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.     

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.     

Photooxidation of vulcanized EPDM/montmorillonite nanocomposites

The photooxidation of a vulcanized ethylene-propylene-diene monomer (EPDM)/montmorillonite nanocomposite as well as EPDM/nanocomposite with stabilizers was studied under accelerated UV-light irradiation (λ ≥ 300 nm, 60 °C) for different times. The development of functional groups during oxidation was monitored by infrared spectroscopy. Photodegradation of the neat polymer and composites took place and the increases of absorbance in hydroxyl and carbonyl groups with irradiation times and also the decreases of the EPDM unsaturations were measured. The data indicated that the photooxidation products were not changed in the presence of the nanofiller. However, the presence of MMt was observed to dramatically enhance the rate of photooxidation of EPDM with a shortening of the oxidation induction time, leading to a decrease of the durability of the nanocomposites. On the other hand, it was observed that addition of stabilizers, either Tinuvin P or 2-mercaptobenzimidazole, was efficient in inhibiting the degradative effect of MMt.     

The effects of annealing in inert atmospheres and of oxygen concentration on chemiluminescence from polypropylene

It has been shown that heating polypropylene powder under a nitrogen atmosphere leads to the significant prolongation of the oxidation induction time measured by chemiluminescence in oxygen at 130 and 140 °C. While heating in nitrogen from 0 to 4 h at 140 °C leads to the linear increase of oxidation induction time, the maximum chemiluminescence intensity I_stat increases with the time of sample annealing until 2 h; then it starts to decay. The different and sometime unknown thermal history of the sample may thus explain the scatter of induction times of oxidation observed with different PPs whether they be pure or stabilised. Maximum chemiluminescence intensity plotted vs. concentration of oxygen in the surrounding atmosphere at 130 and 140 °C also increases linearly; however, this does not correspond with very small reduction of oxidation induction time. The four-parametric “master equations” used in our earlier papers were applied to fit the chemiluminescence runs both in oxygen and in nitrogen. The equation operates with the rate constants of hydroperoxide decomposition and oxidation spreading but at the same time, it takes into account the possible effect of oxidation products on decomposition of hydroperoxides.     

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 (Zn₂-Al-DS, for example) appear to have no inductive effect on polymer oxidation.     

Polymer/carbon nanotube nanocomposites: Influence of carbon nanotubes on EVA photodegradation

The influence of carbon nanotubes on the photodegradation of EVA/carbon nanotube nanocomposites was studied by irradiation under photooxidative conditions (at λ > 300 nm, at 60 °C and in the presence of oxygen). The influence of the nanotubes on both the photooxidation mechanism of EVA and the rates of oxidation of the matrix was characterized on the basis of infrared analysis. On one hand, it was shown that the carbon nanotubes act as inner filters and antioxidants, which contribute to reduction in the rate of photooxidation of the polymeric matrix. On the other hand, it was shown that light absorption could provoke an increase in the local temperature and then induce the photooxidation of the polymer. The competition between these three effects determines the global rate of photooxidation of the polymeric matrix. Several factors are involved, the concentration of the carbon nanotubes, the morphology of the nanotubes and the functionalization of the nanotube surface.     

Influence of MWNT on Polypropylene and Polyethylene Photooxidation

Summary: Nanocomposites containing multiwall carbon nanotubes (MWNT) were prepared by melt compounding with polypropylene and polyethylene. 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 FT-IR spectroscopy.The behaviour of nanocomposites containing MWNT depends from the type of polymer and presence or absence of antioxidant. The presence of MWNT affect photooxidation of the two polymers working as an UV-screener on the other hand the increase of thermal oxidation provoked by conversion of photon energy into thermal partially balance the decrease of positive effects on oxidation rate. The negative effects can be minimized by the presence of antioxidants that show interesting synergistic effects with MWNT.     

Loss of stability by migration and chemical reaction of Santonox R in branched polyethylene under anaerobic and aerobic conditions

Plaques of branched polyethylene stabilized with 0.1 wt.% 4,4′-thiobis(6-tert-butyl-3-methylphenol) [Santonox^® R] were aged at different temperatures between 75 and 95 °C in anaerobic (nitrogen or water) and aerobic (air or water saturated with air) media. Antioxidant concentration profiles were obtained by oxidation induction time (OIT) measurements using differential scanning calorimetry. Results obtained by high performance liquid chromatography of extracts confirmed that the gradual decrease in OIT with increasing ageing time was due to migration of antioxidant to the surrounding medium. The antioxidant concentration profiles along the plaque thickness direction were flat in the plaques aged in the non-aqueous media indicating that the migration of antioxidant to the surrounding medium was controlled by the low evaporation rate at the material boundary. Crystals of antioxidant were detected by optical microscopy on the samples exposed to nitrogen. The similarity of the antioxidant concentration profiles obtained after ageing in nitrogen and in air suggested that the fraction of the antioxidant oxidized is negligible in comparison with the loss of antioxidant by migration to the surrounding media. The antioxidant concentration profiles along the plaque thickness direction obtained after ageing in water were less flat, suggesting faster dissolution in the water phase than evaporation in the case of non-aqueous ageing. The antioxidant diffusivity could be determined from the aqueous experiments and was in reasonable agreement with data reported by Moisan. For the samples exposed to water, the loss of antioxidant was faster from the samples exposed to water saturated with air. This difference is attributed to a faster degradation of the antioxidant in the oxygen-containing water phase increasing the mass transport from the polymer phase boundary to the water phase.     

Intercalation of hydrotalcites with hexacyanoferrate(II) and (III)—a thermoRaman spectroscopic study

Raman spectroscopy using a hot stage indicates that the intercalation of hexacyanoferrate(II) and (III) in the interlayer space of a Mg, Al hydrotalcites leads to layered solids where the intercalated species is both hexacyanoferrate(II) and (III). Raman spectroscopy shows that depending on the oxidation state of the initial hexacyanoferrate partial oxidation and reduction takes place upon intercalation. For the hexacyanoferrate(III) some partial reduction occurs during synthesis. The symmetry of the hexacyanoferrate decreases from O_h existing for the free anions to D_3d in the hexacyanoferrate interlayered hydrotalcite complexes. Hot stage Raman spectroscopy reveals the oxidation of the hexacyanoferrate(II) to hexacyanoferrate(III) in the hydrotalcite interlayer with the removal of the cyanide anions above 250 °C. Thermal treatment causes the loss of CN ions through the observation of a band at 2080 cm⁻¹. The hexacyanoferrate (III) interlayered Mg, Al hydrotalcites decomposes above 150 °C.     

Modelling light stabilizers as thermal antioxidants

Light stabilizers often display some degree of antioxidant activity against thermal degradation of polymers both in the solid state and the melt. Although this capacity to date has been documented in some instances such features have not been kinetically modelled for many light stabilizers. An understanding of the mechanisms of this activity is crucial in polymer materials due to the close link between prior thermal behaviour and post stabilisation.This paper considers the potential antioxidant activity of three representative UV stabilizers using a model system initiated (2,2′-azo-bisisobutyronitrile, AIBN) cumene oxidation. Kinetic measurements of oxidation rates in the presence of the stabilizers showed that the antioxidant activity as well as the mechanism and mode of inhibition was different for each of the stabilizers. Thus, whilst a triazine UV absorber (Cyasorb UV 1164) did not display any antioxidant activity, a hindered phenol (Cyasorb UV 2908) operated as a peroxy radical acceptor, and a hindered amine (Cyasorb 3529) retarded the model reaction without an induction period like many HAS stabilizers.The Cyasorb 2908 revealed weak antioxidant activity with a rate constant for the addition of cumylperoxy RO₂ radicals to the functional group of the stabilizer k₇ = 10^6.2±0.1 e^{−(3900±600)/RT}, however, the inhibition index f (80 °C) is significantly higher than that of the commercial phenolic antioxidant Irganox 1076. Oxidation rate profiles in the presence of Cyasorb 3529 displayed a strong retarding activity by the stabilizer under conditions of the model experiments. The rates were found to depend linearly on the reciprocal square root of the concentration of the stabilizer over a sufficiently wide range thereby fitting the mechanism for the addition of cumylalkyl R radicals to the Cyasorb molecules. The rate constants for the addition of cumyl R radicals to the Cyasorb were determined to be k_(333-353 K) = (2.0 ± 0.8) × 10⁸ M⁻¹ s⁻¹. This value surpasses even the rate constants for other related HAS Chimassorb [Zeynalov EB, Allen NS. Effect of micron and nano-grade titanium dioxides on the efficiency of hindered piperidine stabilizers in a model oxidative reaction. Polym Degrad Stab 2006;91(4):931-9.] stabilizers and it follows that Cyasorb 3529 is a powerful retarder of thermal oxidation.     

The use of inverse gas chromatography to study surface thermal oxidation of polypropylene

Thermo-oxidative effects on the surface energy of polypropylene were measured by inverse gas chromatography as a function of exposure time and temperature. Unaltered polypropylene had a surface energy of 33 mJ/m². Oxidized polypropylene, after exposure to air at temperatures of 100 °C and 110 °C, had a range of maximum surface energies from 38 to 41 mJ/m². Comparisons between FTIR carbonyl peak growth and the surface energy showed that both methods detect oxidation, though the increase in surface energy is detected before the carbonyl peak growth is noticeable. The work of adhesion predicted by the surface free energies obtained in this work between a coated calcium carbonate and polypropylene changes by 10% due to the oxidation of the polymer at 110 °C.     

Preparation and crystallization behaviour of PP/PP-g-MAH/Org-MMT nanocomposite

The melt-direct intercalation method was employed to prepare polypropylene (PP)/maleic anhydride grafted polypropylene (PP-g-MAH)/organic-montmorillonite (Org-MMT), X-ray diffractometer was used to investigate the intercalation effect and crystallite size in composites and TEM micrograph to observe the dispersion of Org-MMT interlayers in polypropylene. The results showed that by introducing maleated polypropylene in PP/Org-MMT composite, macromolecule segments had intercalated into interlayer space of Org-MMT. As a result, Org-MMT interlayers were dispersed evenly in polypropylene and PP/PP-g-MAH/Org-MMT nanocomposite was synthesized. The crystallite size of nanocomposite perpendicular to the crystalline plane such as (0 4 0), (1 3 0), (1 1 1), (0 4 1) is smaller than that of pristine PP, which indicated that the crystallite size of PP in nanocomposite can be diminished by adding PP-g-MAH and Org-MMT in PP. Moreover, the nonisothermal crystallization kinetics of PP and PP/PP-g-MAH/Org-MMT nanocomposite was investigated by differential scanning calorimetry (DSC) with various cooling rates. The Avrami analysis modified by Jeziorny, Ozawa method and a method developed by Liu were employed to describe the nonisothermal crystallization process of these samples. The difference in the exponent n between PP and nanocomposite, indicated that nonisothermal kinetic crystallization corresponded to tridimensional growth with heterogeneous nucleation. The values of half-time, Z_c, F(T) and K(T) showed that the crystallization rate of composites was faster than that of PP at a given cooling rate.     

Effect of oxygen pressure on the oxidation kinetics of unstabilised polypropylene

The thermal oxidation of unstabilised polypropylene films at 80 °C and various oxygen pressures ranging from 0.02 MPa to 5 MPa has been studied by FTIR spectrophotometry (carbonyl growth). The induction time decreases and the maximum oxidation rate increases quasi-hyperbolically when the oxygen pressure increases. The asymptotic behaviour (corresponding to the regime of oxygen excess) is not reached at the highest pressure under study. A kinetic model derived from a classical mechanistic scheme but free of simplifying hypotheses, has been used to simulate the observed behaviour and to determine the elementary rate constants. It is shown that a good simulation of kinetic curves of carbonyl build-up in the whole pressure interval under study can be obtained with a set of physically reasonable rate constant values. The “inverse problem” cannot be, however, totally solved because certain constants are interdependent so that some rate constant values have to be arbitrarily chosen or taken from the literature.     

Chemiluminometric determination of carvedilol in a multi-pumping flow system

In this work a simple, fast, sensitive and selective flow-based procedure for the chemiluminometric determination of carvedilol, a recent non-cardioselective β-blocker with noteworthy antioxidant activity, is proposed. The developed methodology takes advantage of the antioxidant capacity of carvedilol to inhibit the chemiluminescence response resulting from the oxidation of luminol by hypochlorite, by acting as a hypochlorite scavenger. The analytical process was implemented in a multi-pumping flow system that employs multiple solenoid actuated micro-pumps as the only active components. These acted as solution insertion, propelling and commuting units assuring an easily controlled, low cost, compact and reliable analytical system.A linear working range for carvedilol concentrations ranging from 1.2 × 10⁻⁷ to 3.0 × 10⁻⁶ mol l⁻¹ (r > 0.999, n = 6), was obtained, with a detection limit of 8.7 × 10⁻⁹ mol l⁻¹. The system handles about 65 samples per hour yielding precise results (R.S.D. < 1.3%, n = 10). Recoveries within 95 and 104% were obtained.     

The role of specific nucleation in polypropylene photodegradation

The paper focuses on the effects of polymorphism on photodegradation of isotactic polypropylene. The starting polymer was modified by a specific α-nucleating agent, 1,3;2,4-bis(3,4-dimethylbenzylidene)sorbitol, by a specific β-nucleating agent, N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide, or their combination. Samples prepared by compression moulding were then exposed to UV-irradiation in the interval from 0 to 240 h. The differences in morphology were reflected in different photooxidative behaviour. Infrared spectroscopy showed that neat polypropylene was the most sensitive to photooxidation and the sample modified solely by the β-nucleating agent was the least sensitive. The remaining two samples exhibited an intermediate sensitivity. Differential scanning calorimetry revealed that the UV-exposure led to gradual changes in crystallization mechanism specifically asserting in individual materials. This behaviour was ascribed to homogeneous nucleation of partly degraded macromolecules. Possible changes of the nucleating agent itself during UV-exposure were also discussed.     

Thermal oxidation and molding feasibility of cycloolefin copolymers (COCs) with high glass transition temperature

Cycloolefin copolymers (COCs) with high glass transition temperature (T_g = 203 °C) have been synthesized and pelletized by extrusion molding. However, their colors change from transparent to yellow during extrusion molding because of thermal oxidation and generation of alkene groups. We have successfully blended several antioxidants (Irganox 1010, Irgafos 168, Irganox HP2225 and Irganox HP2921) into lab-made COCs to avoid the discoloration. The experimental results show that Irganox HP2921 is the best antioxidant among the antioxidants used and can effectively not only suppress thermal oxidation but also eliminate the color stain.     

Polybutene-1 pipes exposed to pressurized chlorinated water: Lifetime and antioxidant consumption

Pipes of isotactic polybutene-1 were pressure-tested in chlorinated water at a controlled pH (6.5 ± 0.1), and the lifetime was assessed as a function of temperature (95-115 °C) and chlorine content (≤3 ppm). These data were compared with data from pressure testing in hot water (0 ppm chlorine). The lifetime shortening in chlorinated water was significant even at relatively low chlorine contents, 0.5 ppm. A further increase in chlorine content led only to a moderate shortening of the lifetime. The temperature dependence of the lifetime data obeyed the Arrhenius law. The activation energy obtained for failure data in chlorinated water was ∼140 kJ mol⁻¹, which was greater than the value of 108 kJ mol⁻¹ earlier reported for failure data from hot-water pressure testing. A 0.5-mm thick layer of material at the inner wall in the fractured pipes showed depletion of the antioxidant system and the inner wall displayed a large number of surface cracks, confirming that there was a pronounced chemical degradation of the inner wall material. The decrease in the antioxidant concentration was independent of the chlorine concentration in the range 0.5-1.5 ppm. The time to reach depletion of the antioxidant system could be predicted by linear extrapolation in an oxidation induction time (log scale)-exposure time (linear scale) diagram.     

Modified cupric reducing antioxidant capacity (CUPRAC) assay for measuring the antioxidant capacities of thiol-containing proteins in admixture with polyphenols

Proteins are not considered as true antioxidants but are known to protect antioxidants from oxidation in various antioxidant activity assays. This study aims to investigate the contribution of proteins, especially thiol-containing proteins, to the observed overall antioxidant capacity measured by known methods. To determine the antioxidant properties of thiol-containing proteins, the CUPRAC method of antioxidant assay using the oxidizing reagent Cu(II)-neocuproine previously used for simultaneous analysis of cystine and cysteine was adopted. While the CUPRAC method is capable of determining all antioxidant compounds including thiols in complex sample matrices, the Ellman method of thiol quantitation basically does not respond to other antioxidants. The antioxidant quantities in the selected samples were assayed with the ABTS and FRAP methods as well as with the CUPRAC method. In all applied methods, the dilutions were made with a standard pH 8 buffer used in the Ellman method by substituting the Na₂EDTA component of the buffer with sodium citrate. On the other hand, the standard CUPRAC protocol was modified by substituting the pH 7 ammonium acetate buffer (at 1 M concentration) with 8 M urea buffer adjusted to pH 7 by neutralizing with 6 M HCl. Urea helps to partly solubilize and denaturate proteins so that their buried thiols be oxidized more easily. All methods used in the estimation of antioxidant properties of proteins (i.e., CUPRAC, Ellman, ABTS, and FRAP) were first standardized with a simple thiol compound, cysteine, by constructing the calibration curves. The molar absorptivities of these methods for cysteine were: ?_CUPRAC = 7.71 × 10³, ?_Ellman = 1.37 × 10⁴, ?_ABTS = 2.06 × 10⁴, and ?_FRAP = 2.98 × 10³ L mol⁻¹cm⁻¹. Then these methods were applied to various samples containing thiols, such as glutathione (reduced form:GSH), egg white, whey proteins, and gelatin. Additionally, known quantities of selected antioxidants were added to these samples to show the additivity of responses.     

Antioxidants determination with laccase

The spectrophotometric method of antioxidants determination using recombinant laccase Polyporus pinsitus (rPpL) and Myceliophthora thermophila (rMtL) was developed. The method includes simultaneous oxidation of the antioxidant and high reactive laccase substrate producing chromophoric radical cation. As laccase substrates ABTS and other high reactive phenoxazine derivatives: 2-phenoxazin-10-yl-ethanol (PET), 3-phenoxazin-10-yl-propane-1-sulfonic acid (PPSA) and 3-phenoxazin-10-yl-propionic acid (PPA) were used. The kinetic data were analysed using a scheme of simultaneous oxidation of the antioxidant and the substrate.In a range of (0.9-7.3) × 10⁻⁶ M of Trolox the measurings recovered 91 and 99% of the antioxidant if ABTS and both laccases were used. The recovery varied between 82 and 124% if phenoxazine derivatives were used. The antioxidant activity determined in rich with antioxidants food samples, i.e. date-palm, black raisin, golden raisin, skin of red grape, dice of red grape, fitted the literature data.     

Ageing of polyethylene at raised temperature in contact with chlorinated sanitary hot water. Part I - Chemical aspects

In France, hot water quality control inside buildings is occasionally assured by disinfection treatments using sodium hypochlorite (between 0.5 and 1 ppm residual free chlorine). This disinfectant is a strong oxidizer and it could interact with metallic and polymer pipes used in hot water systems. To assess the long-term performance of these pipes, it is then necessary to study the impact of these treatments on the material behaviour, in particular for polymeric materials, even at relatively low disinfectant concentrations as used in potable water treatments. The objective of this work was to study the influence of sodium hypochlorite concentration on PERT/Al/PERT (PolyEthylene Raised Temperature) pipe degradation. Pipe samples were filled with chlorinated water solutions (concentrations ranging between 0 and 100 ppm) and maintained in static conditions during 270 days at 70 °C. The antioxidant depletion profile through a PERT wall was monitored using the oxidation induction time (OIT) method, which is a conventional technique of Differential Scanning Calorimetry. Chemical changes on the aged polymer were checked by Infrared Spectrometry. OIT change showed that the PERT stabilizing system was rapidly chemically consumed by the action of chlorinated water at 25 and 100 ppm concentrations. However, PERT degradation was strictly confined to the immediate inner wall. Only a 0.3 mm thick layer (inner part of the pipe) showed significant antioxidant depletion. An increase of the OH and C-O-C infrared bands was also observed on inner part of any samples during ageing which characterize the oxidation of the PERT on the inner wall.