Dec., 1921
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING C H S M I S T R Y
1133
The Relation between Coefficient of Vulcanization and Mechanical Properties of Vulcanized Rubber’ By 0. de Vries CENTRAL RUBBERSTATION, BUITENZORG, JAVA
Our scientific knowledge of rubber and its vulcanization is still so deficient that even a number of the most fundamental points on which rubber testing should be based are insufficiently cleared up. For instance, it still forms a point of discussion whether the mechanical properties or the coefficient of vulcanization are the most important in judging the state of cure of vulcanized rubber. Some argue that the mechanical properties of tke vulcanized article are the only ones that interest the buyer, while he is indifferent as to how much sulfur has combined with the rubber to reach the degree of elasticity and strength that suits his purpose. Others point out that the mechanical properties are not constant in a colloid like rubber, while the coefficient of vulcanization is not changed by mechanical treatment and is also independent of aging-or a t least nearly so a t ordinary temperatures-so that in general the coefficient is a better indication of the state of cure to which the rubber was originally brought. Discussion on this point may well rest till sufficient experimental material has been gathered to clear it up definitely; elsewherel we have summarized the present situation as follows: “As the determination of the coefficient of vulcanization means a considerable amount of additional work, and gives no actual gain in the routine of our testing of estate samples, we have not included it in our regular tests,” and further, “When one wants to gain as complete an insight as possible into the properties of so complicated a substance as vulcanized rubber, both coefficient of vulcanization and mechanical properties have to be taken into consideration.” EFFECTOF ADDED ACCELERATORS O N COEFFICIENT, OF VULCANIZATION From the latter standpoint, one of the first subjects for further research is the difference in coefficient of vulcanization for different types of rubber, cured to the same mechanical properties (position of the stress-strain curve). It has been remarked by many investigators that, when curing to a fixed cure as judged by the stress-strain curve, the coefficient of vulcanization may vary over a rather wide range. An especially striking example has been recently published by D. F. Cranor,’ who found that the coefficient for a given curve was influenced largely by the addition of accelerators. For instance, when cured to a certain curve, mixtures with and without an accelerator gave the following coefficients: SAMPLE Original smoked sheet.. Same, with 0 . 5 per cent hexamethylenetetramine. Same with 0 . 5 per cent didethylamine derivative..
...
.................
Time of Cure Minutes 198 35 2.4
Coefficient of Vulcanization Per cent 4.9
1.8 0.9
EFFECT OF NATURAL ACCELERATORS The aceelerators increase the rate of cure enormously, and at the same time the coefficient for given mechanical properties decreases markedly. This may be explained in several ways. The organic accelerators may have a stronger accelerating effect on the processes that alter the mechanical properties than on the combining of the rubber with sulfur. 1
Received May 2, 1921.
* 0 de Vries, “Estate Rubber, Its Preparation, Properties and Testing,” 1920, 489, 631. 8 I n d i a Rubbev World, 61, 137; I n d i a Rubber J . , 58, 1199. For further examples see A. van Roesem (Comm. Gov. Inst. DeZft, 1917,213; D.F. Twiss and S. A. Brazier, J . SOC.Chem. I n d . , 39 (1920), 125; H P. Herens, I n d i a Rtlbbw World, 62, 720.
Or, the very short time during which the rubber is exposed to the high temperature of vulcanization may avoid the disintegrating effect of heat (which is opposite to the stiffening effect -of vulcanization). Be this as it may, if powerful organic accelerators give such a marked difference in the coefficient of vulcanization for a fixed curve that the question arises, what may be the effect of the so-called natural accelerators in plantation rubber? These accelerators are either constituents of the latex as it comes from the tree, or decomposition products formed from the non-rubber constituents during preparation. They are supposed to be amines or amino acids, and are undoubtedly very powerful accelerators, since they give a marked increase in rate of cure although present in the rubber in only very small quantities. Ordinary plantation. rubber, tested in pure rubber-sulfur mixtures, indeed shows the phenomenon of differences in coefficient of vulcanization for fixed mechanical properties, but the variation is by no means as large as in the above example. A. van Rossem’ studied this variation for a large amount of statistical material. For our purpose his results may be summarized as follows: Taking ‘as the basis of comparison a stress-strain curve for which the average coefficient of vulcanization is 5.0, out of a number of samples of ordinary first quality crepe or smoked sheet, 50 per cent will not show a larger variation in coefficient of vulcanization than 0.25. For the bulk of plantation rubber, therefore, the relation between stress-strain curve and coefficient is rather close. EFFECTOF TIME OF CURE O N COEFFICIENT OF VULCANIZATION The question further arises, which types of plantation rubber show a coefficient higher than the normal, and which show a lower one? B. J. Eaton and F. W. F. Day2 found the coefficient of vulcanization for standard mechanical properties (their optimum curve) to be: Standard Time of Cure Hours Ordinary crepe.. 3.25 Crepe from matured rubber 1 . 2 5
..........
Coefficient of Vulcanization Per cent 4 . 1 -4.5 4.65-5.1
Matured rubber is prepared by keeping the coagulum in the wet state for some time. A decomposition then sets in, and the protein decomposition products cause an increased rate of cure. Eaton and Day therefore find an increase in coefficient, the more natural accelerator the rubber contains. H. P. Stevens3for three samples of rubber finds the following figures for the coefficient of vulcanization a t fixed positions of the curve. POSITION OF CURVE Load in Rg./Sq. Mm. at 1000 Per cent Length 700 900 1100 1300 COEFFICIENT OB VULCANIZATION Smoked sheet.. 1.8 2.1 2.4 2.7 Crepe.. 2.3 2.65 2.95 3.25 Crepe from matured rubber. , 2.4 2.85 3.1 3.2 TIMEO F CURE (Min.) Smoked sheet.. 77 90 103 115 Crepe.. 113 132 150 168 Crepe from matured rubber.. 57 70 77 81 SAMPLE
.............. ...................... ..
.............. ..................... ..
Up to a coefficient of 3.1 the rapid-curing matured rubber shows higher figures than the ordinary crepe, as in Eaton and Day’s experiments. The sample of smoked sheet, though 1
Comm. Gov. Inst. for Advising the Rubber Trade (Delft), V, 164.
J . SOC.Chem. I n d . , 36 (1917), 1116. * Ibid., 37 (1918). 280C.
T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERIA’G CHEMISTRY
1134
Vol. 13,No. 12
curing more quickly than the crepe, shows throughout a much lower coefficient for the same curves. The author gathered some material to make a further study of this point. I n the first,place, we may mention some samples of ordinary crepe of different rate oE cure. These gave :
I n this case also the coefficicnt of vulcanization is higher than normal for the more quickly curing sample. Though these four types of quick-curing rubber are prepared in totally different manners, and show large differences in composition (both in amount of serum substances and in accelerator formed by maturation) , the excess in coefficient Standard Time of vulcanization is nearIy the same in all cases. of Cure Coefficient of SAMPLE Minutes Vulcanization Plantation rubber therefore seems to show a higher co660 55 5.1 efficient of vulcanization than normal when it is quick curing, en2 9n h n , .. ..94 5.0 1897 independent of whether the quicker cure is caused by un4.4 115 1667 changed latex constituents or by decomposition products 4.75 120 1204 4.4 122.5 1602 formed by maturation. The few exceptions to this rule The coefficient of vulcanization is determined for our stand- deserve further study. I n future investigations of this kind it will be interesting to ard state of cure,’ for which the length a t a load of 1.30 kg. per sq. mm. is 990 per cent. The more quickly curing push the analysis further by determining not only the total crepes, which contain more of the natural accelerators, combined sulfur in the ordinary way,‘but by differentiating show a higher coefficient of vulcanization in accordance with between the sulfur bound by the rubber and,by the nonthe results of Eaton and Day and of Stevens. rubber constituents as proposed by W. J. Kelly.’ Perhaps Some specially prepared samples of abnormal composition a difference as between Samples 7 and 8 might be cleared gave the following figures: up in this way. ,
No.
1 2 3 4 5 6
7 8
SAMPLE 2467 1883 2459 W 2365 D W 2365 BW 2494 A 2494 B 2494 C
DESCRIPTION Matured rubber Latex evaporated t o dryness Ball smoked after Brazilian method Ball smoked after Brazilian method Ball smoked after Brazilian method First clot in partial coagulation Rest after partial coagulation Ordinary coagulation
Standard CoeffiTime of cient of Cure VulcaniMinutes zation 30 4 9 65 5 1 5 25 70 70 5 1 80 5 15 55 5 3 115 4.35 . 115 4.65
The first sample is matured rubber (slab rubber) and may be classed among those with the greatest rate of cure as yet known. Like all matured rubber it contains natural accelerators formed by decomposition of proteins, but less non-rubber constituents than ordinary crepe, as part of the serum substances has been decomposed and escaped in gaseous form (loss in weight about 1.5 per cent). The coefficient of vulcanization is higher than normal. The second sample is also quick curing; it contains all the latex constituents, being prepared by rapid evaporation of latex by hot air (Kerbosch process);z no maturation or decomposition of serum substances has taken place. The coefficient of vulcanization is higher than normal, as in the foregoing case. Samples 3 to 5 have been prepared after the Brazilian method (smoked halls), Sample 3 by a native smallholder and Samples 4 and 5 as part of the output of a large European estate. The samples were 0 . 5 to 1 yr. old when tested, and showed the ordinary rather rapid rate of cure of balls of that age.3 Smoked balls retain somewhat more serum substances than ordinary crepe and smoked sheet, but they contain by no means all latex constituents, as a large amount of serum drips out during and after preparation. Maturation takes place slowly in the balls, bringing the time of cure from 100 to 50 min. in the course of 2 yrs.4 The coefficient of vul, canization for this type is higher than the average and nearly the same as in former cases. Sample 6 is prepared from the first clot obtained in partial coagulation. Such rubber contains more by-substances than ordinary crepe; it cures more quickly (though no maturation has taken place). The properties of Samples 6 to 8 were 5 No Moisture 6 1 40 7 0 64 8 0 93
Ash 0 35 0 17 0 28
Aqueous Extract 0 51 0 13 0 11
Acetone Tensile Extract Viscosity Strength 35 1 40 4 2 1 39 23 2 7 30 1.46 3 2
increase in coefficient of vulcanization for fixed mechanical properties, which amounts to about 0.5 for all types of quick-curing rubber, independent of their composition. The natural accelerators therefore act in exactly the opposite way to the artificial accelerators about which data are now available. The effect of the natural accelerators on the coefficient for a fixed curve seems to be much less pronounced than in the case of those artificial accelerators. Organization of the Natural Resources Production Department of the Chamber of Commerce of the United States is just being perfected. The department is preparing a very active program in an effort to render constructive service to the raw material industries, supplementary to the work of the various trade associations. It is believed that great assistance can be rendered to the industries and to the public by conducting a campaign of education intended to enlighten the public on the vital problems of natural resource activities. The following advisory committee has been appointed: C. S. Keith, Central Coal & Coke Co., Kansas City, chairman; J. H. Ross, Exchange Supply Co., Winter Haven, Fln.; J. E. Spurr, Engineering and Mining Journal, New York City; Christy Payne, Peoples & Hope Natural Gas Cos., New York; E. T. Meredith, former Secretary of Agriculture, Des Moines, Iowa: Sidney J. Jennings, United States Smelting, Mining & Refining Co., Boston, Mass.; R. V. Norris, mining engineer, Wilkes-Barre, Pa.; Van H. Manning, American Petroleum Institute, New York; and William H. Davis, Mid-Continent Oil & Gas Association, Bartlesville, Okla. Canadian pulp and paper exports for September reached a value of $9,457,027, a decline of $7,033,665 as compared with September 1920 and an increase of $51,635 over August 1921. Exports of newsprint for the month amounted to 1,224,136 cwt., compared with 1,212,225 in September 1920. Countries of destination were: Paper United Kingdom $ 116,591 5,249, 548 United States 408,468 Other countries
Pulp $1,221,246 2,175,937 285,328
Comparative figures for the first six months of the fiscal year show:
Slope 33 36 36
See “Estate Rubber, Its Preparation, Properties and Testing,” 463,542. 2 “Rstate Rubber,” p. 436. 8 Ibid., pp, 422-431. 6 Ibid.. pp.422-431 6 Compare A r c h , RubbevcullrruY. 1 (1917), 185; J . Sor: Chem. Ind., 88 (1919),92;I n d i a Rubber J., 67 (1919).1164: “Estate Rubber,” p . 389. 1
SUMMARY The natural accelerator or accelerators in rubber cause an
1921 1920 1919
Paper $33,3?9,508 43,025,764 27,119,246
Pulp $15,804,169 44,217,712 16,626,726
Total $49,185,667 87,243,476 43,745,972
Pulpwood exported to the United States for the first six months of the year were as follows : 1919 1920 1921 1
515,444cords 653,866 421,388
THISJOURNAL, 12 (1920),875.
$5,059,693 7 803 332 5 : 546:785
.
