For years, Dunkin' Donuts has said it would replace its iconic plastic foam cups with cups that are more environmentally friendly.

In a 2010 report, the coffee chain said it considered its use of foam to be "the most prominent sustainability issue we must deal with."

In a follow-up report two years later, the company said it was still searching for an alternative to foam. Dunkin' Brands CEO Nigel Travis said in its 2012 corporate social responsibility report that it hoped to roll out a more sustainable cup in two to three years.

That effort has pretty much stalled.

Six years after declaring that replacing foam cups was the company's "#1 sustainability priority," a majority of Dunkin' Donuts restaurants still serve coffee in cups made of polystyrene, better known as Styrofoam. And the company hasn't made much progress transitioning to a suitable alternative.

"We are not prepared to transition fully out of foam at this time," Christine Riley Miller, Dunkin' Brands' senior director of corporate social responsibility, told Business Insider in a statement.

Like McDonald's golden arches, Dunkin' Donuts' foam cups have become an icon of the brand. The company loves them because they're cheap to buy; customers love them because they insulate hot beverages well. But polystyrene foam is one of the most environmentally unfriendly materials out there, clogging up landfills and choking animals in the wild, among other things.

Because foam is so detrimental to the environment, many municipalities across the country are banning it. In those places, Dunkin' Donuts has introduced cups made of a recyclable plastic called polypropylene, which the company says is "currently the best available alternative to foam."

Polypropylene is much easier to recycle than polystyrene, though it's not as easy to recycle as polyethylene terephthalate, or PET, which is the material used to make most plastic soda bottles.

Polypropylene can be identified by the number 5 that appears on the bottom of cups. A 6 means polystyrene. PET is coded with 1.

Dunkin' Donuts' polypropylene cup was the result of years of research to find an alternative to foam that would keep beverages hot and be accepted in municipal recycling facilities, Miller said. But the company won't be rolling out the cups nationwide anytime soon for a few reasons.

One issue with the polypropylene cups, according to Miller, is the cost. The polypropylene cup and lid cost a lot more than the foam cup and lid combo.

Miller also said customers "are not satisfied with the lid on the new cup." The lid, she said, is still made of polystyrene, which is banned in some places and not recyclable.

As a result, Miller said the company is still working to find a better alternative.

"They're being reactive," said Conrad MacKerron, senior vice president of As You Sow, an organization that has been pushing Dunkin' Donuts to phase out its foam cups. "They're in a comfortable spot because they've made the commitment, and now they say they're just trying the get the logistics right."

One of the main challenges in getting Dunkin' Donuts to adopt sustainability initiatives, like on-site recycling bins, is its ownership structure, MacKerron said. Most Dunkin' Donuts shops are franchises, and, according to MacKerron, there would be a lot of pushback.

McDonald's discontinued the McDLT, which was served in a double-compartment foam container, after phasing out polystyrene food containers in the '90s. theworldsbestever.com

However, MacKerron noted that McDonald's, which also franchises a majority of its restaurants, was able to make the switch from foam to paper cups.

In 2013, McDonald's announced that it would phase out foam cups in favor of paper-based cups at all of its US restaurants. Back in 1990, the fast-food giant ditched its foam food containers, The Los Angeles Times reported at the time.

Starbucks' hot cups are made with 10% recycled paper fiber, and while what is and isn't recyclable varies by municipality, a representative from Starbucks told Business Insider the company pays "local private haulers across the country to collect and recycle hot cups along with our other recyclable products, compost, and trash."

Starbucks has prioritized putting recycling bins in its stores, but that solution only goes so far. After all, many customers take their Starbucks cups to go and then dispose of them in trash cans. As of last year, about 60% of Starbucks stores had front-of-store recycling.

While the coffee chain has been promoting reusable cups by offering a discount to customers who bring in their own, progress has been slower than expected. Starbucks had to drastically lower its goal of serving 25% of its beverages in reusable cups to 5%. Currently, only about 1.6% of Starbucks drinks are served in personal tumblers.

In 2015, when New York City implemented a ban on foam food containers, Dunkin' Donuts made headlines by saying it was "phasing out Styrofoam." The chain temporarily switched to the recyclable polypropylene cups it had been testing in other regions.

But just a few months after the city's foam ban went into effect, a judge overturned it, under pressure from the restaurant industry. And Dunkin' Donuts soon switched back to its foam cups.