Photooxidation Of Polypropylene Research Articles

Influence of Different Environments and Temperatures on the Photo-Oxidation Behaviour of the Polypropylene.

The photo-oxidation of polypropylene at two different temperatures and in three different environments-air, distilled water and sea water-has been followed as a function of the irradiation time. The photo-oxidation kinetic is dramatically dependent on the amount of oxygen available for the oxidation reactions and on the temperature. While the photo-oxidation is very fast in air, the degradation is much slower in the two aqueous media. The degradation in sea water is slightly slower than in distilled water. In all cases, the degradation kinetic increases remarkably with the temperature. This behavior has been attributed to the lower oxygen availability for the oxidation reactions of the polymers. The light difference of the degradation kinetic between the two aqueous media depends on the small difference of the oxygen concentration at the test temperatures of 40 and 70 °C. At the latter temperature, the difference between the degradation kinetic in distilled water and sea water is still less important because increasing the temperature decreases the solubility of the oxygen, and it tends to became very similar in both samples of water.

Optimized design of environmentally-friendly polydopamine nanoparticles for the stabilization of both thermo- and photo-oxidation of polypropylene: Size effects

Despite the huge success of commercial stabilizers (antioxidants, UV absorbers, etc.) on the functionality enhancement and lifetime prolonging of polymers, their toxicity on the environment has been raising severe concerns. Therefore, producing environmentally benign stabilizers via green chemistry has been attracting researchers’ attention. In this work, we demonstrated the potential of mussel-inspired polydopamine nanoparticles (PDA-NPs) with intrinsic biocompatibility for the enhancement of both thermo- and photo-oxidation stability of polypropylene (PP). The effects of PDA-NPs size on the antioxidative performance of PP were systematically explored. PDA-NPs with different sizes of 471 nm, 284 nm, and 98 nm were synthesized first by tuning the pH values of initial dopamine solutions. The antioxidation ability of PDA-NPs for PP increased remarkably with the decreasing size, which was attributed to a higher specific area and more available semiquinone structure of smaller PDA-NPs. Compared with pure PP, the oxidation induction time (OIT) and onset thermal decomposition temperature of optimized PP composites can be increased significantly from 9.8 min to 26.0‍‍ min by 165% with the addition of only 0.5 wt% PDA-NP (98 nm), while the thermo- and photo-oxidative degree (formation of carbonyl products) on the surface can be decreased by 53% and 92% at most, respectively. However, the dependence of UV protection ability of PDA-NPs for PP on their size is non-monotonic. As indicated by cross-sectional fluorescence and modulus imaging, the shallowest degradation depth was observed for PP composites with the PDA-NP size of 284 nm other than that of 98 nm. These results have demonstrated the feasibility of PDA-NPs as robust stabilizers and the importance of size design for optimizing the anti-aging efficiency of PDA-NPs, providing valuable insights into the development of multifunctional stabilizers for environmental sustainability.

A rapid and highly sensitive evaluation of polymer composite aging with linear correlation to real-time aging

Evaluation of polymer aging is very important for the long-term performance of polymer materials, but it remains a challenge to correlate accelerated evaluations with the real-time procedures. Here we develop a novel in-situ aging evaluation system for rapid and sensitive aging evaluations of polymer materials within hours under multiple environmental conditions. It is carried out by in-situ detecting the generation rate of trace gaseous degradation products, e.g. CO2, of polymer materials in a specially designed reaction cell during aging under environmental conditions with various UV irradiation, temperature and humidity. The advantages of this system were demonstrated by applying to evaluate the photo-oxidation of polypropylene (PP)–CaCO3 composites, including stability evaluation, aging status analysis, aging kinetics measurements and study on effects of UV irradiation intensity and humidity. The CO2 generation rate of PP-CaCO3 composites measured in this system is well correlated to carbonyl indices during 120-day natural weathering. A linear relationship was observed between the generation rate of CO2 and the natural logarithm of the carbonyl index. The activation energy of the photo-oxidation of PP-CaCO3 composites was calculated based on generation rates of CO2 at different temperatures in the range of 30–80 °C. The increase of UV irradiation intensity and humidity both enhanced the generation rate of CO2 of PP composites, and the presence of CaCO3 fillers promoted the sensitivity of PP photo-oxidation to both of UV irradiation intensity and humidity. This study provides a new approach to rapid and highly sensitive evaluation of polymer composite aging under multiple environmental conditions.

Chemiluminescence studies on the photooxidation of isotactic polypropylene

Photooxidation of polypropylene (PP) was investigated by chemiluminescence (CL) after UV irradiation of main wavelengths of 254 nm and 365 nm in air at an ambient temperature. From the kinetic analysis of CL intensity change just after irradiation, CL intensity at the time of the cessation of UV irradiation, (I(t=0)), was obtained. The irradiation time dependence of I(t=0) gave a time-course, where the CL intensity increased with time in the first stage and reached a steady state, and after maintaining the same level for a while, it largely increased in the later stage. The steady state intensity, which is considered the rate of peroxy radical formation, was found to be dependent on the crystallinity of PP. The lower the crystallinity is, the higher the steady state intensity is, indicating the preferential photoinitiation reaction to occur in the amorphous phase. The isothermal CL measurement at above the melting point of the samples, which were stored at 25 °C for various periods after UV irradiation, suggests that there are two types of CL emitting sources of different lifetimes; one is quenched in about one day and the other survives for more than 10 days. The temperature rising measurement also indicates the existence of two types of CL emitting sources, too. Further, thermal oxidation before UV irradiation was found to clearly promote the photooxidation. In contrast, thermal oxidation in oxygen was increasingly accelerated by the pre-photooxidation.

Influence of the morphology of ZnO nanomaterials on photooxidation of polypropylene/ZnO composites

In this study, the effect of zinc oxide (ZnO) morphology on photooxidation of polypropylene (PP)/ZnO nanocomposite films was analyzed by characterizations of their structure, morphology and properties. PP/ZnO nanocomposite films containing 0.25, 0.5, 0.75 and 1.0 weight percentage (wt%) of quasi-spherical (0D-ZnO) and rods (1D-ZnO) ZnO particles were prepared by melt processing followed by compression molding and they were exposed under xenon-arc light irradiation for different durations. Photooxidation process of these nanocomposites was monitored by the presence of oxidative products as well as the weight loss observed using infrared spectroscopy (FTIR) and gravimetric analysis respectively. Surface morphology of PP/ZnO nanocomposite films was analyzed by scanning electron microscopy (SEM). Molecular weight of the polymer was used to observe the extent of cleavage promoted by ZnO morphology in the polymer chains. The morphology of ZnO particles showed an important influence on photooxidation of PP/ZnO nanocomposites. A catalytic effect was observed in the photooxidation of PP nanocomposites with 0D-ZnO by an increase in the carbonyl index, weight loss and erosion of the film surface compared to the pristine PP. The 1D-ZnO nanostructures had a UV-screening effect inhibiting the photooxidation of PP and consequently the rupture of polymer chains during longer periods of time with fewer imperfections on the surface. The results obtained show the effects of the semiconductor nanomaterial concentration and morphology on photo-oxidation of polymer-semiconductor nanocomposites, in this case ZnO particles on PP/ZnO nanocomposites.

Is carbonyl index a quantitative probe to monitor polypropylene photodegradation?

For several decades, authors relied upon the carbonyl index to monitor the photo-oxidation of polypropylene and to account for the degradation of the mechanical properties. This paper starts from one question: is the carbonyl band at 1712 cm−1 in infrared spectroscopy really appropriate to quantitatively measure the extent of oxidation of polypropylene? This article brings a negative answer to this question, and the results given in this paper suggest that carbonyl detection by infrared spectroscopy, despite it has been used for years, is not the absolute probe to monitor the photooxidation of polypropylene. Hence, the article aims to provide new relevant and quantitative criteria. These criteria allow the photo-oxidation of PP to be correlated with properties degradation, the one of interest in this case being the gloss loss, which reflects the loss of the surface mechanical properties. These criteria give an early warning of the degradation and permit anticipating the loss of polypropylene functional properties. There are various criteria that can be proposed, the methyl band at 1456 cm−1 being the easiest way to measure the oxidation. It is shown that the modifications of the methyl absorbance fit quite well the increase of crystallinity, molecular weight, and it quite-well reflects the micro-hardness, this last parameter being well correlated to the loss of gloss.

Measurement of peroxides in the volatile degradation products of polypropylene photooxidation

Like many polymers, polypropylene releases volatile degradation products as it oxidizes. Commercially available polypropylene was investigated for the release of volatile species with peroxidic activity upon exposure to ultraviolet radiation at room temperature by a wet-collection and derivatization method. While this method measures the collective nature of volatile products, they could consist of hydrogen peroxide and small hydroperoxides, peroxides, and peroxyacids, as this approach is sensitive to all such species. After an induction period, the rate of release of peroxides was found to increase to a steady value. These results are discussed in relation to the kinetics of the formation of another product of oxidation, carbonyls, a comparison that suggests that the volatile peroxides may be produced from the products of the photolysis of hydroperoxides formed in the polymer during oxidation.

Fourier Transform Infrared (FT-IR) Imaging Coupled with Principal Component Analysis (PCA) for the Study of Photooxidation of Polypropylene

Fourier transform infrared (FT-IR) imaging coupled with principal component analysis (PCA) is used to characterize the photooxidation of polypropylene (PP) and identify the photooxidative products at different oxidation times. PP slices were exposed to ultraviolet (UV) irradiation for times up to 60 hours and spatially resolved spectra were acquired with a transmission FT-IR imaging system in order to view the steric inhomogeneity of the photooxidation process of PP. The evolution of the oxidized products with irradiation time is shown through the application of PCA. Carboxylic acid is the major oxidized product in the initial period from 0 h to 8 h while ketone becomes the major product with the increase of irradiation time. Carboxylic anhydride is identified for the first time to our knowledge in oxidized PP after 16 h irradiation. Carboxylate ester is also observed in the oxidized PP after 32 h irradiation. Possible mechanisms forming these products have been discussed.

Role of β-cyclodextrin nanosponges in polypropylene photooxidation

The interactions between β-cyclodextrin nanosponges and two different UV stabilizers (namely, 2-hydroxy-4(octyloxy)-benzophenone and triphenyl phosphite) in the photooxidation of polypropylene exposed to UV light have been investigated. A significant decrease of the oxidation induction time (OIT) has been observed in presence of β-cyclodextrin nanosponges. On the contrary, the combination of the above nanosponges with 2-hydroxy-4(octyloxy)-benzophenone turned out to be very efficient and advantageous since a remarkable increase of OIT has been registered, namely a threefold increase of polypropylene OIT. This result has indicated a possible synergistic effect between the two species due to the occurring of some chemical or physical interactions, verified by comparing the experimental results and data calculated on the basis of simple additivity rule. Some hypotheses on the interaction mechanism between the two species have been given. Conversely, no effect has been found for the triphenyl phosphite–nanosponge pair.

Photooxidation of polypropylene/layered double hydroxide nanocomposites: Influence of intralamellar cations

The influence of layered double hydroxides (LDHs) on the photooxidation of polypropylene (PP)/LDH nanocomposites was studied under irradiation at long wavelengths ( λ > 300 nm, 60 °C and in the presence of oxygen). The influence of hybrid LDHs containing different divalent cations (Mg, Zn or both Mg and Zn) on the photooxidation mechanism of PP and on the rates of oxidation of the matrix was characterised based on infrared analysis. The presence of LDHs modifies the photoproducts accumulating in the PP and the rates of oxidation of PP were changed depending on the divalent cations in the LDH layers. Whereas natural clays, such as montmorillonite (MMt), can lead to a faster degradation of the materials, LDHs (Zn 2–Al–DS, for example) appear to have no inductive effect on polymer oxidation.

Photochemical behaviour of fire-retarded polymers

The influence of a non-halogenated intumescent fire retardant on the photooxidation of polypropylene is reported. The photooxidation of polypropylene stabilised with a phenolic antioxidant and two redox antioxidants (HALS), without and with the flame retardant has been studied. The chemical modifications resulting from UV-light exposure with wavelengths above 300 nm in the presence of oxygen were followed by IR and UV–visible spectroscopies. Special attention was given to the influence of each component on the rate of oxidation of the polymeric matrix. The photooxidation of the fire-retarded polymer can be described by two independent phenomena: the photooxidation of the intumescent agent and the photooxidation of the polymer. The results obtained offer new insight in the formulation of stabilised fire-retarded PP for outdoors applications.

Influence of nanodispersed hydrotalcite on polypropylene photooxidation

Nanocomposites containing hydrotalcite and prepared by melt compounding with polypropylene were UV-light irradiated in artificial accelerated conditions representative of solar irradiation ( λ > 300 nm) at 60 °C in air. The chemical modifications resulting from photooxidation were followed by IR and UV–visible spectroscopies. The presence of hydrotalcite was shown to change the global rate of photooxidation of polypropylene by reducing the oxidation induction time and increasing the oxidation rate. The differences of the oxidation induction time disappeared after solvent extraction of the antioxidant. They were attributed to a quenching of the antioxidant activity resulting from a migration onto the filler surface induced by the preferential interaction with the polar hydrotalcite. Extracting the antioxidant did not change the oxidation rate at the permanent regime. The increase of the oxidation rate was attributed to transition metal ions, present as impurities in hydrotalcite, which can accelerate the oxidation of the polymer by various mechanisms including a catalysed decomposition of hydroperoxides.

Effect of stabilizer on scission and crosslinking rate changes during photo-oxidation of polypropylene

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of polypropylene samples exposed to ultraviolet irradiation (UV). Results for rubber-toughened polypropylene (PP) containing no photostabilizer are compared with those for the same polymer stabilized using a commercial photostabilizing package (PPS). The samples were in the form of 3 mm thick bars and measurements were obtained at various depths from the exposed surface after different exposure times. The depth profiles for PP and PPS were very different. Reaction rates in the interior of PP showed oxygen diffusion limited behaviour and after prolonged exposure, the rates in the interior of PPS were higher than those in PP. The ratio of scission rate/crosslink rate fell when reaction rate increased. Crosslinking became relatively more likely when reaction rates were low. The low degradation rates obtained with stabilized polymer coupled with the sensitivity of the method of analysis enabled detection of inhibition of photodegradation attributed to residual moulding stresses in the samples.

Influence of nanodispersed boehmite on polypropylene photooxidation

Nanocomposites containing pure or organically modified nanoboehmites of different sizes were prepared by melt compounding with polypropylene. The samples were UV light irradiated in artificial accelerated conditions representative of solar irradiation ( λ > 300 nm) at 60 °C in air. The chemical modifications resulting from photooxidation were followed by IR and UV–visible spectroscopies. The presence of pristine nanoboehmites was shown to change the rate of oxidation of polypropylene by reducing the oxidation induction period due to the presence of residual processing antioxidant. The differences of the oxidation induction periods between the nanocomposites and the pristine polymer disappear after solvent extraction of the antioxidant. The inefficiency of traditional antioxidant in retarding the photooxidation of polypropylene containing nanodispersed boehmite is proved. Antioxidant migration to the boehmite surface induced by the preferential interaction with the polar filler is proposed as an explanation. The oxidative behaviour of the organically modified boehmites was shown to depend on the type of organic substituent. p-Toluenesulfonate reduces the adsorption of antioxidants while the presence of a long-chain alkyl benzensulfonate increased the oxidation rate by generation of radical initiators.

Submicrometric characterization of the heterogeneous photooxidation of polypropylene by microthermal analysis

Photoageing of polymeric materials is known to induce physical and chemical modifications. Until recently the analysis of such modifications was realized in the bulk. The objective of this study is to quantify at a submicrometric level the physical heterogeneity induced by the photooxidation of polypropylene. In a first step, the impact of the photooxidation on the physical and chemical properties are quantified by bulk analysis (DSC and FTIR experiments) of a “homogeneous” photoaged material. In a second step, depth profile experiments are performed by microthermal analysis (μ-TA) and FTIR microscopy (μ-IR). Our main finding is to show that the physical properties of the material can be defined with a sub-micrometric resolution by μ-TA experiments, resolution which cannot be achieved with the usual spectrophotometric measurements.

A new polymeric HALS: preparation of an addition polymer of DGEBA–HALS and its photostabilizing effect

A linear epoxide–amine type of a new polymeric HALS was prepared by an addition polymerization of 2,2-bis(4-glycidyloxyphenyl)propane and 4-amino-2,2,6,6-tetramethylpiperidine (ATMP) in chlorobenzene. Two types of addition polymers, high molecular weight HALS (HMH) and low molecular weight HALS (LMH) were isolated by precipitation in acetone. The structure of HMH was characterized from the NMR and MALDI-TOF spectra. The effects of the reaction conditions on the yield of HMH and LMH were observed by varying the reaction conditions such as monomer concentration, reaction temperature, and reaction time. The yield of HMH increased with the amount of the ATMP concentration, reaction temperature, and reaction time. HMH is fairly miscible with polypropylene (PP), polystyrene (PS), and styrene–butadiene rubber. The photostabilizing effect of HMH on the photooxidation of PP and PS at 254 or 310 nm was studied from the UV and IR absorption spectral changes. The results indicate that the photostabilization effect of HMH was similar to or better than that of Cyabsorb UV-3529, a commercially available polymeric HALS.

Effect of tensile stress on chain scission and crosslinking during photo-oxidation of polypropylene

Scission and crosslink concentrations have been determined in polypropylene samples using gel permeation chromatography molecular weight distributions and computer simulations. Measurements have been made at different depths within injection moulded bars 3 mm thick after 4 weeks ultraviolet (UV) exposure. The scission and crosslink concentrations were found to be 0.025 and 0.006 mol/kg respectively at the surface and 0.004 and 0.001 mol/kg respectively at the centre. The values at the unexposed face were similar to those at the exposed face. Samples exposed to UV while loaded in uniaxial tension (10 MN/m 2) showed markedly increased chain scission but little change in the crosslinking rate.

Surface Photo-Stabilization of Polypropylene by the Use of Different Stabilizers

The surface photo-oxidation of polypropylene in presence of different three antioxidants such as Sanduvor PR-31, Sanduvor 3944 and Tinuvin 770 has been studied by high resolution ESCA. The results show that the additives are effective in protecting also the surface of the material and that, within the three, Sanduvor PR-31 seems to perform better than the others in terms of decreasing the level of the oxidation after the aging.

Photooxidation of fire retarded polypropylene. I. Photoageing in accelerated conditions

The influence of fire retardant additives (decabromodiphenylether, antimony oxide) on the kinetic aspects of the photooxidation of polypropylene is reported. The photochemical oxidation of 60 μm thick films containing different amounts of fire retardant additives was studied and compared with that of pure polypropylene. Monitoring the variations of the infrared carbonyl absorption permitted a comparison of the oxidation curves. A significant reduction of the induction period was observed but, surprisingly, no important variations of the apparent oxidation rate were noted. This latter result was related to the heterogeneous distribution of the oxidation photoproducts in the fire retarded sample. The heterogeneity of the oxidation was resulting from the strong absorption of the flame retardant additives in the range 300–350 nm. The oxidation kinetics monitored in the case of the fire retarded films were then apparent curves that had to be corrected in order to take into account the oxidation profiles. Moreover, considering the high content of fire retardant (12–47% by weight) within the samples, the oxidation curves had to be corrected in order to compare the oxidation of similar concentrations of polymer. On these bases, the influence of the fire retardant on the oxidation kinetics was studied in the case of thin films (thickness near to 10 μm) in order to limit the heterogeneity of the photochemical reactor constituted by the polymeric sample, and the oxidation kinetics were compared by plotting the variation of the ratio OD 1713/ c polymer with the irradiation time. This gave evidence that the oxidation rates increased dramatically in the presence of the fire retardant. Moreover, a significant increase of the rate of oxygen consumption was observed in the case of the fire retarded sample. The oxidation rates were however found to be relatively constant for contents of fire retardant higher than 5%.

Evaluation of some amino- S-triazines and their N-oxyl-derivatives as thermal and UV stabilisers in polypropylene films

Three novel 2,4,6-tri- i-propyl, cyclohexyl and phenyl amino-1,3,5-triazines have been synthesised along with their corresponding N-oxy-derivatives and their effect on the thermal and photo-oxidation of polypropylene examined. The results were compared to samples stabilised with MX [Irganox 3114 (I-3114), 0.1%; Chimassorb 944 (CH-944), 0.1% w/w] and with a 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) (0.01% w/w)+MX, respectively. The hindered phenolic antioxidant, I-3114, was used for base stabilisation of the polymer. The amine triazine derivatives are not efficient stabilisers when added to I-3114, but in the presence of MX (I-3114/CH-944) the thermal stability of the polypropylene (PP) samples with the amine reflects a synergistic effect. Adding an N-oxidised amine to the mixture of stabilisers appears to decrease the oxidation rate with regard to samples containing the same amines in their non-oxidised form, but here the success of the stabilisation depended on the stability of the chain-breaking acceptor which in turn appeared to depend on the nature of the amine and its chemical structure. The N-oxylisopropylamine is less stable than its corresponding amine and consequently the thermal stability is lower. Samples with the N-oxylcyclohexylamine derivative increased stability compared to samples with the amine. This indicates that the thermal stability of PP films can be improved by adding a small amount of some amines in their oxidised form. These amines have no significant effect in controlling the oxidation promoted by UV irradiation. Compared to the stabilisation time of samples with MX, only 43% of that stability is obtained when any of these amines is mixed with Irganox 3114. When the N-oxy-derivatives are added to MX to stabilise PP films the improvement in stabilisation is synergistic. Even at high oxidation levels CI 1714>0.06 the double bond absorbances are practically constant. This double bond pattern is also observed in samples with a lower stability as in samples stabilised with amines and I-3114.