There are two types of plastic produced in the world, Thermoplastics by its very name thermo indicates that when heat is applied it can be melted and re-moulded to create new products, and therefore, recycled. Thermoset plastics comprise of polymers created by an irreversible chemical bond which means they cannot be remelted for recycling purposes.
Even some Thermoplastics end up in landfill and this is primarily because any plastic with food residue on it cannot be recycled unless they go through a cleaning process. To recycle plastic, it must be of good quality and therefore plastic must be rinsed before it is disposed of.
Check out our story on changes to single-use plastics in the UK