Note: Descriptions are shown in the official language in which they were submitted.
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SMB
Heat-Resistant and Refractory Molded Part
The present invention relates to a heat-resistant and refractory molded part
in the
form of an artificial log for fireplaces, a heat-insulating plate or a light-
weight
constructional brick having a density of from 400 to 1000 kg/m3 consisting of
hydraulically cured compositions, and methods for the preparation thereof.
Heat-resistant and refractory molded parts consisting of hydraulically cured
compositions are used, inter alia, as artificial logs for fireplaces, but also
as heat-
insulating plates or light-weight constructional bricks.
Thus, US 4,940,407 describes such artificial logs, which, however, heat up
very
slowly due to their high specific gravity, store a lot of energy and therefore
cool
down also very slowly. Moreover, they are very heavy.
US 5,800,875 describes artificial logs which predominantly consist of mineral
wool
fibers and a high temperature binder. The readily cured form is then suitably
painted, again dried and packaged for sale. Due to the use of mineral wool
fibers,
the temperature resistance is limited. In addition, it is undesirable for
safety
purposes that fragments of such fibers and pieces of fibers can intrude into
the
ambient air of inhabited rooms.
US 5,612,266 describes artificial logs which are prepared by casting a mineral
foam into molds to be cured therein. These products contain organic components
which lead to undesirable smelling when heated.
DE 195 17 267 C1 describes a material having good fire-protection properties
contairiing at least 5% by weight of ettringite together with minor amounts of
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xonotlite and aluminous cement. The ettringite is supposed to decompose upon
heating with consumption of energy to thereby improve the fire-protection
properties. Such a material is not useful for the present invention.
US 3,042,536 relates to a filling material which contains calcium silicate,
for
example, in the form of xonotlite, together with asbestos fibers, in addition
to
aluminous cement. Molded parts made of this material have not been described.
Thus, it has been the first object of the invention to provide artificial logs
for
fireplaces which have as low as possible a density and therefore take up and
release only a relatively small amount of heat energy. Therefore, these
products
heat up very quickly and also cool down quickly. This results not only in a
saving of
heating energy, but also in increased safety, since only for a very short time
after
the switching off of the heating are the artificial logs so hot that they
would cause
burns when touched or could ignite combustible materials when in contact
therewith.
Further, it has been the object of the invention to provide heat-insulating
plates or
light-weight constructional bricks made of such a material, because these must
meet similar demands.
Surprisingly, these objects are achieved particularly well by hydraulically
cured
compositions containing, in an uncured state, in addition to water, aluminous
cement, optionally in admixture with Portland cement, phosphate binders or
water
glass, fillers and, if desired, fibers, curing accelerators, curing delayers,
plasticizing
agents and foaming agents, wherein the uncured composition contains from 10 to
60% by weight of aluminous cement and from 10 to 60% by weight of xonotlite.
Typically, the uncured composition contains from 50 to 200 weight parts of
aluminous cement and from 10 to 250 weight parts of xonotlite.
As the xonotlite, in practice, synthetic xonotlite is used, which is obtained
in the
form of felted globules in the most frequently employed process. However,
according to the invention, a needle-shaped material may also be employed,
which
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is obtained, for example, in the processing of the spherical felted xonotlite
as a by-
product or in the recycling of xonotlite waste and dust.
The use of felted balls is particularly preferred. Such xonotlite can be
prepared by
"heating with stirring" according to US 3,501,324 or GB 1 277 272.
Suitable xonotlites include, in particular, synthetic commercial products,
such as
Promaxon°, produced and sold by the company Promat. By the addition
of major
amounts of xonotlite, the density can be reduced, and the thixotropy of the
uncured composition can be increased. Undesirable deposition phenomena and
thus inhomogeneities are avoided thereby.
The density of the molded parts according to the invention is within a range
of
from 400 to 1000 kg/m3, preferably within a range of from 400 to less than
700 kg/m3.
Preferably employed fillers include wollastonite, tobermorite, finely divided
amorphous silica and/or reactive aluminum oxide.
As light-weight fillers, mainly those selected from the group consisting of
pearlites,
vermiculites, fly ashes and/or glass beads may be employed.
The molding is effected, for example, by introducing into molds, e.g., by
pouring,
but may also be effected by extrusion or compression. Thus, artificial logs
for
fireplaces, but also heat-insulating plates or light-weight constructional
bricks may
be prepared.
Preferred compositions contain from 20 to 40% by weight of synthetic
xonotlites,
from 20 to 40% by weight of aluminous cement, from 0 to 50%, preferably 10%,
by weight of fine amorphous silica, which both acts as a binder and improves
the
flow properties of the composition.
In another preferred mixture, the proportions are from 20 to 40% by weight of
xonotlite and from 40 to 60% by weight of aluminous cement.
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Wollastonite is added in amounts of from 0 to 40%, preferably from 20 to 30%,
by
weight, more preferably 30% by weight. Wollastonite improves the heat
resistance
and resistance to thermal shocks, which often led to cracking and premature
destruction in known products of the prior art. From 0 to 5%, preferably 2%,
by
weight of phosphates also increases the temperature resistance; for example,
suitable materials include sodium polyphosphate glasses as sold by the company
Chemische Fabrik Budenheim under the designation of Budit 8 H. Further, the
mixtures may contain usual curing accelerators, curing delayers, plasticizing
agents and foaming agents. If desired, the material may be mechanically rein-
forced by from 0 to 5%, preferably from 2 to 3%, by weight of fibers, wherein
so
low amounts of even organic fibers do not result in offensive smells or other
undesirable properties. Suitable fibers include, for example, MMMF (man-made
mineral fibers), such as glass fibers. Suitable but less preferred fibers are
cellulose
fibers, organic fibers, such as PVA or PP.
Usual curing accelerators, curing delayers, plasticizing agents and foaming
agents
may be added, especially to facilitate the preparation process. These
additives
include calcium hydroxide, aqueous sodium hydroxide, sodium carbonate, calcium
carbonate, lithium carbonate, borax, citric acid, aluminum hydroxide. Calcium
carbonate is preferred, acting as both a curing accelerator and a suitable
filler.
The molded parts according to the invention are preferably free of asbestos
fibers
and material which decompose upon the action of heat. In contrast to DE
195 17 267 C1, for example, the material according to the invention contains
substantially no ettringite, at least it contains less than 5% by weight. The
material
is preferably free of organic fibers etc. The molded parts according to the
invention
are intended for repeated heating and cooling. Therefore, it is not reasonable
to
add components which will decompose upon the first heating.
The method according to the invention for the preparation of these molded
parts is
effected, in particular, by mixing with water said mixture of aluminous
cement,
optionally in admixture with Portland cement, phosphate binders or water
glass,
fillers and, if desired, fibers, curing accelerators, curing delayers,
plasticizing
agents and foaming agents, and molding and curing the composition, wherein the
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uncured composition should contain from 10 to 60% by weight of aluminous
cement and from 10 to 60% by weight of xonotlite.
Especially when artificial logs for fireplaces are prepared, these are colored
with
refractory pigments either already in the composition, or else later as ready-
cured
logs.
An examination of the products prepared according to the invention has shown
that they have a very good structural and dimensional stability and are highly
resistant towards thermal shocks. They have a high mechanical strength and yet
a
density of only from 400 to 1000 kg/m3. This has the result that little energy
is
stored since the system has a low specific heat. This again results in quick
heating
and quick cooling. Thus, the products can be touched without danger relatively
soon after turning off the heating.
Especially when heat-insulating plates or light-weight constructional bricks
are
prepared from the material, these have good insulation properties, wherein the
so-
called lambda values can be adjusted by varying the ratio of cement to
xonotlite.
These products too do not contain any hazardous fibers or produce undesirable
smells upon heating due to the decomposition of organic components. Both these
heat-insulating plates and light-weight constructional bricks can be employed
for
different purposes, for example, for heat accumulating furnaces or for heat
insulation in the industry in the production of aluminum, ceramics and cement.
Figure 1 shows the structure of xonotlite in the form of needles.
Figure 2 shows xonotlite in the form of felted globules.
Examples
The invention is further illustrated by the following Examples.
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Example 1
21.8% by weight of xonotlite powder, 18.2% by weight of wollastonite and 56.6%
by weight of aluminous cement (Secar 71) were mixed in a dry state.
In addition, a mixture of water, plasticizing agent (0.7% by weight) and glass
fibers (2% by weight) was prepared and added to the mixture of aluminous
cement and xonotlite to give a water-to-solids ratio of 1.18. The mixture was
homogenized. Li2C03 (0.7% by weight) was added and mixed for one minute,
whereupon the product was filled into a mold.
The xonotlite employed was prepared by cutting/milling a xonotlite product
which
had been prepared by casting and hydrothermal treatment. Such a xonotlite is
in
the form of needles (cf. Figure 1). The resistance towards repeated
temperature
variations was tested by quickly cooling a sample after heating for five
minutes at
1000 °C. The product exhibits satisfactory properties.
Example 2
30% by weight of xonotlite, 10% by weight of tobermorite powder and 56.6% by
weight of aluminous cement (Secar 71) were mixed in a dry state. The xonotlite
employed had been prepared by "heating with stirring". As explained in the
description, this yields felted globules. The corresponding structure is
represented
in Figure 2. The further processing was as in Example 1, but using a water-to-
solids ratio of 1.8.
The molded part was also tested according to the testing method as described
in
Example 1. The products did not exhibit any cracks upon heating.
Example 3
A mixture of 40% by weight of globular xonotlite and 56.6% by weight of alumi-
nous cement (Secar 71) was mixed. The further processing and testing was as in
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Examples 1 and z, also with a water-to-solids ratio of 1.8. The product did
not
exhibit any cracks upon heating.
