The effects of water uptake on mechanical properties of viscose fibers

The fact that cellulose materials soften with water uptake is exploited constantly during paper production to form strong bonds between two fibers. Here, we present measurements of surface hardness and reduced modulus of viscose fibers by atomic force microscopy based nanoindentation at varying relative humidity. A home-built setup allowed to access a wide humidity range from 5 % until 95 % relative humidity. The obtained results are compared to those of softwood kraft pulp fibers. Creep effects at a constant load during indentation were observed and found to increase exponentially with increasing humidity. In order to relate mechanical properties to the water content within a fiber, also gravimetric sorption studies were performed. This allowed to extrapolate the mechanical properties of viscose fibers to 100 % relative humidity. Interestingly, hardness and reduced modulus are at this point higher by a factor of 4 and 20, respectively, compared to those of viscose fibers fully swollen in water. Pulp fibers, in comparison, exhibit mechanical properties which are similar to those of viscose fibers. Only when the fibers are swollen, a higher hardness for viscose fibers is evident, whereas the moduli of pulp and viscose fibers are still comparable.