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 °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 °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 °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.
Effects of coagulation temperature, coagulants, and wet-spinning methods on structure and properties of novel regenerated cellulose (RC) fibers prepared from cellulose in 7.5 wt % NaOH/11 wt % urea aqueous solution on pilot scale spinning machine by one- and two-stage coagulation were investigated by tensile testing, optical microscopy, scanning electron micrograph, and wide-angle X-ray diffraction. The results indicated that H2SO4/Na2SO4 and H2SO4 aqueous solutions are potential coagulants for NaOH/urea system and fibers wet-spun from the two-stage coagulation obviously exhibited the better mechanical properties than those from the one-stage coagulation. The optimal coagulation conditions for two-stage coagulation are 10 wt % H2SO4/15 wt % Na2SO4 for the first coagulation bath and 5 wt % H2SO4 for the second bath or 5 wt % H2SO4/15 wt % Na2SO4 for the first coagulation bath and 10 wt % H2SO4 for the second bath. Moreover, the tensile strength of novel fibers increased with a drop in coagulation temperature. The diffusion rate between the coagulant and solvent plays a major role in determining the mechanical properties of the cellulose fibers. Our spinning process was quite different from that of the viscose process, in which orientation and coagulation proceed more or less simultaneously. The production method of this fiber wet-spun could be suitable in the wide range of coagulation conditions, compared to the viscose one.
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