JPH07165969A - Production of styrenic resin expanded sheet - Google Patents

Abstract

PURPOSE:To obtain the sheet, excellent in hot moldability, surface smoothness, etc., and useful as food containers, etc., by extruding and expanding a styrenic resin containing a specific foaming agent from an extruder so as to satisfy a specified relationship between the whole density of the expanded sheet and a surface layer density. CONSTITUTION:This sheet is obtained by extruding and expanding a melt expandable styrenic resin containing a mixture of >30 to <=50wt.% n-butane with >=50 to <70wt.% isobutane as a foaming agent from an extruder so as to satisfy the relationship of formula I [alpha is the whole density (g/cm<3>) of the expanded sheet; beta is the surface layer density (g/cm<3>) of the resultant expanded sheet] and formula II.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing a styrene resin foam sheet.

[0002]

2. Description of the Related Art Styrenic resin foamed sheets are excellent in thermoformability, have a beautiful appearance of the obtained molded product, and are lightweight and have excellent heat insulating properties. It has been used in large quantities in recent years for thermoforming of food containers and the like.

Styrene-based resin foamed sheets are melted by melting a styrene-based resin in an extruder and adding a foaming agent, a foam control agent such as talc or the like, and a lubricant such as liquid paraffin to the molten resin. After kneading, it is produced by a method of extruding from an extruder and foaming.

Industrially available butane is widely used as the foaming agent because it is inexpensive and has excellent secondary moldability when thermoforming a foamed sheet. However,
In the case of a styrene resin foamed sheet using industrial butane as a foaming agent, there is a problem that the rate of escape of the foaming agent in the foamed sheet is high, and the amount of the foaming agent remaining in the foamed sheet decreases in a short period of time. When the decrease in the remaining amount of the foaming agent in the foamed sheet becomes large, the secondary moldability of the foamed sheet decreases and the target thickness during thermoforming (this thickness is hereinafter referred to as "secondary foamed thickness"). A molded product cannot be obtained. For this reason, there has been a problem that a thermoforming suitability period (hereinafter referred to as a life cycle) in which a molded product having a desired thickness can be obtained when the foamed sheet is thermoformed is shortened.

In order to solve this problem, isobutane 7
A mixed butane of 0 to 100% by weight and 0 to 30% by weight of normal butane is added in an amount of 2.5 to 5 parts by weight with respect to 100 parts by weight of the melted polystyrene resin and kneaded to leave butane with respect to 100 parts by weight of the resin. A method has been proposed in which extrusion foaming is performed so that the amount becomes 2.0 to 3.5 parts by weight (Japanese Patent Publication No.
42977).

On the other hand, in a styrene resin foam sheet which has not passed a certain period of time after its production, the surface roughness of the molded product obtained by thermoforming the sheet occurs, and the printability etc. on the surface of the molded product deteriorates. Therefore, thermoforming cannot be practically performed within a certain period after the foamed sheet is manufactured. It is considered that this is because the effect of the plasticizing action of the foaming sheet by the foaming agent becomes large while the amount of the foaming agent remaining in the foaming sheet is large. Therefore, it is necessary to reduce the amount of the remaining foaming agent in the foamed sheet by aging the styrene-based resin foamed sheet for a certain period of time after being manufactured and before being subjected to thermoforming.

However, ensuring a sufficient life cycle of the styrenic resin foam sheet mainly means preventing the foaming agent from escaping from the foam sheet, while the foam sheet can be thermoformed. The shortening of the aging period required until it becomes is that the amount of the remaining foaming agent in the foamed sheet is to be quickly reduced. Therefore, it is necessary to secure a sufficient life cycle and shorten the aging period. Is a contradictory proposition, and the method of Japanese Patent Publication No. 5-42977 mentioned above can prolong the life cycle, but on the other hand, there is a problem that it requires a long aging period until thermoforming becomes possible. In some cases, the above aging was necessary. If the aging period of the foamed sheet is long, a bulky foamed sheet having a foaming ratio of 2 to 20 times must be kept in stock for a long period of time, which causes the selling price of the foamed sheet to increase. Therefore, there has been a demand for the development of an effective method that can satisfy a sufficient life cycle and a shortened aging period at the same time.

The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, foamed butane having a smaller ratio of isobutane to normal butane than the foaming agent used in the method of JP-B-5-42977. By using as an agent and foaming so that a specific relationship is established between the overall density of the foamed sheet and the surface layer density of the foamed sheet, thermoforming is possible if it is aged for about 2 weeks after the production. At the same time, they have found that a sufficient life cycle can be secured, and have completed the present invention.

[0009]

[Means for Solving the Problems] That is, a method for producing a styrene-based resin foam sheet according to the present invention is a styrene-based resin foam sheet in which a melt-foamable styrene-based resin containing butane as a foaming agent is extruded from an extruder to foam. In the production method, butane used as a foaming agent exceeds 30% by weight and 5
0% by weight or less of normal butane and 50% by weight or more, 7
It is a mixture with less than 0% by weight of isobutane, and between the overall density of the resulting foamed sheet: α (g / cm ³ ) and the surface layer density: β (g / cm ³ ) 1.077 α + 0.0031 It is characterized in that extrusion foaming is performed so that the relationship of ≦ β ≦ 1.077α + 0.164 (1) and 0.06 ≦ α ≦ 0.4 (2) holds.

In the present invention, the styrene resin is
Styrene homopolymers such as polystyrene and poly-p-methylstyrene, styrene-maleic anhydride copolymers, styrene-acrylonitrile copolymers, styrene-butadiene copolymers, styrene-acrylonitrile-butadiene copolymers, styrene- Examples thereof include acrylic acid copolymers, styrene-based copolymers such as styrene-methacrylic acid copolymers, and mixtures of polystyrene and polyphenylene oxide. The MFR of the styrene resin used is not particularly limited, but is preferably about 0.1 to 60 g / 10 minutes.

If necessary, inorganic substances such as magnesium carbonate, calcium carbonate, talc, clay and the like can be added. The amount of the inorganic substance added is preferably about 0.2 to 40 parts by weight per 100 parts by weight of the resin.

In the method of the present invention, 30% by weight as a foaming agent
A mixture of more than 50 wt% and less than 50 wt% of normal butane and more than 50 wt% and less than 70 wt% of isobutane is used. Further, in the method of the present invention, the overall density of the obtained foam sheet: α (g / cm ³ ) and the surface layer density of the foam sheet: β (g / g /
cm ³ ), the extrusion is performed so that the relations of 1.077α + 0.0031 ≦ β ≦ 1.077α + 0.164 (1) and 0.06 ≦ α ≦ 0.4 (2) are established. Foam.

According to the method of the present invention, the proportion of isobutane in the foaming agent is regulated within a specific range as described above, and the foaming sheet is foamed so that the overall density and the surface layer density have a specific relationship. As a result, a sufficient life cycle can be secured and the aging period can be set to about 2 weeks.

In the method of the present invention, even if the ratio of normal butane and isobutane in the foaming agent is in the above range, the overall density of the foamed sheet: α and the surface layer density: β are the above (1),
When the formula (2) is not satisfied, or conversely, the overall density of the foamed sheet: α and the surface layer density: β are the above (1),
Even if the formula (2) is satisfied, if the ratio of normal butane and isobutane in the foaming agent is out of the above range, sufficient life cycle cannot be secured, and thermoforming can be performed without surface roughness. There is a problem that it takes a long time to mature the product.

In the method of the present invention, a styrene resin and, if necessary, the above-mentioned inorganic substances and the like are added to the inside of the extruder, and the resin is melted by heating, and then the foaming agent is pressed into and kneaded. A so-called inflation method in which a tubular foam obtained by extrusion from an annular die at the tip is cut open to form a sheet can be adopted. In the present invention, the amount of the foaming agent pressed into the resin is preferably 1 to 5 parts by weight per 100 parts by weight of the resin. In addition, the resin temperature at the time of extruding the foamable styrene-based resin obtained by press-fitting the foaming agent and melt-kneading is 140
It is about ℃ to 170 ℃.

The overall density α of the obtained foamed sheet can be set to a value satisfying the above formula (2) mainly by adjusting the resin temperature at the time of extrusion and the amount of the foaming agent added. On the other hand, the surface layer density: β can be adjusted to a value satisfying the above formula (1) by rapidly cooling the surface of the tubular foam immediately after extrusion foaming. As a method of quenching, there is a method of blowing cooling air to the outer surface of the tubular foam body, and the degree of cooling can be adjusted by the temperature and supply amount of the cooling air. The larger the surface layer density: β, the more beautiful the appearance of the foamed sheet is, which is preferable. However, when the surface layer density: β becomes larger than the upper limit of the above formula (1) by excessive cooling, residual foaming in the foamed sheet occurs. Since the rate of escape of the agent becomes low, the aging time of the foamed sheet necessary to obtain a molded product without surface roughness becomes long. On the other hand, if the cooling is insufficient and the surface layer density: β becomes smaller than the lower limit of the above equation (1),
The appearance of the foamed sheet itself and the molded product obtained therefrom will be poor. Appearances with different surface densities should not be compared by absolute values of surface densities. This is because the required appearance differs depending on the density (overall density) of the foam sheet. Therefore, in the present invention, the surface layer density: β is shown by a relational expression with the overall density: α.

In the present invention, the surface layer density of the foamed sheet is a density within the range of 0.2 mm ± 0.01 mm from the surface of the foamed sheet.

The foamed sheet obtained by the method of the present invention usually has a thickness of about 0.7 to 3.0 mm and a basis weight of 70 to 400 g.
It is of the order of / m ^2, but the thickness and basis weight of the foam sheet obtained by the method of the present invention is not intended to be limited to the above.

[0019]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

Examples 1 to 14 and Comparative Examples 1 to 6 Styrene resin manufactured by Asahi Kasei Corporation: Styron 685 (M
I = 2.1) 1 part by weight of talc per 100 parts by weight of talc was charged into an extruder of 60 mm and heated and melted, and then mixed butane of isobutane and normal butane containing isobutane in a ratio shown in Table 1 was added. Was used as a foaming agent, and the foaming agent was melt-kneaded with the resin by press-fitting into the extruder so that the content in the resin was 3 to 4% by weight. Then, after the melt-kneaded product is cooled to 150 to 155 ° C. in a 90 mm extruder connected to the extruder, it is extruded and foamed from an annular die at the same temperature to foam into a tube, and then this tubular foam is It was cut open to obtain a foamed sheet having a basis weight of 205 g / m ² , a thickness of 2.2 mm, and an average number of cells in the thickness direction of 19 to 21. Table 1 shows the overall density and surface layer density of the obtained foamed sheet. Further, the appearances of the foamed sheets were compared, and those having a good appearance were evaluated as “good” and those having a bad appearance were evaluated as “bad”, and the results are shown in Table 1.
The secondary foam thickness required for this foam sheet is 4.6-
It is 5.2 mm.

The surface density of the foam sheet is Daiwa Koki Co., Ltd.
Using a microtome TU-213 manufactured by Mfg. Co., Ltd., a foamed sheet was sliced to a thickness of 0.2 mm ± 0.01 mm, the weight was measured with a microbalance, and the thickness was measured with a dial gauge. Calculated from the length and width. For the average number of bubbles in the thickness direction, the number of bubbles is counted by observing with a microscope each 5 arbitrary points in the thickness direction of the cut surface of the sample obtained by cutting the foamed sheet in the thickness direction and further slicing the cut surface thinly. , The average value was shown.

Further, the expansion ratio (secondary expansion ratio) when the obtained foamed sheet is heated for 25 seconds with hot air at 140 ° C. (secondary expansion) to obtain a secondary expansion ratio of 4 to 60 days after production. Measured, number of days elapsed: X, and secondary expansion ratio at that time:
From Y and Y, the values of A and B (where A is a constant and B is the coefficient of reduction of the secondary expansion ratio) were determined based on the following formula, log Y = A−B × X. Further, after 40 days from the production, the foamed sheet was subjected to secondary foaming by the above method, and the thickness thereof was measured with a dial gauge. The results are shown in Table 1 together.

After foaming the foamed sheet after 14 days from the production under the same conditions as above, the surface of the test piece was observed. Occurrence "was shown and shown in Table 1.

A tray having a size of 12.5 cm × 19.5 cm and a depth of 1.4 cm was thermoformed using the foamed sheet after 14 days from the production, and the appearance of the obtained tray was observed to give a good appearance. The results are shown in Table 1 by evaluating "good" and "bad" as bad.

[0025]

[Table 1]

Examples 5 to 8 and Comparative Examples 7 to 12 Styrenic resin 1 similar to Examples 1 to 4 and Comparative Examples 1 to 6
1 part by weight of talc per 100 parts by weight was charged into an extruder of 60 mm and heated and melted, and then mixed butane containing isobutane and normal butane in a ratio shown in Table 1 was used as a foaming agent. The content of foaming agent in the resin is 3
The mixture was melted and kneaded with the resin by press-fitting it into the extruder so that the content became 4% by weight. Then, the melt-kneaded product was cooled to 155 to 160 ° C. in a 90 mm extruder connected to the extruder, and then extruded and foamed from an annular die at the same temperature to foam into a tube, and then this tubular foam was It was cut open to obtain a foamed sheet having a basis weight of 250 g / m ² , a thickness of 2.35 mm, and an average number of cells in the thickness direction of 17 to 19. Table 1 shows the overall density, surface layer density and appearance evaluation of the obtained foamed sheet. The secondary foam thickness required for this foam sheet is 4.7 to 5.4 mm.

The change with time of the secondary expansion ratio of the obtained foamed sheet was measured in the same manner as in Examples 1 to 4 and Comparative Examples 1 to 6.
In addition, the presence or absence of surface roughness when the foamed sheet was secondary foamed after 14 days from the production, and the foamed sheet after 14 days from the production were thermoformed, and had a diameter of 14.3 cm and a depth of 7.
The appearance of a 5 cm bowl was evaluated in the same manner as above. The results are also shown in Table 1.

Examples 9 to 12 and Comparative Examples 13 to 18 Styrenic resin 1 similar to Examples 1 to 4 and Comparative Examples 1 to 6
1 part by weight of talc per 100 parts by weight was charged into an extruder of 60 mm and heated and melted, and then mixed butane containing isobutane and normal butane at a ratio shown in Table 1 was used as a foaming agent. The content of foaming agent in the resin is 1.
It was melted and kneaded with the resin by press-fitting it into the extruder so as to be 5 to 2.5% by weight. Then, the melt-kneaded product was cooled to 160 to 165 ° C. in a 90 mm extruder connected to the extruder, then extruded and foamed from an annular die at the same temperature to foam into a tube, and then this tubular foam was formed. It was cut open to obtain a foamed sheet having a basis weight of 200 g / m ² , a thickness of 1.0 mm, and an average number of cells in the thickness direction of 14 to 16. Table 1 shows the overall density, surface layer density and appearance evaluation of the obtained foamed sheet. The secondary foam thickness required for this foam sheet is 1.9 to 2.2 mm.

The change with time of the secondary expansion ratio of the obtained foamed sheet was measured in the same manner as in Examples 1 to 4 and Comparative Examples 1 to 6.
In addition, the presence or absence of surface roughness when the foamed sheet was secondarily foamed after 14 days from the production, and 12.5 cm × 19.5 cm obtained by thermoforming the foamed sheet after 14 days from the production,
The appearance of the 1.4 cm deep tray was evaluated in the same manner as above. The results are also shown in Table 1.

[0030]

As described above, according to the method of the present invention, the aging period required before the thermoforming after the production of the styrenic resin foam sheet is shortened, and the foam sheet is heated at the intended secondary foam thickness. It is possible to simultaneously secure a sufficient moldable life cycle. Therefore, the foamed sheet obtained by the method of the present invention can be thermoformed into an excellent molded article without surface roughness by aging for a relatively short time, and the elapsed time from the production of the foamed sheet to the thermoforming can be performed. It is an excellent material that retains sufficient thermoformability even when the length becomes long.

Claims (1)

[Claims] 1. In a method for producing a styrene resin foam sheet in which a melt-foamable styrene resin containing butane as a foaming agent is extruded from an extruder to foam, butane used as a foaming agent exceeds 30% by weight and 50% by weight. % Or less of normal butane and 50% or more and less than 70% by weight of isobutane, and the resulting foamed sheet has an overall density: α (g / cm ³ ) and a surface layer density: β (g / cm 3 ³ )
And extrusion foaming so that the relationship of 1.077α + 0.0031 ≦ β ≦ 1.077α + 0.164 (1) and 0.06 ≦ α ≦ 0.4 (2) A method for producing a styrene-based resin foam sheet, comprising: