Airgas Inc urged its shareholders on Monday to reject a $5.1 billion tender offer from larger rival Air Products & Chemicals Inc, arguing the offer substantially undervalues the industrial gas supplier.

The move was widely expected, and Airgas investors now have until April 9 to formally consider Air Products's $60-per-share cash offer.

However even if a majority of Airgas shareholders sell their shares, the Airgas board still has a "poison pill" in place to keep any one party from acquiring too large a stake.

Allentown, Pennsylvania-based Air Products took the bid hostile earlier this month by launching the tender after being privately rejected twice by Airgas's board.

If successful, Air Products would become the biggest industrial gas company in North America, allowing Air Products to gain substantial benefits from the economy's resurgence after the recession abates.