Degradation of pre-aged polymers exposed to simulated recycling: Properties and thermal stability

Accelerated thermal and photo-aging of four homopolymers, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) and high-impact polystyrene (HIPS), was performed and the impact of subsequent reprocessing conditions on their properties studied. Polymer samples oven-aged at 100 °C for varying periods of time or UV irradiated in a Weather-o-meter (WOM) at λ = 340 nm were reprocessed in a Brabender plasticorder at 190 °C/60 rpm for 10 min. Chemical changes and the evolution of rheological and mechanical properties accompanying the gradual degradation of the individual polymers were monitored and evaluated (DSC, FTIR, colorimetric method, MFI, tensile impact strength). LDPE and HIPS were found to be more susceptible to thermo-oxidation than HDPE and PP, whereas HDPE and PP were affected to a greater extent by UV exposure; the crucial role here is being played by the stabilization of the studied resins. In HDPE the scission and crosslinking reactions competed both in thermo-and photo-degradation. In the case of LDPE, scission prevailed over branching during thermo-oxidation, whereas photo-oxidation of the same sample led predominantly to crosslinking. Abrupt deterioration of the LDPE rheological properties after one week of thermal exposure was suppressed by re-stabilization. The scission reaction was also predominant for PP during thermo-oxidation, and it took place even faster during UV exposure. In the case of HIPS a slight photo-degradation of PS matrix is accompanied by simultaneous crosslinking of the polybutadiene component.     

Photodegradation of poly(3-hydroxybutyrate)

The effect of ultraviolet radiation on the properties of poly(3-hydroxybutyrate) (PHB) was studied. The PHB investigated is produced from microbial fermentation using saccharose from sugarcane as the carbon source to the bacteria. The material was exposed to artificial UV-A radiation for 3, 6, 9 and 12 weeks. The photodegradation effect was followed by changes of molecular weight, of chemical and crystalline structures, of thermal, morphological, optical and mechanical properties, as well as of biodegradability. The experimental results showed that PHB undergoes both chain scission and crosslinking reactions, but the continuous decrease in its mechanical properties and the low amount of gel content upon UV exposure indicated that the scission reactions were predominant. Molar mass, melting temperature and crystallinity measurements for two layers of PHB samples with different depth suggested that the material has a strong degradation profile, which was attributed to its dark colour that restricted the transmission of light. Previous photodegradation initially delayed PHB biodegradability, due to the superficial increase in crystallinity seen with UV exposure. The possible reactions taking place during PHB photodegradation were presented and discussed in terms of the infrared and nuclear magnetic resonance spectra. A reference peak (internal standard) in the infrared spectra was proposed for PHB photodegradation.     

热稳定剂对PVC紫外光老化过程中微观结构及宏观性能演变的影响

通过UV-Vis、FTIR、DSC、以及色差、力学性能的测试表征,实时追踪分析了在紫外光老化过程中,含Pb、Sn以及Ca-Zn热稳定剂的PVC体系微观结构和宏观性能的演变过程.结果表明,在相同光老化条件下,PVC/Pb、PVC/Sn和PVC/Ca-Zn体系的微观结构变化规律基本一致,过程中主要的化学反应是,大分子吸收光能后,发生脱HCl生成共轭双键的反应、生成羰基的氧化反应、交联反应和降解反应;不同热稳定剂的作用,主要表现在对于微观结构变化的幅度和动力学过程的影响不同.相应地,3种体系的外观色差和力学性能的变化规律也相似,但色差的变化程度和速度以及老化后力学性能的保持率因所含热稳定剂的不同而不同,其中含Sn体系的颜色稳定性最好,含Pb体系的力学性能保持率最高。     

CaCO3/PEEK复合体系的力学行为和热行为研究

以聚醚醚酮和碳酸钙复合体系为研究对象,考察了偶联剂和填料添加量对复合材料力学行为和热行为的影响.发现磺化聚醚醚酮作为偶联剂能有效地改善材料的力学性能,提高基体树脂的玻璃化转变温度,降低基体树脂的熔点,有助于改善聚醚醚酮的加工条件     

Photochemical and thermal behaviours of poly(vinyl alcohol)/graphite oxide composites

The properties of poly(vinyl alcohol)/graphite oxide (PVAL/GO) composites were investigated during UV irradiation using a mercury lamp (λ = 254 nm). The course of photochemical reactions was monitored by FTIR and UV-vis absorption spectroscopies as well as by estimation of insoluble gel amount formed during crosslinking. Changes in average molecular weights resulting from main chain scission in PVAL were measured by gel permeation chromatography. Composite microstructure was characterized by scanning electron microscopy and X-ray diffraction. The thermal behaviour of composites was determined by a thermogravimetric analysis. It was found that 0.1-5.0% GO addition to polymer bulk slightly hampers photooxidative degradation of PVAL. Thermal degradation in PVAL composites starts at somewhat lower temperatures in the presence of GO but this trend is changed in UV-irradiated samples.     

Effects of ionizing radiation on linear aromatic polyesters

The influence of chemical structure on the response of aromatic polyesters to high-energy ionizing radiation was studied. Systematic variations of polymers related to poly(ethylene terephthalate) were subjected to γ radiation, and the competitive chain scission and crosslinking reactions were determined by measuring changes in intrinsic viscosity and molecular weights. It was found that an increase in the paraffinic glycol segment of polyterephthalates facilitated crosslinking, while the protective nature of aromatic groups was demonstrated by modifying the dibasic acid segments. The influence of substituents on the terephthalate moiety was mixed: electronegative groups led to chain scission (as evidenced by decreased viscosities), but electropositive substituents exerted a stabilizing effect on polymer viscosity. In almost all cases, number-average molecular weights were decreased by exposure to γ radiation, regardless of viscosity behavior. Crystalline melting temperatures of the polymers generally were decreased by the combined radiation effects of chain scission and crosslinking.     

Mechanical properties,surface chemistry,and barrier characteristics of electron beam irradiated‐annealed LDPE/PA6/LDPE multi‐layer films at N2

The effects of electron beam irradiation in the nitrogen environment, on chain scission, crosslinking, crystallinity, mechanical performance, and barrier properties of LDPE/PA6/LDPE multi‐layer films were studied. The evaluation of radiation‐induced crosslinking effect by the gel content measurement and Charlesby–Pinner plot suggested more of crosslinking over chain scission, in all the layers, which was more pronounced in polyethylene phase. The FTIR analysis results showed good agreement with those observed by the gel content measurements. It is believed that the crosslinking reaction had occurred through the C? N bonds in polyamide‐6, and vinyl group in polyethylene layers. The evaluation of radiation effect on the crystallinity and crosslinking of films by FTIR technique showed that by increasing the applied doses, the crystallinity in all the layers was decreased and the crosslinking was increased. The differential scanning calorimetry of irradiated samples revealed that due to the crosslinking reaction, the crystallinity was decreased by the applied dose. The tensile strength of the films was increased and the percent elongation at break was decreased, by increasing the applied doses. This study was also indicated that the radiation‐induced crosslinking effect on the tensile properties was dominantly observed up to 50 kGy. The surface free energy analysis of the films using the contact angle measurement and geometric mean equation indicated that the surface polarity was decreased by increasing the absorbed doses. It was found that due to the decline in the surface polarity and the simultaneously formation of crosslinked network in these films, both water vapor transmission rate and oxygen permeability were significantly decreased. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermal and mechanical properties of radiation crosslinked poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The effect of irradiation temperature on the polymer properties was investigated for the fluoroelastomer poly(tetrafluoroethylene-co-perfluoromethylvinyl ether) (TFE/PMVE). TFE/PMVE samples were γ-irradiated to 150 kGy at temperatures ranging from 77 K to 373 K. Analysis of the sol/gel behaviour, tensile properties, and glass transition temperatures indicated that crosslinking commenced in the temperature range 195 to 263 K, for a dose of 150 kGy. The latter temperature was 13 K below the glass transition temperature. Crosslinking remained relatively constant to higher temperatures. Chain scission reactions were found to occur well below the glass transition temperature and increased at higher temperatures. The optimum temperature for the radiation crosslinking of TFE/PMVE, for the temperatures investigated, was 263 K.     

The effect of ions irradiation on the thermal properties of poly(ether ether ketone)

The effect of irradiating amorphous poly (ether ether ketone), PEEK, with ions, 11 MeV proton (H⁺), and 25.6 MeV helium (He²⁺), has been investigated focusing on the changes in thermal properties. The extent of chain scission and crosslinking was evaluated using the Charlesby‐Pinner equation. Crosslinking increased the glass transition temperature (T_g) in line with the DiBenedetto equation from which the crosslinking constant for each ion was calculated. The effect of irradiation on the thermal degradation kinetics was studied in an argon atmosphere at a constant heating rate by mean of the Chang and the second Kissinger methods. Irradiation significantly reduced the thermal stability of the polymer and its service lifetime. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2212–2221, 2008     

Effect of stabilizer on scission and crosslinking rate changes during photo-oxidation of polypropylene

Chain scission and crosslinking rates have been derived from molecular mass distributions obtained by gel permeation chromatography at different stages during photodegradation of polypropylene samples exposed to ultraviolet irradiation (UV). Results for rubber-toughened polypropylene (PP) containing no photostabilizer are compared with those for the same polymer stabilized using a commercial photostabilizing package (PPS). The samples were in the form of 3 mm thick bars and measurements were obtained at various depths from the exposed surface after different exposure times. The depth profiles for PP and PPS were very different. Reaction rates in the interior of PP showed oxygen diffusion limited behaviour and after prolonged exposure, the rates in the interior of PPS were higher than those in PP. The ratio of scission rate/crosslink rate fell when reaction rate increased. Crosslinking became relatively more likely when reaction rates were low. The low degradation rates obtained with stabilized polymer coupled with the sensitivity of the method of analysis enabled detection of inhibition of photodegradation attributed to residual moulding stresses in the samples.     

Synthesis,curing and properties of poly(phthalazione ether sulfone ketone) copolymers crosslinked by click chemistry

Functionalized poly(phthalazinone ether sulfone ketone) was synthesized by successive chloromethylation and azidation, followed by curing reaction with the propargyl end-groups of various molecular weight crosslinking agents in the presence of Cu(Ⅰ) catalyst via the azide-alkyne click reaction. The influences of the chain length of crosslinking agents on the poly(phthalazinone ether sulfone ketone) system were studied. FTIR and DSC tests demonstrated certain crosslinking by azide-alkyne reaction with the formation of triazole ring. DSC results showed that curing temperature shifted to lower temperatures considerably in the presence of Cu(Ⅰ) catalyst. TGA showed cured polymers were of much higher thermal stability, including higher thermal decomposition temperatures and higher char-yielding properties. After being cured, the polymers became insoluble in organic solvents and the gel fraction of the cured polymers exceeded 71%. Wide-angle X-ray diffraction results indicated there was a short distance order in the poly(ether sulfone)(PES) main chain except for the azido methyl poly(phthalazinone ether sulfone ketone) and 4,4'-bis(2-propynyloxy) biphenyl( AMPPESK-BP) system.     

Phlorotannin and antioxidant responses upon short-term exposure to UV radiation and elevated temperature in three south Pacific kelps

Rapid adjustments of the photosynthetic machinery and efficient antioxidant mechanisms to scavenge harmful ROS are physiologic adaptions exhibited by intertidal seaweeds to persist in temperate regions. This study examines short-term (3 h) responses of three large kelps from the cold-temperate coast of Chile, normally adapted to water temperatures <16°C, but exposed abruptly to simultaneous high temperatures and UV radiation during low tide in summer. The kelps were exposed in the laboratory to three temperatures (10, 20 and 28°C) with and without UV radiation, and photochemical reactions, concentration of phlorotannins and antioxidant activity were examined. The exposure to elevated temperature (slightly exacerbated by the presence of UV radiation) decreased photochemical processes (measured as fluorescence kinetics) in the three studied species and increased lipid peroxidation in two of them. The concentration of total soluble phlorotannins was variable and correlated with the antioxidant activity in the presence of UV radiation. Insoluble phlorotannins did not change during the exposure. In all, the downregulation of the photochemical machinery, which was expressed as dynamic photoinhibition, and the rapid induction of soluble phlorotannins triggered by UV radiation minimized the effects of oxidative stress and maintained the operation of photochemical processes during short-term thermal stress.     

Stability of a salicylate-based poly(anhydride-ester) to electron beam and gamma radiation

The effect of electron beam and gamma radiation on the physicochemical properties of a salicylate-based poly(anhydride-ester) was studied by exposing polymers to 0 (control), 25 and 50 kGy. After radiation exposure, salicylic acid release in vitro was monitored to assess any changes in drug release profiles. Molecular weight, glass transition temperature and decomposition temperature were evaluated for polymer chain scission and/or crosslinking as well as changes in thermal properties. Proton nuclear magnetic resonance and infrared spectroscopies were also used to determine polymer degradation and/or chain scission. In vitro cell studies were performed to identify cytocompatibility following radiation exposure. These studies demonstrate that the physicochemical properties of the polymer are not substantially affected by exposure to electron beam and gamma radiation.     

The sulfonation of crosslinked poly(ether ether ketone)—Diffusion‐controlled kinetics

The effect of crosslinks introduced by ion irradiation with 11.7 MeV proton and 30 MeV helium ions on the reactivity of poly(ether‐ether‐ketone) (PEEK) to sulfonation have been investigated following the kinetics of the reaction at room temperature. Concentrated sulfuric acid was used as a swelling and sulfonating agent and the reaction was followed by changes in the FTIR spectrum. The rate of reaction decreased with the degree of crosslinking and the progress with time was consistent with diffusion control of the sulfuric acid into the crosslinked matrix. The results were consistent with the efficiency of the ions in crosslinking PEEK and in particular with the differences in their linear energy transfer (LET). © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 775–783, 2009     

Mechanical properties,surface morphology and failure mode of γ-ray irradiated blends of polypropylene and ethylene-vinyl acetate rubber

The effect of ⁶⁰Co γ-radiation on the mechanical properties, surface morphology and failure characteristics of blends of polypropylene [PP] and ethylene-vinyl acetate rubber [EVA] has been studied with specific reference to the effect of blend ratio, dynamic crosslinking of the rubber phase and absorbed radiation doses. Samples were subjected to radiation in the dose range of 1 to 100 Mrad in air at room temperature at the rate of 0·321 Mrad/h. Both chain scission and crosslinking occur simultaneously in the blend samples. PP and blends containing higher proportions of PP (≥50%) undergo predominant chain scission at lower doses (≤50 Mrad), which causes a drastic drop in tensile strength, followed by a levelling out at higher doses of 100 Mrad. EVA undergoes crosslinking at lower doses resulting in an increase in tensile strength in the dose range 1 to 10 Mrad followed by a decrease in the range 10–25 Mrad. Further increase in radiation dose has little effect on tensile strength. The effect of radiation on stress-strain behaviour, elongation at break, energy at rupture and hardness was also studied. The morphology of the irradiated surfaces after an absorbed dose of 100 Mrad has been examined by scanning electron microscopy (SEM). In order to understand the effect of γ-radiation on the failure mechanism, tensile failure surfaces of both unirradiated and irradiated samples have also been examined by SEM.     

Mechanical and thermal properties of poly-ether ether ketone reinforced with CaCO3

CaCO₃/PEEK (poly-ether ether ketone) composites were prepared on a twin-screw extruder with different mass ratio of CaCO₃/PEEK from 0% to 30%. Four types of particles were used as filler in PEEK matrix. The influence of surface treatment with sulfonated PEEK (SPEEK) of the particles on the mechanical and thermal properties of the composites was studied. The experiments included tensile tests, flexural tests, notched Izod impact tests, TGA, DSC and SEM. The modulus and yield stress of the composites increased with CaCO₃ particles loadings. This increase was attributed to the bonding between the particles and the PEEK matrix, as can be proved by the SEM pictures of tensile fracture surface of the composites. The impact strength of the composites was modified by the SPEEK coated on the CaCO₃ particle surface. DSC experiments showed that the particle content and surface properties influenced the glass transition temperature (T_g) and melting temperature (T_m) of the composites. The T_g increased with the content of fillers while T_m decreased. In this study the fillers treated were found to give better combination properties, which indicated that SPEEK played a constructive role in the CaCO₃/PEEK composites.     

Mechanical properties and heat shrinkability of electron beam crosslinked polyethylene–octene copolymer

The mechanical properties and heat shrinkability of electron beam crosslinked polyethylene–octene copolymer were studied. It was found that gel content increases with increased radiation dose. The analysis of results by the Charlesby–Pinner equation revealed that crosslinking was dominant over chain scission upon irradiation. Formation of a crosslinked structure in the electron beam irradiated sample was confirmed by the presence of a plateau of dynamic storage modulus above the melting point of the polymer. Wide-angle X-ray diffraction revealed that there was little change in crystallinity for the irradiated samples, indicating that radiation crosslinking occurs in the amorphous region of the polymer. The tensile modulus increases, whereas the elongation at break decreases with increased radiation dose. The heat shrinkability of the material increased with an increased radiation dose because the radiation-induced crosslinks serve as memory points during the shrinking process.     

UV-INDUCED PROTEIN ALTERATIONS AND LIPID OXIDATION IN ERYTHROCYTE MEMBRANES

Certain ultraviolet radiation-induced effects in skin may result from primary photochemical alterations in cell membranes. We have studied isolated erythrocyte membranes in order to determine the UV-fluence and wavelength dependence for protein alterations and lipid oxidation. Protein crosslinking was detected as high molecular weight protein (greater than 200,000 DA) on polyacrylamide/agarose gel electrophoresis. Spectrin decreased more rapidly than the other membrane proteins upon exposure to lambda = 250-380 nm radiation. Nitrogen-purging inhibited the UV-induced decrease in spectrin by 60% and decreased crosslinking to an even greater degree. The decrease in spectrin was not inhibited by superoxide dismutase, catalase, or sodium azide. Radiation at 280 nm was most effective for spectrin loss, 265 and 297 nm were less effective and 254 and 313 nm were not effective. Prior irradiation at 280 nm did not sensitize the membranes to subsequent irradiation at 313 nm indicating that photodecomposition products of tryptophan are not involved. Lipid photooxidation was measured with the thiobarbituric acid assay and was induced at higher fluences of UV radiations than those required for loss of spectrin. These results indicate that the major effects of UV radiation on cell membranes are alterations of proteins and suggest that tryptophan is the major chromophore for these alterations.     

Photochemistry of ring-substituted polystyrenes. II. Photolyses of poly(p-fluoro-, p-chloro-, and p-bromostyrene)s

The photodegradation of thin films of p-fluoro (PPFS), p-chloro (PPCS), and p-bromo (PPBS) styrenes brought about by exposure to 254-nm radiation under high vacuum was studied. Mass spectroscopic measurements indicated that hydrogen and hydrogen halides were the only gaseous products because yields of H₂ and HF from poly(p-fluorostyrene) were much smaller than the corresponding yields of chloro- and bromo-substituted polymers. UV and visible spectra of degraded films indicated the presence of unsaturated species, for the initial rates of formation were comparable in PPFS and PS but considerably greater in PPCS and PPBS. Solubility and molecular weight data indicated simultaneous crosslinking and chain scission; both PPCS and PPBS showed an inordinately high susceptibility to crosslinking. These observations can be rationalized in terms of the energetics of abstraction reactions by H and halogen atoms and in terms of scission of the Ph–Br and Ph–Cl bonds which lead to the participation of radicals in the para position in crosslinking. Some qualitative correspondence between the Hammett parameters of the p-substituents and rates of H₂ formation in the substituted polymers was observed. Quantum yields of gaseous product formation and probabilities of crosslinking and chain scission were also determined for the three polymers. Mechanisms of the various reactions are discussed.     

Atmospheric and soil degradation of aliphatic-aromatic polyester films

The degradation of an aliphatic-aromatic biodegradable polyester film was studied under conditions of solar exposure and soil burial in a tropical area. Film samples were evaluated for changes over 40 weeks by visual examination, scanning electronic microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, mechanical properties, molecular weight, gel content, and thermal properties. Photodegradation played a major role in the atmospheric degradation of the film, causing it to lose integrity and mechanical properties after week 8 due to main chain scission and crosslinking. SEM micrographs and FTIR spectra indicated that photodegradation started at the exposed side of the film and propagated through the polymer matrix after week 8. FTIR spectra also indicated that subsequent photooxidation processes took place. The reduction of molecular weight of the soil burial samples was much slower than that of the non-crosslinked portion of solar exposed film samples. The reduction of number average molecular weight of the non-crosslinked solar exposed samples followed a first order reaction, whereas the soil burial samples show a surface erosion biodegradation behavior. The relationship among total solar radiation, gel content and number average molecular weight indicated that an accumulated total solar radiation of 800 MJ/m², reached in approximately 7 weeks at the exposure site, is required for PBAT mulch film integrity loss.