Evaluation of models for the low temperature combustion of alkanes through interpretation of pressure-temperature ignition diagrams

The purpose of this paper is to show the application of global uncertainty analysis to comprehensive and reduced kinetic models as a tool to identify important thermochemical and reaction rate parameters as determinants of the conditions leading to autoignition. Propane oxidation is taken as the test case. The simulation of experimental investigations of the cool flames and two-stage ignitions, via the pressure-temperature ignition diagram, show that existing kinetic models for the low temperature combustion of propane at sub-atmospheric pressures reflect a greater reactivity than seems to be appropriate. That is, the models lead to a prediction of two-stage ignition at pressures somewhat lower and with ignition delays shorter than is found experimentally. The inconsistency between experiment and numerical simulation seems not to be an inherent problem of the qualitative structure of the models, but may derive from uncertainties in the parameters within the mechanism. By use of "brute force", Morris-one-at-a-time and Monte-Carlo simulations, we show that uncertainties in only a small number of parameters, and falling well within the errors that may reasonably be assigned, can shift the response appropriately. Moreover, it appears that in the low temperature combustion regime, thermochemistry is at least as, if not more, important than the reaction rates, yet usually receives less attention within sensitivity studies. In the present case, the main factors controlling the temperature reached in the first stage of two-stage ignition and the time to ignition appear to be connected with the thermochemistry of three specific hydroperoxyalkyl radicals and their derivatives. Other factors, such as heat and mass transport are also addressed, and their effects are mitigated to some extent by evaluation of initial and revised models against experimental data for ignition delay obtained under microgravity. The results highlight more general issues that pertain to the numerical simulation of the combustion of higher hydrocarbons and contribute to the development of the protocol necessary for testing kinetic models before they are ready for use in a predictive capacity.     

Resistivity mapping: An example of aged conducting polymer

A simple multi-contact measurement method, which can be used to map the electrical resistivity of small disc-shaped samples of conducting polymer during temperature ageing, was examined for its resolution and accuracy. The method is based on electrical impedance tomography (EIT), which is used especially in medicine to visualise boundaries between areas having different electrical resistivities. In order to eliminate experimental errors, a computer simulation was used for testing. The time series of resistivity maps were recorded during ageing of real polymer samples. The contour maps and their time development are presented graphically and discussed.     

Long-term performance of poly(vinyl chloride) cables. Part 1: Mechanical and electrical performances

Cables insulated with plasticized poly(vinyl chloride) were aged in air at temperatures between 80 °C and 180 °C and their conditions were assessed by indenter modulus measurements, tensile testing, infrared (IR) spectroscopy and differential scanning calorimetry (DSC). Electrical testing of oven-aged cable samples was performed in order to relate the electrical functionality during a high-energy line break (HELB) to the mechanical properties and to establish a lifetime criterion. The mechanical data taken at room temperature after ageing could be superimposed with regard to ageing time and temperature. The ageing-temperature shift factor showed an Arrhenius temperature dependence. The jacketing material showed an immediate increase in stiffness (indenter modulus and Young's modulus) and a decrease in the strain at break on ageing; these changes were dominated by loss of plasticizer by migration which was confirmed by IR spectroscopy and DSC. The core insulation showed smaller changes in these mechanical parameters; the loss of plasticizer by migration was greatly retarded by the closed environment, according to data obtained by IR spectroscopy and DSC, and the changes in the mechanical parameters were due to chemical degradation (dehydrochlorination). A comparison of data obtained from this study and data from other studies indicates that extrapolation of data for the jacketing insulation can be performed according to the Arrhenius equation even down to service temperatures (20-50 °C). The low-temperature deterioration of the jacketing is, according to this scheme, dominated by loss of plasticizer by migration.     

Degradation of recycled high-impact polystyrene. Simulation by reprocessing and thermo-oxidation

A simulation of the degradation of high-impact polystyrene (HIPS), occurring during service life and mechanical recycling, was performed by multiple processing and thermo-oxidative ageing. All samples were characterized by differential scanning calorimetry (DSC), melt mass-flow rate (MFR) measurements, tensile testing and infrared spectroscopy (FTIR). Multiple processing and thermo-oxidative ageing clearly alter the oxidative stability and the elongation at break of the materials. These changes observed at a macroscopic scale have been related to chemical alterations in the structure of HIPS. The polybutadiene phase was demonstrated to be the initiation point of the degradative processes induced by processing, service life and mechanical recycling. Thermo-oxidative degradation affects more severely the degree of degradation of the material, so it may be deduced that the changes occurring during service life of HIPS are the part of the life cycle that mostly affects its further recycling possibilities and performance in second-market applications.     

Thermal decomposition behaviour of cobalt(II)-picolyl-terminated poly(dimethylsiloxane)

Thermal, chemical and rheological properties of ultraviolet aged asphalt binder were characterized by differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) and dynamic shear rheometer (DSR), respectively. Asphalt binder samples were made with different film thickness (50, 100, 200 and 500 μm) and suffered different ageing time (0, 48, 96 and 144 h), at a certain UV radiant intensity of 20 w m^–2 in a self-made accelerated ageing oven. The results indicate that the UV light ageing would lead to the improvement of thermal behavior and the growth of the glass transition temperature of asphalt binder. This type of ageing can be also reflected from the FTIR spectra in terms of the characteristic peaks of the carbonyl groups and sulphoxides. The UV light ageing can change some rheological parameters of asphalt binder, such as complex modulus and phase angle. The ageing degrees of asphalt binder by this type of ageing test are mainly related to the ageing time and film thickness of the sample.     

Assessment of the EPDM seal lifetime in nuclear power plants

An EPDM seal was qualified for use in nuclear power plants (NPPs). The primary function of the seal is to prevent leakage of unwanted fluids and gases during long-term normal operation, as well as during postulated accidents which may occur at the end of the planned service lifetime. Tested samples were compressed between two massive metal plates (the same compression as under regular service conditions) and exposed to an accelerated ageing procedure to simulate up to 10 years of operation, including accident simulation. Mechanical properties, compression set and a gas leakage test were carried out. The minimum necessary compression to keep the required tightness under all conditions during normal service and during accident conditions was found and the service lifetime was assessed for two seal thicknesses.     

Standard testing methods for mechanical properties and degradation of low density polyethylene (LDPE) films used as greenhouse covering materials: a critical evaluation

LDPE films are currently the most widespread greenhouse covering material in the countries of the Mediterranean region. Taking into account the size of this market, the effect of the material performance on the greenhouse production as well as the related environmental impact problems arising from its disposal, the standardization of the testing of these materials should have already been achieved. However, there are no standard methods available in the European Union for testing LDPE greenhouse films. Furthermore, for predicting the useful lifetime of such films both the critical effect of the various climatic conditions and the effect of the harsh greenhouse micro-environment on their properties should be taken into account. In this paper the various methods for testing the mechanical properties of greenhouse polyethylene films will be presented and discussed critically. Also, the factors affecting ageing of LDPE used as greenhouse covering are presented, including methods for inducing ageing and testing methods in order to probe ageing.     

Measurement of thermo-oxidative stability of isotactic polypropylene by isothermal long-term differential thermal analysis

The thermo-oxidative stability of isotactic polypropylene with different antioxidant concentrations between 0% and 0·1% has been studied using isothermal long-term differential thermal analysis (ILDTA). In the temperature range from 90°C to 210°C the oxidation maximum times were measured up to 2000 h. The Arrhenius plot of the reaction maxima showed straight lines over the whole temperature range, which were not affected by the crystallite melting range. Thus, extrapolations from short-term experiments at elevated temperatures to long-term behaviour at lower temperatures are possible.Ageing tests in a circulating-air oven at 140°C showed that the loss of thermo-oxidative stability with increasing ageing time was nearly linear. So the state of thermo-oxidative deterioration of isotactic polypropylene can be estimated by measurement of residual life time in an ILDTA experiment.The end of the oven life of polypropylene coincided with the loss of mechanical properties which was confirmed by a long-term tensile test at 100°C and 120°C.     

Rejuvenation of PLLA: Effect of plastic deformation and orientation on physical ageing in poly(l‐lactic acid) films

Poly(l ‐lactic acid) (PLLA) is a bio‐degradable polyester which exhibits brittle behaviour due to relatively fast physical ageing of the amorphous phase. This work describes the effects of thermal rejuvenation and molecular orientation of the amorphous phase on this physical ageing process. Uniaxial compression testing showed that physical ageing of the amorphous phase increases the yield stress and the associated strain softening response, both contributing to the observed embrittlement of PLLA in tension. Molecular orientation at constant crystallinity was applied by uniaxial and biaxial plastic deformation just above the glass transition temperature, up to plastic strains of 200% to avoid strain‐induced crystallisation. Using stress‐relaxation experiments combined with tensile testing, both as a function of ageing time, it is shown that both uniaxial and biaxial plastic deformation in excess of 150% plastic strain, decelerates and possibly prohibits the physical ageing process. The oriented monofilaments and films have improved mechanical properties such as stiffness, strength and strain‐to‐break, which were not affected by physical ageing during the whole testing period (40 days). In addition, plastic deformation to higher draw ratios and/or higher temperatures strongly enhanced crystallinity and resulted in PLLA monofilaments and films that also exhibited tough behaviour, not affected by physical ageing. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2233–2244     

Physical model explaining the effect of physical ageing on dynamic mechanical and calorimetric properties of poly(methyl methacrylate)

Using the theory of non-elastic deformation based on a specific model for the mobility in amorphous polymers, an equation was obtained including the real and imaginary parts of the complex modulus and the heat capacity as a function of temperature and ageing time. Thus, the model is based on the calculation of time and temperature evolution of the population of “faults” (corresponding to an excess of enthalpy and entropy) in the structure. We employ the model with only one “characteristic” relaxation time obtained from a concept of hierarchical correlation. The parameters appearing in the equations for G* and c_p are the same in each case and are obtained from classical experiments (mechanical spectroscopy, dilatometry, calorimetry, etc.), thus having a specific physical meaning. Dynamic loss tangent and specific heat of PMMA measured after different thermal histories involving isothermal ageing agrees quite well with the prediction of the model.     

Ageing mechanism and mechanical degradation behaviour of polychloroprene rubber in a marine environment: Comparison of accelerated ageing and long term exposure

Polymers are widely used in marine environments due to their excellent properties and good weathering resistance. Despite this extensive use, their long term behaviour in such an aggressive environment is still not well known. To assess the polymer durability within reasonable durations, it is essential to perform accelerated ageing tests to accelerate the degradation kinetics but without any modification of the degradation process. This study therefore proposes and validates accelerated ageing tests to study marine ageing of a silica-filled chloroprene rubber (CR) used for offshore applications. Several accelerated ageing protocols are investigated for temperatures ranging from 20 to 80 °C in renewed natural seawater. The ageing consequences are characterized using physical measurements (FTIR, solid state NMR) and mechanical testing based on monotonic tension tests. Instrumented micro-indentation tests are also employed, in order to describe accurately the ageing gradients through sample thickness. The measurements obtained on the samples cut from accelerated specimens are compared to those obtained from the topcoat of an offshore flowline aged under service conditions for 23 years. For both kinds of specimens, polychloroprene develops rapid material changes most clearly represented by a considerable increase in stiffness, which allows the accelerated ageing protocols to be validated.     

Durability of Protective Polymers: The Effect of UV and Thermal Ageing

This paper aimed at studying thermal contribution to the UV ageing process, trying to understand which decisive changes reduce the protective effectiveness of polymer coatings. In this paper the effects on the shielding efficacy of different protective polymers, applied on different low porosity stones have been studied, comparing ageing of a 1000 h simulated solar UV radiation with thermal ageing at about 50 °C, which is the temperature usually reached in UV chambers. The aim of the study has been the evaluation of a possible thermal contribution: the testing methods have been suggested by UNI-Normal Italian protocol and include capillary water absorption, static contact angles and colour variation measurements. A reduction in protective efficacy has been highlighted, probably due to both oxidation and a surface rearrangement of the polymeric material; in most cases chemical degradation of the macromolecules did not occur.     

Antioxidant ‘crossover effect’ in oven ageing of polypropylene

Thermal oxidative stabilities of polypropylene formulations are frequently assessed by ageing samples in circulating-air ovens. A 150°C test temperature is almost an accepted standard throughout the polymer industry although it often bears little relation to the actual service temperatures seen during the life of the polypropylene which are frequently below 100°C. Phenolic antioxidants are necessary for the extended service life of polypropylene. When the antioxidants are compared over a range of evaluation temperatures, their relative performance can change quite markedly. As evaluation temperatures are decreased and approach actual use temperature, the chemical kinetics of stabilization and the influencing physical characteristics of the antioxidant appear to change causing a reversal in the relative effectiveness between two high molecular weight antioxidants. This has been defined as the ‘crossover effect’. This understanding should allow for improved predictions of polymer service life.     

Study of artificially degraded woods simulating natural ageing of archaeological findings

Simulation of waterlogged archaeological woods was carried out by immersion of fir and chestnut wood samples into sea water at different temperatures (room temperature and 40°C). The effects of metals in contact with woods were simulated by inserting in some specimens of the two types of wood copper or iron nails, the most important metals from the archaeological point of view. The effects of this ageing simulation on woods were studied by different characterization methods. At first we have performed gravimetric analyses, controlling the mass increase of immersed wood in function of the time of immersion and the temperature of the bath. Then, thermogravimetry, differential thermal analysis, differential scanning calorimetry in oxygen flux were used. The alteration of wood was observed by means of the peak temperatures of DTA, DTG and DSC variation and by the mass losses observed during heating, evaluated on the basis of the measured thermal data. The samples were woods powder obtained by milling. Complementary characterization of the woods was performed by evaluating the crystallinity of cellulose by means of X-ray powder diffraction. The change in colour of woods during ageing was checked by means of spectrophotometric measurements in the visible region. X-ray fluorescence was used to investigate the penetration of metals into wood samples. An artificial ageing treatment with NaOH and O₃ was also performed. Finally, a comparison between the effects of artificial alteration realised in our specimens and natural degradation observed in archaeological woods, was performed.     

Hydrothermal ageing of radiation cured epoxy resin-polyether sulfone blends as matrices for structural composites

The hydrothermal ageing of epoxy-thermoplastic blends, used as matrices for carbon fibre composites, cured by electron beam, has been studied. Two different thermoplastic percentages have been adopted. A suitable choice of both curing process and formulation parameters allows to carry out irradiation at mild temperature with several advantages, coming from a “non thermal” process, for both the final properties of the materials and the environment. Nevertheless the occurring of vitrification phenomena needs the use of a short thermal treatment after irradiation on the already solid materials, in order to complete the cure reactions. Radiation cured epoxy based matrices have been subjected to a thermal and moisture absorption ageing treatment and its influence on the thermal and mechanical properties has been investigated through dynamic mechanical thermal analysis and fracture toughness tests. The results have been interpreted on the basis of the different curing degree reached by the investigated systems and in the light of their morphological structures. Plasticization, thermal curing and degradation reactions occur in different extent depending on the kind of the material. In particular, for fracture properties, a better resistance to ageing is shown by the system at higher thermoplastic concentration.     

Ageing effects and internal stresses in quenched unfilled and clay-filled high density polyethylene

The stress relaxation and creep behaviour of high density polyethylene (HDPE), unfilled or filled with clay particles, were measured after different ageing times after quenching from 120 °C. The measurements were performed at room temperature in the uniaxial extension mode. Ageing time had a pronounced influence on the viscoelastic properties, e.g. the creep curves shifted to longer times with increasing ageing time. The internal stresses, as evaluated from stress relaxation data, were found to increase markedly when the ageing period was extended, and it was suggested that this behaviour may be associated with relaxation of thermal residual stresses. It was furthermore suggested that the change in residual stresses, in addition to the physical ageing process, could also affect the ageing behaviour of HDPE, i.e. the change in viscoelastic properties with ageing time. Clay addition changes the ageing behaviour of HDPE, which could be attributed to a change in the internal stress dependence of the ageing time and/or to a difference in the physical ageing process. The effects of a surface treatment on the ageing of HDPE-clay composites are also evaluated and discussed.     

Thermal studies to determine the accelerated ageing of flares

Summary Thermal analysis is an interesting technique to determine kinetic parameters of separate components, and also of a complete system to receive adequate information on the ageing process of pyrotechnic compositions. The investigated tracer is a tracking tracer of a Swiss missile. It is attached to a missile and produces during burning a red flame. The missile system is already in use for 15 years. Periodic inspections of the system take place every three years. These inspections, however only give information on the actual state of the tracer and not on the future state. The aim of this investigation is to predict the ageing behaviour of the tracer system to give additional information about the future use of the missile system. After two ageing profiles the tracer systems were tested on the test range at Armasuisse in Thun. The results of the testing show that an ageing period of 4 weeks at 60°C gives no changes in performance (light output and burning time). On the other side also a quite heavy ageing period of 4 weeks at 150°C was applied on tracer systems, which gives a dramatically change in burning time. The light output was even higher after this ageing profile, although the intensity changes a lot.     

传递性质的自由体积模型对方阱流体的应用

对简单分子构成的稀薄气体的传递性质已有较严格的分子动力学理论，但对稠密流体，尤其是液体，严格的理论方法尚难以解决实际问题词．实用中仍以经验或半经验模型为主．半经验模型以Eyring的绝对反应速率理论[1，2]和自由体积理论[3-11]为典型代表．由于自由体积模型可以同时反映温度和压力的影响，其应用潜力更大，值得进一步研究．对理论模型的检验一般用实际物系的测定数据．这种检验虽然直接可靠，但往往受到实验条件的限制，例如高温高压；近年来迅速发展的计算机分子模拟为建立和验证理论模型提供了一个有效手段．由于计算机模拟数…     

A critique of lifetime prediction of polymers by thermal analysis

Attempts are made to use kinetics parameters from thermal decomposition experiments at high temperatures to predict service lifetimes of polymeric materials at lower temperatures. However, besides the obvious measurement and extrapolation errors (which can be considerable), there are two fundamental reasons why quantitative long range extrapolations can not be made for complex condensed phase systems. They are: 1) Arrhenius kinetics parameters can not be extrapolated through phase transitions or softening temperatures; 2) Arrhenius kinetics parameters can not be extrapolated through the ceiling temperature region. Satisfactory lifetime prediction methods can be developed only after a thorough analysis of the causes of service failure. A real method has been taken from literature to illustrate the correct procedures. This paper is, in part, based on the Honorary Guest Lecture given at the Sixth Seminar in Memory of Stanislaus Bretsznajder [1].