Application of single hair fibre tensile measurements to create an industry-standard heat damage scale for hair.

The use of heat styling appliances is a rapidly growing segment within the hair care market. However, despite significant levels of academic investigation into the impact of high temperature on the structure and mechanical properties of the hair fibre, the industry does not yet have a standardized method of heat damage measurement to enable consumers and independent evaluation laboratories to objectively compare the performance of different heat styling tools. We discuss the available experimental methods to support such a standardized method and conclude that the well-established approach of measuring single-fibre tensile strength provides the best option. We then explore the sensitivity to heat damage of the various wet and dry single-fibre tensile parameters in order to identify the optimal combination for inclusion in our hair damage metric. Using the selected combination of 8 different wet single-fibre tensile strength parameters, we now propose a robust, reproducible and consumer-relevant 6-point heat damage scale. We demonstrate the relevance of the new damage scale to a range of on-market heated hair straighteners and explore the human perception of hair treated with heated straighteners delivering different damage levels according to the new scale. Based on this, we show a good correlation between the technical damage scale and the human perception of damage on hair tresses.