Catalytic Modification Of Corn Starch By Acid Cellulose Nanocrystalline And Its Application

Starch is a kind of biodegradable and pollution-free natural macromolecular compound.It can be modified by oxidation,etherification and cross-linking to overcome the disadvantages of poor water solubility,high paste viscosity,easy aging and poor thermal stability.In this paper,cellulose nanocrystalline(CNC)was prepared by acid hydrolysis method and used as catalysts for the oxidation and esternfication modification of starch.The prepared oxidized starch(OS)and oxidized acetate starch(OACS)were used for paper surface coating and preparation of composite film.The effect of CNC on the modification of starch and the subsequent coated paper and composite film were systematically studied.Corn starch was used as raw maternal,and CNC/NaCIO was used as a new oxidation system for oxidation modification.The results showed that the nano-scale and needle-like structure of CNC made it easily insert into the swelling starchgranules,and promote the penetration of the oxidant NaClO into the starch granules.The degree of oxidation of the starch was increased,and the traditional catalyst 2,2,6,6-tetramethylpiperidine-l-oxyl could be effectively replaced.When the effective amount of NaCIO was 8.0 wt%and the amount of CNC added was 0.5 wt%,the oxidized starch had the highest carboxyl content at 1.10%.Meanwhile,the morphology of starch granules was seriously damaged,the crystallinity was the lowest and the thermal stability was significantly improved.The coating agent was prepared by mixing starch with PVA/GL.The increasing carboxyl group content could effectively enhance the coating performance and improve the surface and physical strength properties of coated paper.The coated paper showed better properties:the tensile index was 3.90 N·m/g,the tearing index was 5.69 mN.m2/g,the bursting resistance index was 12.49 kPa·m2/g,contact angle was 75.5° and Cobb valuewas 27.0 g/m2.The OS was further modified by CNC/vinyl acetate esterification system to obtain oxyacetate starch(OACS).The results showed that the highest substitution degree of OACS was prepared with 7.0 wt%vinyl acetate and 0.3 wt%CNC,and the substitution degree of OACS was 262.1%higher than vinyl acetate esterified starch and the crystallinity was the lowest at 30.7%,respectively.OACS and polyvinyl alcohol/glycerol(PVA/GL)were blended to prepare the coating agent.The increasing substitution degree of OACS could effectively enhance the coating performance of OACS/PVA/GL.The coating paper showed better properties:the tensile index was 4.58 N·m/g,the tearing index was 5.98 mN·m2/g,the bursting resistance index was 12.49 kPa·m2/g.contact angle was 100 °and Cobb value was 18.15 g/m2.The modified starch was blended with polyvinyl alcohol(PVA)and glycerin(GL)to prepare the composite film.The optical pictures of film-forming solution and viscosity were observed by polarizing microscope and viscometer,respectively.The starch granulesshows bad compatibility with PVA/GL and polarized cross was obvious.OS and OACS had significantly improved compatibility with PVA/GL,and the polarized cross was disappeared.The viscosity of OACS-based film-forming solution showed decreasing trendency with rising temperatureand reached the lowest at 17.40 cp,which was lower than that of film-forming solution based on corn starch and OS.The composite film was characterized by atomic force microscope,scanning electron microscope,ultraviolet spectrophotometer,universal tensile machine.thermogravimetric analyzer,etc.The results showed that:(1)In the oxidation process,increasing content of CNC incorporated into starch was benefical to the water resistance and tensile properties of composite film;when the mass ratio of OS:PVA:GL was 160:24:15,the composite film had the best properties:the contact angle was 98°,tensile stress was 12.89 MPa and Young's modulus was 1597.42 MPa(2)In the dual-modification process,the increasing amount of CNC incorporated into starch enhanced the performance of starch-based composite film;when the mass ratio of OS:PVA:GL was 80:24:15,the composite film had the best properties:the contact angle was 120.5 0,tensile stress was 25.66 MPa and Young's modulus was 2500.85 MPa.