Corundum porcelain composition, processes for its preparation and use

A corundum porcelain composition containing from 60 to 94% by weight of a component A and from 6 to 40% by weight of a component B, wherein component A contains from 0 to 70% by weight of alumina, from 20 to 70% by weight of clay material, from 10 to 50% by weight of glass formers and from 0 to 30% by weight of quartz, and component B comprises comminuted broken porcelain, the chemical composition of the mixture of the components A and B comprises from 20 to 75% by weight of SiO.sub.2, from 15 to 80% by weight of Al.sub.2 O.sub.3 and from 2 to 10% by weight of flux selected from the group consisting of K.sub.2 O, Na.sub.2 O, FeO, MgO, CaO, Li.sub.2 O; BaO, SrO, ZnO and fluoride, and the comminuted broken porcelain has a mean particle size between 25 and 800 .mu.m, useful, for example, in sintered bodies, such as insulators.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to corundum porcelain compositions containing a 
plurality of inorganic starting materials. The invention also relates to 
methods of preparing and using such composition. 
2. Description of Related Art 
The addition of small amounts of comminuted broken porcelain to porcelain 
compositions for producing tableware is known. In the production of 
porcelain tableware, the addition of milled and fired broken porcelain 
increases the shape stability of the shaped bodies during firing. Texture 
effects play virtually no role here. 
While the porcelain tableware items have thin walls and a low weight, the 
components of technical ceramics, particularly high voltage insulators and 
pipes, can have a length up to about 6 meters or a weight of more than 
1000 kg. Depending on their length, their compactness and their weight, 
considerable stresses can arise in the shaped body and in the sintered 
body during shaping, drying, and firing. In the production of high voltage 
insulators, these stresses can lead to the sheds breaking off during 
drying of insulator bodies and to fracture of the shank during suspended 
firing. In use, high voltage insulators are subjected to not only 
considerable electrical stresses, but also mechanical stresses. The 
mechanical properties of the high voltage insulators, therefore, play a 
critical role. In the case of tubes of great length, the shape stability 
of the sintered body is important in use. 
The production of high voltage insulators comprises preparing a plastic 
corundum porcelain composition, extruding a cylindrical body using a 
vacuum extruder, turning, drying, and firing. Extrusion produces textures 
in the green bodies. Texture means the anisotropic alignment of the 
platelet-like clay particles and the other constituents of the composition 
which results from the action of pressing pressure, flow, and shear. 
Distinction can be made between flow texture, screw texture, and cutting 
texture. Particularly the cutting texture, which is produced by the 
flights of the screw of an extruder, is critical for insulator production. 
A thin zone of pronounced texture which runs helically through the shaped 
body is formed. This zone, which often extends over a large area, is a 
weak zone which can cause the shaped body fired in a hanging position to 
break during firing. The flow texture causes an anisotropic shrinkage on 
drying in the shaped body. Since the anisometric raw material particles 
are differently aligned in the outer and inner parts of the shaped body, 
this causes different shrinkage on drying. Drying results in stresses 
which in extreme cases in the drying of high voltage insulators can lead 
to the sheds breaking off from the shank of the shaped body. 
To avoid textures which are too great, a corundum porcelain composition 
which is suitable for high voltage insulators should contain a proportion 
of relatively coarse particles. Up to 5% by weight of the raw materials 
are customarily added as coarse particles having a particle size 
distribution between about 45 and 100 .mu.m, the coarse particles being 
composed of quartz, feldspar, and/or alumina agglomerates. Although 
commercial calcined aluminas have a significantly finer primary particle 
size, they are agglomerated as a result of the method of manufacture. The 
mean particle size of the corundum porcelain composition is then usually 
of the order of 8 .mu.m. 
As constituent of the fired porcelain, quartz has both a 
strength-increasing action and a strength- and lifetime-reducing action 
owing to the .beta.-.alpha.-quartz transformation at 573.degree. C. in the 
cooling phase of firing, which transformation is associated with a volume 
contraction of the quartz grains. The glass phase is already solidified at 
this temperature, resulting in microstructural stresses which increase 
with increasing quartz grain size. Tangential compressive stresses have a 
strength-increasing effect in the vicinity of a quartz grain; this 
microstructural stressing has to be overcome before crack propagation can 
occur. Strength and lifetime are impaired if the compensating radial 
tensile stresses exceed the strength of the quartz and generate 
microcracks in the quartz grains. Microcracks in quartz grains can 
frequently be observed in porcelains on optical microscopic examination. 
It is known that optimum strength is achieved with quartz grains having a 
size of the order of from 10 to 20 .mu.m. However, raw materials able to 
be used on a large scale have such broad particle size distributions that 
proportions of relatively large, crack-prone quartz particles having a 
particle size up to about 80 .mu.m are also always present if these raw 
materials are not milled in an additional process step to particle sizes 
of less than 20 .mu.m for the entire particle size distribution. 
Replacement of quartz by alumina (synthetic aluminum oxide and/or aluminum 
hydroxide powder) avoids the danger of microcrack formation since the 
corundum (.alpha.-Al.sub.2 O.sub.3) retained and/or formed during firing 
has a thermal expansion similar to that of the remaining micro-structural 
constituents of the corundum porcelain. Corundum is the constituent in 
corundum porcelain which has by far the best mechanical properties. The 
strength-increasing action of corundum is essentially dependent on its 
proportion and its grain size. It is generally known that the best 
strength is achieved when processing an alumina whose mean particle size 
after firing in the corundum porcelain is from about 3 to about 9 .mu.m. 
For a corundum porcelain of high strength, such a fine alumina should be 
added or these particle sizes should be produced by milling during 
preparation of the composition; the quartz should be milled as finely as 
possible or avoided entirely. To be sure of excluding quartz particles &gt;20 
.mu.m the mean particle size of the quartz should not exceed 3 .mu.m. Even 
finer milling is advantageous. 
Commercial feldspars too must not be added as a coarse fraction since they 
contain quartz as minor constituent and the feldspars, the alternatively 
usable feldspar substitutes or rocks containing feldspars and/or feldspar 
substitutes, which act as glass formers, have to be homogeneously 
distributed in the composition; these raw materials are, like milled 
broken glass and glass frits, referred to as glass formers in the 
following text. 
The optimum strength can, therefore, only be achieved in conventional 
porcelain production if the starting materials are added in sufficiently 
fine form after prior milling or are finely milled during processing; the 
troublesome texture and the rejects resulting from this texture then have 
to be accepted. If this fine milling is omitted, an additional high 
proportion of the expensive alumina has to be added to achieve a 
sufficient strength. 
In the large-scale manufacture of high voltage insulators, a substantial 
proportion of the rejects is caused by textures. Depending on the process 
procedure and insulator geometry, the rejects caused by texture are from 
about 1 to 3% of the components. Significant rejects resulting from 
textures also occur in the manufacture of other types of components from 
corundum porcelain. These effects are greater, the greater the degree to 
which shear forces occur during shaping and textures can be formed and the 
longer, compactor or heavier the shaped bodies are. Among the extremely 
varied shaping processes for ceramics, the ones which are most sensitive 
to the formation of textures are those which use compositions in a plastic 
state. These in particular include extrusion. 
As indicated in the previous sections, in the production of large sintered 
bodies from conventional porcelain compositions a compromise has to be 
made according to the present state of the art between good behavior in 
production, particularly the reduction of textures, and high strength of 
the fired sintered bodies. The reduction in strength resulting from this 
compromise is occasionally counteracted by an additional high and 
expensive alumina addition, but a significant number of texture flaws 
nevertheless remain and these cannot be counteracted further. 
EP-B-0 189 260 teaches a process for producing a high-strength, 
feldspar-type porcelain in which the nonplastic raw materials of the 
quartz, feldspar and alumina type have, corresponding to FIG. 1 of this EP 
document, a mean particle size of from about 2 to 4 .mu.m, the nonplastic 
raw materials are calcined and subsequently mixed-with the clay material. 
Since the calcination is carried out without clay material, no appreciable 
amount of mullite can be formed. There is, therefore, also an absence of 
the crystal nuclei which can allow a strong framework of mullite needles 
to be formed during firing and thus could increase the shape stability of 
the shaped body during firing. Before or after addition of the clay 
material, the calcined material is milled to a similar particle size to 
that possessed by the nonplastic raw materials prior to calcination. The 
milled mixture to which the clay material has been added is further 
processed by shaping, drying, and firing. 
This document, therefore, teaches the repeated milling of the starting 
materials to an extraordinarily low particle size and the repeated 
carrying out of the firing. The fine-grained porcelain composition leads 
to a lower drying sensitivity. Owing to the fineness of this porcelain 
composition, it is expected that it tends to strong texture formation in 
the production of large bodies. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a process for preparing a 
corundum porcelain composition in which the texture effects are kept so 
small that in the production of sintered bodies substantially no rejects 
occur as a result of the texture, and which requires no additional process 
steps in the production of a high-strength corundum porcelain. 
It is a further object of the invention to use starting materials which are 
as inexpensive as possible in the preparation of the corundum porcelain 
composition. 
It is also an object to provide corundum porcelain compositions having 
those desired characteristics and methods of using them in the formation 
of sintered bodies. 
It is also an object of the invention, to provide sintered bodies, such as 
insulators, having improved characteristics. 
In accordance with the present invention, these objects are achieved by a 
very simple and inexpensive-to prepare corundum porcelain composition 
which is particularly suitable for the mass production of high voltage 
insulators. 
According to the invention, the corundum porcelain composition contains 
from 60% to 94% by weight of a component A and from 6% to 40% by weight of 
a component B, in each case based on the total weight of all inorganic 
starting materials in the dry state. In the composition, component A 
contains from 0 to 70% by weight of alumina, from 20 to 70% by weight of 
clay material, from 10 to 50% by weight of glass formers and from 0 to 30% 
by weight of quartz, in each case based on the total weight of the 
inorganic starting materials of the component A in the dry state, and 
component B comprises comminuted broken porcelain, the chemical 
composition of the mixture of the components A and B comprises from 20 to 
75% by weight of SiO.sub.2, from 15 to 80% by weight of Al.sub.2 O.sub.3 
and from 2 to 10% by weight of flux selected from the group consisting of 
K.sub.2 O, Na.sub.2 O, FeO, MgO, CaO, Li.sub.2 O, BaO, SrO, ZnO and 
fluoride, and the comminuted broken porcelain has a mean particle size 
between 25 and 800 .mu.m, preferably between 32 and 250 .mu.m, in 
particular between 42 and 150 .mu.m. 
In accordance with other aspects of the invention, there is provided a 
method of producing a sintered body from the above compositions, and 
sintered bodies so produced. 
In accordance with further objects of the invention, there is provided a 
sintered body of corundum porcelain produced by shaping, drying, and 
firing a corundum porcelain composition containing comminuted broken 
porcelain, wherein the sintered body has a flexural strength which is at 
least 25 MPa higher than the flexural strength of a sintered body of 
corundum porcelain which is produced without addition of comminuted broken 
porcelain but with an essentially identical chemical composition and with 
the same or higher corundum content in the corundum porcelain, therein 
also providing powers of making and using the composition. 
Further objects, features, and advantages of the invention will become 
apparent from the detailed description of the preferred embodiments that 
follows. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The component A of the corundum porcelain composition of the invention 
preferably contains from 8 to 40% by weight of alumina, from 24 to 65% by 
weight of clay material, from 12 to 28% by weight of glass formers, and 
from about 0.2 to 25% by weight of quartz, in particular from 12 to 30% by 
weight of alumina, from 28 to 60% by weight of clay material, from 14 to 
25% by weight of glass formers and from about 0.8 to 22% by weight of 
quartz. 
The chemical composition of the mixture of the components A and B 
preferably comprises from 30 to 65% by weight of SiO.sub.2, from 25 to 60% 
by weight of Al.sub.2 O.sub.3 and from 2.5 to 8% by weight of flux, in 
particular from 42 to 55% by weight of SiO.sub.2, from 35 to 55% by weight 
of Al.sub.2 O.sub.3 and from 3 to 7% by weight of flux. 
Preferably, in the corundum porcelain composition, the component A as 
mixture has a particle size distribution of from about 80 to 100% by 
weight less than 50 .mu.m, in particular from 97 to 100% by weight less 
than 45 .mu.m. 
For the purposes of the present invention, the alumina used is generally an 
unmilled or milled raw material based on aluminum oxide, transition 
aluminas and/or aluminum hydroxide for which indicated percentages by 
weight are as Al.sub.2 O.sub.3 alone. The fired alumina is here described 
as corundum. To also relate the term corundum porcelain to the raw 
materials added, it is advisable to add an amount of at least 1% of 
alumina. The term clay material denotes plastic raw materials based on 
clay and/or kaolin which, as is usual commercially, can be admixed with 
small amounts of other minerals and impurities. Glass formers is a term 
for the raw materials that contribute to glass formation, such as those 
based on feldspar, feldspar substitutes and/or rock having a high content 
of feldspar and/or feldspar substitutes and/or milled broken glass and/or 
glass frits. They act as fluxes and contribute substantially to formation 
of a glass phase during firing. The term quartz describes amorphous and 
crystalline raw materials based on SiO.sub.2. In many cases quartz is 
present in some of the raw materials like clay material and/or feldspars 
as impurities in a content of at least 0.2 or even more than 0.8 % by 
weight. There is no need for an addition of quartz as separate raw 
material to create a higher strength, although a certain content of it may 
be added. The glass formers and quartz too can contain small amounts of 
other minerals and impurities. Quartz, glass formers and/or clay material 
can also be added in the form of naturally occurring mineral mixtures and 
rock. 
The particle size and the particle size distribution of the coarser 
additives was determined by means of sieving above about 45 .mu.m, while 
the laser granulometer Cilas 920 was used for measurements below 45 .mu.m. 
The comminuted broken porcelain can have a mean particle size greater than 
or equal to 25 .mu.m, 50 .mu.m, 60 .mu.m, 80 .mu.m, 100 .mu.m, or 120 
.mu.m and less than 800 .mu.m. On addition to the corundum porcelain 
composition, it already has a porcelain microstructure containing, apart 
from the glass phase, mullite and possibly corundum and/or quartz. The 
amounts of comminuted broken porcelain can no longer be discerned in the 
microstructure after firing. Preference is given to adding comminuted 
broken porcelain originating from the production of corundum porcelain. In 
the production of high voltage insulators, this is preferably the scrap 
from high voltage insulator manufacture. This can be the comminuted 
offcuts from the end regions of the insulator bodies which are removed 
before fitting the metal caps, and damaged and/or faulty insulators. Tubes 
and other technical ceramic products frequently also comprise corundum 
porcelain whose scrap can be preferably used for the preparation process 
of the invention. 
Fired scrap from conventionally manufactured insulators frequently contains 
quartz grains up to 50 .mu.m and corundum agglomerates up to 70 .mu.m in 
size in the microstructure. Since the quartz grains are usually already 
damaged by microcracks and the corundum agglomerates have a porous and 
inhomogeneous structure, these are greatly reduced in size during 
comminution, processing and firing, since they have weak points. In the 
corundum porcelain of the invention, the oversize quartz grains and 
corundum agglomerates from the comminuted porcelain reach sizes which very 
seldom or never exceed 20 .mu.m, even if the size of the comminuted broken 
porcelain particles exceeds 100 .mu.m or even 500 .mu.m. 
The invention is accordingly based on the idea of breaking the texture 
formed during shaping by means of a coarse particle fraction in the 
corundum porcelain composition, which coarse fraction on firing leaves no 
quartz and corundum grains which impair the strength and lifetime of the 
sintered bodies of corundum porcelain as a result of the grain size or 
initial cracks in the grains. 
Among the raw materials of the component A, alumina preferably has a 
particle size distribution of from 50 to 100% by volume at from 2 to 10 
.mu.m, quartz from 50 to 100% by volume .ltoreq.10 .mu.m and glass formers 
from 50 to 100% by volume .ltoreq.10 .mu.m. 
The component B is generally added with a particle size .ltoreq.2000 .mu.m, 
preferably a particle size .ltoreq.500 .mu.m, in particular a particle 
size .ltoreq.200 .mu.m. As the particle size distribution may be very 
broad such a particle size distribution with a mean particle size e.g. of 
600 .mu.m may show singular particles much about 2000 .mu.m. They are 
preferably added to the mixture in a particle size distribution of from 70 
to 95% by weight at from 30 to 400 .mu.m, in particular a particle size 
distribution of from 70 to 95% by weight at from 30 to 150 .mu.m. The mean 
particle size of the component B is preferably from twice to ten times as 
large as the mean particle size of the component A. The corundum porcelain 
composition preferably contains from 12 to 36% by weight of the component 
B, in particular from 18 to 32% by weight. 
The compositions of the invention may be produced in any desired manner. A 
preferred process for preparing corundum porcelain compositions comprises 
mixing together the starting materials of the component A and, if 
appropriate, also milling the mixture, with the mixture having a particle 
size distribution of from 97 to 100% by weight less than 45 .mu.m, and 
then mixing the component B into the component A. 
A further process for preparing corundum porcelain compositions according 
to the invention comprises producing a mixture of the inorganic starting 
materials of the component A to which a part of the inorganic starting 
materials, in particular a part amount or the entire amount of clay 
material, is not added, homogenizing this mixture and, if appropriate, 
also milling it and subsequently adding the missing parts of the inorganic 
starting materials and the component B to this mixture. In a preferred 
variant of this process, the parts of the inorganic starting materials 
missing from the mixture are combined with the component B, homogenized 
and, if appropriate, also milled, before this second mixture is added to 
the mixture of the other inorganic starting materials of the component A. 
In a particularly advantageous process variant, the part amount of clay 
material missing from the mixture is .gtoreq.50% by weight, in particular 
80% to 95% by weight, of the total amount of clay material added. 
In preferred processes, the total mixture of the components A and B is 
homogenized and, if appropriate, also milled. The homogenization of a 
mixture or the total mixture of the components A and B can be carried out 
in a mixer, but preferably by milling. In these process variants, the 
total mixture is particularly preferably milled to a particle size 
distribution of from 80 to 98% by weight less than 45 .mu.m. 
Those skilled in the art are familiar with the various techniques of 
comminution and preparation of the composition for the purposes of 
porcelain production as well as the specific raw material grades to be 
used here. Any preparation process which ensures that either the prepared 
corundum porcelain composition or the body fired from a corundum porcelain 
composition is free of quartz grains &gt;20 .mu.m is preferred. 
Wet preparation processes in which an amount of water of the order of 50% 
by weight is present in the composition prior to dewatering carried out in 
a filter press are preferable owing to the good homogenization. The 
individual additions can be homogenized and/or milled individually, in 
groups, or in the entire mixture. The clay material can be added at any 
stage of the preparation; when wet milling is used, it is advantageous to 
mix at most 20% by weight of the total amount of clay material to be added 
into the nonplastic raw materials, so as to achieve a more intensive 
milling action. It can be sufficient to homogenize the components A and B 
purely by a mixing procedure, e.g., in a blunger; in many cases it is more 
favorable to also mill this mixture. If greater homogenization of the 
components A and B is to be achieved, it is advisable to add the component 
B essentially in a particle size &gt;30 .mu.m and to carry out stronger 
milling, e.g., to a particle size distribution of from 80 to 98% by weight 
less than 45 .mu.m. 
The comminuted broken porcelain introduces mullite into the corundum 
porcelain composition. This mullite has the effect of crystallization 
nuclei, so that the crystallization of mullite from the glass phase 
commences earlier and more intensively. This results in the formation of 
more mullite needles than in conventional porcelain, so that the viscosity 
of the melt phase appears to be increased owing to fixing, e.g., via the 
interwoven mullite crystals, and the deformation stability during firing 
is improved. Owing to the addition of comminuted broken porcelain, very 
fine milling of the raw materials is not necessary for producing a 
particularly mullite-rich corundum porcelain. In a conventional 
preparation process, the very fine milling would increase the reactivity 
of the raw materials and promote the formation of mullite, but would also 
increase the texture sensitivity of the composition so much that it is not 
usable for insulator manufacture. In addition, the very finely milled 
composition is difficult to process, particularly in shaping. 
The increase in the mullite content was able to be confirmed by X-ray 
diffraction. With the apparently increased viscosity of the melt phase 
resulting from the mullite, the strength of the shaped bodies during 
firing also increases. This makes it possible to produce larger and 
heavier insulators, but also those having smaller diameters and greater 
length. If the insulator bodies are fired standing up, the improved shape 
stability creates firing space in the furnace because the parts used for 
suspension are omitted or can at least be made smaller. 
The oversize quartz particles of the comminuted broken porcelain also 
become smaller under the mechanical stressing during the preparation of 
the composition, since the microcracks in large quartz grains aid 
comminution by acting as preferential points of fracture. During firing, 
the quartz grains are partly dissolved in the melt phase. The strength and 
lifetime of the sintered bodies produced therefrom are not impaired. 
The preparation of the corundum porcelain composition of the invention not 
only avoids repeated firing and repeated milling, but also saves a part of 
the high-cost raw materials, in particular alumina, by addition of 
comminuted broken porcelain. In addition, a higher strength of the 
corundum porcelain of the invention is achieved without having to 
additionally add higher amounts of alumina, since the corundum particles 
present in the comminuted broken porcelain have a fully 
strength-increasing action. 
It has surprisingly been found that a sintered body of corundum porcelain 
according to the invention which is produced by shaping, drying, firing 
and, if desired, further process steps from a corundum porcelain 
composition containing comminuted broken porcelain has a flexural strength 
which is at least 25 MPa higher than the flexural strength of a sintered 
body of corundum porcelain which is produced without addition of 
comminuted broken porcelain but has essentially the same chemical 
composition and the same or higher corundum content in the corundum 
porcelain. The deformation of shaped bodies under their own weight during 
firing is significantly decreased; this additionally opens up the 
opportunity of producing even larger or longer insulators having a greater 
load per unit area and of firing them in a suspended position. 
Surprisingly, the addition of comminuted broken porcelain containing 
oversize quartz and corundum particles resulted in strong size reduction 
and dissolution of these particles during firing, so that these particles 
do not impair the strength and lifetime of the sintered bodies produced 
therefrom. 
The corundum porcelain composition of the invention, can be advantageously 
used, inter alia, for the production of insulators, tubes, rods, catalyst 
supports, laboratory porcelain, porcelain sanitaryware, and porcelain 
tableware and is particularly suitable for high voltage insulators.

EXAMPLES 
The invention is illustrated below by means of the examples according to 
the invention and is compared with the comparative examples. The examples 
are for illustrative purposes only and do not limit the invention. 
In all comparative examples and examples according to the invention the 
corundum porcelain composition was prepared by milling in a ball mill at a 
water content of about 50% by weight and subsequent pressing to a water 
content of about 20% by weight in a filter press. The shaping of the test 
specimens in the form of round rods was carried out using a vacuum 
extruder model V 250 from Netzsch and, for the production of high voltage 
insulators, using a vacuum extruder of appropriate dimensions. Firing was 
carried out in a car bottom furnace using a firing curve as is 
conventionally used for high voltage insulators. The firing temperature 
reached about 1320.degree. C. 
The flexural strength was tested after firing on unglazed and unground 
round rods having a diameter of 12 mm and a span of 100 mm in a 3-point 
flexural testing apparatus in accordance with IEC 672. The round rods were 
made from the respective corundum porcelain composition by extrusion to at 
diameter such that shrinkage during drying and firing resulted in a 
diameter of 12.+-.0.1 mm. Measurements on from 10 to 12 specimens were 
averaged in each case. 
For testing the deformation during firing, unglazed bars having a 
trapezoidal cross section of 23 mm.times.21 mm.times.14 mm height and a 
length of 250 mm were used. They were extruded using a separate die and 
mounted during firing on two knife-edges of refractory material 190 mm 
apart. The deflection of the middle of the bar from the plane of the 
knife-edges was measured. A small amount of sag is a measure of high 
deformation stability during firing. 
For the examination of the microstructure of the fired corundum porcelain, 
polished sections were prepared and these were evaluated using a 
reflected-light microscope. 
Example 1 (Comparative Example) 
For the comparative example 1 and the example 2 according to the invention, 
two batches were made up in such a way that the corundum porcelain 
compositions had essentially identical chemical analyses, but different 
mineralogical compositions. The chemical analysis was (figures in % by 
weight): 
______________________________________ 
Comparative Example 1 
Example 2 
______________________________________ 
SiO.sub.2, 46.91 46.74 
Al.sub.2 O.sub.3 42.29 42.30 
Fe.sub.2 O.sub.3 0.55 0.66 
TiO.sub.2 0.23 0.27 
CaO 0.21 0.25 
MgO 0.43 0.52 
BaO 0.07 0.06 
K.sub.2 O 4.24 4.18 
Na.sub.2 O 0.39 0.40 
Loss on ignition 
4.58 4.52 
______________________________________ 
The batch of the comparative example 1 corresponded to a conventional 
corundum porcelain composition also used for high voltage insulators and 
consisted of 25% by weight of alumina, added as aluminum oxide, 8% by 
weight of quartz, 40% by weight of clay material and 27% by weight of 
feldspar. The mixture of these raw materials was milled for 2.5 hours in a 
ball mill and given a particle size distribution in which 95% by weight of 
the milled material is less than 45 .mu.m, as measured by sieving. The 
flexural strength of the corundum porcelain produced therefrom was 120 
MPa. The microstructure of the comparative example 1 showed quartz grains 
up to 70 .mu.m, corundum grains in agglomerates up to 90 .mu.m and pore 
sizes up to 60 .mu.m. 
Example 2 (According to the Invention) 
For the component A, 17% by weight of alumina as aluminum oxide, 3% by 
weight of quartz, 40% by weight of clay material and 20% by weight of 
feldspar were mixed and milled to a particle size distribution of 99.8% by 
weight &lt;45 .mu.m using a ball mill. 20% by weight of comminuted broken 
porcelain having a particle size distribution essentially between 45 and 
100 .mu.m was then added as component B and the mixture was homogenized by 
milling for 0.5 hours in a ball mill. This likewise gave a particle size 
distribution of 95% by weight &lt;45 .mu.m. The comminuted broken porcelain 
had a corundum content of 25% by weight, so that the alumina and corundum 
content of the batch was 22% by weight. 
In the example 2 according to the invention, a flexural strength of 155 MPa 
was determined; a 35 MPa higher flexural strength was thus achieved, 
although the corundum content of the sintered bodies was 3% by weight less 
than in the case of the sintered bodies of the comparative example 1. 
The microstructure of the corundum porcelain of the invention was, examined 
under the reflected-light microscope, significantly finer and more 
homogeneous than in the comparative example 1. 99.9% of the corundum 
grains were smaller than 30 .mu.m and 99.9% of the quartz grains did not 
exceed 2 .mu.m. The pore size was measured as 99.9% .ltoreq.35 .mu.m. 
X-ray diffraction patterns recorded using a diffractometer showed the 
mullite content in the corundum porcelain of example 2 to be 1.6 times 
that in comparative example 1. Quartz could not be detected in the 
corundum porcelain of example 2, while the corundum porcelain of 
comparative example 1 showed a very distinct quartz peak. 
Comparative Example 3 and Examples 4 to 7 According to the Invention 
In the examples 3 to 7, the amount of comminuted broken porcelain added was 
varied, but the total content of alumina and of corundum from the 
comminuted broken porcelain was kept constant at 20% by weight of Al.sub.2 
O.sub.3 in the corundum porcelain body. The component A used was a mixture 
of alumina, feldspar, quartz, kaolin, and clay. Table 1 shows the added 
amounts of inorganic starting materials of the component A and the added 
amount of the component B. The sum of the individual added amounts is in 
each case 100% by weight based on the total weight of all inorganic 
starting materials in the dry state. Table 2 shows the amounts of alumina 
and corundum added to the corundum porcelain body. The preparation of 
comparative example 3 was carried out as in comparative example 1. The 
mixture of the comparative example 3 was milled to a particle size 
distribution of 95.5% by weight &lt;45 .mu.m. The comminuted broken porcelain 
had the following composition: SiO.sub.2 49.2% by weight; Al.sub.2 O.sub.3 
44.5% by weight; Fe.sub.2O.sub.3 0.5% by weight; TiO.sub.2 0.2% by weight; 
CaO 0.2% by weight; MgO 0.5% by weight; K.sub.2 O 4.5% by weight; Na.sub.2 
O 0.4% by weight. 
The preparation of the examples 4 to 7 according to the invention was 
carried out as in the example 2 according to the invention. Here, the 
component A was milled to particle size distributions of from 99.6 to 100% 
by weight &lt;45 .mu.m. The particle size distributions of the comminuted 
broken porcelain were essentially between 45 and 100 .mu.m. The millings 
of the total mixture gave particle size distributions of from 95 to 96% by 
weight &lt;45 .mu.m. The results of the flexural strength and sag tests for 
the corundum porcelain produced therefrom are likewise reproduced in Table 
2. 
TABLE 1 
__________________________________________________________________________ 
amounts of inorganic starting materials of the component A and amount of 
the component B 
added to the corundum porcelain compositions of the examples and 
comparative examples 
Component B 
Component A 
Example, 
comminuted 
Alumina 
comparative 
broken porcelain 
% by weight 
Feldspar 
Quartz Kaolin Clay 
example 
% by weight 
of Al.sub.2 O.sub.3 
% by weight 
% by weight 
% by weight 
% by weight 
__________________________________________________________________________ 
CE 1 0 25 27 8 30 10 
E 2 20 17 20 3 30 10 
CE 3 0 20 28 12 30 10 
E 4 8 18 25 9 30 10 
E 5 16 16 22 6 30 10 
E 6 24 14 19 3 30 10 
E 7 32 12 16 0 30 10 
E 8 20 12 20 8 30 10 
E 9 20 15 20 5 30 10 
E 10 20 18 20 2 30 10 
E 11 20 21 19 0 30 10 
CE 12 0 25 27 8 30 10 
CE 13 0 29 25 6 30 10 
CE 14 0 33 23 4 30 10 
CE 15 0 37 21 2 30 10 
CE 16 0 41 19 0 30 10 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Amounts of corundum and alumina added to the corundum porcelain 
composition 
and material properties of the corundum porcelain product therefrom 
Addition of broken 
porcelain having Total content 
Example, 
a corundum content 
Alumina 
of alumina and 
Flexural 
comparative 
of 25% by weight 
addition 
corundum 
strength 
Sag 
example 
% by weight 
% by weight 
% by weight 
MPa mm 
__________________________________________________________________________ 
CE 1 0 25 25 120 n.d. 
E 2 20 17 22 155 n.d. 
CE 3 0 20 20 113 19 
E 4 8 18 20 129 20 
E 5 16 16 20 139 15 
E 6 24 14 20 141 12 
E 7 32 12 20 165 10 
E 8 20 12 17 138 15 
E 9 20 15 20 144 14 
E 10 20 18 23 157 13 
E 11 20 21 26 173 12 
CE 12 0 25 25 118 17 
CE 13 0 29 29 134 21 
CE 14 0 33 33 142 18 
CE 15 0 37 37 150 19 
CE 16 0 41 41 163 16 
__________________________________________________________________________ 
The test results on the specimens of examples 3 to 7 clearly show the 
strong rise of the flexural strength and the deformation stability during 
firing with increasing added amount of comminuted broken porcelain, 
although the content of alumina in the form of aluminum oxide and of 
corundum in the total mixtures was kept constant at 20% by weight. 
Examples 8 to 11 According to the Invention and Comparative Examples 12 to 
16 
In the examples 8 to 11 according to the invention, the content of alumina 
and corundum in the corundum porcelain composition was varied, with the 
amount of comminuted broken porcelain added being left unchanged at 20% by 
weight. The component A used as a basis contained 30% by weight of kaolin, 
10% by weight of clay and varying contents of alumina, feldspar and quartz 
(Table 1). The preparation was carried out as in example 2. The component 
A was milled to particle size distributions of from 99.6 to 100% by weight 
&lt;45 .mu.m. The particle size distributions of the component B were 
essentially between 45 and 100 .mu.m. The total mixtures were milled in a 
ball mill to particle size distributions of from 95 to 96% by weight &lt;45 
.mu.m and were thereby thoroughly homogenized. The contents of alumina and 
corundum and also the test results are shown in Table 2. The comminuted 
broken porcelain had the same composition as in examples 4 to 7. The 
example 8 according to the invention was fabricated on a relatively large 
experimental scale; this resulted in neither processing difficulties nor 
texture rejects. 
The preparation of the corundum porcelain compositions of the comparative 
examples 12 to 16 was carried out in a similar manner to that for a 
conventional corundum porcelain. The inorganic starting materials 
including the clay but excluding the kaolin were milled in a ball mill. 
The total kaolin content was added in a blunger to the mixture milled in 
the ball mill. Here, particle size distributions of from 95 to 96% by 
weight &lt;45 .mu.m were set. The major part of the water was subsequently 
removed from these total mixtures in a filter press. To achieve a flexural 
strength as in the examples 8 to 11 according to the invention, 
significantly higher contents of Al.sub.2 O.sub.3 added in the form of 
alumina are required (Table 2). The flexural strength of the comparative 
examples 12 to 16 is, based on the respective corundum content of the 
corundum porcelain, at a significantly different strength level which, for 
a comparable composition, is as much as 35 or more MPa below the 
comparable values of the examples 8 to 11 according to the invention. The 
sag is about one third higher compared with the examples 8 to 11 according 
to the invention and, therefore, worse. 
Although only a few exemplary embodiments of this invention have been 
described in detail above, those skilled in the art will readily 
appreciate that many modifications are possible in the exemplary 
embodiments without materially departing from the novel teachings and 
advantages of this invention. Accordingly, all such modifications are 
intended to be included within the scope of this invention.