Leveling-off Degree Of Polymerization Research Articles

Decreasing the degree of polymerization of microcrystalline cellulose by mechanical impact and acid hydrolysis

Depolymerization of cellulose is often used as a (pre)treatment protocol within the catalytic valorization strategies of cellulose. Typical depolymerization protocols yield polymerization degrees above 70 anhydroglucose units (AGU). However, shorter cellulose fibers are of interest in the search for accessible dietary fiber additives or renewable materials with distinct mechanical properties (bio-composites). In this work, short-polymer microcrystalline celluloses (SMCC) with an average polymerization degree between 29 and 70 AGU were produced with material yields of 95 % and above by combining a planetary ball mill pretreatment with mild acid hydrolysis. By first decreasing the levelling–off degree of polymerization (LODP) with an intensive ball mill treatment, a mild acid hydrolysis protocol was sufficient to ensure high yields of SMCC. Furthermore, the desired polymerization degree could be obtained by tuning the process parameters.

Experimental and molecular level analysis of natural ester delaying degradation of cellulose insulation polymer

Developing green insulation materials has become an important direction for future research. As a new environmental insulation liquid, natural ester can effectively delay cellulose insulation polymer ageing. Mastering the mechanism of natural ester delaying the cellulose ageing could promote its application in power transformers. In this study, based on the accelerated thermal ageing of pressboard immersed in natural ester and 25# mineral oil at 120°C for 221 days, the thermal ageing rate and the levelling off degree of polymerisation (LODP) for two kinds of oil-impregnated pressboards were compared. The molecular level mechanism of natural ester delaying cellulose degradation and keeping higher LODP value was proposed using molecular dynamics simulation. At the early stage of thermal ageing, cellulose with a higher degree of polymerisation (DP) can form many hydrogen bonds with hydronium ions, and their binding energy is stronger, which can accelerate the acid hydrolysis of cellulose, causing the DP of pressboard immersed in natural ester and mineral oil to decrease rapidly. With the breaking of the cellulose molecular chain, the number of hydrogen bonds and binding energy between cellulose and hydronium ions becomes smaller and the thermal ageing rate decreases gradually. At the end stage of ageing, it is difficult for the hydronium ions to enter the crystalline cellulose, acid hydrolysis of crystalline cellulose is hard to conduct, leading to the occurrence of the LODP phenomenon. The degradation rate of cellulose in natural ester is only 50% of that in mineral oil. The cellulose LODP value in natural ester is over 200 larger than that in mineral oil. Compared with the mineral oil-cellulose system, natural ester can form hydrogen bonds with hydronium ions and lead to fewer hydronium ions in cellulose, fewer hydrogen bonds and smaller binding energy between cellulose and hydronium ions, which can reduce the acid hydrolysis reaction of cellulose. This study provides a theoretical explanation for natural esters to delay the ageing of insulation paper.

Impact of hydrothermal and alkaline treatments of birch kraft pulp on the levelling-off degree of polymerization (LODP) of cellulose microfibrils

The levelling-off degree of polymerization (LODP) is typically considered an apodictic number of each cellulosic source that relates to the length of its cellulose crystallites. In this study, we have performed hydrothermal and alkaline treatments on bleached birch kraft pulp to adjust the xylan content, and the resulting pulps have been subjected to acid hydrolysis to determine their LODP. The results show that hydrothermal treatments promote longitudinal alterations in the supramolecular structure of cellulose, represented by a linear decrease in the LODP with lowering the xylan content in the pulp. Moreover, a decrease in LODP is also observed after alkaline extraction of the birch kraft pulp, but the decrease as a function of xylan content differs from that observed after hydrothermal treatments. The results appear to indicate that the LODP is rather an artifact introduced during processing of the material than a true measure of the length of the crystallites in the native cellulose source.

Aging characterization of electrical insulation papers impregnated with synthetic ester and mineral oil: Correlations between mechanical properties, depolymerization and some chemical markers

This article presents a comparative study of two types of transformer solid insulation: standard Kraft and thermally upgraded Kraft papers. The paper samples were impregnated with two different fluids, namely a synthetic dielectric ester fluid, Midel 7131, and a synthesized mineral oil, Luminol Tri. The impregnated papers were submitted to, accelerated aging at 150°C, temperature higher than the temperatures found inside transformers, for extended periods. The results show that for the first hours of aging, the cellulose depolymerization was similar for all the four studied cases. At a specific point, the paper samples aged in the ester fluid exhibited a lower depolymerization exceeding the 8000 hours of aging without reaching the levelling-off degree of polymerization (LODP). A similar trend was observed through the decrease of mechanical properties of papers in the ester-based oil. However, the difference in performance of both fluids was amplified due to the temperature used; at lower temperatures the difference should be smaller. Additionally, a relationship between the degree of polymerization and the mechanical properties accessed by the tensile testing was obtained regardless of the type of paper or oil. The total acidity in the ester fluid was higher than in mineral oil. Methanol, a chemical marker that is closely linked to the rupture of 1,4-ß-glycosidic bonds of cellulose, showed a partial sensitivity to the cellulose aging in the ester fluid. Furan (C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> H <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> O) content was monitored during the aging and its concentration was found to be sensitive to the depolymerization along with the decrease in the mechanical properties of cellulose papers regardless of the type of paper. More importantly, its concentration was found to be dependent on the type of oil. More investigations are needed to further confirm the importance of furan as an indirect indicator of the mechanical performance along with its correlation with the depolymerization of paper aged in ester fluids.

Acid hydrolysis of cellulosic fibres: Comparison of bleached kraft pulp, dissolving pulps and cotton textile cellulose

The behaviour of different cellulosic fibres during acid hydrolysis has been investigated and the levelling-off degree of polymerisation (LODP) has been determined. The study included a bleached kraft pulp (both never-dried and once-dried) and two dissolving pulps (once-dried). Additionally, cotton cellulose from new cotton sheets and sheets discarded after long-time use was studied. Experimental results from the investigation, together with results found in literature, imply that ultrastructural differences between different fibres affect their susceptibility towards acid hydrolysis. Drying of a bleached kraft pulp was found to enhance the rate of acid hydrolysis and also result in a decrease in LODP. This implies that the susceptibility of cellulosic fibres towards acid hydrolysis is affected by drying-induced stresses in the cellulose chains. In cotton cellulose, it was found that use and laundering gave a substantial loss in the degree of polymerisation (DP), but that the LODP was only marginally affected.

The Investigation on Thermal Aging Characteristics of Oil-Paper Insulation in Bushing

Bushing is the key link to connect outer and inner insulating systems and also the essential electric accessory in electric power system, especially in the high voltage engineering (AC 1000kV, DC 800kV). This paper presented the experimental research of thermal aging characteristic of oil- paper insulation used in bushing. A thermally accelerated aging experiment at 90℃ was performed. The bushing models containing five layers of paper were sealed into the aging vessels and further aged for 250 days. Then several important parameters associated with the aging were observed and evaluated. The results showed that the degree of polymerization (DP) of papers gradually decreased. The DP values of outermost layer and middle layer fit well into the second-order kinematic model and first-order kinematic model, respectively. Less deterioration speed of the inter-layer paper than outer layer was confirmed by the variation of DP. Hydrolysis was considered as the main cause to this phenomenon. In addition, the logarithm of the furfural concentrations in insulation oil was found to have good linear relationship with DP of papers. Interestingly, when the aging time is about 250 days and DP is 419, the aging process reaches an inflection point at which the DP approaches the leveling off degree of polymerization (LODP) value. Both tanδ and acid number of oils increased, while surface and volume resistivity of papers decreased. The obtained results demonstrated that thermal aging and moisture absorbed in papers brought great influence to the degradation of insulating paper, leading to rapid decrease of DP and increase of the tanδ . Thus, the bushing should be avoided from damp and real-time monitoring to the variation of tanδ and DP values of paper is an effective way to evaluate the insulation status of bushing.

What is active cellulose in pyrolysis? An approach based on reactivity of cellulose reducing end

The idea of active cellulose is based on the initial induction period that is observed during thermogravimetric analysis of cellulose. The role of the reducing ends of cellulose samples (Avicel PH-101 and Whatman No. 42) was studied in relation to active cellulose formation, because our previous studies suggested that these end groups have lower thermal stabilities. Elimination of the reducing ends from Avicel PH-101 by reduction with NaBH4 substantially prolonged the induction period. These features were only characteristic of Avicel PH-101, with a leveling-off degree of polymerization (DP) of DP¯n 218. Formation of water-soluble oligosaccharides was also lower (240°C/60min). For Whatman No. 42 cellulose with a higher DP (DP¯n 2170), similar elimination of the reducing ends had almost no effect on the weight-loss behavior. The reducing group analysis data showed that a large number of new reducing ends were formed in Whatman cellulose during DP reduction as the initial pyrolysis process. Based on these differences arising from the initial DP of cellulose, a mechanism for active cellulose was proposed: thermal decomposition of reducing end groups, which are originally present or are formed during pyrolysis in crystalline cellulose, activates the following pyrolysis reactions. The molecular mechanism for activation is also discussed; we propose that it includes hydrogen-bond rearrangement in the initiation of pyrolytic reactions such as depolymerization (transglycosylation) and dehydration, based on our previous proposal that proton donation (acting as an acid catalyst) through hydrogen bonding is a fundamental mechanism for these pyrolytic reactions.

Investigation on thermal aging characteristics of vegetable oil-paper insulation with flowing dry air

In order to investigate the thermal aging characteristics of oil-paper insulation in open-type transformers, a thermally accelerated aging experiment with flowing dry air is performed. The vegetable oil, NOMEX paper, pressboard, copper and silicon steel are sealed into the aging vessels and further aged, for 216 hours, by supply of flowing dry air at 170 °C. Several important parameters associated with the aging are observed and evaluated during the aging process. It is shown that the degree of polymerization (DP) of pressboard gradually decreases and fits well into the second-order kinematic model. Little deterioration of the NOMEX paper under 170 ° is confirmed by the tensile strength which stabilizes at a certain level after aging. In addition, the logarithm of the furfural concentration in oil is found to have a good linear relationship with DP of pressboard. Interestingly, when the aging time is about 150 hours and DP is 329, the aging process reaches an inflection point at which the DP approaches the leveling off degree of polymerization (LODP) value, the moisture content and the furfural content of oil begin to fall down while the acid number and the dissipation factor of oil increase with a faster rate. However, the viscosity of the fluid, one of the important parameters of transformer operation, does not change significantly. The obtained results demonstrate that exposure to air leads to serious oxidation of oil and rapid increase of acid and dissipation factor, and this way oxygen should be avoided for the open-type transformer filled with vegetable oil.

Potential of hot water extraction of birch wood to produce high-purity dissolving pulp after alkaline pulping

The potential of hot water extraction of birch wood to produce highly purified dissolving pulp in a subsequent soda-anthraquinone pulping process was evaluated. After intermediate extraction intensities, pulps with low xylan content (3–5%) and high cellulose yield were successfully produced. Increasing extraction intensity further decreased the xylan content in pulp. However, below a xylan content of 3%, the cellulose yield dramatically decreased. This is believed to be due to cleavage of glycosidic bonds in cellulose during severe hot water extractions, followed by peeling reactions during alkaline pulping. Addition of sodium borohydride as well as increased anthraquinone concentration in the pulping liquor increased the cellulose yield, but had no clear effects on pulp purity and viscosity. The low intrinsic viscosity of pulps produced after severe extraction intensities and soda-anthraquinone pulping corresponded to the viscosity at the leveling-off degree of polymerization, suggesting that nearly all amorphous cellulose had been degraded.

Endo-exo Synergism in Cellulose Hydrolysis Revisited

Synergistic cooperation of different enzymes is a prerequisite for efficient degradation of cellulose. The conventional mechanistic interpretation of the synergism between randomly acting endoglucanases (EGs) and chain end-specific processive cellobiohydrolases (CBHs) is that EG-generated new chain ends on cellulose surface serve as starting points for CBHs. Here we studied the hydrolysis of bacterial cellulose (BC) by CBH TrCel7A and EG TrCel5A from Trichoderma reesei under both single-turnover and "steady state" conditions. Unaccountable by conventional interpretation, the presence of EG increased the rate constant of TrCel7A-catalyzed hydrolysis of BC in steady state. At optimal enzyme/substrate ratios, the "steady state" rate of synergistic hydrolysis became limited by the velocity of processive movement of TrCel7A on BC. A processivity value of 66 ± 7 cellobiose units measured for TrCel7A on (14)C-labeled BC was close to the leveling off degree of polymerization of BC, suggesting that TrCel7A cannot pass through the amorphous regions on BC and stalls. We propose a mechanism of endo-exo synergism whereby the degradation of amorphous regions by EG avoids the stalling of TrCel7A and leads to its accelerated recruitment. Hydrolysis of pretreated wheat straw suggested that this mechanism of synergism is operative also in the degradation of lignocellulose. Although both mechanisms of synergism are used in parallel, the contribution of conventional mechanism is significant only at high enzyme/substrate ratios.

Influence of cellulose polymerization degree and crystallinity on kinetics of cellulose degradation

Cellulose was treated in ethylene glycol with p-toluene sulfonic acid monohydrate as a catalyst at different temperatures. At the highest treatment temperature (150 °C) liquefaction of wood pulp cellulose was achieved and was dependant on cellulose polymerization degree (DP). Furthermore, the rate of amorphous cellulose weight loss was found to increase with cellulose degree of polymerization, while the rate of crystalline cellulose weight loss was reciprocal to the size of the crystallites. The cellulose degradation was studied by monitoring of the molecular mass decrease by size-exclusion chromatography. It was revealed that microcrystalline cellulose degrades via a ‘quantum mode’ mechanism, while the degradation of Whatman filter paper no 1. and cotton linters proceeded randomly and were partly dependent on the starting polymerization degree, crystallinity, and treatment temperature. The kinetics of cellulose degradation in heterogeneous media was described by means of a one-stage model, characterised by the consumption of glycosidic bonds in amorphous and crystalline cellulose regions until the levelling-off degree of polymerization is reached.

The reactivity of prehydrolyzed softwood kraft pulps after prolonged cooking followed by chlorite delignification

In a laboratory study high-quality spruce chips were prehydrolyzed to remove hemicelluloses and then kraft cooked to different kappa numbers by varying the cooking time. Each pulp sample was then chlorite delignified to selectively remove the remaining lignin. The reactivities of the pulp samples before and after chlorite delignification were determined by Fock’s test, which is supposed to measure the pulp’s reactivity in the conventional viscose process. A number of analyses were carried out to determine which parameters affected pulp reactivity, as, for example: intrinsic viscosity, kappa number, pulp yield, carbohydrate composition, levelling-off degree of polymerization (LODP), and alkali solubility. The results of the study showed that the pulp reactivity increased with decreasing kappa number, and the highest reactivity was obtained after total lignin removal using chlorite delignification. It was also found that the carbohydrate composition had no influence on the pulp reactivity, but lower intrinsic viscosity either obtained by prolonged cooking or chlorite delignification correlated with higher pulp reactivity. Finally, lower alkali solubility, i.e. higher R18, reduced the reactivity.

Influence of natural ester on thermal aging characteristics of oil–paper in power transformers

AbstractThe cellulosic insulation used in electrical transformers was aged in mineral oil and natural ester to investigate the influence of oil type on the thermal aging properties of oil–paper. Accelerated aging tests were conducted at three different temperatures and the degree of polymerization (DP) of insulation paper was measured to indicate the extent of aging of the paper. Various aging characteristics in oil, i.e., furfural, CO, CO2, water concentration, and oil acidity, were measured during the aging processes. Kinetic models of cellulose degradation in mineral oil and natural ester were established. The correlations between DP and various aging by‐products were also included in our investigation. The results indicate that paper degraded at a significantly faster rate in mineral oil than in natural ester. A leveling off degree of polymerization (LODP) was observed during the thermal aging process. Once the LODP was reached, the aging rate became very slow. Paper impregnated with natural ester has a higher LODP than paper impregnated with mineral oil, so that a longer time is needed to reach its end‐of‐life (DP ≈ 250). In addition, the content of degradation by‐products of insulation paper aged in natural ester was very different from that aged in conventional transformer oil. Thus special attention must be paid when applying conventional diagnostic criteria to aging of composite insulation impregnated with natural ester. Copyright © 2009 John Wiley &amp; Sons, Ltd.

A comparative study of thermal aging of transformer insulation paper impregnated in natural ester and in mineral oil

AbstractAging of two kinds of transformer insulation paper in natural ester is compared to that in conventional transformer oil (mineral oil). Sealed aging vessels containing copper, insulation paper (Kraft paper and thermally upgraded paper), and dielectric fluid (mineral oil and natural ester) were aged at 90, 110, and 130°C for 399, 371, and 329 days. Under the same aging condition, both the degree of polymerization (DP) and the leveling‐off degree of polymerization (LODP) for natural ester aged paper were higher than the ones for mineral oil aged paper, and the micro surface of natural ester aged paper was destroyed less seriously than that of the mineral oil aged paper. The aging time to reach LODP for natural ester aged paper was only 1/4 to 2/3 of that for mineral oil aged paper. When water content in natural ester for natural ester aged Kraft paper samples reached the maximum, acids concentration in natural ester increased abruptly. Water content in natural ester decreased in the whole aging process for natural ester aged thermally upgraded paper samples. Six physical and chemical mechanisms for the slower aging rate of natural ester aged paper are proposed. Evidence for transesterification between natural ester and paper was presented with infrared spectra instrument. Although the acids in natural ester were far more than in mineral oil, the natural ester can significantly slow the thermal aging of the paper. Three mechanisms of this abnormal phenomenon were presented. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Obtaining microcrystalline cellulose from softwood and hardwood pulp

Conditions for obtaining microcrystalline cellulose (MCC) by the thermocatalytic method from hardwood (birch, aspen) and softwood (pine) bleached sulphate pulp have been developed. After thermocatalytic treatment, cellulose polymerization degree has decreased to the so-called levelling-off degree of polymerization (LODP), which, in the case of birch, aspen and pine wood pulp, made up 450, 370 and 250 units, respectively. After grinding the destructed pulp in a ball mill, MCC powder samples were obtained with particles, the major part of which had sizes of 2-20 mm. in terms of physico-chemical indices investigated in this work, the obtained samples conform to the pharmacopoeia requirements. Dispersing the destructed pulp in water medium, at a sufficiently high cellulose concentration (³ 8%), MCC gel samples were prepared, with rheological properties typical for liquid crystalline polymers. The indices of the obtained hardwood and softwood MCC were compared.

Preparation of chitosan powder and investigation of its properties

For the preparation of chitosan powder, the thermocatalytic destruction method developed in our earlier works was applied. Chitosan samples obtained from exoskeletons of crustaceans with a molecular weight of 500 kDalton, deacetylation degree 80% and with a molecular weight of 900 kDalton, deacetylation degree 70% were investigated. For these samples, optimal thermocatalytic treatment conditions for partial destruction of chitosan, which was controlled from the change in intrinsic viscos ity, to the levelling-off degree of polymerization (LODP) were developed. In our case, LODP was ~ 140 units. The destructed chitosan was dissolved in water and precipitated with adding a definite amount of NaOH. The precipitated chitosan was washed off, dried at 60 °C, and ground in a ball mill. As a result, a cream-like powder was obtained, whose major part of particles had sizes of 1-15 µm. The properties of the obtained powder were investigated by the microscopic, X-ray diffraction, IR spectroscopic, and physico- chemical methods.

Influence of xylan on the degradability of laboratory kraft pulps from hardwood and reed canary grass in acid hydrolysis

The depolymerisation of laboratory-prepared kraft pulps from birch, eucalyptus and reed canary grass during acid hydrolysis was studied. The intention was to study especially the influence of xylan content on the levelling-off degree of polymerisation (LODP) and on the dissolution of carbohydrates during the acid hydrolysis. The xylan content in the pulps was varied by prehydrolysis prior to the kraft pulping or by alkali-extraction of the bleached pulps, and the levelling-off degree of polymerisation was compared with the amount of xylan left in the pulps at LODP. The dimensions of the original fibers in the pulps and of the fiber fragments after hydrolysis were also measured. It was found that the fiber fragments after hydrolysis were longer in the pulps containing a higher amount of xylan. Xylan thus appears to prevent degradation during acid hydrolysis, both on the fiber level and on the individual cellulose fibril level.

The Degree of Disorder in Hardwood Kraft Pulps Studied by Means of LODP

The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed kraft pulp. A model explaining the LODP-results is proposed.

Acid hydrolysis of some industrial pulps: effect of hydrolysis conditions and raw material

Dilute acid hydrolysis of pulps was studied by following the decrease in intrinsic viscosity of preparations of microcrystalline cellulose. The decrease in intrinsic viscosity and the loss of weight during hydrolysis at reaction temperatures of 60 and 80 °C was investigated, using acid concentrations from 0.5 to 4 M and two different acids (HCl and H2SO4). The same levelling-off degree of polymerisation (LODP) was reached under all hydrolysis conditions, but a longer time was needed under milder conditions. An appropriate method of determining the intrinsic viscosity at LODP was established and used in this investigation. The greatest difference in LODP was found between a birch prehydrolysed kraft pulp and a mixed hardwood prehydrolysed kraft pulp; the intrinsic viscosities at LODP being 178 and 72 dm3/kg, respectively. The effect of the intrinsic viscosity of a given starting material was also investigated, but only a small difference (10%) in the LODP was found for pulp samples with very different initial intrinsic viscosities.

Periodic disorder along ramie cellulose microfibrils.

Small angle neutron scattering studies have been carried out on cellulose fibers from ramie and Populus maximowicii (cotton wood). Labile hydrogen atoms were replaced by deuterium atoms, in water-accessible disordered regions of the fibers, to increase the neutron scattering contrast between the disordered and crystalline regions. A meridional Bragg reflection, corresponding to a longitudinal periodicity of 150 nm, was observed when scattering collected from hydrogenated and deuterated dry ramie fibers was subtracted. No Bragg reflection was observed with the cotton wood fibers, probably because of lower orientation of the microfibrils in the cell wall. The ramie fibers were then subjected to electron microscopy, acid hydrolysis, gel permeation chromatography, and viscosity studies. The leveling off degree of polymerization (LODP) of the hydrolyzed samples matched exactly the periodicity observed in the diffraction studies. The weight loss related to the LODP was only about 1.5%, and thus, the microfibrils can be considered to have 4-5 disordered residues every 300 residues.