Propylene Glycol

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.

Production

Industrially propylene glycol is produced by propylene oxide . 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 amounts of other polypropylene glycols. 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.

As a humectant food additive, labeled as E number E1520

As an emulsification agent in Angostura and orange bitters

As a moisturizer in medicines, cosmetics, food, toothpaste, mouth wash, and tobacco products

As a carrier in fragrance oils

As an ingredient in massage oils

In hand sanitizers, antibacterial lotions, and saline solutions

In smoke machines to make artificial smoke for use in firefighters’ training and theatrical productions

In electronic cigarettes, it is used to deliver vaporized nicotine

As a solvent for food colors and flavorings

As an ingredient, along with wax and gelatin, in the production of paintballs

As a moisture stabilizer (humectant) for snus (Swedish style snuff).

As a cooling agent for beer and wine glycol jacketed fermentation tanks

As a non-toxic antifreeze for winterizing drinking water systems, and in applications where the used antifreeze eventuallyl be drained into the soil, water, or a septic system.

As a less-toxic antifreeze in solar water heating systems

As a solvent used in mixing photographic chemicals, such as film developers

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.

As an ingredient in UV or blacklight tattoo ink

Propylene glycol has properties similar to those of ethylene glycol (monoethylene glycol, or MEG). (Note: Infrequently propylene glycol may also use the acronym MEG, but as an abbreviation of methyl ethyl glycol- the industry standard acronym for propylene glycol is PG or MPG (monopropylene glycol). The industrial norm is to replace ethylene glycol with propylene glycol when safer properties are desired.

Properties

Propylene glycol is a component in newer automotive antifreezes and de-icers used at airports. Like ethylene glycol, the freezing point of propylene glycol is depressed when mixed with water due to disruption of hydrogen bonding. Unlike ethylene glycol, propylene glycol is much lower in toxicity. Both are readily biodegradable.

Freezing Points of Propylene Glycol – Water Mixtures

Percent Propylene Glycol (wt. %)

Freezing Point (F)

Freezing Point (C)

-3

-7

-12

-18

-5

-20

-10

-23

-20

-29

-30

-34

-40

-50

-46

-60

-51

Safety

Humans

Propylene glycol is metabolized in the human body into pyruvic acid, which is a normal part of the glucose metabolism process and is readily converted to energy.

The oral toxicity of propylene glycol is very low, and large quantities are required to cause perceptible health damage in humans. Serious toxicity will occur only at plasma concentrations over 4g/L, which requires extremely high intake over a relatively short period of time. It would be nearly impossible to reach toxic levels by consuming foods or supplements, which contain at most 1g/kg of PG. Cases of propylene glycol poisoning are related to either inappropriate intravenous use or accidental ingestion of large quantities by children.

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. Because of its low chronic oral toxicity, propylene glycol was classified by the U. S. Food and Drug Administration as “generally recognized as safe” (GRAS) for use as a direct food additive.

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.

Propylene glycol does not cause sensitization and it shows no evidence of being a carcinogen or of being genotoxic.

There is limited evidence that intravenous injection of propylene glycol 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 propylene glycol.[citation needed]

Animals

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).

However, propylene glycol may be toxic to cats in ways not seen in other animals. The U.S. Food and Drug Administration has determined that its presence in or on cat food has not been shown by adequate scientific data to be safe for use. Any such use is considered an adulteration of the cat food and a violation of the Federal Food, Drug, and Cosmetic Act.

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.

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. 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.

Additionally, some electronic cigarette users who inhale propylene glycol vapor may experience dryness of the throat. As an alternative, some suppliers will put Vegetable Glycerin in the “e-liquid” for those who are allergic (or have bad reactions) to propylene glycol.

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. After a few months of treatment, no ill effects were discovered.

Environmental

Propylene glycol is known to exert high levels of biological oxygen demand (BOD) during degradation in surface waters. This process can adversely affect aquatic life by consuming oxygen aquatic organisms need to survive. Large quantities of dissolved oxygen (DO) in the water column are consumed when microbial populations decompose ethylene glycol.

The oxygen depletion potential of airport deicing operation discharges is many times greater than that of raw sewage. For example, before application, Type I propylene glycol-based deicing fluid is generally diluted to a mixture containing approximately 50% propylene glycol. Pure