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The effects of UV irradiation to polyetheretherketone fibres – Characterization by different techniques
Abstract
The effects of UV irradiation on polyetheretherketone (PEEK) fibres were investigated in this study. PEEK fibres were manufactured with a melt spinning system and then artificially aged with simulated solar UV light. Fibres were then characterized by mechanical tests, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), rheology, thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). PEEK, best known for its excellent thermal stability, suffered greatly from the effects of UV irradiation. The low UV stability manifested as embrittlement of the fibres in the mechanical tests, increased crosslinking rate in the rheological tests, formation of carbonyl and hydroxyl groups and changes in the nature of the carbon–hydrogen bonds in the FTIR, diminished thermal properties in TGA, and transverse cracks in the SEM photos. DSC was found to be an inaccurate technique for estimating the degradation level of PEEK fibres, whereas the carbonyl index measured by FTIR was found to be the most convenient technique.
... Because the UVA-340 lamp simulates sunlight in the spectrum area that causes the most polymer degradation, this could theoretically ensure better correlation with outdoor test results than other used light sources [38,51]. The dose rate of the chamber was measured as 0.7 W/ m 2 at the UVB range (290-315 nm), 12.1 W/m 2 at the UVA range (315-400 nm), and 3.1 W/m 2 at the visible range (400-600 nm) [52]. The temperature of the UV chamber reached 31 °C when all lamps were functioning. ...
... The temperature of the UV chamber reached 31 °C when all lamps were functioning. Further technical details on the UV chamber are reported in a previous study [52]. For the UV irradiation tests, samples were exposed for 0, 200, 500, and 1000 h. ...
... Figure 15 compares the carbonyl indices of LIC-C* and LIC-P* coatings at different exposure times to UV irradiation. The carbonyl index represents one of the most used metrics in polymer chemical degradation studies, considering that carbonyl compounds generally constitute the main product of degradation reactions [52]. The carbonyl index of LIC-C* coatings rose with increasing exposure time. ...
Icephobic coatings interest various industries facing icing problems. However, their durability represents a current limitation in real applications. Therefore, understanding the degradation of coatings under various environmental stresses is necessary for further coating development. Here, lubricated icephobic coatings were fabricated using a flame spray method with hybrid feedstock injection. Low-density polyethylene represented the main coating component. Two additives, namely fully hydrogenated cottonseed oil and paraffinic wax, were added to the coating structure to enhance coating icephobicity. Coating properties were characterised, including topography, surface roughness, thermal properties, wettability, and icephobicity. Moreover, their performance was investigated under various environmental stresses, such as repeated icing/deicing cycles, immersion in corrosive media, and exposure to ultraviolet (UV) irradiation. According to the results, all coatings exhibited medium-low ice adhesion, with slightly more stable icephobic behaviour for cottonseed oil-based coatings over the icing/deicing cycles. Surface roughness slightly increased, and wetting performances decreased after the cyclic tests, but chemical changes were not revealed. Moreover, coatings demonstrated good chemical resistance in selected corrosive media, with better performance for paraffin-based coatings. However, a slight decrease in hydrophobicity was detected due to surface structural changes. Finally, paraffin-based coatings showed better resistance under UV irradiation based on carbonyl index and colour change measurements.
... Poly (ether-ether-ketone) (PEEK), a linear and semicrystalline thermoplastic resin, represents a typical kind of high performance polymer with excellent thermal stability, chemical and radiation resistance, and mechanical properties. 7 PEEK matrix composite is generally applied in high-technology products, such as aircraft and space devices, to replace metals and reduce weight. 2 Therefore, to study the degradation mechanism of PEEK composite materials is very important for practical applications mentioned above. ...
... Most degradation studies have been focused on the hightemperature degradation and thermal properties of poly(aryl ether)s, which prove that they have excellent thermal resistance and stability. [8][9][10][11][12] Degradation of pristine PEEK fibers under UV irradiation has been studied from a chemical point of view, 7 and the influence of coupled photodegradation and tensile stress on PEEK sheets was tested as well. 13 As the aromatic polymer groups in PEEK can generally absorb the UV irradiation of wavelength under 380 nm, photochemical oxidation reactions have been observed. ...
... 13 As the aromatic polymer groups in PEEK can generally absorb the UV irradiation of wavelength under 380 nm, photochemical oxidation reactions have been observed. 7 Moreover, UV exposure can cause microcracks on the surface of neat resin and composite surface. [14][15][16] Reduced strength and crystallizability, embrittlement, and discoloration of polymer are also observed due to excessive cross-linking and chain scissions. ...
Durability and damage mechanism of carbon fiber–reinforced poly(ether-ether-ketone) composites (T300/PEEK) have been investigated under ultraviolet (UV) and water condensation conditions for 1560 h. The tensile modulus decreased by 5.4% after 1560 h of exposure, while no significant changes were found in tensile strength. The microhardness and elastic modulus of the resin measured by atomic force microscope–based nanoindentation were found to be dramatically increased after 240 h treatment and then decreased after longer treatment. The thermal decomposition temperature decreased from 549° to 522° after 840 h of exposure due to the formation of side chains and low molecular products induced by UV. The damage of resin was attributed to chain scission and recombined cross-linking by UV irradiation and hydrolytic deterioration by hydrothermal conditioning, where the decomposition led to the formation of carbonyl groups and hydroxyl groups, as well as the reduction of ether groups determined by Fourier transform infrared spectroscope. Scanning electron microscopy analysis on tensile fractures near the exposed surface indicated fiber/matrix debonding. The resin on the surface degraded rapidly, and its roughness increased continuously from 30.8 ± 4.1 nm to 88.8 ± 6.8 nm after 840 h of degradation, with the formation of microholes and microcracks. A degradation mechanism was proposed, and the accelerated weather aging affected only the surface region of T300/PEEK.
... Aiming to use PEEK in an out-off-earth environment and on ISS, irradiation effect on this high-performance polymer must be considered. It has been shown by Mylläri et al. [125] that UV irradiation for 1056 h had a little effect on strength and elastic modulus of PEEK fibres, while it significantly deteriorated elongation at break due to random scission of the chains. There are also other works [126,127] that reported even an increase in yield strength of PEEK sheets caused by crosslinking. ...
... There are also other works [126,127] that reported even an increase in yield strength of PEEK sheets caused by crosslinking. Although an increase in T g and zero shear viscosity of PEEK could be seen by increasing irradiation time, but crystallinity and melting temperature are not influenced by this parameter [125]. ...
Poly (ether ether ketone) (PEEK) is a high-performance engineering thermoplastic polymer with potential for use in a variety of metal replacement applications due to its high strength to weight ratio. This combination of properties makes it an ideal material for use in the production of bespoke replacement parts for out-of-earth manufacturing purposes, in particular on the International Space Station (ISS). Additive manufacturing (AM) may be employed for the production of these parts, as it has enabled new fabrication pathways for articles with complex design considerations. However, AM of PEEK via fused filament fabrication (FFF) encounters significant challenges, mostly stemming from the semi crystalline nature of PEEK and its associated high melting temperature. This makes PEEK highly susceptible to changes in processing conditions which leads to a large reported variation in the literature on the final performance of PEEK. This has limited the adaption of FFF printing of PEEK in space applications where quality assurance and reproducibility are paramount. In recent years, several research studies have examined the effect of printing parameters on the performance of the 3D-printed PEEK parts. The aim of the current review is to provide comprehensive information in relation to the process-structure-property relationships in FFF 3D-printing of PEEK to provide a clear baseline to the research community and assesses its potential for space applications, including out-of-earth manufacturing.
... However, the corresponding wear rate increased by an order of magnitude compared to the non-irradiated sample. According to DSC tests, UV irradiation does not affect the polymer crystallinity and melting point of the tested PEEK fibers, but significantly increases the crosslinking rate, making the fibers brittle [28]. Sheet samples of PEEK with a thickness of 0.22 mm were irradiated with an electron beam with an energy of 10 MeV to doses of 1500 kGy in air at room temperature to study the process of radical formation [29]. ...
In this work, the mechanical and tribological characteristics of polyetheretherketone (PEEK) sheets were enhanced by electron beam irradiation. PEEK sheets irradiated at a speed of 0.8 m/min with a total dose of 200 kGy achieved the lowest specific wear rate of 4.57 ± 0,69 (10−6 mm3/N−1m−1), compared to unirradiated PEEK with a rate of 13.1 ± 0.42 (10−6 mm3/N−1m−1). Exposure to an electron beam at 9 m/min for 30 runs, with a dose of 10 kGy per run for a total dose of 300 kGy, resulted in the highest improvement in microhardness, reaching 0.222 GPa. This may be due to the decrease in crystallite size, as indicated by the broadening of the diffraction peaks in the irradiated samples. According to the results of thermogravimetric analysis, the degradation temperature of the irradiated samples remained unchanged at 553 ± 0.5 °C, except a sample irradiated at dose 400 kGy, where the degradation temperature shifted towards a lower position of 544 ± 0.5 °C. Differential scanning calorimetry results revealed that the melting temperature () of the unirradiated PEEK was about 338 ± 0.5 °C, while a high temperature shift of the was observed for the irradiated samples.
... The chemical-physical characteristics of the utilized micro-and nanoplastic debris have to be determined by appropriate techniques [2]. For example, photoxidation has been recently proposed to utilize the carboxylic index that is determined by attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) [166]. Additional methods to complete the aged plastics surface characterization are the Scanning Electron Microscopy (SEM) coupled with N 2 BET specific surface areas and Differential Scanning Calorimetry (DSC) [14]. ...
The fate, properties and determination of microplastics (MPs) and nanoplastics (NPs) in soil are poorly known. In fact, most of the 300 million tons of plastics produced each year ends up in the environment and the soil acts as a log-term sink for these plastic debris. Therefore, the aim of this review is to discuss MP and NP pollution in soil as well as highlighting the knowledge gaps that are mainly related to the complexity of the soil ecosystem. The fate of MPs and NPs in soil is strongly determined by physical properties of plastics, whereas negligible effect is exerted by their chemical structures. The degradative processes of plastic, termed ageing, besides generating micro-and nano-size debris, can induce marked changes in their chemical and physical properties with relevant effects on their reactivity. Further, these processes could cause the release of toxic oligomeric and monomeric constituents from plastics, as well as toxic additives, which may enter in the food chain, representing a possible hazard to human health and potentially affecting the fauna and flora in the environment. In relation to their persistence in soil, the list of soil-inhabiting, plastic-eating bacteria, fungi and insect is increasing daily. One of the main ecological functions attributable to MPs is related to their function as vectors for microorganisms through the soil. However, the main ecological effect of NPs (limited to the fraction size < than 50 nm) is their capacity to pass through the membrane of both prokaryotic and eukaryotic cells. Soil biota, particularly earthworms and collembola, can be both MPs and NPs carriers through soil profile. The use of molecular techniques, especially omics approaches, can gain insights into the effects of MPs and NPs on composition and activity of microbial communities inhabiting the soil and into those living on MPs surface and in the gut of the soil plastic-ingesting fauna.
... On the other hand, PEEK-5% TiO 2 has the same crack growth surface (Fig. 11 c) and d)). All the evidences allow to assert that probably only the skin surface of PEEK-5% TiO 2 sample is UV aged since as known, the increase in the concentration of carbonyl groups revealed by IR analysis can often be observed well before mechanical properties change [60]. ...
In the paper, extruded filaments of polyetheretherketone (PEEK) filled (1%v, 3%v, 5%v) with sub-micrometric titanium dioxide (TiO2) particles have been manufactured with the aim to increase the UV radiation resistance of PEEK matrix. The TiO2 particles affect crystallinity of PEEK matrix as it increases increasing filler content. The filler presence slightly increases the stiffness and the elastic modulus of resulting composites and does not influence largely the tensile strength but show an influence on the elongation at break enhancing a finger like failure mechanism ascribed to microcracks propagation and interspherulitc fracture. UV aging test showed the effectiveness of TiO2 particles in reducing the photo-degradation effect especially in 5%v filled sample resulting the most effective formulation. For this formulation, after UV aging the mechanical properties and the failure mechanism remains unchanged while the neat PEEK sample shows embrittlement and loss of ductility.
... The elastic modulus and yield stress results confirms the high chemical and structural resistance of the PEEK to the ageing conditions employed. For PEEK at room temperature, Nguyen and Ishida 11 reported an elastic modulus of 4 GPa and a yield stress of 100 MPa, and similar values also were reported in the works of Lai et al. 37 and Myllari et al. 42 . This elevated structural stiffness observed for PEEK is typical of polymeric materials of the poly (aryl ether ketone) class. ...
Special polymers have been used in the manufacture of storage structures and pipelines avoiding corrosive processes during ethanol fuel transport/storage. Therefore, this work investigated comparatively the effects of the ethanol on the physical-mechanical properties of poly (ether ether ketone) (PEEK) and polyamide 11 (PA-11) based on ageing tests. The WAXD and DSC results demonstrated slight reductions on the crystallinity degree of the aged PEEK, contrariwise to what happened with PA-11, where Xcincreased after ageing. However, the results of thermal, thermomechanical and mechanical analysis (TGA, DMTA, tensile and micro-IITs) demonstrated that PEEK is stable and no significant changes were observed in its elastic modulus (Ey≈ 3.4 GPa, E’ andEit≈ 3.7 GPa) or glass transition temperature. PA-11, conversely, was sensitive to ethanol fuel and expressive changes of its physical-mechanical properties were verified. For both materials, a reasonable correlation between crystallinity and mechanical properties was established.
... Nota-se uma similaridade entre os espectros e o aparecimento de uma banda característica da carbonila (1700 cm -1 ) nos espectros do material degradado. Isto sugere a ocorrência de uma modificação estrutural do polímero por meio da cisão de cadeias e o surgimento de compostos carbonilados, como aldeídos e cetonas, característicos da degradação oxidativa [6,10,11] . Sendo assim, fica evidente que a degradação ultravioleta nos tempos estudados ocorre preferencialmente pelo mecanismo de cisão das cadeias, seguido da redução da massa molecular que resulta em uma tenacidade reduzida dos grupos degradados, como já observado pelo comportamento frágil do material no ensaio de fadiga. ...
... Polymer chain break could lead to an increase in chain movement and explain the observed reduction in T g . For UV-treated samples, a small increase in T g was observed, photodegradation process could motive cross-linking that causes decrease in mobility and free size of the polymer chains [72]. ...
In this work, the effect of low-pressure cold plasma treatment, UV and the incorporation of TiO2 nanoparticles on thermal degradation and flammability of titanium dioxide (TiO2)/polyetherimide (PEI) nanofibers was evaluated. The morphology of nanocomposite fibers was studied using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscope (XPS), thermogravimetric analysis (TGA) and differential scanning calorimetry. SEM images of plasma- and UV-treated nanocomposites show that some samples, e.g., those treated at 248 W and 4 min of exposition, seem more brittle compared with other samples. XPS of treated PEI nanocomposite showed that cold plasma treatment breaks C–C/C–H, C–N and C–O bonds at the surface ether groups leading to an increase in carboxyl and O–C bonds. Moreover, UV treatment breaks C–C/C–H and C–N bonds. Surface modification leads also to changes in thermal stability of PEI nanofibers with a decrease of ~ 4 °C in glass transition temperature (Tg) and a reduction of ~ 200 °C in onset temperature under air compared to pristine PEI nanocomposite. Flammability results measured by pyrolysis-combustion flow calorimeter also showed a decrease in initial degradation temperature and a small increase in total heat released. Furthermore, incorporation of TiO2 nanoparticles leads, as expected, to an improved flame retardancy with a reduction of ~ 40% in peak heat release rate as a function of TiO2 content, but no significant difference was observed in onset degradation temperature observed by TGA. These results show that surface treatments do not significantly change thermal behavior of PEI nanocomposites and could be used for applications that require materials with improved characteristics.
... Since degradation reactions initially only occur in a thin layer of the sample, the increase in the concentration of carbonyl groups can often be observed well before the mechanical properties change. 32 The carbonyl index of aged fibers rises with the increasing ozone treatment time and temperature (Table S1). This indicates a larger number of carbonyl groups forming during ozone degradation of the polyamide fiber. ...
The effects of ozone aging on the structure and related properties of Poly(hexamethylene adipamide) (PA66), Poly(p-phenylene dodecanediamine) (PA12T), Poly(m-phenylene isophthalamide) (PMIA) and Poly(p-phenylene terephthalamide) (PPTA) continuous filaments were evaluated. It can be demonstrated that the free radical oxidation reaction occurs in polyamide fibers during ozone aging, resulting in molecular chain degradation, accompanied by color changes. Molecular degradation dominates the subsequent aging process, resulting in a decrease in viscosity, mechanical strength, crystallinity, and thermal stability and an increase in the formation of oxygen-containing group products. PA66, PA12T, PMIA, and PPTA show different descending trends in their ozone aging resistance, due to the different chemical structures of their molecular main chains. The more stable the generated free radicals are, the higher the ozone aging resistance of the corresponding polyamide is. PPTA has the best ozone aging resistance, followed by PA12T and PMIA, with PA66 being the least aging-resistant.
... Indeed triboelectrification and triboemission (emission of electrons) are known to occur during rubbing and have been linked with polymer chain scission [56]. Note that some features in FTIR spectra of PEEK transferred materials are similar to those of plasma treated PEEK in literature [9,31,38,57]. Hence, the effect of triboplasma on the rubbing surfaces and transferred materials must be considered. ...
Polyetheretherketone (PEEK) is a high performance polymer that can be an alternative to metal for some moving components in unlubricated conditions. During rubbing, PEEK is transferred to the counterface. The formation and properties of PEEK transfer films on steel and sapphire are studied by in-situ observations of PEEK wear process, contact temperatures and triboemission, as well as FTIR and Raman spectroscopies ex-situ. Our results suggest that frictional heating alone may not be sufficient to generate PEEK degradation observed in the transfer materials. Triboplasma observed during rubbing, together with mechanical shear, may promote generations of radicals and degradation of PEEK, which subsequently influence the properties of PEEK transfer film and performance of polymer-metal tribopair.
... The degradation products of the cracked phenyl rings give new non-aromatic hydrogen carbons peaks at 3067, 2853 and 2922 cm −1 and from 1365-1465 cm −1 [53]. These findings are surprising because it suggests that the degradation of PEEK at a PEEK-steel contact is associated with UV-photodegradation [53][54][55] as well as thermal degradation. FITR spectra of polymer debris adhered to the steel ball are similar to those of the PEEK transfer layers, with even more broadening of peaks. ...
Polyetheretherketone (PEEK) is one of the most commonly used High Performance Polymers (HPP) although its high temperature performance is poor. In this study, polybenzimidazole (PBI), a HPP with one of the highest glass transition temperatures currently available, is blended to PEEK to form a 50:50 blend (TU60). Tribological performance of the blend (TU60) was investigated by rubbing it against steel at temperatures up to 280 °C. Results obtained are compared to those from neat PEEK and neat PBI. All three polymers were thermally stable during the duration of tests. However chemical analyses on polymeric transfer layers on steel surfaces and polymer debris suggest polymer degradation. The degradation observed is shear-assisted, possibly promoted by shear heating. Indeed the estimated interfacial temperature based on Jaeger model was above the melting point of PEEK in some cases. TU60 outperforms PEEK in all test conditions and PBI at 280 °C. TU60 formed transfer layers on steel similar to that of PEEK. When contact temperature is closed to the melting point of PEEK, PEEK in the TU60 creates a low strength transfer layer which acts as an interfacial lubricant. This reduces friction which in turn reduces PBI degradation in TU60 at high temperature. This work provides a strategy for creating interfacial layers to improve polymer tribological performance while maintaining the integrity of the polymer.
... This is primarily because the process can be conducted under atmospheric pressure, resulting in lower equipment and running costs. The individual and combined effects of UV light and ozone have been studied for various polymer surfaces, including styrene-butadine-styrene rubber (SBS) 16 , polydimethylsiloxane (PDMS) 8,17 , polyethylene (PE) 18 , polyethylene terephthalate (PET) 19 , polypropylene (PP) 20 , polyetheretherketone (PEEK) 21 , as well as other surfaces. Here, we present an experimental and molecular modeling study for the anchoring of the CNT onto the organic polymers polyamide and polypropylene (Figure 1), which in addition to study the design of new surfaces, may lead to the design of sensors and CNT based composites 22 . ...
In this work we study hydroxylated carbon nanotube (CNT) assembly on polyamide (PA) and polypropylene (PP) polymers activated by UV radiation from a theoretical and experimental perspective. Molecular computer simulation was done to understand the stable conformations and bulk properties (molecular dynamics) of the polymers before and after exposure to UV radiation at the molecular level. Our experiments suggest that PA presents more -OH active groups, producing a more hydrophilic surface, whereas PP exhibits less potential UV activation. These results suggest that it is possible a facile covalent functionalization method to tune organic polymer surface properties through SWCNT anchoring for nanotechnological applications requiring defined surface roughness and chemical functionality on inexpensive polymers.
Carbon fiber‐reinforced high‐performance thermoplastic composites have recently become an efficient alternative for aerospace engineering applications. However, the temperature sensitivity of semi‐crystalline carbon fiber/polyether‐ketone‐ketone (CF/PEKK) polymers revealed the necessity of investigating their performance in service conditions. This study aims to evaluate the effect of extreme service conditions on thermomechanical performance and fracture characteristics of CF/PEKK composite laminates. For this, aerospace‐grade composite laminates were manufactured with the automated fiber placement process and were exposed to extreme service temperatures of −50°C (Conditioned I), 180°C (Conditioned II), and initially 180°C following −50°C (Conditioned III), simulating critical service temperature ranges in aerospace applications. According to impact tests, the energy absorbance of CF/PEKK composites decreased in all thermal conditioning scenarios by up to 25%. Additionally, Conditioned I samples represented relatively low glass transition temperature and degree of crystallinity compared to the control samples; however, Conditioned II and Conditioned III samples exhibited an opposite behavior. Dynamic mechanical analysis (DMA) investigations revealed a 13% reduction in the storage modulus for all thermal conditionings. While CF/PEKK composites represented ductile/brittle behavior at room temperature and high/low conditioning temperatures, their brittleness increased at −50°C, and the structure became ductile at 180°C. This study confirms that DMA is a powerful tool for determining the glass transition temperature for fiber‐reinforced composites with higher sensitivity and accuracy than DSC.
Exposing engineering plastics to UV irradiation can easily destroy the original molecular structure of the materials and consequently affect their tribological properties. This study investigated the effects of UV irradiation on the molecular structure of typical engineering plastics, such as polytetrafluoroethylene (PTFE) and polyether ether ketone (PEEK), and on their tribological properties under heavy loads (20 MPa). The surface morphology results showed that the appearance of PEEK changed significantly under UV irradiation. However, the change in PTFE was negligible. Under micromorphology, the processing lines of the two materials gradually became lighter with increasing UV irradiation time. The resulting infrared spectra showed that the molecular chains of both materials were broken, and new functional groups were formed under UV irradiation. Tribology testing demonstrated that with prolonged UV irradiation, the average PTFE coefficient of friction remained relatively stable, whereas that of PEEK was approximately 0.55. As the UV irradiation time increased, the wear rate of PTFE increased significantly, whereas that of PEEK showed no significant change.
On-orbit manufacturing can reduce the cost and time needed by space exploration missions because in-situ maintenance activities can be achieved without the need for additional launches. Furthermore, recent developments in materials science in terms of better mechanical and thermal performance have allowed this application to become a potential reality. However, the deployment of on-orbit manufacturing presents several challenges, including the lack of convective heat transfer and human intervention. This paper proposes an on-orbit 3D printing device capable of operating at a temperature up to 400°C in the vacuum environment. To validate its feasibility for on-orbit manufacturing, we designed four extruders with different characteristics. We investigated the temperature profiles across the extruders under the vacuum condition through a heat transfer model. Based on the thermal analysis, a thermal control method, which combines the Proportion (P) and Fuzzy Proportion and Integration (Fuzzy PI) strategies, is designed to regulate the 3D printing device operation. With the extrusion rate of 8654.6 mm3 /h and the printing temperature at 400°C, the melting and solidification status of the PEEK (Polyether ether ketone) material is verified.
Cette thèse s’inscrit dans une thématique de recherche liée au comportement des polymères en environnement spatial. Elle présente deux objectifs : étudier le vieillissement du PolyEtherEtherKetone (PEEK) sous irradiation électronique et optimiser ses propriétés électriques afin de limiter les phénomènes de charge de surface. Pour cela, des composites PEEK / Fibres Courtes de Carbone ont été élaborés. Le seuil de percolation électrique des fibres a été déterminé à un taux volumique de 9 %. Les applications spatiales du PEEK nécessitant un comportement isolant électrique, le taux de charges de 3 %vol. a été sélectionné. La présence des fibres permet d’améliorer la conductivité électronique à température ambiante, même en-dessous du seuil de percolation électrique. Les matériaux ont alors été soumis à un flux d’électrons de haute énergie afin de simuler leur vieillissement en environnement spatial. L’analyse des échantillons irradiés a mis en évidence deux phénomènes de vieillissement simultanés : une réticulation de la phase amorphe et une amorphisation de la phase cristalline. L’irradiation au voisinage de la transition vitreuse (165 °C) conduit à une densité de réticulation plus importante due à un taux de recombinaison des radicaux plus élevé. Dans les composites, les fibres limitent l’amorphisation et stabilisent le comportement mécanique. Vis-à-vis des propriétés électriques, le vieillissement induit une diminution de la conductivité ionique au-dessus de Tg. Dans les composites, cette diminution est amplifiée. À température ambiante, l’irradiation à 25 °C ou à 165 °C conduit à des évolutions opposées du transport électronique associées à la compétition entre réticulation et amorphisation. Dans les composites, les fibres stabilisent l’évolution de la relaxation de potentiel et permettent toujours un écoulement plus rapide des électrons.
Adhesives for carbon fiber-reinforced poly (etheretherketone) (CFR-PEEK) have attracted the interest of researchers as an effective means for bonding this newly developed lightweight and high-performance composite structures. In this study, we developed a method to overcome the lack of interaction between adherends and adhesives through the modification of glycidyl methacrylate (GMA) brushes via surface-initiated photopolymerization. To achieve a better formation of PGMA brushes, some parameters of the surface-initiated photopolymerization, for example, light intensities (e.g., 2, 4, and 8 mW/cm²) and exposure durations (e.g., 15, 30, 45, 60, 90, 120 min) of ultraviolet (UV) treatment, were optimized. The joints using the adherends with an optimized PGMA layer exhibited more than a two-fold increase in the ultimate lap shear strength compared to that using the bare CFR-PEEK adherends, when a commercially available epoxy film adhesive, FM 309-1, is utilized. The influence of the UV intensities was further revealed via a double cantilever beam test, and the adherends with the modification under 4 mW/cm² of UV irradiation for 2 h exhibited the best performance, owing to the higher grafting density of the PGMA brushes.
Poly(etheretherketone) (PEEK) is a high-performance engineering thermoplastic with high heat deflection temperature. Owing to its low surface free energy, the untreated PEEK adherend may exhibit low bond shear strength with epoxy adhesives. This work presents a method to improve the adhesion performance of PEEK with epoxy adhesives through poly(glycidyl methacrylate) (PGMA) grafting. The PGMA-grafted PEEK (PEEK-g-PGMA) was prepared via surface-initiated photopolymerization under 2 and 8 mW/cm², respectively. The influence of the irradiation duration and the removal of the polymerization inhibitor from the monomer on PGMA grafting are discussed herein. The results showed that (1) the PEEK-g-PGMA is obtained from the photopolymerization under 2 and 8 mW/cm² of UV irradiation; (2) the long durations of the UV exposure promoted the formation of brushes; and (3) the polymerization inhibitor only slightly inhibited the polymerization, but effectively prevented the auto-polymerization of monomer solution during the UV irradiation. The enhanced performance of the epoxy adhesives was demonstrated by the single-lap shear tests using PEEK-g-PGMA joints. Under optimized concoctions, the lap shear strength of PEEK-g-PGMA was significantly enhanced to 15.1 MPa which is seven times higher than that of unmodified PEEK (2.5 MPa).
The interphase size and properties of carbon fiber reinforced poly(ether-ether-ketone) composites (T300/PEEK) were quantitatively characterized using Peak Force Quantitative Nano-Mechanics (PF-QNM) with atomic force microscopy (AFM). The elastic modulus maps of the interfacial region were obtained, and the interphase dimension was determined based on the elastic modulus profile. The interphase thickness of T300/PEEK under coupled ultraviolet (UV) and hydro-thermal degradation decreased from 70.1 ± 8.6 to 18.3 ± 1.8 nm after 1560 h of exposure. The shrinkage of interphase size was attributed to the embrittlement of PEEK after crosslinking reactions induced by UV exposure. This technique shows a great potential in the quantitative measurement of nanomechanics relevant in polymer and polymer composites, where the structure and properties of the interphase are of special interest.
The effect of strain rate on poly(ester-ester-ketone) (PEEK) was investigated using an atomic force microscopy (AFM) based nanoindentation technique and applying a wide range of strain rates (0.012 s−1 ∼ 1 s−1). The test results show that the average hardness and elastic modulus of PEEK follow a linear model with respect to logarithm of strain rate, from 263.9 MPa/1.377 GPa to 323.1 MPa/2.477 GPa. The maximum indentation depth decreased from 287.1 nm to 239.6 nm, indicating an enhanced densification area and shear transform zones under the indentation region. The plasticity index showed a rapid increase at low strain rate and kept stable at ∼0.640 until a strain-rate of 0.037 s−1. Moreover, a ‘pile-up’ phenomenon appeared around the residual indentation area due to more free volume around the cube-corner indenter than a Berkovich indenter during the loading process.
This paper investigates the effects of thermal degradation on polyetheretherketone (PEEK) fibres. PEEK samples were aged at a constant temperature of 250 °C for 1–128 days and characterized with mechanical tests, FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry), rheology, TGA (Thermogravimetric Analysis), SEM (Scanning Electron Microscopy), and UV–Vis diffuse reflectance spectroscopy. The short-term thermal annealing had a positive effect on the mechanical properties, due to the formation and growth of secondary crystals. Crosslinking in the material was verified by rheological inspections. The crosslinking increased the mechanical strength and modulus but reduced the elongation at break of the fibres. FTIR tests showed that carbonyl and hydroxyl groups were slowly formed on the surface of the fibres while ring opening reactions took place. The thermal ageing reduced the thermal stability of PEEK. The decreased stability was observed in the decomposition onset temperature after 8 d and in the melting point and the glass transition temperature after 32 d. The first signs of degradation, crosslinking, embrittlement, and reduced thermal stability, were visible roughly after 8 d of ageing, whereas the deterioration in general usability occurred after 64 d.
Rheology and FTIR spectroscopy are compared as methods to study the degree of photodegradation in polypropylene (PP) and polystyrene (PS) sheets. The materials are hot pressed, artificially photo-aged with fluorescent lights for 4–2048 h and then measured with a rotational rheometer and FTIR. Both materials show a tendency for chain scission which can be seen as a reduction in viscosity. Changes in PP can be observed with both methods after 256 h of irradiation. Changes in PS become significant in rheology after 64 h but in FTIR only after 1024 h of irradiation. Due to the different chemical nature of the materials, the degradation of PS is rather linear with exposure, whereas the degradation of PP is more exponential. Using the zero shear viscosities obtained through extrapolations of the Cole–Cole and Carreau–Yasuda models, relative molecular weights are estimated with the aid of the power–law relationship between these two. These results are compared with the carbonyl indices determined from the FTIR spectra. Rheology is found to be a viable alternative for FTIR in certain situations. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42246.
An attempt has been undertaken to assess the effect of UV radiation on the structure of polyamide and polypropylene fibres, which are characterised by various macroscopic features, colours and additives. Based on the measurements we performed, we were able to conclude that UV radiation under the exposure conditions used brings about changes in both the fibre structure and mechanical properties. The extent of these changes is clearly dependent on the initial fibre structure, added modifiers and macroscopic features.
A review of the literature on the flammability and decomposition of poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK) is presented. This paper provides an overview of the flammability of PEEK and its decomposition mechanisms. Based on this literature, mechanisms have been suggested which attempt to explain the products formed at each stage of PEEK decomposition and indicate the intermediates which should be formed at each of these stages.
The photo-oxidation of poly(α-methylstyrene) (PαMS) has been studied by FTIR and mass spectroscopy. The samples have been submitted to various conditions of irradiation under oxygen, with different irradiation sources and at several temperatures. SF4 and NH3 treatments and photolysis under vacuum have been carried out on the photo-oxidized samples. The results obtained were compared to those obtained in the same conditions of irradiation for polystyrene samples. An identification of the photoproducts is proposed with their main ways of formation.
FTIR-microscopy has become one of the foremost vibrational spectroscopy techniques for problem-solving and analysing and mapping the chemical structure and physical characteristics associated with industrial materials and their fabricated products. Many recent advances have utilised the attributes of reflection techniques, such as specular reflection spectroscopy approaches, while emerging capabilities becoming available to the industrial spectroscopist include both spectral imaging and use of synchrotron radiation as a source. This paper seeks to illustrate each of these recent advances and developments through applications of FTIR-microscopy to industrial problem-solving case studies.
The differential scanning calorimetry (d.s.c.) heating thermograms of 12 poly(aryl ether ether ketone) (PEEK) samples of varying degrees of crystallinity have been recorded. The relation found between the degree of crystallinity as determined by specific gravity measurements, and the melting enthalpy of the polymer, shows that recrystallization is occurring during a heating scan rate of 10-degrees-C min-1. This implies that d.s.c. is not a convenient technique to assess PEEK crystallinity. The infra-red absorbance spectra of the same samples have also been examined in the range from 1030 to 880 cm-1. The 965 cm-1 band, up to now considered as indicative of the PEEK crystallinity, is shown to be practically independent of the degree of crystallinity above 15%. However, there is evidence to support the existence of a true i.r. crystalline band located at 947 cm-1. It is also suggested that the 965 cm-1 band is due to a normal vibration mode of a short segmental conformation, whose presence is favoured in the crystalline phase, but also in the amorphous zones nearest to the crystallite surface.
The rates of degradation of two types of polypropylene (PP) fibres caused by sunlight and xenon tube light were assessed. Changes in the tensile strength and elongation at break of the fibres as well as changes in the fibre surface topography due to their irradiation with the same doses of UV radiation of both types of light were compared. The tensile strength of PP fibres exposed to xenon tube light decreases four up to six times faster than that of PP fibres exposed to sunlight during the summer season. The elongation at break of these fibres decreases about four times faster. The light emitted by an xenon tube causes considerably greater damage to the fibre surface in the form of crosswise cracks, although there are fewer of which than in the case of sunlight.
This study has been carried out to investigate the processing parameters affecting polyetheretherketone's (PEEK) spinnability in a melt spinning process. PEEK has excellent mechanical and thermal properties and fibers made from it could be used in extreme environments. Different PEEK grades were characterized thermally and rheologically to see which one is the most suitable for fiber spinning. The spinning tests made with the most suitable grade (Victrex 151G) show that increased processing temperature, increased capillary diameter or shorter spinning path length improves spinnability. The best fibers made in optimal processing conditions (400°C temperature, 30/1 mm capillary, and 5 cm spinning path) were 18 μm in average diameter. Because of the limitations of the system used, variations in fiber thickness were noticeable and worsened the spinning stability. Scanning electron microscope photos confirmed these variations, and they were also visible in an optical microscope. The selected low‐viscosity PEEK grade provided good spinnability but gave filaments with only mediocre mechanical properties, the tensile strength being around 280 MPa. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
Photo-oxidatative degradation of isotactic polypropylene (PP) has been examined in samples made by injection and compression moulding and using different moulding conditions. Samples were exposed to ultraviolet radiation (UV) in the laboratory for periods of up to 48 weeks. The extent of chemical degradation was assessed by gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FTIR); the specimens were tested in tension and the fracture surfaces were inspected by scanning electron microscopy. The processing pathways defined the structural characteristics of the starting materials, as determined by light microscopy and X-ray diffraction. The investigation conducted here indicates that the fractional crystallinity is the main structural parameter controlling the rate of degradation of polypropylene. The mechanical properties of degraded PP were shown to depend not only on the extent of chemical degradation but also on the character of the polymer physical structure, especially the spherulite size. With most samples studied here a partial recovery in tensile properties was observed after 6–9 weeks exposure. Analysis of the fracture surfaces showed that improvement in the tensile properties coincided with the appearance of a ductile zone inside the surface embrittled layer, indicating that the cracks in the surface layer were arrested on reaching the ductile material in the interior. Surface cracks were formed spontaneously after 9 weeks of UV exposure and the pattern and concentration of these cracks also depends on the processing type and conditions. In injection moulded samples, surface cracks were of the form of circular arcs radiating from the injection gate and they were correlated with the flow lines generated during mould filling.
A novel class of aliphatic polyesters, and their derived copolyesters, have been developed. Beside their specific mechanical and thermal properties, the durability, in terms of biodegradability and photodurability, has been investigated. In particular, the polymers were submitted to natural and accelerated photo-ageing and an original methodology based on melt rheology has been applied to determine molecular changes upon UV weathering. Both scission and recombination reactions, which strongly compete as a function of the exposure time, were found to cause a strong evolution of the molecular structure. The results indicate that chemical structure and stereochemistry of the novel materials define the predominant process and the overall behaviour of the samples upon UV exposure. Moreover, the changes of the molecular structure, induced by UV irradiation, could have a significant role into the further biodegradability of the polymers. Therefore, while the relationships between structure and durability enable to design materials with desired well-adapted performances according to their final destination, the biodegradable character upon lifetime use is considered as really questionable and needs further studies.
This paper describes the vacuum ultraviolet (VUV) radiation durability screening testing of thin (12.7–25.4 μm) polyimide films proposed for use on the Next Generation Space Telescope (NGST) sunshield. Materials included in this screening test were Kapton®E, Kapton®HN, Upilex®S, CP1, CP1 with vapour deposited aluminium (VDA) on its back surface, and CP2 with a VDA coating on its back surface. Samples were exposed to approximately 1000 equivalent sun hours (ESH) of VUV radiation and examined for changes in solar absorptance, thermal emittance, ultimate tensile strength, and elongation at failure. Changes in the solar absorptance were observed for some materials, and, additionally, significant changes in spectral reflectance were observed in the ultraviolet to visible wavelength region for all of the polyimide materials tested. Changes in the ultimate tensile strength and elongation at failure were within the experimental uncertainty for all samples. Longer exposures are needed to verify the observed trends and to develop performance predictions for these materials on the NGST sunshield.
The high-pressure crystallized poly (ether ether ketone) (PEEK) samples were investigated using thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). The results showed that the thermal stability of PEEK could be improved through appropriate high-pressure treatment. Although the lOwt.-% loss temperature and maximum decomposition temperature of the PEEK samples decreased with the increase of crystallization temperature and crystallization time, the effect of the applied pressure on preventing them from further degradation was obviously observed. Furthermore, the amorphous region of certain PEEK samples was eroded off after high-temperature degradation, which made the details of the spherulitic structures revealed more clearly using traditional etching technique. PEEK spherulites with different characteristics were disclosed with SEM, including those with sheaf-like structures, quench haloes or interpenetrating fibrils. The study presented here may be instructive to the applications of the polymer under such ultimate environmental conditions as high pressure and high temperature.
To prepare nuclear track membranes from poly(aryl ether ether ketone) (PEEK) film, we first determined the modifications induced by heavy ion and UV irradiations in this polymer and two of its model compounds, paraphenoxy benzophenone (PPB) and diphenoxy benzophenone (DPB). This article displays the first results obtained by SEC, HPLC, DSC, FTIR, UV and 13C NMR.
The changes in surface chemistry of phenoxy resin and polyetheretherketone during photo-oxidation were monitored by ESCA. Extensive oxygen uptake was observed for both materials and the data showed that oxidation of the phenyl rings had occurred. Comparison of phenoxy resin with its acetylated derivative indicated that the changes in surface chemistry of the former during the initial stages of exposure were not due to the hydroxyl group.
The change in thermal properties induced by irradiation of 10MeV H+, 20MeV He2+ and 2MeV electron were studied for non-crystalline and crystalline poly(ether–ether–ketone) (PEEK), PEEK-a and PEEK-c, by differential scanning calorimetry (DSC). It was revealed from the analysis of thermal parameter changes that crosslinking proceeds by irradiation in all cases studied. The comparison of effects between ion and electron irradiations led to a conclusion that probability of crosslinking in the ion irradiation is extremely large compared with that in the electron irradiation. It was clarified from isothermal crystallization that ion gives more severe damage to crystallite than electron. The tensile properties for PEEK-a were varied in accord with difference in thermal properties between ion and electron irradiations, whereas for PEEK-c the difference in those between ion and electron irradiations was scarcely observed.
The thermodegradative behaviour of blends of poly(ether ether ketone) (PEEK) and poly(aryl ether sulphone) (PES) was studied by dynamic thermogravimetry in order to analyze their thermal stability. The Freeman–Carrol differential approach was used to determine the kinetic parameters i.e. the apparent activation energy (Ea) and order of reaction (n), of the degradation process. The results indicate that the presence of one component influences the thermal stability of the other. Both, temperature for 5% weight loss (T5) and Ea for blends show a negative deviation from the linear behaviour, which signifies a lowering of thermal stability compared to homopolymers. The decrease in the thermal stability at low concentration of PES in PEEK has been explained on the basis of chemical interactions of the degradation products of PES, which has lower induction temperature for degradation, with PEEK and also on the reduction of viscosity of the medium. But the decrease in thermal stability at low concentration of PEEK in PES is unusual and at present, without the complete elucidation of degradation mechanism in these blends, is difficult to explain.
The photolysis and the photooxidation of polyethersulfone (PESF) and bisphenol-A polysulfone (PSF) have been investigated under polychromatic (λ>300 nm) and monochromatic irradiations (254 and 365 nm). Under vacuum exposure, as a result of direct absorption of the incident radiation by the basic backbone structure of polyarylsulfones (PESF and PSF), the photolytic processes that occur involve chain scissions as revealed by UV–visible and high performance liquid chromatography (HPLC) identifications of the fluorescent low molecular weight fragments after methanolic extraction. In parallel, Fourier transform infrared (FTIR) spectroscopy of the irradiated films accounts for the photoreduction of sulfone moieties in benzenesulfinic acid end-groups and for the formation of phenolic extremities after photo-Claisen-type rearrangement of the ether moieties or direct chain scission and subsequent hydrogen abstraction. Phenyl radicals recombination accounts for the pronounced yellowing of the polymer. Under exposure in aerated media, the rapid photooxidation of both polymers involves chain scissions and the formation of numerous carbonylated and hydroxylated oxidative species. The experimental results indicate that most of the oxidation products arise from the photodegradation of the diphenylethersulfone moieties and the cleavage of the aromatic rings; the oxidation of the bisphenol-A moieties occurs only in secondary steps. As a consequence of the chain scissions provoked by the oxidation, the photoproducts are predominantly low molecular weight oxidation products and most of the consumed oxygen appears readily as carbon dioxide.
The photochemical evolution of poly(ether ether ketone) (PEEK) has been investigated under polychromatic irradiation (λ>300 nm) in the absence and in the presence of oxygen. The modifications of the structure of the films resulting from irradiation was monitored by Fourier transform infrared (FTIR) and UV-visible spectroscopy. The various photoproducts formed were identified by HPLC analysis of the low molecular weight fragments after methanolic extraction. Confirmation of the photoproducts nature was obtained by derivatisation reactions, chemical titration and physical treatments. Under vacuum exposure, as a result of direct absorption of the incident radiation by the basic backbone structure of PEEK, the photolytical processes that occur involve pinacolisation reaction, photo-Claisen type rearrangement and direct chain scissions; subsequent H-abstraction leads to the formation of numerous molecular compounds. Photoproducts resulting from inter- and intra-molecular phenylation reactions and the formation of light absorbing transients account for the photodiscoloration of the irradiated films. Under exposure in aerated media, the oxidation of the photoproducts issue from direct phototransformations accounts for the photodegradation of PEEK; the oxidative attack of the phenyl moieties via ring opening reactions is also involved.
Dynamic measurements in a plate−plate system and steady state flow experiments in a capillary die are presented for conventional high-density polyethylenes (HDPEs) and a new type of polyolefin. The latter, the so-called metallocene-catalyzed HDPEs, are characterized by their low polydispersity and the total absence of branching. The metallocene-catalyzed materials show a different rheological behavior than commercial polyethylenes, which can be summarized as follows: (a) Higher viscosities than conventional HDPEs of the same molecular weight. The dependence of the viscosity on the molecular weight follows a power law equation with an exponent of 4.2 for metallocene catalyzed and 3.6 for conventionals. (b) For high molecular weight materials, the storage modulus overcomes the loss modulus (G‘ > G‘‘) at 190 °C in all frequency ranges. However, for conventional HDPEs, G‘‘ > G‘ at the same temperature and frequency range. (c) At long relaxation times, the values of H(τ) spectra of metallocene-catalyzed samples are significantly higher than those which correspond to a conventional sample of practically the same molecular weight. (d) Metallocene-catalyzed HDPEs are difficult to process, as sharkskin and slip-stick effects take place at very low shear rates. The onset of sharskin takes place at σc1 = 0.18 MPa, and the slip-stick regime occurs at σc2 = 0.25 MPa, independently of temperature. The values of the plateau modulus, GN° = 1.6 × 106 Pa, and the corresponding molecular weight between entanglements, Me = 1830, found for the metallocene-catalyzed materials, are very similar to those found for conventional polyethylenes. However, the activation energies of flow of the new polymers (7−9 kcal/mol) are slightly higher than those of conventional HDPEs.
It is found that surface modification of a polymer by an oxygen glow discharge can be modeled by vacuum-UV oxidation. However, during plasma treatment, competing etching/ablation processes also occur which continuously expose unreacted polymer; this does not happen during UV irradiation and therefore a greater depth of oxidation is found with the latter; UV/O 2 -treated PEEK gives the most highly oxidized surface
Depth profiles analyses were performed on a cross section of a photoaged BPAPC substrate by measuring the glass transition temperature and the Young modulus at a microscopic scale. The aim was to define the impact of photochemical modifications induced under UV irradiation upon the physical properties of this polymer. It is well known that ageing of BPAPC begins by a direct phototransformation of the macromolecules according to the photo-Fries rearrangements. These reactions lead to an increase of the free volume and, as a consequence, to a decrease of the temperature associated with the glass transition while in the meantime, the Young modulus of the material is not modified. As ageing proceeds (longer irradiation times), oxidation of the polymer takes place. This oxidative degradation leads to the formation of a cross-linked structure on the exposed surface. This oxidative degradation is associated with an increase of the glass transition temperature and of the Young modulus, whereas in the bulk of the material ageing involves chain scissions characterised by a decreased glass transition temperature. (C) 2003 Published by Elsevier Ltd.
Differential scanning calorimetry (DSC) has been used to study the crystallization kinetics and thermal characteristics of poly(aryl-ether–ether-ketone) (PEEK) samples heated under a variety of conditions. Samples were heated in nitrogen and air at temperatures between 380 and 420°C for times up to 120 min. The results indicate that as the holding time and temperature of the melt increased, the amount of recrystallizable material decreased, especially when heated in air. Isothermal crystallization kinetics confirmed the presence of a two-stage crystal nucleation and growth process with Avrami exponents of the order of about 2.4 and 1.5 for the first and second processes, respectively. Analysis of the primary crystallization process using the Avrami equation revealed that PEEK samples heated above the melt temperature in air crystallized at a much slower rate than samples heated in nitrogen.
Up to now materials were chosen to satisfy specific property(ies) in relation with the required application. Nowadays, a specific attention has to be devoted to the durability of this property regarding to the lifetime duration. It is the reason why, we paid attention about degradability. Thus, a screening of bio- and photodegradability of various selected (co)polyesters has been achieved in order to get better insights about structure / durability relationships. We developed tools allowing the prediction of the behavior of materials upon ageing and the evolution of their properties, regarding to their initial chemical structures. Hence, we could be able to design (co)polyesters characterized by well-adapted physical, thermal and mechanical properties, but also, with high photostability and/or high biodegradability.
Photograph of the photorheometer used in this study. Summary: Dienic elastomers are highly sensitive towards oxidation. During UV exposure, both scission and/or recombination reactions compete which involves an evolution of the molecular structure of the material. We developed an in situ technique to monitor the viscoelastic property changes upon UV irradiation of polymers in a single experiment. We illustrated this photorheology technique in the case of two kinds of elastomers. From our findings, this method is assumed to bring significant new advantages.
Infrared reflection spectroscopy has been used to investigate a series of poly(aryl-ether-ether-ketone) (PEEK) plaques of differing crystallinity. Correlations have been observed between intensity changes in the reflection spectra and the crystallinity as measured by wide angle X-ray scattering (WAXS). These correlations enable estimates to be made of the crystallinity close to the surface and complement the bulk data obtained by WAXS.
The weathering of polymers is reviewed with attention concentrated on the mechanisms of degradation and stabilization, the methods of testing weatherability, and the predictive modelling of weathering behaviour. An introduction to the chemical mechanisms of degradation and stabilization is given and reference made to some of the many reviews available in the literature. Significant emphasis is placed here on engineering aspects, such as the way that weathering influences fracture mechanisms. The difficulties associated with relating accelerated laboratory tests with outdoor service behaviour are discussed. The complexities of the degradation processes limit modelling of the rate of degradation to rather specific systems and the predictions cannot be generalized.
Polypropylene (PP) has achieved a dominating position and hence, their consumption increases thereby littering, which lead
to environmental pollution. Photodegradation seems to be a better choice because of naturally available sunlight as energy
source for degradation. The present work involves the study of the variation of degradation behavior of PP film during tropical
summer and winter seasons. The photodegradation is followed by Fourier transform infrared (FTIR) spectroscopic technique.
Various indices like hydroxyl, carbonyl, vinylidene, lactones, ester, carboxylic acid and crystallinity are calculated and
these values increased after a brief induction period. The variation in the mechanical properties like tensile strength and
elongation at break percentages are determined. The scanning electron microscopic (SEM) images of weathered PP showed surface
cracks when carbonyl index value increases sharply and the mechanical properties show a sudden decrease. Attempted life time
prediction using mathematical models showed that the carbonyl growth is more affected by ultraviolet (UV) and cumulative total
solar radiation for PP weathered during summer. The loss in tensile strength of PP weathered during summer is more dependent
on the average temperature and the UV portion of the total solar radiation whereas, intensity of UV radiation has profound
effect on the tensile strength of PP weathered during winter.
Polypropylene fibers were exposed to short wavelength radiations (λ=253.7 nm). The samples were analyzed by microscopy, staining, FTIR spectroscopy, tensile testing, and X-ray diffraction.
The short-wavelength UV irradiation produces much more reactive radicals such as peroxy and alkoxy groups, which speeds up
the photo-oxidation process. The products were identified by FTIR spectroscopy to be alcohols, peroxides, ketones, aldehydes,
carboxylic acids, and anhydrides. Comparison of the amount of functional groups leads to an estimation of the mechanism of
photo-oxidation. The short-range order increases during the photo-oxidation and the long-range order or crystalline fraction
remains intact. Transverse cracks appeared on the surface of fibers after a long period of exposure to the radiation. A proposed
mechanism for crack formation is the removal of the photo-oxidation products and the restructuring of the residuals. Similar
to the thermal oxidation, mass loss and density increase are the main reasons for the crack formation in photo-oxidation.
KeywordsPhoto-oxidation-Polypropylene-Fibers-UV Radiation-Crack formation
Ultra long duration balloons (ULDB), currently under development by the National Aeronautics and Space Administration (NASA), requires the use of high strength fibers in the selected super-pressure pumpkin design. The pumpkin shape balloon concept allows clear separation of the load transferring functions of the major structural elements of the pneumatic envelope, the tendons and the film. Essentially, the film provides the gas barrier and transfers only local pressure load to the tendons. The tendons, in the mean time, provide the global pressure containing strength. In that manner, the strength requirement for the film only depends on local parameters. The tendon is made of p-phenylene-2,6-benzobisoxazole (PBO) fibers, which is selected due to its high strength to weight ratio when compared to other high performance, commercially available, fibers. High strength fibers, however, are known to degrade upon exposure to light, particularly at short wavelengths. This paper reports the results of an investigation of the resistance of four commercial high strength fibers to ultra violet (UV) exposure. The results indicate that exposing high strength fibers in continuous yarn form to UV led to serious loss in strength of the fibers except for Spectra® fibers. The adverse changes in mechanical behavior occurred over short duration of exposure compared to the 100 day duration targeted for these missions. UV blocking finishes to improve the UV resistance of these fibers are being investigated. The application of these specially formulated coatings is expected to lead to significant improvement of the UV resistance of these high performance fibers. In this publication, we report on the mechanical behavior of the fibers pre- and post-exposure to UV, but without application of the blocking finishes.
Samples of polypropylene were exposed to artificial accelerated photoageing in various conditions of light intensity. The aged samples were analyzed by FTIR techniques coupled with chemical derivatization to determine the degradation structures resulting from UV exposure. Chemical titrations of the hydroperoxides and determinations of the oxygen consumption were carried out to quantify the formation of the oxidation photoproducts. As expected, our results showed that modifying the exposure parameters induced notable changes of the kinetics of the reactions. These changes led also to some modifications of the relative concentrations of the various oxidation photoproducts, particularly those of the carbonylated products including esters and carboxylic acids. This light intensity dependent behaviour permitted the proposal of a mechanism to account for the formation of the final photoproducts resulting from the hydroperoxides. Our results indicated two possible mechanisms of decomposition of the hydroperoxides, whose relative importance is dependent on the light intensity.
The photochemical reaction of poly(ether ether ketone) (PEEK) sheets under tensile loads has been investigated. Two types of UV irradiation tests were carried out in a vacuum environment: with and without a cooling apparatus. Chemical structures, thermal properties, and mechanical properties were measured to clarify photo-deterioration. Chemical analysis based on Fourier Transform Infrared Spectrometry (FT-IR) and X-ray Photoelectron Spectroscopy (XPS) showed photochemical scission caused by UV exposure. Thermal properties, measured by Differential Scanning Calorimetry (DSC), indicated that a crosslinking reaction occurred during the radiation tests. Tensile properties of PEEK sheets after UV radiation clearly showed a tendency to embrittlement affected not only by crosslinking but also by the orientation of molecular chains resulting from the temperature rise of the specimens. Furthermore, applied tensile stress during exposure accelerated molecular scission and disturbed the crosslinking effects of the tensile properties.
The influence of electron irradiation on the glass transition temperature, Tg, of poly(ether ether ketone) (PEEK) has been investigated by differential scanning calorimetry, up to a dose of 100MGy. For amorphous PEEK, the observed Tg increases linearly with absorbed dose at a rate of 0.18°CMGy−1. This indicates the formation of crosslinks, as deduced elsewhere. Above ∼50MGy, these crosslinks prevent crystallization on heating to above Tg, whereas at lower doses, the polymer is able to crystallize to some degree. Qualitatively, the variations in Tg seen in these partially crystalline samples can be explained in terms of a number of factors. The crystals serve to constrain the amorphous fraction, resulting in a direct elevation in Tg. This is reinforced through the rejection of crosslinks from crystalline regions, so resulting in an increase in the local amorphous crosslink density. Conversely, crosslinking can also serve to inhibit crystallization. Quantitatively, models based solely upon the percentage crystallinity are, however, unable to account fully for the variations in Tg that are seen. In contrast, a model based on both overall crystallinity and lamellar thickness gives good agreement with experiment over the complete crystallinity range.
Natural weathering of isotactic polypropylene (PP) plates (2 mm in thickness) was conducted for 15 months in the Philippines. Optical microscopy, scanning electron microscopy, and atomic force microscopy revealed that the surface layer (200–300 nm in thickness) was affected in the first month, and cracks are formed toward the deeper layer. On the basis of the microscopic observations along with FTIR, DSC, GPC, and tensile strength measurements, the following degradation mechanism of PP was proposed. At first, through oxygen introduced into the PP, only the surface layer seems to be affected. Gradually, the surface layer may be removed, exposing the inner layer. Then, molecular chains of the inner layer start to be degraded, accompanying a significant loss of tensile properties. Part of the degraded materials seems to be eroded, leading to the formation of cracks on the surface of the uncovered inner layer. Finally, the original surface layer may be totally eroded, making the structure of the inner layer (which should reflect the spherulitic texture) evident. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 931–938, 2003
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