HAIR DAMAGE 283 In the case of hair, most of these procedures or compositions are not functional since they unacceptably refashion the fiber surface. One viable route avoiding deleterious alterations of the fiber surface was delineated in the patent literature (85). It involves the treatment of weakened hair with reactive silanes such as alkyltrialkoxysilanes in the presence of alcohol and water (0-20%). A preliminary account of silylation with hexa- methyldisilazane was also reported to enhance the stability of straightened hair (86). Reactive trialkoxysilane hydrolyzes in the presence of water to form reactive silanol groups which in turn condense with each other or with the reactive groups on the fiber surface to form a polymeric coating. The process of application is completed by blow drying the hair with hot air. Tensile strength (the force required to break hair) and silicon content in hair were ascertained for hair samples damaged in the process of relaxation by alkali and reducing agents. Control virgin hair is characterized by a tensile strength of 29.5 --- 0.6 G/cross-section, and for alkali-damaged hair this value de- creases to 14.5 + 1.1 G/cross-section. The data exemplified in the patent show a 15-20% increase in the tensile strength of alkali-damaged hair samples after treatment with ethyltriethoxysilane to the value of 14.5-17 --- 0.7 G/cross-section. It was also claimed that treatment with octadecyltrimethoxysilane, 3-aminopropyltriethoxysilane, vinyltriethoxysilane, and N-(triethoxysilylpropyl)urea imparted good settability and curl retention to damaged hair tresses even after post-treatment shampooings. CONCLUSIONS Owing to its unique composition and structure, human hair displays greater resistance to mechanical damage while dry than in the wet state. Structural integrity is derived mainly from the high density of disulfide bridges between cystine components of the protein. Rupture of these crosslinks, attained in cosmetic treatments by reaction with mercaptans, alkalis, or oxidation by hydrogen peroxide leads to weakening of the wet fibers. Matrix-dependent properties such as the torsional modulus, moisture sorption, and swelling are affected to a larger extent than microfibril-determined, longitudinal characteristics. Environmental agents, such as UV irradiation, also contribute to a gradual loss of the original fiber strength and cohesion. Except for reduction with mercaptans, which can be partially remedied by reoxidation, inflicted damage is irre- versible and results in increased swelling, brittleness, and susceptibility to destruction, even during normal handling operations. Surface despoliation, by mechanical abrasion in the wet state or by the chemical action of chlorine, bleaching, or dyeing composi- tions, is an important aspect of hair destruction, resulting in an increase in frictional coefficients and, consequently, combing forces. Efforts to restore the original properties of hair after mild degradation or to protect undamaged hair against structural weakening have been numerous but largely unsuc- cessful. Severely impaired structure, such as from bleaching or relaxation, is probably easier to mend in the sense that any improvement in strength or swellability should be more perceptible in terms of tactile properties, manageability, or settability. Polymer deposition or grafting, in spite of its limitations and side effects such as surface modifi- cation, still has some potential in this respect, provided that new monomer or initiating systems are discovered.

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