What Is Dipropylene Glycol?
Definition of Dipropylene Glycol
Dipropylene glycol (DPG) is a low molecular weight diol, belonging to the propylene glycol family of chemical compounds. It is a clear, viscous, colorless liquid with a slightly bittersweet taste and low vapor pressure. DPG is primarily produced as a co-product during the hydrolysis of propylene oxide to manufacture monopropylene glycol (1,2-propylene glycol or 1,2-propanediol).
Chemical Structure and Isomers
The chemical formula of DPG is C6H14O3, and it consists of three isomeric forms:
- 1,1′-Oxybis-2-propanol
- 2-(2-Hydroxypropoxy)-1-propanol
- 2,2′-Oxybis-1-propanol
Properties of Dipropylene Glycol
Dipropylene glycol (DPG) exhibits the following key properties:
- Low Toxicity and Biocompatibility: DPG has low human toxicity, making it suitable for use in food, cosmetic, and pharmaceutical products. It meets the requirements of the U.S. Food and Drug Administration (FDA) and is classified as generally recognized as safe (GRAS).
- Solubility and Miscibility: DPG is completely miscible with water and has good solvent properties, making it useful as a solvent in various applications.
- Physical Properties:
- Boiling Point: Around 230-240°C
- Freezing Point: Around -60°C
- Viscosity: Relatively low viscosity, typically around 50-60 cP at 25°C
- Odor: Mild odor
- Chemical Stability: DPG exhibits good chemical stability and low corrosivity, making it suitable for various industrial applications.
Uses & Benefits of Dipropylene Glycol
Industrial Applications
Dipropylene glycol (DPG) is widely used as a raw material in the manufacture of unsaturated polyester resins. It imparts desirable properties like flexibility, chemical resistance, and dimensional stability to the resins. DPG is also employed in:
- Functional fluids like aircraft de-icing fluids, antifreezes, lubricants, inks, and heat transfer fluids
- Paints and coatings as a solvent and coalescent
- Plasticizers and cellophane production
Personal Care and Cosmetics
DPG is widely used in personal care products and cosmetics due to its low toxicity, excellent solvent properties, and good emollient characteristics. It acts as a humectant, solvent, and viscosity modifier in:
- Skin and hair care products like moisturizers, shampoos, and conditioners
- Makeup and color cosmetics like lipsticks, mascaras, and foundations
- Sunscreens and sunless tanning products
Food and Pharmaceutical Applications
The U.S. FDA has classified DPG as “generally recognized as safe” (GRAS) for use in foods, cosmetics, and medicine. It is used in:
- Food products as a solvent, humectant, and preservative
- Pharmaceutical formulations as a solvent and stabilizer for drugs and enzymes
Synthesis of Dipropylene Glycol
Synthesis Routes
Propylene Oxide Hydration
- Traditional route: hydration of propylene oxide derived from petrochemical sources
- Catalytic process (150-180°C) with ion exchange resin or acid/alkali catalysts
- Non-catalytic high-temperature process (200-220°C)
Biobased Synthesis from Glycerol
- Acid-catalyzed condensation of bioderived propylene glycol
- Hydrogenation of glycerol over transition metal catalysts
- Produces wholly biobased dipropylene glycol without using propylene oxide
Reaction Conditions and Catalysts
- Acid catalysts: sulfuric acid, ionic liquids
- Base catalysts: alkali compounds
- Transition metal catalysts: copper, copper oxide, zinc oxide, oxides of groups 2-6 and 8-10 elements
- High pressure and temperature for glycerol hydrogenation
Product Purification
- Vacuum distillation for energy-efficient continuous production
- Refining and purification to achieve >99.9% purity and low color (<10)
Safety Information of Dipropylene Glycol
Toxicity and Safety Profile
Dipropylene glycol has a relatively low toxicity profile. The U.S. Food and Drug Administration has determined it to be “generally recognized as safe” (GRAS) for use in foods, cosmetics, and medicine. Toxicology studies on rats and mice found no evidence of carcinogenic effects from dipropylene glycol exposure, even at high doses. However, high doses did increase the incidence of kidney disease, liver inflammation, and nasal tissue atrophy in rats.
Environmental and Occupational Safety
Dipropylene glycol has low volatility and minimal odor, reducing risks of inhalation exposure. It is non-corrosive and has low skin irritation potential, minimizing risks from dermal contact. Proper handling and storage practices should still be followed to mitigate risks of accidental ingestion or eye contact. Appropriate personal protective equipment may be advisable for occupational settings.
Safety in Consumer Products
The low toxicity and mild sensory properties of dipropylene glycol make it suitable for use in consumer products like cosmetics, personal care items, and food additives. However, product formulations should be carefully designed to limit overall exposure levels, especially for vulnerable populations like children. Proper labeling is important to enable informed consumer choices.
Biodegradability and Environmental Fate
Dipropylene glycol is expected to be readily biodegradable in the environment based on its chemical structure. Appropriate disposal methods should still be followed to prevent uncontrolled releases that could impact water sources or ecosystems. Overall, dipropylene glycol has a favorable environmental profile compared to many industrial chemicals.
Latest innovations of Dipropylene Glycol
Synthesis Methods
- Multi-step synthesis from 1,3-dichloropropanol: This method involves dehydration of 1,3-dichloropropanol to 1,3-dichloropropene, followed by hydrolysis to 3-chloro-2-propene-1-alcohol, hydrogenation to 3-chloropropanol, and a final hydrolysis step to produce dipropylene glycol. It offers advantages like mild conditions, low cost, and environmental friendliness.
- Propylene oxide addition to propylene glycol: Dipropylene glycol is synthesized by ring-opening polymerization of propylene oxide with propylene glycol, using catalysts like triphenylphosphine, barium hydroxide, or hydrotalcite-based catalysts. This method allows for high product quality and selectivity.
Continuous Production
Continuous vacuum distillation processes have been developed for efficient dipropylene glycol production. Careful control of temperature, pressure, distillation methods, and catalyst recycling enables low energy consumption, high quality, and high efficiency.
Novel Derivatives
Researchers have explored the synthesis of novel dipropylene glycol derivatives, such as:
- Dipropylene glycol dipropyl ether: Prepared by isomerization of dipropylene glycol monopropyl ether using a phosphotungstic acid quaternary ammonium salt catalyst, enabling high purity.
- Dipropylene glycol monomethyl monoalkyl ether: Synthesized by alkoxidation of dipropylene glycol monomethyl ether with allyl halides, improving yield through recycling of unreacted reactants.
- 2-[2-(2,4-difluorophenyl)-2-propylene-1-yl]-1,3-propylene glycol: A key intermediate in the synthesis of the antifungal drug posaconazole.
These innovations aim to expand the applications of dipropylene glycol and its derivatives in various industries like coatings, pharmaceuticals, and agrochemicals, by improving synthesis efficiency, product quality, and tailoring properties.
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