Water-saving toilet coating

26 November 2019

Interview with 

Tak-Sing Wong, Penn State University

PUBLIC TOILET

PUBLIC TOILET

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Researchers at America’s Penn State University have found a way to dramatically cut the water use of the average toilet, by making the pan super-slippery. This means much less water is needed to flush them and dislodge anything sitting in the bowl. The new treatment, which can be applied to existing as well as new toilets, comprises two layers. The first causes lots of tiny hairs, called nano-hairs, to grow on the bowl like 5 o'clock shadow. These nano-hairs are strongly hydrophobic- they repel water. But to make the bowl even more slippery, a second silicone-based layer is added on top, which should make skid marks impossible, as Adam Murphy heard from the product’s creator Tak-Sing Wong, starting with the nano-hair layer...

Tak-Sing - The nanohair is about one nanometre thick. So you're talking about... it's about a hundred thousand times thinner than the diameter of a human hair. And the way we put this coating on is simply by spray coating. You spray the first part of the coating, which forms the nanohair that is permanently bonded to the toilet surface, and then the second part is spraying a lubricant, and then that completes the process. The whole process only takes within five minutes and you can do it at room temperature.

Adam - What are they made of then to let them grow?

Tak-Sing - Yeah, so the nanohair is made of silicon, it's silicon nanofibres. And the lubricant is also a silicon oil. So you have this solid silicon nanohair infused with silicon oil.

Adam - So they both work together. But how good are they each on their own? Or do they both need to be together to function at all?

Tak-Sing - So with just the nanohair alone it's already fairly liquid-repellant, and can also reduce the adhesion from sticky substances by 70% compared to an untreated surface. With the lubricant, with the silicon oil lubricant, it further enhances the slippery function and can reduce the adhesion by up to 90% of sticky substance compared to our untreated surface.

Adam - And how long do they last, or will you need to be replacing them frequently?

Tak-Sing - For the first layer of the nanohair, that is permanently bonded to the toilet surface, or glass surface. So that you don't need to reapply or replenish. But for the second layer, that is the lubricant which is infused into the nanohair, you have to reapply it from time to time depending on the use case and the frequency of use.

Adam - And the hairs underneath the lubricant- how robust are they? How much punishment do they stand up to?

Tak-Sing - They are very robust. These nanohairs are bonded to the surface through this interaction called covalent bonding. It's among the strongest bonding you can get in nature. And we have done a laboratory test, we use sandpaper to rub against the toilet surfaces, and what we find is that you can't removed this nanohair even after 300 rubbing cycles from the sandpaper. So they are very mechanically robust.

Adam - And why would you want to go about making a super-slippery coating for a toilet? What's the purpose in doing this?

Tak-Sing - If the coated surfaces can prevent adhesion from human waste, first it will look better, and without a human fecal streak on the surfaces it can prevent the bad smell, the unpleasant odour, and also it can reduce bacterial accumulation. So that is more on the health aspect. Another aspect is the actual water saving. Take the toilet in the United States as an example- a typical toilet, we use about 1.6 gallons per flush, or equivalent to six litres per flush. 50% of this fresh water is required to transport human waste through the drain line and the sewage system. But the other 50% of the water, most of the time, you actually use it to clean off or rinse off the surface.

Now imagine if the surface is super non-sticking to human waste. You do not need that 50% of water to clean the surface. So you save that part of the water. And when you think about that, we have probably over a billion flushing toilets or urinals in the world. And if you add up the amount of water that we can save, that's a signifiicant amount of water.

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