Structure-activity Relationship Of Cross-linked Polyvinyl Alcohol And Its Strengthening Mechanism On Paper

Surface sizing has been paid more and more attention in paper industry,which can effectively improve mechanical strength,printability and other properties of paper.It is of great significance to develop a novel surface sizing agent to give it better function.Polyvinyl alcohol is a water soluble polymer containing a large number of hydroxyl groups,which is widely used in specialty paper industry due to its excellent film forming properties,surface sizing and strengthening properties.However,PVA has a high viscosity due to the formation of large amount of hydroxyl group at room temperature.As a surface sizing agent,the penetration of paper is limited and the binding force of ink particles is not strong.What’s more,the waterproof and mechanical properties of paper should be improved.This paper is mainly based on increasing the mutual penetration and chemical cross-linking between PVA and paper fiber.Firstly,self-cross-linking PVA(IPVA)modified by N-(isobutoxy)methylacrylamide(IBMA)is proposed in this paper.On this basis,epoxy cross-linking,cationic cross-linking,ketone-hydrazine cross-linking and aziridine cross-linking are used to modify PVA,respectively.The modified PVA is also mixed with polyethylenimine(PEI)to solve the problems of low strength and poor waterproof of paper,the research work is as follows:(1)Preparation of IPVA by solution copolymerization and alkaline alcoholysis.IPVAc is prepared by using VAc as main monomer,IBMA as functional monomer,azobisisobutyronitrile and benzoyl peroxide as initiator and methanol as solvent.Then IPVA is prepared by alcoholysis under alkaline condition.The factors affecting the synthesis conversion of IPVA,such as initiator type and dosage,solvent type and dosage,reaction temperature and polymerization time,as well as temperature,solvent type,concentration of alkali solution,solvent dosage and alcoholysis time are discussed.The results show that when 1%AIBN is used as initiator,30 mL methanol is used as solvent,the reaction temperature is 75℃,the polymerization is stable and the conversion of monomer is high.The alcoholysis reaction is accelerated with the increase of temperature.However,too high temperature further aggravates the volatilization of methanol,which is not conducive to the stable reaction.The concentration of NaOH as a catalyst directly affects the speed of alcoholysis reaction.The concentration of NaOH and the alcoholysis rate decreases with the increase of time.The amount of solvent affects the phase transition and degree of alcoholysis of the alcoholysis system.Too much or too little methanol is not good for the alcoholysis process and it also affects the energy consumption and cost of solvent recovery.In the process of alcoholysis,it is suitable to keep 70℃ for the temperature,1%for the concentration of alkali solution,40 mL for the dosage of methanol and 3 h for reaction time.When the mass ratio of IBMA/VAc is 20%,the solubility is good.The suitable dissolving temperature is 75℃,the time of complete dissolution is only 180 minutes and the suitable swelling time is only 60 minutes.Considering the gel content,contact angle,water absorption and properties of sizing paper,the best curing temperature is 100-110℃,which is propitious to give full play to the cross-linking effect of IBMA and improve the performance of IPVA product.(2)The cross-linkable epoxy polyvinyl alcohol(G-IPVA)is prepared by using VAc as main raw materials,IBMA and glycidyl methacrylate(GMA)as cross-linked monomer.The structure of G-IPVA is characterized by FT-IR.The effects of GMA on the properties of G-IPVA and the physical properties of paper after sizing are discussed.The mechanisms including cross-linking and performances improvement by G-IPVA are also proposed.The physical properties of the paper increase significantly.The contact angle can be improved by 340.62%to the maximum.The water absorption decreases from 310.2%to 75.2%.The dry and wet tensile indexes of G-IPVA5 sizing paper reach 110.1 N·m/g and 16.8 N·m/g,respectively,which are increased by 239.81%and 290.69%by comparing with base paper.The folding endurance of G-IPVA4 sizing paper is 15.53 times over that of base paper.The results show that the compact structure between the fibers is due to the cross-linking of hydrogen bonds and covalent bonds by a large number of hydroxyl groups,polar groups,self-cross-linking of IBMA segments and epoxy active cross-linking of GMA.(3)Cross-linked cationic polyvinyl alcohol(CPVA)is obtained by solution polymerization using VAc as main raw materials,IBMA and GMA as cross-linked monomer and dimethylaminoethyl methacrylate(DM)as cationic modified monomers.The structure is characterized by HNMR.The effects of DM monomer on the properties of CPVA are investigated.It turns out that CPVA dispersion is a Bingham fluid with good stability.The particle size of CPVA reaches minimum and the value is 147.2 nm,which is in favor of infiltration into the paper.It is more suitable for the DM dosage ranging from 2%to 2.5%in light of the mechanical properties of CPVA film.The obtained CPVA is amorphous and the degree of crystallinity is decreased.When used as surface sizing agent for paper,the water absorption of the sizing paper reaches minimum and the folding resistance reaches maximum when DM is 2.5%.The maximum dry and wet tensile indexes of the paper are increased by 190.71%and 331.09%,respectively.SEM shows that the larger pores between the paper fibers are filled and cross-linked after CPVA sizing,which is beneficial to the improvement of paper strength.(4)A novel modified polyvinyl alcohol(DA-IPVA)used as sizing agent is prepared by using diacetone acrylamide(DAAM)as graft monomer,adihydrazide(ADH)as small molecule cross-linking agent,and N-(isobutoxy)methylacrylamide(IBMA)as self-cross-linking monomer.The effects of the amount of DAAM on the properties of emulsion,film and sizing paper are discussed.The surface micro-structure of the sizing paper is characterized by SEM and AFM.The addition of DAAM/ADH and IBMA endows DA-IPVA with cross-linked active group and increases cross-linking density and hydrophobicity after ADH is added into the emulsion and the cross-linking structure is formed.The enhancing mechanism of surface sizing agent for paper is revealed.DA-IPVA can form a light cross-linking network structure between the paper fibers and improve the binding force between the fibers.It can effectively maintain the movement ability of the chain segments in the network and improve the flexibility and other mechanical properties of the paper.When the mole fraction of DAAM of the cross-linking monomer is 8%,the surface sizing performance of the paper is obviously improved compared with the base paper.The dry and wet strength is increased by 266.5%and 334.3%,respectively and the folding resistance is increased 2946.67%.The contact angle of DA-IPVA5 sizing paper is 83.4°,which is 3.34 times of that of base paper.(5)Carboxylated modified PVA(CIPVA)is prepared by solution free radical copolymerization method and alkaline alcoholysis using VAc,IBMA and BA as main monomers and azobisisobutyronitrile as initiator.Then CIPVA/AZ mixture is modified by aziridine cross-linker(AZ)and used as sizing agent.The structure formation of CIPVA and the cross-linking reaction between AZ with carboxyl groups in the CIP VA are confirmed by NMR and IR.With the increase of AZ content,the gel content,cross-linking density and thermal stability of the AZ-CIPVA film are increased.When the AZ dosage is 3.5%,both tensile strength and modulus of AZ-CIPVA film reach maximum,and the value are 68.34 MPa and 1037.02 MPa,respectively.The water absorption of the film reaches minimum and the folding endurance of the sizing paper reaches maximum when the content of AZ is 2.8%.The maximum dry and wet tensile indexes of the paper are increased by 223.77%and 432.56%,respectively.Roughness of the sizing paper increases slightly,which is helpful to improve the waterproof and water-resistant function of paper.The paper fibers are well filled after sizing,and the synthetic polymers and natural polymers are moderately chemically cross-linked,which is beneficial to improve the mechanical strength and water resistance of paper.And it is a kind of surface sizing agent with excellent performance and low price,which is expected to be used in surface sizing of special paper.(6)In practical application,compound surface sizing agent is often used to show the synergistic effect.Polyethyleneimine(PEI)has the structure of amino polar group and long-chain hydrophobic group.With the help of the cation,high adhesion and adsorption of PEI molecule,it can enhance the strong adhesion between composite surface sizing agent and paper fiber,and the amino group also has good permeability.The hydrophobic group can improve the water resistance of paper-based materials.The hydroxyl group in cross-linked polyvinyl alcohol and the amino group in PEI can form hydrogen bond,which has good blending compatibility.Several cross-linked polyvinyl alcohol(M-PVA)prepared in the above chapters are blended with polyethyleneimine(PEI).The effects of four cross-linked polyvinyl alcohol concentrations and PEI concentrations on the viscosity of mixed sizing solution,sizing amount,mechanical properties including folding resistance,dry strength and wet strength of paper after sizing,waterproof properties including contact angle and sizing degree are discussed.The relationships between structure and performance are concluded.And the advantages and disadvantages of each sizing agent are comprehensively evaluated.M1-PVA has the highest viscosity and M2-PVA has the lowest viscosity.M3-PVA and M4-PVA have similar viscosity.The high viscosity of M1-PVA is related to its epoxy group in the molecule and partial nucleophilic substitution reaction with PEI,while the minimum viscosity of M2-PVA is related to its more cationic groups and better dispersion.The viscosity affects the amount of glue hanging,which shows that under the same concentration and dosage.M2-PVA has the highest amount of glue hanging,while M1-PVA and M3-PVA are relatively low.The results also show that the amount of glue hanging does not completely depend on the viscosity.A smaller viscosity is conducive to the penetration between paper fibers,and the dispersion of sizing solution is good,which is conducive to improving the amount of glue hanging.But if the viscosity is too small,it will also affect the retention of sizing solution on the paper.It is beneficial to improve the hanging amount by appropriately increasing the viscosity of sizing solution.The carboxyl group in M4-PVA molecule has good dispersion and permeability,which is beneficial to improve the glue hanging amount.Therefore,the glue hanging amount of M4-PVA is second only to M2-PVA.M3-PVA has high viscosity due to the introduction of a large number of groups in the molecular side chain,which limits its permeability into the paper and leads to low glue hanging amount.In terms of folding resistance,M3-PVA is the highest,which is related to the cross-linking mode of ketone hydrazine.The molecule combines more PEI flexible chains,which enhances the toughness of the whole molecule.The folding resistance of M4-PVA is second,which is mainly related to the ring opening cross-linking of carboxyl group and aziridine in the molecule.The molecular cross-linking degree is high,the cohesion energy is large,and both dry strength and wet strength reach the highest value.From the index of sizing degree,M3-PVA also reaches the highest,followed by M4-PVA,indicating that their water resistance is better,which is related to the high cross-linking of their molecules and more hydrophobic groups.