The porous structure of paper coatings studied by water diffusion measurements

The porous structure of four paper coating layers with various amounts of k aolin and precipitated calcium carbonate (PCC) pigments were investigated b y studying the diffusion behavior of water absorbed in the coatings. The wa ter diffusion tensor was completely determined by employing the pulsed fiel d gradient (PFG) NMR technique with a tetrahedral held gradient combination set. The trace of the diffusion tensor and the rotation invariant diffusio n anisotropy were quantitatively evaluated. It was demonstrated that the tr ace of the diffusion tensor decreased whereas the diffusion anisotropy rati o increased at higher amounts of kaolin in the coating layers. This indicat es that kaolin tends to give rise to a less porous structure with a higher anisotropy. The NMR results are consistent with data from porosity and pore volume measurements. (C) 1999 Elsevier Science B.V. All rights reserved.