Preparation And Mechanism Of Water-based Terminated Isocyanate/Polyvinyl Alcohol Surface Sizing Enhancer For Special Paper

Waterborne polyurethane is a new type of paper surface sizing enhancer.Waterborne polyurethane as a surface sizing enhancer for special paper needs to use the isocyanate group(NCO)in its macromolecules as a highly reactive group,which can be combined with starch,polyvinyl alcohol or other natural polymers during the sizing process.The isocyanate group is introduced into the chemical crosslinking structure by the reaction with hydroxyl and groups,which greatly increases the physical and mechanical properties of the active material,especially as a surface sizing agent,which can make the paper obtain excellent mechanical properties and other required properties.However,due to the high activity of NCO,it is prone to hydrolysis during dispersion and transfer and loses the enhancement effect.To this end,in this work,the NCO group is protected by end-group blocking,and it can be deblocked and functioned at a specific temperature.There are active functional parts in the main chain of both polyvinyl alcohol and PU.After blending and compounding for paper surface sizing,the isocyanate group NCO can cross-link with the hydroxyl groups of PVA and paper fibers to form a cross-linked network structure,thereby improving the performance of the paper.In this project,the three composite systems of blocked isocyanate,isocyanate/polyvinyl alcohol composite system,isocyanate/organoboron cross-linking agent/polyvinyl alcohol act on the surface of paper,and the deblocking behavior of isocyanate group(NCO),cross-linking The formation mechanism of the network structure was discussed and its enhancement effect on the mechanical properties of paper,especially flexibility and ink adhesion was studied.(1)Using the method of end-capping structure design,the NCO group at the end of the prepolymer chain is protected by forming a weak chemical bond with the sealing agent in order to make it inactive at room temperature,which is convenient for co-polymerization,mixing,compounding and long-term stable storage.Under specific temperature conditions,the deblocking reaction releases the-NCO groups,which makes it play a cross-linking function again,and reacts with-OH on the paper fiber to enhance the mechanical properties of the paper.Three blocked isocyanate polymers were prepared by using DMPA,MDEA and PEG as chain extenders,respectively.The effects of the ratio of soft and hard segments and the R value on the dispersion properties(appearance and stability,particle size and Zeta potential,molecular weight and viscosity,etc.)of TBPU were discussed.When the R value is 1.6,the dispersion stability of TBPU is the best and the viscoelasticity is moderate.The thermal deblocking behavior and thermal properties of TBPU were studied by infrared FT-IR,TG-DSC thermal analysis and toluene-di-n-butylamine titration.The deblocking reaction followed the first-order chemical reaction kinetic law,and the optimal sizing temperature,drying temperature and drying time of surface sizing were determined to be 70℃,110℃ and 15 min,respectively.The end-blocked isocyanate polymer TBPU is used as a paper surface strengthening aid,which can form a primary cross-linked network structure(TBPU-fibers)with paper fibers after unblocking to enhance the comprehensive performance of paper.When the R of TBPU is 1.6 and the mass concentration is about 30%,the comprehensive performance of the paper is the best.Its tensile index,tear index and folding endurance reached 59.69 N·m/g,7.49 mN·m~2/g and 2075 times,respectively,which increased by 68.6%,14.3%and 22.1%compared with the base paper,and remained excellent water resistance.(2)By the method of physical blending,the blocked isocyanate TBPU is used as the main cross-linking component and the water-soluble polyvinyl alcohol macromolecule(PVA)is used as the carrier for surface sizing to prepare a compound with good compatibility and excellent comprehensive performance.Two-component TBPU/PVA water dispersion.The viscoelasticity of TBPU/PVA composite sizing solution and the mechanical properties of its film were studied.Compared with the single-component TBPU sizing solution,the TBPU/PVA composite sizing system contains more hydrogen bonds and covalent bonds,and its composite film has better ductility and greater degree of microphase separation.The overall performance is due to the single-component TBPU and Films formed from PVA.The chemical composition of the paper surface before and after sizing was analyzed by infrared FT-IR and elemental analysis XPS.Since the poly-hydroxyl structure of PVA provides more active-OH cross-linking sites,much new urethane(or urea)bonds are formed between TBPU and PVA and paper fiber groups.These strong polar bonds form an interpenetrating three-dimensional cross-linked network structure(TBPU-PVA-fibers)between the paper fibers,and also form a close physical interaction,which provides the TBPU and PVA macromolecules on the surface and inside of the paper fibers.The localization and binding of TBPU provides higher cohesion and promotes the formation of dual chemically cross-linked network structures(TBPU-PVA-fibers).Through the test of paper application performance,the optimal sizing process scheme was determined as follows:the addition amount of TBPU is 3.0 wt%,and the addition amount of PVA1799 is 4.0 wt%,and the comprehensive performance of the paper is the best at this time.The tensile index,tear index and folding endurance can reach 78.96 N·m/g,11.79 mN·m~2/g and 3956 times respectively,which are 122.9%,79.8%and 132.7%larger than the base paper.The interpenetrating network structure of the urethane groups of the two-component TBPU/PVA system is conducive to the formation of chain entanglement and hydrogen bonding and Coulomb force between rigid segments,which enhances the intermolecular interaction,thus endows the paper with higher hydrophobicity and surface free energy(101.2° and 58.60 mJ·m-2).(3)A one-step bulk copolymerization method was used to successfully synthesize a transparent amber boronic ester cross-linking agent(BGT)without catalyst and solvent.Through the method of physical blending,combined with the design of multiple cross-linked network structures,the borate BGT is used as a delayed cross-linking agent for PVA and cellulose,and the two-component TBPU/PVA sizing system is modified at different addition amounts,a multi-component TBPU/BGT/PVA aqueous dispersion with good compatibility and high cross-linking density was prepared.The adaptability analysis of borate addition amount to multi-component TBPU/BGT/PVA was discussed,and the optimal BGT sample and its applicable amount range were determined as BGT-122 and 0.05-0.20 wt%.The chemical structures and elemental compositions of borate ester(BGT)and TBPU/BGT/PVA composite films were characterized by FT-IR and XPS elemental analysis.The composite film TBPU/BGT/PVA has good mechanical properties(tensile strength is 58.5 MPa,elongation at break is 165.32%).The paper application performance test shows that the multi-component TBPU/BGT/PVA sizing system has a significant reinforcing effect on paper fibers.A large number of borate ions B(OH)4-generated by the multi-stage ionization of boronate BGT form a cross-linked network of strong coordination bonds(PVA-BGT-fibers)with PVA and the hydroxyl(-OH)on the surface of the paper.It interpenetrates and entangles with the TBPU-PVA-fibers cross-linked network to form a denser multiple cross-linked network structure(TBPU-BGT-PVA-fibers),thereby achieving the purpose of enhancing the mechanical properties of paper.When BGT-122=0.15 wt%,the comprehensive properties of the paper are the best.Its tensile index,tear index and folding endurance can reach 88.45 N·m/g,12.64 mN·m~2/g and 5011 times respectively,which are 149.7%,92.8%and 194.7%larger than the base paper.By comprehensively comparing the effects of the one-component sizing system,the two-component sizing system and the component systems on the water resistance of the paper,it can be seen that the paper sized by the multi-component sizing system TBPU/BGT/PVA improves the mechanical properties at the same time.Still maintains good water resistance.(4)The ink coating process of coating and immersion is used to penetrate the ink into the paper sample,and after curing at a constant temperature,the target ink-coated paper samples of different sizing systems are prepared.FT-IR spectroscopy and X-ray photoelectron spectroscopy(XPS)were used to determine the functional group changes and elemental compositions of un-inked paper and inked paper samples.In the multi-component TBPU/BGT/PVA sizing system,the covalent crosslinking between TBPU and PVA and hydroxyl groups of cellulose forms highly polar urethane segments(-NHCOO-).However,the crosslinking effect increases the microscopic roughness of the paper surface and the amount of hydrophobic porous structure,which cooperates with the capillary effect of the fibers to form a high-energy surface(higher than the surface tension of the ink)that firmly grasps the ink particles.The ink adhesion properties of different sizing systems were comprehensively evaluated by tape test and simulated wash test.The two-component TBPU/PVA and multi-component TBPU/BGT/PVA sizing systems have particularly significant improvements in paper ink adhesion,and the final paper ink retention rates in the test can reach 91%and 94%,respectively.