The main universal challenge with using soluble film is water content. Typically, we aim for a maximum of 5% water in a soluble packaging system. This creates a formulation challenge immediately, as the standard make up of cosmetic products and the concentration of the marketed raw materials does not lend itself to such low water percentages.
The next challenge is creating the expected sensory profile for the consumer. In cosmetics we’re typically formulating for either a leave-on or a rinse-off formulation. Typically leave-on formulations are thick and creamy, whereas rise-off are foamy. With most cosmetics, the end user also expects fragrance, colour and even particular skin feel depending on the product. So when we formulate for concentrated, low-water soluble film suppliers products we must be careful to ensure our ingredients impart the right sensory profile so that the resulting product is familiar and understood by the user.
This challenge continues when we look at the concentration and efficacy of soluble packaging. With a 5% water-based formulation intended to be diluted by the end user, you must ensure the end format has the right level of fragrance, colour, actives and preservatives. The concentrated formulation needs to be fairly low viscosity for filling into the film, and so the formulation must thicken on dilution. Powder formulations must be free-flowing for ease of filling, have to be easily dispersible and in some applications may also have to thicken quickly with a minimal amount of shaking. This is challenging but is possible by calculating specific surfactant interaction. We must also ensure that it’s as effective as a ready-to-use product so that it’s competitive.
Of course, all of the above are difficult! Many fragrance and colour ingredients can affect the stability of the PVA film, and in concentrated form this can threaten the PVA structure so ingredients must be chosen carefully. Preservation is also a huge challenge, as the preservative level must be appropriate for both the concentrated form and the resulting diluted form. This may not only threaten the stability of the PVA film but could also result in stringent labelling requirements on packaging.
Similarly, stability testing must be done for both concentrate shelf-life and end use diluted product life. This concept is quite out of the ordinary and so testing raises some new questions for the industry too.