| 1. |
- Pashazadehgaznagh, Sajjad, 1994, et al.
(author)
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Mapping surface defects in highly-filled wood fiber polymer composite extrusion from inline spectral analysis
- 2023
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In: Composites Science and Technology. - 0266-3538. ; 242
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Journal article (peer-reviewed)abstract
- Surface defects in highly-filled wood polymer composites (WPCs) are mapped in single-screw extrusion via inline optical spectral analysis for the first time. The effects of wood fiber content and drying on the dynamics of surface defects are spatio-temporally resolved via space–time inline optical imaging. Surface tearing appeared from the lowest shear rates investigated followed by a gradual decay in spectral intensity with increasing shear rates/slip velocities. This is accompanied by broadening of the surface tearing characteristic frequency while the average wavelength is estimated to remain constant within the experimental conditions. Increasing shear rates, drying and increasing wood fiber content showed mitigating effects on surface tearing. However, surface tearing in undried samples was still present even at the highest shear rates and high wall slip velocities. A regime where the extrudate surface is dominated by bubbles at high shear rates and low wood fiber contents in undried WPCs was identified.
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| 2. |
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| 3. |
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| 4. |
- Moberg, Tobias, 1986, et al.
(author)
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Extensional viscosity of microfibrillated cellulose suspensions
- 2014
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In: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 102:1, s. 409-412
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Journal article (peer-reviewed)abstract
- The extensional properties of micro fibrillated cellulose (MFC)-suspensions at different fibril concentrations and with different amounts of added sodium chloride were evaluated. The MFC-suspensions were obtained by diluting a stock solution consisting of 0.95 wt.% cellulose with either deionized water or sodium chloride solution, giving a series of different concentrations and sodium chloride contents. The extensional viscosities of the suspensions were measured utilizing contraction flow geometry. Here the specimens were forced through a hyperbolic nozzle and the required pressure drop over the nozzle was measured. The extensional viscosity exhibited an extensional-thinning behaviour over the extensional strain rates used. Furthermore the extensional viscosity decreased with decreasing concentration of the suspensions, in similarities with the shear properties of the specimens. For the suspensions containing sodium chloride, the extensional viscosity appeared to increase when the concentration of sodium chloride was increased. But excessive amounts of added sodium chloride promoted an agglomeration of the suspensions.
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| 7. |
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| 8. |
- Moberg, Tobias, 1986, et al.
(author)
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Preparation and Viscoelastic Properties of Composite Fibres Containing Cellulose Nanofibrils : Formation of a Coherent Fibrillar Network
- 2016
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In: Journal of Nanomaterials. - : Hindawi Publishing Corporation. - 1687-4110 .- 1687-4129. ; 2016
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Journal article (peer-reviewed)abstract
- Composite fibres with a matrix of poly(ethylene glycol) (PEG) and cellulose nanofibrils (CNF) as reinforcing elements were produced using a capillary viscometer. Two types of CNF were employed: one based on carboxymethylated pulp fibres and the other on TEMPO-oxidized pulp. Part of the latter nanofibrils was also grafted with PEG in order to improve the compatibility between the CNF and the PEG matrix. The nominal CNF-content was kept at 10 or 30 weight-%. The composite fibres were characterized by optical and scanning electron microscopy in addition to dynamic mechanical thermal analysis (DMTA). Evaluation of the storage modulus indicated a clear reinforcing effect of the CNF, more pronounced in the case of the grafted CNF and depending on the amount of CNF. An interesting feature observed during the DMTA-measurements was that the fibrils within the composite fibres appeared to forma rather coherent and load-bearing network which was evident even after removing of the PEG-phase (by melting). An analysis of the modulus of the composite fibres using a rather simple model indicated that the CNF were more efficient as reinforcing elements at lower concentrations which may be associated with a more pronounced aggregation as the volume fraction of CNF increased.
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| 9. |
- Moberg, Tobias, 1986
(author)
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Rheological and mechanical properties of systems containing nanocellulose
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Nanocellulose is a cellulose entity at the nanoscale. There are several different types of nanocellulose such as cellulose nanofibrils (CNF), which are long, somewhat flexible fibrils and cellulose nanocrystals (CNC), which are shorter and more rod-like particles. These entities are considered to have good mechanical properties; this combined with a low density makes nanocellulose an interesting candidate for several applications, e.g. transparent high strength paper, barrier films, reinforcing elements in composites and rheological modifiers. In the manufacturing of such products the rheological properties of the material are of great importance. A substantial part of this thesis has been devoted to assessment of the rheological behaviour of aqueous suspensions of CNF/CNC as well as of polymer melts containing the same nanomaterials. In general, both systems exhibited, quite a complex behaviour; the suspensions were shear-thinning and also exhibited a viscoelastic character (in agreement with other studies). An increase in the concentration of CNF/CNC generates a corresponding increase in shear viscosity and of the magnitude of the storage and loss moduli, characterizing the viscoelastic behaviour. For the first time, it was also shown that aqueous CNF suspension exhibited an extensional thinning viscosity during contraction flow, which can be of consequence in processing operations. It may be remarked that this complex rheological behaviour was noted already at low concentrations of CNF/CNC and could be attributed to a low percolation threshold for network formation. It is not only the concentration of the nanoelements that is of importance in this context. Here, different types of CNF/CNF were characterized with regard to their dimensions using atomic force microscopy and transmission electron microscopy and relations between the rheological properties and the dimensions were noted and discussed. . In a similar way, presence of charged surface groups and grafted polymer segments on CNF/CNC were found to have a strong effect on the rheology of these suspensions. Another area focussed on in this thesis was the use of CNF as reinforcing elements in polymer matrices. Two polymer matrices were used; poly(ethylene glycol) and an ethylene-acrylic acid copolymer. With the latter matrix, the amount of CNF used was high, up to 70 volume-%. In both systems, the addition of CNF substantially improved the modulus of the materials (and in some cases also the strength). Even at temperatures higher than the melting point of the matrices, the material could carry load indicating the forming of a coherent CNF-network. An analysis of the tensile modulus using the Cox-Krenchel model, pointed to that the fibrils formed aggregates in the composite resulting in stiffness values less than possibly could be expected.
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| 10. |
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| 11. |
- Moberg, Tobias, 1986
(author)
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Rheological Properties of Microfibrillated Cellulose
- 2013
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Licentiate thesis (other academic/artistic)abstract
- Cellulose has for a long time been used by man. It is renewable and an easily recyclable resource. Mostly it has been used in either its native form as wood or in the form of fibres. But in the early 1980s it was shown that it was possible to extract even smaller elements out of the fibres when wood pulp was subjected to intense mechanical shearing. The material that was obtained was called microfibrillated cellulose or abbreviated MFC. It is believed (and to some extent shown) that these microfibrils potentially have better mechanical properties than the cellulose fibres. They can also exhibit other interesting features. It is thus of interest to use microfibrils in certain applications, but, in the form of suspensions, these microfibrils present very different rheological properties than the cellulose fibres. Thus, based on the interest in microfibrillated cellulose for different applications, such as transparent and high toughness papers, there is a need to extend the understanding of the rheological properties of the MFC-suspensions.The rheological properties of two different types of MFC-suspensions with a concentration of 2 weight-% (in water) were determined both in shear and in extension. In shear, the shear storage and shear loss moduli of the suspensions were assessed in terms of their stress and frequency dependence. It was shown that the suspensions exhibited a linear region where the above material parameters were independent of the stress level (linear viscoelasticity). Above a critical stress level, this is no longer valid anymore and the network structure of the suspension starts to deteriorate. Creep measurements confirmed that the suspensions exhibited a linear region in which the creep compliance was independent of the stress applied. This was the case up to ca 10-15 Pa for one of the studied suspensions. The creep measurements was fitted to simple models used to describe viscoelastic behaviour and it was, somewhat unexpectedly, noted that quite simple models could describe the behaviour quite wellThe extensional viscosity of some MFC-suspensions was determined with a contracting flow technique. The material was found to exhibit a strain-thinning behaviour, and the viscosity decreased when the concentration of cellulose decreased (in the concentration range below 1 weight-%), but it increased when the amount of added salt increased. This is believed to be due to screening of the surface charges on the fibrils caused by the salt added to the suspension, increasing the fibril-fibril interactions. The extensional viscosity was much higher than the shear viscosity of the suspensions.
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| 12. |
- Moberg, Tobias, 1986, et al.
(author)
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Rheological properties of nanocellulose suspensions : effects of fibril/particle dimensions and surface characteristics
- 2017
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In: Cellulose. - : Springer Netherlands. - 0969-0239 .- 1572-882X. ; 24:6, s. 2499-2510
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Journal article (peer-reviewed)abstract
- The rheological properties of aqueous suspensions based on three different nanocelluloses were compared. One system was obtained via acid hydrolysis (thus yielding crystalline nanocellulose, CNC) and the other two from mechanical shearing, but from different origins and subjected to different pretreatments. Of the latter two, one was considered to be a rather typical cellulose nanofibril (CNF) suspension whereas the other was a kind of intermediate between CNF and CNC. All three nanocellulose elements differed in dimensions as evident from transmission electron microscopy and atomic force microscopy. With regard to the length of the fibrils/particles, the three nanocelluloses formed three distinct groups with lengths between 200 and slightly more than 800 nm. The three cellulosic elements were also subjected to a TEMPO-mediated oxidation yielding a similar carboxylate content in the three systems. Furthermore, the TEMPO-oxidized elements were grafted with poly(ethylene glycol) (PEG). The amount of grafted PEG was about 35 wt%. The shear viscosity, the storage modulus and the loss modulus of suspensions of the unmodified, the TEMPO-oxidized and the grafted nanocelluloses were determined at room temperature and the solids content of the suspensions was varied between 0.7 and 2.0 wt%. It was concluded that the rheological properties varied significantly between the suspensions depending on the dimensions of the cellulosic elements and their surface characteristics. In this context, the length (or the aspect ratio) of the particles played a very important role.
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| 13. |
- Pashazadehgaznagh, Sajjad, 1994, et al.
(author)
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Surface instability detection in highly-filled biocomposites from inline imaging during extrusion
- 2023
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In: AIP Conference Proceedings. - 1551-7616 .- 0094-243X. ; 2997
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Conference paper (peer-reviewed)abstract
- The processing of wood fiber biocomposites, and in particular, the extrusion, is accompanied by multiple challenges, among which agglomeration, entanglement, slip, or surface instabilities being the most common ones. In the current work, we focus on the dynamics of surface instabilities during the single screw extrusion of highly filled wood fiber biocomposites. The biocomposites are polypropylene based with up to 40 wt% wood fiber content of custom compositions based on the commercial-grade by Stora Enso. To detect and quantify the dynamics of surface instabilities, inline image analysis was applied using an optical visualization system positioned at the die exit. Therefrom, space-time diagrams were constructed, and after that, via 2D-Fourier transform analysis, the spatio-tempoal spectral dynamics of the surface instabilities were determined as a function of the die (apparent) shear rate. The spectral dynamics show that melt instabilities of 40 wt% for example, detected via their characteristic (temporal) frequency and (spatial) wavenumber, dissipate with increasing the shear rate, such that at shear rates above ca. 90 1/s, no characteristic frequency and wavenumber can be distinguished i.e. instabilities can no longer be observed on the surface of the extrudates.
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| 14. |
- Sahlin, Karin, 1991, et al.
(author)
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Nanocellulose in biocomposites
- 2016
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In: Proc Materials for Tomorrow 2016. ; , s. Nr 41-
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Conference paper (other academic/artistic)
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| 15. |
- Sahlin, Karin, 1991, et al.
(author)
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Surface treatment of cellulose nanocrystals (CNC): effects on dispersion rheology
- 2018
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In: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 25:1, s. 331-345
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Journal article (peer-reviewed)abstract
- Cellulose nanocrystals (CNC) were surface modified by grafting azetidinium salts onto the sulphate ester groups on the cellulosic surfaces. The modified CNC were characterized using NMR, FTIR spectroscopy, conductometric titration and measurement of the zeta-potential. Thermal gravimetrical analysis revealed that the onset temperature for the thermal degradation was shifted upwards by almost 100 degrees C as a result of the surface grafting. The rheological properties of dispersions based on unmodified and modified CNC were evaluated in detail. Two solids contents were studied; 0.65 and 1.3 wt%. In general, the grafting of the salts significantly increased the shear viscosity at a given shear rate as well as the storage and loss moduli of the dispersions. The CNC concentration at the gel point (network formation) decreased in a corresponding manner when the nanocellulosic particles were surface modified. This may be associated with pronounced hydrophobic attractive interactions between the grafted substituents.
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| 16. |
- Sjöström, Johan, 1978, et al.
(author)
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Effects of Water Contamination on the Supercooled Dynamics of a Hydrogen-Bonded Model Glass Former
- 2011
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 115:8, s. 1842-1847
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Journal article (peer-reviewed)abstract
- Broad-band dielectric spectroscopy is a commonly used tool in the study of glass-forming liquids. The high sensitivity of the technique together with the wide range of probed time scales makes it a powerful method for investigating the relaxation spectra of liquids. One particularly important class of glass-forming liquids that is often studied using this technique consists of liquids dominated by hydrogen (H) bond interactions. When investigating such liquids, particular caution has to be taken during sample preparation due to their often highly hygroscopic nature. Water can easily be absorbed from the atmosphere, and dielectric spectroscopy is a very sensitive probe of such contamination due to the large dipole moment of water. Our knowledge concerning the effects of small quantities of water on the dielectric properties of these commonly investigated liquids is limited. We here demonstrate the effects due to the presence of small amounts of water on the dielectric response of a typical H-bonded model glass former, tripropylene glycol. We show how the relaxation processes present in the pure liquid are affected by addition of water, and we find that a characteristic water induced relaxation response is observed for water contents as low as 0.15 wt %. We stress the importance of careful purification of hygroscopic liquids before experiments and quantify what the effects are if such procedures are not undertaken.
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| 17. |
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| 19. |
- Venkatesh, Abhijit, 1989, et al.
(author)
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Cellulose nanofibril-reinforced composites using aqueous dispersed ethylene-acrylic acid copolymer
- 2018
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In: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 25:8, s. 4577-4589
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Journal article (peer-reviewed)abstract
- In order to explore the reinforcing capabilities of cellulose nanofibrils, composites containing high contents of cellulose nanofibrils were prepared through a combination of water-assisted mixing and compression moulding, the components being a cellulose nanofibril suspension and an aqueous dispersion of the polyolefin copolymer poly(ethylene-co-acrylic acid). The composite samples had dry cellulose nanofibril contents from 10 to 70 vol%. Computed tomography revealed well dispersed cellulose fibril/fibres in the polymer matrix. The highest content of 70 vol% cellulose nanofibrils increased the strength and stiffness of the composites by factors of 3.5 and 21, respectively, while maintaining an elongation at break of about 5%. The strength and strain-at-break of cellulose nanofibril composites were superior to the pulp composites at cellulose contents greater than 20 vol%. The stiffness of the composites reinforced with cellulose nanofibrils was not higher than for that of composites reinforced with cellulose pulp fibres. Graphical Abstract: [Figure not available: see fulltext.].
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| 20. |
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