Study of thermal oxidation of polyolefins. VI. Change of molecular weight distribution in the thermal oxidation of polyethylene and polypropylene

Abstract

In the thermal oxidation of low density polyethylene (PE) in the melt phase and isotactic polypropylene (PP) in the solid phase, the changes in molecular weight distribution (MWD), weight average molecular weights (Mω), and average number of chain scissions have been studied by gel permeation chromatography (GPC). It has been established that in the case of PP degradation, there is an increase in molecular weight during the induction period both at 80 and 760 torr with simultaneous scission of the macromolecules. In contrast, a similar tendency can be observed in the degradation of PE only at very low (20 torr) pressures, i.e., the degree of molecular weight increases proportional to the length of induction period. For both polymers at the temperature of investigations, considerable molecular weight decrease can be detected both at reduced and atmospheric pressures at the onset of measurable oxygen absorption. In the maximum rate oxidation of PE and PP, the increase of temperature, oxygen pressure, and degradation time each causes a shift of the MWD curves towards lower molecular weights. The average number of chain scissions against the amount of absorbed oxygen was also studied. It has been found that the average number of chain scissions per 1 mole absorbed oxygen is considerably higher for PE than for PP. Molecular weight studies performed in the kinetic range of thermal oxidation show that under given conditions, Intermolecular combination reactions and chain scissions may occur simultaneously.

Impact of oxygen transport properties on polypropylene thermal oxidation, part 1: Effect of oxygen solubility

“…Similarly, the variability of the oxygen solubility enables to simulate a larger cluster of maximal oxidation rates among the compilation of literature data than the effect of the crystallinity ratio alone (see Figure ). Moreover, the highest value of solubility, more consistent with the compilation of iPP data than the previous one, does not allow to simulate the upper range of this compilation, particularly above 110°C.…”

Analysis of Free Energy Increment for Insulation Aging in Crosslinked Polyethylene DC Cable

“…The FEI of XLPE at each aging stage is shown as figure 8. The FEI decreases with aging stage, and this trend is same with the decrease of polymerization degree with aging stage [15], [16]. Figure 9 shows how the degree of polymerization affects the FEI in simulation, and the electric filed in figure 9 is 300 kV/mm.…”

Understanding and predicting the DC voltage endurance coefficient of cross-linked polyethylene insulation under thermal ageing

“…The trend of Δ W in thermal ageing is the same as the trend of the degree of polymerisation (DP) in thermal ageing [23]. Several molecular models with different DPs were built to investigate the relationship between Δ W and the DP.…”