"A process according to claim 1, wherein B is of the formula ##STR2## in which PE is the single-bond unit CH2 CH2 --CH2 O(EO)m Z,Z is hydrogen or a C1-6 alkyl radical, EO is an ethylene oxide unit, and x, y, and z each independently is an integer of at least one, the values of x,y and z being interrelated so that B has a maximum molecular weight of 4,000. 6." . . . .