An Introduction to Propylene Glycol: Properties and Applications
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📋 What You'll Learn
This guide walks you through an introduction to propylene glycol: properties and applications with detailed instructions.
An Introduction to Propylene Glycol: Properties and Applications
The definitive guide to one of the most versatile and safety-conscious chemicals in modern industry — from FDA-approved food additives to industrial-grade heat transfer fluids.
In the vast landscape of industrial and consumer chemicals, few compounds can match the sheer versatility and safety profile of Propylene Glycol (PG). From preserving the texture of our food and stabilizing medications to protecting critical HVAC systems from freezing, PG is a silent workhorse in countless applications. This comprehensive guide provides a deep, technical dive into the properties, grades, and real-world applications of PG — empowering formulators, engineers, procurement specialists, and industry professionals to select and utilize this powerful compound with confidence.
Understanding Propylene Glycol: A Chemical Profile
Propylene Glycol (IUPAC name: propane-1,2-diol) is a synthetic organic compound with the chemical formula C₃H₈O₂. It is a viscous, colorless, and nearly odorless liquid that is completely miscible with water as well as a broad range of organic solvents, including acetone, chloroform, and many esters. Its CAS number is 57-55-6, and it has been a staple in chemical manufacturing for over seven decades.
The molecular structure of PG features two hydroxyl (-OH) groups on adjacent carbon atoms, making it a diol (or glycol). This simple yet powerful architecture gives PG its characteristic ability to form hydrogen bonds with water and numerous other polar and nonpolar molecules. Understanding these interactions is essential for anyone working with industrial solvents or formulating chemical blends.
Its remarkable utility stems from several fundamental physicochemical properties:
- Hygroscopic Nature: PG is an excellent humectant, meaning it attracts and retains water molecules from the surrounding environment. This property is central to its use in personal care, food preservation, and pharmaceutical formulations where moisture control is critical.
- Superb Solvent Capability: PG dissolves a wide array of active pharmaceutical ingredients, flavorings, colorants, and essential oils that are not soluble in water alone, making it an indispensable carrier fluid across multiple industries.
- Low Volatility & High Boiling Point: With a boiling point of 188.2 °C (370.8 °F), PG remains thermally stable across a wide temperature range, which is vital for heat transfer applications and high-temperature processing.
- Excellent Freeze-Point Depression: Like all glycols, PG dramatically lowers the freezing point of water. A 60% PG solution remains liquid down to approximately −48 °C (−55 °F), making it a powerful antifreeze agent.
- Low Toxicity (GRAS Designation): This is PG’s single most commercially important characteristic. The U.S. FDA has classified propylene glycol as “Generally Recognized As Safe” (GRAS) for use in food, pharmaceuticals, and cosmetics — a designation not shared by its cousin, ethylene glycol.
Key Physical Constants
Molecular Formula: C₃H₈O₂ • Molecular Weight: 76.09 g/mol • Density: 1.036 g/cm³ at 25 °C • Viscosity: 42 mPa·s at 25 °C • Flash Point: 99 °C (210 °F) • Refractive Index: 1.432 • pH: Neutral (7.0 in aqueous solution). These properties are important considerations for engineers designing heat transfer systems and for formulators working with high-purity water systems.
Propylene Glycol vs. Ethylene Glycol: A Head-to-Head Comparison
One of the most common questions in the glycol industry is when to choose propylene glycol over ethylene glycol (EG). While both serve as effective antifreeze and heat transfer fluids, they have fundamentally different safety profiles that dictate their appropriate applications. Understanding these differences is essential for chemical safety compliance and making informed procurement decisions.
| Property | Propylene Glycol (PG) | Ethylene Glycol (EG) | Winner |
|---|---|---|---|
| Toxicity | Low — FDA GRAS status | High — toxic if ingested (lethal dose ~100 mL) | PG |
| FDA Status | GRAS for food/pharma | Not approved for food contact | PG |
| Heat Transfer Efficiency | Good — slightly lower thermal conductivity | Excellent — ~5-10% better heat transfer | EG |
| Viscosity (at −20 °C) | Higher — requires more pumping energy | Lower — flows more easily at low temps | EG |
| Freeze Protection (50% solution) | Down to −34 °C (−29 °F) | Down to −37 °C (−35 °F) | EG (marginal) |
| Environmental Impact | Readily biodegradable; low aquatic toxicity | Biodegradable but highly toxic to animals | PG |
| Cost (per gallon) | Moderate — slightly higher | Lower — ~15-20% less expensive | EG |
| Required For | Food/beverage plants, schools, hospitals, potable water systems | Automotive, heavy industrial, closed systems with no food contact | Depends on application |
The bottom line: PG wins decisively in any application where human safety is a factor. For purely industrial, closed-loop systems with no food-contact risk, EG offers marginally better thermal performance at a lower cost. Many professionals in cooling tower and water treatment operations weigh these tradeoffs carefully when specifying glycol-based fluids. For a deeper dive into this comparison, see our dedicated article on Ethylene Glycol vs. Propylene Glycol.
The Mandate for Safety: PG in Food, Pharma, and Cosmetics
In any application where human consumption or direct skin contact is possible, the safety profile of Propylene Glycol USP Grade makes it the unquestionable choice. With its GRAS designation, PG is one of only a handful of industrial chemicals approved for direct use in products consumed by humans. For professionals focused on food-grade propylene glycol applications, understanding the regulatory landscape is critical.
Food & Beverage Industry
PG is a multifunctional food additive (listed as E1520 in the EU) that improves the quality, consistency, and shelf life of processed foods by serving as:
- Humectant & Stabilizer: PG maintains desirable texture and prevents products like baked goods, frostings, marshmallows, and soft candies from drying out. It keeps moisture levels consistent throughout the product’s shelf life.
- Solvent & Carrier: Concentrated natural flavors — including vanilla extract, almond extract, and food-grade colorants — are dissolved in PG for even distribution throughout the final product. It is the preferred carrier over ethanol in many flavoring systems.
- Dough Conditioner: In commercial baking, PG improves dough extensibility and crumb softness, contributing to a more uniform final product.
- Brewery & Winery Cooling: USP-grade PG is mandatory in the jacketed fermentation tanks and glycol chillers used in brewery and winery thermal systems because of the risk of incidental product contact.
Pharmaceutical & Personal Care Products
The pharmaceutical industry relies on PG for drug delivery, stability, and patient compliance. Its versatile functionality includes:
- Solvent in Oral & Topical Medications: PG dissolves active pharmaceutical ingredients (APIs) in liquid syrups, soft-gel capsules, injectable formulations, and topical creams for effective and consistent drug delivery.
- Humectant in Cosmetics: In lotions, moisturizers, serums, and deodorants, PG draws moisture to the skin and helps other ingredients absorb more effectively. It is a cornerstone ingredient in most premium skincare lines.
- Penetration Enhancer: PG can carry active ingredients through the skin barrier, improving the bioavailability of topical medications and transdermal drug patches.
- E-liquid / Vape Base: PG serves as one of the two primary base liquids in e-cigarettes, valued for its ability to carry flavor and produce a satisfying “throat hit.”
Professionals working with pharmaceutical-grade chemicals should always follow PPE and chemical safety protocols and consult the Safety Data Sheet (SDS) for each specific product formulation.
Application Spotlight: Where PG Makes a Difference
Propylene glycol’s unique combination of safety, solvency, and thermal performance opens doors across an extraordinarily diverse set of industries. Here are some of the highest-impact application areas where PG delivers measurable value:
HVAC & Building Systems
Non-toxic antifreeze for hydronic heating, chilled water loops, and cooling tower systems in schools, hospitals, and data centers.
Food & Beverage Processing
Glycol chillers for breweries, dairies, and meat processing where incidental food contact is unavoidable. Explore food-grade PG uses.
Pharmaceuticals
Solvent and co-solvent in oral liquids, injectables, and topical preparations. USP-grade purity is required for all human drug products.
Aviation & De-Icing
Type I and Type IV aircraft de-icing fluids use PG-based formulations at major airports worldwide due to their lower environmental toxicity compared to EG alternatives.
Fog & Special Effects
Theatrical fog machines rely on PG-water blends to create safe, non-toxic atmospheric effects. See our guide on PG in fog generation.
Livestock & Agriculture
PG is administered to dairy cows as an energy supplement to prevent ketosis. Its low toxicity makes it safe for animal consumption in regulated doses.
The Industrial Workhorse: PG in Heat Transfer and Antifreeze Systems
While PG’s safety profile dominates in consumer goods, Technical Grade Propylene Glycol is a robust, high-performance fluid for demanding industrial applications — especially where environmental regulations, incidental food contact, or worker safety are factors.
Non-Toxic Heat Transfer Fluids
PG-based antifreeze and heat transfer fluids are the required choice for any closed-loop heating or cooling system where a leak could potentially contaminate potable water or food products. Critical applications include:
- Glycol chillers in breweries, dairies, wineries, and food processing plants
- HVAC systems in schools, hospitals, assisted living facilities, and residential buildings
- Hydronic radiant floor heating and solar thermal hot water systems
- Data center cooling loops where environmental compliance is mandatory
- Geothermal ground-source heat pump systems
Engineering Note: Viscosity Considerations
PG is more viscous than ethylene glycol, particularly at temperatures below −10 °C. This increased viscosity demands greater pumping energy and can reduce turbulent flow in narrow passages. HVAC engineers should account for a 5–15% increase in pump sizing when switching from EG to PG. Despite this, the safety advantages of PG make this engineering tradeoff worthwhile in virtually all occupied building applications. Properly sized systems using inhibited PG will perform reliably for 10–15 years before fluid replacement is needed.
The Importance of Corrosion Inhibitors
Like all glycols, uninhibited PG degrades over time through thermal oxidation, producing corrosive organic acids (primarily formic and lactic acid) that attack system metals. This is why professional-grade formulations like Inhibited Propylene Glycol are essential for any closed-loop installation. These products contain a sophisticated additive package — typically including phosphates, molybdates, azoles, and buffering agents — that protects steel, copper, brass, aluminum, and solder joints from corrosion and pitting.
For systems where visual leak detection is important, pre-dyed formulations like Arctic Assist allow facility managers to quickly identify even minor leaks before they become costly problems.
Buyer’s Guide: Selecting the Correct Grade of Propylene Glycol
Choosing the correct grade of PG is not merely a performance consideration — it is often a matter of regulatory compliance, legal liability, and public safety. Using the wrong grade can result in product recalls, regulatory fines, or worse. Understanding proper chemical storage and handling protocols for each grade is equally important. The table below breaks down the key distinctions:
| Grade | Purity / Standard | Key Specifications | Required Applications | Price Tier |
|---|---|---|---|---|
| USP Grade | USP-NF & FCC certified; ≥99.5% purity | Meets FDA GRAS; low heavy metals (<5 ppm); residue on ignition <0.007% | Food/beverage, pharmaceuticals, cosmetics, e-liquids, any food-contact system | $$$ Premium |
| Technical Grade | Industrial spec; ≥99.0% purity | Not certified for human consumption; may contain trace impurities above pharma limits | Industrial coolants, de-icing fluids, plasticizers, hydraulic fluids, paint systems | $$ Standard |
| Inhibited PG | Technical Grade + corrosion inhibitor package | Contains phosphate/molybdate/azole inhibitors; pH buffered to 8.5–10.0; protects ferrous & nonferrous metals | ALL closed-loop HVAC, hydronic, solar thermal, and data center cooling systems | $$ Standard+ |
| Dipropylene Glycol (DPG) | Related compound; different MW (134.17) | Higher boiling point (232 °C); lower hygroscopicity; good coupling solvent | Fragrance carrier, plasticizer, polyester resins, industrial cleaning formulations | $$ Standard |
Regulatory Warning
Never substitute Technical Grade PG for USP Grade in any application involving human consumption, pharmaceutical manufacturing, or food-contact systems. This is a violation of FDA 21 CFR regulations, EU food safety directives, and can expose your organization to significant legal liability. When in doubt, always specify USP Grade. For guidance on regulatory compliance and proper chemical disposal, consult with your chemical supplier or a regulatory affairs specialist.
Industry Application Guide: Concentrations & Recommendations
Different industries use propylene glycol at varying concentrations depending on the application requirements. This reference table provides typical concentration ranges, recommended grades, and critical notes for the most common PG applications across sectors. Whether you are formulating products using green chemistry principles or specifying HVAC fluids, these guidelines will help you make informed decisions:
| Industry / Application | Typical PG Concentration | Recommended Grade | Critical Notes |
|---|---|---|---|
| Food & Beverage Processing | 25–50% (in glycol chillers) | USP | Mandatory FDA GRAS compliance; test glycol annually for degradation |
| Pharmaceutical Manufacturing | 5–80% (formulation-dependent) | USP | Must meet USP-NF monograph; certificate of analysis required per lot |
| Cosmetics & Personal Care | 1–15% (typical in formulations) | USP | EU Cosmetics Regulation compliant; patch testing recommended for sensitive skin products |
| HVAC / Hydronic Systems | 25–50% (based on freeze protection needed) | Inhibited | Must include corrosion inhibitors; check pH and reserve alkalinity semi-annually |
| Solar Thermal Systems | 40–50% | Inhibited | High-temperature stability critical; replace every 3–5 years due to thermal cycling |
| Data Center Cooling | 30–40% | Inhibited | Low viscosity critical for server rack cooling; monitor with refractometer monthly |
| Aircraft De-Icing (Type I/IV) | 50–80% | Technical (specialized) | Must meet SAE AMS 1424/1428 specifications; environmentally preferable to EG |
| Fog / Haze Machines | 10–40% (in DI water) | USP | Only use USP grade for indoor venues; ensure adequate ventilation in confined spaces |
| Livestock Feed Supplement | 300–500 mL/day (dairy cattle) | USP | Prevents ketosis in transitioning dairy cows; veterinary supervision required |
When working with any concentration of propylene glycol, proper personal protective equipment should be used. While PG has low toxicity, prolonged skin contact can cause mild irritation in sensitive individuals, and good chemical safety practices should always be observed.
Safety, Handling, and Storage Best Practices
While propylene glycol is one of the safest industrial chemicals available, all chemicals demand professional handling and respect. Implementing proper protocols protects your workers, your products, and your bottom line.
Personal Protective Equipment (PPE)
- Gloves: Nitrile or butyl rubber gloves for routine handling; PG can degrease skin with prolonged exposure
- Eye Protection: Chemical splash goggles or safety glasses with side shields
- Ventilation: Adequate ventilation recommended when heating PG above 100 °C, as it can produce mist/vapor
- Respiratory Protection: Generally not required at ambient temperatures; use NIOSH-approved respirator if mist is generated
Storage Guidelines
Propylene glycol is hygroscopic — it actively absorbs moisture from the air. Proper storage is essential to maintain concentration and purity, as outlined in our chemical storage guide:
- Store in tightly sealed HDPE, stainless steel, or lined carbon steel containers
- Keep in a cool, dry, well-ventilated area away from strong oxidizing agents
- Avoid exposure to temperatures above 50 °C during storage
- Shelf life: 2 years unopened (USP Grade); verify with certificate of analysis for pharmaceutical use
- Label all containers clearly with grade designation, concentration, date received, and lot number
Spill Response
PG is not classified as hazardous under OSHA’s Hazard Communication Standard. However, it creates extremely slippery surfaces when spilled. Clean up promptly with absorbent material (vermiculite, sand, or commercial absorbent pads). Flush the area with water. For large spills, follow your facility’s spill response plan and consult our guide on safe chemical disposal. PG is readily biodegradable and generally does not require special waste disposal, though local regulations should always be verified.
Environmental Profile and Sustainability
One of propylene glycol’s most significant advantages over ethylene glycol is its favorable environmental profile. PG is readily biodegradable in both aerobic and anaerobic conditions, with a biological oxygen demand (BOD) half-life of approximately 1–4 days in most environmental conditions. It has low toxicity to aquatic organisms (LC50 > 10,000 mg/L for most fish species) and does not bioaccumulate.
That said, large-volume releases of PG (such as airport de-icing runoff) can deplete dissolved oxygen in receiving waterways as microorganisms consume it during biodegradation. This is why airports and large industrial facilities implement PG recovery and recycling programs. For organizations committed to green chemistry and sustainability, PG represents a more responsible choice than many alternative glycols and solvents.
Sustainability Fact
Bio-based propylene glycol — produced from renewable glycerin (a byproduct of biodiesel production) rather than petroleum-derived propylene oxide — is now commercially available and offers an even smaller carbon footprint. Companies seeking to reduce their environmental impact should inquire about bio-PG options for compatible applications.
Related Chemicals and Cross-References
Understanding how propylene glycol relates to other chemicals in the Alliance Chemical portfolio helps professionals make better procurement and formulation decisions:
- Ethylene Glycol: PG’s more toxic but thermally efficient cousin — used in automotive and heavy industrial applications. See our complete EG vs. PG comparison.
- Isopropyl Alcohol (IPA): Another versatile solvent often used alongside PG in pharmaceutical and cleaning formulations. IPA evaporates quickly while PG provides lasting moisture retention.
- Hydrogen Peroxide: Used in conjunction with PG in certain sterilization and cleaning protocols, particularly in food processing environments.
- Sodium Hydroxide (NaOH): Used to adjust pH in PG-based systems and in the saponification of PG-containing formulations.
- Industrial Acids: Understanding acid-base interactions with PG is important when formulating cleaning solutions or adjusting system chemistry.
Ready to Source Premium Propylene Glycol?
Alliance Chemical supplies USP, Technical, and Inhibited grades of propylene glycol in quantities from 1 quart to bulk tanker loads. Our technical team is ready to help you select the right grade and concentration for your specific application.
Shop All Glycol ProductsConsult Our Technical ExpertsFrequently Asked Questions
What is propylene glycol and what are its main properties?
Propylene glycol (1,2-propanediol, C₃H₈O₂) is a clear, colorless, nearly odorless liquid with a faintly sweet taste. Key properties: miscible with water, boiling point 370°F (188°C), freezing point -76°F (-60°C, neat), low toxicity (GRAS status), hygroscopic. It serves as an antifreeze, humectant, solvent, and food/pharmaceutical additive.
Is propylene glycol safe for human contact and consumption?
Yes, propylene glycol has GRAS status from the FDA and is widely used in food, cosmetics, and pharmaceuticals. It's fundamentally different from toxic ethylene glycol. The WHO acceptable daily intake is 25 mg/kg body weight. It's metabolized to lactic acid and pyruvic acid—normal metabolic compounds. USP grade is required for human-contact applications.
What is the difference between propylene glycol and ethylene glycol?
The key difference is safety—propylene glycol is non-toxic (FDA GRAS), while ethylene glycol is toxic (lethal dose ~100 mL). Ethylene glycol provides 10-15% better heat transfer and freeze protection. Use propylene glycol wherever leaks could contact food, people, or animals. Use ethylene glycol only in closed industrial systems with no human contact.
What grades of propylene glycol are available?
USP/pharmaceutical grade for food, drugs, and cosmetics. Technical grade for industrial antifreeze and heat transfer. Inhibited grades contain corrosion inhibitors for closed-loop heating/cooling systems. Concentrations range from 30% to 100% (neat). For HVAC and solar thermal, use inhibited USP grade for both safety and corrosion protection.
