**3. Cracking (pyrolysis)**

The cracking of polymers is a process where long polymer chains are broken into products with shorter chains. During heating (at around 350–600°C), molecules start to vibrate until the vibrations are intense enough to overcome van der Waals forces. The short-chain molecules then evaporate. When the energy of the van der Waals force is higher than the enthalpy between carbon atoms in the molecule, the bond in the molecule will break instead of evaporating. In polyolefins' chains, the most unstable bonds will be the first to break, leading to the creation of radicals.

The dissociation energy needed to break the bond between carbon atoms lowers with the order of carbon classification: primary > secondary > tertiary, which means that the first place in a polymer molecule where the bond will break is at the branching. This already indicates that polypropylene cracks at lower temperatures than polyethylene because all carbons in the structure (apart from terminal ones) are tertiary. It also means that LDPE cracks easier than HDPE because of higher branching. All types of polyethylene and polypropylene can be cracked, including those that are challenging for mechanical recycling, like PEX.
