September, 1930
INDUSTRIAL A N D ENGINEERING CHEMISTRY
10" C., or 164 per 10" F. This value is in fair agreement with previous determinations (2, 3, 4). Conclusions 1-Within experimental error, within the range of temperature and in the case of the mix under consideration, the temperature of cure has no effect on the quality of the vulcanized stock. 2-Certain unexplained differences exist in the results obtained by various methods of aging. 3-The temperature coefficient of vulcanization is 2.50 per 10" C.
1007
Acknowledgment
Thanks are due R. P. Dinsmore, of the Goodyear Tire and Rubber Company, for permission to publish this material. Literature Cited (1) Park, Rubber Age (London), 7 , 64 (1926). (2) Sheppard, India Rubber World, SO, 56 (1929). (3) Spence and Young, Z . Chem. I n d . Kolloide, 11, 28 (1912). (4) Twiss and Brazier, 1.SOL.Chem. I n d . , 39, 125T (1920). (5) Vogt, IND.END.CHEM.,17, 535 (1925). (6) Vogt, I b i d . , 17, 870 (1925). (7) Vogt, Ibid., 11, 1015 (1929).
Rubber Mills and Banbury Mixers' Carl F. Schnuck FARRRL-BIRMINGHAM COMPANY, DERBY,CONN.
Roll Mills
HE common sizes of mills have rolls 60- or 84-inch face
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according to the operation and size of batch. Figure 1 shows an %-inch mill driven by a lineshaft from a separate source of power. The majority of rolls are made of chilled cast iron, but there have been introduced in recent years a number of chilled alloy-iron rolls as well as a few caststeel rolls. The latter have not met with great favor owing to inherent disabilities. The working surfaces are often imperfect and become badly marked in use, owing to their softness, sometimes making it difficult for the operator to cut and turn the stock. The softness of the journals also creates a greater tendency for the journals to run hot and start to cut. Attempts to produce steel rolls with hardened surfaces a t competitive prices have not been successful, either by the gas hardening process or the use of hardening steels. Chilled rolls have certain natural advantages, such as hard close-grained iron in the journal bearings and hard wear-resisting surface on the bodies, these qualities existing with a relatively small manufacturing cost. These factors are responsible for the continued use and popularity of chilled iron and chilled alloy-iron rolls in rubber mills. The question is frequently raised as to the desirability of reducing the thickness of the shell of the roll in order to exercise a greater cooling effect on the rubber. Many have the impression that this would eliminate the danger of scorching or overcuring the stock, but unfortunately this is an error. The thickness of the roll shell has some bearing on the cooling efficiency of the rolls, for the internal thermal resistance is inversely proportional to the thickness in inches, while the use of thinner shell and consequent increase in diameter of the hole exposes a greater surface to the action of the cooling water. There are other factors to contend with, however, such as the thermal resistance between the rubber and the roll and between the inner roll surface and the cooling water. Since the thermal resistance of the metal in the average mill roll, with so-called "thick shell" is only about 30 per cent of the total thermal resistance, a reduction of shell thickness from 5 inches to 3 inches would reduce the total thermal resistance less than 12 per cent. According to data secured after months of readings in a large rubber factory, it appears that moderately thin shelled rolls could reduce stock temperature less than 5 degrees. When these tests were made great care was taken to get accurate reading of stock temperatures, but, as is well known, the outer surface of a sheet of rubber on a mill is frequently much cooler than the surface next the roll, and
* Received April 30, 1930. Presented at a meeting of the Chicago Rubber Group, March 21, 1930.
equipment for taking a continuous record of the inner surface was not in existence. Sufficient data were collected, however,
to indicate that the use of thin-shelled rolls or the placing of water ports close to the surface would not completely overcome troubles due to scorching, for stocks which are highly compounded can become scorched on a cold mill. This scorching occurs principally in the bank, which retards interchange, so that sensitive stocks should be worked in small batches. The next matter of interest is the indication of stock temperatures so as to give notice when the danger limit has been reached. Such a device has been perfected and placed in a number of rubber factories for use on mills and calenders. This device as applied to mills consists of a pivoted ploughshare riding on the roll and lifting the stock over itself so that the surface of rubber which had been next to the rollin other words, the hottest portion-will pass over a sensitive element. This device not only indicates the highest temperature but can serve to regulate the flow of water through the rolls. It is, of course, desirable to use refrigerated water if a local supply of cooled water is not available, but sometimes the initial cost proves prohibitive. Refrigerating plants have been installed for supplying water at 45 to 70 " F., the amount of refrigeration having to be determined entirely by local conditions. It is highly desirable to have the interior of the rolls kept free from sediment or scale, for the slightest accumulation of this nature has a tremendous heat-insulating effect. There are records of %-inch mills using 12 to 15 gallons of water per minute at 50" F. and 20 to 30 gallons per minute a t 57" F. There is a wide variation in the practice as to temperature and amount of water used per mill. The use of open discharge funnels seems to have the effect of saving water, one report showing a saving of 30 per cent in quantity of water required to cool the rolls with open discharge, as against the closed type of stuffing box. The deduction is that a closed stuffing box retards the discharge of water sufficiently so that the roll is full of water and, instead of having the incoming water reach the surface intended, the central distributing pipe is in a solid body of water and the incoming stream flows out through the holes and short-circuits back along the pipe, leaving the body of hot water hardly disturbed. If foundry stars or scrap metals are used in a mill roll to keep the interior clear from scale or lime deposit, the shape of the piece is relied upon t o secure its constant change of position. One mill roll which was inspected had a piece of smooth iron occupy a continuous path as the roll rotated and in doing so wore a groove on the interior of the roll, almost to the edge
Vol. 22, No. 9
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1008
of the chilled portion, causing the roll to break at this point. There are many opinions as to the best method of delivering the cooling water to the internalsurface, hut it is generaily conceded that any method used must produce active circulstion. For this purpose internal pipes are applied with holes for the emission of water under pressure, care being taken not to have total area in the boles greater than the capacity of
F18ure I--&(-Inch Roll Mill
the pipes. It might be desirable to have the holes snialler at the end nearest the supply so as to have uniform strength of jet throughout the length of face, but this is seldom followed in practice, the one effort being to have the holes on the upper surface of the pipe and staggered to cover as much snrface as possible. If the rolls of a mill are far enough apart so that there is sufficient thickness in the tongue of rubber between the rolls to exert a strong pull on the mass above, enough wedging pressure can be exerted by cold rubber to break the roll in the body or shear off the neck. If the rolls are set closely together, the effect of thetongue of rubber does not occur. When the rnill is used only for the breaking down of rubber, the rolls are usually set in a fixed position, and since a "column effect" must be produced-that is, rubber must break down on itself-efforts are made to keep t.he mill and the stock as cold as possible. Many expedients have been adopted to improve the efficiency of mills on this work, such as improved cooling
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than when in a plastic condition, due to heat. The Gordon plasticator was produced to accomplish the breaking down of rubber on a high production basis in such an intensive manner that a larger proportion, at least, of the desired plasticization is accomplished before the rubber acquires heat softness due to latent heat. Passing on to the compounding operation, it is found that the differential speed of the rolls is of importance to the working in of the various ingredients to make a uniform blend. If the rolls operated at uniform speed, the powders would be merely pressed into the mass and persist in the iorm of ioelusions rather than dispersions. Each t y p of compounding has its own ideal differential roll speed, hut by t.he construction of experimental mills having separately driven rolls and individual variable-speed motors it was found not to be such a critical matter as to preclude a fixed ratio for general work in a given class of manufacture. The surface differential in general use at present varies from 7 to .50 per cent, depending upon the kind of work being done. Since it is customary to consider a mill by its drive roll speed and many factories have a standard r. p. m. in this respect, let us compare two miils mixing high-grade stocks, having duplicate back roll speeds but one having 40 per cent frictiuii and the other 20 per cent. The lowcr friction mill wiil have a lower intensity of rubbing action but higher a p plication owing to tho increased speed of the front roll carrying the batch. A lower stock temperature should result and a shorter batch period, since the rate of mixing is determined by the speed of the front roll. For this reason there is a trend toward the use of lower friction ratios on mixing mills, although the higher ratios are used for breaking down and for warming mixed stock. The lower frict.ion ratios on mixing do not seem to reduce the kilowatt-hours per pound appreciably. Figure 2 shows a typical power curve of four 84-inch mills compounding tread stock, 165 pounds per mill, 44minute cycle. Mixing aprons are frequently installed where a large volurnc of loose powders is to be incorporated in rubher. The apron is an endless rubber belt, the preferred type having a series of central guiding lugs vulcanized in position, which engage with the rollers and guide the apron by allowing it to expand in either direction from the center. These mixing aprons reduce compounding costa appreciably, as i t is frequently possible for an operator to run two mills equipped with aprons and in any wont, since the loose powders which did not ini-.
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