PROCESS AND APPARATUS FOR PRODUCING DEGRADED POLYOLEPHINE FOAMS Technical Field The invention relates to a process for degrading and foaming degradable polyolefin sheets or polyolefin films or for foaming polyolefin sheets or degraded polyolefin films according to the pre-characterization clause. of Claim 1 of the Patent and also to a particular apparatus for carrying out a process of this type, according to the pre-characterization clause of the Claim. 5 of the Patent. The term polyolefins is used for polymers of the general structure
wherein R1 is hydrogen and R2 is hydrogen or straight or branched chain groups. If R1 and R2 are hydrogen, this gives polymers that have groups of the type
as a basic unit characteristic of the polymer chain The above polymers are called polyetiols. The known polyethylenes can 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 made in particular between the LDPE grades (low density polyethylene), LLDPE (linear low density polyethylene), HDPE high density polyethylene), HDHMWPE (high molecular weight high density polyethylene) and VLDPE (high density polyethylene). low density) . The densities of foams of this type are approximately 25 to 200 kg / m3. The foams have a substantially closed cell structure. Increasing the viscosity and / or hardness of the polymers requires degradation, that is, a reaction in which the linear or branched molecules present, are linked to each other to give three-dimensional polymer networks. Degradation of this type can take place in the exposure to high energy radiation or with the help of peroxides or silanes. The characteristic foam structure arises due to the chemical reactions in the addition of substances known as blowing agents, which during the processing are decomposed at a particular temperature with gas formulation. Examples of conventional blowing agents are the azo compounds and the diazo compounds. In the foam industry, these are called blowing agents that generate cells. When exposed to heat and when appropriate, the catalysts include gases and thus are suitable for producing foamed plastic compositions. The foaming may take place when the product leaves the extrusion instrument, ie directly after extrusion or injection molding or in open molds. Firstly, a polyolefin sheet or a compact 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 slotted nozzles. 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 degrading agent nor the blowing agent reaches its decomposition temperature. After this, the aforementioned degradation of the polyolefin follows at a temperature level that reaches or exceeds the decomposition temperature of the degrading agent but does not yet reach the decomposition temperature of the blowing agent. It is not until the degradation is virtually complete that the decomposition of the blowing agent begins and thus the foaming process is initiated, for example by the additional introduction of heat. This phase of the total production process is exothermic. Prior Art A process and an apparatus of the type mentioned at the beginning are known in the industry. Degradation and foaming, or just foaming, takes place in an oven that consists of several zones installed in succession. A fan, and also a unit known as a blower, is provided to each zone of the furnace. Each blowing chamber has an upper section with openings directed vertically downwards, and also a lower section with openings directed vertically upwards. The two sections of each blower chamber are installed one above the other, at a distance from each other, and are attached to the fan assigned to the respective zone. Between the sections of each of the blowing chambers runs the polyolefin sheet or polyolefin film. This is, for example, unrolled from a roll, passes between the sections installed one on top of the other of the blowing chambers, during which it is heated, degraded and foamed, and finally rolled upwards to a roller rolled Between the furnace and the winding roller there are generally several cooling cylinders for transporting and cooling the polyolefin sheet or polyolefin film. The process is generally continuous, when the sheet or film is generally transported through the furnace and between the sections of the blower chambers with the aid of a circulation maya or conveyor belt. The heating of the sheet or film takes place through the fan and the blowing chambers with the help of hot air. The hot air temperature is adjusted in the individual zones to the respective required value. Normally, the degradation of the polyolefin first takes place, and the foaming of the material takes place in a subsequent hotter zone of the furnace. Furnaces of this type are preferably divided into 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 to generate "heat and control the temperature. therefore, the cost of constructing apparatuses of this type is considerable, and therefore when the known process is used, high capital costs and operating costs must be foreseen Obj ective The objective on which this invention is based, is to create a process of the type mentioned at the beginning, which in particular can carry out the cost-effectiveness for small quantities Another object of the invention is to provide an apparatus of the type mentioned at the beginning which is less expensive to manufacture and operate. Invention The aspects of the process of this objective are achieved by means of a process with the characteristics of Claim 1 of the Patent, and aspects of the apparatus are achieved by r means of an apparatus with the features of Claim 5 of the Patent. According to the invention, the hot air flows along the polyolefin sheet or polyolefin film in the opposite direction to that of the sheet or film transport. When this type of counterflow system is used, the section of the sheet that first makes contact with the hot air is that which has already been in the oven for a relatively long time. The trajectory of the hot air is then taken along the sheet and therefore some of its thermal energy passes to the sheet. Therefore, the temperature of the hot air is reduced 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 driven from I return to the fan. Therefore, a process of this type can be carried out particularly with effective cost, since the air put in contact with the sheet is at a high temperature level in the area where the foaming process takes place and at a lower level of temperature in the area where, for example, degradation of the polyolefins takes place at a lower temperature level. This is advantageous for the hot air to flow along the upper side and along the underside 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 by controlling the flow rate of the hot air, by diverting some of the hot air that flows along the sheet or film, away from these. This is made easy with the constant operation of the fan, in order to influence the temperature level as desired in the direction of hot air flow and thus supply the sheet or film in each area with the precise amount of heat required. in that area for the respective chemical reaction, for example degradation or foaming. It is advantageous for hot air to be driven in a virtually closed circulation system. This has additional favorable effects on the costs of carrying out the process and results in a process that is particularly favorable to the environment, since environmental pollution, for example the content of hazardous materials in the air outlet, is markedly reduced by the closed design. In the aspects of the apparatus of the present invention, the polyolefin sheet or polyolefin film heating equipment is installed and constructed in such a way that the flow direction of the hot air emerging from the equipment runs approximately parallel to the plane of the sheet or film. Thus, the air flows on the sheet or film and therefore can pass its heat content evenly towards the sheet or film. The temperature of the hot air is reduced along its flow path. The heat content of the hot air that has already flowed over a portion of the sheet or film, can therefore be advantageous to the sections of the downstream sheet and supply heat thereto. Therefore, the new apparatus also has different temperatures along the flow path of the hot air, but without requiring the high cost of the prior art equipment. A particular reason for the advantages obtained from the invention is that the manner in which hot air flows over the sheet or film is favorable for heat transfer. The flow direction of the hot air is advantageously the longitudinal direction of the sheet or film and the hot air is preferably conducted countercurrent to the transport direction in which the sheet or film passes through the oven. This also allows the continuous operation of the new apparatus, for example the transportation of a very long polyolefin film at a uniform speed through the apparatus. The advantages of the contraflow method of the invention have already been given in the description of the new 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 virtually over the entirety of that longitudinal section of the sheet or film that is located in the oven. This allows the exothermic energy released - in the foaming process, to be available for hot air and directly to that area of the sheet or film - which is downstream in the hot air flow direction. The design of the heating tunnel according to the invention is simpler to build than several of the blower chambers installed in succession, since the sheet or film is located directly in the heating tunnel and not, as in the prior art has to be pass between the two sections of the blower installed one above the other with a space between them. This results in lower capital costs. In addition, the new apparatus has a lower weight than the conventional apparatus. In another embodiment of the invention, the equipment for heating the polyolefin sheet or film has the equipment for at least partial recirculation of the air-hot before use. That is, preferably at least one fin provided in the heating tunnelWith the help of such a fin, at least some of the hot air flowing through the heating tunnel can be diverted towards the area of the furnace installed on the outside of the heating tunnel. This fin provides a simple way to reduce the volume of flow through the heating tunnel and therefore the flow rate 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 through the flow rate and the temperature of the hot air around the material. This is advantageous for the fin which has been installed in the wall of the heating tunnel and which forms part of it, in such a way that it can rotate this wall. This makes it easy specifically by rotating the flap downward, to drive some of the hot air out of the heating tunnel. In one embodiment of the invention, the fin is installed approximately in the center of the length of the heating tunnel, in that part thereof which is installed above the polyolefin sheet or film. This embodiment of the invention is particularly advantageous if the fan has also been attached to the furnace above the sheet or film, not far from the fin. In another embodiment of the invention, the cooling rollers are provided for transporting and cooling the polyolefin sheet or film. At least one of these is built into a furnace wall in such a way that the spaces formed are small. With the help of this modality the new apparatus is virtually a completely closed system. This reduces the energy consumption of the new device and improves its environmental compatibility. This also substantially prevents outside air from entering the new apparatus. By installing the cooling rollers in this way, the overall length of the new apparatus is also reduced compared to the conventional apparatus. BRIEF DESCRIPTION OF THE DRAWINGS Examples of operation of the subject matter of the invention are described in greater detail below, using the figures. Figure 1 diagrammatically shows a partial longitudinal section through a first embodiment of an apparatus for degrading and / or foaming polyolefin sheets or polyolefin films; and Figure 2 shows in diagrammatic form a partial longitudinal section through a second embodiment of an apparatus; for simplicity the 1
Polyolefin sheet or polyolefin film is not shown herein. MODES OF CARRYING OUT THE INVENTION A first example of operation of a new apparatus 1 for degrading and foaming polyolefin sheets or degradable polyolefin films 2 or for foaming or degrading polyolefin sheets or polyolefin films 2, is shown diagrammatically as a partial longitudinal section in figure 1. For this, the apparatus 1 has a long furnace 3, which in figures 1 and 2 is in the form known as a continuous furnace. On its front face 4 (shown on the left in Figure 1) the furnace 3 has two openings 5 and 6 installed one above the other. The furnace also has two openings 10 and 11 installed one above the other on its rear face 7 and also a space 12 above the upper opening 10 on the rear face 7. A crosslinked conveyor belt 13 driven on the rollers 14 and 15, runs through the openings 5 and 6 in the front face 4 and in the openings 10 and 11 in the rear face 7 of the furnace 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 sheet of polyolefin or polyolefin film 2 called sheet or film then has the shape of a long network and is initially located on a roller 17, whose direction of rotation is indicated by the arrow 20. The film can also pass directly, that is, without using the roller 17, to the conveyor belt 13 coming from the extruder nozzle. The film 2 is located in the upper section of the conveyor belt 13 virtually over the total length of the furnace 3 and is then led over the cooling rollers 21 in a winding roller 22 shown to the right in Figure 1. The direction of The rotation of the winding roller 22 corresponds to that of the roller 17. Both the roller 17 and the winding roller 22 are installed in the frames 23 in such a way that they can rotate. The apparatus 1 also has the equipment 24 assigned to the oven 3, to heat by means of the 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 installed and constructed in such a way that the flow direction of the hot air 25 emerging from the equipment runs approximately parallel to the plane 26 of the sheet or film. The flow direction 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 transport direction of the film 2 passed through the oven 3 with the support of the conveyor belt 13. The equipment 24 for heating the film 2 has a fan 27 and a heating tunnel 30 around the film 2 in the furnace 3. The heating tunnel completely surrounds the film 2 in the furnace 3 in the furnace 3. transverse direction of the film and extends virtually over the entire length section of the film 2 located in the furnace. In figure 1 the heating tunnel 30 encircles, downstream from the fan 27, first a relatively short section 31 on the right hand side and, a relatively long section 32, joins it and extends towards the front face 4 of the furnace 3. The section 31 on the right-hand side of the heating tunnel 30 has flow openings 33, tangentially directed or parallel to the plane 26 of the film. The heating tunnel 30 also has the equipment 34 for at least partial recirculation of the hot air 25 before use. This equipment 34 is a fin 35 provided in the wall of the relatively long section 32 of the heating tunnel 30, and which can be used to deflect a portion indicated by the arrow 36 of the hot air 25 towards a return duct 37 installed on the side outside of the heating tunnel 30. In Figure 1, the flap 35 has been installed in the upper wall of the section 32 of the heating tunnel 30 in such a way that it can be rotated and forms a part of this wall. The fin is installed approximately in the center of the length of the heating tunnel 30. One of the cooling rollers 21 for transporting and cooling the film 2 is constructed in the space 12 on the rear face 7, so that there is only one small space 40 between this cooling roller 21 and the rear face 7. The procedure for operating the new apparatus 1 and carrying out the new process is as follows. The conveyor belt 13 runs at a particular transport speed in the clockwise direction in Figure 1, so that the lower section of the conveyor belt 13 in Figure 1, moves from right to left and the upper section of the conveyor belt moves from left to right. The conveyor belt 13 is driven and driven on the rollers 14 and 15. The film 2 is installed on the upper side of the conveyor belt 13 and is brought into contact with the hot air 25 in the furnace 3. The hot air is driven by the fan 27 initially in the right-hand section 31 of the heating tunnel 30, and from this section emerges through the flow opening 33 along the film 2, tangentially to the plane 26 of the film. The hot air then passes into the relatively long left-hand section 32 of the heating tunnel 30 and from there it flows along the film, up and down, against the conveying direction of the conveyor belt 13 and therefore the film 2. This heats the polyolefin sheet or polyolefin film 2 to cause chemical reactions, and the temperature of the hot air, and therefore the film is adjusted in such a way that a section near the front face 4 of the furnace 3, It is above the temperature required to degrade the film and below that required to foam the film. In a subsequent section of the heating tunnel 30 (in the transport direction of the film), which is directed more towards the rear face 7 of the furnace 3, the temperature of the hot air is in a suitable range for foaming the film 2. The manner of flow of hot air 25 is indicated by arrow 41. In the final area of the furnace (in figure 1 on the right) in the direction of transport of. the film, the film expands three-dimensionally due to the formation of gas by the blowing agent and, with the help of the cooling rollers 21 which also have the function of pulling the rollers, is removed from the conveyor belt 13 by blowing it and / or pulling it out and finally winding it in its foamed and degraded condition on the rolling roller 22. Removal of the film from the conveyor belt by means of blowing is usually achieved with the help of hot air flowing out through the openings of flow 33 installed under the conveyor belt 13. The temperature of the hot air is established and controlled, for example, by means of gas burners in the air circulation system or by the introduction of hot air into the circulation system . The transfer of heat between the hot air and the polyolefin sheet or polyolefin film 2 is adjusted by controlling the flow rate of the hot air 25, by diverting some of the warm air flowing to the long side of the film 2 away from this, with the help of the fin 35 and outside the heating tunnel 30 to the return duct 37. This reduces the flow rate of the remaining hot air in the section of the heating tunnel 30 under the fin 35 in the direction of air flow. Therefore, it is possible to influence the transfer of heat from hot air to the film by controlling the flow rate of the hot air. The stages of "degradation" and "foaming" mentioned above, they clearly depend on the temperature and the residence time of the respective section of the film in a particular area of the oven. Therefore, at a given temperature, the rate of yield can be increased by increasing the residence time, i.e. by lengthening the furnace. The lengthening of the heating tunnel is simple in terms of design and can be carried out at low cost since, unlike conventional ovens, it is not essential to install additional hot air blowers, fans and burners, including control equipment respective. A second example of operation of the invention is shown in Figure 2, where identical or similar characteristics are referred to use the same numbers. For simplicity, Figure 2 does not show the polyolefin sheet or polyolefin film 2 or the roll 17 or the roll 22 with its respective frames 23. It can be seen from FIG. 2 that, in the right-hand section of the oven 3, shown, the equipment 24 shown to heat the sheet has a section on the right and a relatively long section on the left 31 and 32 of the tunnel of heating 30, while the equipment 24 shown 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 furnace 3 that the film is foamed, and it is not until this area that it is removed to pull it away from the conveyor 13. Therefore, the right-hand section 31 of the heating tunnel 30 has also only been provided in this section of the furnace. In this embodiment, the heating tunnel 30 is divided upwards into two sections placed one after the other, to achieve the best possible coupling to the requirements of the product to be produced and / or to increase the rate of return. Clearly, the two heating tunnels placed one after the other shown in figure 2, each more equipment elements to return some of the hot air. There may also be fins 35 in the section of the heating tunnel constructed under the conveyor 13. Another possibility is to allow the lower section of the relatively long section 32 to the right of the heating tunnel 30, to end just before the inner side of the heating tunnel 30. the front face 4 of the furnace 3, unlike FIGS. 1 and 2, in such a way that the air flowing outside the heating tunnel 30 can not only flow away from the section of the heating tunnel below the conveyor belt , as in the case of figures 1 and 2, through the conveyor belt, but also beyond this using the space between the inner wall of the front face 4 of the furnace and the end of the heating tunnel, and towards the return duct 37. This gives a higher cost-efficiency and the most environmentally favorable way of producing foamed polyolefin sheets or foamed polyolefin films.