"Polymers usable for the balloon of the present invention are, for example, polyethylene terephthalate, a polyester (ethylene terephthalate copolymer) obtained from a principal acid or a principal glycol, a mixture of polyethylene terephthalate and a polyester other than polyethylene terephthalate, a mixture of polyethylene terephthalate and polyolefin (for example, polyethylene and polypropylene)," . . . . . . .