| HS Code | |
| Productname | Propylene Oxide |
| Synonyms | Propylene oxide; 1,2-epoxypropane; methyloxirane; propene oxide |
| Iupacname | 2-methyloxirane |
| Chemicalformula | C3H6O |
| Casnumber | 75-56-9 |
| Ecnumber | 200-879-2 |
| Unnumber | 1280 |
| Molecularweight | 58.08 g/mol |
| Appearance | Colorless liquid |
| Odor | Ether-like, sweet, pungent |
| Boilingpoint | 34.3 °C |
| Meltingpoint | -112 °C |
| Flashpoint | -37 °C (closed cup) |
| Autoignitiontemperature | 449 °C |
| Explosivelimits | 2.3–36 vol% in air |
| Density | 0.859 g/cm3 at 20 °C |
| Vapordensity | 2.0 (air = 1) |
| Vaporpressure | 445 mmHg at 20 °C |
| Solubility | Soluble in water, ethanol, ether, acetone, benzene |
| Refractiveindex | 1.3664 at 20 °C |
| Viscosity | 0.28 mPa·s at 25 °C |
| Surfacetension | 22.8 mN/m at 20 °C |
| Criticaltemperature | 209.1 °C |
| Criticalpressure | 4.92 MPa |
| Heatofvaporization | 27.5 kJ/mol |
| Heatofcombustion | -1917 kJ/mol |
As an accredited Propylene Oxide (PO) factory, we enforce strict quality protocols—every batch undergoes rigorous testing to ensure consistent efficacy and safety standards.
| Packing | Propylene Oxide (PO) is packaged in 200 L steel drums or 20,000 L ISO tanks, labeled flammable liquid, UN 1280. |
| Container Loading (20′ FCL) | Container Loading (20′ FCL) for Propylene Oxide (PO): IMDG-approved drums, secured, ventilated, segregated, placarded, with leak checks and emergency preparedness. |
| Shipping | Propylene Oxide (UN 1280) ships as a Class 3 flammable, toxic liquid (Packing Group I). It requires dedicated pressure tanks, tank cars, or ISO containers with nitrogen padding, cooling, and leak monitoring. Transport must follow DOT/IMDG/IATA rules, avoid ignition sources, and protect against polymerization and inhalation hazards. |
| Storage | Propylene oxide (PO) is highly flammable and reactive. Store in a cool, dry, well-ventilated, fire-resistant area away from heat, ignition sources, acids, bases, oxidizers, and reactive metals. Use tightly closed, grounded, clearly labeled containers, preferably under nitrogen, with explosion-proof equipment and secondary containment. Monitor temperature to prevent polymerization, limit quantities, follow local regulations, and ensure ventilation and static control. |
| Shelf Life | Propylene oxide (PO) typically has a 12-month shelf life when stored cool, dry, sealed, and away from heat, acids, bases, and catalysts. |
On continuous flexible slabstock lines, propylene oxide (PO, CAS 75-56-9) is consumed primarily as the dominant oxirane monomer in glycerine-initiated polyether triol production, where reactor sequencing determines hydroxyl number, primary hydroxyl content, and downstream compression set behaviour. The oxide charge is split at 87–92 wt% PO and 8–13 wt% ethylene oxide on a total oxide feed basis, with the ethylene oxide positioned as the terminal cap to raise primary hydroxyl content above 70%. Slabstock-grade polyol is routinely qualified under ASTM D3574-17 for indentation force deflection and resilience, ISO 3386-1 for compression stress-strain, and ISO 8307 for rebound resilience, while REACH registration and automotive interior VOC protocols govern export documentation. Production-scale alkoxylation in stirred loop reactors with external gas-liquid contact is maintained at 105–130°C and 0.3–0.5 MPa absolute, with PO feed staged to limit exotherm excursion and to control molecular weight distribution. After neutralisation with phosphoric acid and magnesium silicate filtration, residual potassium must remain below 10 ppm to prevent reactivity drift in isocyanate-tin systems. Initiator moisture below 0.1 wt% is critical because water initiates propylene glycol side product and broadens polydispersity. The resulting 3,000 g/mol triol typically exhibits hydroxyl number 56 mg KOH/g and viscosity 450–550 mPa·s at 25°C. Terminal finished product types include flexible slabstock foam for bedding, high-resilience moulded foam for automotive seating, and technical polyurethane foam for acoustic absorption in industrial equipment enclosures.
In rigid polyisocyanurate and polyurethane panel lamination, a sucrose- or sorbitol-initiated propylene oxide polyol with hydroxyl number 350–450 mg KOH/g and viscosity 5,000–15,000 mPa·s at 25°C is blended at 100 parts with 8–14 parts cyclopentane or hydrofluoroolefin blowing agent, 2.0–3.5 parts water, 1.5–2.5 parts silicone surfactant, and 1.0–2.0 parts potassium octoate/amine catalyst, with polymeric MDI indexed at 110–115. The polyol itself is built from 70–80 wt% propylene oxide in the alkoxylation charge, the remaining oxide being ethylene oxide for controlled reactivity and blowing agent compatibility. Thermal performance is certified under ASTM C518-21, dimensional stability under ISO 2796, and finished board conformity under EN 13165. High-pressure metering units with impingement mixers deliver component pressures of 120–160 bar before the reacting blend is distributed onto a double-belt laminator held at 40–60°C for the cream, rise, and cure profile. Water above 4.0 parts produces density collapse and friability at the panel edges, while insufficient water reduces flow into narrow appliance wall sections. Terminal finished product types include PIR/PUR insulation boards for flat roofing, pour-in-place refrigerator and freezer insulation, and pre-insulated district heating pipe sections.
Thermal hydration of propylene oxide to monopropylene glycol proceeds without catalyst only when the molar water-to-PO feed is held at 15:1 to 20:1, because lower ratios accelerate secondary etherification to dipropylene glycol and tripropylene glycol. The industrial route uses a continuous tubular or stirred hydration zone at 180–220°C and 12–20 bar, followed by vacuum distillation to separate monopropylene glycol from higher glycols and water. Aircraft deicing fluid formulated from this MPG contains 40–60 wt% monopropylene glycol, 0.1–0.5 wt% tolyltriazole-based corrosion inhibitor, 0.05–0.2 wt% nonionic surfactant, and demineralised water for the balance. Fluid acceptance is governed by SAE AMS 1424 for Type I deicing performance and SAE AMS 1428 for Type II–IV anti-icing rheological stability. Food-grade monopropylene glycol is regulated under 21 CFR 184.1666, while glycol-based heat transfer fluids are tested under ASTM D4171. The production constraint is not distillation alone but the byproduct distribution; dropping the water-to-PO ratio toward 8:1 raises dipropylene glycol output beyond 10 wt% and reduces monopropylene glycol yield below economic thresholds. Terminal finished product types include Type I aircraft deicing fluids, Type IV anti-icing fluids with pseudoplastic thickener packages, industrial heat transfer fluids, and unsaturated polyester resin diluents.
Continuous alkoxylation of methanol with propylene oxide yields propylene glycol monomethyl ether, and subsequent esterification with acetic acid produces propylene glycol monomethyl ether acetate (PMA, CAS 108-65-6). In coil coating solvent systems, PMA addition at 5–15 wt% of the total solvent blend adjusts evaporation rate, surface tension, and electrostatic spray resistance; in photoresist edge bead remover formulations, PMA is compounded at 30–70 wt% with other glycol ether acetates to remove edge build-up without attacking the underlying copper-clad laminate. Compliance testing for flammability uses ASTM D3278-20, while electronics-grade PMA procurement specifications commonly require water below 100 ppm and alkali metal or heavy metal ions below 50 ppb each under RoHS-restricted substance protocols. The alkoxylation step is operated at 120–140°C with methanol excess and basic catalyst, followed by catalytic distillation to PM; esterification proceeds under acid catalysis with azeotropic water removal to drive conversion above 98%. Moisture ingress above 100 ppm in esterification leads to hydrolysis back to PM and acetic acid, shifting solubility parameters and creating residual acid that corrodes microstrip copper surfaces. Terminal finished product types include coil coatings, automotive basecoats, photoresist edge bead removers, solder mask inks, and liquid crystal display cleaning formulations.
When propylene carbonate is formulated into lithium-ion electrolyte blends, its dielectric constant of 64.9 at 25°C and viscosity of 2.5 mPa·s improve salt dissociation and low-temperature conductivity, but its co-intercalation into graphite anodes restricts the addition window to 5–20 wt% of the solvent mass unless sacrificial SEI additives such as vinylene carbonate at 1–3 wt% are present. The cyclic carbonate is manufactured by cycloaddition of propylene oxide with carbon dioxide in a fixed-bed reactor over alkali iodide/quaternary ammonium catalyst at 140–180°C and 5–8 MPa, giving propylene carbonate selectivity above 99% after distillation. Electrolyte blending is executed under dry-room conditions with moisture below 20 ppm and HF below 50 ppm, and cell-level safety qualification references IEC 62619:2022 plus UN 38.3 transport testing. Published data for the exact graphite exfoliation threshold varies by anode source, surface oxidation, and electrolyte salt concentration; therefore high-PC formulations are not simply concentration-limited but anode-chemistry-dependent. Terminal finished product types include lithium-ion battery electrolytes for consumer electronics and electric vehicles, supercapacitor electrolytes, and propylene carbonate-based natural gas sweetening solvents.
Propylene oxide reacts with granular starch under alkaline slurry conditions to form hydroxypropyl starch, with PO charged at 5–10 wt% of dry starch to produce a degree of substitution between 0.02 and 0.20. The etherification proceeds in an aqueous slurry at 35–45% solids with sodium sulfate at 10–15 wt% of dry starch to suppress granule swelling, sodium hydroxide to pH 11.0–11.8, and temperature held at 40–50°C for 18–24 h. Food-grade hydroxypropyl starch must comply with FDA 21 CFR 172.892 and EU Regulation (EC) No 1333/2008 as additive E1440, with JECFA specifying residual propylene chlorohydrin not more than 1 mg/kg. After reaction, the slurry is neutralised with hydrochloric acid, washed to remove sulfate and chloride, and flash-dried to a moisture content below 12 wt%. pH excursions above 12.0 cause irreversible granule swelling and a viscosity spike that makes downstream washing and filtration impractical at production scale. Terminal finished product types include modified food starch for retort sauces and aseptic soups, surface sizing agents for paper, warp sizing compounds for textile weaving, and starch-based viscosifiers for oilfield drilling fluids.
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Propylene Oxide (PO) is the C3 aliphatic epoxide identified by CAS 75-56-9 and EC 200-879-2. The molecule consists of a three-member oxirane ring with a pendant methyl group, giving molecular weight 58.08 g/mol, density 0.829 g/cm³ at 20 °C, boiling point 34.2 °C at 101.3 kPa, closed-cup flash point -37 °C, and vapor pressure 59 kPa at 20 °C. Commercial propylene oxide is supplied as a clear, colorless, highly flammable liquid with a characteristic ethereal odor. It is miscible with most organic solvents but only partially miscible with water; aqueous solubility is approximately 40.5 g/100 g at 20 °C.
Product differentiation in commercial supply is not expressed as discrete model designations; producers classify material by end-use grade such as polyether polyol grade, propylene glycol grade, and glycol ether grade. These grades share the same CAS identity but differ in trace acetaldehyde, propionaldehyde, water, acidity, and non-volatile residue. The product is transported under UN 1280 with Class 3 flammable liquid classification. Under EU CLP, the harmonized classification includes Flam. Liq. 1 H224, Carc. 1B H350, and Muta. 1B H340. This hazard profile determines storage design, relief sizing, and operator exposure controls at production-scale bulk receiving systems.
Commercial polyether-polyol-grade PO is routinely controlled for purity and trace oxygenated impurities because aldehyde content participates in polyol color formation and catalyst deactivation. A representative specification is shown in Table 1; individual producer certificates of analysis may differ slightly in method or limit.
| Property | Typical limit | Test method |
|---|---|---|
| Purity (GC-FID, area-%) | ≥ 99.9 | Producer gas chromatography protocol |
| Water | ≤ 0.010 % (m/m) | ASTM E203-16 |
| Acidity as acetic acid | ≤ 0.002 % (m/m) | ASTM D1613-06 |
| Aldehydes as propionaldehyde | ≤ 0.005 % (m/m) | Hydroxylamine hydrochloride titration |
| Color, platinum-cobalt | ≤ 5 | ASTM D1209-05 |
| Non-volatile residue | ≤ 0.005 g/100 mL | ASTM D1353-13 |
| Density at 20 °C | 0.828–0.831 g/cm³ | ASTM D4052-22 |
| Distillation range, 5–95 % volume | 33.5–35.0 °C | ASTM D1078-11 |
The low water limit is not merely a purity determinant; in polyether polyol polymerization water acts as a competing initiator, altering nominal functionality and broadening molecular weight distribution. At reactor scale, a PO water content above 0.010 % can introduce measurable shifts in hydroxyl number of low-equivalent-weight polyols when the formulation assumes anhydrous PO feed. Aldehydes at levels above 0.005 % have been associated with color development in amine-catalyzed polyol batches, particularly when double metal cyanide catalyst activation is delayed by impurities. Hydroxyl number is measured by ASTM D4274-21.
Vapor-phase hazards govern most receiving-system decisions. The lower flammable limit is 2.3 % (v/v) and the upper flammable limit is 36 % (v/v) in air at 20 °C. Because the normal boiling point is below ambient summer storage temperatures in some locations, bulk storage vessels are designed for pressure/vacuum service, generally 0.5 bar g design pressure, with closed venting to a scrubber or thermal oxidizer. Relief devices are sized for fire exposure using recognized pressure-relief design codes. Ventilation rate in transfer areas is maintained to keep concentrations below 10 % of the lower flammable limit. Personnel exposure is controlled through closed-loop sampling and positive-pressure pump seals, because PO is classified as a potential occupational carcinogen.
Propylene oxide and ethylene oxide both contain the oxirane ring, but the methyl substituent on PO creates steric and electronic differences that influence downstream polymer architecture. Ethylene oxide, with molecular weight 44.05 g/mol, boils at 10.4 °C and has a closed-cup flash point near -18 °C; propylene oxide is less volatile but has a lower flash point. The asymmetric oxirane ring of PO can open at either the primary or secondary carbon. In base-catalyzed polyether synthesis, attack occurs predominantly at the less substituted carbon, yielding a secondary alkoxide; this generates polyether chains with pendant methyl groups and a predominance of secondary hydroxyl termini when PO is the terminal oxide. In ethylene oxide-derived ethoxylates, ring opening yields unbranched oxyethylene repeat units with primary hydroxyl termini, producing greater hydrophilicity and crystallinity. The measured viscosity of PO-based polyols at a given molecular weight is typically higher than that of EO-based ethoxylates of similar molecular weight because methyl substitution restricts chain rotation.
| Property | Propylene Oxide | Ethylene Oxide |
|---|---|---|
| Molecular weight | 58.08 g/mol | 44.05 g/mol |
| Boiling point at 101.3 kPa | 34.2 °C | 10.4 °C |
| Closed-cup flash point | -37 °C | -18 °C |
| Vapor pressure at 20 °C | 59 kPa | 146 kPa |
| Flammable range in air | 2.3–36 % v/v | 3.0–100 % v/v |
These differences make PO the preferred epoxide for rigid and flexible polyether polyols where lower hydrophilicity and liquid handling at ambient temperature are advantageous. Ethylene oxide is used principally as a cap to increase primary hydroxyl content for faster reaction with isocyanate in flexible molded foam; a mixed PO/EO block structure is therefore common. The ratio of EO cap is controlled precisely because excess ethylene oxide raises water absorption in the finished foam; ASTM D2842-19 water absorption and ASTM D3574-17 humid aging protocols are used to evaluate the effect.
Approximately 60–70 % of global propylene oxide demand enters the polyether polyol segment. The dominant process line consists of an alkoxylation reactor charged with a multifunctional initiator such as glycerol, sucrose, sorbitol, or propylene glycol; potassium hydroxide or a double metal cyanide catalyst is dispersed in the initiator at 0.2–0.5 wt% for conventional base catalysis. PO is fed below the liquid surface through a sparger or via a recirculation loop at a rate controlled by reactor pressure, typically 3.0–5.5 bar g for base-catalyzed batch operations. The polymerization is exothermic, and heat is removed by an external shell-and-tube exchanger on a pumped recirculation loop. The reaction mass is maintained at 110–120 °C for KOH-catalyzed batches; double metal cyanide-catalyzed processes may operate at 130–150 °C after initiation. Crude polyol is neutralized, filtered, and stripped under vacuum to remove residual PO. Final polyol quality is characterized by hydroxyl number via ASTM D4274-21, acid number via ASTM D4662-20, viscosity at 25 °C via ASTM D4878-15, and water via ASTM E203-16.
When a continuous polyether polyol line is operated with short hold-up time, PO feed quality directly influences the hydroxyl number trajectory and catalyst productivity. Batch-to-batch variation in water or aldehyde content changes the effective initiator stoichiometry; a water increase of 0.005 % in the PO feed can alter the nominal hydroxyl number of a 56 mg KOH/g rigid polyol by a measurable amount if the control model does not include on-line water analysis. Production lines therefore use on-line near-infrared moisture analyzers in the PO feed line and automated ratio control linked to Coriolis mass flowmeters. The ratio control system typically holds PO:initiator mass flow ratio within ±0.5 % of setpoint. When off-spec aldehyde content is detected, feed rate is reduced and nitrogen stripping increased; the affected batch is segregated for hydroxyl number and color testing by ASTM D4274-21 and ASTM D1209-05. Alkoxylation vessels with external heat exchangers can show hot spots at the tube sheet if feed interruptions create stratified initiator/PO regions.
Bulk receiving of propylene oxide at a polyol plant generally uses carbon steel or stainless steel tanks rated for the maximum vapor pressure at the design temperature. Because PO is a Class 3 flammable liquid with a flash point of -37 °C, transfer pumps are specified with magnetic-drive or canned-motor sealless configurations to reduce seal leaks. Tank blanketing with nitrogen at 0.2–0.5 bar g is standard; pressure/vacuum vents discharge to a closed header rather than directly to atmosphere. Liquid transfer lines are sized for a velocity below 1 m/s in initial fill and below 7 m/s during normal operation to limit static charge accumulation; bonding and grounding are mandatory. Because PO vapor is denser than air, gas detection sensors are placed near grade and in pump pits, with alarm setpoints at 10 % of LFL. Sectionalizing valves and fire-safe shutoff valves are installed at tank nozzles. Closed-loop vapor-balancing lines fitted with flame arresters are used for return vapor to tank trucks or railcars. Published data for PO-specific elastomer compatibility under continuous duty are limited; seal selection therefore relies on supplier immersion testing. Contact with aqueous acids, anhydrous metal chlorides, and amine-based additives is avoided because these materials can initiate polymerization or accelerate corrosion.
In the propylene glycol segment, PO is hydrated at high temperature and pressure, typically 120–200 °C under excess water, to produce a mixture of monopropylene glycol, dipropylene glycol, and tripropylene glycol. The product split is controlled by the water-to-PO molar ratio, with monopropylene glycol yields exceeding 90 % when the water-to-PO ratio is high. USP-grade propylene glycol requires additional purification and must meet the USP monograph for propylene glycol, including a limit on ethylene glycol and diethylene glycol. Glycol ether production involves reaction of PO with methanol, ethanol, or n-butanol; the resulting propylene glycol methyl ether, propylene glycol ethyl ether, and propylene glycol n-butyl ether are used in coatings, cleaners, and inks. The lower toxicity profile of propylene glycol ethers relative to ethylene glycol ethers has led to substitution in formulations where the longer chain or slower evaporation rate is acceptable.
