What Is Propylene Glycol?

Propylene Glycol (PG), also known as 1,2-propanediol (C₃H₈O₂), is a synthetic, water-soluble organic compound produced by the hydration of propylene oxide. It is a colorless, nearly odorless, viscous liquid with a faintly sweet taste. Unlike its close relative ethylene glycol, propylene glycol has very low acute oral toxicity and is considered safe for use in food, pharmaceutical, and personal care products by regulatory agencies including the FDA and EFSA.

Industrial and Commercial Uses

Propylene glycol finds application across an exceptionally wide range of industries, largely because of its combination of low toxicity, good thermal stability, hygroscopic nature, and miscibility with water and many organic solvents.

  • HVAC and Chiller Systems: PG is the preferred heat-transfer fluid in food-processing facilities, breweries, ice rinks, and any application where incidental contact with consumables makes ethylene glycol unacceptable. It is used in closed-loop chilled-water and hot-water systems as an antifreeze and corrosion inhibitor.
  • Antifreeze and De-Icing: PG-based antifreeze is used in aircraft de-icing fluids (Type I/II/IV), solar thermal collectors, geothermal ground loops, and ground-source heat pump systems. It provides freeze protection down to approximately −50 °C at high concentrations.
  • Food and Beverages: Approved as a food additive (E1520 in the EU), PG is used as a humectant, solvent for food colors and flavors, and carrier for antioxidants. It appears in baked goods, salad dressings, flavorings, and beverages.
  • Pharmaceuticals and Cosmetics: PG serves as a solvent and vehicle for oral, injectable, and topical drugs. It is a common ingredient in skin-care creams, shampoos, toothpastes, deodorants, and pharmaceutical tablets as a humectant and preservative.
  • Electronic Cigarettes / Vaping: One of the two primary base fluids (alongside vegetable glycerin) used in e-liquid formulations to carry nicotine and flavor compounds.
  • Plastics and Resins: PG is a monomer in the production of unsaturated polyester resins, used in fiberglass-reinforced composites for boats, automotive parts, and construction panels.
  • Paints, Coatings, and Inks: Used as a coalescing agent and humectant in latex paints and as a solvent in printing inks, improving flow and open time.
  • Animal Feed: PG is used as an energy supplement and propionate precursor for dairy cows and as a carrier for feed additives.
Typical Values — 99.5% Propylene Glycol
Property Value Test Method
AppearanceTransparent LiquidVisual
Boiling Point188 °CASTM D 1120
Freezing Point< −50 °CASTM D 1177
Density (20 °C)1.036 g/mlASTM D 1122
Viscosity (20 °C)52 mPa·sASTM D 445
pH (50% v/v in distilled water)6.5 – 7.5ASTM D 1287
Water content (max)0.05%ASTM D 1123
Cubic Expansion Coefficient0.00062 1/K—
Alkaline Reserve0 ml HCl 0.1NASTM D 1121

Data from "Propylene Glycol Industrial Grade", Carpemar 2016.

Material Compatibility with 99.5% Propylene Glycol

Selection of the proper material for installations, including gaskets and elastomers, is important when designing systems to ensure proper operation and to prevent spills to the environment. The compatibility information in the table below was obtained from proprietary tests and published literature. Conditions of exposure and the presence of other chemicals and trace impurities should always be considered when choosing a construction material.

Glycols are likely to leak past improperly assembled or defective fittings, seals, and block valves.

Material Max Operating Temp. (°F) Max Operating Temp. (°C)
ABS8026.7
CPVC5010.0
Epoxy20093.3
Fluorocarbons FEP400204.4
Fluorocarbons TFE (PTFE)470243.3
Furfuryl Alcohol250121.1
Chlorinated Polyesters10037.8
Polyethylene14060.0
Polypropylene14060.0
Vinyl Ester21098.9
Viton A9032.2
Neoprene GR-M (CR)8026.7
Nitrile Buna N (NBR)8026.7
Corrosion Resistance — Metals & Plastics (Pure 99.5% PG)

The following table provides general guidance on corrosion resistance for various metals and plastics in contact with pure 99.5% propylene glycol. Rating codes are defined below the table. Note that zinc is not compatible with propylene glycol or its water mixtures. Avoid zinc-coated or galvanized reservoirs, fittings, and pipework.

Material Temp. Range (°F) Temp. Range (°C) Rating
Aluminium60 – 17015.6 – 76.72
Brass60 – 9015.6 – 32.23
Bronze60 – 21015.6 – 98.93
Carbon Steel60 – 21015.6 – 98.92
Copper60 – 9015.6 – 32.23
Hastelloy B60 – 9015.6 – 32.22
Inconel60 – 9015.6 – 32.23
Monel60 – 9015.6 – 32.23
Nickel60 – 9015.6 – 32.23
304 Stainless Steel60 – 9015.6 – 32.22
316 Stainless Steel60 – 21015.6 – 98.92
Titanium60 – 9015.6 – 32.21
Rating Code
1≤ 2 mils/year — Excellent
2≤ 20 mils/year — Acceptable
3≤ 20 mils/year — Not Recommended
Corrosion Table — Propylene Glycol – Water Mixtures

Mixtures of propylene glycol and water are generally more corrosive than pure water, so corrosion inhibitor packages should be used to ensure circuit integrity over the service life of the system. The following table shows corrosion values for a 50% v/v propylene glycol – water mixture versus plain water, measured according to ASTM D 1384. Results for plain water are shown for comparison.

Values represent the average change in coupon weight in g/m². A positive number indicates a weight gain due to the formation of a stable, protective oxide layer on the metal's surface.

Material PG 50% v/v (g/m²) Water (g/m²)
Steel−225−76
Aluminium−68−32
Copper−1.2−1.0
Cast Iron−92−192
Brass−2.5−1.0
Solder−136−11

Negative values indicate mass loss (corrosion). Per ASTM D 1384 glassware corrosion test. Inhibited PG formulations will produce significantly better results than uninhibited pure glycol.

Compatible Plastics, Sealants & Elastomers for PG–Water Mixtures

The following materials are compatible with water dilutions of propylene glycol. Data has been gathered from specific bibliography and proprietary tests.

Material Name Abbreviation
Butyl RubberIIR
Chloroprene (Neoprene)CR
Ethylene-Propylene-Diene RubberEPDM
Fluorocarbon Elastomers (Viton)FPM
Natural Rubber (up to 80 °C)NR
Nitrile RubberNBR
PolyacetalPOM
Polyamide (Nylon, up to 115 °C)PA
PolybutenePB
Polyethylene (High/Low Density)PE-LD / PE-HD
Cross-Linked PolyethyleneVPE (PEX)
PolypropylenePP
Polytetrafluoroethylene (Teflon)PTFE
Polyvinyl Chloride, RigidPVC-h
Silicone RubberSi
Styrene-Butadiene Rubber (up to 100 °C)SBR
Unsaturated Polyester ResinsUP
Propylene Glycol Property Calculators

Explore the full set of propylene glycol property calculators on KasperCalc:

Frost Protection Density Specific Heat Capacity Thermal Conductivity Viscosity Prandtl Number Thermal Expansion Coefficient Boiling Point

Cite This Work