Note: Descriptions are shown in the official language in which they were submitted.
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"PROCF~S AND DEVICE FOR PRODUCING CROSS-LINKED POLYOLEFIN FOAMS"
The invention relates to a process for crosslinking and foaming crosslinkable
polyolefin sheets or polyolefin films or for foaming crosslinked polyolefin
sheets or
polyolefin films according to the precharacterizing clause of Patent Claim 1,
and also
to an apparatus in particular for carrying out a process of this type,
according to the
precharacterizing clause of Patent Claim 5.
The term polyolefins is used for polymers of the general structure
Rz
I '
-CHZ - C-
R1
1o n
where R' is hydrogen and R2 is hydrogen or straight-chain or branched groups.
If R'
and RZ are hydrogen, this gives polymers having groups of the type
LCHZ CH2
as a characteristic basic unit of the polymer chain. The latter polymers are
termed
polyethylenes.
Known polyethylenes may be high-pressure or low-pressure polyethylenes,
depending
on the preparation process used. These differ especially with respect to their
degree
of branching and their density. A distinction is drawn in particular between
LDPE (low-
density polyethylenes), LLDPE (linear low-density polyethylenes), HDPE (high-
density
polyethylenes), 'HDHMWPE (high-density high-molecular-weight polyethylenes)
and
VLDPE grades (very-low-density polyethylenes).
The densities of foams of this type are approximately from 25 to 200 kglm3.
The
foams have a substantially closed cell structure.
Increasing the viscosity andlor the hardness of the polymers requires
crosslinking, i.e.
a reaction in which the linear or branched macromolecules present are linked
to one
another to give three-dimensional polymeric networks. Crosslinking of this
type may
take place on exposure to high-energy radiation or with the aid of peroxides
or silanes.
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The' characteristic foam structure arises due to chemical reactions on
addition of
substances known as blowing agents, which during processing decompose at a
particular temperature with gas formulation. Examples of conventional blowing
agents
are azo compounds and diazo compounds. In the foam industry these are called
cell-
s generating blowing agents. When exposed to heat and, where appropriate,
catalysts,
they evolve gases and thus are suitable for producing foamed plastic
compositions.
The foaming may take place when the product leaves the extrusion tool, i.e.
directly
following extrusion or injection molding, or in open molds.
Firstly, a compact polyolefin sheet or polyolefin , film made from a mixture
of a
polyolefin with a blowing agent and a crosslinking agent is preferably
prepared by
extrusion through an extruder with slot dies. This sheet or film is also
called a matrix.
The processing temperature in this production process is adjusted in such a
way that
neither the crossfinking agent nor the blowing agent reaches its decomposition
temperature. There then follows the abovementioned crosslinking of the
polyolefin at a
~5 temperature level which reaches or exceeds the decomposition temperature of
the
crosslinking agent but' does not yet reach the decomposition temperature of
the
blowing agent. Not until the crosslinking is virtually complete is the
decomposition of
the blowing agent and thus the foaming process initiated, for example by
further
introduction of heat. This phase of the total production process is
exothermic.
2o Background Art
A process and an apparatus of the type mentioned at the outset are known in
industry.
The crosslinking and foaming, or the foaming alone, takes place in an oven
which is
composed of a number of zones arranged in succession. A fan, and also a unit
known
as a blower chamber, is allocated to each zone of the oven. Each blower
chamber has
2s an upper section with openings directed vertically downward, and also a
lower section
with openings directed vertically upward. The two sections of each blower
chamber
are arranged one over the other, at a distance from one another, and are
attached to
the fan allocated to the respective zone. Between the sections of each of the
blower
chambers runs the polyolefin sheet or polyolefin film. This is, for example,
unwound
3o from a roll, passed between the sections arranged one over the other of
each of the
blower chambers, during this is heated, crosslinked and foamed, and finally is
wound
up onto a wind-up roller. Between the oven and the wind-up roller there are
generally
a number of chill rolls for transporting and cooling the polyolefin sheet or
polyolefin
film. The process is generally continuous, where the sheet or flm is generally
35 transported through the oven and between the sections of the blower
chambers with
the aid of a circulating mesh or carrier belt.
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The heating of the sheet of film takes place via the fans and blower chambers
with the
aid of hot air. The temperature of the hot air in the individual zones is
adjusted to the
respective value required. Usually, the crosslinking of the polyolefln takes
place first
and the foaming of the material takes place in a subsequent, hotter zone of
the oven.
Ovens of this type are mostly divided into from three to five zones, and, as
mentioned
above, each zone is composed of at least one fan, a blower chamber composed of
two sections, and equipment for generating heat and controlling temperature.
The cost
of constructing apparatuses of this type is therefore considerable, and
therefore high
capital cost and running cost have to be expected when the known process is
used.
1o Object
The object on which the invention is based is to create a process of the type
mentioned at the outset which, in particular, can be carried out cost-
effectively for
small quantities. Another object of the invention is to provide an apparatus
of the type
mentioned at the outset which is less expensive to manufacture and run.
~5 Disclosure of Invention
The process aspect of this object is achieved by means of a process with the
features
of Patent Claim 1, and the apparatus aspect is achieved by means of an
apparatus
with the features of Patent Claim 5.
According to the invention, the hot air flows along the polyolefin sheet or
polyolefin film
2o in a direction opposite to that of the transport of the sheet or film. When
this type of
counterflow system is used, the section of the sheet which contacts the hot
air first is
that which has already been in the oven for a relatively long time. The path
of the hot
air then takes it along the sheet, and it therefore passes some of its thermal
energy to
the sheet. The temperature of the hot air therefore reduces between the time
of its
2s first contact with the sheet and the part of the sheet at the opposite end,
where the hot
air is then at a lower temperature level and is conducted back to the fan.
A process of this type may therefore be carried out particularly cost-
effectively, since
the air brought into contact with the sheet is at a high temperature level in
the area
where the foaming process takes place and at a tower temperature level in the
area
3o where, for example, the crosslinking of the polyolefins at a lower
temperature level is
to take place.
It is advantageous for the hot air to flow along the upper side and along the
lower side
of the polyolefin sheet or polyolefin film. This ensures uniform heating over
the entire
thickness of the sheet or film.
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In an advantageous embodiment of the invention, the heat transfer between the
hot
air and the polyolefrn sheet or polyolefin film is adjusted via control of the
rate of flow
of the hot air, by diverting some of the hot air flowing along the sheet or
film away from
this. This makes it easy, with constant throughput of the fan, to influence
the
temperature level as desired in the direction of flow of the hot air and thus
to supply
the sheet or film in each area with the precise amount of heat which is
required in that
area for the respective chemical reaction, e.g. the crosslinking or foaming.
It is advantageous for the hot air to be conducted in a virtually closed
circulation
system. This has further favorable effects on the costs of carrying out the
process and
o results in a particularly environmentally friendly process, since
environmental pollution,
e.g. the content of hazardous materials in the exhaust air, is markedly
reduced by the
closed design.
fn the apparatus aspect of the invention, the equipment for heating the
polyolefin
sheet or polyolefin film is arranged and constructed in such a way that the
direction of
~5 flow of the hot air emerging from the equipment runs approximately parallel
to the
plane of the sheet or film. The air thus flows over the sheet or film and can
therefore
pass its heat content uniformly to the sheet or film. The temperature of the
hot air
reduces along its flow path. The heat content of the hot air which has already
flowed
over a portion of the sheet or film can therefore be of advantage to sections
of the
2o sheet downstream and supply heat to these. The novel apparatus therefore
also has
different temperatures along the flow path of the hot air, but without
requiring the high
equipment cost of the prior art. A particular reason for the advantages
obtained from
the invention is that the manner in which the hot air flows over the sheet or
film is
favorable for heat transfer.
2s The direction of flow of the hot air is advantageously the longitudinal
direction of the
sheet or film, and the hot air is preferably conducted countercurrently to the
direction
of transport of the sheet or film passed through the oven. This also allows
continuous
operation of the novel apparatus, e.g. transportation of a very long
polyolefin film at a
uniform rate through the apparatus. The advantages of the countertlow method
of the
3o invention have already been given in the description of the novel process.
In an advantageous embodiment of the invention, the equipment for heating the
po(yolefin sheet or polyolefin film has at least one heating tunnel
surrounding the
sheet or film. The heating tunnel completely surrounds the polyolefin sheet or
polyolefin film, preferably in its transverse direction, and extends over
virtually the
35 entirety of that longitudinal section of the sheet or film which is located
in the oven.
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This allows the exothermic energy liberated in the foaming process to be made
available to the hot air and directly to that area of the sheet or film which
is
downstream in the flow direction of the hot air. The design of the heating
tunnel
according to the invention is simpler to build than a number of blower
chambers
arranged in ,succession, since the sheet or film is located directly in the
heating tunnel
and does not, as in the prior art, have to be passed between two sections of a
blower
chamber arranged one over the other with a space between them. This results in
lower capital costs. In addition, the novel apparatus has lower weight than
conventional apparatus.
o In another embodiment of the invention, the equipment for heating the
polyolefin sheet
or film has equipment for at least partial recirculation of the hot air prior
to use. This is
preferably at least one flap provided in the heating tunnel, with the aid of
which flap at
least some of the hot air flowing through the heating tunnel can be diverted
into the
area of the oven arranged outside the heating tunnel. This flap gives a simple
way of
~5 reducing the volume flow through the heating tunnel and therefore the rate
of flow of
the residual part of the hot air in the heating tunnel. This allows control of
the heat
transfer between the hot air and the sheet or film via the rate of flow and
the
temperature of the hot air around the material.
It is advantageous for the flap to have been mounted in the wall of the
heating tunnel
2o and to form a part of in such a way that it can swivel this wall. This
makes it easy,
specifically by swiveling the flap downward, to conduct some of the hot air
out of the
heating tunnel.
In one embodiment of the invention, the flap is arranged approximately at the
longitudinal centre of the heating tunnel in that part of the same which is
arranged
25 above the polyolefin sheet or film. This embodiment of the invention is
particularly
advantageous if the fan has also been attached to the oven above the sheet or
film,
not far from the flap.
In another embodiment of the invention, chill rolls are provided for
transporting and
cooling the polyolefin sheet or film. At least one of these is built into one
wall of the
30 oven in such a way that the gaps formed are small. With the aid of this
embodiment
the novel apparatus is virtually entirely a closed system. This reduces the
energy
consumption of the novel apparatus and improves its environmental
compatability. It
also substantially prevents outside air from penetrating into the novel
apparatus.
Arranging the chill rolls in this way also reduces the overall length of the
novel
35 apparatus compared with conventional apparatus.
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Brief Description of Drawings
Working examples of the subject-matter of the invention are described in
greater
detail below, using the figures.
Fig. 1 shows diagrammatically a partial longitudinal section through a first
embodiment of an apparatus for crosslinking andlor foaming polyolefin sheets
or polyolefin films, and
Fig. 2 shows diagrammatically a partial longitudinal section through a second
embodiment of an apparatus; for simplicity the polyolefin sheet or polyolefin
film
has not been shown here.
o Modes for Carrying Out the Invention
A first working example of a novel apparatus 1 for crosslinking and foaming
crosslinkable polyolefin sheets or polyolefin films 2 or for foaming of
crosslinked
polyolefin sheets or polyolefin films 2 is shown diagrammatically as a partial
longitudinal section in Fig. 1.
~5 For this, the apparatus 1 has a long oven 3, which in Fig. 1 and 2 is in
the form known
as a continuous oven. On its front face 4 (shown on the left in Fig. 1 ) the
oven 3 has
two openings 5 and 6 arranged one over the other. The oven also has two
openings
and 11 arranged one over the other on its rear face 7, and also a space 12
above
the upper opening 10 in the rear face 7.
2o A reticulated carrier belt 13, conducted over rollers 14 and 15, runs
through the
openings 5 and 6 in the front face 4 and the openings 10 and 11 in the rear
face 7 of
the oven 3. The roller 15 is shown as a driven roller in Figure 1 on the right-
hand side.
Its direction of rotation is indicated by the arrow 16.
The polyolefin sheet or polyolefin film 2, called sheet or film below, has the
shape of a
25 long web and is initially located on a roll 17, whose direction of rotation
is indicated by
the arrow 20. The film may also be passed directly, i.e. without using the
roll 17, to the
carrier belt 13 from the extruder die. The film 2 lies on the upper section of
the carrier
belt 13 over virtually the entire length of the oven 3 and is then conducted
over chill
rolls 21 onto a wind-up roll 22, shown towards the right of Fig. 1. The
direction of
3o rotation of the wind-up roll 22 con-esponds to that of the roll 17. Both
the roll 17 and
the wind-up roll 22 are mounted on trestles 23 in such a way that they can
rotate.
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The apparatus 1 also has equipment 24 allocated to the oven 3, for heating, by
means
of hot air 25, the sheet 2 in the oven 3. According to the invention, t<~e
equipment 24
for heating the sheet or film 2 is arranged and constructed in such a way that
the
direction of flow of the hot air 25 emerging from the equipment runs
approximately
parallel to the plane 26 of the sheet or film. The direction of flow of the
hot air 25 in
Fig. 1 runs in the longitudinal direction of the sheet or film 2 and, more
precisely
countercurrently to the direction of transport of the film 2 passed through
the oven 3
with the aid of the carrier belt 13.
The equipment 24 for heating the film 2 has a fan 27 and a heating tunnel 30
1o surrounding the film 2 in the oven 3. The heating tunnel completely
surrounds the film
2 in the oven 3 in the transverse direction of the film and extends over
virtually the
entire section of the length of the film 2 located in the oven. In Fig. 1 the
heating
tunnel 30 encompasses, downstream from the fan 27, firstly a relatively short
section
31 on the right-hand side and, attached to this and extending to the front
face 4 of the
~ 5 oven 3, a relatively long section 32. The section 31 on the right-hand
side of the
heating tunnel 30 has flow apertures 33, directed tangentially or parallel to
the plane
26 of the film. The heating tunnel 30 also has equipment 34 for at least
partial
recirculation of the hot air 25 prior to use. This equipment 34 is a flap 35
provided in
the wall of the relatively long section 32 of the heating tunnel 30, and which
can be
2o used to divert a part indicated by the arrow 36 of the hot air 25 into a
return conduit 37
arranged outside the heating tunnel 30. In Fig. 1 the flap 35 has been mounted
in the
upper wall of the section 32 of the heating tunnel 30 in such a way that it
can swivel,
and forms a part of this wall. The flap has been arranged approximately in the
centre
of the length of the heating tunnel 30.
25 One of the chill rolls 21 for transporting and cooling the film 2 has been
built into the
space 12 in the rear face 7 in such a way that there is only a small gap 40
between
this chill roll 21 and the rear face 7.
The procedure for operating the novel apparatus 1 and carrying out the novel
process
is as follows.
3o The carrier belt 13 runs at a particular transport speed clockwise in Fig.
1, so that the
lower section of the carrier belt 13 in Fig. 1 moves from right to left and
the upper
section of the carrier belt moves from left to right. The carrier belt 13 is
driven and
conducted over rollers 14 and 15.
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The film 2 is arranged on the upper side of the carrier belt 13 and comes into
contact
with hot air 25 in the oven 3. The hot air is conducted by the fan 2~
initially into the
right-hand section 31 of the heating tunnel 30, and from this section emerges
via the
flow apertures 33 along the film 2, tangentially to the plane 26 of the film.
The hot air
s then passes into the relatively long, left-hand section 32 of the heating
tunnel 30 and
there flows along the film, above and below, counter to the direction of the
transport of
the carrier belt 13 and therefore of the film 2. This heats the polyolefin
sheet or
polyolefin film 2 to bring about chemical reactions, and the temperature of
the hot air
and therefore of the film is adjusted in such a way that in a section near to
the front
o face 4 of the oven 3 it is above the temperature required to crosslink the
film and
below that required to foam the film. In a subsequent (in the direction of
transport of
the film) section of the heating tunnel 30, which is directed more towards the
rear face
7 of the oven 3, the temperature of the hot air is in a range suitable for
foaming the
film 2. The manner of flow of the hot air 25 is indicated by the arrow 41.
~5 In the final (in Fig. 1 the right-hand) area of the oven in the direction
of transport of the
film, the film expands three-dimensionally due to formation of gas by the
blowing agent
and, with the aid of the chill rolls 21 which also have the function of pull-
off rolls, is
removed from the carrier belt 13 by blowing andlor pulling and finally is
rolled up in its
crosslinked and foamed condition onto the wind-up roll 22. The removal of the
film
2o from the carrier belt by blowing is usually achieved with the aid of the
hot air flowing
out through the flow apertures 33 arranged under the carrier belt 13.
The temperature of the hot air is established and controlled, for example, via
gas
burners in the air circulation system or via introduction of hot air into the
circulation
system. The heat transfer between the hot air and the polyolefin sheet or
polyolefin
2s film 2 is adjusted via control of the rate of flow of the hot air 25, by
diverting some of
the hot air flowing along the film 2 away from this with the aid of the flap
35 and out of
the heating tunnel 30 into the return conduit 37. This reduces the rate of
flow of the
remainder of the hot air in that section of the heating tunnel 30 downstream
from the
flap 35 in the direction of flow of the air. It is therefore possible to
influence the heat
3o transfer from the hot air to the film via control of the rate of flow of
the hot air.
The abovementioned °crosslinking" and "foaming" steps are clearly
dependent on the
temperature and the residence time of the respective section of the film in a
particular
area of the oven. At a given temperature, therefore, throughput rate can be
increased
by increasing the residence time, i.e by lengthening the oven. Lengthening of
the
35 heating tunnel is simple in engineering terms and can be carried out at low
cost, since,
CA 02319591 2000-08-02
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unlike in conventional ovens, it is not essential to install further blower
chambers, fans
and hot-air burners, including the respective control equipment.
A second working example of the invention is shown in Fig. 2, where identical
or
similar features are refer-ed to using the same numbers. For simplicity, Fig.
2 does not
show the polyolefin sheet or polyolefin film 2 or the roll 17 or wind-up roll
22 with their
respective trestles 23.
It can be seen from Fig. 2 that, in the right-hand section of the oven 3
depicted, the
equipment 24 shown for heating the sheet has a right-hand section and a
relatively
tong, left-hand section, 31 and 32, of the heating tunnel 30, whereas the
equipment 24
o depicted in the left-hand section in Fig. 2 has only the relatively long
section 32. The
reason for these differences is that it is not until the end of the oven 3
that the film is
foamed and not until this area that it is removed by pulling away from the
carrier belt
13. The right-hand section 31 of the heating tunnel 30 therefore also has only
to be
provided in this section of the oven.
~5 In this embodiment, the heating tunnel 30 is divided up into two sections
placed one
after the other, to achieve the best possible matching to the requirements of
the
product to be produced andlor to increase the throughput rate. Clearly, the
two
heating tunnels placed one after the other shown in Fig. 2 could also each
have one
or more items of equipment for returning some of the hot air.
2o There may also be flaps 35 in the section of the heating tunnel constructed
below the
carrier belt 13. Another possibility is to allow the lower section of the left-
hand,
relatively long section 32 of the heating tunnel 30 to terminate just prior to
the inner
side of the front face 4 of the oven 3, unlike in Figs. 1 and 2, in such a way
that the air
flowing out of the heating tunnel 30 can flow away from the section of the
heating
25 tunnel below the carrier belt not only, as in the case of Figs. 1 and 2,
through the
carrier belt but also past this, using the space between the inner wall of the
front face
4 of the oven and the end of the heating tunnel, and into the return conduit
37.
This gives a more cost-effective and more environmentally friendly way of
producing
foamed polyolefin sheets or foamed polyolefin films.
