Vacuum. Drying Apparatus for Unstable Polymeric Materials A. R. KEMP AND W. G. STRAITIFF Bell Telephone Laboratories, Murray Hill, N. J.
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the courbe of research conducted in these laboratories on unsaturated high molecular weight polymers like natural rubber, GR-S, etc., there developed a need for an efficient drying apparatus capable of rapidly and completely removing from these materials volatile substances such as water. ace-
tone, benzene, while ensuring protection against oxidation. The regular laboratory drying methods, such as the use of a high static vacuum in a vacuum desiccator or a constantly changing inert atmosphere a t ordinary temperature, were found slow and inefficient, thus being the source of considerable delay in the
THERMOMETER
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STANDARD LABORATORY
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Figure 1. Diagram of Apparatus 387
Vol. 17, No. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
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prosecution of the research work. Because of the susceptibility of the above materials to oxidation the use of open electrically heated ovens is inadvisable. Experience has shown that the rate of drying of rubber and rubberlike materials is dependent upon temperature, the amount of exposed surface area per unit weight of the material, the type and amount of the sorbed matter, and the concentration of the volatile matter in the atmosphere (humidity) surrounding the surface of the rubber. By making use of presently available laboratory equipment the authors have constructed a highly efficient drying apparatus a t moderate cost which employs both elevated temperatures and a constantly changing inert atmosphere to accelerate the drying process and a t the same time protect the material against oxidation. A sketch of the apparatus is shown in Figure 1. The apparatus is operated a t low pressures for the purpose of obtaining as much as possible an oxygen-free atmosphere and for the most economical use of the inert gas.
nection to the vacuum chamber. The exit valve of the vacuum chamber is connected to a mercury'manometer and to a series of two three-way stopcocks with leads attached to the house vacuum system, to the atmosphere, and to a Duo-Seal vacuum pump. A glass trap is included in the line to remove condensable vapors during the operation of the system, solid carbon dioxide in acetone being used as the coolant. Vibrations are held to a minimum by mounting the pump on a rubber mat 0.6 cm. (0.25 inch) in thickness. All connections are made with heavywalled rubber tubing. For airtightness all rubber-to-glass and rubber-to-metal joints are cemented with shellac. The flowmeter and manometer are supported by metal rods (not shown in sketch) attached to the wooden table top. The apparatus is operated by placing the samples on stainless steel wire-mesh trays in the vacuum chamber and securing the door tightly in place by means of the screw-clamp handle.
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C O N S T R U C T I O N AND OPERATION
The apparatus is supported on a standard 120-cm. (4-fOOt) wooden-top chemical table. The oven is a stainless steel automatic temperature-controlled Webber electric vacuum oven (American Instrument Company, KO.4-158 A) with a temperature range of 20" to 150' C. The vacuum chamber is made of pressed steel and plated with nickel over copper for air-tightness. A small-diameter lead gasket serves as a very effective airtight seal for the removable door. The accessory equipment consists of a 6.3-cu. meter (224-cu. foot) tank of prepurified nitrogen (Air Reduction Company) with an oxygen content of less than 0.002% by volume. The nitrogen gas is dispensed by a double-stage regulator through a flowmeter filled with dibutylphthalate and into the upper con-
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Rate of Removal of Acetone from Smoked Sheet
;Figure 3.
1. In vacuum desiccator under 3-mm. pressure at 25' C. 2. In vacuum-drying apparatus at 50" C.
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Figure 2. 1.
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Rate of Removal of Moisture from Smoked Sheet
In vacuum de9iccator under 3-mm. pressure over P i 0 6 at 25' C. In vacuum-drying apparatus at 55' C. In vacuum-drying apparatus at 80' C.
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Figure 4.
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Rate of Removal of Benzene from Smoked Sheet
1. In vacuum desiccator under 3-mm. pressure at 27' C. 2. In vacuum-drying apparatus at 55' C. 3. In vacuum-drying apparatus at 80' C.
lune, 1945
ANALYTICAL EDITION
The vacuum chamber is evacuated and recharged to atmospheric pressure with the prepurified nitrogen. This is repeated tmce. Khen the mercury manometer becomes steady the valve attached to the gas regulator is opened slightly to where the difference in the two levels of the dibutylphthalate in the flowmeter is 3 mm. with an orifice of about 2.0 mm. This corresponds to a gas flow of 78 liters of expanded gas per hour by the flowmeter which was calibrated at atmospheric pressure with a wet-test gas meter. The samples are removed by shutting off the vacuum and filling the vacuum chamber n-ith nitrogen to slightly above atmospheric pressure. The stopcock arrangement provides for the immediate release of the vacuum outside the pump as a safeguard against drawing the pump oil into the system. After cooling to room temperature, the materials are weighed and the operation is repeated until a constant weight is obtained. The house vacuum system serves well for overnight drying or when a considerable amount of volatile matter is to be removed. FACTORS INFLUENCING DRYING RATE OF RUBBER
The rate of drying of rubberlike materials depends upon a number of factor.. Elevated temperatures, a large exposed sur-
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face, and a rapidly changing low “humidity” atmosphere favor rapid drying. Figures 2, 3, and 4 show, for example, the effect of temperature on the rate of removal of water, acetone, and benzene from a 3.75-cm. (1.5-inch) square 0.3-cm. (0.125-inch) thick piece of Grade A smoked sheet rubber. Curves designated as 1 represent the slow rate of drying under ordinary laboratory conditions, as compared with curves 2 and 3 which show a much more rapid rate when carried out in the drying apparatus a t elevated temperatures. By increasing the temperature to 110’ C. it was found that it required about 0.5 hour to remove 5 % of moisture from the smoked sheet. The same drying rate was found in cases where the drying was carried out a t atmospheric pressure instead of under vacuum. This indicates that the drying process depends principally upon diffusion rather than evaporation from the liquid phase. I n drying delicate biological substances there exists a need for safe and efficient drying procedures and it is hoped that the present work may assist others in this connection.
Instrument for Measuring Thickness of Nonconducting Films Applied over Nonmagnetic Metals ALLEN L. ALEXANDERf PETER KING,
AND
Naval Research Laboratory, Washington,
J. E. DINGER D. C.
The extensive application of camouflage paint to military aircraft, whose paint surfaces are constructed largely from the light metals and their alloys, emphasized the need for a nondestructive method for measuring thickness of paint films applied to nonmagnetic metals. In aircraft painting, control of film thickness is doubly important be-
cause of strict weight allowance and the necessity of providing a durable finish. A n instrument is described for making such measurements and data are presented illustrating its use. The gage satisfactorily measures coatings containing metallic as well as nonmetallic pigments.
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that measurements cannot be made on curved surfaces, and surfaces not providing a level area as much as 1 inch in diameter cannot be measured accurately. The instrument ha. been successfully used over aluminum, various aluminum alloys, copper, brass, and magnesium, and -hould perform adequately over any conducting metal which is nonnizpetic.
S T H E study of the properties of paint films, methods for
determining thickness have been helpful in making more uniform measurements, since some properties of paint films are dependent largely upon the thickness a t vhich they are applied to any given surface. In the past, the most common instruments for measuring dry film thicknesses hare been based on a magnetic principle which resulta in their use for the study of films applied to surfaces of magnetic metals only. Lack of a similar method for studying films applied to the light metals and their alloys has proved a handicap to paint technicians studying finishing materials for aircraft, whose painted metal surfaces are almost invariably constructed of aluminiini and magnesium or their alloys. Paint technolo3ists are familiar irith the ordinary type of gage making use of the magnetic principle, illustrated by the XniincoBrenner AIagnegage ( 1 ) and the G.E. enamel thickness meter (3). These instruments have proved most valuable in aiding the paint technologist to hold film thickness mithin given limitin order to study closely various properties of the film which are largely dependent upon the thickness a t which it is applied. For studying films applied to nonmagnetic metals, resort has been made to the measurement of film thickness by means of dial gages or micrometers which involve either the measurement of the panel thickness before paint is applied or the destruction of at least a portion of the film usually somewhere near the edge of a sample panel. The method discussed herein describes a means for measuring t’he thickness of films applied to nonmagnetic conducting substances without destroying the film a t any point, regardless of the area involved. The outstanding limitation of the instrument in its present stage of develdpment is the fact
DESCRIPTION OF APPARATUS
\Then an alternating current flon-s in a coil near the surface of a nonmagnetic metal, eddy currents are set up in the metal which will affect. the inductance of the coil when placed upon or near the surface of the metal. The instrument was designed n i t h the thought of utilizing this phenomenon ( 2 ) . The measuring instrument makes use of the heterodyne principle in order to compare the variable frequency of one oscillator with the fixed frequency of another. The LC (inductance and capacity) product of the variable oscillator is adjusted to equal the fixed value of the fixed oscillator. The value of L for the variable oscillator depends on the distance of a pickup coil from the nonmagnetic conducting surface. The value of C necessary t o make LC, the required value, is controlled by a variable air condenser. This i$ the usual type of condenser found in a n ordinary radio set. By using a frequency of 50 kc. or greater, the inductance, as the pickup coil approaches the metal, is independent of the thickness of the metal, providing it is not less than 0.02 inch. Consider two oscillators, one of which is fixed whereas the other is variable. The two oscillators beat against each other, producing an audible note having a frequency equal to the difference of the tlvo oscillator frequencies. By adjusting the capacitance in the variable oscillator circuit by means of a condenser,
