Identity and Key Properties
Propylene carbonate (PC) is the cyclic ester of propylene glycol and carbonic acid, CAS 108-32-7, molecular formula C₄H₆O₃, molar mass 102.09 g/mol. Propylene glycol (PG), also called 1,2-propanediol or monopropylene glycol (MPG), is CAS 57-55-6, formula C₃H₈O₂, molar mass 76.09 g/mol. PC is an aprotic solvent with no O–H bond; PG is a protic diol with two hydroxyls. That single structural difference drives almost every performance gap below.| Property | Propylene carbonate | Propylene glycol |
|---|---|---|
| CAS | 108-32-7 | 57-55-6 |
| Formula / MW | C₄H₆O₃ / 102.09 | C₃H₈O₂ / 76.09 |
| Boiling point | ~242°C | ~188°C |
| Melting / freezing | ~ -49°C | ~ -59°C |
| Density (25°C) | ~1.20 g/mL | ~1.036 g/mL |
| Flash point (closed cup) | ~132°C | ~99°C |
| Dielectric constant (25°C) | ~64.9 | ~32 |
| Proton character | Aprotic | Protic (2 × –OH) |
| Water miscibility | Partially miscible | Fully miscible |
Chemical Structure and Why It Matters
PC’s five-membered cyclic carbonate ring gives it high polarity with no acidic proton, so it solvates ions without donating hydrogen-bonding protons that interfere with electrochemistry. PG’s two hydroxyl groups make it hygroscopic and a strong hydrogen-bonder, which is why it holds water in formulations and depresses freezing points. Practical read: reach for PC when you need to dissolve a salt or run a clean electrolyte. Reach for PG when you need to hold moisture, lower a freeze point, or carry an active in a food, drug, or cosmetic.Production Routes
PC is made by inserting carbon dioxide into propylene oxide over a catalyst such as a zinc or quaternary-ammonium system, a CO₂-utilizing route. PG is made by hydrating propylene oxide, either high-pressure non-catalytic or lower-pressure catalytic over ion-exchange resin; a growing share is bio-based PG from glycerol hydrogenolysis. Both start from propylene oxide, so both track propylene and crude pricing. The extra CO₂-insertion step and tighter purity demands make PC the more expensive molecule per kg.Applications, Side by Side
PC’s headline use is as a polar aprotic solvent: lithium-ion and supercapacitor electrolytes, paint stripper and coating-removal formulations (often as a low-odor replacement for NMP or dichloromethane), gas treating, and as a plasticizer and carrier. PG’s volume sits in food and flavor (humectant, carrier), pharmaceuticals (USP solvent), cosmetics (humectant), and engineered heat-transfer fluid and antifreeze where lower toxicity than ethylene glycol is required.| Use case | Better fit | Why |
|---|---|---|
| Li-ion / supercapacitor electrolyte | PC | High dielectric constant, wide electrochemical window, low vapor pressure |
| Low-odor paint / coating stripper | PC | Strong aprotic solvency, high flash point (~132°C) |
| Food / flavor humectant | PG | GRAS under 21 CFR 184.1666 |
| Pharma / cosmetic carrier | PG | USP/EP monograph grade available |
| Lower-toxicity antifreeze / HVAC glycol | PG | Lower oral toxicity than ethylene glycol |
Regulatory Status and Grades
This is where the two diverge most. PG carries a clear food and pharma path: it is affirmed GRAS for direct food use under 21 CFR 184.1666 and is sold to USP and EP monograph grades, plus technical/industrial grade for antifreeze and deicing. Spec the grade to the end use; do not put technical-grade PG into a food or drug line. PC has no equivalent food-additive affirmation. It ships as technical and high-purity (battery/electronic) grades. For electrolyte work, water content is the critical spec, often called out below 20 ppm, since trace water degrades cell performance.Safety and Handling Trade-offs
PG is well known for low oral toxicity, which is why it substitutes for ethylene glycol where accidental exposure is a concern. It is still an industrial chemical: avoid large-scale ingestion, use gloves, and ventilate. PC has low acute toxicity but can cause mild skin and eye irritation, so wear gloves and goggles and keep ventilation adequate. The honest trade-off is cost versus capability. PG is cheaper, broadly available, and food-legal, but it cannot do PC’s electrochemistry. PC is the high-performance aprotic solvent, but it costs more and has no food-grade pathway. Neither is a drop-in for the other.Cost and Sourcing
PG is the higher-volume, lower-cost commodity with a deep, well-established supply chain across USP, food, and technical grades. PC is a smaller, more specialized market; pricing runs higher and is increasingly pulled by battery demand, which can tighten high-purity supply. When you send an RFQ, lead with grade and the one spec that gates your application: for PC, state water content and purity (and whether you need battery grade); for PG, state USP/EP/food/technical grade and whether you need it dyed or inhibited for a coolant loop. Specify packaging (drum, IBC, bulk), and ask for the CoA and SDS.Bottom Line
Choose propylene carbonate when the job is electrochemistry or aprotic solvency: electrolytes, supercapacitors, low-odor strippers, gas treating. Choose propylene glycol when the job is moisture control, freeze-point depression, or a low-toxicity carrier in food, pharma, cosmetics, or HVAC. If you want grade-matched PC or PG quoted to your spec and volume, send your application, target grade, and packaging and we will source against it.FAQs
What are the primary uses of propylene carbonate vs propylene glycol?
Propylene carbonate is a polar aprotic solvent used in lithium-ion and supercapacitor electrolytes, low-odor paint strippers, and gas treating. Propylene glycol is a humectant and low-toxicity carrier used in food, pharmaceuticals, cosmetics, and as an antifreeze and heat-transfer fluid.
How do the chemical properties of propylene carbonate and propylene glycol differ?
Propylene carbonate (CAS 108-32-7) is aprotic with a dielectric constant near 64.9 and a boiling point around 242°C. Propylene glycol (CAS 57-55-6) is a protic diol with a dielectric constant near 32, a boiling point around 188°C, and full water miscibility.
What grades should I specify on an RFQ?
For propylene carbonate, specify technical or battery/high-purity grade and a water-content limit (often below 20 ppm for electrolytes). For propylene glycol, specify USP, EP, food, or technical grade to match your end use, since food and pharma require monograph-grade material.
Which is regulated for food and pharmaceutical use?
Propylene glycol is affirmed GRAS for direct food use under 21 CFR 184.1666 and is available to USP/EP monograph grades. Propylene carbonate has no equivalent food-additive affirmation and is supplied as technical and high-purity industrial grades.
Which chemical is more cost-effective?
Propylene glycol is generally lower-cost and more widely available with a deep supply chain. Propylene carbonate costs more per kg because of its extra CO₂-insertion synthesis step and tighter purity demands, and high-purity grades can tighten with battery demand.
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