Process for producing thermoformed plastic parts from polyethylene terephthalate

Abstract

Process for producing thermoformed plastic parts from polyethylene terephthalate A method for producing thermoformed plastic parts from amorphous polyethylene terephthalate without nucleating agents is described. In order to enable the production of transparent plastic parts suitable for use in microwave ovens, which can be recycled by type, despite economical cycle times and low energy consumption, the following is proposed: In a feeding step, a semi-finished product (1) having a given semi-finished product width is first fed in a machine direction (MD) running parallel to the longitudinal direction of the semi-finished product to a processing section (3) of a thermoforming device consisting of a thermoforming tool, where it is heated in at least one heating step to a stretching temperature of 90 to 180 ° C and stretched in the machine direction (MD) with a stretching degree of 1.2 to 5.0 in at least one stretching step, the at least one heating step being simultaneous with or preceding in time the at least one stretching step, and then, in a shaping step, the semi-finished product (1) stretched in the processing section (3) is shaped in a cooled thermoforming tool, whereby it is quenched to a temperature of at least 30 ° C below the glass transition temperature of the polyethylene terephthalate used.

Description

[Technical field]

[0001] Technical Field The present invention relates to a method for producing thermoformed, fully recyclable plastic parts from nucleating agent-free amorphous polyethylene terephthalate. [Background technology]

[0002] cutting edge technology To produce plastic parts such as plastic cups or plastic containers, a manufacturing process generally known as thermoforming or thermoforming process is used. For example, in the case of polyethylene terephthalate (PET), the plastic semi-finished product in the form of a film or plate is first preheated to 90 ° C and then fed via a conveying system to a thermoforming device where it is usually molded in two steps. In the first step, the PET semi-finished product is gradually heated to above 200 ° C, creating the conditions for thermally induced crystallization and finally achieving the high heat resistance of the container. The semi-finished product is then molded in a thermoforming tool, after which it is rapidly cooled in the cooled thermoforming tool in the second step. As a result of these measures, crystallization of the PET is achieved, which allows the improvement of its mechanical properties, including its heat resistance of at least 120 ° C. However, in order to economically implement such a method, nucleating agents, for example in the form of inorganic filler particles and / or polymer-based nucleating agents, must be added to the PET during the extrusion process of the corresponding semi-finished product in order to minimize the thermally induced crystallization time, which otherwise can be 15 seconds or more depending on the product in question. However, the use of such additives makes recycling more difficult, since it cannot be recycled by material type. Another disadvantage is that, due to crystallization, only opaque plastic parts can be made from PET, not transparent plastic parts.

[0003] Furthermore, thermoforming processes are known that allow the production of transparent plastic parts using amorphous PET, but the plastic parts produced in this way have the disadvantage that they are not suitable for use in microwaves (microwave ovens) due to their low heat resistance up to a maximum of 60-70 °C and the unfavorable tendency to shrink at temperatures above 62 °C.

[0004] Methods exist for producing so-called biaxially oriented PET films, in which the PET film is first preheated to up to 90°C and then stretched biaxially, i.e. in both the machine and transverse directions, by means of a corresponding stretching device. To facilitate the heating of the stretching gap, auxiliary materials are usually added to the PET, which is usually done by means of infrared heaters. The stretching induces crystallization of the PET, which has a special influence on the property profile of the film. In the course of a thermal post-treatment, the PET film is subsequently heated to above 200°C. This can reduce the residual stresses in the film due to crystallization and mitigate the tendency to shrink. However, the required thermal post-treatment and the associated high temperatures require a correspondingly high energy input. Summary of the Invention [Problem to be solved by the invention]

[0005] Description of the invention There is therefore a need to create a method of the above-mentioned type that allows the production of thermoformed, unmixed (pure) recyclable plastic parts that have sufficient and visually appealing transparency, are suitable for use in microwave ovens and have the heat resistance of 120-145°C required for this purpose, despite economical cycle times and relatively low energy consumption. [Means for solving the problem]

[0006] The invention relates to a method for producing a semi-finished product having a given semi-finished product width, in particular a film-like or plate-like semi-finished product, in a feeding step, first in a machine direction running parallel to the longitudinal direction of the semi-finished product, fed to a processing section of a thermoforming apparatus comprising a thermoforming tool, then in at least one heating step, heated there to a stretching temperature of 90-180 ° C and actively stretched in the machine direction with a stretching degree of 1.2-5.0 depending on the set stretching temperature, in at least one stretching step, after which, in a shaping step, the stretched semi-finished product in the processing section is shaped with the aid of a cooled thermoforming tool, whereby it is quenched to a temperature of at least 30 ° C below the glass transition temperature of the polyethylene terephthalate used. Preferably, the thermoforming tool has for this purpose an apparatus temperature of 15-20 ° C. A stretching degree of 1.2-5.0 means that the semi-finished product is stretched in the processing section to 1.2-5 times its original nominal length. Alternatively, the degree of stretching can be specified as a percentage, in which case the degree of stretching will be 120 to 500% when the reference length is defined as 100%.

[0007] Methods for carrying out the invention The invention is based on the realization that, for amorphous and nucleating agent-free PET, the combination of at least one stretching step in the machine direction with a stretching degree of 1.2 to 5.0, preferably 3.0 to 5.0, more preferably 3.5 to 4.5 and particularly preferably 4.4, and at least one heating step to a stretching temperature of 90 to 180° C., preferably 90 to 160° C., more preferably 90 to 145° C., more preferably 120 to 145° C., more preferably 120 to 140° C., more preferably 125 to 135° C. and particularly preferably 130° C., allows favorable crystallization conditions in terms of a low tendency to shrink and sufficient transparency of the plastic parts produced. The stretching temperature in particular denotes the core temperature of the semi-finished product.

[0008] Surprisingly, it has been shown that the stretch-induced crystallization caused by the combination of at least one stretching step in the machine direction with at least one heating step produces a particularly fine layered (lamellar) crystal structure, which can be fixed in a shaping step immediately following at least one stretching step by cooling or quenching to a temperature of at least 30° C. below the glass transition temperature of the PET used. Depending on the thickness or strength of the semi-finished product or the finished plastic part, it is also possible in principle to briefly but intensively heat the stretched semi-finished product at temperatures of 120 to 200° C., preferably 130 to 200° C., more preferably 140 to 200° C., particularly preferably 160 to 180° C., in a post-heating step after at least one stretching step, in order to further improve the crystallization conditions for the heat resistance temperature of the plastic part of 120 to 145° C. Depending on the thickness or strength of the semi-finished product or the finished plastic part, it is also possible in principle to briefly but intensively heat the stretched semi-finished product at temperatures of 120 to 200° C., preferably 130 to 200° C., more preferably 140 to 200° C., particularly preferably 160 to 180° C. In this respect, the higher the temperature selected, the shorter the residence time in the post-heating step. In particular, it can also be provided that the post-heating step follows immediately after at least one stretching step, which is followed immediately by a shaping step. The post-heating step not only further improves the heat resistance of the finished plastic part, but post-heating also allows for better moldability of the semi-finished product in the molding step.

[0009] In principle, the additional active stretching of the semi-finished product in the processing section can also take place in a transverse direction perpendicular to the machine direction or along the width of the semi-finished product. However, preferably, the active stretching takes place only in the machine direction while the semi-finished product is fixed in the processing section with respect to its width. Due to the fact that the semi-finished product remains fixed with respect to its width, the normally occurring shrinkage of the semi-finished product, i.e. the reduction in the width of the semi-finished product due to the longitudinal stretching in the machine direction, is prevented during the active stretching in the machine direction. Thus, in addition to the active stretching in the machine direction, a slight passive stretching in the transverse direction is imposed, which, together with the active stretching in the machine direction, promotes the formation of a fine lamellar crystal structure. Preferably, it can be provided that in at least one stretching step, the semi-finished product is fixed in the processing section at the ends of the semi-finished product that face each other in the transverse direction. In the context of the present invention, active stretching is understood to mean that a stretching force, in particular a tensile force, is introduced into the semi-finished product by active movement of at least one holding element of a stretching device which can be attached to the semi-finished product in such a way that the semi-finished product is stretched parallel to the movement direction of the holding element. In contrast, passive stretching in the transverse direction occurs as an inevitable side effect of active stretching in the machine direction when the semi-finished product is fixed with respect to its width.

[0010] In principle, the semi-finished product can also be initially overstretched in the machine direction in the processing section, after which the semi-finished product is slightly shrunk again in a shrinkage step after the molecular orientation stage before the final shaping step. This measure allows the crystalline chains of the PET material to be aligned in a more ordered structure, so that the advantageous crystallization effect can be further increased, which is further favorable for the material properties of the product. Preferably, the overstretching should be carried out in such a way that the subsequent shrinkage in the machine direction of the semi-finished product before the shaping step is at most 20%, preferably 1-20%, more preferably 5-15%, even more preferably 7-13%, particularly preferably 10-12%. Preferably, at least one stretching step is immediately followed by a shrinkage step, which is immediately followed by a shaping step.

[0011] The selected shrinkage percentage depends in particular on the stretching or stretching speed at which the plate-like semi-finished product is stretched in at least one stretching step and / or on the thickness or thickness of the plate-like semi-finished product after at least one stretching step. If the thickness of the plate-like semi-finished product after stretching is, for example, 1.2 mm, a shrinkage of preferably 10% occurs in the shrinking step. If the thickness of the plate-like semi-finished product after stretching is, for example, 1.5 mm, a shrinkage of preferably 12% occurs in the shrinking step.

[0012] The shrinking step can generally be carried out at a temperature between 120 and 200° C. or according to the preferred temperature ranges specified above for the post-heating step. In particular, the shrinking step can be carried out at a temperature between 120 and 160° C., more preferably at a temperature between 130 and 140° C.

[0013] As mentioned above, the active stretching of the semi-finished product in the machine direction can basically be carried out via a separate stretching device, for example, the relevant stretching force can be introduced via the end face of the semi-finished product, i.e. via the front face of the semi-finished product, which is delimited by the transversely opposite ends of the semi-finished product. However, in particularly preferred processing conditions, the ends of the semi-finished product are fixed and forcibly guided in a clamping clamp (clamping clip) in the processing section, and the stretching force for the active stretching in the machine direction is introduced to the semi-finished product via the clamping clamp. As a result of these measures, both the fixation of the semi-finished product with respect to the semi-finished product width and the introduction of the stretching force for the active stretching in the machine direction take place exclusively via the clamping clamp assigned to the transversely opposite end of the semi-finished product. For example, the clamping clamp can be guided and moved via a known chain-based transport system. The transport system can be designed in such a way that at the start of the processing section, the clamping clamp is positioned against the end of the semi-finished product and held in place. Once the stretching is complete, the clamping jaws are released from the end of the semi-finished product at the end of the processing section and removed via the conveying system. As an alternative to the clamping jaws, it goes without saying that any other suitable removable holding element can also be used, which can be placed on the end of the semi-finished product by means of the conveying system, can hold the semi-finished product and can be released again.

[0014] Plastic parts produced by the method according to the invention can have a shrinkage of up to 0.5% at a service temperature of 120 ° C. As a result, the measures according to the invention make it possible to dispense with an energy-intensive thermal post-treatment after the stretching step to reduce residual stresses due to crystallization. Depending on the processing conditions, plastic parts produced according to the invention have a haze value according to ASTM D 1003 of at most 1%. Overall, the method according to the invention makes it possible to produce plastic parts that have sufficient temperature resistance, especially heat resistance, for microwave use and a visually attractive appearance due to sufficient transparency. It has also been shown that the strain-induced crystallization kinetics according to the invention is sufficiently fast, despite the absence of nucleating agents in the PET, so that cycle times customary in known thermoforming processes can essentially be maintained. Due to the fact that no nucleating agents or auxiliaries are added to the PET used to improve the heating efficiency, the plastic parts obtained by the method according to the invention can be fed to essentially separate recycling processes.

[0015] The method according to the invention can be adapted as required depending on the thickness or thickness of the original semi-finished product, which is 2.5-3.5 mm, for the production of microwave-compatible plastic containers, with regard to the stretching temperature, the degree of stretching, the stretching speed, the cooling speed, the residence time in the thermoforming tool, etc., and, if necessary, with regard to the temperature and residence time during the post-heating step and the temperature, residence time and shrinkage during the shrinking step, the number and timing (temporal sequence) or sequence of the respective heating and stretching steps can also be adapted as required. For example, at least one heating step can be carried out simultaneously with at least one stretching step. In general, however, it is particularly preferred if at least one heating step precedes at least one stretching step, i.e. the heating step is carried out first and then the stretching step.

[0016] To reduce cycle time, the feeding step and at least one drawing step may be performed simultaneously.

[0017] In the case of favorable crystallization conditions, active stretching of the semi-finished product in the machine direction can be carried out in at least one stretching step at a stretching speed of 50-400% per second, preferably 90-350% per second, more preferably 200-350% per second, depending on the original thickness of the semi-finished product. In principle, an increase in the stretching temperature leads to an increase in the stretching speed and vice versa.

[0018] The stretching speed should always be set as fast as the available material allows, depending on the corresponding material thickness etc. The faster the molecular orientation of the PET material after stretching, the higher the thermal energy required to induce a shrinkage process in the final molded part. The heat resistance of the final product can therefore be improved by a higher stretching speed. To carry out the process according to the invention, generally known plastic thermoforming equipment can be used, which, for example, in addition to the thermoforming tool, also comprises a corresponding feeding or conveying device for the semi-finished product and, if necessary, a punching device for punching out the molded plastic parts. [Brief description of the drawings]

[0019] In the drawing, the object of the invention is shown in a schematic top view of a semi-finished product which has been stretched, for example, in a working section of a hot forming device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The method according to the invention is used, for example, to produce thermoformed containers made of amorphous PET without nucleating agents. Such containers have a haze value according to ASTM D 1003 of max. 1% and a shrinkage of max. 0.5% at a use temperature of 120°C. They therefore have visual transparency and are suitable for use in microwaves. As no nucleating agents or additives are added to the PET used to improve the heating efficiency, the cups obtained by the method according to the invention can essentially be recycled without sorting at the end of their useful life.

[0021] The drawing shows a schematic of a semi-finished product 1 in the form of a PET film. The semi-finished product 1 can in principle be supplied as a continuous roll or a continuous film strip. The dotted reference line 2 on the semi-finished product 1 runs in a transverse direction TD to the width of the semi-finished product, in order to illustrate that the semi-finished product 1 is stretched in a machine direction MD running parallel to the longitudinal direction of the semi-finished product in a processing section 3 of a thermoforming device, not shown in detail. The processing section 3 is indicated by two dotted lines that also run in the transverse direction TD. In this embodiment example, the semi-finished product 1 is stretched in the machine direction MD with a stretch degree of 4.4 or 440%, as can be seen from the large distance between the reference lines 2 of the processing section 3 with respect to the reference length 4 defined between the two reference lines 2 of the processing section 3.

[0022] The semi-finished product 1 is held at its end in the processing section 3 via a clamping jaw 5, which is shown diagrammatically, so that the semi-finished product 1 is fixed with respect to its width. A stretching force for active stretching in the machine direction MD is introduced into the semi-finished product 1 via the clamping jaw 5, which can be moved in the machine direction MD, for example, by means of a chain drive in the processing section 3. An exclusive active stretching in the machine direction MD would normally cause a shrinkage of the PET film, i.e. a reduction in the width of the semi-finished product 1, as shown by the dashed waist of the semi-finished product 1 in the processing section 3. However, the fact that the semi-finished product 1 remains fixed with respect to its width prevents such a concavity. As a result, in addition to the active stretching in the machine direction MD, a small passive stretching in the transverse direction TD is forced. At the start of the processing section 3, the clamping jaw is placed against the end of the semi-finished product, which is held by the clamping jaw 5. After the stretching, at the end of the processing section 3, the clamping jaws 5 are released from the end of the semi-finished product. In principle, the number of clamping jaws 5 or removable holding elements used can be freely selected depending on the desired processing conditions and product properties. For example, in some applications it is also advantageous for the processing conditions if as many clamping jaws 5 or removable holding elements as possible are provided in the processing section 3, so that a set overall degree of stretching results from several smaller partial stretches in the machine direction MD.

[0023] A heating step is performed simultaneously with or prior to the stretching step, whereby the semi-finished product 1 is heated to a stretching temperature of about 130° C. Immediately after the stretching step, the semi-finished product 1 is shaped in a forming step using a cooled thermoforming tool (not shown in detail) and quenched to a temperature of at least 30° C. below the glass transition temperature of the PET used. For this purpose, a water-cooled thermoforming tool is adjusted to a tool temperature of 15° C.

[0024] Alternatively, the stretching step can be immediately followed by a post-heating step, in which case the stretched semi-finished product 1 is heated briefly and intensively at a temperature between 120 and 200° C. The shaping step follows immediately after the post-heating step.

[0025] According to a further alternative, the stretching step can be followed by a shrinking step, whereby the semi-finished product 1 undergoes a shrinkage in the machine direction MD of up to 20%. This shrinkage is carried out at a temperature between 120 and 200° C. The shaping step follows immediately after the shrinking step.

[0026] After the forming step, the formed blank 1 is conveyed further in the machine direction MD and fed to a punching device, where finished plastic containers are punched out of the blank 1.

Claims

1. 1. A method for producing thermoformed, completely pure, recyclable plastic parts from nucleating agent-free amorphous polyethylene terephthalate, comprising: In a feeding step, a semi-finished product (1) having a given semi-finished product width, in particular a film-like or plate-like semi-finished product (1), is first fed in a machine direction running parallel to the longitudinal direction of the semi-finished product into a processing section (3) of a thermoforming device comprising thermoforming tools, Then, in at least one heating step, it is heated to a stretching temperature of 90 to 180° C., and In at least one stretching step, the sheet is actively stretched in the machine direction (MD) at a stretch degree of 1.2 to 5.0 depending on the stretching temperature set; said at least one heating step being simultaneous with or preceding in time said at least one stretching step; Then, in a forming step, the stretched semi-finished product (1) in the processing section (3) is formed using cooled thermoforming tools, The process is characterized in that it involves quenching to a temperature at least 30° C. below the glass transition temperature of the polyethylene terephthalate used.

2. 2. The method of claim 1, wherein said at least one stretching step is immediately followed by said shaping step.

3. 2. The method according to claim 1, characterized in that between said at least one stretching step and said shaping step a shrinking step is carried out, the semi-finished product (1) being subjected to a shrinkage of up to 20% in the machine direction (MD).

4. 4. The method according to claim 3, characterized in that the shrinking step is carried out at a temperature of 120 to 200°C.

5. 5. The method according to claim 1, characterized in that the semi-finished product (1) is actively stretched exclusively in the machine direction (MD) in the processing section (3), while the semi-finished product (1) is fixed in the processing section (3) with respect to its width.

6. 6. The method according to claim 5, characterized in that in the stretching step, the semi-finished product (1) is fixed in the processing section (3) at its ends facing each other in a transverse direction (TD) extending transversely to the machine direction (MD).

7. 7. The method according to claim 5 or 6, characterized in that the ends of the semi-finished product are fastened and force-guided in removable holding elements, in particular clamping jaws (5), in the processing section (3), and a stretching force for active stretching in the machine direction (MD) is introduced into the semi-finished product (1) via the holding elements.

8. 8. The method according to any one of claims 1 to 7, characterized in that in the stretching step, the active stretching of the semi-finished product (1) in the machine direction (MD) is carried out at a stretching speed of 50 to 400% / s, preferably 90 to 350% / s, more preferably 200 to 350% / s.

9. 6. A plastic part, in particular a plastic container or a plastic vessel, obtainable by the method according to any one of claims 1 to 5, in which the shrinkage of the plastic part at a use temperature of 120°C is at most 0.5%.

10. 10. Plastic part according to claim 9, characterized in that it has a haze value according to ASTM D 1003 of at most 1%.

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