Method for producing polypropylene-based resin foam particles, method for discriminating polypropylene-based resin, and method for producing polypropylene-based resin

Abstract

This method for producing polypropylene-based resin foam particles includes a foaming step for foaming a polypropylene-based resin composition that contains a base resin composed of a polypropylene-based resin and a foaming agent. The base resin is produced using a recycled polypropylene-based resin (R). The blending amount of the polypropylene-based resin (R) in the base resin is 15 mass% or more. The polypropylene-based resin (R) has a melting point of lower than 160°C. The polypropylene-based resin (R) has an oxidation induction temperature (T3) of 220°C or higher as measured in accordance with ISO 11357-6: 2018.

Description

Method for producing polypropylene resin foam particles, method for identifying polypropylene resin, and method for producing polypropylene resin

[0001] This disclosure relates to a method for producing polypropylene resin foam particles, a method for identifying polypropylene resins, and a method for producing polypropylene resins.

[0002] Polypropylene foam particle molded articles are lightweight and have excellent cushioning and rigidity, making them suitable for various applications such as packaging materials, containers, and cushioning materials. Polypropylene foam particle molded articles are manufactured, for example, by a method called in-mold molding, in which polypropylene foam particles are filled into a mold and heated with steam. In the in-mold molding method, when steam is supplied into the mold to heat the foam particles, the foam particles soften and undergo secondary foaming. As a result, the foam particles in the mold fuse together, and a molded article with a shape corresponding to the shape of the mold cavity can be obtained.

[0003] In recent years, from the perspective of reducing environmental impact and promoting the formation of a circular economy, there has been a desire to recycle waste materials that have been used by end users and utilize them as recycled materials (so-called post-consumer materials). For example, Patent Document 1 describes a method for producing a polyolefin resin foam molded article, which includes the steps of: (a) crushing a waste foamed polyolefin resin molded article to a size of 1 mm to 30 mm and granulating it with an extruder to obtain waste polyolefin resin pellets; (b) mixing the waste polyolefin resin pellets with virgin polyolefin resin and pelletizing them again with an extruder; (c) impregnating the pellets with a foaming agent in an aqueous dispersion system and foaming them to produce pre-foamed particles; and (d) producing a polyolefin resin foam molded article using the pre-foamed particles.

[0004] Japanese Patent Publication No. 2005-297464

[0005] However, recycled polypropylene resins, including those derived from post-consumer materials, sometimes degrade more rapidly than virgin polypropylene resins due to the thermal history experienced during the recycling process. Therefore, the higher the amount of recycled polypropylene resin contained in a polypropylene resin foam particle molded product, the more likely it was that the properties of the molded product, such as tensile strength and compressive strength, would deteriorate over time, depending on the usage environment. This tendency was particularly pronounced when using polypropylene resins derived from post-consumer materials; a higher proportion of polypropylene resin derived from post-consumer materials could lead to premature deterioration of the molded product and a rapid decline in its properties, depending on the usage environment.

[0006] Furthermore, recycled polypropylene resins exhibit significant variations in physical properties between batches. In addition, it was difficult to predict or determine in advance the extent of thermal history a particular recycled polypropylene resin had undergone, or whether the resulting polypropylene resin foam particle molded product would be prone to degradation.

[0007] This disclosure is made in view of the above background and aims to provide a method for producing polypropylene resin foam particles, a method for identifying polypropylene resin, and a method for producing polypropylene resin, which include recycled polypropylene resin, slow down the rate of deterioration of the physical properties of polypropylene resin foam particle molded articles over time, and maintain good physical properties of the molded articles for a long period of time.

[0008] One aspect of the present disclosure relates to a method for producing polypropylene resin foam particles according to the following [1] to

[13] : (1) A method for producing polypropylene resin foam particles, comprising a foaming step of foaming a polypropylene resin composition comprising a base resin composed of a polypropylene resin and a foaming agent, wherein the base resin is made using recycled polypropylene resin (R), the amount of polypropylene resin (R) in the base resin is 15% by mass or more, the melting point of the polypropylene resin (R) is less than 160°C, and the oxidation induction temperature of the polypropylene resin (R), measured in accordance with ISO 11357-6:2018, is 220°C or higher.

[0009] [2] The method for producing polypropylene resin foam particles according to [1], wherein the base resin is made using the polypropylene resin (R) which has been confirmed to have an oxidation induction temperature of 220°C or higher. [3] The method for producing polypropylene resin foam particles according to [1], comprising: an oxidation induction temperature measurement step of measuring the oxidation induction temperature of the polypropylene resin (R) in accordance with ISO 11357-6:2018; a determination step of determining that the polypropylene resin (R) having an oxidation induction temperature of 220°C or higher can be used to produce the foam particles, and that the polypropylene resin (R) having an oxidation induction temperature of less than 220°C cannot be used to produce the foam particles; and a base resin production step of producing the base resin using the polypropylene resin (R) which has been determined to be usable to produce the foam particles in the determination step.

[0010] [4] The manufacturing method according to [3], comprising an antioxidant addition step of adding an antioxidant to the polypropylene resin (R) which has been determined in the discrimination step to be unsuitable for use in the production of foamed particles, such that the oxidation induction temperature is 220°C or higher, and in the foaming step, foaming a base resin containing the polypropylene resin (R) which has gone through the antioxidant addition step and has an oxidation induction temperature of 220°C or higher to obtain foamed particles. [5] The manufacturing method according to [3], wherein when the polypropylene resin (R) which has been determined in the discrimination step to be suitable for use in the production of foamed particles is used, the base resin production step is carried out without adding a phenolic antioxidant, or with a phenolic antioxidant added in an amount of less than 0.01% by mass.

[0011] [6] A method for producing polypropylene resin foam particles according to any one of [1] to [5], wherein the carbon black content in the polypropylene resin (R) is less than 0.5% by mass (including 0). [7] A method for producing polypropylene resin foam particles according to any one of [1] to [6], wherein the carbon black content in the base resin is 0.5% by mass or more and 5% by mass or less. [8] A method for producing polypropylene resin foam particles according to any one of [1] to [6], wherein the carbon black content in the base resin is less than 0.5% by mass (including 0). [9] A method for producing polypropylene resin foam particles according to any one of [1] to [8], wherein the melting point of the polypropylene resin (R) is 150°C or less.

[10] A method for producing polypropylene resin foam particles according to any one of [1] to [9], wherein the amount of phenolic antioxidant blended in the base resin is 0.005% by mass or more and 0.5% by mass or less.

[0012]

[11] A method for producing polypropylene resin foam particles according to any one of [1] to

[10] , wherein the polypropylene resin (R) is derived from a post-consumer material of a polypropylene resin foam molded body.

[12] A method for producing polypropylene resin foam particles according to

[11] , wherein the post-consumer material is obtained by compressing and then crushing a polypropylene resin foam molded body.

[13] A method for producing polypropylene resin foam particles according to any one of [1] to

[12] , wherein the base resin contains a polypropylene resin (A) made of a virgin polypropylene resin, the blending ratio of the polypropylene resin (A) in the base resin is 0.1% by mass or more and 85% by mass or less, and the blending ratio of the polypropylene resin (R) is 15% by mass or more and 99.9% by mass or less (provided that the sum of the blending ratio of the polypropylene resin (R) and the blending ratio of the polypropylene resin (A) is 100% by mass).

[0013] Another aspect of this disclosure is a method for identifying polypropylene resins according to

[14] below.

[0014]

[14] A method for determining whether recycled polypropylene resin can be used to produce polypropylene resin foam particles, comprising: an oxidation induction temperature measurement step of measuring the oxidation induction temperature of the polypropylene resin in accordance with ISO 11357-6:2018; and a determination step of determining that if the oxidation induction temperature of the polypropylene resin measured in the oxidation induction temperature measurement step is 220°C or higher, the polypropylene resin can be used to produce the foam particles, and if the oxidation induction temperature of the polypropylene resin is less than 220°C, the polypropylene resin cannot be used to produce the foam particles.

[0015] Further aspects of this disclosure relate to a method for producing polypropylene resins according to

[15] below.

[0016]

[15] A method for producing polypropylene resin for use in the production of polypropylene resin foam particles, comprising adding 0.005% by mass to 1% by mass of a phenolic antioxidant to a polypropylene resin recovery product derived from post-consumer materials of a polypropylene resin foam molded product, and melt-kneading it in an extruder to produce a polypropylene resin having an oxidation induction temperature of 220°C or higher, as measured in accordance with ISO 11357-6:2018.

[0017] According to the above embodiment, it is possible to provide a method for producing polypropylene resin foam particles, a method for identifying polypropylene resin, and a method for producing polypropylene resin, which include recycled polypropylene resin, slow down the rate of deterioration of the physical properties of a polypropylene resin foam particle molded article (hereinafter also referred to as "molded article") over time, and maintain the good physical properties of the molded article for a long period of time.

[0018] Figure 1 is an explanatory diagram showing an example of the oxidation induction temperature curve of polypropylene resin (R). Figure 2 is an explanatory diagram showing an example of the oxidation induction time curve of polypropylene resin foam particles. Figure 3 is an explanatory diagram showing the method for calculating the heat of fusion of the high-temperature peak.

[0019] (Method for producing polypropylene resin foam particles) The method for producing the polypropylene resin foam particles (hereinafter also referred to as "foam particles") includes a foaming step of foaming a polypropylene resin composition comprising a base resin made of polypropylene resin and a foaming agent. The base resin is made using recycled polypropylene resin (R). The amount of polypropylene resin (R) in the base resin is 15% by mass or more. Furthermore, the melting point of the polypropylene resin (R) is less than 160°C, and the oxidation induction temperature of the polypropylene resin (R), measured in accordance with ISO 11357-6:2018, is 220°C or higher.

[0020] In the above manufacturing method, by using a polypropylene resin (R) as the recycled polypropylene resin, which has a melting point below the specified value and an oxidation induction temperature above the specified value, foamed particles can be easily obtained that maintain the good physical properties of the molded article for a long period of time, even when the blending ratio of the recycled polypropylene resin (R) in the base resin is above the specified amount. In other words, foamed particles in which the oxidation induction time described later is within a specified range can be easily obtained. The method for manufacturing the foamed particles will be described in detail below.

[0021] [Oxidation Induction Temperature Measurement Step] The manufacturing method may further include an oxidation induction temperature measurement step in which the oxidation induction temperature of the polypropylene resin (R) is measured in accordance with ISO 11357-6:2018. Furthermore, when the oxidation induction temperature measurement step is performed, it is preferable to determine whether or not the polypropylene resin (R) can be used in the determination step described later, based on the oxidation induction temperature measured in the oxidation induction temperature measurement step.

[0022] In the above manufacturing method, the oxidation induction temperature of the polypropylene resin (R) can be accurately determined by performing an oxidation induction temperature measurement step. Then, in the discrimination step described later, the suitability of the polypropylene resin (R) is determined based on the oxidation induction temperature measured in the oxidation induction temperature measurement step, and by blending the polypropylene resin (R) that has been determined to be suitable into the base resin of the foamed particles, the effect of maintaining the good physical properties of the molded article can be obtained more reliably.

[0023] In the oxidation induction temperature measurement process, approximately 5 mg of polypropylene resin (R) is used as a sample, and the oxidation induction temperature of the polypropylene resin (R) can be measured by differential scanning calorimetry (DSC) in accordance with ISO 11357-6:2018. More specifically, first, the sample is placed in an open sample pan and then placed on the sample stage of the DSC apparatus. Next, air is supplied into the furnace of the DSC apparatus to replace the atmosphere inside the furnace with air.

[0024] After the furnace is filled with air, the sample is heated at a heating rate of 10°C / min, and the heat flow of the sample is measured until the temperature reaches a point at least 30°C higher than the peak of the exothermic reaction caused by the oxidative decomposition of the sample. The air flow rate is set to 50 mL / min.

[0025] Figure 1 shows a schematic diagram of the oxidation induction temperature curve, with the heat flow obtained in this manner represented on the vertical axis and the heating temperature on the horizontal axis. As shown in Figure 1, the oxidation induction temperature curve has a generally flat shape from the time heating of the sample begins until the sample starts to melt. When the sample melts, an endothermic peak appears with its peak at the melting point T1. After the endothermic peak appears, the oxidation induction temperature curve has a flat shape until the oxidative decomposition of the sample begins. Subsequently, when the sample starts to oxidative decompose at temperature T2, an exothermic peak appears associated with the oxidative decomposition.

[0026] To determine the oxidation induction temperature of polypropylene resin (R), first, the temperature T4 at which the slope of the tangent to the oxidation induction temperature curve is greatest is determined, between temperature T2 and temperature T5, which corresponds to the peak of the exothermic reaction. Next, an extension line L1 of the baseline of the oxidation induction temperature curve extended from temperature T2 and a tangent line L2 to the oxidation induction temperature curve at temperature T4 are drawn on the oxidation induction temperature curve. Then, the temperature T3 corresponding to the intersection of extension line L1 and tangent line L2 is taken as the oxidation induction temperature of the polypropylene resin (R). Note that the oxidation induction temperature is sometimes called dynamic OIT or oxidation onset temperature (OOT). In the above oxidation induction temperature measurement process, measurements are performed using at least three samples, and the arithmetic mean of the oxidation induction temperatures obtained from the three measurements is adopted as the oxidation induction temperature of the polypropylene resin (R).

[0027] The oxidation induction temperature of the polypropylene resin (R) can be adjusted, for example, by adding an antioxidant to the polypropylene resin (R) in the antioxidant addition step described later.

[0028] [Discrimination step] The manufacturing method may further include a discrimination step in which, if the oxidation induction temperature of the polypropylene resin (R) is 220°C or higher, the polypropylene resin (R) can be used to produce the foamed particles, and if the oxidation induction temperature of the polypropylene resin (R) is less than 220°C, the polypropylene resin (R) cannot be used to produce the foamed particles. In the manufacturing method, by blending the polypropylene resin (R) that has been determined to have an oxidation induction temperature of 220°C or higher and can be used to produce foamed particles into the base resin of the foamed particles, the effect of maintaining good physical properties of the molded article can be obtained more reliably.

[0029] If the oxidation induction temperature of the polypropylene resin (R) to be identified in the discrimination step is known, it is possible to determine whether or not the polypropylene resin (R) can be used to produce foamed particles based on the known oxidation induction temperature of the polypropylene resin (R). On the other hand, if the oxidation induction temperature of the polypropylene resin (R) to be identified in the discrimination step is unknown, it is possible to determine whether or not the polypropylene resin (R) can be used to produce foamed particles based on the oxidation induction temperature obtained by performing the oxidation induction temperature measurement step.

[0030] The timing of the discrimination process is not particularly limited as long as it is performed before the polypropylene resin (R) is blended into the base resin. Furthermore, the discrimination process may be performed once or multiple times. For example, in the above manufacturing method, the discrimination process can be performed before the base resin is produced, and the polypropylene resin (R) that has been determined to be usable in the discrimination process can be blended into the base resin.

[0031] Furthermore, for example, if the polypropylene resin (R) is determined to be unusable in the discrimination process, the antioxidant addition process described later can be performed to add an antioxidant to the polypropylene resin (R), and then the discrimination process can be performed again to determine whether the polypropylene resin (R) is usable or not.

[0032] The determination step can also be referred to as a selection step of the polypropylene-based resin (R) that selects a polypropylene-based resin (R) having an oxidation induction temperature of 220 °C or higher among the polypropylene-based resins (R) and uses it for the production of the foamed particles.

[0033] 〔Antioxidant addition step〕The production method may further include an antioxidant addition step of adding an antioxidant to the polypropylene-based resin (R) determined in the determination step as not being able to be used for the production of the foamed particles so that the oxidation induction temperature becomes 220 °C or higher. In the production method, by performing the antioxidant addition step, the range of the polypropylene-based resin (R) that can be used for the production of the foamed particles can be further expanded, and the material recycling of the polypropylene-based resin (R) can be further promoted.

[0034] In the antioxidant addition step, various modes can be adopted for the method of adding an antioxidant to the polypropylene-based resin (R). For example, in the antioxidant addition step, the polypropylene-based resin (R) and the antioxidant may be supplied to an extruder, and the antioxidant may be added to the polypropylene-based resin (R) by melt-kneading the polypropylene-based resin (R) and the antioxidant in the extruder. Further, the form of the antioxidant supplied to the extruder is not particularly limited. For example, in the antioxidant addition step, the antioxidant itself may be directly supplied to the extruder, or a masterbatch containing the antioxidant may be supplied to the extruder. As the base resin of the masterbatch, for example, a thermoplastic resin such as a polypropylene-based resin can be used. The polypropylene-based resin (R) that has undergone the antioxidant addition step becomes, for example, a pellet-shaped raw material.

[0035] Thus, by performing the antioxidant addition step of adding an antioxidant to the polypropylene-based resin (R) so that the oxidation induction temperature becomes 220 °C or higher, and by using the polypropylene-based resin (R) that has undergone the antioxidant addition step to perform the base resin production step and the foaming step to foam the base resin, the effect of slowing down the rate of deterioration of the physical properties of the polypropylene-based resin foamed particle molded body over time can be more surely achieved.

[0036] In the antioxidant addition step, the type of antioxidant added to the polypropylene resin (R) is not particularly limited. For example, known antioxidants used for polypropylene resins such as phenolic antioxidants, sulfur-based antioxidants, and phosphorus-based antioxidants can be used.

[0037] It is preferable that the antioxidant added to the polypropylene resin (R) in the antioxidant addition step contains a phenolic antioxidant. Phenolic antioxidants have a high effect of improving the oxidation induction temperature of the polypropylene resin (R). Therefore, by adding a phenolic antioxidant to the polypropylene resin (R), the oxidation induction temperature of the polypropylene resin (R) can be more easily adjusted within the specific range, and the effect of maintaining good physical properties of the molded body can be more easily obtained. From the viewpoint of obtaining such an effect more reliably, the phenolic antioxidant is preferably added so that the content of the phenolic antioxidant in the polypropylene resin (R) is 0.005% by mass or more and 1% by mass or less, more preferably 0.007% by mass or more and 0.8% by mass or less, still more preferably 0.01% by mass or more and 0.3% by mass or less, and particularly preferably 0.02% by mass or more and 0.2% by mass or less.

[0038] Further, the antioxidant added to the polypropylene resin (R) in the antioxidant addition step may contain a phosphorus-based antioxidant. As described above, in the process of recycling polypropylene resins, a thermal history is applied to the polypropylene resin, so the melt mass flow rate of the recycled polypropylene resin may increase. Also, in the process of manufacturing foamed particles, the polypropylene resin undergoes further thermal history, so the melt mass flow rate of the polypropylene resin tends to increase easily in the process of manufacturing foamed particles. In contrast, phosphorus-based antioxidants are excellent in the effect of suppressing the increase in the melt mass flow rate of polypropylene resins.

[0039] Therefore, by including a phosphorus-based antioxidant in the polypropylene resin (R), it is easier to suppress an excessive increase in the melt mass flow rate of the polypropylene resin (R). As a result, even when the proportion of polypropylene resin (R) in the foamed particles is increased, it is easier to avoid a decrease in foaming properties and moldability during the production of the foamed particles. From the viewpoint of more reliably obtaining this effect, it is preferable that the phosphorus-based antioxidant be added to the polypropylene resin (R) so that the content of the phosphorus-based antioxidant is 0.005% by mass or more and 0.8% by mass or less, more preferably 0.007% by mass or more and 0.5% by mass or less, and even more preferably 0.01% by mass or more and 0.3% by mass or less.

[0040] Furthermore, from the viewpoint of more easily obtaining both the effect of avoiding a decrease in foaming properties and moldability during the production of foamed particles and the effect of maintaining good physical properties of the molded article, it is preferable to add a phosphorus-based antioxidant in the antioxidant addition step such that the content in the polypropylene resin (R) is 0.005% by mass or more and 0.8% by mass or less, and to add a phenol-based antioxidant in the polypropylene resin (R) such that the content in the polypropylene resin (R) is 0.005% by mass or more and 0.8% by mass or less. Moreover, it is thought that by using a phenol-based antioxidant and a phosphorus-based antioxidant in combination as described above, the two will act synergistically, and the effect of the phenol-based antioxidant can be further enhanced. Therefore, in this case, the amount of phenol-based antioxidant added that is necessary to raise the oxidation induction temperature of the polypropylene resin (R) to 220°C or higher can be reduced.

[0041] Furthermore, the antioxidant added to the polypropylene resin (R) in the antioxidant addition step may include a sulfur-based antioxidant. The sulfur-based antioxidant can be added, for example, in such a way that its content in the polypropylene resin (R) is within the range of 0.005% by mass or more and 0.8% by mass or less.

[0042] [Base Resin Manufacturing Process] The above manufacturing method may further include a base resin manufacturing process for manufacturing a base resin using recycled polypropylene resin (R). The form of the base resin manufactured in the base resin manufacturing process is appropriately selected from known forms depending on the foaming method in the foaming process. For example, when foamed particles are obtained by foaming the base resin by an extrusion foaming method in the foaming process, the base resin should be manufactured in the base resin manufacturing process so that the molten resin in the extruder satisfies the requirements of the base resin of this disclosure, and this base resin should be extruded and foamed. Alternatively, for example, when foamed particles are obtained by foaming polypropylene resin particles (hereinafter also referred to as "resin particles"), which are resin pellets in a non-foamed state, in the foaming process, the base resin should be manufactured in the base resin manufacturing process so that the resin particles satisfy the requirements of the base resin of this disclosure. In the following, the base resin manufacturing process for manufacturing resin particles may be referred to as the "pelletizing process".

[0043] The resin particles produced in the pelletizing process have a core layer containing at least a polypropylene resin (R). That is, the resin particles may have a single-layer structure consisting only of a core layer. By foaming such resin particles, foamed particles with a single-layer structure consisting only of a foamed layer containing polypropylene resin (R) can be obtained. Alternatively, the resin particles may have a multilayer structure comprising a core layer and a coating layer made of thermoplastic resin that covers the core layer. By foaming such resin particles, foamed particles with a multilayer structure comprising a foamed layer and a coating layer that covers the foamed layer can be obtained. The more detailed composition of the resin particles will be described later.

[0044] The method for producing resin particles in the pelletizing process is not particularly limited, and a method can be adopted from known resin particle production methods according to the desired resin particle structure. For example, in the pelletizing process, a resin molten mixture containing polypropylene resin (R) is produced in an extruder, and then the resin molten mixture is extruded from the extruder to form strand-shaped extruded products, and resin particles can be produced by cutting these extruded products.

[0045] More specifically, to obtain resin particles with a single-layer structure consisting of a core layer, first, polypropylene resin (R) is supplied into an extruder and a molten resin mixture is produced by melting and kneading it inside the extruder. At this time, if necessary, other resins, bubble nucleating agents, carbon black, and other additives may be supplied to the extruder. Then, the molten resin mixture is extruded through small holes in a die attached to the tip of the extruder to form a strand-shaped extruder. After cooling this extruder, it is cut to the desired length to obtain resin particles with a single-layer structure consisting of a core layer containing polypropylene resin (R).

[0046] Furthermore, for example, when attempting to obtain foamed particles with a multilayer structure comprising a core layer and a coating layer made of thermoplastic resin that covers the core layer, the multilayer resin particles can be produced using a co-extrusion apparatus equipped with a core layer forming extruder, a coating layer forming extruder, and a co-extrusion die connected to these two extruders. In this case, the core layer forming extruder melts and kneads polypropylene resin (R) and other resins and additives as needed to produce a molten resin mixture for core layer formation. The coating layer forming extruder melts and kneads thermoplastic resin constituting the coating layer and additives as needed to produce a molten resin mixture for coating layer formation.

[0047] These molten resin mixtures are co-extruded and merged in a die to form a multilayer composite consisting of a non-foamed core layer and a non-foamed coating layer covering the outer surface of the core layer. This composite is extruded through the small holes of the die to form strand-shaped extruded material. After cooling this extruded material, it is cut to the desired length to obtain multilayer resin particles. This method is called the strand-cut method. However, the method for producing resin particles is not limited to the method described above, and methods such as the hot-cut method or the underwater-cut method may also be used.

[0048] In the pelletizing process, it is preferable to produce resin particles using polypropylene resin (R) that has undergone the discrimination process. The polypropylene resin (R) that has undergone the discrimination process has been confirmed to have an oxidation induction temperature of 220°C or higher. Therefore, by producing resin particles using polypropylene resin (R) that has been confirmed to have an oxidation induction temperature of 220°C or higher in the discrimination process and has been determined to be usable for the production of foamed particles, the effect of maintaining the good physical properties of the molded article can be obtained more reliably.

[0049] Furthermore, if the polypropylene resin (R) that has been determined in the discrimination step to be unsuitable for use in producing the foamed particles is used, it is preferable to perform the pelletizing step after the antioxidant addition step. In this case, the effect of maintaining the good physical properties of the molded article can be obtained more reliably, the range of polypropylene resins (R) that can be used in producing the foamed particles can be expanded, and the material recycling of polypropylene resins (R) can be further promoted.

[0050] In the aforementioned determination step, it is particularly preferable to perform the pelletizing step without adding an antioxidant to the polypropylene resin (R) that has been determined to be usable for producing the foamed particles. However, it is also possible to perform the pelletizing step after adding an antioxidant. In other words, an antioxidant may be further added to the polypropylene resin (R) whose oxidation induction temperature is 220°C or higher. In this case, the type of antioxidant added to the polypropylene resin (R) is not particularly limited, and an antioxidant can be added according to the purpose of addition.

[0051] For example, by further adding a phenolic antioxidant to the polypropylene resin (R) determined in the discrimination step to be usable for the production of foamed particles, the oxidation induction temperature of the polypropylene resin (R) can be further increased, as described above. In this case, the amount of phenolic antioxidant added to the polypropylene resin (R) can be appropriately set from, for example, the specific range described above. From the viewpoint of reducing the production cost of foamed particles while obtaining the effects of the present disclosure described above, it is preferable to add less than 0.01% by mass of the phenolic antioxidant to the polypropylene resin (R) determined in the discrimination step to be usable for the production of foamed particles, more preferably 0.005% by mass or less, and even more preferably 0.001% by mass or less.

[0052] Furthermore, the melt mass flow rate of the polypropylene resin (R) can be adjusted by adding a phosphorus-based antioxidant to the polypropylene resin (R) that has been determined to be usable for the production of foamed particles in the determination step. In this case, the amount of phosphorus-based antioxidant added to the polypropylene resin (R) can be appropriately set from within the specified range.

[0053] [Foaming Process] The above manufacturing method includes a foaming process in which a polypropylene resin composition containing a base resin and a foaming agent is foamed. As for the foaming method of the base resin in the foaming process, an appropriate method can be used from known methods depending on the form of the base resin. For example, if the base resin is a molten resin, an extrusion foaming method can be used in the foaming process. In the extrusion foaming method, a polypropylene resin composition is produced by melting and kneading the molten resin and the foaming agent in an extruder, and a strand-shaped foam is produced by foaming while extruding it from the extruder. Foam particles can be obtained by cutting this foam while cooling it.

[0054] Furthermore, for example, if the base resin is in the form of resin particles, foamed particles can be produced by dispersing the resin particles in a dispersion medium, impregnating the resin particles with a foaming agent, and then releasing the resin particles containing the foaming agent together with the dispersion medium under low pressure to foam the resin particles. This foaming method is sometimes called the "direct foaming method." However, the method of foaming resin particles is not limited to the direct foaming method. For example, if the base resin is in the form of resin particles, an impregnation foaming method can also be employed, in which the resin particles are impregnated with a foaming agent in the gas phase, and then heated by supplying a heating medium to cause foaming.

[0055] Preferably, the resin particles used in the foaming process are made using the polypropylene resin (R) whose oxidation induction temperature has been confirmed to be 220°C or higher. More specifically, the resin particles used in the foaming process may be, for example, resin particles obtained by performing the pelletizing process described above using a commercially available polypropylene resin (R) with an oxidation induction temperature of 220°C or higher, or commercially available resin particles containing a polypropylene resin (R) with an oxidation induction temperature of 220°C or higher. In this case, the oxidation induction temperature of the polypropylene resin (R) may be confirmed by any method. For example, a method for confirming the oxidation induction temperature of the polypropylene resin (R) is to perform the same process as the oxidation induction temperature measurement process and discrimination process described above.

[0056] Furthermore, the resin particles used in the foaming process may be, for example, resin particles obtained by performing the pelletizing process using a polypropylene resin (R) that has been determined to be usable for producing the foamed particles in the determination process. Moreover, the resin particles used in the foaming process may be, for example, resin particles obtained by performing the pelletizing process using a polypropylene resin (R) to which an antioxidant has been added in the antioxidant addition process such that the oxidation induction temperature is 220°C or higher.

[0057] The process of dispersing the resin particles in the dispersion medium may be carried out in the same sealed container used for foaming the resin particles, or in a separate container. From the viewpoint of simplifying the manufacturing process, it is preferable to carry out the resin particle dispersion process and the resin particle foaming process in the same sealed container.

[0058] As the dispersion medium, an aqueous dispersion medium mainly composed of water is used. In addition to water, the aqueous dispersion medium may also contain hydrophilic organic solvents such as ethylene glycol, glycerin, 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.

[0059] It is preferable to add a dispersant to the dispersion medium. By adding a dispersant to the dispersion medium, it is easy to avoid the fusion of resin particles with each other during the foaming process. The amount of dispersant added is preferably 0.001 parts by mass or more and 5 parts by mass or less per 100 parts by mass of resin particles. Organic dispersants and inorganic dispersants can be used as dispersants, but due to their ease of handling, it is preferable to use fine particulate inorganic materials as dispersants. More specifically, as dispersants, for example, clay minerals such as amsnite, kaolin, mica, and clay, or aluminum oxide, titanium oxide, basic magnesium carbonate, basic zinc carbonate, calcium carbonate, iron oxide, etc., can be used. These dispersants may be used alone, or two or more dispersants may be used in combination. Among these, it is preferable to use clay minerals as dispersants. Clay minerals may be natural or synthesized.

[0060] When using a dispersant, it is preferable to use an anionic surfactant such as sodium dodecylbenzenesulfonate, sodium alkylbenzenesulfonate, sodium lauryl sulfate, or sodium oleate as a dispersing aid. The amount of dispersing aid added is preferably 0.001 parts by mass or more and 1 part by mass or less per 100 parts by mass of resin particles.

[0061] After dispersing resin particles in a dispersion medium, the resin particles are impregnated with a blowing agent in a sealed container. The blowing agent used to impregnate the resin particles is preferably a physical blowing agent. Examples of physical blowing agents include inorganic physical blowing agents such as carbon dioxide, air, nitrogen, helium, and argon, and organic physical blowing agents such as aliphatic hydrocarbons such as propane, butane, and hexane, cyclic aliphatic hydrocarbons such as cyclopentane and cyclohexane, and halogenated hydrocarbons such as 1-chloro-3,3,3-trifluoropropene, 1-chloro-2,3,3,3-tetrafluoropropene, 1,3,3,3-tetrafluoropropene, chlorofluoromethane, trifluoromethane, 1,1-difluoromethane, 1-chloro-1,1-dichloroethane, 1,2,2,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride. These physical blowing agents may be used alone or in combination of two or more types. In addition, inorganic and organic physical blowing agents can be mixed and used. From the viewpoint of environmental impact and ease of handling, an inorganic physical blowing agent is preferably used, and carbon dioxide is more preferably used.

[0062] The amount of foaming agent added per 100 parts by mass of resin particles is preferably 0.1 parts by mass or more and 30 parts by mass or less, more preferably 0.5 parts by mass or more and 20 parts by mass or less, and even more preferably 1 part by mass or more and 15 parts by mass or less.

[0063] One method for impregnating resin particles with a foaming agent involves supplying the foaming agent into a sealed container and increasing the pressure inside the container to impregnate the resin particles in the dispersion medium with the foaming agent. In this case, heating the resin particles together with the dispersion medium can further promote the impregnation of the foaming agent into the resin particles.

[0064] 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. Within these ranges, foamed particles can be manufactured safely without the risk of damage or explosion of the sealed container.

[0065] Furthermore, when heating the dispersion medium, the temperature during foaming can be kept within an appropriate range by setting the heating rate of the dispersion medium to a range of 1°C / min to 5°C / min.

[0066] After the foaming agent has impregnated the resin particles, the contents of the sealed container are released into an environment with a lower pressure than the pressure inside the container. This causes the core layer of the resin particles to foam, forming a cellular structure, which is then cooled by the outside air, stabilizing the cellular structure and resulting in foamed particles.

[0067] When impregnating the core layer with a foaming agent, it is preferable to perform heating and foaming in the following manner. Specifically, first, a one-stage holding step is performed in which the temperature is held for a sufficient time, preferably about 10 to 60 minutes, at a temperature of (melting point of resin particles - 20°C) or higher and below the (melting end temperature of resin particles). After that, the temperature is adjusted to between (melting point of resin particles - 15°C) and below (melting end temperature of resin particles + 10°C). Then, if necessary, a second-stage holding step is performed in which the temperature is held for a further sufficient time, preferably about 10 to 60 minutes. After that, it is preferable to release the contents of the sealed container to the outside while the temperature inside the sealed container is at or above (melting point of resin particles - 10°C) to foam the resin particles. It is more preferable that the temperature inside the sealed container during foaming is at or above (melting point of resin particles) and below (melting point of resin particles + 20°C). By heating and foaming the resin particles in this way, secondary crystals are formed in the resin constituting the foamed layer, and foamed particles with excellent mechanical strength and moldability can be easily obtained.

[0068] Whether or not secondary crystals are formed in the resin constituting the foam layer can be determined based on the presence or absence of high-temperature peaks, as described later. Furthermore, the melting point of the resin particles can be determined based on differential scanning calorimetry (i.e., DSC) performed in accordance with JIS K7121-1987, and the obtained DSC curve. First, the resin particles are conditioned according to "(2) When measuring the melting temperature after performing a certain heat treatment". The heating rate and cooling rate in conditioning are set to 10°C / min. The conditioned resin particles are heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the peak temperature of the melting peak that appears in the DSC curve is taken as the melting point of the resin particles. If multiple melting peaks appear in the DSC curve, the peak temperature of the melting peak with the largest area is taken as the melting point of the resin particles.

[0069] In the foaming process, the foamed particles obtained by the direct foaming method described above may be further foamed. When foaming resin particles in this two-stage process, the first foaming stage is called the one-stage foaming stage, and the foamed particles obtained in the one-stage foaming stage are called one-stage foamed particles. The second foaming stage is called the two-stage foaming stage. The foamed particles obtained in the two-stage foaming stage are sometimes called two-stage foamed particles.

[0070] In the foaming process, a method for foaming resin particles in two stages is as follows: First, as a first-stage foaming process, the resin particles are foamed using the direct foaming method described above to obtain first-stage foamed particles. Then, internal pressure is applied to the first-stage foamed particles. More specifically, after placing the first-stage foamed particles in a pressure vessel, the inside of the pressure vessel is pressurized with an inorganic gas such as air or carbon dioxide to impregnate the foamed particles with the inorganic gas. This makes the pressure inside the bubbles of the first-stage foamed particles equal to or greater than atmospheric pressure. Then, the first-stage foamed particles removed from the pressure vessel are heated using a heating medium such as steam or heated air in an environment with a pressure lower than the pressure inside the bubbles to further foam the first-stage foamed particles. In this way, second-stage foamed particles can be obtained.

[0071] (Base resin) [Polypropylene resin (R)] The base resin used in the above manufacturing method contains at least recycled polypropylene resin (R). Furthermore, the oxidation induction temperature of the polypropylene resin (R), as measured in accordance with ISO 11357-6:2018, is 220°C or higher. If the oxidation induction temperature of the polypropylene resin (R) is too low, the deterioration of the molded article will progress easily, and depending on the usage environment, the physical properties of the molded article may deteriorate prematurely. By setting the oxidation induction temperature of the polypropylene resin (R) to 220°C or higher, foamed particles that can maintain good physical properties of the molded article over a long period of time can be easily obtained.

[0072] From the viewpoint of further enhancing this effect, the oxidation induction temperature of the polypropylene resin (R) is preferably 222°C or higher, more preferably 225°C or higher, even more preferably 228°C or higher, particularly preferably 230°C or higher, and most preferably 232°C or higher. On the other hand, from the viewpoint of further improving the in-moldability of the foamed particles, specifically from the viewpoint of more reliably avoiding distortion of the shape of the molded article obtained by in-molding the foamed particles, which leads to deterioration of recovery properties, the oxidation induction temperature of the polypropylene resin (R) is preferably 285°C or lower, more preferably 280°C or lower, even more preferably 270°C or lower, particularly preferably 255°C or lower, even more preferably 250°C or lower, and most preferably less than 250°C.

[0073] In determining the preferred range of oxidation induction temperature for the polypropylene resin (R), the upper and lower limits of the oxidation induction temperature for the polypropylene resin (R) described above can be arbitrarily combined. The preferred range of oxidation induction temperature for the polypropylene resin (R) may be, for example, 220°C to 285°C, 222°C to 280°C, 225°C to 270°C, 228°C to 255°C, 230°C to 250°C, or 232°C to less than 250°C.

[0074] Examples of methods for adjusting the oxidation induction temperature of the polypropylene resin (R) to within the range described above include appropriately selecting the type of polypropylene resin in the polypropylene resin (R), adjusting the temperature at which the polypropylene resin (R) is subjected to thermal history during the recycling process, and adding a phenolic antioxidant to the polypropylene resin (R) as an antioxidant in the antioxidant addition step.

[0075] The base resin may contain one type of polypropylene resin (R), or it may contain two or more types of polypropylene resins (R). In other words, the base resin may contain two or more types of recycled polypropylene resins. The polypropylene resin content in the polypropylene resin (R) is preferably 60% by mass or more, more preferably 70% by mass or more, even more preferably 80% by mass or more, particularly preferably 90% by mass or more, and most preferably 95% by mass or more. Furthermore, if the base resin is resin particles, the polypropylene resin (R) only needs to be contained in the core layer of the resin particles. If the resin particles have a coating layer, the polypropylene resin (R) may or may not be contained in the coating layer.

[0076] The proportion of polypropylene resin (R) contained in the base resin is 15% by mass or more. In the manufacturing method, by using a polypropylene resin (R) having a melting point and oxidation induction temperature within the specified range for the production of the base resin, and by setting the blending ratio of polypropylene resin (R) in the base resin to 15% by mass or more, the effect of maintaining good physical properties of the molded article can be easily obtained. From the viewpoint of maintaining good physical properties of the molded article over a long period of time while further reducing the environmental burden derived from foamed particles, it is preferable that the base resin contains 30% by mass or more of the polypropylene resin (R), more preferably 50% by mass or more, even more preferably 60% by mass or more, even more preferably 70% by mass or more, particularly preferably 80% by mass or more, and most preferably 90% by mass or more.

[0077] In this specification, polypropylene resin means a propylene copolymer having a propylene homopolymer and a propylene component (i.e., a monomer component derived from propylene) content of 50% by mass or more.

[0078] Examples of propylene homopolymers include isotactic polypropylene, syndiotactic polypropylene, and atactic polypropylene. Examples of propylene copolymers include copolymers of propylene with ethylene and / or α-olefins having 4 to 8 carbon atoms, such as propylene-ethylene copolymer, propylene-butene copolymer, and propylene-ethylene-butene copolymer, as well as propylene-acrylic acid copolymer and propylene-maleic anhydride copolymer. The mode of copolymerization in propylene copolymers is not particularly limited. For example, the propylene copolymer may be a random copolymer, a block copolymer, or a graft copolymer. Furthermore, the polypropylene resin is preferably linear. However, the polypropylene resin may contain a branched structure introduced by, for example, reaction with a conjugated diene compound and a radical polymerization initiator.

[0079] The polypropylene resin (R) is preferably composed of a random copolymer, and more preferably composed of one or more random copolymers selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer, and ethylene-propylene-butene random copolymer. In this case, even when the blending ratio of the polypropylene resin (R) in the base resin is increased, a decrease in foaming properties and a decrease in in-mold moldability can be more easily avoided.

[0080] In this specification, recycling refers to the process of making polypropylene resin recovered from waste usable as a raw material for base resins. The process of recovering polypropylene resin from waste may include, for example, waste sorting and volume reduction, and extraction processes to extract polypropylene resin from waste. Furthermore, the recycling process of polypropylene resin may include, for example, processes such as crushing the recovered material, adding additives, and granulation. Therefore, "recycled polypropylene resin (R)" can also be expressed as "polypropylene resin (R) derived from polypropylene resin (R)".

[0081] The polypropylene resin (R) is composed of recovered polypropylene resin. The polypropylene resin (R) may also contain virgin polypropylene resin added during the recycling process of recovered polypropylene resin. Examples of the virgin polypropylene resin include the base resin of the masterbatch used when adding antioxidants in the form of a masterbatch during the recycling process of recovered polypropylene resin or in the antioxidant addition process described above. From the viewpoint of increasing the content of recovered polypropylene resin in the base resin and obtaining more environmentally friendly foamed particles, the blending ratio of recovered polypropylene resin in the polypropylene resin (R) is preferably 90% by mass or more, more preferably 93% by mass or more, and even more preferably 95% by mass or more.

[0082] As recovered polypropylene resin materials, for example, post-consumer materials and pre-consumer materials composed of polypropylene resin can be used. In this specification, post-consumer materials mean "materials discharged from households, or materials generated as products that can no longer be used for their original purpose from commercial facilities, industrial facilities and various other facilities that are end users of the product" as defined in JIS Q14021:2000. Post-consumer materials also include materials returned from distribution channels. Post-consumer materials may be, for example, post-consumer materials of polypropylene resin foam molded products, post-consumer materials extracted from home appliances, post-consumer materials extracted from automobiles, post-consumer materials extracted from automobile crushing residue, etc. In this specification, pre-consumer materials refer to "materials extracted from the flow of waste in the manufacturing process" as defined in JIS Q14021:2000. However, processing-unsuitable products, polishing-unsuitable products, scrap, etc. that can be reused in the same process in which they were generated are excluded from pre-consumer materials.

[0083] In this specification, a polypropylene-based resin foam molded article refers to a foam composed of a polypropylene-based resin. Polypropylene-based resin foam molded articles include, for example, polypropylene-based resin foam particle molded articles obtained by in-mold molding of polypropylene-based resin foam particles, and polypropylene-based resin extruded foams obtained by extruding and foaming a polypropylene-based resin from an extruder.

[0084] The polypropylene resin (R) is preferably derived from post-consumer materials of polypropylene resin foam molded articles, and more preferably from post-consumer materials of polypropylene resin foam particle molded articles. Such polypropylene resin (R) has properties suitable for the production of foam particles. Therefore, in this case, the blending ratio of polypropylene resin (R) in the foam particles can be increased while maintaining good foaming properties. Furthermore, by reusing post-consumer materials of polypropylene resin foam molded articles as raw materials for foam particles, horizontal recycling from polypropylene resin foam molded articles to polypropylene resin foam molded articles can be promoted, which is expected to contribute to the advancement of the circular economy.

[0085] From the viewpoint of increasing the proportion of polypropylene resin (R) in the foamed particles while maintaining good foaming properties and in-moldability, it is more preferable that the polypropylene resin (R) is derived from a post-consumer material of a polypropylene resin foamed molded product, and that the post-consumer material is composed of one or more random copolymers selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer, and ethylene-propylene-butene random copolymer.

[0086] The melting point of the polypropylene resin (R) is less than 160°C. If the melting point of the polypropylene resin (R) is too high, the in-moldability of the foamed particles will decrease, and the molding pressure when manufacturing the molded article may become excessively high. Furthermore, if the amount of polypropylene resin (R) added is increased, it may not be possible to obtain a molded article. From the viewpoint of easily avoiding such problems, the melting point of the polypropylene resin (R) is preferably 158°C or lower, more preferably 155°C or lower, even more preferably 152°C or lower, particularly preferably 150°C or lower, and most preferably less than 150°C. On the other hand, from the viewpoint of more reliably obtaining the effect of improving the heat resistance of the molded article and maintaining the good physical properties of the molded article, the melting point of the polypropylene resin (R) is preferably 130°C or higher, more preferably 132°C or higher, even more preferably 135°C or higher, even more preferably 138°C or higher, particularly preferably 140°C or higher, and most preferably above 140°C.

[0087] In determining the preferred range of the melting point of the polypropylene resin (R), the upper and lower limits of the melting point of the polypropylene resin (R) described above can be arbitrarily combined. The preferred range of the melting point of the polypropylene resin (R) may be, for example, 130°C or more and less than 160°C, 132°C or more and 158°C or less, 135°C or more and 155°C or less, 138°C or more and 152°C or less, 140°C or more and 150°C or less, or more than 140°C and less than 150°C.

[0088] The melting point of polypropylene resin (R) can be determined by differential scanning calorimetry (i.e., DSC) based on JIS K7121-1987 and based on the obtained DSC curve. Specifically, a test specimen made of polypropylene resin (R) is prepared, and the specimen is conditioned according to "(2) When measuring the melting temperature after performing a certain heat treatment". The heating and cooling rates in conditioning are set to 10°C / min, and the temperature range is from 30°C to 200°C. The DSC curve is obtained by raising the temperature of the conditioned specimen from 30°C to 200°C at a heating rate of 10°C / min. The peak temperature of the melting peak that appears in the DSC curve is taken as the melting point of the polypropylene resin (R). If multiple melting peaks appear in the DSC curve, the peak temperature of the melting peak with the largest area is taken as the melting point.

[0089] Based on JIS K7210-1:2014, the melt mass flow rate of the polypropylene resin (R), measured under conditions of a load of 2.16 kg and a temperature of 230°C, is preferably 5 g / 10 min or more and 20 g / 10 min or less. By using such a polypropylene resin (R) to produce the base resin, the blending ratio of the polypropylene resin (R) in the foamed particles can be increased more easily while maintaining good foamability. From the viewpoint of more reliably obtaining this effect, based on JIS K7210-1:2014, the melt mass flow rate of the polypropylene resin (R), measured under conditions of a load of 2.16 kg and a temperature of 230°C, is more preferably 6 g / 10 min or more and 18 g / 10 min or less, and even more preferably 7 g / 10 min or more and 15 g / 10 min or less.

[0090] Examples of methods for adjusting the melt mass flow rate of the polypropylene resin (R) within the above range include controlling the screw rotation speed of the extruder during the recycling process to adjust the amount of shearing the polypropylene resin (R) receives, and adding a phosphorus-based antioxidant to the polypropylene resin (R) as an antioxidant during the antioxidant addition process.

[0091] [Polypropylene Resin (A)] The base resin may include, in addition to recycled polypropylene resin (R), a polypropylene resin (A) composed of virgin polypropylene resin. In this specification, virgin polypropylene resin means a polypropylene resin that has not been processed into products, etc. Virgin polypropylene resin is also sometimes referred to as non-recycled polypropylene resin.

[0092] When the base resin further contains a polypropylene resin (A) made from a virgin polypropylene resin, it is preferable that the blending ratio of the polypropylene resin (A) in the base resin is 0.1% by mass or more and 85% by mass or less, and the blending ratio of the polypropylene resin (R) is 15% by mass or more and 99.9% by mass or less (provided that the sum of the blending ratio of the polypropylene resin (R) and the blending ratio of the polypropylene resin (A) is 100% by mass). Since the polypropylene resin (R) has an oxidation induction temperature within the specific range, even when the blending ratio of the polypropylene resin (R) is relatively high, the effect of maintaining good physical properties of the molded article can be easily obtained.

[0093] From the viewpoint of increasing the proportion of polypropylene resin (R) while maintaining the good physical properties of the molded article, it is more preferable that the proportion of polypropylene resin (A) in the base resin is 0.2% by mass or more and 85% by mass or less, and the proportion of polypropylene resin (R) is 15% by mass or more and 99.8% by mass or less. From a similar viewpoint, it is even more preferable that the proportion of polypropylene resin (A) in the base resin is 0.3% by mass or more and 75% by mass or less, and the proportion of polypropylene resin (R) is 25% by mass or more and 99.7% by mass or less. Furthermore, it is particularly preferable that the proportion of polypropylene resin (A) in the base resin is 0.4% by mass or more and 70% by mass or less, and the proportion of polypropylene resin (R) is 30% by mass or more and 99.6% by mass or less. Furthermore, it is most preferable that the blending ratio of the polypropylene resin (A) in the base resin is 0.5% by mass or more and 50% by mass or less, and the blending ratio of the polypropylene resin (R) is 50% by mass or more and 99.5% by mass or less.

[0094] The polypropylene resin (A) may be a propylene homopolymer or a propylene copolymer. From the viewpoint of more easily avoiding a decrease in foaming properties and further improving moldability, the polypropylene resin (A) is preferably a random copolymer, and more preferably one or more random copolymers selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer and ethylene-propylene-butene random copolymer.

[0095] From the viewpoint of more easily avoiding a decrease in foaming properties and further improving moldability, the melting point of the polypropylene resin (A) is preferably 130°C or more and less than 160°C, more preferably 135°C or more and 155°C or less, even more preferably 138°C or more and 152°C or less, and particularly preferably 140°C or more and 150°C or less. The melting point of the polypropylene resin (A) can be measured by the same method as the melting point of the polypropylene resin (R).

[0096] Based on JIS K7210-1:2014, the melt mass flow rate of the polypropylene resin (A), measured under conditions of a load of 2.16 kg and a temperature of 230°C, is preferably 1 g / 10 min to 12 g / 10 min, more preferably 2 g / 10 min to 10 g / 10 min, and even more preferably 3 g / 10 min to 7 g / 10 min, from the viewpoint of more easily avoiding a decrease in foaming properties and further improving moldability.

[0097] [Other Polymers] The base resin may, in addition to polypropylene resin (R) and polypropylene resin (A), optionally contain other polymers different from polypropylene resins, as long as they do not impair the effect of maintaining the good physical properties of the molded article. Examples of polymers other than polypropylene resins that may be included in the base resin include thermoplastic resins other than polypropylene resins, such as polyethylene resins, polystyrene resins, polyamide resins, and polyester resins, as well as thermoplastic elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers.

[0098] The base resin may contain one polymer other than these polypropylene resins, or it may contain two or more polymers.

[0099] The content of polymers other than polypropylene resins in the base resin is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, even more preferably 3% by mass or less, particularly preferably 1% by mass or less, and most preferably 0% by mass.

[0100] [Additives] The base resin may optionally contain additives such as foam regulators, colorants, antioxidants, antistatic agents, surfactants, light stabilizers, ultraviolet absorbers, and flame retardants. The base resin may contain one of these additives, or two or more. Furthermore, the additives may be contained in the polypropylene resin (R) or in the polypropylene resin (A). Moreover, the additives may be contained in both the polypropylene resin (R) and the polypropylene resin (A).

[0101] As antioxidants, known antioxidants used in polypropylene resins, such as phosphorus-based antioxidants, sulfur-based antioxidants, and phenol-based antioxidants, can be used. The base resin may contain one type of antioxidant or two or more types of antioxidants.

[0102] The content of phosphorus-based antioxidant in the base resin is preferably 0.001% by mass or more and 0.3% by mass or less, more preferably 0.005% by mass or more and 0.2% by mass or less, and even more preferably 0.01% by mass or more and 0.1% by mass or less. As mentioned above, phosphorus-based antioxidants are excellent at suppressing the increase in the melt mass flow rate of polypropylene resins. Therefore, by setting the content of phosphorus-based antioxidant in the base resin within the above-mentioned specific range, it is possible to more easily suppress an excessive increase in the melt mass flow rate of polypropylene resins and more easily avoid a decrease in foaming properties and a decrease in in-mold moldability.

[0103] The content of the phenolic antioxidant in the base resin is preferably 0.005% by mass or more and 0.5% by mass or less, more preferably 0.007% by mass or more and 0.4% by mass or less, and even more preferably 0.01% by mass or more and 0.3% by mass or less. In this case, the effect of maintaining the good physical properties of the molded article can be further enhanced.

[0104] Furthermore, from the viewpoint of more easily obtaining both the effect of avoiding a decrease in foaming properties and the effect of maintaining good physical properties of the molded article, the base resin preferably contains both a phosphorus-based antioxidant in an amount of 0.001% to 0.3% by mass and a phenol-based antioxidant in an amount of 0.005% to 0.5% by mass, and more preferably contains both a phosphorus-based antioxidant in an amount of 0.005% to 0.2% by mass and a phenol-based antioxidant in an amount of 0.007% to 0.4% by mass. From a similar viewpoint, the polypropylene resin (R) preferably contains both a phosphorus-based antioxidant in an amount of 0.005% to 0.8% by mass and a phenol-based antioxidant in an amount of 0.005% to 0.8% by mass, and preferably contains both a phosphorus-based antioxidant in an amount of 0.007% to 0.5% by mass and a phenol-based antioxidant in an amount of 0.007% to 0.5% by mass.

[0105] The base resin may contain carbon black as needed. Carbon black may be added to the base resin, for example, during the base resin manufacturing process, together with a polypropylene resin (R). Furthermore, carbon black may be present in the polypropylene resin (R) or in the polypropylene resin (A). In addition, carbon black may be present in both the polypropylene resin (R) and the polypropylene resin (A).

[0106] The carbon black content in the base resin may be 0.5% by mass or more and 5% by mass or less, 1% by mass or more and 4.5% by mass or less, 1.5% by mass or more and 4% by mass or less, or 2% by mass or more and 3.5% by mass or less. Carbon black is thought to have the effect of capturing radicals generated in the resin and suppressing the degradation of the resin by radicals. Therefore, by setting the carbon black content in the base resin within the above-mentioned specific range, the effect of maintaining the good physical properties of the molded article can be further enhanced. In addition, by foaming a base resin having a carbon black content within the above-mentioned specific range, black foamed particles can be obtained. Therefore, by performing in-mold molding using such foamed particles, a high-quality appearance can be given to the molded article.

[0107] Furthermore, the carbon black content in the base resin may be less than 0.5% by mass (including 0). As mentioned above, since the base resin is made using a polypropylene resin (R) having an oxidation induction temperature within the specified range, the good physical properties of the molded article can be maintained for a long period of time even if carbon black is not present or the carbon black content is relatively low. In addition, foamed particles and molded articles produced from such a base resin can easily avoid problems such as excessive absorption of infrared rays caused by carbon black. Therefore, in this case, the used foamed particles and molded articles can be recycled or reused for a wider range of applications.

[0108] The carbon black content in the polypropylene resin (R) is preferably less than 0.5% by mass (including 0). Polypropylene resins (R) with a carbon black content within the specified range have a relatively bright color tone. Therefore, by using such a polypropylene resin (R) in the production of the base resin, the color tone of the foamed particles can be more easily adjusted to a desired color tone. In other words, in this case, by blending an appropriate colorant with the foamed particles, a desired color other than black can be imparted to the foamed particles. From this viewpoint, the carbon black content in the polypropylene resin (R) is more preferably 0.3% by mass or less (including 0), even more preferably 0.1% by mass or less (including 0), particularly preferably 0.05% by mass or less (including 0), and most preferably 0. Furthermore, in this disclosure, since the oxidation induction temperature of the polypropylene resin (R) is above a predetermined value, the effect of maintaining good physical properties of the molded article can be further enhanced even when the carbon black content in the polypropylene resin (R) is low.

[0109] [Structure of Resin Particles] As described above, the resin particles used in the manufacture of foamed particles may have a single-layer structure consisting only of a core layer, or a multilayer structure comprising a core layer and a coating layer. The core layer of the resin particle contains at least the polypropylene resin (R). The core layer may contain the polypropylene resin (A) in addition to the polypropylene resin (R). When the resin particle has a single-layer structure consisting only of a core layer, the blending ratio of the polypropylene resin (R) in the resin particle is equal to the blending ratio of the polypropylene resin (R) in the core layer. Similarly, when the resin particle has a single-layer structure consisting only of a core layer, the blending ratio of the polypropylene resin (A) in the resin particle is equal to the blending ratio of the polypropylene resin (A) in the core layer.

[0110] Furthermore, the coating layer on the resin particles may cover the entire core layer or only a portion of it. The coating layer may be provided on the surface of the resin particles to improve the fusion properties between foam particles in in-mold molding, or to impart functionality. When a coating layer is provided for the purpose of improving the fusion properties between foam particles in in-mold molding, it is preferable that the thermoplastic resin constituting the coating layer has a melting point or softening point lower than that of the core layer.

[0111] The thermoplastic resin constituting the coating layer may be a crystalline thermoplastic resin or an amorphous thermoplastic resin. Examples of crystalline thermoplastic resins used in the coating layer include polyolefin resins. Examples of amorphous thermoplastic resins used in the coating layer include polystyrene resins. From the viewpoint of adhesion to the core layer, the thermoplastic resin constituting the coating layer is preferably a polyolefin resin, more preferably a polyethylene resin and / or a polypropylene resin, and even more preferably a polypropylene resin.

[0112] Examples of polypropylene resins used in the coating layer include ethylene-propylene copolymer, propylene-butene copolymer, ethylene-propylene-butene copolymer, and propylene homopolymer. Among these, it is particularly preferable that the coating layer be composed of ethylene-propylene copolymer and / or ethylene-propylene-butene copolymer. Furthermore, the polypropylene resin used in the coating layer may be recycled polypropylene resin or virgin polypropylene resin. In addition, the coating layer may contain either recycled polypropylene resin (i.e., polypropylene resin (R)) or virgin polypropylene resin (i.e., polypropylene resin (A)), or it may contain both resins.

[0113] When resin particles have a multilayer structure comprising a core layer and a coating layer, the proportion of polypropylene resin (R) in the resin particles is equal to the ratio of the sum of the amount of polypropylene resin (R) in the core layer and the amount of polypropylene resin (R) in the coating layer to the total mass of the resin particles. Similarly, when resin particles have a multilayer structure comprising a core layer and a coating layer, the proportion of polypropylene resin (A) in the resin particles is equal to the ratio of the sum of the amount of polypropylene resin (A) in the core layer and the amount of polypropylene resin (A) in the coating layer to the total mass of the resin particles.

[0114] The thermoplastic resin constituting the coating layer may contain additives such as nucleating agents, flame retardants, flame retardant aids, plasticizers, antistatic agents, antioxidants, ultraviolet absorbers, light stabilizers, conductive fillers, antibacterial agents, and colorants, to the extent that they do not impair the effects described above. The amount of additives in the coating layer is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less per 100 parts by mass of thermoplastic resin.

[0115] From the viewpoint of improving moldability while maintaining the rigidity of the molded article, the mass ratio (ratio of mass %) of the core layer to the coating layer is preferably 99.5:0.5 to 85:15 (where the sum of the mass of the core layer and the mass of the coating layer is 100% by mass), more preferably 99:1 to 90:10, and even more preferably 98:2 to 92:8.

[0116] The average mass per resin particle is preferably 0.1 mg to 20 mg, more preferably 0.2 mg to 10 mg, even more preferably 0.3 mg to 5 mg, and particularly preferably 0.4 mg to 2 mg. The average mass per resin particle is calculated by dividing the mass of 200 randomly selected resin particles by the number of resin particles.

[0117] (Polypropylene Resin Foamed Particles) Polypropylene resin foamed particles can be obtained by foaming the base resin. The foamed particles obtained by foaming the resin particles have a structure corresponding to the structure of the resin particles. For example, if the resin particles have a single-layer structure consisting only of a core layer, the foamed particles obtained by foaming the resin particles have a single-layer structure consisting only of a foamed layer. Also, if the resin particles have a multilayer structure comprising a core layer and a coating layer covering the core layer, the foamed particles obtained by foaming the resin particles have a multilayer structure comprising a foamed layer and a coating layer covering the foamed layer. The coating layer of the foamed particles may be in a foamed state or a non-foamed state, but it is preferable that it be substantially non-foamed. "Substantially non-foamed" includes a state in which the coating layer does not foam and does not contain air bubbles, and a state in which air bubbles disappear after foaming, meaning that there is almost no air bubble structure in the coating layer.

[0118] [Bulk Density] The bulk density of the foamed particles is 10 kg / m³. 3 More than 200kg / m 3 Preferably, it is 12 kg / m 3 More than 100kg / m 3 It is more preferable that the following conditions apply: 15 kg / m 3 More than 80kg / m 3 It is even more preferable that the following conditions apply: 20 kg / m3 50 kg / m or less 3 It is particularly preferable that it is below this value. In this case, a molded body that is lightweight and has good compressive strength can be easily obtained.

[0119] The method for calculating the bulk density of the expanded particles is as follows. First, the expanded particles are allowed to stand for 24 hours or more in an environment with a relative humidity of 50%, a temperature of 23°C, and an atmospheric pressure of 1 atm to adjust the state of the expanded particles. The expanded particles thus obtained are filled into a graduated cylinder, and the bottom surface of the graduated cylinder is gently tapped several times on the floor surface to stabilize the filling height of the expanded particle group in the graduated cylinder. Then, the bulk volume (unit: L) of the expanded particle group is read from the scale of the graduated cylinder. And by unit conversion of the value obtained by dividing the mass (unit: g) of the expanded particle group in the graduated cylinder by the above-mentioned bulk volume, the bulk density of the expanded particles (unit: kg / m 3 ) can be obtained.

[0120] [Oxidation induction time of expanded particles] The oxidation induction time of the expanded particles at a temperature of 200°C, measured in accordance with ISO 11357-6:2018, is preferably 5 minutes or more and 360 minutes or less. When the oxidation induction time of the expanded particles is 5 minutes or more, good physical properties of the molded body can be maintained over a long period despite containing the recycled polypropylene-based resin (R). From the viewpoint of further enhancing this effect, the oxidation induction time of the expanded particles is more preferably 8 minutes or more, further preferably 10 minutes or more, still more preferably 20 minutes or more, particularly preferably 30 minutes or more, and most preferably 50 minutes or more.

[0121] Furthermore, by limiting the oxidation induction time of the foamed particles to 360 minutes or less, deterioration of the recovery properties of the foamed particles can be more reliably avoided, and the in-moldability of the foamed particles can be further improved. Although the reason for this is not clear, it is thought that foamed particles with a moderately short oxidation induction time do not contain an excessive amount of antioxidant. From the viewpoint of more reliably obtaining this effect, the oxidation induction time of the foamed particles is more preferably 300 minutes or less, even more preferably 240 minutes or less, even more preferably 200 minutes or less, particularly preferably 160 minutes or less, and most preferably 120 minutes or less.

[0122] In determining the preferred range for the oxidation induction time of the foamed particles, the upper and lower limits of the oxidation induction time mentioned above can be arbitrarily combined. For example, the preferred range for the oxidation induction time of the foamed particles may be 8 minutes or more and 300 minutes or less, 10 minutes or more and 240 minutes or less, 20 minutes or more and 200 minutes or less, 30 minutes or more and 160 minutes or less, or 50 minutes or more and 120 minutes or less.

[0123] The oxidation induction time for foam particles is measured by differential scanning calorimetry (DSC) in accordance with ISO 11357-6:2018, using approximately 5 mg of foam particles as a sample. More specifically, first, the sample is placed in an open sample pan and then on the sample stage of the DSC apparatus. Next, nitrogen gas with a purity of 99.99% or higher is supplied into the furnace of the DSC apparatus to replace the atmosphere inside the furnace with nitrogen gas. The flow rate of the nitrogen gas is set to 50 mL / min.

[0124] After the furnace is filled with nitrogen gas, the sample is heated to 200°C at a heating rate of 10°C / min while continuing to supply nitrogen gas. After maintaining the temperature at 200°C for 3 minutes, the supply of nitrogen gas is stopped and air is supplied to the furnace. The air flow rate is set to 50 mL / min. Subsequently, the temperature of 200°C is maintained and the air supply is continued, and the heat flow of the sample is measured until an exothermic peak due to oxidative decomposition of the sample is observed.

[0125] Figure 2 shows a schematic diagram of the oxidation induction time curve, with the heat flow obtained in this manner represented on the vertical axis and the elapsed time from the start of the test represented on the horizontal axis. As shown in Figure 2, since no endothermic or exothermic reactions occur in the sample between the start time t1 of air supply to the furnace and the start time t2 of oxidative decomposition of the sample, the portion of the oxidation induction time curve from time t1 to time t2 has a generally flat shape. On the other hand, when the sample begins oxidative decomposition at time t2, an exothermic peak associated with oxidative decomposition appears.

[0126] To determine the oxidation induction time of foamed particles, first, determine the time t4 at which the slope of the tangent to the oxidation induction time curve is greatest, between time t2 and time t5, which corresponds to the peak of the exothermic reaction. Then, draw an extension line L3 of the baseline of the oxidation induction time curve extended from time t2, and a tangent line L4 to the oxidation induction time curve at time t4, on the oxidation induction time curve. Then, define the elapsed time from time t1 to time t3, which corresponds to the intersection of extension line L3 and tangent line L4, as the oxidation induction time t of the foamed particles. Note that the oxidation induction time is sometimes called isothermal OIT.

[0127] The oxidation induction time of the foamed particles can be easily adjusted to the above range, for example, by setting the oxidation induction temperature of the polypropylene resin (R) blended into the base resin to 220°C or higher. Furthermore, assuming that the oxidation induction temperature of the polypropylene resin (R) blended into the base resin is 220°C or higher, the oxidation induction time of the foamed particles can be further extended by methods such as blending a polypropylene resin (R) with a higher oxidation induction temperature into the base resin, blending carbon black into the base resin, or blending virgin polypropylene resin into the base resin. Furthermore, assuming that the oxidation induction temperature of the polypropylene resin (R) blended into the base resin is 220°C or higher, the oxidation induction time of the foamed particles can be appropriately shortened by methods such as appropriately reducing the amount of antioxidant added to the polypropylene resin (R) in the antioxidant addition step, or appropriately reducing the amount of antioxidant added to the base resin in the base resin manufacturing step.

[0128] [High Temperature Peak] Preferably, the foamed particles have a crystalline structure such that the DSC curve obtained when the foamed particles are heated from 23°C to 200°C at a heating rate of 10°C / min shows a resin-specific peak originating from the melting of crystals inherent to the resin constituting the foam layer, and a high-temperature peak having a peak temperature higher than the peak temperature of the resin-specific peak. Foamed particles having such a crystalline structure have excellent mechanical strength and moldability. The resin-specific peak is caused by endothermic reactions when the crystals inherent to the resin constituting the foam layer melt. On the other hand, the high-temperature peak is presumed to be caused by the melting of secondary crystals formed in the resin constituting the foam layer during the manufacturing process of the foamed particles. In other words, if a high-temperature peak appears in the DSC curve, it is presumed that secondary crystals are formed in the foam layer.

[0129] Whether or not the foamed particles have the aforementioned crystalline structure can be determined based on the DSC curve obtained by performing differential scanning calorimetry (DSC) under the conditions described above, in accordance with JIS K7121:1987. Furthermore, 1 to 3 mg of foamed particles should be used as a sample for the DSC.

[0130] Specifically, when foam particles are heated from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), the resulting DSC curve shows both a high-temperature peak and a resin-specific peak of the resin constituting the foam layer. In contrast, when the foam particles are cooled from 200°C to 23°C at a cooling rate of 10°C / min after the first heating, and then heated again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), the resulting DSC curve shows only the resin-specific peak of the resin constituting the foam layer. Therefore, by comparing the DSC curve obtained during the first heating and the DSC curve obtained during the second heating, the resin-specific peak and the high-temperature peak can be distinguished. The temperature at the peak of this resin-specific peak may differ slightly between the first and second heating, but the difference is usually within 5°C.

[0131] The heat of fusion of the high-temperature peak of the foamed particles is preferably 8 J / g or more and 30 J / g or less, more preferably 10 J / g or more and 28 J / g or less, even more preferably 12 J / g or more and 25 J / g or less, and particularly preferably 15 J / g or more and 22 J / g or less, from the viewpoint of further improving the moldability of the foamed particles and further increasing the rigidity of the molded article.

[0132] The heat of fusion of the aforementioned high-temperature peak is determined as follows. First, 1 to 3 mg of conditioned foam particles are used as a sample, and a differential scanning calorimetry (DSC) curve is obtained by heating from 23°C to 200°C at a heating rate of 10°C / min. An example of a DSC curve is shown in Figure 3. When foam particles have a high-temperature peak, the DSC curve shows a resin-specific peak ΔH1 and a high-temperature peak ΔH2 whose peak is at a higher temperature than the peak of the resin-specific peak ΔH1, as shown in Figure 3.

[0133] Next, draw a straight line L5 connecting point α, which corresponds to 80°C on the DSC curve, and point β, which corresponds to the melting termination temperature T of the foamed particles. Note that the melting termination temperature T is the high-temperature endpoint of the high-temperature peak ΔH2, that is, the intersection point of the high-temperature peak ΔH2 on the DSC curve and the baseline on the side of the high-temperature peak ΔH2 that is higher than ΔH2.

[0134] After drawing the straight line L5, a straight line L6 is drawn parallel to the vertical axis of the graph, passing through the maximum point γ located between the resin-specific peak ΔH1 and the high-temperature peak ΔH2. This straight line L6 separates the resin-specific peak ΔH1 from the high-temperature peak ΔH2. The heat of fusion of the high-temperature peak ΔH2 can be calculated based on the area of ​​the region enclosed by the portion of the DSC curve that constitutes the high-temperature peak ΔH2, the straight line L5, and the straight line L6.

[0135] (Polypropylene-based resin foam particle molded article) After filling the foam particles into a mold, a polypropylene-based resin foam particle molded article can be obtained by supplying a heating medium such as steam into the mold and performing in-mold molding. The density of the molded article is 10 kg / m³ 3 More than 200kg / m 3 Preferably, it is 20 kg / m 3More than 150kg / m 3 It is more preferable that the following conditions are met: 30 kg / m 3 More than 100kg / m 3 The following is even more preferable. In this case, the lightness and rigidity of the molded body can be improved in a balanced manner. The density of the molded body is calculated by dividing the mass of the molded body (in g) by the volume (in L) obtained from the external dimensions of the molded body and converting the units. If, for example, the molded body has a complex shape at least partially and it is not easy to determine the volume from the external dimensions of the molded body, the volume of the molded body can be determined by the immersion method.

[0136] The polypropylene resin foam particle molded article is obtained by in-mold molding foam particles obtained by the foam particle manufacturing method. Therefore, despite containing recycled polypropylene resin, the molded article can maintain its good physical properties over a long period of time. More specifically, for example, deterioration of the molded article is suppressed even in high-temperature environments such as 80°C, so the molded article can be used for a long period of time even in more severe operating environments.

[0137] (Method for identifying polypropylene resin) The method for identifying polypropylene resin includes an oxidation induction temperature measurement step of measuring the oxidation induction temperature of recycled polypropylene resin in accordance with ISO 11357-6:2018, and a determination step of determining that if the oxidation induction temperature of the polypropylene resin measured in the oxidation induction temperature measurement step is 220°C or higher, the polypropylene resin can be used to produce the foamed particles, and if the oxidation induction temperature of the polypropylene resin is less than 220°C, the polypropylene resin cannot be used to produce the foamed particles.

[0138] The aforementioned discrimination method comprises an oxidation induction temperature measurement step and a discrimination step, in which the suitability of using a polypropylene resin is determined based on the oxidation induction temperature measured in the oxidation induction temperature measurement step. Therefore, according to the aforementioned discrimination method, a polypropylene resin that has an oxidation induction temperature of 220°C or higher and maintains good physical properties of the molded article despite being derived from recycled materials can be identified more reliably.

[0139] Conventionally, recycled polypropylene resins have had problems such as large differences in physical properties between lots, difficulty in predicting or determining in advance the extent of thermal history a particular recycled polypropylene resin has undergone, and whether the resulting polypropylene resin foam particle molded product is prone to degradation. On the other hand, the aforementioned discrimination method makes it easy to determine the suitability of recycled polypropylene resin for use in foam particles by using an indicator called the oxidation induction temperature. Therefore, the aforementioned discrimination method has the exceptional effect of eliminating the need to actually manufacture foam particles and their molded products and evaluate their performance when determining whether recycled polypropylene resin can be used as a raw material for foam particles.

[0140] The composition of the polypropylene resin used in the above-mentioned discrimination method is the same as the composition of the polypropylene resin (R) used in the method for producing polypropylene resin foam particles described above. Therefore, the description of the polypropylene resin (R) used in the method for producing polypropylene resin foam particles described above can be appropriately referred to regarding the composition of the polypropylene resin used in the above-mentioned discrimination method.

[0141] Similarly, the configuration of the oxidation induction temperature measurement step and the determination step in the determination method is the same as the configuration of the oxidation induction temperature measurement step and the determination step in the method for producing polypropylene resin foam particles described above. Therefore, the configuration of these steps in the determination method can be appropriately referred to in the description of the corresponding steps in the method for producing polypropylene resin foam particles described above.

[0142] (Method for manufacturing polypropylene resin) Another embodiment of the method for manufacturing polypropylene resin includes: an oxidation induction temperature measurement step of measuring the oxidation induction temperature of recycled polypropylene resin in accordance with ISO 11357-6:2018; a determination step of determining that if the oxidation induction temperature of the polypropylene resin measured in the oxidation induction temperature measurement step is 220°C or higher, the polypropylene resin can be used to produce the foamed particles, and if the oxidation induction temperature of the polypropylene resin is less than 220°C, the polypropylene resin cannot be used to produce the foamed particles; and an antioxidant addition step of adding an antioxidant to the polypropylene resin that has been determined in the determination step to be unusable to produce the foamed particles, such that the oxidation induction temperature becomes 220°C or higher.

[0143] The method for producing polypropylene resin according to the above embodiment includes an oxidation induction temperature measurement step, a discrimination step, and an antioxidant addition step. In the oxidation induction temperature measurement step, the suitability of the polypropylene resin is determined based on the oxidation induction temperature measured. In the antioxidant addition step, an antioxidant is added to the polypropylene resin that was determined to be unsuitable in the discrimination step, such that the oxidation induction temperature becomes 220°C or higher. Therefore, according to the method for producing polypropylene resin according to the above embodiment, it is possible to more reliably obtain a polypropylene resin that has an oxidation induction temperature of 220°C or higher and maintains good physical properties of the molded article despite being derived from recycled materials.

[0144] In the method for producing polypropylene resin according to the above embodiment, the composition of the polypropylene resin whose oxidation induction temperature is measured is the same as the composition of the polypropylene resin (R) used in the method for producing polypropylene resin foam particles described above. Therefore, the composition of the polypropylene resin used in the method for producing polypropylene resin according to the above embodiment can be appropriately referred to in the description of the polypropylene resin (R) used in the method for producing polypropylene resin foam particles described above.

[0145] Similarly, the configuration of the oxidation induction temperature measurement step, the discrimination step, and the antioxidant addition step in the method for producing polypropylene resin according to the above embodiment is the same as the configuration of the oxidation induction temperature measurement step, the discrimination step, and the antioxidant addition step in the method for producing polypropylene resin foam particles described above. Therefore, the configuration of these steps in the discrimination method can be appropriately referred to in the description of the corresponding steps in the method for producing polypropylene resin foam particles described above.

[0146] Furthermore, in another embodiment of the method for producing polypropylene resin, a polypropylene resin having an oxidation induction temperature of 220°C or higher, as measured in accordance with ISO 11357-6:2018, can be produced by adding 0.005% to 1% by mass of a phenolic antioxidant to the recovered polypropylene resin derived from post-consumer materials of a foamed polypropylene resin molded product, and melt-kneading it in an extruder.

[0147] The polypropylene resin obtained by the method for producing polypropylene resin according to the above embodiment is derived from post-consumer materials of polypropylene resin foam molded articles. Therefore, the polypropylene resin obtained by the method for producing polypropylene resin according to the above embodiment has properties suitable for producing polypropylene resin foam particles. In addition, in the method for producing polypropylene resin according to the above embodiment, the specific amount of phenolic antioxidant is added to the polypropylene resin. The polypropylene resin obtained in this way can reliably achieve the oxidation induction temperature within the specific range due to the added phenolic antioxidant and / or the phenolic antioxidant already contained in the recovered polypropylene resin derived from post-consumer materials of polypropylene resin foam molded articles. Therefore, the polypropylene resin obtained by the method for producing polypropylene resin according to the above embodiment is excellent in maintaining the good physical properties of the molded article.

[0148] In the method for producing polypropylene resin according to the above embodiment, it is preferable to use polypropylene resin recovery material obtained by compressing and then crushing a polypropylene resin foam molded body. However, the polypropylene resin foam molded body may be roughly crushed beforehand before compression. The compressed polypropylene resin foam molded body is sometimes called a polypropylene resin foam molded body ingot. By using a polypropylene resin foam molded body ingot as polypropylene resin recovery material, the transportation costs of raw materials can be significantly reduced. Therefore, the manufacturing costs of foam particles can be reduced, and the environmental burden can be further reduced.

[0149] In the method for producing polypropylene resin according to the above embodiment, the method for adding the phenolic antioxidant to the recovered polypropylene resin is not particularly limited and various embodiments can be adopted. For example, in the above production method, the recovered polypropylene resin and the phenolic antioxidant may be melt-kneaded in an extruder to produce a molten resin mixture, and then the phenolic antioxidant may be added to the recovered polypropylene resin by extruding the molten resin mixture from the extruder.

[0150] An example of the method for producing the foamed particles described above will now be explained. In this example, resin particles were prepared as a base resin by the strand cutting method, and then foamed to produce foamed particles.

[0151] (Polypropylene Resin (R)) Table 1 shows the properties of the polypropylene resin (R) used in the production of the foamed particles. The polypropylene resin (R) used in this example all originated from polypropylene resin recovery material recovered from polypropylene resin foam molded articles. More specifically, PP-R1, PP-R2, PP-R4 to PP-R6, PP-R10 and PP-R11 shown in Table 1 are obtained by supplying polypropylene resin recovery material from the lots shown in Table 1 and an antioxidant masterbatch to an extruder in the ratios shown in Table 1, melt-kneading them, and then extruding them from the extruder to granulate.

[0152] More specifically, the polypropylene resin recovery material used in this example originates from a foamed particle molded body composed of a polypropylene random copolymer, and is obtained by compressing and then crushing the foamed particle molded body. Furthermore, the "Lot Number" column in Table 1 shows the lot number assigned when polypropylene resin recovery materials recovered from the same polypropylene resin foam molded body are defined as one lot. Therefore, polypropylene resin recovery materials recovered from different polypropylene resin foam molded bodies are assigned different lot numbers.

[0153] The antioxidant masterbatches used in PP-R1, PP-R2, and PP-R4 to PP-R6 consist of virgin polypropylene resin containing phenolic antioxidants, sulfur-based antioxidants, and phosphorus-based antioxidants. Specifically, BASF's "Irganox® 1010" was used as the phenolic antioxidant, Naugard's "DSTDP" as the sulfur-based antioxidant, and BASF's "Irgafos® 168" as the phosphorus-based antioxidant. The concentration of the phenolic antioxidant in the masterbatch was 2% by mass, the concentration of the sulfur-based antioxidant was 4% by mass, and the concentration of the phosphorus-based antioxidant was 0.6% by mass. Therefore, the amounts of each antioxidant added in PP-R1, PP-R2, and PP-R4 to PP-R6 are shown in Table 1.

[0154] The antioxidant masterbatch used in PP-R10 is composed of a virgin polypropylene resin containing a phenolic antioxidant and a phosphorus-based antioxidant. Specifically, BASF's "Irganox 1010" was used as the phenolic antioxidant, and BASF's "Irgafos 168" was used as the phosphorus-based antioxidant. The concentration of the phenolic antioxidant in the masterbatch is 10% by mass, and the concentration of the phosphorus-based antioxidant is 5% by mass. Therefore, the amounts of each antioxidant added in PP-R10 are shown in Table 1.

[0155] The antioxidant masterbatch used in PP-R11 is composed of a virgin polypropylene resin containing a phenolic antioxidant, a sulfur-based antioxidant, and a phosphorus-based antioxidant. Specifically, BASF's "Irganox 1010" was used as the phenolic antioxidant, Naugard's "DSTDP" as the sulfur-based antioxidant, and BASF's "Irgafos 168" as the phosphorus-based antioxidant. The concentration of the phenolic antioxidant in the antioxidant masterbatch used in PP-R11 was 16% by mass, the concentration of the sulfur-based antioxidant was 4% by mass, and the concentration of the phosphorus-based antioxidant was 0.6% by mass. Therefore, the amount of each antioxidant added in PP-R11 is shown in Table 1.

[0156] PP-R3, PP-R7, and PP-R9 are obtained by melting and kneading the recovered polypropylene resin from a polypropylene resin foam molded product in an extruder, and then extruding and granulating the mixture from the extruder. PP-R8 is obtained by supplying the recovered polypropylene resin from a polypropylene resin foam molded product and carbon black to an extruder in the ratios shown in Table 1, melting and kneading the mixture, and then extruding and granulating the mixture from the extruder. In Table 1, the recovered polypropylene resin from a polypropylene resin foam molded product is referred to as "rEPP".

[0157] [Oxidation Induction Temperature of Polypropylene Resin (R)] The oxidation induction temperature of polypropylene resin (R) was measured using approximately 5 mg of polypropylene resin (R) as a sample, by differential scanning calorimetry (DSC) in accordance with ISO 11357-6:2018. More specifically, first, the sample was placed in an open sample pan and then on the sample stage of the DSC apparatus. Next, air was supplied into the furnace of the DSC apparatus to replace the atmosphere inside the furnace with air. A heat flux differential scanning calorimetry system (TA Instruments, "Device name: DSC Q1000") was used as the measuring device. The air flow rate was set to 50 mL / min.

[0158] After the furnace was filled with air, the sample was heated at a heating rate of 10°C / min, and the heat flow of the sample was measured up to a temperature at least 30°C higher than the peak of the exothermic reaction caused by the oxidative decomposition of the sample. Then, an oxidation induction temperature curve (see Figure 1) was created with the heat flow obtained in this way plotted on the vertical axis and the heating temperature on the horizontal axis.

[0159] In determining the oxidation induction temperature of the polypropylene resin (R), the temperature T4 at which the slope of the tangent to the oxidation induction temperature curve is greatest was determined, between the temperature T2 at which the oxidative decomposition of the sample began and the temperature T5 corresponding to the peak of the exothermic reaction. Then, an extension line L1 of the baseline of the oxidation induction temperature curve extended from temperature T2 and a tangent line L2 to the oxidation induction temperature curve at temperature T4 were drawn on the oxidation induction temperature curve. The temperature T3 corresponding to the intersection of the extension line L1 and the tangent line L2 determined in this way was defined as the oxidation induction temperature of the polypropylene resin (R).

[0160] [Melting Point of Polypropylene Resin (R)] The melting point of polypropylene resin (R) was measured based on a DSC curve obtained by differential scanning calorimetry (i.e., DSC) using polypropylene resin (R) as a sample, in accordance with JIS K7121-1987. Specifically, first, the sample was conditioned according to "(2) When measuring the melting temperature after performing a certain heat treatment". The heating rate and cooling rate in conditioning were set to 10°C / min. The conditioned sample was heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the peak temperature of the melting peak that appeared in the DSC curve was taken as the melting point of polypropylene resin (R). If multiple melting peaks appeared in the DSC curve, the peak temperature of the melting peak with the largest area was taken as the melting point of polypropylene resin (R).

[0161] [Melt Mass Flow Rate of Polypropylene Resin (R)] The melt mass flow rate of polypropylene resin (R) was measured under the conditions of a test temperature of 230°C and a load of 2.16 kg, in accordance with JIS K7210-1:2014.

[0162] (Polypropylene resin (A)) Table 2 shows the properties of the polypropylene resin (A) used in the production of foamed particles. The polypropylene resin (A) used in this example is a virgin polypropylene resin consisting of an ethylene-propylene random copolymer and contains the amount of phenolic antioxidant shown in Table 2.

[0163] [Oxidation induction temperature of polypropylene resin (A)] The method for measuring the oxidation induction temperature of polypropylene resin (A) is the same as the method for measuring the oxidation induction temperature of polypropylene resin (R) described above, except that polypropylene resin (A) is used instead of polypropylene resin (R).

[0164] [Melting point of polypropylene resin (A)] The method for measuring the melting point of polypropylene resin (A) is the same as the method for measuring the melting point of polypropylene resin (R) described above, except that polypropylene resin (A) is used instead of polypropylene resin (R).

[0165] [Melt Mass Flow Rate of Polypropylene Resin (A)] The method for measuring the melt mass flow rate of polypropylene resin (A) is the same as the method for measuring the melt mass flow rate of polypropylene resin (R) described above, except that polypropylene resin (A) is used instead of polypropylene resin (R).

[0166] Next, the composition and manufacturing method of the foamed particles in this example will be explained.

[0167] (Example 1) The foamed particles of Example 1 have a foamed layer containing a polypropylene resin (R) and a coating layer made of virgin polypropylene resin that covers the foamed layer. In producing the foamed particles of this example, a co-extrusion apparatus equipped with a core layer forming extruder, a coating layer forming extruder, and a co-extrusion die connected to these two extruders was used. The composite extruded from the co-extrusion apparatus was cut using a strand-cutting method to produce multilayer resin particles having a coating layer.

[0168] Specifically, PP-R1, PP-A1, a foam regulator, and carbon black were supplied to a core-forming extruder, and a core-forming resin molten mixture was prepared by melting and kneading within the extruder. The mixing ratio of PP-R1 and PP-A1 in the core-forming resin molten mixture was as shown in the "Foam Layer" column of Table 3. Zinc borate was used as the foam regulator. The amount of zinc borate added was 0.05 parts by mass per 100 parts by mass of the total of PP-R1 and PP-A1. The amount of carbon black added was set so that the carbon black content in the core-forming resin molten mixture was 2.8% by mass. In Tables 3 to 5, polypropylene resin (R) is abbreviated as PP(R), and polypropylene resin (A) is abbreviated as PP(A).

[0169] Furthermore, virgin polypropylene resin (specifically, propylene-ethylene random copolymer, melting point 133°C, melt mass flow rate 6 g / 10 min) and carbon black were supplied to the extruder for forming the coating layer, and a molten resin mixture for forming the coating layer was formed in the extruder. The amount of carbon black added was set so that the carbon black content in the molten resin mixture for forming the coating layer was 2.8% by mass.

[0170] These molten resin mixtures were combined in a die to form a composite consisting of a non-foamed cylindrical core layer and a non-foamed coating layer covering the side surface of the core layer. This composite was then co-extruded from the die in a strand shape. The strand-shaped composite was taken up, cooled, and then cut to an appropriate length using a pelletizer to obtain columnar resin particles (i.e., multilayer resin particles) having a core layer and a coating layer covering the outer surface of the core layer. The mass percentage of the coating layer in the multilayer resin particles was 3% by mass.

[0171] Next, the multilayer resin particles were foamed using a direct foaming method. Specifically, 100 kg of multilayer resin particles were first placed in a 400 L container along with 220 L of water as an aqueous dispersion medium. Then, 0.3 parts by mass of dispersant, 0.004 parts by mass of sodium alkylbenzenesulfonate and 0.01 parts by mass of aluminum sulfate were added to the container per 100 parts by mass of multilayer resin particles as dispersion aids, and the multilayer resin particles were dispersed in the aqueous dispersion medium. Kaolin was used as the dispersant.

[0172] Subsequently, carbon dioxide was supplied as a foaming agent into the sealed container while stirring the contents, raising the temperature inside the container to 145°C. The pressure inside the container at this time was 3.4 MPa(G). This temperature was then maintained for 15 minutes to impregnate the multilayer resin particles with the foaming agent and to adjust the crystalline state so that the aforementioned high-temperature peak appeared in the DSC curve of the resulting foamed particles. The container was then opened, and the contents were released into an atmospheric pressure atmosphere to foam the multilayer resin particles. The foamed particles thus obtained were dried for 24 hours in an atmosphere at a temperature of 23°C and a relative humidity of 50%. As a result, foamed particles were obtained having a foamed core layer and a non-foamed coating layer covering the foamed layer.

[0173] (Examples 2-3) The foamed particles of these examples have a configuration that is generally the same as that of the foamed particles of Example 1, except that they contain the polypropylene resin (R) shown in Table 3 instead of PP-R1. The method for producing the foamed particles of these examples is generally the same as the method for producing the foamed particles of Example 1, except that the polypropylene resin (R) shown in Table 3 is used instead of PP-R1.

[0174] (Example 4) The foamed particles of Example 4 have a configuration that is generally the same as that of the foamed particles of Example 1, except that the mixing ratio of PP-R1 and PP-A1 is changed as shown in Table 3, and that carbon black is not included in the foamed layer and coating layer. The method for producing the foamed particles of Example 4 is generally the same as the method for producing the foamed particles of Example 1, except that the mixing ratio of PP-R1 and PP-A1 is changed as shown in Table 3, and carbon black is not added to the molten resin mixture for forming the core layer and the molten resin mixture for forming the coating layer.

[0175] (Examples 5-9, Example 15) The foamed particles of these examples have a configuration that is generally the same as that of the foamed particles of Example 4, except that they contain a polypropylene resin (R) shown in Table 3 or Table 4 instead of PP-R1. The method for producing the foamed particles of these examples is generally the same as the method for producing the foamed particles of Example 4, except that a polypropylene resin (R) shown in Table 3 or Table 4 is used instead of PP-R1.

[0176] (Example 10) The foamed particles of Example 10 have a configuration that is generally the same as that of the foamed particles of Example 1, except that the mixing ratio of PP-R1 and PP-A1 is changed as shown in Table 4. The method for producing the foamed particles of Example 10 is generally the same as the method for producing the foamed particles of Example 1, except that the mixing ratio of PP-R1 and PP-A1 is changed as shown in Table 4.

[0177] (Example 11) The foamed particles of Example 11 have a configuration that is generally the same as that of the foamed particles of Example 3, except that the mixing ratio of PP-R3 and PP-A1 is changed as shown in Table 4. The method for producing the foamed particles of Example 11 is generally the same as the method for producing the foamed particles of Example 3, except that the mixing ratio of PP-R3 and PP-A1 is changed as shown in Table 4.

[0178] (Examples 12 and 14) The foamed particles of these examples have a configuration that is generally the same as that of the foamed particles of Example 1, except that they contain a polypropylene resin (A) shown in Table 4 instead of PP-A1, and the blending ratio of polypropylene resin (R) and polypropylene resin (A) is changed as shown in Table 4. The method for producing the foamed particles of these examples is generally the same as the method for producing the foamed particles of Example 1, except that a polypropylene resin (A) shown in Table 4 is used instead of PP-A1, and the blending ratio of polypropylene resin (R) and polypropylene resin (A) is changed as shown in Table 4.

[0179] (Example 13) The foamed particles of Example 13 have a configuration that is generally the same as that of the foamed particles of Example 1, except that PP-R10 is included instead of PP-R1, and the mixing ratio of PP-R10 and PP-A1 is changed as shown in Table 4. The method for producing the foamed particles of Example 13 is generally the same as the method for producing the foamed particles of Example 1, except that PP-R10 is used instead of PP-R1, and the mixing ratio of PP-R10 and PP-A1 is changed as shown in Table 4.

[0180] (Comparative Example 1) The foamed particles of Comparative Example 1 have a configuration that is generally the same as that of the foamed particles of Example 1, except that PP-R7 is included instead of PP-R1. The method for producing the foamed particles of Comparative Example 1 is generally the same as the method for producing the foamed particles of Example 1, except that PP-R7 is used instead of PP-R1.

[0181] (Comparative Example 2) The foamed particles of Comparative Example 2 have a configuration that is generally the same as the foamed particles of Example 1, except that they contain PP-R8 instead of PP-R1, PP-A2 instead of PP-A1, and the amount of carbon black added is changed so that the content in the resin particles is the value shown in Table 5. The method for producing the foamed particles of Comparative Example 2 is generally the same as the method for producing the foamed particles of Example 1, except that PP-R8 is used instead of PP-R1, PP-A2 is used instead of PP-A1, and the amount of carbon black added is changed.

[0182] (Comparative Example 3) The foamed particles of Comparative Example 3 have a configuration that is generally the same as that of the foamed particles of Example 4, except that PP-R7 is included instead of PP-R1. The method for producing the foamed particles of Comparative Example 3 is generally the same as the method for producing the foamed particles of Example 4, except that PP-R7 is used instead of PP-R1.

[0183] (Comparative Example 4, Comparative Example 5) The foamed particles of these comparative examples have a configuration that is generally the same as that of the foamed particles of Example 10, except that the polypropylene resin (R) shown in Table 5 is used instead of PP-R1. The method for producing the foamed particles of these comparative examples is generally the same as the method for producing the foamed particles of Example 10, except that the polypropylene resin (R) shown in Table 5 is used instead of PP-R1.

[0184] (Comparative Example 6) The foamed particles of Comparative Example 6 have a configuration that is generally the same as that of the foamed particles of Example 12, except that PP-R8 is included instead of PP-R1. The method for producing the foamed particles of Comparative Example 6 is generally the same as the method for producing the foamed particles of Example 12, except that PP-R8 is used instead of PP-R1.

[0185] (Comparative Example 7) The foamed particles of Comparative Example 7 have a configuration that is generally the same as that of the foamed particles of Example 14, except that PP-R9 is included instead of PP-R1. The method for producing the foamed particles of Comparative Example 7 is generally the same as the method for producing the foamed particles of Example 14, except that PP-R9 is used instead of PP-R1.

[0186] Tables 3 to 5 show the properties of the resin particles, foamed particles, and molded articles obtained by in-mold molding of the foamed particles in Examples 1 to 15 and Comparative Examples 1 to 7 as described above.

[0187] (Resin Particles) [Amount of Antioxidant Added] Based on the amount of antioxidant masterbatch added during the granulation of the polypropylene resin (R) used in the production of the resin particles, the amount of polypropylene resin (R) in the resin particles, and the mass ratio of the core layer, the amount of antioxidant contained in the resin particles that was added in the form of a masterbatch during the production of the polypropylene resin (R) was calculated.

[0188] [Melting point and melt mass flow rate of resin particles] The method for measuring the melting point and melt mass flow rate of resin particles is the same as the method for measuring the melting point and melt mass flow rate of polypropylene resin (R) described above, except that resin particles are used as the sample instead of polypropylene resin (R).

[0189] (Foamed Particles) [Heat of Fusion at High Temperature Peak] The heat of fusion at the high temperature peak was calculated based on the DSC curve obtained by performing differential scanning calorimetry (DSC) under the conditions described above, in accordance with JIS K7121:1987. Specifically, first, 1 to 3 mg of foamed particles after conditioning were used as a sample, and a DSC curve was obtained by performing differential scanning calorimetry under the condition of heating from 23°C to 200°C at a heating rate of 10°C / min.

[0190] Next, a straight line L5 was drawn connecting point α, which corresponds to 80°C on the DSC curve illustrated in Figure 3, and point β, which corresponds to the melting end temperature T of the foamed particles. After drawing line L5, a straight line L6 was drawn parallel to the vertical axis of the graph, passing through the maximum point γ, which lies between the resin-specific peak ΔH1 and the high-temperature peak ΔH2. Then, the heat of fusion for the high-temperature peak ΔH2 was calculated based on the area of ​​the region enclosed by the portion of the DSC curve that constitutes the high-temperature peak ΔH2, line L5, and line L6.

[0191] [Oxidation Induction Temperature] The method for measuring the oxidation induction temperature of foamed particles is the same as the method for measuring the oxidation induction temperature of polypropylene resin (R) described above, except that foamed particles are used as the sample instead of polypropylene resin (R), and the amount of foamed particles placed in the sample pan is set to an amount that all foamed particles are in contact with the bottom surface of the sample pan. When all foamed particles are in contact with the bottom surface of the sample pan, the mass of foamed particles in the sample pan is in the range of approximately 3 to 6 mg.

[0192] [Oxidation Induction Time] Approximately 3-6 mg of foamed particles were used as a sample, and the oxidation induction time of the foamed particles was measured by differential scanning calorimetry (DSC) in accordance with ISO 11357-6:2018. More specifically, first, the sample was placed in an open sample pan, and the sample pan was placed on the sample stage of the DSC apparatus without a lid. The sample was filled into the sample pan so that all foamed particles were in contact with the bottom surface of the sample pan and that the foamed particles did not overlap with each other. In other words, the foamed particles were filled into the sample pan so that there was only one layer of foamed particles. Next, nitrogen gas with a purity of 99.99% or higher was supplied into the furnace of the DSC apparatus, and the atmosphere inside the furnace was replaced with nitrogen gas. The flow rate of the nitrogen gas was set to 50 mL / min. A heat flux differential scanning calorimetry system (TA Instruments "DSC Q1000") was used as the measuring device.

[0193] After the furnace was filled with nitrogen gas, the sample was heated to 200°C at a heating rate of 10°C / min while continuing to supply nitrogen gas. After maintaining the temperature at 200°C for 3 minutes, the supply of nitrogen gas was stopped and air was supplied to the furnace. The air flow rate was set to 50 mL / min. Subsequently, the temperature of 200°C was maintained and the air supply was continued, and the heat flow of the sample was measured until an exothermic peak due to oxidative decomposition of the sample was observed. An oxidation induction time curve (see Figure 2) was then created, with the heat flow obtained in this way plotted on the vertical axis and the elapsed time from the start of the test plotted on the horizontal axis.

[0194] To determine the oxidation induction time of the foamed particles, first, the time t4 at which the slope of the tangent to the oxidation induction time curve is greatest was determined, between time t2, when the oxidative decomposition of the sample began, and time t5, which corresponds to the peak of the exothermic reaction. Next, an extension line L3 of the baseline of the oxidation induction time curve, extended from time t2, and a tangent line L4 to the oxidation induction time curve at time t4 were drawn on the oxidation induction time curve. Then, the elapsed time from time t1 to time t3, which corresponds to the intersection of extension line L3 and tangent line L4, was defined as the oxidation induction time t of the foamed particles.

[0195] [Bulk Density] The foamed particles were left to stand for more than 24 hours under conditions of 50% relative humidity, 23°C, and 1 atm pressure to adjust their state. The resulting bulk volume was approximately 500 cm³. 3 The foam particles were filled into a graduated cylinder, and the filling height of the foam particles inside the cylinder was stabilized by lightly tapping the floor several times with the bottom of the graduated cylinder. Then, the exact bulk volume (unit: L) of the foam particles was read from the scale on the graduated cylinder. The bulk density (unit: kg / m³) of the foam particles was then calculated by dividing the mass (unit: g) of the foam particles inside the graduated cylinder by the aforementioned bulk volume and converting the value to units. 3 ) was calculated.

[0196] (Molded Body) A flat molded body measuring 400 mm in length, 300 mm in width, and 80 mm in thickness was produced by in-mold molding using the foamed particles of the Examples and Comparative Examples. Specifically, the foamed particles were filled into the mold by a compression filling method to achieve the filling rate P shown in Tables 3 to 5. The compression filling method is a filling method in which foamed particles are filled into the mold under pressure. The filling rate P is a value expressed by the following formula (1): P = [a / (b × c)] × 100 ... (1)

[0197] However, in formula (1), a is the mass (in kg) of the foamed particles filled in the mold, and b is the bulk density (in kg / m³) of the foamed particles. 3 ) and c is the internal volume of the mold (unit: m 3 )

[0198] Next, in-mold molding was performed by supplying steam into the mold. In the in-mold molding process, first, preheating was performed by supplying steam into the mold for 5 seconds with the drain valve of the mold open. Then, the drain valve was closed, and steam was supplied from one side of the mold to perform the first one-sided heating until the pressure reached 0.08 MPa(G) lower than the molding pressure during the main heating. Next, steam was supplied from the other side of the mold to perform the second one-sided heating until the pressure reached 0.04 MPa(G) lower than the molding pressure during the main heating. After that, the main heating was performed by supplying steam from both sides of the mold to reach the molding pressure shown in Tables 3 to 5 (i.e., the molding pressure during the main heating). After the main heating was completed, the pressure inside the mold was released, and the molded body was cooled inside the mold until the surface pressure due to the foaming force of the molded body reached 0.04 MPa(G). After that, the mold was opened and the foamed particle molded body was removed. The obtained foamed particle molded body was cured in an 80°C oven for 12 hours, and then slowly cooled to room temperature to obtain a foamed particle molded body.

[0199] [Density of the molded body] The mass (in units: g) of the molded body obtained by the method described above is divided by the volume (in units: L) obtained from the external dimensions of the molded body, and then the density of the molded body (in units: kg / m³) is calculated by converting the units. 3 ) was calculated.

[0200] [Tensile Strength] The tensile strength of the molded body was measured in accordance with ISO 1798:2008. Specifically, a 13 mm thick flat plate was cut from the center of the molded body, excluding the skin surface, i.e., the surface that was in contact with the inner wall of the mold during in-mold molding. A No. 1 dumbbell test specimen was cut from this flat plate using a coping saw. After conditioning the specimen by leaving it undisturbed under standard conditions for 24 hours, a tensile test was performed at a tensile speed of 500 mm / min. The maximum load measured during the test was defined as the tensile strength of the specimen. The tensile test was performed using a universal testing machine (Instron® 6800, manufactured by Instron).

[0201] The above tests were performed using five test specimens, and the average value of the tensile strength from the five measurements was used as the tensile strength of the molded body, as shown in Tables 3 to 5.

[0202] [Recovery] In a plan view of the foam particle molded body from the thickness direction, the thickness of the foam particle molded body was measured at four locations 10 mm inward from each vertex toward the center, and the thickness of the foam particle molded body at the center was measured. Next, the ratio (in %) of the thickness of the thinnest location to the thickness of the thickest location among the measured locations was calculated. If the thickness ratio obtained in this way was 95% or more, it was judged to be a pass and the symbol "A" was written in the "Recovery" column of Tables 3 to 5. If it was less than 95%, it was judged to be a fail and the symbol "C" was written in the same column.

[0203]

[0204]

[0205]

[0206]

[0207]

[0208] As shown in Tables 3 and 4, in the manufacturing methods of Examples 1 to 15, foamed particles are produced by foaming polypropylene resin particles containing 15% by mass or more of recycled polypropylene resin (R). Furthermore, the melting point of the polypropylene resin (R) contained in the resin particles is less than 160°C, and the oxidation induction temperature is 220°C or higher. On the other hand, as shown in Table 5, in the manufacturing methods of Comparative Examples 1 to 7, the oxidation induction temperature of the polypropylene resin (R) contained in the resin particles is less than 220°C.

[0209] Of these examples and comparative examples, when comparing Examples 1 to 3 with Comparative Example 1, in which the blending ratio of polypropylene resin (R) in the foamed particles was the same as in Examples 1 to 3, the oxidation induction time of the foamed particles in the examples was longer than that of the foamed particles in the comparative examples. Similarly, in the comparison between Examples 4 to 9 and Comparative Example 3, between Examples 10 to 11 and Comparative Examples 4 to 5, and between Examples 12 to 14 and Comparative Examples 6 to 7, the oxidation induction time of the foamed particles in the examples was longer than that of the foamed particles in the comparative examples.

[0210] Therefore, these results indicate that the oxidation induction time of foamed polypropylene resin particles containing 15% by mass or more of a polypropylene resin (R) having an oxidation induction temperature within the specified range can be extended. The oxidation induction time of foamed particles can be used as an indicator of the rate at which the physical properties of the foamed particles deteriorate over time, and a longer oxidation induction time means that the rate at which the physical properties of the foamed particles deteriorate over time is slower. Therefore, it can be understood that foamed particles with a long oxidation induction time can maintain the good physical properties of the molded article over a long period of time.

[0211] Furthermore, the foamed particles of Comparative Examples 4 and 5 have oxidation induction temperatures similar to those of the foamed particles of Examples 10 and 11, possibly due to the inclusion of antioxidants and a higher proportion of PP-A1, a virgin polypropylene resin. However, as mentioned above, the oxidation induction times of the foamed particles of Comparative Examples 4 and 5 are shorter than those of the foamed particles of Examples 10 and 11. Similarly, although the foamed particles of Comparative Example 6 have oxidation induction temperatures similar to those of the foamed particles of Example 12, the oxidation induction times of the foamed particles of Comparative Example 6 are shorter than those of the foamed particles of Example 12. Also, although the foamed particles of Comparative Example 7 have oxidation induction temperatures similar to those of the foamed particles of Example 14, the oxidation induction times of the foamed particles of Comparative Example 7 are shorter than those of the foamed particles of Example 14.

[0212] Therefore, from these comparisons, it can be understood that in order to lengthen the oxidation induction time of foamed particles containing recycled polypropylene resin (R), it is effective to use polypropylene resin (R) whose oxidation induction temperature is within the specified range. Furthermore, it can be understood that even when the amount of polypropylene resin (R) blended is small, foamed particles with a long oxidation induction time can be easily obtained by using polypropylene resin (R) having an oxidation induction temperature within the specified range.

[0213] Furthermore, one aspect of the present disclosure, a method for identifying polypropylene resins, includes a determination step of determining that a polypropylene resin (R) having an oxidation induction temperature within a specific range is usable. Therefore, these results show that by implementing the determination method, foamed particles with a long oxidation induction time can be easily produced even when a certain amount of recycled polypropylene resin is incorporated into the foamed particles.

[0214] The above describes the methods for producing polypropylene resin foam particles, the method for identifying the polypropylene resin, and the methods for producing the polypropylene resin, based on the examples. However, the specific embodiments of the methods for producing polypropylene resin foam particles, the method for identifying the polypropylene resin, and the methods for producing the polypropylene resin described herein are not limited to those described in the examples, and the configuration can be modified as appropriate without impairing the spirit of this disclosure.

Claims

1. A method for producing foamed polypropylene resin particles, comprising a foaming step of foaming a polypropylene resin composition comprising a base resin composed of a polypropylene resin and a foaming agent, wherein the base resin is made using recycled polypropylene resin (R), the amount of polypropylene resin (R) in the base resin is 15% by mass or more, the melting point of the polypropylene resin (R) is less than 160°C, and the oxidation induction temperature of the polypropylene resin (R), measured in accordance with ISO 11357-6:2018, is 220°C or higher.

2. The method for producing polypropylene resin foam particles according to claim 1, wherein the base resin is made using the polypropylene resin (R) which has been confirmed to have an oxidation induction temperature of 220°C or higher.

3. The manufacturing method according to claim 1, comprising: an oxidation induction temperature measurement step of measuring the oxidation induction temperature of the polypropylene resin (R) in accordance with ISO 11357-6:2018; a determination step of determining that a polypropylene resin (R) having an oxidation induction temperature of 220°C or higher can be used to produce the foamed particles, and that a polypropylene resin (R) having an oxidation induction temperature of less than 220°C cannot be used to produce the foamed particles; and a base resin production step of producing the base resin using the polypropylene resin (R) that has been determined to be usable to produce the foamed particles in the determination step.

4. The manufacturing method according to claim 3, comprising an antioxidant addition step of adding an antioxidant to the polypropylene resin (R) that has been determined in the determination step to be unsuitable for production of foamed particles, such that the oxidation induction temperature is 220°C or higher, and in the foaming step, foaming a base resin containing the polypropylene resin (R) that has gone through the antioxidant addition step and has an oxidation induction temperature of 220°C or higher to obtain foamed particles.

5. The method for producing polypropylene resin foam particles according to claim 3, wherein when the polypropylene resin (R) determined to be usable for producing the foam particles in the determination step is used, the base resin production step is carried out without adding a phenolic antioxidant, or with adding less than 0.01% by mass of a phenolic antioxidant.

6. A method for producing polypropylene resin foam particles according to any one of claims 1 to 5, wherein the carbon black content in the polypropylene resin (R) is less than 0.5% by mass (including 0).

7. A method for producing polypropylene resin foam particles according to any one of claims 1 to 6, wherein the carbon black content in the base resin is 0.5% by mass or more and 5% by mass or less.

8. A method for producing polypropylene resin foam particles according to any one of claims 1 to 6, wherein the carbon black content in the base resin is less than 0.5% by mass (including 0).

9. A method for producing polypropylene resin foam particles according to any one of claims 1 to 8, wherein the melting point of the polypropylene resin (R) is 150°C or lower.

10. A method for producing polypropylene resin foam particles according to any one of claims 1 to 9, wherein the amount of phenolic antioxidant in the base resin is 0.005% by mass or more and 0.5% by mass or less.

11. A method for producing polypropylene resin foam particles according to any one of claims 1 to 10, wherein the polypropylene resin (R) is derived from a post-consumer material of a polypropylene resin foam molded article.

12. The method for producing polypropylene resin foam particles according to claim 11, wherein the post-consumer material is obtained by compressing and then crushing a polypropylene resin foam molded body.

13. A method for producing polypropylene resin foam particles according to any one of claims 1 to 12, wherein the base resin contains a polypropylene resin (A) made of a virgin polypropylene resin, the blending ratio of the polypropylene resin (A) in the base resin is 0.1% by mass or more and 85% by mass or less, and the blending ratio of the polypropylene resin (R) is 15% by mass or more and 99.9% by mass or less (provided that the sum of the blending ratio of the polypropylene resin (R) and the blending ratio of the polypropylene resin (A) is 100% by mass).

14. A method for determining whether recycled polypropylene resin can be used to produce polypropylene resin foam particles, comprising: an oxidation induction temperature measurement step of measuring the oxidation induction temperature of the polypropylene resin in accordance with ISO 11357-6:2018; and a determination step of determining that if the oxidation induction temperature of the polypropylene resin measured in the oxidation induction temperature measurement step is 220°C or higher, the polypropylene resin can be used to produce the foam particles, and if the oxidation induction temperature of the polypropylene resin is less than 220°C, the polypropylene resin cannot be used to produce the foam particles.

15. A method for producing polypropylene resin for use in the production of polypropylene resin foam particles, comprising adding 0.005% by mass to 1% by mass of a phenolic antioxidant to a polypropylene resin recovery product derived from post-consumer materials of a polypropylene resin foam molded product, and melt-kneading the mixture in an extruder to produce a polypropylene resin having an oxidation induction temperature of 220°C or higher, as measured in accordance with ISO 11357-6:2018.

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