Browsing by Subject "aerogeeli"
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(2014)Aerogels are lightweight, porous and dry foams that are produced from gels by replacing the liquid phase by air. When produced from polysaccharide-based hydrogels, potential applications for aerogels could be found as bio-based packaging materials. The literature review focused on the production of polysaccharide-based hydrogels and their chemical crosslinking, as well as the production of aerogels and their properties. In the experimental study the possibilities for utilization of spruce galactoglucomannan (GGM), an abundant but largely unexploited raw material, as aerogel matrix was explored. Nanofibrillated cellulose (NFC) was used as reinforcement and the polysaccharides were crosslinked with ammonium zirconium carbonate (AZC). Hydrogels were prepared from GGM-NFC-suspensions and heat treatment was performed in order to induce crosslinking reaction. Prepared hydrogels were frozen in a bath of carbon dioxide ice and ethanol and subsequently freeze-dried into cubic aerogels. The aim was to investigate the effect of polysaccharide proportions and AZC content on the strength of hydrogels and on the mechanical properties and moisture sensitivity of aerogels. The formation of crosslinks was observed indirectly from the values of storage and loss moduli by dynamic rheological measurement. The strength of hydrogels was highly dependent on the AZC and NFC content. In contrast, the compressive modulus of aerogels instead was affected only by NFC content at relative humidity (RH) of 50% and 23 °C. Hydrogel strength could not be used for prediction of aerogel strength under these ambient conditions. AZC and NFC mainly decreased the sensitivity of aerogels towards moisture by decreasing the water absorption and its plasticizing effect on aerogels. The effect of crosslinking on mechanical and physical properties of aerogels appeared clearly at RH over 50%. GGM was shown to be a suitable aero-gel raw material when combined with NFC. The role of NFC in enhancing the aerogel’s me-chanical properties was significant. The mechanical properties of uncrosslinked aerogels, how-ever, weakened in a humid environment. In particular, AZC is needed to protect aerogels from the plastizicing effect of water. Properties of crosslinked aerogels in a humid environment would be an interesting subject of further studies.
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(2017)The literature review of this master’s thesis dealt with polysaccharide based hydro- and aerogels and their preparation methods and characterization. TEMPO/laccase-catalyzed oxidation was also included in the literature review. The aim of the experimental part of this thesis was to prepare hydrogels by TEMPO/laccase-catalyzed oxidation and study how the oxidation changed the rheological and chemical characteristics of the polysaccharides. The hydrogels were also dried to produce hydrogels via lyophilization and their characteristics were determined. The degree of oxidation of the hydrogels was analyzed by the GC-MS method. The viscoelastic properties of the hydrogels were studied by oscillation measurements. The compressive modulus of the aerogels was determined by means of a compression test. The morphology of the aerogels was studied with the help of a scanning electron microscopy (SEM). The studied polysaccharides were arabinoxylan, glucomannan, galactomannan and xyloglucan. Due to the oxidation, a change in the viscoelastic behaviour could be seen between the native and oxidised polysaccharides. The rheological test revealed that the native polysaccharides were viscous liquids (G' > G'') and oxidized polysaccharides formed elastic hydrogels (G' > G''). The degree of oxidation of the hydrogels varied 3,3–11,7 %. The hydrogels were dried using two different freezing methods, unidirectional and conventional, and they were freeze-dried into aerogels without significant shrinkage. The density of the aerogels varied 0,017–0,030 g/mm3. The compressive modulus of the aerogels was 108‒1184 kPa depending on the polysaccharide. The SEM images revealed that unidirectional freezing resulted in pores that were parallelly oriented with the freezing direction. Unidirectionally frozen aerogels were stronger than conventionally frozen aerogels when the compression was oriented against the freezing direction. This was the first time that TEMPO/laccase catalyzed oxidation was used to prepare arabinoxylan and glucomannan hydro- and aerogels. The arabinoxylan and glucomannan aerogels were mechanically stronger than previously studied aerogels that have been prepared by enzymatic oxidation.
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