Photo-oxidation of thick isotactic polypropylene films I. Characterisation of the heterogeneous degradation kinetics

https://doi.org/10.1016/S0141-3910(00)00123-3Get rights and content

Abstract

Photo-oxidation kinetics of thick isotactic polypropylene films have been compared to thermal-oxidation kinetics of thin films, and noticeable differences have been found. The non-classical kinetic trend of the former can be described as a three step evolution: a typical induction/autoacceleration POOH build-up at the beginning, an intermediate slower POOH content increase and, finally, a gentler POOH increase, which can be better described by quadratic functions of the oxidation time than by a linear dependence. In addition, a series of oscillations appearing from the beginning of the photo-oxidation are found. This kinetic heterogeneity is suggested to be related to the progression of the oxidation to the inside of the strips. However, the FTIR analysis of the evolution of the POOH band position in both photo and thermal oxidation, enables the observed shape changes to be associated to kinetic stages.

Introduction

The main problem in describing the oxidation kinetics of polyolefins is the well-known heterogeneity of the process, which only recently has been taken into account [1], [2], [3], [4], [5], [6], [7], [8] for developing reliable models in predicting the time evolution of either final or intermediate oxidation products.

The problem is still more complicated if the heterogeneity phenomenon is not only due to the semicrystalline nature of the polymer and, as a last resort in the particular case of polypropylene (PP), to the characteristic isotacticity level, but also due to the aggregation state of the material and the mass and the geometry of the sample involved in the oxidation [9], [10].

This latter feature is specially important in the case of films or strips exposed to the UV–Visible light, where the oxidation products depend strongly on thickness and thus, the actual kinetic stage in each inner layer, on depth. Then, besides the well-studied profile of the oxygenated groups content [11], a profile of pseudo-individual kinetic stages should be expected.

Nevertheless, this expected kinetic heterogeneity in thick films photo-degradation has never been characterised, mainly because of two reasons. First, the photo-oxidation kinetics has been measured as the average build-up of the oxygen-containing groups content evolution in the first moments of the process. This analysis reflects the content evolution of these groups in the more oxidised surface layer [11], [12]. Second, because of the appearance of cracks, a phenomenon which takes place shortly after the beginning of the exposition to the light, and which complicates the analysis because it is responsible for the change in the oxygen pressure affecting the oxidation rate in the bulk.

However, it would be very interesting to describe, as far as possible, the kinetic profile in thick films photo-degradation because this analysis would allow to characterise the macroscopic kinetic milestones, through which the whole thickness of the film develops the degradation. In particular, what is the real induction time of the sample, the precise time for the appearance of cracks and their influence in the build-up shape.

In addition, if the kinetics of the whole thickness is known, the relationship between the kinetic stages of the sample and the physico-chemical processes which influence the main parameters controlling the mechanical properties of the film, can be found out. Part I of this work deals with the former aim while part two is focused on the latter.

A review of some results in the literature from several decades ago to the present time, on the kinetic description of photo-oxidation of films and strips [11], [13], [14], [15], [16] (from 100 to 2000 μm thick), reveals that although the oxidation products build-up can be described by using a classical kinetic scheme of a series of induction and auto-acceleration periods, a more accurate fit of the experimental points suggests the existence of sigma-like oscillations, which follow one another all along the average increasing trend. This is schematised in Fig. 1.

Besides the fact that this phenomenon is noticeable in different author's studies far away in time, it is clear also that the milder the degradation conditions [11] and the higher the thickness [14], [15], the more remarkable the oscillations appear. These observations would support the idea of a kinetic profile related to the spatial advance of the photo-degradation to the inside of the strip.

Section snippets

Polypropylene samples

The two polypropylene samples studied are those used in previous works [17]. Sample PP050 is a commercially available polypropylene kindly supplied by Repsol, which has been purified with the pair solvent/non-solvent (o-dichlorobencene/methanol), at 125°C, with the thermal stabiliser Irganox 1010 and under N2. Sample PP7030 is the polypropylene obtained from PP050 in the sixth fractionation step, corresponding to the volume ratio 70/30 of the pair xylene/2-ethoxyethanol, according to the method

Results

Fig. 4 shows the hydroperoxide (POOH) kinetics corresponding to the photo-oxidation of the PP050. It appears clear that an average trend consisting of three stages can be considered. The first one involves an induction stage of about 24 h and an autoaccelerated increase lasting up to 48 h. Next, the POOH content continues to increase though in a slower way, up to roughly 200 h of exposure leading, finally, to a gentle increase, described by Verdu et al. [11] as pseudo-hyperbolic, which spreads

Discussion

The results obtained in previous works on the spectral evolution of chemiluminescence (CL) emitted during the thermo-oxidative degradation of isotactic [17] and atactic PP [19], allow to account for the evolution of the position of IR bands of both CO and POOH functions, in relation to the kinetic stages of the process.

In those works it was found that, in general, the wavelength of the emitted CL holds its characteristic value along the propagation of the thermo-oxidation, this value reflecting

Conclusion

The POOH kinetic shape of thick films iPP photo-oxidation displays noticeable differences, when compared to the classical sigmoid kinetics obtained in thermo-oxidation and in thin films photo-oxidation. The thick film photo-degradation kinetics shows three successive definite periods, which are: first, a 48 h period in which a typical induction and a quick autoacceleration stages follow one another. Then, a slower increasing POOH build-up up to 200 h and, finally, an autoaccelerated increase

Acknowledgements

We are grateful to the Dirección General de Investigación Cientı́fica y Técnica (DGICYT) for the financial support of this research (MAT97-0682) and to the REPSOL plastics company for its collaboration in supplying the polymer.

Cited by (34)

  • Markedly improved photo-oxidation stability of α form isotactic polypropylene with nodular morphology

    2021, Polymer Degradation and Stability
    Citation Excerpt :

    Base on this work, Heude et al. [53] further proposed that the diffusion and consumption of O2 are crucial factors governing the shape of thermal-oxidation profiles in thicker iPP plates. The oxidation of polymer film is regarded as homogeneous in most studies [54], instead of oxygen diffusion-controlled, because of its limited thickness. In the present work, however, the oxidation extent of iPP-ma is heterogeneous along thickness direction, which is consistent with the characteristic of O2 diffusion-controlled degradation reported in semi-crystalline and amorphous polymers [55,56].

  • Stability of UV Stabilizers

    2020, Handbook of UV Degradation and Stabilization
  • Influence of temperature, UV-light wavelength and intensity on polypropylene photothermal oxidation

    2014, Polymer Degradation and Stability
    Citation Excerpt :

    At this standpoint, it is notable that the introduction of the spectral overlap integral in the Schwarzschild's law enables to describe the oxidative behavior of our iPP in both SEPAP and WOM devices. Various results on presumably free-additive polypropylenes have been also compiled from the literature in order to check this relationship ([1,19,53–55]). However, it was only possible to confirm the trend because of the high scattering of induction periods, which was attributed to various amounts of structural defaults (such as catalysis residues) or the uncertainty on input data – e.g. sources' emission spectra.

  • Course of the changes in thick and thin isotactic polypropylene samples subjected to natural aging

    2013, Polymer Degradation and Stability
    Citation Excerpt :

    Rätzsch et al. highlighted that the crosslinks prevail in the PP below 60°, while above this temperature the β-scission is more important [17]. The experiments of artificial aging are normally conducted under isothermal conditions between 30° and 60° [4,10,11,14,16,18,19]. On the contrary, in the natural exposition in a temperate climate the sample temperature oscillates greatly between the extremes of winter nights and summer sunshine.

View all citing articles on Scopus
View full text