Rheology is the study of the flow of materials and the deformation of materials under force and friction. When it comes to personal care products, flow properties count for a lot. For example, consumers typically look for a shampoo that flows from the bottle and spreads easily, but at the same time is not so runny that product is wasted. One main rheological parameter that correlates with the thickness and flow properties of a shampoo is the viscosity (it’s resistance to flow). Viscosity influences the cleaning efficiency, foaming properties, production filling, packaging, storage and long-term stability of the product.
The viscosity and thickness of shampoos and other personal care products such as shower gels are determined by the polymers used in the formulation. Manufacturers can modify the flow behavior of their products by using water-soluble polymers as modifiers to ensure the shampoo meets the expectations of different customer groups. (Read The Rheological Behaviour of Shower Gel – What makes a product acceptable for a specific target customer? to learn how products for these customer groups differ rheologically and how the parameter viscosity can be determined with rheological instruments.)
Even when a shampoo is formulated to have the perfect viscosity- not too thick, not too thin- it still won’t come out of a plastic bottle completely. This is because the surfactants in shampoos and soaps stick to plastic, making it difficult to get that last bit of shampoo from the bottle. This may be a minor concern for consumers, but it’s a big problem as far as recycling the plastic shampoo bottles.
Plastic recycling already faces challenges: the technology is relatively new, and it is difficult to distinguish between recyclable and non recyclable plastic products. In the case of shampoo and other personal care products that come in plastic bottles, the bottle must be completely clean before it can be recycled. That last bit of product stuck in the bottle isn’t just a nuisance, it’s an environmental issue.
That is why researchers at The Ohio State University have developed a patent-pending technology to create a texture inside polypropylene plastic bottles that lets soap products flow freely. Polypropylene is made into bottles and bottle lids for shampoo, soap and detergents, as well as food and medical packaging. As described in an article on EurekAlert! the technique involves lining a plastic bottle with microscopic y-shaped structures that prevent the soap from actually touching the inside of the bottle. The “y” structures are built up using much smaller nanoparticles made of silica or quartz which, when treated further, won’t stick to soap. The university hopes the technique may someday be used for other plastic products that have to stay clean, such as biomedical devices or catheters.
Editor’s Note: Read the rest of the personal care polymers series, Personal Care Polymers: Which Shampoo is Right for You? and Which Shampoo is Right for You? Testing Polymers to Find Out.