Chapter 7 233 Table 2. Effect of treatment on W20 and breaking stress *result statistically significant at p0.05 Temperature % Change W20 % Change Break Stress 60°C -0.8 -2.03 115°C -0.5 -5.4 130°C +3.1 -7.4* 160°C +5.2* -10.2* Table 3. Breaking stress for Asian, African American and relaxer- treated African American hair after five-minute heat treatment Hair Type 25°C 115°C 130°C 160°C Asian 2.07 ± 1.0 1.96 ± 1.1 1.92 ± 0.6* 1.86 ± 0.3* African American (AA) 0.87 ± 0.4 0.79 ± 0.2* 0.79 ± 0.2* 0.77 ± 0.2* AA Relaxed 0.54 ± 0.9 0.52 ± 0.4 0.50 ± 0.2 0.49 ± 0.2* Units on breaking stress are 10-1 GPa * Result of heat significant at p 0.05 As can be seen, at 25°C African American hair had less than half the tensile strength of Asian hair and relaxer treatment further reduced breaking stress by ~40% leading to hair with approximately one-quarter of the breaking stress of virgin Asian hair. Virgin African American hair was slightly more sensitive to heat treatment at 115°C and 130°C but relaxer treatment did not lead to additional heat sensitivity. For more information on ethnic differences in hair see Chapter 6. The data discussed above indicate that the various parameters that can be extracted from stress/strain curves may respond differently to differing damaging treatments. Heat and permanent waving treatments affected post-yield slope and breaking stress more dramatically than yield stress, while oxidative treatment had equivalent effects on these parameters. Hair damage and its assessment are discussed further in Chapter 11 of this book.