Polypropylene resin foam particles

Abstract

The polypropylene resin foam particles contain 50 mass% or more of a recycled polypropylene resin (R). The oxidation induction time (t) of the polypropylene resin foam particles at a temperature of 200° C, as measured in accordance with ISO 11357-6: 2018, is 5-360 minutes. The content of carbon black in the polypropylene resin (R) may be less than 0.5 mass% (including 0). The content of carbon black in the foam particles may be 0.5-5 mass%.

Description

Polypropylene resin foam particles

[0001] This disclosure relates to polypropylene resin foam particles.

[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; when the proportion of polypropylene resin derived from post-consumer materials was high, the molded product was prone to deterioration and its properties could decline prematurely, depending on the usage environment.

[0006] This disclosure is made in view of the above background and aims to provide polypropylene resin foam particles that 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.

[0007] One aspect of this disclosure relates to polypropylene resin foam particles relating to [1] to

[13] below.

[0008] [1] Polypropylene resin foam particles containing 50% by mass or more of recycled polypropylene resin (R), wherein the oxidation induction time at a temperature of 200°C is 5 minutes or more and 360 minutes or less, as measured in accordance with ISO 11357-6:2018.

[0009] [2] Polypropylene resin foam particles according to [1], wherein the carbon black content in the polypropylene resin (R) is less than 0.5% by mass (including 0). [3] Polypropylene resin foam particles according to [1] or [2], wherein the carbon black content in the foam particles is 0.5% by mass or more and 5% by mass or less. [4] Polypropylene resin foam particles according to [1] or [2], wherein the carbon black content in the foam particles is less than 0.5% by mass (including 0). [5] Polypropylene resin foam particles according to any one of [1] to [4], wherein the melting point of the polypropylene resin (R) is 150°C or less.

[0010] [6] Polypropylene resin foam particles according to any one of [1] to [5], wherein the melt mass flow rate of the polypropylene resin (R) measured under the conditions of a load of 2.16 kg and a temperature of 230°C is 5 g / 10 min or more and 20 g / 10 min or less. [7] Polypropylene resin foam particles according to any one of [1] to [6], wherein the content of the phenolic antioxidant in the foam particles is 0.005% by mass or more and 0.5% by mass or less. [8] Polypropylene resin foam particles according to any one of [1] to [7], wherein the content of the phosphorus antioxidant in the foam particles is 0.001% by mass or more and 0.3% by mass or less. [9] Polypropylene resin foam particles according to any one of [1] to [8], wherein the polypropylene resin (R) is derived from a post-consumer material of a polypropylene resin foam molded article.

[0011]

[10] Polypropylene resin foam particles according to any one of [1] to [9], wherein the polypropylene resin (R) is composed of one or more polypropylene resins selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer and ethylene-propylene-butene random copolymer.

[11] Polypropylene resin foam particles according to any one of [1] to

[10] , wherein the foam particles contain a polypropylene resin (A) made of virgin polypropylene resin, the blending ratio of the polypropylene resin (A) in the foam particles is 0.1% 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 99.9% 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).

[12] Polypropylene resin foam particles according to any one of [1] to

[11] , wherein the foam particles contain 75% by mass or more of the polypropylene resin (R).

[13] Polypropylene resin foam particles according to any one of [1] to

[12] , wherein the oxidation induction time of the foam particles, as measured in accordance with ISO 11357-6:2018, is 30 minutes or more and 360 minutes or less.

[0012] According to the above embodiment, it is possible to provide polypropylene resin foam particles that contain 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.

[0013] Figure 1 is an explanatory diagram showing an example of an oxidation induction time curve for polypropylene resin foam particles. Figure 2 is an explanatory diagram showing an example of an oxidation induction temperature curve for polypropylene resin (R). Figure 3 is an explanatory diagram showing a method for calculating the heat of fusion at the high-temperature peak.

[0014] (Polypropylene Resin Foam Particles) The polypropylene resin foam particles (hereinafter also referred to as "foam particles") contain 50% by mass or more of recycled polypropylene resin (R). Furthermore, the oxidation induction time of the foam particles at a temperature of 200°C, as measured in accordance with ISO 11357-6:2018, is 5 minutes or more and 360 minutes or less. By keeping the oxidation induction time of the foam particles within the specified range, the deterioration of the physical properties of the molded article can be suppressed over a long period of time, even though recycled polypropylene resin (R) is the main component.

[0015] If the oxidation induction time for the foam particles is too short, the deterioration of the molded article over time will progress more easily, and depending on the usage environment, the physical properties of the molded article may deteriorate prematurely. By setting the oxidation induction time for the foam particles to 5 minutes or more, the deterioration of the physical properties of the molded article can be suppressed over a long period of time. From the viewpoint of further enhancing this effect, the oxidation induction time for the foam particles is preferably 8 minutes or more, more preferably 10 minutes or more, even more preferably 20 minutes or more, even more preferably 30 minutes or more, particularly preferably 40 minutes or more, and most preferably 50 minutes or more.

[0016] On the other hand, if the oxidation induction time for the foam particles is excessively long, it may lead to a decrease in the in-moldability of the foam particles. More specifically, the shape of the molded article obtained by in-molding the foam particles may be easily distorted, potentially leading to a deterioration in recovery properties. By setting the oxidation induction time for the foam particles to 360 minutes or less, preferably 300 minutes or less, 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, a decrease in the in-moldability of the foam particles can be easily avoided.

[0017] 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 360 minutes or less, 10 minutes or more and 300 minutes or less, 20 minutes or more and 240 minutes or less, 30 minutes or more and 200 minutes or less, 40 minutes or more and 160 minutes or less, or 50 minutes or more and 120 minutes or less.

[0018] 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.

[0019] 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.

[0020] Figure 1 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 1, since no endothermic or exothermic activity occurs 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. The endothermic peak that appears before time t1 in the oxidation induction time curve is a peak that originates from the melting of the polypropylene resin constituting the foamed particles.

[0021] 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 L1 of the baseline of the oxidation induction time curve extended from time t2, and a tangent line L2 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 L1 and tangent line L2, as the oxidation induction time t of the foamed particles. Note that the oxidation induction time is sometimes called isothermal OIT.

[0022] The oxidation induction time of foamed particles can be extended, for example, by incorporating a polypropylene resin (R) with a high oxidation induction temperature, as described later, into the foamed particles. Furthermore, assuming that the polypropylene resin (R) has a high oxidation induction temperature, the oxidation induction time of the foamed particles can be further extended by incorporating a polypropylene resin (R) with an even higher oxidation induction temperature into the foamed particles, incorporating carbon black into the foamed particles, or incorporating virgin polypropylene resin into the foamed particles. Also, assuming that the polypropylene resin (R) incorporated into the resin particles has a high oxidation induction temperature, the oxidation induction time of the foamed particles can be appropriately shortened by appropriately reducing the amount of antioxidant added to the polypropylene resin (R) in the antioxidant addition step described later, or by appropriately reducing the amount of antioxidant added to the resin particles in the resin particle manufacturing process.

[0023] [Polypropylene Resin (R)] The foamed particles contain at least recycled polypropylene resin (R). The foamed particles may contain one type of polypropylene resin (R), or two or more types of polypropylene resin (R). The content of polypropylene resin 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.

[0024] The proportion of polypropylene resin (R) contained in the foamed particles is 50% by mass or more. Since the foamed particles have an oxidation induction time within the specified range, even when they contain a high proportion of polypropylene resin (R) of 50% by mass or more, the deterioration of the physical properties of the molded article can be suppressed over a long period of time. From the viewpoint of further contributing to the reduction of environmental burden while suppressing the deterioration of the physical properties of the molded article over a long period of time, it is preferable that the foamed particles contain 60% by mass or more of the polypropylene resin (R), more preferably 75% by mass or more, even more preferably 80% by mass or more, even more preferably 85% by mass or more, particularly preferably 90% by mass or more, and most preferably 95% by mass or more. The upper limit of the proportion of polypropylene resin (R) contained in the foamed particles is, for example, 100% by mass and, for example, 99% by mass or less.

[0025] 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.

[0026] 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.

[0027] The polypropylene resin (R) is preferably composed of a random copolymer, and more preferably composed of one or more polypropylene resins 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 foamed particles is increased, a decrease in foaming properties and a decrease in in-mold moldability can be more easily avoided.

[0028] In this specification, recycling refers to the process of making polypropylene resin recovered from waste usable as a raw material for foamed particles. 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 recovery."

[0029] 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 later. From the viewpoint of increasing the content of recovered polypropylene resin in the foamed particles 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] From the viewpoint of increasing the proportion of polypropylene resin (R) in the foamed particles while maintaining good foamability during the manufacturing of the foamed particles and good moldability during in-mold molding, 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 polypropylene resins selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer, and ethylene-propylene-butene random copolymer.

[0034] Even when the blending amount of the polypropylene-based resin (R) is large, from the viewpoint of maintaining the in-mold formability of the foamed particles and surely suppressing an excessive increase in the molding pressure when producing a molded article, the melting point of the polypropylene-based resin (R) is preferably less than 160°C, more preferably 158°C or lower, still 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 enhancing the heat resistance of the molded article and more surely obtaining the effect of suppressing the deterioration of physical properties of the molded article over time, the melting point of the polypropylene-based resin (R) is preferably 130°C or higher, more preferably 132°C or higher, still more preferably 135°C or higher, even more preferably 138°C or higher, particularly preferably 140°C or higher, and most preferably exceeding 140°C.

[0035] In constituting the preferable range of the melting point of the polypropylene-based resin (R), the upper limit and the lower limit of the melting point of the polypropylene-based resin (R) described above can be arbitrarily combined. The preferable range of the melting point of the polypropylene-based resin (R) may be, for example, 130°C or higher and less than 160°C, 132°C or higher and 158°C or lower, 135°C or higher and 155°C or lower, 138°C or higher and 152°C or lower, 140°C or higher and 150°C or lower, or exceeding 140°C and less than 150°C.

[0036] The melting point of the polypropylene resin (R) can be determined based on the differential scanning calorimetry (i.e., DSC) performed in accordance with JIS K7121-1987 and based on the obtained DSC curve. Specifically, a test piece made of the polypropylene resin (R) is prepared, and the state of the test piece is adjusted according to "After performing a certain heat treatment, when measuring the melting temperature". The heating rate and the cooling rate in the state adjustment are 10 °C / min, and the temperature range is from 30 °C to 200 °C. The DSC curve is obtained by heating the test piece adjusted in this way from 30 °C to 200 °C at a heating rate of 10 °C / min. Then, the peak temperature of the melting peak appearing in the DSC curve is taken as the melting point of the polypropylene resin (R). In the case where a plurality of melting peaks appear in the DSC curve, the peak temperature of the melting peak with the largest area is taken as the melting point.

[0037] Based on JIS K7210-1:2014, the melt mass flow rate of the polypropylene resin (R) measured under the 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. In this case, while maintaining good foamability, the blending ratio of the polypropylene resin (R) in the foam particles can be more easily increased. From the viewpoint of more surely obtaining such an effect, based on JIS K7210-1:2014, the melt mass flow rate of the polypropylene resin (R) measured under the 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.

[0038] As a method for making the melt mass flow rate of the polypropylene resin (R) within the above range, a method of adjusting the shear amount applied to the polypropylene resin (R) by controlling the screw rotation speed of the extruder and the like in the recycling process described above, a method of adding a phosphorus-based antioxidant as an antioxidant to the polypropylene resin (R) in the antioxidant addition step described later, and the like are exemplified.

[0039] The oxidation induction temperature of the polypropylene resin (R), measured in accordance with ISO 11357-6:2018, is preferably 220°C or higher. In this case, the proportion of the polypropylene resin (R) in the foamed particles can be increased without impairing the effect of suppressing the deterioration of the physical properties of the molded article over time. 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, the oxidation induction temperature of the polypropylene resin (R) is preferably 280°C or lower, more preferably 270°C or lower, even more preferably 260°C or lower, particularly preferably 255°C or lower, and most preferably 250°C or lower.

[0040] 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 280°C, 222°C to 270°C, 225°C to 260°C, 228°C to 255°C, 230°C to 250°C, or 232°C to 250°C.

[0041] The oxidation induction temperature of polypropylene resin (R) is measured by differential scanning calorimetry (DSC) in accordance with ISO 11357-6:2018, using approximately 5 mg of polypropylene resin (R) 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, air is supplied into the furnace of the DSC apparatus to replace the atmosphere inside the furnace with air. The air flow rate is set to 50 mL / min.

[0042] 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 up to a temperature at least 30°C higher than the peak of the exothermic reaction caused by the oxidative decomposition of the sample.

[0043] Figure 2 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 2, 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 that is associated with the oxidative decomposition.

[0044] To determine the oxidation induction temperature of polypropylene resin (R), first, determine the temperature T4 at which the slope of the tangent to the oxidation induction temperature curve is greatest, between temperature T2 and temperature T5, which corresponds to the peak of the exothermic reaction. Next, draw an extension line L3 of the baseline of the oxidation induction temperature curve extended from temperature T2, and a tangent line L4 to the oxidation induction temperature curve at temperature T4, on the oxidation induction temperature curve. Then, define the temperature T3 corresponding to the intersection of extension line L3 and tangent line L4 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).

[0045] One method for raising the oxidation induction temperature of polypropylene resin (R) is to blend an antioxidant, such as a phenolic antioxidant, into the polypropylene resin (R). Specifically, for example, if the oxidation induction temperature of the polypropylene resin (R) is less than 220°C, an antioxidant can be added to raise the oxidation induction temperature to 220°C or higher. In this specification, this step may be referred to as the antioxidant addition step. By performing the antioxidant addition step, it becomes easier to produce foamed particles having an oxidation induction time within the specified range, thereby making it easier to suppress the deterioration of the molded article over time.

[0046] In the antioxidant addition process, various methods can be used to add the antioxidant to the polypropylene resin (R). For example, in the antioxidant addition process, the polypropylene resin (R) and the antioxidant may be supplied to an extruder, and the antioxidant may be added to the polypropylene resin (R) by melt-kneading the polypropylene resin (R) and the antioxidant in the extruder. Furthermore, the form of the antioxidant supplied to the extruder is not particularly limited. For example, in the antioxidant addition process, the antioxidant itself may be supplied directly to the extruder, or a masterbatch containing the antioxidant may be supplied to the extruder. As the base resin of the masterbatch, a thermoplastic resin such as a polypropylene resin can be used.

[0047] [Polypropylene resin (A)] The foamed particles may contain, in addition to recycled polypropylene resin (R), polypropylene resin (A) composed of virgin polypropylene resin. In this specification, virgin polypropylene resin means polypropylene resin that has not been processed into products, etc. Virgin polypropylene resin is also sometimes referred to as non-recycled polypropylene resin.

[0048] When the foamed particles further contain a polypropylene resin (A) made from virgin polypropylene resin, it is preferable that the blending ratio of the polypropylene resin (A) in the foamed particles is 0.1% 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 99.9% 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 foamed particles have an oxidation induction time within the specified range, even when the blending ratio of the polypropylene resin (R) is relatively high, the effect of suppressing the deterioration of the physical properties of the molded article can be easily obtained.

[0049] From the viewpoint of suppressing the deterioration of the physical properties of the molded article while further increasing the proportion of polypropylene resin (R), it is more preferable that the proportion of polypropylene resin (A) in the foamed particles is 0.2% by mass or more and 40% by mass or less, and the proportion of polypropylene resin (R) is 60% 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 foamed particles is 0.3% by mass or more and 25% by mass or less, and the proportion of polypropylene resin (R) is 75% 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 foamed particles is 0.5% by mass or more and 20% by mass or less, and the proportion of polypropylene resin (R) is 80% by mass or more and 99.5% by mass or less. Furthermore, it is most preferable that the blending ratio of the polypropylene resin (A) in the foamed particles is 0.8% by mass or more and 10% by mass or less, and the blending ratio of the polypropylene resin (R) is 90% by mass or more and 99.2% by mass or less.

[0050] 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 polypropylene resins selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer and ethylene-propylene-butene random copolymer.

[0051] 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 higher and less than 160°C, more preferably 135°C or higher and 155°C or lower, even more preferably 138°C or higher and 152°C or lower, and particularly preferably 140°C or higher and 150°C or lower. The melting point of the polypropylene resin (A) can be measured by the same method as the melting point of the polypropylene resin (R).

[0052] From the viewpoint of more easily avoiding a decrease in foaming properties and further improving moldability, the melt mass flow rate of the polypropylene resin (A), measured under the conditions of a load of 2.16 kg and a temperature of 230°C, is preferably 1 g / 10 min or more and 12 g / 10 min or less, more preferably 2 g / 10 min or more and 10 g / 10 min or less, and even more preferably 3 g / 10 min or more and 8 g / 10 min or less, based on JIS K7210-1:2014.

[0053] [Other Polymers] The foamed particles are composed of a polypropylene resin. That is, the foamed particles contain 50% by mass or more of constituent units derived from propylene. However, the foamed particles may, as necessary, contain other polymers different from the polypropylene resin in addition to the polypropylene resin (R) and the polypropylene resin (A), within a range that does not impair the effect of suppressing the deterioration of the physical properties of the molded article. Examples of polymers other than polypropylene resins that may be contained in the foamed particles include thermoplastic resins other than polypropylene resins, such as polyethylene resins, polystyrene resins, polyamide resins, and polyester resins, and thermoplastic elastomers such as olefin-based thermoplastic elastomers and styrene-based thermoplastic elastomers. The foamed particles may contain one of these polymers other than polypropylene resins, or two or more polymers.

[0054] The content of polymers other than polypropylene resins in the foamed particles 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.

[0055] [Additives] The foamed particles may contain additives such as foam regulators, colorants, antioxidants, antistatic agents, surfactants, light stabilizers, ultraviolet absorbers, and flame retardants, as needed. The foamed particles may contain one of these additives, or two or more. The additives may be contained in the polypropylene resin (R), or in the polypropylene resin (A). Furthermore, the additives may be contained in both the polypropylene resin (R) and the polypropylene resin (A).

[0056] As antioxidants, known antioxidants used in polypropylene resins, such as phosphorus-based antioxidants, sulfur-based antioxidants, and phenol-based antioxidants, can be used. The foamed particles may contain one type of antioxidant or two or more types of antioxidants.

[0057] The content of phosphorus-based antioxidants in the foamed particles 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, the polypropylene resin is subjected to a thermal history during the recycling process, which can increase the melt mass flow rate of the recycled polypropylene resin. Furthermore, the polypropylene resin undergoes further thermal history during the manufacturing process of the foamed particles, which tends to increase the melt mass flow rate of the polypropylene resin during the manufacturing process of the foamed particles.

[0058] In contrast, phosphorus-based antioxidants are excellent at suppressing the increase in the melt mass flow rate of polypropylene resins. Therefore, by blending phosphorus-based antioxidants so that their content in the foamed particles is within the specified range, 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 foamability and a decrease in in-mold moldability.

[0059] The phosphorus-based antioxidant in the foamed particles may be added to the resin particles together with the polypropylene resin (R) in the manufacturing process of the resin particles used to produce the foamed particles (this process is sometimes called the "pelletizing process"). From the viewpoint of more reliably obtaining the effects described above, it is preferable that the phosphorus-based antioxidant is added to the polypropylene resin (R) in the antioxidant addition process described above. In other words, it is preferable that the phosphorus-based antioxidant is contained in at least the polypropylene resin (R). Furthermore, the content of the phosphorus-based antioxidant in the polypropylene resin (R) is preferably 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.

[0060] The content of the phenolic antioxidant in the foamed particles 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 suppressing the deterioration of the physical properties of the molded article over time can be further enhanced.

[0061] The reasons why the aforementioned effects can be obtained with phenolic antioxidants are thought to be as follows: Phenolic antioxidants are thought to act efficiently even in the high temperature range of 220°C or higher during the foam particle manufacturing process, capturing degradation factors such as free radicals generated when the polypropylene resin (R) is subjected to thermal history, and suppressing oxidation reactions. Therefore, phenolic antioxidants are thought to be highly effective in improving the oxidation induction temperature of the polypropylene resin (R). Accordingly, by adding a phenolic antioxidant with such an effect to the foam particles and keeping its content within the above range, the oxidation induction time of the foam particles can be extended.

[0062] The phenolic antioxidant in the foamed particles may be added to the resin particles together with the polypropylene resin (R) during the manufacturing process of the resin particles used to produce the foamed particles. However, from the viewpoint of more reliably obtaining the effects described above, it is preferable that the phenolic antioxidant is added to the polypropylene resin (R) during the antioxidant addition process described above. In other words, it is preferable that the phenolic antioxidant is contained in at least the polypropylene resin (R). Furthermore, the content of the phenolic antioxidant in the polypropylene resin (R) is preferably 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.

[0063] Furthermore, from the viewpoint of more easily obtaining both the effect of avoiding a decrease in foaming ability and a decrease in in-mold moldability, and the effect of suppressing the deterioration of the physical properties of the molded article over time, it is preferable that the foamed particles contain 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 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, it is preferable that the polypropylene resin (R) 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 more preferably 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. Furthermore, as described above, by using a phenolic antioxidant and a phosphorus-based antioxidant in combination, it is believed that they act synergistically, further enhancing the effect of the phenolic antioxidant. Therefore, in this case, the amount of phenolic antioxidant added that is necessary to raise the oxidation induction temperature of the polypropylene resin (R) to 220°C or higher can be reduced.

[0064] The foamed particles may optionally contain a sulfur-based antioxidant in an amount of 0.005% to 0.8% by mass. The polypropylene resin (R) may also optionally contain a sulfur-based antioxidant in an amount of 0.008% to 0.5% by mass.

[0065] The foamed particles may contain carbon black as needed. For example, carbon black may be added to the resin particles along with polypropylene resin (R) during the manufacturing process of the resin particles used to produce the foamed particles. Furthermore, carbon black may be contained in polypropylene resin (R) or polypropylene resin (A). In addition, carbon black may be contained in both polypropylene resin (R) and polypropylene resin (A).

[0066] The carbon black content in the foamed particles 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 foamed particles within the above-mentioned specific range, the oxidation induction time of the foamed particles can be more easily adjusted to within the above-mentioned specific range. In addition, foamed particles with a carbon black content within the above-mentioned specific range exhibit a black color. Therefore, by performing in-mold molding using such foamed particles, a high-quality appearance can be given to the molded product.

[0067] Furthermore, the carbon black content in the foamed particles may be less than 0.5% by mass (including 0). Such foamed particles and their molded articles can easily avoid problems such as excessive absorption of infrared rays caused by carbon black. Therefore, in this case, the used foamed particles and their molded articles can be recycled or reused for a wider range of applications. As mentioned above, since the foamed particles have an oxidation induction time within the specified range, even if they do not contain carbon black or have a relatively low carbon black content, the deterioration of the physical properties of the molded article can be suppressed over a long period of time.

[0068] 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 foamed particles, 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 time of the foamed particles is greater than or equal to a predetermined value, even when a polypropylene resin (R) with a low carbon black content is used, the effect of maintaining the good physical properties of the molded article can be further enhanced.

[0069] [Structure of Foamed Particles] The foamed particles may have a single-layer structure consisting only of a foamed layer. The foamed layer of the foamed particles contains at least the polypropylene resin (R). The foamed layer may contain the polypropylene resin (A) in addition to the polypropylene resin (R). When the foamed particles have a single-layer structure consisting only of a foamed layer, the proportion of polypropylene resin (R) in the foamed particles is equal to the proportion of polypropylene resin (R) in the foamed layer. Similarly, when the foamed particles have a single-layer structure consisting only of a foamed layer, the proportion of polypropylene resin (A) in the foamed particles is equal to the proportion of polypropylene resin (A) in the foamed layer.

[0070] Furthermore, the foamed particles may have a multilayer structure comprising a foamed layer and a coating layer made of a thermoplastic resin that covers the foamed layer. The coating layer may cover the entire surface of the foamed layer or cover a part of the foamed layer. The coating layer may be provided on the surface of the foamed particles to improve the fusion properties between the foamed particles in in-mold molding, or to impart functionality. When a coating layer is provided for the purpose of improving the fusion properties between the foamed 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 the melting point of the foamed layer.

[0071] The coating layer may be foamed or non-foamed, 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 contains no air bubbles, and a state in which air bubbles disappear after foaming, meaning that there is almost no bubbly structure within the coating layer.

[0072] 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 foam 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.

[0073] 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.

[0074] When foamed particles have a multilayer structure comprising a foam layer and a coating layer, the proportion of polypropylene resin (R) in the foamed particles is equal to the ratio of the sum of the amount of polypropylene resin (R) in the foam layer and the amount of polypropylene resin (R) in the coating layer to the total mass of the foamed particles. Similarly, when foamed particles have a multilayer structure comprising a foam layer and a coating layer, the proportion of polypropylene resin (A) in the foamed particles is equal to the ratio of the sum of the amount of polypropylene resin (A) in the foam layer and the amount of polypropylene resin (A) in the coating layer to the total mass of the foamed particles.

[0075] In the thermoplastic resin constituting the coating layer, additives such as a nucleating agent, a flame retardant, a flame retardant aid, a plasticizer, an antistatic agent, an antioxidant, an ultraviolet absorber, a light stabilizer, a conductive filler, an antibacterial agent, and a coloring agent may be contained as long as the above-described effects are not impaired. The content of the additive in the coating layer is preferably, for example, 0.01 parts by mass or more and 10 parts by mass or less with respect to 100 parts by mass of the thermoplastic resin.

[0076] The mass ratio (ratio of mass %) of the foamed layer to the coating layer is preferably from 99.5:0.5 to 85:15 (where the total of the mass of the foamed layer and the mass of the coating layer is 100 mass %), more preferably from 99:1 to 90:10, and even more preferably from 98:2 to 92:8, from the viewpoint of enhancing the moldability while maintaining the rigidity of the molded body.

[0077] [Bulk density] The bulk density of the foamed particles is preferably 3 10 kg / m 3 or more and 200 kg / m 3 or less, more preferably 12 kg / m 3 or more and 100 kg / m 3 or less, even more preferably 15 kg / m 3 or more and 80 kg / m 3 or less, and particularly preferably 20 kg / m 3 or more and 50 kg / m

[0078] The method for calculating the bulk density of the foamed particles is as follows. First, the foamed particles are left standing for 24 hours or more in an environment of 50% relative humidity, 23°C temperature, and 1 atm pressure to adjust the state of the foamed particles. The thus-obtained foamed particles are filled into a graduated cylinder, and the filling height of the group of foamed particles in the graduated cylinder is stabilized by gently tapping the floor surface several times at the bottom surface of the graduated cylinder. Then, the bulk volume (unit: L) of the group of foamed particles is read from the scale of the graduated cylinder. And the bulk density (unit: kg / m 3 ) of the foamed particles can be obtained by performing unit conversion on the value obtained by dividing the mass (unit: g) of the group of foamed particles in the graduated cylinder by the above-described bulk volume.

[0079] [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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] (Method for producing polypropylene resin foam particles) The method for producing the foam particles is not particularly limited and can take various forms. For example, the foam particles can be produced by dispersing polypropylene resin particles (hereinafter also referred to as "resin particles") containing polypropylene resin (R) 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. Such a foaming method is sometimes called the "direct foaming method". The foam particles can also be produced by, for example, an extrusion foaming method in which a foaming agent is supplied to a resin molten product containing polypropylene resin (R), the product is melted and kneaded, and then extruded from an extruder and cut while foaming, or by an impregnation foaming method in which a foaming agent is impregnated into the resin particles in the gas phase, and then a heating medium is supplied and heated to cause foaming.

[0087] Resin particles used in the direct foaming method can be produced by the pelletizing process described below. Specifically, the following methods can be used as examples of pelletizing processes. To obtain foamed particles with a single-layer structure consisting of a foamed layer, first, polypropylene resin (R) is supplied into an extruder and a molten resin mixture is produced by melting and kneading it in the extruder. At this time, if necessary, resins other than polypropylene resin (R), such as polypropylene resin (A), or additives such as foam nucleating agents and carbon black may be supplied to the extruder. After that, the molten resin mixture is extruded through small holes in a die attached to the tip of the extruder to form a strand-shaped extruded product. After cooling this extruded product, 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).

[0088] To obtain foamed particles with a multilayer structure comprising a foamed layer and a coating 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 a polypropylene resin (R) with other resins and additives added as needed to produce a core layer forming resin molten mixture. The coating layer forming extruder melts and kneads a thermoplastic resin constituting the coating layer with additives added as needed to produce a coating layer forming resin molten mixture.

[0089] 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.

[0090] 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.

[0091] When the resin particles have a core layer and a coating layer, the mass ratio of the core layer to the coating layer is preferably core layer:coating layer = 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.

[0092] After producing the resin particles as described above, the resin particles are dispersed in a dispersion medium. The dispersion of the resin particles 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 dispersion of the resin particles and the foaming of the resin particles in the same sealed container.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] The melting point of the resin particles can be determined by differential scanning calorimetry (i.e., DSC) based on JIS K7121-1987 and based on the acquired DSC curve. First, the resin particles are 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. 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.

[0104] The foamed particles obtained as described above may be used as is to produce the molded article. Alternatively, the foamed particles obtained by the direct foaming method described above can be further foamed to reduce their bulk density, and the resulting foamed particles can be used to produce the molded article. When foaming resin particles in two stages in this way, 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.

[0105] A method for reducing the bulk density of foamed particles through two-stage foaming is as follows: First, as a one-stage foaming process, resin particles are foamed using the direct foaming method described above to obtain one-stage foamed particles. Then, internal pressure is applied to the one-stage foamed particles. More specifically, after placing the one-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 one-stage foamed particles equal to or greater than atmospheric pressure. Then, the one-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 one-stage foamed particles. By doing so, two-stage foamed particles can be obtained.

[0106] (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 3 More than 150kg / m 3 It is more preferable that the following conditions are met: 30 kg / m 3 More than 100kg / m 3The 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.

[0107] The polypropylene resin foam particle molded article is formed by in-mold molding the foam particles. Therefore, even though the molded article contains a large amount of recycled polypropylene resin, the deterioration of the physical properties of the molded article can be suppressed over a long period of time. More specifically, since the deterioration of the molded article is suppressed even in high-temperature environments such as 80°C, the molded article can be used for a long period of time even in more severe operating environments.

[0108] Examples of the foamed particles described above will be explained.

[0109] (Polypropylene Resin (R)) Tables 1 and 2 show the properties of the polypropylene resin (R) used in the production of 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 shown in Table 1 and PP-R5, PP-R6, PP-R9 shown in Table 2 are obtained by supplying polypropylene resin recovery material from the lots shown in Tables 1 and 2 and an antioxidant masterbatch to an extruder in the ratios shown in Tables 1 and 2, melting and kneading the mixture, and then extruding it from the extruder to granulate it.

[0110] 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 Tables 1 and 2 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.

[0111] The antioxidant masterbatch used in this example is composed of a virgin polypropylene resin containing a phenolic antioxidant, a sulfuric antioxidant, and a phosphorus-based antioxidant. Specifically, BASF's "Irganox® 1010" was used as the phenolic antioxidant, Naugard's "DSTDP" as the sulfuric antioxidant, and BASF's "Irgafos® 168" as the phosphorus-based antioxidant.

[0112] The concentration of phenolic antioxidants in the masterbatch of antioxidants used in PP-R1, PP-R2, and PP-R4 to PP-R6 was 2% by mass, the concentration of sulfur-based antioxidants was 4% by mass, and the concentration of phosphorus-based antioxidants was 0.6% by mass. Therefore, the amounts of each antioxidant added in PP-R1, PP-R2, and PP-R4 to PP-R6 are as shown in Tables 1 and 2.

[0113] In the masterbatch of antioxidants used in PP-R9, the concentration of phenolic antioxidants was 16% by mass, the concentration of sulfur-based antioxidants was 4% by mass, and the concentration of phosphorus-based antioxidants was 0.6% by mass. Therefore, the amounts of each antioxidant added in PP-R9 are as shown in Table 2.

[0114] PP-R3 shown in Table 1 and PP-R7 shown in Table 2 are obtained by melting and kneading the recovered polypropylene resin from a polypropylene resin foam molded product in an extruder, and then extruding it from the extruder to granulate it. PP-R8 shown in Table 2 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 2, melting and kneading them, and then extruding them from the extruder to granulate them. In Tables 1 and 2, the recovered polypropylene resin from a polypropylene resin foam molded product is referred to as "rEPP".

[0115] [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.

[0116] 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 2) was created with the heat flow obtained in this way plotted on the vertical axis and the heating temperature on the horizontal axis.

[0117] To determine 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 L3 of the baseline of the oxidation induction temperature curve extended from temperature T2 and a tangent line L4 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 L3 and the tangent line L4 determined in this way was defined as the oxidation induction temperature of the polypropylene resin (R).

[0118] [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).

[0119] [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.

[0120] (Polypropylene resin (A)) Table 3 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 3.

[0121] [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).

[0122] [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).

[0123] [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).

[0124] Next, the composition and manufacturing method of the foamed particles in this example will be explained.

[0125] (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.

[0126] 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 is as shown in Table 1. 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 4 and 5, polypropylene resin (R) is abbreviated as PP(R), and polypropylene resin (A) is abbreviated as PP(A).

[0127] 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 an extruder for forming the coating layer, and a molten resin mixture for forming the coating layer was prepared 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] (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 4 instead of PP-R1. The method for producing the foamed particles of these examples is generally the same as that for producing the foamed particles of Example 1, except that the polypropylene resin (R) shown in Table 4 is used instead of PP-R1.

[0132] (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 4, 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 that 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, 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.

[0133] (Examples 5-9) 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 the polypropylene resin (R) shown in Table 4 instead of PP-R1. The method for producing the foamed particles of these examples is generally the same as that for producing the foamed particles of Example 4, except that the polypropylene resin (R) shown in Table 4 is used instead of PP-R1.

[0134] (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 that for producing the foamed particles of Example 1, except that PP-R7 is used instead of PP-R1.

[0135] (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 PP-R8 is included instead of PP-R1, PP-A2 is used 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 included instead of PP-R1, PP-A2 is used instead of PP-A1, and the amount of carbon black added is changed.

[0136] (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 that for producing the foamed particles of Example 4, except that PP-R7 is used instead of PP-R1.

[0137] (Comparative Example 4) The foamed particles of Comparative Example 4 have a configuration that is generally the same as that of the foamed particles of Example 4, except that PP-R9 is included instead of PP-R1. The method for producing the foamed particles of Comparative Example 4 is generally the same as that for producing the foamed particles of Example 4, except that PP-R9 is used instead of PP-R1.

[0138] Tables 4 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 9 and Comparative Examples 1 to 4 as described above.

[0139] (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.

[0140] [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).

[0141] (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.

[0142] 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.

[0143] [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.

[0144] [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, "System name: DSC Q1000") was used as the measuring device.

[0145] 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 supply of air 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 1) 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.

[0146] 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 L1 of the baseline of the oxidation induction time curve extended from time t2, and a tangent line L2 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 L1 and tangent line L2, was defined as the oxidation induction time t of the foamed particles.

[0147] [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.

[0148] (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 4 and 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)

[0149] 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 )

[0150] Next, in-mold molding was performed by supplying steam into the mold. In the in-mold molding, 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 until the molding pressure shown in Tables 4 and 5 (i.e., the molding pressure during the main heating) was reached. 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.

[0151] [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.

[0152] [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. A universal testing machine (Instron® 6800, manufactured by Instron Corporation) was used for the tensile test.

[0153] The above tests were performed using five test specimens, and the arithmetic mean of the tensile strength from the five measurements was used as the tensile strength of the molded body, as shown in Tables 4 and 5.

[0154] [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 4 and 5. If it was less than 95%, it was judged to be a fail and the symbol "C" was written in the same column.

[0155] [Heat Aging Resistance] Three cubic test pieces (each with a side length of 50 mm) were cut from near the center of the foam particle molded body, ensuring that the skin surface was not included. These three test pieces were placed in an oven set to a predetermined ambient temperature and removed from the oven after a predetermined test period. The ambient temperature inside the oven and the test period were determined according to the carbon black content in the foam particles used to mold the body, using one of the following two conditions: Condition 1: Used when the carbon black content in the foam particles is 0.5% by mass or more. Ambient temperature inside the oven: 80°C. Test period: 180 days. Condition 2: Used when the carbon black content in the foam particles is less than 0.5% by mass. Ambient temperature inside the oven: 130°C. Test period: 60 days.

[0156] Then, the heat aging resistance was evaluated based on the results of visual inspection of the test specimens after they were removed from the oven. The meaning of the symbols shown in the "Heat Aging Resistance" column in Tables 4 and 5 is as follows: A: None of the three test specimens showed any powdering or color change. B: At least one of the three test specimens showed any powdering or color change. C: At least one of the three test specimens showed significant deterioration and cracks appeared on the surface.

[0157]

[0158]

[0159]

[0160]

[0161]

[0162] As shown in Table 4, the proportion of recycled polypropylene resin (R) in the foamed particles of Examples 1 to 9 is 50% by mass or more. Furthermore, the oxidation induction time for these foamed particles is between 5 minutes and 360 minutes. As a result, these foamed particles exhibit excellent heat aging resistance despite containing recycled polypropylene resin (R) as the main component, and the deterioration of the physical properties of the molded articles was suppressed over a long period of time. Therefore, the molded articles of the examples can be used for a long period of time even in more severe operating environments.

[0163] In contrast, as shown in Table 5, the oxidation induction time for the foamed particles of Comparative Examples 1 to 3 was less than 5 minutes. Therefore, when the molded articles made from these foamed particles were exposed to a high-temperature atmosphere, the physical properties of the articles deteriorated relatively quickly.

[0164] Furthermore, the oxidation induction time for the foamed particles in Comparative Example 4 exceeded 360 minutes. As a result, the molded articles made from these foamed particles exhibited poor recovery properties, and distortion occurred after in-mold molding.

[0165] Although the embodiments of the polypropylene resin foam particles have been described above based on the examples, the specific embodiments of the polypropylene resin foam particles relating to this disclosure are not limited to those of the examples, and the configuration can be modified as appropriate without impairing the spirit of this disclosure.

Claims

1. Polypropylene resin foam particles containing 50% by mass or more of recycled polypropylene resin (R), wherein the oxidation induction time at a temperature of 200°C is 5 minutes or more and 360 minutes or less, as measured in accordance with ISO 11357-6:2018.

2. The polypropylene resin foam particles according to claim 1, wherein the carbon black content in the polypropylene resin (R) is less than 0.5% by mass (including 0).

3. The polypropylene resin foam particles according to claim 1 or 2, wherein the carbon black content in the foam particles is 0.5% by mass or more and 5% by mass or less.

4. The polypropylene resin foam particles according to claim 1 or 2, wherein the carbon black content in the foam particles is less than 0.5% by mass (including 0).

5. Polypropylene resin foam particles according to any one of claims 1 to 4, wherein the melting point of the polypropylene resin (R) is 150°C or lower.

6. Polypropylene resin foam particles according to any one of claims 1 to 5, wherein the melt mass flow rate of the polypropylene resin (R) measured under the conditions of a load of 2.16 kg and a temperature of 230°C is 5 g / 10 min or more and 20 g / 10 min or less.

7. Polypropylene resin foam particles according to any one of claims 1 to 6, wherein the content of the phenolic antioxidant in the foam particles is 0.005% by mass or more and 0.5% by mass or less.

8. Polypropylene resin foam particles according to any one of claims 1 to 7, wherein the content of the phosphorus-based antioxidant in the foam particles is 0.001% by mass or more and 0.3% by mass or less.

9. Polypropylene resin foam particles according to any one of claims 1 to 8, wherein the polypropylene resin (R) is derived from a post-consumer material of a polypropylene resin foam molded article.

10. Polypropylene resin foam particles according to any one of claims 1 to 9, wherein the polypropylene resin (R) is composed of one or more polypropylene resins selected from the group consisting of ethylene-propylene random copolymer, propylene-butene random copolymer, and ethylene-propylene-butene random copolymer.

11. The polypropylene resin foam particles according to any one of claims 1 to 10, wherein the foam particles contain a polypropylene resin (A) made of virgin polypropylene resin, the blending ratio of the polypropylene resin (A) in the foam particles is 0.1% 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 99.9% 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).

12. The polypropylene resin foamed particle according to any one of claims 1 to 11, wherein the foamed particle contains 75% by mass or more of the polypropylene resin (R).

13. Polypropylene resin foam particles according to any one of claims 1 to 12, wherein the oxidation induction time of the foam particles, as measured in accordance with ISO 11357-6:2018, is 30 minutes or more and 360 minutes or less.

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