Porous Polymer Tape

Screening Program

Keywords:

Cables

Tape

Porosity

Testing

Polymer

Permeation

EPRI

EPRI EL-1259

Project 7864

Final Report

December 1979

Prepared by

National Bureau of Standards

Washington, D.C.

Porous Polymer Tape Screening Program

EL- 1259

Research Project 7864

Final Report, December 1979

Prepared by

NATIONAL BUREAU OF STANDARDS

Polymer Science and Standards Division

Washington, D.C. 20234

Principal Investigator

A. J. Bur

Prepared for

Electric Power Research Institute 3412 Hillview Avenue

Palo Alto, California 94304

EPRI Project Manager

Bruce S. Bernstein

Electrical Systems Division
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Prepared by

National Bureau of Standards

Washington, D.C.
ABSTRACT

A screening program has been developed for porous polymer tapes which are considered

to be candidates for a tape-oil insulation system in a high-voltage transmission

cable. The program consists of 21 tests for measuring mechanical, electrical and

physical properties. Descriptions of most of these tests have been published in

ASTM test schedules or in other literature sources. Two of the tests had to be

developed in our laboratories . They are the degree-of-void-filling test and the

solubility test.

For the degree-of-void-filling test, a buoyancy measurement is utilized to calculate

the void volume which is not filled by the permeating oil. Two buoyancy measurements are made: first, the specimen is immersed in a calibration fluid which is

assumed to permeate all voids in the specimen. A true specimen density is calculated and then compared to an apparent density which is obtained from the specimen

immersed in oil. The difference in the two densities is proportional to the

reciprocal of the unfilled void volume.

Four materials and two oils have been used for these experiments. The materials are

a polypropylene synthetic paper (PPSP), a porous polypropyl ene (PP), a polyethylene

synthetic paper (PE) and a paper-polypropylene-paper laminate (PPP). The oils are

an alkyl benzene oil and a polybutene oil. The results show a range of void filling from very little for PPSP to complete filling for PP.

i i i

EPRI PERSPECTIVE

PROJECT DESCRIPTION

The electric power cable industry has always lacked a coordinated way to evaluate

porous tape materials for potential use as paper substitutes in laminar

dielectrics. Although a number of screening tests have been generally employed

by suppliers, there was no consensus on the testing scope, or on the priority to

be assigned to the value of existing tests. Furthermore, it has not been clear

whether existing tape screening tests are fully adequate. Consequently, cable

manuf acturers have found it necessary to test model cables in order to develop

meaningful information. This approach is unnecessarily costly and not fully

revealing. The purpose of this project was to formulate a screening program and

to develop any experimental methods required to evaluate porous tapes, as part of

that screening program.

PROJECT OBJECTIVES

The specific objectives of this project were to (a) establish criteria for

properties that synthetic porous tapes must meet, (b) describe test methods for

measuring these properties, (c) develop new test methods that may be required to

support the test program.

PROJECT RESULTS

A three-phase screening program for porous polymer tapes was developed. The

phases comprise a series of 21 tests (12, 6 and 3 per phase, respectively) , to be

employed sequentially. Phase I is designed to rapidly eliminate unsatisf actory

materials; Phase II focuses on dielectric and mechanical properties of the tape

in oil, and Phase III provides for an aging interval in oil under combined

dielectric, thermal and mechanical stress. Phase I data can be utilized to

develop Pirelli equation data on dry tapes, and Phase II data can be used to develop similar data for oil-immersed tapes. As part of the screening program,

the contractor, the National Bureau of Standards, developed a method to quantitatively measure the void-filling capability of porous polymer tapes. The

method consists of measuring the buoyant force of a porous tape immersed in a

v
dielectric oil relative to immersion in a calibration fluid. The equipment

required for this test is a Cahn electrobalance. The National Bureau of

Standards also developed a test for accurately determining the solubility of

porous tapes in oils, and developed a set of recommended guidelines on acceptable

levels of performance. The program that has been developed and the acceptance

guidelines are recomnended as a guide, and may be further refined as additional

test data are developed by the Industry.

As a result of this work, the Industry now has available a method for reliably

measuring the porosity of tapes for dielectric applications, and a programmed

approach for screening porous tapes prior to preparing model cables.

Bruce Bernstein, Project Manager

Electrical Systems Division

vi
ACKNOWLEDGMENTS

The principal investigator wishes to acknowledge the contributions of Dr. Robert

W. Penn and Mr. William P. Harris. Dr. Penn made measurements of compression

modulus on stacks of paper tapes and helped to develop test Nos. 12 and 16 of

this program. Mr. Harris prepared many of the test descriptions in Section 4 and

carried out most of the editorial work for this report. Also, special acknowledgments are due to Drs. Freddy A. Khoury and Martin G. Broadhurst for many helpful

discussions with the author during the course of this work. The author acknowledges

the consultations and assistance of Mr. Edward D. Eich during the early stages of

this program.

VI 1

CONTENTS

Section Page

1 INTRODUCTION 1-1

2 SELECTION OF PHYSICAL PROPERTIES 2-1

Assumptions on Which Selections of Tests are Based 2-1

Consideration of the Assumptions 2-2

Tests Eliminated from the Program 2-2

Choice of Tests to be Included in the Program 2-4

3 ACCEPTANCE CRITERIA 3-1

The Pirelli Equations 3-1

Application of the Pirelli Equations 3-2

4 THE TEST PROGRAM 4-1

Using the Test Program 4-2

Notes to Table 4-1 4-3

5 VOID-FILLING TEST 5-1

Calculation of Unfilled Void Volume 5-1

Case 1: No Swelling, No Solubility 5_1

Case 2: Swelling Occurs, but no Solubility 5_2

Testing the Hypothesis 5-3

Result 5_4

Experimental Apparatus 5-5

Calibration 5-6

Measurement of Liquid Densities 5-7

Specimen Preparati-on 5-8

Procedure 5- TO

Experimental Results 5-15

6 SOLUBILITY 6-1

7 CONCLUSIONS 7 _!

8 REFERENCES 8-1

ix

ILLUSTRATIONS

Fi qure Page

4-1 Apparatus for Tests 5, 17 and 18 4-5

4-2 Suggested Grip Modification for Tests 12 and 16 4-11

4-3 Suggested Testing Machine for Tests 12 and 16 4-13

4-4 Stacked Specimen in Testing Machine 4-14

4-5 Suggested Oil and Thermal Chamber for Tests 15 and 19 4-15

4-6 Stress Unit for Tests 19 and 20 4-20

4-

7 Grips for Stressing Material for Tests 19 and 20 4-21

5-

1 Schematic Diagram of Cahn RG Automatic Electrobalance 5-5

5-2 Block Diagram of Buoyant -Force-Measuring System 5-7

5-3 Specimens for Void-Filling Test 5-10

5-4 Specimen of PPP Suspended in Oil 5-10

5-5 Dr. A. J. Bur Operating the Void-Filling-Test Apparatus 5-11

5-6 Buoyancy of PPSP in Heptane vs. Time 5-17

5-7 Buoyancy of PPSP in Polybutene vs. Time 5-17

5-8 Buoyancy of PE in Alkyl Benzene Oil vs. Time 5-19

5-9 Buoyancy of PE in Polybutene Oil vs. Time 5-19

5-10 Buoyancy of PP in Alkyl Benzene Oil vs. Time 5-20

5-11 Buoyancy of PP in Polybutene Oil vs. Time 5-20

5-12 Buoyancy of PPP in Alkyl Benzene Oil vs. Time 5-21

I

xi

TABLES

Tabl e Page

3-1 Physical Properties and Criteria - Phase 1 Screening 3-3

3-2 Physical Properties and Criteria - Phase 2 Screening 3-4

3-

3 Physical Properties and Criteria - Phase 3 Screening 3-5

4-

1 The 21-Test, 3-Phase Screening Program 4-25, 4-26

5-

1 Effect of Swelling on Density 5-4

5-2 Densities of Liquids 5-8

5-3 Properties of Dielectric Oils 5-9

5-4 Void Filling at Room Temperature, Alkyl Benzene 5-16

5-

5 Void Filling at Room Temperature, Polybutene 5-16

6-

1 Solubility of Tapes at Room Temperature 6-2

xiii

SUMMARY

PURPOSE AND SCOPE

For many years oil -permeated paper tape has been used for the insulation of highvoltage underground transmission cables. In the future, increased electrical

energy consumption will create new demands on the performance of insulating materials. Synthetic polymer tapes are promising candidates for use as high-performance

alternative materials. But to be a usable cable insulation, a substitute must

possess certain attributes important to the manufacture, installation, and life in

service of a cable. These needed properties include electrical, chemical and

mechanical factors.

The purpose of this study was to assemble a comprehensive screening procedure - a series of tests to be used to eliminate unusable materials from further extensive

testing, without eliminating potentially useful materials.

METHOD OR PROCESS

There are two important elements of this study:

Selection of the properties to be evaluated in the screening

Assembly of appropriate tests for the selected properties

The selection of properties to be evaluated was based on the following assumptions:

1. The most demanding physical properties for this application will

receive the greatest emphasis. These are porosity properties and

mechanical and dielectric properties.

2. The screening tests should simulate the conditions found in the

operating cable.

3. As a substitute for paper, the polymer should perform as well as paper for the same operating conditions.

4. Properties for which acceptance criteria could not be established

are excluded from the measurement program.

5. Properties judged of secondary importance, such as thermal conductivity, should not be measured in the screening program.

6. With the exception of the Pirelli criteria, parameters related to cable construction are not considered in the screening program

or tests.

S-l
7. The overall physical properties of a candidate tape material must

be considered to determine its suitability as cable insulation.

8. Testing of model cables are not included in this program.

9. Economic factors are not considered.

Consideration of the assumptions led to the rejection of tests for some properties,

and the final selection of 21 tests, to be applied in three phases.

Of the 21 tests selected, all but two were available as published tests or descriptions of work done. It remained to develop tests for the degree of void filling

by an oil impregnant, and for solubility in the oils commonly used as impregnants.

The experimental work developing the degree-of-void-fill ing test was a considerable

part of this project.

For each test chosen, an appropriate level of performance had to be established,

on which decisions of acceptance or rejection for further testing could be based.

Setting these levels was necessarily somewhat arbitrary, but were based on the

general premise that performance should equal or better that of the oi 1 -impregnated

cellulosic paper in current use. This follows from the fact that the ultimate

goal, of which this study is one component, is the development of a substitute

for oil -impregnated paper that will out perform the paper.

The performance criteria are set forth in Tables 3-1 to 3-3.

THE RESULT

The screening program is presented in matrix form (Table 4-1) in the report.

The rows are the 21 tests selected; the columns are the eight elements that

comprise a test.

The 21 Tests

Phase 1 Tests.

1. Dielectric Constant vs. Temp.

o to 120°C

2. Loss Tangent vs. temp, to 120°C

3. AC Breakdown, 23°C and 120°C

4. Stress-Strain at 23°C

5. Coefficient of static friction - dry.

6. Solubility at 23, 90, and 120 C

7. Softening Temp.

8. Melting Temp.

9. Degree of Void Filling

10. Dimensional Uniformity

1 1 . Shear Modul us

12. Compression modulus on stack, dry, 23 C.

S-2
Phase 2 Tests.

13. Impulse breakdown in oil at 23 and 120°C

14. AC breakdown on stack of tapes in oil, 23 and 120°C

15. Stress-Strain in oil at 23, 90, 120°C

16. Compression modulus on stack, in oil 23°C

17. Swelling force, stack in oil, 23°C

18. Wet coefficient of friction

Phase 3 Tests.

19. Stress-strain 23°C, sample aged at 120°C, in vacuum-treated oil,

under mechanical and electrical stress.

20. Impulse and AC breakdown 23°C, specimen aged at 120°C, in vacuum- treated oil, under mechanical and dielectrical stress.

21. Creep and recovery 23°C , 90°C.

The Eight Elements of Each Test

1. Specimen Selection, Number and Dimensions

2. Conditioning

3. Apparatus

4. Procedure

5. Precautions

6. Calculation or Interpretation of Results

7. Report

8. Precision and Accuracy

Notes to Table 4-1

The limited space available in the blocks of Table 4-1 were insufficient to hold

all the information needed in many cases. For these, the block carries a reference

to a corresponding note. There are 22 pages of notes.

CONCLUSIONS

It is suggested that this screening program will be used in the following manner:

• A suitable quantity of each candidate material will be made available to the testing laboratory employed to perform the tests.

• Phase 2 tests will be made only on those candidate materials that

successfully meet all Phase 1 criteria.

• Phase 3 tests will be made only on those candidates that meet

Phase 2 criteria.

• Within each phase, agreements will be reached as to the order in

which the tests are to be performed.

• Agreements will be made concerning when to discontinue the testing

due to clear failure of a material to meet the criteria for the tests.

S-3

Section 1

INTRODUCTION

For many years paper tape permeated with dielectric oil has been used for the

insulation of high voltage underground transmission cables. Significant changes

and improvements in the paper-oil system have been made over the years so that

its reliability and durability in the present operating environment is well

recognized. For the future, however, increasing electrical energy consumption

will create new demands on the performance of insulation materials. It is

projected that transmission voltages of 1000 kV or higher will be needed. The

reliable performance of paper-oil cable at such high voltages is in question

because of high-voltage-induced ionization in the cellulose and because forced

cooling would be necessary. New improved insulation materials to meet these new demands are being sought, and synthetic polymer tapes are promising candidates.

A considerable amount of development work on polymer tapes has been carried out over the past fifteen years. Initially solid polymer films were studied for

this application, but recently, in order to facilitate the flow of oil between

tape layers, porous tapes, synthetic papers and paper-polymer film laminates

have been developed (1 -11

)

. In these programs various approaches were taken.

In all cases except (7_), the early development included experiments with model

cables. The various screening programs were not well defined, nor did they

uniformly include the same physical property criteria and tests. Properties

which were of common concern are swelling, solubility, modulus, tensile strength,

dielectric properties, and dielectric strength. Other properties which were

focused upon in some programs are: coefficient of friction, dimensional uniformity

(3j, softening temperature (3_) , stress-strain characteristics (1_) , creep and

recovery ( j_) , wetting (10), stiffness (1), and partial discharge (corona) inception

level (2).

Most notable is the work done at Illinois Institute of Technology Research

Institute (IITRI), to develop polymer tapes from 1962 to 1967 (1). In the IITRI

program, the tests which were employed reflected the demands of the environment

in the cable, i.e. tests were carried out over a wide temperature range in oil.

Mechanical and dielectric tests were emphasized and other properties were given

1-1
secondary emphasis. For example, solubility and swelling were not measured directly

but their effect on mechanical properties was observed. The materials that showed

the most promise as polymer tapes were solid films of polycarbonate, polyphenylene

oxide and polysulfone.

Phelps Dodge Cable and Wire Co., and General Cable Corporation also have had extensive programs to find a polymer substitute for paper tape ( 2,3 ). In both cases,

their screening programs eliminated all synthetic polymer tapes in favor of PPP,

a paper-polypropylene laminate. Some of the polymer tapes were eliminated by

economic factors, which are not be be considered here. The screening programs were not definitive. Among the properties which were mentioned or described were

solubility, swelling and dimensional stability. Swelling larger than 2% was con- sidered to be excessive because oil flow in a model cable was significantly

hindered ( 2,6 ).

The development of two synthetic paper tapes, one of polyethylene (5} and the

other of polypropyl ene ( ]) , note special attention. The polyethylene tape is made

from a highly crystalline fiber and was produced by the Furukawa Electric Co.,

Japan. The polypropylene tape was made by Matsushita Industrial Equipment Co.,

Japan. Both of these materials were made specifically for use as cable insulation.

Samples of these materials were obtained by our laboratory and used in our experimental program.

Recent work has been on porous polymer tapes because it is recognized that tape

porosity provides adequate oil flow in the wrapped cable configurations (j5, 7_, 12).

In this program one of our objectives is to quantitatively measure the degree of

void filling by a dielectric oil and to measure the effect of void filling on other

properties of an oil -immersed tape. Specifically, we have developed a degree of

void filling test by which the amount of void volume unfilled by oil is measured.

Failure to use a screening program to quantitatively evaluate mechanical and electrical properties as well as porosity generated insoluble problems during one study (J_2 ) . The objective of this project has been to develop a test program which

can be used to examine porous polymer tapes and to assess their potential use in a porous polymer tape-oil insulation system. The screening program which we have

developed is designed specifically for porous polymer tapes but it can also be

used for solid polymer films.

1-2
The actual selection of materials is beyond the scope of this study; selection

is to result from the application of this series of tests.

In the course of putting together the test program it has been necessary to develop

two tests which were not adequately described in the existing literature. These

are the degree-of-void-fil 1 ing test and the solubility test. To develop these

tests, laboratory experiments were carried out on several porous polymer films

immersed in a dielectric oil. A buoyant force measurement, described in Section

5, was used to calculate the volume unfilled by the dielectric oil.

1-3

Section 2

SELECTION OF PHYSICAL PROPERTIES

The tests and physical properties which we included in this program are those which

we determined to be essential for assessing the performance of a porous polymer

tape. These tests will obviously include dielectric and mechanical measurements.

Many other physical measurements have been used to test polymer and paper tapes

and it is not so obvious which of these should be included. The selection process

which we used involved making a list of reasonable assumptions and setting intermediate goals for the screening program.

ASSUMPTIONS ON WHICH SELECTIONS OF TESTS ARE BASED

The following assumptions are made:

1. The most demanding physical properties for this application will

receive the greatest emphasis. These are porosity properties and

mechanical and dielectric properties.

2. The screening tests should simulate the environmental conditions

found in the operating cable.

3. As a substitute for paper, the polymer should perform as well as paper for the same operating conditions.

4. Properties for which acceptance criteria could not be established

are excluded from the measurement program.

5. Properties judged of secondary importance, such as thermal conductivity, should not be measured in the screening program.

6. With the exception of the Pirelli criteria, parameters related

to cable construction are not considered in the screening program

or tests.

7. The overall physical properties of a candidate tape material must

be considered to determine its suitability as cable insulation.

8. Testing of model cables are not included in the program.

9. Economic factors are not considered.

2-1
CONSIDERATION OF ASSUMPTIONS

Consideration of assumptions 4, 5, and 6 led to the elimination of tests for the

following physical properties:

• Air permeability

• Wetting angle

t Oil flow

• Thermal conductivity

• Stiffness

• Tearing strength

• Folding endurance

• Partial discharge (corona)

• Swelling

While some of these properties are of importance to the cable designer, they are,

in our judgement, of secondary importance to the screening program.

TESTS ELIMINATED FROM THE PROGRAM

Air Permeability

This test was eliminated because no acceptance criterion could be established.

In the case of paper there is a relationship between this property and cable performance based on many years of experience. Without such experience with porous

polymer tapes, it is not possible to assign a value of air permeability which

indicates acceptance of a tape as cable insulation. Without the criterion of an acceptable value, there is no purpose in performing the test.

Wetting Angle

This is a physical property which we consider to be of secondary importance and

therefore it is not included in the program. Certainly a zero wetting angle for

an oil on a polymer tape will facilitate void filling and oil flow in the cable

configuration. But, in this program we make a more direct measure of oil permeation into the porous tape using the degree-of-void-fil 1 ing test. Thus* a wetting

angle measurement becomes unnecessary.

Oil Flow Through the Porous Polymer Tape

This is not a part of the screening program for the following reasons:

1. There is no information or data available by which oil flow through

a single porous tape can be related to oil flow in the cable.

2-2
2. Swelling of a single free-standing porous tape is different from

swelling of the same tape in an enclosed cable, so that the effect

of pore size is different for each environment. Also, the swellingpore size phenomenon would differ for different polymer materials.

3. While oil flow has been of concern (2_,6J , there are no data available regarding the relationship between oil flow and performance

of porous-polymer- tape cables.

Thermal Conductivity

The I 2 R heat losses in the center conductor must be conducted thru the tape-oil

insulation to the ambient surroundings. It is the composite of tape and oil which

determines the thermal conductivity and it is the thickness of the insulation which

determines the heat flux for a given thermal conductivity. Diffusion as well as convection of oil are the mechanisms by which heat is conducted thru the tape-oil

system. The thermal conductivity of the tape itself is not the single dominant

factor. In our judgement thermal conductivity is of secondary importance.

Stiffness, Tearing Strength, and Folding Endurance

These are tests which have been developed over the years by paper-tape cable

manufacturers. Their chief purpose is as a quality check on incoming paper material.

Long experience with paper has established a correlation between these tests and

cable performance. Folding endurance, for example, is used to detect the degradation of cellulose in paper due to pyrolysis (8). For polymer tapes, correlation

between folding tests and cable performance has not been established.

Partial Discharge Inception Level

After a review of the use of partial discharge (corona) inception level measurements, we have recognized its limitations for detecting voids. In the cable industry partial discharge testing is done on finished cables to detect large flaws

which may have been introduced during manufacture. The state-of-the-art partial

discharge measurement is not developed enough so that it can be used as a routine

test to detect microvoids in insulating tapes.

Swel 1 ing

Swelling of a single tape in the free state has been eliminated from the test

schedule because an acceptance criterion cannot be established. The problem which

a swollen tape causes is not going to be ignored, however. In the cable configuration a swollen tape hinders oil flow and also causes an increase in radial stress.

2-3
If swelling is excessive then it is possible that radial stress can reach a critical level and contribute to fracture ( 4-,6_) . When the tape is physically confined

in space, the swelling is confined to filling spaces between layers and butt gaps

and there is a resultant increase in radial stress (6j. In the Pirelli analysis

(13), the bending force is proportional to the radial stress. Thus, if the radial

stress becomes too high, then the critical buckling stress for the tape can be

exceeded. On the positive side, a sample which swells will allow oil to penetrate

into microvoids which might be less accessible with a non-swelling material. Thus, some swelling can be accommodated, but it is desirable that it be held to a minimum.

A test to measure the increase in radial pressure due to swelling has been described

in the literature (6,14)

.

CHOICE OF TESTS TO BE INCLUDED IN THE PROGRAM

The Goals

The intermediate goals which we established for the screening program indicated

the physical properties and tests which were finally included. These goals are:

1. A test will be devised to measure oil penetration or the degree

of void filling in a porous tape.

2. Consideration will be given to the specific behavioral characteristics of polymer materials, e.g. creep and receovery and softening

temperature.

3. Dielectric and mechanical measurements are the essential core of

the test program.

4. Mechanical measurements on the dry tape must yield enough information to calculate the cable bending stress and critical bending

stress using the Pirelli equations.

5. Likewise, mechanical measurements on the wet tape must yield enough

information to use the Pirelli equations relating to the bending

of an oi 1-permeated cable.

6. Mechanical measurements at elevated temperatures are needed for

assessing the environmental integrity of tape material.

The Choices

The tests, physical properties and acceptance criteria of the screening programs

are shown in Tables 3-1, 3-2 and 3-3. The list contains the essential dielectricand mechanical -property tests, as well as tests which we judge to be necessary

for porous polymer materials, i.e. the softening temperature, the melting temperature, the degree of void filling, solubility and the creep and recovery tests.

2-4
The Three Phases

Note that the screening program is divided into three phases. Each phase has a specific goal:

• Phase 1 is designed to eliminate the most undesirable materials

with simple and inexpensive tests. Also, in Phase 1, all of the

necessary information needed for utilizing the Pirelli equations

for the dry cable is obtained.

• In Phase 2 dielectric and mechanical properties of tapes in oil at

elevated temperature are measured. The data can be used to analyze

wet cable characteristics using the Pirelli equations.

• In Phase 3, longer term effects of temperature, oil, and dielectric

and mechanical stresses will be obtained. Here, specimens are kept

in oil under stress for periods up to several months, after which

their mechanical and dielectric properties are measured at room temperature. The aging time chosen is arbitrary, but felt to be

reasonable for a screening program.

2-5

Section 3

ACCEPTANCE CRITERIA

The criteria of Tables 3-1, 3-2 and 3-3 were determined by reviewing other development programs, by applying the Pirelli equations, by using assumption 3 (See page

2-1), and by applying absolute conditions such as in tests 9, 19, and 20. These

absolute conditions are conservative criteria, and could possibly be relaxed when

more information is available regarding cable performance.

It should be understood that some flexibility with regard to the application o f

these criteria may be appropriate. A material for which a few property values are borderline can sometimes be accomodated by engineering and cable design parameters.

Also, a trade-off between a desirable material property and a marginal property can

be considered in making the judgment about a polymer tape.

orv (1_3) . This theory has been experimentally verified and used in practice for papertape cable manufacturing. The major material difference between paper and polymer

tapes is that a stress relaxation will occur in a polymer tape which does not occur

in paper tapes. In spite of this difference, experience has shown that the Pirelli

equations yield an adequate description for the mechanics of bending a polymer tape

cable. These equations are the basis for the acceptance criteria of the mechanical

properties tests.

According to the Pirelli theory, the force on a tape from bending a cable is

THE PIRELLI EQUATIONS

The mechanics of bending for a wrapped, cable have been described by the Pirelli the-

(3-1)

where $, the radial pressure, is given by

a

-1 (3-2)

c

3-1
y is the coefficient of friction, tape-to-tape.

w is the width of the tape.

T is the taping tension,

s is the tape thickness.

a is the taping angle.

E. is the tensile modulus in the machine or long direction of the tape.

E is the compressive modulus of a stack of tapes in a direction

perpendicular to the surface.

We note that N, , has the dimensions of force per unit length. The force is dibend

rected perpendicular to the edge of a thin tape. If this force becomes too large

it will cause the tape to buckle. The critical force per unit length for buckling

is given by

"crit =_

tff ( 4 02 < 3

- 3 >

R is the cable radius for the tape being considered.

G is the shear modulus which results from an angular displacement

whose direction is perpendicular to the tape.

E

fc is the tensile modulus in the cross machine direction i.e. perpendicular to the long direction of the tape and perpendicular to E c

.

APPLICATION OF THE PIP.ELLI EQUATIONS

A bending force is applied to a tape when a cable is wound on an uptake reel during

manufacture or when it is unwound from the reel during installation. Given the

material moduli E , E c> E^ and G, and the coefficient of friction, y, the other

parameters of Eqs . 3-1, 3-2 and 3-3 are adjusted so that N^^ < N^^ for all

radii o* the cable. Or, from another ooint of view, the moduli and coefficient of

friction must have values such that the other parameters of the equations can be

adjusted within practical limits.

3-2
Table 3-1

PHYSICAL PRETTIES VIC CRITERIA - p HA r E 1 SCREENING

Physical Property Pange of Acceptable

Val ues a Justification of Cange

of Acceotable Values

°uroose—"hat

We Can Learn

1) Dielectric Constant

vs. temp, to 120°C

Less than 3.5 Note b El ectrical

Performance

2) Loss Tangent

vs. temp, to 120°C

Less than 0.22 Note b El ectrical

Performance

3) AC Breakdown,

23°C and 120°C

Greater than

75 kV/mm

Note b El ectrical

Perfo rmance

4) Stress-Strain at 23°C

Note c Note c Mechanical

Performance

5) Coefficient of

static friction

- dry.

Note c Note c Taping Characteristics, cable

behavior.

6) Solubility at

23, 90, 120OC.

Less than 12

(See Note d)

References (2,3,5) Film integrity

7) Softening Temp. Greater than 130°C Note e Film integrity

8) Melting Temp. Greater than 1 30°C Note e Film integrity

9) Degree of Void

Fill ing Note f see text Elimination o^

voi ds

10) Dimensional

Uni formity

1

±52 Reference (3) Quality of manu- factured tane

1 1 ) Shear Modul us 23°C Note c Note c Cable behavior

12) Compression modulus

on stack, dry, 23°C

Note c Note c Cable behavior

guide.

Notes follow Table 3-3

3-3
Table 3-2

PHYSICAL PROPERTIES AMP CRITERIA • 5 HASE 2 SCREENING

Physical Property Range of

Acceptable Values 3 Justi ficationof Acceptable

of Range

Val ues Purpose-What

V'e Can Learn

13) Impulse breakdown

in oil at 23 and

1 20°C

.

Greater than

150 kV/mm

Note b Effect o* temperature on breakdown

14) AC breakdown on stack of tapes i

n

oil 23 and 120°C.

Greater than

50 kV/mm

Note b Series stacking of

faults in porous

tapes

.

15) Stress-Strain

in oil at 23,

90, 1 20°C.

Note c Note c Oil compatibili tv- effect on mechanical

properties

.

16) Compression modulus

on stack, in oil 23°C.

Note c Note c Cable behavior guide

effect of swelling.

17) Swelling force, stack

in oil, 23°C. Note c Note c Effect of swelling

or radial pressure.

18) Wet coefficient of

fricti on

.

Note c Note c Cable behavior guide

effect of swelling.

Notes follow Table 3-3

3-4
Table 3-3

PHYSICAL PROPERTIES AND CRITERIA - PHASE 3 SCREENING

Physical Property Range of

Acceptable

Valuesa

Justi ficati on of Range of

Acceptable Values

Purpose- uhat

We can Learn

19) Stress-Strain

23°C, specimen aged

at 120°C, in vacuum- treated oil under

mechanical and

electrical stress.

No significant

change from

Phase 2 data,

Table 3-2

Note g Loss of

mechani cal properti es

.

20) Impulse and AC breakdown, 23°C

specimen aged at 120°C, in vacuum- treated oil, under

mechanical and dielectric stress.

No significant

change from

Phase 1 and 2 data, Tables

3-1, 3-2.

Note g Loss of

Hi electri

c

properties

.

21) Creep recovery

23, 90°C.

e 3 /ei < 0.2 See note h Reference (1_) Viscous flow properties of

materi al

.

borderline values of these properties may be accommodated by cable design para- meters .

^These values have been measured for paper. As a substitute for paper, the value

for the polymer tape should equal or better the value cited.

r The Pirelli equations for polymer tapes describe the mechanics of the tape in the cable configuration. The equations relate the bending stress and the critical buckling stress to the coefficient of static friction, the modulus of elasticity (fin and CMD), compression modulus of a stack, taping angle, taping tension and tape

dimensions. The physical quantities of an acceptable material, when substituted

in the Pirelli equations, will yield a bending stress which ^oes not cause buckling

or fracture. For this screening program, the acceptable values of modulus, and

coefficient of static friction can be obtained by applyinq the Pirelli equation.

Elongation at fracture should be greater than 3%.

^Design and engineering parameters could be utilized to accommodate higher values

of solubil ity.

e The 130°C minimum softening or melting temperature was chosen in order to accommodate high crystalline polyethylene tapes for which the melting temperature is in the range 1 30- 1 35°C

.

boid filling should be complete, i.e. no unfilled void volume. A material is deemed

to meet the criteria for this test i f either a) all voids are filled, b) it meets

the criterion for Tests 3 and 20.

9Any significant change would indicate that a degradation phenomenon is occurring.

Subsequent investigation of the material would involve aging experiments beyond the scope of this screening program. bere ei is the elastic strain of the tape due to an applied stress and e 3 is the unrecoverable strain after the stress is released.

3-5

Section 4 THE TEST PROGRAM

The three phase test program is presented in matrix form in Table 4-1. The tests

are numbered 1 through 21, and are the row headings. The columns of the matrix

are numbered 1 through 8 and are the elements considered to be necessary in a test

description: Specimen Selection, Number and Dimensions; Conditioning; Apparatus;

Procedure; Precautions; Calculation or Interpretation of Results; Report; Precision

and Accuracy.

The broader subject of sampling plan design is not addressed in this study. It

is assumed that the sample employed for this study is representati ve of the material.

It is also assumed that the amount of material is enough to carry out the tests of

this screening program.

The elements of the matrix are designated in the usual manner. For example, (13*2)

refers to the conditioning of the specimen for the impulse breakdown in oil at

23 and 120°C test. Note that we are using the asterisk (*) to separate the row from the column numbers, to avoid confusion with page numbers.

Whenever possible, we have relied on the ASTM test descriptions. Other test schedules were consulted but the ASTM tests were superior in every category which we

considered. Some tests were not available from ASTM or other test publications,

in which case we relied on published descriptions of experiments. These non-ASTM

experiments were not presented in the thorough, detailed ASTM format, so that the

task remained for us to refine the protocol for these tests.

For precision of reference, the ASTM tests cited include a year designation. Those

cited are the ones that appear in the ASTM Book of Standards for 1978, the latest

available at the time of this study (15).

Those blocks of the matrix which are not sufficiently described by the notation

therein, are set forth in more detail below in the notes to Table 4, with the

exception of the degree of void filling test and the solubility test which are described in Sections 5 and 6 of this report.

4-1
For convenience of handling, the notes to Table 4-1 are placed ahead of the Table,

beginning on page 4-3.

Table 4-1 appears on pages 4-25 and 4-26.

USING THE TEST PROGRAM

The grouping of the tests into the three phases is an essential element of the

program. The tests of Phase 2 should not be executed until the material has suc- cessful ly passed the acceptance criteria for all the tests in Phase 1. Likewise,

the tests of Phase 3 are not carried out until the successful completion of Phase

2 tests. The reason for this is to avoid unnecessary and expensive testing on materials which are not acceptable.

To use Tables 3-1, 3-2 and 3-3 for the purpose of carrying out a test program

some practical judgments are required. With each phase, a decision must be made

concerning the sequence in which the tests are executed. The order in which they

are listed here is somewhat arbitrary. It is neither the sequence of execution

nor the order of importance. For example, in Phase 1, test 3 can not be carried

out until the impregnation procedure is developed in test 9. The impregnation

procedure is also a prerequisite to all of the tests of Phases 2 and 3.

Application of the acceptance criteria of Tables 3-1 thru 3-3, requires judgments

based on experience and knowledge of cable performance and construction. A material

should be labelled unacceptable only after all efforts have been made to accommodate

test failures. As mentioned above, this accommodation can be a trade-off between a good physical property and another which is below the acceptance criterion. Also,

cable engineering design parameters can be used to accommodate borderline physical

properties.

4-2
NOTES TO TABLE 4-1

Test 1. Dielectric Constant

1*3 Apparatus. 1) Means of controlling and measuring temperature.

2)

Capacitance and dissipation factor bridge, or other

suitable circuit

3) Generator or other source of sine-wave excitation,

operating at 60 Hz.

4) Detector having sufficient sensitivity to provide

resolution in dissipation factor of 2 x 10" 5 (0.00002)

or 1 percent of the reading, whichever is higher.

5) Electrodes

Helpful information on apparatus appears in ASTM D 150-74 and in ASTM D202-77,

Sec. 144.

1*4 Procedure. Values of the dielectric constant (permittivity) and loss tangent

(dissipation factor) are to be obtained for the following temperatures: 23,50,70,

90 and 120°C. This can be done by making measurements at these temperatures , or

by interpolating from a curve derived from measurements made at other temperatures

covering this range. The separation of actual measurement temperatures shall not

exceed the intervals given above. No extrapolation is permitted.

Test 3. AC Breakdown at 23°C and 120°C in Oil

3*2 Conditioning. Impregnation is required. The exact procedure for impregnation will be affected by the results of the test for degree of void filling, test 9.

3*3 Apparatus . At first glance, it seems that using one-inch electrodes, instead

of two-inch, would save a lot of material needed for tesing. Not so, for in any

case, normal practice is to use a 15 x 15 cm. square specimen. The extra material

is needed to prevent flashover.

Even when 1/4" electrodes are used, flashover problems dictate as large a specimen

as can be spared.

3*5 Precautions . Keep oil clean. Do not allow carbon particles to increase

conductivity of oil. Replace or filter oil if carbon builds up from repeated

heavy breakdowns. Note whether breakdowns are merely surface flashovers or punctures through the material.

4-3
Breakdowns often cause asperities to develop on one or more of the electrodes.

If this occurs, the asperities must be removed before further use. Polishing or lapping is the usual method for doing this. If the electrodes were properly made,

reapplication of the techniques by which the electrodes were originally polished

is desirable.

3*8 Precision and Accuracy . Precision of breakdown tests is generally low. Only

in the case of single-operator tests performed on the same apparatus can the coefficient of variation be expected to be under 10%. This refers to repeated tests

of five specimens each, with all specimens coming from the same sample.

Many factors other than the intrinsic properties of the material affect the outcome of a breakdown test. These include:

--Thickness and uniformity of thickness of the specimen

--foriegn matter included in the material

--dirt on the surface of the specimen

--asperities on the electrodes

--transient surges (spikes) on the applied voltage waveform

--variations in pressure on the electrodes

--variations in the surrounding medium.

4-4
Test 5. Coefficient of Static Friction, Dry. 23°C.

5 * 1 Specimen Selection, Number and Dimensions. The test for the coefficient of

static friction is described in Reference (J_4). Specimen size is approximately 5 cm square. Four specimens for each direction, MD and CMD, should be used.

5*3 Apparatus. Consists of two flat metal plates between which layers of the

material are sandwiched. The sandwich is compressed by adding weight to the top

plate, so the pressure on the stack is in the range 2xl04 N/m2 to 1 2x1 0^ N/m2

. A film, which is sandwiched between two adjacent films, is attached by grips to a weight

on a cord over a pulley. A sketch of this apparatus is shown in Figure 4-1.

0 Q Pulley System

Stop- WN

Q

Specimen

V

Replace with metai strip

for Test #17 Data

Figure 4-1. Apparatus for Tests 5, 17,18

5*4 Procedure. The measurement of the coefficient of friction is obtained by

measuring the force which is necessary to just initiate movement of the film. The

weight hanging on the cord over the pulley is increased by adding lead shot or other small increments of weight. A motion indicator such as a displacement transducer or microswitch may be used.

Test 7. Softening Temperature

7*8 Precision and Accuracy. Precision and accuracy of the softening temperature

by the ASTM method has not been determined.

4-5
Test 8. Melting Temperature

8*8 Precision and Accuracy. Precision of the melting temperature by the ASTM

method is ±2°CT The accuracy of this test has not been determined.

Test 9. Degree of Void Filling

9*4 Procedure.

1. The calibration fluid shall be chosen according to the following criteria: it must efficiently permeate the polymer under test; it must not

dissolve the polymer at room temperature; and it must be a volatile

liquid which mixes with the dielectric oil.

2. Before undertaking the buoyant force experiments, the solubility of the

tape in the calibration fluid shall be measured. To do this, a specimen, weighing approximately 50 mg, shall be weighed to the nearest 0.01

mg using the electrobalance. Then immerse the specimen in approximately 50 cm 3 of the fluid. Retain the vessel at room temperature and agitate twice daily over a period of several days. Upon removal from the

fluid, the specimen is alternately weighed and vacuum dried until the

specimen weight reaches a constant value. If the final weight of the

specimen is within ± 0.01 mg of the original weight, then, within the

limits of accuracy of the measurement, the specimen is insoluble in the

calibration fluid. This is the ideal case.

In the less-than-ideal case, some weight loss occurs. If the loss is

less than 0.1%, the fluid is considered to be marginally acceptable,

and one or the other of two courses shall be followed:

a) short term solubility tests (for periods up to eight hours) can be

carried out with new specimens, so that solubility vs. time data can be

obtained, or b) the original specimen can be retested in order to determine if the weight loss is due to an initial cleaning which does not

repeat upon further contact with the fluid. If initial cleaning was

the reason for the weight loss, then all subsequent specimens shall be

given an initial cleaning before using them in the buoyant force experiment.

3. Four balance readings are necessary:

A= Weight of wire loop and sinker in air,

B= weight of wire loop and sinker plus specimen in air,

C= weight of wire loop and sinker in fluid,

D= weight of wire loop and sinker plus specimen in fluid.

For convenience, the above measurements are carried out in the order

A, C, B, D. After the measurement of C, the wire loop and sinker are thoroughly cleansed either by allowing the fluid to evaporate or by

rinsing with a volatile solvent.

Prior to the measurement of D, the specimen is slowly immersed into the

fluid, care being taken to avoid dislodging the wire loop from the bal- ance arm or changing the position of the glass rod. Sudden mechanical

jolts to the balance system can disturb the measuring arm and result

in a need for a lengthy recalibration procedure.

4-6
Immediately after the specimen has been immersed in the fluid and

after the vessel is set in place, the bucking voltage is adjusted to cancel out the signal from the electrobalance. A zero reading on the

most sensitive scale of the recorder determines cancellation. As the

fluid permeates into the specimen the change in buoyant force is re- corded with time. If this change is larger than the full scale displacement of the recorder, then the bucking voltage is adjusted ac- cordingly. When the buoyant force has reached a constant value which

persists for a period of at least 8 hours, the DVM reading is recorded

as the value of D.

4. The same specimen that has been immersed in the calibration fluid shall

be dried and used again with the dielectric oil. The procedure as described above for obtaining readings of A, B, and C is followed.

For the measurement of D special attention must be given to the oil

impregnation process and to the solubility of the specimen in oil.

Oil impregnation is monitored using the chart recorder. When a steadystate or near steady-state value of the buoyant force is reached, a preliminary calculation of the percentage of unfilled void volume shall

be carried out using the equation given in 9*6.

If there is a significant amount of unfilled void volume, then vacuum,

temperature and pressure (if it is available) cycling shall be used

in order to achieve the maximum degree of void filling. The cycling

procedures which achieve the optimum are recorded for use in subsequent

impregnation. After all the impregnation procedures are complete, the

steady-state value of the DVM reading is recorded as the value of D.

5. Before a final calculation of the unfilled void volume is made, the

solubility of the specimen in oil must be determined. To do this the

calibration fluid is used as a washing medium after the specimen is

removed from the oil. Since the calibration fluid mixes with the

dielectric oil, it is possible to remove all of the oil from the pores

by repeated washing and drying. Drying can be carried out either by

evaporation at ambient room conditions or under vacuum. The weight

of the specimen is measured after each wash-dry cycle until a constant

weight is reached. If there is no difference between the original and

final weight of the specimen the values of (B-A) and (D-C) can be used

to calculate the unfilled void volume without any adjustment. However,

if there is a specimen weight loss and it is small, less than 1%, then

the value of (B-A) is lowered by the amount of weight loss before

making the calculations in 9*6. Also, for solubility less than 1%,

it can be assumed that the density of oil remains unchanged. If the

solubility is larger than 1%, then the specimen may be judged unsuitable for continued testing, and, if a calculation of the unfilled void

volume is desired it may be necessary to remeasure the density of the

oi 1

.

9*6 Calculations or Interpretations o f Results.

1. The density, specific volume, specific unfilled void volume, and the

percentage of unfilled void volume are calculated from the two sets

of values of A, B, C and D.

4-7
2 . From the calibration fluid data, the specific volume and density of

the porous tape specimen are obtained from the following:

where

v s is the specific volume of the specimen,

df is the density of the calibration fluid at the temperature of the

measurement.

The density of the specimen, p<-, is

p

s - l/vs

3. From the measurements on the oil -immersed specimen, the specific vol- ume of unfilled voids is calculated from the following:

where

v

v is the specific volume of unfilled voids.

d 0 is the density of the oil at the temperature of the measurement.

v s is the specific volume of the specimen.

The apparent density of the specimen in oil, p 0 , is calculated from

B-A

(B-A) - (D-C) pM = d o o

4.

The percentage of unfilled void volume, % v v , is

%v

v =

V + V

. v s

x 100%

9*7 Report .

The report shall contain the following information:

1. Name and location of laboratory,

2. Name of person responsible for the test,

3. Name of person performing the test,

4. Date of experiment,

5. Name and origin of material,

6. Specimen identification,

7. Name of calibration fluid,

8. Name of dielectric oil,

9. Temperature of experiment,

10. Density of calibration fluid at measurement temperature,

11. Density of dielectric oil at measurement temperature,

12. Values of A, B, C, and D for calibration fluid,

13. Values of A, B, C, and D for dielectric oil,

14. Strip chart recording of AW versus time,

15. Impregnation procedure to be used for conditioning specimens for

tests requiring impregnation.

16. Calculation of p,. v v , . r s, s , ro, v and percent unfilled void volume.

4-8
9*8 Precision and Accuracy . In order to establish the precision and accuracy

of this test, errors due to the instrument, temperature variation, different calibration fluids and different dielectric oils need to be considered. Such an ex- tensive analysis requires many measurements and could not be carried out here.

Thus, the overall precision and accuracy of this measurement is not known. The

maximum uncertainty in the measurement of specific volume due to weighing errors alone was calculated to be ±0.0004 g/cm 3

.

Test 11. Shear modulus. Dry, 23°C

11 * 1 Specimen Selection, Number, and Dimensions . The shear modulus measurement

requires a sheet of material approximately 10 cm x 20 cm. This sheet is rolled

into a tube 10 cm long and approximately 2 cm in diameter.

11*3 Apparatus . The apparatus for the measurement of shear modulus will be

the same as that originally described by workers at Pirelli. This apparatus is

described in Appendix B of Reference ( 1_4) . For this test a sheet of material is

rolled into a tubular shape and the starting edge and finishing edge are glued

into place. One end of the tube is held fixed while the other is rotated. This

shear has an angular moment in the direction normal to the plane of the tape surface.

11 * 7 Report . The report should contain the following:

1. Name and location of laboratory where tests were performed

2. Name(s) of persons performing the test

3. Complete identification of material tested

4. Dimensions of test specimen

5. Temperature of test

6. Resultant calculation of shear modulus

11*8 Precision and Accuracy . The precision and accuracy of this test has not

been determined.

Test 12. Compression Modulus, Stack, Dry, 23°C

12 * 1 Specimen Selection, Number and Dimensions, Specimens are stacks of diecut disks, 2.54 cm (1 inch) in diameter. Other diameter specimens can be used

depending on the needs of the testing machine, but the diameter shall not be less

than 1.5 cm. A sufficient number of disks are cut and stacked to make a stack

1 cm high. Three separate specimen stacks shall be measured. The three stacks

may be separate sample material or the disks of one stack may be rearranged and

designated as another specimen stack.

4-9
12*3 Apparatus.

1. Testing machine - The testing machine shall be one of the following

types:

a) A machine which is capable of applying a load increasing at a constant rate either in tension or compression;

b) A machine which operates at a constant rate of crosshead move- ment either in tension or compression.

2. Pressure posts - For the case of a testing machine operating in

compression, the machine shall be fitted with two opposing cylindrical posts with plane and parallel faces. The diameter of these

cylinders shall be the same and shall be equal to the specimen

diameter.

3. Interlocking c-grips - For the case of a testing machine operating

in tesnion, the machine shall be fitted with two interlocking c- grips as shown in Figure 4-2. The opposing surfaces of the c-grips

shall be plane and parallel and shall have a rectangular area larger

than the area of the specimen.

4. Position Transducer - The pressure posts or the interlocking c-grip

shall be fitted with a transducer capable of indicating the relative position of the pair of posts or c-grips to a resolution of

2 ym over a range sufficient to cover the motion resulting from

the application of pressure. A linear variable differential transformer (LVDT) is shown in Figure 4-2.

5. Load indicator - The load-indicating mechanism shall be capable of

showing the total load carried by the test specimen. This mechanism

shall indicate the load with an accuracy of 1% of the indicated

value. The accuracy of the testing machine shall be verified in

accordance with ASTfl E4.

6. Recording device - A recording device with enough sensitivity to record the limit of resulution of both the load indicator and the

position transducer shall be employed. This device can be a chart

recorder or a data processing unit which converts the data to

digital form.

7. Gauge blocks - The position transducer shall be calibrated in

increments of 0.25 mm over the range of the specimen compression.

Gauge blocks shall have thickness accurate to ±1 ym.

12*4 Procedure.

1. Place stack of film disks between pressure posts or interlocking c-grips,

al igning the edges.

2. Bring posts or c-grips in contact with the film stack and apply small

initial pressure.

3. Apply pressure increasing at a constant rate (approximately 3.71 N/sec

(50 lbs/ minute) for those machines so capable, or, apply pressure by

decreasing crosshead separation at a constant rate).

4-10
To Tensile Testing Machine

or Weight-Pulley System

Front View Side View

Figure 4-2. Suggested Grip Modification for Tests 12 and 16

4. Continue to apply pressure and collect output data until a maximum

of 8.27 x 10 5 N/m2 (120 psi) is reached.

5. Release pressure. Data as a function of decreasing pressure is

not used.

6. Remove specimen without di strutting the alignment of the position

transducer.

12*5 Precautions .

1. Care must be exercised to see that the stack does not buckle under

pressure. If buckling occurs, decrease the stack height.

2. The position transducer must remain in alignment so that its calibration remains effective.

4-11
12*6 Calculation or Interpretation of Results .

1. Pressure on the specimen is obtained by dividing the load by the area of the specimen.

2. The thickness of the specimen, t, is equal to the separation between

pressure posts or c-grips and is obtained from the position transducer data.

3. Using the recorded load and transducer data, a plot of pressure

versus specimen thickness is made over the pressure range 0 to 8.27 x 10^ N/m2 (120 psi). On this plot, t is plotted in the

direction of decreasing t.

4. The slope of the pressure versus t curve is obtained at five pressures :

1.38 x 10 5 N/m2 (20 psi)

2.75 x 10 5 N/m2 (40 psi)

4.14 x 10 5 N/m2 (60 psi)

5.52 x 10 5 N/m2 (80 psi)

6.89 x 10 5 N/m 2 (100 psi)

The slope at thickness, t, is calculated either by an automatic data

processing technique or by graphical analysis. It is expressed as:

(Slope)

t At

t

The compression modulus E c at t is given by

E c t(Slope)

t From the three specimens, three compression moduli are calculated at

each of the five pressures. The average and standard deviation of

E c at five pressures are calculated.

12 * 7 Report. The report shall contain :

1. Name and location of laboratory where tests were performed.

2. Name(s) of person(s) performing tests.

3. Date(s) of tests.

4. Complete identification of material tested.

5. Compression modulus calculated per 12 * 6. 12*8 Precision and Accuracy . The precision of the compression modulus measurement by this method should be ±5%. The accuracy of the measurement has not be

determined (16).

4-12
Figure 4-3. Testing machine used by Dr. R. W. Penn at the National Bureau of

Standards to measure the compression modulus of a stack of tapes

(Test Nos. 12 and 16)

.

4-13
Figure 4-4. Stacked specimen in testing machine. The L n'/T is seen mounted on the

posts. This machine is operating in the compression mode, an^ Hoes not require

the c-grips shown in r igure 4-2.

4-14
Test 14. AC Breakdown on Stack of Tapes in Oil, 23 and 120°C.

14*1 Specimen Selection, Number and Dimensions. Specimen selection shall he the

same as for Test 3, except that the number must be increased to provide for stacking

four pieces for each specimen* Ref. (17).

Test 15. Stress-Strain in Oil at 23, 90 and 120°C.

15 * 1 Specimen Selection, Number and Dimensions. The specimen length shall

be 18 cm (7 in.). This is the same as for Test 19. The results of this test

(Test 15) are to be compared later with those of Test 19: thus a standardized

length is desirable.

15*3 Apparatus

.

The apparatus is the same as for Test 4, with the addition of

an oil and thermal jacket. Such chambers are commercially available, or they are easily assembled from standard laboratory apparatus. A sketch of a suggested

design is shown in Figure 4-5.

To Mechanical Testing Machine

or Weight-Pulley System

Thermostat

Fluid

Figure 4-5. Suggested Oil and Thermal Chamber for Tests 15 and 19

4-15
Test 16. Compression Modulus on Stack in Oil, 23°C.

16*3 Apparatus. The apparatus shall be the same as -for Test 12, except modified

to provide for performing the test with the specimen immerse^ in oil. The vessel

of Figure 4-5 can be used eor this test.

16*4 Procedure. The procedure is the same as for Test 12, except that the

specimen is immersed in oil before and during the test.

4-16
Test 17. Swelling, Force, Stack, in Oil, 23 C.

17 * 1 Specimen Selection, Number and Dimensions. Prepare one specimen for each

winding tension being tested. Each specimen consists of a roll of tape prepared

by winding dry tape at constant tension on a 20-mm core until a build of 20-50 mm

is reached. The width of the tape shall be 20-50 mm. The build will depend on the

cable diameter being simulated. At intervals during the winding, metal tabs, des-

.

cribed below, are inserted in such a way as to extend entirely through the roll, and

placed so that they are scattered with respect to a radius of the roll. Tnsert a tab approximately every 5 mm of build. The outer end of the tape is suitably secured

so i t cannot unroll

.

The tabs are pieces of 1 mil stainless steel shim stock, 0.5 inch wide, with holes at one end for attaching a pulling line. It may be necessary to reinforce these holes.

The length of the tabs should be Bnough greater than the width of the tape so that

the tabs will extend completely through the roll and also allow room to attach the,

pulling line.

17 * 2 Conditioning. The impregnation procedure of this test specimen may differ

from that which was developed in test 9, because the specimen is in the wrapped

configuration. It is suggested that a buoyant force measurement be made with the

specimen on the spool. The objective would be to calculate the apparent density in

oil or the specific volume of unfilled voids. To do this, the equation of 9 * 6 is

used where

A is the weight of the empty spool.

B is the weight of the spool plus wound tape.

C is the weight of the empty spool plus wire immersed in oil.

D is the weight the spool plus wire and wound tape immersed in oil.

17*3 Apparatus.

1. Specimen-winding apparatus. - A lathe, or equivalent, shall be used to wind the specimen coils. The lathe shall be capable of running at slow but

steady speed, slow enough to permit insertion of the tabs without stopping.

A supply spool capable of paying off the tape at constant tension shall be used.

Reference (1 4)

.

2. Tab-pulling device. - A support shall be provided capable of supporting

the specimen by the core, with the tabs and pulling holes at the bottom. ® cup

4-17
of adequate size shall be provided, with means of attachment to one of the tabs

at a time. (Weight is then added to the cup until the tab slides out of the coil).

3.

Cup-weighing device. - A scale shall be provided capable of weighing

the cup and its contents. (Contents are that which was just sufficient to pull

the tab from the coil).

17*4 Procedure.

1. Wind specimens as per 17*1

2. Impregnate specimens as per 17*2

3. Measure and record the distance of each tab from the core (along a radius).

4. Hang the specimen by the core from the support described in 17 * 3.

5. Attach the cup (17 * 3) to one of the tabs.

6. Gradually, add weight to the cup by pouring into it water or lead

shot. Arrange to catch the cup without spilling any of the contents

when the tab slips out of the core. Stop adding weight when the

tab slips out.

7. Weigh and record weight of cup and contents, identifying by distance

from core the tab pulled and weight required to pull it.

8. Repeat steps 3-7 for each remaining tab.

17*5 Precautions.

1. Be sure that pull-out force is not affected by roughness or burrs at

edge of the shim stock used for the tabs. If necessary, hone carefully to remove

burrs

.

2. Make sure the coil winds uniformly as the specimen is prepared. Uneven

tape thickness from one edge to the other could produce a misshapen coil.

17*6 Calculation or Interpretation of the Results.

1. The wet coefficient of friction between the material under test and the

metal tabs used must be known. This is determined in Test 18.

2. The swelling force at the radius of each tab is given by:

F = pwVA where u is the wet coefficient of friction

w

F is the force required to pull the tab

A is the area of the tab in contact with the film coil

4> r is the radial pressure in the film coil at radius r.

4-18
17*7 Report

The report shall include:

1. Complete identification of the material under test and oil used.

2. Name and location of laboratory performing the test.

3. Name of person(s) performing the test.

4. Date (s ) of testing.

5. A table giving the radial distance from the core of each tab, the force

required to pull the tab, the radial pressure calculated for each tab.

6. A plot showing radial pressure as a linear function of distance from

the core. 17*8 Precision and Accuracy

1. The accuracy of this test is not established.

2. The plot of radial pressure vs. distance from core will reveal to some degree how precisely the radial pressure can be determined.

Test 18. Wet Coefficient of Friction, 23°C .

18 * 3 Apparatus. The apparatus which was used to obtain the dry coefficient of

friction (Test 5) is used here also. The apparatus is immersed in oil by filling

the pan shown in Figure 4-1.

18 * 4 Precautions. Care should be taken so that slippage of the oil-impregnated

specimen in the grip does not occur.

Test 19. Stress-Strain at 23°C, After Aging Mnder Mechanical and Electrical Stress

in Vacuum-Treated Oil at 120°C.

19 * 1 Specimen Selection, Number and Dimensions. The specimens shall consist of

18 cm x 18 cm square sheets. For materials which are isotropic, one sheet ^or each

aging time period shall be used. For anisotropic materials, two sheets for each

time period shall be used. Since there are two time periods for aging, 4 weeks and

32 weeks, two sheets of isotropic material and four sheets of anisotropic material

are needed. After a sheet specimen has been aged in oil at an elevated temperature,

the sheet is cut into five 18 cm length specimens so that mechanical testing in

accordance with ASTM DS82-75b can be carried out. For anisotropic materials, ten

tensile specimens, five normal and five parallel to the axis of anisotroy, shall

be used.

4-19
19*3 Apparatus .

1. Specimen Stress Unit - This unit shall hold a specimen under mechanical and electrical stress during aging. A design for this unit is

shown in Figures 4-6 and 4-7. A specimen is held extended hy an extension plate inserted between the grips. The size o-f the stress

unit shall be large enough to accommodate the 18 cm x 18 cm square

sheet. An electric field is placed across the specimen using the

high-voltage electrode which is 10 cm in diameter situated in the

center of the stress unit.

2. High Voltage AC Source - This voltage source shall be capable o-f applying a field of 3 kV/mm across the specimen in the stress unit.

3. Temperature Control Oil Bath - a bath capable of maintaining the

stress unit in a dielectric oil at 120°C for 32 weeks shall be

used.

4. Dielectric Oil - The appropriate dielectric oil shall be vacuum treated and dried before being used in the aging test.

5. Testing machine - A testing machine as described in ASTM D882-75b,

Section 5, shall be used.

6. Other apparatus of ASTM D882-75b, Section 5, shall be used as needed.

Assembled Stress Unit

Figure 4-6. Stress Unit for Tests 19 and 20

4-20
To Grip of Tensile

o o o o o o o

Top Grip

eeS

Bottom Grip

Side View

Top Grip

Side View

Figure 4-7. Grips for Stressing Material for Tests 19 and 20

19*4 Procedure .

1. The specimen is placed in the grips of the stress unit with a distance £ between the grips.

2. The grips are attached to the testing machine and the specimen is

gi ven a strain of 1 %=.

3. The specimen is held at 1% strain in the testing machine while the

extension plate is placed in position as in Figure 4-6. The extension plate has a dimension equal to i + 0.0U, thus holding the

specimen at 1 % strain.

4. The assembled stress unit is placed in oil and the specimen is

permeated with oil in accordance with the procedure developed in

test 9. For anisotropic materials two specimens shall be used.

5. After oil impregnation, the high voltage electrode is placed in

posi ti on

.

6. The oil bath is brought to 120°C and the high voltage is applied.

7. At the end of four weeks, the high voltage is turned off, the

specimen is brought to room temperature and removed from the oil bath.

8. For the tensile tests, the sheet is cut into five specimens having

a gage length of 13-15 cm. The shape of the specimens will be in

accordance with ASTM D 882-75b. For anisotropic materials ten tensile specimens, five in each of the perpendicular directions, are needed.

4-21
9.

The dimensions of the specimen are accurately measured.

10. Mechanical testing is carried out in oil at room temperature following the procedure described in test No. 15. The specimens shall be

stressed in the testing machine to yield or fracture, while load anH

extension data are obtained.

11. A calculation of the tensile modulus and strength is made in accordance with 19 * 6.

12. Considering the results of the calculation of modulus and tensile

strength, a .iudqment is made whether or not to continue with a 32-

week experiment, by applying the following criterion: If there is a significant difference between the physical properties as measured

after four weeks and those measured in Test 15, it is concluded that

material deteri oration has begun and further testing is not warranted.

In order to test for a significant difference between the data of Test 15 and

Test 19, a statistical test, a two-sided t-test at the 90% confidence level, is

used. This test, which is described in (1_8) , consists of using the estimated

standard deviations from the two sets of data and a tabular value of t to compute

a test criterion.

We define

X 15 i s

x| to is

s 15 is

s 19 is

U i s

the average value of the measurements from Test 15,

the average value of the measurements from Test 19,

the estimated standard deviation of the values from Test 15,

the estimated standard deviation of the values from Test 19,

the computed test criterion.

If > U

then, it can be concluded that X 15 and X^g are significantly different,

of U is computed from ( 1 8)

where

U = ts

P

t is the statistical t-distribution

,

The value

m 15 is the number of measurements of the mechanical property from

Test 15,

m 19 is the number of measurements of the mechanical property from

Test 19,

4-22
and

( m i5 ~ 1 ) S 15 + ( m 19 ~ 1 ) S m 15 + m 19 -2

19

When m 15 = 5 and m 19 = 5, t = 1.86 at the 90% confidence level and 'J = 1.18 Sp.

13. For the 32-week test, the procedure in steps 1 thru 10 is followed,

except that the specimens are aged at 120OC in oil for 32 weeks.

14. The results of the 32-week test are compared with the results of

Test 15. A check for significant differences is made.

Test 20. . Impulse and AC Rreakdown at 23°C, After Aging Under Mechanica

1

and

Electrical Stress in Vacuum-Treated Oil at 120OC

20 * 1 Specimen Selection, Number and Dimensions. The specimen size shall he

the same as for Test 19, 18 cm x 18 cm square sheets. The number of specimens

shall be five each for the impulse and ac breakdown tests. Since there are two

time periods for aging, 4 weeks and 32 weeks, a total of twenty sheets are needed.

20 * 3 Apparatus. The apparatus of Tests 13 and 14 will be needed in order to

carry out the impulse and ac breakdown tests after the specimens have been aged

in oil at 120°C. In addition, items 1,2 and 3 of 19 * 3, the stress unit, a high voltage ac source, and temperature-controlled oil bath(s) will be needed.

The dielectric oil shall be vacuum treated and dried before using it as an aging

medium at 120°C.

20 * 4 Procedure.

1. The aging procedure shall be the same as •‘"or 19 * 4, steps 1 thru

7, i.e. the specimen sheets are placed in the stress unit and

aged in oil under electrical stress at 120°C.

2. After the 4-week immersion period is finished, the specimens are used to measure impulse and ac breakdown at room temperature. ror impulse breakdown, the procedure followed is the same as 13 * 4 for measurements at 23°C. For ac breakdown, the procedure followed

i s the same as 14*4.

3. Calculations of impulse and ac breakdown are carried out according

to 13 * 6 and 14 * 6 respectively.

4. The 32-week test is carried out only if there is no significant

difference between the results from the 4-week tests and the results

of Tests 13 and 14. To test for significant difference, the procedure outlined in 19 * 4 regarding the application of the twosided t-test, is followed (18).

4-23
5. If there is no significant difference between the results of the

4-week test and the results of Tests 13 and 14, then the 32-week

test is carried out by repeating steps 1 thru 3 above.

6. The results of the 32-week test are compared with the results of

Tests 13 and 14. A check for significant differences is made.

4-24
TABLE 4-1. THREE-PHASE TESTING PROGRAM

PHASE 1

1 Specimen Selection, Number

and Dimensions

7. Conditioning

3 Apparatus Procedure Precautions

6 Calculation or Interpretation

of Results

7 Report

8 Precision and Accuracy

1. Dielectric Constant ASTM D 2305-72, Sec. 42.1 Covered under procedure ASTM 0202-77, Sec. 143

(except 143.4, 143.8. 143.11)

See 1 * 3

ASTM D 202 - 77 Sec. lhi except 1U6.5 and 14.

See 1 * 4

ASTM D 150-74, Appendix A3 ASTM D 150-74, Table 1, according to electrodes used. ASTM D 202-77, Sec. 147, Omit 147.1.1 and 147.1.4 ASTM D 150-74, Sec. 6.5

2. Loss Tangent ASTM D 2305-72, Sec. 42.1 Covered under procedure ASTM D 202-77 ASTM D 202, Sec. 145 ( except

145.5 6 146.6) ASTM D 150-74, Appendix A3 ASTM D 150-74, Table 1, according to electrodes used. ASTM D 202-77, Sec. 147, omit 147.1.1 and 147.1.4 ASTM D 150-74, Sec. 6.5

3. AC Breakdown at 23° c

and 120° C in oil.

ASTM D 2305-72, Sec. 20; ASTM D 149-75, Secs. 9. 1,9.2 Impregnate See 3*2

ASTM 0 149-75; electrodes

Type 1, Table 1. See 3*3 ASTM D IU9 - 75 ASTM D 2305-72, Secs, 18.2, 20. See 3*5

ASTM D 149-75, (Calculate dielectric strength); See also, D 149-75, Appendix A1

ASTM D 2305-72, Sec. 24 See 3*8

4. Stress-Strain at 23°C, dry ASTM D 182-7 Sb, Secs. 6,9 ASTM D 618-61 (1977) ASTM D 882-75b , Sec. 5. ASTM D 882-75b, Secs. 9, 10 ASTM D 882-75b , Notes 6,8,

12 and 13 ASTM D 882-75b , Sec. 11. ASTM D 882-75b, Sec. 12. ASTM D 882-75b , Secs. 5,6, 10 and 11.

5. Coefficient of Static

Friction, Dry 23°C Ref. (13) See 5*1

ASTM D 618-61 (1977) Ref. (13) See 5*3

Ref. (13) See 5*4 ASTM D 1894-75, Notes 2 and 3 ASTM D 1894-75, Sec. 10 ASTM D 1894-75, Sec. 11 ASTM D 1894-75, Sec. 10

6. Solubility at 23, 90 & 120°C

ASTM D 1239-55(1971), Sec. 5 See Text, Sec. 6

Impregnate

See 3*2 ASTM D 1239-55 (1971), Sec. 3 See Text, Sec. 6

See Text, Sec. 6 ASTM D 1239-55(1971), Note 4 ASTM D 1239-55 (1971), Sec. 8 ASTM D 1239-55 (1971). Sec. 9 See Text, Secs. 5 and 6

1. Softening Temperature ASTM D 1637-61 (1976), Sec. 4. ASTM D 1637-61 (1976), Sec. 5. ASTM D 1637-61 (1976), Sec. 3. ASTM D 1637-61 (1976), Sec. 6 ASTM D 1637-61 (1976), Notes 6, 8 and 11. ASTM D 1637-61 (1976), Sec. 7 ASTM D 1637-61 (1976), Sec. 8 ASTM 0 1637-61 (1976) See 7*8

8. Melting Temperature ASTM D 3418-75 ASTM D 618-61 (1977) ASTM D 3418-75. Sec. 7

ASTM D 3418-75, Sec. 10; Omit preliminary cycle (10.1.2) Omit 10.2. Max. Measuring temp,

is 150° C.

ASTM D 3418-75, Sec. 6. ASTM D 3418-75, Sec. 10. ASTM D 3418-75, Sec. 11. ASTM D 3418-75, Secs. 8 & 12 See 8*8

9 - Degree of Void Filling See Text, Sec. 5, Specimen Preparation ASTM D 618-61 (1977) See Text, Sec. 5, Experimental Apparatus See 9 * 4 Covered under Procedure See 9 * 6 See 9*7 See 9

* 8

10. Dimensional Uniformity ASTM D 374-74 ASTM D 618-61 (1977) ASTM D 374-74 ASTM D 374, Sec. 6 Sec. 6.3 6 6.4 not applicable ASTM D 374-74 Calculate average and estimated

standard deviation ASTM D 374-74, Sec. 7 ASTM D 374-74, Secs. 4,5

11. Shear Modulus, Dry, 23°C

See 11*1 ASTM D 618-61 (1977) Refs. (12), (13) See 11*3

Stress-Strain curve observed by adding weight to displace moving

member. Ref . (13)

Refs. (12,13)

Refs. (12, 13) Obtain modulus from slope of stress-strain curve. See 11*7 See 11*8

13. Compression Modulus, Stack.

Dry, 23 C See 12*1 ASTM D 618-61(1977), and ASTM D 1894-75, Sec.

See 12*3 See 12*4 See 12*5 See 12*6 See 12*7 See 12*8

4-25
TABLE 4-1, CONTINUED

PHASE 2

1 Specimen Selection, Number and Dimensions Conditioning3 Apparatus Procedure

5 Precautions

Calculation or Interpretation

of Results

Report

8 Precision and Accuracy

13. Impulse Breakdown In oil at 23° 4 120°C

ASTM D 3426-75,

Secs. 7, 8 Impregnate

See 3*2

ASTM D 3426-75, Sec. 6; Electrodes: ASTM D-149-75, Table 1, Type 1.

ASTM D 3426-75,

Sec. 9 ASTM D 2305-72, Secs. 18.2, 20 See 3*5

ASTM D 3426-75,

Sec. 10 ASTM 0 3426-75, Sec. 11; add indentification of oil. See 3*8

14. AC Breakdown on otack of

23° 4 120°

See 14*1 Impregnate

See 3.2

ASTM D 149-75, Secs, 5, 8, 10, 12 and 13. Electrodes Type 1, Table 1.

ASTM D 149-75, Secs. 6.7. ASTM 0 2305-72, Secs. 18.2-20. See 3*5

ASTM D 149-75. Calculate dielectric strength.

See aslo D 149, Appendix A.

ASTM 0 2305-72, Sec. 24; add indentification of oil. See 3*8

15. Stress-Strain In Oil at

23, 90, and 120°C ASTM 0 882-75b, Secs. 6,8;

See 15 * 1 Impregnate

See 3*2

ASTM D 882-75b , Sec. 5 modified with oil-temperature

chamber.See 15*3

ASTM D 882-75b , Secs, 9, 10 modified for different temp- erature and oil environment.

ASTM D 882- 75b, Notes 6, A,

12 and 13 ASTM D 882-75b , Sec. 11 ASTM D 882-75b, Sec. 12; add identification of oil. ASTM D 882-75b, Secs. 5, 6. 10 and 11.

16. Compression Modulus on Stock

In Oil 23°C

See 12 * 1 Imp regnate

See 3*2 See 16 * 3 See 16 * 4 See 12 * 5 See 12*6 See 12 * 7 add identification of oil. See 12*8

17. Swelling Force, Stack, In Oil 23°C See 17*1 See 17 * 2 See 17*3 See 17*4 See 17*5 See 17*6 See 17*7 See 17*8

lfl. Mot Coefficient of Friction 23°C

Ref (13) See 5 * 1

Impregnate See 3*2

Ref. (13) See 18 * 3 Ref. (13) See 18 * 4

ASTM D 1894-75, Notes 2 and 3 ASTM n 1894-75

Sec. 10

ASTM D 1894-75 ASTM D 1894-75

Sec. 10

PHASE 3

1 Specimen Selection, Number

and Dimensions

2 Conditioning

3 Apparatus Procedure

5 6 Calculation or Interpretation

of Results

7 8 'recisiorr and Accuracy

aging under mechanical and olectrlcnl stress In vncuuoI rented oil nt 120° C.

ASTM D 882-75b , Sec. 6; See 19 * 1 Impregnate

See 3*2

Ref. (1) See 19*3

Ref. (1) See 19 * 4

ASTM D 882- 75b ASTM 0 882- 75b, Sec. 11; Ref. (1)

ASTM D 882-75b, Sec. 12; also include aging time.

ASTM D 882- 75b, Secs. 5,

6, 10 and 11.

23° C, after aging under mechanical and olectrlcnl stress

In vacuum-treated oil at 120°C

See 20*1 Impregnate

See 3*2

ASTM 0 3426-75, Sec. 6; Electrodes: ASTM 0 149-75, Table 1, Type 1. See 20*3

ASTM 0 3426-75, Sec. 9; ASTM 0 149-75. See 20*4

ASTi; D 2305-72, Secs. 18.2, 20. See 3*5 VSTM D 3426-75, Sec. 10; 1STM D 149-75 ASTM D 3426-75, Sec. 11; ASTM D 2305-72, Sec. 24 See 3*8

‘ ' • Croup and Recovery, In Oil 23° and 90°C ASTM D 2990-77, Sec. 8; Throe specimens for each ASTM 0 2990-77, Sec. 9 Impregnate See 3*2

ASTM 0 2990-77, Sec. 6; Ref. (I)

ASTM D 2990-77, Sec. 11; (omit 11.4) ASTM D 2990-77, Secs. 6.1.2;

6.4.1; Note 6; 11.5.1

ASTM 0 2990-77, Sec. 12 (Omit 12.5, 12.6);

Ref. (1)

ASTM D 2990-77, Sec. 13 Ref. (1) ASTM D 2990-77, Sec. 14

4-26
Section 5

DEG REE- OF- VOID- FILLING TEST

In this test, measurements are made for the purpose of calculating the amount of

void volume of a specimen that remains unfilled by the dielectric oil. The experiment consists of measurements of the buoyant force on a porous tape immerse^ suc- cessively in two fluids; one a calibration fluid an^ the other a dielectric oil.

The calibration fluid must satisfy three criteria:

• It must efficiently permeate the polymer material under investigation, including closed (encapsul ated)voi ds

.

• It must not dissolve the polymer at room temperature.

t It must be a volatile liquid which mixes with the dielectric oil.

Having chosen a suitable fluid, it is assumed that the voids of an immersed specimen

are entirely filled by it. From a knowledge of the density of the calibration

fluid, the true specific volume of the polymer is calculated and then compare^ to

an apparent specific volume obtained from the buoyant force measurement of the specimen immersed in oil. The difference in the two specific volumes is proportional to the unfilled void volume.

CALCULATION OF UNFILLED VOID VOLUME

CASE 1: No Swelling, No Solubility

To analyze the experiment, we consider the simple case in which swelling and solubility do not occur. The weight of the specimen in the calibration fluid, W

f

, is

given by

Wf = W

sl

- V *d_ mf f (5-1)

where the subscript f refers to the calibration fluid, V mf

is the volume occupied by

polymer material, d^ is the density of the calibration liquid, and is the weight

of the sample in air. The buoyant force is - V^d^.

5-1
For the polymer tape in oil experiment, another specimen may be used. The weight

of the specimen in oil, W

q

, is

s2

V d - V d mo o vo o (5-2)

where the subscript o refers to the oil, V is the volume o^ the snecimen occuoie H

r mo

by polymer, d

Q

is the density of oil, v vo

is the volume o* unfilled voi H s, an H W ,, is the weight of the specimen in air.

From Eq. 5-1, the specific volume of the specimen, v

s> is obtained by dividing thru

by W

si ,

1 W

f v

s df <’ - s-f >• f sl (5-3)

It is assumed that

Vmf V mo W W. (5-4)

sl s2

A rearrangement of Eq. 5-2 yields the specific volume of unbilled voids in oil, v,

;

:

V , W

vo 1 ,1 0 N V

v

" W

?

" dn

(1 W

~ v

s- S2 0 S2

(5-5)

The percentage of unfilled void volume is

v 100 . (5-5)

Case 2: Swelling Occurs, But No Solubility

We consider now the case in which the specimen swells but there is no solubility.

Generally, swelling can be caused by both the calibration fluid and the oil. I

47 the

density of the swelling liquid which is incorporated in the polymer matrix is the same as the density of the liquid in the free state, then swelling will have no influence on the buoyant force, i.e. the fluid in the polymer exactly displaces itself

and does not contribute to the buoyant force. In the analysis below, the following

symbols are used:

5-2
W

s = weight specimens in air.

M

f

= weight of swelling calibration f1 ui H incorporate^ in the matrix of

s the swelled polymer.

V = volume of the swelling fluid which is incorporated in the matrix of

s the polymer.

Vm

^= volume occupied by polymer material, specimen in calibration fluid.

W.p = weight of specimen in the calibration fluid.

As with Eqs. 5-1 and 5-6, we assume that the calibration fluid permeates effeciently

and fills all the void volume. The weight of the swelled specimen in the fluid is

W

f Ws + Msf

- Vmfd

f

- Vsfd r

(5-7)

The volume V £ can be expressed as PI r /d . where P' . and d , are the weight an^

sf sf sf sf sf

density of the swelling fluid incorporated in the polymer. Dividing thru by '•!

we obtain.

(5-8)

V r where ^ , the specific volume of the specimen. The ratio M

s f/Ws will be

called the fraction of swelling fluid, F^. Eq. 5-8 becomes

v d r +

s f

F , (1 sf v (5-9)

where W,, W and d, are measured quantities. If d = d r , then Eq. 5-9 reduces to

f s f f sf n Eq. 5-1.

Testing the Hypothesis

In order to test the hypothesis that swelling does not influence the buoyant force,

two observations are necessary: one in which no swelling occurs, and the other in

which significant swelling occurs. The proposed test is to determine the specific

volume of the specimen from Eq. 5-3 using a non-swellinq liquid such as water, and

then to compare with the result obtained using a swelling fluid and Eq. 5-3. The

result will indicate the effect of swelling on buoyancy and whether or not it can be

assumed that d^ = d This test need not involve a porous polymer, and in fact it

is easier to use a solid film. Two specimens were used: a solid film of polyethylene and a sheet of cross-linked rubber.

5-3
In our first experiment the density of a crossl inked ruhher was measured in water.

No swelling was detected and the density at 22.6°C was 0.9238 g/cm^. The specimen

was dried and then used in a buoyant force experiment using decane. Initally, this

rubber specimen lost about 4% of its weight in decane, but subsequent drying and

reimmersion in decane caused no observable change in weight, At this point the

specimen was immersed in decane and found to swell by 20%. The density in decane

was calculated to be 0.9253 g/cm^, or an increase of 0.16% over that measured in

water.

Next, a solid film of branched polyethylene was used. Its density, as measured by

the buoyant force in water, was found to be 0.9262 g/crn-^ at 23.0°C. In heptane, the

density at 23.0°C was 0.9270 g/cm 3

, or an increase of 0.09% over the density in water.

The swelling was too small to detect by measurement of the thickness. The specimen

was weighed in air and found to have increased in weight by 6.6%, due to retained

heptane

Resul

Table 5-1 summarizes the results of these measurements.

TABLE 5-1

EFFECT OF SWELLING ON DENSITY

Specimen Temp. Density (q/cm 3

) % A Deqree of Swel 1 inq

Rubber 22.6°C 0.9238 (H 2 0)

0.16%

none

0.9253 (decane) 20 %

Polyethlyene 23.0°C 0.9262 (h2 o)

0.09%

none

0.9270 (heptane) note a

a The thickness change was too small to measure. Weighing in air revealed

that the polyethylene retained 6.6% heptane by weight.

5-4
Because swelling has such a small effect on the buoyancy measurements, this effect

can be neglected in subsequent measurements, except where precision greater than

0.1% is required.

EXPERIMENTAL APPARATUS

The Electrobalance

The equipment which was used for the buoyancy measurements is a Cahn RG Automatic

Electrobalance, shown in a schematic diagram in Figure 5-1. In this instrument the

balance arm is connected to a coil which can rotate in a magnetic field. The current

through the coil necessary to maintain the coil in a stationary position is proportional to the weight on the balance arm. The resolution of the instrument is

Coil

p

Wire-32 Gauge

Sinker Weight

Specimen

immersion Fluid

Figure 5-1. Schematic niagram of Rahn RG Automatic FI ectrobal ance

5-5
10“6 gm. The overall range of the electrobalance is only +100 mg about zero or

200 mg total. A counterbalance arm is provided for a tare weight which must be

used to balance out most of the weight on the measuring arm but allow for the 100

mg swing about zero. Both the tare weight and the wire loop for the specimen are connected to the balance arms by thin glass rods. This isolates electric charges

on the tare weight or specimen from the electrobalance circuitry. The wire loop,

on which the specimen is threaded, is a 32-gauge tinned-copper wire. Also hooked

on the wire is a sinker weight, a small piece of 20-gauge nichrome wire which keeps

the specimen submerged in those cases when it would otherwise float on the liquid.

For these experiments, the 32-gauge wire loop weighs approximately 3 mg and the

nichrome sinker weighs approximately 100 mg.

Although it is not shown in Figure 5-1, the entire electrobalance system is enclosed

in a vacuum vessel. This feature was utilized in several experiments described

below.

The Detecting and Recording Instruments

The current signal from the electrobalance is converted to a voltage which is

applied to two detecting instruments, a digital voltmeter and a potentiometric

recorder. A block diagram is shown in Figure 5-2. The change in buoyant force

with time is detected using the recorder. At the input to the recorder, a bucking

voltage or a zero suppression device is used to cancel most of the signal so that

changes in the electrobalance signal can be recorded on the most sensitive scale

of the recorder (1 mv full scale). The source of the bucking voltage is a 1.35

volt mercury battery in series with a voltage divider. The steady-state value

of the electrobalance signal is detected by the digital voltmeter. The voltage

follower shown in Figure 5-2 is a high-input-impedance operational amplifier,

isolating the DVM from the other devices.

CALIBRATION

Calibration of the balance must be carried out at constant tare weight. If the

tare weight is chanyed, then a recal ibration of the balance is necessary, because

small changes in total weight on the bearing will affect the calibration. Having

established a suitable tare weight, calibration of the instrument was accomplished

by adding 200 mg of calibrated weights to the measuring arm in increments of 5 mg.

The individual weights were calibrated to the nearest 10"6 gm so that, considering

cumulative uncertainties, it was possible to calibrate the balance to the nearest

5-6
Figure 5-2. Block Diagram of Buoyant-Force-Measuring System

10"5 gm. For convenience, the voltage signal from the electrobalance was adjusted

so that a 1 mv change in voltage corresponded approximately to a 1 mg change in

weight. The precision and accuracy of this measurement have not been determined.

The maximum uncertainty in the calculation of specific volume due to weighing

errors alone is ±0.0004 g/cm .

MEASUREMENT OF LIQUID DENSITIES

In order to calculate specimen density and unfilled void volume using Eqs. 5-3

and 5-5, the densities of the calibration fluid and the dielectric oil must be

known. These are measured using a calibrated pycnometer whose temperature is

measured to the nearest 0.1°C. The pycnometer was calibrated by using distilled

water to measure its volume with an accuracy of 0.02%.

The pycnometer is filled with sufficient liquid such that air bubbles are not

trapped when it is closed. The exterior surface of the pycnometer must be cleaned.

Excess liquid on the exterior surface of the pycnometer vessel can be removed by

spraying a stream of acetone and by wiping with a piece of lens tissue. Using

tongs or gloves, the vessel is then handled and weighed to the nearest 0.1 mg.

The results of the density measurements on heptane, decane, alkyl benzene oil and

the polybutene oil are listed in Table 5-2.

5-7
Table 5-2

DENSITIES OF LIQUIDS

Specimen Temperature Oensi ty

Heptane 22.9°C 0.68213 g/cm

23.2 0.68192

Decane 22.6 0.72761

Alkyl Benzene Oil 23.0 0.86679

24.7 0.86604

26.4 0.86497

Polybutene Oil 22.6 0.82922

24.2 0.82854

25.9 0.82798

26.9 0.82745

SPECIMEN PREPARATION

The specimens used in these experiments were cut from a sheet or strip into pieces

1 cm square. A hole 2 mm in diameter was punched in the center of each square, an H three or four squares were then threaded onto the 32-gauge wire loop. This con- stituted the specimen for that experiment. Specimen weights were in the range

30-60 mg

For the experiments on porous tapes, three materials were chosen:

• A polypropylene synthetic paper (PPSP), manufactured by Matsushita

Industrial Equipment Co., Osaka, Japan. See [ 1_)

• A porous polypropylene (PP) manufactured by Armak Co., Chicago, 111.

• A polyethylene synthetic paper (PE), manufactured by Furukawa Electric Co., Yokohama, Japan. See (5).

Two dielectric oils were used, some properties of which are shown in Table 5-3:

• An alkyl benzene oil, manufactured by Chevron Chemical Co., Richmond California, designated Chevron Dielectric Oil 100.

• A polybutene oil, manufactured by Amoco, Wood River, Illinois,

designated Cable Oil 14.

Because of current interest in it, a non-porous laminate o f paper-polypropylenepaper (PPP) was measured. This was obtained from Phel ps-Dodge Cable and Wire Co.,

Yonkers , N.Y. The source wad designated in (17) as "Manufacturer B".

5-8
TABLE 5-3

PROPERTIES OF DIELECTRIC OILS

Alkyl Benzene Pol.ybutene

Dielectric Constant

98. 89°C

2.1 -

Dielectric Breakdown

25°C, by ASTM D877

>35 k

V

>35 kV

Power Factor

100°C

0.02% 0.05%

Viscosity

37.8°C

98 SUS 120 SUS

5-9
Figure 5-3. Specimens for void-filling

test. Clockwise from top: PPP, PP, PE. Figure 5-4. Specimen of PPP suspended inoil, as during the buoyancy change

measurement, showing the ni chrome sinker.

PROCEDURE

Selection of Calibration Fluids

The calibration fluid is chosen according to the criteria stated above, i.e. it must

effeciently permeate the polymer under test; it must not dissolve the polymer at room temperature; and it must be a volatile liquid which mixes with the dielectric

oil. Generally, a low-molecular-weight volatile organic liquid is sought for this

purpose. A knowledge of permeation coefficients of liquids in various polymer

systems can aid in the selection of a calibration fluid. For the polymers investigated in this study, heptane and decane were chosen as calibration fluids because

it is well known that these liquids permeate polyethylene and polypropyl ene with

ease.

5-10
Figure 5-5. Dr. A. J. Bur operating the void-^ill ing-test apparatus. The Dahn

El ectrohal ance is in the horizontal glass cylinder at the top of the photo. The

specimen is suspended in oil in the vertical glass cylinder.

5-11
Solubility in the Calibration Fluid

Before we undertake the buoyant force experiments, the solubility of the tape in

the calibration fluid is measured. To do this, a specimen weighing approximately

50 mg is weighed to the nearest 0.01 mg, using the electrobalance. We then immerse

the specimen in approximately 50 cm ^ of the fluid. The vessel is retained at room temperature and agitated twice daily over a period of several days. Upon removal

from the fluid, the specimen is alternately weighed and vacuum dried until the

weight reaches a constant value. If the final weight is within 0.01 mg of the

original weight of the specimen, then, within the limits of accuracy of the measurement, the specimen is insoluble in the calibration fluid. This is the ideal case.

In the less-than-i deal case some weight loss occurs. If the loss is less than 0.1%

we consider the fluid to be marginally acceptable, and follow one or the other of

two courses of action.

t Short term solubility tests (for periods up to eight hours) can be

carried out with new specimens, so that solubility vs. time data

can be obtained.

• Alternatively, the original specimen can be retested in order to

determine if the weight loss was due to an initial cleaning which

does not repeat upon further contact with the fluid. If initial

cleaning was the reason for the weight loss, then we gave all

subsequent specimens an initial cleaning before using them in the

buoyant force experiment.

fleas urement of the Specific Volume

In order to use Eq. 5-3 for the calculation of the specific volume, four balance

readings are necessary:

A - Weight of wire loop and sinker in air,

B = weight of wire loop and sinker plus specimen in air,

C = weight of wire loop and sinker in fluid,

D = weight of wire loop and sinker plus specimen in fluid.

Here, W of eq. 5-3 is the difference (D-C) and W^ is (B-A). cor convenience, we carried out the above measurements in the order A, C, B, D. After the measurement

of C, the wire loop and sinker were thoroughly cleansed either by allowing the fluid

to evaporate to by rinsing with a volatile solvent.

Prior to the measurement of D, we slowly immersed the specimen into the fluid, being

careful to avoid dislodging the wire loop from the balance arm or changing the position of the glass rod. Sudden mechancial jolts to the balance system can disturb

the measuring arm and result in a need for a lengthy recalibration procedure.

5-12
Immediately after we immersed the specimen in the fluid and after the vessel was set in place, the bucking voltage was adjusted to cancel out the signal from the

electrobalance. A zero reading on the most sensitive scale of the recorder

indicated cancellation. As the fluid permeated into the specimen, the change in

buoyant force was recorded with time. If this change was larger then the full

scale displacement of the recorder, then the bucking voltage was adjusted accordingly. When the buoyant force had reached a constant value which persisted for

a period of at least 8 hours, the DVM reading was recorded as the value of D.

The same specimen that had been immersed in the calibration fluid is dried and

used again with the dielectric oil so that there is no uncertainty regarding the

density of the oil -immersed specimen. The procedure as described above for obtaining readings of A, B and C is followed. For D, special attention must be given

to the oil impregnation process and to the solubility of the specimen in oil.

Impregnation Procedure

Oil impregnation is monitored using the chart recorder. When a steady-state or near steady-state value of the buoyant force is reached, a preliminary calculation

of the percentage of unfilled void volume is carried out using Eqs. 5-5 and 5-6.

If there is a significant amount of unfilled void volume, than vacuum, temperature

and pressure (if available) cycling are used in order to achieve maximum degree

of void filling. Whatever cycling procedures achieve the optimum will be used for

subsequent impregnations.

Sol ubil ity in Oil

Before a final calculation of v is made, the solubility of the specimen in oil must

be determined. To do this the calibration fluid is used as a washing medium after

the specimen is removed from the oil. Since the calibration fluid mixes with the

dielectric oil, it is possible to remove all of the oil from the pores by repeated

washing and drying. Drying can be carried out either by evaporation at ambient

room conditions or under vacuum. The weight of the specimen is measured after each

wash-dry cycle until a constant weight is reached.

If there is no difference between the original and final weight, values of (B-A)

and (D-C) can be used in Eq. 5-5 without any adjustment. However, if there is a specimen weight loss and it is small, less than 1%, then the value of (B-A) is

lowered by the amount of weight loss before using Eq. 5-5. Also, for solubility

less than 1%, it can be assumed that the density of oil remains unchanged.

5-13

i
If the solubility is larger than 1%, then the specimen may be judged unsuitable

for continued testing, and, if a calculation of v

v is desired it may be necessary

to remeasure the density of the oil.

More detail regarding the solubility test is given in Section 6.
EXPERIMENTAL RESULTS

The results of the buoyancy experiments on the three porous tapes and the PPP

laminate are given in Tables 5-4 and 5-5 and in Figures 5-6 thru 5-12. The

Tables and Figures show that the three specimens of tape cover the extremes from

very limited oil penetration (PPSP) to rapid and complete penetration (PP). Void

filling of the PE specimen lies between these extremes in that it is not very

rapid nor is it complete. Vacuum and temperature cycling were used but were

found to have very little effect.

Results - PPSP

Figure 5-6 shows the change in buoyant force, AW, versus time for PPSP in heptane

the calibration fluid. The curve is typical of the calibration data obtained for

the PP and PE porous tapes. The weight in air of the PPSP specimen was 43 mg.

In heptane, the buoyant force gradually changed by 1 7 mg over a period of 2 hrs.

This change is interpreted as the effect of diffusion of heptane into the pores

and microvoids of the specimen. The steady-state buoyant force was used to calcu

late the density of PPSP at 23.0°C which is 0.9390 g/cm.

Neither the alkyl benzene oil nor the polybutene oil was able to penetrate the

PPSP material to any appreciable degree. Vacuum cycling was used but was found

to have very little effect. The purpose was to withdraw trapped gases from the

specimen and then open the vessel to atmospheric pressure. A AW versus time curve for PPSP in alkyl benzene oil is not shown because little change with time

occurs, i.e., no oil permeation occurs.

The behavior in polybutene oil, which is shown in Figure 5-7, demonstrated

evidence of specimen dissolution in the time range from 7 minutes to 80 minutes.

On this plot, decreasing AW indicates that the specimen is being dissolved in

the oil and increasing AW indicates pore filling. After the PPSP specimen was retrieved from the oil, washed in heptane and vacuum dried, a weight loss of

0.07 mg or 0.4% of the original weight was observed.

Both vacuum and temperature cycling were used for the purpose of improving the

oil penetration into the PPSP specimen. The temperature of the immersed specimen

was raised to 50°C with and without vacuum, and then returned to room temperature

The cycle was repeated using 80°C as the maximum temperature. In both cases, the

void filling was only slightly improved. We have concluded that temperature and

5-15
Table 5-4

VOID FILLING AT ROOM TEMPERATURE, ALKYL BENZENE

Specimen

Density in

Heptane or Decane

Density in

Alkyl Benzene

Unfilled Void

Volume-Alkyl Benzene

g/cm3 g/cm 3 %

PPSP 0.9390 0 . 647 4 ( 4h rs

)

0.6738(85hrs

)

31.0

28.2

PPSP after vacuum 0 . 6623(6hrs

)

29.5

PE 0.9607 0.9525(22hrs

)

0.87

PE after

vacuum 0.9581 (28hrs

)

0.26

PP 0.9116 0.9114 0

PPP 1 .0499 1 .0451 see text

Table 5-5

VOID FILLING AT ROOM TEMPERATURE, POLYBUTENE

Specimen

Density in

Heptane or Decane

Density in

Pol.ybutene

Unfilled Void

Vol ume-Pol.ybutene

g/cm3 g/cm 3 %

PPSP 0.9390 0.6232(4hrs

)

33.5

PPSP after vacuum

0.6542(20hrs

)

30.2

P

D SP after

vacuum, temp. 0.6853(48hrs

)

27.0

PE 0.9607 0.9267(3hrs

)

0.9562(67hrs

)

3.54

0.47

PE after vacuum 0.9573(90hrs) 0.35

PP 0.9116 0.9087 0.31

5-16
time minutes

Figure 5-6. The Change in Buoyant Force vs. Time for PPSP in Heptane.

Figure 5-7. The Change in Buoyant Force vs. Time for PPSP in Polybutene Oil;

(a) 0-200 minutes ; (b) 200-380 minutes

.

5-17
vacuum cycling can only be of value for those oil-polymer combinations for which

the oil spreads on the polymer surface. Using either kind of cycling, it is possible to enhance an already good penetration or to bring the permeation to completion, but it is not possible to induce a permeation which is not thermodynamical ly

favorable on mixing the two materials.

Results - PE

Curves of AW versus time for the PE specimen in the two oils are shown in Figures

5-8 and 5-9. As the data in Tables 5-4 and 5-5 indicate, unfilled void volume

is present. Vacuum treatment decreased the volume of unfilled pores, but filling

remained incomplete. In Figure 5-9, it appears that pore filling with the polybutene oil proceeds in a stepwise manner. The discontinuity at t = 79 minutes

is interpreted as a sudden release of gas bubbles from the specimen.

Visual inspection of the PE specimens yielded some qualitative information. For

example, the porous tapes are opaque in the dry unfilled condition. As the tapes

fill with liquid they become transparent. During the filling process, those re- gions of the tape which have not been penetrated by the oil remain opaque. In the case of the Furukawa polyethylene, gas-filled voids could be observed. These

regions expanded when the vacuum was applied and bubbles of gas escaped from those

which were near the edge of the specimen. Two voids remained encapsulated and

gas was unable to escape from these. Thus, for the PE tapes it could be visually

observed that the pore filling was not complete.

Results - PP

The most efficient oil permeation occurred for the PP specimens. The data of Fig- ures 5-10 and 5-11 show that pore filling for both oils was nearly complete in

20 minutes. Within the limits of accuracy of our measurement, there were no unfilled

pores for the alkyl benzene oil and only 0.31% for the polybutene oil.

Results - PPP

The AW versus time curve for PPP is shown in Figure 5-12. Initially, oil permeated

rapidly into this specimen, but, at t

= 5 minutes, penetration slowed significantly

and continued slowly until equilibrium was reached in approximately 24 hours. Good

oil permeation was achieved, but, because of variations in density over the area of

the original sample sheet, an accurate measure of unfilled void volume can not be

obtained from this single measurement. The 0.5% difference in the numbers of Table

5-4 is probably due to the difference in density of the two samples used in the

decane and alkyl benzene. More data, which would permit a statistical analysis

of the experiment, is needed for a more precise measurement.

5-18
Figure 5-8. The Change in Buoyant Force vs. Time For P E in Alkyl Benzene Oil.

Figure 5-9. The Change in Buoyant Force vs. Time for PE in Polybutene Oil,

5-19
Figure 5-10. The Change in Buoyant Force vs. Time for PP in Alkyl Renzene Oil.

Figure 5-11. The Change in Buoyant Force vs. Time for PP in Polybutene Oil.

5-20
1.4

1.2

1.0

0.8

AW

mg

0.6

0.4

0.2

0.0

0 40 80 120 160 200 240 280 320 360 400

time, minutes

Figure 5-12. The change in Buoyant Force vs. Time for DDD in Mkyl Benzene °i 1

5-21

SECTION 6

SOLUBILITY

In this test solubility of a porous sample in a dielectric oil is measured. Some

features of the measurement procedure are described in Section 5. Use of the

calibration fluid to wash the sample after immersion in the oil is an essential

step in the procedure.

ASTM D 1239-55 (1971) will be followed for the measurement of solubility with

two modifications: a) the specimen must be impregnated with the oil in accordance

with the impregnation- time profile which is measured in the degree-of-void-filli ng test (test 9); b) the specimen is washed in the calibration fluid in order to remove the dielectric oil. The weighing apparatus can be either a precision

analytical balance or an electrobalance as described. If an electrobalance is

used, specimen size must be tailored so as not to exceed the range of the instrument.

After the original weight of the specimen is obtained, the specimen is immersed in

the dielectric oil for the impregnation period. If vacuum cycling was found to be

necessary to achieve impregnation in the degree-of-void-fi 1 1 i ng test, then the

it should be used here also. After impregnation the specimen is placed at one of

the testing temperatures: 23, 90 or 120°C. The container should be agitated

twice daily for three days. The specimen is then removed from the oil and washed

with the calibration fluid at room temperature. Drying is achieved either by

evaporation at ambient room conditions or by vacuum drying. The specimen is

given as many wash-dry cycles as is necessary for its weight to reach a constant

value. When a constant value is obtained, it is compared with the original

specimen weight to find a value for the weight lost. The weight lost divided

by the original weight multiplied by 100 gives the percent weight loss, or solubility.

Table 6-1 gives the results for the four materials tested.

6-1
TABLE 6-1

Solubility of Tapes at Room Temperature

Specimen

Weight Loss

Alkyl Benzene Oil

Weight Loss

Polybutene

PPSP 0 0.4%

PE 0 0 PP 0 0

PPP 0 •

6-2
Section 7

CONCLUSIONS

On the basis of previous work on porous polymer tapes and on the basis of a set

of reasonable assumptions, it was possible to choose those properties and tests

which are of primary importance for application of porous tapes as insulation in

underground transmission cables, and to eliminate those properties which are of

secondary importance. The essential physical properties of the test program are electrical, mechanical and those properties which are peculiar to porous polyrer

materials i.e. solubility, melting and softening temperature, degree of void filling, and creep and recovery.

The application of the acceptance criterion for each test requires judgement decisions regarding materials with borderline properties. A trade-off between a desirable property and a borderline property is possible by considering engineering

design parameters. For the degree-of-void filling test, the ideal result is that

all voids be filled. If, however, it is demonstrated that a small fraction of

void volume does not result in deterioration of ac and impulse breakdown, then

the tape can be considered acceptable.

Of the 21 tests in this screening program it was found that all but two tests were

either fully described in published literature or small modifications could be

made in a published test without extensive laboratory development.

The feasibility of the buoyant force measurement for obtaining a quantitative measure of the unfilled void volume was demonstrated. This test is used to describe

the permeation procedure that is necessary in order to achieve maximum void filling.

In those cases where oil permeation is not complete, vacuum and teroerature cycling

should be employed. The recipe for void filling, which includes the tine needed

for oil permeation and the necessary cycling procedures, will be used for all tests

which require impregnated specimens.

Although we did not use pressures greater than atmospheric to enhance the oil impregnation, the conclusions which we reached from our observation of vacuum cycling

applied here also. That is, pressure cycling can improve the kinetics of oil

7- 1
impregnation for a tape-oil system which spontaneously mixes. It will not, however, cause oil permeation in a tape when it is not thermodynamical ly favorable. In our judgement, positive pressure is a tool to be used for impregnating model cables

or full size cables after it has been demonstrated by the void-filling test that

the oil permeates the material under investigation.

If was found that the calibration fluid served two purposes: (a) Using this fluid

the true specimen density was obtained from the buoyant force measurement; and

(b) the fluid was used as a washing medium in order to remove all traces of oil

from the specimen prior to a solubility determination. Generally for organic poly- mers the calibration fluid is a low-molecular-weight organic liquid.

The three porous tapes which we used in the development of the solubility and the

degree-of-void-fi 11 ing tests displayed a varied range of void filling. For the

PPSP material, we observed that very little oil permeation occured and that the

characteristic change in buoyant force with time (indicating oil permeation) did

not occur. Although vacuum and temperature cycling were used, we observed only

a small change in the unfilled void volume. For PE material, a small fraction

of unfilled void volume remained. We were able to use vacuum cycling to improve

oil penetration but complete void filling was not achieved for the PE specimens.

Only the PP specimen in alkyl benzene oil showed complete oil permeation. We con- cluded from these experiments with porous tapes that thermodynamically favorable

mixing of oil and tape is essential for good permeation.

Although the PPP material does not belong in the category of a porous polymer tape,

we measured its void filling characteristic because of current interest in this

material. Variations in spatial density of this material were responsible for

a higher uncertainty in the measurement of unfilled void volume. A more precise

determination requires extended experimentation and statistical analysis. We have

concluded that good oil permeation does occur for PPP in alkyl benzene oil.

7-2
Section 8

REFERENCES

1. Polymers Designed for Extra High Voltage Applications . Illinois Institute

Of Technology Research Institute Report No. I ITRI-43001 -57 (1967).

2. 765 kV Pipe Type Cable. Quarterly Reports by Phelps-Dodge Wire and Cable

Co., EPRI Contract RP7812, (1972-1978).

3. Development of 500 kV AC Cable Employing Laminar Insulation Other Than

Conventional Cellulosic Paper , Final Report by General Cable Corp., EPRI

Contract RP7810-1, (1977).

4. Polymer and Polymer/Paper Laminated Tapes for EHV Oil -Filled Cables , by

D. R. Edwards, J. Counsel, J. Gibbons and R. Scarisbrick, CIGRE, Paper

15-05, International Conference on Large High Tension Electric Systems, (1972).

5 . New Modified Polyethylene Paper Proposed for UHV Cable Insulation , by

Z. Iwata, Et.al., IEEE Trans, Power App. Sys., PAS-96, 1573 (1977)

.

6 . Development of Polypropylene Laminated Paper Insulated EHV Cables , by

K. Matsuura, H. Kubo, T. Miyazaki, Underground Transmission and Distribution Conference, 1976.

7. A New Synthetic Paper-MPSP for Insulation, by. A. Tomago, T. Suzuki, A. Iami

,

and I. Yamauchi, IEEE Trans. Elect. Insul., EI-12, 301 (1977).

3. Synthetic Paper for Extra High Voltage Cable , by T. Yamamoto, S. Isshiki,

S. Nakayama, IEEE Trans. Power App. Sys. PAS-91, 2415 (1972).

9.

Tenax, A Hew Low-Loss High-Temperature Resistant Synthetic Paper for EHV

Cables and Other Electrical Equipment , by J. Vermeer et.al., 1970 Annual

Report, Conference on Electrical Insulation and Dielectric Phenomena ,p. 56.

10. E. J. McMahon and J. Punderson, IEEE Trans, Elec. Insul., No. 3 (1973).

11. Electrical Cable With Polymer-Oil Insulation , by E. J. McMahon, J. 0. Punderson, W. P. Pitcusky, U. S. Patent No. 3,748,372, (1973).

12. A Program To Evaluate, Develop and Test Poroplastic Materials as Improved

Electrical Insulation For High Power Underground Cables , EPRI Research Project 7842-1, Final Report by Moleculon Research Corp.

13. A Brief Review of the Theory of Paper Tapping of a Single-Core High Voltage

Cable , by P. Gazzana-Priagrogia, E. Occhini, and N. Palmieri, Proc. IEE (London)

Monograph 3905, 108C , 25 (I960).

14. Some Theoretical Considerations Affecting the Design of Lapped Plastic Insulation for Super-Conducting Power Transmission Cables , Prepared by Brookhaven

National Laboratory, EPRI EL-269, Research Project 7844-1, Apoendices A and

B, (1976).

8-1
15. Annual Book of ASTM Standards, Published by the American Society for Testing

and Materials, Philadelphia, Pa. 19103 (1973).

16. Private Communication, Robert W. Penn, National Bureau of Standards, Washington, D.C. 20234, (1979).

17. E. M. Allam, W. H. Cortelyou and H. C. Doepkin, Low Loss 765 kV Pipe-Type

Power Cable. IEEE Trans. Power App. Sys . , PAS-97, 2019 (1978).

18. M. G. Natrella. Experimental Statistics. Handbook 91 Published by U.S. Dept.

Commerce, National Bureau of Standards, page 3-24, (1963).

8-2


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Below are five index cards that allow for filing according to the

four cross-references in addition to the title of the report. A brief

abstract describing the major subject area covered in the report

is included on each card. For information regarding index card

subscriptions to past and future EPRI publications contact the

Research Reports Center, P.0. Box 50490, Palo Alto, California

94303. Telephone (415) 965-4081.

EPRI EL-1259 RP7864

Final Report

December 1979

Porous Polymer Tape Screening Program

Contractor: National Bureau of Standards

EPRI

This report discusses the development of (1) a three-phase screening pro- gram for porous polymer tapes, (2) a method to measure quantitatively the

void-filling capability of such tapes, and (3) a test for accurately determining

the solubility of porous tapes in oils. A method for reliably measuring the

porosity of tapes for dielectric applications and a programmed approach for screening porous tapes prior to preparing model cables are described. EPRI Project Manager: B. Bernstein

Cross-references:

1. EPRI EL-1259 2. RP7864 3. Underground Transmission Program 4. Cables

ELECTRIC POWER RESEARCH INSTITUTE

Post Office Sox 10412, Palo Alto, CA 94303 415-855-2000

EPRI EL-1259

Porous Polymer Tape Screening Program

Contractor: National Bureau of Standards

This report discusses the development of (1) a three-phase screening pro- gram for porous polymer tapes, (2) a method to measure quantitatively the

void-filling capability of such tapes, and (3) a test for accurately determining

the solubility of porous tapes in oils. A method for reliably measuring the porosity of tapes for dielectric applications and a programmed approach for screening porous tapes prior to preparing model cables are described. EPRI Project Manager: B. Bernstein

Cross-references:

1. EPRI EL-1259 2. RP7864 3. Underground Transmission Program 4. Cables

ELECTRIC POWER RESEARCH INSTITUTE

Post Office Box 10412. Palo Alio, CA 94303 415-855-2000

RP7864

Porous Polymer Tape Screening Program

Contractor: National Bureau of Standards

This report discusses the development of (1) a three-phase screening pro- gram for porous polymer tapes, (2) a method to measure quantitatively the

void-filling capability of such tapes, and (3) a test for accurately determining

the solubility of porous tapes in oils. A method for reliably measuring the porosity of tapes for dielectric applications and a programmed approach for screening porous tapes prior to preparing model cables are described. EPRI Project Manager: B. Bernstein

Cross-references:

1. EPRI EL-1259 2. RP7864 3. Underground Transmission Program 4. Cables

EPRI EL-1259 RP7864

Final Report

December 1979

EPRI EL-1259 RP7864

Final Report

December 1979

ELECTRIC POWER RESEARCH INSTITUTE

Post Office Box 10412. Palo Alto. CA 94303 415-855-2000