Daimler refuses to use the refrigerant, made by Honeywell and DuPont, on the grounds it can emit toxic hydrogen fluoride gas when it burns.
"The tests that Daimler did were static and don't reflect the course of a real accident," Honeywell European government affairs manager Tim Vink told Germany's Handelsblatt newspaper.
Instead, the carmaker is continuing to use an older refrigerant called R134a, a potent global warming agent that was banned throughout the European Union from the start of this year.
France halts registrations
Daimler's refusal to stop using R134a prompted France to halt registrations of Mercedes-Benz A-class, B-class and SL cars assembled after June, while EU governments agreed on July 17 that steps should be taken to bring all vehicles within the rules.
Honeywell has said there is no significant risk from its refrigerant and that it is the most cost-effective and environmentally friendly alternative.
"We are asking ourselves why Daimler doesn't try to constructively resolve the problem instead of going it alone in refusing to use R1234yf," Vink told Handelsblatt.
Honeywell said slight changes to the air-conditioning system could be made to allow the gas to escape quickly in the event of an emergency, and that could resolve Daimler's concerns.
"It would cause only minimal costs per year, other manufacturers who have already taken that step tell us," Vink said.
While R1234yf is more expensive than R134a, which has been in use for 25 years, it is better for the environment, he said.
"For us this is a purely Daimler phenomenon," Vink said.
Daimler decided not to use R1234yf after Germany said it would allow it to continue using R134a. The European Commission has given the German government until Aug. 20 to explain its decision -- the first step before possible EU infringement proceedings.