Effects of thermal-oxidative aging on the flammability and thermal-oxidative degradation kinetics of tris(tribromophenyl) cyanurate flame retardant PA6/LGF composites

The influence of thermal-oxidative aging on the flame retardancy of the flame retardant long-glass-fiber reinforced polyamide 6 composites (FR/PA6/LGF) with different thermal-oxidative exposure times at 160 °C were studied in this work. The flammability and flame-retardant properties of FR/PA6/LGF were investigated by means of the limiting oxygen index (LOI), vertical burning test (UL-94), cone calorimeter test (CONE), and scanning electronic microscopy (SEM), before and after thermal-oxidative aging. The thermal-oxidative stability and degradation kinetics of the unaged and aged composites were studied by thermogravimetric analysis (TGA) with the methods of Kissinger and Ozawa in dynamic measurements (10 °C/min–40 °C/min). The results indicated that the flammability properties mirrored the degradation behaviors of these FR/PA6/LGF composites whatever their forms (aged or not). The Ozawa method showed that the causes of the first peak in the heat release rate change by CONE measurement corresponded to the apparent activation energies of the first stage degradation of aged FR/PA6/LGF composites, and the same conclusion with respect to the other heat release rate peak. Moreover, this aging slightly enhanced the solid phase flame-retardant mechanism by a char-promotion function, but had no effect on the gaseous flame-retardant mechanism and the decrease of harmful gas release rates. The existence of a surface migration effect on the flame retardant would endow FR/PA6/LGF composites with better LOI values, a more protective char layer structure, and excellent UL-94 ratings.     

The chemical and dielectric properties of epoxy/(Ba0.8Sr0.2)(Ti0.9Zr0.1)O3 composites for embedded capacitor application

The characteristics of epoxy/(Ba_0.8Sr_0.2)(Ti_0.9Zr_0.1)O₃ (BSTZ) composites are investigated for the further application in embedded capacitor device. The effects of BSTZ ceramic powder filler ratio on the chemical, physical and dielectric properties of epoxy/BSTZ composites are studied. Differential scanning calorimeter (DSC) thermal analysis is conducted to determine the optimum values of hardener agent, curing temperature, reaction heat, and glass transition temperature (T_g). The hardener reaction process starts at about 115 °C and completes at about 200 °C, for that it is appropriate to process of epoxy/BSTZ composites in the range of temperature. The highest glass transition temperature (T_g) of 155 °C is obtained at one equivalent weight ratio (hardener/epoxy). Only the BSTZ phase can be detected in the XRD patterns of epoxy/BSTZ composites. The more BSTZ ceramic powder is mixed with epoxy, the higher crystalline intensity of tetragonal BSTZ phase are revealed in the XRD patterns. The dielectric constant measured at 1 MHz increases from 5.8 to 23.6 as the content of BSTZ ceramic powder in the epoxy/BSTZ composites increases from 10 to 70 wt%. The loss tangents of the epoxy/BSTZ composites slightly increase with the increase of measurement frequency.     

The effect of ozone and high temperature on polymer degradation in polymer core composite conductors

The next generation High Temperature Low Sag Polymer Core Composite Conductors can experience harsh in-service environments including high temperature and highly concentrated ozone. In some extreme cases, it is possible that the conductors will experience temperatures of up to 180 °C and ozone concentrations as high as 1% (10,000 ppm). Therefore, the goal of this work was to understand the degradation mechanisms in a high temperature epoxy, which could be used in the conductors at temperatures as high as 140 °C in the presence of 1% ozone. Then, the combined aging data for the epoxy were compared to the aging results from room temperature aging in 1% ozone and aging in air at 140 and 180 °C. In addition, important but limited aging testing was also performed on a set of PCCC rods to verify some of the observations from the neat resin experiments. It was determined that the mass loss, volumetric shrinkage, and flexural strength reductions of the epoxy aged at 140 °C were driven almost entirely by temperature and that the effect of 1% ozone at that temperature can be thought of as insignificant for aging times up to 90 days. The composite rods displayed postcuring at 140 °C and were also unaffected by the presence of ozone at aging time lengths of 90 days. Up to this time aging the polymer and composite specimens in atmospheric 180 °C resulted in the most drastic changes in both physical and mechanical properties, except viscoelasticity where the polymer specimens aged at 140 °C with 1% ozone showed the greatest increase in the storage modulus. The least amount of degradation to the materials was found to occur after aging at room temperature in 1% ozone.     

Thermal stability and degradation kinetics of novel organic/inorganic epoxy hybrid containing nitrogen/silicon/phosphorus by sol-gel method

Hybrids containing silicon, phosphorous and nitrogen were prepared by the sol-gel method and compared with pure epoxy. The silicon, phosphorous and nitrogen components were successfully incorporated into the networks of polymer. Thermogravimetric analysis (TGA) was used for rapid evaluation of the thermal stability of different materials. The integral procedure decomposition temperature (IPDT) has been correlated the volatile parts of polymeric materials and used for estimating the inherent thermal stability of polymeric materials. The IPDT of pure epoxy was 464 °C and the IPDTs of hybrids were higher than that of pure epoxy. The thermal stability of hybrids increased with the contents of inorganic components. The inorganic components can improve the thermal stability of pure epoxy.Two methods have been used to study the degradation of hybrids containing silicon, phosphorous and nitrogen hybrid during thermal analysis. These investigated methods are Kissenger, Ozawa's methods. The activation energies (E_a) were obtained from these methods and compared. It is found that the values of E_a for modified epoxy hybrids are higher than that of pure epoxy. The hybrids of high activation energy possess high thermal stability.     

Durability of a polymer with triple-shape properties

A series of cyclic thermo-mechanical measurements was conducted on segregated poly(ester urethane) to study substantial changes in triple-shape properties as a result of hydrolytic aging (80 °C). Prior to the analysis of aging effects, a concept of triple-shape testing was elaborated, starting with the implementation of two distinct programming units. The first one included a deformation at 60 °C to ?_m1 = 100% (temporary shape B) and its fixing through soft segment crystallization by cooling to −20 °C under constant strain. The second one consisted of a deformation at −20 °C to ?_m2 = 200% (temporary shape A) and its stabilization through soft segment vitrification as achieved by cooling to −60 °C under fixed strain constraint. Then, gradual heating of the polymer from below to above its thermal transition temperatures gave two independent shape recovery responses in the reverse order of shape fixing: A → B through passing the glass transition by heating from −60 to 23 °C and B → C (back to the permanent shape), when heating the material from 23 to 60 °C and thus above its soft segment melting temperature. In a progressive approach, the storage of loading history through the sequential fixing of two temporary shapes was proven by the development of shape recovery stresses under constrained environment. With the implementation of the two testing methods several aging-related effects could be detected. Good shape fixing abilities ≥90% for both shapes were found and contrasted by significant changes in shape recoverabilities and stress storage capacities. Further insights derived from differential scanning calorimetry (DSC) measurements, indicating an aging-related growth in soft segment crystallinity, and dynamic mechanical analysis (DMA), suggesting a plasticizer effect of water onto the polymer matrix and that aging favoured an increase in cross-linking density.     

Application of isotopic labeling,and gas chromatography mass spectrometry,to understanding degradation products and pathways in the thermal-oxidative aging of Nylon 6.6

Nylon 6.6 containing ¹³C isotopic labels at specific positions along the macromolecular backbone has been subjected to extensive thermal-oxidative aging at 138 °C for time periods up to 243 days. In complementary experiments, unlabeled Nylon 6.6 was subjected to the same aging conditions under an atmosphere of ¹⁸O₂. Volatile organic degradation products were analyzed by cryofocusing gas chromatography mass spectrometry (cryo-GC/MS) to identify the isotopic labeling. The labeling results, combined with basic considerations of free radical reaction chemistry, provided insights to the origin of degradation species, with respect to the macromolecular structure. A number of inferences on chemical mechanisms were drawn, based on 1) the presence (or absence) of the isotopic labels in the various products, 2) the location of the isotope within the product molecule, and 3) the relative abundance of products as indicated by large differences in peak intensities in the gas chromatogram. The overall degradation results can be understood in terms of free radical pathways originating from initial attacks on three different positions along the nylon chain which include hydrogen abstraction from: the (CH₂) group adjacent to the nitrogen atom, at the (CH₂) adjacent the carbonyl group, and direct radical attack on the carbonyl. Understanding the pathways which lead to Nylon 6.6 degradation ultimately provides new insight into changes that can be leveraged to detect and reduce early aging and minimize problems associated with material degradation.     

Predicting the lifetime of fluorosilicone o-rings

Long-term (up to 1000 days) accelerated oven-aging studies on a commercial fluorosilicone o-ring seal are used to predict the sealing lifetime at room temperature (23 °C). The study follows force decay (relaxation) on squeezed o-ring material using isothermal compression stress relaxation (CSR) techniques. The relaxation is normally a complex mix of reversible physical effects and non-reversible chemical effects but we utilize an over-strain approach to quickly achieve physical equilibrium. This allows us to concentrate the measurements on the chemical relaxation effects of primary interest to lifetime assessment. The long-term studies allow us to access a fairly broad temperature range (80-138 °C) which results in improved modeling of the temperature dependence of the accelerated data. Non-Arrhenius behavior is observed with evidence of a significant lowering of the activation energy at the lowest accelerated aging temperature (80 °C). This observation is consistent with numerous recent accelerated aging studies that probed temperature ranges large enough to observe similar non-Arrhenius behavior. The extrapolated predictions imply that significant loss of sealing force requires on the order of 50-100 years at 23 °C. Field aging results out to ∼25 years at 23 °C are shown to be in reasonable accord with the significant change in Arrhenius slope observed from the accelerated aging study.     

Photoluminescence in the CaxSr1−xWO4 system at room temperature

In this work, a study was undertaken about the structural and photoluminescent properties, at room temperature, of powder samples from the Ca_xSr_1−xWO₄ (x=0-1.0) system, synthesized by a soft chemical method and heat treated between 400 and 700 °C. The material was characterized using Infrared, UV-vis and Raman spectroscopy and XRD. The most intense PL emission was obtained for the sample calcined at 600 °C, which is neither highly disordered (400-500 °C), nor completely ordered (700 °C). Corroborating the role of disorder in the PL phenomenon, the most intense PL response was not observed for pure CaWO₄ or SrWO₄, but for Ca_0.6Sr_0.4WO₄. The PL emission spectra could be separated into two Gaussian curves. The lower wavelength peak is placed around 530 nm, and the higher wavelength peak at about 690 nm. Similar results were reported in the literature for both CaWO₄ and SrWO₄.     

Structures and Magnetic Properties of New Inserted Uranium Tellurides: U3Te5Zx (Z=Ge, Sn)

The two new compounds U₃Te₅Ge_0.7 and U₃Te₅Sn_0.5 were prepared by heating the binary compound U₃Te₅ and the corresponding group 14 element at 850°C in a fused-silica tube. Single crystals have been grown by chemical vapor transport using iodine as transporting agent in temperature gradients of 870-840°C and 840-800°C for U₃Te₅Ge_0.7 and U₃Te₅Sn_0.5, respectively. They have been characterized by single-crystal X-ray diffraction measurements and by energy dispersive X-ray analysis. The two isostructural compounds crystallize with two formula units in the orthorhombic space group Pmmn in cells of dimensions: U₃Te₅Ge_0.7a=4.2764(1) Å, b=13.1029(3) Å, c=8.9104(2) Å; U₃Te₅Sn_0.5, a=4.3160(1) Å, b=13.1999(4) Å, c=8.9128(2) Å. The crystal structures comprise two independent U atoms with two different coordination geometries. Atom U(1) is surrounded by eight Te atoms in a bicapped trigonal prismatic geometry. Atom U(2) is in a seven coordinate environment of Te atoms, with an arrangement usually described as a 7-octahedron. The three-dimensional packing results in distorted hexagonal cavities where the metalloid atoms are inserted. Magnetic measurements reveal that both compounds U₃Te₅Ge_0.7 and U₃Te₅Sn_0.5 are hard ferromagnets with ordering temperature of 135 and 140 K, respectively. At low temperature, they display large magnetocrystalline anisotropy with origin on the domain wall pinning at the magnetic domain boundaries.     

Compatibility evaluation between La2Mo2O9 fast oxide-ion conductor and Ni-based materials

The chemical reactivity of La₂NiO_4+δ and nickel metal or nickel oxide with fast oxide-ion conductor La₂Mo₂O₉ is investigated in the annealing temperature range between 600 and 1000 °C, using room temperature X-ray powder diffraction. Within the La₂NiO_4+δ/La₂Mo₂O₉ system, subsequent reaction is evidenced at relatively low annealing temperature (600 °C), with formation of La₂MoO₆ and NiO. The reaction is complete at 1000 °C. At reverse, no reaction occurs between Ni or NiO and La₂Mo₂O₉ up to 1000 °C. Together with a previous work [G. Corbel, S. Mestiri, P. Lacorre, Solid State Sci. 7 (2005) 1216], the current study shows that Ni-CGO cermets might be chemically and mechanically compatible anode materials to work with LAMOX electrolytes in solid oxide fuel cells.     

Synthesis and characterization of a novel arylacetylene oligomer containing POSS units in main chains

A novel arylacetylene oligomer containing octamethyl POSS units in main chains was prepared from difluoride octamethyl POSS (diexo-(CH₃)₈Si₈O₁₁F₂) and diethynylbenzene (DEB) by Grignard reaction and characterized by FT-IR, NMR, WAXD, GPC, DSC, and TGA. The curing reaction kinetic of the oligomer was studied by Kissinger and Ozawa methods and the kinetic parameters were obtained. The cured polymer had good thermal and thermal-oxidative properties. TGA analyses demonstrated that the thermal decomposition temperature (T_d5) of the cured polymer in nitrogen and air were 503 and 479 °C, respectively.     

Preparation and thermal properties of diglycidylether sulfone epoxy

A diglycidylether sulfone monomer (sulfone type epoxy monomer, SEP) was prepared from bis(4-hydroxyphenyl) sulfone (SDOL) and epichlorohydrin without any NaOH or KOH as basic catalyst. FT-IR, ¹H NMR, ¹³C NMR and mass spectroscopic instruments were utilized to determine the structure of the SEP monomer. The cured SEP epoxy material exhibited not only a higher T_g (163.81 °C) but also a higher T_g than pristine DGEBA (from 111.25 °C to 139.17 °C) when the SEP monomer moiety had been introduced into the DGEBA system. The thermal stability of cured epoxy herein was investigated by thermogravimetric analysis (TGA). The results demonstrated that the sulfone group of the cured SEP material decomposed at lower temperatures and formed thermally stable sulfate compounds, improving char yield and enhancing resistance against thermal oxidation. Additionally, the IPDT and char yield of the cured SEP epoxy (IPDT = 1455.75, char yield = 39.67%) exceeded those of conventional DGEBA epoxy (IPDT = 667.27, char yield = 16.25%).     

Structure and thermal degradation of poly(vinyl chloride) synthesized by various polymerization catalyst

Relationship between the structure and the thermal stability of poly(vinyl chloride) synthesized by various polymerization catalysts was investigated. The Cp∗Ti(OPh)/MAO catalyst, n-butyllithium (n-BuLi), the Cu(0)/TREN/CHBr₃/DMSO catalyst, benzoyl peroxide/N,N-dimethylaniline (BPO/DMA), 2,2’-azobis(2.4-dimethylvaleronitrile) (V-65) was used as the polymerization catalyst. The temperature of 5% weight loss was in the following order; Cp∗Ti(OPh)₃/MAO (280 °C) > n-BuLi (264 °C) > V-65 (249 °C) > Cu(0)/TREN/CHBr₃/DMSO (215 °C) > BPO/DMA (209 °C), and the rate of weight loss was the reverse order of T_−5% in the isothermal degradation of the polymer from 160 °C to 220 °C. The T_−5% value of the polymer obtained from the polymerization with Cp∗Ti(OPh)₃/MAO catalyst increased with an increase of the molecular weight of PVC, in contrast to that PVC obtained with the radical initiator did not depend on the molecular weight of the polymer. The T_−5% value of PVC macromonomer was 285 °C, while the temperature of non-functionalized PVC was 262 °C, respectively. It is clear that the PVC macromonomer had a good thermal stability regardless of low-molecular weight.     

Nylon 6.6 accelerating aging studies: II. Long-term thermal-oxidative and hydrolysis results

Long-term (greater than 5 year exposures), low-temperature (as low as 37 °C) accelerated oven aging results were obtained for Nylon 6.6 fibers under thermo-oxidative conditions (air aging with an oxygen partial pressure of 13.2 cmHg in Albuquerque). To assess the importance of humidity on aging, experiments were also conducted under a combination of 100% RH plus 13.2 cmHg of oxygen partial pressure at temperatures ranging from 138 °C to 64 °C plus an additional experiment at 70% RH and 80 °C. The low-temperature tensile strength results showed that the Arrhenius activation energy under the pure oxidative degradation conditions dropped from ∼96 kJ/mol above ∼100 °C-∼30 kJ/mol below this temperature, indicative of a transition in the oxidative chemistry at low temperatures. Earlier work by our group on the same material concluded that hydrolytic degradation effects dominated oxidation effects at higher aging temperatures. However, the current long-term, low-temperature comparisons lead to the conclusion that humidity is not an important aging factor below ∼50 °C. By extrapolating time-temperature superposed oxidative degradation data using the low-temperature activation energy, we obtain predictions at 21 °C. At this temperature, we estimate that a tensile strength loss of 50% takes on the order of 70 years. The 21 °C predictions are shown to be reasonably consistent with long-term (up to 38 year) ambient results on similar Nylon materials removed from field-aged parachutes. Although the estimated average exposure temperature varies from parachute to parachute, the highest average temperature is estimated to be on the order of 21 °C.     

Highly selective transformation of poly[(R)-3-hydroxybutyric acid] into trans-crotonic acid by catalytic thermal degradation

Highly selective transformation of poly[(R)-3-hydroxybutyric acid] (PHB) into trans-crotonic acid was achieved by thermal degradation using Mg compounds: MgO and Mg(OH)₂ as catalysts. Through catalytic action, not only the temperature and E_a value of degradation were lowered by 40-50 °C and 11-14 kJ mol⁻¹, respectively, but also significant changes in the selectivity of pyrolyzates were observed. Notably, Mg(OH)₂ showed nearly complete selectivity (∼100%) to trans-crotonic acid. Kinetic analysis of TG profiles revealed that the catalytic thermal degradation of PHB was initiated by some random degradation reactions, followed by the unzipping β-elimination from crotonate chain-ends as a main process. It was suggested that the Mg catalysts promote the totality of the β-elimination reactions by acting throughout the beginning and main processes, resulting in a lowering in the degradation temperature and the completely selective transformation of PHB.     

Bio-oil production from Onopordum acanthium L. by slow pyrolysis

In this study, the usability of the plant thistle, Onopordum acanthium L., belonging to the family Asteraceae (Compositae), in liquid fuel production has been investigated. The experiments were performed in a fixed-bed Heinze pyrolysis reactor to investigate the effects of heating rate, pyrolysis temperature and sepiolite percentage on the pyrolysis product yields and chemical compositions. Experiments were carried out in a static atmosphere with a heating rate of 7 °C/min and 40 °C/min, pyrolysis temperature of 350, 400, 500, 550 and 700 °C and particle size of 0.6 < D_p < 0.85 mm. Catalyst experiments were conducted in a static atmosphere with a heating rate of 40 °C/min, pyrolysis temperature of 550 °C and particle size of 0.6 < D_p < 0.85 mm. Bio-oil yield increased from 18.5% to 27.3% with the presence of 10% of sepiolite catalyst at pyrolysis temperature of 550 °C, with a heating rate of 40 °C/min, and particle size of 0.6 < D_p < 0.85 mm. It means that the yield of bio-oil was increased at around 48.0% after the catalyst added. Chromatographic and spectroscopic studies on the bio-oil showed that the oil obtained from O. acanthium L. could be used as a renewable fuels and chemical feedstock.     

Thermal stability of microencapsulated epoxy resins with poly(urea-formaldehyde)

A series of microcapsules filled with epoxy resins with poly(urea-formaldehyde) (PUF) shell were synthesized by in situ polymerization, and they were heat-treated for 2 h at 100 °C, 120 °C, 140 °C, 160 °C, 180 °C and 200 °C. The effects of surface morphology, wall shell thickness and diameter on the thermal stability of microcapsules were investigated. The chemical structure and surface morphology of microcapsules were investigated using Fourier-transform infrared spectroscope (FTIR) and scanning electron microscope (SEM), respectively. The thermal properties of microcapsules were investigated by thermogravimetric analysis (TGA and DTA) and by differential scanning calorimetry (DSC). The thermal damage mechanisms of microcapsules at lower temperature (<251 °C) are the diffusion of the core material out of the wall shell or the breakage of the wall shell owing to the mismatch of the thermal expansion of core and shell materials of microcapsules. The thermal damage mechanisms of microcapsules at higher temperature (>251 °C) are the decomposition of shell material and core materials. Increasing the wall shell thickness and surface compactness can enhance significantly the weight loss temperatures (T_d) of microcapsules. The microcapsules with mean wall shell thickness of 30 ± 5 μm and smoother surface exhibit higher thermal stability and can maintain quite intact up to approximately 180 °C.     

Cure behavior of epoxy resin/montmorillonite/imidazole nanocomposite by dynamic torsional vibration method

Flory’s gelation theory, the non-equilibrium thermodynamic fluctuation theory and the Avrami equation have been used to predict the cure behavior of epoxy resin/organo-montmorillonite (Org-MMT)/imidazole intercalated nanocomposites at various temperatures and Org-MMT loadings. The theoretical prediction is in good agreement with the experimental results obtained by a dynamic torsional vibration method. The results show that the addition of Org-MMT reduces the gelation time t_g and increases the rate of the curing reaction, the value of the kinetic constant k. The half-time t_1/2 of cure after the gel point decreases with increasing of cure temperature, and the value of n is around 3 at lower temperature (<90 °C) and decreases to ∼2 as the temperature increases. The addition of Org-MMT has no obvious effect on the apparent activation energy of the cure reaction. There is no special curing process required for the formation of an epoxy resin/Org-MMT/imidazole intercalated nanocomposite.     

A procedure for precise determination of thermal stabilization reactions in carbon fiber precursors

Thermal stabilization of polyacrylonitrile (PAN) fibers is an important step in production of carbon fibers. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap. In order to separate the stabilization reactions, we combined the results of FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry), TGA (Thermogravimetry analysis), TMA (Thermomechanical analysis), and densitometry. It was shown that combination of DSC and TGA allowed separation of reactions regardless of the composition of initial PAN fibers. FTIR, TMA and densitometry results verified the validity of proposed method. Accordingly, three special and commercial grade PAN fibers with different chemical composition were studied. FTIR results indicated that during thermal stabilization of PAN fibers chemical reactions including cyclization, oxidation and dehydrogenation occurred in the fibers and a ladder polymer was formed. According to DSC and TGA curves, initiation temperature, temperature range and order of occurrence of these reactions were a function of chemical composition of initial fibers. In fibers containing itaconic acid plus methyl acrylate comonomers, oxidation reactions already started at 175 °C. Cyclization started above 210 °C, and reactions occurring above 250 °C were mainly dehydrogenation. In fibers containing only itaconic acid cyclization initiated above 210 °C, dehydrogenation started after 242 °C and oxidation occurred only after 284 °C. In fibers containing vinyl acetate comonomers, the initial reactions above 240 °C were attributed to cyclization. Oxidation occurred below 290 °C and dehydrogenation started above 290 °C.     

Unusual change in molecular weight of polyhydroxyalkanoate (PHA) during cultivation of PHA-accumulating Escherichia coli

This study aimed to investigate the factors affecting molecular weight of poly[(R)-3-hydroxybutyrate] [P(3HB)] when polyhydroxyalkanoate (PHA) synthase (PhaRC_Bsp) from Bacillus sp. INT005 was used for P(3HB) synthesis in Escherichia coli JM109. It was found that the molecular weight of P(3HB) decreased with time in mid- and late-phase of culture and was strongly affected by culture temperature. At 37 °C culture temperature, the molecular weight of P(3HB) rapidly decreased from 4.4 × 10⁵ to 4.8 × 10⁴ with culture time, whereas it was almost unchanged at 25 °C. Kinetic analysis suggested that the decrease in molecular weight of P(3HB) was due to random scission of the polymer chain. The decrease in molecular weight of P(3HB) was not observed when PHA synthases other than PhaRC_Bsp were expressed. This study sheds light on the unique behaviour in molecular weight change of P(3HB) that is synthesized by E. coli expressing PhaRC_Bsp.