Effect of hindered piperidine light stabilizer molecular structure and UV absorber addition on the oxidation of HDPE. Part 2: Mechanistic aspects—molecular modeling and electron spin resonance spectroscopy study

The first in this series of papers explored the effect of the structural characteristics of 2,2,6,6‐tetramethylpiperidine‐based hindered amine light stabilizers (HALS) on the thermal and photostabilization of high‐density polyethylene. In the second part, the energies (stabilities) of the nitroxyl radicals and various intermediate species have been predicted using AccuModel® and related to stabilization performance and electron spin resonance (ESR) spectral data. Nitroxyl radicals with low predicted stability generally afforded improved thermal and photostabilization. ESR spectra were used to obtain values of nitroxyl radical concentration ([>N? O·]) and g‐factor as a function of pre‐aging time for combinations of pre‐aged HDPE and >N? H HALS. Demethylation reactions of > N‐methyl HALS resulted in uselessly weak ESR spectra. The HALS that afforded poor thermal oxidative stabilization gave rise to pronounced minima in [>N? O·] that coincided with a maximum in hydroperoxide concentration. The g‐factor values indicated that a predominant nitroxyl canonical form generally promoted superior thermal oxidative stabilization, whereas a predominance of the dipolar N·⁺?O^? form promoted superior photo‐oxidative stabilization. These trends may be related to greater radical‐scavenging and peroxide‐trapping effects, respectively. Molecular modeling and ESR spectra can therefore provide valuable insight into the effectiveness of HALS and stabilization mechanisms. J. Vinyl Addit. Technol. 10:159–167, 2004. © 2004 Society of Plastics Engineers.     

Effect of penta- and tetramethyl HALS on the radiation resistance of polypropylene

2,2,6,6-Tetramethyl-4-piperidinyl methacrylate (TMPM), a new pentamethyl HALS 1,2,2,6,6-pentamethyl-4-piperidinyl methacrylate (PMPM), their homopolymers (PTMPM and PPMPM), and their copolymers, [i.e., TMPM or PMPM copolymerized with monomers methyl methacrylate (MMA), methacrylic acid (MAA), acrylic acid (AA), and styrene (St) separately] were synthesized. PMPM shows an extremely excellent stabilizing effectiveness on improving the radiation resistance of polypropylene (PP) in comparison with other low molecular weight (LMW) HALS or polymeric HALS. Polymeric HALS shows a much higher thermal stability and better extraction resistance than that of LMW HALS. It is found that the Eb values of the PP-added pentamethyl HALS series are higher than those of PP with the corresponding tetramethyl HALS series at a given irradiation dose. A conclusion can be drawn that the pentamethyl HALS series has a higher stabilizing effectiveness than that of the tetramethyl HALS series. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 69: 2649–2656, 1998     

Photostabilization of polypropylene by surface modified rutile‐type TiO2 nanorods

Rutile‐type titanium dioxide (TiO₂) nanorods were prepared, superficially modified and tested for the protection of polypropylene (PP) from the UVB and UVC irradiations. The silica coating blocked the active sites on the nanorods and the following calcination further reduced the amount of surface hydroxyl groups and thus, made the TiO₂ nanorods more efficient against the photodegradation. Compared with spherical TiO₂ nanoparticles, the calcined silica‐coated TiO₂ nanorods demonstrated good photostabilization efficiency due to the excellent shielding effect and the improved dispersion of the nanoparticles in PP matrix. When used in combination with the conventional hindered amine light stabilizer (HALS), CHIMASSORB^® 944, the surface modified TiO₂ nanorods revealed strong synergistic effect during the photo‐oxidation of the PP composites. The capacity of photostabilization was much higher than the combination with the commercial spherical TiO₂ nanoparticles and even higher than the typical HALS photostabilization system containing hindered phenol TINUVIN^® 328. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40601.     

受阻胺光稳定剂对PVC的光稳定作用

传统上,聚氯乙烯（ＰＶＣ）防光老化主要用于二苯甲酮类光稳定体系,一般认为在聚烯烃中应用效果较好的受阻胺光稳定剂,不适用于ＰＶＣ体系,将受阻胺光稳定剂试用于ＰＶＣ薄膜,结果表明,该光稳定剂用于ＰＶＣ中,其光,热稳定效果与二苯甲酮类紫外光稳定剂相近,为使ＰＶＣ具有防尘功能的丙烯酸酯类聚合物表面涂层,对ＰＶＣ的紫外光稳定有益。     

A Comparative Study of the Efficiency of a Typical Hindered Amine Stabilizer in Different Hydrocarbon Polymers

Abstract

Inhibition of the photo-oxidation of three different polymers by the same HALS has been studied: an ethylene-propylene random copolymer (EPM), a styrene-butadiene-styrene block copolymer (SBS) and the same block copolymer previously hydroperoxidized by reaction with singlet oxygen. Two techniques have been used and compared: oxygen absorption measurements and infra-red spectroscopy in the carbonyl region. The results indicate that HALS inhibits more efficiently the propagation of the photo-oxidation in the unsaturated polymers studied. However, it also catalyses a new type of termination that in EPM delays and in SBS promotes the production of ketones. It is suggested that the practical efficiency of HALS could be mainly dependent of that last effect.     

聚烯烃光稳定化技术进展

本文介绍了聚烯烃光稳定化的研究进展,并就受阻胺光稳定剂新的发展趋势及受阻胺光稳定剂与其它稳定剂之间的相互作用进行了较详细的讨论。     

Photostabilization of commercial polypropylene by a hindered amine stabilizer

The photostabilization of commercial polypropylene by a hindered amine stabilizer is examined by luminescence and ESR spectroscopy. The presence of the amine stabilizer inhibits the photolysis of the α,β unsaturated carbonyl impurity groups present in the polymer. Examination by ESR, of the stabilized polymer before and after irradiation shows the development of a highly stabilized free radical indicating that the stabilizer can act as an ultraviolet stable antioxidant. Possible mechanisms whereby the hindered amine inhibits the photolysis of the α,β unsaturated carbonyl groups are discussed.     

Synthesis and characterization of hindered amine light stabilizers based on end functionalization of polypropylene

A polymeric hindered amine light stabilizer (HALS), where HALS moiety was attached at the terminal end of the polypropylene chain via end‐functionalized vinylidine PP through simple organic reactions, was synthesized. It comprises the synthesis of vinylidine‐terminated polypropylene by using Cp₂ZrCl₂/MAO as catalyst system and epoxidation of vinylidene polypropylene. The final product was synthesized by carrying out the reaction between epoxy end functionalized polypropylene and 4‐amino‐2,2,6,6‐tetrametyl piperidine. The final product was characterized by using ¹H NMR, ¹³C NMR, and FT‐IR spectra. Functionality was calculated by using vapor phase osmometry and ¹H NMR. The solubility and diffusion coefficient of the product were calculated and also its stabilization performance was checked. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1596–1602, 2007     

Triplet-state quenching by photostabilizers of polyolefins

Paramagnetic nickel(II) complexes which are used as commercial photostabilizers of polyolefins are shown in laser photolysis studies to quench the triplet state of benzophenone in dilute benzene solution. The efficiency of quenching observed is less than that of diamagnetic square-planar nickel(II) complexes which are more effective photostabilizers of polypropylene in “Xenotest” experiments. The observed rate constants for quenching of triplet benzophenone are compared with those observed by other workers for quenching of singlet oxygen, O₂^*(¹Δ_g), in dilute solution. The relevance of these studies to the understanding of photodegradation in polyolefins is discussed.     

Ischemic and Ozone Oxidative Preconditioning in the Protection Against Hepatic Ischemic-Reperfusion Injury

It has been demonstrated that ozone, probably through a mechanism of oxidative preconditioning (OP), protected the liver against the damage mediated by reactive oxygen species. Taking into account that iscehmic preconditioning (IschP) is also a protective mechanism, a comparative study between both preconditioning settings was performed in order to study the effectiveness of both protective procedures. Rats were randomly divided into 4 groups: 1- control, sham operated (anesthesia and laparotomy plus surgical manipulation); 2- I/R (ischemia for 90?min followed by 90?min reperfusion); 3- Ischp+I/R, as group 2 but submitted to a previous ischemic preconditioning (ischemia 10?min and reperfusion 10?min); 4- OzoneOP+I/R, as group 2 but submitted to a pervious oxidative preconditioning with 15 sessions, daily, of ozone by rectal administration (dose of 1?mg/kg). The comparison between both preconditionings showed no biochemical differences for the parameters evaluated. Nevertheless, the histological study demonstrated that the protective effect produced by the OzoneOP is superior to that achieved with the IschP.     

Gaseous ozone decomposition over high silica zeolitic frameworks

For several decades, it has been known that ozone emissions are harmful to humans, plants, and animals. Heterogeneous catalytic decomposition is an efficient process for removing ozone from air. This study examines the effect of the zeolite's framework and pore width on efficiency for decomposing gaseous ozone. Four highly hydrophobic zeolites are used: a large cavity zeolite (Faujasite/H‐FAU), a medium pore zeolite with parallel channel (Mordenite/H‐MOR), and two medium pore zeolites with interconnected channels (H‐ZSM‐5/H‐MFI and Na‐ZSM‐5/Na‐MFI). Experiments were conducted in fixed‐bed flow reactors loaded with zeolite at ambient conditions (20 °C and 101 kPa). Zeolite surfaces were analyzed during the experiments in order to understand the influence of physical and chemical surface properties on the ozone decomposition mechanism. A higher amount of ozone is eliminated using H‐MOR, compared with the zeolite samples H‐FAU, H‐MFI, and Na‐MFI. Pore width and micropore framework size distribution (channel and cages) appear to be key factors. A narrow channel or cage, slightly larger than the ozone molecule size, seems to promote ozone interactions with Lewis acid sites. Fourier transform infrared spectroscopy shows that Lewis acid sites (LAS), located on the walls of zeolite pores, decompose ozone. This leads to the formation of atomic oxygen species that could react with another ozone molecule to form dioxygen. Hence, LAS are regenerated, ready to decompose another ozone molecule once more.

     

Ozone formation with (V)UV‐enhanced dielectric barrier discharges in dry and humid gas mixtures of O2, N2/O2, and Ar/O2

The effect of (V)UV illumination at 172 run and 253.7 run on ozone formation with dielectric barrier discharges in air‐like mixtures of nitrogen/oxygen and argon/oxygen as function of the water concentration is presented. Although (V)UV at these wavelengths efficiently cleaves ozone, the ozone concentration in a combined (V)UV/dielectric barrier discharge in oxygen‐containing gases is reduced only very little. This corresponds to an enhanced concentration of atomic singlet oxygen, which, in presence of water, increases the production of hydroxyl radicals. This is confirmed by measurements of the removal rates of 2‐propanol and of its byproducts in dry and humid air in a combined (V)UV/dielectric barrier discharge treatment.     

Study of some parameters responsible for the efficiency of hindered amine light stabilizers

The photooxidation of a styrene-butadiene-styrene block copolymer previously hydroperoxidized by reaction with singlet oxygen has been studied by measuring the increase of the carbonyl absorbance at 1720 cm^?1. Five different hindered amine light stabilizers (HALS) have been introduced into the polymer by diffusion from methanolic solutions, and their efficiencies as inhibitors of the photooxidation have been compared. The sequence of efficiency observed is: tert. amine > sec. amine > amino ether Under the experimental conditions used HALS are localized preferentially in oxidized regions of the polymer, and their efficiency can be correlated with their ability to catalyze the decomposition of clustered hydroperoxides. As a consequence, HALS are oxidized to nitroxy radicals that are the actual inhibitors of the photooxidation. A high local concentration of HALS in oxidized regions of the polymer also contributes to their efficiency, and this effect justifies the observation that some tertiary amines are more efficient than others.     

某些高分子受阻胺对聚丙烯稳定化作用的研究

众所周知,受阻胺光稳定剂对聚烯烃有良好的光稳定效率,已引起世界的注意。然而,小分子量的受阻胺易于挥发,从而影响它的效率发挥,为了克服这个缺点,使小分子受阻胺的高分子化是近年来受阻胺光稳定剂发展的方向,本文主要是研究三种商品化高分子受阻胺光稳定剂对聚丙烯的稳定化作用及其效率和机理。     

HALS stabilization of LDPE films used in agricultural applications

The mechanism of stabilization of low density polyethylene (LDPE) films used in agricultural applications with hindered amine light stabilizer (HALS) has been studied under the influence of natural weather for 750 d. The structural modifications in the exposed films were monitored by Fourier transform infrared spectroscopy (FTIR), while the examination of the surface behavior was performed by scanning electron microscopy (SEM). Finally, the stability of the material was evaluated by measuring the percent elongation at break. The performance of HALS in the films was analyzed through a comparative study carried out on unstabilized LDPE samples exposed to the same weathering conditions for 170 d. The FTIR results have shown that HALS reduces significantly the rate of formation of ketone groups in the exposed films due to deactivation of the excited ketonic carbonyls. This indicates the role of HALS to prevent the occurrence of Norrish I and II reactions. On the other hand, the development of vinyl groups at 909 cm^–1 during the exposure period of 750 d is attributed to the photolysis of the ester group of the HALS localized at 1 734 cm^–1 in the film and the photooxidation of the polymer. The decrease observed in the ester band suggests the loss of the stabilizer. For the unstabilized LDPE film, the results have revealed a fast increase in both ketonic and vinyl groups attributed mainly to Norrish II reactions. This resulted in chain scission responsible for both the rapid drop in elongation at break and the considerable deterioration of the surface of the exposed film. On the contrary, the presence of HALS in the film resulted in a large retention of the elongation at break. Moreover, SEM analysis reveals the role of HALS to prevent the surface damage by reducing the number and size of microcracks.     

Synthesis of new polymeric hindered amine light stabilizers: Performance evaluation in styrenic polymers

Polymeric hindered amine light stabilizers (HALSs), in which the HALS functionality was attached to the terminal isocyanate chain end of poly(styryl‐co‐styryl isocyanate), were synthesized by a two‐step process. First, cinnamoyl azide was prepared and copolymerized with styrene by a free‐radical copolymerization method. Polymeric low‐molecular‐weight and high‐molecular‐weight 2,2,6,6‐tetramethyl‐4‐pipridinol‐graft‐poly(styryl‐co‐styryl isocyanate) and 4‐amino‐2,2,6,6‐tetramethyl piperidine‐graft‐poly(styryl‐co‐styryl isocyanate) were synthesized by a grafting method. The photodegradation and stabilization of different grades of high‐impact polystyrene (HIPS) were studied at 55°C in air at different time intervals, and the photostabilizing efficiency of polymeric HALSs was compared with conventional light stabilizers, such as 2,2,6,6‐tetramethyl‐4‐pipridinol and bis(2,2,6,6‐tetramethyl‐4‐piperidinyl)sebacate. Polymeric HALSs showed significant improvements in the photostabilization of HIPS. The solubility and diffusion coefficient of polymeric HALSs were studied. The morphological changes in HIPS caused by photooxidation were also studied. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1126–1138, 2003     

臭氧尾气回用对曝气池污泥影响研究

将活性污泥分别在14.68、30.72、43.84 mg/L 3种臭氧浓度下进行氧化,同时以纯氧曝气作为平行实验,考察臭氧尾气对活性污泥的影响。结果表明,污泥经纯氧曝气后,在反应时间内并无明显变化,但在臭氧氧化过程中,污泥ATP下降,同时上清液中COD、TP、TN上升,且臭氧浓度越高,变化速率越大,其原因为污泥发生了溶胞现象;臭氧氧化后,污泥MLSS、MLVSS减少,污泥呼吸速率下降,但沉降性得到改善,同样随着臭氧浓度的升高,变化更为显著。经14.68、30.72 mg/L臭氧氧化后的污泥,其硝化能力、生化能力与反应前并无明显差距,而经43.84 mg/L臭氧氧化后污泥的硝化能力、生化能力明显弱于反应前。     

Reaction of sodium perborate with a chlorinated cyclic hindered amine (TMP-Cl): I. Quantitative evaluation of active oxygen species in fabric bleaching

The reaction of N-chloro-4-hydroxy-2,2,6,6-tetramethylpiperidine (TMP-Cl) with sodium perborate (PB) was investigated with special reference to the generation of singlet oxygen and the possible application to a new oxidative bleaching process. Generation of the singlet oxygen (¹O₂), the hydroxyl radical (HO·) and superoxide anion radical (O₂·⁻) in the PB/TMP-Cl mixed solution was confirmed by the trapping reagent method. From the results of another experiment, in which the bleaching abilities of each active oxygen species were confirmed, the main active oxygen species contributing to the bleaching of purpurogallin, the skeleton of black tea pigment, in the PB/TMP-Cl system was concluded to be¹O₂.     

Uncharged atomic oxygen in oxidative conversion of C1-C2 alkanes

The role of uncharged atomic oxygen species in oxidative conversion of CH₄ and C₂H₆ has been considered. It is shown that active atomic oxygen species participate in C₁---C₂alkanes conversion in the presence of NO on zeolites, in the CH₄N₂O¹ and CH₄---O₂ systems on transition metal silicides and nitrides, as well as hydrocarbon oxidation during N₂O and O₃ photolysis by excimer laser light. Participation of uncharged oxygen species in conversion of light alkanes makes it possible to explain not only hydrocarbon formation pathways, but those of oxygen-containing compounds as well.     

Migration of hindered amine light stabilizers in automotive clearcoats

The diffusion behavior of two hindered amine light stabilizers (HALS) [bis(1-octyloxy-2,2,6,6-tetramethyl-4-piperidinyl)-sebacate and bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-sebacate] in an automotive clearcoat has been quantified via time-of-flight secondary ion mass spectrometry. Diffusion couples made from clearcoats with and without HALS allow for the estimation of the diffusion coefficient of one of the HALS. The high diffusion coefficient of one of the HALS contradicts the presence of HALS gradients in weathered coating systems. This discrepancy is ascribed to a decreasing diffusion coefficient of the HALS in the coating matrix due to a weathering-induced increase in the T _g of the clearcoat.