Bottle caps are made of a different plastic resin than the containers. They are usually made of number 4 polypropylene PP because this material is more rigid than the other resins and works well for the strength needed for a beverage cap.
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The little recycling triangle on the bottom of most plastic containers and many other plastics is called an Society of the Plastics Industry SPI code. This code was developed by the Society of the Plastics Industry (SPI) to help consumers and businesses easily identify the resin type. This helps for recycling purposes but the code identifier is not meant to indicate that the material is "recyclable." The industry chose to put symbols on the six most common resins plus an "other" category to capture the numerous other resins and the blends that are used.
Even though the plastic resin in a milk jug and a butter tub may be the same, the molding process to make them is quite different. A milk jug is blow molded, like blowing a bubble, and a tub is injection molded, like stamping two molds together. To use each molding technique, different chemicals must be added to each plastic resin to make it melt and flow correctly. These added chemicals make the resins incompatible in most recycling processes because they no longer melt at the same rate. Melting is an important phase of the plastics recycling process.