+
* &OLD-RESISTANT SYNTHETIC RUBBER
I
R. H. CROSSLEY AND C. G. CASHION
+,
y.
Xylos Rubber Company, Akron, Ohio
oil-wing Butaprene NF, a new oil-resisting synthetic rubber with strength, especially after oven excellent flexibilily at low temperature, ir dixusaed. Dab aging, as the stearic acid is innow the are presented showing the effect of several compounding creased, this trend was not convariables on c e h i n propartias. The effects of varying United States, the types msde by firmed in other related work done the copolymerisation of acrylonithe 11..rIc acid, zinc oxide, lead oxide, magnesium oxide, in this laboratory and cormM e and butadiene have attained and sulfur concenbations, the types and connnkations of quently is not regarded as si@ considerable importance. Many acceleraton, RII*n, and dtcnen, and the time of cure a n cant. The stearic acid variation had no d e c t on the other properrepeikd. The effect of t h a e variations on cold mistance, vsriations of these types, dm known as Buna N, are on the hdl. sbength (before and after oven aging), compression ties measured, and it may be 14, and milience are &own. Swelling data in various mar!& under the trade names omitted entirely without adverse gawlina and oils a n praented. Typical compounds e5ects. Itisthougbtthatstearic of H y c a r O R , P e r b u n a n , having superior resistance to low temperatures an r e p o d . acid in Bome c%w may bloom Chwigum, and Bntaprene. At to the surface of uncured mixes pRsent nearly all of these synand thus interfere with proper thetic rubbers 818 going into the “healing” or ”knitting” during molding, although this would manufacture of essential equipment and accessories for aircraft be difficult to demonstrate. Since it does not appear e&nand other combat vehicles, including bullet--ling fuel tanks and fittings,MAE, d i s p h m g ~ ~hose, ~ , pacldng, and other e q u i p tin1 in Bntaprene N F vulcaniaates. it has been omitted from ment operating in contact with oils and fuels. subsequent test recipes in this paper. hlthoU& resiatsncs to Oils and fuels has been the p r i w Effect of Metalllc Oxides. The e5ect of varying the amounts W w n t Of Products made from B u m N rubbers, it has been of sinc oxide, aide, and litharge in formula 11 is m e e d that they must a h be able to fundon d e r the shown in figure2. It is e+dent that a metallicoxide necessary extreme temperatures likely to be met in service. since -70’ F. in a ~~b~~~ NF compound con&ng a thiaaoletype aWiera t conceivably be encountered, fl&b*tY in this temperature *tor if tensile strength is to be attained. when 3 to be an ?Pbt me has Of Oil-parts per 100 of hydrocarbon are used, there is little to c h o m pE& for aircraft. between the three oxides in their e5ect on physical properties. Buna N types generally have poor %&bfitY at low h p e m The minimumamount newas not determined, and it is tures, dtbOugh this Property Can be improved bY Proper COmpogsible that leas than 3 parte per 100 of hydmwhon might be pounding, particularly by the choice and amount of PbtiCiser Increasing the oxides from 3 to 6 parts did not (8, 5, 6, 8, 7). A speoial type of Buna N, however, hss been the results significantly, except that 6 parta of magnesium p m d u d which pe x d e n t cold reaistsnce, at Some oxide appeared to improve cold A a n c e . There is Limited sacrSce Of Oil resistance and tensile This rubber is evidence tbat maguegium oxide may be somewhat more “scorchy” B u t a p m e NF, a modified copolymer of butadiene and mrYl0in p r ~ i n g zinc oxide, and since the latter is mast comnitrile. Ita vulcanisates are c h c t e r i d by %exibilitYSt VerY monly in compounding, 3 parts of oxide 100 parts and their properties can be low by variaof hydrocarbon should be included in Butaprene N F recipes tions in compounding. employing a thiasoletype accelerator. MONG the
Tad Formulas and M W r The data were obtained in the test formulas shown in Table I. Formula V represents the standard basic recioe from which each of the others was obtained by v 4 one or more inmedients. AU comwunds were cured
Effect of Sulfur Concmbation. The deet of varying the sulfur concentration in formula 111 from 1 to 3.5 parta per 100 of Bntaprene N F is shown in Figure 3. As the sulfur was increased, there was a corresponding increase in hardneas and rebound resilience at 212’ F. and a decrease in elongation. Tensile strength, original and after heat aping, appeared to reach optimum values at 2.5 to 3 parts, 88 did compression set, but higher elongations after heat aging were obtained with somewhat lower sulfur. The cold resistance was d e c t e d by variation in sulfur content of 1 to 2.5 parts, but 3 and 3.5 parts raised the brittle point slightly. From all data presented, 2 to 2.5 parts of sulfur oer 1M) arts Of ButaDrene NE a D W be 88tkfaCbIY COUCeUirations:
&d m&&inaccordance w l b standard practiEe as recommended by A. S. T. M. ( I ) , except that low-temperature %exibility was measured bv the brittle-mint method of 8elker. Winsoear. and
steelbd i / , inch in & m e t e r falling 25.4 inches on a cylindrical
sample 1.59 inches in diameter and a/, inch hgh. &bound and wmpresaion set samples were cured 50 minutes at 312’ F. Normal tensile nrooenies were obtained on lea cured 20, 30, and 40 m i n k e i a t 312’ F. but for simplicitj, only the 3 0 minute cures were reportad. other tests w m obtained on samples cured 30 minutea at 312’ F. Compression set was dstsmined by A.S.T.M. Method B after aging the samples for 70 hours at 212’ F. The S ~ D I E Sw e e deflected according to hardnew, as specilied in the A:S.T.MM.4.A.E. claasdicatioui of rubber compounds ( 1 ) . Thus, a 50durometer sample would be de%ected 30 m cent of its orininal thickncss. a 70dummeter
hl
ERca of Accelerator Concentration. Several types of accelerator were tried with Butaprene NF, and best all-round d t s are obtained with the thiazoles or their derivatives. The de& o f . .varvinc --.--the m o u n t of beneothiaavl disulfide in formula IV is shown in hgure 4; benzothiasyl disd6de wm chosen because satisfactory results have always been obtained with it. Figure 4 indicates that varying the accelerator had no great e5ect on the ~
Effect of Stearic Acid. The d e c t s of variable amounts of stearic acid on Bntaprene N F in form& I is shown in Figure 1. while there appears to be a tendency toward improved ten&
85
INDUSTRIAL AND ENGINEERING CHEMISTRY
56
Vol. 36, No. 1
I400 51300
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Effect of Stearic A c i d on Butaprene Formula I
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1944
FOR BUTAPRENB NF TABLE I. TESTFORMULAS
Formula No, I" Butaprene N F 100 6. R. black 50 Bardol 5 Dibutyl phthalate 5 Zina oxide 3 Sulfur 2.5 Benzothiazyl disulfide 1 6 6
116
100 50 5 5
...
2.5
1
I11 100
IV 100 50
V 100
3
6 3 2.5
50 5 5 3 2.5
1
Varied
1
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VI= VIId VI116 100 100 100
... 50 5 ... 5 ... 3 3 2.5 2.6
1
50 5
5 3
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Stearic acid content varied. b Metallic oxide content vsried. Carbon black loading varied. d Softener varied. e Accelerator varied.
properties were chosen (Figure 5 ) . The rates of cure of the sulfurless compounds were approximately the same as t h a t of the benzothiazyl disulfide-sulfur control, and for simplicity only the 30-minute cures at 312" F. were reported. Both sulfurless compounds were lower in original tensile than the control, but both were superior in retention of tensile and elongation after heat aging, and increased less in hardness. Both sulfurless compounds were greatly superior t o the control in compression set. The tetramethylthiuram disulfide recipe was measurably better than the control in cold resistance, and equivalent in rebound resilience. The dipentamethylenethiuram tetrasulfide compound appeared slightly poorer in cold resistance than the control but superior in rebound resilience. G O M P RE SS r0 N
57
Therefore, Butaprene NF compounds vulcanized with the thiuram-polysulfide type of accelerator without added sulfur are superior t o the conventional recipe containing sulfur; the retention of tensile and elongation after heat aging and the compression set and cold resistance are better, and these formulas are recommended wherever lower original tensile strength can be tolerated. Effect of Time of Cure. The physical properties of Butaprene N F in formula V, cured from 10 to 160 minutes a t 312" F., are shown in Figure 8. Tensile strength reaches a maximum in 20 to 40 minutes, and drops slightly to a value which then remains constant. There is surprisingly little change over the whole range of cures in the other properties measured, indicating a very flat type of cure. Effect of Loading. Like many synthetic rubbers, Butaprene N F requires the addition of reinforcing fillers to attain optimum physical properties. Carbon blacks are most commonly used and have been found to impart maximum reinforcement to Butaprene NF. The physical properties obtained by loading 100 parts of Butaprene N F in formula VI mith 25 to 150 parts (14 to 84 volumes) of easy-processing channel black, semireinforcing black, fine thermal black, and medium thermal black are shown in Figure 7. The highest tensile strength is obtained with easy-processing channel black a t a loading of 75 to 100 parts (42 to 56 volumes) per 100 parts hydrocarbon. The next best tensile strength was imparted by semireinforcing black a t a loading of about 150 parts (84 volumes), followed by fine thermal black at 150 parts (84 volumes). Durometer hardness was highest with easy-processing channel black, and elongation lowest. Compression set appeared to be independent of type or amount of black. Cold resistance decreased in each case as the loading (and durometer hardness) increased; easy-processing channel and medium thermal blacks were slightly better than the others. Resiliency decreased in each case as the loading increased; easyprocessing channel black caused greatest decreases and gave the poorest resilience at all loadings, while medium thermal black was best.
Effect of Softeners. Unlike natural rubber, the Buna N types require comparatively high concentrations of softener to be processed in the factory. Such high softener concentrations naturally play a significant part in modifying the physical properA S c A 8 C ties of the vulcanizates; it is not surprising, therefore, that a tremendous amount of work in evaluating softeners has been Figure 5. Comparison of Sulfur and Sulfurless Cures on done by many laboratories. Excellent and exhaustive softener Butaprene N F in Formula VI1 CURATIVLS PARIS PER 100 PARTS BUTAPRENE NF studies have been published in booklet form and widely cirA. Sulfur 2.5, b e n r o t h i k 4 disulflde 1. E . Tetramethylthlursm disulfide 4. culated (4, 9). C. Dipontamothylanothlursm tetrarulflde 3. Three general types of softeners have been found satisfactory' for use with Butaprene N F t ] I I I I I 0 1600 -namely, coal tar derivatives, organic esters, vl and organic ether-esters. Although it is beE 1500 yond the scope of this paper to evaluate the E 1400 T E N SI LE STRENGTH many softeners commercially available, data are included showing the effect of these three types on the physical properties of Butaprene N F vulcanizates. Accordingly, the effect of varying amounts of Bardol (coal tar derivative), dibutyl phthalate (ester), and tributoxy ethyl phosphate (ether-ester) on Butaprene N F in formula VI1 are plotted in Figures 8 and 9. Varying amounts of a 50-50 blend of Bardol and dibutyl phthalate are compared to tributoxyethyl phosphate in Figure 8. With both softeners a gradual reduction of tensile strength and hardness was realized as the softener was increased. The blend Figure 6. Effect of Time of Cure on Butaprene N F in Formula V
INDUSTRIAL A N D ENGINEERING CHEMISTRY
58
Vol. 36, No. 1
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Effect of Carbon Black Loading on Butaprene NF in Formula VI
produced higher tensile and elongation a t all softener concentrations than the ether-ester. Compression set became poorer as the softener was increased for both types, the ether-ester being superior a t all concentrations. Both softeners improved rebound resilience a t room temperature, the ether-ester being slightly better. At 212' F. the results were somewhat variable, but again the ether-ester was slightly superior. Both softeners
25 PER
IO0 PARTS
1
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I
12 5 SOFTENER-PARTS
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37 5 BUTAPRENE
Effect of Softeners on Butaprene
NF
NF in Formula VI1
improved cold resistance, the ether-ester being markedly superior to the blend. A comparison of 25 parts each of Bardol, dibutyl phthalate, tributoxyethyl phosphate, and a 50-50 blend of Bardol and dibutyl phthalate in formula VI1 is shown in Figure 9. Bardol, dibutyl phthalate, and a blend of the two are superior to tributoxyethyl phosphate in tensile and elongation, have higher Shore hardness, are inferior in compression set and cold resistance, and are equivalent in rebound resilience. Swelling Data. Although Butaprene N F should not be regarded as a highly oil-resisting material, its behavior when
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1944
59
plications requiring high resistance to aromatic hydrocarbons. The behavior of Butaprene N F with different types of softeners when exposed to petroleum base oils is shown in Figure 11. A comparison was made of no softener with 25 parts each of dibutyl phthalate, Bardol, and tributoxy ethyl phosphate in formula VII. Immersion was 70 hours a t 300' F. in A.S.T.M.-S.A.E. rubber processing oil (aniline point 159" F.) and in aircraft engine lubricating oil (aniline point 259' F.). Comparatively high swelling was obtained with all compounds in the low aniline point oil as would be expected. I n the aircraft engine oil a slight volume increase was obtained with the compound containing no softener as well as with that containing 25 parts of Bardol; but all other compounds displayed shrinkage, probably indicating a leaching out of the plasticizer. A Figure 9.
B
C
D
A
B
C
D
Effect of Softeners on Butaprene N F in Formula VI1 PARTSSOFTENER PER 100 PARTSB U ~ A P R E NNF E
A. E. C.
D.
Bardol1P.5,dlbutvlphthalate 12.5
Bardol 25
Dibutyl phthalate 25 Tributoxyethyl phorphate 25
I-
E 20 u a a w '0 0
Figure 1Q. Effect of Various Gasolines on Butaprene N F in Formula V
exposed to hydrocarbons of various types is worthy of mention. Swelling characteristics in three kinds of gasoline are shown in Figure 10. Formula V was used, and volume increase data were obtained by A.S.T.M. S t a n d a r d Method B (1) after immersion for 70 hours at 80" F. Moderate swelling was obtained in the 65 and 100 octane gasolines, compared to very high swelling in the aromatic blended fuel. B u t a p r e n e N F should not be used in ap-
TABLE 11. TYPICAL BUTAPRENE NF COMPOUXDS Formula Butaprene N F Tributoxyethyl phosphate Semireinforcing black Zinc oxide Polymerized trimethyldihydroquinoline (antioxidant) Sulfur Benzothiasvl disulfide Cured 30 min. at 312O F. Normal teats Elonption. % Tensile, Ib./sq. in. Shore hardness A ed 70 hf. a t 212' F. in air oven %ongation, % Tensile, Ib./sq. in. Shore hardness Immersed 70 hr. a t 80° F. in 05-octane gasoline Volume increase, yo Immersed 70 hr. ? t 212' F. in petroleum-base hydraulic fluid ' , Elongation % Tensile I b j s p in. Shore h'ardness Volume increase, Yo Cold resistance A.S.T.M.-S.A.E. bent-looxI test (5 hr. a t -70" F.) Brittle point, O F. Cured 50 min a t 312" F. compression set tested by A.S.T:M. Method B (70hr. a t 212O F.) Deflection, % Set, Yo of de5ection
A 100 14.5 77 3 2 2.5 1
B 100 25 60 3
... 2.5 1
-
-
200.0
181.6
310 1770 59
345 1000 45
200 1640 73
230 1275 53
23 210 1080 51 I8
+
OK
..
.. OK
- 63
- 83
30 45
30 40
Typical Formulas. The compounding data presented make possible the formulation of compounds from Butaprene N F which have interesting physical properties. Recipes varying in durometer hardness from 35 to 85 are possible, and are characterized by excellent cold resistance together with moderate swell in aliphatic hydrocarbons. Typical 45- and 60-durometer compounds are illustrated in Table IT. Formula A (59-durometer) combines intermediate tensile strength and moderate swell in 65-octane gasoline and petroleum-base hydraulic fluid (aniline point 189' F.) with excellent low-temperature flexibility as judged by the A.S.T.M.S.A.E. bent loop test ( 1 ) of 5 hours at -70" F. and a brittle point (8) of -63" F. Formula B (45-durometer) was lower in tensile strength and also withstood the bent loop test after 5 hours a t -70" F. Its cold resistance was considerably better than that of formula A , judged 5 +5 by its brittle point of -83" F., doubtless because of the higher amount of softener and % lower black loadings. w z+2 Swelling data on formula B were not obtained. Conclusions. B u t a prene N F is a special5 ," purpose Buna N type rubber, and is characterized by its resista ance to stiffening at low Figure 11. Effect of Softeners on temperatures. T h e Swelling of Butaprene N F in properties of Butaprene Formula VI1 N F vujcanizates can be modified by variations in type and amount of compounding ingredients. Butaprene N F compounds may be prepared which have intermediate tensile strength, moderate swelling in petroleum base hydrocarbpns, and excellent low temperature flexibility. LITERATURE CITED
(1) -4m. SOC.of Testing Materials, Standards on Rubber Products. Feb., 1943. (2) Cole, Trans. Am. SOC.M e c h . Engrs., 65, 15 (1943). E N G . CHBM.,33, 602 (1941). (3) Garvey, Juve, and Sauser, IND. (4) Hycar Chemical Co.,'%oftener Study for Hycar OR", 1941; "Softener S t u d y 2A for Hycar OR-15", 1942. ( 5 ) Kemp, Malm, and Winspear, IND. E N G .CHEM., 35,488 (1943). (6) Moll, Howlett, and Buckley, Zbid., 34, 1384 (1942). (7) Morris, James, and Werlcenthin, I b i d . , 35, 864 (1943). (8) Selker, Winspear, and Kemp, Zbid., 34,157 (1942). (9) Stanco Distributors, Inc., "Perbunan Compounding and Processing", 1943. PR~SENTED before the fall meeting of the Division of Rubber Chemistry, AMERICANCXEMIOAL SOCIETY, in New York, N. Y.,1943.
