Dipropylene glycol is produced as a byproduct of the manufacture of propylene glycol, which is also called 1,2-propanediol. Propylene glycol is synthesized by hydrating propylene oxide. The final product contains 20 percent propylene glycol and 1.5 percent dipropylene glycol, as well as other compounds. Pure propylene glycol is finally obtained after a chemical process called rectification. Propylene glycol and dipropylene glycol have more similarities than differences, since they are both glycols, which are organic compounds belonging to the alcohol family. However, some differences between them can be listed.
Formula and Composition
A single molecule of propylene glycol contains three atoms of carbon, eight of hydrogen and two of oxygen, represented by the chemical formula C3H8O2. On the other hand, dipropylene glycol contains twice as many carbon atoms as well as fourteen hydrogen atoms and three oxygen atoms in each molecule, and has the chemical formula C6H14O3.
Propylene glycol is used in foods as a preservative, as a moisture-retaining agent in cosmetics and as a solvent in oral hygiene products. It is also used in anti-freezing formulations. When used as a humectant in the food industry, propylene glycol is labeled as E number E1520. Dipropylene glycol is used in pesticides, hydraulic brake fluids, polyester resins, cutting oils and as a plasticizer, which is is a substance added to a plastic to improve its flexibility and resilience.
Propylene glycol is miscible with water, methanol, ethanol, acetone, diethyl ether and chloroform. The compound's boiling point is 188.2 degrees C or 370.76 degrees F, while its freezing point is -39 degrees C or -38 F. Dipropylene glycol is miscible with water and ethanol; it boils at about 236 degrees C or 456.8 degrees F and freezes at the same temperatures as propylene glycol.
Both propylene glycol and dipropylene glycol have low toxicity to humans. However, propylene glycol can cause skin irritation to more than two percent of people suffering from eczema, an inflammatory skin condition. Dipropylene glycol also has a low potential to cause allergic skin reactions in humans. However, dipropylene glycol administered at high concentrations caused kidney damage and behavioral changes in laboratory animals.