JP5331272B2 - Polyester film for in-mold transfer foil - Google Patents

Description

  The present invention relates to a polyester film for in-mold transfer foil that has a good thermal dimensional change in a printing process from shallow drawing to medium drawing, excellent printability, and good moldability that easily stretches with low stress in molding.

Biaxially stretched polyester film represented by polyethylene terephthalate film has good mechanical strength, thermal characteristics, humidity characteristics and many other excellent characteristics, so it can be used in a wide range of fields such as industrial materials, magnetic recording materials and packaging materials. It is used.

  Particularly in an in-mold transfer foil, the polyester film is useful as the substrate. This transfer foil is configured by sequentially laminating a transfer layer such as a release layer, a design printing layer, and an adhesive layer on one side, and a hard coat layer or a metal vapor deposition layer is laminated on the transfer layer according to the purpose. Further, a functional agent such as an antistatic agent or an antibacterial agent is added to the release layer or the transfer layer to give a function as a transfer foil.

  In these transfer foil transfer methods, the transfer foil is set in an injection mold and a resin molded product is molded. At the same time, a transfer foil sheet is integrally bonded to the surface of the transfer foil, thereby forming a resin molded product. A so-called in-mold transfer method (simultaneous molding transfer method) in which a pattern is transferred and decorated is widely known (Patent Documents 1 to 5).

  Such a transfer method is a method of transferring to a transfer object via an adhesive layer using a plurality of laminated transfer foils including an adhesive layer on the surface of a polyester base material. It is used in a wide range of applications for the purpose of surface treatment such as decoration and surface protection on the surface of various resin molded products such as automobile parts, cosmetic containers, and toys.

  Conventionally, a polyester biaxially stretched film has been used as a base film for a transfer foil used by printing and molding (see, for example, Patent Document 1 and Patent Document 2). Further, it has been proposed to scratch a polyester biaxially stretched film having an intrinsic viscosity and density within a specific range and use them as a matte transfer foil film (see Patent Document 3).

  In addition, as a deep drawing film, it has been proposed to use a polyester film having a molding stress within a specific range (see Patent Document 4).

  Further, it has been proposed to use a polyester film polymerized from a monomer composition containing two or more glycol components selected from butanediol and the like as a film for process such as molding.

  On the other hand, in the molding application, the diaphragm in the molding process varies greatly depending on various applications, such as cellular phones and electrical products, which have a narrow diaphragm during molding and those with a large diaphragm during molding such as automobiles. The drawing in this forming process is classified into shallow drawing, medium drawing, and deep drawing, and a suitable base film is selected and used according to the intended application.

  Important requirements for the base film include thermal dimensional changes due to stretch deformation and width shrinkage of the base film due to the influence of drying temperature and tension during coating and printing processes such as release layers, pattern printing, and adhesive layers. Since there arises a problem of occurrence of printing misalignment and harmful flatness deterioration, the substrate film is required to have certain thermal dimensional stability and mechanical properties. However, since the transfer foil laminated using this base film retains mechanical strength, the deformation stress is high at the time of in-mold molding, the followability with the mold is poor, and the printing is clear. There is a problem that chipping phenomenon and molding tearing are likely to occur.

  In this base film for deep drawing, since the base material itself has soft characteristics in terms of film design, it is processed at a low temperature such as 80 to 100 ° C., which does not hinder elongation deformation in consideration of this point. In many cases, the pattern to be painted is one-step full surface printing such as woodgrain, and it is used for transfer foils that do not interfere with the quality of the pattern even if some thermal dimensional change or elongation deformation occurs. Yes.

  However, in many applications, such as shallow drawing to medium drawing, patterns to be painted are often printed in multiple colors, such as three to seven colors, and stretch deformation due to the influence of drying temperature and processing tension due to such high temperatures. And width shrinkage, printing deviation and flatness (single tarmi, etc.) are deteriorated, resulting in a fatal defect in the quality of the transfer foil.

  For this reason, the base film for shallow drawing to medium drawing is given mechanical strength so as to suppress elongation deformation in the longitudinal direction in coating and printing. However, the transfer foil obtained using such a base film also has high mechanical strength, so the deformation stress is high at the time of in-mold molding, the followability with the mold is poor, and the printing is clear. There is a problem that chipping phenomenon and molding tearing are likely to occur.

  In recent years, with the expansion of applications for shallow and medium drawing, such as mobile phones and personal computers, excellent mechanical elongation and thermal dimensional stability are achieved in coat lamination processing and printing of transfer foil for shallow drawing to medium drawing. In addition, the in-mold molding process is required to have a property of easily extending with low stress.

Japanese Patent Laid-Open No. 7-196821 JP 7-237283 A JP 2007-181978 A JP 2004-9596 A JP-A-6-210799 JP 2000-344909 A Japanese Patent Publication No. 60-11628 Japanese Patent No. 3090911 JP 2002-97261 A

  The present invention has been made in view of the above-described circumstances, and the problem to be solved is that the deformation stress in the laminated region of the transfer foil retains the mechanical strength, the thermal dimensional stability is good, and in-mold molding Polyester for in-mold transfer foil suitable for shallow drawing to medium drawing, satisfying both contradictory mechanical strength properties of a base film that deforms with low stress and has high elongation in the deformation region of To provide a film.

  As a result of intensive studies in view of the above circumstances, the present inventor has found that the above problems can be easily solved by a film having a specific configuration, and has completed the present invention.

That is, the gist of the present invention is a polyester film obtained by heat-treating a polyester film after longitudinal and transverse stretching and simultaneously relaxing in the width direction, and the thermal shrinkage in the film longitudinal direction after treatment at 180 ° C. for 5 minutes is 3. 0% or less, the thermal shrinkage in the transverse direction of the film under the same conditions is 0.5% or less, the 5% elongation stress in the longitudinal direction of the film at 25 ° C. is 90 to 120 MPa, The polyester film for in-mold transfer has a 100% elongation stress of 200 MPa or less in both longitudinal and transverse directions, and a longitudinal elongation at break of the film of 150% or more.

Hereinafter, the present invention will be described in detail.
The polyester film used in the present invention is a polyester obtained using aromatic dicarboxylic acid or its ester and glycol as main starting materials, and 80% or more of the repeating structural units are ethylene terephthalate units or ethylene- A polyester having 2,6-naphthalate units. And you may contain other 3rd components on the conditions which do not deviate from said range. As the aromatic dicarboxylic acid component, for example, in addition to terephthalic acid and 2,6-naphthalenedicarboxylic acid, for example, isophthalic acid, phthalic acid, adipic acid, sebacic acid, oxycarboxylic acid (for example, p-oxyethoxybenzoic acid, etc.) ) Etc. can be used. As the glycol component, in addition to ethylene glycol, for example, one or more of diethylene glycol, propylene glycol, butanediol, 1,4-cyclohexanedimethanol, neopentyl glycol and the like can be used. Can be used.

  Examples of the polymerization catalyst include antimony compounds such as antimony trioxide and antimony pentoxide, germanium compounds, and titanium compounds. Titanium compounds include, for example, tetraalkyl titanates, tetraaryl titanates, titanyl oxalate salts, titanyl oxalate, chelate compounds containing titanium, titanium tetracarboxylates, and specifically tetraethyl titanate, tetrapropyl titanate, tetraphenyl Examples thereof include titanate or a partial hydrolyzate thereof, titanyl ammonium oxalate, potassium titanyl oxalate, titanium triacetylacetonate and the like.

  In addition, inorganic particles, organic salt particles and crosslinked polymer particles can be added to the polyester film of the present invention.

  Inorganic particles include calcium carbonate, kaolin, talc, magnesium carbonate, barium carbonate, calcium sulfate, barium sulfate, lithium phosphate, calcium phosphate, magnesium phosphate, aluminum oxide, silicon oxide, titanium oxide, zirconium oxide, lithium fluoride, etc. Is mentioned.

  Examples of the organic salt particles include terephthalate such as calcium oxalate, calcium, barium, zinc, manganese, and magnesium.

  Examples of the crosslinked polymer particles include homopolymers or copolymers of vinyl monomers of divinylbenzene, styrene, acrylic acid, methacrylic acid, acrylic acid or methacrylic acid. In addition, organic particles such as polytetrafluoroethylene, benzoguanamine resin, thermosetting epoxy resin, unsaturated polyester resin, thermosetting urea resin, and thermosetting phenol resin may be used.

  The polyester film of the present invention is a method in which a coating layer is formed on one or both sides of a film in order to impart functions such as adhesion, antistatic property, slipperiness, releasability to the film during and / or after the stretching process. Or surface treatment such as corona treatment.

  The polyester film of this invention can use the polyester raw material with little oligomer content as a method of suppressing the oligomer of a surface. Such a raw material can be obtained by solid-state polymerization in which a polyester chip obtained by a usual melt polycondensation reaction is kept at 180 to 240 ° C. for about 1 to 20 hours under reduced pressure or under the flow of an inert gas. it can. A single layer polyester film may be formed by mixing this raw material alone or this raw material with a normal raw material, or a multilayer structure of two or more layers, and this raw material is applied only to the surface layer opposite to the transfer layer. It may be used. In the case of a multilayer structure, normal polyethylene terephthalate may be used for the inner layer, and for simultaneous molding transfer, for the purpose of improving moldability, copolymerized polyester or polybutylene containing isophthalic acid and terephthalic acid as a copolymerization component. Terephthalate may be used.

  The polyester film used as the base material of the present invention may be either a single layer or a multilayer structure, but if it is a multilayer structure, it is possible to design differently for the inner layer and the outer layer, for example, containing particles only in the fill surface layer This can reduce manufacturing costs.

  The total thickness of the polyester film for transfer foil of the present invention is not particularly limited because it is appropriately selected depending on the use for which the polyester film for transfer foil of the present invention is used, but it has points such as mechanical strength, handling properties and productivity. Therefore, it is preferably 12 to 100 μm.

  The surface roughness of the polyester film for transfer foil of the present invention is not particularly limited because it is appropriately selected according to the application, but in consideration of the surface gloss after transfer and the winding property and slit property at the time of lamination, The value of the center plane average roughness (SRa) is preferably 50 nm or less.

  In the present invention, from the viewpoint of heat resistance, molding processability, and dimensional stability, the melting peak temperature Tm measured with a differential scanning calorimeter is preferably 220 to 260 ° C, and preferably 230 to 255 ° C. When Tm is less than 220 ° C., the heat resistance and dimensional stability are inferior, and thus wrinkles are generated in the printing process or the film surface after molding is swollen, so that the design of the design pattern is impaired. Problems may occur. On the other hand, when Tm exceeds 260 ° C., moldability and productivity tend to deteriorate.

  In the present invention, the secondary transition temperature Tg obtained from the differential scanning calorimeter is preferably 50 to 90 ° C, more preferably 55 to 80 ° C. When Tg is 50 ° C. or lower, heat resistance tends to be inferior, and when it exceeds 90 ° C., formability tends to be inferior.

  The film of the present invention is preferably used as a base film having an antistatic layer having both antistatic ability and blocking prevention ability.

Details of the antistatic agent will be described below.
When an anionic antistatic agent having a low molecular weight is used as the antistatic agent, the antistatic agent is transferred to the surface of the transfer layer where the release layer is coated while the polyester film is wound into a roll, and the release layer is coated. The antistatic agent is transferred to the adhesive layer when the transfer layer is rolled up after processing, and the adhesive cannot exhibit the desired performance. In order to prevent such transfer of the antistatic agent, a high molecular weight anionic compound is preferably used. In the case of a cationic antistatic agent, it is desirable to use a high molecular weight cationic compound.

The surface specific resistance of the antistatic layer is 1 × 10 ¹³ Ω / □ or less, preferably 1 × 10 ¹² Ω / □ or less. If the surface specific resistance exceeds 1 × 10 ¹³ Ω / □, the antistatic performance is inferior, and problems in the process may not be improved.

  As described above, the antistatic layer is characterized by little or no transfer of the antistatic agent, but at the same time, it should not block the adhesive of the transfer layer. As a guideline that does not cause blocking, the peel strength of the pressure-sensitive adhesive tape (Cellotape (registered trademark)) from the pressure-sensitive adhesive layer is preferably 2.4 N / cm or less, more preferably 2.0 N / cm or less, and particularly preferably 1 0.7 N / cm or less. If this value exceeds 2.4 N / cm, the effect of improving the blocking property may not be obtained.

  Examples of the antistatic agent satisfying such characteristics include a compound having a quaternary ammonium base. This refers to a compound having a component containing a quaternary ammonium base in the main chain or side chain in the molecule. Examples of such constituents include, for example, a pyrrolidinium ring, an alkylamine quaternized product, a copolymer of these with acrylic acid or methacrylic acid, an N-alkylaminoacrylamide quaternized product, a vinylbenzyltrimethylammonium salt, 2-hydroxy 3-methacryloxypropyltrimethylammonium salt etc. can be mentioned. Further, these may be combined or copolymerized with other resins. In addition, examples of anions that serve as counter ions for these quaternary ammonium salts include ions such as halogen, alkyl sulfate, alkyl sulfonate, and nitric acid.

  In the present invention, the compound having a quaternary ammonium base is preferably a polymer compound. When the molecular weight is too low, the antistatic agent is transferred from the antistatic layer to the adhesive layer, and a desired adhesive effect is not obtained, or it adheres to a heating roll or a mold during transfer. In order not to cause such a problem, it is desirable that the number average molecular weight of the compound having a quaternary ammonium base is usually 1000 or more, more preferably 2000 or more, particularly 5000 or more. On the other hand, when such a compound has a molecular weight that is too high, problems such as excessively high viscosity of the coating solution may occur. In order not to cause such a problem, the number average molecular weight is preferably 500,000 or less.

  In order to reduce the peel strength of the adhesive tape from the adhesive to 2.4 N / cm or less, not only the selection of the antistatic agent is important, but in order to reduce the peel strength, a polyolefin resin and / or a fluorine-based resin is used. It is effective to mix the resin actively. As the polyolefin-based resin and the fluorine-based resin, it is preferable to blend those having little transfer, preferably no transfer, like the antistatic agent.

  The coating layer in the present invention is preferably provided by in-line coating. In-line coating is a method of coating within the process of producing a polyester film. Specifically, it is a method of coating at any stage from melt-extrusion of polyester to biaxial stretching, heat setting and winding. is there. Usually applied to either a substantially amorphous unstretched sheet obtained by melting and quenching, a uniaxially stretched film stretched in the longitudinal direction (longitudinal direction), or a biaxially stretched film before heat setting. To do. In these, the method of extending | stretching to a horizontal direction after apply | coating to a uniaxially stretched film is excellent. According to such a method, since film formation and coating / drying can be performed simultaneously, there is a merit in manufacturing cost, thin film coating is easy to perform stretching after coating, and heat treatment performed after coating is not limited. Since the high temperature is not achieved by this method, the coating film and the polyester film are firmly adhered.

  The thickness of a coating layer is 0.001-10 micrometers normally as thickness after drying, Preferably it is 0.010-5 micrometers, More preferably, it is 0.015-2 micrometers. When the thickness of the coating layer is less than 0.001 μm, the antistatic effect may not be sufficiently improved. When the thickness of the coating layer exceeds 10 μm, so-called blocking may occur in which the coating layer acts like an adhesive and the films wound up on the roll adhere to each other.

In the present invention, the heat shrinkage in the longitudinal direction of the film after heat treatment at 180 ° C. for 5 minutes is required to be 3.0% or less, and the heat shrinkage in the transverse direction is required to be 0.5% or less. The direction is 2.5% or less, and the lateral direction is 0.3% or less. When the thermal contraction rate in the vertical direction is larger than 3.0%, the flatness (single tarmi) considered to be affected by the thermal contraction difference in the width direction is likely to deteriorate. On the other hand, if the thermal contraction rate in the horizontal direction exceeds 0.5%, the width shrinkage becomes large, and a printing misalignment problem occurs in the width direction.

  In the film of the present invention, from the viewpoint of processability to transfer foil, the 5% elongation stress in the film longitudinal direction at 25 ° C. needs to be in the range of 90 to 120 MPa, and preferably in the range of 95 to 115. If the 5% elongation stress is lower than 90 MPa in the tensile test in the longitudinal direction, film elongation occurs in a processing step such as printing, and problems such as printing misalignment tend to occur in the longitudinal direction. On the other hand, if the 5% elongation stress exceeds 120 MPa in the longitudinal direction, the mechanical strength of the base film finished on the transfer foil is also maintained, so that the deformation stress during in-mold molding is also high, and good moldability is ensured. It becomes difficult.

  From the viewpoint of formability, the film of the present invention has a tensile strength test in which the 100% elongation stress in both the longitudinal and transverse directions at 25 ° C. is 200 MPa or less and the breaking elongation in the longitudinal direction of the film is 150% or more. There must be. Preferably, the 100% elongation stress is 180 MPa or less, and the breaking elongation is 160% or more. If the 100% elongation stress in the vertical direction or the horizontal direction at 25 ° C exceeds 200 MPa, the deformation stress also increases at the time of in-mold molding, the followability with the so-called mold deteriorates, and the decorative printed surface is clear. This is not preferable because a phenomenon lacking in the thickness and molding breakage are likely to occur. Moreover, even if the 100% elongation stress is 200 MPa or less, if the elongation at break is 150% or less, the molding elongation suitable for various applications may not be obtained. Many defects are likely to occur.

  Polyester for transfer foil is required to ensure the mechanical strength necessary for printing processing and to be easily deformed in molding processing and satisfy both of these contradictory mechanical strength properties. You can achieve that.

  That is, in order to secure the 5% elongation stress of the present invention in a specific range and to reduce the 100% elongation stress and to have high elongation properties, the film after longitudinal and transverse stretching has a relaxation rate in the width direction simultaneously with the high temperature heat treatment. By raising, the above-mentioned characteristics of the present invention can be satisfied, and a target polyester film for in-mold transfer foil with excellent printability and moldability can be obtained.

  According to the present invention, it is possible to provide a polyester film for in-mould transfer foil that is excellent in printability in a printing process and has extremely good moldability in applications of shallow drawing to medium drawing, and the industrial value of the present invention. Is very big.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a following example, unless the summary is exceeded.

(1) Melting peak temperature (Tm)
Accompanied by melting obtained when a temperature difference of about 5 mg of a polyester resin is raised from 0 ° C. to 300 ° C. at a rate of 20 ° C./minute using a differential scanning calorimeter “MDSC 2920 type” manufactured by TA Instruments Inc. The temperature of the endothermic peak was Tm.

(2) Second order transition temperature (Tg)
Using a differential scanning calorimeter “MDSC2920 type” manufactured by TA Instruments Inc., about 5 mg of polyester resin is heated from 0 ° C. to 300 ° C. at a rate of 20 ° C./min, and held at 300 ° C. for 5 minutes. After that, the glass was rapidly cooled to 0 ° C. or lower, and then the transition point accompanying the glass transition observed when the temperature was raised from 0 ° C. to 300 ° C. at 300 ° C. at 20 ° C./min was defined as Tg.

(3) Surface resistivity (Ω / □)
Yokogawa, Hewlett-Packard's inner electrode 50mm diameter, outer electrode 70mm diameter concentric circular electrode 16008A (trade name) is placed on the sample in an atmosphere of 23 ° C, 50% RH, and a voltage of 100V is applied. The surface resistivity of the sample was measured using 4329A (trade name), a high resistance meter manufactured by the same company.

(4) Center plane average roughness (SRa)
Using a non-contact type three-dimensional roughness meter made by Micromap Co., Ltd. using a light interference method with a measurement wavelength of 554 nm, the average surface roughness SRa value of the film surface in a measurement region of 232 μm × 177 μm is averaged by 10 points. It was.

(5) 5% elongation stress Using an Intesco tensile tester Intesco Model 2001, a sample film having a length of 50 mm and a width of 15 mm at a temperature of 25 ° C. is subjected to a tensile test at a speed of 200 mm / min. The stress at 5% elongation in the longitudinal direction was determined.

(6) 100% elongation stress By the measuring method of said (5), the sample piece was pulled to the vertical direction and the horizontal direction, and the stress at the time of 100% elongation was calculated | required.

(7) Elongation at break The sample piece was pulled in the vertical direction and the horizontal direction by the measurement method of (5) above, and the elongation at break of the sample piece was determined.

(8) Heat shrinkage rate of 180 ° C. Using a hot air circulating furnace (manufactured by Tajii Mfg. Co., Ltd.), the film in a non-tension state is heat-treated for 5 minutes in an atmosphere of 180 ° C. Was measured by the following formula and expressed as an average value for each of five samples.
Thermal contraction rate (%) = (L ₀ −L ₁ ) × 100 / L ₀
In the above formula, L ₀ represents the sample length (mm) before the heat treatment, and L ₁ represents the sample length (mm) after the heat treatment. However, when L ₀ is smaller than L ₁ (when the film expands), the value of the thermal contraction rate is represented by − (minus).

(9) Printability <Print misalignment>
A roll-shaped film sample was unwound at a tension of 8 MPa, subjected to four-color gravure printing, and then dried at 180 ° C. for 30 seconds to prepare a pattern printing film.
The printing misalignment of the resulting pattern printing film was visually observed and judged according to the following criteria.
A: No occurrence of printing misalignment (elongation and shrinkage of the film) is observed. O: Slight printing misalignment is observed, but is practically usable level. X: Printing misalignment is observed, and practically unusable. (failure)

<Flatness (single tarmi)>
The created pattern printing film was drawn out to a length of 2 m from the roll shape, and the flatness of the one piece was visually observed and judged according to the following criteria.
◎: The flatness of the piece tarmi is hardly observed on the pattern printing film. ○: The piece tarmi is slightly observed but is practically usable. ×: The piece tarmi is slightly conspicuous and the sheet-like appearance is poor. (failure)

(10) Formability The pattern printing film prepared in the above (8) was continuously formed at a rate of 100 pieces / min into a cylindrical shape having a bottom diameter of 50 mm and a depth of 5 mm using a male and female mold. The state of the obtained sample was visually observed and judged according to the following criteria.
A: 95 or more out of 100 films are uniformly formed without tearing of the film. O: 80 or more of 100 films are uniformly formed without generating a film tear. X: 21 out of 100 films. Film tear occurs at more than one piece, and many defective parts are observed (failed)

Next, the polyester raw materials used in Examples and Comparative Examples will be described.
<Polyester 1>
Using terephthalic acid as the dicarboxylic acid component and ethylene glycol as the polyhydric alcohol component, a polyester chip containing no lubricant particle size and having an intrinsic viscosity of 0.66 dl / g by a conventional melt polymerization method was produced.

<Polyester 2>
Using terephthalic acid as the dicarboxylic acid component and ethylene glycol as the polyhydric alcohol component, 0.60 part of amorphous silica having an intrinsic viscosity of 0.66 dl / g and an average particle diameter of 2.5 μm by a conventional melt polymerization method A polyester chip was produced.

<Polyester 3>
The polyester 1 chip was heated in a nitrogen stream at 220 ° C. for 10 hours to produce a polyester chip having a low oligomer content. The amount of oligomer contained in polyester 1 was 0.83% by weight, whereas the amount of oligomer contained in polyester 3 was 0.24% by weight.

<Polyester 4>
Using diphthalic acid components as isophthalic acid and terephthalic acid, and polyhydric alcohol components as ethylene glycol, raw material chips polymerized by a conventional melt polycondensation method were produced. The isophthalic acid content in the dicarboxylic acid component of this raw material was 22 mol%.

Comparative Example 1:
Polyester 2 and polyester 4 are blended at a weight ratio of 1: 4, melted in an extruder, supplied to a single layer die, extruded into a film, cast on a cooling drum at 35 ° C., rapidly cooled and solidified. A stretched film was prepared. Next, after preheating with a heating roll at 80 ° C., using an infrared heater and a heating roll in combination, the film is stretched 3.0 times in the vertical direction between 85 ° C. rolls, and then charged to a thickness of 5 μm with a gravure coater on one side of the film. After performing the prevention coating, the film end is held by a clip and guided into a tenter, and stretched 3.5 times in the horizontal direction while being heated at a temperature of 100 ° C., and subjected to a heat treatment at 195 ° C. for 10 seconds, A polyester film composed of a single layer film having a thickness of 50 μm was obtained by relaxing 3% in the width direction at 170 ° C. The properties of the obtained film were as shown in Table 1. From this result, in printability, the printing displacement in the longitudinal direction and the printing displacement in the width direction due to width shrinkage are conspicuous due to the lack of 5% elongation stress in the longitudinal direction. Furthermore, since the vertical shrinkage was somewhat large, a slight deterioration in flatness was observed. In terms of moldability, a good molded product was obtained without any problem of mold breaking.

Comparative Example 2:
By applying the same manufacturing conditions as in Comparative Example 1, only the vertical magnification is increased from 3.0 times to 3.5 times, and the relaxation in the width direction is changed from 3% to 4%. A polyester film adjusted to a thickness of 50 μm was obtained. The properties of the obtained film were as shown in Table 1. From this result, the printability improved, but the flatness deterioration was observed due to the increase in the vertical shrinkage rate. As for the moldability, a good molded product was obtained without any problem as in Comparative Example 1.

Comparative Example 3:
Polyester 2 and polyester 3 are blended at a weight ratio of 1: 3, melted in an extruder, supplied to the outer layer A of the laminated die, polyester 1 is supplied to the inner layer of the laminated die, and each extruder is supplied. The extrusion amount ratio of the outer layer A and the inner layer B is supplied at a ratio of 1: 4, and a two-layer / three-layer laminated polyester resin composed of the outer layer A / inner layer B / outer layer A is extruded into a film shape at 35 ° C. An unstretched film cast on a cooling drum and rapidly solidified was produced. Next, after preheating with a heating roll at 80 ° C., the film is stretched 3.8 times in the longitudinal direction between 95 ° C. rolls using an infrared heating heater and a heating roll, and then charged to a thickness of 5 μm with a gravure coater on one side of the film. After performing the prevention coating, the film edge is held by a clip and guided into a tenter, stretched 4.2 times in the transverse direction while heating at a temperature of 110 ° C., and subjected to a heat treatment at 230 ° C. for 10 seconds, 3% relaxation was applied in the width direction at 190 ° C. to obtain a polyester film having a thickness of 50 μm.
The properties of the obtained film were as shown in Table 1. From this result, the printing misalignment was very good, but many breaks occurred in the moldability, and the molded article was partially observed to have a defect of lack of clear printing at the corner.

Comparative Example 4:
Applying the same manufacturing conditions as in Comparative Example 3 above, the longitudinal magnification was changed from 3.8 times to 3.2 times, the transverse magnification was changed from 4.2 times to 3.8 times, and only the draw ratio was changed to a low magnification, A polyester film adjusted to a thickness of 50 μm was obtained. The properties of the obtained film were as shown in Table 1. From these results, both printing misalignment and flatness were good, but molding breakage due to insufficient formability occurred.

Example 1:
Applying the same production conditions as in Comparative Example 4 above, the heat treatment temperature was increased from 230 ° C. to 235 ° C., and then the relaxation portion in the width direction was performed simultaneously with the heat treatment to give 10% relaxation, and a 50 μm thick polyester film was obtained. Obtained. The properties of the obtained film were as shown in Table 1. From these results, both printing misalignment and flatness were good, and the moldability also tended to improve.

Example 2:
Applying the same production conditions as in Example 1 above, the heat treatment temperature was further increased from 235 ° C. to 238 ° C., and the relaxation in the width direction was changed from 10% to 15%. A polyester film having a thickness of 50 μm was obtained. The properties of the obtained film were as shown in Table 1. From this result, both printing misalignment and flatness were good, and a very good result was also obtained in moldability.

  The film of the present invention can be suitably used as, for example, an in-mold transfer film.

Claims (2)

A polyester film obtained by heat-treating the polyester film after longitudinal and transverse stretching and simultaneously relaxing in the width direction, wherein the heat shrinkage rate in the machine direction of the film after treatment at 180 ° C. for 5 minutes is 3.0% or less. The thermal contraction rate in the horizontal direction of the lower film is 0.5% or less, the 5% elongation stress in the film longitudinal direction at 25 ° C. is 90 to 120 MPa, and the 100% elongation stress under the same conditions is in both the longitudinal and lateral directions. A polyester film for in-mold transfer, which is 200 MPa or less and has a longitudinal elongation at break of 150% or more. The polyester film for in-mold transfer according to claim 1, wherein the temperature when relaxing in the width direction simultaneously with the heat treatment is 235 ° C or higher, and the relaxation rate is 10% or more.