Determination of Modulus of Elasticity of Varnish Films Effect of W a t e r Immersion HERBERT G. STEFFENS',
Material Laboratory, Navy Yard, Brooklyn,
N. Y.
A method is described for measuring tho change in modulus of elasticity of detached varnish films during water immersion, using an improvised tensiometer of simple design. Special reference is made to insulating varnishor, and d a b on immersion characteristics of a typical sir-drying varnish w e included, together with a discussion of the precision of tho measurements.
THE
use or reconditioning of electrical equipment which has been submerged in water for any length of time frequently involves handling of the insulation. Therefore, the mechanical as well as the electrical resistance of 'the insulating varnish film is an important considemtion. Many insulating varnishes which meet the X&vyrequirements (2) of a minimum insulation resistance of 1 megohm after water immersion show evidences of change in optical and mechanical properties, characterized by hazing or whitening and softening of the fihn. Exploratory tests described herein were part of a general investigation conducted to determine the effect of water immersion on the physical properties of insulating varnishes.
Various types of tensiometers of rigid construction are available (1). However, for the purpose of these tests i t was desired t o construct an inexpensive and simple loading device, equipped with a reasonably accurate means of measuring deflections, which might be used to m e a s w elongation of films at very light losdings and which could be immersed in water or placed in a temperature-controlled enclosure.
PREPARATION OF FILMS
for whitening on &ter immersion.
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25" C: and 50% relacive'humidity. during 8 hours of which the
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sometimes the case using &algamated p k e s .
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Present address. 280 W m Jerser St., Eliraheth 2,
. N. J.
Figure
I Figure
1.
Tensiometer
The apparatus shown in Figure 2 was constructed from spare pieces of molded phenolic, brass, and copper, and consists of tnbe A to which me d d e d accurately measured volumes of liquids, a balanced pulley E, fitted with ball bearings to reduce starting friction, over which stress from A is transferred to clamp C by means of line D, and wire lead E t o which is attached a crossbar forming a rest for the actuated arm of the optical lever, F. D e flections in clamp C are noted by reflection of a suitable scale viewed through mirror G by means of a crow-hair telescope (not shown).
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b.
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Apparatus for Preparation of Detached Varnish Films
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I N D U S T R I A L A N D E N G I N E E R IiN G C H E M I S T R Y
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Vol. 17, No. 7
Measurements n w e made on films clamped into position as shown in Figure 2, using plastic adapters. The reference distance was taken as the distance between the edges of the adapters. Sufficient weight, previously determined, was added to the loading tube so that motion of the upper clamp was impending. Measured volumes of distilled water (about 0.3-ml. portions) were added then to tube A , and the deflection of the scale was noted 4 minutes thereafter. The scale was so situated that an observed deflection of 0.1 inch corresponded to a strain of 0.3 mil in the test specimen. The observed scale was read t o the nearest 0.02 inch. The test 15 as conducted at 28" C., the apparatus bcing placed i n :i lnrgc beaker to avoid the effects of drafts. RESULTS OF TESTS
Figure 3 shows the stress-strain relationship for a film iiiiniersed 72 Iiours in distilled water a t 25" C. (dotted line) as compared with an unimmersed control specimen (solid line). The unit stress was calculated using the film thickness of spccimens before immersion, since, as a rule, the dimensional changes of short-oil insulating varnishes after \ d e r immersion for periods up to 240 hours rarely csxceed aboiit lo%, and since the film is usually softened to the r s t m t that accuratc thickness measurements are not possible. Figure 4 sho\va thc effect of various inimcrsion periods on the elasticity of test specimens, the modulus of elasticity being plotted again.;t thr inimcrsion time. -
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Figure 5. Reproducibility of Elongation Measurements on Films CONCLUSIONS
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licsults of the tests show a measurable change in tlie inociuiw of elmticity for the particular varnish studied, and indicate that the change may be determined with accuracy, even with irnprovised equipment. The fact that this change curve is not a linear function of the immersion time has significance in the testing of the elastic properties of the film after immersion, since thcse tcsts are best made at times whcn the modulus of elasticity is changing least rapidly with respect to time. This point can bc found only by nicaiis of tests similar to those described here. Slight variations in the slope of control-specimen curves using diffcrmt i,ing specimens have been noted. This is probably due to slight variations in film thickness, which would exert an influence on the drying of the test spcviincms. For this reason it is considcrcd nc'ccssxry to iimkc control tvsts on each ring specimen. The errors in thr method, apart from errors i n measurement, are chiefly due to improper 1o:itling (initial) of the specimen, variation in temperature during th(h test, and incomplete elimination of vibration. However, it is possible, with sufficient care, to rncasflre with pwcision tensile propertics on adjacent portions of the ring specimens, as is shown by the stress-strain graph of Figure 8. The solid line here represents an average of three tests. I n this case, three test specimens slightly smaller than those described in this papci \wre cut from the same ring spccimen.
004 005 &e INCH
3. Stress-Strain Diagram1 for Film Immersed 7 9 Hours in Water
ACKNOWLEDGMENTS
Acknoaledgrnent is hereby madc to Lt. Commander George Barsky, U.S.S.R., and to Charles E. Fraser, principal inaterials engineer, both of the Material Laboratory, for their assistance and advice in the preparation of this p:ipc~. LITERATURE CITED
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Figure 4,
Change in Modulus of Elssticity of Films during Water Immersion
(1) Gerdner, H. A., "Physical and Chemical Examination of Paints, Varnishes, Lacquers and Colors", 9th ed., p. 119, Wsshington, D. C., Institute of Paint and Varnish Research, 1939. (2)
Navy Dept., Specification 52V13e, Varnish, Insulating (Electrical), March 1, 1944.
TEE views presented in this paper are those of the writer a n d are not to he construed ae the official views of the Navy Department.
