A vehicle seat core material includes a molded article that includes thermoplastic resin expanded beads. The molded article has a substantially rectangular shape in a top view, and has a front side and a rear side opposite the front side. The molded article has an average density (Z) of 20 kg/m.sup.3 to 50 kg/m.sup.3. The molded article includes, on the rear side of the molded article, a rear thin portion having a thickness of 10 mm to 40 mm, the thickness of the rear thin portion being smaller than an average thickness of the molded article. A ratio (Y/Z) of a density (Y) of the rear thin portion to the average density (Z) of the molded article is 1.05 to 3. The rear thin portion of the molded article has a fusion rate of 70% or more.

A vehicle seat core material includes a molded article that includes thermoplastic resin expanded beads. The molded article has a substantially rectangular shape in a top view, and has a front side and a rear side opposite the front side. The molded article has an average density (Z) of 20 kg/m.sup.3 to 50 kg/m.sup.3. The molded article includes, on the rear side of the molded article, a rear thin portion having a thickness of 10 mm to 40 mm, the thickness of the rear thin portion being smaller than an average thickness of the molded article. A ratio (Y/Z) of a density (Y) of the rear thin portion to the average density (Z) of the molded article is 1.05 to 3. The rear thin portion of the molded article has a fusion rate of 70% or more.

An expandable thermoplastic resin particle including a polymer obtained by a process including polymerizing monomers including 60 to 80 parts by weight of α-methylstyrene and 40 to 20 parts by weight of acrylonitrile. A content of residual monomeric components in the expandable thermoplastic resin particle is not more than 0.5% by weight. When an expanded mold article is produced from the expandable thermoplastic resin particle at an expansion ratio of 10 times, foam cells in a cross section of the expanded mold article have an average chord length of from 20 μm to 60 μm.

A process for forming voided latex particles is improved by combining swelling and polymerization of an outer shell into a single step. The process includes contacting multi-stage emulsion polymer particles comprising a core, at least one intermediate shell, with a swelling agent, and polymerizing an outer shell after said contacting with swelling agent wherein the core and the at least one intermediate shell are contacted with swelling agent in the presence of less than 0.5% monomer based on the weight of the multi-stage emulsion polymer particles, and substantially all of the swelling occurs during polymerization of the outer shell.

A process for forming voided latex particles is improved by combining swelling and polymerization of an outer shell into a single step. The process includes contacting multi-stage emulsion polymer particles comprising a core, at least one intermediate shell, with a swelling agent, and polymerizing an outer shell after said contacting with swelling agent wherein the core and the at least one intermediate shell are contacted with swelling agent in the presence of less than 0.5% monomer based on the weight of the multi-stage emulsion polymer particles, and substantially all of the swelling occurs during polymerization of the outer shell.

A modified expanded bead that includes an expanded bead which includes a specific base polymer (β) and a coating layer which includes a specific base polymer (α). The coating layer covers at least part of an outer surface of the expanded bead. The melting point Tα of the base polymer (α) and the melting point Tβ of the base polymer (β) satisfy a relationship of 0≤Tα−Tβ≤20. When the modified expanded bead is cut into two halves along a cutting plane to form two half portions and thereafter one of the two half portions is cut along planes perpendicular to the cutting plane to form ten small pieces, outermost pieces among the ten small pieces have a xylene-insoluble content (B) which is less than a xylene-insoluble content (A) in the half portion and the xylene-insoluble content (A) is 10-80% by mass.

A modified expanded bead that includes an expanded bead which includes a specific base polymer (β) and a coating layer which includes a specific base polymer (α). The coating layer covers at least part of an outer surface of the expanded bead. The melting point Tα of the base polymer (α) and the melting point Tβ of the base polymer (β) satisfy a relationship of 0≤Tα−Tβ≤20. When the modified expanded bead is cut into two halves along a cutting plane to form two half portions and thereafter one of the two half portions is cut along planes perpendicular to the cutting plane to form ten small pieces, outermost pieces among the ten small pieces have a xylene-insoluble content (B) which is less than a xylene-insoluble content (A) in the half portion and the xylene-insoluble content (A) is 10-80% by mass.

An expanded polypropylene resin particle is obtained from a base material resin having a melting point of 140° C. to 150° C., wherein the base material resin includes a polypropylene resin A including 3 weight % to 15 weight % of 1-butene and having a melting point of 130° C. to 140° C.; and a polypropylene resin B having a melting point of 145° C. to 165° C., and wherein the expanded polypropylene resin particle has an average cell diameter of 100 μm to 340 μm.

An expanded bead having a tubular shape with a through hole and a method for producing the same are provided. The expanded bead includes a foamed core layer and a covering layer. A polyolefin-based resin included in the covering layer has a melting point lower than a melting point of a polypropylene-based resin included in the foamed core layer. An average hole diameter d of the through hole of the expanded bead is less than 1 mm, and a ratio d/D of the average hole diameter d to an average outer diameter D of the expanded bead is 0.4 or less. The polypropylene-based resin for the foamed core layer has a flexural modulus of 1,200 MPa or more and a melting point of 158° C. or lower.

The present invention relates to methods for the welding of foam particles, by electromagnetic waves, wherein foam particles with a surface modification are welded in a molding tool by electromagnetic waves, wherein the surface modification is produced by contacting the foam particles with a solution containing polymers which are capable of forming intra- or inter-molecular covalent bonds, under the conditions required for this purpose.