JP2006063186A - Method for producing polypropylene film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a biaxially stretched polypropylene film by which biaxial stretching properties are excellent with excellent thickness uniformity and thermal dimensional stability of the resultant biaxially stretched film and even breakdown voltage can be maintained at a high level at high temperatures while raising the regularity of the polypropylene resin. <P>SOLUTION: The method for producing the polypropylene film is carried out as follows. A polypropylene resin (A) having ≤5 ratio (Mw/Mn) of the weight-average molecular weight (Mw) to the number-average molecular weight (Mn) and a polypropylene resin (B) having high melt tension are melt kneaded and the resultant polypropylene resin is melt extruded. After cooling and casting, biaxial stretching is performed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a biaxially stretched polypropylene film, particularly for a biaxially stretched polypropylene film that is used for capacitor applications, mold release applications, etc. and requires high uniformity of heat resistance, thickness and film properties. It relates to a manufacturing method.

  Polypropylene has a high melting point among polyolefin resins, excellent heat resistance, excellent electrical properties and releasability, and its biaxially stretched film is widely used for packaging materials such as foods, snacks, sundries, adhesive tapes, and print lamination. It is used. In particular, there is a need for further improvements in heat resistance for capacitor applications that make use of its electrical characteristics and process paper for manufacturing optical materials such as liquid crystal materials that make use of releasability. Many proposals have been made.

Specific examples include a method of defining the melting point of polypropylene and the amount of low molecular weight (Patent Document 1), a method of increasing the stereoregularity of polypropylene resin (Patent Documents 2 to 4), and the like.
JP 2001-146536 A (Claims) JP H10-156938 PR (Claims) JP H10-156939 PR (Claims) However, when such a method is used, the crystallinity of the polypropylene resin is unnecessarily increased, and biaxial stretching becomes difficult or can be stretched. However, the film thickness may be inferior in uniformity, or voids may occur frequently, resulting in a film having a low dielectric breakdown voltage, resulting in practical problems.

  The present invention is excellent in biaxial stretchability while improving the regularity of the polypropylene resin, excellent in uneven thickness and thermal dimensional stability of the obtained biaxially stretched film, and can maintain high withstand voltage at high temperature. I would like to propose a method for producing such a biaxially oriented polypropylene film.

In order to solve the above problems, the present invention
(1) Melting and kneading a polypropylene resin (A) having a ratio (Mw / Mn) of number average molecular weight (Mn) to weight average molecular weight (Mw) of 5 or less and a high melt tension polypropylene resin (B) A method for producing a polypropylene film obtained by melt-extrusion of the obtained polypropylene resin, cooling casting, and biaxial stretching.
(2) The method for producing a polypropylene film as described in (1), wherein the mesopentad fraction of the polypropylene resin (A) is 0.945 to 0.995.
(3) The production of the polypropylene film as described in (1) or (2), wherein 0.2 to 50 parts by weight of the polypropylene resin (B) is added to 100 parts by weight of the polypropylene resin (A). Method.
(4) Item (1) to (3) characterized in that the film temperature at the time of stretching at least uniaxially is from the melting point −15 (° C.) to the melting point +5 (° C.) of the polypropylene resin (A). The manufacturing method of the polypropylene film in any one.
(5) The method for producing a polypropylene film according to any one of (1) to (4), wherein the thickness of the polypropylene film is 2.5 to 6 μm.
(6) The method for producing a polypropylene film according to any one of (1) to (4), wherein the thickness of the polypropylene film is 30 to 100 μm.
This is a proposal.

  In the present invention, by adding a high melt tension polypropylene resin while having a highly ordered polypropylene resin as its constituent requirements, it has excellent biaxial stretchability, excellent thickness uniformity, high thermal dimensional stability, and high withstand voltage. A polypropylene film having the following can be obtained. Of course, it is also suitable for general polypropylene film applications such as packaging applications, print lamination applications, adhesive tape applications, etc., but is particularly suitable as a method for obtaining biaxially stretched polypropylene films suitable for capacitor applications and mold release applications.

  Hereinafter, the present invention will be described in detail together with preferred embodiments.

  The basic requirement of the present invention is that it comprises at least a polypropylene resin (A) and a polypropylene resin (B).

  The polypropylene resin (A) needs to have a ratio (Mw / Mn) of the number average molecular weight (Mn) to the weight average molecular weight (Mw) of 5 or less, more preferably 4 or less. . Mw / Mn is a parameter indicating the degree of dispersion of the molecular weight distribution of the polypropylene resin, that is, the molecular weight distribution, and is obtained by a GPC (gel permeation chromatography) method. This means that the smaller the Mw / Mn is, the smaller the molecular weight distribution is. However, if the Mw / Mn is too small, the uniformity when the molten polymer is formed into a sheet shape decreases, or the biaxial stretchability decreases. Cause problems. However, on the other hand, since there are few low molecular weight components which do not contribute to a structure, it can be said that it is resin excellent in heat resistance. In the present invention, when Mw / Mn exceeds 5, the effect of improving the heat resistance is small or not exhibited at all, and the withstand voltage at high temperatures and the mechanical properties are inferior. On the other hand, the lower limit of Mw / Mn is 1. However, this means a resin having a single molecular weight, which is difficult to obtain industrially by the current catalyst technology. 2 or more, preferably 2.5 or more.

  In order to obtain a polypropylene resin having such a molecular weight, it is possible to optimize the catalyst configuration, and it can be optimized with a known Ziegler-Natta catalyst or metallocene catalyst. Another method is to polymerize a polypropylene resin having a high molecular weight to some extent, and obtain by heat loss with peroxide or the like at the time of pelletization. The resin powder once obtained is a solvent such as n-heptane or xylene, or a propylene monomer. The method etc. which are set to predetermined Mw / Mn by washing | cleaning are illustrated.

  As a more preferable property of the polypropylene resin (A), the mesopentad fraction, which is an index of stereoregularity, is preferably 0.945 to 0.995, and more preferably 0.955 to 0.990. If the mesopentad fraction is too low, heat resistance and mechanical strength may be reduced. On the other hand, the upper limit of the mesopentad fraction is defined by the upper limit value obtained industrially at present, but this is not the case when a resin having higher regularity at an industrial level is developed in the future.

Next, the polypropylene resin (B) is a so-called high melt tension polypropylene.
Here, it is a high melt tension polypropylene or a polypropylene resin having a branched structure. The characteristic of high melt tension polypropylene is the relationship between melt tension (MS) and melt flow rate (MFR) when measured at 230 ° C. (1)
log (MS)> − 0.56 · log (MFR) +0.74 (1)
(However, MS: melt tension (cN) measured at 230 ° C., MFR: melt flow rate (g / 10 min))
Is to satisfy. Specific examples of the polypropylene resin having such a branched structure include “HMS-PP” (PF-814, PF-633, PF-611, SD-632, etc.) manufactured by Basell, “HMS-” manufactured by Borealis. PP "(WB130HMS etc.) is illustrated.

  By adding a polypropylene resin having such a branched structure, the melt crystallization temperature of the polypropylene resin can be increased and there is almost no decrease in stretchability. In some cases, stretchability is improved and thickness uniformity is also improved. Since it improves, it is preferable.

  In the present invention, it is necessary to add 0.2 to 50 parts by weight of the polypropylene resin (B) to 100 parts by weight of the polypropylene resin (A), preferably 0.5 to 30 parts by weight, Preferably it is 1-10 weight part. If the added amount of the polypropylene resin (B) is too small, the processing suitability is not improved and the film has poor thickness spots. On the other hand, when the addition amount is too large, only a film having a high heat resistance, particularly a large thermal shrinkage can be obtained.

  In the present invention, the stretching temperature in the uniaxial direction when stretching the sheet-formed unstretched film is preferably set to −15 to + 5 ° C. of the melting point of the polypropylene resin (A). This is preferable because not only the thickness uniformity is increased but also the thermal shrinkage is small and the withstand voltage at high temperature is excellent.

  In particular, the sequential biaxial stretching method by the tenter method is a method in which a resin is melt-extruded from a slit die such as a T die, cooled on a cooling drum, an unstretched sheet is obtained, and the sheet is preheated with a heated metal roll or the like. It is a method of obtaining a biaxially stretched film by stretching in the longitudinal direction between a plurality of rolls provided with a difference in peripheral speed, further gripping both ends with clips by a tenter, and stretching in the transverse direction in an oven. In such a case, it is preferable to control the temperature of the sheet so as to be in the temperature range of the melting point of the polypropylene resin (A) from -15 to + 5 ° C., particularly in the longitudinal stretching. This is preferable because the thermal dimensional stability of the film is further improved and the uniformity of the film thickness in lateral stretching is improved. Here, the uniformity of the film thickness in the transverse stretching is to measure a substantial stretching ratio by performing marking at regular intervals on the film after stretching in the longitudinal direction and measuring the spacing of the markings after transverse stretching. It can be evaluated by the method. Specifically, there is exemplified a method in which square squares are transferred, the area before and after transverse stretching is measured, and the extent to which the area ratio (= effective magnification) before and after stretching varies in the film width direction is exemplified.

  The film thickness after biaxial stretching that is preferable as a method for producing the polypropylene film of the present invention is preferably a thin film of 2.5 to 6 μm or a thick film of 30 to 100 μm. That is, by using the resin configuration of the present invention, it is possible to stably form a thin sheet. Furthermore, the stretching stress at the time of stretching can be stabilized and the uniformity can be further increased. On the other hand, when a resin having a narrow molecular weight distribution is cast in a thick film as described above, spherulites develop, so that not only the formation of voids at the time of stretching becomes extremely inferior to the haze, but also the sequential biaxial stretching method is used. In this case, it becomes difficult to obtain uniformity during transverse stretching, and the film tends to be inferior in thickness unevenness.

  For the purpose of improving slipping property, antistatic property, oxidation deterioration resistance, etc., known organic / inorganic slip agents, antistatic agents, primary and / or secondary antioxidants, etc. are added to the film of the present invention. be able to. As other functional additions, UV absorbers, fillers, other polyolefins, polyesters, polyamides, and other resins can be added for light resistance and concealment.

  It goes without saying that the film of the present invention can be co-extruded and co-stretched with at least one other resin layer within a range not departing from the object.

Hereinafter, embodiments of the present invention will be described based on examples, but the present invention is not limited to the examples.

  Next, measurement methods and evaluation methods used in the examples of the present invention will be described.

(1) Molecular weight distribution (Mw / Mn)
It calculated | required by the monodisperse polystyrene reference | standard using the gel permeation chromatography (GPC).

  The number average molecular weight (Mn) and the weight average molecular weight (Mw) are defined by the following formulas based on the molecular number (Ni) of the molecular weight (Mi) at each elution position of the GPC curve obtained through the molecular weight calibration curve. Is done.

Number average molecular weight: Mn = Σ (Ni · Mi) / ΣNi
Weight average molecular weight: Mw = Σ (Ni · Mi ² ) / Σ (Ni · Mi)
Molecular weight distribution: Mw / Mn.

The measurement conditions were as follows (() indicates the manufacturer)
Apparatus: Gel permeation chromatograph GPC-150C (Waters)
Detector: Differential refractive index detector RI Sensitivity 32 ×, 20% (Waters)
Column: Shodex HT-806M (2) (Showa Denko)
Solvent: 1,2,4-trichlorobenzene (BHT 0.1 w / v% added) (Ardrich)
Flow rate: 1.0 ml / min
Temperature: 135 ° C
Sample: Dissolution condition 165 ± 5 ° C. × 10 minutes (stirring)
Concentration 0.20w / v%
Filtration Membrane filter pore size 0.45μm (Showa Denko)
Injection volume: 200 μl
Molecular weight calibration: The molecular weight of polypropylene was determined using the relationship between molecular weight and retention time obtained by measuring monodisperse polystyrene (Tosoh) under the same conditions as the specimen. Data processing which is a relative value based on polystyrene: According to a GPC data processing system manufactured by Toray Research Center.
(2) Measurement of mesopentad fraction Mesopentad fraction (mmmm) The sample was dissolved in a solvent, and the mesopentad fraction (mmmm) (100 fraction) was determined under the following conditions using 13C-NMR.
Measurement conditions Device: Bruker DRX-500
Measurement nucleus: 13C nucleus (resonance frequency: 125.8 MHz)
Measurement concentration: 10 wt% (sample 10 wt%, solvent 90 wt%)
Solvent: benzene / heavy orthodichlorobenzene = 1: 3 mixed solution (volume ratio)
Measurement temperature: 130 ° C
Spin rotation speed: 12Hz
NMR sample tube: 5 mm tube Pulse width: 45 ° (4.5 μs)
Pulse repetition time: 10 seconds Data point: 64K
Number of conversions: 10000 times Measurement mode: complete decoupling
-Analysis conditions Fourier transform was performed by setting LB (line broadening factor) to 1.0, and the mmmm peak was set to 21.86 ppm. Peak splitting is performed using WINFIT software (manufactured by Bruker). At that time, the peak splitting is performed from the peak on the high magnetic field side as shown below, and the soft automatic fitting is performed to optimize the peak splitting. The sum of the peak fractions is defined as the mesopentad fraction (mmmm).
The measurement is performed with n = 5, and the average value is obtained.
・ Peak (a) mrrm
(B) (c) rrrrm (divided as two peaks)
(D) rrrr
(E) mrmm + rmrr
(F) mmrr
(G) mmmr
(H) ss (mmmm spinning sideband peak)
(I) mmmm
(J) rmmmr.

(3) Intrinsic viscosity ([η])
0.1 mg of a sample was dissolved in 100 ml of tetralin at 135 ° C., and this solution was measured using a viscometer in a constant temperature bath at 135 ° C., and the intrinsic viscosity [η] was determined from the specific viscosity S according to the following formula (unit: : Dl / g).

    [Η] = (S / 0.1) × (1 + 0.22 × S).

(4) Melting point (° C)
The measurement was performed under the following conditions using a Seiko RDC220 differential scanning calorimeter.
Preparation of sample: 10 mg of polypropylene resin is sealed in an aluminum pan for measurement.
Measurement conditions The endothermic peak observed when the temperature was raised from room temperature to 280 ° C. at a rate of 20 ° C./min was defined as the melting point (Tm (° C.)). When there are a plurality of peak values, the melting peak having the largest peak area is adopted. The above measurement was repeated 5 times, and the average value of the remaining 3 points, excluding 2 points of the maximum value and the minimum value, was defined as Tm (° C.).

(5) Stretching Uniformity Using a tenter sequential biaxial stretching method, biaxial stretching properties were evaluated as follows.
(A) Biaxial stretching An unstretched sheet formed on a sheet-forming cooling drum from a T-shaped die having a slit width of 300 mm from an extruder having a screw diameter of 65 mmφ is guided to a roll stretching device at a temperature shown in Table 2 in the longitudinal direction. After stretching 4.6 times, it was stretched 7 times at a mechanical magnification at a temperature of 158 ° C. in the width direction with a tenter. The cooling drum temperature was 60 ° C. when the thickness of the biaxially stretched film was 15 μm or more, and 90 ° C. when the thickness was 10 μm or less.
(B) Stretching uniformity By a grid-like engraving (15 mm x 15 mm) having a square mm of 10 mm x 10 mm at the entrance of the tenter, each side of the square is parallel to the longitudinal direction and the width direction of the film. The film was transferred onto a uniaxially stretched film so that the area (mm ² ) of each square of the obtained biaxially stretched film was determined, and the stretch uniformity was determined by the following equation. In addition, the cell to measure measured 1 row | line | column (film width direction) with the good transferability of the cell pattern, and measured 15 line | wire of the row | line | column.

The draw ratio Xi at each square is determined by the area after stretching (mm ² ) / the area before stretching (100 mm ² ), and the stretch uniformity index is calculated from the average value X, maximum value Xmax, and minimum value Xmin of 15 mm Asked. The stretching uniformity index is preferably 90% or more.

        Stretching uniformity index = 100− (Xmax−Xmin) / X × 100 (%).

(6) 140 ° C. Heat Shrinkage Based on JIS-Z-1712, the rate of dimensional change when the sample film is held in a hot air oven at 140 ° C. for 15 minutes under the following conditions is defined as the heat shrinkage. The MD (longitudinal direction) of film formation is preferably 5% or less, and the TD (width direction) is preferably 2% or less.

(A) Sample width 10mm x length 200mm
(B) Oven conditions: 140 ° C., load 3 g
(C) The measurement length is calculated by the following formula using the read value of the film length L1 (mm) before and after the processing based on L0 = 100 mm before the processing, and the thermal contraction rate (%) = (L0−L1) / L0 × 100.

(7) Dielectric breakdown voltage (V / μm)
According to JIS C2330 (2001 edition) 7.4.11.2 B method (plate electrode method). When the film thickness is 4 μm, the dielectric breakdown voltage is preferably 600 V / μm or more.

(8) Haze (%)
It measured based on JIS-Z-1712.

  Next, description will be made based on an embodiment of the present invention.

  When polymerizing the polypropylene resin, the polymerization catalyst was changed to prepare three types of polypropylene resins (A-1, A-2, A-3) having the characteristics shown in Table 1. Further, as a high melt tension polypropylene resin, PF-814 (B-1) manufactured by Sun Allomer and WB130HMS (B-2) manufactured by Borealis were prepared.

When these resins are melt-extruded, those obtained by chip blending in advance with the combinations shown in Table 2 are guided to an extruder and formed into a sheet having a predetermined thickness. Table 2 summarizes the results of evaluating the stretching uniformity and thermal shrinkage of the film.
Hereinafter, an evaluation result is demonstrated for every Example and a comparative example.
(Examples 1-3)
As shown in Table 2, a biaxially stretched film having a film thickness of 4 μm was obtained using polypropylene resins (A-1, A-2) and high melt tension polypropylene resins (B-1, B-2). All of them were excellent in stretchability, and the thermal shrinkage in the transverse (TD) direction was as small as 1%.
(Comparative Example 1)
As Comparative Example 1, an attempt was made to stretch under the conditions of Example 1 without adding high melt tension PP. However, the film was broken at the time of MD stretching, and thus the film was stretched at 145 ° C., which was the MD stretching employed in Example 2. As a result, the stretch uniformity of the film obtained was slightly inferior, and at the same time, the heat shrinkage ratio was increased.
(Comparative Example 2)
A biaxially stretched film was obtained in the same manner as in Example 2 except that type A-1 was used as the polypropylene resin. A product having a large heat shrinkage rate was obtained.
Example 4
A-1 and B-1 were used as the polypropylene resin to obtain a film having a film thickness of 60 μm after biaxial stretching. Excellent stretching uniformity and excellent heat resistance with small heat shrinkage.
(Comparative Example 3)
Stretching was performed in the same manner as in Example 4 using only A-1 as the polypropylene resin, but the film was inferior in stretching uniformity, high in heat shrinkage, and inferior in transparency.

  Since the polypropylene film obtained by the present invention is excellent in heat resistance and excellent in uniformity in thickness and film physical properties, it can be preferably used for food packaging applications and adhesive tapes. In particular, applications that require high uniformity, heat resistance, and withstand voltage characteristics such as capacitor applications and mold release applications are particularly preferred because a film that satisfies the required characteristics can be obtained.

Claims (6)

  Polypropylene obtained by melt-kneading a polypropylene resin (A) having a ratio (Mw / Mn) of number average molecular weight (Mn) to weight average molecular weight (Mw) of 5 or less and a high melt tension polypropylene resin (B) A method for producing a polypropylene film, in which a resin is melt-extruded, cooled and cast, and then biaxially stretched.   The method for producing a polypropylene film according to claim 1, wherein the mesopentad fraction of the polypropylene resin (A) is 0.945 to 0.995.   The method for producing a polypropylene film according to claim 1 or 2, wherein 0.2 to 50 parts by weight of the polypropylene resin (B) is added to 100 parts by weight of the polypropylene resin (A).   The polypropylene film according to any one of claims 1 to 3, wherein the film temperature at the time of stretching in at least one axial direction is a melting point of the polypropylene resin (A) -15 (° C) to a melting point +5 (° C). Manufacturing method.   The method for producing a polypropylene film according to any one of claims 1 to 4, wherein the thickness of the polypropylene film is 2.5 to 6 µm.   The method for producing a polypropylene film according to any one of claims 1 to 4, wherein the thickness of the polypropylene film is 30 to 100 µm.