Changes in mechanical properties of impregnated Nomex papers 410 and 910 during accelerated aging

New materials are currently being developed for applications in transformer design. With the useful life of transformers now determined by solid insulation conditions, a better understanding of aging kinetics is important in order to improve electrical system management and planning from the technical and economic points of view. This paper summarizes an investigation of the effects of impregnating aramid and cellulose/aramid papers (Nomex 410 and 910) with insulation fluids (Nynas Polaris and Luminol Tri) at thermally accelerated conditions (170 °C) on their mechanical properties.It was found that Nomex 410 (100% aramid) showed only a small change in tensile strength (~5% decrease) after accelerated aging (around 7500 h). However, its elongation capacity was significantly reduced (~45–70% decrease for dry and wet Nomex 410, respectively) by the end of the aging process, probably due to hydrolysis. In addition, the interaction between water and aramid hydrogen bonds at high temperatures produced the rupture and then, the randomly rebuilt of these bonds in meta-aramid fibres, thereby reducing its plastic deformation capacity.In the case of Nomex 910 (aramid enhanced cellulose), its mechanical properties were maintained for a longer time than those of thermally upgraded Kraft paper (TUK), as measured by the retained percentage of tensile index. However, when the tensile index was used instead of the retained percentage, TUK showed a higher tensile index value than Nomex 910 during the initial stage, while the values for both papers became similar during the second stage. It is only at the end of the aging process that Nomex 910 presented an advantage over TUK paper due to the addition of the aramid fibres.It was also found that the inception fractures in Nomex 910, as a fibrous layered composite paper, start in the weakest part of the composite, probably in the central cellulose layer. The fracture line follows the weakest path, avoiding the aramid fibres.The results presented in this paper can be used as a benchmark for improving our understanding of aging and changes in the mechanical properties of these relatively new materials used in the solid insulation of power transformers. A better understanding of the aging characteristics (thermal degradation) of aramid-based papers should help better assess the condition of the new generation of power transformer fleets.     

Electrical–thermal aging characteristic research of polymer materials by infrared spectroscopy,

Traditional insulation paper (pressboard) used in power transformers has weaknesses such as poor thermostability, low breakdown voltage, and high permittivity, which leads to its degradation or even breakdown over time. For this paper, in order to judge if they could be used as new insulation materials for transformers, polycarbonate and polyester films were selected for electrical–thermal aging tests in temperatures of 110°C and 130°C in comparison with the insulation paper. Several infrared spectral tests were carried out on the three materials under a scanning electron microscope to analyze their electrical–thermal aging characteristics, mechanical behaviors, and degrees of polymerization. The functional group whose absorption peak intensity decreased drastically with aging was referred to as the characteristic functional group, and its peak absorption intensity was used to reflect its aging level. This paper found that the polycarbonate had a better aging resistance than both the insulation paper and PET. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal lifetime of cellulose insulation material evaluated by an activation energy based method

A rapid method, based on a logarithmic degradation model of insulation material, is proposed to reduce the test duration in lifetime assessment of cellulose paper insulating materials. This method proposes the determination of the activation energy from a non-isothermal measurement made by differential scanning calorimetry or another thermal analysis technique and an aging test at a single elevated temperature. The use of the onset temperature of the exothermal peak at ca. 300 °C is proposed for evaluation of the activation energy of degradation. For comparison, the thermal aging of Kraft cellulose paper for power transformer insulation was performed according to the general standard IEC 60216-1/2001 at three different temperatures: 155, 135 and 115 °C, and subsequently, the lifetimes at different service temperatures were estimated. The experimental data proved to have good agreement between the applied methods, the differences being <10 % in terms of the estimated lifetime across the range of service temperatures. The novel proposed method is effective in terms of both energy and manpower costs as compared to the current method: a factor of around 10 in the case of reducing the aging time, a factor of 3 for the time needed for measurements, and a factor of 10 for the reduction of power intake.     

GC Methods for the Determination of Methanol and Ethanol in Insulating Mineral Oils as Markers of Cellulose Degradation in Power Transformers

Methanol and ethanol in transformer oils have been recently proposed as new markers of thermal and mechanical degradation of cellulose (the solid insulation in power transformers). In this work, we optimized and compared the performance of two headspace gas chromatographic methods based on flame ionization (HS–GC–FID) and mass spectrometry detection (HS–GC–MS) to determine methanol and ethanol in insulating mineral oil. For methanol and ethanol, the detection limits were 12 and 27 μg kg^?1 (HS–GC–FID) and 1.3 and 3.1 μg kg^?1 (HS–GC–MS). Repeatability was evaluated in transformer oils for both the methods at different concentration levels of analytes and RSD values were found to lie between 1.8 and 16 %. The accuracy of the methods was assessed under a proficiency test (Cigré JWG A2/D1.46). The methods were compared by a F-test and a one-sided paired t test performed on 21 transformer oils in service. Correlations of methanol and ethanol content in sampled oils against their actual time of service are provided. For each sample, the content of traditional markers (furan-2-carbaldehyde and CO₂) was also measured, finding a correlation between light alcohols and CO₂ content. This indicates that methanol and ethanol determination may be helpful in providing further information on the thermal degradation conditions of transformers’ solid insulation. The method developed is currently routinely applied by the laboratories of Sea Marconi Technologies for the assessment of transformers’ conditions.     

Comparison of the degradation behavior of cotton, linen, and kozo papers

The long term degradation behavior of cotton, linen, and kozo papers was studied to compare changes to the chemical and physical properties with time. The elemental composition, α-content, β-content, γ-content, and lignin content (K number) of the three unaged controls were determined. The papers were then degraded at 90 °C and 50 % relative humidity for several thousand hours. Changes to the pH, carbonyl content, yellowness index (YI), moisture content, molecular weight, and tensile strength with aging were monitored. The general trends in degradation behavior of linen and kozo papers were similar to cotton in that all three showed decreases in pH, molecular weight, and strength as well as increases in carbonyl content and YI during hydrolysis. However, the kinetics of degradation differed between the three papers. The cellulose component of all three papers dominated measured changes to the molecular weight while the presence of hemicellulose in the linen and kozo papers led to unique measured moisture contents, carbonyl group, and YI values relative to cotton after the same amount of degradation had occurred.     

A novel life management model consists of chemical aging model and electrical-thermal aging model for power transformers using a new activation energy calculation method

Power transformers aging is investigated by a chemical aging model and an electrical-thermal aging model. In the chemical aging model, oil and cellulose chemical status are considered, but transformer load is not considered, while in the electrical-thermal model, only the effects of load and hotspot temperature are considered. The primary purpose of this paper is to combine both aging models to achieve a transformer life management model that considers the effect of all chemical parameters and the transformer load current simultaneously. Combining one chemical and one electrical-thermal aging model to reach a life management model for the transformer means that the remaining life of the transformer is first estimated using the chemical aging model by the provided equations for pre-exponential factor and activation energy in this paper. For this estimation, after conducting scientific studies, including experimental studies and computation on measurement results, an empirical mathematical equation will be presented to calculate the activation energy. Then using the presented equation, the remaining life will be estimated more accurately. Then, in the next step, this estimated life will be used in the electrical-thermal aging model and finally, a hotspot temperature will be calculated for the transformer. Finally, using the proposed hotspot temperature value and presented equations in this paper, a load current will be determined, which is recommended that the transformer load should not exceed that value. Finally, the proposed equation for activation energy calculation and the presented life management model validation will be verified using some transformers measurement results.

Graphical abstract

     

Functional Properties and Molecular Degradation of Schizostachyum Brachycladum Bamboo Cellulose Nanofibre in PLA-Chitosan Bionanocomposites

The degradation and mechanical properties of potential polymeric materials used for green manufacturing are significant determinants. In this study, cellulose nanofibre was prepared from Schizostachyum brachycladum bamboo and used as reinforcement in the PLA/chitosan matrix using melt extrusion and compression moulding method. The cellulose nanofibre(CNF) was isolated using supercritical carbon dioxide and high-pressure homogenisation. The isolated CNF was characterised with transmission electron microscopy (TEM), FT-IR, zeta potential and particle size analysis. The mechanical, physical, and degradation properties of the resulting biocomposite were studied with moisture content, density, thickness swelling, tensile, flexural, scanning electron microscopy, thermogravimetry, and biodegradability analysis. The TEM, FT-IR, and particle size results showed successful isolation of cellulose nanofibre using this method. The result showed that the physical, mechanical, and degradation properties of PLA/chitosan/CNF biocomposite were significantly enhanced with cellulose nanofibre. The density, thickness swelling, and moisture content increased with the addition of CNF. Also, tensile strength and modulus; flexural strength and modulus increased; while the elongation reduced. The carbon residue from the thermal degradation and the glass transition temperature of the PLA/chitosan/CNF biocomposite was observed to increase with the addition of CNF. The result showed that the biocomposite has potential for green and sustainable industrial application.     

Molecular weight parameters of cellulose nitrates modified with alcohols

We have studied molecular weight parameters of the products prepared via chemical modification of cellulose nitrates with aliphatic alcohols. Proceeding from the quantum-chemical simulation data the most probable pathways of degradation of cellulose nitrate macromolecules have been elucidated.     

Polymer matrix influence on stability of wood polymer composites

The aim of the presented work is to show the influence of the various polymer matrices and the different amounts of the cellulose filler on the composites properties. Samples based on polypropylene, polystyrene, polyoxymethylene, acrylonitrile butadiene styrene, polyester resin, and polylactic acid with different contents of cellulose fibers were prepared by injection molding process. The mechanical and dielectric properties of these composites were studied in order to check whether investigated wood polymer composites fulfill requirements for their application in electrical devices. For all tested composites, a linear increase of modulus with cellulose content was observed. Addition of cellulose to the tested polymers significantly reduces strain at break. In the case of polypropylene and polyoxymethylene composites, the tensile strength increases with the content of the filler. For other materials, there is an inverse relationship, namely the addition of cellulose decreases the tensile strength. The electrical strength decrease was observed with increased cellulose content for the majority of the investigated composites. Polar groups incorporated by cellulose fibers have led to dielectric constant increase. Furthermore, aging of composites in mineral oil and evaluation of water uptake for wood–plastic samples were performed. Wood polymer composites have changed significantly after aging. The water diffusion coefficients were determined, and the significant influence of the amount of cellulose on the water absorption was shown. Copyright © 2015 John Wiley & Sons, Ltd.     

Porcine Pancreas Lipase Catalyzed Ring‐Opening Polymerization of ϵ‐Caprolactone

Abstract

Extensive studies on lipase catalyzed polymerization of ?‐caprolactone showed that porcine pancreas lipase (PPL) gave good conversions and molecular weights of the order of 11,000. Various attempts were also made to prepare higher molecular weight polycaprolactone esters and to increase the molecular weights of polycaprolactone esters by further polymerizing it in the presence of other potent bifunctional monomers. Blends of enzymatically prepared polycaprolactone ester with polystyrene and cellulose acetate yielded very good films, which were characterized in terms of tensile strength, elongation, and optical properties.     

Identification of a chemical indicator of the rupture of 1,4-β-glycosidic bonds of cellulose in an oil-impregnated insulating paper system

In this study, headspace gas chromatography/mass spectrometry has been used to assess the volatile by-products generated by the ageing of oil-impregnated paper insulation of power transformers. Sealed-glass ampoules were used to age under oxidative conditions 0.5-g specimens of insulating paper in 9 mL of inhibited mineral oil in a temperature range of 60–120 °C and moisture of 0.5, 1 and 2% (w/w). A linear relationship between one of the oil-soluble degradation by-products, i.e. methanol, and the number of ruptured 1,4-β-glycosidic bonds of cellulose, regardless of the type of paper (ordinary Kraft or thermally-upgraded (TU) Kraft paper), was established for the first time in this field. Ageing at 130 °C of model compounds of the Kraft paper constituents (α-cellulose, hemicellulose and lignin) and two cellulosic breakdown by-products (D-(+)-glucose and 1,6-anhydro-β-d-glucopyranose) confirmed that the α-cellulose degradation was mostly responsible for the presence of this molecule in the system. Furthermore, additional 130 °C-tests with six different papers and pressboard samples under a tight control of initial moisture indicated that at least one molecule of methanol is formed for each rupture of 1,4-β-glucosidic bond of the molecular chains. Stability tests showed that the ageing indicator is stable under the oxygen and temperature conditions of open-breathing transformers. The presence of methanol was detected in 94% of oil samples collected from over than 900 in-service pieces of equipment, confirming the potential for this application. Lastly, the tests have shown that oil-oxidation by-products and TU-nitrogenous agents modify the methanol partitioning coefficients in the paper/oil/air system, which makes their study essential over a range of field conditions encountered by power transformers. Results are presented and discussed in comparison with 2-furfuraldehyde, which is the current reference in the domain.     

Iron gall ink-induced corrosion of cellulose: aging,degradation and stabilization. Part 2: application on historic sample material

Degradation of cellulose in historic paper by iron gall ink is a synergistic process of both, acid hydrolysis caused by acidic ink ingredients and oxidation catalyzed by free iron and/or copper ions. The interplay of both reactions was studied according to the CCOA method on historic paper samples. Only minute amounts (few mg) of the samples were required to obtain profiles of naturally present and oxidatively introduced carbonyl groups, which was done by group-selective fluorescence labeling in combination with determination of the molecular weight distribution by GPC-MALLS. In the present study naturally occurring degradation pathways in historic sample papers have been investigated. Different extents of oxidatitive degradation were shown for paper with and without ink. A typical pattern of the molecular weight distribution in naturally aged papers was identified, the peculiar feature being a distinctive shoulder in the region of low molecular weight, roughly between 25,000 and 5,000 g/mol corresponding to a DP between 150 and 30. This pattern was a typical attribute of degraded natural samples: any artificial aging procedures aimed at modeling natural aging processes must thus attempt to reproduce this feature. Although the historic samples had been more severely oxidized than model papers, the inhibition of further oxidation and hydrolysis by the calcium phytate/hydrogen carbonate treatment was evident and could be proven for the first time on the molecular level. Also on plain paper without ink application the oxidation was suppressed and the molecular weight was stabilized on a high level.     

Preparation and Characterization of Optically Active Polyurethane from Rotatory Binaphthol Monomer and Polyurethane Prepolymer

Optically active polymers are promising multifunctional materials with great application potentials. Herein, environmentally friendly optically active polyurethanes (OPUs) were obtained by introducing rotatory binaphthol monomer to polyurethane. The influence of binaphthol monomer content on the structure, mechanical properties, infrared emissivity, and thermal insulation of OPUs was studied intensively. Structure characterization indicated that the optically active polyurethanes have been successfully synthesized. The OPU synthesized with BIMOL and BDO at the mole ratio of 1:1 presented better thermal resistance. In addition, OPUs showed enhanced tensile strength and stretchability with the increase of BINOL content to a certain extent due to its rigid structural features and high molecular weight. The optically active polyurethanes showed lower infrared emissivity values (8–14 μm) than waterborne polyurethane (WPU), and the infrared emissivity decreased from 0.850 to 0.572 as the content of the BINOL monomer increased. Moreover, OPU4 exhibited the best heat insulation and cooling ability with about a 7 °C temperature difference. The thus-synthesized optically active polyurethanes provide an effective solution for developing highly effective thermal insulation materials.     

Simultaneous determination of furfural and its degradation products,furoic acid and maleic acid,in transformer oil by the reversed‐phase vortex‐assisted liquid–liquid microextraction followed by high‐performance liquid chromatography

To explore why the use of furfural as a transformer oil‐paper insulation aging characteristic is problematic in real world application, we developed a method for the simultaneous determination of furfural, furoic acid, and maleic acid in transformer oil by reversed‐phase vortex‐assisted liquid–liquid microextraction combined with high‐performance liquid chromatography. The conditions for the proposed method were optimized, and the obtained extract can be directly analyzed by high‐performance liquid chromatography. The detection limits (signal‐to‐noise ratio = 3) of the method ranged from 1.0 to 4.6 μg/L, the enrichment factors for furfural, furoic acid, maleic acid, and fumaric acid were 4.6, 25.1, 15.6, and 17.5, respectively, and the recovery rates for three analytes (fumaric acid was undetected) range from 82.1 to 106.2%. The contents of furfural, furoic acid, and maleic acid resulted from accelerated aging of transformer insulation oil‐paper were measured using the present method for the first time, and the aging samples were analyzed by liquid chromatography with mass spectrometry for the identification of furoic acid and maleic acid in the aging transformer oil samples. Using the optimal method, the target products of samples at different aging time were tracked and measured.     

Studies into the Early Degradation Stages of Cellulose by Different Iron Gall Ink Components

Selective fluorescence labelling of oxidized cellulose functionalities followed by GPC-MALLS was used to get a deeper insight into ink-induced degradation processes. As the method is very sensitive towards oxidation and molecular weight changes, slight variations at the very beginning of aging processes, e.g. during ink corrosion of cellulose, can be studied. Five different ink modifications were applied on model papers and underwent mild accelerated aging at 55 °C and cycling humidity (7 days) followed by a short period of static humid aging at 80 °C (2 days). Pure ink constituents like tannic acid or iron sulphate do not result in the same degree of oxidation or chain scission as complete inks. Balanced ink degrades paper more than single compounds, but less than unbalanced inks. Interestingly, some degradation occurs already during or shortly after the application process of unbalanced inks on paper. It could be demonstrated that this oxidation proceeded in a rather high Mw area, while the subsequent aging steps affected predominantly regions of shorter cellulose chains.     

Introduction to pyrolysis of biomass

The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicelluloses and lignin and their minor components including extractives and inorganic materials. Pyrolysis of these materials proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products.Pyrolysis cellulose at lower temperatures below 300° C involves reduction in molecular weight, evolution of water, carbon dioxide and carbon monoxide and formation of char. On heating at higher temperature 300–500° C, the molecule is rapidly depolymerized to anhydroglucose units that further reacts to provide a XXXX pyrolyXXXXA: still higher temperatures, the anhydrosugar compounds undergo fussion, dehydration disproportionation and decarboxylation reaction to provide a mixture of low molecular weight gaseous and volatile produces. The composition of these produces and mechanism and kinetics of their production are reported.     

Is cellulose degradation due to β-elimination processes a threat in mass deacidification of library books?

As papers become acidic and brittle over time, libraries apply mass deacidification processes to their collections in order to neutralize acids and deposit an alkaline reserve in the paper. Books commonly treated by mass deacidification have undergone natural aging of up to 150?years. The risk of alkali-induced degradation of cellulosic material upon mass deacidification remains uncertain. In the present study, the extent of β-elimination-type degradation reactions was investigated by comparing deacidified and non-deacidified counterparts, using deacidified library materials and identical issues of non-deacidified books from second-hand book shops. The study dealt with only naturally-aged papers focusing on investigation of immediate effects of mass deacidification rather than a long-term impact. Gel permeation chromatography coupled with carbonyl group labeling gave insight into cellulose chain cleavage as well as into the behavior of oxidized functionalities. Processes occurring under natural aging conditions were compared to those upon artificial oxidation of model pulps. Library books did not show a significant reduction in molecular weight after mass deacidification compared to the non-deacidified controls, which stands in contrast to oxidized model pulps. The models showed a more pronounced loss of molecular weight upon deacidification treatments. A decrease in carbonyl groups other than reducing ends was found to occur. Thus, oxidized functionalities were found to be reactive in mass-deacidification reactions; the different behavior was traced down to particular regions of oxidative damage along the cellulose chains. In general, β-elimination processes did not pose a large risk factor upon mass deacidification treatments of the naturally-aged library material tested.     

Accelerated and Natural Aging of Cellulose-Based Paper: Py-GC/MS Method

Samples of papers artificially (2 to 60 days) and naturally (10, 45, and 56 years) aged were studied by the Py-GC/MS method to identify decomposition products. Possible reaction scenarios for cellulose degradation were developed. One of the degradation products is acetic acid, which can (auto)catalyze the cleavage of cellulose β(1→4)-glycosidic bonds of cellulose polymer chains. However, during 20 s of Py-GC/MS analysis, temperatures of up to 300 °C did not significantly increase or modify the formation of decomposition products of paper components. At 300 °C, the amount of several cellulose decomposition products increased regularly depending on the number of days of artificial aging and natural aging, demonstrated mainly by the generation of 2-furancarboxaldehyde, 5-hydroxymethylfurfural, and levoglucosan and its consecutive dehydration products. No correlation between the amount of lignin decomposition products and the time of aging was found when the pyrolysis was performed at 300 °C and 500 °C. Compounds present in the products of decomposition at 500 °C bear the imprint of the chemical composition of the sampled paper. Pyrograms taken at 300 °C using the Py-GC/MS method can give additional information on the changes in the chemical structure of paper during natural or artificial aging, mainly about the cleavage of β(1→4)-glycosidic bonds during aging.     

Deterioration of ancient cellulose paper,Hanji: evaluation of paper permanence

Hanji paper, the paper material traditionally used in Korea, is in the focus of the present aging and mechanistic study. As raw materials and historic recipes for paper making are still available for Hanji today, specimen resembling historical material at the point of production can be prepared. While from that starting point, historical material had taken the path of natural aging, newly prepared samples—prepared according to both historic and current recipes—were artificially aged, and both aging modes can be compared. For the first time, an in-depth chemical and mathematical analysis of the aging processes for Hanji is presented. The aging of Hanji paper, resulting in hydrolysis and oxidation processes, was addressed by means of selective fluorescene labeling of oxidized groups in combination with gel permeation chromatography, providing profiles of carbonyl and carboxyl groups relative to the molar mass distribution. Starting Hanji showed the highest molecular weight (>1,400 kDa) ever reported for paper. We have defined two critical parameters for comparison of the paper samples: half-life DP (the time until every chain is split once on average) and life expectancy (the time until an average DP of failure is reached and no further mechanical stress can be tolerated). The two values were determined to be approximately 500 and 4,000 years, respectively, for the Hanji samples, provided there is no UV radiation. The rate of cellulose chain scission under accelerated aging (80 °C, RH 65 %), was about 600 times faster than under natural conditions. In addition, cellulose degradation of Hanji paper under accelerated aging condition was about 2–3 times slower than that of historical rag paper as those used in medieval Europe.     

Characterization and pyrolysis behaviour of different paper mill waste materials

Paper industry generates a considerable amount of wastes. Their composition mainly depends on the type of paper produced and the origin of cellulose fibres. Nowadays, in Spain, 40% of solid wastes generated by the paper and pulp industry are deposited directly in landfill, 25% are used in the agriculture, 13% in the ceramic industry and 7% in the concrete production. In the last years, thermal treatment methods like combustion, pyrolysis and gasification have been widely study as alternative techniques for the valorization of different organic waste materials. The main objective of the present work is to study the pyrolysis behaviour of different paper mill waste materials. For this reason, a wide characterization of eight paper mill waste materials from different origins was performed using SEM, FTIR, DRX and thermogravimetric techniques. Paper mill sludges from recycled paper, mainly wastes obtained from deinking process, showed high CaCO₃ and clays contents. Compared with the elevated total organic matter content (TOM) of paper mill waste materials their low organic carbon content determined by Cr₂O₇²⁻ oxidation reveals the elevated chemical stability of organic matter, due to high content on cellulose fibres. Analysis of samples by SEM indicates that successive recycled processes of paper leads to paper mill waste materials with more degraded fibres. XRD analyses show as crystalline cellulose was present in reject and primary sludge from paper mills that produced paper from virgin wood. However, crystalline cellulose content significantly decreased in waste materials from recycled paper. Finally, thermogravimetric analysis indicates that presence or mineral matter and degradation of cellulose significantly influences their pyrolysis behaviour. In general, weight loss of paper mill waste materials started at lower temperatures than pure cellulose. In waste materials from recycled paper weight loss continues at temperatures highest than 500 °C due to kaolinite dehydration and carbonates decomposition.