Help, science of why/how brake fluid damages paint - PeachParts Mercedes-Benz Forum

whunter · #1 02-11-2013, 11:52 AM

I am arguing with several local (lurker) forum members regarding brake fluid damaged automotive paint.

Scientific Documentation of the why/how would help.

Thank you.
.

Stretch · #2 02-11-2013, 01:33 PM

Well here they use the magic word "Glycol"

Brake fluid - Wikipedia, the free encyclopedia

Which then leads on to this kind of information

http://www.dow.com/webapps/lit/litorder.asp?filepath=oxysolvents/pdfs/noreg/110-00965.pdf

Is that sufficient information to sort out the lurkers?

jmk · #3 02-12-2013, 04:32 PM

This will explain the mechanism on how brake fluids, which contain various combinations of glycol ethers, destroy paint.

Anyone familiar with the art and science of paint making knows that materials that affect paint can be generally classified as hydrophilic and hydrophobic. From Greek, those terms literally mean "water loving" and "water fearing (or hating)." For example, ethanol (the stuff you drink), is hydrophilic. It readily dissolves into water. Cooking oil is hydrophobic. The commonly known phrase is "oil and water do not mix."

What mixes is what causes failure. If your coating is hydrophobic, then polar (hydrophilic) chemicals, such as ethanol, will not attack your coating. They won't mix, so the coating will not dissolve. Conversely if your coating is hydrophilic, then oily chemicals like gasoline (not gasohol!) will not attack the coating.

It is very easy to design a coating that resists hydrophilic or hydrophobic chemicals. It is very difficult to design a coating that resists both. This is the main reason brake fluid is so damaging to coatings. The glycol ethers that brake fluid is made from are both hydrophilic and hydrophobic. One end of the molecule will attack the hydrophilic part of the coating, the other end of the glycol will attack the other end. That in combination with some of the other materials makes these types of solvents very aggressive when attacking paint coatings.

Other solvents that are known to be good at damaging coatings are Skydrol (used in aircraft hydraulic systems), gasohol (hydrophobic gasoline mixed with hydrophilic ethanol), suntan lotion, and the active ingredient in bug spray, DEET.

The most common method of creating resistance to both types of chemcials is to increase the tortuosity (the definition on Wikipedia is pretty good.) The most common method is through crosslinking. The larger, the more complex the resin matrix, the harder it is for attacking chemicals to solvate (or dissolve). The extra connections between the polymers in the coating make dissolving them much harder.

That is one reason why POR-15 is so chemical resistant. It is made with aromatic urethane crosslinkers. The relatively large aromatic structure in the urethane in combination with the high crosslinking potential of the material creates good chemical resistance. Just one note: the aromaticity of that material gives it poor UV resistance. Don't use it where it is exposed to light.

If you really want to the basics of designing a chemical resistant coating, then you can refer to my US patent #6,869,996 titled "Waterborne Coating Having improved Chemical Resistance." Here is the link:
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-adv.htm&r=1&f=G&l=50&d=PALL&S1=6869996.PN.&OS=PN/6869996&RS=PN/6869996