1NDUSTRIAL i l N D ENGILTTEERINGCHEMISTRY
162
their hardness, they are also very nearly arranged according Note-Hardness was judged by stiffness, brittleness, and strength. Permeability was judged from K', the rate of diffusion for a sheet 0.130 cm. thick.
to their permeability, the hard rubbers being the least permeable. No quantitative relationship was sought, but ac21
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cording to the degree of hardness and permeability, the thirteen rubbers were divided into five groups: The sulfuric acid-treated rubber and the hard rubber were by far the hardest, being in fact brittle. They were also the least permeable to water. The vulcanized sulfuric acid-treated rubber was semi-hard, tough mechanically, and somewhat more permeable. The third group was composed of the commercial dental rubber and the representative inner-tube stock, both of which were tougher and stronger and less per-
Vol. 21, No. 2
meable (allowing for the effect of thickness) than the main body of rubbers-vulcanized pale crepe, vulcanized commercial smoked sheet, vulcanized rewashed smoked sheet, sulfurrubber compound, low-protein rubber, and silica-filled insulating compound-that formed group 4. (The silica-filled rubber may seem t o be out of place in this classification, but as determined by actual handling tests i t was found to be softer than the rubbers of group 3.) Group 5 contained only the pure rubber hydrocarbon and the crude raw ribbed smoked sheet, which were the most permeable and the softest. As composition affectsthe hardness of a rubber, i t obviously plays a role in the diffusion problem. Thus, in general, pale crepe rubber is tougher than smoked sheet and from Figure 5 vulcanized pale crepe (curve 1)was found to be less permeable than vulcanized smoked sheet (curve 2). Rewashing the smoked sheet softens it, indicating the probable cause of the greater permeability shown by curve 3. Other variations in the composition of the rubbers, such as vulcanization, fillers, and low protein content, do not seem to greatly affect the permeability. The relation between hardness and permeability to water is very useful in explaining other phenomena. A moderate increase in the temperature of a rubber sheet lessens its hardness while a t the same time it becomes much more permeable. The increase in permeability is so great that it seems reasonable to believe that part of it may be due to the decrease in hardness. Then again, the loosening in structure, which was described in a preceding paragraph as possibly resulting from the saturation of rubber with water, might, by its softening effect, be the chief cause of the accompanying increase in permeability.
Practical Method for Obtaining Dry Air for Humidity Control in a Rubber Laboratory' F. S. Conover THBNsw JERSEY ZINC COMPANY, PALMERTON, PA.
HE effect of relative humidity on rubber-testing has been the subject of much recent investigation. Stringfield2 and Conover and Depew3 have published papers on this subject. The last-named authors recommended that the rubber be stored in dry cabinets before milling, between milling and vulcanization, and between vulcanization and testing, a t a temperature of 75" *5' F. A short time later the Physical Testing Committee of the Rubber Division of the AMERICANCHEMICALSOCIETYrecommended that all laboratory testing be carried out a t 45 per cent relative humidity and 82' *5' F. While both methods have undoubted merit, it was believed that for physical testing laboratories, particularly such as this one, zero humidity was both more conducive to reliable results and easier to maintain. Accordingly, equipment was installed for maintaining zero humidity and its performance has been consistently good. Since several of the larger rubber laboratories have shown interest in the equipment, it has been decided to present this description of the installation and its operation.
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1 Presented before the Division of Rubber Chemistry at the 76th Meeting of the American Chemical Society, Swampscott, Mass., September 10 to 14, 1928. 2 IND.ENO.CHBM.. 17, 833 (1925). 8 Rubber Age, 21, 401 (1927).
Storage Cabinets
Three storage cabinets were built. One is used for the storage of crude rubber and master batches, the second for uncured samples after milling, and the third for vulcanized samples before testing. These cabinets are ventilated by a slow stream of dry air. They are each 1.066 by 0.915 by 0.61 meters (31/2 by 3 by 2 feet). They contain a series of 6.35-mm. (1/4-inch) mesh wire screen shelves on which the samples and rubber are placed. (Figure 1) The air enters each box a t the bottom and circulates upward and out a t the top. (Figure 2) Silica Gel Towers
In order to secure the necessary stream of dry air silica gel was used, mainly because of the ease of operation secured by this method. Silica gel is an 8- to 10-mesh silica composition having the properties of adsorbing and condensing gases a t low (room) temperatures and releasing them again a t high temperatures (140" t o 170" C.). This property lends itself to a very convenient arrangement, for, while air is being desiccated by being passed through one dehydrator, the other dehydrator can be reactivated by heating. Two silica gel dehydrators were built. (Figure 2) They were arranged so that one could be used to supply dry air
INDUSTRIAL A N D E.VGIArEERING CHfi.WST&Y
February, 1929
while the other was k i n g reactivated. Each gel tower contains 27.2 kg. (60 pounds) of silica gel, which is theoretically enough to desiccate 0.05663 cubic meter (2 cubic feet) of saturated air per minute for 4 days at 99 per cent efficiency. Since silica gel is most efficient at low temperatures, a coil of 12.7-mm. (%-inch) copper pipe was placed inside each gel tower so that water could be circulated through during the actual working periods of the apparatus.
163
activation and dehydration is supplied by the same blower, the air for dehydration being by-passed out of the main line vhile the larger remaining volume goes on into the tower which is being reactivated. If the reactivation period ai one tower is finished before the tower in use is exhausted, the excess air is passed out into the room. After reactivation is finished the direction of the air current is always reversed so that dehydration proceeds in the opposite direction. Air Heaters
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