Propylene glycol: why the grade decides everything
Technical article · Eapearl Chemical ·
Propylene glycol is one of the few chemicals that a buyer can get badly wrong while ordering exactly the right substance. The CAS number is the same for the drum that goes into aircraft de-icing fluid and the drum that goes into a cough syrup. Everything that separates them sits in the grade, the monograph and the certificate.
What it is
A small, colourless, nearly odourless diol with a faintly sweet taste, miscible with water in all proportions and hygroscopic enough to be used specifically for that property. It is the mildest of the commodity glycols and the only one with a large food and pharmaceutical market.
| CAS number | 57-55-6 |
|---|---|
| Molecular formula | C3H8O2 |
| Molecular weight | 76.09 g/mol |
| Also sold as | MPG · monopropylene glycol · 1,2-propanediol · E1520 |
| Harmonised CLP classification | None — no Annex VI entry applies |
Why the grade is the whole decision
Ask for “propylene glycol, 99.5%” and you have specified almost nothing. Assay tells you how much of the drum is propylene glycol. It does not tell you what the other half a percent is, how the material was made, what it was stored in, or whether anyone tested for the two contaminants that matter.
Industrial grade
Made and sold against a commercial specification. Appropriate for resins, coolants, de-icing fluids and solvent duty. It is not made to a pharmacopoeial monograph and carries none of the assurances that monograph provides.
Pharmacopoeial and food grade
Made and controlled against a monograph — USP, Ph. Eur. or the equivalent — with identity, purity and limit tests defined by that monograph rather than by the supplier. Two of those tests exist for one reason: a specific limit for diethylene glycol and ethylene glycol on every batch.
That test is not paperwork. Diethylene glycol substituted for propylene glycol or glycerine has caused repeated mass poisonings, because the substitution survives an inspection that checks appearance and density but not identity. We set out how that happens, and what it means for incoming inspection, in the article on diethylene glycol.
The operational rule that follows: if the material will end up in something a person eats, drinks, inhales or puts on their skin, the grade is decided before the price is discussed, and the certificate of analysis for the batch shipped has to show the glycol limit test. A supplier who cannot produce it on request has not supplied that grade, whatever the label says.
What propylene glycol is used for, by grade
Unsaturated polyester resins
One of the largest industrial outlets. As a diol in the polycondensation it gives resins with good clarity and mechanical balance — the backbone of glass-reinforced panels, marine laminates and cultured marble.
Antifreeze, coolants and de-icing
Chosen over ethylene glycol wherever accidental contact or environmental release is part of the risk picture: food plant cooling circuits, breweries, HVAC systems and aircraft de-icing. The performance is slightly lower; the reason for the switch is the toxicology, not the thermodynamics.
Solvents, paints and inks
A slow, high-boiling, water-miscible solvent used to hold formulations open and to stop water-based systems from drying at the wrong moment.
Food, flavour and tobacco
As a carrier and humectant — E1520 in the EU system — where it dissolves flavour compounds that water cannot carry and keeps a product from drying out. Food grade only.
Pharmaceutical and cosmetic formulation
A solvent and co-solvent for actives with poor water solubility, and a humectant in topical products. Pharmacopoeial grade only.
MPG, DPG, TPG: one family, three jobs
Each additional propylene oxide unit raises boiling point and viscosity and lowers volatility. The chemistry is continuous; the applications are not.
| Monopropylene glycol | CAS 57-55-6 · the food and pharmaceutical member of the family, and the general-purpose one |
|---|---|
| Dipropylene glycol | CAS 110-98-5 · higher boiling, low odour; the standard carrier in fragrance work and a plasticiser intermediate |
| Tripropylene glycol | CAS 24800-44-0 · higher still; coupling agent, coalescent and intermediate for acrylate esters |
A formulation written for one of these does not transfer to another by changing the drum. Volatility differences show up as drying time, odour profile and retained solvent.
Classification, stated precisely
No entry in Annex VI to the CLP Regulation applies to propylene glycol, so there is no harmonised classification for it. Hazard statements published for this substance — including on supplier websites and aggregated databases — come from self-classification or from notification data. That is evidence, not law, and it should carry a visible label saying which it is.
The practical consequence for a buyer: do not resolve a question about this material from a pictogram found on a web page. Ask for the safety data sheet for the specific grade, from the supplier who will ship it, and read section 2 together with section 3. For a comparison, ethylene glycol does carry a harmonised classification (H302), which is exactly the kind of distinction that gets lost when the two names are used loosely.
Storage, and the two things that go wrong
Propylene glycol is stable, but it is hygroscopic and it is a good growth medium once diluted. Water pickup in a part-used drum lowers the assay and, in food or pharmaceutical service, opens a microbiological question that did not exist in the sealed container. Store closed, dedicate the transfer equipment for high-grade material, and do not return drawn material to the drum.
What to tell us in an inquiry
- The grade, explicitly — industrial, food or pharmacopoeial, and which monograph if the last.
- The end use — it decides the grade, and for food and pharmaceutical service it decides the documentation.
- Whether you need the glycol limit test on the certificate — ask for it in writing at the inquiry stage rather than discovering its absence at incoming inspection.
- Packaging and destination — drums, IBC totes or ISO tank; see logistics and packaging.