Flushing the system

Materials World magazine
3 Jan 2020

A repellent coating has been developed to reduce residue sticking to toilets, in a bid to minimise how much water they need to dispose waste. Anthony Caggiano reports.

A coating to help avoid stains on toilets and reduce the amount of water consumed when flushing has been created by researchers at Penn State University, USA.

The coating, named liquid-entrenched smooth surface (LESS), is designed to repel aqueous fluids, bacteria and viscoelastic solids. It is sprayable onto the surface to be protected and is claimed to reduce water consumption by about 90% over uncoated surfaces.

Reported in the paper, Viscoelastic solid-repellent coatings for extreme water saving and global sanitation, published in Nature Sustainability, the development of the coating comprises a two-step method, which can be applied to materials including ceramic, steel and vitreous china.

Making the product

There are two layers to the product – one that coats the bowl to shield it, and a second that acts as a lubricant to repel matter. Forces between the two layers hold them together to make the system. The first layer consists of a series of molecularly grafted polymer nanofibres, which is sprayed directly onto a clean toilet bowl.

A silicone-based silane was used to chemically create a nanofibre chain. ‘The silane molecules can react to each other and form a chain. One silane molecule is one unit of the nanofibre chain,’ Postdoctoral Fellow and first author, Jing Wang, told Materials World. ‘In addition, one end of the chain is chemically bonded to the surface, leaving the other end free.’

The coating co-developer and Wong Laboratory for Nature Inspired Engineering Principal Investigator, Tak-Sing Wong, added that, ‘Dimethydimethoxysilane (DMDEOS) is polymerised into polydimethylsiloxane (PDMS) through condensation polymerisation reaction – it is like building Lego pieces one by one from bottom-up’.

Once the DMDEOS is sprayed onto the surface, the hydroxyl groups in silica-based materials such as glass, silicon and china, react together to grow polydimethylsiloxane nanohairs.

‘The PDMS nanohair will form covalent bonds with the hydroxyl groups in silica-based substrates,’ Wong said.

This layer acts as a substrate for the second material. The second layer is a silicone-based lubricant, which can infuse into the nanofibres and form strong inter-molecular forces between the two layers. Wang said the lubricant can physically and chemically remain stable on the first layer.

‘The silicone oil is interacting with the PDMS nanohair through van der Waals interactions, due to the similarity in the chemical composition of the silicone oil and PDMS nanohair, they can interact with each other,’ Wong added.

The first layer could last for a long time as it is permanently bonded to the toilet surface through covalent bonding. But the second layer will wear off depending on frequency of use. Wang and researchers believe for a typical toilet with a 6L flush, the lubricant would need to be reapplied after about 500 flushes.

Water usage

According to the paper, toilet flushing accounts for 24% of all indoor wastewater-producing activities and is the largest contributor of indoor household wastewater production, in the USA.

‘A number of approaches have been proposed to reduce freshwater consumption for toilet flushing that range from the use of rainwater for flushing to the use of self-contained dry toilets,’ the paper said.

‘Owing to a great variety of complex factors, such as local environment, resource availability and user preference, none of these approaches can completely address the water consumption issue. A relatively unexplored approach is to engineer the material interface of toilet surfaces to significantly weaken the adhesion of human faeces and urine to reduce the amount of flushing water for waste removal.’

Wang said tests had shown about 3L of water is needed to flush bulk waste – half the amount used in standard toilets.

Influence of toilet design

Water usage can change, depending on the design of the toilet. For example, Cranfield University, UK, has invented a waterless toilet system, named the Nano Membrane Toilet project.

Wang said the UK team reached out to the USA in search of a non-sticky self-cleaning surface for their toilet bowl. ‘The toilet design would address the water consumption for flushing bulk waste, while the LESS coating would reduce the water consumption for cleaning’ Wang said.

LESS coating technology is being commercialised by start up company spotLESS Materials, and the products are available online.