research New Method of Dyeing Keratin Fibers Using Poly(Ethylene Imine) and Its Coloring Mechanism 6 citations, October 2003 in “Journal of applied polymer science” A new method using poly(ethylene imine) improves hair dyeing at lower temperatures with better color retention.
research Influence of Urea on the Coloring Ability of a Low-Temperature Coloring Method of Keratin Fibers Using Polyethyleneimine 5 citations, January 2004 in “Journal of applied polymer science” Urea helps hair dye penetrate better and last longer.