JPH0577695B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a polyester film containing countless microscopic bubbles on its surface and inside. Specifically, the present invention relates to a stretched polyester film which has a reduced weight per unit volume by containing fine closed cells and has excellent hiding properties and flatness. <Prior Art and Problems to be Solved by the Invention> Conventionally, polyester films have been widely used as various industrial base materials because of their excellent mechanical properties, electrical properties, chemical resistance, and the like. In particular, biaxially oriented polyethylene terephthalate film has superior flatness and dimensional stability compared to other films, and is available on the market at a relatively low price, making it an indispensable material for the information industry and plate-making. It is. For example, the base material for the board surface of electronic blackboards used in offices and conference rooms, etc.
High-grade shielded polyethylene terephthalate film is used as the base material for magnetic cards, such as NTT's telephone card and JR's Orange Card, and fully exhibits the excellent properties mentioned above. However, although the high-hidden film has excellent properties, in order to increase the hiding power to a high degree, it contains extremely large amounts of inorganic particles with a large specific gravity such as titanium dioxide, which causes various problems. ing. In other words, since the weight per unit volume of the film is 20 to 40% larger than that of ordinary film, if it is used vertically for a long period of time as the board surface of an electronic blackboard, the film itself will sag very quickly, causing the product to sag. This is a factor that significantly reduces the value. In addition, the presence of inorganic particles in the film can significantly shorten the life of the knife during film slitting and cutting processes such as cards, reducing productivity, and may cause cuts to the hands due to the edges of the film. There were also problems in handling. Therefore, in order to solve these problems, the present inventors recognized that the most important point for improvement was how to reduce the weight per unit volume of the film, that is, the apparent density of the film. We started research to create a structure. In the past, many methods for producing polyester foams have been proposed, such as Japanese Patent Application Laid-Open No. 50-38765.
Publication No. 57-46456 or Japanese Patent Application Publication No. 1987-46456
A method of adding a gas or a vaporizable substance to form a foam as described in Japanese Patent Publication No. 57-34931, etc., and a method described in Japanese Patent Application Laid-open No. 52-43871 and Japanese Patent Publication No. 58-50625, etc. A method of forming a foam by adding a substance that chemically decomposes and generates gas, such as that described in Japanese Patent Application Laid-Open No. 51-34963 and Japanese Patent Publication No. 52
A method is known, such as that described in Japanese Patent No. 27666, in which after molding, the material is impregnated with a liquid and a substance soluble in a solvent is extracted to form a foam. However, it is extremely difficult to simply apply these foaming methods to biaxially oriented polyester films. In other words, these methods have been proposed for producing molded bodies, and cannot be said to be methods that can be simply applied to films with a size of several hundred microns or less. Moreover, it is extremely difficult to biaxially orient an extruded sheet without breaking it. This is supported by the fact that there have been almost no concrete proposals for foaming biaxially oriented polyester films. Although it is extremely difficult to employ conventional foaming methods, it goes without saying that it is effective to reduce the apparent density if a large number of fine air bubbles can be incorporated into the polyester film. If the bubbles can be made as fine as several tens of microns or even a few microns, the scattering of light by the bubbles can incidentally improve the hiding property. Therefore, the amount of inorganic particles added to impart hiding properties can naturally be reduced, which helps reduce the apparent density of the film and helps prevent sagging due to its own weight, as well as reducing film production during slitting. Improved properties and handling properties can also be achieved at the same time. Therefore, the present inventors investigated a method of reducing the apparent density of the film by incorporating countless such fine closed cells without being bound by the conventional foaming method, and as a result, they previously proposed Japanese Patent Application No. 313,896/1986. . That is, by stretching a sheet made of aromatic polyester blended with a specific polymer, a reduction in the apparent density of the film was achieved. Moreover, unlike conventional foaming methods, this is a simple method that does not require any new equipment, making it an epoch-making method. However, when the film obtained by this invention is reviewed from the viewpoint of surface characteristics, it is difficult to say that it necessarily maintains sufficient flatness, and it is also difficult to say that the film has poor productivity, that is, continuous film formation. Because of their inferiority, it was necessary to improve them. <Means for Solving the Problems> In view of the above circumstances, the present inventors first investigated the factors that inhibit flatness, and found that different types of polymers, which should originally be finely dispersed throughout the entire layer of the polyester film, are dispersed in the surface layer of the film. It has been found that not only are the fibrous polymers present in the film and do not form uniform bubbles, but also that some of the fibrous polymers protrude onto the film surface, severely deteriorating the surface shape.
Therefore, if the polymer to be finely dispersed in polyester is extruded so as not to become fibrous, and air bubbles are included so that the surface layer is of the same quality as the inner layer, the flatness will be greatly improved, and the film can be formed continuously. The present invention was achieved by discovering that it is possible to simultaneously improve properties. That is, the gist of the present invention is to use, as a void-forming polymer, a liquid crystalline polyester or polyphenylene sulfur having a melt viscosity that crosses that of polyester at a shear rate of 20 to 1000 sec ^-1 when heated and melted. A film containing 3 to 40% by weight of id and stretched in at least one axis, where the density A (g/cm ³ ) in a portion of the film up to a depth of 5 μm from the surface layer and the portion up to a depth of 5 μm from the surface layer of the film. Density B in the central part excluding
(g/cm ³ ) satisfies the following formulas (1) to (3) at the same time. 0.5≦A≦1.3 (1) 0.5≦B≦1.3 (2) A−B≦0.5 (3) The present invention will be described in detail below. Polyester as used in the present invention refers to aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, and naphthalene dicarboxylic acid or their esters, ethylene glycol, diethylene glycol, 1,4
- It is a polyester produced by polycondensing glycols such as butanediol and neopentyl glycol. These polyesters are manufactured by directly reacting aromatic dicarboxylic acids and glycols, and
It can also be produced by carrying out a transesterification reaction between an alkyl ester of an aromatic dicarboxylic acid and a glycol, followed by polycondensation, or by polycondensing a diglycol ester of an aromatic dicarboxylic acid. Typical examples of such polyesters include polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate. This polyester may be a homopolymer or may be one copolymerized with a third component.
In any case, in the present invention, polyesters having 70 mol% or more of ethylene terephthalate units and/or ethylene 2,6-naphthalate units and/or butylene terephthalate units are preferred, more preferably 80 mol% or more, particularly preferably 90 mol% or more. It is a polyester having mol% or more. In addition, in the present invention, if the degree of polymerization of the polyester is too low, the mechanical strength will decrease, so the intrinsic viscosity is preferably 0.4 or more, more preferably 0.5 to 0.5.
1.2 Particularly preferably 0.55 to 0.85. Normally, when polyester is made into a film, a polyester containing an appropriate amount of fine inert particles is used to provide slipperiness between the films or between the film and a metal roll, but the polyester used in the present invention contains such fine particles. Preferably, those containing no . This is because the presence of the fine particles may impede control of the color tone and hiding power of the obtained film. However, there is no problem in using polyester containing such inert fine particles as long as it does not interfere with the color tone and degree of hiding required for the resulting film. In the present invention, a specific polymer, that is, a void-forming polymer is blended with the aromatic polyester. The void-forming polymer in the present invention refers to a polymer that is dissolved in the aromatic polyester or immediately subjected to a reaction such as transesterification. It is a thermoplastic polymer that disperses in the form of fine particles in the polyester (sea component) during melt mixing to form island components, and when the sea-island forming sheet is stretched at a temperature higher than the glass transition temperature of the polyester, it forms an island component. Refers to polymers that easily form voids or voids at interfaces due to low compatibility. Examples of such polymers include liquid crystalline polyester and polyphenylene sulfide polypropylene, which have a melt viscosity of 20 to 20% when melted by heating compared to polyester.
It is a polymer with a melting characteristic that crosses between 1000 sec ^-1 . That is, it is a polymer with shear rate dependence such that at the temperature during melt mixing, the melt viscosity is higher than that of polyester at low shear rates and lower at high shear rates. This is an indispensable characteristic in polyester for finely dispersing the polymer and uniformly forming island components. In the present invention, a void-forming polymer having such characteristics is blended into the aromatic polyester in an amount of 3 to 40% by weight, preferably 5 to 30% by weight, and more preferably 5 to 20% by weight. is within the range of If the blending amount is less than 3% by weight, the amount of microbubbles generated in the film is small and the apparent density of the film cannot be sufficiently reduced, while if it exceeds 40% by weight, the number of bubbles generated is extremely large, which may cause problems during film stretching. This is undesirable because it causes frequent breakage and makes the producers extremely inferior. In the present invention, a void-forming polymer is blended with the above-mentioned aromatic polyester to form a film, but the final objective of the present invention is to contain fine closed cells on the surface and inside of the film to reduce the apparent density. In order to achieve this, it is necessary to stretch an unstretched sheet of such a blend in at least one direction. That is, sufficient closed cells are not yet generated on the surface and inside of a sheet obtained by simply extrusion molding such a compound, and numerous closed cells effective in reducing the apparent density are formed only after the stretching process. That is,
By stretching, voids are forcibly formed around the finely dispersed void-forming polymer in the form of fine particles as an island component in the polyester (sea component). The stretching conditions themselves are not special, but the stretching is carried out at a temperature higher than the glass transition point of polyester. This is not preferred because the polyester having a temperature below the glass transition point cannot be stretched uniformly. More specifically, a mixture of aromatic polyester and void-forming polyester is used as raw material in an extruder.
The mixture is melt-kneaded at a temperature of 250 to 320°C, extruded into slits through a die, and cooled to a temperature of about 70°C or less to form a substantially amorphous sheet. Next, the sheet is stretched in the longitudinal and/or transverse directions at a temperature equal to or higher than the glass transition point of the aromatic polyester to an area magnification of 4 times or more, preferably 9 times or more, and further heat treated at 120 to 250°C. do. There are no particular limitations on the method of blending the void-forming polymer; for example, polyester chips and void-forming polymer chips may be mixed in advance and then fed into the extruder hopper. A method of quantitatively feeding polyester chips and void-forming polymer chips is preferably adopted because it is simple and causes less unevenness in blending. In the present invention, basically, by using an aromatic polyester and a void-forming polymer, the film has an apparent density of 0.5 to 1.3 g/cm ³ and a hiding degree of 0.2.
Although the film described above can be obtained, third components such as other additives may be added as long as the above physical properties are satisfied. As such third component,
Examples include antioxidants, antistatic agents, ultraviolet absorbers, pigments, dyes, lubricants, matting agents, optical brighteners, etc., and they can be added in the required amount by an appropriate method as needed. In this way, a film with an apparent density of 0.5 to 1.3 g/cm ³ and a hiding degree of 0.2 or more, which is the object of the present invention, is obtained. Apparent density A
is 0.5 to 1.3 g/ ^cm3 , and the apparent density B of the central part excluding the part from the surface layer to a depth of 5 μm is 0.5 to 1.3.
g/ ^cm3 , and the apparent density difference between A and B is
It is necessary that it is 0.5 g/cm ³ or less, preferably 0 to 0.3 g/cm ³ , and more preferably 0 to 0.2 g/cm ³ , and these conditions must be satisfied at the same time. That is, if A and B are less than 0.5 g/cm ³ , the mechanical strength of the film will be significantly reduced, which is undesirable, while if it exceeds 1.3 g/cm ³ , it will not be effective in suppressing sagging due to the film's own weight. Undesirable. Furthermore, if the difference in apparent density between A and B exceeds 0.5 g/cm ³ , the surface roughness of the film surface will become extremely large due to the phenomenon described below, forming undulating surface irregularities and impairing the flatness of the film. This is not desirable as it becomes extremely inferior. On the other hand, there is no problem even if the difference between A and B is less than 0, that is, B is larger than A, but in reality, it is difficult to produce such a film. Now, the feature of the present invention is that it finally obtains a polyester film having a uniform foam structure with little difference in apparent density between the surface layer and the center of the film, that is, no difference in the amount of fine bubbles contained. However, in order to produce such a film, the extrusion conditions are important when blending a void-forming polymer with an aromatic polyester and melt-extruding it to form a substantially amorphous sheet. That is, when the molten polymer passes between the lips of the slit-shaped die, the shear rate γ is usually controlled to 200 sec ^-1 or less, preferably 5 to 100 sec ^-1 , and the draft from the die lip exit to the point where the molten polymer contacts the cooling drum is controlled. The ratio is usually controlled between 1 and 20. Also, γ〓
The value obtained by multiplying the square of the draft ratio by the draft ratio is normally controlled to be 80,000 or less, preferably 40,000 or less. γ〓 is
If an amorphous sheet is obtained under conditions where the temperature exceeds 200 sec ^-1 , the draft ratio exceeds 20, or the value obtained by multiplying the square of γ by the draft ratio exceeds 80,000, an island component exists in the sheet. Since the void-forming polymer exists in the surface layer in the form of elongated fibers, when the sheet is then stretched in at least one direction, voids can no longer be formed, which only hinders the reduction of the film's apparent density. Alternatively, a portion of the fibrous polymer protrudes onto the film surface, deteriorating the surface properties. Further, even if the fibrous polymer does not protrude from the surface of the film, the presence of the fibrous polymer in the surface layer induces uneven stretching, resulting in a rough surface of the film and extremely deteriorating the flatness. Furthermore, the presence of the fibrous polymer causes frequent breakage during transverse stretching in a tenter, resulting in extremely low productivity. The thinner the film to be obtained, the greater the effect of this problem, and the extrusion molding conditions are such that the polymer, which is finely dispersed in the amorphous sheet, exists in a uniform shape from the surface layer to the center. It is an important point in the present invention to employ this. Although the film of the present invention is obtained in this manner, various surface treatments may be carried out to improve the surface properties of the film, such as adhesiveness and antistatic properties. Examples of surface treatments include primer treatment, corona treatment, plasma treatment, solvent treatment,
Examples include ultraviolet treatment and sandblasting treatment, which can be applied to one or both sides of the film of the present invention at an appropriate time and in the required amount. <Example> The present invention will be specifically explained below with reference to Examples.
The present invention is not limited to the following examples unless it exceeds the gist thereof. Measurement and evaluation of various physical properties in the present invention were performed by the following methods. (1) Film apparent density (g/cm ³ ) Five points were sampled from any part of the film, the weight per unit volume of each sample was determined, and the average value was taken as the film apparent density value. Next, both sides of the film were shaved to a thickness of 5 μm each using a microtome, and the weight per unit volume of the sample was determined, and the density of the central portion was determined as B. The density A of the surface layer portion was calculated from the density of the entire film and the density B of the central portion. (2) Hiding degree Transmission density in visual light was measured using a Macbeth densitometer model TR-927. Measurements were made at three points, and the average value was taken as the mildness value. The larger this value is, the higher the hiding power is. (3) Surface roughness Ra (μm) Measured using a surface roughness measuring instrument SE-3F model manufactured by Kosaka Institute Co., Ltd. according to the method of JISB-0601-1976. The detailed conditions were a stylus diameter of 2 μm, a stylus pressure of 30 mg, a cut-off value of 0.8 mm and 2.5 mm, and the number of measurements was 12 each.
The average value of 10 times excluding the maximum and minimum values was taken as center line average roughness Ra ^0.8 and Ra ^2.5 value. (4) Evaluation of average appearance The surface condition of the film was visually observed and evaluated as × if the surface had roughness and clearly poor flatness, and 〇 if it was hardly observed. (5) Evaluation of film formability When an amorphous sheet is stretched in the longitudinal direction and then laterally stretched in a tenter, the state of breakage of the film is observed, and it is found that the film often breaks at the stretched part and other areas, which clearly indicates poor productivity. Items were evaluated as ×, and items with almost no breakage and good productivity were evaluated as ○. Example 1 Liquid crystal polyester a was added to a polyethylene terephthalate chip with an intrinsic viscosity (hereinafter abbreviated as [η]) of 0.66.
[Terephthalic acid/isophthalic acid/paraoxybenzoic acid/ethylene glycol polymer; Melt viscosity (hereinafter abbreviated as η) at 290°C is 5000 poise at a shear rate (abbreviated as 下下γ〓) of 50sec ^-1 , γ〓1000sec ^{- 1}
The uniformly mixed raw materials containing 30 ^wt . An amorphous sheet with a thickness of about 370 μm was obtained by extruding it into a sheet on a cooling drum at ℃. Next, the sheet is stretched 3 times in the longitudinal direction and 3 times in the transverse direction, and heat-treated at 220°C for 5 seconds to finalize the film thickness.
A biaxially stretched film having a diameter of 75 μm, an apparent density of 076 g/cm ³ and a degree of opacity of 0.55 was obtained. There was almost no breakage during film formation, and productivity was good. The density of the surface layer of this film is 0.77 g/cm ³ , and the density of the central portion is 0.76 g/cm ³ , and the Ra ^0.8 of the film surface is 0.29 μm, and the Ra ^{2.5 is 0.76 g/cm 3 .}
was 0.62 μm, and no roughness was observed even by visual observation, indicating good flatness. Example 2 Instead of liquid crystal polyester a used in Example 1, liquid crystal polyester b with different melting properties (η at 290°C is 10000 poise under γ 50 sec ^-1 , γ 1000 sec ^-1
Same as Example 1 except that 15% by weight of 2000 poise polymer) was blended, and an amorphous sheet of about 330 μm was formed under the conditions of γ = 100 sec ^-1 and draft ratio of 3 during melt extrusion molding. A film was made in the same way, and the final film thickness was 50μm and the apparent density was 1.02g/
A biaxially stretched film was obtained with a thickness of cm ³ and a degree of hiding of 0.42. Similar to Example 1, there was almost no breakage during film formation. The density of the surface layer of the obtained film was 1.06 g/cm ³ and the density of the central portion was 1.01 g/cm ³ , and there was almost no difference in density between the two, and the flatness of the film surface was Ra ^0.8 0.25 μm.
It was good with Ra ^2.5 0.58 μm. Example 3 Polyphenylene sulfide (η at 290°C) was used instead of the liquid crystal polyester a used in Example 1.
is 4000 poise under γ = 50 sec ^-1 . 15% by weight of a polymer exhibiting 800 poise under γ = 1000 sec ^-1 is blended, and γ = 150 sec ^-1 during melt extrusion and the draft ratio is 2. The final thickness was 50 μm and the apparent density was obtained in the same manner as in Example 1 except that the amorphous sheet was approximately 355 μm thick.
A biaxially stretched film with a hiding degree of 0.43 of 1.10 g/cm ³ was obtained with almost no breakage. The density of the surface layer of such a film is 1.18 g/cm ³ ,
The density of the central part is 1.08g/ ^cm3 , and the density of the surface is 1.08g/cm3.
Ra ^0.8 was 0.26 μm, Ra ^2.5 was 0.68 μm, and although there was a slight density difference, good flatness was shown. Comparative Example 1 The compound used in Example 1 was not used at all, and the γ value during melt extrusion molding was 125 sec ^-1 and the draft ratio was 1.5.
Example 1 except that the amorphous sheet is approximately 680 μm thick.
Finally, the thickness is 75μm and the apparent density is 1.4.
A biaxially stretched film with a density of g/cm ³ and a degree of hiding of 0.07 was obtained.
As for the physical properties of the obtained film, as shown in Table 1 below, there was no problem with the difference in density between the surface layer and the inner layer and the surface properties, but it was a transparent film with no hiding properties at all. Comparative Example 2 Example except that 15% by weight of the liquid crystal polyester a used in Example 1 was blended, γ = 250 sec ^-1 during melt extrusion molding, draft ratio was 1, and an amorphous sheet with a thickness of about 705 μm was made. In the same manner as in Example 1, a biaxially stretched film having a film thickness of 150 μm and an apparent density of 0.73 g/cm ³ and a hiding degree of 0.62 was finally obtained. During film formation, breakage sometimes occurred, and productivity was slightly inferior. The surface density of the obtained film was 1.30 g/cm ³ and the center density was
The difference in density between the two is 0.69g/cm ³ and 0.61g/cm ³ .As a result, the surface of the film is 0.51μm at Ra ^0.8 , but it is extremely large at 5.2μm at Ra ^2.5 , resulting in inferior flatness. It was hot. In addition, visual observation also revealed that the surface had a noticeable rough feel. Comparative Example 3 Using the same raw materials as in Example 3, γ = 200 sec ^-1 during melt extrusion molding, draft ratio 3, and about 230 μm.
The final film thickness was 38 μm and the apparent density was
A biaxially stretched film with a weight of 1.12 g/cm ³ and a degree of hiding of 0.31 was obtained. The film often broke during film formation, resulting in poor productivity. The central density of the obtained film was 1.0 g/cm ³ , but the surface density was as high as 1.41 g/cm ³ , and the film surface had poor flatness, with Ra ^0.8 of 0.37 μm and Ra ^2.5 of 4.37 μm. It was hot. Visual observation also showed an extremely poor appearance similar to Comparative Example 2. Comparative Example 4 Same as Example 1 except that 45% by weight of the liquid crystal polyester b used in Example 2 was blended, γ = 5 sec ^-1 draft ratio was 30 during melt extrusion molding, and an amorphous sheet with a thickness of about 150 μm was made. A film was formed in the same manner, but the final biaxially stretched film could not be obtained due to frequent breakage during film formation. The film physical properties and film formability evaluation results of Examples 1 to 3 and Comparative Examples 1 to 4 described above are summarized in Table 1 below.

【table】

【table】 ·
*2 r = 5 at 290℃ using Koka type float tester
Melt viscosity value at 0 and 1000 (unit: poise)
<Effects of the Invention> As detailed above, the polyester film containing microcells of the present invention contains countless fine closed cells to reduce the film's apparent density and impart hiding properties. It is possible to significantly delay the change in talumi over time. In addition, film slitting properties have been improved, which not only improves productivity during film processing, but also reduces accidents caused by film edges. Furthermore, by reducing the difference in density between the surface layer and the center of the film, the flatness of the film was significantly improved, resulting in a significant improvement in film quality.

Claims (1)

[Claims] 1. As a void-forming polymer, the melt viscosity when melted by heating is higher than that of polyester at a shearing rate of 20~
A film containing 3 to 40% by weight of liquid crystalline polyester or polyphenylene sulfide having melting characteristics that intersect in the range of 1000 sec ^-1 and stretched in at least one axis, the film being 5 μm deep from the surface layer of the film. Density A in the part between
(g/cm ³ ) and the density B (g/cm ³ ) in the central part excluding the part from the surface layer to a depth of 5 μm is calculated by the following formula (1) ~
A polyester film containing microcells, which satisfies (3) at the same time. 0.5≦A≦1.3 …(1) 0.5≦B≦1.3 …(2) A−B≦0.5 …(3)