JPH09100364A - Foamed styrene resin sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a foamed styrene resin sheet which has a wide allowance of heating temp. at the second foaming and molding step and hence can be deeply drawn into a deep container free from appearance defects such as due to partial breaking or burn mark. SOLUTION: This sheet comprises a styrene resin having a strain hardening index of stretch viscosity of 1.5 or higher at 130 deg.C and pref. has a density of 0.2-0.5g/cm and a thickness of 1.3-3.0mm. A deep container is produced by deeply drawing a laminated foam sheet obtd. by sticking a nonfoamed resin film to at least one side of this sheet.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a styrene resin foam sheet having excellent secondary foaming moldability. More specifically, the range of heating conditions at the time of secondary foam molding is wide, and when molding a deep container by deep drawing, it is possible to suppress the occurrence of appearance defects such as pear and burn and widen the range of molding conditions. The present invention relates to a styrene resin foam sheet having excellent secondary foam moldability.

[0002]

2. Description of the Related Art Food containers using foamed styrene resin sheets are used in various forms such as trays, cups and bowls, and discussions on global environmental issues such as plastic recyclability are currently being held in various places. However, there is still a great demand due to its light weight, heat insulation, high resistance to moisture absorption and the like.

In general, in the secondary foam molding of a thermoplastic resin foam sheet such as this styrene resin foam sheet, the foam sheet is heated and softened by a heating means such as an oven, and immediately after that, it is introduced between the male and female molds. After that, it is molded into a desired shape by match mold molding in which the mold is sandwiched.
However, when forming a deep-type container by so-called deep drawing, in which the drawing ratio (container height / container diameter) is 1.0 or more, the amount of tensile deformation of the foamed sheet is particularly large on the side surface of the container. Therefore, the foamed sheet in this portion is partially broken at the time of molding, and a defective appearance of the container due to this (hereinafter, this phenomenon is referred to as “Naki”) is likely to occur.

Therefore, in order to make such a large deformation possible, the heating temperature and the heating time immediately before the secondary foam molding are increased to soften and thicken the foamed sheet before the secondary molding. It is known how to do it. However, when this is done, there is a problem that the surface layer portion becomes keloid-like due to the breakage of cells on the surface of the foamed sheet and the deterioration of the resin (hereinafter, this phenomenon is referred to as “burning”).

In order to solve these problems, so-called high-impact polystyrene in which a rubber component is added to a styrene resin is used, or the absolute value of the molecular weight or the molecular weight distribution has been adjusted (for example, a special method). JP-A-62-22834, JP-A-6-57028, etc.). However, none of these techniques has provided a molded container that has good physical properties and is inexpensive, such as insufficient container strength or heat resistance after molding or high cost.

[0006]

Further, as described above,
It is important to select a resin having high tensile strength against large tensile deformation in order to prevent partial breakage of the foamed sheet at the time of molding the container and defective container appearance (naked) due to the resin modification. The selection of such a resin has heretofore been made by measuring melt shear viscosity typified by melt flow rate, tensile strength at room temperature, and Vicat softening point. However, when we examined each of these evaluation factors from the viewpoint of preventing molding defects such as pearing and burning when molding a deep container by deep drawing,
It was not possible to explain what caused the above-mentioned molding failure and what did not. Generally, the surface temperature of the foamed sheet during secondary foam molding of the styrene resin foamed sheet is 10
It is said that the temperature is about 0 to 130 ° C, and naki occurs at temperatures lower than 100 ° C, and burns occur at temperatures higher than 130 ° C. However, particularly in the molding of a deep container by deep drawing, as described above, since the amount of tensile deformation of the foamed sheet forming the side surface of the container becomes large, naki easily occurs, and the moldable temperature range is increased. When the technology becomes narrower and the conventional technique is used, depending on the specifications of the original fabric, naki may occur even if heated up to 130 ° C., resulting in impossible molding. There was even that. In order to secure such a range of molding conditions, conventionally, the foam weight per unit area (hereinafter, this parameter is referred to as "unit weight") has been increased to increase the rigidity of the foam sheet. However, increasing the basis weight of such a foamed sheet leads to an increase in the cost of forming the container, which is not preferable.

[0007]

Means for Solving the Problems Accordingly, the present inventors have
As a result of earnest research in view of the above problems, a foamed sheet made of a styrene resin having improved extensional viscosity characteristics at 130 ° C., which is the upper limit of the temperature range in which the styrene resin foamed sheet can be molded, is used. It has been found that by performing deep drawing container molding, a deep drawing container having an industrially sufficient molding condition width can be secured and a good appearance and physical properties can be obtained without increasing the basis weight.

That is, in the elongational viscosity measurement at 130 ° C., a styrene resin foam sheet made of a styrene resin having a strain hardening index of elongational viscosity of 1.5 or more, particularly, a density of 0 is obtained immediately before heating in secondary foam molding. .2-
When a so-called deep-drawing container having a drawing ratio of 1 or more is formed using a styrene resin foam sheet having a thickness of 0.5 g / cm ³ and a thickness of 1.3 to 3.0 mm, heating conditions during secondary foam molding. The inventors have found that the above-mentioned problems can be solved with a wide range and completed the present invention. Further, a non-foamed film may be laminated on one side or both sides of the foamed sheet to be used as a laminated foamed sheet.

By using the styrene-based resin foam sheet according to the present invention as described above, the above-mentioned styrenic resin foam sheet can be used as a molding container during the secondary foam molding of a deep container by so-called deep-draw molding having a drawing ratio of 1.0 or more. It has become possible to provide a molded product that does not suffer from a defective appearance such as scorch and pear and has good physical properties such as strength and heat insulation.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Polystyrene is most preferably used as the styrene resin in the present invention.
-Styrene monomers such as methylstyrene and p-methylstyrene, acrylonitrile, methyl acrylate,
Those in which methyl methacrylate, acrylic acid, methacrylic acid, maleic anhydride, etc. are copolymerized may be used.
The proportion of the other vinyl-based monomer is 0 to 30% by weight based on the total vinyl-based monomer including styrene.

Of these styrene resins,
In particular, when measuring the extensional viscosity at 130 ° C., when a deep-drawing container is formed by using a foamed sheet obtained by extruding and foaming a styrene resin having a strain hardening index of the extensional viscosity of 1.5 or more, a good secondary It has moldability.

The extensional viscosity means the melt flow rate,
Unlike the melt viscosity (= shear viscosity) of resin, which has been conventionally measured such as initial relaxation elastic modulus, both sides of a rod-shaped or plate-shaped measurement sample were supported by pinch rollers or tracks under a constant temperature atmosphere. After that, the pinch roller or caterpillar is rotated, tensile deformation is applied to the measurement sample so as to maintain a constant strain rate, and the sample cross-sectional area during deformation and the torque applied to the pinch roller or caterpillar are detected. .

Usually, the detected torque during the measurement of the extensional viscosity decreases gently after the peak is formed at the initial stage of measurement because the sample cross-sectional area sharply decreases as the amount of strain increases. However, some resins have a so-called strain hardening property in which the peak of the detected torque has two peaks, and as a result, the extensional viscosity in the high strain region rapidly increases. The parameter indicating the degree of strain hardening is the strain growth index.
Is known (Bradley and Phillips, ANTEC'90,71
7).

That is, the strain hardening index of this extensional viscosity is
In the uniaxial elongational viscosity measurement at a constant strain rate (= ε), the following formula (1)

[0015]

(Equation 1)

Η _e ⁺ defined by here,
F (t) is the detected torque, A ₀ is the initial cross-sectional area of the sample,
t is time. When the elongational viscosity (logarithm) is plotted against time (logarithm) in the case of a resin having no strain-hardening property, the elongational viscosity η _e becomes a curve asymptotic to 3 times the zero-shear viscosity. _e ⁺ becomes 1. In the case of a resin having a strain hardening property, the plot does not become an asymptotic curve, and η _e rises sharply in a particularly high strain region to form a peak and then break. In the present invention, the same strain (time) as η _e at the peak of the resin having strain hardening property
The strain hardening index η _e ⁺ is defined as the ratio with respect to η _e of the resin having no strain hardening property.

In the present invention, 13 of the styrene resins are used.
In measurement of elongational viscosity at 0 ° C., a styrene resin having a strain hardening index η _e ^{+ of} elongational viscosity of 1.5 or more is used. Styrenic resins having a strain hardening index of 1.5 or more of this viscosity are disclosed in, for example, JP-A-6-57028 and JP-A-6-57028.
Among the styrene-based resins described in JP-A-256420, those having a strain hardening index of 1.5 or more in the above-mentioned elongational viscosity measurement can be selected and used. The foamed sheet according to the present invention has talc, indigo, calcium carbonate, higher fatty acid metal salts (barium stearate, magnesium stearate, zinc stearate, etc.) against styrene resins having such a strain hardening index.
Etc., a foam regulator such as liquid paraffin, and a moldability regulator such as liquid paraffin are appropriately added, melted and mixed by using an extruder, and a foaming agent is press-fitted, and then extruded and foamed from a die having a cylindrical shape, a flat plate shape or the like. It is manufactured by forming a sheet.

The styrenic resin foam sheet may be used alone, and in addition to the styrene resin, an ethylene-vinyl alcohol copolymer, an ethylene-vinyl acetate copolymer may be used on at least one surface of the foam sheet. It may be used as a laminated sheet formed by laminating non-foamed films of a polymer, polyvinylidene chloride, polypropylene, polyethylene, polybutadiene, polyisoprene and the like. These non-foamed resin films are appropriately determined depending on the shape of the molding container and the required physical properties (strength, heat resistance, etc.). Especially when molding a deep container by deep drawing, in addition to the styrene resin, ethylene- Vinyl alcohol copolymer and polypropylene are particularly preferably used.

In addition, the styrene resin foam sheet,
Particularly when subjected to deep drawing, it is preferable that the density of the foamed sheet is 0.2 to 0.5 g / cm ³ and the thickness is 1.3 to 3.0 mm. When the density is less than 0.2 g / cm ³ , the foam strength at the time of molding a deep-drawing container is insufficient and the molding condition width is substantially lost, which is not preferable.
If it exceeds 5 g / cm ³ , not only is the cost increased, but the lightness and heat insulating properties of the foam container are also reduced, which is not preferable. On the other hand, if the thickness is less than 1.3 mm, the thickness of the obtained deep-drawn container is too thin, and the heat insulation becomes insufficient,
If it exceeds 3.0 mm, the molding condition width becomes narrow, which is not preferable.

[0020]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

(Examples 1 to 6 and Comparative Examples 1 to 4) Four kinds of styrene resins having a strain hardening index at 130 ° C., a melt flow rate and a Vicat softening point as shown in Table 1 were used, and Using the foamed sheet produced by extrusion foaming by the method as a foaming layer, a foamed sheet having a structure, density and thickness shown in Examples 1 to 6 and Comparative Examples 1 to 4 in Table 2 was produced. The strain hardening index of the styrene-based resin forming the foamed layer of the foamed sheet is calculated by the above formula (from the measurement result of the elongational viscosity at 130 ° C. using the elongational viscometer “Melten Rheometer” manufactured by Toyo Seiki Seisakusho Ltd.). It was determined by 1). When measuring the extensional viscosity, the rod-shaped sample (φ5 to 7 mm) is 1
It was heated in silicone oil adjusted to 30 (± 0.3) ° C. for about 10 minutes, and the strain rate was set to 0.01 [s ⁻¹ ].

[0022]

[Table 1]

[0023]

[Table 2]

Next, by deep drawing of these foamed sheets, a deep container having a container height of 105 mm, a container diameter of 96 mm and a drawing ratio of 1.1 was formed. As a result, the appearance of the obtained molded product was inspected. Table 2 also shows the results of measuring the surface temperature of the foamed sheet immediately before molding in the secondary foam molding by an infrared thermometer.

[0025]

INDUSTRIAL APPLICABILITY As described above, the styrene resin foam sheet according to the present invention has a wide heating temperature range at the time of secondary foam molding and can be deep-drawn by deep drawing without causing defective appearance such as pear and burn. Molded containers can be molded.

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Claims (4)

[Claims] 1. A styrene-based resin foam sheet suitable for deep drawing, comprising a styrene-based resin having a strain-hardening index of elongation viscosity of 1.5 or more in elongation viscosity measurement at 130 ° C. 2. The styrene resin foam sheet has a density of 0.
2 to 0.5 g / cm ^Three, The thickness is 1.3 to 3.0 mm
Styrenic resin foam according to claim 1, characterized in that
Sheet. 3. A laminated foam sheet obtained by laminating a non-foamed resin film on one side or both sides of the styrene resin foamed sheet according to claim 1 or 2. 4. A deep container formed by deep-drawing the foam sheet according to any one of claims 1 to 3.