propylene glycol

Propylene glycol[1]
IUPAC name	propane-1,2-diol
Other names	propylene glycol, 1,2-propanediol, 1,2-Dihydroxypropane, methyl ethyl glycol (MEG), methylethylene glycol, PG, Sirlene, Dowfrost
Identifiers
CAS number	[57-55-6]
RTECS number	TY6300000
ATCvet code	QA16QA01
SMILES	CC(O)CO
ChemSpider ID	1003
Properties
Molecular formula	C3H8O2
Molar mass	76.09 g/mol
Density	1.036 g/cm³
Melting point	-59 °C
Boiling point	188.2 °C
Solubility in water	fully miscible
Solubility in ethanol	fully miscible
Solubility in diethyl ether	fully miscible
Solubility in acetone	fully miscible
Solubility in chloroform	fully miscible
Thermal conductivity	0.34 W/m-K (50% H2O @ 90°C)
Hazards
MSDS	External MSDS
S-phrases	S24 S25
NFPA 704	1 0 0
Related compounds
Related glycols	Ethylene glycol
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references

Propylene glycol, known also by the systematic name propane-1,2-diol, is an organic compound (a diol alcohol), usually a faintly sweet, and colorless clear viscous liquid that is hygroscopic and miscible with water, acetone, and chloroform.

Chirality

Propylene glycol contains an asymmetrical carbon atom, so it exists in two stereoisomers. The commercial product is a racemic mixture. Pure optical isomers can be obtained by hydration of optically pure propylene oxide.^[2]

Production

Industrially propylene glycol is produced by propylene oxide hydration.^[3] Different manufacturers use either non-catalytic high-temperature process at 200 °C (392 °F) to 220 °C (428 °F), or a catalytic method, which proceeds at 150 °C (302 °F) to 180 °C (356 °F) in the presence of ion exchange resin or a small amount of sulfuric acid or alkali.

Final products contain 20% 1,2-propanediol, 1.5% of dipropylene glycol and small amount of other polypropylene glycol.^[4] Propylene glycol can also be converted from glycerol, a biodiesel byproduct.

Applications

Propylene glycol is used:

• As a solvent in many pharmaceuticals, including oral, injectable and topical formulations. Notably, diazepam, which is insoluble in water, uses propylene glycol as its solvent in its clinical, injectable form.^[5]
• As a moisturizer in medicines, cosmetics, food, toothpaste, mouth wash, and tobacco products
• In electronic cigarettes to deliver vaporized nicotine
• As an emulsification agent in Angostura and orange bitters
• As a solvent for food colors and flavorings
• As an ingredient, along with wax and gelatin, in the production of paintballs
• As a humectant food additive, labeled as E number E1520
• As a cooling agent for beer and wine glycol jacketed fermentation tanks
• As a carrier in fragrance oils
• As an ingredient in massage oils
• As a less-toxic antifreeze
• As a solvent used in mixing photographic chemicals, such as film developers
• In smoke machines to make artificial smoke for use in firefighters' training and theatrical productions
• In hand sanitizers, antibacterial lotions, and saline solutions
• In cryonics
• As a working fluid in hydraulic presses
• As a coolant in liquid cooling systems
• To regulate humidity in a cigar humidor
• As the killing and preserving agent in pitfall traps, usually used to capture ground beetles
• As an additive to pipe tobacco to prevent dehydration.
• To treat livestock ketosis
• As the main ingredient in deodorant sticks.
• To de-ice aircraft.^[6]
• UV Blacklite Tattoo Ink

Propylene glycol has properties similar to those of ethylene glycol (monoethylene glycol, or MEG). (Note: propylene glycol may also use the acronym MEG, but as an abbreviation of methyl ethyl glycol.) The industrial norm is to replace ethylene glycol with propylene glycol.

Safety

Potential health effects

Eye

Causes mild eye irritation. Contact may cause irritation, tearing, and burning pain.

Skin

Causes moderate skin irritation. Contact with the skin may cause erythema, dryness, and defatting.

Ingestion

May cause gastrointestinal irritation with nausea, vomiting and diarrhea. Low hazard for usual industrial handling. May cause hemoglobinuric nephrosis. May cause changes in surface EEG.

Inhalation

Low hazard for usual industrial handling. May cause respiratory tract irritation.

Chronic

May cause reproductive and fetal effects. Laboratory experiments have resulted in mutagenic effects. Exposure to large doses may cause central nervous system depression. Chronic ingestion may cause lactic acidosis and possible seizures.

The oral toxicity of propylene glycol is very low, and large quantities are required to incur the dangerous effects described above. The potential for long-term toxicity is also low. In one study, rats were provided with feed containing as much as 5% PG over a period of 104 weeks and they showed no apparent ill effects.^[7] Because of its low chronic oral toxicity, propylene glycol is generally recognized as safe (GRAS) for use as a direct food additive. Cases of propylene glycol poisoning are related to either inappropriate intravenous use or accidental ingestion of large quantities by children.^[8]

Serious toxicity will occur only at plasma concentrations over 4g/L, which requires extremely high intake over a relatively short period of time.^[9] It would be nearly impossible to reach toxic levels by consuming foods or supplements, which contain at most 1g/kg of PG. Propylene glycol is metabolized into pyruvic acid, which is a normal part of the glucose metabolism process and is readily converted to energy.

Veterinary data indicates that propylene glycol is toxic to 50% of dogs at doses of 9mL/kg, although the figure is higher for most laboratory animals (LD₅₀ at levels of 20mL/kg).^[10] However, it may be toxic to cats in ways not seen in other animals. The U.S. Food and Drug Administration has determined that propylene glycol in or on cat food has not been shown by adequate scientific data to be safe for use. If propylene glycol is used in or on cat food, the food is considered adulterated and in violation of the Federal Food, Drug, and Cosmetic Act.^[11]

Prolonged contact with propylene glycol is essentially non-irritating to the skin. Undiluted propylene glycol is minimally irritating to the eye, and can produce slight transient conjunctivitis (the eye recovers after the exposure is removed). Exposure to mists may cause eye irritation, as well as upper respiratory tract irritation. Inhalation of the propylene glycol vapors appears to present no significant hazard in ordinary applications. However, limited human experience indicates that inhalation of propylene glycol mists could be irritating to some individuals. Therefore inhalation exposure to mists of these materials should be avoided. Some research has suggested that propylene glycol not be used in applications where inhalation exposure or human eye contact with the spray mists of these materials is likely, such as fogs for theatrical productions or antifreeze solutions for emergency eye wash stations.^[12]

Propylene glycol does not cause sensitization and it shows no evidence of being a carcinogen or of being genotoxic.^[13][14]

There is limited evidence that intravenous injection of PG can cause adverse responses in a small number of people. A Clinical Journal of Medicine article describes two cases of adult men experiencing psychosis from use of injected phenytoin that contained PG as a solvent. Their symptoms resolved when they were switched to a phenytoin formulation without PG.^{[citation needed]}

Allergic reaction

Research has suggested that individuals who cannot tolerate propylene glycol probably experience a special form of irritation, but that they only rarely develop allergic contact dermatitis. Other investigators believe that the incidence of allergic contact dermatitis to propylene glycol may be greater than 2% in patients with eczema.^[15]

Patients with vulvodynia and interstitial cystitis may be especially sensitive to propylene glycol. Women struggling with yeast infections may also notice that some OTC creams can cause intense burning.^[16]Post menopausal women who require the use of an estrogen cream may notice that brand name creams made with propylene glycol often create extreme, uncomfortable burning along the vulva and perianal area. In these cases, patients can request that a local compounding pharmacy make a "propylene glycol free" cream.

Possible air germicide

Studies conducted in 1942 by Dr. Oswald Hope Robertson of University of Chicago's Billings Hospital showed vaporized propylene glycol inhalation in laboratory mice may prevent pneumonia, influenza, and other respiratory diseases. Additional studies in monkeys and other animals were undertaken to determine longterm effects, especially the potential for accumulation in the lungs.^[17]

References

See also

• Ethylene glycol
• 1,3-Propanediol

External links

• Propylene glycol website
• WebBook page for C3H8O2
• ATSDR - Case Studies in Environmental Medicine: Ethylene Glycol and Propylene Glycol Toxicity U.S. Department of Health and Human Services (public domain)
• Propylene Glycol - chemical product info: properties, production, applications.