Effects of Coagulation Conditions on the Properties of Regenerated Cellulose Films Prepared in NaOH/Urea Aqueous Solution

Effects of coagulant components and coagulation conditions on the structure and properties of regenerated cellulose (RC) films prepared from cellulose in 7.5 wt % NaOH/11 wt % urea aqueous solution were investigated by 13C NMR, X-ray diffraction, scanning electron micrograph, tensile testing, etc. The uniform design method based on theoretical accomplishments in the number-theoretic method was used to optimize the coagulation conditions of H2SO4 aqueous solution with various concentrations (from 1 to 15 wt %), time (from 1 to 15 min), and temperature (from 25 to 55 (degree)C). Moreover, a series of RC films coagulated, with 5 wt % H2SO4/Na2SO4, Na2SO4, HOAc, and (NH4)2SO4, respectively; different concentrations (from 1 to 20 wt %) and times (from 1 to 20 min) at 25 (degree)C were also investigated. The results indicated that the optimal coagulant concentrations and coagulation times of the RC films are 5 wt % H2SO4 for 5 min, 5 wt % H2SO4/5 wt % Na2SO4 for 5 min, 5 wt % Na2SO4 for 15 min, 3 wt % HOAc for 5 min, and 5 wt % (NH4)2SO4 for 3 min, respectively, at 25 (degree)C. The RC film that coagulated with 5 wt % H2SO4/5 wt % Na2SO4 aqueous solution exhibited a higher optical transmittance, more homogeneous structure, and better mechanical properties than that coagulated with others on the whole. Moreover, the tensile strength of the RC films in the wet and dry states increased simultaneously with a drop in coagulation temperature. The coagulation mechanism can be described as a two-phase separation, namely a cellulose-rich phase in the gel and a cellulose-poor phase in solution.