| 12651. |
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| 12652. |
- Logenius, Louise, et al.
(författare)
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The influence of high temperature on the ageing of cellulose impregnated with metal chlorides under dry and moist conditions.
- 2005
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Ingår i: Proceedings, 13th International Symposium on Wood, Fibre and Pulping Chemistry, 16-19 May, Auckland, New Zealand. - 0958554897 ; , s. 243-249
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Cellulose sheets impregnated with metal chlorides were treated at high temperatures in either a dry or a humid environment. The influence of Mg2+, Ca2+ and Fe3+ on the heat-induced discolouration and on the viscosity of cellulose was investigated. Addition of metal ions to cellulose sheets did not significantly influence the colour of the un-aged sheets. At 120 and 150C there are only small increases in the discolouration during heat-ageing. The samples with iron are somewhat more discoloured than the others. After 3,5 hours at 150C Mg2+ also showed an increased discolouration. When the sheets are aged at 180C for short time no large difference between the cellulose reference and the metal impregnated cellulose can be observed. After longer time there is a significant discolouration caused by Ca2+ and Mg2+. Samples with Ca2+ and Mg2+ are 4 and 10 times, respectively, more discoloured than cellulose reference and cellulose impregnated with Fe3+. Under humid conditions at 180C and after 30 minutes Ca2+ and Mg2+ impregnated cellulose are more discoloured than the cellulose reference and the Fe3+ impregnated cellulose. Moisture more than doubles the effect of temperature. pH has no large influence on the discolouration of the cellulose but acid in combination with Mg2+ increases the discolouration 20 times compared to the un-aged reference. The k-values are related to the amount of chromophore formed during ageing. The largest increase in k-values was observed at 285 nm, which was due to the formation of 5-hydroxymethyl-2-furaldehyde (HMF) from wood hexoses formed during hydrolysis of cellulose. Mg2+ and Ca2+ caused a large decrease in DP and increased formation of HMF, compared to the cellulose reference with subsequent formation of chromophores upon heating.
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| 12653. |
- Logenius, Louise
(författare)
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The influence of high temperature on the discoloration and degradation of carbohydrates and on hydrogen peroxide bleaching of spruce TMP
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When processing wood to obtain pulp, the fibers are subjected to high temperatureat moisture levels and to the influence of metal ions. Many process developmentshave resulted in utilization of higher temperatures that induce chemical reactionsin the wood fibers. This thesis deals with some aspects of using high temperaturesin pulping processes. The first part of this work concerns how carbohydrates areaffected, in terms of discoloration and degradation, by high temperature, moisture,and the presence of metal ions. The next part deals with the metal ion profiles inthermomechanical pulp (TMP) processes and the effects of the high temperaturetreatment of native wood under TMP processing conditions. The third sectionconcerns processing conditions present in the high‐temperature hydrogenperoxide bleaching of TMP.In experiments using cotton linter sheets impregnated with glucuronic acid anddifferent metal ions, increased 5‐(hydroxymethyl)furfural and 2‐furaldehydeconcentrations were accompanied by decreased viscosity and increaseddiscoloration when Mg2+ ions and, to some extent, Ca2+ ions were present. Thisindicates that under certain conditions, in this case, dry heat‐induced ageing underslightly acidic conditions and with access to oxygen, the presence of alkaline earthmetals can cause substantial degradation and discoloration of cellulose. Thisindicates that Mg2+ ions can promote the Lewis‐acid‐catalyzed degradation ofcellulose. When glucuronic acid was added to the cotton linter sheets, the presenceof Ca2+ or Mg2+ ions decreased the discoloration caused by the dry heat‐inducedageing of glucuronic acid and also led to a smaller decrease in viscosity comparedto that in cotton linter sheets containing only Ca2+ or Mg2+ ions. This may explainwhy the negative effect of Ca2+ or Mg2+ ions on viscosity is less pronounced incertain pulps. In mechanical pulping processes, the addition of sodium silicate and kaolin clayand re‐circulation of water from the paper machines increased the levels of ironand aluminum ions. In a mill using dithionite bleaching, the levels of iron andaluminum ions in the final pulp were approximately ten times higher than in a millusing hydrogen peroxide bleaching. This was probably due to extraction of moreiron and aluminum ions from the clay to the water due to low pH and to thecorrosion of process equipment induced by thiosulfate ions derived fromdecomposed dithionite. In laboratory trials, it was impossible to completelyremove iron from these pulps using diethylenetriaminepentaacetic acid orextraction with hydrochloric acid. This was probably due to the strong binding ofiron to the wood fiber or its presence as oxide/hydroxide clusters. The mostimportant issue is to avoid contaminating the pulp with iron ions; process waterand process chemicals with low iron contents should therefore be used. Especiallyin the dithionite bleaching process, iron contamination increases thedecomposition of the dithionite, causing corrosion of the process equipment andadding further iron to the pulp.The shavings method is a straightforward method for studying the formation andelimination of chromophores in wood. Light absorption coefficients, lightscattering coefficients, and brightness can easily be obtained using this method.The method was used for studying how temperature in the absence of mechanicalenergy affects wood brightness. This provided useful information about the effectof process conditions in mechanical pulping and its relationship to chromophoreformation. An advantage of using thin shavings is the rapid penetration ofchemicals into the fibers. The shavings method was therefore also used in studyingthe kinetics of hydrogen peroxide bleaching at high temperature.The high‐temperature (HT) hydrogen peroxide bleaching of a thermomechanicalpulp was carried out in a pressurized laboratory‐scale wing defibrator. Theexperiments demonstrate two ways to obtain high brightness under HT conditions.One way is two‐stage peroxide bleaching with a low total alkali/peroxide ratio inthe first stage to avoid alkaline darkening. The second way is to pre‐impregnatethe pulp with hydrogen peroxide before adding alkali in two stages. Themaximum brightness was reached after only 2.5 minutes at low alkali charge.Thus, the bleaching time can be short and the total alkali charge low under hightemperatureperoxide bleaching conditions. To achieve very high brightness usingHT hydrogen peroxide bleaching, other measures than increasing the alkali chargeand prolonging the bleaching time are necessary.
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| 12654. |
- Logenius, Louise, et al.
(författare)
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The influence of temperature and moisture on the optical properties of cellulose in the presence of metal chlorides and glucuronic acid
- 2008
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Ingår i: Nordic Pulp & Paper Research Journal. - 0283-2631 .- 2000-0669. ; 23:1, s. 72-80
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Tidskriftsartikel (refereegranskat)abstract
- The influence of Mg2+, Ca2+, Fe3+ ions and glucuronic acid on the dry and humid heat-induced (120180C) discoloration of cotton linter sheets was investigated. The quantity of the added metal ions was similar to that of metal ions found in wood and pulp. Heat-induced ageing (120150C, pH 5) under dry conditions caused only slight discoloration of the reference sheets and of the sheets impregnated with the metal ions. However, during ageing at 180C, the presence of Mg2+ ions accelerated the discoloration. An initial pH in the range of 38 had no great influence on the discoloration of the reference sheets, but a low pH in combination with Mg2+ ions increased the discoloration drastically. Under humid ageing conditions at 180C (<30 min), the sheets discolored approximately twice as much as under dry conditions; the difference in discoloration caused by the metal ions was less than in dry ageing. Adding glucuronic acid to the sheets caused extensive discoloration under dry ageing conditions (120180°C), while Ca2+ and Mg2+ ions decreased the glucuronic acid induced discoloration. Under humid conditions (180°C), however, Mg2+ and Ca2+ ions did not prevent the discoloration caused by glucuronic acid. An increase in the absorption coefficient was observed at 285 nm, most likely due to the formation of 5-hydroxy-2-methyl-furaldehyde (HMF) and 2-furaldehyde (furfural). GC-MS analysis reveals increased formation of HMF in the sheets impregnated with Mg2+ ions. Degradation of the cellulose in which the alkaline earth metals act as Lewis acids is proposed.
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| 12655. |
- Logenius, Louise, et al.
(författare)
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The influence of temperature and moisture on the physical properties of cellulose in the presence of metal chlorides and glucuronic acid
- 2008
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Ingår i: Nordic Pulp & Paper Research Journal. - 0283-2631 .- 2000-0669. ; 23:1, s. 81-86
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the influence of Mg2+, Ca2+, and Fe3+ ions and glucuronic acid on the viscosity, after dry or humid ageing, of two different cellulosic materials, cotton linters and dissolving pulp. The quantity of added metal ions was similar to that found in pulp. In the cotton linters sheets, especially in those containing Mg2+, increased 5-hydroxy-2-methyl furfural (HMF) and 2-furaldehyde (furfural) concentrations were accompanied by decreased viscosity under dry conditions at 180C (pH 5). This indicates that Mg2+ ions can promote the Lewis-acidcatalyzed degradation of cellulose. For cotton linters and dissolving pulp sheets, adding Ca2+ and Mg2+ ions is detrimental when ageing at 180C under dry conditions (pH 5). Adding glucuronic acid increases the viscosity of cotton linters sheets, especially when Ca2+ and Mg2+ ions are present. Due to formation of complexes between the Ca2+ and Mg2+ ions and glucuronic acid, these ions reduce pulp viscosity to a smaller degree.
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| 12656. |
- Loginov, Y. N., et al.
(författare)
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Compression deformation and fracture behavior of additively manufactured Ti–6Al–4V cellular structures
- 2022
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Ingår i: International Journal of Lightweight Materials and Manufacture. - : Elsevier BV. - 2588-8404. ; 5:1, s. 126-135
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Tidskriftsartikel (refereegranskat)abstract
- Corresponding research was carried out to assess if the porous structures with modified diamond-shaped lattice cells can provide better integrity of the constructions in the case of overloading. The aim of the study is designing the structures with high porosity for the biomedical applications (implants) having good load bearing capacity. Studied lattice structures are based on the modified tetrahedral beam-based cells with spherical reinforcements at the beam joints and variable beam diameter. Samples with a porosity of 50–80% were studied in present research. Structures were additively manufactured from a titanium alloy Ti–6Al–4V using SLM. Sample compression tests were carried out according to the ISO 13314 standard. Loading experiments were carried out and critical parameters extracted from the stress-strain curves. Finite element modeling was carried out for the analysis of the stress and assessment of the potential failure mechanisms. Corresponding hypothesis explaining the appearance of shear bands in porous structures under compression is formulated. Obtained results show that when the sample porosity rises from 50% to 80%, corresponding plateau stress decreases by 13 times, first maximum compressive strength decreases by 12 times, and compression offset stress decreases by 12 times, while the plateau end does not change significantly. The experiments revealed the barrel distortion of the samples geometry, which corresponds to the general knowledge how the friction between the solid compressing surfaces (anvils of the compression testing machine) and the lattice affects the sample deformation. Compression experiments also revealed the formation of shear bands during sample deformation. The stochastic nature of their development suggests that the main reason of shear bands appearing is the initial inhomogeneity of the boundary conditions of the experiment. Suggested modifications of the basic cells show a good potential for achieving regular beam-based lattice structures with high porosity and increased load bearing capacity. More experiments are needed for statistical analysis, and improvements of the loading experiments methodology for better failure mode analysis are planned for the future.
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| 12657. |
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| 12658. |
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| 12659. |
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| 12660. |
- Lohmander, Peter, et al.
(författare)
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Optimal forest management based on growth data from the Iranian Caspian Forest
- 2016
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Konferensbidrag (refereegranskat)abstract
- Forest management in the Iranian Caspian Forest is optimized via harvesting decisions based on individual trees. The theory of optimal control in discrete time, stochastic dynamic programming, is applied. The optimization program utilizes new basal area growth and volume functions for beech, estimated via data from the Iranian Caspian forest.Recent information about the degree of price variation is used in the optimizations. The optimal control function is defined as a reservation price function, which is determined via backward recursion. The optimal reservation price function and the optimal expected present value function are determined for alternative levels of interest rate and risk. Furthermore, the optimal harvest year and optimal harvest diameter frequency distributions are determined for different degrees of risk and rate of interest.
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