Thermal Degradation of Polyesters
Poly (ethylene terephthalate) decomposes upon heating through a series of different reactions. These run either concurrently or consecutively. The result is a complex mixture of volatile and nonvolatile products. It was found that when poly (ethylene terephthalate) is maintained in molten condition under an inert atmosphere at 282–323C, it slowly converts to a mixture of gaseous low molecular weight fragments [581]. The major products from pyrolysis of poly (ethylene terephthalate) are carbon dioxide, acetaldehyde and terephthalic acid. In addition, there can be detected trace amounts of anhydrides, benzoic acid, p-acetyl benzoic acid, acetophenone, vinyl benzoate, water, methane, ethylene, acetylene, and some ketones [505]. The following mechanism of degradation was postulated [505]:
The vinyl end groups that from cleavage of the ester groups decompose fur the r in a number of ways:
The thermal degradation of poly(butylene terephthalate) was examined with the aid of a laser microprobe and mass spectrometry [506]. A complex multistage decomposition mechanism was observed that involves two reaction paths. The initial degradation takes place by an ionic mechanism. This results in an evolution of tetrahydrofuran. This is followed by concerted ester pyrolyses reactions that involve intermediate cyclic transition states and result in formation of 1,3-butadiene. Simultaneous decarboxylations occur in both decomposition paths. The latter stages of decomposition are characterized by evolutions of carbon monoxide and various aromatic compounds, like toluene, benzoic acid, and terephthalic acid. The first step can be shown as follows [506]:
The subsequent decomposition, shown below, can actually take place at lower temperatures:
There are indications that there is moisture among the decomposition products. This may imply that acid hydrolysis plays a part in tetrahydrofuran formation:
At higher temperatures, the reaction path involves elimination of 1,3-butadiene [506, 507];
In oxidations of hydrocarbons, oxygen is believed to act as a diradical in the ground state. This would explain radical combination reactions:
and the subsequent hydrogen abstraction reaction:
The rate of formation of the peroxy radical is much higher than is the rate of hydrogen abstraction [521]. The overall rate of oxidation of polymeric materials by atmospheric oxygen is strongly affected by light, heat, oxygen concentration, moisture, and the presence of traces of impurities. The impurities, however, can act as either catalysts or as inhibitors of oxidation. The degradation of poly-a-esters was studied on poly (isopropyl dine carboxylate) [518] over a range of temperatures, from 200 to 800C. Among the decomposition products were found tetramethyl glacolide, acetone carbon monoxide, and to a lesser extent methacrylic acid. The primary decomposition product appears to be tetramethylene glacolide that becomes an intermediate upon further pyrolysis:
Decomposition of polycarbonates was studied on poly[2,20-propane-bis-(4-phenyl) carbonate] [519]. It was concluded that a rearrangement mechanism is the main intermediate material in subsequent formation of carbon dioxide and volatile phenolic compounds [519]:
