Abstract
The aim of this work was to evaluate the photo degradation of PP films containing photo stabilizers, a ultraviolet absorber and a hindered amine light stabilizer (HALS), and a red pigment. The films were produced by extrusion and exposed to the UV radiation in the laboratory for up to 15 weeks. The results obtained from FTIR, UV-vis and colorimetry showed that the pigment reduced the rate of chemical degradation both in non-stabilized and photo stabilized films, and the influence was more significant for the combination of pigment and HALS. However, the color shift was more evident when the pigment was present, suggesting that this additive may suffer chemical rearrangements during exposure but do not generate free radicals that can initiate degradation of the polymer. The mechanical properties of the films followed the same trend as the other results, but the unexposed films showed a peculiar behaviour, with much higher tensile strength when the pigments were present. X-ray diffraction and DSC analyses suggested that this might be related to differences in crystal structure.
Highlights
Polypropylene is one of the most used plastic materials, with a vast number of applications, varying from general propose items to high performance components
For the neat PP, two absorption bands are relevant: (i) at 222nm which is due to a diene group, and (ii) at 267nm, due to a chromophore triene[26]. Both are supposed to be formed during synthesis and processing, and may act as starting sites for the initiation of chemical degradation during UV exposure[2]
The results of this work showed that when photo stabilizers like Tinuvin 326 (UV absorber) and Chimassorb 2020 (HALS) are present, the lifetime of PP films are increased, especially for the hindered amine light stabilizer (HALS) additive, with a lower rate of degradation and much better mechanical properties after 15 weeks of UV exposure in the laboratory
Summary
Polypropylene is one of the most used plastic materials, with a vast number of applications, varying from general propose items to high performance components. Even though the cost of product rise when stabilizers are added, they are mandadory for the vast majority of polymers. The free radicals generated by the initiation reacts with oxygen, give rise to an autocatalytic mechanism of oxidation that results in both chain scission and the formation of chemical groups like carbonyls and hydroperoxides. This mechanism is very well established in the literature, with many studies involving virtually all types of polymers including polypropylene[1,2]. Other types of additives can act either (i) reducing the rate of degradation by limiting the oxygen diffusion into the product or blocking the passage of ultraviolet light[3] or even acting chemically with the degradation mechanism[4]
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