Radioisotope Technique in Sea Water Sorption d y Rubber
Dm,
World War 11, many marine propeller shaft failures from galvanic corrosion were reported. Waster rings as sacrificial electrodes were tried; but under and near the ends of these, cracks occurred. Therefore, because of its success in stern-tube shafts. rubber was used as a protective coating. Corrosion protection from rubber relates to amount of water absorbed, and of course, to its resistance to sea water penetration. A common test is immersion in boiling water for 96 hours and noting volume change. However, this excessive temperature, far above that found in practice, may radically alter properties of the rubber. Types of rubber representative of propeller shaft coatings were investigated. GR-S for solid and hollow shafts was cured at low and higher temperatures, respectively, and buffed to 67-mil thickness. Vulcanized coldbonded neoprene was included because of strong interest in drydock applications, and a rubber gasket material, Buna-N, more susceptible to water leaching than the others, was included for comparison. Synthetic sea water was prepared by dissolving Sea-Rite (Lake Products Co.), a commercial mix containing all components of sea water except rarer elements. For radioactive sea water, 1 ml. of sodium-22 chloride solution, specific activity of 0.27 millicurie per milliliter, was diluted to 100 ml. with inert sea water. Specific activity of this solution was 5.72 X 106 disintegrations per minute per milliliter; with a sodium iodide well-crystal scintillation counter, sensitivity greater than 0.5 y of sodium chloride was obtained. For nonradioactive studies, immersion was in a jar and the water at a temperature of 25O & 1' C., was agitated with a n air-driven stirrer. For water sorption studies at 1 atm. or less, 50-ml. twonecked flasks were used, connected to a vacuum pump and manometer. For immersion at high pressures, a Parr oxygen bomb was used, pressurized with argon and containing a 200-ml. beaker insert for the radioactive sea water. For preparing thin samples and shav. ing surfaces, a horizontal sliding microtome with a carbon dioxide freezing attachment was used. Strong gamma emission was measured with a Model DS-3 scintillation counter (Nuclear Intrument and Chemical Corp.).
9 18
INDUSTRIAL AND
Effect of pressure on sorption of water and salt is not large. The increase at 0.02 and 50 atm., about 30% over that at 1 atm., can be explained by a tendency of these pressures to clear adhering air bubbles and permit closer contact between water and rubber. hTeoprene was less sorptive under all pressure than GR-S. At elevated temperatures and atmospheric pressur-, volume swell was used to indicate water pickup. Samples were buffed on both faces; mechanical operations were in accordance with Navy's Bureau of Ships, MIL-S-15058D. Refluxing in radioactive sea water at 101' C. for 584 hours increased salt sorption. This and the fact that GR-S showed shrinkage while neoprene continued to swell are attributable to leaching which increases surface area. GR-S left considerable particulate matter in the water that becamo turbid ; but water from neoprene was clearer. Boiling for 96 hours in distilled water produced greater swells for all three products. For room temperature under different pressures, 10-mil samples were first dried in a vacuum desiccator, weighed, and after immersion, rinsed in distilled water, blotted dry, redried in a vacuum desiccator overnight and weighed again. Thus, both material leached and water sorbed could be determined. With thicker samples, a 90' C. oven was used for quicker drying. After 4 weeks' immersion at 25' C., salt had penetrated less than 20 microns -about half the thickness of the thin samples. When 20-micron slices were removed from both faces, more than 95% of adsorbed salt was removed. Comparison between buffed and smooth surfaces shows that salt uptake is adsorptive instead of absorptive and that rubber is a barrier to its entrance. When GR-S having a thickness of 1.3, 10: and 67 mils was immersed in sea water at 25' C., equilibrium for water sorption was obtained after 1, 3, and 30 or more days, respectively. Water sorption is a linear function of thickness. Under the experimental conditions, leaching for GR-S was negligible but Buna-N required correction. When GR-S of varying thickness is immersed in radioactive sea water at 25' C. for 1 week, it is evident that water and not sea water penetrates. Sodium chloride is adsorbed either on
ENGINEERING CHEMISTRY
or close 10 the surface. A sample of GR-S 36 microns thick picked up 51 y of water per square centimeter and 0.55 y of sodium chloride. A sample 251 microns thick picked u p 396 y of water but only 0.54 y of sodium chloride. The osmotic theory can be applied. Rubber itself can be considered a semipermeable membrane permitting passage of water but not solutes either in itself or the surrounding solution. If i t is assumed that a liquid is incompressible, this can be restated in an equation. Where 7r is osmotic pressure; V solv., partial molar volume; AF soh., molar free energy; f% and fs solvent fugacities of internal and external solution, respectively, then
-
7r
V solv.
= -
F solv.
= -
R T In fi/Jd
Assuming further that vapors are ideal gases, their pressures may be substituted for fugacity. Where Pi and P, are internal and external pressures, respect ively, then T
v solv.
=
- R T In P , / P ,
Osmotic pressure of the system is that which must be applied to the internal solution to raise the vapor pressure of its solvent to that of the external solution. This pressure is supplied by resistance to deformation. Low water absorption reported for high modulus rubber can be explained through smaller water uptake producing the necessary osmotic pressure to equalize vapor pressures of internal and external solutions rapidly. Deficiencies of the military specification's volume swell test can be eliminated by applying a weight instead of volume index to thin specimens immersed at room temperature. The amount of material leached and water sorbed can be simultaneously determined gravimetrically . Acknowledgment
Cooperation of the Rubber Development Section of the U. s. Naval Material Laboratory in volume swell measurements is acknowledged, and the encouragement of A. R. Allison of this laboratory and T. A. Werkenthin and E. A. Bukzin, Bureau of Ships, in fostering the use of radioisotopes in developing naval material is appreciated.
NORMAN MICHAEL1, BERNARD L. GILBERT2, and JOSEPH L. KALINSKY Material Laboratory, New York Naval Shipyard, Brooklyn, N.Y. Copies of the complete manuscript m a y b e obtained from J. I.. Kalinsky a t the above address. 1 Present address, Atomic Energy Engineering, Alco Products Inc., Schenectady,
N. Y . 2 Present address, Food Machinery and Chemical Corp., Princeton, N. J .
