A CRYO-SEM METHOD TO FIND STARCH BASED BIO-LATEX DISTRIBUTION DURING PAPER COATING DRYING

摘要

Cryo-SEM is a powerful tool for characterizing the hydrated microstructures of organisms and materials,and can also be used to observe the microstructure development by sampling at different time point during the process.Being sustainable,eco-friendly and comparatively cheap,starch based bio-latex has been increasingly used topartially substitute traditional petroleum based synthetic latex in paper coatings.During wet coating drying,the binder may move relatively with pigment particles leading to non-uniform distributions along the coating thickness,thus impairing coated paper properties and performance.In this study,a visible and direct method to analyze the binder distribution in the coating during consolidation was established.Coating specimens were prepared by drying in hot air oven or ambient environment.At specific time points during drying process,specimens were removed and plunged into liquid nitrogen to quickly freeze.After being fractured,sublimed and sputtered in the Cryo-SEM,coatings were imaged to characterize the respective distribution of binder and pigment at different points throughout the cross section.Model paper coatings mainly comprised of bio-latex and kaolin clay pigment were prepared,and Cryo-SEM technique was utilized to determine the transient bio-latex distributions at the surface and inner of coatings.The results showed continual changes of bio-latex distribution throughout the coating layer in the drying process,i.e.,bio-latex tended to accumulate at coating surface initially,while be deprived from surface finally.This surprising redistribution phenomenon may be attributed to the hydrophilic property of starch based bio-latex.