Cavity-containing polyester film and method for manufacturing the same

Abstract

This invention enables the application of thermoplastic resins recycled from injection-molded products to film products, and further provides a void-containing polyester film with excellent void formation, opacity, and whiteness. [Solution] A cavity-containing polyester film containing a layer (layer A) that contains a cavity inside, wherein layer A comprises at least a polyester resin, thermoplastic resin a and thermoplastic resin b, and thermoplastic resin a is a thermoplastic resin recycled from an injection-molded product.

Description

[Technical Field]

[0001] This invention relates to a cavity-containing polyester film using a thermoplastic resin recycled from injection-molded products as a non-compatible resin used as a cavity-inducing agent. [Background technology]

[0002] Synthetic paper, a paper substitute primarily composed of synthetic resins, offers superior water resistance, moisture absorption, dimensional stability, and surface stability compared to natural paper. It is widely used in labels, stickers, posters, recording paper, and packaging materials. While polyethylene, polypropylene, and polyester resins are commonly used as raw materials for synthetic paper, polyester resins, particularly polyethylene terephthalate, are widely used due to their excellent mechanical and thermal properties.

[0003] Methods for obtaining films with functions similar to paper generally include incorporating a large number of microscopic cavities into the film, or roughening a flat film by performing surface treatments such as sandblasting, chemical etching, or matting. Among these, the former method of incorporating a large number of microscopic cavities into the film is widely adopted because it not only provides paper-like opacity and whiteness, but also reduces the cost per unit area by making the film itself lighter, and provides appropriate flexibility and cushioning, resulting in superior image clarity during printing.

[0004] A common method for creating fine cavities within a film involves mixing an incompatible thermoplastic resin (hereinafter referred to as the incompatible resin) with a polyester resin to obtain a sheet in which the incompatible resin is dispersed in the polyester resin, and then stretching the sheet in at least one axial direction to create cavities through interfacial delamination between the polyester resin and the incompatible resin. For the incompatible resin used to create cavities in the polyester resin, polyolefin resins such as polyethylene resins, polypropylene resins, and polymethylpentene resins (see, for example, Patent Documents 1-3) and polystyrene resins (see, for example, Patent Documents 4 and 5) are preferably used. Among these, polypropylene resins are particularly preferred in terms of cavity creation properties and cost-effectiveness.

[0005] With the enforcement of the Plastic Resource Recycling Promotion Act in 2022, the use of recyclable materials and biomass materials has been increasingly demanded in recent years. Recycled materials are being developed for injection-molded products (e.g., CD cases, DVD cases), which are the most common products using thermoplastic resins. One known method of utilization is to add them to injection-molded products, as shown in Patent Documents 6 and 7. Furthermore, as shown in Patent Documents 8 and 9, injection-molded products with improved physical properties of recycled materials have been developed by using additives.

[0006] However, the recycled products described in Patent Documents 6-9 are limited to injection-molded products and are insufficient for the widespread use of recycled materials. Recycled materials for injection-molded products made of thermoplastic resins have low viscosity and a wide molecular weight distribution due to degradation, which has limited the products in which they can be used. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Unexamined Patent Publication No. 49-134755 [Patent Document 2] Japanese Patent Application Publication No. 2-284929 [Patent Document 3] Japanese Patent Application Publication No. 2-180933 [Patent Document 4] Special Publication No. 54-29550 [Patent Document 5] Japanese Patent Application Publication No. 11-116716 [Patent Document 6] Japanese Patent Publication No. 2024-159325 [Patent Document 7] Japanese Patent Publication No. 2024-144954 [Patent Document 8] Japanese Patent Publication No. 2018-87294 [Patent Document 9] Japanese Patent Publication No. 2024-137917 [Overview of the project] [Problems that the invention aims to solve]

[0008] The inventors have begun investigating the application of recycled thermoplastic resin from injection-molded products to film products, and in particular, the use of recycled thermoplastic resin as a cavity-forming agent in cavity-containing polyester films. However, thermoplastic resins recycled from injection-molded products have a relatively low molecular weight in order to ensure fluidity during injection molding and to suppress stress and strain in the injection-molded products to improve dimensional stability. As a result, it was sometimes difficult to create sufficient voids to achieve high opacity and whiteness in film products.

[0009] This invention has been made in view of the above circumstances, and its purpose is to provide a cavity-containing polyester film that uses a thermoplastic resin recycled from injection-molded products as a cavity-forming agent, and which exhibits excellent cavity formation properties, as well as excellent opacity and whiteness. [Means for solving the problem]

[0010] The inventors of the present invention have found that in a hollow-containing polyester film, by adding a thermoplastic resin separately from the thermoplastic resin recycled from an injection molded product as a void-forming agent, the void-forming property can be improved, and a hollow-containing polyester film with excellent concealability and whiteness can be obtained.

[0011] That is, the hollow-containing polyester film of the present invention has the following composition. [Item 1] A layer (layer A) containing voids inside, wherein the layer A contains at least a polyester resin, a thermoplastic resin a, and a thermoplastic resin b, and the thermoplastic resin a is a thermoplastic resin recycled from an injection molded product. [Item 2] The hollow-containing polyester film according to item 1, wherein a layer (layer B) made of a polyester resin containing inorganic particles is laminated on at least one surface of the layer (layer A) containing voids inside. [Item 3] The hollow-containing polyester film according to item 1, wherein the weight average molecular weight of the thermoplastic resin b is higher than the weight average molecular weight of the thermoplastic resin a, or the melt viscosity of the thermoplastic resin b is higher than the melt viscosity of the thermoplastic resin a. [Item 4] The hollow-containing polyester film according to item 1, wherein the thermoplastic resin a is an olefin resin. [Item 5] The hollow-containing polyester film according to item 4, wherein the olefin resin a satisfies the following (1) and (2). (1) The weight average molecular weight Mw measured by gel permeation chromatography is in the range of 100,000 to 500,000, and the molecular weight distribution represented by weight average molecular weight Mw / number average molecular weight Mn is 4 to 8. (2) The melt viscosity measured by a capillary graph is 50 to 100 PA·s. [Item 6] The hollow-containing polyester film according to item 1, wherein the thermoplastic resin b is an olefin resin. [Item 7] The cavity-containing polyester film according to item 6, wherein the olefin resin b satisfies the following (1) and (2). (1) The weight average molecular weight Mw measured by gel permeation chromatography is within the range of 500,000 to 700,000, and the molecular weight distribution represented by weight average molecular weight Mw / number average molecular weight Mn is 4 to 8 (2) The melt viscosity measured by a capillary graph is 200 to 700 Pa·s [Item 8] The cavity-containing polyester film according to item 1, wherein the injection molded product includes at least a CD case or a DVD case. [Item 9] The apparent density is 0.7 to 1.1 g / cm 3 The cavity-containing polyester film according to item 1. [Item 10] The cavity-containing polyester film according to item 1, wherein the inorganic particles in the B layer are titanium oxide. [Item 11] The cavity-containing polyester film according to item 1, having a total light transmittance of less than 55% and an optical density of 0.15 or more. [Item 12] A method for producing a cavity-containing polyester film, comprising the following steps. (Step a) A step of preparing a thermoplastic resin a and a thermoplastic resin b (Step b) A step of melt-mixing the thermoplastic resin a and the thermoplastic resin b to obtain a polyester resin composition (Step c) A step of extruding the polyester resin composition into a sheet shape to obtain a sheet-like material (Step d) A step of stretching the sheet-like material in at least one direction (However, the thermoplastic resin a is a thermoplastic resin recycled from an injection molded product) [Advantages of the Invention]

[0012] The inventors have found that in a cavity-containing polyester film, by using a thermoplastic resin recycled from injection-molded products and a thermoplastic resin different from the recycled thermoplastic resin as cavity-forming agents, a cavity-containing polyester film with excellent cavity-forming properties, opacity, and whiteness can be obtained while maintaining environmental friendliness. [Modes for carrying out the invention]

[0013] The present invention will be described in detail below. First, in this specification, the notation "○○~△△" (where ○○ and △△ are numbers) indicates that it is greater than or equal to ○○ and less than or equal to △△. Furthermore, ○○ and △△ are not necessarily fixed combinations; they are independent as greater than or equal to ○○ and less than or equal to △△, and can be arbitrarily combined with the numerical value of "~" in the same item, representing greater than or equal to and less than or equal to. In addition, "greater than or equal to ○○" means that it is the same as ○○ or greater than ○○, and "less than or equal to △△" means that it is the same as △△ or less than or equal to △△. The cavity-containing polyester film of the present invention has at least one layer (layer A) containing a cavity inside, and layer A is composed of a composition containing at least a polyester resin, a thermoplastic resin a which is a thermoplastic resin recycled from an injection-molded product, and a thermoplastic resin b. Thermoplastic resin b is a thermoplastic resin different from thermoplastic resin a, and here, different thermoplastic resins include those that are the same type of resin but have different molecular weights or melt viscosity.

[0014] Furthermore, a layer (layer B) made of a polyester resin containing inorganic particles may be laminated on at least one side of the layer A. Laminating layer B can further improve opacity.

[0015] In the cavity-containing polyester film of the present invention, the polyester resin that forms the main component of layers A and B is a polymer synthesized from a dicarboxylic acid or its ester-forming derivative and a diol or its ester-forming derivative. Typical examples of such polyester resins include polyethylene terephthalate, polybutylene terephthalate, and polyethylene-2,6-naphthalate, with polyethylene terephthalate being preferred from the viewpoint of mechanical properties, heat resistance, and cost.

[0016] Furthermore, these polyester resins may be copolymerized with other components, as long as the objectives of the present invention are not impaired. Specifically, examples of copolymerized components include isophthalic acid, naphthalenedicarboxylic acid, 4,4-diphenyldicarboxylic acid, adipic acid, sebacic acid and its ester-forming derivatives as dicarboxylic acid components. Examples of diol components include ethylene glycol, hexamethylene glycol, neopentyl glycol, and cyclohexanedimethanol. Polyoxyalkylene glycols such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol are also examples. The copolymerization amount is preferably 10 mol% or less per constituting repeating unit, and more preferably 5 mol% or less. Alternatively, a homopolyester such as polyethylene terephthalate may be blended with a copolymerized polyester. In this case, the amount of copolymerized component in the blended polyester resin composition is the same as described above.

[0017] One method for producing polyester resins involves first using the aforementioned dicarboxylic acid or its ester-forming derivative and a diol or its ester-forming derivative as the main starting materials, carrying out esterification or transesterification reactions according to conventional methods, and then further carrying out polycondensation reactions under high temperature and reduced pressure.

[0018] The intrinsic viscosity of the polyester resin is preferably in the range of 0.50 to 0.9 dl / g, and more preferably in the range of 0.55 to 0.85 dl / g, from the viewpoint of film-forming properties.

[0019] Next, in the present invention, thermoplastic resin a can be used as a cavity-forming agent. Examples of injection-molded products include electrical appliance casings, industrial products such as smartphone covers, as well as daily necessities such as cups and plates, and CD and DVD cases. Preferred types of thermoplastic resins include olefin resins, polyester resins, polyamide resins, polyvinyl chloride, polystyrene, and ABS resin. Olefin resins are particularly suitable and provide sufficient effectiveness as cavity-forming agents. In the present invention, a cavity-containing polyester film using recycled olefin resin (olefin resin a) from injection-molded products as a cavity-forming agent can have its cavity-forming properties adjusted and enhanced by blending it with thermoplastic resin b, even when using recycled raw materials. This allows for sufficient lightness and the production of a cavity-containing polyester film with superior opacity and whiteness. The injection-molded product described above may be crushed, washed, dried, and then further finely ground if necessary. The resulting pulverized product may be used directly in the manufacture of film, or it may be melted and processed into pellets.

[0020] As thermoplastic resin a, styrene-based resins and olefin-based resins are preferred, with olefin-based resins being more preferred. Examples of olefin-based resins include polymethylpentene (TPX), polypropylene (PP), and polyethylene (PE). Among these, PP is particularly preferred from the viewpoint of industrial use because it is cheaper and more versatile. The same applies to thermoplastic resin b; thermoplastic resin a and thermoplastic resin b do not need to be the same type of resin, but they are preferably the same type of resin. That is, thermoplastic resin b is also preferably an olefin-based resin, and polypropylene is even more preferred. In the following, if thermoplastic resin b is an olefin-based resin, it may be referred to as olefin-based resin b, and if it is polypropylene, it may be referred to as polypropylene b. Note that for resins to be of the same type, it is not necessary for the resins to be completely identical at the monomer level. For example, in the case of polypropylene, in addition to propylene monomer, small amounts of ethylene or other α-olefins may be copolymerized, but if the copolymerization composition is 90% or more, preferably 95% or more, it can be considered the same type of resin. Specifically, polypropylene 1 with propylene / ethylene / other α-olefins = 95 / 2 / 3 and polypropylene / ethylene / other α-olefins = 97 / 2 / 1 have a copolymer composition that is 99% (95+2+1) identical, and can therefore be considered to be the same type.

[0021] Thermoplastic resin b may be unused chipped material after polymerization, or recycled material, as long as it possesses the properties described below. If recycled, materials recycled from calendered products, extruded products, films, etc., are preferable as they have relatively high molecular weights. In the case of polypropylene, recycled materials from unstretched or stretched polypropylene films and sheets, or by-products from their manufacturing processes, are preferred examples.

[0022] It is preferable that thermoplastic resin b has a weight-average molecular weight higher than that of thermoplastic resin a, or a melt viscosity at 280°C higher than that of thermoplastic resin a. By using a thermoplastic resin b with a higher weight-average molecular weight or melt viscosity than thermoplastic resin a, the cavity-forming agent is less likely to collapse during the stretching process that generates cavities, thereby improving cavity formation.

[0023] The following explanation will use olefin resins, which are preferred thermoplastic resins, as an example. Note that the numerical values ​​for olefin resins in the following explanation are examples for polypropylene, but they can also be applied to other olefin resins. The olefin resin (olefin resin a) recycled from injection-molded products used in this invention is originally used in injection-molded products, so its weight-average molecular weight (Mw) is preferably 100,000 to 500,000, and more preferably 200,000 to 400,000. When the weight-average molecular weight (Mw) of olefin resin a is within the above range, olefin resin a can be used for film applications and can also be used as a cavity-forming agent to create cavities. Furthermore, for olefin resin b blended to obtain a film with excellent cavity-forming properties and excellent opacity and whiteness, a higher molecular weight than olefin resin a is preferable, specifically, a weight-average molecular weight (Mw) of 500,000 to 700,000 is preferable. A weight-average molecular weight of 500,000 or more is preferable because it makes it easier to improve cavity formation. When the weight-average molecular weight (Mw) is 700,000 or less, the dispersibility of the olefin-based dispersed particles is good, fine cavities are formed, and sufficient opacity is obtained, which is preferable.

[0024] Furthermore, the olefin resin a used in this invention preferably has a molecular weight distribution (Mw / Mn), which is the ratio of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn), of 4 to 8, and more preferably 5 to 7. Mw / Mn is an indicator that represents the breadth of the molecular weight distribution, and the larger this value, the wider the molecular weight distribution. If Mw / Mn is 8 or less, low molecular weight components are suppressed and cavity formation is good, which is preferable. Also, if Mw / Mn is 4 or more, it is suitable for industrial production from a cost standpoint. Note that the weight-average molecular weight (Mw) and number-average molecular weight (Mn) are values ​​measured by gel permeation chromatography (GPC).

[0025] The olefin resin a used in this invention is originally used in injection molded products, so a melt viscosity of 50 to 100 Pa·s is preferable. When the melt viscosity of olefin resin a is within the above range, olefin resin a can be used for film applications and can also create cavities as a cavity-forming agent. Furthermore, for olefin resin b blended to obtain a film with excellent cavity-forming properties, opacity, and whiteness, a higher melt viscosity than olefin resin a is preferable, specifically 200 to 700 Pa·s is preferable, 300 to 600 Pa·s is more preferable, and 400 to 500 Pa·s is even more preferable. A melt viscosity of 200 Pa·s or higher is preferable because it makes it easier to improve cavity formation. When the melt viscosity is 700 Pa·s or lower, the dispersibility of olefin-based dispersed particles is good, fine cavities are formed, and sufficient opacity is obtained, which is preferable. Note that the melt viscosity is a value measured by a capillary graph.

[0026] The blend ratio of olefin resin a and olefin resin b is preferably such that the proportion of olefin resin a is 1 to 50% by mass, more preferably 5 to 40% by mass, and even more preferably 10 to 30% by mass, of the total olefin resin, in order to balance the environmental benefits of using recycled resin with the benefits of enhancing cavity formation by using different olefin resins. If the proportion of olefin resin a is 50% by mass or less, it is preferable because a film with better cavity formation, higher opacity, and whiteness can be obtained. The higher the proportion of olefin resin b (the lower the proportion of olefin resin a), the higher the cavity formation, the lower the apparent density, and the higher the opacity and whiteness can be. The appropriate range for the amount of olefin resin a to be added can be determined by considering environmental benefits and the characteristics of the resulting film.

[0027] For example, from the viewpoint of environmental protection, the proportion of olefin resin a is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more. From the viewpoint of enhancing cavity formation, it is preferably 30% by mass or less, more preferably 25% by mass or less, and particularly preferably 20% by mass or less.

[0028] In the cavity-containing polyester film of the present invention, the content of the olefin resin is preferably 3 to 35% by weight, and more preferably 5 to 30% by weight, relative to the total amount of each component in layer A, from the viewpoint of cavity formation and film-forming properties. By setting the olefin resin content to 3% by weight or more, cavities can be formed to obtain sufficient lightness and cushioning properties. On the other hand, by setting the olefin resin content to 35% by weight or less, good film-forming properties can be obtained.

[0029] Furthermore, other miscible resins besides olefin resins may be included as long as they do not impair the objectives of the present invention. However, it is preferable that the olefin resin is present in an amount of 90% by weight or more, more preferably 95% by weight or more, and most preferably 100% by weight, relative to the total amount of miscible resins in layer A. In addition, it is preferable that dispersants such as polyethylene glycol and surfactants are not included from the viewpoint of whiteness and cavity formation.

[0030] Furthermore, to the extent that the objectives of the present invention are not impaired, these polyester resins or olefin resins may contain small amounts of other polymers, antioxidants, heat stabilizers, matting agents, pigments, UV absorbers, fluorescent whitening agents, plasticizers, or other additives. In particular, it is preferable to include antioxidants or heat stabilizers to suppress oxidative degradation of the olefin resin. The types of antioxidants and heat stabilizers are not particularly limited, but examples include hindered phenols, phosphorus, and hindered amines, which may be used individually or in combination. The amount added is preferably in the range of 1 to 50,000 ppm relative to the entire film. In this invention, excellent whiteness can be ensured even without adding a fluorescent whitening agent to the film.

[0031] In the present invention, the cavity-containing polyester film may contain inorganic particles in the polyester resin or olefin resin as needed to improve opacity and whiteness. Examples of such inorganic particles include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, titanium dioxide, and zinc sulfide, but titanium dioxide, calcium carbonate, and barium sulfate are preferred from the viewpoint of opacity and whiteness. These inorganic particles may be used individually or in combination of two or more types. These particles can be incorporated into the film by adding them to the polyester resin or olefin resin in advance.

[0032] In the present invention, the method of mixing inorganic particles with a polyester resin or an olefin resin is not particularly limited, and examples include a method of dry blending the polyester resin and the olefin resin and then directly feeding them into a film-making machine, or a method of dry blending the polyester resin and the olefin resin and then melt-kneading them using various general kneaders to form a masterbatch.

[0033] The cavity-containing polyester film of the present invention has a layer structure consisting of a composition containing a polyester resin and an olefin resin, and has a layer (layer A) containing a cavity inside, and a layer (layer B) made of polyester resin containing inorganic particles may be laminated on at least one side of layer A. If layer A containing the olefin resin is exposed to the surface, some of the exposed olefin dispersion particles may cause process contamination such as roll staining. Furthermore, covering layer A containing recycled olefin resin from injection molded products with layer B containing inorganic pigment has the effect of preventing a decrease in whiteness.

[0034] The thickness of layer B (the sum of the thicknesses of both outermost layers) is preferably in the range of 1 to 40% of the total film thickness, and more preferably in the range of 5 to 30%, from the viewpoint of cavity formation and suppression of olefin resin exposure. When the thickness of layer B is 1% or more, it is preferable because it can suppress the exposure of the olefin resin. On the other hand, when the thickness of layer B is greater than 40%, it tends to be difficult to form cavities necessary to obtain sufficient lightness and cushioning.

[0035] In the present invention, examples of inorganic particles to be contained in layer B include silica, kaolinite, talc, calcium carbonate, zeolite, alumina, barium sulfate, titanium dioxide, and zinc sulfide. However, from the viewpoint of opacity and whiteness, titanium dioxide, calcium carbonate, and barium sulfate are preferred, and titanium dioxide is particularly preferred. Furthermore, these inorganic particles may be used individually or in combination of two or more types. These particles can be incorporated into the film by adding them to a polyester resin in advance.

[0036] The amount of inorganic particles added to layer B is not particularly limited, but is preferably 1 to 35% by weight of the entire layer B, and more preferably 2 to 30% by weight. When the amount added is 1% by weight or more, the opacity and whiteness can be improved, and when the amount added is 35% by weight or less, the film-forming properties and the mechanical strength of the film are good, which is preferable.

[0037] Furthermore, the cavity-containing polyester film of the present invention may have a coating layer on at least one side in order to improve the wettability and adhesion to printing inks, coatings, and the like. A polyester resin is preferred as the compound constituting the coating layer, but other compounds known as means of improving the adhesion of ordinary polyester films, such as polyurethane resins, polyester urethane resins, and acrylic resins, can also be applied.

[0038] Commonly used methods for applying the coating layer include gravure coating, kiss coating, dip coating, spray coating, curtain coating, air knife coating, plate coating, and reverse roll coating. The coating process can be carried out before film stretching, after longitudinal stretching, or on the film surface after stretching.

[0039] An example of a method for producing a cavity-containing polyester film according to the present invention will be described. First, polyester resin and olefin resin, including recycled olefin resin from injection-molded products, are fed into an extruder and melted and mixed. Subsequently, this molten mixture is extruded in a sheet-like manner from a T-shaped die onto a casting drum or the like to obtain a sheet-like material. The layer consisting of this composition containing polyester resin and olefin resin is layer A. If layer B is to be provided, polyester resin and inorganic particles are fed into a separate extruder and melted and mixed. Layer A and layer B are then laminated together in the die and extruded to obtain a laminated sheet-like material. When obtaining a sheet-like material, it is preferable to bring it into close contact with the casting drum using methods such as electrostatic application. Next, the unstretched film is stretched and oriented. Below, the most commonly used sequential biaxial stretching method, particularly the method of stretching the unstretched film longitudinally and then transversely in the width direction, will be explained as an example. First, in the longitudinal stretching step, the film is heated and stretched 2.5 to 5.0 times between two or more rolls with different peripheral speeds. The heating method at this time may be a method using heated rolls or a method using a non-contact heating medium, or a combination of both, but it is preferable to keep the film temperature in the range of (Tg-10℃) to (Tg+50℃). Next, the uniaxially oriented film is introduced into a tenter and stretched 2.5 to 5 times in the width direction at a temperature of (Tg-10℃) to (Tm-10℃ or lower) to obtain a biaxially oriented film. Hereinafter, Tg is the glass transition temperature of the polyester resin, and Tm is the melting point of the polyester resin. Furthermore, it is preferable to subject the film obtained from the above to heat treatment as needed, and the treatment temperature is preferably in the range of (Tm - 60°C) to Tm.

[0040] In the longitudinal stretching process, the lower limit of the stretching temperature is preferably 65°C, and more preferably 75°C. A stretching temperature of 65°C or higher provides sufficient opacity. The upper limit of the stretching temperature is preferably 125°C, more preferably 115°C, even more preferably 105°C, and particularly preferably 95°C. A stretching temperature of 125°C or lower can suppress deformation of the dispersed particles, and performing the process at a lower temperature tends to result in a lower apparent density.

[0041] The cavity-containing polyester film in this invention has an apparent density of 0.7 to 1.1 g / cm³ from the viewpoint of cushioning and lightness. 3 It is preferable that the ratio be 0.8 g / cm³ from the standpoint of recyclability and strength. 3 It is also acceptable if it is greater than or equal to 0.9 g / cm³. 3 It may be greater than or equal to 0.7 g / cm³. 3By doing so, excessive void formation is suppressed, and handling is improved during post-processing such as printing and during use. 1.1 g / cm³ 3 However, sufficient lightness and cushioning can be obtained under the following conditions. Note that the apparent density is a value obtained from the measurement method described in the evaluation method below.

[0042] In the present invention, when only layer A is present in the cavity-containing polyester film, the optical density (OD value) is preferably 0.15 or higher, and more preferably 0.20 or higher. When the OD value is 0.15 or higher, sufficient opacity is obtained, making it suitable for printing applications such as labels. Furthermore, by providing layer B as a skin layer on both sides of layer A, the opacity can be improved. In this case, the OD value is preferably 0.43 or higher, more preferably 0.45 or higher, and even more preferably 0.50 or higher. When the OD value is 0.43 or higher, sufficient opacity is obtained, and when used for labels, the clarity of the printed image is excellent, which can enhance the product's value.

[0043] In the present invention, when only layer A is present in the cavity-containing polyester film, the total light transmittance is preferably 55% or less, and more preferably 45% or less. When the total light transmittance is 55% or less, sufficient opacity is obtained, making it suitable for printing applications such as labels. Furthermore, opacity can be improved by providing layer B as a skin layer on both sides of layer A. In this case, the total light transmittance is preferably 28% or less, and more preferably 25% or less. When the total light transmittance is 28% or less, sufficient opacity is obtained, and when used for labels, the clarity of the printed image is excellent, which can enhance the product's value.

[0044] The thickness of the cavity-containing polyester film of the present invention is arbitrary, but is preferably 20 to 300 μm.

[0045] The resulting cavity-containing polyester film is lightweight, has good opacity and whiteness, and is suitable for use as a base material for labels, cards, packaging materials, and the like. [Examples]

[0046] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the examples described below. The evaluation items in the examples and comparative examples were measured by the following methods.

[0047] (1) Intrinsic viscosity [η] The viscosity was measured at 30°C using an Ostwald viscometer after dissolving in a phenol / tetrachloroethane mixed bath medium of 60 / 40 (weight ratio).

[0048] (2) Weight-average molecular weight Mw and molecular weight distribution (Mw / Mn) The weight-average molecular weight (Mw) and molecular weight distribution (Mw / Mn) were determined using Gerber emission chromatography (GPC) and converted to polystyrene equivalents. The measurement conditions for GPC are as follows: Measurement time: 60 minutes per sample Equipment: HLC-8321GPC / HT (manufactured by Tosoh) Measurement temperature: 140℃ Columns: Styragel HT6E, HT4, HT3 (Waters brand, 4.6mm x 300mm, 3 strips) (Fractional range: 500-10,000,000) Solvent: Orthodichlorobenzene (ODCB) with dibutylhydroxytoluene (BHT) Added (BHT concentration: 0.5g / L). Sample concentration: 1 mg / 1 ml Flow rate: 0.3mL / min Injection volume: 200μl Detection: RI (Polarity (-))

[0049] (3) Melt viscosity Using a Capillograph 1DPMD-C manufactured by Toyo Seiki Seisakusho, the extrusion speed was 10 mm / min and the shear speed was 1.216 × 10⁻⁶. 2 sec -1 The melt viscosity was measured at a temperature of 280°C using a φ1 × 10 mm capillary tube.

[0050] (4) Apparent density The film was cut into a 10.0 cm square, and using a micrometer, nine points were measured with four significant figures at different locations of the total thickness, and the average value of the thickness was obtained and taken as the average thickness per sheet (t: μm). The weight (w: g) was measured with four significant figures using an automatic top-pan balance, and the apparent density was determined from the following formula. Apparent density (g / cm 3 ) = w / (10.0 × 10.0 × t × 10 -4 )

[0051] (5) Total light transmittance (TT) It was measured with a haze meter NDH-7000II manufactured by Nippon Denshoku Industries Co., Ltd. Note that the lower the value of the total light transmittance, the greater the hiding power.

[0052] (6) Optical density (OD value) It was measured using a transmission densitometer "IhAc-T5 type" manufactured by Ihara Electronics Industry Co., Ltd. Note that the higher the value of the optical density, the greater the hiding power.

[0053] (7) Film-forming property The evaluation was carried out as follows based on the number of breakages when manufacturing for 2 minutes of film-forming time. 〇: No breakage ×: Frequent breakage, film-forming impossible

[0054] The following resins were used in the examples. [Olefin resin] Pellets shown in Table 1 were used as the olefin resin. Note that olefin resin a is obtained by recycling (crushing, melting, washing, and pelletizing) a polypropylene-made DVD case recovered from the market, and olefin resins b1 and b2 are unused polypropylene pelletized after polymerization. [Polyester resin] Pellets of polyethylene terephthalate (PET) with an intrinsic viscosity of 0.62 were used. [Titanium oxide master pellet] A mixture of 50% by weight of PET resin and 50% by weight of anatase-type titanium dioxide with an average particle size of 0.3 m (by electron microscopy) was supplied to a vented twin-screw extruder and kneaded to produce a master pellet (M1).

[0055] [Table 1]

[0056] (Example 1) [Manufacturing of unstretched film] 79.8% by weight of PET resin, 18.7% by weight of a dry blend of olefin resin a and olefin resin b1 in a 1:9 ratio, and 1.5% by weight of master pellet (M1) were dry blended and vacuum dried to produce the raw material for the cavity-containing polyester A layer. On the other hand, 30% by weight of master pellet (M1) and 70% by weight of PET resin were dry blended and vacuum dried to produce the raw material for the inorganic particle-containing polyester B layer. These raw materials were supplied to separate extruders, melted at 280°C, and laminated so that the cavity-containing polyester A layer and the inorganic particle-containing polyester B layer were in the order B / A / B. They were joined with a feed block so that the thickness ratio was 10 / 80 / 10, and extruded from a T-die onto a cooling drum adjusted to 20°C to produce an unstretched film with a 2-type, 3-layer structure.

[0057] [Fabrication of cavity-containing polyester films] The obtained unstretched film was uniformly heated to 105°C using a heated roll and longitudinally stretched 3.2 times between two pairs of nip rolls with different peripheral speeds. The uniaxially oriented film thus obtained was guided to a tenter, heated to 110°C and transversely stretched 3.8 times, the width was fixed, and heat treatment was performed at 240°C to relax it by 3% in the width direction, thereby obtaining a cavity-containing polyester film with a thickness of 50 μm. The results for apparent density, total light transmittance, and film-forming properties are shown in Table 2. The cavity-containing polyester film of Example 1 contains sufficient cavities and is a film with excellent opacity and whiteness, and can also contribute to reducing environmental impact.

[0058] (Example 2) In the production of the unstretched film of Example 1 described above, a cavity-containing polyester film was obtained in the same manner as in Example 1, except that a dry blend of olefin resin a and olefin resin b1 in a ratio of 3:7 was used as the olefin resin for layer A. The results are shown in Table 2. The cavity-containing polyester film of Example 2 contains sufficient cavities by dry blending 70% high molecular weight material with 18.7% total polypropylene resin, and is a film with excellent opacity and whiteness. Furthermore, since 5.6% of the raw materials are recycled from injection-molded products, it can also contribute to reducing environmental impact.

[0059] (Example 3) In the production of the unstretched film of Example 1 described above, a cavity-containing polyester film was obtained in the same manner as in Example 1, except that 28.1% of the olefin resin for layer A was a dry blend of olefin resin a and olefin resin b1 in a ratio of 1:9. The results are shown in Table 2. The cavity-containing polyester film of Example 3 contains sufficient cavities by dry blending 90% high molecular weight material with 28.1% total polypropylene resin, and is a film with excellent opacity and whiteness. Furthermore, since it uses recycled raw materials from injection-molded products, it can also contribute to reducing environmental impact.

[0060] (Example 4) In the production of the unstretched film of Example 1 described above, a cavity-containing polyester film was obtained in the same manner as in Example 1, except that a dry blend of olefin resin a and olefin resin b2 in a ratio of 1:9 was used as the olefin resin for layer A. The results are shown in Table 2. The cavity-containing polyester film of Example 4 contains more cavities by using a polypropylene resin with a higher molecular weight, and is a film with excellent opacity and whiteness. Furthermore, since 1.9% of the raw material is recycled from injection-molded products, it can also contribute to reducing the environmental impact.

[0061] (Example 5) In the production of the unstretched film of Example 1 described above, a cavity-containing polyester film was obtained in the same manner as in Example 1, except that a dry blend of olefin resin a and olefin resin b2 in a ratio of 3:7 was used as the olefin resin for layer A. The results are shown in Table 2. The cavity-containing polyester film of Example 5 contains more cavities by using a polypropylene resin with a higher molecular weight, and is a film with excellent opacity and whiteness, and can also contribute to reducing environmental impact.

[0062] (Reference example 1) Reference Example 1 is an example of a conventional, known cavity-containing polyester film that does not use recycled resin. In the production of the unstretched film of Example 1, a cavity-containing polyester film was obtained in the same manner as in Example 1, except that only olefin resin b1 was used as the olefin resin for layer A. The results are shown in Table 2. It can be seen that the cavity-containing films of Examples 1 to 5 have excellent properties equivalent to those of the cavity-containing film of Reference Example 1.

[0063] (Comparative Example 1) In the production of the unstretched film of Example 1 described above, a cavity-containing polyester film was obtained in the same manner as in Example 1, except that only olefin resin a was used as the olefin resin for layer A. The results are shown in Table 2. The cavity-containing polyester film of Comparative Example 1 was MD-stretched at a low temperature, and therefore contained cavities and was a film with excellent opacity and whiteness. However, compared to the cavity-containing polyester films of each example, its opacity and whiteness were lower.

[0064] (Comparative Example 2) In the production of the unstretched film of Example 1 described above, only olefin resin a was used as the olefin resin for layer A, and in the production of the cavity-containing polyester film, the heating roll was uniformly heated to 95°C and the film was longitudinally stretched 3.2 times between two pairs of nip rolls with different peripheral speeds. The cavity-containing polyester film was obtained in the same manner as in Example 1. The results are shown in Table 2. The cavity-containing polyester film of Comparative Example 2 was MD-stretched at a lower temperature and therefore contained more cavities than the cavity-containing polyester film of Comparative Example 1, and was a film with superior opacity and whiteness. However, compared to the cavity-containing polyester films of each example, its opacity and whiteness were lower.

[0065] (Comparative Example 3) In the production of the unstretched film of Example 1 described above, only olefin resin a was used as the olefin resin for layer A, and in the production of the cavity-containing polyester film, the heating roll was uniformly heated to 115°C and the film was longitudinally stretched 3.2 times between two pairs of nip rolls with different peripheral speeds. The same method as in Example 1 was used to obtain the cavity-containing polyester film. The results are shown in Table 2. The cavity-containing polyester film of Comparative Example 3 was MD-stretched at a higher temperature than Comparative Examples 1 and 2, and therefore was a film that was slightly inferior in cavity formation, opacity, and whiteness.

[0066] [Table 2] [Industrial applicability]

[0067] According to the present invention, by blending a low molecular weight, low melt viscosity olefin resin recycled from injection molded products with a high molecular weight, high melt viscosity olefin resin, it is possible to provide a void-containing polyester film with excellent lightness and opacity without impairing the film's properties.

Claims

1. A cavity-containing polyester film comprising a layer (layer A) containing internal cavities, wherein layer A comprises at least a polyester resin, thermoplastic resin a, and thermoplastic resin b, and thermoplastic resin a is a thermoplastic resin recycled from an injection-molded product.

2. The cavity-containing polyester film according to claim 1, wherein a layer (layer B) made of a polyester resin containing inorganic particles is laminated on at least one side of the layer (layer A) which contains a cavity inside.

3. The cavity-containing polyester film according to claim 1, wherein the weight-average molecular weight of thermoplastic resin b is higher than the weight-average molecular weight of thermoplastic resin a, or the melt viscosity of thermoplastic resin b is higher than the melt viscosity of thermoplastic resin a.

4. The cavity-containing polyester film according to claim 1, wherein the thermoplastic resin a is an olefin-based resin.

5. The cavity-containing polyester film according to claim 4, wherein the olefin resin a satisfies the following (1) and (2). (1) The weight-average molecular weight Mw measured by gel permeation chromatography is in the range of 100,000 to 500,000, and the molecular weight distribution expressed as weight-average molecular weight Mw / number-average molecular weight Mn is 4 to 8. (2) The melt viscosity measured by capillary graph is 50 to 100 PA·s.

6. The cavity-containing polyester film according to claim 1, wherein the thermoplastic resin b is an olefin-based resin.

7. The cavity-containing polyester film according to claim 6, wherein the olefin resin b satisfies the following (1) and (2). (1) The weight-average molecular weight Mw measured by gel permeation chromatography is in the range of 500,000 to 700,000, and the molecular weight distribution expressed as weight-average molecular weight Mw / number-average molecular weight Mn is 4 to 8. (2) The melt viscosity measured by capillary graph is 200 to 700 Pa·s.

8. The cavity-containing polyester film according to claim 1, wherein the injection-molded product includes at least a CD case or a DVD case.

9. Apparent density is 0.7–1.1 g / cm³ ３ The cavity-containing polyester film according to claim 1.

10. The cavity-containing polyester film according to claim 1, wherein the inorganic particles in the B layer are titanium dioxide.

11. The cavity-containing polyester film according to claim 1, wherein the total light transmittance is less than 55% and the optical density is 0.15 or higher.

12. A method for producing a cavity-containing polyester film, comprising the following steps. (Step a) Step of preparing thermoplastic resin a and thermoplastic resin b, (Step b) A step of melting and mixing the thermoplastic resin a and thermoplastic resin b to obtain a polyester resin composition. (Step c) Step of extruding the polyester resin composition into a sheet to obtain a sheet-like material. (Step d) Step of stretching the sheet-like material in at least one direction. (However, thermoplastic resin a is a thermoplastic resin recycled from injection-molded products.)

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