Oxidation Induction Time Measurements Research Articles

Thermal aging of electrical cable insulation made of cross‐linked low density polyethylene for under‐hood application in automotive industry

AbstractThe thermal aging of two electrical cable insulations made of cross‐linked low density polyethylene (XLPE) only differing by their stabilization system was studied in air between 125 and 200°C. The chemical consumption of the two types of antioxidants (phenol and sulfide), the build‐up of oxidation products and the consequences of thermal oxidation on the mechanical properties were respectively determined by measurement of the oxidation induction time (OIT), Fourier transform infrared spectroscopy (FTIR) and uniaxial tensile testing. It was found that the chemical consumption of each antioxidant occurs in turn almost independently, starting with the sulfide antioxidant and involving small but well detectable antagonistic effects. As soon as the antioxidant concentration becomes very low, that is, practically undetectable by OIT measurements, the thermal oxidation of XLPE is detected by FTIR spectroscopy causing a catastrophic decrease in the fracture properties. It was shown that the complete consumption of antioxidants coincides with the end of the oxidation induction period and the onset of the sudden decrease in the elongation at break. Based on all these experimental results, a simplified kinetic model was developed to account as faithfully as possible for the consumption kinetics of antioxidants, evaluate their antagonistic effects and, using a relevant end‐of‐life criterion, predict the lifetime of the two XLPE insulations. It was found a satisfying agreement between theory and experiment.

Evaluation of the Durability of PE-HD Geosynthetic Barriers Using High-Pressure Autoclave Tests

To evaluate the resistance of a 2 mm thick polymeric geosynthetic barrier (GBR-P) made of high-density polyethylene (PE-HD), oxidation tests at 80 °C using high-pressure autoclaves at three different oxygen pressures were carried out. After different times, 18 different evaluations were performed on the aged test specimens of the GBR-P. The following characteristics were considered: thermoanalytical methods like oxidation induction time (OIT), crystallinity and oxidation onset temperature (OOT), viscosity measurements like melt mass-flow rate (MFR) and viscosity, mechanical properties of the short term tensile test as well as stress crack resistance methods with the notched constant tensile load (NCTL) test and the strain hardening test (SHT). The failure times extrapolated to atmospheric conditions at the test temperature of 80 °C were in the range of 450–550 days for the stress crack tests. For the analytical methods, the failure times were 620–780 days and for the tests in the short-time tensile test, the failure times were between 700 and 900 days, depending on the property. To evaluate the OIT measurements, investigations were carried out on the influence of the sampling, preparation and treatment of the OIT measurement samples. Furthermore, the influence of oxidation on the thickness of the test samples was investigated. For this purpose, test specimens with thicknesses of 1 mm, 2 mm, 4 mm and 10 mm were produced and tested with the same material. The consumption of antioxidants on the surface of the test specimens was twice as high as inside the specimens. It was shown that a wide range of parameters is suitable for representing the ageing states of a PE-HD GBR-P. With very few exceptions, it was possible to determine the time transitions up to which the properties hardly changed and from which a steep rise or steep fall occurred. The most reliable and practicable test methods for ageing tests in the study were: elongation at break in the tensile test, the strain hardening modulus, OIT and OOT.

Assessing the integrity of in service polyethylene pipes

The service life of Polyethylene (PE) pipes is influenced by many factors including type of PE resin used, time in service as well as service location. Assessing the integrity of PE pipes which have been in service using standard test methods is complicated by the requirement that samples should be sectioned directly from the used pipe wall. In this current study, a new fatigue test method, termed the Cyclic PENT (CPENT), is described that allows direct testing of unaltered material sectioned from the pipe wall. The CPENT was combined with oxidation induction time measurements to provide information on the relative ranking of pipes which have been in service for a time period of 20–40 years from 3 different service locations. It was found that both service location and in-service time are important factors when assessing the remaining life of PE pipes.

Synthesis, characterization and stabilization effect of antioxidant-functionalized celluloses

Phenolic antioxidants were immobilized on cellulose nanocrystals and microcrystalline cellulose to reduce their loss. The products were synthesized with antioxidant acid chlorides forming ester bonds, and some additionally grafted with aliphatic acid chlorides to improve their solubility in non-polar polymeric matrices. The products were fully characterized using FT-IR, solid-state NMR, X-ray diffraction, and thermogravimetric analysis. The quantity of grafted groups was determined through alkaline hydrolysis and subsequent titration of the excess of alkali. The stabilization effect of the products was verified by oxidation induction time measurements in squalane. Different mixing ratios to test the influence of stabilizer amount were used. The products are compared and influences on the stabilization effect discussed.

Synthesis and oxidative stability of phenolic antioxidants immobilized by cellulose nanocrystals

Antioxidants are immobilized by attaching them as functional groups to cellulose nanocrystals. The synthesis route includes a carboxymethylation step followed by covalent grafting of di-tert-butylphenol moieties (first product). To improve solubility within non-polar polymeric matrices additional octadecyl groups are added (second product). Both products are fully characterized using FT-IR, solid-state NMR, and thermogravimetric analyses. The degree of substitution in each functionalization step is determined by conductometric titration. The syntheses are successful, with moderate degrees of functionalization due to limited accessibility of the hydroxyl groups, which correlates with the geometric shape of cellulose nanocrystals. The stabilization effect is determined by oxidation induction time measurements in squalane. Further, the influence of additional octadecyl moieties grafted to cellulose nanocrystals is assessed.

Time-resolved rheology as a tool to monitor the progress of polymer degradation in the melt state – Part II: Thermal and thermo-oxidative degradation of polyamide 11/organo-clay nanocomposites

Thermal and thermo-oxidative degradation of nanocomposites based on polyamide 11 (PA11) and organo-modified clay (Cloisite® 30B) are studied in the melt state (T = 215 °C) via time-resolved mechanical spectroscopy (TRMS). The goal is assessing the potentiality of rheological analysis for studying polymer degradation in complex systems such as polymer nanocomposites, whose rheological response stems from the combination of the contributions of polymer and nanoparticles. We prove that a thorough analysis grounded on TRMS allows to isolate the effect of degradation of the polymer matrix, whose progress can be hence profitably monitored. Essentially the same degradation mechanisms as in neat PA11 are identified for the nanocomposite, but the kinetics of the thermo-oxidation processes are much faster in the presence of organo-clay. In particular, rheology promptly identifies cross-linking reactions since the early stage of the treatment in air. Matrix-assisted laser desorption/ionization and oxidative induction time measurements corroborate the conclusions drawn on the basis of rheological analyses, shedding light on the chemical aspects of PA11 degradation. Overall, rheological analysis confirms to be a valuable tool for monitoring polymer degradation even in case of inherently complex systems such as nanocomposites. In this case, however, the analysis can be difficult if the dynamics of the nanoparticles prevail over those of the polymer matrix, which is the case of nanocomposites at high filler contents.

Chlorine dioxide resistance of different phenolic antioxidants in polyethylene

A series of polyethylene tape samples containing 8 different phenolic antioxidants (concentration = 0.1 ± 0.01 wt.%) were exposed to water containing 10 ppm chlorine dioxide buffered to pH = 6.8 at 70 °C for different periods of time. The degradation rate and depletion time of the antioxidants in the polyethylene were obtained by oxidation induction time measurements using DSC. The majority of the tape samples (6 out of 8) showed a simple behaviour: the rate of antioxidant loss decreased and the antioxidant depletion time increased in linear fashion with increasing initial molar concentration of phenolic groups in the polymer. The tape that contained Hostanox O3 had a high initial phenolic concentration but it exhibited a short antioxidant depletion time due to the limited solubility of this antioxidant in polyethylene. Tapes containing Irganox 1330 and Cyanox 1790 showed antioxidant depletion times that were almost twice that of the other antioxidants with the same initial molar concentration of phenolic groups.

Preparation of silica-supported 2-mercaptobenzimidazole and its antioxidative behavior in styrene-butadiene rubber

A novel type of rubber antioxidant, silica-supported 2-mercaptobenzimidazole (SiO2-s-MB), was prepared by reacting 2-mercaptobenzimidazole (MB) with chlorosilane-modified silica (m-SiO2). Raman spectroscopy, FT-IR, XPS and TGA confirmed that MB was chemically bonded onto the surfaces of silica particles. SEM observation showed that SiO2-s-MB was homogeneously dispersed in the styrene-butadiene rubber (SBR) matrix. Based on the measurement of oxidation induction time (OIT) of SBR/SiO2-s-MB and SBR/m-SiO2/MB composites containing equivalent antioxidant components, it was found that the antioxidative efficiency of SiO2-s-MB was superior to that of the corresponding low molecular MB. By determining the changes of tensile strength, elongation at break and crosslinking density of SBR composites during long-term accelerated aging, it was shown that the thermal oxidative stability of SBR/SiO2-s-MB composites was much higher than that of SBR/m-SiO2/MB composites. Furthermore, the color contamination, migration and volatility of SiO2-s-MB were lower than those of MB, indicating that SiO2-s-MB is environmentally friendly.

Natural antioxidants for polypropylene stabilization

A study on the efficiency of bio-based compounds as stabilizers for polypropylene (PP) is reported. A water extract from French maritime pine bark (Pycnogenol ®), a by-product containing polyphenols obtained from wine production, and a carotenoid-containing oleoresin from processing of tomatoes were used. Their stabilizing activity was compared with that of a commercial phenolic antioxidant. Thermogravimetric analysis and Oxidative Induction Time measurements performed on unaged samples, as well as infrared spectroscopy on samples aged at 70 °C, provided evidence for the effectiveness of the natural stabilizers. Mechanical characterization was carried out on aged films and injection moulded samples. Experimental results indicated that particularly grape extract could provide long-term stabilization to PP under conditions of oxidative degradation. Therefore, it could be used as efficient and high value-added additive for polypropylene. Pycnogenol ® also showed antioxidant activity, however the achievement of a more homogeneous dispersion in the polymer matrix could improve the mechanical performance of aged samples.

Melt stabilisation of Phillips type polyethylene, Part II: Correlation between additive consumption and polymer properties

Phillips type polyethylene stabilised with combinations of 700 ppm phenolic antioxidant and different amounts of various phosphorous stabilisers (sterically hindered aryl phosphite [Hostanox PAR 24], phosphonite [Sandostab P-EPQ], and aryl–alkyl phosphine [PEPFINE]) was processed by six consecutive extrusions. The polymer was characterised by FT-IR spectroscopy, rheological (melt flow index, creep compliance), colour and oxidation induction time measurements. Films were prepared by blowing and their mechanical strength was determined by Elmendorf and dart drop tests. The consumption of the antioxidants was compared to the characteristics of the polymer and to the strength of the films. The consumption rate of both the phenolic and the phosphorous antioxidants is reduced in their combinations compared to single antioxidants. The chemical structure of the polymer is modified considerably in the first extrusion even at high antioxidant levels. The mechanism of stabilisation is determined by the type of the antioxidant(s) in further processing steps. The phenolic antioxidant does not prevent the formation of long chain branches. The phosphonite and the phosphine hinder efficiently hydrogen abstraction from the polymer chain and long chain branching. Their efficiency is similar, but the phosphonite is consumed fast, while the phosphine oxidises slowly. The investigated phosphite is less reactive; the contribution of the phenolic antioxidant to the inhibition reactions is significant in phenol/phosphite combinations, therefore long chain branching increases continuously with increasing number of processing steps.

ANTIOXIDANT DEPLETION AND OIT VALUES OF HIGH IMPACT PP STRANDS

The service life of a polyolefin product depends to large extent on the type and amount of the antioxidants added. During the manufacturing, storage and use of the product the antioxidants are depleted by physical processes and chemical degradation, and this impairs its long-term performance. The initial and in-use oxidation stability is often characterized and monitored by the measurement of the oxidative induction time (OIT), and service life predictions are based on the rate of decrease of the OIT value. To study the correlation between the OIT value and the actual antioxidant concentration, eight random arrays of high-impact polypropylene (PP) strands stabilized using six different antioxidant packages (composed of Irgafos 168, Irganox 1010 and 1330, Chimassorb 944) were immersed in hot water (80°C and 90°C) and oven aged in air (80°C) for more than two years, and the change of OIT values and antioxidant concentrations was measured. For phosphitic and phenolic stabilizers water immersion is generally a more critical aging condition than oven aging in air, while for the hindered amine stabilizers (HAS) the opposite was observed. As expected, a linear correlation between OIT values and concentrations was found for the "classical" package of phosphitic and phenolic stabilizers. In the case of Irganox 1330, the change of the OIT values during aging was considerably slower than the change of concentration. Even when zero concentration was reached according to the vanishing peak height in the chromatogram, a considerable OIT value was still measurable. This may be due to unidentified degradation products of Irganox 1330 which still act as antioxidants in the long run. Addition of HAS (Chimassorb 944) seems to enhance the initial OIT of stabilized phenolic samples and the long-term effectiveness of the phenolic stabilization under air oven aging conditions. However, the marked long-term effectiveness of the HAS itself and the slow change with time of the concentration were not detectable by OIT measurements above the PP melting point of about 170°C.

Description of the O2pressure influence on the oxidation time of motor oils

A characteristic index for the oxidation stability this is the oxidation induction time (OIT) which is defined by the time between the start of oxygen exposure and the onset of oxidation. Pressure DSC is required to increase oxygen concentration in order to achieve faster reactions at lower temperatures. OIT measurements of reference engine oils have been used to study the influence of oxygen pressure in the range from 0.1 to 10 MPa. A power law relationship was derived to describe this correlation between OIT and oxygen pressure. From this a quantitation factor is proposed to represent the influence of stabilizer. The exponent describes the sensitivity of the oxidation reaction of the oil towards the oxygen pressure and the term 'inherent stability' is proposed for that.. This relationship characterizes in more details the oxidation behavior. Extrapolation to higher pressures indicates, that the stabilization effects of additives can be overcome by the inherent stability. This signifies, that the ranking of the oils can be affected by the oxygen pressure.

Interlaboratory test on polymers: determination of oxidation induction time and oxidation induction temperature by differential scanning calorymetry

The results of the oxidation induction time (OIT) tests indicate that the determination of OIT involves a high degree of uncertainty in respect of the measured data, particularly for low OIT values. This would seem to show that OIT measurements have an extremely critical significance for quality control purposes or lifetime predictions of polyolefin parts. Determining oxidation induction temperature (OIT*) could therefore be a valuable alternative for less stabilized polyolefins (low OIT values) in particular. However, OIT* measurement clearly indicates that the ability to distinguish between different samples decreases drastically as OIT* data increase. Generally, the evaluated data can be used to estimate the results of in-house and external OIT/OIT* measurements and their compatibility. This report should therefore support day-to-day work in analytical laboratories where oxidation induction values are measured by DSC.

Interlaboratory tests on polymers by differential scanning calorimetry (DSC): determination and comparison of oxidation induction time (OIT) and oxidation induction temperature (OIT∗)

This report presents results obtained from two interlaboratory tests: 14 and 16 participants, respectively, mainly from industry and research institutes, measured oxidation induction times (OIT) and oxidation induction temperatures (OIT∗) on four different grades of polyethylene by differential scanning calorimetry (DSC). The measured data were collected by EMPA and evaluated using a robust statistical method. Repeatability and reproducibility data were of special interest. The results of the OIT tests indicate that the determination of OIT involves a high degree of uncertainty in respect of the measured data, particularly for low OIT values. This would seem to show that the significance of OIT measurements for quality control purposes or lifetime predictions of polyolefin parts has to be viewed critically. Determining OIT∗ could therefore be a valuable alternative for less stabilised polyolefins (low OIT values) in particular. However, OIT∗ measurement clearly indicates that the ability to distinguish between different samples decreases drastically as OIT∗ data increase. One interlaboratory test participant uses chemiluminescence instead of DSC to determine OIT values. In the context of this study it was shown that chemiluminescence could also provide OIT results within the range of DSC interlaboratory test data. Overall, the evaluated data can be used to estimate the significance of in-house and external OIT/OIT∗ measurements and their compatibility. This report should therefore support day-to-day work in analytical laboratories where oxidation induction values are measured by DSC.

지중 배편케이블의 열화에 미치는 온수의 영향

In this study, we investigated the effects of hot water on the aging of URD power cables due to the hot water ingress to power distribution lines. Hot water contacted and non-contacted cables extracted from 2 distribution lines were characterized by the measurement of oxidation induction time(OIT) and chemical structure. In OIT measurement, hot water contacted cables showed the shorter OIT than non-contacted ones. Especially, hot water contacted insulation layer near insulation shield showed the shortest OIT. In chemical structure analysis, the antioxidant content was decreased and the acid concentration was increased by hot water ingress. From this results, it can be concluded that the hot water ingress to power cable may be accelerate the aging by means of antioxidant consumption.

Polypropylene compounding with post-consumer material: II. Reprocessing

We have investigated extrusion moulding for 13 processing cycles, using formulations containing virgin polypropylene and 1:1 mixtures of virgin and post-consumer polypropylene containing, or not, the antioxidant Recyclostab 411™. The main effect of reprocessing for polypropylene was the decrease of molar mass due to degradation with chain scission, which was detected by the melt flow rate and the increase in crystallinity. Antioxidant consumption and material stability reduction with processing cycles were observed by the reduction of the thermo-oxidative stability detected by oxidative induction time measurements. The addition of antioxidants minimized degradation and maintained the stability of the material.

Irradiation ageing of CSPE/EPR control command electric cables. Correlation between mechanical properties and oxidation

In this paper, correlations between the elongation at break and the oxidation of chlorosulfonated polyethylene and ethylene propylene rubber (EPR) polymers in instrumentation and control cables irradiated at different dose rates are brought to evidence. During irradiation, the following phenomena are observed: an increase of oxygen consumption, a degradation of the mechanical properties and a reduction of the oxidation induction time (OIT) measured for EPR. A correlation between the mechanical properties and the OIT of the EPR has only been established in the case of irradiation at low dose rate. This reveals a difference in the oxidative degradation process at low and high dose rates. This study shows the possibility to assess the ageing of electric cables installed inside nuclear power plants by OIT measurements.

Comparison of oxidation induction time measurements with values derived from oxidation induction temperature measurements for EPDM and XLPE polymers

AbstractOxidative stability and retained operational utility of polymers used as insulation for electrical cables, such as ethylene propylene diene monomer (EPDM) and crosslinked polyethylene (XLPE), may be assessed by oxidation induction time (OIT) analysis. OIT is measured directly with a differential scanning calorimeter. Using a simplified kinetics model, Gimzewski demonstrated that it is possible to calculate the OIT from measured values of oxidation induction temperature and the activation energy for petroleum lubricants. In the present research, directly measured OITs are compared with OITs calculated from measured oxidation induction temperatures and activation energies for EPDM and XLPE cable insulation. Good agreement between the two methods was demonstrated for these materials.

Molecular structure, morphology, and antioxidant consumption in medium density polyethylene pipes in hot‐water applications

AbstractChanges in polymer structure and antioxidant concentration have been systematically studied as functions of temperature, hoop stress, exposure time, and location in pipe wall on pressure tested pipes of medium density polyethylene. The pressure tests have been performed with water as the internal medium and air as the external medium at temperatures in the range 80 to 105°. Infrared spectroscopy shows that oxidation is initiated at the inner wall surface just prior to the onset of the so‐called stage III fracture. X‐ray diffraction and size exclusion chromatography show that oxidation involves only the amorphous phase and results in a significant molar mass reduction. The near‐inner‐wall material exhibits a 10% reduction in mass average molar mass before the onset of stage III fracture and thereafter a more dramatic decrease. Oxidation induction time measurements by differential scanning calorimetry show that the antioxidant concentration is almost twice as high in the center of the wall as in the near‐inner‐wall and outer‐wall material of the unexposed pipe, that the loss of antioxidant is anomalously rapid at the beginning of the high temperature exposure, and that the antioxidant concentration profile gradually becomes more skewed towards the outer wall on prolonged exposure. The data presented in this paper are used in a parallel paper for modeling purposes.

Thermal endurance evaluation of XLPE insulated cables

Thermal endurance characterization of XLPE cable models is realized by applying conventional procedures and an analytical method. The latter method is based on oxidation induction time measurements to estimate the activation energy of the degradation reaction and then the slope of the thermal endurance line. The conventional life test at an intermediate temperature is used to determine the ordinate intercept of the same line. The estimates of temperature indices, obtained by different diagnostic properties and end-point selections on the basis of both conventional and analytical techniques, are fairly close, thus revealing the ability of appropriate analytical procedures to characterize reliably the thermal endurance of XLPE cables, with the advantage of shortening test times with respect to conventional methods. A discussion is presented on the choice of diagnostic properties and end points, in order to achieve thermal endurance lines and indices which provide actual information on cable performance under service stresses. Analysis of the possible choices is performed with reference to the results of multiple-stress tests which simulate service stresses and by investigating the aging mechanisms. Finally, considerations of the compensation effect applied to life lines, with reference to both Eyring and Arrhenius models, are made. Checks relevant to different property end points reveal very good agreement of the experimental results with the aging compensation effect.