Huntsman Corp., the world's largest manufacturer of epoxy adhesive, has extended a merger agreement with Apollo Management LP's Hexion Specialty Chemicals unit by three months as it seeks a legal route to force Hexion to honor the deal.

Huntsman's board allowed it to extend the merger agreement through Oct. 2, the company said in a statement Wednesday. Huntsman added that it filed documents in Delaware to force Hexion to honor its $6.54 billion takeover of the adhesive maker.

Hexion has tried to back out of the transaction. It sued Huntsman in Delaware last month to terminate the deal, saying the combined company would be insolvent. Huntsman responded by suing New York-based Apollo and partners Leon Black and Joshua Harris in a Texas court, seeking more than $3 billion. Salt Lake City-based Huntsman runs its operations from The Woodlands, Texas.

"We continue to believe the combined companies are solvent," Huntsman's Chief Executive Officer Peter Huntsman said Wednesday in an interview. "This is all just a smoke screen by Apollo to try to get out of this deal, and we are not going to let them do that."

The combined company would be one of the world's largest specialty-chemical makers, with annual sales exceeding $14 billion, 21,000 employees and 180 facilities, according to Apollo. Hexion, led by Chief Executive Officer Craig O. Morrison, is the top producer of adhesives used in plywood.

Hexion on Wednesday reiterated its opinion that the merged companies wouldn't be viable.

"We do not understand how Huntsman's board of directors could in good faith make that determination," Morrison said in the statement. "There is no factual basis to conclude that the combined company would be solvent."

Hexion won European antitrust approval to purchase Huntsman after agreeing to sell assets in Germany and the United States, the European Commission, the 27-nation European Union's antitrust regulator, said Tuesday.

Huntsman said Wednesday in court filings that Hexion and Apollo wrongly ordered a solvency opinion from an outside consultant that didn't seek information from Huntsman.

Hexion is underestimating Huntsman's earnings by 20 percent to 30 percent over the next four years, using the stable economic growth and stable energy prices assumed in the solvency opinion, Peter Huntsman said.