Polyester film for use in a laminate with a metal plate

Abstract: A polyester film for use in a laminate with a metal plate comprises a polyester-group resin containing a white pigment having a mean particle diameter of 0.01 to 1.8 µm at a content of 10 to 30% by weight. The film has (a) a void ratio of 4 to 30%, (b) a melting peak temperature of 150 to 245 °C , (c) an amount of oligomer of not more than 3% by weight and (d) an orientation degree of 0.7 to 4.0. When the film is laminated on a metal, it can provide excellent light interruptive, lamination, forming and processing properties.

Description: The present invention relates to a white polyester film for use in a laminate with a metal plate excellent in heat resistance, wet heat resistance and forming property, and more specifically to a white polyester film suitable to use for forming after heat bonding it to a metal. Various white polyester films are known. For example, (1) a film wherein copolymerization components are present in an amount of 2 to 50 mol% and inorganic additives are added at a content of 1 to 20% by weight (for example, JP-A-SHO 52-13542), (2) a film wherein the heat of crystalline melting is suppressed to not more than 6.0 cal/g and white inorganic particles are added at a content of 5 to 30% by weight (for example, JP-A-HEI 1-241492), and (3) a film having added to it particles with a diameter of 0.1 to 25 µ m in a number of about 1x10 8 /mm 3 whose melting point is in the range of 210 to 245°C (for example, JP-A-HEI 5-170942), are known. However, because the heat and wet heat resistances of such white films described in (1), (2) and (3) above are poor, they are not sufficiently useful in practice, particularly for uses wherein the films are heat bonded to a metal and formed and thereafter the metal-film laminates are heated. Polyester films which contain white inorganic pigments are also disclosed in EP-A-0 288 315 and EP-A-0 386 707. Accordingly, it would be desirable to provide a white polyester film for use in a laminate with a metal plate which is excellent in heat resistance and wet heat resistance and which is less subject to soiling during processing and excellent in forming property. A polyester film suitable for use in a laminate with a metal plate according to the present invention is characterized in that the polyester film comprises a polyester-group resin containing a white pigment having a mean particle diameter of 0.01 to 1.8 µ m at a content of 10 to 30% by weight, and the film has (a) a void ratio of 4 to 30%, (b) a melting peak temperature of 150 to 245°C, (c) an amount of any oligomer which may be present of not more than 3% by weight and (d) an orientation degree of 0.7 to 4.0. Further, the polyester film according to the present invention may be formed as a laminated film of layers A and B. Namely, a polyester film for use in a laminate with a metal plate according to the present invention may comprise (i) a layer B comprising a polyester-group resin containing a white pigment having a mean particle diameter of 0.01 to 1.8 µ m at a content of 10 to 30% by weight, the layer B having (a) a void ratio of 4 to 30%, (b) a melting peak temperature of 150 to 245°C, (c) an amount of oligomer "O B " of not more than 3% by weight and (d) an orientation degree of 0.7 to 4.0; and (ii) a layer A laminated on the layer B, the layer A comprising a polyester-group resin containing a white pigment at a content of 10 to 30% by weight, the layer A having an amount of oligomer "O A " (% by weight) satisfying the equation: O A < (O B - 0.2). A metal-film laminate may be made by heat bonding such a white polyester film to a metal plate, in the case of a single layer film, by directly heat bonding the film to a metal plate, and in the case of a laminated film, by heat bonding the layer B to a metal plate. In such a white polyester film, in uses wherein a metal-film laminate is formed after the film is heat bonded to a metal, excellent light interruptive property, lamination property, forming property and processing property (less soil, less oligomer) as well as excellent heat resistance and wet heat resistance can be achieved. Further objects, features, and advantages of the present invention will be understood from the following more detailed description of preferred embodiments of the present invention. In the polyester-group resin forming the white polyester film according to the present invention, as the acid component, for example, an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid and naphthalene dicarboxylic acid and an aliphatic dicarboxylic acid such as adipic acid, azelaic acid, sebacic acid and decanedicarboxylic acid can be used. As the alcohol component, for example, an aliphatic diol such as ethylene glycol, diethylene glycol, butanediol and hexanediol and a cycloaliphatic diol such as cyclohexanedimethanol can be employed. In order to improve the forming property and heat bonding property in such a polyester-group resin, it is preferred that the sum total of any aliphatic acid component having not less than 4 carbon atoms which may be present and/or any alcohol component having not less than 4 carbon atoms which may be present is suppressed to an amount not more than 5.0 mol%, preferably not more than 1.5 mol%, more preferably not more than 0.8 mol%. If the sum is more than 5.0 mol%, the heat resistance and the wet heat resistance of the white polyester film tend to deteriorate. Moreover, it is further preferred that the sum of any aliphatic acid component and any alcohol component having not less than 3 carbon atoms is suppressed to an amount not more than 3.0 mol%. In the film according to the present invention, a white pigment having a mean particle diameter of 0.01 to 1.8 µ m is added at a content of 10 to 30% by weight. If the content is less than 10% by weight, the desired whitening becomes difficult to achieve, and if the content is more than 30% by weight, although the desired whitening can be achieved, the forming property deteriorates and in addition the film becomes fragile because of the high content. As a typical white pigment, titanium oxide, barium titanate, barium sulfate and calcium carbonate particles can be used. Particularly, as such a white pigment, titanium oxide particles having a mean particle diameter of 0.1 to 1.2 µ m and a loss on drying of not more than 0.8% are preferred. Such titanium oxide particles may be added at a content of 10 to 30% by weight to form a white polyester film. The mean particle diameter of the titanium oxide particles is preferably in the range of 0.1 to 1.2 µ m from the viewpoint of achieving a desired whitening while ensuring a desired forming property. As the titanium oxide particles, for example, rutile type, bruckite type and anatase type titanium oxide particles can be used. In particular, rutile type titanium oxide particles subjected to treatment with an organic material such as silane treatment (silane coupling agent treatment) or polyfunctional alcohol treatment is effective to achieve the above-described range of loss on drying. The loss on drying of the particles in the present invention was determined based on JIS-K5101 for a sample weight of 2g. In JIS-K5101, a sample of a pigment (particles) is heated in a dryer at a temperature of 105 °C ± 2 °C for two hours, and the loss on drying "W" (%) is calculated by the following equation. W = [(m 0 - m 1 )/m 0 ] x 100 Where, "m 0 " represents the weight of a sample before heating and "m 1 " represents the weight of the sample after heating In the determination, the loss on drying is preferred to be not more than 0.8%. If the loss on drying is more than 0.8%, the heat resistance and the wet heat resistance deteriorate. In order to control the loss on drying to not more than 0.8%, it is effective to perform a surface treatment on the titanium oxide particles. For example, the following treatments are effective. 1 ○ treatment by silane coupling agent or isopropyltitanate 2 ○ hydrophobic treatment by an organic compound after treatment by two or more kinds of hydrate oxides In the film according to the present invention, the void ratio must be controlled so as to lie in the range of 4 to 30%. Although the void ratio might be lower than the above-described range from the viewpoint of only voids, if the void ratio is lower than the above-described range, the amount of oligomer precipitated on the film surface increases and this is not preferred. On the other hand, if the void ratio is higher than the above-described range, the film becomes fragile and it becomes difficult to impart mechanical properties such as strength required to the film. The white polyester film according to the present invention must have a melting peak temperature of 150 to 245 °C. The melting peak temperature is preferably in the range of 180 to 230 °C. If the melting peak temperature is lower than 150 °C, a sufficiently high heat resistance cannot be obtained. If the melting peak temperature is higher than 245 °C, the forming property and the bonding property become poor. The melting peak temperature is determimed by using a differential scanning calorimeter (DSC) produced by Perkin Elmer Corporation and measuring it at a temperature elevation speed of 10 °C/min. In the film according to the present invention, if a component having a molecular weight of not more than 2,000, which may be an oligomer as described below, but could be, for example, a lower molecular weight component such as a monomer, is present to a content of not less than 1.5% by weight, the advantages according to the present invention may be further improved remarkably. Both during polymerization and subsequent extrusion into films (especially after recycling raw material) ring trimer oligomer may he formed. In a film according to the present invention, the amount of such oligomer must be not more than 3% by weight. The amount of oligomer is preferably not more than 1.5% by weight, more preferably not more than 1.0% by weight from the viewpoint of process stability. If the amount of oligomer is more than 3% by weight, when the film is subjected to a process for a long period of time, oligomer is likely to precipitate on the film surface and soil the film surface. Therefore, if the film is applied to, for example, cans for food, the oligomer may, for some reason, enter into food stored in the cans and various problems such as soiling of the food may occur as a result. The film according to the present invention may be formed as a laminated film of a layer A and a layer B. The layer B is composed of the above-described film. The layer A is composed of a polyester-group resin containing a white pigment at a content of 10 to 30% by weight. The amount of oligomer in the layer A is suppressed to a small amount, that is, so as to satisfy the equation: O A < (O B - 0.2) where, O A : amount of oligomer in layer A (% by weight) O B : amount of oligomer in layer B (% by weight). When a metal-film laminate is formed using such a laminated film having layers A and B, the layer B may be heat bonded to the metal and the layer A is placed opposite to the metal. In such a laminated film having layers A and B, a mixture of two or more kinds of polyester having intrinsic viscosities different from each other can be used as the resin for the layer B. For example, a mixture of a virgin raw material and a recycled raw material can be used. Even if the amount of oligomer in the layer B becomes relatively great, because the layer B is covered with the layer A having a smaller amount of oligomer, the precipitation of oligomer particularly to the film surface of the layer A side can be suppressed small. Therefore, troubles accompanying with precipitation of oligomer can be prevented in various processes, and the precipitation of oligomer to the outer surface of the film after lamination with a metal plate can be suppressed so as to be small. Namely, a recycled raw material having a relatively large amount of oligomer can be effectively reused. The above-mentioned intrinsic viscosity is determined at a temperature of 25 °C using o-chlorophenol. In the film according to the present invention, the orientation degree must be in the range of 0.7 to 4.0. If the orientation degree is lower than 0.7, the lamination property slightly deteriorates. If the orientation degree is higher than 4.0, the forming property slightly deteriorates. Although the film according to the present invention may be a uniaxially stretched film, a biaxially stretched film is preferred from the viewpoint of strength, durability, strength after forming, etc. In the case of a laminated film of layers A and B, although the thicknesses of the layers A and B are not particularly restricted, the thickness ratio of the layers A and B (layer A/layer B) is preferably in the range of 1/100 to 100/1. Further, the thickness of the layer A is preferably controlled to be in the range of 1.0 to 10 µ m for effectively suppressing the precipitation of oligomer in a process. Further, in the white polyester film according to the present invention, the surface glossiness is preferably in the range of 20 to 70% and the optical density at a thickness of 20 µ m is preferably in the range of 0.2 to 0.6 particularly from the viewpoint of printing appearance in use in a laminate with a metal. Where, the optical density represents a degree of opacity of the film, and it is defined by determining the ratio of intensity of an incident light (I 0 ) to intensity of a transmitted light (I) as "log(I 0 /I)". Particularly in the case of a laminated film of layers A and B, the content of the white pigment of the layer B may be less than the content of the white pigment in the layer A. In the preparation of the polyester-group resin according to the present invention, a catalyst for transesterification . such as Ca, Li, Mn, Zn or Ti can be used. Alternatively, terephthalic acid may be directly transesterified. As the polymerization catalyst, Sb 2 O 2 or germanium oxide can be used. Further, as needed, other additives, for example, fluorescent whitening agent, anti-oxidant, thermal stabilizer, ultraviolet absorbent or antistatic agent may be added in conventional amounts provided that the objects and advantages of the present invention are not adversely affected to any serious extent. In particular, it is preferred that an anti-oxidant is added, because the wet heat resistance can be further improved. Next, a typical method for producing the polyester film according to the present invention will be explained. The polyester-group resin to which has been added titanium oxide particles and other various additives is made molten and delivered out from a die to be cooled and formed into a sheet. In the case of a two-layer laminated film, two kinds of polyester-group resin are made molten by two extruders, respectively, the molten polymers are laminated in a feed block for lamination or a die for lamination, and the laminated polymers are delivered out from a die to be cooled and formed into a sheet. In a case where the formed sheet is stretched uniaxially or biaxially, the longitudinal stretching temperature is preferably in the range of 60 to 120°C, the longitudinal stretching draw ratio is preferably in the range of 2.6 to 5.5 times, the transverse stretching temperature is preferably in the range of 70 to 160°C, and the transverse stretching draw ratio is preferably in the range of 2.6 to 5.0 times. The thermosetting is conducted preferably at a temperature of 110 to 240°C and a relax ratio of 0 to 10%. The titanium oxide particles may be added either at the time of polymerization or at the time of film formation by mixing and dispersing the particles in the molten polymer. Alternatively, a method for preparing a high-concentration master pellets and diluting the master pellets may be employed. The white polyester film thus formed is heat bonded to a metal plate or a metal sheet by a regular method to form a metal-film laminate. As a typical metal plate laminated with the film according to the present invention, a tin plate or a tin-free steel or aluminum plate can be used. The surface of these metal plates may be appropriately treated by an organic or inorganic compound. Next, the methods for determining the characteristics in the present invention and estimating the effects according to the present invention will be explained. (1) Void ratio (%): Amount of addition of white pigment/specific gravity of the white pigment + [(1 - the amount of addition of the white pigment)/specific gravity of polyester portion] = 1/ δ was calculated. Using the calculated "δ ", the void ratio was defined by the following equation. Void ratio = [1 - (specific gravity of film/δ)] x 100 (%) For this calculation, the polyester portions were all regarded as crystallized, and 1.515 was used as the value of the specific gravity. (2) Melting peak temperature: Using a differential scanning calorimeter (DSC) produced by Perkin Elmer Corporation, the melting peak temperature was determined at a heating speed of 10°C/min. and an amount of sample of 10 mg. (3) Amount of oligomer (% by weight): A filtered solution was prepared by dissolving and depositing a sample using hexafluoroisopropanol/acetone. /acetone. Using a gel osmotic chromatography (GPC244, produced by Waters Corporation), the amount of ring trimer in the solution was determined at a detecting wave length of 254 nm, using chloroform (1 ml/min.) as a solvent. The determined amount of ring trimer was defined as the amount of oligomer. (4) Orientation degree: The following Fourier-transformation infrared spectrophotometer (FT-IR) was used. Measuring apparatus: FTS-15E/D (FT-IR produced by Bio Rad Digilab Corporation) Light source : Special ceramic detector "MCT" Attached apparatus : Micro-ATR measuring attachment (produced by Wilks Corporation) incident angle: 45 degrees IRE: KRS-5, using a polarizer Measuring conditions: resolution: 4 cm -1 cumulative times: 500 times The ratio of absorption of light having a frequency of 970 cm -1 due to ν C-O transband to absorption of light having a frequency of 790 cm -1 due to benzene ring was determined and this was defined as the orientation degree. (5) Mean particle diameter: After dissolving a polyester-group resin and separating particles from the resin, the mean particle diameter of the particles were determined with the particles dispersed in ethanol using a decanter (CAPA-500, produced by Horiba Seisakusho Corporation). (6) Light interruptive property: A film sample having a thickness of 15 µ m was prepared, and the light interruptive property was determined by optical density based on JIS-K7605. In JIS-K7605, an illumination light is radiated uniformly on a circlular area of the sample having a diameter of 1 mm or more. The optical density is determined by the following equation. Optical density = log 10 (F 0 /F) Where, "F" represents the amount of the light transmitted through the measuring area (the circular area) of the sample and measured by a light receiver behind the sample, and "F 0 " represents the amount of the transmitted light determined under the same conditions when the sample is removed. The light interruptive property was determined by the following ranks. ○ : Optical density/thickness ( µ m) is not less than 0.031. Δ: Optical density/thickness ( µ m) is less than 0.031 and not less than 0.026. ×: Optical density/thickness ( µ m) is less than 0.026. Rank "○" and rank "Δ" were found to be of practical use. (7) Lamination property: A film sample was laminated onto a hot tin-free steel plate of a thickness of 0.3 mm and having a temperature of a melting peak temperature (in a case where two or more peaks are present, arithmetic mean determined using melting energy ratio) plus 8 °C. The metal-film laminate was then rapidly cooled, and after dry treated at a temperature of 210 °C for five minutes, a weight or 250g was dropped onto tne metal-film laminate from a height of 6 cm to provide an impact. Thereafter, the sample was put in 1% NaCl water, a voltage of 6V was applied, and development of an electric current of not more than 0.2 mA was considered to be good " O " and higher currents were considered not to be good "×". (8) Forming property: The film sample was laminated in the same manner as in (7) above, and the metal-film laminate was then formed by drawing at a draw ratio of 1.37 times. The formed metal-film laminate was then dry treated at a temperature of 210°C for ten minutes. Where the film was delaminated from the metal, the film was whitened by deformation or delamination or the edge of the film shrinked remarkably, it was regarded as not good "×" and otherwise, it was regarded as good "○", (9) Processing property (Stability in a process): The film sample was laminated in the same manner as in (7) above, and the metal-film laminate was then dry treated at a temperature of 160°C for two hours (except that where melting peak temperature was not higher than 170°C, the sample was heated at [the melting peak temperature - 10°C l). The sample was washed by isopropyl alcohol, and the amount of oligomer precipitated on the film surface was determined. Where the amount of precipitated oligomer was more than 1/3 of the amount of contained oligomer or the amount of precipitatod oligomor was more than 2% by weight this was considered not to be good " ×", and where the amount of precipitated oligomer was not more than 1/3 of the amount of contained oligomer or the amount of precipitated oligomer was not more than 2% by weight, this was considered to be good "○". Where the amount of precipitated oligomer satisfied the level of rank " ○ " but the amount was in the range of 75 to 100% of the upper limit specified by the present invention, this was classed as rank "Δ". Examples and Comparative Examples: Preferred examples will be hereinafter explained together with comparative examples. The results of these examples and comparative examples are shown in Tables 1 to 6. Example 1: As shown in Table 1, a polyester-group resin whose acid component was composed of terephthalic acid (TPA) and isophthalic acid (IPA) and whose alcohol component was composed of ethylene glycol (EG) and diethylene glycol (DEG) was prepared by solid phase polymerization, and rutile type titanium dioxide particles (white pigment) having a mean particle diameter of 0.4 µ m whose loss on drying was controlled to 0.4% by providing silane treatment to the surfaces were added to the resin at a content of 16% by weight. The resin was supplied to an extruder and molten and extruded at a temperature of 280 °C (Table 6), and the molten polymer was delivered out from a die in the form of a sheet. The polymer sheet was cast onto a cooling drum and cooled and solidified to form a sheet. The non-stretched sheet was stretched in the longitudinal direction at a temperature of 102 °C and a draw ratio of 3.0 times, and thereafter, the uniaxially stretched film was stretched in the transverse direction at a temperature of 110 °C and a draw ratio of 3.1 times, and then the biaxially stretched film was heat treated at a temperature of 190°C and a relax of 1.0% to form a biaxially oriented polyester film having a thickness of 15 µ m (Table 6). The film thus prepared was laminated on a tin-free steel plate having a thickness of 0.4 mm at a temperature of 240 °C, and the obtained metal-film laminate was determined. The resulting data are shown in Tables 1, 2 and 6. Examples 2-4, Comparative Examples 1-5: Conditions were changed as shown in Tables 1 and 6. The characteristics of the films prepared and the metal-film laminates obtained are shown in Tables 1 and 2. With respect to loss on drying of the pigment, the loss on drying due to silane treatment was 0.4%, the loss on drying due to alcohol treatment was 0.7% and the loss on drying with no treatment was 1.4%. Examples 5-10, Comparative Examples 6-8: In these Examples and Comparative Examples, recycled raw material was used at a content of 40% by weight for forming the layer B, and the film was formed as a laminated film of layers A and B. Conditions were as shown in Tables 3, 4, 5 and 6. The characteristics of the films prepared and the metal-film laminates obtained are shown in Tables 3, 4 and 5. As shown in Tables 1 to 5, the films and the metal-film laminates embodying the present invention manifested excellent light interruptive properties, lamination properties, forming properties and processing properties, but those outside the present invention did not manifest such excellent properties. Light interruptive property Void ratio (%) Lamination property Forming property Processing property Example 1 ○ 9 ○ ○ ○ Example 2 ○ 11.5 ○ ○ ○ Comparative Example 1 ○ 21.2 × × ○ Comparative Example 2 ○ 9 × × ○ Comparative Example 3 × 3.1 × × ○ Comparative Example 4 × 5.6 × × × Comparative Example 5 × 6.8 ○ × × Example 3 ○ 8.6 ○ ○ ○ Example 4 ○ 20.9 ○ ○ ○ Extrusion temperature (°C) Longitudinal stretching Transverse stretching Heat treatment Examples 1,2,4-10 Comparative Examples 1,4 & 8 280 102 °C 3.0 times 110 °C 3.1 times 190 °C 1.0 % Comparative Example 2 280 95 °C 3.0 times 110 °C 3.1 times 190 °C 1.0 % Comparative Example 3 280 102 °C 3.8 times 110 °C 4.3 times 180 °C 0 % Example 3 280 80 °C 3.0 times 93 °C 3.1 times 140 °C 1.0 % Comparative Example 5 280 40 °C 1.15times none none none none Comparative Example 6 280 105 °C 2.9 times 115 °C 3.0 times 200 °C 3.0 % Comparative Example 7 280 85 °C 4.1 times 100 °C 4.1 times none none

Claims: A polyester film comprising a polyester-group resin containing a white pigment having a mean particle diameter of 0.01 to 1.8 µ m at a content of 10 to 30% by weight, which film has (a) a void ratio of 4 to 30%, (b) a melting peak temperature of 150 to 245 °C, (c) an amount of oligomer of not more than 3% by weight and (d) an orientation degree of 0.7 to 4.0; each of which parameters (a) to (d) is as defined herein. A polyester film according to claim 1, wherein the white pigment comprises titanium oxide particles having a mean particle diameter of 0.1 to 1.2 µ m and a loss on drying of not more than 0.8%. A polyester film according to claim 2, wherein the titanium oxide particles are rutile type titanium oxide particles treated by a silane or an alcohol. A polyester film according to any preceding claim, wherein the sum total amount, in the polyester-group resin, of any aliphatic acid component and/or any alcohol component having not less than 4 carbon atoms is not more than 5.0 mol%. A metal-film laminate comprising a metal plate and a polyester film as claimed in any one of claims 1 to 4 heat bonded to the metal plate. A metal-film laminate according to claim 5, wherein the metal plate is composed of a tin-free steel. A laminated polyester film comprising: a layer B comprising a polyester-group resin containing a white pigment having a mean particle diameter of 0.01 to 1.8 µ m at a content of 10 to 30% by weight, the layer B having (a) a void ratio of 4 to 30%, (b) a melting peak temperature of 150 to 245 °C, (c) an amount of oligomer "O B " of not more than 3% by weight and (d) an orientation degree of 0.7 to 4.0, wherein each of which parameters (a) to (d) is as defined herein ; and a layer A laminated on the layer B, the layer A comprising a polyester-group resin containing a white pigment at a content of 10 to 30% by weight, the layer A having an amount of oligomer "O A " (% by weight) satisfying the equation: O A < (O B - 0.2). A laminated polyester film according to claim 7, wherein the content of the white pigment of the layer B is less than the content of the white pigment in the layer A. A laminated polyester film according to claim 7 or 8, wherein the white pigment of the layer B and/or the white pigment of -the layer A comprises titanium oxide particles having a mean particle diameter of 0.1 to 1.2 µ m and a loss on drying of not more than 0.8%. A laminated polyester film according to claim 9, wherein the titanium oxide particles are rutile type titanium oxide particles treated with a silane or an alcohol. A laminated polyester film according to any one of claims 7 to 10, wherein the sum total amount, in the polyester group resin, of any aliphatic acid component and/or any alcohol component of the layer B having not less than 4 carbon atoms is not more than 5.0 mol%. A metal-film laminate comprising a metal plate and a laminated polyester film as claimed in any one of claims 1 to 11, wherein layer B of the laminated polyester film is heat bonded to the metal plate. A metal-film laminate according to claim 12, wherein the metal plate is composed of a tin-free steel.

Inventors: KUROME HIROKAZU, OOE WATARU, SHIBATSUJI KUNIO, SUGIURA SEIYA, SUMIYA TAKASHI, TANAKA IWAO, KUROME, HIROKAZU, OOE, WATARU, SHIBATSUJI, KUNIO, SUGIURA, SEIYA, SUMIYA, TAKASHI, TANAKA, IWAO
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1995
Main classification: C08J5
IPC codes: B32B15, C08J5, B32B27, C08K3