JP6377911B2 - Polyester film - Google Patents

Description

  The present invention relates to a polyester film used for industrial purposes and a method for producing the same. Specifically, the present invention relates to a polyester film having excellent heat whitening prevention properties.

Polyester films represented by polyethylene terephthalate are superior in cost performance while having excellent mechanical strength, dimensional stability, flatness, heat resistance, chemical resistance, optical properties, etc. It is used.
Polyester is a linear polymer usually produced from a dicarboxylic acid component and a glycol component by a polycondensation reaction. However, known polyesters contain about 1% by weight of cyclic oligomers. Such a cyclic oligomer has a problem that when the polyester film is heat-treated, it is deposited on the film surface and the film is whitened. In particular, as the use of polyethylene terephthalate film diversifies, the processing conditions and use conditions of the film also diversify. Oligomer precipitation on the film surface is particularly important for applications that require high transparency, such as optical applications. This is a serious problem when used for cast supports and the like that require high surface flatness. In recent years, there is a tendency that the heat treatment temperature applied to the post-processing for the purpose of improving the function of the product is increased, and oligomer precipitation due to the heat treatment is becoming more serious.

As a method for suppressing the precipitation of oligomers due to heating, as described in Patent Document 1, a proposal has been made to suppress the heated precipitation oligomers by applying a specific coating layer and modifying the surface of the polyester film. However, when oligomer deposition is suppressed in the coating layer, even slight scratches that do not affect the quality lack the precipitation suppression function, and the precipitation may occur intensively there. It cannot be.
Then, in order to reduce the oligomer in a polyester film, reducing the amount of cyclic oligomers of a polyester raw material by the solid-phase polymerization method is proposed (patent documents 2-5). As another method for reducing the amount of cyclic oligomer in polyester, Patent Document 6 proposes a method of adjusting the flow rate of the inert gas to 1 to 500 liters / kg · hour during heat treatment. 7 proposes a method of adjusting the degree of vacuum during solid-phase polymerization to 15 to 300 mmHg. Further, Patent Document 8 proposes that the hydroxyl (OH) terminal amount of the polyester resin is set to a predetermined amount or less.

JP 2005-336394 A JP-A-9-99530 JP 2000-141570 A JP 2003-191413 A Japanese Patent Laid-Open No. 2003-301057 Japanese Patent Publication No.62-49294 Japanese Patent Publication No.62-49295 JP 2011-252128 A

  However, in the methods proposed in Patent Documents 2 to 5, although the amount of cyclic oligomer in the polyester can be reduced by solid phase polymerization, the polycondensation reaction of the polyester also proceeds at the same time, and the degree of polymerization of the obtained polyester becomes high. For this reason, the intrinsic viscosity of the polyester is increased, the load during extrusion molding is increased, and the temperature of the polyester is increased by shearing heat generation, causing thermal decomposition. For this reason, a high melting point product is generated, which may cause a problem of deterioration of transparency of the obtained molded body or the like and fluctuation of the crystallization speed.

  On the other hand, in the methods proposed in Patent Documents 6 and 7, the amount of the cyclic oligomer can be reduced while suppressing the progress of the polycondensation reaction of the polyester, but there is a problem that the cyclic oligomer is regenerated upon subsequent melting. That is, it is necessary to melt the raw material polyester in the film formation, and even if the amount of the cyclic oligomer in the film raw material is reduced by a conventionally known method, the cyclic oligomer is formed as a by-product due to the thermal history in the film melting film formation. It was inevitable to generate. For this reason, efforts have been made to reduce the amount of cyclic oligomers in the film raw material as much as possible, but there is a limit to such measures from the viewpoint of productivity. Therefore, a sufficient low oligomer film has not been realized by regenerating the cyclic oligomer in the melt extrusion process during film formation.

In addition, a decrease in the hydroxyl (OH) terminal amount of the polyester resin means an increase in one terminal carboxyl (COOH) terminal, impairing heat resistance during melt molding and durability in long-term use. It was scarce.
Further, even when such a resin with a low oligomer content is used, oligomer precipitation may be observed in a very severe environment, for example, when the temperature of the heat treatment is very high or the time is long.
An object of the present invention is to solve the above-mentioned problems of the conventional methods and to provide a polyester film that is excellent in transparency after heat processing and has less precipitation of cyclic oligomers even under more severe conditions.

  As a result of intensive studies in view of the above circumstances, the present inventors have found that the above problems can be solved by setting the surface energy of the film made of a specific resin within a specific range, and the present invention has been completed. It was.

That is, the present invention is a film comprising a polyethylene terephthalate resin, wherein the intrinsic viscosity of the polyethylene terephthalate resin is 0.60 dl / g or more and 0.68 dl / g or less, and the weight of the cyclic trimer oligomer in the film. the ratio WCy3 / WCy4 the fraction (WCy3) and weight fraction of the cyclic tetramer oligomer (WCy4) is 3 or less, and der least the surface energy at one surface is 50 mN / m or more films is, the display characterized Rukoto using a polyester film.

Further, the present invention preferably further has a specific structure, that is, the film is a laminated film of at least three layers obtained by a coextrusion method, and the inherent property of the resin constituting the outermost layer. It is preferable that the viscosity is higher than the intrinsic viscosity of the resin constituting the inner layer.
Furthermore, it is more preferable that the film has a coating layer on at least one surface thereof, and the surface energy on the surface of the coating layer is 50 mN / m, and it is particularly preferable to form the coating layer on the film within the film manufacturing process. preferable.

The polyester film of the present invention is excellent in transparency after heat processing under harsher conditions, and since the oligomer precipitation is small, it can be post-processed at a high temperature and for a long time, so high quality is required. It can be suitably used in industrial applications including optical applications.
Moreover, since it is difficult to accompany the increase of the carboxyl terminal of the polyester resin, it has heat resistance at the time of melt molding and durability in long-term use, and is excellent in practicality.

Hereinafter, the present invention will be described in more detail.
[Polyester film]
(Polyethylene terephthalate resin)
The polyester film in the present invention is a film made of polyethylene terephthalate resin.
The polyethylene terephthalate resin in the present invention is a polyester having ethylene terephthalate as a main repeating unit. Here, the “main repeating unit” means 80 mol% or more, preferably 90 mol% or more, particularly preferably 95 mol% or more of all repeating units constituting the polyester. That is, the polyester may be a copolyester. In that case, examples of the copolymer component include acid components such as isophthalic acid and naphthalenedicarboxylic acid, and glycol components such as diethylene glycol and 1,4-butanediol.

The polyethylene terephthalate resin in the present invention is preferably a polyester polymerized using a germanium compound or a titanium compound as a polymerization catalyst, and among these, it is particularly preferable to use a germanium compound as a polymerization catalyst. When a germanium compound is used as a polymerization catalyst, the polyester can contain, for example, 0.1 to 100 ppm, preferably 1 to 70 ppm, more preferably 10 to 50 ppm of germanium element. Here, the germanium element is derived from a germanium compound used as a polymerization catalyst for polyester. Examples of such germanium compounds include germanium dioxide, germanium tetroxide, germanium hydroxide, germanium oxalate, and germanium chloride.
In this way, the polyethylene terephthalate resin obtained from the final polycondensation reactor is usually formed into granules (chips) by a melt extrusion molding method and then supplied to the solid phase polycondensation step.

The granular polyethylene terephthalate resin supplied to the solid phase polycondensation step may be preliminarily crystallized by heating to a temperature lower than the temperature at which solid phase polycondensation is performed, and then supplied to the solid phase polycondensation step. Good. Such a precrystallization step can be performed by heating granular polyethylene terephthalate in a dry state, usually at a temperature of 120 to 200 ° C., preferably 130 to 180 ° C. for 1 minute to 4 hours. It can also be carried out by heating to a temperature of 120 to 200 ° C. for 1 minute or longer in a steam atmosphere or a steam-containing inert gas atmosphere.
The solid phase polycondensation step in which such granular polyethylene terephthalate resin is supplied comprises at least one stage, and the polycondensation temperature is usually 190 to 230 ° C., preferably 195 to 225 ° C., and nitrogen gas, argon gas, carbon dioxide gas The solid phase polycondensation reaction is performed under an inert gas atmosphere such as. Of these inert gases, nitrogen gas is preferred.

  The polyester resin (the granular polyethylene terephthalate resin) as a raw material for forming a film in the present invention, as a result of undergoing the polymerization step, preferably has an intrinsic viscosity of 0.65 or more, more preferably 0.70 or more, More preferably, it should be 0.75 or more. By doing in this way, it becomes easy to make the intrinsic viscosity of a film into the range prescribed | regulated by this invention.

The polyester film referred to in the present invention is a polyester film extruded by a so-called extrusion method in which the above-mentioned polyester is usually melt-extruded from an extrusion die, and if necessary, in the biaxial direction of the vertical direction and the horizontal direction. An oriented film. Although the stretching in the machine direction and the stretching in the transverse direction may be performed separately, they are manufactured by the simultaneous biaxial stretching method because there are fewer opportunities for contact with the roll and it is difficult to generate scratches on the surface that cause oligomer precipitation. It is preferable to do.
The total thickness of the film is usually 25 to 200 μm, preferably 38 to 188 μm. If it is less than 25 μm, the mechanical strength and heat resistance of the film may be insufficient, and problems such as wrinkles may occur in subsequent processing steps. On the other hand, if the thickness of the film exceeds 200 μm, the film may be too stiff and handleability in the subsequent process may be poor.

(Intrinsic viscosity of film)
The intrinsic viscosity (unit: dl / g) of the resin constituting the polyester film of the present invention is 0.60 or more. The intrinsic viscosity is preferably 0.62 or more, more preferably 0.64 or more. In order to suppress the precipitation of the oligomer, both the effect of suppressing the oligomer regeneration and the effect of suppressing the oligomer movement are important. If the intrinsic viscosity of the resin constituting the film is less than 0.60, both effects are It is damaged and appearance defect (whitening) occurs during heat processing. In the present invention, in consideration of such a mechanism, the intrinsic viscosity of the film exhibits the above-described effects when the intrinsic viscosity of the entire polyester film is within the above numerical range. Therefore, when the film is an embodiment of a laminated film described later, the intrinsic viscosity of the laminated film as a whole may be in the above numerical range. In the aspect of the laminated film, the intrinsic viscosity of the polyethylene terephthalate resin constituting the outermost layer of each of the front and back layers constituting the laminated film is preferably 0.60 or more, more preferably 0.62 or more, further preferably The aspect which is 0.64 or more, Most preferably, it is 0.66 or more is preferable. In the most preferred embodiment, the polyethylene terephthalate resin constituting the outermost layer of the two layers satisfies the above preferred range, and the intrinsic viscosity of the polyethylene terephthalate resin constituting the inner layer is preferably 0.60 or more, more preferably 0.62 or more. More preferably, the aspect is 0.64 or more. Here, when the inner layer is a plurality of layers, the intrinsic viscosity as a bulk of the entire inner layer may be in the above-mentioned preferable range.

  Moreover, it is preferable that the intrinsic viscosity of resin which comprises the polyester film of this invention is 0.72 or less. Thereby, the load concerning resin in extrusion molding can be made small, and shear heat_generation | fever can be suppressed. Thereby, thermal decomposition of the resin due to such heat generation can be suppressed. From this viewpoint, the intrinsic viscosity is more preferably 0.70 or less, and still more preferably 0.68 or less. According to the present invention, oligomers can be suppressed while using a resin whose intrinsic viscosity is not excessively high.

(Laminated film)
The polyester film of the present invention can be preferably a laminated polyester film of two or more layers having one outermost layer and another layer, or three or more layers having two outermost layers and an inner layer. Here, the inner layer may be a single layer or a plurality of two or more layers. In order to obtain such a lamination mode, a laminate structure using a so-called coextrusion method using two or more extruders is preferable.

  As the structure of the layer, an A / B structure using an A raw material and a B raw material, or an A / B / A structure, an A / B / C structure using a C raw material, or a layer having a larger number of layers other than the above. It can be set as the film of a structure. Here, for example, A and C are the outermost layers, and B is the other layers and the inner layers. More specifically, for example, the surface flatness can be designed using a raw material that does not contain particles as the A raw material, and the raw material containing particles can be A / B as the B raw material. Moreover, it can also be set as the film of A / B / A structure using the same raw material, an easy slipping layer can be formed in the surface of one A layer, and the surface defect by film manufacture can also be suppressed. In this case, since the raw material of B layer can be selected freely, a cost advantage is large. Further, even if the recycled material of the film is blended in the B layer, the surface roughness can be designed by the surface A layer, so that the cost advantage is further increased.

  In the present invention, the polyester film is a laminated film, and the intrinsic viscosity of the resin constituting the outermost layer (sometimes referred to as the surface layer) is the inner layer (sometimes referred to as the core layer. In addition, the two-layer configuration. It is preferable that the other layers in the above are higher than the intrinsic viscosity of the resin constituting the layer). This difference is preferably 0.01 or more, more preferably 0.02 or more, further preferably 0.03 or more, and particularly preferably 0.04 or more. Here, the intrinsic viscosity of the resin constituting the inner layer refers to the intrinsic viscosity as a bulk of the entire inner layer when there are a plurality of inner layers. If the intrinsic viscosity of the resin that forms the outermost layer is lower than the intrinsic viscosity of the resin that forms the inner layer, the effects of both the oligomer regeneration inhibiting effect and the oligomer migration inhibiting effect tend to be reduced. Precipitation tends to increase and the film may impair the appearance.

  The thickness of the outermost layer of the film (the thickness of one layer) is preferably 0.5 μm or more and 30 μm or less, more preferably 1.0 μm or more and 25 μm or less, and further preferably 3 μm or more and 20 μm or less. When the outermost layer is less than 0.5 μm, the effect of suppressing the oligomer movement is reduced, and as a result, the precipitation of the cyclic oligomer after heat processing tends to increase, and the film may impair the appearance. On the other hand, the upper limit of the thickness may be 30 μm or less because the outermost layer of the film may be a thickness that can suppress the migration of the oligomer, and a sufficient effect is exhibited even if it is 20 μm or less.

(Lubricant)
Particles can be added to the polyester film of the present invention as long as the effects of the present invention are not hindered. Examples of the particles added to the film include inorganic particles such as silicon dioxide, alumina, zirconium oxide, kaolin, talc, calcium carbonate, titanium oxide, barium oxide, carbon black, molybdenum sulfide, antimony oxide, and the like, and hybrids thereof. . Among these, silicon dioxide is easy to use because it is inexpensive and has various particle sizes.

  Examples of the organic particles include polystyrene or polyacrylate, polymethacrylate, and organic / inorganic hybrid products in which a crosslinked structure is achieved by a compound containing two or more carbon-carbon double bonds in one molecule (for example, divinylbenzene). It is done. In the present invention, when the particles are blended with the polyester, the method is not particularly limited, and a known method can be adopted. For example, it can be added at any stage of producing the polyester, but it is preferably added as a slurry dispersed in ethylene glycol or the like at the stage of esterification or before the start of the polycondensation reaction after completion of the transesterification reaction, The polycondensation reaction may proceed. Also, a method of blending a slurry of particles dispersed in ethylene glycol or water with a vented kneading extruder and a polyester raw material, or a blending of dried particles and a polyester raw material using a kneading extruder. It is done by methods.

(Additive)
The polyester film of the present invention can be mixed with other thermoplastic resins such as polyethylene naphthalate and polytrimethylene terephthalate as long as the effects of the present invention are not impaired. Moreover, you may mix | blend coloring agents, such as a ultraviolet absorber, antioxidant, surfactant, a fluorescent whitening agent, a lubricant agent, a light-shielding agent, a matting agent, and a dye, a pigment.

(Oligomer fraction)
The polyester film of the present invention has a ratio of weight fraction of cyclic trimer oligomer (WCy3) (unit: wt%) to weight fraction of cyclic tetramer oligomer (WCy4) (unit: wt%) in the film ( (Weight fraction ratio) WCy3 / WCy4 must be 5 or less.
When this weight fraction ratio exceeds 5, deterioration in appearance (whitening) in the heating step becomes particularly remarkable. From this viewpoint, the weight fraction ratio is preferably 4 or less, more preferably 3.5 or less, and still more preferably 3 or less. In order to set the weight fraction ratio of the cyclic oligomer within the above range, for example, water treatment described in JP-A-3-47830 is performed to sufficiently reduce the activity of the polymerization catalyst, and the resin is sufficiently dried. Can be achieved by providing a suitable thermal history and then using it to form a film. As a method of giving a thermal history to the resin, (1) a method in which a chip is once melted and extruded (for example, in the form of a strand) to re-chip, and (2) after the chip is melt-extruded, for example, in a film forming apparatus Examples of the method include forming a film or the like, then crushing and remelting the molded product, and extruding (for example, in the form of a strand) to rechip. The method described in (2) is particularly preferable, and the weight fraction ratio of the cyclic oligomer is efficiently adjusted to a specified range by mixing the resin with and without heat history in an appropriate ratio. I can do it. That is, since the ratio of WCy3 / WCy4 tends to be small after passing through an appropriate thermal history, the tendency of the ratio of WCy3 / WCy4 to be small as a whole should be utilized when the content of such resin is increased. . Hereinafter, a specific method is illustrated as an example. This method is a method for producing a film through three steps from step 1 to step 3.

[Step 1]
First, as the process 1, the preferred polyester resin in the present invention described above is used, and it is melt-extruded to produce a resin composition 1. Here, the polyester resin is the aforementioned polyethylene terephthalate resin. In the present invention, “by melt extrusion” refers to discharging a molten resin from a die or the like.
The resin composition 1 may be, for example, a fiber, a film, or another three-dimensional solid shape. In the present invention, the other three-dimensional solid shape is neither a fiber shape nor a film shape, and may be a polyhedron such as a cube, a curved surface such as a sphere, a box shape, or a film shape. Including non-sheet-like or plate-like ones.
The resin composition 1 contains 90 to 100% by weight of a polyester resin. Here, content is content with respect to 100 weight% of weight of the resin composition 1 obtained. By setting such a weight ratio range, it is possible to reduce the content of oligomers by the heat treatment in the subsequent step 2. Finally, in step 3, the weight fractions of the cyclic trimer and the cyclic tetramer are within the range defined by the present invention. The resin molded body 4 can be obtained.

  In the melt extrusion of the resin composition 1, the melt extrusion conditions may be appropriately determined according to the melting point of the polyester resin to be used and the shape and characteristics of the resin composition 1 to be obtained. In addition, the resin composition 1 obtained here is indirectly a raw material of the resin molding 4 to be finally obtained, and thus characteristics such as appearance are not so important. From the viewpoint of deterioration of the resin, it is preferable that there is less deterioration and less polymer chain scission by hydrolysis. Therefore, it is preferable that the melt extrusion temperature in this process is not too high. Further, if the productivity is too low, the raw material for producing the resin molded body 4 is insufficient, but a certain degree of productivity is necessary. Therefore, it is preferable that the temperature condition is not too low. Further, the melt extrusion time may be appropriately set in consideration of the tendency that the deteriorated product increases if it is too long and the unmelted product increases if it is too short. For example, 5 to 30 minutes.

[Step 2]
Subsequent to Step 1, as Step 2, the resin composition 1 obtained in Step 1 is melt-kneaded at a temperature of Tm to Tm + 60 ° C., where Tm is the melting point of the polyester constituting the resin composition 1. The resin composition 2 is manufactured by melt extrusion. Here, “melt kneading” includes a mode in which the resin is moved forward while kneading the resin in the screw portion of the extruder.
In addition, if the resin composition 1 obtained in the step 1 is in the form of pellets, it can be used as it is in the step 2 by being put into an extruder as it is. When the resin composition 1 has a fibrous shape, a film shape, or other three-dimensional solid shape, the resin composition 1 may be put into the extruder after being formed into a shape that can be put into the extruder by pulverization or the like. In addition, the pulverized product can be used after being granulated into a pellet by applying pressure.

In the melt-kneading and melt-extrusion of the resin composition 2, those conditions satisfy the above-described conditions, and the other conditions are the melting point of the polyester resin to be used, the shape and characteristics of the resin composition 2 to be obtained. It may be determined appropriately according to the situation. In addition, the resin composition 2 obtained here is a raw material of the resin molded body 4 to be finally obtained, and thus characteristics such as appearance are not so important, but the deterioration of the resin From the viewpoint, it is preferable that there are few deteriorated products and the polymer chain is not broken by hydrolysis. From this viewpoint, it is preferable that the temperature condition is not too high. Further, if the productivity is too low, raw materials for producing the resin molded body 4 are insufficient, and therefore a certain degree of productivity is necessary. From this viewpoint, it is preferable that the temperature condition is not too low. Further, the melt extrusion time may be appropriately set in consideration of the tendency that the deteriorated product increases if it is too long and the unmelted product increases if it is too short. For example, 5 to 30 minutes.
As a specific example of the step 2, the film 1 as the resin composition 1 obtained in the step 1 is a temperature in the temperature range of more than Tm and Tm + 60 ° C. or less, where Tm is the melting point of the polyester constituting the film 1 A step of melt-kneading at t, melt-extruding, and producing pellets 2 as the resin composition 2 can be mentioned.

[Step 3]
Subsequent to Step 2, as Step 3, a resin composition 3 containing the resin composition 2 obtained in Step 2 above is prepared, and the melting point of the polyester constituting the resin composition 1 is defined as Tm. The film 4 as the resin molded body 4 is manufactured by melt-kneading at a temperature of 0 ° C. or less and melt-extruding.
The content of the resin composition 2 in the resin composition 3 may be set as appropriate. Further, when the resin composition 2 is a recovered and recycled raw material, the content of the resin composition 2 in the resin composition 3 is a recovery rate, and by adding as much as the characteristics of the resulting resin molded body 4 allow, This is preferable because it reduces costs and improves productivity. From this viewpoint, the content of the resin composition 2 in the resin composition 3 is, for example, 15% by weight or more, preferably 30% by weight or more, more preferably 35% by weight or more, and further preferably 40% by weight or more. . The upper limit is 100% by weight. In addition, content is content with respect to the weight of the resin composition 3 obtained here.

  The resin composition 3 contains the resin composition 2, but the other components will be obtained as long as the main component is a polyester resin and does not impair the object of the present invention as a subordinate component. A component appropriately selected according to the configuration and characteristics of the resin molded body 4 can be contained. Here, the “main component” means that it is usually 50% by weight or more, preferably 70% by weight or more, more preferably 90% by weight or more in the remaining components. Examples of the polyester resin and appropriately selected components include a polyester resin constituting the resin composition 1. The polyester resin as the remaining component constituting the resin composition 3 and the polyester resin constituting the resin composition 1 may be the same or different. When the polyester resin is employed, the polyester resin may be used in the form of pellets.

In addition, if the resin composition 2 obtained in the step 2 is in the form of pellets, it can be used as it is in the step 3 by being put into an extruder as it is. When the resin composition 2 has a fibrous shape, a film shape, or other three-dimensional solid shape, the resin composition 2 may be put into the extruder after being formed into a shape that can be put into the extruder by pulverization or the like. In addition, the pulverized product can be used after being granulated into a pellet by applying pressure.
Thus, the film 4 as the resin molded body 4 can be manufactured. The film 4 thus obtained satisfies the above-described ratio range of WCy3 / WCy4. In addition, in the aspect of the laminated film, the content of the resin that has undergone such a thermal history is adjusted in each layer, and the thickness ratio between the outermost layer and the inner layer is adjusted as necessary, so that WCy3 / WCy4 as a whole. The ratio can be adjusted to be within the range defined by the present invention.
In the present invention, WCy3 is preferably 1% by weight or less, more preferably 0.85% by weight or less.

(Surface energy of film)
The polyester film of the present invention has a surface energy on at least one surface of 50 mN / m or more, more preferably 53 mN / m or more, still more preferably 55 mN / m or more, and particularly preferably 60 mN / m or more. Even if the surface energy of the polyester film of the present invention is less than 50 mN / m, the oligomer film is sufficiently precipitated under conditions used in post-processing such as hard coat processing (up to about 1 hour at 150 ° C.). Although it can be suppressed, a slight increase in haze may be observed in processing at a temperature of 180 ° C. or higher or heat processing for several hours. Accordingly, the present invention is capable of preventing the haze from increasing even at higher temperatures and longer conditions by setting the surface energy of the film to 50 mN / m or more. This increase in haze is considered to occur because the oligomer diffusion rate increases at a very high temperature such as 180 ° C. or higher, so that slight oligomer deposition occurs on the surface, which deposits on the film surface. . Therefore, the present invention finds that by increasing the surface energy of the film, deposition of oligomers on the film surface can be prevented if the amount is small, thereby suppressing an increase in haze.
From the above viewpoint, in the present invention, it is particularly preferable that the surface energy of both surfaces of the polyester film is in the above range.

  The polyester film made of polyethylene terephthalate resin usually shows a surface energy of less than 50 mN / m in a state where nothing is processed, so that some surface processing is necessary to make the surface energy 50 mN / m or more. The method is not limited as long as the object of the present invention is not violated. Processing methods for increasing the surface energy are generally roughly classified into physical methods and chemical methods. Examples of physical methods include corona treatment, plasma treatment, flame treatment, ultraviolet ray treatment, electron beam / radiation treatment, etc., and from the point of maintaining a clean surface state without impairing the properties of the polyester film, Plasma treatment can be exemplified more preferably. On the other hand, chemical treatment includes chemical treatment, steam treatment, surface grafting treatment, atmospheric pressure plasma treatment in a specific gas atmosphere, electrochemical treatment, and coating treatment. Among these, a method of forming a coating layer on one side or both sides of a polyester film by a coating treatment or the like can be preferably exemplified from the viewpoint that surface processing can be performed without impairing the characteristics of the film, which will be described in detail below.

(Coating layer)
The formation of the coating layer is not particularly limited, but can be formed by coating, which is preferable. Note that the coating layer in this case may be referred to as a coating layer.
There are two types of coating on polyester film: a method of coating a coating using a coater such as a roll coater or die coater after manufacturing the film, and a method of using a coater in the process of manufacturing the film. Among them, the method of coating in the film production process is preferable from the viewpoint that oligomer precipitation / deposition on the film surface in the film production process can be suppressed. Even in the case where the coating is applied after the film is manufactured, it is generally preferable that the undercoating is separately performed in the film manufacturing process in order to ensure the adhesion durability of the coating layer.

  In the present invention, the coating layer is mainly composed of a binder component that has good adhesion to the polyester film and can increase the surface energy of the film. Moreover, the crosslinking component for improving durability of a coating film, the easy-slip component which provides handling property to a film, etc. can be mix | blended. Here, “mainly” means 50% by weight or more, preferably 55% by weight or more, more preferably 60% by weight, based on the solid content of the coating liquid for forming the coating layer. The upper limit of the content of the binder component is not limited, and an optional component described later may be arbitrarily contained in the solid content of the coating liquid, and the content may be selected so that the remainder becomes the binder component.

  Preferable examples of the binder component for increasing the surface energy include polyester resins, acrylic resins, and polyurethane resins. Said resin shall also contain those derivatives, respectively. The derivative here refers to a resin obtained by reacting a reactive compound with a copolymer or a functional group with other components. In any case, it is more preferable to contain a hydrophilic functional group for the purpose of increasing the surface energy. Among these, a polyester resin binder is preferable from the viewpoint of superior adhesion to a polyester film. In this case, in order to contain a hydrophilic functional group, it is preferable to copolymerize an acid component having a metal base or to copolymerize a polyalkylene glycol component such as a diethylene glycol component or triethylene glycol. The copolymerization amount is preferably 0.1 to 8 mol%, more preferably 2 to 6 mol%, based on 100 mol% of the total acid component or total alcohol component of the polyester. When the amount of hydrophilic functional groups is small, the effect of increasing the surface energy is low. On the other hand, when the amount is large, the surface energy tends to be high, but the adhesion durability of a hard coat or the like tends to be poor in a humid and hot atmosphere. In particular, when the binder component is water-soluble, the adhesion durability under a moist heat atmosphere may be greatly deteriorated. Therefore, the binder component is preferably water dispersible.

  Melamine-based, epoxy-based, and oxazoline-based resins are generally used as a crosslinking agent to be added for the purpose of enhancing durability, and among these, oxazoline-based resins are particularly preferable in terms of coatability and durable adhesiveness. As content of a crosslinking agent, 1 to 40 weight% is preferable with respect to solid content of the coating liquid for forming a film layer, Furthermore, 2 to 30 weight% is preferable.

  On the other hand, it is preferable to contain inorganic particles or organic particles as the slippery component. Examples of the inorganic particles include silicon dioxide, alumina, zirconium oxide, kaolin, talc, calcium carbonate, titanium oxide, barium oxide, carbon black, molybdenum sulfide, antimony oxide and the like and hybrid products thereof. Among these, silicon dioxide is easy to use because it is inexpensive and has various particle sizes. Examples of the organic particles include polystyrene or polyacrylate polymethacrylate and organic / inorganic hybrid products in which a crosslinked structure is achieved by a compound containing two or more carbon-carbon double bonds in one molecule (for example, divinylbenzene). The compounding amount of the particles in the coating layer is usually 0.1 to 10% by weight, preferably 0.1 to 5% by weight. When the blending amount is less than 0.1% by weight, the blocking resistance and the slipperiness may be insufficient. When the blending amount exceeds 10% by weight, the transparency of the film may be hindered, which may hinder online inspection. is there.

Moreover, the coating layer may contain an antistatic agent, an antifoaming agent, a coating property improving agent, a thickener, an antioxidant, an ultraviolet absorber, a foaming agent, a dye, a pigment, and the like as necessary.
As the coating method of the coating layer, for example, reverse roll coater, gravure coater, rod coater, air doctor coater or other coating apparatus as shown in Yuji Harasaki, Tsuji Shoten, published in 1979, “Coating Method” Can be used.

The coating layer of the present invention is preferably applied within the film production process, but is preferably applied before film stretching or after longitudinal stretching in the case of stretching by a longitudinal-lateral sequential biaxial method.
The thickness of the coating layer is usually in the range of 0.01 to 0.5 μm, preferably 0.02 to 0.3 μm, as the final dry thickness. When the thickness of the coating layer is less than 0.01 μm, it may be difficult to obtain a uniform coating layer, and oligomer precipitation may not be sufficiently suppressed. When the thickness of the coating layer exceeds 0.5 μm, the films tend to stick to each other, and particularly when the coated film is re-stretched to increase the strength of the film, it adheres to the roll in the process. There is a tendency to become easy. The above problem of sticking appears particularly when the same coating layer is formed on both sides of the film.

(Production method of biaxially oriented polyester film)
Hereinafter, although an example is given and demonstrated concretely about the manufacturing method of the polyester film of this invention, as long as the summary of this invention is satisfied, this invention is not specifically limited to the following illustrations.
In the above-described Steps 1 to 3, after the melt extrusion in Step 3, the film formation as the resin molded body 4 is preferably biaxially stretched to obtain a biaxially oriented polyester film. The raw material obtained according to the above steps 1 to 3 is dried and then supplied to a melt extrusion apparatus, and heated to a temperature equal to or higher than the melting point of each polymer and melted. Next, the molten polymer is extruded from a die and rapidly cooled and solidified on a rotary cooling drum so that the temperature is equal to or lower than the glass transition temperature to obtain a substantially amorphous unoriented sheet. In this case, in order to improve the flatness of the sheet, it is preferable to improve the adhesion between the sheet and the rotary cooling drum. In the present invention, an electrostatic application adhesion method and / or a liquid application adhesion method is preferably employed.

  In the present invention, the sheet thus obtained is preferably stretched in the biaxial direction to form a film. Specifically describing stretching conditions, the unstretched sheet is preferably 2 to 6 times in the longitudinal direction (in the film forming machine axis direction; sometimes referred to as longitudinal direction or MD) at 70 to 145 ° C. To a longitudinally uniaxially stretched film, and then in the transverse direction (a direction perpendicular to the film-forming machine axis direction in the film plane; sometimes referred to as the width direction or TD) at 90 to 160 ° C. It is preferable to perform 2-6 times stretching. At this time, a film for forming a coating layer is formed on the film before stretching or on the film before stretching in the biaxial direction (preferably the transverse direction) after stretching in the uniaxial direction (preferably the longitudinal direction). It is preferable to form a coating layer by applying a liquid.

Next, heat treatment is performed at 150 to 240 ° C. for 1 to 600 seconds. Further, at this time, a method of relaxing 0.1 to 20% in the longitudinal direction and / or the transverse direction in the maximum temperature zone of the heat treatment and / or the cooling zone at the heat treatment outlet is preferable. Further, it is possible to add re-longitudinal stretching and re-lateral stretching as necessary.
In the present invention, two or three or more polyester melt extruders may be used to form a laminated film having two or more layers as described above by a so-called coextrusion method.

(Film characteristics)
The polyester film of the present invention preferably has an initial haze of 1.00% or less, more preferably 0.80% or less, still more preferably 0.50% or less, and particularly preferably 0.30% or less. Thus, it can be suitably used for applications requiring transparency, particularly optical applications.

Further, the difference between the haze after holding the film at a temperature of 180 ° C. for 240 minutes (haze after heating) and the initial haze (haze increase width when heating the film, ΔHz) may be 2.00% or less. Preferably, the lower the value, the better the whitening effect due to oligomer precipitation. ΔHz is more preferably 1.00% or less, further preferably 0.50% or less, particularly preferably 0.20% or less, and most preferably 0.15% or less.
It is generally difficult to satisfy ΔHz while satisfying the initial haze, but it can be achieved according to the present invention.
The haze after heating is preferably 3.00% or less, more preferably 2.0% or less, and still more preferably 1.5% or less. Furthermore, it is preferably 1.00% or less, more preferably 0.80% or less, further preferably 0.60% or less, and particularly preferably 0.40% or less.

  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. In the examples and comparative examples, “parts” means “parts by weight”. The measuring method used in the present invention is as follows.

(1) Measurement of intrinsic viscosity of polyester 0.6 g of a sample was dissolved in 50 ml of orthochlorophenol by heating and then cooled once, insoluble matter was removed by a centrifuge, and the solution was removed at 35 ° C. using an Ostwald type viscosity tube. It calculated from the solution viscosity measured on the temperature conditions. For the measurement of the intrinsic viscosity of only the outermost layer or the inner layer, in the process of co-extrusion from the melt extruder, the intrinsic viscosity of the sampled polyester is measured by extruding only the resin to be collected at the same discharge rate as the film formation. Or only the outermost layer was removed from the film using a suitable tool such as a knife, and the sample (outermost layer) and the remaining sample (inner layer) were removed for measurement.

(2) Initial haze Measured with a Nippon Denshoku haze meter NDH-2000 according to JIS K7361. Measurements were made at any five points in the film plane, and the average value was determined.

(3) Haze rise during film heating The film sample was held for 240 minutes in a hot air oven heated to 180 ° C., and the haze of the heated film was measured according to the method described in (2) above. The initial haze value measured in (2) was subtracted from this measured value, and the increase width (ΔHz) of haze accompanying the heat treatment was determined.

(4) Weight fraction of cyclic oligomer A mixed solvent of hexafluoroisopropanol / chloroform is added to 0.05 g of a film to dissolve it, and then this solution is put into acetonitrile to precipitate a polymer component. Filter the precipitate and dry the supernatant. The dried product was dissolved in 2 ml of acetonitrile to obtain a sample solution for liquid chromatogram.
Using a chromatogram LC20A manufactured by Shimadzu Corporation, using Develosil ODS-MG3 manufactured by Nomura Chemical Co., Ltd. as a column, using a water-acetonitrile mixed solution as a developing solution, a chromatogram is obtained with UV light having a wavelength of 254 nm, and circular The oligomer was quantified by substituting a calibration curve prepared with dimethyl terephthalate.

(5) Surface energy The contact angles of water and methylene iodide with known surface energy to the sample surface: θw and θy were measured using a contact angle meter (“CA-X type” manufactured by Kyowa Interface Science Co., Ltd.) at 25 ° C. And 50% RH. Using these measured values, the surface energy γs was calculated as follows.
The surface energy γs is the sum of the dispersive component γsd and the polar component γsp. That is,
γs = γsd + γsp (Formula 1)
From the Young equation,
γs = γsw + γw · cos θw (Formula 2)
γs = γsy + γy · cos θy (Formula 3)
Here, γsw is the tension acting between the sample surface and water, γsw is the tension acting between the sample surface and methylene iodide, γw is the surface energy of water, and γy is the surface energy of methylene iodide.
From the Fowkes equation,
γsw = γs + γw-2 × (γsd · γwd) 1/2 -2 × (γsp · γwp) 1/2 ( Formula 4)
γsy = γs + γy−2 × (γsd · γyd) ^1/2 −2 × (γsp · γyp) ^1/2 (Formula 5)
It is. Where γwd is the water surface energy dispersive component, γwp is the water surface energy polar component, γyd is the methylene iodide surface energy dispersive component, and γyp is the methylene iodide surface energy polar component. .
The surface tension γs = γsd + γsp can be calculated by solving the simultaneous equations of Formulas 1-5. At this time, the surface energy of water (γw): 72.8 mN / m, the surface energy of methylene iodide (γy): 50.5 mN / m, the dispersive component of the surface energy of water (γwd): 21.8 mN / m , Polar component of surface energy of water (γwp): 51.0 mN / m, Dispersive component of surface energy of methylene iodide (γyd): 49.5 mN / m, Polar component of surface energy of methylene iodide (γyp) : 1.3 mN / m was used.

(6) Durability evaluation Using a strip-shaped sample piece cut to a length of 100 mm in the vertical direction and a width of 10 mm in the horizontal direction, the film is left in an environmental test machine set at a temperature of 121 ° C. and a humidity of 100% RH. Five samples were aged at 4 conditions of 20, 30, and 40 hours, and the longitudinal elongation at break of the sample was measured at n = 5 for each condition, and the average value was obtained. The tensile test was performed using Toyo Baldwin (trade name “Tensilon”), and the initial chuck distance was 50 mm and the tensile speed was 50 mm / min. Similarly, the breaking elongation was measured at five points for the sample pieces before being left in the environmental testing machine, and the average value of the initial breaking elongation was obtained from the average value thereof. The value obtained by dividing the average value of the five points under each aging condition by the average value of the initial breaking elongation was defined as the breaking elongation retention [%]. Based on the obtained value, a breaking elongation deterioration curve was prepared, and durability was evaluated according to the following criteria.
◎: Breaking elongation retention half-life of 40 hours or more ○: Breaking elongation retention half-life of 30 hours to less than 40 hours ×: Breaking elongation retention half-life of less than 30 hours

The production method of the polyester used in the following examples and comparative examples is as follows.
<Manufacture of polyester>
[Method for producing ester (A)]
100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol are used as starting materials, 0.09 parts by weight of manganese is used as a catalyst in the reactor, the reaction start temperature is 150 ° C., and the reaction temperature is gradually increased as methanol is distilled off. The temperature was 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. After adding 0.04 part of ethyl acid phosphate to this reaction mixture, 0.04 part of antimony trioxide was added, and a polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.70 dl / g due to a change in stirring power in the reaction tank, and the polymer was discharged under nitrogen pressure to obtain a polyethylene terephthalate resin as polyester (A). . The intrinsic viscosity of the obtained polyester (A) was 0.70 dl / g.

[Production method of polyester (B)]
100 parts by weight of dimethyl terephthalate and 60 parts by weight of ethylene glycol are used as starting materials, 0.09 part by weight of germanium oxide is used as a catalyst in the reactor, the reaction start temperature is 150 ° C., and the reaction temperature is gradually increased as methanol is distilled off. The temperature was raised to 230 ° C. after 3 hours. After 4 hours, the transesterification reaction was substantially terminated. After adding 0.04 part of ethyl acid phosphate to the reaction mixture, a polycondensation reaction was carried out for 4 hours. That is, the temperature was gradually raised from 230 ° C. to 280 ° C. On the other hand, the pressure was gradually reduced from normal pressure, and finally 0.3 mmHg. After the start of the reaction, the reaction was stopped at a time corresponding to an intrinsic viscosity of 0.55 dl / g due to a change in stirring power of the reaction tank, and the polymer was discharged under nitrogen pressure to obtain a polyethylene terephthalate resin as polyester (B). . The intrinsic viscosity of the obtained polyester (B) was 0.56 dl / g.

[Production method of polyester (C)]
In order to reduce the oligomer contained in the polyester (B) after the production of the polyester (B), the intrinsic viscosity was improved by solid phase polymerization. The polyester resin after the solid phase polymerization was heated to a temperature of 150 ° C. for 3 minutes or more under a steam-containing nitrogen gas atmosphere to obtain a polyester (C). The intrinsic viscosity of the obtained polyester (C) was 0.77 dl / g.

[Production method of polyester (D)]
Polyester C was used and melt extruded at a resin temperature of 290 ° C. to obtain a polyester sheet. Next, this sheet is pulverized, heat-treated for 4 hours while blowing hot air at 150 ° C. in a metal container, remelted at a temperature of 280 to 310 ° C., extruded into a strand, and converted into a chip. Obtained. The intrinsic viscosity of the obtained polyester (D) was 0.64 dl / g.

[Production method of polyester (E)]
Polyester A was used and melt extruded at a resin temperature of 290 ° C. to obtain a polyester sheet. Next, this sheet is pulverized, heat-treated for 4 hours while blowing hot air at 150 ° C. in a metal container, remelted at a temperature of 280 to 310 ° C., extruded into a strand shape, and converted into a chip. Obtained. The intrinsic viscosity of the obtained polyester (E) was 0.63 dl / g.

Moreover, the coating liquid used by the following Example and the comparative example was manufactured as an aqueous coating liquid with a density | concentration of 8 weight% of the following composition (Table 1).
<Manufacture of coating liquid>

Resin component: Copolyester (Tg = 80 mol% of terephthalic acid / 15 mol% of isophthalic acid / 5 mol% of 5-sodium sulfoisophthalic acid and 60 mol% of ethylene glycol / 40 mol% of diethylene glycol) 43 ° C.) (water dispersibility).
Crosslinking agent: polymer having an oxazoline group composed of 30 mol% methyl methacrylate / 2 mol% 2-isopropenyl-2-oxazoline / 10 mol% polyethylene oxide (n = 10) methacrylate / 30 mol% acrylamide (Tg = 50 ° C.).
Filler: Silica filler (average particle size 40 nm)
Additive: Carnauba wax wetting agent 1: Polyoxyethylene (n = 7) lauryl ether wetting agent 2: Sodium dodecylbenzenesulfonate

[Example 1]
A polyester raw material in which the above polyesters (C) and (D) are mixed at a ratio of 80:20 (weight ratio, the same applies hereinafter) as the raw material for the A layer, and the above polyesters (C) and (D) as the raw material for the B layer. After the polyester raw materials mixed at a ratio of 50:50 were respectively fed to two extruders and melted at 285 ° C., the A layer was the outermost layer (surface layer) and the B layer was the inner layer (core layer). On the casting drum cooled at 40 degreeC, it coextruded by the layer structure of 2 types and 3 layers (A / B / A), and it was made to cool and solidify, and the unoriented sheet was obtained. Subsequently, the coating liquid A was uniformly apply | coated to both surfaces of the film with the aqueous coating liquid roll coater, and it dried at 90 degreeC. The coated film was continuously stretched 3.2 times in the longitudinal direction and 3.6 times in the transverse direction at a stretching temperature of 100 ° C. using a simultaneous biaxial stretching machine, heat-treated at 225 ° C., and then in the longitudinal direction. A laminated polyester film having a thickness of 100 μm was obtained by relaxing 1% and 2% in the lateral direction. The thickness of each layer of the obtained film was A layer / B layer / A layer = 15/70/15 μm, and the thickness of the coating layer was 0.08 μm on both sides. Moreover, the surface energy of the obtained film was 62 mN / m.

[ Comparative Example 6 ]
The polyester (D) is used as the raw material for the A layer, the polyester raw material obtained by mixing the polyesters (C) and (D) in a ratio of 30:70 is used as the raw material for the B layer, and the melt extrusion temperature is 290 ° C. A laminated polyester film was obtained in the same manner as in Example 1 except that.

[ Comparative Example 7 ]
A polyester raw material in which the above polyesters (C) and (D) are mixed in a ratio of 90:10 as a raw material for the A layer, and a polyester raw material in which the above polyesters (C) and (D) are mixed in a ratio of 60:40 as a raw material for the B layer. A laminated polyester film was obtained in the same manner as in Example 1 except that the obtained polyester raw material was used and the melt extrusion temperature was 280 ° C.

[Example 4]
A laminated polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the coating liquid applied to both surfaces of the film with an aqueous coating liquid roll coater was coating liquid B. The thickness of the coating layer was the same as in Example 1.

[Example 5]
A laminated polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the coating liquid applied to both surfaces of the film with an aqueous coating liquid roll coater was changed to the coating liquid C. The thickness of the coating layer was the same as in Example 1.

[Example 6]
After obtaining the non-oriented sheet, a laminated polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the coating liquid was not applied. The thickness of each layer of the obtained film was the same as in Example 1. Both sides of this laminated polyester film were subjected to corona treatment with a corona treatment machine, and the corona discharge intensity was adjusted to obtain a laminated polyester film having a surface energy of 54 mN / m on both sides.

[Comparative Example 1]
The polyester (A) is used as the raw material for the A layer, the polyester raw material obtained by mixing the polyesters (A) and (E) at a ratio of 60:40 is used as the raw material for the B layer, and the melt extrusion temperature is 280 ° C. A laminated polyester film was obtained in the same manner as in Example 1 except that.

[Comparative Example 2]
A polyester raw material in which the polyester (C) and (D) are mixed in a ratio of 50:50 as a raw material for the A layer, and a polyester raw material in which the polyester (C) and (D) are mixed in a ratio of 20:80 as a raw material for the B layer. A laminated polyester film was obtained in the same manner as in Example 1 except that the obtained polyester raw material was used and the melt extrusion temperature was set to 305 ° C.

[Comparative Example 3]
Using a polyester raw material in which the polyesters (A) and (E) are mixed at a ratio of 60:40, the mixture is melt-extruded as a single layer at 285 ° C. with an extruder, and cooled and solidified on a casting drum cooled to 40 ° C. A non-oriented sheet was obtained. Thereafter, a polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1.

[Comparative Example 4]
A laminated polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the coating liquid applied to both surfaces of the film with an aqueous coating liquid roll coater was the coating liquid D shown below.

[Comparative Example 5]
After obtaining the non-oriented sheet, a laminated polyester film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the coating liquid was not applied.
The evaluation results of the film obtained above are shown in Table 2. As shown in Table 2, the polyester film of the present invention was excellent in transparency after heating.

  The polyester film of the present invention is excellent in transparency after heat processing, and can be post-processed at a high temperature because there is little oligomer precipitation, and it has practical heat resistance and durability, so high quality is required. It can be suitably used in various industrial applications, including display applications and optical applications such as casting films when manufacturing display peripheral members.

Claims (4)

A film made of polyethylene terephthalate resin, wherein the intrinsic viscosity of the polyethylene terephthalate resin is 0.60 dl / g or more and 0.68 dl / g or less, and the weight fraction (WCy3) of the cyclic trimer oligomer in the film; the ratio of the weight fraction of the cyclic tetramer oligomer (WCy4) WCy3 / WCy4 is 3 or less, and Ri der least a surface energy at one surface is 50 mN / m or more films, and characterized in be used in actual display A polyester film.   The film is a laminated film of at least three layers obtained by a coextrusion method, wherein the intrinsic viscosity of the resin constituting the outermost layer is higher than the intrinsic viscosity of the resin constituting the inner layer, Item 2. The polyester film according to Item 1.   The polyester film according to claim 1 or 2, wherein the film has a coating layer on at least one surface thereof, and the surface energy on the surface of the coating layer is 50 mN / m or more.   4. The polyester film according to claim 3, wherein the film layer is formed on the film within the film production process.