Penetration of different molecular weight hydrolysed keratins into hair fibres and their effects on the physical properties of textured hair

OBJECTIVE: To investigate the effects of different molecular weight (MW), wool derived hydrolysed keratins (i.e. peptides) on the physical properties of relaxed textured hair. METHODS: Very curly hair of African origin was relaxed using sodium hydroxide‐based treatment. Relaxed hair was treated with different MW peptides derived from keratin protein and an amino acid, L‐Leucine. The low‐MW keratin peptides were 221 Da, the mid‐MW keratin peptides were approximately 2577 Da, and the high‐MW keratin peptides were approximately 75 440 Da. The penetration of these different peptides into relaxed hair was evaluated using a laser scanning micrometre and by fluorescence microscopy. The effect of these compounds on single‐fibre mechanical properties and thermal properties was evaluated using tensile and DSC testing, respectively. RESULTS: Low‐ and mid‐MW compounds were able to penetrate deep into the hair cortex. High‐MW peptide adsorbed onto the hair surface and possibly slightly penetrated into the outer layers of the fibre surface. Both mid‐ and high‐MW keratin peptides, increased Young’s modulus and reduced hair breakage at 20% and 80% relative humidity. With the exception of mid‐MW peptide, other peptides and amino acid were not able to modify thermal properties of relaxed textured hair. CONCLUSIONS: Our data suggest that low‐MW compounds may increase hair volume, and high‐MW peptides may repair damage on freshly relaxed textured hair.