Polypropylene resin expanded particles and their manufacturing method

Abstract

To provide polypropylene resin foam particles capable of suppressing accumulation of deposits to a molding die, while keeping favorable adhesiveness between the foam particles during molding in the die, and to provide a production method thereof.SOLUTION: Polypropylene resin foam particles comprise a foamed core layer 2 made of a polypropylene resin as the base resin and an adhesive layer 3 covering the core layer 2. The adhesive layer 3 contains silica particles and a higher fatty acid amide. The adhesive layer 3 has a total content of the silica particles and the higher fatty acid amide of 0.05 wt.% or more and 3 wt.% or less. The adhesive layer 3 has a weight ratio of the silica particles and the higher fatty acid amide of 1:0.2-1:8.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to expanded polypropylene resin beads and a method for producing the same. [Background technology]

[0002] Expanded polypropylene resin bead moldings obtained by molding expanded polypropylene resin beads in a mold are used in various applications such as packaging materials, automobile components, building materials, etc. The expanded beads that constitute such expanded bead moldings are produced, for example, as follows.

[0003] First, polypropylene-based resin particles are dispersed in an aqueous medium containing an inorganic dispersant in a container. Next, the polypropylene-based resin particles are impregnated with a blowing agent in the container, and the resin particles containing the blowing agent (i.e., expandable resin particles) are released from the container together with the aqueous medium and expanded. In this way, expanded polypropylene-based resin particles are produced.

[0004] A fusion layer may be formed on the surface of expanded beads to enhance fusion between the expanded beads during molding in a mold (see, for example, Patent Document 1). Expanded beads with a fusion layer can be obtained, for example, by preparing polypropylene resin beads having a core layer and a fusion layer provided on the surface of the core layer, and expanding the resin beads. In this case, the fusion layer is made of a crystalline polyolefin resin having a melting point lower than that of the polypropylene resin constituting the core layer, or an amorphous polyolefin resin having a low softening point. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6757871 Summary of the Invention [Problem to be solved by the invention]

[0006] Although expanded beads having a fusion layer improve the fusion between the expanded beads during in-mold molding, when expanded beads having a fusion layer are used to produce expanded bead moldings over a long period of time by in-mold molding, there is a risk that deposits originating from the expanded beads (specifically, deposits originating from the fusion layer) may accumulate, for example, in areas of the molding die that are easily heated. The accumulation of deposits may have an adverse effect on, for example, the surface smoothness of the expanded bead moldings.

[0007] The present invention has been made in view of the above background, and provides expanded polypropylene resin beads that can maintain good fusion between the expanded beads during in-mold molding while suppressing the accumulation of deposits on the molding die, and a method for producing the same. [Means for solving the problem]

[0008] One aspect of the present invention is an expanded polypropylene-based resin bead having an expanded core layer made of a polypropylene-based resin as a base resin and a fusion layer covering the core layer, the fusing layer contains silica particles and a higher fatty acid amide, The higher fatty acid amide includes erucic acid amide, the total content of the silica particles and the higher fatty acid amide in the bonding layer is 0.1% by weight or more and 1% by weight or less and The weight ratio of the silica particles to the higher fatty acid amide in the bonding layer is 1: 0.3 ~1: 5 The polypropylene resin foam particles are

[0009] Another aspect of the present invention is a method for producing a dispersion liquid, comprising: a dispersing step of dispersing polypropylene-based resin particles in an aqueous medium in a container; a blowing agent impregnation step of impregnating the polypropylene-based resin particles with a blowing agent in the container; and an expansion step of releasing the polypropylene-based resin particles containing the foaming agent from a container together with an aqueous medium to expand the particles, the polypropylene-based resin particles have a core layer containing a polypropylene-based resin as a base resin and a fusion layer covering the core layer, the fusion layer of the polypropylene-based resin particles contains silica particles and a higher fatty acid amide, The higher fatty acid amide includes erucic acid amide, The total content of the silica particles and the higher fatty acid amide in the fusion layer is 0.1% by weight or more and 1% by weight or less and The weight ratio of silica particles to higher fatty acid amide in the fusion layer is 1: 0.3 ~1: 5 The present invention relates to a method for producing expanded polypropylene resin beads. [Effects of the Invention]

[0010] The expanded polypropylene resin beads can maintain good fusion between the expanded beads during molding in a mold, while suppressing the accumulation of deposits on the molding die. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram of a cross section of an expanded bead. [Figure 2] FIG. 2 is a schematic diagram of a cross section of a multilayer resin particle. [Figure 3] FIG. 3 is an explanatory diagram showing a method for calculating the area of ​​the high-temperature peak. DETAILED DESCRIPTION OF THE INVENTION

[0012] In this specification, when a numerical value or physical property value is enclosed before and after the symbol "~", the values ​​before and after the symbol are included. Furthermore, when a numerical value or physical property value is expressed as a lower limit, it means that the value is equal to or greater than that numerical value or physical property value, and when a numerical value or physical property value is expressed as an upper limit, it means that the value is equal to or less than that numerical value or physical property value. Furthermore, "wt%" and "mass%", "parts by weight" and "parts by mass" are essentially synonymous. Furthermore, in this specification, expanded polypropylene resin beads are referred to as "expanded beads" as appropriate, and expanded bead molded articles are referred to as "molded articles." Expanded beads having an expanded core layer made of polypropylene resin are generally referred to as expanded polypropylene resin beads.

[0013] The expanded beads can be used to produce expanded bead molded articles. Specifically, a molding step is carried out in which a large number of expanded beads are filled into a mold and a heating medium such as steam is supplied to fuse the expanded beads together, thereby producing a molded article. In other words, a molded article can be obtained by molding the expanded beads in the mold.

[0014] FIG. 1 illustrates an example of an expanded bead, but the present invention is not limited to these drawings. As shown in FIG. 1, the expanded bead 1 has an expanded core layer 2 and a fusion layer 3, and the fusion layer 3 covers the core layer 2. The expanded core layer of the expanded bead is appropriately referred to as the "expanded core layer." The shape of the expanded bead 1 is, for example, cylindrical or spherical in appearance. The concept of a sphere includes not only those whose cross section is a perfect circle, but also those whose cross section is an ellipse, a circle, or a shape close to an ellipse.

[0015] The fusion layer 3 is a layer that covers all or part of the foamed core layer 2 (see FIG. 1). The expanded beads 1 have, for example, a so-called core-sheath structure. The fusion layer 3 is a layer that, when foamed beads having a single foamed core layer without a fusion layer (i.e., single-layer foamed beads) are molded in a mold, enables molding of an expanded bead molding of a quality equivalent to the quality single-layer foamed bead molding at a steam pressure lower than the minimum steam pressure at which a foamed bead molding in which the single-layer foamed beads are well fused together (i.e., a quality single-layer foamed bead molding). Note that an expanded bead molding in which the foamed beads are well fused together means that the molding has a fusion rate of 80% or more. A method for measuring the fusion rate of a molding will be described later. Note that the fusion layer 3 can also be said to be made of a resin that is more likely to fuse to each other at a lower temperature (specifically, a lower steam pressure) than the resin that constitutes the foamed core layer 2. Examples of the resin constituting the fusion layer include a resin having a lower melting point than the resin constituting the core layer, and a resin having a lower softening point.

[0016] The fusion layer uses, for example, a polyolefin resin as the base resin. Examples of the polyolefin resin include polyethylene resins such as linear low-density polyethylene, low-density polyethylene, and high-density polyethylene, polypropylene resins such as propylene-ethylene copolymers, propylene-butene copolymers, and propylene-ethylene-butene copolymers, and mixtures thereof. From the perspective of adhesion to the foamed core layer, the polyolefin resin of the fusion layer is preferably a polypropylene resin and / or a polyethylene resin, more preferably a polypropylene resin. The base resin constituting the fusion layer is more preferably at least one selected from propylene-ethylene copolymers, propylene-butene copolymers, and propylene-ethylene-butene copolymers. In this case, the adhesion between the foamed core layer and the fusion layer is further improved, and a molded body having good mechanical properties can be stably obtained. Note that the base resin composed of at least one selected from the group consisting of propylene-ethylene copolymers, propylene-butene copolymers, and propylene-ethylene-butene copolymers may be composed of a single copolymer or a mixture of a plurality of copolymers.

[0017] When the base resin constituting the fusion layer is a polyolefin resin, the melting point Tms of the polyolefin resin is preferably lower than the melting point Tmc of the polypropylene resin constituting the core layer in the foamed state. That is, for example, it is preferable that Tms < Tmc. In this case, the fusibility between the foamed particles during molding is improved, and a molded body with excellent fusibility at a low temperature (specifically, low steam pressure) during in-mold molding can be produced. From the perspective of further improving this effect, it is preferable that Tmc - Tms ≧ 1, more preferably Tmc - Tms ≧ 3, and even more preferably Tmc - Tms ≧ 5. From the perspective of further suppressing the peeling between the foamed core layer and the fusion layer and the adhesion of the resin to the mold during in-mold molding, it is preferable that Tmc - Tms ≦ 30, more preferably Tmc - Tms ≦ 25, and even more preferably Tmc - Tms ≦ 20.

[0018] From the viewpoint of easily suppressing adhesion of the resin to the molding die, the melting point Tms of the polyolefin resin constituting the fusion layer is preferably 100°C or higher, more preferably 110°C or higher, and even more preferably 120°C or higher. On the other hand, from the viewpoint of easily increasing the fusion properties of the expanded beads during molding, the melting point Tms of the polyolefin resin constituting the fusion layer is preferably 150°C or lower, more preferably 145°C or lower, and even more preferably 140°C or lower. The melting point of the polyolefin resin constituting the fusion layer is determined in accordance with JIS K7121:1987. Specifically, it is determined under the same conditions and by the same method as for the polypropylene resin constituting the foamed core layer described below.

[0019] The bonding layer contains silica particles and higher fatty acid amide. The sum of the content of silica particles and the content of higher fatty acid amide in the bonding layer is 0.05% by weight or more and 3% by weight or less. The weight ratio of silica particles to higher fatty acid amide in the bonding layer (weight of silica particles:weight of higher fatty acid amide) is 1:0.2 to 1:8. In other words, the content of higher fatty acid amide per part by weight of silica particles in the bonding layer is 0.2 to 8 parts by weight. Because the bonding layer contains silica particles and higher fatty acid amide in the above-mentioned predetermined ratio, the expanded beads exhibit good fusion properties during molding in a mold, while preventing the accumulation of deposits on the molding die. The content of silica particles and the content of higher fatty acid amide in the fusion layer are roughly the same as the amount of silica particles and the amount of higher fatty acid amide blended into the fusion layer during the production of the expanded beads.

[0020] Because the fusion layer is composed of a resin with a lower melting point than the foam core layer, for example, when conventional expanded beads having a fusion layer are molded in a mold, the components (specifically, the resin composition) constituting the fusion layer soften or melt and tend to adhere to the mold interior. Furthermore, when molding is performed in a mold over a long period of time, deposits accumulate in the mold, causing defects such as depressions on the surface of the molded article and impairing the smoothness of the molded article's surface. In particular, when a mold is used that has complex-shaped portions, such as areas with a high vent hole density or thin-walled portions, deposits tend to accumulate in the complex-shaped portions of the mold. This is thought to be because, during molding, heating is typically performed to ensure that areas that are difficult to heat are sufficiently molded, making the complex-shaped portions prone to overheating. As described above, the expanded beads of the present disclosure contain silica particles and higher fatty acid amides in the fusion layer in the specified ratio, thereby suppressing the accumulation of deposits while maintaining the fusion properties of the expanded beads. The occurrence of such deposits can be suppressed, for example, by properly cleaning the mold. However, if the frequency of cleaning the casting mold becomes too high, productivity will decrease, so it is preferable to clean the casting mold as infrequently as possible.

[0021] If the fusion layer does not contain one or both of silica particles and higher fatty acid amides, the components that make up the fusion layer will be more likely to adhere to parts with complex shapes, for example, making it difficult to prevent the accumulation of deposits inside the molding die.

[0022] If the total content of silica particles and higher fatty acid amide in the fusion layer is too low, the components constituting the fusion layer will easily adhere to parts with complex shapes, making it difficult to prevent the accumulation of deposits inside the molding die. On the other hand, if the total content of silica particles and higher fatty acid amide in the fusion layer is too high, the foamed particles will be less likely to fuse together during molding inside the mold, which may result in insufficient fusion of the molded product. From the viewpoint of further preventing the accumulation of deposits and further improving fusion, the total content of silica particles and higher fatty acid amide in the fusion layer is preferably 0.1% by weight or more and 2% by weight or less, more preferably 0.2% by weight or more and 1% by weight or less.

[0023] Furthermore, if the content of higher fatty acid amide per 1 part by weight of silica particles is too low, the content of silica particles in the bonding layer will be excessive, which may result in insufficient bonding. Furthermore, if the content of higher fatty acid amide per 1 part by weight of silica particles is too high, the content of higher fatty acid amide will be excessive, which may result in insufficient bonding. From the viewpoint of further improving bonding, the content of higher fatty acid amide per 1 part by weight of silica particles in the bonding layer is preferably 0.8 to 6 parts by weight, more preferably 1 to 5 parts by weight. In other words, the weight ratio of silica particles to higher fatty acid amide in the bonding layer (weight of silica particles:weight of higher fatty acid amide) is preferably 1:0.8 to 1:6, more preferably 1:1 to 1:5.

[0024] The content of silica particles in the fusion layer is preferably 0.01% by weight or more and 1% by weight or less. In this case, the fusion property of the molded body can be sufficiently increased while further suppressing the accumulation of deposits on the molding die. From the viewpoint of further improving this effect, the content of silica particles in the fusion layer is more preferably 0.01% by weight or more and less than 0.5% by weight, even more preferably 0.05% by weight or more and 0.4% by weight or less, and even more preferably 0.08% by weight or more and 0.3% by weight or less.

[0025] Furthermore, the content of the higher fatty acid amide in the adhesive layer is preferably 0.05% by weight or more and 1% by weight or less. In this case, the adhesion of the molded body can be sufficiently increased while further suppressing the accumulation of deposits on the molding die. From the viewpoint of further improving this effect, the content of the higher fatty acid amide in the adhesive layer is more preferably 0.08% by weight or more and 0.8% by weight or less, and even more preferably 0.1% by weight or more and 0.5% by weight or less.

[0026] As the silica particles, synthetic silica such as dry silica or wet silica, and / or natural silica can be used. Wet silica is silica produced by a wet method, and is produced by a method such as neutralizing a sodium silicate solution with a mineral acid such as sulfuric acid or hydrochloric acid, or by hydrolysis of alkoxysilane.

[0027] Commonly known methods for producing wet silica include the precipitation method and the gelation method. Examples of silica particles produced by the precipitation method include "Nipsil E-200A" manufactured by Tosoh Silica Corporation. Examples of silica particles produced by the gelation method include "NIPGEL BY-601" and "NIPGEL BY-001" manufactured by Tosoh Silica Corporation. Furthermore, silica particles whose surfaces have been treated with a silane coupling agent or the like can be used, if necessary.

[0028] The number-based arithmetic mean particle size of the silica particles is preferably 0.1 μm to 30 μm, more preferably 0.5 μm to 20 μm, and even more preferably 1 μm to 10 μm, in which case expanded beads can be stably obtained that maintain fusibility and suppress the accumulation of deposits on the molding die. The number-based arithmetic mean particle diameter of silica particles can be determined by converting the volume-based particle size distribution measured by a laser diffraction scattering method into a number-based particle size distribution by assuming that the particles have a spherical shape, and then calculating the arithmetic mean of the particle diameters based on this number-based particle size distribution. Note that the particle diameter refers to the diameter of a hypothetical sphere having the same volume as the particle.

[0029] In this specification, higher fatty acid amide refers to a fatty acid amide having a hydrocarbon group with 12 or more carbon atoms. The carbon number of the hydrocarbon group of the higher fatty acid amide is preferably 12 or more and 30 or less. In this case, the fusion property of the expanded beads is maintained while the accumulation of deposits on the molding die is easily suppressed. From the viewpoint of improving this effect, the carbon number of the hydrocarbon group of the higher fatty acid amide is more preferably 16 or more and 26 or less, and even more preferably 18 or more and 24 or less. Note that the carbon number of the hydrocarbon group of the higher fatty acid amide is the carbon number of the hydrocarbon group excluding the carbon atoms constituting the amide group. For example, when the higher fatty acid amide is a primary amide, the higher fatty acid amide is represented by the general formula RCONH2 and is a compound having a hydrocarbon group (specifically, a long-chain fatty acid group) and an amide group. In the general formula RCONH2, R is a hydrocarbon group.

[0030] The higher fatty acid amide may be a saturated fatty acid amide or an unsaturated fatty acid amide. Unsaturated fatty acid amides are preferred from the viewpoint of easily obtaining a molded article having good mechanical properties while suppressing the accumulation of deposits on the molding die. Furthermore, the higher fatty acid amide may be a primary amide, a secondary amide, or a tertiary amide. Primary amides are preferred from the viewpoint of easily dispersing the higher fatty acid amide well in the resin constituting the foamed particles and the fusion layer.

[0031] Specific examples of higher fatty acid amides include saturated fatty acid amides such as lauric acid amide, palmitic acid amide, stearic acid amide, and behenic acid amide; and unsaturated fatty acid amides such as oleic acid amide, erucic acid amide, and nervonic acid amide. The fusion layer may contain one or more higher fatty acid amides. From the viewpoint of stably obtaining expanded beads that maintain fusion properties and suppress the accumulation of deposits on the molding die, it is preferred that the higher fatty acid amide contains at least erucic acid amide. Furthermore, when the higher fatty acid amide contains erucic acid amide, the proportion of erucic acid amide in the higher fatty acid amide is preferably 50% by weight or more, more preferably 60% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more.

[0032] The proportion of the fusion layer in the expanded beads is preferably 0.5% by weight or more and 10% by weight or less. In this case, the in-mold moldability of the expanded beads can be improved, and an expanded bead molding having excellent mechanical properties can be stably obtained. From the viewpoint of improving this effect, the proportion of the fusion layer in the expanded beads is more preferably 1% by weight or more and 8% by weight or less, and even more preferably 2% by weight or more and 6% by weight or less.

[0033] The core layer of the expanded beads is made of a polypropylene resin as a base resin, which may be a propylene homopolymer or a propylene copolymer containing 50% by weight or more of structural units derived from propylene. Examples of polypropylene copolymers include copolymers of propylene with ethylene or an α-olefin having 4 or more carbon atoms, such as propylene-ethylene copolymer, propylene-butene copolymer, and propylene-ethylene-butene copolymer; propylene-acrylic acid copolymer; and propylene-maleic anhydride copolymer. These copolymers may be block copolymers, random copolymers, or graft copolymers. The above-mentioned polymers may be crosslinked, but are preferably non-crosslinked.

[0034] The base resin constituting the foam core layer may contain, in addition to the polypropylene-based resin, other resin materials such as other resins and elastomers, as long as they do not deviate from the spirit of the present invention. Examples of other resin materials include thermoplastic resins and / or elastomers other than polypropylene-based resins. Examples of thermoplastic resins other than polypropylene-based resins include at least one resin selected from polyethylene-based resins, polystyrene-based resins, polyamide-based resins, polyester-based resins, etc. Examples of elastomers that can be used include olefin-based thermoplastic elastomers and / or styrene-based thermoplastic elastomers.

[0035] The proportion of polypropylene resin in the base resin is 50% by weight or more, preferably 70% by weight or more, more preferably 80% by weight or more, even more preferably 90% by weight or more, and particularly preferably 100% by weight.

[0036] The melting point Tmc of the polypropylene resin constituting the core layer of the expanded beads is preferably 135 to 160°C. In this case, expanded bead molded articles having an excellent balance between the in-mold moldability of the expanded beads and the mechanical properties of the resulting molded articles can be stably obtained. From the viewpoint of improving the mechanical properties of the resulting molded articles, the melting point Tmc of the polypropylene resin constituting the core layer is preferably 138°C or higher, and more preferably 140°C or higher. Furthermore, from the viewpoint of improving in-mold moldability under low molding pressure conditions, the melting point Tmc of the polypropylene resin constituting the core layer is preferably 155°C or lower, more preferably 150°C or lower, and even more preferably 146°C or lower.

[0037] The melting point of polypropylene resins is determined in accordance with JIS K7121:1987. Specifically, the conditioning method is "(2) Measuring the melting temperature after a certain heat treatment," and the conditioned test piece is heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, with the melting point being the apex temperature of the melting peak. If multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is taken as the melting point.

[0038] The foam core layer may contain a higher fatty acid amide. Examples of the higher fatty acid amide include those similar to those used for the above-mentioned adhesive layer. The higher fatty acid amide preferably contains erucic acid amide. In this case, the proportion of erucic acid amide in the higher fatty acid amide is preferably 50% by weight or more, more preferably 60% by weight or more, even more preferably 80% by weight or more, and particularly preferably 90% by weight or more. It is also preferred that the foam core layer contains a higher fatty acid amide and that the content of the higher fatty acid amide in the fusion layer is greater than that in the foam core layer, in which case the secondary expandability of the foamed beads is maintained during in-mold molding, a molded product with excellent fusion properties can be obtained, and the accumulation of deposits inside the mold can be stably suppressed. When the foam core layer contains a higher fatty acid amide, the content of the higher fatty acid amide in the foam core layer is preferably 0.01% by weight or more and 0.5% by weight or less, more preferably 0.02% by weight or more and 0.2% by weight or less, and even more preferably 0.03% by weight or more and 0.1% by weight or less. The content of the higher fatty acid amide in the foamed core layer is approximately equal to the amount of the higher fatty acid amide blended into the foamed core layer during the production of the expanded beads.

[0039] The expanded beads are multilayered particles having a foamed core layer and a fusion layer. The foamed core layer is made of a foamed polypropylene resin, and the fusion layer is made of a foamed or non-foamed polyolefin resin. The fusion layer is preferably in a substantially non-foamed state. "Substantially non-foamed" means that there is almost no cellular structure.

[0040] The expanded beads may contain additives. Specifically, the foamed core layer and the fusion layer of the expanded beads may contain additives such as a flame retardant, a flame retardant assistant, a cell regulator, an antioxidant, a weathering agent, a crystal nucleating agent, a colorant, a conductive material, and an antistatic agent.

[0041] Preferably, the foamed core layer contains carbon black. In this case, the foamed beads are colored black. From the viewpoint of imparting a good black color to the molded article while maintaining the moldability of the foamed beads, the content of carbon black in the foamed core layer is preferably 0.5% by weight or more and 5% by weight or less, more preferably 1% by weight or more and 4% by weight or less, and even more preferably 2% by weight or more and 3% by weight or less. It is also preferable that the fusion layer of the expanded beads contains carbon black. In this case, the carbon black content in the fusion layer is preferably 0.5% by weight or more and 5% by weight or less, more preferably 1% by weight or more and 4% by weight or less, and even more preferably 2% by weight or more and 3% by weight or less. In this case, the fusion properties of the expanded beads can be ensured while the accumulation of deposits inside the molding die can be further suppressed. Furthermore, when the foam core layer and the fusion layer contain carbon black, the color of the molded product can be more easily made uniform. The content of carbon black in the expanded beads and the fusion layer is approximately equal to the amount of carbon black blended into the expanded beads and the fusion layer during the production of the expanded beads.

[0042] Examples of carbon black that can be used include channel black, roller black, furnace black, thermal black, acetylene black, etc. Among these, furnace black is preferred as the carbon black used in the expanded beads because it has an excellent balance between dispersibility in polypropylene-based resins and material costs.

[0043] The bulk density of the foam particles is 10 kg / m 3 More than 500kg / m 3 In this case, the weight of the molded body can be reduced while increasing the rigidity of the molded body. From the viewpoint of improving this effect, the bulk density of the expanded beads is preferably 15 kg / m or less. 3 More than 100kg / m 3 More preferably, it is 18 kg / m or less. 3 More than 50kg / m 3 It is even more preferable that:

[0044] The bulk density of expanded beads is measured using the following method. First, the expanded beads to be measured are left to stand for at least 24 hours in an environment with a temperature of 23°C, relative humidity of 50%, and 1 atm. After that, a measuring cylinder is filled with expanded beads of weight W (unit: g), and the bottom of the measuring cylinder is lightly tapped on the floor several times to stabilize the filling height of the expanded beads in the measuring cylinder. The bulk volume V (unit: L) of the expanded beads indicated on the measuring cylinder is read, and the weight W of the expanded beads is divided by the bulk volume V of the expanded beads (i.e., W / V). The value obtained in this way is kg / m 3 The bulk density of the expanded particles (unit: kg / m 3 ) can be obtained.

[0045] The expanded beads preferably have a crystalline structure in which a melting peak specific to the polypropylene resin (i.e., a peak specific to the resin) and one or more melting peaks (i.e., high-temperature peaks) appear on the higher temperature side of the DSC curve obtained when heated from 23°C to 200°C at a heating rate of 10°C / min. The DSC curve is obtained by differential scanning calorimetry (DSC) in accordance with JIS K7121:1987 using 1 to 3 mg of the expanded beads as a test sample. The resin-specific peak is an endothermic peak due to the melting of crystals specific to the polypropylene-based resin that constitutes the expanded beads. It is believed to be due to the endothermic heat generated by the melting of crystals typically found in polypropylene-based resins. On the other hand, the endothermic peak on the higher temperature side of the resin-specific peak (i.e., the high-temperature peak) is an endothermic peak that appears higher than the resin-specific peak on the DSC curve. The appearance of this high-temperature peak suggests the presence of a high-melting secondary crystalline component in the resin. Note that, as described above, a DSC curve obtained by heating from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), followed by cooling from 200°C to 23°C at a cooling rate of 10°C / min, and then heating again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating) shows only the endothermic peak due to the melting of crystals specific to the polypropylene-based resin that constitutes the expanded beads, making it possible to distinguish between the resin-specific peak and the high-temperature peak. The temperature at the apex of this resin-specific peak may differ slightly between the first and second heatings, but the difference is usually within 5°C.

[0046] From the viewpoint of further improving the moldability of the expanded beads and obtaining molded bodies with superior rigidity, the heat of fusion of the high-temperature peak of the expanded beads is preferably 5 J / g or more and 40 J / g or less, more preferably 7 J / g or more and 30 J / g or less, and even more preferably 10 J / g or more and 20 J / g or less. The total heat of fusion of the expanded beads is preferably 25 J / g or more and 150 J / g or less, more preferably 35 J / g or more and 100 J / g or less, and even more preferably 50 J / g or more and 90 J / g or less.

[0047] The heat of fusion of each peak in the DSC curve of expanded beads is determined as follows. First, approximately 2 to 4 mg of expanded beads are sampled from the group of expanded beads after conditioning. These expanded beads are used as test pieces, and a DSC curve is obtained by heating the test piece from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter. Note that multiple expanded beads may be used as test pieces. Figure 3 shows an example of a DSC curve. As illustrated in Figure 3, the DSC curve exhibits a resin-specific peak ΔH1 and a high-temperature peak ΔH2 that peaks at a higher temperature than the peak of the resin-specific peak ΔH1. Next, a line L1 is obtained by connecting point α at 80°C on the DSC curve with point β at the melting end temperature T of the expanded beads. Next, a line L2 parallel to the vertical axis of the graph is drawn from point γ on the DSC curve, which corresponds to the valley between the resin-specific peak ΔH1 and the high-temperature peak ΔH2, and the point where lines L1 and L2 intersect is designated as δ. Point γ can also be considered a maximum point between the resin-specific peak ΔH1 and the high-temperature peak ΔH2. The area of ​​the resin-specific peak ΔH1 is the area surrounded by the curve of the resin-specific peak ΔH1 portion of the DSC curve, the line segment α-δ, and the line segment γ-δ, and is defined as the heat of fusion of the resin-specific peak. The area of ​​the high-temperature peak ΔH2 is the area surrounded by the curve of the high-temperature peak ΔH2 portion of the DSC curve, the line segment δ-β, and the line segment γ-δ, and this is the heat of fusion of the high-temperature peak (i.e., the high-temperature peak heat quantity). The area of ​​the total melting peak is the area surrounded by the curve of the resin-specific peak ΔH1 part of the DSC curve, the curve of the high-temperature peak ΔH2 part, and the line segment α-β (i.e., straight line L1), and this is the heat of fusion of the total melting peak (total heat of fusion).

[0048] Next, an example of a method for producing expanded beads will be described. Expanded beads are produced by carrying out a dispersion step, a blowing agent impregnation step, and a foaming step. In the dispersion step, polypropylene-based resin particles are dispersed in an aqueous medium in a container. In the foaming agent impregnation step, the polypropylene-based resin particles are impregnated with a foaming agent in the container. In the foaming step, the polypropylene-based resin particles containing the foaming agent (i.e., expandable resin particles) are released from the container together with the aqueous medium and foamed. Hereinafter, an embodiment of the production method will be specifically described.

[0049] The dispersion step is a step of dispersing polypropylene-based resin particles in an aqueous medium. The polypropylene-based resin particles used are resin particles having a core layer and a fusion layer covering the core layer. The polypropylene-based resin particles having a core layer and a fusion layer are hereinafter referred to as "multilayer resin particles" as appropriate.

[0050] 2, the multilayer resin particle 10 has a core layer 20 made of a polypropylene-based resin and a fusion layer 30 that covers the core layer 20. The core layer 20 is a substantially non-foamed layer, and the fusion layer 30 is as described above.

[0051] Multilayer resin particles are produced, for example, as follows. Two extruders, a core layer extruder and a fusion layer extruder, are connected to a co-extrusion die. The core layer extruder melts and kneads a polypropylene resin for the core layer with optional additives to produce a molten core layer resin composition. The fusion layer extruder melts and kneads a polyolefin resin for the fusion layer with silica particles, a higher fatty acid amide, and optional additives to produce a molten fusion layer resin composition. The melts are then extruded and merged in the die to form a sheath-core composite consisting of a non-foamed core layer and a non-foamed fusion layer covering the outer surface of the core layer. In the composite, the core layer is composed of the core layer resin composition, and the fusion layer is composed of the fusion layer resin composition. The composite is then extruded from the extruder and pelletized by various cutting methods, such as with a pelletizer. The cutting method can be selected from strand cutting, hot cutting, underwater cutting, etc. In this manner, a multilayer resin particle 10 consisting of a core layer 20 and a fusion layer 30 covering this core layer 20 can be obtained (see FIG. 2).

[0052] Multilayer resin particles can also be produced, for example, as follows. First, a polypropylene resin for forming a core layer and optional additives are fed into an extruder and melt-kneaded to produce a melt of the resin composition for forming the core layer. Next, the melt is extruded, for example, in the form of strands to form a non-foamed core layer (specifically, a non-foamed strand-like material). The non-foamed core layer is then pelletized, for example, while being extruded from the extruder, by various cutting methods using a pelletizer or the like to obtain a particulate core layer (specifically, resin particles composed of the resin composition for forming the core layer). Next, the particulate core layer is fed to, for example, a mixing device having mixing and heating functions and heated, thereby heating the surface portion of the core layer. Next, a resin composition for forming a fusion layer, including a polyolefin resin for forming a fusion layer, silica particles, a higher fatty acid amide, and optional additives, is fed to the mixing device. The heated core layer particles are then mixed with the resin composition for forming a fusion layer, and the surface of the core layer is coated with the resin composition for forming a fusion layer. In this manner, multilayer resin particles are obtained. This mixing method is preferred because it makes it easy to form the fusion layer 30 over the entire surface of the core layer 20 (see FIG. 2).

[0053] The particle diameter of the multilayer resin particles is preferably 0.1 to 3.0 mm, more preferably 0.3 to 1.5 mm. The length / outer diameter ratio of the resin particles is preferably 0.5 to 5.0, more preferably 1.0 to 3.0. The average mass per multilayer resin particle (calculated from the masses of 200 randomly selected particles) is preferably 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and particularly preferably 0.4 to 2 mg. The mass ratio of the core layer to the fusion layer in the multilayer resin particle (specifically, the mass of the core layer:the mass of the fusion layer) is preferably 99.5:0.5 to 90:10, more preferably 99:1 to 92:8, and even more preferably 98:2 to 94:6.

[0054] The particle size and average mass of the multilayer resin particles can be adjusted, for example, by cutting the resin melt while appropriately changing the extrusion speed, take-up speed, cutter speed, etc. when the resin melt is extruded from the extruder.

[0055] An aqueous medium is used as a dispersion medium (specifically, a liquid) for dispersing the multilayer resin particles obtained as described above in a sealed container. The aqueous medium is a dispersion medium (specifically, a liquid) whose main component is water. The proportion of water in the aqueous medium is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. Examples of dispersion media other than water in the aqueous medium include ethylene glycol, glycerin, methanol, and ethanol. Water is particularly preferred as the aqueous medium.

[0056] In the dispersion step, an inorganic dispersant can be added to the aqueous medium. Additives other than the inorganic dispersant may also be added to the aqueous medium as appropriate. Examples of inorganic dispersants that can be used include inorganic fine particles such as aluminum oxide, tricalcium phosphate, magnesium pyrophosphate, zinc oxide, kaolin, and mica. These inorganic fine particles may be used alone or in combination of two or more. Additives other than the inorganic dispersant include dispersing aids and surfactants. Examples of dispersing aids include aluminum sulfate. Examples of surfactants include anionic surfactants such as sodium alkylbenzenesulfonate, sodium dodecylbenzenesulfonate, and sodium alkanesulfonate. Surfactants may be used alone or in combination of two or more.

[0057] The blowing agent impregnation step is a step of impregnating the multilayer resin particles with a blowing agent. The blowing agent impregnation step may be performed after the dispersion step described above, or part or all of the step may be performed overlapping with the dispersion step.

[0058] In the blowing agent impregnation step, for example, a container containing the multilayer resin particles is sealed, and a blowing agent is added to the sealed container to impregnate the multilayer resin particles with the blowing agent. This allows for the production of expandable resin particles containing the blowing agent in the multilayer resin particles. Examples of the blowing agent include physical blowing agents. The physical blowing agent can be added to the sealed container at any time before the multilayer resin particles are expanded. A solid blowing agent may be added to the aqueous medium together with the resin particles during the dispersion step, and then the gaseous blowing agent may be impregnated into the multilayer resin particles by heating or the like. Alternatively, a gaseous blowing agent may be pressure-injected into the sealed container during or after the dispersion step to impregnate the multilayer resin particles with the blowing agent. For example, when carbon dioxide is used as the physical blowing agent, a solid blowing agent (specifically, in the form of dry ice) may be added to the aqueous medium together with the multilayer resin particles during the dispersion step, or gaseous carbon dioxide may be pressure-injected into the sealed container during or after the dispersion step. When the blowing agent is impregnated into the multilayer resin particles, the impregnation of the blowing agent into the multilayer resin particles can be promoted by heating and / or pressurizing the inside of the closed container.

[0059] To adjust the crystalline state of the expanded beads, the heating rate of the sealed container may be adjusted during the dispersion step and / or the blowing agent impregnation step, or the temperature and time for maintaining the sealed container at a predetermined temperature may be adjusted. For example, adjustments can be made so that an endothermic peak (i.e., a high-temperature peak) appears at a higher temperature than the endothermic peak (i.e., the resin-specific peak) of the main component of the base resin constituting the core layer of the expanded beads in a DSC curve obtained by heat flux differential scanning calorimetry. Expanded beads exhibiting a high-temperature peak are preferred from the viewpoint of broadening the range of molding conditions (specifically, molding temperature and molding pressure) that can produce good molded articles. Adjustments to obtain the high-temperature peak can be made, for example, with reference to the method described in Japanese Patent No. 4077745.

[0060] The foaming agent is appropriately selected from among the general foaming agents used to obtain expanded beads. The blowing agent is preferably a physical blowing agent. The physical blowing agent may be an inorganic physical blowing agent and / or an organic physical blowing agent. Examples of inorganic physical blowing agents include carbon dioxide, air, nitrogen, helium, argon, and water. Examples of organic physical blowing agents include aliphatic hydrocarbons such as propane, butane, and hexane; alicyclic hydrocarbons such as cyclopentane and cyclohexane; and halogenated hydrocarbons such as methyl chloride, ethyl chloride, methylene chloride, 2,3,3,3-tetrafluoropropene, trans-1,3,3,3-tetrafluoropropene, and trans-1-chloro-3,3,3-trifluoropropene.

[0061] These physical blowing agents may be used alone or in combination of two or more. Of these blowing agents, preferably, blowing agents containing an inorganic physical blowing agent such as carbon dioxide, nitrogen, or air as the main component are used, and more preferably, carbon dioxide is used. In this specification, "containing an inorganic physical blowing agent as the main component" means that the physical blowing agent contains 50 mol% or more of the inorganic physical blowing agent. Preferably, the physical blowing agent contains 70 mol% or more, more preferably 90 mol% or more of the inorganic physical blowing agent, and even more preferably, the physical blowing agent consists solely of the inorganic physical blowing agent.

[0062] The amount of physical foaming agent added is determined appropriately depending on the type of base resin constituting the multilayer resin particles, the type of foaming agent, the desired bulk density of the expanded particles, etc. In particular, the amount of physical foaming agent added is preferably determined depending on the desired bulk density. For example, when carbon dioxide is used as the physical foaming agent, the amount of carbon dioxide added is preferably 0.1 to 30 parts by weight, more preferably 0.5 to 15 parts by weight, and even more preferably 1 to 10 parts by weight, per 100 parts by weight of the multilayer resin particles.

[0063] The expansion step is a step in which multilayer resin particles containing a blowing agent (i.e., expandable resin particles) are released from a container together with an aqueous medium and expanded to obtain expanded particles. More specifically, the expandable resin particles are expanded by releasing the expandable resin particles together with the aqueous medium under a pressure lower than the internal pressure of the container (specifically, a sealed container). This expansion method makes it easy to obtain expanded particles with a low bulk density.

[0064] The bulk density of the expanded particles can be adjusted by, for example, appropriately changing the expansion conditions, such as the temperature and pressure in the sealed container when the content of the sealed container is released in the expansion step. To produce expanded beads with a lower bulk density, a multi-stage expansion process can be performed, such as the two-stage expansion process described below. In the two-stage expansion process, the expanded beads obtained as described above are first stored in a pressurizable sealed container, and a gas such as air is injected into the sealed container to increase the pressure inside the cells of the expanded beads (i.e., the internal pressure). The expanded beads with increased internal pressure are then removed from the sealed container and heated with steam or hot air to expand the expanded beads. This completes the two-stage expansion process. The two-stage expansion process can produce expanded beads with a lower bulk density (two-stage expanded beads). Furthermore, compared to a method of obtaining expanded beads with a low bulk density through a single expansion process, the method of obtaining expanded beads with a low bulk density through the two-stage expansion process makes it easier to increase the cell diameter of the final expanded beads. Expanded bead moldings produced using the expanded beads obtained through the two-stage expansion process can further suppress the occurrence of color unevenness. Therefore, the two-stage expansion process is preferred when producing expanded beads containing a colorant.

[0065] The expanded beads are used, for example, to produce a molded article, which is produced, for example, by molding the expanded beads in a mold.

[0066] Specifically, foamed beads are first filled into a mold having a cavity corresponding to the shape of the molded article to be produced. The foamed beads filled in the mold are then heated with a heating medium such as steam. The foamed beads in the cavity further expand due to the heating and fuse together. This unifies the foamed beads, resulting in a molded article corresponding to the shape of the cavity. Molded articles produced using the expanded beads have excellent fusion properties, and therefore exhibit the mechanical properties that are inherently required of expanded bead molded articles, making them suitable for a variety of uses, such as packaging materials, automotive components, and building materials.

[0067] The density of the compact is 10 kg / m 3 More than 500kg / m 3 In this case, the weight of the molded body can be reduced while increasing the rigidity of the molded body. From the viewpoint of improving this effect, the density of the molded body is preferably 15 kg / m or less. 3 More than 100kg / m 3 More preferably, it is 18 kg / m or less. 3 More than 50kg / m 3 The density of the molded body is calculated by dividing the weight of the molded body by the volume calculated based on the dimensions of the molded body.

[0068] The expanded beads of the present invention may be produced by a production method (hereinafter also referred to as a second production method) different from the production method of the present invention described above. The second production method is a method in which resin particles for forming an expanded core layer are first prepared, and then the resin particles are expanded to obtain the expanded core layer, and the surface of the expanded core layer is then coated with a fusion layer to obtain the expanded beads. For example, expanded beads can be produced by the second production method shown below. First, a polypropylene resin for forming the core layer and optional additives are fed into an extruder and melt-kneaded to produce a melt of the resin composition for forming the core layer. The melt is then extruded, for example, into strands to form a non-foamed core layer (specifically, a non-foamed strand-like material). The non-foamed core layer is then pelletized, for example, by a cutting method using a pelletizer while being extruded from the extruder, to obtain a particulate core layer (specifically, resin particles composed of the resin composition for forming the core layer). The particulate core layer is then expanded by a known expansion method to form an expanded bead body (specifically, a foamed core layer). Next, the foamed core layer is fed to a mixing device having mixing and heating functions and heated, for example, to heat the surface of the core layer. A resin composition for forming the fusion layer, including a polyolefin resin for forming the fusion layer, silica particles, a higher fatty acid amide, and optional additives, is then fed to the mixing device. The heated expanded core layer and the resin composition for forming a fusion layer are mixed together to coat the surface of the core layer with the resin composition for forming a fusion layer, thereby obtaining the expanded beads of the present invention. [Example]

[0069] Example 1 (Production of multilayer resin particles) A manufacturing apparatus was prepared, including a core layer-forming extruder with an inner diameter of 50 mm, a multilayer strand-forming die attached downstream of the core layer-forming extruder, and a coating layer-forming extruder with an inner diameter of 30 mm. The manufacturing apparatus was configured so that the downstream side of the coating layer-forming extruder was connected to the multilayer strand-forming die, allowing for lamination of melts to form each layer within the die and co-extrusion. A propylene-ethylene random copolymer was used as the base resin constituting the core layer. The propylene-ethylene random copolymer used was a polypropylene-based resin with a melting point (i.e., peak melting temperature) of 142°C measured according to JIS K7121:1987 and a melt mass-flow rate of 7 g / 10 min measured according to JIS K7210-1:2014 at 230°C and a load of 2.16 kg.

[0070] The core layer forming materials (specifically, base resin, cell control agent, carbon black, and higher fatty acid amide) for forming the core layer were fed into the extruder for forming the core layer and melt-kneaded. This produced a melt of the resin composition for forming the core layer, which contained 0.1 wt% of cell control agent (specifically, zinc borate) and 2.7 wt% of carbon black (specifically, furnace black), as well as higher fatty acid amide (specifically, erucic acid amide: Kao Corporation's "Fatty Acid Amide E") in the amounts shown in Table 1.

[0071] A propylene-ethylene-butene random copolymer was used as the base resin constituting the adhesive layer. The propylene-ethylene-butene random copolymer was a polypropylene-based resin having a melting point (i.e., peak melting temperature) of 135°C measured according to JIS K7121:1987 and a melt mass-flow rate of 7g / 10min measured according to JIS K7210-1:2014 at 230°C under a load of 2.16kg. The weight ratio of the propylene, ethylene, and butene in this polypropylene-based resin was 94:3.5:2.5.

[0072] The materials for forming a bonding layer (specifically, base resin, carbon black, silica particles, and higher fatty acid amide) were fed into the extruder for forming a bonding layer and melt-kneaded. This produced a melt of a resin composition for forming a bonding layer, containing 2.7% by weight of carbon black (specifically, furnace black), silica particles, and higher fatty acid amide (specifically, erucic acid amide: Kao Corporation's "Fatty Acid Amide E") in the amounts shown in Table 1.

[0073] The melts for forming each layer were introduced into a multilayer strand-forming die and merged inside the die, and a composite (specifically, a multilayer strand) with a two-layer structure (a sheath-core structure consisting of a fusion layer and a core layer) was extruded from the small holes in the die. The extruded strand was water-cooled and cut with a pelletizer to obtain multilayer resin particles with an average weight of 1.0 mg per particle.

[0074] (Production of expanded beads) 1 kg of the multilayer resin particles was supplied to a sealed container having a capacity of 5 L together with 3 L of water as an aqueous medium. Furthermore, 0.3 parts by weight of kaolin as an inorganic dispersant and 0.2 parts by weight (amount as an active ingredient) of a surfactant (trade name: NEOGEN, manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were added to the sealed container per 100 parts by weight of the multilayer resin particles.

[0075] Next, carbon dioxide was injected into the sealed container as a blowing agent and the pressure was increased to 2.0 MPa (G) in gauge pressure. In this specification, the pressure indicated by (G) refers to the gauge pressure, i.e., the pressure value relative to atmospheric pressure. The contents of the sealed container were then heated at a rate of 2°C / min while stirring until the foaming temperature (specifically, 149.5°C) was reached, and the temperature was maintained for 15 minutes. This resulted in the foamed particles being adjusted to have a high-temperature peak in the endothermic curve measured by DSC.

[0076] Thereafter, the contents of the sealed container (specifically, resin particles and water) are released under atmospheric pressure to a bulk density of 60 kg / m 3 The same steps as those described above were repeated several times to obtain expanded beads to be subjected to the evaluation described below.

[0077] The first-stage expanded beads obtained as described above were left to cure for 24 hours in an environment of 23°C temperature, 50% relative humidity, and 1 atm. The cured first-stage expanded beads were then filled into a pressurizable airtight container, and the pressure inside the airtight container was increased from normal pressure to pressurize the expanded beads. The pressurized state of the expanded beads was maintained for a predetermined time, increasing the pressure inside the cells of the expanded beads. The first-stage expanded beads were then removed from the airtight container, and first-stage expanded beads with an internal cell pressure of 0.5 MPa (G) were obtained. The first-stage expanded beads were then fed into a second-stage expansion device. Steam was supplied into the device to expand the first-stage expanded beads, resulting in a bulk density of 27 kg / m. 3 The expanded beads obtained by the two-stage expansion were used for the following measurements and for producing expanded bead moldings. The results are shown in Table 1.

[0078] Bulk density of foamed particles First, the expanded particles to be measured were left for 24 hours or more in an environment with a temperature of 23°C, relative humidity of 50%, and 1 atm. After that, a weight W (unit: g) of expanded particles was filled into a measuring cylinder, and the bottom of the measuring cylinder was lightly tapped on the floor several times to stabilize the filling height of the expanded particles in the measuring cylinder. The bulk volume V (unit: L) of the expanded particles indicated on the measuring cylinder's scale was read, and the weight W of the expanded particles was divided by the bulk volume V of the expanded particles (i.e., W / V). The value thus obtained was converted into units to determine the bulk density (unit: kg / m 3 ) was obtained.

[0079] Total melting heat and high-temperature peak heat Approximately 2 mg of expanded particles were collected from the group of expanded particles after conditioning. These expanded particles were used as test specimens, and a DSC curve was obtained by heating the specimen from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (specifically, a DSC.Q1000 manufactured by TA Instruments). The area under all melting peaks in the DSC curve was calculated, and this was taken as the total heat of fusion. In addition, the area under the high-temperature peak in the DSC curve was calculated, and this was taken as the high-temperature peak heat. The above measurements were performed on five expanded particles, and the arithmetic average values ​​were used as the measurement results for the total heat of fusion and the high-temperature peak heat.

[0080] (Production of Molded Body) Expanded beads (specifically, two-stage expanded beads) were filled into a pressurizable sealed container, and the pressure inside the sealed container was increased from normal pressure to pressurize the expanded beads. The pressurized state of the expanded beads was maintained for a predetermined time to increase the pressure inside the cells of the expanded beads. The expanded beads were then removed from the sealed container, and expanded beads with an internal cell pressure of 0.1 MPa (G) were obtained. These expanded beads were filled into a molding die (specifically, a metal mold) having a molding cavity capable of molding a box-shaped molded product having dimensions of 200 mm length x 150 mm width x 50 mm height, with bottom and side wall thicknesses of 10 mm, and heated using the following heating method. The heating method consisted of preheating (exhaust step) by supplying steam to the mold with drain valves on both sides of the mold open. Steam was then supplied from one side of the mold to heat it, and then further heated from the other side of the mold. Next, steam was supplied from both sides of the mold at a molding heating steam pressure of 0.26 MPa (G) to heat it. After heating, the pressure was released and the molded product was water-cooled until the surface pressure due to the foaming force of the molded product reached 0.04 MPa (G), after which the mold was opened and the molded product was removed. This molded product was cured in an oven at 80°C for 12 hours and then slowly cooled to room temperature to obtain a box-shaped molded product with an open top. This molded product was a box-shaped open-top molded product with external dimensions of 200 mm length x 150 mm width x 50 mm height, with a bottom wall and side walls 10 mm thick. When the 200 mm length x 150 mm width surface was considered the bottom surface, the top surface was open.

[0081] Productivity evaluation Productivity was evaluated based on the presence or absence of resin adhesion to the mold when repeated in-mold molding was performed using expanded beads. Specifically, 30 cycles of in-mold molding were performed continuously using the same mold and molding conditions. After molding, the mold was visually inspected for the presence or absence of resin adhesion, and the evaluation was based on the following criteria. ○: When no adhesion was found. ×: When adhesion was observed at least partially.

[0082] ·density The density was calculated by dividing the weight of the compact by the volume calculated based on the dimensions of the compact.

[0083] -Fusing ability The fusion property of the molded body was determined based on the ratio of the number of foamed beads that were broken among the foamed beads exposed on the fracture surface when the molded body was broken. Specifically, test pieces (100 mm long x 100 mm wide x thickness: thickness of molded body) were first cut out from the molded body, and each test piece was cut with a cutter knife in the thickness direction of about 5 mm, and then the test piece was broken at the cut. Next, the number of foamed beads (n) present on the fracture surface of the molded body and the number of foamed beads that were broken (b) were measured, and the ratio (b / n) of (b) to (n) was expressed as a percentage to obtain the fusion rate (%), which was evaluated as follows: ○: When the fusion rate is 80% or more. △: The fusion rate is less than 80% and 40% or more. ×: The fusion rate is less than 40%.

[0084] <Examples 2 to 4> Examples 2 to 4 are examples in which the contents of silica particles and higher fatty acid amide were changed from those in Example 1. Specifically, expanded beads and molded articles were produced in the same manner as in Example 1, except that the contents of silica particles and higher fatty acid amide in the fusion layer of the resin beads were changed to the contents shown in Table 1, and a melt of the resin composition for forming a fusion layer was prepared. The expanded beads and molded articles of Examples 2 to 4 were evaluated and measured in the same manner as in Example 1. The results are shown in Table 1.

[0085] <Example 5> In this example, the weight proportion of the fusion layer in the expanded beads was changed from that in Example 1. Specifically, expanded beads and a molded article were produced in the same manner as in Example 1, except that the proportion of the fusion layer was reduced when extruding the multilayer strand, thereby changing the ratio of the core layer to the fusion layer as shown in Table 1. The expanded beads and molded article of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 1.

[0086] Example 6 In this example, the bulk density of the expanded particles was changed from that of Example 1. Specifically, the conditions for the second-stage expansion in Example 1 were changed to a bulk density of 18 kg / m 3The expanded beads were obtained. A molded article was produced using these expanded beads in the same manner as in Example 1. The expanded beads and molded article of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 1.

[0087] <Comparative Example 1> This example is an example in which silica particles and higher fatty acid amide were not blended into the fusion layer. Specifically, expanded beads and molded articles were produced in the same manner as in Example 1, except that silica particles and higher fatty acid amide were not blended into the fusion layer-forming material. The expanded beads and molded articles of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 2.

[0088] <Comparative Example 2> In this example, no higher fatty acid amide was added to the fusion layer, and the content of silica particles was changed from that in Example 1. Specifically, expanded beads and molded articles were produced in the same manner as in Example 1, except that no higher fatty acid amide was added to the fusion layer-forming material, and the content of silica particles in the fusion layer of the resin particles was changed to the content shown in Table 2, and a melt of the resin composition for forming a fusion layer was produced. The expanded beads and molded articles of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 2.

[0089] <Comparative Example 3> In this example, silica particles were not blended into the fusion layer, and the content of higher fatty acid amide was changed from that in Example 1. Specifically, silica particles were not blended into the fusion layer-forming material, and the content of higher fatty acid amide in the fusion layer of the resin particles was changed to the content shown in Table 2, and a melt of the resin composition for forming a fusion layer was prepared. Expanded beads and molded articles were produced in the same manner as in Example 1. The expanded beads and molded articles of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 2.

[0090] <Comparative Example 4> In this example, the content of silica particles was changed from that in Example 1. Specifically, expanded beads and a molded article were produced in the same manner as in Example 1, except that a melt of the resin composition for forming a fusion layer was prepared by changing the content of silica particles in the fusion layer of the resin beads to the content shown in Table 2. The expanded beads and molded article of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 2.

[0091] <Comparative Example 5> This example is an example in which the content of higher fatty acid amide was changed from that of Example 1. Specifically, expanded beads and a molded article were produced in the same manner as in Example 1, except that a melt of a resin composition for forming a fusion layer was prepared by changing the content of higher fatty acid amide in the fusion layer of the resin beads to the content shown in Table 2. The expanded beads and molded article of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 2.

[0092] <Comparative Example 6> In this example, the contents of silica particles and higher fatty acid amide were changed from those in Example 1. Specifically, expanded beads and molded articles were produced in the same manner as in Example 1, except that a melt of a resin composition for forming a fusion layer was prepared by changing the contents of silica particles and higher fatty acid amide in the fusion layer of the resin beads to the contents shown in Table 2. The expanded beads and molded articles of this example were evaluated and measured in the same manner as in Example 1. The results are shown in Table 2.

[0093] [Table 1]

[0094] [Table 2]

[0095] As can be seen from Table 1, the expanded beads of Examples 1 to 6 can maintain good fusion between the expanded beads during molding in a mold while suppressing the accumulation of deposits on the molding die.

[0096] As can be seen from Table 3, resin deposits were observed on the molding die in Comparative Example 1, because the fusion layer of the expanded beads did not contain silica particles or higher fatty acid amide. Also, in Comparative Example 2, in which the fusion layer of the expanded beads did not contain higher fatty acid amide, and in Comparative Example 3, in which the fusion layer of the expanded beads did not contain silica particles, resin deposits were observed on the molding die.

[0097] In Comparative Examples 4 and 5, the weight ratio of silica particles to higher fatty acid amide was poorly balanced, resulting in insufficient fusion between the expanded particles in the molded product.In Comparative Example 6, the total amount of silica particles and higher fatty acid amide was excessive, resulting in insufficient fusion between the expanded particles in the molded product. [Explanation of symbols]

[0098] 1. Foam particles 2 core layers 3 Fusion layer

Claims

1. A polypropylene-based resin expanded particle having a foamed core layer made of a polypropylene-based resin as a base resin and a fusion layer covering the core layer, the fusing layer contains silica particles and a higher fatty acid amide, the higher fatty acid amide includes erucic acid amide, the total content of the silica particles and the higher fatty acid amide in the bonding layer is 0.1% by weight or more and 1% by weight or less, The expanded polypropylene resin particles have a weight ratio of the silica particles to the higher fatty acid amide in the fusion layer of 1:0.3 to 1:

5.

2. 2. The expanded polypropylene resin beads according to claim 1, wherein the content of the silica particles in the adhesive layer is 0.01% by weight or more and less than 0.5% by weight.

3. 3. The expanded polypropylene resin particles according to claim 1, wherein the content of the higher fatty acid amide in the adhesive layer is 0.05% by weight or more and 1% by weight or less.

4. the core layer contains a higher fatty acid amide, 4. The expanded polypropylene resin beads according to claim 1, wherein the content of the higher fatty acid amide in the adhesive layer is greater than the content of the higher fatty acid amide in the core layer.

5. The expanded polypropylene resin particles according to any one of claims 1 to 4, wherein the base resin constituting the fusion layer is at least one selected from a propylene-ethylene copolymer, a propylene-butene copolymer, and a propylene-ethylene-butene copolymer.

6. The expanded polypropylene resin beads according to any one of claims 1 to 5, wherein the proportion of the fusion layer in the expanded polypropylene resin beads is 0.5% by weight or more and 10% by weight or less.

7. The bulk density of the polypropylene resin foam particles is 10 kg / m ３ More than 500kg / m ３ The expanded polypropylene resin particles according to any one of claims 1 to 6, wherein:

8. a dispersing step of dispersing polypropylene-based resin particles in an aqueous medium in a container; a blowing agent impregnation step of impregnating the polypropylene-based resin particles with a blowing agent in the container; and an expansion step of releasing the polypropylene-based resin particles containing the foaming agent from a container together with an aqueous medium to expand the particles, the polypropylene-based resin particles have a core layer containing a polypropylene-based resin as a base resin and a fusion layer covering the core layer, the fusion layer of the polypropylene-based resin particles contains silica particles and a higher fatty acid amide, the higher fatty acid amide includes erucic acid amide, the total content of the silica particles and the higher fatty acid amide in the fusing layer is 0.1% by weight or more and 1% by weight or less, The method for producing expanded polypropylene resin beads, wherein the weight ratio of silica particles to higher fatty acid amide in the fusion layer is 1:0.3 to 1:5.

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