Difference Between Dipropylene Glycol and Propylene Glycol

The key difference between dipropylene glycol and propylene glycol is that dipropylene glycol is a mixture of three isomers, whereas propylene glycol is an organic compound having two alcohol groups.

Propylene glycol is very useful in the production of many polymer materials. Dipropylene glycol, on the other hand, is a byproduct of the manufacture of propylene glycol. Since they both are glycols, these compounds have more similarities than differences. However, this article focuses on the difference between dipropylene glycol and propylene glycol.

CONTENTS

1. Overview and Key Difference
2. What is Dipropylene Glycol 
3. What is Propylene Glycol
4. Side by Side Comparison – Dipropylene Glycol vs Propylene Glycol in Tabular Form
5. Summary

What is Dipropylene Glycol?

Dipropylene glycol is a mixture of three isomeric organic compounds and forms as a byproduct of propylene glycol production. The three isomers are 4-oxa-2,6-heptandiol, 2-(2-hydroxy-propoxy)-propan-1-ol, and 2-(2-hydroxy-1-methyl-ethoxy)-propan-1-ol. It occurs as a colourless liquid and is odourless. Moreover, it has a high boiling point and low toxicity. However, we can state that dipropylene glycol is a combination of two propylene glycol molecules which occurs in three isomeric forms; thus, the chemical formula is C₆H₁₄O₃.

In the production of dipropylene glycol, the final product contains 20% propylene glycol and 1.5% dipropylene glycol. When considering the uses of this compound, it is useful in the production of pesticides, hydraulic brake fluids, polyester resins, cutting oils, etc. Further, this compound is miscible with water and ethanol. Its melting point is equal to that of propylene glycol while its boiling point is 236°C.

What is Propylene Glycol?

Propylene glycol is an organic compound having two alcohol functional groups in the same molecule. Its chemical formula is C₃H₈O₂. Further, this compound occurs as a colourless and nearly odourless liquid, but it has a faintly sweet taste. It is soluble in many solvents, including water, acetone, chloroform, etc. The melting point of this compound is -39°C while the boiling point is 188.2°C.

We can produce this compound industrially from propylene oxide by the following reaction;

Here, this reaction gives a mixture of compounds which has 20% propylene glycol and 1.5% dipropylene glycol. Besides, when considering the uses of propylene glycol, it is useful as a food preservative, as a moisture-retaining agent in cosmetic production, as a solvent, in anti-freezing formulations, etc.

Difference Between Dipropylene Glycol and Propylene Glycol

Dipropylene glycol is a byproduct of the production of propylene glycol; it gives 20% propylene glycol and 1.5% dipropylene glycol. The key difference between dipropylene glycol and propylene glycol is that the dipropylene glycol is a mixture of three isomers, whereas propylene glycol is an organic compound having two alcohol groups. The chemical formula of dipropylene glycol is C₆H₁₄O₃ while the chemical formula of propylene glycol is C₃H₈O₂.

Furthermore, the dipropylene glycol is useful in the production of pesticides, hydraulic brake fluids, polyester resins, cutting oils, etc., while the propylene glycol is useful as a food preservative, as a moisture-retaining agent in cosmetic production, as a solvent, in anti-freezing formulations, etc.

Below infographic summarizes the difference between dipropylene glycol and propylene glycol.

Summary – Dipropylene Glycol vs Propylene Glycol

Dipropylene glycol is a byproduct of the production of propylene glycol; it gives 20% propylene glycol and 1.5% dipropylene glycol. The key difference between dipropylene glycol and propylene glycol is that the dipropylene glycol is a mixture of three isomers, whereas the propylene glycol is an organic compound having two alcohol groups.

Reference:

1. “Propylene Glycol.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, Available here.

Image Courtesy:

1. “Dipropylene glycol” By en:User_talk:Ronhjones – Own work (Public Domain) via Commons Wikimedia
2. “1,2-Propandiol Synthesis V1” By Jü – Own work (CC BY-SA 4.0) via Commons Wikimedia