How Heat Affects Human Hair: Thermal Characterization and Predictive Modeling of Flat Ironing Results
Many people with curly hair experience heat damage—loss of curls and structural degradation of hair—after repetitive use of flat irons. While an array of relevant studies provide insight into thermochemical processes behind the phenomenon, practical tools for flat iron users are unavailable. As a result, people shun heat for fear of unpredictable amount of heat damage while adopting other laborious methods to satisfy a persevering need for temporary hair straightening. Thus three overarching research projects emerge to address the problem. In Part 1, I develop an empirical approach to mathematically correlate four flat ironing parameters (a temperature setting, gliding speed, the number of passes, and exposure time) with three metrics of flat ironing results (reduction in fatigue strength, straightening efficacy, and permanent curl loss). The objective is to establish user-friendly predictive models for flat ironing results to help users make informed decisions. Hair samples are exposed to various flat ironing conditions to evaluate the impact of each parameter thereby formulating predictive models. In the subsequent study, the impact of heat protectants on the flat ironing results is exclusively investigated to provide insight into better utilizing the widely marketed products for protecting hair from heat damage. In Part 2, thermal characterization of human hair and heat transfer modeling serve as a practical tool for predicting the amount of heat damage due to flat ironing in conjunction with the previously developed predictive models. To measure thermal diffusivity of hair, I develop and validate a non-contact infrared thermography measurement technique based on the Angstrom Method. Then, these properties are integrated into a 2D heat transfer model of the thermal transport between a hair bundle and flat iron utilizing the finite difference method. Experimental validation of the model follows to complete the overarching goal of providing practical tools for decision making before flat ironing. This work provides a practical tool that assists flat iron users in making decisions regarding the use of flat irons. It also introduces novel empirical and modeling approaches for understanding the effects of flat ironing. Furthermore, it presents a novel measurement technique for thermal characterization of polymer fibers.
Marconnet, Purdue University.
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