Recovery of valuable products from polytetrafluoroethylene (PTFE) waste

Abstract

The disposal of PTFE waste presents a global problem from both an ecological and economical perspective. This study was performed to address this problem as well as to understand the mechanism according to which PTFE is depolymerised to form the different products, namely C2F4, C2F6, C3F6, C3F8 and c-C4F8. Different conceptual reactors were designed and built for different tests on filled and unfilled PTFE. These included a laboratory scale Rotating-kiln Reactor for testing the depolymerisation properties in a horizontal reactor, a Drop-tube Reactor for the continuous depolymerisation of unfilled PTFE, a Paddle Reactor for the continuous depolymerisation of filled as well as unfilled PTFE and a Vibrating Reactor for the evaluation of other fluoropolymers like THV, HTE, ETFE and PFA. Analytical techniques such as GC, GC/MS, TGA and SEM were used to evaluate the different types of PTFE and analyse the gaseous products from the depolymerisation process. A mechanism by which depolymerisation and product formation takes place was proposed on the basis of experimental data obtained during experimentation on various PTFE depolymerisation systems, utilising different analytical techniques, including a thermodynamic study, a kinetic study and molecular modeling. For the product formation mechanism a two-route approach was proposed. Route 1 dominated at temperatures < 700 °C, producing mainly C2F4, C3F6 and c-C4F8. Route 2 dominated at temperatures between 700 and 900 °C, producing mainly C2F4) C2F6, C3F6, C3F8, PFIB and c-C4F8. A thermodynamic study and molecular modeling was used to evaluate possible product formation reactions in order to propose scientifically sound mechanisms for Routes 1 and 2.

A kinetic study revealed that the order of the depolymerisation reaction is 0.54, and that the reaction proceeds at a rate of 4.03 x 10"3 s"1 at 600 °C. The activation energy for the depolymerisation process was calculated to be 260 kJ-mol"1. A Rotating-kiln Plant was designed for the production of HFP from PTFE on a commercial scale. An economic analysis indicated that a 500 kg-h"1 filled PTFE depolymerisation plant, selling HFP as a product at R55 per kg, is highly profitable.