Diethylene glycol (DEG, CAS 111-46-6) is a linear aliphatic diol with the formula C₄H₁₀O₃ and molar mass 106.12 g/mol. The substance is recovered as a co-product during the non-catalytic thermal hydration of ethylene oxide to monoethylene glycol; the mass ratio of DEG to monoethylene glycol rises as the molar water-to-oxide feed ratio is reduced. At 20 °C, bulk release specifications list density at 1.1150–1.1170 g/cm³ by ASTM D4052, dynamic viscosity near 35.7 mPa·s, and closed-cup flash point of 124 °C by ASTM D93 Procedure A. The normal boiling point is 244–246 °C at 101.325 kPa, the freezing point is approximately -10.4 °C, and the vapor pressure at 25 °C is below 0.01 mmHg.

DEG is fully miscible with water and lower alcohols because the molecule contains two terminal hydroxyl groups and an internal ether linkage. The product is hygroscopic; at relative humidity above 60 %, storage under dry nitrogen is required to maintain water content below 0.10 wt%. Carbon steel tanks are used at ambient temperature, while 316L stainless steel is specified where color stability is critical because iron leaching promotes oxidative degradation. Copper and zinc alloys are avoided because dissolved transition metals catalyze oxidation to acidic degradation fractions.

The material is not interchangeable with propylene glycol in food-contact or pharmaceutical service. Propylene glycol is a three-carbon diol with lower acute oral toxicity and holds direct food additive clearance, whereas DEG is not permitted as a direct additive. Equipment cleaning between propylene glycol and DEG is performed with hot water at 70–80 °C because the product is water-miscible; rinse validation is conducted by conductivity or total organic carbon analysis.

What Limits DEG Purity in Oxirane-Derived Byproduct Streams?

The principal purity limitation is the homolog distribution from ethylene oxide hydration. Crude glycol liquor contains monoethylene glycol, DEG, triethylene glycol, and trace tetraethylene glycol; the relative fractions are a function of water-to-ethylene oxide molar ratio and reactor recycle. Published industrial distributions shift from approximately 85:12:3 to 90:9:1 for MEG:DEG:triethylene glycol as the water ratio increases. Separation is performed in a multi-column vacuum distillation sequence: a water column, a monoethylene glycol column, a DEG column, and a triethylene glycol column. The DEG column is operated at overhead pressure below 10 kPa absolute and bottom temperature below 160 °C to suppress thermal decomposition. A side-draw fraction is sent to a thin-film evaporator for final removal of light ends and color precursors.

Analytical release for DEG is normally performed by gas chromatography using ASTM E2409. Water is measured by ASTM E203 Karl Fischer titration because even 0.05 wt% water shifts the hydroxyl number and affects stoichiometric calculations in downstream esterification. Specification drift during ocean freight can result from water ingress through unlined carbon steel tank vents; shipments are sampled at top, middle, and bottom levels because water contamination is not always uniform in viscous static cargo.

When Diethylene Glycol Replaces Monoethylene Glycol in High-Temperature Solvent Service

Solvent selection between monoethylene glycol and DEG is governed by flash point, vapor pressure, and viscosity trade-offs. DEG offers a higher normal boiling point and lower vapor pressure, which reduces evaporative losses in dye and resin dissolution. The penalty is viscosity at 20 °C: DEG is approximately 2.1 times more viscous than monoethylene glycol. A pumped loop designed for monoethylene glycol must be re-evaluated for line pressure drop and net positive suction head margin when the fluid is switched to DEG. Heat transfer coefficients in shell-and-tube exchangers decline because the Prandtl number rises; the magnitude depends on tube-side geometry and Reynolds number.

Relative to triethylene glycol, DEG cannot sustain identical reboiler temperatures in gas dehydration. Thermal degradation of DEG becomes measurable above 160 °C; triethylene glycol tolerates 177–204 °C under vacuum-assisted regeneration. This temperature gap lowers the achievable equilibrium water dew-point suppression. Absorber designs using DEG therefore target dew-point depressions below 28 °C, whereas TEG contactors may target 45 °C or greater depending on tray efficiency and stripping gas rate.

Field gas dehydration skids with DEG are most often used where pipeline inlet temperatures stay below 38 °C and dew-point depression requirements do not exceed 28 °C. Lean DEG at 95–96 wt% is fed to the absorber top tray at 28–32 °C. The rich glycol is flashed in a three-phase separator at 350–500 kPa to remove entrained hydrocarbons. Regeneration uses a still column with stripping gas injection; the reboiler is fired to maintain a liquid temperature not exceeding 160 °C. The resulting lean DEG water content is typically 2.0–3.5 wt%; further water removal requires either lower pressure or increased stripping gas flow. Published field data for DEG-specific contactor performance is limited compared with TEG.

Brake Fluid Equilibrium Boiling Point Constraints

Brake fluid formulations using DEG as a solvent are constrained by FMVSS 571.116 and SAE J1703. The dry equilibrium reflux boiling point of the blended fluid is determined by distilling a 60 mL sample under specified conditions. The wet boiling point is evaluated after humidifying the fluid to 3.5 wt% water. A DOT 3 fluid must have a dry boiling point of at least 205 °C and a wet boiling point of at least 140 °C; DOT 4 requires 230 °C dry and 155 °C wet. DEG contributes to the high-boiling fraction, but low-temperature viscosity at -40 °C is controlled by the glycol ether distribution. Excess DEG raises cold viscosity and can reduce rubber swell compatibility with EPDM seals unless balanced with borate ester and anti-corrosion additive packages.

Unsaturated polyester resin synthesis uses DEG as a partial replacement for propylene glycol in the reaction with maleic anhydride and phthalic anhydride. The esterification kettle is operated at 180–220 °C with a nitrogen sparge; acid number is monitored until 15–25 mg KOH/g is reached. DEG lengthens the chain between unsaturation sites and shifts the cured network toward lower tensile modulus. Mechanical tests under ASTM D638-14 show reduced tensile strength relative to rigid propylene glycol formulations, while elongation at break increases. The addition level is normally limited to 20–30 mol% of total diol because higher levels reduce heat deflection temperature in unfilled castings.

Catalytic conversion of DEG to morpholine uses a fixed-bed hydrogenation catalyst in a plug-flow reactor. The process operates at 150–200 °C and 5–15 MPa hydrogen partial pressure. Excess ammonia is used to shift selectivity toward morpholine; the main byproducts include aminoethoxyethanol and higher oligomers. Catalyst selectivity is sensitive to liquid hourly space velocity, and published data for specific catalyst configurations is limited.

Printing ink and textile lubricant uses rely on the moisture-retention and dye-solubility properties of DEG. In flexographic water-based inks, DEG is charged at 2–8 wt% of the formulation and viscosity is checked at 25 °C by ASTM D2196. The use rate is limited because excessive DEG slows drying and increases blocking of printed film. In textile fiber finishes, DEG is blended with ethoxylated fatty acid esters at 5–15 wt%; the formulation is applied by roller kiss-coat to maintain 0.5–1.5 % oil pickup by fiber mass.

Industrial handling of DEG requires closed equipment and local exhaust ventilation. The material is classified as harmful if swallowed and must not be used in food, drug, or cosmetic formulations. For indirect food-contact adhesives and coatings, migration limits are established under 21 CFR 175.300 and 21 CFR 177.1390; these approvals apply only to specified polymer matrices and do not permit direct addition. In the European Union, REACH registration requires exposure scenarios; occupational exposure limits are derived from DNEL values rather than consumer product exemptions. The substance must not be combined with strong oxidizers, concentrated nitric acid, or elemental sodium. Uncontrolled bulk addition to isocyanates is also incompatible because the resulting exotherm can exceed the thermal stability of the mixing vessel.

Cement grinding aid dosage of DEG is typically 0.01–0.05 wt% of clinker feed. It is added at the mill belt or sprayed into the second compartment of a ball mill. The polar diol adsorbs on fresh clinker surfaces and reduces agglomeration of particles below 45 µm. Published mill audits show Blaine surface area gains of 200–500 cm²/g at constant specific energy, although the response is nonlinear and depends on clinker mineralogy and separator efficiency.