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 EP2026052891_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 decreasing 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] 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 80-100°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 degree of drawing of 1.2-12.0, 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 degree of drawing of 0.9-1.2.wherein at least one preheating step occurs simultaneously with or preceding at least one stretching step, that 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, that 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 that a forming step occurs after at least one post-heating step.wherein the semi-finished product, stretched during the processing section, is formed using a cooled hot forming tool and quenched to a temperature at least 30 °C below the glass transition temperature of the polyethylene terephthalate used. Preferably, for this purpose, the hot forming tool has a tool temperature of 15–35 °C, particularly 15–20 °C. The degree of stretching of 1.2–12.0 means that the semi-finished product is stretched to 1.2–12 times its original reference length during the processing section. Alternatively, the degree of stretching can also be expressed as a percentage.where, with a reference length defined as 100%, the degree of stretching is accordingly 120–1200%. For the purposes of the invention, relaxation in the relaxation step is understood to mean the absolute percentage reduction in the degree of stretching of the semi-finished product previously stretched in the machine direction in at least one stretching step. If, for example, the initial length of the semi-finished product is 100 mm and stretching in the machine direction occurs at a degree of stretching of, for example, 4.55% or 455% to a stretched length of 455 mm, then a subsequent relaxation of 45% in the relaxation step means that the degree of stretching resulting from stretching and relaxation decreases by 45% to 410% (or 4.1%), which in the aforementioned example corresponds to a resulting length of the semi-finished product of 410 mm after the relaxation step. Preferably, the temperature of the semi-finished product, i.e., the core temperature of the semi-finished product, decreases.From the initial preheating until the forming step, the temperature must not fall below the glass transition temperature of the polyethylene terephthalate used.
[0016] 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 stretch ratio of 1.2–12.0, preferably 3.0–5.0, more preferably 4.0–4.5, and particularly preferably 4.55, with at least one heating step to a stretching temperature in the range of 80–100°C, preferably 83–95°C, and more preferably 90–95°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. The orientation of PET within a defined temperature window leads to orientation-induced nucleation and thus to the formation of a finely dispersed, 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. As a result, the transparency of the PET is maintained, while at the same time the degree of crystallinity is increased, thus improving the thermal and mechanical stability of the material.
[0017] Surprisingly, it has been 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.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 temperature is increased in three successive steps, with the first post-heating temperature in the range of 135–145 °C, the second in the range of 165–185 °C, and the third in the range of 195–220 °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 post-heating step at a temperature of 220–235 °C and optionally a fifth at a temperature of 220–255 °C may also be performed. At even higher post-heating temperatures up to 270 °C, the indexed semi-finished product, especially if it is a film, must be kept under constant tensile stress.
[0018] 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%, even more preferably 1 to 45%, even more preferably 5 to 45%, even more preferably 5 to 30%, even more preferably 15 to 25%, and even more preferably 5 to 25%.
[0019] Preferably, the at least one stretching step is immediately followed by the relaxing step, after which the at least one post-heating step and then the forming step immediately follow.
[0020] The relaxation to be selected depends in particular on the drawing step(s), the drawing rate(s), and especially the drawing temperatures used in at least one drawing step to draw the sheet or film-shaped semi-finished product, and / or on the thickness of the sheet or film-shaped semi-finished product after the at least one drawing step. The relaxation value is higher at lower drawing temperatures in the range of 80 to 90 °C 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 relatively low drawing temperature in the range of 85–90 °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.
[0021] 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. Description of the invention
[0022] 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.
[0023] 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 a stretching device that can be attached to the semi-finished product, 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.
[0024] 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 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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 a semi-finished product section to be concealed in the machine direction and engage the semi-finished product in such a way that, through a corresponding relative movement, stretching forces, in particular tensile forces, are introduced into the semi-finished product, causing it to stretch parallel to the relative direction of movement of the clamping strips. Furthermore, edge-mounted clamping clips can absorb the initial and tensile forces and move the semi-finished product synchronously with its orientation.
[0029] 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.
[0030] 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.
[0031] semi-finished product section, in machine and / or transverse direction, upon its transfer to the corresponding transport system.
[0032] 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 for at least 90 minutes. Consequently, the measures according to the invention eliminate the need for energy-intensive thermal post-treatment after the stretching step to reduce any residual stresses caused by crystallization. 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 according to ASTM D 1003 of at most 20% is accepted, a plastic part can be obtained, according to 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 for at least 20 minutes.Overall, the inventive process enables the production of plastic parts that are sufficiently temperature-resistant, particularly heat-resistant, for microwave use and also exhibit an aesthetically pleasing appearance due to their sufficient transparency and gloss. Furthermore, it has been shown that the strain-induced crystallization rate according to the invention proceeds at a sufficient speed 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 inventive process can be subjected to essentially pure recycling. According to the invention, a purity of at least 97% is considered essentially pure.
[0033] 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., as well as, if necessary, 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.
[0034] To reduce cycle time, the feeding step and at least one stretching step can, in principle, also be carried out simultaneously.
[0035] Under 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.
[0036] 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 subsequent heat energy requirement to induce shrinkage in the finished product. Accordingly, higher stretching rates can increase the heat resistance of the final product. To carry out a process according to the invention, known plastic thermoforming devices can be used, which, for example, include a thermoforming tool, a suitable feeding or transport system for the semi-finished product, and optionally a die-cutting device for cutting out the formed plastic parts.
[0037] A method according to the invention is used, for example, for the production of hot-formed containers made of germ-forming-free, amorphous PET.
[0038] 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.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.
[0039] 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.
[0040] Brief description of the invention
[0041] The invention is illustrated in the drawing, for example, in a schematic top view of a semi-finished product stretched in a processing section of a hot forming device.
[0042] Ways to implement the invention
[0043] The drawing shows a schematic representation of a semi-finished product 1, designed as PET film. The semi-finished product 1 can basically be supplied as an endless roll or...
[0044] Continuous foil tape is fed in. 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, are intended to illustrate that the semi-finished product 1 is stretched in a machining section 3 of a hot forming device (not shown in detail) in a machine direction MD parallel to the longitudinal direction of the semi-finished product. The machining 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 machining section 3 with respect to a reference length 4 defined between two reference lines 2 at a degree of stretching of 4.5% or 455% in the machine direction MD, as can be seen from the larger distance between the reference lines 2 in the machining section 3.
[0045] 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 ensures uniform force application 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 jaws are positioned against the semi-finished product edges at the beginning of machining section 3, so that these are held by the clamping jaws 5. After stretching, the clamping jaws 5 release the semi-finished product edges, 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 machining section 3, so that a new indexing step of the semi-finished product 1 can take place to initiate the subsequent stretching step.
[0046] In principle, the number of clamping jaws 5 or detachable holding elements used is freely selectable depending on the desired process conditions and product properties. For example, depending on the application, it may be advantageous for the process conditions to have as many clamping jaws 5 or detachable holding elements as possible in processing section 3, so that the set total degree of stretching results from several smaller partial stretchings in the machine direction MD. As schematically indicated, the clamping jaws 5 are connected to a drive unit 6 based on a pantograph. Just as the lateral clamping jaws 5 can be freely positioned to define the starting position for a stretching step, the width of the transverse holding device running parallel to the transverse direction TD can also be adjusted.The clamping strips 7 should be adjustable, in particular analogous to the transverse holding devices used in conventional thermoforming machines, so that different semi-finished product widths and stretching configurations can be realized.
[0047] 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...
[0048] The semi-finished product 1 is heated to 90 °C. Subsequently, it undergoes a relaxation step of 45%, 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 220 °C. Immediately after the last post-heating step, the semi-finished product 1 is formed using a cooled hot-forming tool (not shown) and quenched to a temperature at least 30 °C below the glass transition temperature of the PET used. For this purpose, the water-cooled hot-forming tool is heated to a temperature of 15–35 °C.
[0049] 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.
[0050] 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.
[0051] 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 using Fourier-transform infrared spectroscopy (FTIR) or birefringence measurements.
[0052] The combination of crystallinity, crystallite size, degree of orientation, and relaxation state resulting from these processes 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. As a result of monoaxial or biaxial orientation followed by thermal fixation, the PET plastic containers produced according to the invention exhibit a defined morphology characterized by the following parameters:
[0053] The degree of crystallinity X determined by DSC c is according to the formula
[0054] 100
[0055]
[0056] Calculated, where 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.
[0057] 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.
[0058] 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) depending on the set stretching temperature at a degree of stretching of 1.2-12.0, while the semi-finished product (1) is fixed in the processing section (3) with respect to its semi-finished product 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 cooled hot forming tool and is thereby quenched to a temperature of at least 30 °C below the glass transition temperature of the polyethylene terephthalate used.
2. Method according to claim 1, characterized in that at the beginning and / or at the end of the processing section (3) the semi-finished product (1), in particular during the at least one stretching step, is stress-decoupled.
3. Method according to claim 1 or 2, characterized in that the method is carried out within a continuous production line, wherein the processing of the individual method steps is discontinuous.
4. Method according to one of claims 1 to 3, 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.
5. A method according to any one of claims 1 to 4, characterized in that only one drawing step is carried out at a drawing degree of 3.0 - 5.0, preferably 4.0 to 5.0, more preferably 4.0 - 4.5, in particular 4.55, and a drawing temperature in the range of 80 - 100°C, preferably 83 - 95°C, preferably 90-95°C, after which the semi-finished product (1) undergoes a relaxation of 5 - 45%, preferably 45%, in the relaxation step, after which the post-heating temperature increases stepwise in three successive post-heating steps, wherein the first post-heating temperature is in the range of 135 - 145°C, preferably at 145°C, the second post-heating temperature is in the range of 165 - 185°C, preferably at 185°C, and the third post-heating temperature is in the range of 195 - 220 °C, preferably at 220 °C, and the forming step takes place immediately thereafter.
6. The method of claim 5, 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, preferably 220–235 °C.
7. The method of claim 6, characterized in that the post-heating temperature increases stepwise in five successive post-heating steps, wherein the fifth post-heating temperature is 220–255 °C.
8. Method according to one of claims 1 to 7, 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).
9. Method according to one of claims 1 to 7, 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.
10. Method according to one of claims 1 to 9, 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.
11. Plastic part, in particular a plastic container or plastic vessel, obtainable by a method according to any one of claims 1 to 10, wherein the shrinkage of the plastic part at a service temperature of 190 °C for 90 minutes is at most 0.5%.