Propylene Glycol: A Versatile Humectant, Solvent, and Antifreeze

CAS Registry Number: 57-55-6

Overview

Propylene glycol (also known as propane-1,2-diol) is a synthetic organic compound with the chemical formula C₃H₈O₂ and a molecular weight of 76.09 g/mol. It is a clear, colorless, slightly syrupy liquid that is practically odorless and tasteless. The compound is hygroscopic, meaning it readily absorbs water from the surrounding environment.

Key Properties

• Boiling Point: 187–188 °C

• Melting Point: –60 °C

• Density: Approximately 1.036 g/cm³ at 20 °C

• Solubility: Miscible with water, acetone, ethanol, and chloroform; soluble in ether

• Stability: Stable at cooler temperatures in sealed containers; oxidizes at higher temperatures, producing lactic acid, pyruvic acid, and acetic acid

Major Applications

Food and Beverage Industry

The U.S. Food and Drug Administration (FDA) has classified propylene glycol as Generally Recognized as Safe (GRAS) for use in food. It is used to:

• Absorb extra water and maintain moisture in products

• Serve as a solvent for food colors and flavors

• Function as an emulsifier, stabilizer, and anticaking agent

Pharmaceutical and Medical Applications

• Used as a solvent and diluent for intravenous medications and cough preparations

• Employed as a humectant and preservative in topical ointments (e.g., corticosteroids)

• Functions as a moisturizing agent in various pharmaceutical formulations

Cosmetics and Personal Care

Propylene glycol is widely used in skin care products, hair care products, color cosmetics, and soaps as an:

• Effective humectant preventing water loss

• Emollient and lubricant

• Solubilizer for other ingredients

Industrial Applications

• Antifreeze and Deicing: Used as a less toxic alternative to ethylene glycol in automotive antifreeze, aircraft deicing fluids, and windshield washer fluids

• Heat Transfer Fluids: Employed in HVAC systems, ice-rink refrigeration, and cold storage units

• Plastics and Resins: Used as a starting material in the production of polyester compounds

• Special Effects: A main ingredient in artificial smoke or fog for theatrical productions and fire-fighting training

• Paint and Plastics Industry: Serves as a solvent

Safety and Toxicology

Regulatory Status

The FDA has classified propylene glycol as GRAS for use in food. It is not currently classified as carcinogenic by the U.S. DHHS, IARC, or U.S. EPA.

General Safety

Propylene glycol has low toxicity for humans and is generally considered safe for consumption at normal levels. It is metabolized in the body to lactic acid, a normal constituent of the citric acid cycle. However, large doses over short periods of time can be toxic.

Species-Specific Considerations

• Cats: Cats are particularly susceptible to propylene glycol toxicity due to the unique properties of their hemoglobin and limited glucuronidation capacity. Dietary propylene glycol at levels as low as 5% can cause Heinz body formation and anemia in cats. This has led to its removal from many commercial cat foods.

• Dogs: Chronic administration at 5 g/kg/day may cause mild hematologic changes but no overt clinical signs of toxicosis.

• Cattle and Horses: Overdoses can lead to ataxia, depression, and metabolic acidosis.

Incompatibilities

Propylene glycol reacts with strong oxidizing agents, which can create a fire hazard. It reacts with hydrofluoric acid, nitric acid, and silver nitrate to form silver fulminate, an extremely explosive compound.