March, 1925
INDUSTRIAL AND ENGINEERING CHEMISTRY
295
Plasticity Control in Rubber Mixing' By Paul Beebe a n d R. B. Stringfield GOODYEAR TIRE& RUBBERCo., AKRON,Orno
HE theory and details of plasticity measurements on
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unvulcanized rubber and rubber compounds have been adequately covered by Williams,2 and Vogt3 has brought out clearly the effect of the time and temperature of milling on the plasticity of the raw stock, as well as on the physical properties of the cured stock. The present paper, therefore, does not pretend to describe any new properties, but merely illustrates some of the ways in which plasticity measurements can be used to prevent trouble in the factory. Adaptation of Plasticity Measurement t o Factory Control First a word as to modification of the method devised by Williams to adapt it to rapid factory control. Although to determine the characteristics of any unknown stock it is necessary to determine several points on the plasticity curve and even to run determinations a t different temperatures, with any stock whose general characteristics are known it is sufficient for most factory purposes to determine one suitably selected point a t a definite temperature. Accordingly, for control purposes on most stocks we are able to take as a plasticity measurement for comparative purposes the gage in centimeters shown by a 2-cc. sample of the stock after being subjected for 3 minutes to a load of 5 kg. between parallel flat surfaces a t 70" C. For this purpose a plasticity press (substantially as described by Williams) is used, this being mounted in a de Khotinsky constant-temperature oven held a t 70" C. The operator keeps five samples warming up in the oven a t all times, and one in the press, placing a cold sample in the oven as each warmed sample is placed in the press and cutting out a fresh sample during the 3-minute intervals between readings. By this system one operator, even with normal interruptions, can turn out over one hundred samples per day. A recent improvement has been the use of a small arbor press carrying two hemispherical dies for cutting out samples.
2-cc. sphere of stock within an accuraiy of about 2 per cent. This has so speeded up the preparation of samples that it is now possible for the operator to use two plasticity presses simultaneously and thereby turn out over two hundred samples per day. Essential Features Affecting Plasticity Vogt's report3 brought out the essential features affecting the plasticity of uncured rubber stock, but the illustrations to follow will be clearer if these features are reviewed briefly: I-When crude rubber or a rubber stock is milled, it softens very rapidly a t first, but soon reaches a point where the additional milling produces only small changes in plasticity. 2-For any given time of milling, the cooler the stock during milling the softer the finished product. 3 4 t h e r factors being held constant, the power consumption on a mill is less for any given plasticity of the finished stock the cooler it has been kept on the mill-i. e., plasticity is not a function of the power consumed in breaking down except at constant temperatures and mill ratio. 4-A stock softened by heating without mechanical working regains its original toughness on cooling. The plasticity of a rubber stock is largely determined at the mixing mill. Although until recently the industry has had no numerical method of evaluating this property, it has long recognized that smooth operation of calenders, tube machines, etc., depends on having stock of uniform consistency, ' and most of the large companies, a t least, have milled their stocks to definite specifications, giving weight of batch, time, order of mixing, etc. That some of the factors not ordinarily considered in mixing specifications may largely affect the condition of stock will be shown in this paper. Mill Water Control In Figure 1 is plotted the average daily temperature of the water used for cooling mixing mills against the average
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These dies are vented with a 1-mm. hole in the center of each depression to permit escape of air while cutting, and yield a 1 Presented before the Division of Rubber Chemistry at the 68th Meeting of the American Chemical Society, Ithaca, N. Y.,September 8 t o 13,1924. * THISJOURNAL, 16,363 (1924). * Report of Physical Testing Committee, Division of Rubber Chemistry, 67th Meeting of the American Chemical Society, Washington, D. C., April 21 to 26, 1924.
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daily plasticity of a pure gum coat stock. Unfortunately the plant from which these data were secured is dependent for its water supply chiefly on a shallow stream the temperature of which fluctuates rapidly with the weather. As the temperature of the mill is vitally affected by the temperature of the cooling water, it is to be expected that the plasbicity of the stock will be affected, but the close agreement between the curves is rather remarkable. With a stock of this type, dif-
I N D U S T R I A L A N D ENGINEERILVG CHEMISTRY
296
ferent batches mixed in succession by the same mill man check very closely as to plasticity, the batch being what the mill man calls a “busy batch”-i. e., requiringahis attention prartically the whole time, and consequently each batch receiving almost identical working. At the calenders it is found that above a plasticity of 0.370 scorching troubles begin to appear. Consequently, we now aim to alter the milling
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specifications whenever necessary and, by methods to be mentioned later, to keep the plasticity of this stock as close to the optimum figure of 0.360 as possible. I n this connection the fact should not be overlooked that it is possible to scorch a stock by running a t too thick a gage or by piling up slabs without sufficient cooling, and to vary the plasticity by varying the warming-up conditions; but these points are much ’more easily controlled than the milling itself. Similar agreement between temperature and plasticity is shown in Figure 2. This stock is a very highly loaded beadwire insulation, of a type with which it is difficult for a mill man to produce uniform stock, plasticities from consecutive batches sometimes varying by 30 to 40 points. With such a variation it is remarkable that the average plasticity follows the temperature as closely as it does. Work with this stock was started with the idea t h a t the softer the stock within reasonable limits the better was the subsequent handling. Consequently, as the water temperature went up and the plasticities increased, it was decided to change the proportions of softener in the batch to pull down the plasticity] which result was effectively accomplished. Almost a t once, however, the amount of poorly insulated wire began to increase, and on checking back and plotting the figures it was shown that, with the method of handling used, the tougher the stock up to the scorching limit, which is a plasticity of about 0.425, the better the insulation. Further checking showed that some mill men were consistently producing softer stock than others, and the net result has been the reduction of milling time for a batch from 50 to 35 minutes, an increase in production of 43 per cent. Similar examples could be cited with treads and other stocks, and the work is still so new that fresh illustrations are appearing constantly. The compounders a t last have a laboratory test which gives them a definite numerical answer showing whether or not a milling change has toughened a stock, whether or not one stock is tougher than another, and whether or not any given stock is coming of uniform consistency. It is no longer possible for the production department to call up and insist that a stock has gone to the bad and must be changed immediately. A check usually shows that a new man has been put on the tube machine or that the calender gang which usually runs ply stock tried to run sidewalls last night a t the same temperature.
Vol. 17, Eo. 3
Effect of “Working” Stock
In most cases the uniformity of the stock is found to be more important than the absolute plasticity, which usually means that the tougher a stock is run, as long as it is kept safely inside the scorching limit, the less money is spent for milling. This brings up the question as to what happens a t the various stages of milling and what can be done to vary the plasticity of a given batch. The effect of the mill temperature on plasticity has already been shown. Referring to Figure 3, another feature within the control of the mill man becomes evident. These figures, taken on a batch mixed a t 0.5 inch gage and carrying a large bank, show how rapidly the temperature of the stock on the face of the roll rises when it is allowed to run without being cut into. I n this case all the power goes into heating a small portion of the batch and, the gage being fairly thick and the rubber a poor conductor of heat, more heat is generated than can be carried away and the temperature rises rapidly. As a result this portion of the batch is heat-softened and receives less permanent softening by mechanical working than it should; moreover, is very likely to scorch if any curing agents are present. This is contrary to the ideas most of us have held, it being commonly believed t h a t scorching usually occurs in the bank rather than on the roll. This is often the case on a calender or with small mill batches, the foregoing referring particularly to large batches. The effect of a thinner gage in reducing the stock temperature is also plainly shown. It should be remembered that these temperatures represent the average of a small sheet taken from the center of the roll, and that the actual temperature as the stock passes the nip of the rolls is far higher than the figures shown. I n fact, some sensitive stocks, which would withstand a temperature of 200” F. for a long period without any indication of scorch, will scorch badly a t an average temperature on the mill of 200’ F. or less. Effect of Adding Water on the Batch From Figure 3 it will be seen that the mill man who mixes and “works” his batch thoroughly instead of letting ,it run will produce softer stock in the same length of time. Figure
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