Optimization of the process variables for treating cellulose fiber with NaOH/urea aqueous solution for improved water retention value and paper strength

Cellulose fibers swell significantly in NaOH/urea solutions, and swelling increases with a decrease in temperature and an addition of urea. The combined effects of the factors of a pretreatment procedure (sodium hydroxide concentration, urea concentration, temperature, and time) and post-treatment conditions (medium, stirring speed, and time) of preparing fibers on the properties of pulp and the resulting paper sheets were investigated and optimized using single factor experiments. The optimum sodium hydroxide concentration (1 %), urea concentration (8 %), precooling temperature 0 °C, pretreatment time (12 h), post-treatment medium (7 % ( NH 4 ) 2 SO 4 {({\mathrm{NH}_{4}})_{2}}{\mathrm{SO}_{4}} ), post-treatment stirring speed (1500 rpm), and time (30 min) were obtained. Under the optimal conditions, the water retention values and tear, tensile, and burst indexes increased about 54 %, 277 %, 394 %, and 98 %, respectively. This work demonstrates that this technical route can effectively improve fiber swelling capacity and paper strength.