BIAXIALLY ORIENTED POLYESTER FILM CONTAINS A POST CONSUMER OR POST INDUSTRIAL RECYCLED FROM A COATED POLYESTER FILM AND PROCESS FOR MANUFACTURING THE FILM AND THE RECYCLED FOLLOWING

Description

[0001] A biaxially oriented polyester film containing at least one post-consumer or post-industrial reclaimed material, which was produced from at least 10 wt. % of a coated polyester film, as well as a process for its production and in particular a process for producing said reclaimed material. Also claimed is a reclaimed material consisting of at least 10 wt. % coated polyester film (post-consumer or post-industrial) produced by the process according to the invention. Summary

[0002] The present invention relates to a biaxially oriented polyester film containing at least one post-consumer or post-industrial reclaimed material, which was produced from at least 10 wt. % of a coated polyester film. The coated polyester film is generally a siliconized polyester film or a coating with crosslinked or partially crosslinked components such as hard coats or the like. The coated film used to produce the reclaimed material has undergone at least one additional process step that differs from the actual polyester film production and finishing, or the film has been used in its intended end application and accrued there as "waste." It is therefore post-consumer or post-industrial reclaimed material.This film is collected after its intended use and then mechanically recycled after any non-polyester film components have been separated. This recycling process involves chopping the film, melting it in an extruder, and finally granulating the melt strands. The extruder used is a multi-screw extruder. The resulting granulate has a significantly better quality in terms of inclusions than a standard single-screw extruder. When reused in polyester film production, this leads to a significantly lower level of specks and a significantly improved filter life compared to the use of a recyclate made from the same starting material that was produced using a single-screw extruder. Description

[0003] The increasing importance of recycling in the context of the circular economy discussion also requires solutions in PET film production that allow the use of recycled material. In the PET bottle sector, this has been an industrial reality for 20 years. The used PET bottles are collected and shredded, other plastics (caps, labels, etc.) are subsequently separated using various processes (including float-sink processes). The flakes are also surface-cleaned using an alkaline wash, then melted in an extruder and then granulated again. Finally, liquid-phase condensation takes place either while still in the melt or subsequent condensation in a tumble dryer to restore the chain length, which has been reduced during the process, to an SV of at least 1000, which is required for the production of new bottles.The extruders used are typically single-screw extruders (e.g., from market leader Erema, or NGR, and others), as these are cheaper to purchase and more robust and cheaper to operate than twin- or even multi-screw extruders. Twin- and multi-screw extruders are generally only used when the mixing (compounding) of other substances (such as white pigments in masterbatches) is desired, or following melting in the single-screw extruder in liquid-phase condensation.

[0004] Polyester films are frequently used in laminates (which would have to be laboriously separated before mechanical recycling) or in long-term applications (e.g., in electronic components, where the polyester content is low, so recycling is not yet worthwhile). These cannot currently be economically reused at the end of their life cycle. The situation is different for polyester films used as "process films." Examples include films onto which other films or lacquers are coated, and these other films or lacquers are then peeled off the polyester film (e.g., PU casting) or transferred to another surface (e.g., lacquers in the furniture industry). A particularly large application of this type is siliconized films, which are used as liners for all types of labels. The applied labels are usually peeled off the film by machine and applied to the target substrate (e.g.,beverage or detergent bottles). What remains is a largely clean polyester film with a silicone layer and is contaminated with small amounts of adhesive residue and possibly individual, non-transferred labels.

[0005] A process for recycling such a film is described in EP2524015 and was also used commercially on an industrial scale by the applicant (MITSUBISHI POLYESTER FILM INC [US]) for a time.

[0006] A disadvantage of using the raw materials manufactured as described in EP2524015 in film production is the relatively short filter service life on the film production line, which can be as low as 10% by weight of the usual filter service life (when using post-consumer, regenerated-free raw materials). The reduction in filter service life depends on the total amount of siliconized film in the regenerated material used (or calculated based on the proportion of recycled siliconized film in the newly produced film) that is incorporated into the new film. Another disadvantage is the significantly increasing proportion of inclusions (specks) in the film, which increases the likelihood of film tears and can cause problems in the film's end applications. Shorter filter service lives and more tears significantly reduce the economic efficiency of film production.

[0007] Similar problems also arose when reusing regrind produced from process films onto which previously crosslinked or partially crosslinkable coatings had been applied and removed.

[0008] The object of the present invention was therefore to provide a raw material and a process for producing the same, which consists at least partially of siliconized films, or films with cross-linked or partially cross-linked layers, which does not have the disadvantages mentioned when the pressure increases, and it is furthermore the object of the present invention to provide a polyester film containing this raw material, which can be produced with good economic efficiency and has an acceptable level of inclusions.

[0009] This task is solved by a process in which the coated polyester film to be recycled is melted in a twin- or multi-screw extruder, the melt is filtered, and the melt is then cooled and pelletized in strands. A biaxially oriented polyester film is then produced, consisting entirely or partially of this raw material.

[0010] Where: 1. The extruders according to the invention for melting the polyester are twin-screw or multi-screw extruders. 2. The mass flow fed to the extruder contains at least 10% by weight of a coated polyester film. 3. The melt is filtered before granulation. 4. A biaxially stretched film is produced which consists of at least 10% by weight of the aforementioned polyester raw material.

[0011] The extruders used in the process according to the invention are exclusively twin- or multi-screw extruders. Such extruders are manufactured commercially, among others, by Japan Steel Works.

[0012] The use of single-screw extruders is unsuitable for the purposes of the invention, as they surprisingly lead to significantly poorer results in the film production process. When single-screw extruders (such as the commercially available Erema extruders for processing film offcuts or bottle shreds) are used, the filter service life on the polyester film production line is reduced so significantly that commercial operation is no longer possible over an extended period. This applies noticeably when at least 10 wt.% of a raw material is used that was produced from at least 10 wt.% coated polyester film waste. Furthermore, the proportion of inclusions increases significantly over the production period, leading to an intolerable number of film breaks.This is particularly surprising, as single-screw extruders are the most widely used standard for processing waste generated during the film process in the polyester film industry. Even the process waste (edge ​​trim, start-up and exit rolls, material from film tears) generated during the production of siliconized film itself can be processed with good quality on single-screw extruders and can be reused. Therefore, the significant advantage offered by twin-screw extruders in processing post-consumer waste from siliconized film was highly unexpected.

[0013] Twin-screw extruders are preferred over multi-screw extruders, as the latter are more expensive to purchase and operate and offer no advantage in the application according to the invention over twin-screw extruders. Less preferred within the meaning of the invention are combinations consisting of an initial single-screw extruder for melting the polyester waste, followed by a twin- or multi-screw extruder, which typically serves as the discharge unit of a unit for increasing the melt viscosity. While such a combination provides better processability of the produced product compared to a single-screw extruder alone, it is inferior to twin- or multi-screw extruders alone.

[0014] The use of twin-screw extruders also reduced the level of specks in the film compared to the use of single-screw extruders. Disturbing specks are generally inclusions with a total defect size of over 20 µm. The number of such inclusions depends on the type of filtration, the residence time of the melt in the extrusion system, and the type of raw materials used. For the purposes of the invention, a film produced without post-consumer regrind can be considered standard. When using post-consumer regrind produced using a single-screw extruder, the detection rate increased by a factor of 1.3 with only 10% of post-consumer recyclate from siliconized film (based on the total weight of the film). When using post-consumer regrind made from the same starting material in the same quantity, which was produced using a twin-screw extruder, only an increase of a factor of 1.05 was observed.The specks are visually detected and marked using polarized light. Structures smaller than 20 µm are virtually undetectable visually, so it can be assumed that almost all defects found have a surface area of ​​approximately 20 µm or more. To be on the safe side, the marked defects are measured again under a light microscope, and defects with a length of 20 µm in one spatial direction are counted as specks. At least three pieces of 5 m² of film must always be examined.

[0015] The coated films to be recycled are films that, after their intended use, become waste at the end user. This film waste must be removed from any foreign material (such as beverage cans, paper, etc.) before being fed to the extruder and ideally collected separately at the end user's site. The film must be shredded before being fed to the extruder.

[0016] The coated films are siliconized films, which means they have a silicone layer. This silicone layer is made up of poly(organo)siloxanes, which are made up of individual siloxane units. The composition of the siloxane unit is calculated taking into account the fact that each oxygen atom acts as a bridge between two silicon atoms: R n SiO (4-n) / 2 (n=0, 1, 2, 3), i.e. a siloxane unit can have one to four additional substituents, depending on the number of free valences on the oxygen atom. Siloxane units can therefore be mono-, di-, tri- and tetrafunctional. This is represented symbolically by the letters M (mono), D (di), T (tri) and Q (quatro): [M]=R 3 SiO 1 / 2 , [D]=R 2 SiO 2 / 2 and [T]=RSiO 3 / 2 .

[0017] As with organic polymers, the multitude of possible compounds is based on the fact that different siloxane units can be linked together within the molecule. Based on the classification of organic polymers, the following groups can be distinguished: Cyclic polysiloxanes are ring-shaped and composed of difunctional siloxane units. Structure [D n ]. Linear polysiloxanes have the structure [MD n M] or R 3 SiO[R 2 SiO] n SiR 3 (e.g., poly(dimethylsiloxane)). Branched polysiloxanes have trifunctional or tetrafunctional siloxane units as branching elements. Structure [M n D m ​​T n ]. The branching point(s) is / are incorporated either into a chain or a ring. Crosslinked polysiloxanes in this group are chain- or ring-shaped molecules linked to planar or three-dimensional networks using tri- and tetrafunctional siloxane units. Chain formation and crosslinking are the dominant principles for the construction of high-molecular-weight silicones.

[0018] The polyester films used in the recycling process according to the invention can have any type of silicone coating. By way of example, reference is made to US Pat. No. 5,672,428 A. This document describes siliconized films that can be used according to the present invention. The siliconized films described therein are polyester films provided with a silicone coating. The coating composition of the silicone coating comprises a vinyl-containing alkylvinylpolysiloxane, a catalyst that is a platinum complex or a tin complex, a glycidoxysilane, and an alkylhydrogenpolysiloxane that contains 1.5 to 2 mol% hydrogen.

[0019] Other coated polyester films, such as the lacquer transfer films described above, which still contain residues of the transferred lacquers, are less preferred.

[0020] Depending on the type of coating used and the remaining amount, undesirable odor development or discoloration of the film may occur during film production. Therefore, in a preferred embodiment, such other coated polyester films are present in the input material at less than 20 wt.%, preferably less than 10 wt.%, and ideally not at all.

[0021] According to the invention, the fraction of siliconized films has an average silicone layer thickness of < 0.5 µm, preferably < 0.4 µm, particularly preferably < 250 nm and ideally < 180 nm. The higher the average layer thickness of the silicone, the higher the number of inclusions in the reclaimed film and the shorter the filter service life during production. This means that within the meaning of the invention it is preferred if the siliconized film is predominantly one in which the siliconization was applied during film production (in-line), since in this case the silicone layer thicknesses are usually below 200 nm. Such products are available, for example, under the brand name Hostaphan RN 23 2SLK from Mitsubishi Polyester Film GmbH Germany, but also from Siliconature (Italy).If offline siliconized film waste (silicone layers of typically 0.5 - 3 µm) is to be recycled, then in this preferred embodiment, it must be ensured by mixing it with inline siliconized film waste that the average silicone layer thickness in the feed material does not exceed the stated limits. The silicone layer thickness of the respective liner (term for siliconized film as a carrier for labels) can be determined for all liner types contained in the waste using known methods (e.g. cutting the film and measuring the layer thickness under an electron microscope or using optical methods such as ellipsometry). After determining the layer thickness for the liner types contained in the waste, their ratios in the process must be adjusted so that, as mentioned above, the preferred ranges for average layer thicknesses are not exceeded.

[0022] Within the context of the circular economy discussion, the "cradle to cradle" approach is the most preferred. This means that a product is regenerated (recycled) at the end of its life cycle, and this regenerated material is used to manufacture the same product again. This creates a true circular economy. Therefore, the goal is to use as much post-consumer regenerated siliconized film as possible, with the highest possible proportion of this material being used in the production of new siliconized film.

[0023] The proportion of siliconized film in the input material for recycling is therefore at least 10 wt.%, preferably at least 50 wt.%, particularly preferably at least 90 wt.% and ideally > 99 wt.%.

[0024] In addition to the siliconized film, the input material may contain other post-consumer / post-industrial polyester waste, such as other polyester film waste (coated or uncoated), or shredded polyester bottles and trays. However, since these are not recycled into their original application, they are less preferred within the above-mentioned limits within the meaning of the invention.

[0025] The addition of "virgin" polyesters or process waste from polyester film production that has not yet been used as a product by a private or industrial end customer is also not preferred. Such polyesters are present in the input material at less than 15 wt.%, preferably less than 5 wt.%, and ideally not at all.

[0026] In addition to other polyester waste, individual labels are usually still present on the film that were not properly transferred to the target substrate during the end-user process. In addition to the label material itself, these introduce printing ink and adhesives into the melt. These can lead to discoloration in the melt and in the subsequently produced film, or block the filters during reclaim production or film production. There is also a significant risk of intolerable odor nuisance during the production of the reclaim and especially later during film production. Therefore, remnants of labels must be removed before reclaim production. This can be done, for example, by manually sorting affected film webs or by automated sorting of the chopped material on a conveyor belt using color recognition and blowing off the affected chopped material.The following limits therefore apply to the following materials, regardless of whether they entered the chippings as leftover labels or from another source.

[0027] Paper content in the chopped material fed into the extruder may not exceed 1% by weight, preferably 0.3% by weight, and ideally less than 0.05% by weight. Since paper burns in the extruder and clogs the melt filters, this leads to black inclusions, which can subsequently cause film tears. If these limits are exceeded, economical production of reclaim and film is no longer possible.

[0028] Printed polyesters which consist of more than 70 wt.% PET (the other maximum 30 wt.% can be other monomers such as isophthalic acid, cyclohexanedimethanol, diethylene glycol, naphthalenedicarboxylic acid, propanediol, butanediol, etc.) are contained in the chopped material at a maximum of 7 wt.%, preferably at a maximum of 4 wt.%, particularly preferably at a maximum of 1 wt.% and ideally below 0.5 wt.%.

[0029] Polypropylene is contained in the shredded material at a maximum of 2 wt.%, preferably at a maximum of 1 wt.%, particularly preferably at a maximum of 0.5 wt.%, and ideally at less than 0.1 wt.%. Polypropylene generally does not block filters to a significant degree, but it does lead to significant odor nuisance and a significant, undesirable clouding of the film subsequently produced from the regenerated material.

[0030] Any other polymers that may be present are contained in the chopped material at a maximum of 0.8 wt.%, preferably at a maximum of 0.1 wt.% and ideally at less than 0.05 wt.%.

[0031] The melt is filtered before final granulation. Typically, metal mesh filters or sintered metal filters are used for this purpose. However, self-cleaning filters, such as those available from Gneuss, can also be used. The specified nominal pore size of the final filtration stage is less than or equal to 100 µm, preferably less than or equal to 80 µm, particularly preferably less than 50 µm, and ideally less than or equal to 30 µm. Another filter (pore size larger than 80 µm) can be placed before this final stage, which is particularly advantageous if the chopped material still contains paper above the ideal range. The smaller the pore size of the final filtration stage, the longer the filter service life in the film production plant. Since filter changes on the film production plant are significantly more expensive than those on the extruder for reclaim production, this is an advantage.However, the lower the pore size on the reclaim extruder, the shorter the filter life. Therefore, low pore sizes can generally be used when the aforementioned contamination of the chopped material with foreign material is at the low levels mentioned above.

[0032] In the context of this invention, filter service life is understood to be the period of time a filter can be operated for production purposes from the time of installation until a maximum permissible pressure value is exceeded. The absolute level of the maximum pressure value is system- and filter-specific, so the filter service life also varies depending on the system used (but is reproducible for each selected system). Filter systems that allow high pressure differences before and after filtering lead to longer filter service lives, but are also more complex to implement due to the higher absolute pressures. As a rule, an attempt is made to find a compromise between the longest possible filter service life and, at the same time, avoiding high absolute pressures in order to keep technical complexity to a minimum.

[0033] Filter service life depends primarily on the degree of contamination of the raw materials by foreign matter. Post-consumer reclaim, in particular, typically leads to a significant reduction in filter service life, down to just a few days, making film production economically unviable.

[0034] The recyclate can be condensed either in the melt (liquid-phase condensation) or after granulation in a tumble dryer. Liquid-phase condensation is preferred because it occurs in the same process step as the initial extrusion and removes volatile substances from the melt more effectively than a tumble dryer from the solid granules. This can significantly reduce odor nuisance during film production. Increasing the viscosity itself is only beneficial in film production when the proportion of post-consumer reclaim exceeds 30% by weight, since the viscosity can be more easily adjusted beforehand using the other raw materials used.

[0035] Since the regrind is largely transparent unless condensed in a tumble dryer and largely clear siliconized films were the input material, it has proven advantageous to inspect the granules before use by means of an optical (or X-ray) automated control and to sort out granules with large inclusions (> 30 µm) (e.g. at Sikora AG, Bremen, Germany).

[0036] This granulate can then be reused in the production of a biaxially oriented polyester film, or less preferably for other polyester applications.

[0037] In a preferred embodiment, the post-consumer regenerates used have a filter test rating of acceptable or better.

[0038] In the film production according to the invention, at least 10 wt. %, preferably at least 25 wt. %, and ideally at least 31 wt. % of the regenerated material according to the invention is added, based on the total weight of the film. The film can be produced from 100 wt. % of the regenerated material, but dosages above 50 wt. % are less preferred, since this then leads to a noticeably reduced filter service life in film production, even when using a twin-screw extruder. Furthermore, the preferred target application of the film is again a siliconized film as a liner for labels. Since this is then also to be recycled after use according to the process described here, the silicone remaining on the film accumulates in the process at regenerated material contents above 50 wt. %, which then leads to reduced filter service life and more inclusions in the resulting films.

[0039] If post-consumer regenerated material contents of more than 50 wt.% are nevertheless to be achieved, it has proven advantageous to additionally use a post-consumer recyclate from uncoated polyester films, or preferably a regenerated material from polyester bottles, or particularly preferably a chemically recycled polyester from post-consumer polyester waste, in order to close the gap to the target regenerated material content.

[0040] The other polymer raw materials added to the film are at least 95 wt.%, particularly preferably at least 98 wt.% thermoplastic polyesters. A polyester made from ethylene glycol and terephthalic acid (= polyethylene terephthalate, PET) is preferred. Polyesters containing naphthalene-2,6-dicarboxylic acid as a repeating unit are less suitable, as these lead to a higher price for the film without offering any further advantages in the primary target application of label liners. The polyester can contain other monomers, such as isophthalic acid, cyclohexanedimethanol, or ethylene glycol. Based on the total weight of the film, the proportion of monomers other than ethylene glycol and terephthalic acid is < 7 wt.%, as otherwise the mechanical stability of the film would be reduced, which is undesirable in most target applications.

[0041] Other polymers besides thermoplastic polyesters are added in amounts of less than 5 wt.%, preferably less than 1 wt.%, and ideally not at all. In addition to the polymers mentioned, the film may contain particles such as calcium carbonate, silicon dioxide, or aluminum trioxide. Such particles are added to improve slip behavior or for matting and are typically present in the film in amounts of less than 2 wt.%, preferably less than 1 wt.%, and ideally less than 0.5 wt.%.

[0042] In a less preferred embodiment, the film may contain whitening particles such as titanium dioxide and barium sulfate; these are present in the film at a maximum of 19 wt.%. If whitening particles are present, the above-mentioned information on the polymer content refers only to the polymer content of the film and does not take into account the inert white pigment. White polyester films are less preferred within the meaning of the invention because they cannot currently be recycled themselves. Separate collection is not currently worthwhile due to the small total quantity. And they cannot be mixed with the transparent coated films, since the color of the resulting film cannot then be adjusted due to fluctuating contents of white films in the input material.

[0043] The film may contain additional additives, such as radical scavengers (e.g., Irganox 1010) or dyes (preferably blue dyes to compensate for the yellow component). The proportion of such additives is < 0.5 wt.% and preferably < 0.3 wt.%.

[0044] The film can be single-layer or multi-layer. In a preferred embodiment, the film has at least three layers. In a preferred embodiment with at least three layers, the post-consumer reclaim is added to only one or more inner layers, while the two outer layers are made from virgin polymer. This has the advantage that any inclusions and impurities present in the post-consumer reclaim have a less pronounced impact on the film surface and thus pose less of a problem in the final application.

[0045] The total film thickness is at least 12 µm and preferably at least 15 µm. If the film is thinner than 12 µm, inclusions in the post-consumer reclaim lead to film tears significantly more frequently than with thicker films.

[0046] For the production of the film according to the invention, the SV value of the polyester is expediently selected such that the film has an SV value of > 600, preferably > 650, and ideally > 700. The SV value of the film is preferably < 950 and particularly preferably < 850. If the SV value is below 600, the film becomes so brittle during production that frequent tears occur. Furthermore, the mechanical strengths mentioned below can no longer be reliably achieved at lower SV values. If the film has an SV higher than 950, the polymer becomes so tough in the extruder that excessively high flows occur and pressure fluctuations occur during the extrusion. This leads to poor runnability. Furthermore, the abrasion on the extrusion dies and cutting tools becomes disproportionately high.

[0047] These values ​​are achieved by adjusting the arithmetic mean of all raw materials (including post-consumer reclaim), weighted according to their weight percentages, so that it is approximately 10-100 SV units above the respective target value for the film. Since the degradation of the SV value during film production depends heavily on the plant, the value applicable to the specific film plant must be determined.

[0048] The film according to the invention further exhibits a modulus of elasticity in both film directions (TD and MD) of greater than 3000 N / mm 2 , preferably greater than 3500 N / mm 2 , and particularly preferably (in at least one film direction) of > 4000 N / mm 2 in the longitudinal and transverse directions. The F5 values ​​(force at 5% elongation) are preferably greater than 80 N / mm 2 , and particularly preferably greater than 90 N / mm 2 , in the longitudinal and transverse directions. These mechanical properties can be adjusted and maintained by varying the parameters of the biaxial stretching of the film within the process conditions specified below.

[0049] Films with the mechanical properties mentioned above are not excessively stretched under tension, remain easy to handle and have the stiffness desired in the end application.

[0050] In a preferred embodiment, the film is siliconized in-line. Examples of in-line siliconization are given, for example, in US5672428 or in EP3283295.

[0051] The dry silicone layer thickness is adjusted to a thickness of at least 60 nm, preferably at least 80 nm, and ideally at least 90 nm. If the layer is thinner than 60 nm, the adhesives commonly used in the label industry adhere too strongly to the film, making the label difficult to remove. The layer thickness is < 180 nm and preferably < 170 nm. If the layer thickness is greater than 180 nm, the problems described above during regeneration and reuse of the post-consumer regenerated material are more likely to occur. Process for producing the film

[0052] The polyester polymers of the individual layers are obtained as regenerates, such as the post-consumer regenerate according to the invention, or produced by polycondensation, either starting from dicarboxylic acids and diol or—although less preferably—starting from the esters of the dicarboxylic acids, preferably the dimethyl esters, and diol. Usable polyesters preferably have SV values ​​in the range of 500 to 1300, whereby the individual values ​​are less important, but the average SV value of the raw materials used must be greater than 700 and is preferably greater than 750. These values ​​are expediently used to achieve the SV values ​​of the film described above. If the SV value is too low, the film becomes brittle; if it is too high, the stretching forces during the process increase sharply, and economical production is thus significantly impaired.

[0053] First, the polyester of the individual layers is compressed and liquefied in extruders. In a preferred embodiment, the melt temperatures (temperature measured in the melt at the extruder outlet) are between 290°C and 300°C. At temperatures above 300°C, the yellowness index increases; at temperatures below 290°C, the risk of unmelted polymer components, which can lead to undesirable surface elevations, increases. This temperature is adjusted via the throughput to revolutions ratio of the extruder or via the temperatures of the extruder heater. These respective ratios and temperatures depend on the type of extruder used and can be adjusted by a person skilled in the art using the parameters mentioned. The melt is filtered upstream of the die through a sintered metal or metal flow filter; the nominal pore size of the filter is < 50 µm, preferably less than 35 µm, ideally < 21 µm and preferably > 10 µm.Pores larger than 50 µm result in too many inclusions in the film and an increased number of film breaks. Pores smaller than 10 µm result in an insufficient filter life. The melts are then formed into flat melt films in a multi-layer die, pressed through a slot die, and drawn off onto a chill roll and one or more take-off rolls, where they cool and solidify.

[0054] The film according to the invention is biaxially oriented, i.e., biaxially stretched. Biaxial stretching of the film is most commonly carried out sequentially. Stretching is preferably carried out first in the longitudinal direction (i.e., in the machine direction, = MD direction) and then in the transverse direction (i.e., perpendicular to the machine direction, = TD direction). Longitudinal stretching can be carried out using two rolls running at different speeds, depending on the desired stretch ratio. A suitable tenter frame is generally used for transverse stretching. By selecting suitable stretching parameters, the skilled person can adjust the mechanical properties of the film, such as its modulus of elasticity, stiffness, and extensibility.

[0055] The temperature at which stretching is carried out can vary within a relatively wide range and depends on the desired properties of the film. In general, stretching is carried out in the longitudinal direction in a temperature range of 80°C to 130°C (heating temperatures 80°C to 130°C) and in the transverse direction in a temperature range of 90°C (start of stretching) to 140°C (end of stretching). The longitudinal stretching ratio is in the range of 2.5:1 to 4.5:1, preferably 2.8:1 to 4.0:1. A stretching ratio above 4.5 leads to significantly impaired manufacturability (tears). The transverse stretching ratio is generally in the range of 2.5:1 to 5.0:1, preferably 3.2:1 to 4.0:1. To achieve the desired film properties, it has proven advantageous if the stretching temperature (in MD and TD) is below 125°C and preferably below 118°C.Before transverse stretching, one or both surfaces of the film can be coated in-line using conventional methods.

[0056] During the subsequent heat-setting process, the film is held under tension at a temperature of 150°C to 250°C for a period of approximately 0.1 s to 10 s. The film is then wound up in the usual way. Analytics

[0057] The following measurements were used to characterize the raw materials and the films: SV value (standard viscosity)

[0058] The standard viscosity SV (DCE) was measured, based on DIN 53 726, at a concentration of 1% in dichloroacetic acid in an Ubbelohde viscometer at 25°C. The dimensionless SV value is determined from the relative viscosity (η rel ) as follows: SV = η rel − 1 × 1000

[0059] Film or polymer raw materials were dissolved in DCE. The particle content was determined by ash analysis and corrected by additional weights. This means: Einwaage = Einwagemenge laut Vorschrift / 100 − Partikelgehalt in Gew . - % / 100 Mechanical properties

[0060] The mechanical properties were determined by tensile testing based on DIN EN ISO 527-1 and -3 (test specimen type 2) on 100 mm x 15 mm film strips. Filter test

[0061] The raw material is melted in a single-screw extruder at a throughput of 2.4 kg / h and pressed at 290°C through a filter with a 10µm metal fiber fleece (e.g., from Bekaert) and a filter area of ​​50cm². The pressure upstream of the filter is continuously measured throughout the entire extrusion time.

[0062] The filtration quality is assessed based on the pressure difference between 60 and 15 min of extrusion time. Very good < 2 bar pressure rise Good 2 - 8 bar pressure rise Acceptable 9 - 20 bar pressure rise Poor 21 - 30 bar pressure rise Very poor > 31 bar pressure rise Examples

[0063] The used siliconized polyester films were collected by type from various bottling plants (beverages / detergents) after use.

[0064] The bags containing the liners were emptied onto a conveyor belt, and any foreign material was removed manually. Visible labels were also removed where possible. The film was then chopped, and every two hours a 1 kg sample of the chopped material was examined for the remaining amount of label material (label material was separated and weighed). The average silicone layer thickness was also determined on these samples (the layer thickness was measured using ellipsometry on 50 flakes and averaged). The values ​​given in the table represent the average of four such samples. For the comparative examples with increased residual label content, the labels were not removed.

[0065] The chopped material was then melted in a twin-screw extruder from Japan Steel Works, and the similarly processed chopped material was melted in a single-screw extruder from Erema for comparison. The chopped material for Comparative Example 6 was melted in a single-screw extruder from NGR with a connected liquid-phase condensation system and multi-screw discharge system. It was then filtered through a 30 µm metal fleece filter and then granulated.

[0066] The resulting post-consumer reclaim was then mixed with the other polymers as indicated in the table and melted at 292°C, filtered through a 20 µm (pore volume) metal fleece filter, and electrostatically applied through a slot die onto a chill roll heated to 50°C. It was then stretched longitudinally and then transversely under the following conditions: Longitudinal stretching Heating temperature 75-115 °C Stretching temperature 115 °C Longitudinal stretch ratio 3,8 Transverse extension Heating temperature 100 °C Stretching temperature 112 °C Transverse stretch ratio (including stretching of the first fixation field) 3,8 Fixation temperature 237 - 150 °C Length of time 3 S Fixation Temperature 1. Fixing field 170 °C

[0067] Before transverse stretching, a silicone coating was applied as in Example 7 of US5672428. The thickness of the silicone layer on the finished fixed film was 110 - 125 nm.

[0068] The following raw materials are used in the examples: PET1 = polyethylene terephthalate raw material made from ethylene glycol and terephthalic acid with an SV value of 820 (isophthalic acid content < 0.1 wt.%, DEG 1 wt.%).

[0069] PET2 = Polyethylene terephthalate raw material made from ethylene glycol and terephthalic acid with an SV value of 790 (isophthalic acid content < 0.1 wt.%, DEG 1.1 wt.%). With 1 wt.% Sylysia 320 from Fuji Sylysia Japan.

[0070] PET3 = Polyethylene terephthalate raw material made from ethylene glycol and terephthalic acid with an SV value of 1020 (isophthalic acid content < 0.1 wt.%, DEG 1 wt.%).

[0071] In film production, a maximum of one tear-off every 8 hours is acceptable; anything else is uneconomical. If the pressure in the filter of base layer B increases more than normal (in production without post-consumer recyclate, the filter service life is typically > 14 days), the economic efficiency decreases because the filter reaches the pressure limit more quickly and must be replaced. A sharp pressure increase can lead to the pressure limit being reached within less than 2 days, for example, which is clearly uneconomical. Significant clouding of the film is undesirable for the intended application. A clearly perceptible odor development is undesirable.

[0072] The term "sharp pressure increase" means that the pressure increases faster than normal and the maximum pressure is reached within a shorter time.

[0073] The following Tables 1 (Table of the produced regenerates) and 2 (Table of the produced films using the regenerates contained in Table 1) summarize the recipes and resulting raw material and film properties:

Claims

1. Process for recycling a polyester film, comprising (a) comminuting a used polyester film to shreds, the used polyester film comprising a polyethylene terephthalate polymer and a silicone coating, the layer thickness of the silicone coating being less than 0.5 µm, (b) melting the shreds in a twin-screw or multi-screw extruder, (c) filtering the melt, (d) pelletizing.

2. Process according to Claim 1, characterized in that the shreds in (b) are melted in a twin-screw extruder.

3. Process according to Claim 1, characterized in that the layer thickness of the silicone coating is less than 250 nm, more preferably less than 180 nm.

4. Process according to one or more of Claims 1 to 3, characterized in that in (c) metal-fabric filters or sintered-metal filters are used.

5. Process according to one or more of Claims 1 to 4, characterized in that the nominal pore size of the final filtration stage is ≤ 100 µm, preferably ≤ 80 µm, more preferably ≤ 30 µm.

6. Process for producing a biaxially oriented polyester film, comprising: (a) comminuting a used polyester film to shreds, the used polyester film comprising a polyethylene terephthalate polymer and a silicone coating having a layer thickness of less than 0.5 µm, melting the shreds in a twin-screw or multi-screw extruder, filtering the melt and pelletizing, (b) melting PET pellets in an extruder, at least 10% by weight of the PET pellets being pellets obtained according to step (a), (c) extruding the resulting melt through a flat-film die, (d) cooling on a chill roll and solidifying to a film, (e) biaxially stretching the film.

7. Process according to Claim 6, characterized in that at least 25% by weight, preferably at least 31% by weight, of the pellets have been obtained by a process according to Claim 1.

8. Process according to Claim 6 or 7, characterized in that the mean SV of the PET pellets is > 700, preferably > 750.

9. Process according to one or more of Claims 6 to 8, characterized in that the biaxial stretching in (d) is carried out sequentially.