GLASS FIBRES
Glass fiber is one of the versatile fibers and
possesses many unique properties with divergent applications, which other
fibres cannot have. Glass is a man-made fibre and can be manufactured either in
filament form or in staple form.
RAW MATERIALS OF GLASS FIBRE
The principal ingredient of
glass fibre is silica or silicon dioxide. However, depending upon the end-use,
other materials like lime, magnesia, alumina, soda, potash or boric oxide are
used. Based on this, there are three types of glass fibre. Those are :
- Alkali type Glass or Glass 'A' (A = Alkali)
- Electrical type Glass or Glass 'E' (E = Electrical)
- Chemical type Glass or Glass 'C' (C= Chemical) %3D
The approximate compositions
of different type of glass, fibers are shown in Chemical
Compositions of Glass Fibres
Glass A Glass E Glass
C
- Silica% 72 52-56 (%) 62-65
- Lime 10 16-25 6
- Magnesia 3 0.6 -
- Alumina 2 12-16 1
- Soda 13 0.1 11-15
- Potash - 0.1 1.3
- Boric oxide - 8-13 3-4
The principal ingredient
i.e., silicon dioxide is generally obtained from sands or sandstones. The
second important ingredient is lime, which is used for a stabilizing influence.
Magnesium oxide has similar effects. Alumina is usually added to improve the
strength, durability, and resistance
to weathering. The other ingredients are
generally added to improve the end-use applications. Sometimes zinc oxide is
used to improve acid durability. The addition of titanium oxide reduces the
viscosity of the melt. Barium oxide improves weathering characteristics and
also it increases the melting rate.
FIBRE FORMATION OF GLASS FIBRE
The fibre formation from
glass generally consists of the following processes.
(a) Preparation of the glass
marbles
(b) Melting and Extrusion of
the glass
(c) Filament or staple fibre
formation.
PREPARATION OF THE GLASS MARBLES
PREPARATION OF THE GLASS MARBLES
The raw materials as per are mixed in a mixer. The exact amount of
feeding can be done by means of weighing hoppers, which will transport the
predetermined quantity of the raw material to the mixer. The mixer simply mixes
all the materials homogeneously and uniformly. After mixing, the chemicals are
generally transferred to a melter. In the melter, all the chemicals are melted.
The molten materials are then converted into marble form and solidified.
The diameter of the marbles
is approximately 2 cm. Before further processing, the marbles are generally
inspected to check the defects, which may interrupt subsequent processing.
These marbles are then transferred to the spinning hopper.
MELTING AND EXTRUSION
The marbles are present in
the hopper transferred to the spinning unit by means of transfer pipe. The
marbles are then melted in an electrical furnace. The temperature of the
furnace is in the range of 800°C or above depending upon the type of the glass
to be produced. The molten material is extruded through small orifices for the
thread formation.
FILAMENT OR STAPLE FIBRE
FORMATION
Glass is a supercooled
liquid and thus is totally amorphous. The fibre can be drawn and/or stapled
without much difficulty. The fine stream of liquid extruded through the
spinnerette is generally deformed or drawn and collected in a winding tube after
proper size application. The speed of the take up is around 1000 meters/minute.
The filament thus formed can be further twisted and plied for their end uses.
For staple fibre formation, the molten glass streams are converted into a staple
fibre below the spinnerette by means of high-pressure air.
The air blower pulls the
glass streams into fibres. The fibres are then collected together over a
revolving drum. From the drum, a thin veil of fibers is pulled out just like
pulling of carded webs from the doffer. The strand of fibres can be converted
in sliver form by means of a ring guide and collected on a winding speed. The
sliver can further be drafted, twisted like other fibres.
STRUCTURE OF LASS FIBRE
The chemical structure of the
fibre is shown. Glass fibres are formed from the complex mixture of silica’s,
oxides of sodium, potassium, calcium, aluminum, magnesium and other salts in
varying composition. Owing to this, the structure of the glass fibre is more
complex. The cation of silicon and oxygen ion consists of a network structure,
where the silicon cation is surrounded by four oxygen ions. These are arranged
in a tetrahedron with a cache of the four oxygen atoms at a corner and identical
with the corner of the adjoining tetrahedron. In this manner, a continuous
network structure is formed with only silicon and oxygen. In spite of this, the
structure of the glass is of an irregular network having holes or interstices in it.
The holes are filled with
other captions like calcium, sodium, potassium, etc as shown in fig These
cations have relatively large ionic radii with small charges in comparison with
the radius value of 0.14 micron of oxygen and 0.04 of silicon. The properties
of the glass fibre continuously change with temperature because of these
cations although glass fibres do crystallize and do not have any characteristic
transitions. The presence of more cations decreases the value of the Si:O ratio.
In a
pure silica, the ratio is 0.5. Further it can be reduced to 0.25 if an equal
amount of cation is introduced in glass structure i.e., equal amounts of silica
and other oxides. A hard glass generally approaches to high Si:O ratio of 0.5
and it forms a pure polymeric chain of silicon bridging oxygen. On the other
hand, a soft glass will have a low Si:0 ratio with more amount of non-bridging
oxygen. In general, a high Si:O ratio is generally viewed as a high degree of
polymerization and exhibits high softening temperature with a low coefficient
of thermal expansion.
PHYSICAL PROPERTIES OF GLASS FIBRE
The properties of the glass
fibre change as per the chemical composition or Si:O ratio. The fibre, is
extremely dense, having a density of 2.5 to 2.6. The density of glass C is slightly
higher than that of glass E. The fibre has no affinity to water and so the
moisture regain of the fibre is maximum 0.5% or lesser. The fibres are
extremely strong. The strength depends upon both the composition and the method of production. Fibers
with boric oxide or borosilicate type of glass fibres i.e., glass E is the
strongest fibre.
The tenacity in the dry state
varies from 6 to 10 g/d, which reduces to 5 to 8 g/d when wet. The breaking
elongation is only 3 to 4% but is perfectly elastic up to their breaking point.
Glass fibres are quite stiff, brittle, break on bending and so exhibit poor
abrasion resistance. The fibre softens, melts and does not burn upon heating.
The fibre can be used continuously up to 500°C. The fibre becomes slightly brash
and embrittled when passed over metal surfaces in the temperature range of
450°C - 480°C. The melting point of the fibre is around 750°C. The electrical
resistance of the fibre is very high. The fibre exhibits excellent electrical
insulation properties.
CHEMICAL PROPERTIES OF GLASS FIBRE
The chemical properties of
the glass fibre depend upon their composition. The fibres are chemically inert
to oxidizing agents, biological agents, heat, and sunlight under normal
conditions. Alkali containing fibres are less resistant to weathering, have
lower insulation resistance and dielectric strength than glass 'E'. However, most of the mineral acids like hydrofluoric, hydrochloric, sulphuric and
phosphoric acids attack glass fiber. Also, hot solutions of weak bases and cold
solutions of strong bases deteriorate the fibre. The fibre exhibits excellent
corrosion resistant behavior. It is difficult to dye glass fibre.
APPLICATIONS OF GLASS FIBRE
High tensile strength, low
moisture absorption, higher utilization temperature, non-compatibility, high
heat conductivity, better electrical resistance, higher corrosion resistance,
better drapability, all contribute to using of this fibre extensively in
furnishing fabric and industrial fabrics. These fibres are used in electrical the industry as the insulating material, fiberglass reinforced plastics for trucks and
car bodies, thermal insulating materials, tire cord, industrial filters, protective
clothing, decorative materials like curtains, and draperies, protective
clothing against radiation, defense equipment, shipbuilding, etc.
