This article is the second of two discussing instrumental testing methods with applicability to the hair styling area. The previous article1 discussed consumer language associated with these products—in particular, the term hold. Instinctively, there is a tendency to equate this word with style longevity, but also discussed was the strong link of hair stiffness to consumer perception of this attribute. In short, while consumers profess the desire for products that do not adversely affect the feel properties of hair, this stiffness appears to act as the cue that the products are working.
In keeping with this discussion, the earlier article described the two most commonly used methods for evaluating styling products in the laboratory: high humidity curl droop experiments for style longevity, and mechanical bending experiments to assess hair stiffness. However, it is worth pointing out that the properties of polymer films deposited by styling products are not constant, and performance criteria can change dramatically as a result of formulation and environmental factors. This article discusses additional evaluation gauges to help hair product development scientists with formulation optimization.
Perhaps the simplest and undoubtedly one of the most insightful exercises involves casting polymer films from prototype and finished formulations. This is achieved by spraying or pouring the product into a suitable receptacle, then setting it aside for a few days to allow for the evaporation of volatile ingredients and drying of the film (see Figure 1). At the end of this duration, the visual properties of the film, i.e., color and clarity, can be observed while simultaneously gaining an appreciation of its hardness.
The mechanical properties of the resulting film are dictated by a number of variables. Most obvious is the selection of the polymer itself, but these characteristics can be significantly modified by formulation ingredients. The most common means of manipulating stiffness are through the selection and use levels of a neutralizing agent. The solubility of polymers containing carboxylic acid functionality can be achieved and facilitated by raising the pH, which generates the carboxylate ion and produces charged groups on the molecule. This is generally accomplished through the use of small organic bases, e.g., amino methypropanol (AMP), which also become entrapped in the polymer coating and in doing so, act as plasticizers that soften the resulting film.