Journal of Scientific & Industrial Research

Vol. 62, December 2003, pp 1145- 1151

Development of Specialty Papers is an Art: Electrical Insulation Paper from Indigenous Raw Materials - Part IX

Dharm Dutt· , Vikram Singh and A K Ray

Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247 00 I

and

S Mukherjee

Department of Electrical Engineering, Indian Institute of Technology Roorkee, Roorkee 247 00 I

Received: 16 April 2003; accepted: 25 September 2003

Electrical insulation papers have wide applications in the most different branches of engi neering viz. , high tension power cable, telephone cable, wire winding, and capaci tor or condenser. Paper is the cheapest source and has outstanding electrical properties and dielectric constant among known insulating materials. It needs to develop certain physical, chemical , and electrical properties. In order to develop electrical insulation paper, it is imperative to modi fy process water quality as well as paper manufacturing process. As the O2 delignified pulp is refined extremely far, up to more than 9011 Schopper­Reigler, so long fibred pulp of good mechanical strength is preferred for the development of electrical grade insulation paper. In this study possibility of Cannabis sativa hemp ribbons were explored to develop various electrical insulation papers like, hi gh tension power cable paper, telephone cable paper, and capacitor tissue paper.

Keywords: Speciality paper, Electri cal insulation paper, Indigeneous raw material

In trod uction

The rapid development of very long power transmission lines, electronics devices and electrical engineering result an augment use of electrical insulation materials particularly capacitor ti ssue. However, technological development has now surpassed them, and they are gradually being replaced by other materials. Further petroleum cost and availability conditions may make the use of intermediate-voltage paper-insulated and sheathed cables advantageous again. The most common insulators are organic polymers, which are covalently bonded. When electrical insulation system is being de~i gned , the most common insul ating materials chosen are the covalently bonded polymers. Electrons are being tightly shared between atoms, generally are not available to move over the molecule and hence applied elec tric field does not cause current flow. Pure cellulose has outstanding electrical properties.

* Au thor for correspondence Tel: 01322711740

Cellulose is the cheapest, polar in nature and having proper dielectric constant among the known on electrical insulation materials. Paper cable permits transmission of larger power loads through given size ducts compared to other insulations . Based use the electrical insulation paper may be categorized as high tension power cable paper, telephone cable paper, and capacitor tissue paper. The most important properties for electrical insulation paper are as follows :

• Physical Properties - It includes density, shrin­kage, moisture and oil absorption . The high density paper increases the dielectric and impulse strength of the cable. Impregnation of paper with mineral oil or polyisobutylene lowers the die lectric loss, oxidation and value of di ssipation factor to some extent, which closely approximates the power factor. Loss factor is the product of dissipation factor and dielectric constane. Non-impregnated paper is used for telephone cables. Almost all insulation systems have voids somewhere in the structure due to entrapped gases, delamination, shrinkage or lack of adhesion. At voltage below

1146 J SCI IND RES VOL 62 DECEMBER 2003

break down, gases in the voids ionized causing generation of and electrical discharges. Therefore, it is essential to know partial discharges or corona prior to use for insulation2

.

• Mechanical Properties - The important mecha­nical properties are tensile strength, elongation, compression strength, TEA (Tensile Energy Absorption) and fibre length. Thermal conductivity controls heat transfer from the conductor te. the ambient environment. Addition of resins like, epoxy resin , phenol formaldehyde, vinyl formaldehyde or urea formaldehyde' into the long fibred stock, improves their thermal conductivity . Thermal conductivity may be correlated with electric bursting strength4

.

• Chemical Properties -- The important chemical properties are ash, pH, conductivity, cr, S04--, and degree of polymerization. It is important to note that the conductivity of the polymers vary wi th the environmental conditions like moisture and temperature. The increased temp causes softening of the polymer, which results in an increase in conductivity. The ionic impurities conduct under voltage stress depending upon their concentration and mobility. The mobility is greatly influenced by the environment. Water from high humidity environments diffuses in to the polymer and increases the ionic conduction. At temperature above glass-transition temperature of the polymers the flexibility of the polymer increases and causes the polymer to relax, which results in great ion mobil ity . It is, therefore, common to run the paper machine with chemjcally deionized water when producing higher grades of electrical insul at ion papers.

• Electrical Properties - Tbe most common elec­trical properties are dielectric constant, i.e., specific inductive capacity, high dielectric strength, i.e., electrica l resistance, low power factor, i.e. , dielectric loss, freedom from conductive materials. Every insulator possesses a few electrons. Under the stress of an electric field, these electrons tend to move between the electrodes. A voltage level is reached, wherein the e lectrons impact with atoms and molecules, thereby dislodging other electrons and causing an electric break down . This is an arbitrary value associated with a device that describes the abi ll ity of the insulating system to withstand impulses5

.

• The pulp should be free from blemishes, specks, shives, and slivers. It is essential to use deculator, foam killers, pulp selector, and centricleaners.

In present investigation high tension power cable paper, telephone cable paper, and capacitor tissue paper were made from Cannabis saliva hemp ribbons . The important characteristics for these grades of paper are as follows:

High Power Tension Cable Paper - It is manufactured in the basis weight range of 65 to 155 g/m2 on twin ply paper machine to improve formation and eliminate the risk of holes. The paper has to be clean for avoiding leaking currents because power transmission occurs at very high tensions up to 400 kY. The advantage of using paper in cables is that in the case of leakage the paper will swell and prevent water from flowin g along the conduct of the cable.

Telephone Cable Paper - It is made up of huge number of threads and individually isolated by paper. It is being manufactured in the basis weight range of 30 to 40 g/m2 to avoid excessive thickness of cable.

Capacitor Tissue - It is extensively used in the capa­citors required for electric and radio communication, television, automatic control and electrical measuring instruments as well as for improving the power factor (cos ine), power take-off from high voltage power transmission lines, protection against over voltage, in construction of powerful impul se ligh t sources , hi gh voltage switches and transformers . Power capacitors are required for induction furnaces, longitudinal compensation of medium and high voltage power transmjssion lines, electron microscope and X-ray apparatus. It is manufactured in the basis weight range of 6 to 12 g/m2, density 0 .8 to 1.3 g/cm 1 and thickness 4 to 3011.

Experimental Methodology

Morphological Characteristics - A piece of hemp ribbon was heated with potassium chlorate and hydrochloric acid solution in order to separate cellulosic fibres to gather by di ssolving middle lamella. The separated fibres were washed with hot water and stained with Graff 'C' stain. Fibre length and width were measured as per BIS: 5285- 1998. The results are reported in Table I.

Proximate Chemical Analyses -Prox imate chemjcal analysis was carried out as per Tappi test methods. The results are reported in Table 2.

DUT[ el aL.: ELECTRICAL INSULATION PAPER 1147

Demineralized Water Analysis - Demineralized water was collected from D M water plant of Star Paper Mills Ltd, Saharanpur. Demineralized water is prepared by ion exchange resin method. In this process one tank is filled with resin of 'X ' type contains large number of organic molecules combined with acidic group (-COOH) . Ca++, Mg++, Na+ and K+

Table I - Morphological characteristics of Cannabis saliva hemp ribbons

SI No .

2

3

4

5

6

7

8

Particulars

Colou r

Mo isture, per cent

Di a of stalk, cm

Bast tibres , per cent

Central wood part, per cent

Bulk density, kglm3

Fibre length , mm

Min

M ax

Avg

Fibre width , I-lm

Min

Max

Avg

Hemp ribbons

Greenish white

30

1.5 to 4 .0

28.70

71.30

185

5.5

51.2

20.5

10.5

31.3

19.6

Table 2 - Proximate chemical analysis of Cannabis saliva hemp ribbons

SI No . Parti cul ars Hemp ribbons,

(per cent)

Cold water solu ble 3.94

2 Hot water soluble 8 .06

3 Alcohol-benzene soluble 1.20

4 One per cent NaOH soluble 29.0

5 Lignin 7.30

6 Cross and Bevan cellulose 6 1.2

7 Pentosan 6.30

8 Pentosan in Cross and Bevan cellulose 2.40

9 A-cellulose

10 Ash 5.30

II DP at maturity of hemp ribbons 9300

DP of early harves ted hemp ribbons 8746

DP of delayed harvested hemp ribbons 9345

ions present in the hard water are absorbed by these groups. After that stage water is passed through the bed of resin 'Y' type contains large number of organic molecules combined with NH40H particles containing base groups (-OH) . It absorbs negati ve ly charged ions like cr, S04--, etc. The purity of water can be checked by conductivity meter. Demineralized water is analyzed for conductivity, hardness, Ca++, Mg++, Na+, K+, cr, silicic acid, pH, oxygen demand and silica as per BIS :3025 Part 11-1983, 14-1984, 15-1984, 17-1984, 21-1983,24-1986,32- 1988,35-1988, 38-1989, 40-1991 and 45-1993. The results are reported in Table 3 and 4.

Pulping Studies - Hemp ribbons having moisture content about 10 to 12 per cent were hand-chopped to

Table 3 - Analysis of deminerali zed water for high tension cable paper and capaci tor ti ssue

SI No. Particulars Test Specified values values

Specific electrical 1.0 1.0 conductivity

2 Hardness, mg-equ/L traces Negli gib le

3 Na+ content, mglL traces Neglig ible

4 cr content, mglL 7.99 Neglig ible

5 Silicic acid content , mglL 45 50

6 pH 6.8 6.0-7.0

7 Oxygen consumed, mglL 1.0 Not over 1.0

8 Silica, kglL 0 .2

9 Chromacity on platinum- 15° Not cobalt scale mentioned

Table 4 - Analysis of demineralized water fo r te lephone cable paper

SI No. Particulars Test Specified values values

Suspended matter, mglL nil Nil

2 Hardness, mg-equ/L 1.8 2 .0

3 Na+ content, mg/L 10 Not over 10

4 pH 7 . 1 6.5-7 .5

5 Oxygen consumed, mglL 1. 8 Not over 1.0

6 Silica, kglL 0 .7

7 Chromacity on platinum- 20tl Not cobalt scale mentioned

1148 J SCI [NO RES VOL 62 DECEMBER 2003

length 40 to 50 rnm. Chopped ribbons were cooked by alkali oxygen delignification process in WEVERK rotary electrically heated digester of capacity 0.02 m

3•

Cooking conditions and cooking results are reported in Table 5.

Pulp Beating and Stock Preparation - The alkali-02 delignified was beaten to 45 DS R in laboratory WEVERK make digester at consistency of 1.57 per cent. After subsequent swelling of the stock, it was

Table 5 - Results of alkali-02 deJigni fication of Cannabis saliva hemp ribbons

SI No. NaO H, per cent (as NaOH) 9

O2 pressure, kg/cm2 " .J

2 MgS04, as carbohydrate stabilizer 0.1

3 Max pulping temp. "C 160

4 Time at max pulping temp, min 120

5 Liquor to wood ratio 5.0:]

6 Screened pulp yield, per cent 62 .&

7 Kappa number II

further beaten to 60 DSR. The beaten pulp was passed through WEVERK rotary screen having slot size 0.15 rnm in order to remove lumps. The pulp was treated with 0.1 per cent EDT A at 12 per cent consistency at 90 DC for 90 min and then washes with hot water properly to eliminate d block elements from the pulp . The pulp was treated with I per cent melamine formaldehyde to develop cross linkage, 0. 1 per cent wax emulsion, 1 per cent fortified rosin and a lum free rosin to get pH 6.0. Laboratory made hand-sheets of 75 and 90 g/m2 were prepared on Mesmer sheet former. The laboratory made hand-sheets were pressed at 8 kg/cm2 for 5 mjn and dried by sintering at 120 DC. It is important to note that high tension cable and capacitor papers were prepared by using demineralized water of specificati ons as lay down in Table 3. While te lephone cable paper was prepared by using deminera li zed water of specifi cations as lay down in Table 4 . Finally the laboratory made sheets were impregnated with polyi sobuty lene oi l to determine dielectric loss and power factor of paper.

Table 6 - Chemical, mechani cal and electrical properties of laboratory made capacitor and hi gh tension cable paper

SI No. Particulars

I Degree Schopper-Reigler

2 Basis weight, g/m2

3

4

5

6

7

8

Thickness, pm

Bulk density, g/cm3

Breaking length , m

Burst index, kPa M2/g

Ash content, per cent

Specific electri c conductivity of aqueous extract at 25 "c, ~l moh/cm

At modulus 1:50

At mod ulus 1:20

':) Potassium content in paper, mglkg

10 Sodium content in paper, mg/kg

I I Total content of iron and copper in paper, mg/kg

12 Chl oride content in paper mg/kg

13 Su lphate content in paper, mglkg

14 Moisture , per cent

15 Porosity Bendtsen, mUmin

16 Aqueous extract pH

17 Tangent angle of di electric losses at 120 "c, per cent

I 8 Power factor

19 Dielectri c constant

Capacitor and high tension cable paper

60

75 '

94.0 '

1.25'

7985

6.10

0.20

22

40

1.2

8.5

8.7

11.2

16

15

95

6.5

0.27

0.002

1.11

S peci ticat ions

75±2 per cent

1.1 8-1.26

At least 7000

Not over 0.30

23

46

Not over 2

Not over 10

Not over 10

Not over 10

Not over 15

25 ± 5 per cent

6.3 - 7.0

0.28

DUTT et al.: ELECTRI CAL INSU LATION PAPER 11 49

Table 7 - Chemical, mechanical and electrical properti es of laboratory made telephone cable paper

SI No. Particu lars

2

3

4

Degree Schopper-Reigler

Basis weight, glm2

Thickness, ~llll

Bulk density, g/cm·1

5 Breaking length , m

MD

CD

Avg

6 Burst index , kPa M2/g

7 Stretch, per cent

MD

CD

Avg

Telephone cable paper

60

90

125

0.74

7885

6.35

3.7

Spec ifications

90 ± 2 per cent

0.75 ± 0.05

10000

4000

7000

2.5

4.5

3.5

8 Specific electric conducti vit y of aqueous ex tract at 25°C, 11 moil/cill

At mod ulus 1:50

9 Tangcnt angle of diel ectri c losses at 50 <lC, per cent

10 Ash content, per cent

I I Air permeability, s/l 00 mL

Finall y the results Cl.re compared with standards and the results are reported in Table 6 and 7.

Results and Discussion

Table I shows the morphological characteri stics of Call/wbis sativa hemp ribbons. In full y matured plant , bast fibres constitute about 28.70 per cent part of the plant. The indi vidual fibers when complete are very long measuring from 5.5 to 5 1.2 mm with an average of 20.5 mm and moderately wide ranging fro m 10 .5 to 31.3 Illn with an average of 19.6 ILI11. The fi bres are moderately th in to thi ck wa lled, the lumen bei ng not uni fo rm in wid th , whi ch may equal to greater than the wall thickness. They do not exhibit any twist, but joints or compressed areas with transfer fractures. longitudinal stnatlons and somewhat swollen fis sures are conspicuous .

Tab le 2 shows the prox imate chemica l ana lysis of Canl!ahis sativa hemp ribbons. Water and alcohol­benzene soluble are very low in hemp ribbons. Hence the possi bili ty of impact of soluble on process equipments in terms of chocking of Fourdri ni er wire and press felt and pulp quantity and quality is very

18 20

0.22 0 24

0.38 Vcry low

372 300

low. Lignin in hemp ribbons is 7.30 per cent onl y; it means Cannabis sativa hemp ribbons require lesser cooking chemicals and shorter cooking cyc le. However, Cross and Bevan cellulose is 6 1.2 per cent. It is important to note that degree of polymeri zati on of Cannabis sativa hemp ribbons is greatl y influenced by harvesti ng time. In earl y harvesting degree of polymerizati on is determined to about 8746 by ligh t scattering measuring technique, whereas in delayed harvesting the degree of po lymeri zati on is about 9345. However the degree of polymeri zat ion in delayed harvesting is comparative ly more but the fibers are harsher and rigid in nature. On the other hand the degree of polymeri zation at matur ity is 9300 and fib res show texture and flex ibility.

Tab le 3 and 4 shows the analys is of demine­ra li zed water for hi gh tension cable pal er. capac it or ti ssue and telephone ca ble paper. All the parameter of deminera lized waler meets the specifications st ri ctl y except sili c ic acid which is s lightl y on hi gher <; ide. The deminera li zed water is used in pUlping, washing. refin ing, and sheet mak in g of high tens ion cab le paper, capacitor ti ssue and telephone cab le paper.

11 50 J SCI IND RES VOL 62 DECEMBER 20m

Tabl e 5 shows the results of a lka li oxygen de li gnificati on o f Cannabis saliva hemp ribbons. 9 pe r cent NaOH, 5 kg/cm2 O2 pressure, 0.1 per cent eposm s<l lt <l S carbohydrate st<l bilize r, max pu lping time 120 min at 160 DC and bath rati o I :5.0 were found optimum to produce pulp y ie ld 62.8 per cent at ka ppa numbe r I I.

Ta bl e 6 shows chemi cal , mec hanical and e lec­tri cal prope rti es of laboratory made capac itor and high tens ion cabl e pape r and co mparison with spec i­fi cati ons prescribed by BIS and GOST. The pulp is beaten to 60 °SR in o rde r to inc rease surface area for bonding and miti gate voids vo lume . The bulk density at 75 ± 2 pe r cent should be about 1. 18 to 1.26 g/cm, . A bulk dens ity o f 1.25 is obtained by beating the furnace to 60 °SR and pressing at 8 kg/cn{ The reaso n is that the entrapped gases in vo ids ioni ze below vo lt age break down and e lec tri ca l discharges occur. As h content should not over to 30 mg/kg of pa per. Wate r from hi gh humid environments diffuses in to the po lymer and inc reases the ionic conduc tion . At temperature above g lass-transiti on of the po lymers, the fl ex ibility of the polymer increases and cause ' the po lymer to re lax, wh ich results in g rea t ion mobility. In o rde r to miti gate d block e le ments and alkali ea rth meta ls in the form ash from raw mate rial s, pulp is treated eth ylene diamine tetra acetic ac id and washed w ith demine rali zed water. The spec ific conduc tivity o f aqueous ex trac t of pape r at 25 °C at modulus 1:50 and I :20 is 22 and 40 ~l moh/cm, respecti vely , compared to spec ifi ed values 23 and 46 ~l moh/cm respec tive ly. Potass ium, sodium, iron, and coppe r and sulphate contents are within the range o f specifi ed va lues except chloride content which is s lightly on hi ghe r s ide. The moi stu re content is 15 pe r cent again st spec ified value 25 ± 5 pe r cent and Bendtsen poros ity is 95 mLimin . It means that ioni zati on of gases in vo ids take p lace at s li ghtly hi ghe r temperature and accord ing ly vo ltage breakdown will increase. All these prope rties can be corre lated w ith tangent ang le of die lec tric losses at 120 °C, p wer fac to r and di e lec tri c constant which are observed to in the vic init y o f spec ified val ues i. e. 0.27 pe r cent , 0.002 and 1. 11 , respective ly , whereas, tangent ang le of di e lectri c loss at 50 DC is 0 .22 against 0.24 pe r cent. Ash content and a ir pe rmeability a re in cl ose prox imity to spec ifi ed va lues .

T able 7 shows the che mica l, mechanical and e lec tri cal prope rti es of laborato ry made te lephone ca bl e pape r. The bulk dens ity of pape r is 0.74 g/c m2

aga inst spec ified value of 75±0.05 at a bas is weight range of 40 ± 2 pe r cent. In te lepho ne cable pape r huge numbers o f thread s are iso lated with pape r, hence thickness, breaking leng th and s tretch a re the mos t important prope rties and all these prope rti es fulfill the spec ificati ons. The spec ific co nducti vity of aqueous extract of paper at 25 °C at modulu s I :50 is 18 pe r cent aga in st 20 pe r cent.

Conclusions

o A morphological characteris ti c o f Cann({bis sariv({ hemp shows that the ave rage fibre length is 20.5 mm and ave rage fibre width is 19.6 ~Ul1 . T he fib res are moderate ly thin to thic k wa lled the lumen be ing not uniform in width , w hi ch may equal to g reater than the wall thickness. They clo not ex hibit any twi st, but j o ints o r compressed a reas with transfe r frac tures, long itudin a l striati ons and somewhat swo llen fi ssures a re conspi cuous.

.. Wate r and a lcoh o l-benzene so lu ble are on lower side the refore hemp ribbon pulp poses less problems like, fe lt and wire ch oc king and pitch formation during paper formati on. T he bast fibres obtained afte r early ha rvesting shows sli ghtl y lower degree o f polymeriza ti on and bast fibres obtained afte r de layed harvesting a re harshe r and ri g id in nature . The bast fibres obtained at maturit y are flexibl e and shows good lusti el·.

• 9 pe r cent NaOH, 5 kg/cm2 0 1 pressure, 0. 1 pe r cent eposm salt as carbohydrate stabilize r, max pulping time 120 min at temp 160 (Ie and bath ratio I :5 .0 were found optimum to produce pulp yie ld 62.8 pe r cent at kappa numbe r I I.

• 0 .1 per cent EDT A at 12 pe r cent cons is tency and at 90 °C for 90 min dosing is fou nd suitabl e to e liminate d block e le ments from the pulp .

• In order to control dimensiona l in s tability, to preserve we t strength and to cont ro l mo isture I pe r cent me lamine fo rmalde hyde, O. I pe r cent wax e mul sion , I pe r cent fo rtifi ed ros in and iro n free a lum to get pH 6.0 are found sati s f<lc to ry to so lve the purpose.

• All the mechanical prope rties required fo r capac itor and hi gh tens ion cab le pape r are de ve loped at a freeness level of 60 °SR and press ing at 8 kg/c m2

. In orde r to deve lop optimum e lectrical prope rti es pape r is impregnated with po lyisobutylene o il to minimi ze d ie lec tri c loss and po we r facto r. All the che mi ca l, mec hani ca l and

Du n et a l. : ELECTRICAL INSULATION PAPER 1151

e lectrical properties are closely resembl ed with specifi cati ons .

• All the chemical, mechanical and e lectrical properties of laboratory made te lephone cable paper fulfill a ll the requirements of te lephone cable paper.

Recommendations

• If textil e waste, i .e ., conlber I S used for development o f e lec trical insulati on paper du sting

pri or to pulping is essenti a ll y required.

• In each batch Bauer McNett fibre c lass ificati on o f pulp is essentially required and based on the surface area of pulp chemical dos ing should be monitored . It will he lp to eradicaie vari ous

problems assoc iated with manufac turing process like press picking and dusting e tc and paper defects like, fuzziness due to loose bonding and dimensional instability.

References

Kirk- Orhmer. F:;lI cyciopediu of chemical t{'chllo lo/-:.I' . 3"1 ed (A John Wiley Sons Pu bli cati on, New YO I'k) L3, 19X I. p.536.

2 Swiss J & Dekin T W, Westill ghuuse E II I:{ , 14 ( 1954) I I·+'

3 Harper C A, Fundamentals of electri cal in sul at ion materials. Ind Res Dell, 20 ( 1978) 12.

4 Moses G L, In sula tion eng illeerillg /i llldwlIl'lI fa ls (Lake Publi shing Co. Lake Forest) III 1958, P 8.

5 Paschen F, A IIII Phys, 37( 1989) 69.