Rheological characterisation of coating colours - comparison between conventional techniques and a slit die system

An experimental rheological evaluation of coating colours has been performe d. The pigments studied were three kaolin grades (Brazilian, English and Am erican delaminated clay) and two aragonite grades (precipitated calcium car bonate, PCC). The aspect ratios of the clay pigments ranged from quite low to very high values. Two different grades of thickener (carboxymethyl cellu lose, CMC) with different molecular weights were also studied. The Brookfie ld viscosity and dynamic-mechanical properties. were measured using a rotat ional viscometer, but the focus has been on the rheological properties at h igh shear rates (> 100 000 s(-1)), which were measured using a capillary vi scometer equipped with both capillaries and a slit die. Capillaries of the same diameter but different lengths were used in order to correct for entra nce pressure losses (EPL). The geometry of the slit die resembled the area in the vicinity of the blade tip during blade coating. The apparent viscosity values obtained using capillaries and the slit die w ere compared at the same apparent shear rate (1 000 000 s(-1)), in order to enable the non-Newtonian EPL to be estimated. The ratio between the appare nt viscosity obtained using the slit die and the shear viscosity was quite large, about 2, but it was roughly independent of the clay pigment used and thus independent of the aspect ratio of the pigment particles. For aragoni te-based colours, however, the ratio was much higher than for clay-based co lours, indicating that the non-Newtonian EPL had a pronounced influence. Co lours based on aragonite or delaminated day showed significant non-Newtonia n EPL not only in capillary viscometry but also particularly when the slit die was used. The influence of water-soluble polymers on the shear viscosit y at a shear rate of 1 000 000 s(-1) was small. In the case of coating colo urs based on English clay, the non-Newtonian EPL over the slit increased in the presence, of polymer, but it decreased significantly in the case of co lours based on a rather coarse aragonite pigment. The pressure needed to at tain a shear rate of 1 000 000 s(-1) using the slit die was also significan tly lower in the presence of polymer in the case of colours based on the co arse aragonite pigment, whereas the opposite was observed in the case of co lours based on English clay.