Let's Make Rain-X
Updated: Feb 10
I had taken an interest in ceramic coatings for the automotive industry putting together some formulations based on silazane chemistry. They work well, but a good deal of caution needs to be taken in preparation as the raw materials are quite reactive..... I also didn't feel the longevity met my expectations. You can see how I made a ceramic coating below.
I appreciate the protective properties it adds to your vehicle but what I liked most was the water repelling property... So I looked for something that would be both easier and cheaper and repel water the same way. Rain-X.
Rain-X is a product used to make your windshield repel water. It's about $6 for a few ounces and can be purchased at AutoZone among other stores. I have always appreciated the water repellency that Rain-X offered my windshield. It does not boast diamond-like hardness (which is really nothing more than marketing), but it still works great to help visibility during heavy rainstorms. So, I decided to peak under the hood and determine how complex this chemistry is. In this short article I will walk you through its preparation and use. I have also included a link to a short video where you can watch me prepare and test it out.
Without further ado, let's make some Rain-X!
Here is the recipe for 200 grams which is about 7 ounces. This will be enough for me to coat several windshields when completed.
Recipe (In order!):
2g fuming sulfuric acid (H2SO4)
172g ethanol or methanol
8g isopropyl alcohol
Using a 250mL beaker, add 20 grams dimethicone. Add 2g fuming sulfuric acid and mix this for 20 minutes. Use a teflon stir bar and stir plate. You will notice the viscosity decrease as the dimethicone (polydimethylsiloxane) hydrolyzes. You can apply heat to accelerate the process, but I don't recommend going higher than 50-70C. Next dilute with 172g of ethanol and 8g of isopropyl alcohol. You will likely end up with a hazy emulsion. I like to tap off a small vial for observation and put the rest into whatever bottle you want to use to store it. I chose an empty spray bottle. Thats it! You're done.
Clean your vehicle windshield very well with soapy water as necessary.
Rinse and dry with a terrycloth towel.
Clean your windshield with Windex and dry thoroughly.
(optional) if you still have sap or other spots, you can clean with isopropyl alcohol.
Allow windshield to dry thoroughly.
SHAKE UP your repellent emulsion just prior to use.
Spray your new repellent on windshield and rub in using a circular motion and terry cloth towel. Cover your entire windshield.
Allow repellent to dry. It will look like a cloudy haze across your windshield.
Using a clean terrycloth towel, gently wipe windshield in circular motion until it is crystal clear.
You now have a microscopic coating a water repellent bound to your windshield via the hydroxy groups you created.
A couple of comments on the method above. Its critical you clean your glass REALLY well. The reason why is you want the key ingredient, hydrolyzed polydimethylsilicone to bond to it. Oils, sap, dust, dirt, salt etc. will interfere with it and reduce the effectiveness and how long it the effect will last. Remember your repellent is an emulsion and may separate so give that bottle a good shake before application! When the repellent dries to a hazy film remember, THIS HAZY RESIDUE IS THE KEY MATERIAL. The hydrolyzed polydimethylsilicone is the haze. When you rub it off with a clean terry cloth, you are essentially polishing your windshield, leaving a micron layer thick coating on the windshield. Once you have coated your windshield with a form of PDMS, you have essentially reduced the surface energy to around 20 mJ m^-2. For comparison an auto windshield (untreated) may range in surface energy from 40-60 mJ/m^-2.
Here is the video and good luck if make some yourself!