Commercial rotational rheometers

Abstract

Materials generally show a broad spectrum of behaviour, the rheological properties lying between the extremes of a truly elastic and an ideally viscous response. The rheological testing procedures involve measuring the response of a material to an applied deformation but, given that ideality in response is rarely encountered, a straightforward rheological test is not usually sufficient for a complete material characterization. Also, materials often show considerable time dependent changes as well as markedly non-linear responses to an applied stress or strain, so the rheological tests need to be chosen carefully. The deformation, or strain, can be applied in a variety of ways but the aim of any study, in establishing a relationship between material properties and the stress-strain-time conditions, means that only relatively simple and well characterizable shear conditions are used. Understandably, the most usual approach is to shear the material between two surfaces. Now, since it is only the relative movement within the material that is important, the approach used does not require that any one surface should be stationary. However, from a practical viewpoint, a simple arrangement is to move the surfaces in contact with the material instead of moving the material itself. Here it is important to have good coupling between the material and the boundary surfaces so that’ slip’ does not occur, since it is vital for the material in contact with the boundary surfaces to be undergoing a motion identical with the motion of the surfaces themselves. By satisfying this requirement any measure will be a reflection of the properties of the bulk material, not those of an indeterminate part.