| 1. |
- Hietala, Maiju, et al.
(författare)
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Bionanocomposites of thermoplastic starch and cellulose nanofibers manufactured using twin-screw extrusion
- 2013
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Ingår i: European Polymer Journal. - : Elsevier BV. - 0014-3057 .- 1873-1945. ; 49:4, s. 950-956
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to investigate if cellulose nanofiber (CNF) gels with high water contents can be processed to nanocomposites with starch powder using continuous twin-screw extrusion and to improve the mechanical properties and moisture sensitivity of thermoplastic starch. Nanocomposites with 0, 5, 10, 15 and 20 wt% cellulose nanofiber content were prepared. The characterization methods were conventional tensile testing, UV/Vis spectroscopy, scanning electron microscopy and moisture absorption. The cellulose nanofiber gel with high water content was mixed with starch powder, fed to the extruder as powder, performing the gelatinization of starch as well as the mixing of CNF in one step. The microscopy study showed that the CNF aggregated during the process and that the screw configuration needs to be more distributive and dispersive to get homogeneous material. The results showed that the addition of CNF improved the mechanical properties and had a positive effect on moisture uptake of the thermoplastic starch. Also, the translucency of the TPS/CNF composite films remained, even with high CNF content (20 wt%).
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| 2. |
- Hietala, Maiju, et al.
(författare)
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Extrusion processing of green biocomposites: Compounding, fibrillation efficiency, and fiber dispersion
- 2014
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Ingår i: Journal of Applied Polymer Science. - : Wiley. - 0021-8995 .- 1097-4628. ; 131:6
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Tidskriftsartikel (refereegranskat)abstract
- The efficiency of twin-screw extrusion process to fibrillate cellulose fibers into micro/nanosize in the same step as the compounding of green bionanocomposites of thermoplastic starch (TPS) with 10 wt % fibers was examined. The effect of the processing setup on micro/nanofibrillation and fiber dispersion/distribution in starch was studied using two types of cellulose fibers: bleached wood fibers and TEMPO-oxidized cellulose fibers. A composite with cellulose nanofibers was prepared to examine the nanofiber distribution and dispersion in the starch and to compare the properties with the composites containing cellulose fibers. Optical microscopy, scanning electron microscopy, and UV/Vis spectroscopy showed that fibers were not nanofibrillated in the extrusion, but good dispersion and distribution of fibers in the starch matrix was obtained. The addition of cellulose fibers enhanced the mechanical properties of the TPS. Moisture uptake study revealed that the material containing TEMPO-oxidized fibers had higher moisture absorption than the other composites
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| 3. |
- Hietala, Maiju
(författare)
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Extrusion processing of wood-based biocomposites
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Interest in biocomposite materials and their use in various applications has been growing steadily over the past 10-15 years. Increasing environmental awareness and lower material costs are the main driving forces for using renewable materials, such as wood and cellulose fibers, as reinforcement in polymer composites. In addition to the materials used in the composite preparation, the processing has a large impact on the final properties of the composite. Therefore, the main thrust of this work has been on processing of wood-based biocomposite materials using twin-screw extrusion.In the first part of the work (Papers I, II and III), wood-plastic composites were manufactured using twin-screw extrusion. Currently, wood flour consisting of small wood particles with low aspect ratio is used as the main wood raw material in commercial woodplastic composites. A better reinforcement is achieved by using wood fibers with a higher aspect ratio, but individual fibers are seldom used in composite manufacturing due to processing problems and higher cost. Therefore, the objective of the first part of this work was to study the use of wood chips as raw material in wood-plastic composites and the possibility to separate individual fibers from the wood chips during the composite manufacturing process. The effect of extrusion parameters and raw materials on the aspect ratio of the wood particles/fibers and on the mechanical properties of the composites was evaluated. The study showed that wood chips can be used as raw material in a one-step process for manufacturing wood-plastic composites, and that it is possible to separate individual fibers with a higher aspect ratio than wood flour from the wood chips under suitable processing conditions. The second part of the work (Papers IV and V) focused on the twin-screw extrusion of cellulose nanocomposites. The use of nanosized cellulose fibers to reinforce polymer matrices has many benefits over the macrosized fibers, such as the high surface area and large aspect ratio. However, the preparation of cellulose nanocomposites is more complicated due to the high hydrophilicity and aggregation tendency of nanocellulose, meaning that drying of the nanofibers is not recommended when good dispersion of nanofibers is needed. Therefore, the aim was to study the processing of green cellulose nanocomposites with twin-screw extrusion using thermoplastic starch as the matrix polymer and cellulose nanofibers with high water content as the reinforcement (Paper IV). In addition, the effect of twin-screw extrusion on separating micro/nanoscale fibers from cellulose fibers during the compounding of biocomposites was studied (Paper V). The fibrillation of nanocellulose is a highly energy intensive process; therefore, it would be very beneficial if it could be done at the same step as the compounding of the composites. The preparation of thermoplastic starch and composite compounding was performed in one step, and the effects of extrusion compounding on the dispersion of the cellulose nanofibers, on the micro/nanofibrillation of cellulose fibers, and on the composites’ mechanical, optical and moisture absorption properties were studied. The results showed that some aggregation of cellulose nanofibers occurred during the extrusion process, but that the addition of cellulose nanofibers had a positive effect on the properties of the prepared bionanocomposites. Nanofibrillation of cellulose was not accomplished using the selected processing conditions; however, dispersion of the fibers was enhanced.
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| 4. |
- Hietala, Maiju
(författare)
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Extrusion processing of wood raw materials for use in wood-polymer composites
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interest in wood-polymer composites and their use in different applications has been growing over the last 10-15 years. Environmental issues and demands on lower material costs are the driving forces behind the increasing use of renewable materials such as wood and other natural fibres as reinforcement in polymer composites. Wood flour consisting of small wood particles is currently used as the main wood raw material in commercial wood-polymer composites. However, the reinforcing potential of wood flour is limited. A better reinforcement could be achieved by using wood fibres with a higher aspect ratio as raw material, but individual fibres are seldom used in composite manufacturing due to processing problems and high cost. Therefore, the objective of the work was to study the possibility to use wood chips as raw material and separate individual fibres with higher aspect ratios from the wood chips during the composite manufacturing process. First, the effect of the extrusion process only on wood raw material was studied without a matrix polymer, and then composites using polypropylene as matrix were made. The main goal was to produce wood particles/fibres with high aspect ratio during the manufacturing of wood polymer composites. The effects of extrusion parameters, different screw configurations, raw materials and raw material pre-treatments were evaluated. The size of the separated wood particles and fibres was measured using optical fibre analysis methods. Microstructure of wood particles as well as the fractured surfaces of prepared composites were examined using electron microscopy. The mechanical properties of the composites were measured using flexural and impact testing. The results showed that wood chips can be used as raw material in a one-step manufacturing process of wood-polymer composites. Also, individual fibres with a higher aspect ratio than wood flour were separated from the wood chips with suitable processing conditions
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| 5. |
- Hietala, Maiju, et al.
(författare)
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Highly redispersible sugar beet nanofibers as reinforcement in bionanocomposites
- 2017
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Ingår i: Cellulose. - : Springer. - 0969-0239 .- 1572-882X. ; 24:5, s. 2177-2189
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Tidskriftsartikel (refereegranskat)abstract
- A simple method for preparing redispersible nanofibers from sugar beet residue and their use as a well-dispersed reinforcement for a polyvinyl alcohol (PVA) matrix is reported. It is known that the redispersion of dried cellulose nanofibers is difficult because of the formation of strong hydrogen bonds between the nanofibers. The results show that the properties of the initial sugar beet nanofiber suspension can be recovered without the use of chemical modification or additives with higher pectin and hemicellulose content. Undried and redispersed nanofibers with and without pectin were used as nanocomposite reinforcement with PVA. The redispersed nanofibers were as good reinforcements as the undried nanofibers. The tensile strength and elastic modulus of the nanocomposites with the redispersed sugar beet nanofibers were as good as those of the nanocomposites with undried nanofibers. Interestingly, the nanofiber dispersion in the PVA matrix was better when sugar beet nanofibers containing pectin and hemicellulose were used as reinforcements.
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| 6. |
- Hietala, Maiju, et al.
(författare)
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Pelletized cellulose fibres used in twin-screw extrusion for biocomposite manufacturing : Fibre breakage and dispersion
- 2018
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier. - 1359-835X .- 1878-5840. ; 109, s. 538-545
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Tidskriftsartikel (refereegranskat)abstract
- Pelletizing is effective in compacting cellulose fibres, but it also causes fibre breakage and poor dispersion due to increased hydrogen bonding. This study investigated whether fibre dispersion and length could be improved by the addition of a lubricant, a commonly used composite processing aid, into cellulose pellets, or by using pelletized fibres that have not been completely dried to reduce hydrogen bonding. Cellulose pellets with different lubricant and moisture contents were prepared and compounded using twin-screw extrusion with polypropylene with 5 wt% fibre and 50 wt% fibre contents. The fibre dispersion, morphology and mechanical properties of the prepared composites were analysed. Dispersion and composite strength were improved with the addition of 4–6 wt% of lubricant while moisture had a negative effect on both properties. This study demonstrated that pelletization in the presence of a lubricant is a promising way to compact cellulose fibres and enable their continuous processing into biocomposites with improved mechanical properties.
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| 7. |
- Hietala, Maiju, et al.
(författare)
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Potential of municipal solid waste paper as raw material for production of cellulose nanofibres
- 2018
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Ingår i: Waste Management. - : Elsevier. - 0956-053X .- 1879-2456. ; 80, s. 319-326
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Tidskriftsartikel (refereegranskat)abstract
- When aiming for higher resource efficiency, greater utilization of waste streams is needed. In this work, waste paper separated from mixed municipal solid waste (MSW) was studied as a potential starting material for the production of cellulose nanofibres (CNFs). The waste paper was treated using three different techniques, namely pulping, flotation and washing, after which it was subjected to an ultrafine grinding process to produce CNFs. The energy consumption of the nanofibrillation and nanofibre morphology, as well as properties of the prepared nanofibers, were analysed. Despite the varying amounts of impurities in the waste fibres, all samples could be fibrillated into nanoscale fibres. The tensile strengths of the CNF networks ranged from 70 to 100 MPa, while the stiffness was ∼7 GPa; thus, their mechanical strength can be adequate for applications in which high purity is not required. The contact angles of the CNF networks varied depending on the used treatment method: the flotation-treated networks were more hydrophilic (contact angle 52.5°) and the washed networks were more hydrophobic (contact angle 72.6°).
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| 8. |
- Hietala, Maiju, et al.
(författare)
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Processing of wood chip-plastic composites : effect on wood particle size, microstructure and mechanical properties
- 2011
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Ingår i: Plastics, rubber and composites. - 1465-8011 .- 1743-2898. ; 40:2, s. 49-56
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Tidskriftsartikel (refereegranskat)abstract
- Wood chips were used as raw material in extrusion of wood-plastic composites. Wood-plastic composites with similar to 50 wt-% wood content were manufactured by using two different compounding methods. Dried and undried wood chips were used to investigate the effect of wood moisture content on the wood particle size and whether the drying process could be carried out in the same step. Wood particle properties were measured using optical fibre analysis. Microscopical methods were used to examine the microstructure of wood particles. Furthermore, the prepared composites' mechanical properties were studied. The particle size of wood chips was significantly reduced during extrusion in both processing methods. The undried wood chips had higher aspect ratios in comparison with the dried wood chips after extrusion. Despite the higher aspect ratio, the mechanical properties of composites manufactured with undried wood chips were not better than the properties of composites with dried wood chips
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| 9. |
- Hietala, Maiju, et al.
(författare)
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Technologies for Separation of Cellulose Nanofibers
- 2014
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Ingår i: Handbook of Green Materials. - Singapore : World Scientific and Engineering Academy and Society. - 9789814566452 - 9789814566476
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Bokkapitel (refereegranskat)
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| 10. |
- Hietala, Maiju, et al.
(författare)
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The effect of pre-softened wood chips on wood fibre aspect ratio and mechanical properties of wood-polymer composites
- 2011
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Ingår i: Composites. Part A, Applied science and manufacturing. - : Elsevier BV. - 1359-835X .- 1878-5840. ; 42:12, s. 2110-2116
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this work was to study the effect of chemical pre-treatment and moisture content of wood chips on the wood particle aspect ratio after compounding in a twin-screw extruder and on the mechanical properties of wood-polymer composites (WPC). Composites with 50 wt% wood content were manufactured using pre-treated and untreated wood chips. The effect of wood moisture content on composite properties was studied by using dried and undried wood chips. The mechanical properties and fracture surfaces of the composites as well as the microstructure and aspect ratio of wood particles after compounding were studied. The highest wood particle aspect ratio after extrusion was achieved by using pre-treated, undried wood chips as raw material. The chemical pre-treatment was found to enhance the defibration of wood chips as well as the mechanical properties of the composites.
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