Recycling by product massive number of types of plastic, they each carry a resin identification code, and must be sorted before they can be recycled. This can be costly; while metals can be sorted using electromagnets, no such 'easy sorting' capability exists for plastics. In addition to this, while labels do not need to be removed from bottles for recycling, lids are often made from a different kind of non-recyclable plastic. To help in identifying the materials in various plastic items, resin identification code numbers 1-6 have been assigned to six common kinds of recyclable plastic resins, with the number 7 indicating any
Polystyrene the backbone were to be laid as a flat elongated zig-zag chain, each phenyl group would be tilted forward or backward compared to the plane of the chain. The relative stereochemical relationship of consecutive phenyl groups determines the tacticity, which has an effect on various physical properties of the material. The diastereomer where all of the phenyl groups are on the same side is called "isotactic" polystyrene, which is not produced commercially. The only commercially important form of polystyrene is "atactic", in which the phenyl groups are randomly distributed on both sides of the polymer chain. This random positioning prevents