Products, quality is often defined by a range of functional and sensory attributes, some of which may be easily measured - such as color - and others which may not. Visual or textural appearance and ease of application exemplify properties in this second category. Ultimately these attributes are assessed subjectively via user feedback, but relying on such information to guide formulation is somewhat impractical, particularly in the early stages of product development. Metrics that reliably correlate with product performance are therefore essential.
Introduction
Developing a detailed specification that defines and safeguards product quality through development and into manufacture is an essential aspect of product formulation. For personal care products quality is often defi ned by a range of functional and sensory attributes, some of which may be easily measured - such as color - and others which may not. Visual or textural appearance and ease of application exemplify properties in this second category. Ultimately these attributes are assessed subjectively via user feedback, but relying on such information to guide formulation is somewhat impractical, particularly in the early stages of product development. Metrics that reliably correlate with product performance are therefore essential. Rheology adds value in product formulation and benchmarking studies by providing these product performance metrics. The resulting data can be used either to replicate the quality of an existing product, to compare formulations or competitor products, or to develop a specifi cation for a brand new product. Rheological properties such as viscosity, yield stress and viscoelasticity can be used to determine how easy it is to squeeze toothpaste or ointment from its packaging, to spread a body lotion over skin, or the feel of skin cream when the user dips their fi ngers into the pot. In addition it can also be used to assess and predict product stability and help to resolve process issues (1). In this article we look specifically at the application of rheological tests to quantify aspects of consumer product performance focusing on which tests are best for which application and the selection of appropriate test conditions. Knowing which test to apply and how to make relevant measurements is the key to success.Understanding the role of rheology in formulation
Formulation has traditionally been an empirical process involving the addition and processing of a range of components on a somewhat trial and error basis to arrive at a product with the required attributes. Today though, the value of formulated products has become suffi ciently substantial to support a more scientifi c approach. Rheology is widely recognized as being critical in this regard, but there remains uncertainty of how to apply it in an effi cient and relevant way.
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