Can a union insist on keeping a zipper clause in a collective bargaining agreement during contract negotiations?

A union can insist on keeping a zipper clause in a contract during contract negotiations. However, the company would have to agree to do this.

A zipper clause is when both the union and the management agree not to bargain during the time the agreement is in effect. There are two kinds of zipper clauses. One is a comprehensive zipper clause. This is very uncommon because it means there can be no negotiations on any topic while the contract is in existence. This could allow the company to implement some changes, and the union couldn’t do anything about it. The other kind of zipper clause is one where items that have been negotiated and other items that were taken off the table during  the negotiations cannot be discussed while the contract is in effect. This would allow for discussions about issues that are not currently covered in the contract to occur while the contract is in place.

It should be noted that no zipper clauses would be included in a contract unless both sides agree to include it in the contract.