from 0.1 to 10% by {TAIL ~ weight} of a compound of formula : ##STR3## wherein the individual radicals R are identical are each {HEAD ~ a C1 C12 alkyl group} or a polyoxyethylene radical which contains 1 to 6 ethylene units and whose terminal OH group is methylated; and 
from 0.1 to 10 % by { TAIL ~ weight } of a compound of formula : # # STR3 # # wherein the individual radicals R are identical are each { HEAD ~ a C1 C12 alkyl group } or a polyoxyethylene radical which contains 1 to 6 ethylene units and whose terminal OH group is methylated ; and
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH