Aug 28 (Reuters) - U.S. chemical and seeds company Dow Chemical Co said on Monday it plans to buy an additional 15 percent stake in Sadara Chemical Co, its joint venture with Saudi Aramco .

Dow, which currently owns a 35 percent stake in Sadara, said it signed a non-binding agreement with Aramco to boost that interest to 50 percent.

The deal is expected to occur after Dow closes its merger with DuPont .

(Reporting by Ahmed Farhatha in Bengaluru; Editing by Sai Sachin Ravikumar) ((Ahmed.Farhatha@thomsonreuters.com; within U.S. +1 646 223 8780, outside U.S. +91 806749 6568; Reuters Messaging: ahmed.farhatha.thomsonreuters.com@reuters.net))