Thermal Depolymerization of Polyoxymethylene

Safety precautions Before this experiment is carried out. Sect. 2.2.5 must be read as well as the material safety data sheets (MSDS) for all chemicals and products used. Because of the formation of gaseous formaldehyde the experiments have to be carried out in a closed hood.The decomposition of polyoxymethylene can be conveniently performed in a thermobaiance, respectivelyTGA-apparatus (see Sect.2.3.4.7). 

100 mg of each of the following samples are weighed into small test tubes with an as constant as possible internal diameter  

The tubes are now placed in the decomposition vessel which is then evacuated and filled with pure nitrogen three times. A slow stream of nitrogen is passed through the vessel which is heated in an oil bath or air thermostat to 190 °C. At intervals of 1 h the decomposition vessel is taken out of the hot bath, the tubes are allowed to cool under nitrogen for 15 min.They are then individually weighed and heated again as described above.The wt% residue is plotted against time. 

The thermal depolymerization of polyoxymethylene starts from the unstable hydroxy end groups, but the oxidative and acid-catalyzed hydrolytic degradation takes place within the main chain. Flence, if polyoxymethylene is heated in air or in the presence of strong acids samples with blocked end groups will also degrade. 

Oxidative Degradation of Poly(vinyl Alcohol) with Periodic Acid 

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Polyoxymethylenes have a marked tendency to undergo thermal depolymerization with loss of formaldehyde. To prevent thermal depolymerization, polyoxymethylenes are structurally modified, the two possibilities being acetylation to block the reactivity of the end groups of co-polymerization with cyclic ethers, e.g., ethylene oxide. Polyacetals are also sensitive towards autoxidation, which invariably leads to depolymerization as a result of chain scission. The formaldehyde released by depolymerization is very likely to be oxidized to formic acid, which can catalyze further depolymerization. 

Polyoxymethylene is susceptible to depolymerization, or unzipping, under molding conditions. To improve thermal stability, end capping is essential. The capping of the hydroxyl end groups is achieved by etherification or, preferably, by esterification using acetic anhydride ... 

The main difference between homo- and copolymers is the fact that copolymers exhibit higher thermal-oxidative resistance. Figure 5.196. Thermal-oxidative degradation in polyoxymethylene leads to the formation of unstable end groups that initiate depolymerization under the formation of formaldehyde. In homopolymers, this results in total decomposition. In copolymers, depolymerization proceeds only to the next comonomer unit (mostly polyethylene, comonomer content approx. 0.5 to 5 wt.%). That widens the processing window for copolymers and reduces the risk of mold fouling [771]. 

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