U.S. patent application number 12/994004 was filed with the patent office on 2011-04-07 for cellular ceramic plates with asymmetrical cell structure and manufacturing method thereof.
Invention is credited to Hans Strauven.
Application Number | 20110082023 12/994004 |
Document ID | / |
Family ID | 39616021 |
Filed Date | 2011-04-07 |
United States Patent
Application |
20110082023 |
Kind Code |
A1 |
Strauven; Hans |
April 7, 2011 |
CELLULAR CERAMIC PLATES WITH ASYMMETRICAL CELL STRUCTURE AND
MANUFACTURING METHOD THEREOF
Abstract
A method for the continuous production of a cellular ceramic
plate having asymmetric cells comprising thermally treating ceramic
particles and a blowing agent in a foaming furnace while conveying
said ceramic particles and said blowing agent at a first speed
thereby forming a cellular ceramic plate, and annealing said
cellular ceramic plate in an annealing lehr by cooling it down
while conveying it at a second speed, larger than said first speed,
thereby stretching and cooling said cellular ceramic plate.
Inventors: |
Strauven; Hans; (Lummen,
BE) |
Family ID: |
39616021 |
Appl. No.: |
12/994004 |
Filed: |
May 25, 2009 |
PCT Filed: |
May 25, 2009 |
PCT NO: |
PCT/EP2009/056329 |
371 Date: |
November 22, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61055733 |
May 23, 2008 |
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Current U.S.
Class: |
501/80 ; 264/54;
425/4C |
Current CPC
Class: |
C03B 25/08 20130101;
C03B 19/08 20130101; C03B 35/163 20130101 |
Class at
Publication: |
501/80 ; 264/54;
425/4.C |
International
Class: |
C08J 9/12 20060101
C08J009/12; B29C 44/28 20060101 B29C044/28; C04B 38/00 20060101
C04B038/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 23, 2008 |
GB |
0809441.9 |
Claims
1-16. (canceled)
17. A method for the continuous production of a one-piece
continuous cellular ceramic plate comprising: a) thermally treating
ceramic particles and a blowing agent in a foaming furnace while
conveying said ceramic particles and said blowing agent at a first
speed thereby forming a one-piece continuous cellular ceramic
plate, and b) annealing said one-piece continuous cellular ceramic
plate in an annealing lehr by cooling it down while conveying it at
a second speed, larger than said first speed, thereby stretching
and cooling said one-piece continuous cellular ceramic plate.
18. The method according to claim 17 wherein prior to step (b), the
one-piece continuous cellular ceramic plate is transferred from
said foaming furnace to said annealing via an intermediate conveyor
at a third speed higher or equal to said second speed.
19. The method according to claim 17 or 18, wherein the difference
between the second speed and the first speed is 25% or less of the
first speed.
20. The method according to claim 18, wherein the difference
between the third and second speed is between 0 and 10% of the
first speed.
21. The method according to 17, wherein the one-piece continuous
cellular ceramic plate is a one-piece continuous glass foam
plate.
22. An apparatus for the continuous production of a one-piece
continuous cellular ceramic plate comprising: a) a foaming furnace
arranged to treat in one continuous piece ceramic particles and a
blowing agent, said foaming furnace comprising a first conveyor
adapted to convey at a first speed while heating said one-piece
continuous ceramic particles and said blowing agent to form a one
piece continuous cellular ceramic plate, and b) an annealing lehr
arranged to anneal said one-piece continuous cellular ceramic plate
by cooling it down, said annealing lehr being located downstream
from said foaming furnace and comprising a second conveyor adapted
to convey said one-piece continuous cellular ceramic plate at a
second speed, larger than said first speed.
23. The apparatus according to claim 22 further comprising an
intermediate conveyor prior to the second conveyor arranged to
transfer the one-piece continuous cellular ceramic plate from said
foaming furnace to said annealing lehr.
24. The apparatus of claim 23, wherein the intermediate conveyor is
adapted to convey at a third speed higher or equal to said second
speed.
25. The apparatus according to claim 22, wherein said first and
second conveyors are driven in such a way that the difference
between the second speed and the first speed is 25% or less of the
first speed.
26. The apparatus according to claim 24 or claim 25, wherein the
difference between the third and second speed is between 0 and 10%
of the second speed.
27. The apparatus according to claim 23, wherein the first conveyor
is resistant to higher temperatures than said second conveyor.
28. The apparatus according to claim 27, wherein the first conveyor
is resistant to a temperature up to 900.degree. C. and wherein said
second conveyor is resistant to a temperature up to 600.degree.
C.
29. The apparatus according to claim 23, wherein the intermediate
conveyor comprises rolls.
30. The apparatus according to claim 23 wherein the intermediate
conveyor is located at the beginning of the annealing lehr or in an
intermediate lehr located between the foaming furnace and the
annealing lehr.
31. The apparatus according to claim 23 wherein the intermediate
conveyor is resistant to temperatures in the range 600.degree.
C.-800.degree. C.
32. A ceramic plate comprising a one piece continuous cellular
ceramic plate that has been stretched whole hot and cooled in
stretched condition.
33. A one piece continuous cellular ceramic plate made by the
process according to claim 17.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to methods and apparatus for
the continuous production of cellular ceramic products (e.g.
cellular ceramic plates such as glass foam plates). The present
invention also relates to cellular ceramic products obtained by
such method.
TECHNICAL BACKGROUND
[0002] There are several methods known for the manufacture of
cellular ceramic materials involving foaming. Examples are:
[0003] a) insertion (incorporation e.g. injection) and mechanical
distribution of gases in a low viscosity melt.
[0004] b) Release and expansion of dissolved gases in a low
viscosity melt under vacuum.
[0005] c) Insertion (incorporation) of foaming agents in a
melt.
[0006] d) Mixing glass powder with a foam agent and subsequent
heating.
[0007] In the case of the last glass foaming process, the
manufacturing equipment typically comprises a foaming furnace with
a belt carrying the glass powder and foam, and a powder loading
apparatus. The foaming involves the foaming of either thick or thin
plates.
SUMMARY OF THE INVENTION
[0008] The present invention results from the observation that
stretching of cellular ceramic products (such as foam glass) during
production brings about changes in the physical properties of the
final material such as the thermal insulation of the cellular
ceramic product (such as glass foam). Adaptation of these physical
properties is therefore possible. This is advantageous as it
permits the properties of the glass to be tailored to customer
needs (e.g. stretching lowers the thermal conductivity, i.e.
improves insulation properties) In a first aspect, the present
invention relates to a method for the continuous production of a
cellular ceramic plate (e.g. a one-piece ceramic plate)
comprising:
[0009] a) thermally treating ceramic particles and a blowing agent
in a foaming furnace while conveying said ceramic particles and
said blowing agent at a first speed to thereby form a cellular
ceramic plate, and
[0010] b) annealing said cellular ceramic plate in an annealing
lehr by cooling it down while conveying it at a second speed,
larger than said first speed, thereby stretching and cooling said
cellular ceramic plate.
[0011] In an embodiment of the first aspect, the present invention
relates to a method wherein prior to step (b), the (one-pieced)
cellular ceramic plate is transferred from said foaming furnace to
said annealing lehr via an intermediate conveyor at a third speed
higher or equal to said second speed. This is advantageous as it
enables the stretching of the cellular ceramic plate to be
performed in a zone of relatively high temperature without the need
for the annealing conveyor (second conveyor) to be resistant to
said relatively high temperature. Stretching at relatively low
temperatures induces more stress in the foam than stretching at
relatively high temperatures. Preferably, only the intermediate
conveyor, which can be shorter (e.g. much shorter) than the
annealing conveyor, is adapted to be resistant to said relatively
high temperature. Since the annealing requires a relatively long
conveyor, it is economical to use an annealing conveyor which is
not adapted to be resistant to relatively high temperatures (e.g.
when an intermediate conveyor is used, a second conveyor resistant
to a temperature up to 600.degree. C. is enough and there is no
need to use a second conveyor resistant to a temperature up to
800.degree. C. or 900.degree. C. in the long annealing lehr).
[0012] In an embodiment of the first aspect, the present invention
relates to a method wherein the difference between the second speed
and the first speed may be 25% or less of the first speed,
preferably between 1 and 25%, more preferably between 2 and 20%,
most preferably between 3 and 15%. Stretching in these ranges
provides improvement in heat insulation (lower k-values) while
simultaneously keeping the amount of breakage relatively low. In
general, to decrease breakage at high speed differences, a higher
stretching temperature is helpful.
[0013] In an embodiment of the first aspect, the present invention
relates to a method wherein the difference between the third and
second speed is between 0 and 10% (or between 1 and 10%), 0 to 5%
(or 1 to 5%) being preferred. Some pre-stretching is advantageous
as it allows the one-pieced cellular ceramic plate to shrink during
the annealing, thereby releasing stress and reducing the fracture
tendency.
[0014] In an embodiment of the first aspect, the present invention
relates to a method wherein the stretching is between 3 and 15%.
This is advantageous as it is in this range that the ceramic
cellular plate formed has simultaneously acceptable compressive
strength and improved insulation properties in comparison with an
otherwise identical non-stretched cellular ceramic plate.
[0015] In an embodiment of the first aspect, the present invention
relates to a method wherein the cellular ceramic plate may be a
glass foam plate.
[0016] The foaming step may produce open or closed cells. For
insulating purposes closed cells are preferred. In the case of
foamed glass, open cells can be obtained by addition of some
crystalline material (such as e.g. TiO.sub.2) to the amorphous
glass powder. For instance, adding around 1% TiO.sub.2 during
grinding (e.g. in a ball mill) of the glass can lead to 100% open
cells in a glass foam. When closed cells are required, the addition
of TiO.sub.2 or similar crystalline material is preferably
avoided.
[0017] In a second aspect, the present invention relates to an
apparatus for the continuous production of a cellular ceramic plate
comprising:
[0018] a) a foaming furnace for thermally treating ceramic
particles and a blowing agent while conveying at a first speed to
thereby form a cellular ceramic plate, and
[0019] b) an annealing lehr for annealing said cellular ceramic
plate by cooling it down while conveying it at a second speed,
larger than said first speed, thereby stretching and cooling said
cellular ceramic plate.
[0020] In other words, the second aspect of the present invention
relates to an apparatus for the continuous production of a cellular
ceramic plate comprising:
[0021] a) a foaming furnace for thermally treating ceramic
particles and a blowing agent, said foaming furnace comprising a
first conveyor adapted for conveying at a first speed (i.e. linear
speed) while heating said ceramic particles and said blowing agent
to form a cellular ceramic plate, and
[0022] b) an annealing lehr for annealing said cellular ceramic
plate by cooling it down, said annealing lehr being downstream from
said foaming furnace and comprising a second conveyor adapted for
conveying said cellular ceramic plate at a second speed (i.e.
linear speed), larger than said first speed.
[0023] For the purpose of obtaining a second linear speed higher
than the first linear speed, independent driving means may be
provided for said first and said second conveyor.
[0024] In an embodiment of the second aspect, the apparatus further
comprises an intermediate conveyor prior to the second conveyor for
transferring the cellular ceramic plate from said first conveyor
(e.g. from said foaming furnace) to said second conveyor (e.g. from
said annealing lehr). The presence of the intermediate conveyor
between said first and second conveyors permits the use of a second
conveyor with lower heat resistance than were said second conveyor
to be directly adjacent to the first conveyor. This is particularly
advantageous in view of the resulting reduced costs per unit
length, which is important in view of the relatively large length
of the second conveyor when compared to the intermediate
conveyor.
[0025] In embodiments of the second aspect, where an intermediate
conveyor is present, it may be adapted for conveying at a third
linear speed higher or equal to said second speed. In other words,
it may be driven at said third speed higher or equal to said second
speed. This is advantageous as it permits the stretching of the
cellular ceramic plate to be performed in a zone of relatively high
temperature without the need for the annealing conveyor (second
conveyor) to be resistant to relatively high temperatures. A
stretching performed at a relatively high temperature leads to less
stress than a stretching performed at a relatively low
temperature.
[0026] For the purpose of obtaining a third linear speed higher
than said second speed, independent driving means may be provided
for said third conveyor.
[0027] In embodiments of the second aspect of the present
invention, the first and second conveyors may be adapted for being
driven in such a way that the difference between the second speed
and the first speed is 25% or less of the first speed, preferably
between 1 and 25%, more preferably between 2 and 20%, most
preferably between 3 and 15%. In other words, the first and second
conveyors may be driven in such a way that the difference between
the secand speed and the first speed is 25% or less, preferably
between 1 and 25%. In some embodiments this difference can be
between 5 and 25%.
[0028] In embodiments of the second aspect where an intermediate
conveyor is present, the difference between the third and second
speed may be between 0 and 10%, with between 0 and 5% being
preferred.
[0029] In embodiments of the second aspect of the present
invention, the first conveyor may be adapted to be resistant to
higher temperature than the second conveyor. This is advantageous
as the temperature in the foaming zone is higher than the
temperature in the annealing zone.
[0030] In embodiments of the second aspect of the present
invention, the first conveyor may be adapted to be resistant to a
temperature up to 800.degree. C., preferably up to 900.degree. C.
and the second conveyor may be adapted to be resistant to a
temperature up to 600.degree. C. These temperatures are typical
maximal temperatures for the foaming and the annealing step
respectively.
[0031] In embodiments of the second aspect of the present
invention, the intermediate conveyor may comprise rolls. The
preferred distance between two rolls can vary in function of many
parameters and is preferably set via trial and error. Typically,
this distance can be from 0.2 m to 1.5m. In some embodiment, the
distance between two rolls can be from 0.2 to 0.4 m. In other
embodiments, the distance between two rolls can be at least 0.8 m
and less than 1.5 m.
[0032] This is advantageous because within this range, the distance
is large enough to reduce the number of rolls and therefore the
number of friction zones between the cellular ceramic plate and the
conveyors. This friction causes dust. For larger distances between
the rolls, the risk of jam-up in case of breakage of the cellular
ceramic plate becomes significantly higher. A roll conveyor is
advantageous because it is easier to construct than belt conveyors
and it leads to less jam up of broken cellular ceramic plates when
used at relatively high temperature (for foam glass, this is
especially true above 450.degree. C.).
[0033] In embodiments of the second aspect of the present
invention, the intermediate conveyor may be situated at the
beginning of the annealing lehr or in an intermediate lehr situated
between the foaming furnace and the annealing lehr.
[0034] It is advantageous to adapt the apparatus in such a way that
substantially no temperature gradient (e.g. at least
perpendicularly to the conveying direction) exists in the zone of
the apparatus where stretching occurs (e.g. between the first and
the second conveyor if no intermediate (third) conveyor is present
or between the first and the intermediate (third) conveyor if an
intermediate conveyor is present). This way, fracture of the
cellular ceramic plate during and after production is
minimised.
[0035] In embodiments of the second aspect of the present invention
where an intermediate conveyor is present, the intermediate (third)
conveyor may be resistant to temperatures in the range 600.degree.
C.-800.degree. C., preferably in the range 600.degree.
C.-900.degree. C.
[0036] This is advantageous as the presence of an intermediate
(third) conveyor ensures a more reliable transition between the
first conveyor and the second conveyor.
[0037] In a third aspect, the present invention relates to cellular
ceramic plate having a cell structure, whereby the cells are
asymmetrical, e.g. elongated. In an embodiment, the longest
dimension of the cells (e.g. the length) may on average be larger
than the shortest dimension of the cell (e.g. its dimension
perpendicular to the surface of the plate (height)). In embodiments
of the present invention this ratio between the average largest
dimension of the cells and the average shortest dimension of the
cells may be from 1.2 to 2.5. An advantageous ratio has been found
to be between 1.2 and 1.6 as it provides a trade off between
insulation properties and mechanical properties. For the purpose of
the present invention, this difference in length between the
average largest dimension and the average smallest dimension has
been measured with ultrasonic measurements. For instance, the ratio
of ultrasonic transit times measured lengthwise (in the direction
of the conveying and stretching) on ultrasonic transit times
measured heightwise (perpendicular to the glass foam surface) has
been found to be about 1.4 for glass foam exhibiting a thermal
conductivity of 0.042 W/mK at 10.degree. C. with a density of 115
kg/m.sup.3.
[0038] In an embodiment of the third aspect, the cellular ceramic
plate may have an asymmetrical cell structure and may be obtainable
by any of the methods of the first aspect of the present
invention.
[0039] In an embodiment of the third aspect, the cellular ceramic
plate may be a glass foam plate and/or be a closed cell foam.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040] FIG. 1 schematically shows an apparatus according to an
embodiment of the present invention.
DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0041] Although the present invention will be described in
connection with certain embodiments, it is not intended to be
limited to the specific form set forth herein. Rather, the scope of
the present invention is limited only by the accompanying claims.
In the claims, the term "comprising" does not exclude the presence
of other elements or steps. Additionally, although individual
features may be included in different claims, these may possibly be
advantageously combined, and the inclusion in different claims does
not imply that a combination of features is not feasible and/or
advantageous. In addition, singular references do not exclude a
plurality. Thus, references to "a", "an", "first", "second" etc. do
not preclude a plurality. Furthermore, reference signs in the
claims shall not be construed as limiting the scope.
[0042] Definitions:
[0043] Types of "Cellular Ceramics" include but are not limited to
carbon foams, glass foams, and cellular concrete. Glass foams have
a combination of unique qualities including rigidity, compressive
strength, thermal insulation, non-flammability, chemical inertness,
water/steam resistance, insect/rodent resistance, and are generally
lightweight. Glass foams are generally formed by the action of a
gas-generating agent (e.g. foaming agent), which is mixed with
ground glass (i.e; glass particles). This mixture is heated to a
temperature at which the evolution of gas from the foaming agent
occurs within the softened glass. The gas evolved creates bubbles
which form the cells (e.g. pores) in the final glass foam. Cellular
ceramics according to the present invention preferably have a
density which is from 2 to 45%, preferably 3 to 25%, more
preferably 4 to 10%, of the density of the corresponding plain
(non-cellular) ceramic. In the case of foam glass, the density of
the glass obtained is preferably from 50 to 1000 Kg/m.sup.3,
preferably from 75 to 600 Kg/m.sup.3, most preferably from 90 to
250 Kg/ m.sup.3 or100 to 250 Kg/m.sup.3.
[0044] As used herein and unless provided otherwise, the term
"plate", when relating for instance to cellular ceramic plates,
relates to a three-dimensional object, wider than thick and of any
length. In the context of the first aspect of the present
invention, the term "plate" refers to a continuous one-pieced plate
until it is cut after or at the end of the annealing step. The term
"plate" as used in the context of the third aspect of the present
invention, refers either to said continuous one pieced plate or to
a shorter plate as obtained after transversal cutting of said
continuous plate.
[0045] As used herein and unless provided otherwise, the term
"resisting" when relating to a temperature applied to a conveyor,
means that the conveyor does not substantially deform when exposed
to said temperature for an extended period. For instance, when the
conveyor is a belt conveyor, the elongation of the belt should
preferably not be more than 1% over a 120 days period at said
temperature.
[0046] The term foaming furnace, as used in disclosing the present
invention, means a furnace in which cellular ceramic (e.g. foamed
glass) is produced.
[0047] In a first aspect, the present invention relates to a method
for the continuous production of a (one-piece) cellular ceramic
plate. By continuous production, is meant the production of a
one-piece (continuous) cellular ceramic plate, as opposed to
batch-wise production methods such as e.g. moulding. In the
continuous production processes according to the first aspect of
the present invention, a single piece of cellular ceramic plate is
produced which is cut into plates of well-defined length at the end
of the annealing step or after the annealing step.
[0048] In embodiments of the present invention, said method
comprises the steps of: (a) thermally treating ceramic particles
and a blowing agent in a foaming furnace thereby forming a cellular
ceramic plate, and (b) annealing said cellular ceramic plate in an
annealing lehr by cooling it down. In a preferred embodiment, the
cellular ceramic plate is a glass foam plate.
[0049] In an embodiment, the thermal treatment can be heating up
the ceramic particles and the blowing agent to a temperature high
enough to induce the formation of a cellular ceramic material. This
temperature can, for instance, be comprised between 650 and
850.degree. C. in the case of glass foam, with from 700 to
800.degree. C. being preferred. The ceramic particles may have any
shape, size or aspect ratio known in the art to permit the
production of cellular ceramic bodies. The ceramic particles
specific surface area is preferably from 0.5 m.sup.2/g to 1
m.sup.2/g as measured by Brunauer Emmett Teller (BET)-analysis.
Blowing agents usable in embodiments of the first aspect of the
present invention comprise any blowing agent known in the art
enabling the production of cellular ceramic bodies. They can
comprise but are not limited to carbon black and carbonates (e.g.
calcium carbonate or sodium carbonate). The proportion of blowing
agent to ceramic particles can be any proportion known in the art
to permit the production of cellular ceramic bodies. Preferably, it
is between 0.1% and 2%. For carbon black, it is preferably from 0.2
to 0.6% and particularly preferably from 0.3 to 0.5%. For
carbonates, it is preferably between 0.7 and 1.3%, preferably from
0.8 to 1.2%.
[0050] In embodiments of the present invention, the annealing step
may be a slow temperature decrease according to a prescribed
temperature profile.
[0051] Although this is not the object of the present invention,
the temperature profile of the whole foaming and annealing process
has an influence on the appearance of defects in the foamed glass
product obtained via a stretching procedure as described herein.
The fine-tuning of this temperature profile is a question of trial
an error and is well within the skills of the person skilled in the
art without undue experimental effort. In general, the purpose of
these temperature profiles is to relax at a smaller temperature
gradient to reduce residual strain. It is also advantageous to have
smooth temperature transitions between the different sections of
the apparatus.
[0052] In a preferred embodiment of the first aspect of the present
invention, step (a) is performed while conveying said ceramic
particles and said blowing agent at a first speed thereby forming a
ceramic plate and step (b) is performed while conveying said
cellular plate obtained in (a) at a second speed, larger than said
first speed, thereby stretching said cellular plate. The first
speed can, for instance, range from 1 to 100 cm/min depending upon
the ceramic material foamed and the plate thickness to be produced.
For instance, it can range from 1 to 15 cm/min in some embodiments.
In embodiments of the present invention, the use of a first
conveyor conveying the foam at a first speed in the foaming zone
and the use of a second conveyor in the annealing zone conveying
the foam at a second speed higher than said first speed, permits
the cellular ceramic foam, e.g. the glass foam, to be
stretched.
[0053] In an embodiment of the present invention, the speed of the
second conveyor, i.e. the speed of the annealing conveyor, is
higher than the speed of the foaming conveyor (i.e. the first
conveyor). Especially during the production start (e.g. when the
production has been interrupted and must be restarted), it is very
advantageous to have a distinctly higher (e.g. between 3 and 20%
higher, preferably between 4 and 20% higher, more preferably 7%-20%
higher, for instance 8% or higher) second conveyor speed in the
lehr than in the first conveyor in the foaming zone. After the
production start, the second conveyor speed can be reduced to be,
for instance, around 3%. The difference between the first speed and
the second speed is preferably 25% or less, more preferably between
from 3 to 25% or from 5 to 25%. As an example, it has been shown
that stretching a continuous glass foam string during production by
up to 20%, e.g. up to 10%, reduces the k-value and decreases the
compressive strength. In embodiments of the present invention, the
preferred stretching is between 3 and 25% (e.g. between 5 and
25%).
[0054] In an embodiment of the present invention, the stretching of
the foam is obtained by using separate conveyors for the foaming
and for the annealing of the foamed glass.
[0055] In an embodiment of the first aspect of the present
invention, prior to step (b) and after step (a), the cellular plate
is transferred from said foaming furnace to said annealing lehr via
an intermediate conveyor. Preferably, said intermediate conveyor
conveys at a third speed higher or equal to said second speed.
Preferably, the difference between the third speed and the second
speed is between 0 and 10% of the second speed. In another
embodiment of the present invention, the intermediate conveyor
(e.g. the rolls of the intermediate conveyor when it comprises
rolls) may be coupled with the second conveyor in such a way that
the linear speed of the intermediate conveyor equals that of the
second conveyor. As a consequence, the stretching occurs between
the first conveyor (e.g. a foaming belt) and the intermediate
conveyor (e.g. the 1.sup.st roll of the intermediate conveyor),
where the temperature is higher. As an optional feature, a
pre-stretch of the foam by using a speed for the intermediate
conveyor up to a value a few % of the second speed higher (e.g.
between 1 and 10%, for instance 5%) than the value in the annealing
lehr is performed between the first and the intermediate conveyor.
This allows the foam to shrink in the lehr and the final stretching
is the stretching due to the difference in speed between the first
conveyor and the second conveyor. Shrinking of the ceramic cellular
material in the lehr reduces stress and breakage.
[0056] For all embodiments of the first aspect of the present
invention, it is preferred that the temperature distribution across
the width of the cellular ceramic plate is as uniform as possible
in the zone where the stretching occurs. In embodiments, the
temperature distribution across the width of the cellular ceramic
plate span 20.degree. C. or less in the zone where the stretching
occur. This can be obtained, for instance, by isolating said zone
from the rest of the apparatus (avoiding drafts (e.g. air and flue
gas currents)) and/or by adapting the position of heaters with
individual temperature control. In an embodiment, the zone where
the stretching occurs (e.g. the zone in between the first conveyor
and the intermediate conveyor) is adapted to experience a local
minimum of currents. This means that zones situated directly
upstream or downstream from said zone where the stretching occurs,
experience more drafts than said zone where the stretching
occurs.
[0057] In a second aspect, the present invention relates to an
apparatus for the continuous production of a cellular ceramic
plate. This apparatus is adapted to perform the steps of the method
of the first aspect. The apparatus of the present invention
comprises a foaming furnace and an annealing lehr. The foaming
furnace is suitable for thermally treating ceramic particles and a
blowing agent. The treatment temperature may vary depending upon
the nature of the particles used. For instance, in the case of
glass particles it can be between 600.degree. C. and 950.degree. C.
and is preferably between 650.degree. C. and 800.degree. C. during
most of the foaming process. The annealing lehr is suitable for
annealing the cellular ceramic plate by cooling it down in a
controlled manner. The annealing lehr is downstream from the
foaming furnace. The apparatus also comprises at least two
conveyors: a first conveyor and a second conveyor. The conveyor
used for the foaming will be hereafter referred to as the first
conveyor. The first conveyor is situated in the foaming zone (e.g.
it is comprised in said foaming furnace). A suitable conveyor for
this purpose is an endless metallic belt with holes filled with a
suitable ceramic material. The conveyor used for the annealing will
be hereafter referred to as the second conveyor. The second
conveyor is comprised in the annealing lehr. A suitable conveyor
for the annealing lehr can for instance be a belt or rolls.
[0058] The length of the foaming conveyor can be for instance (in
the case of glass foaming) from 35 to 75 m, e.g. from 45 to 55 m.
The length of the annealing conveyor can be for instance (in the
case of glass foaming) from 150 to 300 m, preferably from 200 to
280 m. In general, these dimensions can be made smaller or larger
by decreasing or increasing the conveying speed respectively. Much
smaller (see the example below on a pilot size line) or larger
dimension are therefore useable. In embodiments of the present
invention, the ratio between the length of the second conveyor and
the length of the first conveyor is from 2 to 8.
[0059] In a preferred embodiment of the second aspect of the
present invention, the first conveyor is adapted to convey at a
first speed while the second conveyor is adapted to convey at a
second speed, higher than said first speed. Preferably, the first
conveyor and the second conveyor are adapted for being driven in
such a way that the difference between the second speed and the
first speed is 25% or less of the first speed, more preferably from
1 to 25% and most preferably from 3 to 5%. In embodiments, this
difference can be from 5 to 25%. In embodiments of the present
invention, stretching implies a higher speed for the second
conveyor than for the first conveyor and therefore a longer lehr
than were there to be no stretching. If 20% of stretching is
required, a 20% longer second conveyor is preferably used. In other
words, the length of the conveyor is preferably proportional to the
required stretching.
[0060] In an embodiment of the second aspect of the present
invention, the first conveyor is preferably adapted to be resistant
to higher temperatures than said second conveyor. More preferably,
the first conveyor is adapted to be resistant to a temperature up
to 800.degree. C. or even 900.degree. C. or 950.degree. C. A
suitable first conveyor can for instance be a metallic mesh belt
filled in with a suitable ceramic (e.g. a ceramic resistant to said
temperature without shrinking substantially). More preferably, the
second conveyor is resistant to higher temperatures up to e.g.
800.degree. C., preferably 900.degree. C. if no intermediate
conveyor is used between the first conveyor and the second conveyor
and to a lower temperature (e.g. up to 600.degree. C.) if an
intermediate conveyor is used between the first conveyor and the
second conveyor. In an embodiment wherein an intermediate conveyor
is used, the second conveyor may be adapted to be resistant to a
temperature up to 600.degree. C.
[0061] The second conveyor being relatively long, and the
temperature at the end of the foaming zone (i.e. foaming furnace)
being relatively high (up to 800.degree. C. or even up to
900.degree. C. or 950.degree. C. in the case of glass foam plates),
it is advantageous to have an intermediate conveyor between the
foaming zone and the annealing conveyor, which is resistant to
relatively high temperature (e.g.
[0062] in the range 600.degree. C-800.degree. C.). Hence, in some
embodiments of the present invention, it is advantageous to use one
or more intermediate conveyors between the first conveyor and the
second conveyor. In embodiments of the second aspect of the present
invention, the apparatus further comprises at least a third
conveyor (also referred to as intermediate conveyor(s) in the rest
of the text). Preferably a single intermediate conveyor is used and
although the rest of the description will refer to a single
intermediate conveyor, it applies mutatis mutandis to multiple
intermediate conveyors. The presence of this intermediate conveyor
permits the use of a less temperature-resistant and therefore
cheaper second conveyor (e.g. one only resistant to temperatures up
to 600.degree. C. in the case of glass foam plates production).
This is particularly advantageous in view of the relatively long
length and therefore high cost of the second conveyor. Said
intermediate conveyor is preferably adapted to convey at a third
speed equal or higher than the second speed. In an embodiment of
the present invention, a separate driving system is provided on the
intermediate conveyor (e.g. on the rolls of the intermediate
conveyor). As a consequence, the equipment is able to generate a
different linear speed for the intermediate conveyor than for the
first or second conveyor. More preferably, the difference between
the third and second speed is between 0 and 10%, preferably between
0 and 5%. In a preferred embodiment of the present invention, when
an intermediate conveyor comprising rolls is used between the first
and the second conveyor, the rolls may be driven at such a speed
that the conveying speed of the intermediate conveyor is the same
or up to 10%, preferably 5% faster than the conveying speed of the
second conveyor. Preferably, the intermediate conveyor is resistant
to temperatures in the range 600.degree. C-800.degree. C., i.e. up
to 800.degree. C., more preferably up to 850.degree. C. The length
of the intermediate conveyor can for instance be from 2% to 30% of
the length of the second conveyor, preferably being from 3% to 20%
of the length of the second conveyor. The intermediate conveyor
preferably comprises rolls. This is advantageous as it is easier
and cheaper to build and breakage is less likely with rolls when
the ceramic (e.g. glass) is at a temperature high enough for it to
be viscoelastic. The preferred distance between two rolls is better
determined by trial and error as it depends upon many parameters.
Typically it can range from 0.2 to 1.2 m. A preferred distance is
from 0.2 to 0.4 m, with in some embodiments 0.6 m or more and less
than 1.5 m being used. In other embodiments, 0.8 m or more and less
than 1.2 m can be used. In yet other embodiments, between 0.9 and
1.2 m can be used. In some embodiments, a useful value has been
found to be 0.3 m. In other embodiments, a useful value has been
found to be 1 m. The intermediate (third) conveyor is placed prior
to (i.e. upstream from) the second conveyor. It is preferably
situated at the beginning of the annealing lehr or in an
intermediate lehr situated between the foaming zone/furnace and the
annealing lehr. Said intermediate conveyor is suitable for
transferring a cellular ceramic plate from the foaming furnace to
the annealing lehr. In an embodiment of the present invention,
transversal temperature gradients (temperature differences across
the plate) where stretching occurs are preferably avoided.
Preferably, the transversal temperature gradient (temperature
difference across the plate) where stretching occurs is of
20.degree. C. or less. This can for instance be achieved by
installing heaters with separate temperature control at the
appropriate places.
[0063] In a third aspect, the present invention relates to cellular
ceramic plates having a cell structure, whereby the cells are
asymmetrical. The plate obtained in the method of the first aspect
is a one-piece continuous plate that can be cut in any desired
dimensions after or at the end of the annealing step. Due to
stretching, properties in the final material are different from
those of non-stretched plates. The dimension and shape of the cells
within a cellular ceramic plate (e.g. a glass foam plate) stretched
according to an embodiment of the present invention are as follows:
the average diameter of the cells is preferably smaller than 1mm
and the cell shape will on average be asymmetrical with one
dimension larger than the other. Preferably, one dimension is
larger than the other by a factor in ultrasonic transit time
comprised between 1.2 and 1.6, preferably 1.3 and 1.5, e.g. about
1.4.
[0064] In an embodiment of the third aspect, the present invention
relates to cellular ceramic plates obtainable by any of the methods
of the first aspect of the present invention.
EXAMPLES
Example 1
Pilot Line
[0065] A glass foam plate was produced according to the first
aspect of the present invention. For this example, various glass
foam plates were produced with an apparatus comprising a powder
loading apparatus, a foaming furnace including a first conveyor, an
intermediate zone including an intermediate (third) conveyor and an
annealing lehr including a second conveyor. The glass powder was
applied on the foaming conveyor in an amount of 8000 cm.sup.2/g.
The foaming oven was 10 m long. The first conveyor was a
powder-tight refractory steel belt filled with a heat-stable
ceramic material. Its linear speed was ca. 3 cm/min. The
temperature in the foaming furnace was between 650 and 670.degree.
C. at the beginning of the furnace and between 750 and 770.degree.
C. at the end of the furnace. The intermediate (third) conveyor was
a set of water cooled rolls. The temperature in the intermediate
zone was between 650 and 680.degree. C. at the beginning of the
intermediate zone and reached a maximum of 800.degree. C. between
the beginning and the end of the zone and was around 700.degree. C.
at the end of the intermediate zone. The length of the intermediate
conveyor was 1 meter. Its rolls were driven at a speed about 5%
higher than the speed of the second conveyor. The second conveyor
was another set of rolls (the use of a belt would also have been
suitable) and the temperature in the annealing lehr was about
600.degree. C. at the beginning of the lehr down to room
temperature (20-40.degree. C.) at the end of the lehr. Its length
was about 22 m. The second conveyor had a linear speed 5, 10 and
15% above the speed of the first conveyor leading to foam-glass
plates having a density of 105 Kg/m.sup.3 with stretching of 5, 10
and 15% respectively. The foam-glass plates could thereafter be
sawed laterally and/or horizontally and/or transversally.
[0066] The relative speeds of the first, second and intermediate
conveyors in these examples were as follow:
[0067] The first speed was always about 3 cm/min. For a stretch of
5, 10 or 15%, the second speeds were respectively 5, 10 or 15%
higher than the first speed. The third speed (i.e. the speed of the
intermediate conveyor) was 5% higher than the second speed. For a
stretching up to 15%, we obtained the following results in table 1
below.
TABLE-US-00001 TABLE 1 Stretch [%] compressive strength
[N/mm.sup.2] k-value [W/mK] 5 0.9 0.0415 10 0.77 0.0413 15 0.7
0.0408
[0068] The results in table 1 show that stretching brings about a
decrease in the k-value and a decrease in the compressive strength.
Other densities or ceramics would give different results.
[0069] We obtained improvement in the mechanical properties with a
conveying speed for the second conveyor 20% higher than for the
conveying speed in the foaming furnace. This led to a stretch of
20%. In this way, we obtained foams with an approximate thickness
of 16 cm at 120 kg/m.sup.3.
[0070] With a set-up wherein an intermediate conveyor is used at a
speed higher than the speed of the second conveyor and therefore a
pre-stretching up to a first value, e. g. 25% with a net stretching
equal to a second value lower than the first value, e.g. 20%, it
was possible to anneal a 16cm thick foamed glass sheet with a 120
kg/m.sup.3 density without lehr breakage, only 10% delayed breakage
and a defect free bottom at 3.18 cm/min for the first conveyor.
[0071] FIG. 1 schematically shows an apparatus according to an
embodiment of the present invention.
[0072] In this figure, a first conveyor 1, a second conveyor 2, and
an intermediate conveyor 5 are shown. The first conveyor 1 conveyed
the foaming glass through the foaming furnace 3 and transferred the
foamed-glass ribbon to the intermediate conveyor 5. The
intermediate conveyor 5 conveyed the foamed glass ribbon through
the intermediate lehr 6 and transferred the foamed-glass ribbon to
the second conveyor 2. The second conveyor 2 conveys the foamed
glass ribbon through the annealing lehr 4.
* * * * *