JP7554628B2 - Method for producing thermoplastic resin film - Google Patents

Description

The present invention relates to a method for producing a thermoplastic resin film that can suppress the occurrence of horizontal stripes.

In a manufacturing method of a thermoplastic resin film that involves uniaxial roll stretching in the longitudinal direction, horizontal steps and wrinkles and scratches can occur at the edges, so various methods for manufacturing a thermoplastic resin film that has excellent smoothness and suppresses the occurrence of these problems have been investigated.
For example, Patent Document 1 proposes a method of heating a thermoplastic resin sheet to a temperature higher than its glass transition temperature, uniaxially stretching the sheet, and then cooling the thermoplastic resin film to a temperature equal to or lower than its glass transition temperature in a cooling oven. Patent Document 2 describes a method of reducing the surface temperature of a mirror-finished roll or belt that sandwiches a resin film to 130° C. or lower as a method of eliminating horizontal striations.
Patent Document 3 proposes a method for producing an optical film by a solution casting film formation method, characterized in that a dope discharge speed _V1E from a casting width end of a casting die, a dope discharge speed _V1C from a casting width center of the casting die, and a support moving speed _V2 satisfy the relationships (1) _V2 > _V1C and (2) ( _V2 / _V1E ) > ( _V2 / _V1C ).
Furthermore, Patent Document 4 proposes a method for producing an optical film having a step of nipping and pressing between an elastic roll and a cooling roll, characterized in that the surface temperature Tr1 of the cooling roll and the glass transition temperature Tg of a resin mixture constituting the optical film satisfy Tg<Tr1≦Tg+40° C.

JP 2010-99946 A International Publication No. WO 2016/157908 International Publication No. 2018/074019 JP 2011-68005 A

According to the manufacturing methods disclosed in Patent Documents 1 to 4, the occurrence of horizontal stripes can be suppressed to some extent, but this is not sufficient, and improvements are desired.
The present invention has been made in consideration of the above circumstances, and has an object to provide a method for producing a thermoplastic resin film that produces less horizontal striations.

As a result of extensive research conducted by the inventors to achieve the above object, they discovered that by squeezing a thermoplastic resin composition melt-extruded from a T-die between an elastic metal roll and a rigid metal mirror-finish roll, and then passing the composition through rolls arranged in a specific positional relationship, it is possible to suppress the occurrence of horizontal striations and obtain a thermoplastic resin film with excellent smoothness, thus completing the present invention.

That is, the present invention relates to the following [1] to [7].
[1] A method for producing a thermoplastic resin film, comprising a step of sandwiching a thermoplastic resin composition melt-extruded from a T-die between an elastic metal roll (I) and a rigid mirror-surface metal roll (II), and then passing the film through a rigid mirror-surface metal roll (III) and winding the film, wherein the distance Gth (mm) between the rigid mirror-surface metal roll (II) and the rigid mirror-surface metal roll (III) and the thickness Fth (mm) of the thermoplastic resin film satisfy the relationship of the following formula (1).
5.0≦Gth/Fth≦30.0 (1)
[2] The method for producing a thermoplastic resin film according to [1], wherein the thickness Fth of the thermoplastic resin film is 0.02 to 0.20 mm.
[3] The method for producing a thermoplastic resin film according to [1] or [2], wherein the gap Gth is 0.4 to 2.0 mm.
[4] The method for producing a thermoplastic resin film according to any one of [1] to [3], wherein the line speed is 9 to 60 m/min.
[5] The method for producing a thermoplastic resin film according to any one of [1] to [4], wherein the temperature difference between the rigid mirror-surface metal roll (II) and the rigid mirror-surface metal roll (III) is 10° C. or less.
[6] The method for producing a thermoplastic resin film according to any one of [1] to [5], wherein the thermoplastic resin composition is a (meth)acrylic resin composition.
[7] The method for producing a thermoplastic resin film according to [6], wherein the (meth)acrylic resin composition contains a (meth)acrylic polymer having a multilayer structure.

The present invention provides a method for producing a thermoplastic resin film with minimal horizontal stripes.

FIG. 2 is a diagram showing an example of a film forming apparatus in the production method of the present invention.

[Method of manufacturing thermoplastic resin film]
The present invention relates to a method for producing a thermoplastic resin film, comprising the steps of: nipping a thermoplastic resin composition melt-extruded from a T-die between an elastic metal roll (I) and a rigid mirror-finished metal roll (II); and further passing the film through a rigid mirror-finished metal roll (III) and winding the film,
The distance Gth (mm) between the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III), and the thickness Fth (mm) of the thermoplastic resin film satisfy the relationship of the following formula (1).
5.0≦Gth/Fth≦30.0 (1)

The present inventors have studied a method for producing a thermoplastic resin film by passing a thermoplastic resin composition melt-extruded from a T-die through an elastic metal roll (I), a rigid mirror-finished metal roll (II) and a rigid mirror-finished metal roll (III). As a result, they have found that if the thermoplastic resin film is not smoothly transported from the rigid mirror-finished metal roll (II) to the rigid mirror-finished metal roll (III), the thermoplastic resin film will remain stuck to the rigid mirror-finished metal roll (II) for a long time, and then it will be transported to the rigid mirror-finished metal roll (III) so as to peel off from the rigid mirror-finished metal roll (II) all at once, and at that time, horizontal steps will occur in the thermoplastic resin film due to bending or wrinkles. Therefore, the present invention suppresses the occurrence of horizontal steps by adjusting the interval Gth so that the thermoplastic resin film is smoothly transported from the rigid mirror-finished metal roll (II) to the rigid mirror-finished metal roll (III). Specifically, from the viewpoint of suppressing the occurrence of the above-mentioned film sagging and wrinkles, when the thickness of the thermoplastic resin film is Fth, the gap Gth/Fth must be 5.0 or more, preferably 6.0 or more, more preferably 7.0 or more, and even more preferably 8.0 or more, and must be 30.0 or less, preferably 25.0 or less, more preferably 21.0 or less, and even more preferably 18.0 or less.
Specifically, the gap Gth is preferably 0.4 mm or more, more preferably 0.5 mm or more, and even more preferably 0.6 mm or more, while the gap Gth is preferably 2.0 mm or less, more preferably 1.9 mm or less, and even more preferably 1.6 mm or less.
In this specification, Gth refers to the distance between the side of the rigid mirror-surfaced metal roll (II) and the side of the rigid mirror-surfaced metal roll (III) on a straight line connecting the center of the rigid mirror-surfaced metal roll (II) and the center of the rigid mirror-surfaced metal roll (III).

The production method of the present invention may be carried out in any apparatus as long as it satisfies the above-mentioned requirements, but can be carried out, for example, in the apparatus shown in FIG.
In the apparatus of FIG. 1, the thermoplastic resin composition as the raw material is first melted in an extruder 1. Next, the molten thermoplastic resin composition passes through a gear pump 2, is filtered through a polymer filter 3, and is then discharged in the form of a sheet from a T-die 4. The discharged molten resin (molten thermoplastic resin composition) is pressed between a roll 5 (elastic metal roll (I)) and a roll 6 (rigid mirror-finished metal roll (II)) to be molded to a desired thickness, and is cooled by the rigid mirror-finished metal roll (II). Next, it is further cooled by a roll 7 (rigid mirror-finished metal roll (III)), and then further pressed between nip rolls 8, 8', and finally wound into a roll by a winder 9.

In the present invention, from the viewpoint of suppressing the occurrence of horizontal steps, it is preferable that the temperature difference between the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) is 10° C. or less. If the temperature difference is 10° C. or less, the thermoplastic resin film is not cooled excessively, so that it is possible to more efficiently suppress the occurrence of horizontal steps caused by temperature changes. From this viewpoint, the temperature difference between the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) is more preferably 8° C. or less, even more preferably 6° C. or less, and even more preferably 5° C. or less. The temperature of each roll is a set temperature.
Specifically, the temperatures of the rigid mirror-surface metal roll (II) and the rigid mirror-surface metal roll (III) are preferably 50 to 105° C., more preferably 60 to 100° C., and even more preferably 70 to 95° C. When the temperatures of the rigid mirror-surface metal roll (II) and the rigid mirror-surface metal roll (III) are within the above ranges, the thermoplastic resin film is sufficiently cooled by the rolls, making it easier to suppress the occurrence of horizontal striations.
In relation to the glass transition temperature of the thermoplastic resin composition used in the present invention, the temperatures of the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) are preferably (glass transition temperature of thermoplastic resin composition)-70 to (glass transition temperature of thermoplastic resin composition)-15°C, more preferably (glass transition temperature of thermoplastic resin composition)-60 to (glass transition temperature of thermoplastic resin composition)-20°C, and even more preferably (glass transition temperature of thermoplastic resin composition)-50 to (glass transition temperature of thermoplastic resin composition)-25°C.

In the present invention, the acute angle of the angle formed by the line (A) (not shown) connecting the center of the elastic metal roll (I) and the center of the rigid mirror-finished metal roll (II) and the line (B) (not shown) connecting the centers of the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) is preferably 0 to +45°, more preferably 0 to +30°, and even more preferably 0 to +20°, assuming that the line (A) and the line (B) are aligned is 0°. If the acute angle of the angle formed by the line (A) and the line (B) is within the above range, the effects of the present invention are easily achieved, the molten resin is sufficiently cooled, and the film surface is less likely to become rough.
In the present invention, in addition to the elastic metal roll (I), the rigid mirror-finished metal roll (II), and the rigid mirror-finished metal roll (III), one or more rolls can be used for cooling or transport. It is preferable that these rolls also have the above-mentioned angle in terms of their positional relationship with the adjacent rolls, but it is preferable to further adjust these angles appropriately in consideration of the space of the production line.

The production apparatus and conditions suitable for carrying out the present invention will be described in detail below.
<Extruder>
The extruder for melting and extruding the thermoplastic resin composition may be, for example, a single screw extruder, a twin screw extruder, a multi-screw extruder, etc. In the present invention, the extruder is preferably equipped with a vent mechanism in order to remove volatile matter generated when the thermoplastic resin composition is melt-kneaded.
As the screw of the extruder, a screw with a barrier flight or a mixing section can be used. The L/D of the screw (L represents the cylinder length of the extruder, and D represents the cylinder inner diameter) is preferably 10 or more, more preferably 20 or more, even more preferably 25 or more, and preferably 100 or less, more preferably 50 or less, and even more preferably 40 or less, from the viewpoint of obtaining sufficient plasticization and kneading of the thermoplastic resin composition. By setting the L/D to the lower limit or more, sufficient plasticization and kneading of the thermoplastic resin composition can be obtained. In addition, by setting the L/D to the upper limit or less, kneading is possible while suppressing decomposition of the thermoplastic resin composition due to shear heat generation.

The cylinder temperature of the extruder when the thermoplastic resin composition is in a molten state depends on the thermoplastic resin used and its glass transition temperature, but is preferably 150°C or higher, more preferably 180°C or higher, and preferably 310°C or lower, more preferably 280°C or lower. By having a cylinder temperature equal to or higher than the lower limit, the thermoplastic resin composition can be sufficiently melted. On the other hand, by having a cylinder temperature equal to or lower than the upper limit, low-boiling decomposition products, scorching, and gelation that occur due to decomposition caused by thermal degradation of the thermoplastic resin composition can be suppressed.

<Gear pump>
In the present invention, a gear pump may be installed after the extruder in order to compensate for pressure loss in a polymer filter or a static mixer, which will be described later. There are no particular limitations on the gear pump, but an inverter-controlled gear pump is preferred. By using an inverter-controlled gear pump, pulsation of the flow rate of the molten resin discharged from the extruder can be suppressed.
The resin pressure at the inlet of the gear pump is preferably 10 MPa or less, and more preferably 8 MPa or less. When the resin pressure at the inlet of the gear pump is equal to or less than the upper limit, extrusion defects are less likely to occur even if the number of elements of the static mixer is increased.

<Polymer filter>
In the present invention, it is preferable to use a polymer filter between the gear pump and the static mixer. The polymer filter preferably comprises a filter element part for filtering the thermoplastic resin composition and a housing part for introducing and discharging the molten resin.
The filter element may be of a disk type or a cylindrical type.
A cylindrical filter element usually comprises a filtration section that filters a fluid from its outer peripheral surface, a hollow section through which the filtered fluid flows, a discharge section at an end section that discharges the fluid from the hollow section, and a tip section of the filter element. Examples of cylindrical filter elements include tube types and candle types, and among these, candle-type filter elements are preferred.
There is no particular limitation on the shape of the candle-type filter element, and a wave type, pleated type, etc. can be used. The pleats in the pleated type may extend in the radial direction of the filter element, or may be so-called spiral pleats that extend obliquely to the radial direction and have a curved or arched cross-sectional shape.

The filtration accuracy of the filter element is preferably 5 μm or more, more preferably 7 μm or more, and preferably 50 μm or less, more preferably 30 μm or less. By having a filtration accuracy within the above range, it is possible to improve the balance between productivity and foreign matter size. In particular, by having a filtration accuracy equal to or greater than the above lower limit, it is possible to suppress thermal deterioration due to shear heat generated when passing a molten thermoplastic resin composition. On the other hand, by having a filtration accuracy equal to or less than the above upper limit, it is possible to effectively remove foreign matter.

<Static mixer>
In the present invention, a static mixer may be used for the purpose of efficiently stirring and mixing the thermoplastic resin composition while reducing pressure loss.
When a static mixer is used, the number of elements (the number of unit mixing elements) is preferably 5 to 30, more preferably 8 to 25, and even more preferably 11 to 20. By having the number of elements within the above range, the temperature uniformity of the thermoplastic resin composition and the uniformity of each blended component are improved. In addition, when the number of elements is equal to or less than the upper limit, the surface area of the elements is reduced, making it possible to prevent the deteriorated resin from adhering to the element surface and to prevent the inclusion of foreign matter.

<T-die>
In order to uniformize the thickness of the film produced, it is preferable to use a T-die such as a manifold die, a fishtail die, or a coat hanger die. In order to stably control the thickness, it is preferable to use an automatically adjusting die having a mechanism for automatically adjusting a die lip clearance adjustment bolt.

The discharge rate of the thermoplastic resin composition discharged from the T-die depends on the size of the extruder, but is preferably 90 to 600 kg/hour, more preferably 100 to 400 kg/hour, and even more preferably 110 to 250 kg/hour. By having the discharge rate within the above range, the state of the molten resin from the extruder to the T-die is stable, and unevenness in the discharge rate at the T-die is suppressed, making it easier to achieve the effects of the present invention.

From the viewpoint of improving film thickness accuracy and facilitating the production of a film with few die lines and horizontal stripes, the die lip clearance is preferably 0.2 to 2.0 mm, more preferably 0.3 to 1.8 mm, and even more preferably 0.5 to 1.5 mm.
From the same viewpoint as for the die lip clearance, the die lip width is preferably 300 to 4,000 mm, more preferably 1,000 to 3,000 mm, and further preferably 1,500 to 2,500 mm.

<Air gap>
The air gap, which represents the distance between the resin discharge portion of the T-die and the contact point between the molten resin and the casting roll (roll 5 in FIG. 1), is preferably 50 mm to 150 mm, more preferably 50 mm to 100 mm, and even more preferably 50 mm to 90 mm. If the air gap is equal to or less than the upper limit, the contact time between the molten resin and the air is relatively short, so that the surface temperature of the thermoplastic resin composition does not decrease, and the mirror transferability by the roll is improved. On the other hand, by setting the air gap to equal to or more than the lower limit, time for the volatile components from the thermoplastic resin composition to diffuse can be secured, and the volatile components are less likely to adhere to the casting roll or the like.

<Roles>
In the manufacturing method of the present invention, from the viewpoint of suppressing the occurrence of horizontal steps and further improving the smoothness and thickness accuracy of the thermoplastic resin film, the extruded molten material (molten resin) is compressed between an elastic metal roll (I) and a rigid mirror-finished metal roll (II) and cooled by the rigid mirror-finished metal roll (II), and the thermoplastic resin composition is further cooled by the rigid mirror-finished metal roll (III). In particular, in the present invention, the distance Gth (mm) between the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) and the thickness Fth (mm) of the thermoplastic resin film satisfy the relationship of formula (1), so that the occurrence of horizontal steps is suppressed and a thermoplastic resin film with excellent smoothness can be obtained.

The linear pressure of each roll used in the present invention is preferably 5 N/mm or more, more preferably 10 N/mm or more, and even more preferably 15 N/mm or more, from the viewpoint of improving the smoothness of the thermoplastic resin film. The upper limit of the linear pressure of each roll is usually 50 N/mm, and may be 40 N/mm.

In the present invention, it is preferable to adjust the line speed (the transport speed of the film after solidification. However, if a stretching process is performed, the transport speed of the film before the stretching process) by adjusting the rotation speed of the roll, and specifically, it is preferably 9 to 60 m/min, more preferably 11 to 50 m/min, and even more preferably 15 to 35 m/min. By setting the line speed at or above the lower limit, the thermoplastic resin film is less likely to stick to the rigid mirror metal roll (II), and the transport of the thermoplastic resin film from the rigid mirror metal roll (II) to the rigid mirror metal roll (III) becomes smooth, which makes it easier to suppress the occurrence of horizontal streaks. On the other hand, by setting the line speed at or below the upper limit, the thermoplastic resin film can be produced while preventing breakage.

<Thickness (Fth)>
The manufacturing method of the present invention is particularly effective in manufacturing a thermoplastic resin film having a thickness of 0.02 to 0.20 mm, and can obtain a desired thermoplastic resin film with high thickness accuracy. Furthermore, from the viewpoint of improving the handleability of the thermoplastic resin film and keeping costs low, the thickness of the thermoplastic resin film is preferably 0.03 to 0.18 mm, more preferably 0.04 to 0.15 mm, and even more preferably 0.05 to 0.13 mm. When the thickness is equal to or greater than the lower limit, the rigidity is increased, and the handleability of the thermoplastic resin film is improved. Furthermore, when the thickness is equal to or less than the upper limit, the balance between the strength of the thermoplastic resin film and the manufacturing cost is improved. The thickness is determined by measuring the thickness at any three points in the width direction of the film with a micrometer and averaging the measured values.

<Thermoplastic resin composition>
The thermoplastic resin composition used in the present invention is not particularly limited as long as it contains a thermoplastic resin. However, from the viewpoint of easily suppressing the occurrence of die lines and from the viewpoint of producing a film suitable for optical applications, etc., A resin composition containing a (meth)acrylic resin, that is, a (meth)acrylic resin composition, is particularly preferred.

[(Meth)acrylic resin]
The (meth)acrylic resin used in the present invention is not particularly limited, but preferably contains a structural unit derived from methyl methacrylate and, if necessary, a structural unit derived from an acrylic acid ester. Examples of acrylic acid esters that can be used in the (meth)acrylic resin include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, s-butyl acrylate, t-butyl acrylate, amyl acrylate, isoamyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, pentadecyl acrylate, dodecyl acrylate, cyclohexyl acrylate, norbornenyl acrylate, isobornyl acrylate, benzyl acrylate, phenoxyethyl acrylate, 2-hydroxyethyl acrylate, 2-ethoxyethyl acrylate, glycidyl acrylate, allyl acrylate, and phenyl acrylate. Among these, alkyl acrylates in which the alkyl group has 1 to 6 carbon atoms are preferred.

The amount of structural units derived from methyl methacrylate in the (meth)acrylic resin is preferably 85 to 100% by mass, and more preferably 90 to 100% by mass, from the viewpoint of improving the mechanical strength of the thermoplastic resin film.
Furthermore, when the (meth)acrylic resin contains a structural unit derived from an acrylic ester, the amount thereof in the (meth)acrylic resin is preferably 0 to 15 mass %, and more preferably 0 to 10 mass %, from the viewpoint of improving the thickness accuracy of the thermoplastic resin film.

The glass transition temperature of the (meth)acrylic resin used in the present invention is preferably 95° C. or higher, more preferably 100° C. or higher, and even more preferably 105° C. or higher. When the glass transition temperature is within the above range, the thickness precision of the thermoplastic resin film is improved. The glass transition temperature of the (meth)acrylic resin used in the present invention is usually 130° C. or lower.
The glass transition temperature of the (meth)acrylic resin can be measured in accordance with JIS K7121:2012.

The weight average molecular weight of the (meth)acrylic resin is preferably 60,000 to 150,000. If the weight average molecular weight is equal to or greater than the lower limit, the mechanical properties are improved, and if it is equal to or less than the upper limit, the melt viscosity is lowered and processability is improved. From the above viewpoint, the weight average molecular weight is more preferably 70,000 to 120,000, and even more preferably 80,000 to 100,000.

The molecular weight distribution (ratio of weight average molecular weight (Mw) to number average molecular weight (Mn), Mw/Mn) of the (meth)acrylic resin is preferably 1.0 to 1.8, more preferably 1.0 to 1.4, and even more preferably 1.0 to 1.3. When the molecular weight distribution (Mw/Mn) is within the above range, the mechanical strength of the film is improved. Mw/Mn can be controlled by adjusting the type and amount of the polymerization initiator used during production.
Mw and Mn are values obtained by converting a chromatogram measured by gel permeation chromatography (GPC) into the molecular weight of standard polystyrene, and specifically, can be determined by the method described later in the Examples.

The stereoregularity of the (meth)acrylic resin is not particularly limited, and for example, a (meth)acrylic resin having stereoregularity such as isotactic, heterotactic, or syndiotactic can be used.
In the present invention, two or more kinds of (meth)acrylic resins differing in the monomers constituting the (meth)acrylic resins, weight average molecular weight, stereoregularity, etc. may be used in combination.

The method for producing the (meth)acrylic resin is not particularly limited, and the resin can be produced by known polymerization methods such as radical polymerization and anionic polymerization. There are no particular limitations on the production conditions, and the desired (meth)acrylic resin can be obtained by appropriately adjusting the polymerization temperature, polymerization time, the type and amount of the chain transfer agent, the type and amount of the polymerization initiator, etc.

The content of the (meth)acrylic resin in the thermoplastic resin composition of the present invention is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 75% by mass or more, from the viewpoint of improving the transparency of the thermoplastic resin film.

[(Meth)acrylic polymer having a multilayer structure]
The thermoplastic resin composition used in the present invention may contain a (meth)acrylic polymer having a multilayer structure.
The (meth)acrylic polymer having a multilayer structure is not particularly limited as long as it has a multilayer structure, and for example, a (meth)acrylic polymer having a core-shell multilayer structure can be mentioned. In addition, the number of layers constituting the multilayer structure is not particularly limited, and it may be two layers or three or more layers.
Among these, from the viewpoint of improving the impact resistance of the thermoplastic resin film, a (meth)acrylic polymer having a core-shell multilayer structure is preferred, more specifically, a (meth)acrylic polymer having a core-shell multilayer structure consisting of three layers, i.e., a core (inner layer), an inner shell (middle layer), and an outer shell (outer layer) is preferred. In the present invention, a (meth)acrylic polymer having a core-shell multilayer structure consisting of three layers refers to one in which the core and the inner shell, and the inner shell and the outer shell are each composed of different polymers.
When a resin composition containing the (meth)acrylic polymer having the three-layer core-shell multilayer structure is melt-kneaded, all or a part of the outer shell is fused and united with the (meth)acrylic resin to form a matrix, and the matrix contains core-shell particles each consisting of two layers, a core and an inner shell.

Hereinafter, a (meth)acrylic polymer having a core-shell multilayer structure consisting of three layers, a core (inner layer), an inner shell (middle layer), and an outer shell (outer layer), will be described in detail. The polymer constituting the core will be referred to as "polymer (a)", the polymer constituting the inner shell as "polymer (b)", and the polymer constituting the outer shell as "polymer (c)".

(Polymer (a): Polymer Constituting the Core)
The polymer (a) is preferably a polymer containing a structural unit derived from methyl methacrylate, a structural unit derived from an alkyl acrylate, a structural unit derived from a grafting agent, and, if necessary, a structural unit derived from a crosslinking agent. In the present invention, the grafting agent means a monomer having two or more different polymerizable groups, and the crosslinking agent means a monomer having two or more of the same type of polymerizable groups (excluding the grafting agent).

The alkyl acrylate used in the polymer (a) is not particularly limited, but the number of carbon atoms in the alkyl group is preferably 1 to 8, more preferably 1 to 6. When the number of carbon atoms in the alkyl group in the alkyl acrylate is within the above range, the thermal decomposition resistance of the (meth)acrylic polymer having a multilayer structure is improved, and the hot water whitening resistance and boiling water whitening resistance of the thermoplastic resin film are improved. Specific examples of the alkyl acrylate include methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, s-butyl acrylate, t-butyl acrylate, n-butyl methyl acrylate, n-heptyl acrylate, 2-ethylhexyl acrylate, and n-octyl acrylate. These can be used alone or in combination of two or more.
Among these, methyl acrylate is particularly preferred from the above viewpoint.

It is preferable that the polymer (a) contains a structural unit derived from a grafting agent for the purpose of chemically bonding the polymer (a) and the polymer (b) and for the purpose of assisting the formation of a crosslinked structure of the polymer (a).
Examples of the grafting agent used in the polymer (a) include allyl methacrylate, allyl acrylate, mono- or di-allyl maleate, mono- or di-allyl fumarate, crotyl acrylate, and crotyl methacrylate. These grafting agents can be used alone or in combination of two or more.
Among these, allyl methacrylate is preferred from the viewpoints of improving the bonding ability between the polymer (a) and the polymer (b) and improving the stress whitening resistance and transparency of the thermoplastic resin film.

The polymer (a) may contain a structural unit derived from a crosslinking agent for the purpose of forming a crosslinked structure in the polymer (a) and for the purpose of forming a graft structure between the polymer (a) and the polymer (b).
Examples of the crosslinking agent used in the polymer (a) include diacryl compounds, dimethacrylic compounds, diallyl compounds, divinyl compounds, diene compounds, trivinyl compounds, etc. More specifically, ethylene glycol di(meth)acrylate, butylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, divinylbenzene, trivinylbenzene, ethylene glycol diallyl ether, propylene glycol diallyl ether, butadiene, etc. These crosslinking agents can be used alone or in combination of two or more.

The amount of structural units derived from methyl methacrylate in polymer (a) is preferably 40 to 98.99% by mass, and more preferably 45 to 96.9% by mass, of the total structural units of polymer (a). When the amount of structural units derived from methyl methacrylate is equal to or greater than the lower limit, the weather resistance of the thermoplastic resin film is improved, and when the amount is equal to or less than the upper limit, the impact resistance of the thermoplastic resin film is improved.

The amount of structural units derived from alkyl acrylate in polymer (a) is preferably 1 to 59% by mass, and more preferably 3 to 55% by mass, of the total structural units of polymer (a). When the amount of structural units derived from alkyl acrylate is equal to or greater than the lower limit, the thermal decomposition resistance of the multilayered (meth)acrylic polymer is improved, and when the amount is equal to or less than the upper limit, the hot water whitening resistance and boiling water whitening resistance of the thermoplastic resin film are improved.

The amount of structural units derived from the grafting agent in polymer (a) is preferably 0.01 to 1 mass %, more preferably 0.1 to 0.5 mass %, of the total structural units of polymer (a). When the amount of structural units derived from the grafting agent is equal to or greater than the lower limit, the bonding strength between polymer (a) and polymer (b) is improved, and when the amount is equal to or less than the upper limit, the impact resistance of the thermoplastic resin film is improved.

The amount of structural units derived from the crosslinking agent in polymer (a) is preferably 0 to 0.5% by mass, and more preferably 0 to 0.2% by mass, of the total structural units of polymer (a). When the amount of structural units derived from the crosslinking agent is within the above range, the impact resistance of the thermoplastic resin film is improved.

It is preferable that polymer (a) and polymer (b) described later are insoluble in a solvent such as acetone, i.e., are grafted. When polymer (a) and polymer (b) are grafted, polymer (a) and polymer (b) are present as particles with a two-layer structure in the matrix, which is preferable because it improves the impact resistance of the thermoplastic resin film.

(Polymer (b): Polymer constituting the inner shell)
The polymer (b) is preferably a polymer containing a structural unit derived from an alkyl acrylate, a structural unit derived from a grafting agent, and, if necessary, a structural unit derived from an aromatic vinyl compound and a structural unit derived from a crosslinking agent.

As the alkyl acrylate used in polymer (b), the alkyl acrylates exemplified for polymer (a) above can be mentioned, and n-butyl acrylate is particularly preferred from the viewpoint of improving the thermal decomposition resistance of the (meth)acrylic polymer having a multilayer structure and improving the hot water whitening resistance and boiling water whitening resistance of the thermoplastic resin film.

Examples of aromatic vinyl compounds used in polymer (b) include aromatic vinyl compounds such as styrene, α-methylstyrene, p-methylstyrene, and m-methylstyrene, and styrene is preferred from the viewpoint of improving the thermal decomposition resistance of the (meth)acrylic polymer having a multilayer structure.

The grafting agent used for polymer (b) may be any of the grafting agents exemplified for polymer (a), and allyl methacrylate is preferred from the viewpoint of improving the bonding ability between polymer (a) and polymer (b) and improving the stress whitening resistance and transparency of the thermoplastic resin film.

The crosslinking agent used for polymer (b) may be any of the crosslinking agents exemplified for polymer (a) above. From the viewpoint of forming a crosslinked structure in polymer (b) and between polymer (a) and polymer (b), diacrylic compounds, dimethacrylic compounds, diallyl compounds, divinyl compounds, etc. are preferred.

The amount of structural units derived from alkyl acrylate in polymer (b) is preferably 70 to 99.5% by mass, and more preferably 80 to 99% by mass, of the total structural units of polymer (b). When the amount of structural units derived from alkyl acrylate is equal to or greater than the lower limit, the impact resistance of the thermoplastic resin film is improved, and when the amount is equal to or less than the upper limit, the stress whitening resistance and transparency of the thermoplastic resin film are improved.

The amount of structural units derived from aromatic vinyl compounds in polymer (b) is preferably 0 to 29% by mass, and more preferably 0 to 20% by mass, of the total structural units of polymer (b). When the amount of structural units derived from methyl methacrylate is within the above range, the impact resistance of the thermoplastic resin film is improved.

The amount of structural units derived from the grafting agent in polymer (b) is preferably 0.5 to 30% by mass, and more preferably 1 to 20% by mass, of the total structural units of polymer (b). When the amount of structural units derived from the grafting agent is equal to or greater than the lower limit, the stress whitening resistance of the thermoplastic resin film is improved. On the other hand, when the amount is equal to or less than the upper limit, the impact resistance of the thermoplastic resin film is improved.

The amount of structural units derived from the crosslinking agent in polymer (b) is preferably 0 to 5 mass % of the total structural units of polymer (b), and more preferably 0 to 2 mass %. When the amount of structural units derived from the crosslinking agent is within the above range, the impact resistance of the thermoplastic resin film is improved.

In the present invention, it is preferable that polymer (b) is softer than polymer (a) and polymer (c). Since polymer (b) is softer than polymer (a) and polymer (c), impact resistance is improved.

(Polymer (c): Polymer Constituting the Outer Shell)
The polymer (c) is preferably a polymer containing a structural unit derived from methyl methacrylate and a structural unit derived from an alkyl acrylate.
Examples of the alkyl acrylate used in the polymer (c) include the alkyl acrylates exemplified for the polymer (a). From the viewpoints of improving the thermal decomposition resistance of the (meth)acrylic polymer having a multilayer structure and improving the hot water whitening resistance and boiling water whitening resistance of the thermoplastic resin film, methyl acrylate is particularly preferred.

The amount of structural units derived from methyl methacrylate in polymer (c) is preferably 80 to 99% by mass, and more preferably 85 to 98% by mass, of the total structural units of polymer (c). When the amount of structural units derived from methyl methacrylate is equal to or greater than the lower limit, the stress whitening resistance of the thermoplastic resin film is improved. On the other hand, when the amount is equal to or less than the upper limit, the thermal decomposition resistance of the thermoplastic resin film is improved.

The amount of structural units derived from alkyl acrylate in polymer (c) is preferably 1 to 20 mass %, more preferably 2 to 15 mass %, of the total structural units of polymer (c). When the amount of structural units derived from alkyl acrylate is equal to or greater than the lower limit, the thermal decomposition resistance of the (meth)acrylic polymer having a multilayer structure is improved, and when the amount is equal to or greater than the lower limit, the stress whitening resistance of the thermoplastic resin film is improved.

It is preferable that polymer (c) is soluble in a solvent such as acetone, i.e., is not crosslinked. When polymer (c) is not crosslinked, if a (meth)acrylic polymer having a multilayer structure consisting of polymer (a), polymer (b) and polymer (c) is used as part of the thermoplastic resin composition in the present invention, polymer (c) forms part of the matrix, in which particles consisting of polymer (a) and polymer (b) are present, improving the ease of production of the thermoplastic resin film.

(Mass Ratio of Polymer (a), Polymer (b) and Polymer (c))
The amount of polymer (a) in the (meth)acrylic polymer having a multilayer structure is preferably 1 to 50% by mass, more preferably 5 to 45% by mass, and even more preferably 10 to 40% by mass. When the amount of polymer (a) is equal to or more than the lower limit, the thermal stability and productivity of the thermoplastic resin film are improved. On the other hand, when the amount of polymer (a) is equal to or less than the upper limit, the impact resistance and flexibility of the thermoplastic resin film are improved.

The amount of polymer (b) in the (meth)acrylic polymer having a multilayer structure is preferably 20 to 60% by mass, more preferably 25 to 55% by mass, and even more preferably 30 to 50% by mass. When the amount of polymer (b) is equal to or greater than the lower limit, the thermal stability and productivity of the thermoplastic resin film are improved. On the other hand, when the amount of polymer (b) is equal to or less than the upper limit, the impact resistance and flexibility of the thermoplastic resin film are improved.

The amount of polymer (c) in the (meth)acrylic polymer having a multilayer structure is preferably 5 to 40% by mass, more preferably 10 to 35% by mass, and even more preferably 15 to 30% by mass. When the amount of polymer (c) is equal to or greater than the lower limit, the flowability of the (meth)acrylic polymer having a multilayer structure and the moldability of the thermoplastic resin film are improved. When the amount of polymer (c) is equal to or less than the upper limit, the impact resistance and stress whitening resistance of the thermoplastic resin film are improved.

(Method for producing a (meth)acrylic polymer having a multilayer structure)
The method for producing the (meth)acrylic polymer having a multilayer structure used in the present invention is not particularly limited. For example, the polymer can be obtained by successively forming a polymer (a) in a primary polymerization, a polymer (b) in a secondary polymerization, and a polymer (c) in a tertiary polymerization, by a seed emulsion polymerization method.

(Polymerization initiator)
The polymerization initiator used in the production of a (meth)acrylic polymer having a multilayer structure is not particularly limited, and examples thereof include water-soluble inorganic initiators such as potassium persulfate and ammonium persulfate; redox initiators obtained by combining an inorganic initiator with a sulfite, a thiosulfate, or the like; and redox initiators obtained by combining an organic peroxide with a ferrous salt, a sodium sulfoxylate, or the like.
The polymerization initiator may be added to the reaction system all at once at the start of polymerization, or may be added to the reaction system in portions at the start of polymerization and during polymerization, taking into consideration the reaction rate, etc. The amount of the polymerization initiator used can be appropriately set, for example, so that the average particle size of the (meth)acrylic polymer having a multilayer structure falls within the range described below.

(emulsifier)
From the viewpoint of obtaining the (meth)acrylic polymer having a multilayer structure as a latex, it is preferable to use an emulsifier in the production of the (meth)acrylic polymer having a multilayer structure used in the present invention. There is no particular limitation on the emulsifier, but examples thereof include anionic emulsifiers such as long-chain alkyl sulfonate, sulfosuccinic acid alkyl ester salt, and alkylbenzene sulfonate; nonionic emulsifiers such as polyoxyethylene alkyl ether and polyoxyethylene nonylphenyl ether; polyoxyethylene nonylphenyl ether sulfate such as sodium polyoxyethylene nonylphenyl ether sulfate, polyoxyethylene alkyl ether sulfate such as sodium polyoxyethylene alkyl ether sulfate, and alkyl ether carboxylate such as sodium polyoxyethylene tridecyl ether acetate. The amount of the emulsifier used can be appropriately set so that the average particle size of the (meth)acrylic polymer having a multilayer structure falls within the range described below.

(Chain Transfer Agent)
In the production of the (meth)acrylic polymer having a multilayer structure used in the present invention, a chain transfer agent can be used in each polymerization step for the purpose of adjusting the molecular weight of the (meth)acrylic polymer having a multilayer structure. In particular, in the third polymerization step, a chain transfer agent is added to the reaction system to adjust the molecular weight of the polymer (c), thereby adjusting the weight average molecular weight of the (meth)acrylic polymer having a multilayer structure to the above-mentioned range.
The chain transfer agent used is not particularly limited, and examples thereof include alkyl mercaptans such as n-octyl mercaptan, n-dodecyl mercaptan, t-dodecyl mercaptan, and n-hexadecyl mercaptan; xanthogen disulfides such as dimethyl xanthogen disulfide and diethyl xanthogen disulfide; thiuram disulfides such as tetrathiuram disulfide; and halogenated hydrocarbons such as carbon tetrachloride and ethylene bromide.

The amount of chain transfer agent used can be set appropriately within a range that allows the polymer to be adjusted to a desired molecular weight in each polymerization. The amount of chain transfer agent used in the tertiary polymerization varies depending on the amount of polymerization initiator used in the tertiary polymerization, but is preferably 0.05 to 2 parts by mass, and more preferably 0.08 to 1 part by mass, per 100 parts by mass of the total amount of monomers used in the tertiary polymerization, specifically methyl methacrylate and alkyl acrylate.

In the production of the (meth)acrylic polymer having a multilayer structure used in the present invention, the first polymerization, the second polymerization, and the third polymerization may be carried out sequentially in one polymerization tank, or the first polymerization, the second polymerization, and the third polymerization may be carried out sequentially in a different polymerization tank, but it is preferable to carry out each polymerization sequentially in one polymerization tank. In addition, the temperature of the reaction system during the polymerization is preferably 30 to 120°C, and more preferably 50 to 100°C.

Each of the above polymerizations can be carried out by mixing the above monomers, specifically methyl methacrylate, alkyl acrylate, grafting agent and crosslinking agent in the above ratio and supplying the mixture to the reaction system. There is no particular restriction on the rate at which the above monomers are supplied to the reaction system, but it is preferable to supply the monomers at a rate of preferably 0.05 to 10 mass%/min, more preferably 0.1 to 8 mass%/min, and even more preferably 0.2 to 7 mass%/min, based on the total amount of monomers used in each polymerization. Supplying the monomers at the above rates can prevent the formation of undesirable polymer aggregates and adhesion of polymer scale to the reaction vessel, and can prevent appearance defects such as fish eyes that can occur due to the inclusion of polymer aggregates or polymer scale.

The latex obtained by the above method can be coagulated by a known method. Examples of the coagulation method include freeze coagulation, salting out coagulation, and acid precipitation coagulation. Among these, the freeze coagulation method is preferred because it does not require the addition of a coagulant that is an impurity for the (meth)acrylic polymer having a multilayer structure.

It is preferable to thoroughly wash and dehydrate the slurry obtained by solidification. By washing and dehydrating the slurry, water-soluble components such as emulsifiers and catalysts can be removed from the slurry. The washing and dehydration of the slurry can be performed, for example, using a filter press, a belt press, a Ginner type centrifuge, a screw decanter type centrifuge, etc. From the viewpoint of productivity and washing efficiency, it is preferable to use a screw decanter type centrifuge. It is preferable to wash and dehydrate the slurry at least twice. The more times washing and dehydration are performed, the lower the remaining amount of water-soluble components. From the viewpoint of productivity, it is preferable to wash and dehydrate the slurry three times or less.

The obtained slurry is preferably dried. The slurry is preferably dried so that the moisture content (moisture content) in the (meth)acrylic polymer having a multilayer structure obtained by the above-mentioned method is preferably less than 0.2% by mass, more preferably less than 0.1% by mass. The higher the moisture content, the more likely it is that an ester hydrolysis reaction will occur in the (meth)acrylic polymer having a multilayer structure during melt extrusion molding, and carboxyl groups will be generated in the molecular chains, which will result in the occurrence of streaks in the thermoplastic resin film.

The average particle size of the (meth)acrylic polymer having a multilayer structure is preferably 0.01 μm or more, more preferably 0.04 μm or more, even more preferably 0.05 μm or more, particularly preferably 0.07 μm or more, and preferably 0.35 μm or less, more preferably 0.30 μm or less. If the average particle size is too large, the stress whitening resistance of the thermoplastic resin film tends to decrease. The average particle size is determined by the method described in the Examples based on the light scattering method.

The content of the (meth)acrylic polymer having a multilayer structure is preferably 5 to 50 parts by mass, more preferably 10 to 40 parts by mass, and even more preferably 15 to 30 parts by mass, per 100 parts by mass of the (meth)acrylic resin, from the viewpoint of improving the mechanical strength and processability of the thermoplastic resin film.

[Optional components]
The thermoplastic resin composition used in the present invention may contain known additives such as ultraviolet absorbers, antioxidants, light stabilizers, plasticizers, polymer processing aids, lubricants, dyes, and pigments, as necessary.
When the thermoplastic resin composition contains an additive, the content thereof in the thermoplastic resin composition is preferably 20 mass % or less.
The additives may be added, for example, to a thermoplastic resin composition molten in a film forming machine, or may be dry blended with a pelletized thermoplastic resin composition, or may be added when a (meth)acrylic polymer and/or a (meth)acrylic resin having a multilayer structure is pelletized (masterbatch method).

(Ultraviolet absorber)
The thermoplastic resin composition used in the present invention preferably contains an ultraviolet absorber as an additive. The ultraviolet absorber is a compound capable of absorbing ultraviolet rays, and mainly has the function of converting light energy into heat energy. Examples of organic ultraviolet absorbers include ultraviolet absorbers based on benzophenone, salicylate, benzotriazole, triazine, benzoate, cyanoacrylate, anilide oxalate, malonate, and formamidine.

Examples of the benzophenone-based ultraviolet absorbers include 2,4-dihydroxybenzophenone, 4-n-octyloxy-2-hydroxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2-hydroxy-4-n-octyloxybenzophenone, bis(5-benzoyl-4-hydroxy-2-methoxyphenyl)methane, and 1,4-bis(4-benzoyl-3-hydroxyphenone)-butane.
An example of a salicylate-based ultraviolet absorber is p-tert-butylphenyl salicylate, etc. An example of a benzoate-based ultraviolet absorber is 2,4-di-tert-butylphenyl-3',5'-di-tert-butyl-4'-hydroxybenzoate, etc.

Benzotriazole-based ultraviolet absorbers include, for example, 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol], 2-(3,5-di-tert-butyl-2-hydroxyphenyl)-5-chlorobenzotriazole, 2-(2H-benzotriazol-2-yl)-p-cresol, 2-(2H-benzotriazol-2-yl)-4,6-bis(1-methyl-1-phenylethyl)phenol, 2-benzotriazol-2-yl-4,6-di-tert-butylphenol, 2-[5-chloro(2H)-benzotriazol-2-yl]-4-methyl-6-(tert-butyl)phenol, 2-(2H-benzotriazol-2-yl)-4,6-di-tert-butylphenol, 2-(2H-benzotriazol- 2-yl)-4-(1,1,3,3-tetramethylbutyl)phenol, 2-(2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalimidylmethyl)phenol, methyl 3-[3-(2H-benzotriazol-2-yl)-5-tert-butyl-4-hydroxyphenyl]propionate/polyethylene glycol 300 reaction product, 2-(2H-benzotriazol-2-yl)-6-dodecyl-4-methylphenol, 2-(5-methyl-2-hydroxyphenyl)benzotriazole, 2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxy-C7-9 alkyl ester, etc.

Examples of triazine-based ultraviolet absorbers include 2,4-diphenyl-6-(2-hydroxy-4-methoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-ethoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-(2-hydroxy-4-propoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-butoxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-hexyloxyphenyl)-1,3,5-triazine, and 2,4-diphenyl-6-(2-hydroxy-4-octyloxyphenyl). [0113] Examples of such compounds include 2,4-diphenyl-6-(2-hydroxy-4-dodecyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-benzyloxyphenyl)-1,3,5-triazine, 2,4-diphenyl-6-(2-hydroxy-4-butoxyethoxy)-1,3,5-triazine, 2,4-bis(2-hydroxy-4-butoxyphenyl)-6-(2,4-dibutoxyphenyl)-1,3-5-triazine, and compounds having a 2,4-bis(2,4-dimethylphenyl)-6-[2-hydroxy-4-(3-octyloxy-2-hydroxypropyloxy)-5-α-cumylphenyl]-s-triazine skeleton.
The ultraviolet absorbents may be used alone or in combination of two or more kinds in any ratio.

The ultraviolet absorbent used in the present invention is particularly preferably a benzotriazole ultraviolet absorbent from the viewpoints of compatibility and weather resistance, and among these, 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol] is preferred.
The content of the ultraviolet absorber is preferably 0.05 to 5 parts by mass per 100 parts by mass of the total amount of the thermoplastic resin and the (meth)acrylic polymer having a multilayer structure.

(Polymer processing aid)
As the polymer processing aid, it is preferable to use polymer particles having a particle size of 0.05 to 0.5 μm, which can be usually produced by emulsion polymerization. The polymer particles may be single-layer particles made of a polymer with a single composition ratio and a single intrinsic viscosity, or multi-layer particles made of two or more polymers with different composition ratios or intrinsic viscosities. Among these, particles with a two-layer structure having an inner polymer layer with a low intrinsic viscosity and an outer polymer layer with a high intrinsic viscosity of 5 dL/g or more are preferred.

The polymer processing aid used in the present invention preferably has an intrinsic viscosity of 3 to 6 dL/g. When the intrinsic viscosity is within this range, the molding processability of the resin composition is improved.
Commercially available polymer processing aids include, for example, Paraloid K125 and K125P (both trade names, manufactured by Dow Chemical Japan, Ltd.), Metablen P-530A and P-550A (both trade names, manufactured by Mitsubishi Chemical Corporation), and Kane Ace PA-20 and PA-30 (both trade names, manufactured by Kaneka Corporation).

When the thermoplastic resin composition contains a polymer processing aid, the content is preferably 0.1 to 15 parts by mass, more preferably 0.5 to 10 parts by mass, and even more preferably 1.0 to 5 parts by mass, per 100 parts by mass of the total of the thermoplastic resin and the (meth)acrylic polymer having a multilayer structure. When the content of the polymer processing aid is within the above range, moldability is improved.

There are no particular limitations on the method for producing the thermoplastic resin composition, and it can be produced, for example, by thoroughly mixing a thermoplastic resin, a (meth)acrylic polymer having a multilayer structure, and any optional components using a known mixing method.

The glass transition temperature of the thermoplastic resin composition used in the present invention is preferably 70 to 135°C, more preferably 80 to 130°C, and even more preferably 90 to 125°C. By adjusting the glass transition temperature of the thermoplastic resin composition used in the present invention to an appropriate range, the effects of the present invention can be more easily obtained.

<Applications of thermoplastic resin films>
The thermoplastic resin film produced by the production method of the present invention can be used in the following various applications. For example, it can be used in the medical equipment field such as automobile interior and exterior, personal computer interior and exterior, mobile phone interior and exterior, solar cell interior and exterior, solar cell back sheet, glasses, contact lenses, endoscope lenses, medical supplies requiring sterilization, road signs, bathroom equipment, flooring materials, road light-transmitting plates, lenses for paired glazing, skylight windows, carports, lighting covers, building material sizing, and other construction and building materials fields, microwave cooking containers (dishes), housings for home appliances, toys, sunglasses, stationery, etc. It can also be used as an alternative to molded products using transfer foil sheets.
In particular, it is suitable for optical films because of its excellent heat resistance and optical properties, and can be used for various optical members. For example, it can be applied to known optical applications such as imaging fields such as taking lenses, finders, filters, prisms, Fresnel lenses, lens covers for cameras, VTRs, and projectors; lens fields such as optical disk pickup lenses for CD players, DVD players, and MD players; optical recording fields for optical disks such as CDs, DVDs, and MDs; front panels of liquid crystal screens for terminals such as mobile phones, smartphones, and tablets; vehicle fields such as automobile headlight tail lamp lenses, inner lenses, instrument covers, and sunroofs; lighting lenses; liquid crystal display peripherals such as liquid crystal light guide plates, diffusion plates, backsheets, reflective sheets, polarizing films, transparent resin sheets, retardation films, light diffusion films, prism sheets, optically isotropic films, polarizer protective films, and transparent conductive films; information equipment fields such as surface protective films; organic EL peripherals such as organic EL films; optical communications fields such as optical fibers, optical switches, and optical connectors; optical lenses; optical disks; and the like.

The resin film obtained by the present invention can also be used as a laminated film or the like in which other layers, such as a layer made of a metal and/or metal oxide, a layer made of a resin that is cured by heat or active energy rays, or other thermoplastic resin layers, are laminated on at least one surface. The method for laminating the other layers is not particularly limited, and they can be produced by bonding directly or via an adhesive layer, or by co-extrusion. One or more other layers can be laminated.

The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. The measurement methods in these examples are as follows.

[Measurement method]
<Film thickness (Fth)>
The thickness of the films produced in each of the Examples and Comparative Examples was measured at three positions, namely, the center in the width direction and two points 600 mm away from the center in the width direction, using a micrometer (manufactured by Mitutoyo Corporation, model number: MDH-25), and the average value was regarded as the film thickness.

<Weight average molecular weight (Mw)>
A sample solution was prepared by dissolving 4 mg of the resin to be measured in 5 ml of tetrahydrofuran (THF) and filtering through a filter with a pore size of 0.1 μm. The column used was a "HLC-8320" manufactured by Tosoh Corporation. Two "TSKgel Super Multipore HZM-M" columns and a "Super HZ4000" column, both manufactured by Tosoh Corporation, were connected in series. Tetrahydrofuran was used as the eluent. (THF) was used. The temperature of the column oven was set to 40° C., and 20 μL of the sample solution was injected into the device at an eluent flow rate of 0.35 ml/min to measure the chromatogram. 1 is a chart in which the electric signal value (intensity Y) derived from the refractive index difference between a solution and a reference solution is plotted against the retention time X.
GPC was performed using 10 standard polystyrenes with molecular weights ranging from 400 to 5,000,000, and a calibration curve was created showing the relationship between retention time and molecular weight. The average molecular weight (Mw) was determined. The baseline of the chromatogram is the point where the slope of the peak on the high molecular weight side of the GPC chart changes from zero to positive as viewed from the early retention time, and the point where the slope of the peak on the low molecular weight side changes from zero to positive as viewed from the early retention time. The line connects the points where the slope of the peak changes from negative to zero, starting from the earliest retention time. If the chromatogram shows multiple peaks, the slope of the peak with the highest molecular weight changes from zero to positive. The line connecting the point where the change occurs and the point where the slope of the peak on the lowest molecular weight side changes from negative to zero was defined as the baseline.

<Glass transition temperature>
The glass transition temperatures of the (meth)acrylic resin and the thermoplastic resin composition were measured in accordance with JIS K7121:2012.

<Average particle size of (meth)acrylic polymer having multilayer structure>
The average particle size of the (meth)acrylic polymer having a multilayer structure was measured by diluting a latex containing sample particles 200 times with water and analyzing the diluted solution at 25° C. using a laser diffraction scattering type particle size distribution measuring device (manufactured by Horiba, Ltd., device name "LA-950V2") In this case, the absolute refractive indexes of the (meth)acrylic polymer having a multilayer structure and water were set to 1.4900 and 1.3333, respectively.

[Evaluation method]
<Horizontal line>
A length of 500 mm was cut out from the thermoplastic resin film rolls produced in the Examples and Comparative Examples, and placed on a black sheet. The pitch of the distortion of the fluorescent lamp reflected on the film under fluorescent light was visually evaluated as a horizontal stripe. If the distortion pitch of the horizontal stripe is short, it is thin and has low visibility, but if the pitch is long, it is strong and highly visible, and cannot be used as a product. If the horizontal stripe pitch is less than 2 mm (including cases where no horizontal stripe was observed), it was evaluated as "◎", if the horizontal stripe pitch is 2 mm or more but less than 10 mm, it was evaluated as "○", and if the horizontal stripe pitch is 10 mm or more, it was evaluated as "×".

<Film-forming ability>
Thermoplastic resin films often suffer from a phenomenon called edge bead, where the thickness of the edge portions at both ends of the film increases and the film bulges in a bead-like (string-like) shape. If the edge bead is larger than the gap between rolls 6 and 7, when the film passes through, wrinkles will form starting from the edge bead and the film will not be able to be used as a product. Furthermore, when the wrinkles pass through rolls 6 and 7, they may damage the rolls and destroy the equipment. The case where the wrinkles do not occur for 5 minutes or more after the gap between rolls 6 and 7 is adjusted is evaluated as "good", and the case where the wrinkles occur within 5 minutes after the gap between rolls 6 and 7 is adjusted is evaluated as "poor".

[Production Example]
<Production Example 1: Production of (meth)acrylic resin>
99.3 parts by mass of methyl methacrylate and 0.7 parts by mass of methyl acrylate were added with 0.008 parts by mass of a polymerization initiator [2,2′-azobis(2-methylpropionitrile), hydrogen abstraction ability: 1%, 1-hour half-life temperature: 83° C.] and 0.26 parts by mass of a chain transfer agent (n-octyl mercaptan) and dissolved to obtain 3,000 kg of a raw material liquid.
100 parts by mass of ion-exchanged water, 0.03 parts by mass of sodium sulfate, and 0.45 parts by mass of potassium polymethacrylate were mixed to obtain 6000 kg of a mixed liquid. The mixed liquid and the raw material liquid (total 9000 kg) were charged into a pressure-resistant polymerization tank, and the temperature was raised to 70° C. while stirring under a nitrogen atmosphere to start the polymerization reaction. After 3 hours had elapsed after the start of the polymerization reaction, the temperature was raised to 90° C., and stirring was continued for 1 hour to obtain a liquid in which a bead-like copolymer was dispersed. Although some polymer adhered to the walls of the polymerization tank or the stirring blades, there was no foaming, and the polymerization reaction proceeded smoothly.
The obtained copolymer dispersion was washed with an appropriate amount of ion-exchanged water, and the bead-like copolymer was taken out using a bucket centrifuge and dried for 12 hours in a hot air dryer at 80° C. to obtain bead-like (meth)acrylic resin.
The resulting (meth)acrylic resin had a methyl methacrylate unit content of 99.3 mass %, a methyl acrylate unit content of 0.7 mass %, a weight average molecular weight (Mw) of 92,000 and a glass transition temperature of 120°C.

<Production Example 2: Production of (meth)acrylic polymer having multilayer structure>
According to the following procedure, an acrylic polymer having a core-shell multilayer structure consisting of three layers, a core (inner layer), an inner shell (middle layer), and an outer shell (outer layer), was produced.
(1) Synthesis of the inner layer In a reactor equipped with a stirrer, a thermometer, a nitrogen gas inlet tube, a monomer inlet tube and a reflux condenser, 1050 parts by mass of ion-exchanged water, 0.3 parts by mass of sodium polyoxyethylene tridecyl ether acetate and 0.7 parts by mass of sodium carbonate (total 2100 kg) were charged, and the inside of the reactor was fully replaced with nitrogen gas. Then, the inside temperature was set to 80°C, 0.25 parts by mass of potassium persulfate was added, and the mixture was stirred for 5 minutes. 245 parts by mass of a monomer mixture consisting of 95.4% by mass of methyl methacrylate, 4.4% by mass of methyl acrylate and 0.2% by mass of allyl methacrylate was continuously dripped into the mixture over 60 minutes. After the dripping was completed, the polymerization reaction was carried out for another 30 minutes so that the polymerization conversion rate was 98% or more.

(2) Synthesis of intermediate layer Next, 0.32 parts by mass of potassium persulfate was added to the reactor and stirred for 5 minutes. After that, 315 parts by mass of a monomer mixture consisting of 80.5% by mass of n-butyl acrylate, 17.5% by mass of styrene, and 2% by mass of allyl methacrylate was continuously dripped over 60 minutes. After the dripping was completed, the polymerization reaction was continued for another 30 minutes so that the polymerization conversion rate was 98% or more.

(3) Synthesis of outer layer Next, 0.14 parts by mass of potassium persulfate was added to the reactor and stirred for 5 minutes. Then, 140 parts by mass of a monomer mixture consisting of 95.2% by mass of methyl methacrylate, 4.4% by mass of methyl acrylate, and 0.4% by mass of n-octyl mercaptan was continuously added dropwise over 30 minutes. After the dropwise addition was completed, the polymerization reaction was continued for another 60 minutes so that the polymerization conversion rate was 98% or more.

After obtaining a latex containing an acrylic polymer having a multilayer structure by the above operations, the latex containing an acrylic polymer having a multilayer structure was frozen and coagulated. It was then washed with water and dried to obtain an acrylic polymer having a multilayer structure. The average particle size of the particles was 0.23 μm.

[Example 1]
A thermoplastic resin film was produced using the apparatus of Fig. 1. Specifically, 78.4 parts by mass of a (meth)acrylic resin, 19.6 parts by mass of a (meth)acrylic polymer having a multilayer structure, and 2.0 parts by mass of Adeka STAB LA-31 (manufactured by Adeka Corporation) as an ultraviolet absorber were mixed in a Henschel mixer, and melt-kneaded using a vented twin-screw extruder (screw diameter: 58 mm, cylinder temperature: 260°C) to obtain a thermoplastic resin composition ((meth)acrylic resin composition).
The (meth)acrylic resin composition was melted using a vented single-screw extruder (screw diameter: 75 mm, cylinder temperature: 265° C., L/D=33) and extruded into a film shape from a T-die with a die lip width of 1850 mm and a die lip clearance of 0.5 mm at a discharge rate of 180 kg/hour. Next, the mixture was pressed between an elastic metal roll (I) and a rigid mirror-finished metal roll (II), cooled by the rigid mirror-finished metal roll (II), and further cooled by the rigid mirror-finished metal roll (III) to obtain a thermoplastic resin film ((meth)acrylic resin film) having a thickness of 60 μm. The acute angle of the angle formed by the straight line connecting the center of the elastic metal roll (I) and the center of the rigid mirror-finished metal roll (II) and the straight line connecting the center of the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) was 0° (i.e., both straight lines were in a straight line). The evaluation results are shown in Table 1.
The gear pump used was a spur-tooth external gear pump (manufactured by Toshiba Machine Co., Ltd.), and the pump was delivered at an inlet pressure of 5 MPa. Other conditions are shown in Table 1.

[Example 2 and Comparative Examples 1-2]
A thermoplastic resin film ((meth)acrylic resin film) was obtained in the same manner as in Example 1, except that the resin composition and the conditions for each roller were as shown in Table 1. The evaluation results are shown in Table 1.

In Examples 1 and 2, films with few horizontal striations were produced with good film-forming properties. On the other hand, in Comparative Example 1, wrinkle defects occurred due to edge beads, and a film that could be used for evaluation of horizontal striations was not obtained, and in Comparative Example 2, a film with good film-forming properties was obtained, but the occurrence of horizontal striations could not be suppressed.
As is clear from the results of the Examples and Comparative Examples, according to the present invention, a thermoplastic resin film with few horizontal striations can be produced.

Reference Signs List 1 Extruder 2 Gear pump 3 Polymer filter 4 T-die 5 Roll (elastic metal roll (I))
6 Roll (rigid mirror-finished metal roll (II))
7 Roll (rigid mirror-finished metal roll (III))
8, 8' Nip roll 9 Winder

Claims (4)

A method for producing a thermoplastic resin film, comprising the steps of: nipping and compressing a thermoplastic resin composition melt-extruded from a T-die between an elastic metal roll (I) and a rigid mirror-finished metal roll (II); and further passing the film through a rigid mirror-finished metal roll (III) and winding the film,
the thermoplastic resin composition is a (meth)acrylic resin composition containing a (meth)acrylic polymer having a multilayer structure,
The distance Gth (mm) between the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III), and the thickness Fth (mm) of the thermoplastic resin film satisfy the following formula (1),
The thickness Fth of the thermoplastic resin film is 0.05 to 0.13 mm,
The gap Gth is 0.4 to 2.0 mm,
The line speed is 15 to 35 m/min.
A method for producing a thermoplastic resin film, characterized in that the temperature difference between the rigid mirror-surface metal roll (II) and the rigid mirror-surface metal roll (III) is 10°C or less .
5.0≦Gth/Fth≦30.0 (1) The method for producing a thermoplastic resin film according to claim 1, wherein the rigid mirror-finished metal roll (II) and the rigid mirror-finished metal roll (III) each have a temperature of 50 to 105°C. The method for producing a thermoplastic resin film according to claim 1 or 2, wherein an acute angle formed by a straight line (A) connecting the center of the elastic metal roll (I) and the center of the rigid mirror-surface metal roll (II) and a straight line (B) connecting the centers of the rigid mirror-surface metal roll (II) and the rigid mirror-surface metal roll (III) intersect is 0° to 45°. The method for producing a thermoplastic resin film according to any one of claims 1 to 3 , wherein the discharge rate of the thermoplastic resin composition discharged from the T-die is 90 to 600 kg/hour .