Air Products took its year-long, $5.9 billion fight for Airgas back to court on Tuesday in a case that could shape the way big hostile takeovers are carried out.

Air Products wants a Delaware judge to sweep away Airgas' legal defenses and allow it to buy the company for $70 per share. A takeover would transform Air Products into a dominant North American supplier of industrial gases, which are used in hospitals, factories and by welders.

Wall Street's dealmakers will be watching closely as the ruling in the case could shape the use of the "poison pill" defense against hostile takeovers.

"If (the judge) allows the pill to remain, it's a big case. If he removes it, it's a big case, too" said Charles Elson, a professor at the University of Delaware. "It defines the legality of a pill."

Under Delaware law, which governs the majority of major U.S. companies, poison pills are meant to give directors time to evaluate a deal and make their case for alternatives. They have been criticized as a way for boards to entrench themselves by defeating a hostile bid that might provide a handsome return to shareholders.

The judge overseeing the case, William Chandler, is expected to rule quickly, possibly on Thursday when the hearings are scheduled to wrap up.

Air Products has argued that Airgas board has had enough time to evaluate the deal and explain its analysis.

"My view is we've been at this for 14 months and at this point in time there is enough information and shareholders are sophisticated enough to decide on their own," Air Products Chief Executive John McGlade said at a hearing on Tuesday.

Airgas attorneys countered that the current $70 per share offer has only been on the table for a month.

Chandler weighed in with his own line of questioning, asking Airgas CEO Peter McCausland about his bullish outlook for his company and why there had not been another bidder for his company.

"I have had contacts from time to time with other bidders," said McCausland. "There are a couple things holding other companies back (from bidding). One would be we are only in the early days of the recovery."

Air Products has been negotiating with Airgas since late 2009 to reach a consensual deal, and has raised its offer several times to win over the Airgas board.

Airgas shareholders voted in September to oust McCausland as chairman, a move widely considered supportive of Air Products. Shareholders also voted to change the company's bylaws to bring forward Airgas's annual meeting by half a year, which would have given Air Products a shot of gaining board control in early 2011. Chandler approved the bylaw change, but it was overturned by the state's Supreme Court.

Observers said this week's hearings may be a way for Chandler to build the record to support an opinion that could have far-reaching impact.

"The one thing the courts are worried about is if you draw down a bright-line rule, like 'this is the circumstance in which the pill will be pulled,' is that everyone will structure a deal to get there," said Brian Quinn, a professor at Boston College Law School. "The court enjoys the ambiguity. So courts have tried to avoid this day."

Chandler's ruling is widely expected to be appealed, particularly if he sweeps away the poison pill defense.

The state's high court is considered more protective of directors, and many observers expect the Delaware Supreme Court to come to the defense of the poison pill.

"Either Chandler will uphold the pill or it will get reversed on appeal," said J. Robert Brown, a professor at the University of Denver's Sturm College of Law.

The case is Air Products & Chemicals Inc v Airgas Inc, Delaware Court of Chancery, No. 5249.