Earth 911, the preeminent source for all things recycling declared in a 2010 article that the verdict is in: keep the bottle caps on. The reason for some removing plastic caps stemmed from the fact that the plastic bottle is typically made of polyethylene terephthalate, also known as PET and plastic No. 1, or polyethylene, also known as HDPE and plastic No. 2. The cap is typically made of polypropylene, also known as PP and plastic No. 5. It turns out plastics have to be separated based on melting points. The Association of Plastic Recyclers recommends crushing empty bottles and placing the cap back on the bottle, though rinsing them isn’t necessary. Put old 2-liter plastic bottles to use as plastic bag storage in your kitchen.