JPS63168441A - Micro-cellular polyester film - Google Patents

Abstract

PURPOSE:To obtain the title film which is inexpensive and light and is excellent in hiding property and whiteness, by mixing a crystalline polypropylene homopolymer having a melt flow index in a specified range with an aromatic polyester, extruding the mixture and stretching the extrudate. CONSTITUTION:3-20wt.% crystalline polypropylene homopolymer of a melt flow index of 0.2-120 is mixed with an aromatic polyester (e.g., polyethylene terephthalate or polybutylene terephthalate). This aromatic polyester is fed to an extruder, and melt-kneaded and extruded through a die to form a sheet. The obtained sheet is stretched at least uniaxially to form innumerable fine closed cells in the inside and surface of the film. In this way, the purpose micro- cellular polyester film can be obtained. This film can be suitably used for reverse-printed packaging materials, window decorating materials, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a polyester film containing numerous microscopic bubbles on its surface and inside. More specifically, the present invention relates to a sub-stretched polyester film which has a reduced weight per υ unit area by containing fine closed cells, and which has excellent hiding properties and whiteness.

<Problems to be solved by conventional technology and invention> Conventionally,
It is well known that polyester films are widely used in various industries because of their excellent mechanical properties, electrical properties, chemical resistance, weather resistance, heat resistance, etc. Biaxially oriented poly(ethylene terephthalate) film has excellent dimensional stability, flatness, elastic modulus, etc. compared to other films, and is available on the market at a relatively low price, so it is suitable for plate making or It is now an indispensable material for the information industry.

Now, the information industry has made remarkable progress in recent years, and the development of various new products is astounding.For example, the appearance of electronic whiteboards is one of them, and it can be said that the image of the conventional blackboard has been completely changed. . In other words, it is an epoch-making product that helps keep offices and conference rooms clean by eliminating dust caused by white MK, and also contributes to more efficient meetings by adding a copying function. Furthermore, with the advent of the cashless era, magnetic cards such as cash cards and telephone cards are becoming increasingly popular.

Conventionally, as a major component of such products, a 92-axis stretched poly(ethylene terephthalate) film has been used, which has been blended with a white pigment such as fine particles of titanium oxide to impart white color and high hiding properties.

Because of its excellent flatness and dimensional stability, such films are used in the most important parts of final products, such as the board part of electronic whiteboards and the base material of magnetic cards, and their excellent properties are used as the board part of electronic whiteboards and as the base material of magnetic cards. He is performing without regret.

However, even in the case of such a film having excellent properties, various disadvantages arise because a large amount of white pigment is blended in order to increase its hiding power. In other words, in the case of such a film, since it contains 70% by weight or more of inorganic particles having a large specific gravity, such as fine titanium oxide particles, the apparent specific gravity of the film is 797 or more, and thus X
, Ot /- or more, and the weight per unit volume is 2 to 5
Because of this, in order to obtain a film product with the required area, heavy labor during manufacturing operations such as slitting, transporting, or packaging was required.

In addition, due to the increase in weight per unit volume, when used in electronic whiteboards, the film loosens over time, a so-called aging phenomenon, which significantly reduces product value.

On the other hand, since a large amount of inorganic particles are blended, the rigidity of the polyester film itself is greatly increased, and when handling the film, such as cutting one's hand on the edge of the film when slitting the film or cutting products such as magnetic cards, There were also safety issues. Therefore, in order to solve these problems, the present inventors first focused on the point that the most significant modification point was how to reduce the apparent specific gravity, and the material itself was made into a foam material. Started research on manufacturing.

In other words, if gas (bubbles) is mixed into a thermoplastic resin and a foam structure is created by applying the foam manufacturing recipe used for insulation and cushioning materials, it should be possible to reduce the weight of the material.

Conventionally, many formulations for producing foams related to polyester have been proposed, such as JP-A-5O-3♂765, JP-A-Shoj 7-4'44-6, or JP-A-Sho.
A method of manufacturing a foam by adding a gas or a vaporizable substance as described in Publication No. 7-3 Hiro 3/3, etc., Japanese Patent Application Publication No. 1975-1AJr7/4j5 or Japanese Patent Application Publication No. 1972-1982. -! 062! A method of foaming by adding a substance that chemically decomposes to generate gas, as described in Japanese Patent Publication No. JP-A-Sho J'/-! IIri No. 53 Publication or Special Publication Show! Examples include a method of impregnating a molded material with a liquid or extracting a substance soluble in a solvent after molding, as described in Kou No. 276.

However, it is extremely difficult to simply apply these foaming formulations to biaxially oriented polyester films. After all, such a formulation is for manufacturing a molded body, and no matter which formulation is adopted, the bubble diameter should be several tens of microns or less when the thickness of the film is taken into account, and the bubbles should be controlled to be uniform. It is difficult to say that this formulation is possible, and even more so, it is too risky to biaxially stretch an extruded sheet containing bubbles without breaking it. This can be inferred from the fact that there have been almost no examples proposed in patent publications regarding biaxially stretched foamed polyester films.

Even though it is extremely difficult to adopt conventional foam manufacturing formulations, if it is possible to incorporate fine air bubbles into a biaxially oriented polyester film, it will be effective in reducing the apparent specific gravity. do not have.

Moreover, if it is possible to contain an infinite number of bubbles with diameters ranging from several microns to several tens of microns, it is expected that the bubbles will naturally scatter the transmitted light, resulting in a film with high hiding properties. Naturally, the amount added can also be reduced, which will apparently help reduce specific gravity and also lead to suppression of rigidity. Therefore, if such a film is obtained, the heavy labor during handling will be reduced and it will be effective against loosening due to its own weight, so-called aging. Moreover, if such a film could be obtained by a simple method without being bound by conventional foaming formulations,
Its value is extremely great.

<Means for Solving the Problems> In view of the above circumstances, the present inventors have conducted extensive research in order to obtain a polyester film containing countless microbubbles, and as a result, the present inventors have found that aromatic polyester is combined with polyolefin polymer. The present inventors have discovered that a film suitable for such purposes can be obtained by simply blending the following, and have thus arrived at the present invention. This invention falls within the scope of the method already proposed in Japanese Patent Publication No. 5/724432;
This method was originally proposed to impart writability and a matte finish to the film surface, and it does not necessarily result in the microbubble-containing film intended by the present inventors. In order to produce the desired film, the present inventors discovered that K could only be achieved using a very specific specific polyolefin polymer, leading to the present invention.

That is, the gist of the present invention is to extrude an aromatic polyester containing 3 to 6% by weight of a crystalline polyglopylene homopolymer with a melt flow index (M, F, engineering) of 00λ to /-θ into a sheet shape. Then, by stretching the sheet in at least one axial direction, the polyester film contains fine closed cells on the surface and inside of the film.

By using such a specific crystalline polypropylene homopolymer, it is possible to industrially and inexpensively produce a polyester film with a reduced apparent specific gravity, which is the objective of the present inventors.

The present invention will be explained in detail below.

The aromatic polyester referred to in the present invention refers to terephthalic acid,
It is a polyester produced by polycondensation of aromatic dicarboxylic acids such as isophthalic acid and naphthalene dicarboxylic acid, or their nisterates, and glycols such as ethylene glycol, diethylene glycol, dibutanediol, and neopentyl glycol.

These polyesters are produced by directly reacting aromatic dicarboxylic acids and glycols, or by polycondensation after transesterification of alkyl esters of aromatic dicarboxylic acids and glycols, or by polycondensation of diglycols of aromatic dicarboxylic acids. It can also be produced by methods such as polycondensation of esters. Typical examples of such polyesters include poly(ethylene terephthalate) and poly(ethylene-terephthalate).
2,6-naphthalate) or poly(butylene terephthalate).

This polyester may be a homopolymer or may be a copolymer of a third component. In any case, in the present invention, the content of ethylene terephthalate units and/or ethylene co, 6-7 thale units and/or butylene terephthalate units is 70 m01% or more, preferably /0m01% or more, and more preferably Omo1%.
Polyesters containing the above are preferred.

Furthermore, in the present invention, if the degree of polymerization of the polyester is too low, the mechanical strength will decrease, so the intrinsic viscosity of the polyester will be 0. Hiroshi or higher, preferably 0. j~/, 2, more preferably 0.
! j~0. ! ! Preferably.

Normally, when polyester is made into a film, a film surface roughening agent is used to reduce the slipperiness between the films or between the film and metal rolls, that is, the coefficient of friction, so as not to impair productivity and handling workability. That is, a polyester containing an appropriate amount of fine inert particles is used, but the polyester used in the present invention preferably does not contain such fine inert particles. This is because the inert fine particles may interfere with the control of the whiteness and hiding properties 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 required properties of the desired film.

Polypropylene, which is an essential component in the present invention, is a crystalline polypropylene polymer and must have a specific melt flow index (M, ?, M). It is also preferable that the amount of polypropylene blended is within a specific range. In addition, homopoly i- of such polypropylene is at least ? s mot
s - h, preferably 9? The polymer has mot% or more of propylene units.

That is, when the polypropylene used in the present invention is amorphous polypropylene, although it can be blended into polyester to form an amorphous sheet, it is possible to It cannot be used because parts bleed out and adhere to and contaminate the surfaces of casting drums, stretching rolls, etc., and furthermore, the clips come off during transverse stretching in a tenter and/or the clips become contaminated.

In addition, if such polypropylene is a polymer copolymerized with ethylene units in a random or block form, for example, at a moz% or more, it is possible to form and contain countless microbubbles inside the finally obtained stretched film, which is the object of the present invention. Therefore, the film does not seem to achieve a sufficient reduction in specific gravity. By the way, the polypropylene used to manufacture polyolefin films and molded products is now commonly copolymerized to improve impact strength and J1 crack strength, but there are many different brands of these. Of these, the present invention cannot be achieved without a homopolymer.

Now, the melt flow index of the polypropylene of the present invention is 0.2~/2θ, preferably 0.5~! '
0 is preferred. That is, if the melt flow index is less than 0.2, the bubbles generated are too large, and breakage occurs frequently during stretching, making it impossible to obtain the final film.On the other hand, if the melt flow index exceeds 720,
This is not preferable because the clips frequently come off during lateral stretching using a tenter, resulting in poor productivity.

The amount of polypropylene used in the present invention is 3 to 20% by weight. That is, if the blending amount is less than 3% by weight, the final film obtained will have a small amount of microbubbles, and will no longer be effective in reducing the apparent specific gravity. If the stretching temperature exceeds 100, the generation of fine bubbles increases, but this is not preferable, as the number of breaks during stretching increases, probably due to the excessive number of bubbles generated.

In the present invention, the above-mentioned aromatic polyester and crystalline polypropylene homopolymer are blended.

In order to contain numerous fine closed cells on the surface and inside of the final film, which is the object of the present invention, it is essential to further stretch the film in at least one direction.

In other words, such fine closed cells have not yet been formed on the surface or inside of a sheet obtained by simply extrusion molding such a compound, and although the mechanism of their formation is not necessarily clear, it is only after the stretching process that such fine closed cells form. generate. Therefore, it can be said that the formulation is clearly different from conventional foaming formulations. The stretching recipe itself is not special, and the final method for producing a normal polyester film may be followed as it is.

That is, using such a blend as a raw material, an extruder produces 250
The mixture is melt-kneaded at a temperature of ~300°C, extruded through a die into a sheet, and cooled to a temperature of about 70°C or less to form a substantially amorphous sheet. The sheet-like material can then be stretched in the longitudinal and/or transverse directions by an area magnification of at least 3 times, preferably at least 1 times, and further heat-treated at a temperature of 2Q-120°C.

The blending method of such compound raw materials is not limited to %, but polyester chips and polypropylene chips may be mixed and blended in advance and charged into the extruder hopper. A method of quantitatively feeding chips and polypropylene chips is preferably adopted because it is the simplest method and there is less unevenness in the blending ratio.

Now, the present invention is basically achieved by blending polyester and polypropylene, but there is no problem in blending third components such as other additives as long as the effects of the present invention are not impaired.

Examples of such third components include antioxidants, antistatic agents, ultraviolet absorbers, pigments, dyes, lubricants, matting agents, and whitening agents. Just add it.

<Effects of the Invention> The polyester film of the present invention obtained in this manner is a film containing numerous fine closed cells of several microns to several tens of microns on the surface and inside of the film, and therefore is superior to conventional polyester films. The apparent specific gravity has been reduced by about 50% compared to the original film, which eliminates heavy labor during handling, and also reduces accidents caused by film etching, probably due to the buffering effect of the two bubbles it contains.

In addition, since the polyester film of the present invention contains numerous fine air bubbles, even without the addition of a white pigment such as titanium oxide to provide concealing properties, the polyester film of the present invention provides considerable concealment, as is clear from the examples described later. The fact that the film has excellent properties and whiteness, and the amount of pigment added can be considerably reduced does not appear to be a mere contribution to the reduction of specific gravity, but is also a great advantage in terms of improving productivity. That is, 7 pigments
Conventional production of white films, which contain more than 0% by weight, is forced to lower the draw ratio compared to normal polyester film production, and the yield rate is also low due to breakage, etc. However, if the present invention is adopted, the productivity will be improved to the same level as that of ordinary films.

Furthermore, the polyester film of the present invention is a matte film with high surface roughness and low surface gloss;
This was not possible with conventional white films. The film has a unique texture, has comparable heat shrinkage rate to regular polyester film, and although its mechanical strength is somewhat inferior, it is durable enough to withstand use in the future. It can be used in a wide range of fields such as back printing packaging materials and window decoration materials. Furthermore, since the polyester film of the present invention contains air bubbles, it can also be expected to be used in the field as a heat insulating material.

<Examples> Hereinafter, the present invention will be specifically explained using Examples, but the present invention is not limited by the following Examples unless the gist thereof is exceeded. Incidentally, the method for measuring the overhead value and the method for evaluating film formability in the present invention were carried out by the following methods.

(1) Appearance is specific gravity (f/ct/I); Cut out a 20 cm x 20 CIn piece from any part of the obtained film, measure the average thickness at any gluing point with a micrometer, then weigh it and calculate the per unit volume. The weight of was calculated. The number of measurements is n=5
The average value was taken as the measured value.

(2) Hiding degree: The density of transmitted light under the G filter was measured using a Mamepeno densitometer TD-TAO Hiro model. The larger this value is, the higher the hiding power is.

(3) Whiteness value; Japanese military color ■ Color difference meter ND-K
Measured using J type. The number of measurements was 11=J, and the average value was taken as the measured value. If this value is large toward the → side, it indicates a strong yellowish tint, and is usually used as an index to indicate whiteness.

(4) Glossiness (%): Measured using a Nippon Denshoku ■Share crossmeter model VG-/θ7 according to the method of J. Ko. 5Z-1'7. Using a black standard plate as a reference, the values at an incident angle and a reflection angle of 60'' were measured.The number of measurements was n = J,
The average value was taken as the measured value.

(5) Surface roughness R5L (μm); 1,08B-〇6 using a -JP type surface roughness measuring instrument 8 manufactured by Koita Institute.
θ／−／? It was measured according to the method of No. 74. Detailed conditions are stylus diameter 2μm, stylus pressure 30■, cutoff value oJtm
The measurement length was Jjfi, the number of measurements was n=72, and the average value of n=/θ excluding the maximum and minimum values was taken as the centerline average roughness Ra value.

(6) Dynamic friction coefficient μ(L; A137M D-/7?
≠Place two films on top of each other on a smooth glass plate, place a load on top of it, and apply contact pressure between the two films to 2.
2/d, the upper film is II Orran /M
IX was run, and the frictional force between the films was measured. jW
The friction coefficient at the point where the vehicle ran in the R direction was defined as the dynamic friction coefficient μd value.

The number of measurements was n=70, and the average value was taken as the measured value.

(7) Tensile strength at break (/y/md): Using an Intesco tensile tester, sample width/! At request, strain rate 700
%/min, and the stress at break was measured. The number of measurements is n
= s, and the average value was converted into the strength per unit cross-sectional area (kg/Ld) to obtain the tensile strength at break.

(Heat shrinkage rate (%): Calculated using the following formula by heat-treating for 1 minute in an atmosphere of 0.degree. C. under no tension and measuring the length of the sample before and after the treatment.

(9) Evaluation of clip detachment: When an amorphous sheet is longitudinally stretched and then laterally stretched, the situation in which the longitudinally stretched film slips and comes off from the clips that fix the film ends at the stretching point of the horizontal stretching machine (tentter) is evaluated. Judgment and evaluation were performed according to the following three ranks.

This evaluation, like the next (to) item, indicates continuous film forming performance, and is an important item for determining the quality of productivity.

Rank ○: Good productivity with almost no clips coming off 27262 Occasional lip coming off, poor productivity Rank ×: The film slithers, causing clips coming off, and no productivity at all.

(4) Evaluation of breakability: As in the previous item (9), the situation in which the film breaks during stretching or heat setting was evaluated using the following three ranks in a tenter.

Rank ○: Good productivity, almost always causing film breakage Rank Δ: Occasional film breakage, poor productivity Rank: Always causing film breakage, no productivity αυ Melt wisdom flow tube index M, ? , engineering (y/
20rrtipr); JIS K-t7re-/
yr/K was measured alone. The higher this value is, the lower the melt viscosity of the polymer is.

Example / Intrinsic viscosity [η) OJ, melt flow index (
M, F, Engineering,)! 70% by weight of crystalline polypropylene homopolymer chips were mixed, uniformly blended, melted in an extruder at 0°C, cooled on a cooling drum at 1A0°C, and had an apparent thickness of 650 μm. An amorphous sheet with a specific gravity of 7.27 was obtained.

Then vertically 2. After stretching 3.2 times in the transverse direction, heat treatment was performed for 6 seconds at 2≠j℃, and the final film thickness was 20θμ.
A biaxially stretched film of m was obtained.

Table 7 shows the evaluation results of film formability during stretching and the cost performance values of the finally obtained films.

As shown in Table 7, the clip removal and breakage properties during film formation were both ranked O, and productivity was not impaired. On the other hand, the apparent specific gravity of the obtained film was that of the comparative example/
Ordinary polyester film shown in /, 4A Oo? Met.

The surface and cross section of the film t - As a result of microscopic observation, the diameter / θ~! e' indicates that there are countless closed cells dedicated to θμ.
I published a magazine. Note that no closed cells were observed in the amorphous sheet sampled during the film forming process.

The properties of the obtained film include a hiding degree of 0 JOsb value +
7. Compared to the normal polyethylene terephthalate film of Comparative Example, which had a gloss level of 37%, the film had high concealment and white color, and had surface irregularities of Raθ and 34Lβ, and had suppressed surface gloss. The film had a smooth surface with a coefficient of dynamic friction of 0.23, and had a tensile strength at break of 72 and a supple feel to the touch of 9/-. The heat shrinkage rate was almost the same as that of the comparative example, and the film had excellent dimensional stability.

M, F, in place of the polypropylene used in Examples
I, 75 crystalline polyglobylene homopolymer of 27
A biaxially stretched film was obtained in the same manner as in Example except that θ and θ/weight % of the fluorescent brightener were blended. The film formability evaluation results and various physical property values of the obtained film are shown in Table 1. Film 1! ! The JIJ property was the same as in Example, and both clip removal and breakage properties were good/good.

The apparent specific gravity of the obtained film was OJj, and it was observed under a microscope @
As a result of observation, numerous closed cells with a diameter of 70 to 60 μm were observed, as in Example. The film had almost the same characteristics as Example/. More specifically, the film has an extremely high whiteness compared to the b value + /, / of Example /, and the film surface Ra O, 6J'μ
m, the gloss level was 25%, and the surface gloss was further suppressed compared to Example.

Example 3 Substance of polypropylene used in Example/F) KM, ?
, Engineering, crystalline polypropylene homopolymer to eight! fk% by weight and j% by weight of titanium oxide with an average particle size of 0.3 μm.
A biaxially stretched film was obtained in the same manner as in Example except for blending. The evaluation results of the film formability of the obtained film and the values of various physical properties of the film are shown in Table 1. As shown in Table 1, the film formability of the film was ranked ○ in terms of clip release and breakage properties, as in Examples 1 and 2. In addition, the apparent specific gravity of the obtained film was 0, and as in Example 1, as a result of microscopic observation, a large number of closed cells with a diameter of 20 to 200 μm were observed, and in addition, a large number of closed cells with a diameter of 20 to 200 μm were observed. Ultrafine protrusions of approximately 1-=μm were observed at the same time. The film had almost the same characteristics as Example/, and more specifically, compared to Example/, it had a higher degree of hiding and a lower degree of surface gloss.

Comparative Example/A poly(ethylene terephthalate) chip containing the polypropylene used in Example/ without any of the polypropylene used, but containing Calcium Carbonate Q having an intrinsic viscosity [η]O9≦50 and an average particle size of 0.7 μm, and a % by weight, was used. A biaxially stretched film was obtained in the same manner as in Example except for this. The obtained film, as shown in Table 1, is a very common polyester film with an apparent specific gravity of 1. At No. 3, no closed cells as described in Example were observed.

The appearance of the film was slightly cloudy and translucent, compared to the white film of Examples/-3.

M and F were substituted for the homopolypropylene used in Comparative Example and Example.
, ■, 22? A biaxially stretched film was obtained in the same manner as in Example 1, except that 70% by weight of mo1% ethylene random copolymer polypropylene was blended. As shown in Table 1, the obtained film had no problems in film formability as in Examples/--3, but the apparent specific gravity was 7.30, and it was observed under a microscope that there were no closed cells. The formation of the polypropylene was only very sparsely scattered, and the polypropylene was only finely dispersed. Moreover, the film was extremely inferior in whiteness and hiding power compared to Examples/--3.

Comparative Example J A biaxial stretching process was performed in the same manner as in Example 1, except that 5% by weight of an amorphous polyglobylene homopolymer was added in place of the crystalline polypropylene used in Example. The clip came off in the tenter with a rank of ×, and it slipped, so no sample could be obtained.

Comparative example ≠ M, F, 1 in place of the polypropylene used in Example 1
．． / j O crystalline polypropylene homopolymer 2
A biaxial stretching process was performed in the same manner as in Example except that 0 weight i% was added. The film formability in the film forming process is shown in Table 1.
As shown in , the clip detachment rank Δ and breakability rank Δ
Unfortunately, productivity was poor.

Comparative example! In place of the polypropylene used in Example, 5 layers of crystalline polypropylene homopolymer of M, F, engineering, 0.7 were added.
A biaxial stretching process was carried out in the same manner as in Example except that the composition was blended in %. The film formability in this film formation is ranked ○ in terms of clip detachment, but the breakability at the exit of the tenter is rank ×, and the film often breaks at the center and edges, and the biaxially stretched film is not good. I couldn't.

Comparative Example 6 Instead of the polypropylene used in Example /, M, F, engineering, /θ crystalline polyglobylene homopolymer was added /wt%
A biaxially stretched film was obtained in the same manner as in Example 1 except for blending. Although there were no problems with the film-forming properties during the film-forming process, the obtained film had an apparent specific gravity of 7.3♂, and although the formation of closed cells was observed by microscopic ψ observation, they were only scattered. The appearance was translucent and the whiteness was extremely poor.

Comparative Example 7 In place of the polypropylene used in Example /, M, F, Engineering, /! When the biaxial stretching process was carried out in the same manner as in Example except that the crystalline polypropylene homopolymer was blended with 30 layers of yellow, the lip detachment phenomenon occasionally occurred at the clip detachment rank Δ during film formation, and the clip When no separation occurred, the film was broken frequently, that is, the breakability was ranked x, and the final biaxially stretched film could not be obtained.

Comparative example! M, F, ■ instead of polypropylene used in Example 1
, 7.3 crystalline low density polyethylene homopolymer /
A biaxially stretched film was obtained in the same manner as in Example except that Q% by weight was added. The film-forming property during the film-forming process was ranked △ due to clip detachment, and the productivity was poor. The apparent specific gravity of the obtained film was 7.23, and when observed under a microscope, closed cells were only scattered very sparsely, as in Comparative Example C, indicating that the polyethylene was only finely dispersed. Ta. Therefore, like Comparative Example 1, the film was semitransparent and had poor hiding power.

Claims (1)

[Claims] (1) By extruding an aromatic polyester containing 3 to 20% by weight of a crystalline polypropylene homopolymer having a melt flow index of 0.2 to 120 into a sheet shape, and then stretching the sheet in at least one direction, A polyester film containing fine closed cells on the surface and inside the film.