434
INDUSTRIAL AND ENGIKEERING
Figure 1A shows plots of Equations 15 and 21 and data on condensing diphenyl (1). While the data scatter considerably, they show, in general, fair agreement with the predicted line for the turbulent region for a value of c p / k = 5 . Inasmuch as the value of c p / k of water is equal to 1.8 at 100” C. and 1.0 a t 190’ C., data on condensing steam (under am-forming conditions rather than dropwise) would be expected to lie near the lower curve shown on the figure. This plot is convenient to use where the number and diameter of tubes can be conveniently assumed, thus fixing the value of 4C;p and the length is left variant. Alternatively, Figure 1B shows a plot of Equations 16 and 22 and the same data. This plot is convenient t o use where the condenser length is fixed, and the number and diameter of tubes are left variant. I n the latter case it is necessary to estimate the mean temperature difference across the condensate layer, which may require cut-and-try calculations, based on the
CHEMISTRY
Vol. 26, No. 4
value for the over-all logarithmic mean temperature difference. Since condenser lengths are more often fixed than the number and diameter of the tubes, Figure 1B will be preferred by many engineers. High vapor velocities in a downward direction will decrease the film thickness and increase the heat transfer coefficient thereby; they may also cause a transition from viscous to turbulent flow before the value of 4C/p becomes 1600. It is emphasized also that these equations do not hold under conditions causing drop-forming condensation ( 6 ) , but the evidence available indicates that film-forming condensation predominates with organic materials. It is therefore believed that the figures presented make possible a conservative determination of the hest transfer surface.
LITERATURE CITED Badger, W. L., Monra4, C. C., and Diamond, H. W., IND.ENQ. C H E U . , 22, 700-7 (1930). Colburn, A. P., Ibid., 25, 873-7 (1933). Colburn, A. P., Trans. A m . Inst. Chem. Engrs., 29, 174-239 (1933). Cooper, C. M., Drew, T. B., and McAdams, W. H., IND.ENG. CHEV., 26, 428 (1934). Kirkbride, C. G., Ibtd., 26, 425 (1934). Nagle, W. M., and Drew, T. B., paper presente3 a t Roanoke, Va., meeting, Am. Inst. Chem. Eigrs., Dec. 12 to 14, 1933. Susselt, W., 2.Ver. deut. Ing., 60, 541-6, 559-75 (1916).
RECEIVEDJ a n u a r y 20, 1931. This paper is Contribution 142 from the Experimental Station of E. I. d u Pont de Nemours & Company, Inc.
The Chemistry of Soft Rubber Vulcanization 111. Comparison of Vulcanized Rubber with Unmilled Crude Rubber Reclaims, and Unvulcanized Stocks Containing Stiffeners or Gas Black B. S. GARVEY,JR., The B. F. Goodrich Company, Akron, Ohio Tough unmilled crude rubber, reclaims, and uncured stocks containing gas black or stiffeners have some of the properties of more or less vulcanized rubber. By suitable tests, however, they can be differentiated clearly f r o m vulcanized rubber. T h e s imilar it ies complicate the measurement of vulcanization and make it desirable to exclude these materials f r o m the compounds used in a study of vulcanization. T h i s exclusion is justified by evidence that the
I
h’ PART I (1) of this series’ it was pointed out that the
measurement of vulcanization is complicated by the use of unmilled rubber (e. g., latex stocks), stiffeners, gas black, and reclaims. Therefore limitations in compounding were accepted and a standard method of processing was adopted. These limitations were accepted in recognition of the fact that compounds containing these materials show some properties akin to those of vulcanized rubber. For example, some crude rubbers have stress-strain characteristics almost identical with those of certain types of vulcanizate. Uncured gas black stocks likewise show some of the characteristics of vulcanized rubber, and the reenforcing action of gas black is sometimes spoken of as a sort of vulcanization ( 2 ) . Such limitations are desirable only if it can be shown that the phenomena excluded are different from those being studied. 1
P a r t I1 appeared in IND.ENG.CHEM., 26, 1292 (1933).
phenomena they produce are different f r o m those resulting f r o m vulcanization. The different combinations of properties suggest that there are at least three distinct types of structure incolved: a crude rubber structure, a pigment structure, and a vulcanized structure. Stiffeners tend either to retain the crude rubber structure or to build up a similar one. I n reclaims the vulcanized structure seems to be only partly broken down. The experiments2 reported in this paper show how the tough, hard, crude rubbers can be differentiated from vulcanized rubber and how the effect of gas black and stiffeners can be differentiated from that of vulcanization. The generally accepted view that reclaims are distinct in character from either vulcanized or unvulcanized rubber is supported by this investigation. The experiments also show why it is desirable, for the present, to exclude these materials from the compounds used to study vulcanization. CRUDERUBBER Moderately milled crude rubbers showed no signs of vulcanization by any of these tests. They gave results similar to those of unvulcanized rubber-sulfur compounds (1). With tough crude rubber which was unmilled, or only slightly 2
The tests used were described in detail in the previous paper ( 1 ) ,
INDUSTRIAL AND ENGIKEERING CHEMISTRY
April, 1934
433
I 210
30
8
9
/od
+TTLNi-/WTY
milled, the properties were in some respects very similar to those of milled rubber which had been slightly vulcanized. The test data for four such rubbers are given in Table I. I n the hot water test, crude Para and sprayed latex did not break. After slight mastication all crude rubbers broke in this test. On the mill, tough crude and very slightly vulcanized milled rubbers behaved much the same although differences were readily detected in samples in which vulcanization had progressed a little farther. The values for thermoplasticity, hysteresis set, and hysteresis/set modulus for crude rubber are about the same as those for very slightly vulcanized rubber. The tensile strength and modulus figures for crude rubber, on the other hand, sometimes reach values which can be attained with milled rubber only after a rather high degree of vulcanization. I n fact some stocks which were fairly well vulcanized when judged by d l other tests showed tensile values lower than those for crude hard Para or sprayed latex rubber. Apparently tough crude rubbers cannot be distinguished from slightly vulcanized rubbers by any of these tests. The retentivity softness relations a t 30' and 100' C., however, differentiate sharply between tough crude and scorched rubbers. This is shown b y Figures I and 2 . The slope of the curve for crude rubber is considerably less, and
it increases when the rubber is milled. The slope of the curve for the slightly vulcanized sheets is considerably greater and it decreases when the rubber is milled. By this relation tough crude rubber can be classified positively as unvulcanized in spite of the similarity of some of its properties to those of scorched, milled rubber. STIFFENERS Stiffeners, such as benzidine and magnesia, were added to crude rubber with varying degrees of milling. The following compounds were tested : 1. First latex crepe, 100; benzidine, 1. The stock was mixed on the mill with Iight mastication of the rubber. It was also tested after remilling for 10 minutes. 2 . Smoked sheets No. 1, 100; benzidine, 1. The stock was mixed on a very hot mill with minimum mastication of the rubber and a tensile sheet molded 30 minutes a t 105' C. 3. Smoked sheets No. 1, 100; light calcined magnesia, 1. The stock yas mixed in an internal mixer with minimum mastication of the rubber. It was also tested after being remilled for 10 minutes. 4. Smoked sheets KO.1, 100; light calcined magnesia, 1. The stock was mixed on a very hot mill with minimum mastication of the rubber and a t,ensile sheet molded 30 minutes a t 105' C.
TABLE I. VULCANIZATIOK TESTSox UNMILLED CRUDERUBBER RUBBER
QUALITATIYE TESTS ---PLABTICITY SoluIce Hot Mill- bilwaterwater inp: itv R30 Rioo S3o
H a r d Para 0 1 0 Smoked sheets 0 Crepe 0 0 Soraved 0 1 . . latex a Measurements suitable
1 1
2 20.5 28.45 6 . 7 18.9 33.0" 6 . 4 1 1 17.3 28.4G 7 . 0 1 21.3 28.5a 6 . 2 for use without the others. 1 1
.
TESTS
Sioo
Pa0
8.7 11.9 12.6 8.6
1.4 1.2 1.2 1.3
Pioa 2.5 3.9 3.6 2.5
PZO lO P30
-HYSTERESIS Modulus at Set 300%
6.25'" 90a 1 2 . 7 0 130" 10 8'1 100a 6 . 7 a 100a
4.5 4.9 4.2 5.1
64n 70a 60a 73"
TESTSSet/ modulus 20.2 19.5 26.4 23.8
--STRESS-STRAINTESTS-Modulus at Ultimate tensile
1.41: 1.37 1.85' 1.67a
10.5 3.5 3.5 8.8
150a 50a 50a 125a
79 14.6 31.4 88
1125" 208' 446a 1250°
TABLE 11. VULCANIZATIONTESTSON Gas BLACK STOCKS (A. rubber 100. ea8 black 50. factorv mixed: B, rubber 100, gas black 50, sulfur, 3)
,----HYBTERESISTESTS---STRESS-QTR.~~N TESTS-Modulus Modulus at Set/ at Ultimate Set 300y0 modulus 500% tensile KQ.( 4 b . i KO./ Lb./ K Q . ~Lb./ c m . zn. Kg. Lb. cm. zn. cm. in.'
QUALITATIVETESTS SoluCUREAT Ice H o t ?fill- bilSTOCK142' C.a water watering i t y A
None 30 120 240 None 30 60
0
4 3 3 . 9 41.86 3 . 7 1 0 . 9 4 3 7 . 4 37.06 5 . 7 11.8 4 36.5 39.06 4 . 9 9.2 4 37.5 34.06 4 . 6 8.0 4 3 8 . 4 42.36 6 . 8 2 6 . 1 4 30.7 29.86 4 . 0 6 . 6 3 4 3 9 . 8 30.56 2 . 7 5.6 4 4 1 9 . 4 1 8 . 4 b 2.0 2 . 4 = 40 pounds steam pressure. b Measurements suitable for use without the others. C Stock broke a t 480 per cent elongation.
0 0 0 0 B 0 0 0 240 2 142' C. = 287' F.
0 0 0 0 0 1 1 1
0 0 0 0 2
1.2 2.15 1.8 1.8 2.6 1.23 1.16 0.40
4.6 4.4 3.8
2.8 11.0 1.99
1.70
0.46
17.66 9.06 i 9 i h 7 . 2 b 185b 4 . 3 6 l6Ob 46.5b 3.26 i i i b 2 . 5 ' ~ lOOb 0.46b0.356
l8:i 23.9 44
248b 340b 625b
46Ob
1.506 1.066
19.'36 2 i 5 2 1 . 9 b 325 2 8 . l b 400
28' 30 35
4256
0:505 4.1 0.296 0 . 8 5 0.066
45',7b 890
56:2 63.3 140.6
9006 2000b
9 , ' i 1 3 0 6 21'.4 1 2 . 2 174b 15.1 9.1 1 8 . 4 2626
7:l
0.646
.. ..
C
5OOb 8005
Vol. 26, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
436
I
1
Y
I
q.rrmnv/rr
I n all the tests these stocks gave results like those obtained with crude rubber which has been milled to some extent. Since the most sensitive test for vulcanization appears to be retentivity-softness curve, this is shown in Figure 3 for several stocks with stiffeners added. It will be noted (conipare with Figures 1 and 2) that these curves are those for crude and not for vulcanized rubber.
GAS BLACK With low loadings of black the stocks did not resemble vulcanized rubber but with higher loadings they did in some respects. I n Table I1 data are given for a factory-mixed master batch containing 50 parts by weight of black on 100 of rubber. I n the ice water, hot water, and milling tests the behavior was that of milled crude rubber. The solubility was that of a well-vulcanized rubber. The plasticity data were confusing and showed only that, if the gas black had any vulcanizing action, it was very small. The hysteresis set values were in the range given by milled crude rubbers. The modulus a t both 300 and 500 per cent elongation was considerably higher than those found for any crude rubber and the tensile strength was much above that of milled crude rubber. The same gas black master batch was then heated with 3 parts of sulfur so as to vulcanize the rubber slightly. The testing data are given in Table 11. As compared with the unvulcanized master batch, marked differences were noted in the hot water test, the behavior on the mill, plasticity factors, set, modulus, and tensile strength.
The ice water test and solubility indicate a fairly high degree of vulcanization. I n the hot water test these reclaims were similar to unvulcanized rubber. I n the milling test a special class, 6, was necessary for the classification of reclaims. They behaved quite differently from crude rubber. They were all sticky, especially on a hot mill, and were either grainy and short or rough and nervy, The plasticities and thermoplasticities were mostly in the range of crude rubbers or slightly vulcanized rubbers. The hysteresis set and set/ modulus values are in the range of slightly vulcanized rubbers as are the values for modulus and tensile strength. The retentivity-softness relations were different from those for either vulcanized or unvulcanized rubber as is shown by the curves in Figure 4.
DISCUSSION
Some of the tough crude rubbers resemble milled rubber with a rather high degree of vulcanization with regard to tensile and modulus characteristics. However, their behavior on the mill, solubility, plasticity, and hysteresis set are similar to those of milled rubber which has been only slightly vulcanized. Behavior on the mill, hysteresis set, and plasticity are all similar in that they involve what might be called the L L f l ~characteristics" w of the rubber. Comparing milled vulcanized rubber with tough, unmilled, crude rubber it is seen that the relation between the solubility and flow characteristics and the tensile properties are quite different. It has been shown that the two types can be sharply differentiated by the retentivity-softness relations. I n the case of the slightly vulcanized rubber the effect of raising RECLAIMS the temperature is predominantly to increase the softness with Four types of reclaim were tested: (1) whole tire reclaim, comparatively little effect on the retentivity. With crude alkali process, rubber value (estimated rubber content) 50 rubber, on the other hand, raising the temperature causes per cent; ( 2 ) tube reclaim, heater process, rubber value 62 per a considerable increase in the retentivity. In the earlier paper (1) the characteristics of vulcanized rubcent; (3) tube reclaim, special process, rubber value 68 per cent; and (4) boot and shoe reclaim, acid process, rubber ber vere attributed t o a mechanical structure developed by value 38 per cent. The testing data are given in Table 111. chemical reactions in the hydrocarbon. Similarly the peTABLE QUALITATIVE
TESTS
SoluIce Hot Mill- bilNO. water water ing ity
111. VULCASIZATION TESTSAPPLIEDTO R E C L A I ~ ~
-
PLABTICITY TEST
P
-HYsrEmxm
PA0
Ru
Rioo
530
Sioa
Pa
PIW
Pm
Set
~fodulus at 300%
Kg./ Lb.1 cm. in.' 3 2
0
6
0
6
3
47.0 45= 16.5 30.5 5 1 . 5 O 22.0 2 0 6 21.6 19.0a 25.0 4 2 0 6 3 17.0 23.0a 3.7 5 Measurements auitable for use without the others. b Broke at 375 per cent. 1 2 3
3 3
26.5 59.2 59.0 10.8
7.7 S.6 5.5 0.66
11.9
30.5 11.2 2.5
77" 10S5 22.W 9.55
llO5
105a 60"
125"
6.6
94a
575 4.0 570 1 1 . 2 5 160a 4.0
TESTSSet modulus
Kp.
Lb.
-STRESS-STRAIN
15 odulus 50%
Kg./ Lb.[
1.17"
cm.2 6.7= 4.5O
In. 96
1.0.Y 11.1 0 . 7 S a
8.4O
120
16.6 26.2 15.0
1.845
...
64 b
TESTS-
Ultimate tensile Ks.< Lb;/ cm. In. 6.7 96a 8.0 115" 17.5 250' 14.0 200a
April, 1934
I NDUSTR IAL X SD
EN GIN EE R I N G CH E M ISTR Y
culiar properties of unmilled rubber may be attributed to a “crude rubber structure.” This structure appears to be essentially different from the vulcanized structure. The data indicate that stiffeners help to retain the crude rubber structure or possibly to build up a similar one. No evidence n-as found to show that they act as vulcanizing agents. Reclaims obviously differ from both crude rubber and from unreclaimed vulcanized rubber. The softeners and pigments present undoubtedly have considerable effect on the results obtained in some of these tests. The experiments seem to indicate that, during reclaiming, the vulcanized structure is partly broken down and the flow characteristics are increased by softeners added or formed during the process. When uncured gas black stocks are compared with vulcanized rubber in a similar manner, it is found that the black stocks resemble unvulcanized rubber in their behavior on the mill, and in the hot water and the hysteresis set tests. These are all “flow characteristics.” The solubility and modulus are similar to those of well-vulcanized rubber while the tensile strength is similar to that of slightly vulcanized rubber. Here is a combination of properties which is different from that in either vulcanized or crude rubber. It seems,
437
therefore, that there exists here a third type of structure, the “pigment structure.” Investigators who have studied the reenforcing action of gas black generally attribute the effect to interfacial phenomena between the individual pigment particles and the rubber matrix (9). There are, therefore, a t least three distinct types of structure: (1) a crude rubber structure, possibly having its origin in the latex particles; (2) a pigment structure established by interfacial phenomena between the hydrocarbon and the pigment particles; and (3) a vulcanized structure built up within the hydrocarbon by chemical reaction. Each of these three structures causes distinctive combinations of solubility, flow characteristics, and tensile properties. LITERATURE CITED (1) Garvey, B. S., and White, W. D., IND.ENQ.CHEM.,25, 1042
(1933). (2) Kindscher, E. (in K. Memmler’s Handbuch der Kautschukwissenschaft), p. 379, S. Hirzel, Leipsig, 1930; Stevens, W. H., J. SOC.Chem. Ind., 48,60T (1929). (3) Shepard, N. A. (in Alexander’s “Colloid Chemistry”). Vol. IV, p. 309, et seq., Chemiral Catalog, 1932. RECEIYEDSepteniber 28, 1933.
...*.
IV. Vulcanizing Agents Other than Sulfur The vulcanizing action of sulfur chloride, mdinitrobenzene, selenium, tetramethylthiuram disulfide, and benzoyl peroxide has been inz~estigated by the use of a set of tests developed ,for measuring the degree of vulcanization with sulfur-cured compounds. These tests show that all of these malerials are true vulcanizing agents. Comparison of the various vulcan izates with different types of unculcanized rubber
P
REVIOUS papers in this series have covered the development of a set of tests for measuring vulcanization (IO) and the use of these tests for studying the function of sulfur during vulcanization (Q), and for comparing vulcanized rubber with unmilled rubber and uncured gas black stocks (8). This report covers the use of the same set of tests for a comparison of vulcanization by sulfur with that by other materials generally accepted as vulcanizing agents. The recipes for the compounds used are as follows: SULFURCHLORIDE Compound 1 First latex crepe was Compound 2. First latex cre e was calendered in the lahocalendered an$ cured ratory to 0.038 em (vapor process) in the (0.015 inch) and cured factory. the sheet was in the factory by the 0.023 ek. (0009 inch) vapor process followed thick. by treatment with ammonia.
-
123
COMPOUND 5, TETRAMETHYLTHIURAM DISULFIDE
-
COMPOEXD 4. SELENIUM [Cures: 15, 30, 60, 120, 240, aud 480 min. a t 149’ C. (300’ F.)1 First latex crepe 100 Vandex (selenium) 28 @-Naphthylamine 4 P a r a 5 n wax 4 Litharge 25 Zinc oxide 50 COMPOUND 6, BENZOYL PEIIOXIDE
104
109
The dinitrobenzene compound is based on a formula used by Fisher and Gray (7) ; that for selenium is based on a recom-
shows that the outstanding characteristic of the vulcanized structure is its resistance io flow under u wide variety of conditions. The vulcanized structure appears to be a composite one which varies with the rate of a reaction catalyzed by the culcanizing agenf, fhe rate of combination of the vulcanizing agent, the nature of the addition product, and possibly with differences in the nature of the catalyzed reaction. inendation by Boggs (5). The mixing, curing, and testing procedure was the same as that described in detail in the earlier paper (IO). I n Table I are given the testing data for each compound on the uncured stock and on the cured sheet haring the highest tensile strength in the range selected.
DISCESSION OF RESULTS Comparison of these data with the corresponding data on sulfur compounds, reported in the earlier paper, shows that all of these compounds have undergone the changes characteristic of vulcanization with sulfur. Therefore, according to the criteria used here, the materials tested are true vulcanizing agents. A comparison of the properties of the different types of vulcanized rubber with each other and with tough crude rubber and uncured gas black stocks shows that all of the vulcanizates have a marked resistance to flow in all of the tests. On the other hand, both the gas black stocks and crude rubber show a comparatively small resistance to flow in one or more of the following tests: hot water, milling, retentivity, thermoplasticity, hysteresis set. With regard to tensile and modulus, both of these types give values as high or higher than do some types of vulcanizate. Uncured gas black stocks, like vulcanized rubber, show low solubility in benzene. In ice water, some vulcanized, high accelerator-low sulfur compounds freeze as badly as crude rubber. It is thus apparent that the outstanding characteristic of the vulcanized structure is its resistance to flow under a wide variety of conditions, especially a t high temperatures. Vulcanization, then, is a reaction which primarily snp-
