Accelerated aging and aging mechanism of acrylic sealant

One-component acrylic latex sealants with different moduli were subjected to accelerated aging tests, heat aging at different temperatures, xenon arc and UV exposure. The effects of various aging factors on sealant properties were studied by reviewing the appearance and mechanical properties. The aging mechanism was evaluated by thermal analysis, FTIR and solvent swell. The results show that a good relation exists among mechanical properties after aging; mild accelerated aging conditions and short aging times are enough to lead to obvious degradation of acrylic sealants with low modulus. Fillers with ingredients within high modulus may resist UV radiation effectively and enhance their durability but the quantity must be proper. The degree of chain breaking caused by UV is larger than that by heat aging. The study also shows that there are the same or similar aging mechanisms for different aging conditions and that the cure reaction by water volatilisation dominates the initial period of aging while degradation will be induced when aging time is longer and it can result in changes of molecular structure.     

Rubber aging in tires. Part 2: Accelerated oven aging tests

The kinetics of oxidation of wedge and skim rubber from tires aged at different oven temperatures with various fill gases have been measured for 5 different tires and compared to field results. We demonstrate that oven aging tires mounted on wheels and inflated to the maximum sidewall pressure closely reproduce the aging behavior measured for tires collected after customer use. Temperatures as high as 70 °C can be used to accelerate aging. Use of 50/50 blend of N₂/O₂ as a fill gas accelerates the oxidative aging by 30-40% relative to air. By combining elevated temperatures with oxygen enriched fill gas, it is possible to oven age tires to an age equivalent to 6 years in Phoenix in 8 weeks or less.     

Mechanical strengthening of TMOS-based alcogels by aging in silane solutions

It has been shown that aging of tetramethoxysilane (TMOS)-based alcogels in solutions of tetraethoxysilane (TEOS)/methanol (MeOH) provides new monomers to the alcogel and favorably increases the strength and stiffness of the alcogel and hence reduces the shrinkage during the subsequent drying. Load relaxation experiments have been performed to determine the shear modulus (G), Poisson's ratio (), and the permeability of wet gel rods as a function of aging time in the TEOS/MeOH solution. The modulus of rupture (MOR) and G have also been obtained from 3-point bending tests. Aging the gels in 70 vol% TEOS/MeOH causes an increase in G from 0.48 MPa to 1.8 MPa and 7.4 MPa after aging for 24 hours and 144 hours, respectively.It is shown that the drying stress is actually increased by the aging treatment, but the increase in strength of the gel is even greater; hence, strengthening of the alcogels dramatically reduces the probability of cracking during drying. Unaged gels with higher TMOS concentrations corresponding to the silica content of gels aged in TEOS solution, however, showed large shrinkage and severe cracking.     

Physical aging and structural relaxation in polymer nanocomposites

Molecular dynamics simulations of a coarse‐grained polymer nanocomposite model are used to study the impact of nanoparticles on physical aging. The physical aging rate of the composites is obtained from measurements of the per‐particle pair energy, while the (segmental) mean‐squared displacement and creep compliance are used to probe simultaneously the dependence of structural relaxation times on waiting time elapsed since the glass was formed. Although bulk regions behave similarly to a neat polymer glass, interfacial regions exhibit a reduction in the physical aging rate for attractive polymer–nanoparticle interactions. Repulsive interactions lead instead to a significant increase. This change in physical aging rate is found to be proportional to the local mobility of the polymer atoms. By contrast, aging exponents obtained from time‐waiting time superposition of mean‐squared displacements or compliance curves are much less affected by the nanoinclusions. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1789–1798, 2009     

Modeling mechanical aging shift factors in glassy polymers during nonisothermal physical aging. I. Experiments and KAHR‐ate model prediction

This article presents experimental results and model predictions of the mechanical response of polymers during nonisothermal physical aging. The nonisothermal temperature history leads to a complex evolution in the aging behavior of the material. To characterize this response, sequential creep tests of polyether‐ether‐ketone (PEEK) and polyphenylene sulfide (PPS) films are performed at various aging times using a dynamic mechanical analyzer. The resulting strain histories are analyzed to determine discrete aging shift factors (a_te) for each of the creep tests. The nonisothermal aging response is then predicted using the KAHR‐a_te model, which combines the KAHR model of volume recovery with a suitable linear relationship between aging shift factors and specific volume. The KAHR‐a_te model can be utilized to both predict aging response or to determine necessary model parameters from a set of aging shift factor data. For the PEEK and PPS materials considered in the current study, predictions of mechanical response are demonstrated to be in good agreement with the experimental results for several thermal histories. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 340–352, 2009     

Morphological study of aging phenomena in XLPE by TEM technique

The long-term dielectric performance of underground power cable XLPE (cross-linked polyethylene) insulation suffers from poorly understood aging phenomena. A study of the morphological modifications of XLPE due to electrical aging may provide insight for a better understanding of aging mechanisms. The TEM technique has been used to study the XLPE morphology of unaged, laboratory-aged, and field-aged cable samples. A suitable image contrast enhancing and a stabilization of radiation damage were both successfully achieved by staining the sections with ruthenium tetroxide (RuO₄), as individual XLPE lamellae were neatly and reproducibly resolved. Image analysis was used to help in the determination of any aging-induced morphological changes. XLPE samples from an unaged cable (D1) has been exposed to a low-energy electron beam, in an attempt to simulate certain conditions thought to occur in electric field-stressed dielectrics. © 1996 John Wiley & Sons, Inc.     

Growth and aging phenomena in silica gels

Growth and aging of silica aggregates are influenced both by temperature and by catalyzing fluorine ions as shown by SAXS and BET. It was found that both fluorine and increased temperature slightly increased the fractal dimension Df during aging, but the fluorine catalyzed system showed a lower BET surface area.To understand the effect of fluorine and increased temperature on the aggregates, 2D aggregations and SAXS simulations were carried out using two new programs GRASP and DALAI. In agreement with experiments it was found that binary RLCCA aggregates have a slightly higher Df value compared to DLCCA aggregates and that branch-flexibility during aging increases Df even further.     

Alterations in the peptidergic regulation of energy balance in the course of aging

With advancing age most aspects of the peptidergic regulation of energy balance are altered. The alteration involves both the peripheral peptides derived from the adipose tissue or the gastrointestinal tract and the peptides of the central nervous system (brainstem and hypothalamus). In general, the expression of orexigenic peptides and their receptors decreases with age, while that of the anorexic ones rather increases, but not simultaneously and not in a linear fashion. Apart from such quantitative changes, the efficacy of the related peptides may also change with age. These changes are not necessarily linear, either: instead of continuous decline or increase of its effects, the effects of a peptide may become less pronounced in some phases of aging and much enhanced in other ones. Comparing the individual peptides, the phasic alterations in their anabolic or catabolic roles in the regulation of energy balance may exhibit dissimilar time-patterns. In addition, within the overall anabolic or catabolic effects, the feeding and metabolic actions of certain peptides may not change simultaneously. Altogether, as compared with young adults, in middle-aged animals or individuals the anabolic processes (increased food intake with decreased energy expenditure) seem to prevail, which processes may contribute to the explanation of age-related obesity, while in the old ones the catabolic processes (anorexia with enhanced metabolic rate) dominate, which possibly explain the aging anorexia, frailty and sarcopenia.     

Comparative G-Protein-Coupled Estrogen Receptor (GPER) Systems in Diabetic and Cancer Conditions: A Review

For many patients, diabetes Mellitus and Malignancy are frequently encountered comorbidities. Diabetes affects approximately 10.5% of the global population, while malignancy accounts for 29.4 million cases each year. These troubling statistics indicate that current treatment approaches for these diseases are insufficient. Alternative therapeutic strategies that consider unique signaling pathways in diabetic and malignancy patients could provide improved therapeutic outcomes. The G-protein-coupled estrogen receptor (GPER) is receiving attention for its role in disease pathogenesis and treatment outcomes. This review aims to critically examine GPER’ s comparative role in diabetes mellitus and malignancy, identify research gaps that need to be filled, and highlight GPER’s potential as a therapeutic target for diabetes and malignancy management. There is a scarcity of data on GPER expression patterns in diabetic models; however, for diabetes mellitus, altered expression of transport and signaling proteins has been linked to GPER signaling. In contrast, GPER expression in various malignancy types appears to be complex and debatable at the moment. Current data show inconclusive patterns of GPER expression in various malignancies, with some indicating upregulation and others demonstrating downregulation. Further research should be conducted to investigate GPER expression patterns and their relationship with signaling pathways in diabetes mellitus and various malignancies. We conclude that GPER has therapeutic potential for chronic diseases such as diabetes mellitus and malignancy.     

Dilatometric studies of physical aging of polyetherimide

Physical aging of polyetherimide (PEI) was studied using a bellows dilatometer based on Zoller's design. A linear variable differential transformer (LVDT) is used to measure the displacement of the bellows. The voltage output of the LVDT is interfaced to a computer for automated data collection. Isothermal aging experiments were carried out at temperatures near the glass temperature (206–209 C) using a constant temperature oil bath maintained at the desired aging temperature. The time required to reach equilibrium and the reduced curve produced by aging time-temperature superposition are given. The results compare well with data obtained by capillary dilatometry for the same material.     

Mechano‐reversible physical aging of elastic oriented syndiotactic polypropylene

Syndiotactic polypropylene (sPP), obtained at 0 °C in the trans‐planar mesophase, was drawn at room temperature up to λ = 6, and left at room temperature for 1 year with fixed or relaxed ends. Data analysis allowed the clarification of the structure of the crystalline phases and their transformations during the aging. In both oriented samples similar structural changes were observed, although they were due to different aging mechanisms. The physical aging led to the crystallization in both samples of an oriented helical form, due to a partial transformation of the mesophase and of the amorphous phase. In the oriented sample aged with fixed ends, a small fraction of the crystalline trans‐planar form III became stable even by releasing the tension after 1 year. This last sample did not undergo the large shrinkage, always observed by unloaded sPP after drawing, and therefore was no more elastic. Also, the sample aged with free ends for 1 year showed a reduced elasticity in terms of both dissipated energy and permanent set. However, after a new deformation up to λ = 6, the fiber recovered its previous elasticity. Indeed a mechano‐reversibility was apparent for the oriented elastic sample of sPP aged at room temperature with free ends, leading to a renewed elasticity. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 599–606, 2008     

Physical aging of syndiotactic polypropylene

Syndiotactic polypropylene (sPP) was quenched from the melt in an ice‐water bath, and changes in the structural organization, during the aging time, were followed by X‐rays, differential scanning calorimetry, and transport properties of dichloromethane at low activities. After 1 month, an increase of crystallinity from 19 to 26% was observed. In addition, the results of sorption and diffusion indicated a consistent increase of an intermediate phase not crystalline, yet impermeable to the vapors. The study of the mechanical properties showed that there is a remarkable increase of all the mechanical parameters with the aging time, and this effect was associated to the increase of the intermediate phase. The elastic modulus increased three times in the first hours of aging. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 173–180, 1999     

Prediction of paper permanence by accelerated aging I. Kinetic analysis of the aging process

The validity of accelerated aging tests to predict and rank papers on their permanence has been under question, preventing the development of performance-based standards for permanent paper. We conducted a general kinetic analysis to investigate the aging process of paper. A general kinetic model is proposed to describe the depolymerization of cellulose. Experimentally it was shown that in the case of aging, cellulose degradation follows classic first-order kinetics as a special case of our general kinetic model. The Arrhenius equation was critically re-examined for the case of a multiple reaction system. It was shown analytically that the Arrhenius equation is still applicable when certain conditions are met. This was convincingly supported by experimental results. We also analysed the dependence of the degradation rate on the moisture content and hydrogen ion concentration. By conducting systematic experiments on these two factors, a general and quantitative relationship was established to explain the contribution of each factor and their interactions. Finally, based on this kinetic analysis, the effects of storage conditions on the life expectancy of paper were estimated.     

Depletion mechanism of antioxidants in MDPE-clay nanocomposites under thermal aging

The depletion behavior of two types of hindered phenolic antioxidants (AO), Irganox^® 1010 (I-1010) and Irganox^®1076 (I-1076), in medium density polyethylene (MDPE)/nanoclay composite was evaluated by incubating samples in a forced air oven at 85 °C. The presence of 4 wt% nanoclay accelerated the depletion of both types of AO, particularly at the surface region of the sample. However, the depletion mechanism in the interior of sample was governed by the AO molecular structure. For samples containing the bulky Irganox®1010, OIT decreased exponentially with aging time consistent with a first order reaction. In contrast, an increase of OIT was detected in first 60 days of heat aging for sample containing I-1076 and afterward the OIT decreased slowly with aging time. The hypothesis for the initial increase of OIT is that the relatively small and linear structure of I-1076 may enable it to be trapped inside the nanoclay galleries and then subsequently released into the polymer matrix during heat aging.     

Rubber aging in tires. Part 1: Field results

Oxidative aging of skim and wedge rubber inside the tire results in a loss of peel strength and tensile properties of these rubber materials, which has been found to increase the likelihood of tread separations in certain tires. In order to develop an accelerated laboratory tire aging test, we have carried out extensive field and laboratory studies of rubber property change in tires. This paper describes the analysis of rubber oxidation in a specific set of tires collected from the field. In particular, we determine the rate of property loss under worst-case environmental conditions and analyze the implications of variability in aging results. The analysis is used in a companion paper to develop acceleration factors for different laboratory tests.     

Hygrothermal aging of Nafion

The membrane durability is a critical issue for the development of Proton Exchange Membrane Fuel Cells (PEMFC). Since PEMFC in situ tests were not conclusive to determine Nafion^® membrane degradation mechanism, ex situ aging tests were performed on Nafion^® 112 in practical fuel cell usage conditions. The polymer chemical structure evolution was investigated by infrared spectroscopy (IR) and Nuclear Magnetic Resonance (NMR) while its hydrophilicity, directly linked to its protonic conductivity, is established through sorption isotherms by Dynamical Vapour Sorption (DVS). Durability studies over a period of 400 days revealed membrane degradation through a modification of sulfonic acid end-groups. Formation of sulfonic anhydride (from the condensation of sulfonic acids) was strongly demonstrated by IR spectroscopy and, indirectly, by NMR. The substitution of ionic end-groups by less hydrophilic anhydrides leads to a significant decrease of water uptake and thus of its hydrophilicity. Surprisingly, kinetic study reveals that the hygrometric level accelerates this condensation reaction.     

Prediction of paper permanence by accelerated aging II. Comparison of the predictions with natural aging results

Accelerated aging tests are credible and useful to predict paper permanence only if such tests can be shown to correlate with natural aging. In the first part of this study, a kinetic model was developed based on the accelerated aging results. In this report, we have shown that this kinetic model can indeed predict the natural aging results of lignin-free sheets with a statistical confidence. This is the first quantitative comparison of accelerated aging with natural aging.     

SANS study of the aging of xTiO2-SiO2 gels

Silica-titania gels, prepared in acidic conditions, with contents of TiO₂ up to 6 mol%, have been studied by small angle neutron scattering (SANS) as wet gel with increasing aging times. In all samples, small primary particles have been found with a diameter of about 3 nm. At the gel point, cylindrical clusters, ca. 10 nm long, were observed as well. These grow with aging time, becoming branched. The cluster-cluster diffusion limited aggregation model is found to be consistent with the SANS results. The growth process is independent of the TiO₂ content. It is mainly influenced by the ratio of the aging time to the gelation time.     

Cyclooxygenase-2 inhibitors modulate skin aging in a catalytic activity-independent manner

It has been proposed that the pro-inflammatory catalytic activity of cyclooxygenase-2 (COX-2) plays a key role in the aging process. However, it remains unclear whether the COX-2 activity is a causal factor for aging and whether COX-2 inhibitors could prevent aging. We here examined the effect of COX-2 inhibitors on aging in the intrinsic skin aging model of hairless mice. We observed that among two selective COX-2 inhibitors and one non-selective COX inhibitor studied, only NS-398 inhibited skin aging, while celecoxib and aspirin accelerated skin aging. In addition, NS-398 reduced the expression of p53 and p16, whereas celecoxib and aspirin enhanced their expression. We also found that the aging-modulating effect of the inhibitors is closely associated with the expression of type I procollagen and caveolin-1. These results suggest that pro-inflammatory catalytic activity of COX-2 is not a causal factor for aging at least in skin and that COX-2 inhibitors might modulate skin aging by regulating the expression of type I procollagen and caveolin-1.