Method for producing thermoformed plastic parts, which are substantially recyclable by material type, from polyolefins and / or polystyrene
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 EP2026052890_13082026_PF_FP_ABST
Abstract
Description
[0001] Procedures for
[0002]
[0003] and essentially single-variety
[0004]
[0005] and / or
[0006]
[0007] Technical field
[0008] The invention relates to a method for producing hot-formed and essentially single-material recyclable plastic parts from polyolefins and / or polystyrene.
[0009] State of the art
[0010] 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 120 °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-stream 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.
[0011] 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 microwave use 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.
[0012] 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 input is required. WO 2023178369 A1 discloses processes for the production of hot-formed PET plastic parts that exhibit a heat deflection temperature of up to 145 °C.
[0013] Furthermore, processes are known from EP4209331A1 in which biaxially oriented polyethylene films (BOPE) are thermoformed.
[0014] Description of the invention
[0015] The invention is therefore based on the objective of creating a process of the type described above which, despite economical cycle times and relatively low energy consumption, enables the production of hot-formed, sorted recyclable plastic parts made of polyolefins and / or polystyrene, which, with sufficient, visually appealing gloss and only slight turbidity, are also suitable for use in microwaves or for applications requiring a heat resistance of at least 150 °C.
[0016] The invention solves the stated problem by first feeding a semi-finished product, in particular a film 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 in at least one preheating step to a drawing temperature of 70–180 °C, in particular 80–150 °C, and then actively drawing it in at least one drawing step, depending on the set drawing temperature, at a drawing degree of 1.2–12.0 exclusively in the machine direction, 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 drawing step, that a relaxation step occurs after at least one drawing step, wherein the semi-finished product undergoes a relaxation of at most 100%, preferably at most 50%, and in particular at most 20% 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 100–200 °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, drawn in the processing section, is formed using a cooled hot forming tool and thereby quenched to a temperature of at most 40 °C. When hot forming oriented polypropylene,For polyethylene and polystyrene semi-finished products, the quenching temperature, depending on the material and process, typically ranges from approximately 5 to 80 °C, particularly from 5 to 40 °C. Lower temperatures generally result in a particularly advantageous fixation of the orientation and thus increased heat resistance, while higher temperatures allow for increasing relaxation of the orientation and associated, potentially desired, dimensional and property changes. For example, the hot forming tool has a tool temperature of 15 to 40 °C, particularly 15 to 20 °C. A degree of stretching of 1.2 to 12.0 means that the semi-finished product is stretched to 1.2 to 12 times its original reference length during the machining process. The degree of stretching can alternatively 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 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 decreases by 34% to 336% (or 3.36%), which in the aforementioned example corresponds to a resulting length of the semi-finished product of 336 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 does not fall below the glass transition temperature of the plastic used. A complete process, starting from the initial heating of a semi-finished product, for example, a section of a plastic film, through at least one stretching step and at least one post-heating step, up to the forming step in which the semi-finished product is formed and quenched while retaining the hot-formed plastic part, is possible within a time window of 60 seconds using the measures according to the invention.
[0017] The invention is based on the finding that, in the case of polyolefins such as polypropylene or polyethylene (for example, HDPE, MDPE, or LDPE), as well as in the case of polystyrene, the combination of at least one stretching step in the machine direction at a degree of stretch of 1.2–12.0, preferably 3.0–5.0, more preferably 3.0–4.5, and particularly preferably 3.2, with at least one heating step to a stretching temperature of 80–170°C, in particular 80–150°C, and even more preferably 80–120°C, enables advantageous conditions with regard to low shrinkage tendency and negligible opacity of the manufactured plastic part. The stretching temperature refers in particular to the core temperature of the semi-finished product.
[0018] Surprisingly, it has been shown that in polypropylene and polyethylene, 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. This structure can be fixed by the cooling or quenching in the forming step immediately following the at least one stretching step, to a temperature of at most 80 °C, preferably at most 40 °C. 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, preferably in a single post-heating step, at a post-heating temperature in the range of 100–200 °C, thus promoting the crystallization conditions required for increased heat resistance.The higher the temperature chosen for this purpose, the shorter the residence time in the post-heating step. Post-heating in at least one step also improves the formability of the semi-finished product during the forming step. It is particularly advantageous in this context if the forming step immediately follows the at least one post-heating step.
[0019] In the case of polypropylene, the molecular orientation achieved according to the invention increases toughness and resistance to brittle fracture at low temperatures by improving load transfer along the aligned polymer chains. As a result, plastic parts based on polypropylene oriented according to the invention can withstand freezing and icing conditions significantly better than plastic parts based on non-oriented polypropylene.
[0020] In polyethylene, the orientation achieved according to the invention noticeably increases tensile strength and heat resistance. In certain highly oriented polyethylene grades (especially oriented HDPE and UHMW-PE structures), the thermal stability under load can extend to the range of approximately 140-150 °C due to the restricted chain mobility.
[0021] 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 in the case of polypropylene and polyethylene, because this measure aligns the crystalline chains of the material into even more ordered structures, which are therefore even more advantageous for the material properties of the product. Preferably, the overstretching should be carried out such that the relaxation of the semi-finished product in the machine direction before the forming step is at most 50%, more preferably 1 to 30%, more preferably 2 to 20%, more preferably 5 to 18%, more preferably 7 to 13%, more preferably 9 to 12%, and particularly preferably 10%.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.
[0022] 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 up to 90 °C than at higher drawing temperatures above 90 °C. If the thickness of the sheet or film-shaped semi-finished product before embedding is, for example, 1.2 mm, a relaxation of preferably 20% occurs in the relaxation step at a comparatively low drawing temperature. For a thickness of the sheet or film-shaped semi-finished product...For sheet-like semi-finished products of, for example, 1.5 mm thickness, a relaxation of preferably 24% occurs during the relaxation step before drawing, also at a comparatively low drawing temperature. If, however, the sheet-like or film-like semi-finished product is drawn at a higher drawing temperature, the required relaxation is reduced accordingly, particularly to values of 10% to 15%, for the same thickness. In the case of polyethylene, drawing at temperatures near the melting range leads to a lower stored internal stress, so that relaxations of approximately 15–20% can be achieved at a thickness of 1.2 mm, while at lower drawing temperatures, relaxations of up to 30% are possible.For polystyrene drawn above its glass transition temperature, particularly high stored orientations occur at low drawing temperatures, so that relaxations of preferably 25–35% can occur at a thickness of 1.2 mm, while at higher drawing temperatures relaxations of 12–18% are sufficient. For polypropylene, due to its semi-crystalline structure, relaxations of 20–30% are common at low drawing temperatures, while at higher drawing temperatures relaxations of 10–15% are sufficient, in each case based on a comparable semi-finished product thickness.
[0023] 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.
[0024] According to the invention, active stretching occurs exclusively in the machine direction, while the semi-finished product is fixed with respect to its width in the processing section. 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 the fine-grained lamellar crystal structures in polypropylene and polyethylene. 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 is understood to mean that, through the active movement of at least one holding element of a stretching device that can be attached to the semi-finished product, stretching forces, in particular tensile forces, are introduced into 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. Particularly favorable conditions arise when each index or feed step of the method according to the invention, i.e., the at least one stretching step, the relaxation step, and the at least one post-heating step, each takes place within a time window in the range of 0.5 s to 5 s.Preferably, the post-heating steps in the discontinuous process are carried out material-specifically at a post-heating temperature in the range of 100–200 °C, and the number of post-heating steps is selected such that a cumulative post-heating time results, which is in the range of 20 s to 60 s for polypropylene at a post-heating temperature of 160 °C to 190 °C, particularly up to 170 °C; in the range of 15 s to 45 s for polyethylene at a post-heating temperature of 125 °C to 160 °C, particularly up to 135 °C; and in the range of 25 s to 90 s for polystyrene at a post-heating temperature of 120 °C to 145 °C, particularly up to 130 °C. This achieves an effective reduction of residual stresses caused by orientation and, if applicable, crystallization, which ultimately leads to improved heat deflection temperature.
[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, be carried out using 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 tension-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] Additionally, it can be provided that the edges of the semi-finished product are fixed and guided by clamping jaws in the machining section, whereby tensioning forces for active stretching in the machine direction are also introduced into the semi-finished product via the clamping jaws. As a result of these measures, the semi-finished product is fixed with respect to its width by clamping jaws assigned to the edges opposite each other in the transverse direction. In contrast, the majority of the tensioning forces required for active stretching in the machine direction are introduced into the semi-finished product via the transverse holding device, which is tension-coupled to the semi-finished product, so that the clamping jaws essentially only perform a position-locking function.
[0030] 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 or a separate 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.
[0031] In general, it is recommended that when removing a 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, in the machine and / or transverse direction, during its transfer to the corresponding transport system.
[0032] The measures according to the invention make it possible to reduce or completely eliminate the otherwise necessary, energy-intensive thermal post-treatment after the stretching step for reducing residual stresses caused by crystallization and / or orientation. 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 20%. If a haze value according to ASTM D 1003 of at most 50% is accepted, a plastic part can be obtained, according to a process with four post-heating steps as described above, which exhibits a shrinkage of at most 5% at a service temperature of 150 °C for at least 90 minutes.Overall, the inventive method 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 gloss and negligible opacity. Even in applications involving frozen products, it has been shown that the inventive plastic parts retain sufficient heat resistance upon subsequent heating following microwave exposure. In principle, the inventive plastic parts can also be colored or mixed with color pigments. Since no nucleating agents or additives are preferably added to the plastic used to improve heating efficiency, the plastic parts obtained by the inventive method can be recycled in a substantially single-material manner.According to the invention, a varietal purity of at least 95% is considered to be essentially pure.
[0033] The inventive method can be adapted as needed, depending on the initial semi-finished product thickness, which for example is 0.3–3.5 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.
[0034] To reduce cycle time, the feeding step and at least one stretching step can, in principle, also be carried out simultaneously.
[0035] For favorable crystallization conditions in the case of polypropylene and polyethylene, depending on the initial semi-finished product thickness, 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 50–500% per second, preferably 90–350% per second, and even more preferably 200–350% per second. In principle, increasing the stretching temperature also allows for an increase in the stretching rate.
[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 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 process according to the invention is used, for example, for the production of thermoformed containers made of a polyolefin and / or polystyrene. Because no nucleating agents or additives are added to the plastic used to improve heating efficiency or for optical enhancement with regard to increased transparency, the cups obtained by the process according to the invention 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 up to 200 °C, containers can be obtained that exhibit a haze value according to ASTM D 1003 of at most 50% and a shrinkage of at most 5% at a service temperature of 150 °C for at least 90 minutes. Accordingly, the containers have an optically appealing gloss and negligible opacity and are suitable for use in microwave ovens.
[0038] Brief description of the invention
[0039] 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.
[0040] Ways to implement the invention
[0041] The drawing shows a schematic representation of a semi-finished product 1 made of polypropylene film (PP 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 width of the semi-finished product, illustrate that the semi-finished product 1 is stretched in a processing 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 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 processing section 3 with respect to a reference length 4 defined between two reference lines 2 at a degree of stretching of 12 or 12.Stretched by 1200% in the machine direction MD, as can be seen from the larger distance between the reference lines 2 in machining section 3.
[0042] 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 clamps 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 tension-coupled 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 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 PP 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 width 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.
[0043] 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 stretching operations 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.
[0044] Simultaneously with or preceding the drawing step, a preheating step takes place, in which the semi-finished product 1 is heated to a drawing temperature of 120 °C. Subsequently, in a relaxation step, the semi-finished product 1 undergoes a relaxation of 20%, after which, in a post-heating step, it is heated to a post-heating temperature of 160 °C. Immediately after the post-heating step, in a forming step, the semi-finished product 1 is formed using a cooled hot forming tool (not shown in detail) and quenched to a temperature of approximately 10–80 °C. For this purpose, the water-cooled hot forming tool is maintained at a tool temperature of 15–40 °C.
[0045] In the case of a semi-finished product 1 based on a polyethylene film, the stretching temperature in the preheating step is preferably 80 °C.
[0046] The semi-finished products or plastic parts made of polypropylene and polyethylene produced according to the invention exhibit a defined, increased crystallinity as well as a pronounced molecular orientation as a result of stretching and subsequent thermal treatment, whereas in polystyrene-based semi-finished products a defined molecular orientation is achieved without the formation of a crystalline structure.
[0047] 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.
[0048] 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.
[0049] The aforementioned analytical methods can be used independently of any coloring of the semi-finished product or the plastic part, since the determination of the crystalline structure and / or the molecular orientation is essentially color-independent, so that the structural features set according to the invention can be clearly identified and reproducibly verified even in colored or pigmented materials.
Claims
Patent claims 1. A method for producing hot-formed and essentially single-material recyclable plastic parts made of polyolefins and / or polystyrene, characterized in that, in a feeding step, a semi-finished product (1), in particular in the form of a film or sheet, 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 drawing temperature of 70–180°C, and in at least one drawing step, depending on the set drawing temperature, is actively drawn exclusively in the machine direction (MD) at a drawing degree 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 100-200 °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 quenched to a temperature of at most 80 °C.
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. Method according to one of claims 1 to 4, characterized in that the semi-finished product is based on polypropylene, and that only one stretching step is carried out at a stretching degree of 8 and a stretching temperature of 120 °C, after which the semi-finished product (1) undergoes a relaxation of 20% in the relaxation step, after which the semi-finished product (1) is heated in a single post-heating step to a post-heating temperature in the range of 160 - 190 °C, preferably 160 - 170 °C, more preferably 160 °C, and after which the forming step is carried out immediately.
6. Method according to one of claims 1 to 4, characterized in that the semi-finished product is based on polyethylene, and that only one stretching step is carried out at a degree of stretching of 6 and a stretching temperature of 80 °C, after which the semi-finished product (1) undergoes a relaxation of 10% in the relaxation step, after which the semi-finished product (1) is heated in a single post-heating step to a post-heating temperature in the range of 125 - 160 °C, preferably 125 to 140 °C, more preferably 125 - 135 °C, and after which the forming step is carried out immediately.
7. Method according to one of claims 1 to 4, characterized in that the semi-finished product is based on polystyrene, and that only one stretching step is carried out at a degree of stretching of 3 to 5, preferably 4, and at a stretching temperature in the range of 105 - 120 °C, after which the semi-finished product (1) undergoes a relaxation of 10 % to 20 %, preferably 15 %, after which the semi-finished product (1) is heated in a single post-heating step to a post-heating temperature in the range of 120 - 145 °C, preferably 120 to 130 °C, and after which the forming step is carried out immediately.
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 8, 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 plastic 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 150 °C is at most 5%.