Paper Coating Rheological Testing and Influencing Factors

The effect of pigments, adhesives, and auxiliary adhesives on the rheology of coatings. Finally, the future research direction of coating rheology is introduced, which provides reference for research on coating rheology.

Paper coating is a mixed system composed of a variety of inorganic materials and polymers with different structures. Its performance is an important factor in determining the final product quality of the coated paper and the coating process. In the research and evaluation methods for the properties of paper coatings, the rheology of the coatings is considered to be the most important parameter in the coating operation. The coating must have an appropriate low shear viscosity, and the viscosity is too small, which may result in difficulty in feeding and leaking. The viscosity is too large to increase energy consumption, which is not conducive to the gravity filtration and dehydration of the coating. Paper coatings generally exhibit "shear-thinning" properties during the coating process. It seems to be said that once the shear expansion occurs, the practical solid content of the coating mixture and the practical speed of the coating machine have reached the upper limit. The rheological properties can not only be used to measure the advantages and disadvantages of different formulations, but also provide research and development direction, help to improve the technical content and level of coatings.

1 Coating rheological detection and indication method

The apparent viscosity of the coating generally indicates the rheological properties of the coating system. Specifically divided into low shear viscosity and high shear viscosity. The apparent viscosity of a coating at low shear rates is determined by the interaction of the various components that make up the coating. These include pigment flocculation, the colloidal nature of the particles, and the association of a small amount of rheological additives. The apparent viscosity of a coating at high shear rates is very important and is determined by the fluid mechanics of the coating, such as the average particle size, particle size distribution, shape and surface charge of the pigment particles, and the adhesive properties.

The apparent viscosity is measured by a viscometer, including a rotational viscometer and a capillary viscometer, wherein the capillary viscometer is suitable for measuring the high-shear viscosity of the paint. Three major rheometers are described below: Brookfield Viscometer, Hercules Viscometer, and Capillary Viscometer.

The Brookfield Viscometer is easy to use and inexpensive, providing low shear viscosity of the coating. Since only a single point can be measured, high shear viscosity cannot be predicted and it is suitable for daily monitoring during production stability.

Hercules viscometers have accurate shear rates, provide complete rheograms and high shear viscosities, but are insensitive to measurements at lower shear rates and sometimes provide only qualitative information rather than quantitative results, making them suitable for predictions. Rheological properties during coating to high shear rate.

The capillary viscometer has a definite shear rate and provides a very high shear rate viscosity, but multiple measurements are required to correct it, which is suitable for simulating high-speed coating processes. However, this simulation process is only approximate, for example, in the actual blade coating process, the shear rate under the blade is large (up to 106s), and the shear deformation time of the coating is very short (10s), so it is under the blade. The shear strain of the paint is small.

Obviously, each rheometer has a certain range of shear rate and application conditions, as shown in Figure 1. The results of several rheometer measurements can be combined to meet the requirements of most coating process predictions.

2 Factors Affecting Coating Rheology

pigment

Pigment is one of the main components of the coating, accounting for about 90% of the coating quality and 80% of the volume.