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Comment by TYFYC | Hacker News Reader

Barium Sulphate is nothing new. It's used widely on integrating spheres, and I have painted some myself. The video that they show painting it on a piece of glass and comparing that to commercial paint is very misleading. It is not the easiest thing to work with, and is not at all durable. Once on a surface, it dries to a white chalky texture and just touching it will get it dirty and diminish its reflective properties. You would have to have so many additives to make it weatherproof and durable enough for outside that it would end up being no better than standard white paint. When I used it to coat an integrating sphere, it had to be thinned with 100% alcohol (impossible to obtain unless your a pharmacist because all non-drinking alcohol is poisoned so you can't get drunk without paying a liquor tax) so that all the thinning agent would dry out without leaving any residue behind. Anything left in the Barium Sulphate will diminish its reflectivity.

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That's too dismissive take of the paper. The whole innovation here is the formulation of BaSO4 based acrylic paint that retains most of the reflective properties while being practical as a paint instead of using pure BaSO4 which would be too fragile.

Select quotes:

> Overall, the BaSO4 film reached a solar reflectance of 97.6% and an emissivity of 0.96 in the sky window (Figure 2b). Its solar reflectance is significantly higher than that of the commercial white paint (400 μm thickness), especially in the UV and NIR ranges. [...] For the BaSO4 paint, a standalone paint sample of 400 μm reached similar optical properties (98.1% solar reflectance, 0.95 sky window emissivity)

> To demonstrate the reliability of our BaSO4 paint, we conducted abrasion test, outdoor weathering, and viscosity characterizations. [...] The BaSO4 paint reached a wear index of 150, comparable to the commercial exterior paint with a wear index of 104. The weathering test was conducted by exposing the BaSO4 paint outdoors for 3 weeks (Figure 4b). The solar reflectance remained the same within the experimental uncertainty.

100% this.

I cringe every time a HN comment starts with “this is nothing new; not ground breaking; people should’ve expected this”.

So much for a discussion forum where you’re meant to read the post and spark insightful commentary.

Restore balance to the force, you must.

3 weeks of outdoor exposure means absolutely nothing. The minimum for most paint manufacturers is at least 2 years in a sub tropical area like Florida.

> Barium Sulphate is nothing new (…) It is not the easiest thing to work with, and is not at all durable

I only read the abstract, but it seems like they’ve considered your stated limitations.

“we experimentally demonstrate remarkable full-daytime subambient cooling performance with both BaSO4 nanoparticle films and BaSO4 nanocomposite paints”

“providing great reliability, convenient paint form, ease of use, and compatibility with the commercial paint fabrication process.”

Also, you can always get commercial licenses for restricted substances like ethanol; that’s not an actual problem for house paint jobs.

> all non-drinking alcohol is poisoned

...in the USA.

In many (most?) other countries pure ethanol can be bought easily over the counter, and where I live even at any gas station. Yes, you could drink the ethanol from the gas station here (it has no additives, but better dilute it first), but nobody does because the cheapest cachaça isn't any more expensive.

It's a peeve of mine. One of those inconvenient truths of history that people don't like to talk about or don't know about. We (the US government) started poisoning alcohol WITHOUT TELLING PEOPLE. The fact that the practice continues is ridiculous.

My father worked for Saudi petrochemical company SABIC in the 80s. Some bright spark decided to go to the ethanol factory and get some high purity hooch from a tap and make an end run around Saudi alcohol restrictions. Unfortunately he made a wrong turn and went to the methanol factory instead…

SAme for the UK, it should have some percentage of methanol or some sort of difficult to remove bittering agent

I can't talk for all countries, but in Austria the cheapest alcohol you can buy is "Spiritus", and they add something to make it taste bitter. You can use it as fuel, for degreasing, cleaning, etc.

You can get pure, food grade ethanol at the pharmacy.

Definitely not the case in the EU.

You can buy it, but it'll be taxed like drinking alcohol.

However, I'm surprised that other alcohols (e.g. isopropanol) that don't suffer from this cannot be used instead of ethanol.

> 100% alcohol (impossible to obtain unless your a pharmacist because all non-drinking alcohol is poisoned so you can't get drunk without paying a liquor tax)

Only ethanol needs to be denatured. You can easily get 99.953% pure isopropyl on Amazon, and it isn't poisoned because isopropanol is already poison.

I mean ethanol is denatured by adding methanol, which is also an alcohol, so technically the mixture is still 100% alcohol :-) Except for drawing water due to being hydrophilic etc.

I think sometimes they use acetone.

They don't add methanol. You can check this easily by looking up a Safety Data Sheet. They will add a bittering agent to make it extremely unpalatable.

> 100% alcohol (impossible to obtain unless your a pharmacist because all non-drinking alcohol is poisoned so you can't get drunk without paying a liquor tax

Actually, its because ethanol is a aziotroph with water at about 95%. To distill it further, you need to add dangerous and toxic organic compounds. 95% ethanol is the highest that is still safe to drink.

Would zinc oxide or titanium dioxide be a better option? They use that white in everything. Might be durable on a roof.

Possibly, possibly not. One thing they really like with this is that it radiates heat photons in wavelengths that go through the sky rather than being reflected back to the ground by the greenhouse effect.

Woah! Now that is definitely a special attribute.

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