JP2010037432A - Polypropylene resin pre-expanded particle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide polypropylene resin pre-expanded particles that gives an expansion-molded product having a high expansion magnification, a glossy surface and a small dimensional shrinkage even when an inorganic gas expanding agent is used. <P>SOLUTION: The polypropylene resin pre-expanded particles are obtained by dispersing polypropylene resin particles in an aqueous dispersion medium in a pressure-resistant container, introducing an inorganic gas as an expanding agent, heating and pressurizing the container to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, and releasing the particles to a zone at a pressure lower than the inner pressure of the pressure-resistant container. The pre-expanded resin particles contain, with respect to 100 parts by weight of a polypropylene resin, (a) 0.001 to 0.5 part by weight of an inorganic substance, water of crystallization of which shows a dehydration start temperature of 200°C or higher, (b) 0.005 to 0.5 part by weight of a fatty acid amide, (c) 0.0001 to 0.5 part by weight of a silicate compound, and (d) 0.005 to 0.5 part by weight of a trivalent organic phosphorus compound. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a polypropylene resin pre-expanded particle which can be suitably used for the production of an in-mold foam-molded product of a polypropylene resin used for buffer packaging materials, boxing, heat insulating materials, automobile members, etc. The present invention relates to a polypropylene resin foam molded article obtained by filling a mold and heating.

  Conventionally, polypropylene resin particles are stirred together with dispersants such as poorly water-soluble inorganic substances, dispersion aids such as surfactants, and inorganic gas foaming agents such as volatile organic foaming agents and carbon dioxide, nitrogen and air as necessary. While being dispersed in an aqueous medium, the temperature is raised and the resin particles are impregnated with a foaming agent at a constant pressure and a constant temperature, and then released into a low-pressure atmosphere to obtain polypropylene resin foamed particles (for example, And Patent Documents 1 to 3).

  Propane, butane, pentane, etc. are known as volatile blowing agents, but many of these are not only dangerous due to flammability, but also cause global warming when released to the atmosphere. He also had environmental problems.

  On the other hand, inorganic gas foaming agents such as carbon dioxide, nitrogen, and air have the advantage of being nonflammable and environmentally friendly, but have a high foaming ratio that can be obtained when using volatile foaming agents. There was a problem that it was difficult to obtain the pre-expanded particles. In order to solve this problem in the inorganic gas foaming agent, methods have been proposed in which polypropylene resin particles contain an inorganic substance such as aluminum hydroxide or borax (Patent Documents 4 and 5). However, when producing a foam molded article using the pre-expanded particles obtained by this method, not only is the appearance of the foam molded article uneven due to the large variation in cell diameter, but the shrinkage ratio of the foam molded article is large, There was a problem that the dimensional stability of the foam molded article was poor.

  In order to solve such a problem, a method in which a metal borate such as zinc borate is present in polypropylene resin particles has been proposed (Patent Document 6). According to this method, the uniformity of the bubble diameter is improved and the uneven appearance is also eliminated, but in order to obtain a high expansion ratio, it is necessary to increase the amount of addition, in which case the bubble diameter becomes fine, There was a problem that the foaming ability of the pre-foamed particles at that time was lowered, and a large number of particle gaps remained on the surface of the molded body, which deteriorated the surface beauty.

In addition, for the purpose of preventing shrinkage when the expansion ratio is high, 2,2-methylenebis (4,6-di-phosphate), which is a pentavalent organic phosphate compound generally used as a crystal nucleating agent for polypropylene, is used. A method using tert-butylphenyl) sodium has been proposed (Patent Documents 7 and 8). However, when this method is used, there is a problem that the cell diameter is reduced, the particle gap on the surface of the foamed molded article is increased, and the surface beauty is impaired.
JP 52-77174 A JP-A-60-245650 JP 60-229936 A JP 61-4738 A JP-A-3-223347 WO 98/25996 Japanese Patent Laid-Open No. 5-156065 JP-A-6-192462

  An object of the present invention is to provide a polypropylene resin pre-expanded particle that provides a foamed molded article having a high expansion ratio, a beautiful surface, and a small dimensional shrinkage even when an inorganic gas foaming agent is used.

  As a result of intensive studies for solving the above-mentioned problems, the present inventor has found that the polypropylene resin pre-expanded particles are 3 in addition to the inorganic substance, fatty acid amide and silicate compound whose dehydration start temperature of crystal water is 200 ° C. or higher. By including a polyvalent organic phosphorus compound on the surface and / or inside, it is possible to obtain pre-expanded polypropylene resin particles that give a foamed molded article having a high surface expansion ratio and a small dimensional shrinkage ratio while having a high expansion ratio. The headline, the present invention has been completed.

  That is, in the first aspect of the present invention, polypropylene resin particles are dispersed in an aqueous dispersion medium in a pressure vessel, an inorganic gas is introduced as a foaming agent, and heated and pressurized to a temperature equal to or higher than the softening temperature of the polypropylene resin particles. Thereafter, the polypropylene resin pre-expanded particles obtained by discharging into a pressure region lower than the internal pressure of the pressure vessel are inorganic in which (a) the dehydration start temperature of crystal water is 200 ° C. or more with respect to 100 parts by weight of the polypropylene resin. 0.001 to 0.5 part by weight of the substance, (b) 0.005 to 0.5 part by weight of the fatty acid amide, (c) 0.0001 to 0.5 part by weight of the silicate compound And (d) a polypropylene resin pre-expanded particle comprising 0.005 part by weight or more and 0.5 part by weight or less of a trivalent organophosphorus compound.

As a preferred embodiment,
(1) The inorganic substance whose dehydration start temperature of crystal water is 200 ° C. or higher is zinc borate,
(2) The fatty acid amide is one or more selected from erucic acid amide and ethylene bis stearic acid amide,
(3) The silicate compound is one or more selected from talc, mica, and kaolin.
(4) Trivalent organophosphorus compound is tris (2,4-di-t-butylphenyl) phosphite, tetrakis (2,4-di-t-butylphenyl) [1,1-biphenyl] -4 , 4'-diylbisphosphonite, bis (2,4-di-t-butylphenyl) pentaerythritol diphosphite, bis [2,4-bis (1,1-dimethylethyl) -6-methylphenyl] One or more selected from ethyl ester phosphorous acid,
(5) In the pressure vessel, polypropylene resin particles are dispersed in an aqueous dispersion medium, an inorganic gas is introduced as a foaming agent, heated and pressurized to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, Obtained by giving foaming ability to the polypropylene resin pre-expanded particles obtained by releasing into a pressure range lower than the internal pressure, and heating,
The present invention relates to the above-mentioned polypropylene resin pre-expanded particles.

  A second aspect of the present invention relates to a polypropylene resin foam molded article obtained by filling the polypropylene resin pre-expanded particles described above in a mold and heating.

  Even if the polypropylene resin pre-expanded particles of the present invention are filled in a mold and heated to produce a polypropylene resin foam molded article, the surface is beautiful and the dimensional shrinkage ratio is small even though the expansion ratio is high.

  The polypropylene resin pre-expanded particles of the present invention are obtained by dispersing polypropylene resin particles in an aqueous dispersion medium in a pressure vessel and heating and pressurizing to a temperature equal to or higher than the softening temperature of the polypropylene resin particles. Is obtained by discharging to a low pressure region, and (a) the inorganic substance whose dehydration start temperature of crystal water is 200 ° C. or higher is 0.001 part by weight or more and 0.5% by weight with respect to 100 parts by weight of the polypropylene resin. Part (b) fatty acid amide 0.005 part by weight or more and 0.5 part by weight or less, (c) silicate compound 0.0001 part by weight or more and 0.5 part by weight or less, (d) a trivalent organophosphorus compound 0.005 part by weight or more and 0.5 part by weight or less.

  As the polypropylene resin constituting the polypropylene resin pre-expanded particles in the present invention, polypropylene polymerized using a Ziegler catalyst or a metallocene catalyst, an ethylene-propylene random copolymer, a propylene-butene random copolymer, an ethylene- Examples thereof include polypropylene resins such as a propylene block copolymer and an ethylene-propylene-butene terpolymer, and these are used alone or in combination. In addition, these polypropylene resins are preferably non-crosslinked, but crosslinked resins can also be used.

  A polypropylene resin is made into the shape of a polypropylene resin particle using a known method. For example, it is melted by using an extruder, a kneader, a Banbury mixer (trademark), a roll or the like, and processed into polypropylene resin particles with a weight of preferably 0.2 to 10 mg, more preferably 0.5 to 6 mg. Is done. In general, it is preferable to melt by using an extruder and to manufacture by a strand cutting method. For example, polypropylene resin particles having a desired shape can be obtained by cutting and solidifying polypropylene resin extruded in a strand form from a circular die with water, air, or the like. If necessary, an underwater cut method in which a molten polypropylene resin is extruded into water and then cut and granulated in water may be used.

  In the present invention, (a) an inorganic substance having a crystal water dehydration start temperature of 200 ° C. or higher is used. In the present invention, the dehydration start temperature of crystal water refers to a temperature at which weight reduction starts in thermogravimetry (TGA). Examples of such substances include aluminum hydroxide (crystal water dehydration start temperature: 200 ° C.), magnesium hydroxide (crystal water dehydration start temperature: 300 ° C.), and zinc borate (crystal water dehydration start temperature: 290 ° C.) and the like, and these can be used alone or in combination. Among these, it is preferable to use zinc borate.

  The amount of the inorganic substance having a crystal water dehydration start temperature of 200 ° C. or more is 0.001 part by weight or more and 0.5 part by weight or less, preferably 0.01 part by weight or more with respect to 100 parts by weight of the polypropylene resin. 0.3 parts by weight or less, more preferably 0.01 parts by weight or more and 0.1 parts by weight or less. If the amount is less than 0.001 part by weight, a sufficient expansion ratio cannot be obtained. If the amount exceeds 0.5 part by weight, the polypropylene resin pre-expanded particles are filled in a mold and heated to obtain a surface of the polypropylene resin foam molded body. Particle gaps remain.

  In the present invention, (b) fatty acid amide is used. Examples of the fatty acid amide include monoamides such as erucic acid amide, oleic acid amide, and stearic acid amide, and diamides such as methylene bis stearic acid amide and ethylene bis stearic acid amide, which are used alone or in combination. I can do it. Among these, it is preferable to use one or more selected from erucic acid amide and ethylenebisstearic acid amide, and it is most preferable to use erucic acid amide.

  The amount of the fatty acid amide used is 0.005 part by weight or more and 0.5 part by weight or less, preferably 0.01 part by weight or more and 0.3 part by weight or less, more preferably 0 with respect to 100 parts by weight of the polypropylene resin. 0.01 parts by weight or more and 0.1 parts by weight or less. If the amount is less than 0.005 parts by weight, the filling property of the polypropylene resin pre-expanded particles in the mold is deteriorated. If the amount exceeds 0.5 parts by weight, the amount of the inorganic dispersant remaining on the surface of the polypropylene resin pre-expanded particles after foaming increases. However, the fusion between the polypropylene resin pre-expanded particles becomes worse during molding.

  In the present invention, (c) a silicate compound is used. Examples of the silicate compound include talc, mica, kaolin, pyroxene, olivine, amphibole and the like, and these can be used alone or in combination. Among these, at least one selected from talc, mica, and kaolin is preferable, and kaolin is most preferable.

  The usage-amount of a silicate compound is 0.0001 weight part or more and 0.5 weight part or less with respect to 100 weight part of polypropylene resins. If the amount is less than 0.0001 part by weight, the bubble diameter varies and the expansion ratio becomes small. If the amount exceeds 0.5 part by weight, the bubble diameter of the polypropylene resin pre-expanded particles becomes fine, and the polypropylene resin pre-expanded particles are made of gold. Particle gaps remain on the surface of the polypropylene resin foam molding obtained by filling the mold and heating.

  In the present invention, (d) a trivalent organic phosphorus compound is used. Examples of the trivalent organic phosphorus compound include tris (2,4-di-t-butylphenyl) phosphite and tetrakis (2,4-di-t-butylphenyl) [1,1-biphenyl] -4. , 4'-diylbisphosphonite, bis (2,4-di-t-butylphenyl) pentaerythritol diphosphite, bis [2,4-bis (1,1-dimethylethyl) -6-methylphenyl] Examples thereof include ethyl ester phosphorous acid, and these can be used alone or in combination. Among these, it is particularly preferable to use tris (2,4-di-t-butylphenyl) phosphite.

  The amount of the trivalent organic phosphorus compound used is 0.005 parts by weight or more and 0.5 parts by weight or less with respect to 100 parts by weight of the polypropylene resin. If it is less than 0.005 parts by weight, the effect of addition is not recognized, and if it exceeds 0.5 parts by weight, the effect is saturated.

  Furthermore, when manufacturing the polypropylene resin particles, various additives can be added as necessary within the range not impairing the properties of the polypropylene resin. Examples of additives include colorants such as carbon black and organic pigments, antistatic agents such as alkyldiethanolamide, alkyldiethanolamine, hydroxyalkylethanolamine, fatty acid monoglyceride, fatty acid diglyceride, FLAMESTTAB (registered trademark) NOR116 (Ciba), Non-halogen flame retardants such as MELAPUR (registered trademark) MC25 (Ciba) are exemplified.

  In the present invention, that the polypropylene resin pre-expanded particles contain (a) to (d) means that these compounds are present on the surface and / or inside of the polypropylene resin pre-expanded particles.

  The polypropylene resin pre-expanded particles of the present invention are obtained by dispersing polypropylene resin particles in an aqueous dispersion medium in a pressure vessel and heating and pressurizing to a temperature equal to or higher than the softening temperature of the polypropylene resin particles. Is also obtained by discharging to a low pressure range. Further, the polypropylene resin pre-expanded particles obtained by the above method may be given foaming ability and heated to form polypropylene resin pre-expanded particles. Thus, there exists a tendency which can obtain the pre-expanded particle | grain with a higher expansion ratio by providing foaming ability and heating. The method is not particularly limited. For example, after the polypropylene resin particles are dispersed in an aqueous dispersion medium in a pressure vessel and heated to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, the pressure is lower than the internal pressure of the pressure vessel. The polypropylene resin pre-expanded particles obtained by discharging into the pressure region are pressurized with an inorganic gas such as air in a container to give foaming ability, and then heated with steam or a heater.

  Hereinafter, after the polypropylene resin particles are dispersed in an aqueous dispersion medium in the pressure vessel and heated and pressurized to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, they are discharged to a pressure range lower than the internal pressure of the pressure vessel. When the polypropylene resin pre-expanded particles obtained are referred to as single-stage expanded particles, and the polypropylene resin pre-expanded particles obtained by adding foaming ability to the single-stage expanded particles and heating are referred to as two-stage expanded particles. There is.

  The pressure vessel to be used is not particularly limited as long as it can withstand the pressure in the vessel and the temperature in the vessel at the time of producing the pre-foamed particles, and examples thereof include an autoclave type pressure vessel.

  The aqueous dispersion medium in the present invention is not particularly limited as long as it contains water as a main component, and it is usually preferable to use water. The amount of the aqueous dispersion medium used is from 100 parts by weight to 500 parts by weight with respect to 100 parts by weight of the polypropylene resin particles in order to improve the dispersibility of the polypropylene resin particles in the dispersion medium. Is preferred.

  If necessary, a dispersant and a dispersion aid can be used in a dispersion in which polypropylene resin particles are dispersed in an aqueous dispersion medium. There is no restriction | limiting in particular as a dispersing agent used by this invention, The inorganic type dispersing agent generally used can be used.

  Specifically, aluminosilicates such as kaolin and talc, mainly composed of barium sulfate and silica-alumina, calcium phosphates such as aluminum oxide, titanium oxide, tricalcium phosphate and hydroxyapatite, calcium carbonate, magnesium pyrophosphate, phosphorus Examples include magnesium acid, basic magnesium carbonate, and basic zinc carbonate.

  Among these, aluminosilicate, calcium phosphate, and magnesium phosphate mainly composed of barium sulfate and silica-alumina are preferable from the viewpoint of having a dispersion effect with a small amount of use and a small wastewater treatment load.

  The addition amount of such a dispersant is not particularly limited and is appropriately adjusted so that the dispersion stabilizing effect is manifested, and is appropriately adjusted in consideration of the addition ratio with the dispersion aid. However, it is preferably 0.01 to 5 parts by weight, more preferably 0.05 to 4 parts by weight, most preferably 100 parts by weight of the polypropylene resin particles. Is 0.1 to 3 parts by weight. If the amount is less than 0.01 parts by weight, the dispersibility of the polypropylene resin particles tends to decrease at a temperature higher than the softening point temperature of the polypropylene resin particles. If the amount exceeds 5 parts by weight, a dispersant is added to the surface of the pre-expanded polypropylene resin particles. As a result, the melt-bonding property of the foamed molded product tends to decrease.

  As the dispersion aid used in the present invention, a surfactant can be used. As the surfactant, commonly used anionic surfactants, nonionic surfactants, cationic surfactants, and amphoteric surfactants can be used.

  Specifically, (A) alkyl sulfonate (higher alcohol sulfate ester salt), alkyl sulfonate, alkane sulfonate, alkyl benzene sulfonate, alkyl naphthalene sulfonate, sulfosuccinate, α-olefin sulfonic acid Salt, N-acyl sulfonate, alkyl sulfate, alkyl ether sulfate, alkyl allyl ether sulfate, alkyl amide sulfate, alkyl phosphate, alkyl ether phosphate, alkyl allyl ether phosphate, alkyl ether carvone Anionic surfactants such as acid salts and N-acyl amino acid salts, (B) alkyl and alkylallyl polyoxyethylene ethers, alkylallyl formaldehyde condensed polyoxyethylene ethers, polyoxyethylene polyoxypropyl alkyl ethers Glycerin ester polyoxyethylene ether, sorbitan ester polyoxyethylene ether, sorbitol ester polyoxyethylene ether, polyethylene glycol fatty acid ester, glycerin ester, higher fatty acid glycerin ester, polyglycerin ester, sorbitan ester, propylene glycol ester, Nonionic surfactants such as sucrose ester, aliphatic alkanolamide, polyoxyethylene fatty acid amide, polyoxyethylene alkylamine, amine oxide, (c) aliphatic amine salt, hydroxyalkyl monoethanolamine salt, aliphatic quaternary Cationic surfactants such as ammonium salts, amphoteric surfactants such as (d) carboxybetaine, imidazolinium betaine, aminocarboxylate Agents, etc.

  From the viewpoint of the stability of the dispersion, the surfactant is preferably an anionic surfactant, more preferably an alkyl sulfonate, alkane sulfonate, alkyl benzene sulfonate, alkyl naphthalene sulfonate, sulfosuccinic acid. Sulfonates such as salts, α-olefin sulfonates, N-acyl sulfonates, alkyl sulfates, alkyl ether sulfates, alkyl allyl ether sulfates, alkyl amide sulfates, most preferably alkyl sulfonates , Alkane sulfonate, α-olefin sulfonate, and alkylbenzene sulfonate.

  The addition amount of such a dispersion aid is not particularly limited and may be appropriately adjusted so that the dispersion is stable, but is 0.001 part by weight or more and 0.5 part by weight based on 100 parts by weight of the polypropylene resin particles. The amount is preferably not more than parts by weight, more preferably not less than 0.003 parts by weight and not more than 0.3 parts by weight, and most preferably not less than 0.005 parts by weight and not more than 0.2 parts by weight. If the amount is less than 0.001 part by weight, the dispersibility of the polypropylene resin particle tends to be lowered at a temperature higher than the softening point temperature of the polypropylene resin particle. If the amount exceeds 0.5 part by weight, foaming of the dispersion becomes intense and wastewater treatment is difficult. The load tends to increase.

  In the present invention, an inorganic gas is used as the foaming agent. Examples of the inorganic gas include carbon dioxide (carbon dioxide), air, oxygen, nitrogen, water, and the like. Among these, foaming property and the resulting polypropylene resin foamed particles have a small variation in magnification and a variation in bubble diameter. From the viewpoint of reducing the size, it is preferable to use carbon dioxide or a combination of carbon dioxide and water.

  Specifically, polypropylene resin particles, if necessary, a dispersant and a dispersion aid are dispersed in an aqueous dispersion medium in a pressure vessel, an inorganic gas is introduced as a foaming agent, and the mixture is heated to a predetermined pressure with stirring. And heated to a predetermined temperature equal to or higher than the softening temperature of the polypropylene resin particles and held for a certain period of time, preferably 5 to 180 minutes, more preferably 10 to 60 minutes. Polypropylene resin pre-expanded particles can be produced by opening a valve provided in the lower part of the pressure vessel and releasing it under a low-pressure atmosphere (usually under atmospheric pressure).

  In addition, when using the water which comprises an aqueous dispersion medium as a foaming agent, it is preferable to pressurize the inside of a pressure-resistant container with inorganic gas, such as nitrogen, air, and a carbon dioxide.

  When the dispersion containing the polypropylene resin particles is discharged under a low-pressure atmosphere, it can also be discharged through an opening orifice of 2 to 10 mmφ for the purpose of adjusting the flow rate and reducing variation in magnification. In some cases, the low-pressure atmosphere is filled with saturated steam for the purpose of increasing the expansion ratio.

  The temperature at which the inside of the pressure vessel is heated (hereinafter sometimes referred to as the foaming temperature) varies depending on the melting point [Tm (° C.)] of the polypropylene resin used, the type of foaming agent, etc. The temperature is equal to or higher than the softening temperature of the resin, and is preferably in the range of Tm-30 (° C) to Tm + 10 (° C). Further, the pressure for pressurizing the inside of the pressure vessel (hereinafter sometimes referred to as foaming pressure) varies depending on the type of polypropylene resin used, the type of foaming agent, and the desired expansion ratio of the pre-expanded particles, and cannot be specified unconditionally. Is determined from a range of approximately 1 to 8 MPa (gauge pressure).

  The melting point of the polypropylene resin here refers to melting the polypropylene resin by heating the sample 5 to 6 mg from 40 ° C. to 220 ° C. at a rate of 10 ° C./min using a differential scanning calorimeter. From the DSC curve obtained when the temperature was further increased from 40 ° C. to 220 ° C. at 10 ° C./min after crystallization by lowering the temperature from 220 ° C. to 40 ° C. at 10 ° C./min. This is a value obtained as the melting peak temperature at the time of temperature rise.

  The average cell diameter (L (av)) of the polypropylene resin pre-expanded particles of the present invention is preferably 130 μm or more and 500 μm or less, more preferably 160 μm or more and 400 μm or less, and further preferably 210 μm or more and 350 μm or less. When the average cell diameter (L (av)) is less than 130 μm, the resulting polypropylene-based resin foam molded article may suffer from problems such as poor fusion properties, distorted shape, and wrinkles on the surface. In the case of exceeding the above, there is a tendency that the buffer characteristics of the obtained polypropylene resin foam molded article are lowered. The average bubble diameter (L (av)) and the bubble diameter variation (S) in the present invention are performed by cutting substantially the center of the expanded particle and observing the enlarged cross section, which will be described in detail later.

  Although there is no restriction | limiting in particular in the expansion ratio of the polypropylene resin pre-expanded particle obtained by this invention, 50 times or less are preferable. When the expansion ratio exceeds 50 times, when the foam of the polypropylene resin pre-expanded particles obtained is broken or the polypropylene resin foam molded article is molded, the dimensional accuracy, mechanical strength, heat resistance, etc. are insufficient. Tend to be.

  The polypropylene resin pre-expanded particles obtained as described above can be made into a polypropylene resin foam molded body by a conventionally known molding method. For example, a) Pre-expanded particles are pressurized with an inorganic gas, such as air or nitrogen, impregnated with the inorganic gas in the pre-expanded particles to give a predetermined internal pressure of the pre-expanded particles, filled in a mold, (B) A method in which pre-expanded particles are compressed by gas pressure and filled in a mold, and a heat-fusing method is carried out with steam using the recovery force of the pre-expanded particles. A method such as a method in which pre-expanded particles are filled in a mold without being heated and heat-sealed with water vapor can be used.

  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. The evaluation in the examples and comparative examples was performed by the following method.

(Foaming ratio)
Take 3 to 10 g of polypropylene resin pre-expanded particles, dry at 60 ° C. for 6 hours, measure the weight w, put in a graduated cylinder filled with water, submerge, measure volume v from rising water surface, expand The true specific gravity ρb = w / v of the particles was determined, and the expansion ratio K = ρr / ρb was determined from the ratio to the density ρr (= 0.9 g / cm ³ ) of the raw resin composition.

(Average bubble diameter (L (av)))
For each of the 20 randomly selected expanded particles, the center is cut and the cross section that appears is magnified. Here, the X axis and the Y axis perpendicular to each other at the approximate center of the cross section are drawn, the point where the X axis and the Y axis at the approximate center of the cross section intersect with each other is the center O, and the point where the X axis intersects with the end of the cross section. The points where the Y axis intersects the end of the cross section are designated as B and B ′.

  Next, the number of bubble walls intersected by the line segment OA is counted, and the value obtained by dividing the length of the line segment OA by the number of bubble walls is further divided by 0.616 to obtain the bubble diameter L (OA).

  The same processing is performed for the line segment OA ', the line segment OB, and the line segment OB', and L (OA '), L (OB), and L (OB') are obtained, respectively. In addition, when there was the center O on the bubble wall, it counted as a bubble wall.

  Four arithmetic average values L ′ (av) of L (OA), L (OA ′), L (OB), and L (OB ′) are calculated, and L ′ (av) of 20 polypropylene-based resin expanded particles is calculated. ) Was further arithmetically averaged to obtain an average cell diameter L (av).

(Molded product fusion rate)
After a crack with a depth of about 5 mm is made on the surface of the foamed molded product with a knife, the foamed molded product is divided along the crack, the fracture surface is observed, and the ratio of the number of broken particles to the total number of particles observed is obtained. The molded product fusion rate was used.

(Surface beauty)
○: Less wrinkles, less intergranular (dents and holes between foam particles), beautiful △: Less wrinkles, but slightly intergranular (dents and holes between foamed particles) ×: wrinkles, Or, the intergranularity is noticeable, and there are sink marks etc.

(Dimension shrinkage)
Measure the length amm, width bmm, and thickness cmm of the molded body for the mold dimensions 400mm (length) x 300mm (width) x 50mm (thickness) Was calculated.
Dimensional shrinkage (%) = [(400−a) / 400 + (300−b) / 300 + (50−c) / 50] / 3 × 100

(Examples 1-9, Comparative Examples 1-3)
Table 1 shows 100 parts by weight of an ethylene-propylene random copolymer having an ethylene content of 4% by weight (density 0.90 g / cm ³ , melt flow rate (MFR) 7.3 g / 10 min, melting point 138 ° C.). A predetermined amount of the indicated additives was added and then blended in a tumbler mixer. Next, the blended product was supplied to a 50 mmφ single screw extruder, melt-kneaded, extruded from a cylindrical die having a diameter of 1.8 mmφ, cooled with water, cut with a cutter, and cylindrical polypropylene resin particles (1.2 mg / Grain).

  100 parts by weight of the obtained polypropylene resin particles were put into a pressure vessel together with 200 parts by weight of pure water, 1.0 part by weight of tricalcium phosphate and 0.05 part by weight of sodium dodecylbenzenesulfonate, and then deaerated and stirred. Then, 6 parts by weight of carbon dioxide gas was put in a pressure vessel and heated to 143 ° C. The pressure at this time was 3 MPa. Immediately after opening the valve at the bottom of the pressure vessel, the contents were discharged under atmospheric pressure through an orifice with a diameter of 4 mmφ 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 container did not decrease. Table 2 shows the expansion ratio and average cell diameter of the obtained single-stage expanded particles.

  The single-stage expanded particles obtained here were dried at 60 ° C. for 6 hours and then introduced into a pressure resistant container. When the container was pressurized with 0.6 MPa air for 15 hours, the air pressure inside the expanded particles was as shown in Table 2. It became the value shown. The foamed particles were subjected to two-stage foaming by contacting with steam at a pressure shown in Table 2 to obtain two-stage foamed particles having a foaming ratio shown in Table 2. Next, the two-stage expanded particles are again introduced into the pressure resistant container and pressurized with 0.3 MPa air for 17 hours, and the internal pressure of the expanded particles is set to the value shown in Table 2, and then inside the 300 × 400 × 50 mm mold. In-mold foam molding was performed using pressurized steam of 0.25 MPa. Table 2 shows the evaluation results of the obtained foamed molded product.

Claims (7)

  In a pressure vessel, polypropylene resin particles are dispersed in an aqueous dispersion medium, an inorganic gas is introduced as a foaming agent, heated to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, and then the internal pressure of the pressure vessel is exceeded. The polypropylene resin pre-expanded particles obtained by releasing in a low pressure range are (a) 0.001 part by weight or more of inorganic substance whose dehydration start temperature of crystal water is 200 ° C. or more with respect to 100 parts by weight of polypropylene resin. .5 parts by weight or less, (b) fatty acid amide 0.005 part by weight or more and 0.5 part by weight or less, (c) silicate compound 0.0001 part by weight or more and 0.5 part by weight or less, (d) trivalent organic A polypropylene resin pre-expanded particle comprising 0.005 part by weight or more and 0.5 part by weight or less of a phosphorus compound.   The polypropylene resin pre-expanded particles according to claim 1, wherein the inorganic substance having a crystal water dehydration start temperature of 200 ° C or higher is zinc borate.   The polypropylene resin pre-expanded particles according to claim 1 or 2, wherein the fatty acid amide is at least one selected from erucic acid amide and ethylenebisstearic acid amide.   The polypropylene resin pre-expanded particles according to any one of claims 1 to 3, wherein the silicate compound is one or more selected from talc, mica, and kaolin.   Trivalent organophosphorus compounds are tris (2,4-di-t-butylphenyl) phosphite, tetrakis (2,4-di-t-butylphenyl) [1,1-biphenyl] -4,4 ′. -Diylbisphosphonite, bis (2,4-di-t-butylphenyl) pentaerythritol diphosphite, bis [2,4-bis (1,1-dimethylethyl) -6-methylphenyl] ethyl ester It is one or more chosen from phosphoric acid, The polypropylene resin pre-expanded particle as described in any one of Claims 1-4.   In a pressure vessel, polypropylene resin particles are dispersed in an aqueous dispersion medium, an inorganic gas is introduced as a foaming agent, heated to a temperature equal to or higher than the softening temperature of the polypropylene resin particles, and then the internal pressure of the pressure vessel is exceeded. The polypropylene resin pre-expanded particles according to any one of claims 1 to 5, which are obtained by imparting foaming ability to the polypropylene resin pre-expanded particles obtained by releasing in a low pressure range and heating.   A polypropylene resin foamed molded article obtained by filling the polypropylene resin prefoamed particles according to any one of claims 1 to 6 in a mold and heating.