Testing – taking rheological control

Samiul Amin, Stephen Carrington & John Duffy examine the application of advanced rheology measurement in personal care formulations

Samiul Amin, Stephen Carrington & John Duffy examine the application of advanced rheology measurement in personal care formulations

Formulating personal care products presents numerous challenges, from shampoos that grow runny after the addition of scent to body lotions too thick to dispense once shear is applied. Similar issues abound when designing or engineering in new product cues or benefits. Why does product A suspend coloured beads while product B fails to? How to ensure the long-term stability of emulsion C? Fundamental to resolving these issues is the need to understand and engineer the underlying complex fluid that constitutes a particular personal care formulation. Modern rheological measurement approaches can help here.


Figure 1: The derivation of optimised design rules


Complex fluids

Personal care formulations are often structured complex fluids, materials with structural organisations at various length scales that tend to exhibit complex responses when subjected to external stimuli such as shear, temperature and pH. While these structured fluids produce the interesting rheological properties seen in a wide variety of products, they frequently challenge the formulation scientist.

R&D programmes in personal care are geared to delivering well defined performance for a specific application. Conventionally this involves testing a large number of new materials and proceeding empirically, through trial and error towards development goals. Experience suggests that only rarely does this produce a more generally applicable design rule. The preferred alternative is a knowledge-based approach, which requires an understanding of the link between performance and rheological response (if there is one), and between the rheological response and the underlying microstructure of the formulation. Knowing how existing and new components of a formulation affect the underlying microstructure can also support performance optimisation over the longer term (figure 1).


Optimising rheology & measurements

Generating broadly applicable, optimised design rules demands expert knowledge in a variety of fields including rheology, complex fluid microstructure and physics, colloid science and others. Such expertise is not always readily available in commercial companies, especially small to medium sized organisations, and so taking a knowledge-based approach can seem a daunting task. However this is being aided by continuing developments in rheometry.

Shampoos, body washes and shower gels, for example, tend to be structured using a combination of anionic surfactant-sodium laurel ether sulphate (SLES) and a zwitterionic surfactant cocoamidopropyl betaine (CAPB). In the presence of an electrolyte, such as sodium chloride, these form elongated cylindrical worm-like micelles that become entangled and influence viscosity and viscoelasticity.

The rheology of worm-like micelles is controlled by the structural parameters of the system, and the rheological response of an entangled worm-like micelle is now well understood. Rheometry can therefore be used as a tool to extract the rheology controlling structural parameters, and to evaluate the influence of different components on them.

Entangled worm-like micelles can be described as single relaxation time Maxwell fluids, and their response in an oscillation frequency measurement, a routinely applied rheological test, is described by the semicircular shape of the plot of the viscous modulus G" versus the elastic modulus G' (Cole-Cole plot). Any deviation from this response suggests that the system is not fully entangled or is branched. Oscillation frequency measurements are therefore an efficient way of probing the controlling microstructure of these fluids.

Clearly this description of behaviour and the measurement of rheological parameters can quickly become quite technical, and setting up the experiment and performing the Maxwell fluid analysis may be even more of a challenge. However, new generation rheometers promise to simplify this process, offering the ability to run test sequences under Standard Operating Procedure (SOP)-type interfaces which link appropriate rheological tests and required data analysis directly, as well as providing user guidance at every stage of the measurement process.

Personal care formulations contain many different functional additives, and rheological tests on the base complex fluid often need to be repeated to determine the effects of increasing the concentration of a particular additive (by following the changes in Cole-Cole plot signature or in structural parameters). This has particular relevance when developing optimised design rules (figure 1), typical examples being: investigating the impact of perfume on microstructure and rheology (perfume can have a major effect on viscosity); assessing the effectiveness of structuring on changing a surfactant base; or understanding the levels or type of additional structurant required to suspend visual cues. Overall such an approach leads to clear understanding and maps the effect of additives in relation to microstructure and rheological performance. To examine how this can work in practice, consider some comparative work carried out on body wash formulations.


Figure 2: Yield Stress test to investigate ability of formulations to suspend particles or bubbles


Example: understanding body wash structure

Body wash 1 does not suspend bubbles, yet body wash 2 does. Rheological differences between the two formulations can be identified by running a yield stress analysis, the results of which are presented in figure 2. Body wash 2 shows a clear maximum in a stress ramp test, one of the most common protocols for establishing yield stress, while body wash 1 does not, implying that body wash 2 has a yield stress that is absent in body wash 1. Acting against gravity, yield stress prevents the sedimentation of particles, or maintains bubbles in suspension.

While such rheological analysis delivers some understanding of why one formulation suspends bubbles, it does not, in isolation, offer enough information to adopt a knowledge-based approach to structure modification based on a generally applicable design rule. That requires correlation of the formulation and microstructure differences between these two products.


Figure 3: Elastic modulus G\' and viscous modulus G\" as a function of frequency for body wash 1 and body wash 2 formulations


Having the ability to set up a dedicated sequence (or SOP-type test) on the rheometer then enables an appropriate test for microstructural characterisation of worm-like micellar systems to be developed and used. Such a rheological test sequence involves both a frequency sweep and Cole-Cole plot analysis to extract the structural parameters. The sequence set-up can also include full user guidance at all stages of the measurement as required, driving robust and best practice rheological test protocols. The data from such a sequence are presented in figure 3, and marked differences between the two products are clearly evident.

Body wash 1 has a relatively strong frequency dependence of G' and G", the moduli determined in a frequency sweep and used to quantify the viscoelastic character of a material; G' is the elastic (or storage) modulus and G'' is the viscous (or loss) modulus. In body wash 2 on the other hand this frequency dependence is weak. Body wash 2 also has a higher elastic modulus and no crossover between G' and G". This weak frequency dependence of the body wash 2 formulation suggests a behaviour that does not follow the single relaxation time Maxwell model response expected for worm-like micelles but this can be verified through the automated generation of a Cole-Cole plot (see figure 4).


Figure 4: Cole-Cole plots for body wash 1 and body wash 2 formulations
Data for a third formulation (body wash 3) is also shown to provide a comparison with another worm-like micellar microstructure


An entangled worm-like micelle system should give a semicircular plot; body wash 1 does so, especially at lower frequencies. Its deviation from ideal behaviour at higher frequencies is probably the result of other functional additives affecting the base surfactant microstructure, a hypothesis that could be confirmed through further systematic rheological investigation, ie by using the same sequence as above but starting with base surfactant only and/or including fewer functional additives than the final product formulation.

So, even in the fully formulated product, the dominant worm-like micelle microstructure signature is clearly observed in body wash 1. Such a response is absent in body wash 2. Its departure from worm-like micelle behaviour is evidence of microstructural differences between the two formulations, attributable to their different constituents and composition. In fact, although both formulations contain the same surfactants, body wash 2 also contains an associative polymer.

This associative polymer is likely to be forming a network with the worm-like micelle and/or with itself, producing the observed yield stress, the weak frequency dependence and higher elastic modulus. This network effect appears to dominate the rheology of body wash 2, giving it the necessary structure for bubble suspension, a structure that body wash 1 lacks. The effectiveness of the associative polymer in providing the necessary yield stress will depend on how much bridging exists between the worm-like micelle and the polymer, and the network forming capabilities of the polymer itself. Changing the hydrophobic nature of the polymer and/or its concentration in the formulation are likely to be efficient strategies for further microstructural tuning.

Kinexus (Malvern Instruments) brings an intelligent SOP-driven sequence approach to rheology testing

In the Cole-Cole plot of figure 4 the microstructural fingerprint of a third formulation (body wash 3) is also included. This product shows a more defined worm-like micelle semicircular signature than body wash 1, which suggests a more entangled structure. The level of hydrophobic modification, and concentration of the polymer required to build the necessary network and yield stress in this system, would therefore be very different from the body wash 1 formulation because of this difference in the starting worm-like micelle microstructure.

Here then rheological analysis begins to inform the formulation process. The characterisation techniques applied quantify the properties of the base formulations (body wash 1 and 3 respectively) and point up the structural changes promoted by the inclusion of the associative polymer (body wash 2). Knowing that the associative polymer modifies the microstructure makes it easier to tune polymer properties to achieve a desirable outcome. Furthermore, knowledge of how to make microstructural changes is more generic than how to develop a yield stress for a specific formulation; a more generally applicable design rule.


Knowledge-based approach

The example here shows how formulators can start with an application problem and from there understand formulation parameters by applying a knowledge-based approach supported by advances in test development (SOP-based sequences) on the latest rheometer platforms. The resulting design rules are likely to be robust, widely applicable and useful as the basis for specifying new materials for optimised performance.

Most personal care products, their applications and the corresponding microstructures lend themselves to this type of detailed analysis. Dedicated rheometer measurement sequences can be tuned to meet specific product format, microstructure and/or applications for individual companies or organisations. For global multinationals the approach provides the means to standardise tests and analysis across R&D sites worldwide.


References
1. Larson RG, The Structure and Rheology of Complex Fluids, Oxford University Press (1999)
2. Eguchi K, Kaneda I, Hiwatari Y, Masunaga H & Sakurai K, Journal of Applied Crystallography, 40, 264-268 (2007)

Authors
Samiul Amin, strategic technology group manager, Malvern Instruments
Stephen Carrington, product marketing manager, Malvern Instruments
John Duffy, product technical specialist rheometry, Malvern Instruments

Contact
Malvern Instruments Ltd, UK
tel +44 1684 892456
www.malvern.com

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