I get asked some version of this question ALL THE TIME and I really liked Umbras's answer. You can link directly to it or read below. (It's featured with other questions, so I just pasted the one I was interested in here)
Also, here's a more in-depth explanation (also linked from Umbra's article)
Q. Dear Umbra,
I can understand that a recycling station might accept only plastics type 1 and 2. What I can’t understand is the common situation that they’ll accept only types 1 and 2, and only if the plastic is in the shape of a narrow-necked bottle. If it’s made of the same stuff, and in the recycling process will be ground up and melted down (or so I assume), why does the shape of the plastic item matter? Thanks!
Jonathan T.Norwich, Vt.
A. Dearest Jonathan,Recycling rules do often seem arbitrary, don’t they? We’ll take mayonnaise jars, but only if there’s a dab of mustard on the lid and a pickle on the side ... it can be mind-boggling at times. In this case, however, there actually is a logical reason.
That reason being that narrow-necked bottles (such as soda, shampoo, and dish soap often come in) are manufactured differently than wide-necked items like yogurt containers. The narrow-necked ones are blow-molded, while the others are injection-molded. Even if the types of plastic are the same, the rigamarole they go through during manufacture leaves them with different melting points, which means they must be handled differently in the recycling process. So it is sometimes difficult to find a buyer who wants or can handle both. For a much more detailed look at this topic, and a refresher on why you’re glad you don’t oversee a recycling program, see this plastics explainer from the good folks at the Central Virginia Waste Management Authority.
Plastic recycling continues to be a confusing topic for many readers. My best advice is to first buy as little plastic as possible, then do what Jonathan here has done: pay close attention to your town’s recycling guidelines and recycle what you can. Should you have time for a little light reading, I also recommend the eye-opening Plastic Recycling Facts at Earth911, and the Ecology Center’s Seven Misconceptions About Plastic Recycling. You might never look at a tub of butter the same way again.