Photo-oxidative Stabilization Research Articles

Photo-oxidative resistance and adjustable degradation of poly-lactic acid (PLA) obtained by biomass addition and interfacial construction

Based on the improved performance by interface construction, poly-lactic acid (PLA)/biomass composites have attracted much attention and received wide application as a completely degradable material. This work aims to study the effect of biomass components and the designed interface on the photo-degradation behavior of PLA/biomass composites exposed to the simulated sunlight irradiation, which could further alleviate the restriction caused by the poor ultraviolet (UV) stability of PLA. The biomass shows a significant delay effect in the degradation rate of the PLA matrix by the absorption of visible and UV ranges, acting as a photo-oxidative stabilizer during the irradiation. The investigation of the surface chemical changes reveals a dual mechanism for the degradation of the composites, consisting of a typical Norrish II photochemical and photo-oxidation mechanism. The interface in the composites is constructed by the surface treatment of biomass through the in situ polymerization of lactic acid, which could effectively alleviate the shielding effect of biomass. In our view, this work provides a novel strategy to prepare the PLA/biomass composites that possessed excellent photo-oxidative stabilization provided by biomass, adjustable degradation cycle offered by the interface construction, and potential application in more advanced packaging and agriculture fields.

Photo-Oxidative Stabilization Effect of a Reactive Hindered Amine on Monomer Casting Nylon-6

Monomer casting (MC) nylon-6 was polymerically modified by grafting 4-amino-2,2,6,6-tetramethylniperidine (TEMP), as a reactive hindered amine, onto its chain so as to improve the compatibility and stability of the stabilizer in the polymer matrix. The long-term aging of MC nylon-6 under UV irradiation indicated that the grafted sample of MC nylon-6 had lower gel content than that of the pure sample, and only a slow change in the reduced viscosity was observed. The carboxyl concentration of the grafted sample was lower than that of the pure sample, and its terminal amine group concentration was relatively higher during the whole aging process, presenting a more stable chemical structure. The delayed oxidation exothermic peak and increase of oxidation induction time (OIT) suggested that the grafted TMEP was stable in the matrix of nylon-6 after long-term UV aging. Decreasing crystallization ability after aging was observed for the pure sample of MC nylon-6, while a relatively stable crystallization behavior was observed for the grafted samples, indicating that TEMP can inhibit the structural change of MC nylon-6 during UV irradiation.

Photo-oxidative stabilization of carbon nanotubes on polylactic acid

This paper aims to study the effect of multiwall carbon nanotubes (MWCNTs) on the photo-degradation behavior of polylactic acid (PLA) composites exposed to UV-light. The MWCNT dispersion state within the PLA matrix was analyzed by electrical conductivity measurements. From gel permeation chromatography it was verified that the rate of photo-degradation of PLA/MWCNT composites is lower than that of the unfilled PLA. The surface morphology modifications induced by UV have been analyzed by optical and scanning electron microscopy. Thermal analysis revealed an increase in the polymer crystallinity and a decrease in the degradation temperature during the UV treatment. The mechanical properties (Young's modulus and tensile strength at the yield point) were significantly increased by the addition of MWCNTs. However, the tensile strength and strain to failure slightly decreased with an increase in irradiation time. This complex behavior was attributed to a molecular reorganization in the first period of photo-aging followed by a severe macromolecular chain scission.

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

AbstractThe 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.

Photodegradation and stabilization of styrene-butadiene-styrene rubber

Photooxidative degradation and stabilization of a polystyrene–block–polybutadiene–block–polystyrene thermoplastic elastomer using a polychromatic UV light in air at 60°C has been studied by monitoring the appearance of the hydroxyl and carbonyl groups in Fourier transform infrared spectroscopy. The extent of photooxidative degradation in different samples has been compared. The rate of photooxidation was also estimated in the presence of different concentrations of 2,6-di-tert-butyl-4-methylphenol [BHT], 2-(2′-hydroxy-5′-methylphenyl)benzotriazole [Tinuvin P] and tris(nonylphenyl) phosphite [Irgafos TNPP], and 1,2,2,6,6-pentamethyl piperidinyl-4-acrylate was grafted onto the surface of the SBS film. The kinetic evolution of the oxidative reaction was determined. The morphological changes upon irradiation in the solution cast SBS films were studied by scanning electron microscopy. Based on the experimental data a suitable photooxidative degradation mechanism also has been proposed. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1097–1102, 2000

Functionalization of polyorganophosphazenes. VII. Photooxidative stabilization of aryloxy-substituted polyorganophosphazenes induced by grafting HALS groups through anhydride moieties

In this article, the general procedure for stabilizing polyorganophosphazenes against the damage induced in these substrates by photooxidation reactions under accelerated conditions is explored. This method is based on the preventive grafting of succinic anhydride groups onto selected polyphosphazenes induced by light excitation, followed by the reaction of the functionalized phosphazene substrates with amino-terminated HALS groups. HALS moieties, in fact, are able to strongly inhibit the photooxidation ofphosphazene films under accelerated conditions even when randomly attached to the polyphosphazene materials in very low amounts.

Grafting reactions onto poly(organophosphazenes). VI. Thermooxidative stabilization of poly[bis (4-benzylphenoxy)phosphazene] by grafting acrylate polymers containing HALS groups

Poly[bist 4-benzylphenoxy phosphazene] is heated at 200 C in air and the oxidation of the 4-diphenylmethane side substituent to phosphazene-supported benzophenone, benzhydrol, and hydroperoxides is observed. The thermo-oxidative stabilization of the polymer is obtained by light-induced grafting copolymerization of the acrylic acid 2.2.6.6-tetramethyl-piperidin-4-yl ester onto the polyphosphazene substrate. The HALS moieties grafted onto the polyphosphazene films prevent the oxidative modification of the substituents and preserve the polymer against thermal damage. This process, investigated by IR. UV, and EPR spectroscopy, is basically dependent on the amount of HALS groups grafted onto the polyphosphazene matrix and on the composition of the reaction medium used to carry out the grafting process. The comparison between the thermooxidative stabilization process and the corresponding photooxidative stabilization reaction is also discussed.

Thermal and photooxidative behaviour of novel 4-substituted alicyclic amino-2-hydroxybenzophenone stabilizers in polypropylene and high density polyethylene films

The thermal and photooxidative stabilization activities of four light stabilizers based on 2-hydroxybenzophenone with alicyclic amine groups substituted in the 4-position have been examined and compared with those of 2-hydroxy-4- n-octoxybenzophenone (HOBZ) in polypropylene and high density polyethylene (HDPE) films at 0·1% w/w concentration infra-red absorption spectroscopy. From both carbonyl index and embrittlement times, both alkylpiperazine derivatives were found to be ineffective on exposure to QUV irradiation, whereas the propoxymorpholinyl and the hindered piperidine compounds were found to be more effective than HOBZ. All the stabilizers were synergistic with the commercial hindered phenolic antioxidant, Irganox 1010, especially the HOBZ. On the thermal ageing all the light stabilizers behaved as antioxidants, including the HOBZ. However, the hindered piperidine derivative was the most effective, exhibiting a stabilizing effect close to that of the Irganox 1010. In the presence of the Irganox 1010 weak antagonism was observed, with the HOBZ exhibiting the most effective stabilization. In HDPE the effects were different on irradiation, with all the 4-substituted amino benzophenones being effective. In this case the 4-amino hindered piperidine stabilizer was more effective than HOBZ. In the presence of Irganox 1010 strong synergism is common except for the bis- N, N-octylpiperazine derivative. On oven ageing all the stabilizers exhibited antioxidant behaviour with the 4-amino-piperidine derivative being the most effective. The antioxidant effect of Irganox 1010 was not impaired in HDPE apart from that with the propoxy-morpholino derivative. The steric hindrance, molecular size and oxygen scavenging ability of the cyclic amine are all concluded to be important factors in controlling the antioxidant behaviour of the stabilizer with hydroperoxides playing a key role in the latter case. The results also demonstrate the importance of the ortho hydrogen bonded hydroxyl group in 2-hydroxybenzophenones as an active chain-breaking donor in photostabilization and not UV absorption. It is further concluded that the results show the potential for the development of a single stabilizer entity which will behave as an effective photothermal antioxidant.

Synthesis and thermal and photooxidative behaviour of 1, n-bis(3-hydroxy-4-benzoylphenoxy) alkanes in polyolefin films

A series of light stabilizers based on the 2,4-dihydroxybenzophenone unit linked at each end of an alkane chain have been prepared via the Williamson ether synthesis and characterized. Six homologues were prepared in this way from propane to octane. The thermal and photooxidative stabilization activities of all six compounds have been examined and compared with those of 2,4-dihydroxybenzophenone (DHBZ) and 2-hydroxy-4- n-octoxybenzophenone (HOBZ) in medium density polyethylene and polypropylene films at 0·1% w/w concentration using IR absorption spectroscopy. In medium density polyethylene all the bis-benzophenone homologues are weak antioxidants, the propane derivative being the most effective. The commercial light stabilizers 2,4-dihydroxybenzophenone (DHBZ) and 2-hydroxy-4- n-octoxybenzophenone (HOBZ) were also weak antioxidants but less effective than the bisbenzophenone alkane homologues. In polypropylene, similar results were obtained but in this case the DHBZ was the most effective antioxidant and HOBZ was the least effective. In the presence of a hindered phenolic antioxidant, Irganox 1010, all the stabilizers except the HOBZ were synergistic in the thermal oxidation of polyethylene whereas in polypropylene the effects were antagonistic. In both polymers, with Irganox 1010, the BHB-alkane structures with odd numbered alkane links were superior to those with even numbered alkane links. On photooxidation, the effects were reversed with antagonism being observed in the case of polyethylene and synergism in polypropylene. Surprisingly, all the bis-benzophenone homologues were less effective as light stabilizers than the two simple commercial benzophenone stabilizers HOBZ and DHBZ despite a marked increase in their extinction coefficients. In polypropylene the bis-benzophenone stabilizers with an even numbered alkane chain were the most effective whereas the reverse was the case in polyethylene. In the presence of Irganox 1010, however, the effect was reversed in polypropylene but not in the case of polyethylene. Initial hydroperoxide concentrations in the polypropylene films with and without Irganox 1010 were found to match the trends in light stabilization, confirming earlier theories on the radical scavenging behaviour of these compounds as chain-breaking donors. The thermal and light stability data are discussed in terms of possible cis and trans conformational arrangements of the benzophenone terminal groups on the alkane chains. In the former case this would allow some degree of self quenching as well as the possibility of association with the hindered phenolic hydroxyl groups on Irganox 1010 to give improved stabilization efficiency.

Synthesis and thermal and photo-oxidative behaviour of novel 4-substituted alicyclic amino-2-hydroxybenzophenone stabilizers in polypropylene films

Four light stabilizers have been synthesized and characterized, based on the substitution of an alicyclic amine group into the 4-position of 2-hydroxybenzophenone. The compounds prepared were bis- N, N′-alkylpiperazines, N-propoxymorpholinyl and 4-amino-2,2,6,6-tetramethylpiperidinyl derivatives. The thermal and photo-oxidative stabilization activities of the three compounds have been examined and compared with those of 2-hydroxy-4-n-octoxybenzophenone (HOBZ) in polypropylene film at 0.1% w/w concentration using i.r. absorption spectroscopy, hydroperoxide analysis and yellowness index. Through both carbonyl index and embrittlement times, both alkylpiperazine derivatives were found to be ineffective on exposure to QUV irradiation but both the propoxymorpholinyl and the hindered piperidine compounds were found to be more effective than HOBZ. These stabilizing effects followed closely their ability to inhibit hydroperoxide formation before and during oven ageing. With the commercial hindered phenolic antioxidant, Irganox 1010, all the stabilizers were synergistic, especially HOBZ. On thermal ageing, all the light stabilizers including HOBZ behaved as antioxidants. The hindered piperidine derivative was the most effective, exhibiting a stabilizing effect close to that of the Irganox 1010. In the presence of Irganox 1010, weak antagonism was observed with HOBZ exhibiting the most effective stabilization. Both piperazine derivatives discoloured markedly on irradiation while the alkylmorpholinyl exhibited an initial photobleaching effect. The steric hindrance, molecular size and oxidizability of the cyclic amine are all concluded to be important factors in controlling the antioxidant behaviour of the stabilizer, with hydroperoxides playing a key role in the latter case. The results also demonstrate the importance of the ortho hydrogen bonded hydroxy group in 2-hydroxybenzophenones as an active chain-breaking donor and not the extinction coefficients. It is further concluded that the results show the potential for the development of a single stabilizer entity which will behave as an effective photothermal antioxidant.

Synthesis and thermal and photo-oxidative behaviour of novel aminated 2-hydroxybenzophenone stabilizers in polyolefin films

Three light stabilizers have been prepared and characterized, based on the substitution of a tertiary amine group into the 3- and 5-positions of 2,4-dihydroxybenzophenone (DHBZ) and 2-hydroxy-4-n.octoxybenzophenone (HOBZ) via the Mannich reaction. The derivatives prepared by this method were, the 3- N, N-diethylaminomethyl of the former and the 5- N, N-diethylaminomethyl and 5-morpholinomethyl derivatives of the latter. The thermal and photo-oxidative stabilization activities of the three compounds have been examined and compared with those of DHBZ and HOBZ in medium density polyethylene and polypropylene films at 0.1% w/w concentration using i.r. absorption and fluorescence spectroscopy and hydroperoxide analysis. In medium density polyethylene, all three aminated derivatives are weak antioxidants with the diethylaminomethyl derivative of DHBZ being the most effective. The commercial light stabilizers DHBZ and HOBZ were also weak antioxidants but less effective than the aminated derivatives. In polypropylene, similar results were obtained but in this case both the Mannich derivatives of HOBZ were effective. In the presence of a hindered phenolic antioxidant, Irganox 1010, all the stabilizers except the HOBZ were synergistic in the thermal oxidation of polyethylene whereas in polypropylene the effects were antagonistic. On photo-oxidation, the effects were reversed with antagonism being observed in the case of polyethylene and synergism in polypropylene but only in the case of the morpholinomethyl derivative of HOBZ. Surprisingly, the three aminated derivatives were less effective as light stabilizers than the two simple commercial benzophenone stabilizers HOBZ and DHBZ. Hydroperoxide formation is markedly reduced on oven-ageing of polypropylene by the aminated derivatives and is consistent with the ability of the tertiary amine group to scavenge oxygen and free radicals. Fluorescence quenching studies on the same polymer also show that the aminated derivatives are more effective quenchers than the corresponding commercial HOBZ and DHBZ stabilizers.

Photooxidative Destruction and Stabilization of Polycaproamide by Bis-Aroilenbenzimidazole Derivatives

Abstract Some derivatives of bis-naphthoylenebenzimidazoles have been studied as light stabilizers of polyamide-6 and polyethylene terephtalate. These stabilizers are stable in the melts of these polymers for long times at 260–280°. The light-stabilizing effect of compounds studied is discussed.

Kinetics of photo-oxidative degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide) by metal dialkyl dithio phosphate

The kinetics of photo-oxidative degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide) have been studied in the temperature range 313-363 K using monochromatic light of 365 nm with a constant intensity flux of 8.7 × 10−9 einstein s−1 cm−2 in the absence and presence of Ni(II) O,O′-diisopropyldithiophosphate. The course of degradation and stabilization was determined by means of light scattering and spectroscopic techniques. The extent of photodegradation was followed by carbonyl formation, gel content and activation energy. The incorporation of 1.2 wt% stabilizer in the matrix of the polymer films exercises the maximum protective influence on the photodegradation of the polymer.

Protection of poly(2,6-dimethyl-1,4-phenylene oxide) against u.v. irradiation

The photo-oxidative degradation and stabilization of poly(2,6-dimethyl-1,4-phenylene oxide) films have been examined in the temperature range 263–313 K in air with u.v. light of 253.7 nm. The changes in weight-average molecular weight, quantum efficiencies, carbonyl index, hydroperoxide concentration and i.r. spectra have been followed in the absence and presence of the stabilizer. The heats of activation of the systems have been calculated and the mechanism of stabilization has been postulated. A saturation limit in photostabilization of the polymer was achieved beyond 0.6 wt% of zinc di- n-hexyl dithiophosphate.

Photo-oxidative transformations and stabilization of mineral oils

1. 1. A study was made of photo-oxidation of mineral oils. It was established that at the initial stages the process takes place with molecules of singlet oxygen, formed as a result of photo-sensitizing action of aromatic polycyclic hydrocarbons. 2. 2. Parameters k r characterizing the reactivity of petroleum hydrocarbons in relation to 1O 2 were determined. The valud of A k is about 10 5 mole/sec for atomatic nad 10 4 for petroleum-paraffinic hydrocarbons. 3. 3. Effective stabilizers were found for the photo-oxidative processe of mineral oils and preserving oils.

Effect of copper chelates on U.V. stability of butyl rubber and SBR rubber

An investigation of the photo-oxidative degradation and stabilization of IIR and SBR in the temperature range of 258 to 313°K in air with a monochromatic light of 366 nm in the absence and presence of various concentrations of the different copper chelates has been described. The stabilizer performance in IIR and SBR was accessed by carbonyl index and quantum yield measurements. The changes of IIR and SBR during the irradiation with an ultra-violet light (366 nm) have been conducted by viscometrically and actinometric techniques. Irradiations were conducted on the films at 0, 2, 4, 6, 8, 10, 12 and 14 hours and various parameters were obtained.

Effect of Fillers on the Photooxidative Stabilization of Polyethylene

Effect of Fillers on the Photooxidative Stabilization of Polyethylene