Using thin-crystal engineered kaolins to enhance mechanical properties of coatings, 2010 TAPPI International Conference on Nanotechnology for the Forest Product Industry

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Structures in nature display hierarchical features which span nano to macroscopic length scales. In paper coatings, the mineral pigment particles themselves form the building blocks, and together with the binder are the limiting factor in determining the structural properties of the coating. The mechanical properties of paper coating layers are very important in converting and printing operations. These include stiffness, resistance to fold cracking, dusting and pick resistance. In this work, we show the potential gains in mechanical properties (such as elastic modulus, tensile strength and stiffness) which can be realised by reducing the z-direction size of the building blocks to nanoscale dimensions. This can be done using kaolins having thinner crystals than have been mined hitherto. For a given coat weight, compared to conventional clays or carbonates, these thin particles give greater energy transfer under tensile deformation and so increase the elastic modulus of the coating layer. This leads to gains in sheet stiffness which allow savings in raw material and energy costs. A coating layer containing thin crystal clay has an elastic modulus 7-8 times higher than one containing only ground calcium carbonate. Future developments in binder design need to be pursued to further improve the modulus of the coating and accelerate dematerialisation of the sheet.