Polyethylene resin foamed particles, polyethylene resin in-mold foam molded article, and method for producing polyethylene resin foamed particles

A method for producing polyethylene resin foamed particles which have a Z-average molecular weight of not less than 40×104 to not more than 70×104, an average cell diameter of not less than 180 μm to not more than 450 μm, and an open-cell ratio of not more than 12%, the method comprising a first-stage foaming step of:

• (i) dispersing polyethylene resin particles to be foamed in an aqueous dispersion medium in a closed vessel together with a foaming agent, the polyethylene resin particles to be foamed including a polyethylene resin composition which contains, in an amount of not less than 1000 ppm to not more than 4000 ppm in total, compounds selected from the group consisting of antioxidant, metal stearate, and inorganic substance;
• (ii) pressurizing and heating up to or above a softening temperature of the polyethylene resin particles to be foamed;
• and
• (iii) releasing the polyethylene resin particles to be foamed into a zone whose pressure is lower than an internal pressure of the closed vessel,
• wherein the antioxidant includes a phosphorus-based antioxidant and a phenol-based antioxidant, an amount of the phosphorus-based antioxidant contained in the polyethylene resin composition is not less than 500 ppm to not more than 1500 ppm, and a ratio of the amount of the phosphorus-based antioxidant to an amount of the phenol-based antioxidant in the polyethylene resin composition (the amount of the phosphorus-based antioxidant/the amount of the phenol-based antioxidant) is not less than 2.0 to not more than 7.5, and
• wherein the inorganic substance is in an amount of not less than 300 ppm to not more than 2500 ppm.

A method for producing polyethylene resin foamed particles which have a Z-average molecular weight of not less than 40×104 to not more than 70×104, an average cell diameter of not less than 180 μm to not more than 450 μm, and an open-cell ratio of not more than 12%, the method comprising:

• (A) a first-stage foaming step of:
• (i) dispersing polyethylene resin particles to be foamed in an aqueous dispersion medium in a closed vessel together with carbon dioxide, the polyethylene resin particles to be foamed including a polyethylene resin composition which contains, in an amount of not less than 1000 ppm to not more than 4000 ppm in total, compounds selected from the group consisting of antioxidant, metal stearate, and inorganic substance,
• (ii) pressurizing and heating up to or above a softening temperature of the polyethylene resin particles to be foamed, and
• (iii) releasing the polyethylene resin particles to be foamed into a zone whose pressure is lower than an internal pressure of the closed vessel so as to obtain polyethylene resin foamed particles;
• and
• (B) a second-stage foaming step of:
• (i) putting the polyethylene resin foamed particles obtained in the first-stage foaming step into a pressure-resistant vessel,
• (ii) impregnating the polyethylene resin foamed particles with an inorganic gas containing at least one type of gas selected from the group consisting of air, nitrogen, and carbon dioxide to impart an internal pressure to the polyethylene resin foamed particles, and
• (iii) further foaming the polyethylene resin foamed particles through heating,
• wherein the antioxidant includes a phosphorus-based antioxidant and a phenol-based antioxidant, an amount of the phosphorus-based antioxidant contained in the polyethylene resin composition is not less than 500 ppm to not more than 1500 ppm, and a ratio of the amount of the phosphorus-based antioxidant to an amount of the phenol-based antioxidant in the polyethylene resin composition (the amount of the phosphorus-based antioxidant/the amount of the phenol-based antioxidant) is not less than 2.0 to not more than 7.5, and
• wherein the inorganic substance is in an amount of not less than 300 ppm to not more than 2500 ppm.