Hygrothermal Stability Research Articles

Design optimization of multi-coupled trapezoid laminates

Novel pavement design and size design are important factors to improve the mechanical properties and applicability of composite structures. Based on the multi-coupling of laminates, the universal analytical conditions for multi-coupled laminates are derived. Then four types of laminates with extension-shear coupling capable of forming bending-twisting coupled structure are designed, including three types of multi-coupled laminates. The shape of laminate is no longer limited to rectangular laminate, which is considered as a variable in the design process. Through the combination of theoretical calculation and numerical simulation, the optimal design of trapezoidal laminates with multiple coupling is achieved. The effect of the multi-coupling effect on the stiffness performance has been explored for free-layer laminates, and it has been found that the introduction of the multi-coupling can greatly improve the stiffness performance of laminates. Finally, the hygro-thermal stability performance is verified by numerical simulation and the robust analysis is obtained for stiffness performance.

Influence of aggressive environmental aging on mechanical and thermo-mechanical properties of Ultra Violet (UV) Cured in Place Pipe liners

Ultraviolet light cured-in-place pipe is a popular trenchless rehabilitation technology used to repair existing buried pipelines without soil excavation. A polymer composite liner is manufactured directly in the field. After curing is complete, the new liner is placed in service to convey flowing water. Long-term water exposure has the potential to degrade the fiber reinforced cured-in-place pipe liners. Like other composites, liner hygrothermal stability could be a major challenge, especially for undercured materials. The objective of this study was to investigate the effect of aggressive environmental conditions such as water, salt solution, and simulated concrete pore solution at 50℃ on cured-in-place pipe liner mechanical and thermo-mechanical performance. Liners were exposed to aqueous solutions ranging from a typical exposure environment to a very aggressive alkaline environment. Cured-in-place pipe liner samples were collected from New York storm sewer installation sites. Specimens were assessed for the degree of curing as determined by differential scanning calorimetry and TGA. Samples were then immersed in aforementioned solutions for investigating the stability of the liners as a function of degree of cure. Both interlaminar shear strength and thermo-mechanical properties significantly differed based on the degree of cure. Cured-in-place pipe liner mechanical and thermo-mechanical properties were significantly impacted by water, salt, and pore solutions exposure. Long-term exposure to salt and alkaline environments may have a detrimental effect on the performance of cured-in-place pipe liners and desires additional scrutiny.

Hygrothermal assessment of a traditional earthen wall in a dry Mediterranean climate

ABSTRACT In this paper, we define the hygrothermal properties of a rammed earth wall through experimental analysis, relating thermal parameters to moisture content. These tests were conducted in Campo de Criptana (Ciudad Real, Spain), and the wall examined in this study is a 70-cm-thick north-facing rammed earth wall that is part of an occupied traditional dwelling Ambient and surface temperature and humidity values were monitored, along with the temperature and humidity at several points inside the wall and the heat flux on both sides of the wall. We recorded the wall’s behaviour regarding ambient comfort conditions for one year and obtained transmittance and conductivity values. Results show that it is quite difficult to analyse the hygrothermal behaviour of a rammed earth wall throughout the year, since the properties of the wall vary enormously. We found that, to evaluate the thermal behaviour of the wall, it was not appropriate to consider the same conductivity value for whole sections of the wall, as this value varies according to moisture content (generally between 0.39 and 0.55 Wm−1 K−1): we propose seasonal values for conductivity. Finally, the hygrothermal stability that these structures provide to the spaces they enclose has been shown, especially during hot and dry periods.

A green chemistry approach to leather tanning process: Cage-like octa(aminosilsesquioxane) combined with Tetrakis(hydroxymethyl)phosphonium sulfate

The octa(aminopropylsilsesquioxane) (POSS–NH2) was synthesized by hydrolytic and condensation of γ-aminopropyltriethoxysilane in isopropyl alcohol solvent with tetraethylammonium hydroxide as catalyst. The structure and morphology of products was characterized by Nuclear magnetic resonance (1H, 13C-NMR), fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). A novel chrome-free tanning process based on combination tanning with cage-like octa(aminosilsesquioxane) and tetrakis(hydroxymethyl) phosphonium sulfate (THPS) has been proposed. Hygrothermal stability test showed that the shrinkage temperature (Ts) of THPS tanned leather was obviously increased with the introduction of POSS-NH2, implying that THPS and POSS nanoparticles had synergistic effect in the combination tanning process. The tanning process is optimized as 6.0% POSS-NH2 combination with 2.5% THPS, endowing the leather with Ts above 83.0 °C and thickness rate 44.6%. FT-IR analysis showed that hydrogen bond were formed between amino groups of POSS and hydroxyl groups of collagen during tanning process. The results of scanning electron microscope and energy dispersive X-ray spectroscopy (SEM-EDX) results reveal that POSS and THPS tanning agents could penetrate and bind evenly in the leather matrix, thus promoting the weaving of collagen fibers into a tight network structure.

Bending-twisting coupled structures based on composite laminates with extension-shear coupling effect

To improve the mechanical properties of bending-twisting coupled structures, the necessary and sufficient conditions for composite laminates with extension-shear coupling effect which contribute to the bending-twisting coupled structures are deduced. Then the analytical expressions for torsional distortion of bending-twisting coupled structures are deduced. The Differential Evolution algorithm is used to optimize the bending-twisting coupled structures for the purpose of maximizing torsional distortion, the stacking sequences of two extension-shear coupled laminates are obtained through calculation. Lastly, the applicability of simplified model is verified. The hygro-thermal stability, bending-twisting coupling effect and buckling load of bending-twisting coupled structures are verified via simulation. The yield strength is analyzed based on Tsai-Wu criterion.

Osmotic Tension, Plasticization and Viscoelastic response of amorphous Poly-Ether-Ether-Ketone (PEEK) equilibrated in humid environments

Dynamic-Mechanical Analysis (DMA), Differential scanning Calorimetry (DSC) and water moisture sorption-desorption kinetics and equilibria have been parallely investigated to monitoring the environmental and thermal sensitivity of amorphous thermoplastic Poly-Ether-Ether-Ketone (PEEK). Morphological modification occurring in PEEK during the calorimetric and thermo-mechanical characterizations and the hygrothermal stability are presented. An interpretation based on the free energy of mixing of a polymer chains with penetrant molecules and pseudo-equilibrium Flory-Huggins interaction parameter χ and related osmotic tension is proposed. The correlation between dynamic mechanical measurements and water saturation is discussed in terms of different levels of osmotic tension generated in the amorphous PEEK. From the analysis of the water diffusion coefficients in amorphous PEEK equilibrated at low and high activity environments, it has been found that even samples with very low water contents may undergo bulk matrix relaxation above 40°C.

Understanding storage environmental stability during power outages: A key issue in sub-tropical climates

Efficient air-conditioning systems facilitate climate control in modern museums, libraries, and archives. The climate in Hong Kong is typically hot and humid, so the survival of collections is highly dependent on 24-hour air-conditioning systems to maintain the desired environment in repositories of archives and books, or artefact stores. The continuous operation of such systems in turn relies on a number of factors, such as uninterrupted electricity supply, an active maintenance programme, daily monitoring of their performance and quick response to irregularities as well as backup or alternative systems to support the air-conditioning load in case they are down for servicing. From the risk management point of view, unlike the latter four factors, electricity supply is an external factor that cannot be guaranteed. Fluctuation in the power level that results in stoppage of air-conditioning equipment is not uncommon even in well-developed countries. Long power outages could be catastrophic as they might lead to irreversible damage to collection items as a result of fast, large, changes in temperature and relative humidity (RH) towards the external environment if the air-tightness or hygrothermal stability of the storage are not good enough. Understanding storage environmental stability, especially during electrical power disruptions, is an essential part of the preservation measures and disaster preparedness necessary to protect valuable collections in cultural heritage institutions in the region.

Humidity response of poly(butylene adipate-co-butylene terephthalate) copolyesters and their composites with wood flour determined by dynamic mechanical analysis

This study investigated the mechanical and viscoelastic properties of poly(butylene adipate-co-butylene terephthalate) copolyesters, P(BA-co-BT)s and their composites, P(BA-co-BT)–WF with 20 vol.% of beech wood flour (WF) at dry (N2) and in humid atmosphere (50% RH) using a large temperature range from 20 to 90 °C. The melting behaviour of P(BA-co-BT) copolyesters were studied with differential scanning calorimetry (DSC) and wide-angle X-ray analysis. The increase of the aromatic moiety as well as reinforcement with WF improved stiffness of copolyesters. The ambient testing environment with the increased temperature negatively impacted the stiffness and strain properties of P(BA-co-BT)s as well as P(BA-co-BT)–WF composites. The loss of mechanical properties was accelerated in humid atmosphere due to hydrolytic degradation of the ester linkage in copolymers. It was determined that wood flour used as a reinforcement slightly contributed to the hygrothermal aging of P(BA-co-BT) composites. Understanding the effect of humid atmosphere combined with dynamic changed temperature provides useful information for the assessment of the hygrothermal stability of biodegradable polyesters.

Water Absorption and Hygrothermal Aging Behaviors of SEBS-g-MAH Toughened Poly(lactic acid)/Halloysite Nanocomposites

Poly(lactic acid)/halloysite nanoclay composites (PLA/HNC) containing maleic anhydride grafted styrene-ethylene/butylene-styrene (SEBS-g-MAH) were exposed to hygrothermal aging at three different temperatures, i.e., 30, 40 and 50°C. The analysis was focused on the water diffusion kinetics and physical changes induced by the hygrothermal degradation. The water absorption kinetics of the PLA/HNC composites at immersion temperature of 30 and 40°C conforms to Fick's law. The equilibrium water absorption (Mm ) and diffusion coefficient (D) values are dependent on the SEBS-g-MAH content and immersion temperatures. Hydrolytic chain scission provoked significant molecular weight reduction. Nevertheless, adding SEBS-g-MAH improved hygrothermal stability of the PLA/HNC nanocomposites.

Viscoelastic phase diagram of fluorinated and grafted polymer films and proton‐exchange membranes for fuel cell applications

ABSTRACTThe influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)‐based proton‐conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion® NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 °C for 10% < RH < 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion®. Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power‐law behavior with exponent equal to −0.04 ± 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power‐law behavior at RH < 60% was also observed. However, a markedly different behavior was evident at RH > 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH ≥ 70% and 30 °C. The general behavior of grafted films observed at 30 °C was also obtained at 50 °C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 °C when compared with 30 °C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time‐dependent behavior, from power‐law scaling to exponential scaling. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1139–1148

Structure and transport properties of solution-cast Nafion® membranes subjected to hygrothermal aging

Nafion®, a perfluorosulfonated ionomer used as a proton conducting membrane in fuel cells, is usually considered as highly chemically stable. However, the extruded Nafion® 112 membrane was shown to be sensitive to hygrothermal aging when stored for very long periods at 80°C under humid atmospheres. The aging process was interpreted as a condensation of the sulfonic groups to form anhydrides. The hygrothermal stability of solution-cast and chemically stabilized Nafion® (NRE212CS) commercialized to replace Nafion® 112 is investigated through the determination of water sorption, conductivity, ion-exchange capacity and mechanical properties. The evolution of the chemical and physical structure was analyzed by IR, NMR, SAXS and WAXS. The lower rate of degradation for solution cast membranes compared to extruded membranes is attributed to a lower concentration of divalent cation contaminants such as Cu2+. The hygrothermal aging process is faster at high water content but it does not occur in liquid water. It is shown to progress from the surface to the bulk without major modifications of the local microstructure and crystallinity. The results are analyzed in the framework of the polymer ribbon model and it is suggested that the aging mainly acts within the bundles while the interbundle areas appear as the main conductive pathways.

Unhairing animal hides using probiotic Bacteria bacillus subtilis

The most efficient technology of processing natural raw materials into skin and fur is the use of enzyme products for soaking and liming processes. Therefore, the use of bacterial products, which produce enzymes of various functional effects, is considered to be very promising for the above mentioned processes. Soaking and liming of flint-dried rabbit hides were carried out using probiotic bacreria Bacillus subtilis on 4 samples in a laboratory centrifuge at soaking temperature 36-38°С and working solution рН 6,7-7. Unhairing was made by painting method, except one sample, unhairing of which was made mechanically after having soaked the hides in solution for 36-48 hours. Due to active action of Bacillus subtilis bacteria, the soaking process was reduced, unhairing was conducted without lime-sulfide painting paste, and the period of semi-finished product softening was decreased by 30 minutes. According to chemical analysis data, the concentration of chrome (III) oxide in spent solution is lower, and according to physical-mechanical tests of hides, the hygrothermal stability reaches its maximal value. The obtained results prove the prospects of Bacillus subtilis bacteria use in developing effective technologies of soaking and liming processes of leather and fur production. The technology elaborated allows combining soaking, unhairing, liming and desalting into a one-phase technological process. Advantages of this technology are 2 times faster soaking, comparing to the existing technology, with leather output increased by 20%, and reduction of chromium compounds in spent solutions by 34% that decreases the anthropogenic impact on the environment during its implementation.

The challenge of achieving hygrothermal stability in composite laminates with optimal couplings

The necessary and sufficient conditions for hygrothermal curvature stability of composite laminates have been derived previously, and their development is summarized in this work. These conditions are shown to be material independent. A constrained optimization routine is implemented to arrive at hygrothermally stable stacking sequences that are optimal for each of bend-twist and extension-twist coupling. The necessary and sufficient conditions for hygrothermal stability are used as constraints, while the compliance coefficients from classical lamination Theory are used as the objective functions. Both sequential quadratic programming and ant-colony optimization routines are used to ensure global optimality. Laminates consisting of two through ten plies are considered. Laminates consisting of five through ten plies are manufactured and tested to demonstrate the achievable level of extension-twist or bend twist coupling. A geometrically nonlinear model is presented to predict the response of bend-twist-coupled laminates. Comparison with the previous state-of-the-art stacking sequence for achieving extension-twist coupling with hygrothermal stability demonstrates a nearly 80% increase in the level of coupling.

Aging of Toughened Polylactic Acid Nanocomposites: Water Absorption, Hygrothermal Degradation and Soil Burial Analysis

The environmental aging behaviour of montmorillonite (MMT) filled polylactic acid (PLA) nanocomposites (PLA/MMT) and linear low density polyethylene (LLDPE)-toughened PLA (PLA/LLDPE ratio = 90/10) nanocomposites (PLA/LLDPE/MMT) were investigated in this study. The nanocomposites were subjected to water absorption, hygrothermal degradation and soil burial analysis. Both PLA/MMT and PLA/LLDPE/MMT nanocomposites were immersed in distilled water at three different temperatures (room temperature, 60, and 90 °C) and the weight difference before and after immersion was calculated. The kinetics of water absorption for both nanocomposites followed the Fick’s second law of diffusion, where a linear relationship exists between the initial moisture absorption at any time t and t1/2 (the square root of time), followed by a horizontal plateau (saturation). The equilibrium moisture content (Mm) and diffusion coefficient (D) of PLA nanocomposites increased with the addition of MMT (2 phr) and LLDPE. However, the D values of both nanocomposites decreased by increasing MMT (4 phr). The Mm for PLA/MMT and PLA/LLDPE/MMT nanocomposites increased by increasing immersion temperature (60 °C) and prolonged immersion resulted in hygrothermal degradation of both nanocomposites. The hygrothermal degradation studies showed that PLA degrades much faster at 90 °C as compared to 60 °C in both the nanocomposites. The addition of MMT and LLDPE improved the hygrothermal stability of PLA in both nanocomposites. Soil burial test revealed deterioration of impact strength in all samples while the rate of biodegradation was retarded in the presence of MMT and LLDPE.

New Families of Hygrothermally Stable Composite Laminates with Optimal Extension-Twist Coupling

The necessary and sufficient conditions for hygrothermal stability of laminated composites are presented. A thorough survey of hygrothermally stable families is performed to identify stacking sequences that produce maximum extension-twist coupling. This is achieved through a constrained optimization routine, where the objective function is derived from the constitutive law given in classical lamination theory and the constraints are the necessary conditions for hygrothermal stability. A representative sample of these optimized laminates are constructed and tested to demonstrate significant improvement in coupling over the previously known optima. Comparisons are made with a nonlinear model and finite element analysis. An investigation of the robustness of the optimal stacking sequences to errors in ply angle typical of hand-layup manufacturing is presented.

Hygrothermal Aging of Silane-Laced Epoxy Coatings

The impact of silane on the hygrothermal stability of epoxy coatings was investigated by specular neutron reflectivity (NR). By comparing the hygrothermal degradation behavior of neat novolac epoxy coating and corresponding bis[3-(triethoxysilyl)propyl]tetrasulfide-laced epoxy coating, the role of silane was elucidated. Accelerated aging was achieved by exposing the samples to 80°C liquid water. For the pure epoxy coating, degradation occurs at the coating–substrate interface, which makes the coating vulnerable to adhesion failure. For epoxy–silane coating the addition of silane imparts resistance to the interfacial degradation observed in the neat epoxy coating.

Families of Hygrothermally Stable Asymmetric Laminated Composites

Necessary and sufficient material-independent conditions are derived for hygrothermal stability of a laminated composite plate with plies made of the same specially orthotropic material based on classical lamination theory. The minimum number of plies required to obtain an asymmetric hygrothermally stable stacking sequence is proven to be five, and families of stable laminates are identified for six, seven, and eight plies. A sensitivity analysis demonstrates the robustness of hygrothermally stable solutions to small errors in ply orientation. A finite element analysis of geometric and ply orientation imperfections was performed to verify the robustness of the classical lamination theory results. Hygrothermally stable asymmetric laminate specimens were fabricated to confirm the analytical predictions.

Hygrothermal Stability of Electrical Contacts Made from Silver and Graphite Electrically Conductive Pastes

The hygrothermal stability of electrical contacts made from silver and graphite electrically conductive pastes is comparatively evaluated by measurement of the increase in contact electrical resistance during immersion in water at 15°C and 40°C. The pastes are silver paint, silver paint with a nonconductive epoxy overcoat, silver epoxy, and graphite colloid. Each electrical contact is made between a seven-strand tin-coated copper wire and the surface of a carbon fiber epoxy-matrix composite. Silver paint and graphite colloid penetrate the spaces among the 130-μm-diameter strands, but silver epoxy does not. Partly due to its low penetrability and the silver flake (15 μm) preferred orientation, silver epoxy gives contacts of significantly higher resistance than silver paint. Graphite colloid is comparable to silver epoxy in the resistance. Among the four pastes, silver paint with an epoxy overcoat is most durable, though it gives slightly higher resistance than silver paint without epoxy. Silver epoxy is less durable than silver paint without an epoxy overcoat, particularly at 40°C, due to the low hygrothermal stability of epoxy. Graphite colloid is even less durable than silver epoxy, due to its being water based.

Influence of the network structure and void content on hygrothermal stability of resol resin modified with epoxy–amine

AbstractEpoxy–amine was blended with two resol resins to study the influence of the final network and void content on the water absorption of these systems. To characterize the network structure of the blends, infrared analyses were performed. It was observed that a diminution in the crosslink density of the network and the reaction between the epoxy and resol have been occurred. From dynamic–mechanical analyses, a diminution in the Tg value and an increment in the height of the tan δ peak with the epoxy–amine content added to the resol were observed. Thermogravimetric analysis showed that the thermal stability of the resol was reduced by the addition of epoxy–amine as well as a lesser crosslink network for the blends with higher epoxy–amine content. The maximum water uptake and the diffusion coefficient were related with the chemical structure and void content of the specimens. On one hand, the maximum water uptake was observed to depend on the void content of the blends. On the other hand, the diffusion coefficient appears to be related with the local motions of the polymeric chains and with the hydrophilic character of the materials. This behavior was observed for the two resols studied. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 588–597, 2006

Hygrothermal aging of polyimide matrix composite laminates

Two polyimide-matrix systems (Avimid R K3B/IM7 and PETI-5/IM7) were subjected to a series of hygrothermal aging experiments at various temperatures and relative humidities. For each aging condition, we measured water absorption rate and the microcracking toughness as a function of aging time. The toughness decreased with time. The rate of decrease increased with temperature and relative humidity. All experimental results could be fit to a simple first-order kinetics analysis where the degradation rate was assumed to be proportional to the total exposure to water above a threshold water level. Below the water threshold, no degradation occurred. The kinetics analysis was used to construct a hygrothermal aging master plot that can be used to make predictions of toughness degradation due to arbitrary hygrothermal environments. The master plot can be constructed from a few experiments on specimens immersed in water and thus is a potential accelerated test method for characterizing hygrothermal stability of composite laminates.