Offerings identified for pension financing have almost quadrupled from $750 million in the same period of 2012, Bloomberg data show. The actual figures may be much higher, with some proceeds sometimes coded to general corporate purposes rather than retirement plans, said Lonski of Moody's.

“They just won't reveal it,” he said. Goodyear, which has contributed $1.4 billion to its U.S. plans over the past five years, still faced a $2.7 billion funding gap as of Dec. 31, up $200 million from a year earlier, according to Fitch.

The bond sale by the largest U.S. tire-maker was part of a strategy to pre-fund and hedge the risk in its pension plans, which provide benefits for people who are living longer, as it closes them to new entrants, Chief Financial Officer Darren Wells said on a Feb. 12 conference call to discuss fourth-quarter earnings.

Merck Bonds
Ford, whose $18.7 billion pension deficit is the second- largest outstanding, according to UBS research, sold $2 billion of 4.75 percent, 30-year debt on Jan. 3 with some of the proceeds going toward accelerating contributions to its pension plans, according to Bloomberg data and a filing at the time from the Dearborn, Michigan-based automaker. Merck & Co. issued $2.5 billion of bonds in three parts with coupons ranging from 1.1 percent to 3.6 percent on Sept. 10 to help fund its pension obligations and repay debt, among other general corporate purposes, according to Bloomberg data and a company filing at the time.

Eoin Treacy's view Pension liabilities represent a significant headache for corporations and a potential source of industrial action. It is therefore logical that companies would tap the lowest borrowing costs on record as a means of recapitalising pension funds because it is a more expedient solution than cutting benefits or funding them directly.

The challenge for pensions, which assume a 7.5% annual return, is that reaching this goal in the credit markets is particularly difficult with yields as low as they are. The only alternative would be to invest directly in equities. However, the capital requirements that have been imposed on most funds means they are unable to hold a large overweight position in equities not least because of the perception of high volatility.

This means that either pension funds will need to gain the right to invest more of their capital in equities or the money raised today from bond issuances will need to be followed, a few years from now, with additional capital raising exercises.