in which R1 and R2, independently of one another, are selected from linear or branched C4-C20 alkyl radicals, linear or branched C4-C20 alkenyl radicals, and linear or branched C4-C20 hydroxyalkyl radicals R3, R4, R5 and R6, independently of one another, are selected from hydrogen, hydroxyl, C1-C4 alkyl radicals, C1-C4 alkenyl radicals, C1-C4 alkoxy radicals, and C1-C4 hydroxyalkyl radicals.