JP2009256409A - Method for producing polypropylene-based resin foam particle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing polypropylene-based resin foam particles using a starting resin produced by adding a hydrophilic material to a polypropylene-based resin having a small Mw/Mn ratio and giving foam particles excellent in secondary foaming ability, using water as a foaming agent, and giving a foam molded product having good surface properties, melt bonding property and low contractility. <P>SOLUTION: The method for producing polypropylene-based resin foam particles having an expansion ratio of ≥20 times with water as a foaming agent includes: dispersing polypropylene-based resin particles having a ratio of weight-average molecular weight to number-average molecular weight of ≤5.0 in an aqueous dispersion medium in a pressure-resistant container; applying heat and pressure to a softening temperature of the polypropylene-based resin particles or above; and releasing to a pressure area below the internal pressure of the pressure-resistant container, wherein the polypropylene-based resin particles contain a hydrophilic material having a molecular weight of ≤600 and a water content of polypropylene-based resin foam particles immediately after foaming is 0.7-10 wt.%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing expanded polypropylene resin particles. More specifically, for example, the present invention relates to a method for producing expanded polypropylene resin particles that can be suitably used as a raw material for an in-mold foam molded article.

  An in-mold foam molded product obtained by filling polypropylene resin foam particles in a mold and heat-molding with water vapor or the like is an advantage of the in-mold foam molded product, such as arbitrary shape, lightness, heat insulation, etc. Has characteristics. This in-mold foam molded article is superior in chemical resistance, heat resistance, and strain recovery rate after compression, compared to the in-mold foam molded article obtained using polystyrene resin foam particles. In addition, the dimensional accuracy, heat resistance, and compressive strength are superior to the in-mold foam molded article using polyethylene resin expanded particles. Due to these characteristics, in-mold foam molded articles obtained using polypropylene resin foam particles are used in various applications such as heat insulating materials, shock-absorbing packaging materials, automobile interior members, and automobile bumper core materials. Polypropylene resin foamed particles are obtained by dispersing polypropylene resin particles in water in a pressure-resistant container, adding a foaming agent, and maintaining the temperature at a constant temperature near the melting point of the polypropylene resin under high pressure and impregnating the foaming agent. It can be produced by a method of releasing in a low pressure atmosphere. This method is called a decompression foaming method or an autoclave method.

  As important characteristics of the in-mold foam molded body obtained from the polypropylene resin foam particles, (1) excellent surface smoothness of the in-mold foam molded body, and (2) individual foam particles in the in-mold foam molded body. The fusion property between them is good, and (3) the shrinkage ratio of the dimension of the in-mold foam molded product to the mold dimension is small.

  Among these, the smoothness of the surface of the foamed molded product (hereinafter also referred to as surface property) is based on whether one of the contours of the foamed particles on the surface of the in-mold foamed molded product is fused with the adjacent particles. Be evaluated. One of the factors affecting the smoothness of the surface of the in-mold foam molded product is said to be foamability (secondary foaming ability) at the time of molding the foamed particles.

  Further, the fusing property is such that the foam particles adjacent in the molded body are fused on the surface. When the in-mold foam molded body is broken, the more the expanded particles are broken on the fracture surface, the better the fusing property is. If the fusing property is excellent, the strength of the molded product tends to increase.

  Furthermore, in-mold foam molded articles obtained from polypropylene resin expanded particles usually shrink after being molded and removed from the mold. The shrunk in-mold foam molded body recovers in size over time, but does not recover to the mold size. In particular, an in-mold foam molded product with a high foaming ratio causes a large shrinkage because the cell membrane is thin. For this reason, a so-called curing process is required in which the in-mold foamed molded product is held in a warm atmosphere for a certain period of time. Therefore, in order to shorten the curing time, there is a demand for foamed particles that can produce a foamed molded article having a small shrinkage rate (hereinafter also referred to as low shrinkage).

  In Patent Document 1, foamed particles using a polypropylene resin having a small ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw / Mn) as a raw material resin has excellent secondary foaming ability, and the obtained in-mold It is disclosed that the foamed molded article has an excellent appearance with few inter-particle gaps on the surface and inside (many particles are fused with the particles adjacent to all the contours of the foamed particles). Patent Document 1 also discloses that a foamed molded article obtained from expanded particles using a polypropylene resin having a small Mw / Mn as a raw material resin has a small shrinkage rate.

  On the other hand, in the production of polypropylene resin expanded particles, fluorine gas or butane gas has been used as a foaming agent. However, a method of using water as a foaming agent has been proposed from the viewpoint of environmental protection and ensuring safety during production. For example, Patent Document 2 discloses a method for obtaining expanded particles having a high expansion ratio if a polypropylene resin copolymerized with ethylene is used as a raw material resin when producing expanded particles using water as a foaming agent. ing. Patent Documents 3 to 6 disclose hydrophilic substances such as hydrophilic polymers and hydrophilic low-molecular compounds in order to increase the amount of impregnated moisture in the resin when producing foamed particles using water as a foaming agent. A method is disclosed in which a polypropylene resin to which is added is used as a raw material resin.

From the above, a polypropylene resin in which ethylene is copolymerized, a polypropylene resin having a small ratio of weight average molecular weight (Mw) to number average molecular weight (Mn) (Mw / Mn), and a hydrophilic substance is added. When the expanded resin is produced using water as a raw material resin and water as a foaming agent, it is possible to obtain expanded particles with a high expansion ratio. However, in spite of the use of a polypropylene resin having a small Mw / Mn, depending on the type of the hydrophilic substance, foaming which gives a foamed molded article having poor surface smoothness, poor fusion property and high shrinkage rate It has been found that particles may form.
JP-A-3-152136 JP-A-60-212440 International Publication No. WO 97/38048 Japanese Patent Laid-Open No. 10-306179 JP-A-11-92599 JP 2004-67768 A

  The problem of the present invention is that a polypropylene resin having a small Mw / Mn that gives foamed particles having excellent secondary foaming ability and having a hydrophilic substance added is used as a raw material resin, and water is used as a foaming agent. An object of the present invention is to provide a method for producing foamed particles, which is an in-mold foam-molded product having both surface properties, fusion properties and low shrinkage.

  The present inventors have found that the above problem can be solved by using a substance having a molecular weight of 600 or less as a hydrophilic substance. That is, the present invention relates to the following method for producing polypropylene-based resin expanded particles, polypropylene-based resin expanded particles, and a polypropylene-based resin in-mold foam molded product.

(1) Polypropylene resin particles in which the ratio (Mw / Mn) of the weight average molecular weight (Mw) to the number average molecular weight (Mn) is 5.0 or less are dispersed in an aqueous dispersion medium in a pressure resistant container, and the polypropylene resin particles In the method for producing polypropylene-based resin expanded particles having a foaming ratio of 20 times or more with water as a foaming agent, released to a pressure region lower than the internal pressure of the pressure vessel after being heated and pressurized to a temperature equal to or higher than the softening temperature of The polypropylene-based resin particle comprises a hydrophilic substance having a molecular weight of 600 or less, and the water content in the expanded polypropylene-based resin particle immediately after foaming is 0.7 wt% or more and 10 wt% or less. A method for producing resin foam particles.
(2) The method for producing expanded polypropylene resin particles according to (1), wherein the hydrophilic substance having a molecular weight of 600 or less is polyethylene glycol.
(3) The method for producing expanded polypropylene resin particles according to (2), wherein the molecular weight of polyethylene glycol is 200 or more and 600 or less.
(4) The method for producing expanded polypropylene resin particles according to any one of (1) to (3), wherein carbon dioxide gas is used in combination as a foaming agent.
(5) The method for producing expanded polypropylene resin particles according to any one of (1) to (4), wherein the polypropylene resin is a polypropylene resin containing ethylene as a copolymerization component.
(6) Polypropylene resin foam particles obtained by the method for producing polypropylene resin foam particles according to any one of (1) to (5), wherein the average cell diameter is 50 to 800 μm, and in differential scanning calorimetry, 2 Polypropylene resin expanded particles having a crystal structure exhibiting one or more melting points.
(7) An in-mold foam-molded article obtained by foam-molding the polypropylene resin expanded particles according to (6).

  The polypropylene resin expanded particles obtained by the production method of the present invention can provide an in-mold expanded molded product having both surface properties, fusion properties and low shrinkage.

The polypropylene resin constituting the polypropylene resin particles used in the present invention is not particularly limited as long as it contains propylene as a monomer component. For example, propylene homopolymer, α-olefin-propylene random copolymer, An α-olefin-propylene block copolymer is exemplified. These may be used alone or in combination of two or more. Particularly preferred is a polypropylene resin containing ethylene as a comonomer component, wherein the α-olefin is ethylene. The ethylene content is preferably 1 to 10% by weight, more preferably 1 to 7% by weight, further 2 to 7% by weight, further 3 to 7% by weight, and 5% by weight or more and 6% by weight or less, particularly 3.5% by weight or more and 5% by weight or less. In addition, content of the comonomer component based on ethylene in a polypropylene resin can be measured using ¹³ C-NMR.

  The polypropylene resin used in the present invention may contain a monomer other than ethylene as a copolymerization component. Moreover, the polypropylene resin containing ethylene as a comonomer component may contain a monomer other than ethylene as a comonomer component. As comonomer components other than ethylene, 1-butene, isobutene, 1-pentene, 3-methyl-1-butene, 1-hexene, 4-methyl-1-pentene, 3,4-dimethyl-1- Α-olefins having 4 to 12 carbon atoms such as butene, 1-heptene, 3-methyl-1-hexene, 1-octene, 1-decene; cyclopentene, norbornene, tetracyclo [6,2,11,8,13,6 ] Cyclic olefins such as 4-dodecene; 5-methylene-2-norbornene, 5-ethylidene-2-norbornene, 1,4-hexadiene, methyl-1,4-hexadiene, 7-methyl-1,6-octadiene, etc. Diene: vinyl chloride, vinylidene chloride, acrylonitrile, vinyl acetate, acrylic acid, methacrylic acid, maleic acid, ethyl acrylate, acrylic Butyl, methyl methacrylate, maleic anhydride, styrene, methyl styrene, vinyl toluene, vinyl monomers such as divinylbenzene and the like, may be used these one or two or more.

  As the polypropylene resin used in the present invention, either a random copolymer or a block copolymer can be used. In particular, it is preferable to use an ethylene-propylene random copolymer or an ethylene-propylene-butene random terpolymer having high versatility. The ethylene content is 1% to 7% by weight, 3% to 7% by weight, more preferably 3.5% to 6% by weight, particularly 3.5% to 5% by weight. Some ethylene-propylene random copolymers or ethylene-propylene-butene random terpolymers are preferred.

  In addition to polypropylene resins, other thermoplastic resins such as low density polyethylene, linear density polyethylene, polystyrene, polybutene, ionomer, etc. may be mixed and used within a range that does not lose the properties of the polypropylene resin. Polypropylene resin particles may be used.

  The ratio (Mw / Mn) of the weight average molecular weight (hereinafter sometimes referred to as Mw) and the number average molecular weight (hereinafter sometimes referred to as Mn) of the polypropylene resin used in the present invention is 5.0 or less. It is. Mw / Mn is preferably 4.5 or less, more preferably 4.0 or less, and particularly preferably 1.5 or more and 4.0 or less. When Mw / Mn exceeds 5.0, the surface property and low shrinkage of the in-mold foamed molded product are deteriorated.

Mn and Mw are measured under the following conditions.
measuring equipment:
Waters Alliance GPC 2000 Type Gel Permeation Chromatography (GPC)
column:
2 TSKgel GMH6-HT,
Two TSKgel GMH6-HTL (each inner diameter 7.5mm x length 300mm, manufactured by Tosoh Corporation)
Mobile phase: o-dichlorobenzene (containing 0.025% BHT)
Column temperature: 140 ° C
Flow rate: 1.0 mL / min
Sample concentration: 0.15% (W / V) -o-dichlorobenzene Injection amount: 500 μL
Molecular weight calibration: Polystyrene conversion (calibration with standard polystyrene)

  The polypropylene resin used in the present invention can be produced, for example, by subjecting a polypropylene resin to oxidative decomposition (degradation treatment) with an organic peroxide. A polypropylene resin having a desired Mw / Mn can be obtained by adjusting the kind of the original polypropylene resin, the kind and amount of the organic peroxide, the oxidative decomposition temperature and the time. The amount of the organic peroxide used is preferably 0.001 part by weight or more and 0.1 part by weight or less with respect to 100 parts by weight of the polypropylene resin. In order to oxidatively decompose the polypropylene resin, for example, a polypropylene resin added with an organic peroxide can be heated and melted in an extruder.

  Examples of the organic peroxide that can be used include 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, t-butylperoxylaurate, 2,5-dimethyl2,5-di ( Benzoylperoxy) hexane, t-butylperoxybenzoate, dicumyl peroxide, 1,3-bis (t-butylperoxyisopropyl) benzene, t-butylperoxyisopropyl monocarbonate and the like.

  The polypropylene resin used in the present invention can also be obtained by adjusting the polymerization conditions using a catalyst such as a metallocene catalyst or a post metallocene catalyst. It is preferable to use a method in which a general-purpose polypropylene resin is oxidatively decomposed with an organic peroxide because a polypropylene resin having desired characteristics such as molecular weight and melt index can be easily obtained. It is also possible to use a method in which a polypropylene resin obtained using a catalyst such as a metallocene catalyst or a post metallocene catalyst is further oxidized and decomposed with an organic peroxide.

  The polypropylene resin used in the present invention is preferably in an uncrosslinked state, but may be crosslinked by treatment with an organic peroxide or radiation. Two or more polypropylene resins may be mixed.

  The melting point of the polypropylene resin particles used in the present invention is preferably 130 ° C. or higher and 165 ° C. or lower, and more preferably 135 ° C. or higher and 155 ° C. or lower. When the melting point is less than 130 ° C., heat resistance and mechanical strength tend to be insufficient. Moreover, when melting | fusing point exceeds 165 degreeC, there exists a tendency for it to become difficult to ensure the melt | fusion at the time of in-mold foam molding. Here, the melting point is a temperature of 10 to 10 ° C./min from 40 ° C. to 220 ° C., and then cooled to 40 ° C. at a rate of 10 ° C./min with a differential scanning calorimeter. The peak temperature of the endothermic peak in the DSC curve obtained when the temperature is increased again to 220 ° C. at a rate of 10 ° C./min.

  The melt index (hereinafter referred to as MI value) of the polypropylene resin particles that can be used in the present invention is preferably 0.5 g / 10 min to 30 g / 10 min, and more preferably 2 g / 10 min to 20 g / 10. Minutes or less are preferred. If the MI value is less than 0.5 g / 10 minutes, it may be difficult to obtain polypropylene-based resin expanded particles with a high expansion ratio. If it exceeds 30 g / 10 minutes, the bubbles of the polypropylene-based resin expanded particles are likely to break. The open cell ratio of the polypropylene resin expanded particles tends to increase. The MI value is measured according to JIS K7210 at a temperature of 230 ° C. and a load of 2.16 kg.

  In the production method of the present invention, polypropylene resin particles containing a hydrophilic substance having a molecular weight of 600 or less are used. When the hydrophilic substance is an inorganic salt or the like, the formula weight is used instead of the molecular weight. Specific examples of hydrophilic substances include water-soluble inorganic substances such as sodium chloride, calcium chloride, magnesium chloride, borax and zinc borate, water-absorbing organic substances such as melamine, isocyanuric acid and melamine / isocyanuric acid condensate, polyethylene glycol, polyethylene oxide Polyethers such as polyethers, adducts of polyethers to polypropylene and the like, alloys thereof, alkali metal salts of ethylene (meth) acrylic acid copolymers, alkali metal salts of butadiene (meth) acrylic acid copolymers, carboxylated nitriles Examples include hydrophilic polymers such as alkali metal salts of rubber, alkali metal salts of isobutylene-maleic anhydride copolymer, and alkali metal salts of poly (meth) acrylic acid. International publications WO 97/38048, JP-A-10-306179, JP-A-11-92599, and JP-A-2004-67768 describe hydrophilic substances in detail. Two or more of these water-soluble inorganic substances, water-absorbing organic substances and hydrophilic polymers may be used in combination.

  When the molecular weight of the hydrophilic substance exceeds 600, a larger amount of hydrophilic substance is required to obtain expanded particles having the same expansion ratio as compared with the case where a hydrophilic substance having a molecular weight of 600 or less is used. The surface property, fusing property or low shrinkage of the obtained foamed molded product is lowered. When the hydrophilic substance is a polymer, its average molecular weight can be measured using a liquid chromatograph mass spectrometer such as LCQ Advantage manufactured by Thermo Fisher Scientific. Among these hydrophilic substances, polyethylene glycol, melamine or zinc borate is preferable.

  The addition amount of the hydrophilic substance is preferably 0.005 part by weight or more and 2 parts by weight or less, more preferably 0.005 part by weight or more and 1 part by weight or less, further preferably 100 parts by weight of polypropylene resin. Is 0.01 parts by weight or more and 0.5 parts by weight or less. Here, the addition amount of the hydrophilic substance refers to the weight of the hydrophilic substance in a state where water is not absorbed. When the addition amount of the hydrophilic substance is less than 0.005 parts by weight, the expansion ratio of the polypropylene resin expanded particles cannot be improved or the effect of uniforming the bubbles tends to be reduced. When the addition amount exceeds 2 parts by weight, shrinkage of the expanded polypropylene resin particles tends to occur, or the hydrophilic substance tends to be insufficiently dispersed in the polypropylene resin. In addition, when a hydrophilic substance is a hydrophilic polymer, it is preferable to use 0.1 to 0.5 weight part with respect to 100 weight part.

  In addition to the hydrophilic substance, a foaming nucleating agent (cell nucleating agent) may be added to the polypropylene resin. A foam nucleating agent is a substance that promotes the formation of cell nuclei during foaming. Examples of the foam nucleating agent include inorganic substances such as talc, calcium carbonate, silica, kaolin, barium sulfate, calcium hydroxide, aluminum hydroxide, aluminum oxide, and titanium oxide. Among hydrophilic substances, particularly inorganic substances, there are substances that act as a foam nucleating agent. Among these, talc and calcium carbonate are preferable because they are easy to obtain foamed particles having good dispersibility in a polypropylene resin and having uniform cells. A foam nucleating agent may be used independently and may use 2 or more types together.

  The addition amount of the foam nucleating agent varies depending on the foam nucleating agent to be used and cannot be generally determined, but is preferably 0.005 parts by weight or more and 2 parts by weight or less with respect to 100 parts by weight of the polypropylene resin. More preferably, the content is 0.01 parts by weight or more and 1 part by weight or less. When the addition amount of the foam nucleating agent is less than 0.005 parts by weight, the expansion ratio of the polypropylene resin foamed particles may not be increased or the uniformity of the bubbles may be deteriorated. When the addition amount of the foam nucleating agent is more than 2 parts by weight, the average cell diameter of the polypropylene resin foamed particles becomes too small, and the in-mold foam moldability tends to be poor.

  For example, when talc is used as the foam nucleating agent, the addition amount is preferably 0.005 parts by weight or more and 1 part by weight or less, more preferably 0.01 parts by weight with respect to 100 parts by weight of the polypropylene resin. The amount is 0.5 part by weight or less and more preferably 0.02 part by weight or more and 0.2 part by weight or less.

  Furthermore, when producing polypropylene resin particles, if necessary, colorant, antistatic agent, antioxidant, phosphorus processing stabilizer, lactone processing stabilizer, metal deactivator, benzotriazole UV absorber, benzoate Add additives such as light stabilizers, hindered amine light stabilizers, flame retardants, flame retardant aids, acid neutralizers, crystal nucleating agents, amide additives, etc. within a range that does not impair the properties of the polypropylene resin. be able to. When an additive is added to the resin, it is preferably mixed with the polypropylene resin using a blender or the like before the production of the polypropylene resin particles. Moreover, you may add an additive in the molten polypropylene resin.

  Polypropylene resins are usually melted using extruders, kneaders, Banbury mixers, rolls, etc. to produce expanded particles, and processed into resin particle shapes such as cylindrical, elliptical, spherical, cubic, and rectangular parallelepiped shapes. To do. Other resins and additives that are added as necessary can also be added in this step. As for the size of the polypropylene resin particles, the weight of one particle is preferably 0.1 mg or more and 30 mg or less, and more preferably 0.3 mg or more and 10 mg or less. The weight of one polypropylene resin particle is the average resin particle weight obtained from 100 randomly selected polypropylene resin particles, and is hereinafter expressed in mg / grain.

  The polypropylene resin particles are dispersed in an aqueous dispersion medium in a pressure vessel, heated and pressurized to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, and then released into a pressure region lower than the internal pressure of the pressure vessel. Polypropylene resin expanded particles can be obtained.

  In the present invention, water is used as the foaming agent. Whether or not water acts as a foaming agent can be determined by measuring the moisture content described later. As another method, it is also possible to measure the expanded particles immediately after expansion with a polymer moisture meter or a Karl Fischer moisture meter.

  If water is used as a foaming agent, other physical foaming agents may be used in combination. Examples of other physical foaming agents include saturated hydrocarbons such as propane, butane and pentane, ethers such as dimethyl ether, alcohols such as methanol and ethanol, and inorganic gases such as air, nitrogen and carbon dioxide. Among them, it is desirable to use carbon dioxide gas in combination because it has a particularly low environmental load and no danger of combustion. By using water and carbon dioxide gas in combination, it is easy to increase the foaming power, so even when obtaining a high expansion ratio, the amount of foam nucleating agent added can be reduced, resulting in expanded particles having a large average cell diameter. And secondary foamability tends to be good.

  More specifically, the polypropylene resin particles can be made into polypropylene resin expanded particles by the following method.

  Polypropylene resin particles are dispersed in an aqueous dispersion medium in a pressure vessel, and another physical foaming agent is added as necessary. Next, the polypropylene resin particles have a softening temperature or higher, preferably the melting point of the polypropylene resin particles −25 ° C. or higher and the melting point of the polypropylene resin particles + 25 ° C. or lower, more preferably the melting point of the polypropylene resin particles −15 ° C. or higher. The resin particles are heated to a temperature in the range of the melting point of the resin particles + 15 ° C. or lower and pressurized to impregnate the polypropylene resin particles with water as a foaming agent and, if necessary, other foaming agents. Thereafter, one end of the pressure vessel is opened, and the polypropylene resin particles are produced by releasing the polypropylene resin particles into a lower pressure atmosphere than in the pressure vessel.

  There is no particular limitation on the pressure-resistant container in which the polypropylene resin particles are dispersed, and any pressure-resistant container that can withstand the pressure in the container and the temperature in the container at the time of producing the foamed particles may be used.

  Water is preferable as the aqueous dispersion medium. A dispersion medium in which methanol, ethanol, ethylene glycol, glycerin or the like is added to water can also be used.

  In order to prevent coalescence of polypropylene resin particles in the aqueous dispersion medium, it is preferable to use a dispersant. Examples of the dispersant include inorganic dispersants such as tricalcium phosphate, tribasic magnesium phosphate, basic magnesium carbonate, calcium carbonate, barium sulfate, kaolin, talc, and clay.

  Further, it is preferable to use a dispersion aid together with the dispersant. Examples of dispersing aids include N-acyl amino acid salts, alkyl ether carboxylates, carboxylate types such as acylated peptides, alkyl sulfonates, alkyl benzene sulfonates, alkyl naphthalene sulfonates, sulfosuccinates, etc. Sulfate type, sulfate oil, alkyl sulfate, alkyl ether sulfate, sulfate ester type such as alkylamide sulfate, phosphoric acid such as alkyl phosphate, polyoxyethylene phosphate, alkyl allyl ether sulfate An anionic surfactant such as an ester type can be exemplified. Also, polycarboxylic acid type polymer surfactants such as maleic acid copolymer salts and polyacrylates, polyvalent anionic polymer surfactants such as polystyrene sulfonates and naphthalsulfonic acid formalin condensate salts, etc. Can be used.

  As the dispersion aid, it is preferable to use a sulfonate type anionic surfactant, and it is also preferable to use one or a mixture of two or more selected from alkyl sulfonates and alkylbenzene sulfonates. Preferably, an alkyl sulfonate is more preferably used, and using an alkyl sulfonate having a linear carbon chain having 10 to 18 carbon atoms as a hydrophobic group reduces the dispersant adhering to the expanded particles. This is particularly preferable because it can be performed.

  Among these, it is preferable to use one or more selected from tricalcium phosphate, tribasic magnesium phosphate, barium sulfate or kaolin as a dispersant and n-paraffin sulfonic acid soda as a dispersion aid.

  The amount of the dispersant and the dispersion aid varies depending on the type and the type and amount of the polypropylene resin used, but usually 0.2 parts by weight or more and 3 parts by weight of the dispersant with respect to 100 parts by weight of the aqueous dispersion medium. It is preferable to mix 0.001 part by weight or more and 0.1 part by weight or less of the dispersion aid. The polypropylene resin particles are usually preferably used in an amount of 20 to 100 parts by weight with respect to 100 parts by weight of the aqueous dispersion medium in order to improve the dispersibility in the aqueous dispersion medium. .

  The example of the manufacturing method of a polypropylene resin expanded particle is as follows. Polypropylene resin particles containing 0.05 to 2 parts by weight of a hydrophilic substance having a molecular weight of 600 or less and 100 parts by weight of a polypropylene resin and containing a foam nucleating agent are dispersed in an aqueous dispersion medium in a pressure vessel. The polypropylene resin particles are impregnated with water, which is a blowing agent, by heating and pressurizing to a temperature equal to or higher than the softening temperature of the polypropylene resin. Further, the internal pressure in the pressure vessel is increased by press-fitting nitrogen or air, and then released into a pressure region lower than the internal pressure of the pressure vessel to produce polypropylene resin expanded particles. Before releasing into the low-pressure region, nitrogen or air is injected to increase the internal pressure in the pressure-resistant container, thereby adjusting the pressure release speed during foaming and adjusting the foaming ratio and the average cell diameter.

  When using a gaseous physical foaming agent such as carbon dioxide at room temperature, after introducing polypropylene resin particles and an aqueous dispersion medium into a pressure vessel, a physical foaming agent such as carbon dioxide may be introduced into the pressure vessel. . Specifically, for example, the following procedure can be used.

  After preparing polypropylene resin particles, aqueous dispersion medium, dispersant, etc. in a pressure vessel, evacuating the inside of the pressure vessel and introducing carbon dioxide of about 1 to 2 MPa to a temperature above the softening temperature of the polypropylene resin Heat. By heating, the pressure in the pressure vessel rises to about 1.5 MPa to 3 MPa. Carbon dioxide gas is further added near the foaming temperature to adjust to a desired foaming pressure, and the temperature is further adjusted, and then released into a pressure range lower than the internal pressure of the pressure resistant container to obtain polypropylene resin foamed particles. Alternatively, after preparing polypropylene resin particles, an aqueous dispersion medium, a dispersant, etc. in a pressure vessel, evacuating the pressure vessel as necessary, and then heating to a temperature equal to or higher than the softening temperature of the polypropylene resin Carbon dioxide gas may be introduced while being introduced.

  In the present invention, the polypropylene resin particles are dispersed in an aqueous dispersion medium, heated and pressurized to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, and then released into a pressure region lower than the internal pressure of the pressure vessel. The moisture content of the expanded particles is 0.7 wt% or more and 10 wt% or less. A preferred moisture content is 1% by weight or more and 8% by weight or less, and a more preferred moisture content is 1% by weight or more and 5% by weight or less. When the water content is less than 0.7% by weight, only low foaming ratio can be obtained, and when it exceeds 10% by weight, the foamed particles are easily shrunk because the inside of the foamed particles after foaming has a low internal pressure. Shrinkage may remain even after oven curing after foaming.

The moisture content is measured as follows. The water adhering to the surface of the obtained polypropylene-based resin expanded particles is dehydrated with an air stream, and the weight (W1) is measured. Furthermore, the weight (W2) when the expanded particles are dried in an oven at 80 ° C. for 12 hours is measured. The moisture content is calculated by the following formula.
Moisture content (%) = (W1-W2) / W2 × 100

  In addition, when foaming agents other than water and nitrogen are used in combination, the conditions such as foaming temperature and foaming pressure should be the same as when foaming agents other than water and nitrogen are used. Expanded particles are produced and measured similarly.

  The polypropylene resin foamed particles obtained by the above-mentioned method are pressurized with an inorganic gas such as air in a pressure-resistant container, and after applying an internal pressure, the foam is further foamed by heating, and the foaming ratio is further increased. Good. It should be noted that polypropylene resin particles are dispersed in an aqueous dispersion medium in a pressure vessel, impregnated with a foaming agent at high temperature and high pressure, and released into a pressure region lower than the internal pressure of the pressure vessel to cause foaming. The foamed particles obtained by single-stage foaming may be referred to as “single-stage foamed particles”. Furthermore, pressurizing the first-stage foamed particles with an inorganic gas such as air in a pressure-resistant container, applying internal pressure, and then further foaming by heating is called “two-stage foaming” and is obtained by two-stage foaming. The obtained expanded particles may be referred to as “two-stage expanded particles”.

  In the present invention, the expansion ratio of the finally obtained expanded particles is 20 times, preferably 30 times or more, more preferably 32 times or more. The expansion ratio of the finally obtained expanded particles is preferably 45 times or less. If the expansion ratio is less than 20 times, the advantage of weight reduction cannot be obtained, and the flexibility and buffering properties of the obtained in-mold foam molded product tend to be insufficient. There is a tendency that the dimensional accuracy, mechanical strength, heat resistance and the like of the in-mold foam-molded product are insufficient. In order to obtain polypropylene resin expanded particles having an expansion ratio of 20 times or more, it is preferable to obtain by two-stage expansion. A method for measuring the expansion ratio of the polypropylene resin expanded particles will be described later.

  The average cell diameter of the expanded polypropylene resin particles obtained by the present invention is preferably 50 μm or more and 800 μm or less, more preferably 100 μm or more and 600 μm or less, and further preferably 200 μm or more and 500 μm or less. When the average cell diameter is less than 50 μm, there are cases where the shape of the obtained in-mold foam molded product is distorted and the surface is wrinkled. When the average cell diameter exceeds 800 μm, the buffer properties of the in-mold foam molded product to be obtained May decrease. The average cell diameter is measured in accordance with ASTM D3576 with respect to the cut surface of the polypropylene-based resin expanded particles, except for the surface layer portion.

  The open cell ratio of the polypropylene resin expanded particles of the present invention is preferably 0 to 12%, more preferably 0 to 8%, and still more preferably 0 to 5%. When the open cell ratio exceeds 12%, the foamability by steam heating is inferior at the time of in-mold molding, and the obtained in-mold foam molded product tends to shrink.

  The expanded polypropylene resin particles of the present invention preferably have a crystal structure showing two or more melting points in a DSC curve obtained by differential scanning calorimetry. In the case of a polypropylene resin foamed particle having a crystal structure having two or more melting points, in-mold foam moldability is good, and an in-mold foam molded article having good mechanical strength and heat resistance tends to be obtained. Here, the DSC curve obtained by differential scanning calorimetry of the polypropylene resin expanded particles refers to 1 to 10 mg of polypropylene resin expanded particles from 40 ° C. to 220 ° C. at a temperature increase rate of 10 ° C./min by a differential scanning calorimeter. It is a DSC curve obtained when the temperature is raised. In this DSC curve, the temperature indicated by the melting peak that appears is the melting point.

  As described above, the polypropylene resin expanded particles having a crystal structure exhibiting two or more melting points can be easily obtained by setting the temperature in the pressure resistant container at the time of expansion to an appropriate value. The temperature is selected from the melting point of the polypropylene resin used as the base material, preferably the melting point + 3 ° C. or more, and less than the melting end temperature, preferably the melting end temperature −2 ° C. Here, the melting end temperature is 1 to 10 mg of polyolefin resin by a differential scanning calorimeter at a rate of 10 ° C./min from 40 ° C. to 220 ° C., and then at a rate of 10 ° C./min to 40 ° C. This is the temperature at which the melting peak curve obtained when cooling and again raising the temperature to 220 ° C. at a rate of 10 ° C./min returned to the baseline position on the high temperature side.

  When the polypropylene resin expanded particles obtained in the present invention are used for in-mold foam molding, the following conventionally known methods can be used. B) A method to use as it is, b) A method in which an inorganic gas such as air is press-fitted into polypropylene resin foam particles in advance to give foaming ability, and c) A polypropylene resin foam particle is filled in a mold in a compressed state and molded. how to.

Among these, the method (b) in which an inorganic gas such as air is press-fitted into the polypropylene resin foamed particles in advance to impart foaming ability is preferable. Specifically, an in-mold foam molded product can be obtained by the following in-mold foam molding method.
1) The foaming ability is imparted by pressurizing the polypropylene resin foamed particles with air in a pressure resistant container and pressing the air into the polypropylene resin foamed particles.
2) The obtained polypropylene resin expanded particles are filled into a molding space composed of two molds, which can be closed but cannot be sealed.
3) Molding is performed using steam or the like as a heating medium at a steam pressure of about 0.2 to 0.4 MPa (G) and a heating time of about 3 to 30 seconds to fuse the polypropylene-based resin expanded particles.
4) Cool the mold with water 5) Open the mold and take out the in-mold foam molded body.

  The foaming ratio of the in-mold foam molded product to be obtained is not particularly limited, but the foaming ratio is 30 to 60 times, further 35 to 55 times, further 35 to 50 times, and further 40 to 50 times. Is useful.

  The in-mold foam-molded article using the polypropylene resin foam particles obtained in the present invention can be used for applications such as a heat insulating material, a buffer packaging material, an automobile interior member, and a core material for an automobile bumper. It is particularly desirable to use the in-mold foamed foam using the polypropylene resin foam particles obtained in the present invention for the buffer packaging material in which the in-mold foam molded body with a high expansion ratio is often used. is there.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to such examples. In addition, evaluation in Examples and Comparative Examples was performed by the following method.

(Foaming ratio)
After drying 3 to 10 g of expanded particles at 60 ° C. for 6 hours and measuring the weight w (g), the volume v (cm ³ ) is measured by a submerging method, and the true specific gravity ρ _b = w / v of the expanded particles is set. The expansion ratio K = ρ _r / ρ _b was determined from the ratio with the density ρ _r of the polypropylene resin particles before foaming.

(Open cell rate)
Using an air comparative hydrometer (manufactured by Tokyo Science Co., Ltd., model 1000), the closed cell volume of the obtained foamed particles was obtained, and this was obtained by dividing this by the apparent volume obtained separately by the submerged method. The bubble rate (%) was determined by subtracting from 100.

(Bubble uniformity, average bubble diameter)
The foamed particles are cut almost at the center with great care so that the bubble film is not destroyed, and the cut surface is enlarged with a microscope, and the surface layer part having a thickness corresponding to 5% of the diameter of the foamed particles from the surface of the foamed particles The following measurement was performed on the portion (A) excluding. Some arbitrary direction and x-direction, a direction orthogonal thereto is taken as the y direction, x of a single cell, y direction Feret's diameter d _x, measured d _y, the one bubble by the following equation The diameter d _i is obtained.
d _i = (d _x + d _y ) / (2 × 0.785)

As no bias in the radial direction in the portion (A), to measure the d _i for more than 40 bubbles continuously adjacently. The average bubble diameter d of one expanded particle and the variation coefficient u of the bubble diameter are calculated by the following equations. Here, n is the number of bubbles was measured d _i, sigma is the standard deviation of the cell diameter.
d = Σ (d _i ) / n
u = σ / d × 100

U is obtained for three or more foamed particles, and the average is U. Bubble uniformity was evaluated according to the following criteria.
◎: U is 30 or less ○: U exceeds 30 and 35 or less ×: U is more than 35

(Moisture content)
When foaming agents other than water and nitrogen are used, the foamed particles should be the same as when foaming agents other than water and nitrogen are used. Manufacturing. The water adhering to the surface of the obtained particles is dehydrated with an air stream, and the weight (W1) is measured. Furthermore, the weight (W2) when the expanded particles are dried in an oven at 80 ° C. for 12 hours is measured. The moisture content is calculated by the following formula.
Moisture content (%) = (W1-W2) / W2 × 100

(Surface properties of molded products)
The surface property was evaluated using an in-mold foam molded product obtained using a rectangular parallelepiped mold having a design outer dimension of 400 mm × 300 mm × 20 mm. After molding, the mixture was allowed to stand at 23 ° C. for 2 hours, then cured at 65 ° C. for 6 hours, and then allowed to stand in a room at 23 ° C. for 4 hours. . Evaluation of fusing property and low shrinkage was also performed using this in-mold foam molded product.
◎: Wrinkles, less intergranular, beautiful ○: Slight wrinkles, intergranular but good x: Wrinkles, sink marks and poor appearance

(Low shrinkage of molded product)
After molding, it is allowed to stand at 23 ° C. for 2 hours, then cured at 65 ° C. for 6 hours, and then measured for the longitudinal dimension of the in-mold foam molded product obtained by leaving it in a room at 23 ° C. for 4 hours. The ratio of the difference between the mold dimension and the dimension of the in-mold foam molded body with respect to the mold dimension was defined as the mold dimension shrinkage rate, and was evaluated according to the following criteria.
◎: Dimensional shrinkage ratio against mold is 4% or less ○: Dimensional shrinkage ratio against mold exceeds 4% and 7% or less ×: Dimensional shrinkage ratio against mold is larger than 7%

(Fusibility of molded product)
After a crack with a depth of about 5 mm is made with a knife on the surface of the in-mold foam molded body, the in-mold foam molded body is divided along the crack, the fracture surface is observed, and the fracture particles for the total number of particles in the fracture surface The ratio of the number was determined and used as the compact fusion rate.

(Example 1)
100 parts by weight of polypropylene resin (propylene-ethylene random copolymer: ethylene content 3.0% by weight, Mw / Mn = 4.7, MI = 6 g / 10 min, melting point 143 ° C., die swell ratio 1.087) In contrast, 0.5 parts by weight of polyethylene glycol (average molecular weight 300, manufactured by Lion Co., Ltd.) is pre-blended, and then talc (manufactured by Hayashi Kasei Co., Ltd., Talcan Powder PK-S) 0.05 wt. Part was blended. After supplying to a 50 mm single screw extruder and melt-kneading, it was extruded from a cylindrical die having a diameter of 1.8 mm, cooled with water, and cut with a cutter to obtain cylindrical polypropylene resin particles (1.2 mg / particle).

  100 parts by weight of the obtained polypropylene resin particles were put into a pressure vessel together with 200 parts by weight of water, 1.0 part by weight of tricalcium phosphate and 0.05 parts by weight of sodium dodecylbenzenesulfonate, and then deaerated and stirred. However, 6 parts by weight of carbon dioxide gas was put in a pressure vessel and heated to 148 ° C. The pressure at this time was 3 MPa. Immediately after opening the valve at the lower part of the pressure vessel, an aqueous dispersion containing polypropylene resin particles and an aqueous dispersion medium was discharged under atmospheric pressure through an orifice having a diameter of 4 mm to obtain expanded particles (single-stage expanded particles). At this time, during discharge, the pressure was maintained with carbon dioxide gas so that the pressure in the pressure vessel did not decrease.

  The obtained first-stage expanded particles showed two melting points at 138 ° C. and 157 ° C. in differential scanning calorimetry, and the expansion ratio, the open cell ratio, and the average cell diameter were measured. As a result, the expansion ratio was 19 times and the open cell ratio was 0. .6%, excellent bubble uniformity, and average bubble diameter of 340 μm. The moisture content was 3.3% by weight when the pressure inside the pressure vessel was set to 148 ° C., the pressure inside the pressure vessel was 3 MPa with nitrogen, and water foaming was performed.

  After drying the one-stage expanded particles expanded together with carbon dioxide gas at 60 ° C. for 6 hours, impregnating with pressurized air in a pressure resistant container to make the internal pressure about 0.4 MPa, about 0 MPa Two-stage foaming was performed by contacting with 0.08 MPa vapor to obtain two-stage foamed particles with an expansion ratio of 30 times. The two-stage expanded particles showed two melting points in differential scanning calorimetry, and had excellent bubble uniformity with an open cell ratio of 1.3% and an average cell diameter of 435 μm. As a result of observing the surface of the two-stage expanded foam particles with an electron microscope, it was found that the foam diameter of the surface portion was uniform, the surface was not rough, and the thickness of the foam particle surface film was small. . Next, the two-staged foamed particles were again pressurized with air in a pressure resistant container to an air pressure of about 0.19 MPa, and in-mold foam molding was performed. The surface of the in-mold foam molding obtained has excellent smoothness, no wrinkles, small dimensional shrinkage of the molding, little distortion of the molding, excellent fusion between particles, and beautiful in-mold foaming It was a molded body.

（実施例２）
添加剤のポリエチレングリコール（平均分子量３００）を０．２重量部、タルクを０．１重量部とした他は実施例１と同様に発泡、二段発泡、型内成形した。一段発泡粒子は２つの融点を示し、発泡倍率１５倍、連泡率０．７％、気泡の均一性に優れ、平均気泡径２７０μｍであった。含水率は２．０重量％であった。次に、実施例１と同様に発泡倍率３０倍の２段発泡粒子を得た。２段発泡粒子は、示差走査熱量計測定において２つの融点を示し、連泡率０．８％、平均気泡径３７５μｍで気泡の均一性に優れていた。型内成形評価の結果、得られた型内発泡成形体の表面は平滑性に優れ、しわの発生も無く、成形体の寸法収縮が小さく、成形体の歪が少なく、粒子どうしの融着に優れ、美麗な型内発泡成形体であった。

(Example 2)
Foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that the additive polyethylene glycol (average molecular weight 300) was 0.2 parts by weight and talc was 0.1 parts by weight. The first-stage expanded particles exhibited two melting points, the expansion ratio was 15 times, the open cell ratio was 0.7%, the cell uniformity was excellent, and the average cell diameter was 270 μm. The water content was 2.0% by weight. Next, two-stage expanded particles having an expansion ratio of 30 times were obtained in the same manner as in Example 1. The two-stage expanded particles showed two melting points in differential scanning calorimetry, and had excellent bubble uniformity with an open cell ratio of 0.8% and an average cell diameter of 375 μm. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article is excellent in smoothness, no wrinkles are generated, dimensional shrinkage of the molded body is small, distortion of the molded body is small, and the particles are fused. It was an excellent and beautiful in-mold foam molding.

(Example 3)
Foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that the additive polyethylene glycol (average molecular weight 300) was 0.1 parts by weight. The single-stage expanded particles obtained by the single-stage expansion exhibited two melting points, an expansion ratio of 11 times, a continuous bubble ratio of 0.7%, excellent bubble uniformity, and an average cell diameter of 275 μm. The water content was 1.3% by weight. Next, similarly to Example 1, two-stage expanded particles having an expansion ratio of 30 times were obtained. The two-stage expanded particles showed two melting points in differential scanning calorimetry, and had excellent bubble uniformity with an open cell ratio of 0.8% and an average cell diameter of 420 μm. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article is excellent in smoothness, no wrinkles are generated, dimensional shrinkage of the molded body is small, distortion of the molded body is small, and the particles are fused. It was an excellent and beautiful in-mold foam molding.

Example 4
The additive polyethylene glycol (average molecular weight 300) was 0.5 parts by weight and talc 0.1 parts by weight. Carbon dioxide gas as a blowing agent was not used, nitrogen gas was introduced, and the mixture was heated to 151 ° C. Others were evaluated in the same manner as in Example 1, one-stage foaming, two-stage foaming, and in-mold molding. The internal pressure of the pressure vessel for the first stage foaming was set to 3.0 MPa. The expanded particles obtained by the single-stage expansion had two melting points, the expansion ratio was 12 times, the open cell ratio was 1.1%, and the average cell diameter was 235 μm. The uniformity of the bubbles was almost uniform, although somewhat inferior to Examples 1-3. The water content was 3.3% by weight. Next, similarly to Example 1, two-stage expanded particles having an expansion ratio of 30 times were obtained. The two-stage expanded particles showed two melting points in the differential scanning calorimeter measurement, and had excellent bubble uniformity with an open cell ratio of 2.3% and an average cell diameter of 355 μm. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article is excellent in smoothness, no wrinkles are generated, dimensional shrinkage of the molded body is small, distortion of the molded body is small, and the particles are fused. It was an excellent and beautiful in-mold foam molding.

(Example 5)
One-stage foaming, two-stage foaming, and in-mold molding were evaluated in the same manner as in Example 4 except that the additive polyethylene glycol (average molecular weight 600) was 0.5 parts by weight. The internal pressure of the pressure vessel for the first stage foaming was set to 3.0 MPa. The expanded particles obtained by the single-stage expansion had two melting points, the expansion ratio was 10 times, the open cell ratio was 1.2%, and the average cell diameter was 225 μm. The uniformity of the bubbles was almost uniform, although somewhat inferior to Examples 1-3. The water content was 3.0% by weight. Next, two-stage expanded particles having an expansion ratio of 30 times were obtained in the same manner as in Example 1. The two-stage expanded particles showed two melting points in differential scanning calorimetry, and had excellent bubble uniformity with a continuous bubble ratio of 2.5% and an average bubble diameter of 345 μm. Molded in-mold. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article is excellent in smoothness, no wrinkles are generated, dimensional shrinkage of the molded body is small, distortion of the molded body is small, and the particles are fused. It was an excellent and beautiful molded body.

(Example 6)
The same as in Example 1, except that 0.1 parts by weight of melamine pulverized instead of 0.5 parts by weight of polyethylene glycol and 0.03 parts by weight instead of 0.05 parts by weight of talc were used. One-stage foaming, two-stage foaming, and in-mold molding were evaluated. The internal pressure of the pressure vessel for the first stage foaming was set to 3.0 MPa. The expanded particles obtained by the single-stage expansion had two melting points, the expansion ratio was 15 times, the open cell ratio was 0.9%, and the average cell diameter was 240 μm. The uniformity of the bubbles was almost uniform, although somewhat inferior to Examples 1-3. The water content was 2.4% by weight. Next, two-stage expanded particles having an expansion ratio of 30 times were obtained in the same manner as in Example 1. The two-stage expanded particles showed two melting points in differential scanning calorimetry, and had excellent bubble uniformity with an open cell ratio of 1.0% and an average cell diameter of 350 μm. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article is excellent in smoothness, no wrinkles are generated, dimensional shrinkage of the molded body is small, distortion of the molded body is small, and the particles are fused. It was an excellent and beautiful molded body.

(Example 7)
One-stage foaming, two-stage foaming, and in-mold molding were evaluated in the same manner as in Example 1 except that 0.1 part by weight of zinc borate was used instead of 0.5 part by weight of polyethylene glycol and talc was not used. The internal pressure of the pressure vessel for the first stage foaming was set to 3.0 MPa. The expanded particles obtained by the single-stage expansion had two melting points, the expansion ratio was 13 times, the open cell ratio was 1.1%, and the average cell diameter was 235 μm. The bubbles were almost uniform. The water content was 2.0% by weight. Next, two-stage expanded particles having an expansion ratio of 30 times were obtained in the same manner as in Example 1. The two-stage expanded particles showed two melting points in differential scanning calorimetry, and had excellent bubble uniformity with an open cell ratio of 1.0% and an average cell diameter of 350 μm. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article is excellent in smoothness, no wrinkles are generated, dimensional shrinkage of the molded body is small, distortion of the molded body is small, and the particles are fused. It was an excellent and beautiful in-mold foam molding.

(Comparative Example 1)
Polyethylene glycol was not used, and foaming was performed in the same manner as in Example 1 under the conditions shown in the table. Only a low expansion ratio of 6 times was obtained, and the average bubble diameter was as small as 150 μm. In the two-stage foaming, a high vapor pressure is required to increase the expansion ratio to 30 times, and many so-called sticks with foam particles adhering to each other were observed. When the two-stage foamed particles were used and subjected to in-mold foam molding, the resulting in-mold foam molded article had a large dimensional shrinkage, wrinkles were observed, and the appearance was poor.

(Comparative Example 2)
One-stage foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that 1 part by weight of a crosslinked polyalkylene oxide was used instead of polyethylene glycol. The obtained in-mold foam molded article was characterized by large dimensional shrinkage and inferior fusion between particles.

(Comparative Example 3)
Foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that the additive polyethylene glycol (average molecular weight 6000) was 1.0 part by weight and talc 0.1 part by weight. The first-stage expanded particles obtained by the first-stage expansion showed two melting points, the expansion ratio was 12 times, the open cell ratio was 1.3%, and the average cell diameter was 260 μm. The uniformity of the bubbles was almost uniform, although somewhat inferior to Examples 1-3. The water content was 2.2% by weight. Next, similarly to Example 1, two-stage expanded particles having an expansion ratio of 30 times were obtained. The two-stage expanded particles showed two melting points in the differential scanning calorimeter measurement and had excellent bubble uniformity with an open cell ratio of 2.0% and an average cell diameter of 390 μm. As a result of in-mold molding evaluation, the surface of the obtained in-mold foam molded article was excellent in smoothness, free of wrinkles, small dimensional shrinkage of the molded article, less distortion of the molded article, and a beautiful molded article. It was. In the fusion of the particles, a slightly unfused portion was observed as compared with Examples 1 to 3. Despite the addition of a large amount of polyethylene glycol, the expansion ratio of the obtained expanded particles was relatively small.

(Comparative Example 4)
One-stage foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that 0.5 parts by weight of sodium polyacrylate was used instead of polyethylene glycol. The bubbles of the first-stage expanded particles were inferior in uniformity because large bubbles and small bubbles were mixed. When the two-stage foamed particles were used to obtain an in-mold foam molded product, wrinkles were observed on the surface of the molded product, the dimensional shrinkage was large, and the particles were poorly fused.

(Comparative Example 5)
One-stage foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that 0.3 parts by weight of sodium carboxymethylcellulose was used instead of polyethylene glycol and 0.1 parts by weight of talc was used. The bubbles of the first-stage expanded particles were inferior in uniformity because large bubbles and small bubbles were mixed. When the two-stage foamed particles were used to obtain an in-mold foam molded product, wrinkles were observed on the surface of the molded product, the dimensional shrinkage was large, and the particles were poorly fused.

(Comparative Example 6)
One-stage foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that 1.0 part by weight of zeolite A type was used instead of polyethylene glycol and talc was not used. The air bubbles of the first-stage expanded particles are a mixture of coarse and small bubbles, and are inferior in uniformity. In the two-stage foaming, a high vapor pressure is required to increase the expansion ratio to 30 times, and a slight amount of adhesion between the expanded particles was observed. When the two-stage expanded particles were used to obtain an in-mold foam molded product, wrinkles were conspicuous on the surface of the molded product, and the dimensional shrinkage was large.

(Comparative Example 7)
Single-stage foaming, two-stage foaming, and in-mold molding were performed in the same manner as in Example 1 except that 0.2 parts by weight of polypropylene glycol (average molecular weight 2000) and 0.1 parts by weight of talc were used instead of polyethylene glycol. Only a low expansion ratio of 9 times was obtained, and the average bubble diameter was as small as 100 μm. In the two-stage foaming, a high vapor pressure is required to increase the expansion ratio to 30 times, and many sticks to which the foamed particles adhere are observed. When the two-stage expanded particles were used and subjected to in-mold foam molding, the compact had a large dimensional shrinkage, wrinkles were observed, and the appearance was poor.

Claims (7)

  In a pressure vessel, polypropylene resin particles having a weight average molecular weight (Mw) to number average molecular weight (Mn) ratio (Mw / Mn) of 5.0 or less are dispersed in an aqueous dispersion medium, and the softening temperature of the polypropylene resin particles In the method for producing expanded polypropylene resin particles having a foaming ratio of 20 times or more using water as a foaming agent, after heating and pressurizing to the above temperature, the pressure is released to a pressure range lower than the internal pressure of the pressure vessel. The expanded polypropylene resin particles, wherein the resin particles comprise a hydrophilic substance having a molecular weight of 600 or less, and the water content in the expanded polypropylene resin particles immediately after foaming is 0.7 wt% or more and 10 wt% or less Manufacturing method.   The method for producing expanded polypropylene resin particles according to claim 1, wherein the hydrophilic substance having a molecular weight of 600 or less is polyethylene glycol.   The method for producing expanded polypropylene resin particles according to claim 2, wherein the polyethylene glycol has a molecular weight of 200 or more and 600 or less.   The method for producing expanded polypropylene resin particles according to any one of claims 1 to 3, wherein carbon dioxide gas is used in combination as a foaming agent.   The method for producing expanded polypropylene resin particles according to any one of claims 1 to 4, wherein the polypropylene resin is a polypropylene resin containing ethylene as a comonomer component.   A polypropylene resin foamed particle obtained by the method for producing a polypropylene resin foamed particle according to any one of claims 1 to 5, wherein an average cell diameter is 50 µm or more and 800 µm or less, and in differential scanning calorimetry, 2 Polypropylene resin expanded particles having a crystal structure exhibiting one or more melting points.   An in-mold foam-molded article obtained by foam-molding the polypropylene resin expanded particles according to claim 6 in-mold.