A polymer is a very large molecule made of many small repeating units, called monomers, linked together in long chains. Plastics are synthetic polymers: polyethylene is a chain of ethylene units, polypropylene a chain of propylene units.
The type of monomer, the length of the chains and how they are branched determine a plastic’s properties, from the stiffness of HDPE to the stretch of LLDPE. Most packaging plastics are thermoplastics, polymers that soften when heated and harden when cooled, again and again. That is what makes mechanical recycling possible. Each heating cycle shortens the chains slightly, which is why recyclers control temperature, filter the melt and use additives to protect the material. Different polymers do not mix well, so they are identified by resin identification codes and sorted before recycling.
