Trade union members have started 24-hour strikes at two Dutch plants of chemicals giant AkzoNobel in an attempt to force the employer to pay an additional €400m into its Dutch pension fund.

The unions have demanded the extra contribution as, in their opinion, staff should also benefit from the €7.5bn sale – paid for in cash – of its Specialty Chemicals branch to US private equity firm Carlyle.

The board of the €5.3bn company scheme, APF, and a pensioners’ association also backed the request for a €400m cash injection into the pension fund.

However, the company has reiterated that the money was not available, as it had already promised the proceeds of the sale to its shareholders.

It also argued that the accounting rules of IFRS did not allow for such an additional payment, and that it would jeopardise the fixed premium for the current collective defined contribution scheme, which replaced the scheme’s defined benefit plan in 2005.

The unions, however, disagreed, citing the expert opinion of Jaap Koelewijn, professor of finance at Nyenrode Business University. Koelewijn argued that IFRS allowed for a “non-mandatory and one-off contribution”, as long as it was properly accounted for, according to Dutch financial newspaper FD.

It said that the unions countered AkzoNobel’s argument that the pension fund’s current coverage ratio of almost 111% was above average, by contending that indexation in arrears for the scheme’s participants had increased to 14% following 10 years without indexation.

In a webcast for its staff, Knut Schwalenberg, director of AkzoNobel Chemie Nederland, said that the company couldn’t meet the unions’ demands.

“I would like to ask to think properly before they cause damage to the company, mutual relationships as well as our reputation among our clients,” he added.

The unions have indicated that strikes at other plants elsewhere in the Netherlands were possible during the coming days.