Study On The Synthesis Of Blocked Waterbome Polyurethane And Its Application In Paper Strengthening

Paper and paper-based products are natural network polymeric materials, which aremainly composed of plant fibers linked each other by hydrogen bonds. However, therelative low physical strength limits its wide application. In the present research, theco-polymerization characteristics of lignocellulose in paper with polyurethanepre-polymer was investigated in detail. The paper agents of blocked waterbornepolyurethane (pre-polymer) was synthsized. The obtained imidazole-blocked TDI-PEGpre-polymer was analyzed with TGA and DCS to understand the unblockedtemperature. The study proves the unblocked temperature was between109-112°Cwhich is suitble in papermaking. Through the change of molecule weight ofpolyethylene glycol (PEG), effects of the ratio of hard segments to soft segments of thepre-polymers on paper properties were investigated. It was found that the dry tensileindex and the stiffness decrease with the increase of the soft segment ratio. The surfacetreatment can enhance the physical strength of paper significantly. Takeimidazole-TDI-PEG (LTP400) as example, When the coating weight of LTP400was8gram per sq. m, the wet tensile index of the treated paper was increased by10.67times.Dry tensile index of the paper was enhanced by2.07times. The folding endurance wasincreased by53.60times. Moreover, the tear index was also increased by1.51times.The stiffness was enhanced by1.32times.Using three mixed emulsifiers to emulsify the polyurethane pre-polymer, Apolyurethane pre-polymer emulsion with good stability and high efficient was obtained.The emulsifiers are span40, span80and tween80with the proportion was3.5:1:3.The emulsion was283.81nm size and-67.32mV. The process of addition ofpolyurethane emulsion was also optimized. The retention of polyurethane and physicalstrength of paper were increased significantly. Al2(SO4)3was more effective in retentionof polyurethane.In order to understand the effect of microfibril content on the synthesis ofpaper-based polyruethane composite, physical properties, surface feature, chemicalbonds of the composite were analyzed by physical test, SEM observation and XPSdetermination. Reaction mechanism of imidazole-TDI-PEG pre-polymer with fiber andlignin was studied by FT-IR and CP/MAS13C-NMR determination. In addition, theeffect of beating degree of fibers on properties of paper treated with polyurethane wasalso investigated. The results showed that carbomate bonds were formed betweenfibers after reaction of unblocked polyurethane and hydroxyl groups of cellulose,hemicellulose and lignin in cell wall of fibers. The hydrogen bonds between fibers couldbe changed into covalent bonds. Therefore, the physical strength of the paper treatedwere improved, especially the wet strength. The content of free hydroxyl groupsincreased with the enhancement of beating degree of fibers. The base-paper can beeasily treated by polyurethane with significant improvement of physical strength. Thecontent of urethane bond was1.63%when the beating degree of composite was75°SR, which was6.30times more than composite of10°SR. On the basis of these results, theapplication of blocked waterborne polyurethane as paper agents in coating process andwet-end part in papermaking were also investigated in this work. Some noveltechnologies of production of high strength functional paper by polyurethanetreatment were also developed successfully. We also find that residual lignin from pulpwas reactive with the pre-polymer. Poplar lignin and wheat straw lignin which containsmore G&H lignin type were more reactive. Poplar lignin-polyurethane and wheat strawlignin-polyurethane had better heat resistance than fir lignin-polyurethane in300°C.