Browsing by Author "Nenkova S."

Now showing 1 - 7 of 7
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    Adsorption of Ag+ ions on hydrolyzed lignocellulosic materials based on willow, paulownia, wheat straw and maize stalks
    (2016-05-01) Vassileva P.; Radoykova T.; Detcheva A.; Avramova I.; Aleksieva K.; Nenkova S.; Valchev I.; Mehandjiev D.
    In the present work, the adsorption of Ag+ ions on hydrolyzed plant biomass (willow, paulownia, wheat straw and maize stalks) was investigated. Chemical analyses were performed to establish the composition of the obtained materials. Adsorption mechanism, adsorption sites and specific surface areas of these materials were examined by BET analysis, IR spectroscopy, XPS and EPR. The effects of contact time, acidity of initial solutions and Ag+ ion concentrations were followed. Pseudo-first-order, pseudo-second-order and intra-particle diffusion models were used to analyze kinetic data. In all cases, the adsorption was significantly affected by the pH value. Different types of adsorption isotherms of Ag+ ions (either Langmuir or Freundlich) were registered depending on the adsorbing material. The adsorption mechanism is complex, and the process passes through different stages as clustering of Ag+ ions and formation of elemental Ag. The maximal adsorption capacities varied from 2.05 to 6.07 mg g−1. The obtained results revealed that the examined waste lignocellulosic materials are promising adsorbents for Ag+ ions.
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    Black liquor lignin products, isolation and characterization
    (2013-11-05) Radoykova T.; Nenkova S.; Valchev I.
    By extraction with toluene and ethyl acetate monomeric lignin compounds: 2-methoxyphenol, 2,6-dimethoxyphenol, 4-hydroxy-3-methoxybenzaldehyde, 4-hydroxy-3,5-dimethoxybenzaldehyde, 1-(4-hydroxy-3-methoxyphenyl) ethanone, 1-(4-hydroxy-3,5-dimethoxyphenyl) ethanone, etc. were isolated from black liquor and sulfate lignin, and identified through GC/MS analysis. The identified low-molecular mass lignin compounds do not contain organic bonded sulphur. The sulfate lignin, separated from black liquor after acidification with acid was subjected to an alkaline treatment (4 h; 180oC; 5 % NaOH; lignin/NaOH ratio -1:8).
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    Methylcellulose/SiO2 hybrids: Sol-gel preparation and characterization by XRD, FTIR and AFM
    (2011-03-21) Rangelova N.; Radev L.; Nenkova S.; Salvado I.M.M.; Fernandes M.H.V.; Herzog M.
    Methylcellulose (MC)/SiO2 organic/inorganic hybrid materials have been prepared from MC and methyltriethoxysilane or ethyltrimethoxysilane, and characterized by XRD, FTIR and AFM. XRD showed peak shifts. FTIR shows intermolecular hydrogen bonding between MC and SiO2. AFM depicts surface roughness which depends on the silica precursor and MC content. © Versita Sp. z o.o.
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    Polycarbonate diols to produce elastic polyurethane foams - A method of immobilization of carbon dioxide into a polymer structure
    (2017-01-01) Dimitrov K.; Todorova D.; Nenkova S.; Herzog M.
    Using carbon dioxide as feedstock for polymers is a big challenge and a chance for our sustainable future. It has an immense potential for the coming decade, which comes much faster than expected. Scientists are very active lately in carbon dioxide chemistry research, especially in the field of carbon dioxide based chemicals and polymers. We report here on the stability of higher molecular weight polycarbonate diols and the formation of specialty polyurethanes such as UV stable elastomers, semi-rigid foams, and elastic sealants. These polyurethanes are produced from polycarbonate diols (ETERNACOLL®) and aromatic 4,4'-diphenyl-methane diisocyanate, from aliphatic hexane-1,6-diisocyanate, or the trimer, diisocyanates. We use butane-1,4-diol or pentane-1,5-diol as a chain extender. The thermal and mechanical properties of the polyurethanes obtained are determined by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The data obtained show no phase segregation or any type of crystallinity. Any possible changes in the molecular mass distribution of the samples are investigated with the application of gel permeation chromatography (GPC). It is evidenced that only heat treatment brings about molecular mass distribution shifts to lower values.
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    Preparation, Characterization, and Antibacterial Properties of Cu-Fibreboards
    (2023-11-01) Aleksandrov L.; Rangelova N.; Lazarova-Zdravkova N.; Georgieva N.; Dragnevska M.; Nenkova S.
    In the present study, copper modified fibreboards were prepared and their existing phase, morphology, and antibacterial behaviour were investigated. The copper content and the physical and mechanical properties of fibreboards (thickness, bending strength, and swelling) were determined. X-ray diffraction analysis (XRD) showing diffraction peaks typical for cellulose, Cu2S, and Na2SO4, depended on the preparation conditions. The average size of the Cu2S crystals varied between 20 and 50 nm. The morphology of the obtained fibreboards, as well as the size and shape of copper particles, were observed by scanning electron microscopy (SEM) and transition electron microscopy (TEM). The antibacterial activity was tested against Gram-positive (Bacillus subtilis 3562) and Gram-negative (Escherichia coli K12 407) bacteria. The tests showed that the materials had higher antibacterial activity against E. coli, which depended on their preparation conditions. Based on these results, the obtained copper fibreboards can be used as antibacterial agents in the packaging and building industry.
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    Shape memory polyurethanes based on recycled polyvinyl butyral II. mechanical and shape memory properties
    (2014-01-01) Tsonev T.; Nenkova S.; Herzog M.
    Shape memory polyurethanes (SMPUs) based on recycled polyvinyl butyral and 4,4'-diphenylmethane diisocyanate, hexane-1,6-diol, and polypropylene glycol of varying hard segment content were investigated. Their mechanical properties and shape memory effect in particular were examined defining the amorphous soft segment domain as a reversible phase, while the crystalline hard segment domain as a frozen phase. Effects of hard segment content (30-45 %) and maximum strain on the cyclic tensile and mechanical properties below (25°C) and above (70°C) material glass transition temperatures were studied. The properties reversibility observed on repeated deformation was improved by decrease of the deformation magnitude and the increase of the hard segment content.
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    Shape memory polyurethanes based on recycled polyvinyl butyral. I. Synthesis and morphology
    (2013-10-17) Tsonev T.; Herzog M.; Nenkova S.
    Shape memory polyurethanes (SMPUs) were synthesized by 4,4′- diphenylmethane diisocyanate (MDI), hexane-1,6-diol (HD), polypropylene glycol (PPG), and recycled polyvinyl butyral (PVB). Dynamic mechanical analysis, differential scanning calorimetry and Fourier transformation infrared attenuated total reflection spectroscopy was used to characterize the poly (vinylbutyral-urethanes). Micro-phase domain separation of hard and soft segments and phase inversion were investigated. Increasing the hard segment content, i.e., average hard segment molecular weight, leads to an increase in the degree of micro-phase separation, hard domain order and crystallinity. The crystalline hard segment structures combined with the elastic nature of soft segment matrix provide enough physical and chemical crosslinks to have shape memory effect. © 2013 Versita Warsaw and Springer-Verlag Wien.