Wipe On, Wipe Off: Make Your Own Rain Repellent

Once upon a time, we drove an old six-volt VW Beetle. One sad day, the wiper motor went out, and as this happened before the Internet heyday, there were no readily-available parts around that we were aware of. After briefly considering rubbing a potato on the windshield as prescribed by the old wives’ tale, we were quite grateful for the invention of Rain-X — a water-repelling chemical treatment for car windshields.

Boy would we have loved to know how to make it ourselves from readily-available chemicals. As you’ll see in the video below, it doesn’t take much more than dimethicone, sulfuric acid, and a cocktail of alcohols. [Terry] starts with dimethicone, which he activates with a healthy dose of concentrated sulfuric acid, done under the safety of an exhaust hood. After about 20 minutes on the stir mix-a-lot plate, [Terry] added ethanol and isopropyl alcohols. Finally, it was off to the garage with the mixture in a spray bottle.

After meticulously cleaning the windshield, [Terry] applied the solution in small areas and rubbed it in with a towel to create a thin bond between it and the glass. This creates a perfectly normal haze, which can be removed after a bit with a clean towel.

If you just love listening to your windshield wipers, at least make them move to a beat.

Thanks for the tip, [askcheese]!

56 thoughts on “Wipe On, Wipe Off: Make Your Own Rain Repellent

  1. I used to use Rain-X liquid. Applying it was a PITA. Then I discovered Rain-X washer fluid, and haven’t bought any different washer fluid ever since.

    This is a cool science experiment, but the dangers of working with acids strong enough to need a fume hood probably make it not worth repeating when you can buy RAIN-X washer fluid for a few bucks.

        1. I’ve also had it effectively “glue” bug guts to my windshield. I kept hitting bugs on a long summer road trip and they weren’t coming off. I stopped at a gas station, scraped them off with the damn ice scraper in the trunk, bought some normal cheap washer fluid, refilled, and never had dead bug sticking problems again the entire trip.

    1. Why are people so paranoid about strong acids?

      When I was in middle school the local science hobby store _refused_ to sell me glycerin and fuming nitric and sulphuric acids. Bloody fascists!

      Fortunately dad directed me to the local gun store’s reloading isle.
      USA! USA! USA!

      The fake ID came in handy a couple of years later when I wanted beer. All good fun!

  2. I’ve seen a very good water beading effect from the paint some car dealers used to put prices on windshields. Wash it off and where the paint was is visible when it rains because it cannot sheet on it nor form large drops. It balls up into tiny drops then blows away.

  3. Professional chemist here. The instructions included here are exceedingly dangerous. Much worse than the acid, this creates diethyl sulfate, which is known to be genotoxic and highly likely to be carcinogenic (look it up on wikipedia to verify what I’m saying about how bad it is). If you follow the instructions as they are presented here, you run the risk of killing yourself and anyone else exposed to this. Diethyl sulfate is hazardous even at ppm levels, so it is not worth trying or even modifying this experiment without the analytical tools to verify that diethyl sulfate isn’t present.

    If you’ve already made this (mixing the acid with ethanol and/or isopropanol), don’t throw this away or flush it down any drains. That will only move the hazard on to someone else who wasn’t ignorant enough to do this. WEARING 2 PAIRS OF NITRILE GLOVES: put your container in a heavy-duty ziplock bag (think freezer rated), remove the outer gloves, then put the outer gloves and the first ziplock bag into another heavy-duty ziplock bag (preferably one-size larger). Then take this to your local hazardous waste disposal unit. Search “X county household hazardous waste”, where X is the name of your county for details.

    1. Professional Engineer here: I’m pretty sure you’re wrong.
      Afaik and according to the handbook I had closest (Ulmman’s Encyclopedia), sulfuric acid + alcohols should lead to an equilibrium with alkylsulfuric acid and water unless you really crank up the heat which should lead to dehydration products (not recommended by the author). Where’d you get the idea this would lead to dialkyl sulfates?

    2. I’ve been (and still am) a pharmaceutical process chemist for the past 24 years. A significant part of my job is identifying impurities and designing reactions and processes to avoid them. Sulfuric acid plus ethanol gives diethyl sulfate + water. This is a known reaction in my industry, and there are many papers discussing this. You don’t have to take my word for it. Do a search for diethyl sulfate synthesis. I’ll get you started with this link from the NIH (https://pubchem.ncbi.nlm.nih.gov/compound/Diethyl-sulfate#section=Methods-of-Manufacturing).

      This is an equilibrium reaction. Given that there’s very little water present and a vast excess of ethanol (183 molar equivalents in this recipe), the equilibrium will be shifted towards diethyl sulfate. The monoethyl hydrogen sulfate you reference above is an intermediate. The reactions look like this:
      1. EtOH + H2SO4 H2O + EtOSO3H
      2. EtOH + EtOSO3H H2O + (EtO)2SO2
      translated to English:
      1. ethanol + sulfuric acid reversibly react to give water + monoethyl hydrogen sulfate.
      2. ethanol + monoethyl hydrogen sulfate reversibly react to give water + diethyl sulfate.

      When you start out with 183 parts ethanol for every part sulfuric acid (with very little water), the equilibrium gets pushed pretty hard to the final product of diethyl sulfate.

      As stated before, this chemical is extremely hazardous at ppm levels, so it doesn’t matter if you only make a little bit. It’s still bad. (fyi: the process described on this page does not make just a little bit…it makes a lot). You also can’t fix this by just adding a bunch of water. That will shift the equilibrium back to the left, but you’ll still have dangerous levels present (remember, ppm levels are still genotoxic).

      As others have also pointed out: why do all this when actual Rain-X is available and cheaper?

      And Matt, why are you actively trying to refute this? By your own admission, it isn’t your area of expertise, yet you took the trouble to look up a single reference and try to tell others I’m wrong about my actual real-life job. Why? At least the original poster can claim ignorance. You hear an expert say “something is dangerous, please don’t do this” and think: I know better than that. Just stop.

      1. And I thought that water would decompose it. Was more concerned about leaving the sulphuric acid in the solution without neutralizing it, it really likes to destroy steel. Maybe adding some ammonia at the end could help?
        BTW, back in the lab days we were once preparing diethyl ether by heating a mixture of 98% sulphuric acid and ethanol. Remember that they specifically told us to not use more concentrated acid, and that 95% was also ok. Just saying.

      2. Thanks Bryce. I haven’t done API (active pharmaceutics ingredient) synthesis for 20 yrs and even then I definitely listened when someone was saying there was even a chance of super toxic byproducts.
        You also accurately summed up my feelings when I comment on medical stuff, my current profession, here at HaD and then people get all confrontational. It’s hard to ignore.

      3. Im general I’m actively trying to get you to back up your claims with more than “trust me, I’m an expert”, after all the OP also claims to be a PhD Chemist (did you check [Terry]’s about me page?).

        1. You seem to want to appear to be technically inclined. So here’s more you can look at to back up my claims (as you apparently desire). Search for “dimethyl sulfate synthesis mechanism.” Yeah, there’s more information about methanol than ethanol, but they’re both primary alcohols and behave analogously. (But you knew that, since you’re an engineer). The wikipedia page is rather helpful in backing me up, although you’re also likely to get a lot of hits under the category “Scholarly articles”, which are usually behind paywalls. Sorry about that. You can probably tell from some of the titles (“Assay at low ppm level of dimethyl sulfate in starting materials for API synthesis using derivatization in ionic liquid media and LC–MS”) that ppm-level control of these types of impurities are an ongoing concern.

          I did see that Terry Clayton (the guy in the YouTube video) was a PhD chemist. I don’t doubt that he is. In fact, his LinkedIn page leads me to believe he’s a rather accomplished chemist in a variety of industries. He’s also currently a professor. He probably hasn’t come across the hazards of forming dialkyl sulfates, because it’s not a focus in any of the work he’s done. Believe it or not, chemistry is a big field and nobody knows all of it. Kinda like engineering that way. I’ve reported his video on YouTube as being dangerous, and I’ve reached out to him personally through his website, explaining the situation and kindly asking him to voluntarily remove the video. Because just like Kristina Panos (author of this HaD article), he’s merely unaware of the hazards.

          You, on the other hand, seem intent on attacking me, and you’re having a bit of a revisionist moment. Did you ask me to back up my claims? No, you said “I’m pretty sure you’re wrong.” Did I ever say “trust me, I’m an expert”? No, I pointed out what was wrong, urged others not to do this, and gave a safe remediation strategy for those who may already have done it. I didn’t get into the technical details of how this was dangerous in that first message, because that would glaze people’s eyes over and just look like showboating. Had you simply asked for references, synthetic mechanism, or an explanation I would have given you so.

          Reply all you want, if getting the last word makes you feel like you’ve won. I’m unsubscribing from this page so I can move on with my life. Haven’t been in a pointless internet fight in a while, and regretting getting into this one. I’ve presented the hazard, and explained enough that anyone curious can look into it more to satisfy themselves. If somebody else wants to try to further refute me, and that leads someone else to injure/kill themselves or someone else, that’s on them.

        2. So, if a bomb tech told you a hack about how to do disarm a bomb then another bomb tech told you not to do this and why, would you still try the hack or would you leave it to the professionals? Just because someone says something can be done doesn’t mean that it’s a good idea. Or, that it’s safe. Obviously, Terry has experience and expertise that make this a hack that he can do. The rest of us should just buy rain-x and ca) the bomb squad.

      4. Professional aviation electrician here. Seems like Bryce knows what he’s talking about. Maybe the chemist knows a little something about chemicals. I don’t even have to Google it to know that I’m going to listen to a chemist when he says, “Don’t do this”, about anything having to do with chemistry.

  4. I once looked into making my own bourbon at home. In addition to being, very charitably, a legal gray area, it turns out that the effort needed to make, distill then age it for years left me with the conclusion that the stuff is readily available at the grocery store, all the time, and way better than I could ever do at home.
    Feels relevant to this somehow…

      1. Instead I spent obscene money on upgrading my home beer making operation lol. The realization that even a small amount of law enforcement questioning of home distilling could cost me my professional license and livelihood wasn’t worth it at all.

          1. My current rig is, briefly, a 24 gal brew kettle fired off of plumbed in natural gas with a custom fabricated SS mesh basket that is used to do full volume mashes. I can brew a half-barrel of normal strength beer at a time. It is all plumbed and pumped through sanitary fittings and hoses. Cold end is a pair of SS conicals in nominal 5 and 10 gallon sizes, each with heaters and glycerol chillers for 4 seasons brewing in the garage. They are pressure rated so I can spund (ferment under pressure) or simply carbonate in the fermenter prior to packaging, which I typically keg using an oxygen-free transfer rig I built. I also have a “kegmenter” made by kegland Australia for full half-barrel ferments but there isn’t much benefit to making 15.5 gal over 10 gal. I literally can’t give the beer away fast enough (10 gal is 20 growlers) sometimes. My very best advice though is get some brewing software (I use Brewsmith) that allows you to take very good notes that won’t get lost or forgotten. If I started from scratch right now, I would recommend a 5 gallon all electric set up (brewzilla, Grainfather etc) for ease of use and good initial results.

          2. I’m officially jealous of Craig. I want conicals for fermenting cider. Ideally, a couple of 15-25G as I got enough apples for 32G last harvest. Dealing with 5 or 6 different batches is a pain. They’re rather pricey though.

      2. Add oak chips then microwave it. You can make it in minutes. Obviously different to barrel aging. I also read you can use electrolysis to age it. Given smaller barrels have a larger surface to volume ratio I’m tempted to say the home distiller can do it quicker than the industrial scotch.

  5. Just buffing a bit of silicone oil onto the windshield does the trick. Get the stuff sold for lubricating treadmills. About $6 for 4oz. Solvating it helps with the application but it is only weakly soluble in IPA or ethanol. I’m still looking for a good non toxic solvent.

      1. I believe most people have missed the point here. The reason for treating the dimethicone (a silicone oil – polymethylsiloxane) with sulfuric acid is probably to make it slightly polar so that it will dissolve in alcohols and to increase it’s bonding with the substrate. There is no reacting of alcohol and sulfuric acid. Ideally all the acid should have reacted with the silicone oil. You can use just pure treadmill silicone oil directly onto cleaned glass by using a feltpad and buffing it off with elbow greas. By solvating the oil it is a bit easier to initially spread it around on the windshield. Rain-ex MSDS says the solvent is a mix of water and acetone. Looking at a mix of patents in the area, the idea of reacting the oil with acid seems to be claimed to make the substance stick a little better to the inside of aluminum cans for making the can water-repellent. It may also improve longevity of adhesion to the window glass. I’ve experimented with pure oil and it works quite similiarly to rain-ex itself. No need in my opinion for doing the extra acid step, especially if you can make gallons of the stuff for a few bucks. Just apply it every time you squeegy the window. I did my tests on a few drops borrowed from my girlfriends treadmill kit. I’m getting a bottle of my own and I’ll try using Acetone instead of IPA or ethanol. Neither of these would dissolve the oil at all. I was able to get a bit of a mayonnaise like effect wtih small droplets and then dispersed these on the windo prior to buffing the oil off.

  6. I reported Bryce’s comment just to make HaD editors aware of the risks here as leaving an article like this up with no warnings about the health risks seems irresponsible at best.

    I’d have hoped that HaD at least pass articles/links through some sort of sanity check before publishing them to avoid cases like this.

    1. Did you see mentioned in the commenyd that the original author is also a chemist? In fact, a PhD. So, maybe HaD was OK trusting him, as much as we should trust Bryce.

      A discussion between him and Bryce could be enlightening…

    2. Some time ago I gave HaD some sh*t in the comments section on several consecutive occasions regarding publishing (or reposting) some, IMO, super dangerous articles. The editors actually wrote an article about the topic of journalistic responsibility, kinda, and the comments section turned really ugly, quickly. I don’t think a whole lot was resolved, but typically articles do now include some passing nods to safety. This article actually states “under the safety of an exhaust hood” which, prior to the sh*tshow article mentioned above, I don’t think they would have included.

  7. @Chemists: My BMW X5 has an “aquaphobic” windshield. Rarely need the wipers when it rains. How does it actually work? Until today I had never thought about it, but now I’m curious. One thing is certain: potatoes are not involved. At least I think so ;-)

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