Method for producing thermoformed plastic parts, which are substantially recyclable by material type, from nucleating-agent-free, amorphous polyethylene terephthalate
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-02-04
- Publication Date
- 2026-08-13
Smart Images
Figure EP2026052893_13082026_PF_FP_ABST
Abstract
Description
[0001] Procedures for
[0002]
[0003] and essentially single-variety parts from germination-forming-free
[0004]
[0005]
[0006] Technical field
[0007] The invention relates to a method for producing hot-formed and essentially single-material recyclable plastic parts from germ-forming free amorphous polyethylene terephthalate.
[0008] State of the art
[0009] The production of plastic parts, such as plastic cups or containers, typically utilizes manufacturing processes known as hot forming or thermoforming. In the case of polyethylene terephthalate (PET), for example, a film- or sheet-shaped plastic semi-finished product is first preheated to up to 90 °C, then fed via a transport system to a hot forming machine, where it is usually formed in two steps. In the first step, the PET semi-finished product is gradually heated to over 200 °C to create the conditions for heat-induced crystallization and thus ultimately achieve high heat resistance in the containers. The semi-finished product is then formed in a hot forming tool, after which, in a second step, it is rapidly cooled in a cooled hot forming tool.These measures enable PET to crystallize, improving its mechanical properties, including a heat deflection temperature of at least 120 °C. However, to make such processes economically viable, nucleating agents, such as inorganic filler particles and / or polymer-based nucleating agents, must be added to the PET during the extrusion process of the corresponding semi-finished products. This is necessary to minimize the heat-induced crystallization time, which would otherwise exceed 15 seconds, depending on the target product. The use of such additives, however, complicates recycling because single-material recycling is not possible. A further disadvantage is that, due to the crystallization process, only opaque, but not transparent, plastic parts can be produced from PET.Furthermore, PET products containing nucleating agents are not suitable for hot applications in the food sector for single-stream or circular recycling, as the added inorganic or organic nucleating agents cannot be removed during the recycling process. On the contrary, there is an increased risk that nucleating agents will accumulate in the plastic with each recycling cycle, uncontrollably altering the crystallization kinetics, transparency, melt viscosity, and migration characteristics of such recycled PET and thus preventing high-quality, material-identical reuse in a closed PET cycle.
[0010] Furthermore, thermoforming processes are known that, using amorphous PET, also enable the production of transparent plastic parts. However, a disadvantage of these processes is that plastic parts produced in this way are unsuitable for use in microwave ovens or for hot applications in the food industry due to their low heat resistance (up to a maximum of 60-70 °C) and their undesirable tendency to shrink at temperatures above 62 °C.
[0011] Furthermore, processes exist for the production of so-called biaxially oriented PET films, in which a PET film is first preheated to up to 90 °C and then stretched biaxially, i.e., both in the machine direction and in the transverse direction, using a suitable stretching device. Additives are typically mixed with the PET to facilitate the heating of the stretching gap, which is usually achieved using an infrared heater. The stretching process induces crystallization of the PET to selectively influence the film's property profile. In a subsequent thermal post-treatment, the PET film is heated to up to 200 °C or higher. This reduces internal stresses in the film caused by crystallization, thereby reducing its tendency to shrink.However, due to the necessary thermal post-treatment and the associated high temperatures, a correspondingly high energy expenditure is required.
[0012] Procedures according to the general term also include, for example, those arising from the
[0013] WO 2023178369 A1 is known, wherein plastic parts are obtained which have a heat resistance of up to 145 °C.
[0014] There is therefore a need to create a process of the type described above that, despite economical cycle times and relatively low energy consumption, enables the production of hot-formed, sorted, recyclable plastic parts which, with sufficient, visually appealing transparency, are also suitable for use in microwaves or for applications requiring a heat resistance of at least 190 °C.
[0015] Description of the invention
[0016] The invention solves the stated problem by first feeding a semi-finished product, in particular a foil or sheet-shaped product with a predetermined width, into a processing section of a production plant in a machine direction parallel to the longitudinal direction of the semi-finished product in a machine direction and subsequently heating it to a drawing temperature of 90-100°C in at least one preheating step in the processing section and actively drawing it exclusively in the machine direction in at least one drawing step, depending on the set drawing temperature and at a drawing degree of 1.2-5.0, while the semi-finished product is fixed in the processing section with respect to its width such that it is passively drawn in a transverse direction perpendicular to the machine direction at a drawing degree of 0.9-1.2.wherein at least one preheating step occurs simultaneously with or preceding at least one stretching step, wherein a relaxation step occurs after at least one stretching step, wherein the semi-finished product undergoes a relaxation of at most 100%, preferably at most 50%, in the machine direction, wherein at least one post-heating step occurs simultaneously with or following the relaxation step, in which the semi-finished product is heated to a temperature in the range of 120–270 °C, wherein the temperature of successive post-heating steps increases stepwise, and wherein a forming step occurs after at least one post-heating step.
[0017] A stretching ratio of 1.2 to 5.0 means that the semi-finished product is stretched to 1.2 to 5 times its original reference length during the processing step. Alternatively, the stretching ratio can also be expressed as a percentage, where, with a reference length defined as 100%, the stretching ratio is accordingly 120 to 500%. For the purposes of the invention, relaxation in the relaxation step is understood to mean the absolute percentage reduction in the stretching ratio of the semi-finished product previously stretched in the machine direction in at least one stretching step. If the initial length of the semi-finished product is, for example, 100 mm and stretching in the machine direction occurs at a degree of stretching of, for example, 3.7 or 370% to a stretched length of 370 mm, then a subsequent relaxation of 34% in the relaxation step means that the degree of stretching resulting from stretching and relaxation increases by 34% to 336% (or 370% to ...3.36) reduced, which in the example given corresponds to a resulting length of the semi-finished product of 336 mm after the relaxation step.
[0018] Preferably, the semi-finished product temperature, i.e., the core temperature of the semi-finished product, does not fall below the glass transition temperature of the polyethylene terephthalate used from the initial preheating until the forming step. The invention is based on the finding that, in the case of amorphous, nucleating agent-free PET, the combination of at least one stretching step in the machine direction at a degree of stretch of 1.2–5.0, preferably 2.7–4.2, more preferably 3.2–3.8, and particularly preferably 3.7, with at least one heating step to a stretching temperature of 80–100°C, preferably 83–95°C, more preferably 90–95°C, and particularly preferably 93°C, enables advantageous crystallization conditions with regard to low shrinkage tendency and sufficient transparency of the manufactured plastic part. The stretching temperature refers in particular to the core temperature of the semi-finished product.Orienting PET within a defined temperature window leads to orientation-induced nucleation and thus to the formation of a finely distributed, homogeneous crystalline structure. Due to the controlled orientation and limited crystal growth dynamics, the resulting crystallites remain far below the wavelength range of visible light in terms of size, so that no significant light scattering occurs. Consequently, the transparency of the PET is maintained, while at the same time the degree of crystallinity is increased, thereby improving the thermal and mechanical stability of the material.
[0019] For stretching degrees above 4.2, it was surprisingly shown that the strain-induced crystallization triggered by the at least one stretching step in the machine direction, in combination with the at least one heating step, results in a particularly fine-grained lamellar crystal structure, which can be fixed to a temperature of at least 30 °C below the glass transition temperature of the PET used by the cooling or quenching immediately following the at least one stretching step in the forming step.
[0020] At lower degrees of stretching, as indicated above, it has been shown that, compared to the less pronounced strain-induced crystallization effects, cold crystallization effects, induced by the various preheating and postheating steps, now play a more significant role. To stabilize these crystal structures formed as a result of cold crystallization and thus achieve high heat resistance, the invention provides that, during the forming step, the semi-finished product, stretched during the machining process, is tempered using a hot forming tool at a temperature range of 100–140 °C, preferably 110–120 °C, and finally demolded. Utilizing the residual heat of the semi-finished product at the beginning of the forming step, the hot forming tool typically has a tool temperature of 70–100 °C, particularly 80–90 °C. The demolding temperature of the plastic part is generally 70–90 °C.In principle, it can also be provided that the hot forming tool is cooled after tempering, whereby the semi-finished product can also be quenched to a temperature of at least 30 °C below the glass transition temperature of the polyethylene terephthalate used.
[0021] To further improve the heat resistance of the manufactured plastic part, the invention provides that, after the relaxation step, the semi-finished product is heated in at least one post-heating step, in particular in at least three, at least four, or at least five post-heating steps, in a stepwise increasing temperature to a range of 120–270 °C, thus promoting the crystallization conditions required for increased heat resistance. The higher the temperature selected for this purpose, the shorter the residence time in the post-heating step. Post-heating in at least one post-heating step also enables better formability of the semi-finished product during the molding step. In this context, it is particularly advantageous if the molding step immediately follows the at least one post-heating step.Preferably, the post-heating process involves three successive reheating steps in which the reheating temperature increases incrementally, with the first reheating temperature in the range of 135–145 °C, the second in the range of 165–185 °C, and the third in the range of 190–200 °C. To achieve an even higher heat deflection temperature of up to 220 °C, accepting a certain degree of cloudiness in the plastic part (which is negligible with regard to customer acceptance), a fourth reheating step at a temperature of 200–235 °C and, optionally, a fifth reheating step at a temperature of 220–255 °C may be included. At even higher reheating temperatures up to 270 °C, the indexed semi-finished product, particularly if it is a film, must be kept under constant tensile stress.
[0022] During the processing step, the semi-finished product is also overstretched in the machine direction. After the molecular orientation phase, the semi-finished product is then slightly relaxed or shrunk in a relaxation step before the final forming step. This further enhances the advantageous crystallization effects because the crystalline chains of the PET material align in even more ordered structures, which are even more beneficial for the product's material properties. Preferably, the overstretching should be such that the relaxation of the semi-finished product in the machine direction before the forming step is at most 100%, preferably at most 50%, more preferably 1 to 45%, more preferably 5 to 45%, and more preferably 5 to 35%. Depending on the application, the relaxation can also be in the range of 5 to 25%, more preferably 15 to 25%.Preferably, the at least one stretching step is immediately followed by the relaxation step, after which the at least one post-heating step and then the forming step immediately follow.
[0023] The relaxation to be selected depends in particular on the drawing intensity(s) and the drawing rate(s) used to draw the sheet or film-shaped semi-finished product in at least one drawing step, and / or on the thickness of the sheet or film-shaped semi-finished product after the at least one drawing step. At lower drawing temperatures in the range of 80 to 90 °C, the relaxation is higher than at higher drawing temperatures above 90 °C. If the thickness of the sheet or film-shaped semi-finished product before drawing is, for example, 1.2 mm, a relaxation of 20–35%, preferably about 25%, can be provided in the relaxation step, especially at a comparatively low drawing temperature in the range of 85–89 °C.For sheet-like semi-finished products, for example 1.5 mm thick, relaxations of 25–40%, preferably about 30%, can be provided before drawing at a drawing temperature in the range of 85–86 °C. If the sheet-like or foil-like semi-finished product is drawn at a higher drawing temperature, the required relaxation is reduced accordingly for the same thickness, in particular to values of 8–20%. For particularly high degrees of orientation, low drawing temperatures, and / or greater semi-finished product thicknesses, relaxations of up to 100%, in particular up to 50%, can also be provided for effective stress reduction.
[0024] The relaxation step can generally be carried out at a temperature of 80 - 200 °C, or according to the preferred temperature ranges as specified above in the post-heating step.
[0025] According to the invention, active stretching occurs exclusively in the machine direction, while the semi-finished product is fixed with respect to its width during the machining process. Because the semi-finished product remains fixed with respect to its width, the normally occurring shrinkage of the semi-finished product due to longitudinal stretching in the machine direction is prevented during active stretching. Thus, in addition to active stretching in the machine direction, a slight passive stretching in the transverse direction is forced, which, together with the active stretching in the machine direction, promotes the formation of fine-grained lamellar crystal structures.
[0026] Preferably, it can be provided that in at least one stretching step, the semi-finished product is fixed at its transversely opposite edges in the processing section. For the purposes of the invention, active stretching means that stretching forces, in particular tensile forces, are introduced into the semi-finished product by the active movement of at least one holding element of its stretching device, such that the semi-finished product is stretched parallel to the direction of movement 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.
[0027] Particularly favorable conditions arise when each index or feed step of the inventive process, i.e., the at least one stretching step, the relaxation step, and the at least one post-heating step, is performed within a time window or index time in the range of 0.5 s to 30 s. Preferably, the respective index steps are carried out within a common cycle with a respective index time of at most 5 s, preferably about 3 s, and particularly preferably less than 3 s. With an index duration of about 3 s, a mold throughput of approximately 20 complete process cycles per minute can be achieved after the first completion of a process cycle, which enables a particularly economically advantageous overall process. The number of post-heating steps is selected, in particular, such that a cumulative post-heating time of at least 15 s is obtained within the selected temperature range.This process selectively reheats or tempers the PET, resulting in heat-induced crystallization and orientation fixation to further increase heat resistance. Assuming an index time of 3 seconds, a cumulative reheating time of, for example, 15 seconds can be achieved through 5 reheating steps.
[0028] The process according to the invention is preferably carried out within a production line that forms a continuous production route, wherein the individual process steps are executed discontinuously. The individual process steps are thus processed batch by batch.
[0029] Plastic films are preferably used as semi-finished products. These can be fed into the production plant as a continuous roll, so that, in principle, no cutting or separating of individual film sections is necessary within the production line. The film is therefore only cut for the first time during the forming step, for example, due to a specific process.
[0030] Die-cutting the finished plastic part from the film. To prevent negative impacts on the thermal and mechanical properties of the finished plastic part, and to avoid undesirable bowing of the semi-finished product or film, which would be particularly pronounced with die-induced stretching solely in the machine direction (MD), it is recommended that the semi-finished product be stress-decoupled at the beginning and / or end of the processing section, especially during at least one stretching step. This effectively prevents the stretching applied to the semi-finished product during the processing step from affecting film sections located before and / or after the processing section.In particular, this largely prevents undesirable internal stresses, inhomogeneities, or geometric deformations of the semi-finished product. For this purpose, for example, appropriate crossbars running transversely to the machine direction can be provided, positioned at the beginning and / or end of the processing section, which lift or guide the film roll while relieving film tension.
[0031] The active stretching of the semi-finished product in the machine direction can, as described above, generally be carried out via a separate stretching device, which introduces the relevant stretching forces, for example, via the end face of the semi-finished product, or, particularly in the case of a continuous film roll, via a transverse front of the semi-finished product bounded by the transverse edges opposite it. It is preferably provided that, in at least one stretching step, the semi-finished product is stress-coupled at a transverse front of the semi-finished product in the processing section to introduce stretching forces. For this purpose, a transverse holding device can be provided, which, for example,The clamping strips run transversely to the machine direction, and thus parallel to a corresponding transverse front of the semi-finished product. These strips are positioned upstream and downstream of the semi-finished product section to be stretched in the machine direction and engage the semi-finished product in such a way that stretching forces, particularly tensile forces, are introduced into the semi-finished product through a corresponding relative movement, causing the semi-finished product to be stretched parallel to the relative direction of movement of the clamping strips. Furthermore, edge-mounted clamping jaws can absorb the initial and tensile forces and move the semi-finished product synchronously with its orientation.
[0032] For example, the clamping clamps can be guided and moved via a known chain- or linear motor-based transport system. This transport system can be designed such that the clamping clamps are positioned against the edges of the semi-finished product at the beginning of the processing section and hold them in place, with the transverse holding device being tension-coupled to the semi-finished product. After stretching, the clamping clamps release the edges of the semi-finished product, or the transverse holding device releases the transverse front of the semi-finished product, at the end of the processing section, and are then removed via the transport system. It goes without saying that, as an alternative to the clamping clamps, other suitable releasable holding elements can also be used, which can be positioned against the edges of the semi-finished product by means of a transport system, hold them in place, and then release them.
[0033] In general, it is recommended that when removing the stretched semi-finished product or semi-finished product section from the processing section, the tension already applied to the semi-finished product section in the machine direction be maintained at least until the semi-finished product or semi-finished product section in question has been transferred to a transport system for removal. This prevents undesired relaxation of the semi-finished product or semi-finished product section.
[0034] semi-finished product section, in the machine and / or transverse direction, during its transfer to the corresponding transport system. The plastic parts produced by the process according to the invention can exhibit a shrinkage of at most 0.5% at a service temperature of 190 °C sustained for at least 90 minutes. Consequently, the measures according to the invention make it possible to dispense with energy-intensive thermal post-treatment after the stretching step to reduce any crystallization-related residual stresses. Depending on the process conditions, plastic parts produced according to the invention can exhibit a haze value according to ASTM D 1003 of at most 5%.If a haze value of at most 20% according to ASTM D 1003 is accepted, a plastic part can be obtained using a process with four to five post-heating steps as described above, which exhibits a shrinkage of at most 0.5% at a service temperature of 220 °C sustained for at least 20 minutes. Overall, the process according to the invention enables the production of plastic parts that are both sufficiently temperature-resistant, and in particular heat-resistant, for microwave use, and that also have an optically appealing appearance due to their sufficient transparency and gloss. It has also been shown that the strain-induced crystallization rate according to the invention proceeds at a sufficiently high rate despite the absence of nucleating agents in the PET, so that the cycle times typical for known thermoforming processes can be essentially maintained.Because no nucleating agents or additives are added to the PET used to improve heating efficiency, the plastic parts obtained by the process according to the invention can be subjected to essentially pure recycling. According to the invention, a purity of at least 97% is considered essentially pure.
[0035] The inventive method can be adapted as needed, depending on the initial semi-finished product thickness, which for example is 1.0–3.5 mm for the production of microwave-safe plastic containers, with regard to the stretching temperature, degree of stretching, stretching rate, cooling rate, residence time in the thermoforming tool, etc., and optionally with regard to the temperature and residence time during the at least one post-heating step and the temperature, residence time, and relaxation during the relaxation step. Likewise, the number and timing of the respective pre-heating and stretching steps can be adapted as needed. For example, the at least one pre-heating step can be carried out simultaneously with the at least one stretching step.However, particularly favorable conditions generally arise if at least one preheating step precedes at least one stretching step in time, i.e., if the preheating step takes place first and then the stretching step.
[0036] To reduce cycle time, the feeding step and at least one stretching step can, in principle, also be carried out simultaneously.
[0037] For favorable crystallization conditions, depending on the initial thickness of the semi-finished product, active stretching of the semi-finished product in the machine direction can be carried out in at least one stretching step at a stretching rate of 50–400% per second, preferably 90–350% per second, and even more preferably 200–350% per second. As a general rule, an increase in the stretching temperature should also be accompanied by an increase in the stretching rate, and vice versa.
[0038] The stretching rate should generally be set as fast as the material allows, depending on its thickness, etc. The faster the molecular orientation of the PET material occurs due to stretching, the higher the heat energy required later to induce shrinkage in the finished product.
[0039] Accordingly, the heat resistance of the final product can be increased by higher stretching rates. To carry out a process according to the invention, generally known plastic thermoforming devices can be used, which, for example, include not only a thermoforming tool but also a corresponding feeding or transport system for the semi-finished product and, optionally, a die-cutting device for cutting out the formed plastic parts. A process according to the invention is used, for example, for the production of thermoformed containers made of germ-forming-free, amorphous PET.
[0040] Such containers exhibit a haze value according to ASTM D 1003 of at most 5% with a shrinkage of at most 0.5% and a service temperature of 190 °C. Accordingly, the containers have an aesthetically pleasing transparency and gloss and are also suitable for microwave use. Because no nucleating agents or additives to improve heating efficiency are added to the PET used, the cups obtained by the inventive process can be recycled in a substantially pure manner at the end of their service life. If, as described above, a final post-heating step is carried out at a temperature of 235–255 °C, containers can be obtained that exhibit a haze value according to ASTM D 1003 of at most 20% with a shrinkage of at most 0.5% and a service temperature of 220 °C.
[0041] The measures according to the invention make it possible to obtain plastic parts that are entirely thermoformed from pure amorphous PET and, as a result of the process according to the invention, are completely migration-free up to service temperatures of up to 220 °C, whereby all migration-relevant organic, inorganic and heavy metal-based tests according to the relevant test guidelines - for example Singapore Food Regulations 37(2) under Sale of Food Act (Chapter 283, Section 56(1)), GN No. S444 / 2012 - yield undetectable values.
[0042] Furthermore, the inventive method can be used to produce plastic parts with a degree of crystallinity of 30% to 40% or more, whereby these plastic parts remain transparent due to the fine-crystalline structure produced by the orientation and thermal treatment. The crystallization can be specifically controlled by selecting the drawing and post-heating temperatures, so that, alternatively, plastic parts with a reduced degree of crystallinity, a correspondingly lower haze value, and lower temperature resistance can also be produced, allowing, for example, the production of completely clear, but less temperature-resistant cups.
[0043] Brief description of the invention
[0044] The invention is illustrated in the drawing as an example. It shows
[0045] Fig. 1 is a schematic top view of an embodiment of a fixing device according to the invention,
[0046] Fig. 2 shows a comparison of a reference container and a test container subjected to a thermal aging test after temperature treatment to 150 °C, wherein both containers were produced by a method according to the invention.
[0047] Fig. 3 shows a comparison corresponding to Fig. 2 after the test container has been heated to 210 °C, and
[0048] Fig. 4 shows a comparison corresponding to Fig. 3 after the test container has been heated to 220 °C.
[0049] Ways to implement the invention
[0050] Fig. 1 shows a schematic representation of a semi-finished product 1 made of PET film. The semi-finished product 1 can generally be supplied as a continuous roll or continuous film strip. The dashed reference lines 2 on the semi-finished product 1, running in a transverse direction TD with respect to the semi-finished product width, illustrate that the semi-finished product 1 is stretched in a machine direction MD parallel to the longitudinal direction of the semi-finished product in a processing section 3 of a hot forming device (not shown in detail). The processing gate 3 is indicated by two dashed lines, also running in the transverse direction TD. In the present embodiment, the semi-finished product 1 is stretched in the processing section 3 with respect to a reference length 4 defined between two reference lines 2 at a degree of stretching of 3.7% or 370% in the machine direction MD, as can be seen from the larger distance between the reference lines 2 in the processing section 3.
[0051] The semi-finished product 1 is held in machining section 3 by means of schematically indicated clamping jaws 5 at its edges, thus fixing its width. Tensioning forces for active stretching in the machine direction MD are partially introduced into the semi-finished product 1 via the clamping jaws 5, which can be moved in machining section 3, for example by means of a chain drive.While the clamping jaws 5 primarily serve a position-locking function to prevent inward shrinkage, the majority of the stretching forces acting in the machine direction (MD) are introduced into the semi-finished product 1 via a corresponding transverse holding device by means of clamping strips 7 connected to the semi-finished product 1. This transverse front of the semi-finished product is assigned to the respective clamping strips 7, thus enabling uniform force introduction and stress distribution as well as a reduction of local stress peaks in the semi-finished product 1 during MD stretching. Active stretching exclusively in the machine direction (MD) would normally cause inward shrinkage of the PET film, i.e., a decrease in the width of the semi-finished product 1, as indicated by the dashed line marking the waist of the semi-finished product 1 in processing section 3.Because the semi-finished product 1 remains fixed with respect to its width, such a jump in is prevented. Consequently, in addition to the active stretching in the machine direction MD, a slight passive stretching in the transverse direction TD is forced. The clamping clips are positioned against the edges of the semi-finished product at the beginning of processing section 3, so that these are held by the clamping clips 5. After stretching, the clamping clips 5 release the edges of the semi-finished product, and the transverse holding device releases the two transverse fronts of the semi-finished product assigned to the clamping strips 7 at the end of processing section 3, so that a new indexing step of the semi-finished product 1 can take place to initiate the subsequent stretching step. In principle, the number of clamping clips 5 or releasable holding elements used is freely selectable depending on the desired process conditions and product properties.Depending on the application, it may be advantageous for the process conditions, for example, to provide as many clamping clips 5 or detachable holding elements as possible in processing section 3, so that the set overall degree of stretching results from several smaller partial stretches in the machine direction MD. As schematically indicated, the clamping clips 5 are connected to a drive unit 6 based on a pantograph. Just as the lateral clamping clips 5 can be freely positioned to define the starting position for a stretching step, the width of the transverse holding device or clamping strips 7 running parallel to the transverse direction TD can also be adjustable, particularly analogous to transverse holding devices used in conventional thermoforming machines, so that different semi-finished product widths and stretching configurations can be realized.
[0052] Simultaneously with or preceding the stretching step, a preheating step takes place, whereby the semi-finished product 1 is heated to a stretching temperature of approximately...
[0053] The semi-finished product 1 is heated to 93 °C. Subsequently, it undergoes a relaxation step of 34%, after which the post-heating temperature is increased in three successive post-heating steps. The first post-heating temperature is 145 °C, the second 185 °C, and the third 200 °C. Immediately after the last post-heating step, the semi-finished product 1 is formed using a cooled hot forming tool (not shown in detail). During this forming step, the product is tempered to a temperature of 110–120 °C to stabilize the crystal structures resulting from cold crystallization effects. For this purpose, the hot forming tool is tempered to a tool temperature of 85–90 °C.
[0054] Alternatively, it may also be provided that the third post-heating step is followed by a fourth post-heating step at a post-heating temperature in the range of 200-235 °C and, if necessary, a fifth post-heating step at a post-heating temperature in the range of 220-255 °C.
[0055] Figures 2 to 4 show comparisons of a reference container with a test container subjected to a thermal aging test at different test temperatures.
[0056] The underlying thermal aging test was performed in a multi-stage process. First, the test container underwent a visual inspection upon delivery to ensure the material was free of pre-existing damage, particularly cracks or mechanical deformations. The actual thermal cycle began with the storage of the test container in a conditioned oven at a starting temperature of 100 °C. The test container remained in the oven until thermal equilibrium was reached and was then stabilized at this temperature for one hour. After this holding period, the test container was removed from the oven and cooled to room temperature under controlled conditions. A final visual inspection was then performed to check for any signs of melting or morphological changes.Provided no thermally induced defects or melting events were detected, the procedure was continued iteratively. The test temperature was increased in increments of 10 °C from the previous level. This cycle of heating, one-hour holding time, cooling, and inspection was repeated until the target temperature of 220 °C was reached.
[0057] As can be seen from the comparison in Fig. 1 after tempering to 150 °C, the test container shows no clouding and no significant deformation, indicating sufficient heat resistance for microwave applications. Even after tempering to 210 °C and 220 °C as shown in Figs. 3 and 4, the test container showed no melting and remained largely structurally intact. Following the thermal aging test, a migration test was carried out in accordance with Singapore Food Regulations 37(2) under Sale of Food Act (Chapter 283, Section 56(1)), GN No. S444 / 2012, and none of the tested migrating substances (lead, arsenic, antimony, cadmium) were detected.
[0058] A combined thermal aging test, consisting of cryogenic and hot storage, was also performed. The first phase of the test involved cryogenic storage. For this, the test container was conditioned in a cold chamber at a constant temperature of -40 °C until thermal equilibrium was reached. Once this state was achieved, the temperature was stabilized for 24 hours. Immediately afterward, the test container was subjected to hot storage in an oven. Here, the test container was exposed to a temperature of 120 °C. After reaching thermal equilibrium, this temperature level was maintained for one hour. Following the completion of the hot storage phase, the test container was removed from the oven and cooled to room temperature under controlled conditions.The procedure concluded with a final visual inspection, during which the material was specifically examined for signs of melting, deformation, or other structural changes. Apart from a certain degree of cloudiness, no significant changes were detected. The subsequent migration test (as described above) also failed to detect any of the tested migrating substances.
[0059] To determine the seal integrity, comparative tests were also conducted between conventional CPET containers and the test containers obtained using the inventive method. A dedicated high-temperature sealing film, specified for PET sealing surfaces, was used. The samples were sealed at a sealing temperature of 197 °C and a contact time of 0.7 s, followed by a controlled cooling phase at 125 °C in accordance with established CPET test protocols. After sealing, the samples remained at room temperature for a defined period of 40 hours to ensure complete stabilization of the seal and the completion of any relaxation processes before mechanical testing. The seal strength was determined on a dedicated peel testing machine using precisely cut sealing strips with a sample width of 1 inch or 25.4 mm.To ensure exact comparability, identical, surface-optimized strips were peeled from both the CPET containers and the test containers obtained according to the invention. The peel test was performed at room temperature with a constant peel speed of 7.5 inches / min or 190.5 mm / min. The resulting measurement data confirmed that, under these identical conditions, peel forces were measured on the test containers obtained according to the invention that were approximately ten times higher than those measured on the CPET containers. The results indicated a significantly wider and more efficient sealing window for the sealing of containers obtained according to the invention, which enables an acceleration of sealing cycles in industrial applications.
[0060] Furthermore, the comparatively high sealing integrity creates the prerequisite that corresponding food-filled and sealed containers can be subjected to an internal gas pressure of up to 10 bar, which can subsequently lead to an increase in the shelf life of the packaged food.
[0061] The crystalline and oriented structure achieved by the inventive method can be clearly and reproducibly detected using standard laboratory analytical methods. Differential scanning calorimetry (DSC) is used to determine the degree of crystallinity and the relaxation state.
[0062] The molecular morphology, in particular the crystal structure, crystallite size, and orientation distribution in the machine direction (MD) and transverse direction (TD), can be quantified using X-ray diffraction techniques such as wide-angle (WAXS) and small-angle X-ray scattering (SAXS). Additionally, the molecular orientation can be determined by Fourier-transform infrared spectroscopy (FTIR) or birefringence measurements. The combination of crystallinity, crystallite size, orientation degree, and relaxation state resulting from these methods represents a process-specific structural characteristic. This enables the technical identification and traceability of any plastic containers produced according to the inventive process on the market. These structural features are detectable regardless of any coloring or pigmentation of the semi-finished products, since the analysis of the crystalline phase and the molecular orientation is selective and color-independent.
[0063] The PET plastic containers produced according to the invention have a defined morphology as a result of monoaxial or biaxial orientation followed by thermal fixation, which is characterized by the following parameters:
[0064] The degree of crystallinity X determined by DSC c is according to the formula
[0065] 100
[0066]
[0067] calculated, whereby AH m ...on the enthalpy of fusion, AH CC refers to the enthalpy of cold crystallization and AH^ to the theoretical enthalpy of fusion or formation for 100% crystalline PET. At AH^ = 140 J / g, the material exhibits an X c -value of preferably 30-40%. The net enthalpy of fusion (H m - H CC ) is particularly in the range of 42-56 J / g.
[0068] In a WAXS measurement (CuKa), PET-typical crystalline reflections are detectable at 20° angles of approximately 16.0°, 17.5°, 22.5°, and 25.5°. The azimuthal intensity distribution of these reflections represents the specific mono- or biaxial orientation state.
[0069] The orientation state can be quantified via the refractive indices. In biaxially oriented versions, the refractive indices lie in the plane (n). x and n y ) preferably in the range of 1.65, while the refractive index in the thickness direction (n z ) is in the range of 1.50.
Claims
Patent claims 1. A method for producing hot-formed and essentially single-material recyclable plastic parts made of germ-forming-free, amorphous polyethylene terephthalate, characterized in that, in a feeding step, a semi-finished product (1), in particular film- or sheet-shaped, with a predetermined semi-finished product width, is first fed to a processing section (3) of a production plant in a machine direction (MD) running parallel to the longitudinal direction of the semi-finished product, and in the processing section (3) is subsequently heated in at least one preheating step to a stretching temperature of 80-100°C and in at least one stretching step is actively stretched exclusively in the machine direction (MD) at a degree of stretching of 1.2-5.0, depending on the set stretching temperature, while the semi-finished product (1) is fixed in the processing section (3) with respect to its width.that the semi-finished product (1) is passively stretched in a transverse direction (TD) perpendicular to the machine direction (MD) at a degree of stretching of 0.9 - 1.2, wherein the at least one preheating step occurs simultaneously with or preceding the at least one stretching step, that a relaxation step is performed after the at least one stretching step, wherein the semi-finished product (1) undergoes a relaxation of at most 100%, preferably at most 50%, in the machine direction (MD), that at least one post-heating step is performed simultaneously with or following the relaxation step, in which the semi-finished product (1) is heated to a temperature in the range of 120-270 °C, wherein the temperature of successive post-heating steps increases stepwise, and that a forming step is performed after the at least one post-heating step.wherein the semi-finished product (1) stretched in the processing section (3) is formed using a hot forming tool and tempered in a temperature range of 100 - 140 °C.
2. Method according to claim 1, characterized in that in the forming step the hot forming tool is cooled after tempering z such that the semi-finished product (1) is quenched to a temperature of at least 30 °C below the glass transition temperature of the polyethylene terephthalate used.
3. Method according to claim 1 or 2, characterized in that at the beginning and / or at the end of the processing section (3) the semi-finished product (1) is stress-decoupled, in particular during the at least one stretching step.
4. Method according to one of claims 1 to 3, characterized in that the method is carried out within a continuous production line, wherein the processing of the individual method steps is discontinuous.
5. Method according to one of claims 1 to 4, characterized in that the at least one stretching step, the relaxation step and the at least one post-heating step each take place in a time window in the range of 0.5s to 30s.
6. A method according to any one of claims 1 to 5, characterized in that only one drawing step is carried out at a drawing degree of 2.7 to 4.2, preferably 3.2 to 3.8, in particular 3.7, and a drawing temperature in the range of 80 to 100°C, preferably 83 to 95°C, more preferably 90 to 95°C, after which the semi-finished product (1) undergoes a relaxation step of 5 to 45%, preferably 5 to 35%, more preferably 25 to 35%, more preferably 35%, after which the post-heating temperature is increased stepwise in three successive post-heating steps, wherein the first post-heating temperature is in the range of 135 to 145°C, preferably at 145°C, the second post-heating temperature is in the range of 165 to 185°C, preferably at 185°C, and the third post-heating temperature is in the range of 190-200 °C, preferably at 200 °C, and the forming step follows immediately. 7.Method according to claim 6, characterized in that the post-heating temperature increases stepwise in four successive post-heating steps, wherein the fourth post-heating temperature is in the range of 200 - 235 °C.
8. Method according to claim 7, characterized in that the post-heating temperature increases stepwise in five successive post-heating steps, wherein the fifth post-heating temperature is in the range of 220 - 255 °C.
9. Method according to one of claims 1 to 8, characterized in that in the stretching step the semi-finished product (1) is fixed in the processing section (3) at the semi-finished product edges opposite each other in a transverse direction (TD) running perpendicular to the machine direction (MD).
10. Method according to one of claims 1 to 9, characterized in that the semi-finished product temperature from the initial preheating until the forming step does not fall below the glass transition temperature of the polyethylene terephthalate used.
11. Method according to one of claims 1 to 10, characterized in that in the stretching step the active stretching of the semi-finished product (1) in the machine direction (MD) takes place at a stretching rate of 50-400% per second, preferably 90-350% per second, more preferably 200-350% per second.
12. Plastic part, in particular a plastic container or plastic vessel, obtainable by a method according to any one of claims 1 to 11, wherein the shrinkage of the plastic part at a service temperature of 190 °C for 90 minutes is at most 0.5%.