Electrical Properties of Certain
Bakelite Materials W. A. ZINZOW AND THOMAS HAZEN Bakelite Corporation, Bloomfield, N. J.
AKELITE materials have many forms and uses, and one of their important applications is in electrical insulation. The electrical properties of an insulator that interest the average engineer may be placed in one of three groups-viz., dielectric strength, direct-current resistances, and power or alternating-current losses. There are a few characteristics-e. g., arcing resistance-which might not be properly placed in these three groups, but generally such a classification will cover the electrical tests made on an insulating material.
Dielectric Strength The dielectric strength of an insulating material tor dielectric may be defined as the voltage gradient a t which a continuous electrical discharge will take place between twcl electrodes when the dielectric in question is placed between these electrodes and a potential difference is applied to them. This voltage gradient is usually expressed in terms of volts per mil or volts per centimeter of thickness. The results of any dielectric strength test will depend to a large extent on the test conditions. Such factors as shape and size of the electrode used, thickness of the dielectric under test, rate of change of applied potential, temperature of dielectric, nature of surrounding medium, have an effect on the results of any test. I t is of utmost importance that all of these variables be conqidered in making dielectric strength tests. Most of the variables mentioned are controlled in any standard dielectric strength test. In this country the most commonly accepted standards for measurements of this type are those set up by the American Society for Testing Materials. A description of practically all commonly accepted tests together with the conditions governing them is described in the bulletin published annually by this society under the name of a committee report-viz., Committee D-9 on Electrical Insulating Materials. While most of the testing conditions are described in this report, a few factors affecting dielectric strength test results mav need more discussion. The effect of thickness of the test specimen and the eff ect of elevated temperatures on the test results are such factors. It is generally well known that the dielectric strength of a particular t y p e of dielectric tested und e r c e r t a i n conditions will vary considerably with t h e thickness of the test specimen. This is t r u e i n general of FIGGRE l . OF DIELECTRIC a l l dielectrics, i n STRENGTH WITH THICKNESS OF TEST cluding B a k e l i t e . PIECE BY STEPTEST METHOD
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The curves in Figure 1 illustrate this variation for several different types of Bakelite materials. These curves are based on observed values obtained when using the step-by-step method of increasing the applied voltage. Thus a standard molding material having a dielectric strength of 350 volts per mil for a test piece of 100-mil thickness will show a dielectric strength of approximately 100 volts per mil when the ter;t piece is 0.5 inch thick, This fact must always be taken into consideration when making dielectric strength tejts. For this reason the standard test piece is definitely set a t 0.125 * 0.005 inch. However, frequently it is nece-sary to make tests on materials which are not of a standard thickne3s. In order to make comparisons with standard thicaknesyes, it 13 p o s s i b l e to apply a correction factor. Figure 2 shows a curve f r o m ivhich c o r r e c t i o n factors may be obtained for v a r i o u s thickneqse. and types of materials. K e c e s s a r i l y these factors are only approximate but will usually give a value of L the dielectric btrerigtli $ a t standard thicknewithin *10 per cent of obqerved v a l u e s . 8 It serTes as a guide only and may be used t o avoid such absurdities as have occurred / .I00 ,200 ,300 .400 .5oO .6W ,700 when values of 1200 FIGURE 2. CORRECTIO'.FKTORS volts per mil for one FOR VIRIITIO\ OF DIELECTRIC m a t e r i a l have been STRENGTH WITH THICKYESS OF TEST c o m D a r e d with 700 SIMPLE B'r S T E P T E S T h'fETHOD volts per mil for another when both would have been the same if the teats had been made on the same thickness of tebt piece. Another important factor affecting the results of dielectric tests is the temperature a t which the test is made. We may expect in a general way that the dielectric strength of nearly all dielectrics will be lower a t elevated temperatures than they are a t ordinary room temperatures. By elevated temperatures, reference is made to temperatures below the point a t which any noticeable physical disintegration takes place. Figure 3 shows some characteristic variations in dielectric strength with temperature. The dielectric strength remains practically constant until certain temperatures are reached ; then the values of dielectric strength decrease with increased temperatures until a flattening tendency in the curve3 is again noted. This seems to be characteristic of molding materials only. The effect is not found with laminated materials. The temperature a t which the dielectric strength drops off seems to vary with different materials and for a given material nil1 depend to a certain extent on its previous 899
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INDUSTRIAL AND ENGIXEERING CHEMISTRY
hi-tory, particularly the molding time and afterbaking time. The effect of afterbaking on the dielectric strength of the finished product is well shown by curve A , Figure 3. The decided decrease in the dielectric strength with a n increase in test temperature previously noted is not evident when the material iq afterbaked. .4n afterbaking period of 10 to 20 hours a t 135" to 140" C. (275" to 284" F.) is usually enough to produce thiq change. The apparent rise in die l e c t r i c strength with i n c r e a s i n g t e m p e r a t u r e s is p r o b a b l y a false effect d u e t o t h e change in conductivity of the oil in which the test is made. Any change in the resistance of t h e medium surrounding the test FIGURE3. VARIATIONOF DIELEC- electrodes i n a n y dielectric strength TRIC STREUGTH V I T H TESTTEMPER.4T U R E FOR B31-120 M . 4 T E R I A L test causes a redist r i b u t i o n of the electrical stresses that may result in breakdown values which might possibly be misleading. It is possible that, the apparent rise of dielectric strength with increasing temperatures as shown by curve A is due to this effect. At best, as far as test conditions are concerned, considerable variation may be expected in results of the dielectric strength test because it is basically a "weak spot" test. That is, the breakdown always occurs a t a spot or small area, within the area under test or near the edge of the electrodes, where the material is weakest dielectrically. Hence, if there are variations in the material, and no composite material is perfectly homogeneous throughout, the point or small area having the lowest dielectric strength will usually break down, and that value will of necessity be used as the breakdown value for the whole test piece. The dielectric strength test is frequent'ly, and often erroneously, used as the sole criterion for determining whether or not a part'icular material is satisfact,ory as a n insulating medium for some particular use. The reason is that ultimately most electrical failures of insulat'ing materials are manifested in some sort of dielectric breakdown. The contributing factors to this breakdown niay not have been an inherently low or poor dielectric strength but some other elect,rical defect such as low resistance or high power factor which finally shows up as a dielectric breakdown. For this reaqon a knowledge of other electrical properties of the insulator may also be desirable.
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Direct-Current Resistance The direct-current resistance of a dielectric is usually measured by determining the current that will pass through the test piece when a potential difference is applied to two different points or arem of the te.st piece. The well-known Ohm's law relationship is then used to determine the actual value of the resistance. The resistance of any test piece is usually expressed in either ohms or megohms. From this measured value of the resistance, the value of either the surface or volume resistivity is sometimes determined. The surface rehistivity is expressed in either ohms or riiegohms while the value of the volume resistivity is usually given in ohm-centimeters (or megohm-centimeters) or in ohm-inche3 (or meg-
VOL!. 27, NO. 8
ohm-inches). It should be kept in milid that, the resistance is a property determined by the nature of the test piece a i well as hg the characteristics of the material. The residivitiej are characteristics of materials which should be independent of the shape and size of the test piece. Frequently a coniparison between different materials is all tliat i, needed; therefore the relatire resistances of two or more inaterial< in t'erms of a standard test piece are sufficient.. .Again t,he conformation of the test piece mag make calciilatio~i~ of the resistirities difficult or impractical, and then comparative values niay be used as long as test' conditicinb are uniform t'hroughout. A typical test ii that used and described and known variously as the insulation r Electric resistance, or surface resistance. Tlie test piece, in the case of laminated materials, consists of a flat sheet. 2.5 X 14 inches, with holes drilled along the center line with 1.25 inches between centers. Copper washere approximately 0 . 5 inch in diameter are held in place under preseure on both sides of the plates by means of 8-32 screw parsing through the holes. This test piece when assembled is then placed in a temperature- and humidity-controlled chamber for 4 day,+. The temperature is 35" C. (95" F.) with a relative humidity of 90 per cent'. .4t the end of this 96-hour exposure the resistance between adjacent electrodes is measured and recorded. Here obviously the shape and size of t.he test piece, together with the effect of t'he 4-day conditioning, are iniportant factors in determining the value of the resistance between electrodes. With these conditions fixed, any variations between different materials will depend largely on the inherent surface resistivity of the material as well as the amount of m o i s t u r e a b sorbed by the sirf a c e d u r i n g the c o n d i t i o 11 i 11 g tperiod. * 10 S i m i l a r l y , in LL making studieq on the effect of temE p e r a t u r e on t,lre t v o l u m e resistivities of niaterialr, a teht piece con-istFIGURE 4. RE~ISTANCE-TEMPER %TUBE illg Of 2-illch disk CH.4RACTERISTICS OF MOLDING31.4W'i t'h t W 0 bra>.; TERIALS electrodes rnolded into t)he piece wa,? used. -4s long as the same dimensions were employed, results for different materials could be compared simply by using the nieaiured values of reiktances without calculating the resistivities in each case. In any resistance measurements the teinperature of the test, material niust be carefully controlled. Along with practically all organic materials, Bakelite has a negative resiztance temperature coefficient. That is, t,he re>istaire of the material decreases as the temperature rise.. This change in resistance with temperature is quite large arid is approxiniatelp a logarithmic relationship Tlie curves show11 in Figure 4 give the effect of changing temperature;: on the resistance of various types of materials. The change of resistance with temperature shows an approximately linear relationship between the logarithm of the re.sistance and the temperature. The relationship might be expressed in tjhe general form: R = ae--hT
y
(L
4
The fact that these materials have such a large temperature coefficient of resistance may explain inany electrical failures under adverse teinperature conditions. For example,
IYDUSTRIAL AND EXGINEERIXG CHEMISTRY
AUGUST, 1935
1
30 TEMPERATURE
3.
'C.
V.4RIATION OF POWER FACTOR WITH TEMP E R A T U R E .4T .4 FREQUEZCY OF 60 CYCLES
FIGURE
TEMPERATURE 'C, 50 60 70
40
FIGURE6,
\'.4RIATION OF P O W E R F.4CTOR \\-ITH TEMPERATURE FOIt XM-262 xf.4TERIAL
a Ltandard material may have a sufficiently high resistance a t room temperature so that the leakage current for a particular use is negligible. If, however, the temperature is raised to 100" C. (212" F.) the resistance may drop enough to permit the flow of a leakage current of 500 to 1000 times its former value; this leakage might conceivably cause damage or a t least be a possible source of heat, which in turn mould produce a cumulative effect and finally result in bome 3ort of failure.
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compared, since it gives a bet'ter measure of the losses per unit volume of the dielectric than does the power factor. Many properties affect the power factor of an insulating material. The value of the power factor for a giren material will vary considerably with temperature and with tlie frequency of the applied voltage. Figure 5 s h o w the manner in 11-hich the power fact,or changes with the teiiiperature for some typical Bakelite molded produck when t!ie niessurements are made a t commercial frequencies. Curve .I i- not characteristic of this material and is discussed lster. IC the values of power factor were to be expressed logaritluiiically and plotted in a similar manner, practically a linear relation;hip would be shown. This finding is in line with the result? obtained on direct-current resistance measurements. , Curvm 1 and B of Figure 5 are of bpecial interest as illustrating what may occur if ordinary precautions are not taken in molding parts when using a standard type of iiiolding material. Curve A shows results obtained oti a particularly poor test piece; this test piece was then baked for 65 hours a t 120" C. (248" F.) and the power factor was again measured with bhe results shown by curve B. The results as sho\\-ri by curve B are generally characteristic of this inaterial. If the power factor measurements are made a t different frequencies on a given material and at some definite teniperature, there will be considerable variation among results obtained for power factors a t these different frequencie3. Figures 6 and 7 show some typical variations of power factors with measuring frequencies.
Power Factor R h e n dealing n-ith direct current', the loaaes in an insulator exposed to an electric field can be expressed in terms of the resistance, or its reciprocal, conductance, and tjhe leakage current passing through the material. In the case of alternating-current circuits the losses are expressed as power factors. The power factor of an insulat'ing material may be defined as the ratio of the total power 1 0 s (watts) in the niaterial to the product' of voltage (volts) and current' (amperes) in a capacitor in which that material is the dielect,ric. This factor is often expressed a i a percentage instead of a ratio. Let us suppose a condenser in a circuit in such a position that an alternating voltage is applied to it,. If the dielectric in the condenser has zero power factor, the c u r r d will be passed by the condenser without' any energy losses. There will be no heat,ing effects in the dielectric. If, however, the dielectric has a pover factor of 0.10 or 10 per cent and an alternating voltage is applied, some of the energy (10 per cent) n-ill be "lost" in passing through the condenser. This energy will usually appear in the form of heat in the dielectric. Frequently, instead of using the value of tlie power factor, engineers prefer to use the so-called loss factor because it gives information relative to widely differing types of materials. The loss factor of any dielectric a t a certain frequency is approximately equal to the product of its polver factor and dielectric const,ant, provided the power factor is not more than about 0.10 bo 10 per cent. Actually the loss factor is equal to the product of the cotangent of the phase angle and the dielectric constant. The dielectric constant or specific inductive capacity is expressed as a ratio which is determined by the capacity of a given condenser with the inaterial in question as a dielectric, as compared to the capacity of the same condenser with dry air or a vacuum as the dielectric. The loss factor is of particular interest when niat'erials having widely different dielectric constants are
The poJ\-er factor-temperature characteristics of Bakelite materials a t higher frequencies, such as 1,000,000 cyrles, do not s h o the ~ large variations found a t the lower frequencies. The results of some studies on laminated paper-base material indicate in some cases a small decrease in power f'act,or with rising temperatures. This is not surprising when me consider the trend a t the lower frequencies. Undoubtedly the presence of mater in the finished product is the most important factor in determining what the power factors of the different materials are to be a t the lower frequencies. Figure 8 shows the results of variation of power factor at 1000 cycles with loss of weight of the molding powder on vacuum-drying. This loss of weight is probably largely the moisture content of the molding powder. The powder was vacuum-dried at room temperature for 48 hours over concentrated sulfuric acid a t a pressure of less than 1 cm. of
INDUSTRIAL AND ENGINEERING CHEMISTRY
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mercury. This curve is of particular interest in that it shows how slight variations in moisture content may cause large variations in power factor of the resulting finished product. It also e3plains why preheating the molding material for a short time before molding will have such a large effect on the electrical properties of the finished product. Afterbaking the molded product improves these characteristics largely for the same reason. Any inherently good molding material can be ruined, as far as electrical qualities of the finished product are concerned, by exposing it for a relatively short time to atmospheric conditions involving high humidities such as frequently exist in molding shops, particularly if there are a few steam leaks in the lines or presses. On the other hand, many apparently poor electrical materials can be greatly improved by some treatment of the molding material, such as preheating, under conditions which tend to dry out any excess moisture. Generally, anything that affects one electrical characteristic of Bakelite materials will affect all of the electrical properties in the same way. This does not mean, however, that determining one of various electrical qualities will tell the whole story as to the suitability of the material for electrical uses. Apparently if only one property is to be measured, the power factor a t low frequencies will serve best as an indication of the insulation qualities of the material. Probably the dielectric strength tests could be considered the least satisfactory as a single test for determining electrical qualities. Even so, i t is necessary that all test conditions be carefully
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Selenium in Soils
VOL. 27, NO. 8
controlled in order to secure satisfactory results from any testing procedure. Other types of tests may be applied to determine the suitability of an insulator for some particular use, but most of them are special tests designed to simulate working conditions which frequently cause failure of some kind in the field. Such tests are sometimes applied for determining acceptance or rejection of materials for that particular use and do not always indicate the general insulating qualities of the material.
Samples and Tests Curves shown are based on data obtained on various types of Bakelite materials molded under correct conditions. The laminated materials were standard paper-base laminated products. Samples BM-021 and BM-120 are standard, general-purpose, wood-flour filled, molding compounds. Sample BM-250 is a general-purpose, mineral-filled, molding material with higher heat resistance and lower moisture absorption than the wood-flour filled materials. Sample XM262 is a low-loss, mineral-filled compound used for special electrical purposes. Sample XM-1000 is another specialpurpose, cellulose-filled material with improved characteristics such as arc resistance and higher direct-current resistance a t elevated temperatures. All of the results of tests given were obtained by the standard methods of measurement of the American Society for Testing Materials unless otherwise specified. R E C E I Y EMarch D 8, 1935.
w**
In Relation to Its Presence
in Vegetation HORACE G. BYERS
AND
HENRY G. KNIGHT
Bureau of Chemistry and Soils, Department of Agriculture Washington, D. C.
A
T A MEETING of the Associationof Offi-
cial Agricultural Chemists in Washington on October 30,1934, one of the authors presented an address on “The Selenium Problem.” Attention was called t o various phases of this new agricultural question and in particular to the following facts which were developed as a result of cooperative effort by various bureaus of the Department of Agriculture and the experiment stations of South Dakota and Wyoming. Selenium has been shown to be present in a wide variety of plants growing upon certain soil areas. It is present in the plants in concentrations which range from traces up to quantities which are lethal (to animals). I n many cases the selenium present produces chronic diseases which may ultimately cause death. Quickly lethal results and chronic disease have both been produced by addition of inorganic selenium compounds or of seleniferous vegetation to the normal diet of both large and small animals. It has been found possible to trace the source of the selenium in the plants to the soil upon which they are grown and to certain shales which are the parent materials of the seleniferous soils. From the available data it seems probable that the primary source of the selenium is iron pyrites or other sulfide ores containing selenium. I t should therefore be present to
Selenium occurs in soils to a varying extent over extremely wide areas. In certain limited areas it is present in quantities sufficient to produce toxic vegetation. The quantity of selenium in vegetation grown upon a soil depends not alone upon the concentration but also upon a variety of other factors which include the plant species, the composition of the soil, the moisture relations in the soil, the stage of development of the plant, and the portion of the plant examined. The nature of the general selenium problem is briefly discussed. some degree in all soils derived from pyritiferous materials in areas where rainfall is not sufficient to leach it from soils. The serious character of the situation as well as the novelty of the problem has attracted widespread interest. It was also pointed out in the address mentioned that, while there exists a wide variation in the quantities of selenium in vegetation grown upon seleniferous soils, the variation is not due alone to differences in the soil content. It is influenced by a number of factors, among which are the sulfur content of
