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- Elamin, Khalid, 1977, et al.
(författare)
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Calorimetric and relaxation properties of xylitol-water mixtures
- 2012
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 136:10
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Tidskriftsartikel (refereegranskat)abstract
- We present the first broadband dielectric spectroscopy (BDS) and differential scanning calorimetry study of supercooled xylitol-water mixtures in the whole concentration range and in wide frequency (10(-2)-10(6) Hz) and temperature (120-365 K) ranges. The calorimetric glass transition, T-g, decreases from 247 K for pure xylitol to about 181 K at a water concentration of approximately 37 wt. %. At water concentrations in the range 29-35 wt. % a plentiful calorimetric behaviour is observed. In addition to the glass transition, almost simultaneous crystallization and melting events occurring around 230-240 K. At higher water concentrations ice is formed during cooling and the glass transition temperature increases to a steady value of about 200 K for all higher water concentrations. This Tg corresponds to an unfrozen xylitol-water solution containing 20 wt. % water. In addition to the true glass transition we also observed a glass transition-like feature at 220 K for all the ice containing samples. However, this feature ismore likely due to ice dissolution [A. Inaba and O. Andersson, Thermochim. Acta, 461, 44 (2007)]. In the case of the BDS measurements the presence of water clearly has an effect on both the cooperative a-relaxation and the secondary beta-relaxation. The a-relaxation shows a non-Arrhenius temperature dependence and becomes faster with increasing concentration of water. The fragility of the solutions, determined by the temperature dependence of the a-relaxation close to the dynamic glass transition, decreases with increasing water content up to about 26 wt. % water, where ice starts to form. This decrease in fragility with increasing water content is most likely caused by the increasing density of hydrogen bonds, forming a network-like structure in the deeply supercooled regime. The intensity of the secondary beta-relaxation of xylitol decreases noticeably already at a water content of 2 wt. %, and at a water content above 5 wt. % it has been replaced by a considerably stronger water (w) relaxation at about the same frequency. However, the similarities in time scale and activation energy between the w-relaxation and the beta-relaxation of xylitol at water contents below 13 wt. % suggest that the w-relaxation is governed, in some way, by the beta-relaxation of xylitol, since clusters of water molecules are rare at these water concentrations. At higher water concentrations the intensity and relaxation rate of the w-relaxation increase rapidly with increasing water content (up to the concentration where ice starts to form), most likely due to a rapid increase of small water clusters where an increasing number of water molecules interacting with other water molecules.
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- Johansson, Nils, et al.
(författare)
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Behaviour of an intumescing system subjected to different heating conditions
- 2013
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Ingår i: Fire and Materials 2013 - 13th International Conference and Exhibition, Conference Proceedings. ; , s. 187-196
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Konferensbidrag (refereegranskat)abstract
- Previous studies have indicated that the expansion of fire seals and fire stoppers will be dependent on the heat exposure. Standardized methods for classification includes exposure to a rapid heat exposure of the product that is tested, but this might not always be the worst case scenario for the product. In this paper a series of tests are presented in order to study how a graphite based intumescing system, intended for cavities, reacts when subjected to fire conditions different form that in the standardised method EN 1366-4. Two different experimental setups, a cone calorimeter at Lund University and a small-scale furnace at SP Fire Technology, were used in the tests. The start of expansion reaction in the tested fire stopper was around 180°C. The start of reaction temperature is rather independent of the incident heat flux, while expansion rate is clearly dependent on the incident heat flux. Furthermore, the studied fire stopper performed well in the small-scale furnace because the graphite expanded enough to give the same protection, although it is preheated or wet, as when subjected to a heat exposure similar to ISO 834. These results are good but they are only considered valid for the studied system and for the given situation. Future research is needed in order to study how other types of intumescing fire protection systems work when subjected to realistic fire environments.
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