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When silicate rock or flint is melted (1650° C)
with a small quantity of calcareous matter and
the liquid is blown by steam jet (Fig.15.3), it
splashes out in the form of small globules which
are hurled in a large container at a great speed to
cause them to be drawn into very soft and flexible
fibres of not more than 10 micron diameter. The
source material is glass bottle waste melted at
1300–1400° C temperature. Also known as rock
wool it can be packed into small pads or formed
into (5–6) boards or blankets. The fibers are
chemically inert and have small apparent
densities (130–240 kg/m^3 ) and a low coefficient
of heat conductivity (0.43 to 0.78 mW/cm° C).
Glass wool has high tensile strength and chemical
resistance and, low sound and heat conductivities.
Water absorption should not exceed 0.5%. They
contain air in the pores forming a useful filter
media for air conditioners and an insulating
material for heat and sound. Glass wool may
also be used as an aggregate in the manufacture of asbestos-cement items and as a fine
aggregate for plastering and finishing mortars. Glass wool is also used to produce glass plastics
by mixing it with polymers.
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The main constituent of ceramic materials is silicate: Portland cement, glass, tiles, vitreous
enamels, reinforcing glass fibres, etc. The primary unit of silicates is the SiO 4 , tetrahedron, in
which one silicon atom fits between four oxygen atoms with either ionic or covalent bond
mechanism.
The tetrahedral units of silica are arranged in a hexagonal pattern at room temperature. But
at 875° C the stable arrangement of silica-tetrahedral breaks to a new structure of a hexagonal
pattern. Above 1940° C a third stable solid arrangement of tetrahedral in cubic pattern is
formed. These three modifications are named as quartz — the low temperature mineral;
tridymite — the intermediate temperature mineral and cristobalite — the high temperature
mineral. However, quartz may exist as metastable phase at high temperature and tridymite
and cristobalite may remain as metastable phases at room temperature.
A second type of inversion which may occur in silica is shown in Fig. 15.4. At 573° C the
straightening of bonds across the oxygen takes place. This change which is rapid and
spontaneous produces instantaneous volume expansion. As the quartz cools the process is
reversed. Figure 15.4 represents thermal expansion of quartz. Tridymite shows two such
changes in its metastable range, one at 117° C and the other at 163° C, whereas cristobalite
undergoes a change similar to that of quartz at 200° C.
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