Oxidation Of Isotactic Polypropylene Research Articles

Acyl-chloride functionalized graphene oxide chemically grafted with hindered phenol and its application in anti-degradation of polypropylene

In this study, graphene oxide (GO) nanosheets were first functionalized by the acyl-chloride reaction and then reacted with hindered phenol (HP) to obtain HP grafted GO (GO-g-HP). The GO-g-HP prepared under the optimized condition exhibited a high efficiency against both thermal and photo oxidation of isotactic polypropylene (iPP). GO-g-HP exhibited a much better anti-aging efficiency than that of HP and GO used individually or that of their physical mixture. The outstanding anti-aging effect of GO-g-HP was attributed to the excellent synergistic effect produced between the chemically bonded GO and HP, which resulted in an improved oxygen barrier property of GO and excellent utilization of the free radical scavenging capability of GO and HP. Also, thanks to the grafting of HP on GO, the physical loss of HP, which was one of the main shortcomings for traditional small molecular antioxidants, was largely suppressed. It is expected that the GO-g-HP prepared can well meet the requirement of long-term applications of polymers in some harsh service conditions.

Effects of graphene on thermal oxidation of isotactic polypropylene

Nanocomposites containing graphene nanoplateletes (GNP) and isotactic polypropylene (PP) were obtained by in situ polymerization. The kinetics of peroxyl radical termination in the obtained nanocomposites was measured by decay of photoinduced chemiluminescence of PP. The concentration of peroxyl radicals was found to decrease significantly, and the rate of their decay was found to increase substantially in the presence of GNP. This shows that GNP is able to inhibit radical processes. At the same time, the additives of GNP were found to act as an effective catalyst, significantly decreasing oxidation induction times and increasing the rates of nanocomposite thermal oxidation. The treatment of GNP by ultrasound can reduce the catalytic activity of GNP, as a result of which the inhibitory effect of GNP becomes predominant. Possible mechanisms of catalysis and inhibition involving GNP are discussed.

γ-Initiated Oxidation of Isotactic Polypropylene: Effect of γ-Dose Rate on Durability

The effect of γ-irradiation dose rate on the chemical, molecular, and mechanical properties of isotactic polypropylene (iPP) films was investigated using three different 60Co sources with dose rates of 600, 660, and 780 Gy h−1. Fourier transform infrared spectroscopy coupled with chemical derivatization with NO and SF4, and measurements of the elongation at break, toughness, and molecular weight were used to describe the effects of γ-dose rate on iPP. It was found that the formation of oxidation products depended significantly on the dose rate. However, the kinetic accumulation of ketones were more sensitive to the dose rate of gamma source. Degradation induced by γ-radiation depends on the dose rate—the degradation of mechanical properties was more significant at a low dose rate than at a high dose rate. It was found that the lower the dose rate, the higher the degree of oxidation in terms of the formation of γ-products. The mechanism of γ-degradation is discussed based on the correlation between the changes in chemical and physical properties upon exposure to γ-radiation.

The effect of some thiosemicarbazide compounds on thermal oxidation of polypropylene

The effect of some 1–4 disubstituted thiosemicarbazide compounds (TSC), including metal complexes, on thermal oxidation of isotactic polypropylene, has been studied, based on the knowledge that TSC contain various chemical groups such as CS, –NH– that act as radical scavengers. All compounds exhibit antioxidant properties that represent around half of the effectiveness of Irganox 1076 (a strong phenolic antioxidant) during the induction period and around a quarter of this during the propagation period. The influence of X- atom (as a substituent in the 1st position) decreases in the following order: H > Cl > Br. R-group (as a substituent in 4th position) seems to exhibit lower influence than X (R = cyclohexyl, or phenyl ethyl). TSC complexes with Ni are more effective antioxidants as compared to TSC ligands. The activation energy of oxidation of isotactic polypropylene (i-PP) in the presence of Ni-complexes is very low as compared to the values found in the case of those free of additive i-PP or with other metal complexes. A compensation effect seems to be involved in these cases. A synergistic effect has been observed in the case of Ni(A2) 2–Irganox 1076 couples, while with similar A5 and A6 couples, additive and antagonistic effects were observed, respectively.

Oxidation fronts in polypropylene as studied by imaging chemiluminescence

The oxidation of isotactic polypropylene (PP) films containing three different commercial stabilisers has been studied at 150°C in air using imaging chemiluminescence (ICL). The oxidation was initiated using unstabilised PP powder, UV-irradiation or copper. It was shown that the initiated local oxidation spread as a propagating oxidation front in the stabilised PP films. By comparing the speed with which the propagating oxidation front spread, the performance of different stabilisers could be assessed. Furthermore, the influences of temperature, antioxidant concentration and sample thickness on the front speed were compared. The advantage of this technique is a controlled initiation of the oxidation that minimises the influence of weak points in the films. Furthermore, the experimental time is considerably shortened. The position of the oxidation front has been monitored using both ICL and infrared-microscopy. It was concluded that the technique is effective when evaluating stabiliser performance. Furthermore, it allows the study of oxidation phenomena such as oxidative spreading. The technique was also applied to polyamide 6, however, in this polymer the oxidation did not show the same spreading behaviour.

Chemiluminescence from stabilised polypropylene. The effect of annealing on the induction time of oxidised polypropylene stabilised with Irganox 1010 and Irganox 1076

Chemiluminescence under isothermal and non-isothermal conditions has been used for a more detailed study of the oxidation of isotactic polypropylene stabilised with Irganox 1010 and Irganox 1076. Annealing of the stabilised films of polypropylene at lower temperatures prolongs the induction time of oxidation measured at higher temperatures. Arrhenius plots of induction time over a wide temperature interval have been derived from non-isothermal chemiluminescence runs.

Efficiency assessment of additives in thermal degradation of i-PP by chemiluminescence I. Triazines

In this paper the kinetics of thermal oxidation of isotactic polypropylene with some new mercaptotriazines is presented. Isothermal degradation of i-PP was studied by chemiluminescence in air at 180°C on polymer samples containing various concentrations of stabilizers (0.15, 0.25, 0.50 and 0.75 wt%). The antioxidant efficiencies in thermal protection of i-PP were assessed by means of temporal parameters (ti, t12 and tmax), oxidation rates (Voxmax), relative activities (A) and relative stabilities (S) relative to a standard (Irganox 565). The concentration constants that characterize the stabilization effect of triazinic antioxidants were calculated and linear correlation between OIT and σ-Hammett constants could be pointed out.

Photochemical behavior of isotactic-polypropylene irradiated in 32O 2– 36O 2

The photochemical oxidation of isotactic polypropylene (iPP) in an atmosphere composed of a blend of 32O 2 and 36O 2 has been investigated using mass spectroscopy (MS) and infra-red spectroscopy (IR). The samples were irradiated with an artificial accelerated photoageing unit at 60°C. MS analysis reveals the formation of several oxidation photoproducts previously identified, and unambiguously shows that photooxidation of iPP generates 34O 2. A possible interpretation is that, according to the results proposed in the literature, a pseudo-termination reaction by recombination of peroxy radicals POO occurs and undergoes a head-to-head reaction, which produces tetroxides. These intermediate species decompose to give molecular oxygen and two alkoxy macroradicals. On a kinetic point of view, two successive stages are observed. In the first, at low levels of oxygen incorporation, recombinations of POO are not favoured and peroxy radicals mostly generate hydroperoxides. In the second phase, the local concentrations of peroxy radicals are high enough to favour recombination.

Average bond conformations of polymer chains

In this paper, the average bond conformations of polypropylene chains in the second-order interaction approximation, and of isotactic polypropyleneoxide chains in the third-order interaction approximation are investigated. We calculate the a priori probabilities using numerical value calculation method. The average bond conformations may be used as a tool to assess the energies of various rotational isomeric stages, especially for chains that have poorly defined rotational isomers.

Effect of polydimethylsiloxane on oxidation of isotactic polypropylene

Addition of liquid polydimethylsiloxane to isotactic polypropylene retards its non-inhibited and inhibited oxidation by molecular oxygen. We propose that the reasons for this retardation are the migration of mobile free radicals and the reversible transition of antioxidant into inclusions of liquid siloxane in the polymer matrix.

Radiation induced oxidation of isotactic polypropylene and mechanism of action of HALS stabilizers

Radiation induced oxidation of isotactic polypropylene and mechanism of action of HALS stabilizers

Gamma‐, photo‐, and thermally‐initiated oxidation of isotactic polypropylene

AbstractThe detailed oxidation products have been identified and compared from the γ‐, photo‐, and thermally‐initiated oxidation of unstabilized polypropylene films. Products were identified and quantified by a combination of iodometric analysis and infrared spectroscopy. Spectral resolution was enhanced by derivatization reactions which allow the quantification of primary, secondary, and tertiary hydroperoxide and alcohol groups as well as more reliable analysis of carbonyl species. In contrast to polyethylene oxidation which yields predominantly ketone with lesser amounts of secondary hydroperoxide and carboxylic acid, polypropylene oxidizes to give predominantly tertiary hydroperoxide and lesser quantities of secondary hydroperoxide and ketone. In addition carboxylic acid groups are a minor product except at high degrees of thermal and photoinitiated oxidation. © 1993 John Wiley & Sons, Inc.

ESR study of the molecular dynamic of peroxyradicals in the post‐irradiation oxidation of isotactic polypropylene

AbstractBy adopting the modified Bloch equations, the analysis of the motion of macroperoxyradicals during the post‐irradiation oxidation of isotactic polypropylene at room temperature has been performed. Rotations about the chain axis and cubic jumps are the motion of prominent importance in the early stages of reaction involving mostly radicals trapped at defects or in amorphous phase; on the other hand rotations about the CO bonds at decreasing rates are dominant in the latter stages. Clear evidence of the progressive immobilization of peroxyradicals, which may be reckoned with monomolecular termination mechanism and of the trapping of peroxyradicals in the crystalline phase, have been obtained.

Evolution of H2O and CO2 during the copper‐catalyzed oxidation of isotactic polypropylene

AbstractThe kinetics and mechanism of H2O and CO2 evolution during uncatalyzed and copper(oxide)‐catalyzed (Cu, CuO, CuO0.67) oxidation of isotactic polypropylene have been investigated in detail for various catalysts over a range of temperatures (90–150°C). These volatiles were determined chromatographically; H2O and CO2 represent the main volatiles of the oxidation, comprising about 80 mol % of all volatiles. Uncatalyzed oxidation evolves ca. 1 mol of H2O and 1 mol of CO2 for each unit mole of polymer oxidized, while catalyzed oxidation produces 2 mol of H2O and ca. 1.2 mol of CO2 for each unit mole of polymer. These results indicate that secondary as well as tertiary H atoms on the polymer chains are involved in hydroperoxide formation and decay. The oxidation mechanism has been formulated and evaluated on this basis. It consists essentially of two parallel oxidation reactions involving tertiary and secondary groups (H atoms and hydroperoxides), respectively. The mechanism can be represented by first‐ and pseudo‐first‐order reactions in series: (1) oxygen absorption showing induction periods; (2) hydroperoxide formation and decay (plateaus are reached); (3) H2O evolution from the decay of hydroperoxides; and (4) subsequent CO2 production involving chain scission. Arrhenius parameters for all oxidation reactions (uncatalyzed and catalyzed) are also presented. It appears that CuO0.67 is the most efficient catalyst of those investigated.

Kinetic modelling of the post-irradiation oxidation of isotactic polypropylene

A kinetic model for the post-irradiation oxidation of isotactic polypropylene films has been obtained giving concentration profiles of some reactants and products as a function of time and of the spatial co-ordinates. The diffusion parameters for oxygen and the rate constants for the main steps in the hydroperoxidative cycle were determined for the reaction within film depth of 12–40 μm.

Hydrogen peroxide determination by I.R. specular reflectance spectroscopy during uncatalyzed and copper (oxides)‐catalyzed oxidation of isotactic polypropylene

AbstractIR specular reflectance spectra with respect to hydrogen peroxide formation during uncatalyzed and Cu (oxide)‐catalyzed oxidation of isotactic Polypropylene films have been measured as function of time and temperature (90–130°C). Energies of activation for the various cases have been obtained. The originally proposed kinetic scheme based on oxygen‐absorption measurements has been modified in order to accommodate the spectroscopical results. The amount of ROOH groups present at any time on the polymer is very small, indicating relatively slow rates of ROOH formation and fast rates of their decomposition. The kinetic scheme fits well the experimental data. However, the reasons for the variations of the relevant energies of activation obtained for the catalyzed oxidation in absence and presence of the main volatile reaction products, H2O and CO2, are not yet understood, i.e., the mechanism needs further investigations.

Zno‐photocatalyzed oxidation of isotactic polypropylene

AbstractThe ZnO‐photocatalyzed oxidation of isotactic propylene appears to be a superposition of a photocatalytic oxidation of the pigmented zones and an oxidative photochemical reaction occurring in the unpigmented zones. The photoactivity of various zinc oxides produced at high and low temperatures varies at the most within a factor of 5 and is fairly independent of the surface area of the pigment. High activation energy and low initial quantum yield have been determined measuring the appearance of carbonyl compounds. The behaviour of ternary systems composed of isotactic polypropylene, zinc oxide and a typical antioxidant or hydroperoxide reducer is described. Direct interaction between the pigment and the reducing agent explains the inhibition of the oxidation of polypropylene, the inhibition of the chemical consumption of the reducer and the variations of the photoreactivities due to reheating.

The diffusional regime in the initiated oxidation of polymers

The kinetics of chain oxidation of a polymer have been analysed theoretically for chain termination in which either alkyl or peroxide radicals participate, for the case when the rate of diffusion of oxygen is commensurate with the rate of oxidation v. By means of a computer, a number of numerical solutions of the diffusion equation have been obtained for a non-linear dependence of v on the oxygen concentration c: v= ac√ v i /( b+ c). The oxidation of isotactic polypropylene initiated by dibenzoylperoxide has been studied at 366 K. The experimental data were in agreement with approximate expressions.

Thermal oxidation of polypropylene close to industrial processing conditions

AbstractThermal oxidation of isotactic polypropylene (PP) at 220 and 280°C in air was studied. Gas chromatography–mass spectrometric analysis was used to separate and identify the volatile products of PP oxidation. Twenty‐three products were identified and 15 substances were quantified. The aldehydes are the principal products formed, followed by ketones, acids, and alcohols. The main organic product quantified was acetaldehyde. The volatile organic products formed constitute about 3% of the amount of water formed. The relative amounts of the most volatile products formed in the range 220–280°C are practically independent on the temperature of oxidation. The mechanism of formation on the oxidation products is discussed.

Kinetic characteristics of the inhibiting effect of 4-tert-butyl-2,6-dimercaptophenol on oxidation of polypropylene

A study was made of the inhibiting effect of 4-tert-butyl-2,6-dimercaptophenol on initiated oxidation of isotactic polypropylene at 388°K. Mercaptophenol terminates chains by reaction with peroxide radicals with fk 7/ k 2=7·7 ×10 2; mercaptophenol and its products ensure repeated chain termination, the stoichiometric coefficient f⩾10. Products of interaction of mercaptophenol with hydroperoxide terminate chains in polypropylene with f=1, but in cumene, with f⩾10.