Monday, March 15, 2010

Viscoelasticity while Painting

Paints are a classic example of a viscoelastic fluid – shear thinning is needed so that the paint can be easily applied to the wall (a high shear situation), but not run down it after it is applied (a low shear situation). But there are so many more viscoelastic materials to consider when painting.

Ceiling paints have a different need entirely, in that gravity is now pulling normal to the film, not along the film. Hence there is a need for controlling the extensional viscosity. Painting a ceiling is difficult enough without having to worry about the paint dripping back down on you.

But even before the first brush stroke is made, another viscoelastic material is on the job – the pressure-sensitive adhesive with a crepe-paper backing, otherwise known as masking tape. Masking tape typically uses a natural rubber based adhesive that has been modified with a whole gamut of additives to modify the adhesion, not only the initial adhesion, but also the build of adhesion over time, which is why there are so many options available. The adhesive properties of these materials can be related to viscoelastic measurements by the Dahlquist criteria.

Paint brushes for latex paints have bristles from nylon or polyester or a combination of the two. Regardless, the bristles were formed from a drawn, molten polymer, an obvious example of a non-Newtonian fluid. During the fiber forming operation, the fluid was exposed to a large range of time-dependent shear and extensional stresses, all with a thermal background of varying states.

(These thoughts were brought to you courtesy of a weekend DIY project. I had to have something to think about while watching paint dry.)

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