JPH10305475A - Forming method of thermoplastic resin sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a thermoplastic resin sheet with fine appearance by preventing fine air bubbles from being mixed in a thermoplastic resin sheet in an extrusion forming method that becomes a leading process in a forming method of a thermoplastic rein sheet. SOLUTION: In forming thermoplastic resin into a sheet by an extrusion machine, molten resin is stayed at the screw tip end of an extrusion machine for feeding out molten resin to a sheet extrusion die, and molten resin is fed out by controlling a feeding quantity of molten thermoplastic resin by the number of screw rotation of the extrusion machine for producing a resin pressure of 10-200 kgf/cm<2> at that time, thereby forming into a sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a thermoplastic resin sheet having an excellent appearance.

[0002]

2. Description of the Related Art Sheet molding of a thermoplastic resin generally includes a cast molding method, a compression molding method, and an extrusion molding method. The cast molding method is a method of obtaining a sheet by polymerizing and solidifying a thermoplastic resin monomer or prepolymer in a sheet-shaped mold such as a glass cell, and the compression molding method is to heat and melt the thermoplastic resin to form a sheet. After compressing
This is a method of obtaining a sheet by cooling and solidifying.
The extrusion molding method is a method in which a thermoplastic resin is heated and melted in a sheet extruder, extruded into a sheet shape from a die at the tip of the extruder, extruded, and cooled and solidified with three to four rolls to obtain a sheet. . Among these, the extrusion molding method is becoming mainstream because it is possible to form a large number of sheets of the same quality continuously and at low cost.

A major problem in extrusion molding of a thermoplastic resin sheet is the incorporation of air bubbles into the sheet. Bubbles are defective phenomena generated by heating foaming of moisture contained in a resin, entrainment of air in a molding process, decomposition products of a resin, and the like. Therefore, in the above-mentioned molding method, a deaeration step is generally provided to prevent air bubbles from being mixed. In casting and compression molding, degassing is performed by repeating pressurization and depressurization when the thermoplastic resin is in the molten state, and in these degassing processes, air bubbles are almost certainly avoided. . On the other hand, in the extrusion molding method, which is becoming mainstream in sheet molding, a deaeration port is provided in a sheet extruder and deaeration is performed by reducing the pressure.
At present, the mixing of minute air bubbles of μm or less continues.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermoplastic resin sheet having a good appearance by preventing minute air bubbles from being mixed into the thermoplastic resin sheet in an extrusion molding method.

[0005]

Means for Solving the Problems In the deaeration by the casting molding method or the compression molding method, bubbles are completely removed by repeating pressurization and depressurization at the time of hot melting. This is performed only at the time when the resin passes through the deaeration port, and the air bubbles that cannot be removed at that time are sealed in the molten resin. In addition, in extrusion molding, a mechanism and molding conditions that do not allow molten resin to stay in order to prevent thermal deterioration of the thermoplastic resin are adopted, and the mixed air bubbles do not stay in the extruder, but in the extruded product It will be enclosed as it is.

As a result of intensive studies, the present inventor has found that the molten resin containing minute air bubbles that are not completely defoamed at the deaeration port is retained at the tip of the screw, and the amount of the molten resin fed from the screw is adjusted to make the appropriate adjustment. By applying pressure, it is found that the molten resin containing more fine bubbles flows backward along the inner wall of the extruder cylinder as a backward flow, and defoams from the degassing port which has been decompressed, thereby completing the present invention. Reached.

That is, according to the present invention, when a thermoplastic resin is formed into a sheet shape by an extruder, the molten thermoplastic resin is retained at the tip of an extruder screw for feeding the molten resin to a sheet extrusion die, and the resin pressure at that time is 10 times. ~ 2
The thermoplastic resin sheet is characterized in that the molten resin is fed to a sheet extrusion die and formed into a sheet shape by adjusting the feed rate of the molten thermoplastic resin by the number of rotations of an extruder screw so as to be 00 kgf / cm ² . It is a molding method.

Hereinafter, the present invention will be described in detail. As the thermoplastic resin used in the present invention, for example, acrylic resin, polycarbonate resin, styrene resin, polyolefin resin, polyphenylene ether resin, vinyl chloride resin, vinylidene chloride resin, fluorine resin, polyester resin, polyurethane Resins, polyacetal resins, silicone resins, polybutadiene resins, polyisoprene resins, polyamide resins, polyamideimide resins, polyetherester resins, and the like. One or more of these resins may be used, and a copolymer or a mixture may be used.

If necessary, these thermoplastic resins may contain ultraviolet absorbers, antioxidants, antistatic agents, heat stabilizers, light stabilizers, flame retardants, mold release agents, surfactants, dispersants, lubricants, Blocking agent, light diffusing agent, coloring agent, inorganic filler,
Various additives such as a glass fiber, a cross-linking agent, a plasticizer, and an organic cross-linked body can be blended. With the extrusion molding method of forming a thermoplastic resin into a sheet, the thermoplastic resin is heated and melted in a sheet extruder, fed into a die installed at the tip of the extruder with a screw, expanded in the die, and extruded into a sheet shape This is a molding method in which a plastic resin is guided into three to four rolls, and the thickness is adjusted, the surface is formed, and the sheet is cooled and solidified by the rolls to obtain a sheet.

The sheet extruder used here may be either a single-screw or multi-screw extruder. The extrusion screw for heating and melting the thermoplastic resin is also not particularly limited, and may be any of a full flight screw, a screw with various mixing mechanisms, a multi-stage screw, and an arbitrary screw design. The installation position of the deaeration port is also arbitrary, and there may be two or more ports.

The die for widening the molten thermoplastic resin into a sheet shape is not particularly limited, and examples thereof include a fish tail type T die, a coat hanger type T die, a straight manifold type T die, and a laminated die. Any of these can be selected. The thickness of the sheet obtained using this method is usually 0.25 to 20 mm,
Preferably it is 0.5 to 15 mm.

[0012] The tip of the sheet extruder and the die are connected by a single tube or the like having a smooth inner surface. A screen for removing foreign substances and gels in the molten resin, and a gear pump for quantitatively feeding the molten resin to the die. Is preferably connected therebetween, and an auxiliary mixing device such as a static mixer for further kneading the molten resin may be connected.

As a method of retaining the molten thermoplastic resin at the tip of the sheet extruder screw, a method of narrowing the inner diameter of a single pipe attached to the tip of the sheet extruder to narrow the flow path, and a method of comparing the discharge amount from the gear pump are described. It can be performed by a method such as increasing the discharge amount from the sheet extruder. The present invention is characterized in that pressure is applied to the retained molten thermoplastic resin, and the retained resin pressure is 10 to 200 kgf /
cm ² is preferred. More preferably, 30 to 180 kgf
/ Cm ² . If the retained resin pressure is less than 10 kgf / cm ^2, the air bubbles that have passed through the deaeration port cannot be accumulated in the retained portion, and are sent to the die together with the molten resin, and the air bubbles are mixed into the sheet. Therefore, the target sheet cannot be obtained. Conversely, the higher the pressure applied to the stagnant portion, the better for deaeration, but
If it exceeds kgf / cm ² , the molten resin accelerates thermal deterioration under high temperature and high pressure, and causes problems such as coloring and decomposition.

The method of applying pressure to the retained molten thermoplastic resin can be arbitrarily changed by adjusting the rotation speed of the screw of the sheet extruder and adjusting the amount of the molten thermoplastic resin sent out by the screw. Most of the air bubbles that have passed through the deaeration port of the extruder during the extrusion molding of the sheet and are mixed in the sheet are microscopic bubbles having a diameter of 50 μm or less as a result of microscopic observation. When the molding method of the present invention is used, it is possible to prevent the entry of the fine bubbles having a size of 50 μm or less, and to obtain a thermoplastic resin sheet having a good appearance.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The effects of the present invention will be described more specifically with reference to examples and comparative examples below, but the present invention is not limited to these examples. 1. Method for measuring and judging microbubbles in sheet 1) 20cm * randomly poured out from molded sheet
A 20 cm sheet and a total of three sheets can be measured with a transmission laser type visual inspection device with a foreign matter counter (LS-LS manufactured by Keyence Corporation).
3033), the number of foreign substances is counted, and at the same time, the location of the foreign substances is specified.

2) Next, using a transmission optical microscope (BH3-MJL9 manufactured by Olympus Corporation), the specified foreign matter is observed, and the size of bubbles is measured. 3) Judgment method The arithmetic results of the three measurement results were arithmetically averaged, and 4 or less microbubbles having a size of 50 μm or less were determined to be acceptable.

4 or less / sheet: No practical problems such as appearance and physical properties Note that bubbles exceeding 50 μm or more are also measured by the above-described evaluation method, but such bubbles are easily found visually. Therefore, it is not usually commercialized as a poor appearance product because it causes a decrease in strength and the like.

[0018]

Example 1 Screw diameter 5 set to 260 ° C.
Acrylic resin (Delpet LP-1 manufactured by Asahi Kasei Kogyo Co., Ltd.) is heated and melted by a sheet extruder with a degassing opening of 0 mm, conveyed to a gear pump connected to the extruder tip by screw rotation, and further coated by a gear pump. The molten acrylic resin was quantitatively conveyed to a hanger-type T-die, the molten resin was widened in the die, and extruded into a sheet.
The molten acrylic resin extruded into a sheet is guided to three mirror-finished rolls of 200 mm in diameter, adjusted to a sheet thickness of 2 mm, transferred to a mirror surface, cooled and solidified, and then trimmed to a width of 290 mm. Thus, an intended acrylic resin sheet was obtained. One deaeration port is provided at a position of about 200 mm from the tip of the sheet extruder, and the pressure is reduced by a vacuum pump to deaerate. In extrusion molding, with the rotation speed of the gear pump kept constant, the rotation speed of the extruder screw is gradually increased, the discharge amount from the extruder exceeds the discharge amount from the gear pump, and the acrylic resin in the molten state at the tip of the screw Was adjusted so that a resin pool could be formed. Further, the rotational speed of the screw was finely adjusted, and the feeding amount of the molten resin was adjusted so that the resin pressure of the retained acrylic resin became 50 kgf / cm ² , and molding was performed. The acrylic resin sheet thus formed was found to have only 2 microbubbles of 50 μm or less per sheet, and a sheet having a good appearance was obtained.

[0019]

Embodiment 2 With the rotation speed of the gear pump kept constant,
The procedure was performed in the same manner as in Example 1 except that the rotation speed of the extruder screw was adjusted and the feeding amount of the molten resin was adjusted so that the resin pressure of the retained acrylic resin became 100 kgf / cm ² . The resulting acrylic resin sheet had one microbubble of 50 μm or less per sheet as in Example 1.

[0020]

Embodiment 3 With the rotation speed of the gear pump kept constant,
The procedure was performed in the same manner as in Example 1 except that the rotation speed of the extruder screw was adjusted and the feeding amount of the molten resin was adjusted so that the resin pressure of the retained acrylic resin became 150 kgf / cm ² . The resulting acrylic resin sheet had one microbubble of 50 μm or less per sheet as in Example 1.

[0021]

Embodiment 4 With the rotation speed of the gear pump kept constant,
The procedure was performed in the same manner as in Example 1 except that the number of rotations of the extruder screw was adjusted and the feeding amount of the molten resin was adjusted so that the resin pressure of the retained acrylic resin became 200 kgf / cm ² . As in the case of Example 1, the resulting acrylic resin sheet contained only one microbubble of 50 μm or less per sheet, but slightly discolored due to thermal deterioration.

[0022]

[Comparative Example 1] With the rotation speed of the gear pump kept constant,
By adjusting the number of rotations of the extruder screw so that the discharge amount of the gear pump and the discharge amount from the extruder become almost the same, resin accumulation at the screw tip is not possible, and the acrylic resin in the molten state smoothly flows from the extruder to the gear pump, Extruded to die. At this time, the resin pressure of the acrylic resin flowing at the tip of the screw was about 5 kgf / cm ² . The conventional acrylic resin sheet was molded under such conditions, and the obtained acrylic resin sheet certainly appeared at first glance to have a good appearance and no problem. However, when observed in detail, it was 50 μm or less, particularly 30 to 40 μm. It was found that there were as many as 20 microbubbles per sheet, and the bubbles could not be completely removed only by degassing.

[0023]

[Comparative Example 2] With the rotation speed of the gear pump kept constant,
The procedure was performed in the same manner as in Example 1 except that the rotation speed of the extruder screw was adjusted and the feeding amount of the molten resin was adjusted so that the resin pressure of the retained acrylic resin became 300 kgf / cm ² . In the acrylic resin sheet obtained in this way,
The number of microbubbles having a size of 50 μm or less was one per sheet, but the discoloration was large due to thermal degradation, and a desired sheet having a good appearance was not obtained.

[0024]

According to the present invention, in the extrusion molding method, which is becoming the mainstream of the thermoplastic resin sheet molding method, it is possible to prevent fine bubbles from being mixed into the thermoplastic resin sheet and to provide a thermoplastic resin sheet having a good appearance. be able to.

Claims (2)

[Claims] When a thermoplastic resin is molded by an extruder, the molten thermoplastic resin is retained at an extruder screw tip for feeding the molten resin to an extrusion die, and the resin pressure at that time becomes 10 to 200 kgf / cm ² . A method for molding a thermoplastic resin, comprising: adjusting the feed rate of the molten thermoplastic resin by the number of revolutions of an extruder screw, sending the molten resin to an extrusion die, and molding. 2. The extruded product is a sheet.
the method of.