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.     

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.     

Storage Stability Study of Salicylate-based Poly(anhydride-esters)

Storage stability was evaluated on a biodegradable salicylate-based poly(anhydride-ester) to elucidate the effects of storage conditions over time. The hydrolytically labile polymer samples were stored in powdered form at five relevant storage temperatures (−12 °C, 4 °C, 27 °C, 37 °C, 50 °C) and monitored over four weeks for changes in color, glass transition temperature, molecular weight, and extent of hydrolysis. Samples stored at lower temperatures remained relatively constant with respect to bond hydrolysis and molecular weight. Whereas, samples stored at higher temperatures displayed significant hydrolysis. For hydrolytically degradable polymers, such as these poly(anhydride-esters), samples are best stored at low temperatures under an inert atmosphere.     

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.     

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.     

Microencapsulation Preservation of the Stability and Efficacy of Citrus Grandis Oil-Based Repellent Formulation against Aedes aegypti during Storage

Essential oils have been widely used as an active ingredient in mosquito repellent products. However, essential oils are highly unstable and prone to degradation when exposed to the environment during storage. Microencapsulation techniques help to maintain the stability of molecules in essential oils that are sensitive to environmental stress, and therefore improve shelf life. In this study, the physical stability and efficacy of a repellent formulation consisting of encapsulated Citrus grandis essential oil (CGEO) were evaluated under different storage conditions over a 12-month period by comparing the formulation with a non-encapsulated formulation. The formulations were both stored under two different storage conditions, i.e., 25 ± 2 °C/60% ± 5% relative humidity (RH) and 40 ± 2 °C/75% RH ± 5%, for 12 months. Droplet size, zeta potential, and pH value were measured after 1, 6, and 12 months of storage to determine their stability. For the study of efficacy, each formulation was tested against Aedes aegypti under laboratory conditions. We found that the microencapsulated formulation’s physical characteristics showed insignificant changes as compared with the non-encapsulated formulation during storage. The microencapsulated formulation demonstrated better repellent effects, sustaining high protection (>80%) for 4 more hours of exposure after 12 months of storage as compared with the non-encapsulated formulation that demonstrated high protection for only an hour post application. Microencapsulation helped to preserve the stability of the formulation, which resulted in high protection being maintained for over 12 months of storage.     

Studies on enhancing the keeping quality of peach (Prunus persica Bausch) Cv. Elberta by gamma-irradiation

The effect of gamma-irradiation on keeping quality of peach fruit was studied. The fruit, after harvesting at proper maturity stage, was irradiated in the dose range of 1.0–2.0 kGy, stored under ambient (temp. 25±2 °C, RH 70%) and refrigerated (temp. 3±1 °C, RH 80%) conditions and evaluated periodically for firmness, total soluble solids (TSS), anthocyanins, water-soluble pectic fractions, loss in weight and decay percentage. The anthocyanin evaluation of the fruits revealed that irradiation enhanced the colour development under both the storage conditions. The gamma-irradiation dose range of 1.2–1.4 kGy proved effective in maintaining higher TSS concentration, reducing weight loss and significantly (p⩽0.05) delaying the decaying of the fruit by 6 days under ambient conditions and by 20 days under refrigerated storage conditions.     

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.     

Effect of gamma-irradiation and refrigerated storage on the improvement of quality and shelf life of pear (Pyrus communis L., Cv. Bartlett/William)

Gamma-irradiation alone and in combination with refrigeration was tested consecutively for 3 years for extending the shelf life of pear. Matured green pears were irradiated in the dose range of 0.8–2.0 kGy and stored under ambient (temperature 25±2 °C, RH 70%) and refrigerated (temperature 3±1 °C, RH 80%) conditions. Dose range of 1.5–1.7 kGy extended the storage life of pear by 14 days under ambient conditions. Control unirradiated pears were almost fully ripe within 8 days, while as the pears irradiated in the dose range of 1.5–1.7 kGy were fully ripe within 22 days of ambient storage. Irradiation dose of 1.5–1.7 kGy significantly inhibited the decaying of pears upto 16 days of ambient storage. Irradiation in combination with refrigeration prevented the decaying of pears upto 45 days as against the 35% decay in unirradiated samples. Irradiation dose of 1.5–1.7 kGy also gave an extension of 8 and 4 days during additional ambient storage of the pears following 30 and 45 days of refrigeration, respectively.     

Data quality in tissue analysis using desorption electrospray ionization

There has been a recent surge in applications of mass spectrometry (MS) to tissue analysis, particularly lipid-based tissue imaging using ambient ionization techniques. This recent growth highlights the need to examine the effects of sample handling, storage conditions, and experimental protocols on the quality of the data obtained. Variables such as time before freezing after organ removal, storage time at −80 °C, time stored at room temperature, heating, and freeze/thaw cycles were investigated for their effect on the data quality obtained in desorption electrospray ionization (DESI)-MS using mouse brain. In addition, analytical variables such as tissue thickness, drying times, and instrumental conditions were also examined for their impact on DESI-MS data. While no immediate changes were noted in the DESI-MS lipid profiles of the mouse brain tissue after spending 1 h at room temperature when compared to being frozen immediately following removal, minor changes were noted between the tissue samples after 7 months of storage at −80 °C. In tissue sections stored at room temperature, degradation was noted in 24 h by the appearance of fatty acid dimers, which are indicative of high fatty acid concentrations, while in contrast, those sections stored at −80 °C for 7 months showed no significant degradation. Tissue sections were also subjected to up to six freeze/thaw cycles and showed increasing degradation following each cycle. In addition, tissue pieces were subjected to 50 °C temperatures and analyzed at specific time points. In as little as 2 h, degradation was observed in the form of increased fatty acid dimer formation, indicating that enzymatic processes forming free fatty acids were still active in the tissue. We have associated these dimers with high concentrations of free fatty acids present in the tissue during DESI-MS experiments. Analytical variables such as tissue thickness and time left to dry under nitrogen were also investigated, with no change in the resulting profiles at thickness from 10 to 25 μm and with optimal signal obtained after just 20 min in the dessicator. Experimental conditions such as source parameters, spray solvents, and sample surfaces are all shown to impact the quality of the data. Inter-section (relative standard deviation (%RSD), 0.44–7.2%) and intra-sample (%RSD, 4.0–8.0%) reproducibility data show the high quality information DESI-MS provides. Overall, the many variables investigated here showed DESI-MS to be a robust technique, with sample storage conditions having the most effect on the data obtained, and with unacceptable sample degradation occurring during room temperature storage.     

Softeners for Rubber and Corrosion-Resistant Coatings Based on Shale and Petroleum Raw Materials

A broad spectrum of softeners for vulcanized rubber and corrosion-resistant materials are obtained by mechanical mixing of kerogen shale (Kerogen-70¹) with commercial products of petroleum refining and shale chemistry such as oil tar, bitumens BND 90/130 and G, softener A-10, residue of atmospheric distillation of crude shale oil boiling above 350°C, and also by thermal treatment of the indicated mixtures at 360-400°C.     

Heterogeneous platinum catalytic aerobic oxidation of cyclopentane-1,2-diols to cyclopentane-1,2-diones

A method for the aerobic oxidation of cyclopentane-1,2-diols to the corresponding diketones over a commercial heterogeneous Pt/C catalyst is described. Unsubstituted and 3- or 4-substituted cyclopentane-1,2-diols are oxidized to 1,2-dicarbonyl compounds in good yields under the reported optimized reaction conditions (atmospheric air, 1 mol % of catalyst, 1 equiv of LiOH, aqueous solvents and 60 °C temperature). The method is applicable for producing cyclopentane-1,2-diketones in a scalable manner.     

Gamma irradiation induced enhancement of phenylalanine ammonia-lyase (PAL) and antioxidant activity in peach (Prunus persica Bausch,Cv. Elberta)

Effect of medium dose gamma irradiation on PAL and antioxidant activity of peach fruit was investigated. Peach fruit after harvest at commercial maturity was irradiated in the dose range 1.0–2.0 kGy, stored under refrigerated conditions (3±1 °C, RH 80%) and evaluated at intervals of 7 days. The antioxidant activity as determined by DPPH and FRAP methods revealed significant (p≤0.05) increase particularly in the dose range 1.6–2.0 kGy. During storage, maximum increase in both PAL and antioxidant activity was observed after 21 days. Positive correlation (r=0.75) existed between antioxidant activity and total phenols. EC₅₀ values as obtained from DPPH and FRAP experiments were significantly (p≤0.05) lower in irradiated fruits compared to control.     

Post-deposition ageing reactions of plasma derived polyterpenol thin films

Owing to the structural flexibility, uncomplicated processing and manufacturing capabilities, plasma polymers are the subject of active academic as well as industrial research. Polymer thin films prepared from non-synthetic monomers combine desirable optical and physical properties with biocompatibility and environmental sustainability. However, the ultimate expediency and implementation of such materials will dependent on the stability of these properties under varied environmental conditions. Polyterpenol thin films were manufactured at different deposition powers. Under ambient conditions, the bulk of ageing occurred within first 150 h after deposition and was attributed to oxidation and volumetric relaxation. Films observed for further 12 months showed no significant changes in thickness or refractive index. Thermal degradation behaviour indicated thermal stability increased for the films manufactured at higher RF powers. Annealing the films to 405 °C resulted in full degradation, with retention between 0.29 and 0.99%, indicating films' potential as sacrificial material.     

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.     

Accelerated weathering of polyaramid and polybenzimidazole firefighter protective clothing fabrics

Exposure to simulated ultraviolet sunlight at 50 °C and 50% relative humidity caused a significant deterioration in the mechanical performance of polyaramid and polyaramid/polybenzimidazole based outer shell fabrics used in firefighter jacket and pants. After 13 days of exposure to these conditions the tear resistance and tensile strength of both fabrics decreased by more than 40%. The polybenzimidazole containing fabric was less impacted by these conditions as it maintained approximately 20% more of its mechanical properties. These conditions also significantly degraded a water repellant coating on the fabric, which is critical to the water absorption performance of the outer shell fabrics. However, these conditions had little impact on the ultraviolet light protection of the outer shell as both fabrics still blocked 94% of ultraviolet light after 13 days of exposure. Confocal microscopy showed these conditions caused significant surface decomposition of and the switch from ductile to brittle failure of the polyaramid fibers. Cleavage of the amide linkages and the formation of oxidation species (as observed by Infrared spectroscopy) suggested these conditions caused photo-oxidation of the polyaramid fibers. There was little evidence of polybenzimidazole fiber degradation.     

Accelerated ageing to study the degradation of cellulose nitrate museum artefacts

Cellulose nitrate is susceptible to hydrolysis as well as loss of plasticiser when left in a humid atmosphere. A comparison of the ageing behaviour of cellulose nitrate samples prepared from cotton linters was used to simulate the artefacts studied in a previous study. Certain artefacts were also subjected to accelerated ageing at 12%, 55% and 75% relative humidity at 70 °C. The rate of degradation was observed to vary with the RH, indicating the connection between the absorption of moisture and the hydrolysis process. The effect of varying the sulphate level on the rate of hydrolysis was studied using concentrations similar to those detected in artefacts. The study was carried out using <0.1 mg g⁻¹, 2 mg g⁻¹ and 5 mg g⁻¹ of sulphate and distinct differences were observed which is consistent with the conclusions drawn for a study of a number of artefacts. The observed rates of degradation are consistent with previous studies on cellulose nitrate. The degradation was studied using a combination of infrared spectroscopy, ion chromatography and gel permeation chromatography. The analysis was complemented by a study of the weight changes which occur during ageing.     

Compression set of thermoplastic polyurethane under different thermal-mechanical-moisture conditions

Elastomeric materials are used in the manufacture of structural dampeners due to their high damping coefficient and ease of production. However, elastomers, and in particular thermoplastic polyurethanes (TPU), are susceptible to degradation from environmental conditions. Samples of TPU were investigated, in terms of their mechanical properties, under the influence of four factors; time (up to 10 weeks thermal exposure), temperature (20-80 °C), strain (10% and 25%) and moisture (pre-soak/testing in water). Compression, hardness and compression set tests were used to determine the major contributors to the degradation process. It was found that pure thermal loading at 70 °C for 10 weeks did not result in any changes in material properties, other than an initial drying phase causing an increase in hardness of 2-3 Shore D. The compression set values were found to be heavily dependent on the test temperature, with a significant increase in compression set being seen between 70 and 80 °C. The presence of water (introduced by testing in water) acted as a plasticiser and resulted in a larger amount of compression set, than testing in the absence of water. The level of compression set was shown to be insensitive to the strain level. Overall, it was found, for the conditions tested, that temperature was the major driving force behind the compression set of the TPU material.     

A study on the degradation of polylactic acid in the presence of phosphonium ionic liquids

In this overview study, two ionic liquids (IL) with different anions (decanoate, tetrafluoroborate) but with the same phosphonium-based cation that showed promising plasticizing/lubricating behavior in polylactic acid (PLA) were screened for their effects on the polymer degradation under thermomechanical, thermo-oxidative (at 160 °C), hydrolytic (100% humidity, 60 °C), conditions, and during soil immersion. Depending on the particular medium and conditions used, degradation was followed by changes in molecular weight, melt viscosity, sample weight and appearance, morphology, crystallinity, acid number, and pH. The effects of the IL containing a decanoate anion were more pronounced on lubrication and also on degradation as evidenced by reduced melt viscosities and accelerated thermomechanical, isothermal, hydrolytic, and soil degradation. The IL containing the tetrafluoroborate anion showed higher thermal stability compared with the IL containing decanoate anion as also confirmed from thermal degradation rate constants which were calculated from random chain scission statistics. Accelerated hydrolytic degradation was observed in PLA containing the tetrafluoroborate based IL but to a lesser extent than the decanoate based IL. The catalytic role of the decanoate anion in hydrolytic degradation was confirmed through experiments with model compounds. X-ray diffraction (XRD) data on the materials exposed to soil degradation provided evidence that the initially amorphous polymer attained a certain degree of crystallinity as a result of the significant MW reduction.     

Thermal oxidative degradation of additive-free polypropylene pellets investigated by multichannel Fourier-transform chemiluminescence spectroscopy

Thermal oxidative degradation of additive-free polypropylene pellets heated isothermally in dry air at 150 and 180 °C (below and above the melting point of 163 °C) was investigated by multichannel Fourier-transform chemiluminescence spectroscopy. The initial peak wavelength of chemiluminescence emission at 490 nm remained constant during the early stages of thermal degradation, but new emissions developed with time in the red spectral region over an extended oxidation period. The time-dependent luminescence spectra were deconvoluted into three emission bands by least-squares fitting using Gaussian curves. We concluded that at least three groups of luminescent species (luminophores), having different conjugation lengths, were generated by thermal oxidation over extended periods and show luminescence around 490, 660, and 740 nm.