Evaporation temperature/rate of methanol in windshield washer fluid

Quick chemistry Q: when methanol is “bound” in windshield-washing fluid, what is its evaporation temperature and rate?

Not sure what you mean by “bound”. Its a mixture.

Again not sure what you mean by evaporation temperature because most liquids evaporate at all temperatures. They have a vapor pressure which is a function of temperature. Boiling occurs when the vapor pressure equals the pressure on the liquid (atmospheric pressure if boiling in open air).

So for methanol windshield fluid, the weight percent of methanol is about 37% for -20 F fluid. (Cite : http://www.houghton.com/docs/productdata/Windshield%20Washer%20Fluid%20Product%20Data.pdf)

For the 37% methanol, the boiling point (more correctly the bubble point) is about 175 F. Is this what you are looking for ?

Thanks. I guess I was looking more for something like, “does the methanol in windshield washing fluid take 30 minutes to evaporate at 90 F?” Some sort of plug-and-play equation.

Thats a hard one because it depends so much on the surface area of contact and if the air is still or blowing past. One can approximately find some answers by considering how much heat transfer can happen to the vessel from the ambient.

Here is a link for water evaporation rates. Its a paid publication :


The published data shows that at around 85 F (28.9 C) ambient temperature, the evaporation rate of water is about 0.7 mg/min cm2. Lets round it off to 1 mg/min cm2.

Now the vapor pressure of pure methanol at 90F is about 3.5 psi while water at the same temperature is about 0.7 psi . A ratio of about 5.

So, pure water at 90 F is about 1 mg/min.cm2 and pure methanol is estimated to be about 5 mg/min.cm2 (roughly - we should use specific heats and molar values though)

So although I have not answered your question, you have the bounds to your answer. Hope this helps.

ah, OK, thanks. I was scientifically curious today how long windshield washing fluid, when sprayed on the exterior of my car, took to dry. FWIW, it was a hot Texas sunny day, 95 F, and I was driving for about 8 minutes after it was sprayed on the car exterior. I had forgotten to take the wind factor into account; the air wind blowing past the car must certainly speed up the drying. But when it dries, does the fluid leave behind a thin crust of methanol or is the methanol evaporated?

Methanol doesn’t leave a “thin crust of methanol” behind when it evaporates. It all evaporates. If there is a thin crust left behind, it’s composed of dust and dirt that didn’t get completely washed/wiped away. Lather, rinse, repeat.

Any methanol or water sprayed on your windshield on a hot day at highway speed and then spread into a thin film by the wipers is going to evaporate in a hurry. Where the wipers have gathered the fluid into larger droplets at the edges of the windshield, these will take longer to evaporate because they have a larger surface-area-to-volume ratio than the thin film in your central field of view.