Michael Caves, India Business Development Manager, Malvern Instruments, elaborates on how viscosity measurements if carried out over an appropriate range can be highly informative
Viscometry has been a mainstream technique for formulation, but those leading the way have long since transitioned to the broader capabilities of rheometry. The choices associated with replacing your ageing viscometer may, therefore, be a long way from like-for-like. Here we provide insight to support an upto-date assessment of the value of upgrading.
A standard rotational viscometer is an efficient tool for measuring viscosity, at a single point or across a moderate range of shear conditions. However, viscosity is far from being the only parameter that can usefully be measured to determine product performance, or indeed commercial appeal. Other defining rheological properties include: viscoelasticity; yield stress; thixotropy; and tackiness. Rotational rheometry extends access to the comprehensive measurement of all of these parameters and in so doing has the potential to deliver significant value to formulators. In simple terms, the capabilities of a rotational rheometer allow formulators to develop a detailed understanding of performance across all the changing conditions to which a product may be subjected, during manufacture and use.
Complete flow curve measurement
Viscosity measurements remain the most widespread rheological analysis and if carried out over an appropriate range can be highly informative. Non-Newtonian materials either shear thin i.e. exhibit lower viscosity at higher shear rates, or, less commonly, shear thicken, meaning that viscosity increases with applied shear rate. Generating a flow curve – a plot of viscosity as a function of shear rate – over a representative shear rate is therefore a valuable core strategy for formulation studies. A particular strength of rheometers, relative to viscometers is that they enable measurement across a broader shear rate range, in particular at the extremely low shear rates associated with storage, and can therefore provide insight into long-term product stability (Figure 1).
The majority of viscosity measurements involve the application of a rotational shear force or rate, but rheometers, by enabling the precise control of normal force and the gap between upper and lower geometries, also provide the means to carry out axial testing, a technique that is inaccessible with a standard rotational viscometer. This permits pull away or tack testing – to quantify stickiness – and squeeze flow measurements – to extend flow curve measurement for samples with high solids loadings which can fracture during standard viscosity testing.
A further primary difference between viscometers and rheometers is that the latter permit oscillatory testing. As the term suggests, oscillatory testing involves subjecting the sample to a relatively small shear force, or displacement, applied in the form of a sinusoidal wave. It therefore calls for bi-directional movement of the upper geometry, relative to the lower, a feature uniquely associated with rotational rheometers. This testing is usually performed in the Linear Viscoelastic Region (LVR) where stress-strain is linearly dependent and the microstructure remains intact. Oscillatory testing probes the viscoelasticity of a material – the extent to which it exhibits viscous (liquid-like) or elastic (solid-like) behaviour, or indeed transitions between the two under different conditions. Key test variables are the frequency of oscillation (v), which correlates with the timescale over which deformation is carried out (t ˜ 1/v), and temperature, which is similarly tailored to reflect in-use conditions. The metrics measured include the complex modulus, G*, which indicates total material stiffness and which can be broken down in to its elastic and viscous moduli, G’ and G’’ by utilising the phase angle or phase difference, c, between the applied stress and measured strain.
Oscillatory testing supports the development of a microstructural fingerprint that provides insight into important aspects of product performance such as stability by identifying conditions under which structure develops in the product (Figure 2). It can also quantify characteristics such as spreadability, as a function of temperature for example, and provides textural information by quantifying stiffness (via G*) and springiness (via phase angle).
In recent decades formulation has transformed from black art to advanced science, a process that continues as productive analytical strategies emerge. Formulating to fully meet consumer expectations is vital for commercial success and there is intense time
pressure on formulation and reformulation programmes. By offering enhanced sensitivity, and a wider range of test capabilities, rotational rheometers answer far more fully to current formulation requirements than traditional viscometers, more effectively supporting a knowledge-led approach to product development.
Upgrading to a rotational rheometer can therefore pay significant dividends in the form of faster, more secure formulation and, ultimately, competitive product performance.
Malvern Aimil Instruments
A-8 Mohan Co-operative
New Delhi – 110044