the cured hard rubber cylinders and fractured for examination. The specimens vulcanized a t 148' C. were dense and free from porosity, whereas the 150" C. vulcanizing temperature yielded a porous product, permeated with hydrogen sulfide. The curves in Figure 1 show t h a t the time required for the beginning of the exothermic reaction is considerably longer for the GR-S than for the natural rubber. The reaction, when i t does occur, produces a much higher rise in temperature in the case of 'the GR-S containing 45 and 50 parts sulfur and vulcanized at 150" C.
C.100 CR-SI 50 SULFUR
D. 100 SMOKED
SHEET, 47 SULFUR
PHY S1C.4 L PROPERTIES
T I M E IN M I N U T E S
Figure 1.
Vol. 38, No. 7
Heat of E x o t h e r m i c R e a c t i o n of GR-S and N a t u r a l R u b b e r E b o n i t e s during Vulcanization
The A.S.T.M. procedures (3)for testing hard rubber products were followed in determining tensile strength. elongation. ffexural strength. impact strength (Izod), cord flow, a n d kockwell hardness. Measurkments of deformation under the conditions of the cold flow test were made a t 71" C., and elastic moduli were calculated from load-deflection data taken in conjunction with flexural strength tests. The data for GR-S and Hycar OS-10 ebonites are listed in Tables I and 11, respectively. These results show the variations obtainable through changes in sulfur content and time of cure. Changes produced by continued heating are particularly evident in the GR-S ebonites compounded with less sulfur than is required for theoretical saturation. There is a striking similarity between the over-all properties of the GR-S ebonites compounded with 40 or more parts of sulfur per hundred of whole polymer, and the corresponding data for a fully cured natural ebonite. The fully cured Hycar OS-10 ebonite compounded with 40 parts of sulfur appears to be superior. -
zation. None of these compounds mixed on a n open mill was sufficiently tacky or free from nerve to be suitable for calendering, plying, and tin plating according to the method employed in processing natural rubber. Compression-type molds faced with tin foil were used to make uniform test sheets. The foil facilitated removal from the molds and assured uncontaminated surfaces for electrical tests. Sheets l / 2 , 1/8, and 1/16 inch thick and 6 inches square were vulcanized in steam platen presses with the temperature carefully controlled. The sheets were cured individually and positive pressure was maintained during vulcanization, to compensate for shrinkage and t o eliminate the necessity of surface grinding in the preparation of test specimens. Two-hour intervals were used t o study the time effect on the vulcanization reaction and on the resulting products. The lowest vulcanizing time, producing a Rockwell hardness above HR-80 in a half-inch sheet, was utilized to establish the range of cures for each compound. HEAT OF EXOTHERMIC REACTION
The effects of sulfur content and temoerature of vulcanization on the exothermic reaction in GR-9 ebonites are shown in Figure 1. Cylinders ( l l / a X l3/8 inches) of GR-S ebonite mixtures, containing 40, 45, and 50 parts by weight of sulfur, were cured in a steel mold placed in the center of steam platens and shielded from external air currents. Iron-constantan were inserted into th? center of each specimen through a transite plug in the side of the mold. Similar.data were obtai~edforanaturalrubberebonite of 68: 32 composition, because, although this reaction In natural hard rubber has been the subject of a number of investigations, the most recent O f which lVasreported by Church and Daynes ( 6 ) ,i t was considered advisable to obtain a direct comparison under identical conditions. Platen temperatures of 148" and 150" C. were used for curing all samples; a cure also was run on the 100:45 GR-S ebonite a t 153" C. With natural rubber and sulfur, com-
I
PROPERTIES OF GR-S EBONITES TABLE I. PHYSICAL Impact
Strength
Sulfur Compd. 100:25 100:30
as Compounded,
%
20.0 23.07
% Cold Flow ( h o d ) , Cure ~ r 0 . c. 16 153 20 24 8 153 10
24 100:35 io0:40
25.92 28.57
6 8 10
153
6 8
153
io
~.
100:45
31.03
8 10 12
148
100:50
33.33
8 10 12
148
10
148
Natural rubber ebonite, 100:47 a
E = Modulus.
RockFlexural Properties. Tensile well .Lb. Per sq. In.. Strength, Elonga- HardE' X Lb. per tion; ness, Strength 103 6q. In. % HR 9,130 242 5,980 5.8 83 10,100 263 6,525 4.0 89 10,230 295 7,380 3.9 94 255 6.678 7.2 83 9,300 265 7,560 6.0 88 9,830 301 8,288 5.4 104 12,770
49O
c.
7Io
39 3 35 4 26 2 35.8 34.6 2.8
73.3 48.0 39.4 43.7 45.7 21.6
Ft.-Lb. per In. 1.119 0.963 0.759 0.871 0,932 0,684
18.0 3 3 . 2 17.4 35.5 5 . 0 27.1
0.732 0,661 0.583
10,570 11,800 11,800
319 296 305
8,28*
7,920
j.3 5.8 3.9
92 95 101
3 . 0 20.5 1.4 9.9 1.4 9.5
0.552 0,523
13,070 12,470 14,000
323 317 325
8,740 7,870 9,100
7.0 5.2 8.5
103 107 107
4.7 2.2 2.2
26.6 17.2 17.8
0.517 0.501 0.496
12.0 27.5 1.5 11.2 1.8 8 . 4
0.547 0.662 0.563
13,300 13,700 13,870 13,030 14,270 14,700
318 323 325 332 337 333
7,570 8,420 8,180 8,325 8,130 7,418
7.2 4.8 3.2 5.7 3.1 2.4
103 108 io9 101 109 111
0.53
15,000
330
10,680
4.0
99
2.5
c.
18.0
0.515
8,380
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
July, 1946
689
a sharp bend over a constricted area t o test sprcimens x l / l b inch in cross section. Talde I11 impact Flexural ~ ~ ~ l i compares the bending properties of natural ami Glt-S Properties ~ ~ ~ , ~ i uell l ~ % Cold Flow Lb. per Sq. In. s t r e n g t h , ~ l ~ ~~ ~ ~ ebonites d ~- as ~made -b:- this method. Another type of Cure 49' 71' Ft.-Lb. Lb. per tion ness. Compd, H r , C, e, C. per In, s t r e n g t ) , ";o" sq,I n , 5' HR equipment used f o r tests was the 'Isen1o0330 153 2 1 , 3 4 4 , 8 48j 11,200 368 9,310 Tour-Slarshall stiffliess tester (23). Although it 6 1.4 21.9 0.491 15,800 364 8.670 5 0 109 was primarily designed for measuring modulus in 10,700 6.7 107 0.483 15,300 360 I 7 26.7 8 flexure of a rigid material, this tester was adapted 9,350 4.0 92 341 0.502 11,500 100:35 4 148 3 7 . 3 54. 6 3.6 . 2 8 . i 0.500 ii,500 363 9,400 4.1 104 t o the quantitative measurement of brittleness by 10.850 6.3 113 392 2 7 0.441 12,200 8 0 8 the use of a 7/1&inch span. Figure 3 shows typical 100:4" 6 148 1 $ ~ ~ ~ results comparing the stiffness, A , and brittleness, 8 0.4 1 4 0.507 li,100 359 10,610 4.3 116 B , of natural and GR-S ebonites. 100:46 4 148 1 . 4 12 4 0 418 16,400 399 8,540 5.0 110 It appears t h a t the ability t o withstand a sharp 6 0 5 1.7 0 332 17,800 339 10,740 5.4 116 8 0.3 0.7 0.352 12,400 387 11,340 6.0 118 bend is characteristic of natural ebonites and is not obtainable in GR-S ebonites, even if the latter have a relatively low sulfur content. , UnsuccessTABLE 111. BRITTLENESSAND P E S E T R 4 T I O S RECOVERYO F ful attempts were made t o improve this property of GR-S GR-S ASD NATURAL RCBBER EBOKITES ebonites by the use of selenium, which is claimed t o produce an Brittleness Tests with l/s-In. Speci~ ~ -knele ~ ~ k i ~ ~ elastic phase incapable of conversion t o the hard state, and by the mens, and l/d-In. Ball f o r I / ~ ~ - I , ,s. t r i p s Deflected 6'/ Residual deuse of polymeric plasticizers such as polyisobutylerie, Butyl rubCompn. S e e . , Degrees Penetration'. formation ber, and a sulfur vulcanizable polyester (Paraplex 5-200). I t is GR-S: fulfur of interest t o note, however, that exposure a t low temperatures 108:35 39 121.5 26 ioe:40 30 115 0 21 reverses the order of brittleness in the natural and GR-S ebonites. 27 115 0 22 100345 100,513 26 108 0 19 After a 2 4 h o u r conditioning period at -30" C. the natural Natural ebonite: ebonite fractured when it was bent I?", while the brittleness of a 900 116.0 31 aulfur, 100:47 GR-S ebonite was unchanged. 0 Specimen did n o t break. The heat deformation of natural and synthetic ebonites was b One scale division = 0.002 mm. inve-tigated by means of a parallel platk plastometer with electrically heated platens, arid by a deforming load of 900 pounds per square inch applied for one hour. SIaterials examined included natural anti GR-Sebonites of 100:40.5 and 100:45 composition. The Rockwell hardness values for the GR-S ebonites are higher than.those commonly observed in natural ebonites, and a reduction in impact strength is evident in tlie Hycar OS-10 ebonite of 100345 composition; but none of the other measured properties furnishes any indication of the brittleness mentioned by European investigators. Brittleness is knoxn t o exist in these materials through observations made in the course of preparing test specimens. T o add further significance t o the differences in Rockwll hardness, penetration and recovery measurements after 15-second intervals \\-ere made on a series of GR-S ebonites compounded with different sulfur contents, in comparison with a natural ebonite of 100:47 composition. Using tlie conditions specified by the Rockwell hardness test for hard rubber, the data in Table I11 were obtained. They demonstrate that GR-S ebonites containing 40 and 45 parts of sulfur are penetrated to approximately the Bame depth as the natural ebonite. Their tendency toward instantaneous recovery ,when the load is removed is of a higher order than that of the natural ebonite. The GR-S ebonite of 100: 35 composition is somewhat less rkistant t o penetration than the natural ebonite but neverthelesh exhibits a greater rccovery upon removal of the deforming load. Incrca3ing the sulfur coiitent t o 50 parts per hundred of GR-S increaxs the re.-iGtance to penetration t o a markc:d ilt~grw and a1.o effect, a furtlier reduction in tlie deformable phase. The similarity of impact srrengtli value5 for the natural a ~ i t l GR-S ebonites might lead to the assumption that s t r i p of nai urn1 and synthetic ebonite:, of equal thickness can be Lent t o the same degree before breaking. -1discussion in tlie literature ness of ebonite ( 4 ,\\-as I d on the results of Clinrpy . If. ho\\-cver, brittlcn s defined as the ticagree of bending t h a t a given material will n.itlista~ida t 25' r, bcforc breaking, instead of as the energy a supported sample \vi11 shsorb from a pendulum before breaking, a fundamental differcnce tlctween the natural and synthetic products Iiccoines evident. Figure 2 . Apparatus for Brittlenes. Testing of Hart1 Several mcthods wwe investigated for detcrmining t hi: deyrce Rubbers to which natural and synthetic ebonites can be bent before irncStrips 1 / 1 inch w i d e and 1 / 1 6 inch t h i c k are b r o k e n b y bending turc occur'. The apparatus shon-n in Figure 3 \\-a? r l . ~ dto apply at 6' per s e c o n d
TABLE11. PHYSICAL PROPERTIES OF HYCAR OS10 EBONITES
s$t$h
0
,
:,::!:: it;
ii;:;:
i;z
45:;
;z
690
5
INDUSTRIAL AND ENGINEERING CHEMISTRY
PO
iliis material occurrcd a t 99" C.: after 9.3 liourb at 149' C. the weight of the precipitate was 0.174 gram. The relative chemical statrilltioh of l i n t urai and GI1-S hard rubber dusts were tested by storing sarriplt's of similar particle size in a closed container in the presence of, but not in contact with, lead acctate-inipregn:ited filter paper; the samples were examined at intervals during a 2-month period. I'rogro-sive darkcriirig of the paper indicated :hat the rate of hydrog i ~ nsulfide evolution was decidedly morc rapid f r o m the natural hard rubber dust. After one hour a rioriwable reaction was apparent on the edges and exp .side of the paper over the natural dust, but no (ii c.jlorntion was observed in that over the GR-S. .\ftc.r 2 monyhs the reaction of hydrogen sulfide n i t h i c a t l acetate over the GR-S dust was barely evident, w I i i l ( ~the same exposure to the natural dust produced a 1u;trous gray-black formation over the*entire exposed ArLction of the indicator paper.
Y
4 za
60
Y
m I
z
2
30
I
b
8 0
A.
30
60 90 0 30 ANGULAR DEFLECTION IN DEGREES
Stiffness. S p a n (L)= Linchea. Capacity ( C ) = 6 i n c h - p o u n d s
Figure 3 .
60
90
B . Brittienesa. Span ( L ) = inch. Capacity (C) = 10 inchpound8
Tour-3Iar-hall Tests on GR-S and Natural R u b b e r Ebonites
rr?spertivcly, and nitrile ebonites of 100:40 and 100: 45 composition. T h e latter were compounded with a nitrile copolymer containing 26y0acrylonitrile. I n view of the difficulties encountered in the fabrication of nitrile ebonites, one of these materials containing 10 parts by weight of Xaftolen R-100 was included in the series. The temperature-deformation curves in Figure 4 show that the natural ebonite is r;uperior to the GR-S hard product below 60" C., hut above this temperature the reverse is true. The superior heat deformation of the nitrile ebonites also is evident, but the adverse effect of using a plastieizcr t o improve the working properties is obvious. CHE3IICAL STABILITY
The effect of heat on the rate of hydrogen sulfide evolution from natural and G R S ebonitcs was determined according to a procedure followed by previous investigators (24, 25). The apparatus consisted of a glass tube with a n enlarged section for tht. samples, and connecting inlet and outlet tubes of smaller diameter for thepassage of nitrogen over the cbcnite. The outlet tubc, was fused t o the sample container, and the inlet tube connected by a ground glass joint. .Li glass wool filter pad a t thc. cxit end of the sample tube prevented any of the material from being swept into the precipitation bottle. Thc apparatus was mounted in an oven, and commercially pure nitrogen was passed through a 25foot coil of stainless steel tubing which was also endosed in the oven. A Fisher-blilligan bubble tower, containing a saturated solution of lead acetate acidified v i t h acetic acid, was connected to the exit end. The temperatures at which perceptible lead sulfide precipitates formed during a 2-hour exposure wcre determined with this assembly; 2.5-gram samples mere removed with a wood rasp from fully vulcanized half-inch-thick ebonite sheets. The natural ebonite (100 smoked sheet, 46.5 sulfur, cured 11 hours a t 148' C.i exhibited no appreciable signs of decomposition during 2-hour periods a t 93" and 99" C. At 104" C., however, a lead sulfide precipitate was visible a t the tip of the exit tube in the bottom O f the bubble tower. At 116" and 127" C. lead sulfide appeared in appreciable quantities throughout the toiwr. .4 fresh samplr placed in the heating tube for 2 hours a t 138' C . resulted in the formation of 0.040 gram of lead sulfide. A similar examination of GR-S ebonite (100 GR-S, 45 sulfur, cured 11 hours a t 148" C.) produced only a slight viciblc formalion of lead sulfide. A f 143' C'. some decompositioii iva. evident in a two-hour heating period, an(i at 149 "C. thc lead sulfide precipitate from a 9.5-hour tert \vciyIied 0.039 gram, approximately tlic ticcomposition oh.qcxrved for natural cboiiitc in 2 hours at 138" C. .i nitrile ebonite sample (100 Pubtinan, 42 sii!fur, cured 10 iiours a t 142' C , ) was mor(. un-t:il)l~~ than tlir, riatriral and GR-S ebonitcs. The first noticea1)lc lead .-ulfide foi.nxttion from
Vol. 38, No. 7
DUST-FILLED G R - S HARD RUBBERS
Hard iubber dust ha> lorig txeii u,wd in the fabrication of t1iic.k articles, mainly hecaust, it dilutcs a given mixture and rcduccs tile exothermic heat of reartion dur)ig vulcanization. Its use results in materials which are satidactory dielectrics and can be niachined easily, although they are of reduced physical quality. Hard rubber dust also lowers the cost of articles when reduced quality is commensurate with ultimate application. At first, commercial-type hard rubbers containing GR-S were compounded with whole-tire reclaimed rubber and natural hard rubber dust (Table IT', compound A ) . Vulcanizates of this kind, however, proved to be extremely brittle and subject to excesbive deformation under the conditions of t h e cold flow test. As GR-S became morc available and the supply of natural hard rubber dust was exhausted, formulas were investigated in which thc rcclainied rubber was deleted and GR-S ebonite dust replaced the [,orresponding natural product (Table IV, compound F ) , The composition listed contains the minimum quantity of dust required t o impart satisfactory p r o c c 4 n g characteristics to an opcn-roll-mill mix: the sulfur content sho~vnproduces a suitable compromise betwcen cold flow and impact strength. 30,
DEGREES C.
Figure 1..
Flow Characteristics of Natural and Synthetic Hard Rubbers
INDUSTRIAL AND ENGINEERING CHEMISTRY
July, ,1946
TABLE Is'. TECHSICAL FORMULATIOSS OF SYSTHETIC HARDRCBBERS
Ingredients, Parts hy Wt. GR-S, t y p e C.4 GR-S, type AC Nitrile rubber, 26% acr\-lonitrile Nitrile rubber, 39.5% a c w Ionitrile Reclaimed rubber, 627, rubber hydrocarbon Sulfur, tire brand Sulfur, micronized SC Magnesium oxide, L C Hard rubber dust, natural Hard rubber d u s t , G R - S Atomite whiting Celite 270 Naftolen R-100 O.E.I. (dodecyl m e r c a p t a n ) Synpep N G-25 plasticizer Total Vulcanization Time, hr. Temperature,
C.
Physical properties Cold flow, 70 490 71' C . Flexural strength, lb./'sq. in. .\lodulus, Ib./sq. in. Izod impact strength, ft.-lb./ in. of notch Brittleness, degrees t o break 0.062 X 0 50 in. 0.031 X O . 5 0 i n .
c.
GR-S Reclaim DustLoaded Hard Rubber, A 1oo:o
... ... I2l,585.1a
...
200'0
...
... 32'3
... ...
GR-S Hard
Flexible ebonite,
B
Rubbers Whiting- CeliteEbonite, filled, filled, C D E
100.0
100.0
... ...
..,. ... ...
. .
37.5
...
400 3.0
... ... . .
...
'2'0
...
... . . ...
1oo:o
1oo:o
...
...
100.0
...
...
...
...
...
100.0
4O:O . .
42.5
3.0
10.0
'3'0
...
...
io0
::.
... ...
...
...
...
10'0
50.0 12.0
12.0
...
..
..
145.0
11 148
118
11
142 5
235.5
201
11 148
11 148
11 Id8
1.8
o
..
~ 2 . 5 148
11 148
in 142
14,800 312,000
11.0 13,900 298,000
1.4 15.9 11,800 303,000
2.53 13.5 14,300 402,000
7.6 27.1 9,000 355,000
0.265
0.465
0.415
0.463
0.410
30 55
25
20
12 32
IO 0
205.0
16.6 40.0 10,600 369,000
2.2 14.8
42 > 90
...
...
...
,..
... ... ... .. ... _ _ _ - _ _ _ _ _ ~ - - -
539 5 5 .
3.0
... , .
l5,O
...
..
...
5o:oo
...
35'0
...
...
...
. .
,
...
39 0
1oo:o
... 2.0
..
...
3i.5
. .
. . ...
H ... ...
G
...
...
,..
3.0
F
Nitrile Hard Rubbers Celitefilled, Ebonite,
1oo:o
...
...
Dustfilled,
0 46 1.38
o
IO
142
.. ..,
0.50 0 85 11,700 335,000
0.313
...
0.398
20
16
22
,
...
...
, . .
691 a t 30" C. and 90% relative humidity. The surface re&tivity of a GR-S ebonitc of 1 0 0 : 4 5 c o m p o s i t i o n previously had becn determined as 3.3 X lo6 ohms after 110 hours, a t which time the test had becn discontinucd because of the low value. hftcr 800-hour exposure, the whitingfilled natural and whitingfilled GR-S hard rubbers cxhibited surface r e s i s t i v i t y values of 3.4 X 108 and 3.8 X 10'. ohms, respectively, as compared t o 4.8 X lo5 ohms for the natural ebonite after 800 hours and 3.3 X 106 ohms for the GR-S ebonite after 110 hours. On the basis of these tests, although the surface r e s i s t i v i t i e s of b o t h t h e w h i t i n g - f i l l e d GR-S hard rubber and the GR-8 ebonite are lower than t h a t of the natural products, the improvrment brought about by whiting is significant. DIELECTRIC PROPERTIES
Further studies on GR-S dust-filled hard rubbers, using hot processed copolymers, indicated t h a t commercial compounds can be produced adequately with smaller amounts of dust, in applications where retention of shape and elimination of blowing during vulcanization require its use. However, GR-S dust in quantities as low as 5 parts by weight causes a n appreciable increase in brittleness. MINERAL-FILLED G R - S HARD RUBBERS
Experimerltal compounds, primarily t o investigate the possibilities of improving the working properties of GR-9 hard rubbers, covered the rangr of mineral fillers commonly used in the rubber industry. With the cool open-roll-mill mixing technique, only one ingredient, a diatomaceous earth designated as Celite 270, had a favorable effect on processing characteristics, The minimum quantity required to produce a stock that could be calendered . without excessive shrinkage was 50 parts by weight on t h e whole polymer. Further compounding experiments with formula9.1, tions based-on GR-S and Celite in these 2% I proportions resulted in the development of 3.0 commercially' adaptable compounds which d 5 2.9 have found extensive use. 4 typ'ical formula
Equipment and methods described in a n earlier publication ( I S ) were used in most of t h e dielectric tests. T h e power faetor and dielectric constant of GR-S and Hycar OS-10 ebonites were measured a t 1, 10, and 100 kilocycles, with the test temperature a t 25' C. and the relative humidity equal t o or less than 40%. The variation of diclcctric constant and power factor with frequency and sulfur content of the GR-S and Hycar OS-10 ebonitcs is shown in Table s'. D a t a on nitrile hard rubbcrs Bs well as d a t a of Scott, McPherson, and Curtis (19) on natural rubber ebonite are included for comparison. The sulfur content of GRS,Hycar os-10,and natural rubber ebonites appears t o have only a small effect on dielectric properties under the given conditions. The change in dielectric constant and power factor with time of cure of the GR-S and Hycar OS-10 ebonites also is slight, as indicated in Table VI. Dielectric constant increases between 4- and %hour cures probably are due to increases in combined sulfur. The power factor of the Hycar OS-10 ebonites is lower than t h a t
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of this type is listed in Table I V (compound E ) . A GR-S whiting-filled hard rubber compound suitable for fabrication of sheets (compound D, Table IV) was examined for electrical surface stability after exposure to ultraviolet light, in comparison with a whiting-filled natural hard rubber and the corresponding natural ebonite. Expowre was by a General Electric S, sunlamp a t a distance of 12 inches; the surface resistance was measured with a high sensitivity galvanometer by the direct deflection method
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Vol. 38, No. 7
IN MINUTES
RIilling ' i t i r d y of llitrile Rubber for Ebonite Fabrication 1000-cc. batches processed in ho. 0 Banbury
The synthetic cbonitcs prepared and examined as described Serve to define the over-all properties obtainable; the fact t h a t special techniques were required t o produce them, honc>vci,,led to an intensive study of the proccssing charactcristics ot GI1-S and nitrilc copolymers, to determine \vhicIi of these base materials were best suited for commercial ebonite fabrication. Since Banbury breakdown is factory prwtice, this method was used in studies of the effcct of time and temperature as well as addition of organic compounding ingredients as processing aids. The Banbury-proce4eed materials werc worked for 5 minutes in 325-cc. portions from 1000-cc. batches on a 6 X 12 inch roll mill set for 0.040-inch clearance under constant conditions; t h e materials were sheeted on the front roll by gradual opening until the hank disappeared. The samp1t.s thcn were cut squarely acro1-5, placed on a talceli surface, and measured 5 minutes after removal t o determine shrinkage. .i 20-minute breakdown at 149-163" C. with Xaugatuck GR-S produced a base material having a shrinkage figure of 27y0. One of the softest GR-S copolymers available, which forms a hole-free band in several passes through the open roll.
