January, 1934
INDUSTRIAL AND EKGINEERING
alliyield product< that are deficient in extencibility. Another specific effect is that due to phenyl. P-Phenylbutadiene polymerizes quite rapidly, but the product is mostly the crystalline dimer. The small amount of higher polymer formed is soft and probably has a relatively low molecular weight. (This effect of phenyl is further illustrated by observations on other aryl butadienes made by Xhitby and Gallay, 11 .) The formation of dimers is ah-ays a competing reaction in the synthesis of rubber from dienes, and it rapidly becomes more serious the higher the temperature used. At ordinary temperatures the rate of dimer formation from isoprene and chloroprene is roughly the same, but the temperatures required t o obtain 50 per cent polymerization of the two dienes in 10 days are respectively about 90" and 25' C., and the percentages of dimer in the products at these temperatures are about 40 and < l . This fact gives addi-
CHEXlISTKY
33
tional emphasis to the importance of a high rate of polymerization. (1)
LITERATURE CITED Berchet and Carothers, J. Am. Chem. Soc., 55, 2034
-
(1933).
(2) Carothers, Chem. Rev., 8, 364 (1931). (3) Carothers and Berchet, J . Am. Chem. Soc., 55, 2507 (1933).
(4) Ibid.,55, 2813 (1933). ( 5 ) Carothers and Coffman, Ibid., 54, 4071 (1932). (6) Carothers, Collins, and Kirby, Ibid.,55, 786 (1933). (7) Carothers, Williams, Collins, and Kirby, Ibid., 53, 4203 ( 1 9 3 1 ) . (8) Jacobson and Carothers, Ibid., 55, 1622 (1933). 19) Sieuwland, Calcott, Downing, and Carter, Ibid., 53, 4197 (1931). (10) TThitby and Crozier, Can. J . Research, 6, 203 (1932). (11) Whitby and Gallag, Ibid., 6, 280 (1932).
RECEIVED November 23, 1933. Thie paper is Contribution 138 from the Experimental Station of E. I . du Pont de Kernours h Company.
Physical Properties of DuPrene Compounds E. R. BRIDGWATER E. I. du Pont de Nemours & Company, Wilmington, Del. black per 100 volumes of rubber HERE is perhaps no Although DuPrene differs chemically f r o m or DuPrene. The two DuPrene e n g i n e e r i n g material natural rubber, it bears a closer physical resemcompounds each contain 2 per that has so wide a range blance to rubber than any of the previously cent of the antioxidant, phenylof properties and so great a diknown synthetic rubbers. I t differs f r o m nafural / ? - n a p h t h y l a m i n e . This is versity of uses as v u l c a n i z e d rubber in inany respects which makes it more suitperhaps the best antioxidant for rubber. By appropriate selecDuPrene compounds, and 2 per tion and proportioning of acable f o r certain purposes and less suitable f o r cent of it is completely soluble in celerators, antioxidants, reenothers. Its physical properties are susceptible DuPrene, both before and after forcing agents, etc., and by varyof wide vuriation depending on the nature and vulcanization. However, this ing conditions of vulcanization amounts of vulcanizing agents, reenforcing piga m o u n t of phenyl-/?-naphthylone can modify the properties of ments, etc., that are compounded with it, and it amine is not completely soluble rubber in a l m o s t any desired in rubber and, if used in rubber, direction. S e r v i c e conditions fills the need f o r a rubber-like material having would bloom to the surface after that a few years ago could not greater resistance to the action of oils and solvents vulcanization. T h e r e f o r e , an be fulfilled with any known type inand greater resistance to the deteriorating equal weight of phenyl-a-naphof rubber compound are today fluences oj' heat and oxjdation than natural rubthylamine was used in the rubbeing met with modern rubber ber. ber c o m p o u n d s since it is an compounds which may not even eauallv efficacious antioxidant involve any new raw materials but merely an artful combination of compounding ingredients for rubber and has the advan&ge"of being more soluble in rubber and, consequently, nonblooming. The DuPrene that have long been in use. The rubber chemist has even greater latitude in selecting and the corresponding rubber compounds also differ in other compounding ingredients for DuPrene than for rubber. respects. For example, compound D-10 contains rosin and Consequently, there is no one ideal DuPrene compound, but magnesia which are desirable ingredients for DuPrene comrather there are hundreds of DuPrene compounds, each of pounds but are not of particular value in a rubber compound which represents some chemist's effort to develop the composi- of this type. R-10, on the other hand, contains the accelertion best suited to meet a particular set of service conditions. ator mercaptobenzothiazole, stearic acid, which functions Likewise, results that we are today unable to obtain with as an activator for the accelerator, and, of course, sulfur. DuPrene we may next year be able to realize, not through im- TABLEI. COMPOSITION OF DUPRENE AND NATURAL RWB~R provement in DuPrene itself but through growth in our knowlCOMPOONDS edge of DuPrene compounding. D-10 R-10 D-11 R-11 Duprene type D 100.0 100.0 The purpose of this paper is to compare the physical propSmoked sheets 100.0 100.0 erties of DuPrene and natural rubber. The most one can do Channel carbon black 36.0 45.0 Zinc oxide. 10.0 6.0 10.0 6.0 is to compare the properties of certain popular types of DuLight-calcined magnesium oxide 10.0 10.0 Prene compounds with similarly compounded rubber stocks. Phenyl-B-naphthylamine 2.0 2.0 Phenyl-a-naphthylamine 2.0 2.0 The tests reported in this paper were conducted on two DuWood rosin 5.0 5.0 Sulfur 3.0 1.0 3.0 Prene compounds, D-10 and D - l l , and on two rubber comhlercaptobenaothiazole 0.5 0.7 pounds, R-10 and R-11, whose compositions are given in Stearic acid 1.0 4.0 Pine tar 2.0 Table I. Compounds D-10 and R-10 are of the pure gum Cottonseed oil 3.0 type; that is, they contain only such compounding ingrediCURINGRANGE ents as are essential for vulcanization. Compound R-11 is of the type commonly used for tire treads and D-11 is its One of the most desirable properties of a rubber compound DuPrene counterpart, both containing 25 volumes of carbon is the ability to produce a good vulcanizate over a wide range
T
'
.
IN DUSTR IA L A N D EN GI N EERI N G CH E M I STRY
34
of time and temperature of vulcanization. This is particularly essential for bulky articles which, on account of the low heat conductivity of rubber, are vulcanized a t a higher temperature on the outside than in the center. 1IW
UXIO
-4
-2
6w
8
c:
mse 400
WmSrntn
3Omm
lhr
2hrs
4hn
8hm
l6iu~z.lhn
T l n E OP CUPe AT Ul'C.
FIGURE 1. TENSILE STRENGTH AND ELONGATION OF CoxPOUNDS us. TIMEOF CUREAT 141' C.
Figure 1 shows the tensile strength and elongation of these four compounds plotted against time of vulcanization a t 141" C. The vulcanizing ingredients have been so proportioned that all four compounds reach their optimum cure in approximately 45 minutes. The range of cure of all four appears to be good, bearing in mind that the plateau of the vulcanization curves has been foreshortened by plotting the time of cure on a geometrical scale. It is noteworthy that the addition of carbon black has but little effect on the maximum tensile strength of DuPrene but nearly doubles the maximum tensile strength of the rubber compound. This is a characteristic behavior of DuPrene as compared with rubber.
AGINGPROPERTIES One might conclude from study of Figure 1 that rubber has as broad a range of cure as DuPrene, but the fallacy lies in the fact that compounds R-10 and R-11 have poor tear resistance and aging properties when vulcanized more than 1 hour a t this temperature, and when vulcanized for more than 2 hours would be commercially almost useless. Figure 2 shows tensile-time and elongation-time curves on compounds D-10 and R-10 after aging in the oxygen bomb for 4 days at 70" C. under 300 pounds per square inch (21.1 kg. per sq. cm.) oxygen pressure. It is possible to compound rubber in such a manner as to make it age better than R-10 when badly overcured, but it is quite impossible to produce a rubber compound whose aging properties will be SO completely unaffected by variations in cure as is the DuPrene compound D-10. Neither rubber nor DuPrene is ever cured in such thick masses that the outside of the article gets twentyfour times as much cure as the center, but the aging properties of the long cures are of practical interest to the extent that they indicate the effect on the aging properties of DuPrene or rubber of prolonged exposure to high temperatures in service. The general experience of this laboratory with compounds of the R-10 type indicates that the 40-minute cure, which is just short of the optimum tensile, has the best aging properties. Consequently, this cure was chosen for further aging tests in the oxygen bomb and in air a t 100" C. DuPrene compounds, on the other hand, generally age best when cured to the point of maximum tensile strength, and, as pre-
Vol. 26, No. 1
viously shown, prolonging the cure beyond that point causes no deterioration in aging properties. Therefore, compound D-10 (cured 60 minutes) was chosen for prolonged oxygen bomb and 100" oven-aging tests. The results of the oxygen bomb aging tests are shown in Figure 3. The properties of compound D-10 were affected hardly a t all by aging for 28 days, whereas R-10 deteriorated badly in 5 days and retained practically no strength a t all after aging 4 days. Results of the 100" oven aging tests are shown in Figure 4. I n this test the DuPrene compound deteriorated to some extent but a t a far slower rate than the rubber compound. Again, it should be pointed out that rubber compounds can be produced to withstand this test better than R-10, but no rubber compound can be produced that will withstand oxygen bomb aging as well as properly compounded DuPrene.
PHYSICAL PROPERTIES OF VULCANIZATES Figure 5 shows stress-strain curves on these four compounds when cured 45 minutes a t 141" C. The DuPrene compounds take up load more rapidly during the early stages of extension but more slowly toward the end of the extension. Having observed this difference in a great many comparative tests, one is inclined to regard i t as a general characteristic of DuPrene as compared with rubber, although tomorrow a type of compound may be found that will disprove the generalization.
280 ZW
240
Ern
w m 880
l 1 1 1 l I l l l l l I l l l l l 401hr2hn rnm
8hn
4hn
lbhn
z4lns.
l l % E OB C U R E A I 141.C
FIGURE 2. TENSILE STRENGTH AND ELONGATION OF COMPOUNDS D-10 AND R-10 us. TIMEOF CUREAFTER 4 DAYS IN OXYGENBOMB
It is even less safe to draw general conclusions as to the comparative abrasion resistance of DuPrene and rubber because so many different types of abrasion are encountered in service and also because temperature, degree of distortion, etc., have such a profound effect. Consequently, there is no laboratory abrasion test that can be relied upon to enable one to predict exactly the difference in abrasion resistances under any given set of service conditions. Table I1 shows results of abrasion tests on compounds D-11 and R-11 that were run on the du Pont abrasion machine. TABLE11. ABRASIONTESTS COMPOUND
AT
CURE 141° C.
Minutes
R-11 D-11 R-11
D-11
50 60 50
60
TEYP. TEST
OF
O
C.
28 28
50 50
ABRABION
Loss
Cc./h.p.lhour 177 184 Sticky 176
The rubber and DuPrene compounds were cured 50 and 60 minutes, respectively, which seemed to be the cures that would abrade best a t 28" C. The DuPrene compound showed
January, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
slightly greater abrasion loss, but the difference was within experimental error. At 50' C. the rubber compound softened t o such an extent that it could not be tested on this machine, although i t is undoubtedly a practical compound for a tire tread a t an operating temperature of 50" C. I n general the abrasion resistance of rubber declines sharply with increasing temperature, but it appears from this test and from observations on other compounds that temperatures up to 50" C., and possibly higher, have but little effect on the abrasion resistance of DuPrene compounds. A common cause of failure of rubber compounds is flexcracking. Since this type of deterioration is generally conceded to be intimately associated with oxidation, it is not surprising to find that DuPrene compounds resist flexing many times better than similarly compounded rubber stocks. Compounds D-11 and R-11 were compared on a flexing machine which makes use of a test piece with grooves molded in the outer surface and with a tire fabric reenforcement on the back (Figure 6). The test piece is oscillated under a pulley supporting a 50-pound (22.7-kg.) weight. Under these testing conditions, compound R-11 showed slight nicks a t the base of the design in 8 hours and definite cracks about 1 mm. deep in 14 hours, and was broken through to the fabric base at the end of 24 hours. This is average behavior for a properly compounded tire tread stock under these testing conditions. The test on compound D-11 was continued for 80 hours, a t the end of which time no indication of failure was observed. Figure 6 was photographed after 24 hours of running on compound R-11 and 80 hours on 11-11.
OILRESISTANCE One of the most striking differences between DuPrene and rubber is the greater resistance of DuPrene to the swelling and deteriorating action of oils and solvents that have a notoriously bad effect on rubber. The resistance of rubber to IlOO IWO 400
z
m8
7M 6
bag
mi: 4a)* xx)
35
giving it what is technically known as a very tight cure. Consequently, oil immersion tests were run on samples cured 45 minutes at 141' C. The test samples were 2.5 cm. wide, 12.5 cm. long, and 2 mm. thick. These 1100 samples were immersed Iwo Qm in v a r i o u s s w e l l i n g media for one week a t 28" C. and in some cases a d a t 100" C. Tests were -4 not run at the higher rimw 3m@temperature i n lowa0 boiling materials such 100 as gasoline, both because they would have little p r a c t i c a l value and because of the difficulty and danger of cond u c t i n g s u c h a test. No test was r u n in petrolatum a t 28" C. because it is a solid a t that temperature and DAYS AGED IN AIR AT UQ'C consequently it would FIGURE 4. EFFECT OF looo C. OVEN-AQINQ ON TENSILE be difficult to o b t a i n STRENGTH AND ELONGATION OF perfect contact of the COMPOUNDS D-10 AND R-10 sample with the swelling medium. The results of these tests are shown in Table IV. It is clear that the higher the temperature the greater is the difference between the degree of swelling of a rubber compound and of a similar DuPrene compound in any given swelling medium. Rubber can be compounded SO as to resist the action of some oils and solvents very satisfactorily a t normal temperature, but there is no rubber compound that will not ultimately disintegrate completely when immersed in any petroleum distillate a t 100' C. TABLE111. FORMULAS OF OIL-~SISTINQ STOCKS
zg
R-1 DuPrene Smoked sheets Soft whiting (40 vol.) Gastex (20 vol.) Glue (10vol.) Zinc oxide. . Light-calcined magnesium oxide Cottqnseed oil Stearic acid Wood rosin Phenyl-@-naphthylamine Phenyl-a-naphthylamine Sulfur D u P o n t accelerator 808
...
100 112 38 15 10
...
1
1 ... ...
2
6
1
D-1 100
...
85 28.5 11.5 10 10 2
..
5 2
... 1.5 ...
TABLEIV. EFFECTOF IMMERSING COMPOUNDS D-1 IN SWELLING MEDIAFOR ONE WEEK
AND
R-1
70INCREASE IN VOLUME At 28' C. At 100° C. D-1 R-1 D-1 R-I 1AYS AGED IN OXYGEN BUHB AT 70.C
FIGURE3. EFFECTOF OXYGENBOMB AGING ON TENSILESTRENGTH AND ELONGATION OF COJ~POUNDS D-10 AND R-10
these deteriorating influences may be greatly improved by loading it heavily with reenforcing pigments, especially certain types of carbon black. Table 111 shows the formula for a typical oil-resisting rubber stock and for a comparable DuPrene compound containing the same fillers in the same volume relationship. These compounds contain vulcanizing ingredients in such proportions that the DuPrene compound reaches its maximum tensile strength when cured 15 minutes at 141' C., and the rubber compound reaches its maximum tensile strength when cured 5 to 7 minutes a t 141O C. It is well known that the oil resistance of rubber is improved by
Satd. petroleum derivatives: Casing-head gasoline 9 58 ... Kerosene 22 78 37 Dissdlved Paraffin-base motor oil5 1 11 11 150 Pennsylvania crude oil 10 59 21 Dissolved 17 178 Petrolatum Unsatd. petroleum derivativese Midcontinent crude oil 20 61 75 Disaolved Coastal crude oil 18 54 102 Dissolved 33 88 ... ... Cracked motor gasoline Benzeneb 174 122 ... Carbon tetrachloridec 176 172 ... ... Cottonseed oil 2 15 ... ... Linseed oil 4 7 ... ... Turpentine 88 135 ... ... Lard oil 5 12 ... ... 0 Hydrogenated lubricants from unsaturated crudes give similar results. b Toluene, xylene, a n d solvent naphtha give similar results. C Other chlorinated solvents give similar resulta.
...
...
It has long been known that unsaturated hydrocarbons are somewhat more active swelling agents for rubber than the corresponding saturated hydrocarbons. This is true of DuPrene to a much greater extent than of rubber. It follows
36
IKDUSTRIAL AND ENGINEERING CHEMISTRY
that the difference between the swelling of a DuPrene compound and of a similar rubber compound is uniformly greater in saturated than in unsaturated hydrocarbons. I n fact, the aromatics, such as benzene which represents the highest possible degree of unsaturation, actually swell DuPrene to a greater extent than rubber at room temperature. It is also true that the difference between compounds D-1 and R-1 is much greater in cottonseed oil which is nondrying than in linseed which is a drying (i. e., unsaturated) oil. Also, turpentine, which has a cyclic structure, swells 340 b oth DuPrene and 320 900 rubber quite badly, and m the difference between the s w e l l i n g of Du260 Prene and of rubber in *€ 2 4 0 r”” this m e d i u m is relatively slight. !im The most important difference between 2the b e h a v i o r of Du2m Prene and rubber in Eeo oils and s o l v e n t s lies M not in the d e g r e e of 40 20 s w e l l i n g b u t in t h e t o u g h n e s s and other PFeCENTAGE SWUGATION physical pro p e r t ies FIGURE5. STRESSSTRAIN CURVES a f t e r e x p o s u r e t o OF COMPOUNDS D-10AND R-10 t h e swelling agent. Rubber c o m p o u n d s when badly swollen by oils, etc., become quite tender and tend to slough off in tiny particles. DuPrene compounds have no such tendency even though they may be swollen to three or four times their original volume. I n fact, a DuPrene compound such as D-1 will retain approximately half of its tensile strength after immersion in kerosene a t room temperature for six months, whereas a rubber compound such as R-1 will have only about one-eighth of its original tens’le strength after immersion in kerosene for a few days. Tensile strength after immersion in swelling agents is usually calculated with reference to the original cross section before immersion, and the preceding statements are based on that method of calculation. . I
g?
OTHERPHYSICAL PROPERTIES One of the most important properties of rubber is its hysteresis loss or its mechanical efficiency as a medium for storing and releasing energy. Another important and closely related property is its permanent set which is a measure of the extent to which it fails to return to its original shape after deformation. A detailed study of these properties of DuPrene as compared with rubber is being made and will be reported in a subsequent paper. I n general, unpigmented DuPrene stocks containing no sulfur have higher permanent set than sulfurvulcanized rubber, but the power loss of DuPrene compounds is inclined to be lower than for similarly compounded rubber stocks. Moreover, the addition of sulfur to DuPrene compounds tends to reduce the permanent set or cold flow under compression and to reduce the hysteresis loss. These conclusions are purely tentative and must be accepted Kith caution because the choice of vulcanizing agents and the conditions of vulcanization appear greatly to affect these properties. Sun-checking and corona-checking are types of failure of rubber that are believed to be caused by ozone. DuPrene can be made to show these effects under severe conditions but the development of cracks is so slow that under normal conditions DuPrene is not subject to sun-checking and is much less subject to corona-checking than rubber.
Vol. 26, No. 1
DuPrene is less permeable to gases than natural rubber. This is presumably due to the fact that it contains none of the proteins, resins, and other so-called nonrubber constituents of natural rubber which appear to assist in the diffusion of gases. Although DuPrene can be burned if a sufficiently hot flame is applied to it, it will not support combustion after the source of heat is removed, whereas natural rubber will. I n general, it is chemically more stable than natural rubber, by which is meant that i t is more resistant to oxidation and to acids and alkalies and other chemicals that react on rubber. Rubber cements are made by swelling natural rubber in gasoline and other solvents. DuPrene cements may be prepared in a similar manner, but the range of available solvents is limited to the aromatic hydrocarbons and the chlorinated solvents such as carbon tetrachloride. DuPrene is not soluble in gasoline and other petroleum fractions, even in the unvulcanized form. DuPrene cements are, in general, better adhesives than rubber cements because unvulcanized DuPrene is tougher and less extensible than unvulcanzied rubber and, moreover, the properties of DuPrene are unaffected by dissolving it and removing the solvent by evaporation, whereas rubber appears to undergo some form of deaggregation. In rubber manufacturers’ parlance, DuPrene cements have “shorter legs” than rubber cements. Since the molecule of chloroprene, of which DuPrene is a polymer, contains a chlorine atom, i t is not surprising that DuPrene has a higher specific gravity than natural rubber (DuPrene, 1.21; natural rubber, 0.93). No attempt has been made in this paper to discuss the art of DuPrene compounding. No evidence is presented to s u p port such statements as the one that DuPrene allows the chemist greater latitude in his choice of compounding ingrediFlex-cracks
$ 3 .
FIGURE 6. FLEX-CRACKING TESTS
ents than-.he has when working with rubber, but a few examples may not be amiss. Sulfur is, of course, essential for the vulcanization of rubber and not essential for the vulcanization of DuPrene although it may in some cases be used to ad\-antage. The vulcanization of natural rubber is retarded by most acids stronger than the higher fatty acids such as stearic. On the other hand, acidic ingredients such as leather dust may be used to good advantage in DuPrene compounds without affecting the rate of vulcanization. More detailed information concerning the influence of various compounding ingredients may be found elsewhere.’ KECEIVEDAugust 23, 1933. 1
Bridgwater and Krismann, IND. ENG.CAEM.,26, 280 (1933).
