Oxidative degradation of polyolefins. I. Thermal oxidative degradation of atactic polypropylene in solution

Atactic polypropylene was oxidized in solution with radical initiator in the absence and presence of metal catalyst to considerably high conversion over the temperature range of 70° to 125°C, and oxidation products and the change in molecular weight distribution were measured. The unoxidized polypropylene had no ultraviolet absorption at 253.7 nm, whereas the oxidized polypropylene showed distinct absorption over a wide range of molecular weights. It was found that oxygen was incorporated into the polymer chain as hydroperoxide, acid, carbonyl, and hydroxy groups. Much of the absorbed oxygen was found to be involved in smaller fraction of low molecular weight products. Although the average molecular weight of the oxidized polypropylene decreased significantly, the formation of low-boiling products was quite small.     

Catalytic action of metallic salts in autoxidation and polymerization. VI. The inhibitive effects of transition-metal N,N-diethyldiselenocarbamates on the thermal oxidative degradation of isotactic polypropylene

Transition-metal (Zn, Ni, Pd, Cu, Cr, Co) N,N-diethyldiselenocarbamates were prepared, and the inhibitive effects of these compounds on the thermal oxidative degradation of isotactic polypropylene, both in the presence and absence of copper stearate as oxidation accelerator, were examined by conventional oxygen uptake. In both cases, all metal selenocarbamates showed a pronounced inhibitive effect on the degradation of the polymer, especially Zn and Ni selenocarbamates. The inhibitive action of these metal selenocarbamates was discussed from the viewpoint of viscosity change and carbonyl and hydroperoxide contents.     

Thermooxidative reactions of polypropylene wax in the molten state

Thermooxidation of polypropylene wax was investigated in the molten state under an air‐flowing atmosphere. Calorimetric measurements were performed to investigate the effect of the inclusion of 1‐dodecanol as an efficient primary alcohol and to reveal the typical outlines of thermooxidation, that is, the induction time and oxidation rate. The same parameters were obtained and compared on the basis of the Fourier transform infrared (FTIR) peaks of major oxidation products, such as ketones and hydroxyl groups. The results showed that 1‐dodecanol caused a change in the shape of the calorimetric thermogram and also yielded oxidized products with more clarity in color. With the Arrhenius equation, the apparent activation energy was calculated on the basis of the induction time and oxidation rate for carbonyl and hydroxyl groups individually. The obtained apparent activation energy was in agreement with the proposed mechanism based on bimolecular hydroperoxide decomposition involving dodecanol. With FTIR spectroscopy, no changes in the chain helical conformation or double‐bond concentration were observed during oxidation up to 500 min. An acid value of up to 6 mg of KOH per gram was obtained at this time. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Chemiluminescence studies of the degradation and stabilization of polymers

In order to predict the safe service life of a polymer from its rate of oxidation, a detailed understanding of degradation and stabilization reactions in the induction period is required. The mechanism by which an oxidizing polymer emits weak visible light (chemiluminescence) is reviewed and the use of the technique to study the early stages of the oxidation of nylon 66 and polypropylene is considered. Steady-state chemiluminescence experiments are described to measure the activation energy of initiation of nylon 66 oxidation and to study the mechanism of the dark reaction of a hindered amine light stabilizer (and its nitroxyl radical) with polypropylene hydroperoxide. Non-stationary experiments, in which the steady state is perturbed by UV irradiation and gas switching, demonstrate the reactions of long lived alkyl radicals in the induction period. The observation of stress-induced chemiluminescence from nylon 66 shows the effect of even a low fiber load is to greatly increase the rate of oxidation.     

Oxidative stability of sunflower oils differing in unsaturation degree during long-term storage at room temperature

The objective of this work was to study the evolution of oxidation in sunflower oils differing in unsaturation degree during long-term storage at room temperature. For this purpose, a combination of adsorption and size-exclusion chromatographies was used for quantification of oxidized triacylglycerol (TG) monomers, dimers, and polymers. Conventional sunflower oil, genetically modified high-oleic sunflower oil, and a 1∶1 mixture of the two were used. Results showed that oxidized TG monomers were the only group of oxidation compounds increasing during the early oxidation stage, and an excellent correlation was found between amounts of oxidized TG monomers and PV during the induction period, independently of the degree of oil unsaturation. Both the rate of formation and the amount of oxidized TG monomers accumulated at the end of the induction period increased as the unsaturation degree of the oils tested was higher. The end of the induction period was marked by the initiation of polymerization and exhaustion of tocopherol. Therefore, the concomitant determination of oxidized TG monomers and polymerization compounds provided a complete picture of the oxidation process.     

Synthesis of polyblends from polypropylene and poly(R,S)‐β‐hydroxybutyrate,and their characterization

Blends of modified polypropylene (PP) with poly(R,S)‐β‐hydroxybutyrate (PHB) were prepared by casting polymer solutions, followed by compression molding into thin films. The modified polypropylene was obtained by oxidation with hydrogen peroxide. Oxidation of polypropylene produced new functional groups such as carbonyl and hydroxyl groups on the polymer chain, and a decrease in molecular weight and crystallinity of the polymers. Maximum crystallinity and mechanical properties of the polyblends were found with a PP/PHB ratio of 90/10 (w/w), and then decreased with increasing PHB content in the polyblends. Biodegradability of the polyblends was lower than that of bacterial and synthetic PHBs. Furthermore, an increase of PHB proportion in the polyblends resulted in highly non‐compatible polyblends. Hence only PHB and small parts of the polyblends were decomposed by microorganisms. Copyright © 2006 Society of Chemical Industry     

Effect of water activity on secondary products formation in autoxidizing methyl linoleate

The role of water activity on the formation of peroxides and carbonyl compounds during lipid oxidation is important to know because there could be either beneficial or detrimental effects of water activity on lipid oxidation in stored foods. Therefore, methyl linoleate was chosen as a model lipid and was autoxidized to 1% at water activity ranging from 0.02 to 0.79 at 37°C. Oxygen uptake was monitored manometrically. The peroxide and carbonyl contents were determined upon termination of the autoxidation studies. Methyl linoleate autoxidation was characterized by three phases: i) an initial induction period of no oxygen absorption; ii) a slow rate of oxygen absorption, up to 0.15% oxidation; and iii) a relatively faster rate of oxygen absorption beyond 0.15% up to 1% oxidation. Water activity had considerable influence during the first phase. There was no induction period at or below water activity 0.22. The induction period begins at water activity 0.32 and could be extended to a limit with increase in water activity. Once the induction period was passed water activity had no influence on the rate of oxidation. However, during the second and third phases water activity becomes important in the stabilization of peroxides/hydroperoxides and decides the course of secondary reactions that follow peroxide decomposition. Higher water activity values, particularly water activity 0.67, tended to stabilize peroxides. Water activity had considerable influence on the formation of secondary products of autoxidation as evidenced by the variation in the type and quantity of carbonyl compounds at different water activity values.     

Structural interpretation of plasma-polymerized propargyl alcohol

Vapor of propargyl alcohol under reduced pressure was glow-discharged for plasma polymerization by which hydrophilic polymer film could be deposited onto a solid substrate. The hydrophilic property was more enhanced with use of the same monomer containing water. An interpretation of chemical structures of the polymer molecules was therefore attempted to discover the reason of the hydrophilic property. The polymer materials were subjected to IR spectroscopy, ¹³C-NMR and ¹H-NMR spectroscopies, elemental analysis, number average molecular weight determination, mass spectrometry, and other wet chemical analysis for . The IR spectra of the polymers showed the presence of OH, C?O, and C?C groups with a disappearance of C?C group of the monomer. It was also found that the molecular formula of a solvent extract of the polymer derived from the plain monomer was C₂₆H₃₆O₉ ≒ 490, involving four hydroxyl, one carbonyl, and two to three olefinic unsaturation groups within a unit molecule. On the other hand, the polymer from 60% water-containing monomer showed its molecular formula as C₄₄H₆₆O₁₈ ≒ 890, containing seven to eight hydroxyl, two carbonyl, and one to two olefinic unsaturation groups. This information was used to establish the symbolic chemical structures of the polymer molecules. It has been found that both polymers were not as much crosslinked as other plasma polymers because of having side chains with fewer branching. The oxygen atoms were retained in the polymer not only as the forms of OH and C?O but also as a member of the principal frame of the polymers.     

Photo‐oxidation of polymers: Validation of oxygen uptake and relationship with extent of hydroperoxidation

In this piece of work we focused our attention on the peroxidation step and alongside considered hydroperoxides as probes for the overall oxidative process. This technique of oxygen uptake actually monitors the amount of oxygen consumed during polymer degradation. Our present work aimed to investigate this technique to evaluate the oxygen uptake during in situ photoirradiation, under controlled atmosphere. Our experimental results clearly elucidate that the oxygen consumption data accounts for a very early stage of aging during the photo‐oxidation of polymers and provides us with an accurate and sensitive diagnosis about the formation of hydroperoxides. A straight‐line relationship was exhibited under our experimental conditions while observing a relation between oxygen pressure drop and hydroperoxide content in the polymer. This correlation was dependent upon the nature of polymer and of course on the aging conditions (temperature, irradiation, oxidative atmosphere). The impact of environmental atmosphere on aging was particularly kept in mind. To conclude, we emphasized that oxygen uptake is a promisingly powerful tool to identify the impact of the overall environmental parameters on the polymer photoaging. An important implication was on the understanding of atmospheric factors (including pollutants such as O₃, NO_x, etc.), which is usually given minor importance, on the degradation of polymer upon outdoor weathering. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Localization of oxidation in polypropylene

The distribution of oxidative degradation in isotactic polypropylene has been monitored using ultra-violet microscopy. Reaction of carbonyl groups in the oxidized polymer with 2,4-dinitrophenylhydrazine allows the oxidation to be seen with illumination at 350 nm. There is much extraction of oxidized material during the staining process. It is shown that oxidation initially favours the atactic fraction of the polymer. With increasing oxidation prior to crystallization rejection of oxidized material to the spherulite boundaries leads to increasing weakness in these regions although there is no evidence that their subsequent oxidation rate is enhanced. Localized oxidation in diluent-phase polymer is shown to correlate strongly with the distribution of catalyst residues in the polymer. Similar localization can be observed in polymer formed on a high efficiency, gas phase, catalyst although catalyst residues are not directly visible in this polymer.     

Autoxidation of ethyl eicosapentaenoate and docosahexaenoate

The extent of oxidation of ethyl esters of eicosapentaenoic (EPA) and docosahexaenoic acids (DHA) was compared quantitatively with that of ethyl linoleate (Lo) and ethyl linolenate (Ln) by oxygen uptake and formation of conjugated diene, hydroperoxide and secondary oxidation products. EPA and DHA esters were oxidized rapidly even at 5 C in the dark after an induction period of 3–4 days, while the induction periods of Ln and Lo esters were 20 days and more than 60 days, respectively. Oxygen uptake of EPA and DHA esters after the induction period was 5.2 and 8.5 times faster than that of ethyl Ln, respectively. Hydroperoxides of EPA and DHA esters are much less stable than those of ethyl Lo. The peroxide value is not necessarily a good indication of oxidation in these polyenoic acids because a considerable amount of secondary products is formed at the early stage of oxidation. Polymers were found to be major secondary products in these polyenoic esters.     

Kinetic effects of silica nanoparticles on thermal and radiation stability of polyolefins

The effects of γ-irradiation as the accelerated degradation procedure were analyzed for the evaluation of material stability. Low density polyethylene, polypropylene and ethylene–propylene terpolymer were studied in formulations with SiO₂ nanoparticles (2 and 5 wt%) or as neat materials. High energy irradiation (up to 100 kGy) has revealed a faster increase in the absorption of carbonyl band in comparison with the corresponding change in hydroperoxide band. The three studied materials present increased oxidation rates as the received energy transferred from incidental rays is enhancing, because the thermal regime of degradation depends on the structural characteristics, namely initial number of tertiary carbon atoms and unsaturation level. The fate of hydroperoxides as oxidation initiators is discussed.     

环己烷液相非催化/催化氧化反应动力学特性比较

分别以不同氧浓度气体及纯氧作为氧源,利用鼓泡塔反应器在1.1 MPa、不同温度、不同气体流量条件下比较了环己烷液相非催化氧化和催化氧化反应特性。通过比较分析,讨论了环己烷液相氧化的链式反应机理。理论分析和实验数据表明,环己烷液相催化氧化反应诱导期短,其表观速率与环己烷浓度和氧气浓度都为一级;而非催化氧化反应诱导期长,是由烃直接氧化引发过程控制,只有当过氧化氢积累到一定浓度以后,过氧化物分解产生自由基速率变得显著,反应才表现出自催化反应特征。而在催化剂存在的情况下,环己基过氧化氢分解非常迅速,环己基过氧化氢浓度在整个反应过程中稳定在一个很低的水平,反应由氧化生成酮的链传递反应过程控制。     

Catalytic action of metal salts in autoxidation and polymerization. X. The effect of various metal stearates on the thermal oxidation of 2,6,10,14-tetramethylpentadecane

The various metal stearate-catalyzed thermal oxidations of 2,6,10,14,-tetramethylpentadecane (TMPD, a model compound of polypropylene) and the decomposition of TMPD hydroperoxide (TMPD-HPO) were carried out. The results obtained were compared with those of the oxidation of polypropylene reported previously;¹⁸ and the initial tion process of the oxidation of the polymer is discussed. The catalytic activity of metastearates in the oxidation of polypropylene differed from that in the oxidation of pure TMPD and was fairly similar to that in the oxidation of TMPD with TMPD-HPO. The more effective the catalyst for the decomposition of TMPD-HPO, the more effective it was for the oxidation of both TMPD with TMPD-HPO and polypropylene. The results apparently indicate that metal catalysts enhance the decomposition of hydroperoxides which presumably contaminate the polymer and form during the oxidation, and also imply that the initiation of the oxidation of polypropylene is primarily affected by the presence of hydroperoxide contamination of the polymer.     

Oxidative degradation of poly(vinyl chloride)

Rates of oxygen consumption and formation of oxidation products were determined in γ-initiated oxidation of poly(vinyl chloride) (PVC) at 25°C. Data are given for five dose rates and correlated on the basis that the overall oxidation is the sum of two reactions that are first order and half order in rate of initiation. The principle oxidation product is hydroperoxide, formed by the half-order reaction, whereas alcohol and carbonyl compounds result mostly from first-order reactions. These products account for 77–80% of the total oxygen absorption. The presence of oxygen was found to increase twofold the rate of dehydrochlorination of irradiated PVC. A reaction scheme is developed which assumes that a significant fraction of the interactions of tertiary peroxy radicals is nonterminating and yields alkoxy radicals which propagate or decompose by β-scission. This decomposition occurs mostly by splitting off a chlorine atom with formation of a ketone and, to a lesser extent, by C? C cleavage which accounts for the observed decrease in molecular weight of the oxidized PVC. Polystyrene was found to be much more resistant to oxidation than PVC. A classification of some commercial polymers in function of their susceptibility to oxidation is proposed.     

Radiation processing of EPDM/PP blends

The effect of ionizing radiation on thermal oxidation of ethylene-propylene copolymer/polypropylene (EPDM/PP) was evaluated over the range of total gamma doses up to 250 kGy. The influence of irradiation dose on oxidation induction periods was investigated by oxygen uptake and thermal analysis on polymer samples containing various concentrations of components (100/0, 80/20, 40/60, 60/40, 20/80 and 100/0 w/w ). Drastic decrease in oxidation induction time was observed for low doses. The competition between crosslinking and scission has been examined on the basis of radical recombination on post-irradiation time. The influence of specimen formulations on oxidation induction time is discussed regarding to the contribution of antagonistic processes: crosslinking and oxidative degradation.     

Formation of peroxides in fatty esters. II. Methyl linoleate: application of the polarographic and direct oxygen methods

Summary This study of the prolonged autoxidation of methyllinoleate at 80°C. has included polarographic identification and determination of hydroperoxides, the direct determination of oxygen contents, and catalytic micro-hydrogenation for the determination of unsaturation. The polarographic method has further substantiated the observations of other workers that the principal peroxidic substance formed during the autoxidation of methyl linoleate at 80°C. is a hydroperoxide. The direct oxygen measurements have shown that most of the oxygen absorbed in the initial stages of autoxidation can be accounted for as hydroperoxide. During the latter stages there was a continuous increase of oxygen uptake, half of which can be accounted for as free acid and half as forms other than hydroperoxide, ester, or free acid. By means of the catalytic micro-hydrogenation method it has further been shown that as the autoxidation progresses there is a continuous decrease in unsaturation.     

A new technique for determining the oxidative stability of molten polymers

A new method and apparatus for determining the oxidative stability of molten polymers, especially polyolefins, is described. The apparatus consists of a laboratory scale mixer modified to allow for the continuous determination of the oxygen absorbed by the polymer. The oxygen uptake curve observed for polyolefins is of the type normally associated with the autocatalytic oxidation of hydrocarbons and yields an induction period—a traditional measure of stability. Typical results from experiments concerning the antioxidant type, antioxidant level and temperature on polyethylene stabilization are presented to illustrate the usefulness of the technique in both quality control and research applications.     

IR spectroscopy study of polypropylene natural aging

Structural modifications of a commercial polypropylene (PP) sample are studied by IR spectroscopy. Aging this sample under an ambient atmosphere with or without the presence of ambient light shows the oxidation process to be the most predominant. Spectral analysis reveals that the commercial sample is isotactic and also indicates that, for the new sample, oxygen establishes single bonds with carbon. However, through the aging process, spectral changes essentially occur in the regions of ∼3400, ∼1712, and 1170 cm⁻¹ which correspond to hydroxyl and/or hydroperoxide groups, a carbonyl group, and C O, respectively. The deconvolution of the bands corresponding to a carbonyl group reveals the presence of a complicated mixture of oxidation products: aldehydes, ketones, esters, acids, peresters, and peracids. Spectral analysis also shows that the most favorable site for oxidation is at the methylene group. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1742–1748, 2000     

The weatherability of polypropylene monofilaments. Effects of fiber production conditions

From a comparison of the photo-and γ-irradiation-initiated oxidations of monofilaments and films, polypropylene oxidation rates and product ratios were found to be independent of sample morphology and orientation. Filament sensitivity to photo-oxidation was, however, drastically affected by extrusion and draw conditions, photosensitivity increasing with increasing draw speed and decreasing draw temperature. Draw effects were minimized by the exclusion of oxygen, indicating that free radicals produced by backbone cleavage during draw react with oxygen to give chromophoric oxidation products. The most important product detectable after drawing was probably the polypropylene hydroperoxide. A phenolic antioxidant reduced hydroperoxide formation, although sufficient hydroperoxide was still produced to accelerate photodegradation as compared with a similarly stabilized undrawn filament. Melt oxidation within the extruder was concluded to be much more important than thermal oxidation of the extruded filament as it cooled on the spinline.