JPS6222834A - Polystyrene foam having excellent post-expansion moldability under heating - Google Patents

Abstract

PURPOSE:To provide the titled form which has a wide heat molding condition range, excellent post-expansion moldability under heating and good appearance and strength, by specifying a molecular weight distribution. CONSTITUTION:A PS resin base having a weight-average MW (Mw) of 100,000-700,000 as measured by gel permeation chromatography and a molecular weight distribution (Mw/Mn) of 3.0 or above [in terms of a ratio of the Mw to the number-average MW (Mn)], composed of at least 50wt% arom. vinyl compd. such as styrene or alpha-methylstyrene and not more than 50wt% other vinyl compd. such as acrylonitrile or methyl acrylate, is melt-kneaded with a blowing agent such as propane or dichloromethane. The mixture is extruded through a T-die or a circular die to obtain a sheet-form PS foam having a thickness of 0.2-5mm and a density of 0.05-0.3g/cc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polystyrene foam having excellent secondary heat foaming moldability. More specifically, the present invention relates to a polystyrene foam that has excellent secondary foaming moldability under a wide range of molding and heating conditions.

[Prior art/problems to be solved by the invention] Polystyrene foam undergoes secondary foaming by heating and can be easily molded into a desired shape, and the molded product is lightweight, has excellent mechanical strength, and is beautiful. Because it is highly hydrophobic, sanitary, and has excellent heat retention and insulation properties, it is formed into boxes, plates, cups, etc., and is often used in various food packaging materials and simple containers.

However, if the heating conditions during the secondary foaming are varied, a molded article with a poorly defined shape or a molded article with a keloid-like surface is produced, making it difficult to manage the heating conditions during molding. Particularly, in the production of a heated secondary foam molded article, in which a sheet-like polystyrene foam is heated in a heating furnace to foam, then taken out from the heating furnace and molded in a mold.

Due to the unevenness of the temperature inside the heating furnace and fluctuations in the temperature inside the furnace due to the outside temperature, part or all of the sheet may be underheated.1. Phenomena such as the sheet being torn during molding, poor shapeability, excessive heating resulting in a keloid-like surface, and reduction in the strength and thickness of the molded product are likely to occur. Therefore, it is a polystyrene foam that has good shapeability even when heating is insufficient, and does not easily become keloid-like on the surface even when heated excessively, and has excellent secondary foaming moldability over a wide range of forming heating conditions. development is desired.

In order to prevent the surface from becoming keloid-like due to excessive heating, it is necessary to increase the molecular weight of the base polystyrene resin, apply a film to the sheet surface of sheet-like polystyrene foam, or add a layer with high resin density, the so-called skin. There are methods such as forming layers, but such methods make it difficult to form a well-defined foam, and tend to cause sheet tearing when molding deep molded products.

On the other hand, in order to obtain a foam with a good shape, there are methods such as lowering the molecular weight of the base polystyrene resin or adding plasticizers, lubricants, etc. to the base polystyrene resin. Keloids on the surface that occur when overheating become more likely to appear, and the strength of the molded product also decreases. Furthermore, if the foam cell diameter is increased and the cell film thickness is increased, keloids are less likely to appear on the surface and the shapeability becomes better, but the appearance of the surface deteriorates.

In view of the above circumstances, the present invention has been developed to prevent the formation of keloids on the surface even if the heating conditions during the secondary foaming process are changed, and to form a deep molded product with good shapeability and good strength and appearance. This was done in order to obtain a polystyrene foam whose molding and heating conditions can be changed over a wide range.

Means for Solving Problem E] The present invention is based on the discovery that the above objectives can be achieved by using a resin with a wide molecular weight distribution as the base polystyrene resin of the Bostyrene foam. The weight average molecular weight of the base polystyrene resin of the polystyrene foam was measured by gel permeation chromatography (hereinafter referred to as GPC).
w) and number average molecule! This invention relates to a polystyrene foam having excellent secondary foaming moldability over a wide range of molding and heating conditions, characterized by having a molecular weight distribution (Rs/Rn) defined as the ratio of 1 to 3.0.

[Example] In the present invention, a resin having a ratio of Flw to Rn, that is, Fm/Rn measured by GPC, of 3.0 or more is used as the base polystyrene resin of the polystyrene foam.

As the base polystyrene resin, for example, a homopolymer of an aromatic vinyl compound such as styrene or α-methylstyrene or a copolymer thereof, 50% of an aromatic vinyl compound such as styrene or α-methylstyrene, etc. Examples include copolymers containing the above (% weight, same hereinafter) and polymerized to contain remaining amounts of other vinyl compounds such as acrylonitrile and methyl acrylate, and these may be used alone. 2f! ! The above may be used in combination.

Rw measured by GPC of the base polystyrene resin
/ There are no particular limitations on Flw, Rn, etc. other than Rn being 3.0 or more, but Rn measured by GPC is 100,000 or more.
It is preferably 700,000 in order to match the moldability and strength of the molded product during secondary foam molding to practical molding usage conditions, and more preferably 150,000 to 400,000.

It is preferable that the Rw/Fm is 3.0 or more, preferably 3.5 or more, that is, the molecular weight distribution is wide, because the range of molding and heating conditions for the resulting polystyrene foam is wide.

Although the cause is not clear at present, it is thought as follows. In other words, the thermal conductivity of polystyrene foam is very poor, so when it is placed in a heating furnace, there is a large difference between the surface temperature and internal temperature of the polystyrene foam, and the surface tends to be overheated. It is thought that while the blubber fluid tends to form into keloids, the interior is not heated enough to soften and deform easily. However, when the molecular weight distribution is wide, there are many high molecular weight resins and low molecular weight resins, and the heat resistance of the high molecular weight resins prevents the resin on the surface from flowing and forming keloids, and the plasticity of the low molecular weight resins. I think this works to make it easier to soften and deform the inside, improving moldability. Therefore, a resin with a wide molecular weight distribution in which many high molecular weight resins and low molecular weight resins coexist has good moldability, and a thick polystyrene foam to be subjected to secondary foaming molding has a wide molecular weight distribution. It is preferable to use polystyrene resin as the base material. For example, the wall thickness is 1.0 to 1.5 m, such as natto containers and lunch boxes.
In the case of heating and secondary foam molding a sheet-like polystyrene foam of about m, ~/Fi of the polystyrene foam
11 should be 3.0 or more, but in the case of forming plates, bowls, etc., a sheet-shaped bowl with a thickness of 1.51 or more, usually 2 mm or more.

Since a restyrene foam is used, the Rw at that time
/FIn is desirably 3.5 or more.

Polystyrene resins with FJM/F111 of 3.0 or higher are mainly produced using suspension polymerization, which involves changing the polymerization temperature according to the progress of the polymerization reaction, adding a catalyst or chain transfer agent during polymerization, or using difunctional polymerization. It is manufactured using methods such as using a chemical catalyst. It can also be produced simply by mixing resins with different molecular weights.

The polystyrene resin is impregnated with a blowing agent such as a lower hydrocarbon such as butane, pentane, or hexane, or a halogenated hydrocarbon such as methyl chloride, dichloromethane, trichloromonofluoromethane, or dichlorodifluoromethane, and then supplied to an extruder. Alternatively, after supplying polystyrene resin in an extrusion manner, the above-mentioned foaming agent, etc. is press-fitted into an extruder, the foaming agent and polystyrene resin are melted and kneaded, and extruded from a T-die or circular die. A polystyrene foam, preferably a sheet-like polystyrene foam having a thickness of 0.2 to 51 mm and a density of 0.05 to 0.3 (1/CC) is produced by a conventional method.

When resins are melted and kneaded in an extruder, decomposition and scission of molecular chains occur, mainly in high-molecular-weight polystyrene resins, resulting in a decrease in molecular weight and a decrease in L/H (molecular weight distribution narrows).
Therefore, it is desirable to select a polystyrene resin in advance, taking into account the narrowing of the molecular weight distribution.

When foaming polystyrene resin and coating the polystyrene foam, we add nucleating agents such as talc and calcium carbonate to control the foam cell diameter, plasticizers, lubricants, pigments, antistatic agents, flame retardants, etc. to the raw resin. They may be used in combination, or an antistatic agent or a lubricant such as silicone may be applied to the surface to change the surface properties of the roughened polystyrene foam.

In order to perform secondary foam molding on the polystyrene foam obtained as described above, the polystyrene foam is placed in a heating furnace, heated, softened and subjected to secondary foaming, taken out of the heating furnace, and immediately pressed. Generally, it is press-molded using a mold to form a molded product.

The polystyrene foam of the present invention has a wide range of forming and heating conditions, so even if there are temperature fluctuations in the heating furnace or temperature distribution within the heating furnace, keloid formation on the surface is unlikely to occur, and it has a well-defined shape.
You can change the molded product to look good. As a result, molding conditions can be easily set, workability is improved, and the occurrence of defective products is suppressed.
The molding yield is improved, the strength of the molded product is strong, and the variation in strength is reduced.

Next, the polystyrene foam of the present invention will be explained based on Examples.

Example 1 and Comparative Example 1 The polystyrene resin shown in Table 1 was extruded and foamed using Freon-12 as a foaming agent, and the resulting foam had a thickness of 1.30 mm and a density of 0.0.
A sheet-like polystyrene foam of 96 g/cc was obtained.

Note that the specific viscosity η8 in Table 1. is the value in a 100 mg/10 CC toluene solution at 30°C, and was measured in a 5.0 mg/dTHF solution using a Ding SK HLC-801 model (Toyo Soda Kogyo, Monosei) as a GPC window measuring device.

1st After curing the exposed polystyrene foam for one week, it was heated to 165°C.
Figure 1 shows the results of evaluating the relationship between the thickness and heating time and the presence or absence of surface keloids when heated in an infrared heating furnace for secondary foaming, and heated in the same manner as above.
After secondary foaming, a bento box was made using a press mold, and the shape was evaluated by evaluating the relationship between the protrusion pattern and heating time, as well as the presence or absence of surface keloids.
As shown in the figure.

7 From the results in Figure 1, it is clear that in Example 1, which has a wide molecular weight distribution of 3.0, keloids are less likely to form on the surface even when heated for a long time than in Comparative Example 1, which has a narrow molecular weight distribution of 2.4. It can be seen that the thickness does not decrease rapidly even if the length is increased.

It should be noted that the heating time for which the appearance of the pattern becomes 0.51 or more, which is acceptable for the product, and the heating time until the appearance of surface keloids are within the molding and heating conditions that allow the product to be obtained, and from the results shown in Figure 2, It can be seen that the molding and heating conditions for Example 1, which has a large molecular weight distribution of 3.0, are wider than those of Comparative Example 1, which has a small molecular weight distribution of 2.4.

Examples 2 to 4 and Comparative Example 2 Polystyrene resins having the characteristics shown in the raw resin items in Table 2 were produced by a suspension polymerization method.

The obtained resin was extruded and foamed using Freon-12 as a foaming agent to obtain a sheet-like polystyrene foam having a thickness of 2.3 mm and a density of 0.087 (1/CC).

The molecular weight distribution and specific viscosity ηsp of the resin after being made into a foam are also shown in Table 2.

From the comparison of the characteristics of the raw resin and foam resin in Table 2,
It can be seen that the extrusion foaming operation narrows the molecular weight distribution and lowers the molecular weight.

The obtained sheet-shaped polystyrene foam was heated in a heating furnace at 175°C for the time shown in Table 2, and then taken out from the furnace, and the length x width x height was 199+am x 99mm x 27mm.
A tray was press-molded, and the molded product was visually evaluated. The results are shown in Table 2.

[Left below] From the results in Table 2, it can be seen that the broader the molecular weight distribution of the polystyrene foam resin, the wider the range of heating conditions under which a good molded product can be obtained. As shown in Comparative Example 2, it is preferable that the width of 3.2 is wider than that of 2.5, and furthermore, it can be seen that 3.5 is more preferable.

In addition, the strength of the tray molded with a heating time of 9.0 seconds was calculated by the load required to compress both sides of the tray in the width direction by 5%, as shown in Table 3. there were. Note that X in Table 3 is the average intensity of 30 measurements, and σ is the standard difference at that time.

[Margin below] Table 3 From the results in Table 3, it can be seen that the larger the molecular weight distribution, the higher the strength and the smaller the variation (τ), and furthermore, the variation becomes smaller when the molecular weight distribution is 3.5 or more.

[Effects of the Invention] The polystyrene foam of the present invention can be molded and heated over a wide range of conditions, and when the foam is used, keloids are less likely to occur on the surface even if the heating conditions during secondary foaming are changed, and shape-defining properties are improved. It is possible to mold a deep molded product with good strength and good appearance.

4. Brief explanation of the surface FIG. 1 was obtained in Example 1 and Comparative Example 1.

FIG. 2 is a graph showing the relationship between the thickness and heating time when the polystyrene foam is subjected to secondary foaming, and the results of evaluating the presence or absence of surface keloids.
It is a graph showing the results of evaluating the relationship between the appearance of protrusion patterns and heating time and the presence or absence of surface keloids when a bento box was produced using a press mold after the subsequent foaming.

~% Pei C ≧■ (g)

Claims (1)

[Scope of Claims] 1 Weight average molecular weight of base polystyrene resin measured by gel permeation chromatography (@M@w
) and number average molecular weight (@M@n), the molecular weight distribution (@M@w/@M@n) is 3.0 or more. Superior polystyrene foam. 2. The polystyrene foam according to claim 1, wherein the base polystyrene resin has a molecular weight distribution of 3.5 or more. 3. The polystyrene foam according to claim 1 or 2, wherein the polystyrene foam is in the form of a sheet having a thickness of 0.2 to 5 mm and a density of 0.05 to 0.3 g/cc.