Recycled polyester film, laminated recycled polyester film

Abstract

To manufacture recycled polyester film with superior quality even when using a large amount of recycled polyester raw material. [Solution] A recycled polyester film in which the amount of 4,4'-stilbendicarboxylic acid contained in the polyester film is 0.1 ppm or more and 100 ppm or less.

Description

Technical Field

[0001] The present invention relates to a recycled polyester film.

Background Art

[0002] Recently, as an international effort, the SDGs Promotion Headquarters has been established as a sustainable development goal, and efforts to recycle resins are actively underway as an international goal. In Japan, since around 2000, the Container and Packaging Recycling Law and the Law for the Promotion of Effective Use of Resources have been implemented, and the recycling technology of polyester resin has been almost established. However, in 2022, due to international efforts, the Plastic Resource Recycling Law incorporating the concept of "3R+Renewable" was implemented, and it is attempting to shift to a circular economy technology that minimizes the recycling of waste.

[0003] In the resource recycling of plastics, the recycling of PET bottles mainly composed of polyethylene terephthalate resin, which is representative of polyester, is in the lead. In recent years, the scope of recycling technology has expanded from PET bottles to other industrial materials, such as packaging materials, process materials such as release and protection materials, and functional materials. Also, in recycling methods, there are material recycling (hereinafter sometimes referred to as mechanical recycling) and chemical recycling. Although chemical recycling can obtain recycled polyester raw materials comparable to virgin raw materials, the number of processes required for recycling increases, resulting in higher costs and input energy. Therefore, recycled polyester resin by material recycling is widely adopted.

[0004] Material recycling of polyester raw materials that have already been molded can easily affect the properties of the recycled product. Furthermore, it is difficult to determine whether the product used for recycling already contains recycled polyester raw materials, and if recycled products are manufactured without this determination, there is a problem of product quality degrading. For example, when polyester resin is melted and molded into a product, it may become more yellowish than the original polyester resin, and when it is melted and molded again, the resulting molded product may also have a yellowish tint.

[0005] As a method for suppressing yellowing using recycled polyester resin, Patent Document 1 proposes a technique that utilizes the thickness of each layer in the laminated structure of the film for mechanical recycling, thereby suppressing the crystallization of the polyester resin within the layers and suppressing the increase in the film's color b value. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2024-110921 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the laminated film described in Patent Document 1 does not investigate what causes the color b value of the melt-molded polyester resin to increase, which limits the selection of the amount and type of recycled polyester resin to be used. Furthermore, since this technology requires a specific die-cutting device, it incurs modification costs for the film manufacturing equipment, limiting its applicability to various manufacturing devices.

[0008] The objective of this invention is to produce a recycled polyester film with superior quality even when using a large amount of recycled polyester raw material. [Means for solving the problem]

[0009] In other words, a preferred embodiment of the present invention consists of the following configuration. (1) A recycled polyester film containing 4,4'-stilbendicarboxylic acid in an amount of 0.1 ppm or more and 100 ppm or less. (2) The recycled polyester film described in (1), wherein the color b value is 5.0 or less. (3) The recycled polyester film described in (1) or (2), wherein the film's color b value per 1 μm of film thickness is 0.150 or less. (4) A recycled polyester film as described in any of (1) to (3), having a haze value of 2.0% or less. (5) A recycled polyester film as described in any of (1) to (4), having a thickness of 1 μm or more and 500 μm or less. (6) A recycled polyester film according to any one of (1) to (5), comprising 50% by mass or more of recycled polyester raw material. (7) A recycled polyester film according to any of (1) to (6) that is substantially free of particles. (8) A laminated recycled polyester film having a functional layer on at least one surface of the recycled polyester film described in any of (1) to (7). [Effects of the Invention]

[0010] The present invention allows for the use of a large amount of recycled polyester resin, provides a recycled polyester film with good film quality, and furthermore, allows for the use of multiple types of recycled polyester resin. [Modes for carrying out the invention]

[0011] The recycled polyester film of the present invention is a polyester film mainly composed of recycled polyester resin. The polyester resin of the recycled polyester resin is a polyester obtained using an aromatic dicarboxylic acid or an aliphatic dicarboxylic acid and a diol or a derivative thereof. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, and 4,4'-diphenylsulfondicarboxylic acid. Examples of aliphatic dicarboxylic acids include adipic acid, suberic acid, sebacic acid, dimer acid, dodecanedionic acid, cyclohexanedicarboxylic acid and their ester derivatives. Terephthalic acid and 2,6-naphthalenedicarboxylic acid are particularly preferred. These acid components may be used individually, in combination of two or more, or partially copolymerized with oxyacids such as hydroxybenzoic acid. Examples of diol components include ethylene glycol, 1,2-propanediol, 1,3-propanediol, neopentyl glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, polyalkylene glycol, 2,2-bis(4-hydroxyethoxyphenyl)propane, isosorbate, and spiroglycol. Among these, ethylene glycol is preferably used. These diol components may be used individually or in combination of two or more. Of the above polyesters, it is preferable to use a polyester selected from polyethylene terephthalate and its copolymers, polyethylene naphthalate and its copolymers, polybutylene terephthalate and its copolymers, polybutylene naphthalate and its copolymers, polyhexamethylene terephthalate and its copolymers, and polyhexamethylene naphthalate and its copolymers.

[0012] The recycled polyester film of the present invention contains recycled polyester resin. Recycled polyester resin refers to resin that has been melt-molded from polyester resin. The recycling method may be either chemical recycling or material recycling. It is preferable that the recycled polyester resin be mainly composed of materially recycled resin, as this can be manufactured at low cost. Here, recycling refers to recycling a polyester resin composition that has been molded into a desired shape according to the target product, or a molded product that has been further processed. Specific molded products include fibers, nonwoven fabrics, films, bottles, etc. In particular, molded products that have been distributed to general consumers often suffer from significant deterioration and contamination of the resin. Therefore, as the materials to be recovered for the recycled polyester resin of the present invention, unused film scraps generated in the film manufacturing process within the factory, or films that have been shipped out of the factory and used in other factories, are preferred.

[0013] Methods of mechanical recycling include using recycled polyester resin obtained by cutting molded products into flakes, or by directly melt-extruding and cutting flakes or films and solidifying them into pellets. It is preferable to use products that have been peeled and separated from the film and washed with water. Mechanical recycling can be pre-consumer, where waste generated in a company's own factory is reprocessed and reused as material for different products, or post-consumer, where used products have been provided to the market.

[0014] One method of chemical recycling involves first removing and washing away functional layers and foreign substances, then depolymerizing the polyester using chemical equipment to return it to monomers and oligomers, and finally repolymerizing it to produce a polyester composition. The main methods of chemical recycling include the metanolysis method, glycolysis method, and hydrolysis method. The metanolysis method involves adding methanol to depolymerize the polyester, decomposing it into dimethyl terephthalate and ethylene glycol, and using these as raw materials. The glycolysis method involves adding ethylene glycol to polyethylene terephthalate, decomposing it at high temperature to produce bishydroxyethyl terephthalate (hereinafter simply referred to as BHET), and using this BHET as a raw material to produce PET. Furthermore, there is also a method in which methanol is added to this BHET, decomposing it into dimethyl terephthalate and ethylene glycol by transesterification, and purifying these to use as raw materials.

[0015] In both mechanical and chemical recycling, if functional layers or foreign matter are attached to the molded product before recycling, it is preferable to immerse it in an alkaline solvent or the like to remove the functional layers and foreign matter, wash it with water, and then extract only the film to increase its purity as recycled polyester resin. Examples of functional layers include smoothing layers provided for the running of the film, hard coat layers such as the functional layer of an optical film, refractive index adjusting layers, adhesive layers, layers for easy adhesion to other components, adhesive layers, and release layers made of silicone or non-silicone, such as release sheets. Examples of foreign matter include dust and dirt, residue from dry film resist, and residue from the material to be released.

[0016] In the present invention, it is preferable to use mechanical recycling for the recycled polyester resin, from the viewpoint of the number of processes and cost.

[0017] The recycled polyester film of the present invention contains 4,4'-stilbenedicarboxylic acid in an amount of 0.1 ppm or more and 100 ppm or less in the polyester film. The content of 4,4'-stilbenedicarboxylic acid is the content with respect to the whole recycled polyester film. When the film is composed of a plurality of layers, it is calculated based on the entire layers constituting the recycled polyester film (excluding the functional layers described later). 4,4'-Stilbenedicarboxylic acid is a substance generated by the side reaction of polyester and is represented by the following chemical formula.

[0018] [Chemical formula]

[0019] By setting the content of 4,4'-stilbenedicarboxylic acid contained in the polyester film to 0.1 ppm or more and 100 ppm or less, the quality of the recycled polyester film, particularly the b value of the color tone, is good. Note that ppm is a mass unit. The yellowness of the polyester is due to the side reaction of the polyester. The carbonyl oxygen is activated by the metal catalyst in the polyester, and the β hydrogen is extracted, generating vinyl end group components (H2C=CH-). It is considered that the polymer is colored yellow by the formation of polyene, which is a hydrocarbon group having a large number of ethylene bonds (-CH=CH-) in the molecule, by this vinyl end group. However, it is difficult to detect the amount of these polyenes. In the present invention, the yellowness of the recycled polyester film can be controlled by 4,4'-stilbenedicarboxylic acid, which is considered to be a product of the vinyl end group component. 4,4'-Stilbenedicarboxylic acid itself has no absorption in the visible light region and thus does not directly affect the b value of the color tone. Since 4,4'-stilbenedicarboxylic acid has seven conjugated double bonds, the compound can be easily detected by measurements such as liquid chromatography and fluorescence emission intensity measurement described later.

[0020] If the content of 4,4'-stilbendicarboxylic acid is less than 0.1 ppm, it is close to the detection limit and difficult to detect, and the amount of polyene generated is also thought to be small, so it does not affect the control of the color b value of the recycled polyester film of the present invention. If the content of the 4,4'-stilbendicarboxylic acid compound exceeds 100 ppm, a large amount of polyene is generated, the color b value of the recycled polyester film also increases, and it tends to take on a yellowish tint. Therefore, the present invention can be easily achieved by adjusting the recycled polyester raw material, which is prone to the generation of polyenes due to side reactions of polyester, to the desired amount of 4,4'-stilbendicarboxylic acid in the production of the recycled polyester film. It is preferable that the resin constituting the film in the recycled polyester raw material is 30% by mass or more and 100% by mass or less, more preferably 50% by mass or more and 100% by mass or less, based on the total mass. Furthermore, as can be seen from the above description, this 4,4'-stilbendicarboxylic acid is difficult to form in virgin raw materials and chemically recycled raw materials because they are not in a melt-molded state.

[0021] The b value of the color tone of the recycled polyester film of the present invention is preferably 5.0 or less. When the b value of the color tone exceeds 5.0, it becomes yellowish and the transparency is inhibited. Since the main purpose of the present invention is to suppress the yellowish color of the recycled polyester film by polyene, the lower limit is not particularly limited. However, when the b value of the color tone is -5.0 or less, it becomes a bluish color, so the lower limit is -5.0. Preferably, the b value of the color tone is -5.0 or more and 5.0 or less. By using a large amount of recycled polyester resin, the polyene content increases, and the b value of the color tone of the recycled polyester film tends to increase. Therefore, it is preferable to appropriately adjust the distribution of the resin types constituting the polyester film. Preferably, the b value of the color tone is 3.5 or less, more preferably 3.0 or less, still more preferably 2.5 or less, and even more preferably 2.0 or less. The b value of the color tone of the film tends to increase as the thickness of the film increases. The b value of the color tone of the film per 1 μm of the film thickness, that is, the value obtained by dividing the b value of the color tone of the film by the film thickness of 1 μm is 0.150 or less, more preferably 0.100 or less, and still more preferably 0.050 or less. The value obtained by dividing the b value of the color tone of the film by the film thickness of 1 μm is a calculated value rounded to four decimal places.

[0022] The recycled polyester film of the present invention preferably has a haze value of 2.0% or less. A haze value of 2.0% or less results in superior transparency quality of the film. One way to achieve a haze value of 2.0% or less is to omit additives other than polyester resin from the film. The haze value of recycled polyester resin is caused by the composition that constitutes its haze value. For example, when using polyester resin as a recycled raw material, it is preferable to remove resins other than polyester resin. For example, if a functional layer is laminated, it is preferable to peel it off by immersing it in an alkaline cleaning solution and then washing it with water. Furthermore, during processing or storage of the film, components of additives such as ultraviolet absorbers or polyester oligomers that precipitate and cloud the surface of the film should be removed by washing the film in the same way as the functional layer. To further lower the haze value of the recycled polyester film, it is even more preferable to use a polyester resin that contains as few additives as possible, such as particles or colorants. Furthermore, polyester resin raw materials generally contain a considerable amount of cyclic polymers, such as cyclic trimers, as by-products, and the formation of cyclic polymers through cyclization-depolymerization reactions from the polymer chain ends is also known. In particular, cyclic trimers, among cyclic polymers, have a lower molecular weight compared to other polymers, making them prone to precipitation on the surface of molded products. Due to heating and environmental factors over time, some of them precipitate and adhere to the surface. Molded products that have been shipped to consumers often undergo various heating processes, and this tendency is higher compared to pre-consumer molded products from the company's own factory. If these molded products are used as recycled polyester raw materials as they are, they tend to contain cyclic trimers and other substances, resulting in a high haze value. Therefore, it is preferable to include a process of washing the molded product before recycling, either in its pre-recycling state or after crushing the molded product into flake raw materials, to remove the cyclic trimers and other substances from the surface before melting it as recycled polyester raw material, in order to suppress the increase in the haze value of the recycled polyester film. This also allows for the removal of other foreign substances such as dust in addition to cyclic trimers. The haze value of the recycled polyester film is preferably 1.5% or less, and more preferably 1.0% or less.The haze value tends to increase when the diffuse transmittance is high relative to the total light transmittance of the film, i.e., the sum of diffuse transmittance and directional transmittance. This suggests that there are many particles, by-products, foreign substances, and catalyst residues in the film that contribute to diffuse transmission. In addition to determining the transparency quality of the recycled polyester film of the present invention, the haze value can also serve as a guideline when selecting a film to be used as a raw material for recycled polyester resin.

[0023] The recycled polyester film of the present invention has polyester resin as its main component, and at least one or more types of polyester resin can be used as the component. Preferably, at least one or more types of polyester resin are recycled polyester resins, and more preferably, multiple types of recycled polyester resins are used. Furthermore, it is preferable that the molded product used as the raw material is a film, as this makes it easier to determine which recycled polyester resin to use.

[0024] The layer structure of the recycled polyester film of the present invention can be a single layer or a structure of two or more layers, as long as it does not impede the effects of the present invention. In the case of a single layer, it consists only of layer A, and in the case of two or more layers, it has layer A and other layers, for example, a two-layer structure of layer A / layer B, layer B / layer A / layer B, layer A / layer B / layer C, or layer B / layer A / layer C, in which layer A is used in some of the layers of a multilayer film, or a pseudo-single-layer structure in which the other layers, i.e., layer B, layer C, etc., have the same composition as layer A. In the case of a composite film, it is preferable that the thickness of layer A is 70% or more of the total thickness of the film.

[0025] When the recycled polyester film of the present invention is used in applications requiring transparency, it is preferable that it is substantially free of particles. "Substantially free of particles" means that the amount of particles is 0.01% or less of the total mass excluding the functional layer laminated on at least one side of the film, and this configuration allows the film to be imparted with transparency. Here, "total mass excluding the functional layer" refers only to the base material of the polyester film, specifically the composition constituting the base material of the extruded film.

[0026] The recycled polyester film of the present invention is a laminated recycled polyester film having a functional layer on at least one surface of the film. The functional layer is one of the layers mentioned above, and if it is provided by coating, such as a release layer, a smooth layer, an easy-adhesion layer, or a hard coat layer, it can be applied and laminated during the film manufacturing process, or it can be applied and laminated offline on the surface of the recycled polyester film after the film has been formed, or in the case of a vapor-deposited layer, it can be deposited and laminated by methods such as sputtering.

[0027] The thickness (μm) of the recycled polyester film of the present invention is preferably 1 μm or more and 500 μm or less. This value excludes the thickness of functional layers, vapor-deposited layers, etc., provided on the film surface. If the thickness of the polyester film is less than 1 μm or more than 500 μm, thickness variations are likely to occur. Also, if the thickness is less than 1 μm, the polymer chain tends to become shorter due to the degradation of the recycled polyester resin, and the film may tear during the manufacturing process. Furthermore, the color tone b value of the film tends to decrease as the film thickness decreases, and the film thickness is more preferably 10 μm or more and 200 μm or less, and even more preferably more than 15 μm and 100 μm or less.

[0028] A preferred embodiment or example of the method for producing the polyester film of the present invention will be described below.

[0029] The recycled polyester resin is not particularly limited as long as it has polyester as its main component. Furthermore, the film of the present invention can be made by blending other polyester resins in addition to recycled polyester resin. Therefore, multiple types of recycled polyester resin can be used. When a functional layer is laminated on the polyester film to be recycled, methods such as immersing the film in an alkaline or other chemical to dissolve and remove only the functional layer, or scraping off the functional layer by matting, can be used. In these methods, it is preferable to wash with water to remove the chemical, polymer powder from scraping, and foreign matter that was originally attached when the film was reused. Furthermore, it is preferable to pulverize the film from which the functional layer has been removed and dry it at around 160°C, the glass transition temperature of the polyester resin, to remove moisture. It is preferable to reduce the moisture content after drying to about 100 ppm or less. The dried pulverized film is melted in an extruder and extruded in strand form from the die, and then rapidly cooled and solidified with cold water. It is desirable to shorten the melting time in the extruder to suppress thermal degradation. Furthermore, after the solidified strands are cut into pellet shapes with a strand cutter, it is also preferable to crystallize the surface of the pellets using a hot air dryer or the like to prevent fusion during the drying process when forming the film. By pre-measuring the amount of 4,4'-stilbendicarboxylic acid contained in each polyester resin, as well as the haze value and presence or absence of particles in the film after washing away functional layers and foreign matter that will become the raw material for the recycled polyester, and selecting the resin types and proportions that make up the recycled polyester resin, it becomes easier to adjust the amount of 4,4'-stilbendicarboxylic acid and the haze value of the recycled polyester film to the range specified by the present invention.

[0030] Next, the recycled polyester resin and other polyester resins are mixed in a predetermined ratio and dried. As an example of layer formation, the molten polyester resin is supplied to extruder A corresponding to polyester resin layer A to form a single layer, or, in the case of a two-layer composite film, the molten polyester resin is supplied to two extruders, extruder A corresponding to polyester resin layer A and extruder B corresponding to polyester resin layer B. The molten thermoplastic resin from each channel is melt-extruded into a sheet using a T-type die or the like, and then cooled and solidified on a casting drum to obtain an unstretched multilayer laminated film. In the case of a composite film with three or more layers, the resin can be supplied to the corresponding other extruder in the same manner. The melt-extrusion conditions for the polyester resin in each extruder are preferably such that the inside of the extruder is heated to the melting point (Tm) + 20°C or higher, and the area from the extruder outlet to the slit die is heated to the melting point (Tm) + 15°C or lower. The polymer melted and extruded in the extruder is filtered through a filter. Even extremely small foreign particles can become large protrusion defects if they enter the film, so it is effective to use a high-precision filter that can capture more than 95% of foreign particles larger than, for example, 3 μm. Next, the material is extruded into a sheet through a slit-shaped slit die and cooled with a casting roll to create an unstretched film.

[0031] The stretching method may be sequential biaxial stretching or simultaneous biaxial stretching. Simultaneous biaxial stretching is preferable because it does not involve stretching by rolls, thus suppressing localized heating unevenness on the film surface and obtaining uniform quality, as well as suppressing the speed difference at the contact points between the film and rolls during stretching, and the occurrence of scratches due to the transfer of minute scratches on the rolls. Sequential biaxial stretching is preferable because it allows for individual and precise control of the characteristics in the longitudinal and width directions.

[0032] In simultaneous biaxial stretching, the unstretched film is first stretched simultaneously in the longitudinal and width directions at a stretching temperature of 80°C to 125°C, preferably 85°C to 120°C. If the stretching temperature is lower than 80°C, the film is prone to tearing, and if the stretching temperature is higher than 125°C, sufficient strength may not be obtained. Furthermore, from the viewpoint of preventing uneven stretching, the total stretching ratio in the longitudinal and width directions is 4 to 20 times, preferably 6 to 15 times. If the total stretching ratio is less than 4 times, it is difficult to obtain sufficient strength. On the other hand, if the ratio is greater than 20 times, film tearing is more likely to occur, and it may be difficult to manufacture a stable film. To obtain the required strength, it is preferable to perform another stretching at a temperature of 140°C to 200°C, preferably 160°C to 190°C, to a length of 1.02 to 1.5 times, preferably 1.05 to 1.2 times, in the longitudinal and / or widthwise directions, resulting in a total stretching ratio of 3.0 to 4.5 times, preferably 3.2 to 4.0 times, in the longitudinal direction, and 3.2 to 5.0 times, preferably 3.5 to 4.3 times, in the widthwise direction. After that, heat setting is performed at 205°C to 240°C, preferably 220°C to 240°C, for 0.5 seconds to 20 seconds, preferably 1.0 second to 15 seconds. If the heat setting temperature is lower than 205°C, the thermal crystallization of the film will not progress, which may make it difficult to stabilize the target dimensional change rate, etc. Furthermore, in order to stabilize the physical properties of the film, the temperature difference between the top and bottom of the film during heat treatment should be 20°C or less, more preferably 10°C or less, and even more preferably 5°C or less. If the temperature difference between the top and bottom of the film is greater than 20°C, it may cause deterioration of flatness during heat treatment. Subsequently, a relaxation treatment of 0.5% to 7.0% is performed in the longitudinal and / or widthwise directions during the slow cooling section.

[0033] On the other hand, the film of the present invention can also be manufactured using sequential biaxial stretching. For the initial stretching in the longitudinal direction, the stretching temperature is 80°C to 125°C, preferably 85°C to 120°C. If the stretching temperature is lower than 80°C, the film is prone to tearing, and if the stretching temperature is higher than 125°C, the film surface may be susceptible to thermal damage. The stretching ratio is 3.0 to 4.5 times in the longitudinal direction, preferably 3.2 to 4.0 times. For the subsequent stretching in the width direction, the stretching temperature is 80°C to 125°C, preferably 85°C to 120°C. The stretching ratio is 3.2 to 5.0 times in the width direction, preferably 3.5 to 4.3 times. If the temperature and stretching ratio are outside this range, problems such as uneven stretching or film tearing may occur, making it difficult to obtain a film that meets the characteristics of the present invention. In particular, when the stretching ratio during longitudinal stretching is high, the dimensional change rate in the longitudinal direction at a heating temperature of 140°C tends to exceed 0.20%. The film, which has been sequentially biaxially stretched in this manner, is then subjected to re-longitudinal stretching similar to the simultaneous biaxial stretching described above, or it is stretched transversely as is, then passed through an intermediate cooling section, and then heat-set at 205°C to 240°C, preferably 210°C to 230°C, for 0.5 seconds to 20 seconds, preferably 1.0 second to 15 seconds. If the heat-set temperature is lower than 205°C, the crystallization of the film will not progress, resulting in an unstable structure, and the target dimensional change rate and other characteristics may not be obtained. After that, a relaxation treatment of 0.5% to 7.0% in the longitudinal and / or width directions is performed in a slow-cooling section.

[0034] In sequential biaxial stretching, the longitudinal stretching process is a process in which the film and rolls come into contact, and scratches are likely to occur due to the difference in peripheral speed between the rolls and the film. Therefore, a drive system that allows the peripheral speed of each roll to be set individually is preferred. In the longitudinal stretching process, the material of the transport rolls is selected based on whether the unstretched film is heated to above its glass transition point before stretching, or whether it is transported to the stretching zone while kept at a temperature below its glass transition point and then heated all at once during stretching. If the unstretched film is heated to above its glass transition point before stretching, it is preferable to select from non-stick silicone rolls, ceramics, or "Teflon" (registered trademark) to prevent adhesion due to heating. If the unstretched film is transported to the stretching zone while kept at a temperature below its glass transition point and then heated all at once during stretching, it is preferable to use metal rolls with a surface roughness Ra of 0.2 μm or more and 0.6 μm or less, with a surface treatment of hard chromium or tungsten carbide, for the transport rolls in the preheating zone. Furthermore, during heat treatment, relaxation may occur in the longitudinal and / or transverse directions.

[0035] The film obtained in this manner exhibits excellent color tone (b-value), making it suitable for a variety of applications, including optical films for polarizing plate protection and conductive panel substrates, release films, and electronic component protective films. Furthermore, it offers advantages in terms of environmental protection due to the large amount of recycled polyester resin that can be used. [Examples]

[0036] The present invention will be described in more detail below based on comparative examples and embodiments. However, the present invention is not limited to the embodiments described below.

[0037] [Method for evaluating physical properties] (1) Film color tone, b value and evaluation Five 10cm x 10cm squares were cut from the polyester film and used as sample pieces. The color tone of the film was measured using the transmission method (JIS-Z8722 (2009)) with a NIPPON DENSHOKU ZE-2000 spectrophotometer. Five measurements were taken from each side of the film, and all 10 data points were averaged and rounded to two decimal places to obtain the color tone b value. The color tone was then evaluated as follows. S:2.0 or less A: 2.5 or less B: 3.0 or less C: exceeds 3.0.

[0038] Furthermore, the b-value obtained from the above was divided by the thickness determined by the film thickness in (2) below, and the result was rounded to four decimal places to obtain the b-value per μm.

[0039] (2) Film thickness The sample pieces cut out in (1) were measured according to the micrometer method (JIS-C-2151 (2019)). If the thickness was 50 μm or less, 10 pieces were stacked and measured, and the thickness per piece was obtained by dividing by the number of pieces. The thickness was rounded to one decimal place to obtain an integer thickness.

[0040] (3) Methods for detecting chemical substances The detection of chemical substances contained in recycled polyester resin compositions and polyester resin products was performed by high-performance liquid chromatography (HPLC) on bis(2-hydroxyethyl) terephthalate (BHET) compositions obtained by depolymerizing recycled polyester resin compositions and polyester resin products.

[0041] (4) Depolymerization of recycled polyester resin composition The depolymerization of the recycled polyester resin composition was carried out by the following method.

[0042] 100 parts by mass of recycled polyester resin composition, 200 parts by mass of ethylene glycol, and 0.5 parts by mass of sodium hydroxide were placed in a reaction vessel, and depolymerization was carried out while gradually increasing the temperature. When the internal temperature reached 203°C, depolymerization was stopped, and the mixture was allowed to cool to 25°C. A depolymer product containing impurities as solid content was obtained using solid-liquid separation.

[0043] To this depolymerized product, 1000 parts by mass of water were added per 100 parts by mass of the recycled polyester resin composition used, and after dissolving at 100°C, the mixture was filtered using a 1.0 μm filter. The resulting filtrate was cooled to crystallize the BHET composition, and wet BHET composition crystals were obtained as solid matter by filtration using 5B filter paper. The obtained wet BHET composition crystals were dried to obtain the BHET composition.

[0044] (5)HPLC analysis HPLC analysis was performed using the following method. Equipment: Ultimate3000 (Thermo Fisher Scientific) Column: Reverse-phase column Mobile phase: A: 19 mmol / L ammonium acetate aqueous solution, B: acetonitrile grandient conditions The compound content was calculated using a calibration curve created by HPLC.

[0045] (6) Film haze value Five 10cm x 10cm squares were cut from the sample, and these were used as sample pieces. Surface haze was measured on these sample pieces using a HZ-V3 haze meter (manufactured by Suga Test Instruments Co., Ltd.) (old JIS-K-7105 (1981)). Measurements were taken once from one side and once from the other side of each sample piece. The data from all 10 samples (5 pieces) was averaged, rounded to two decimal places, and this was obtained as the haze value (%), which was then evaluated as follows. S:1.0 or less A: 1.5 or less B: 2.0 or less.

[0046] (7) Determination of particle content in the film After removing the functional layer and foreign matter from the film by alkaline cleaning, the film is dissolved in a solvent selected that dissolves the polymer but not the particles. Subsequently, the particles are centrifuged from the polymer, and films in which the ratio of particles to the total mass (mass%) is 0.01% by mass or less relative to the polymer are determined to be substantially particle-free.

[0047] [Preparation and physical properties of polyethylene terephthalate resin] (Polyester resin a (virgin raw material)) A slurry consisting of 86 parts by mass of petroleum-derived terephthalic acid and 37 parts by mass of petroleum-derived ethylene glycol (1.15 times the molar amount of terephthalic acid) was gradually added to an esterification reaction apparatus containing 105 parts by mass of petroleum-derived bishydroxyethyl terephthalate melted at 255°C, and the esterification reaction was allowed to proceed. The temperature in the reaction system was controlled to be between 245 and 255°C, and the esterification reaction was terminated when the reaction rate reached 95%.

[0048] The 105 parts by mass of the esterified reaction product at 255°C (equivalent to 100 parts by mass of the polyester resin composition) obtained in this way were transferred to a polymerization apparatus. 0.01 parts by mass of antimony trioxide was added as a polymerization catalyst, based on the amount to be added when 100 parts by mass of the polyester resin composition is used. The temperature inside the apparatus was then gradually increased to 290°C while the pressure inside the apparatus was reduced to 1 Torr or less. As the polymerization reaction progressed, the viscosity of the reactants increased, and the reaction was terminated when the increase in stirring torque of the reactants reached the target value for the end of polymerization. The polyester was then discharged from the polymerization reactor into a water tank. The discharged polyester was cooled in the water tank and chipped with a cutter to obtain a polyester resin composition. This was designated as polyester resin a. The amount of 4,4'-stilbendicarboxylic acid in the obtained polyester resin a was 0 ppm.

[0049] [Reference Example 1 (Method for Manufacturing Polyester Film)] A polyester resin a (virgin raw material) that was substantially free of inert particles was prepared, dried at 180°C for 5 hours under reduced pressure of 3 torr, and then put into a melt extruder and melted at 290°C. After filtration through five leaf disc filters with a filtration accuracy of 8 μm, the material was extruded into a sheet from a T-shaped die. The extruded sheet was cooled and solidified on a mirror-finished casting drum at a surface temperature of 20°C by electrostatic casting to obtain a substantially amorphous unstretched film.

[0050] This unstretched film was first preheated to 75°C using a continuously arranged group of rolls, then heated to 90°C using rolls, and stretched longitudinally (in the direction of film travel) at a stretching ratio of 3.3 times. Next, it was cooled using a group of cooling rolls to obtain a uniaxially oriented (uniaxially stretched) film.

[0051] Subsequently, the film was placed in a stent oven and stretched transversely to 4.0 times its original length (in a direction perpendicular to the film's direction of travel) under conditions of 100°C. Following this, heat treatment and 5% widthwise relaxation were performed in the same oven at 230°C, and after cooling to 100°C, a biaxially oriented polyester film with a thickness of 50 μm was obtained. The physical properties of the obtained film are shown in Tables 1 and 2.

[0052] (Recycled polyester resin b (flake raw material)) The polyethylene terephthalate film described in [Reference Example 1 (Method for Manufacturing Polyester Film)] above was crushed, the crushed recovered polyester was melted in an extruder, and the resulting strand was extruded from a die and rapidly cooled and solidified with cold water. The solidified strand was cut into pellets using a strand cutter to obtain polyethylene terephthalate resin. This was designated as recycled polyester resin b. The amount of 4,4'-stilbenecarboxylic acid in the obtained polyethylene terephthalate resin was 1.0 ppm.

[0053] (Recycled polyester raw material c-1) A used polyethylene terephthalate film α, 50 μm thick, was collected. It consisted of a silicone release layer and a base layer (easy-adhesion layer) laminated sequentially on one side of the film, and an easy-slip layer laminated on the other side. The film was immersed in a 1 mol / L NaOH aqueous solution at 80°C for 15 minutes to remove the silicone release layer, base layer, and easy-slip layer. After washing and rinsing with water, the film was dried by blowing hot air. The haze value of the dried film was 1.3%, and no particles were present. The dried film was broken to obtain a flake-like polyester resin composition, which was designated as recycled polyester resin c-1. The 4,4'-stilbenecarboxylic acid content of the obtained polyester resin was 4.0 ppm.

[0054] (Recycled polyester raw material c-2) A used polyethylene terephthalate film β, in which a silicone release layer and a base layer (easy-adhesion layer) were sequentially laminated on one side of the film and an easy-slip layer was laminated on the other side, was recovered, and a polyester resin composition was obtained in the same manner as in c-1. This was designated as recycled polyester resin c-2. The amount of 4,4'-stilbenecarboxylic acid in the obtained polyester resin was 19.5 ppm. The haze value of the film after drying in the regeneration process of the recycled polyester resin was 1.4%, and no particles were contained in the film.

[0055] (Recycled polyester raw material c-3) A used polyethylene terephthalate film δ for process release, in which a silicone release layer and a base layer (easy-adhesion layer) were sequentially laminated on one side of the film and an easy-slip layer was laminated on the other side, was recovered, and a polyester resin composition was obtained in the same manner as in c-1. This was designated as recycled polyester resin c-3. The amount of 4,4'-stilbenecarboxylic acid in the obtained polyester resin was 3.0 ppm. The haze value of the film after drying in the regeneration process of the recycled polyester resin was 0.9%, and no particles were contained in the film.

[0056] [Examples 1-12] A polyester film was obtained in the same manner as in [Reference Example 1 (Method for Manufacturing Polyester Film)], except that the content of the polyester resin constituting the film and the film thickness were as shown in Table 1. The detected amounts of compounds in the obtained polyester film, as well as the film's color b value and haze value, are shown in Table 2.

[0057] [Table 1]

[0058] [Table 2]

[0059] Considering the results in Tables 1 and 2, it can be seen that, when comparing films of the same thickness, the color b value of the film increases as the amount of 4,4'-stilbendicarboxylic acid increases. It can also be seen that increasing the thickness increases the color b value. Therefore, by checking the amount of 4,4'-stilbendicarboxylic acid in the recycled polyester resin used in the recycled polyester film in advance and adjusting the type, composition, and thickness of the polyester resin, it is possible to obtain a recycled polyester film in which the increase in color b value is suppressed, even when using a large amount of recycled polyester resin and multiple types of recycled polyester resin.

Claims

1. A recycled polyester film in which the amount of 4,4'-stilbendicarboxylic acid contained in the polyester film is 0.1 ppm or more and 100 ppm or less.

2. The recycled polyester film according to claim 1, wherein the color b value is 5.0 or less.

3. The recycled polyester film according to claim 1, wherein the color tone b value of the film per 1 μm of film thickness is 0.150 or less.

4. The recycled polyester film according to claim 1, wherein the haze value is 2.0% or less.

5. The recycled polyester film according to claim 1, wherein the thickness is 1 μm or more and 500 μm or less.

6. The recycled polyester film according to claim 1, comprising 50% by mass or more of recycled polyester resin.

7. A recycled polyester film according to claim 1, which is substantially free of particles.

8. A laminated recycled polyester film having a functional layer on at least one surface of the recycled polyester film according to any one of claims 1 to 7.

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