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Expanded, substantially non-crosslinked polypropylene resin beads capable of producing a high rigidity foamed molding at a relatively low temperature. The beads are produced by a process including a step of dispersing substantially non-crosslinked polypropylene resin particles in a dispersing medium

Expanded, substantially non-crosslinked polypropylene resin beads capable of producing a high rigidity foamed molding at a relatively low temperature. The beads are produced by a process including a step of dispersing substantially non-crosslinked polypropylene resin particles in a dispersing medium containing an organic peroxide to obtain a dispersion, a step of heating the dispersion to decompose the organic peroxide and to modify the surface of the surface-modified polypropylene resin particles, and a step of expanding the non-crosslinked, surface-modified polypropylene resin particles using a blowing agent.

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1. A process for the preparation of expanded polypropylene resin beads, comprising the steps of:(a) dispersing substantially non-crosslinked polypropylene resin particles in a dispersing medium containing an organic peroxide to obtain a dispersion; (b) maintaining said dispersion at a temperature lo

1. A process for the preparation of expanded polypropylene resin beads, comprising the steps of:(a) dispersing substantially non-crosslinked polypropylene resin particles in a dispersing medium containing an organic peroxide to obtain a dispersion; (b) maintaining said dispersion at a temperature lower than the melting point of said polypropylene resin but sufficient to decompose said organic peroxide, thereby obtaining substantially non-crosslinked, surface-modified polypropylene resin particles; and (c) expanding said non-crosslinked, surface-modified polypropylene resin particles using a blowing agent to obtain expanded, substantially non-crosslinked polypropylene resin beads. 2. A process as claimed in claim 1, wherein, in step (b), said dispersion is maintained at a temperature not lower than the glass transition point but not higher than the Vicat softening point of said polypropylene resin.3. A process as claimed in claim 1, wherein said blowing agent is a physical blowing agent.4. A process as claimed in claim 3, wherein said physical blowing agent comprises at least one inorganic blowing agent selected from the group consisting of nitrogen, oxygen, carbon dioxide and water.5. A process as claimed in claim 1, wherein step (c) is performed so that the expanded polypropylene resin beads have an apparent density of 10 g/L to 500 g/L and a high temperature endothermic peak, in a DSC curve thereof, in addition to an intrinsic endothermic peak located at a lower temperature side of said high temperature peak.6. A process as claimed in claim 5, wherein said high temperature endothermic peak has an area corresponding to a calorific value in the range of 2-70 J/g.7. A process as claimed in claim 1, wherein the expanded polypropylene resin beads have a MFR value which is not smaller than that of the non-crosslinked polypropylene resin particles before step (b) and which is in the range of 0.5-150 g/10 min.8. A process as claimed in claim 1, wherein a surface region of the expanded polypropylene resin bead has a melting point lower than that of an inside region thereof.9. A process as claimed in claim 1, wherein each of said expanded polypropylene resin beads has a surface region and an inside region, wherein each of said surface and inside regions shows a high temperature endothermic peak, in a DSC curve thereof, in addition to an intrinsic endothermic peak located at a lower temperature side of said high temperature peak, and wherein said high temperature endothermic peaks of said surface region and said inside region have areas that correspond to calorific values of Hs and Hi, respectively, and wherein Hs and Hi have the following relationship:Hs<0.86×Hi 10. A process as claimed in claim 1, wherein said organic peroxide generates oxygen radicals when decomposed.11. A process as claimed in claim 1, wherein said organic peroxide is a substance half the amount of which decomposes when maintained for 1 hour at a temperature Th and wherein Th is not lower than the glass transition point but not higher than the Vicat softening point of said polypropylene resin.12. A process as claimed in claim 10, wherein said organic peroxide is a carbonate.13. A process as claimed in claim 1 wherein said polypropylene resin particles dispersed in step (a) and said expanded polypropylene resin beads have a degree of crosslinking of 5% or less.14. A process as claimed in claim 1 wherein said polypropylene resin particles dispersed in step (a) and said expanded polypropylene resin beads have a degree of crosslinking of 3% or less.15. A process as claimed in claim 1 wherein said polypropylene resin particles dispersed in step (a) and said expanded polypropylene resin beads have a degree of crosslinking of 1% or less.16. A process as claimed in claim 8 wherein the melting point of the surface region is at least 0.3° C. lower than the melting point of the inside region.17. A process as claimed in claim 16 wherein said polypropylene resin particles dispersed in step (a) and said expanded polypropylene resin beads have a degree of crosslinking of 5% or less.18. A process as claimed in claim 1 wherein the surface-modified propylene resin particles have a surface region and an interior region and wherein the surface region has a greater oxygen content per unit weight than that of the interior region.19. A process as claimed in claim 8 wherein the surface region has a greater oxygen content per unit weight than the inside region.20. A process as claimed in claim 16 wherein the surface region has a greater oxygen content per unit weight than the inside region.21. A process as claimed in claim 1 wherein the decomposed organic peroxide attacks only a surface region of the polypropylene resin particles but not an inside region of the polypropylene particles.22. A process for the preparation of expanded resin beads, comprising the steps of:(a) dispersing substantially non-crosslinked particles of a base resin including a polypropylene resin in a dispersing medium containing an organic peroxide to obtain a dispersion; (b) maintaining said dispersion at a temperature lower than the melting point of said base resin but sufficient to decompose said organic peroxide, thereby obtaining substantially non-crosslinked, surface-modified resin particles; and (c) expanding said non-crosslinked, surface-modified resin particles using a blowing agent to obtain expanded, substantially non-crosslinked resin beads. 23. A process as claimed in claim 22, wherein, in step (b), said dispersion is maintained at a temperature not lower than the glass transition point but not higher than the Vicat softening point of said base resin.24. A process as claimed in claim 22, wherein said blowing agent is a physical blowing agent.25. A process as claimed in claim 24, wherein said physical blowing agent comprises at least one inorganic blowing agent selected from the group consisting of nitrogen, oxygen, carbon dioxide and water.26. A process as claimed in claim 22, wherein step (c) is performed so that the expanded resin beads have an apparent density of 10 g/L to 500 g/L and a high temperature endothermic peak, in a DSC curve thereof, in addition to an intrinsic endothermic peak located at a lower temperature side of said high temperature peak.27. A process as claimed in claim 26, wherein said high temperature endothermic peak has an area corresponding to a calorific value in the range of 2-70 J/g.28. A process as claimed in claim 22, wherein the expanded resin beads have an MFR value which is not smaller than that of the non-crosslinked resin particles before step (b) and which is in the range of 0.5-150 g/10 min.29. A process as claimed in claim 22, wherein a surface region of the expanded resin bead has a melting point lower than that of an inside region thereof.30. A process as claimed in claim 22, wherein each of said expanded resin beads has a surface region and an inside region, wherein each of said surface and inside regions shows a high temperature endothermic peak, in a DSC curve thereof, in addition to an intrinsic endothermic peak located at a lower temperature side of said high temperature peak, and wherein said high temperature endothermic peaks of said surface region and said inside region have areas that correspond to calorific values of Hs and Hi, respectively, and wherein Hs and Hi have the following relationship:Hs<0.86×Hi 31. A process as claimed in claim 22, wherein said organic peroxide generates oxygen radicals when decomposed.32. A process as claimed in claim 22, wherein said organic peroxide is a substance half the amount of which decomposes when maintained for 1 hour at a temperature Th and wherein Th is not lower than the glass transition point but not higher than the Vicat softening point of said base resin.33. A process as claimed in claim 31, wherein said organic peroxide is a carbonate.34. A process as claimed in claim 22 wherein said base resin particles dispersed in step (a) and said expanded resin beads have a degree of crosslinking of 5% or less.35. A process claimed in claim 22, wherein said base resin particles dispersed in step (a) and said expanded resin beads have a degree of crosslinking of 3% or less.36. A process as claimed in claim 22, wherein said base resin particles dispersed in step (a) and said expanded resin beads have a degree of crosslinking of 1% or less.37. A process as claimed in claim 29 wherein the melting point of the surface region is at least 0.3° C. lower than the melting point of the inside region.38. A process as claimed in claim 37 wherein said base resin particles dispersed in step (a) and said expanded resin beads have a degree of crosslinking of 5% or less.39. A process as claimed in claim 22 wherein the surface-modified resin particles have a surface region and an interior region and wherein the surface region has a greater oxygen content per unit weight than that of the interior region.40. A process as claimed in claim 29 wherein the surface region has a greater oxygen content per unit weight than the inside region.41. A process as claimed in claim 37 wherein the surface region has a greater oxygen content per unit weight than the inside region.42. A process as claimed in claim 22, wherein the decomposed organic peroxide attacks only a surface region of the base resin particles but not an inside region of the base resin particles.