Kinetics of the thermal degradation of linear polyethylene

Abstract

For a system of equivalent bonds undergoing random bond scission it is reasonable to assume that the rate of bond breaking (hence the rate of creation of new molecules) reported in the literature and data presented here demonstrate that the number of molecules created is not proportional to the time of thermal treatment hence they seem to belie this reasonable hypothesis. Other authors have adduced the presence of some non‐equivalent bonds in order to account for the observed cruvature and still retain the hypothesis. Implicit in these arguments is the assumption of a steady‐state concentration of reactive fragments. Our analysis explores the consequences of abandoning the steady‐state assumption and shows that a quantitative explanation of the observed degradation behavior may be had by this means wihile still retaining the hypothesis of a constant probability of a bond being broken per unit time.

Reexamination of the Pyrolysis of Polyethylene: Data Needs, Free-Radical Mechanistic Considerations, and Thermochemical Kinetic Simulation of Initial Product-Forming Pathways

“…95,96 However, f will not be simply related to macroscopic melt viscosity and is difficult to predict. 97 Cage recombination achieves no chemical change, but cage disproportionation would give the same stoichiometric result 44 as the "concerted" process noted above and, if f , 1, could make a nonnegligible contribution to the RS pathway even with a sizable kinetic chain length (KCL). 98 One propagation pathway available to R p • is -scission to form monomer (the UZ pathway) and regenerate itself, except for a decrease in degree of polymerization (DP) by one (step 2).…”

“…To compare these estimates of initiation rates to the behavior of PE in the T regime below the onset of volatilization but above that of "enlargement", we converted 21 data points 44,47,49 1a,c,52 We assign no mechanistic significance to the derived Arrhenius parameters because of serious scatter but used them to extrapolate an approximate value at 400 °C of k MW ≈ 1 × 10 -6 s -1 . Hence for illustration, to achieve an R ) k MW t value of 0.001 for the probability of breaking any given backbone bond would require 1 × 10 3 s. Yet in this time period, the probability of bond breaking in the butane model in Table 1 would be only 0.000 000 024, i.e., (…”

“…An alternate and more attractive explanation 44 is that the rate decreases as degradation increases, even though the rate of initiation likely increases (Table 1), because decreases in viscosity will strongly accelerate the diffusion-controlled k t processes; 1f such an increase in k t would be reinforced by the increasing diffusivity of the polymer radicals that are steadily decreasing in average size (see below). 126a…”

Increase of long-chain branching by thermo-oxidative treatment of LDPE: Chromatographic, spectroscopic, and rheological evidence

“…Polyethylene is one of the important commodity polymers in the market, and it is therefore not surprising that many efforts were focused on understanding the thermooxidative degradation of polyethylene, since degradation is of crucial relevance for processing [Hinsken et al (1991); Johnston and Morrison (1996); Epacher et al (2000)], for performance under service conditions [Gijsman (2008)], and for recycling [Hakkarainen and Albertsson (2004); Roy et al (2011)]. Chain scission and cross-linking [Arnett and Stacy (1966); Meltzer and Supnik (1964)] as well as long-chain branching (LCB) [Holmstr€ om and S€ orvik (1974)] have been identified as the main structural modifications in polyethylene as a consequence of thermal degradation. Details on the chemical reactions occurring during thermo-oxidative degradation can be found, e.g., in Holmstr€ om and S€ orvik (1978), T€ udos and Iring (1988), and Gugumus (2002).…”

Distribution Kinetics of Plastics Decomposition

“…The reverse reaction is termination of the chain reaction by recombination of two radicals, which is assumed to occur via recombination. The ratio of recombination/disproportionation is stated as about 2-10 27), 28) . (18) The rate expressions for this reaction are…”