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Wet-strength paper production is of great significance due to its prominent importance in everyday consumer activities. Recently, market demands for paper with specific properties and purposes are increasing. Production of these paper types is very challenging for the manufacturer since they have to meet very specific demands. Wet-strength paper products include bags that come in contact with moisture and wet products, hygienic and kitchen towels, paper towels, packaging products (packages for liquids, aseptic dishes, and a variety of packaging for food), and other special purposes items (tea bags, coffee filters, paper money, etc.) Throughout this research, the focus was on the effects a wetstrength agent has on the tensile strength of paper. The wet strength property of paper is the ratio between the strength of paper in the wet and dry state. The strength in the dry state comes from the hydrogen bonds present in natural cellulose, while the strength of paper in the wet state requires waterproof (covalent) bonds created by the use of a wet strength agent. The wet strength agent used is a cationic PAE resin. The aim of the work is to determine the optimal amount of agent needed in relation to the required value of the tensile strength property of the paper. The investigation of these influences was carried out on Semi Extensible Standard Kraft type of paper in Natron-Hayat. [ABSTRACT FROM AUTHOR]

Abstract Temperature and humidity are the main factors that cause the decomposition of Nomex paper widely used as turn‐to‐turn insulation material of mining dry‐type transformers. In this study, for understanding the effect of ambient humidity and thermal aging on the properties of Nomex paper, specimens with the initial moisture content of 7% were aged in the oven for 168, 504, 672 h at 180°C, respectively. After thermal aging, corona discharge experiments of unaged and aged specimens with different aging degrees were carried out at different relative humidity (RH) levels from 10 to 70%. The characteristic parameters of corona discharge, such as the average discharge amplitude and the number of discharges, were measured. Then, the surface conductivity and the surface potential decay rate were measured. The morphology and attenuated total internal reflectance Fourier transform infrared spectroscopy were also analysed for explaining the difference of properties between unaged and aged specimens. The experiment results indicate that the average discharge amplitude decreases with the increase of RH or the deepening of the aging degree of specimens, but the number of discharges shows the reversal tendency. The changes in the physical structure and chemical composition, which is induced by the hydrolysis reaction of Nomex paper, accelerate the surface potential decay and increase the surface conductivity.

The condition assessment of oil–paper insulation is a vital issue in transformer management. To investigate the effect of moisture content on the accuracy of the prediction model, accelerated ageing experiments were performed with different initial moisture content (IMC) in laboratory condition. Two types of multi-parameter lifetime prediction models, e.g. with and without consideration of IMC, were presented for comparison purpose. The authors found that a better accuracy for its prediction results could be achieved by establishing the prediction models with the consideration of IMC. Besides, models considering IMC have better economic feasibility than that without considering IMC(non-IMC) as its total cost is cheaper according to life cycle cost (LCC) calculations. In addition, failure probability based on the proposed prediction models was also calculated as a direct indicator for evaluating the oil–paper insulation status. [ABSTRACT FROM AUTHOR]

A computationally efficient method to study the in-plane and out-of-plane dimensional instability of thin paper sheets under the influence of moisture changes is presented. The method explicitly resolves the bonded and the free segments of fibers in the sheet, capturing the effect of anisotropic hygroexpansion at the fiber level. The method is verified against a volumetric model. The importance of longitudinal fiber hygroexpansion is demonstrated in spite of the absolute value of longitudinal hygroexpansion being an order of magnitude lower than the transverse hygroexpansion component. Finally, the method is used to demonstrate the formation of macroscopic sheet curl due to a moisture gradient in structurally uniform sheets in the absence of viscoelastic or plastic constitutive behavior and through-thickness residual stress profiles. [ABSTRACT FROM AUTHOR]

The dielectric properties of oil-paper insulation degrade due to moisture ingression and ageing. This degradation significantly impacts the space charge accumulation and charge trapping behaviour in the insulation, which are vital parameters for insulation health under the high-voltage direct current environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of an oil pressboard under the influence of moisture and ageing. The conventional IRC model considers the total relaxation current because of charge de-trapping only. However, in the case of a composite dielectric like oil pressboard, dipolar relaxation also affects the relaxation current. In this work, a methodology has been proposed to investigate charge de-trapping behaviour of oil-pressboard insulation considering the dipole relaxation process from IRC measurements. For this purpose, frequency domain spectroscopy measurements and IRC measurements have been performed on oil-impregnated pressboard specimens carefully prepared in the laboratory having different ageing conditions and moisture contents. Results presented in this work depict that charge trapping parameters, i.e. the trap depth and trapped charge distribution are highly affected both by ageing and moisture. It was observed that ageing leads to the generation of deeper traps, while moisture mainly enhances the density of shallow traps.

Epoxy resin impregnated paper (RIP) bushings are widely used as transformer bushings and wall bushings, but this type of bushing is easy to be dampened. In order to study the effect of moisture on the bushings' insulation performance, a test device was established to simulate the moisture absorption process in the air or N(2) gas. Based on the frequency-domain dielectric spectroscopy (FDS) method, FDS curves that indicate the dielectric loss factor (tan δ) and capacitance (C) under different frequencies were tested and analysed. The results demonstrated that the FDS curve moves to the high frequency direction with the increase of the moisture content, and there exist minus tan δ and a negative peak in low-frequency band. Since the moisture content is much lower than in the air, tan δ and C under power frequency of the bushing almost unchanged in the N(2) gas. However, the tan δ under 0.1 Hz increases sharply with the increase of trace moisture, and the higher moisture content, the greater increase rate. A damp diagnosis method for RIP bushings is proposed based on the FDS curves' frequency translation characteristics and tan δ variation law. Also the results can give references for RIP bushings' insulation diagnosis.

The measurement of water inside a transformer is essential to maintain its reliable operation, as the water content can increase as the transformer ages. This measurement is frequently performed by a utility as a part of its periodic maintenance activities. In this paper, the application of optical fiber sensors as an online method to measure water content of paper inside an operating transformer is presented. There are several advantages of using these sensors, including their size, lightweight, flexibility, multiplexing, no-electromagnetic interference, etc. A unique benefit of fiber optic technology is that sensors can be embedded into the winding structure and operate in areas of high electric field strength. For this work, fiber optic water and temperature sensors are investigated by comparing their measurements to a commercial water activity probe immersed in oil. The study found that the determination of water content by fiber optic sensors compared closely with other techniques, which lead to the conclusion that they can be an effective technology for water measurement in an operational transformer. [ABSTRACT FROM AUTHOR]

The performance of oil-impregnated paper significantly affects the safety and reliability of transformers. Previous studies on water reducing the breakdown strength of oil-impregnated paper have been mainly due to increasing charge carriers and conductivity. However, the influence of water on the cellulose crystal structure of oil-impregnated paper is still not clear; in particular, its role on breakdown strength remains ambiguous. In this study, the AC breakdown strength and crystal structure of vegetable and mineral oil-impregnated paper with different moisture contents were investigated. The results show that with increasing moisture content, the cellulose crystallinity and crystal size increases linearly, while the breakdown strength decreases. The breakdown strength of vegetable oil-impregnated paper is higher than that of mineral oil-impregnated paper and is less affected by moisture. Herein, a model is proposed on the effects of water on the crystal structure of oil-impregnated paper and the correlation of breakdown strength with crystal structure is reported. [ABSTRACT FROM AUTHOR]

Abstract A number of problems associated with dimensional stability of paper products have to do with hygroexpansion in response to changes in humidity or moisture content. The main underlying mechanism of hygroexpansion in paper is the effect of the change of fiber cross-sections transferred through fiber bonds. In fact, the transverse expansion of fibers can be an order of magnitude greater than the longitudinal expansion. Addressing such problems using modeling on the microscale is associated with large computational costs since both the bonds and the fibers need to be resolved. We present a method for modeling the hygro or thermal expansion of interconnected fiber networks modeled with beam elements and connected through beam-to-beam contact. Being a line structure, beams can only support pointwise contact, which poses a challenge for modeling the force transfer induced by the deformation of the cross-sections at the contact point. The idea of implementing the stress transfer is to use a concurrent multiscale approach in which the bond level is resolved in detail using the configuration of the fibers and the computed strains are passed over to the beam elements. We verify and prove the applicability of this approach by comparing it with continuum models. We demonstrate the advantage of using this approach in terms of its tremendous saving in time. The use of beam models for modeling the hygro- or thermal expansion of fiber networks enables considering relevant sizes in the problems involving dimensional stability, in particular those associated with embedded inhomogeneities. We will show the applicability of the model by providing insights into published experimental observations on the hygroexpansion properties of paper products. Finally, we will demonstrate that the use of a 2D model to simulate the inter-fiber bonds in a network, not only leads to underestimation of out-of-plane deformations, but also to overestimation of the contribution of the transverse deformation of fibers to the in-plane dimensional change of the network. [ABSTRACT FROM AUTHOR]

Epoxy resin impregnated paper (RIP) bushings are widely used as transformer bushings and wall bushings, but this type of bushing is easy to be dampened. In order to study the effect of moisture on the bushings' insulation performance, a test device was established to simulate the moisture absorption process in the air or N2 gas. Based on the frequency-domain dielectric spectroscopy (FDS) method, FDS curves that indicate the dielectric loss factor (tan δ) and capacitance (C) under different frequencies were tested and analysed. The results demonstrated that the FDS curve moves to the high frequency direction with the increase of the moisture content, and there exist minus tan δ and a negative peak in low-frequency band. Since the moisture content is much lower than in the air, tan δ and C under power frequency of the bushing almost unchanged in the N2 gas. However, the tan δ under 0.1 Hz increases sharply with the increase of trace moisture, and the higher moisture content, the greater increase rate. A damp diagnosis method for RIP bushings is proposed based on the FDS curves' frequency translation characteristics and tan δ variation law. Also the results can give references for RIP bushings' insulation diagnosis. [ABSTRACT FROM AUTHOR]

Study of alternate fluids for transformer and their combination with traditional ones is an interesting area of research. The intent of this work is to investigate the thermal ageing performance of ester oils (synthetic and natural), and mixed oil vis‐à‐vis mineral oil. All the samples were thermally stressed at 110, 140, 160, and 185°C for 2 weeks each. Dissolved decay contents in oil and linear thermal expansion coefficients of solid insulation have been studied using ultraviolet visible infrared spectroscopy and dilatometry analysis, respectively, as per ASTM standards. Change in functional groups of the insulation oils with ageing have been understood by Fourier transform infrared spectroscopy analysis. Additionally, diffusion of oil–moisture mixture into paper with ageing and the corresponding effect on dielectric strength of paper has been also examined using ASTM standards. It is observed that the use of synthetic ester (SE) and addition of SE to mineral insulating oil lead to improved performance of oil–paper insulation system with retarding the rate of degradation of the insulation system. [ABSTRACT FROM AUTHOR]

Nowadays, reliability of power system is of great importance. Therefore, the condition of transformers as the expensive and critical components of a power system should be monitored and assessed. The dominant factor that causes limitation in transformers lifetime is the paper insulation. This paper aims to investigate the conspicuous influence of moisture content, especially in low oxygen and temperature on lifetime estimation under constant conditions, based on the results of previous experiments. Degree of polymerization (DP) is employed to quantify the effects of detrimental parameters with respect to their time-varying nature. This paper proposes a novel lifetime estimation-approach considering a yearly moisture growth for transformers. The results indicate different trend of DP profiles, which lead to an accurate lifetime estimation in comparison with DP obtained through ideal conditions. Furthermore, oscillations in the hot-spot temperature of transformers are demonstrated to have a negative effect on the lifetime. [ABSTRACT FROM AUTHOR]

This paper studied the impact of moisture on the correlated characteristics of the condenser bushings oil-paper insulation system. The oil-impregnated paper samples underwent accelerated thermal aging at 130 °C after preparation at different initial moisture contents (1%, 3%, 5% and 7%). All the samples were extracted periodically for the measurement of the moisture content, the degree of polymerization (DP) and frequency domain dielectric spectroscopy (FDS). Next, the measurement results of samples were compared to the related research results of transformer oil-paper insulation, offering a theoretical basis of the parameter analysis. The obtained results show that the moisture fluctuation amplitude can reflect the different initial moisture contents of insulating paper and the mass ratio of oil and paper has little impact on the moisture content fluctuation pattern in oil-paper but has a great impact on moisture fluctuation amplitude; reduction of DP presents an accelerating trend with the increase of initial moisture content, and the aging rate of test samples is higher under low moisture content but lower under high moisture content compared to the insulation paper in transformers. Two obvious "deceleration zones" appeared in the dielectric spectrum with the decrease of frequency, and not only does the integral value of dielectric dissipation factor (tan δ) reflect the aging degree, but it reflects the moisture content in solid insulation. These types of research in this paper can be applied to evaluate the condition of humidified insulation and the aging state of solid insulation for condenser bushings. [ABSTRACT FROM AUTHOR]

Significant dimensional variations may occur in paper-based materials when subjected to changes in moisture content. Moisture induced deformations are governed by the swelling of individual fibres, which is transferred through inter-fibre bonds to the entire fibrous network. Complex interactions between mechanical and hygro-expansive properties take place in the bonding areas, affecting the overall material response. In most network models, the role of these inter-fibre bonds is not explicitly incorporated. This work presents a periodic meso-structural model for the discrete fibrous network, which considers the free-standing fibre segments and inter-fibre bonds. Despite its simplicity, the reference unit-cell enables the incorporation of relevant micro- and meso-structural features such as network structure, fibres and bond geometry and hygro-elastic properties. The proposed model is solved analytically through a proper homogenization strategy, allowing to recover the paper’s anisotropic hygro-mechanical response in terms of effective elastic constants and effective hygro-expansive coefficients, exploiting the coupling at the meso-structural level between hygroscopic and mechanical behavior. A comparison with experimental results obtained from the literature shows that the presented approach is quite accurate in predicting the overall paper response, thereby revealing the influence of several meso-scale parameters (e.g. fibre orientation, dimensions, mechanical strength). [ABSTRACT FROM AUTHOR]

To model advanced 3-D forming strategies for paper materials, the effects of environmental conditions on the mechanical behavior must be quantitatively and qualitatively understood. A tensile test method has been created, verified, and implemented to test paper at various moisture content and temperature levels. Testing results for one type of paper for moisture contents from 6.9 to 13.8 percent and temperatures from 23 to 168 degrees Celsius are presented and discussed. Coupled moisture and temperature effects have been discovered for maximum stress. Uncoupled effects have been discovered for elastic modulus, tangent modulus, hardening modulus, strain at break, tensile energy absorption (TEA), and approximate plastic strain. A hyperbolic tangent function is also utilized which captures the entire one-dimensional stress-strain response of paper. The effects of moisture and temperature on the three coefficients in the hyperbolic tangent function may be assumed to be uncoupled, which may simplify the development of moisture- and temperature-dependent constitutive models. All parameters were affected by both moisture and temperature with the exception of TEA, which was found to only be significantly dependent on temperature. [ABSTRACT FROM AUTHOR]

Abstract: In this work we present a mathematical model describing the coupled heat and moisture transport in paper. The model is solved numerically and the numerical solution is used to study the interdependency of the moisture and temperature distribution in paper. The results show that variation with temperature of the saturated water vapor concentration and the sorption isotherm parameters are both important for inducing moisture desorption. It is also found that for steep relative humidity ramps moisture sorption generates temperature increments that slow down the sorption process itself. The model is also used to study the moisture gradients in a paper sheet inside a printer from Océ Technologies, which contains a warm print surface. The results predict changes in moisture content of only 0.2%, which suggests that no deformations are induced on the printed sheet. [Copyright &y& Elsevier]

We present theory for the shrinkage on drying of the cross-sectional area of cellulosic fibres and show that this depends on their initial moisture content only. For the linear shrinkage of cross-sectional dimensions, an additional parameter is required, which we estimate from comparison of the theory with experimental data from the literature. We proceed to combine our model of fibre shrinkage with probabilistic theory for fibre contacts in random fibrous networks to obtain a model for the unrestrained shrinkage of isotropic paper sheets. The expression obtained depends only on the moisture content of fibres and the extent of inter-fibre contact and exhibits good agreement with experimental data for laboratory-formed paper sheets. The results have relevance to the commercial manufacture of paper and potential for application to the study of cellulose nano-composites. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a method to predict the through-thickness moisture content distribution and associated induced deformations of paper and cardboard sheets as they are subjected to relative humidity changes. The transient moisture diffusion problem is solved using a “natural” analytic approach that has previously been applied for solving transient heat conduction in multi-layer solids. The deformation behaviour of the sheet during the moisture diffusion process is predicted using a semi-analytical approach based on a Rayleigh–Ritz minimization of the total potential energy. Geometrically nonlinear effects are taken into account. Curvatures of the originally flat sheet are predicted as a function of time, as are the shapes of the sheet for steady-state condition. As multiple solutions exist, stability is studied. The developed model was used to study the deformation behaviour of one paper and two cardboard sheets. Comparisons with finite-element results demonstrate that the developed model provides accurate results. The displacements obtained for steady-state conditions are within +6%. Comparisons with previous steady-state analyses reveal important differences in the shape of one cardboard sheet. This suggests that the moisture diffusion process may influence the configuration assumed by the sheet at steady-state equilibrium. Hence, it may be necessary to take the moisture diffusion into account in the analysis to accurately predict the hygro-mechanical behaviour of paper or cardboard sheets. [Copyright &y& Elsevier]

Abstract: In this paper a simplified network model for mechano-sorptive creep is presented, which is a further development of an earlier paper [Strömbro, J., Gudmundson, P., 2008. Mechano-sorptive creep under compressive loading – a micromechanical model. International Journal of Solids and Structures 45 (9), 2420–2450.]. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys a constitutive law that is non-linear. Fibre kinks are included in order to capture experimental observations of larger mechano-sorptive effects in compression than in tension. Moisture dependent material parameters and anisotropy in the fibre distribution have been introduced. Theoretical predictions based on the model are compared to experimental results for an anisotropic paper both under tensile and compressive loading at varying moisture content and it is found that the important features in the experiments are captured by the model. Different kinds of drying conditions have also been examined. [Copyright &y& Elsevier]

Abstract: Cockling of paper is a common problem occurring in the production, storage and end-use of paper. It is usually induced by a moisture content change. In many cases, cockling is an irreversible phenomenon; i.e. the initial shape is not obtained although the initial moisture content is restored. This kind of moisture content change occurs in copying machines and in the printing process, for example. In this paper, we present a continuum mechanical model, which is used to study the irreversible cockling of paper. In the model, paper is treated as an orthotropic elasto-plastic material and the model takes into account the small-scale variation of fibre orientation. The model is used to show the importance of the through-thickness moisture gradient on the cockling phenomenon during a cyclic moisture content change. The results suggest that the moisture gradient is a crucial factor for the irreversible cockling. [Copyright &y& Elsevier]

Abstract: The creep of paper is accelerated by moisture cycling, an effect known as mechano-sorptive creep. It has also been observed that the mechano-sorptive effects are larger in compression than in tension. In this paper a simplified network model for mechano-sorptive creep is presented. It is assumed that the anisotropic hygroexpansion of the fibres leads to large stresses at the fibre–fibre bonds when the moisture content changes. The resulting stress state will accelerate creep if the fibre material obeys constitutive laws that are non-linear in stress. Geometrical fibre effects are included in the model in order to capture experimental observations of the differences between paper loaded in tension and compression. Theoretical predictions based on the developed model are compared to experimental results for paper both under tensile and compressive loading at varying moisture content. The important features in the experiments are captured by the model, i.e. the creep is accelerated by the moisture cycling and the mechano-sorptive effects are larger in compression than in tension. [Copyright &y& Elsevier]

This study presents experimental analysis and numerical modeling of hygroscopic moisture buffering by paper and books. First, a literature review of moisture transport properties of paper is presented. Experimental work on two paper types includes SEM analysis of the paper structure, determination of sorption isotherms and water vapor permeability measurements. A hysteretic model for paper is presented, which is based on the measurement of the main adsorption and desorption curves. It is shown that the water vapor permeability in a hysteretic model is dependent on the moisture content and not on the relative humidity. Books consist of several paper sheets with air layers between the sheets. To take the air layers into account, a parallel transport model is proposed to determine the effective moisture transport properties of books taking into account the air layers. The dynamic hygroscopic behavior of small book samples was measured. It is shown that, although the water vapor permeability of different paper types can be quite different, the effusivity of a book highly depends on the presence of the air layers and can therefore remain comparable for different paper types. [ABSTRACT FROM AUTHOR]

The effects of moisture, oxygen, and acidity upon the aging of Kraft and thermally upgraded ("Insuldur®") papers have been studied in detail. The results for Kraft are consistent with the kinetic model and parameters proposed earlier by Emsley et al., and with the principle that moisture promotes acid-hydrolysis by causing carboxylic acids to dissociate. Moisture is released during the aging of Kraft and, hence, its aging is auto-acceleratory. In contrast, Insuldur consumed moisture under the same conditions, aged more slowly, and its aging was less accelerated by added moisture. 2-Furfuraldehyde (2FAL), which is a dehydration product of pentosans and, hence, an index of moisture production, is also released from Kraft during aging, but not from Insuldur. Acids are, however, produced from both types of paper. Theoretical explanations for these findings are proposed, and their practical implications for transformer maintenance are discussed. [ABSTRACT FROM AUTHOR]

The dynamic mechanical responses of Scots pine, paper, PA6, cellophane, PVAc and PUR samples subjected to changes in the relative humidity of the surrounding air from 5 to 85% and vice versa have been analysed semiquantitatively on the basis of coupled non-linear rate equations for the moisture concentration in the sample as a function of the time. Important characteristics of the diffusion of moisture into and out of the samples have been studied by measuring the sample weight as a function of time. Moisture sorption results in sample swelling in Scots pine, paper, PA6, cellophane and PUR, for all of which mechanical loss peaks were detected. For PVAc, which does not bind moisture at load-bearing hydrogen bonding sites, no mechanical loss peak could be found. Characteristic of the mechanical loss at low vibration frequencies (0.01 to 1 Hz) is a peak immediately following a change in relative humidity for all studied sample materials except PVAc. This peak is almost certainly due to modulation of the number of load-bearing hydrogen bonds in the material. The relation between the width of the mechanical loss peak and the duration of the moisture sorption and desorption processes is interpreted according to an accepted model of two water molecule binding modes, one in which load-bearing hydrogen bonds are broken unimolecularly by water molecules and one in which further water molecules form clusters on the already unimolecularly bound water. © 1998 Chapman & Hall [ABSTRACT FROM AUTHOR]