Effect of molecular weight of polyethylene on its flammability

Abstract

Polyethylene has been widely applied in our society and is known to burn well. Many researchers have been studying to control the flammability of polyethylene when adding several catalysts. However, this study has confirmed the effect of the molecular weight on the flammability of polyethylene. This article displays combustion states of two different molecular weights of polyethylene. The low molecular weight of polyethylene did not ignite and the high molecular weight of polyethylene ignited and burned well. From this result, it was difficult for polyethylene to continue combustion under limited conditions, such as the vertical configuration of the materials, air circumstance, and low molecular weight. To analyze such behaviors, the relationship between combustion state and degradation behavior of polyethylene was compared to that of polypropylene (PP) and polystyrene. PP and polystyrene decomposed into small molecules directly through chain-end scission and that low-molecular weight scission products went to gas phase and reacted with oxygen and consequently fire around specimens during combustion. Polyethylene decomposed into relatively higher molecular weight scission products through random scission at primary degradation. The scission products further decomposed into lower molecular weight products with drip, which caused the combustion under specimens. There were no big differences in a weight loss temperature and scission products at high temperature between a low molecular weight of polyethylene and a high molecular weight of polyethylene. Therefore, the degradation and combustion behavior of scission products of polyethylene in primary degradation warrant further research on flame retardant of polyethylene.

Effects of ageing on the fire behaviour of flame-retarded polymers: a review

“…Dripping is obviously more severe for low‐viscosity materials. As a surprising example, Nakashima et al have shown that highly flammable polyethylene does not burn in a UL‐94 vertical test when its molecular weight is 35 000 g mol −1 or lower, due to severe dripping. Viscosity influences many other phenomena involved in flame retardancy: accumulation and migration of fillers at the surface, formation of a protective layer, intumescence, bubbling, etc.…”

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.…”

“…The temperature at which a sample weight decreases to 50% is the lowest for PS and the highest for PE. In addition, the decomposition products of PP and PE are mainly aliphatic, but many decomposition products of PS contain aromatic rings 19 . Therefore, the formation of soot particles proceeds easily for PS, which results in a red flame spreading rapidly and pronounced oscillations 41 .…”

“…The viscosities of the droplets during combustion were much lower than those of the precombustion samples for PE and PP; however, they were negligibly lower than those of the precombustion samples for PS. For PE with of 76,000 and PP with any molecular weights, because the molecular weight gradually decreased through multiple stages of decomposition before volatilization and the viscosity gradually decreased, a continuous melt flow was formed, and the flame was located below rather than around the specimen body 19 . For the ultrahigh molecular weight PE, the viscosity remains high even after being heated; however, the viscosity decreased rapidly through multiple stages of decomposition.…”

“…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.…”