Rheologic Modifiers in Novacolor Products

Among the ingredients of most Novacolor Products, users will find the term 'rheologic modifiers'. But what exactly does this mean? Here is a brief explanation of rheologic modifiers and why it is important for Novacolor Finishes to contain them.

Definition of Terms

It is perhaps best to begin by defining some of the terms used in the course of this post.

Rheologic, rheology - Derived from the Greek word 'rheos', meaning flow or stream, rheology is in essence the study of the flow and deformation of substances. The term 'rheologic' subsequently refers to substance flow/ deformation.

Viscosity - This is defined as the frictional forces within a system and the resulting resistance in liquids to flow. An important property, it contributes to the performance of a liquid.

Newtonian Flow - At different rates of shear, the viscosity of any given substance is constant.

Pseudoplastic Flow - As shear rates increase, viscosity decreases. This process is also described as shear thinning.

Thixotropic Liquids - These liquids display time dependent rebuilding (recovery) of viscosity after the removal of shear force.

Liquids and Force

During the application of force to a liquid, this liquid will relieve the strain of the applied force by flowing. Some systems (liquids) resist this flow to a greater measure than others. The measure of this resistance provides the measurement of the system's viscosity.

Newtonian Shear Model

A basic liquid flow measurement model first introduced by Isaac Newton is based on the idea of a liquid situated between a static base plate and second plate (A) that is moving at a specific velocity. According to this model, a liquid's viscosity, ɳ, may be calculated using the shear stress, Ƭ, and the shear rate, ẏ, using the following equation:

ɳ[Pa·s]=Ƭ[Pa]/ẏ[s-1]

Shear Stress - This is the force that is applied to the surface A when shear strain causes deformation.

Shear Rate - This is basically defined by the distance, h, between the two plates and the velocity, v, of the liquid flowing between them, and can be calculated using this equation:

ẏ[s-1]=v[m·s-1]/h[m]

During application of coatings (and depending on the method of application), shear rates (measured in reciprocal seconds, s-1) can vary significantly and may range from a few hundred to thousands. Assuming a brush speed of v = 0.8 m/s (m·s-1) and a 'distance' of h = 80 x 10-6 m, a simple brush application, for example, would generate a shear rate of:

0.8/80 x 10-6 = 1 x 10-4 s-1 = 10.000 s-1

That, of course, is an impressive shear rate by anyone's standards.

Viscosity in Paints/ Coatings

Typically rendered pseudoplastic, paints/ coatings display high stability during periods of storage. Due to support of high viscosity at low shear conditions, settling is effectively avoided. Comparatively high viscosity at lower shear rates also enables high quantities of paint/ coating to be held on brushes without dripping.

General mixing/ pumping of paints/ coatings is usually carried out at medium to low shear rates, while shear conditions during application (spraying, brushing, etc) are generally fairly high. Here, it is obviously beneficial to have lower viscosity.

Paints/ Coatings, Viscosity and Rheologic Modifiers

Often requiring levelling to a certain degree following the initial application, paints/ coatings obviously should not recover their viscosity too quickly. At the same time, a viscosity recovery that is too slow may cause dripping or sagging. This is where rheologic modifiers come into play. Depending on the type of modifier used, the addition of rheologic modifiers adjusts the flow behaviour of paints/ coatings, providing them with pseudoplastic flow; Newtonian flow and/ or thixotropic properties and ultimately ensuring optimal application properties.

Summary

In essence, rheologic modifiers provide Novacolor paints/ coatings with the best possible properties/ flow behaviours required for clean drip, sag and drag free application by brush, rollers, sprayers, etc.