The Science Behind “The Paint That Pees Back”

Another week, another story about urine. You’re welcome, you guys! This one begins in Hamburg, in the St. Pauli district. St. Pauli, like many areas famous for their nightlife, had a problem with public urination. This left the streets smelling and stained, just like other areas. It was not pleasant. It was “pee back time”. They reached out to the creators of Ultra Dry (r),. This product claimed to repel water-based (and some oil based) liquids from all surfaces. It was sprayed on the walls of the district with signs that said “Wir pinkeln zuuck” (“We pee back”), and the story made national headlines around the globe. Soon after, San Francisco, a small city that already had a human waste problem, followed suit.

I’ll let my Forbes contributor Janet D. Stemwedel discuss this approach. Spoiler: It probably isn’t. But, for now, we can talk about how paint works. Let’s begin with the things that we don’t know.

  1. The paint’s chemical composition. It is mostly acetone, which is also used in nail varnish remover. A few percent of silica (siliconium dioxide) can be found in the material safety sheets. This component of sand is a major constituent. The 1% remaining is the interesting part – the unnamed additive which repels liquids. The company states that the final product will be “susceptible environmental conditions such as ultraviolet”. This could indicate that the additive is a type polymer. However, I am unsure so please don’t quote me.
  2. It repels liquids. This coating is applied in two steps. The basecoat is like a make-up primer. It smoothens the surface to allow the topcoat to adhere to. This second layer repels liquids. However, it is essential. Let’s talk about hydrophobic material to better understand why.

This was an area that I studied during my time at the National Physical Laboratory. We were interested in lubrication and how we could reduce friction through printing tiny patterns onto various materials. The shape of the droplet of liquid on a surface can be used to measure its effect. A hydrophilic surface (literally, water-loving), is one where water sticks easily to it. Therefore, a droplet of liquid on it will look somewhat like a dome. Drop some water onto your desk or on a plate nearby and it will spread out flatten. A hydrophobic surface (water-fearing), will keep a droplet of water roughly spherical. This type of surface is best illustrated by the lotus leaf. It remains clean and matte, but water collects on it to form jewel-like blobs. Hydrophobic materials can be difficult to wet. Sometimes, it is even impossible. That’s how you make water- or pee-friendly surfaces. – repellent.

Hydrophobicity of lotus leaves is due to their complex pattern of small bumps that are many times thinner than human hairs. A water droplet can’t penetrate the leaf because of the tiny bumps. Instead, H 2O molecules stick together to form a ball that is able to roll off the leaf when it is tilted. This is the same idea that Ultra-Ever Dry (r), used to make their anti-pee wall coating. The basecoat has a particular chemical composition. The topcoat interacts with these molecules when it is applied. According to their FAQs forms a pattern of “high points”, and “lower regions” that together prevent liquids (not just water) from sticking. This material is extremely good at repelling liquids. A surface coated with it becomes superhydrophic.

This means that pee-ers who are trying to get out of the street won’t see warm water running down the walls. Instead, their urine would bounce off the wall and not leave a stain. This is great. However, it wouldn’t get on their shoes. You can see why city managers are so eager to get rid of the smell and sight of urine on the streets. It is also relatively inexpensive – according the BBC Hackney, London, spends PS100,000.00 a year cleaning out urine from walls and footpaths. However, the Washington Post states that you can purchase enough Ultra-Ever Dry for six square meters for $700. It’s not magical. The coating’s hydrophobicity is dependent on a very specific combination of small peaks and valleys. It becomes less effective when rubbed, abraded, or eroded. According to the manufacturers, this paint would need to be reapplied at least every year to outdoor surfaces.

Many other cities now follow San Francisco and Hamburg in spraying anti-pee paint on walls. It’s an effective deterrent for people who are too drunk or lazy to look for alternatives, but these cities recognize it’s only a temporary solution and that it isolates the homeless. Building more public toilets is the best way to get rid of human waste from the streets.

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