Title of Invention

A PROCESS FOR CROSSLINKING AND FOAMING CROSSLINKABLE POLYOLEFIN SHEETS

Abstract

The present invention relates to a process for crosslinking and foaming crosslinkable polyolefin sheets or polyolefin films or for foaming crosslinked polyolefin sheets or polyolefin films by thermal decomposition of a crosslinking agent and/or a blowing agent mixed with the polyolefin, in which the continuously transported polyolefin sheet or polyolefin film is brought into contact with hot air, wherein the hot air flows along the polyolefin sheet or polyolefin film in the direction opposite that of the transport of the sheet or film.

Full Text

Process and apparatus for producing crosslinked polyolefin foams Technical Field 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. 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), HOPE (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 firom 25 to 200 kg/m^. The foams have a substantially closed cell structure. Increasing the viscosity and/or 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. 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-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 exttuder 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 crosslinking agent nor the blowing agent reaches its decomposition temperature. There then follows the abovementioned crosslinking of the polyolefin at a 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. 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 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 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 film is generally transported through the oven and between the sections of the blower chambers with the aid of a circulating mesh or canier belt. 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 polyolefin 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. 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. 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 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 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 lower temperature level in the area 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. In an advantageous embodiment of the invention, the heat transfer between the hot air and the polyolefin 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 firom 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 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. In 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 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 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. 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 counterflow method of the invention have already been given in the description of the novel process. In an advantageous embodiment of the invention, the equipment for heating the polyolefin 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 entirety of that longitudinal section of the sheet or film which is located in the oven. 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. 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 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 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 above the polyolefin sheet or film. This embodiment of the invention is particulariy 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 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. An^anging the chill roils in this way also reduces the overall length of the novel apparatus compared with conventional apparatus. 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 and/or 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. 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 crossiinked polyolefin sheets or polyolefin films 2 is shown diagrammatically as a partial longitudinal section in Fig. 1. 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 an-anged one over the other. The oven also has two openings 10 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. 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 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 rotation of the wind-up roll 22 conresponds 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. 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, the 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 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 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 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. 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. The canier 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 canier belt 13 is driven and conducted over rollers 14 and 15. The film 2 is arranged on the upper side of the canier belt 13 and comes into contact with hot air 25 in the oven 3. The hot air is conducted by the fan 27 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 tlien 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 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 an-ow 41. 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 and/or pulling and finally is rolled up in its crosslinked and foamed condition onto the wind-up roll 22. The removal of the film fi-om the canier belt by blowing is usually achieved with the aid of the hot air flowing out through the flow apertures 33 an-anged 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 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 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 heating tunnel is simple in engineering temns and can be carried out at low cost, since, 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 referred 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 long, left-hand section, 31 and 32, of the heating tunnel 30, whereas the equipment 24 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 canier 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. 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 and/or 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. 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 tunnel below the carrier belt not only, as in the case of Figs. 1 and 2, through the canier 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. WE CLAIMS 1. Process for crosslinking and foaming crosslinkable polyolefin sheets or polyolefin films or for foaming crosslinked polyolefin sheets or polyolefin films, in which the essentially continuously transported polyolefin sheet or polyolefin film is brought into contact with hot air, characterized in that the hot air flows along the polyolefin sheet or polyolefin film in the direction opposite that of the transport of the sheet or film. 2. Process according to Claim 1, characterized in that the hot air flows along the upper side and along the lower side of the polyolefin sheet or polyolefin film. 3. Process according to Claim 1 or 2, characterized in that the heat transfer between the hot air and the polyolefin 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. 4. Process according to one of the preceding claims, characterized in that the hot air is conducted in a virtually closed circulation system. 5. Apparatus for crosslinking and foaming crosslinkable polyolefin sheets or polyolefin films or for foaming crosslinked polyolefin sheets or polyolefin films, in particular for carrying out a process according to one of the preceding claims, with an oven (3) and equipment (24) allocated thereto for heating the polyolefin sheet or polyolefin film (2) in the oven (3) by means of hot air (25), characterized in that the equipment (24) for heating the polyolefin sheet or polyolefin film (2) has been arranged and constructed in such a way that the direction of flow of the hot air (25) emerging from the equipment (24) runs approximately parallel to the plane (26) of the sheet or film. 6. Apparatus according to Claim 5, characterized in that the direction of flow of the hot air (25) runs in the longitudinal direction of the sheet or film (2). 7. Apparatus according to Claim 5 or 6, characterized in that the hot air (25) is conducted countercurrently to the direction of transport of the sheet or film (2) passed through the oven (3). Apparatus according to one of Claims 5 to 7, characterized in that the equipment (24) for heating the polyolefin sheet or polyolefin film (2) has at least one heating tunnel (30) surrounding the sheet or film (2). Apparatus according to Claim 8, characterized in that the heating tunnel (30) completely sunrounds the polyolefin sheet or polyolefin film (2) in its transverse direction and extends over virtually the entirety of that longitudinal section of the sheet or film (2) which is located in the oven (3). ■ Apparatus according to one of Claims 5 to 9, characterized in that the equipment (24) for heating the polyolefin sheet or polyolefin film (2) has equipment (34) for recycling at least some of the hot air (25) prior to its use. Apparatus at least according to Claims 8 and 10, characterized in that the equipment (34) for recycling at least some of the hot air (25) prior to use is at least one flap (35) provided in the heating tunnel (30), with the aid of which flap at least a part (36) of the hot air (25) flowing through the heating tunnel can be diverted into the area of the oven (3) arranged outside the heating tunnel (30). Apparatus according to Claim 11, characterized in that the flap (35) has been mounted in the wall of the heating tunnel (30) in such a way that it can swivel and forms a part of this wall. Apparatus according to Claim 11 or 12, characterized in that the flap (35) has been arranged appnoximately at the longitudinal centre of the heating tunnel (30) in that part of the same which is arranged above the polyolefin sheet or polyolefin film. Apparatus according to one of Claims 5 to 13, characterized in that chill rolls (21) have been provided for transporting and cooling the polyolefin sheet or polyolefin film (2), and at least one of these has been built into a wall (7) of the oven (3) in such a way that the gaps (40) fonmed are small. 15. Process for crosslinking and foaming crosslixikable polyolefin sheets or polyolelin films, substantially as hereinabove described and illustrated with reference to the accompanying drawings.

Documents:

in-pct-2000-235-che-abstract.pdf

in-pct-2000-235-che-assignment.pdf

in-pct-2000-235-che-claims filed.pdf

in-pct-2000-235-che-claims grand.pdf

in-pct-2000-235-che-correspondence others.pdf

in-pct-2000-235-che-correspondence po.pdf

in-pct-2000-235-che-description complete filed.pdf

in-pct-2000-235-che-description complete grand.pdf

in-pct-2000-235-che-drawings.pdf

in-pct-2000-235-che-form 1.pdf

in-pct-2000-235-che-form 26.pdf

in-pct-2000-235-che-form 3.pdf

in-pct-2000-235-che-form 5.pdf

in-pct-2000-235-che-pct.pdf

in-pct-2000-235-che-priority documents.pdf