Scission Products and Molecular Weight Effects on the Combustion of Polyethylene

Abstract

The mutual relations between the flame positions during the vertical combustion test and the thermal degradation of polyethylene (PE), polypropylene (PP) and polybutene (PB) as polyolefin and polystyrene (PS) were studied. The flame developed downward for PE and upward for PS. The flames of PP and PB remained in the middle. The combustion of polymer materials has been thought to occur on the surface of the material after having been ignited. However, from the result of the vertical combustion test, it was understood that a flame exists on the surface of the material that drips from PE. The vaporization of the sample is not thought to occur in the primary degradation, but in the secondary degradation phase, when it reacts with oxygen. In order to analyze the phenomena, the scission products of those polymers were measured by Py-GC-MS. When the combustibility of PE of different molecular weight was evaluated by the vertical combustion test, PE of lower molecular weight did not burn where as PE of higher molecular weight did. PE of the lower molecular weight only melted without ignition. Even if the Bunsen flame of the burner was applied to the thinly dripping melted material, it would not ignite. It was then concluded that PE with lower molecular weight does not degrade and vaporize in the secondary degradation.

Image analysis of flame behavior for polyolefins and polystyrene in vertical flame test

“…Factors that cause the fuel production rate to differ depending on the molecular structures include a difference in the thermal decomposition process of polymers. Two major thermal decomposition processes have been well characterized: random scission and chain‐end scission 19–22,40 . For PE, because carbon–carbon bonds in the main chain decompose dominantly by random scission, a significant amount of main chain scission is required to generate a sufficient quantity of volatile products.…”

“…Two major thermal decomposition processes have been well characterized: random scission and chain-end scission. [19][20][21][22]40 For PE, because carbon-carbon bonds in the main chain decompose dominantly by random scission, a significant amount of main chain scission is required to generate a sufficient quantity of volatile products. By contrast, because the chain-end scission is dominant for PS, monomers, dimers, and trimers are generated from the end in a single scission reaction, easily generating volatile products.…”

“…To develop novel flame‐retardant materials, it is necessary to gain a deeper understanding of the polymer combustion behavior, including its thermal decomposition. Different combustion behaviors for polyethylene (PE), polypropylene (PP), and polystyrene (PS) have been previously reported using the UL‐94 vertical flame test, and the relationship between polymer combustion and thermal decomposition has been investigated 19–22 . The UL‐94 method, more generally known as the vertical flame test, provides data such as the ignition time and combustion duration in a vertical flame test 23–24 ; however, it is insufficient for quantitative evaluations.…”

“…Different combustion behaviors for polyethylene (PE), polypropylene (PP), and polystyrene (PS) have been previously reported using the UL-94 vertical flame test, and the relationship between polymer combustion and thermal decomposition has been investigated. [19][20][21][22] The UL-94 method, more generally known as the vertical flame test, provides data such as the ignition time and combustion duration in a vertical flame test [23][24] ; however, it is insufficient for quantitative evaluations. Hence, we developed a novel method and instrument for measuring combustion behaviors, such as the heat release rate (HRR) and drip amount in the vertical flame test.…”

Effect of molecular weight of polyethylene on its flammability

“…The most notable result of these experiments was that when the molten sample created by directly heating of the PE specimen (M w ¼ 35,000) with a flame of a butane burner was reheated, it did not burn at all. As the flame did not change the characteristics of the molten sample, this could become a new molding method by using open fire 26 . A major difference in the pyrolysis process on the combustion of polymer was expected, because the polymer did not react directly with oxygen, but rather the scission products reacted with oxygen in gas phase.…”