When different types of plastics are melted together, they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications. This is in part why the plastics industry has developed the resin identification codes. The two most widely manufactured plastics, polypropylene and polyethylene, behave this way, which limits their utility for recycling. Each time plastic is recycled, additional virgin materials must be added to help improve the integrity of the material. Therefore, even recycled plastic has new plastic material added in. Moreover, the same piece of plastic can only be recycled about 2–3 times. Thus, even when plastics have a resin code, or are collected for recycling, only a small portion of that material is actually recycled. For example, as of 2017, only 8% of US plastic was recycled.