Shampoo bottle empties to last drop

ENGINEERS at Ohio State University, US, have come up with a unique coating for the inside of plastic bottles for liquid detergent products such as shampoo, hand soap and laundry liquid.

Lead researcher, mechanical engineering professor Bharat Bhushan, admits that this is something of a “first world problem”, but it does have serious sustainability issues, with bottles ending up in the waste still containing product. In order to be recycled, the bottles must first be completely clean, so a bottle from which all the product can simply pour out, would be useful.

Bhushan and Philip Brown used a spray-coating process to cover the inner surface of a bottle with ultra-fine silica nanoparticles. The nanoparticles, which are just a few micrometers high and somewhat spiky and Y-shaped, are dissolved in a solvent which softens the surface of the bottle slightly and allows the nanoparticles to embed there. The nanoparticles end up a few micrometers apart, and the main branches of the Y-shape are too steep to allow them to support a droplet that could fall between them. This creates an air pocket underneath, meaning that any liquid inside the bottle never actually touches the polymer sides. The liquid repellency can be further increased by coating it with fluorosilane.

“Compared to soaps, getting ketchup out of a bottle is trivial. Our coating repels liquids in general, but getting it to repel soap was the hard part,” says Bhushan.

The difficulty comes because the surface tension of liquid detergents is so low. They are designed to bond to other surfaces to allow grease and dirt removal, which also means they stick to plastics. Water molecules, on the other hand, have a high surface tension and are more likely to stick to each other, meaning they adhere less strongly to surfaces and roll off more easily, as do products high in water, like ketchup.

So far the coating has been applied to polypropylene and polycarbonate surfaces. Polypropylene is not the most common bottle material, but in the US alone, more than 80,000 t/y of polypropylene bottles and caps are made. The researchers say that with further development, the coating could also be used for other applications, such as medical catheters.

Philosophical Transactions of the Royal Society A DOI: 10.1098/rsta.2016.0193