JPH0717778B2 - Polyester film containing fine bubbles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention relates to a polyester film having a large number of fine bubbles on its surface and inside. More specifically, the present invention relates to a white stretched polyester film containing fine cells, which has a highly improved light resistance and is lightweight and has high hiding power.

<Problems to be Solved by Prior Art and Invention> Conventionally, polyester films, especially polyethylene terephthalate films, have excellent mechanical properties, electrical properties, flatness, chemical resistance, etc., and are therefore used for information recording materials and capacitors. It is widely used in various application fields such as printing, plate making, packaging, and electrical insulation. In recent years, taking advantage of the excellent properties of such a polyester film, for example, a white highly concealable biaxially stretched polyester film is used as a whiteboard portion base material of an electronic whiteboard used in offices and conference rooms, and, The white film is also used as a base material in magnetic cards typified by NTT's telephone cards and JR's orange cards, and its range of use is expanding further in the future.

However, since the white film must be blended with a large amount of inorganic particles having a large specific gravity, such as titanium dioxide, in order to highly enhance the hiding property, the weight per unit volume of the film is 3 times as much as that of a normal polyester film. ~ 50% larger. The increase in the weight per unit volume of such a film causes the whiteboard surface of the electronic whiteboard to slacken and age due to its own weight when used for a long period of time, which is a factor that significantly reduces the commercial value. Further, when a large amount of these inorganic particles is contained in the film, not only the life of the knife is significantly shortened in the film slitting process and the cutting process of the card and the productivity is impaired, but also the hand is cut at the film edge to improve the handleability. Also had a problem. Therefore, in order to solve these problems, the present inventors have proposed, for example, Japanese Patent Application No. 61-31.
As proposed in Japanese Patent No. 3896, the film per unit volume was reduced by making the film have a foam structure, that is, a film containing innumerable fine closed cells, and the aging deterioration due to the weight of the film was significantly improved. Furthermore, since the film can also have a hiding property due to innumerable finely dispersed closed cells, it is possible to suppress the amount of pigment added, the life of the knife is extended, the productivity is improved, and the film handling property is also improved. Was done.

However, although the microbubble-containing polyester film exerts such an excellent effect, it is inferior in concealing property to the conventional white film, and therefore uses requiring a very high concealing power, for example, the board of the electronic white board described above. In the present situation, it is inevitable to use a white pigment together.

Since such an electronic white board is used for a long period of time while being affected by ultraviolet rays from sunlight or a fluorescent lamp, especially a fluorescent lamp incorporated in the electronic white board, a white polyester film used as a board portion base material is Therefore, the problems that it yellows over time and that the mechanical strength decreases are still left.

Particularly, the yellowing phenomenon of the film is a serious problem because it significantly lowers the commercial value, and there is a strong demand for improvement. It has been found that such a yellowing phenomenon is based on the yellowing of titanium dioxide particles contained as a white pigment in the polyester film, and in general, a measure to prevent yellowing by incorporating an ultraviolet absorber is used. It is taken.

However, conventionally used UV absorbers are organic UV absorbers such as benzophenone-based, benzotriazole-based, and hydroquinone-based UV absorbers, and such UV-absorbers are generally susceptible to thermal decomposition or oxidative decomposition, and at high temperatures. Many sublime. For example, when a film is formed by adding an ultraviolet absorber related to polyethylene terephthalate, it is extruded at a high temperature of 270 to 300 ° C,
The ultraviolet absorbent has a significantly reduced ultraviolet absorbing ability due to thermal decomposition or sublimation. Further, in order to obtain a desired ultraviolet absorbing ability, if the addition amount of the ultraviolet absorber is increased, the color tone of the obtained film is lowered, or the ultraviolet absorber is sublimated and accumulated around the extruder, the tenter, etc. It was causing trouble. Thus, in the method of simply blending the organic ultraviolet absorber, it is very difficult to impart sufficient whiteness and light resistance without impairing the film color tone, so it is necessary to find a radical light resistance improving method. there were.

<Means for Solving Problems> In view of such circumstances, the present inventors have earnestly studied to improve the light resistance of a fine bubble-containing high concealing white polyester film, and as a result, have been used as a white pigment in combination. The inventors have found that the use of anatase type titanium dioxide containing a zinc compound or an aluminum compound as titanium particles dramatically improves the light resistance and has reached the present invention.

That is, the gist of the present invention is characterized by containing 1 to 20% by weight of at least one of the following two types of titanium dioxide (a) and (b) and 3 to 40% by weight of crystalline polypropylene. In a fine bubble-containing polyester film.

(A); Anatase type titanium dioxide containing zinc element (b); Anatase type titanium dioxide containing aluminum element The present invention will be described in detail below.

The aromatic polyester referred to in the present invention is polycondensed with an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid or naphthalenedicarboxylic acid or an ester thereof and a glycol such as ethylene glycol, diethylene glycol, 1,4-butanediol or neopentyl glycol. It is a polyester produced by These polyesters are produced by directly reacting an aromatic dicarboxylic acid with a glycol, or by subjecting an alkyl ester of an aromatic dicarboxylic acid and a glycol to an ester exchange reaction, followed by polycondensation, or with an aromatic dicarboxylic acid. It can also be produced by a method such as polycondensation of diglycol ester.
Typical examples of such polyesters include polyethylene terephthalate, polyethylene-2,6-naphthalate and polybutylene terephthalate. This polyester may be a homopolymer or a copolymer of a third component. In any case, in the present invention, ethylene terephthalate unit and / or ethylene-2,
A polyester having 6-naphthalate units and / or butylene terephthalate units in an amount of 70 mol% or more, preferably 80 mol% or more, more preferably 90 mol% or more is preferable.

Further, in the present invention, if the polymerization degree of the polyester is too low, the mechanical strength is lowered, so that the intrinsic viscosity thereof is 0.4 or more, preferably 0.5 to 1.2, more preferably 0.55 to 0.85.

By the way, when polyester is usually made into a film, slipperiness between films or between film and metal rolls,
That is, in order to reduce the friction coefficient and not to impair the productivity and handling workability, a film surface roughness imparting agent, that is, a polyester containing an appropriate amount of fine inert particles is used, but the polyester used in the present invention is Those which do not contain such inert fine particles are preferably used. This is because the addition of the inert fine particles may interfere with the control of whiteness and hiding power of the film obtained. However, a polyester film containing such inert fine particles may be used as long as the whiteness and the hiding power required for the obtained film are not hindered.

In the present invention, for such an aromatic polyester,
Anatase type titanium dioxide particles (a) containing a zinc element and / or anatase type titanium dioxide particles (b) containing an aluminum element are blended.

In the titanium dioxide (a), the content of zinc element is preferably 0.01 to 5% by weight, more preferably 0.1 to 2% by weight, based on the titanium dioxide. If the content of the zinc element is less than 0.01% by weight, there is no light resistance improving effect which is the main object of the present invention, while if it exceeds 5% by weight, the thermal stability of the polyester is deteriorated. It is not preferable because it increases the amount and hinders the productivity. Also in the titanium dioxide (b), the content of aluminum element is preferably in the range of 0.01 to 5% by weight, and more preferably in the range of 0.1 to 2% by weight, based on the titanium dioxide. If the content of the aluminum element is less than 0.01% by weight, the light resistance improving effect is insufficient. On the other hand, if it exceeds 5% by weight, the thermal stability of the polyester is deteriorated, and the film often breaks during film formation. It is not preferable because it is inferior in productivity. Further, in the titanium dioxide (b), elemental silicon is 5% by weight or less, preferably 0.1% by weight based on the titanium dioxide.
It is preferable to coexist in an amount of 05 to 3% by weight. This is because titanium dioxide (b) tends to agglomerate into particles in polyester,
This is because the hiding property is lowered and the film is broken, so that aggregation is prevented and particle dispersibility is improved. If the content of such elemental silicon exceeds 5% by weight based on titanium dioxide, the color tone of titanium dioxide, and thus the color tone of the polyester film, is delicately changed, which is not preferable.

The average particle size of titanium dioxide (a) and (b) is 0.1-1μ
It is preferably in the range of m, more preferably 0.2 to
It is in the range of 0.5 μm. If the average particle size is less than 0.1 μm, the dispersibility in the aromatic polyester is poor, and coarse agglomerates are scattered in the film to form pinholes in the film, which significantly reduces the product value. It is not preferable because the frequency of breakage during crystallization increases and the productivity decreases. On the other hand, if the average particle size exceeds 1 μm, the hiding property of the film is poor, which is not preferable.

The titanium dioxides (a) and (b) used in the present invention are not limited in their production method as long as they satisfy the above-mentioned conditions. As a method of incorporating the zinc element or the aluminum element into the anatase titanium dioxide particles, a predetermined amount of a zinc compound (for example, zinc acetate, zinc benzoate, zinc oxide, zinc carbonate, zinc chloride, hydroxide) is previously prepared during the production of the anatase titanium dioxide. Zinc, zinc lactate, zinc phosphate, zinc sulfate, zinc nitrate, etc. are not specified and are included) or aluminum compounds (for example, aluminum acetate, aluminum sulfate, aluminum oxide,
Aluminum carbonate, aluminum chloride, aluminum hydroxide, aluminum lactate, aluminum phosphate, aluminum formate, aluminum nitrate, aluminum stearate, aluminum monostearate, aluminum laurate, aluminum zinc sulfate and the like are not specified) May be added and compounded, or the anatase type titanium dioxide particles produced in advance may be surface-treated. Therefore, the zinc element or the aluminum element may be contained inside the anatase type titanium dioxide particles or may be present on the surface, but it is more preferable to be present on the surface in order to improve the light resistance and the initial whiteness. preferable.

The content of titanium dioxide must be in the range of 1 to 20% by weight with respect to the polyester, preferably 2 to 10%.
It is in the range of% by weight. When the content is less than 1% by weight, it is not sufficient for imparting the hiding property to the film. On the other hand, when it exceeds 20% by weight, the polyester apparent density of the film, which is a characteristic of the polyester film, is reduced, and at the same time, The film rigidity is extremely increased, which is not preferable.

The method of blending titanium dioxide with the aromatic polyester is not particularly limited, and it can be blended at any time from the stage of producing the aromatic polyester to the final production of the polyester film. For example, the titanium dioxide (a) and / or (b) may be blended as a glycol slurry or powder as it is at any stage before the start of polycondensation, during the polycondensation reaction, and after the end of polycondensation. Alternatively, the aromatic polyester and titanium dioxide (a) and / or (b) may be melt-mixed by an extruder. In any case, in the present invention, a method in which a so-called master chip, which is blended in an aromatic polyester at a high concentration, is manufactured in advance, and which is diluted to a desired blending amount before being formed into a film is preferably adopted.

In the present invention, a specific amount of crystalline polypropylene is further blended. The crystalline polypropylene in the present invention means a polymer in which at least 95 mol% or more, preferably 98 mol% or more has a propylene unit. Examples of the other structural units included include ethylene units, butene units, and isoprene units. For example, when a copolymer obtained by copolymerizing ethylene units in an amount of more than 5 mol% is used, formation of fine bubbles in the film is generated. Is extremely small,
It is not preferable because the reduction of the apparent density of the film is not sufficiently achieved.

The melt flow index (hereinafter abbreviated as MFI) of such crystalline polypropylene is usually 0.2 to 120 g / 10 min, preferably 0.5 to 50 g / 10 min. Ie such MFI
When it is less than 0.2 g / 10 min, the diameter of closed cells generated in the film becomes extremely large, so that breakage frequently occurs during stretching.
On the other hand, when the MFI is more than 120 g / 10 min, clip detachment frequently occurs in the stretching process in the tenter and the like, which is inferior in productivity, which is not preferable.

Further, the blending amount of such crystalline polypropylene needs to be in the range of 3 to 40% by weight, preferably 5 to 30%.
%, More preferably 5 to 20% by weight. That is, if the compounding amount is less than 3% by weight, the amount of fine bubbles generated in the film is small, so that it is not effective in reducing the apparent density of the film. It is not preferable because the mechanical strength is extremely lowered and breakage frequently occurs during stretching, which extremely hinders productivity.

In the present invention, the above-mentioned titanium dioxide (a) and / or (b) and crystalline polypropylene are blended with an aromatic polyester and the mixture is extruded. In order to contain a large number of fine closed cells inside, the extruded sheet is usually stretched in at least a uniaxial direction. That is, in the extruded sheet, sufficient formation of closed cells is not yet observed, and sufficient bubbles are formed by the stretching process.

The stretching method itself does not need to use a special method,
A usual method for producing an aromatic polyester film can be adopted. That is, using such a compound as a raw material, it is melted and kneaded in an extruder at a temperature of 250 to 320 ° C., extruded in a sheet form from a die and cooled to a temperature of about 70 ° C. or less to obtain a substantially amorphous sheet. Next, the sheet-like material is stretched longitudinally and / or laterally with an area ratio of 4 times or more, preferably 8 times or more,
Further, it is manufactured by performing heat treatment at a temperature of 120 to 250 ° C.

In the present invention, the method of blending the raw materials for the blend is not particularly limited, and titanium dioxide (a) and / or (b), crystalline polypropylene chips and aromatic polyester chips are uniformly mixed and blended in advance. It may be charged into the extruder hopper as described above, but as described above, so-called master chips, crystalline polypropylene chips, and aromatic polyester chips in which the titanium dioxide (a) and / or (b) is premixed in the aromatic polyester in a high concentration. Is preferred.

Further, the method in which the titanium dioxide particle master chip and the aromatic polyester chip are pre-mixed and the mixed chip and the crystalline polypropylene chip are quantitatively supplied to the extruder inlet, respectively, is the simplest method and there is little mixing unevenness. It is particularly preferably adopted.

The present invention is basically achieved by blending an aromatic polyester with crystalline polypropylene and titanium dioxide (a) and / or (b), but other additives may be added as long as the effects of the present invention are not impaired. There is no problem even if a third component such as a substance is added. Examples of such a third component include an antioxidant, a lubricant, a matting agent, a brightening agent, a surfactant, an ultraviolet absorber, a pigment, a dye, an antistatic agent, etc. The required amount can be added.

Further, the film of the present invention can be subjected to various surface treatments in order to improve necessary properties in various applications, for example, adhesion to a magnetic layer or printing ink. Examples of such surface treatments include various primer coating treatments, flame treatments, solvent treatments, corona discharge treatments, plasma treatments, ultraviolet treatments, ion plating treatments, radiation treatments, and sandblast treatments, but such surface treatments are required. The treatment can be carried out on one or both sides of the film of the invention at the appropriate time and in the required amount. Further, these treatment methods may be performed plural times, and when they are performed on both sides, the same treatment method may be performed or different treatment methods may be performed.

<Examples> Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited to the following Examples unless it exceeds the gist. The methods for measuring and evaluating various characteristics in the present invention are shown below.

(1) Apparent density of film (g / cm ³ ) Five 10cm × 10cm squares are cut out from an arbitrary part of polyester film, and the thickness of each sample is measured at 9 arbitrary points with a micrometer, and the average of each is measured. After obtaining the volume using the thickness, calculate the weight per 1 cm ³ by weighing the weight of each cut sample,
The average value of the five samples was determined and used as the film apparent density value.

(2) Concealment Degree The transmission density by visual light was measured using a Macbeth densitometer TR-927 type. The measurement was performed at 3 points, and the average value was used as the hiding value. The larger this value is, the higher the concealing property is.

(3) Color tone (a value and b value) The ND-K5 type manufactured by Nippon Denshoku Co., Ltd. was used to measure the a value and the b value. The measurement was performed at 3 points, and the average value of each was used as the measured value. In the case of a value, a large value on the + side indicates a reddish color tone, and on the negative side, a large color tone indicates a greenish color tone. Indicates that.
Usually, b value is compared and evaluated as an index of whiteness.

(4) Evaluation of light resistance UV long life fade meter manufactured by Suga Test Instruments Co., Ltd.
Using a FAL-5 type, after irradiating with ultraviolet rays at 63 ± 3 ° C. for 100 hours, the b value was measured in the same manner as in the above (3). The difference between the b value of the sample before ultraviolet irradiation and the b value after 100 hours was calculated as the Δb value and used as the degree of color tone change. The smaller the Δb value, the better the light resistance.

Further, the film before and after the ultraviolet irradiation was visually observed, and the change in color tone was visually evaluated by the following ranks.

(5) Evaluation of filter life 10 kg / hr using a 1000 mesh filter with an excess area of 30 cm ^2.
The amount of polymer extruded until the pressure difference between the inlet and the outlet of the filter reached 150 kg / cm ² was obtained by extruding the molten polymer, and the quality of productivity was evaluated and judged according to the following ranks. If the amount of polymer extruded is small, the replacement frequency of the filter increases, and thus the productivity is significantly impaired.

(6) Evaluation of film-forming property When an amorphous sheet is stretched in the longitudinal direction and then transversely stretched by a tenter, the stretching state of the film is observed, and the fixing state of a clip for fixing the film end and the film breakage due to stretching The situation was evaluated according to the following ranks. This evaluation is for judging the continuous film forming property, and is an important item for judging the quality of the productivity.

(7) Average particle size d ₅₀ (μm) Centrifugal sedimentation type particle size distribution analyzer SA-CP3 manufactured by Shimadzu Corporation
The equivalent spherical distribution was measured using a mold, and the equivalent spherical particle size value corresponding to 50% of the integrated value (weight basis) in the distribution was taken as the average particle size value.

(8) Melt flow index MFI (g / 10min) Measured according to JIS K-6758-1981. The smaller this value is, the higher the melt viscosity of the polymer is.

Example 1 (Production of Zinc Element-Containing Titanium Dioxide) To 1,000 ml of water, 3.74 g of zinc oxide was added to a slurry liquid in which 500 g of anatase type titanium dioxide particles were dispersed, and well mixed with a high shear mixer, and then sand milled. Further finely mixed and mixed. Water was removed from the finely dispersed slurry liquid by an evaporator and sufficiently dried by a dryer to prepare anatase type titanium dioxide particles (A) containing 0.6% by weight of zinc element. The average particle size of the particles was 0.31 μm.

(Manufacture of Masterbatch Chip) Polyethylene terephthalate chip 60 having 40 parts by weight of anatase type titanium dioxide particles (A) containing 0.6% by weight of zinc element thus obtained and an intrinsic viscosity (hereinafter abbreviated as [η]) 0.721 60 A masterbatch containing 40 parts by weight of titanium dioxide (A) is blended with 1 part by weight, melt-kneaded at 290 ° C with a vented twin-screw extruder, extruded into a strand shape, cooled with water, and then the strand is chipped with a pelletizer. A chip (A ') was obtained.

(Production of polyester film) Masterbatch chip (A ') 12.5 parts by weight and MFI 10g / 1
Add 10 parts by weight of 0 min crystalline polypropylene chip to [η].
670 polyethylene terephthalate chips 77.5 parts by weight and uniformly blended raw materials were melt-mixed at an extruder at 290 ° C and extruded in a sheet form on a 40 ° C cooling drum to give a sheet of about 0.65.
An amorphous sheet having a thickness of mm was obtained. Then, the sheet was stretched 3 times in the longitudinal direction and 3.3 times in the transverse direction, and heat-treated at 235 ° C. for 5 seconds to finally obtain a film thickness of 100 μm and a film apparent density of 1.01 g / c.
A polyester film containing m ³ fine bubbles was obtained.

Table 1 below shows the filter life and film-forming property evaluation results in the film-forming process and various properties of the obtained film.
However, the filter life and the film-forming property were almost satisfactory, and the productivity was good.

The obtained film has a hiding power of 0.87, a value of -1.1, b
It was a film having a high hiding property of 0.5 and an extremely excellent whiteness, and was a film having extremely excellent light resistance with almost no yellowing after a 100-hour fade meter treatment.

Example 2 The same procedure as in Example 1 was repeated except that zinc chloride was used instead of the zinc oxide used in Example 1, and finally the elemental zinc was 0.9%.
Anatase type titanium dioxide particles (B) having an average particle size of 0.37 μm contained in an amount of 10% by weight were prepared, and a master batch chip (B ′) containing 40% by weight of the particles (B) was prepared in the same manner as in Example 1. It was made.

25 parts by weight of such a master batch chip (B ') and MF
I5 g / 10 min of crystalline polypropylene chips (5 parts by weight) were mixed with 70 parts by weight of [η] 0.670 polyethylene terephthalate chips, and a uniform blend was used. Thickness 100 μm, film apparent density 1.22 g / cm ³ , hiding factor 0.
A polyester film having a whiteness of 91 and a b value of 0.3 was obtained. The productivity in forming such a film is good as shown in Table 1, and the light resistance of the obtained film is Δb.
At a value of 1.0, excellent characteristics were exhibited as in Example 1.

Example 3 In the same manner as in Example 1 except that aluminum hydroxide and silicon dioxide were used in place of the zinc oxide used in the above Example 1, finally, 0.3% by weight of aluminum element and 0.3% by weight of elemental silicon were contained. Anatase type titanium dioxide particles (C) having an average particle size of 0.33 μm were prepared, and a masterbatch chip (C ′) containing 40% by weight of the particles (C) was prepared in the same manner as in Example 1.

7.5 parts by weight of such masterbatch chips (C ') and MFI
18 parts by weight of 25 g / 10 min crystalline polypropylene chip [η] 0.670 polyethylene terephthalate chip 74.5
Film formation was carried out in the same manner as in Example 1 except that raw materials mixed in parts by weight and uniformly blended and mixed were used to finally form a film having a film thickness of 102 μm and a film apparent density of 0.78 g / cm ³ ,
A polyester film having an opacity of 0.86 and ab value of 0.4 and having an excellent whiteness was obtained.

In forming such a film, the light resistance of the obtained film was as excellent as that of Example 1 as shown in Table 1, although polypropylene had a large MFI and very rarely resulted in clip detachment and slightly inferior productivity. It was a film.

Example 4 Only aluminum chloride was used in place of the aluminum hydroxide and silicon dioxide used in Example 3, and finally 0.6% by weight of aluminum element was contained in the same manner as in Example 1 to obtain an average particle size of 0.41 μm. Anatase type titanium dioxide particles (D) were prepared, and a master batch chip (D ′) containing 40% by weight of the particles (D) was prepared according to Example 1.

12.5 parts by weight of such master batch chips (D ') and MF
15 parts by weight of 10g / 10min crystalline polypropylene chip was added to polyethylene terephthalate chip 72.5 of [η] 0.670.
A film was formed in the same manner as in Example 1 except that the raw materials blended in parts by weight and uniformly blended were used, and finally the film thickness was 103 μm, the film apparent density was 0.83 g / cm ³ , the hiding power was 0.95, and the b value was 0.3. A polyester film having excellent whiteness was obtained.

In such a film formation, since silicon dioxide was not added to the anatase type titanium dioxide particles in combination, the filter life became a little short and the productivity was a little inferior, but the light resistance of the obtained film was as shown in Table 1. Similar to Example 1, it showed excellent characteristics.

Example 5 Masterbatch chip (A ′) 6.25 produced in Example 1
Parts by weight, 6.25 parts by weight of the masterbatch chips (C ') prepared in Example 3 and 15 parts by weight of crystalline polypropylene chips of MFI 10g / 10min were mixed with 72.5 parts by weight of polyethylene terephthalate chips of [η] 0.670 and blended uniformly. A film is formed in the same manner as in Example 1 except that the above is used as a raw material, and finally a polyester film having a film thickness of 100 μm, a film apparent density of 0.83 g / cm ³ , a concealment degree of 0.94 and a whiteness of b value of 0.4 is excellent. Obtained.

In such a film, there is no problem with filter life,
Also, the film productivity was good as in Example 1 with no clip detachment or breakage during stretching. The light resistance of the obtained film was Δb value 0.9 as shown in Table 1, and almost no discoloration was visually observed, which was extremely good as in Example 1.

Comparative Example 1 In place of the masterbatch chip (A ′) used in Example 1, a masterbatch chip containing 40% by weight of anatase type titanium dioxide particles having an average particle size of 0.32 μm and containing no zinc element or aluminum element was used for 12.5. A film was formed in the same manner as in Example 1 except that the weight part was used, and finally the film thickness was 102 μm, the film apparent density was 0.99 g / cm ³ ,
A white film having a hiding power of 0.85 and ab value of 2.3 was obtained. The productivity of such a film was the same as in Example 1, but the whiteness of the obtained film was more yellowish than that of Example 1 as shown in Table 1, and was yellowish brown after 100 hours of the fade meter. The film was markedly deteriorated and was extremely inferior in light resistance, which can be easily visually discerned.

Comparative Example 2 Masterbatch chip (D ') 0.5 used in Example 4
Parts by weight and 5 parts by weight of crystalline polypropylene chips of MFI 5 g / 10 min were mixed with 94.5 parts by weight of polyethylene terephthalate chips of [η] 0.670 and uniformly blended raw material was formed into a film according to Example 1 to obtain a film having a thickness of 101 μm. A white film was obtained. As for various characteristic values of the obtained film, as shown in Table 1 below, a sufficient film hiding degree was not obtained because the amount of titanium dioxide particles blended was small.

Comparative Example 3 Master batch chip (G ') 62.5 used in Comparative Example 4
1 part by weight and 2 parts by weight of a crystalline polypropylene chip of MFI 10 g / 10 min and 35.5 parts by weight of a polyethylene terephthalate chip of [η] 0.721 were uniformly blended to form a film according to Example 1 to obtain a white film having a thickness of 103 μm. Obtained. The various characteristic values and the productivity evaluation results of the obtained film are shown in Table 1 below. Although the light resistance and the hiding property of the film were sufficiently imparted, the apparent density of the film was 1.46 g / cm because the compounding amount of polypropylene was small. ^{It was 3} , which was a film lacking in flexibility. In addition, the filter life was short and the breakage during stretching was frequent, resulting in poor productivity.

Comparative Example 4 The masterbatch chip (B ′) used in Example 2 was replaced with 5
45 parts by weight of crystalline polypropylene chips having 25 g / 10 min of MFI and 50 g of MFI and 50 parts by weight of polyethylene terephthalate chips having [η] 0.670 were blended and uniformly blended to form a film according to Example 1. Due to the large amount of the compound, the clip was detached during the stretching and the sample was broken, and even the sample could not be removed, resulting in extremely poor productivity.

Comparative Example 5 Instead of the anatase type titanium dioxide particles used in Example 1, rutile type titanium dioxide particles were used and zinc oxide was contained in an amount of 0.6% by weight in terms of zinc element, and the rutile type titanium dioxide particles having an average particle size of 0.06 μm (I ) Was prepared according to Example 1, 12.5 parts by weight of a masterbatch chip (I ′) containing 40% by weight of the particles (I), 10 parts by weight of a crystalline polypropylene chip having MFI of 10 g / 10 min, and [Η] 0.670
77.5 parts by weight of the polyethylene terephthalate chip was mixed and uniformly blended, and a film was formed in the same manner as in Example 1. The productivity evaluation at the time of forming the film and the characteristics of the obtained film are shown in Table 1 below. However, since the titanium dioxide particles used were extremely small in particle size, they were not finely dispersed in the molten polymer and formed many large aggregates. The filter life was short and the breakage during stretching was frequent, resulting in poor productivity. As for the film characteristics, the light resistance was good because it contained the zinc element as in Example 1, but the whiteness b value of the obtained film was 2.1 because of the rutile type titanium dioxide. It was inferior to the comparison.

Table 1 below shows the results of the evaluation of the film-forming productivity and the results of the film characteristics of Examples 1 to 5 and Comparative Examples 1 to 5 described above.

<Effects of the Invention> As described above in detail, the microbubble-containing polyester film of the present invention has a high hiding power and an improved light resistance of the film without impairing the properties such as the apparent specific gravity of the film and the flexibility of the film. Has been achieved. Since the light resistance is improved, for example, as a board surface of an electronic white board, even when it is used for a long time while receiving a shadow of ultraviolet rays such as a fluorescent lamp or sunlight, yellowing of the board surface over time is extremely solved. The product image can be significantly improved.

At the same time, the whiteness of the film itself is further improved, which is useful in applications where it is used outdoors.

Claims (1)

[Claims] 1. Fine cells characterized by containing 1 to 20% by weight of at least one of the following two types of titanium dioxide (a) and (b) and 3 to 40% by weight of crystalline polypropylene. Containing polyester film. (A); Anatase type titanium dioxide containing zinc element (b); Anatase type titanium dioxide containing aluminum element