JP2005329617A - Method for producing thermoformed article of styrene resin, molding, and container - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a thermoformed article of a styrene resin which is excellent in oil resistance and appearance and maintains a balance with strength, its molding, and its container. <P>SOLUTION: In the method for producing the molding obtained by thermoforming a sheet of the styrene resin, the molding after the thermoforming is radiation-heated by a heater of the maximum energy wavelength being in a range of a wavelength zone where the absorption wavelength of its used resin becomes maximum ±1 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a thermoformed product of a styrenic resin having excellent oil resistance and appearance, and having a good balance between strength, and a molded body and container thereof.

  Styrenic resin thermoformed containers are used in a variety of applications, and are widely used as molded products, especially lightweight containers for food packaging, because they are transparent, rigid, and relatively inexpensive. In particular, a thermoformed product using a biaxially stretched sheet has high rigidity. In addition, these containers are required to have oil resistance and heat resistance due to their use, and as a method for imparting their functions, a method using a copolymer of styrene and acrylic acid, methacrylic acid or maleic anhydride is generally used. (For example, see Patent Document 1). However, since the methods using these copolymers are fragile in material compared to molded products made of polystyrene alone, it is necessary to increase the orientation relaxation stress when creating biaxially stretched sheets. Residual strain tends to remain in the product, which contributes to a decrease in oil resistance. In order to solve this problem and the like, a biaxially stretched laminated sheet having a sheet having a multilayer structure, a surface layer having a copolymer of styrene and acrylic acid, methacrylic acid or maleic anhydride, and a middle layer having a polystyrene layer Has been proposed (see, for example, Patent Documents 2 and 3). However, this method has a problem in the appearance of the sheet because the melt fracture at the layer interface tends to occur during the feed block T-die multilayer coextrusion method at the time of producing the sheet. The melt fracture referred to here occurs at the interface between the middle layer and the surface layer in the T-die during coextrusion. Because the temperature at the T-die is constant, the middle polystyrene layer has a lower viscosity than the surface copolymer layer at the same temperature, and due to this viscosity difference, the flow does not go smoothly and the striped appearance is poor. Appear as.

JP-A-55-56112 JP-A-2-239933 JP 7-156342 A

  The present invention has been made in view of such circumstances, and has a method for producing a thermoformed product of a styrene-based resin having excellent oil resistance and appearance, and having a good balance between heat resistance and strength, and a molded body. The object is to provide a container.

  That is, the present invention relates to a method for producing a molded product obtained by thermoforming a sheet of a styrene resin, and the maximum energy within the wavelength band ± 1 μm in which the absorption wavelength of the resin used is maximized. When the heating time is 10 seconds or less by the heater of the wavelength and the longitudinal orientation relaxation stress is σM and the horizontal orientation relaxation stress is σT, the average thickness is 0. It is a manufacturing method of the single layer thermoformed product characterized by being 2 mm or more.

  In the production method described above, the styrenic resin used for thermoforming is a copolymer of styrene and at least one monomer selected from acrylic acid, methacrylic acid, and maleic anhydride. A method for producing a single-layer thermoformed product. A molded product and a container obtained by the production method described above.

  The present invention provides a method and a container for producing a styrene-based container having a good balance between oil resistance and strength, and the container obtained is particularly suitable for applications such as lightweight containers for food packaging. .

The present invention is described in detail below.
First, in the method for producing a thermoformed product of a styrene resin in the present invention, a molded product after thermoforming is radiated by a heater having the same maximum energy wavelength as the maximum wavelength band in which the absorption wavelength of the resin used is within a range of ± 1 μm. Must be heated. Here, as a method for measuring the intensity of the absorption wavelength of the resin used, an infrared absorption spectrum may be measured by a known infrared spectrophotometer, and the method is shown in JIS-K0117. In addition, the maximum energy wavelength of the heater can be obtained from the temperature of the heating element of the heater according to Wien's law (for example, see Non-Patent Document 1), and the wavelength where the absorption wavelength of the resin used is 100% is the maximum energy wavelength. If the molded product is radiantly heated by the heater, most of the energy will be absorbed on the surface of the molded product, and the surface will be selectively heated rather than the middle side in the sheet thickness direction of the molded product. . The heated portion can relieve distortion caused by deformation during sheet preparation and thermoforming, and the oil resistance on the surface of the molded product that is actually in contact with food or the like is improved. However, when both wavelengths exceed the range of ± 1 μm, the entire sheet thickness direction of the molded product is uniformly heated, and although the oil resistance is improved, the strength of the molded product is lowered.

Japan Society of Mechanical Engineers "Mechanical Engineering Handbook 5th revised edition" (1968) P11-10

Even if the maximum energy wavelength of the heater matches the wavelength at which the absorption wavelength of the resin used is the maximum, other wavelength components are also emitted from the heater, and heat conduction from the surface makes it possible to produce molded products for a long time. When heated, the entire thickness direction tends to be heated to some extent uniformly. Even when the thickness of the molded product is thin, selective heating of only the surface is difficult for the same reason. As a result of intensive studies by the inventors, the heating time is preferably 10 seconds or less, and the thickness of the molded product is preferably 0.2 mm or more.
Further, the heater used is preferably one having a sharp wavelength characteristic as much as possible, and in addition, one having a fast rising speed is preferable. Considering these and the absorption wavelength of polystyrene, it is preferable to use a carbon heater.

The sheet of styrene resin used for thermoforming is not particularly limited, but a biaxially stretched sheet is preferably used. In general, the purpose of stretching a styrene resin is to align molecular chains and increase strength. However, when stretched, the strength increases and the residual strain also increases, which contributes to a decrease in oil resistance.
By applying the manufacturing method of the present invention, the residual strain in the vicinity of the surface of the molded product that is actually in contact with food or the like is selectively reduced, so that the oil resistance is improved and at the same time the strength as a molded product is increased in the thickness direction. Can keep.

  In addition, in the styrene-based sheet used in the present invention, when a biaxially oriented sheet is used, assuming that the longitudinal orientation relaxation stress is σM and the lateral orientation relaxation stress is σT, | σM−σT | ≦ 0.2 It is preferable to satisfy [MPa]. The orientation relaxation stress here is measured using a test piece cut out from the sheet along the sheet extrusion direction (longitudinal direction) or the direction perpendicular to it (lateral direction), and can be measured according to ASTM D1504. . When the difference between the orientation relaxation stresses in the vertical direction and the horizontal direction is large, there is a tendency that the strength against tearing in one direction tends to be weak because the directionality of the sheet exists strongly. This is because the cracks tend to grow in a concentrated manner in a low strength, that is, low orientation direction, and therefore easily break.

  The production method of the present invention is also suitable for a material using a copolymer of styrene and acrylic acid, methacrylic acid or maleic anhydride as a raw material for the styrene resin sheet to be used. When these copolymers are used, they are more fragile than polystyrene alone due to their material properties. Therefore, it is necessary to increase the orientation relaxation stress when creating a biaxially stretched sheet. It tends to remain and contributes to a decrease in oil resistance. However, if the production method of the present invention is applied, the residual strain in the vicinity of the surface of the molded product that is actually in contact with food or the like is selectively reduced, so that the oil resistance is improved and the strength of the molded product can be maintained.

As an apparatus for obtaining the sheet used in the present invention, a conventional apparatus may be used. For example, by an extrusion molding method in which a resin is melt-kneaded by an extruder, extruded from a T-die in a flat shape, and passed through a roll. The sheet is formed, and the obtained sheet is stretched (biaxially stretched) in the longitudinal and lateral directions as necessary, for example, by a stretching method such as a tenter method, and cooled.
At this time, as a method for adjusting the temperature, there is a method in which water or oil refrigerant is circulated inside the roll and cooled by heat exchange during longitudinal stretching. It is preferable to use a sensor that automatically opens and closes a solenoid valve or the like depending on the difference from the set temperature when the temperature sensor senses the actual temperature. During transverse stretching, hot air heated by a heater is blown into the tenter to raise the ambient temperature, and by sensing the actual temperature of a temperature sensor such as a thermocouple, the heater is turned on / off depending on the difference from the set temperature. It is common to let them.
As a method for controlling the orientation relaxation stress, the orientation relaxation stress of the obtained sheet can be changed by changing the set temperature and the line speed at this time.

  Moreover, as a method of thermoforming, it can obtain using a commercially available general hot-plate pressure forming machine. The molding machine to be used is preferably of a type in which the time during which the sheet is pressed against the hot plate, the time during which the sheet is formed by compressed air, the time lag for switching from the sheet pressure welding to the pressure forming, the molding cycle, etc. can be set. reference).

The Society of Polymer Science “Plastic Processing Technology Handbook”, Nikkan Kogyo Shimbun (1995)

The production method of the present invention is applied to the thermoformed product obtained as described above, and it is not preferable to apply it at the stage of the sheet or at the time of thermoforming. This is because if the surface of the surface heated is thermoformed, it will be fused to the hot plate or water drop-like irregularities called rain drops are likely to occur.

  The sheet used in the present invention is an antioxidant, a lubricant, a mold release material, a plasticizer, a pigment, as necessary, as long as it does not impair the purpose of the present invention, at the time of melt kneading or raw material production for forming a sheet. Known additives such as dyes, foaming agents, foam core materials, inorganic fillers, antistatic agents, and light diffusing agents can be contained.

  Hereinafter, the embodiment of the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited thereto. The evaluation and test equipment used are shown below.

1. The infrared absorption spectrum of each resin used was measured in advance below.
Apparatus name: IR spectrophotometer IR-810 manufactured by JASCO Corporation
Test method: According to JIS-K0117. The test piece was melted at 230 ° C. in a press molding machine to obtain a film having a thickness of about 50 μm, and then left in a constant temperature and humidity chamber of 23 ° C. × 50% for 24 hours to adjust the state.

2.2 Extrusion / stretching equipment used for preparing the axially stretched sheet is as follows.
Extruder: PLASTIC EXTRUDER NVC65 manufactured by Nakatani Machinery Co., Ltd.
Longitudinal stretching machine: 400 type longitudinal stretching roll unit manufactured by Tanabe Plastics Machinery Co., Ltd. Horizontal stretching machine: SK-WE A88-027 manufactured by Kobayashi Machinery Co., Ltd.

3. Using the obtained biaxially stretched sheet, according to ASTM D1504, σM and σT, which are the maximum values of orientation relaxation stress in the sheet extrusion direction (longitudinal direction) and the direction perpendicular to the extrusion direction (transverse direction), were measured. The test piece was cut into 20 mm × 135 mm from the sheet, and the average value measured five times was adopted as the measurement value.

4). Using the obtained biaxially stretched sheet, hot plate compression molding was performed under the following conditions to obtain a container molded product.
(1) Mold:
Top: 90 x 170 [mm]
Height: 50 [mm]
Bottom: 120 x 200 [mm]
(2) Molding conditions:
Molding machine: PK400 manufactured by Kansai Automatic Molding Machine
Molding temperature: Changed depending on the resin used Pressure welding pressure delay: 0.8 [sec]
Pressure welding vacuum delay: 1 [sec]
Pressure welding time: 4 [sec]
Molding pressure time 3.5 [sec]

5). Thickness was measured using Mitutoyo's Digimatic Micrometer BMD-25DM at 8 arbitrary positions on the top surface of the obtained container molded product and 4 arbitrary positions on the side surface. Was the thickness of the molded product.

6). The obtained container molded product was heated from a position 10 cm above the surface of the molded product using a medium wavelength carbon heater CZB4600 / 600G manufactured by Heraeus. At this time, in addition to changing the heating time, the temperature of the heating element was changed by adjusting the current, and the maximum energy wavelength was calculated from the displacement side of Vienna.
Vienna displacement side: λ = 2897 / T (λ: maximum energy wavelength [μm], T: heating element temperature [° C.])

7). As an evaluation of oil resistance, a 50 × 30 [mm] test piece was collected from the center of the top surface of the obtained container molded product, taken out after being immersed in coconut oil at 75 ° C. for 5 minutes, and visually checked for white turbidity. Observed.
○: Good △: Some cloudiness ×: Some cloudiness

8). As an evaluation of strength, a weight is dropped on the center of the top surface of the obtained container molded product, and energy at which cracking occurs ([J] = weight height [m] × weight weight [kg]) is measured. Is the drop weight strength. The portion of the weight that contacts the molded product is the same, and the weight can be changed. The tip of the weight that is in contact with the weight is a hemisphere having a diameter of 15 mm. Increase the height in 1cm increments until cracks occur and conduct a preliminary test until the sheet breaks. If the sheet breaks, repeat the test to lower the drop height by 1 cm, and if it does not break, increase the drop height by 1 cm. The average value measured 10 times was adopted as the measurement value.
○: 30 [J] or more Δ: 20 [J] or more and less than 30 [J] ×: Less than 20 [J]

9. Appearance was evaluated by visually observing the appearance of the obtained container molded product, ○ indicating that there was almost no melt fracture, Δ indicating that there was a slight melt fracture, and × indicating that the melt fracture was clearly recognized. Evaluation of sex.
10. As evaluation of heat resistance, the deformation of the container was visually observed after putting the obtained container molded article into a hot air dryer set at 110 ° C. for 10 minutes.
○: Deformation is not known or is minute Δ: Deformation is present, but the outer dimensions are not significantly changed ×: Deformation is large and dimensions are also changing

The resins used in the examples and comparative examples are shown below.
Resin A: HRM61C manufactured by Toyo Styrene Co., Ltd.
Resin B: To an autoclave having an internal volume of 210 L, 100 g of polyvinyl alcohol was added to 90 kg of pure water and stirred. Next, 48.5 kg of styrene, 1.5 kg of methacrylic acid, 55 g of t-butylperoxy-2-ethylhexanoate as a polymerization initiator, 10 g of ethyl-3,3-di (t-butylperoxy) butyrate, a chain transfer agent Was charged with 45 g of α-methylstyrene dimer, and the temperature was raised to 112 ° C. for 6 hours, and then held at 132 ° C. for 4.5 hours for polymerization. The obtained beads were washed, dehydrated and dried, and then extruded to obtain a pellet-shaped resin.
Resin C: The amount of styrene charged was 46 kg, and the amount of methacrylic acid charged was 4 kg. Others were the same as Resin B.
As a result of measuring the infrared absorption spectrum of each resin, all resins showed 100% absorption at 3.43 μm.

[Example 1]
Resin A is coextruded with a T-die at 230 ° C in an extruder at a set temperature of 105 ° C at a set temperature of 105 ° C in the flow direction 2.2 times, and then at a set temperature of 105 ° C at a set temperature of 105 ° C in the width direction. To 2.2 times to obtain a biaxially stretched sheet. The line speed at this time is 8 m / min. When the obtained biaxially stretched sheet was measured for σM and σT, which are the maximum values of orientation relaxation stress, by the above method 3, σM = 0.57 MPa and σT = 0.50 MPa.
The obtained biaxially stretched sheet was hot-plate molded into a container at 125 ° C. by the method 4 described above. After the thickness of the molded product was measured by the method 5 in the obtained container molded product, the molded product was heated for 10 seconds by the method 6 above. The heating element temperature at that time is adjusted to 570 ° C., and the maximum energy wavelength is about 3.4 μm according to the Wien's displacement law. Thereafter, the oil resistance test was evaluated by the above method 7, and the falling weight strength was evaluated by the above method 8.
Table 1 shows the above results. It can be seen that the oil resistance test and drop weight strength are good and balanced.

[Examples 2 to 7] and [Comparative Examples 1 to 5]
Implemented in the same manner as in Example 1 except that the resin used, the set temperature during longitudinal stretching, the set temperature during transverse stretching, the hot plate molding temperature, and the part heating conditions were appropriately changed as shown in Table 2. did. In Comparative Example 4, the container molded product is not heated. The above results are summarized in Table 1. It can be seen that all the examples are good and balanced in oil resistance, drop weight strength, appearance, and heat resistance.
[Comparative Example 6]
The above procedure was carried out in the same manner except that PLASTIC EXTRUDER NVC65 manufactured by Nakatani Machinery Co., Ltd. was used as the middle layer extruder, and PLASTIC EXTRUDER VSK40 manufactured by Nakatani Machinery Co., Ltd. was used as the surface layer extruder. The results are shown in Table 1. It can be seen that the appearance is poor in multiple layers.

Sheet with melt fracture

Claims (5)

In the manufacturing method of a molded product formed by thermoforming a sheet of styrene resin, the molded product after thermoforming is heated by a heater having a maximum energy wavelength that falls within a wavelength band ± 1 μm in which the absorption wavelength of the resin used is maximum. When the time is 10 seconds or less and radiation heating is performed, and the longitudinal orientation relaxation stress is σM and the horizontal orientation relaxation stress is σT, the following equation 1 is satisfied and the average thickness is 0.2 mm or more. A method for producing a single-layer thermoformed product.
2. The method for producing a single-layer thermoformed product according to claim 1, wherein the styrenic resin sheet used for thermoforming is a biaxially stretched sheet. The styrenic resin used for thermoforming is a copolymer composed of at least one monomer out of acrylic acid, methacrylic acid, and maleic anhydride and styrene. A method for producing a single-layer thermoformed product. A styrene-based resin molded article obtained by the production method according to any one of claims 1 to 3. A styrene-based resin container obtained by the production method according to any one of claims 1 to 3.