MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
this fact, a series of oils of Gulf Coast origin was prepared; each had the same amount of aromatic unsaturation (8.08.1 per cent), but each contained a different amount of olefinic unsaturation. Standard stamp capacitors impregnated 2.1 2.0 I.9
1.8 1.7 1.6
1.5
1.4
1.3 1.2 1.1
1.0 9
8 7 6
5
A 3 2
I 0 1 2 3 4 5 6 7 8 9 1 0 1 1
WEEKS
Ir
75.c.-
81e V.P.M.
FIGURE13.. EFFECTOF MINERALOIL-DISSOLVED DIPHENYL ON THE DIELECTRIC STABILITY OF TREATED INSULATION
with such oils were tested on 75" C. life test a t 812 volts per mil for power factor stability in the usual manner. Figure 11 relates the 10-week, 60-cycle, treated insulation power factor value to the original olefinic unsaturation present in the impregnating oil. It is evident that, within the limits examined, the presence of olefinic unsaturation contributes to insulation instability. Olefinic unsaturated products present in the oil in an amount greater than 5 per cent produce a marked power factor instability in the treated paper.
Effect of Aromatic Unsaturation The presence of normally occurring aromatic unsaturation in mineral impregnating oils was shown to give advantageous results (3). Whether the effect of such oil-soluble products concerns an increase in oil resistance to oxidation, a difference in the type of oxidation products, or an increased degree of fiber wettability by the oil, with resultant increased efficiency of impregnation, has not been determined. That the presence of aromatic unsaturates does promote increased insulation stability, however, is demonstrated in Figure 12. The treated insulation stability is here related to the aromatic unsaturation of the impregnating oil used. The oils are all of Gulf Coast origin and are refined to give a widely varying degree of aromatic unsaturation with, in all cases, an olefinic unsaturation of less than 0.5 per cent. After vacuum drying and oil impregnation, the standard stamp capacitors are subjected to a 10-week life test run at 75" C. and 812 volts per mil. The 10-week, 60-cycle, 30" C. power factor value is related in Figure 12 to the degree of original oil aromatic unsaturation. It is apparent that the presence of normally occurring aromatic oil unsaturation gives increased oil-treated insulation stability. The optimum stability with the type of mineral oil under discussion is obtained with an aromatic unsaturation between approximately 4 and 8 per cent. The application of such a generalization, however, is impossible without due regard to the effect of the varying amounts of olefinic unsaturation which may be present. The effect of such olefinic unsaturation may predominate, with the result
333
that the oil is unsuited for high-voltage use, although what normally would be an optimum amount of aromatic unsaturation is present.
Addition of Aromatic Hydrocarbons It has been suggested that the addition of certain synthetic aromatic hydrocarbons-for example, diphenyl-decreases the gas evolution from mineral insulating oils subjected to electrical discharge (1). The fact that such an effect is produced is generally accepted. However, the addition of such hydrocarbons to mineral oil demands a study of the effect produced on the stability of the impregnated dielectric, The effect of the addition of synthetic aromatic hydrocarbons of the benzenoid type to impregnating oil was studied by means of the hydrocarbons diphenyl and dibenzyl. Diphenyl was selected as representative of a polyphenyl hydrocarbon with the phenyl groups directly united. Dibenzyl was selected to demonstrate the effect, if any, produced by the presence of a carbon chain. The results are illustrated in Figures 13 and 14. I n all tests the standard vacuum-dried and vacuum-impregnated linen paper stamp capacitor was used. The presence of oil-dissolved diphenyl on the dielectric stability of the oil-treated insulation is negligible unless the amount of diphenyl exceeds its solubility limit in the oil. Figure 13 indicates that 20 per cent diphenyl by weight in the mineral oil produces some dielectric instability in the treated insulation. Such a quantity of dissolved diphenyl produces crystal separation at the lower testing temperatures. IO
9 8 5 7
L
26
E:
k 3 E 2 B I 0 0
I 2 3 4 5 6 7 8 WEEKS A T 75.C.- 812 V P M
9
1011 12 1314
15
OF OILFIGURE 14. COMPARATIVE EFFECT DISSOLVED DIBENZYL AND DIPHENYL ON DIELECTRIC STABILITY OF TREATED INSULATION
In contrast to the inertness of oil-dissolved diphenyl, the presence of dibenzyl in the impregnating oil gives definite evidence of dielectric instability for the treated dielectric. Figure 14 shows the dielectric instability produced by the addition of 1, 5, and 7 per cent dibenzyl. Figure 14 also demonstrates for comparison the negligible effect produced by 10 per cent diphenyl in the same basic oil used for the dibenzyl investigation. Within the limits of complete oil solubility over the whole range of operating temperature to which the oil-impregnated insulation will be subjected, it can be concluded from the data submitted only that the selection and addition of synthetic aromatic hydrocarbons to mineral impregnating oil for the purpose of reducing gas evolution under electric discharge must be carefully considered. Not all aromatic hydrocarbons are inert with respect to the dielectric stability of the treated insulation. The presence of a carbon chain in the aromatic addition promotes dielectric instability.
Literature Cited (1) Berberich, L. J., IND.ENQ.CHEM., 30, 280-6 (1938). (2) Clark, F. M., Ibid., 29, 698-702 (1937). (3) Clark, F. M., U. S.Patent 2,112,735 (March 29, 1938). (4) Kattwinkel, R., Brennstoff-Chem.,8, 353 (1927). RECH~XVBID July 13, 1938.
ELECTRICAL APPLICATIQN
OF POLYSTYRENE L, A. MATHESON AND W. C. GOGGIN The Dow Chemical Company, Midland, Mich. Polystyrene, formed by the thermal polymerization of monomeric styrene, CBH&H:CHz, has been known for many years to be an excellent dielectric solid. Polystyrene possesses outstandingly low electrical power factor, high dielectric strength, great arcing resistance, and low water absorption. The material has recently become available in the United States at a price which will promote much more extensive application. The electrical, mechanical, thermal, and other properties of the polymer are outlined. Mention is made of the methods of applying styrene in the electrical field as the monomer and as the polymer in the cast, molded, and solution forms. The paper includes a discussion of the properties of the material with respect to many applications. Graphs of interest in electrical applications are shown.
low dielectric constant, 2.6-2.8. The low power factor and the low water absorption seem t o arise from the lack of polarity and the strong binding of all the components in the chain. The polymer consists only of carbon and hydrogen atoms, the majority of which are in benzene ring structures. Polystyrene is transparent and of low density, .and exhibits a tensile strength of 5,000-7,000 pounds per square inch. It may be manufactured at a cost comparable to that of other plastics, The properties of polystyrene, compression molded from ground molding powder, are shown in Tables I, 11, 111, and IV. Polystyrene possesses good tensile strength, modulus of elasticity, and hardness, and fair impact strength. The maintenance of the impact strength at very low temperatures is noteworthy. The one per cent tensile elongation of polystyrene before break is smaller than usual for plastics. However, this low elongation does have some advantage in that the full strength of the material is reached in only a small displacement. That is, the Young's modulus of the material is higher than normal for thermoplastics. Polystyrene forms an exceedingly rigid material without the yield to which most other thermoplastic materials are subject. This, coupled with the close tolerances with which polystyrene can be molded, results in good dimensional accuracy and stability. The distortion temperature of polystyrene under load is 89-90' C. This value is lower than that for the thermosetting materials-i. e., phenol-formaldehyde and urea-formaldehyde-but is higher than is usual for thermoplastics. This particular property, low compared to phenolics, has tended to prevent full use of polystyrene in the electrical and radio field. Discoloration or yellowing on long continued exposure to ultraviolet light or sunshine is one characteristic of previous polystyrenes which has been quoted against their use. Recent improvements in the treatment and purification of the polymer have resulted in greatly increased light resistance and have considerably widened the possibilities of continuous outdoor applications. The properties of polystyrene, in general, are good for most purposes. Further discussion on electrical and water absorption properties will be given later in the paper. A major variable in polystyrene is the molecular weight, or the number of monomer units per polymer molecule. This is dependent mainly upon conditions of polymerization-i, e., temperature, purity of monomer, and presence of inhibitors. The latter may be naturally present or added. A change in molecular weight of the polymer does not influence the electrical properties appreciably but does change the mechanical properties to a great extent. Higher polymerization temperatures result in more brittle polystyrene and affect particularly the tensile and impact strength and the softening temperature.
P
OLYSTYRENE, a transparent, thermoplastic solid, is formed by the polymerization of monomeric styrene, CeH$H:CH2, a colorless liquid of boiling point 146" C. Polystyrene has been known for many years to be an exceedingly excellent electrical insulator (7). However, the application of the material has been severely restricted in the past by unavailability, impurities, price, and certain limitations in use. These restrictions are gradually being overcome, and domestic supplies of styrene have been, made available at a price which will foster many new applications. Industry now has a thermoplastic insulator with the electrical properties equal to those of fused quartz, with extreme water resistance, and with dimensional stability a t temperatures up to 70" C. The manufacture of monomeric styrene and its polymerization were reported by Stanley (9). The resultant polymer is probably a long-chain carbon compound with a phenyl side group for every pair of carbon atoms of the chain, as in the following structure: H H H H
-LLA-AI
I
I
I
This structure, continued on to the order of thousands of carbons in a chain and with many long chains intermingled, results in a plastic material of excellent electrical properties with a power factor of only 0.01-0.02 per cent and a relatively 334
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
Although the data of Table I were determined on compression-molded test pieces, they characterize, with some exceptions, the properties of the material in other useful forms, such as polymerized in place, injection-molded, and applied from a solution.
Cast Polymerized Styrene Polymerization of the monomer in place is increasing in application. This allows a solid or porous article to be incased in a strong, transparent, electrically insulating structure, and provides protection from weather, water, and handling. Several factors must be considered when styrene is polymerized in place. The heat of polymerization (about 20,000 calories per gram mole of monomer) tends to restrict the thickness which may be polymerized in one piece. Polymerization in a reasonable length of time requires temperatures of 100" C. or higher. These temperatures approach the boiling point of the monomer, 146' C. The volume of the material contracts about 15 per cent during polymerization and cooling. During polymerization and subsequent cooling, styrene tends to form bubbles or voids, and if air is present, there may be slight surface oxidation which results in color. For ornamental or exposed situations the resultant polymer should have very little volatile material-for instance, monomer; otherwise, exposed surfaces will blush or check,
TABLEI. MECHANICAL PROPERTIES OF POLYSTYRENE" Tensile strength, lb./sq. in. (A. S. T. M. D48-37) Elongation, % Modulus of elasticity lb sq. in. Impact stren th (A. 8. M. D-256-34T Izod notched t a r ) , ft.-lb.: 25' C. -70' C. Rockwell hardness No. ('/n-in. ball 60-kg. load) Rockwell superficial hardness (I/,&. diam. ball, 15kg. load)
i!
5,500-7,000 1
5.5-7.0 X 108 0.2-0.3 0.2-0.3 R90-RQ7 15 X 89-15 X 92
a Determined on compression-molded test pieces.
TABLE11. ELECTRICAL PROPERTIES OF POLYSTYRENE Surface resistivity, ohms .t,ivitv. ohm-om. !:M. ,?495-38T), sea.
>10'0 >io17
240-250
."", _._...
On 0.125411, section Dielectric properties at various frequencies: Frequency, Dielectric cycles constant 60 2.6-2.7 2.6-2.7 1,000 2.6-2.8 1,000,000
>
2500 500
Power factor, % 0.02 0.02 0.04
TABLE111. THERMAL PROPERTIES OF POLYSTYRENE g c i f i c heat, cal./gramK.C. ermal exDansion coe cient
0.324 1:
TABLEIV. GENERAL PROPERTIES OF POLYSTYRENE Refractive index (n",") Light transmission through 0.10-in. thickness % Water absorption in 48 hr. (A. 9. T. M. D48:37 immersion), % Specific gravity Specific volume, cu. in./lb. Burning rate
1.59 90 0.00 1.05 26.3 Low
Light: Slight coloration on long exposure t o sunlight Weak acids or alkalies: No effect Strona acids: Discolor Strong alkalies: No effect Solubility: Sol. in aromatic hydrocarbons, chlorinated hydrocarbons, aliphatic esters, and most ketones; insol. in alcohols, paraffin hydrocarbons, vegetable oils, and waxes Toxicity: Chronic oral feeding and skin exposure show none
335
However, methods have been evolved as outlined in the succeeding paragraphs which give satisfactory results. Polymerized blocks are shown in Figure 1. A machined part from such a block is illustrated in Figure 2. One such use of styrene polymerized in place has been for the inspection of the mechanical uniformity of high-voltage underground cables. Wyatt, Smart, and Reynar (IS) made perfect styrene-impregnated sections of cable with all components literally "petrified" in position. The insulating oil was extracted from short sections of the cable by a reflux of liquid styrene boiling a t a low pressure in the absence of air. The styrene-impregnated lengths were then sealed in cans and polymerized at 120' C. for several days. After polymerization, the petrified cable was sawed into sections about 1 mm. thick, which were mounted on transparent plates and projected like a slide or viewed as a transparency. The method allowed a study of the mechanical uniformity of oil-impregnated paper cable insulation of various constructions and history. The publication of this work contains an excellent account of extraction and polymerization procedure. Polystyrene has been used in high-voltage cables in England. A number of satisfactory cable joints, plugs, and ends have been made by polymerizing monomeric styrene in place. The monomer was forced by pressure into a section of the cable between two nipples (8) and later Polymerized by heat. Other cable joints have been made oil impassable by impregnating the tape for the joint with monomeric styrene before winding, followed by polymerization after assembly (2, 8). Scott and Webb (8) claim that their most satisfactory procedure is that of impregnating tape before winding with monomeric styrene containing a plasticizer. The styrene was polymerized and the tape wetted with monomeric styrene during winding. The resulting joint was claimed to be better than the rest of the cable and to improve with age as the styrene completed its polymerization under operating conditions. Impregnation, followed by polymerization of such items as coils and capacitors, is an indicated possibility. Polymerized or cast blocks of polystyrene may be machined easily to produce a great variety of parts. Simple shapes may be formed bubble-free, and the polymer may be easily machined or molded to shape. Where only a few parts are required, machining from the solid is the more economical procedure. Machining operations should be so conducted as to minimize the heat production and to remove the heat from the polystyrene as quickly as possible. The use of a heat transfer fluid, such as soapy water, played over the cutting edge increases the possible cutting speed.
Molded Polystyrene Molded shapes of polystyrene are of increasing interest to the trade. Polystyrene, in the form of powder or pellets (Figure 3) is easily molded by heat and pressure to give parts of excellent dimensional stability and hard glossy surface, as well as crystal clarity. The flow is such that the thinnest mold sections may be readily filled a t temperatures and pressures within the ranges used for other thermoplastics. Polystyrene is well adapted to injection molding procedures, but parts should be designed to provide uniform thickness walls in so far as possible. Molding conditions are as follows: Compression molding temp. F. (" C.) Compression molding pressdre, lb./sq. in. Compression ratio (approx.) Mold shrinkage Injection molding temp., F. ( O C.) Injection molding pressure, lb/sq. in. Color possibilities
240-350 (116-177) 300-2,000 2.5 0.002-0.0025 380-420 (193-216) 3,000-30,000 Unlimited
Effect of Orientation on Properties Fabricating processes which impart flow during molding result in improved mechanical properties in the direction of
336
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 31, NO. 3
ing member (3). This construction uses polystyrene in the combined stretched and helical form. I n this manner no short surface or volume leakage paths are provided between conductors, and only a small amount of dielectric is present in the electric field. A number of patents have been issued on methods of usage of polystyrene in concentric cables for highfrequency or high-voltage work ( 1 ) . Such cables are of great importance for television (Figure 4).
Polystyrene Films Polystyrene films are now available for use as condenser foil or as cable wrapping. Solutions containing polystyrene as the major nonvaporizahle component have been advocated as electrical insulating varnishes. Such solutions have been applied as a “dope” for coils in the radio industry. Pla8ticizers may be added to polystyrene to increase its elongation and pliability. To ensure the best electrical properties, only nonpolar agents such as hydrocarbon type compounds should he used. Plasticizers in general decrease the
MARCH, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
tensile strength and the heat distortion temperature when more than a few per cent are added. For that reason it seems best to work with unplasticized polystyrene if possible.
Frequency 60 1,000 50,000 1,000,000 20,000,000
Power Factor, % ' 0.02 0.015 0.022 0.016 0.028
337 Dielectric Constant
2.55 2.57 2.58 2.60 2.60
Losa
Factor 0.0005
0.0004 0.0006 0.0004
0.0007
Electrical Properties Some measurements which illustrate the electrical properties and applications of polystyrene are as follows: DIELECTRIC STRENGTH.The variation of dielectric strength of polystyrene with thickness is shown in Figure 5. Samples molded from the ground material were subjected to 60-cycle
2000 -
I
l
l
/
In fact, there seems to be very little dielectric absorption of polystyrene from zero frequency, even into the range of visible radiation. The dielectric constant a t visible light frequency is the square of the index of refraction (n2 = 1.592 = 2.51). This is almost equal to the dielectric constant of the material in the commercial frequency range (about 2.6). The difference of these values arises from the dielectric loss in the intermediate frequency region (14). This difference is small compared to that found in other plastics. The power factor has been measured for ultrahigh radio frequencies (6). Polystyrene showed a power factor of only 0.04 per cent a t wave lengths of 60 to 150'cm. The material has been used as low-loss insulation a t wave lengths of 1 to 2 em. and for far infrared transparent windows at wave lengths of 0.005 cm. The dielectric loss of polystyrene a t 1,000,000 cycles with a high voltage gradient across the sample was measured by Race (6) by means of a calorimetric method. He found a power factor of 0.04 per cent as observed with lower voltage gradients; i. e., the power factor of styrene is affected little by increased voltage gradient, even at radio frequency.
THICKNESS IN INCHES
1000
.010
,005
,015
FIGURE 5 . DIELECTRIC STRENGTH OF COMPRESSION MOLDED POLYSTYRENE
10 5
a. c. voltage, increasing a t the rate of 2,500 volts per second. inch in diameter with edges rounded on a Electrodes, inch radius, were subjected to a pressure of about 5 pounds per square inch against the sample submerged in oil. The thickness was measured with a micrometer. To guard against the effect of oil films between the electrodes, the measurements shown in curve A were repeated using metal rings filled with mercury. The flat portion of the face of each electrode was mercury, so that no spuriously high dielectric strength should be observed. The results are substantially the same as shown on curve B. The dielectric strength is about equivalent to that of excellent mica-namely, 5,500 volts per mil-and high compared to that observed with other plastics. Curve C shows data taken after 48-hour water immersion at 25" C.; it illustrates the small effect of water on polystyrene and the maintenance of its good electrical properties even under adverse conditions. POWER FACTOR. Dielectric constant and power factor have been determined over a range of temperatures and frequencies. The following table shows data obtained a t 60 cycles with about 50 volts per mil applied to the sample (supplied through the courtesy of the General Electric Company) : Temp.,
Power Factor, % '
30 BO 90
0,035 0.044 0.09
*
c.
Dielectric Constant 2.55 2.57 2.60
These data show that polystyrene maintains its low losses .at temperatures approaching the softening point of the material. This property is especially desirable in an insulator ,operating in a high-voltage gradient where increased losses a t higher temperatures tend to increase the temperature indefinitely. The dielectric constant varies little with frequency, a definite indication of low losses (4):
ID .5
.IO
.05
.01
0
OF WATER BY SEVERAL FIGURE6. SORPTION PLASTICS os. TIME
WATERABSORPTION. The water absorption of polystyrene is very ,small and usually quoted a t 0.00 per cent for the standard A. S. T. M. immersion of 48 hours in 25" C. water. The percentage changes in weight when polystyrene, previously conditioned for several days a t 50 per cent relative humidity, is immersed in water at 70" C. are shown in Figure 6. Some measurements of the Bell Telephone Laboratories ( l a ) on water absorption of other plastics and rubber are shown on the same scale. A comparison of the data for polystyrene and for other materials shows the exceedingly small water absorption of polystyrene. The electrical properties of polystyrene are affected slightly by what water absorption does take place. Exposure of polystyrene to atmospheres of various relative humidities is found to affect its properties less than water immersion.
338
INDUSTRIAL AND ENGINEERING CHEMISTRY
SURFACE ARC RESISTAKCE.The ability of an insulating material to withstand electrical breakdown on its surface is essential in some applications. Polystyrene surfaces withstand electrical discharges well. The polymer naturally repels water and thus tends to prevent breakdown across its surface unless water is present in appreciably large quantities. Also, the depolymerization of polystyrene above 300” C. and subsequent evaporation of the monomer tends to prevent carbonization of the surface by neighboring or surface discharges. The result is that polystyrene has an excellent rating of 240 seconds before breakdown, due to an intermittent arc playing over the surface (A. S. T. M. D495-38T). It also withstands, better than other resins, another type of arcing test in which salt solution containing a wetting agent is dripped onto the surface between concentric rings charged with 110 volts a. c. or higher. These behaviors show polystyrene to withstand incipient and prolonged arcing across its surface in a satisfactory manner. SURFACEAND VOLUMERESISTIVITY.The surface and volume resistivities of normal polystyrene are so large that the measurements necessitated an electrometer. A gold leaf electrometer was fitted with polystyrene insulation and used alone or in parallel with various insulating shapes. These measurements of leakage currents indicate that the surface
VOL. 31, NO. 3
resistivity is greater than 1OI6 ohms and the volume resistivity is greater than lo1’ ohm-cm. The excellent electrical properties shown for polystyrene in the previous paragraphs indicate a bright future in the electrical field and foreshadow improved electrical apparatus through its application.
Literature Cited (1) Engler, J., and Studt, E., U.S. Patent 2,101,386(1937); Klimmer, F.,Ibid., 2,116,267(1938); Klimmer, F.,Ibid., 2,116,268 (1938); Rost, H., Ibid.,2,116,643(1938): Unterbrusche. F., and Bodemann, E., Ibid., 2,118,907 (1938). (2) McCulloch, L.,Ibid., 2,106,850(1938). (3) Mayer. H.F., and Fischer, E., Elektrotech. Z., 56, 1245 (1935). (4) Morgan, 8. O.,INIJ.ENG.CHEM.,30,273 (1938). (5) Race, H.H., and Leonard, S. C., Elec. Eng., 55,1347(1936). (6) Rohde, L.,2.tech. Physik, 16,637 (1935). (7) Scott, T. R.,Electrical Communication, 16,51 (1937). (8) Scott, T. R., and Webb, J. K.. Ibid., 16,174(1937); 17,88(1938). (9) Stanley, H.M., Chemistry &Industry, 57,93 (1938). (10) Staudinger, H.P.,and Stanley, H. M., Ibid., 57, 141 (1938). (11) Studt, E., and Meyer, U.,U. 5.Patents 1,992,678(1936) and 2,074,285(1937); Fischer, E., Ibid., 2,047,554(1936). (12) Taylor, R. L., and Kemp, A. R., IND.ENQ.CHEM.,30, 409 (1938). (13) Wyatt, K. S., Smart, D. L., and Reynar, J. M., Elec. Eng., 57, 141 (1938). (14) Yager, W.A.,Phulsics, 7,434 (1936). RECDIVED September 20,C1938.
L‘INCANTATION By Felicien Raps Thanks to the cooperation of the late Dr. Allan F. Ode11 we are able to bring a new artist, Felicien Raps, into the Berolzheimer Series of Alchemical and Historical Reproductions. In presenting this as No. 99 in the series, we note that here the Alchemist is dabbling in necromancy and visualizes the symbol of a dream which is tied up with the idea of the elixir of life and perpetual youth, as in Rabinovitch’s picture, “The Alchemist” which is No. 79 in the series. While the latter evokes the female of his dreams from an alembic, we here see the main figure in a painting brought to life. Rembrandt also portrays the visualizing of the symbol of an idea in his Dr. Faustus (No. 21). Felicien Raps, a Belgian, was born in Namur in 1833 and died at Essonnes in 1898. He was a painter, engraver, and lithographer, and did the illustrations for many books, made’during a sojourn in Paris, also many political Carica. tures. D. D. BBROLZHBIMBR* 50 East 41st Street New York, N. Y.
A completc list of rhc Brsr 96 reproductions appcarcd in our January, 1939, iSSuc, pagc 124. An additional rcproductton nppcars cach month.
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