JP6698566B2 - Method for producing expanded beads and method for producing expanded molded article - Google Patents

Description

  The present invention relates to a method for producing expanded beads and a method for producing an expanded molded article. More specifically, the present invention relates to a method for producing expanded beads and a method for producing an expanded molded article, which can reduce the amount of a hydrocarbon-based blowing agent having a high environmental load.

  It is known that a foam-molded product made of a polystyrene resin is excellent in rigidity, heat insulation, lightness, water resistance and foam-moldability. Therefore, this foamed molded article is widely used as a cushioning material or a heat insulating material for building materials. However, the foamed molded product made of polystyrene resin has a problem that it has poor chemical resistance and impact resistance.

  On the other hand, it is known that a foamed molded article made of a polyolefin resin has excellent chemical resistance and impact resistance. Therefore, this foamed molded product is used for automobile-related parts. However, since the polyolefin resin is inferior in the retention of the foaming agent, it is necessary to precisely control the foam molding conditions. Therefore, there is a problem that the manufacturing cost is high. In addition, this foamed molded article has a problem that its rigidity is inferior to that of the foamed molded article made of polystyrene resin.

  In order to solve the problems of the foam-molded article made of the polystyrene-based resin or the polyolefin-based resin, various foam-molded articles obtained from composite resin particles of the polystyrene-based resin and the polyolefin-based resin have been reported (JP-A-2015). -189911 gazette: patent document 1). This foamed molded product has both the excellent rigidity and foaming moldability of polystyrene resin and the excellent chemical resistance and impact resistance of polyolefin resin.

JP, 2005-189911, A

  Since the method for producing expanded beads described in the above publication uses a hydrocarbon-based foaming agent, it has a high environmental load, and it is desired to reduce the amount used.

Thus, according to the present invention, a step of impregnating 100 parts by mass of resin particles containing a polyolefin resin and a polystyrene resin with 5 to 9 parts by mass of a hydrocarbon foaming agent to obtain primary expandable particles,
Foaming the primary expandable particles to obtain primary expanded particles;
A step of introducing an inorganic gas of 0.1 to 1.0 MPa into the primary expanded particles as a foaming agent to obtain secondary expandable particles;
Foaming the secondary expandable particles to obtain expanded particles,
Provided is a method for producing expanded beads, wherein the expanded beads have a bulk expansion factor of 2.5 to 10 times the bulk expansion factor of the primary expanded particles.
Further, according to the present invention, there is provided a method for producing a foamed molded article, which comprises the step of in-mold foaming the foamed particles obtained by the above-mentioned method for producing expanded beads to obtain a foamed molded article.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the foaming particle which can reduce the usage-amount of the hydrocarbon type foaming agent with high environmental load can be provided. Further, it is possible to provide a method for producing an expanded molded article using the expanded particles.

5 is a SEM photograph of a cross section of the primary expanded beads of Example 4. 5 is a SEM photograph of a cross section of the primary expanded beads of Example 5. 5 is a SEM photograph of a cross section of the primary expanded beads of Comparative Example 1.

(Method for producing expanded particles)
The expanded beads produced by the method of the present invention contain a polyolefin resin and a polystyrene resin as base resins.
(I) Polyolefin-based resin The polyolefin-based resin is not particularly limited, and known resins can be used. Further, the polyolefin resin may be crosslinked. For example, polyethylene resins such as branched low-density polyethylene, linear low-density polyethylene, medium-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, and cross-linked products of these polymers. , Polypropylene homopolymers, ethylene-propylene random copolymers, propylene-1-butene copolymers, ethylene-propylene-butene random copolymers and the like. In the above example, low density is preferably 0.91~0.94g / cm ^3, more preferably 0.91~0.93g / cm ^3. High density is preferably 0.95~0.97g / cm ^3, more preferably 0.95~0.96g / cm ^3. Medium density is a density between these low density and high density.

The polyolefin resin preferably has a melting point of 95 to 150°C. When the melting point is lower than 95°C, heat resistance may be deteriorated. When the temperature is higher than 150°C, foaming becomes nonuniform and it may be difficult to obtain uniform expanded particles. A more preferable melting point is 100 to 145°C, and a still more preferable melting point is 105 to 145°C.
The polyolefin resin preferably has an MFR of 0.3 to 15 g/10 minutes. When the MFR is less than 0.3 g/10 minutes, foaming variation may occur during foaming. If it is higher than 15 g/10 minutes, the heat resistance may be lowered or the foamed article may be shrunk. A more preferable MFR is 0.5 to 10 g/10 minutes, and even more preferably 0.5 to 5 g/10 minutes.

(Ii) Polystyrene-based resin As the polystyrene-based resin, polystyrene, a polymer of substituted styrene (substituent includes lower alkyl, halogen atom (especially chlorine atom), etc.), styrene as a main component, and styrene Examples thereof include copolymers with other polymerizable monomers. The main component means that styrene accounts for 70% by mass or more of all monomers. Examples of the substituted styrene include chlorostyrenes, vinyltoluenes such as p-methylstyrene, and α-methylstyrene. As other monomers, in addition to substituted styrene, acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, acrylic acid alkyl ester, methacrylic acid alkyl ester, maleic acid mono- or dialkyl, divinylbenzene, ethylene glycol mono- or di( Examples thereof include (meth)acrylic acid ester, polyethylene glycol dimethacrylate, maleic anhydride, N-phenyl maleide and the like. In the examples, alkyl means alkyl having 1 to 8 carbon atoms.

(Iii) Content Ratio of Polyolefin Resin and Polystyrene Resin The base resin preferably contains 100 parts by mass of the polyolefin resin and 100 to 500 parts by mass of the polystyrene resin. If the content of the polystyrene-based resin is more than 500 parts by mass, the crack resistance of the foamed molded product may decrease. On the other hand, if the amount is less than 100 parts by mass, the crack resistance is significantly improved, but the rigidity may be lowered. The content of the preferred polystyrene resin is 100 to 400 parts by mass.
(Iv) Other Resin The base resin may contain a resin other than the polyolefin resin particles and the polystyrene resin. Examples of other resins include acrylic resins derived from acrylic monomers such as acrylonitrile, methacrylonitrile, acrylic acid, methacrylic acid, alkyl acrylate, and alkyl methacrylate.

(V) Other Additives Expanded particles include coloring agents, nucleating agents, stabilizers, fillers (reinforcing agents), higher fatty acid metal salts, flame retardants, antistatic agents, lubricants, natural or synthetic oils, waxes, and UV absorption. Additives such as agents, weathering stabilizers, antifogging agents, anti-blocking agents, slip agents, coating agents, neutron shielding agents may be included.

(Vi) Manufacturing method The expanded particles are obtained by impregnating resin particles with a foaming agent to obtain expandable particles and expanding the expandable particles.
(1) Resin Particles Resin particles containing a base resin obtained by simply mixing a polyolefin-based resin and a polystyrene-based resin can be used as the resin particles for forming expanded particles, but the polyolefin-modified styrene-based resin described below is used. Particles are preferred. More preferable resin particles are polyethylene-modified styrene resin particles.
The polyolefin-modified styrene-based resin particles (also referred to as modified resin particles) are obtained by adding a styrene-based monomer to an aqueous medium in which the polyolefin-based resin particles are dispersed and held, and polymerizing the styrene-based monomer. The method for producing the modified resin particles will be described below.
The polyolefin resin particles for producing the modified resin particles can be obtained by a known method. For example, the polyolefin-based resin particles can be produced by melt-extruding the polyolefin-based resin using an extruder and then granulating by underwater cutting, strand cutting, or the like. The polyolefin-based resin particles may have, for example, a spherical shape, an elliptic spherical shape (egg shape), a cylindrical shape, a prismatic shape, a pellet shape, a granular shape, or the like. Hereinafter, the polyolefin-based resin particles are also referred to as micropellets.

  The polyolefin resin particles may contain a radical scavenger. The radical scavenger may be added to the polyolefin resin in advance, or may be added simultaneously with melt extrusion. As the radical scavenger, a compound having a radical scavenging action such as a polymerization inhibitor (including a polymerization inhibitor), a chain transfer agent, an antioxidant, a hindered amine light stabilizer, etc., which is difficult to dissolve in water, is preferable. .

  Examples of the polymerization inhibitor include t-butylhydroquinone, paramethoxyphenol, 2,4-dinitrophenol, t-butylcatechol, sec-propylcatechol, N-methyl-N-nitrosoaniline, N-nitrosophenylhydroxylamine, triphenyl. Phosphite, tris(nonylphenylphosphite), triethylphosphite, tris(2-ethylhexyl)phosphite, tridecylphosphite, tris(tridecyl)phosphite, diphenylmono(2-ethylhexyl)phosphite, diphenylmonodecylphosphite Phenol-based polymerization inhibitors such as fight, diphenylmono(tridecyl)phosphite, dilaurylhydrogenphosphite, tetraphenyldipropyleneglycol diphosphite, tetraphenyltetra(tridecyl)pentaerythritol tetraphosphite, nitroso-based polymerization inhibitors, Examples thereof include aromatic amine-based polymerization inhibitors, phosphite-based polymerization inhibitors, thioether-based polymerization inhibitors, and the like.

Examples of chain transfer agents include β-mercaptopropionic acid 2-ethylhexyl ester, dipentaerythritol hexakis(3-mercaptopropionate), tris[(3-mercaptopropionyloxy)-ethyl]isocyanurate and the like. To be done.
As the antioxidant, 2,6-di-t-butyl-4-methylphenol (BHT), n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, pentaerythrium Lytyl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 1,3,5-tris(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, 3,9-bis[2-{3-(3-t- Butyl-4-hydroxy-5-methylphenyl)propionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane, distearyl pentaerythritol diphosphite, tris (2,4-di-t-butylphenyl)phosphite, bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite, tetrakis(2,4-di-t-butylphenyl)4,4 '-Biphenylenediphosphonite, bis(2-t-butyl-4-methylphenyl)pentaerythritol diphosphite, 2,4,8,10-tetra-t-butyl-6-[3-(3-methyl -4-Hydroxy-5-t-butylphenyl)propoxy]dibenzo[d,f][1,3,2]dioxaphosphepin, phenyl-1-naphthylamine, octylated diphenylamine, 4,4-bis(α) , Α-Dimethylbenzyl)diphenylamine, N,N′-di-2-naphthyl-p-phenylenediamine, and other phenolic antioxidants, phosphorus antioxidants, amine antioxidants, and the like.

Examples of the hindered amine light stabilizer include bis(2,2,6,6-tetramethyl-4-piperidyl) sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, and bis(1 , 2,2,6,6-pentamethyl-4-piperidyl)-2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butylmalonate and the like.
The amount of the radical scavenger used is preferably 0.005 to 0.5 parts by mass with respect to 100 parts by mass of the polyolefin resin particles.

Polyolefin resin particles include talc, calcium silicate, calcium stearate, synthetic or naturally produced silicon dioxide, a foam nucleating agent such as ethylenebisstearic acid amide, and a methacrylic acid ester copolymer, hexabromocyclododecane. , A flame retardant such as triallyl isocyanurate 6 bromide may be included.
Next, the micropellets are dispersed in an aqueous medium in a polymerization container, and polymerization is performed while impregnating the styrenic monomer into the micropellets.
Examples of the aqueous medium include water and a mixed medium of water and a water-soluble solvent (for example, alcohol).

A solvent (plasticizer) such as toluene, xylene, cyclohexane, ethyl acetate, dioctyl phthalate, or tetrachloroethylene may be added to the styrene-based monomer.
The amount of the styrene-based monomer used is 100 to 500 parts by mass based on 100 parts by mass of the polyolefin-based resin particles. It is more preferably 150 to 360 parts by mass. This usage amount substantially corresponds to the contents of the polyolefin-based resin and the polystyrene-based resin constituting the foamed molded product.
If the amount of the styrene-based monomer used exceeds 500 parts by mass, the particles of the polystyrene-based resin alone may be generated without being impregnated with the polyolefin-based resin particles. In addition, not only the crack resistance of the foamed molded article may decrease, but also the chemical resistance may decrease. On the other hand, if the amount is less than 100 parts by mass, the ability of the expandable particles to retain the foaming agent may decrease. If it decreases, it becomes difficult to achieve high foaming. Also, the rigidity of the foamed molded product may be reduced.

The impregnation of the polyolefin resin particles with the styrene monomer may be carried out during the polymerization or before the polymerization is started. Of these, it is preferable to carry out the polymerization. When the polymerization is carried out after the impregnation, the styrene monomer is likely to be polymerized in the vicinity of the surface of the polyolefin resin particles, and the styrene monomer not impregnated in the polyolefin resin particles is polymerized by itself. However, a large amount of fine particle polystyrene resin particles may be generated.
When performing the impregnation while polymerizing, the polyolefin-based resin particles in the case of calculating the content, the styrene-based monomer impregnated with the polyolefin-based resin, the particles composed of the polystyrene-based resin further impregnated and already polymerized Means
The styrenic monomer can be added continuously or intermittently to the aqueous medium in the polymerization vessel. Particularly, it is preferable to gradually add the styrene-based monomer to the aqueous medium.

  An oil-soluble radical polymerization initiator can be used for the polymerization of the styrene-based monomer. As this polymerization initiator, a polymerization initiator commonly used for the polymerization of styrene-based monomers can be used. For example, benzoyl peroxide, lauroyl peroxide, t-butyl peroxy octoate, t-hexyl peroxy octoate, t-butyl peroxy benzoate, t-amyl peroxy benzoate, t-butyl peroxy vivarate, t- Butyl peroxy isopropyl carbonate, t-hexyl peroxy isopropyl carbonate, t-butyl peroxy-3,3,5-trimethylcyclohexanoate, di-t-butyl peroxy hexahydroterephthalate, 2,2-di-t- Examples thereof include organic peroxides such as butylperoxybutane, di-t-hexyl peroxide, and dicumyl peroxide, and azo compounds such as azobisisobutyronitrile and azobisdimethylvaleronitrile. The oil-soluble radical polymerization initiators may be used alone or in combination.

As a method of adding the polymerization initiator to the aqueous medium in the polymerization container, various methods can be mentioned. For example,
(A) A method in which a polymerization initiator is dissolved and contained in a styrene-based monomer in a container different from the polymerization container, and the styrene-based monomer is supplied into the polymerization container,
(B) A solution is prepared by dissolving a polymerization initiator in a part of a styrene-based monomer, a solvent such as isoparaffin, or a plasticizer. A method of simultaneously supplying this solution and a predetermined amount of styrene-based monomer into the polymerization container,
(C) A dispersion liquid in which a polymerization initiator is dispersed in an aqueous medium is prepared. Examples include a method of supplying this dispersion liquid and a styrene-based monomer into a polymerization container.

It is preferable to add 0.02 to 2.0 mass% of the total amount of the styrene-based monomer to the polymerization initiator.
A water-soluble radical polymerization inhibitor is preferably dissolved in the aqueous medium. The water-soluble radical polymerization inhibitor not only suppresses the polymerization of the styrene-based monomer on the surface of the polyolefin-based resin particles, but also prevents the styrene-based monomer floating in the aqueous medium from polymerizing alone, and thus the polystyrene-based resin The generation of fine particles can be reduced.

  As the water-soluble radical polymerization inhibitor, a polymerization inhibitor capable of dissolving 1 g or more in 100 g of water can be used, and examples thereof include thiocyanate such as ammonium thiocyanate, zinc thiocyanate, sodium thiocyanate, potassium thiocyanate and aluminum thiocyanate. Acid salts, sodium nitrite, potassium nitrite, ammonium nitrite, calcium nitrite, silver nitrite, strontium nitrite, cesium nitrite, barium nitrite, magnesium nitrite, lithium nitrite, nitrite such as dicyclohexylammonium nitrite, mercapto Water-soluble sulfur-containing organic compounds such as ethanol, monothiopropylene glycol, thioglycerol, thioglycolic acid, thiohydroacrylic acid, thiolactic acid, thiomalic acid, thioethanolamine, 1,2-dithioglycerol, 1,3-dithioglycerol Further, ascorbic acid, sodium ascorbate and the like can be mentioned. Among these, nitrite is particularly preferable.

The amount of the water-soluble radical polymerization inhibitor used is preferably 0.001 to 0.04 parts by mass with respect to 100 parts by mass of water in the aqueous medium.
In addition, it is preferable to add a dispersant to the aqueous medium. Examples of such a dispersant include organic saponifiers such as partially saponified polyvinyl alcohol, polyacrylate, polyvinylpyrrolidone, carboxymethylcellulose, and methylcellulose, magnesium pyrophosphate, calcium pyrophosphate, calcium phosphate, calcium carbonate, magnesium phosphate. Inorganic dispersants such as magnesium carbonate and magnesium oxide. Of these, inorganic dispersants are preferred.
When an inorganic dispersant is used, it is preferable to use a surfactant together. Examples of such a surfactant include sodium dodecylbenzene sulfonate, sodium α-olefin sulfonate, and the like.

The shape and structure of the polymerization vessel are not particularly limited as long as they have been conventionally used for suspension polymerization of styrene-based monomers.
The shape of the stirring blade is not particularly limited, and specifically, paddle blades such as V-type paddle blades, furdler blades, inclined paddle blades, flat paddle blades, and pull margin blades, turbine blades, fan turbine blades, etc. Examples thereof include turbine blades and propeller blades such as marine propeller blades. Among these stirring blades, the paddle blade is preferable. The stirring blade may be a single-stage blade or a multi-stage blade. A baffle may be provided in the polymerization container.

Further, the temperature of the aqueous medium when polymerizing the styrene-based monomer in the micropellets is not particularly limited, but it is in the range of −30 to +20° C. of the melting point (measured by the DSC method) of the polyolefin-based resin used. Is preferred. More specifically, 70 to 140°C is preferable, and 80 to 130°C is more preferable. Furthermore, the temperature of the aqueous medium may be a constant temperature or may be increased stepwise from the start to the end of the polymerization of the styrene-based monomer. When increasing the temperature of the aqueous medium, it is preferable to increase the temperature at a temperature rising rate of 0.1 to 2° C./min.
Further, when using particles made of a cross-linked polyolefin resin, the cross-linking may be carried out in advance before impregnating the styrene monomer, or while impregnating and polymerizing the styrene monomer in the micropellets. Or after impregnating and polymerizing the styrene-based monomer in the micropellets.

Examples of the cross-linking agent used for cross-linking the polyolefin resin include 2,2-di-t-butylperoxybutane, dicumyl peroxide, and 2,5-dimethyl-2,5-di-t-butylperoxy. An organic peroxide such as hexane may be used. The crosslinking agents may be used alone or in combination of two or more. In addition, the amount of the crosslinking agent used is usually preferably 0.05 to 1.0 parts by mass with respect to 100 parts by mass of the polyolefin resin particles (micropellets).
Examples of the method for adding the crosslinking agent include, for example, a method of directly adding to the polyolefin resin particles, a method of dissolving the crosslinking agent in a solvent, a plasticizer or a styrene monomer, and a method of adding the crosslinking agent to water. The method of adding above, etc. are mentioned. Among these, a method in which the crosslinking agent is dissolved in the styrene-based monomer and then added is preferred.
Modified resin particles are obtained by the above method.

(2) Foaming Agent Impregnation Step and Foaming Step In the impregnation step, the resin particles are impregnated with a foaming agent in an aqueous medium to obtain expandable particles (wet impregnation method) or impregnated in the absence of a medium. It can be performed by a method of obtaining expandable particles (dry impregnation method). Further, the foaming step can be performed by a method of obtaining foamed particles by heating the expandable particles with a heating medium such as water vapor as necessary to foam them to a predetermined bulk density.

In the present invention, the impregnation step and the foaming step are performed as follows. That is, the impregnation step and the foaming step are
A step of impregnating 100 parts by mass of resin particles containing a polyolefin-based resin and a polystyrene-based resin with 5 to 9 parts by mass of a hydrocarbon-based foaming agent to obtain primary expandable particles (primary impregnation process),
A step of foaming the primary expandable particles to obtain primary expanded particles (primary expanding step),
A step of introducing an inorganic gas of 0.1 to 1.0 MPa as a foaming agent into the primary expanded particles to obtain secondary expandable particles (secondary impregnation step),
It includes four steps including a step of foaming the secondary expandable particles to obtain expanded particles (hereinafter, also referred to as secondary expanded particles) (secondary expansion step). In these four steps, impregnation and foaming are performed twice in order to obtain expanded particles from the expandable particles. In addition, the foaming agent for the first impregnation is a hydrocarbon-based foaming agent (organic foaming agent), and the foaming agent for the second impregnation is an inorganic gas. By using the organic foaming agent in the first impregnation step, the resin particles can be finely foamed without applying excessive stress. Further, by using the inorganic gas for the second time, the amount of the organic foaming agent having a relatively high global warming potential is reduced, so that the environmental load can be suppressed.

In the above four steps, it is preferable to adjust the conditions so that the secondary expanded beads have a bulk expansion ratio of 2.5 to 10 times the bulk expansion ratio of the primary expanded particles. Since the primary expanded particles and the secondary expanded particles have the bulk expansion ratio within this range, it is possible to suppress the environmental load during the production of the expanded particles. The secondary expanded particles may have a bulk expansion ratio of 3 to 9 times or 3 to 8.5 times the bulk expansion ratio of the primary expanded particles.
The hydrocarbon-based foaming agent is not particularly limited, and any known foaming agent can be used. In particular, an organic compound which has a boiling point not higher than the softening point of the polystyrene resin and is gaseous or liquid at normal pressure is suitable. For example, propane, n-butane, isobutane, n-pentane, isopentane, neopentane, cyclopentane, cyclopentadiene, n-hexane, hydrocarbons such as petroleum ether, ketones such as acetone and methyl ethyl ketone, methanol, ethanol, isopropyl alcohol, etc. Examples thereof include low boiling point ether compounds such as alcohols, dimethyl ether, diethyl ether, dipropyl ether, and methyl ethyl ether. These foaming agents may be used alone or in combination of two or more kinds. Among these, it is preferable to use a hydrocarbon having a boiling point of −45 to 40° C. from the viewpoint of quickly substituting with air and suppressing the change with time of the foamed molded product, and more preferably butane.
The inorganic gas is not particularly limited, and any known one can be used. For example, carbon dioxide (carbon dioxide), inorganic gases such as nitrogen and ammonia, etc. may be mentioned. These foaming agents may be used alone or in combination of two or more kinds.

(2-1) Primary Impregnation Step In the primary impregnation step, primary expandable particles are obtained by impregnating resin particles with a hydrocarbon-based foaming agent as a foaming agent in a pressure resistant container.
In the primary impregnation step, if the impregnated amount of the hydrocarbon-based foaming agent is less than 5 parts by mass, the foamability may decrease. When the amount of impregnation is more than 9 parts by mass, the effect of reducing the environmental load may be insufficient. The impregnation amount may be 5 to 9 parts by mass, 6 to 9 parts by mass, or 7 to 9 parts by mass. The "impregnation amount" in the present specification means the amount to be impregnated, that is, the charging amount for so-called impregnation.
The hydrocarbon-based foaming agent is, for example, a predetermined amount of the hydrocarbon-based foaming agent, resin particles, water, and a dispersant charged in an autoclave, which are kept at an impregnation temperature of 60 to 75° C. for 1 to 3 hours while stirring. Thus, the resin particles can be impregnated.

(2-2) Primary foaming step In the primary foaming step, foaming can be performed, for example, by heating the primary expandable particles for 5 to 300 seconds with a heat medium at 105 to 150°C. The vapor pressure (gauge pressure) of the heat medium may be, for example, 0.001 to 0.2 MPa. Steam is generally used as the heat medium.
The primary expanded beads may be aged, for example, at normal pressure before the secondary impregnation step.
(2-3) Secondary impregnation step In the secondary impregnation step, the secondary foamable particles are obtained by introducing the primary foamed particles into the pressure resistant container with an inorganic gas of 0.1 to 1.0 MPa as a foaming agent. ing.
In the secondary impregnation step, if the introduction pressure (gauge pressure) of the inorganic gas is less than 0.1 MPa, the foamability may decrease. When the introduction pressure is higher than 1.0 MPa, the surface of the primary expanded particles may be depressed. The introduction pressure may be 0.1 to 0.6 MPa or 0.1 to 0.5 MPa.

(2-4) Secondary foaming step In the secondary foaming step, foaming can be performed by heating the secondary expandable particles for 5 to 50 seconds with a heat medium at 105 to 150°C, for example. The vapor pressure (gauge pressure) of the heat medium may be, for example, 0.01 to 0.2 MPa. Steam is generally used as the heat medium.
The secondary expanded beads may be aged under normal pressure, for example, before the foam molding step for producing the foam molded article.

(Method for producing foamed molded body)
If the secondary foamed particles are fused and integrated by filling the secondary foamed particles in a mold of a foam molding machine and heating the foamed product in the mold, a foamed molded product having a desired shape can be obtained. it can. As the foam molding machine, an EPS molding machine or the like used when manufacturing a foam molded article from polystyrene-based resin pre-expanded particles can be used. The multiple of the foamed molded product can be prepared, for example, by adjusting the filling amount of the foamed particles in the mold.
The foamed molded product preferably has a multiple of 5 to 70 times. If the multiple is more than 70 times, the strength of the foamed molded product may decrease. On the other hand, if it is less than 5 times, the mass of the foamed molded product may increase. A more preferable multiple is 10 to 60 times.
The foamed molded product can be used as a cushioning material or a packaging material. Specifically, it is used as a cushioning material (cushioning material) for home appliances, electronic components, various industrial materials, applications such as food containers, shock absorbers such as vehicle bumper core materials and door interior cushioning materials. It can be suitably used for the purpose.

Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.
The methods for measuring various physical properties in the following examples are described below.
<MFR>
Melt mass flow rate (MFR) is a semi-auto melt indexer 2A manufactured by Toyo Seiki Seisakusho Co., Ltd. JIS K 7210:1999 "Plastics-Test for melt mass flow rate (MFR) and melt volume flow rate (MVR) of thermoplastics" Method "b" described in method B) was measured by a method of measuring the time required for the piston to move a predetermined distance. The measurement conditions were as follows: sample 3 to 8 g, preheat 270 seconds, load hold 30 seconds, test temperature 190° C., test load 21.18 N, piston movement distance (interval) 25 mm. The sample was tested three times, and the average was taken as the value of melt mass flow rate (g/10 minutes).

<Melting point>
It was measured by the method described in JIS K7121:1987 "Method for measuring transition temperature of plastics". However, the sampling method and temperature conditions were as follows.
A differential scanning calorimeter DSC 6220 (manufactured by SII Nano Technology Co., Ltd.) was used to fill about 6 mg of a sample so that the bottom of an aluminum measuring container had no gap. After the filling, the temperature was lowered from 30° C. to −40° C. under a nitrogen gas flow rate of 20 mL/min, and then the temperature was maintained for 10 minutes. After the holding, the temperature was raised from −40° C. to 220° C. (1st Heating) and held for 10 minutes. Then, the temperature was lowered from 220° C. to −40° C. (Cooling), the temperature was kept for 10 minutes, and the temperature was raised from −40° C. to 220° C. (2nd Heating) to obtain a DSC curve. It should be noted that all the temperature rises/falls were performed at a rate of 10° C./min, and alumina was used as a reference substance. In the present specification, the melting point is a value obtained by reading the temperature at the top of the melting peak observed in the 2nd Heating process using the analysis software attached to the apparatus.

<Bulk expansion ratio of primary and secondary expanded particles>
The bulk expansion ratio of the expanded particles was measured by the following procedure. First, the graduated cylinder was filled with the expanded particles up to a scale of 500 cm ³ . However, when the graduated cylinder was visually observed from the horizontal direction and if even one expanded particle reached the scale of 500 cm ³ , filling was completed. Next, the mass of the expanded particles filled in the graduated cylinder was weighed with an effective number of two decimal places, and the mass was defined as W (g). The bulk expansion ratio of the expanded beads was calculated by the following formula.
Bulk expansion ratio (times) of expanded particles = 500/W

<Multiple of foam molded product>
Effective numbers for the mass (a:g) and volume (b:cm ³ ) of the test piece (Example 75×300×35 mm) cut out from the foamed molded product (which has been dried at 50° C. for 4 hours or more after molding). The measurement was carried out so that it would be 3 digits or more, and the multiple (times) of the foamed molded product was obtained by the formula (b)/(a).

<Heat dimensional change rate of foam molded article: heat resistance>
The heating dimensional change rate of the foamed molded product was measured by the B method described in JIS K6767:1999 "Expanded plastic-polyethylene-Test method". Specifically, a test piece measuring 150 mm in length×150 mm in width×20 mm in height was cut out from the foamed molded product dried at 60° C. for 7 days. On the surface of the test piece, three straight lines having a length of 50 mm oriented in the longitudinal direction are written in parallel at intervals of 50 mm, and three straight lines having a length of 50 mm oriented in the lateral direction are in parallel with each other at 50 mm. I filled in every interval. Then, the test piece was left in a hot air circulation dryer at 80° C. for 168 hours and then taken out, and then in a standard condition (20±2° C., humidity 65±5%) for 1 hour. I left it. Next, the lengths of the six straight lines drawn on the surface of the test piece were measured, and the arithmetic mean value L1 of the lengths of the six straight lines was calculated. The degree of change S was calculated based on the following formula, and the absolute value of the degree of change S was defined as the heating dimension change rate (%).
S=100×(L1-50)/50

<Compressive strength of foam molding>
A test body of 50 mm in length×50 mm in width×25 mm in height was cut out from the foamed molded body dried at 60° C. for 7 days. The compressive strength was measured by the method described in JIS K7220:2006 "Hard foamed plastic-Determination of compression characteristics". That is, using a Tensilon universal testing machine UCT-10T (manufactured by Orientec Co., Ltd.), the compression strength at 25% compression (at 10 mm displacement) at a compression speed of 10 mm/min was measured for a test body of 50 mm×50 mm×25 mm. It was measured.

Example 1
2000 g of polyethylene resin (manufactured by Nippon Polyethylene Co., Ltd., trade name “LV-115”, melting point: 108° C., MFR: 0.3 g/10 min) is supplied to an extruder, melt-kneaded, and pelletized by strand cutting. The resin particles were obtained by converting into a resin.
Next, 40 g of magnesium pyrophosphate as a dispersant and 0.6 g of sodium dodecylbenzenesulfonate as a surfactant were dispersed in 2 kg of pure water in a 5 L autoclave equipped with a stirrer to obtain a dispersion medium. 700 g of resin particles were dispersed in this dispersion medium at 30° C., held for 10 minutes, and then heated to 60° C. to obtain a suspension.
Next, 300 g of styrene monomer in which 0.7 g of dicumyl peroxide was dissolved was dropped into this suspension in 30 minutes, and after the dropping, it was held for 30 minutes to allow the resin particles to absorb the styrene monomer. It was
Next, the temperature of the reaction system was raised to 130° C. and kept for 1 hour and 30 minutes to polymerize the styrene monomer in the resin particles (first polymerization).

Next, the reaction solution of the first polymerization was brought to 90° C., 1.5 g of sodium dodecylbenzenesulfonate was added to this suspension, and then 3.6 g of dicumyl peroxide was dissolved as a polymerization initiator. 1000 g of styrene monomer was added dropwise over 4 hours. Further, 6 g of ethylene bisstearic acid amide as a bubble control agent was added dropwise over 30 minutes from 3 hours after the start of styrene monomer addition, and polymerization (second polymerization) was carried out while being absorbed by the resin particles.
After completion of this dropping, the temperature was raised to 143° C. and the temperature was maintained for 2 hours and 30 minutes to complete the polymerization and obtain modified resin particles.
Next, the temperature was cooled to room temperature, and the modified resin particles were taken out from the 5 L autoclave. After the removal, 1 kg of the modified resin particles and 3 L of water were charged again into a 5 L autoclave equipped with a stirrer. Further, 2 g of sodium dodecylbenzene sulfonate and 80 g of butane as a foaming agent (8 parts by mass of the foaming agent impregnated/100 parts by mass of the modified resin particles) were poured into a 5 L autoclave equipped with a stirrer. After the injection, the temperature was raised to 70° C. and stirring was continued for 2 hours.
Then, it was cooled to room temperature, taken out from the 5 L autoclave, dehydrated and dried to obtain primary expandable particles.

Next, 1000 g of the obtained primary expandable particles was put into a pre-expanding machine, and steam having a gauge pressure of 0.005 MPa was introduced into the can of the pre-expanding machine for 50 seconds and heated to a bulk expansion ratio of 3.8 times. Pre-expansion was performed to obtain primary expanded particles.
Next, the obtained primary expanded beads were stored in an oven at 70° C. for 1 week, and the remaining butane gas was scattered so that the content was 0.05 mass% or less. Then, 500 g of the primary expanded particles in which butane gas was scattered were put into a 5 L autoclave, nitrogen was introduced, and the internal pressure was maintained at 0.3 MPa at room temperature for 24 hours to obtain secondary expandable particles. Then, 500 g was put into the pre-foaming machine, and steam having a gauge pressure of 0.015 MPa was introduced into the can of the pre-foaming machine for 60 seconds and heated to foam at a bulk expansion ratio of 32 times to obtain secondary expanded particles.
Furthermore, the obtained secondary expanded particles were filled in the cavity of a molding die having a cavity of 400 mm×300 mm×30 mm, and 0.1 MPa of steam was introduced into the molding die for 50 seconds to heat, The foamed molded product was cooled until the maximum surface pressure of the foamed molded product dropped to 0.05 MPa to obtain a foamed molded product.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Example 2
The amount of the primary expandable particles impregnated with the foaming agent was 5 parts by mass/100 parts by mass of the modified resin particles, and the steam introduction time was 55 seconds to obtain primary expanded particles having a bulk expansion ratio of 5 times. Example 1 except that the foaming agent species and impregnation pressure to be used were carbon dioxide and 0.8 MPa, and the introduction pressure and introduction time of steam were 0.02 MPa and 90 seconds to obtain secondary expanded particles having a bulk expansion ratio of 16 times. A foam molded article was obtained in the same manner as in.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Example 3
The amount of the primary expandable particles impregnated with the foaming agent was 5 parts by mass/100 parts by mass of the modified resin particles, and the steam introduction time was 55 seconds to obtain primary expanded particles having a bulk expansion ratio of 5 times. In the same manner as in Example 1 except that the foaming agent impregnation pressure was set to 0.5 MPa, the steam introduction pressure and the introduction time were set to 0.02 MPa and 120 seconds, and secondary expanded particles having a bulk expansion ratio of 28 times were obtained. A foam molded product was obtained.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Example 4
The foaming agent species and impregnated amount of the primary expandable particles were pentane and 5 parts by mass/100 parts by mass of the modified resin particles, and the steam introduction time was 50 seconds to obtain primary expanded particles having a bulk expansion ratio of 4 times. Example 1 except that the foaming agent impregnation pressure for impregnating the foamed particles was set to 0.5 MPa, the steam introduction pressure and the introduction time were set to 0.02 MPa and 100 seconds to obtain secondary expanded particles having a bulk expansion ratio of 20 times. In the same manner, a foamed molded product was obtained. A SEM photograph of a cross section of the primary expanded beads is shown in FIG.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Example 5
The steam introduction time was 60 seconds to obtain primary expanded particles having a bulk expansion ratio of 10 times, the impregnation pressure of the foaming agent for impregnating the primary expanded particles was 0.5 MPa, and the introduction pressure and the introduction time of steam were 0.02 MPa and 120. A foamed molded product was obtained in the same manner as in Example 1 except that secondary expanded particles having a bulk expansion ratio of 40 times were obtained per second. A SEM photograph of the cross section of the primary expanded beads is shown in FIG.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Example 6
The amount of primary foamable particles impregnated with the blowing agent was 5 parts by mass/100 parts by mass of the modified resin particles, and the steam introduction time was 40 seconds to obtain primary expanded particles having a bulk expansion ratio of 3.5. In the same manner as in Example 1 except that the foaming agent impregnation pressure to be impregnated in was set to 0.8 MPa, the steam introduction pressure and the introduction time were set to 0.02 MPa and 90 seconds to obtain secondary expanded particles having a bulk expansion ratio of 24 times. Thus, a foamed molded body was obtained.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Reference example 1
The amount of the primary foamable particles impregnated with the blowing agent was 15 parts by mass/100 parts by mass of the modified resin particles, and the introduction pressure and the introduction time of water vapor were 0.01 MPa and 65 seconds to obtain the primary expanded particles having a bulk expansion ratio of 25 times. Except that the foaming agent impregnation pressure for impregnating the primary expanded beads was set to 0.8 MPa, the steam introduction pressure and the introduction time were set to 0.02 MPa and 70 seconds, and secondary expanded particles having a bulk expansion ratio of 55 times were obtained. A foam molded article was obtained in the same manner as in Example 1.
Under these molding conditions, a foamed molded product having a good appearance and fusion was obtained.
Then, the obtained foamed molded article was used to measure the expansion ratio, the rate of change in heating dimension (heat resistance), and the compression strength.

Comparative Example 1
The amount of the primary foamable particles impregnated with the blowing agent was 2.5 parts by mass/100 parts by mass of the modified resin particles, and the steam introduction time was 30 seconds to obtain primary expanded particles having a bulk expansion ratio of 1.6. The foaming agent impregnation pressure for impregnating the expanded beads was set to 0.8 MPa, the steam introduction pressure and the introduction time were set to 0.02 MPa and 120 seconds to obtain secondary expanded particles having a bulk expansion ratio of 1.9. A SEM photograph of the cross section of the primary expanded beads is shown in FIG.
However, with this secondary expanded particle, a good expanded molded article could not be obtained.

Comparative example 2
The primary foaming was not performed, and the foaming agent impregnation pressure for impregnating the resin particles was 0.9 MPa, and the steam introduction pressure and the introduction time were 0.02 MPa and 120 seconds. , Could not be foamed.

Comparative Example 3
Although the primary foaming was not performed, the foaming agent impregnation pressure for impregnating the resin particles was set to 0.9 MPa by using carbon dioxide gas as the foaming agent, and the introduction pressure and the introduction time of water vapor were set to 0.02 MPa and 120 seconds, respectively. As a result, expanded particles with a bulk expansion ratio of 1 were obtained, and expansion could not be performed.
The obtained results are summarized in Table 1.

  From Table 1, by setting the impregnated amount of the hydrocarbon-based foaming agent, the impregnation pressure of the inorganic gas foaming agent, and the bulk expansion ratio of the primary expanded particles and the secondary expanded particles within the specific ranges, It can be seen that the amount of the hydrocarbon-based foaming agent, which has a high environmental load, can be reduced during production.

Claims (5)

A step of impregnating 100 parts by mass of resin particles containing a polyolefin resin and a polystyrene resin with 5 to 9 parts by mass of a hydrocarbon foaming agent to obtain primary expandable particles;
Foaming the primary expandable particles to obtain primary expanded particles;
A step of introducing an inorganic gas of 0.1 to 1.0 MPa into the primary expanded particles as a foaming agent to obtain secondary expandable particles;
Foaming the secondary expandable particles to obtain expanded particles,
The method for producing expanded beads, wherein the expanded beads have a bulk expansion ratio of 2.5 to 10 times the bulk expansion ratio of the primary expanded particles.   The method for producing expanded particles according to claim 1, wherein the inorganic gas is nitrogen or carbon dioxide gas.   The method for producing expanded particles according to claim 1, wherein the resin particles contain 100 to 500 parts by mass of a polystyrene resin with respect to 100 parts by mass of a polyolefin resin.   A method for producing an expanded molded article, which comprises a step of in-mold expanding the expanded beads obtained by the method for producing expanded beads according to any one of claims 1 to 3 to obtain an expanded molded article.   The method for producing a foam molded article according to claim 4, wherein the foam molded article is used as a cushioning material or a packaging material.