Chapter 1: The Chemistry of Thermal Decomposition
To appreciate the danger, one must move beyond the simplified view of "melting plastic" and understand the violent, chaotic chemical reality of pyrolysis.
1.1 Fundamentals of Polymer Pyrolysis
Pyrolysis is the thermochemical decomposition of organic material at elevated temperatures in the absence of oxygen. Unlike combustion (exothermic), pyrolysis is endothermic—it requires a massive, continuous input of heat to break chemical bonds.
The Bond Vibrational Limit
Polymers like Polyethylene (PE) consist of chains with thousands of carbon atoms. Between 350°C and 500°C, the vibrational energy exceeds the bond dissociation energy of the carbon-carbon backbone. This causes the chains to scission (snap) randomly.
1.2 The Heat Transfer Problem: "The Pot Still"
In an industrial setting, heat is managed via fluidized sand beds that ensure every particle reaches cracking temperature simultaneously. In a home "pot still" reactor, heat transfer is notoriously poor.
Simulation: The Operator's Dilemma
Try to heat the reactor. Notice how the walls overheat before the center melts.
Plastic melts into a viscous, insulating "goo". The plastic against the reactor wall may be 600°C (over-cracking to gas), while the plastic in the center is only 300°C (melting but not cracking). This thermal gradient is the first major safety flaw: it leads to unpredictable pressure spikes and "waxing out," where uncracked plastic clogs the system.
1.3 The Free Radical Mechanism: Chaotic Chain Reaction
The cracking of plastic is driven by free radical mechanisms. When a carbon bond breaks, it forms highly reactive fragments possessing an unpaired electron. These radicals are desperate to stabilize themselves.
- Initiation: The bond breaks, creating radicals.
- Propagation: A radical steals a hydrogen atom from a stable neighbor ("Hydrogen Abstraction") or breaks further ("Beta Scission").
- Termination: Two radicals collide and bond, forming a stable molecule.
1.4 The Myth of "Clean" Feedstock
A primary fallacy is that one is pyrolyzing "pure plastic". Consumer plastics contain up to 50% additives by weight.
Often contain bromine or chlorine. When pyrolyzed, they release HBr and HCl acid gas, or form brominated dioxins.
Frequently rely on heavy metals like lead, cadmium, tin, and zinc. These concentrate in the char residue.
Even a small fraction of incompatible plastic—such as a single PVC blister pack in a batch of milk jugs—can generate enough acid gas to corrode the reactor.