Study On The Nanotribological Properties Of Stretched Keratin Fibers

Friction property is a very important aspect of surface properties of the fibers, which not only has influence on the fiber processing, but also effects the handle of the products made from it. In addition, friction also causes abrasion and deformation of fibers, resulting in mass transfer, heat and electrostatic phenomena. Because of the special surface scales structure of the keratin fibers, the friction performances of the fibers show some particular features. With the rapid development of nanotechnology and a series of scanning electron microscope, nanotribology becomes one of the hottest areas of tribology. It can reveal the mechanisms of friction, abrasion and lubrication from the atomic and molecular scale, and establish structural relationships and quantitative criteria between microstructure and macroscopic properties of materials. On one hand, the investigation of nanoscale friction for keratin fibers is able to help further understand the phenomenon and mechanism of fiber friction and improve fiber friction theories. On the other hand, it contributes to provide theoretical guidance to the actual production and further development of keratin fibers.In this paper, surface morphology changes of keratin fibers after stretching treatment are studied by two-dimensional, three-dimensional images and surface roughness using atomic force microscope. The results show that when the keratin fibers are stretched, the scales are obviously elongated and scale edges become blurred. Furthermore, the scales appear cracking and fracture damages in high stretching ratios. With the increase of stretching ratios, the surface roughness is nonlinear change. When the Caucasian hair is stretched100%, the surface roughness is the largest. However, for Asians hair and alpaca fiber, when the stretching ratios reach40%, the surface roughness is the largest.The nanoscale and macroscale friction properties of keratin fibers are researched and compared. The results indicate that for Caucasian hair, when the stretching ratio is20%, the nanoscale coefficient of friction is0.038, which is the largest in all stretching treatment samples. However, when the stretching ratio reaches40%, the nanoscale coefficient of friction is0.012, which is the smallest in all samples. When the Asian hair is stretched20%, the nanoscale coefficient of friction is0.008, which is the smallest in all samples, but when Asian hair is stretched40%, the nanoscale coefficient of friction is0.033, which is the largest in all samples. Moreover, when the stretching ratio is less than60%, the nanoscale coefficient of friction has larger fluctuations and opposite changes with the increase of stretching ratios. While, when the stretching ratio is more than60%, it is observed that the nanoscale coefficient of friction has the same changes with the increase of stretching ratios. The nanoscale coefficient of friction for alpaca fiber is larger than two human hairs and gradually increases with the increase of stretching ratios. When the alpaca fiber is stretched60%, the coefficient of friction reaches0.056. It is important to note that the macroscale coefficient of friction is distinct larger than nanoscale. The results demonstrate that the friction of fibers has scale effects. In addition, the directionality dependence of friction is discussed in macroscale and nanoscale. It is found that the mechanisms are different in macroscale and nanoscale. Directionality dependence effect of macroscale friction attributes to the overall orientation of the surface scales. However, the directionality dependence effect of nanoscale friction mainly reflects edge effect of surface scales.The adhesion properties of keratin fibers are investigated by using AFM force-distance curve. The adhesive force and adhesive energy based on JKR (Johnson-Kendal-Roberts) contact theory of keratin fibers with different stretching ratios are calculated. The results illustrate that the adhesion force of keratin fibers is between15nN and55nN. Meanwhile, when the keratin fibers are stretched20%, the adhesive force and adhesive energy are the largest in all samples.