Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
_C GROUND_ F THE _NVENTION
This invention relates to a polyolefin composition for electrical
insulation which is far less vulnerable to the deterioration of insulation
that occurs as dendroid voids (referred to as "water trees" hereunder) in
polyolefin or crosslinked polyolefin employed as an electrical insulation,
particularly For high-tension power cables which are operated under water.
It has been known that water trees grow from voids and foreign mat-
ters, such as metals, in electrical insulation made of polyolefin or cross-
linked polyolefin composition when the insulation is subjected to a high
voltage under water. In spite of continued efforts for improvement of the
manufacturing process, the complete elimination of foreign matters and
voids is still difficult to achieve.
As a means for preventing water trees, incorporation of a small
quantity of strong electrolyte into polyolefin was proposed in Japanese
Patent Publication No. 24938/1977. ~n the other hand, there is a strong
theory that the preSence of metal ions is a cause of water trees.
The idea of adding a voltage stabilizer or alcohol to polyolefin
.was proposed several times as a common countermeasure for water trees. For
example, Japanese Patent Laid-open No. 16746/1978 teaches that the addition
of monohydric higher alcohol having 6 to 24 carbon atoms is effect;ve, How-
ever, such low-molecular weight alcohol is mobile through polyolefin mole-
cules and blooms easily to the surface of the insulation as the power cable
ge-ts hot, with the result being that the bloomed alcohol evaporates and the
e~ect of the alcohol is soon lost. Furthermore, Japanese Patents Laid-
open Nos. 119937/197~, 40547/1977, ~0553/1977~ and 40554/1977 describe sup-
plementarily the incorporation of a polyhydric alcohol into an electric in-
sulation which is rnade resistant by adding as an indispensable ingredient a
voltage stabilizer such as ferrocene, dihydronaphthalene, dialkoxynaphtha-
lene, phosphorane, and halogenated p-nitroaniline. These patents clairn that
the dielectric strength is further improved by the addition of a polyhydric
alcohol, but they treat the polyhydric alcohol as a mere additive similar -to
dispersants, surfactants, unsaturated polymers, etc. However, the signifi-
cance oF addiny a polyhydric alcohol is barely recognized in the examples,
and the polyhydric alcohols rnentioned in these patents broadly vary in their
molecular weight. What is given great importance in these patents is not
water trees which readily occur in an insulation placed under water, but
rather the conventional treeing that is attributable to high voltage alone,
no-t associated with the presence of water.
Another countermeasure for water treeing disclosed in Japanese
Patent Laid-open No. 145799/1977 covers the addition of a large quan-t-ity of
inorganic filler. In this patent, the addition of inorganic filler together
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with diethylene glycol was practiced in some exarnples, However, the
reason why it was added is not described. Examples without diethylene
glycol were rather superior to examples with this substance. Japanese
Patent Laid-open No. 3700/1978 discloses the incorporation of a water
tree inhibitor. The example of this patent illustrates a composition to
which polyethylene glycol distearate is added in small quantities. However,
the effect of this substance as a water tree inhibitor is just as mediocre
as the effect of a metal salt of stearic acid. This Laid-open patent
enumerates fatty acid, fatty acid metal salt, fatty acid ester, etc. in the
same category. This Fact suggests that fatty acid is an indispensable moi-
ety which must have a group that can catch water readily. Such a group,
however, is not identified in the specification, and, needless to say,
nothing is mentioned about polyethylene glycol.
Japanese Patent Laid~open No~ 60346/1979 discloses a modifier which
is very effective for a crosslinked polyethylene composition. Such a modi-
fier should be a compound of less than 24 carbons containing in the molecule
both a carboxyl group or its metal salt with a high affinity for foreign
matters such as metals and a double bond that forms chemical bonding with
the polyolefin.
After a series of experiments on the prevention of water treeing, it
was concluded that, contrary to the conventional teachings, a small quanti-
ty o-~ high-molecular weight polyethylene glycol is all that is required,
~nd no voltage stabilizer nor carboxylic acid or its salt is necessary. As
pointed out before, the prior art teaches the use of ethylene glycol or di-
ethylene glycol as an adjuvant For a voltage stabilizer. Other prior art
discloses polyethylene glycol end-blocked by stearic acid as a water tree
inhibitor. ~hat differentiates the present invention from the prior art
is that the problem can be solved simply by adding a small quantity of
high-molecular weight polyethylene glycol, which is one of the cheapest
compounds, to polyolefin.
One of the above-mentioned items of prior art maintains that a poly-
hydric alcohol is effective secondarily. In this prior art, however, poly-
ethylene glycol is treated as the same category as monomeric ethylene gly-
col, propylene glycol, butane diol, etc. and alicyclic and aromatic poly-
hydruxyl compounds. In contrast with these teachings, the present invention
discloses that only high-molecular weight polyethylene glycol is effective
arld polypropylene glycol is not. Accordingly, it is believed that high-
mol~cular weight polyethylene glycol is quite different in its functional
mechanism from polyhydric alcohols which have hereto-fore been used second-
arily in combination with a voltage stabilizer and the like.
SUMMARY OF THE INVENT~ON
This invention relates to a polyolefin composition for electrical
insulatjon which comprises polyolefin or crosslinked polyolefin and a
small quantity o~ high-rnolecular weight polyethylene glycol. This cornpo-
sition is far less vulnerable to the deterioration that occurs as den-
droid voids when employed as an electrical insulation of high-tension pow-
er cables which are operated under water. The polyethylene glycol has
more than 44 carbon atoms and is used in an amount of from 0.3 to 10 wt%,
based on the polyolefin or crosslinked polyolefin.
RIEF 3ESCRIPTION OF THE DRA~INGS
Figure 1 is a view of the test specimen.
Figure 2 is an enlarged view of an inverted conical hole in the test
specimen .
Figure 3 is a perspective view of the test apparatus.
Figure 4 is an enlarged view of a water tree in a test specimen.
DETAILED DESCRIPTION OF THE INVENTION
The composition according to this invention is especially effective
For preventing water trees which appear in an insulation immersed in water
when deterioration takes place under high-tension. ~Water trees are differ-
ent frorn cornmon electric trees not only in appearance, but also in the fact
that the former occur only when a high-tension is applied to an insulation
llnder water.) The composition of this invention will find applications in
electrical insulating rnaterials to be used under water. Such applications
-include power cable insulation, insulating tape for cable splicing, and
other electric and electronic parts which might be exposed to water or
w~ter vapor.
We will discuss how water -trees are different from electric trees
which have so Far been terrned simply trees , and chemical trees which
occur under spec-ific environments. ~ater trees occur when an organic poly-
meric insulating material like a polyolefin is subjected to an electric
field under water for a long time. Water trees were named from their den-
droid configuration they usually assume. Actually, they are fine cavities
or voids, which look white when filled with water but which are almost in-
visible when dry. Therefore, specimens should be boiled in water or dyed
-ror observation of water trees. On the other hand,electric trees which
are referred to conventionally as trees grow and propagate along the
pllh o~ electric discharge, leaving carboni2ed resin therein. This is the
reason why they usllally look black. Meanwhile, chemical trees stem From
copper sulFide and copper oxide which are formed on a copper conductor by
reaction of copper with hydrogen sulfide and other reactive gases which
have diffused to copper conductor through the resin insulation. These
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reaction products grow into black or reddish brown needle crystals that
look like trees. Chemical trees are characterized by ~he fact that they
occur even under tension-free conditions.
The polyolefin as used in this invention includes polyethylene,
polypropylene, ethylene-propylene copolymers, polybutene, polyisobutylene,
poly-4-methylpentene, copolymers thereof~ and crosslinked products ther-
of. High-molecular weight polyethylene glycol means polyethylene glycol
having more than 44 carbon atoms, preferably more than 80 carbon atoms.
The polyethylene glycol is incorporated in an amount of 0.3 to 10 % by
weight, based on the polyolefin or crosslinked polyolefin. Under 0.3 %,
no satisfactory effect is attained, particularly in the case where poly-
ethylene glycol of relatively low molecular weight is employed. On the
other hand, when over 10 % is used, some difficulties are encountered in
achieving uniform compounding without slight discoloration in the result-
ing composition, although the inexpensive polyethylene glycol has no eco-
nomic limitation in the quantity to be added.
Whether or not the polyolefin is crosslinked does not matter in
this invention, and the use of stabilizers~ inorganic fillers, and other
additives is within the scope of this invention.
EXAMPLES
The polyolefin used as the major constituent of the composition
as selected From low-density polyethylene and ethylene-vinyl acetate co-
polymer which are most popular as electrical insulating materials. High-
molecular weight polyethylene glycol which characterizes this invention
was selectrd from a series ranging From 1,000 to 20,000 in the molecular
weight. It was confirrned that the present invention can be practiced
For polyolefin with or without crosslinking. The results obtained are
$hown in Table 1. ~lso shown in Table 1~ for comparison, are the results
of Comparative Examples that follow.
Comparative Example 1 employed no polyethylene glycol and water
trees were observed at all the measuring points. In Comparative Example
2, 0.25 parts by weight of polyethylene glycol having a molecular weight
of 1000 was incorporated into 100 parts by weight of polyethylene. Water
trees were observed at 25 % of the measuring points.
In Example 1, the occurrence of water trees was reduced to zero by
adding 0.5 parts by weight of polyethylene glycol having a moleclllar weight
oP 1000 to 100 parts by weight of crosslinked polyethylene. Good resul-ts
werr also obtained in Example 2, in which the major constituent was ther-
rnoplastic polyethylene. Examples 3 to 6 were intended to determine how
the molecular weight and quantity of polyethylene glycol affect the occur-
rence of water trees. It was found that the higher the molecular weight
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andthe larger the quantity added, the better the results, although the
effect levels off beyond 0.5 %,
Tests methods employed in the Exarnples are as follows:
(1) Preparation of test specimens: Tray-shaped specimens, each having
twelve inverted conical holes on the botton, were cornpression molded from
polyolefin compositjons into which specified quantities of polyethylene
glycol had been incorporated by kneading. The section and dimensions of the
specimen are shown in Fig. 1. An enlarged view of an inverted conical hole and
dimensions of the hole are shown in Fig. 2.
(2) Test apparatus: The tray-shaped specimen was filled with O.Ol N sod-
ium chloride solution~ and the bottom of the specimen was placed in O.Ol N
sodium chloride solution. Voltage (as specified below) was applied to the
saline solutions inside and outside the specimen through platinum wires.
The perspective view of the apparatus is shown in Fig. 2, wherein 1 is a
transparent test vessel, 2 is a wire leading to high voltage power source,
3 is a polyethylene lid, 4 is a transparent container, 5 is a platinum
wire electrode, 6 is a specimen, 7 is an earthing wire, and 8 is an
electrolyte.
(3) Occurrence and observation of trees: The presence or absence of
2~ water trees that had grown from the apex of the inverted conical hole in
the specirnen after application of an alternating voltage, 5 kV and 5 kHz,
For ~8 hours, was examined and the lerlgth of the tree was measured using
an optical microscope. In order to facilitate observation, the specimen
was sliced into thin chips near the apex of the inverted conical hole
along the conical axis, and the sliced chips were dyed by boiling in a
methylene blue aqueous solution. Figure 4 schematically shows an enlarged
Yiew of a water tree as represented by 9.
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