Dipropylene glycol (25265-71-8) is a hygroscopic, medium-viscosity diol produced from propylene oxide hydrolysis. The commercial substance is a mixture of oxydipropanol isomers, principally 1,1'-oxybis-2-propanol, with a molecular weight of 134.2 g/mol. At 20 °C, density is reported in the range 1.020–1.024 g/cm³ by ASTM D4052; the boiling range at 101.3 kPa is 228–236 °C by ASTM D1078. The equilibrium vapor pressure is lower than that of propylene glycol, typically near 1–2 Pa at 25 °C in supplier safety data sheets. Dipropylene glycol is not the same as diethylene glycol (111-46-6) and is not subject to the same acute oral toxicity classification under GHS in bulk supplier safety data sheets. It is sold in technical, low-odor, polymer, and fragrance grades that differ in water content, acidity, color, and trace carbonyl control.

Supplier-specific grade names are not harmonized; a grade suffix indicating low odor may reflect reduced unsaturated carbonyls, but the buyer must verify the isomer ratio and water limit because these parameters control downstream stoichiometry. In polymer applications, total glycol purity is more significant than individual isomer purity unless the esterification reactor is sensitive to primary-secondary hydroxyl distribution.

What Purity and Water Limits Are Applied to Bulk Dipropylene Glycol?

Bulk specifications are set around the failure modes observed in heated processing equipment. Residual propylene glycol lowers the average hydroxyl number and increases top-phase losses in vacuum esterification; tripropylene glycol raises melt viscosity and can delay acid number reduction. Polymer-grade DPG is therefore typically supplied with total glycol purity not less than 99.0 wt% by gas chromatography. Water is controlled to ≤ 0.10–0.20 wt% by ASTM E203 because water consumes isocyanate in polyurethane intermediates and reverses esterification equilibria. Color is limited to ≤ 15 Pt-Co by ASTM D1209 because high color transfers to clear resin systems and can indicate oxidative degradation during storage.

Propylene oxide hydration produces a distribution of monopropylene glycol, dipropylene glycol, tripropylene glycol, and higher oligomers. Vacuum fractionation separates DPG from the higher glycols; the isomer ratio in the distilled product depends on reactor water-to-oxide ratio and temperature. Batch-to-batch variation in the proportion of primary and secondary hydroxyls changes esterification rates in polyester polyol reactors. In a production-scale continuous esterification line, a shift toward secondary hydroxyl-rich DPG slows acid number reduction under fixed residence time, so the control system requires a higher diol-to-diacid ratio or a longer finishing stage. For this reason, polyol producers request lot-specific hydroxyl number certification by ASTM D4274 and do not rely on nominal purity alone.

Low-Vapor-Pressure Solvent Function in Fragrance and Cosmetic Formulations

Dipropylene glycol is used as a carrier and solvent for fragrance oils in alcoholic and aqueous systems. Its low vapor pressure, on the order of 1–2 Pa at 25 °C, reduces evaporation rate compared with ethanol and increases fragrance retention on skin; the trade-off is a heavier initial wet feel. The hydroxyl groups provide hydrogen-bonding solvency for polar fragrance components, while the propylene oxide backbone retains moderate affinity for hydrophobic aroma chemicals. Use levels are formulation-dependent and are typically optimized by headspace gas chromatography rather than fixed by specification. In cosmetic formulations, DPG is listed in the European Commission Cosmetic Ingredient Database; formulators should confirm the specific grade complies with the cosmetic product safety assessment under EC 1223/2009. High solvent loading can flatten the headspace concentration of low-boiling top notes, and this effect is not reversible once the fragrance-solvent mixture is blended.

Low-odor fragrance-grade DPG is controlled for carbonyl compounds that cause off-notes and for peroxides that can form during prolonged storage. Peroxide formation is limited by storage under dry nitrogen and by avoiding ultraviolet light exposure; supplier certificate analyses may include a peroxide limit expressed as meq/kg, but the limit is supplier-specific. The material is not suited to undiluted aerosol propellant service because its viscosity and boiling point are outside the range of common propellant systems.

In aqueous hard-surface cleaners and architectural coatings, DPG functions as a coupling agent for oil-soluble actives and as a medium-boiling coalescent. In water-based acrylic coatings, the selection of DPG over propylene glycol shifts minimum film formation temperature reduction less efficiently on a mass basis because of the higher molecular weight, but it lowers evaporation rate during open time. Formulators measure minimum film formation temperature by ASTM D2354 and scrub resistance by ASTM D2486 when comparing coalescent packages. In cleaning concentrates, DPG couples nonionic surfactants and hydrophobic fragrances at ambient temperatures, but the addition must be limited to avoid viscosity inversion in high-electrolyte systems.

Unsaturated Polyester Resin Synthesis Is Sensitive to Diol Isomer Ratio and Water Content

In unsaturated polyester resin production, DPG is charged with maleic anhydride, phthalic anhydride, and other glycols in a heated reactor, typically between 180 °C and 230 °C, with water removal through a partial condenser and vacuum finishing. Compared with propylene glycol, DPG has a higher boiling point and lower vapor pressure, which reduces glycol loss in the overhead water stream but increases resin melt viscosity at a given molecular weight. The use of DPG decreases the molar concentration of ester groups per kilogram of resin, generally lowering water absorption and tensile strength of the cured casting; tensile properties are compared by ASTM D638-14. The isomer distribution affects the rate of cis-trans isomerization of maleate to fumarate unsaturation, which in turn influences styrene crosslink density and reactivity. Reactor operators use acid number and hydroxyl number tracking rather than fixed time; acid number is commonly monitored by ASTM D1639 or an equivalent harmonized method. Water is controlled below 0.10 wt% in the diol feed to avoid slower esterification and foaming during vacuum pull-down.

When a Secondary Coolant Must Offer Lower Acute Oral Toxicity Than Ethylene Glycol

Aqueous DPG solutions are evaluated in closed-loop secondary cooling and heating systems where ethylene glycol is restricted by site safety review. DPG has a higher molecular weight than propylene glycol; at equivalent mass fraction, the freezing point depression is lower because molality is lower. The higher boiling point of DPG can reduce vapor-phase glycol carryover in vented expansion tanks, but the higher liquid viscosity increases pump energy and lowers heat transfer coefficient at the same flow rate. Corrosion inhibitor packages are selected and validated using ASTM D1384 or ASTM D8040, with pH commonly maintained between 8.0 and 9.0. Published data for DPG-specific heat transfer coefficients in compact plate exchangers is limited; therefore, design calculations should use measured density, viscosity, thermal conductivity, and specific heat for the actual dilution instead of extrapolating from propylene glycol data. Systems with aluminum components require inhibitor review because glycols can corrode aluminum under thermal stress.

The diol is also esterified with benzoic acid to produce dipropylene glycol dibenzoate, which is used as a high-solvating plasticizer in polyvinyl chloride and in water-based adhesive latex. Batch esterification to the dibenzoate is carried out with excess benzoic acid and an acid catalyst; the water of reaction is removed under reduced pressure until acid number falls below 0.5 mg KOH/g. The higher molecular weight of DPG compared with propylene glycol yields a plasticizer with lower volatility than propylene glycol dibenzoate, but it also raises the glass transition temperature depression efficiency less on a molar basis. Plasticizer compatibility in PVC is evaluated by loop migration tests against ASTM D3291, depending on the end-use specification.

Differentiation from Propylene Glycol and Tripropylene Glycol Is Based on Hydroxyl Number and Volatility

The propylene oxide oligomer family separates by molecular weight and functionality. Dipropylene glycol contains two hydroxyl groups per molecule, but the higher molecular weight reduces hydroxyl number compared with propylene glycol. This reduction is useful when a polyester or polyurethane formulator needs a longer flexible segment between crosslinks without raising vapor pressure. Tripropylene glycol has a still higher molecular weight and lower hydroxyl number but also higher viscosity and lower water solubility. The table below lists representative values from public safety data sheets and manufacturer technical bulletins; these are not product specification limits.

In polyurethane polyester polyols, DPG is selected over propylene glycol when the target polyol hydroxyl number is below 56 mg KOH/g and a liquid intermediate is required at room temperature. The longer DPG backbone increases segmental motion and reduces water absorption in the cured elastomer, but it also raises the viscosity of the resulting polyol, which must be handled with heated transfer lines. Before reaction with diisocyanates, the polyol is dehydrated to ≤ 0.05 wt% water; residual water generates carbon dioxide and changes the isocyanate index. Storage of DPG in open or vented tanks in high-humidity locations can increase water content by more than 0.1 wt% per 24 h, so dry nitrogen blanketing is required. The product should not be stored with strong oxidizing agents, acid chlorides, or isocyanates; carbon steel and 304 stainless steel are acceptable for ambient storage, while aluminum is avoided in heated service. In indirect food-contact uses, formulators verify the specific application under 21 CFR 175.105 or 21 CFR 176.180 as applicable.