3.6 million parents use the most expensive federal student loan to pay for their kids' college. Here's why it can push off retirement.

• Parent PLUS loans are student loans parents can take out to pay for their kids' educations.
• But those loans have the highest interest rate - 6.28% - allowing debt to build quicker for parents.
• Parents told Insider they would still do whatever it takes to give their kids an education.

The $1.7 trillion student debt crisis is falling on the shoulders of 45 million Americans, causing tremendous financial burdens for many. For 3.6 million parents who took out loans to pay for their kids' college, debt could jeopardize retirement.

That's because they took out parent PLUS loans - the most expensive type of student loan with the highest interest rate.

A direct PLUS loan, commonly referred to as a parent PLUS loan, is a type of federal student loan parents can take out to pay for their kids' educations. To receive a PLUS loan, according to Federal Student Aid (FSA), all parents needs to do is prove they have good credit history and meet the general eligibility requirements for federal student aid, and the government will then lend them money that can cover up to the cost of attendance for their child minus any other financial aid the child already received.

Compared to other types of student loans, though, PLUS loans can accumulate quickly if the parents doesn't have the financial means to pay them off immediately. Insider reported in June on the new interest rates for different types of federal student loans that will be in effect until July of next year: direct loans for undergraduates have a rate of 3.73%, direct loans for graduates and professionals have a rate of 5.28%, and PLUS loans hold the highest rate of 6.28%.

The problem with PLUS loans is that they are too easy to take out, per a report for The Texas Public Policy Foundation. That's because the amount parents receive is based on cost of attendance instead of how much the parents can actually afford. It can create a "dangerous mentality" that leads to increased, and unchecked, borrowing, Andrew Gillen, author of the report, told Yahoo Finance.

Insider previously spoke to two parents who are pushing off their retirement thanks to interest rates on PLUS loans that are making it difficult to pay off the debt. Reid Clark, 57, unexpectedly became the sole provider for his five children and his debt load now stands at over $550,000.

"I'm looking at paying $3,000 a month for the better part of the rest of my life," Clark told Insider. He estimates he'll have to keep making those payments for at least three more decades.

For 64-year-old Robert Pemberton who has $265,000 in PLUS loans he took out for his two children, debt is an "endless cycle where the loan can never be paid off unless I have a windfall and pay it all, or I die and it goes away."

"I don't know if I'll be able to work into my 80s," he said.

Pemberton told Insider the process to take out the loans was strangely easy, and he described it as being "on autopilot" and all he had to do was "sign a paper."

PLUS loans have been included in the pandemic student-loan payment freeze, but once the freeze lifts on February 1, parents and other federal student-loan borrowers will have to resume paying off their debt. But it's unclear whether PLUS loans are included in the federal student-loan forgiveness conversation, and while both Pemberton and Clark told Insider they're not holding out hope for any loan forgiveness, they don't regret doing whatever was necessary to make sure their kids received educations.

"For those of us who want to see our kids do better, we understand that you better yourself, and you better your chances for success, with education," Clark said. "And I'm just not going to take the chance on not sending my kids to school, even though it's going to create a tremendous financial burden. It's not an option."