Propylene Glycol, Monopropylene Glycol, Methylethylene Glycol, E1520, MPG, PG, 57-55-6, 4254-14-2, 4254-15-3

Propylene Glycol, Monopropylene Glycol, Methylethylene Glycol, E1520, MPG, PG, 57-55-6, 4254-14-2, 4254-15-3

PROPYLENE GLYCOL (Mono Propylene Glycol, MPG)

1. PRODUCT DEFINITION AND CHEMICAL IDENTITY

Chemical Name (IUPAC): Propane-1,2-diol
Common Name: Propylene Glycol, Mono Propylene Glycol
Food Additive Code: E1520
CAS Number: 57-55-6 (also referenced as 4254-14-2, 4254-15-3 for racemic/enantiomeric forms)
EC No: 200-338-0
Molecular Formula: C₃H₈O₂
Molecular Weight: 76.10 g/mol
Appearance: Clear, colourless, viscous, hygroscopic liquid
Purity (Standard Grades): ≥99.5% (USP/EP, Food Grade), also available as industrial and fiber grades.

2. PHYSICAL AND CHEMICAL PROPERTIES

Melting Point: −59 °C (pourable below this temperature; tends to supercool)
Boiling Point: 188–189 °C (at 760 mmHg)
Density: ~1.036 g/cm³ at 25 °C
Solubility: Completely miscible with water, ethanol, acetone, chloroform, and many organic solvents. Insoluble in fixed oils, but miscible with essential oils.
pH (100 g/L in water, 20 °C): 6.0–8.0
Flash Point (closed cup): ~98–101 °C
Autoignition Temperature: ~371 °C
Explosive Limits (% v/v in air): 2.4–17.4
Refractive Index (n₂₀/D): ~1.432
Vapour Pressure: ~0.08 mmHg at 20 °C
Viscosity (dynamic, 20 °C): ~56 mPa·s
Dielectric Constant: ~32.0 at 20 °C
Water Hazard Class (WGK, Germany): 1 (low hazard to water)
Harmonized System Code (HS / GTIP): 2905.32.00

3. OTHER NAMES AND SYNONYMS

• 1,2-Propanediol

• α-Propylene Glycol

• Methylethylene Glycol

• 1,2-Dihydroxypropane

• MPG (Mono Propylene Glycol)

• Monopropylene Glycol USP / Pharma Grade

• E1520 (European food additive number)

• Common terms: “Food-grade glycol”, “aroma carrier solvent”, “humectant additive”, “PG solvent”

4. INDUSTRIAL APPLICATIONS, USAGE RATES, AND EXAMPLE FORMULATIONS

4.1. Food and Beverage Industry
Purpose: Humectant to retain moisture in baked goods, desserts, and pet foods; solvent and carrier for flavours, colours, antioxidants, and vitamins; emulsifier aid in sauces and dressings.
Usage Rate:

• General food products: 0.1–5% w/w (depending on formulation and local regulations).

• As a flavour solvent: 1–10% of the flavour concentrate.

• As a colour solvent: 2–15% in liquid colour preparations.
  Example Formulation – Flavour Emulsion Concentrate:

• 10% propylene glycol (solvent/carrier)

• 15% flavour oil (e.g., lemon oil)

• 10% gum arabic (emulsifier)

• 65% water
  Mix PG and flavour oil; add to gum arabic solution under high-shear mixing. Homogenize to a stable emulsion.

4.2. Pharmaceuticals and Veterinary Medicine
Purpose: Solvent and co-solvent for oral, injectable, and topical drug formulations; humectant in antiseptic hydrogels; cryoprotectant in biological samples; oral drench for ketosis prevention in dairy cattle.
Usage Rate:

• Oral/injectable solutions: 10–50% v/v (as part of the solvent system, often with water/ethanol/glycerol).

• Topical gels and creams: 5–20% w/w.

• Veterinary ketosis drench: 300–400 mL per cow per day (oral administration of pure or diluted PG).
  Example Formulation – Antiseptic Hydrogel:

• 20% propylene glycol (humectant/solvent)

• 1% hydroxyethyl cellulose (gelling agent)

• 0.05% chlorhexidine digluconate

• Water to 100%
  Disperse hydroxyethyl cellulose in water; dissolve chlorhexidine in PG and blend into the gel under stirring.

4.3. Cosmetics and Personal Care
Purpose: Humectant and moisturizer in skin creams, lotions, shampoos, hair conditioners, and makeup; solvent for fragrances and active ingredients; enhances spreadability and feel.
Usage Rate:

• Moisturizing creams/lotions: 1–5% (often combined with glycerin).

• Shampoos and body washes: 1–3%.

• Makeup and baby wipes: 1–10% as a humectant.
  Example Formulation – Light Moisturizing Lotion:

• 4% propylene glycol

• 2% glycerin

• 5% caprylic/capric triglyceride

• 2% cetyl alcohol

• 0.5% Carbomer, water to 100%
  Neutralize Carbomer with triethanolamine to pH ~6; PG and glycerin are added to the water phase for improved skin hydration.

4.4. Industrial Antifreeze and Heat Transfer Fluids
Purpose: Used in chillers, closed-loop cooling systems, ground-source heat pumps, and aircraft de-icing/anti-icing fluids. Its non-toxic profile makes it the glycol of choice for food-processing plants, breweries, and ice rinks where incidental contact with products or people may occur.
Usage Rate:

• Freeze protection (−15 °C to −35 °C): 30–50% v/v propylene glycol in water. A 50% aqueous solution has a freeze point of approximately −33 °C; a 30% solution around −13 °C.

• Aircraft de-icing (SAE Type I fluid): usually >50% PG with corrosion inhibitors, surfactants, and thickeners, diluted with water to match required freeze point buffer.
  Example Formulation – Food-Grade Coolant Concentrate:

• 90% propylene glycol

• 3% water

• 7% corrosion inhibitor package (food-grade phosphates, azoles)
  Dilute on-site with deionized water to a 35–40% solution for use in brewery fermenter cooling jackets.

4.5. Polymer and Resin Production
Purpose: A key diol monomer in the synthesis of unsaturated polyester resins (UPR), alkyd resins, and polyols for polyurethanes. Provides flexibility, hydrolysis resistance, and good solubility with styrene.
Usage Rate: 30–70% by weight of the glycol component in polyester resin formulations, depending on the desired resin properties.
Example Formulation – General-Purpose Unsaturated Polyester Resin:

• 1.0 mol phthalic anhydride

• 1.0 mol maleic anhydride

• 2.2 mol propylene glycol
  Esterify at 180–220 °C until acid value reaches ~20–30 mg KOH/g. Cool and dissolve in styrene (30–35% w/w) to obtain the final liquid resin.

4.6. Artificial Smoke and Fog Fluids
Purpose: Primary ingredient in theatrical fog/smoke machines and fire-fighter training. Propylene glycol vaporizes upon contact with the heat exchanger to produce a dense, non-toxic fog.
Usage Rate: Typical fluid composition is 20–35% propylene glycol, with the remainder being deionized water (sometimes a small fraction of glycerol is added to increase hang time).
Example Formulation – Standard Fog Fluid:

• 25% propylene glycol

• 75% deionized water
  Stir until homogeneous; filter before use. Adjust PG ratio to tune fog density (higher PG = denser, longer-lasting fog).

4.7. Other Applications

• Tobacco: Humectant in pipe tobacco and cigarette manufacture (prevents drying, softens cut tobacco).

• Printing Inks: Solvent and wetting agent in water-based flexographic and gravure inks.

• Non-ionic Detergents: Intermediate in the production of non-ionic surfactants when alkoxylated.

• De-icing/Anti-icing of Aircraft: For ground de-icing, Type I fluids often contain >50% PG, Type IV anti-icing fluids use PG with pseudoplastic thickeners for holdover protection.

• Cryonics: Used in cryoprotectant formulations due to its ability to prevent ice crystal formation.

• Ultrasonic Gel: Humectant and coupling agent in ultrasound transmission gels.

5. ALTERNATIVES AND COMPARISON

5.1. Glycerol / Glycerin (E422)
Pros: More “natural” perception; higher moisturizing capacity (higher hydroxyl number); sweeter taste; non-volatile; excellent humectant.
Cons: Significantly more viscous; lower solvent power for some organic compounds; can feel sticky on skin at high concentrations.
Comparison with PG: PG is preferred when low viscosity, rapid absorption, and better solvency for flavours/actives are required. Glycerol is often preferred in “natural” cosmetic formulations.

5.2. Ethylene Glycol (MEG)
Pros: Lower cost, lower viscosity, slightly better heat transfer efficiency, widely used in automotive antifreeze.
Cons: Toxic to humans and animals (metabolizes to glycolic and oxalic acid causing metabolic acidosis and kidney damage); cannot be used in food, pharmaceutical, or personal care applications.
Comparison with PG: PG is the non-toxic replacement wherever human or animal contact is possible (food, pharma, HVAC in occupied spaces, airport deicing near waterways).

5.3. Dipropylene Glycol (DPG)
Pros: Higher boiling point (~230–235 °C) and lower vapour pressure; less hygroscopic; often preferred as a fragrance carrier and in personal care products for reduced skin feel.
Cons: Higher molecular weight (134.17 g/mol), different solvent characteristics; higher cost.
Comparison with PG: DPG is used where lower volatility and reduced hygroscopicity are needed, e.g., fine fragrances and colour cosmetics. PG is more universal as a humectant and general solvent.

5.4. Butylene Glycol (1,3-Butylene Glycol)
Pros: Slightly better skin feel, often considered less irritating; has a moderate antimicrobial effect; similar humectant performance.
Cons: More expensive; less widely available.
Comparison with PG: Butylene glycol is frequently used in high-end cosmetics as a premium humectant. PG remains the cost-effective, workhorse humectant.

5.5. 1,3-Propanediol (Bio-PDO)
Pros: Produced from renewable corn sugar via fermentation; 100% bio-based; has a strong “natural” marketing appeal; similar properties to PG but with a slightly different dipole.
Cons: Typically more expensive; limited availability in some regions.
Comparison with PG: Both can be used interchangeably in many cosmetic and cleaning products. The choice is often driven by natural-origin claims and sustainability goals.

6. FREQUENTLY ASKED QUESTIONS

Q1. Is propylene glycol safe for human consumption?
Yes. The U.S. FDA classifies it as GRAS (Generally Recognized As Safe). The European Food Safety Authority (EFSA) has set an Acceptable Daily Intake (ADI) of 25 mg per kg of body weight per day for its use as a food additive (E1520). It is metabolized via normal pathways into lactic and pyruvic acid.

Q2. What is the key difference between propylene glycol and ethylene glycol?
Toxicity. Ethylene glycol is highly toxic if ingested and can cause severe kidney damage. Propylene glycol is much less toxic and is the recommended glycol for applications where incidental ingestion or environmental release might occur (e.g., food-processing coolant, RV antifreeze, pet-safe deicers).

Q3. Is propylene glycol suitable for vegan, halal, or kosher products?
Yes. Standard propylene glycol is of synthetic mineral origin (propylene oxide hydrolysis) and does not contain animal-derived ingredients. Bio-based PG (from glycerol) is also plant-derived. Certifications can be obtained from manufacturers.

Q4. What is the typical shelf life of propylene glycol?
When stored in a tightly closed container in a cool, dry place away from direct sunlight, propylene glycol has a recommended shelf life of at least 24 months from the date of manufacture. After opening, it should be kept sealed to prevent moisture absorption.

Q5. How should propylene glycol be handled and stored?
Store at ambient temperature (<40 °C) in stainless steel, aluminium, or appropriate plastic (HDPE, PP) containers. Mild steel is acceptable for short-term storage but not for long-term due to slight acidity. Avoid open flames – it is a combustible liquid (flash point ~100 °C). For spills, absorb with inert material (sand, vermiculite) and dispose according to local regulations. Prevent entry into drains and watercourses.

Q6. Is propylene glycol flammable?
It is classified as a combustible liquid, not a flammable one. The flash point is around 100 °C, so it will not readily ignite at room temperature. However, when heated above flash point and in the presence of an ignition source, vapours can ignite. Storage drums should be grounded during transfer.

Q7. Why is propylene glycol used in e-cigarette (vaping) liquids?
It serves as a carrier for flavours and nicotine, produces a visible vapour (“smoke”), and provides a “throat hit” sensation. The inhalation of PG is considered safe for use as a theatrical fog additive; however, long-term health effects of repeated deep lung inhalation in vaping are the subject of ongoing study.

Q8. Can propylene glycol cause skin irritation or allergic reactions?
True allergy (contact dermatitis) to propylene glycol is rare but has been reported. It is generally considered a mild irritant at very high concentrations, which is why cosmetic formulators typically use it at 1–10%. Patch tests can identify sensitivity in a very small percentage of individuals.

Q9. What is the difference between mono propylene glycol (MPG) and dipropylene glycol (DPG)?
MPG is a single, well-defined molecule (propane-1,2-diol) with a molecular weight of 76.10 g/mol. DPG is a mixture of isomers (primarily 1,1'-oxybis-2-propanol etc.) with a higher molecular weight (~134.17 g/mol). This gives DPG a higher boiling point, lower volatility, and different solvency. They are not directly interchangeable in all applications.

Q10. How is propylene glycol produced?
The primary industrial route is the high-temperature hydrolysis of propylene oxide with water, either non-catalytic (200–220 °C) or catalytic using an ion-exchange resin (150–180 °C). A growing bio-based method involves the hydrogenolysis of glycerol, which utilizes a biodiesel by-product. This yields a bio-based PG that is chemically identical, often used in cosmetics and cleaning products for its sustainability profile.