Lifetime predictions for semi-crystalline cable insulation materials: I. Mechanical properties and oxygen consumption measurements on EPR materials

Long-term accelerated aging studies (up to 7 years of aging) were conducted on four typical EPR materials used as cable insulation in nuclear power plant safety applications with the goal of establishing lifetime estimates at typical aging conditions of ∼50 °C. The four materials showed slow to moderate changes in mechanical properties (tensile elongation) until just before failure where abrupt changes occurred (so-called “induction-time” behavior). Time-temperature superposition was applied to derive shift factors and probe for Arrhenius behavior. Three of the materials showed reasonable time-temperature superposition with the empirically derived shift factors yielding an approximate Arrhenius dependence on temperature. Since the elongation results for the fourth material could not be successfully superposed, consistency with Arrhenius assumptions was impossible. For this material the early part of the mechanical degradation appeared to have an Arrhenius activation energy E_a of ∼100 kJ/mol (24 kcal/mol) whereas the post-induction degradation data had an E_a of ∼128 kJ/mol. Oxygen consumption measurements were used to confirm the 100 kJ/mol E_a found from early-time elongation results and to show that the chemistry responsible before the induction time is likely to remain unchanged down to 50 °C. Reasonable extrapolations of the induction-time results indicated 50 °C lifetimes exceeding 300 years for all four materials.     

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.     

Effects of ultraviolet radiation, temperature and moisture on aging of coatings and sealants - A chemical and rheological study

Photodegradation of polymeric materials leads to significant modifications in both chemical properties and mechanical-rheological behaviors over time. Thus, it is important to characterize both properties to gain a better understanding of the durability of the materials. In this contribution, the chemorheological tools based upon Fourier transform infrared (FTIR) spectroscopy and dynamic mechanical thermal analysis (DMTA) were used to study the effects of temperature and moisture on photodegradation of a model sealant/coating system based upon a styrene-butadiene-styrene triblock copolymer. Specimens were exposed coincidentally to ultraviolet-visible radiation between 295 nm and 600 nm, and one of four different combinations of temperature and relative humidity (RH), i.e., (a) 30 °C and <1% RH, (b) 30 °C and 80% RH, (c) 55 °C and <1% RH, and (d) 55 °C and 80% RH. The rate of photodegradation was examined in terms of formation of oxidation species and evolution of mechanical-rheological data, including glass transition temperatures, moduli, and the number of effective crosslinked butadiene chains per unit volume per exposure time. Environmental exposure resulted in similar degradation modes for all four environments but the rate of photodegradation was found to depend strongly on temperature. Conversely, the role of moisture on photodegradation was not significant. The study shows that chemical modification can be directly related to the corresponding rheological modifications. In addition, the relative stability of styrene and butadiene against photodegradation as a function of temperature and moisture was compared.     

Investigation of the influence of different commercial softeners on the stability of poly(lactic acid) fabrics during storage

We have studied the potential degradation of poly(lactic acid)-based fabrics treated with commercial softeners and stored under two sets of conditions for one year. Initial wet-processing caused a fall in molecular weight of about 28%, irrespective of after-treatment. Storage at 40 °C and 80% RH produced further degradation which, with few exceptions, was aggravated by the presence of softeners. Ultimately, all samples degraded beyond the point of commercial usefulness. No clear distinction could be made between the effects of softeners having differing compositions. In contrast, fabrics stored under milder conditions of 23 °C and 50% RH showed no significant time-dependent polymer degradation, irrespective of the treatment applied. There were slight changes in tensile properties and some evidence of physical structural effects having occurred, which we attribute to physical aging. However, we do not believe these to be so serious as to call into question the long-term viability of PLA-based textile products.     

A simple method for the evaluation of heat resistant property of elastomers

The paper shows a simple method that provides us the induction period, rate of weight increase at an earlier stage of thermal oxidation of elastomer and the rate of weight loss by oxidative degradation from a certain period of aging time. Ethylene-propylene elastomer (EPR) was aged in air at various constant temperatures ranged from 90 °C to 130 °C. The weight of samples was measured periodically at room temperature. The weight increased after a certain period of induction period at the early stage of aging. The activation energy obtained from the reciprocal of the induction period and that of the rate of weight increase was the same. The values were 113 kJ/mol. The weight started to decrease from the maximum point. The activation energy for the tangent of decrease curve was 60.3 kJ/mol. This method was applied to study the effect of pre-irradiation of EPR in air on the heat resistant property of the sample. The relatively low dose of 40 kGy decreased the induction period.     

Influence of temperature on the thermo-oxidative degradation of polyamide 6 films

The thermo-oxidative degradation of polyamide 6 (PA6) was studied at relative high temperatures (between 120 and 170 °C) using oxygen uptake and hydroperoxide determination methods, chemiluminescence, FT-IR and UV-VIS spectroscopy as well as solution viscosity and tensile property measurements.The relation between the results of the different analytical techniques and influence of temperature on these relations was determined. Arrhenius plots of the degradation determined with the different methods are linear; however the activation energies determined from these plots depend on the analytical method used. For oxygen uptake measurements and changes in UV absorbance (at 280 nm) and solution viscosity an activation energy of about 120 kJ/mol was calculated, for the increase in carbonyl index of about 80 kJ/mol and for the decrease in elongation at break of about 150 kJ/mol.The changes in oxygen uptake UV absorbance and solution viscosity are probably due to the same chemical process. The lower activation energy from changes in the carbonyl index is attributed to the formation of gaseous products, which play a larger role at higher temperatures. The higher activation energy from the elongation at break measurements was ascribed to the contribution of physical changes that play the largest role at the highest temperatures.     

The influence of biotic and abiotic factors on the rate of degradation of poly(lactic) acid (PLA) coupons buried in compost and soil

Poly(lactic) acid (PLA) is a compostable biopolymer and has been commercialised for the for the manufacture of short-shelf life products. As a result, increasing amounts of PLA are entering waste management systems and the environment; however, the degradation mechanism is unclear. While hydrolysis of the polymer occurs abiotically at elevated temperature in the presence of water, potential catalytic role for microbes in this process is yet to be established. In this study, we examined the degradation of PLA coupons from commercial packaging at a range of temperatures (25°, 37°, 45°, 50° and 55 °C) in soil and compost and compared with the degradation rates in sterile aqueous conditions by measuring loss of tensile strength and molecular weight (Mw). In addition, in order to assess the possible influence of abiotic soluble factors in compost and soil on degradation of PLA, degradation rates in microorganism-rich compost and soil were compared with sterile compost and soil extract at 50 °C. Temperature was determined to be the key parameter in PLA degradation and degradation rates in microorganism-rich compost and soil were faster than in sterile water at temperatures 45° and 50 °C determined by tensile strength and Mw loss. Furthermore, all tensile strength was lost faster after 30 and 36 days in microorganism-rich compost and soil, respectively, than in sterile compost and soil extract, 57 and 54 days, respectively at 50 °C. Significantly more Mw, 68% and 64%, was lost in compost and soil, respectively than in compost extract, Mw, 53%; and in soil extract, 57%. Therefore, degradation rates were faster in microorganism-rich compost and soil than in sterile compost and soil extract, which contained the abiotic soluble factors of compost and soil at 50 °C. These comparative studies support a direct role for microorganisms in PLA degradation at elevated temperatures in humid environments. No change in tensile strength or Mw was observed either 25° or 37 °C after 1 year suggesting that accumulation of PLA in the environment may cause future pollution issues.     

Thermal-oxidative aging of DGEBA/EPN/LMPA epoxy system: Chemical structure and thermal-mechanical properties

The evolvement of chemical structure and thermal-mechanical properties of diglycidyl ether of bisphenol-A and novolac epoxy resin blends cured with low molecular polyamide (DGEBA/EPN/LMPA system) during thermal-oxidative aging were investigated by Attenuated Total Reflectance Fourier Transform Infrared spectrometry (ATR-FTIR) and Dynamic Mechanical Thermal Analysis (DMTA). The results revealed that the chemical reactions during thermal-oxidative aging contained oxidation and chain scission. Some possible chemical reaction processes were given. There was a new compound formed during aging processes and the change of its glass transition temperature (T_g) with aging time followed an exponential law. In addition, the changes of dynamic mechanical behavior of this epoxy system aged at four different temperatures (110 °C, 130 °C, 150 °C, 170 °C) were compared. An empirical formula was obtained through kinetic analysis and this formula can be used to predict the oxidative degree of the surface at different aging temperature.     

Laboratory accelerated and natural weathering of styrene-ethylene-butylene-styrene (SEBS) block copolymer

Indoor accelerated and outdoor weathering of polystyrene-b-(ethylene-co-butylene)-b-styrene (SEBS) was studied by infrared spectroscopy. Accelerated conditions involved simultaneous exposure of specimens to ultraviolet-visible radiation between 295 nm and 450 nm and each of four temperature/relative humidity (RH) environments, i.e., (a) 30 °C ± 1 °C at <1% RH, (b) 30 °C ± 1 °C at 80% RH, (c) 55 °C ± 1 °C at <1% RH, and (d) 55 °C ± 1 °C at 80% RH. Outdoor exposure was conducted in Gaithersburg, MD, in two different time periods. Similar photooxidative mechanisms were operative under all conditions. In the case of indoor accelerated exposure, the rate of photooxidation was found to depend strongly on temperature. Unlike the exposure at 55 °C, moisture-assisted photooxidation was insignificant at 30 °C. A quantitative study on the synergistic effect of environmental stressors revealed that the degrading effect of combined temperature and moisture on photooxidation was greater than the sum of the two effects exerted independently. Outdoor weathered specimens exhibited significantly slower photooxidation. Acceleration of photooxidation ranged from 2.5 to 10 times in comparison to the outdoor exposure, depending on the indoor accelerated conditions.     

Correlation of antioxidant depletion and mechanical performance during thermal degradation of an HTPB elastomer

Thermal degradation studies of a stabilized HTPB based elastomer were conducted at temperatures from 50 °C to 110 °C. The concentration of extractable antioxidant (AO2246) in the polymer was quantified via AO extraction and a gas chromatography-based method using internal standards. The decrease in extractable AO levels as a function of time and temperature was evaluated and correlated with mechanical property changes. Most importantly, AO depletion features were found to be temperature dependent. At elevated temperatures (>80 °C) extractable AO levels decreased rapidly and faster than the concurrent loss in mechanical properties. While extractable AO concentrations decrease quickly, the material is able to maintain some useful mechanical properties, perhaps via non-extractable or grafted AO species formed during degradation providing additional protection. At lower aging temperatures extractable or free AO levels decreased more slowly than the mechanical properties. Therefore, for condition monitoring purposes a universal correlation between AO levels and aging state or material condition could not be established. Most importantly, however, loss of mechanical properties and oxidative degradation is observed at lower temperatures despite significant levels of free antioxidant in the material. The antioxidant appears to be limited in its effectiveness to completely prevent degradation reactions, or only fractions of the total AO available are actually involved in the inhibition process.     

Reinforcement of natural rubber/high density polyethylene blends with electron beam irradiated liquid natural rubber-coated rice husk

Coating of rice husk (RH) surface with liquid natural rubber (LNR) and exposure to electron beam irradiation in air were studied. FTIR analysis on the LNR-coated RH (RHR) exposed to electron beam (EB) showed a decrease in the double bonds and an increase in hydroxyl and hydrogen bonded carbonyl groups arising from the chemical interaction between the active groups on RH surface with LNR. The scanning electron micrograph showed that the LNR formed a coating on the RH particles which transformed to a fine and clear fibrous layer at 20 kGy irradiation. The LNR film appeared as patches at 50 kGy irradiation due to degradation of rubber. Composites of natural rubber (NR)/high density polyethylene (HDPE)/RHR showed an optimum at 20–30 kGy dosage with the maximum stress, tensile modulus and impact strength of 6.5, 79 and 13.2 kJ/m², respectively. The interfacial interaction between the modified RH and TPNR matrix had improved on exposure of RHR to e-beam at 20–30 kGy dosage.     

Sorption and regeneration performance of novel solid desiccant based on PVA-LiCl electrospun nanofibrous membrane

Electrospun PVA (polyvinyl alcohol)-LiCl composite membranes were prepared as novel solid desiccants. Experimental results show that nanofibrous membranes (NFMs) exhibit notable advantages in sorption capacity, sorption rate and low-temperature desorption rate, as compared with the solution-cast PVA-LiCl membranes (SCMs). The PVA NFM with 15 wt% LiCl can sorb 1.04 g g⁻¹ water at 25 °C and 90% relative humidity (RH), which is more than twice of the reported capacity of silica gel. Due to the nano-structure and small diffusion distance, the desiccant membranes have fast sorption and desorption rates. The desorption isobars show that about 90% of the sorbed moisture can be removed at temperatures between 40 °C and 60 °C, which makes it promising to utilize solar energy or exhaust heat for air dehumidification. The composite desiccant membranes can also be recycled without the degradation of sorption and desorption performance.     

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.     

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.     

Effects of hydrolysis-induced molecular weight changes on the phase separation of a polyester polyurethane

A polyester polyurethane, was subjected to humid and dry aging conditions at 70 °C with 75% and 0% relative humidity, respectively. Differences in molecular weight and quasi-static tensile strength between humid- and dry-aged samples are attributed to hydrolysis of the humid-aged polymers. A phase-separation study was performed on selected samples from the aging matrix. Polymer samples were subjected to 110 °C for 10 min, by mixing the polyester (soft) and the polyurethane (hard) domains, then rapidly cooled to room temperature, initiating the phase-separation process. Uniaxial tension, dynamic shear and infrared spectra of these samples were measured as a function of time providing insight into the effects of hydrolytic degradation and the relationship of mechanical and molecular-level properties. An Avrami-type analysis shows two distinct processes whose characteristics vary as a function of increased hydrolysis. LA-UR 04-6447.     

Thermal analysis of poly(2-methylpentamethylene terephthalamide)

Poly(2-methylpentamethylene terephthalamide) (Nylon M5T) is a new high temperature aromatic polyamide developed by Hoechst Celanese. In this paper thermal properties of Nylon M5T chips, as well as as-spun and drawn fibers were studied by DSC, DMA, hot stage microscopy and WAXS.T _g of the fully amorphous Nylon M5T is 143°C when measured by DSC;T _g increases with crystallinity to 151°C. The temperature dependence of the solid and melt specific heat capacities has also been determined. The heat capacity increase at the glass transition of the amorphous polymer is 103.9 J °C^–1 mol^–1.T _g by DMA for the as-spun fiber is 155°C, for a drawn fiber is 180°C. Three secondary transitions were observed by DMA in addition to the glass transition. These correspond to a local mode relaxation of the methylene groups at –120°C, onset of rotation of the amide-groups at –65°C and the onset of the rotation of the phenylenegroups (at 63°C). The crystallinity of Nylon M5T strongly depends on the rate of cooling from the melt. The isothermal crystallization data are melt temperature dependent: two-dimensional crystallization takes place when the samples are crystallized from higher melt temperatures, and this phase changes into a spherulitic structure during cooling to room temperature. Spherulitic crystallization occurs when lower melt temperatures are used. This polymer has three crystal forms as indicated by DSC, DMA and WAXS data. The crystal to crystal transitions are clearly visible when amorphous samples are heated in the DSC, or the DMA curves of as-spun fibers are recorded. It is experimentally shown that a considerable melting of the lower temperature crystal forms takes place during the crystal to crystal transitions. The equilibrium melting point as measured by the Hoffman-Weeks method, has been determined to be 339°C.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

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.     

Evaporative loss kinetics of di(2-ethylhexyl)phthalate (DEHP) from pristine DEHP and plasticized PVC

The migration of di(2-ethylhexyl)phthalate (DEHP) from poly(vinyl chloride) (PVC) to a surrounding gas phase at temperatures below 120 °C kinetically is controlled by evaporation. The effects on the DEHP loss rate of nitrogen flow rate, relative humidity and degradation of the plasticizer at 100 °C was assessed. The sample mass decreased linearly with time for both pristine DEHP and plasticized PVC at comparable rates, suggesting that a thin film of DEHP was present on the jacketing insulation during desorption. The latter hypothesis was supported by infrared spectroscopy and by the fact that DEHP is an amphiphilic molecule that will tend to aggregate at the surface with the hydrophobic 2-ethylhexyl units at the air interface. The effect on the migration rate of moisture present in the gas phase was negligible. The DEHP loss rate increased in a retarding non-linear fashion with increasing gas flow rate. In one of the experiments, DEHP was accidently degraded as revealed by discoloration, the presence of low molar mass degradation products (liquid chromatography) containing additional carbonyl groups (infrared spectroscopy) and an increase in the evaporation rate at temperatures between 100 and 130 °C.     

Enhanced hydrolysis resistance of biodegradable polymers and bio-composites

This study investigated the hydrolysis of biodegradable polymers and bio-composites at 50 °C and 90% relative humidity (RH). With increasing hydrolysis time, the mechanical properties of the biodegradable polymers and bio-composites significantly decreased due to the easy hydrolytic degradation of the ester linkage of the biodegradable polymers. With increasing hydrolysis time, the tensile strength of the polybutylene succinate (PBS) treated with anti-hydrolysis agent or with trimethylolpropane triacrylate (TMPTA) significantly increased compared to the non-treated PBS. The same results were observed for the PBS-based bio-composites. This result was confirmed by the Fourier transform infrared-attenuated total reflectance (FTIR-ATR) and X-ray photoelectron spectroscopy (XPS) spectra, which exhibited a less eroded surface, small cracks and fewer holes due to the reduced surface hydrolysis and erosion under high humidity condition.     

Thermal degradation kinetics of the biodegradable aliphatic polyester, poly(propylene succinate)

The preparation of the biodegradable aliphatic polyester poly(propylene succinate) (PPSu) using 1,3-propanediol and succinic acid is presented. Its synthesis was performed by two-stage melt polycondensation in a glass batch reactor. The polyester was characterized by gel permeation chromatography, ¹H NMR spectroscopy and differential scanning calorimetry (DSC). It has a number average molecular weight 6880 g/mol, peak temperature of melting at 44 °C for heating rate 20 °C/min and glass transition temperature at −36 °C. After melt quenching it can be made completely amorphous due to its low crystallization rate. According to thermogravimetric measurements, PPSu shows a very high thermal stability as its major decomposition rate is at 404 °C (heating rate 10 °C/min). This is very high compared with aliphatic polyesters and can be compared to the decomposition temperature of aromatic polyesters. TG and Differential TG (DTG) thermograms revealed that PPSu degradation takes place in two stages, the first being at low temperatures that corresponds to a very small mass loss of about 7%, the second at elevated temperatures being the main degradation stage. Both stages are attributed to different decomposition mechanisms as is verified from activation energy determined with isoconversional methods of Ozawa, Flyn, Wall and Friedman. The first mechanism that takes place at low temperatures is auto-catalysis with activation energy E = 157 kJ/mol while the second mechanism is a first-order reaction with E = 221 kJ/mol, as calculated by the fitting of experimental measurements.