Method for producing polypropylene-based resin foamed particles and method for producing foamed particle molded body

Abstract

A method for producing polypropylene-based resin foamed particles has a dispersing step, a primary foaming step, and a secondary foaming step. In the dispersing step, resin particles are dispersed in a dispersing medium, the resin particles having: a core layer having a predetermined proportion of carbon black blended with respect to 100 parts by mass of a polypropylene-based resin; and a coating layer having a predetermined proportion of carbon black blended with respect to 100 parts by mass of a polyolefin-based resin, and coating the core layer. In the primary foaming step, a primary foamed particle is obtained in which the core layer of the resin particle is foamed to have a volume expansion ratio of 5 to 25 times. In the secondary foaming step, the primary foamed particle is further foamed to obtain a foamed particle. The ratio M2 / M1 of the volume expansion ratio M2 of the foamed particle to the volume expansion ratio M1 of the primary foamed particle is 1.2 to 3.0.

Description

Technical Field

[0001] This invention relates to a method for manufacturing polypropylene resin foamed granules and a method for manufacturing foamed granule molded articles. Background Technology

[0002] Foamed granule molded bodies made of polypropylene resin foamed granules are used in various applications due to their lightweight nature and excellent cushioning and rigidity. These molded bodies are manufactured, for example, by an in-mold molding method, in which polypropylene resin foamed granules are filled into a mold, and then heated by supplying a heating medium such as steam into the mold. In the in-mold molding method, when a heating medium is supplied into the mold, the foamed granules undergo secondary foaming, and their surfaces melt. As a result, the foamed granules within the mold fuse together, producing a molded body with a shape corresponding to the shape of the mold cavity. Since the molded body expands easily through secondary foaming immediately after molding, it is cooled within the mold using water, air, or other means before being demolded.

[0003] Polypropylene resin foamed granules used in the manufacture of foamed granule molded articles are mostly manufactured by the following method: after impregnating polypropylene resin granules dispersed in a dispersion medium in a closed container with an inorganic physical foaming agent, the resin granules and dispersion medium are released from the closed container together into an environment with a lower pressure than the closed container. Furthermore, the above foaming method is sometimes referred to as the direct foaming method.

[0004] For example, Patent Document 1 describes a method in which polypropylene resin colored particles with added coloring pigment are foamed using a direct foaming method to produce foamed particles, and then the foamed particles are in-mold molded to produce a polypropylene resin colored foamed particle molded body. From the viewpoint of giving the molded body a high-end feel, carbon black is sometimes used as the coloring pigment added to the polypropylene resin colored particles.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 7-300537 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] In recent years, the applications of foamed granule molded articles have expanded, with a growing demand for foamed granule molded articles that offer higher blackness and less color unevenness. In the method of Patent Document 1, since blackness is reduced or color unevenness occurs, there is room for improvement in blackness and color unevenness. Furthermore, if the amount of carbon black is increased to improve blackness and color unevenness, from a weldability perspective, obtaining a good foamed granule molded article requires increasing the molding temperature or lengthening the cooling time, leading to decreased productivity.

[0010] Furthermore, in the method of Patent Document 1, there is room for improvement in the filling performance of the foamed particles into the molding die. With an increased amount of carbon black, depending on the desired shape of the foamed particle molded body and the molding conditions, insufficient filling of the foamed particles into the molding die occurs, leading to deterioration of the surface properties of the foamed particle molded body. Moreover, when using foamed particles with a large volume ratio for in-mold molding in order to obtain a lighter foamed particle molded body, the aforementioned problems with blackness and filling performance become more pronounced.

[0011] The present invention was made in view of the above background, and its object is to provide a method for manufacturing polypropylene resin foamed particles that can form good foamed particle molded articles with excellent filling properties, high blackness, and inconspicuous color unevenness, as well as a method for manufacturing foamed particle molded articles using the polypropylene resin foamed particles.

[0012] means for solving problems

[0013] One aspect of the present invention is a method for manufacturing polypropylene resin foamed granules, wherein the method for manufacturing polypropylene resin foamed granules includes:

[0014] The dispersion process involves dispersing polypropylene resin particles in a dispersion medium. The polypropylene resin particles comprise: a core layer containing a polypropylene resin as the base resin and 0.1 to 5 parts by weight of carbon black relative to 100 parts by weight of the polypropylene resin; and a coating layer containing a polyolefin resin as the base resin and 0.1 to 5 parts by weight of carbon black relative to 100 parts by weight of the polyolefin resin, and coating the core layer.

[0015] In the primary foaming process, after the polypropylene resin particles are impregnated with an inorganic physical foaming agent in a dispersion medium within a sealed container, the polypropylene resin particles and the dispersion medium are released from the sealed container together into an atmosphere at a lower pressure than that inside the sealed container, thereby causing the core layer of the polypropylene resin particles to foam and obtain primary foamed particles with a volume ratio of 5 times or more and 25 times or less; and

[0016] In the secondary foaming process, after the pressure inside the bubbles of the primary foaming particles increases, the primary foaming particles are heated, thereby causing the primary foaming particles to further foam to obtain polypropylene resin foamed particles.

[0017] The ratio of the volume ratio M2 of the polypropylene resin foamed particles to the volume ratio M1 of the primary foamed particles, M2 / M1, is greater than 1.2 and less than 3.0.

[0018] Another aspect of the present invention is a method for manufacturing a foamed granule molded body, wherein after filling a molding mold with polypropylene resin foamed granules obtained by the above-described method, a heating medium is supplied to the molding mold to perform in-mold forming, thereby producing a foamed granule molded body.

[0019] Invention Effects

[0020] According to the above method, it is possible to provide a method for manufacturing polypropylene resin foamed particles that have excellent filling properties, high blackness, and good color uniformity that are not easily noticeable when forming foamed particles, as well as a method for manufacturing foamed particle molded articles using the polypropylene resin foamed particles. Attached Figure Description

[0021] Figure 1 This is an explanatory diagram illustrating the calculation method for the heat of the high-temperature peak. Detailed Implementation

[0022] (Manufacturing method of polypropylene resin foamed granules)

[0023] As previously described, the method for manufacturing the polypropylene resin foamed particles (hereinafter referred to as "foamed particles" or "secondary foamed particles") comprises: a dispersion step in which the specific polypropylene resin particles (hereinafter referred to as "resin particles") are dispersed in a dispersion medium; a primary foaming step in which the core layer of the resin particles is foamed to obtain primary foamed particles with a volume ratio M1 of 5 times or more and 25 times or less; and a secondary foaming step in which the primary foamed particles are foamed to obtain secondary foamed particles with a volume ratio M2 of the secondary foamed particles to the volume ratio M1 of the primary foamed particles, M2 / M1 of 1.2 or more and 3.0 or less. In this way, by foaming the specific polypropylene resin particles in two stages, foamed particles with high blackness, small hue deviation, and excellent filling properties can be easily obtained. Furthermore, based on the foamed particles obtained in this way, it is easy to obtain lightweight foamed particle molded bodies (hereinafter referred to as "molded bodies") with excellent surface properties, high blackness, and inconspicuous color unevenness.

[0024] In the manufacturing method described above, resin particles are used, comprising a core layer with a polypropylene-based resin as the base resin and a coating layer covering the core layer. A predetermined amount of carbon black is incorporated into both the core layer and the coating layer as a black colorant. Without the coating layer, it is difficult to mold the foamed particles at low molding temperatures. Furthermore, in this case, the filling properties of the foamed particles may decrease, and the surface properties of the molded foamed particle body may be reduced due to molding conditions, etc.

[0025] The amount of carbon black in the core layer is 0.1 parts by mass and less than 5 parts by mass per 100 parts by mass of polypropylene resin. Similarly, the amount of carbon black in the coating layer is 0.1 parts by mass and less than 5 parts by mass per 100 parts by mass of polyolefin resin.

[0026] If the amount of carbon black in the core layer is too small, there is a risk of a decrease in the blackness of the foamed particle molded body obtained by using foamed particles and an increase in the deviation in color tone. By setting the amount of carbon black in the core layer to 0.1 parts by mass or more, preferably 0.5 parts by mass or more, more preferably 1.0 parts by mass or more, and even more preferably 2.0 parts by mass or more, relative to 100 parts by mass of polypropylene resin, it is easy to obtain foamed particles that can be molded into foamed particle molded bodies with the desired color tone.

[0027] On the other hand, if the amount of carbon black in the core layer is too large, it may be difficult to mold the foamed particles at a low molding temperature. Furthermore, in this case, the time required for cooling the molded article within the molding die may increase. These problems can be easily avoided by setting the amount of carbon black in the core layer to less than 5.0 parts by weight relative to 100 parts by weight of the polypropylene resin, preferably to 4.5 parts by weight or less, more preferably to 4.0 parts by weight or less, and even more preferably to 3.5 parts by weight or less.

[0028] Furthermore, if the amount of carbon black in the coating layer is too small, there is a risk of reduced blackness and increased color deviation in the foamed particle molded article obtained using the foamed particles. Additionally, in this case, there is a risk of reduced filling properties of the foamed particles. By setting the amount of carbon black in the coating layer to 0.1 parts by weight or more, preferably 0.5 parts by weight or more, more preferably 1.0 parts by weight or more, and even more preferably 2.0 parts by weight or more, relative to 100 parts by weight of the polyolefin resin, it is possible to mold a foamed particle molded article with the desired color, and foamed particles with excellent filling properties can be easily obtained.

[0029] On the other hand, if the amount of carbon black in the coating layer is too high, it may be difficult to mold the foamed particles at low molding temperatures. Furthermore, this could lead to a reduction in the filling properties of the foamed particles. These problems can be easily avoided by setting the amount of carbon black in the coating layer to less than 5.0 parts by weight relative to 100 parts by weight of the polyolefin resin, preferably to 4.5 parts by weight or less, more preferably to 4.0 parts by weight or less, and even more preferably to 3.5 parts by weight or less.

[0030] From the viewpoint of easily obtaining foamed particles with high blackness and low hue deviation with less carbon black, and from the viewpoint of further improving the filling properties of foamed particles, it is preferable that the carbon black is uniformly dispersed throughout the resin components constituting the coating layer.

[0031] Furthermore, it is preferable that the amount of carbon black in the coating layer is the same as the amount of carbon black in the core layer. More specifically, the mass ratio of the amount of carbon black in the coating layer to the amount of carbon black in the core layer is preferably 0.8 or more and 1.2 or less, preferably 0.9 or more and 1.1 or less, and most preferably 1. In other words, it is most preferable that the amount of carbon black in the core layer is equal to the amount of carbon black in the coating layer. By reducing the difference between the amount of carbon black in the core layer and the amount of carbon black in the coating layer, the blackness of the foamed particles can be further improved, and the color deviation can be further reduced.

[0032] The carbon black used in the core layer and coating layer of the foamed particles is a black colorant, and as mentioned earlier, it is a different material from conductive carbon black in that it can achieve its effect with a relatively small amount. Furthermore, by setting the amount of carbon black in the coating layer to 0.1 parts by mass or more and less than 5 parts by mass relative to 100 parts by mass of the polyolefin resin, it is possible to obtain an effect that improves the filling properties of the foamed particles while maintaining good formability.

[0033] Examples of carbon black that can be used include channel black, roller black, furnace black, pyrolytic black, acetylene black, and Ketjen black. From the viewpoint of achieving an excellent balance between dispersibility in polypropylene resins and material cost, furnace black is preferred as the carbon black.

[0034] The oil absorption of the dibutyl phthalate (DBP) in the carbon black is preferably less than 150 mL / 100 g, more preferably less than 140 mL / 100 g, further preferably less than 130 mL / 100 g, particularly preferably less than 120 mL / 100 g, and most preferably less than 110 mL / 100 g. By using the above-mentioned carbon black, foamed granules with high blackness can be obtained with a relatively small amount of carbon black, and foamed granules with excellent filling properties can be obtained more easily. Furthermore, the aforementioned DBP oil absorption is a value measured according to ASTM D2414-79.

[0035] In addition, the BET specific surface area of ​​the carbon black is preferably 200 m². 2 / g or less, more preferably 150m 2 / g or less, more preferably 100m 2 / g or less. By using the above carbon black, foamed particles with the desired blackness and excellent filling properties can be easily obtained. Furthermore, the BET specific surface area of ​​the carbon black is a value determined by the BET method according to ASTM D-3037.

[0036] In the primary foaming process, the core layer of the resin particles is foamed so that the volume ratio M1 of the primary foamed particles is 5 times or more and 25 times or less. In this primary foaming process, the core layer of the resin particles is foamed to form a foamed layer, thereby improving the filling properties of the foamed particles and the blackness of the final foamed particle molded body, and reducing color deviation. On the other hand, if the coating layer is foamed, the aforementioned effects may be compromised. Therefore, in the primary foaming process, it is preferable not to substantially foam the coating layer.

[0037] That is, the primary foamed particles obtained through the primary foaming process preferably consist of a foamed layer in a foamed state, which uses polypropylene resin as the base resin and contains the specified amount of carbon black, and a non-foamed coating layer covering the foamed layer. The base resin of the coating layer is a polyolefin resin, and the specified amount of carbon black is added to the coating layer. Furthermore, the aforementioned "non-foamed state" includes a state where the coating layer is not foamed, does not contain air bubbles, and has disappeared after foaming, referring to a state where there is almost no air bubble structure within the coating layer.

[0038] When the volume ratio M1 of the primary foaming particles is too low, in order to obtain foaming particles with the desired volume ratio, the primary foaming particles need to be foamed at a high ratio in the secondary foaming process. As a result, there is a risk of a decrease in the independent bubble rate of the secondary foaming particles and the formation of agglomeration between foaming particles in the secondary foaming process. By setting the volume ratio M1 of the primary foaming particles to 5 times or more, preferably 8 times or more, more preferably 10 times or more, and even more preferably 12 times or more, these problems can be easily avoided.

[0039] On the other hand, when the volume ratio M1 of the primary foaming particles is too high, it becomes difficult to obtain the effect achieved by foaming the resin particles in two stages. Specifically, there is a risk of a decrease in the blackness of the final foamed particle molded article and an increase in the deviation in color tone. These problems can be easily avoided by setting the volume ratio M1 of the primary foaming particles to 25 times or less, preferably 20 times or less, and more preferably 18 times or less.

[0040] When the volume ratio M2 / M1 is too low, it becomes difficult to achieve the desired effect of foaming the resin particles in two stages. As a result, there is a risk of reduced blackness and increased color deviation in the foamed particles. These problems can be easily avoided by setting the volume ratio M2 / M1 to 1.2 or higher, preferably 1.4 or higher, more preferably 1.8 or higher, and even more preferably 2.0 or higher.

[0041] On the other hand, if the volume ratio M2 / M1 is too high, there is a risk of reducing the independent bubble rate of the secondary foaming particles and causing agglomeration between foaming particles in the secondary foaming process. These problems can be easily avoided by setting the volume ratio M2 / M1 to 3.0 or less, preferably 2.8 or less.

[0042] The volume ratio M1 of the primary foamed granules is determined by the density (unit: kg / m³) of the polypropylene resin that forms the foaming layer of the primary foamed granules, in other words, the layer composed of the core layer of resin granules. 3 Divide by the bulk density of the primary foamed particles (unit: kg / m³) 3 The value after () is used. Additionally, the volume ratio M2 of the foamed particles is calculated using the density (unit: kg / m³) of the polypropylene resin that constitutes the foaming layer of the foamed particles, in other words, the layer corresponding to the core layer of the resin particles. 3 Divide by the bulk density of the foamed particles (unit: kg / m³) 3 The value after () is given. Furthermore, the method for determining the bulk density of primary foamed particles and foamed particles is described later.

[0043] In the manufacturing method described above, foamed particles are manufactured by performing a two-stage foaming process (in other words, two-stage foaming) and a primary foaming process, adjusting the volume ratio of the foamed particles obtained in each foaming process to the predetermined range and relationship mentioned above. The foamed particles obtained through the above-mentioned two-stage foaming process can produce molded bodies with high blackness and minimal color unevenness. The reason for this is considered to be as follows.

[0044] For a long time, foamed granules manufactured using polypropylene resin foamed granules produced by direct foaming have tended to exhibit a decreasing blackness and noticeable color unevenness. This trend is particularly pronounced when aiming to achieve the desired volume ratio in a single foaming process. One reason is believed to be the formation of multiple bubbles during foaming; another is that bubbles near the surface of the foamed granules are strongly affected by cooling, resulting in an excessive increase in the number of bubbles near the surface.

[0045] In contrast, in the manufacturing method described above, by employing a direct foaming method as the first-stage foaming process, while simultaneously reducing the volume ratio M1 of the first-stage foamed particles in this process, and by setting a second-stage foaming process, the volume ratio M2 of the foamed particles can be increased to the desired ratio within a range where the relationship with the volume ratio M1 satisfies the specific relationship. Furthermore, by manufacturing foamed particles through such a two-stage foaming process, excessive increase in the number of bubbles on the surface can be suppressed, while obtaining foamed particles with the desired volume ratio.

[0046] As a result, the manufacturing method described herein can produce foamed particles with high blackness and small hue deviation. In particular, the manufacturing method described herein can suppress the reduction of blackness and the occurrence of color unevenness, even for foamed particles with a volume ratio exceeding 30 times.

[0047] When manufacturing foamed granules with a desired volume ratio M2, from the viewpoint of easily obtaining foamed granules that can form molded articles with higher blackness and less color unevenness, it is preferable to further reduce the volume ratio M1 of the primary foamed granules and further increase the volume ratio ratio M2 / M1 in the secondary foaming process. Specifically, it is preferable that the volume ratio M1 of the primary foamed granules is 5 times or more and 20 times or less, and the volume ratio M2 / M1 is 1.4 or more and 3.0 or less; more preferably, the volume ratio M1 of the primary foamed granules is 10 times or more and 18 times or less, and the volume ratio M2 / M1 is 1.8 or more and 3.0 or less.

[0048] Furthermore, in the manufacturing method, by manufacturing foamed particles using a method including the aforementioned primary foaming step and secondary foaming step, the filling properties of the obtained foamed particles are improved, for example, compared to the case where foaming is performed in a single step to achieve the desired volume ratio. The reason for this effect is not currently clear, but it is considered to be due to factors such as improved flowability of the foamed particles through a non-foamed coating layer including the specific amount of carbon black, or changes in the unevenness of the foamed particles during the aforementioned secondary foaming step.

[0049] The following provides a detailed description of each step in the manufacturing method of the foamed granules.

[0050] <Distributed Processes>

[0051] In the dispersion process, the polypropylene resin particles are dispersed in a dispersion medium.

[0052] The core layer of the resin particles is composed of a polypropylene-based resin. The aforementioned polypropylene-based resin refers to a polypropylene copolymer containing 50% by mass or more of a homopolymer derived from propylene monomers and the constituent units of propylene. The polypropylene-based resin is preferably a polypropylene copolymer copolymerized from propylene and other monomers. Examples of preferred polypropylene copolymers include ethylene-propylene copolymers, propylene-butene copolymers, hexene-propylene copolymers, and ethylene-propylene-butene copolymers, which are copolymers of propylene with α-olefins having 4 to 10 carbon atoms. These copolymers are, for example, random copolymers and block copolymers, with random copolymers being preferred. Furthermore, the core layer may contain various polypropylene-based resins. From the viewpoint of further improving the formability of the foamed particles, the polypropylene-based resin constituting the core layer is preferably any one of an ethylene-propylene random copolymer, an ethylene-propylene-butene random copolymer, or a propylene-butene copolymer.

[0053] Furthermore, the core layer may also contain polymers other than polypropylene resins, within a range that does not impair the aforementioned effects. Examples of other polymers include thermoplastic resins other than polypropylene resins such as polyethylene resins and polystyrene resins, as well as elastomers. The content of other polymers in the core layer is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, and particularly preferably 0% by mass. In other words, it is particularly preferred that the core layer, as a polymer, substantially contains only polypropylene resin.

[0054] The polypropylene resin constituting the core layer is preferably an ethylene-propylene random copolymer, and the ethylene content in the copolymer is preferably 0.5% by mass or more and 5.0% by mass or less. By foaming the above-mentioned resin particles into foamed particles, the rigidity of the molded body can be further improved, and a molded body with good surface properties can be formed at a lower molding temperature. Furthermore, the total ethylene and propylene content in the ethylene-propylene random copolymer is 100% by mass.

[0055] From the viewpoint of further improving the rigidity of the molded article, the ethylene content in the copolymer is more preferably 4.5% by mass or less, more preferably 4.0% by mass or less, and particularly preferably 3.5% by mass or less. On the other hand, from the viewpoint of suppressing excessive rise in molding pressure of the foamed particles, the ethylene content in the copolymer is preferably 0.5% by mass or more.

[0056] Furthermore, from the viewpoint that a good molded article can be formed at an even lower molding temperature (in other words, a lower molding pressure), the content of ethylene in the copolymer is more preferably 1.0% by mass or more, more preferably 1.2% by mass or more, even more preferably 1.5% by mass or more, and particularly preferably more than 2.0% by mass.

[0057] Furthermore, from the viewpoint of further improving the formability of the obtained foamed particles and further shortening the water cooling time during molding, it is more preferable that the polypropylene resin used as the base resin of the core layer is an ethylene-propylene-butene copolymer or a propylene-butene copolymer, and the butene content in these copolymers is preferably 2% by mass or more and 15% by mass or less, more preferably 5% by mass or more and 12% by mass or less.

[0058] From the same perspective, it is preferable that the polypropylene resin used as the base resin for the core layer is an ethylene-propylene-butene copolymer, wherein the total content of butene and ethylene in the copolymer is preferably 2% by mass or more and 15% by mass or less, and the mass ratio of butene content to ethylene content (butene content / ethylene content) is preferably 0.5 or more. The mass ratio of butene content to ethylene content (butene content / ethylene content) is preferably 2 or more, more preferably 5 or more, and even more preferably 10 or more. Furthermore, the upper limit is preferably 30, more preferably 20. In addition, the total content of butene, ethylene, and propylene in the copolymer is set to 100% by mass.

[0059] Furthermore, the content of monomer components in the copolymer can be determined by IR spectroscopy. The ethylene, propylene, and butene components in the copolymer refer to the constituent units derived from ethylene, propylene, and butene, respectively. Additionally, the content of each monomer component in the copolymer refers to the content derived from the constituent units of that monomer in the copolymer.

[0060] As mentioned above, in the polypropylene resin constituting the core layer, 0.1 parts by weight and less than 5 parts by weight of carbon black are added relative to 100 parts by weight of the polypropylene resin. The polypropylene resin may also contain additives such as bubble modifiers, crystallizing nucleating agents, flame retardants, flame retardant additives, plasticizers, antistatic agents, antioxidants, UV stabilizers, light stabilizers, and antibacterial agents, without impairing the aforementioned effects. The content of additives in the core layer is preferably, for example, 0.01 parts by weight and less than 1 part by weight relative to 100 parts by weight of the polypropylene resin.

[0061] In the polypropylene resin constituting the core layer, it is preferable to contain one or more borate metal salts selected from zinc borate and magnesium borate as bubble conditioners.

[0062] The aforementioned zinc borate is a general term encompassing metal salts containing boron and zinc bonded to oxygen. Examples of zinc borate include zinc metaborate [Zn(BO₂)₂] and basic zinc borate [ZnB₄O₇·2ZnO]. Furthermore, substances represented by chemical formulas such as 2ZnO·3B₂O₃·3.5H₂O and 3ZnO·2B₂O₃·5H₂O can be listed, but are not limited to these.

[0063] The aforementioned magnesium borate is a general term encompassing metal salts of boron and magnesium bonded to oxygen. Examples of magnesium borate include orthoborate [Mg3(BO3)2], magnesium diborate, magnesium pyroborate [(Mg2B2O5) or (2MgO·B2O3)], magnesium metaborate [MgO·B2O3], trimagnesium tetraborate [(Mg3B4O9) or (3MgO·2B2O3)], and pentamagnesium tetraborate [Mg5B4O9]. 11 Magnesium hexaborate [MgB6O] 10 Furthermore, as magnesium borate, substances represented by chemical formulas such as 2MgO·3B2O3·nH2O (where n is a positive integer), MgO·4B2O3·3H2O, and MgO·6B2O3·18H2O can be listed, but are not limited to these.

[0064] Among these borate metal salts, zinc borate, represented by chemical formulas such as 2ZnO·3B2O3·3.5H2O and 3ZnO·2B2O3·5H2O, is particularly preferred.

[0065] Preferably, the arithmetic mean particle size (hereinafter, also simply referred to as the average particle size) based on the number of borate metal salts is 1 μm or more, and the proportion of particles with a particle size of 5 μm or more in the borate metal salt is 20% or less. With the arithmetic mean particle size based on the number of borate metal salts within this range, and the particle size distribution within this range, the foamed particles can become foamed particles with better bubble uniformity, and foamed particle molded bodies with a better appearance and less color unevenness can be obtained.

[0066] From the perspective of being able to suppress the coagulation of borate metal salts and further improve the uniformity of bubbles in foamed particles, the average particle size of borate metal salts is preferably 1 μm or more and 5 μm or less, more preferably 1.5 μm or more and 4 μm or less, and even more preferably 2 μm or more and 3 μm or less.

[0067] Furthermore, from the perspective of being able to better suppress bubble deviation of foamed particles, the proportion of borate metal salt particles with a particle size of 5 μm or more is more preferably 15% or less, and even more preferably 12% or less.

[0068] A number-based particle size distribution can be obtained by converting a volume-based particle size distribution, determined using laser diffraction scattering, into a number-based particle size distribution, assuming the particle shape is spherical. Then, the arithmetic mean particle size based on this number-based particle size distribution can be calculated by taking the arithmetic mean of the particle sizes. Furthermore, the proportion of particles with a diameter of 5 μm or larger can be determined from the number-based particle size distribution. Note that the aforementioned particle size refers to the diameter of a virtual sphere with the same volume as the particle.

[0069] The content of the borate metal salt is preferably 0.001% by mass or more and 3% by mass or less, relative to 100 parts by mass of the polypropylene resin constituting the core layer. By making the content of the borate metal salt in the resin particles 0.001% by mass or more, it functions more effectively as a bubble modifier, and the foamed particles have a more uniform bubble structure. By making the content of the borate metal salt in the resin particles 3% by mass or less, excessive shrinkage of the bubbles in the foamed particles can be suppressed. To further improve these effects, the content of the borate metal salt in the resin particles is more preferably 0.01% by mass or more and 1% by mass or less, and more preferably 0.03% by mass or more and 0.5% by mass or less, relative to 100 parts by mass of the polypropylene resin constituting the core layer.

[0070] The melting point (Tmc) of the polypropylene resin constituting the core layer is preferably 158°C or lower. By foaming the aforementioned resin particles, it is easy to obtain a molded article with good surface properties and excellent rigidity at a lower molding temperature (in other words, a lower molding pressure). From the viewpoint of improving this effect, the melting point (Tmc) of the polypropylene resin constituting the core layer is preferably 155°C or lower, more preferably 150°C or lower. On the other hand, from the viewpoint of further improving the heat resistance and mechanical strength of the molded article, the melting point (Tmc) of the polypropylene resin constituting the core layer is preferably 135°C or higher, more preferably 138°C or higher, and even more preferably 140°C or higher.

[0071] The melting point of polypropylene resin can be determined based on JIS K7121:1987. Specifically, firstly, test pieces made of polypropylene resin are prepared, and the state of the test pieces is conditioned according to "(2) after certain heat treatment, the melting temperature is measured". The heating rate and cooling rate during the state conditioning are both set to 10°C / min. A DSC curve is obtained by heating the state-conditioned test piece from 30°C to 200°C at a heating rate of 10°C / min, and the peak temperature of the melting peak appearing in the DSC curve is set as the melting point Tmc. In addition, if multiple melting peaks appear in the DSC curve, the peak temperature of the melting peak with the largest area is set as the melting point Tmc.

[0072] The melt flow rate (MFR) of the polypropylene resin constituting the core layer is preferably 5 g / 10 min or more, more preferably 6 g / 10 min or more, and even more preferably 7 g / 10 min or more. In this case, the foaming properties and formability of the foamed particles can be further improved. On the other hand, from the viewpoint of further improving the rigidity of the molded body, the MFR is preferably 12 g / 10 min or less, more preferably 10 g / 10 min or less. Furthermore, the MFR of the polypropylene resin is a value measured based on JIS K7210-1:2014, under test conditions of 230°C and a load of 2.16 kg.

[0073] The flexural modulus of the polypropylene resin constituting the core layer is preferably 800 MPa or higher and 1600 MPa or lower. By foaming the above-mentioned resin particles into foamed particles, the rigidity of the molded body can be further improved, and a molded body with good surface properties and high rigidity can be formed at a lower molding temperature.

[0074] From the viewpoint of further improving the rigidity of the molded article, the flexural modulus of the polypropylene resin constituting the core layer is preferably 850 MPa or more, more preferably 900 MPa or more, and even more preferably 950 MPa or more. On the other hand, from the viewpoint of suppressing excessive increase in the molding pressure of the foamed particles, the flexural modulus of the polypropylene resin constituting the core layer is preferably 1600 MPa or less.

[0075] Furthermore, from the viewpoint of being able to mold articles with excellent surface properties and rigidity at even lower molding temperatures (in other words, lower molding pressures), the flexural modulus of the polypropylene resin constituting the core layer is preferably 1550 MPa or less, more preferably 1500 MPa or less, and even more preferably less than 1200 MPa. Moreover, the flexural modulus of the polypropylene resin can be determined based on JIS K7171:2008.

[0076] The core layer of the resin particles is coated with a coating layer. The coating layer may cover the entire surface of the core layer or a portion of the surface of the core layer. From the viewpoint of more reliably obtaining the aforementioned effects, the coating layer preferably covers 50% or more of the surface area of ​​the resin particles relative to 100%, more preferably 60% or more, and even more preferably 70% or more. For example, the resin particles may also have a multilayer structure having a columnar core layer and a coating layer covering the side peripheral surfaces of the core layer.

[0077] The preferred mass ratio of the core layer to the coating layer in the resin particles is core layer:coating layer = 99.5:0.5 to 80:20, more preferably 99:1 to 85:15, and even more preferably 97:3 to 90:10. In other words, the ratio of the mass of the coating layer to the total mass of the core layer and the coating layer (i.e., the total mass of the resin particles) is preferably 0.5% or more and 20% or less, more preferably 1% or more and 15% or less, and even more preferably 3% or more and 10% or less. In this case, it is easier to obtain foamed particles with high blackness, small hue deviation, and excellent filling properties.

[0078] Examples of polyolefin resins constituting the coating layer include polyethylene resins, polypropylene resins, and polybutene resins. From the viewpoint of adhesion to the core layer, the polyolefin resin constituting the coating layer is preferably a polyethylene resin or a polypropylene resin, and more preferably a polypropylene resin. Examples of polypropylene resins include ethylene-propylene copolymers, ethylene-butene copolymers, ethylene-propylene-butene copolymers, and propylene homopolymers.

[0079] From the viewpoint that carbon black is not unevenly distributed in the coating layer and can be easily and well dispersed, the content of polypropylene resin in the coating layer is preferably 95% by mass or more, more preferably 97% by mass or more, further preferably 99% by mass or more, particularly preferably more than 99.5% by mass, and most preferably 100% by mass. In other words, it is most preferably that the polyolefin resin constituting the coating layer is composed of only polypropylene resin.

[0080] The coating layer may contain polymers other than polyolefin resins, within a range that does not impair the aforementioned effects. Examples of other polymers include thermoplastic resins and elastomers other than polyolefin resins, such as polystyrene resins. From the viewpoint that carbon black will not be unevenly distributed in the coating layer and will be easily and well dispersed, the content of other polymers in the coating layer is preferably 20% by mass or less, more preferably 10% by mass or less, further preferably 5% by mass or less, particularly preferably less than 0.5% by mass, and most preferably 0% by mass. In other words, it is most preferable that the coating layer, as a polymer, substantially contains only polyolefin resins.

[0081] Furthermore, as mentioned above, carbon black is added to the polyolefin resin constituting the coating layer at a rate of 0.1 parts by weight or more and less than 5 parts by weight relative to 100 parts by weight of the polyolefin resin. The polyolefin resin may also contain additives such as crystallizing nucleating agents, flame retardants, flame retardant accelerants, plasticizers, antistatic agents, antioxidants, UV stabilizers, light stabilizers, and antibacterial agents, within a range that does not impair the aforementioned effects. The content of additives in the coating layer is preferably, for example, 0.01 parts by weight or more and 1 part by weight or less relative to 100 parts by weight of the polyolefin resin.

[0082] The polyolefin resin particles constituting the above-mentioned coating layer preferably include silica particles and higher fatty acid amides, and the total content of the silica particles and the higher fatty acid amides in the coating layer is 0.05% by mass or more and 3% by mass or less. In this case, it is possible to suppress the accumulation of deposits originating from the foamed particles in the mold during in-mold molding, resulting in a foamed particle molded article with better surface properties. From the viewpoint of further improving this effect, the total content of silica particles and the higher fatty acid amides in the coating layer is more preferably 0.1% by mass or more and 2% by mass or less, and even more preferably 0.2% by mass or more and 1% by mass or less.

[0083] Furthermore, from the same point of view, the mass ratio of the silica particles to the higher fatty acid amide in the polyolefin resin particles constituting the above-mentioned coating layer is preferably 1:0.2 to 1:8, more preferably 1:0.8 to 1:6, and even more preferably 1:1 to 1:5.

[0084] The aforementioned higher fatty acid amides refer to fatty acid amides in which the hydrocarbon group has 12 or more carbon atoms. Specifically, examples of higher fatty acid amides include saturated fatty acid amides such as lauryl amide, palmitamide, stearamide, and benzyl amide; and unsaturated fatty acid amides such as oleamide, erucamide, and ceramide.

[0085] The melting point (Tms) of the polyolefin resin constituting the coating layer is preferably 120°C or higher and 145°C or lower, more preferably 125°C or higher and 140°C or lower. By foaming the above-mentioned resin particles, foamed particles with excellent weldability during molding can be easily obtained.

[0086] Furthermore, it is preferable that the melting point Tms of the polyolefin resin constituting the coating layer is lower than the melting point Tmc of the polypropylene resin constituting the core layer. By foaming the aforementioned resin particles into foamed particles, it is easy to obtain molded articles with good surface properties and excellent rigidity at lower molding temperatures (in other words, lower molding pressures). From the viewpoint of improving this effect, the difference between the melting point Tmc of the polypropylene resin and the melting point Tms of the polyolefin resin, Tmc-Tms, is preferably 5°C or more, more preferably 6°C or more, and even more preferably 8°C or more. On the other hand, from the viewpoint of suppressing the peeling of the foamed layer from the coating layer and the adhesion between the foamed particles, the difference between the melting point Tmc of the polypropylene resin and the melting point Tms of the polyolefin resin, Tmc-Tms, is preferably 35°C or less, more preferably 20°C or less, and even more preferably 15°C or less.

[0087] Furthermore, the method for determining the melting point of the polyolefin resin constituting the coating layer is the same as that for determining the melting point of the polypropylene resin constituting the aforementioned foaming layer, except that a test piece made of polyolefin resin is used instead of a test piece made of polypropylene resin. Specifically, when multiple melting peaks appear in the DSC curve, the temperature of the lowest melting peak is set as the melting point Tms.

[0088] The MFR of the polyolefin resin constituting the coating layer is preferably the same as that of the polypropylene resin constituting the core layer. Specifically, it is preferably 2 g / 10 ppm or more and 15 g / 10 ppm or less, more preferably 3 g / 10 ppm or more and 12 g / 10 ppm or less, and even more preferably 4 g / 10 ppm or more and 10 g / 10 ppm or less. Based on the foamed particles formed by foaming the above-mentioned resin particles, the delamination between the foamed layer and the coating layer can be reliably suppressed. Furthermore, when the polyolefin resin is a polypropylene resin, its MFR is a value measured based on JIS K7210-1:2014 at a test temperature of 230°C and a load of 2.16 kg; when the polyolefin resin is a polyethylene resin, its MFR is a value measured based on JIS K7210-1:2014 at a test temperature of 190°C and a load of 2.16 kg.

[0089] In the production of resin granules, a co-extrusion apparatus can be used, for example, which includes a core-forming extruder, a coating-forming extruder, and a co-extrusion die connected to both extruders. Resin granules can be produced, for example, by wire cutting. Specifically, in the core-forming extruder, a polypropylene resin for core-forming, carbon black, and additives added as needed are melt-blended to produce a core-forming melt blend. In the coating-forming extruder, a polyolefin resin for coating-forming, carbon black, and additives added as needed are melt-blended to produce a coating-forming melt blend. By co-extruding these melt blends and converging them within the die, a multi-layered composite consisting of a non-foamed columnar core and a non-foamed coating covering the outer surface of the core is formed. After extruding this composite through a small orifice in the die, the extrudate is cooled by passing it through a water bath. Then, by cutting the extrudate to the desired length, resin granules can be obtained. Furthermore, the methods for manufacturing resin particles are not limited to those mentioned above; thermal cutting, underwater cutting, and other methods can also be used.

[0090] <Distributed Processes>

[0091] In the dispersion process, the resin particles are dispersed in the dispersion medium by stirring after being added to it. The dispersion process can be carried out in a closed container used in the subsequent primary foaming process, or in a different container than the closed container used in the primary foaming process. From the viewpoint of simplifying the manufacturing process, it is preferable to carry out the dispersion process in the closed container used in the primary foaming process.

[0092] As the dispersion medium, an aqueous dispersion medium with water as the main component is used. In addition to water, the aqueous dispersion medium may also contain hydrophilic organic solvents such as ethylene glycol, glycerol, methanol, and ethanol. The proportion of water in the aqueous dispersion medium is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0093] It is preferable to add a dispersant to the dispersion medium. By adding a dispersant to the dispersion medium, the melting of resin particles heated in the container during the primary foaming process can be suppressed. The amount of dispersant added is preferably 0.001 parts by weight or more and 5 parts by weight or less per 100 parts by weight of resin particles. Organic dispersants and inorganic dispersants can be used as dispersants, but from the perspective of ease of processing, particulate inorganic substances are preferred. More specifically, dispersants such as alumina, kaolin, mica, clay minerals, alumina, titanium dioxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, and iron oxide can be used. These dispersants can be used alone or in combination of two or more dispersants. Among them, clay minerals are preferred as dispersants. The clay minerals can be natural clay minerals or synthetic clay minerals.

[0094] Furthermore, when using a dispersant, it is preferable to use an anionic surfactant such as sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, sodium dodecyl sulfate, or sodium oleate as a dispersing aid. The amount of dispersing aid added is preferably 0.001 parts by weight or more and 1 part by weight or less per 100 parts by weight of resin particles.

[0095] <First-stage foaming process>

[0096] In the primary foaming process, firstly, an inorganic physical foaming agent is impregnated in resin particles within a dispersion medium in a sealed container. Carbon dioxide, air, nitrogen, helium, argon, etc., can be used as the inorganic physical foaming agent for foaming the resin particles. From the viewpoint of environmental impact and operability, carbon dioxide is preferred. The amount of foaming agent added is preferably 0.1 parts by weight or more and 30 parts by weight or less, and more preferably 0.5 parts by weight or more and 15 parts by weight or less, relative to 100 parts by weight of resin particles.

[0097] As a method for impregnating resin particles with an inorganic physical foaming agent in the primary foaming process, a method can be adopted by supplying the foaming agent into a closed container, increasing the pressure inside the closed container, and thus impregnating the resin particles in the dispersion medium with the foaming agent. In this case, heating the resin particles and the dispersion medium together can further promote the impregnation of the foaming agent into the resin particles.

[0098] The pressure inside the sealed container during foaming is preferably 0.5 MPa (G) or higher in gauge pressure. On the other hand, the pressure inside the sealed container is preferably 4.0 MPa (G) or lower in gauge pressure. As long as it is within the above range, foamed granules can be manufactured safely without the risk of damage to the sealed container or explosion.

[0099] In addition, the temperature during foaming can be kept within an appropriate range by increasing the temperature of the dispersion medium in the primary foaming process at a rate of 1 to 5 °C / min.

[0100] After impregnation of the foaming agent into the resin particles, the contents of a sealed container are released into an environment at a lower pressure than the sealed container. This causes the core layer of the resin particles to foam, forming a bubble structure, which is then stabilized by cooling with an external gas (in other words, atmosphere), resulting in primary foamed particles. From the viewpoint of easily suppressing the adhesion between the primary foamed particles immediately after primary foaming, and from the viewpoint of easily suppressing the shrinkage of the primary foamed particles, the temperature Tu of the atmosphere used to release the resin particles is preferably adjusted to a low temperature. Specifically, the temperature Tu of the atmosphere used to release the resin particles is preferably less than 80°C, more preferably less than 75°C.

[0101] Traditionally, when the temperature Tu of the atmosphere releasing resin particles is low, the blackness of the resulting foamed particles tends to be low, making it difficult to obtain lightweight molded articles with high blackness. In contrast, according to the manufacturing method described above, even when the temperature Tu of the atmosphere releasing resin particles is adjusted to a low temperature, for example, less than 80°C, the blackness of the final molded article can be improved. From the viewpoint of suppressing excessive decrease in the blackness of the primary foamed particles, the temperature Tu of the atmosphere releasing resin particles is preferably 40°C or higher, more preferably 60°C or higher.

[0102] Furthermore, from the viewpoint of further improving the blackness of the primary foamed particles and further suppressing the adhesion and shrinkage of the primary foamed particles, the difference between the melting point Tmc of the polypropylene resin constituting the core layer of the resin particles and the temperature Tu of the atmosphere releasing the resin particles [Tmc-Tu] is preferably 65°C or higher and 85°C or lower.

[0103] When heating resin particles while impregnating them with a foaming agent, heating and foaming are preferably performed in the following manner: First, a primary holding process is conducted, where the temperature is maintained at a level above (melting point of polypropylene resin - 20°C) and below (melting completion temperature of polypropylene resin) for a sufficient time, preferably about 10 to 60 minutes. Then, the temperature is adjusted from (melting point of polypropylene resin - 15°C) to below (melting completion temperature of polypropylene resin + 10°C). Next, a secondary holding process is conducted, where the temperature is maintained at this level for a further sufficient time, preferably about 10 to 60 minutes. Afterward, the contents of a sealed container are preferably released while the temperature inside the sealed container is set above (melting point of polypropylene resin - 10°C), causing the resin particles to foam. The temperature inside the sealed container during foaming is more preferably above (melting point of polypropylene resin) and below (melting point of polypropylene resin + 20°C). By heating the resin particles in this way to foam them, foamed particles with excellent mechanical strength and formability can be easily obtained.

[0104] It is believed that the improved mechanical strength and formability of the foamed particles due to heating and foaming under the aforementioned conditions is due to the formation of secondary crystals in the polypropylene resin constituting the core layer. Whether secondary crystals form in the polypropylene resin can be determined by the presence or absence of a high-temperature peak in the DSC curve. Furthermore, the method for determining the presence or absence of the high-temperature peak will be described later.

[0105] From the above, it is possible to obtain primary foamed particles having a foamed core layer, a non-foamed coating layer, and a volume ratio M1 within the specified range. The obtained primary foamed particles can be dried, for example, by standing at 23°C and a 50% atmosphere for at least 12 hours. The volume ratio M1 of the primary foamed particles can be adjusted, for example, by the type of base resin of the core layer, the amount of foaming agent added in the primary foaming process, the foaming temperature, the temperature of the atmosphere at which the contents are released from the sealed container, and the pressure difference between the pressure inside the sealed container and the pressure of the environment at which the contents are released from the sealed container.

[0106] The volume ratio M1 of primary foamed granules is the density (unit: kg / m³) of the polypropylene resin that makes up the core layer of the resin granules. 3 Divide by the bulk density of the primary foamed particles (unit: kg / m³) 3The value after () is used to calculate the bulk density of primary foamed granules. The calculation method is as follows: First, allow the primary foamed granules to stand for at least 24 hours in an environment with a relative humidity of 50%, a temperature of 23℃, and an air pressure of 1 atm to adjust their state. Next, fill a graduated cylinder with the adjusted primary foamed granules in a natural stacking manner, and read the bulk volume (unit: L) of the primary foamed granule group according to the graduated cylinder's scale. Then, convert the unit by dividing the mass (unit: g) of the primary foamed granule group in the graduated cylinder by the aforementioned bulk volume to obtain the bulk density (unit: kg / m³). 3 ).

[0107] <Secondary foaming process>

[0108] In the secondary foaming process, firstly, internal pressure is applied to the primary foaming particles. More specifically, after the primary foaming particles are placed in a pressure-resistant container, the container is pressurized with inorganic gases such as air or carbon dioxide, causing the inorganic gases to impregnate the primary foaming particles. This sets the pressure inside the bubbles of the primary foaming particles to above atmospheric pressure. Then, the primary foaming particles removed from the pressure-resistant container are heated using a heating medium such as steam or heated air at a lower pressure than the pressure inside the bubbles, thereby allowing the primary foaming particles to further foam. The result is that foaming particles (secondary foaming particles) with a volume ratio of M2, where M2 is 1.2 times or more but less than 3.0 times the volume ratio of M1, can be obtained.

[0109] For example, the volume ratio M2 of the foamed particles can be adjusted by factors such as the type of base resin of the foaming layer of the primary foamed particles, the pressure difference between the pressure inside the bubbles in the primary foamed particles under internal pressure and the pressure of the heating environment, the heating temperature, and the heating time. The method for determining the volume ratio M2 of the foamed particles will be described later.

[0110] According to the manufacturing method of the present invention, by having a predetermined secondary foaming process, it is possible to obtain foamed particles that can achieve improved productivity in the above-mentioned primary foaming process while having excellent filling properties, high blackness, and inconspicuous color unevenness in foamed particle molded bodies.

[0111] From the viewpoint of easily obtaining foamed particles with the desired volume ratio M2, in the secondary foaming process, the internal pressure applied to the primary foamed particles is preferably atmospheric pressure or higher, and in gauge pressure, preferably 0.1 MPa(G) or higher, more preferably 0.2 MPa(G) or higher, and even more preferably 0.3 MPa(G) or higher. On the other hand, the upper limit of the internal pressure applied to the primary foamed particles is approximately 1 MPa(G), and preferably 0.8 MPa(G) or lower.

[0112] From the viewpoint of suppressing the agglomeration of primary foaming particles while easily obtaining foaming particles with the desired volume ratio M2, the heating time of the primary foaming particles in the secondary foaming process is preferably set to a range of 3 seconds or more and 60 seconds or less. From the same viewpoint, the temperature of the heating medium is preferably a range of 80°C or more and 120°C or less.

[0113] (Polypropylene resin foamed granules)

[0114] The polypropylene resin foamed particles obtained above have: a foamed layer, which uses polypropylene resin as a base material; and a coating layer, which uses polyolefin resin as a base resin and coats the foamed layer. Furthermore, the foamed layer contains 0.1 parts by weight or more and less than 5 parts by weight of carbon black relative to 100 parts by weight of the polypropylene resin, and the coating layer contains 0.1 parts by weight or more and less than 5 parts by weight of carbon black relative to 100 parts by weight of the polyolefin resin. From the viewpoint of further improving the appearance and rigidity of the obtained molded article, it is preferable that the foamed particles do not have through-pores.

[0115] Since the polypropylene resin constituting the foaming layer of the foamed particles is the same as the polypropylene resin constituting the core layer of the resin particles, the description related to the aforementioned polypropylene resin can be appropriately referred to. Similarly, since the polyolefin resin constituting the coating layer of the foamed particles is the same as the polyolefin resin constituting the coating layer of the resin particles, the description related to the aforementioned polyolefin resin can be appropriately referred to.

[0116] The independent bubble rate of the foamed particles is preferably 88% or more, preferably 90% or more, and more preferably 95% or more. In this case, high in-mold formability can be obtained more stably. In addition, based on the above-mentioned foamed particles, foamed particle molded articles with good surface properties and excellent rigidity can be easily obtained.

[0117] The independent bubble rate of the foamed particles can be determined using an air comparison hydrometer based on ASTM-D2856-70 procedure C. Specifically, the determination is as follows: First, the foamed particles are left to stand for at least 24 hours in an environment with a relative humidity of 50%, a temperature of 23°C, and an air pressure of 1 atm to adjust their state. This is done so that the bulk volume, in other words, the value of the mark when naturally packed into the graduated cylinder, is approximately 20 cm³. 3 After selecting a sample for testing from the state-conditioned foamed particles, the apparent volume of the sample is measured. Specifically, the apparent volume of the sample is the volume equivalent to the rise in liquid level when the sample is submerged in a graduated cylinder containing ethanol at 23°C.

[0118] After thoroughly drying the test sample whose apparent volume was measured, the true volume of the test sample was determined using an Accupyc II 1340 manufactured by Shimadzu Corporation, following step C as described in ASTM-D2856-70. Then, using these volume values, the independent bubble rate of the test sample was calculated based on the following formula (1). The above operation was performed five times using different test samples, and the arithmetic mean of the independent bubble rates obtained from these five measurements was taken as the independent bubble rate of the foamed particles.

[0119] Independent bubble rate (%) = (Vx - W / ρ) × 100 / (Va - W / ρ) ···(1)

[0120] Wherein, Vx (unit: cm) in the above formula (1) 3 Va is the actual volume of the foamed granules (in other words, the sum of the volume of the resin constituting the foamed granules and the total volume of the individual air bubbles within the foamed granules), in cm³. 3 ) is the apparent volume of the foamed particles (in other words, the volume measured by the rise in liquid level when the foamed particles are submerged in a graduated cylinder containing ethanol), W (unit: g) is the mass of the sample used for the measurement, and ρ (unit: g / cm³) is the mass of the sample used for the measurement. 3 ) is the density of the polypropylene resin that makes up the foam layer.

[0121] The foamed particles preferably have a crystalline structure in the first DSC curve obtained when heating from 23°C to 200°C at a heating rate of 10°C / min, exhibiting an endothermic peak based on the inherent melting of the polypropylene resin constituting the foam layer, and one or more melting peaks located on the higher temperature side than the endothermic peak. Foamed particles possessing this crystalline structure exhibit excellent mechanical strength and formability. Furthermore, in the following, the endothermic peak based on the inherent melting of the polypropylene resin appearing in the DSC curve will be referred to as the "resin-inherent peak," and the melting peak appearing on the higher temperature side than the resin-inherent peak will be referred to as the "high temperature peak." The resin-inherent peak is generated due to the endothermic melting of the polypropylene resin inherently constituting the foam layer. On the other hand, it is inferred that the high temperature peak is generated due to the melting of secondary crystals formed in the polypropylene resin constituting the foam layer during the manufacturing process of the foamed particles. That is, it is inferred that when a high temperature peak appears in the DSC curve, secondary crystals are formed in the polypropylene resin.

[0122] Whether the foamed particles possess the aforementioned crystalline structure can be determined by performing differential scanning calorimetry (DSC) under the aforementioned conditions according to JIS K7121:1987, and obtaining the resulting DSC curve. Furthermore, 1–3 mg of foamed particles can be used as a sample when performing DSC.

[0123] Furthermore, as described above, after heating from 23°C to 200°C at a heating rate of 10°C / min (in other words, the first heating), 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 (in other words, the second heating), only the endothermic peak of melting inherent to the polypropylene resin constituting the foam layer can be seen in the DSC curve. Therefore, it is possible to distinguish between the inherent resin peak and the high-temperature peak. The temperature of the peak of this inherent resin peak may sometimes differ slightly between the first and second heating, but typically, the difference is within 5°C.

[0124] From the viewpoint of further improving the formability of foamed particles and obtaining molded bodies with better rigidity, the melting heat of the high-temperature peak of the foamed particles 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.

[0125] The heat of fusion of the aforementioned high-temperature peak was determined as follows. First, by using 1–3 mg of state-conditioned foamed particles as a sample, differential scanning calorimetry (DSC) was performed under the aforementioned conditions to obtain the DSC curve. An example of a DSC curve is shown below. Figure 1 .like Figure 1 As shown, when the foamed particles have a high temperature peak, the DSC curve shows the resin-specific peak ΔH1 and a high temperature peak ΔH2 with a peak on the high temperature side of the peak of the resin-specific peak ΔH1.

[0126] Next, draw a straight line L1 connecting point α on the DSC curve, which corresponds to 80°C, and point β, which corresponds to the melting completion temperature T of the foamed particles. Furthermore, the melting completion temperature T is the endpoint of the high-temperature side of the high-temperature peak ΔH2; in other words, it is the intersection of the high-temperature peak ΔH2 on the DSC curve and the baseline on the higher-temperature side of ΔH2.

[0127] After drawing straight line L1, draw straight line L2 parallel to the vertical axis of the graph, passing through the maximum point γ between the resin intrinsic peak ΔH1 and the high-temperature peak ΔH2. The resin intrinsic peak ΔH1 and the high-temperature peak ΔH2 are separated by this straight line L2. The heat of fusion of the high-temperature peak ΔH2 can be calculated based on the area enclosed by the portion of the high-temperature peak ΔH2 constituting the DSC curve, straight line L1, and straight line L2.

[0128] The volume ratio M2 of the foamed particles is preferably 10 times or more and 75 times or less, more preferably 20 times or more and 75 times or less, even more preferably 30 times or more and 75 times or less, and particularly preferably 35 times or more and 60 times or less. By using the above-mentioned foamed particles, lightweight foamed particle molded articles with high blackness, inconspicuous color unevenness, and good surface properties can be easily obtained. Historically, foamed particles with high volume ratios are more prone to a decrease in blackness and a deterioration in filling properties. According to the manufacturing method of this disclosure, even foamed particles with a volume ratio of 30 times or more can suppress the decrease in blackness and the deterioration in filling properties.

[0129] The volume ratio M2 of the foamed granules is the density (unit: kg / m³) of the polypropylene resin that makes up the core layer of the resin granules. 3 Divide by the bulk density of the foamed particles (unit: kg / m³) 3 The bulk density of the foamed granules is determined as follows: First, the foamed granules are placed in an environment with an air temperature of 23℃, a relative humidity of 50%RH, and 1 atm for more than 24 hours to adjust their state. The adjusted foamed granules are then naturally piled into a graduated cylinder, and the bulk volume (unit: L) of the foamed granule group is read from the graduated cylinder's scale. Then, by dividing the mass (unit: g) of the foamed granule group in the graduated cylinder by the aforementioned bulk volume and converting the units, the bulk density (unit: kg / m³) of the foamed granules can be obtained. 3 ).

[0130] (Foamed granule molded body)

[0131] By in-mold forming the foamed particles, a molded foamed particle body can be obtained. The density of the molded body is preferably 10 kg / m³. 3 Above and 100kg / m 3 The following applies. In this case, a good balance can be achieved between the lightweight and rigidity of the molded article. From the viewpoint of further improving the rigidity of the molded article, a density of 20 kg / m³ is more preferable. 3 The above is further optimized to 25 kg / m 3 That's all. From the viewpoint of further improving the lightweight nature of the molded article, a density of 80 kg / m³ is more preferable. 3 The following is a further preferred value: 50 kg / m 3 The following is particularly preferred: 35 kg / m 3 The density of a molded body can be calculated by dividing its mass (in grams) by its volume (in liters) derived from its external dimensions, and then performing unit conversion. If it is not easy to determine the volume from the external dimensions of the molded body, the volume can be determined using the water immersion method.

[0132] The manufacturing method of the foamed granule molded body is as follows. First, foamed granules for making the molded body are prepared. In the manufacturing of the molded body, the foamed granules obtained by the aforementioned manufacturing method can also be used directly. Alternatively, internal pressure can be applied by impregnating the foamed granules with an inorganic gas such as air in a pressure vessel, and in-mold forming can be performed using the foamed granules that increase the pressure inside the bubbles of the foamed granules.

[0133] Next, foamed particles are filled into a molding die having a cavity corresponding to the shape of the desired molded body. After the foamed particles are filled, a heating medium is supplied into the molding die to heat the foamed particles. For example, steam can be used as the heating medium. The foamed particles in the molding die are heated by the heating medium, undergoing secondary foaming and fusing together. This allows the foamed particles in the molding die to be integrated, forming a molded body.

[0134] After the foamed granules are heated, the molded body inside the mold is cooled to stabilize its shape. Then, the molded body is removed from the mold, completing the in-mold molding process. Alternatively, after in-mold molding, the molded body can be cured by standing in an atmosphere of approximately 60–80°C for at least 12 hours. Curing the molded body after it is removed from the mold helps to suppress shrinkage and deformation.

[0135] The foamed particles can be formed into well-formed foamed particle bodies with excellent filling properties, high blackness, and minimal noticeable color unevenness. In particular, even foamed particles with high volume ratios can suppress the reduction of blackness and the deterioration of filling properties.

[0136] The foamed granule molded body obtained by in-mold molding the polypropylene resin foamed granules contains, for example, 0.1 parts by mass and less than 5 parts by mass of carbon black relative to 100 parts by mass of the resin constituting the foamed granule molded body, and the molded body density of the foamed granule molded body is 10 kg / m³. 3 Above and 35kg / m 3 Hereinafter, the L* value of the foamed granule molded body is less than 24. Further, a 100mm × 100mm square is drawn at the center of the surface of the foamed granule molded body, and the number of gaps (gaps) of 1mm × 1mm or larger existing along the diagonal line drawn from one corner of this square is three or fewer. The L* value of the foamed granule molded body is preferably 23 or less, more preferably 21 or less.

[0137] The L* value of the foamed granule molded body can be determined by the following method. Five measurement locations are randomly selected from the surface of the molded body, and the L* values ​​are measured using a spectrophotometer (e.g., the "SE2000" manufactured by Nippon Denshoku Kogyo Co., Ltd.). The arithmetic mean of the L* values ​​obtained from these five measurement locations is then taken as the L* value of the molded body. Furthermore, the measurement range is set to 30 mmΦ, and the measurement method is the reflection method.

[0138] Example

[0139] The following describes an example of the method for manufacturing polypropylene resin foamed granules. The polypropylene resins and polyolefin resins used in this example are shown in Table 1. Furthermore, the ethylene-propylene copolymer and ethylene-propylene-butene copolymer used in this example are both random copolymers. Additionally, the polypropylene resins (PP1, PP2, and PP3) shown in Table 1 have a density of 900 kg / m³. 3 .

[0140]

[0141] In addition, the methods for determining the physical properties of the resins shown in Table 1 are as follows.

[0142] • Comonomer content of polypropylene resins

[0143] The monomer content of polypropylene resins (specifically, ethylene-propylene copolymers and ethylene-propylene-butene copolymers) was determined using a known method determined by IR spectroscopy. Specifically, the content was determined using the method described in the Handbook of Polymer Analysis (edited by the Polymer Analysis Research Conference of the Japan Society for Analytical Chemistry, published January 1995, Kinokuniya Shoten, page numbers and titles: 615-616 "II.2.3 2.3.4 Propylene / Ethylene Copolymers", 618-619 "II.2.3 2.3.5 Propylene / Butene Copolymers"). In other words, the content was determined quantitatively by adjusting the absorbance of ethylene and butene by predetermined coefficients and considering the relationship between the absorbance and the thickness of the film-like test piece. More specifically, firstly, polypropylene resin was hot-pressed at 180°C to form a film, and multiple test pieces of varying thicknesses were prepared. Then, the IR spectrum of each test piece was measured, and the absorbance at 722 cm⁻¹ derived from ethylene was read. -1 And 733cm -1 absorbance at (A) 722 A 733 ), 766cm derived from butene -1 absorbance at (A) 766Next, for each test piece, the ethylene content in the polypropylene resin was calculated using the following equations (2) to (4). The arithmetic mean of the ethylene content obtained for each test piece was taken as the ethylene content in the polypropylene resin (unit: mass%).

[0144] (K′ 733 ) c =1 / 0.96{(K′) 733 ) a -0.268(K′ 722 ) a}···(2)

[0145] (K′ 722 ) c =1 / 0.96{(K′) 722 ) a -0.268(K′ 722 ) a}···(3)

[0146] Ethylene content = 0.575{(K′) 722 ) c +(K′ 733 ) c}···(4)

[0147] In equations (2) to (4), K′ a It refers to the apparent absorbance coefficient (K′) at each wavenumber. a =A / ρt), K′ c This refers to the corrected absorbance coefficient, where A is the absorbance and ρ is the density of the resin (unit: g / cm³). 3 ), where t refers to the thickness of the film-like test piece (unit: cm). In addition, the above formulas (2) to (4) can be applied to random copolymers.

[0148] In addition, the butene content in the polypropylene resin was calculated using the following formula (5) for each test piece. The arithmetic mean of the butene content obtained for each test piece was taken as the butene content in the polypropylene resin (unit: mass%).

[0149] Butene content = 12.3 (A) 766 / L)···(5)

[0150] In equation (5), A refers to absorbance and L refers to the thickness of the film-like test piece (unit: mm).

[0151] · Flexural modulus

[0152] The resin shown in Table 1 was hot-pressed at 230°C to produce a 4mm sheet. Test pieces measuring 80mm long × 10mm wide × 4mm thick were cut from this sheet. The flexural modulus of elasticity of the test piece was determined according to JIS K7171:2008. Furthermore, the radius of the indenter R1 and the radius of the support platform R2 were both 5mm, the distance between the support points was 64mm, and the test speed was 2mm / min.

[0153] Melting point

[0154] The melting points of the resins shown in Table 1 were determined based on JIS K7121:1987. Specifically, firstly, the state of the test piece made of resin was adjusted based on "(2) the condition of measuring the melting temperature after a certain heat treatment" as described in JIS K7121:1987. The DSC curve was obtained by heating the state-adjusted test piece from 30°C to 200°C at a heating rate of 10°C / min. Then, the peak temperature of the melting peak appearing in the DSC curve was set as the melting point. In addition, a differential scanning calorimeter (manufactured by SII Nano Technology Co., Ltd., model: DSC7020) was used as the measuring device.

[0155] Melt flow rate

[0156] The melt flow rate (MFR) of the resins shown in Table 1 was determined according to JIS K7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0157] Next, the manufacturing methods of the foamed particles in Examples 1 to 7, Comparative Examples 1 to 5, and Reference Examples 1 to 2 will be described.

[0158] (Example 1)

[0159] In producing the foamed granules of Example 1, firstly, a co-extrusion apparatus equipped with a core-forming extruder, a coating-forming extruder, and a co-extrusion die connected to both extruders was used. The extrudate extruded from the co-extrusion apparatus was cut by wire cutting to produce multilayer resin granules. Specifically, PP1 as shown in Table 1, carbon black in proportions shown in Table 2 relative to PP1, and a bubble modifier were supplied to the core-forming extruder, and melt-blended within the extruder to obtain a core-forming melt blend. Furthermore, furnace black (DBP with an oil absorption of 100 mL / mg and a BET specific surface area of ​​80 m²) was used as the carbon black. 2 / g, with an average particle diameter of 20nm), zinc borate is used as a bubble conditioner, and the amount of zinc borate added is set to 500 ppm by mass relative to the polypropylene resin.

[0160] For zinc borate, Borex's "Firebrake ZB Fine" was used. Furthermore, the arithmetic mean particle size of this zinc borate was 2.8 μm, and the proportion of particles with a diameter of 5 μm or larger was 10%. The particle size distribution of zinc borate was determined using a Microtrac MT3000, based on the aforementioned method. 1 g of zinc borate and 1 g of a 1% aqueous solution of sodium dodecylbenzenesulfonate were added to 100 g of water, and the resulting mixture was dispersed using an ultrasonic vibrator for 5 minutes. The sample refractive index was set to 1.81, and the sample shape was set to non-spherical.

[0161] Furthermore, a melt-blended compound for coating layer formation was obtained by melt-blending PO1 (as shown in Table 1), carbon black (in proportions shown in Table 2 relative to PO1), silica particles, and advanced fatty acid amide in an extruder for coating layer formation. The same furnace black used as the carbon black was employed in the aforementioned core layer. The amount of silica particles added to the polyolefin resin was set to 0.1% by mass, and the amount of advanced fatty acid amide added was set to 0.1% by weight. Erucamide ("Fatty Acid Amide E" manufactured by Kao Corporation) was used as the advanced fatty acid amide.

[0162] By co-extruding these molten compoundes and converging them within a die, a composite is formed consisting of a non-foamed cylindrical core layer and a non-foamed coating layer covering the side circumferences of the core layer. After extruding this composite from the die, the extrudate is stretched and cooled in water, then cut to appropriate lengths using a granulator, thereby obtaining multilayer resin particles consisting of a core layer and a coating layer covering the side circumferences of the core layer. Furthermore, the multilayer resin particles do not have through-holes. The mass ratio of the core layer to the coating layer in the multilayer resin particles is set to core layer:coating layer = 95:5 (in other words, the mass ratio of the coating layer is 5%). Additionally, the mass of each multilayer resin particle is set to approximately 1.0 mg.

[0163] <Distributed Processes>

[0164] 100 kg of the multilayer resin particles obtained in this manner were added together with 220 L of water as the dispersion medium into a sealed container with an internal volume of 400 L. Next, 0.3 parts by mass of a dispersant relative to 100 parts by mass of the multilayer resin particles, 0.004 parts by mass of sodium alkylbenzene sulfonate, and 0.01 parts by mass of aluminum sulfate were added as dispersing aids to the sealed container, and the multilayer resin particles were dispersed in the dispersion medium. Kaolin was used as the dispersant.

[0165] <First-stage foaming process>

[0166] Subsequently, while stirring in a sealed container, carbon dioxide, acting as an inorganic physical foaming agent, was supplied to the container, raising the temperature inside to the foaming temperature shown in Table 2. The pressure inside the container at this point (in other words, impregnation pressure, carbon dioxide pressure) is shown in the "Foaming Agent Pressure" column of Table 2. By maintaining the temperature inside the container at the foaming temperature shown in Table 2 for 15 minutes, the foaming agent impregnates the multilayer resin particles. After impregnation with the foaming agent is complete, the sealed container is opened, and the contents are released into an atmospheric atmosphere at 75°C, thereby foaming the core layer of the multilayer resin particles. No adhesion between the primary foamed particles was observed, and no shrinkage was observed. The primary foamed particles were dried for 24 hours in an atmosphere at 23°C and 50% relative humidity. Thus, primary foamed particles were obtained, comprising a foamed layer formed by core layer foaming and a non-foamed coating layer covering the foamed layer. In addition, the temperature Tu of the atmosphere from which the resin particles are released is adjusted by introducing cooling air into the space directly below the sealed container.

[0167] <Secondary foaming process>

[0168] Next, the primary foamed granules were placed in a pressure vessel, which was then sealed. In this state, air, an inorganic gas, was used to pressurize the pressure vessel, immersing the air within the bubbles so that the pressure within the bubbles of the primary foamed granules reached the value shown in the "Internal Pressure" column of Table 2. After applying the internal pressure to the primary foamed granules, they were removed from the pressure vessel and placed into a metal drum. Then, steam with the pressure shown in the "Drum Pressure" column of Table 2 was supplied to the primary foamed granules, and they were heated at atmospheric pressure. This process further foamed the primary foamed granules to obtain secondary foamed granules. The various properties of the foamed granules obtained in this way are shown in Table 2.

[0169] (Example 2, Example 3)

[0170] The manufacturing methods of the foamed granules in Examples 2 and 3 are the same as those in Example 1, except that the pressure in the sealed container in the first foaming process (in other words, the foaming agent pressure) and the steam pressure in the second foaming process (in other words, the roller pressure) are changed to the values ​​shown in Table 2.

[0171] (Example 4)

[0172] The method for manufacturing the foamed granules in Example 4 is the same as that for manufacturing the foamed granules in Example 1, except that the resin constituting the coating layer is changed from PO1 to PO2.

[0173] (Example 5)

[0174] The method for manufacturing the foamed granules in Example 5 is the same as that for manufacturing the foamed granules in Example 1, except that the resin constituting the core layer (in other words, the foaming layer of the foamed granules) is changed from PP1 to PP2.

[0175] (Example 6)

[0176] The method for manufacturing foamed granules in Example 6 is the same as that in Example 5, except that the pressure (in other words, the pressure of the foaming agent) in the sealed container in the first foaming process, the foaming temperature, and the pressure of the steam (in other words, the pressure of the roller) in the second foaming process are changed to the values ​​shown in Table 2.

[0177] (Example 7)

[0178] The manufacturing method of the foamed granules in Example 7 is the same as that of Example 1, except that the resin constituting the core layer of the resin granules (in other words, the foaming layer of the foamed granules) is changed from PP1 to PP3, and the pressure (in other words, the foaming agent pressure), foaming temperature, and steam pressure (in other words, the roller pressure) in the closed container in the first foaming process and in the second foaming process are changed to the values ​​shown in Table 2.

[0179] (Comparative Example 1)

[0180] In the manufacturing method of the foamed granules in Comparative Example 1, firstly, multilayer resin granules were prepared using the same method as in Example 1. Then, the resin granules were foamed to a volume ratio of 35 times by a single-stage foaming process alone, thereby producing foamed granules. In the manufacturing method of the foamed granules in Comparative Example 1, a second-stage foaming process was not performed. The foaming temperature and foaming agent pressure of the first-stage foaming process in Comparative Example 1 are shown in Table 3.

[0181] (Comparative Example 2)

[0182] The manufacturing method of the foamed particles in Comparative Example 2 is the same as that of the foamed particles in Comparative Example 1, except that the amount of carbon black compounded in the core layer and the coating layer is changed to the values ​​shown in Table 3.

[0183] (Comparative Example 3, Comparative Example 4)

[0184] In the manufacturing methods of the foamed granules in Comparative Examples 3 and 4, firstly, single-layer resin granules consisting only of a core layer were produced. Specifically, PP1 as shown in Table 1, carbon black in proportions shown in Table 3 relative to PP1, and a bubble modifier were supplied to an extruder, and a melt-mixed compound was obtained by melt mixing in the extruder. Then, by extruding the melt-mixed compound from the extruder, an extrudate consisting of a cylindrical core layer in a non-foamed state was formed. While stretching the extrudate, the extrudate was cooled in water and cut to an appropriate length using a granulator, thereby obtaining resin granules consisting only of a core layer. Subsequently, by performing a primary foaming process and a secondary foaming process under the conditions shown in Table 3, the resin granules were foamed to obtain foamed granules.

[0185] (Comparative Example 5)

[0186] The method for manufacturing the foamed particles in Comparative Example 5 is the same as that for manufacturing the foamed particles in Example 3, except that carbon black is not incorporated into the coating layer.

[0187] (Refer to Example 1)

[0188] In the method for manufacturing the foamed granules of Reference Example 1, resin granules were first prepared using the same method as in Example 1, except that carbon black was not incorporated into the core layer and the coating layer. Then, the resin granules were foamed by performing a primary foaming process and a secondary foaming process under the conditions shown in Table 3 to obtain the foamed granules.

[0189] (See Example 2 for reference)

[0190] In the method for manufacturing the foamed granules of Reference Example 2, resin granules were first prepared using the same method as in Comparative Examples 3 and 4, except that carbon black was not incorporated into the core layer. Then, the resin granules were foamed by performing a primary foaming process and a secondary foaming process under the conditions shown in Table 3 to obtain the foamed granules.

[0191] Next, the evaluation methods for the various properties of the foamed particles obtained above will be explained.

[0192] • Volume ratio M1 of primary foamed granules

[0193] The primary foamed granules were left to stand for at least 24 hours in an environment with a relative humidity of 50%, a temperature of 23°C, and an air pressure of 1 atm to adjust their state. The adjusted primary foamed granules were then naturally packed into a graduated cylinder, and the bulk volume (in L) of the granule group was recorded according to the graduated cylinder's scale. Subsequently, the bulk density (in kg / m³) of the primary foamed granule group was calculated by dividing the mass (in g) of the granule group in the graduated cylinder by the aforementioned bulk volume and performing unit conversion. 3Then, the volume ratio M1 of the primary foamed particles was calculated by dividing the density of the polypropylene resin constituting the foam layer by the bulk density of the primary foamed particles. The volume ratio M1 of the primary foamed particles in the examples, comparative examples, and reference examples is shown in Tables 2 and 3.

[0194] • Volume ratio of foamed particles M2

[0195] The bulk density (unit: kg / m³) of the foamed granules was determined using the same method as for the determination of the bulk density of primary foamed granules. 3 Then, the volume ratio M2 of the foamed particles was calculated by dividing the density of the polypropylene resin constituting the foam layer by the bulk density of the foamed particles. The volume ratio M2 and bulk density of the foamed particles in the examples, comparative examples, and reference examples are shown in Tables 2 and 3.

[0196] Independent bubble rate

[0197] The independent bubble ratio of the foamed particles was determined using an air comparison hydrometer according to ASTM-D2856-70 procedure C. Specifically, it was calculated as follows. The packing volume after conditioning was approximately 20 cm³. 3 The foamed particles were used as the test sample. As described below, the apparent volume Va could not be accurately measured using ethanol. After the test sample with the measured apparent volume Va was thoroughly dried, the true volume Vx of the test sample was measured using an Accupyc II 1340 manufactured by Shimadzu Corporation, according to step C as described in ASTM-D2856-70. Then, using these volume values ​​Va and Vx, the independent bubble rate of the test sample was calculated based on the following formula (6). The operation was performed five or more times with different test samples, and the arithmetic mean of the independent bubble rates of the five test samples (N=5) was taken as the independent bubble rate of the foamed particles.

[0198] Independent bubble rate (%) = (Vx - W / ρ) × 100 / (Va - W / ρ) ···(6)

[0199] The symbols in equation (6) above have the following meanings.

[0200] Vx: The true volume of the foamed particles as determined by the above method, i.e., the sum of the volume of the resin constituting the foamed particles and the total volume of the individual air bubbles within the foamed particles (unit: cm). 3 )

[0201] Va: Apparent volume of the foamed particles, measured by the rise in water level when the foamed particles are submerged in a graduated cylinder containing ethanol (unit: cm). 3 )

[0202] W: Mass of the sample used for the determination of foamed particles (unit: g)

[0203] ρ: Density of the resin constituting the foamed particles (unit: g / cm³) 3 )

[0204] The independent bubble ratios of the foamed particles from the examples, comparative examples, and reference examples are shown in Tables 2 and 3.

[0205] High temperature peak heat

[0206] Differential scanning calorimetry (DSC) curves were obtained by using 1–3 mg of state-conditioned foamed particles and performing differential scanning calorimetry (DSC) according to JIS K7121:1987. The DSC start temperature was set to 23°C, the measurement completion temperature to 200°C, and the heating rate to 10°C / min. A TA Instruments "DSC.Q1000" was used as the measuring apparatus. The area of ​​the high-temperature peak in the DSC curves obtained by the aforementioned method was calculated, and this value is presented as the high-temperature peak calorific value in Tables 2 and 3.

[0207] • Filling properties

[0208] The filling performance of foamed granules was evaluated based on the angle of repose and the filling performance when filling with a molding die. The method for measuring the angle of repose is as follows. In measuring the angle of repose of the foamed granules, the angle of repose measuring device "Flow Surface Angle Measuring Instrument FSA-100S" manufactured by Rigaku Machinery Co., Ltd. was used, and the angle of repose was measured by the cylindrical rotation method. Specifically, firstly, 200 mL of foamed granules were placed into a cylindrical container with a volume of 500 mL. Next, the rotation speed was set to 1 revolution and 26 seconds, and the container was rotated for 3 minutes to adjust the packing state of the foamed granules. After the state of the foamed granules was adjusted, the rotation of the container was continued. Then, the rotation of the container was stopped before the upper part of the foamed granule packing in the container deformed. An angle gauge was set to align the upper and lower ends of the inclined plane in the foamed granule packing formed by this operation, and the angle of the inclined plane was measured. The angle of the inclined plane obtained in this way is defined as the angle of repose of the foamed granules.

[0209] In the "Angle of Repose" column of the "Filling Properties" section of Tables 2 and 3, cases with an angle of repose less than 27° are marked with the symbol "A", cases greater than 27° but less than 30° are marked with the symbol "B", cases greater than 30° but less than 33° are marked with the symbol "C", and cases greater than 33° are marked with the symbol "D". The angle of repose is a value related to the ease of flow of foamed particles, meaning that the smaller the angle of repose, the easier the foamed particles flow and the higher their filling properties.

[0210] Furthermore, the evaluation method for filling performance when filling the molding die with foamed particles is as follows. Specifically, except that the amount of cracking in the evaluation of the minimum molding pressure described later is set to 0% and foamed particles are filled, the molded body is manufactured using the same method as described in the evaluation of the minimum molding pressure. A 100mm × 100mm square is drawn in the center of the obtained molded body, and a diagonal line is drawn from one corner of the square. The number of voids (gap) of 1mm × 1mm or larger present on the diagonal line is counted. In the "Molded Body Evaluation" column of the "Filling Performance" section of Tables 2 and 3, the case with less than three voids is recorded as symbol "A", the case with three or more but less than five voids is recorded as symbol "B", the case with five or more but less than eight voids is recorded as symbol "C", and the case with eight or more voids is recorded as symbol "D". In addition, the weldability and resilience of the obtained molded body are not evaluated in this evaluation.

[0211] Minimum forming pressure during in-mold forming

[0212] After filling a mold with foamed particles into a cavity having internal dimensions of 300 mm long × 250 mm wide × 60 mm thick, in-mold forming was performed by supplying steam as a heating medium into the mold. Before forming, a pretreatment pressurization was performed, applying an internal pressure of 0.1 MPa (G) to the foamed particles, and forming was performed with the decomposition rate set to 10% (in other words, 6 mm). More specifically, in-mold forming was performed through the following steps: First, after closing the mold, a preheating venting process was performed by supplying steam for 5 seconds from both sides of the mold in the thickness direction. Then, steam was supplied from one side of the mold to a pressure 0.08 MPa (G) lower than the forming pressure during the formal heating (described later) for one-sided heating. Next, steam was supplied from the other side of the mold to a pressure 0.04 MPa (G) lower than the forming pressure during the formal heating for one-sided heating. Finally, steam was supplied from both sides of the mold for formal heating. After the formal heating is completed, the pressure inside the molding die is released, and the molded body is cooled within the molding die until the surface pressure based on the foaming force of the molded body reaches 0.04 MPa (G). Then, the molded body is removed from the molding die. The demolded molded body is then placed in an oven at 80°C for 12 hours for a curing process. The resulting molded body measures 300 mm in length, 250 mm in width, and 60 mm in thickness.

[0213] In the above forming method, the minimum forming pressure at which a well-formed molded body can be obtained is defined as the minimum forming pressure, as shown in Tables 2 and 3, when the steam pressure during formal heating (in other words, the forming pressure) varies by 0.02 MPa each time from 0.20 MPa(G) to 0.32 MPa(G). Furthermore, a lower minimum forming pressure indicates better formability. A well-formed body is defined as one that passes any of the evaluations of weldability, surface properties, and resilience described later.

[0214] <Weldability>

[0215] The molded body was bent and fractured. The number of foamed particles present on the fracture surface (C1) and the number of foamed particles after fracture (C2) were calculated. The ratio of the number of foamed particles after fracture to the number of foamed particles present on the fracture surface was then calculated (in other words, the material breakage rate). The material breakage rate was calculated using the formula C2 / C1×100. The above measurement was performed five times using different test pieces, and the material breakage rate was calculated for each. Then, cases where the arithmetic mean of these material breakage rates was 90% or higher were considered acceptable.

[0216] <Surface>

[0217] Draw a 100mm x 100mm square in the center of the molded body, and draw a diagonal line starting from one corner of the square. Count the number of gaps (voids) larger than 1mm x 1mm that exist along the diagonal line. If the number of gaps is less than three, it is considered acceptable.

[0218] <Restoration>

[0219] The thickness of the molded body was measured at four locations 10 mm inside each vertex in the center direction, and the thickness of the molded body in the center, when viewed from above in the thickness direction. Then, the ratio (in %) of the thickness of the thinnest position to the thickness of the thickest position was calculated, and a thickness ratio of 95% or higher was considered acceptable.

[0220] Water cooling time during in-mold forming

[0221] During the in-mold forming under the aforementioned minimum forming pressure, the time from the moment the formal heating was completed to the moment when the surface pressure based on the foaming force of the molded body reached 0.04 MPa (G) was measured, and this time was set as the water cooling time.

[0222] • Density of molded body

[0223] The density of the molded body (unit: g) was calculated by dividing the mass of the molded body obtained by in-mold forming under the aforementioned minimum forming pressure by the volume (unit: L) obtained based on the external dimensions of the molded body, and then performing unit conversion. 3 ).

[0224] ·tone

[0225] The evaluation of hue is based on the judgment of the depth of color of the molded body and the degree of color unevenness of the molded body, using the L* value based on the CIE 1976 L*a*b* color system. The method for measuring the L* value is as follows. First, five locations are randomly selected from the surface of the molded body, that is, from the surface that contacts the mold during in-mold molding, and the L* value is measured using a spectrophotometer (SE2000 manufactured by Nippon Denshoku Kogyo Co., Ltd.). Furthermore, the measurement range is set to 30 mmΦ, and the measurement method is the reflectance method. Then, the arithmetic mean of the L* values ​​at these five measurement locations is taken as the L* value of the molded body. The L* values ​​of the molded bodies composed of foamed particles from the examples and comparative examples are shown in the "L* value" column of the "Hue" section in Tables 2 and 3. In addition, in the "Hue" column of Tables 2 and 3, the "Color Depth" column is marked with the symbol "A" for L* values ​​less than 24, "B" for L* values ​​greater than 24 but less than 28, and "C" for L* values ​​greater than 28. The L* value is an indicator of brightness; the lower the value, the higher the blackness and the deeper the black.

[0226] In addition, the degree of color unevenness of the molded body was evaluated based on visual observation. Specifically, under visual inspection, color unevenness was evaluated in five stages: from no color unevenness on the surface of the molded body, presenting a uniform black color (5 points), to obvious color unevenness, with gray areas visible everywhere (1 point). The average score (rating) of the evaluations from five observers was calculated. In the "Color Unevenness" column of the "Color Tone" section in Tables 2 and 3, scores of 4 points or higher were recorded as symbol "A", scores of 3 points or higher but less than 4 points were recorded as symbol "B", and scores less than 3 points were recorded as symbol "C". Furthermore, since no carbon black was incorporated into the foamed particles in the reference example, color unevenness was not evaluated in the reference example. Therefore, the color unevenness column in the reference example is recorded as symbol "-".

[0227]

[0228]

[0229] As shown in Table 2, the foamed particles obtained by the manufacturing methods of Examples 1 to 7 exhibit excellent filling properties. Furthermore, by in-mold molding these foamed particles, a good foamed particle molded body with high blackness and minimal color unevenness can be obtained. Even with high bulk density, the foamed particles obtained by the manufacturing methods of the examples are foamed particles with good filling properties, high blackness, and small hue deviation. Based on the comparison between Examples 1 and 3, it can be understood that when the volume ratio M2 of the secondary foamed particles to the volume ratio M1 of the primary foamed particles is M2 / M1 or higher, a molded body with a deeper black color can be manufactured. Additionally, in Examples 1 to 7, the water cooling time of the foamed particles in Examples 5 to 7 is particularly short.

[0230] On the other hand, in the manufacturing method of Comparative Example 1 shown in Table 3, the foamed particles having the same volume ratio as those in Examples 1 and 3 to 6 were not subjected to a secondary foaming process, but were instead produced by primary foaming. Therefore, compared to the foamed particles obtained by the manufacturing methods of Examples 1 and 3 to 6, the molded articles obtained from the foamed particles obtained by the manufacturing method of Comparative Example 1 have lower blackness and more noticeable color unevenness. Furthermore, the independent bubble rate of the foamed particles is also more likely to decrease. Furthermore, the filling properties of the foamed particles deteriorate.

[0231] In the manufacturing method of Comparative Example 2, due to the excessive amount of carbon black incorporated into the core layer and the coating layer, the minimum forming pressure became higher, and the water cooling time also became longer. In addition, the filling properties of the resulting foamed particles decreased.

[0232] The foamed particles obtained by the manufacturing method of Comparative Example 3 have poor filling properties compared to the foamed particles of the Examples because they do not have a non-foamed coating layer.

[0233] The foamed particles obtained by the manufacturing method of Comparative Example 4 have a low blackness and a tendency for large color deviations in the resulting molded articles due to the excessively high volume ratio M1 of the primary foamed particles. Furthermore, the foamed particles obtained by the manufacturing method of Comparative Example 4 lack a non-foamed coating layer, resulting in poor filling performance compared to the foamed particles of the embodiments. Moreover, based on the comparison between Comparative Example 3 and Comparative Example 4, it can be understood that the foamed particles of Comparative Example 4, with a higher volume ratio M2 than Comparative Example 3, have further reduced filling performance compared to Comparative Example 3. Additionally, it is known that the filling performance of the foamed particles of Comparative Example 4 is lower than that of Reference Example 2 described later; therefore, the carbon black-containing foam layer degrades the filling performance.

[0234] In the manufacturing method of Comparative Example 5, no carbon black was incorporated into the coating layer. Therefore, the foamed particles obtained by the manufacturing method of Comparative Example 5 had a slightly lower blackness and a more pronounced color deviation compared to the foamed particles obtained by the manufacturing methods of Examples 1 and 3 to 5. Furthermore, since the foamed particles obtained by the manufacturing method of Comparative Example 5 did not contain carbon black in the coating layer, their filling properties were poor compared to the foamed particles obtained by the manufacturing methods of Examples 1 and 3 to 5.

[0235] Reference Examples 1 and 2 are examples in which no carbon black is incorporated into either the foaming layer or the coating layer. The foamed particles of Reference Examples 1 and 2 are white in color. Comparing Reference Examples 1 and 2, the filling properties of the foamed particles obtained by Reference Example 1 are slightly improved compared to those obtained by Reference Example 2. In contrast, in the examples incorporating carbon black, comparing Example 1 and Comparative Example 3, which have the same composition except for the presence or absence of a coating layer, it is clear that the filling properties of the foamed particles obtained by Example 1 are significantly improved compared to those obtained by Comparative Example 3. Furthermore, comparing Example 1 with Comparative Example 5, which does not incorporate carbon black into the coating layer, it is also clear that the filling properties of the foamed particles obtained by Example 1 are improved compared to those obtained by Comparative Example 5.

[0236] Therefore, based on the comparison of the embodiments, comparative examples and reference examples, the following effect can be understood: by not only providing a non-foamed coating layer on the surface of the foamed layer, but also providing a non-foamed coating layer containing the specific amount of carbon black, and manufacturing foamed particles through a two-stage foaming process as described above, the filling properties of the foamed particles are significantly improved.

[0237] The above describes the specific method for manufacturing polypropylene resin foamed particles according to the present invention based on the embodiments. However, the specific method for manufacturing polypropylene resin foamed particles according to the present invention is not limited to the embodiments, and the configuration can be appropriately modified within the scope of the present invention without affecting the spirit of the present invention.

Claims

1. A method for manufacturing polypropylene resin foamed granules, wherein, The method for manufacturing the polypropylene resin foamed granules includes: The dispersion process involves dispersing polypropylene resin particles in a dispersion medium. The polypropylene resin particles comprise: a core layer containing a polypropylene resin as the base resin and 0.1 to 5 parts by weight of carbon black relative to 100 parts by weight of the polypropylene resin; and a coating layer containing a polyolefin resin as the base resin and 0.1 to 5 parts by weight of carbon black relative to 100 parts by weight of the polyolefin resin, and coating the core layer. In the primary foaming process, after the polypropylene resin particles are impregnated with an inorganic physical foaming agent in a dispersion medium within a sealed container, the polypropylene resin particles and the dispersion medium are released from the sealed container together into an atmosphere at a lower pressure than that inside the sealed container, thereby causing the core layer of the polypropylene resin particles to foam and obtain primary foamed particles with a volume ratio of 5 times or more and 25 times or less; and In the secondary foaming process, after the pressure inside the bubbles of the primary foaming particles increases, the primary foaming particles are heated, thereby causing the primary foaming particles to further foam to obtain polypropylene resin foamed particles. The ratio of the volume ratio M2 of the polypropylene resin foamed particles to the volume ratio M1 of the primary foamed particles, M2 / M1, is greater than 1.2 and less than 3.

0.

2. The method for manufacturing polypropylene resin foamed granules according to claim 1, wherein, The ratio of the volume ratio M2 of the polypropylene resin foamed particles to the volume ratio M1 of the primary foamed particles, M2 / M1, is greater than 1.8 and less than 3.

0.

3. The method for manufacturing polypropylene resin foamed granules according to claim 1 or 2, wherein, In the primary foaming process, the temperature Tu of the atmosphere released by the polypropylene resin particles is above 40°C and below 80°C.

4. The method for manufacturing polypropylene resin foamed granules according to claim 1 or 2, wherein, The melting point Tms of the polyolefin resin used as the base resin of the coating layer is lower than the melting point Tmc of the polypropylene resin used as the base resin of the core layer.

5. The method for manufacturing polypropylene resin foamed granules according to claim 1 or 2, wherein, The volume ratio M2 of the polypropylene resin foamed particles is more than 30 times and less than 75 times.

6. The method for manufacturing polypropylene resin foamed granules according to claim 1 or 2, wherein, The carbon black is a coloring carbon black with a dibutyl phthalate oil absorption of less than 150 mL / 100 g.

7. The method for manufacturing polypropylene resin foamed granules according to claim 1 or 2, wherein, The polypropylene resin used as the base resin for the core layer is an ethylene-propylene-butene copolymer or a propylene-butene copolymer, and the butene content in these copolymers is 2% by mass or more and 15% by mass or less.

8. A method for manufacturing a foamed granular molded body, wherein, After filling a molding mold with polypropylene resin foam particles obtained by the manufacturing method of polypropylene resin foam particles according to any one of claims 1 to 7, a heating medium is supplied to the molding mold to perform in-mold forming, thereby producing a foam particle molded body.

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