Influence of temperature and relative humidity on aging of atmospheric plasma jet treatment effect on ultrahigh-modulus polyethylene fibers

The aging effects of atmospheric pressure plasma treated fiber surfaces are important for storage and processing of the fibers. One of the high-performance fibers, ultrahigh modulus polyethylene (UHMPE) fiber, was chosen as a model system to investigate the aging process of atmospheric pressure plasma jet (APPJ) treated fibers surfaces 0, 7, 15 and 30 days after initial plasma treatment. The fiber was first plasma-treated and then stored at temperatures varying from ?80 to 80°C on the same relative humidity (RH, 0%) and on RH of 0%, 65% and 100% at the same temperature of 20°C. Immediately after the plasma treatment, scanning electron microscope (SEM) showed the roughened fiber surface. X-ray photoelectron spectroscopy analysis showed changed surface chemical compositions. Contact-angle measurement showed increased surface wettability and microbond test showed an increase in IFSS. With increasing relative humidity or decreasing temperature, the IFSS value decreased and the contact angle increased more slowly. However, after 30 days, the IFSS values and contact angles reached a similar level for all groups. Moisture showed no effect on the single fiber tensile strengths during aging. The reasons for the observed aging behavior could be that decreasing temperature or increasing relative humidity hindered the surface rearrangement of polymer chains after plasma treatment.     

Influence of hygrothermal aging on dimensional stability of thin injection‐molded short glass fiber reinforced PA6 materials

The hygrothermal aging of short glass fiber reinforced polyamide 6 materials (PA6/GF) is a major problem for thin‐walled components used in the automotive sector. In this work, the thickness and glass fiber content of PA6/GF materials were varied and exposed to hygrothermal aging. The temperature and relative humidity were chosen to range from ?40 to 85°C and 10% RH to 85% RH respectively, according to automotive requirements for components in the passenger compartment. For the absorption of moisture, the diffusion behavior could not be generally described by Fick's law. However, the results indicate that the diffusion behavior is dependent on the relative humidity and thickness of the PA6/GF material. The morphology of the test specimen, which is influenced by injection molding, was also found to affect the diffusion behavior. The states of equilibrium for moisture absorption are strongly dependent on the relative humidity during hygrothermal aging and less dependent on the temperature. The maximum absorbed humidity was found at a temperature of 65°C and 85% RH, which was higher than at 85°C and 85% RH because of reduced contrary aging processes, such as postcrystallization. In certain climatic conditions and test specimen thicknesses, there was a characteristic overshoot in the mass change. This behavior could be attributed to a different degree of crystallization and lower glass fiber content. Both moisture absorption and an overshoot of the mass affected the dimensional stability of the test specimens. This effect on dimensional stability could be correlated with the glass fiber orientation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42245.     

Hygrothermal aging studies of short carbon fiber reinforced nylon 6.6

A study of hygrothermal aging, in terms of the kinetics of moisture absorption by nylon 6.6 and its carbon fiber-reinforced composites, has been undertaken. The diffusion of water into the various materials was investigated at 100% relative humidity, by immersion of specimens in water at temperatures of between 25 and 100°C. A mathematical treatment used in analyzing the data was that of a single free phase model of diffusion, which assumed Fickian diffusion and utilized Fick's second law of diffusion. Good agreement was observed between the experimental and theoretical values. The equilibrium moisture content and the apparent diffusion coefficient of the various composites were evaluated. Hygrothermal aging has reduced the tensile properties of both unreinforced and reinforced nylon 6.6, albeit a better recoverability of the properties was achieved by the former after drying. This behavior is explained in terms of the combined action of moisture-induced plasticization and interfacial degradation. © 1994 John Wiley & Sons, Inc.     

Effect of moisture absorption on the tensile properties of short glass fiber reinforced poly(butylene terephthalate)

Injection molded short glass fiber reinforced poly(butylene terephthalate) was subjected to hygrothermal aging at two different relative humidities—81.2% and 100% RH. A single free phase model of diffusion has been used to analyze the data obtained from the kinetics of moisture absorption study. The diffusion coefficient and the equilibrium moisture content were found to be dependent on the volume fraction of fibers and relative humidity. Incorporation of short glass fibers into a poly(butylene terephthalate) matrix has led to a significant improvement in the retention and recoverability of the tensile properties. Examination of fracture surfaces using a scanning electron microscope (SEM) has revealed some evidence for the hydrolysis of the polymer matrix. The hydrolysis resulted in the formation of microvoids, the absence of a plastic deformation process, and degradation at the fiber-matrix interface.     

Indentation deformation and cracking behavior of hydrated aluminoborate glasses

Aluminoborate glasses have recently been found to feature high resistance to crack initiation during indentation due to a highly flexible network structure. In cesium aluminoborate glasses, it has been found that the use of a simple post-treatment, namely aging in a humid atmosphere, can further improve this resistance. To better understand the mechanical properties of this glass family upon humid aging, we here study the effect of aging conditions on the structure and mechanical properties of Li,K,Cs-aluminoborate glasses. As expected, we find that higher humidity and longer aging time cause more pronounced permeation of atmospheric water into the glasses. Due to their denser structure and stronger modifier-oxygen bonds, the humid aging has a relatively smaller effect on the mechanical properties of Li- and K-containing glasses relative to Cs-containing glasses, with the latter achieving an ultrahigh crack resistance. We find that the humid aging leads to the formation of a hydration layer in the Cs-aluminoborate glass surface, with a thickness of around 26 μm upon aging at 23 °C with 40% relative humidity for 7 days. Moreover, a remarkable indentation behavior, that is, the observation of μm-sized shear bands inside the imprint of the Cs-glass upon aging at 60% relative humidity is reported. Taken as a whole, the work provides guidelines for how to control the humid aging rate as a function of relative humidity and temperature to form a hydration layer and thus achieve improved crack resistance in such glasses.     

Retarding hydrolytic degradation of polylactic acid: Effect of induced crystallinity and graphene addition

The combination of elevated temperature and humidity leads to rapid degradation of polylactic acid (PLA) because of hydrolysis. Consequently, PLA, which is a bio‐derived and biodegradable polymer, is not currently used for durable applications since properties cannot always be maintained over time. In this work, the ability of polymer crystals to reduce the rate of degradation during accelerated aging tests was studied. Also examined was the influence of addition of 2 wt % graphene nanoplatelets to act as moisture transport barriers in the polymer. PLA samples were immersed in aqueous media of different pH or exposed to 100% relative humidity at 50 °C for different lengths of time to study the hydrolytic degradation behavior. In addition to monitoring the loss in mass of the samples, the values of crystallinity, melt viscosity, and mechanical properties, among others, were measured as functions of aging time using techniques such as DSC and rheometry. It was found that both crystallization and graphene addition are able to slow down the rate of degradation at short times, but significant degradation of PLA still occurs at long times. This is because PLA crystallites and graphene nanoplatelets can only reduce, but not eliminate, moisture diffusion into the polymer sample. Between the use of nanoplatelets and crystals, though, the former approach may be the better choice since enhanced crystallization tends to make PLA brittle. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44166.     

Influence of alkaline delignification on moisture uptake behavior and bonding enthalpies of hemp

Raw hemp fibers are alkaline delignified to evaluate the effect of lignin on the moisture uptake behavior of hemp. Samples are subjected to moisture absorption/desorption isotherms from 5% to 95% relative humidity at three different temperatures (15, 25, and 35°C). The sorption ratio between delignified and untreated hemp is lowered by 25%. Hysteresis makes evident the greater influence of lignin on moisture binding at low relative humidity while at high relative humidity, cellulose plays the most important role. The GAB, Hailwood-Horrobin and Dent models are fitted to absorption/desorption isotherms. Delignification and temperature decrease the size of the monolayer. The energy constants of the monolayer differ according to the model used. Lignin causes great differences between absorption and desorption. The greater the temperature is, the lower the monolayer energy constant. The energy constants of the multilayer are identical for the three models, showing a greater range of variation in delignified samples than in the untreated samples. Based on primary and secondary energy constants, bonding enthalpies are determined. Estimations in desorption show higher correlation coefficients with the energy constants, which enables to conclude that measures in desorption better explain the effect of delignification on the water uptake behavior of hemp.     

Lifetime Prediction and Aging Mechanism of Glass Fiber Reinforced Acrylate-Styrene-Acrylonitrile/Polycarbonate Composite under Hygrothermal Conditions

The development of fifth-generation technology has resulted in increased demand for materials with low dielectric losses and superior thermal and mechanical properties. However, ensuring the widespread use of such materials by investigating their aging mechanisms and operating lifetimes remains challenging. In this study, a glass-fiber (GF)-reinforced acrylate-styrene-acrylonitrile/polycarbonate (ASA/GF/PC) composite is designed and comprehensively investigated its aging behavior, mechanism, and service lifetime under long-term hygrothermal conditions. Based on the general Peck model, the composite maintains a high level of quality for over 10 years, including under harsh conditions of 40 °C and 80% relative humidity. The aging mechanism is primarily ascribed to cracking between the GF fibers and matrix, the breaking of chemical bonds, the generation of new cross-linked domains, and physical aging. These findings provide valuable insights into the long-term utilization of ASA/GF/PC composites in harsh environments.     

The Drying Kinetics and Equilibrium Moisture Content of MDF Fibers

An experimental device was constructed to study the drying kinetics of wood fibers under controlled conditions. The device consisted of a drying chamber in which a net basket filled with the fiber material was connected to a load cell. The drying medium was then forced through the basket at controlled levels of humidity and temperature.

Experiments were performed with spruce fibers and the drying medium at varying temperature (50–170°C) and relative humidity (1–86%). In general, the drying rate increased with increasing temperature and decreasing relative humidity. A constant drying rate period was observed in all cases. The critical moisture content was approximately 1.25. The characteristic drying curve has a slight downward concave shape. The equilibrium moisture contents obtained at ambient temperature agree well with data in the literature.     

Effect of layering pattern on the water absorption behavior of banana glass hybrid composites

Hybridization of Banana fibers with glass fibers has been found to reduce the water absorption behavior of the composites in an earlier work by us. Banana fibers were hybridized with glass and different layering patterns were followed in the preparation of the composites. The effect of the various layering patterns on the water absorption of the composites was studied. It was found that water diffusion occurs in the composite depending on the layering pattern as well as the temperature. In all the experiments, it has been found that composites with an intimate mixture of glass and banana show the maximum water uptake except for temperature of 90°C. At 90°C the maximum water uptake is found to be for composites where there is one layer of banana and another layer of glass. The water uptake follows the same trend as that in all other temperatures till a time span of 4900 min is reached. The kinetics of diffusion was found to be Fickian in nature. The various thermodynamic parameters like sorption coefficient, diffusion coefficient. Enthalpy change, entropy change, and activation energy of the various composites were calculated. From all the calculations it has been concluded that layering pattern is an important parameter which controls the water absorption of the composites. The layering pattern Cg‐b‐g was found to have the lowest water uptake. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Humidity regulation by stretched PP and PLA films with dispersed CaCl2

Polymer materials that regulate the relative humidity in their environment are relevant for applications in the packaging and building sectors. By integration of salts in polymer structures, such materials are able to absorb and desorb high amounts of water vapor. In this study, films of polylactic acid and polypropylene with dispersed calcium chloride (2 and 4 wt %) were produced and biaxially stretched to induce the formation of cavities. The resulting cavities in these films account up to 10 vol % and are able to contain emerging calcium chloride solution formed by water vapor absorption. These films absorb reversibly up to 15 wt % water vapor at 75% relative humidity at 23 °C. This absorption behavior is described by effective diffusion and effective sorption coefficients. Using a simple model, the effective water vapor diffusion coefficient of these films can be estimated from the permeation coefficient of the polymer and the sorption coefficient of the absorber. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45713.     

Effect of composition and configuration on hindered diffusion of residue fractions through polycarbonate membranes

Diffusion behavior of narrow fractions of three residues and subfractions of a residue through four polycarbonate membranes was investigated using a diaphragm cell at 308 K. The results show that the diffusivities of fractions with similar molecular weight behaved differently. Comparisons among SAR subfractions (saturates, aromatics, and resins) of the same fraction show that saturates has the largest diffusion coefficient, followed by aromatics, and then resins. The diffusion coefficients of fractions fall among that of their subfractions. The regular variation of diffusivity along with the properties for fractions with similar molecular weight is an indication of the difference in their diffusivities resulting from the difference in their compositions and structures. Hindrance factors of the subfractions through membranes with small pores have a similar variation trend as their diffusion coefficients. The hindrance factors of the feedstocks in 50 and 80 nm pore size membranes are mostly larger than 0.90, but the values in 15 nm membranes range from 0.55 to 0.81. The study not only indicates that the diffusional limitations are significant in pores with the typical hydrotreating catalyst size but composition and configuration have an effect on the hindered diffusion of residue molecules. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1369–1377, 2013     

Degradation of a poly(ester urethane) elastomer. IV. Sorption and diffusion of water in PBX 9501 and its components

In preparation for studying the hydrolytic degradation of Estane® 5703 in the plastic‐bonded explosive PBX 9501, the sorption (solubility) and diffusion of water in PBX 9501 and each of its components are studied experimentally and modeled theoretically. Experiments are reported that measure the weight gain or loss due to a change in the relative humidity (RH). For all of the components, the equilibrium amount of water sorbed per gram of sample is linear in the RH at low relative humidities but curves upwards at higher relative humidities. This behavior is modeled with a water cluster model. Diffusion coefficients are determined by modeling the time dependence of the water concentrations assuming Fickian diffusion, and that fits the data for some of the materials. However, all the samples that contain the explosive HMX show much more complicated behavior at high relative humidities, and that is presented and discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Aging of hydrophobized surfaces of ramie fibers induced by atmospheric pressure plasma treatment with ethanol pretreatment

In order to investigate hydrophilic recovery of hydrophobic treatment of cellulose fibers, ramie fibers are ethanol-pretreated followed by atmospheric pressure plasma jet (APPJ) treatment using helium as the treatment gas and age for up to 150?days in 20?°C and 65% relative humidity. Scanning electron microscopy shows the fiber surfaces of the ethanol-pretreated?+?APPJ-treated group of freshly prepared, aged for 30?days, and aged for 150?days are covered with polypropylene matrix after fiber pullout tests. X-ray photoelectron spectroscopy shows that the freshly prepared ethanol-pretreated?+?APPJ-treated group has a 31% reduction in atomic ratio of oxygen to carbon and maintains at a similar level even after 150?days of aging. Water contact angle measurement demonstrates that the wettability of fiber surface of the freshly prepared ethanol-pretreated?+?APPJ-treated group drastically decreases and remains at the same lever after aging. Interfacial shear strength test reveals that the interfacial adhesion between PP matrix and ramie fiber for the freshly prepared ethanol-pretreated?+?APPJ-treated group increases 26% and remains substantially higher than that of the control group over time. These results indicate that the ethanol pretreatment followed by APPJ treatment is a permanent surface treatment with negligible aging for at least five months.     

Bending stress relaxation and recovery of wool,nylon 66, and terylene fibers

The bending stress relaxation and subsequent recovery behavior were determined for merino wool, nylon, and Terylene fibers. The effect of four experimental parameters were investigated, viz., the level of bending strain (0.5–4%), the time of stress relaxation before release (1–1000 min), the relative humidity (0–85%), and the temperature (20°–60°C). For small strains the merino and nylon fibers displayed behavior characteristic of linear viscoelastic materials, while Terylene exhibited a degree of nonrecoverable set. It was possible to construct master recovery curves for fibers held bent for different times before release. These curves can be used as a more convenient means of presenting the results. A relationship was found, for each fiber type, between the percentage stress relaxation and the time taken to recover to a given level of set. This relationship appeared to be independent of the experimental conditions employed. Although the fibers were not linear viscoelastic under all conditions, recovery could be roughly predicted from their stress relaxation behavior at the particular test conditions using the Boltzmann superposition principle.     

Kinetics of property change associated with atmospheric humidity changes in alumina powder granules with PVA binder

A microcompression testing machine was used on single Al₂O₃ powder granules to study their stress–strain behavior as a function of relative humidity. The test granules were prepared by spray-drying slurries containing 3 mass% poly(vinyl alcohol) and Al₂O₃ powder. The stress–strain curves and granule strength were determined at regular time intervals, after step changes in atmospheric humidity. When stress was applied, deformation increased rapidly with the stress. This stress corresponded to the fracture strength of the granule. In a dry atmosphere, the granules deformed linearly with increasing stress, and a rapid change in strain was noted above a certain critical value of increased stress. In a wet atmosphere, the granules deformed continuously at lower stress. The mechanical properties of the granules changed rapidly with time when the atmospheric relative humidity was changed. Within 3 min of exposure to a new atmosphere, the mechanical properties of the granules reached equilibrium values. The change in properties with humidity was controlled by moisture diffusion in the granules and was reversible.     

About the influence of temperature and environmental relative humidity on the longitudinal and transverse mechanical properties of elementary alfa fibers

Composites reinforced with plant-based fibers present a high potential for valorization in new industrial applications due to their good specific mechanical characteristics, renewability, and recyclability. In order to accelerate their wide industry adoption, it is critical to assess their behavior and durability in heat and humid environments. This article aims at investigating the effects of temperature and relative humidity (RH) on the longitudinal and transverse mechanical properties of the lignocellulosic fibers extracted from alfa plant (Stipa tenacissima L). For this purpose, tensile and nanoindentation tests were performed on elementary alfa fibers subjected to a thermal cycle of 200°C. The fibers were held at various periods of 15, 30, 60 and 120 min. The test results showed that the longitudinal and transverse Young's moduli are moderately affected by short thermal cycles having duration of 15–30 min. However, for longer thermal cycle (i.e., 2 hr), a degradation of 21% for the transverse modulus was recorded. This degradation doubled for the longitudinal modulus (43 vs. 21%). A similar trend was observed for the breaking strength. This study also showed that the RH strongly affects the mechanical performances of alfa fibers.     

Study of the Effects of Aging under Humidity Control on the Thermal Decomposition of NC/NG/BTTN/RDX Propellants

The effects of aging under controlled humidity on the thermal decomposition of two nitrate ester/RDX propellants are examined. Propellant samples are artificially aged at 60 °C and 70 °C at various levels of relative humidity. Differential scanning calorimetry is used to study the post aged samples to determine the activation energy of the peak exotherm and peak temperature of these events. For each humidity level, differential scanning calorimetry is conducted on an aged and an unaged sample that is conditioned to the humidity of the environment to distinguish between aging effects and residual moisture effects. Nitrate ester stabilizer depletions rates are presented as a function of aging and humidity. The results of the study show that moisture level during aging irreversibly alters the activation energy of the primary heat release events and significantly influences the stabilizer depletion rates.     

Moisture uptake and resulting mechanical response of bio‐based composites. II. composites

The durability of entirely bio‐based composites with respect to the exposure to elevated humidity was evaluated. Different combinations of bio‐based resins (Tribest, EpoBioX, Envirez) and cellulosic fibers (flax and regenerated cellulose fiber rovings and fabrics) were used to manufacture unidirectional and cross‐ply composite laminates. Water absorption experiments were performed at various humidity levels (41%, 70%, and 98%) to measure apparent diffusion coefficient and moisture content at saturation. Effect of chemical treatment (alkali and silane) of fibers as protection against moisture was also studied. However, fiber treatment did not show any significant improvement and in some cases the performance of the composites with treated fibers was lower than those with untreated reinforcement. The comparison of results for neat resins and composites showed that moisture uptake in the studied composites is primarily due to cellulosic reinforcement. Tensile properties of composites as received (RH = 24%) and conditioned (RH = 41%, 70%, and 98%) were measured in order to estimate the influence of humidity on behavior of these materials. Results were compared with data for glass fiber reinforced composite, as a reference material. Previous results from study of unreinforced polymers showed that resins were resistant to moisture uptake. Knowing that moisture sorption is primarily dominated by natural fibers, the results showed that some of the composites with bio‐based resins performed very well and have comparable properties with composites of synthetic epoxy, even at elevated humidity. POLYM. COMPOS., 36:1510–1519, 2015. © 2014 Society of Plastics Engineers     

Influence of Composition,Humidity, and Temperature on Chemical Aging in Epoxies: A Local Study of the Interphase with Air

The developing chemical depth profile in an epoxy adhesive bulk (with varying amine content) is monitored during aging by FTIR microspectroscopy on sample cuts prepared with low angle microtomy. Three aging regimes are applied in order to separate the role of temperature and water: dried or moist air (90% rel. humidity) at 60°C and dried air at 120°C for up to 300 days. Quantitative evaluation of the IR spectra shows: thermo-oxidative aging (= dried air) is controlled by the diffusion of atmospheric oxygen. It affects a gradient region of more than 200 μm in depth. At given aging time, the depth profiles depend on temperature, humidity, and on the epoxy-amine ratio. Humidity mainly affects the IR band intensities. The plasticizing effect of water promotes the loss of small network fragments. At 120°C, autoxidation of α-CH₂ at ether and amine groups and the oxidative attack on tertiary amines dominate aging. At 60°C in dried air, these processes proceed only very slowly. In the case of amine excess, aging is extended by the additional oxidation of primary and secondary amines to carbonyls. Carbonyls undergo consecutive reactions, especially in the presence of water. Hence, increasing temperature does not simply accelerate the aging mechanisms but it reduces their selectivity and changes their hierarchy. Thus, the long-term aging behavior at moderate temperatures cannot be predicted safely from accelerated aging tests.