JPH0717779B2 - Polyester film containing fine bubbles - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to a polyester film having innumerable fine bubbles on the surface and inside thereof. More specifically, the present invention relates to a stretched polyester film that contains fine closed cells and has a reduced weight per unit volume and is excellent in hiding power and whiteness.

<Prior Art and Problems to be Solved by the Invention> Conventionally, polyester films have been widely used as various industrial base materials because they have excellent mechanical properties, electrical properties, chemical resistance, and the like. In particular, the biaxially oriented polyethylene terephthalate film is superior to other films in terms of flatness and dimensional stability, and is relatively inexpensive to be offered to the market, so it is an indispensable material for the information industry and plate making base material. Is. For example, base materials for board surfaces of electronic blackboards used in offices and conference rooms and NT
Highly concealed polyethylene terephthalate film is used as a base material for magnetic cards typified by telephone cards of T and orange cards of JR, and the excellent characteristics described above are fully exhibited.

However, such a high concealing film, although having excellent properties, since it contains a large amount of inorganic particles having a large specific gravity, such as titanium dioxide, in order to enhance the concealing power to a high degree, it has various adverse effects. Has occurred. That is, the weight per unit volume of the film is increased by 20 to 40% compared to the normal film, so if it is used as a board surface of an electronic blackboard in a vertical state for a long period of time, tarmi due to the film's own weight occurs extremely quickly, It is a factor that significantly reduces the product value. Also, due to the presence of inorganic particles in the film, the life of the knife is significantly shortened in the slitting process of the film and the cutting process of cards, etc., and the productivity is reduced, and the hand is cut at the edge of the film. There was also a problem in handleability.

Therefore, in solving the various problems, the present inventors
First, from the recognition that the most important improvement point is how to reduce the weight per unit volume of the film, that is, the apparent density of the film, we started research into making the film into a foam structure.

Many methods for producing foams related to polyester have been proposed in the past, for example, JP-A-50-38765 and JP-B-57-4.
No. 6456 or JP-A-57-34931, a method of forming a foam by adding a gas or a vaporizable substance, JP-A-52-43871 and JP-B-58-5.
A method of forming a foam by adding a substance that chemically decomposes to generate a gas, as described in JP-A-0625, etc., and further, JP-A-51-34963 and JP-B-52-27666. There is known a method, such as the method described in the publication, in which a liquid after molding is impregnated 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 the oriented polyester film. That is, these methods are proposed for producing a molded body, and it is difficult to say that these methods can be simply applied to a film having a size of several hundreds of microns or less. Moreover, it is very difficult to stretch and orient the extruded sheet without breaking. This is supported by the fact that there is almost no concrete proposal of foaming in the conventional stretch-oriented polyester film.

Thus, it is difficult to adopt the conventional foaming method, but it is needless to say that it is effective in reducing the apparent density if it is possible to include innumerable fine bubbles in the polyester film. If the bubbles to be made can be miniaturized to several tens of microns or less than several microns, the hiding property can be concomitantly improved by the scattering of light by the bubbles.
Therefore, the blending amount of the inorganic particles blended to impart the concealing property can be naturally reduced, which contributes to the reduction of the apparent density of the film and contributes to the prevention of tarmi due to the weight of the film, and the productivity of the film during slit processing. In addition, the handling property can be improved at the same time.

Therefore, the present inventors have studied the method of reducing the apparent density of the film by containing a large number of such fine closed cells without being confined to the conventional foaming method, and previously described in JP-A-63-168441. Suggested a method. That is, the apparent density of the film was reduced by stretching a sheet obtained by blending aromatic polyester with a specific polypropylene. Moreover, unlike the conventional foaming method, it does not require a new device and is a revolutionary method because it is an extremely simple method that can be produced within the range of conditions for producing ordinary polyester films. It was

However, since polypropylene used in such a method has a lower melting point than polyester and is inferior in heat resistance, it is inevitable that a low molecular weight product is generated due to heat deterioration in the film manufacturing process, particularly in the melt extrusion process, and contamination or stretching during the process occurs. There were problems such as frequent breakage. Further, the obtained film does not necessarily have a sufficient whiteness, probably because of the heat deterioration, and a drastic improvement means was required.

<Means for Solving the Problems> In view of the above circumstances, the inventors of the present invention have completed the present invention as a result of earnestly examining improvement means thereof without impairing generation of fine closed cells. .

That is, the gist of the present invention resides in a polyester film formed by stretching in at least a uniaxial direction, wherein the polyester film contains 3 to 40% by weight of polymethylpentene, and the polyester film contains fine bubbles.

Hereinafter, the present invention will be described in detail.

The polyester referred to in the present invention means an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid or an ester thereof and ethylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol, cyclohexane-1,4-dicarboxylic acid. It is a polyester produced by polycondensation with a glycol such as methanol.

These polyesters are produced by a method such as transesterification of aromatic dicarboxylic acid and glycol and then polycondensation, or polycondensation of diglycol ester of aromatic dicarboxylic acid. Typical examples of such polyesters include polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate.

Further, the polyester may be a homopolymer,
It may be a copolymer of the third component. In any case, in the present invention, the polyester having an ethylene terephthalate unit and / or an ethylene-2,6-naphthalate unit and / or a butylene terephthalate unit is preferably 70 mol% or more, more preferably 80 units of these units. A polyester having a mol% or more, particularly preferably 90 mol% or more of these units.

In the present invention, when the degree of polymerization of such polyester is too low, the mechanical strength is lowered, so that the intrinsic viscosity [η] is
It is preferably 0.4 or more, more preferably 0.5 to 1.2, and particularly preferably 0.55 to 0.85.

Usually, when a polyester is used as a film, a polyester containing a suitable amount of fine particles inert to the polyester is used in order to impart slipperiness between films and between the film and metal rolls. The polyester used in (1) is preferably one that does not contain such fine particles. This is because the presence of the fine particles may hinder the control of the color tone and the degree of hiding of the film obtained. However, polyesters containing such inert fine particles may be used as long as the color tone and the degree of hiding required for the obtained film are not hindered.

The polymethylpentene referred to in the present invention is preferably a polymer having a unit derived from 4-methylpentene-1 in which 70 mol% or more, more preferably 80 mol% or more, and most preferably 90 mol% or more, Examples of other components include ethylene units, propylene units, butene-1 units, 3-methylbutene-1 and the like derived units.

The melt flow rate of such polymethylpentene used in the present invention (hereinafter abbreviated as MFR) is preferably in the range of 1 to 100 g / 10 minutes, more preferably 5 to 80 g / 10 minutes,
Particularly preferably, it is in the range of 10 to 70 g / 10 minutes. That is,
If the MFR is less than 1 g / 10 minutes, the bubbles generated in the finally obtained film may be extremely large, and thus breakage frequently occurs during stretching.On the other hand, if the MFR exceeds 100 g / 10 minutes, the tenter is increased. In such a case, the clip is detached at the time of lateral stretching or the roll or the like in the film forming step is contaminated, and productivity is impaired in any case, which is not preferable.

In the present invention, polyester contains polymethylpentene, and the content thereof is in the range of 3 to 40% by weight, preferably 5 to 30% by weight, more preferably 5 to 20% by weight.
It is in the range of% by weight. When the content is less than 3% by weight, the amount of bubbles generated in the finally obtained film is small and the apparent density cannot be sufficiently reduced. On the other hand, if it exceeds 40% by weight, the amount of generated bubbles becomes extremely large and the bubble diameter also becomes large, so that breakage frequently occurs during film stretching and productivity is extremely deteriorated, which is not preferable.

In the present invention, a film is obtained by containing polymethylpentene in such polyester, but in order to finally generate fine closed cells on the surface and inside of the film, which is the object of the present invention, to reduce the apparent density. It is necessary to stretch an extruded sheet of polymethylpentene-containing polyester in at least a uniaxial direction. That is, sufficient closed cells are not yet generated on the surface and inside of the sheet, and a large number of bubbles effective for reducing the apparent density are formed only after the stretching step.

The stretching condition itself in the present invention does not need to employ a special method, and it can be performed under the conditions for producing an ordinary polyester film.

Next, the method for producing the film of the present invention will be described more specifically.

250 polyester raw materials blended with polymethylpentene
After melt-kneading at a temperature of up to 320 ° C., it is extruded in a slit form from a die and cooled to a temperature of about 70 ° C. or lower to obtain a substantially amorphous polyester sheet. Next, the obtained sheet is subjected to an area magnification of 4 times or more, preferably 9 times or more in the longitudinal and / or lateral directions within the range of the glass transition point of polyester to the crystallization temperature.
The film is stretched at least twice and then heat-treated at a temperature of 120 to 250 ° C.

In the above method, the method of blending polymethylpentene with polyester is not particularly limited,
For example, polyester chips and a desired amount of polymethylpentene chips may be uniformly mixed and charged into the raw material supply hopper of the extruder. Further, the method of quantitatively supplying the polyester chips and the polymethylpentene chips to the raw material charging port of the extruder is particularly preferable because it is particularly simple and has little mixing unevenness.

The present invention basically uses polyester and polymethylpentene as raw materials, so that the final film apparent density is
A film having a hiding degree of 0.4 to 1.3 g / cm ³ and a hiding degree of 0.2 or more can be obtained, but a third component such as an additive may be contained as long as the physical properties are satisfied. Examples of such a third component include an antioxidant, an antistatic agent, an ultraviolet absorber, a pigment, a dye, a lubricant, a matting agent, and a fluorescent whitening agent. Depending on the required characteristics of various applications, an appropriate time and The required amount can be added by the method.

Further, the addition of a surfactant as the third component is particularly preferable because it is possible to appropriately control the diameter of bubbles generated in the film finally obtained. The term "surfactant" as used herein refers to a compound that changes the properties at the interface between polyester and polymethylpentene, and is a compound that has the effect of increasing or suppressing the compatibility of the two. In the present invention, it is not specified as a surfactant, and examples thereof include anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants and the like, but preferably nonionic surfactants. Surfactants, especially silicone-based surfactants are used, and the amount of addition thereof is appropriately 0.001 to 3% by weight based on the polymer raw material.

In this way, the microbubble-containing polyester film of the present invention can be obtained, but various surface treatments can be performed to improve the surface properties of the film, such as adhesiveness and antistatic property. Examples of such a surface treatment include various primer treatments, corona discharge treatments, plasma treatments, solvent treatments, ultraviolet treatments, sandblast treatments, flame treatments, etc. It can be done on both sides at the right time and in the required amount. Further, these treatment methods may be performed in plural, and when they are performed on both sides, the same treatment may be performed or different treatments may be performed.

<Examples> Examples will be described more specifically below, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded. The 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 10 cm × 10 cm squares were cut out from an arbitrary part of the obtained film, and the thickness of each sample was measured with a micrometer to determine the volume. The weight of each sample was measured and the weight per cm ³ was calculated. The average value of the five samples was taken 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) Whiteness b value The b value was measured using ND-K5 type manufactured by Nippon Denshoku Co., Ltd. The measurement was carried out at 3 points, and the average value was used as the b value. This value is +
A large value on the side indicates a strong yellowish tint, which is usually used as an index of whiteness.

(4) Evaluation of film forming When the amorphous sheet is stretched in the longitudinal direction and then transversely stretched by a tenter, the fixing state of the clip that fixes the film end, the state of film breakage during stretching, and the long-term post-formation process The contamination status was observed and evaluated according to the following ranks. These evaluations are important items for determining the quality of film productivity.

(5) Melt flow rate MFR (g / 10 minutes) Measured at 260 ° C. under a load of 5 kg according to ASTM D-1238.

Example 1 A polyethylene terephthalate chip having an intrinsic viscosity [η] of 0.685 was added to a polymethylpentene chip (4: MFR 25 g / 10 min.
The unit derived from methyl pentene-1 is 90 mol% or more) 10% by weight, and the uniformly mixed raw materials are melt-kneaded at 290 ° C. in an extruder and extruded in a slit shape on a 40 ° C. cooling drum, An amorphous sheet having a thickness of about 0.6 mm was obtained. Then, the sheet was stretched 3 times in the longitudinal direction and 3.2 times in the transverse direction, and heat-treated at 240 ° C. for 5 seconds to finally obtain a film having a thickness of 100 μm.

The obtained film has an apparent density of 0.85 g / cm ³ and a hiding power of 0.5.
It was 0, b value + 0.6, and the apparent density was extremely reduced as compared with a normal white polyester film. Further, the film-forming property was also extremely improved, and the whiteness of the film was remarkably good.

Example 2 Instead of the raw materials used in Example 1, 15% by weight of polymethylpentene (90 mol% or more of units derived from 4-methylpentene-1) having an MFR of 15 g / 10 min and a silicone-based surfactant were used. Example 1 except that 0.1% by weight of raw materials were used
A film was formed in the same manner as in (1) to finally obtain a white film having a thickness of 50 μm. As shown in Table 1, the obtained film was a highly concealable film having an extremely low density and excellent whiteness as in Example 1.

Examples 3 and 4 A raw material containing 20% by weight of the polymethylpentene used in Example 1 and 3% by weight of titanium oxide particles having an average particle size of 0.3 μm, or MFR 70 g / 10 min polymethylpentene (4-methylpentene- (Units derived from 1 is 90 mol% or more) 5% by weight and 5% by weight of barium sulfate particles having an average particle size of 0.7 μm are used in the same manner as in Example 1 except that a raw material is formed into a film. 250 μm thick and 75
A white film with a thickness of μm was obtained.

As shown in Table 1, the obtained films were highly concealable films each having a highly reduced density and excellent whiteness.

Comparative Example 1 A white film having a final thickness of 100 μm was prepared in the same manner as in Example 1 except that polyethylene terephthalate chips having an intrinsic viscosity of 0.670 containing 15% by weight of titanium oxide particles having an average particle size of 0.3 μm were used as a raw material. Got
As shown in Table 1, the obtained film had a high concealing property, but had an apparent density of 1.57 g / cm ³ , which was a very high density film as compared with a normal polyester film.

Comparative Examples 2 and 3 The same procedure as in Example 1 was repeated except that the polymethylpentene used in Example 1 was replaced with polypropylene having a melt flow index of 10 g / 10 min or low density polyethylene of 2 g / 10 min. A film having a thickness of 100 μm was obtained. The physical properties of the obtained films are shown in Table 1. Although the polypropylene compounding system achieves low density and high hiding, it is not only poor in productivity due to soiling of rolls in process during film formation, The whiteness was inferior to that of No. 1. In addition, the polyethylene compounding system is shown in Table 1.
As can be seen from the above, the film was not only inferior in film-forming property, but also was not sufficiently reduced in density and was inferior in hiding property.

Comparative Examples 4 and 5 A film having a thickness of 100 μm was finally formed in the same manner as in Example 1 except that the compounding amount of polymethylpentene used in Example 1 was 2% by weight or 50% by weight. As shown in Table 1, the situation at the time of forming a film and the physical properties of the film are 50% by weight because the 2% by weight system cannot achieve a sufficiently low density and high hiding.
In the system, the final film was not obtained due to continuous breakage during film formation.

The results obtained above are summarized in Table 1 below.

<Effects of the Invention> As described in detail above, the fine bubble-containing polyester film of the present invention is excellent in whiteness of the film and has extremely improved productivity, and is used in various applications such as electronic white boards and magnetic cards. It has a high industrial value.

Claims (1)

[Claims] 1. A polyester film containing at least one uniaxially stretched film, wherein the film contains 3 to 40% by weight of polymethylpentene.