Propylene glycol (PG; mono-propylene glycol, MPG; INCI Propylene Glycol; CAS 57-55-6) is a clear, viscous, hygroscopic diol used industrially as a humectant, solvent, antifreeze and heat-transfer fluid, and a carrier for flavors, fragrances, and active ingredients. Its chemical formula is C3H8O2. The same molecule turns up in food and cosmetic formulations, HVAC loops, polyester resin kettles, and de-icing fluids — which is why buyers shop it by grade rather than by name. A USP/EP lot and a technical lot are the same compound at different purity and impurity profiles, and the price gap is real. This page maps where PG gets used, how the commercial grades differ, and how to write an RFQ that gets you the right material.
Chemical Properties of Propylene Glycol
PG is propane-1,2-diol: a three-carbon backbone carrying two hydroxyl groups. Those two -OH groups drive almost every industrial use. They hydrogen-bond with water (hence the hygroscopic, water-miscible behavior) and react in polycondensation to build polyester resins. It is a low-volatility liquid with a high boiling point and near-absence of odor, which is why it works as a carrier where you do not want the solvent to flash off or impart a smell.
| Property | Typical value / behavior |
|---|---|
| Molar mass | 76.09 g/mol |
| Chemical formula | C3H8O2 (propane-1,2-diol) |
| Appearance | Clear, colorless, viscous liquid; practically odorless; faintly sweet taste |
| Boiling point | ~188 °C |
| Freezing / pour behavior | Does not freeze cleanly; PG-water blends give strong freezing-point depression (basis of antifreeze use) |
| Viscosity | ~48 mPa·s at 25 °C — markedly more viscous than water |
| Hygroscopicity | Strongly hygroscopic — absorbs and holds atmospheric moisture (humectant function) |
| Water miscibility | Fully miscible with water; also miscible with many polar organics (ethanol, acetone, glycerin) |
| Toxicity / regulatory note | Low acute oral toxicity in mammals; affirmed GRAS by the U.S. FDA for specific food uses (21 CFR 184.1666). Confirm grade and use for your application. |
One practical trade-off worth flagging up front: PG’s hygroscopicity is the feature you buy it for, but it is also a storage liability. Open or poorly sealed drums pull water from the air, which dilutes the active concentration and can push a high-purity lot out of spec. Keep totes and drums closed and blanketed where moisture pickup matters.
Propylene Glycol Grades: USP/EP vs Industrial
The single most important sourcing decision is grade. The molecule is identical; the specification, impurity control, and documentation are not. Pharma- and food-facing work needs a USP/EP lot with a Certificate of Analysis and traceability; coolant, resin, and paint work generally runs on industrial/technical grade at a lower cost. Buying USP for a de-icing loop wastes money; buying technical for a cosmetic formulation creates a compliance problem.
| Grade | Typical purity | Where used |
|---|---|---|
| USP / EP (pharmaceutical, food, cosmetic) | ≥99.5%, tight impurity limits, CoA + monograph compliance | Cosmetic and personal-care formulation, food/flavor carrier, pharmaceutical excipient applications |
| Industrial / Technical | ~99.5% typical, looser impurity and documentation requirements | Antifreeze and heat-transfer fluid, unsaturated polyester resins, paints/coatings, liquid detergents, de-icing |
| Dipropylene glycol (DPG) — related, not interchangeable | Varies by grade (fragrance vs industrial) | Fragrance carrier, plasticizers, resins; a distinct molecule — see dipropylene glycol |
Do not confuse propylene glycol with ethylene glycol (EG, CAS 107-21-1). They are different compounds with different toxicity profiles, covered in the comparison section below. The recommendation: specify the grade explicitly on every purchase order (“USP/EP” or “industrial/technical”), because vendors stock both and the wrong one ships when the PO is vague.
Industrial Applications of Propylene Glycol by Sector
PG earns its place across industries because one molecule covers four jobs at once: it holds moisture, dissolves what water and oils will not, depresses freezing points, and carries flavors, dyes, and actives evenly through a batch. Below is where that translates into real demand.
Food & Flavor (carrier and humectant)
In food applications, USP/food-grade PG functions as a humectant that retains moisture in baked goods, icings, and soft confections, and as a solvent-carrier that distributes flavoring compounds and colors uniformly through a batch. The FDA affirms it as GRAS for specific uses under 21 CFR 184.1666, with use levels capped by food category. In frozen products it interferes with ice-crystal growth, supporting a smoother texture. This is informational reference, not a recommendation for any specific formulation; confirm the grade, use level, and jurisdiction for your product.
Cosmetics & Personal Care (humectant and solvent)
Listed by its INCI name, Propylene Glycol, PG is one of the workhorse humectants and solvents in personal care formulation. It draws and holds water at the surface to support skin and hair feel, dissolves botanical extracts and actives that water alone cannot, and improves the spreadability of creams, lotions, and serums. It also depresses the freeze point of water-based formulas, which protects products through cold shipping and storage. Formulators typically specify USP/EP grade here for documentation and impurity control.
Pharmaceutical Applications (excipient)
In pharmaceutical manufacturing, USP/EP propylene glycol is used as an excipient: a solvent and carrier for active ingredients in oral, topical, and some injectable formulations, where its water miscibility and ability to dissolve poorly water-soluble compounds matter. This is informational description of an established excipient function only; it is not medical advice or a claim about any drug product, and excipient suitability, grade, and compliance must be confirmed against the applicable monograph and regulatory requirements.
Antifreeze, De-icing & HVAC Heat-Transfer Fluid
This is one of PG’s largest industrial markets. Blended with water, propylene glycol depresses the freezing point and raises the boiling point of the loop fluid, making it a heat-transfer medium for HVAC chillers, closed-loop hydronic systems, refrigeration, breweries, and food-plant cooling. It is the diol of choice over ethylene glycol specifically where the fluid may contact people, food, or potable systems — because PG has markedly lower acute mammalian toxicity than EG, not because it is “non-toxic.” Inhibited PG-based fluids also carry corrosion inhibitors; an uninhibited PG charge will still corrode a system, so specify an inhibited heat-transfer formulation, not bulk PG, for a working loop. PG-water blends are also used in aircraft and surface de-icing fluids.
Unsaturated Polyester Resins (UPR)
PG is a primary glycol feedstock for unsaturated polyester resins. It reacts with maleic and phthalic anhydrides in polycondensation to build the resin backbone used in fiberglass-reinforced composites, gel coats, marine and automotive parts, and construction panels. PG-based UPRs are valued for water resistance and clarity relative to some alternative glycols. Resin producers typically run industrial-grade PG here; the trade-off versus cheaper glycols is balanced against the cured resin’s performance requirements.
Paints, Coatings & Inks
In water-based coatings, PG acts as a coalescing aid and open-time extender. It slows drying so the film levels properly, and it provides freeze-thaw stability so a can of latex paint survives a cold warehouse and re-disperses on thaw. It also helps control viscosity and pigment wetting. Typical addition rates are a few percent; the trade-off is that PG contributes to the formulation’s VOC accounting in some jurisdictions, so coatings formulators balance its open-time benefit against VOC limits.
Plastics, Detergents, E-liquids & Hydraulic Fluids
Beyond resins, PG appears as a plasticizer and intermediate in various plastics and polymer chemistries. In liquid detergents and household cleaners it serves as a solvent and freeze-point depressant that keeps concentrated formulas pourable in cold storage. It is a base humectant/carrier in e-liquid formulations, paired with vegetable glycerin, and is used in certain lower-toxicity hydraulic and brake-fluid formulations where leakage into sensitive environments is a concern. Across all of these, grade selection follows the end use: technical for industrial fluids, USP for anything contacting people or food.
Propylene Glycol vs Ethylene Glycol
The two glycols are routinely compared because both are antifreeze and heat-transfer diols, but they are not interchangeable on safety. The decisive difference is toxicity: ethylene glycol has significant acute mammalian toxicity (ingestion causes serious harm), whereas propylene glycol has markedly lower acute mammalian toxicity and is the diol selected wherever the fluid may contact people, food, or potable systems. To be precise: lower acute toxicity is not the same as “non-toxic” or “safe,” and PG should still be handled per its SDS.
| Factor | Propylene glycol (PG) | Ethylene glycol (EG) |
|---|---|---|
| CAS | 57-55-6 | 107-21-1 |
| Acute mammalian toxicity | Markedly lower | Significant — ingestion causes serious harm |
| Heat-transfer efficiency | Slightly lower; higher viscosity | Slightly higher; lower viscosity |
| Typical antifreeze use | Food-plant, HVAC, potable-adjacent, RV/marine | Automotive engine coolant, industrial loops with no contact risk |
The trade-off is real: EG transfers heat slightly more efficiently and runs at lower viscosity, so an EG loop can be cheaper to pump. PG is chosen when the lower-toxicity profile outweighs that efficiency margin — which, in food, HVAC, and any contact-risk application, it usually does.
Propylene Glycol Supplier — Bulk USP & Industrial Grade
RawSource supplies propylene glycol in bulk for industrial buyers — both USP/EP grade for food, cosmetic, and pharmaceutical-excipient work, and industrial/technical grade for coolant, resin, coatings, and detergent production. Because the two grades are the same molecule at different specifications and price points, a clean RFQ moves faster and prices better. Tell us three things up front:
- Grade: USP/EP (with CoA and monograph compliance) versus industrial/technical — driven by your end use.
- Volume and packaging: drums, IBC totes, or bulk tanker, with annual or per-order quantity.
- Delivery: ship-to location, cadence (spot versus contract), and any documentation or compliance requirements.
New to sourcing diols at volume, or comparing suppliers? Our comprehensive guide to chemical procurement walks through specifying grade, vetting suppliers, and structuring volume contracts. For the related fragrance- and resin-grade diol, see dipropylene glycol.
Frequently Asked Questions
What is propylene glycol used for?
Propylene glycol is used as a humectant and solvent in cosmetics and food, as a low-toxicity antifreeze and HVAC heat-transfer fluid, as a glycol feedstock for unsaturated polyester resins, and as a carrier for flavors, fragrances, dyes, and active ingredients. It also appears in paints, liquid detergents, e-liquids, and certain hydraulic fluids.
Is propylene glycol safe?
Propylene glycol has markedly lower acute mammalian toxicity than ethylene glycol, and the U.S. FDA affirms it as GRAS for specific food uses under 21 CFR 184.1666. That is not the same as “non-toxic.” It should be handled, stored, and used according to its current Safety Data Sheet, and the correct grade must be confirmed for your specific application and jurisdiction.
What is the difference between propylene glycol and ethylene glycol?
They are different compounds. Propylene glycol (CAS 57-55-6) has markedly lower acute mammalian toxicity and is chosen where the fluid may contact people, food, or potable systems. Ethylene glycol (CAS 107-21-1) transfers heat slightly more efficiently but has significant toxicity on ingestion, so it is used mainly in automotive coolant and closed industrial loops with no contact risk.
Is propylene glycol the same as antifreeze?
Not exactly. Propylene glycol is an ingredient used to make antifreeze and heat-transfer fluids; blended with water it depresses the freezing point. A working antifreeze charge is an inhibited formulation that adds corrosion inhibitors and other additives. Bulk PG on its own is not a finished coolant — for a working loop, specify an inhibited PG-based heat-transfer fluid.
What is the difference between USP and industrial-grade propylene glycol?
They are the same molecule at different specifications. USP/EP grade meets pharmacopeial monograph limits with tight impurity control and a Certificate of Analysis, and is used for food, cosmetic, and pharmaceutical-excipient work. Industrial/technical grade has looser documentation and impurity requirements at a lower cost, and is used for antifreeze, resins, coatings, and detergents. Specify the grade on every purchase order.
Is propylene glycol natural or synthetic?
Commercial propylene glycol is produced synthetically, most commonly by the hydration of propylene oxide, which is derived from propylene. A bio-based route from glycerin or other renewable feedstocks also exists and yields chemically identical material. So the same molecule reaches the market by both petrochemical and bio-based manufacturing paths.
What is the chemical formula of propylene glycol?
The molecular formula of propylene glycol is C3H8O2, with a molar mass of 76.09 g/mol. Its systematic name is propane-1,2-diol, reflecting a three-carbon chain carrying two hydroxyl (-OH) groups. Those two hydroxyl groups give it the water miscibility, hygroscopicity, and reactivity that underpin its industrial uses. Its CAS number is 57-55-6.
Where can I buy propylene glycol in bulk?
RawSource supplies propylene glycol in bulk for industrial buyers in both USP/EP and industrial/technical grade, packaged as drums, IBC totes, or bulk tanker. Send an RFQ specifying grade, volume, and delivery location through the propylene glycol product page, and see our procurement guide for help structuring a volume contract.
Disclaimer: This article is provided for general industrial reference and is compiled from authoritative public sources. It is not medical, health, or efficacy advice, and nothing here is a claim that any product is “safe” or “non-toxic.” Always consult the current Safety Data Sheet (SDS) before handling, storage, transport, or disposal, and confirm grade, regulatory status, and suitability for your application and jurisdiction. Property values are typical reference figures, not a guaranteed specification.
