JPH069850B2 - Method for forming polypropylene resin film - Google Patents

Description

The present invention relates to a method for forming a polypropylene resin, and more specifically, a sheet-like material such as a polypropylene film excellent in transparency, uniformity of front and back surfaces, slipperiness, and stretchability can be obtained. The present invention relates to a method for forming a polypropylene resin film.

[Conventional technology]

Conventionally, as a method for cooling and molding a sheet material obtained by melting polypropylene resin and extruding it by a T-type die method, the sheet material is brought into contact with a cooling drum, and air is blown to a grounding point of the sheet material to forcibly adhere to the drum. There is an air knife method aimed at cooling from the air and air cooling, and a roll method in which a roll is brought into close contact with a cooling drum to perform cooling molding. Also known as the inflation method is a cooling method in which air is blown onto a molten tube, or a water cooling method in which water is caused to flow in a ring shape and is brought into contact with the outer circumference of the tube.

[Problems to be Solved by the Invention]

However, in the air knife method and the roll method by the T-type die method, there is a difference between the cooling speed from the cooling drum and the cooling speed from the air blowing side and the roll side, and the polymer crystals on the front and back of the formed polypropylene film are different. There are different things. This becomes more prominent as the thickness of the polypropylene film increases and the film forming speed increases.

If the crystallization of the front and back of the polypropylene film is different,
As a matter of course, the crystal size and the crystallinity change, and the spherulite deformation amount and the deformation degree in the subsequent stretching process are different between the front and back of the film. For this reason, the surface morphology of the front and back of the film may be different, or curling may occur. In addition, the film may have many voids with poor transparency.

For this reason, a method has been devised, as disclosed in JP-A-59-49937, in which a large number of foam particles are made from a water-soluble or water-dispersed liquid and the foam particles are brought into contact with molten plastic to cool the plastic particles. ing. However, in this method, there is no means for uniformizing the film thickness, which is important for film quality. For example, in cooling molding in the T-type die method, the distance from the die to the cooling drum is long, so that the sheet-like material that occurs during this period. Vibration and sagging of the sheet-like material due to its own weight cause deterioration of the thickness unevenness of the obtained polypropylene film. Further, also in the inflation method, the outer surface of the tube is cooled rapidly, but the inner surface is not cooled, and the crystallization of the inner and outer surfaces of the film is different.

Further, Japanese Utility Model Application Laid-Open No. 53-134262 discloses a device for imparting water droplets to a film after melt extrusion with an air mixing spray nozzle in order to obtain a film having a uniform appearance without flow marks or boil marks on the film surface. It is shown.

According to the means disclosed in this apparatus, the flow mark and the boil mark on the film surface are eliminated, but there is a problem that transparency, uniformity, stretchability and the like cannot be obtained yet.

Further, in the case of a polypropylene resin, crystallization progresses in the process of cooling after melt extrusion, but if the cooling efficiency is poor, the film is gradually cooled, resulting in a problem that the film becomes larger and the film has poor transparency.

On the other hand, in a polypropylene-based stretched film, as disclosed in JP-A-60-88049, inorganic particles and a nucleating agent are added for the purpose of imparting slipperiness, transparency and anti-blocking property to the stretched film. Is done. However, there is a drawback in that the transparency is deteriorated due to the scattering of transmitted light due to the addition of the inorganic particles when only the inorganic particles and the nucleating agent are used.

Further, Japanese Patent Publication No. 56-53489 discloses that a polypropylene film containing a nucleating agent is cooled with a mirror-finished cooling roll to impart transparency and smoothness. However, this method is still inadequate in terms of transparency because the cooling efficiency is poor.

Therefore, an object of the present invention is to provide a method for forming a polypropylene resin, which solves the above-mentioned drawbacks, is excellent in transparency, slipperiness, and stretchability, and obtains a polypropylene film having uniform front and back surfaces and no curl. It is a thing.

[Means for Solving the Problems]

The present invention has the following configuration in order to achieve the above object. That is, a polypropylene resin is melt-extruded from a die to form a sheet, and the sheet is brought into contact with a cooling drum to be cooled and solidified to form a polypropylene resin. Inorganic particles having an average particle diameter of 0.1 to 10 μm are added in an amount of 0.05 to 1.
After the addition of 0% by weight, the sheet-like material comes into contact with a mirror-finished cooling drum, and then water is sprayed on the surface of the sheet-like material opposite to the surface in contact with the cooling drum to atomize fine particles. It is a method for forming a polypropylene resin film, which comprises spraying.

The polypropylene-based resin in the present invention means a resin containing a polypropylene component in an amount of 80% by weight or more.

In the present invention, it is necessary to add a small amount of a nucleating agent having a crystal nucleation effect of the resin to the polypropylene-based resin, which will significantly improve the transparency of the polypropylene film as described later. The type of nucleating agent is aluminum benzoate, which has the crystal nucleating effect of polypropylene resin, 1,3,2,4-dibenzilidine-D-sorbitol, but is not limited thereto. The amount added is preferably about 0.01 to 1.0% by weight.

Further, the inorganic particles referred to in the present invention are silica, siloid, kaolinite, zeolite, titanium oxide, calcium carbonate and the like. From the viewpoints of slipperiness and transparency of the polypropylene film, the average particle diameter of the inorganic particles is set to 0.1 to 10 μm, and the addition amount is set to 0.05 to
It is necessary to set it in the range of 1.0% by weight.

The cooling drum must be mirror-finished to give the polypropylene film excellent transparency and uniformity. As the material, a material in which hard chrome (HCr) is wound on the drum surface or a material obtained by processing ceramic, silicone rubber or the like to form the drum surface can be used.

As a method for cooling the surface of the drum, it is preferable to cool the inside of the drum by passing ordinary water, chilled water or warm water, and the surface temperature of the cooling drum in the present invention is preferably in the range of 5 ° C to 80 ° C. Surface temperature of the cooling drum is 5 ℃
When it is less than the above, dew drops are generated due to the temperature difference from the outside air, and remain as a defect on the film surface. Further, when the surface temperature of the cooling drum exceeds 80 ° C., the film is poor in cooling effect and has many voids, resulting in poor transparency.

In the present invention, although not particularly limited, the sheet-like material melt-extruded from the die can be adhered to the drum by an air knife method or a roll method.

At this time, the distance from the die to the cooling drum should be close as long as there is no hindrance in molding, so that the vibration and fluttering of the sheet-like material can be eliminated and the crystallization time can be shortened.

In addition, the air velocity of the air knife method and the roll temperature of the roll method are preferably set on the cooling side as long as there is no hindrance in molding.

By an air knife method or a roll method, water is atomized into fine particles and sprayed on the non-cooling drum surface side of the sheet-like material that is in close contact with the cooling drum. As a method for atomizing water into fine particles, which can be applied to the present invention, for example, used in an electrostatic coating machine, air atomization, electrostatic atomization, electrostatic atomization with air, hydraulic atomization and spraying. There are a gun type and a swirling airflow type, and further atomization by an ultrasonic humidifier. The diameter of water particles to be sprayed is not particularly limited, but is preferably 100 μm or less. If the water particle size exceeds 100 μm, liquid droplets tend to be formed and remain as a defect on the film surface. Further, the cooling by the latent heat of vaporization, which is the function of the present invention, is difficult to achieve. That is, in cooling with running water, the amount of heat is transferred only by absorbing heat of water, but when water is evaporated, heat transfer due to heat absorption and latent heat of evaporation occurs. This has a cooling effect 10 times or more compared with the cooling by running water. Therefore, it is preferable to make the water particles small so that they have a large surface area and are easily evaporated.

In the method of the present invention, it is necessary to add a crystal nucleating agent, but the temperature range in which the polypropylene resin is crystallized is high and the crystallization is fast. Therefore, by spraying water in the form of fine particles onto the non-cooling drum surface side of the sheet-like material adhered to the cooling drum to intensify the cooling, the nuclei of crystals generated in the cooling process after melt extrusion are further improved. In many cases, the crystallization becomes faster, so that the crystals become finer and the transparency is significantly improved. At the same time, the difference in cooling and crystallization between the front and the back becomes small, resulting in a film with small curl.

The sheet is sprayed at a position after the sheet is grounded on the cooling drum, but as close as possible to the grounding point, crystallization can be suppressed and the difference in crystallization between the front and back of the film can be reduced.

The spraying method according to the present invention is preferably a uniform spray in the width direction of the slit type nozzle from the atomization generator through a joint such as a hose pipe.

When a slit type nozzle is used, the spraying angle θ (angle between the nozzle direction and the gravity direction) is preferably in the range of 45 to 90 °. If θ is out of this range, agglomerated liquid beads may fly onto the sheet-like material or the cooling effect may be weakened. For example, if θ is 105 °, the liquid droplets that have agglomerated at the tip of the nozzle will fly, and the liquid droplets will adhere to the surface of the sheet-like material, and the places where the liquid droplets adhere will be surface defects of the film after film formation. There is.

The sheet material in the present invention includes a laminate obtained by extrusion lamination. Also in this case, after the laminate comes into contact with the cooling drum, water is sprayed in the form of fine particles onto the surface of the laminate opposite to the surface in contact with the cooling drum.

Further, it is preferable to appropriately select the spray flow rate and the spray flow rate by changing the cooling drum speed, the film forming speed, and the film thickness.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the following examples.

Example 1 FIG. 1 shows an example of an apparatus used in the method of the present invention.
DESCRIPTION OF SYMBOLS 1 is an extruder, 2 is a T-shaped die of the extruder, 3 is a cooling drum having a mirror-finished hard chrome on its surface, 4 is an air knife for adhering a sheet-like material to the cooling drum, 5 is an atomization generator, 6 is a water tank, 7 is a slit type nozzle, 8 is a hose pipe connecting the atomization generator and the slit type nozzle, 9 is a polypropylene resin sheet, and the arrow indicates the direction of gravity. In this figure, the angle θ of the slit type nozzle with respect to the direction of gravity is 80 ° C.

Using the apparatus shown in FIG. 1, polypropylene was formed into a film as follows. A resin obtained by adding 0.5% by weight of inorganic particles (silica, average particle diameter 3.5 μm) and 0.1% by weight of aluminum benzoate as a nucleating agent to 100 parts by weight of polypropylene was charged into the extruder 1 from a hopper, Melting temperature 260
It was extruded from the T-die 2 onto the cooling drum 3 at ℃. The thickness of this taste-like sheet material 9 was 1 mm.

On the sheet-shaped material 4 on the cooling drum 3, 8
At an angle of 0 °, water from the slit type nozzle 7 has a particle size of about 1
It was made into a mist of μm and sprayed, and then cooled and solidified.

Then, it was stretched 5 times in the longitudinal direction at a stretching temperature of 120 ° C., and then this uniaxially stretched film was introduced into a stenter kept at 160 ° C. and stretched 7 times in the transverse direction.

The film-forming property in the stretching process was good, and a film with uniform crystallization on the front and back sides of the film, with different front and back surface shapes and less voids was obtained. When the visual transparency of the obtained film was measured, the NAS was 3.8, and the L
SI was 1.8 and it was excellent in transparency.

Here, the visual transparency is a measure of the degree of haze. Compared with the conventional cloudiness value measurement (JIS K6714 etc.), the light receiving angle is closer to the human eye. The incident angle of the human eye is 0.7 ° in a bright place and 1.4 ° in a dark place, and the NAS (narrow-angle transmitted light property value) used here is -0.4 ° to
0.4 ° transmitted light, LSI (narrow angle diffuse transmission value)-
The scattered light of 1.2 ° to −0.4 ° and 1.2 ° to 0.4 ° was measured, and the measurement was performed using a visual transparency tester manufactured by Toyo Seiki Seisakusho.

Comparative Example 1 The procedure of Example 1 was repeated, except that water was not atomized and sprayed, and that no nucleating agent was added.

The visual transparency of the obtained film was measured and found to be NA
The S was 6.4 and the LSI was 6.7, and the transparency was considerably inferior to the film of Example 1.

Comparative Example 2 The procedure of Example 1 was repeated, except that the water was not atomized and sprayed. That is, a film was obtained by melt-extruding, cooling-molding and biaxially stretching a resin containing 0.1% by weight of aluminum benzoate as a nucleating agent. The resulting film has a NAS of 4.5 and is an LSI
Is 3.1, which is inferior to that in Example 1 and has a large curl as compared with Comparative Example 1 although the transparency is improved.

〔The invention's effect〕

 The present invention has the following effects.

(1) On the front and back of a polypropylene film, a film having uniform front and back surfaces with uniform crystallization can be obtained.

(2) Since the cooling effect is high, a polypropylene film having good transparency can be obtained.

(3) Since the front and back surfaces of the polypropylene film are uniformly crystallized, the orientation of the front and back surfaces of the film does not change even when biaxially stretched, and a curl-free film having excellent stretchability is obtained.

[Brief description of drawings]

FIG. 1 shows an embodiment of a film forming apparatus to which the present invention is applied. 1 ... Extruder, 2 ... T-die, 3 ... Cooling drum, 4
…… Air knife, 5 …… Atomization generator, 6 …… Water tank, 7 …… Slit nozzle, 8 …… Hose pipe, 9 ……
Polypropylene resin sheet

Claims (1)

[Claims] A polypropylene resin is melt extruded from a die to form a sheet, and the sheet is brought into contact with a cooling drum to solidify by cooling to form a polypropylene resin into a film. Nucleating agent and inorganic particles having an average particle size of 0.1 to 10 μm are used in an amount of 0.05 to 1.0.
% By weight, and the sheet-like material comes into contact with a mirror-finished cooling drum, and then water is atomized into fine particles on the surface of the sheet-like material opposite to the surface in contact with the cooling drum. A method for forming a polypropylene-based resin film, which comprises spraying.