Insulating Paper in the Telephone Industry

more important types of paper insulations used by the telephone industry, and will show the relation the manufacturing pro- cedures bear to the initia...
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Insulating Paper in the Telephone Industry J. M. FINCH, Bell Telephone Laboratories, New York, N. Y.

specification control of paper will be discussed with emphasis on the simplification of chemical test methods and on minimizing the number of such tests. Finally, mention will be made of some of the modified forms of cellulose, which possess insulating characteristics superior to paper and which are already replacing it for some uses.

This article will discuss briefly a few of the more important types of paper insulations used by the telephone industry, and will show the relation the manufacturing procedures bear to the initial properties, the permanence, and the uses of the product. Special emphasis is placed on chemical properties as criteria of permanence. The

IXCE the beginning of experimentation with electricity,

cable ends; and after the splice is completed, the exposed insulation is dried by the application of molten paraffin wax heated well above 100" C. before the sheath is closed with the lead splicing sleeve. That ordinary impregnation of paper with wax or bituminous compounds does not prevent the absorption of water by paper is shown by Figures 2 and 3. These curves demonstrate that, under given conditions of temperature and humidity, paper or cellulose fibers impregnated with Superla wax (8) or with bitumen-wax compound (4)absorb as much moisture as unimpregnated materials. Some users of waxed papers do not realize this fact, and the assumption that dried and waximpregnated paper will remain dry without additional protection leads to results not anticipated. Wax impregnation, however, does afford a measure of protection from high humidity because the absorption rate is appreciably slower than is the case with unimpregnated material. The natural result is that the saturation point of the paper is not reached unless the high humidity period persists for some time. To maintain the necessary high degree of dryness of a dried, impregnated condenser unit, it is enclosed in a metal container which is filled with a molten waterproofing wax or bituminous compound. I n this way access to the paper is blocked by a thick layer of protective material, so that the absorption rate is reduced to extremely low values which for practical purposes is zero. Many coils, such as small transformers and loading coils which use paper in some form as insulation, are potted in a similar manner. Methods such as these are necessary in order to maintain a moisture content of a sufficiently low value to ensure good performance and long service life of the apparatus.

S

paper and fabrics have constituted one of the most important classes of thin, flexible, sheet insulating materials. Because of its cheapness and because of its much greater range of easily obtainable physical and electrical characteristics, paper is used much more generally than fabric, which is confined to purposes which require, for instance, unusually high mechanical strength. Progress in the electrical industry has imposed an increasingly severe burden upon all insulating materials, including paper. This has required a constant improvement in the material in order to maintain a satisfactory level of performance and reasonably long service life.

Properties of Cellulose Related to Insulating Paper Characteristics Since insulating paper is composed of cellulosic fibers, its electrical properties are essentially those of callulose. Although the electrical characteristics of cellulose are excellent as long as it is kept dry, it is very hygroscopic, and for any but the most uncritical uses absorption by paper insulation of atmospheric moisture must be prevented. I n general, it is desirable to maintain a low moisture content in the paper insulation of condensers, and in some special cases a moisture content as low as 0.1 per cent is necessary (6). The ssnsitivity of insulation resistance of cotton to moisture is illustrated by Figure 1 (14),which compares washed and unwashed cotton fiber. The insulation resistance of both decreases steadily with increasing relative humidity, and washing of cotton produces a definite elevation in insulation resistance. Included in Figure 1 is a curve for cellulose acetate rayon which shows that this material maintains a higher resistance level than cotton. The conductors of telephone cables are insulated with an unimpregnated paper which is protected from moisture by the lead alloy sheath. To attain the necessary dryness of the paper, the insulated conductors are given a thorough vacuumdrying treatment to reduce the moisture content to a low value. Except by accidental p-rforation of the sheath, the paper insulation is never exposed to moisture. During cablesplicing operations, precautions are taken to seal the opened

Other Waterproofing Treatments Many less critical uses of paper insulation permit of less effective waterproofing. Typical of this class is the paper impregnated with various waterproofing compounds and resins and used as insulation in some types of coils which, because of space and functional considerations, cannot be potted. The impregnated paper in this type of apparatus is more or less directly exposed to the atmosphere because the 1021

IKDUSTRIAL AND ENGINEERING CHEMISTRY

1022 10 0

crease by a double varnish treatment. These data show that the effectiveness of the treatments is dependent on the degree to which the paper surface is separated from the electrodes by the impregnating material. I n other words, in an insulator of this kind, paper functions principally as strong flexible support for the impregnating material.

WASHED C O T T O N

/

I

VOL. 32, NU. 8

I

Influence of Impurities While absorption of moisture is one of the major mu he,^ oi undesirable changes in electrical properties of paper. the presence of electrolytic impurities is also objectionable. I n addition, sometimes there are resinous impurities in paper which produce undesirable effects on the electrical propertie5 of certain impregnating compounds.

8

7

n

6

m U VI

4

5

Y U

I

50 PER

I

1

I

60 70 80 C E N T RELATIVE HUMIDITY

I

90

I

too

FIGURE1. EFFECT OF MOISTUREox THE IXXJLATIOX RESISTANCE OF WASHEDAND UNWASHED COTTONAND OF CELLULOSE ACETATERAYON

paper substance is, under the best conditions of impregnation, covered with only a thin layer of wax. varnish, or resin, all of which are more or less permeable to water vapor. Hence, unless the surface fibers of the paper are completely covered by a layer of appreciable thickness of impregnating compound and remain so in service, practically no permanent improvement in insulation resistance is effected by ordinary impregnation. Table I shows the relatively small improvement in insulation resistance resulting from the impregnation of paper with various materials. Because the relatively high conductivity of this kind of insulation leads to electrolytic corrosion of fine wire windings of unpotted coils, impregnated paper is being replaced in these cases by other sheet materials such as cellulose acetate. Cellulose acetate maintains a relatively high insulation resistance under high humidity conditions, as shown by Figure 1 and Table I where the insulation resistance values were determined a t 90 per cent relative humidity and 35' C. A relatively small increase in insulation resistance is obtained by wax impregnation, a greater increase by a single baked varnish treatment, and a still greater in-

t I-4 U W W U

c-3

I I

I

1 5

10

15 TIME

FIGURE2 . ASD

20

25

I

30

35

IN H O U R S

&IOISTURE ABSORPTIOKOF ~ N I M P R E G S A T E D SUPERLA-WAX-IMPREGSATED PAPER

Xnioiig the more common electrolytic impurities are chlorides, hydroxides, sulfates, and sulfides from cooking and bleaching operations performed on paper pulp during the papermaking process. Another common source of such impurities is the excessive re-use of paper machine water as an economy measure in paper mills having costly or inadequate pure water supply. This practice builds up the electrolyte content of the machine water, with the result that the effectiveness of the previous washing of the pulp is partially destroyed. Chloride contamination is often the result of inadequate washing of bleached pulps, but in other cases it may TABLE I. EFFECT O F I M P R E G N A T I O S O S INSCL.4TION RESISTANCE be due to natural waters of high chloride content. I n either OF PAPER COMPARED TO SHEET CELLULOSE ACETATE" case the result will be an increase in conductance of the inOver-all Insulation Specimen Impregnant Thickness Resistance sulating paper, which may reduce the life of apparatus in Inch ( m m . ) .Megoh?ns which the contaminated paper is used. I n addition, a given Kraft paper Sone 0.001 1 (0.0279) 100 concentration of chloride contamination will aggravate elecWax 0,0013 (0.0330) 140 Baking varnish trolytic corrosion of copper to a much greater degree than the Single impregnation 0.0015 (0.0381) 434 same concentration of sodium hydroxide. Consequently it Double impreanation 0.0025 (0,0635) 4595 - Cellulose acetate is particularly important to avoid the use of chloride-consheet None 0.0010 (0.0254) 70,000 taminated paper in fine wire coil windings. a Specimen 1 inch ( 2 . 5 4 em.) wide; electrode separation, 0.5 inch (1.27 Figure 4 illustrates the decrease in insulation resistance cm.). Specimens conditioned 48 hours a t 917, relative humidity and 35' C . which excessive chloride contamination in the base paper will

INDUSTRIAL AND EKGINEERING CHEMISTRY

AUGUST. 1940

produce in phenolic-resin laminated insulation. Material containing 0.3 per cent of chlorides may give rise to excessive electrolytic corrosion. The data in Figure 4 indicate the marked decfease in insulation resistance which results from successively increasing moisture content by equilibrating the material at several different relative humidities. Table I1 shows the improvement in insulation resistance effected in a fabric treated with phenol-formaldehyde resin by washing out the soluble salts from the fabric before impregnation with the resin.

1023

-

c--4

UNWASHED COTTON THREAD P H E N O L FIBRE < O.Ol% C 1

8 0

11. EFFECTOh' INSULATIOh' RESISTASCE OF ~ A S H I S G ELECTROLYTES OUT OF THE FABRIC BASEOF PHEXOLIC LAMISATED FABRIC

T.4BLE

Bakelite-Impregnated Muslin

F /O

Megohms 9

Unwashed Washed

* At

Ash Content of L7ninipregnated lluslin

Insulation Resistancea

0.9 0 04

102

SOo C . and 85% relative humidity.

The reduction of the life of condensers by the action of excessive amounts of chlorides in the paper is illustrated by the observation that a condenser made of paper purposely contaminated with 0.19 per cent chlorides (total in ash) failed under exaggerated conditions of impressed d. c. voltage in about 1.5 hours; under the same conditions a condenser made of paper containing 0.03 per cent chlorides had a life of about 60 hours, and one made with paper containing 0.01 per cent chlorides showed a life of 280 hours. Because the presence of chlorides is so objectionable in all kinds of electrical insulation, it is considered good practice to avoid the use of paper made from bleached fiber unless special precautions are taken to remove residual chlorides.

5

100

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$ $g +

3u.o

80 60

5 40

5 2 a ?

;2 0 2

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8

4

FIGURE 3.

12

16

20

24

28 32 36 T I M E IN H O U R S

40

44

48

52

56

60

MOISTURE ABSORPTIOXO F cNIJlPREGS.4TED .4ND BITUiVIXOCS-W.4X-IMPREGNATED JUTE

Resinates from rosin sizing are objectionable where chlorinated compounds are used for impregnation, because those substances increase the conductivity of this type of compound. The increase in conductivity of chlorinated diphenyl produced by sodium or aluminum resinates is shown by Figure 5 which also contains a curve indicating the much smaller effect reaulting from the use of rosin. These data illustrate the importance of eliminating the use of rosin-sized papers and also TABLE 111.

RELATIOS OF

ACIDITYAND (:OPPER

Cable Paper

Kormal Slight Moderate High T h e strength

Thickness, Inch (Mm.)

Apparent Density, Gram/Cc.

5; .Ish

"",'WING TO DIMENSIONAL DIFFERENCES I N TEST SPECIMENS, THE RESISTANCE

Acidity, milliequivalent/ gram

'I\. I \ I

LEVELS Of COTTON A N D PHENOL FIBRE SHOWN HERE ARE NOT DIRECTLY COMPARABLE

the necessity of taking steps to prevent the containination of unsizecl paper with residues of rosin size when the unsized paper is made with equipment previously used for making sized paper. I n other words, these facts show that it is advantageous in the manufacture of insulating paper to employ equipment which is not used on occasions for making either sized or bleached papers. A great many kinds of insulating paper are subject,etl to high-temperature treatment for the purpose of reducing the nioisture content, of hardening drying-oil-varnish impregnation, or of curing the resin in phenolic-resin laminated insulation. Moreover, a considerable proportion of paper insulation must withstand elevated temperatures developed in the apparatus in which i t is used. Ability t o withstand these exposures to heat without embrittlement is almost entirely dependent upon the absence of more than minute fractions of acids or acid salts such as aluminum sulfate. Percentages too small to have a serious effect on conductance or corrosion characteristics are objectionable from a heat-aging standpoint. Table I11 shows the relation between acidity and brittleness of paper induced by heating in a ventilated oven. The data also show that' in

XCYBER TO INCREASE IN

r-

.4ciditg. of

I

BRITTLENESS OF

Unbaked Copper number

BrittleAcidity, Tensile ness millistrength, strengtha equivalent/ kg./0.5 in. k d 0 . 5 in: gram

0 0052 (0.1321) 0.69 0.58 Seutral 0.48 10.4 16.7 0.005 0.0052 (0 1321) 0.69 1.10 0.0003 0.3s 9.7 16.5 0.006 0.0063 (0.1346) 0.69 1.80 0.0094 0.39 9 1 16 2 0.043 0.0052 (0.1321) 0.6i 1.58 0.0240 0.42 8.8 14.9 0.059 of a paper strip broken over the rounded edge of a thin beam 0.004 inch (0.1016 m m . ) thick (9).

HEAT-AGED P A P E R

Baked 48 Hr. Copper number

0.98

0.89 1.30 1.51

(136" C.)--

BrittleTensile ness strength strength0 kgJ0.5 i i . kg./0.5 id. 10.: 9.1

7.9 A 1

12.5 1 2 . .5 5 s 1 6

1024

INDUSTRIAL AND ENGINEERING CHEMISTRY

VOL. 32, NO. 8

more can be made. Condenser paper has this sort of structure, and its high density and extreme thinness with practical absence of holes are made possible by this gelatinizing property of cellulose. Figure 6 shows the difference in structure of paper made from lightly beaten fiber not gelatinized, and of paper made from fiber strongly beaten to develop considerable gelatinous material. The dense and strong insulating boards having high dielectric strength are other examples of paper insulation made with fiber largely altered to the gel state. Finally, the properties of the finished paper are much affected by the treatment the sheet receives on the paper machine. Thus the paper density is controlled by the use of fiber gelatinized to the correct degree combined with the proper calendering pressure.

Relation between Chemical, Physical, and Electrical Properties

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l 0.06

R O S'I N 6 8 O~ C.

T

l 0.08

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1 0.10

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1 0.12

P E R CENT R E S I N

OF SMALL ADDITIONS OF ALUMIFIGURE 5. EFFECT NUM AND SODIUX RESINATES AND ROSIN ON THE CONDUCTIVITY O F AROCLOR

these papers the acidity rather than the copper number of the unaged paper is the significant criterion of heat stability. It is also evident that the increase in copper number is an approximate measure of the amount of physical degradation produced by the heating treatment (2, 10, 11).

Properties Dependent on Structure The physical and certain of the electrical properties of paper are directly related to its structure which, in turn, can be varied over rather wide limits by the proper control of the papermaking process and to some extent by the choice of the raw materials. Consideration of these relations directs attention to the fact that paper is essentially a felted fibrous structure which can be made so that i t has an apparent density as low as 0.3 or as high as about 1.3, which value approaches the density (1.6) of cellulose. The dielectric constant of the unimpregnated paper may, in consequence of the variable proportion of air and cellulose in a given volume, vary over wide limits. By varying the density, the dielectric strength as well as the absorbency of paper for such waterproofing impregnants as waxes, varnishes, and synthetic resins can be controlled. The ability of cellulose to take up water and become gelatinized under certain conditions is utilized to increase the latitude of physical characteristics of paper. For instance, the use of relatively long fibers or fiber particles gelatinized to only a slight degree will produce, with proper paper machine conditions, a medium-density, strong, flexible paper. I n such a paper the thin layer of gel-like cellulose on the fiber surfaces functions as an adhesive and thus augments strength. At the other end of the scale, an appreciable proportion of the fiber substance may be transformed to the gel form. With such pulp, papers having apparent densities of 1.0 or

Inasmuch as telephone equipment is designed to function for a period of years, it is of interest to know what properties of the paper are significant for defining its suitability for use, particularly those properties which will determine its expected useful life span in apparatus in service. Knowledge of its physical, electrical, and, to some extent, its chemical p r o p erties will show whether a paper will successfully endure the mechanical stresses incident to fabrication into the forms suitable for insulation as well as the stresses involved in a p plication to the apparatus. The physical and electrical prop~erties' also define its initial electrical performance. On the other hand, the chemical properties of paper furnish us with information which permits an effective appraisal of the stability of the insulation under adverse service conditions which may entail intense electrical stresses and elevated temperatures. I n a general way the chemical characteristics of paper insulation intended for use in telephone apparatus are the same as for that used in power apparatus, because long service life is desirable in both cases. On the other hand, the physical properties may be quite different because very difierent electrical properties which are largely dependent on the physical properties are sometimes required for the two uses. Such differences between the two industries are illustrated by the telephone cable operating a t a voltage of about 100 volts maximum and by the power cable with service potentials of up to 50,000 or 100,000 volts.

Paper Manufacture To show the relation of insulating paper characteristics to the papermaking process, it will be of interest to examine the principal steps in its manufacture. I n a general way the operations involved in producing the felted structure of cellulosic fibers constituting the finished product are as follows: The raw materials in the form of coniferous wood chips, worn out RIanila hemp rope, and cotton and linen in the form of rags or cordage (and others) are subjected to some form of chemical treatment (cooking) usually with a dilute alkaline solution a t about 125" C. for a sufficient period to dissolve and render water soluble the noncellulosic components of the raw materials making the cellulosic papermaking fibers available. After the removal of the major part of the cooking solution, the cooked fibers (pulp) are then subjected to more or less prolonged severe maceration in water. The treatment, called '[beating", not only reduces the fibers to the proper sized fragments but it also transforms more or less of the fiber substance into the gel-like form of cellulose, depending on the duration of beating and on the beating pressure used. Following beating and washing out of cooking liquor, the pulp is freed from unbeaten fibers and fiber bundles by pass-

AUGUST, 1910

INUUS'TLIIAL AND ENGINEERING CHEMISTRY

iiig it through screens. The licavy particles, including rriet~al, are removed by ceiitrifiigal refiners which operate on the beaten stock just hefore it is delivered to the paper machine. From a water suspension of about 0.1 to 1 or 2 pr cent weight of fiber to water, the fiber is deposited by filtratiori on some form of moving fino-mesli wire screcii which constitutes what is known as the \ret end of the paper maelline. Water is drawn from tho layer of wet fibers by suction box rolls. By thc time the wet sheet has rcaclied the end of the nrovirig endless screen belt of the Foiirdriiiier-type pappcr machine, it possesses sufficient slrengtli to support itself across a narrow gap 011 its ~ a t,o y the steam-hcate(1 drying rolls. Leaving the drying rolls, the sliect is Ciilli~J:l~t,ei~ to a greater or less degree hy passing through heavy calciidor rolls arranged in a vertical stack. Very little or no c;ileodering is done on lowilensity papers such as ahSlJrbC!nt papers for plieiiolic-resin laminated insulation and telephone c:tbl(! pager ; hut on Irigli-density papers such as condenser tissues, caltsrdering is perfonned at very high pressures.

1025

ture requires the exercise of the greatest sliill. l'apermakers consider it one of the most difficult papers to produce. That the structure of coiidenser paper has an import.ant bearing on condenser c1iara.cteristics appears to be a safe assumption to make in view of the large proportional variations in thickness shown by Figure 7 8 . The cross section is of paper with a nominal thickness of 0.0004 inch. The threeor fourfold variatioii in thickness from spot to spot shows that the effect.ive thickness of tire 0.4-mil paper is about 0.1 mil. When ciamirier1 microscopically with transmitted light a t a magnification of from 100 to 200 times, tlie paper in the thin areas appears practically structureless. This fact confirms the idaa that the thin areas are made up of fibers hcatsn morc

I

.I

Condenser Paper Condenser paper is made from either iiew linen rags, lineii curdage waste, or coniferous wood fiber produced by the sulfate process (kraft pulp). If mede from linen, the rags or cordage are cut into small pieces and subjected to a mild caustic soda caok, which is followed by heating and washing. With kraft paper the raw material is cooked kraft pulp supplied to the paper mill in very tliick sheets. Whiehever fiber is used, a seT-ere beating treatment is required, first to reduce the length mid diameter of the individual fibers by frapnentation until tlic diameter is about equal to or less than the thickness of the paper which is 0.0003 or 0.0004 inch, and secondly, to prodiicc a considerable amount of finely divided gelatinized ~elldi~tie whiclr is practically devoid of fihrous structure. The latter material serves the purpose of bridging the s p a c ~ sbetween tlie fiber fragments in the fiber lattice formed on the paper machine wire, of clusing spaces which would otlienvisc be hales in finished sheet, arid (Jf fiilirig voids within the sheet striictorik (13). After drying arid caleudering at moileriite pressures on the paper machilie calender stack, coiideriser paper is colripacted atill more in the "supercalender" rolls, where the pressure iirtensity is very high anrl is just below that whiclr woiild crud, the paper. As mentioned before, the'appnreot densit,y of cnnileiiser pa,per, calculatcd from the measured thickness anrl weight, is 0.9 to 1.10, Because it is i:xtri.mdy tliiii arid m o s t he praetically free from holes, coiirleriscr paper rmmufac-

severely than those comprising tlie tliick areas. Sitice the slirinkape of paper during dryirig i ~ i itlie paper machine ini:rcascs with scvcrity of bcatiiig, it is t o be expected that the rel:latively structureless areas sliould shrink most and hence should be thinnest (6, 15). Contrast,ed witli the nonuniform paper is a cross section of 0.0004-inch glycerolfree cellophane (Vigure 7 B ) . The noniinifomiity of condciiser paper leads to the reconsideration of a rruniber of fact,ors canirected with the evaluation of its characteristics as well as some related t,o its use. This information will uniloubtedly prove an important addition to tho kno\vledge ihat t,hn condenser designer I I O W has at Iris disposal. It is obvious from Figure 7A that tlie apparent deiisity of t.tie sheet. calculated as grams per cc. from the measured thickness and weight, is definitely Icss tliarr the true density OS the palm substitiice and considerably less tlian could he obtained in a sheet frec from surface irregularities. It also appears that the commonly calciilated ''air space" in condenser tissue coiisists iargely of the air occupying the spaccs lying between the actual paper surfaces ~ 1 x 1the two planes contacting.., the liigh on each sur., sriots . face. In other 1 \ 0 1 . the calcrilated air space, liitlierto considered to consist largely of vriirls witliiii the paper structure, is really mostly outsiilc of the shect. That t h e apparent density of the sheet suhstance is greater tlian the range of 0.9 to 1.1, when ealculnted on the thickness ineasiired with a niicsorneter, is substilntiated by the fact that apparont ilensities of 1.3 are ohtaineil on tlrick (0.015 to 0.025 inch) insulating hoards wliere thc caleridcring pressure intensity is proliahly lower tliaii is the case with 0.0003 t,o 0.0004 iiicli condenser paper, hut wlrere the siisEacc air spmx volurnc is proportionally mucli less. Since tho iliclcctric strength of coiideiiser papm is ealciilated on t,he measured tiiick3. I3i.rtcn so t h a t P rolntively Inigo amount 01 gdatinircd nmterinl was pmduocd ness, it is obviims that our ideas of the ilielecr l i i c b closed ail tho holes between fibers. tric strength oi celluli~scin this form must he revised. 'ER MADE FROM BEATEN FIBER( X 50)

INDUSTRIAL AND ENGINEERING CHEMISTRY

1026

VOL. 32, NO. 8 ~~

TABLE IV. TYPICAL SPECIFICATION REQUIREMENTS FOR INSULATING PAPERS"

Paper Type Tele hone o a b g paper Saturating papers

Cond en Jer tissue Pressboard

Apparent Density Gram&. 0.70(max.)

Fiber Hemp, cotton, sulfate wood Cotton-rag,

0.5-0.8

K3th"

0.5-0.8

Linen, k r a f t

0.89 (min.)

Thickness Range, Inch ( M m . )

Max. WaterTensile Sol. Strength ConKg./0.5 In:/ tent, Mil %

0.00175-0.014 (0.0444-0.3556) 0.004-0.010 (0.1016-0,254) 0.004-0.010 (0.1016-0.254) 0.0003-0.0004 (0.0076-0.0101) 0.015-0.125 (0.381-3.175)

1.2-2.0

Max. AlcoholSol. Content,

7%

2.5

2.0

1.5

...

1.5

...

.., ...

...

...

... ...

.\lax. Max. S p . Total Conductivity Chloride Max. Reaction, of Water Ext. Content Max. Milli!.Mhos X 10-5) on Ash, . h h . % equivalent/Gram per Gram % 3 0 0 005 acid, ... 0 0175 alk ... 0 005 acid 3 . .i 0 020

...

0.005 acid

2 . .j

0.01.;

...

0 001 3 acid

2.0

0 008

0.8-1.3 K r a f t , cotton... ... ... 0 005 acid 2.0 0 015 rag a These requiFements are npt qomplete but are intended t o illustrate what may be considered general practice in the electrical industry, particularly with rmpect t o chemical characteriaations.

Telephone Cable Paper

Specifications

Unlike condenser paper, telephone cable paper must have an apparent density of not over 0.70 and a t the same time must be very strong and tough to withstand the severe mechanical stresses incident to winding it in ribbon form (at some 3000 r. p. m.) onto the conductor. The fiber make-up of cable paper is usually a combination of sulfate wood pulp (kraft fiber) and Manila hemp (Musa teztilis) obtained from old Manila hemp rope and cordage. As with kraft condenser paper, sulfate wood pulp is used as supplied by the kraft mill in the form of thick folded sheets. After hand-sorting, the old Manila hemp rope is cut into pieces a few inches long and is given a n 8- or 10-hour cook in 6 to 8 per cent caustic soda solution a t about 125-130" C. T o form a sheet structure of the low density and high strength required, the beating treatment is comparatively mild so that the fibers are shortened only slightly, retain their original diameter, and are gelatinized to only a slight degree. The sheet is gently calendered on the paper machine.

The function of specifications is to describe as accurately as possible in terms of electrical, physical, and chemical properties, the insulating quality and the stability of the paper for any specific use. I n addition to the various specification requirement limits to which a paper must conform, a test method is prescribed for determining each characteristic covered by the specification, because test result values are usually dependent upon the methods used. Such specifications are of use to the design engineer and to the purchasing and inspection organizations. Twenty-five years or more ago specifications for insulating papers were rare, and the quality of the paper available was, in general, very inferior when judged by present-day standards. It was recognized, however, that certain paper characteristics, such as the water-soluble salt content and acidity, should be controlled if a fairly satisfactory permanence under continuously applied electrical potential was to be attained. At that time specification requirements were established by selecting the papers which showed the best performance records and writing a specification which would permit their acceptance. Although this method left much to be desired, it did eliminate papers of a quality appreciably lower than that of the average product made in accordance with the best papermaking practices of the time.

Paper Pulp Insulation One of the most interesting and revolutionary developments in papermaking is the pulp insulation process ( 7 , 12), in which a sulfate wood pulp paper insulation is formed around the conductor on an ingeniously modified cylinder paper machine. To prevent a highdensity insulation due to shrinkage while drying, the wet fiber cover on the wire is dried with extreme rapidity, starting with a temperature of approximately 850" C. and finishing a t 450" C. The resulting evolution of steam aids in the formation of an open structure having a low dielectric constant. The insulating machine simultaneously coats sixty conductors a t the rate of 100 or more feet :30 meters) per minute and has resulted in outstanding economies in the production of small-sized insulated wire ranging from 22 to 26 American wire gage, inclusive. For long-line cables where voice current attenuation must be kept a t the lowest practical level, rather thick paper (about 0.010 inch) is still used because only with it can the mutual capacity between conductors be kept a t the necessary low values. Enlarged photographs of pulp and of paper inmlation are shown in Figure 8.

Saturating Paper For the manufacture of phenolic-resin laminated sheet insulation, an absorbent paper of low density and moderately high strength is required. Cotton, purified sulfite wood (alpha) pulp, or sulfate wood (kraft) pulp are in common use. The papermaking procedure is similar to that for cable paper except that the beating is even milder and produces practically no gelatinization effect. The calendering also is minimized so that the paper will absorb up to 65 per cent phenolic resin.

FIGURE8. TELEPHONE CONDUCTOR INSULATED WITH A SPIRALLY WOUNDPAPERSTRIPCONTRASTED WITH A TELEPHONE CONDUCTOR INSULATED WITH KRAFTPULPINSULATION FORMED o s THE COSDUCTOR

Since then apparatus operating conditions have become increasingly exacting; they require an improved product and emphasize the importance of a more complete knowledge of the relation between the physical and chemical properties of the paper and its performance. Progress along these lines has been made so that the chemist can now furnish the

AUGUST, 1940

INDUSTRIAL AND ENGINEERING CHEMISTRY

papermaker with much sounder advice as to how to make a paper with better service life characteristics. Along with these advances improvements have been made in testing technique, and a t the same time new test methods have been developed to define properties not previously controlled but now known to be important. All of these factors combine to make possible a much sounder approach to the formulation of specification requirements and limits. The ideal procedure would obviously consist of carrying out research to relate every paper characteristic with performance of apparatus under accelerated tests designed t o simulate years of actual service conditions. In most practical cases, however, the magnitude of such a task is tremendous, because of the great number of paper variables, the difficulty of obtaining material in which one component characteristic is

1027

definite weight of paper is digested under controlled conditions in a known volume of Aroclor. The drop of the Aroclor resistivity produced by this treatment is limited by the specification requirement. As important as adequate specifications are, other factors are equally important in obtaining a uniformly good product. For instance, the mill where the paper is made must have a personnel conversant with modern chemical control procedure and appreciative of the importance of maintaining a high degree of chemical purity in the finished paper. Location of the mill with respect to an adequate supply of pure water is another important item. If a normally good water supply is subject to periodic or seasonal contamination, the condition should be recognized and appropriate safeguards established. Unless similar precautions are taken, the product is apt, t o

FIGURE9. APPARATUSFOR DETECTING CONDUCTING PARTICLES IN CONDENSER PAPER

varied to the exclusion of all others, and the wide variety of service conditions. However, a workable approach to such a n ideal is to do sufficient research to evaluate by performance tests those characteristics which general experience and chemical knowledge indicate to be important. These physical and chemical characteristics can then be controlled by choice of suitable specification requirements. Illustrative of how the requirements differ for paper insulation intended for various specific uses, typical specification limits for telephone cable paper, condenser paper, saturating paper, and pressboard are given in Table IV (1).

Test Methods In the interest of economical inspection testing, simplicity and accuracy of test methods are important. Progress has been made in simplification by the development of methods which define two or more paper characteristics, each of which formerly required a separate test and method. An example is the conductivity of water extract which is a measure of the water-soluble electrolytes. This method, together with a n appropriate requirement, has replaced the water-soluble, alkalinity, and ash methods and requirements with an appreciable reduction of testing time. Requirements and methods for chlorides and acidity are retained because the conductivity of water extract method is not sufficiently sensitive in its reaction to these impurities. A qualitative rosin test has replaced the longer, more involved determination of alcohol-soluble material. Wherever possible, use is made of test methods established by recognized technical organizations, such as the Technical Association of the Pulp and Paper Industry or the American Society for Testing Materials. Tests associated with requirements not listed in Table IV include a method for determining the effect on the resistivity of Aroclor (chlorinated diphenyl) produced by immersion of condenser insulation in it. In this test a

vary to an extent too great to control by inspection testing. The result is that poor quality paper will sometimes find its way into apparatus. It is evident, therefore, that before contract relations are established with a mill, i t is well to have some knowledge of its equipment, its personnel, and its water supply.

Substitute Materials Marked improvements in the performance characteristics of paper insulation have been made during recent years, and we may expect similar or greater advances in the near future as the result of work which has been stimulated by the recent advances in the development of paper substitutes. An example of attempts to improve insulating properties is the partial acetylation of fibers which are then made into paper ( 3 ) . Such a paper appears to have superior electrical properties, but it is evident that further development work is necessary to improve the mechanical properties. Although paper is still the best relatively thin flexible insulating material, some of the new substitute products are replacing it for special uses. Cellulose acetate sheet is an outstanding example of such a material, and for interleaving in fine-wire magnet coils its electrolytic corrosion characteristics are better than those of paper. Other substitutes are being scrutinized as they become available. They include Pliofilm, ethylcellulose, and cellophane. They are used in sheet form by themselves or laminated with paper, to which they add resistance to abrasion and toughness, and in some cases they increase the insulation resistance. In addition to these sheet materials, there are a number of synthetic resins which are being investigated in the form of coatings on paper. They are applied either from solution or in the form of the melted material. Such treatments also improve the mechanical and electrical characteristics of the paper base.

INDUSTRIAL AND ENGINEERING CHEMISTRY

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Conclusion

VOL. 32, NO. 8

Literature Cited

During the progressive improvement of the performance characteristics of paper, testing technique must necessarily undergo refinement in precision and reproducibility in order to define adequately paper which will function satisfactorily under increasingly exacting service conditions. I n doing so the chemist must continue to fulfill his obligations by furnishing better and more economical means t o the engineer and purchasing agent for selecting paper insulation of satisfachory quality.

Acknowledgment Assistance in the form of data and suggestions was received from R. R. Williams, G. T. Kohman, D. A. McLean, E. J. Murphy, J. H. Ingmanson, H. G. Walker, and W. J. Kiernan.

Brit. Plastics, 6 , No. 1, 364-70 (Jan., 1935). Burton, J. O., Bur. Standards J . Research, 7, 429-39 (1931). Electrician, 117, 657-8 (Nov. 27, 1936). Ingmanson. J. H., and Vacca, G: N., ISD. ESG. CHEY.,26, 1274-5 (1934). Jahn, E. C., Paper Trade J., 107, No. 9 , 38-40 (1938). Kohman, G. T., IKD. ENG.CHEY.,31, 807 (1939). Little. J . S.,Paper Trade J , 96, No. 8, 29-32 (1933). hlclean, D. A., unpublished rept. Peek, E. L., and Finch, J. M., Paper Trade J . , 88, No. 6, 56-62 (1929). Rasch, R. H., BUT.Standards J . Research, 7 , 465-75 (lcl31). Richter, G . A., IXD. ESG.CHEM.,26, 1154-7 (1934). Walker, H . G., and Ford, L. S., Bell System Tech. J . , 12, 1-21 (1933). Weil, C., Paper Ind., 16, N o . 12, 842 (1935). Williams, R. R., and Murphy, E. J., Trans. Am. Insl. Elec. Engrs., 48,No. 4, 568-75 (1929).

Materials for Vacuum Tube Manufacture A. J. MONACK RCA Manufacturing Company, Inc., Harrison, N. J.

The requirements of metals and alloys for cathodes, anodes, and grids are presented, and a brief treatment of thermionic emission in the cases of oxide-coated, thoriatedtungsten, and tungsten filaments is given. Metals and alloys for use in glass-metal seals are enumerated. The function of getters is explained in connection with the exhaust process. Types of photoelectric tubes and their uses are discussed. Silicates, sulfides, and tungstates are treated in terms of persistence and color of luminescence when used for fluorescent screens in cathode-ray tubes. Miscellaneous materials and parts are listed, and cleaning of vacuum tube parts is discussed in terms of methods.

HE term “vacuum tube” is used to identify a large variety of devices which have in common an outer en-

T

velope of glass or metal (occasionally quartz), into which one or more electrodes are sealed, and from which the air has been mostly exhausted or has been replaced by some other gas. Mercury rectifiers, gas-filled rectifiers, high-vacuum rectifiers, amplifiers, oscillators, modulators, detectors, photoelectric tubes, cold-cathode tubes, cathode-ray tubes, iconoscopes for television pickup, vacuum thermocouples used for measuring small alternating currents, electron multipliers, ionization gages for measuring the degree of vacuum are among the numerous types that are made. “Electron tubes” is a term considered synonymous with “vacuum tubes”; and the ex-

pression “thermonic tubes” is applicable in all cases where the primary electrons are produced thermally.

Filaments and Cathodes The passage of electric currents between the electrodes of vacuum tubes produces effects which make these tubes useful in various ways; and the cathode is the heart of the tube, since electron emission from this electrode forms the current which determines the characteristics of the tube. The cathode must satisfy three requirements: The required rate of electron emission must be available; the cathode must have a satisfactory life; and the structural strength a t high temperatures must be sufficient for the cathode to retain its shape. Since the first two requirements are in opposition, careful choice of materials and design is necessary. Metals for cathodes must emit electrons easily (low work functions) ; must have low vapor pressures so that evaporation does not cause early failure; must have high melting points, low thermal conductivities to prevent rapid heat conduction away from the cathode, high tensile strength, and stiffness; and must not fail by creep. Some compensation for failure to meet these requirements completely is possible by changes of cathode shape-. g., the use of ribbon cathodes. A heated tungsten filament was one of the early commercial sources of electrons, and tungsten is still the filament material in nearly all large power tubes. The wire must be pure and uniform in properties and dimensions. A local reduction in diameter, for instance, would cause overheating and subsequent burnout at that point. The electron emission efficiency (emission current per watt of heating power) increases as the temperature increases, but the evaporation rate of tungsten sets an upper limit. I n order to obtain greater emission efficiency without exceeding the safe operating temperature of the filament, tungsten wire to which has been added 1 to 2 per cent of thoria is often used. The emission efficiency a t the