Abstract
For the purpose of improving the coloring ability of keratin fibers at a lower temperature, we investigated the influence of urea on the coloring ability of a low‐temperature coloring method using polyethyleneimine (PEI) as a counter ion reagent (the human hair was previously treated with a PEI solution, and then was colored with acid dye). The coloring and color fastness to shampooing of the hair pretreated with a PEI solution containing urea clearly improved compared with those pretreated with a PEI solution not containing urea. Also, we prepared cross‐sectional samples of the treated hair and investigated the penetration of PEI and Orange II into human hair by optical microscopy. The results showed that the penetration of PEI and Orange II into human hair pretreated with a PEI solution that contained urea clearly increased compared with those pretreated with a PEI solution that did not contain urea. From these experiments, we concluded that urea acts as a penetration accelerator for PEI; and PEI, which penetrates deeper into human hair by adding urea, exerts counter ionization on Orange II, thus increasing the penetration of Orange II into human hair and thereby improving the coloring ability. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3827–3834, 2004