U.S. patent number RE37,012 [Application Number 09/291,087] was granted by the patent office on 2001-01-09 for froth system for continuous manufacture of polyurethane foam slab-stocks.
This patent grant is currently assigned to Foaming Technologies, Cardio BV. Invention is credited to Carlo Fiorentini, Anthony C. M. Griffiths.
United States Patent |
RE37,012 |
Fiorentini , et al. |
January 9, 2001 |
Froth system for continuous manufacture of polyurethane foam
slab-stocks
Abstract
A process and a system for the continuous manufacture of
polymeric foams. Reactive chemical components and additives
comprising a low boiling blowing agent are mixed under pressure;
the mixture is then frothed before chemical reaction takes place by
feeding the mixture through a pressure equalizing and frothing
device having a pressure-drop zone opening into a frothing cavity
having an output aperture to discharge the froth onto a moving
substrate.
Inventors: |
Fiorentini; Carlo (Saronno,
IT), Griffiths; Anthony C. M. (Paphos,
CY) |
Assignee: |
Foaming Technologies, Cardio BV
(NL)
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Family
ID: |
26331021 |
Appl.
No.: |
09/291,087 |
Filed: |
April 14, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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271918 |
Jul 8, 1994 |
5665287 |
|
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Reissue of: |
480242 |
Jun 7, 1995 |
05620710 |
Apr 15, 1997 |
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Foreign Application Priority Data
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|
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Jul 14, 1993 [IT] |
|
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M193A1546 |
Sep 30, 1993 [IT] |
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M193A2090 |
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Current U.S.
Class: |
425/4C; 425/224;
425/817C; 425/89 |
Current CPC
Class: |
C08J
9/30 (20130101); B29C 44/462 (20130101); B29C
44/348 (20130101); C08J 2375/04 (20130101) |
Current International
Class: |
C08J
9/00 (20060101); B29C 44/46 (20060101); C08J
9/30 (20060101); B29C 44/34 (20060101); B29C
044/28 (); B29C 044/50 () |
Field of
Search: |
;425/4C,89,224,817C |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1169648 |
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May 1964 |
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DE |
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61613 |
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May 1968 |
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DE |
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1504091 |
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Jul 1969 |
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DE |
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1956419 |
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Jun 1970 |
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DE |
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058553 |
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Aug 1982 |
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EP |
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084384 |
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Jul 1983 |
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EP |
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114741 |
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Aug 1984 |
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EP |
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1524032 |
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May 1968 |
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FR |
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2023961 |
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Aug 1970 |
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FR |
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2517591 |
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Jun 1983 |
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FR |
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2116574 |
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Sep 1983 |
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GB |
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1190554 |
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Feb 1988 |
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IT |
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WO 91/08243 |
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Jun 1991 |
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WO |
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WO 91/12287 |
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Aug 1991 |
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WO |
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Primary Examiner: Davis; Robert
Attorney, Agent or Firm: Pillsbury Madison & Sutro
LLP
Parent Case Text
This is a division of application Ser. No. 08/271,918, filed Jul.
8, 1994.Iadd., which is now U.S. Pat. No. 5,665,287.Iaddend..
Claims
What is claimed is:
1. A device for controlling frothing of a polyol, isocyanate and
liquid CO.sub.2 blowing agent mixture .[.comprised of.].
.Iadd.comprising .Iaddend.a housing having a pressure equalization
chamber provided therein and an inlet for the mixture leading into
said pressure equalization chamber, at least one outlet aperture
leading away from said pressure equalization chamber and a frothing
cavity attached to said housing and positioned to receive the
mixture flowing from said at least one outlet aperture, said at
least one outlet aperture being dimensioned to maintain back
pressure on the upstream mixture to keep the .[.liquid blowing
agent.]. .Iadd.mixture .Iaddend.in a liquid state and to initiate
frothing under pressure controlled conditions to avoid turbulent
evaporation of the blowing agent upon discharge of the mixture from
the outlet aperture, said frothing cavity .[.being comprised of.].
.Iadd.comprising .Iaddend.a series of interconnected and
differently oriented wall members that deflect the direction of
flow of the mixture flowing from said at least one outlet
aperture.
2. A device as in claim 1 wherein said at least one outlet aperture
comprises a slot having a length elongated in the direction of
flow.
3. A device as in claim 1 wherein said frothing cavity forms the
mixture flowing therethrough into a progressively non-reactive
expanding frothing material by progressively releasing the blowing
agent into the frothing material as the frothing material flows
along the frothing cavity.
4. A device as in claim 1 wherein said frothing cavity further
includes at least one substantially right angle turn positioned
downstream from said at least one outlet aperture.
5. A device as in claim 4 wherein said substantially right angle
turn is positioned directly adjacent said at least one outlet
aperture.
Description
FIELD OF THE PRESENT INVENTION
The present invention relates to the production of polymeric foams
by using the frothing technique, and more particularly is directed
to a process and a system for the continuous production of flexible
and rigid slab-stocks, normally used for providing panelling, soft
cushioning, and the like.
BACKGROUND OF THE PRESENT INVENTION
Polymeric foams in particular polyurethane foams are well known. In
general their preparation requires the mixing of reactive chemical
components, such as a polyol and an isocyanate, in the presence of
normally used additives such as a suitable catalyst, a surfactant
or cell control agent, and water which chemically reacts with the
isocyanate to produce the carbon dioxide for blowing the foam.
In the continuous production of flexible foams and particularly in
the production of flexible foams in slab-stocks, as currently
practised on conventional machines, it is common practice to spread
or pour a thin layer of the mixture in a liquid state onto a moving
sheet substrate provided on a slightly sloped conveyor and then the
foam is allowed to rise freely, due to reaction between the
chemical components, until the total expansion of the foam is
obtained. The foam is then allowed to cure and thereafter is
cross-sawn into blocks. Convention process and apparatus are
described, for example, in U.S. Pat. Nos. 3,325,823 and
4,492,664.
In order to avoid a situation where the liquid mix underrunning the
foam, and to allow the production of uniform blocks, use of a small
slope, and high speed for the conveyor and high chemical out put
are usually required. This results in costly and large space
consuming machines, as well as in an excessively high production
rate and very large scale plants.
In an attempt to partially remedy the problems and disadvantages
involved by a conventional process, U.S. Pat. No. 3,786,122
suggests an alternative foaming procedure, in which liquid
reactants are mixed and introduced in the liquid state, at the
bottom of a prefoaming trough. This allows the mixture to expand
upwardly causing the pre-expanded mixture to flow out of the
vessel, on a channel-shaped sheet material travelling on a conveyor
device. Although this process eliminates the use of reciprocating
mixing heads in the production of a continuous slab of a polymeric
foam, nevertheless some problems arise due to "build-up" of the
foam in the trough which causes a progressive narrowing or
reduction of the useful volume of the trough and consequently a
reduction in the residence time in the through. The partly expanded
foam is still of a relatively high density and low viscosity and
limits the slope angle that can be used for the foaming conveyor.
Consequently, the risk of foam underrunning, therefore, still
exists.
Although the main object of the U.S. Pat. No. 3,786,122 was to
employ a conveyor shorter in length, running at a speed slower than
the conveyor in a conventional machine, the fact that the mixture
emerging from the vessel is still in a liquid state practically
prevents the speed and the length of the conveyor to be reduced to
any substantial extent. Therefore the resulting slab-stock foaming
machine was large and still required large spaces. A Publication PU
Handbook G. HOERTEL Ed. Carl Hanser Verlang--1985 pages 162-168
describes a further variation of the pouring technique in the
attempt to achieve an uniform lay-down of a mixture of polyurethane
components by a fixed mixhead. This mixture being discussed by
Hoertel is not a froth nor does blowing occur. According to Hoertel
the mixture is spread onto the whole width of the conveyor by the
use of a distribution bar through which the mixture is delivered
across a broad front. In the same manner the resulting flow is
similar to that resulting from the trough used in the U.S. Pat. No.
3,786,122; depending on the volume of the distribution bar and the
chemical reactivity of the foam system, the mixture is delivered
onto a substrate as a clear (no reaction) or an already creamy
(reaction started) liquid. Consequently, density and viscosity of
the mixture still depend on the volume of the distribution bar in
which chemical foaming takes place. Therefore, the distribution bar
according to Hoertel does not substantially differ from the trough
system of U.S. Pat. No. 3,786,122.
The frothing process is a well known technique in polyurethane
technology but not in the production of slab stock foam. When
frothing, a non-reactive inert gas, or blowing agent, is mixed
under pressure in a liquid state or in solution with the
polyurethane chemical components in a mixer. The pressure is
subsequently released causing the frothing or pre-expansion. The
vaporization of the blowing agent causes the cells to grow and to
foam the liquid reaction mixture which cures to form an
elastomer.
Typical blowing agents are the various chlorofluorocarbons (CFC),
however certain environmental problems are associated with the use
of CFC materials. Therefore, many attempts have been made to
produce foamed polyurethane materials, by frothing with carbon
dioxide (CO.sub.2).
Carbon dioxide (CO.sub.2) as non-reactive blowing agent, in the
frothing technique, has been suggested for example by U.S. Pat.
Nos. 3,184,419 and 5,120,770.
According to these two patents, the reaction mixture is subjected
to a pressure during mixing, to maintain the blowing agent in the
liquid state. Thereafter the mixture is ejected at atmospheric
pressure causing a turbulent vaporisation of the blowing agent.
Therefore, while the froth technique and the use of an inert
blowing agent incorporated in a liquid state into the reaction
mixture, allows the manufacture of a foam of reduced density,
nevertheless the cell structure is of very inconsistent quality due
to irregular shaped and oversized cells or bubbles being
present.
Although frothing with inert gas, in particular CO.sub.2, is a well
known potential technique, up to now no successfully predicable
frothing process and system have been suggested or discovered for
use with and in slab stock foam production.
In an attempt to solve the problem of slab stock production without
use of chlorofluorocarbon blowing agents, and by using the frothing
techniques, it has been now discovered that the suitable release of
the mixture under pressure must take place under controlled
conditions. Use of controlled conditions in the production of
polymeric foams positively influences the growth of the cells
during initial frothing expansion of the mixture, which is of
importance in the production of slab stock.
Presently, the need for a new foaming process and system in the
continuous manufacturing of flexible slab-stock or other continuous
foam production lines, in which the frothing technique and a
non-reactive blowing agent could be practically usable, still
exists.
OBJECTS AND SUMMARY OF THE PRESENT INVENTION
It is an object of the present invention to provide a foaming
process as referred above, by which it is possible to
advantageously use a non-reactive liquid blowing agent, preferably
carbon dioxide, to froth, without negatively affecting the cellular
structure of the foam, to provide an appropriate bubble free
commercial product.
Another object of the present invention is to provide a process and
a system as referred to above, for continuous foaming, in the
production of flexible or rigid slab-stocks, in which the frothing
of the mixture may be usefully performed under controlled
conditions allowing the spreading of the mixture on a moving
substrate in the form of a high viscous froth, without involving
turbulent vaporisation of the blowing agent. This also permits
production of a low density foam on the order of 14 kg/m.sup.3
(0.87 pcf).
A further object of the invention is to provide a foaming process
and system as referred to above, which make possible the use of a
low output plant running at very low speeds thereby permitting foam
lines that are much shorter in length, in comparison to
conventional or known machines or production lines or plants,
avoiding limitations of the conventional systems while maintaining
the advantageous features thereof. This also results in a dramatic
reduction of exhaust, from a shortened line operating at slower
conditions, so That the volume/hour of exhaust fumes that require
scrubbing or other cleaning effort prior to release is similarly
reduced. Equally important is the lessening of the amount of
factory air that needs to be conditioned thereby reducing energy
costs associated with slab stock production.
The main objectives of the present invention are therefore the
elimination of cloro-fluoro carbons (CFC) and volatile organic
compounds (VOC) from the formulation and substitution thereof with
a less expensive component, as well as production of soft, low
density foam, with a very homogeneous cell structure, free from
large bubbles, pinholes and visible defects.
Contrary to general trends of conventional prior art, the invention
resides in mixing under pressure a reactive blend of polymeric
chemical components and an inert, low boiling blowing agent,
followed by frothing the mixture, prior to the start of any
reaction, under pressure-controlled conditions through a pressure
equalising and frothing device having an elongated pressure-drop
zone extending cross-wise to the moving direction of a substrate.
Then the frothing mixture is restrained along a frothing passageway
or cavity. Thereafter, reaction of the frothed mixture begins. That
frothing passageway or cavity preferably has an output aperture of
larger area relative to the area of the aperture or outlet of the
pressure-drop zone. According to the invention, the pressure during
mixing ranges preferably from 5 to about 18 bar.
The present invention is unique in the use of a frothed form of the
mixed chemical ingredients comprising the polymeric foam to form
slab stock. The present invention is unique too in that it uses an
environmentally safe blowing agent to develop the froth in the
manufacture of slab-stock foam. Such foam slab stock can be either
flexible or rigid foam. The invention employs a specially designed
discharge or lay-down head, called a gate bar, that uniquely aids
the mixed foam ingredients to froth. This frothing occurs in a very
controlled way by use of an elongated pressure-drop zone, as part
of the gate bar, and then by having the mixed, frothing material
pass through a frothing passageway or cavity wherein gaseous
blowing agent is gradually released into the frothing mixture prior
to discharge onto a moving substrate or conveyor. The discharge
head assures the frothed material is distributed across the width
of the machine, either across a substantial portion of the slab
stock machine or across a desired portion thereof. The slab stock
machine can include a complete line or plant in which the frothed
foam will be permitted to chemically react, fully rise, cure and
then be cut into desired pieces.
From experimental tests has been also noted that critical factors
for producing a large foam block are the equilibrium between foam
profile angle, metering machine's throughout, conveyor speed and
formulation characteristics, such as viscosity build-up reactivity,
etc. According to present invention, because the mixture, when
reaction begins, is already viscous and supports a steep rise
angle, the foam's rising angle is no longer a limitation on the
process conditions. The froth being discharged from the frothing
cavity is a homogeneous pre-expanding mixture with a sufficiently
high viscosity to avoid rise angle problems associated with prior
production equipment and flexible slab stock lines. The viscosity
is enough to sustain the production of high blocks, that have fully
reacted, even at very slow speeds with steep fall plate angles.
This condition is accomplished by controlling the expansion phase
of the mixture, after the mix head, and allows for the progressive
release of the blowing agent in the reacting mass. Accordingly, the
four critical factors can be varied to achieve the desired density
rather than having to be linked to a rigid set of parameters as was
the case with prior foam processes. The speed and size of a line or
plant can be tailored to the needs of the foam manufacturer with
speeds from 1 to 5 meters per minute, and lengths as short as 20
meters or less, rather than the more conventional length of about
100 meters. This also permits a smaller volume per hour of exhaust
to be dealt with and removed or scrubbed, simpler metering and
plant fabrication, foams made with CO.sub.2, a smaller volume of
air to be conditioned, and very low densities down to about 14
kg/m.sup.3 or less.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention and preferred embodiments thereof, will be now
described in great detail, with reference to the accompanying
drawings, in which:
FIG. 1 shows a system which operates according to the frothing and
foaming process of the present invention;
FIG. 2 is an enlarged view of a frothing device embodying the
features of the present invention;
FIG. 3 shows a second system embodying the features of the present
invention.
FIG. 4 shows a further system embodying a modified form of a
frothing device;
FIG. 5 is an enlarged view of the frothing device of previous FIG.
4;
FIG. 6 shows an enlarged detail of a further frothing device;
FIG. 7 shows another embodiment of the frothing device according to
the invention.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The essential features of the invention are now described in great
detail with reference first to the appended FIGS. 1 and 2. As
shown, the apparatus for performing the process according to the
invention, is provided with two side walls 4 arranged to be
perpendicular to the conveyor 1. Continuous side papers 3 are
arranged to run and move along the inside of each side wall 4 in
the production direction P (see the arrow) of the foam. A
continuous paper sheet 2 slightly wider than the distance between
the side walls 4, is fed onto the conveyor 1 to provide a foaming
path. The excess layer at each side is turned up so as to form a
seal against the bottom edge of the side papers 3. The bottom paper
2 and the side papers 3 effectively form a continuous open-topped
trough, which could be, for example, about two meters wide by 1
meter high. The conveyor 1 may be substantially arranged with a
horizontal disposition or, alternatively, it may be formed with a
small angle such as six degrees.
The apparatus furthermore comprises a mixer 11 having an outlet 12
positioned at a level above the conveyor 1, for feeding a mixture
of reactive chemical components to a frothing device, generally
indicated at 13, and schematically shown in the detail of FIGS. 2
and 5.
A pressure gauge 12' is provided at the outlet spout 12 of the
mixing device 11 to indicate the pressure of mixed chemicals which
leave the mixing chamber through the outlet 12. Tanks 5, 6 and 7
are provided to hold the foam forming chemical components such as a
polyol and an isocyanate as well as conventionally use chemical
additives comprising a low boiling blowing agent such as CO.sub.2.
From each of the tanks, the chemical components and blowing agent
are fed to the mixer 11 by pipes and respective metering pumps 8, 9
and 10.
According to the embodiment as shown in FIG. 2, the outlet spout 12
of the mixer 11 is connected by tubing 18 to the mixture frothing
device 13 transversally extending across the conveyor 1. The
frothing device 13 includes drop-pressure generating means
comprising an elongated drop-pressure zone in the form of an
elongated slot 17. It should be understood that the purpose of the
elongated pressure drop zone is to provide a back-pressure and
allow the mixing under pressure of chemical components into the
mixer 11 as well as an equalization of the pressure in the same
frothing device, before the pressure reduction, to prevent
turbulent evaporation of the blowing agent during frothing of the
mixture. At the same time the frothing device 13 allows frothing of
the mixture under restrained and pressure controlled conditions.
The frothing device 13 also assures that the resulting froth is
smoothly delivered onto the substrate 2, 3 while the same mixture
is flowing in the moving direction of the conveyor device.
As referred above, and shown in the example of FIG. 2, the pressure
equalizing and frothing device 13 can, for example, comprise a
tubular member defining an elongated pressure equalizing chamber 21
having a longitudinal axis. Chamber 21 is connected at one or more
feeding points, to the mixer 11 by at least one tubing arrangement
18. The pressure equalizing or tubular chamber 21 is provided with
a pressure-drop zone, for example, in the form of the elongated
slot 17 which extends longitudinally along one side of chamber 21.
However, the pressure drop zone could be formed from other suitably
shaped pressure-drop apertures for flowing the mixture before
frothing. As is shown in FIGS. 6 and 7, the discharge or gate bar
could include a series of apertures of circular, oblong or
rectangular shape, or a series of elongated, but shorter, slots so
long as the controlled conditions were produced. The slot or more
precisely said pressure-drop zone 17 has a restricted
cross-sectional area sufficient to cause a pressure reduction in
the mixture emerging from the chamber 21 during frothing, and a
corresponding back pressure in the mixing device 11 for the purpose
mentioned above.
Froth control, to obtain the controlled conditions that are desired
during lay down of the froth, is accomplished by use of an enlarged
cavity, such as is shown at 19 having an outlet aperture 20 from
which the frothing mixture is delivered onto the moving
substrate.
The enlarged frothing cavity 19 has a suitable configuration that
will provide flow control over the frothing mixture and in
particular the cross-sectional area of the output aperture 20
should be greater than the cross-sectional area of pressure-drop
zone, such as the elongated slot 17, which provides communication
between the pressure equalizing chamber 21 and the frothing cavity
19. The sectional area of the output aperture 20 should range from
ten to hundreds of times, or more, the sectional area of the
pressure-drop zone in the form of the elongated slot 17.
In the embodiment shown in FIG. 1, the frothing device 13 is
delivering the frothed mixture onto the substrate 2/3 which is
moving on the conveyor 1 along a substantially horizontal path. The
conveyor may be operated at a rate of about 1 to 5 meters per
minute, so that the full block height may be achieved within a
range of about 1 to 8 meters from the deposit point of the
froth.
The above example of FIG. 1 refers to a system in which the foam
rise upwardly from the bottom of the conveyor.
The example of FIG. 3 refers to a different system in which the
foam is allowed to froth onto a downwardly sloping path which
allows the length of the machine to be greatly reduced. According
to the embodiment, the sloping angle of the conveyor, onto which
the frothing mixture is discharged, is not critical due to the high
viscosity of the froth emerging from the frothing cavity.
Therefore, underrunning does not occur.
The continuous slab-stock machine of FIG. 3 continues to employ a
main conveyor 1, together with the associated side walls 4 and side
and bottom papers 2 and 3, and that conveyor 1 is in a horizontal
position.
This embodiment also includes a second driven conveyor 25 inclined
at an angle .alpha., for example of about 30 degrees to the
horizontal, and positioned between the side walls and side papers
in such a manner that its lower end is immediately above the main
conveyor 1. The pressure equalizing and frothing device 13, such as
the one described in Example of FIG. 1 has been positioned
immediately above the upper surface of the conveyor 25 at its
highest point. As an alternative to the conveyor 25, it is possible
to use a slide surface, such as a fall-plate for establishing the
downwardly directed path. The angle .alpha. preferably can vary
from about 10.degree. to about 40.degree. with a range of about
25.degree. to about 30.degree. being the preferred angle.
The bottom paper 2 in this case, runs onto and down the upper
surface of the slanted conveyor 25 and then onto and along the main
conveyor 1.
A second continuous paper 22 runs under a roller 23 so that it just
clears the upper surface of the frothing cavity 19 and in such a
manner that the paper 22 rests on and runs with the top surface of
the expanding foam 16.
Optional pressure plates 24 are fitted so as rest on the top side
of the paper 22, if required, to assist in shaping of the expanding
foam.
Comparative tests have been conducted in the continuous manufacture
of slab-stocks by means of a process and a system according to
present invention, in comparison with a conventional system.
EXAMPLES
Example 1
According to this comparative example, a conventional continuous
slab-stock machine as described in FIG. 1 was used except for the
frothing device 13 which had been eliminated. A blend of chemicals,
identified as Blend A in the following Table 1, was made and
introduced into tank 5. The 80:20 TDI, Part B, was introduced into
tank 6. Tank 7 held stannous octoate catalyst, Part C.
The pumps 8, 9 and 10 were set to give the outputs as defined in
Table 1 for Parts A, B and C respectively. Conveyor 1 was set to
run at a speed of 5 meters per minute.
The foam-forming chemicals, coming from the mixer, were allowed to
pour from the mixing head outlet 12 directly onto the paper 2. The
reading on the pressure gauge 12' was zero.
Expansion of the foam 16 started at a point along the length of the
conveyor 1 about 0.8 meters from the mixing head 11. This
represented a time of about 12 seconds from mixing.
The foam was fully expanded by a point about 8 meters from the
mixing head 11. This represented a time of about 105 seconds from
mixing. The height of the foam block after it had cured was 0.8
meters. The density of a sample of foam cut from the block was 21.5
kg/m.sup.3. The cell structure of the foam was of a consistent
quality with few irregular shaped cells, oversized cells or voids
being present.
TABLE 1 Examples Example 1-3 4 Parts Output Output by in in weight
kg/min kg/min PART A 120 24 Polyether Polyol, 3500 mw 100.00 Water
4.50 Amine catalyst, Niax A1 0.10 Silicone, Tegostab B2370 1.20
PART B 62.4 12.5 80:20 TDI 55.2 PART C 0.25 0.05 The catalyst,
stannous octoage 0.22 NIAX is a Trade mark of Union Carbide
TEGOSTAB is a Trade Mark of TH Goldschmidt (blowing agent having
comparatively low cost)
Example 2
According to this comparative example, the same slab-stock foam
machine as used in Example 1 was utilized. In this case the blend
of chemicals designated Part A in Table 1, which was in the tank 5
was saturated with carbon dioxide gas at pressure, by addition of a
quantity of liquid carbon dioxide. Sufficient liquid carbon dioxide
was added to achieve a pressure in tank 5, as indicated by the
pressure gauge 14, of 6 bar.
The slab-stock foam machine was operated in the same manner as
described in Example 1.
This time, the chemicals left the mixing head outlet tube 12 in the
form of a turbulent froth. It was also noticed that large bubbles
of gas were forming in the foam after being deposited on the bottom
paper 2. The pressure indicated by the pressure gauge 12' on the
outlet 12, was zero.
The foam expansion was completed at a distance of about 7 meters
from the mixing head 11.
When the foam block had cured, it was found to have a height of 0.9
meters, indicating that more expansion had occurred than in Example
1.
The density of a sample of foam cut from the block was found to be
19 kg/m.sup.3.
The cell structure of the foam was inconsistent, there being
present in the foam large ovaloid voids or pin holes up to 30
millimeters high by up to 10 millimeters diameter. The presence of
these irregularities in the cell structure made the foam
commercially unacceptable.
Example 3
According to the invention the same slab-stock frothing foam
machine as used in Example 1 and 2 was employed, except that a
frothing device 13 according to the invention, was fitted to the
mixing head outlet (12) so as to equalize the pressure of the foam
forming chemicals across the width of the conveyor (1) before
allowing them to froth by reducing their pressure on passing
through pressure drop zone 17 of the pressure equalising chamber
21.
The pressure equalizing, frothing device illustrated
diagrammatically in FIG. 2, consisted of a 1.9 meter length of
steel tube 21 of internal diameter 30 millimeters. Along the length
of the pressure equalizing chamber 21, in the form of a steel tube
was cut a slot 17 of a height of 0.5 millimeter, and a length of
1.85 meters. An inlet tube 18 was fitted onto the slotted tube 21,
which was in turn connected to the mixing head outlet 12.
A frothing cavity 19 in the form of a diverging diffuser/baffle 19
was attached to the outside of the gate bar or the pressure
equalization chamber 21 so as to form a diverging enclosed path
from the elongated slot 17 to the rectangular outlet 20 of the
diffuser/baffle. The dimensions of the outlet aperture of frothing
cavity were 1.85 meters wide by 0.2 meters high. The length of the
diffuser baffle form of the frothing cavity from the slot to the
outlet aperture was 0.5 meter.
The slotted tube, with the diffuser/baffle, was fitted onto the
slab-stock foam machine so that the outlet aperture 20 was just
above the bottom paper 2 and facing down the conveyor 1 in the
direction of production as shown by arrow P.
The gate bar 60 could have the configuration shown in FIGS. 6 and
7. In FIG. 6 the bar had a rectangular outer shape as well as a
rectangular cross-sectional inner chamber 62. A series of elongated
slots, as are shown at 64, 66 and 68, for example, could be used to
provide the desired outlet from the gate bar 60 and the desired
pressure drop. In FIG. 7 the gate bar 70 is provided with a
circular cross-sectioned interior chamber 72 from which a series of
tubular outlet apertures, as shown at 74, 76 and 78, for example,
axially extend in the flow direction to provide the desired
openings and pressure drop.
The slab-stock foam machine was run in the same manner as for
Example 2. The pressure in the tank 5 was again 6 bar as shown by
pressure gauge 14; the pressure in the mixing chamber was about 18
bar.
We have found that the pressure drop in the gate bar, or the
pressure equalizing chamber 21 should be a significant part of the
total pressure drop from the mixing chamber onwards.
This time, the foam emerged from the rectangular aperture outlet 20
as a smooth froth. There was no evidence of large bubbles being
present in the expanding foam. The pressure indicated by the
pressure gauge 12' on the mixing head outlet tube 12 was 6 bar.
Foam expansion was completed at a distance shorter than 7 meters
from the outlet aperture of the outlet 20. In consequence of the
high viscosity of the froth, according to the invention it is
possible to reduce the velocity of the conveyor and to dramatically
reduce the overall length of the line and therefore of the entire
plant in comparison to a conventional one, without causing
underrunning problems.
When the foam block had cured, it was found to have a height of 1
meter. The density of a sample of foam cut from the block was found
to have a density of 17 kg/m.sup.3. The cell structure was fine and
contained no large voids. The foam quality was judged to be
commercially acceptable.
Example 4
According to the invention the same chemical recipe was used in
Example 3, with 6 bar pressure in the tank 5 containing the polyol
blend/carbon dioxide mixture, in pressure gauge 12'.
In this case, however, the outputs of the pumps 8, 9 and 10 were
reduced to one fifth of the outputs used in Example 3, as shown in
Table 1 and the slot 17 reduced to a height of 0.4 mm.
The conveyors 1 and 21 were both run at a surface speed of 1
meter/minute.
The expanding foam 16 in this case reached full expansion at a
distance of about 1.2 meters from the outlet 20.
The height of the cured foam block was 1 meter. The density of a
sample of foam cut from the block was measured to be 17 kg/m.sup.3.
The cell structure was fine and similar to the foam made in Example
3. It contained no large voids. The quality was judged to be
commercially acceptable.
The examples 3 and 4 in this specification clearly demonstrate the
efficiency of the process according to the present invention by
controlling the pressure drop and the frothing of the mixture in
continuous production of polymeric foam material.
Example 5
The same chemical recipe has been used in the Example 4, except for
the CO.sub.2 content that has been increased as described below.
The embodiment has been modified in order to pump the liquid
CO.sub.2 in a continuous way rather than pre-mixing it in the tank
with the polyol. A pump 54, shown in FIG. 4, has been added in
order to mix the liquid CO.sub.2 from the tank 55 in the polyol
stream with the aid of a static mixer 53. A pressure reducing valve
56 has been introduced in order to assure that the pressure in the
polyol line is maintained that will keep the CO.sub.2 in a liquid
state at the working temperature before the static mixer 53. The
liquid CO.sub.2 has been pumped in the polyol stream at an output
to correspond to a weight ratio of 4% (CO.sub.2 on polyol). The
liquid CO.sub.2 could be pumped as well in the isocyanate stream.
The slot 17 has been further reduced in height to about 0.3 mm. The
pressure gauge 12' indicated a pressure of 15 bars. All the other
parameters were maintained as in Example 4.
The height of the cures block was 1.2 meters and the density was 14
kg/m.sup.3. The cell structure was good.
Experimental tests conducted for a long time demonstrated the real
possibility to use CO.sub.2 as primary blowing agent in an
effective manner, having a smooth and homogeneous frothing of the
reactant mixture on the moving substrate, as well as the foaming of
the material while running on the conveyor at a comparatively low
velocity, thus resulting in a machine of substantially reduced
length and output in respect of machines which make use of a
conventional process or frothing technique.
In conventional mechanical mixing of flexible polyurethane foams,
it is well known that it is necessary to add small amounts of
nucleating gas to the liquid reactants during mixing. The purpose
of the nucleating gas is to provide nucleation sites for cell
formation at the start of foaming. Typically, nucleating gas such
as air or nitrogen would be added at a rate of 0.3 to 3N liters per
minute for a mixing throughput of 100 kg/minute of chemical
reactants.
According to the invention we have also found that when liquid
carbon dioxide is introduced into the liquid reactants as an
auxiliary blowing agent, it is still advisable to add a nucleating
gas. The gas, as would be expected, must be introduced into the mix
at a sufficiently high pressure to overcome the pressure in the
mixer. This pressure, as previously mentioned, can be about 5 to 18
bar.
We have further found that the quantity of nucleating gas can be
considerably higher than in the case of conventional mechanical
mixing of flexible polyurethane foam. At a pressure of 5-18 bar in
the mixing head and a chemical throughput of 100 kg/minute, we have
found it possible to add nucleating gas (e.g. nitrogen) at the rate
of 10-40N liters/minute. If the lower addition rates typical of
conventional mechanical mixing are used, the foam cell structure is
very coarse, of low porosity and the foam product is not of
commercial quality.
With reference now to FIGS. 4 and 5 an alternative form of the
apparatus and the frothing device are shown.
According to the example of FIG. 4, in accordance with the present
invention, the liquid CO.sub.2 contained in the tank 55, by means
of a metering pump 54, is directly injected in the flow of polyol
fed from the tank 5 as in the example of FIG. 1. More precisely the
liquid CO.sub.2 is fed into the flow of polyol upstream a static
mixer 53 connected to the high pressure mixer 11 for the
polyurethane components by means of a pressure-reducing valve 56.
The valve 56 serves to ensure that the pressure in the polyol line
be such to maintain the CO.sub.2 in the liquid state, at the
working temperature. The liquid CO.sub.2 could be differently
introduced into the flow of isocyanate fed from tank 6, in FIG.
1.
Reference 13 in FIG. 4 indicates an alternative embodiment of the
pressure equalising and frothing device for the polyurethane
mixture, shown in detail in FIG. 5. The tube 18 in FIG. 5, still
comes from the mixing chamber 11, as in the embodiments shown in
FIGS. 1-3 and the pressure euqalization chamber or gate bar 21 is
employed. Here the chamber 21 has a rectangular cross-sectional
shape and shapes other than circular or rectangular could be used.
The gate bar also includes a pressure reduction aperture 17.
Aperture 17 opens into a modified frothing cavity, generally
indicated at 40. Frothing cavity 40 is comprised of a top wall 42
that is attached to the gate bar and extends outwardly for a
distance of about 10 mm. Wall 42 terminates at wall 44 positioned
at an angle of about 90.degree. with respect to wall 42. Wall 44
extends downwardly for about 30 mm and extends past the outlet of
slot 17. Accordingly, the emerging frothing mixture will intersect
walls 42 and 44 and have its direction turned 90.degree. relative
to the flow through slot 17. We have found that ending the frothing
cavity at this point, that is at the end of wall 44, and a portion
of the rear wall 48, can produce results that are satisfactory for
controlling frothing and for providing suitable back pressure on
mixing chamber 11. We prefer, however, to continue the frothing
cavity by continuing rear wall 48 for a distance of about 40 mm, by
having a bottom wall 50 extend away from rear wall 48, at an angle
of about 90.degree., for a distance of about 50 mm, and by having
top wall 46 extend from wall 44 at an angle of 90.degree. and for a
distance of about 40 mm. Walls 46 and 50 are positioned to be
substantially parallel although they could diverge at a slight
angle of about 10.degree. to about 20.degree..
By employing flow diverting walls 46 and 50 the frothing mixture
makes another 90.degree. turn, this time relative to the direction
the frothing mixture is flowing by reason of walls 42, 44 and the
top portion of rear wall 48. Passage of the frothing mixture
through the frothing cavity allows frothing to begin and occur,
initially, under pressure controlled conditions. Passage by the
frothing mixture through the frothing cavity helps the initial
frothing process to develop without the turbulence associated with
direct injection systems. The resulting froth emerges in a smooth
flowing manner from the outlet and produces a smooth, free flowing
transition onto the moving substrate in non-turbulent condition as
a creamed froth. Distribution is enhanced and the further
transition from frothing to reaction of the chemical ingredients,
and foam growth as in generally indicated at 52, also occurs
smoothly and more completely.
As FIG. 3 shows, the frothing cavity can be used with the moving
continuous paper sheet and, where desirable, can also be used with
the top side paper sheet 22.
It is however evident that different or equivalent solutions are
possible, in respect to the examples previously described, without
departing from the innovative principles of the present invention
as claimed.
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