Closing the performance gap

Synthetic polymers, derived from petrochemicals, have long been the first choice due to their versatility and efficacy¹. However, there are growing concerns about their environmental impact as they are not biodegradable and may be categorised as Synthetic Polymeric Microparticle (SPM). The importance of environmental impact is also reflected in consumers’ desires for more sustainable products. In 2022, around 50% of product launches incorporated sustainability claims², highlighting the importance of exploring natural alternatives. As a result, there is a growing demand for bio-based polymers that match the high performance of synthetic polymers while minimising the environmental footprint of the product.

However, the development of biopolymers that match the performance of synthetic polymers is a significant challenge due to their inherent complexity. Factors including raw material source, extraction method, and processing influence the characteristics such as rheology, compatibility, solubility, and stability. And these characteristics are also affected by external influences, in particular potential interactions with other ingredients present in a formulation, such as surfactants, emulsifiers, preservatives, and active ingredients. A new biopolymer has been developed to balance the requirements on performance, versatility and sustainability, and to provide an effective alternative to synthetic polymers. It is based on Hydroxypropyl Starch Phosphate, a chemically modified starch that is becoming increasingly popular.³ 

Hydroxypropyl starch can differ in many ways, including its specific properties, source and manufacturing process, resulting in variations in performance and applicability. This biopolymer (INCI name Hydroxypropyl Starch Phosphate)⁴ is a modified starch and is derived from non-GMO potatoes, sourced and produced in Europe, is readily biodegradable according to OECD standards and formulated without the use of preservatives. 96% of this biopolymer is derived from renewable feedstocks. The product has a Natural Origin Index (NOI) according to ISO 160128 of 0.96.5  The versatility nature of this newly developed ingredient is underlined by its vegan-friendly6, kosher, halal and cruelty-free attributes. As its structure is non-ionic, it is compatible with cationic, anionic and amphoteric agents, allowing flexible use in cold and hot processes and combinations in a broad range of pH values (3–9). 

Rheology benefitss

Rheological measurements aiming to characterise the flow behaviour and viscoelastic properties of the investigated materials were performed using an HR 20 Discovery Hybrid Rheometer at 23°C and geometries as stated in the graphs. The flow properties were obtained by recording apparent viscosity values when shearing the samples at increasing shear stress (logarithmically). Viscoelastic behaviour of the materials was studied using dynamic oscillatory tests. Oscillatory Frequency Sweep measurements were performed at a constant strain amplitude and increasing frequency ranging from 0.1 to 10.0 Hz. The elastic modulus (G’), viscous modulus (G’’) and tan (o) were recorded as a function of the frequency. 

Study of the Aqueous solutions

Figures 1 and 2 present the results obtained with aqueous solutions of Hydroxypropyl Starch Phosphate at 5% and the synthetic benchmark (Sodium Polyacrylate at 0.5%). Hydroxypropyl Starch Phosphate shows shear thinning behaviour as does the synthetic benchmark, with slightly different flow behaviour. At high shear stress, Hydroxypropyl Starch Phosphate shows smooth breaking of the polymer structure, whereas Sodium Polyacrylate shows a slightly faster and more abrupt breaking of the polymer structure. Shear thinning behaviour (viscosity decreases when shear stress increases) gives an easy spreading and light skin feel to formulas⁷.

According to result illustrated in Figure 2, Hydroxypropyl Starch Phosphate has a similar rheology profile to Sodium Polyacrylate and shows dominant elastic properties when subjected to slow or moderate shear stress (frequency 1Hz). Both synthetic benchmark (Sodium Polyacrylate) and Hydroxypropyl Starch Phosphate show elastic properties which describe a gel-like behaviour that promotes and actively contributes to the stabilisation of emulsions (i.e. oil stability).

Study of the behaviour in emulsions

The stabilisation properties were also confirmed by the rheology study. The stabilising performance of the Hydroxypropyl Starch Phosphate in comparison with Xanthan Gum was tested by emulsion stability.

Hydroxypropyl Starch Phosphate shows shear thinning behaviour with a high yield value conforming the good emulsion stability during storage. The emulsion comprising only the biopolymer Xanthan Gum (Emulsion 2) is unstable and shows an immediate oil phase separation, while emulsions formulated with Hydroxypropyl Starch Phosphate alone (Emulsion 1) or in combination with Xanthan Gum (Emulsion 3) improve oil stabilisation. The results, shown in Figures 3 and 4, confirmed that the new biopolymer contributes to oil stabilisation, reducing oil droplet size.

Synergetic effect of thickening and 
texturizing performance

A synergistic effect of thickening and texturising performance was observed when combining Hydroxypropyl Starch Phosphate with another biopolymer8 (INCI: Xanthan Gum) as shown in Figure 5. The viscosity of the formulation was measured using a Brookfield RVDV-I+ viscometer at pH 5, with a rotation speed of 10 rpm.

The experiments were carried out as follows. While maintaining a concentration of 3% of the new biopolymer Hydroxypropyl Starch Phosphate, the level of Xanthan Gum was altered in the range of 0.1 to 1%. In each case of the addition of 0.1, 0.3 or 0.5% Xanthan Gum, a substantial increase in viscosity by a factor ranging from approx. 2.3 to 5 was observed in comparison with the same levels of Xanthan Gum alone constituting a synergetic effect. Formulations comprising only Hydroxypropyl Starch Phosphate or Xanthan Gum do not provide desired parameters, such as texture and flow behaviour.

As an ideal concentration for improved viscosity properties and attractive appearance, a formulation comprising 3% Hydroxypropyl Starch Phosphate and 0.3% Xanthan Gum was identified and is thus recommended. 

Sensory Profile

Although the sensory profile of a formulation is typically influenced by a combination of different ingredients rather than a single component. A head-to-head comparison was investigated between two simplex formulations (one made by cold and another one made by hot process) exchanging a synthetic reference material (Sodium Polyacrylate) with the biopolymer Hydroxypropyl Starch Phosphate in order to investigate the impact. For the evaluation of the sensory perception during and after the usage of leave on products, different formulations were investigated by volunteers in a direct comparison. A defined amount (150 µl) of the product was applied on the inside of each forearm and distributed there in 20 big circles. Defined attributes of the formulations were assessed during and after total absorption by a five-point-scale from -1 to +1 (compared to the reference, in its properties the test product could be more-or-less highly/slightly pronounced or be the same). The panelists evaluated the products individually. The results, as shown in Figure 6, indicate that the formulations with Hydroxypropyl Starch Phosphate had the same sensory profile as the same formulations with synthetic polymer.

Conclusion

The new Hydroxypropyl Starch Phosphate was shown to meet the functional requirements in the same way or even better than the synthetic benchmark. It showed dominant elastic properties when subjected to slow or moderate shear stress and exhibited a gel-like behaviour that promotes and actively contributes to the stabilisation of emulsions. 

The flow behaviour is close to that of the synthetic benchmark (sodium polyacrylate). Although both exhibit shear thinning, the new biopolymer shows a smoother breaking of the polymer structure under high shear stress, resulting in improved spreadability and sensory experience. It has sufficient thickening and texturising profiles in both leave on and rinse off applications.  The introduction of Hydroxypropyl Starch Phosphate offers a plethora of benefits for formulators. The biopolymer’s ability to meet the performance standards of synthetic counterparts ensures that formulators can maintain the desired functionality and properties in their products. Its compatibility with a wide range of ingredients, including cationic, anionic, and amphoteric, combined with its wide viscosity range and pH flexibility, allows formulators to use Hydroxypropyl Starch Phosphate in a variety of products in personal care that have different requirements. This flexibility can help to simplify manufacturing processes. Its sustainable sourcing and biodegradability contribute to reducing the environmental impact of the industry. Additional ongoing investigations are showing promising results in different application fields such as styling and will be published in an upcoming styling study.

References:

1. Lochhead, Robert Y. (2007), The Role of Polymers in Cosmetics: Recent Trends, Cosmetic Nanotechnology, 3-53.
2. 2023 Consumer Trends: North America, www.mintel.com/beauty-and-personal-care-market-news/beauty-trends.
3. The global hydroxypropyl starch phosphate market was estimated at $ 97.60 million and is expected to reach $142.92 million by 2029, exhibiting a CAGR of 5.60% during the forecast period. Global Hydroxypropyl Starch Phosphate Market Research Report 2023 (Status and Outlook), www.marketresearch.com/Bosson-Research-v4252/Global-Hydroxypropyl-Starch-Phosphate-Research-34162052/
4. Cosmedia HP Starch, further information available online: https://www.personal-care.basf.com/products-formulation/products/products-detail/Cosmedia%20HP%20Starch/30832555 (last access August 3, 2023).
5. Calculation of Natural Origin Index according to ISO 16128 excludes implications from manufacturing processes.
6. BASF provides customers with information for vegan suitability assessment. Compliance with a particular vegan label is unintended and responsibility of the cosmetic product manufacturer.
7. Brummer, R. (2006), Rheology essentials of cosmetic and food emulsions.
8. Verdessence Xanthan, further information available online: https://www.personal-care.basf.com/verdessence (last access August 3, 2023).