Steven Deans believed he was being treated as the "enemy within" after 25 years of working at the plant, according to Unite.
Mr Deans accused the company of leaking emails to the media to "smear his reputation", in his resignation latter to Ineos.
He told the Unite meeting: "The hurt to my family is unforgivable and I will not allow management to take away my dignity and self-respect any further."
The union said it is considering "what steps can be taken on Mr Deans' behalf" without diminishing its commitment to the survival plan workers signed with Ineos last week.
Grangemouth owners Ineos are in last ditch talks with the workers' union, but the workforce is just one of the problems facing the plant.
Thousands of jobs are under threat after a dispute over conditions led to a shock decision to close the petrochemical site in Grangemouth.