JP2017149114A - Method for producing polypropylene sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a highly transparent sheet to be produced even by using a single material of a crystalline polypropylene resin in a melt extrusion method.SOLUTION: A method for producing a polypropylene resin sheet, in which a polypropylene resin sheet is produced with a polypropylene resin as a single raw material by a melt extrusion method, includes: a discharge step of discharging the raw material in a molten state, in a sheet form, from a flat die; and a cooling step of bringing the discharged raw material in the sheet form into contact with plural cooling rollers in turn to cool the raw material. In the cooling step, the plural cooling rollers are rotated so that the rotation directions of the cooling rollers are inverted in turn; and cooling by the first cooling roller is ended when a face, coming in contact with the first cooling roller, of the raw material in the sheet form has a temperature that is less than a melting point but does not reach a crystallization temperature.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a method for producing a transparent polypropylene sheet.

  Polypropylene resin belongs to crystalline polyolefin resin. In the process of producing a sheet by extruding a molten polypropylene resin above the melting point from a flat die and solidifying it on a cooling roll (hereinafter referred to as “melt extrusion method”), the sheet temperature is the crystallization temperature on the cooling roll. Passing through the process, the generation of crystal nuclei and crystal growth proceed, and the sheet becomes a translucent product.

  The polypropylene film on the market for general packaging (in this specification, the “film” has a thickness of less than 100 μm and the “sheet” has a thickness of 100 μm or more) has a very thin thickness of 10 to 30 μm. A polypropylene film is produced by biaxially stretching a sheet that has been cooled and solidified from a molten state, but polypropylene has a property of increasing transparency when biaxially stretched, and the transparency of the product film after stretching can be ensured. Therefore, there is no problem even if the cooling sheet itself before stretching is translucent.

  On the other hand, in order to ensure transparency in the production of unstretched films that do not go through the stretching process, the crystal growth is suppressed by rapid cooling that rapidly passes through the crystallization temperature of polypropylene, and a highly transparent product. Can be manufactured. In the production of non-stretched film, because of the thin base material, both sides of the film can be sufficiently rapidly cooled only by cooling and solidifying with a single large-diameter roll, and therefore it is relatively easy to ensure transparency. . On the other hand, in the production of unstretched sheets, since the sheets are thick, it is difficult to increase the cooling rate when the sheet temperature passes the crystallization temperature, and thus crystal growth proceeds, and as a result, it is difficult to ensure transparency. Become. For example, even if the cooling rate when the sheet surface temperature in contact with the roll passes the crystallization temperature can be increased, the temperature of the sheet surface not in contact with the roll passes through the crystallization temperature. It is difficult to increase the cooling rate at the time.

  A technique related to a method for producing a transparent polypropylene sheet is known. Patent Document 1 discloses a method for producing a sheet by a rolling method called an inconstant speed method for the purpose of improving the strength and transparency of a polyolefin resin sheet. In this manufacturing method, the sheet material is passed through a pair of rolling rolls having sequentially different peripheral speeds to manufacture a sheet. However, since this method is a manufacturing method based on a rolling method in which a pair of roll gaps are passed, the manufacturing process itself is different from the melt extrusion method targeted by the present invention.

  Patent Document 2 relates to a method for producing a transparent polypropylene sheet having a cooling step in the melt extrusion method and a subsequent heat treatment step. However, in this method, a material obtained by adding a metallocene-based ethylene-α-olefin copolymer to polypropylene is used. That is, this document does not refer to a method for producing a transparent sheet of pure polypropylene resin.

  Patent documents 3, 4 and 5 also relate to a transparent polypropylene sheet. However, these also propose materials in which several kinds of additives are added to polypropylene resin.

  Patent Document 6 describes a method of cooling a flat plastic product using a large-diameter roll and a small-diameter roll.

Japanese Patent No. 3451306 Japanese Patent No. 5563972 JP-A-8-134291 JP-A-9-71696 JP 2001-163998 A US Patent Application Publication No. 2010/0109180

  According to Patent Documents 2 to 5, in order to produce a highly transparent sheet by melt extrusion using polypropylene, which is a crystalline resin, the raw materials are combined (addition of other polymers and additives). It is effective. However, a method for producing a transparent sheet by a melt extrusion method using only a single material of 100% crystalline polypropylene resin as a raw material is not known.

  Patent Document 6 does not propose a method for producing a transparent sheet.

  An object of the present invention is to enable the production of a highly transparent sheet while using a single material of crystalline polypropylene resin in a melt extrusion method.

According to one aspect of the invention,
In a method for producing a polypropylene resin sheet, a polypropylene resin sheet is produced using a polypropylene resin as a single raw material by a melt extrusion method.
A discharge step of discharging the molten raw material into a sheet form from the flat die;
And cooling the discharged sheet-like raw material by sequentially contacting a plurality of cooling rolls,
In the cooling step,
The plurality of cooling rolls are rotating with their rotation directions reversed in order,
Production of a polypropylene resin sheet characterized in that the cooling by the first cooling roll is finished before the surface of the sheet-like raw material in contact with the first cooling roll reaches a temperature below the melting point but does not reach the crystallization temperature. A method is provided.

  According to the present invention, a highly transparent sheet can be produced in the melt extrusion method while using a single material of crystalline polypropylene resin.

It is the schematic which shows the example of the sheet manufacturing apparatus which can implement this invention. It is a graph which shows the DSC curve of the polypropylene resin used for the Example of this invention. It is the schematic which shows the example of the conventional sheet manufacturing apparatus. 6 is a graph showing a sheet temperature in Comparative Example 1. 4 is a graph showing a sheet temperature in Example 1. It is the schematic which shows another example of the sheet manufacturing apparatus which can implement this invention. 6 is a graph showing a sheet temperature in Example 2. It is the photograph which copied the white cloud which floats in the blue sky through a polypropylene resin sheet (the sheet manufactured in Comparative Example 2 is arranged on the left side, and the sheet manufactured in Example 3 is arranged on the right side).

  The present invention relates to a method for producing a polypropylene resin sheet using a polypropylene resin as a single raw material. In this specification, the sheet surface that contacts the first cooling roll (No. 1 roll described later) is called the “front surface”, and the other surface, that is, the sheet surface that does not contact the first cooling roll. May be referred to as the “back side”.

〔raw materials〕
Only one type of polypropylene resin is used as a single raw material. Do not use raw materials other than polypropylene resin.

  As a polypropylene resin, a well-known polypropylene resin can be used suitably, and it can select from a well-known material conventionally as a raw material of a polypropylene sheet conventionally.

[Discharge process]
In the present invention, a polypropylene resin sheet is produced by a melt extrusion method. The melt extrusion method has a discharge step and a cooling step.

  In the discharging step, a molten raw material, that is, a polypropylene resin is discharged from a flat die into a sheet shape. As the flat die, a conventionally known flat die can be appropriately used in the field of polypropylene resin sheet production by melt extrusion. As a machine used for discharging, a known machine in the field can be used as appropriate.

[Cooling process]
In the cooling step, the sheet-like raw material discharged in the discharge step is cooled by sequentially contacting a plurality of cooling rolls. That is, for example, the sheet-like raw material is first cooled by contacting the first cooling roll, then cooled by contacting the second cooling roll, and then cooled by contacting the third cooling roll (3 When the first cooling roll is present). In some cases, the i-th cooling roll (i represents a positive integer) is referred to as “No. Indicated as i-roll. In other words, the roll that cools the sheet by bringing the molten resin discharged from the flat die into contact is called “No. 1 roll” (generally, about 180 ° of this roll is brought into contact with the sheet). No. An auxiliary roll existing on the opposite side through the resin discharged to one roll is referred to as a “touch roll”, but the touch roll is not necessarily required. No. The cooling roll installed downstream of the first roll is sequentially expressed as “No. 2 roll” and “No. 3 roll”.

  The plurality of cooling rolls are rotated with their rotation directions being sequentially reversed. For example, no. When one roll is rotating clockwise, no. Two rolls are counterclockwise, no. Three rolls rotate clockwise. With this configuration, one side and the other side of the sheet-like raw material obtained from the discharge process can be alternately cooled by a plurality of cooling rolls. For example, the surface of the sheet-like raw material is No. Cool with 1 roll, then backside. After cooling with 2 rolls, the surface was again no. It can be cooled with 3 rolls.

  The number of cooling rolls is preferably four or more from the viewpoint of finely adjusting the temperature of the sheet-like raw material, but may be three or two.

  The first cooling roll, i.e. Cooling with one roll is the No. of sheet raw material. The surface in contact with one roll reaches a temperature lower than the melting point, but ends before the crystallization temperature is reached. That is, no. At the end point of cooling by one roll, the sheet raw material No. The temperature of the contact surface with one roll is less than the melting point of the raw material. And No. In the cooling process with one roll, the sheet raw material No. The temperature of the contact surface with one roll does not reach the crystallization temperature of the raw material.

  When the crystalline resin sheet is cooled by the melt extrusion method, the sheet temperature (particularly the temperature of the surface not in contact with the roll) is crystallized even if the sheet is rapidly cooled using a large-diameter roll as described above. It is difficult to increase the cooling rate when passing through the temperature, so that crystal growth proceeds, and as a result, the transparency of the sheet tends to decrease.

  In order to avoid such a phenomenon, in the present invention, no. Cooling of one surface of the sheet-like raw material by one roll was finished early, specifically, the sheet-like raw material No. No. 1 is terminated before the temperature of the contact surface with the roll reaches the crystallization temperature. The opposite surface of the sheet raw material is cooled by two rolls.

  In addition, melting | fusing point and crystallization temperature of a raw material can be calculated | required by differential scanning calorimetry (DSC) based on JIS-K7121.

[Example of sheet manufacturing equipment]
Hereinafter, the present invention will be described with reference to the drawings, but the present invention is not limited thereto.

  FIG. 1 shows an outline of an example of a sheet manufacturing apparatus capable of carrying out the present invention. In this apparatus, small diameter rolls are continuously arranged. Thereby, it becomes easy to cool one side and the other side of the sheet alternately in a relatively short time. Therefore, it is easy to end the cooling by the first cooling roll before the surface of the sheet-like raw material in contact with the first cooling roll reaches a temperature below the melting point but does not reach the crystallization temperature. As a result, it is easy to suppress the progress of crystal growth in the sheet and produce a highly transparent sheet.

  As described above, only polypropylene resin is used as the sheet raw material. The touch roll 10 is a sheet in which the molten state discharged from the flat die 11 is No. It is used for the purpose of sufficiently adhering to the surface of one roll 1. If the touch roll 10 is excluded from consideration, in this apparatus, four cooling rolls (No. 1 to No. 4 rolls) are installed.

  More specifically, this apparatus has a diameter equivalent to that of the touch roll 10 (diameter 300 mm). 1 roll 1 and No. 1 2 roll 2, No. of slightly smaller diameter (diameter 250 mm) than these. 3 roll 3 and No. 3 Four rolls 4 are provided. No. One roll rotates counterclockwise on the paper surface, and the second and subsequent cooling rolls rotate in the opposite direction to the immediately preceding roll. A series of cooling rolls alternately cool one side and the other side of the sheet. No. The polypropylene resin sheet 12 cooled from 4 rolls is obtained.

  Note that the rotation axes of all the cooling rolls and also the touch rolls are arranged in parallel to each other on the same plane.

[Cooling roll set temperature]
It is desired to suppress deformation of the sheet when the sheet is peeled off from the cooling roll (particularly, No. 1 roll). This can be achieved by applying a known appropriate method in the production of a polypropylene resin sheet. For this purpose, for example, the set temperature of the first cooling roll (No. 1 roll) is set to crystallize the raw material resin. It is preferable that the temperature is lower by 50 ° C. or more than the temperature, and the setting temperature of the second and subsequent cooling rolls is equal to or lower than the setting temperature of the roll immediately before each roll. On the other hand, for example, the set temperature of all the cooling rolls can be about 100 to 110 ° C. lower than or higher than the crystallization temperature of the raw material resin.

  In general, the temperature of the cooling roll is adjusted by circulating a temperature-controlled liquid medium (such as water or oil) in the roll. The management temperature of the liquid medium is the set temperature of the cooling roll.

[Cooling roll peripheral speed]
Regarding the rotation speed of the cooling roll, unlike the rolling method, the peripheral speeds of all the rolls are basically the same. However, the resin sheet may be slightly stretched by pulling or may be contracted as it is cooled. When such a phenomenon occurs, the sheet-like raw material may slide with respect to the cooling roll. In order to avoid such sliding, the peripheral speeds of all the cooling rolls do not necessarily have to be exactly the same, and may be approximately equal. For example, the peripheral speed of the second and subsequent cooling rolls can be adjusted within a range of 0.98 to 1.02 (within an increase / decrease ratio of ± 2%) with respect to the peripheral speed of the immediately preceding cooling roll.

[Sheet floating area between rolls]
An area where the sheet floats (area where the sheet is not in contact with any of the rolls) may occur between one cooling roll and the roll immediately after that. Since the sheet is not substantially cooled in this region, this region should be as narrow as possible, and the distance is preferably 1/10 or less of the circumferential length of the roll immediately before that region. The lower limit value of the distance of the sheet floating region is 0 (zero).

[Cooling roll diameter]
It is preferable that the diameters of the cooling rolls are all the same. This is because it is easy to uniformly cool the front and back surfaces of the sheet, and thus it is easy to obtain a sheet having excellent surface properties and transparency.

[Others]
For example, in the apparatus shown in FIG. 1 to No. Cooling to a temperature slightly above the crystallization temperature of the resin with 2 rolls, The temperature of the entire sheet can be cooled below the crystallization temperature with 3 rolls, and further cooling can be performed with subsequent rolls.

  After cooling by the cooling roll, various processes (trimming and the like) known in the production of conventional polypropylene resin sheets and the accompanying sheet conveyance can be appropriately performed. Usually, a polypropylene resin sheet finally becomes substantially room temperature during these various processes or conveyance, and is wound up in a roll shape as necessary.

  According to the present invention, a highly transparent sheet having a minimum haze (ratio of parallel light transmittance and diffuse light transmittance with respect to incident light), which is an index of transparency of a film or sheet, is 10% or less, preferably 8% or less. Can be obtained. A sheet having a haze value of 10% or less means a sheet having at least one region having a haze value of 10% or less. Moreover, a haze measurement can be performed based on JIS-K7136.

  The method of the present invention is extremely effective for producing a transparent polypropylene resin sheet having an average thickness in the entire width direction of 100 μm or more and 1000 μm or less, particularly a transparent polypropylene resin sheet having an average thickness in the entire width direction of 100 μm or more and 500 μm or less. Is preferred.

  Examples of the present invention are shown below, but the present invention is not limited thereto.

  In FIG. 2, the DSC measurement result of the polypropylene resin used as a raw material in each example is shown. From this graph, it can be seen that this material has a melting point of 164.0 ° C. and a crystallization temperature of 117.7 ° C.

[Comparative Example 1]
FIG. 3 is a schematic view of a configuration of a typical polypropylene resin sheet manufacturing apparatus in the prior art. This apparatus includes a large diameter (diameter 700 mm) cooling roll (No. 1 roll 1), a touch roll 10 (diameter 350 mm), and a small diameter (diameter 350 mm) No. 1 roll. Two cooling rolls 2 are provided.

  In the sheet manufacturing by this apparatus, the sheet-shaped molten resin extruded from the flat die 11 is rotated in the counterclockwise direction. No. 1 when 1 roll 1 is contacted and cooling is started. Resin No. 1 with a touch roll 10 that rotates clockwise at the same peripheral speed as one roll. Securely adhere to one roll. No. After cooling and solidification of the sheet-shaped resin proceeded on one roll, the sheet was No. No. 1 was peeled off, The sheet 2 is brought into contact with the roll, and the sheet back surface (the surface opposite to the surface in contact with the No. 1 roll) is cooled.

  About the sheet manufacturing apparatus which has this structure, the temperature analysis of the sheet | seat cooling process was performed by simulation. FIG. 4 shows the result.

  As assumed manufacturing conditions, the temperature of the molten polypropylene extruded from the flat die 11 is set to 250 ° C., which is common in the field, and all the rolls are rotated at a peripheral speed of 20 m / min. The set temperature of 1 roll 1 is 30 ° C. The set temperature of the two rolls 2 was 15 ° C., and a sheet 12 having a thickness of 0.6 mm was obtained.

  As shown in FIG. The temperature of the sheet surface in contact with the surface of one roll not only falls below the melting point, but also reaches the crystallization temperature and further decreases. This temperature drops rapidly at first and falls below the melting point, but since the cooling rate gradually becomes slow, it cannot be said that the cooling rate is fast when passing through the crystallization temperature. No. The temperature of the back surface of the sheet not in contact with the roll 1 gradually decreased, and the sheet was no. 2 rolls, no. The result is below the crystallization temperature on 2 rolls. Furthermore, the temperature of the sheet intermediate surface also gradually decreases near the crystallization temperature.

  In such a temperature change situation, it is estimated that crystal growth proceeds particularly from the sheet surface (No. 1 roll contact surface) to the vicinity of the sheet intermediate surface.

  The region where the cooling time is about 3.5 to 3.8 seconds is No. 1 roll and no. This is the area where the sheet floats between the two rolls.

[Example 1]
With respect to the sheet manufacturing apparatus having the configuration shown in FIG. 1, the temperature analysis of the sheet cooling process was performed by simulation. FIG. 5 shows the result.

  As assumed manufacturing conditions, all rolls rotate at a peripheral speed of 20 m / min. 1 The set temperature of the cooling roll 1 is 55 ° C. The set temperature of 2 roll 2 is 30 ° C. 3 roll 3 and No. 3 The set temperatures of the four rolls 4 were all 20 ° C., and a sheet 12 having a thickness of 0.6 mm was obtained.

  From FIG. 5, it can be seen that the sheet is quickly and sequentially conveyed to each cooling roll, and the entire sheet is cooled and solidified while the front and back surfaces are alternately cooled. The temperature of the sheet surface rises once when the sheet surface is cooled by contacting with the surface of a certain cooling roll and then transferred to the next cooling roll and shifted to the opposite roll surface (side not contacting the cooling roll). On the other hand, the cooling of the sheet intermediate surface progressed at a substantially constant rate. The result is below the crystallization temperature on 3 rolls.

  The first cooling roll, i.e. Although the temperature of the sheet surface (the surface in contact with the No. 1 roll) becomes less than the melting point due to cooling by 1 roll 1, it does not reach the crystallization temperature. No. Although the temperature of the sheet back surface (the surface not in contact with the No. 1 roll) is lower than the crystallization temperature due to the cooling by the 2 roll 2, the cooling rate at that time is relatively fast. No. Although the sheet surface (the surface that was in contact with the No. 1 roll) is below the crystallization temperature due to cooling by the 3 roll 3, the cooling rate at that time is also relatively fast.

  In the case of such a temperature change, it is presumed that the progress of crystal growth is suppressed, and thus the obtained polypropylene resin sheet is excellent in transparency.

[Example 2]
FIG. 6 shows an outline of another example of a sheet manufacturing apparatus that can implement the present invention. In the apparatus configuration of Example 1, there is no sheet floating region between adjacent cooling rolls. On the other hand, in this apparatus, no. Each of the second and subsequent rolls is set a little away from the immediately preceding roll to provide a sheet floating region between the rolls. Specifically, the sheets float in the sections of 111.4 mm, 119.9 mm, and 87.9 mm, respectively, from the upstream side. Except for this point, the apparatus configuration of this example is the same as the apparatus configuration of the first embodiment.

  About the sheet manufacturing apparatus which has this structure, the prediction analysis of the sheet | seat temperature was implemented by the same manufacturing conditions as Example 1 by simulation. The result is shown in FIG.

  From this figure, the cooling rate of the sheet slows down in the floating region after the roll is peeled (temperature rise is seen in some cases), and the cooling near the crystallization temperature is slightly slower than in Example 1. Yes. From the viewpoint of the transparency of the sheet, the faster the cooling near the crystallization temperature, the better. Therefore, although it is ideal that there is no sheet floating region between the rolls, this example is superior to Comparative Example 1 in terms of transparency. is there.

  Further, in the roll layout of this example, the floating distance between rolls is more than 1/10 of the circumference of the immediately preceding roll (the float distance between the rolls with respect to the circumference of the immediately preceding roll is 0.12, 0 in order from the upstream side. .13, 0.11), however, the temperature of the sheet intermediate surface is gradually approaching a certain value in each floating region. Therefore, it can be said that the floating distance between rolls is preferably 1/10 or less of the immediately preceding roll circumferential length in order to increase the cooling efficiency even when a floating region is unavoidably generated due to the apparatus configuration. For example, no. 2 rolls and No. The floating distance of the sheet between the three rolls is no. No. 2 so as to be 1/10 or less (94.2 mm or less) of the circumferential length of 2 rolls (942 mm). It is preferable to arrange 3 rolls.

[Comparative Example 2]
Based on the sheet manufacturing conditions used in Comparative Example 1, a sheet was actually manufactured to obtain a sheet sample. The set temperature of the roll is the same as the analysis condition of Comparative Example 1, except that the peripheral speed of the cooling roll is No. One roll is 20.00 m / min. Two rolls were 20.20 m / min.

Example 3
Based on the sheet manufacturing conditions used in Example 1, a sheet was actually manufactured. However, the peripheral speed of the small-diameter continuous cooling roll is No. One roll is 20.00 m / min. 2 rolls are 19.99 m / min. 3 rolls were 19.97 m / min. Four rolls were 19.93 m / min.

Example 4
Based on the sheet manufacturing conditions used in Example 2, a sheet was actually manufactured. However, the peripheral speed of the small-diameter continuous cooling roll was the same as in Example 3.

  FIG. 8 is a photograph of a white cloud floating in the blue sky through the produced polypropylene resin sheet. The sheet of Comparative Example 2 is arranged in the left half of the figure, and the sheet of Example 3 is arranged in the right half. Has been. From this figure, it can be seen that the transparency of the right sheet is clearly high.

  Table 1 shows the average thickness and haze in the full width direction of the sheets obtained in Examples 3 and 4 and Comparative Example 2. In Comparative Example 2, it was difficult to lower the haze to 15% or less. However, the haze of the sheet produced according to the present invention was about 11%, and further about 8% was achieved. Therefore, it has been found that the sheet produced according to the present invention is superior in transparency to the sheet produced by the prior art.

Example 5
A comparatively thin sheet having a thickness of 0.3 mm was manufactured in the same manner as in Example 3 except that the amount of resin discharged from the flat die was halved. It was possible to obtain a sheet having a very high haze of 6.5%.

  As described above, in the embodiment, first, by predicting the sheet surface temperature by simulation, No. No. 1 of sheet-like raw material is cooled by one roll. The conditions for producing a polypropylene resin sheet were estimated such that the surface in contact with one roll reached a temperature lower than the melting point but ended before reaching the crystallization temperature. And the polypropylene resin sheet was actually manufactured on the manufacturing conditions, and it confirmed that a highly transparent sheet | seat was obtained.

1 No. 1 roll 2 No. 2 roll 3 No. 3 roll 4 no. 4 roll 10 touch roll 11 flat die 12 polypropylene resin sheet

Claims (7)

In a method for producing a polypropylene resin sheet, a polypropylene resin sheet is produced using a polypropylene resin as a single raw material by a melt extrusion method.
A discharge step of discharging the molten raw material into a sheet form from the flat die;
And cooling the discharged sheet-like raw material by sequentially contacting a plurality of cooling rolls,
In the cooling step,
The plurality of cooling rolls are rotating with their rotation directions reversed in order,
Production of a polypropylene resin sheet characterized in that the cooling by the first cooling roll is finished before the surface of the sheet-like raw material in contact with the first cooling roll reaches a temperature below the melting point but does not reach the crystallization temperature. Method. The set temperature of the first cooling roll is a temperature that is 50 ° C. or more lower than the crystallization temperature of the raw material,
The set temperature of the second and subsequent cooling rolls is less than or equal to the set temperature of the roll immediately before each roll,
The method of claim 1.   The method according to claim 1 or 2, wherein the circumferential speed of the plurality of cooling rolls is set to be approximately equal so that the sheet-like raw material does not slide with respect to the cooling roll.   The method according to any one of claims 1 to 3, wherein the diameters of the cooling rolls are all the same.   The method according to any one of claims 1 to 4, wherein a sheet floating distance between the rolls is 1/10 or less of a circumference of the immediately preceding roll.   The method as described in any one of Claims 1-5 whose sheet | seats manufactured are 100 micrometers or more and 1000 micrometers or less in average thickness of the full width direction, and the minimum value of a haze is 10 or less.   The method as described in any one of Claims 1-6 whose sheet | seats manufactured are 100 micrometers or more and 500 micrometers or less in average thickness of the full width direction, and the minimum value of haze is 8 or less.