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Title: Thermal properties of Poly(ethylene terephthalate) recovered from municipal solid waste by steam autoclaving

item Holtman, Kevin
item Kodama, Alice
item Klamczynski, Artur
item FLYNN, ALLISON - Lapol
item Bozzi, David
item TORRES, LENNARD - Lapol
item Franquivillanueva, Diana
item MAO, JINGDONG - Old Dominion University
item Glenn, Gregory - Greg
item Orts, William

Submitted to: Journal of Applied Polymer Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/5/2012
Publication Date: 5/11/2012
Citation: Holtman, K.M., Kodama, A.B., Klamczynski, A., Flynn, A., Bozzi, D.V., Torres, L., Franquivillanueva, D.M., Mao, J., Glenn, G.M., Orts, W.J. 2012. Thermal properties of Poly(ethylene terephthalate) recovered from municipal solid waste by steam autoclaving. Journal of Applied Polymer Science. 126(5):1698-1708. doi: 10.1002/app.36752.

Interpretive Summary: Almost three tons of poly(ethylene terephthalate), PET, drinking bottles are deposited in U.S. landfills annually that could be recovered and reused for other products. PET can be recovered from municipal solid waste (MSW) as part of a waste recovery regime involving steam and heat, putting the PET at risk for thermal and chemical decomposition. To assess the impact of the steam autoclaving process on the PET bottles, virgin PET and unused post-consumer bottles were included in the study. Thermal analysis (TGA, DSC) and molecular weight analysis (GPC) were performed to analyze the materials. The results showed that successive heat cycling led to a decrease in molecular weight and thermal properties for all PET samples. Steam autoclaving adds an additional heat cycle to the thermal history of the PET, therefore autoclave-recovered PET has degraded to a further extent. All PET having been through multiple heat cycles is better suited for downcycling; i.e., such as fibers or injected molded products.

Technical Abstract: The goal of this study was to evaluate the effects of steam autoclaving on the properties of PET, data which could ultimately be applied to determine the most likely end use of this potentially huge waste stream. Through the course of the study it was determined that stretch blow molding in bottle manufacture has little impact on Mw or intrinsic viscosity, but causes some change in morphology as witnessed by the decrease in Tc from 134°C to 125°C. A substantial degree of crystallinity persists through the blow molding process. Autoclaving results in a decrease in Tc and an increase in Tm. Apparently, this processing temperature, which exceeds the Tg of PET is high enough to allow for some local reorganization of the polymer. Incorporated debris may act as a nucleating agent inducing crystallinity at lower Tc (120°C) and increasing Tm to higher temperatures (253°C) as a result of more stable crystallites. The steam autoclaving process utilized in this study represents an additional heat cycle affecting both molecular weight and intrinsic viscosity, as well as polymer morphology. Specifically, Mw was reduced from 61,700 g/mol for post-consumer PET to 59,700 g/mol for autoclaved post-comsumer PET and 57,400 g/mol for reclaimed PET. IV is also decreased from 0.749 dL/g for post-consumer PET to 0.735 and 0.709 dL/g, respectively, for autoclaved post consumer PET and recovered PET. Considering that this process maintains the PET in a moist, acidic environment, these modest losses in Mw are not surprising. These changes imply that it is more than likely that recovered PET will be used for downcycling uses. Color and contamination will likely dictate that non-food contact blow molding applications be the primary end use of these products