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Yi-kuang Chen

The Colour Change of Flowing Paint when Subjected to Shear

This project was concerned with the way shear can influence the colour of paints. The problem is of industrial relevance and may be able to give insight into the structrue and mesophase organisation of paint during shear.

Paint is composed of two different phases: a continuous and a discontinuous phase. There are two main components in the continuous phase: the binder and the solvent or diluent. Binders are mainly polymers or resins, which form the main structure of the paint film. The main function of the solvent is to dissolve the binders and it can be used to adjust the viscosity for the desired application. In the discontinuous phase, there are three major types of pigments: primary pigments, additives, and extenders. Primary pigments control the optic effects on the paint; additives make a marked effect on the functions of the paint; extenders are used in matt paint to generate desirable surface properties of a paint (see e.g. Lambourne et al., 1987).

The commercial relevance of shear influence of colour change relates to the spraying of car body panels. In some cases, different spraying conditions can lead to different sprayed colour although the starting paint is the same. This is thought to be due to the shear that results from the spray process as in a spray gun. Shear rates of about 104-105s-1 can be reached (Ansdell, 1987). Therefore, this project was concerned with developing an experimental apparatus and protocol that could help achieve an understanding of this colour change.

In this project, an acrylic basin based paint with red iron oxide was used as the sample. Colour measurements were carried out using a X-Dap spectrophotometer. Two shearing systems were using to generate shear in the samples: the Linkam Cambridge Shearing System and the Multi-Pass Rheometer. The rheological properties of the samples were characterised using a Rheometrics Dynamic Spectrometer RDSII. A Maxwell model with a non-linear Wagner damping function (Wagner, 1976) was used to predict the steady shear behaviour of the sample.

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