Is Antifreeze Flammable? (A comprehensive overview)

Is Antifreeze Flammable?

There are antifreeze that are flammable and that are non-flammable. Flammable antifreeze are the ones composed of ethylene glycol or other volatile alcohols. Once they are mixed with water their fire hazards decrease considerably.

Ethylene glycol by itself or in antifreeze formulations (usually composed of ethylene glycol and a small portion of additives) can promptly catch on fire if heated to high temperature or in contact with a spark or naked flame. 

When ethylene glycol is mixed with water its fire hazards are considerably reduced. The same is true for other antifreeze such as propylene glycol or ethanol.

What’s the Definition of Antifreeze?

Antifreeze is a substance, or mixture of substances, that when mixed with water causes the freezing point of the new water-based solution to decrease. For instance, a water solution containing around 50% ethylene glycol will freeze at -36 ºC.

When a water based liquid is mixed with another substance forming an homogeneous mixture the freezing point of the liquid will, in most cases, decrease relative to the pure liquid. Colligative properties are the main cause for this phenomenon.

Regular salt, sodium chloride (NaCl), is another example of an antifreeze substance. Other salts such as potassium chloride or calcium chloride are also sometimes used as antifreeze agents.

Polar organic liquids such as alcohols can act well as antifreeze agents. Alcohols have a hydroxyl group (OH) in their molecular structures. This structural feature allows alcohols to interact well with water molecules through hydrogen bonds.

The effectiveness of a given antifreeze agent can vary a great deal.

The amount of antifreeze that is used in common application also varies. Too much antifreeze can drastically diminish the viscosity of water or turn it too corrosive. Too little antifreeze will have a low effect on the freezing point.

General View of Antifreeze Agents

Antifreezes are used in automobiles, air-conditioning, some fire sprinklers, as an additive in cement. The most easily accessed antifreeze agent is probably sodium chloride, mixing it with water to form brine. Brine freezing point can be as low as -21 ºC.

Brine solutions pose pretty much no immediate fire hazard (the only possible hazard would be that brines are strong conductors of electricity). However, brines are highly corrosive to metals and can cause accumulation of the salt over time.

Alcohols (including glycols) are less corrosive than salts and do not accumulate over time. The main disadvantage of alcohol is their fire hazards.

Methanol is sometimes used as antifreeze; methanol is also highly toxic if ingested or inhaled. Ethanol is less toxic and has similar flammability. Solutions of water with ethanol or methanol are still flammable, mainly because these alcohols can still evaporate if heated.

Glycols are alcohols that contain two hydroxyl groups in distinct carbon atoms. They are slightly less effective in reducing the freezing point of water. 

But, they evaporate at temperatures higher than water, meaning heating a solution of glycols and water will normally not cause glycols to evaporate.

Common Flammable Antifreeze Chemicals

The two most commonly used flammable antifreeze raw materials are ethylene glycol and propylene glycol. But in some cases ethanol, methanol and other alcohols may also be used.

Alcohols such as the ones mentioned previously are highly flammable substances. When burned in the presence of air they produce carbon dioxide and water, but hazardous gases such as carbon monoxide and others are also formed in smaller quantities.

Below are some important thermal properties of alcohols that are commonly used as antifreeze agents.

Antifreeze substanceThermal properties of the chemical in pure form
Flash pointAutoignition temperatureExplosive limits in airMaximum effect on the water freezing point
Ethanol14 ºC365 ºC3.3% to 19.0%-38 ºC
Ethylene glycol111 ºC410 ºC3.2% to 15.2%-36 ºC
Glycerol199 ºC390 ºC2.7% to 19.0%-22 ºC
Methanol52 ºC470 ºC6.0% to 36.0%-50 ºC
Propylene glycol101 ºC371 ºC2.6% to 13.0%-32 ºC

Below are the molecular structures of some alcohols that are used as antifreeze.

Ethylene Glycol

One of the most commonly used antifreeze chemicals is ethylene glycol (OHCH2CH2OH). This alcohol is slightly less flammable than ethanol. Naked flames or sparks can easily cause ethylene glycol to catch on fire.

Most antifreeze formulations that use ethylene glycol contain more than 90% of it.

A mixture consisting of ethylene glycol and water is considerably less flammable than pure ethylene glycol.

Ethylene glycol is an odorless, colorless liquid at normal conditions. This substance degrades over time and corrodes rubber and similar materials.

If pure ethylene glycol or antifreeze made from it is excessively heated or exposed to sparks or flames it will most likely catch on fire. In such a case, carbon monoxide gas will possibly be formed along with carbon dioxide and water.

To extinguish fires caused by alcohols in general, ethanol-proof foams are recommended. Carbon dioxide can also be used.

Ethylene Glycol is slowly absorbed by some elastomers (e.g. rubber) causing them to swell and possibly be damaged.

Ethylene glycol is very toxic to humans. Ethylene glycol is considered teratogenic, this substance can also cause dysrhythmias and heart failure. If acute exposure to ethylene glycol does occur, nephrotoxicity takes place within 1 to 3 days.

Propylene glycol

SImilarly to ethylene glycol, pure propylene glycol or antifreeze formulations containing it are also flammable (some thermal properties are shown in the table above). An aqueous solution containing propylene glycol is not flammable, unless in very severe conditions.

If pure propylene glycol is burned, the produced gases will be carbon dioxide and water vapor along with toxic carbon monoxide.

Propylene glycol is a less efficient antifreeze agent than ethylene glycol but it has very low toxicity. Therefore this glycol is used in food-processing or in water pipes, since in those applications the antifreeze may end up ingested.

For instance, propylene glycol is used in baked goods, frozen custard, ice cream and in salad dressings.

Propylene glycol is oxidized to lactic acid, which is also non toxic to humans.

When a solution with propylene glycol starts to become reddish or balck it is a sign that the solution has started to corrode parts of the system. Propylene glycol can be oxidized in the presence of oxygen and metals.

Glycols Can Cause Corrosion to Some Metals

Under some conditions glycols will start to corrode some metallic systems. Glycols will be slowly oxidized to carboxylic acids over time (the higher the temperature and the more oxygen, the higher the oxidation rate).

One way to help prevent the action of such acids is by adding small amounts of bases such as KH2PO4 and Na2HPO4 in very low amounts ( ~0.4% relative to the glycol weight). 

Another additive that can help prevent this type of corrosion is borax addition to the metallic surfaces.

 In systems where there is a high movement of the antifreeze solution, corrosion can affect the layers in contact with the antifreeze over time. Additives that can help prevent this type of corrosion are borates, molybdates, nitrates, phosphates, silicates.

In order to help prevent incidents involving corrosion, monitoring some factors is recommended. The factors to be monitored include: pH, the density, the color and the biological contamination.

Antifreeze in Fire Sprinklers Can Pose High Hazards

Antifreeze solutions have been used in fire sprinklers for decades until 2011, when new regulations started to be implemented.

The main reason for this change in regulations was the evidence that when antifreeze fire sprinklers were activated in a fire, this would worsen the fire.

According to the NFPA, up until 2010 when a residential fire would happen, the average number of fatalities would be 83% higher when antifreeze fire sprinklers systems were used.

On September 30, 2022 NFPA requirements for wet fire sprinkler systems will change. 

Wet fire sprinkler systems that were installed prior to 2012 will have to be reevaluated to meet the requirements of NFPA changes made in 2013.

Systems that were sold and installed after 2012 were designed in such a way that antifreeze was not necessary.

Non-Flammable Antifreeze Chemicals General Characteristics

Although far less versatile than glycol based antifreeze, salt based antifreeze can be used in some less demanding applications. Salt-based antifreeze is not flammable.

Examples of sat based antifreeze are: Calcium chloride* CaCl2, Potassium Chloride KCl, Sodium chloride NaCl and sea water.

Conclusions

An overview of the main antifreeze agent and their general characteristics and fire hazard was presented.

With the exception of common salt-based antifreeze, antifreeze by themselves are flammable. When in use, that is mixed with water, they are nearly non-flammable.

Frequently Asked Questions (FAQ): Is Antifreeze Flammable?

Can antifreeze catch on fire?

An alcohol or glycol based formulation will catch on fire and burn if subjected to sparks or naked flames. When mixed with water, the flammability will decrease substacianly.

At what temperature does antifreeze ignite?

Ethylene glycol based antifreeze formulation will autoignite (catch on fire spontaneously) at around 410 ºC.

When mixed with water the temperature or general conditions would need to be more drastic for the solution to spontaneously catch on fire.

Is antifreeze a hazardous material?

If ingested, antifreeze can be very hazardous to human health. Ethylene glycol is highly harmful if ingested; antifreeze formulations will also contain additives that are highly toxic.

Propylene glycol has a low toxicity, however they also are accompanied by toxic additives.

Is coolant the same as antifreeze?

Not exactly, they have similar characteristics but are not the same. Antifreeze is used to make it harder for water to freeze. A coolant helps prevent a system from heating. An antifreeze can keep a system at lower temperatures than it would be possible without it. Therefore antifreeze can have a coolant like effect, but its main purpose is not that.

At what temperature does antifreeze freeze?

Ethylene glycol, the most commonly used antifreeze material, freezes at -13 ºC. A solution of ethylene glycol and water will freeze below that temperature, the exact temperature will depend on the concentration of antifreeze. The lowest temperature such a solution can freeze is around -36 ºC.

Does antifreeze evaporate?

Every liquid evaporates to some degree. Water evaporates at normal temperatures at very low rate. Most antifreeze evaporates even slower than water.

What are antifreeze proteins?

They are proteins that aid certain animals, plants and microorganisms survive at temperatures below 0 ºC.

References

Fink, J. K. 2011. Engineer’s Guide to Oil Field Chemicals and Fluids. Chapter 14, pages 427 – 424. Published by GPP.

https://www.cdc.gov/niosh/npg/npgd0272.html (Accessed March 28th, 2022)

http://www.hydratechglobal.net/technical/Ethylene+Glycol/58/en (Accessed March 28th, 2022)

Fowles J. R, Banton M. I., Pottenger L. H. A toxicological review of the propylene glycols. Crit Rev Toxicol. 2013 Apr;43(4):363-90. doi: 10.3109/10408444.2013.792328

https://www.nfpa.org/-/media/Files/Public-Education/By-topic/Home-fire-sprinklers/AntifreezesafetyalertAug2010.ashx (Accessed March 28th, 2022)

https://vanguard-fire.com/antifreeze-fire-sprinklers-systems-what-to-know-before-september-30-2022/ (Accessed March 28th, 2022)

https://www.monarchchemicals.co.uk/Information/News-Events/700-/The-difference-between-Propylene-Glycol-and-Ethylene-Glycol-in-antifreeze (Accessed March 28th, 2022)

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