Expanded polypropylene resin beads, method of producing foam molding of expanded polypropylene resin beads and foam molding obtained by the method

Expanded beads comprising cell walls which define a plurality of cells and which comprise a base resin comprising a polypropylene resin,

• each of said beads having a through hole extending therein from one end to the other end,
• said beads having a maximum diameter D0 mm, an average cell diameter LCV in a radial direction, an average cell diameter LCH in a circumferential direction and a maximum through hole diameter HD mm, wherein D0, LCV, LCH and HD are as defined hereinafter and measured as described hereinafter, and
• wherein the ratio LCV/LCH is at least 1.05, the ratio HD/D0 is 0.08 to 0.4 and the ratio HD/LCV is 0.1 to 10;
• and
• wherein D0, LCV, LCH and HD are measured and defined as follows:
• (a) ten beads are arbitrarily sampled;
• (b) each sampled bead is cut by a plane perpendicular to a straight line connecting the one end and the other end of the through hole at the center of said straight line to give an annular cross-section having an outer circumference and an inner circumference;
• (c) the annular cross-section is displayed or photographed using a microscope;
• (d) on the cross-section thus displayed or photographed, a first straight line is drawn such that the first straight line intersects the outer circumference at two intersection points D01 and D02 and the inner circumference at two intersection points and that the distance Dn0 between the two intersection points D01 and D02 is the maximum diameter of the annular cross-section;
• (e) the distance Dn0 mm between the two intersection points D01 and D02 is measured;
• (f) a second straight line perpendicular to the first straight line is drawn such that the second straight line intersects the outer circumference at two intersection points d01 and d02 and the inner circumference at two intersection points and that the distance between the two intersection points d01 and d02 is maximum;
• (g) a circle C1 having a radius of Dn0/4 is drawn with its center at the intersection point P1 between the first and second straight lines;
• (h) a number NnH of intersection points between the circumference of the circle C1 and cell walls appearing on the displayed or photographed cross-section is counted;
• (i) two circles C2 and C3 each having a radius of Dn0/8 are drawn with their centers at the two intersection points P2 and P3, respectively, between the first straight line and the circle C1;
• (j) numbers of intersection points between the first straight line and cell walls located on the circle C2 and between the first straight line and cell walls located on the circle C3 are counted, and the greater of the two numbers Nnv is selected;
• (k) an average cell diameter LnCV is calculated by the following formula:
• LnCV=0.405×(Dn0/NnV);
• (l) an average cell diameter LnCH is calculated by the following formula:
• LnCH=0.810×Dn0×sin(π/NnH);
• (m) maximum diameter HnD mm is the greatest distance between two points on the inner circumference defining the through hole in the displayed or photographed cross-section is measured;
• (n) D0 is a maximum diameter calculated as an arithmetic mean of Dn0 of the ten sampled beads;
• (o) LCV is an average cell diameter calculated as an arithmetic mean of LnCV of the ten sampled beads;
• (p) LCH is an average cell diameter calculated as an arithmetic mean of LnCH of the ten sampled beads;
• and
• (q) HD is a maximum through hole diameter calculated as an arithmetic mean of HnD of the ten sampled beads.