The contract chemical manufacturer has entered a six-week consultation period with its employees which could see the current workforce of 156 cut to around 50 by next summer.
But Unite Scotland has pledged to fight 'every step of the way’ to save the jobs.
The losses would also mean the end of manufacturing on the former ICI site.
Calachem’s announcement came as the company announced it was moving out of contract chemical manufacturing – which it says is no longer sustainable - and into managing estates for tenants on its site.
The Earls Road based company blamed the loss of a major European contract, COVID disruption and overseas competition for its decision to cull the jobs.
Scott Foley, Unite industrial officer, urged bosses to talk to them.
He said: “We believe that there is a viable manufacturing future within the CalaChem business at Grangemouth.
“The company has also returned to profitability over the last few years and it is showing a healthy balance sheet.
“It’s vital that CalaChem engage with us, the Scottish Government and Falkirk Council to retain as many jobs as possible. “
And he warned: “Closure of the manufacturing unit is simply unacceptable and we will fight these proposals every step of the way to ensure CalaChem explores every viable alternative option in order to retain a manufacturing capacity.”
In 2010, the Munich based investment company Aurelius AG acquired KemFine UK Ltd, and established CalaChem Ltd.
The Grangemouth business returned to profitability over the last two years running a profit after tax of £390,000 year ending December 2020, and £391,000 year ending December 2019.
Unite says manufacturing sales and profitability have shown an upward trend during this period pointing towards a viable future for the manufacturing services unit.
Colin Loudon, managing director, said: “Over the past few months, the board has been working tirelessly to develop a sustainable strategy for the business, and in the current climate we are forced to focus on the areas that are profitable.
“Unfortunately, this is not contract chemical manufacturing – through no fault of our talented workforce."