JP2013194125A - Polystyrene-based foam-molded article - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polystyrene-based foam-molded article having favorable surface hardness and tensile strength.SOLUTION: A polystyrene-based foam-molded article comprises a fused body of polystyrene-based foam particles, wherein the foam-molded article contains isobutane satisfying expressions of 0.6X≤Y≤1.0X (Expression 1) and 0.5≤Y≤2.5 (Expression 2), where X (wt.%) represents an amount of isobutane in the inner part of the foam-molded article and Y (wt.%) represents an amount of isobutane in the surface layer of the article.

Description

  The present invention relates to a polystyrene-based foam molded article. More specifically, the present invention relates to a polystyrene-based foam molded article having improved surface hardness and tensile strength.

Synthetic resin foamed molded articles such as polystyrene have heat insulating properties, light weight and excellent shock absorption. The foamed molded product can be formed by a relatively easy method of filling the foamed particles in a mold and heating the mold to fuse the foamed particles together. For this reason, a foaming molding is widely used as a container used when accommodating, storing or transporting vegetables, fruits, meats, seafood, industrial products and the like.
The properties of the foamed molded product vary greatly depending on how many times the expandable resin particles as a raw material are expanded, or by changing the expansion ratio of the so-called foamed molded product. For this reason, the foaming molding is used with the appropriate expansion ratio according to a use.
Furthermore, the more the expandable resin particles are expanded, the smaller the amount of the synthetic resin material. For this reason, a product can also be manufactured cheaply. However, the greater the product multiple, the lower the surface hardness. Therefore, Patent Document 1 proposes a foamed molded article having improved surface hardness by making the foaming ratio of the surface layer of the foamed molded article lower than the internal foaming ratio.

JP 2011-68776 A

  However, although the foam molded article described in Patent Document 1 is effective in improving the surface hardness, it has not been satisfactory in terms of weight reduction because the foaming ratio of the surface layer is lowered. For this reason, it is desired to provide a foamed molded article having excellent surface hardness without changing the magnification.

In view of such problems, the inventor of the present invention has intensively studied, and as a result, found that a specific amount of isobutane is left in the surface layer and the inside of the foam molded article, thereby having good surface hardness. It came to complete.
Thus, according to the present invention, it is a polystyrene-based foam molded article composed of a fused product of polystyrene-based foamed particles,
When the foamed molded product has an isobutane content of X wt% inside and an isobutane content of the surface layer defined as Y wt%, the following formula 0.6X ≦ Y <1.0X (formula 1)
0.5 ≦ Y ≦ 2.5 (Formula 2)
There is provided a polystyrene-based foam-molded product characterized by containing isobutane so as to satisfy the above requirements.

  ADVANTAGE OF THE INVENTION According to this invention, the polystyrene-type foaming molding provided with favorable surface hardness and tensile strength can be provided by leaving isobutane by a specific amount in a surface layer and an inside.

Further, the polystyrene-based foam molded body is a container having an opening on the upper surface, a bottom on the lower surface, and a side wall located around the opening and connected to the bottom, and the side wall has a thickness of 15 to 30 mm. When it has a height of 150 to 400 mm, it is possible to provide a polystyrene-based foam molded article having better surface hardness and tensile strength.
Furthermore, when the polystyrene-based foam molded article is formed by an in-mold foam molding method using a bead method, a polystyrene-based foam molded article having better surface hardness and tensile strength can be provided.

(Polystyrene foam molding)
The inventor of the present invention tried to analyze the structure of a foamed molded product in order to provide a polystyrene-based foamed molded product (hereinafter also referred to as a foamed molded product) having good surface hardness and tensile strength. As a result, the inventor has unexpectedly realized that a certain amount of isobutane, which is a kind of foaming agent that has been conventionally considered to be replaced with air, remains in the foamed molded product. Therefore, the inventor examined the remaining amount of isobutane, and it was found that if isobutane is contained in the foam molded body and the surface layer within a specific range, not only the tensile strength but also the surface hardness can be improved. We found empirically from the experimental results. The reason why the tensile strength and the surface hardness can be improved is not clear, but the inventor is that the foam breakage of the foam molded product skin is suppressed even slightly by increasing the amount of isobutane inside the foam molded product from the surface layer. I guess. The surface hardness is a factor indicating the degree of prevention of scratches on the foamed molded product, and is also a factor that affects the feel of the surface.

The foamed molded product of the present invention is composed of a fused product of polystyrene-based expanded particles. When the amount of isobutane in the interior is X wt% and the amount of isobutane in the surface layer is Y wt%, the following formula 0.6X ≦ Y <1.0X (Formula 1)
0.5 ≦ Y ≦ 2.5 (Formula 2)
Contains isobutane to satisfy

When the Y weight%, which is the amount of isobutane in the surface layer of the foam molded article, is less than 0.6 X weight% and less than 0.5 weight%, the surface hardness may not be sufficient. Moreover, when it exceeds 1.0X weight% and 2.5 weight%, the external appearance of the foaming molding surface may deteriorate. A more preferable amount of isobutane is a range having a relationship satisfying the following formula.
0.7X ≦ Y ≦ 0.9X (Formula 3)
1.0 ≦ Y ≦ 2.0 (Formula 4)
Here, the surface layer means a portion from the surface to t / 3 when the thickness t of the foamed molded article is 24 mm or less, and when the thickness t exceeds 24 mm, it means a portion from the surface to 8 mm. . Further, the inside means a portion from t / 3 to 2t / 3 when the thickness t of the foamed molded product is 24 mm or less, and when the thickness t exceeds 24 mm, the depth is 8 mm and 16 mm from the surface. Means the part in between.

The foam molded article can be suitably used for a foam molded article having a side wall portion and a bottom portion connected to the side wall portion.
The side wall portion generally has a plate shape, and may include a handle for facilitating gripping and a reinforcing portion for further improving the strength.
The side wall portion preferably has a thickness of 15 to 30 mm. If the thickness is less than 15 mm, sufficient surface hardness may not be obtained. When it is larger than 30 mm, although sufficient surface hardness can be obtained, the raw material cost of the foamed molded product may be increased. A more preferable thickness is 17 to 25 mm, and a still more preferable thickness is 18 to 23 mm.

The side wall portion preferably has a height of 150 to 400 mm. When the height is smaller than 150 mm, the amount of articles that can be stored in the foamed molded product may be reduced. If it is larger than 400 mm, sufficient tensile hardness may not be obtained. A more preferred height is 150 to 370 mm, and a still more preferred height is 160 to 300 mm.
Moreover, when a side wall part is high, it is preferable that it is thick from a viewpoint of ensuring sufficient tensile strength. On the other hand, when it is low, it is preferably thin from the viewpoint of reducing the raw material cost. It is preferable that the thickness and the height have a ratio of 1: 5 to 30.

The thickness of the bottom is not particularly limited, and can be set as appropriate according to the weight of the article held in the foam molded body and the area of the bottom. The thickness of the bottom can be set to 12 to 30 mm, for example. The area of the bottom is usually ^{3 × 10 4 ~20 × 10 4} mm 2.
Furthermore, the side wall is preferably thicker than the bottom. The surface hardness can be improved by being thick. Specifically, the side wall portion preferably has a thickness of 1.0 to 1.5 times the thickness of the bottom portion, and has a thickness of 1.0 to 1.2 times. It is more preferable.

The foamed molded article preferably has a foaming factor of 60 to 80 times. By having the expansion ratio in this specific range, it is possible to provide a more lightweight foam-molded article having sufficient surface hardness. A more preferable expansion ratio is 65 to 75 times.
Moreover, it is preferable that the average cell diameter of the expanded polystyrene-type resin particle which comprises a foaming molding is the range of 50-100 micrometers. By having an average cell diameter in this range, it is possible to achieve both weight reduction of the foamed molded product and improvement of surface hardness.

  The polystyrene resin contained in the expanded polystyrene resin particles constituting the expanded molded article is not particularly limited as long as it has a weight average molecular weight in the above range. For example, as the polystyrene resin, homopolymers of styrene monomers such as styrene, α-methylstyrene, paramethylstyrene, vinyltoluene, chlorostyrene, ethylstyrene, i-propylstyrene, dimethylstyrene, bromostyrene, or the like A copolymer etc. are mentioned. The polystyrene resin preferably contains 50% by weight or more of a component derived from styrene, and more preferably consists of polystyrene.

The polystyrene resin may be a copolymer of a styrene monomer and a vinyl monomer copolymerizable with the styrene monomer. In the case of a copolymer, it is preferable that a component derived from a styrene monomer occupies a main component (50% by weight or more, preferably 80% by weight or more, more preferably 99.8 to 99.9% by weight). Examples of such vinyl monomers include alkyl (meth) acrylates having 1 to 8 carbon atoms such as methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, and cetyl (meth) acrylate, (meth) In addition to acrylonitrile, dimethyl maleate, dimethyl fumarate, diethyl fumarate and ethyl fumarate, bifunctional monomers such as divinylbenzene and alkylene glycol dimethacrylate, maleic anhydride, N-vinylcarbazole and the like can be mentioned.
The foamed molded product can be used as a container used for housing, storing or transporting vegetables, fruits, meats, seafood, industrial products and the like.

(Method for producing foamed molded article)
The foamed molded product can be obtained by heat-molding a mold filled with expanded polystyrene resin particles with steam. Although the heat molding time by steam is not particularly limited, it is, for example, 10 to 30 seconds. Here, the expanded polystyrene resin particles are generally referred to as pre-expanded particles.
More specifically, this thermoforming is performed as follows. That is, the pre-expanded particles are filled into a closed mold having a large number of small holes. The pre-expanded particles are expanded by heating the mold with steam having a specific vapor pressure (water vapor or the like). As a result, a gap between the pre-expanded particles is filled and the pre-expanded particles are fused and integrated with each other, whereby a foam-molded article can be manufactured. At that time, the density of the foamed molded product can be adjusted, for example, by adjusting the filling amount of the pre-expanded particles in the mold.

The pre-expanded particles may be aged, for example, at normal pressure before forming the foamed molded product. The aging temperature of the pre-expanded particles is preferably 5 to 50 ° C. When the aging temperature is low, the aging time of the pre-expanded particles may be long. On the other hand, if it is high, the foaming agent in the pre-expanded particles may be dissipated and the moldability may deteriorate.
The pre-expanded particles can be produced, for example, by the following method. First, polystyrene resin particles are impregnated with a foaming agent to obtain expandable polystyrene resin particles. Next, pre-expanded particles can be obtained by pre-expanding the expandable polystyrene resin particles.

Expandable polystyrene resin particles are, for example,
(I) Polystyrene resin seed particles (hereinafter referred to as seed particles) are dispersed in an aqueous medium, and a styrene monomer is continuously or intermittently supplied thereto, and suspension polymerization is performed in the presence of a polymerization initiator. Particles obtained by so-called seed polymerization method, or (ii) a styrenic monomer is continuously or intermittently supplied into an aqueous medium and subjected to suspension polymerization in the presence of a polymerization initiator, and a foaming agent Particles obtained by a so-called suspension polymerization method can be used, and the production method (i) is preferred in terms of the uniformity of the surface layer of the molded body.
As the styrenic monomer, the styrenic monomer mentioned in the column of the polystyrene resin is used. Moreover, you may add the vinyl monomer quoted in the column of the said polystyrene-type resin to a styrene-type monomer.

  In addition, it does not specifically limit as a polymerization initiator used in the said suspension polymerization method and seed polymerization method, For example, benzoyl peroxide, lauryl peroxide, t-butyl peroxybenzoate, t-butyl peroxide, t- Butyl peroxypivalate, t-butyl peroxyisopropyl carbonate, t-butyl peroxy-2-ethylhexyl monocarbonate, isopropyl carbonate, t-butyl peroxyacetate, 2,2-bis (t-butylperoxy) butane, Organic peroxides such as t-butylperoxy-3,3,5 trimethylhexanoate and di-t-butylperoxyhexahydroterephthalate, and azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile These are mentioned Alone may be also alone, or two or more are used.

  The aqueous suspension obtained by dispersing polystyrene resin particles in an aqueous medium may be obtained by using the reaction liquid after polymerization by the suspension polymerization method or seed polymerization method as an aqueous suspension, or by using the suspension described above. The polystyrene resin particles obtained by the turbid polymerization method or the seed polymerization method may be separated from the reaction solution, and the polystyrene resin particles may be suspended in a separately prepared aqueous medium to form an aqueous suspension. In addition, it does not specifically limit as an aqueous medium, For example, water, alcohol, etc. are mentioned, Water is preferable.

In the suspension polymerization method or the seed polymerization method, a suspension stabilizer may be used to stabilize the dispersibility of the styrene monomer droplets or the polystyrene resin seed particles when the styrene monomer is polymerized. Good. Examples of such suspension stabilizers include water-soluble polymers such as polyvinyl alcohol, methylcellulose, polyacrylamide, and polyvinylpyrrolidone, and poorly water-soluble inorganic salts such as tribasic calcium phosphate and magnesium pyrophosphate. When a water-soluble inorganic salt is used, an anionic surfactant is usually used in combination.
Examples of the anionic surfactant include alkyl sulfates such as sodium lauryl sulfate, alkylbenzene sulfonates such as sodium dodecylbenzene sulfonate, higher fatty acid salts such as sodium oleate, and β-tetrahydroxynaphthalene sulfonate. And alkylbenzene sulfonates are preferred.

In the suspension polymerization method or seed polymerization method, a water-soluble polymerization inhibitor may be added for the purpose of preventing the generation of fine powder due to polymerization of styrene dissolved in water when the styrene monomer is polymerized. Good.
Then, the polystyrene resin particles dispersed in the aqueous suspension are impregnated with a foaming agent to obtain expandable polystyrene resin particles. The blowing agent contains at least isobutane. Furthermore, as a result of various investigations on the foaming agent, the present inventor has found that the foaming property and the time during which the surface pressure of the foamed molded product is equal to or lower than a predetermined pressure during foam molding in the mold (hereinafter also referred to as a molding cycle) are reduced. Butane is the most excellent foaming agent in terms of long-term storage stability of the foamed particles, especially when normal butane and isobutane are contained in a predetermined ratio and used as a foaming agent, the surface hardness, molding cycle, and appearance of the foamed molded product are improved. We have found that it can be improved.

  Specifically, the content of butane (total amount of normal butane and isobutane) in the foaming agent is preferably 4.0 to 15.0 parts by weight with respect to 100 parts by weight of the polystyrene resin particles, and is preferably 5.0 to 12.0 parts by weight is more preferred. When the content of butane is less than 4.0 parts by weight, the foamability of the expandable polystyrene resin particles is lowered, and a foamed molded article having a desired magnification cannot be obtained, or the foamed pre-expanded particles are obtained. While the thermal fusion between the foamed particles to be obtained becomes insufficient and the mechanical strength of the resulting foamed molded product is reduced, when the amount exceeds 15.0 parts by weight, the resulting foamed molded product is contracted or foamed. This is not preferable because the molding cycle becomes longer.

  The butane is preferably composed of isobutane and normal butane. Furthermore, the content of isobutane in butane is preferably 40 to 90% by weight, more preferably 50 to 80% by weight, and particularly preferably 55 to 70% by weight. If the content of isobutane is less than 40% by weight, the molding cycle may become long or sufficient surface hardness may not be obtained. On the other hand, if it is used in excess of 90% by weight, the molding cycle may become longer or the appearance of the foamed molded product may deteriorate.

As the foaming agent, a foaming agent other than butane may be contained within a range not affecting the present invention (for example, 0 to 0.1% by weight). Examples of blowing agents other than butane include hydrocarbons such as normal pentane, isopentane, neopentane, cyclopentane, cyclopentadiene, normal hexane, petroleum ether, and low-boiling ether compounds such as dimethyl ether, diethyl ether, dipropyl ether, and methyl ethyl ether. Inorganic gas such as carbon dioxide and nitrogen.
Additives such as air conditioners, fillers, flame retardants, flame retardant aids, lubricants, colorants, solvents, plasticizers, etc., as necessary, in the expandable polystyrene resin particles as long as the physical properties are not impaired Can be added.

  The average particle diameter of the polystyrene resin particles is the filling of the pre-expanded particles obtained by pre-expanding the expandable polystyrene resin particles into the cavity when in-mold foam molding is performed using the expandable polystyrene resin particles. From a viewpoint of property, 0.3-2.0 mm is preferable, 0.6-1.4 mm is more preferable, 0.85-1.1 mm is still more preferable.

  Furthermore, when the polystyrene-based weight average molecular weight of the polystyrene-based resin constituting the polystyrene-based resin particles is small, the mechanical strength of the foamed molded product obtained by foaming the expandable polystyrene-based resin particles may decrease, If it is large, the foamability of the expandable polystyrene resin particles is lowered, and a foamed molded article having a high expansion ratio may not be obtained, so 200,000 to 400,000 are preferable, and 240,000 to 350,000 are more preferable.

EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited by these Examples. Various measurement methods in the examples are described below.
(Weight average molecular weight)
The weight average molecular weight is measured using GPC (gel permeation chromatography). The measuring method is as follows. In addition, a weight average molecular weight means a polystyrene (PS) conversion weight average molecular weight.
A 50 mg sample is dissolved in 10 ml of tetrahydrofuran (THF), filtered through a non-aqueous 0.45 μm chromatographic disk, and measured using a chromatograph. The chromatographic conditions are as follows.
Liquid chromatograph: manufactured by Tosoh Corporation, trade name “Gel Permeation Chromatograph HLC-8020”
Column: Tosoh Corporation, trade name “TSKgel GMH-XL-L” φ7.8 mm × 30 cm × 2 Column temperature: 40 ° C.
Carrier gas: tetrahydrofuran (THF)
Carrier gas flow rate: 1 ml / min, pump temperature: 35 ° C
Detection: RI
Injection amount: Standard polystyrene for 100 microliter calibration curve: Showa Denko Co., Ltd., trade name “shodex” Weight average molecular weight: 1030000 and Tosoh Corporation, weight average molecular weight: 54480, 3840000, 355000, 102000, 37900, 9100, 2630, 870

(Amount of isobutane)
The amount of isobutane is measured using a gas chromatograph. The measuring method is as follows.
The surface layer part and the inside are cut out from the side wall of the foamed molded article using a ham slicer, each sample 20 to 50 mg is placed in a 50 ml vial, precisely weighed, sealed, and gas chromatograph with an autosampler (gas chromatograph manufactured by Shimadzu Corporation). GC-2010, HS autosampler (TurboMatrix HS40, manufactured by PerkinElmer), heated at 160 ° C. for 30 minutes, the gas in the upper space is introduced into the gas chromatograph and measured by the absolute calibration curve method. The calibration curve is created using a standard gas (isobutane: about 1%, normal butane: about 1%, balance gas: nitrogen) manufactured by GL Sciences. The conditions for the gas chromatograph are as follows.
Column: J & W, trade name “DB-1” 1.0 μm × 0.25 mmφ × 60 m
Column temperature: Hold for 6 minutes at an initial temperature of 50 ° C., increase to 200 ° C. at 40 ° C./minute, increase to 270 ° C. at 15 ° C./minute, and then hold at 270 ° C. for 17 minutes.
Inlet temperature 200 ° C., carrier gas He, pressure control mode, pressure 124.2 kPa, purge flow rate 3.0 ml / min. Gas pressure: held at initial pressure 124.2 kPa for 10 minutes and increased to 165.6 kPa at 3.5 kPa / min. And hold for 10 minutes.
Detector: FID, detection temperature 280 ° C., makeup gas He, makeup flow rate 30 ml / min, H ₂ flow rate 40 ml / min, Air flow rate 400 ml / min Autosampler: oven temperature 160 ° C., heat retention time 30 minutes, pressurized gas The pressure is 172.5 kPa, the pressurization time is 3 minutes, the drawing time is 0.2 minutes, the needle temperature is 165 ° C., the transfer line temperature is 180 ° C., and the sample introduction time is 0.05 minutes.

(surface hardness)
Two test pieces (100 × 300 × 20 mm) cut out from the long side wall of the foam molded article were used to select each of 15 locations arbitrarily selected using a surface hardness meter (trade name “ASKER TYPE CS” manufactured by Kobunshi Keiki Co., Ltd.). The arithmetic average value was measured as the surface hardness of the foamed molded product.

(Tensile strength)
A 1000N load cell is connected to a universal testing machine (product name “Tensilon UCT-10T” manufactured by Orientec Co., Ltd.), and a tensile test measurement is performed at a speed of 1000 mm / min by sandwiching the long wall of the box with a U-shaped jig. The primary maximum point load value is taken as the tensile strength.

(Feel evaluation)
Tactile evaluation is performed on the boxes of all the Examples and Comparative Examples by 10 molding workers. Criteria are based on whether it is hard or soft. The box that all 10 people felt hard was rated as 100%, and the box that all 10 people felt soft was rated as 0%.

(Appearance evaluation)
Appearance evaluation is performed on all the boxes of the examples and comparative examples by 10 molding workers. Criteria based on whether the fused part between the foamed particles is smooth or uneven is used as a criterion. 100% is a box in which all 10 people are smooth. Assess as 0%.

(Product multiple)
The weight (a) and volume (b) of the test piece (100 × 300 × 20 mm) cut out from the long side wall of the foamed molded product are measured so that each of them has three or more significant figures, and the formula (b) / (a) Obtain the expansion factor (times).

(Average bubble diameter)
The average cell diameter is measured according to the test method of ASTM D2842-69. Specifically, the side wall portion of the foamed molded product is cut in the vertical direction, and the cut surface is photographed with a scanning electron microscope (trade name “JSM-6360LV” manufactured by JOEL) magnified 100 times.
Next, the photographed image is printed on A4 paper, a straight line with a length of 60 mm is drawn at an arbitrary position, and the average chord length (t) of the bubbles is calculated from the number of bubbles existing on the straight line by the following formula. To do.
Average string length t = 60 / (number of bubbles × photo magnification)
When drawing a straight line, the straight line should be penetrated as much as possible without making point contact with the bubbles. Also, if some of the bubbles come into point contact with a straight line, this bubble is included in the number of bubbles, and if both ends of the straight line are located in the bubble without penetrating the bubbles Includes the bubbles in which both ends of the straight line are located in the number of bubbles.
And based on the calculated average chord length t, the bubble diameter can be calculated by the following equation.
Average bubble diameter (μm) D = t × 1000 / 0.616
Furthermore, the average cell diameter is calculated in the same manner as described above at any five locations in the photographed image, and the arithmetic average value of these average cell diameters is taken as the cell diameter of the foam molded article.

(Molding cycle)
The time from the start of the molding machine through the filling process of the expandable polystyrene particles into the mold, the heating process and the cooling process until the surface pressure of the foamed molded product reaches 0.003 MPa and the mold opening is taken as the molding cycle. taking measurement.

Example 1
(Manufacture of seed particles)
In an autoclave with a stirrer with an internal volume of 100 liters, 120 g of tricalcium phosphate, 1 g of sodium dodecylbenzenesulfonate, 140 g of benzoyl peroxide (purity 75% by weight), 30 g of t-butylperoxy-2-ethylhexyl monocarbonate, ion exchange 40 kg of water and 40 kg of styrene monomer were added and stirred at a rotation speed of 100 rpm to prepare a suspension.
Next, the temperature in the autoclave was raised to 90 ° C. while stirring the stirring blade at a rotation speed of 100 rpm, and then held at 90 ° C. for 6 hours. After that, the temperature in the autoclave is raised to 120 ° C. and held at 120 ° C. for 2 hours, then the temperature in the autoclave is cooled to 25 ° C., polystyrene particles are taken out from the autoclave, and washing and dehydration are repeated. After drying and classification, polystyrene particles having a particle diameter of 0.5 to 0.7 mm and a weight average molecular weight of 302,000 were obtained.

(Manufacture of expandable polystyrene resin particles)
In another autoclave with a stirrer having an internal volume of 100 liters, 11 kg of the polystyrene particles, 30 kg of distilled water, 100 g of magnesium pyrophosphate and 6 g of sodium dodecylbenzenesulfonate were stirred and suspended.
Thereafter, in a dispersion prepared by dispersing 20 g of magnesium pyrophosphate and 2 g of sodium dodecylbenzenesulfonate in 6 kg of prepared distilled water, 130 g of benzoyl peroxide having a purity of 75% by weight and t-butylperoxy- 10 g of 2-ethylhexyl monocarbonate and 2 g of distearyl 3,3-thiodipropionate are dissolved in 5 kg of styrene monomer and added, and stirred uniformly with a homomixer to prepare a suspension. It supplied in the said autoclave hold | maintained at 75 degreeC.

After maintaining the suspension in the autoclave at 75 ° C. for 1 hour with stirring, the polystyrene resin particles were allowed to absorb styrene monomer, benzoyl peroxide and t-butylperoxy-2-ethylhexyl monocarbonate, 28 kg of styrene monomer was continuously supplied into the autoclave at a rate of 9333 g / hr for 3.0 hours, and the temperature in the autoclave was continuously increased so that the suspension became 108 ° C. when the supply of styrene monomer was completed. Subsequently, the inside of the autoclave was heated to 120 ° C. and held for 30 minutes.
On the other hand, after adding 880 g of cyclohexane as a foaming aid to a dispersion obtained by adding 13 g of magnesium pyrophosphate and 0.8 g of sodium dodecylbezenesulfonate to 2 kg of distilled water, the suspension was stirred uniformly with a homomixer. The suspension was press-fitted into the autoclave.

Thereafter, the inside of the autoclave is cooled to 100 ° C., 1720 g of isobutane and 1140 g of normal butane are pressed into the autoclave, and the inside of the autoclave is held at 100 ° C. for 3 hours. After cooling to 20 ° C., the expandable polystyrene particles were taken out, washed and dehydrated, and then dried.
Further, the expandable polystyrene particles were aged at 15 ° C. for 3 days until the bubble diameter after the foaming was completely stabilized to obtain expandable polystyrene resin particles having an average particle diameter of 1.0 mm (weight average molecular weight). (312,000). Next, the surface of the obtained expandable polystyrene resin particles is coated with 0.1% by weight of zinc stearate, 0.05% by weight of hydroxystearic acid triglyceride, and 0.05% by weight of monoglyceride stearate as a surface treatment agent. As a result, expandable particles coated with the surface treatment agent were obtained.
The obtained coated expandable particles were pre-expanded and then aged at 20 ° C. for 24 hours to obtain pre-expanded particles.

(Manufacture of foam moldings)
Spare in the cavity of a foam bead automatic molding machine (trade name “ACE 11 Model” manufactured by Sekisui Koki Co., Ltd.) equipped with a mold having a box-shaped cavity of 320 mm × 540 mm × height 170 mm to which a surface pressure gauge is attached. The foamed particles were filled and thermoformed at a vapor pressure of 0.07 MPa for 20 seconds. Next, the foamed molded body in the cavity of the mold was water-cooled for 15 seconds, then allowed to cool to 0.003 MPa under reduced pressure, taken out of the resin container from the mold, and then dried in a constant temperature room at 30 ° C. for 24 hours. Thus, a container-like foamed molded article having a wall thickness of 20 mm and a foaming factor of 65 times was obtained.

(Example 2)
A foam molded article was obtained in the same manner as in Example 1 except that the particle diameter after classification of the seed particles was 0.4 to 0.6 mm and the average particle diameter of the expandable polystyrene particles was 0.9 mm.
(Example 3)
A foamed molded article was obtained in the same manner as in Example 1 except that 2110 g of isobutane as a foaming agent and 570 g of normal butane were used.

Example 4
A foamed molded article was obtained in the same manner as in Example 1 except that 2290 g of isobutane as a foaming agent and 1410 g of normal butane were used.
(Example 5)
A foamed molded article was obtained in the same manner as in Example 2 except that 1320 g of isobutane as a foaming agent and 1320 g of normal butane were used.

(Example 6)
A foamed molded article was obtained in the same manner as in Example 1 except that 2640 g of isobutane as a foaming agent and 880 g of normal butane were used.
(Example 7)
A foamed molded article was obtained in the same manner as in Example 1 except that the expansion ratio was 75 times.

(Example 8)
A foamed molded article was obtained in the same manner as in Example 1 except that distearyl 3,3-thiodipropionate was changed to 1.6 g during the production of the expandable polystyrene resin particles.
Example 9
The automatic foam bead forming machine has been changed to an automatic foam bead forming machine (trade name “Ace 30” manufactured by Sekisui Koki Co., Ltd.) equipped with a mold having a box-shaped cavity of 350 mm × 580 mm and a height of 260 mm. Except for the above, a foam molded article having a side wall thickness of 22 mm was obtained by molding in the same manner as in Example 1.

(Comparative Example 1)
A foam molded article was obtained in the same manner as in Example 1 except that 1980 g of isobutane as a foaming agent and 4620 g of normal butane were used.
(Comparative Example 2)
A foamed molded article was obtained in the same manner as in Comparative Example 1 except that the particle diameter after classification of the seed particles was 0.4 to 0.6 mm and the average particle diameter of the expandable polystyrene particles was 0.9 mm.

(Comparative Example 3)
Comparative example except that the particle diameter after classification of the seed particles is 0.3 to 0.5 mm, the average particle diameter of the expandable polystyrene particles is 0.7 mm, isobutane as a blowing agent is 790 g, and normal butane is 1850 g. A foamed molded product was obtained in the same manner as in Example 1.
(Comparative Example 4)
A foamed molded product was obtained in the same manner as in Example 1 except that 3170 g of isobutane as a foaming agent and 350 g of normal butane were used.
The results obtained from the examples and comparative examples are shown in Table 1.

From the examples, it can be seen that when the amount of isobutane is in a specific range, a foamed molded article having improved surface hardness, tensile strength, feel and appearance can be obtained in a short time.
From Comparative Examples 1 to 3, it can be seen that when the surface isobutane amount Y is equal to or less than the internal isobutane amount X × 0.6, a foamed molded article having inferior surface hardness, tensile strength and feel can be obtained.
It can be seen from Comparative Example 3 that when the amount of isobutane Y in the surface layer is less than 0.5% by weight, a foamed molded article having inferior surface hardness, tensile strength and feel can be obtained.
From Comparative Example 4, it can be seen that when the amount of isobutane Y in the surface layer exceeds 2.5% by weight, a foamed molded article having inferior tensile strength and appearance can be obtained.

Claims (3)

A polystyrene-based foamed molded article composed of a fused product of polystyrene-based expanded particles,
When the foamed molded product has an isobutane content of X wt% inside and an isobutane content of the surface layer defined as Y wt%, the following formula 0.6X ≦ Y <1.0X (formula 1)
0.5 ≦ Y ≦ 2.5 (Formula 2)
A polystyrene-based foam molded article comprising isobutane so as to satisfy   The polystyrene-based foam molded body is a container having an opening on the upper surface, a bottom on the lower surface, and a side wall located around the opening and connected to the bottom, and the side wall is 15 to 30 mm. The polystyrene-based foam molded article according to claim 1, having a thickness and a height of 150 to 400 mm.   The polystyrene-based foam molded product according to claim 1 or 2, wherein the polystyrene-based foam molded product is formed by an in-mold foam molding method using a bead method.