Photo-oxidative Degradation Process Research Articles

Characterization of photo-oxidative degradation process of polyolefins containing oxo-biodegradable additives

The effects of oxo-biodegradable additives on the photo-oxidation of four types of polyolefin polymers, namely high-density polyethylene (HDPE), low-density PE (LDPE), linear low-density PE (LLDPE), and isotactic polypropylene (itPP) were investigated. Scanning electron microscopy, infrared spectroscopy, differential scanning calorimetry, and wide- and small-angle X-ray scattering were employed to gain insights into the chemical and structural modifications induced by the degradation process. The photo-oxidation preferentially occurred at the sample surface and was favored in the amorphous region of the polymers. The carbonyl index calculated for the neat and additive-containing samples suggested that the additives promoted the photo-oxidative degradation, particularly of the PE polymers, which are intrinsically resistant to oxidation, without altering their degradation pathways. The overall degradation rate of the additive-containing polymers was in the order of itPP > LDPE > LLDPE > HDPE.

Microheterogeneous Triplet Oxidation of Hydrophobic Organic Contaminants in Dissolved Black Carbon Solutions under Simulated Solar Irradiation.

Dissolved black carbon (DBC) is proven to accelerate the triplet-mediated photodegradation of hydrophobic organic contaminants (HOCs). However, its photosensitization mechanisms are not clear. In this study, five HOCs including 2,4,6-trimethylphenol, N,N-dimethylaniline, 17β-estradiol, 17α-ethinylestradiol, and bisphenol A were selected as model compounds to explore the triplet-mediated phototransformation of HOCs in illuminated DBC solutions. All five HOCs presented high organic carbon-water partition coefficient (KOC) values in DBC solutions, indicating the strong sorption capacity of DBC for HOCs. When reaching sorption equilibrium, the apparent pseudo-first-order rate constants of HOCs vs log[DBC] were well fitted with a sorption-enhanced phototransformation model (R2 > 0.98). Using the sorption-enhanced phototransformation model, the degradation rates of HOCs determined at intra-DBC (kDBC,HOCs') were 1-2 orders of magnitude higher than those observed in aqueous bulk solution (kHOCsaq). Moreover, typical triplet quenchers (2,4,6-trimethylphenol and oxygen) exhibited a microheterogeneous quenching effect on the triplet-mediated photodegradation of 17β-estradiol. Therefore, our results suggested that HOCs underwent a microheterogeneous photooxidative degradation process in DBC solutions. Furthermore, a sorption-enhanced phototransformation mechanism was proposed to elucidate the microheterogeneous photooxidative behavior of HOCs in DBC solutions. This study provides new insights into the fate and transport of HOCs in aquatic environments.

Modifications of Polymers through the Addition of Ultraviolet Absorbers to Reduce the Aging Effect of Accelerated and Natural Irradiation.

The photooxidative degradation process of plastics caused by ultraviolet irradiation leads to bond breaking, crosslinking, the elimination of volatiles, formation of free radicals, and decreases in weight and molecular weight. Photodegradation deteriorates both the mechanical and physical properties of plastics and affects their predicted life use, in particular for applications in harsh environments. Plastics have many benefits, while on the other hand, they have numerous disadvantages, such as photodegradation and photooxidation in harsh environments and the release of toxic substances due to the leaching of some components, which have a negative effect on living organisms. Therefore, attention is paid to the design and use of safe, plastic, ultraviolet stabilizers that do not pose a danger to the environment if released. Plastic ultraviolet photostabilizers act as efficient light screeners (absorbers or pigments), excited-state deactivators (quenchers), hydroperoxide decomposers, and radical scavengers. Ultraviolet absorbers are cheap to produce, can be used in low concentrations, mix well with polymers to produce a homogenous matrix, and do not alter the color of polymers. Recently, polyphosphates, Schiff bases, and organometallic complexes were synthesized and used as potential ultraviolet absorbers for polymeric materials. They reduced the damage caused by accelerated and natural ultraviolet aging, which was confirmed by inspecting the surface morphology of irradiated polymeric films. For example, atomic force microscopy revealed that the roughness factor of polymers’ irradiated surfaces was improved significantly in the presence of ultraviolet absorbers. In addition, the investigation of the surface of irradiated polymers using scanning electron microscopy showed a high degree of homogeneity and the appearance of pores that were different in size and shape. The current work surveys for the first time the use of newly synthesized, ultraviolet absorbers as additives to enhance the photostability of polymeric materials and, in particular, polyvinyl chloride and polystyrene, based mainly on our own recent work in the field.

Role of Organo-Modifier and Metal Impurities of Commercial Nanoclays in the Photo- and Thermo-Oxidation of Polyamide 11 Nanocomposites.

The photo-oxidative degradation processes of bio-based PA11 nanocomposites containing montmorillonite (MMT) and the organo-modified Cloisite®30B were investigated to discriminate the influence of organo-modified components on the polymer durability. Indeed, despite the extensive studies reported, there are still ambiguous points to be clarified from the chemical point of view. To this aim, UV-aged materials were analyzed by Size Exclusion Chromatography (SEC), Inductively Coupled Plasma–Mass Spectrometry (ICP-MS) and Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS). This enabled determining changes in both chemical structure and Molar Masses (MMs) induced by light, heat and oxygen exposure. The addition of organo-modified nanoclays strongly affected the PA11 light durability, triggering the macromolecular chains scission due to the typical αH, Norrish I and II mechanisms. However, the main contribution in boosting the photo-oxidative degradation is induced by iron impurities contained into the clays. Conversely, thermo-oxidation process performed at 215 °C was unambiguously affected by the presence of the organo-modifiers, whose presence determined an enhancement of crosslinking reactions.

Halloysite nanotubes and thymol as photo protectors of biobased polyamide 11

The formulation of cost-effectiveness sustainable materials is extensively growing and becoming an attractive approach for both industrial and academic fields. In this context, the bio-based aliphatic polyamide 11 (PA11) has acquired significant interest as environmentally friendly thermoplastic option. In addition, its formulation with selected natural antioxidant and/or reinforced compounds through green processing methods, might improve physical and mechanical properties without sacrificing the intrinsic bio-based nature of the matrix. In this work, we have investigated and compared the photo-oxidative degradation processes occurring on PA11 composites based on thymol and halloysite nanotubes prepared by using ball-milling method. In particular, halloysite nanotubes were used as green nano-container of a natural antioxidant molecule, and reinforced nano-filler as well. Molecular and structural information of photo-exposed samples were obtained by using size exclusion chromatography (SEC) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS). Thermal and mechanical properties were also tested as well as thymol release. Data collected confirmed that PA11 filled with HNTs-Thymol nano-hybrid showed superior durability performance if compared to both pure PA11 and PA11 blend realized by simply adding thymol and HNTs to polymer matrix. Furthermore, we found that HNTs and thymol combined in the nano-hydrid form exhibited an active and synergic role to achieve a major photo stabilization of PA11 biocomposite, without sacrificing its mechanical properties.

Photo-oxidative degradation of TiO2/polypropylene films

Photo-oxidative degradation of polypropylene films with TiO2 nanoparticles incorporated was studied in a chamber of weathering with Xenon lamps as irradiation source. TiO2 powder with crystalline structure of anatase was synthesized by thermal treatments at 400 and 500°C starting from a precursor material obtained by sol–gel method. Composites of TiO2/polypropylene were prepared with 0.1, 0.5 and 1.0wt% of TiO2. The mixture of components was performed using a twin screw extruder, the resulting material was pelletized by mechanical fragmenting and then hot-pressed in order to form polypropylene films with TiO2 dispersed homogeneously. Photo-oxidative degradation process was followed by visual inspection, weight loss of films, scanning electron microscopy (SEM), infrared spectroscopy with Fourier transformed (FTIR), and gel permeation chromatography (GPC).

Effects of sterically hindered N-alkoxyamines on photooxidative stability of reinforced polypropylene

AbstractIn the present paper the influence of Flamestab® NORTM 116 (abbrev. FNOR), a sterically hindered N-alkoxyamine stabilizer, on the photo-oxidative stability of calcium carbonate reinforced polypropylene (abbrev. CCPP) used for production of plastic collars has been investigated. The samples, prepared by melt compounding, were exposed to artificial accelerated photo-ageing carried out at λ>300 nm and 45 °C in air. FT-IR and UV-VIS analysis revealed that the deterioration of neat CCPP takes place very rapidly in comparison with the composites containing FNOR. Hydroperoxides and various carbonyl species are formed as the main degradation products during the photo-oxidation process. Moreover, the intensity of the absorption bands grows with increasing of UV exposure time indicating that the material suffers a photo-oxidative degradation process. Indeed, addition of only 0.1 wt.-% FNOR did not significantly modify the length of the induction period of neat CCPP, but a strong efficiency was obtained by increasing the amount of FNOR (0.5 wt.-%) which shows an increase of eight times more as compared with neat CCPP. Therefore, it may be supposed that OIT is linked to the level of concentration of the stabilizer. This remarkable increase in composite stability demonstrated by a lengthening of OIT is typical for sterically hindered N-alkoxyamine stabilizers since they are very efficient at scavenging freeradicals formed as the polymer degrades. In other words, these UV stabilizers are converted to nitroxide radicals (see Scheme 1) which readily trap the polymer degradation species to produce alkoxyamines. Then, the formed alkoxyamines do subsequently react with peroxy radicals to regenerate the active nitroxide radical, thereby this process (called Denisov cycle) proceed in a regenerative cycle, thus retarding oxidative polymer degradation. The increase of absorbance in the carbonyl region as a function of irradiation time was used to determinate the photooxidation rate. It was found out that the photooxidation rate measured from the slope of linear part of the curves is significantly decreased for the CCPP/FNOR 0.5 composite compared to neat CCPP. In the light of the above mentioned findings, FNOR could be taken into consideration as an UV stabilizer for CCPP.

Weathering of low‐density polyethylene grafted with itaconic acid in laboratory tests

AbstractPolyethylene grafted with itaconic acid was subjected to weathering under laboratory accelerated conditions. The course of the photo‐oxidative degradation process of that material was studied by FTIR spectroscopy both through quantitative measurements of changes in absorbance values at selected wave numbers and through measurements of surface area values for absorption bands which were separated by means of deconvolution. The use of both those procedures of quantitative determinations resulted in a general conclusion that the oxidation process was initiated from the very first moment of irradiation, and it produced ketones, acids, esters (intramolecular and of acetate type), peracids, peresters, hydroperoxides, and alcohols. The molecular weight values and gel number values, which were established as well, pointed out that oxidation was accompanied by cracking of polymer macromolecules, and also by polymer crosslinking to a limited degree. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Potential Iatrogenic Tetracycline Staining of Endodontically Treated Teeth via NaOCl/MTAD Irrigation: A Preliminary Report

This study reported red-purple staining of light-exposed, root-treated dentin when root canals were rinsed with 1.3% NaOCl as initial rinse followed by the use of BioPure MTAD as final rinse. This intrinsic dentin staining occurred irrespective of whether the root canals were filled, and could be observed even in crown dentin when the latter was sequentially immersed in NaOCl and MTAD. Bench-top reproductions of the phenomenon revealed that the exothermic reaction is not an acid-base reaction. Conversely, the reaction is of a redox nature that highly resembled the previously reported mechanism of tetracycline staining, in which photo-oxidation of tetracycline resulted in a red-purple tetracycline degradation product that has a high affinity for hydroxyapatite. This photo-oxidative degradation process is probably triggered by the use of NaOCl as an oxidizing agent, and may be prevented by rinsing the NaOCl-treated dentin with ascorbic acid, a reducing agent, before the application of MTAD.

The effect of ultraviolet light on the mechanical properties of polyethylene and polypropylene films

AbstractMeasurements were made of dynamic mechanical response spectra and stress–strain properties at room temperature on films of isotactic polypropylene and low‐density polyethylene prior and after ultraviolet irradiation in a Xenotest 450 apparatus. The period of irradiation that caused a deep deterioration of ultimate mechanical properties influenced the dynamic mechanical properties only insignificantly. This is attributed to the heterogeneous nature of the photo‐oxidative degradation process which is concentrated in a finite number of sites, thus forming crack precursors rather than changing the material properties in bulk. For a biaxially oriented tubular film of low‐density polyethylene, anisotropic embrittlement after exposure in Xenotest 450 was observed. This even reversed the order of strain‐at‐break values in the two main directions of the film. This is remarkably similar to the effect of artificial incisions introduced into the specimens.