| 1. |
- Nordgren, Niklas, 1975-
(författare)
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Probing Interactions between Bio-Fibre Components
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- I den här avhandlingen har atomkrafts mikroskopi (AFM) och kvartskristall mikrovåg med dissipation (QCM-D) används för att undersöka interaktioner i bio-fiber modell system av cellulosa och xyloglukan (XG); en naturligt förekommande polysackarid viktig för tillväxt av träd och andra växter. Dessutom har det avgörande momentet av friktionskrafts-kalibrering studerats för att erhålla noggranna och kvantitativa resultat med kolloidal prob AFM. Fyra olika cellulosa modell ytor med varierande morfologi har utvärderats med kolloidal prob AFM. De normala ytkrafterna för alla undersökta substrat var kvalitativt likadana. Däremot, för en given ytråhet leder adsorption av XG till en konsekvent reduktion av friktions-koefficienterna för alla ytor, vilket antyder att effekten på friktion av antingen ytråhet eller kemi kan separeras. För att kunna undersöka interaktionen mellan enbart cellulosa och xyloglukan ändmodifierades först XG enzymatiskt med en tiol grupp. Dessa makromolekyler ympades sedan till guld, där de bildade ett tiol-bundet borst-liknande lager. QCM-D experiment bekräftade att det ympade lagret var biologiskt tillgängligt för enzymatisk nedbrytning genom användning av växtenzymet endo-xyloglukanas TmNXG1. Friktions-koefficienterna för den orörda ympningen och den enzymexponerade visade samma trend med ytråheten som i fallet med cellulosaytorna. Adhesions mätningar på denna modellyta visade att den ursprungliga specificiteten mellan xyloglukan och cellulosa bevarades. Ett antal AFM kantilevrar har kalibrerats med olika tekniker för att erhålla både normal- och friktionsfjäder-konstanter med mål att utvärdera den relativa noggrannheten mellan de olika metoderna. Överensstämmelsen var god mellan teknikerna för att bestämma fjäder konstanterna i normalled. Något högre avvikelse mellan teknikerna för att bestämma torsions fjäder konstanterna förekom. Men denna avvikelse leder inte till kvantitativt olika resultat och baserat på behändighet borde den termiska tekniken av Sader et al. tillämpas. Vidare har det viktiga momentet att kalibrera fotodetektorn behandlats, och en ny teknik föreslås för att bestämma detektor-känsligheten genom att kombinera olika metoder. Slutligen beräknades en konverterings faktor, som gör det möjligt att utföra kalibrering av kantilevern i luft, men att själva friktionsmätningen sedan kan utföras i vätska.
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| 2. |
- Koklukaya, Oruc
(författare)
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Flame-Retardant Cellulose Fibre/Fibril Based Materials via Layer-by-Layer Technique
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- According to an analysis conducted by the Swedish Chemicals Inspectorate in 2006, the approximate numbers of fire injuries per year in Sweden are 100 deaths, 700 major and 700 minor injuries.1 Observations also show that there has been an increase in the number of house fires during recent years. One possible explanation can be the increased use of plastics in the building industry and in furniture. The advantages of easy processing, light weight and low cost make plastic materials most prevalent in the market. However, plastics behave significantly differently from natural materials in the case of fire. Polymeric materials, including rigid polyurethane foams (PU) which are widely used in the building industry due to their insulating properties, are highly flammable and they release heat at a very high rate. In addition, polymeric materials release more harmful smoke, toxic gases and combustion products than natural materials. A house fire typically starts with the ignition of a combustible material. Flames then spread to nearby materials and shortly thereafter the heat radiation generated reaches a point where the contents of the room suddenly and simultaneously ignite. This stage is called a flash over. After this stage, the fire is fully developed and it continues until everything is consumed. The higher rate of heat and smoke production from plastic materials reduces the time to flash over and hence the time to escape from a fire. The traditional flame-retardant treatments are based mainly on halogenated compounds which are classified as gas phase flame-retardants. The halogenated flame-retardants are under severe investigation due to their adverse effect on health and on the environment since they release toxic gases during combustion and they may leach out and accumulate in the food chain.2-3 The restrictions due to growing environmental concerns have been a driving force to develop alternative flame-retardants by using natural and renewable resources. In recent years, the layer-by-layer (LbL) technique has been used as a simple and versatile surface engineering technique to construct functional nanocoatings through the sequential adsorption of polyelectrolytes and charged nanoparticles in an effort to impart flame-retardant characteristics by inhibiting the combustion cycle.4-5 This thesis presents the physical modification of cellulose fibre/fibril based materials as a means of improving flame-retardant properties.In the first part of work described in this thesis, the adsorption of polyelectrolyte multilayers onto pulp fibres was investigated as a way to impart flame-retardant characteristics to paper-based materials. It was found that intumescent nanocoatings consisting of nitrogen and phosphorus containing polyelectrolytes such as chitosan (CH) and poly(vinylphosphonic acid) (PVPA) were able to significantly improve the thermal stability and flame-retardant properties of sheets made of LbL-treated fibres, and were able to self-extinguish the flame in the horizontal flame test (HFT). High magnification images revealed that this improvement in flame-retardancy was due to the formation of a coherent char layer on the fibres (Paper I).6 In addition to imparting flame-retardancy by the LbL-coating of polyethylenimine (PEI) and sodium hexametaphosphate (SHMP), it was also possible to improve the mechanical properties of the paper material with this treatment (Paper III).7In the second part of the work, wet-stable porous cellulose fibril-based aerogels were developed by freeze-drying and used as a template for the build-up of intumescent nano-brick wall assemblies. The formation of multilayers of CH, PVPA and montmorillonite clay (MMT) was investigated as a function of solution concentration, and it was found that five quadlayers (QL) of CH/PVPA/CH/MMT treated aerogels using 5 g/L solutions of the respective components were able to self-extinguish the flame in HFT and that they showed no ignition under the heat flux of 35 kW/m2 used in cone calorimetry (Paper II).8 In a different application, a novel low density, porous, wet-stable cellulose fibre network was developed using chemically modified cellulose fibres by solvent exchange from water to acetone followed by drying at room temperature. The fibre networks (FN) were modified using the LbL technique to construct a flame-retardant nanocoating consisting of CH, SHMP, and inorganic particles (i.e., MMT, sepiolite (SEP), and colloidal silica (SNP)). The influence of the shape of the nanoparticles on flame-retardancy was investigated and it was found that plate-like and rod-like clays with a high aspect ratio showed self-extinguishing behaviour in HFT. A 5 QL of CH/SHMP/CH/SEP reduced the peak heat release rate and total smoke release by 47% and 43%, respectively, with an addition of only ~8 wt% to FN (Paper IV).Finally, non-crystalline cellulose gel beads were used as a substrate for the LbL assembly of CH and SHMP in model studies aimed at identifying the molecular mechanisms responsible for the fire-retardant properties of the LbL structures. The beads were formed by precipitating the dissolved cellulose-rich fibres according to an earlier described procedure,9 and it was shown that these smooth cellulose beads can be utilized as a model substrate to study the influence of LbL chemistry and nanostructure on flame-retardancy. These new types of model systems thus constitute a new important tool for clarifying the mechanism behind flame-retardant nanocoating systems (Paper V).
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| 3. |
- Ahmadi Svensson, Mozhgan
(författare)
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Sampling and Analysis of Tars by Means of Photo Ionization Detection and Solid Phase Micro Extraction
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Gasification of biomass will likely play an important role in the production of energy and chemicals in a future sustainable society. However, during gasification impurities, such as tars, will be formed. Tars may cause fouling and blockages of equipment downstream the gasifier. It is therefore important to minimize the formation of tars, alternatively to remove the formed tars. These processes need to be monitored, which makes it necessary to develop tar analysis methods suitable for this task.This work describes the development of two tar analysis methods, an on-line method based on a photoionization detector (PID) and an off-line method based on solid phase microextraction (SPME). Both methods were successfully validated against the established solid phase adsorption (SPA) method.The method based on PID was shown to have a very fast response time. Furthermore, the PID method is selective towards tar, but only limited information will be obtained regarding the composition of the tar compounds. The PID method is suitable for applications where it is important to detect fast changes of the tar concentration, i.e. process monitoring.The SPME method was shown to be a very sensitive method for qualitative and quantitative tar analysis. The sampling temperature was shown to be crucial for obtaining analysis results with the wanted detection limit. The SPME method is suitable for applications where extremely low detection and quantification limits are needed, i.e. for syngas production.
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| 4. |
- Akhlaghi, Shahin
(författare)
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Degradation of acrylonitrile butadiene rubber and fluoroelastomers in rapeseed biodiesel and hydrogenated vegetable oil
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Biodiesel and hydrotreated vegetable oil (HVO) are currently viewed by the transportation sector as the most viable alternative fuels to replace petroleum-based fuels. The use of biodiesel has, however, been limited by the deteriorative effect of biodiesel on rubber parts in automobile fuel systems. This work therefore aimed at investigating the degradation of acrylonitrile butadiene rubber (NBR) and fluoroelastomers (FKM) on exposure to biodiesel and HVO at different temperatures and oxygen concentrations in an automated ageing equipment and a high-pressure autoclave. The oxidation of biodiesel at 80 °C was promoted by an increase in the oxygen partial pressure, resulting in the formation of larger amounts of hydroperoxides and acids in the fuel. The fatty acid methyl esters of the biodiesel oxidized less at 150 °C on autoclave aging, because the termination reactions between alkyl and alkylperoxyl radicals dominated over the initiation reactions. HVO consists of saturated hydrocarbons, and remained intact during the exposure. The NBR absorbed a large amount of biodiesel due to fuel-driven internal cavitation in the rubber, and the uptake increased with increasing oxygen partial pressure due to the increase in concentration of oxidation products of the biodiesel. The absence of a tan δ peak (dynamical mechanical measurements) of the bound rubber and the appearance of carbon black particles devoid of rubber suggested that the cavitation was caused by the detachment of bound rubber from particle surfaces. A significant decrease in the strain-at-break and in the Payne-effect amplitude of NBR exposed to biodiesel was explained as being due to the damage caused by biodiesel to the rubber-carbon-black network. During the high-temperature autoclave ageing, the NBR swelled less in biodiesel, and showed a small decrease in the strain-at-break due to the cleavage of rubber chains. The degradation of NBR in the absence of carbon black was due only to biodiesel-promoted oxidative crosslinking. The zinc cations released by the dissolution of zinc oxide particles in biodiesel promoted reduction reactions in the acrylonitrile part of the NBR. Heat-treated star-shaped ZnO particles dissolved more slowly in biodiesel than the commercial ZnO nanoparticles due to the elimination of inter-particle porosity by heat treatment. The fuel sorption was hindered in HVO-exposed NBR by the steric constraints of the bulky HVO molecules. The extensibility of NBR decreased only slightly after exposure to HVO, due to the migration of plasticizer from the rubber. The bisphenol-cured FKM co- and terpolymer swelled more than the peroxide-cured GFLT-type FKM in biodiesel due to the chain cleavage caused by the attack of biodiesel on the double bonds formed during the bisphenol curing. The FKM rubbers absorbed biodiesel faster, and to a greater extent, with increasing oxygen concentration. It is suggested that the extensive biodiesel uptake and the decrease in the strain-at-break and Young’s modulus of the FKM terpolymer was due to dehydrofluorination of the rubber by the coordination complexes of biodiesel and magnesium oxide and calcium hydroxide particles. An increase in the CH2-concentration of the extracted FKM rubbers suggested that biodiesel was grafted onto the FKM at the unsaturated sites resulting from dehydrofluorination.
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| 5. |
- Alin, Jonas
(författare)
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Microwave heating effects on degradation and migration of additives from polypropylene packaging
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The effect of different food types, polymer qualities and microwaves on the overall and specific migration during microwave heating of plastic packaging was investigated to better understand the packaging-food interactions and the effect of microwaves on food packaging. This work focuses on the migration of chemical compounds to food simulants from commercially available polypropylene packages. Packages used were made of polypropylene homopolymer (PP), co-polymer (PP-C) and random co-polymer (PP-R). Polymers matrix changes were monitored by following possible changes in crystallinity after microwave heating. Antioxidants Irgafos 168 and Irganox 1010 were present in all the three PP packages. Other volatiles, primarily degradation products of antioxidants, were also detected and identified in the unaged packages. Significant antioxidant degradation took place during microwave heating of the packages in the fatty food simulants 90/10 isooctane/ethanol and ethanol resulting in the formation and migration of degradation products while no degradation of antioxidants was detected during conventional heating of the packages in the fatty food simulants. Antioxidant Irgafos 168 and Irganox 1010 migration rates were otherwise similar during microwave heating as during conventional heating to the fatty food simulants and antioxidant diffusion coefficients were similar to earlier established values obtained during conventional heating. Antioxidant migration rates from the three polymers to fatty food simulants differed largely with respect to PP type and increased with decreasing degree of crystallinity in the materials, PP-R showing the highest migration rate. Swelling in isooctane food simulant caused the antioxidant diffusion coefficients to increase by factors of 100-1000 at 80 ºC and decreased the temperature dependence of antioxidant migration. It also increased the overall migration to above established overall migration limits during both microwave and conventional heating. Electrospray ionization mass spectrometry (ESI-MS) was shown to be a valuable new tool for additive migration analysis of compounds not detectable by HPLC or GC-MS.
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| 6. |
- Alipour, Nazanin, 1978-
(författare)
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Structure and Mechanical/Transport properties of Single and Multilayer Polyethylene-based Materials
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The current study discusses the structure, mechanical and transport properties of polyethylene-based materials into two parts. The first part deals with the migration and chemical depletion of active substance such as insecticides from moulded polyethylene sheets. Deltamethrin (DM) and synergist piperonyl butoxide (PBO) are often used for insect control purpose. It was found that DM as a powder was incapable of recrystallization and remained in liquid state after cooling to room temperature, and that the evaporation of a DM/PBO solution was greater than that predicted from the evaporation rates of pristine separate material components. Infrared spectroscopy and liquid chromatography showed that the loss of DM and PBO through polyethylene sheets was negligible over 30 days, when aged in air at 80 °C (60 and 80 %RH). However, significant migration of the active species was observed in aged polyethylene sheets which were exposed in liquid water (at 80 and 95 °C). In the second part, the structure and properties of multi–layered polymer films were studied in terms of crystallization kinetics, mechanical and transport properties. Previously, it has been shown that when the layer thickness decreases from micrometre-scale to nanometre-scale, leading to improvement of the film performance such as crack propagation and oxygen barrier properties. In this work, two multi-layered systems were considered based on compatible (i) or incompatible layers (ii). In the first case (i), metallocene polyethylene (mPE) and low-density polyethylene (LDPE) where investigated as 2, 24, and 288 adjacent layers. In the second case (ii) poly(ethylene-co-vinyl alcohol) (EVOH) and polyethylene adhesive was evaluated as 5 and 19 layers. The crystallization kinetic studies showed that the crystallization rate was retarded as the layers became thinner with increasing number of layers in the multi-layered films as compared to the reference films (2 and 5 layers). The observation was suggested to stem from greater association between layers (inter layer mixing) in the case of mPE/LDPE films with 2 layers. Furthermore, the crack growth resistance increased with increasing number of layers. The x-ray scattering and tensile testing showed that the films were orientated more in extrusion direction than in the transverse direction, besides the EVOH films (the incompatible system) showed higher orientation in the extrusion direction than mPE/LDPE films. The uptake of n-hexane was reduced significantly in multi-layered EVOH films due to the effective protective role of EVOH. Furthermore, it was revealed that non-homogenous swelling causing a folding/curling of bilayer films when exposed to the vapour of the solvent.
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| 7. |
- Alipour, Yousef, 1979-
(författare)
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Furnace Wall Corrosion in a Wood-fired Boiler
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of renewable wood-based fuel has been increasing in the last few decades because it is said to be carbon neutral. However, wood-based fuel, and especially used wood (also known as recycled wood or waste wood), is more corrosive than virgin wood (forest fuel), because of higher amounts of chlorine and heavy metals. These elements increase the corrosion problems at the furnace walls where the oxygen level is low.Corrosion mechanisms are usually investigated at the superheaters where the temperature of the material and the oxygen level is higher than at the furnace walls. Much less work has been performed on furnace wall corrosion in wood or used wood fired boilers, which is the reason for this project. Tests are also mostly performed under simplified conditions in laboratories, making the results easier to interpret. In power plants the interpretation is more complicated. Difficulties in the study of corrosion processes are caused by several factors such as deposit composition, flue gas composition, boiler design, and combustion characteristics and so on. Therefore, the laboratory tests should be a complement to the field test ones. This doctoral project involved in-situ testing at the furnace wall of power boilers and may thus contribute to fill the gap.The base material for furnace walls is a low alloy steel, usually 16Mo3, and the tubes may be coated or uncoated. Therefore tests were performed both on 16Mo3 and more highly alloyed materials suitable for protective coatings.Different types of samples exposed in used-wood fired boilers were analysed by different techniques such as LOM (light optical microscopy), XRD (X-ray diffraction), SEM (scanning electron microscopy), EDS (energy dispersive spectroscopy), WDS (wavelength dispersive spectroscopy), FIB (focused ion beam) and GD-OES (glow discharge optical emission spectroscopy). The corrosion rate was measured. The environment was also thermodynamically modelled by TC (Thermo-Calc ®).The results showed that 16Mo3 in the furnace wall region is attacked by HCl, leading to the formation of iron chloride and a simultaneous oxidation of the iron chloride. The iron chloride layer appeared to reach a steady state thickness. Long term exposures showed that A 625 (nickel chromium alloy) and Kanthal APMT (iron-chromium-aluminium alloy) had the lowest corrosion rate (about 25-30% of the rate for 16Mo3), closely followed by 310S (stainless steel), making these alloys suitable for coating materials. It was found that the different alloys were attacked by different species, although they were exposed in the boiler at the same time in the same place. The dominant corrosion process in the A 625 samples seemed to be by a potassium-lead combination, while lead did not attack the APMT samples. Potassium attacked the alumina layer in the APMT samples, leading to the formation of a low-protective aluminate and chlorine was found to attack the base material. The results showed that stainless steels are attacked by both mechanisms (Cl- induced attack and K-Pb combination).Decreasing the temperature of the furnace walls of a waste wood fired boiler could decrease the corrosion rate of 16Mo3. However, this low corrosion rate corresponds to a low final steam pressure of the power plant, which in not beneficial for the electrical efficiency.The short term testing results showed that co-firing of sewage sludge with used wood can lead to a reduction in the deposition of K and Cl on the furnace wall during short term testing. This led to corrosion reduction of furnace wall materials and coatings. The alkali chlorides could react with the aluminosilicates in the sludge and be converted to alkali silicates. The chromia layer in A 625 and alumina in APMT were maintained with the addition of sludge.
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| 8. |
- Alipour, Yousef
(författare)
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High temperature corrosion in a biomass-fired power boiler : Reducing furnace wall corrosion in a waste wood-fired power plant with advanced steam data
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of waste (or recycled) wood as a fuel in heat and power stations is becoming more widespread in Sweden (and Europe), because it is CO2 neutral with a lower cost than forest fuel. However, it is a heterogeneous fuel with a high amount of chlorine, alkali and heavy metals which causes more corrosion than fossil fuels or forest fuel.A part of the boiler which is subjected to a high corrosion risk is the furnace wall (or waterwall) which is formed of tubes welded together. Waterwalls are made of ferritic low-alloyed steels, due to their low price, low stress corrosion cracking risk, high heat transfer properties and low thermal expansion. However, ferritic low alloy steels corrode quickly when burning waste wood in a low NOx environment (i.e. an environment with low oxygen levels to limit the formation of NOx). Apart from pure oxidation two important forms of corrosion mechanisms are thought to occur in waste environments: chlorine corrosion and alkali corrosion.Although there is a great interest from plant owners to reduce the costs associated with furnace wall corrosion very little has been reported on wall corrosion in biomass boilers. Also corrosion mechanisms on furnace walls are usually investigated in laboratories, where interpretation of the results is easier. In power plants the interpretation is more complicated. Difficulties in the study of corrosion mechanisms are caused by several factors such as deposit composition, flue gas flow, boiler design, combustion characteristics and flue gas composition. Therefore, the corrosion varies from plant to plant and the laboratory experiments should be complemented with field tests. The present project may thus contribute to fill the power plant corrosion research gap.In this work, different kinds of samples (wall deposits, test panel tubes and corrosion probes) from Vattenfall’s Heat and Power plant in Nyköping were analysed. Coated and uncoated samples with different alloys and different times of exposure were studied by scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), X-ray diffraction (XRD) and light optical microscopy (LOM). The corrosive environment was also simulated by Thermo-Calc software.The results showed that a nickel alloy coating can dramatically reduce the corrosion rate. The corrosion rate of the low alloy steel tubes, steel 16Mo3, was linear and the oxide scale non-protective, but the corrosion rate of the nickel-based alloy was probably parabolic and the oxide much more protective. The nickel alloy and stainless steels showed good corrosion protection behavior in the boiler. This indicates that stainless steels could be a good (and less expensive) alternative to nickel-based alloys for protecting furnace walls.The nickel alloy coated tubes (and probe samples) were attacked by a potassium-lead combination leading to the formation of non-protective potassium lead chromate. The low alloy steel tubes corroded by chloride attack. Stainless steels were attacked by a combination of chlorides and potassium-lead.The Thermo-Calc modelling showed chlorine gas exists at extremely low levels (less than 0.1 ppm) at the tube surface; instead the hydrated form is thermodynamically favoured, i.e. gaseous hydrogen chloride. Consequently chlorine can attack low alloy steels by gaseous hydrogen chloride rather than chlorine gas as previously proposed. This is a smaller molecule than chlorine which could easily diffuse through a defect oxide of the type formed on the steel.
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| 9. |
- Almgren, Karin M., 1980-
(författare)
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Wood-fibre composites : Stress transfer and hygroexpansion
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wood fibres is a type of natural fibres suitable for composite applications. The abundance of wood in Swedish forests makes wood-fibre composites a new and interesting application for the Swedish pulp and paper industry. For large scale production of composites reinforced by wood fibres to be realized, the mechanical properties of the materials have to be optimized. Furthermore, the negative effects of moisture, such as softening, creep and degradation, have to be limited. A better understanding of how design parameters such as choice of fibres and matrix material, fibre modifications and fibre orientation distribution affect the properties of the resulting composite material would help the development of wood-fibre composites. In this thesis, focus has been on the fibre-matrix interface, wood-fibre hygroexpansion and resulting mechanical properties of the composite. The importance of an efficient fibre-matrix interface for composite properties is well known, but the determination of interface properties in wood-fibre composites is difficult due to the miniscule dimensions of the fibres. This is a problem also when hygroexpansion of wood fibres is investigated. Instead of tedious single-fibre tests, more straightforward, macroscopic approaches are suggested. Halpin-Tsai’s micromechanical models and laminate analogy were used to attain efficient interface characteristics of a wood-fibre composite. When Halpin-Tsai’s model was replaced by Hashin’s concentric cylinder assembly model, a value of an interface parameter could be derived from dynamic mechanical analysis. A micromechanical model developed by Hashin was used also to identify the coefficient of hygroexpansion of wood fibres. Measurements of thickness swelling of wood-fibre composites were performed. Back-calculation through laminate analogy and the micromechanical model made it possible to estimate the wood-fibre coefficient of hygroexpansion. Through these back-calculation procedures, information of fibre and interface properties can be gained for ranking of e.g. fibre types and modifications. Dynamic FT-IR (Fourier Transform Infrared) spectroscopy was investigated as a tool for interface characterization at the molecular level. The effects of relative humidity in the test chamber on the IR spectra were studied. The elastic response of the matrix material increased relative to the motion of the reinforcing cellulose backbone. This could be understood as a stress transfer from fibres to matrix when moisture was introduced to the system, e.g. as a consequence of reduced interface efficiency in the moist environment. The method is still qualitative and further development is potentially very useful to measure stress redistribution on the molecular level.
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| 10. |
- Álvarez-Asencio, Rubén, 1978-
(författare)
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Nanotribology, Surface Interactions and Characterization : An AFM Study
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When two surfaces achieve contact, then contact phenomena such as adhesion, friction and wear can occur, which are of great interest in many disciplines, including physics, physical chemistry, material chemistry, and life and health sciences. These phenomena are largely determined by the nature and magnitude of the surface forces such as van der Waals, capillary and hydration forces. Moreover these forces are length-dependent, and therefore when the system scales down, their contribution scales up, dominating the interaction between the surfaces.A goal of my PhD work was to investigate fundamental contact phenomena in terms of the surface forces that regulate their properties. The primary tool applied in this PhD thesis work has been the atomic force microscopy (AFM), which (with all of its sub-techniques) offers the possibility to study such forces with high resolution virtually between all types of materials and intervening media. Therefore, in this work it was possible to study the long ranged interactions presented in air between different industrially relevant materials and how these interactions are shielded when the systems are immersed in an ionic liquid.Also investigated was the influence of microstructure on the tribological properties of metal alloys, where their good tribological properties were related with the vanadium and nitrogen contents for a FeCrVN tool alloy and with the chromium content for a biomedical CoCrMo alloy. Moreover, the effect of the intervening media can significantly affect the surface properties, and when the biomedical CoCrMo alloy was immersed in phosphate buffer saline solution (PBS), repulsive hydration forces decreased the friction coefficient and contact adhesion. On the other hand, with the immersion of the FeCrVN tool alloy in the NaCl solution, small particles displaying low adhesion were generated in specific regions on the surface with low chromium content. These particles are assumed to be related to a prepitting corrosion event in the tool alloy.The mechanical properties of stratum corneum (SC), which is the outermost layer of the skin, were also studied in this work. The SC presents a highly elastic, but stiff surface where the mechanical properties depend on the nanoscale. A novel probe has been designed with a single hair fibre in order to understand how the skin deforms locally in response to the interaction with such a fibre probe. This study revealed that is mostly the lateral scale of the deformation which determines the mechanical properties of the SC.Finally, important achievements in this work are the developments of two new techniques - tribological property mapping and the Hybrid method for torsional spring constant evaluation. Tribological property mapping is an AFM technique that provides friction coefficient and contact adhesion maps with information attributed to the surface microstructure. The Hybrid method is an approach that was originally required to obtain the torsional spring constants for rigid beam shaped cantilevers, which could not be previously determined from their power torsional thermal spectra (conventional method). However, the applicability is shown to be general and this simple method can be used to obtain torsional spring constants for any type of beam shape cantilever.
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