The Chemistry of Soft Rubber Vulcanization I. Measurement of Vulcanization B. S.GARVEY,JR., AND W. D. WHITE,The B. F. Goodrich Company, I k r o n , Ohio the same degree as the milled For a series of rubber-sulfur compounds and ULCANIZATIOS is unirubber heated with sulfur. versally recognized as a f o r four commercial, high-gum accelerated comExperiments w h i c h will be p r o c e s s for improving pounds the changes during cure hme been measreported elsewhere show that the properties of rubber for variured by a series of physical tests. The change unmilled rubber, gas black, and ous uses, generally by treatment in combined sulfur has also been meusured f o r stiffeners produce some effects with sulfur or sulfur chloride. similar to rulcanization. This the rubber-sulfur compounds. Experimental The tests made to determine the makes the choice of a suitable degree of vulcanization approach conditions hare been so controlled that all results set of tests for all types of stock in multiplicity the variety in the are comparable. r a t h e r complicated. At t h e uses of rubber. Based on the experimental results, a set of present time a too comprehenBruni ( 2 ) c o m m e n t s that criteria has been selected with approximate sire treatment seems of doubtful t h e r e i s still no satisfactory value. It is better t o accept limiting calues f o r unvulcanized and for culmethod of determining when the necessary restrictions and rubber is v u l c a n i z e d . Deficanized rubber. Conditions are specified under t o work within the limits imnitions of vulcanization such as which the criteria can be used to measure the posed by them. I n this study those of Weber ( I S ) , Schiddegree of tdcanization. The selection qf these the limitations have been obrowitz ( I O ) , Kindscher (7), and criteria is, in effect, a definition in terms of the served by e s t a b l i s h i n g the Shepard ( I I ) , all specify treatfollowing standard procedure: changes i n physical properties which limits the ment w i t h sulfur or sulfurb e a r i n g compounds. Bridgmeaning of the term “i~ulcanization” i n a manner The rubber is moderately masticated by following the mixing prowater ( I ) defines vulcanization useful for the study of the phenomenon. cedure of the AMERICANCHEMIas a change in t h e p h y s i c a l The difference in the rate of change of the CAL SOCIETY (9). The use of stifstate of the rubber which is acfeners, reclaims, and large amounts seseral properties is interpreted as evidence that companied b y certain changes of softener or pigment, particularly during vulcanization a mechanical strucfure is gas black, is avoided. If these in physical properties. I n most precautions are observed, the uncases no d e f i n i t i o n is given, built up. cured batch stock is definitelv unbut the coefficient of vulcanizavulcanized by every test applied. tion o r t h e c h a n g e in some All of the changes can then be atphysical characteristic such as solubility or tensile strength tributed to vulcanization and dependable comparisons can be is used t o measure the degree of vulcanization. Thus TF‘ie- made between different types of high-gum stock. rubber for all the experiments reported was taken from gand (14) considered coefficient of vulcanization, elongation oneThe batch of selected, first latex crepe which was blended on an at break, tensile strength, modulus, tensile product, and 84-inch mill in the factory. All of the batches were given as resilient energy in selecting optimum cure. Williams (15) nearly the same mechanical treatment as was possible without and Dieterich and Davies (4) used plasticity measurements extreme precautions. I n several cases good checks were obtained in duplicate batches, which shows that the errors due t o t o detect scorching or incipient vulcanization. processing were insignificant. For a study of the chemistry of vulcanization it is necessary After a consideration of the theoretical and practical to define what phase of vulcanization is t o be considered. Such a definition can best be made in terms of the change aspects of the problem four types of tests were selected: in a specified set of physical characteristics, which should as (1) qualitative, ( 2 ) plasticity, (3) hysteresis, and (4) streesfar as possible characterize all types of vulcanization. The strain tests. Certain of the measurements have been selected first step in the study is a n examination of the changes as being suitable for use without the others, and in the tables these are marked *. Discussion of the tests is based on a brought about b y curing rubber with sulfur. The physical properties of vulcanized rubber depend on large number of experiments, and can be verified by an so inany variables that a comparison of the results obtained examination of the data given in the tables and curves. a t different times and in different places is unsatisfactory QUALIT.4TIVE TESTS unless all experimental details are reported. I n the present Qualitative tests considered by practical rubber men to study the changes in physical properties and in combined sulfur for a series of rubber-sulfur compounds have been show characteristic differences between vulcanized and unmeasured under conditions which make all of the results vulcanized rubber are included in this work because of their comparable. Physical testing data are also included for four relation to manufacturing operation. Testing conditions accelerated compounds of commercial, high-gum types t o were standardized and a semi-quantitative method of classishow t h a t the same changes take place in the presence of fication developed so that the results are comparable to those of the other tests. Cnvulcanized rubbers are put into class 0. accelerators. The choice of the test, or tests, used to measure the degree With increasing degree of vulcanization the stocks are of vulcanization may materially affect the conclusions to be classified as 1, 2, 3, 4, or 5 . The tendency of stretched rubber to freeze is measured drawn. Thus, as judged b y tensile strength alone, n-e might conclude from the examination of a tough crude rubber, the in the ice water test. When unvulcanized rubber isstretched, same rubber after milling, and the same rubber after heating immersed in ice water, and released, it does not recover as it with sulfur t h a t vulcanization was exactly the reverse of does a t room temperature. This is called freezing. When milling and that the tough crude rubber n-as vulcanized to stretched frozen rubber is again warmed, it recovers. This
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I S DU STR I A L A KD E N G IN EER I N G CH E MI STR Y
test is not sensitive during the early stages of cure. I n some cases, as with high accelerator-low sulfur stocks, the rubber may seem well vulcanized b y every other test and yet freeze badly in ice TTater. There are five classes based on this test. The hot water test measures the tendency of stretched rubber to break by plastic flow under the influence of heat. Although unvulcanized rubber breaks readily, after only a slight cure the samples no longer break. Thus, the test can distinguish only betn-een unvulcanized rubber and rubber which has a definite, eyen if slight, cure. There are two classes based on thiq test.
Behavior on the mill appears t o give a composite measure of those changes which are most characteristic c:)f vulcanization. The measurement, however, is not sufficiently exact t o permit quantitative use. There are six classes based on the milling test. Solubility differences in the rubber before and after milling are measured by the solubility test. The change shown here is largely complete in the early stages of cure. There are five classes based on the solubility in benzene: ICEWATERTEST. In applying this test a thin strip of stock was cut from the sheet and stretched almost to the limit six times. While stretched it was immersed in ice water for 20 seconds and then released under water. Uncured rubber froze very badly and showed practically no recovery on release. With increasing cure there was more tendency to recover, and the recovery was more rapid. Finally, with the well-cured rubber the stock recovered just as rapidly in ice water as it did at room temperature. Between the two extremes were all gradations of freezing as indicated by the rate of rt'covery. By this test stocks can be divided into five classes from unvulcanized to n-ell-vulcanized: Class 0. Class Class Class Class
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Class 0.
Stock behaves like milled crude rubber; these stocks quickly become smooth and plastic Class 1. Stock mills like tough crude rubber; these stocks are initially somewhat harder and rougher and take longer to break down and mill smoothly Class 2. Stock is rather hard and dry and somewhat grainy; it usually smooths out on longer milling, especially on a cool mill Class 3. Stock is rough and d r y and does not smooth out satisfactorily Class 4. Stock mumbles somewhat b u t after a few minutes hangs together well enough t o form a rough sheet Class 5 . Stock crumbles very badly and does not eheet out even after 10 t o 15 minutes on the mill
The standard procedure was modified in some cases for convenience. Thus for soft, sticky stocks a much cooler mill was used. In some cases it was necessary (for reasons not connected with the investigation) to use different mills. In many cases it was not necessary to leave the rubber on the mill for the full 10 minutes, as the classification was obvious after 3 to 5 minutes. Such variations in procedure do not affect the classification. SOLUBILITY TEST. For the solubility test sufficient compound to contain one gram of rubber was cut up into thin strips and put into 100 cc. of benzene. It was allowed to stand for 24 hours with intermittent shaking by hand. Solubility was determined both before and after the milling test. The uncured stocks were completely soluble before and after milling. With increasing times of cure, fiolubilities decreased, and on this basis the stocks could all be divided into five classes. The time of milling will influence solubility but within the limits used for the milling tests consistent results can be obtained:
3
BEFORE MILLING Dissolves completely Pieces lose their shape but d o not dissolve completely Pieces retain their shape Pieces retain their shaDe
4
Pieces retain their shape
CLass 0 1 1
PLa45TIClTY
AFTER MILLIXG Dissolves completely Dissolves completely Dissolves completely A few undissolved flocs are present Stock is swollen rather t h a n dissolved
TESTS
The retentivity and softness were measured on the Goodrich plastometer, and the plasticity was calculated from them by the procedure described by Karrer, Davies, and Dieterich (6). Determinations were made a t both 30" and 100" C. The
(a) Practically no recovery
( b ) Stock is too weak t o test 1. Some recovery, but very slow
2. 3. 4.
Recovery is fasier t h a n in class 1 but elol\-er than in class 3 Recovery is slower t h a n in air Recovery is as +ast as in air
HOT WATERTEST. thin strip of rubber was cut from the sheet, stretched, and held under the hot water faucet. The water temperature was close to 90" C. The sample was kept under a tension insufficient to cause a sharp break but sufficient to cause plastic flow. The uncured stocks flowed and broke under the influence of the heat. After even a slight cure, the stocks no longer broke: Class 0 Stock breaks or is t o o ueak to test Class 1. Stock does not break
MILLING TEST. The milling tests xere carried out on a 12-inch (30.5-cm.) mixing mill (9). I t was found that sharper differentiations could be made on a hot mill than on a cold one. Hence the mill was kept at temperatures between 90" and 110" C. The standard time for milling was 8 to 10 minutes. During most of this period the mill was kept tight [opening about 1/32 inch (0.79 mm.) or less], but toward the end it was opened wider and the stock sheeted thicker. At this point scorched rubber shows more roughness than crude rubber. According to their behavior on the mill the stocks were divided as follon-s:
values are given as Ra, Rlw, S30, Slc0,P30, and Pl0o, with the subscripts indicating the temperature of measurement. The heating period for the
