CA1110055A - Voltage stabilized solid polyolefin dielectric composition - Google Patents

Abstract

VOLTAGE STABILIZED SOLID POLYOLEFIN DIELECTRIC COMPOSITION

ABSTRACT OF THE DISCLOSURE:

A voltage stabilized solid polyolefin dielectric com-position is disclosed. The dielectric composition comprises a polyolefin containing a voltage stabilizing amount of an additive selected from the class of dyes identified in the Color Index as being of the solvent type possessing either an azo or a quinoid type structure, or having a xanthene structure exhibiting fluo-rescence, The solvent dyes must be soluble in or melt miscible with the polyolefin. The solvent dyes must also be stable at the processing temperature and under the conditions of use.

Description

~ss This invention relates to additives for improving the voltage stability of polyolefin based dielectric compositions for use at high voltages.
It has been repeatedly observed that polyolefinic insu-lation of high voltage cables often fails as a result of prefe-rentially conductive paths being formed in the insulation. These conductive paths grow with time and electrical stress and ultima-tely bridge the high voltage conductors to ground. These conduc-tive paths are often referred to as "trees" because their physi-cal shape often resembles microscopic trees. Such trees are oftenassociated with defects (voids or inclusions) in the insulation.
It has been repeatedly demonstrated that voids, inclusions or other defects initiate tree growth and, as a result, a series of tests for resistance to tree growth have been based on introdu-cing "standard defects" into the insulation (c.f. "An Accelerated Screening Test for Polyethylene High Voltage Insulation", AIEE
Transactions Paper No. 62-54 (1962) by D.W. Kitchin and O.S.
Pratt, and "Laboratory Studies of Treeing in Solid Dielectrics and Voltage Stabilization of Polyethylene", IEEE Conference Re-cord of 1976, IEEE International Symposium on Electrical Insula-tion, Montreal, June 14-16, 1976, p. 213, IEEE Publication No.
76 CHl 088-4-El, by A.C. Ashcraft, R.M. Eichhorn and R.G. Shaw).
These test methods have been shown to correlate well with life tests on activated cables and consequently give a suitable and quick method of screening modified insulations for "treeing re-sistance". As a consequence of test methods such as these, a number of materials have been discovered for use as additive agents for increasing the resistance to treeing of the electri-cally insulating resins, and a number of patents have been issued in this area. ~ypical examples of such patents are U.S. Patent No. 3,522,183 issued July 28, 1970 to L.J. Heidt, U.S. Patent ~Jo.
3,632,680 issued January 4, 1972 to G.H. Hunt et al., U S. Patent - 1 - ,.~

l~lW55 l~o. 3,346,500 issued October 10, 1967 to G.H. Hunt and Canadian Patent No. 875,024 issued July 6, 1971 to G.EI. Hunt, covering the use of such materials as halogenated polycyclic aromatic com-pounds and substituted aromatic hydrocarbon compounds, particular-ly the nitro-substituted aromatic hydrocarbon compounds; U.S. Pa-tent No. 3,577,346 issued May 4, 1971 to J.J. McKeown covering the use of organo metallic compounds; U.S. Patent No. 3,499,791 issued March 10, 1970 to D.E. Maloney covering the use of quater-nary ammonium salts and Canadian Patent No. 919,331 issued January 16, 1973 to F. Wuerstlin et al. covering the use of various sub-stituted anilines. Many of these compounds have problems asso-ciated with their use as voltage stabilizers, such as: being in-jurious to health and/or explosive; causing degradation of other polymer characteristics such as flammability, mechanical proper-ties, electrical loss factor etc.; and/or being difficult to pro-cure and use. Consequently, other voltage stabilizers without these and other disadvantages have been sought with the capability of being used in grades of polyolefins used commercially for the insulation of high voltage cables.
Applicant has surprisingly found that certain classes of dyes, which are not normally recommended for coloring polyole-fins and for which no enhancement of electrical properties was previously known, can effect substantial improvements in the re-sistance of polyolefins to treeing. The classes of dyes which have been found to be effective as voltage stabilizers are iden-tified in the Color Index as being of the solven~ type and possess either an azo or a quinoid type structure, or have a xanthene structure exhibiting fluorescence. The solvent dyes must be solu-ble or melt miscible with the polyolefin. The solvent type dyes must also be stable at the processing temperature and under the conditions of use.

Some members of the azo dyes which have been found par-~llOC~S5 ticularly good are the monoazo solvent yellow 2 and 56 identified in the Color Index as Nos. 11020 and 11021, respectively, the monoazo solvent orange 7 (C.I. No. 12140) and the diazo solvent red 19 and 26 (C.I. Nos. 26050 and 26120 respectively). Other suitable members of the azo family are: the solvent yellow mono-azo dyes identified by the Color I,ndex Nos. 11000, 11129, 11160, 11350, 11380, 11390, 11800, 11810, 11830,11840, 11850, 11855,11860, 12055, 12700, 12740, 13900:1, 14070 and 18690; the solvent yellow diazo dyes identified by the Color Index Nos. 20010, 21230 and 21240; the solvent orange monoazo dyes identified by the color Index Nos. 11005, 11270:1, 11320:1, 11700, 11920, 12100, 12125, 18745:1 and 18736:1; the solvent orange diazo dyes identified by the Color Index Nos. 20020, 26020 and 26075; the solvent orange azo dye identified by the Color Index No. 26080; the solvent red monoazo dyes identified by the Color Index Nos. 11215, 11385, 12005, 12010, 12150, 12155, 12156, 12159, 12170, and 12715; the solvent red diazo dyes identified by the Color Index Nos. 21250, 21264, 26100, 26105, 26110, 26125, 26030, 26766, 26705 and 27306;
the solvent brown monoazo dyes identified by the Color Index Nos.
11285, 11330, 11360, 12000 and 12020; the solvent brown diazo dyes identified by the Color Index Nos. 21000:1 and 21010:1, and the solvent black diazo dyes identified by the Color Index ~los.
26040 and 26150.
A member of the quinoid dyes which has shown good re-sults is the anthraquinone solvent dye violet 13 identified in the Color Index as No. 60725. Other suitable members of the same family are: the anthraquinone solvent yellow 100, solvent orange 55, solvent orange 64, solvent orange 65 and solvent orange 66;
the anthraquinone solvent red 52 (Color Index No. 68210), solvent violet 11 (Color Index No. 61100), solvent violet 12 (Color Index a~ss No. 61105), solvent violet 14 (Color Index No. 61705), solvent violet 26 (Color Index No. 62015), solvent blue 11 (Color Index No. 61525), solvent blue 12 (Color Index No. 62100), solvent blue 14 (Color Index No. 61555), solvent blue 18 (Color Index No.
64500), solvent blue 26 (Color Index No. 61561), solvent blue 63 (Color Index No. 61520, solvent b~ue 68 (Color Index No. 61110), solvent blue 69 (Color Index No. 62500), solvent blue 78 (Color Index No. 61500), and solvent green 3 (Color Index No. 61565).
The xanthene dyes exhibiting fluorescence which have been found suitable are, for example, the xanthene solvent green 4 identified by the Color Index No. 45550, and the xanthene dye identified by the Color Index No. 45555.
The dyes are present in a voltage stabilizing amount which is preferably from about 0.1 to about 10~ by weight based upon the weight of the polyolefin. The solid dielectric composi-tion may also contain other conventional additives used in elec-trical insulation materials to efect crosslinking of the polyole-fin during processing or to otherwise render the material more suitable for use in cables,such as thermal degradation stabili-zers, light stabilizers, carbon black and other pigments or fil-lers.
Before proceeding further with the description, let us mention that the test method used by Ashcraft, A.C., Eichhorn, R.M. and Shaw, R.G., as reported in pages 213-218 of the IEEE
Conf. Record of the 1976 IEEE International Symposium on Electri-cal Insulation identified above, has been used by the applicant for all the voltage stabilization testsreported in the description with one modification. The "Standard Defects" used by Ashcraft et al. were made by specially sharpening No. 7 Sharp sewing needles, whereas the present tests have employed commercially available extra sharp needles fabricated by the Ogura Jewel Company of To-kyo, Japan and identified by them as part No. X-253-3. Using these needles and the method of Ashcraft et al., a double needle characteristic voltage (DNCV) of 12.4 + 1.0 kV has been obtained for a low density polyethylene having a specific gr~vity OL 0 . 92 and a melt index of 0.2 (this polyethylene is identified as type DY~K-2 in the above paper and in the following description).
The solvent dye additive, is incorporated into the base resin while the resin is heated to a temperature above the melting point of the base resin. The dye must be soluble in or at least melt miscible with the base resin at that temperature in order to improve the voltage stabilization. Dyes which are not soluble or melt miscible with the polyethylene do not offer the same degree of improvement in voltage stability. Indeed, the diazo pigment identified as pigment yellow 13 (Color Index No. 21100) and having the chemical formula:

H C.OH Cl Cl COH H

H3C- ~ N.C-C-N=N

which is a recommended coloring vehicle for polyolefins does not dissolve to any appreciable extent in polyethylene and does not melt at or below normal polyethylene processing temperatures.
When mixed into the polyethylene melt, the pigment remains, for the most part, as a discrete second phase even at the low concen-tration of 0.1 wt~. The double needle characteristic voltage of a DYNK-2 polyethylene containing this loading (0.1 wt~) of pigment yellow 13 was determined to be 13.0 + 0.7 kV which is substantial-ly the same as the value obtained for the base resin. The same loadiny of the melt miscihle azo dye ide~tifiedas solvent yellow 56, Color Index No. 11021, however, produced DNCV values of 16.1 +
1.7 kV and of 14.4 + 1.4 kV in two separate determinations. These lllQQ55 values are significantly higher than that of the base resin in spite of the very low loading of dye present. The importance of solubility in or melt miscibility with the base resin has also been observed when using quinoid type dyes. Quinoid vat dyes, for example, do not effect the same improvements in voltage stability.
Applicant has found, for example,,tnat a dispersion in DYNK-2 polyethylene of vat orange 3 (Color Index 59,300) having the following formula:

0 B~

which contains the quinoid grouping actually caused a decrease in the D~CV to 10.9 + 0~3 kV. A similar result was reported by Ash-craft et al. for flavanthrone, also known as yat yellow 1, and having the chemical structure:

~ N

and for violanthrone, having the structure:

Similar limitations are expected for the quinoid pigments, such as pigment red 122, which is a linear quinacridone of the general formula:

~lO~S5 ~ o R ~ R where R represe~ts alkyl grOups.

This material does not appear to be soluble in or melt miscible with polyethylene, but nevertheless is a good and recom-mended coloring agent.
The solvent dye additive may be incorporated into the base resin using any conventional mixing and blending apparatus such as twin-roll mills, Banbury mixers, or compounding extru-ders, provided that such apparatus produce an homogeneous melt.
In one method used by the applicant for making samples for tes-ting, the solvent dye was compounded into a base resin by adding the requisite weight of the additive to the base resin which was being heated in a twin-roll mill to a temperature just above the melting point of the base resin and then continuing to mill the mix until a uniform color was achieved. The correct incorporation of the dye into the base resin can be determined by examining the surface or a section of the processed resin optically, at a magni-fication of lOOX, by means of a microscope. A uniform colour, without the presence of discernible areas of high coloration, at this magnification, can be taken as indicative of suitable melt visibility and mixing. Should discrete particles or highly colo-red areas remain visible under such an examination, then addition-nal processing, higher processing temperatures or an incompatible system are indicated.
EXAMPLES
-Test samples of DYNK-2 polyethylene having a melting point of approx. 110C and containing 0.5 wt% of various azo dyes as voltage stabilizers were made using a twin-roll mill heated )OS5 to 160C. The samples were prepared from the material by hot pressing in a suitably shaped mold. The values recorded for the double needle characteristic voltage of such samples are given in the following Table I:

, lS~ )SS

~i P~
3 ~G~ ~,~ ~ u~
U~ Z~1 ~ O O O
o ~+l +l +l +l +l , J~ u~
,,~ , . . . . .
~ ~ O ~ 1-,.~ ~ ~ ~ ,~, . æ ~
.~ ~1 .
. ~;O ~0 ~ ~ r~
a~
5~ ~ ~ ~
~ , _ .--1 N U Z ~
~i _ _, 1~ ~ 11 ll o z z _~ Z fr æ
. Hl ~ ~ ~ T ~D ~,~

" æ æ ~ z Z z , o o E~ O O O N N
N N N t~ (~
f ~ ~ ~ .~ .

O
~ Z O ~ O O O
O ~I ~ ~r ,1 ~ ~`I
~I X o o ~1 O r~l O '~U ~ ~ - ~ ~D ~9 ~ ~ ~ ~ ~ '~`J f~
H
.

o ~o x ~,~
~ o~I, ~ ~ ~ ~u c 'v~ ~ o ~
H ~J J_) S~ ~~ '~L~ ~ f~ ~flJ
O ~ ~ ~ ~
O ~1, O O O O O
C.) ~ U~ U~ U~ U~ U~

~l~ooss In all cases, a substantial improvement of the voltage necessary to initiate treeing is observed as compared to a DYNK-2 polyethylene without a stabilizer which was used as a control and which has been found to have a double needle characteristic vol-tage (DNCV) of 12.4 + 1 kV.
Similar improvements in DNCV's have been observed for additions of quinoid type dyes and dyes having a xanthene struc-ture exhibiting fluorescence and meeting the criteria of solubi-lity in and/or melt miscibility with the base resin. Some exam-ples of observed DNCV's for dyes in these categories are given inthe following Table II:

111~55 i ~ o (n ~ ~ a) U~ i a) ~ ~ ~

_-- ~D ~ G~
~ o ~ +l Z G' ~ ~

1~ Q ~1 ~ ~`I
(~ o~~ 1s~ U~ 11~
Q 3~o O
U _ ~ ~i z ~ z Q ~ ~ P~ ~ ~
_ . ~ .
P~ U ~ ~ S~ ~ ~ ~ 0 o n ~ ~ ~r ~ (`l ~I Q.-IJ u~.-l ,_~
H
~ ~ ~C
r~E Z o~

~'~ ~ Z~_ ~
'~ ~ o~ o U
~U ~

. o oZ In O
H O Lt') _.
03 ~ I ~r x ~ ~ o a)_. I
~ O P~ .~ ~ ~
H 1~ ~J a) 0 ~ > ~ ~ S~
U~ ~1 ~1 ~ IIS
O ~ O O O U~
U ~ I tr) U~ U~-- I
! .

Solvent red 138 listed in Table II has a melting point of 252C which is higher than the processiny temperature (160C) and when the sample was examined under the microscope, a non-homogeneous colour could be seen although some solution of the dye in the base resin had obviously occurred. This explains the lower DNCV (16.4 + 0.6) obtained for this material. This is a good example of a material of a suitable chemical type which has been incorporated into the base resin at a temperature which is lower than the critical temperature.
The amounts of the above mentioned solvent dyes, of the azo, quinoid and xanthene types, which should be incorporated into polyolefins in order to effect improvements in resistance to tree-ing are governed by different considerations controlling the low and high limits. The lower limit of addition of the voltage sta-bilizing additive is controlled by the level at which any voltage stabilization effect can be seen. This is typically at 0.1 wt%
of additive. The upper limit is controlled by economic conside-rations and by the effect large additions of voltage stabilizers might have on other important chemical, physical or electrical properties of the insulation. The upper level limit thus changes with the nature of the application and the required degree of stabilization. Weight percentages over 10 wt~ would be excessive for many applications. From the above, it can be seen that the amount of the stabilizing agent is preferably within the range of about 0.1 to about 10 wt~, most preferably 0.1 to 2 wt~, although not necessarily restricted to this range.
Although the invention has been disclosed with reference to polyethylene, it is to be understood that other polyolefin re-sins suitable for making solid dielectric compositions are also envisaged. Furthermore, the invention is not limited to the use of the additives given in Tables I and II as non-limitative spe-cific examples of suitable stabilizing dyes according to the pre-~OQ55 sent invention, but extended to all additives claimed in the ap-pended claims.

Claims (8)

What is claimed is:

1. An electrical insulating composition containing a polyolefin and an additive for improving the voltage stability of the electrical insulating composition selected from the class of dyes identified in the Color Index as being of the solvent type possessing either an azo or a quinoid type structure, or having a xanthene structure exhibiting fluorescence, which dyes are soluble in or melt miscible with the polyolefin.

2. An electrical insulating composition as defined in claim 1, wherein the polyolefin is polyethylene.

3. An electrical insulating composition as defined in claims 1 or 2, wherein the additive is a solvent dye possessing an azo type structure identified in the Color Index as Nos. 11021, 11020, 12140, 26120 or 26050.

4. An electrical insulating composition as defined in claims 1 or 2, wherein the additive is a solvent dye possessing a guinoid type structure identified in the Color Index as No.
60725.

5. An electrical insulating composition as defined in claims 1 or 2, wherein the additive is a solvent dye possessing a xanthene type structure exhibiting fluorescence identified in the Color Index as No. 45550.

6. An electrical insulating composition as defined in claims 1 or 2, wherein the dye is present in an amount of from about 0.1 to about 10% by weight based upon the weight of the polyolefin.

7. An electrical insulating composition as defined in claims 1 or 2, wherein the dye is present in an amount of from 0.1 to 2% by weight based upon the weight of the polyolefin.

8. An electrical insulating composition as defined in claims 1 or 2, which comprises other conventional additives used in electrical insulation materials.