Propylene glycol: properties, production and applications / Articles

Propylene glycol (1,2-propanediol, monopropylene glycol, MPG, CAS 57-55-6) is the simplest representative of the dihydric alcohol (glycol) class based on propylene. It is a clear, colourless, slightly viscous, hygroscopic liquid with virtually no odour and a slightly sweet taste. Due to its low toxicity, ability to retain moisture and miscibility with water in all proportions, propylene glycol has become one of the most versatile and in-demand chemical products, finding application in the production of polymers, heat transfer fluids, cosmetics, pharmaceuticals and food products. Global propylene glycol production is constantly growing, and its consumer properties make it the preferred alternative to more toxic ethylene glycol in a number of industries.

Chemical Nature and Physicochemical Properties

The molecular formula of propylene glycol is C₃H₈O₂, with the structural formula CH₃–CH(OH)–CH₂–OH. Its molecular mass is 76.09 g/mol. The presence of two hydroxyl groups and a methyl radical gives the molecule a unique balance between polarity and lipophilicity, ensuring its compatibility with both water and organic compounds.

The key physical characteristics of propylene glycol determine its industrial application:

Boiling point: 187–189 °C. The high boiling point ensures low volatility and stability when heated.
Freezing point: below minus 60 °C. This property makes it the basis for low-freezing heat transfer fluids and antifreezes.
Density: 1.035–1.037 g/cm³ at 20 °C.
Viscosity: 60 cP at 20 °C, higher than water but ensuring good circulation in systems.
Solubility: miscible with water, ethanol and most organic solvents.
Hygroscopicity: actively absorbs moisture from the air, which is valuable for moisturising compositions but requires hermetic packaging during storage.
Flash point: about 107 °C (open cup). Propylene glycol is classified as a combustible liquid, but its high flash point makes it relatively safe.

Regulatory Framework and Grades

Requirements for propylene glycol in Russia and CIS countries are regulated by technical specifications, as well as pharmacopoeial monographs and international standards. Propylene glycol is produced in several grades differing in purity and application. According to typical technical specifications, propylene glycol grade A is divided into three classes. The highest grade is intended for the electronics industry, the first grade — for the pharmaceutical industry, and the second grade — for technical purposes and antifreeze preparation. The mass fraction of the main substance is at least 99.7% for the highest grade, 99.5% for the first and 99.0% for the second.

For food applications, propylene glycol must comply with the requirements of food additive E1520 and safety standards. In pharmaceuticals, propylene glycol complying with pharmacopoeial monographs (e.g., USP or the State Pharmacopoeia) with a main substance content of at least 99.5% is used. The product must be transparent, colourless, free of mechanical impurities, with a Hazen colour not exceeding 10–20 units.

Production Technology

The main industrial method for producing propylene glycol is the liquid-phase hydration of propylene oxide. The process involves the reaction of propylene oxide with a large excess of water at elevated temperature (180–220 °C) and pressure (15–25 bar). The reaction can proceed in both non-catalytic and catalytic modes. The traditional technology with a large excess of water (water:propylene oxide molar ratio of 15–25:1) requires significant energy consumption for subsequent water evaporation.

A modern and more economical solution is reactive distillation technology. In a reactive distillation column, water and propylene oxide simultaneously react and separate: the propylene glycol formed is removed from the trays in a timely manner, which suppresses side reactions. This allows the water:propylene oxide molar ratio to be reduced to 1.5–4:1, produces a reaction mixture with a high propylene glycol concentration (50–80%) and significantly reduces energy consumption by eliminating the multi-stage evaporation step. The process selectivity reaches 93% with almost complete conversion of propylene oxide.

An alternative direction is the production of propylene glycol by hydrogenation of glycerol — a by-product of biodiesel production. This method allows "bio-propylene glycol" to be produced and reduces dependence on petroleum feedstock, aligning with sustainable development trends.

Applications

Production of unsaturated polyester resins. This is the largest consumption area for propylene glycol. It is used as a key monomer in the synthesis of polyester resins, which find application in the production of fibreglass, artificial stone, gelcoats, pipes, tanks, boats and construction panels. Propylene glycol imparts flexibility, elasticity and hydrolysis resistance to the resins.

Antifreezes and heat transfer fluids. Propylene glycol is the basis for the production of low-freezing coolants and heat transfer fluids. Unlike ethylene glycol, it is less toxic, making it preferred for systems where contact with food or people is possible: in the food industry, pharmaceutical production, residential heating systems, chillers and heat pumps. Aqueous propylene glycol solutions (typically 40–50%) provide freeze protection down to minus 30–35 °C and can be used as refrigerants.

Food industry. Propylene glycol is registered as food additive E1520 and classified as generally recognised as safe (GRAS) for use in food products. It is used as a humectant, solvent, anticaking agent, stabiliser and carrier for flavourings. It is added to beverages, sauces, seasonings, confectionery, packaged foods and animal feed to maintain moisture and texture. Due to its low toxicity, it is also used as an antifreeze in systems where leakage may lead to contact with food.

Cosmetics and personal care. Propylene glycol is widely used in cosmetic formulations due to its humectant and emollient properties. It helps retain moisture in the skin, improves the texture of creams and lotions, promotes the dissolution and even distribution of active and aromatic substances, and regulates the viscosity of emulsions. Propylene glycol is found in creams, lotions, serums, shower gels, shampoos, conditioners, deodorants, toothpastes and decorative cosmetics. It is also used as a solvent in perfumery and as a component of e-liquids.

Pharmaceuticals. In the pharmaceutical industry, propylene glycol is used as a solvent, co-solvent and carrier for active pharmaceutical ingredients, especially for poorly water-soluble drugs. It is used in the production of injectable solutions, syrups, ointments, gels, capsules and transdermal patches. Propylene glycol is also a component of medical antifreezes and heat transfer fluids for medical equipment.

Other applications. Propylene glycol is used in the production of liquid detergents, hydraulic fluids, plasticisers, printing inks, and in the tobacco industry to maintain tobacco moisture. In agriculture, it is used as an energy supplement in cattle feed (for the prevention of ketosis) and as a component of veterinary preparations. In industry, it serves as a heat transfer fluid in cooling systems for laser and electronic equipment.

Safety and Storage

Propylene glycol belongs to substances with low toxicity. It is classified as generally recognised as safe (GRAS) for food use and is approved for use in cosmetics and pharmaceuticals. However, ingestion of large quantities may have a toxic effect, so it is not intended for consumption in pure form. When working with propylene glycol, personal protective equipment (gloves, goggles) is recommended, and contact with eyes and prolonged skin contact should be avoided. Propylene glycol vapours at high concentrations may irritate the respiratory tract.

Propylene glycol is hygroscopic, so it should be stored in hermetically sealed containers made of stainless steel or polymeric materials. Manufacturers recommend storage under a nitrogen blanket to prevent moisture absorption and oxidation. The product is combustible (flash point about 107 °C), so fire safety measures must be observed during storage and transportation.

Market and Trends

The global propylene glycol market demonstrates steady growth, driven by demand from producers of polyester resins, antifreezes, cosmetics and food products. Key industry trends include the transition to bio-propylene glycol produced from glycerol or renewable feedstock, the expansion of its use as a less toxic alternative to ethylene glycol in heat transfer fluids and antifreezes, and growing demand for high-purity grades for pharmaceuticals and electronics. The Asia-Pacific region remains the largest market, due to large-scale production of polyester resins and packaging.

Conclusion

Propylene glycol remains one of the key products of the global chemical industry, combining the functions of a polymer monomer, heat transfer fluid, solvent, humectant and food additive. Its low toxicity, hygroscopicity and miscibility with water make it indispensable in cosmetics, pharmaceuticals, food and cooling systems. Organising supplies of propylene glycol, including grade selection (technical, food, pharmaceutical), quality confirmation and logistics, requires an individual approach that takes into account specific requirements and the regulatory norms of the destination country.