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On Wikipedia, it says that polyethylene terephthalate (PET) decomposes at temperatures above 350°C. What are these decomposition products? I read further that acetaldehyde is produced, but is that the only decomposition product?

Also, what compounds are produced when PET is burned? ($\ce{+O2->?}$)

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This is a partial answer to your query. According to the paper Degradation and Recyclability of Poly (Ethylene Terephthalate) (Venkatachalam, 2012), there are a few degradation products, from the article:

Apart from aldehyde, $\ce{CO}$, $\ce{CO2}$, ethylene, benzene, biphenyl are also identified as degradation products

(The article also shows diagrms of the chemical processes of PTE degradation).

an important aspect to note is experiments/observations have

shown that rate of thermal degradation in the closed system is about three times higher than that in open system.

In regards to burning of PET, the products can vary according to the page EPB 433- Health and Environmental Effects of Burning Waste Plastics (Saskatchewan Ministry of Environment) stating that

the composition of byproducts of plastic combustion as to their type and concentration depends on the combustion temperature and the flame residence time.

According to the Peiyu Plastics page (a reference from Wikipedia), if complete combustion occurs, then the products would be $\ce{CO2}$ and $\ce{H2O}$, incomplete combustion could produce soot and carbon monoxide.

Further reference:

Here is the MSDS for PET

A study of thermal decomposition and combustion products of disposable polyethylene terephthalate (PET) plastic using the High Resolution Fourier Transform Infrared Spectroscopy, Selected Ion flow Tube Mass Spectrometry and Gas Chromatography Mass Spectrometry (Sonova et al. 2008).

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