United Technologies Corp.'s $9.8 billion offering, the largest in more than three years, sent U.S. corporate bond sales higher last week even as speculative-grade issuance plummeted to the lowest levels of 2012.

Overall issuance soared 38 percent from the previous week to $19.4 billion while junk bond sales declined 60 percent to $1.9 billion, according to data compiled by Bloomberg. Hartford, Connecticut-based United Technologies, which last tapped the market more than two years ago, sold debt in six portions ranging from 18-month floating-rate notes to 30-year bonds.

Bond buyers are being lured by higher-rated debt like that of the maker of Sikorsky helicopters and Pratt & Whitney engines as turmoil in Europe sends them to safer assets. While investment-grade returns in the U.S. are down 0.2 percent this month through yesterday, speculative-grade bonds have declined 1.3 percent, according to Bank of America Merrill Lynch index data.