Styrofoam™ or EPS
Styrofoam is a trademarked brand name that is owned by Dow Chemical. Since we don't want Dow Chemica mad at us, we will not use that term when referring to expanded polystyrene. Whats the difference between expanded polystyrene and other types of polystyrene? Air. They pump up the goo up to take up over 10 times as much space by injecting air into it, which also creates the lightweight little "balls" that come apart when you break up a piece of expanded polystyrene. These little balls will break down further over time, but not completely go away. They stay around as microplastics. Perhaps you've heard of the Great Pacific Garbage Patch, also known as the Pacific trash vortex?
PS-6
Anything with a recycling symbol and the number 6 is polystyrene and can be broken down from a solid with the use of acetone. You know those red Solo™ cups? Despite Toby Keith's declaration, they are not "decomposable" in about 14 years. In fact they could take from 500 to a million years to decompose. Styrofoam and other plastics currently make up about 30 percent of the landfill volume in the United States.
Acetone and the break down
Acetone, is an organic compound with the formula (CH₃)₂CO. It is the simplest and smallest ketone. It is a colorless, highly volatile and flammable liquid with a characteristic pungent odor. Acetone is miscible with water and serves as an important organic solvent in its own right, in industry, home, and laboratory. Wikipedia