Method for the preparation of an air-permeable plug member

A method for the preparation of an air-permeable plug member having a cellular structure of a foam rubbery material consisting of a closed-cell structure and an open-cell structure which comprises the steps of:

• a) shaping a foamed body of a rubbery material having a closed-cell structure only, the diameter of the closed-cells being the range from 0.03 to 1 mm;
• and
• b) subjecting the foamed rubber body obtained from step a) to a foam-breaking treatment so as to break the walls of a part of the closed cells to create an apparent open cell ratio in the range from 20% to 90% by passing the foamed rubber body through a gap between a pair of rollers or between two oppositely facing pressure plates, the width of the gap being the range from 10% to 50% of the thickness or diameter of the foamed rubber body having the closed-cell structure, wherein the peripheral velocity of the rollers and pressure plates is in the range from 3.0 to 70 meters per minute.