Cellular Ebonites from NitrileTYPe Synthetic Rubbers RICHARD A. CLARK AND LAVERNE E. CHEYKEY; Battelle Memorial Institute, Columbus, Ohio Compounding studies are presented, showing the effect of several variables on the compression properties of cellular ebonites cured at 325” F., including choice of nitrile-type rubber. amounts of sulfur, diazoaminobenzene (blowing agent), and several selected plasticizers, pigments, and accelerators. Effects of some of these variables on the exothermic heat of vulcanization were also hriefly studied.
weight of sulfur combined nrith 100 parts by weight of polymer can be calculated on the basis of one atom of sulfur per double bond of polymer (8,fS, 20). T h e reaction is violently exothermic a serious factor in overheating the product which leads to poroqit: of structure or possibly explosion (6, 20, 23). This tendency for overheating can be greatly reduced by lowering the temperature of vulcanization, diluting the stock with pigments, reducing the amount of acceleration, and decreasing the thickness of the prodiit>t qr nthern-ise aiding i n t h r~r m o r a l of the heat formed (?, 7 i.? i
CELLUI,r\H ebunite is
expanded hard rubber, produced by combining the techniques for producing a hard rubber product with those for “blowing” a soft rubber or plastir sponge or foam. Cellular ebonite was made from natural rubber ds early as 1931, when Beclimann ( 1 ) reported on the electrical properties of accumulator separator diaphragms made from ’,mieroporous” hard rubber. Little technical information has been published on the application of nitrile-type synthetic rubbers t o rhe production of cellular ebonites, although such products have been used for radomes, flotation material, t,herrqal insulation, and sandwich-type structures. T h e Germans have been credited Kith niaking the first ebonite from a synthetic rubber, 1Iethyl Rubber H, during World War I ( 2 1 ) . I n 1938-39 they began to manufacture ebonite from Buna 55, a butadiene-sodium polynier. I n 1940 Gartner (9) reported on various types of German synthetic rubbers for use in hard rubber linings. His data s h o r e d t h a t Buna 85 ebonite was superior t o the natural rubber product in elongation, Buna SS ebonite was superior to Buna S a n d t o natural rubber ebonites in tensile strength, and Perbunan and Perbunan-Estra ebonites n-ere superior to natural rubber ebonite in both tensile strength and shock resistance. Garvey and Sarbach (10) reported concerning ebonites from liycar @R-15, a nitrile-type rubber. Physical properties of the vulcanizates were found to vary with choice and amourit of accelerator, pigment, and softener (8). Cheyney and Robinson (4)reported t h a t Buna S, a butadiene-styrene copolymer. could be reacted with sulfur alone to form ebonites, with a gradual increase in tensile strength during the transition from IO\\ -sulfur soft rubber to high-sulfur hard rubber. This is unlike tht. t4ecis found earlier with natural rubber, although a similar effect hati brei1 briefly reported for Perbunan ( $ 1 ) . GR-S compounding studies made by JIorris, Mitton, Sregnian: and Werkcnthin ( 1 6 ) indicated that 50 parts of sulfur gave optimum physical results, and t h a t by the proper choice of pigments, softeners, and accelerators, the tim? of cure could be shortened and ease of processing could be improved. It appeared difficult, however, to improve the physical properties beyond that of the base stock containing only GR-S and sulfur. Tensile strengths were reported slightly under 10,000 pounds per square inch, which is comparable t o those of ebonites from natural rubber (8). Two distinct advantages of ebonites produced from nitrile-type synthetic rubbers are higher tensile strength and higher thermal softening point, nhich has been reported to be in excess of 250” F. (15, 19). I n the vulcanization of natural and synthetic rubber ebonites, it has been found t h a t the vulcanization coefficient (parts by 2
Present address, Pollock Paper Corporation, Middletown, Ohio.
PREPARAl’lOh OF CEL1.I: L 4 H M.4TERIAL4
In the production of soft rubber sponge, tn-0 general nirthudr are employed: ( a ) the foaming of rubber lates, followed 0 - gelation and cure, and ( b ) the exparision of slab rubber stocks, foilowed by cure requiring special compounding (8,6, 1 1 , 1 4 , 16, 2.2;. The lates method is not generally considered applicable for the production of cellular ebonite. The expansion of slab rubber stock is usually obtained by chemically generating gases within the st’nck--for example, by thermal decomposition of diazonminobenZene, n-hich begins to liberate nitrogen gas a t about %MoF. ( y j . Other methods involve impregnating the stock with a gas ( 1 8 ) or solvent ( 1 9 ) and securing expansion by pressure release 07 thermal means. Both the chemical and impregnation mrthods are adaptable to, and are being i1aed for, the production of c ~ l l u - lar ebonites. I n the production of cellular etmiitw, the balance betweeii the “bloffing” pressure and the rwirtancxe of the stock to espansion 1s even more critic;il than it is in that of soft rubbers. This coiidltion results from the greater range of stiffness throughout Cht curing cycle of cellular ebonite, and the necessity for obtaining complete espansion early in the cure. Problems also assovi:ited with the curing of noncellular ebonite, such as the esothrr,mic. heat of vulcaniz:-rtion, complicate still further the production of orllulnr ebonite. The scope of this study \?:is limited largely to conipouidinp studies directed toward developing a cellular ebonite having high cwniprcwinn strength, relative irisensitivit>- to thermal ch:tngeb ( -BO” to 180‘ F.‘I. and capable of being formed “in place’‘ without sllrinliage. The nitrile types were believed t o be best suited for t h i s application, in view of the superior properties reported tiy (ithPv ior noncellular ebonites from this type of synthetic rubher. PROCEDURES AZID TECHNIQUES
Cellular ebonite batches were compounded on a Iaboratory-.*ize rubber mill, with a 15-minute normal breakdown time for the base rubber used. The molds had four 3 X 3 X I inch cavities and two /b-incli steel top and bottom plates. After the uncured specimens were loaded into the molds and the assembly was fitted together by C-clamps, the loaded molds were placed in a thermostatically controlled, circulated-air oven for the designated cure time. I n some cases thermocouples were placed in the center of the specimen, and the t r m p c ~ a t u r rwas recorded on a Brown recorder. I n most iristaricrs approrimstely 1.25 hour6 were required for the specimen to reach a cure temperature of 325” F. Cured specimens were not removed from the mold until cool. After removal, the test specimens were trimmed to 2.5 X 2.5 X 1 before tasting. (30nipreqeion tests on the individual test specimens were run in a 2252
INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1949
Baldwin-Southwark Universal testing machine (head traveling rate, 0.04 inch per minute); strain readings were taken with ii dial-type micrometer. Stress-strain plots were made, and the compression modulus (slope) and yield stress (stress a t which a deflection occurred in the stress-strain curve) were obtained from the curves. Compression st,rength was taken as the stress a t which the specimen either completely failed or began to show,a relatively large increase in strain for a relatively small increase i n *tress. Figure 1is a typical compression stress-strain curve.
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CHOICE OF NITRILE-TYPE RUBBER
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Table I reports results obtained n-ith seven commercial nitrile~ x p erubbers, all cured the same time in the same recipe. The ,Jmii;.;ion of a plasticizer in some pre1imin:iry work indicated that 5lon.s without a plasticizer n-ere incomplete. Hence, castor oil, a 3uccessful plasticizer employed commercially in products of this :ype, in the amount of 10 parts per 100 parts of rubber, n-as ill+luded in the basic recipe. Quantitiw for the other ingredients in the recipe were arbitrarily +elected. These cures shon-ed that 'he best expansion and kaat shrink:ieP on cooling were ohtained
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Effect of Density on Compression Properties o f Cellular Ebonite
nith the batch containing Chemigum K3 as the base polymer. On the basis of these screening tests, future work was limited to Chemigum K3 and later to Chemigum S3XS ( a nonstaining variety), when the 1 3 variety WRS not available. EFFECT O F D E S S I T Y A K D TIXIE O F CURE
Figure 2 s h o w the effect of density on compression properties of a typical product. .I commercial uncured stoclr was used. 4 s z R-ould be expected, both the compression strength and the com0 m pression modulus increase with density. For comparative pur' m poses, the compounding studies n-hich follow are all presented on 2 . I n the same density basis-i.e., a mold loading density of 20 pounds v) W per cubic foot, Tvhich gave product densities in the range of 18 t o v) I20 pounds per cubic foot. Ailarge laboratory batch !vas made up according to the recipe given in Table I for Chemigum 53. Half of the batch was cured a t varying times the following dsy. and the other half was similarly cured one week later. Figure 3 shows the results of compres10 20 30 40 50 60 70 sion tests on the vulcanizates. The ?-ield stres? was higher for the STRAIN, INCHES DER INCH X 10 ' batch aged only one da!-. This indicates that fresh stock would Figure 1. Typical Compression be used Then possible. Lncured stocks tended to blow as much Stress-Strain Curve for Cellular Ebonite as 100% as a result of shelf aging alone, which suggested that the differences in yield stress obtained for the 1-day and 7-day aged stock might be TABLEI. EFFECTO F CHOI('E OF BASERUBBER03 P H Y 5 I C A L P R O P E R T I E - O F CELLCLAR rGaused by differences in cell EBONITE structure. The best length of 2 7 ti , Kacipe, parts by w t . cure is believed to be in the Perbunan 18" Perbunan 26a range 6 to 8 hours, although i 00 . , Hycar OR-25 h ... the I-hour cures gave comHycar OR-15 h .. Chemigum S3 e ... LO(! . .. prcssion properties of about Chemigum A-4 ... 100 the same order of magnitude Butaprene S T d , , . .. . 5. 106 Zinc oxide 5 5 5 for the 1-day aged stock. Beniothiaayl disulfide (.%Itax) * 1 1 1 1 Sulfur 3 .i 3.5 35 35 Cures made a t 300" F. gave Diaaoaminolirnacne (Unicel)! 20 20 20 20 results in approximately the Castor oil ( . i . i ) b ' 10 10 10 10 1Iooney visrwity a t 21Y0 F. 29.0 8 , .j ,5 . 0 2.0 aame order, evcept that conCURE: 1 HR. .%'I d 7 A J €..I H R . A T 300' F., I ' O L L O R ~E'L U 2 HH.A P 325O E .iderably lower compression L'ercentage of mold (2 inch i.d. X I ini,lii filled f o r loading o f : properties were obtained for 10 lb./cu. ft. 20 30 50 40 75 40 60 15 Ib./cu. ft. the 4 h o u r cures. To reduce 33 40 73 70 100 60 90 20 lb./cu. f t . 50 80 I00 LOO 100 100 100 the curing time as much as Diam., inches, of specimeu. from Irlr)ld icir h a d i n g of: possible, a curing temperature 10 Ib./cu. ft. ... .. 1.99 ... 1.81 15 Ib./cu. ft. 1.91 , . 1.91 1.97 1.99 1.91 1.87 of 325" F. was adopted a8 20 Ib./cu. f t . ,. .. 1 91 1.94 1.99 1.90 1.87 3tandnrd Properties a t mold loadirip of 20 ll,., c u . i t
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Compression modulu?. I h . / s q . in. 540u h Lltimate compression .rrength. Ih. .
