Study On The Crosslinking Performance Of Enzymatic Starch And The Mechanism For Improving The Strength Of Papers Via Surface Sizing

Higher requirements for paper workers are put forward with the increasing cost and demand for the fiber resources.The way of adding fillers into paper can reduce the cost,but also destroy the bond strength between fibers,which results in the decreased paper strength.In traditional methods,the addition of strength agent in wet-end system is limited by its addition dosage.As for the surface sizing,starch could not only enhance the strength,but also improve the surface properties of paper.However,due to the limited addition,the enhancement of paper strength needs to be further improved,especially for the packaging paper with high requirements for strength and stiffness.The penetration of cross-linking agent and the enzymatic starch into the paper,connecting the fibers via the covalent bond through cross-linking,is of great significance to form an interpenetrating network structure with the fibers and strengthen the paper,improve the wet strength as well as the corresponding performance in high humidity environment.The physicochemical properties of enzymatic starch and its permeability in paper,the cross-linking method and mechanism of starch with small molecular crosslinker,the strengthening of paper via small molecular co-cross-linking system with starch,and the surface sizing through high/small molecular co-cross-linking system with starch were mainly carried out.Results show that the destroyed crystallinity structure and increased proportion of amorphous region is companied with the reduced molecular weight of starch,which is helpful for its penetration into papers.The viscosity molecular weight of 9.52×10~4 exhibits satisfied penetration performance;the starch sizing significantly increases the pressure at the bubble point and decreases the maximum diameter of the through-pore.The starch with lower molecular weight is readily to penetrate the open pores and holes with small size and thus makes the pore distribution more concentrated.When the molecular weight of starch decreased from 20.15×10~4 to 9.52×10~4,the main distribution of the pore size increased from 4?m to 4.8?m.The glyoxal addition(molar ratio of Gly/AZC=3:1)into the AZC aqueous solution could increase the steric hindrance of AZC and inhibit its self-polymerization upon heating.In near-neutral conditions,the water resistance of the starch film co-crosslinked by Gly/AZC is significantly enhanced,compared with the native starch,glyoxal crosslinked starch and AZC crosslinked starch.Also,the crosslinking significantly decreased the amount of crack on the surface of the resultant starch films.The addition contents of glyoxal plays a great role in forming a uniform and dense structure,due to the inter-molecular strong interaction between glyoxal and AZC as well as the crosslinking between the crosslinkers and starch.It is significantly important to enhance the water resistance.As the AZC and glyoxal dosage is 5%and 20%,the swelling ratio of the as-prepared co-crosslinked film at 60 min is 10%.In near neutral conditions,the paper strength is improved via the Gly-AZC co-crosslinking system and the Gly-AZC exhibits synergistic crosslinking action.The villous protuberance formed on the fiber surfaces increases the contact area between fibers.The effective limit of glyoxal on the self-polymerization of AZC(mass ratio of Gly/AZC=1:1)impedes the formation of AZC crosslinked starch gel(starch concentration is 10%),and thus the surface sizing system of Gly/AZC-starch could meet the requirement of surface sizing.The glyoxal and AZC co-crosslinked system could enhance the strength of papers with starch.It could be attributed to the intra-fiber crosslinking mode for the small-molecular crosslinkers within the network.As compared with the starch sizing paper,when the AZC dosage is 0.3%(wt)and starch dosage is3%(3 g/m~2,3mPa.s),the glyoxal addition content at 0.9%(wt)could make the tensile index,ring crush index and stiffness index increased by 7.91%,6.06%and 9.1%,respectively;and the elastic modulus increases to 0.96 GPa.PAE is a kind of high molecular crosslinker with remarkable crosslinking performance.The PAE/Gly crosslinking system shows significantly enhanced strength and stiffness with starch.Specifically,the corresponding performance are further improved in relatively high humid environment.As compared with the starch sizing paper(single-sized sizing),when the glyoxal dosage is 0.3%(wt)and starch dosage is 3%(3 g/m~2,3mPa.s),the PAE addition content at 0.5%(wt)could make the tensile index,ring crush index and stiffness index increased by 9.73%,13.30%and 13.8%,respectively;the elastic modulus increases to 1.03GPa.It could be ascribed to the inter-fiber bridging and crosslinking for the PAE and the intra-fiber crosslinking of glyoxal,which effectively improve the ability to transfer the stress when the papers is stretched under force.In this work,the enhancement on papers via the crosslinking system with starch is studied.Firstly,the starch with uniform distribution and suitable molecular weight is selected.The Gly/AZC co-crosslinking system is proposed for the first time and shows significantly enhanced crosslinking performance on the starch.A good strengthening performance on papers via this co-crosslinking system of Gly/AZC and starch is also achieved.The small and high molecular crosslinking system of PAE/Gly is proposed and utilized to enhance the strength and stiffness through their intra-fiber and inter-fiber synergistic crosslinking.The dramatically improved performance provides valuable theoretical reference for preparing light weight and high strength papers via penetration and crosslinking using the surface sizing approach.