I N D U S T R I A L A N D E;VGINEERI-YQ CHE.WISTRY
1202
Vol. 17, No. 11
Measurement of Crude Rubber Consistency' Tentative Method Proposed by the Crude Rubber Committee of the Division of Rubber Chemistry of the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 t o 10, 1925 HE two most important properties of crude rubber are its
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ability to give a satisfactory cured product and its workability in the factory operations. The workability of rubber is recognized as related to its plasticity, but the evaluation of true plasticity of rubber is so involved with theoretical considerations that it seems hopeless a t this time to attempt to develop any simple test making use of plasticity factors which can be put to direct practical use. A study of true plasticity values of rubber must be considered a research problem of great importance but one that will take time to work out and on which i t is beyond the scope of this committee to offer recommendations. The committee does wish, however, to start more or less standardized work in determining some factor of crude rubber that will be a measure of its workability. Such a test must be simple to run, it must truly represent the quality of the rubber being tested, and it must have a fair degree of accuracy. There are three methods by which this property might be determined. The first is by measuring the viscosity of rubber solutions. This is rather a long procedure and demands accurate control of operations. Second, there is a penetration method by which a weighted plunger is allowed t o penetrate a block of rubber. This might be developed into a practical method, but at present seems less desirable than the last method. The third method is the use of the Williams plasticity press. Since its introduction a year ago the Williams press has become of quite general use in the industry, its operation has become generally known, and in general it seems to offer the most satisfactory means of measuring this workability, or consistency factor, of crude rubber. With the three essentials in mind-that it must be simple to run, it must represent the rubber being tested, and it must be accurate-the following is offered as a start towards a standard test of consistency with the hope that after a trial the test as recommended, or some modification of it, or possibly some other better test, may be generally adopted by the industry. Apparatus The press as described by Williams is used. A weight of 5 kg. is placed in the upper plate and the entire press is placed in a controlled oven a t 100' C.
Sampling The 2-cc. block of rubber as recommended by Williams is used. For milled rubber these can be cut out direct from a slab. In this case, however, we are dealing with unmilled crude rubber often in loose, uneven sheets full of holes, and it is, therefore, necessary to prepare the sample in such a way as to secure a compact solid block for the test, which should, moreover, be a representative sample of the bale or lot of rubber being tested. This latter point means a blending of sections from different points of the bale or lot. Any milling of the rubber greatly affects the consistency, and as conditions of milling are very hard to standardize it is extremely difficult to get check results if the sample is milled enough to sheet out. The following, however, has been found to give satisfactory results. Procedure
A 200-gram sample of rubber, including small pieces from different portions of the bale or lot, is cut up into pieces not Method developed by C. W. Sanderson and presented in connection with the report of the committee for the year. 1
over 13 mm. (0.5 inch) in diameter. These small pieces are then put through the rolls of a small mill eight times, doubling over the sheet twice each time and not allowing the sheet t o cling to the rolls. The rolls are kept hot and the opening between them is kept a t 2 to 3 mm. The more or less blended mass of rubber is then pressed out between holland to a thickness of 6 mm. (0.25 inch) under hydraulic pressure in an ordinary platen press, the platens being kept a t 100' C. for one hour. The platen plates are kept apart by two bars of 6-mm. (0.25inch) iron, thus allowing the rubber between them to flow freely in a t least two directions. This gives a homogeneous sheet, practically free from air bubbles, which can be plied up to the required thickness. Although values secured from these samples are undoubtedly somewhat low due t o the slight amount of work done on the rubber, this work has been reduced to the minimum consistent with a thorough blending of the sample. The pressing out a t 100' C. has been found to affect the readings but very slightly. Evaluation of Results The Williams formula, YX"-K, is disregarded as an unnecessary complication for this purpose. Instead, the thickness of the test piece in millimeters after 5 minutes' pressing istaken as a measurement of consistency. The figures thus obtained are called flow numbers (F.N.). As crude rubber is a poor conductor of heat a certain time must be allowed for the rubber sample to come up to a uniform temperature. The necessary preheating period has been found to vary somewhat with different rubbers, but 20 minutes appears to be a safe period. It was not found satisfactory merely t o set the test pieces in the oven at 100' C. during the warming-up period, as irregular results were then secured. The test piece is placed in the press with the plates held apart by a small block of steel 13 mm. (0.5 inch) on a side, so that the upper plate but lightly touches the sample during the 20 minutes' warming-up period, and a reading is then taken 5 minutes after the steel block is withdrawn which lets the weight fall on the sample. Readings thus obtained can usually be checked with a variation of less than -0.05 mm. for samples cut from the same slab of rubber, and less than -0.10 mm. for samples of the same original rubber put through entire process of preparation. De Vries M e t h o d De Vries has recently recommended a method for obtaining the consistency of crude rubber that involves no milling or preparation of the sample. He cuts a thin strip from the bale of rubber, warms it slightly, and then rolls it in his fingers to a ball which he cuts with scissors to a volume of 0.4 cc. This ball is put in the Williams press in the usual way and after 30 minutes he reads the thickness of the rubber sample in millimeters. This method is simple lo run and involves no working of the rubber. It does, however, give a figure that represents only a very small piece of the rubber. The variations between two different rubbers is not great because of the smallness of the readings and the figures thus obtained cannot be correlated with readings for milled samples of rubber or mixed rubber stocks, which are generally obtained from 2-cc. samples. D. F. CRANOR, Chairma# c. w. S A N D g l S O N E. A. VANVALKXNBURGE R. P . ROSE
w. L. STURTZVANT
