The Solubility Difference of Four Celluloses in NaOH/Urea Aqueous System

The solubility of four celluloses(cotton linter pulp,bamboo pulp,hardwood pulp and softwood pulp) in NaOH/urea aqueous was studied.Cotton linter pulp and bamboo pulp were soluble and formed an uniform liquid system,whereas a layered solution was formed for hardwood pulp and softwood pulp,indicating that the solubility of hardwood pulp and softwood pulp were lower in the aqueous system.The degree of polymerization and the degree of crystallinity of all four celluloses were decreased after dissolving-regeneration.The degree of polymerization of cotton linter pulp and bamboo pulp celluloses decreased from 549 and 624 to 474 and 555,a reduction by 13.6% and 11.1%,respectively;whereas the degree of polymerization of hardwood pulp and softwood pulp celluloses decreased from 1067 and 1460 to 989 and 1419,a reduction by only 7.3% and 2.8%,respectively.The degree of crystallinity of cotton linter pulp and bamboo pulp celluloses decreased from 66.5% and 79.7% to 18.8% and 31.8%,a reduction by 71.7% and 60.1% respectively;whereas those of hardwood pulp and softwood pulp celluloses decreased from 76.4% and 70.4% to 62.5% and 54.7%,a reduction by 18.2% and 22.3%,respectively.These results indicated that the solubility difference of four celluloses can be attributed to the fact that the solvent system can not reduce the crystallinity of hardwood pulp or softwood pulp celluloses because of the high degree of polymerization.The infrared spectra of these materials showed that new groups do not appear after dissolving-regeneration,indicating that the NaOH/urea system is not a cellulose derivative solution.X-ray diffraction(XRD) showed that the regenerated cellulose is converted from cellulose I to cellulose II,this system can cause a change of cellulose crystal structure.