With an increased need for digitalization and automation of production processes, monitoring critical to quality (CTQs) parameters can improve the production performance by providing valuable information regarding the process metrics.
In the paint industry, process viscosity plays a critical role in determining the coatings’ end-user performance. A coating must be stable during storage, easy to apply, and ideally, the final surface finish should be smooth and dip-free. Control of the flow characteristics of paints during production is therefore essential for their successful application.
Paints and Coatings
Paints or surface coatings are complex colloidal systems that are applied as a continuous layer to a surface. Paints typically consist of binders, pigments, solvents, and additives. The pigments give the paint color and opacity; solvents make it easier to apply and flow; binders help it form a thin film, and additives alter the physical properties to suit different applications. Additives can serve as everything from rheology modifiers to anti-fungicidal agents.
Paints can be classified based on the industry (e.g., architectural, automotive, industrial paints, etc.), functionality (e.g., primers, finishes, base coat, top coat, etc.), chemistry, or formulation (e.g., acrylic, epoxy paints, waterborne/solvent-borne paints).
Coatings and Rheology
Rheological measurements can provide information relating to the materials’ fundamental structural properties and material behavior under a given set of processing or application conditions.
In the paint and coating industry, viscosity is an essential rheological parameter for intermediate and final product characterization. Viscosity can be directly affected by temperature, process shear rate, thixotropy (time-dependent viscosity), particle size, presence of microbubbles, or activation/induction time for the paint components.
Viscosity has a significant influence on steps throughout storage, production, application, and film formation of paints. Paints are typically formulated to be shear thinning, i.e., thick under storage conditions and get thinner during mixing, pumping, or transportation. During processing and application, paints are subjected to a variety of different shear regimes. For example, in the production process, paint is pumped around the plant, subjecting it to moderate to high shear, whereas storage and transportation are low-shear processes. An application can vary from moderate (dipping) to high shear (roller coating, spraying).
Lambourne and Strivens  provide the following examples where viscosity is essential during application.
- Transfer of paint from the bulk container to the tool used
- Transfer of paint from the applicator (brush or roller) to the surface to form a thin and even film
- Loss of solvent by evaporation
During each of these steps, the paint’s flow characteristics and its time relaxation produce interesting rheological responses.
RheoStream® FC in Paint Production
RheoStream® FC is a process analyzer that can monitor process viscosity in near-real-time during paint production. RheoStream® FC draws representative paint samples from the production line (e.g., process/let-down tank, recirculation pipe, etc.), conditions the sample to the desired temperature, and measures the sample viscosity at pre-defined shear rates, and discards the sample.
RheoStream® FC can aid in paint production optimization by
- Providing continuous viscosity monitoring and thus the Production Support can be prepared for any unforeseen changes in the process
- Having a reliable value for process control, thereby providing a possibility of changing product parameters automatically
- Saving time and lab costs and speeding up the process as samples do not have to be taken to a lab viscometer, requiring proper and accurate handling for a reliable viscosity value.
1. R. Lambourne and T. A. Strivens, Paint and Surface Coatings: Theory and Practice, Woodhead Publishing, 2 ed., 1987.