PHB

PHB was a material composed of fermented natural ingredients such as sugar and starch. Unlike the petroleum-based plastic PES, PHB was biodegradable.

When the First Rani (disguised as Mel) asked whether PHB or PES would work as a suitable heat conducting material in the assembly of her Time Manipulator, the Seventh Doctor recommended using PHB due to its biodegradability, advising "Mel" against littering up Lakertya with non-destructible waste like they're doing on her planet. Ultimately, the Rani would elect to use PES, against the Doctor's advice. (TV: Time and the Rani)