HAIR DAMAGE 281 Z¸

282 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS densation, is claimed to produce polymer deposits that improve hair settability, al- though no numerical data regarding the extent of grafting were quoted (82). The low- molecular-weight system comprising resorcinol and heximinium salts impregnates untreated hair, resulting in 5-15% add-ons which reduce fiber moisture uptake by a few percent (81). Peroxides or hydroperoxides were found to be particularly effective as initiators for vinyl polymerization, mainly due to the formation of redox couples with thiol groups (72-80). Analysis of the literature on grafting of vinyl monomers on wool reveals the successful application of both chemical and radiation methods of initiation (71). Transi- tion and non-transition metal ions in their higher valence states have also been used as initiators (71). Generally, deposition of synthetic polymers results in small changes in the mechanical properties of hair. The process of reduction results in weakening of the fibers, as re- flected by the measurements of the wet extension properties (about 15% decrease in work to 20% extension, elastic limit, or force to elastic limit after mild TGA reduc- tion), and impregnation with polymers affords insignificant recovery of the tensile properties (75). Only for heavily weakened fibers, after extensive bleaching, consider- able increase in tensile modulus was reported (77). In contrast to this, remarkable enhancement of the mechanical properties of reduced and bleached hair was noted after polymerizing vinyl monomers within the fiber and subsequently reacting them with metal salts (75). Improvement of wet-load extension properties in the range of 16-48% was observed after the deposition of poly(dimethylaminoethylmethacrylate) (PDEM), polyacrylonitrile, or poly(ethylene glycol monomethacrylate), followed by the complex- ation with mercuric acetate. Relatively small effects were brought out by the same treatment to carboxylic acid-containing hair. The amelioration in the wet-load exten- sion properties of PDEM-containing hair appears to be related to the amount of syn- thetic polymer in the fibers. Other metals, e.g., Cu(II) and Ni(II) were found to pro- duce similar, albeit smaller effects. An increase in cross-sectional area (80), split-end mending (80), water sorption reduc- tion, and improvement of the curl retention (81,82) are quoted as advantages for hair fiber modification by polymer deposition. These are, however, sometimes offset by deleterious coating of the fiber surface, negatively affecting both dry and wet comb- ability, especially at high polymer uptakes (80). 3. SURFACE POLYMERIZATION AND DIRECT APPLICATION OF SURFACE POLYMER FILMS Interfacial polymerization can be used to deposit linear polyamides, polyurethanes, or polyureas on the surface of wool fibers (84). The modification protocol calls for impreg- nating wool with an aqueous solution of diamine and then immersing it in a solution of a diacyl chloride, bischloroformate, or diisocyanate in an inert organic solvent. This procedure results in the formation of polymer at the boundary of the solvents, that is, at the fiber surface. In an alternative process, pre-formed polymers are applied directly to the fiber surface to form a shrink-resistant coating. These polymer systems usually contain reactive groups such as carboxylic acid chloride, azetidinium, thiol, isocyanate, carbamoylsulfonate, N-methylolamide, or silanol, which can form both polymer- polymer or polymer-fiber crosslinks.