Highly Dispersible Kaolin for Printing Inks - poster
Anabelle Elton-Legrix, Andrew Curtis
Imerys Minerals Ltd, St Austell, UK
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The inherent hydrophilic nature of kaolin particles make them well suited for incorporation into water borne liquid systems, but it is also of interest to disperse fine kaolin particles for solvent based applications, in particular non polar and oil based offset inks. Here we focus on using fine platy kaolins (with median particle size down to 0.2 micron by sedimentation) in heatset inks used for printing high gloss magazines. The fine kaolin particles need to be well dispersed with absence of aggregates or agglomerates in order to reduce the surface roughness of the printed ink layer and maintain high final printing gloss (1).

Mechanical solutions including suitable grinding and milling of fine kaolin particles are commonly used in the minerals industry, allowing easier dispersion of kaolin particles to a suitable and acceptable fine size and reduced aggregates when incorporated in the oil or solvent. However, agglomerates can still be present due to the incompatibility of surface chemistry between the hydrophilic kaolin and the oleophilic dispersion medium.  Suitable surface treatments can be used to increase the compatibility of the kaolin surface in oil and previous literature studies (for example 2) have shown promising results using long chain amines and polyisobutylene-based stabiliser with a succimide/amine head group for dispersion in non polar solvents.

Here we have initially used a simple and effective sedimentation test in the ink petroleum distillate to allow screening of a wider range of amine and silane chemistries, with the aim to find the optimum dosage and treatment for effective dispersion of particles and sedimentation of treated kaolin into a hard compact layer. The most promising treated kaolins were incorporated into the full ink varnish (based on phenolic modified rosin, alkyd, veg oil and mineral distillate) up to 15wt% addition and printed onto polyester film to measure print gloss compared to an unfilled ink varnish. High print gloss was confirmed for well dispersed ultrafine kaolins and examination of the printed surfaces using Scanning Electron Microscopy tilted images showed a significant reduction in surface micro-roughness.  Full pigmented inks also gave better dispersion on the rollers of a triple roll mill and high print gloss at high kaolin content. Work is ongoing to consider the health and safety hazard of the treatments, minimise the viscosity for ease of addition onto dry kaolin and optimise the cost performance of the treated kaolin by considering blends of long chain and bulky amines as suggested in publications on other minerals (for example 3).