Propylene glycol and Dipropylene glycol are derived from the same chemical process, adding water to propylene oxide. However, the molecular structure and chemical composition of each is very different. Though both are used in the cosmetics industry, dipropylene glycol is the preferred fragrance carrier oil for a number of reasons.
Chemical Properties
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Propylene glycol has a low boiling point and moderate freezing point. It is a diol, or double alcohol, which means that people who are not permitted to use alcohol would not be able to use any fragrance oils with it as a carrier oil. Dipropylene glycol has a high boiling point and low freezing point. It can act as a solvent and stabilizer for many products containing oils. It is non-alcoholic and therefore can be used without restriction for fragrance oil, incense and so on.
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Skin Sensitivity
Propylene glycol is generally well tolerated by most humans. However, those with eczema have been known to show allergic reactions such as contact dermatitis and hives. Dipropylene glycol has not been shown to cause allergic reactions.
Toxicity if Ingested
Propylene glycol has a sweet taste. It is toxic to dogs and cats, and therefore human toiletries and fragrances should never be left within reach of pets. Dipropylene glycol exposure has been shown under lab conditions to result in minor corneal irritation in rabbits and kidney or renal system changes in small rodents.