Here's why:

* Structure: Propylene glycol has a hydroxyl (-OH) group attached to each of its carbon atoms. These hydroxyl groups are polar, with the oxygen atom carrying a partial negative charge and the hydrogen atom carrying a partial positive charge.

* Hydrogen Bonding: The hydrogen atoms in the hydroxyl groups can form hydrogen bonds with the lone pairs of electrons on the oxygen atoms of neighboring propylene glycol molecules. These hydrogen bonds are relatively strong intermolecular forces.

* Other Interactions: While hydrogen bonding is the dominant force, there are also dipole-dipole interactions due to the polarity of the molecule, and London dispersion forces which are present in all molecules.

Therefore, while propylene glycol contains covalent bonds within its molecule, the primary type of bonding responsible for its properties is hydrogen bonding.