Sept 7 (Reuters) - A successful sale of debt backing several large leveraged buyouts (LBOs) being sounded to investors is expected to spark a rush of new junk-bond supply this month from companies looking to fund acquisitions and refinance debt.

U.S. junk debt issuance in September could total $20 billion or more, compared to a total $17 billion in the previous two months, according to several high yield investors and Informa Global Markets data.

That tally would easily outpace the $9 billion in September 2022, when bond issuance slowed and market volatility spiked on worries that default rates could rise in a higher interest rate environment.

But default rates have so far been lower than expected, and investors' demand for junk-rated debt has risen on higher yields, creating an issuance opportunity for companies.

Total year-to-date returns on the Morningstar LSTA U.S. Leveraged Loan 100 Index and ICE BofA High Yield Index , at 8.98% and 6.68%, have outpaced the 1.75% year-to-date returns on the ICE BofA U.S. Corporate Index , which tracks high-grade corporate bonds.

"Investor demand should be strong for most new high-yield debt because they are paying high coupons in an environment where default rates have been arguably much lower than expected," said Manuel Hayes, senior portfolio manager at asset manager Insight Investment.

"You could miss this current phase of nearly 9% yields on new junk bonds if you wait for market conditions to settle down," he added.

On Wednesday, a Goldman Sachs-led bank group said it will market $1.7 billion in senior secured notes next week to help fund drugmaker Syneos Health's $7.1 billion buyout by Elliott Investment Management, Patient Square Capital and Veritas Capital.

Banks led by JPMorgan and Goldman Sachs this week began pre-marketing $9.4 billion in debt backing merchant servicer Worldpay's July takeover by buyout firm GTCR, according to people familiar with the matter.

JPMorgan-led banks have also begun pre-marketing talks on $1.75 billion in debt that helped fund drugmaker Bausch+Lomb's purchase of eyecare assets from Novartis, according to two of the people familiar with the matter.

JPMorgan and Goldman Sachs declined to provide comment.

"We'd expect about half of the total supply to finance M&A and half for refinancing purposes," said Scott Macklin, director of leveraged loans at asset manager AllianceBernstein.

Investors are also expecting more secured tranches rather than those that were not secured by any collateral.

"Traditionally within the high yield bond market, the issuance has skewed sort of one-third secured versus two-thirds unsecured," said Brian Gelfand, co-head of global credit at asset manager TCW. "In late 2022 and so far this year, it’s really skewed the other way." (Reporting by Matt Tracy; Editing by Shankar Ramakrishnan and Nick Macfie)