The Fire Statistics Monitor (a body within Communities and Local Government) estimates that 312 people are killed and 7,400 are injured in fires in England alone (April ’10 to March ’11). In 2008 the economic cost of fire damage was estimated at £8.30 billion.212 The majority of these deaths, injuries and losses occur in buildings where fire and smoke protection measures are inadequate. The installation of effective fire resistant glasses can save lives and reduce the costs associated with fire damage. Several global manufactures produce complex fire products that are installed in the commercial and industrial sector. These are clear materials, which are laminated structures comprising a transparent sodium-silicate/polyol hydrogel interlayer sandwiched by sheets of ordinary float glass:
Figure 1.13 Images of the product before and after a firing test. From a front view, the product appears to be ordinary glass but tilted on its side, one can see the hydrogel/float glass laminates making up the product.
The mechanism of fire protection is based on the foaming and swelling of the hydrogel when exposed to temperatures in excess of 100°C, thereby producing endothermic cooling and physically impeding the transverse transfer of radiant heat, smoke and fire through the structure. The advantage of this product range over other fire resistant materials is that it stops the spread of flames, hot gases and heat and can offer up to 120 minutes of integrity and insulation during a fire.
1.13.1 Temperature Regimes
Figure 1.14 represents the structural and visual changes that occur within the hydrogel as a function of temperature. There are essentially two temperature regimes.
1. High temperature: excess of 150°C induces the fire response as mentioned above
2. Lower temperature regime, 40-100°C leading to crystallisation of the hydrogel into makatite. The crystallisation process generates a hazing throughout the product that becomes progressively worse with time. The rate of crystallisation is dependent upon temperature and the composition of the hydrogel. More importantly the crystallisation process is detrimental to the performance of the product.
Figure 1.14 Effect of temperature on the structural changes occurring within the hydrogel interlayer and corresponding visual observations
The formation of an organic-inorganic hydrogel is a complex process requiring a careful control over synthesis conditions. The hydrogels are amorphous, structurally heterogeneous and show a lack of long range order, which precludes the use of standard diffraction techniques making their
characterisation more challenging. Solid-state NMR has shown to be an ideal technique in the study of amorphous systems and is therefore the primary spectroscopic tool used throughout the project. The overall aim of the project is to characterise the local environment of the hydrogel and correlate this to the products longevity and thermal stability. In order to achieve this goal, the project has been divided into four areas:
(1). molecular characterisation of the materials
(2). investigation into the crystallisation of makatite as a result of thermal treatment
(3). methodologies to improve the product i.e. retardation of makatite formation
(4). insight into makatite formation and interlayer reactivity via the synthesis of a makatite/glycerol hydrogel
In chapter 4 the molecular level characterisation of a series of hydrogels with different weight ratios are investigated by means of 1D MAS NMR experiments. This includes a detailed analysis of the local structure of silicon, sodium and proton environments. As water is a major constituent in the hydrogels, variable temperature relaxation methodologies are employed to characterise water mobility and domains. The influence of ethylene glycol or glycerol on directing properties on the molecular level is investigated. The homogeneity of glycerol is also studied using spin-lattice relaxation measurements. The results are used to generate structural models for the systems and allow for a greater understanding of the materials.
The formation of makatite is detrimental to the performance of the product, therefore it is important to understand the mechanism of its formation in the potentially motionally restricted hydrogel environment. The crystallisation of makatite from the hydrogel with or without polyol is followed using optical measurements and X-ray diffraction techniques. With the aid of
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H-29Si CP MAS, 23Na MQMAS and variable temperature heteronuclear correlation NMR experiments we aim to show the change in silica and sodium speciation, the thermal instability of the ethylene glycol hydrogel over the glycerol counterpart and relate this to differences in crystallisation rates.
Chapter 6 deals with methodologies to inhibit makatite nucleation and growth by modifying the starting sodium-silicate solutions. A synthetic analogue to one of the commercial solutions is prepared and dried to form a hydrogel, with and without glycerol. The synthetic system greatly inhibits makatite formation. The constitution of the synthetic and commercial solutions are analysed using 29Si NMR and Lentz-GC analysis. The local environments of the hydrogels are investigated using 1H-29Si CP MAS and
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Na MQMAS NMR experiments. A discussion follows as to possible reasons as to why the synthetic systems significantly inhibits makatite nucleation and growth.
In the final chapter we ‘reverse’ the hydrogel process, that is we synthesise a makatite hydrogel containing the same water and glycerol contents to those in the initial amorphous based hydrogel. The local structure of this composite is compared to the initial hydrogels as to gain a deeper understanding between the interactions occurring between silica, water and glycerol. On a separate theme, we also demonstrate the difficulty in synthesising makatite hydrothermally and its resilience towards intercalation chemistry.