Preparation And Solution Properties Of Low-substituted Hydroxyalkyl Cellulose

The growing shortage of petroleum resources has limited the development of chemical fiber industry.Cellulose has many advantages such as abundant stock,numerous sources and good biocompatibility.It is of great significance for the sustainable development of chemical fiber industry to develop and utilize cellulose.However,cellulose is difficult to be dissolved and reprocessed in common solvents because of its complex hydrogen bond network structure and high crystallinity.Etherification modification of cellulose can solve this problem.The hydrogen bond network structure is destroyed due to the introduction of hydroxyethyl or hydroxyl propyl substituent groups on the cellulose ring respectively,which makes the etherified low substituted hydroxyethyl cellulose(HEC)or hydroxylpropyl cellulose(HPC)soluble in weak alkali solution,which solution has good film-forming property.It breaks the limitations of the alkali solvent system for dissolving cellulose,such as low solid content,easy gelation,and low polymerization degree of soluble cellulose.In this paper,two groups of HEC and HPC with the same molar substitution degree(MS)were prepared.The effects of different substituted functional groups on the structure,solubility behavior,rheological properties and film-forming properties of cellulose were studied.It provided a theoretical basis for industrial production and application of low substituted hydroxyalkyl cellulose products.The main research contents were given as follows:(1)The L9(3~4)orthogonal experiment was designed by the liquid phase method to determine the relatively optimal process conditions for preparing low-substituted HEC and low-substituted HPC.The results showed that:when the alkalization temperature was 20?,the alkalization time was 1 h(or 2 h),the alkalization concentration was 20 wt%,and the etherification temperature was 70°C(or 85°C),HEC(or HPC)could be prepared with relatively optimal solubility in the alkali solvent.Two sets of HEC and HPC with MS values of 0.14 and0.25 were prepared and labeled as HEC-0.14,HEC-0.25,HPC-0.14,and HPC-0.25.The structures and properties of HEC and HPC were characterized by gas chromatography(GC),FT-IR and XRD.The results showed that the etherification modification of HEC and HPC were weak etherification modification,basically retaining the original structure of cellulose.From cellulose to HEC or HPC,the crystal structure of the fiber had changed from cellulose I to a crystal structure close to cellulose II.The crystallinity of HEC or HPC decreased with the increase of MS.And under the same MS values,HPC had lower crystallinity than HEC.(2)The dissolution behavior of two groups of HEC and HPC with the same MS in Na OH solution was investigated.By measuring the turbidity value of the solution and comparing the polarized light photos,the relative optimal dissolution temperature of HEC and HPC in 8%Na OH solution was explored.The relative optimal dissolution temperature of HEC or HPC was-6?or 0?respectively.Dynamic light scattering(DLS)was used to explore the size of undissolved crystals and solution homogeneity in HEC or HPC solutions.It was found that the size of undissolved crystals in the HEC and HPC solutions were in the range of 30-70?m,and showed a normal distribution trend.Under the same MS values,HPC had smaller crystal size than HEC solution and better solution uniformity and transparency.The thermal properties of HEC or HPC solutions were studied by DSC.It was found that when the melting enthalpy of the eutectic was zero,the number of Na OH interacting with a single macromolecule of HEC and HPC had reached at least 5.27.The true limit solid content of the solution could reach more than 6 wt%.(3)The steady state and dynamic rheological properties of HEC and HPC solutions with the same MS values were investigated.It was found that temperature,MS and the type of solution had important effects on the fluidity and spinnability of the solution.The steady-state rheology showed that both the HEC solution and the HPC solution were shear-thinning pseudoplastic fluids.Increasing temperature and MS would reduce the structural viscosity index(??)and viscous flow activation energy(E?)of the solution.Under the same MS values,the HPC solution had lower??and E?than the HEC solution.It proved that HPC solution had better fluidity than HEC solution and was more sensitive to temperature.Dynamic rheology showed that the increase in temperature and MS was beneficial to delay the gel phenomenon of HEC and HPC solutions.Under the same MS values,HPC solution tended to appear gel spots in the high frequency region more than HEC solution.It was proved that HPC solution was more stable than HEC solution.(4)The structures and properties of HEC and HPC films with the same MS values were investigated.The surface morphology of HEC membrane or HPC membrane was observed by SEM.The surface and cross section of HEC membrane and HPC membrane were smooth.And the surface of the HPC film of the same MS values was smoother than that of the HEC film.The structure changes of the regenerated membrane were studied by FTIR and XRD.The regeneration process of HEC and HPC films was a physical change accompanied by the decrease of crystallinity.The crystallinity of HPC films was slightly lower than that of HEC films.The mechanical properties and hydrophilicity and hydrophobicity of the film were studied by mechanical stretching and contact angle tests.From dry to wet state,the membrane changed from strong and brittle to low strength and toughness.Under the same MS values,the mechanical properties of HPC film were worse than that of HEC film,and the hydrophilicity was slightly weaker.