A method using direct flame ionization detector (FID) measurement was developed to study total volatile organic compounds (VOCs) emissions during thermal degradation of polymers. This was used to estimate organic emissions from both virgin polymer resins and commingled plastics. The effects of process parameters, i. e., temperature, heating rate and residence time, were also studied. Significant VOCs emissions were observed at normal processing temperatures, particularly from recycled polymers. Each polymer showed a distinct evolution pattern during its thermal degradation. Kinetics of VOCs emissions were also studied using a non-isothermal technique. The kinetic parameters were in agreement with data from the literature.
Polypropylene, as a commodity recyclable thermoplastic, was studied in this research to evaluate the potential environmental impact resulting from VOCs emitted during multiple melt reprocessing. Unstabilized and stabilized PP homopolymers, referred to as U-PP and S-PP, were used to simulate recycled materials prone to degradation. They were evaluated for total VOCs emissions generated during multiple melt reprocessing by injection molding and extrusion respectively. Results show that the maximum amount of total VOCs from each cycle (up to six cycles for extrusion and up to ten for injection molding) did not significantly change, while the cumulative VOCs increased with increasing processing cycle for both materials. A good correlation was obtained between the cumulative VOCs increase and the Melt Flow Index increase for the U-PP, and the MW decrease for the S-PP. Reprocessing in all cases was accompanied by decreases in molecular weight and melt viscosity as a result of thermo-oxidative degradation. Corresponding structural changes were investigated using FTIR, and the data showed increases in carbonyl content and degree of unsaturation with the increase of processing cycle number. At equivalent cycle numbers, degradation appeared to be more severe for the extruded material in spite of the longer oxidative induction time of the "as received" pellets used in extrusion. The onset and type of structural changes was shown to depend on cycle number and reprocessing method. A simulation study was also performed by multiple heating and cooling of a single U-PP sample under static conditions, and under different gaseous atmospheres. The results indicate that the actual reprocessing conditions generated emissions whose levels, and rate of generation were closer to a mild thermo-oxidative degradation rather than a pure thermal one.
Continuous nonmethane organic carbon (C-NMOC) analysis was considered to be a more accurate and on-line method for monitoring emissions during polymer processing. An improved version of the C-NMOC system was developed in this research. A multibed microtrap was developed to prevent the breakthrough of small molecules such as propane and methanol. Two novel sampling configurations were also developed, and were referred to as sequential valve with backflushed microtrap (SV-BM) and multiinjection sequential valve with backflushed microtrap (MSV-BM). By combining the multi-bed microtrap with the SV-BM and MSV-BM configurations, ideal performances were obtained in terms of linearity, reproducibility of multiple injections and separation of background gases. Both small molecules and large molecules could be effectively collected and desorbed with the optimized microtrap.
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