John Kettle

Structural and Nanomechanical Properties of Paperboard Coatings Studied by Peak Force Tapping Atomic Force Microscopy

Paper coating formulations containing starch, latex, and clay were applied to paperboard and have been investigated by scanning electron microscopy and Peak Force tapping atomic force microscopy. A special focus has been on the measurement of the variation of the surface topography and surface material properties with a nanometer scaled spatial resolution. The effects of coating composition and drying conditions were investigated. It is concluded that the air-coating interface of the coating is dominated by close-packed latex particles embedded in a starch matrix and that the spatial distribution of the different components in the coating can be identified due to their variation in material properties. Drying the coating at an elevated temperature compared to room temperature changes the surface morphology and the surface material properties due to partial film formation of latex. However, it is evident that the chosen elevated drying temperature and exposure time is insufficient to ensure complete film formation of the latex which in an end application will be needed.

Short timescale inkjet ink component diffusion: An active part of the absorption mechanism into inkjet coatings

The structures of inkjet coatings commonly contain a high concentration of fine diameter pores together with a large pore volume capacity. To clarify the interactive role of the porous structure and the coincidentally occurring swelling of binder during inkjet ink vehicle imbibition, coating structures were studied in respect to their absorption behaviour for polar and non-polar liquid. The absorption measurement was performed using compressed pigment tablets, based on a range of pigment types and surface charge polarity, containing either polyvinyl alcohol (PVOH) or styrene acrylic latex (SA) as the binder, by recording the liquid uptake with a microbalance. The results indicate that, at the beginning of liquid uptake, at times less than 2 s, the small pores play the dominant role with respect to the inkjet ink vehicle imbibition. Simultaneously, water molecules diffuse into and within the hydrophilic PVOH binder causing binder swelling, which diminishes the number of active small pores and reduces the diameter of remaining pores, thus slowing the capillary flow as a function of time. The SA latex does not absorb the vehicle, and therefore the dominating phenomenon is then capillary absorption. However, the diffusion coefficient of the water vapour across separately prepared PVOH and SA latex films seems to be quite similar. In the PVOH, the polar liquid diffuses into the polymer network, whereas in the SA latex the hydrophobic nature prevents the diffusion into the polymer matrix and there exists surface diffusion. At longer timescale, permeation flow into the porous coating dominates as the resistive term controlling the capillary driven liquid imbibition rate.

Comparison of Young-Laplace pore size and microscopic void area distributions in topologically similar structures: a new method for characterising connectivity in pigmented coatings

Scanning electron microscopy (SEM) combined with image analysis can provide a quantitative description of the area distribution of a porous structure, such as a paper coating. This is one of the few techniques where one can limit the measurement area strictly to the coating layer, fully excluding the base paper. It has been found that SEM cross-sectional porosity, defined as visible relative void area, and mercury porosimetry results agreed qualitatively to some degree but differed quantitatively. From an understanding of the differences in observations provided by the two methods, it is realised that comparison of void area distribution and intruded pore volume distribution, the latter including effects of entrance geometry to pores (mercury porosimetry and pore shielding), effectively describes the 2D to 3D transformation between the cross section and the pore network structure, i.e. a description of connectivity, in the unique case where the topology of the pore structure skeleton remains similar. Such structures are termed homeomorphisms. By studying the pore structural parameters of pigmented tablet structures, consisting of natural ground calcium carbonate with progressively increasing dose of latex binder, it is shown that the pore structural parameter of connectivity, and, hence, effective tortuosity/permeability, derived independently using the pore network model, Pore-Cor, can be deduced by forming the differences and correlations (convolution) between the two pore size distribution methods.