* Winning shareholder approval "not trivial" - Linde chairman
* Not prepared to divest more than $3.7 bln sales without rethink
* Linde shares up 1.8 pct, Praxair up 0.9 pct (New throughout)
MUNICH, June 2 (Reuters) - German industrial gases company Linde turned its attention on Friday to winning over investors to its planned $75 billion merger with U.S. peer Praxair, a task that Chairman Wolfgang Reitzle said was not straightforward.
A day after securing board approval in the face of unexpectedly tough trade union opposition, executives hailed the "historic" deal that will create the world's biggest gases group and reunite a company split 100 years ago by World War One.
"We have taken an important first step towards realizing this once-in-a-lifetime merger opportunity," Praxair CEO Steve Angel told a news conference in Linde's home city of Munich.
The deal agreed after a marathon Linde supervisory board meeting on Thursday will create a global leader to overtake France's Air Liquide with combined market value of $75 billion, revenue of $30 billion and 88,000 staff.
Angel and Linde CEO Aldo Belloni told Reuters they would at once head off on a roadshow to sell the merits of the deal. Linde needs 75 percent of shareholders to tender their shares to the new company, while Praxair needs a simple majority vote at a shareholder meeting.
Executives said they were prepared to make the divestments needed to satisfy anti-trust regulators in 25 countries, particularly in the United States, but had set a "pain threshold" beyond which the deal might no longer make sense.
Angel said the merger agreement set these thresholds at $3.7 billion in sales or $1.1 billion in core earnings (EBITDA). Above these limits, either party could walk away from the deal without penalty, he told analysts.
Executives from both companies said they did not expect regulators' demands to approach this level.
"It could happen that we have to give up more than expected but even that would be below the pain threshold," Reitzle told Reuters, adding that convincing shareholders to accept the offer was more of a concern. "The 75 percent is not trivial."
$100 BILLION TARGET
Linde estimates that individual retail investors own about 15 percent of its shares, while 10-13 percent may be held by index-tracker funds, some of whose rules would forbid them from tendering until acceptance reaches a certain level.
Tracking down retail investors will be tough, Reitzle said. "They may be sitting in retirement somewhere on Majorca and don't even know that Linde is doing a merger."
Shares in Linde hit a 22-month high on Friday, rising by 2 percent to 176.55 euros by 1500 GMT. Praxair shares hit fresh records, rising 1.9 percent to $135.98.
Reitzle said the merged company should have an "aspirational target" to hit a market value of $100 billion within five years after completing its all-share merger of equals.
He added that Thursday's supervisory board meeting, at which one labor representative broke ranks and abstained from voting - handing victory to shareholder representatives in favor of the merger - had ended surprisingly amicably.
Labour representatives had initially given their blessing for Linde to pursue merger talks but trade unions concerned about losing the influence they enjoy under German law once the new company's headquarters are established in Dublin intervened.
The head of trade union IG Metall in Bavaria on Friday criticized Linde for going ahead with the merger without winning broad consensus or seeking more talks.
"It is a break with German industrial history to push through such a close decision with brute force rather than seeking a consensus," Juergen Wechsler said in a statement.
(Reporting by Georgina Prodhan; Editing by Harro ten Wolde and Maria Sheahan)