Effect of temperature on the lifetime of stabilized and unstabilized PP films

doi:10.1016/S0141-3910(98)00059-7

Polymer Degradation and Stability

Volume 63, Issue 1, January 1999, Pages 41-52

https://doi.org/10.1016/S0141-3910(98)00059-7 Get rights and content

Abstract

Unstabilized and phenolic antioxidant stabilized PP films were aged in draft air ovens at temperatures between 150 and 40°C. The Arrhenius plots of the failure times reveal quite distinctive features. Thus, unstabilized PP films show a marked downward curvature of the plot at temperatures below 80°C. The role of Ti catalyst residues in this behavior is discussed.Phenolic antioxidant stabilized films show a similar bent in the Arrhenius plot at significantly higher temperatures. Furthermore, these plots can be superposed onto the curve obtained for the unstabilized film by a shift parallel to the reciprocal absolute temperature axis. The numerical value of the shift is a function of the chemical nature of the phenolic antioxidant and of its concentration. Remarkably, the phenol stabilized films show a second curvature in the Arrhenius plot at temperatures below 80°C. This second curvature is an upward bent so that the Arrhenius plot becomes a typical sigmoid. This low temperature curvature depends also on the nature of the phenolic antioxidant and its concentration. It is attributed to some kind of complexation of the Ti catalyst residues by the phenols. Comparison with PP films stabilized with Hindered Amine Stabilizers (HAS) and with PE-LD films lends additional support to the conclusions concerning the role of transition metals.

Introduction

The use of the Arrhenius equation to predict the lifetime of polymer samples at low temperature from data generated at high temperature seems rather straightforward1, 2, 3, 4, 5, 6, 7, 8, 9. However, it was shown already a long time ago, that such an extrapolation is not valid, not even for unstabilized polypropylene[1]. The deviations from linearity observed at low temperature were attributed to the activity of transition metal impurities, e.g. iron from the processing equipment.

The responsibility of transition metals in this curvature was confirmed by additional work2, 3. More precisely, the titanium catalyst residues were shown to be responsible for this effect. The curvature in the Arrhenius plot is attributed to a change in the hydroperoxide decomposition mechanism. This mechanism is thought to be a pure thermal mechanism at high temperature and Ti-catalyzed hydroperoxide decomposition at low temperature2, 3. The curvature of the Arrhenius plot was also confirmed independently for various unstabilized PP homopolymers and copolymers[4]. Furthermore, a similar curvature was found with phenolic antioxidant stabilized PP films [4]. However, in this instance a second one in the other direction follows the first curvature so that a typical S-shaped curve results. In the following, additional experimental data will be presented and discussed.

Section snippets

Experimental

The polymers used for sample preparation were unstabilized commercial resins stored in a refrigerated room to minimize oxidation. The characteristics of the polypropylene grades used in this work are given in Table 1.

Unstabilized PP powder was dry-blended with 0.1% Ca stearate (Merck) and extruded at 260°C into 120μm thick films. These melt-cast films were cooled in a water bath maintained at 20°C. The procedure was overall the same for the stabilized films, the stabilizers being dry-blended

Effect of temperature on the lifetime of unstabilized PP films

As already mentioned the Arrhenius plot of the lifetime of PP shows deviations from linearity at low temperature. This is shown once more for 120μm thick melt-cast PP films in Fig. 1. It can be seen in Fig. 1, that second generation PP and third generation PP show comparable behavior in this respect. In the plot, the data for third generation PP are on a slightly higher level than those obtained with third generation polymer. So far it cannot be concluded to general validity of these results.

Results and discussion

The data obtained with various phenolic antioxidants are plotted in Fig. 3Fig. 4Fig. 5Fig. 6 The plots show once more the typical curvature for the unstabilized control sample. The plots show also that, the phenolic antioxidants tested all show the typical S-shape already published for AO-1. This phenomenon was observed with all the phenolic antioxidants tested so far and, therefore, can be considered as quite general.

To check if some artefact that would be inherent to the Arrhenius plot did

Conclusions

The bent in the Arrhenius plot relative to thermooxidative degradation of unstabilized PP films already described previously for second generation PP is also found with third generation PP. The results from previous work as well as from this work point to prime responsibility of the Ti catalyst residues present in the polymer. However, taking into account the possible effect of titanium on the rate of hydroperoxide decomposition does not yield a quantitative explanation of the pronounced

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