Human scalp hair is exposed to water for a significant portion of its lifetime. Acting as an ion-exchange resin, hair can extract cations of water hardness metals, calcium and magnesium, from the tap water used during hygiene practices and chemical treatments. The characteristics and consequences of the interaction between water hardness metals and inert substrates such as pipes and bathroom fixtures are well understood and recognized. However, less work has been conducted to elucidate the nature of the interaction between water hardness metals and human hair, a chemically reactive substrate. Thus, we endeavored to investigate this interaction, and our research efforts and findings are summarized in this dissertation.
First, the variables that influence the interaction between water hardness metals and human hair were examined. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was employed to quantify the metal content of hair, which was studied as a function of the following variables: hair condition (oxidative damage), level of water hardness, and water pH. It was found that the condition of the hair, a key representation of the binding capacity (available anionic sites), was most influential. Water hardness level had a smaller effect than one would expect; hair became saturated with notable amounts of water hardness metals even after repeated exposure to soft water depending on the level of damage. Water pH was positively correlated with metal uptake. A pilot study was conducted to determine the strength of the laboratory data vs. human subject data. The study consisted of thirty-five women who were grouped based on their residential water hardness (soft, hard, or very hard) and hair type (virgin, colored/bleached). The data from this study agreed with the laboratory experiments, and also provided information on consumer perception of water hardness effects on hair.
Next, the hair structural implications of water hardness metal uptake were explored. Hair mechanics and styling were examined by technical measures of single fiber torsional and tensile properties, combability, and style retention as a function of the calcium and magnesium content of virgin and bleached hair. Fiber stiffening was induced by the presence of water hardness metals in the fibers of both virgin and bleached hair, and this was accompanied by a reduction in combing forces. The long-term style retention of virgin hair was significantly improved by water hardness metals, while that of bleached hair was directionally reduced.
Finally, the key technical approaches to the management of water hardness metal uptake, chelation and acidic treatment, were evaluated. This work was completed through the testing of three commercial products that utilized one or both of these approaches and acidic surfactant solutions with and without a chelant. It was concluded that the most effective way to control the water hardness metal uptake was to first reduce the levels in hair with an extended contact, rinse-off product, and then use a shampoo that can either maintain the reduced levels or slow down uptake during subsequent wash cycles. Both products should be acidic (pH < 6) and contain chelants.