Hybrid coating pigments of poly(styrene-co-maleimide)/kaolin/alumina trihydrate for paper coating

Novel organic–inorganic hybrid pigments were prepared by in situ imidization of poly(styrene-co-maleic anhydride) (SMA) aqueous dispersion with ammonia in the presence of inorganic pigments, i.e. kaolin (K) or a mixture of kaolin (K)/alumina trihydrate (ATH). The objective was to allow the organic nano-particles of poly(styrene-co-maleimide) (SMI) to precipitate during their formation onto the surface of the inorganic pigment(s) and to investigate their potential in paper coatings. The produced hybrid pigments were characterized by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT-FTIR), scanning electron microscopy (SEM), wide-angel X-ray diffraction (WXRD) spectroscopy and their particle sizes were determined by dynamic light scattering technique. By FTIR measurements the formation of maleimide was confirmed and the existence of strong hydrogen bonding between organic and inorganic components could also be detected. Whereas, the SEM images revealed that the surface of silicate layers had successfully been to a large extent covered by the organic nano-particles to give raise to novel hybrid particles. In addition, the particle size measurements support the observation that the organic nanoparticles for the most part had been chemically or physically attached to the larger inorganic particles. Thus, this in situ imidization process in the presence of clay particles provides a neat method of dispersing the formed nanoparticles onto the inorganic surface through hydrogen bonding. The produced hybrids were applied as auxiliary pigments in paper coating where they improved gloss without roughening the paper surface.