HAIR DAMAGE 277 tween protein chains. Exposure of hair fibers to formaldehyde vapor resulted in 20øC upscale shifts in low and high temperature transitions as detected by TMA and DTA (21). The stiffening of formaldehyde-treated hair is also reflected in a small increase of W2o from 350 to 389 at 25øC (11). The plot of the hysteresis ratio versus temperature for formaldehyde-treated fibers showed no change from that of control fibers. Feugh- elman and Watt also observed only a moderate increase in the viscosity ratio (wet/dry) in torsional measurements of formaldehyde-modified wool (68). 2. POLYMERIZATION OR POLYCONDENSATION OF MONOMERS WITHIN THE KERATIN FIBERS Pioneering work on the deposition of polymers inside the structure of keratin fibers was done by Speakman (69,70). The field has been extensively explored in recent years, especially in relation to wool fibers, and the use of a variety of monomers, initiating systems, and solvents has been described. Several reviews and a large number of papers are available on this subject (71). Table I lists examples of systems applied for graft copolymerization or polymer deposi- tion in hair. It was established that the process of deposition is diffusion-controlled (73) and, consequently, to enhance the accessibility of the fiber to initiators and monomers the first step of the modification procedure is the reduction of cystine crosslinks (72). This is especially important in the case of hair keratin, where the high density of cystine crosslinks considerably impedes the diffusion of reagents. Reduction is usually per- formed by the use of thioglycolic acid (73-75), bisulfite, or tetrakis(hydroxymeth- yl)phosphonium chloride (72), which can also act as an effective oxygen scavenger in persulfate-initiated vinyl polymerizations. Under acidic conditions, tris(hydroxy- methyl)phosphine, the product of THPC dissociation, reacts readily with hydrogen peroxide or dissolved oxygen but was reported to be inert towards persulfates. Disulfide reduction produces two cysteine residues which can be utilized to form a redox couple with suitable oxidizing agents such as the persulfate ion. A further advantage of THPC is that this reagent, unlike bisulfite or TGA, should not interfere with the process of initiation by the formation of a redox system. For polymerizing into bleached hair, the amount of graft copolymer increases with the degree of bleaching. Since bleaching destroys crosslinks inside the hair, it opens up the hair structure, and the enhanced diffusion augments the overall grafting yield (76,80). Only a limited amount of monomers and initiating systems have been tested in graft copolymerizations involving hair fibers (Table I). Methacrylamide, salts of acrylic and methacrylic acid, were shown to be deposited with excellent yield (20-80% add-on) without appreciable, undesired modification of the fiber surface properties (72,76,77). Nonionic N-ethylmethacrylamide and N-vinylpyrrolidone, as well as cationic methacryl- amidopropyltrimethylammonium chloride, were shown to produce only modest gains (4-5%) in fiber weight (83). Contradictory reports exist on the deposition of amine- containing toohomers N,N-dimethylaminoethylmethacrylate and N,N-dimethyl- aminopropylmethacrylamide. The former was shown to produce 20.9% add-ohs (74,75), while the latter formed only 4-8% deposits in reduced fibers (83). It was suggested that both steric and electrostatic factors might determine the rate and extent of the deposition, although this interpretation does not find consistent support in the existing data (83). Application of an aqueous solution containing an oligomeric precon- densate of glyceraldehyde and resorcinol, under conditions promoting in situ polycon-

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