Moisture Sorption Kinetics Research Articles

Kinetics and Equilibrium Isotherms of Water Vapor Adsorption/Desorption in Cement-Based Porous Materials

The kinetics and equilibrium of water vapor adsorption/desorption isotherm (WVSI) are fundamental information for solid–moisture interaction in the microstructure of cement-based porous materials (CBPM). This paper presents firstly the experimental data for WVSI of CBPM with a ternary binder. Mass changes in specimen are recorded in sorption tests for materials with different aggregate contents and water-to-binder (w/b) ratios under controlled ambient relative humidity. Both the sorption equilibrium and sorption kinetics are investigated through some established physical models, and the corresponding intrinsic parameters of moisture sorption are determined. For sorption kinetics, the mass change during sorption is interpreted through Elovich model, power model and pseudo-first-order and pseudo-second-order models. The mass change in sorption occurs mainly in the first 40 days, and the pseudo-second-order model shows the best adaption to all adsorption/desorption processes. For sorption equilibrium, Guggenheim–Anderson–de Boer (GAB), modified Halsey and Oswin models are used to interpret the moisture sorption capacity. The hysteresis between adsorption and desorption processes is attributed, respectively, to the mechanisms of snap-through, energy instability and pore constrictivity for low, middle and high relative humidity ranges. From the analysis, it is found that (1) through the interpretation of pseudo-second-order model the moisture sorption kinetics can be controlled by the sorption between the water molecules and the pore wall in addition to vapor diffusion; (2) the sorption capacity is sensitive to aggregate content and w/b ratio; incorporating aggregates, decreasing w/b ratio and increasing humidity gradient tend to decrease the sorption capacity of CBPMs; (3) the sorption isotherm hysteresis is well described by GAB-H model, both the extent of hysteresis and the energy constant related to pore surface tension showing clear correlation with the aggregate content and w/b ratio of CBPMs.

Moisture Sorption Kinetics of Switchgrass, Big Bluestem, and Bromegrass Biomass

<abstract><title><italic>Abstract.</italic></title> Moisture content of biomass is the most influential factor in biomass storage. Moisture sorption kinetics control the dynamic moisture condition of the biomass, thus affecting biomass storage, processing operations, and final utilization applications. Moisture sorption characteristics of switchgrass, big bluestem, and bromegrass, potential biomass feedstocks for the Northern Great Plains of the U.S., were studied. Study objectives were to determine the moisture sorption kinetics, mathematically model the sorption process using standard models, and evaluate the effect of temperature on moisture sorption. Moisture sorption experiments were conducted at temperatures of 20°C, 40°C, and 60°C and a fixed high relative humidity of 95% using a controlled-environment chamber. Standard moisture sorption kinetics models (exponential, Page, and Peleg) were used to analyze the experimental sorption characteristics of the feedstocks. Bromegrass had the highest moisture sorption rates and final moisture contents, followed by big bluestem and switchgrass. On average, at 20°C, 50% of moisture sorption completion occurred at 1.5, 1.9, and 1.7 h and 90% completion occurred at about 8.5, 13.4, and 12.8 h for switchgrass, big bluestem, and bromegrass, respectively. For the temperatures studied, on average 41% ±3%, 40% ±5%, and 39% ±1% of moisture sorption completion occurred in 1 h and 82% ±2%, 76% ±4%, and 73% ±1% completion occurred in 5 h for switchgrass, big bluestem, and bromegrass, respectively. Moisture sorption rates decreased very sharply during the first hour (≥78%) from their initial values and quickly plateaued thereafter. Increase in temperature increased the moisture sorption rates for all the biomass types tested. Both the Page and Peleg models effectively described the observed sorption characteristics for the selected biomass types (R² ≥ 0.96). The Arrhenius equation adequately described the temperature dependence of the model parameters (0.77 ≤ R² ≤ 1.00). Based on this study, the Peleg model in combination with the Arrhenius equation is recommended for moisture sorption predictions. Fitted moisture sorption kinetics models, developed nomograms, and combined prediction equations (R² ≥ 0.83) form baseline data essential for storage of the selected biomass types and various handling, conditioning, and processing operations.

Isotherm moisture absorption kinetics in natural-fiber-reinforced polymer under immersion conditions

In natural-fiber-reinforced polymer, absorption of water or moisture is a significant issue in maintaining strength and stiffness. To enhance the understanding of water and moisture sorption behavior, the kinetics of moisture sorption in natural-fiber-reinforced polymers are investigated under immersion conditions. Samples of hemp-bast-fiber-reinforced polyethylene are prepared using an injection molding technique at different hemp fiber volume fractions (vf). The samples are then immersed in water for 274 days. Moisture content and uptake rate are analyzed at different fiber volume factions and matrix crystallinity percentages. A simplified two-dimensional contraction model is developed to investigate the contraction effect on the moisture uptake; it shows that a matrix with high crystallinity has more stiffness contraction on the reinforcing natural fibers, which limits the maximum amount of the absorbed moisture. The Fickian diffusion is found to be the dominant mechanism, shifting toward pseudo-Fickian or anomalous diffusion depending on the natural fiber volume fraction and the crystallinity percentages of the matrix. The natural-fiber-reinforced polymers diffusivity is evaluated and modeled to characterize the ability of liquid molecules to diffuse into these composites at different hemp fiber volume fractions. Both the crystallinity percentage of the matrix material and the volume fraction of the reinforcing fibers were found to interactively affect the sorption kinetics of the tested natural-fiber-reinforced polymers.

Analysis and modeling of moisture sorption behavior for antimicrobial composite protein films.

The WPI-NaCas-GLY antimicrobial film takes full advantage of the controlled release of active or antimicrobial agents as well as demonstrates a great potential for functioning as an alternative biodegradable polymer in practical applications. The moisture sorption kinetics of the film as an important carrier of active agents was investigated at various relative humidities (RH). The results indicated that the moisture sorption characterization and procedure of this film can be described well by the empirical Peleg model with higher confidence and concordance. The model could predict the film's moisture content at any time (Mt), the time to reach any given level of R (tR), the equilibrium moisture at any RH condition (Me), and isotherm trend based upon experimental data and modeled constants k(1), k(2), a, b, c, and d without giving consideration to their physical meaning. The water vapor transmission rate of the WPI-NaCas-GLY antimicrobial film increased exponentially with increasing RH due to its hydrophilicity, which was primarily caused by the presence of glycerol in a higher content. The results also suggested that aw predominately affects the film's Me values compared with the temperature factor by fixed nonlinear multiple regression analyses.

Fabrication and Characterisation of Microgel/Polymer Composite Microfibres

AbstractIn this work microgel‐PVA composite fibres are prepared by means of an electrospinning process using aqueous solutions of poly(vinyl alcohol) (PVA) and poly(N‐vinylcaprolactam)‐based microgels. The optimal parameters are defined to fabricate homogenous composite fibres in which the microgels are embedded into a PVA matrix. It is demonstrated that the fibres can be physically and chemically crosslinked. The fibre morphology is visualised and its properties investigated by differential scanning calorimetry, DSC, thermo‐gravimetric analysis, TGA, and isothermal moisture sorption analysis. The results show that the thermo‐sensitive properties of the microgels are retained inside the fibres and that the degree of swelling and the moisture sorption kinetics can be tuned by changing the temperature.

Empirical modeling of moisture sorption characteristics and mechanical and barrier properties of cassava flour film and their relation to plasticizing–antiplasticizing effects

Moisture sorption kinetics of cassava flour film was investigated at various relative humidities (RH). Their curves were fitted to a Peleg model. This film was also studied for its sorption isotherm. Empirical models were tested to fit the experimental data. Moisture sorption was more rapid in the initial stages; less moisture was adsorbed as adsorption time increased. GAB and Lewicki models were found to be better-fitted models. Effects of sorbitol concentration and RH on mechanical properties of films were examined. The tensile strength (TS) of films vastly decreased with increasing sorbitol concentration. The percentage of elongation increased significantly with increasing sorbitol content; however, it declined when sorbitol content was above 40g/100g. Modulus of elasticity (E) of films displayed a similar trend to TS. The E of this film was inversely linear to RH, whereas its elongation was exponentially proportional to RH. Effects of sorbitol concentration on barrier properties of films were also examined. The water vapor transmission rate of film was gradually reduced with increasing sorbitol content; however, it dropped dramatically with sorbitol content above 40g/100g. The oxygen transmission rate of film significantly decreased with rising sorbitol content. This is helpful in understanding the performance of edible films under varying RH conditions and plasticizer contents.

Mechanical properties of pultruded glass fiber-reinforced plastic after moistening

The kinetics of moisture sorption under immersion in water at room and elevated temperatures and flexural characteristics of dry (conditionally initial) and wet (moistened up to saturation level) composite material were investigated on flat specimens of polyester based glass fiber-reinforced plastic, cut from I-beam pultruded profile. It was found that the coefficients of diffusion and swelling are different in three principal axis of the composite. The former have the largest value in fiber axis direction, but the latter – in transverse to fiber axis direction out of plane of the layers. The observed difference in kinetics of mass gain and change of volume strain for the specimens of various sizes allowed evaluate the volume of pores, filled with water. The large data spread for the specimens, cut from the flanges, was found. Edge specimens have lower density and showed lower elastic and strength characteristics. The material specimens moistened up to saturation level showed the decrease in flexural modulus and strength for 8% and 16.5%, respectively. No effect of sorbed moisture on tensile modulus and strength of the composite (tension along the fiber axis direction) was found.

MECHANICAL PULPING: Influence of beating and chemical additives on residual stresses in paper

Fibre modification might be a way to improve the performance of paper, to increase its cost competiveness and enable new paper-based products to be developed. Therefore, the influence of fibre modification (with polyelectrolytes or by fibre cross-linking) on the mechanical properties of special importance for packaging paper grades was studied. Creep deformation under varying humidity conditions (i.e. mechano-sorptive creep) is of outmost importance for the stacking life of paper-based boxes. The influence on creep behaviour of adsorbing polyallylamine (a cationic polyelectrolyte) to fibre surfaces or throughout the fibre walls was studied. Adsorption to fibre surfaces reduced the creep at constant humidity. The mechano-sorptive creep was not however influenced. The use of polyelectrolytes did not thus appear to be a feasible strategy for reducing mechano-sorptive creep. Polyelectrolytes can however be efficient in improving other mechanical properties. The use of multilayers consisting of polyallylamine (PAH) and polyacrylic acid (PAA) was for example shown to significantly increase the strength of paper with much less densification and build-up of residual stress than is obtained by beating. Cross-linking by oxidation with periodate radically decreased the mechano-sorptive creep of sheets made from the oxidised fibres. The basic mechanism behind the reduction in mechano-sorptive with cross-linking was found to be that the cross-linking slowed down the moisture sorption kinetics. A lower sorption rate led to smaller moisture content variations during the mechano-sorptive creep testing, and thus less sorption-induced swelling and stress concentrations at fibre/fibre joints. However, for cross-linking to be a practical way to reduce creep, the large problem of embrittlement must be solved. The shear strength of couched sheets was measured to study the interaction between the sheets at different solids content. The shear strength was low until a solids content of approximately 60−70% was reached, which suggests that interactions important for the strength at complete dryness start to develop at this solids content. The effect of different fibre modifications and additives on how the fibres interact during the consolidation process is not always well understood. The method of shear strength determination could in the future be applied to modified fibres to hopefully increase the understanding of how different modifications influence the fibre/fibre interactions. A deeper understanding might reduce the time for the development of new and improved fibre modifications.

Shelf life prediction of packaged cassava-flour-based baked product by using empirical models and activation energy for water vapor permeability of polyolefin films

Moisture sorption kinetics and isotherms of cassava-flour-based baked product were investigated. Empirical models were tested to fit the experimental data. Textural changes of the product were investigated. In addition, activation energies ( E p ) for water vapor permeability (WVP) of polyolefin films were determined. Finally, the product was packaged in low-density polyethylene (LDPE) or oriented polypropylene (OPP) pouches, and stored at 30 ± 1 °C and 50 ± 2% RH to simulate actual storage conditions and to determine shelf life. This actual shelf life was compared to the predicted shelf life by using empirical models and E p for WVP. Moisture sorption kinetics was more rapid during the initial stage, while a lesser amount of moisture was adsorbed as adsorption time increased. The higher the relative humidity used, the more pronounced the effect. The sigmoidal moisture sorption isotherms of this product can be classified as type II. The GAB model was found to be the best-fit model for this product. Once the product hardness or work reached the maximum and began to reduce at moisture content (MC) ≈6%, the product texture began to be detected as becoming slightly soft. This implies that hardness and work at the maximum level could be used to identify the critical MC which causes a loss of crispness to an unacceptable degree. The predicted shelf lives – estimated by employing E p for WVP of LDPE and OPP, and the GAB model – were close to the actual shelf lives. Therefore, the estimation by empirical models and activation energy was found to be applicable for rapid and accurate shelf life prediction.

Hygroscopic Moisture Sorption Kinetics Modeling of Corn Stover and its Fractions

Corn stover, a major crop-based lignocellulosic biomass feedstock, is required to be at an optimum moisture content for efficient bioconversion processes. Environmental conditions surrounding corn stover, as in storage facilities, affect its moisture due to hygroscopic sorption or desorption. The measurement and modeling of sorption characteristics of corn stover and its leaf, husk, and stalk fractions are useful from utilization and storage standpoints, hence investigated in this article. A benchtop low-temperature humidity chamber provided the test environments of 20°C, 30°C, and 40°C at a constant 95% relative humidity. Measured sorption characteristics with three replications for each fraction were obtained from instantaneous sample masses and initial moisture contents. Observed sorption characteristics were fitted using exponential, Page, and Peleg models. Corn stover fractions displayed a rapid initial moisture uptake followed by a slower sorption rates and eventually becoming almost asymptotic after 25 h. Sorption characteristics of all corn stover fractions were significantly different (P 0.05) on these fractions. The initial 30 min of sorption was found to be critical due to peak rates of sorption from storage, handling, and processing standpoints. The Page and Peleg models had comparable performance fitting the sorption curves (R2 = 0.995), however the exponential model (R2 = 0.91) was not found suitable because of patterned residuals. The Arrhenius type relationship (P < 0.05; R2 = 0.80) explained the temperature variation of the fitted sorption model parameters. The Peleg model fitted constants, among the sorption models studied, had the best fit (R2 = 0.93) with the Arrhenius relationship. A developed method of mass proportion, involving individual corn stover fraction dry matter ratios, predicted the whole corn stover sorption characteristics from that of its individual fractions. Sorption characteristics models of individual corn stover fractions and predicted whole corn stover including a nomogram can be used for direct and quick estimation. Developed sorption characteristics find application in several fields of corn stover biomass processing, handling, and transport.

Reduced water vapour sorption in cellulose nanocomposites with starch matrix

The effects of microfibrillated cellulose nanofibers from wood on the moisture sorption kinetics (30% RH) of glycerol plasticized and pure high-amylopectin starch films were studied. The presence of a nanofiber network (70 wt% cellulose nanofibers) reduced the moisture uptake to half the value of the pure plasticized starch film. The swelling yielded a moisture concentration-dependent diffusivity. Quite surprisingly, the moisture diffusivity decreased rapidly with increasing nanofiber content and the diffusivity of the neat cellulose network was, in relative terms, very low. It was possible to describe the strong decrease in zero-concentration diffusivity with increasing cellulose nanofiber/matrix ratio, simply by assuming only geometrical blocking using the model due to Aris. The adjusted model parameters suggested a “simplified” composite structure with dense nanofiber layers oriented in the plane of the film. Still, also constraining effects on swelling from the high modulus/hydrogen bonding cellulose network and reduced amylopectin molecular mobility due to strong starch–cellulose molecular interactions were suggested to contribute to the reductions in moisture diffusivity.

Water vapor absorption and permeability of films based on chitosan and sodium caseinate

AbstractThe objective of this work was to characterize the moisture sorption and water vapor permeation behavior of edible films made from sodium caseinate and chitosan for future applications as protective layers on foods. Glycerol was used as a plasticizer, and the films were obtained by a casting/solvent‐evaporation method. The moisture sorption kinetics and water vapor permeability (WVP) were investigated. The effect of the addition of glycerol on the WVP characteristics of the films was determined at 25°C with a relative humidity (RH) gradient of 0–64.5% (internal to external). Experimental data were fitted with an exponential function with two fitting parameters. WVP increased with increasing glycerol content in both films, chitosan samples being much more permeable than caseinate ones at any glycerol content. WVPs of sodium caseinate, chitosan, and chitosan/caseinate films with 28 wt % glycerol were also determined for two RH gradients, 0 to 64.5% and 100 to 64.5%, higher WVPs being measured at higher RHs. The moisture sorption kinetics of caseinate films prepared with various glycerol contents were determined by the placement of the films in environments conditioned at 20°C and 75% RH. Peleg's equation and Fick's second law were used to predict the moisture sorption behavior over the entire time period. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Moisture permeability of a polymer nanocomposite containing unmodified clay

Results of an investigation into the kinetics of moisture sorption by an unfilled styrene-acrylate copolymer and a nanocomposite containing unmodified montmorillonite clay are reported. The sorption tests were performed at 25°C and different levels of the relative humidity of air up to 80%. The moisture diffusivity was calculated using Fick's equation for one-dimensional diffusion. Data on the influence of montmorillonite concentration on the coefficients of diffusion, solubility, and permeability are obtained. The experimental values of the permeability coefficient are compared with those calculated by using a model taking into account the increased path of a diffusing water molecule caused by the shielding effect of plate like filler particles.

A comparative analysis of moisture transport models as applied to an epoxy binder

The results of experimental and theoretical investigations into the kinetics of moisture sorption by a neat epoxy resin obtained from RAE Industries (Reapox 520, D523) are reported. The sorption process was realized in atmospheres with a constant relative humidity of 33, 53, 75, 84, and 97% and a temperature of 50°C. The results obtained showed that the diffusion behavior of epoxy resin did not obey Fick’s law under the experimental conditions considered. Consequently, the application of a non-Fickian diffusion model was necessary. For this purpose, two-phase moisture sorption models, a model with a time-dependent diffusivity, a two-phase material model, as well as relaxation and convection models of anomalous diffusion, were considered. The model parameters were obtained from the approximation of experimental sorption data. A comparative analysis of the sorption models was performed, and the specific features of their applications were estimated. The two-phase material model and the model with varying diffusivity were found to be the most suitable ones due to a good agreement between calculation results and experimental data and the rather small (three or four) number of parameters, which make them more flexible and physically more justified than the classical Fick’s model with its two parameters.

Effective moisture diffusivity modelling versus food structure and hygroscopicity

Effective moisture diffusivities ( D eff) for model food systems presenting distinct structure and water sensibility were identified at various levels of hydration. Experimental moisture sorption kinetics with a controlled atmosphere microbalance were analyzed using distinct Fickian models with specific assumptions. Taking into account the deformation and the external resistance to mass transfer, higher values of D eff were identified specially for hygroscopic dense products in the range of high water activity. The difference of impact of the model assumptions on D eff depending on the investigated water activity range and food material was related to the structure and the equilibrium moisture sorption properties.

Lactose/beta-lactoglobulin interaction during storage of model whey powders.

The objective of this study was to evaluate the presence or absence of interaction between lactose and beta-lactoglobulin during storage of model whey powders at different water activities (a(w)). Model whey powders were prepared by colyophilization of lactose with increasing quantities of beta-lactoglobulin. These colyophilized beta-lactoglobulin:lactose powders, assigned as BL powders, were stored from 0.11 to 0.95 a(w). The water sorption behavior of BL powders was studied gravimetrically, and the state of lactose was investigated using differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Before storage, BL powders were amorphous. After storage, a loss of water was observed on moisture sorption isotherms of BL powders. It was related to the formation of lactose crystals, detected by DSC and SEM analysis, and to the structural collapse of the powders. Water loss due to lactose crystallization was shifted to higher a(w) with increasing beta-lactoglobulin content in BL powders. Moreover, kinetics of moisture sorption demonstrated that beta-lactoglobulin was also responsible for a slower crystallization process in BL powders. Then, the water sorption behavior of BL powders was very different from the behavior of the 2 compounds mixed after separate lyophilization. All these results pointed out interaction between lactose and beta-lactoglobulin, which appeared during lyophilization and still occurred during storage. This lactose/beta-lactoglobulin interaction stabilized model whey powders against lactose crystallization.

The Plasticisation Effect of Glycerol and Water on the Gelatinisation of Wheat Starch

AbstractIn the extrusion manufacture of starch‐based thermoplastics, such as biodegradable packaging materials, glycerol is an effective additive as a plasticiser, that is, to diminish the brittle nature of the product and provide the desired extent of flexibility. However, the addition of glycerol may also affect the gelatinisation behaviour of the starchwater mixture, and hence the required processing conditions for producing a homogeneously gelatinised starch‐based material. The effect of glycerol on the gelatinisation of wheat starch was studied using differential scanning calorimetry (DSC). Mixtures of starch, water and glycerol were investigated with a water content ranging from 12 — 40% and a glycerol concentration up to 75%. Dependent on composition, the enthalpy of gelatinisation ranged from 1.7 — 12.6 J/g (on a dry starch basis), while the onset and peak temperatures varied from 54 to 86 °C and 60 to 90 °C, respectively. As expected, water acted as a plasticiser in that the onset temperature for gelatinisation (T0) decreased with increasing moisture content. Glycerol, however, increased T0. It is shown that the T0 of starch‐glycerol‐water mixtures may be predicted on the basis of the effective moisture content of the starch fraction of these mixtures resulting from the relative speed of moisture absorption by glycerol and starch, respectively. Moisture sorption kinetics of wheat starch and glycerol in 100% relative humidity were determined and used to predict the preferential water absorption by glycerol in starch‐glycerol‐water mixtures and hence the resulting T0 of the system.

Water absorption in EVOH films and its influence on glass transition temperature

Moisture sorption kinetics of nonoriented ethylene vinyl alcohol copolymer (EVOH) film (EF-E15) were studied at 25, 35, and 45°C. Anomalous diffusion was observed for the polymeric film at high relative humidities (RH) and higher temperatures. Diffusion and solubility coefficients of water were found to be concentration dependent. The moisture sorption isotherms of three types of EVOH films (EF-E15, EF-F15, and EF-XL15) determined at 25, 35, and 45°C, were well described using the GAB equation. Glass transition temperatures (Tg) of the EVOH films, as influenced by RH, were measured using differential scanning calorimetry. Tg values decreased with increasing RH due to the plasticization effect of water, and were found to be dependent on ethylene content and orientation of the EVOH films. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 691–699, 1999

Moisture Sorption Characteristics of Freeze Dried Blueberries

ABSTRACTMoisture sorption isotherms of freeze dried lowbush blueberries at 4, 16, 25, 35, and 45°C were determined using indirect gravimetric method. Moisture sorption kinetics at these temperatures and 75% RH were also investigated. The isotherms followed a type III shape with a temperature inversion effect at 0.9 water activity. They were best described by the GAB equation. Net isosteric heat of moisture sorption in freeze dried blueberries, determined using the Clausius Clapeyron equation, varied from 5.5 to 0.25 kJ/mol as moisture content changed from 8 to 36% (dry basis). Moisture adsorption kinetics in freeze dried blueberries were well described by the first order kinetics equation. The rate constant followed the Arrhenius relationship with an activation energy of 38.6 kJ/mol.

Effect of moisture on the properties of Togilen fibre

The sorption properties of Togilen fibre were determined. Desorption of Togilen is similar to desorption of viscose fibres with respect to the equilibrium moisture absorption and sorption kinetics. It was found that the curve of the strength characteristics of Togilen as a function of the humidity decreases monotonically. The maximum decrease in the strength is 50% of the conditioning value. The change in the deformation-strength properties of the fibre in water as a function of the duration of its exposure was investigated. The time of establishment of equilibrium with respect to the strength and elongation was determined. The loss of strength of the wet fibre attained 60–70% and the elongation increased by two times; the properties of the fibre were almost totally restored during drying. A comparison of the properties of textile fibres with elevated sorption properties and Togilen suggested that Togilen is a hydrophilic fibre.