Electric insulation with a silicic acid additive made by a melt process

The invention relates to electric insulation with a polyolefin base for medium and high voltage from about 10 kV up. In insulation of this type the formation of so-called water trees is to be prevented on the one hand and on the other hand, the dielectric losses are to be kept as low as possible. For this purpose it is provided that silicic acid prepared in the following manner is added to the insulation: SiO.sub.2 -containing mineral substances are melted with additions of aluminum oxide, boron oxide and carbonates and/or oxides of alkali and earth-alkali metals to form a homogeneous glass phase; from the glass melt, glass bodies with a large surface are made; the glass bodies are subjected to a leaching process with mineral acid; the porous SiO.sub.2 bodies produced are washed and dried and optionally milled. The electric insulation according to the invention is particularly well suited for use in cables and wires.

BACKGROUND OF THE INVENTION 
The invention relates to electric insulation for use especially in cables 
and wires for medium and high voltage from about a minimum of 10kV. The 
insulation is based upon polyolefin with an additive of 0.05 to 10% by 
weight (with a particle size of up to 50 .mu.m or an agglomerate size of 
up to 100 .mu.m) which retards formation of water trees. 
In electrically stressed pololefin insulation, processes can occur which 
are called "electrochemical treeing" (ECT) or "water treeing". These 
processes, which are serious problems particularly in respect to the 
operating safety of plastic-insulated medium and high-voltage cables lead 
to the formation of tree-like structures, the so-called ECT structures. 
The optical appearance of ECT structures which are visible with 
particularly high contrast and detail after suitable dyeing is manifold. 
Fundamentally, two forms can be distinguished: 
"vented trees" which start from the surface of the insulation and extend 
into the insulation, and 
"bow-tie trees" which are generated in the interior of the insulation. 
The mechanism of the ECT formation has not been clarified to date. It is 
generally assumed, however, that an electric field and the presence of a 
polar liquid, especially water, are required for the formation of the ECT 
structures; the ECT structures are therefore also called water trees. The 
initiation points of the water trees always seem to be faults such as 
impurities, aggregate admixtures, voids, gaps, cracks or boundary 
surfaces, of which, however, only a part leads to the formation of water 
trees. The faults cannot be avoided completely in the case of insulation 
produced in a large technical scale. From these faults, the tree-like 
structures extend in the direction of the electric field. 
Since ECT structures represent local changes of the insulating material, 
they can cause damage to the insulation especially with respect to the 
dielectric strength. Numerous attempts have therefore already been 
undertaken to prevent or at least retard the growth of water trees. In 
particular, additives to the insulation layer have been made. 
Suitable additives with which the ECT formation can be prevented 
effectively and permanently are in particular: 
barbituric acid and 2-thiobarbituric acid as well as derivatives thereof 
(DE-OS No. 32 02 828 or U.S. Pat. No. 4,458,044, 
water-soluble alkali and earth-alkali phosphates as well as hydrolyzable 
phosphoric-acid esters (DE-OS No. 32 02 896 or U.S. Pat. No. 4,581,290, 
substances with a certain particle or agglomerate size which are 
adsorption-active for heavy-metal ions or bind them in an ion exchange 
(DE-OS No. 33 18 988 or U.S. Pat. No. 4,623,755), 
alcoholates of magnesium, calcium and aluminum (DE-OS No. 33 21 268 or U.S. 
Pat. No. 4,574,111), 
potassium and sodium stannate as well as titanium oxysulfate (DE-OS No. 35 
03 998), and 
derivatives of pyrimidine and hexahydropyrimidine (DE-OS No. 35 16 971). 
From DE-OS No. 33 18 988 or U.S. Pat. No. 4,623,755, cited above, it is 
known to use in a polyolefin base electric insulation as an additive for 
retarding water trees, a homogeneous distribution of 0.05 to 10% by weight 
of a substance which is adsorption-active for heavy metal ions or binds 
heavy metal ions in an ion exchange, the substance having a particle size 
of up to 50 .mu.m or an agglomerate size of up to 100 .mu.m. As additives 
can be used here aluminum oxide and oxide hydrates with a large active 
surface and/or aluminum silicates; preferably, however, the electric 
insulation contains pyrogenic and/or precipitated silicic acid. The 
additives are preferably synthetic products; however, minerals found in 
nature can also be used. 
Although additives of the above-mentioned type have been found to be 
effective for retarding water trees, electric insulation, particularly 
cable and wire insulation, must exhibit a number of further important 
properties for practical employment. Among them are particularly 
thermally-oxidative stability, sufficient mechanical strength, low 
dielectric losses, optical transparency and storage stability as a 
processable insulating compound. Special attention must be given to the 
dielectric losses because they can be increased if additives are present 
in the insulation. 
It is an object of the invention to further develop electric insulation of 
the type mentioned at the outset in such a manner that it meets the 
requirements of practice in every respect, ECT-retarding effectiveness and 
low dielectric losses being sought in particular. 
SUMMARY OF THE INVENTION 
These and other objects are achieved according to the invention, by an 
electrical insulation composed of a polyolefin base and a silicic acid 
additive which prevents or retards water-treeing, wherein the silicic acid 
is prepared in the following manner: 
(a) melting mineral substances containing SiO.sub.2 with additives of 
aluminum oxide, boron oxide and carbonates and/or oxides of alkali and 
earth alkali metals to form a homogeneous glass phase, 
(b) preparing from the glass melt particulate glass bodies with a large 
surface area, i.e., finely divided particulate, 
(c) leaching the glass bodies with mineral acid, and 
(d) washing and drying as well as optionally milling the porous SiO.sub.2 
bodies formed.

DETAILED DESCRIPTION OF THE INVENTION 
Silicic acid (chemical composition: SiO.sub.2) prepared in the foregoing 
manner is characterized by the feature that it is not crystalline but 
amorphous to X-rays. In addition, it is highly purified. This silicic acid 
is used in the electric insulation according to the invention as an 
extremely small (micro) size of particulate i.e., finely powdered form 
which is obtained if required by milling coarser-grained SiO.sub.2 bodies. 
Silicic acid of the above-mentioned kind is known per se (DE-OS No. 33 23 
844). It serves as a filler for plastics which are used for enclosing or 
covering integrated semiconductor circuits. For this purpose, however, it 
is essential that the filler, i.e., the silicic acid exhibits a low alpha 
particle radiation activity. To this end, it is necessary to use a silicic 
acid which is free of impurities, in particular such as the elements 
thorium and uranium which are responsible for the alpha emission. 
It was therefore surprising and could also not be foreseen that electric 
insulation with such silicic acid as an additive such as the electric 
insulation according to the invention represents, but only has excellent 
ECT-retarding effectiveness but also low dielectric losses. Such electric 
insulation is therefore particularly well suited for practical employment. 
The silicic acid used in the electric insulation according to the invention 
is prepared from SiO.sub.2 containing mineral starting materials; this 
purpose is served particularly by quartz sand. The starting material is 
converted here, together with the additives, into a homogeneous glass 
phase by melting (temperature: about 1100.degree. to 1500.degree. C.), 
where the SiO.sub.2 content in the glass melt is preferably adjusted to a 
value between 50 and 70% by weight. As additives are used, besides 
aluminum oxide and boron oxide, preferably the oxides or carbonates of 
sodium, potassium, magnesium and/or calcium. From the glass melt, 
particulate glass bodies having in a composite sense a large surface area, 
i.e., finely divided, are then prepared. This can either be done in such a 
manner that from the glass melt, glass fibers with uniform diameter of 
less than 100 microns are drawn or that the solidified glass melt is 
comminuted and milled to form grains of defined grain size. The glass 
bodies are subsequently subjected to a leaching process in mineral acid, 
for which purpose preferably hydrochloric acid heated to at least 
95.degree. C. serves (3 n to 6 n; duration: 4 to 96 hours). In the 
process, the impurities and the additives are dissolved from the glass 
matrix, whereby an amorphous and high-purity particulate silicic acid 
(SiO.sub.2) is produced. This silicic acid is then washed until free of 
acid and is dried (temperature: about 150.degree. C.) and if required, is 
finally milled so that it has the desired particle size. 
As already explained, the silicic acid content in the electric insulation 
is about 0.05 to 10% by weight, according to the invention, relative to 
the total weight (of the insulation), and the particle size of the silicic 
acid is up to 50 microns. Advantageously, the silicic acid content is 0.1 
to 4% by weight and preferably 0.5 to 2% by weight. The particle size of 
the silicic acid is preferably up to 20 microns. The silicic acid is added 
to the insulating material. In cables and wires, the silicic acid can be 
added, besides the insulting layer proper, also the field-limiting layers, 
i.e., to the inner and/or outer semiconducting layers. 
In addition to cables and wires, the electric insulation according to the 
invention can also be used in bushings and fittings. 
In the insulating composition of the invention, polyolefins serve as the 
base, in cross-linked or noncross-linked form. Polyethylene (PE) and 
cross-linked polyethylene (VPE) are used in particular in the insulation 
according to the invention. In addition, however, also ethylene copolymers 
such as ethylene-propylene copolymers (EPR), ethylene-vinylacetate 
copolymers (EVA) and ethylene-alkylacrylate copolymers (for instance, 
ethylene-ethylacrylate and butyacrylate copolymers), or 
ethylene-propylene-diene terpolymers and mixtures (blends) of these 
ethylene copolymers and terpolymers with polyolefins, especially 
polyethylene and polypropylene, can be used. As already mentioned, the 
foregoing polymers or polymer mixtures can be used cross-linked as well as 
noncross-linked. The cross-linking takes place preferably peroxidically or 
by high-energy radiation. To the insulating compositions can also be added 
customary additives such as oxidation stabilizers. 
In the electric insulation according to the invention, the polyolefin is 
preferably polyethylene or an ethylene copolymer with a polar comonomer or 
a polymer blend of polyethylene and the ethylene copolymer, the content of 
the polar comonomer in the copolymer or polymer blend being 1 to 5% by 
weight; see in this connection the U.S. patent application entitled 
"Electric Insulation with a Silicic Acid Additive" which has as inventors 
Drs. Peter Fischer, Hans-Joachim Henkel and Norbert Muller and is based 
upon priority German application No. P 3626409.1, the disclosure of which 
is incorporated herein by reference. The polar comonomer of the ethylene 
copolymer is here preferably an alkylacrylate, especially butylacrylate, 
i.e., an ethylene-butylacrylate copolymer (EBA) serves as the copolymer. 
In addition, however, methyl- and ethylacrylate can also be considered as 
well as vinyl acetate, vinyl alcohol, acrylonitrile and alkyl maleinate as 
further polar co-monomers. 
The invention will be explained in greater detail with the aid of the 
following example. 
EXAMPLE 
Plates 3 mm thick were prepared of low-density, high-purity polyethylene 
(LDPE) which contained 4.4'-thiobis(6-tert-butyl-3-methyl phenol) as an 
oxidation stabilizer and dicumyl peroxide as a cross-linking agent. The 
preparation took place by extruding part of the foregoing polymer 
composition under pressure and at elevated temperature together with 
pyrogenic silicic acid with a large specific surface (for instance, about 
300 m.sup. 2/g) which is commercially available under the name 
Aerosil.RTM., whereby the cross-linking of the insulating material took 
place. Into the other part of the foregoing polymer composition was mixed 
a silicic acid prepared according to DE-OS 33 23 844, by the so-called 
leaching method, in a concentration of 1% by weight. The plate-shaped test 
pieces were then stressed electrically with 10 kV/50 Hz, both surfaces 
being in direct contact with a 3-% sodium chloride solution heated to 
70.degree. C. The duration of the stress was 130 hours. 
The test results show, first, that none of the plate-shaped test pieces 
exhibited ECT structures after the electrical stress, i.e., those which 
correspond to the electric insulation according to the invention as well 
as those corresponding to the electric insulating according to DE-OS No. 
33 18 988. As already explained, it is also important for electrical 
reasons, besides the ECT retardation, that the electric losses and thereby 
also tan .delta. are as low as possible. 
The test performed showed in detail the following: 
______________________________________ 
Insulating Material 
ECT Structures 
tan.delta. 
______________________________________ 
100% LDPE + 1% Aerosil .RTM. 
None 28 .times. 10.sup.-4 
100% LDPE + 1% silicic acid 
None 10 .times. 10.sup.-4 
as per DE-OS 33 23 844) 
______________________________________ 
While it is found that the tan .delta.-values of all test pieces are below 
the value 40.times.10.sup.-4 which is specified in IEC Publication No. 502 
(1978 Edition) and thereby meet the standard requirements, an insulating 
material with a silicic acid prepared from mineral starting materials by 
the leaching method is to be preferred over other types of silicic acid 
such as pyrogenic silicic acid because it has the lower dielectric losses. 
Generally it is therefore found that the electric insulation according to 
the invention which contains silicic acid prepared by the leaching method 
has low dielectric losses as well as suppresses the ECT formation 
effectively and permanently.