Starch Graft Copolymers Latex For Coated Paper

This dissertation focuses on preparation and application of starch graft copolymerlatexes for coated paper. The cassava starch was modified by α-amylase or oxidationbefore copolymerization. The enzymatically degradedstarch-graft-poly(styrene-co-butyl acrylate)(EDS-g-P(St-BA)) latex and oxidizedstarch-graft-poly(styrene-co-butyl acrylate)(OS-g-P(St-BA)) latex were prepared bygraft copolymerization of modified starch with styrene and n-butyl acrylate. Thecopolymers were characterized by FT-IR, NMR, TG, DSC, SEM, TEM, GPC, anddynamic light scattering. The application properties of EDS-g-P(St-BA) andOS-g-P(St-BA) latex were also investigated.(1) The cassava starch was degraded by α-amylase before copolymerization. TheEDS-g-P(St-BA) latex was prepared by surfactant-free emulsion polymerization,ammonium persulfate as initiator and N,N’-methylenebisacrylamide as crosslinkingagent. The results showed that the molecular weight of degraded starch and apparentviscosity of gelatinizing solution decreased greatly and the percentage of reducingsugar increased. The degraded starch not only replaced part of monomers, but alsoshowed good properties of dispersion and stability. The EDS-g-P(St-BA) latex werestabilized by degraded starch without any surfactant. The graft level decreased and theparticle size increased when the degradation time increased. The results of DSCshowed that the glass-transition temperature (Tg) became higher thanpoly(styrene-butyl acrylate)(P(St-BA)) polymer without starch. The results of TGshowed that the thermal stability of EDS-g-P(St-BA) was improved by the addition ofstarch. The optimal conditions for preparation of EDS-g-P(St-BA) latex were asfollows: concentration of starch solution1mol/L, amount of α-amylase5U/g,concentration of monomers3.6mol/L, degradation time10min, concentration of initiator7×10-3mol/L, reaction temperature80oC, reaction time4h. The maximumpercentage of monomer converting, percentage of graft (PG), graft efficiency (GE) andthe percentage of starch in copolymers could reach98%,238%,72%, and35%,respectively. The graft level and percentage of starch were higher than previous workpublished.(2) The oxidized starch was prepared and characterized, H2O2as oxidation agentand Cu2+as catalyst. The apparent viscosity of gelatinizing solution decreased and thecontent of starch in solution increased. The OS-g-P(St-BA) latex was prepared byemulsion polymerization, ammonium persulfate as initiator and Tween80as surfactant.The results of characterization revealed that the concentration of oxidized starchaffected the graft level, particle size, and Zeta potential. The particle size decreasedand Zeta potential increased when the concentration of oxidized starch increased. Thelatex particle carried negative charge because of carboxyl groups of oxidized starch.The colloida stability of latex was improved by combination of the electrostaticrepulsion and steric stabilizing effects. The results of TG and DSC showed that the Tgbecame higher and thermal stability was improved by the addition of oxidized starch.The reaction conditions were optimized using a4-factor3-level Box-Behnken designbased on response surface methodology. The analysis revealed that concentration ofoxidized starch, concentration of monomers, concentration of initiator, and reactiontemperature significantly influenced the PG and GE. The optimum values ofparameters predicted through RSM were as follows: concentration of oxidized starch6.82%, concentration of initiator0.31%, concentration of styrene or butyl acrylate20%,and reaction temperature70oC, the maximum PG of293%were obtained. For theoptimum value GE of40%, the predicted levels of factors were concentration ofoxidized starch10%, concentration of initiator0.27%, concentration of monomer18.09%, and reaction temperature70oC.(3) The EDS-g-P(St-BA) latex was used as cobinders with conventional P(St-BA)latex for paper coating. The coating color and coated paper were characterized. Theresults showed that with increasing the percentage of EDS-g-P(St-BA) latex incobinders, the brightness, gloss, and Cobb value kept constant, the dry surface strengthwas improved and the smoothness, printing gloss, the ink absorb and wet surfacestrength became worse. When the percentage of EDS-g-P(St-BA) latex in cobinderswas less than70%, the properties of coated paper deteriorated insignificantly. Theaddition of crosslinking agent improved the properties of coated paper. When the amount of crosslinking agent increased from1%to3%, the properties of coated paperimproved significantly. The printing properties of coated paper were not affected bythe EDS-g-P(St-BA) latex. The paper coated with EDS-g-P(St-BA) latex as solebinders also showed good printing results.(4) The EDS-g-P(St-BA) latex were used as cobinders with conventional P(St-BA)latex for paper coating. The coating color and coated paper were characterized. Theresults showed that with increasing the percentage of OS-g-P(St-BA) latex incobinders, except the brightness kept constant, all the other properties deteriorated.When the percentage of OS-g-P(St-BA) latex in cobinders was less than50%, theproperties of coated paper deteriorated insignificantly. When the percentage ofOS-g-P(St-BA) latex in cobinders was less than70%, the paper coated withOS-g-P(St-BA) latex showed good printing results.