In what condition should undiluted propylene glycol not be used?

Undiluted propylene glycol should not be used in non-precipitation conditions due to its hygroscopic nature, which means it can absorb moisture from the air. When propylene glycol takes up moisture without sufficient precipitation, it may not effectively perform its intended functions, such as de-icing or anti-icing, particularly if environmental conditions lead to the formation of ice. In the absence of precipitation, the likelihood of ice formation increases, making the use of propylene glycol potentially ineffective.

Conditions like sunny weather, high humidity, or heavy precipitation might influence its application, but they do not inherently compromise the functionality of the propylene glycol as non-precipitation conditions do. It’s the lack of moisture in the ambient environment that is crucial to its performance. Thus, the answer highlights an important aspect of when undiluted propylene glycol may not be suitable for use.