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Quality Assurance - Rheological Testing of the Sagging Behavior of Coatings

Introduction

Ms. C., head of the testing laboratory and responsible for the company's viscometers and rheometers, has a problem. The QA tests performed recently on her coating samples do not provide useful information: test results fail to show clear differences between the water-based coatings that are qualitatively good and those that cause problems.

From a practical point of view, the following three criteria are especially important for coatings:

  1. Separation and sedimentation behavior, which reflect their long-term stability in storage [1]
  2. Pump and flow behavior during application (here: spray paint for vehicles)
  3. Behavior after application, which can be described as the combination of the following processes: leveling and sagging, layer thickness formation, degassing behavior, leveling and fixing of effect pigments. These particular processes are often referred to as thixotropic behavior [2] [3]

Customers using these coatings have been reporting problems relating to the first two points (sedimentation as well as pump and flow behavior). However, Ms. C. is also looking for a test method for QA that she could use to examine sagging behavior.

Here, we show that a combination of rotational and oscillatory tests performed on an Anton Paar Modular Compact Rheometer (MCR) and analyzed with an Anton Paar software enable the full characterization of the sagging behavior of coatings.

[1] Mezger, T.; Stellrecht, P.: The behavior at rest. Europ. Coat. Journal (09/2000)

[2] Mezger, T.; Stellrecht, P.; Wollny, K.: Thixotropic behavior / structure recovery of coatings (Congress Papers: European Coatings Show). PRA: Teddington, and Vincentz: Hannover, 2001

[3] Mezger, T.; Will, S.: Rheologische Pruefung des Ablaufverhaltens von Lacken. Journal Welt der Farben (11/2001) 

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