Moisture Absorption Process Research Articles

Pretreatments of natural fibers and their application as reinforcing material in polymer composites—A review

AbstractIn recent years, natural fibers reinforced composites have received much attention because of their lightweight, nonabrasive, combustible, nontoxic, low cost and biodegradable properties. Among the various natural fibers; flax, bamboo, sisal, hemp, ramie, jute, and wood fibers are of particular interest. A lot of research work has been performed all over the world on the use of natural fibers as a reinforcing material for the preparation of various types of composites. However, lack of good interfacial adhesion, low melting point, and poor resistance towards moisture make the use of natural fiber reinforced composites less attractive. Pretreatments of the natural fiber can clean the fiber surface, chemically modify the surface, stop the moisture absorption process, and increase the surface roughness. Among the various pretreatment techniques, graft copolymerization and plasma treatment are the best methods for surface modification of natural fibers. Graft copolymers of natural fibers with vinyl monomers provide better adhesion between matrix and fiber. In the present article, the use of pretreated natural fibers in polymer matrix‐based composites has been reviewed. Effect of surface modification of natural fibers on the properties of fibers and fiber reinforced polymer composites has also been discussed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers

Modeling moisture absorption process of wood-based composites under over-saturated moisture conditions using two-part equations

The objective of this study was to investigate the moisture absorption process for wood-based composites subjected to over-saturated moisture conditions. Two stages are comprised in the moisture transfer process at the over-saturated moisture conditions, an initial stage which is the moisture transfer process mainly under fiber saturation point (FSP), and a second stage which is the moisture transfer process beyond the FSP. A model was developed based on two-part equations to describe the process, from which three coefficients (k1, k21, and k22) can be used to quantitatively describe the moisture transfer process under the conditions. Two different wood-based composites, wood fiberboard and wood fiber/polymer composites (polymer content: 30%), were used to test the model at four different ambient temperatures (30, 45, 62, and 80°C). It was shown that the two-part equation can accurately describe the moisture absorption process under over-saturated moisture conditions. The moisture absorption rate in the initial stage was about 30–60% greater than that in the second stage for most of the cases evaluated in this study. The higher the temperature, the greater moisture absorption parameters were obtained. At both moisture absorption stages (below FSP and above FSP), the calculated activation energy for the moisture absorption rate of wood fiberboard was very close to that of wood fiber/polymer composites.

Experimental and Analytical Investigation of Moisture Movement in Clothing

In order to design an efficient air-conditioning system, the specifics of the transient thermal and physiological responses of a human body to the hygrothermal environment should be taken into account. This response will strongly depend on the behavior of heat and moisture transport, and moisture accumulation in the clothing, especially when sweat exists in and around the clothing. As a basic information on this problem, a model of the moisture transfer in cloth is necessary. In this article, a model of the liquid moisture movement in clothing under gravity is developed and applied, based on a diffusion model. Firstly, the moisture diffusivity is determined from the investigation of the moisture absorption process in the horizontal direction where gravity has no influence on the moisture movement. Secondly with the use of the moisture diffusivity thus determined, a moisture absorption process in the upward direction against gravity is investigated. The validity of the parameter related to the gravity effect is examined by comparing the results of numerical calculations with the experimental results obtained from the gamma-ray method. Since the two agree generally well, the parameters namely liquid moisture diffusivity and sorption isotherm derived here for a sample of broadcloth are considered reliable.

Diffusion model based on Fick’s second law for the moisture absorption process in wood fiber-based composites: is it suitable or not?

The diffusion model based on Fick’s second law was applied to the moisture absorption process in wood fiber-based composites. The data of moisture absorption process on wood fiberboard and wood fiber/polymer composites with different densities were analyzed using the diffusion model. Specimens were subjected to different relative humidities and temperatures. The diffusion coefficients (D) and surface emission coefficients (S) along the thickness direction were calculated by the unsteady-state method using a nonlinear curve fitting algorithm. The experimental results showed that the S obtained from Fick’s second law diffusion model did not provide an external resistance when the moisture is absorbed into the wood. While the diffusion model did not provide the best fit of the moisture absorption data, the calculated internal moisture transfer rate, D, still shows physical meanings. The temperature had a significant effect on the D. The higher the temperature, the greater the D obtained. The D showed a decreased trend as the RH (or EMC) increased. No specific trends were found on the D as a function of board density. Other limitations of the diffusion model for the moisture movement in wood are also discussed.

Measurement of Diffusion Coefficients in Building Materials Using the Initial Rate of Sorption

This work presents experimental values of molecular diffusion coefficient, obtained with samples of building materials, using a non-stationary process of moisture absorption. The experiments covered four different types of building materials (with two selected ranges of relative humidity) and were carried out in the range of temperatures between 20°C and 40°C. The results show that the increase in temperature resulted in an increase in molecular diffusion coefficient and the relative humidity also influenced the molecular diffusion in a uniform manner. Finally, a good agreement with literature values was found.

Hygrothermal and Stacking Sequence Effects on Carbon Epoxy Composites with Molded Edges

Moisture and high temperature may cause a variety of effects on the mechanical and thermophysical properties of polymeric composites. The diffusion of moisture in the laminate occurs primarily by capillarity until a state of equilibrium is reached. Moisture affects the matrix and the fiber/matrix interface. The stacking sequence and the free edge finishing may affect the laminate tensile strength causing a free edge delamination; they can also affect the moisture absorption process. Laminates produced with molded edges do not require machining operations for the trimming and finishing of the free edges causing a gain in productivity. However, the mechanical behavior of laminates with molded edges saturated with moisture has not been characterized. This work presents a comparative study of the tensile strength of cross ply laminates with molded and machined edges under dry condition and saturated with moisture.

Chemical modification of kenaf fibers

The interest in using natural fibers in composites has increased in recent years due their lightweight, non-abrasive, combustible, non-toxic, low cost and biodegradable properties. However, lack of good interfacial adhesion, low melting point and poor resistance to moisture absorption make the use of natural fiber reinforced composites less attractive. Chemical treatment of the fiber can clean the fiber surface, chemically modify the surface, stop the moisture absorption process and increase the surface roughness. In this study, kenaf bast fibers, supplied by MARDI, for use in fiber-reinforced composites, were modified using NaOH of different concentrations. Morphological and structural changes of the fibers were investigated using scanning electron microscopy (SEM). A series of fiber bundle tensile tests were also performed to evaluate the effect of the treatments on the fiber tensile strength. It has been found that the alkalization treatment has improved the mechanical properties of the kenaf fiber significantly as compared to untreated kenaf fiber. It is also interesting to note that 6% NaOH yields the optimum concentration of NaOH for the chemical treatment.

Water absorption of hemp fiber/unsaturated polyester composites

The water absorption of hemp fiber/unsaturated polyester composites was determined by immersing the samples in water or by exposing them to air with a relative humidity of 94%. The water absorption increased with increasing fiber content. By using images obtained with a magnetic resonance imaging (MRI) system, the moisture absorption process was shown to follow a diffusion mechanism and to be more important in the longitudinal than in the transverse direction. The longitudinal diffusion coefficient was computed to be about 3 × 10–11 m2/s. Composite samples immersed in water reached saturation levels after about eight months and showed no signs of cracking due to swelling. Fibers reached the same saturation limit whether submerged in water or exposed to saturated air when fiber content was less than 21 vol%. Various fiber treatments were tested but none resulted in a substantial increase of the resistance to water absorption. The most effective technique to enhance moisture resistance was to properly seal all the fibers within the matrix. POLYM. COMPOS. 26:509–525, 2005. © 2005 Society of Plastics Engineers

Hygroscopic Behavior of Woven Fabric Carbon-Epoxy Composites

The hygroscopic behavior of a woven fabric carbon-epoxy composite and its effect on the viscoelastic properties and glass transition temperature was investigated. The mechanical and thermal properties of the material had been previously fully determined. An experimental study was conducted at full immersion in water and at a specific temperature condition. The moisture absorption process was found to be reversible with a low-saturation moisture uptake. The absorption through-the-thickness was determined to be lower than in the in-plane directions. The coefficients of moisture expansion or hygroelastic coefficients were determined and found to be similar in the warp and fill directions and much lower than through-the-thickness direction. The results were correlated to the behavior of a unidirectional composite with the same fiber and matrix. The viscoelastic properties were not affected during the process but the plasticization was very pronounced.

Effects of hygrothermal ageing and thermal shock on reliability of interfacial adhesion between black oxide coated copper substrate and epoxy resin

Black oxide is a conversion coating applied onto the Cu substrate to improve the interfacial adhesion with polymeric adhesives. A comprehensive study was made to characterize the black oxide coating and the corresponding interfacial adhesion with various types of polymeric resin, aiming to optimize the oxide processing conditions. The reliability of adhesion performance of the coating was evaluated before and after accelerated hygrothermal ageing, such as temperature cycling, pressure cooker test, and moisture sensitivity test followed by thermal shock. The moisture resistance of the substrate with black oxide coating was much higher than the bare Cu substrate, during both the moisture absorption and desorption processes. Thermal cycling alone did not change significantly the adhesion performance of any of the substrates studied. Pressure cooker test was detrimental to adhesion performance of oxide coated Cu substrates. Nevertheless, the residual interfacial bond strengths were consistently much higher for the black oxide coated substrates than the bare Cu surface. Significant delamination occurred between the bare Cu and the moulding compound after the moisture sensitivity test followed by thermal shock, whereas there was virtually no delamination on the black oxide coated samples under the same ageing condition, confirming the higher reliability of interfacial adhesion performance for the latter.

Transient hygrothermal stresses in fiber reinforced composites: a heterogeneous characterization approach

Transient hygrothermal stresses induced in fiber-reinforced composites are studied in detail by adopting a novel heterogeneous characterization approach. This approach incorporates two distinct features: transient moisture absorption analysis of actual composite materials exposed to a humid environment, and highly detailed computational analyses that capture the actual heterogeneous microstructure of the composite. The latter feature is carried out by modeling a uniaxial laminate having more than one thousand individual carbon fibers that are randomly distributed within an epoxy matrix. Results indicate that these computational models are essential in capturing the accurate moisture absorption process of the actual specimen. In the analysis, the evolutions of thermal residual stresses and moisture-induced stresses within the humidity and thermal exposed composites have been analyzed. It was observed that high stress concentration develops in the epoxy phase where high fiber density or fiber clustering exists and its magnitude increases as the moisture content saturates. Large stresses can potentially initiate epoxy damage or delamination of epoxy and fibers. Furthermore, due to opposing effects of thermal and moisture exposure, lower stresses are found in the laminate when both are considered simultaneously.

Inverse analysis for transient moisture diffusion through fiber-reinforced composites

A new approach has been developed, based on an inverse analysis technique, to determine critical moisture diffusion parameters for a fiber-reinforced composite. This technique incorporates two distinct features: direct experimental observations of the weight gained by a composite material exposed to a humid environment, and highly detailed computational analyses that capture the actual heterogeneous microstructure of the composite. The latter feature was carried out by modeling more than 1000 individual carbon fibers that are randomly distributed within an epoxy matrix. The verification and efficacy of this technique was established by conducting an experiment on a high-grade IM7/997 carbon fiber-reinforced epoxy to determine the maximum moisture content at saturation and the diffusivity of epoxy. With the inverse analysis, the time duration required to estimate these moisture diffusion parameters could be drastically reduced as compared to conventional procedures. Subsequently, the established models were employed to characterize transient moisture absorption process within the composite. Here, it was demonstrated that modeling the heterogeneous microstructure of the composite is critical for obtaining accurate diffusion parameters, and an analytical model with effective properties does not produce correct transient moisture absorption behavior. Furthermore, the evolution of stress fields due to moisture induced volumetric expansion was quantified. It was observed that high stress concentrations develop in regions of fiber concentration. These regions then act as potential failure initiation sites that can lead to lower damage tolerance.

The Influence of Air Flow Rate and Its Temperature on The Moisture Absorption and Regeneration Processes.(Dept.M)

Production of fresh water by absorption of moisture from atmospheric air may be the only choice in sone places and circumstances. This can be performed by introducing the air into a packed bed of clothes saturated with a suitable desiccant to absorb water vapour from air with subsequent regeneration and condensation of water vapour. In this work, the effect of air flow rate and its temperature on the rate of moisture absorption and ine regeneration process is experimentally investigated. The power consumed in heating or cooling the air, and the blowing power are considered. For this purpose, an experimental setup is designed and constructed. A three honeycomb packed bed units are arranged in the flow direction, such that the face of the upstream units is perpendicular to the flow. The middle and down stream units are fixed in the same way. The three beds are saturated with Calcium Chloride (CaCl2)as a desiccant. The Experimental work is divided into two group s during the absorption and regeneration processes. The first group is carried out for constant air flow rate and different air temperatures, while the second group is carried out at constant air temperature and different flow rates. The power consumption is caleulated according to the measured data. Results have shown that increasing the air flow rate during absorption process is more effective, while increasing its temperature in the regeneration process is preferable. Results of this work are presented in dimensioless and graphical form.

Growth Kinetics and Deposition‐Related Properties of Subatmospheric Pressure Chemical Vapor Deposited Borophosphosilicate Glass Film

Growth kinetics of borophosphosilicate glass (BPSG) films prepared by the chemical reaction of tetraethylorthosilicate (TEOS), triethylborate, and triethylphosphate, with ozone‐oxygen mixture has been studied using a single‐wafer deposition chamber at 373–458°C under reduced pressure or subatmospheric pressure condition (SABPSG). Based on the comparison of the effective reaction rate constants and other kinetic features, the similarity of BPSG deposition kinetic for different types of precursors and deposition conditions are shown. Fourier transform infrared spectroscopy, X‐ray fluorescence analysis, secondary ion mass spectrometry, stress measurement, optical microscopy, scanning electron microscopy, atomic force microscopy, surface laser scanning, capacitance‐voltage, current voltage, and surface charge analysis techniques have been used for the characterization of SABPSG film properties. Moisture absorption and surface defect formation phenomena have been studied for both as‐deposited and furnace annealed films with boron and phosphorus ranging from 3 to 6 wt %. The liquid state, solid state, and crystalline BPSG surface defects have been characterized and their relationships have been shown. A multipath and multistep reaction mechanism scheme of TEOS‐ozone BPSG deposition process with the formation of active intermediate products, a scheme of porous TEOS‐ozone BPSG film with embedded dopant‐rich clusters, and a scheme of defect formation and growth on BPSG films are discussed. Empirical equations of quantitatively optimized BPSG dopant concentration range are presented by taking into account flow properties, defect formation, and moisture absorption processes in the film. © 1999 The Electrochemical Society. All rights reserved.

Transfer function method to calculate moisture absorption and desorption in buildings

A transfer function approach has been described to calculate dynamic moisture absorption and desorption by building materials in the hygroscopic range, and the dynamic moisture absorption and desorption processes have been theoretically modeled. Their surface vapor pressure and moisture absorption and desorption flux can be evaluated by this approach. It has turned out mathematically that the material surface attains instantaneous moisture equilibrium with the surroundings at high Biot number (Bi → ∞), and that the material moisture behavior can be described through a lumped-parameter modeling at low Biot number (Bi → 0). For building materials at intermediate Biot number, comparisons with experimental results and numerical solutions have shown satisfactory agreement with the proposed approach. Using this approach, the dynamic response of moisture absorption and desorption in interior building materials to the variation of indoor air humidity and the dynamic effect of moisture absorption and desorption by building interior materials on indoor air humidity and space latent cooling load can be simultaneously calculated.

The use of elevated temperature in the structural testing of FRP components for simulating the effects of hot and wet environmental exposure

Abstract The natural process of moisture absorption in epoxy resin matrices is normally slow and this makes it is very difficult to achieve a realistic degree of moisture-related degradation in an FRP structural test element in a practical time. One of the effects of absorbed moisture is to lower the glass transition temperature of the matrix, thereby causing a reduction in the temperature-related properties. It is postulated that an increase in test temperature, equivalent to that of the change in the glass transition temperature, could be used to represent the degradation due to moisture. Experimental evidence to support the use of such an alternative is presented. The evidence comprises an examination of three matrix-dependent mechanical properties and their failure modes. Results show that the compression strength, the ± 45° tension strength and the interlaminar shear strength, together with the failure mode characteristics of the hot-wet structure, can be duplicated using a temperature increment over and above the normal temperature requirements for airworthiness clearance to achieve an equivalent effect of moisture.

Modelling water vapour conditions in buildings

This paper proposes a 'moisture admittance' model which accounts for the moisture absorption and desorption processes for predicting humidity levels in buildings. The experimental method whereby the required coefficients were derived is described and some examples of the results are given. The approaches to real buildings, as opposed to test rooms, and possible modifications to BREDEM are briefly discussed.

Modelling of the moisture transfer process in particleboards

There were a number of studies on moisture transfer modelling of wood, but no systematic study for moisture transfer in particleboard with respect to usage or storage. The physical parameters of four kinds of particleboards were determined in this study. The unsteady-state diffusion coefficients and surface emission coefficients of moisture in particleboards were separated in one experimental period by using the method of linear regression. Then the moisture transfer processes in particleboard were analysed by using Finite Element Method (FEM), and the moisture absorption processes of four kinds of particleboards were observed experimentally. By comparing the computed results with the experimental results, it showed that the error was within 10%. Therefore, we came to the conclusion that the processes of moisture transfer in particleboard can be described by using FEM:

Modelling the process of moisture absorption in three dimensions by wood samples of various shapes: cubic, parallelepipedic

Modelling the process of absorption of moisture by wood samples of various shapes and dimensions: cube and parallelepipedes, has been studied. The mathematical model is based on numerical method with finite differences, and takes into account all the facts: the external resistance with the coefficient of moisture transfer at the atmosphere-wood interface, the internal resistance with the diffusion of moisture within the wood. The problem is rather complex, because the rate of diffusion varies with the direction chosen in the wood, and because of the concentration-dependent diffusivities. Not only the model is able to obtain the kinetics of absorption of moisture in good agreement with experiments done with samples of various shapes, but it is also capable of calculating the profiles of concentration of moisture developed within the sample, and so affording a further insingt on the process of moisture absorption.

Interrelation between moisture absorption, mechanical behavior, and extent of the boundary interphase in particulate composites

AbstractThe influence of moisture absorption on the extent of the boundary interphase in particulate composites is thoroughly studied. It was found that, during the process of moisture absorption there is a variation of the extent of the boundary interphase, closely related to the degradation of the mechanical behavior of the composite, as well as to the percentage amount of moisture absorbed. An explanation of the observed relationship was advanced, based on a theoretical mechanism of absorption. This study complements a previous one, where the observed degradation of the thermomechanical properties of particulate composites due to moisture absorption was shown to be intimately interrelated with the state and extent of the interphase between fillers and matrix.