Subject: Time:4:08 PM
OFFICE MEMO Brake Fluid and Elastomers Date:6/7/94
The following is an article I had prepared for the Classic Jaguar Association
bi-monthly bulletin, as they were having a snail mail discussion about silicone
brake fluid, but it seems like it is relevant to this group as well. Somebody
save it and put it in the FAQ file.
Warning: a bit long winded and pedantic
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More on brake fluid and elastomers (rubber). by Rob Reilly
I stumbled across this information while looking for something else (work
related, actually). I thought it might be useful to add to the discussion about
silicone brake fluid. I included some stuff about rubber products you might use
for sealing oil and fuel. All this is gleaned from a catalogue put out by the
Parker Seal Group, Lexington, Kentucky 40512, a reputable manufacturer of
O-rings.
In choosing rubber parts to seal against the various fluids in a car, one would
be well advised to ask exactly which rubber compound is used in the parts.
There are many different elastomers, all of which are good for some fluids but
are attacked by others. Elastomers also have a shelf life, which can depend on
exposure to sunlight, heat, ozone, and even water. The rubber parts should be
packed in some way to minimize exposure and increase shelf life.
Silicone fluids are chemically very stable. .....all types of seal polymers
except silicone rubber (Si) may be used for silicone oils. Silicone fluids have
a great tendency to remove plasticizer from some compounds that have them,
causing them to shrink. The effect is most severe with low viscosity silicone
fluids and high temperatures. Because of this, military nitrile compounds, and
any other nitriles with a low temperature limit below -40deg F should not be
used to seal silicone fluids, as such low temperature nitriles contain
plasticizers.
Butyl Rubber (IIB) contains isobutylene and isoprene. It is used for inner
tubes and the inside layer of tubeless tires. It is recommended for silicone
fluids. It is not recommended for petroleum oils.
Epichlorohydrin Rubber (CO,ECO) has excellent resistance to hydrocarbon oils,
fuels and ozone.
Ethylene Acrylic componds have good resistance to petroleum oils and automatic
transmission fluids.
Ethylene Propylene Rubber (EPM,EPDM) was invented in 1961. It is recommended
for silicone oils and automotive brake fluids. It is not recommended for
petroleum oils.
Fluorocarbon Rubber (FKM), trade names Kel-F and Viton, is recommended for
petroleum oils and silicone fluids.
Silicone Rubber (Si) trade name Silastic is good in high temperatures but has
poor strength and abrasion resistance, making it unsuitable for moving seals.
Silicone rubber (Si) is rated 3 (doubtful) in contact with silicone fluids. The
poor rating is given because silicone rubber tends to absorb silicone fluids,
resulting in swelling and softening of the rubber. So RTV silicone sealant
would not be good for sealing a brake fluid reservoir if using silicone brake
fluid.
Fluorosilicone (FSi), trade name Silastic LS, is used in fuel systems and where
resistance to petroleum oils is needed.
Isoprene Rubber-Synthetic (IR) is a synthetic equivalent to natural rubber.
Natural Rubber-Natural Polyisoprene (NR) comes from trees. Petroleum oils are
the greatest enemy of natural rubber compounds. Synthetics have all but
completely replaced natural rubber for seal use. It is recommended for
automotive brake fluid. It is not recommended for petroleum products.
Neoprene Rubber (Chloroprene, CR) is unusual in that it has resistance to both
oxygen and to petroleum oils.
Nitrile or Buna N (NBR) is recommended for petroleum oils and silicone oils. It
is not recommended for automotive brake fluids.
Polyacrylate Rubber (ACM) is used primarily in automatic transmissions and
power steering gear using Type A fluid.
Polyurethane Rubber (AU, EU) has good resistance to petroleum oils and
hydrocarbon fuels and is used where toughness and abrasion resistance is
required.
Styrene Butadiene Rubber (SBR, Buna S, GRS) was developed during WW2 as a
substitute for NR. Together with NR, it accounts for 90% of total world rubber
consumption, primarily for tires. Recommended for automotive brake fluid. Not
recommended for petroleum oils.
For automotive brake fluid (non-petroleum kinds), including Delco, Girling,
Mopar, and Wagner 21B, Parker recommends EPM ethylene propylene, and says SBR
is also satisfactory. All other elastomers are rated fair to unsatisfactory, or
are not rated due to insufficient data. There was no mention here of any
"petroleum kind" of brake fluid. I wonder if that would be the cheapie DOT 3
flammable kind. If its like a petroleum oil, that might explain how it might
ruin brake seals intended for non-petroleum brake fluids.
For SF96, SF1153, and SF1154 GE Silicone Fluids, Parker recommends EPM ethylene
propylene, and SBR and almost everything else is rated #1 satisfactory except
NBR nitrile. I believe one of these is silicone brake fluid.
For SF1147 GE Silicone Fluid, Parker recommends FKM fluorocarbon rubber, but
NBR nitrile, EPM ethylene propylene, and IIB butyl are only fair to doubtful,
and nothing else is rated.
For silicone oils and greases, Parker recommends EPM ethylene propylene, and
everything else is rated #1 satisfactory except Si silicone and FSi
fluorosilicone, which are rated #3 doubtful.
For gasoline, Parker recommends NBR nitrile, with FKM fluorocarbon also
satisfactory, and all others only fair to unsatisfactory.
For automatic transmission fluid, including Dexron, Esso Type A, Mobil WA200
type A, and Texamatic A, Parker recommends NBR nitrile, and says FKM and ACM
are also satisfactory. Most everything else is rated unsatisfactory.
For motor oil, Parker recommends NBR nitrile, with FKM fluorocarbon and ACM
polyacrylate also rated #1. All others are unsatisfactory.
For SAE 90 EP and Hypoid 90 lube, Parker recommends NBR nitrile, with FKM, ACM
and AU also satisfactory.
For water/ethylene glycol, Parker recommends NBR nitrile, with EPM ethylene
propylene, FKM fluorocarbon and SBR styrene butadiene also satisfactory.
Reference: Parker O-Ring Handbook, copyright 1982 by Parker Seal Group,
Lexington KY
Some may have seen the Wall Street Journal article of September 29, 1993, in
which a Jaguar was described as having "the brakes burst into flame". I became
suspicious of this report, wondering what is there in the brake system that can
"burst into flame"? Brake pads can get stuck and smoke, ok. Rubber hoses and
seals could cook and smoulder with unpleasant black smoke, yes, but flame? Not
likely. What about the fluid?
I tried Castrol LMA DOT 4 and also some silicone DOT 5 fluid, turning the
propane torch on a small amount of each. Neither one burned, flamed, or even
discolored. Not having any DOT 3 on hand, I couldn't try it, but I went to a
store and read the label on a can of DOT 3. It had an NYFD warning on it, so
that may mean some brands of DOT 3 are flammable.
My conclusion is that either:
1) the Wall Street Journal writer was exaggerating and turning a case of
smoking pads into a more journalistically interesting "burst of flame", or
2) an uninformed mechanic or the owner of the Jag with the burning brakes may
have put DOT 3 in it, which may have ruined the seals and allowed a leak, which
may have ignited.
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